IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 5, MARCH 1, 2010 269

A Two-Wafer Approach for Integration of OpticalMEMS and Photonics on Silicon Substrate

Qingxin Zhang, Jing Zhang, Mingbin Yu, Chee Wei Tan, Guo-Qiang Lo, and Dim-Lee Kwong

Abstract—This letter reports a novel two-wafer approach whichdemonstrates an integration of optical microelectromechanicalsystem (MEMS) devices and photonics on a silicon substrate.The great advantage of this novel wafer bonding scheme is theability to maintain the optical axis of the optical MEMS device atthe same axis as the optical components. The bonded two waferswhich are partially processed, which allows for further processingon the wafer after bonding. Thus, the critical alignment issue isresolved for devices requiring precise alignment in x-/y-/z-axis.Individual functionalities of optical MEMS device and opticalcoupling between silicon waveguide, fibers and ball lens aredemonstrated. This technology shows the potential for integratingsilicon photonics integrated circuit and MEMS components withreconfiguration functions on a single silicon substrate.

Index Terms—Integration, optical microelectromechanicalsystem (MEMS), Si-photonics.

I. INTRODUCTION

I N THE past decade, microelectromechanical system(MEMS) technology has attracted much attention from the

research community for potential application in optical com-munication systems [1], [2]. A variety of optical components,such as mirrors, switches, filters, lenses, and other fundamentalelements have been successfully developed using MEMStechnology. Also various subsystems with reconfigurationfunctions such as tunable lasers, high-speed optical modulators,reconfigurable wavelength add–drop multiplexers, and opticalcross-connect have been demonstrated using optical MEMS.MEMS technology builds up individual optical componentson a single silicon substrate which enables not only advancedoptical performance, but also the system-level integration thatallows electrical devices, mechanical structures, and photonicscomponents integrated in a silicon substrate.

Typically, two approaches are used to integrate opticalMEMS components, electrical and photonics devices [3], [4].The single-wafer scheme refers to a postintegrated circuit(IC) process where MEMS devices are fabricated on the samewafer that consists of photonics and/or electrical devices. Thedual-wafer scheme involves two prefabricated wafers, i.e.,MEMS and photonics IC (PIC) device wafers that are bonded

Manuscript received June 01, 2009; revised November 23, 2009; acceptedNovember 24, 2009. First published January 12, 2010; current version publishedFebruary 03, 2010.

The authors are with the Institute of Microelectronics, Agency forScience, Technology and Research (A-STAR), Singapore 117685, Sin-gapore (e-mail: qingxin@ime.a-star.edu.sg; zhangj@ime.a-star.edu.sg;mingbin@ime.a-star.edu.sg; tancw@ime.a-star.edu.sg; logq@ime.a-star.edu.sg; kwongdl@ime.a-star.edu.sg).

Digital Object Identifier 10.1109/LPT.2009.2038236

Fig. 1. Schematic cross-sectional view of stacked optical MEMS device andphotonics on silicon substrate.

together for consequence processes. The active devices in com-plimentary metal–oxide–semiconductor (CMOS) IC or PIC areoften fabricated within a thin silicon surface layer m ,but the MEMS devices (e.g., mirror) are often fabricated witha rather thick structure m . It is difficult to arrangethese two devices to be co-axis as desired in most integratedphotonics if the single-wafer approach is employed. While inthe dual-wafer approach, the precise alignment between thetwo wafers is the major challenge. For instance, the x-/y-axisalignment between devices on two wafers is limited by thecoarse wafer-to-wafer bonding process while bonding of twoseparate devices induces difficulties in z-axis alignment control.

This letter reports a novel platform technology for integra-tion of the optical MEMS and photonics devices. The opticalaxes of photonics and optical MEMS are made to be coaxial bybonding two partially processed wafers together, and then fur-ther processing the wafer. High-precision alignments of the twodevices in all three axes have been achieved. The potential ap-plication of this technology is the hybrid integration of siliconPIC and optical MEMS components with reconfiguration func-tions on silicon substrate.

II. INTEGRATION PROCESS

Fig. 1 depicts a schematic of the two-wafer bonding approachwhere the optical MEMS device and photonics are aligned toeach other. There are two major components in this design. Thefirst wafer has the MEMS structure which consists of a drivingcomb, folded beam, mass structure, and a vertical mirror. Thesecond wafer has the optical coupling module which consists offiber trench, ball lens cavity, and silicon waveguide. The siliconmirror serves as a switch to control the light in the couplingmodule by moving in/out of the optical path. This applicationrequires the silicon waveguide and the MEMS mirror on thesame optical axis.

Fig. 2 shows the major steps in the integration process. It in-volves two wafers, a bulk silicon wafer (top wafer) and a sil-icon-on-insulator (SOI) wafer with 320-nm silicon on 1- moxide (bottom wafer). The silicon waveguide is patterned to

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270 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 5, MARCH 1, 2010

Fig. 2. Integration process of optical MEMS devices and photonics on siliconsubstrate.

create an optical mode converter structure on the bottom SOIwafer using an ultraviolet (UV) 248 scanner. It is processedin an inductively coupled plasma (ICP) system for good con-trol of critical dimension and vertical/smooth sidewall. After2- m plasma-enhanced chemical vapor deposition (PECVD)oxide coverage on the silicon waveguide, a second mask is usedto create the fiber trench, ball-lens cavity, and silicon recesseson the bottom wafer. By the deep reactive ion etching (DRIE)method, trenches with different depths are fabricated and twoadditional masks are used to achieve precise trench depth. Inparallel, silicon island structures are formed on the top waferusing wet silicon etch or the DRIE process. The two wafers arethen bonded using benzocyclobutene (BCB) as an intermediatelayer at a temperature of 250 C to avoid damages on the elec-tronic and photonics devices [5]. After mechanical thinning ofthe top wafer, the metal pad layer Cr–Au 50 nm 500 nmis sputtered and etched by wet process. In the final step, op-tical MEMS structures are defined and etched by DRIE on thetop silicon layer. The silicon layer over the fiber trenches andlens cavities on the bottom wafer are etched during the DRIEprocess.

III. RESULTS AND DISCUSSION

Fig. 3 shows the top view of the structures fabricated on thebottom wafer. The silicon waveguide, 400 nm in width with a150-nm tip, is patterned from the device layer of the SOI wafer.The waveguide tip, 2 m away from the optical coupling inter-face, has a vertical sidewall and smooth surface, as shown inthe enlarged image. Several recesses with various shapes anddepths are formed on the bottom wafer using the DRIE process.The shallower rectangle trench is used to hold optical fiber whilethe deeper circular trench is used to hold an optical lens. The

Fig. 3. Fabricated Si waveguide and deep trench structures on the bottomwafer; the right-top inset is an enlarged scanning electron microscope (SEM)image of the Si waveguide tip.

Fig. 4. Cross-sectional view of top wafer with islands bonded with bottomwafer with recesses.

fiber holding trench is 62.5 m deep for fiber with a diameterof 125 m while the lens holding trench is 150 m in depth fora ball lens of 300 m in diameter. The optical fiber, lens, andsilicon waveguide are aligned accurately in both Z- (normal towafer surface) and Y-directions. In order to achieve high cou-pling efficiency, a misalignment of less than 0.1 m in the Y-di-rection is achieved by combining all the recess patterns on onemask, and aligns this mask to the silicon waveguide pattern onthe wafer surface. As for the alignment in the z-axis, additionalmasks with 5- m tolerance are employed to selectivelyopen the predefined recesses and control their depths with anaccuracy of 0.5 m.

Fig. 4 shows the bonding structure which consists of a topwafer with islands and a bottom wafer with recesses after thin-ning of top silicon. The silicon recess on the bottom wafer is100 m deep etched by the DRIE process. The silicon island

ZHANG et al.: TWO-WAFER APPROACH FOR INTEGRATION OF OPTICAL MEMS AND PHOTONICS 271

Fig. 5. (a) SEM images of integrated optical MEMS mirror and optical benchon Si substrate, (b) the enlarged images showing the thick spring structure con-structed on Si island and the thin Si as anchor, and (c) the thick folded beamover Si recess.

on the top wafer, 60 m in height, is etched in potassium hy-droxide (KOH) solution. The thin silicon layer on the top wafer,

50- m thickness, is thinner than the island. This height dif-ference is desired to make the silicon waveguide (bottom wafer)and the central part of the MEMS structure to be coaxial con-sidering the thickness of the intermediate bonding layer. Thetwo wafers are aligned in an electronic visions group (EVG)aligner and bonded in an EVG bonder with alignment accuracyof 5 m in both X-/Y-directions. This misalignment caninduce optical coupling loss of larger than 10 dB in a fiber-balllens-Si waveguide coupling system. As the MEMS components,defined on the thin silicon layer, are aligned with structures onthe bottom wafer, the misalignment can be controlled within

1 m with equivalent optical coupling loss of less than3 dB.

The integrated MEMS structure, silicon waveguide, and sil-icon recesses on a bonded silicon substrate are shown in Fig. 5.The thick MEMS structures, such as combs, springs, mirrors,and net structures are constructed on the thick silicon ( 60 m)over the recess while its anchors are mainly formed on the thinsilicon ( 20 m) layer on the bottom wafer. The silicon mirrorgoes into the silicon trench/recess, making the waveguide on thebottom wafer and the central part of the mirror on the same op-

tical plane. The recesses for holding optical fibers and ball lensesare exposed during the DRIE process, as shown in Fig. 5. Thevertical mirror, located in between the fiber trench and the balllens cavity, is perpendicular to the optical path from optical fiberthrough the ball lens into the silicon waveguide. The movementof the mirror along its longitudinal direction will adjust the op-tical coupling between the fiber and the silicon waveguide.

The MEMS mirror as an optical switch is characterized. Themirror is displaced for 5 m in plane at a driving voltage upto 30 V. The displacement deviates from the designed value of25 m. It was observed that the gap between the silicon combfingers is extended probably due to the notching effect in theDRIE process. This can be solved through optimization of maskdesign and fine-tuning of the process parameters. The opticalcoupling between silicon waveguide and the optical fiber is mea-sured to investigate the alignment accuracy between the opticalcomponents and to verify the effect of the DRIE process. Afterassembly of the ball lens and fibers, the single-mode fiber (SMF)is connected to a laser source at a wavelength of 1550 nm, whilethe multimode fiber (MMF) is connected to a detector. Couplingloss from the waveguide to the MMF is ignored. The propa-gation loss of the waveguide is determined to be about 34 dB/cm. In this experiment, a 1-mm-long waveguide introducesabout 0.4-dB propagation loss. The total insertion loss betweenthe source fiber and the silicon waveguide is as small as 2.4 dB.Further works will be focused on the optimization of the MEMSdesign, process, and the demonstration of MEMS reconfigura-tion function in the silicon photonics system.

IV. SUMMARY

A two-wafer approach has been developed for integration ofMEMS and photonics devices on a silicon substrate. The flex-ible and low-temperature BCB bonding process is employedto bond the two preprocessed wafers which consist of siliconislands and recesses. Then, the MEMS and photonics devicesare realized on the same optical plane without trade-off in po-sitional accuracies at both the lateral ( 1 m) and vertical( 0.5 m) directions in the postbonding process.

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