the electro-thermal properties of integrated circuit microbolometers

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The electro-thermal properties of

integrated circuit microbolometers

* M. du Plessis, J. Schoeman, W. Maclean ** C. Schutte

* Carl and Emily Fuchs Institute for Microelectronics, University of Pretoria* * Detek, Denel Aerospace Systems

CEFIMCarl and Emily Fuchs Institute

for Microelectronics

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Carl and Emily Fuchs Institute for Microelectronics - CEFIM

CEFIMCarl and Emily Fuchs Institute for Microelectronics

T & M 2010 - 8 Nov 2010

3CEFIMCarl and Emily Fuchs Institute for Microelectronics

Infrared thermal images T & M 2010 - 8 Nov 2010


Principal types of infrared (IR) detectors

1) Photon detectors - [ Cooled ] In photon detectors the absorbed photons directly produce free electrons and holes, to generate a photon-induced current or voltage, either in a photoconductive or photovoltaic mode.

2) Thermal detectors - [ Uncooled ] In thermal detectors the absorbed photons produce a temperature change, which is then indirectly detected by measuring a temperature dependent property of the detector material.

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Two classifications of thermal sensors

1) Direct sensors Direct sensors convert thermal signals (temperature or heat) to electrical signals.

2) Indirect sensors Indirect sensors are based on thermal actuation effects, such as thermo-mechanical (thermal expansion) effects.


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1) Bolometers A bolometer changes its resistance according to the change of the

temperature, and thus a high temperature coefficient of resistance (TCR) is needed for high sensitivity.

2) Pyroelectric effects

The pyroelectric effect is exhibited by ferro-electric crystals that exhibit electric polarization. They have no direct current (DC) response and therefore must employ radiation modulators.

3) Thermoelectric effects Two junctions made of two different materials are at different temperatures, and the magnitude of the voltage generated across the thermopile junction depends on the type of materials and the temperature difference between the junctions.

Three types of direct thermal detectors


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Thermal sensors based on CMOS technology became feasible when CMOS micromachining (MEMS) was established.

Micromachining makes it possible to remove thermally conducting material for the thermal isolation of heated microstructures.

While thermal effects are intuitively considered to be slow, the small size of CMOS microsensors brings about thermal time constants in the millisecond range.

CMOS integration of microbolometers


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Sensorabsorbsradiation

Temperatureincreases

Circuit tomeasure

resistance

Infraredradiation

Temperature sensitiveresistive material

thermally isolated from ambient

Principle of bolometer IR detection

ROICReadout

integrated circuit


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Two approaches used to thermally isolate the sensor structure or part of it from the bulk silicon:

Bulk-machined sensors

Machining the silicon substrate, either from the front or the back. Membranes can be released by surface etching of the silicon

Surface machined sensors Using stacked thin films on the front surface. The mechanical structure is released by removing a sacrificial layer underneath it.

A. Hierlemann,O. Brand, C. Hagleitner and H. Baltes, “Microfabrication techniques for chemical/biosensors”, Proceedings of the IEEE, Vol. 91, No. 6, pp. 839-863, June 2003.

Micro-machined silicon sensors


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Bulk machined device - CEFIM, UPT & M 2010 - 8 Nov 2010

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First ever submicron support beamsStiffness enhancement techniquesU-profile – 100 nm sensor thickness – 2.5 ms thermal time constant

SEM picture of 50 x 50 micron polySiGe bolometer

Surface machined bolometer - IMEC, Belgium


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Important considerations in choosing bolometer material A high TCR, Low noise, especially 1/f noise, Not too high resistivity, and Compatibility with post processing IC

fabrication.

Many materials have been used for bolometers, such as Metals ( Pt, Ti), and Semiconductors (VOx, amorphous silicon).

The semiconductor materials exhibits a TCR ofapproximately –2 %/K, which is 10 times that of metals.

Bolometer thermosensitive materials


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CfG

PT O

22241

= fraction IR power absorbedPO = incident IR power, WG = thermal conductance, W/K = GGAS + GSOL

f = frequency of modulation, Hz

= thermal time constant = H/G, secH = thermal capacitance, J/K

GSOL

Solid thermalconductance

of supporting leg

Gaseous thermalconductance

of suspended plateGGAS

Thermal capacityof suspended plate

H

TB

TSTB = Plate (bolometer) temperatureTS = Substrate (ambient) temperature

T

Heat

PO

Membrane or Plate

Bolometer thermal analysis

CTTT SB

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Bolometer electrical analysis

KdT

Rd

dT

dR

RTCR

B

B

B

B

B

%)(ln1IB

VB

IB

tton <<

WV

GRI

dP

dT

dT

dR

dR

dV

dP

dVR BB

O

B

B

B

B

B

O

BV

Voltage sensitivity RV :

For RV and NETD , we need , , G , Ad

KA

GvNETD

d

n

With noise voltage vn in system, we candefine the system performance parameter NETD,the Noise Equivalent Temperature Difference.

with Ad = bolometer area, m2

Trade-off

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A. Rogalski, “Optical detectors for focal plane arrays”, Opto-Electronics Review, Vol. 12, No. 2, pp. 221-245, 2004.

Thermal conductivity vs. fill factor trade-offSingle level surface machining

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Advanced detector structuresSurface machining

Higher fill factors than bulk machining

Double level machiningHigher fill factors than single level machining

Single level Double levelSurface micromachining

D. Murphy et al, “Resolution and sensitivity improvements for VOx microbolometer FPAs”, Proceedings of SPIE Vol. 5074, Infrared Technology and Applications XXIX, pp. 402-413, 2003.

Surface micromachining


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Comparing single and double level devices


Single level , 50 m pixel Double level , 25 m pixel


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Single level2 m design rules

Single level1 m design rules

Double level2 m design rules

Double level1 m design rules

10 20 30 40 50 60Pixel dimension (m)

1000

100

10

1

Rel

ativ

e pe

rfor

man

ceConstant thickness, TCR, absorption and ROIC design

Bolometer performance vs. pixel dimension


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C.M. Hanson et al, “Small pixel a-Si/a-SiGe bolometer focal plane array technology atL-3 Communications”, Proc. of SPIE Vol. 7660, 76600R-2, 19 May 2010.

Surface machined bolometer atL-3 Communications, USA

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L-3 Communications, USATCR = 3.9 % / K 17μm pixel technology 1024 768NETD8-12μm ~ 35mK Thermal time constant ~10ms

Gth ~ 5 nW/K 0.35μm photolith IR absorptance ~ 90%

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60 nm Ti600 nm Au

100 nm SiO2

200 nm SiO2

900 nm Si 3N4

2 μm Cavity 2 μm Al

Si bulk

1 μm SiO2

Ti

5 µm

Cavity

Membrane

73 μm

97 μm

Si 3N4

29 μm

9 μm i i

iiiSOL l

dWG 2

Wi = width of supporting legdi = thickness of supporting legli = length of supporting legi = thermal conductivity of supporting leg material

s

dgasGAS d

AG

Ad = device areads = cavity separationgas = gas thermal conductivity = 0.026 W/mK , 1 atm N2

= 0 in vacuum

Our experimental device


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GSOL

G

10-7

10-6

10-5

10-4

10-4 10-2 100 102 104

Pressure (Torr)

The

rmal

con

duct

ance

(W

/K)

Thermal conductance vs. pressure

M. Ou-Yang and J. Shie, “Measurement of effective absorbance on microbolometers”,IEEE Tran. Instr. and Meas., Vol. 55, No. 3, pp.1012-1016, June 2006.


GGAS

Conventional model: G = GSOL + GGAS

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We

de

ds

Cross section

Substrate

Heat flow

Our improved analytical model 1,2

Sidewall thermalgaseous conduction

P

T1 T2 T3 T4 T(x)

x

exp(-x/Lth)

x

rsol x

ggas x

x

Equivalent thermallength Lth

mddgr

L segas

e

gassolth

131

1.

2.


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Our improved analytical model 3,4

x

T1 T3T2

xx=L

PE

ppp p

PS

PS+PE

T0T(L)

L

rsol x

x

OEESsol

TxTL

xPxPP

LGxT

)(

2

1)(

2

T(x)

Distrubuted thermalconduction in legs

4

1ln2

1

1ln

11 2

eeC d

R

p

e

W

W

N. Topaloglu, P.M. Nieva, M. Yavuz, J.P. Huissoon, “Modeling of thermal conductance in an uncooled microbolometer pixel”, Sensors and Actuators A, Vol. 157, 2010, pages 235 to 245.

Wp

We

Spreading resistance

Plate and leg

3.

4.


RC

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0

10

20

30

40

50

0 20 40 60 80 100 120 140

Distance x μm

K

Long section Plate

Short section

Conventional model

Modified model

CoventorWaresimulation

Lth

RC

Fsw

T(x)

Thermal modeling and simulation at atmospheric pressure

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Thermal modelling and simulation under vacuum conditions

0

10

20

30

40

50

0 20 40 60 80 100 120 140

Conventional modelModified modelCoventorWare

Long section Plate

Short section

Distance μm

K

T(x)

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5.5

5.6

5.7

5.8

5.9

6.0

6.1

0 0.5 1.0 1.5 2.0

IB2 A2 ( 106)

RB1

1

( 1

04 )

a / G = 17.5

a = 0.1 % / K)1()( TRTR BOB

CG

RI

G

PT BB

2

G

I

RRB

BOB

211

Experimental determination of the thermal conductance at atmospheric pressure

IB

VB

RB = VB / IB

DC

Self heating of device:

G = 60 μW/K

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Experimental determination of the thermal time constant at atmospheric pressure

Bolometer current iB

IH

Time

IL

Bolometer voltage vB Time

tf

iB

vB

Bolometer

VL = IL RL

VT = IH RL

VH = IH RH

V = IH ( RH RL ) = IH R

V(RL)

(RH)

The bolometer resistance will rise exponentially during tf withthe exponential time constant equal to the thermal time constant

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Thermal time constant transient curve

V = 80 mV

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 100 200 300 400 500 600

Time μs

stt

BB eevtv 16008.0)0()(

Thermal time constant = 160 μsThermal capacitance H = G = 9.5 nJ/K

(Atmospheric pressure)

vB(t)

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Predicted thermal parameters under vacuum conditions for our device

Atmospheric pressure Vacuum

G 60 μW/K 600 nW/K

160 μs 16 ms

H 9.5 nJ/K 9.5 nJ/K

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“Acid test” for IR room temperature system

State of the art microbolometer NETD ≈ 30 mK

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Conclusions

Theory/modeling and design of IR bolometers well understood

Improvements to analytical modeling – atmospheric pressure

Experimental determination of thermal parameters

THANKS TO AMTS (TIA)


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the electro-thermal properties of integrated circuit microbolometers

Documents

thermal effects

emily fuchs institute

direct sensors direct

bulkmachined sensors

thermal signals temperature

thermal actuation effects

pyroelectric effects

bolometer material