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Simulation and Fabrication of MEMS based Remote Pressure Sensor

A Project Report

submitted by

HARSH NAIKEE03B106

Under guidence of Dr.Nandita DasGupta in partial fullment of the requirements for the award of the degree of

BACHELOR OF TECHNOLOGY

DEPARTMENT OF ELECTRICAL ENGINEERING INDIAN INSTITUTE OF TECHNOLOGY, MADRAS. MAY 2007

THESIS CERTIFICATE

This is to certify that the thesis titled Simulation and Fabrication of MEMS based Remote Pressure Sensor, submitted by Harsh Naik, to the Indian Institute of Technology, Madras, for the award of the degree of Bachelor of Technology, is a bona de record of the research work done by him under our supervision. The contents of this thesis, in full or in parts, have not been submitted to any other Institute or University for the award of any degree or diploma.

Prof.Nandita DasGupta Research Guide Professor Dept. of Electrical Engineering IIT-Madras, 600 036

Place: Chennai Date: 10th May 2007

ACKNOWLEDGEMENTSThere are many people who contributed to this thesis that I would like to thank.I would rst like to convey my heartfelt gratitude and thanks to my research guide, Dr.Nandita DasGupta for her invaluable encouragement, support and assistance.Her patience and guidance have pulled me through the challenges of working on this project. I would also like to thank Hari Krishna for his constant help during the course of the project.Thanks to all those at Microelectronics Lab including Madhavi,Sharmaji,Uday,Hareesh,Sheeja, Sachin and Somshekhar Bhatt who have made working here,a truly enjoyable experience.I would also like to thank Amit Mittal who helped during the initial stages of the project. And last but not the least I would like to thank all my friends and seniors at Tapti Hostel who have made my stay here at IIT Madras truely a memorable one,an experience whose memories will be etched in my heart forever.

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ABSTRACTKEYWORDS: MEMS ; Pressure Sensors; Capacitive sensors; Piezoresistive Sensors:Micromaching.

Since the discovery of piezoresistivity in silicon in the mid 1950s, siliconbased pressure sensors have been widely produced. Micromachining technology has greatly beneted from the success of the integrated circuit industry, borrowing materials, processes, and toolsets. Because of this, microelectromechanical systems (MEMS) are now poised to capture large segments of existing sensor markets and to catalyse the development of new markets. Given the emerging importance of MEMS, it is instructive to review the history of micromachined pressure sensors, and to examine new developments in the eld. Pressure sensors,especially capacitive,will be the focus of this thesis. Micromachined pressure sensor typically uses a Silicon membrane as the sensing element and piezoresistors or capacitors for data retrieval.Remote sensing of data indicating variation in pressure allows inplementation of battery free devoces with indenite lifetime and is thus very attractive for bio-medical applications. In this research work capacitive pressure sensors have been simulated and fabricated.Due to pressure variation membrane deects and the value of capacitance changes which can be detected by an external circuit.First an optimum structure was proposed based on results from simulation and a process ow and masks were designed for fabrication of this structure.

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TABLE OF CONTENTSACKNOWLEDGEMENTS ABSTRACT LIST OF FIGURES 1 Introduction 1.1 1.2 1.3 2 Background and purpose of the research . . . . . . . . . . . . . Statement of the problem and objectives . . . . . . . . . . . . . Organization of the thesis . . . . . . . . . . . . . . . . . . . . . i ii vi 1 1 2 3 4 6 6 7 9 12 14 14 15 18 19 19 21 22 25

Processes for Micromachining 2.1 2.2 2.3 2.4 2.5 Epitaxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

Capacitive Pressure Sensors 3.1 3.2 Capacitive Sensing Techniques . . . . . . . . . . . . . . . . . . Micromachined Pressure Sensors . . . . . . . . . . . . . . . . . 3.2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Previous Work 4.1 4.2 4.3 Piezo-resistive Pressure sensor . . . . . . . . . . . . . . . . . . Capacitive Pressure sensor . . . . . . . . . . . . . . . . . . . . . Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Results and Discussion

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5.1 5.2 5.3

Simulation Results and Discussion . . . . . . . . . . . . . . . . Mask and Process steps Design . . . . . . . . . . . . . . . . . . Fabrication steps . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 5.3.2 5.3.3 5.3.4 5.3.5 Wafer Cleaning . . . . . . . . . . . . . . . . . . . . . . . Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . Photolithography . . . . . . . . . . . . . . . . . . . . . . Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . .

25 28 32 34 34 35 36 40 41 42

5.4 6

C-V Measurements . . . . . . . . . . . . . . . . . . . . . . . . .

Summary and Further work

LIST OF FIGURES1.1 2.1 Concept of a Resonant Absolute Pressure Sensor . . . . . . . . Illustration of the basic process ow in micromachining: Layers are deposited; photo resist is lithographically patterned and then used as a mask to etch the underlying materials. The process is repeated until completion of the microstructure. . . . . . . . . An illustration of proximity and projection lithography. . . . . Double sided alignment scheme. . . . . . . . . . . . . . . . . . Etching proles for different types of etchants. . . . . . . . . . Illustration of the anisotropic etching of cavities in 100-oriented silicon: (a) cavities, self-limiting pyramidal and V-shaped pits, and thin membranes; and (b) etching from both sides o the wafer can yield a multitude of different shapes including hourglassshaped and oblique holes. . . . . . . . . . . . . . . . . . . . . . Illustration of a wafer bonding process. (a)The two wafers to be bonded.(b)Pressing Surfaces. (c)Binding Si-O bonds.(d)Binding Si-Si bonds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Examples of simple capacitance displacement sensors: (a) moving plate, (b) variable area, and (c) moving dielectric. . . . . . A schematic cross section of a typical pressure sensor diaphragm. Dotted lines represent the undeected diaphragm. . . . . . . . A cross section schematic diagram of a bulk-micromachined, capacitive pressure sensor. . . . . . . . . . . . . . . . . . . . . . . 2

5 8 9 10

2.2 2.3 2.4 2.5

11

2.6

13

3.1 3.2 3.3 3.4 3.5

14 16 16

A cross section schematic diagram of a bulk-micromachined, contactmode pressure sensor. . . . . . . . . . . . . . . . . . . . . . . . 17 A comparison of deection shapes for uniform-thickness (left) and bossed (right) diaphragms. . . . . . . . . . . . . . . . . . . Structure Simulated in CoventerWare . . . . . . . . . . . . . . Circuit Equivalent of the Structure . . . . . . . . . . . . . . . . Results of the simulation of the structure in Figure 4.1 . . . . . Capacitors connected in Parallel . . . . . . . . . . . . . . . . . 17 20 21 22 22

4.1 4.2 4.3 4.4

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5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9

A meshed model of the device before simulation . . . . . . . . Results of simulation showing (a)Sensitivity v/s Pressure and (b)Delta C v/s Pressure for different structures. . . . . . . . . . The structure simulated for further analysis. . . . . . . . . . . The nal meshed structure simulated with parallel capacitors. The displacement prole from mechanical analysis. . . . . . . Process ow designed for the fabrication of the pressure sensor. Anisotropic etching of < 100 > silicon. . . . . . . . . . . . . . . (a)The three layer mask.(b)Layer 1-Bottom wafer.(c)Layer 2-Top wafer.(d)Layer 3-Metallization mask. . . . . . . . . . . . . . . . Mask used for fabrication purpose. . . . . . . . . . . . . . . . .

26 27 29 30 30 31 32 33 34 38 39 40 41

5.10 (a)A two dimensional etching prole of the top wafer(b)X-prole of the etched windows showing the etch depth. . . . . . . . . . 5.11 (a)A two dimensional etching prole of the bottom wafer(b)Xprole of the etched windows showing the etch depth. . . . . 5.12 A three dimensional etching prole of the bottom wafer. . . . 5.13 C-V curve of the device at 1KHz. . . . . . . . . . . . . . . . . .

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CHAPTER 1

Introduction

1.1

Background and purpose of the research

Sensors can be considered the eyes and ears of any system that requires information about its environment. In many cases human beings take part in such a system. They can act as a sensor to provide a machine with data (entering visually observed date into a computer) or even control the response of a machine (driving a car)(Nandor, 1997). In other cases sensors are required to provide human beings with information they can not directly observe, like radiation. Pressure is a common parameter used in biomedical research as well as in clinical care ; these measurements are essential in many patient management situations, eg; intracranial pressure in neurosurgery, blood pressure in surgery and intensive care, air pressure in respiratory diseases, intrauterine pressure in obstetrics, abdominal and urinary pressure for diagnosis of respective disorders etc . Besides external and internal catheter tip measurements, it is frequently desirable to use implanted pressure measurem