Luận án tiến sĩ: Cảm biến gia tốc piezoresistive siêu nhỏ cho y sinh học

ENCAPSULATED SUB-MILLIMETER PIEZORESISTIVE ACCELEROMETERS FOR BIOMEDICAL APPLICATIONS A DISSERTATION SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Woo-Tae Park December 2005 UMI Number: 3197493 Copyright 2006 by Park, Woo-Tae All rights reserved. INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted.

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ProQuest Information and Learning Company 300 North Zeeb Road P. Box 1346 Ann Arbor, MI 48106-1346 © Copyright by Woo-Tae Park 2006 All Rights Reserved lì I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Kenny, Principal Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Pruitt I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.

Khuri-Yakub Approved for the University Committee on Graduate Studies. Micromachined accelerometers have been introduced in the late 1970s and have been used in various applications. The applications range from inertial navigation and data logging in wells to body activity monitoring for pacemakers. Although the size of the accelerometers was sufficient for their applications, there were not many efforts in pushing the limits of accelerometer miniaturization.

In this study, we utilized film deposition packaging technology and other modern microfabrication techniques to miniaturize the size and mass of the packaged accelerometers, two orders of magnitude smaller than any accelerometers ever reported. We used these ultra miniature accelerometers to offer sensing capabilities for biomedical applications which was not possible with any other means. | A novel design of the accelerometer and packaging has been developed for miniaturization. The accelerometer consists of a proof mass suspended by a single high- aspect-ratio beam attached to the substrate.

Piezoresistors are implanted on the sidewall of the beam to sense the maximum stress applied on the beam. A thick layer of epitaxial silicon is deposited on the accelerometer to form a mechanically robust yet compact package. The new packaging method enables reduction in die area up to 70% compared to conventional wafer bonded package. A new polyimide flexible circuit is also developed to route the signals from the ultra-miniature accelerometers to a conventional package.

The new technology is used in experimental biomedical applications. The accelerometer is evaluated as an implantable sound sensor for cochlear implants which can possibly replace the externally worn microphones. It is also used as an electrical stethoscope to measure respiratory and heart signal of neonatal mice. There are many other possible applications in the biomedical field such as imaging artifact reduction for live animal microendoscopy.

This technology has the potential to open up new realms of motion sensing in the biomedical science and engineering. iv Acknowledgements The work presented in the dissertation would not have been possible without the help from a number of people. I would like to start by thanking my PhD advisor Prof. Tom Kenny who had the most influence on my graduate studies.

Our relation started from my first day at Stanford, since Tom was also my masters’ degree academic advisor. In spite of his busy schedule, Tom always finds time to meet with his students, in the office, the lab, or even the golf course. I am truly grateful for his persistent guidance and faith in my abilities. I would like to thank Prof.

She was initially my mentor in the Kenny group, and later as a professor in the department served as the reading committee of my thesis. I would also like to thank the rest of the thesis committee; Prof. Pierre Khuri-Yakub, Prof. Sunil Puria, and Prof.

I especially want to thank Sunil for his help in the middle ear measurements which is the highlight application of my research. Tá like to thank my collaborators. First, in the wafer level packaging project, I’d like to thank Dr. Aaron Partridge, Markus Lutz, Rob Candler, and Gary Yama.

Aaron was my mentor in building accelerometers or any piezoresistive devices in general. Second, in the middle ear measurements, I need to thank Kevin O’Connor, Dr. Toshiki Maetani, Dr. Joe Roberson, and Jaehoon Sim.

And finally, in the mouse stethoscope project, Matt Beasley, Berit Jacobson, Dr. Richard Bland, and Ian Chen helped me throughout the measurements. I’d like to thank the past and present Kenny group members, Dr. Amy Herr, Dr.

Olaleye Ajakaiye, Anu Tewary, Dr. Eugene Chow, Dr. Lian Zhang, Dr. Michael Bartsch, Roxanne Daniels, Dr.

Yoshi Hishinuma, Dr. Robert Rudnitsky, Hyeun-su Kim, Dr. Dan Laser, Evelyn Wang, Ginel Hill, Dr. Holden Li, Jennifer Bower, Kevin Lohner, Jeffrey Li, Dan Soto, Vipin Vitikkate, Kuan-Lin Chen, Parmita Dalal, Andrew Graham, Wesley Smith, Matt Hopcrof, Manu Agarwal, Bongsang Kim, Saurabh Chandorkar, Chandra Jha, Kwan Kyu Park, Suhrid Bhat, and Renata Melamud.

It was a great pleasure to work in such an enthusiastic group. Each group member was a great collaborator or a mentor. I especially want to thank Kuan-Lin and Vipin for all the help and discussions on the encapsulated accelerometer. I'd like to thank Robert Jung of Altaflex® for fabricating the Kapton flex circuits used in this work for free.

I think his generous support reduced about a year of flex circuit development. I’d like to thank the Stanford Nanofabrication Facility staff, Maurice Stevens, Gladys Sarmiento, Uli Thumser, Cesar Baxter, Nancy Latta, Mahnaz Mansourpour, Elmer Enriquez, and finally a non-staff but resident ‘fabgod’ Dr. I’d like to thank the people who supported my life outside of the lab. The Korean mechanical engineer society was a big presence in my Stanford life and I’d like to thank all of them.

Sangkyun Kang, Dr. Sukwon Cha, Dr. Byeongchan Lee, Dr. Suhong Kim, Dr.

Jaemo Koo, Dr. Simon Song, Tonghun Lee, Taegyeong Yang, and Kwonsoo Chun. I’d also like to thank all the members of the micro-engineering labs, especially Dave Huber, Dr. Shuhuai Yao, and Dr.

I'd like to thank my funding agencies. This work was supported by DARPA HERMIT (ONRN66001-03-1-8942), the Robert Bosch Corporation Palo Alto Research and Technology Center, a CIS Seed Grant, The National Nanofabrication Users Network facilities funded by the National Science Foundation under award ECS-9731294, and The National Science Foundation Instrumentation for Materials Research Program (DMR 9504099). Finally, I’d like to express my deepest gratitude towards my parents for always supporting me in various ways. And last but not the least, I'd like to thank my wife, Yu Kyung, for supporting me and loving me for the man I am.

vi Table of Contents Abstract. sesssessee LV - ACKknowl€dØ€ITIIÉS.0009 0090666096660 068 V Table of Content ú. 16098 6009 56669466090668806 Vii List Of EÏØTFS.90684 x ba)at 1) (eee xvi IN0oIme€nncÌÏAfUIF€.ooco o5 25 52559093059 9569969650898956936936909369694648846960894604 xvii Chapter 1 Infr0oduCfÏOI.1 MEMS accelerometer h1SfOFY. - cọ HH HH ng ng ng ưiện 1 1.2 MEMS Packaging BackgTOunid.- - co s91 ng 1g ng cư.3 Film encapsuÌafiOT.- su ng kg gu 11 1.3 Biomedical Sensors Background.1 Biomedical applications of micromachined sensors .2 Biomedical applications of micromachined accelerometers.

15 Chapter 2 Design and ÁIaÌÏySÌS.2 Piezoresistive Accelerometer Design.1 Flexure & piezoresistor €SIET.2 Proof mass CeSIQN 0n .3 Squeeze film damping analysis .1 Electrical & mechanical noise .2 Sensitivity, resolution, bandwidth, and range.3 Device Dimension s©Ï€CfIOTI.4 Encapsulation DeS1gTn.1 Encapsulated acceÏ€rOT€fT.2 Design for min1afUT1ZAfIOTA. s- c H TH n nHh n gnện 42 2.3 Wafer level self-test of release. HH ng HH ng gu 45 2.5 ca nh hố. 0G Go SH cọ nH n 096889464896680 08686 49 KNNsov.

vn ng:::adiaiia.1 Accelerometer faDr1CAfiOTI. Án TH HH ng ng ng 49 3.2 Flexible CITCUI{ WITITIE. Q1 ng ng ng TH nàn 60 Chapter 4 Characterization .2 Frequency ReSponse am. 66 KP song o0 nh .-- GG cv ngưng 71 Chapter 5 Implantable Sound Sensors for Cochlear Implants .1 Overview of devices for hearing 1mpairment.2 Previous and current efforts for sound sensing .3 Loudness and sound pressure level (SPL) .4 Requirements for an implantable accelerometer.1 Sample pT€D4äTAfIOT\.

«L1 191031181101 11 11g ng HH ng 84 5. - LH HH TH KH KH KH HH 00 K01 17T 85 5.3 Results from First Generation DeViCeS.1 Noise mm€aSuT€TT€HIÍ.2 Artifact me€aSUT€TTeTIÍ.- G5 S1 KH ng ke 89 5.3 Ïncus €aSUT€IT€TIE.4 Results from Second Generation DeviC€s.1 Flexible circuit stiffness evaluation on middle ear ossicles .2 Noise me€aSUT€TN€TIE.3 Stapes measurement with fused incudo-stapedial Joint.4 Stapes I€aSUT€TTII. ác HH ng ng kg 98 5.5 Acoustic coupling 1TneaSUTEIN€TIÍ.5 Conclusions and future WOTĂ.- án HH ng ng re 102 Chapter 6 Other Ultraminiature Accelerometer Applications .1 Neonatal Mice Electronic Stethoscope Study .- --- cọ HH net 104 6. - 4ó «cọ HT TH ng rà 105 6.2 Traumatic Optic Neuropathy Studyy.

Go HH ke 107 6. 107 Chapter 7 Conclusions and Future Work .ccssscseessssssesecers —— 109 9u oi nh. 0G G00 Họ cọ TH 000000004 0000008006488 0660406096 113 1X List of Figures Figure 1. Example of a piezoresistive accelerometer.

Most of them are sensitive out of plane with a variation in the number of ÍeXUT€S. Example of capacitive accelerometers. Conditioning circuit is fabricated in a separate CHIp. -- Ăn HH KH ng HH ng ng 0k4 4 Figure 1.

Packaging process flow of ‘die level packaging’. The singluated device is then attached to a package (c). The device is then wirebonded to the package (d) and finally the package is sealed ›41:8:8 0/2) 0117777. Packaging process flow of ‘wafer level packaging’.

Encapsulated wafer is diced to singulate each device (c). This form can be suitable for PCB mounting. Alternatively, the device can be mounted, wirebonded and molded on a plastic molded package 6). Electrical feedthough connections for encapsulation.

(a) Lateral connections have signal lines running beneath the seal ring to bondpads on the die periphery. (b) Vertical connections have signal path through the encapsulation cap to bondpads on top of the encapsulaf1OT. --- + ch HH TH HT TH Hàng Hung 10 Figure 1. Process flow of film encapsulation.

A micromachined silicon strain gauge used for chronic physiologic studies [46]. The device consisted of a doped resistor, two gold electrical pads, and two suture rings for attachment to tissue. Two platinum-iridium wires were attached to the pads by conductive epoxy and the wires are terminated with a silicone rubber block. The whole device is coated by Parylene®.

Simple model of an accelerometer. It is a second order system. Every accelerometer consists of proof mass (m), spring (k), and damper (b). The displacement (x) is proportional to the acceleration (A) and they are in the same direction.

The range of the proof mass movement is limited by the end stops which prevent the device from shock aT486. Amplitude of the displacement in respect to the frequency. The displacement is “OQ” times larger at the natural Íf€QU€TCY. ó0 t0 9111 nh ngay 19 Figure 2.

Illustration of the piezoresistive accelerometer. Accelerometer shape and important dimensions. The flexure, proofmass, and end stop are defined in a single DRIE SI€P. Gà“ HH HT TH HH ng TT TH Hà Hán iệt 20 Figure 2.

Piezoresistance factor P(N, T) as a function of impurity concentration and temperature for n-type silicon. P-type silicon behaves similarly [8§0]. Wheatstone bridge configuration for (a) single strain gauge, and for (b) dual SUTAIN QAUGE.