Ghodssi R., Lin P., MEMS Materials and Processes Handbook

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MEMS Materials and Processes Handbook

MEMS Reference Shelf

Series Editors:

Stephen D. Senturia Roger T. Howe

Professor of Electrical Professor Department

Engineering, Emeritus of Electrical Engineering

Massachusetts Institute Stanford University

of Technology Stanford, California

Cambridge, Massachusetts

Antonio J. Ricco

Small Satellite Division

NASA Ames Research Center

Moffett Field, California

For other titles in this series, go to:

www.springer.com/series/7724

Reza Ghodssi · Pinyen Lin

Editors

MEMS Materials

and Processes Handbook

123

Editors

Reza Ghodssi

Department of Electrical and Computer

Engineering

Institute for Systems Research

MEMS Sensors and Actuators Laboratory

University of Maryland

College Park, Maryland

USA

ghodssi@umd.edu

Pinyen Lin

Touch Micro-system Technology Corp.

Taoyuan

Taiwan

and

Walsin Lihwa Corporation

Taipei

Taiwan

pinyen_lin@alum.mit.edu

ISSN 1936-4407

ISBN 978-0-387-47316-1 e-ISBN 978-0-387-47318-5

DOI 10.1007/978-0-387-47318-5

Springer New York Dordrecht Heidelberg London

Library of Congress Control Number: 2011921730

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Foreword

The ﬁeld that is affectionately known as “MEMS” (an acronym for Micro-Electro-

Mechanical Systems) is a descendant of the integrated circuits industry, but a

descendent that has developed in ways and directions never anticipated by its par-

ent. Now a highly specialized discipline in its own right, MEMS draws not only on

all of conventional microelectronics but also on novel fabrication methods and uses

of non-microelectronic materials to create devices that are mechanical, or ﬂuidic, or

biochemical, or optical, many without any transistors in sight. The key words are

sensors and actuators, sometimes combined with (or without) microelectronics to

create complete microsystems. MEMS devices and microsystems are now found

everywhere – in automobiles, in ink-jet printers, in computer games, in mobile

telephones, in forensic labs, in factories, in sophisticated instrumentation systems

launched into space, in the operating r oom and in the clinic. The genie is out of the

bottle. MEMS devices are everywhere.

Because of this immense diversity, no single book can capture the essence of

the entire ﬁeld. But all MEMS devices represent highly speciﬁc answers to two

critical questions: “How shall I make it? And from what materials shall I make it?”

Processes and materials. Materials and processes. Because these two challenges are

common to all MEMS devices, it makes sense to gather the wisdom of the least

several decades on “how to make it” and “from what materials” into a single data-

rich, process-detail-rich compendium. That is the raison d’etre for this volume, and

that is its goal: to document MEMS processes and materials at a sufﬁcient level of

detail to be of signiﬁcant practical use.

I congratulate the co-Editors, Reza Ghodssi and Pinyen Lin, as well as our con-

sulting editors and all of the contributors, for their diligence, persistence, care, and

skill in bringing this material to published form, and I invite the MEMS commu-

nity world-wide to beneﬁt from the knowledge, wisdom and cumulative expertise

gathered into these pages.

Brookline, Massachusetts Stephen D. Senturia

June 2010

This is Blank Page Integra vi

Preface

Throughout the relatively short history of microelectromechanical systems

(MEMS), there have been numerous advances and inventions directly related to

device fabrication. From humble beginnings using borrowed and modiﬁed IC fabri-

cation techniques to current MEMS-speciﬁc tools such as deep reactive ion etching

(DRIE) using inductively coupled plasma (ICP) sources, MEMS r esearchers have

continually advanced and augmented the capabilities of wafer-based fabrication

technologies. These advances have been instrumental in the demonstration of new

devices and applications – Texas Instruments’ Digital Micromirror Device, the MIT

microturbine, Analog Devices’ accelerometers – and even in the creation of new

ﬁelds of research and development: bioMEMS, microﬂuidic devices, and optical

MEMS.

To date, a number of books have been written about these new fabrication tech-

nologies and materials in general, but discussion of their relationship to MEMS

design has been minimal. As a particularly diverse and multidisciplinary area of

research, the ﬁeld of MEMS offers a vastly different set of challenges relative to

typical IC fabrication and design. Much effort is often focused on characterization

runs and developing in-house recipes and speciﬁc processes to develop and man-

ufacture MEMS structures, each time at the risk of wasting research efforts and

“reinventing the wheel.” A wealth of knowledge exists in the MEMS community,

but much of this expertise is most readily accessed by informal, nonmethodological

means such as discussions with colleagues at conferences. The authors of this book

have observed an unnecessarily steep learning curve for the development of com-

mon MEMS processes, and believe the time spent traversing this curve would be

better spent brainstorming new ideas and uncovering new applications. This book

was conceived and born of this belief.

A fundamental and comprehensive MEMS-focused reference book will be an

important asset for current and future research scientists and engineers. It was

decided early in the brainstorming sessions for this book to include materials as

well as processes in the discussion, as MEMS utilizes a wide variety of each in

common applications. We intend this book to provide the reader with the basics of

MEMS materials and processes, but beyond this goal, we intend for it to give prac-

tical insight into the workings and standard procedures carried out in research labs

vii

viii Preface

and production facilities on a daily basis. To this end, each chapter has an extended

section with case studies, giving step-by-step examples and recipes prepared by

experts in industry and academia. Particularly, the effect of processing conditions

on material properties are covered where applicable, illustrating the interdepen-

dence and multidisciplinary nature of MEMS fabrication. The chapters are meant

to be a springboard of sorts, providing basic information about each topic, with a

large number of classic and contemporary literature references to provide in-depth

knowledge. Ultimately, it is our goal to provide a useful design reference volume for

the seasoned researcher and the MEMS newcomer alike. We hope this book consol-

idates important information for readers and thereby spurs the creation of many new

devices and processes.

MEMS devices are essentially microsystems that have structures and empty

space built together. The authors of this book view the materials and processes

as the fundamental building blocks for making those structures and empty spaces.

Keeping this in mind, the book is divided into two main sections: Chapters 2, 3, 4,

5, and 6 covering materials and Chapters 7, 8, 9, 10, 11, 12, and 13 covering fab-

rication techniques. These two general thrusts are bookended by Chapter 1, which

discusses general MEMS design, and Chapter 14, which deals with MEMS process

integration.

Chapter 1 provides a basic framework for the design of MEMS systems and pro-

cesses, which we highly recommend reading before diving into the materials and

process sections of the book. Chapter 2 presents an overview of the recipes and

methods used in the deposition of semiconductor and dielectric thin-ﬁlms, partic-

ularly those most commonly used in the fabrication of MEMS. The basics here

include chemical vapor deposition, epitaxy, physical vapor deposition, atomic layer

deposition, and spin-on techniques. Additive processes for depositing metal ﬁlms

are discussed in detail in Chapter 3, where particular attention is paid to thick

metal deposition with signiﬁcant coverage devoted to electrochemical and electro-

less plating processes that are often required for MEMS fabrication. The entirety of

Chapter 4 is devoted to the use of polymeric materials for MEMS. Polymers, such as

polydimethylsiloxane (PDMS), are important materials for a vast array of devices,

as encapsulants for tactile sensors and as an integral enabling technology for the

emerging ﬁeld of bioMEMS. The piezoelectric ﬁlms detailed in Chapter 5 are an

important part of MEMS technology, serving as both sensor and actuator elements.

The basic properties of these materials and the physics of operation are described

in detail as well as practical deposition and fabrication methods. Chapter 6 focuses

on the fabrication and integration of shape memory alloy (SMA) materials, which

provide high-force and high-displacement actuator mechanisms for MEMS.

Chapter 7 begins the section on processing of materials for MEMS applications

by covering the very important area of dry etching methods (including DRIE),

particularly the inﬂuence of different parameters on the etch recipe development

process. Complementing the coverage of dry etching, wet etching processes for

MEMS micromachining are covered in Chapter 8 with a comprehensive recipe and

reference list included in this chapter to aid in ﬁnding etch rates and etch selectivities

for a wide range of materials from silicon to III–V compound semiconductors.

Preface ix

Chapter 9 describes the technology of lithography and related techniques, cov-

ering traditional contact lithography, projection and X-ray lithography, and more

exotic direct-write and printing lithographic techniques. Doping processes typical in

and for MEMS applications for electrical purposes and etching control are reviewed

in Chapter 10, along with diagnostic techniques for these methods. Wafer bond-

ing, a crucial fabrication technique for silicon MEMS encapsulation and structure

fabrication, is covered in detail in Chapter 11 with emphasis placed on direct and

intermediate layer bonding methods.

Chapter 12 discusses the still-evolving ﬁeld of MEMS packaging, pointing out

differences with current microcircuit packaging techniques; this chapter in partic-

ular highlights how MEMS devices present very unique challenges as compared

to traditional microcircuits. Surface treatments for MEMS devices are discussed in

Chapter 13, covering antistiction and planarization coatings, functionalization of

surfaces for biological and optical applications, and chemical mechanical polishing

(CMP). Chapter 14 concludes the book with a discussion of the integration of any

number of the above processes and materials into a compatible and efﬁcient pro-

cess ﬂow, referred to here as process integration. The ﬁnal chapter also discusses

economic and practical aspects of process integration, citing some commercially

successful examples of MEMS devices.

This reference volume would not have been possible without the help of many of

our colleagues in the MEMS ﬁelds, from both academia and industry. We would like

to extend words of thanks and gratitude to Stephen (Steve) Senturia for providing

the vision, support, and guidance to our team over the past ﬁve years in navigating

the completion of the chapters and for reviewing them diligently, and to the Series

Associate Editors Roger Howe and Antonio (Tony) Ricco for carefully reviewing

the chapters, providing helpful comments, and for recommending prospective con-

tributing authors. We thank Steven (Steve) Elliot, from Springer, for his patience in

dealing with thirty-ﬁve unique and independent experts, and his persistence in con-

tacting each of us in order to develop the logistics for the book publishing process.

Last but not least, we acknowledge all thirty-ﬁve contributing authors, who self-

lessly gave time, expertise, effort, and creativity to make this book a one-of-a-kind

contribution to the current and future MEMS community, including industry and

government professionals, academic faculty and staff, and students.

The idea for this book was born at the Transducers 2005 Conference in Seoul,

South Korea, and it was ﬁnalized and ﬁnished at The Hilton Head 2010 Workshop

on Hilton Head Island, South Carolina. The ﬁve years of cooperative activity that

culminated in this handbook prove that great ideas can become reality when col-

leagues work collaboratively to achieve a common goal. This message, which we

have tried to convey by writing this book, is what the greater MEMS community is

all about.

College Park, Maryland Reza Ghodssi

Taoyuan, Taiwan Pinyen Lin