Poly-SiGe for MEMS-above-CMOS Sensors

210 Pages·2014·6.81 MB·English

by  Pilar Gonzalez Ruiz

Springer Series in Advanced Microelectronics 44
Pilar González Ruiz
Kristin De Meyer
Ann Witvrouw
Poly-SiGe for
MEMS-above-
CMOS Sensors
Springer Series in Advanced Microelectronics
Volume 44
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Kiyoo Itoh, Kokubunji-shi, Tokyo, Japan
Thomas H. Lee, Stanford CA, USA
Takayasu Sakurai, Minato-ku, Tokyo, Japan
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Doris Schmitt-Landsiedel, Munich, Germany
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Pilar González Ruiz Kristin De Meyer
•
Ann Witvrouw
Poly-SiGe for
MEMS-above-CMOS
Sensors
123
PilarGonzález Ruiz AnnWitvrouw
CMOSSystem Department Department of Metallurgy
IMEC and Material Engineering
Leuven KULeuven
Belgium Leuven
Belgium
Kristin DeMeyer
Corporate TechnologyOffice
IMEC
Leuven
Belgium
ISSN 1437-0387
ISBN 978-94-007-6798-0 ISBN 978-94-007-6799-7 (eBook)
DOI 10.1007/978-94-007-6799-7
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Acknowledgments
CompletingmyPh.D.degreeisprobablythe mostchallengingactivityofthefirst
30 years of my life. My Ph.D. journey has been a continuous learning path with
ups and downs, successes but also setbacks. These years have been rich of pro-
fessional and personal experiences, both good and bad ones. These experiences
havenotonlycontributedtomyprofessionalgrowth,butalsoshapedthepersonI
am today.
Many people contributed to this dissertation in innumerable ways, and I am
grateful to all of them. First, in this acknowledgments section, I would like to
thankmypromotorProf.KristinDeMeyerwhogavemetheopportunitytopursue
my Ph.D. at K.U.L and imec under her supervision and guidance, and supported
me always during this period. I would also like to thank the members of my
examination committee for their valuable suggestions and comments on this
manuscript, as well as to Prof. Etienne Aernoudt for chairing the defense.
Ihavereceivedtremendousguidanceandhelpduringmyresearchatimec.Iam
particularlygratefultoAnnWitvrouw,whofollowedupmyworkonadailybasis
and provided me with timely guidance, motivation, valuable advices, and con-
structivesuggestions.Thankyouforyourhelp,continuoussupport,andpatience.I
am also indebted to Stefaan Decoutere and Simone Severi, who managed the
MEMSteaminimecduringtheinitialandfinalpartofmyPh.D.,respectively,for
alwaysprovidingmewiththeresourcesIneededtocarry outmyresearch.NextI
want tothankthemembers oftheMEMS teamforallthehelptheygave me:Bin
Guo, Philippe Helin, Bert Du Bois, Agnes Verbist, and Rita Van Hoof. A lot of
thanks go also to many other people in imec: David San Segundo, for his help
during the circuit design; Muriel de Potter de ten Broeck, Ann op de Beeck and
Morin Dehan for the split definition and processing of the CMOS wafers; Piotr
CzarneckiforteachingmehowtousethePMVandforhishelpwiththecapacitive
measurements, Olalla Varela Pedreira for her help measuring membrane deflec-
tion, Mireille Matterne for teaching me how to use the PAV and Kris Vanstreels
for his help with the nanoindentation measurements. I am also thankful to Gillis
Winderickx, Christophe Soonekindt, Guy Lepage, who were always available
whenever I needed help with SEM or FIB. I also have to acknowledge George
Bryce,themembersoftheContamteamand,ingeneral,alltheP-linepeoplewho
have contributed to the success of my experiments.
v
vi Acknowledgments
Leuven, however, has been to me much more than imec, and I would like to
show my gratitude to all the friends I met here during the years. As you are far
fromyourcountry,yourfriendsbecomeyoursecondfamilysomehow.Thankyou
Joumana, Karolina, Carolina, Francesca, Robin, Samer, Andrea, Antonio, Mireia,
Santi, Germain, Mihaela, Salvatore… To mention you all would not fit into a
singleAcknowledgmentsection.Youreallymademylifeeasierduringtheseyears
in Leuven, and were always a source of laughter, joy, and support.
If there is a person who can rightfully stake a claim to my thesis more than
anyone else, then that is Michal Rakowski, ‘‘mój mały misiu’’. A mere word of
thankshereisinsufficienttoexpressmygratitudeforthecontributionhehasmade,
fromhelpduringthecircuitdesign,layoutandmeasurements,totheformattingof
this text. Thank you for your support, motivation, and patience. Thank you for
lettingmebeangry,happy,nervous,stressed,relaxed,orcalmwheneverIneeded
it. Thank you for being my rock. I hope I can return the favor when you are
completing your Ph.D.
Elagradecimientomásintensoesparamihermano,misobrinoysobretodo,mis
padres. Papá, mamá, gracias por vuestro apoyo y amor incondicional, por ens-
eñarmeymimarme,porserejemplodesacrificioysuperación,yporsiemprecreer
enmí.Portodoloquemehabéisdadoosdedicoestatesis.Esperoquemitrabajo
os haga sentir orgullosos.
Leuven, September 2012 Pilar González Ruiz
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Motivation and Goal of This Work . . . . . . . . . . . . . . . . . . . . . 1
1.2 MEMS: Definition, Technologies and Applications. . . . . . . . . . 2
1.3 CMOS-MEMS Integration: Why, How and What? . . . . . . . . . . 5
1.4 Polycrystalline SiGe for MEMS-above-CMOS Applications. . . . 8
1.4.1 SiGe MEMS Demonstrators. . . . . . . . . . . . . . . . . . . . . 8
1.4.2 Poly-SiGe Deposition Technology. . . . . . . . . . . . . . . . . 10
1.4.3 Selected Poly-SiGe Structural Layer . . . . . . . . . . . . . . . 11
1.5 A Poly-SiGe Based MEMS Pressure Sensor. . . . . . . . . . . . . . . 13
1.5.1 Applications of MEMS Pressure Sensors. . . . . . . . . . . . 13
1.5.2 Technologies for MEMS Pressure Sensors. . . . . . . . . . . 14
1.5.3 Why a Poly-SiGe Based MEMS Pressure Sensor? . . . . . 15
1.6 Outline of the Book. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2 Poly-SiGe as Piezoresistive Material. . . . . . . . . . . . . . . . . . . . . . . 25
2.1 Introduction to Piezoresistivity . . . . . . . . . . . . . . . . . . . . . . . . 25
2.1.1 Single Crystalline Materials. . . . . . . . . . . . . . . . . . . . . 26
2.1.2 Polycrystalline Materials . . . . . . . . . . . . . . . . . . . . . . . 29
2.1.3 Definition of the Gauge Factor. . . . . . . . . . . . . . . . . . . 30
2.2 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.2.1 Layout and Fabrication Process . . . . . . . . . . . . . . . . . . 30
2.2.2 Poly-SiGe Layers Studied . . . . . . . . . . . . . . . . . . . . . . 32
2.3 Measurement Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.3.1 The Four Point Bending Method. . . . . . . . . . . . . . . . . . 36
2.3.2 Calculation of the Piezoresistive Coefficients. . . . . . . . . 37
2.4 Results and Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.4.1 Standard Poly-SiGe Layer and Comparison to Poly-Si. . . 38
2.4.2 Optimization of Boron Doped Poly-SiGe Layers
for Piezoresistive Sensing Applications . . . . . . . . . . . . . 41
2.4.3 Piezoresistivity and Electrical Properties of Poly-SiGe
Deposited at CMOS-Compatible Temperatures. . . . . . . . 45
vii
viii Contents
2.5 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3 Design of a Poly-SiGe Piezoresistive Pressure Sensor . . . . . . . . . . 51
3.1 A Piezoresistive Pressure Sensor: Definition
and Important Performance Parameters . . . . . . . . . . . . . . . . . . 51
3.1.1 Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.1.2 Important Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.2 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.2.1 Membrane Area and Thickness. . . . . . . . . . . . . . . . . . . 59
3.2.2 Piezoresistor Placement. . . . . . . . . . . . . . . . . . . . . . . . 62
3.2.3 Piezoresistor Dimensions. . . . . . . . . . . . . . . . . . . . . . . 65
3.2.4 Piezoresistor Shape. . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.2.5 Membrane Shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.2.6 Effect of Supports. . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
3.3 Summary and Conclusions of the Sensor Design. . . . . . . . . . . . 71
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4 The Pressure Sensor Fabrication Process . . . . . . . . . . . . . . . . . . . 75
4.1 The Pressure Sensor Fabrication Process:
A Generic Technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.2 Pressure Sensor Schematic Process Flow . . . . . . . . . . . . . . . . . 80
4.3 Process Developments and Challenges. . . . . . . . . . . . . . . . . . . 87
4.3.1 Piezoresistive Layer . . . . . . . . . . . . . . . . . . . . . . . . . . 89
4.3.2 Piezoresistor Patterning . . . . . . . . . . . . . . . . . . . . . . . . 89
4.3.3 Release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4.3.4 Sealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
4.3.5 Piezoresistor Contact. . . . . . . . . . . . . . . . . . . . . . . . . . 93
4.4 Discussion on the Poly-SiGe Pressure Sensor Process . . . . . . . . 95
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
5 Sealing of Surface Micromachined Poly-SiGe Cavities. . . . . . . . . . 101
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
5.2 Fabrication Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
5.3 Direct Sealing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
5.3.1 Sealing with Si-Oxide . . . . . . . . . . . . . . . . . . . . . . . . . 106
5.3.2 Sealing with AlCu. . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
5.4 Intermediate Porous Cover . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
5.5 Measurement Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5.6 Analytical Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
Contents ix
5.7 Results and Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5.7.1 Membrane Behavior Under 0-Pressure Difference. . . . . . 119
5.7.2 Cavity Pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
5.7.3 Long-Term Hermeticity. . . . . . . . . . . . . . . . . . . . . . . . 123
5.8 Summary and Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
6 Characterization of Poly-SiGe Pressure Sensors . . . . . . . . . . . . . . 127
6.1 Measurement Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
6.2 Measurement Results: Pressure Response. . . . . . . . . . . . . . . . . 129
6.2.1 Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
6.2.2 Comparison to Simulations . . . . . . . . . . . . . . . . . . . . . 134
6.2.3 Offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
6.2.4 Nonlinearity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
6.2.5 Thermal Behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
6.3 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
6.4 Capacitive Pressure Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 144
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
7 CMOS Integrated Poly-SiGe Piezoresistive Pressure Sensor . . . . . 149
7.1 The Sensor Readout Circuit: An Instrumentation Amplifier . . . . 149
7.1.1 Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7.1.2 Layout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
7.1.3 Fabrication. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
7.1.4 Measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
7.2 Fabrication of a CMOS Integrated Pressure Sensor . . . . . . . . . . 161
7.3 Effect of the MEMS Processing on CMOS. . . . . . . . . . . . . . . . 167
7.4 Evaluation of the CMOS-Integrated Pressure Sensor . . . . . . . . . 169
7.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
8 Conclusions and Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
8.1 Conclusions and Contribution of the Dissertation . . . . . . . . . . . 175
8.2 Future Research Directions and Recommendations . . . . . . . . . . 179
Appendix A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
Appendix B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
Appendix C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
Appendix D. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197