Open Access
Add to BibSonomyAbstract
We present the design, fabrication, and measurement results of an electromagnetic biaxial microscanner with mechanical amplification mechanism. A gimbaled scanner with two distinct single-crystal silicon layer thicknesses and integrated copper coils has been fabricated with combination of surface and bulk micromachining processes. A magnet assembly consisting of an array of permanent magnets and a pole piece has been placed under the substrate to provide high strength lateral magnetic field oriented 45° to two perpendicular scanning axes. Micromirror has been supported by additional gimbal to implement a mechanical amplification. A 1.2mm-diameter mirror with aluminum reflective surface has been actuated at 60Hz for vertical scan and at 21kHz for horizontal scan. Maximum scan angle of 36.12° at 21.19kHz and 17.62° at 60Hz have been obtained for horizontal and vertical scans, respectively.
© 2015 Optical Society of America
1. Introduction
Micro-Electro-Mechanical Systems (MEMS) technology has been widely researched in the field of optical MEMS where various optical and microphotonic components have been combined with MEMS technology to provide devices and functionalities that were infeasible in the past. MEMS-based microscanners are good examples as they provide equal or superior performance compared to conventional galvano mirrors and polygon mirrors while consuming much less volume and electric power. Moreover, MEMS scanners can be equipped with 2-dimensional (2-D) scanning capability which can be very challenging for conventional macro scale actuators. Most of the optical MEMS devices including the microscanners are being widely used and researched due to small size and weight, low production cost, high performance and high energy efficiency. These characteristics have promoted the utilization of microscanners in applications such as laser-based pico projectors and automotive head up display systems [1, 2]. Furthermore, as the new technologies such as Light Detection And Ranging (LIDAR) and Optical Coherence Tomography (OCT) progress to be commercialized in a mass consumer market, demand for high performance MEMS scanners is ever increasing [3].
To implement a high resolution display and to obtain information from a wider area using laser beam source and MEMS scanner, performance requirements for wide scan angle and high scan speed are becoming more pronounced. Moreover, biaxial scan capability, small form-factor, and low power consumption are also required in realizing a commercially viable scanning system. For a MEMS-based microscanner, these and other requirements can be fulfilled with various actuation mechanisms and fabrication methods, where each of these approaches has distinctive advantages and shortcomings [4–12]. In general, electrostatic actuators can meet the requirements such as low power consumption, fast response time, and simple fabrication process, but the difficulty in providing high enough actuation voltage with a compact integrated system remains as an issue to be resolved. Considering the need for long-term stability and improved quality factor, use of hermetic or vacuum packaging with an optically transparent window is an attractive but expensive option for comb-driven actuators. Schenk et al. have developed a comb-driven biaxial scanning mirror which provides a large deflection angle without the need for vertical comb electrodes with large separation [13]. Milanović et al. have proposed a vertical comb-driven biaxial scanner based on mechanical linkages and mechanical rotation transformers to obtain large deflection angle and fast speed [14]. Piezoelectric actuators can achieve high power density due to large piezoelectric constants of the material [9, 10]. However, relatively small stroke which complicates the design of the actuator, complex fabrication process, reliability issue and repeatability of the thin-film piezoelectric material should be taken into account for piezoelectric actuators. In contrast, electromagnetic actuators can produce relatively high output force and cost-effective polymer housing can be used in place of the hermetic package which takes up major portion of the production cost in typical MEMS devices. However, electromagnetic actuators require the integration of either metallic coil or magnetic material in the MEMS mirror and housing to generate Lorentz force. In general, electroplated metal coil is formed on the mirror and high strength rare earth magnets are installed in the housing to maximize the magnetic force without substantial increase in moment of inertia of the mirror. Also, optimization of pole piece design can further improve the externally applied magnetic field strength. Ji et al. have developed an electromagnetic biaxial scanner which utilizes a radial magnetic field generated by two concentric permanent magnets and pole piece placed under the silicon substrate [11].
To maximize the scan range while maintaining the fast scan speed, mirrors are typically actuated at their resonant modes. Schenk et al. have utilized the resonant mode actuation in both scan axes to generate Lissajous pattern, whereas Yalcinkaya et al. have used combination of resonant mode and forced actuations to generate raster scanned pattern [12, 13]. Also, angular deflection amplification mechanism based on dual spring-mass-damper system has been utilized to increase the deflection angle at resonance [12, 13, 15, 16].
In this research, we have designed, fabricated, and analyzed the performance of an electromagnetic biaxial scanning micromirror which utilizes the lateral magnetic field oriented 45° to two perpendicular scanning axes and angular deflection amplification mechanism. In contrast to the previously reported magnet configuration where magnetic field between geometrically separated magnetic components have been utilized, lateral field generated on top of the flat surface of the rectangular magnet array has been used [12]. Uniquely designed magnet assembly, which is a combination of an array of permanent magnets and a pole piece, has been placed under the substrate to provide high strength lateral magnetic field with minimum package volume. In contrast to conventional gimbaled 2-D microscanners, mirror has been supported by additional gimbal to amplify the deflection angle of the mirror.
Design of the 2-D microscanner including the development of new magnet assembly and gimbaled 2-D scanner architecture using finite element analysis (FEA), characterization and measurement results of the fabricated MEMS scanner are presented.
2. Design
Figure 1 shows the perspective view of the designed microscanner. A 1.2mm-diameter micromirror is supported by reinforcement rim and connected to inner gimbal through torsion beams with spring constant k. Elliptically-shaped reinforcement rim relieves the dynamic deformation of the micromirror through unique shape and position of the connectors which support the micromirror. Outer gimbal which supports the inner gimbal is connected to the substrate by torsion beams with spring constant k2. Torsion beams with spring constant k1 connect the inner and outer gimbals. Fabricated microscanner die is flipped during assembly with the magnet assembly to make use of the reflective surface formed at the backside of the micromirror [Fig. 1(b)]. Horizontal scan of the reflected beam can be achieved by rotation of the micromirror along x-axis, and vertical scan is obtained by rotation of outer gimbal along y-axis shown in Fig. 1(a).
Coils for horizontal and vertical scans are formed separately on the inner and outer gimbals, respectively, and are fed through the torsion beams for outer gimbal [Fig. 2(a)]. Also, feed lines are designed to have no effect on the actuation of each gimbal. Number of turns for each coil is six. As shown in Fig. 2(b), x and y axes represent the rotational axes for horizontal and vertical scans, respectively. Vertical scan is achieved by current flowing in the outer coil (Iv). As shown in Fig. 2(b), coils formed on the two sides of the gimbal that are perpendicular to the magnetic field direction are mainly responsible for the actuation. Rotational direction of the outer gimbal (θv) is determined by direction of the field (B) and the current (Iv). Horizontal scan is achieved by rotation of the micromirror which is actuated indirectly by current applied to the inner coil (Ih) at resonance. Although same working principle applies to the determination of rotational direction of the mirror (θh), crosstalk cannot be avoided due to rocking mode actuation of the outer gimbal at non-resonant operational modes. As the micromirror is actuated by coil formed separately on the inner gimbal, deformation of the mirror due to heat can be minimized and amplification of angular deflection can be implemented.
Fig. 2 Schematics of the coil geometry actuation mechanism: (a) coil geometry, (b) directions of applied magnetic field, driving current, and corresponding torque on each axis.
Magnetic field oriented 45° to two perpendicular scanning axes is applied by a permanent magnet assembly attached at the bottom of the substrate. As the magnetic field intensity near the coil has to be maximized for a more efficient actuation, a unique magnet assembly design has been proposed. As shown in Fig. 3(a), the magnet assembly comprises two pairs of inner and outer rectangular magnets and a pole piece. The magnet assembly is designed to maximize the lateral magnetic flux density at two parallel regions, dashed box in Fig. 3(a), on top of which the coils shown in Fig. 2 will be situated. Figure 3(c) shows the relative position of the micromirror die and magnet assembly. Magnet geometry and coil dimensions have been optimized by 3-D and 2-D FEA tools. As shown in Fig. 3(d), a 2-D FEA on a planar symmetric geometry has been performed with FEMM (Finite Element Method Magnetics), whose results have been verified with a 3-D FEA using Ansys Workbench [Fig. 3(c)]. Nd-Fe-B (neodymium iron boron) magnets with maximum energy product of 52MGOe have been used in the simulation and direction of the magnetization for each magnet is shown in Fig. 3(c). Figure 3(e) shows the lateral magnetic field intensity (Hr) simulated by 2-D FEA at various gaps between the magnet and coil. According to the simulation results, a maximum lateral magnetic field intensity of 37,734A/m can be achieved when the gap between the magnet assembly and coil is 500μm. Also, torque component which is proportional to Hr and square of the distance between coil and axis of rotation (R) has been shown in dashed lines. Due to the square-dependence on R, distance between the coil and axis of rotation has to be slightly larger than the inner magnet length (lmi). Width (wm) and length (lm) of the designed magnet assembly are both 10mm, and thickness of the magnet (tm) and pole piece (tp) are both 1mm. Length of inner and outer magnets (lmi and lmo) are 1mm and 5mm, respectively. Besides the advantage of providing an appropriate lateral magnetic field distribution to maximize the driving torque for both axes, proposed magnet assembly design offers an easy-to-handle structure for assembly processes.
Fig. 3 Schematics of the magnet design and simulation results: (a) magnet assembly, (b) 3-D FEA result (arrows indicate the magnetization direction of the magnets), (c) relative position of the silicon die and magnet assembly, (d) 2-D FEA result, (e) 2-D simulated magnetic field intensity and torque component at various 400, 500, and 600μm gaps.
In comparison to the conventional magnet configurations where the magnetic field between the two separated magnets or cores are utilized, proposed geometry provides a mechanically stable architecture and high magnetic field intensity at small footprint. As the magnetic flux lines form a closed loop inside the magnets and the pole piece, assembly process for the magnets can be completed by minimal adjustment of magnet position after assembly. Integration of magnet assembly with housing is also easy as the magnetic flux line coming out of the assembly is focused on the topside.
Independent biaxial scan with minimum crosstalk can be achieved by combination of forced actuation in vertical direction and resonant actuation in horizontal direction. Vertical scan is achieved by applying a 60Hz input current to the outer coil, while the horizontal scan is obtained by resonant actuation of the inner gimbal using inner coil. Modal analysis of the designed device has been performed using Ansys Workbench. As shown in Fig. 4(a), resonant frequency of the vertical scan mode is 463Hz. Among the three different resonance modes for the horizontal scan (3rd, 7th, and 8th modes), 8th mode is used where the mirror and outer gimbal are rotated in phase.
Fig. 4 Modal analysis result: (a) 1st mode 463Hz, (b) 3rd mode 3,743Hz, (c) 7th mode 14,693Hz, (d) 8th mode 20,875Hz.
By appropriate design of the moment of inertia of the mirror and gimbals in relation with spring constants, angular deflection of the mirror can be maximized while minimizing the deflection angle of the inner and outer gimbals as shown in Fig. 4(d) [10, 11, 14]. Rocking mode of the outer gimbal is also minimized in 8th mode to minimize the crosstalk. Assuming that the proposed scanner is utilized in a laser projection display system by applying a 60Hz sawtooth waveform to vertical scan axis and utilizing the horizontal scan at resonant frequency of 21kHz, designed device is capable of generating SVGA resolution (800x600) image when the duty ratio of the sawtooth wave is approximately 86%.
For a better understanding of the resonant actuation for horizontal scanning, harmonic analysis has also been performed. As shown in Fig. 5(a), torque (4.42μNm) has been applied to the inner coil and frequency response of the deflection in z direction has been obtained for three representative points for mirror, inner and outer gimbals. Empirically determined constant damping ratio of 0.000645 has been used in the simulation. Figures 5(b) and 5(c) show the obtained frequency response of mechanical half scan angle for the three measurement points. Maximum deflection angle of 6.65° has been obtained at 20,755Hz for the mirror and amplification ratio of 19.29 has been achieved compared to the deflection angle of the inner gimbal. Expected deflection angle of the outer gimbal is 0.063° which is 0.95% of the mirror deflection angle. As the damping was not considered in modal analysis, resonant frequencies obtained with harmonic analysis had slight differences with those obtained by modal analysis.
Fig. 5 Harmonic analysis results: (a) simulated model and measurement points, (b) frequency response, (c) magnified view of the frequency response near 8th mode.
For a 1.2mm-diameter mirror to be capable of realizing an SVGA resolution image, mechanical deflection angle of the mirror should be above ± 7.8°. As the mirror undergoes a large angular deflection at high speed, dynamic deformation of the mirror has to be taken into account. Dynamic deformation of the mirror has been estimated by 3D FEA using Ansys Workbench. As shown in Figs. 6(a) and 6(b), an 80μm-thick single crystal silicon mirror with 1.2mm-diameter without the rim undergoes a peak-to-peak deformation of 300nm and root-mean-square deformation of 60nm at mechanical scan angle of ± 7.7° at scan frequency of 21kHz. Utilizing a unique reinforcement rim structure, dynamic deformation of the mirror, and thus the reflective surface, has been minimized. At the driving frequency of 21kHz and mechanical scan angle of ± 7.7°, peak-to-peak deformation of 44nm and root-mean-square deformation of 12.5nm have been achieved as shown in Figs. 6(c) and 6(d).
Fig. 6 Dynamic deformation of the mirror at driving frequency of 21kHz and mechanical scan angle of ± 7.7° (simulation result): (a) perspective view (model without rim), (b) top view (model without rim), (c) perspective view (model with rim), (d) top view (model with rim).
3. Fabrication
Fabrication process of the proposed scanning micromirror is based on a combination of surface and bulk micromachining processes. Device has been fabricated using Senplus’ proprietary metal SOI (silicon-on-insulator) process which enables the realization of multiple thickness single crystalline silicon structure with two metal layers on top (Senplus Inc., Gyeonggi-do, Korea) [17]. A 6-inch SOI substrate is used whose thicknesses of the device layer, buried oxide, and handle wafer are 80μm, 1μm, and 400μm, respectively. After a 400μm-deep cavity formation on the handle wafer, 60μm-deep trenches are formed at the bottom side of the 80μm-thick device layer, which results in a 20μm-thick silicon layer for the fabrication of torsion beams for vertical scan [Fig. 7]. All the other features are fabricated with 80μm-thick device layer. A 10μm-thick copper layer for the coil is electroplated on top side of the device layer. Details of the fabrication process are explained in [11], except for different layer thicknesses and coil formation process using two metal layers. Before the bulk etching process on the backside of the wafer for cavity formation, stack of aluminum and silicon oxide layers are formed on the front side for the interconnection of electroplated copper layer. Thicknesses of the aluminum and oxide layer are both 2μm. Inset in Fig. 6 shows the cross-sectional details of the copper winding, where the electrical connection of the inner turn is made through the aluminum layer formed at the bottom. Shapes of the mirror, gimbals, and torsion beams are defined by Bosch process on front side of the substrate at the final stage of fabrication process. Figure 8 shows the scanning electron microscope (SEM) images of the fabricated scanning micromirror.
Fig. 8 SEM images of the fabricated micromirror: (a) bottom side, (b) top side (torsion beams for horizontal scan), (c) top side (torsion beams for vertical scan).
After releasing the suspended structures by Bosch process and die separation, electrical connections to the silicon die are made by anisotropic conductive film (ACF) bonding of flexible printed circuit board (FPCB) directly on the pads formed on the silicon die. Magnet assembly is prepared by attaching individual Nd-Fe-B permanent magnet components and pole piece fabricated with electrogalvanized cold-rolled steel [Fig. 9(a)]. Silicon die and magnet assembly are integrated in a polymer housing fabricated with polycarbonate. Silicon die and magnet assembly are inserted into the slots formed on the cap and bottom housing, respectively [Fig. 9(b)]. Manual assembly process is completed by joining the cap and bottom housing at which the lateral alignment and gap between the mirror and magnets are determined passively as designed. A two-part epoxy adhesive (3M DP-460) is used in all the insertion and joining processes for the fixation of each part. Total volume of the housing is approximately 0.77cc.
Fig. 9 Magnet assembly and fabricated microscanner package: (a) magnet assembly, (b) schematics of the packaged device, (c) fabricated scanning micromirror package.
4. Experimental results and discussion
4.1 Frequency response of the device
Frequency response of the mirror has been measured using measurement setup shown in Fig. 10. Driving current for the micromirror has been applied through the voltage follower board and laser beam (638nm) incident on the reflective surface of the mirror has been reflected towards the position sensitive detector (PSD). Output from the PSD has been collected with computer equipped with LabVIEW via field programmable gate array (FPGA).
As the mirror is actuated in a resonant mode for horizontal scan, corresponding resonant frequency [Fig. 4(d)] of the device has been analyzed. Figure 10 shows the frequency response of the mirror at various driving voltages. Angular deflection of the mirror in horizontal direction has been measured while input frequency has been swept from 20 to 22kHz at given amplitude. As shown in Fig. 11, reduction of resonant frequency has been observed as the input signal was increased. As shown in Fig. 11(b), measured resonant frequency at maximum input voltage and thus maximum scan angle was 21.19kHz. As the input voltage was increased from 1 to 10V, horizontal resonant frequency was reduced from 21.32 to 21.19kHz. Variation of resonant frequency suggests that a feedback control based on the measurement of actual scan angle or deflection of the mirror is required for the horizontal scan.
Fig. 11 Frequency response of the mirror (horizontal scan): (a) bode plot for horizontal scan at various input voltages, (b) variation of horizontal resonant frequency at various input voltages.
4.2 Measurement of angular deflection
Angular deflection of the mirror has been measured using the setup shown in Fig. 12. Driving current for the micromirror has been applied through the voltage follower board and laser beam incident on the reflective surface of the mirror has been reflected towards the screen to estimate the scan angle by measuring the length of scan line. Experimental setup shown in Fig. 12 was also used in the calibration of setup shown in Fig. 10.
Figure 13 shows the measured angular deflection of the mirror in horizontal and vertical directions. For horizontal scan, maximum deflection angle of 36.12° has been obtained at peak-to-peak input voltage and current of 10V and 515.17mA, respectively. Horizontal scan angle has been measured while supplying sinusoidal input signal at 21.19kHz. For vertical scan, maximum deflection angle of 17.62° has been obtained at peak-to-peak input voltage and current of 2.5V and 196.97mA, respectively. Vertical scan angle has been obtained by measuring the scan length at 60Hz sinusoidal input.
Figure 14 shows the captured images of the lines scanned on the screen. As shown in each figures, independent scans in horizontal and vertical directions, as well as the raster mode 2-D scan have been achieved successfully. Figure 14(c) shows the scanned 2-D image when 21.19kHz and 60Hz sinusoidal input signals have been fed to the inner and outer coils, respectively. Although not demonstrated 2-D image of SVGA resolution can be obtained by modulation of laser beam incident on the micromirror.
5. Conclusion
We have successfully designed, fabricated, and tested a new type of electromagnetic biaxial microscanner with mechanical amplification mechanism. Proposed device utilizes a lateral magnetic field oriented 45° to both scanning axes provided by an array of rectangular permanent magnets and a pole piece. Geometry of the magnet array has been optimized to maximize the lateral magnetic field and to minimize the total height of the assembled device. Scanning micromirror has been fabricated using SOI substrate with 90μm-thick device layer. A 10μm-thick electroplated copper with 2μm-thick buried aluminum interconnection layer has been used in the fabricated of two electrically separated windings, which have been formed on inner and outer gimbals for vertical and horizontal scanning, respectively. Fabricated scanning micromirror has been packaged using a non-hermetic polymer housing based on polycarbonate which measures approximately 0.77cc. Maximum horizontal scan angle of 36.12° has been obtained at 21.19kHz in response to a peak-to-peak input current of 515.17mA. For vertical scan, maximum scan angle of 17.62° has been obtained at 60Hz sinusoidal input with peak-to-peak amplitude of 196.97mA. A raster mode 2-D scan has been successfully implemented using the realized scanner. Proposed device structure and actuation principle can be applied to various types of laser scanning applications.
Acknowledgments
Authors would like to thank Byung-Min Yoon, Seung Lee, Taeha Kim, Yeontae Chung, and Si-Hong Ahn of Senplus Inc., for the device fabrication and their guidance and support with the characterization. This work was supported by the Industrial Technology Innovation Program (No.10047785) funded by the Ministry of Trade, Industry & Energy (MI, Korea), Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A1A2057721), and by the Space Core Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (MSIP) (NRF-2013M1A3A3A02042410).
References and links
1. W. O. Davis, R. Sprague, and J. Miller, “MEMS-based pico projector display,” in Proceedings of IEEE/LEOS International Conference on Optical MEMS and Nanophotonics (IEEE/LEOS, 2008), pp. 31–32. [CrossRef]
2. M. O. Freeman, “MEMS scanned laser head-up display,” Proc. SPIE 7930, 79300G (2011).
3. T. Sandner, M. Wildenhain, C. Gerwig, H. Schenk, S. Schwarzer, and H. Wöfelschneider, “Large aperture MEMS scanner module for 3D distance measurement,” Proc. SPIE 7594, 75940D (2010).
4. J. T. Nee, R. A. Conant, M. R. Hart, R. S. Muller, and K. Y. Lau, “Stretched-film micromirrors for improved optical flatness,” in Technical Digest of IEEE International Conference on MEMS (IEEE, 2000), pp. 704–709. [CrossRef]
5. U. Krishnamoorthy, D. Lee, and O. Solgaard, “Self-aligned vertical electrostatic combdrives for micromirror actuation,” J. Microelectromech. Syst. 12(4), 458–464 (2003). [CrossRef]
6. D. Hah, P. R. Patterson, H. D. Nguyen, H. Toshiyoshi, and M. C. Wu, “Theory and experiments of angular vertical comb-drive actuators for scanning micromirrors,” IEEE J. Sel. Top. Quantum Electron. 10(3), 505–513 (2004). [CrossRef]
7. Y.-C. Ko, J.-W. Cho, Y.-K. Mun, H.-G. Jeong, W.-K. Choi, J.-H. Lee, J.-W. Kim, J.-B. Yoo, and J.-H. Lee, “Eye-type scanning mirror with dual vertical combs for laser display,” Proc. SPIE 5721, 14–22 (2005).
8. C.-H. Ji, M. Choi, S.-C. Kim, S.-H. Lee, S.-H. Kim, Y. Yee, and J.-U. Bu, “Electrostatic scanning micromirror with diaphragm mirror plate and diamond shaped reinforcement frame,” J. Micromech. Microeng. 16(5), 1033–1039 (2006). [CrossRef]
9. Y. Yasuda, M. Akamatsu, M. Tani, T. Iijima, and H. Toshiyoshi, “Piezoelectric 2D-optical micro scanners with PZT thick films,” Integr. Ferroelectr. 80(1), 341–353 (2006). [CrossRef]
10. M. Tani, M. Akamatsu, Y. Yasuda, H. Fujita, and H. Toshiyoshi, “A 2D-optical scanner actuated by PZT film deposited by arc discharged reactive ion-plating (ADRIP) method,” in Proceedings of IEEE/LEOS International Conference on Optical MEMS (IEEE/LEOS, 2004), pp. 188–189.
11. C.-H. Ji, M. Choi, S.-C. Kim, K.-C. Song, J.-U. Bu, and H.-J. Nam, “Electromagnetic two-dimensional scanner using radial magnetic field,” J. Microelectromech. Syst. 16(4), 989–996 (2007). [CrossRef]
12. A. D. Yalcinkaya, H. Urey, D. Brown, T. Montague, and R. Sprague, “Two-axis electromagnetic microscanner for high resolution displays,” J. Microelectromech. Syst. 15(4), 786–794 (2006). [CrossRef]
13. H. Schenk, P. Dürr, T. Haase, D. Kunze, U. Sobe, H. Lakner, and H. Kück, “Large deflection micromechanical scanning mirrors for linear scans and pattern generation,” IEEE J. Sel. Top. Quantum Electron. 6(5), 715–722 (2000). [CrossRef]
14. V. Milanović, G. A. Matus, and D. T. McCormick, “Gimbal-less monolithic silicon actuators for tip–tilt–piston micromirror applications,” IEEE J. Sel. Top. Quantum Electron. 10(3), 462–471 (2004). [CrossRef]
15. A. Arslan, D. Brown, W. O. Davis, S. Holmström, S. K. Gokce, and H. Urey, “Comb-actuated resonant torsional microscanner with mechanical amplification,” J. Microelectromech. Syst. 19(4), 936–943 (2010). [CrossRef]
16. M. Yoda, K. Isamoto, C. Chong, H. Ito, A. Murata, and H. Toshiyoshi, “Design and fabrication of a MEMS 1-D optical scanner using self-assembled vertical combs and scan-angle magnifying mechanism,” in Proceedings of IEEE/LEOS International Conference on Optical MEMS and Their Applications (IEEE/LEOS, 2005), oral B2. [CrossRef]
