LETTER OpenAccess Abulk-micromachinedcornercube

Park and ParkMicro and Nano Systems Letters 2013, 1:7http://www.mnsl-journal.com/content/1/1/7

LETTER Open Access

A bulk-micromachined corner cuberetroreflector with piezoelectricmicro-cantileversJongcheol Park and Jae Yeong Park*

Abstract

A piezoelectrically actuated corner cube retroreflector (CCR) has been investigated for free space opticalcommunications. The proposed CCR consisted of two mutually orthogonal bulk-micromachined mirror assembledwith piezoelectrically actuated horizontal mirror. The vertical mirrors were fabricated by using anisotropic wet-etchingof double silicon-on-insulator (SOI) wafer and horizontal mirror was supported by two stress-compensating and oneactuating lead zirconate titanate (PZT) micro-cantilevers. The fabricated CCRs exhibited angular displacement of 1.87°at 5 volts and switching times of 276 μs. It also exhibited a good cut-off frequency of 2.5 kHz which can be digitallymodulated up to about 5 kb/s.

Keywords: Corner cube retroreflectors (CCR); Optical mirrors; Piezoelectric actuators; Micro-cantilevers;Anisotropic silicon etching

IntroductionA corner cube retroreflector (CCR) has been developedas an optical passive transmitter in wireless optical com-munication with low power consumption [1]. While theCCR does not have a light source, it can transmit thedata to the source by digitally modulated reflection of theincident light. It is comprised of two mutually orthogonalvertical mirrors and horizontal mirror with the mag-netic or electro-static actuator. The actuator is utilizedto form the angular displacement of the horizontal mir-ror. The electrostatic actuators have been commonly useddue to their simple working principles and structures[2-5]. However, the electro-static actuator still needs highdriving voltage to obtain the large angular displacement.Two mutually orthogonal vertical mirrors for the CCRshave successfully been fabricated using surface micro-machining technique [2-5]. However, the alignment oftwo mirrors has been limited due to the curvature ofthe fabricated mirrors from the asymmetric film stressesand a manual assembling of two mirrors. In order to

*Correspondence: [email protected] of Electronic Engineering, Kwangwoon University,447-1 Wolgye-dong, Nowon-gu, Seoul 139-701, Republic of Korea

improve the flatness and alignment of themirrors, bondedsilicon-on-insulator (BSOI) with structurally-assisted andassembled or self-assembled structure was utilized [3,4].While they have presented good feasibility, it is not easyto obtain the accurate angular alignment to form mutu-ally orthogonalmirror surfaces. In this study, a silicon bulkmicromachined CCRwas investigated with ultra-low volt-age operation and negligible power consumption [6]. Itwas comprised of the bulk-micromachined silicon verticalmirror and silicon nitride horizontal mirror with piezo-electric cantilever actuator. For achieving good surfaceroughness, accurate angular alignment, and mass produc-tivity of the vertical mirror, a new fabrication process wasdeveloped using a double-SOI wafer and anisotropic KOHetching technique. For obtaining a large displacement atlow induced voltage and minimizing the initial angu-lar displacement of the horizontal mirror, the piezoelec-tric micro-cantilever actuator and supports were new1yapplied.

FindingsThe PZT micro-cantilever was utilized as an actuatorfor the horizontal mirror to obtain large angular dis-

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placement. However, the PZT cantilever has an initialbending due to its asymmetric film stress in multilayerstructure. The initial bending of PZT cantilever intro-duces the angular misalignment between the horizontaland vertical mirrors. Since there is difficulty in the con-trol of the residual stress in the deposition of PZT thinfilm, it might be serious problem for the MEMS CCRbecause the colinear differential scattering cross section(CDSCS) ofMEMSCCR is affected by the radius of curva-ture and angular misalignment of mirrors [5]. Especially,the maximum angular misalignment of mirror should bebelow 0.055° to communicated over free space and 0.11°or more angular misalignment is sufficient to switch offthe CCR. Therefore, the stress compensated structureof the PZT cantilevers was investigated for the hori-zontal mirror. Figure 1 presents the schematic drawingof the proposed horizontal mirror with the PZT micro-cantilevers for MEMS CCR. As shown in Figure 1 (a),the horizontal mirror was equally suspended by two sup-porting and one actuating PZT cantilevers with torsionalmeander springs. These two supporting PZT cantileverswere utilized to improve the angular alignment of thehorizontal mirror by compensating the initial bending ofthe actuating PZT cantilever after the fabrication. Thus,the proposed CCR has three mutually orthogonal mir-rors to reflect the incident light to the source as shown inFigure 1 (a). When the angular misalignment is occuredby actuating cantilever as shown in Figure 1 (b), theincident light is scattered away from the source. There-fore, the proposed MEMS CCR can transmit the on-off-keyed digital siginal to the source as a passive opticaltransmitter.The vertical mirror should have two mutually orthog-

onal reflective surfaces with accurate angular alignmentand good surface roughness. Figure 2 (a - d) shows

the fabrication procedure to obtain the two mutuallyorthogonal reflective surfaces with accurate angular align-ment. A double-SOI wafer with a silicon spacer wasused to fabricate the cross shaped vertical silicon mir-ror. The double-SOI wafer was comprised of two siliconwafers with thickness of 300 μm, buried oxide layersof 1 μm in thickness, and a silicon spacer with thick-ness of 30 μm. On the double-SOI wafer, silicon nitridewas deposited as a masking layer for KOH wet etch-ing. Firstly, top and bottom SiNx layers were sequen-tially patterned using the same mask with parallel linesto <111> direction. In order to obtain the symmetri-cally formed vertical comb structure with high aspectratio, the double-SOI wafer was etched down to 300 μmas far as by using KOH solution and the buried oxidewas used as an etch stop layer. The KOH etchant wasoptimized to have 40 wt% of concentration and 70°C ofprocessing temperature to minimize surface roughnessof the fabricated silicon mirror structure. To obtain thecross shaped vertical mirror, the etched double-SOI waferwas carefully sawed in the perpendicular direction of theformed structures and finally rotated by 90°. In order toimprove the reflectivity of the vertical mirror, 800 Å ofgold thin film was sputtered on the vertical silicon mirrorsurfaces.Figure 2 (e - f ) shows the fabrication process for the

horizontally actuated mirror. Firstly, low stress SiNx layerof 1μm in thickness was deposited on a silicon substrateand then Ti/Pt bottom electrode, PZT, and Pt top elec-trode were sequentially deposited to have a thickness of20 nm / 120 nm, 500 nm, and 100 nm, respectively. ThePZT film was formed using spin-casting and annealingprocesses. The supporting and actuating PZT cantileverswere defined through the dry etching of Pt/PZT/Pt thinfilm. The horizontal mirrors with a size of 150×150 μm2

Figure 1 Schematic drawing of the proposed CCR with horizontal mirror using PZT cantilever actuator for large deflection at low inducedvoltage and supports for minimizing an initial tilting angle: (a) on-state and (b) off-state.


Figure 2 Fabrication sequences of proposed MEMS CCR with silicon cross shaped vertical mirrors and horizontal mirrors with PZTcantilevers: (a) deposition of SiNx on a double SOI wafer, (b) formation of mask for KOHwet etching, and (c) KOHwet etching, (d) dicingand rotation, (e) deposition of SiNx/Ti/Pt/PZT/Pt on a silicon wafer, (f) formation of PZT cantilevers, horizontal mirrors, torsional hingesprings, and SU-8 micro-holder, (g) KOHwet etching for release, and (h) dicing and assembly of the vertical mirrors onto the horizontalmirrors.

or 250×250 μm2 and torsional meander spring with 5μm in width were formed by using low stress SiNx layerand Au was then deposited on the horizontal mirror byusing lift-off technique. The SU-8 holders with thick-nesses of 100 μm were patterned to accurately align andhold the fabricated vertical mirror. Finally, the PZT can-tilevers and the horizontal mirror were released by usingKOH wet etching technique. The fabricated cantilevershave width and length of 70 μm and 100 μm, respectively.

Finally, the proposedMEMS CCRwas fabricated by align-ing and inserting the vertical mirror manually into themicro-holder formed on the horizontal mirror as shownin Figure 2 (h).Figure 3 shows scanning electron micrograph (SEM)

pictures of the fabricated MEMS CCR. It is comprised offour CCRs where each device works independently. Thefabricated vertical silicon mirror with 300 μm (length) ×300 μm (height) × 30 μm (thickness) were well aligned


Figure 3 SEM pictures and photomicrograph of the fabricated MEMS CCR with two supporting and one actuating PZT cantileversmounted on a PCB substrate: (a) top view of four MEMS CCRs, (b) side view of vertical mirror assembled on horizontal mirror with PZTcantilever, (c) SU-8 holder to align the vertical mirror, and (d) MEMS CCR on PCB test jig.

with the cross angle of 90°. The angular misalignmentof two vertical mirrors measured by optical microscopewas bounded within 0.32° and the surface roughnessmeasured by AFM was within 3.523 nm rms (root-mean-square). Figure 4 presents the surface topographof fabricated horizontal mirrors. The surface roughnessand radius of curvature of fabricated horizontal mirrormeasured by Nanofocus uSurf 3D non-contact profilerwere approximately 5.75 nm rms and 54.6 mm, respec-tively. While the fabricated PZT cantilever exhibited

large angular displacement of approximately 2.29° withradius of curvature of 4.16 mm due to its residual stress,the angular misalignments (δ) were within 0.28° and0.13° for the horizontal mirrors with area of 150×150μm2 and 250×250 μm2, respectively, due to the useof two supporting cantilevers. The smaller angular mis-alignment can be achieved by optimizing the length ofcantilevers or residual stress of PZT cantilevers. Thehorizontal mirror exhibited an angular misalignmentbelow 0.05° through the FEM simulation using Coventor-

Figure 4 Surface topograph of fabricated horizontal mirrors using PZT cantilever actuator with area of 150×150 μm2 (a) and 250×250 μm2 (b) by using Nanofocus uSurf 3D non-contact profiler.


Figure 5 The simulated (dotted line) andmeasured (symbol)angular displacement of the fabricated MEMS CCRs with thehorizontal mirrors of 150×150 μm2 and 250×250 μm2.

Ware at the overall residual stress of 200 MPa in PZTcantilevers.Figure 5 shows the angular displacement of the fab-

ricated MEMS CCRs with two different horizontal mir-rors. The fabricated CCRs with the horizontal mirrorsof 150×150 μm2 and 250×250 μm2 exhibited the angu-lar displacement of 1.37° and 1.87° at 5 volts, which aregood enough for on-off keying, respectively. As shown inFigure 5, the measured angular displacement was also ingood agreement with the simulated one.The fabricated CCR was demonstrated to check the

feasibility as a passive optical transmitter. Firstly, thereflective pattern was observed to confirm that the threemirrors were well aligned orthogonally to each other.Figure 6 (a) shows the reflective pattern of an unactuatedCCR with diagonal illumination (λ = 632.8 nm). As shownin Figure 5, the patterns exhibit similar “star” patterns dueto six effective reflective regions [3]. Figure 6 (b) and (c)present the captured images by CMOS image sensor in

Figure 7 The measured frequency responses of the fabricatedMEMS CCRs with two different horizontal mirrors with diametersof 150 μm and 250 μm.

the off-state and on-state of the fabricated CCR. As shownin Figure 6 (b) and (c), there is a clear difference betweenthe “on” and “off” states. In order to detect a He-Ne laserbeam reflected from the CCR, a silicon photodiode wasutilized as a receiver. The installed distance between theCCR and receiver was 50 cm because the fabricated CCRhad large angular misalignment. As shown in Figure 7,the output voltage at the photodiode was significantlydecreased as the driving frequency increases above 1 kHz.It might be attributed to the finite switching time, so thatthe mirror does not undergo full angular displacementand creates reduced peak-to-peak voltage. The 3-dB cut-off frequencies were approximately 2.5 kHz and 1 kHz forthe CCRwith the horizontal mirrors of 150×150μm2 and250×250 μm2, respectively. It also exhibited good switch-ing characteristics with an off-to-on-state transition of163 μs and on-to-off-state transition of 276 μs at a rect-angular input voltage and switching frequency of 10 V and1 kHz, respectively.

Figure 6 Photograph of the reflective pattern of the fabricated MEMS CCR from diagonally illuminated lay (λ = 632.8 nm) (a), andcaptured images in off-state (b) and on-state (c) by using a CMOS image sensor.


ConclusionsSilicon bulk micromachined CCRs have been presentedfor free space optical communications. They were com-prised of two vertical silicon mirrors and one piezo-electrically actuating horizontal mirror. The fabricatedvertical mirror exhibited an accurate angular alignmentof three mutually orthogonal reflective surfaces by usinganisotropic wet etching technique of (110) Si wafer. Thefabricated horizontal mirror with PZT cantilever actuatorexhibited large angular displacement and low switchingvoltage for on-off keying. The alignment of the horizontalmirror with the vertical mirror was significantly improvedby applying the supporting PZT cantilevers and meandersprings.

Competing interestsThe authors declare that they have no competing interests.

Authors’ contributionsJYP and JP carried out the design & simulation and drafted the manuscript. JPcarried out the fabrication of MEMS device and experimental measurements.Both authors read and approved the final manuscript.

AcknowledgementsThe authors are grateful to acknowledge the support from the Basic ScienceResearch Program (2010-0024618) through the National Research Foundationof Korea (NRF) funded by the Ministry of Education, Science and Technology,Korea.

Received: 28 September 2013 Accepted: 25 November 2013Published: 18 December 2013

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doi:10.1186/2213-9621-1-7Cite this article as: Park and Park: A bulk-micromachined corner cuberetroreflector with piezoelectric micro-cantilevers. Micro and Nano SystemsLetters 2013 1:7.

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