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570 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 10, NO. 3, MAY/JUNE 2004

A 5-V Operated MEMS Variable Optical Attenuatorby SOI Bulk Micromachining

Keiji Isamoto, Kazuya Kato, Atsushi Morosawa, Changho Chong, Member, IEEE, Hiroyuki Fujita, Member, IEEE,and Hiroshi Toshiyoshi, Member, IEEE

Abstract—We report the design, fabrication, and successfuldemonstration of microelectromechanical variable optical atten-uator (VOA) using an electrostatic microtorsion mirror (0.6 mmin diameter) combined with a fiber-optic collimator. The VOA op-erates at low voltages (dc 5 V or less) for large optical attenuation(40 dB, corresponding to mirror angle of 0.3 ) and a fast responsetime (5 ms or faster). The mirror made of a bulk-micromachinedsilicon-on-insulator wafer has been designed to be shock resistantup to 500 G without any mechanical failure. We also have sup-pressed temperature dependence of optical performance to be lessthan 0.5 dB at 10-dB attenuation in the range of 5 C–70 Cby mechanically decoupling the parasitic bimorph effect from theelectrostatic operation.

Index Terms—Microelectromechanical systems (MEMS),optical MEMS, variable optical attenuator (VOA).

I. INTRODUCTION

VARIABLE optical attenuators (VOAs) are indispensablecomponents in the fiber-optic network and are used to

control the optical signal intensity after laser diodes, beforefiber-optic amplifiers, and photo detectors. A large numberof VOAs are needed particularly in the wavelength-divisionmultiplexing (WDM) system, where transmission powers ofmultiple channels of different wavelengths are individuallycontrolled at the wavelength multiplexing (MUX) and demulti-plexing (DEMUX) nodes, as shown in Fig. 1.

Most conventional VOAs are made up with an assembly of aprism or a mirror driven by a solenoid coil or a motor. Despiteexcellent optical performance, they have not fully satisfiedescalating demand of customers in terms of device size, powerconsumption, mechanical reliability, and cost. Several micro-electromechanical systems (MEMS) approaches, therefore,have been proposed to meet their requirements. Fig. 2 showsthree different architectures of MEMS. The shutter insertiontype [1]–[3] shown in Fig. 2(a) can be easily integrated withsurface or bulk micromachined actuators, but polarizationdependent loss (PDL) is generally greater than those of othertypes. Rotating or sliding a mirror near a coupled pair of lensfibers [4]–[6] [Fig. 2(b)] can also control coupling efficiency,but it occupies a relatively large area to hold the fibers onthe substrate. Besides mirrors, electromechanically operated

Manuscript received November 11, 2003; revised January 26, 2004.K. Isamoto, K. Kato, A. Morosawa, and C. Chong are with the Santec Cor-

poration, Aichi 485-0802, Japan (e-mail: [email protected]).H. Fujita and H. Toshiyoshi are with the Institute of Industrial Science, Uni-

versity of Tokyo, Tokyo 153-8505, Japan.Digital Object Identifier 10.1109/JSTQE.2004.828475

Fabry–Pérot interferometers [7] can control attenuation, butcomplete optical blockout (over 40-dB attenuation) is difficultin general. Hence, we employed a torsion mirror coupledwith fibers through a collimator as shown in Fig. 2(c). Theadvantages of this optomechanical design are small PDL, smallpackage size, and compatibility to arrayed multichannels. Inthis work, we also developed a MEMS design for low-voltageoperation (dc 5 V or less) in spite of large shock tolerance andtemperature insensibility.

II. MEMS VOA DESIGN

Considering electromechanical stability and high productionthroughput as well as low operation voltage, we have concludedthat the simple parallel-plate electrostatic torsion mirror wouldbe the most suitable design. Fig. 3(a) shows the schematic viewof the mirror. A circular mirror with electrostatic parallel platesis suspended with torsion bars over a shaped through-hole. Thewhole upper structure is made of silicon-on-insulator (SOI).Only the right-hand-side parts of the SOI actuator are backedup with the GND substrate via a 2- m air gap, where electro-static attraction torque rotates the actuator plates with a smallangle when driving voltage is applied to the SOI layer; the di-mensions of the supporting suspensions are made to yield tiltangle rather than vertical or lateral motion at a given voltage.There is a large through-hole under the mirror to prevent themirror from hitting the substrate. The optical design requestsa mechanical rotating angle of only 0.3 for 40-dB attenuationwhen the mirror and a pair of single-mode optical fiber (placedwith 125- m pitch) are placed at the focal length of the colli-mator lens (1.8-mm focal length). As shown in Fig. 3(b), themaximum angle to block out the light is determined by the con-tact angle of the actuator, , while controllable angle rangeis limited by the electrostatic pull-in angle, at which stable con-trol becomes impossible because electrostatic attraction torqueexceeds the restoring torque of the suspensions; considering op-eration margin, it is safe to set the maximum usable angle to beat one-third of the contact angle.

Simplicity of actuation mechanism and fabrication processhas been our first priority in designing the device for the sakeof low production cost and high mechanical reliability. Fig. 4illustrates fabrication steps using two deep-reactive ion etching(DRIE) processes. The active layer of SOI wafer (30- m sil-icon–2- m oxide–400- m-silicon substrate) is first patternedinto the mirror shape by DRIE, and the backside is etched tothe buried oxide (BOX) using a 100-nm-thick aluminum mask.

1077-260X/04$20.00 © 2004 IEEE

ISAMOTO et al.: 5-V OPERATED MEMS VOA BY SOI BULK MICROMACHINING 571

Fig. 1. VOAs used at various locations in the fiber network system.

Fig. 2. Three different mechanisms of MEMS VOAs. (a) Microattenuator is inserted between fibers placed in the groove. (b) Micromirror is rotated (or shifted)between fibers placed in the grooves. (c) Coupling through a collimator lens is controlled by a tilt mirror.

After releasing the structure in hydrofluoric acid, 30-nm-thickchromium and 50-nm-thick gold layers are deposited by vacuumevaporation through a stencil mask. Finally, individual chips aresnapped off the wafer for packaging by using the same techniquedescribed elsewhere [8]. The total packages size is as small as5.6 mm in diameter and 23 mm in length, as shown in Fig. 5,which has been already commercially released.

Fig. 6(a) shows a scanning electron microscope (SEM) viewof the finished torsion mirror chip (2.4 mm 2.4 mm 0.5 mm)mounted on a package stem. The center mirror is 600 m indiameter, and the actuator plates on the sides are each 700 mlong. The typical radius of curvature of the mirror (metal coated)measured by confocal microscope observation is 2 m or longer.Fig. 6(b) is a close-up view of the suspension (2 m wide and400 m long), whose supporting parts are intentionally rounded( m) for mechanical reinforcement. Thanks to thisshape, the suspension would not break even after mechanicalshocks of 500 G. Tiny bars extending out from the torsion bar arefor preventing the SOI membrane from rupturing during the fab-rication process. The slits seen on the actuator plates are release

holes for HF sacrificial etching. In spite of such delicate torsionbars supporting the relatively large mirror, fabrication yield wasnearly 100% for 250 pieces of devices out of a 4-in SOI wafer.Besides the dimensions shown in Fig. 6(b), we have tried sev-eral different designs of the suspension and actuator plates andoptimized the design parameters as shown in Table I after im-proving temperature and vibration tolerance of the electrostaticoperation.

Single-crystalline silicon has orientation-dependent elasticconstants [9]. In a (100) wafer, the Young’s modulus alongthe direction is larger (169.2 GPa) than that along the

direction (130.2 GPa). To the contrary, the shear modulusof rigidity of silicon around the direction is smaller(62.1 GPa) than that around the direction ( 79.6 GPa).We have taken advantage of the anisotropic feature and havedesigned the torsion bars in parallel with the directionin order to lower driving voltage for rotational motion. Atthe same time, the suspension rigidity has been made largefor higher tolerance to the external vibration in the unwantedup-and-down motion of the mirror.


Fig. 3. (a) Schematic view of the MEMS VOA with a tilt mirror. (b) Cross section showing the mirror and actuator side views.

III. OPTOMECHANICAL PERFORMANCE

A curve with circular dots in Fig. 7(a) shows typical mirror-angle motion as a function of applied dc voltage measured bylaser Doppler vibrometer (theoretical curves in Fig. 7(b) will bediscussed in the following section). Spontaneous electrostaticpull-in of the mirror was observed at a driving voltage of 5.1 V,after which the edge of the actuator was brought into contactwith the counterelectrode (contact angle 1.12 ); in other words,maximum usable angle was limited to the pull-in angle of 0.53 .After pull-in, the actuator edge stuck to the substrate until thedriving voltage was lowered to 2.2 V. Despite the electrostaticpull-in, the VOA was found to operate more than 10 cycleswithout causing permanent stuck. Given a collimator lens of

1.8-mm focal length, mirror angle for 40-dB attenuation was de-signed to be 0.3 , which could be well controlled before causingpull-in. Driving voltage could be tailored in the range of 3–10 Vby changing the suspension dimensions in relation with the ac-tuator plate width. In spite of such low-voltage operation, themirror’s rotational resonance was still as high as 1 kHz, whichgave typical response time in a several-millisecond range asshown in Fig. 8. The overshoot oscillation at the rising edge wasfaster than that of the falling edge because of negative spring-constant effect associated with the electrostatic force of the par-allel-plate actuator.

After assembling the mirror chip with an optical collimatorand a pair of single-mode optical fibers, we have measuredoptical attenuation at a wavelength of 1.55 m as a function


Fig. 4. Fabrication process using DRIE of SOI wafer using only twophotolithography steps. After pattering the front surface by DRIE in step 1,the backside substrate is trench etched by DRIE in step 2. The mirror part isCr–Au coated by vacuum evaporation through a stencil mask before each chipis snapped off the wafer in step 3.

Fig. 5. Photograph of the assembled VOA device. Package size is 5.6 mm indiameter and 2.3 mm in length.

of applied dc voltage as shown in Fig. 9. Typical attenuationrange of 40 dB or more was achieved by driving voltage of5 V or less. Saturation of the attenuation around 40 dB is dueto the sensitivity limit of the photodetector used. Insertion lossof 0.8 dB at 0 V was mainly due to alignment error, whileideal minimal loss was estimated to be 0.2 dB consideringreflection loss at the surfaces of lens, mirror, and fiber facets.Resolution in controlling attenuation was measured to be0.1 dB, when driving analog voltage was controlled by a12-b digital-to-analog converter at the maximum attenuationslope (40 dB/V) around 4.2 V in Fig. 9. Attenuation levelby open-loop control exhibited long-term drift of typical

0.1 dB/h at a 10-dB attenuation level under a constanttemperature and asymptotically reached stationary value withinone hour, after which the attenuation level showed repeatabilitywithin 0.1 dB. For drift associated with temperature change,

Fig. 6. (a) SEM views of fabricated VOA mirror mounted on a TO-3 package.(b) Close-up view of the suspension part.

TABLE IMEMS VOA DIMENSIONS FOR SMALL TEMPERATURE DEPENDENCE AND

LARGE VIBRATION TOLERANCE

we used a lookup table for adjusting driving voltage. Opticalperformance of VOA is summarized in Table II.

IV. DISCUSSION

A. Analytical Model for Electrostatic Operation

We have developed an analytical model for calculating mirrorangle as a function of applied voltage after modifying a modelin [10] and [11] by taking the mirror’s up-and-down motion( -motion) as well as the rotational motion into account.


Fig. 7. Electrostatic angle-voltage curve. (a) Experimental result exhibitedelectrostatic pull-in at 5.1 V. (b) Theoretical angle–voltage curves (at differentBOX thicknesses).

As we have described the actuation principle using Fig. 3,the right-hand-side parts of the upper structure is attracted bythe electrostatic torque and pull-down force normalto the substrate

(1)

(2)

where is the dielectric constant of vacuum, 8.85 10 F/m,and and are the length and width of the actuator plates,respectively [9], [10]. The axis of coordinates has beentaken along the actuator’s width with its origin

Fig. 8. Step response of mirror angle measured by laser Doppler vibrometer.Mirror angle settles down within 3 ms.

Fig. 9. Typical optical attenuation curve as a function of driving dc voltage.

TABLE IIMEMS VOA PERFORMANCE


Fig. 10. Temperature effect on electromechanical performance of VOA in terms of pull-in voltage.

located on the rotational axis. A parameter is the coeffi-cient representing the fill factor of the actuator plate, whichhas through holes for sacrificial etch: as we have designedthe holes to be a 5- m 60- m rectangle located in every15- m 90- m section of the actuator plates, we used a valueof m m m m m m

for .On the other hand, mechanical restoring torque of the torsion

bars is written as

(3)

(4)

where is the modulus of rigidity of silicon (62.1 GPa) and, and are the width, length, and height of the torsion

bar, respectively. Mechanical restoring force in the directionis written as

(5)

(6)

(7)

where is Young’s modulus (169.2 GPa).One can find values of and under the electromechanical

equilibrium condition by numerically solving the following si-multaneous equations:

(8)

The solid curves in Fig. 7(b) shows theoretical angle–voltagecurves by using dimensions listed in Table I. To show the varia-tion that depends on the air gap, we have used 1.9, 2.0 ( BOXthickness), and 2.1 m for the air gap . The analytical modelhas been found to agree the experimental results fairly well.The maximum rotation angle (1.1 ) of the experimental result is

Fig. 11. WYKO confocal image of tilt mirror. The bridge structure of mirrorand suspension has buckled up due to the built-in stress of the SOI layer.

smaller than the simulated value (1.3 ). Also judging from theexperimental pull-in voltage, which is smaller than the simula-tion, it is plausible to explain that the actual gap would have be-come smaller than the initial BOX thickness due to the mechan-ical strain in the SOI layer associated with the residual stressof the deposited metal layers, as will be addressed in the nextsection.

B. Temperature Dependence of VOA

Thermal stability of the electrostatic tilt mirror is highly re-quired to make the VOA operated by the open loop control.However, a slight change of attenuation level was observed onthe packaged VOA (approximately 0.5 dB at 10-dB attenua-tion in the 5 C–70 C range). We also have developed a ther-momechanical model to explain the temperature dependence.

Fig. 10 plots the temperature-dependent deviation of pull-involtages with respect to the values at room temperature (23~ C)measured on three different mirror designs: a bare silicon mirror,a mirror with confined metallization, and a mirror with blanketmetallization. When no metal was deposited on the bare siliconmirror, the pull-in voltage remained at almost a constant value;


Fig. 12. Front and side views of released tilt mirror. The initial electrostatic gap changes by the bimetal effect of the deposited metal on the silicon mirror.

Fig. 13. Temporal fluctuation of attenuation under mechanical impact of 25 G.

thus, the mirror’s angle–voltage curve exhibited minimum tem-perature dependence. Once the mirror is blanket coated with30-nm Cr and 50-nm Au, pull-in voltage increased with in-creasing temperature; in other words, mirror angle at a constantdriving voltage decreased with increasing temperature.

The drift of pull-in voltage was attributed to the thermal de-formation of the mirror and the actuator plates. A confocal mi-croscope image of the bare silicon mirror in Fig. 11 shows acircular mirror that has been buckled upward (by 0.5 m acrossa 1.3-mm span) along the suspension axis after sacrificial re-lease. The active silicon layer of a bonded and etched SOI usu-ally has built-in compressive stress as illustrated in Fig. 12(a),which usually deforms released structures after removing theburied oxide, as shown in Fig. 12(b). When Cr–Au is evapo-

rated on the mirror at a high temperature, relatively tensile stressis induced in the metal when cooled down to room tempera-ture, which pushes down the released structure to the substrateas shown in Fig. 12(c). The electrostatic gap between the actu-ator plate and the substrate thus becomes small at room tempera-ture and it slightly increases with increasing temperature, whichexplains the fact that the mirror angle at a constant voltage de-creases with temperature elevation.

After this analysis, we have altered the fabrication process tohave a metal layer deposited on the circular mirror only. The newmirror design has exhibited smaller temperature dependence asshown by the middle curve in Fig. 10. Typical mirror radius ofcurvature was measured to be 2 m or longer when 30-nm Crand 50-nm Au are deposited on a 30- m SOI layer. Evaporating


another metal layer on the backside of the mirror was found tofurther improve thermal stability to 1-dB fluctuation or less at20-dB attenuation.

C. Vibration Tolerance of VOA

Another crucial requirement to VOAs is tolerance to mechan-ical vibration. The VOA on a shock-test bench has shown no me-chanical failure up to 500 G. The optical attenuation level underapplied bias voltage, however, was found to fluctuate by morethan 7 dB in the first design model (suspension length 200 m)as shown in Fig. 13. Coupling between the mirror angle and ex-ternal vibration can be explained by the up-and-down motion ofthe actuator plate, which alters the mean electrostatic gap andthus the mirror angle.

Mechanical analysis tells that the suspension rigidity in thetorsional motion is proportional to while that in the di-rection (normal to substrate) bending is proportional to .Hence, the suspension rigidity in the direction can be al-most doubled by shortening the length to 75% (200–150 m)and thinning the width to 80% (2.0–1.6 m) without stiffeningthe rotational rigidity (or without increasing driving voltage).By using the shorter suspension design shown in Table I, wehave reduced attenuation fluctuation to almost 50% as shown inFig. 13. We have also found that mechanical damping has enor-mous effect to the vibration tolerance.

V. CONCLUSION

We proposed a very reliable design for MEMS variable op-tical attenuator made by DRIE of an SOI wafer. The attenuationmechanism employed an electrostatic tilting mirror controllingthe coupling efficiency of optical fibers through a collimatorlens. Driving voltage to producing 40-dB attenuation was lowerthan dc 5 V. Even at such low-voltage operation, dynamic re-sponse as fast as several milliseconds was achieved. Tempera-ture-dependent loss was suppressed by decoupling the bimorpheffect from the electrostatic tilting operation of the mirror. Fab-rication process of the VOA was very simple, which requiresonly two steps of photolithography. We also developed an an-alytical model for the electromechanical actuation and thermo-elastic behavior of the mirror. The simulation results agreed verywell with the experimental results of mirror angle versus appliedvoltage. By localizing the metal coating only on the mirror, wecould improve thermal stability of the electromechanical actua-tion. By optimizing the suspension dimensions while keepingthe driving voltage low, we could also improve the mirror’svibration tolerance. The optomechanical performance MEMSVOA as well as manufacturing cost was found to be fairly com-petent to replace our conventional VOA products that used elec-trical stepping motors for controlling attenuation, and we havecommercially released MEMS version VOAs. A minor changeof VOA designs is under development for faster settling timeand yet higher mechanical tolerance for customers requestingbetter performance.

REFERENCES

[1] V. Aksyuk, B. Barber, C. R. Giles, R. Ruel, L. Stulz, and D. Bishop,“Low insertion loss package and fiber connectorised MEMS reflectiveoptical switch,” Electron. Lett., vol. 34, no. 14, pp. 1413–1414, 1998.

[2] C.-H. Ji, Y. Yee, J. Choi, and J.-U. Bu, “Electromagnetic variable opticalattenuator,” in 2002 IEEE/LEOS Int. Conf. Optical MEMS, pp. 49–50.

[3] S.-S. Yun, Y.-Y. Kim, H.-N. Kown, W.-H. Kim, J.-H. Lee, Y.-G. Lee,and S.-C. Jung, “Optical characteristics of a micromachined VOA usingsuccessive partial transmission in a silicon optical leaker,” in 2002IEEE/LEOS Int. Conf. Optical MEMS, pp. 51–52.

[4] C.-H. Kim, N. Park, and Y.-K. Kim, “MEMS reflective type variableoptical attenuator using off-axis misalignment,” in 2002 IEEE/LEOS Int.Conf. Optical MEMS, pp. 55–56.

[5] C. Marxer, B. de Jong, and N. de Rooij, “Comparison of MEMS vari-able optical attenuator designs,” in 2002 IEEE/LEOS Int. Conf. OpticalMEMS, pp. 189–190.

[6] W. Noell, P.-A. Clerc, L. Dellmann, B. Guldimann, H.-P. Herzig, O.Manzardo, C. R. Marxer, K. J. Weible, R. Dändliker, and N. de Rooij,“Applications of SOI-based optical MEMS,” IEEE J. Select. TopicsQuantum Electron., vol. 8, pp. 148–154, Jan.–Feb. 2002.

[7] J. E. Ford and J. A. Walker, “Dynamic spectral power equalizationusing micro-opto-mechanics,” IEEE Photon. Technol. Lett., vol. 10, pp.1440–1442, Oct. 1998.

[8] H. Toshiyoshi, M. Goto, M. Mita, H. Fujita, D. Kobayashi, G.Hashiguchi, J. Endo, and Y. Wada, “Fabrication of micromechanicaltunneling probes and actuators on a silicon chip,” Jpn. J. Appl. Phys.,pt. 1, vol. 38, pp. 7185–7189, Dec. 1999.

[9] M. A. Meyers and K. K. Chawla, Mechanical Behavior of Mate-rials. Englewood Cliffs, NJ: Prentice-Hall, 1999.

[10] H. Toshiyoshi and H. Fujita, “Electrostatic micro torsion mirrors for anoptical switch matrix,” J. Microelectromech. Syst., vol. 5, pp. 231–237,Dec. 1996.

[11] H. Toshiyoshi, W. Piyawattanametha, C.-T. Chan, and M. C. Wu, “Lin-earization of electrostatically actuated surface micromachined 2D op-tical scanner,” J. Microelectromech. Syst., vol. 10, pp. 205–214, June2001.

Keiji Isamoto received the B.S and M.S degreesin electrical engineering from Toyohashi Universityof Technology, Aichi, Japan, in 1994 and 1996,respectively.

He joined the Central Research and DevelopmentLaboratory, Omron Corporation, in 1996, and wasengaged in the development of microlens arrays.He joined the Santec Corporation, Aichi, Japan, in2001, where he develops optical fiber components.

Kazuya Kato received the B.S. and M.S. degrees inmaterial science and engineering from the NagoyaInstitute of Technology, Nagoya, Japan, in 1998 and2000, respectively.

In 2000, he jointed the Santec Corporation, Aichi,Japan, where he has been engaged in the research anddevelopment of fiber-optic components.

Atsushi Morosawa received the B.S. degree in preci-sion mechanics from Chuo University, Tokyo, Japan,in 1997.

He joined the Central Research and DevelopmentLaboratories, Nidec Corporation, in 1997 to developelectrical motors for hard-disk drives. Since 2001, hehas been engaged in the development of fiber-opticcomponents at Santec Corporation, Aichi, Japan.


Changho Chong (M’02) was born in Seto, Japan, in1970. He received the B.S. degree from YokohamaNational University, Yokohama, Japan, in 1993 andthe M.S. degree in electrical engineering from theUniversity of Southern California, Los Angeles, in1995.

From 1995 to 1998, he was with Omron CentralLaboratories, involved in research and developmentin optoelectronics. In 1997, he was also a VisitingResearcher with the Technology Research Institute ofOsaka (TRI) in 1997. He is currently a Researcher at

the Santec Corporation, Aichi, Japan, and has been involved in research and de-velopment of optical components.

Hiroyuki Fujita (S’76–M’80) received the B.S.,M.S., and Ph.D. degrees in electrical engineeringfrom the University of Tokyo, Tokyo, Japan, in 1975,1977, and 1980, respectively.

Since 1980, he has been with the Institute of In-dustrial Science, University of Tokyo, as a Lecturer(1980–1981), Associate Professor (1981–1993), Pro-fessor (1993-present). Since 2000, he has also beenDirector of the Center for International Research onMicroMechatronics (CIRMM), Tokyo, Japan. He hasbeen engaged in the investigation of microelectrome-

chanical systems fabricated by integrated circuit-based processes and applica-tions to optics, hard-disk drives, and bio- and nanotechology. He is also inter-ested in autonomous distributed microsystems.

Hiroshi Toshiyoshi (M’97) received the M.Eng. andPh.D. degrees in electrical engineering from the Uni-versity of Tokyo, Tokyo, Japan, in 1993 and 1996,respectively.

From 1999 to 2001, he was a Visiting AssistantProfessor at the University of California, LosAngeles, for his sabbatical years. Since 2002, hehas been an Assistant Professor with the Instituteof Industrial Science (IIS), University of Tokyo. Heis currently a Co-Director of the first Japan-Franceinternational research collaboration Laboratories for

Integrated Micro-Mechatronic Systems (LIMMS) with IIS. His main researchinterest is MEMS for free-space optics.

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