Thomas M. Adams Richard A. Layton. Introductory MEMS. Fabrication and Applications

Transcription

1 Introductory MEMS

2 Thomas M. Adams Richard A. Layton Introductory MEMS Fabrication and Applications 123

3 Thomas M. Adams Department of Mechanical Engineering Rose-Hulman Institute of Technology 5500 Wabash Ave. Terre Haute IN USA Richard A. Layton Department of Mechanical Engineering Rose-Hulman Institute of Technology 5500 Wabash Ave. Terre Haute IN USA ISBN e-isbn DOI / Springer New York Dordrecht Heidelberg London Library of Congress Control Number: c Springer Science+Business Media, LLC 2010 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Printed on acid-free paper Springer is part of Springer Science+Business Media (

4 Dedication For Diedre, whose love and companionship is no small thing. Thom To Gary, my first guitar teacher. Thanks, Dad. Richard

5 Contents Preface xiii Part I Fabrication Chapter 1: Introduction What are MEMS? Why MEMS? Low cost, redundancy and disposability Favorable scalings How are MEMS made? Roadmap and perspective 12 Essay: The Role of Surface to Volume Atoms as Magnetic Devices Miniaturize 12 Chapter 2: The substrate and adding material to it Introduction The silicon substrate Silicon growth It s a crystal Miller indices It s a semiconductor Additive technique: Oxidation Growing an oxide layer Oxidation kinetics Additive technique: Physical vapor deposition Vacuum fundamentals Thermal evaporation Sputtering Other additive techniques 57

6 viii Introductory MEMS: Fabrication and Applications Chemical vapor deposition Electrodeposition Spin casting Wafer bonding 58 Essay: Silicon Ingot Manufacturing 59 Chapter 3: Creating and transferring patterns Photolithography Introduction Keeping it clean Photoresist Positive resist Negative resist Working with resist Applying photoresist Exposure and pattern transfer Development and post-treatment Masks Resolution Resolution in contact and proximity printing Resolution in projection printing Sensitivity and resist profiles Modeling of resist profiles Photolithography resolution enhancement technology Mask alignment Permanent resists 89 Essay: Photolithography Past, Present and Future 90 Chapter 4: Creating structures Micromachining Introduction Bulk micromachining processes Wet chemical etching Dry etching Surface micromachining Surface micromachining processes Problems with surface micromachining Lift-off Process integration A surface micromachining example 115

7 Contents ix Designing a good MEMS process flow Last thoughts 124 Essay: Introduction to MEMS Packaging 126 Chapter 5: Solid mechanics Introduction Fundamentals of solid mechanics Stress Strain Elasticity Special cases Non-isotropic materials Thermal strain Properties of thin films Adhesion Stress in thin films Peel forces 149 Part II Applications Chapter 6: Thinking about modeling What is modeling? Units The input-output concept Physical variables and notation Preface to the modeling chapters 163 Chapter 7: MEMS transducers An overview of how they work What is a transducer? Distinguishing between sensors and actuators Response characteristics of transducers Static response characteristics Dynamic performance characteristics MEMS sensors: principles of operation 178

8 x Introductory MEMS: Fabrication and Applications Resistive sensing Capacitive sensing Piezoelectric sensing Resonant sensing Thermoelectric sensing Magnetic sensing MEMS actuators: principles of operation Capacitive actuation Piezoelectric actuation Thermo-mechanical actuation Thermo-electric cooling Magnetic actuation Signal conditioning A quick look at two applications RF applications Optical applications 207 Chapter 8: Piezoresistive transducers Introduction Modeling piezoresistive transducers Bridge analysis Relating electrical resistance to mechanical strain Device case study: Piezoresistive pressure sensor 221 Chapter 9: Capacitive transducers Introduction Capacitor fundamentals Fixed-capacitance capacitor Variable-capacitance capacitor An overview of capacitive sensors and actuators Modeling a capacitive sensor Capacitive half-bridge Conditioning the signal from the half-bridge Mechanical subsystem Device case study: Capacitive accelerometer 250

9 Contents xi Chapter 10: Piezoelectric transducers Introduction Modeling piezoelectric materials Mechanical modeling of beams and plates Distributed parameter modeling Statics Bending in beams Bending in plates Case study: Cantilever piezoelectric actuator 276 Chapter 11: Thermal transducers Introduction Basic heat transfer Conduction Convection Radiation Case study: Hot-arm actuator Lumped element model Distributed parameter model FEA model 306 Essay: Effect of Scale on Thermal Properties 310 Chapter 12: Introduction to microfluidics Introduction Basics of fluid mechanics Viscosity and flow regimes Entrance lengths Basic equations of fluid mechanics Conservation of mass Conservation of linear momentum Conservation equations at a point: Continuity and Navier-Stokes equations Some solutions to the Navier -Stokes equations Couette flow Poiseuille flow Electro-osmotic flow Electrostatics 340

10 xii Introductory MEMS: Fabrication and Applications Ionic double layers Navier-Stokes with a constant electric field Electrophoretic separation 357 Essay: Detection Schemes Employed in Microfluidic Devices for Chemical Analysis 362 Part III Microfabrication laboratories Chapter 13: Microfabrication laboratories Hot-arm actuator as a hands-on case study Overview of fabrication of hot-arm actuators Cleanroom safety and etiquette Experiments 377 Experiment 1: Wet oxidation of a silicon wafer 377 Experiment 2: Photolithography of sacrificial layer 384 Experiment 3: Depositing metal contacts with evaporation 388 Experiment 4: Wet chemical etching of aluminum 392 Experiment 5: Plasma ash release 395 Experiment 6: Characterization of hot-arm actuators 397 Appendix A: Notation 405 Appendix B: Periodic table of the elements 411 Appendix C: The complimentary error function 413 Appendix D: Color chart for thermally grown silicon dioxide 415 Glossary 417 Subject Index 439

11 Preface We originally wanted to call this book Dr. Thom s Big Book about Little Things, but, apart from being perhaps too playful a title, we didn t like the word big in it. That s because the book was intended to be an introduction to the world of science and engineering at the microscale, not a comprehensive treatment of the field at large. Other authors have already written books like that, and they have done a wonderful job. But we wanted something different. We wanted an introductory MEMS text accessible to any undergraduate technical major, students whose common background consists of freshman level physics, chemistry, calculus and differential equations. And while a little book about little things might not suffice to that end, we at least desired a somewhat compact book about things micro. When we taught an introductory MEMS course for the first time in the spring of 2002 to just such an audience, it was, at least to our knowledge, a unique endeavor. We attempted to cover way too much material though, and we threw one of those big comprehensive books at the students. It nearly knocked them out. In subsequent installments, we cut back on the material and started using instructor notes in lieu of a text. Those notes, outlines, bulleted lists, and fill-in-the-blank handouts became the skeleton around which this text was formed. As creating microstructures requires such a different set of tools than those encountered in the macro-world, much of learning about MEMS rests squarely in learning the details of how to make them. Part I of this text therefore deals mainly with introducing the reader to the world of MEMS and their fabrication. Actuation and sensing are also treated from a generic standpoint, with MEMS devices used as examples throughout. In Chapter 7 of Part II, an overview of some of the most common MEMS transducers is given from a mainly qualitative, non-mathematical standpoint. Hence, the first seven chapters should suffice for the majority of introductory courses. Following Chapter 7 are specific treatments and modeling strategies for a handful of selected MEMS. The mathematical modeling is more detailed than in previous chapters, covering a number energy domains. Though the models given can be a bit involved, the necessary tools are

12 xiv Introductory MEMS: Fabrication and Applications developed for the reader. Part II is therefore better suited for a follow-up course, or perhaps a standalone course for students with the appropriate background. Alternatively, an introductory course covering Chapters 1-7 could culminate with one modeling chapter from Part II. The last chapter on microfluidics is in some sense a standalone treatment of the field. Just as no one is able to design a functioning power plant after having taken an introductory course in thermodynamics, no one will be able to design, build and test a successful MEMS device after only reading this text. However, the reader will have acquired the new skill of considering microtechnology-based solutions to problems, as well as the ability to speak intelligently about MEMS and how they are modeled. The text can therefore serve as both a springboard for further study or as an end in itself. One of the challenges in writing such a text is that it is a bit like writing a book entitled Introduction to Science and Engineering, as this is what MEMS really is science and engineering at the microscale, that is. It can therefore be quite difficult to keep it truly general. By making the intended audience an undergraduate technical major in any field, and therefore not assuming any other specialized background, we have avoided slanting the text in some preferred direction. That is to say, we have done our best to keep the text a true introduction to MEMS as a whole rather than an introduction to, say, dynamic systems modeling of MEMS devices, or materials engineering aspects of MEMS. The opposite danger, of course, is not including enough material to really understand MEMS. To address this, where needed we have included introductions to fields that are generally not part of the common experience of all technical majors. The introductions are kept brief, as they are intended to give the reader just enough background to understand the field in context of the MEMS device(s) at hand. Naturally these sections can be omitted when tailoring a course for specific majors. In reading the text, most readers will find themselves outside of their comfort zone at some point. At other times the reader may find themselves reading things that seem obvious. Which things are which will be different for different readers, depending on their backgrounds. What s more, readers may occasionally find themselves a trifle disoriented even within a field in which they have heretofore considered themselves well-versed. A prime example is Chapter 12 on microfluidics, in which electro-osmotic flow is treated. In electro-osmotic flow the traditional fields of fluid mechanics and electrostatics, topics usually thought of as having little to do with each other, are coupled and of equal importance. Throw in a smattering of mass transfer and chemistry and you have a topic in which very few of us can hit the ground running.

13 Preface xv Such is the world of MEMS. Scientists and engineers of all fields are necessarily drawn to one another in order to make things work. The cliché that the world is getting smaller has found new metaphorical meaning with the miniaturization of technology. It is for this very reason that we feel so strongly that this text and the types of courses it is designed to accompany are vitally necessary in the education of scientists and engineers. Gone are the days when technical professionals could pigeonhole themselves into not venturing outside of narrow areas of expertise. Multidisciplinary endeavors are all the buzz anymore, and rightfully so. For MEMS they are its very lifeblood. ACKNOWLEDGEMENTS Such a book is never truly the work of only it authors. Countless people have come together to make this happen. Many thanks go to the members of the Rose-Hulman MiNDS Group for their numerous selflessly given hours to make MEMS more assessable to a general audience: Jameel Ahmed, Daniel Coronell, Pat Ferro, Tina Hudson, Elaine Kirkpatrick, Scott Kirkpatrick, Michael McInerney, Daniel Morris, Jerome Wagner and Ed Wheeler. Thanks also to the students over the years with whom we ve worked and learned. We wish we had delivered you this text earlier. Special thanks to Azad Siahmakoun, director of the Micro-Nano Devices and Systems Facility at Rose-Hulman Institute of Technology, without whom undergraduate MEMS education would not exist at all at our institution, let alone a textbook. Thom extends extra special thanks to Diedre Adams for all the corrections and wonderful suggestions, and also for simply putting up with him. That s for both the little things and the big things. THOMAS M. ADAMS RICHARD A. LAYTON TERRE HAUTE, INDIANA