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Muller, R.S. “Microdynamic Systems in the Silicon Age”

Handbook of Micro/Nanotribology.

Ed. Bharat Bhushan

Boca Raton: CRC Press LLC, 1999

Microdynamic Systems

in the Silicon Age

Richard S. Muller

13.1 Introduction

Origins

13.2 Micromachining

Substrate Micromachining • Surface Micromachining •

Polycrystalline Silicon Properties • Tribology in MEMS

13.3 MEMS Structures and Systems

MEMS Actuation Forces • MEMS for Microphotonics •

Coupling Efﬁciency

13.4 Berkeley Microphotonics Research

Acknowledgments

References

ABSTRACT

With the proven record of accomplishment in very large integrated circuits (VLSI) brought about by

batch-fabrication technology for electronic devices of ever-decreasing size, there is widespread enthu-

siasm about emerging opportunities to design mixed micromechanical and microelectronic systems.

New development, based heavily on integrated-circuit-related technologies, have led to rapid progress

in the development of

microdynamic systems

. These systems are based upon the science, technology,

and design of

moving

micromechanical devices, and are a subclass of what is known in the U.S. as

microelectromechanical systems (MEMS). The dimensions of the micromechanical devices in MEMS

are typically smaller than 100 µm. Recent rapid progress gives promise for new designs of integrated

sensors, actuators, and other devices that can be combined with on-chip microcircuits to make possible

high-performing, compact, portable, low-cost engineering systems. Mechanical materials for some of

the microdynamic systems that have thus far been demonstrated consist of deposited thin ﬁlms of

polycrystalline silicon, silicon nitride, aluminum, polyimide, and tungsten among other materials. To

make mechanical elements using thin-ﬁlm processing, microstructures are freed from the substrate

by etching a

sacriﬁcial layer

of silicon dioxide. First demonstrated as a means to produce electrostatically

driven, doubly supported beam bridges, this sacriﬁcial-layer technology has proved very versatile and

has been used to make, among other structures, laterally vibrating doubly folded bridges, gears, springs,

and impacting microvibromotors. Recently, micromirrors that consist of multiple hinged plates which

Updated, expanded, and revised version of article ﬁrst published in

Micro/Nanotribology and Its Applications

(B.

Bhushan, ed.), NATO ASI Series, Kluwer Academic Publishers, Dordrecht, 1997.

fold out of the surface plane in which they are formed (reaching vertical heights of millimeter dimen-

sions) have been demonstrated. The polycrystalline silicon cross section for these mirrors is thinner

than 2 µm. The mirrors can be moved using electrostatic comb drives or microvibromotors. Continued

research on the mechanical properties of the electrical materials forming microdynamic structures

(which previously had exclusively electrical uses), on the scaling of mechanical design, on tribological

effects, on coatings, and on the effective uses of computer aids is now under way. This research promises

to provide the engineering base that will exploit this promising technology.

13.1 Introduction

Microdynamic systems, that is, systems in which micromechanical elements undergo controlled motion,

are, from one very useful perspective, a logical consequence in the continued evolution of microelectronic

systems. Microdynamics offer special opportunities for the production of extremely miniaturized, highly

complex systems. The opportunities presented by microdynamics can be compared with those that

enabled the ﬁeld of electronics to be revolutionized by the achievement of large-scale electronic-device

integration. The VLSI revolution has, of course, affected all of society and its multi-billion-dollar indus-

trial component continues robust growth after three decades of development. The linkage between

micromechanical methods and typical integrated-circuit processes has been developing strongly since it

was ﬁrst demonstrated in seminal work done in 1982 when R. T. Howe demonstrated means to make

microbeams from polycrystalline silicon ﬁlms (Howe and Muller, 1982). Following this demonstration,

Howe built the prototype polysilicon MEMS to be used as a chemical vapor sensor (Howe and Muller,

1986); see Figure 13.1.

Through the 15 ensuing years, engineers have focused on employing the key steps of batch processing

and self-assembly that underlie modern microelectronics for the development of systems composed of

micromechanical as well as microelectronic components to make microelectromechanical systems, now

typically designated by the acronym MEMS. The impact of MEMS is very broad, potentially affecting

engineering design in areas as diverse as sensing, biological, environmental, and process instrumentation,

robotics, engine control, and guidance. MEMS techniques also make possible the introduction of new

test specimens and new testing protocols that can uncover fundamental information about material

properties. As specimen dimensions shrink toward lower and lower multiples of single-crystal and

molecular sizes, their controlled fabrication and manipulation in MEMS begin to offer exciting prospects

for proving fundamental theory. The MEMS ﬁeld promises to impact a wide swath of research and

industry and affect not only material scientists, but also engineers, physicists, chemists, and biologists.

13.1.1 Origins

The modern integrated circuit (IC) is the direct descendant of the planar process which, when introduced

in the 1950s, combined the arts of photolithography and silicon chemical technology to extraordinary

advantage. The productivity of this combination was made evident very early by the rapidity with which

circuits of ever-increasing complexity appeared. It was therefore easy to justify large and repeated capital

expenditures for technology and equipment and to put new ideas into development shortly after their

conception so that ICs advanced rapidly. Today, it is commonplace to ﬁnd ICs built with millions of

devices; in fact IC engineers speak of the immediate future as

the Gigabit Era.

There is no doubt that

integrated microelectronics

are keystone elements of information processing in the engineering systems

of our modern silicon age.

For the complete design of an engineering system, however, it is not only necessary to process infor-

mation. The information itself must be exchanged between a largely nonelectrical world and the electrical

processing medium of the IC. This is the job of the

sensors

and

actuators

or, taken as one, of the

transducers

In transducers we ﬁnd the formidable partnership between micromechanics and microelectronics taking

shape to make possible the new level of integration into MEMS.

Some basic MEMS ideas had surfaced very early in the IC era. In fact, among the creative ideas tried

out within the ﬁrst decade of IC history was the employment of microfabrication technologies to produce

a microdynamic element by a group of researchers at the Pittsburgh Westinghouse Research Laboratories.

This group employed the silicon planar process to fabricate a resonant-gate, ﬁeld-effect transistor (FET)

(Nathanson et al., 1967) which consisted of a conventional silicon FET having a metal gate that was

cantilevered over a surface channel covered only by silicon dioxide. An electrostatic ﬁeld pumped the

cantilever and its mechanical resonant frequency had applications as a timing source. The Westinghouse

team also designed a large-screen optical projector based upon electrostatically driven cantilever reﬂec-

tors. This work was very early in the development cycle of silicon microfabrication which was then (in

the 1960s) revolutionizing all aspects of circuit design. The technology for the Westinghouse micrody-

namic elements was not reﬁned sufﬁciently for widespread commercial adoption and the research can

now be seen as an early harbinger rather than as a catalyst for new directions in microfabrication.

In the early 1980s, with two decades of IC development completed, the sophistication of electronics

had advanced markedly. Circuit performance that would have been astonishing in the 1960s had become

commonplace. The

interfaces

of these circuits with the largely nonelectrical world had become the logical

point of focus for engineering design. The advantages of silicon micromechanics had begun to show

themselves in its applications to ink-jet printing at companies like IBM, Hewlett-Packard, Canon, and

Texas Instruments and in miniature pressure sensors such as those made at Honeywell Corporation. The

impressive review of applications of silicon as a mechanical material published by Kurt Petersen (1982)

focused the interest of many engineers on this area. Petersen’s review paper established itself almost

immediately as a prime reference for the practice of silicon micromechanics.

FIGURE 13.1

Detail from the ﬁrst polysilicon MEMS. SEM of an apertured microbridge that is resonated by

electrostatic forces under the control of on-chip circuits. (From Howe. R. T. and Muller, R. S., 1982,

Extended Abstracts

of the 1982 Spring Meeting of the Electrochemical Society,

Montreal, Canada, 82-1, May 9–14. With permission.)

13.2 Micromachining

The earliest methods used to build structures from silicon making use of lithography and etch technology

that was mastered as a part of solid-state device research was via

substrate micromachining

13.2.1 Substrate Micromachining

The technology for substrate micromachining is based upon orientation-dependent chemical etching of

the silicon substrate. When used inventively together with etch-stopping techniques and masking ﬁlms,

this directed-etch technique can produce surprisingly complex structures. Some signiﬁcant applications

have been to a number of different sensing mechanics, such as silicon diaphragms (for pressure sensors)

and cantilever beams (for accelerometers). Details about surface micromachining are not included in

this chapter.

13.2.2 Surface Micromachining

In the early 1980s, research being carried out at Berkeley showed that polycrystalline silicon had use as

a mechanical material with very good characteristics for compatible IC processing. The design and

fabrication by Howe of polysilicon resonant beams together with on-chip MOS circuitry demonstrated

the practicality of what has become known as

surface micromachining

(Howe, 1985). Howe designed his

beams to be driven by electrostatic forces like those at Westinghouse, mentioned earlier. The polysilicon

beams were fabricated from patterned thin ﬁlms by etching an underlying

sacriﬁcial

silicon dioxide layer.

The sacriﬁcial layer was heavily doped with phosphorus to enhance its etch rate in hydroﬂuoric acid. To

carry out surface micromachining with polysilicon, it is necessary that the sacriﬁcial silicon dioxide be

etched considerably faster than is the polysilicon mechanical material, itself. Laboratory studies have

shown that polysilicon is etched in hydroﬂuoric acid (HF) at a negligible rate while silicon dioxide is

etched at rates of 100 nm/min to 1 µm/min, depending on composition. This high ratio of etch rates is

the key to successful surface micromachining. In contrast to substrate micromachining in which mechan-

ical parts are sculpted from the wafer itself, surface micromachining makes mechanical elements out of

materials deposited on the wafer surface.

13.2.3 Polycrystalline Silicon Properties

For surface micromachining with polycrystalline silicon (polysilicon) deposited by low-pressure chemical

vapor deposition (LPCVD) techniques, the polysilicon is often also used as a mechanoelectric transducer

through its piezoresistivity. The use of CVD polysilicon strain gauges has advantages because the trans-

ducing layer is dielectrically isolated from the substrate by a silicon nitride layer. Gauges made of

polysilicon layers typically exhibit higher coupling factors and superior temperature characteristics to

those made of single-crystal silicon.

Electrical Properties

. The electrical properties of polysilicon have been thoroughly studied and can be

reviewed in several reference works (Kamins, 1988). Piezoresistance in polysilicon is a consequence of

strain effects on the passage of carriers across the barriers between the crystalline grain boundaries, as

well as the contributions of the bulk piezoresistivity of the silicon crystallites (Seto, 1976; French and

Evans, 1985).

Mechanical Properties

. To use polysilicon effectively as a mechanical material, it is necessary to know

well its mechanical properties. For most applications, the most important of these are the Young’s

modulus, Poisson’s ratio, residual strain, and ultimate strength. In work over the past 10 years, consid-

erable dependence has been found in these properties dependent on the details of the polysilicon fabri-

cation process.

At typical IC-LPCVD conditions, polysilicon ﬁlms are in a residual compressive state after cool

down to room temperature. This is generally undesirable and can cause multiply constrained

structures, such as diaphragms, plates, or doubly ﬁxed bridges to buckle. The compressive strain

can have several possible sources including crystallites impinging during growth (Guckel and Burns,

1986) or entrapped gases such as oxygen (Muraka and Retajczyk, 1983). Studies of residual strain

have been carried out using measurements of substrate curvature. The strain can be reduced signif-

icantly by annealing (typically, at 850°C) (Choi and Hearn, 1984). Residual strain can be reduced

signiﬁcantly and even brought to a tensile state if LPCVD polysilicon is grown at temperatures very

near the amorphous boundary (580°C). By annealing for 3 h at 1150°C, very low residual strains

can be achieved (a typical value is –1.4

–4)

(Guckel et al., 1987).

Young’s modulus

and Poisson’s ratio

for polysilicon have been measured using the curvature of

different substrates coated with a given thin ﬁlm over a range of temperatures. From these measurements,

the ratio

/(1 –

) and the thermal expansion coefﬁcient

are found. Some reported results from

polysilicon heavily doped with phosphorus are

/(1 –

)

≈

140 Gpa, about 70% as large as is found in

single-crystal silicon.

13.2.4 Tribology in MEMS

Tribology takes center stage when one considers the motions of very small bodies in which micrograms

are proper units of mass. At this scale, as we know from observing the insect world, inertial effects play

far smaller (in many cases negligible) roles in the relationships between driving force and motion. The

nature of friction on this tiny scale is in need of fundamental study as systems for a multitude of

applications are explored.

A major challenge for MEMS designers employing surface micromachining is to understand and

overcome the effects of stiction. The term

stiction

is used to describe two circumstances: (1) sticking of

a “freed” member together with the newly exposed surface underneath the sacriﬁcial layer after the ﬁnal

“freeing” etch step of the micromachining process, and (2) adherence of two mechanical microparts that

approach or touch one another when the microsystem is in operation.

MEMS engineers employing surface micromachining frequently encounter fatal stiction effects when

they attempt to release structures in the ﬁnal etch step. Research into this problem has identiﬁed the

major role of surface tension during the drying stage after a ﬁnal liquid-etch release step (Mulhern et al.,

1993). As MEMS designs have incorporated larger-area, very thin, low-stiffness members (lateral surfaces

tens to hundreds of a micrometer on a side and 1 or 2 µm thick), the release-stiction problem becomes

of ﬁrst order. When sacriﬁcial layers are dissolved by the etch, adjacent surfaces within micrometer

dimensions result in menisci that lead to attractive forces. These forces can, in turn, buckle the “freed”

member, and possibly cement it to the underlying surface. Mechanical analysis of release-stiction effects

has been performed (Mastrangelo, 1997). By using mechanisms to sublimate the ﬁnal etch, release stiction

can be avoided, and large surface-micromachined structures processed successfully (Guckel et al., 1990;

Lebouitz et al., 1995).

Already with the ﬁrst microdynamic devices in which surfaces slide past one another (Fan et al., 1998),

stiction was observed to account for the major retarding forces, and techniques, such as the production

of “dimples” in moving polycrystalline silicon elements, were demonstrated. Recently, methods that

enable quantitative study of stiction effects have been described. Mastrangelo and co-workers have shown

a method for quantifying the post-release stiction of structures using an array of released cantilever beams

having graduated lengths (Mastrangelo, 1997). By applying electrostatic forces, the beams are attracted

to a surface. When the force is removed, beams longer than a critical “detachment” length remain adhered

to the surface while shorter beams are freed because of their internal stiffness (Mastrangelo and Hsu,

1992). The detachment length can be related to an adhesion energy. By observing the inﬂuences of

different surface coatings on adhesion-energy values, one can form comparative evaluations about the

coatings (Houston et al., 1995, 1996).

The MEMS ﬁeld has developed sufﬁciently to produce micromechanisms as complex as gear boxes

capable of two-and-three levels of speed reduction (Sniegowski, 1997). For such structures to enter

commercial applications, surface-lubricating ﬁlms are a necessity. At Texas Instruments, Inc., surface

micromachining has been used to produce a micromirror array composed of movable aluminum struc-

tures. Results with lubricants used on this digital micromirror array are reported by Henck (1997).

Research results on ﬁlms for polysilicon MEMS mechanisms are discussed by Maboudian and Howe

(1997).

13.3 MEMS Structures and Systems

Both surface and bulk micromachining have been employed to make devices and systems having very

surprising complexity. Some of these are now incorporated into commercial products, while others are

still the subject of research. Because of the particular interests of the author and the need to limit the

length of this discussion, the following examples will be drawn exclusively from surface-micromachined

MEMS.

An impressive example of a commercial MEMS that makes use of surface micromachining to build a

fully integrated accelerometer is the AD-XL50 produced by Analog Devices, Inc., for use in automobile

airbag-deployment systems. This accelerometer integrates a tiny seismic mass in the midst of formidable

bipolar-CMOS circuitry. The system employs force feedback using coulombic force to hold the seismic

mass ﬁxed and it senses position making use of a differential capacitance measurement. The AD-XL50

has been widely described in the popular literature (Goodenough, 1991) and it now accounts for a large

and growing share in a very competitive marketplace. Figure 13.2 lists some of the surprising properties

of this monolithic MEMS.

A coulombic actuating force (as employed in the AD-XL50) is used frequently in the MEMS ﬁeld,

especially for surface-micromachined embodiments because coulombic force (alternatively, electrostatic

force) is easily incorporated into the system and employs only an imposed voltage between two elements.

The force increases linearly with the capacitance between the electrodes and quadratically with the applied

voltage. For typical surface-micromachined parts and spacings, the force is quite small (of the order of

micronewtons) but still adequate for many applications. To increase the range of achievable forces in

MEMS, a number of alternative actuation mechanisms have been used or are under study at this time.

Most of these alternatives to coulombic force introduce complication into the fabrication technology for

the MEMS. Nonetheless, to obtain millinewton forces and higher, it is likely that these other forcing

FIGURE 13.2

Facts about the Analog Devices AD-XL50 accelerometer. (Courtesy of Richard Payne, Analog Devices,

Inc.)