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magnetostatic 2-axis MEMS scanner

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  • 日期: 2015-06-07
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W3P.099 A MAGNETOSTATIC 2-AXIS MEMS SCANNER WITH I-SECTION RIB-REINFORCEMENT AND SLENDER PERMANENT MAGNET PATTERNS Hsin-Yu Huang1, Tsung-Lin Tang1, Wei-Lun Sung1, Hung-Yi Lin3, and Weileun Fang1,2 1 Dept. of Power Mechanical Engineering, 2 Inst. of NanoEngineering and MicroSystems, National Tsing Hua University, Hsinchu, Taiwan 3 Touch Micro-system Technology Corporation, Taoyuan, Taiwan ABSTRACT This study demonstrates the 2-axis epitaxial silicon scanner driven by the coil-less magnetostatic force using electroplated permanent magnet (CoNiMnP) film. The present approach has four merits: (1) the process employs the cheap silicon wafer with epitaxial layer; and the electrochemical etching stop technique is used to precisely control the thickness of scanner; (2) the I-section rib-reinforced structure is implemented to provide high stiffness of the mirror plate; (3) the magnetostatic driving force on scanner is increased by electroplated permanent magnet film with slender patterns; (4) the size of packaged scanner is reduced since the assembled permanent magnets are not required. KEYWORDS Optical scanner, Magnetostatic actuation, I-section rib-reinforcement, permanent magnet film, Electroplating INTRODUCTION The 2-axis MEMS scanner is a key enabling component for the compact display system [1], laser printer [2], spectroscopy [3], and barcode scanning [4]. In general, the 2-axis MEMS scanner employs micro-actuators to manipulate incident light in two orthogonal directions. Thus, the design considerations of the scanner include high scanning speed, high resolution, small feature size, and low power consumption requirements in such demanding application. The SOI substrate with deep reactive ion etching (DRIE) process is extensively employed to implement MEMS scanners [5]. Nevertheless, cost is a major concern for such technique. In [6], the low cost p-type silicon wafer with n-type epitaxial silicon layer (named epi-wafer) has been employed to realize a large size flat mirror plate using the electrochemical etching stop (ECES) technique. However, the stiffness and moment of inertia are trade-off for the design of mirror thickness. The electromagnetic forces, such as Lorentz force and magnetostatic force, have been exploited to drive MEMS scanner [1, 4, 5]. However, the process issues for electrical routing, and the problem of joule heat can not be ignored for Lorentz force scanner [7]. On the other hand, the magnetostatic force can be easily induced by coating a ferromagnetic material on the scanner [5, 8]. Thus, the electrical routing is not required. However, the permanent magnet is required to ensure the magnetization direction of the ferromagnetic material. Hence, it remains a challenge to shrink the package size for scanners driven by magnetostatic force. This study exploits the simple ECES process to implement the scanner with I-section rib-reinforced structure on an epi-wafer. Hence, the stiffness of mirror plate is increased. Moreover, this study also implements the electroplated permanent magnet film with slender patterns on the scanner, and the magnetization direction of the hard magnet material (CoNiMnP) is fixed. Thus, the assembled permanent magnet in [5] is no longer required, and the package size of the scanner is reduced. CONCEPT AND DESIGN This study employs the epitaxial silicon (epi-Si) to realize the MEMS scanner. The ECES technique is thus used to release single crystal silicon scanner and precisely control its thickness [6]. Fig. 1 illustrates the schematic design concept of the scanner. Fig. 1(a) shows the front side view of the 2-axis scanner on epi-wafer. The scanner consists of the mirror plate connects to the supporting frame through one pair of torsional springs (fast scan spring), and the supporting frame connects to the anchor through another pair of torsional springs (slow scan spring). The permanent magnet (CoNiMnP) film is electroplated on the mirror plate and supporting frame for magnetic actuation. Fig. 1(b) illustrates the backside view (a) Fast scan spring Permanent magnet film Mirror plate Supporting frame (b) Slow scan spring Epi-Si diaphragm I-section rib Figure 1: Schematic diagram of (a) front side, and (b) backside views of the scanner design on the epi-wafer. 978-1-4577-0156-6/11/$26.00 ©2011 IEEE 2366 Transducers’11, Beijing, China, June 5-9, 2011 Fast scan axis (a) (b) Vertical pattern y y xz xz Slow scan axis Horizontal pattern Figure 2: Orthogonal electroplated slender permanent magnet patterns for the fast and slow scan axes. of the scanner. It shows the scanner with I-section ribs suspended on a cavity etched by anisotropic ECES process. The I-section rib structures made of the Si substrate are employed to increase the stiffness of mirror plate. The ECES process is also employed to precisely control the epi-Si diaphragm. As indicated in Fig. 1(a), the permanent magnet film is patterned to slender rectangles to increase the shape anisotropy so as to further enhance magnetization [5]. This study designs permanent magnet patterns to induce the magnetostatic force for the actuation of the 2-axis scanner. As indicated in Fig. 2, the mirror plate is covered with horizontal permanent magnet patterns for the actuation of fast scan axis; and the supporting frame is covered with vertical permanent magnet patterns for the actuation of slow scan axis. Fig.3 shows the driving scheme for the present 2-axis scanner. A time varying magnetic field was produced when an ac current with driving frequency is introduced into the solenoid. Moreover, the solenoid can be driven by two function generator, so that it can provide the ac magnetic field with multiple excitation frequencies. The 2-axis scanner is actuated by the solenoid with input ac signals consisting of two frequencies respectively for slow scan and fast scan. Scanning pattern Laser Magnetic field Solenoid Function generator Adder Function generator Figure 3: Schematic illustration of the experiment setup for scanner driving test. Figure 4: Modal analysis results by Finite-element (ANSYS), (a) 1st mode for slow scan: 1,078 Hz, and (b) 6th mode for fast scan: 21,119 Hz. Fig. 4 shows the modal analysis result of the designed scanner. The first mode is the slow scan mode at 1,078 Hz, where the mirror plate rotates about the slow scan springs (Y-axis scanning). The sixth mode of the scanner with a frequency of 21,119 Hz is employed for the fast scan mode. The mirror plate rotates about the fast scan springs (X-axis scanning). FABRICATION AND RESULTS The process steps shown in Fig. 5 have been established to realize the presented scanner. The process started with a (100) epi-wafer which had a 50µm thick n-type silicon epitaxial layer on top of a 400µm thick p-type silicon substrate. As shown in Fig. 5(a), a SixNy layer (150nm) was deposited on the wafer by low pressure chemical vapor deposition (LPCVD). After that, a 200nm thick Cr layer was deposited onto the backside of the epi-wafer by electron beam evaporation. As illustrated in Fig. 5(b), the backside SixNy and Cr layers were simultaneously patterned to define the etching window for the subsequent wet etching and DRIE processes. As depicted in Fig. 5(c), a lift-off process was employed to pattern a 100nm thick evaporated Al layer to increase the reflectivity of the mirror plate. Fig. 5(d) shows the 20nm thick Ti adhesion layer and the 200nm thick Au seed layer were deposited onto the top side of an epi-wafer for the following electroplating process. These Ti/Au layers also acted as the electric conduction layers for the subsequent ECES process. A 12m thick photoresist was patterned to act as a mold, and then a 10µm thick CoNiMnP film was electroplated and molded using the photoresist [9], as illustrated in Fig. 5(e). As depicted in Fig. 5(f), a time-controlled DRIE process was employed to define the thickness of the p-type Si layer (30m in this study) for the I-section rib structure. As shown in Fig. 5(g), the anisotropic ECES process was employed to precisely control the thickness of epi-Si diaphragm [6] and to form the I-section rib structure as well. Finally, as indicated in Fig. 5(h), the epi-Si diaphragm was patterned by a DRIE process to release the scanner. The scanning electron microscope (SEM) micrographs in Fig. 6 show a typical fabricated scanner. Fig. 6(a) shows the front side view of the whole device consisting of mirror plate, supporting frame and torsional springs. The zoom-in micrograph in Fig. 6(b) shows the distribution of slender CoNiMnP patterns. The vertical 2367 (a) (e) (b) (f) (c) (g) (d) (h) p-type Si n-type Si SixNy Cr Al Ti/Au magnet Figure 5: Fabrication process steps of the 2-axis scanner with electroplated permanent magnet film and I-section rib-reinforced structure. (a) (b) shown in Fig. 7(a), the material composition of electroplated permanent magnet film was analyzed by Energy Dispersive Spectrometer (EDS). The magnet film is composed of Co: 74.5 %, Ni: 19.4 %, P: 3.9 % and Mn: 2.2 % in weight percentage scale. It indicates the major composition of this material is cobalt. The magnetic properties of the permanent magnet film are characterized with a vibrating sample magnetometer (VSM). The typical demagnetization hysteresis loops for the lateral direction (in-plane) is shown in Fig. 7(b). During electroplating, external magnetic field was applied parallel to the substrate plane, and the alloy with a magnetic coercivity of 550 Oe, a retentivity of 0.58 Teslas, and a maximum energy density of 8.8 kJ/m3. Fig. 8 shows the driving test of the scanner to demonstrate the 2-axis scanning. Fig. 8(a) shows the driving stage established in this study for scanning test. The scanner fixed to the stage is observed. The solenoid embedded in the stage provides the magnetic field to drive the scanner. The magnetostatic force is induced by electroplated hard magnet (CoNiMnP) film on the scanner. Since permanent magnets on the stage are not required, the size of packaged scanner is relatively small, as compared with the scanner in [5]. Fig. 8(b) shows the typical measured frequency response of the 2-axis scanner. The two orthogonal scanning resonant modes were 1.1 kHz (slow scan) and 21.4 kHz (fast scan), respectively. It indicates the orthogonal permanent magnet patterns respectively induce magnetostatic force on the mirror plate and the supporting frame. This study also demonstrated the scanning pattern using the 2-axis (a) (c) (d) Figure 6: SEM image of the fabricated scanner, (a) front side view, (b) vertical and horizontal electroplated permanent magnet patterns, and (c-d) I-section rib structure. and horizontal patterns are electroplated on the supporting frame and the mirror plate respectively. The SEM micrographs in Fig. 6(c)-(d) show the I-section rib-reinforced structure at the backside of mirror plate. EXPERIMENTS AND DISCUSSIONS Fig. 7 shows the characterization of the electroplated permanent magnet (CoNiMnP) films in this study. As (b) 1.5 1.0 4M (Teslas) 0.5 0.0 -0.5 -1.0 -1.5 -4000 -2000 0 2000 4000 H (Oersted) Figure 7: Characterization of the electroplated magnet, (a) Energy dispersive spectrum (EDS), and (b) 4πM-H hysteresis loop. 2368 (a) (c) -120 1st mode: -130 1.1 kHz Magnitude (dB) -140 5mm -150 (c) -160 6th mode: 21.4 kHz -170 -180 10mm 0 5 10 15 20 25 Frequency (kHz) Figure 8: Driving test for the magnetostatic 2-axis scanner with electroplated permanent magnet film, (a) the scanner packaged in the driving stage with embedded magnetic coils, (b) the frequency response, and (c) typical 2D scanning pattern. scanner, as shown in Fig. 8(c). Such typical 2D scan pattern was produced after applying sinusoidal ac currents (root-mean-square value of Irms = 400 mA) with frequencies of 60 Hz and 21.4 kHz to the solenoid. The optical scan angles are respectively 7.8 in horizontal direction and 6.9 in vertical direction. CONCLUSIONS This study presents the design and implementation of the magnetostatic 2-axis epi-Si scanner driven by the electroplated permanent magnet (CoNiMnP) film. The CoNiMnP film is electroplated on the top side of the 2-axis scanner with orthogonal slender patterns respectively for the fast scan and slow scan axes. Since the assembled permanent magnets are not required to drive the presented scanner, the package size is significantly reduced. Moreover, the thickness of the epi-Si diaphragm and I-section rib-reinforced structure at the backside of the mirror plate are implemented by ECES process. In application, this study demonstrates the 2D scan pattern of 7.8 in horizontal direction and 6.9 in vertical direction after applying harmonic excitations of Irms = 400 mA (root-mean-square value) and frequencies of 60 Hz and 21.4 kHz to the solenoid. ACKNOWLEDGMENTS This study was partially supported by the National Science Council of Taiwan under grant of NSC-96-2628E-007-008-MY3. The authors would like to appreciate the Center for Nanotechnology, Materials Science and Microsystems of National Tsing Hua University, the Nano Facility Center of National Chiao Tung University, the National Nano Device Laboratories, and the NEMS Research Center of National Taiwan University for providing the fabrication facilities. REFERENCES [1] A. D. Yalcinkaya, H. Urey, D. Brown, T. Montague, and R. Sprague, “Two-axis Electromagnetic Microscanner for High Resolution Displays,” IEEE Journal of Microelectromechanical Systems, vol. 15, pp. 786-794, 2006. [2] W. O. Davis, D. Brown, M. Helsel, R. Sprague, G. Gibson, A. Yalcinkaya, and H. Urey, “High-performance silicon scanning mirror for laser printing,” Proc. of SPIE, vol. 6466, 64660D, 2007. [3] C. Ataman, H. Urey, and A. Wolter, “A Fourier transform spectrometer using resonant vertical comb actuators,” Journal of Micromechanics and Microengineering, vol. 16, pp. 2517-2523, 2006. [4] A. D. Yalcinkaya, O. Ergeneman, and H. Urey, “Polymer magnetic scanners for bar code applications,” Sensors and Actuators A, vol. 135, pp. 236-243, 2007. [5] T.-L. Tang, C.-P. Hsu, W.-C. Chen, and W. Fang, “Design and implementation of a torque-enhancement 2-axis magnetostatic SOI optical scanner,” Journal of Micromechanics and Microengineering, vol. 20, 025020, 2010. [6] C. Lee, “Design and fabrication of Epitaxial Silicon Micromirror Devices,” Sensors and Actuators A, vol. 115, pp. 581-590, 2004. [7] T. Mitsui, Y. Takahashi, and Y. Watanabe, “A 2-axis optical scanner driven nonresonantly by electromagnetic force for OCT imaging,” Journal of Micromechanics and Microengineering, vol. 16, pp. 2482-2487, 2006. [8] A. D. Yalcinkaya, H. Urey, and S. Holmstrom, “NiFe Plated Biaxial MEMS Scanner for 2-D Imaging,” IEEE Photonics Technology Letters, vol. 19, pp. 330-332, 2007. [9] H. J. Cho, J. Yan, S. T. Kowel, F. R. Beyette, Jr., and C. H. Ahn, “A scanning silicon micromirror using a bi-directionally movable magnetic microactuator,” Proc. of SPIE, vol. 4178, pp. 106-115, 2000. CONTACT * W. Fang, Tel: +886-3-5742923; 2369




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