MEMS Packaging for Micro Mirror Switches

Transcription

1 MEMS Packaging for Micro Mirror Switches Long-Sun Huang', Shi-Sheng Lee*, Ed Motamedi', Ming C. Wu* and C.-J. Kim University of California, Los Angeles (UCLA) Mechanical and Aerospace Engineering Department, *Electrical Engineering Department , Engineering V Building, Los Angeles, CA , USA 'Rockwell Science Center, Thousand Oaks, CA 'Tel: , seas.ucla.edu Abstract A new packaging architecture is developed for the hybrid integration of free-space MOEMS (micro-opto-electromechanical systems) chip with a silicon micromachined submount. The submount is designed to accommodate various free-space MOEMS chips with minimal active optical alignment, thus reducing the packaging cost. The silicon submount has a central recess to place the MOEMS chip in, four V-grooves for optical fibers, and micropits for micro ball lenses, all bulk micromachined at the same time by a single anisotropic wet etching step. A corner compensation technique was empolyed to prevent erosion of the convex corners, where different geometries meet. With this packaging scheme, a "pick-and-drop'' passive hybrid packaging of MOEMS devices becomes possible. The packaged MOEMS device can then be assembled into final product using standard integratedcircuit packages, such as pin-grid-array packages. The vibration test of a packaged micro mirror switch chip was performed to investigate the robustness of the packaging. Neither mechanical degradation or optical error was observed for vibrations up to 100 g's and frequencies from 200 Hz to 10 khz for over 24 hours.. ntroduction The silicon micromachining technology has opened up many new possibilities in implementing optical and optoelectronic systems in micro scales. t offers unparalleled capabilities in extending the functionality of optical devices and miniaturizing optical systems. Movable structures, microactuators, micropositioners and micro optical elements can be monolithically integrated on the same substrate using batch processing technologies. A free space optics can be defined as any optical component that has aiddevice interface [ 11. With the micromachining techniques, free-space micro optical systems can be realized on a silicon wafer using freespace micro optical bench (FS-MOB) technology [2]. MEMS micro mirror switches are very attractive for freespace fiber-optic switch applications because of their compactness and potential low-cost. Fiber optical switches are used to reconfigure the network and/or increase its reliability. For example, the FDD (Fiber Distributed Data nterface) fiber optical ring network employs optional 2x2 fiber-optic switches, called optical bypass switches, to bypass failed nodes. Figure l(a) shows the FDD dual ring architecture and l(b) shows operation of an optical bypass switch. Since the switch introduces additional optical loss, fiber-optic switches should be designed to minimize the insertion loss. The switch can be realized by free-space approach or waveguide approach. The free-space approach [3,4] offers a number of advantages over the conventional waveguide approach, such as lower coupling loss and smaller cross talk. However, the packaging of these free-space MOEMS devices is difficult using conventional packages such as C packages because free space optical elements often need to be elevated to operating level. n this paper, we report a novel packaging scheme for free-space MOEMS devices, especially, vertical micro mirror switches. This new packaging scheme involves the development of a bulk micromachined silicon submount that aligns the free-space micro mirror switches with all the nesssary optical elements. (a) Dual ring architecture. Optical (b) Operation Principle Figure 1 FDD optical bypass switch Although silicon V-grooves have long been used to package optoelectronic waveguide devices with fibers, packaging of free-space MOEMS device presents unique /98 $ EEE Electronic Components and Technology Conference

2 challenges due to its free space nature. n this paper, we present the concept of hybrid integration approach for MOEMS devices with a silicon submount. This packaging scheme can potentially reduce the total manufacturing cost of free-space MOEMS devices. 11. Design A. Packaging Scheme A free-space micro optical system can be realized on a single chip by the Multi User MEMS Processes (MUMPS), the three-layer polysilicon surface-micromachining process offered by MEMS Technology Application Center at North Carolina. The surface-miromachined free-space fiber-optic switch [5] (figure 2), consisting of a sliding, vertical micro mirror and integrated microactuators, is very attractive because of its compactness and potential low-cost. -Guides \\ MOEMS chips of different thicknesses can be accommdated with only minor modification. With this hybrid packaging scheme, we can fabricate the MOEMS device and the silicon submount separately, thus avoiding the complexity associated with fabricating all the features on a single chip. The silicon submount assembled with the MOEMS device can then be placed in the common packages, such as PGA package, enhancing feasible commercialization. B. Corner Compensation For free-space fiber-oplic switches, light emitted from the input fiber is transmitted to the opposite fiber or redirect by the mirror to the orthogonal fiber. Micro ball lenses can be used to reduce the coupling loss and to increase the working distance. n order to assemble optical elements, the silicon submount is required to have proper mechanical structures to hold the optical components. The silicon structures required for optical elements on the (100) silicon submount are V- grooves for fibers, micropits for micro ball lenses, and a central recess for a micro mirror chip. The V-grooves, micropits and central recess can be realized by slow etching on { 11 1 } plane. However, it is known that convex corner on a (100) silicon is attacked in an anisotropic wet etch process making definition of coinvex corners difficut. A proper compensating structure design is necessary to obtain the final convex patterns reasonablly close to the desired. nnn Figure 2. Schematic drawing of the micro mirror switch The packaging idea in this paper is to pick and drop the free-space MOEMS device into a submount that has optical components already aligned on it. Figure 3 shows the cross section view of our MOEMS package - a free-space micro mirror chip placed in the silicon submount. The submount has aligned on it and is covered with a glass piece. and optical coupling elements. The submount is composed of two silicon substrates bulk-micromachined and bonded together. Optical Fiber(D=l25pm) Micro Ball (D=300P) Figure 4. Various corner compensation designs Silicon 15" Submount Figure 3. Cross section view of a MOEMS packaging The top silicon substrate has a central opening for the mirror chip, V-grooves for optical fibers, and micropits for micro ball lenses. The bottom substrate bonded with the top substrate has a central recess area. With the two substrate approach, Various corner compeinsation designs [6,7,8,9] have been proposed as shown in figure 4. Using the corner compensation, the convex corners may be preserved at the end of the etching. For current design, the extension of mask patterns from the corners is oriented to the 41 lo> direction as shown in figure 5. The length design should correspond to the etch ratio between the corner attack and that of { 100) plane. Using 30% KOH at 70 C, the etch ratio was found about for our condition. n other words, to prevent the corner Electronic Components and Technology Conference

3 attack, the length for corner compensation is required to be times greater than the etched depth in vertical <loo> direction. To etch through the wafer ( around 500 pm thick), the compensation beam over 1 mm in total length is needed per each convex corner. Since the necessary compensation beam was too long to fit inside the 454 pm x 454 micropit (See Appendix), a folded configuration is proposed as shown in figure 5. Second, total etching time is reduced by etching the central hole from both top and bottom sides of the wafer. With these approaches, the desired structures on the silicon submount can be obtained. thermal oxide LPCVD nitride RE opening at both sides KOH etching A lift-off TOP substrate Bottom substrate rm Adhesive bonding thermal oxide LPCVD nitride KOH etching assemble fibers, ball lenses and MOEMS device MOEMS device Figure 6. Process flow for the silicon submount Figure 5. Mask pattern including the corner compensation and the final microstructure pattern desired Fabrication Figure 6 shows the.fabrication procedures. Both top and bottom substrates are (100) silicon wafers, the top being a double-side polished wafer. First, 1000 A thermally grown silicon dioxide and 1500 A low-pressure chemical-vapordeposition (LPCVD) silicon nitride layers are used as the silicon etch mask during the anisotropic wet etching. To minimize the effect of silicon crystal lattice-to-mask misalignment during etching, determination of the true crystal direction within 0.1" accruacy [ 111 is desired for the top wafer. Reference marks are etched to reveal the true crystal direction at the beginning of the process. After reactive ion etching (RE) (CF4/02) using double-side photolithography (k 2 pm front-to-back alignment accuracy), the top wafer is etched in KOH. Once the central hole is opened, the wafer is taken out of the etching solution. All the features are fabricated in a single anisotropic wet etching step. Then a 5000 fi aluminum pad is deposited for wire bonding by a lift-off process. The bottom silicon substrate is prepared separately with a much simpler process of making a recess. V. Assembly Figure 7 shows the schematic drawing of the silicon submount with optical fibers and micro ball lenses in place. First, the top and bottom wafers are diced to the size of 15 mm x 15 nun. The two substrates are then put together while four dummy fibers (125 pm) in the V-grooves on the facing surfaces snap them in position easily [12]. Thermally cured adhesive applied along the perimeter of the bottom surface bonds the two substrates together. Perpendicularly arranged dummy fibers lock the two substrates against translational and rotational sliding during the curing step when pressure is applied to the mating substrates. Optical Fibers / - Dummy Fiber Micro Ball Lenses -/ L A<Y V Bottom Si substrate Figure 7 Schematic drawing of the silicon submount The micropits (for ball lenses) are self-aligned with V-grooves (for fibers) during the processing since they are made Electronic Components and Technology Conference

4 ~ simultaneously in one step. The contraction between the pit and the central hole is a stopper preventing the ball lens from rolling over to the central hole as shown in figure 8. The 300 ym micro ball lenses are picked and dropped in the micropits. The fibers are close to the ball lenses and glued on the V- grooves. Figure 9 shows the assembly of the fiber and ball lens on the V-groove and micropit. package, such as PGA (pin grid array) packages, to complete the final product. Figure O. SEM of a packaged micro mirror device Figure 8 V-groove, micropit and central hole after etching Figure 9. SEM of the fiber and ball lens assembly The central hole is designed to accommodate various types of micro optical devices. The example in this paper presents a fully actuated mirror chip [5], which is designed for FDD optical bypass switch. At present, the chip is diced along the mark line within 12 pm error. The opening of the central hole corresponds to the size of the mirror chip (See Appendix). Therefore, the mirror chip is picked and dropped in the central hole to first align the side walls of the central opening. Currently, a slight adjustment needs to be made following the initial alignment to the side walls. The recess on the bottom surface ensure a good vertical alignment. Thermally cured adhesive bonds the mirror chip to the submount. The submount-packaged MOEMS device, shown in figure 10, can then be assembled into a standard C V. Vibration Test The vibration test of the packaged device was conducted to examine the robustness and reliability of the packaging and the micro mirror switch. Quantitative measurement in terms of the data bit error rate (BER) of the switch has been obtained. A schematic diagram of the experiment setup is shown in figure 11. n this experiment, we prepared the switch in the reflection mode. Two multimode optical fibers are attached to the silicon submount in the PGA packaging for in situ monitoring of the optical signals during vibration. The device packaging is mounted on the Unholtz-Dickie vibration-testing machine with the hinged micromirror facing the axis of the vibration. Since the hinged micromirror is most sensitive to vibration in this direction, the measurement result should be considered as the worst case of vibrations in three axes. An HP bit error rate tester is used to drive the optical transceiver and measure the error rate, and a Tektronix real-time digital oscilloscope is used to record the eye diagram. The performance of the: packaged device under vibration is evaluated by measuring ithe bit error rate of the received optical signal in the reflection mode. A bit long psudorandom test patterns at 125 MHz clock rate are used to investigate the vibration sensitivity at a wide range of frequencies and g-levels. No error was observed for the entire period of the test (24 hours) of the switch and the package for vibrations up to 100 g's and frequency between 200 Hz to 10 khz (error rate < 10'13). Comparison of the receiving sensitivity with and without vibration shows that there is virtually no effect of vibration of this scale. No mechanical failure observed throughout the entire test. Clear open eye diagram shown iin figure 12 was observed during the testing Electronic Components and Technology Conference

5 Bit Error Tester PackagedMOEMS device, Tektronix Digital Oscilloscope Figure 1 1. Schematic diagram of the experiment setup walls in the pit rather than the bottom plane whose depth is hard to control. However, slight corner attack is intended to allow for wider light passage. By etching the central hole from both sides of silicon, the total etching time is cut in half, reducing the total length of corner compensation beam in half as well. Optical alignment in plane is ensured by fabricating all the features on top substrate at the same time in a single anisotropic wet etching process, while the vertical alignment is provided independently by controlling the depth of central recess in the bottom substrate for a given chip type. The pick-and-drop approach, based on the silicon submount using passive optical alignment, promotes an effective assembly of all micro optical elements. The error of the features on top substrate is 5 1 pm for fibers and pm for ball lenses. The error for the mirror chip is around 12 pm due to the limitation of the dicing machine we used. Slight adjustment under the microscope was needed to align the predetermined guidelines on the mirror chip to the ball lenses and fibers. The final goal is a batch-packaging capability for MOEMS devices by precise dicing or etching without any further adjustment of the chip in the central hole. V. Summary We have demonstrated the new MEMS packaging based on the hybrid integration of the silicon submount and MOEMS device. The two substrate approach for the silicon submount offers great flexibility to accommodate various free-space MOEMS devices. Using corner compensation technique and double-side etching on (100) silicon, we can achieve all the features for optical elements on the top substrate in one etching step. The pick-and-drop scheme makes the assembly procedure much more effective. The ruggedness and reliability of the fully packaged device has been verified against vibration up to 100 g, frequency between 200 Hz to 10 khz and operation over 24 hours. Eb# 200mVn Figure 12. Measured eye diagram of the device at the vibration of 100 g's and 200 Hz. V. Discussion Since silicon is a brittle material, the top substrate, which has a central hole and V-grooves, tends to break along the grooves under external forces. The problem has been solved after the top is bonded with the bottom substrate as the silicon submount. As for the corner compensation design, the width of the <110> oriented beams extended out of the corners does not play an important role during the etching. Beams of 40 pm and 20 pm width of the <110> oriented beams were used in the central hole and inside the micropits, respectively. The corner compensation beams are eventually removed and the side walls for the micropit are confined by the { 111 } plane. t has been designed such that the ball lens is held by the side Acknowledgement This project was supported by the Defense Advanced Research Projects Agency (DARPA) MEMS Program. Appendix Dimensional relation of among key features of submount fiber diameter : DF = 125 pm ball lens diameter: DB = 300 pm mirror chip width: WM top silicon thickness: ts dummy fiber for alignment : DF = 125 pm angle: 8 =54.7" Mask opening for optic components on (100) silicon fiber opening: WF = DF CSC~ + D~cOt8 ball lens opening: WB= DBcsc@ + DF cot8 central hole opening : WC= WM + ts cot8 the Electronic Components and Technology Conference

6 dummy fiber opening for alignment: WAF = DF csce Fiber Ball lens F WC 10. Zanini, M, Mikkor, la, Elder, R.C., Cathey, L.W., Bomback, J.L., and Artz, B.E., Edge Erosion Effects in Double Sided Silicon Micromachining, nternational Solid-state Sensors and Actuators Conference, Hilton Head, South Carolina, June 6-9, 1988, pp Ensell, G., Alignment of Mask Patterns to Crystal Orientation, Proc. 8 nternational Conference on Solid- State Sensors and Actuators - TRANSDUCERS 95, Eurosensors X, vol. 1, 1995, pp Smith, R.L. and Collins, S.D., A Wafer-to-Wafer Alignment Technique, Sensors and Actuators, vol. 20, no. 3, 1989, pp References 1. Walker, S.J., Optics & MEMS: An Overview of Current Technology, EEE/LEOS, EEJ/SAMS, nternational Conference on Optical MEMS and Their Application (MOEMS97), NOV , 1997, pp Wu, M.C., MicroMachining for Optical and Optoelectronic System, Proceedings of the EEE, vol. 85, no. 11, 1997, pp Dautartas, M.F., Benzoni, A.M., Chen, Y.C., and Blonder, G.E., A Silicon-based Moving-Mirror Optical Switch, J. Lightwave Technology, vol. 10, no. 8, August, 1992, pp Toshiyoshi, H. and Fujita, H., Optical Crossconnection by Silicon Micromachined Torsion Mirrors, Digest. EEE/LEOS, 1996 Summer Topical Meeting, Keystone, CO,, Aug. 5-9, 1996, pp Lee, S.S., Motamedi, E., and Wu, M.C., Surfacemicromachined Free-Sapce Fiber Optic Switches With ntegrated Microactuators for Optical Fiber Communication Systems, nternational Solid-state Sensors and Actuators Conference - TRANSDUCERS 97, Chicago, June 16-19, 1997, pp Wu, X.-P and KO, W.-H, A Study on Compensating Corner Undercutting in Anisotropic Etching of (100) Silicon, nternational Solid-State Sensors and Actuators Conference - TRANSDUCERS 87, Tokyo, Japan, 1987, pp Sandmaier, H., Offereins, H.L., Kuhl, K., and Lang, W., Corner Compensation Techniques in Anisotropic Etching of (100) Silicon Using Aqueous KOH, nternational Solid-state Sensors and Actuators Conference - TRANSDUCERS 91, San Francisco, CA, 1991, pp Enoksson, P., Stemme, G., and Stemme, E., Vibration Modes of a Silicon Tube Density Sensor, J. Microelectromecchanical Systems, vol. 5, no. 1, Mar. 1996, pp Enoksson, P., New Structure for Corner Compensation in Anisotropic KOH Etching, J. Micromechanics and Microemgineering, vol. 7, no. 3, 1997, pp Electronic Components and Technology Conference