MEMS Material Handbook

Reza Ghodssi·Pinyen Lin Editors

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

Table of contents

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 Chapter4isdevotedtotheuseofpolymericmaterialsforMEMS.

Polymers,suchas 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 referencelistincludedinthischaptertoaidinﬁndingetchratesandetchselectivities for a wide range of materials from silicon to III–V compound semiconductors.

Chapter 9 describes the technology of lithography and related techniques, cov- ering traditional contact lithography, projection and X-ray lithography, and more exoticdirect-writeandprintinglithographictechniques.Dopingprocessestypicalin 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.