微机电系统（MEMS）工程基础电子书概览

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"本书是关于微电子机械系统（MEMS）工程的入门读物，涵盖了MEMS技术的基本概念、材料和制造工艺。" MEMS，全称微电子机械系统，是一种集成了微型机械结构和电子元件的技术，它将机械、光学、化学和生物功能与微电子学相结合，形成微型化、智能化的器件。这项技术源于微米尺度的精密加工，旨在创造具有广泛应用潜力的微小设备。书中提到，MEMS的主要吸引力在于其能够实现传统大型机械系统无法达到的性能、成本和尺寸优势。MEMS器件可以应用于各种领域，包括但不限于传感器（如加速度计、陀螺仪）、执行器（如微型泵和阀）、微镜、生物医疗设备、微型能源系统以及光学通信设备等，这些市场正在迅速增长。在考虑是否采用MEMS技术时，书中讨论了制定标准的重要性，因为这有助于确保互操作性、降低成本并促进技术发展。然而，MEMS领域的标准化进程面临挑战，因为它涉及到跨学科的复杂性和技术的独特性。在介绍MEMS材料时，书中详述了硅作为基础材料，包括硅本体、氧化硅和氮化硅，它们是MEMS制造中的常用材料。此外，还提到了金属层、聚合物、玻璃和石英基底、碳化硅、金刚石、III-V族化合物半导体，以及形状记忆合金等。每种材料都有其特定的物理属性和效应，如压阻效应、热电效应，这些特性对于MEMS器件的设计和功能至关重要。接着，书中探讨了微加工过程的基本工具，如外延生长、氧化、溅射沉积、蒸发、化学气相沉积、旋涂方法、光刻和蚀刻等。这些工艺是构建MEMS器件的关键步骤。同时，也介绍了高级工艺工具，如阳极键合、硅直接键合、研磨和抛光等，这些都是制造微型结构所必需的精细工艺。这本书为读者提供了一个全面的MEMS技术入门指南，涵盖了从基本概念到材料选择，再到制造工艺的全过程，是理解和探索这一前沿领域的宝贵资源。通过深入学习，读者将能够理解MEMS技术的核心原理，并了解如何利用这些知识来设计和制造微型器件。

A-n

Intrncrluctinr! tn Micro~l~rtrnmechanical Systems Engineerin9

What

micromachining?

Micromachining is the set of design and fabrication tools that precisely

machine and form structures and elements at a scale well below the limits

of our human perceptive faculties-the microscale. Micromachining is

the underlying foundation of MEMS fabrication; it is the toolbox of

MEMS.

Arguably, the birth of the first micromachined components dates

back many decades, but it was the well-established integrated circuit

industry that indirectly played an indispensable role in fostering an envi-

ronment suitable for the development and growth of micromachiniig

technologies. As the following chapters will show, many tools used in

the design and manufacturing of MEMS products are "borrowed" from

the integrated circuit industry. It should not then be surprising that

micromachining relies on silicon as a primary material, even though the

technology was certainly demonstrated using other materials.

Applications and markets

Present markets are primarily in pressure and inertial sensors and inkjet

print heads, with the latter dominated by Hewlett Packard Company of

Palo Alto, California. Future and emerging applications include high-

resolution displays, high-density data storage devices, valves, and fluid

management and processing devices for chemical microanalysis, medicai

diagnostics, and drug delivery. While estimates for MEMS markets vary

considerably, they all show significant present and future growth, reach-

ing aggregate volumes in the many billions of dollars by the year 2004

[2-4].

The expected growth is driven by technical innovations and accep-

tance of the technology by an increasing number of end users and

customers.

However, because of the lack of a single dominant application-the

"killer

app"-and the diverse technical requirements of end users, there is

no single MEMS market, but rather a collection of markets, many of

which are considered niche markets--especially when compared to their

kin semiconductor businesses. It is true that unit volumes in a few

segments, including automotive, are substantial, running in the tens of

millions, but

the corresponding dollar volumes tend to be modest. Fur-

thermore, occasional poor forecasting of emerging applications poses

MEMS:

Technologv from Lilliput

additional risks and difficulties to companies engaged in the development

and manufacture of MEMS products. For instance, the worldwide market

for

airbag crash sensors-thought by many to be a considerable mar-

ket-is estimated today at $1 50,000,000, even as these components

become standard on all 50,000,000 vehicles manufactured every year

around the globe. Market studies conducted in the early 1990s incorrectly

estimated the unit asking price of these sensors, neglecting the effect of

competition on pricing, and artificially inflating the size of the market to

$500,000,000 (Table

1.2).

To MEMS or not to MEMS?

Like many other emerging technologies with significant future potential,

MEMS is subject to a rising level of excitement and publicity. As it

evolves and end markets develop, this excessive optimism is gradually

Table

1.2

Analysis and Forecast of U.S. MEMS Markets (in Millions of U.S. ~ollars)'

Information

Technology Military

Year

~utomotive~ Medical

~ndurtxial~ Ikospace Total

ooo,ooo)

($OOO.ooo)

ooo,ooo)

ooo,OOO)

CAGR

16% 21% 16% 16% 17%

Data prior to 1997

actual. (The projected compound

annual

growth rate

(CAGR)

averages 17% across the dominant market sectors. Source: Frost

Sullivan

[4].)

Airbag systems and

MAP

sensors constitute 90%

the automotwe

MEMS

market.

In 1998, the market division

was:

wets 75.6Oh. displays 5.4%. and industrial 19%.

moderated with a degree of realism reflecting the technology's strengths

and capabilities.

Any end user considering developing a MEMS solution or incorporat-

ing one into a design invariably reaches the difficult question of "Why

MEMS?" The question strikes at the heart of the technology, particularly

in view of competing methods, such as conventional machining or plastic

molding techniques that do not have recourse to micromachining. For

applications that can benefit from existing commercial MEMS products

(e.g., pressure or acceleration sensors), the answer to the above question

relies on the ability to meet required specifications and pricing. But the

vast majority of applications require unique solutions that often necessi-

tate the funding and completion of an evaluation or development

program. In such situations, a clear-cut answer is seldom easy to establish.

In practice, a MEMS solution becomes attractive if it enables a new

function, provides significant cost reduction, or both. For instance, medi-

cal applications generally seem to focus on added or enabled functionality

and improved performance, whereas automotive applications often seek

cost reduction. Size reduction can play an important selling role, but is

seldom sufficient as the sole reason unless it becomes enabling itself.

Naturally, reliability is always a dictated requirement. The

decision-

making process is further complicated by the fact that MEMS is not

a single technology, but a set of technologies (e.g., surface vs. bulk

micromachining). At this point, it is beneficial for the end user to become

familiar with the capabilities and the limitations of any particular MEMS

technology selected for the application in mind. The active participation

of the end user allows for the application to drive the technology develop-

ment, rather than the frequently occurring opposite situation.

Companies seeking MEMS solutions often contract a specialized facil-

ity for the design and manufacture of the product. Others choose first to

evaluate basic conceptual designs through existing foundry services. A

few companies may decide to internally develop a complete design. In

the

latter case, there is considerable risk that manufacturing considerations

are not properly taken into account, resulting in significant challenges in

production.

Regardless of how exciting and promising a technology may be, its

ultimate realization is invariably dependent on economic success. The

end user will justify the technology on the basis of added value, increased

productivity,

and/or cost competitiveness, and the manufacturer must

MEMS:

Techno10g.v

from

Lilliput

show revenues and profits. On both tracks, MEMS technology is able to

deliver within a set of realistic expectations that may vary with the end

application. A key element in cost competitiveness is "batch fabrication,"

which is the practice of simultaneously manufacturing hundreds or thou-

sands of identical parts, thus diluting the overall impact of fixed costs,

including the cost of maintaining expensive cleanroom and assembly

facilities. This is precisely the same approach that has resulted, over the

last few decades, in a dramatic decrease in the price of computer memory

chips. Unfortunately, the argument works in reverse too: Small manufac-

turing volumes will bear the full burden of overhead expenses, regardless

of how "enabling" the technology may be (Figure

1.1).

Standards

Few disagree that the burgeoning MEMS industry traces many of its

roots to the integrated circuit industry. However, the two market dynam-

ics differ greatly with severe implications, one of which is the lack

of standards in MEMS. Complementary metal-oxide semiconductor

(CMOS) technology has proven itself over the years to be a universally

accepted manufacturing process for integrated circuits, driven primarily

by the insatiable consumer demand for computers and digital electronics.

In contrast, the lack of a dominant MEMS high-volume product, or fam-

ily of products, combined with the unique technical requirements of

Figure

1.1

Volume manufacturing is essential for maintaining

profitability.

Introduction to Microelectromechanical Systems Engineering

each application have resulted in the emergence of multiple fabrication

and assembly processes. The following chapters will introduce these

processes. Standards are generally driven by the needs of high-volume

applications, which are few in MEMS. In turn, the lack of standards feeds

into the diverging demands of the emerging applications.

The psychological barrier

It is human nature to cautiously approach what is new because it is for-

eign and untested. Even for the technologically savvy or the fortunate

individual living in high-tech regions, there is a need to overcome the

comfort zone of the present before engaging the technologies of the

future. This cautious behavior translates to slow acceptance of new tech-

nologies and derivative products as they are introduced into society.

MEMS acceptance is no exception. For example, demonstration of the

first micromachined accelerometer took place in 1979 at Stanford Uni-

versity. Yet it took nearly fifteen years before it became accepted as

a device of choice for automotive

airbag safety systems. Naturally, in the

process, it was designed and redesigned, tested and qualified in the

laboratory and the field before it gained the confidence of automotive

suppliers. The process can be lengthy, especially for embedded systems

(see Figure 1.2).

Today, MEMS and associated product concepts generate plenty of

excitement, but not without skepticism. Companies exploring for the first

time the incorporation of MEMS solutions into their systems do so with

trepidation, until an internal "MEMS technology champion" emerges

to educate the company and raise the confidence level. With many

micromachined silicon sensors embedded in every car and in numerous

critical medical instruments, and with additional MEMS products finding

their way into our daily lives, the height of this hidden psychological bar-

rier appears to be declining.

Journals, conferences, and

Web sites

The list of journals and conferences focusing on micromachining and

MEMS continues to grow every year. There is also a growing list of on-

MEMS:

Technology from Lilliput

Percent

100

Figure

1.2

Chart illustrating the percent household penetration of

new electronic products.

takes

years before new

technologies gain wide acceptance

[5].

line Web sites, most notably the MEMS Clearinghouse hosted by the

Information Sciences Institute (ISI), Marina del Rey, California, and

the European Microsystems Technology On-line (EMSTO), Berlin, Ger-

many, sponsored by the ESPRIT program of the European Commission.

The sites provide convenient links and maintain relevant information

directories (Table 1.3).

List

journals and magazines

Several journals and trade magazines published

the

U.S.

and Europe

cover research and advances in the field. Some examples are:

Sensors and Actuators

(A,

C): a peer-reviewed scientific journal

published by Elsevier Science, Amsterdam, The Netherlands.

Journal ofMicromechanica1 Systems

(JMEMS): a peer-reviewed scien-

tific journal published by the Institute of Electrical and Electronic

Engineers (IEEE), Piscataway, New Jersey, in collaboration with

the American Society of Mechanical Engineers

(ASME), New York,

New York.

Journal of Micromechanics and Microengineering

(JMM): a peer-

reviewed scientific journal published by the Institute of Physics,

Bristol, United Kingdom.

Introduction to Microelectromechanical Systems Engineering

Table

1.3

List of a Few Government and Nongovernment

Organizations With Useful On-line Resources

Organization Address Description Web Site

Marina del Rey, CA U.S. Clearinghouse

merns.isi.edu

Berlin, Germany European

www.nexus-emsto.

Clearinghouse corn

VDINDE

Teltow, Germany Association of www.vdivde-it.de/

German Engmeers MST

DARPA Arlington, VA Sponsored U.S. web-ext2.darpa.rmV

government projects

MTO

NET Gaithersburg,

Sponsored

U.S.

www.atp.nist.gov

government projects

Institute of Defense Alexandria, VA Insertion

military

www.ida.org/MEMS

Analyses applications

AIST

MITI

Tokyo. Japan The "Micromachine www.aist.go.jp

Project"

Japan

Sensors Magazine: a trade journal with an emphasis on practical and

commercial applications published by Helmers Publishing Inc.,

Peterborough, New Hampshire.

MSTnews: an international newsletter on microsystems and MEMS

published by VDIIVDE Technologiezentrum Informationstechnik

GmbH, Teltow, Germany.

Micromachine Devices: a publication companion to R&D Magazine

with news and updates on MEMS technology published by Cahners

Business Information, Des Plaines, Illinois.

List of conferences and meetings

Several conferences cover advances in MEMS or incorporate program

sessions on micromachined sensors and actuators. The following list gives

a few examples:

International Conference on Solid-state Sensors and Actuators

(Transducers): held on odd years and rotates sequentially between

North America, Asia, and Europe.

MEMS:

Techno10,qy from Lilliput

Solid-state Sensor and Actuator Workshop (Hilton Head): held on

even years in Hilton Head Island, South Carolina, and sponsored by

the Transducers Research Foundation, Cleveland, Ohio.

MicroElectroMechanical Systems Workshop (MEMS): an interna-

tional meeting held annually and sponsored by the Institute

of Electrical and Electronics Engineers (IEEE), Piscataway, New

Jersey.

International Society for Optical Engineering (SPIE): regular

conferences held in the United States and sponsored by SPIE, Bel-

lingham, Washington.

MicroTotalAnalysis Systems (MTAS): a conference focusing on

microanalytical and chemical systems. This conference was held on

alternating years, but will become annual beginning in the year

2000.

It alternates between North America and Europe.

Summary

Microelectromechanical structures and systems are miniature devices

that enable the operation of complex systems. They exist today in many

environments, especially automotive, medical, consumer, industrial, and

aerospace. Their potential for future penetration into a broad range of

applications is real, supported by strong developmental activities at many

companies and institutions. The technology consists of

large portfolio of

design and fabrication processes (a toolbox), many borrowed from the

integrated circuit industry. The development of MEMS is inherently

interdisciplinary, necessitating an understanding of the toolbox as well as

the end application.

References

[I]

Dr. Albert Pisano, in presentation material distributed by the United States

Defense Advanced Research Program Agency (DARPA), available at

http://web-ext2.darpa.mil.

[2]

System Planning Corporation, "Microelectromechanical Systems (MEMS):

An SPC Market Study," January 1999, 1429 North Quincy Street, Arlington,

Virginia 22207.

Tntroduction

MicroelectromechanicaI Systems Enqineering

[3]

Frost

Sullivan, 'World Sensors Market: Strategic Analysis," Report

#5509-32, February 1999, 2525 Charleston Road, Mountain View, California

94043, http:llwww.frost.com.

[4]

Frost

Sullivan, "U.S. Microelectromechanical Systems (MEMS)," Report

#5549-16, June 1997, 2525 Charleston Road, Mountain View, California

94043, http:Ilwww.frost.com.

[5]

Mercer Management Consulting, Inc., In

Business Week,

April 19, 1999,

Selected bibliography

Angell, J.

B.,

Terry, S. C., and Barth,

W.,

"Silicon Micromechanical Devices,"

ScientificAmerican,

Vol. 248, No. 4, Apr. 1983, pp. 44-55.

Gabriel,

J.,

'Engineering Microscopic Machines,"

Scientific American,

Vol. 273,

No. 3, Sept. 1995, pp. 150-153.

Petersen,

E., 'Silicon as a Mechanical Material,"

Proceedings of the

IEEE,

Vol. 70,

No. 5, May 1982, pp. 420-457.

'Nothing but light,"

ScientificAmerican,

Vol. 279, No. 6, Dec. 1998, pp. 17-20.

Contents

Silicon material system

Other materials and

substrates

Important material

properties

and

physical

effects

Summary

The

Sandbox:

Materials for MEMS

You

can't

see

it,

but

it's

everywhere you go.

Bridget Booher, journalist, on silicon.

we view micromachining technology as a

set of generic tools, then there is no reason

to limit its use to one material. Indeed,

micromachining was demonstrated in silicon,

glass, ceramics, polymers, and compound

semiconductors made of group 111 and

ele-

ments, as well as a wide variety of metals

including titanium and tungsten. However,

silicon remains the primary material of

choice for microelectromechanical systems.

Unquestionably, this popularity arises from

the large momentum of the electronic

integrated circuit industry and the derived

economic benefits, not the least of which is

the extensive industrial infrastructure. The

object of this chapter is to present the proper-

ties of silicon and a few other materials while

emphasizing that the final choice

materials

is determined by the type of application and

economics.

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