eth diss 12675

yieval, pho-right

DISS. ETH No. 12675

CMOS Microsystems forThermal Presence Detection

A thesis submitted to theSWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH

for the degree ofDOCTOR OF NATURAL SCIENCE

presented by

NIKLAUS WERNER SCHNEEBERGER

Dipl. Phys. ETH ZurichBorn December 24, 1967

Citizen of Schoren bei Langenthal, Bern, Switzerland

accepted on the recommendation of

PROF. DR. H. BALTES, examinerDR. U. DILLNER, co-examiner

PROF. DR. O. PAUL, co-examiner

1998

Copyright © 1998 by Niklaus Schneeberger, Physical Electronics LaboratorAll rights reserved. All parts of this book may be reproduced, stored in a retrsystem, or transmitted, in any form or by any means, electronic, mechanicaltocopying, recording or otherwise, without the written permission of the copyholder, if properly referenced by name and institution.

array

f a

Published by:

Physical Electronics LaboratorySwiss Federal Institute of Technology (ETH) ZurichETH-Hoenggerberg, HPT H6CH-8093 ZurichSwitzerland

Printed in Switzerland

Cover: Foreground, thermal image of a person aquired with an infrared sensormicrosystem (cf. chapter 6). Background, scanning electron micrograph omicromachined sensor array (cf. chapter 3).

Contents

5

30

8

5

Contents

Contents 4

Abstract 6

Zusammenfassung 7

1 Introduction 8

1.1 Infrared Radiation 9

1.2 Infrared Sensor Types 10

1.3 Applications 13

1.4 CMOS Fabrication of IR Sensors 1

1.5 Previous Art 15

1.6 Outline of the Thesis 17

2 Theory 18

2.1 Infrared Radiation 18

2.2 Sensor Principle 23

2.3 Heat Transfer 25

2.4 Seebeck Effect 27

2.5 Imaging Optics 28

2.6 Sensor Characteristics and Figures of Merit

3 Device Fabrication 35

3.1 CMOS Processes 35

3.2 Post-Processing Method 3

3.3 Fabricated Device Types 44

3.4 Post-Processing Restrictions 6

4

85

4 Characterization 73

4.1 Sensitivity Measurements 73

4.2 Array Characterization 83

4.3 Spectral Absorptance Measurements

5 Modeling 100

5.1 Analytical Model 103

5.2 Variational Model 107

5.3 Finite Element Model 108

5.4 Comparison of Models 112

5.5 Device Optimization 112

6 Demonstrators 117

6.1 Presence Detector 117

6.2 Thermal Imager 121

7 Summary and Outlook 126

Appendix 128

References 128

Acknowledgments 136

Curriculum Vitae 138

List of Abbreviations 139

5

Abstract

6

Abstract

In this thesis we report the development, fabrication, characterization, and model-ing of thermoelectric infrared sensor microsystems. These devices are intendedfor the detection of the presence of persons by means of the emitted infrared radi-ation.

The presented microsystems consist of dedicated signal conditioning circuitrycointegrated on a chip with a pair or an array of sensors. The microsystem chipsare fabricated in a commercial CMOS process. The sensor devices consist of ther-mopiles integrated in a thermally isolated structure made from the dielectricCMOS layers. Thermal isolation is achieved by removing silicon under the sensorstructure using bulk-micromachining after completion of the industrial CMOSprocess. The sensor structure absorbs incident infrared radiation and thereforeheats up. The resulting temperature increase is converted into an electrical signalby thermopiles. The thermopiles consist of two of the conducting CMOS layers,namely, n

+

-polysilicon/aluminium, p

+

-polysilicon/aluminium, or n

+

-polysili-con/p

+

-polysilicon. This approach allows mass production and cointegration ofsensors and of state-of-the-art signal conditioning circuitry in a mature low-costtechnology.

We fabricated devices with sizes of the sensitive area ranging from 0.015 mm

2

to1.05 mm

2

. Sensitivities of 45.8 V/W and normalized detectivities of 6.69 10

7

cm

√

Hz/W were achieved. Array microsystems with up to 240 pixels and anon-chip low-noise amplifier were fabricated and integrated in a demonstratorsystem for thermal imaging. With an array of 100 pixels we demonstrated a noiseequivalent temperature difference of 715 mK on the target with a signal band-width of 50 Hz, and a frame rate of 0.49 Hz.

We developed models of the sensor performance using finite element analysis.Comparison of modeled and measured sensor performance shows a deviation of21% in the worst case. These models allow the optimization of sensor designs forthe intended application prior to fabrication.

7

Zusammenfassung

Diese Dissertation befasst sich mit der Entwicklung, Herstellung, Charakterisie-rung und Modellierung von thermoelektrischen Infrarotsensor-Mikrosystemen.Diese Systeme detektieren die Anwesenheit von Personen anhand der emittiertenInfrarot-Strahlung.

Die vorgestellten Mikrosysteme bestehen aus einem Sensor-Paar oder einem Sen-sor-Array das zusammen mit geeigneter Schaltungen zur Signalverarbeitung aufeinem Chip integriert wurde. Die Chips mit den Mikrosystemen werden in einemkommerziellen CMOS-Prozess hergestellt. Die Sensoren bestehen aus Thermo-säulen die in einer thermisch isolierten Struktur aus den dielektrischen Schichtendes CMOS-Prozesses eingebettet sind. Thermische Isolation wird durch Mikro-strukturierung anschliessend an den CMOS-Prozess erreicht. Einfallende Infrarot-strahlung wird in der Sensorstruktur absorbiert die sich dadurch erwärmt. Derresultierende Temperaturanstieg wird durch die Thermosäulen in ein elektrischesSignal verwandelt. Die Thermosäulen bestehen aus einem Paar der elektrisch lei-tenden Dünnschichten des CMOS-Prozesses, nämlich n

+

-Polysilizium/Alumi-nium, p

+

-Polysilizium/Aluminium oder n

+

-Polysilizium/p

+

-Polysilizium. DieseHerstellungsmethode ermöglicht die Massenproduktion und Integration mitmodernen Signalverarbeitunsschaltungen.

Die Sensoren haben eine sensitive Fläche von 0.015 mm

2

bis 1.05 mm

2

. Eswurden Empfindlichkeiten bis 45.8 V/W und normalisierte Detektivitäten bis6.69 10

7

cm

√

Hz/W erreicht. Sensor-Arrays mit bis zu 240 Pixeln und integrierterVerstärkerschaltung wurden hergestellt und in ein Wärmebild-Demonstrationssy-stem eingebaut. Mit einem Array von 100 Pixeln wurde eine rauschequivalenteTemperaturdifferenz am Objekt von 715 mK bei einer Signalbandbreite von50 Hz und einer Bildwiederholrate von 0.49 Hz erreicht.

Wir haben Finit-Element-Modelle der Sensoren zur Vorhersage der Empfindlich-keit entwickelt. Der Vergleich zwischen gemessener und simulierter Empfind-lichkeit ergab im schlechtesten Fall 21% Abweichung. Diese Modelle ermögli-chen eine Optimierung der Sensoren für die geplante Anwendung vor der Herstel-lung.

1 Introduction

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1 INTRODUCTION

Integrated circuits (IC) are ubiquitous in modern life. They perform various tin credit cards, computers, cars, and communication networks. This has bepossible due to the drastic drop of the price-to-performance ratio of silicon Ithe last three decades, enabled by batch fabrication, integration, and miniation. In the late sixties, silicon IC technology has been expanded by silicon mmachining technology, i.e., ways to selectively structure the IC materials.added new, mechanical [1] and thermal [2] functionality to previously purely etrical devices; hence the current term “micro electro mechanical syste(MEMS). A large variety of such systems is fabricated and investigated umethods borrowed from silicon IC technology [2-4]. These methods enablefunctionality at a low device cost through miniaturization and mass producThe devices presented in this thesis were fabricated in commercial IC produfacilities with subsequent post-processing (IC MEMS) [5]. This approach lithe choice of available process steps, materials, and layer thicknesses. Hoit builds on a mature low-cost technology and benefits from the possibility to tegrate state-of-the-art circuitry with the sensors [6-10]. This improves pemance and minimizes system size and cost [11]. Such integrated MEMS syare called iMEMS or CMOS MEMS.

This thesis presents the development of thermoelectric infrared sensor mictems fabricated using commercial CMOS processes with subsequent micchining. These microsystems comprise infrared sensor pairs or sensor arraytegrated with dedicated signal conditioning circuitry on a single chip. Theyintended to detect the presence of a person in their vicinity by means of its inradiation. This application is generally referred to as intrusion, motion, or pence detection. Most intrusion detectors applied today sense only moving peand thus, are also called motion detectors. In contrast, the presence detectems presented in this thesis also sense a person at rest. Intrusion detector comprising a two-dimensional array of sensors allow to obtain an infrared imand are referred to as thermal imagers.

8

1.1 Infrared Radiation

trumergiesatmo-1 µm, aren, andrrorsr, alu-n of

isibleingy andal andd typ- forlarly appli-


Infrared (IR) radiation is the part of the electromagnetic radiation specextending from 0.8 µm to 1000 µm wavelengths. The respective photon enrange from 1.5 eV to 1.2 meV. Figure 1.1 shows the IR transmittance of the sphere at sea level over a distance of 2 km. The bands from 0.8 µm to 1.3 µm to 1.8 µm, 2 µm to 2.6 µm, 3 µm to 5.5 µm, and from 8 µm to 14 µmtransparent to IR radiation. Some solid materials such as germanium, silicozinc selenide, transmit IR radiation and allow the fabrication of IR lenses. Mifor IR radiation are easily accessible since many metals such as gold, silveminium, and copper are excellent IR reflectors. This allows the constructioimaging optics for IR cameras and imagers similar to their analogues for vlight. While reflected light is used for imaging in the visible range, IR imagrelies on the thermal radiation emitted by the imaged objects. The intensitspectral composition of the emitted thermal radiation depends on the materiits temperature. At room temperature the highest radiation intensity is emitteically in the band from 7 µm to 14 µm. Thermal radiation can be exploitedimaging, even in the absence of illumination. This makes IR vision particuattractive where the observer wants to be unnoticed such as in surveillancecations.

Fig. 1.1: Atmospheric transmittance over 2 km at sea level [12].

9

1 Introduction

es. Inrtant

mea-

in

d thusmplesalized

e high corre-r longtron3]. To

n, the

rbingergy is

1.2 Infrared Sensor Types

Two main types of IR sensors are distinguished: photonic and thermal devicthe following the two types and their performance are summarized. An impofigure of merit of IR sensors is their normalized detectivity D*. It describes thesignal-to-noise ratio that can be obtained at a given radiation intensity and issured in cm√Hz/W. A more detailed definition and explanation is givensection 2.6.

Photonic sensors

In photonic sensors IR photons excite electrons to higher energy states anmodulate some electronic property of the sensor. The most important exaare listed in table 1.1. Photonic sensors are inherently fast and have a norm

detectivity in the range of 1010 to 1012 cm√Hz/W. Their sensitivity stronglydepends on the photon energy. The energy of an incident photon must benough to excite an electron to an available state. For longer wavelengths,sponding to lower energy photons, the sensitivity decays rapidly to zero. Fowavelength applications low-band-gap detector materials with a low elecexcitation energy are applied such as InSb, PbSe, PbS, and doped silicon [1avoid large background signals and noise due to thermal carrier excitatiosensor has to be cooled to cryogenic temperatures.

Thermal sensors

Thermal sensors are “two-stage transducers”, including a radiation absomaterial and a temperature measurement. In the first stage the photon en

Type Excited state Sensor property

Photodiode electron-hole pair photo-current

Photoconductor mobile electron resistance

Charge coupled device tunnelling electron gate charge

Tab. 1.1: Popular types of photonic sensors.

10

1.2 Infrared Sensor Types

rature existsimizelatedughly

e ther-ermal

ectralorber.over a

bsorber.ermal

orbss heatsn pres-ge of

pres-ng and

pendenta biaseters

converted to thermal energy in the absorber, which heats up. This tempechange is converted to an electrical signal by the second stage. A trade-offbetween the sensitivity and response time of thermal sensors [14]. To maxsignals for a given radiation power the absorbing structure is thermally isofrom the ambient. On the other hand, the response time of the device is roproportional to the product of the thermal conductance to the ambient and thmal mass of the absorber. Shrinking the device dimensions reduces the thmass. Thus miniaturization is very attractive for thermal IR sensors. The spsensitivity of thermal sensors is entirely defined by the properties of the absSpecialized absorbers are available with a uniform spectral absorptance wide wavelength band [15].

The second stage of the device measures the temperature increase of the aDepending on which physical property is exploited for the measurement, thsensors are grouped in the types listed in table 1.2.

In a Golay cell, gas is confined in a fixed volume [16]. Either the gas itself absthe radiation or it is in thermal contact with an absorber. In both cases the gaup when the cell is irradiated and the pressure in the volume increases. Thesure is measured. The normalized detectivity of Golay cells is in the ran109 cm√Hz/W, but the cells are highly sensitive to sound and atmospheric sure variations [18,19]. These problems are addressed by a very rigid housiventing the gas cell through a small leak [20].

In a bolometer temperature changes are measured via the temperature deresistance of a resistor that is in thermal contact with the absorber. Usually current is applied to the resistor and the potential drop is measured. Bolom

Type Measured property

Golay cell gas pressure

Bolometer electrical resistance

Pyroelectric sensor electrical polarization

Thermoelectric sensor Seebeck voltage

Tab. 1.2: Types of thermal IR sensors

11

1 Introduction

ateri-

largedetec-

e. This

riza-rature elec-ffer-ts flow Thus, obtain

-apsu-

etrics mea-A ther-

sink atrber asp to a

ials.eebecke heat

with sizes down to 40 µm have been fabricated. By operating specialized mals, e.g., VO2 or superconductors, close to a phase transition temperature,temperature coefficients of resistance are obtained. Resulting normalized tivities are in the range of 109 cm√Hz/W [21]. Bolometers always have a largoffset due to the potential drop over the resistor in the absence of radiationissue can be addressed by, e.g., a Wheatstone bridge.

In pyroelectric sensors, a film of material with a spontaneous electrical polation is in close thermal contact with the absorber. The polarization is tempedependent. Its direction is usually perpendicular to the film. This results in antrical potential difference between top and bottom faces of the film. This dience is measured by electrodes on the two faces. However, leakage currenbetween the two electrodes and tend to equalize the potential difference.pyroelectric sensors are insensitive to steady-state (dc) radiation signals. Toa defined decay behavior a high ohmic resistor (GΩ) is used as an intentional leakage path. Uncontrolled leakage due to humidity is avoided by hermetic enclation. Normalized detectivities of 108 to 109 cm√Hz/Ware typical [22].

Some authors use the term “thermoelectric” also for pyroelectric and bolomsensors. For distinction, we use it only for sensors based on thermocouplesuring the temperature difference between the absorber and the ambient. mocouple consists of two stripes of different conducting materials A and B extend-ing from the absorber to the supporting structure. The latter acts as a heat ambient temperature. The two conductors are connected near the absoshown in fig. 1.2. This is called the hot contact. When the absorber heats utemperature T2, while the heat sink is at T1, the so called Seebeck voltage UCarises. It is given by

, (1.1)

where α is the relative Seebeck coefficient of the two thermocouple materSeveral thermocouples can be connected in series to add their respective Svoltages. This is called a thermopile. The thermocouples are connected on th

UC α T1 T2–( ) α T∆==

12

1.3 Applications

esce the isola-

motetionsnsors, andonicsompo- mayon the

les con-

sink to form the so-called cold contacts. The voltage UT of a thermopile contain-ing N thermocouples is given by

. (1.2)

Normalized detectivities of 108 cm√Hz/W have been reported [23]. Thermopilare inherently offset-free and capable of measuring dc signals. However, sinthermocouples connect the absorber and the support, they limit the thermaltion of the absorber.

1.3 Applications

There are mainly two motives to apply detectors for thermal radiation: Retemperature measurement and vision without visible light [16]. In all applicathe detector system contains at least the following parts: Imaging optics, seelectronics, signal output, and housing. The optics collects the radiationfocuses it on the sensors which convert it into electrical signals. The electrprocesses the signal and feeds it to the output. The housing supports the cnents and protects them from environmental influences. Additional functionsbe necessary such as cryogenic cooling, or radiation chopping. Depending

Fig. 1.2: Schematic view of a thermopile consisting of several thermocoupconnected in series. Each thermocouple consists of two different ducting materials.

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Material A

TAbsorber

THeat sink

Material BU

C

Cold contact Hot contact

1 2

UP Nα T∆=

13

1 Introduction

d into

pere for

roduc-rmal

xam-

class,vices

ded forensors twond tem-, suchIn case

complexity and cost of the components the detector systems can be dividethree main classes [17]:

• Consumer products characterized by large volume production (>10’000year), price below $1’000, and room temperature operation. An examplthis class is a intrusion detector.

• Professional and equipment products characterized by medium scale ption and a price of $1’000 up to $50’000. Examples are cameras for theimaging.

• New products with top performances and price at very low quantities. Eples are research equipment or space qualified instruments.

Development of CMOS infrared sensors focuses on applications in the first since the strength of this method is mass production at low cost. The dereported in this thesis, presence detectors and thermal imagers, are intenintrusion detection. The presence detector systems feature two identical s“staring” at different areas as shown in fig. 1.3. Only the difference of thesensor signals is processed by the electronics. Thus homogenous backgrouperature changes cancel. In contrast, the radiation of a localized IR sourceas an intruder, is focussed only on one sensor, and thus, provides a signal.

Fig. 1.3: Typical application of a motion detector.

Sensors Optics Surveyed Areas

2.5 m

10 m

14

1.4 CMOS Fabrication of IR Sensors

jacentation

rangeure ofptics

sed onsing.tiond toingle

main and thisricated,cuitrysignalthermalicated during,rmedi-it bearslogy.eathssium

ion.

pyro-n, and

of a thermal imager the pixels in the sensor array “stare” at a number of adareas and their signals are processed individually. Thus they provide informon the shape, position, and movement of the intruder. The typical working of a low-cost intrusion detector is 10 m. With the average surface temperata clad human being of 24°C [17] the total radiation power collected by the oand transmitted to the sensors is in the order of 1 µW.

1.4 CMOS Fabrication of IR Sensors

The production of the IR sensor microsystems discussed in this thesis is bacommercial, standard CMOS IC fabrication with subsequent post-procesCMOS IC technology is a set of highly sophisticated, well developed fabricasteps of approved reliability [24]. Hundreds of individual steps are performefabricate transistors and their interconnections. They are carried out on scrystal silicon wafers producing numerous identical chips per wafer. The steps are implantation and diffusion of dopants into silicon, thin film growthdeposition, and patterning by photolithography and etching. By exploitingprocess sequence, not only electronic circuits, but also sensors can be fabsimultaneously, on the same chip [25]. The cointegration of sensors and cirallows to reduce the system size, pin count, and length of noise sensitive paths. Nevertheless, some features required for sensor operation, such as isolation or deposition of specialized materials can only be obtained by dedprocessing steps. Such additional process steps can be performed before,or after the standard CMOS process [26,27] and are referred to as pre-, inteate-, or post-processing, respectively. We chose post-processing because the smallest risk of interference with the sophisticated CMOS process technoThe thermal isolation is obtained by bulk micromachining: the silicon undernthe sensor area is removed with an anisotropic silicon etchant, e.g., potahydroxide (KOH) or ethylene-diamine pyrocatechol (EDP) in aqueous solut

1.5 Previous Art

Today’s low-cost segment for thermal radiation detectors is dominated by electric sensors. Due to their high performance, room temperature operatio

15

1 Introduction

nabil-themf mea-y.

y was]. In sili-d the

goldtec-mo- wasconear

e andhileusingd the

cop-rs are

s

e ther-tureithpen-

low cost, these have been widely used in motion detectors. However, their iity to measure dc radiation signals without mechanical chopping makes unsuitable for presence detection. Thermopiles, in contrast, are capable osuring dc signals and are readily accessible with micromachining technolog

Early research on infrared thermopile sensors using silicon planar technologcarried out by Lahiji et al. [29,30], Shibata et al. [31], and Sarro et al. [321982 Lahiji et al. reported an infrared sensor fabricated on a 2 mm by 2 mmcon membrane with bismuth/antimony thermocouples. In 1982 he proposefabrication of a similar sensor with higher performance using polysilicon andfor the thermopile. In 1987 Sarro et al. [32] reported the fabrication of IR detors using silicon structures with integrated p-doped silicon/aluminium therpiles. The first thermoelectric IR sensor using dielectric thin film membranesreported by Völklein et al. [23] in 1991. This sensor consists of a silioxide/nitride membrane with Bi-Sb-Te thermopiles. In the same yLang et al. [28] proposed a thermoelectric sensor with dielectric membranpolysilicon/aluminium thermopile that can be produced in a CMOS line, wBaltes et al. [33] reported the fabrication of thermoelectric infrared sensors commercial CMOS technology. In 1993 Lenggenhager et al. [34] reportefabrication of thermoelectric sensors using n

+

-poly/p

+

-poly thermopiles in a com-mercial CMOS process. Srinivas et al. [35] demonstrated a free-standingper/constantan thermopile. Today, various infrared thermoelectric sensocommercially available. Examples of manufacturers are:

•

Meggitt Avionics, Inc.

, Manchester, New Hampshire, USA.•

EG&G Heimann Optoelectronics

, Wiesbaden, Germany.•

IPHT

, Institut für Physikalische Hochtechnologie, Jena, Germany.

IR detectors of

Meggitt Avionics Inc.

rely on thermopiles deposited on mylar filmin various sizes and thermopile materials.

EG&G Heimann

offers a silicon micro-machined membrane sensor from a reduced CMOS process [36] where thmopile materials polysilicon and aluminum are optimized for a low temperacoefficient of sensitivity. The

IPHT

specializes in custom sensor designs w

optimized (non-CMOS) thermopile materials and absorbers on stress comsated silicon nitride/silicon oxide membranes.

16

1.6 Outline of the Thesis

e opti-etryand Theyithicgen-singanno

eration995, in a

inte-R radi-ensors, fol-or fab- the sensorpecialth theracter-ctralectralset ofls for

el are dem-pter 6

r sys-

In the last seven years a significant number of contributions were made to thmization of thermopile IR sensors in the choice of material [23], geom[37-42], and signal conditioning circuitry [8,10]. The cointegration of circuits thermoelectric IR sensors was first demonstrated by Baer et al. [43] in 1991.fabricated a linear array of 32 sensors with multiplexing circuit. The monolcointegration of circuits for signal conditioning was reported in 1993 by Lenghager et al. [44,45] and in 1995 by Müller et al. [7]. With integrated addrescircuit, large two-dimensional arrays of sensors became feasible. In 1994 K et al. [46] reported an array of 128 by 128 sensor elements, designed for opin vacuum and fabricated using a custom silicon process. Later, in 1Oliver et al. [47] reported a sensor array with 32 by 32 pixels, fabricatedresearch CMOS facility.

1.6 Outline of the Thesis

In this thesis we report the fabrication, characterization, and modeling of grated IR sensor microsystems. In chapter 2 we present the basic theory of Iation, thermoelectric sensor operation and explain the figures of merit for sperformance. Chapter 3 starts with a description of a generic CMOS proceslowed by a description of the specific processes and materials used for sensrication. Then an introduction is given to bulk micromachining used forpost-processing of the sensor devices. Finally we list the fabricated sensors,arrays, and sensor systems with descriptions of their physical layout and sfeatures. In chapter 4 we report the sensor characterization. We begin wisetup used to measure the sensitivity, followed by the measured sensor chaistics. Next we introduce a method to determine in-situ the relative speabsorptance of thin film absorber sandwiches. The measured relative spabsorptance in the range from 2 µm to 14.6 µm is reported for a complete absorbing CMOS layer sandwiches. In chapter 5 we discuss different modethe sensor performance. An analytical, variational, and finite element modreported and compared. To show the power of the finite element model, weonstrate the optimization of a sensor device for a presence detection. In chawe report the fabricated motion detector and thermal imager demonstratotems. The thesis is summarized with chapter 7.

17

2 Theory

in theailednder-ging per-

. Theuthorsfini- to µmd byrum.

ringegel

script

antity.

r-

2 T

HEORY

This chapter summarizes the theoretical background of the work describedfollowing chapters. It starts with a discussion of infrared radiation. Then a detdescription of the sensor principle is given, followed by a discussion of the ulying physical effects, viz. heat transfer and the Seebeck effect. Next, imaoptics is briefly reviewed. Finally, the figures of merit used to characterize theformance of sensors and detector systems are introduced.


Infrared radiation extends over the range from 0.8 µm to 1 mm wavelengthrespective photon energies range from 1.5 eV to about 1.2 meV. Various adistinguish four subbands of the infrared spectrum, varying slightly in the detion: The near infrared from 0.8 µm to 3 µm, the middle infrared from 3 µm6 µm, the far infrared from 6 µm to 16 µm, and the extreme infrared from 16to 1000 µm. Infrared radiation, invisible to the human eye, was discovereWilliam Herschel (1738 - 1822) in 1800 in an experiment on the solar spect

Radiometry

In this section a few terms and definitions of radiometry are compiled. Diffeterminologies abound in the literature. The one given here follows Siet al. [48].

A fundamental concept of radiometry is the spectral

intensity

i'λ. It describes theenergy flow at a point in space, per unit time, of specific wavelength λ, and in aspecific direction through a unit surface area normal to the direction. The subλ and the prime, respectively, denote a spectral and direction dependent qu

In contrast to i'λ the spectral emissive power e'λ usually refers to a point on a suface A. It describes the radiation power density through A, in a specific direction

18


ies are

e, or

he

ee

its spe-

relative to the surface, of a specific wavelength λ. If the radiation from a surfaceis isotropic, i'λ is a constant for all directions (ϑ , ϕ), and Lambert’s law,

(2.1)

holds. Such surfaces are called Lambert radiators. Three important quantitderived from e'λ by integration over the wavelength spectrum, the hemispherboth. They are, respectively, the directional total emissive power e' defined as

, (2.2)

the hemispherical spectral emissive power eλ given by

, (2.3)

and the hemispherical total emissive power e, i.e.,

. (2.4)

The absorptivity or absorptance α 'λ of a surface is defined as the ratio of tabsorbed power density pabs(λ) and the incident intensity e'λ(ϑ , ϕ),

. (2.5)

As proposed in [48] the ending -ity in absorptivity will be used for the intensivproperty, i.e. for opaque materials where α is independent of sample size. Thabsorption behavior of a partly transparent sample, in contrast, depends on cific size, and thus, is extensive. In this case the ending -ance will be used.

e'λ ϑ ϕ,( ) i'λ ϑcos=

e' ϑ ϕ,( ) e'λ ϑ ϕ,( ) λd

0

∞

∫=

eλ e'λ ϑ ϕ,( ) ϑd ϕd

ϕ 0=

2π

∫ϑ 0=

π 2⁄

∫=

e eλ λ e' ϑ ϕ,( ) ϑd ϕd

ϕ 0=

2π

∫ϑ 0=

π 2⁄

∫=d

0

∞

∫=

α'λ T ϑ ϕ, ,( )pabs λ( )

e'λ ϑ ϕ,( )--------------------=

19

2 Theory

- thislack,

ity

re. Theation

andields a lawscon-r

pec-rbed

mittedhe

Similar to eqns. (2.2), (2.3), and (2.4) the hemispherical absorptivity αλ, totalabsorptivity α', and total hemispherical absorptivity α can be defined by respective integrations over eqn. (2.5). The largest possible absorptivity is unity. Incase all incident radiation is absorbed. A body with this property is called bor a blackbody.

The hemispherical reflectivity ρλ is defined as the ratio of the radiation intensreflected from the sample and the one impinging on it. It is given by

, (2.6)

where denotes the reflected intensity and is integrated over the hemispheabsorptivity and reflectivity of an opaque surface satisfy the energy conservcondition

. (2.7)

Thermal Radiation

All matter constantly radiates IR radiation. The spectral energy distributionintensity depends on the material and its temperature. This phenomenon yconnection between electrodynamics and thermodynamics. Its principalwere first formulated by Kirchhoff [49] in 1859. They can be understood by sidering a model system of two flat surfaces of equal area A facing each other ovea small gap where radiation is the only means of energy exchange. Let α1, α2 ande1, e2 denote their total hemispherical absorptivity and emissive power, restively. Then Ae1 denotes the power emitted from body 1 and the power absoby body 2 is Ae1α2. In thermal equilibrium the equation

(2.8)

holds. The same argument applies separately for all components of the transpower having different wavelengths λ. From eqn. (2.8) it becomes clear that t

ρλ iλˆ iλ⁄=

iλˆ

αλ ρλ+ 1=

e1α2 e2α1=

20


le

y was

on-

µm

ratio eλi/αλi is a constant ebλ for all surfaces, depending on temperature T andwavelength only. This leads to Kirchhoff’s law

. (2.9)

As a consequence from eqn. (2.9) a blackbody with αλ = 1 has the largest possibemissive power eλ = ebλ. This leads to the interpretation of ebλ as the emissivepower of a blackbody. The total hemispherical emissive power of a blackbodfound experimentally by Stefan [50] and theoretically by Boltzmann [51] as

, (2.10)

where

(2.11)

is the Stefan-Boltzmann constant. The spectral emissive power ebλ, however, wasfirst correctly described by Planck [52] in 1900. It is given by

. (2.12)

where c, k, and h denote the speed of light and Boltzmann’s and Planck’s cstants, respectively. Figure 2.1 shows plots of ebλ for T = 300 K and T = 400 K. Abody at “terrestrial” temperatures radiates mostly in the range from 2to 40 µm. Hence the name thermal radiation.

eλ λ T,( ) αλ λ T,( ) ebλ λ T,( )⋅=

eb ebλ T( ) λd

0

∞

∫ σT4= =

σ 5.670 8–×10W

m2K4--------------=

ebλ T( )2πhc

2

λ5--------------- 1

e

hckTλ----------

1–

-------------------⋅=

21

2 Theory

Black surfaces are always Lambertian, i.e. eqn. (2.1) holds, and thus e'bλ, and e'can be deduced from eqn. (2.12). The values are

, (2.13)

and

. (2.14)

The emissivity ε of a surface is defined as the ratio of its emissive power e and thatof a blackbody eb

. (2.15)

Fig. 2.1: Spectral emissive power ebλ of a blackbody at 300 K and 400 K.

0

20

40

60

80

100

120

140

1 10 100

Em

issi

ve p

ower

[W

/m2 µm

]

Wavelength [µm]

400 K

300 K

e'bλ ϑ T,( )2hc

2

λ5----------- ϑcos

e

hckTλ----------

1–

-------------------⋅=

e'b ϑ T,( )σT

4

π---------- ϑcos=

ε eeb-----=

22

2.2 Sensor Principle

sur-

ransfer

e sup-ver, asyers.m,re.

ear the

e lowghlyThusstrate.output

By definition (see eqn. (2.9)) the emissivity is equal to the absorptivity of theface.

As a consequence of eqns. (2.9), (2.10), and (2.15) the net radiative heat tpem from a unit surface area with emittance ε at temperature T1 to the surroundingsat T2 is given by

, (2.16)

where the approximation is valid for

. (2.17)

2.2 Sensor Principle

The sensitive area of the sensor consists of a thermally isolated structurported by the silicon substrate. This can be a membrane, bridge, or cantileshown in fig. 2.2. These structures are composed of the dielectric CMOS laThermopiles made from two of the conducting CMOS thin films (aluminun-poly, n+-poly, p+-poly) are integrated in the layer sandwich of the structuTheir cold contacts are on the silicon substrate and their hot contacts are nabsorber.

IR radiation incident on the sensor is absorbed in the membrane. Due to ththermal conductivity of the dielectric layers and their small thickness (rou4 µm), the free parts of the structure are well isolated from the bulk silicon. the temperature of the structure is increased with respect to the silicon subThe temperature increase is measured with the thermopiles providing the voltage UP given by

, (2.18)

pem εσ T14

T24–( ) 4εσT1

3T1 T2–( )≈=

T1 T2–

T1--------------------- 1«

UT γ∆Tii 1=

N

∑=

23

2 Theory

effi- con-cts. Itn.

signal, inte-

re

where N, γ, and ∆Ti denote the number of thermocouples, their Seebeck cocient, and the individual temperature differences between their hot and coldtacts, respectively. A heating resistor may be integrated near the hot contaallows to dissipate a controlled heating power for test purposes or calibratio

To protect the sensor from mechanical damage, contamination, and stray the sensor is packaged in a closed housing. An IR transparent filter windowgrated in the package provides access for the radiation to be sensed.

Fig. 2.2: Schematic view of a thin membrane, bridge and cantilever structuwith integrated thermopiles.

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAA

ASubstrate

Thermopile

Membrane

Substrate

ThermopileBridge

Substrate

ThermopileBeam

24

2.3 Heat Transfer

tion

ty oferm eat loss has toound-g

ultinggh of the

ion of

2.3 Heat Transfer

The thermopile signal originates from a non-uniform temperature distribu governed by the heat transfer equation

, (2.19)

where ρ, cV, and κ, denote the density, heat capacity, and thermal conductivithe medium, respectively. The right hand side of eqn. (2.19) is the source tI.The source term takes into account heat generation due to absorption and hdue to thermal radiation. To describe the behavior of the sensor, eqn. (2.19)be analyzed in the domain of the sensor chip and the surrounding air. The bary condition is given by the package surface S0 surrounding the sensor and actinas a heat sink at T0

, (2.20)

and the initial condition

. (2.21)

The typical power incident on the sensor structure of area 1 mm2 is 1 µW. Theresulting temperature increase is in the order of 1 mK. The radiation loss resfrom this is roughly 5 mW/m2 or 0.5% of the absorbed power density. This rouestimate shows that the radiation losses can be neglected in the calculationtemperature distribution.

The solution of the heat transfer problem (see e.g. [53]) involves an expansthe solution of eqn. (2.19) in terms of the orthonormal eigenfunctions Ψi of thedifferential operator

, (2.22)

T x t,( )

ρcV t∂∂T ∇ κ T∇( )⋅– I x t,( )=

T x t,( ) T0 x t,( )= for x S0∈

T x 0,( ) f x( )= at t 0=

∇ κ∇Ψ i µi2ρcVΨi=⋅

25

2 Theory

ndary

nd

gen- doneirectpertiesina-

Sec-mpo-

nentsues

y state

where the µi2 denote the respective eigenvalues with the homogenous bou

condition

. (2.23)

The general solution then has the form

, (2.24)

where

, (2.25)

is the decomposition of the initial condition in terms of the eigenfunctions, a

, (2.26)

accounts for the boundary condition and the production term. Evaluating thiseral solution involves finding all the eigenfunctions. In our case, this can beonly numerically at large computational expense, which makes this dapproach unpractical. Nevertheless eqn. (2.24) reveals some interesting proof the temperature distribution. First, the spatial distribution is a linear combtion of the eigenfunctions, each of which evolves independently with time. ond, in the case of steady sources and boundary conditions, individual conents essentially relax exponentially to the steady-state. Different compodecay with different time constants τm. These are the inverses of the eigenvalµi

2 of the associated eigenfunctions.

The response of the sensor to steady radiation is described by the steadproblem associated with eqn. (2.19)

, (2.27)

Ψi 0= x S0∈

T x t,( ) eµi

2– t

Ψi x( ) f i˜ gi t'( )e

µi2t'

t'd

0

t

∫+i 1=

∞

∑=

f i˜ ρcVΨi f vd

V∫=

gi t( ) T0 x t,( )κn∂

∂ Ψi x( ) sdS∫– Ψi x( )I x t,( ) vd

V∫+=

∇ κ T∇( )⋅ I=

26

2.4 Seebeck Effect

, as the. Therature ele-with

r two

t ,

is a

motive, the.g., in

t,

and the boundary condition (2.20). The steady state problem can be solveddynamic problem, by the expansion of eqn. (2.27) with the eigenfunctionseigenfunction to the eigenvalue 0 is proportional to the steady-state tempedistribution. However, solutions can be found more efficiently using the finitement method (FEM). Numerical calculation of the temperature distribution FEM is discussed in chapter 5.

2.4 Seebeck Effect

The Seebeck effect is one of the three thermoelectric effects [54]. The otheare the Thomson and Peltier effects. It occurs when a conducting material A, i.e.a material with mobile charge carriers, is subject to a temperature gradienwhich leads to an electric field E given by

, (2.28)

where γA is the Seebeck coefficient of the material. Since the temperaturepotential field, the integral of the related gradient field E along any closed pathvanishes. Thus the Seebeck effect cannot be used to generate an electroforce in a closed circuit of one material. However, in case of two materialsgeneration of an electromotoric force is possible. This effect is employed, ethermocouples. They consist of two different conducting materials A and B thatare in electrical contact as shown in fig. 2.3. If T2 is the temperature of the contac

Fig. 2.3: Schematic view of a thermocouple.

T∇

E γA T∇=

Material A

Material BU

C

Cold contact, T Hot contact, T1 2

27

2 Theory

ltage

This and ontowhichn imageing ine fol-

array.

esfined.trance

a voltage UC is generated between two points on materials A and B at temperatureT1. With the Seebeck coefficients γA and γB of the two materials the voltage is

. (2.29)

With the relative Seebeck coefficient

(2.30)

of the two materials, and for sufficiently small temperature differences the voon the thermocouple is

. (2.31)

2.5 Imaging Optics

Any complete detector system comprises some form of imaging optics.optics may consist of a stop, a light guide, or complex system of mirrorslenses. It collects the infrared radiation from the environment and directs itthe sensors. In the simplest case the optics just defines a field of view from the sensor receives radiation. In most cases, however, the optics transfers aof the scene on the sensors. Here we discuss the implications of the imagsimple systems with fixed focus and sensors in a focal plane geometry. Thlowing calculations apply for both single sensors and sensor pixels in an The setup is shown in fig. 2.4.

The sensor is placed at the focal distance f from the optical element. This assurthat the image of an object far away from the optical element is sharply deThe area in the object space that is imaged onto the sensor is called en

UC γA Td

T1

T2

∫ γB Td

T2

T1

∫+ γA γB–( ) Td

T1

T2

∫= =

γAB γA γB–=

UC γAB T1 T2–( ) γAB T∆==

28

2.5 Imaging Optics

ce

anglees theonly

ndow.

window of the sensor. Its size x is related to its distance from the detector l, thesize x' of the sensitive area, and focal length by

. (2.32)

The radiation power P captured by the optics from an object filling the entranwindow of the sensor can be approximated [55] by

, (2.33)

where D denotes the diameter of the optics or the aperture stop and ε is the objectsemissivity. The first term on the right-hand side in eqn. (2.33) is the spatial of the optics aperture with respect to the object. The second term denotpower emitted by the object. The size of the optics aperture is commdescribed by the f-number nf, defined as

. (2.34)

Fig. 2.4: Schematic view of a optics system with sensor and its entrance wi

Optics SensorEntrance window

l f

x

x’

xl-- x'

f---=

PD

2π4l

2---------- x

24εσT3∆T

π------------------------------⋅ x'2D

2

f2

------------- εσT3∆T⋅= =

nffD----=

29

2 Theory

ans-

the

char-e, and-noise tem-everal

nted.

The collected power P is focused on the sensor but reduced by the nonideal trmission efficiency E of the optics. The power available on the sensor P' is then

. (2.35)

For sensor arrays the x' in eqns. (2.32) through (2.35) refers to the extent of sensitive pixel area A rather than the pitch y of the array. The ratio of the area Aand the area y2 occupied in the array is called the fill factor F = A/y2.

The field of view angle Θ covered by an array with n × m quadratic pixels is givenby

. (2.36)


Thermoelectric infrared sensors can be described by a small set of primaryacteristics. These are their size, sensitivity, output resistance, signal noisresponse time. From these, a set of additional characteristics, i.e., signal-toratio, noise equivalent power, normalized detectivity, and noise equivalentperature difference can be derived. Depending on the application, one or sof these characteristics are suitable as figures of merit.

In the following these sensor characteristics are defined and briefly commeThe measurement of these characteristics is described in chapter 4.

P' Ex'2

nf2

--------εσT3∆T=

Θ 2y n

2m

2+2 f

-------------------------

atan=

30


tatic

plified

essure,

tsltagehavior

resis-l noise

Sensitivity

The sensitivity S is the most frequently used property. It describes the sresponse to radiation. It is defined as the change in signal voltage ∆U per changein total incident radiation power ∆P on the absorbing sensor area [56]

. (2.37)

Assuming a linear dependence and neglecting offset, eqn. (2.37) can be simto

. (2.38)

The sensitivity, e.g., depends on the sensor temperature, ambient gas prradiation wavelength, and incidence angle.

Thermopile Resistance

Besides to being the source of the voltage UP a thermopile is also a resistor. Irepresentation in electrical models is a resistor with a thermoelectric vosource in series. This model describes both its noise performance and its bewhen driving a load.

Noise

The noise in thermopile sensors is due to Johnson noise from the thermopiletance. The frequency spectrum of Johnson noise is white and the spectravoltage density of the thermopile resistance RT is

. (2.39)

SU∆P∆

--------=

SUP----=

ν

ν 4kT RT=

31

2 Theory

nt sen-result-

gauss-ignal

pe-ignal

entireo-

ther than

al-is

If several noise sources are present in a system’s signal path, such as differesors or circuits, the noise powers of the different sources superimpose. The ing total spectral noise voltage density is

. (2.40)

For white noise and a limited signal frequency bandwidth ∆f the total noise volt-age V becomes

. (2.41)

If the output of such a system is sampled, the measured amplitudes have aian distribution. The mean value of the distribution corresponds to the sincluding offset. The standard deviation of the distribution is V. Thus 68% of thesamples lie within ±V/2 of the mean value.

Signal-to-Noise Ratio

The signal-to-noise ratio Q is a figure to describe the quality of a signal in a scific situation rather than a sensor property. It is defined as the ratio of the sand noise voltages, i.e.,

. (2.42)

It can be used to describe the signal quality of a stand-alone sensor or ansystem. From eqn. (2.42) it is obvious that Q depends on the radiation power prducing the signal U and the relevant bandwidth. The definition of Q given herefollows the literature on IR sensors [55,56]. A different definition common inelectrical engineering literature is the ratio of signal and noise power, rathevoltage.

Noise Equivalent Power

The noise equivalent power NEP describes, like the signal-to-noise ratio, the quity of the sensor or system signal. In contrast to Q it characterizes the system. It

ν ν i2∑=

V ν f∆=

Q U V⁄=

32


ual to

s

i-ncen with

e noisem

blackubsti-

only

defined as the incident radiation power producing a signal-to-noise ratio eqone at the output. It can be calculated as

. (2.43)

If nothing else is stated, the noise refers to a bandwidth ∆f of 1 Hz. For systemswith a non-white noise spectrum the NEP also depends on frequency.

Normalized Detectivity

Like the NEP the normalized detectivity D* describes the signal quality. It iderived from the detectivity D which is the inverse of the NEP. The normalizeddetectivity is defined as

, (2.44)

where A is the sensitive sensor area, ∆f refers to the bandwidth used in the defintion of the NEP. The normalized detectivity is applied to compare the performaof sensors of different size and type. This is possible because in comparisoD and NEP, the dependence of D* on size is reduced.

Noise Equivalent Temperature Difference

In contrast to the previous characteristics describing sensor performance, thequivalent temperature difference NETD is a figure to describe a detector systeincluding imaging optics. It is defined as the temperature difference on a object that produces a signal-to-noise ratio of one at the system output. By stuting eqn. (2.35) into eqn. (2.43) and solving for ∆T the NETD becomes

. (2.45)

The first term on the right-hand side of eqn. (2.45) depends on the opticswhile the other term describes the performance of the sensor.

NEPVS---- P

Q----= =

D∗ A ∆f⋅NEP

-------------------=

NETDnf

2

E-------- NEP

A------------ 1

σT3

----------⋅=

33

2 Theory

abruptponse staten time

.on with

xel ixel

ion

Response Time

The response time of the sensor characterizes how fast its signal reacts to anchange in the incident radiation power. As describes in section 2.3 the resconsists of a superposition of exponential relaxations to the new steadyresponse. In practice the component associated with the longest relaxatiohas the largest amplitude and all others are neglected. Thus the responseU(t) ofthe sensor to a sudden change in radiation power ∆P at is given by

, (2.46)

where denotes the sensor signal for , and τ defines the response timeThe dynamic response can also be described using the response to radiatia harmonic time dependence of angular frequency ω

. (2.47)

Then the sensor response is also of the form

, (2.48)

and the frequency response is then given by

. (2.49)

This is the typical behavior of a low-pass filter with cut-off frequency (2πτ)-1.

Crosstalk

If a pixel A within an array is irradiated, and therefore heats up, a nearby piBmay also heat up due, e.g., to heat conduction. This leads to a signal from pBalthough it is not irradiated. The ratio of the signals from pixel UB and the irradi-ated pixel UA is called crosstalk tAB. The crosstalk depends on the relative positof the two pixels within the array and is usually given in percent.

t 0=

U t( ) U 0( ) P∆ S 1 et– τ⁄–( )⋅+=

U 0( ) t 0≤

P t( ) Pωeiωt–=

U t( ) Uωeiωt–=

UωPω-------

1

1 ω2τ2+-------------------------=

34

3.1 CMOS Processes

mmer--pro-ith af thessing. dis- pro-

wellinte-hip. ori-icationvolveilme forf twoemi-, and name wellsistorsed tolectricy iso-ontact

3 DEVICE FABRICATION

The IR sensors and IR sensor arrays in this thesis were fabricated using cocial standard CMOS IC technology with subsequent CMOS compatible postcessing. In the following we describe the main fabrication steps. We start wdescription of a general CMOS process followed by the special features othree processes we used. Next we discuss CMOS compatible post-proceThen a physical description of the devices we fabricated is given. Finally wecuss the critical steps and pitfalls we encountered in micromachining CMOScessed silicon wafers and chips.

3.1 CMOS Processes

CMOS technology for integrated circuits is a set of highly sophisticated, developed fabrication steps [24] of high reliability. Modern very large scale gration (VLSI) circuits contain more than 100 million transistors on a single cThe fabrication of the CMOS devices is carried out on single crystal (100)ented silicon wafers. Several hundred process steps are required for the fabrof the transistors and their interconnections. The main fabrication steps inimplantation and diffusion of dopants into silicon, oxidation of silicon, thin fdeposition, and patterning by photolithography and etching. The typical devica CMOS process is the inverter schematically shown in fig. 3.1. It consists ofield effect transistors (FET) and their interconnections. Their metal oxide sconductor (MOS) structure is realized with the gate polysilicon, gate oxidesubstrate. The complementary action of the two transistors, which gave theto CMOS (complementary MOS), is achieved by placing one transistor in aof doping type opposite to that of the substrate. The active area of the tranis defined by an opening in the field oxide. Two metallization layers are usconnect the active devices. They are made from an aluminum alloy. The dielayers include field, contact, and intermetal oxides and the passivation. Thelate the conducting layers from each other. When opened, they allow the c

35

3 Device Fabrication

cor-ds and in thee listedriety

e three

l lay-

n-erallectricebeck were

AAAAAAA

between different conducting layers.The passivation protects the circuit fromrosion and mechanical damage. Openings in the passivation are called paare used for connections between the chip and the package. The layerscross-section 3.1, and their typical materials, thicknesses, and purposes arin table 3.1. While the basic structure is similar for all CMOS processes, a vaof specialized processes exists. We now describe the special features of thprocesses ECPD 10, alp2lv, and alp1mv used for this thesis.

ECPD10

The ECPD 10 process of Atmel ES2 in Aix-en-Provence, France, is a digitaCMOS process. It features a minimum gate length of 1 µm, two metallizationers, and one polysilicon layer. The process was derived from the ECPD 10 of Phil-ips. Table 3.2 lists the materials of the ECPD 10 process that were used for IR sesors with their thickness and thermal conductivity. For the dielectrics, the ovthickness and their average thermal conductivity are listed. The thermoelproperties of the conducting layers are listed in table 3.3. The relative Secoefficient of polysilicon and metal 1 is given. The data in tables 3.2 and 3.3measured by von Arx [57].

Fig. 3.1: Schematic cross-section of a CMOS inverter.

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAPassivation

SubstrateWell

Field oxideContact oxide

Intermetal oxide

Metal 2

Gate Metal 1Gate oxide

Pad

36

3.1 CMOS Processes

cal

[57].

Layer Purpose Material Thickness

Passivation circuit protection Si3N4 2 µm

Metal 2 interconnect aluminum alloy 1 µm

Intermetal oxide

isolation spin-on-glass 1 µm

Metal 1 interconnect aluminum alloy 1 µm

Contact oxide isolation reflow glass 1 µm

Gate transistor polysilicon 0.4 µm

Field oxide isolation SiO2 0.5 µm

Gate oxide isolation SiO2 0.04 µm

Source/drain contact doped Si 0.3 µm

Well transistor doped Si 4 µm

Substrate substrate Si 700 µm

Tab. 3.1: Layers contained in the CMOS inverter shown in fig. 3.1 with typithicknesses.

Layer Material Thickness κ [W/mK]

Passivation oxynitride

3.15 µm(overall)

1.06(average)

Intermetal oxide spin-on-glass

Contact oxide boro-phosphoroussilicate glass

Field oxide SiO2

Metal 2 aluminum alloy 1 µm 180

Metal 1 aluminum alloy 0.5 µm -

Poly n+-polysilicon 0.35 µm 23.7

Tab. 3.2: Thicknesses and thermal conductivities of the layers of ECPD 10

37


th oficonstors,ut on.

tapes to a

ted in

es 3.2or

s theth the

queouscholof the

alp2lv

The alp2lv process of EM Microelectronic-Marin SA in Marin, Switzerland, is ananalog, low-power, low-voltage IC process. It features a minimum gate leng2 µm, two metallizations, and two polysilicon layers. One of these polysillayers is available with three different doping levels, used in capacitors, resipoly-diodes, tunnel-diodes, and EEPROM cells. The process is carried o6” wafers. The thermal material properties of alp2lv layers are listed in table 3.4An option of the alp2lv process are gold bumps. These are normally used forautomated bonding. They are electroplated on top of the passivation or padheight of 25 µm. The electrical parameters of the conducting layers are listable 3.5.

alp1mv

Like the ECPD 10 process of Atmel ES2, the alp1mv process of EM Microelec-tronic-Marin SA in Marin, Switzerland, was derived from the ECPD 10 of Philips.The two processes are electrically equivalent and the material data in tabland 3.3 were also used for the alp1mv process. Gold bumping is also available fthe alp1mv process.

3.2 Post-Processing Method

Post-processing is the crucial step in CMOS IR sensor fabrication. It providethermal isolation necessary for the sensor operation. The silicon underneaactive sensor area is removed by an anisotropic etching step. We used asolutions of potassium hydroxide (KOH) or ethylene-diamine pyrocate(EDP). KOH is suitable for the etching of sensor membranes from the rear

Layer Sheet resistance Rsq [ΩΩΩΩ/sq] Seebeck coeff. γ [µV/K]

Metal 2 0.06 -

Metal 1 0.06 -

Poly 30.8 -108

Tab. 3.3: Electrical properties of the conducting layers of ECPD 10 [57].

38


m thehile

,59].

wafer, while EDP is used for beam and bridge type structures etched frowafer front. KOH would destroy the metal pads at the front of the wafer, wthey are preserved in EDP.

Layer Material Thickness κ [W/mK]

Bump Au 25 µm 312

Passivation Si3N4/SiO2 1 µm 1.1

Metal 2 aluminum alloy 1 µm 174

Intermetal oxide spin-on-glass 0.9 µm 1.0

Metal 1 aluminum alloy 0.7 µm 197

Contact oxide borophosphorous glass 0.7 µm 1.2

Poly 2 n+-polysilicon 0.38 µm 18

n-polysilicon 0.38 µm 22

p+-polysilicon 0.38 µm 14

Poly 1 n+-polysilicon 0.38 µm 18

Field oxide SiO2 1 µm 1.2

Tab. 3.4: Thicknesses and thermal conductivities of the layers of alp2lv [58

Layer Sheet resistance Rsq [ΩΩΩΩ/sq] Seebeck coeff. γ [µV/K]

Metal 2 0.03 0

Metal 1 0.044 0

n+-poly 2 29 -88

n-poly 2 2613 -454

p+-poly 2 427 270

n+-poly 1 29 -92

Tab. 3.5: Electrical properties of the conducting layers of alp2lv [58,59].

39


dif- solu-respec-loitedof these the of a

aticd 3.3,. The form of etch

tal theshapettice.

ch asn array

cture

Bulk Micromachining by Anisotropic Etching

Bulk micromachining of silicon with KOH and EDP is based on the fact thatferent planes of the silicon monocrystal have different etch rates. In thesetions the (111) and (100)-planes have the slowest and fastest etch rates, tively. Etch rate ratios up to 35:1 are reported [60]. This property can be expto etch cavities of diverse forms into the (100)-oriented wafers. The surface wafer is protected by an etch mask. The mask is patterned to locally exposilicon bulk. For a rectangular mask opening the cavity grows in the shape

inversed truncated wedge restricted laterally by (111)-planes. A schemcross-section and SEM picture of such a cavity are shown in figs. 3.2 anrespectively. The size of the mask opening in fig. 3.3 is 1800 µm by 1300 µmbottom of the etch pit is a (100)-plane and is etched until the cavity has theof a v-groove. As indicated in fig. 3.2 the comparatively low etch rate(111)-planes leads to a small undercut of the mask. If the opening in themasks contains convex corners or line segments not aligned with the crysmask is undercut accordingly. After a sufficient etching the undercut has the of the rectangular envelope of the mask opening aligned with the crystal laThe undercutting allows the formation of laterally supported structures subeams and bridges composed of the masking material. Figure 3.4 shows a

Fig. 3.2: Schematic cross-section of an etched groove and top view of a strureleased by the underetching of convex mask corners.

AAAAAA

AAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEtch mask<111>

<100>

Silicon

Etch groove

Undercutstructure

Convex corner

40


m andopen-

joinhapedm by

e rearby an

size

m.

of such beams. The length and width of these cantilever beams is 300 µ100 µm, respectively. Bridges are obtained from the combination of several

ings in the etch mask. As shown in fig. 3.5 the individual cavities overlap anddue to the undercutting. An example is shown in fig. 3.6. It consists of an s-sbridge obtained with two etch mask openings. The size of the bridge is 100 µ150 µm.

If the mask opening is large enough the etched cavity eventually reaches thof the wafer and an opening is formed. If the rear of the wafer is protected

Fig. 3.3: SEM picture of a cavity etched into rectangular mask opening. Theof the mask opening is 1800 µm by 1300 µm.

Fig. 3.4: Array of micromachined beams. The length of the beams is 300 µ

Etch mask

(111)-planes

(100)-plane

41


in a

tainszine.

fer.

etch resistant film, the etching stops at the silicon/film interface. This resultsclosed membrane as shown in fig. 3.7.

EDP

We used EDP type S [61] at 95°C to fabricate beams and bridges. It con1000 ml ethylene-diamine, 160 g pyrocatechol, 133 ml water, and 6 g pyra

Fig. 3.5: Schematic top view of two underetched bridge structures.

Fig. 3.6: S-shaped bridge obtained with two etch mask openings.

Fig. 3.7: Cross-section of a membrane obtained by etching through the wa

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

Opening

Bridge

Opening

Opening

Bridge

Opening

Etch mask

Silicon

Membrane

42


etch. Tohere insolu-e and

sg step,nings,

inter-ds is etchf the

e and size

etch-pprox-

etch [6],liters thenpro- rear

ranesd toh theaferafer

staticking

The etch rate in (100) direction for fresh solutions at 95°C is 27 µm/h. Thebehavior varies significantly with water, oxygen, and silicon concentrationobtain a predictable etch, the solution has to be kept under nitrogen atmospa reflux condenser [62]. After 10 g of silicon have been dissolved per liter of tion, solid reaction products are deposited on the surfaces [63]. The field oxidpassivation layers are etched more than 104 times more slowly than silicon. Thiallows the fabrication of beams and bridges in a maskless post-processinusing the CMOS dielectrics as the etch masking layers [64]. The access opewhere the EDP attacks the silicon, are defined by a cut in the passivation,metal oxide, contact oxide, and field oxide. The unprotected aluminum of paetched 180 times slower than silicon in the (100) direction. This provides anwindow of 4 h during which bondable pads are preserved [65]. The details oetching procedure are given in [66].

The advantages of EDP are the simplicity of the post-processing procedurthe possibility to process a single chip for prototype fabrication. However, theof the fabricated structures is limited by mechanical stability and maximum ing time of 4 h. Stress in the dielectric layers causes structures larger than aimately 300 µm to break during fabrication [66].

KOH

We used 6 M solution of KOH at 95°C to fabricate dielectric membranes. Itsrate for silicon in (100) direction, silicon nitride, and field oxide are 150 µm/h0.1 nm/h [67], and 0.12 nm/h [67], respectively. Up to 190 g of silicon per can be dissolved with only a 7% change in (100) etch rate [68]. KOH removepad metallization within seconds, and thus cannot be used in contact with utected pads. Thus, KOH is preferentially used for micromachining from theof wafers.

We processed entire 6” wafers to obtain hundreds of micromachined membin one process step [6]. A wafer fixture developed by Linder [69] was usemechanically protect the wafer front during the etch. A cross-section througwafer fixture is shown in fig. 3.8. It consists of two stainless steel parts. The wis clamped between them with two sealing rings. The cavity formed by the wand the fixture is rinsed with diluted ascorbic acid. This balances the hydropressure from the KOH on the wafer and neutralizes KOH possibly lea

43


mem-aving

teps

nicalmbrane

mask.idualsbtainr is

ationnce ise were

fer

through broken membranes. We etched until the silicon underneath the branes was completely removed and the etching stopped at the field oxide, lemembranes composed of all dielectric CMOS layers.

The preparation of the wafers for the KOH etch consists of the following s[6,70]:

• Deposition of stress compensating passivation.• Deposition of additional dielectric front protection layer.• Polishing of rear side of the wafers.• Deposition of the etch mask.• Electrochemical growth of gold bumps.• Patterning of the etch mask.

The membrane yield after etching is increased by controlling the mechastress in the membranes. This is done by compensating the stress of the melayers with a customized passivation [70].

The surface state of the wafer back is crucial for the adhesion of the etch After the CMOS process the back of the wafers are covered with process resand scratched from handling. An isotropic etch of the wafer back is used to othe required surface quality [6]. After etching, the front protection layeremoved, the wafers are cleaned, and finally diced.

3.3 Fabricated Device Types

In this section we describe the fabricated IR sensors, i.e., the layout, fabricprocess, choice of materials, and geometry. Their measured performareported in chapter 4. IR sensors of the beam, bridge, and membrane typ

Fig. 3.8: Cross-section through the fixture used for the protection of the wafront side during KOH etching.

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAWafer

FixtureSeal ring Cavity

44


arraysd with

ems.

ned inof therangedr ou- theouples.

fabricated. Sensors of the bridge and membrane type were integrated into with up to ten rows and columns. Single sensors and arrays were cointegrateon-chip signal conditioning electronics, in order to form integrated microsystThe devices are reported in the following order:

• Single sensors, beam type.• Single sensors, bridge type.• Single sensors, membrane type.• Sensor arrays, composed of several sensors.• Sensor systems, composed of sensors and electronics.

CMOS thermopiles for beam, bridge, and membrane sensors can be desigtwo ways. In the first, the two thermocouple legs are stacked one on top other as shown in fig. 3.9 a. In the second, the two thermocouple legs are arlaterally as shown in fig. 3.9 b. Both layouts are characterized by the numbeN ofthermocouples, the spacing w0 between them, and the widths of both thermocple legs, i.e. w1 and w2. The spacing between the two thermocouple legs insecond type was designed to be equal to the spacing between the thermoc

Fig. 3.9: Layout of stacked (a) and neighboring (b) thermopile layout.

Material 1Material 1

Material 2Material 2

(a) (b)

w 1

w 2

w 0

w 1

w 2

w 0

w 0

45


tigatedmask- thatfabri-ility

tralctures. They

ndiationferentsted iniches

nt

sorber

Beams

Beam and bridge type thermoelectric sensors have previously been invesby Sarro et al. [32] and Lenggenhager et al. [34,71]. They are obtained by less bulk-micromachining with EDP. The advantage of these types ispost-processing is simple and can be done with a single chip for prototype cation. However, their size is limited to roughly 300 µm by mechanical staband a maximum etching time of 4 h [66].

A series of test structures test1 to test10 was designed to measure the specabsorptivity of layer sandwiches applicable for IR sensors. These test struare simple thermoelectric IR sensors of the beam type as shown in fig. 3.10

were fabricated in the ECPD 10 process. The cantilevers are 300 µm long a150 µm wide. A 150 µm by 150 µm square area at the tip serves as radabsorber. It consists of the layer sandwich under test. Nine versions with difabsorber sandwiches were fabricated. The various layer sandwiches are litable 3.6. These combinations are a complete list of the absorbing sandwwhich can be fabricated reproducibly using the ECPD 10 process and subsequemicromachining with EDP. From the seven layers deposited in ECPD 10,127combinations can be inferred, however, most of them can be excluded as absandwiches by the following considerations:

Fig. 3.10: Schematic top view and cross-section of a test structure.

Absorber Thermopile

Bulk silicon

46


arbi- and

ittedise it

uc-

tack.moved final

sed toiched

o IReonlyalent

layer

test

• The fabrication of absorbing stacks by the CMOS process does not allowtrary layer combinations. During fabrication all layers are first depositedthen etched away locally, where necessary. Thus, if a layer is to be omfrom the stack, the underlying layer has to offer a clear etch stop. Otherwis also removed (overetched). As an example, metal 2 may not be omitted ontop of metal 1. Dielectric layers may usually be removed only above condtors.As an exception, this rule does not apply to the lowest layer in the sOveretching of this layer attacks the substrate. Since the substrate is reby the subsequent micromachining, the overetch does not affect theabsorber sandwich.

• During the post-processing the top and bottom layer of the stack are expothe etchant. Polysilicon is attacked by EDP and thus has to be sandwbetween other layers.

• Aluminum with a thickness larger than 100 nm is completely opaque tradiation [73]. Thus, if metal 1 or metal 2 is included in the sandwich, only thlayers on top of the metallization absorb radiation. Sandwiches differing in the layers below the metallization, were thus considered as equivabsorbers. However, they still may differ by thermal conductance.

Systematic application of these rules results in a list of the nine absorbingsandwiches test1 to test9 listed in table 3.6. The absorber sandwich test10 isequivalent to test3 according to the last rule. Nevertheless it was included to

Layer

test

1

test

2

test

3

test

4

test

5

test

6

test

7

test

8

test

9

test

10

Passivation

Metal 2

Intermetal

Metal 1

Contact

Poly

Field oxide

Tab. 3.6: Absorbers layer compositions of the test structures.

47


theirctureorber

reb-

ingpilesm and. The

sted in

the validity of this rule. A micrograph of the structures test5, test7, and test9 isshown in fig. 3.11. The different absorber squares can be distinguished bydifferent degree of transparency. A thermopile is integrated in the test struwith cold contacts on the bulk silicon and hot contacts adjacent to the absarea. The thermopile consists of 30 thermocouples of poly and metal 1 with aspacing of 2 µm. The width of the metal 1 thermocouple legs is 1.5 µm. They astacked on top of the 2.5 µm wide poly legs. A reference structure without absoring area was also fabricated. Its schematic is shown in fig. 3.12.

The sensor named ECPD-I consists of two identical cantilever beams extendfrom two opposite sides of an etch cavity as shown in fig. 3.13. The thermoof both beams are connected in series. The beam length and width are 145 µ160 µm, respectively. The separation between the tips of the beams is 10 µmwidth of the lateral etch access openings is 70 µm. Sensor layout data are litable 3.7.

Fig. 3.11: Optical micrograph of the structures test5, test9, and test7.

48


oma-s areby the

d toges:

ding.or. one

Bridges

IR sensors of the bridge type are fabricated, like beams, by front-side micrchining with EDP. In comparison with beams the doubly supported bridgemore rugged and can be made larger. Nevertheless, their size is limited maximum etching duration of 4 h [65].

Five types of s-shaped bridge sensors were fabricated. One is based on theECPD10 process, the others on the alp1mv technology. When these sensors are usecreate an array, the distinctive s-shape shown in fig. 3.6 has these advanta

• The s-shape ensures complete underetching.• The in-plane stress in the structure may partially relax by out-of-plane ben• The rectangular bridge fits into the rectangular cavity, giving a high fill fact• A large number of identical bridges can be placed in a row, resulting in

long etched groove spanned by parallel bridges.

Fig. 3.12: Schematic top view of the reference structure.

Fig. 3.13: SEM micrograph of the sensor ECPD-I.

49


idges.

er. Itheavity.s areta arerfec-t-sideussed

ave

SEM inta for

• In the above configuration wires can run across the groove over the brThis is important for array addressing.

The sensor ECPD-II is shown in fig. 3.6. The central area serves as absorbcontains an integrated metal 2 reflector embedded in the dielectric layers. Tabsorber is connected by two arms to opposite rims of the micromachined cEach arm contains a thermopile with five thermocouples. The thermopileconnected in series. The overall size of the bridge is 100 µm by 150 µm. Daprovided in table 3.8. Along the border of the etch openings in fig. 3.6 impetions can be observed. These “stringers” are a severe problem for fronpost-processing. The origin of stringers and methods to avoid them are discat the end of this chapter.

The sensors alp1-I to alpI-IV were fabricated using the alp1mv process. They han s-shape similar to that of ECPD-II. In contrast to ECPD-II, they have no inte-grated reflector. The thermopiles extend to the center of the structure. Anmicrograph of alp1-III is shown in fig. 3.14. A polysilicon resistor is integratedthe middle of the bridge for testing and calibration purposes. The layout da

Sensor name ECPD-1

Process ECPD 10

Cavity size length [µm] 300

width [µm] 300

Etch opening width [µm] 70

Number of thermocouples 70

Thermo-couple

material A poly

width [µm] 2.5

spacing width [µm] 1.5

material B metal 1

width [µm] 1.5

layout stacked

Tab. 3.7: Layout parameters of sensor ECPD-I.

50


ed foropti-

d

the three sensors are listed in table 3.8. The sensors alp1-I, alp1-III, and alp1-IVare similar in bridge size and thermopile layout. All three sensors are designintegration in arrays for thermal imaging. The variations were introduced to mize the performance and to improve the post-processing yield.

Sensor name ECPD-II alp1-I alp1-II alp1-III alp1-IV

Process ECPD 10 alp1mv

Bridge size

length [µm] 150 182 258 192 192

width [µm] 100 188 325 195 195

Etchopening width [µm] 20 22 22 30 45

Number of th. couples 10 18 40 8 8

Thermo-couple

material A poly poly poly poly poly

width [µm] 2.5 2.5 2.5 10 10

spacing width [µm]

1.5 1.75 1.75 1.5 1.5

material B metal 1 metal 1 metal 1 metal 1 metal 1

width [µm] 1.5 1.5 1.5 1.5 1.5

layout stacked stacked stacked stacked stacke

Tab. 3.8: Layout parameters of bridge type sensors.

Fig. 3.14: SEM micrograph of alp1-III.

51


uthorsigher pro-

were

own iners of pas-em-an be

ane as.

g the

brane were were

h

izetheir

Membrane Sensors

Sensors of the membrane type have been investigated by various a[7,33-47]. Their advantage, in comparison with beams and bridges, is the hmechanical stability. This allows post-processing on the wafer level for massduction and fabrication of larger sensors. Membranes up to 14 by 16 mmsuccessfully etched [70].

All membrane sensors discussed in this thesis have the general layout shfig. 3.15. The membrane is rectangular and consists of all the dielectric laythe CMOS process, namely field oxide, contact oxide, intermetal oxide, andsivation. Two identical thermopiles are symmetrically integrated into the mbrane. They are connected in series. This layout is highly symmetric and cdescribed by a few parameters:

• The length lm and width wm of the membrane.• The number N of thermocouples.• The material of the thermocouple legs, their widths w1 and w2, and their spac-

ing w0.• The margin wb of the thermopile to the border of the membrane.

On some sensors a polysilicon heating resistor is integrated on the membrshown schematically in fig. 3.15. It is used for test and calibration purposes

Thirteen different membrane sensor designs were fabricated, three usinECPD 10 process, six using the alp2lv technology, and four using the alp1mv pro-cess. The membrane lengths range from 1500 µm to 200 µm. The memwidths are between 800 µm and 200 µm. The sensors larger than 600 µmfabricated for motion detector systems. The sensors smaller than 345 µmdesigned for application in arrays.

The sensor ECPD-III is similar to the beam sensor ECPD-I. They have a similarsize, but a different thermopile layout. The ECPD-III has 148 thermocouples wita 1.5 µm wide poly leg, while ECPD-I has 70 thermocouples with wpoly = 2.5 µm.All design data are shown in table 3.9.

The two sensors ECPD-IV and ECPD-V are based on large membranes. Their sis 630 µm by 625 µm and 720 µm by 720 µm, respectively. The detail of

52


ility of

sis-mo-

anes,

ow-

layout is given in table 3.9. These sensors were designed to test the feasiblarge membranes fabricated with the ECPD 10 process.

The sensors alp2-I to alp2-IV are all designed to have the same thermopile retance of 300 kΩ. However, they vary in size, thermocouple materials, and therpile layout. The sensors alp2-III to alp2-VI make use of the p+-poly available inthe alp2lv process. The design data are listed in table 3.10.

Three membrane based sensors were fabricated using the alp1mv process. Theirdesign data are shown in table 3.11. The sensor alp1-V has a size of 1500 µmby 700 µm. It is intended for a motion detection system. The smaller membri.e., alp1-VI and alp1-VII, are intended for a sensor array. The sensor alp1-VII wasfabricated in four versions. All versions have the same thermopile layout. H

Fig. 3.15: General layout of a membrane sensor.

wl

Membrane

Thermopiles

Marging

Resistor

Silicon substrate

w bm

m

53


ective

arly asraysannocuumhich

nte

iodic

ever, they vary in margin and consequently in size. The margins and respdimensions are listed in table 3.11.

Arrays

Linear arrays of integrated thermoelectric sensors have been studied as e1985 by Choi et al. [74] and later by Sarro et al. [75]. Two-dimensional arbecame feasible with the cointegration of sensors and circuitry. In 1994 Ket al. [46] reported an array of 128 by 128 sensor elements designed for vaoperation. In 1995 Oliver et al. [47] reported a 32 by 32 element array wallows operation in air at ambient pressure.

We have fabricated arrays using the ECPD 10 and the alp1mv process. Three dif-ferent arrays were fabricated with the ECPD-IV membrane sensors. Six differearrays were fabricated with the alp1-VI and alp1-VII membrane sensors from thalp1mv process. Another four arrays were made with the bridge sensors alp1-I,alp1-II, alp1-III, and alp1-IV using the same process. These arrays are per

Sensor name ECPD-III ECPD-IV ECPD-V

Process ECPD 10

Membranesize

length [µm] 345 630 720

width [µm] 325 625 720

Margin width [µm] 20 145 200

Number of th. couples 148 64 46

Thermo-couple

material A poly poly poly

width [µm] 1.5 7.5 11

spacing width [µm]

1.5 2 2

material B metal 1 metal 1 metal 1

width [µm] 1.5 1.5 1.5

layout stacked stacked stacked

Tab. 3.9: Layout of membrane sensors fabricated using the ECPD 10 process.

54


titutes a col-

within exam-

ally iso-

linesl sep-l lines

al

cess.

arrangements of the same sensor in columns and rows. Each sensor conspixel of the array. The array layout is described by the number of rows andumns and two pitches. Further issues are how to address individual pixels the array and how to suppress cross-talk between neighboring pixels. As anple, a close-up of an array of alp1-III pixels is shown in fig. 3.16.

The arrays fabricated using the ECPD 10 process are ArrECPD-I to ArrECPD-III.They consist of two membrane pixels ECPD-III. The two pixels are united on single membrane as shown in fig. 3.17. Since they share the same thermalated structure a large cross-talk is to be expected. Broad lines of the metal 1 and2 were thus integrated into the membrane, between the two pixels. Theseprovide a thermal separation between the pixels. To investigate this thermaaration scheme, different layouts were explored in the three arrays: No metain ArrECPD-I; one line, 10 µm wide, of stacked metal 1 and 2 in ArrECPD-II.;two lines, 4 µm wide and 2 µm apart, in ArrECPD-III.

Sensor name

alp2

-I

alp2

-II

alp2

-III

alp2

-IV

alp2

-V

alp2

-VI

Process alp2lv

Membranesize

length [µm] 1200 1000 1200 1000 1200 1500

width [µm] 800 500 800 500 700 700

Margin width [µm] 100 100 90 90 100 100

Number of th. couples 190 54 40 42 112 118

Thermo-couple

material A n+-poly p+-poly n+-poly n+-poly n+-poly n+-poly

width [µm] 8 27 11 7 3 4

spacing width [µm]

2.5 2.5 3 3 3 3

material B metal 1 metal 1 p+-poly p+-poly p+-poly p+-poly

width [µm] 2 2 34 26 9 12

layout stacked stacked lateral lateral lateral later

Tab. 3.10: Layout data of membrane sensors fabricated using the alp2lv pro

55


v

Sensor name alp1-V alp1-VI alp1-VII a/b/c/d

Process alp1mv

Membranesize

length [µm] 1500 216 200/240/260/250

width [µm] 700 216 200/240/260/250

Margin width [µm] 124 28 5/25/35/30

Number of th. couples 44 64 12

Thermo-couple

material A poly poly poly

width [µm] 55 2.5 30

spacing width [µm]

2 2 2

material B metal 1 metal 1 metal 1

width [µm] 2 1.5 1.5

layout stacked stacked stacked

Tab. 3.11: Layout data of membrane sensors fabricated using the alp1mprocess.

Fig. 3.16: SEM micrograph of an array of alp1-III sensors.

Grooves

Bulk silicon rim

PixelSwitch

Signal line

56


leermal

these cold

goldrown

i-here,port.

eeidthrray

nted byl

erentnepa-

ne.

Six arrays of membrane sensors were also fabricated using the alp1mv process.Like ArrECPD-I to ArrECPD-III they consist of pixels integrated on a singmembrane. In contrast to these, however, they are two-dimensional. The thseparation is provided by a similar scheme as with ArrECPD-III. A path with highthermal conductance is provided between the pixels by metal lines. In two-dimensional arrays the separation lines also provide a heat sink for thecontacts of the thermopiles. Instead of the CMOS metallization, 25 µm thicklines were used as shown in fig. 3.18. These lines are electrochemically gusing the gold bumping service of the alp1mv process. The signal lines of all indvidual pixels run under the gold lines to the border of the membrane. Taddressing switches are integrated in the bulk silicon of the membrane sup

The array ArrAlp1-VI consists of alp1-VI sensor pixels. The array consists of thrcolumns and four rows with pitches of 350 µm. The pixel is square with a wof 216 µm. The two gold lines are 50 µm wide with a spacing of 34 µm. The alayout data are listed in table 3.12.

The array ArrAlp1-VII was fabricated in five different versions. Like iArralp1-VI all pixels are located on one membrane and are thermally separagold lines. All versions consist of alp1-VII pixels with the same pitch and pixelayout. Four different versions of the thermal separation lines, and two diffarray sizes were implemented. Versions a, b, c, and d consist of seven by sevepixels, whereas version f consists of ten rows and ten columns. The thermal s

Fig. 3.17: ArrECPD-II with two ECPD-III sensors integrated on one membra

Membrane

Absorber

Thermopile

Thermalseparation

57


. Thes thecom-

s

EMus-rallelilicon

s rim.using are

linesidgesw ared out.me is

lly

ration in versions a, b, and c is provided by double gold lines, while versions d andf have only one line. The layout data of the five arrays are listed in table 3.12total width of the thermal separation varies, but the pitch is constant. Thupixels of the four versions have different sizes. These four versions allow to pare the cross-talk of different thermal separation layouts.

Four additional arrays were fabricated using the alp1mv process. These arrayconsist of front-etched, s-shaped bridge pixels. The array ArrAlp1-I to ArrAlp1-IVconsist of alp1-I to alp1-IV sensors, respectively. Figure 3.16 shows an Smicrograph of ArrAlp1-III . Each column in these arrays consists of pixels spended over a long micromachined groove. The array thus consists of paadjacent grooves. The cold contacts of the pixels are located on the bulk srim between the columns. A single signal line runs along each column on thiThe signals from the columns pixels are selectively connected to this line switching transistors integrated in the bulk silicon of the rim. The switchescontrolled by an addressing line as shown in fig. 3.19. For clarity the signalare highlighted in the SEM micrograph. The addressing line runs over the bralong the row. By selecting an address line, all pixels of the corresponding roconnected to their respective column signal line where their signal can be reaA schematic of this arrangement is shown in fig. 3.20. This addressing sche

Fig. 3.18: SEM micrograph of a pixel in ArrAlp-VII. Double gold lines thermaseparate neighboring pixels.

Pixel

Gold lines

58


ArrAlp1-VI Arralp1-VII

a b c d f

Pixel name alp1-VI alp1-VII

width [µm] 216 200 240 260 250 250

length [µm] 216 200 240 260 250 250

Columns number 4 7 10

pitch [µm] 350 330 330

Rows number 3 7 10

pitch [µm] 350 330 330

Thermalseparationlines

number 2 2 2 2 1 1

width [µm] 50 50 25 20 80 80

spacing [µm] 34 30 40 30

Tab. 3.12: Layout data of ArrAlp1-VI and five versions of ArrAlp1-VII.

Fig. 3.19: Addressing scheme used in ArrAlp1-I to ArrAlp1-III.

Pixel output

Signal line

Switch

Addressing line

59


s beenetersctricir gap.

0 µm.

. The

yield. are

and

-III.

extendable to arrays of arbitrary size. The same addressing scheme haapplied by Cole et al. [76] and Tanaka et al. [77] for micromachined bolomand by Oliver et al. [47] for an array of 32 by 32 micromachined thermoelesensors. The pixels within the arrays are separated from each other by an aThis ensures thermal separation and therefore a relatively small cross-talk.

The ArrAlp1-I consists of the smallest pixels alp1-I. Their size is 182 µm by188 µm. They are arranged in three columns and six rows with a pitch of 21The largest array is ArrAlp1-IV. It consists of 240 alp1-IV pixels arranged in six-teen columns and fifteen rows. The dimensions of alp1-IV are 190 µm by 200 µmThe pitch of the rows and columns is 217 µm and 243 µm, respectively.alp1-IV pixel is similar in layout to alp1-I and alp1-III. The alp1-IV is the newestversion and the small changes were made to improve the post-processingThe pixels alp1-II of the ArrAlp1-II are larger than the others. The dimensions258 µm by 325 µm. There are three rows and columns with a pitch of 347 µm287 µm, respectively. The layout data of arrays ArrAlp1-I to ArrAlp1-IV are givenin table 3.13.

Fig. 3.20: Schematic of the addressing scheme used in ArrAlp1-I to ArrAlp1

Pixel

Address lines

Signal lines

Switch

60


coin-tedhager

n andntial

Theocks. arraygram

Systems

Integrated IR sensor microsystems consist of one or more IR microsensorstegrated on a chip with signal conditioning circuitry. The first CMOS integramicrosystem using thermoelectric IR sensors were reported by Lenggenet al. [44] in 1993, and by Müller et al. [7] in 1995.

We report the fabrication of eight IR sensor microsystems using the alp2lv andalp1mv process. Six systems are designed for motion or presence detectioconsist of two IR sensors connected in parallel to the inputs of a differelow-noise amplifier. The circuit schematic of this setup is shown in fig. 3.21.systems vary in sensors and amplifiers. Some include additional circuit blTwo systems were fabricated for thermal imaging. They consist of a sensorcointegrated with addressing circuit and a low-noise amplifier. The block dia

ArrAlp1-I ArrAlp1-II ArrAlp1-III ArrAlp1-IV

Pixel name alp1-I alp1-II alp1-III alp1-IV

width [µm] 182 258 190 190

length [µm] 188 325 195 200

Columns number 3 3 10 16

pitch [µm] 210 287 210 217

Rows number 6 3 10 15

pitch [µm] 210 347 224 245

Tab. 3.13: Layout data of ArrAlp1-I, ArrAlp1-II, ArrAlp1-III., and ArrAlp1-IV.

Fig. 3.21: Schematic circuit of motion detection systems.

+

-

+ Sensor -

+ Sensor -

Amplifier

61


their
of these systems is shown in fig. 3.22. Table 3.14 lists all eight systems withsensors and low-noise amplifier blocks.
Fig. 3.22: Block diagram of the thermal imaging systems.

Name Sensor/Array Amplifier

Number Name PrincipleImpedance

[MΩ]Size [µm]

Noise [nV/√Hz]

SysAlp2-I 2 alp2-I

Bipolar 0.6860210

107SysAlp2-II 2 alp2-II

SysAlp2-III 2 alp2-III

SysAlp2-IV 2 alp2-IV

SysAlp2-V 2 alp2-VAuto-zero 100

850600

317SysAlp2-VI 2 alp2-VI

SysAlp1-IV 1 ArrAlp1-IVChopper 100

12601110

15SysAlp1-VII 1 ArrAlp1-VII

Tab. 3.14: Components of the sensor microsystems.

-

+

Sensor Array

Differential AmplifierColumn Multiplexer

Column Address

Row Address

Row

Sel

ect

Differential Sensor Signals

62


lockral

ffer-lator,-noise

ith a

tech-with theand- 3.6 V.hich

andkHz. sin-ows a

The systems SysAlp2-I to SysAlp2-IV consist of two sensors, alp2-I to alp2-IV,respectively, and the same low-noise amplifier. This is a standard circuit bfrom the analog cell library of alp2lv process. Its input stage consists of latebipolar transistors with an input resistance of 600 kΩ and gain of 60. Its size is860 µm × 210 µm.

The systems SysAlp2-V and SysAlp2-VI consist of two sensors alp2-V andalp2-VI, respectively, and five circuit blocks. These blocks are a low-noise diential amplifier, low-pass filter, bandgap reference voltage generator, osciland output stage. The systems are configured as shown in fig. 3.23. The low

amplifier was designed by Malcovati [8]. It has a MOSFET input stage wresistance larger than 100 MΩ, and a gain of 2000. Its size is 920 µm × 590 µm.To suppress the large flicker noise inherent in FETs it employs the auto-zeronique in a switched capacitor implementation. A white noise spectrum 317 nV/√Hz was obtained. To minimize clock feed-through and optimizepower supply rejection ratio, a fully differential architecture was used. The bgap reference generates an analog ground from the power supply of 0 andThe anti-aliasing filter limits the signal and noise bandwidth of the sensors, wis necessary for the sampling of the amplifier. The clock for the amplifieroutput stage are provided by the on-chip oscillator with a frequency of 40 The output stage converts the differential signal of the primary stage into agle-ended output and provides the necessary impedance. Figure 3.24 sh

Fig. 3.23: Block diagram of the SysAlp2-V and SysAlp2-VI.

+

- -

++ Sensor -

+ Sensor -

63


. Theytput.

y-eenue,

thanathr isf theere

micrograph of SysAlp2-V. The systems are integrated on a chip 3.5 mm × 3.5 mmin size, small enough to be packaged in a standard TO-5 transistor headerrequire three pins for operation, two for the power supply and one for the ou

The systems SysAlp1-IV and SysAlp1-VII are intended for thermal imaging. Theconsist of a sensor array ArrAlp1-IV and ArrAlp1-VII, respectively, with addressing and multiplexing circuits and a low-noise amplifier. The amplifier has bdeveloped by Menolfi [10]. He applied the chopper stabilization techniqobtaining a low-frequency noise-level of 15 nV/√Hz and an offset below 1 µVwith a transistor-only CMOS implementation. The input impedance is larger100 MΩ. To allow operation in a mixed-signal environment the signal pthroughout the amplifier was kept fully differential. The size of the amplifie1260 µm by 1110 µm. To take full advantage of the differential architecture oamplifier, the signal paths throughout the array and multiplexing circuits wkept differential.

Fig. 3.24: Micrograph of SysAlp2-V.

Bandgap

Sensors

Amplifier

Filter

Output stage

Oscillator

reference

64

3.4 Post-Processing Restrictions

plifier anss. Am by

dtem

tech-ized

The system SysAlp1-IV consists of ArrAlp1-IV with 240 alp1-IV pixels in fifteenrows and sixteen columns. Each individual pixel can be connected to the amthrough the multiplexing circuit. The multiplexing is controlled by applyingeight-bit binary address with four bits each for the row and column addremicrograph of the system is shown in fig. 3.25. The size of the chip is 5.5 m6.2 mm.

The system SysAlp1-VII consists of ArrAlp1-VII with 100 alp1-VII pixels in tenrows and columns. Like in SysAlp1-IV the signals from the pixels are multiplexeon-chip and directly fed to the amplifier. The chip with the integrated microsysis shown in fig. 3.26. Its size is 5.5 mm by 6.2 mm.


While commercial CMOS processes offer a reliable and well documented nology for circuit fabrication, they are usable, but neither intended nor optim

Fig. 3.25: Micrograph of SysAlp1-IV.

65


thisfrontsible

the yields are steps con- stress

lax byss may

for, the fabrication of micromachined structures. Two difficulties arise from “misuse” of CMOS technology: broken membranes and stringers in the access openings. In the following, we analyse their origin and report posmethods to avoid them.

Mechanical Stability

In the fabrication of microelectronic circuits the mechanical properties ofapplied materials are a minor issue, but they are crucial for the stability andof micromachined structures. Only the initial stress of the deposited thin filmroutinely monitored in CMOS processes and tuned in view of film adhesion,coverage, and electromigration. Generally, a minimal compressive stress isidered optimal [78]. For beams, bridges, and membranes both the in-planeand the stress gradient perpendicular to the plane are of importance.

Cantilever beams are singly clamped structures. Their in-plane stresses remotion of the unclamped end. Nevertheless, during the release etch the stre

Fig. 3.26: Micrograph of SysAlp1-VII d.

66


radientre fab-them byding

planeengthophic

beams struc-e stresse crit-n the

re thess for

-

cause the structures to break as discussed in [66]. An out-of-plane stress gcauses the structures to bend as shown in fig. 3.27. The beams in fig. 3.27 aricated in the ECPD 10 process. They consist of all dielectric layers, except field oxide. The length and width of the beams are 200 µm by 100 µm, 400 µ200 µm, 600 µm by 300 µm, and 300 mm by 600 µm, respectively. The benradius is inversely proportional to the stress gradient [79]. Thus the out-of-displacement at the tip is larger for long structures. The beam with 400 µm lshows a tip deflection of 80 µm. Although bending does not cause catastrfailure, it is often undesirable and limits the maximum length of beams.

Bridges and membranes do not react to stress gradients as strongly asbecause they are clamped on two respectively four sides. Doubly clampedtures also react to in-plane stresses. For homogenous, tensile or compressivbelow a critical value, they remain flat. If the compressive stress exceeds thical value they buckle out of the plane. The critical stress level depends olength to thickness ratio of the structure. The longer and thinner a structulower the buckling threshold stress. Jaeggi [6] measured the critical stresquare membranes composed of the dielectric layers from the alp2lv process. Heobtained a value of -13.2 a-2 Pa m-2, where a is the dimension of the square membrane.

Fig. 3.27: Beams bending due to a stress gradient in their layer sandwich.

67


thentrastarraysthe

e wasing.rmly

of the etch The end,nding in the littlere the prop-

These considerations explain the following failure we observed. Duringpost-processing, membranes with sides larger than 6 mm broke. In cosmaller membranes did not fail. Membranes of such size are required for with more than 20 alp1-VII pixels. The breaking occurred along the border of

membrane as shown in fig. 3.28. Associated with the crack in the membrana delamination in the dielectric layers. Closer examination showed the followIn KOH the etching of membranes larger than 1 mm does not proceed unifoacross the membrane area. The etch rate is higher along the perimeter(100)-etch front and its corners. Figure 3.29 shows the profile of such anfront. It is obtained from a 6 mm by 6 mm membrane after 4 h of etching.height of this profile is approximately 15 µm. As the etching approaches thethe strongly compressive membranes buckled. The buckling introduces a bestress along the borders of the membrane. At this stage the remaining siliconmiddle of the membrane is 10 µm thick, the corners are clear of silicon andsilicon remains along the sides. Cracks in the silicon occurred in areas whebending stress is concentrated due to the thinner silicon. These cracks then

Fig. 3.28: Location of the cracks on the etched membranes.

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

Silicon remains

DielectricMembra

ne

Etch mask

Cracks

68


roughielec-

ws ater ofin on

agated into the dielectric layers as shown in fig. 3.30. They propagated ththe lowest membrane layers, allowing the KOH to attack the intermediate dtrics.

This interpretation is supported by the micrograph shown in fig. 3.31. It shocrack in the dielectric layers along a sidewall of the etched cavity. The centhe membrane is still covered with silicon. Separate patches of silicon rema

Fig. 3.29: Profile of the etch front from a membrane 6 mm by 6 mm in size.

Fig. 3.30: Buckling membrane towards the end of the etching process.

54

32

10

65

43

21

0

20

10

0

Hei

ght [

µm]

y [mm]x [mm]

CracksSilicon substrate

Membrane Silicon remains

69


to the

layerin filmn be

f pro-mpen-

ing flat

ed by fieldreatesrefore

ectionr theotoli-r highce of

les inng is

each side of the crack. Their borderlines are parallel and in equal distancecrack.

The problem of breaking dielectrics has been solved by finding an oxynitride which serves both as passivation and stress compensation. Stress in thoxynitride is largely determined by the deposition conditions. This stress cacontrolled over a range from 300 MPa to -300 MPa by appropriate control ocess parameters [70]. We chose a passivation with a small tensile stress, cosating the compressive stress of the other membrane layers, and achievmembranes.

Etch Access Openings

The access opening required for front side bulk micromachining are obtainstacked openings in the passivation, intermetal oxide, contact oxide, andoxide [66]. Such a structure never occurs in a standard CMOS circuit and ca non-standard situation in the patterning of these layers. Special care is therequired. Insulating dielectric layers are usually patterned to enable a connbetween the two conducting layers below and above the insulator. Aftedielectric is deposited on the lower conductor, an opening is obtained by phthography and subsequent etching. The etching process is optimized foselectivity to the underlying conductor. Thus the etching stops at the interfathe two layers. Timing is not critical.

Two problems may occur when this procedure is used to obtain stacked hodielectric layers. They are schematically shown in fig. 3.32. First, if an openi

Fig. 3.31: Micrograph of a crack in the silicon and lower dielectric layers.

Cavity wall

Si remains

Membrane

Crack

70


fter to theunder- for the layerfficienturing

. Theng the

resultngs ismaskin thexideof theure is

ers.

etched in the dielectric with no underlying conductor, overetching occurs. Aetching through the top dielectric is complete, the subjacent layer is exposedetchant. Instead of stopping at the interface, the etching proceeds into the lying layer. Second, when two stacked openings are present the step heightnext layer in the stack becomes unusually high. As a consequence this thirdand the resist on the steps are thicker than usual. Standard exposure is insufor resist of such thickness. Along the steps some resist is not dissolved ddevelopment. It eventually prevents the underlying layer from being etchedresulting structures are called stringers. Such stringers can be observed aloborder of the etched cavity in figs. 3.6 and 3.13.

Stringers can be minimized by proper design of the dielectric openings, the of which can be seen on fig. 3.14. The layout used to obtain these openishown schematically in fig. 3.33. The contact oxide is not opened by the design. The intermetal oxide opening is shifted with respect to the opening field oxide. With an intentional, long overetch of the passivation the contact oand gate oxide is opened. No stringers occur, and a well defined border etched structure is obtained. A cross-section through the resulting structshown in fig. 3.34.

Fig. 3.32: Photolithography problems from stacked openings in dielectric lay

Insulators

Overetch

Resist

StringersThick resist Resist residuals

71


.

AAAAAA

Fig. 3.33: Staggered openings in the dielectric layers to minimize stringers.

Fig. 3.34: SEM cross-section through the border of an etch access opening

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AAAAAAAAAAAA

AAAAAAAAAAAA

AAAAAA

Passivation etch

Silicon

Field oxideContact oxide

Intermetal oxide etchContact oxide deposition

Passivation Contact Oxide

Field Oxide

Intermetal Oxide

Metal 2

Metal 1

Poly

Silicon

72

4.1 Sensitivity Measurements

spectiveecondents.

ovidesitivityrum is

ea-

res setupposedm the

4 CHARACTERIZATION

This chapter reports the setups used to characterize the sensors and the reexperimental results. The first part describes sensitivity measurements, the sthe characterization of arrays, and the third spectral absorptance measurem


The sensitivity of IR sensors depends on the radiation wavelength. To prmeasurement results relevant for motion detection, we measured the senusing a broadband radiation spectrum similar to that of a person. This spectprovided by a blackbody at controlled temperature.

Measurement Principle

According to eqn. (2.38) the sensitivity S is defined as

. (4.1)

In practice it is more reliable to measure responses U1 and U2 of the sensor to twodifferent radiation power levels P1 and P2. The sensitivity is then calculated as

, (4.2)

where ∆U = U1 - U2 and ∆P = P1 - P2. This method reduces errors such as msuring equipment offset. To provide a controlled power difference ∆P we built ameasurement setup consisting of two blackbodies at respective temperatuT1and T2, a chopper with reflecting blade, a mirror, and an aperture stop. The is configured as shown in fig. 4.1. The sensor under test is alternatingly exto radiation from either blackbodies. When the chopper is open, radiation fro

SUP----=

SU∆P∆

--------=

73

4 Characterization

the at the

axis

blackbody at T1 is reflected from the fixed mirror through the open chopper todevice under test. With the chopper closed, radiation from the blackbody atT2 isreflected onto the sensor. The aperture stop is designed in such a way th

Fig. 4.1: Schematic view of the blackbody measurement setup.

Fig. 4.2: Arrangement of sensor, aperture stop, and blackbody with optical unfolded.

Blackbody, T

Blackbody, T

Chopper

Mirror

Sensor

Aperture Stop

1

2

Aperture Stop, rA

Blackbody, rB

Sensor, rS

dSA

dAB

74


trates

sor,and fromalter-

hand differ-s

,80].onicalpen-lack

angehape andy the

sensor views only the two blackbodies through the aperture. Figure 4.2 illusthis requirement. It can be stated as

, (4.3)

where rA, rB, and rS denote the radii of the aperture, blackbody, and senrespectively. The symbols dSA and dAB denote the distance between sensor aperture, and blackbody and aperture, respectively. If condition (4.3) holds,the sensor’s point of view the aperture stop is equivalent to a blackbody of nating temperature. The radiation power density pin incident on the sensor fromthe blackbody at T1 is

, (4.4)

where ε denotes the emissivity of the blackbody and the last term on the right-side is the spatial angle of the aperture stop with respect to the sensor. Theence in intensity ∆pin between the open and closed chopper phases results a

. (4.5)

Blackbody

In our realization of the blackbodies we followed a standard approach [13Each blackbody consists of a conical cavity in a massive metal body. The ccavity is closed by a flat stop with circular opening as shown in fig. 4.3. This oing acts as the black surface. The inside wall of the cavity is coated with 3M Bpaint. The spectral emissivity of the paint is high and uniform over a wide rin the infrared [80]. The emissive power of the opening is influenced by the sof the cavity as well [80, 81]. The length and radius of the cavity are 93 mm50 mm, respectively. The radius of the opening is 25 mm. With this geometrtheoretical cavity emittance is 0.997.

rS r B+

dSA dAB+-----------------------

rS r A+

dSA----------------≤

pin dSA T1,( ) εσT14 r A

dSA--------

2=

pin dSA T1 T2, ,( )∆ εσ T24

T14–( )

r A

dSA--------

2=

75

4 Characterization

l wasircu-

ownade

iral

The cavity was machined from a massive copper cylinder. A spiral channecut into the surface of the cylinder as shown in fig. 4.4. It is used for water c

lation. A copper tube, fitting over the cavity cylinder, covers the spiral as shin fig. 4.3. The cavity is fixed in its cylindrical steel housing by two supports m

Fig. 4.3: Schematic cross section through the blackbody in its housing.

Fig. 4.4: Photographs of the cavity machined from a copper cylinder. The spcut into the cylinder is used for a water channel.

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AAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAA

A

AAAAAAAA

AA

AA

AAAA

AA

AA

AAAA

AAAAAAAAAAAAA

AAAAAAAAAAAAA

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AAAAAA

AA

AAAAAA

AAAAAAAA

AAAAAAAAAA


AAAAAA

Copper body

Hollow cone

Support

Housing

Water channel

Temperature sensors

Black aperture

76


withh thesing ailiza-n the

rature00°C. shown

withA andl seg-

dies.

out of “Delrine”. The space between the copper body and the housing is filledglass wool. Water from a thermalized bath (Julabo HC4) is pumped througwater channel around the body. The temperature of the bath is regulated utemperature sensor directly inserted into the blackbody. After the initial stabtion the bath achieves a temperature stability better than ±0.02°C. In additiotemperature of the copper body is monitored by an independent tempesensor with an absolute error smaller than 0.03°C in the range from 0° to 1Two such blackbodies were made and placed in the measurement setup asin fig. 4.5.

The mirror and the reflecting chopper blade were made from PMMA coatedchrome and gold. Chrome was used as an adhesion layer between PMMgold. Two openings were machined into the blade in the shape of 90° radiaments giving it a duty cycle of 50%.

Fig. 4.5: Photograph of the sensitivity measurement setup with two blackboThe optical axis is indicated by a dotted line.

BlackbodiesChopper MirrorReflectingChopper Blade

SensorMounting Sensor Stop Temperature Monitor

Aperture Stop

77

4 Characterization

.5. Itsce fromckedsor is

ing the, and

ro-ts withnce in

or the ar dis-

bound

The sensor stop used for the adjustment of the setup is also shown in fig. 4aperture is larger than the sensitive area of the sensor under test. Its distanthe aperture stop was adjusted to satisfy the condition (4.3). Its validity is cheby looking through the sensor stop. Keeping the stop fixed in place, the senthen mounted directly behind it.

Characterization

We determined the performance of the measurement setup by characteriztemperature stability of the blackbodies, the time used for initial stabilizationthe achieved emissive power.

The total emissive power e was measured using an electrically calibrated pyelectric radiometer (ECPR) as a reference. We compared the measurementheoretical values obtained from eqn. (4.5). The ECPR measures the differeabsorbed power density ∆p between the two chopper phases. This power ∆p isgiven by

, (4.6)

where αλ denotes the absorptance of the ECPR. This parameter is known frange from 0.1 µm to 14 µm. The uncertainty about αλ outside this range leavesrange of possible power density readings for a given incident spectral powetribution ∆pλ. The lower bound ∆pmin

, (4.7)

corresponds to no absorption outside the 0.1 - 14 µm range, and the upper∆pmax corresponds to αλ = 1

. (4.8)

p∆ pλ dSA T1 T2, ,( )∆ αλ λd

0

∞

∫=

p∆ min pλ dSA T1 T2, ,( )∆ αλ λd

0.1 µm

14 µm

∫=

p∆ max p dSA T1 T2, ,( )∆=

78


5 mm tem-

nsity

s been stablevia-

The radiation intensity was measured with an aperture stop diameter of 9.from a distance of 235 mm. The diameter of the ECPR sensor is 8 mm. Theperature of the first blackbody T1 was kept at 24°C and T2 was varied from 24°Cto 74°C. Figure 4.6 shows the comparison of measured ∆p, and the calculated∆pmax and ∆pmin. The measurements agree with theory and the measured intecorresponds to a blackbody emittance of at least 0.98.

Figure 4.7 shows the blackbody temperature after the thermalized bath haswitched on and set to 30°C and 70°C. The temperature is reached andwithin 45 min and 140 min respectively. After stabilization, the measured detions from the mean value are smaller than 0.01°.

Fig. 4.6: Measured radiation power density ∆p, and theoretical values ∆pmin and ∆pmax for T1 = 24°C as a function of temperature difference ∆T = T2 - T1.

J

J

J

J

J

0

0.5

1

1.5

2

0 5 10 15 20 25 30 35 40 45 50

Po

we

r d

en

sity

[kW

/m2 ]

Temperature difference [°]

∆pmax

∆pmin

∆p

79

4 Characterization

e alsoimatedidgews the

Measurement Results

Table 4.1 lists the measured sensor area A, thermopile resistance RT, sensitivity S,and response time τ of the beam and bridge type sensors. The NEP and D*, calcu-lated from the measured results according to eqns. (2.43) and (2.44), arlisted.The measurement error for the sensitivity and response time are estat 5% to 10% due to the small signals(< 1 µV) that are involved. The bralp1-II with the largest area, and the largest thermopile resistance also sholargest sensitivity of 30.4 V/W. The best NEP and D* however, is obtained withthe alp1-III bridge sensor. The changes introduced in the design of alp1-III and

Fig. 4.7: Blackbody settling from room temperature to 30°C and 70°C.

Sensor A RT S τ NEP D*

[mm2] [kΩ] [V/W] [ms] [nW] [cm √Hz/W]

ECPD-I 0.090 161 20.9 0.83 2.5 1.21 107

ECPD-II 0.015 9.1 13.1 0.063 0.9 1.31 107

alp1-I 0.034 57 27 0.012 1.14 1.63 107

alp1-II 0.084 181 30.4 0.22 1.80 1.61 107

alp1-III 0.037 7.0 11.6 0.005 0.93 2.08 107

alp1-IV 0.037 7.9 11.1 0.005 1.03 1.88 107

Tab. 4.1: Sensor characteristics of beam and bridge type sensors.

20

40

60

80

0 60 120 180

Tem

pera

ture

[°C

]

Time [min]

70°C

30°C

80


vity.

ors fab-nce,

en-

s.

alp1-IV with respect to alp1-I decreased the thermopile resistance and sensitiAs a consequence of the lower resistance the NEP and D* are improved.

Table 4.2 lists the measured sensor characteristics for the membrane sensricated using the ECPD 10 process. The membrane with the highest resistai.e., ECPD-III shows the highest sensitivity of 20.7 V/W. The better NEP and D*of 2.5 nW and 2.85 107 cm√Hz/W, respectively, are obtained with ECPD-Vwhich has a lower sensitivity and resistance.

The sensor membranes listed in table 4.3 are all fabricated in the alp2lv process.The four sensors with n+-poly/p+-poly thermopiles, alp2-III to alp2-VI, show thehigher sensitivity than alp2-I and alp2-II. The largest membrane, alp2-VI with aresistance of 2.2 MΩ shows the highest sensitivity of 45.8 V/W. Among the ssors from the alp2lv process the largest D* of 4.33 107 cm√Hz/W is obtained withthe alp2-III sensor. An even higher D* of 6.69 107 cm√Hz/W is obtained with



ECPD-III 0.11 546 20.7 1.8 4.6 0.73 107

ECPD-IV 0.39 100 15.6 6.5 2.7 2.37 107

ECPD-V 0.52 50 11.4 0.72 2.5 2.85 107

Tab. 4.2: Characteristics of membrane sensors from the ECPD 10 proces



alp2-I 0.96 319 21.1 15.4 3.5 2.84 107

alp2-II 0.50 262 26.4 4.8 2.5 2.83 107

alp2-III 0.96 274 29.8 25.4 2.3 4.33 107

alp2-IV 0.50 276 27.5 9.8 2.5 2.88 107

alp2-V 0.84 2700 40.9 19.5 5.2 1.77 107

alp2-VI 1.05 2260 45.8 13.1 4.2 2.43 107

Tab. 4.3: Characteristics of membrane sensors from the alp2lv process.

81

4 Characterization

d

r-

s:

hich struc-

sensi-s inin the] aree totalivities,orber

son ist of

istedwing any

alp1-V. The measured sensor characteristics of alp1-V are listed in table 4.4 withthe those from the other sensors fabricated using the alp1mv process. As expectethe four versions of alp1-VII show very similar performance. The version c withthe largest area shows the highest D*. The highest sensitivity is observed for vesion d.

Comparison of these figures of merit with values reported in literature show

• The performance of our devices is similar to those reported in [7,36,71], ware fabricated using CMOS processes and include a dedicated absorbingture covering less than a quarter of the membrane. The numbers for thetivity reported in [7,36] are up to five times larger than the numbertables 4.3 and 4.4. This seeming discrepancy is due to a difference method to calculate the sensitivity: the figures of merit reported in [7,36calculated with respect to the much smaller absorber area rather than thmembrane area used in this work. Lenggenhager [71] reported the sensitcalculated with both methods. The values with respect to the smaller absarea are up to 4.5 times larger for the same device. While direct comparidifficult, we conclude that our devices yield a performance similar to tha[7,36], even without dedicated absorbers.

• Völklein et al. [23] achieved sensitivities up to ten times larger than those lin tables 4.3 and 4.4 using optimized thermopile materials (Bi-Sb-Te) shoa high thermoelectric efficiency [72]. These materials are not included insilicon IC process and would have to be added on.



alp1-V 1.050 10.7 8.7 14.6 13.3 6.69 107

alp1-VI 0.047 91.2 12.0 0.192 10.2 0.67 107

alp1-VII a 0.040

3.6

4.1 0.006 5.9 1.07 107

alp1-VII b 0.058 4.1 0.007 5.9 1.28 107

alp1-VII c 0.068 4.2 0.010 5.8 1.42 107

alp1-VII d 0.063 4.3 0.009 5.0 1.56 107

Tab. 4.4: Characteristics of membrane sensors from the alp1mv process.

82

4.2 Array Characterization

e per-d the ofand aver-

n rms

pro-pixels

could pixelsss linece, the

4.2 Array Characterization

Two aspects are important for the performance of sensor arrays apart from thformance of the pixels, namely the cross-talk between neighboring pixels anuniformity of the pixel performance over the array. The uniformitiesArrAlp1-VI and ArrAlp1-VII were investigated by measuring the resistance sensitivities of their pixels. Results are shown in figs. 4.8, 4.9, and 4.10. The

age resistance in ArrAlp1-VII is 3.6 kΩ with a root mean square (rms) deviatioof 1.1%. The sensitivities of these pixels are shown in fig. 4.9. They have adeviation of 5.5% from the average of 4.3 V/W. The pixel yield after post-cessing and dicing was 98%. The thermopile resistance of the front-etched in ArrAlp1-IV is shown in fig. 4.10. Their average resistance is 7.9 kΩ with a rmsdeviation of 7.2%. The missing values in the figure correspond to pixels that not be characterized. The address lines in this array are integrated in thewhich are s-shaped bridges (cf. fig. 3.14). If a pixels is damaged the addreis broken. Therefore a number of other pixels can not be addressed. Hen

Fig. 4.8: Resistance of thermopiles in ArrAlp1-VII d.

4

3

2

1

0

Res

ista

nce

[kΩ

]

RowsColumns

83

4 Characterization

Fig. 4.9: Sensitivities of pixels in ArrAlp1-VII d.

Fig. 4.10: Resistance of thermopiles in ArrAlp1-IV.

6

5

4

3

2

1

0

Sen

siti

vty

[V/W

]

RowsColumns

15

10

5

0

Res

ista

nce

[kΩ

]

RowsColumns

84


onal5%.

gratedhbors.

a-

ceacked by

ts hass a func-rrow0 µm.rptancetanceponse

t struc-cludedsorber

half-column of missing resistance values in fig. 4.10. The yield of operatipixels is 93%. Their average sensitivity is 11.1 V/W with a rms deviation of 1

Cross-Talk

The cross-talk in the arrays was measured by heating a pixel with the interesistor and comparing the response of the heated pixel with that of its neigThe cross-talk in ArrECPD-I, ArrECPD-II, and ArrECPD-III is 5.5%, 4.8%, and4.7%, respectively. Broad lines of metal 1 and metal 2 are used as thermal sepration between the pixels of these arrays: No metal lines in ArrECPD-I; one line,10 µm wide, of stacked metal 1 and 2 in ArrECPD-II.; two lines, 4 µm wide and2 µm apart, in ArrECPD-III . The large cross-talk and the small differenbetween the different versions shows that the thermal separation with stmetal 1 and metal 2 is inefficient. In contrast to this, the thermal separationgold lines is very effective. A cross-talk of 1% was measured in the versiond ofArrAlp1-VII with one gold line. In the versions a, b, and c with the pairs of thermalseparation lines, the cross-talk was below 0.2%.


A previous measurement setup [71] for spectral reflectance measuremenbeen expanded to determine the response of sensors and test structures ation of radiation wavelength. Devices can be exposed to radiation of a nawavelength band, whose center wavelength can be scanned from 2 µm to 2Specialized test structures were used to measure the relative spectral absoαλ of the layer sandwiches applicable for IR sensors. The spectral absorpwas determined by measuring both absolute radiation power and device resas a function of wavelength.

In this section the measurement setup, radiation power measurement, testures, and evaluation of the acquired data are explained.The section is conwith the experimental results, i.e., the relative spectral absorptance of the absandwiches.

85

4 Characterization

, radi-wn in

resis-

par-ter oftween of the2. Ittion

l raysction

ith

Measurement Setup

The measurement setup consists of a monochromator (Jobin-Yvon HR250)ation source, radiation chopper, imaging optics, and sample holder as shofig. 4.11. The source, the so-called globar, is a silicon carbide rod heated

tively to 1550°K. Depending on its configuration the monochromator is transent to a narrow wavelength band of the radiation from the globar. The centhe band can be adjusted between 2 µm and 20 µm, its width varies be0.04 µm and 0.15, µm depending on the wavelength. The monochromator isCzerny-Turner type. Its operating principle is shown schematically in fig. 4.1consists of an entrance slit, two flat mirrors, two parabolic mirrors, a reflecgrating, and an exit slit. The first parabolic mirror shapes a bundle of parallefrom the light emerging from the entrance slit. The parallel rays hit the reflegrating and are reflected according to

, (4.9)

where θ denotes the angle of incidence on the grating, θ'n the angle of reflectionof order n, and d is the period of the grating. Two gratings are available w

Fig. 4.11: Schematic view of the spectral measurement setup.

AAAAAAAAAAAA

Chopper SampleGlobar LensMonochromator Vacuum chamber

AmplifierOscilloscopeComputer

θ'nsin θsin– nλd---=

86


cusesy dif-ngths

oma-nt

150 lines/mm and 60 lines/mm, respectively. The second parabolic mirror fothe parallel rays forming an image of the entrance slit, with images formed bferent wavelength radiation and reflection order separated spatially. Wavelecan be chosen by turning the reflection grating, thus varying θ and θ'. For a fixedorientation of the grating the images of wavelengths λn satisfying

. (4.10)

are projected onto the exit slit. A long-wavelength-pass filter at the monochrtor exit is used to block all λn with n > 1. Several filters can be chosen for differewavelength ranges. These are

• Filter 0: 2.0 µm - 3.0 µm• Filter 1: 2.4 µm - 4.8 µm• Filter 2: 3.5 µm - 7.0 µm• Filter 3: 4.3 µm - 8.6 µm• Filter 4: 7.3 µm - 14.6 µm

Fig. 4.12: Schematic view of the monochromator.

θ θ’

Parabolic Mirrors

Turnable grating Exit slitEntrance slit

Mirrors

λn

λ1

n-----=

87

4 Characterization

r. Theinate

d by The

ac-n. The

-

n, the

nt on 4.13m

The lens is used to image the exit slit onto the sample in its test chambechamber is evacuated to increase the sensitivity of the sample and to eliminfluence from moving air and sound. The signal of the sample was amplifiea low noise amplifier and registered by a digital oscilloscope LeCroy 9420.signal data is then transferred to a computer for evaluation.

Radiation Power Measurement

The spectral radiation power PR(λ) incident on the device under test was charterized using the ECPR reference detector placed into the sample positiorelation between PR(λ) and the power incident on the sample under test PS(λ) isgiven by the following considerations. With eλ(x) denoting the intensity distribution in the image and AR the aperture area of the ECPR, the power PR(λ) is givenby

, (4.11)

where x denotes the position in the aperture plane. Assuming no dispersiospectral dependence in eλ(x) can be separated from the spatial variations f(x)

, (4.12)

and eqn. (4.11) can thus be written as

. (4.13)

Thus, the power PS incident on the sample under test with area AS is

, (4.14)

where C is a constant independent of the wavelength. Thus the power incidethe sample is proportional to the power measured with the ECPR. Figureshows the measured spectral radiating power PR(λ). The segments in the spectru

PR λ( ) eλ x( ) AdAR

∫=

eλ x( ) eλ f x( )=

PR λ( ) eλ f x( ) AdAR

∫=

PS λ( ) eλ f x( ) AdAS

∫ CPR λ( )= =

88


The 4.5.

ngth. rat-

correspond to different filters and gratings applied in the monochromator.segments, filters, and gratings used for the measurement are listed in table

Fig. 4.13: Radiation power measured with the ECPR as a function of waveleThe segments of the spectrum correspond to different filters and gings applied in the monochromator.

Wavelength range [µm]

Filter No. Grating

[lines/mm]

2.0 - 3.0 0

1502.9 - 4.1 1

4.0 - 7.0 2

6.9 - 7.7 3

7.6 - 14.6 4 60

Tab. 4.5: Measured wavelength ranges with the applied filters and gratings.

0

10

20

30

40

50

60

70

2 3 4 5 6 7 8 9 10 11 12 13 14

Wavelength [µm]

Po

we

r [µ

W]

89

4 Characterization

bsorp-cturessand-

pective

e isf such

er-

aria-uctionacuums the

erptionr

Test Structures

Specialized test structures were devised to measure the spectral relative atance αλ of the layer sandwiches applicable for IR sensors. These test struand their absorbing sandwiches are described in section 3.3. A list of the wiches is given in table 3.6.

To obtain the absorptance of the absorber material we measured the resresponses of the test structure ∆U(λ) and a reference structure ∆U0(λ) to the inci-dent radiation power PS(λ) as a function of wavelength. The reference structuridentical to the test structures but has no absorbing area. A schematic view oa reference structure is shown in fig. 3.12. To calculate the absorptance αλ fromPS(λ), ∆U(λ), and ∆U0(λ) we used the following analytical model of the tempature distribution along the cantilever.

In view of the small thickness (3 - 5 µm) of the cantilevers the temperature vtions perpendicular to the beam plane are neglected. Heat loss by condthrough the surrounding gas can also be neglected under the experimental vconditions. Because of the highly symmetrical layout of the test structuretemperature is a function of the distance from the support of the beam.

The incident radiation PS(λ) causes a temperature elevation ∆T(λ) of

, (4.15)

where K is the thermal conductance of the beam. It is defined as the sum

(4.16)

over the component layers (indexed with n), where κnand an denote the respectivethermal conductivities and cross-sectional areas and lb denotes the length of thbeam. The first term on the right hand side of eqn. (4.15) results from absoin the absorber area and ∆T0(λ) is the contribution from the radiation powe

∆T λ( )αλ λ( ) PS λ( )⋅

K------------------------------- ∆T0 λ( )+=

K1l b---- κnan

n∑=

90


refer-

asured

d-d the

valelow ange. (PSD)ous

absorbed in the thermopile area. The latter term can be determined with theence structure (αλ = 0), for which eqn. (4.15) reduces to

. (4.17)

Thus the absorptance of the absorber area can be calculated from the metemperature difference as

. (4.18)

The temperature differences ∆T(λ) and ∆T0(λ) are obtained from the corresponing thermopile signals ∆U(λ) and ∆U0(λ). Combining eqns. (2.31), (4.14) an(4.18) allows αλ to be obtained from the measured thermopile signals andECPR measurement via the proportionality

. (4.19)

Data Evaluation

To measure the response ∆U(λ) of the test structures a specialized signal retriemethod was required. Since the radiation intensity on the samples is b1 W/m2 and their area is 22×10-9 m2, only a few nanowatts are absorbed. Withsensitivity in the range of 10 V/W the resulting signals are in the nanovolt raTo distinguish the signals from the noise we used a phase sensitive detectortechnique [82]. It employs radiation chopping in combination with synchronsignal averaging.

The oscilloscope was configured to repeatedly record the signal U(t) during acycle of the chopper. From the acquisitions over N chopper periods of length tc theaverage of the form

(4.20)

∆T λ( ) ∆T0 λ( )=

αλ λ( )∆T λ( ) ∆T0 λ( )–

PS λ( )K--------------------------------------=

αλ λ( )∆U λ( ) ∆U0 λ( )–

PR λ( )----------------------------------------∼

U

U t( )1N---- U t ntc+( )

n 0=

N

∑=

91

4 Characterization

d withcess.e ratios then

o sensor

pared

pproxi--state

e doess of the

was calculated. Because the noise, in contrast to the signal, is not correlatethe chopping, its amplitude is reduced by the factor in the averaging proThus, by taking the average over thousand chopper cycles the signal-to-noiscan be improved by more than a factor of 30. The averaged waveform waanalyzed to find the signal amplitude ∆U.

To find the steady-state signal amplitude ∆U from the dynamic response tchopped radiation two cases have to be distinguished. The response of a(section 2.6) to a sudden change in radiation power at t = 0 is given by

. (4.21)

Two cases may be distinguished. In the first case, the chopping is slow comwith the sensor response time, i.e.

. (4.22)

In this case, at the end of each open or closed chopper phase the signal amates its steady-state value with sufficient accuracy. Thus finding the steadyamplitude from U(t) is straightforward. The amplitude ∆U is calculated as

, (4.23)

where the chopper cycle starts at t = 0.

In the second case, i.e. if condition (4.22) is not satisfied, the sensor responsnot reach the steady-state value, in neither the open nor the closed phasechopper. Thus ∆U is obtained from the dynamic response U(t). It is described by

, (4.24)

N

U

U t( ) U 0( ) U 1 et– τ⁄–( )∆+=

τ tc«

U∆ U tc 2⁄( ) U tc( )–=

U t( ) U∆ 1 et– τ⁄–

1 et– c 2τ⁄

+------------------------= 0 t tc 2⁄< <

92


fple of

ntical struc-

differ-e of theet thempleicularlane.ioned

ne).

for the open chopper and by

(4.25)

for the closed chopper. The amplitude ∆U is thus obtained by fitting this model othe dynamic behavior to the experimental data. Figure 4.14 shows an examsuch a measured dynamic response and the model fit.

Measurement Results

The measurements of the spectral absorptance αλ performed in this work providerelative values. They involve the response of different sensors under ideradiation conditions, i.e., the device under test, the ECPR, and the referenceture. For successful measurements two conditions have to be met: First, theent sensors have to be placed at the same position with respect to the imagexit slit, and second, the radiation intensity has to be stable with time. To mefirst requirement we kept the distance from the exit slit to the lens and safixed and moved the lens with a micropositioning stage in a plane perpendto the optical axis, thus moving the image of the exit slit in the sample-pScanning the image intensity distribution with the sensor, the lens was posit

Fig. 4.14: Example of measured sensor response (dots) and fitted model (li

U t( ) U∆ et th–( ) τ⁄–

1 etc 2τ⁄–

+------------------------= tc 2⁄ t tc< <

X

XXX

XXX

XX

X

XXXXXXX

XXXXXX

XX

XXXXXXXXXX

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

XXXXXXXX

XX

X

XXX

X

XXXXXX

XXX

X

XX

XXXXX

XXXXXXXXXX

XXXXXXXXXXXXXXXXX

XXXXXXXXXXX

X

XXX

XXXX

XXXXXXXXXXXX

XXX

XXXXX

X

XXXX

XX

XXXXXX

XXXX

Res

pons

e

Time

93

4 Characterization

in theithin

12 h

wn in

te was the

cturesradia-re by theectral

d

to give the maximum response, corresponding to the location of the samplecenter of the image. With this method sample readings were reproducible w2%. The stability of the radiation intensity was measured over a period ofwith the ECPR. A maximum deviation of 0.3% was found.

The results of the relative spectral absorptance measurements are shofigs. 4.16 to 4.19. The spectra are calculated from the measured values of PS(λ),∆U(λ), and ∆U0(λ) according to eqn. (4.19). The results αλ are given in percenof the largest absorptance found for all test structures. This maximum valufound for test3 at a wavelength of 12.45 µm. The spectra are labeled with

layers of the stack exposed to radiation. The absorbing layers of the test struare listed in table 4.6. Since aluminum thicker than 100 nm is opaque to IR tion, layers below metal 1 and metal 2 are shielded from the radiation and aassumed not to contribute to the absorptance. This assumption is confirmedspectra shown in fig. 4.15. The figure shows the measured relative sp

Test structures Absorbing layers

Test1 passivation 26%

Test2 metal 2 12%

Test3 passivation, metal 2 59%

Test4 passivation, intermetal oxide 31%

Test5 passivation, intermetal oxide, metal 1

55%

Test6 passivation, intermetal oxide, contact oxide

32%

Test7 all dielectrics 35%

Test8 passivation, intermetal oxide, poly, field oxide

57%

Test9 all dielectrics, poly 63%

Tab. 4.6: Test structures with their absorbing layers (see also table 3.6) anaverage absorptance.

α

94


e

ce is

m. To aver-

.

absorptance of test3 and test10. Both consist of all dielectrics and metal 2, in addi-tion the absorber sandwich of test10 includes metal 1. Nevertheless, the averagdeviation between both spectra is only 2%.

In test1 and test2 single layers act as absorbers. Their relative absorptanshown in fig. 4.16. The topmost layer of test2 is metal 2. Pure aluminum is a goodIR reflector [73] and a weak absorptance can be expected from metal 2. In agree-ment with this it shows an absorptance below 24% over the entire spectrucharacterize the absorption of thermal radiation we introduce the weightedage absorptance

, (4.26)

for radiation of a blackbody at 296°C with respect to a background at 293°C

Fig. 4.15: Relative absorptance of test3 and test10 absorbers.

0%

25%

50%

75%

100%

2 3 4 5 6 7 8 9 10 11 12 13 14

Wavelength [µm]

Re

lativ

e A

bso

rpta

nce

test10

test3

α

ααλ eλ 296K( ) eλ 293K( )–[ ] λd

2µm

14.6µm

∫eλ 296K( ) eλ 293K( )–[ ] λd

2µm

14.6µm

∫---------------------------------------------------------------------------------------=

95

4 Characterization

he. No µm.

inm to

absorp-

e rel-

e

with

The weighted average absorptance of test2 is 12%. The absorber of test1 con-sists of the oxynitride passivation. It shows 30% to 50% relative absorption in tband from 8 µm to 13 µm. An absorption peak of 10% is observed at 2.95 µmsignificant absorption was measured from 2 µm to 2.85 µm and 3.15 µm to 7The average weighted absorptance is 26%. The absorber of test3 consists of thepassivation and metal 2. Its relative absorptance spectrum is also shownfig. 4.16. An absorptance of 50% to 100% is observed in the band from 8 µ14.5 µm. A 37% absorptance peak was measured at 2.95 µm. The average tance is 55%. The increased absorption in test3 with respect to test1 is explainedby the reflecting properties of metal 2. Radiation transmitted by the passivation isreflected back by the metal 2 layer and passes again through the passivation.

A similar effect is observed with the spectra in fig. 4.17. The figure shows thative absorptance of test4 composed of passivation and intermetal oxide and test5consisting of the same composition with a metal 1 reflector. For comparison therelative absorptance of the passivation is also shown. The combination of thintermetal and passivation layer shows a similar spectrum to the passivation

Fig. 4.16: Relative absorptance of the passivation, metal 2, and passivationmetal 2.

0%

25%

50%

75%

100%

2 3 4 5 6 7 8 9 10 11 12 13 14

Wavelength [µm]

Re

lativ

e A

bso

rpta

nce

test1:passivation

test2: metal 2

test3:passivation,metal 2

α

α

α

96


etal,

s.

Fig. 4.17: Relative absorptance of absorbers containing passivation, intermand metal 1.

Fig. 4.18: Relative absorptance of absorbers containing only dielectric layer

0%

25%

50%

75%

100%

2 3 4 5 6 7 8 9 10 11 12 13 14

Wavelength [µm]

Re

lativ

e A

bso

rpta

nce

test1:passivation

test5:passivation, intermetal, metal 1

test4:passivation, intermetal

0%

25%

50%

75%

100%

2 3 4 5 6 7 8 9 10 11 12 13 14

Wavelength [µm]

Re

lativ

e A

bso

rpta

nce

test1:passivation

test4:passivation, intermetal

test6:passivation, intermetal, contact oxide

test7:all dielectrics

97

4 Characterization

eased.bsorp-

reased 50%.

tance issharp sharp

offacts

ayers.

alone. Absorption bands between 8 and 10 µm and 11.5 and 13 µm is incrMaximum absorption of 62% occurs between 8.5 and 9 µm. The average atance is 31%. The absorption of the same sandwich with a metal 1 reflector isenhanced over the entire spectrum. The absorptance peak at 2.95 µm is incto 31% and the absorptance in the band from 8 µm to 14.5 µm is higher thanThe highest absorptance of 96% occurs at 8.5 µm and the average absorp55%. The peak at a wavelength of 4.4 µm is an artifact. It is due to the decrease in the radiation power at the same wavelength (cf. fig. 4.13). Thisdecrease in combination with a wavelength error in the radiation spectrum PR(λ)relative to the spectral response ∆U(λ) leads to large error signals. The peak18% at 4.4 µm is explained by a wavelength error of 0.025 µm. Similar artiare also observed in the spectra of test3, test8, and test9.

Figure 4.18 shows the relative absorptance of test1, test4, test6 and test7. Theabsorber sandwiches of these four test structures contain only dielectric lThe absorber stack of test6 consists of the passivation, intermetal oxide, and con-tact oxide. Its spectrum differs only slightly from the spectrum of the passivation

Fig. 4.19: Relative absorptance of absorbers containing the poly layer.

0%

25%

50%

75%

100%

2 3 4 5 6 7 8 9 10 11 12 13 14

Wavelength [µm]

Re

lativ

e A

bso

rpta

nce

test8:passivation,intermetal, poly, field oxide

test9:all dielectrics, poly

test5:passivation, intermetal, metal 1

98


her,sorber

from 35%.

which

of aver-d for a

t arum is

layer

by

by

nge

e

with intermetal oxide. In the band from 6.8 µm to 7.8 µm its absorptance is hignamely more than 20%. The average weighted absorptance is 32%. The ablayer stack of test7 contains the field oxide in addition to the layers of test6. Theeffect of the additional layer is a slight increase of the absorption in the band7 µm to 9 µm. The maximum increase is 10%. The average absorptance is

Finally, fig. 4.19 shows the relative absorptance of the absorber layer stackscontain poly. These are the absorber sandwiches of test8 and test9. For compari-son the sandwich composed of passivation, intermetal oxide, and metal 1 is alsoshown. All dielectric layers and poly are contained in the absorber sandwichtest9. It shows an absorption of at least 24% over the entire spectrum and anage absorptance of 63%. The strongest absorptance of 99% is observewavelength of 8.25 µm. Similarly the spectral absorptance of test8 is higher than23% for a range from 2.6 µm to 14.6 µm. It consists of the same layers astest9with the exception of the contact oxide. Its peak absorptance of 85% occurs awavelength of 8.3 µm. The average weighted absorptance over the spect57%. The largest difference between the spectrum of test8 and test9 is 21%, theaverage difference is 7%.

As a conclusion the effect of the individual materials on absorption of the sandwich is summarized:

• Passivation absorbs uniformly in the range from 8 µm to 12 µm.• The intermetal layer increases absorptance in the band from 8 µm to 10 µm

more than 10%.• The contact oxide increases absorptance in the range from 6.8 µm to 7.8 µm

roughly 20%.• No significant change in the absorptance is observed due to the field oxide.• The absorptance of metal 2 is below 25% over the entire spectrum. Combini

an absorbing layer with metal 2 or metal 1 below strongly enhances the relativabsorptance.

• In contrast to the dielectric layers poly shows significant absorption in thrange from 2 µm to 7 µm.

99

5 Modeling

entsressed actu-

ctricse of of thesed innalyt-ent

oured the

als.

tical,n for

tionsw bested to

o the the

elec- for asors.

5 MODELING

To keep up with today’s short product cycles and time-to-market requiremthere is a need for accelerated microsensor development. This need is addby sensor models predicting the performance of the devices before they areally fabricated. Various models [23,37-42] for the optimization of thermoelesensors have been reported. The models vary in complexity, flexibility, eause, and accuracy. The most accurate model would be an analytical solutionsteady-state heat transfer problem given by eqn. (2.27). Although, as discussection 2.3, a general solution to this problem exists, it cannot be obtained aically. Approximations to the solution can be found using the finite elemmethod (FEM). We used the SOLIDIS FEM modeling toolkit developed atlaboratory [83] to study the steady-state response of IR sensors. We checkaccuracy of this method by comparing measured and modeled sensor sign

In this chapter we discuss three models of thermoelectric sensors: AnalyVariational, and FEM. We conclude with an examples of a sensor optimizatioa presence detection application using the FEM.

Optimizing System Performance

Optimizing an IR detector system means varying its layout within the restricdictated by the fabrication process and the application in order to reach theperformance. Depending on the application, different figures of merit are uscharacterize performance. We chose the NEP for presence detector systems.

All models allow to calculate the sensor resistance R and the sensitivity S. Thenormalized detectivity D* and noise equivalent power NEP of the sensor can becalculated from these. If, in addition, the optics parameters are known, alsNETD can be obtained. To calculate the performance of the system fromsensor characteristics, a simple analytical model of the signal conditioningtronics is used. Figure 5.1 shows the equivalent electric circuit model usedpresence detector system with a differential low-noise amplifier and two sen

100

e
The thermopiles are described as ideal voltage sources U with a series resistancR. The amplifier is characterized by its input impedance RA, gain g, and noise volt-age density VA. The sensitivity Ssys and noise voltage density Vsys of the system,are then calculated as
(5.1)

and

. (5.2)

Each thermopile contributes the Johnson noise density of 4kTR. Substituting thisresult into eqn. (2.43) yields

. (5.3)

Fig. 5.1: Schematic circuit of a thermoelectric presence detector system.

AmplifierThermopile

g

V

Ra

RU

RU

a

Ssys SgRA

RA 2R+--------------------=

Vsys g f∆ VA2 8kTR f∆+=

NEPsys

Vsys

Ssys----------

RA 2R+

RAS-------------------- f∆ VA

2 8kTR f∆+⋅= =

101

5 Modeling

of theratureence ise total

mpera-

of

meterepos-

,tionsopile ises are

Similarly, the normalized detectivity

(5.4)

and noise equivalent temperature difference

(5.5)

are obtained by substitution into eqns. (2.44) and (2.45).

Thermoelectric sensors are two-stage transducers. The first stage consiststhermally isolated absorber structure that converts radiation into a tempeincrease. Its efficiency is given by α/K, where, α and K denote the absorptancand thermal conductance of the sensor, respectively. The thermal conductadefined as the ratio of the average hot contact temperature increase and thabsorbed power. The second stage is the thermopile which converts the teture increase into an electrical signal. Its efficiency is given by γN, where γ denotesthe Seebeck coefficient. The sensitivity S of the complete sensor is the productthese two efficiencies

. (5.6)

This expression suggests that increasing α, 1/K, γ, and N would improve thesensor performance. However, this is misleading. Often, increasing one paramay lead to reduce another. For example, if a special IR absorbing layer is dited onto the membrane to increase α, at the same time 1/K is decreased. Similarlyan increase in the number of thermopiles N improves the thermopile efficiencybut simultaneously increases its thermal conductance. Thus, for optimizathese two stages can rarely be considered separately because the thermintegrated in the supporting structure and thus, the performance of both stagintertwined.

Dsys∗ A ∆f⋅

NEPsys-------------------=

NETDsys

nf2

E--------

NEPsys

A------------------ 1

σT3

----------⋅=

SUP----

αK---- γN⋅= =

102

5.1 Analytical Model

cribingese thenctric is givenefined

rs (cf.

struc-er-

oper-e sur- heat

ighlyy in the

AAA

AAAAAAA

The approach followed here is to use a set of independent parameters desthe layout. The efficiencies α/K and γN are then expressed as a function of thparameters. The optimal sensor design for the use with a given amplifier isfound by combining eqns. (5.3), (5.4), or (5.5) with eqn. (5.6). For thermoelesensors based on a given CMOS process, a natural set of such parametersby the thermopile materials and the sensor and thermopile dimensions as din section 3.3. The four thermopile dimensions are the widths w1 and w2 of the twothermocouple materials, their separation distance w0, and for membrane sensothe lateral distance wb between the thermopile and the membrane edgefig. 3.15). The thermopile efficiency γN and resistance R are determined by thechoice of materials and the number of thermocouples fitting onto the sensorture. The absorptance α is given by the materials of the CMOS process. The thmal conductance K was calculated using three different models.


Analytically the simplest case is the beam-type structure shown in fig. 5.2 ated in vacuum. In this case only the heat conduction in the thermopile and throunding dielectrics must be considered. As discussed in section 4.3 the

transfer equation can be simplified to a one-dimensional problem in this hsymmetrical sensors. We assume that radiation is absorbed homogeneousl

Fig. 5.2: Sensor of the beam type.

A

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAPackage

Substrate

103

5 Modeling

ture

duc-5.7)

e ther-

. 5.3

thef

aking

A

area of the beam and that the support of the beams is at ambient temperaT0.Then eqn. (2.27) reduces to

, (5.7)

where tb, κ, and p denote the thickness of the beam, its average thermal contivity, and the incident radiation intensity, respectively. The solution of eqn. (shows that the temperature distribution is parabolic along the beam and thmal conductance K of the beam is given by

, (5.8)

where a denotes the cross-sectional area of the beam, and P = plTwT the total inci-dent radiation power. In view of the cross-section of the beam shown in figthe product aκ can also be written as

, (5.9)

where κ1, κ2, t1, and t2 denote the thermal conductivities and thicknesses oftwo thermopile materials, respectively, and ad and κd the cross-sectional area othe dielectrics in a thermocouple and their average thermal conductivity. T

Fig. 5.3: Cross-section of a thermopile.

tbκ∇ x2T x( ) αp–=

KαPT∆ lT( )

---------------- 2aκα P

αplT2wT

------------------- 2aκlT

----------= = =

aκ N κdad κ1w1t1 κ2w2t2+ +( )=

AA AA AA A AA


AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAwd

w1

w2

ad

104


con-

tys

e two

into account that both the thickness and thermal conductivity of the layers arestant for a given CMOS process we define the thermal sheet conductivity for both thermocouple materials and for the dielectrics with

. (5.10)

Then K is given by

. (5.11)

Substitution of eqn. (5.11) into eqn. (5.6) yields

, (5.12)

where i runs over 1, 2, and d. Equation (5.12) implies that for maximum sensitiviall thermocouple dimensions w0, w1, and w2 should be minimal. However, thialso causes a maximum resistance R. It is given by

, (5.13)

where j runs over 1 and 2, and and denote the sheet resistances of ththermocouple materials. With eqns. (5.12) and (5.13) the NEP and D* are givenby

(5.14)

κ κ t=κd κdad wd⁄=

wd w1 w2 2w0+ +=

K 2NlT---- κdwd κ1w1 κ2w2+ +( )=

Sαγ lT

2 κ iwii∑

-----------------------=

R NlTρ j

wj-----

j∑=

ρ1 ρ2

NEP4 κ iwi

i∑αγ

-----------------------

f∆ kTNρ j

wj-----

j∑lT

-----------------------------------⋅=

105

5 Modeling

kf the

ir. The to

pack-tes,

and

. (5.15)

From eqn. (5.15) it is obvious that for maximum D* the absorptance, Seebeccoefficient, and length of the beam should be maximal. If the contribution odielectrics is neglected D* is a function of the ratio q = w1/w2, viz.

. (5.16)

For a given set of material parameters the optimal x is easily found. For the alp1mvprocess, for instance, the optimal ratio wpoly/wmetal is 108.4.

This model has been expanded to approximate the sensor operation in aapproach followed by Elbel et al. [37], Völklein et al. [38], and Dillner [40] isexpand eqn. (5.8) to

, (5.17)

where Gcond describes the heat flow from the beam to the substrate and the age at T0. Approximating this flow by the heat conduction between parallel plaGcond is

, (5.18)

D∗ f∆ wTlTNEP

-----------------------αγ lT

4 κ iwii∑

-----------------------wi

i∑kT

ρ j

wj-----

j∑-----------------------⋅= =

D∗ q( )αγ lT

4 qκ1 κ2+( )----------------------------- 1 q+

kTρ1

q----- ρ2+

------------------------------⋅=

tbκ∇ x2T x( ) T x( ) T0–( )Gcond αp–=

Gcond

κair

ds---------

κair

dp---------+=

106

5.2 Variational Model

pack-

try thed cir-orre-

klein

then in

thee

nsid-opile

where ds and dp denote the distance between the beam and the substrate andage, respectively. The resulting temperature distribution is of the form

, (5.19)

where C is a constant depending on material parameters and geometry.

The membrane type sensors require more complex models. Due to symmeheat transfer problem is essentially two-dimensional. Only for quadratic ancular symmetric membranes a reduction to one dimension is possible. Csponding analytical models and solutions have been reported by Völet al. [38] and Dillner [40].

5.2 Variational Model

To find the thermal conductance K of rectangular membrane sensors we usedvariational method to calculate the two-dimensional temperature distributiothe membrane. A parametrized set of test functions TC(x) is used to approximatethe temperature distribution, where C stands for a set of parameters. Then parameter values C whose respective TC(x) best approximates the solution of thheat transfer equation are calculated. These value satisfy

, (5.20)

where I is the variational integral corresponding to eqn. (5.17), i.e.,

. (5.21)

Due to symmetry, only a quarter of the membrane, as shown in fig. 5.4, is coered. The quarter membrane area is divided into two parts, namely the therm

T x( ) T0CxsinhClTsinh

--------------------+=

δCI 0=

I12---– tκ TCx∇( )2 1

2---αpTC

2– GcondTC+ xd∫=

107

5 Modeling

e. The

nsitiv-n

e sur-del-

area and the neighboring region between thermopiles and membrane edgtest functions TC(x,y) with the parameters C = (C0,C1,C2) are

(5.22)

in the thermopile area, and

, (5.23)

in the neighboring region. To validate the model we compared measured seities of alp2-I, alp2-II, alp2-III, and alp2-IV with calculated values. A deviatioof -6%, -55%, -2% and 17% was found for the four sensors, respectively.

5.3 Finite Element Model

To model the three-dimensional heat flow in the membrane sensors and throunding air, we carried out numerical simulations with the finite element mo

Fig. 5.4: Schematic view of the quarter membrane.

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA Membrane

Thermopiles

TC x( ) T0 C0

C1xsinh

C1lTsinh----------------------+=

TC x y,( ) T0 C0

C1xsinh

C1lTsinh----------------------

C2ysinh

C2wbsinh------------------------⋅+=

108


ving

tosi-med to

e sur- werein.

ated.

rep-

ing toolbox SOLIDIS [83]. Thermopile temperatures were obtained by solthe static, three-dimensional heat transfer equation

, (5.24)

where κ is the thermal conductivity tensor, IA denotes the heat generation duethe absorption of IR radiation, and Iel denotes the electrical power density dispated for calibration purposes in the integrated heater. The radiation is assube absorbed homogeneously over the entire membrane area and thicknessd. Thus

.

Equation (5.24) was solved in the domain defined by the sensor chip and throunding air in the encapsulation. The surfaces of the substrate and filtertaken as heat sink at T0 (cf. eqn. (2.20)). Figure 5.4 shows the simulation domaIn view of the symmetry of the device only a quarter of the volume was simul

Fig. 5.5: Simulation domain with discretization mesh. For clarity the mesh resenting air is not shown.

∇ κ T∇( )⋅ I A I el+=

I A αpd1–=

Package lid

Substrate

Silicon Chip

Membrane

Thermopile

109

5 Modeling

h wasining

aniso-eelent

sured

rated

ted in. The

imu-

mely

andstedation mate-

10% sim-

e cal--

e

o

To reduce the number of finite element nodes, an effective-medium approacchosen to simulate the integrated thermopiles: Membrane volumes contathermocouples were replaced by an equivalent homogenous material with tropic thermal conductivity κ. The respective thermal conductivities in the thrprincipal directions were calculated similar to eqn. (5.11) using equivalumped thermal resistance circuits [42].

Validation of the Model

To validate the finite element simulations we compared modeled and mearesponsivities of nine sensors in three situations:

• The sensor is in vacuum. An electrical power is dissipated in the integheater and the thermopile output is measured. The sensitivity Svac isdefined as .

• The sensor is at ambient pressure in air. An electrical power is dissipathe integrated heating resistor and the thermopile output is measuredsensitivity Sair is defined as .

• The sensitivity S is measured with the setup described in section 4.1 and slated assuming an absorptance α = 1.

All material parameters used in the numerical simulation of and , nathe thermal conductivity κ and Seebeck coefficient γ, were known from previousmeasurements. Table 5.1 shows simulated and measured responsivities

. A maximum deviation of 21% was found. We explain the deviations liin table 5.1 by the inaccuracy of the sensitivity measurement, the simulmodel, and the material parameters. Recent, improved measurements of therial parameters and film thicknesses [57] have shown deviations of 2% tofrom the values used for the simulations. We estimate the inaccuracy of theulation model at 5%, mainly due to the lumped element analysis used for thculation of the anisotropic thermal conductivity κ in the thermopile area. The inaccuracy of the sensitivity measurement is estimated at 5% to 10%.

In the simulation of the responsivity S, the absorptance α is the only unknownparameter. The ratio of measured and modeled S provides an experimental valuof the true absorptance. Measured and calculated responsivities S are listed intable 5.2 together with the deduced absorptances α. These range from 0.40 t

PelUvac

Svac Uvac Pel⁄=Pel

UairSair Uair Pel⁄=

Svac Sair

Svac

Sair

110


mem-s;

f thesere

ed

0.76. To check the validity of the deduced absorptances α we compare them withthe measured relative absorptance reported in section 4.3. The sensorbranes consist, in different amounts, of three absorbing layer sandwichealldielectrics and poly or passivation, intermetal, metal 1 in thermopile area and alldielectrics on the rest of the membrane. The average relative absorptance othree sandwiches with ECPD 10 materials in the range from 2 µm to 14.6 µm a

Sensor [V/W] [V/W]measured simulated % measured simulated %

alp2-I 63.2 67.3 6 44.6 49.8 12

alp2-II 59.7 66.1 11 45.1 48.7 8

alp2-III 121.5 125.4 3 64.0 70.1 10

alp2-IV 104.4 105.4 1 68.3 68.9 1

alp2-V 314.8 330.2 5 162.6 197.3 21

alp2-VI 294.0 292.6 0.5 162.0 182.6 13

ECPD-III 62.8 59.7 -5 61.3 54.0 -12

ECPD-IV 106.5 89.9 -15 76.8 66.3 -14

ECPD-V 102.0 91.0 -11 65.7 60.9 -7

Tab. 5.1: Measured and simulated sensor responsivities.

Sensor S [V/W]measured simulated α

alp2-I 21.1 30.1 0.70

alp2-II 26.4 35.7 0.74

alp2-III 29.8 52.3 0.57

alp2-IV 27.5 36.7 0.75

alp2-V 40.9 103.1 0.40

alp2-VI 45.8 90.1 0.51

ECPD-III 20.7 27.3 0.76

ECPD-IV 15.6 22.7 0.69

ECPD-V 11.4 18.0 0.63

Tab. 5.2: Measured and simulated sensor responsivities and deducabsorptances.

Svac Sair

α

111

5 Modeling

ed fored from

y con-na-ener-

imen-

aturee sur-. Thiseric

nduc-

dis-modeltimeon-th this of the

ceranes.stem

f

63%, 55%, and 35%, respectively. Taking into account the spectral range usthe measurements and the estimated accuracies, the absorptances deducthe simulation and the spectral measurement are in agreement.

5.4 Comparison of Models

The three models for the thermal conductance of the thermopile sensors varsiderably in complexity, flexibility, computational effort, and accuracy. The alytical model is the most transparent and enables the formulation of some galized rules independent on sensor layout. This model is practical for one-dsional heat flow and useful mainly for beams and bridges.

The variational model makes it possible to calculate two-dimensional temperdistributions in sensor membranes. The heat conduction through air to throunding package is modeled by a heat flow perpendicular to the membranemodel can be implemented with most existing tools for algebraic and numcomputation as e.g. Mathematica™. The time to calculate the thermal cotance K of a sensor was approximately an hour on a Sun Sparc 5 workstation.

The FEM model allows the calculation of the three-dimensional temperaturetribution in the sensor membrane and surrounding air. To implement such a an FEM tool such as SOLIDIS™ or ANSYS™ is required. The computation on a Sun Sparc 5 workstation is approximately a minute. The validation demstrates that the most reliable thermal conductance values are obtained wimodel. Based on these results we chose to use the FEM for the optimizationsystem performance.

5.5 Device Optimization

To find the optimum layout for the alp2-VI sensors for application in a presendetector microsystem we performed FEM simulations of the sensor membThe NEPsys was then calculated according to eqns. (5.3) and (5.6). The syincludes an auto-zero amplifier and was fabricated in the alp2lv technology. Thesize of the sensor membrane was restricted to 1500 µm × 700 µm for reasons omechanical stability and system size.

112


twoasn-d for ther-

rmo-

detec-he

Figure 5.6 shows the modeled sensitivity of the alp2-VI sensor withp+-poly/n+-poly thermocouples. In the simulations we varied the width of the thermocouple legs wn and wp. The margin of the membrane in this simulation w100 µm while the thermocouple spacing w0 was chosen at the technological miimum of 3 µm. As expected from eqn. (5.12) the largest sensitivity is obtaineminimal thermocouple widths. The thermopile resistance as a function of the

mocouple dimensions is shown in fig. 5.7. The sensor with the minimal thecouple widths has the largest resistance.

The calculated NEP of the sensor alone is shown in fig. 5.8. The smallest NEP of6.5 nW is obtained for wp = 18 µm and wn = 6 µm. The corresponding modeleNEPsys is shown in fig. 5.9. The optimum value of 11.5 nW for the presence dtor system is found for wp = 12 µm and wn = 4 µm. These results were used for tlayout of the alp2-VI sensor integrated in the SysAlp2-VI system. If we had opti-mized the stand-alone sensor for NEP, the resulting NEPsys of SysAlp2-VI would

Fig. 5.6: Modeled sensitivity of alp2-VI as a function of wn and wp.

1814

106

2

48

1216

20

200

150

100

50

0

Sen

siti

vit

y [

V/W

]

wp [µm] wn [µm]

113

5 Modeling

imiza-
have been 7% higher, namely 12.3 nW. This example demonstrates that opttions have to take into account the entire system.
The layout of alp1-V was optimized in the same manner as alp2-VI for a systemwith a chopper amplifier. Similarly the pixels of SysAlp1-VII were optimized withrespect to the NETD of the system.

Fig. 5.7: Calculated thermopile resistance of alp2-VI as a function of wn and wp.

1814

106

2

48

1216

20

25

20

15

10

5

0

Res

ista

nce

[M

Ω]

wp [µm] wn [µm]

114


Fig. 5.8: Modeled NEP of alp2-VI as a function of wn and wp with minimum at wp = 18 µm and wn = 6 µm.

1814

106

2

48

1216

20

30

25

20

15

10

5

0

NE

P

[nW

]

wp [µm] wn [µm]

115

5 Modeling

Fig. 5.9: Modeled NEPsys of SysAlp2-VI as a function of wn and wp with mini-mum at wp = 12 µm and wn = 4 µm.

1814

106

2

48

1216

20

30

25

20

15

10

5

0

NE

PS

ys [

nW

]

wp [µm] wn [µm]

116

6.1 Presence Detector

builtile theld, bat-ance oftured.pli-temsial26). Amputerulting

ther--d oner an6.1. Itdelec-ircuits. The

nsordiationgnalsperson

6 DEMONSTRATORS

To show the potential of the microsystems reported in chapters 3 and 4 wetwo demonstrator systems. The first addresses presence detection, whsecond demonstrates IR thermal imaging. The presence detector is handhetery operated, and able to detect the presence of a human being at a distapproximately four meters. A small series of 32 demonstrators was manufacTheir core is a SysAlp2-VI microsystem with two sensors and a low-noise amfier on the same chip. The thermal imager is based on the microsysSysAlp1-IV and SysAlp1-VII. These systems contain a low-noise differentamplifier, a sensor array, and addressing electronics (see figs. 3.25 and 3.lens is used to project an IR image of the scene onto the sensor array. A cois used to control the acquisition of the pixel signals and displays the resimage.


The presence detector demonstrator is a small instrument (27 mm × 30 mm ×62 mm) detecting and indicating the presence of a person in its vicinity. Themal radiation of the person is sensed by the SysAlp2-VI microsystem. Upon detection of such an IR radiation source a red light emitting diode (LED) is turneand an electrical signal is provided on a connector allowing, e.g., to triggalarm. A photograph of the presence detector demonstrator is shown in fig. consists of a housing, a battery, and a SysAlp2-VI microsystem packaged anmounted on a printed circuit board (PCB) with additional signal processing tronics. The microsystem contains two IR sensors and signal conditioning cas described in section 3.3. The two sensors cover two different spatial anglesignal conditioning circuit amplifies and filters the difference of the two sesignals. In the absence of a localized heat source, both sensors receive rafrom the background. If the temperature of the background is uniform the siof both sensors are equal the output signal of the circuit vanishes. When a

117

6 Demonstrators

y tem-als donding elec-shold,on bytem,gital

atelyo sec-

m capomes onhowstor. The sym-

ack).

is present, its radiation is projected on one sensor only. Presumably the bodperature differs from the background temperature and thus, the sensor signnot cancel. The output of the microsystem is either positive or negative, depeon the location of the person in either sector. This output is processed by thetronics on the PCB. If the absolute value of the output exceeds a certain threeither positive or negative, the detector indicates the presence of the persswitching on the LED. Figure 6.2 shows the analog signal of the microsysamplified and filtered by the processing electronics, together with the dioutput of the demonstrator when a person walks by at a distance of approxim2 m. The positive and negative peaks occur when the person crosses the twtors.

Microsystem Packaging

The microsystem chip is packaged in a standard TO-5 header with a custoshown in fig. 6.3. An IR filter window is integrated in the cap covered by a chrlayer structured to form a slit diaphragm. The slit and the two sensorSysAlp2-VI are arranged as shown in fig. 6.4. This schematic cross-section sthat each of the two sensors receives radiation from a separate space secsectors on both sides extend approximately from 2° to 40° with respect to themetry plane.

Fig. 6.1: Photograph of the presence detector system with PCB (front and b

118


als isfirstatelyto thisced on

f the

Signal Processing

The microsystem has two outputs. The difference of the two sensor signamplified by the differential amplifier and output stage. This provides the output signal. The voltage of the on-chip bandgap reference of approxim1.3 V is provided as the analog ground. The amplified sensor voltage refers reference level. These signals are processed and displayed by the circuit plathe PCB. The block diagram of the circuit is shown in fig. 6.5. The offset o

Fig. 6.2: Processed microsystem signal and resulting digital output when aperson walks by at a distance of 2 m.

Fig. 6.3: View of the package cap with IR filter window and slit diaphragm.

0

1

2

3

0 1 2 3

Res

pons

e [V

]

Time [s]

Processed

Digital output

Threshold levels

microsystem signal

Slit diaphragm

IR filter

Package cap

119

6 Demonstrators

offsethiftedtablel levellifierrenceal is

givenltered

.

microsystem signal with respect to the analog ground is canceled with the compensation block. This block generates an adjustable reference level swith respect to the analog ground. It consists of an amplifier with an adjusgain of approximately 1.1. The reference level is adjusted to the output signacorresponding to equal radiation power on both sensors. A differential ampis used to amplify the difference between microsystem output and the refelevel. Its output voltage is filtered by a 10 Hz low-pass filter. This analog signanalysed with a window comparator. It checks whether the signal is within a range around the reference level. The digital output of the comparator is fi

Fig. 6.4: Schematic cross-section of the packaged microsystem.

Fig. 6.5: Block diagram of the demonstrators signal processing electronics

Sensors

Active sectors

Package

Chrome layer

IR filter

AAAA

AAAAAAAAA

AAAAAAAAA

AAAAA AAAAAA

AAAAAA

AAAAAAAAAAAA

AAA +

-

FilterMicrosystem Amplifier

Offset compensation

Comparator Trigger

Optocoupler

LEDSignal

Analog ground Reference Connector

120

6.2 Thermal Imager

rator dis-upler

var-wn in

orger.r is

idth.

rosys-

by a Schmitt trigger. Its output remains high for 100 ms after the compadetected an out-of-window condition. The signal from the Schmitt trigger isplayed by a LED and provided as a digital output on a connector. An optocois used to decouple connector and system potentials.

The processing electronics is realized with two ICs, two potentiometers, andious resistors and capacitors on the double-sided printed circuit board sho

fig. 6.6. The IC AD T9631 contains four differential amplifiers. One is used fsignal amplification, one for offset compensation, and two for the Schmitt trigThe IC LT1042 is the window comparator. One high resolution potentiometeused for offset compensation, the other to adjust the comparator window w

6.2 Thermal Imager

The thermal imager consists of an array microsystem, a housing for the mictem chip, a computer, and a multimeter. Either the SysAlp1-IV or SysAlp1-VII can

Fig. 6.6: Photograph of front and rear of the demonstrators PCB.

Microsystem

Window

On/Off switch

Window widthadjustment

OptocouplerLED

Outputconnector

comparator IC

Potentiometer for offset

compensation

IC with fourdifferentialamplifiers

121

6 Demonstrators

ntrol

a. Thento the mm,

m a

thecom-gitalpleted the

iming

be plugged into the imager. As shown in fig. 6.7 the computer is used to co

the operation of the demonstrator and to acquire and display the image dathousing contains an IR Fresnel lens casting an image of the thermal scene osensor array. The lens is made from polyethylene with a diameter of 12.710 lines/mm, a focal length of 9.4 mm, and nf of 0.74. With the pitch of 245 µmand 330 µm the angular resolution is 1.5° for of ArrAlp1-IV and 2° forArrAlp1-VII. The polyethylene sheet transmits 53% of the IR radiation froperson at 24°C.

The signals from the individual pixels are multiplexed and amplified byon-chip circuit. The multiplexer is controlled by the address supplied by the puter. The differential output signal of the microsystem is converted to diform by a multimeter and transmitted to the computer. To acquire a comimage, all pixel addresses are sequentially supplied to the multiplexer, anrespective signals are synchronously converted with the multimeter. The t

Fig. 6.7: Block diagram of the thermal imager.

+-

Addressing

Control/Readout

Lens Microsystem

Multimeter

Computer& Display

HousingObject

122

6.2 Thermal Imager

roni-o pro-ion iset thes theesses

mings per,5.4 s,framever, armore,-as not

us the

of the addressing and A/D conversion is shown in fig. 6.8. To achieve synchzation between the addressing and the A/D conversion, the computer alsvides a trigger whenever a new address is supplied. The A/D conversdelayed by 1 ms with respect to the trigger signal. This delay is required to lamplifier output signal settle to the new value. The multimeter measureoutput value by integrating the signal during 20 ms. The integration suppr

noise and signal components with frequencies above 50 Hz. With this tischeme the acquisition of a pixel signal lasts for 21 ms. Thus, 47.6 pixelsecond can be read. For SysAlp1-VII and SysAlp1-IV with hundred and 240 pixelsrespectively, the acquisition of a complete image frame lasts 2.1 s and respectively. This corresponds to a frame rate of 0.49 Hz and 0.18 Hz. The rate can be increased by reducing the delay and integration time. Howeshorter delay increases the electrical cross-talk between the pixels. Furthea decreased integration time deteriorates the NETD through larger signal bandwidth and thus, increased noise. Cross-talk occurs because the amplifier hyet completely settled to the new pixel signal when the integration starts. Thsignal from the previous pixel affects the next reading.

Fig. 6.8: Timing of the addressing and A/D conversion by the multimeter.

Trigger

Output Signal

Multimeter Delay Measurement Delay Measurement

Time

Address i Address i+1Address

123

6 Demonstrators

band-d andds to

byitivity. Therded.z of the-

per-

Figure 6.9 shows an image acquired with the microsystem SysAlp1-IV. A delay of1 ms and an integration time of 20 ms was used, corresponding to a signalwidth of 50 Hz and a frame rate of 0.49 Hz. The picture shows a person, heachest with arms up at a distance of approximately 2.5 m. Black corresponambient temperature, while white indicates 16°C above ambient.

The NETD achieved with this configuration is 715 mK. This was determinedthe following measurements. The thermal imager was placed in the sensmeasurement setup described in section 4.1, viewing the two blackbodiessignal from the pixel viewing the centers of the blackbodies was then recoFigure 6.10 shows the signal of an alp1-IV pixel with a signal bandwidth of 10 Hand blackbody temperatures of 23°C and 28°C. The temperature sensitivityimager with the SysAlp1-IV and SysAlp1-VII is 1.5 mV/K and 1 mV/K, respectively. The standard deviation of the signal with both blackbodies at equal tematures is 380 µV and 320 µV, respectively. This corresponds to NETDs of

Fig. 6.9: Thermal image of a person at a distance of 2.5 m.

0°

16°

12°

8°

4°

∆T

124

6.2 Thermal Imager

-

bod-

715 mK for SysAlp1-IV and 560 mK for SysAlp1-VII with respect to a signal bandwidth of 50 Hz.

Fig. 6.10: Signals of a single pixel from SysAlp1-IV alternately viewing blackies with a temperature difference of 5 K and 0 K.

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35 40

Time [s]

Sig

na

l [m

V]

∆T = 5 K

∆T = 0 K

125

7 Summary and Outlook

ance, usingmer-

y con-chip

ed one area fab-

d innsors.. Theated bys and

r sen-plica-pectralcturesMOSCMOS

e theed andd intrated

7 SUMMARY AND OUTLOOK

In this thesis we demonstrated the feasibility, and characterized the performof fully integrated IR sensor microsystems for presence detection fabricatedCMOS IC technology. The systems were fabricated using three different comcial CMOS processes with subsequent wafer-by wafer post-processing. Thesist of pairs or arrays of thermoelectric sensors, combined with on-state-of-the-art signal conditioning circuitry.

We reported the fabrication of a variety of micromachined IR sensors basmicromachined beams, bridges and membranes. The size of their sensitivvaries from 150 µm by 100 µm up to 1500 µm by 700 µm. Thermopiles werericated using n+-polysilicon/aluminium, n+-polysilicon/p+-polysilicon, andp+-polysilicon/aluminium. Six of these sensors have been integratetwo-dimensional arrays. Four arrays were fabricated using bridge type seSeven arrays were fabricated using two different membrane type pixelspixels in these arrays are located on one membrane and are thermally separ25 µm thick gold lines. The largest array has 240 pixels arranged in 15 row16 columns.

We built a measurement setup for reproducible measurement of IR detectositivity with a radiation spectrum representative for presence detection aptions. A second measurement setup was developed to determine the sresponse of IR sensor devices in the range from 2 µm to 14.6 µm. Test struwere fabricated for measuring in-situ the relative spectral absorptance of Clayer sandwiches and the relative absorptance spectra of a complete set of IR absorbing layer sandwiches were measured.

A FEM model was applied for the simulation of sensor performance beforfabrication. The accuracy of this model was tested by comparing the measursimulated sensitivity for nine different sensors. A deviation of 21% was founthe worst case. Optimization of a sensor layout with this model was demonsfor a presence detection system.

126

tems.econdrs the

sys-

ith thereatly

largerns toion of

ountatis-the-

loped.ationckage

pera-brica-lso be

re thanesti-left to

We built two demonstrators to show the potential of the fabricated microsysThe first, using two separate sensors, is for presence detection while the sdemonstrates IR thermal imaging with sensor arrays. With both demonstratopresence of a person at a distance of several meters is clearly detected.

To fully exploit the potential of this technology for integrated sensor microtems, future research will have to address several tasks:

• Two-sensor presence detection microsystems should be integrated also wchopper amplifier. The performance of these systems would benefit gfrom its very low noise performance.

• For thermal imaging larger arrays with more pixels are needed to cover a field of view and to improve spatial resolution. These arrays will need meaincrease the frame rate. A simple, though costly, solution is the cointegratseveral amplifiers.

• For arrays with large numbers of devices the fabrication yield is of paramimportance. In view of this, the fabrication of the bridge type pixels is not sfactory, while the yield of the membrane pixels is very promising. Neverless, the latter must be improved before full large-scale production starts.

• Novel methods of packaging the integrated IR sensors have to be deveThese packaging methods should be compatible with modern PCB fabricmethods such as surface mount technology. Above all, the price of the pahas to be reduced in comparison with traditional TO headers.

• Other fields of application than the presence detection such as remote temture sensing should be investigated. The thermoelectric sensors, their fation technology, and the simulation methods reported in this thesis can aused in different application areas.

If some of these challenges are met, a room-temperature IR camera with mo1000 pixels and a price below $500 will be possible within a few years. An mate of the possible applications and market volume for such a product is the imagination of the reader.

127

Appendix

70,

sed

ss,

ew

A,

TH

Atedpers

esis

s”,

ctricoc.

Appendix

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Sin-ron

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Acknowledgments

First of all I would like to thank Prof. Dr. Henry Baltes for making this work psible, for creating a professional, friendly and productive atmosphere at the ical Electronics Laboratory (PEL), for introducing me to microsystems, andhis generous support of my work, from drafting the original research plan tcritical review of this thesis.

It is my pleasure to thank Prof. Dr. Oliver Paul, now with the UniversityFreiburg im Breisgau, who was my supervisor throughout this work. To meestandards has always been a challenge, and I learned a lot trying to do so.to thank him for his guidance and readiness to assist whenever needed. I alsto thank him for uncompromisingly proof-reading this thesis like so many otexts before.

I wish to thank Dr. Ulrich Dillner for co-examining and proof-reading this the

I want to thank Dr. René Lenggenhager for his work on CMOS thermoelectrsensors which was the starting point of my work.

I am indebted to Dr. Dominik Jaeggi, Dr. Piero Malcovati, Christian MenoUlrich Münch, Andri Schaufelbühl, and Marc Wälti who worked with me on thermoelectric IR sensor microsystems. Many of the results presented ithesis are a result of their work and their support of my work.

I owe special thanks to my office-mate and friend Felix Mayer for his supduring periods of frustration or hard work and for sharing the good times.

It was a pleasure to share the office with Dr. Johannes Bühler, Michael MayeRolf Frei.

I want to thank Donat Scheiwiler for his work on the demonstrator systemsthe outstanding performance in keeping the equipment up and running. I wlike to thank Max Schlapfer for his friendly companionship and assistancimage editing.

I am grateful to Dr. Christophe Fumeaux for assisting me with his IR laser.

136

rcelonure to

r. M.

e thisanielnd,Jörg

err,ink,tz,

Ric-ider,raut-rd

oingwork.

s ofroject

I would like to thank Serge Déteindre, Verena Dubacher, Kristian Haller, MaHübscher, Phillip Ludwig, Karl Przibilla, and Marcel Vogel for the contributithey made to this thesis during their student projects, which I had the pleassupervise.

I am indebted to the staff of our industrial partners, notably Dr. P. Ryser, DForster, Dr. K. Müller, and Dr. M. Loepfe of Cerberus AG, and Mr. E. Doeringand Dr. A. Descombes of EM Microelectronic-Marin SA and P. Sagnol of AtmelES2.

I want to thank my colleagues at PEL whose enthusiasm and friendship maklaboratory a very special place to work. These are Dr. Martin Bächtold, Dr. DBolliger, Dr. Thomas Boltshauser, Dr. Frank Bose, Dr. Oliver BraDr. Ruggero Castagnetti, Christian Cornila, Dr. Michael Dammann, Dr. Funk, Markus Emmenegger, Liselotte Glasl, Dr. Andreas Häberli, Dr. Egon HErna Hug, Mark Hornung, Andreas Koll, Stefan Koller, Prof. Dr. Jan KorvDirk Lange, Dr. Stefan Linder, Igor Levak, Christoph Maier, Matthias MeHeidi Moser, Dr. David Moser, Thomas Müller, Luca Plattner, Dr. Concetta cobene, Jaques Robadey, Dr. Berthold Rogge, Michael SchneDr. Franz-Peter Steiner, Ralph Steiner, Stefano Taschini, Dr. Stephan Tweiler, Yelena von Allmen, Martin von Arx, Dr. Rolf Vogt, Prof. Dr. GerhaWachutka, Dr. Rolf Wohlgemuth, Volker Ziebart, and Martin Zimmermann.

I owe special thanks to my parents and to my wife Flavia. Through their ongsupport, love, and understanding they have contributed substantially to my

This work has been supported by the LESIT and MINAST priority programthe Board of the Swiss Federal Institute of Technology and the ESPRIT pDEMAC of the European Community.

137

Appendix

onS

icals)

Curriculum Vitae

Niklaus Werner Schneeberger

Born December 24, 1967

Citizen of Schoren bei Langenthal, BE, Switzerland

Married

Apr. 1984 - Sept. 1987 Technical Gymnasium Thun.

Oct. 1987 Matura type C.

Feb. 1988 - Nov. 1988 Swiss army service.

Nov. 1988 -Nov. 1993 Student of physics at ETH Zurich. Diploma thesismechanical material properties of dielectric CMOthin films.

Nov. 1993 Dipl. Phys.ETH.

Sept. 1995 Marriage with Flavia Camastral.

Nov. 1993 - Apr. 1998 Work towards a doctoral degree at the PhysElectronics Laboratory directed by Prof. Dr. H. Balteat the Swiss Federal Institute of Technology (ETHZurich.

138

139

List of Abbreviations

alp1mv Analog, Low-Power, 1 µm, Medium Voltage

alp2lv Analog, Low-Power, 2 µm, Low Voltage

CMOS Complementary Metal Oxide Semiconductor

ECPR Electrically Calibrated Pyroelectric Radiometer

EDP Ethylene-Diamine Pyrocatechol

FEM Finite Element Method

FET Field Effect Transistors

IC Integrated Circuit

IPHT Institut für Physikalische Hochtechnologie

IR Infrared

KOH Potassium hydroxide

LED Light Emitting Diode

MEMS Micro Electro Mechanical System

MOS Metal Oxide Semiconductor

NEP Noise Equivalent Power

NETD Noise Equivalent Temperature Difference

PCB Printed Circuit Board

PMMA PolyMethyl-Methacrylat

PSD Phase Sensitive Detector

VLSI Very Large Scale Integration

eth diss 12675

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