Bhushan B. Handbook of Micro/Nano Tribology, Second Edition

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techniques will be necessary. A listing of other actuation means that have already been applied is given

below.

13.3.1 MEMS Actuation Forces

13.3.1.1 Electrostatic Comb Drive

As already discussed, actuation in MEMS using coulombic force has distinct advantages for compatibly

processed MEMS. Howe, for example, made use of coulombic force to power his resonating beam in the

ﬁrst polysilicon micromechanical structure (Howe and Muller, 1982). In further work with electrostatic

forcing, Howe and student W. Tang ﬁrst constructed a polysilicon comb-drive resonant actuator which

employs coulombic force in a ﬂexure structure to cause it to move parallel to the substrate surface (Tang

et al., 1989). The layout of this comb drive, which is supported by only two pedestals attached to the

substrate, is shown in Figure 13.3.

The comb drive has become a very frequently used microactuator both for resonant and nonresonant

systems. Because the actuated part is a ﬂexing structure, there is no surface friction in its motion, and

it can be driven in resonance with a very high mechanical

value (100,000 in vacuum). Much work is

continuing on comb-drive resonators and their applications and they have been made by substrate as

well as by surface micromachining. Figure 13.4 compares aspects of the behavior of the comb-drive to

that of a parallel-plate actuator. A photograph of one of the original moving comb structures undergoing

oscillation is shown in Figure 13.5.

13.3.1.2 Vertical vs. Lateral Oscillation

Vibrating micromechanical structures are useful for a variety of sensors and actuators. For sensing, one

can make use of the dependence of the frequency of a mechanical resonator to physical or chemical

parameters which affect the vibrational energy. Microfabricated resonant structures for sensing pressure,

acceleration, and vapor concentration have been demonstrated. An elegant example of resonant drive

for actuators is provided by the successful Bulova accutron wristwatch movement in which an electronic

tuning fork is coupled mechanically to a rotating mechanism. Pisano has pointed out the possible

applications of this design to microdynamics (Pisano, 1989). Recently, some very spectacular mechanisms

have been made using polysilicon surface micromachining at the Sandia Laboratories in New Mexico.

The Sandia process makes use of chemical-mechanical polishing in order to planarize wafers after the

FIGURE 13.3

Comb-drive resonator. (Courtesy of William Tang, Jet Propulsion Laboratory.)

micromechanisms have been made in a recessed area of the chip. Then, in a subsequent CMOS process,

very high quality electronic circuits can be fabricated in a continuous-batch process.

Figure 13.6 shows a gear assemblage driven by resonant comb drives that was made at Sandia. The

tiny rotor is 55

m in diameter and it has achieved rotation rates as high as 300,000 rpm. It can be

rotated either counterclockwise or clockwise. To gain a measure of the size of these structures, Figure 13.7

FIGURE 13.4

Comparative behavior of a vertical resonator with a comb-driven substrate-parallel resonator. (Cour-

tesy of William Tang, Jet Propulsion Laboratory.)

FIGURE 13.5

Resonating comb drive. (Courtesy of William Tang, Jet Propulusion Laboratory.)

shows a photograph of the tiny driven rotor of Figure 13.6. Next to the rotor in Figure 13.7 are clumps

of red blood cells and, adjacent to the side arms, a grain of sand.

The MEMS ﬁeld has begun to expand in several directions where full miniature “systems on a chip”

are being realized. Important commercial MEMS applications to accelerometry, to environmental sensing,

and to display have received fairly wide coverage in the technical and industrial product literature. An

area in which the author has worked that is just now developing into fully engineered systems is the area

of miniature integrated optical systems for communications, instrumentation, and sensing applications.

The remainder of this chapter will focus on developments in this area called

microphotonics

13.3.2 MEMS for Microphotonics

The incorporation of micromechanical structures into ﬁber-optic systems holds promise of reducing

costs and providing new opportunities for systems applications. Research groups around the world are

FIGURE 13.6

Batch-fabricated polysilicon assemblage made at Sandia Laboratories. (Courtesy of P. McWhorter.)

FIGURE 13.7

Rotor driver on the wheel assemblage of Figure 13.6 made at Sandia Laboratories. (Courtesy of P.

McWhorter.)

instituting programs to meet these opportunities using a range of technologies. Silicon-based microma-

chining, both surface and bulk, has been exploited. Work at the Berkeley Sensor & Actuator Center

(BSAC) at UC Berkeley and at UCLA has concentrated on surface micromachining with polysilicon and

has developed actuation techniques appropriate both for beam switching and for scanning using micro-

mirrors made by folding micromachined polysilicon structures out of the surface plane and covering

them with a gold reﬂecting surface. Figure 13.8 shows such a folded mirror suitable for use in an

interferometer.

The electrostatic comb drive has been used to move the micromirror at frequencies ranging from DC

to more than 1 kHz. Impact-actuated linear vibromotors allow mirrors to travel over large (>100 µm)

ranges with smaller than 1 µm positioning precision. These actuated mirrors have been built and operated

as components on a laser-to-ﬁber coupling chip and in a bar-code reader. As an alternative to electrostatic

actuation, a magnetically actuated mirror responsive to an off-chip ﬁeld has also been demonstrated at

Berkeley. The mirror can be conveniently deactivated even in the presence of the ﬁeld by clamping it

with an on-chip electrostatic clamp. By this means one can address individual mirrors among an array

of them (Judy and Muller, 1996).

The use of MEMS technologies has the potential of adding very signiﬁcant impetus to the already

strong market for ﬁber-optic systems. With their proven high performance for communications and

sensing, ﬁber-optic systems are in high demand despite typical high costs and relatively large size. The

positive impact of MEMS on both cost and size reduction will open new opportunities that are now

unreachable because of economical or portability limitations. Researchers around the world have sensed

the opportunities in this area and have launched a research ﬁeld that can broadly be labeled

micropho-

tonics

. By using silicon substrates and well-known IC technologies, such as anisotropic etching, “silicon-

optical bench” assembly is already an important commercial activity. Until now, however, the term

silicon-

optical bench

has referred to a passive assemblage of optical components that remains ﬁxed as conﬁgured

in the ﬁnal step of manufacture. Recent advances in MEMS technologies make possible a major advance

toward the design of dynamic microphotonic systems.

To build a dynamic “optical bench on chip,” high-aspect-ratio structures are required to interact with

laser beams that typically travel in free space on paths that are parallel to the surface of the silicon

substrate. To make high-aspect-ratio structures from the typically planar elements produced in surface-

FIGURE 13.8

Micromirror made from polysilicon folded into an upright position. An attached comb-driven

actuator makes the structure useful in a Fabrey–Perot interferometer. (Courtesy of M.-H. Kiang, UC Berkeley.)

micromachining technology, we use hinges (Pister et al., 1992) that permit the rotation of optical elements

out of the plane of the substrate (Tien et al., 1995; Kiang et al., 1996). Figure 13.9 shows the important

steps in the fabrication process for the hinged mirrors.

To fabricate the structures as shown in Figure 13.9, three layers of structural polysilicon above a

polysilicon ground plane are used. A layer of sacriﬁcial LPCVD phosphosilicate glass (PSG) separates the

polysilicon layers. After deposition of sequential passivation layers of silicon oxide and silicon nitride, a

0.5 µm

polysilicon ground plane is deposited and patterned. The ﬁrst sacriﬁcial layer of PSG is then

deposited and patterned. Depressions are wet etched into the PSG using 5:1 hydroﬂuoric acid to create

dimples under the mirror (for reduced surface-to-surface contact). The dimples lessen stiction between

these structures and the substrate. Openings are also etched (usually in a ﬂuorine-based, CF

plasma)

for anchors between the next structural polysilicon and the substrate. Next, the ﬁrst structural polysilicon

layer is deposited by LPCVD and patterned using a chlorine-based plasma etch. This layer forms the

comb-drive actuator. A second sacriﬁcial layer is then deposited and patterned. After deposition, the

second structural polysilicon layer is etched to form the pins of the hinges. The third sacriﬁcial layer and

the third structural layer are then deposited and patterned to complete the hinge structures and form

the mirror. The polysilicon structural layers are 2 to 5

m thick, and the sacriﬁcial silicon dioxide layers

range from 1 to 2 µm in thickness. The sacriﬁcial layers are removed through wet etching in hydroﬂuoric

acid to release the movable structures. The last step is the evaporation of 50 nm of gold to increase mirror

reﬂectivity—resulting in measured reﬂectivities of 85% at 1.35

m. The reduction from the theoretical

96% reﬂectivity of a perfect gold mirror is due to scattering caused by polysilicon surface roughness and

by etch holes and dimples in the mirror.

Movable Micromirror

. Applications of these folded mirrors are for beam steering, optical alignment,

scanning, and switching functions. Figure 13.10 illustrates the concept of the optical bench on a chip.

FIGURE 13.9

Processing steps to form micromirrors. (Courtesy of M. Daneman, UC Berkeley.)

Typical micromirror sizes are 0.5

0.5 mm

. If they are driven directly using comb drives as shown in

Figure 13.8, maximum movement is in the order of 10 µm. This drive is useful for harmonic cavities as,

for example, in an external-cavity laser or to scan a light source in a bar-code reader.

Precision

. The positioning precision of the micromirrors by the integrated actuators has been measured

using visual observation of the micromirror displacement as the voltage applied to the comb drive is

varied. The measurements were made for two identical mirrors having differing lengths of restoring

springs in their actuators. The standard deviation of the mirror displacement with respect to applied DC

bias was lower than 0.2 µm (limited by the resolution of the measuring equipment) for the longer (and,

therefore, more ﬂexible) spring. In either case, this mirror system is capable of submicron positioning

precision. Figure 13.11 shows results obtained with the mirrors used as scanners. Tests run at Berkeley

have exhibited 13 cycles of constructive and destructive interference in a Fabry–Perot interferometer at

a 1.3-µm wavelength. The corresponding displacement of the mirror is 8.45 µm.

The frequency response of these comb-driven mirrors in air extends to about 1 kHz. Although studies

are yet to be completed, it is expected that this upper frequency will increase markedly for mirrors

encapsulated in a low-pressure ambient. By using several comb drives to move mirrors at more than one

support point, it is possible to scan beams at angles situated in planes parallel to the substrate as well as

perpendicular to it. An example of a mirror with these capabilities is shown in Figure 13.12.

Larger movements are required for some of the other applications shown in Figure 13.10. For a laser-

to-ﬁber coupler or for switching applications, typical requirements are total displacements in the order

of 100 µm or greater with submicron precision. The program at Berkeley has addressed this need by

building microvibromotors — motors in which resonant energy is delivered to the load in the form of

collisions. Earlier research at BSAC had shown that polysilicon could withstand multiple collisions

without fracture and that elements could be made to slide on a silicon surface when actuated by a

microvibromotor (Houston et al., 1996). The driving energy for the vibromotors is derived from comb

drives which are built in a similar process to that described earlier. The motor and mirror carriage are

built using only two layers of polysilicon.

FIGURE 13.10

Concept and uses for a silicon optical bench on a chip using folded micromirrors. (Courtesy of M.

Daneman, N. Tien, O. Solgaard, and K. Lau, UC Berkeley.)

The impact force in a vibromotor turns out to be a useful way to overcome the sticking forces between

two polysilicon layers. These forces dominate over any inertial effects and lead to results that can be

surprising to the MEMS neophyte. Tests conducted at Hewlett-Packard Company facilities by Berkeley

researchers showed, for example, that the tiny carriages holding the folded micromirrors would not move

even under 500

loading in shock tests. Direct hammering on the carriage, however, is able to move it

FIGURE 13.11

Measured performance of folded micromirrors used as scanners. (Courtesy of M. Daneman, O.

Solgaard, and M. H. Kiang, UC Berkeley.)

FIGURE 13.12

Scanning micromirror with comb drives for scanning around three axes. (Courtesy of M. H. Kiang,

and O. Solgaard, UC Berkeley.)

in steps that are only a fraction of a micrometer per blow. In the BSAC design, pairs of vibromotors

strike the carriage at 45° relative to its guiding keyway slot.

Figure 13.13, shows the concept for a microvibromotor system to drive a slider (ultimately the mount

for a micromirror) over about 100 µm displacement. Pairs of these vibromotors are used to position the

support vane of a folded mirror in the structure shown in Figure 13.14. Some details of the microvibro-

motor-driven micromirror system are indicated in Figure 13.15.

FIGURE 13.13

Microvibromotors design using four drivers to move a slider 100 m. (Courtesy of M. Daneman, O.

Solgaard, and N. Tien, UC Berkeley.)

FIGURE 13.14

Folded micromirror driven by four microvibromotor drivers. (Courtesy of M. Daneman, O. Solgaard,

and N. Tien, UC Berkeley.)

13.3.3 Coupling Efﬁciency

Coupling to single-mode optical ﬁber using the actuated for microreﬂector ﬁne alignment should provide

coupling efﬁciencies as high as 70% according to theory.

The research on microphotonics at BSAC has been reported in a series of papers that are listed

separately in the following bibliography.

13.4 Berkeley Microphotonics Research

1. O. Solgaard, M. Daneman, N.C. Tien, A. Friedberger, R.S. Muller, and K.Y. Lau, “Optoelectronic

Packaging Using Silicon Surface-Micromachined Alignment Motors,”

IEEE Photonics Technol. Lett.

7, 41–43, January 1995.

2. M.-H. Kiang, O. Solgaard, R.S. Muller, and K.Y. Lau, “Surface Micromachined Micromirrors with

Integrated High-Precision Actuators for External-Cavity Semiconductor Lasers,”

IEEE Photonics

Technol. Lett.

, 8, 95–100, January 1996.

3. M. Daneman, O. Solgaard, N.C. Tien, K.Y. Lau, and R.S. Muller, “Laser-to-Fiber Coupling Module

Using A Micromachined Alignment Mirror,”

IEEE Photonics Technol. Lett.

, 8, 396–398, March 1996.

4. O. Solgaard, M. Daneman, N.C. Tien, A. Friedberger, R.S. Muller, and K.Y. Lau, “Micromachined

Alignment Mirrors for Active Optoelectronic Packaging,” in

Late News Paper, 1994 Conference on

Lasers and Electro-Optics

(

CLEO

), 9–13 May, Anaheim, CA, 1994.

FIGURE 13.15

Features of an actuated microreﬂector system. (Courtesy of M. Daneman, UC Berkeley.)

5. N.C. Tien, O. Solgaard, M. Daneman, A. Friedberger, K. Lau, and R.S. Muller, “Movable, High-

Aspect Ratio Micromirror for Optoelectronic Applications,”

1994 Sensor and Actuator Workshop,

Late News Poster Paper

, Transducer Research Foundation, Hilton Head Island, SC, 13–16 June, 1994.

6. N.C. Tien, M. Daneman, O. Solgaard, K.Y. Lau, and R.S. Muller, “Impact-Actuated Linear Microvi-

bromotor for Micro-Optical Systems on Silicon,”

1994 IEEE Int. Electr. Dev. Mtg.,

San Francisco,

CA, Tech Digest 924–926, 12–14 December, 1994.

7. M.J. Daneman, N.C. Tien, O. Solgaard, A.P. Pisano, K.Y. Lau, and R.S. Muller, “Linear Microvi-

bromotor for Positioning Optical Components,” MEMS ‘95, 1995 Int. Conference on Microelec-

tromechanical Systems, Amsterdam, The Netherlands, January, 1995.

8. O. Solgaard, M. Daneman, N.C. Tien, A. Friedberger, R.S. Muller, and K.Y. Lau, “Precision and

Performance of Polysilicon Micromirrors for Hybrid Integrated Optics,” 1995 International Sym-

posium on Optoelectronic, Microphotonics, and Laser Technologies, Conference 2383A, SPIE,

The International Society for Optical Engineering, San Jose, CA, 4–10 February, 1995.

9. M. Daneman, O. Solgaard, N.C. Tien, R.S. Muller, and K.Y. Lau, “Integrated Laser to Fiber

Coupling Module using a Micromachined Alignment Mirror,”

Conf. on Lasers and Electro-Optics

(

CLEO

), Baltimore, MD, May, 1995.

10. M.-H. Kiang, O. Solgaard, M. Daneman, N.C. Tien, R.S. Muller, and K.Y. Lau, “High-Precision

Silicon-Micromachined Micromirrors with On-Chip Actuation for External-Cavity Semiconductor

Lasers,”

Conf. on Lasers and Electro-Optics

(

CLEO

), pp.

248–249, Vol. 15, May 1995, Baltimore, MD.

11. N.C. Tien, O. Solgaard, M.-H. Kiang, M. Daneman, K.Y. Lau, and R.S. Muller, “Surface-Micro-

machined Mirrors for Laser-Beam Positioning,”

1995 IEEE Int. Conf. on Solid-State Sensors and

Actuators, TRANSDUCERS ‘95, Tech. Digest.

352–355, 25–29 June, 1995, Stockholm, Sweden.

12. N.C. Tien, M.-H. Kiang, M.J. Daneman, O. Solgaard, K.Y. Lau, and R.S. Muller, “Actuation of

Polysilicon Surface-Micromachined Mirrors,”

SPIE Proc.

, 2687, SPIE Conf., 27 Jan.–2 Feb. 1996,

San Jose, CA.

13. M.-H. Kiang, O. Solgaard, R.S. Muller, and K.Y. Lau, “Surface Micromachined Electrostatic-Comb-

Driven Scanning Micromirrors for Barcode Scanners,” IEEE MEMS ‘96 Workshop, 11–15 February,

1996, San Diego, CA.

14. M.-H. Kiang, O. Solgaard, R.S. Muller, and K.Y. Lau, “Design and Fabrication of High-Performance

Silicon Micromachined Resonant Microscanners for Optical Scanning Applications,”

Integrated

Photonics Research, 1996 Technical Digest Series

, Vol. 6, pp. 545–548, April 29–May 2, 1996, Boston,

MA.

15. M.J. Daneman, N.C. Tien, O. Solgaard, K.Y. Lau, and R.S. Muller, “Actuated Micromachined

Microreﬂector with Two Degrees of Freedom for Integrated Optical Systems,” Integrated Photonics

Research Conference, Boston, MA, May, 1996.

16. M.J. Daneman, N.C. Tien, O. Solgaard, K.Y. Lau, and R.S. Muller, “Linear Vibromotor-Actuated

Micromachined Microreﬂector for Integrated Optical Systems,” 1996 Sensor and Actuator Work-

shop, 109–112, Transducer Research Foundation, Hilton Head Island, SC, 2–6 June 1996.

17. M.-H. Kiang, O. Solgaard, R.S. Muller, and K.Y. Lau, “High-Precision Silicon Micromachined

Micromirrors for Laser-Beam Scanning and Positioning,” 1996

Sensor and Actuator Workshop

, (late

news) Transducer Research Foundation, Hilton Head Island, SC, 2–6 June 1996.

Acknowledgments

Many colleagues have contributed their thoughts, suggestions, and work to this brief review of the MEMS

ﬁeld. The author especially thanks R.T. Howe, W. Tang, N. Tien, K. Lau, K. Pister, P. McWhorter, R.

Payne, O. Solgaard, M. Daneman, M.-H. Kiang, J. Judy, A. Friedberger, C. Keller, and the staff of the

Berkeley Micro Laboratory. The author’s work and time has been partially supported by the National

Science Foundation through A. Schwartzkopf, D. Crawford and M. White, by the Defense Advanced

Research Projects Agency through K. Gabriel, and by the Hewlett-Packard Company through W. Ishak,

K. Carey, and G. Trott.