Optical MEMS in Bonded Silicon on Insulator

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1 Optical MEMS in Bonded Silicon on Insulator Richard R. A. Syms OSD Group, EEE Dept., Imperial College London, Exhibition Road, London, SW7 2BT, UK phone ; fax Abstract Bonded silicon on insulator (BSOI) material provides an excellent solution for optical MEMS devices, which typically require optical surfaces with high flatness and low roughness to be combined with high quality mechanical parts and low power, high force microactuators. This paper will review a number of different approaches to optical MEMS, including surface tension self-assembly for 3D components such as fixed mirrors and lenses, torsion mirror scanners and active corner cube reflectors, and deep reactive ion etching for variable attenuators, blazed diffraction gratings and tuneable external cavity lasers. Current performance and limitations of tuneable components will be given. Methods of mounting additional macroscopic components such as filters and optical fibres will also be described, together with latching actuators for linear, rotary and tilt alignment. 1 Optical MEMS Since the mid-1990s, there has been an explosion of interest in microelectromechanical systems (MEMS) for optical fibre communications [1]. Precisely etched alignment features have been used for many years to assemble single-mode systems. The additional availability of movable components has allowed the development of variable attenuators, tuneable filters, tuneable lasers, crossconnect switches, scanners and variable delay lines. Although resonant mechanical systems have speed limitations, optical MEMS still offer performance advantages, including wavelength and polarization insensitivity, and scalability to large port counts based on the use of miniature free-space optics [2]. Anisotropic wet etching, surface micromachining and deep reactive ion etching (DRIE) of bonded silicon-on insulator (BSOI) material are all used for fabrication. DRIE of BSOI is particularly promising, since it allows thick layers of high-quality single crystal material, of essentially arbitrary thickness [3]. Despite limitations on process flexibility, 3D optical MEMS may also be formed in BSOI by surface tension self-assembly. 2 Deep Etching of Bonded Silicon Arbitrary features may be formed in silicon with high verticality by deep reactive ion etching in a high density, inductively coupled fluorine plasma. Two methods are used. The first is based on the cyclic etch-passivate process developed by Bosch [4, 5]. In the etch step, SF 6 is used to remove silicon. Lateral erosion is prevented by deposition of a polymer (C x F y ) in the passivation step. To begin etching again, fluorine radicals first etch the passivation base, and then the silicon itself. High etch rates (4 µm/min) and depths of > 500 µm may be achieved, with wall angles of > 89 o. Masking is performed with thick layers of resist or SiO 2, and mask selectivities of > 60 are obtained. Because of this high value, buried oxide can act as an etch-stop. High sidewall verticality is also obtained in the cryogenic process by reducing of the substrate temperature [6]. All DRIE processes suffer from lag effects, which lower the etching rate between closely spaced, high-aspect ratio structures. There are also loading effects, which cause the etch-rate to be affected by the area of silicon exposed to the plasma. Finally, the cyclic DRIE process generates sidewall scallops, with a typical periodicity of 0.25 µm. Bonded silicon-on-insulator (BSOI) consists of a thermally oxidised silicon wafer fused to another silicon wafer. The additional device layer is then polished back to the desired thickness. After fabrication of mechanical parts by deep reactive ion etching, the oxide layer may be isotropically etched in buffered HF, to free suspended parts for motion.

Freeze-drying may then be used to avoid surface tension collapse; however, stiction effects are generally less significant in bonded silicon, due to the large feature height.

2 Freeze-drying may then be used to avoid surface tension collapse; however, stiction effects are generally less significant in bonded silicon, due to the large feature height. Dry process options include vapour-phase etching of the oxide layer, XeF 2 etching of the silicon substrate beneath the device layer, and deep reactive ion etching of the substrate beneath the device. DRIE of BSOI allows optical components and elastic supports to be combined with electrostatic [7, 8] or electro-thermal [9, 10] actuation, without additional functional materials. In each case, one limit to operating speed is set by the mechanical time constants of any spring-suspended masses. Electrostatic actuators do not require a holding power; however, high (> 200 V) may be needed for parallel plate operation with large electrode gaps. Parallel plate and comb electrodes may be used, for both in-plane and out-of-plane motion. Electrothermal actuators consume power continually, and also a further speed limitation set by a thermal time constant; however, they typically use low drive voltages and allow large actuation forces. Material bimorphs, which operate by differential thermal expansion in a bilayered cantilever, can generate both out-of-plane and in plane motion. Shape bimorphs, which operate by differential expansion between parts of different length or crosssection, are normally used for plane motion, as are buckling mode actuators, which operate by differential thermal expansion between suspended beams and the substrate. In in-plane optical MEMS, the beam travels parallel to the substrate and is acted on by optical components formed from the etched surfaces themselves. This configuration only allows small diameter beams, which diffract quickly, but devices may be fabricated simply by etching and undercutting the device layer. However, control of the optical quality of the etched surface is important. In out-ofplane optical MEMS, the beam is generally perpendicular to the substrate, and the optical components are based on the device layer surface. This configuration allows much larger beam diameters, and is largely unaffected by the quality of the etched surfaces. However, it may require removal of the substrate, to allow the beam to pass through and a stacked electrode assembly may be required to construct a complete electrostatic actuation system. In 3D components, the beam travels parallel to the substrate again, but the optical components are now formed in the surface of the device layer, and then rotated out-of-plane and fixed in position. 3 Bonded Silicon Optical MEMS High quality, vertically etched mirrors for in-plane optical systems may be fabricated by DRIE [11]. Mirrors and shutters have been combined with mechanisms, electrostatic drives and fibre alignment springs to form optical switches [12], using the insertion of small 45 o mirrors into the nodes of orthogonal optical paths. Variable optical attenuators (VOAs) based on the partial insertion of an etched shutter into a gap between two co-linear optical fibres have also been developed [13]. The alternative principle of image translation by a movable mirror has been used to improve the polarization and wavelength dependence of loss [14]. Elastic [15] and rack-and-tooth [16, 17] clamps have also been developed, to maintain the attenuation in the unpowered state. Figure 1 shows a VOA with a compound latch, which uses one shape bimorph electrothermal actuator to control the position of a small shutter, and additional actuators to operate two rack-and-tooth latches for coarse and fine control. The racks have a precision of 10 µm, which is limited by the resolution of the deep-etched tooth pattern. A mechanical lever is used to increase the the precision of one stage ten-fold, to allow suitable attenuation states to be achieved with the 8 µm MFD beam obtained from a single mode fibre. Fig stage latching VOA [17]. Other in-plane optical MEMS include

More recently, vertical etching has been used to fabricate high-order blazed gratings directly [19].

3 tuneable lasers. The Iolon Apollo TM laser uses a hybrid-integrated mirror mounted on a deep etched electrostatically-drive rotation stage and arranged in a Littman configuration with a fixed grating [18]. More recently, vertical etching has been used to fabricate high-order blazed gratings directly [19]. These gratings have reasonable optical characteristics, but improvements to overall reflectivity and resolution are required to obtain competitive performance when used in lasers. Deep-etched gratings have been combined with elastic suspensions in both the Littrow [20, 21] and the Littman [22] configurations. Figure 2 shows the layout of an electrostatic tuning actuator for a Littrow external cavity laser [20]. The grating is mounted on a compound flexure, consisting of a cantilever in series with a portal frame, allowing the linear and angular positions of the grating to be adjusted independently. Single-mode output powers up to 500 µw have been obtained from fixed gratings. Peak fibre-coupled powers of 100 µw have been obtained from tunable gratings, with a sidemode suppression ratio of > 20 db, and electrostatic tuning over a range of 20 nm using a 50 V drive has been demonstrated. The main limitation on performance is the difficulty in increasing the height of a deep-etched structure combining a fine-period, blazed grating and comb-electrodes. This difficulty is exacerbated in the Littman configuration, which requires a longer optical path with multiple passes through an etched mirror and an etched grating. As a result, lower powers have been obtained from such devices [22]. Fig. 2. External cavity tuning element with flexure, drive and grating [20]. Out-of-plane components include variable attenuators acting on beams travelling perpendicular to the substrate. Single-blade [23] and multiple-blade devices have been constructed. Figure 3 shows a 4- blade iris, which uses the synchronous motion of four triangular elements driven by buckling mode electrothermal actuators and sliding together to create a variable square aperture [24]. The potential advantage of this approach is reduced polarization dependence of loss. Fig. 3. Iris variable attenuator [24]. Devices with similar geometry include Fabry-Perot filters for use as channel monitors [25] and as tunable reflectors in Fabry-Perot external cavity lasers [26]. However, the majority of throughwafer devices are torsion mirror switches. Arrays of two-axis tilt mirrors have been used in optical crossconnects with large (1000 x 1000) port counts [27]. Improved optical performance has been obtained by replacing polysilicon mirrors with single crystal optical surfaces [28]. Electrical drift has been reduced and shock resistance increased [29], and more stable electromechanical performance has been obtained by replacing parallel plate electrostatic drives with staggered comb electrodes [30]. 3D polysilicon MOEMS have been formed by out-of-plane rotation of flat parts mounted on micromachined hinges. Surface tension self-assembly is a mass-parallel assembly method in which rotation is powered by compact pads of meltable material. No hinge is required, and the method has been used to construct BSOI MOEMS using photoresist [31] and solder [32]. The fixed and movable parts are formed in the bonded layer, and linked by the meltable pad. The parts are undercut, and the pad is melted to

activate the assembly sequence. Self-assembled components demonstrated to date include scanners [33, 24], lens arrays [35] and corner cube reflectors [36].

Staggered electrostatic comb drives have also been formed by self-assembly [37]. Assembly angles are accurate to a few minutes of arc.

Self-assembled corner cube retroreflection modulator [36]. Bonded silicon is also increasingly being used to construct alignment systems for hybrid integrated micro-optical components.

4 activate the assembly sequence. Self-assembled components demonstrated to date include scanners [33, 24], lens arrays [35] and corner cube reflectors [36]. Figure 4 shows a recently demonstrated corner cube reflector, containing two torsion mirror scanners to allow operation as a retroreflection modulator. Staggered electrostatic comb drives have also been formed by self-assembly [37]. Assembly angles are accurate to a few minutes of arc. being improved beyond the limit of pattern transfer using latches operating on the Vernier principle. Fig. 5.Latching translation stage [41]. Fig. 4. Self-assembled corner cube retroreflection modulator [36]. Bonded silicon is also increasingly being used to construct alignment systems for hybrid integrated micro-optical components. Fibre-based devices include switches [38], and alignment devices [39, 40]. A key requirement is for a high force actuator capable of deflecting a short cantilevered section of optical fibre, and this may be provided in BSOI because of the large structural height. Mechanisms for aligning and fixing trains of micro-optical components such as lenses and dielectric filters during opto-hybrid assembly are now being developed. The mechanisms must support the weight of macroscopic components, which typically lie in the milligram range, so that high-aspect ratio structures are required. The simplest mechanisms are electrically driven latching translation stages [41]. These methods are now being generalised to rotation and tilt stages [42]. Figure 5 shows a latching translation stage. Motion of the central table is achieved by actuating the stage with a buckling mode electrothermal drive. Latching is achieved using a pair of rack-and-tooth clamps operated by shape bimorph actuators. Stable latching has been demonstrated, and precision is Figure 6 shows a similar rotation stage, in which rotary motion is achieved by using a linear stage to drive a small elastically supported central table via a tangential drive pin [42]. Stages of this type are now being combined with clamps for mounting external components. Recently developed clamps are operated electrically, and some are even bistable [43]. Fig. 6. Latching rotation stage [42]. 4 Conclusions Deep reactive ion etching of bonded silicon-oninsulator is well suited for optical MEMS, since it allows high aspect ratio structures with high mechanical and optical quality to be co-integrated in a simple and flexible manner. Major applications include tuneable components and alignment systems.

5 5 Acknowledgements The Author is extremely grateful for the assistance of Dr John Stagg, Dr Helin Zou, Dr Anke Lohmann, Dr Huang Weibin, Youngki Hong and Hadi Veladi with device fabrication. The financial support of EPSRC is also gratefully acknowledged. 9 References 1. Walker J.A. The future of MEMS in telecommunications networks J. Micromech. Microeng. 10, R1-R7 (2000) 2. Syms R.R.A. "Scaling laws for MEMS mirror rotation optical cross-connect switches" IEEE J. Lightwave Tech. 20, (2002) 3. Klaassen E.H., Petersen K., Noworolski J.M., Logan J., Maluf N.I., Brown J., Storment C., McCulley W., Kovacs T.A. Silicon fusion bonding and deep reactive ion etching: a new technology for microstructures Sensors and Actuators A52, (1996) 4. Laermer F., Schilp A. Method of anisotropically etching silicon US Patent Mar 26 (1996) 5. Hynes A.M., Ashraf H., Bhardwaj J.K., Hopkins J., Johnston I., Shepherd J.N. Recent advances in silicon etching for MEMS using the ASE TM process Sensors and Actuators 74, (1999) 6. Jansen H., de Boer M., Wensink H., Kloeck B., Elwenspoek M. The black silicon method. VIII. A study of the performance of etching silicon using SF 6 /O 2 -based chemistry with cryogenical wafer cooling and a high density ICP source Microelectr. J. 32, (2001) 7. Tang W.C., Nguyen T. - C.H., Howe R.T. "Laterally driven polysilicon resonant microstructures" Sensors and Actuators 20, (1989) 8. Selvakumar A., Najafi K. Vertical comb array microactuators IEEE/ASME J. Microelectromech. Syst. 12, (2003) 9. Guckel H., Klein J., Christenson T., Skrobis K., Laudon M., Lovell E.G. "Thermo-magnetic metal flexure actuators" Proc. IEEE Solid-State Sensor and Actuator Workshop, Hilton Head, SC, June 22-25, pp (1992) 10. Sinclair M.J. A high force low area MEMS thermal actuator Proc. 7 th Intersoc. Conf. on Thermal and Thermomechanical Phenomena, Las Vegas, May 23-26, pp (2000) 11. Marxer C., Gretillat M.-A., de Rooij N.F., Battig R., Anthamatten O., Valk B., Vogel B. Vertical mirrors fabricated by reactive ion etching for optical fiber switching applications Proc. 10 th Workshop on MEMS, Nagoya, Japan, Jan 26-30, pp (1997) 12. Marxer C., de Rooij N.F. Micro-optomechanical 2 x 2 switch for single-mode fibers based on a plasma-etched silicon mirror and electrostatic actuation IEEE J. Lightwave Tech. LT-17, 2-6 (1999) 13. Marxer C., Griss P., de Rooij N.F. A variable optical attenuator based on silicon micromechanics IEEE Photon. Tech. Lett. 11, (1999) 14. Kim C.-H., Park N., Kim, Y.-K. "Development and characterization MEMS reflective type variable optical attenuator using off-axis misalignment" IEEE/LEOS Int. Conf. on Optical MEMS, Lugano, Switzerland, Aug 20-23, pp (2002) 15. Dhuler V.R., Hill E.A. "Microelectromechanical devices having brake assemblies therein to control movement of optical shutters and other movable elements" European Patent EP Oct. 4 (2001) 16. Syms R.R.A., Zou H., Stagg J., Moore D.F. Multi-state latching MEMS variable optical attenuator IEEE Photon. Tech. Lett. 16, (2004) 17. Syms R.R.A., Zou H., Stagg J., Moore D.F. "MEMS variable optical attenuator with a compound latch" Microelectr. Engng , (2004) 18. Berger J.D., Zhang Y., Grade J.D., Lee H., Hrinya S., Jerman J.H. "Widely tunable external cavity diode laser based on a MEMS electrostatic rotary actuator" OFC 2001, Anaheim, CA, Mar , Paper Tuj2 (2001) 19. Lohmann A., Syms R.R.A. External cavity laser with a vertically-etched silicon blazed grating IEEE Photon. Tech. Lett. 15, (2003) 20. Syms R.R.A., Lohmann A. MOEMS tuning element for a Littrow external cavity laser IEEE/ASME J. Microelectromech. Syst. 12, (2003) 21. Liu A.Q., Zhang X.M., Li J., Lu C. Single-

6 /multi-mode tunable lasers using MEMS mirror and grating Sensors and Actuators A108, (2003) 22. Huang W., Syms R.R.A., Stagg J., Lohmann A. "Precision MEMS flexure mount for a Littman tunable external cavity laser" IEE Proc. Sci. Meas. Techol. 151, (2004) 23. Wood R., Dhuler V., Hill E. A MEMS variable attenuator IEEE/LEOS Int. Conf. on Optical MEMS, Kauai, Hawaii, Aug , pp (2000) 24. Syms R.R.A., Zou H., Stagg J., Veladi H. "Sliding-blade MEMS iris and variable optical attenuator" J. Micromech. Microeng. 14, (2004) 25. Flanders D.C., Whitney P.S., Miller M.F. "Silicon on insulator optical membrane structure for Fabry-Perot MOEMS filter" WIPO Patent WO Sept 9 (2001) 26. Tayebati P., Wang P., Azimi M., Maflah L., Vakshoori D. Microelectromechanical tunable filter with stable half-symmetric cavity Elect. Lett. 34, (1998) 27. Laor H. Construction and performance of a 576 x 576 single-stage OXC Proc. LEOS 99, San Francisco, CA, Nov 8-11, Vol 2, pp (1999) 28. Greywall D.S., Busch P.A., Pardo F., Carr D.W., Bogart G., Soh H.T. Crystalline silicon tilting mirrors for optical cross-connect switches IEEE/ASME J. Microelectromech. Syst 12, (2003) 29. Gasparyan A. et al. Drift-free, 1000G mechanical shock tolerant single-crystal silicon two-axis MEMS tilting mirrors in a 1000 x 1000 port optical cross-connect OFC 2003, Atlanta GA, March 23-28, paper PD36-1 (2003) 30. Krishnamoorthy U., Lee D., Solgaard O. Selfaligned vertical electrostatic combdrives for micromirror actuation IEEE/ASME J. Microelectromech. Syst. 12, (2003) 31. Syms R.R.A. Surface tension powered selfassembly of 3-D micro-optomechanical structures IEEE/ASME J. Microelectromech. Syst. 8, (1999) 32. Harsh K.F., Kladitis P.E., Michalicek M.A., Zhang J.L., Zhang W., Tuantranont A., Bright V. M., Lee Y.C. "Solder self-alignment for optical MEMS" Proc LEOS Ann. Meet, San Francisco, CA, Nov. 8-11, pp (1999) 33. Syms R.R.A. Operation of a surface-tension self-assembled 3-D micro-optomechanical torsion mirror scanner Elect. Lett. 35, (1999) 34. McCarthy B., Bright V.M., Neff J.A. A solder self-assembled torsional micromirror array Proc. 16 th IEEE Ann. Int. Conf. on Microelectromechanical Systems, Kyoto Japan, Jan 19-23, pp (2003) 35. Syms R.R.A. Refractive collimating microlens arrays by surface tension selfassembly IEEE Photon. Tech. Lett. 12, (2000) 36. Hong Y.K., Syms R.R.A., Pister K.S.J., Zhou L.X. Corner cube reflectors by surface tension self-assembly IEEE/LEOS Optical MEMS 2004, Kagawa, Japan, Aug 22-26, paper H2 (2004) 37. Syms R.R.A. Self-assembled 3D silicon microscanners with self-assembled electrostatic drives IEEE Photon. Tech. Lett. 12, (2000) 38. Hoffmann M., Kopka P., Nusse D., Voges E. "Fibre-optical MEMS switches based on bulk silicon micromachining" Microsyst. Technol. 9, (2003) 39. Haake J.M., Wood R.L., Duhler V.R. "Inpackage active fiber optic micro-aligner" Proc. SPIE 3276, (1998) 40. Syms R.R.A., Zou H., Yao J., Uttamchandani D., Stagg J. Scalable electrothermal MEMS actuator for optical fibre alignment J. Micromech. Microeng. 14, (2004) 41. Syms R.R.A., Zou H., Stagg J. Robust latching MEMS translation stages for microoptical systems J. Micromech. Microeng. 14, (2004) 42. Syms R.R.A., Zou H., Stagg J. "MEMS angular positioning stage with a Vernier latch" 15 th European Micromechanics Workshop, MME '04, Sept 5-7, Leuven, Belgium, paper A6 (2004) 43. Boyle P., Moore D.F., Breen R., Syms R.R.A., Zou H., Stagg J. "MEMS bistable clamp with electrical locking and release" 15 th Micromechanics Workshop, MME '04, 5-7 Sept., Leuven, Belgium, paper A10 (2004)

SURFACE TENSION POWERED SELF-ASSEMBLY OF 3D MOEMS DEVICES USING DRIE OF BONDED SILICON-ON-INSULATOR WAFERS INTRODUCTION

SURFACE TENSION POWERED SELF-ASSEMBLY OF 3D MOEMS DEVICES USING DRIE OF BONDED SILICON-ON-INSULATOR WAFERS R.R.A Syms, C. Gormley and S. Blackstone Dept. of Electrical and Electronic Engineering, Imperial