Electromagnetic Two-Dimensional Scanner Using ... - IEEE Xplore

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 16, NO. 4, AUGUST 2007

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Electromagnetic Two-Dimensional Scanner Using Radial Magnetic Field Chang-Hyeon Ji, Member, IEEE, Moongoo Choi, Sang-Cheon Kim, Ki-Chang Song, Jong-Uk Bu, Member, IEEE, and Hyo-Jin Nam

Abstract—In this paper, we present the design, fabrication, and measurement results of a two-dimensional electromagnetic scanning micromirror actuated by radial magnetic field. The scanner is realized by combining a gimbaled single-crystal-silicon micromirror with a single turn electroplated metal coil, with a concentric permanent magnet assembly composed of two concentric permanent magnets and an iron yoke. The proposed scanner utilizes the radial magnetic field rather than using a lateral magnetic field oriented 45 to the horizontal and vertical scan axes to achieve a biaxial magnetic actuation. The single turn coil fabricated with electroplated copper achieves a nominal resistance of 1.2 . A two-dimensional scanner with mirror size of 1.5 mm in diameter was fabricated. Maximum optical scan angle of 8.8 in horizontal direction and 8.3 in vertical direction were achieved. Forced actuation of the gimbal at 60 Hz and resonant actuation of the micromirror at 19.1–19.7 kHz provide slow vertical scan and fast horizontal scan, respectively. The proposed scanner can be used in raster scanning laser display systems and other scanner applications. [2006-0081] Index Terms—Displays, lasers, mirrors.

I. INTRODUCTION

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ICROMACHINED scanning micromirrors have become a promising candidate in a wide variety of microphotonic applications due to their high performance, low production cost, small size, and low power consumption. Raster scanning displays [1]–[4], barcode scanners [5], laser printers [6], and confocal microscopy [7] are some of the well-known applications to which high-performance scanning micromirrors can add significant value. With advances in display technology, high-quality laser projection display systems are widely researched in various fields of application due to the exceptionally large color gamut, unprecedented smooth image, and potential for small form-factor. Furthermore, the advent of the mobile era has motivated the development of a portable projection system. The micromirror-based laser scanning display system can be readily applied to these applications because of inherent advantages such as high optical efficiency, relatively simple optics, and potentially small form-factor. Several research groups have successfully demonstrated scanning micromirrors using various actuation mechanisms,

Manuscript received May 1, 2006; revised November 20, 2006. Subject Editor H. Zappe. The authors are with the Devices/Materials Laboratory, LG Electronics Institute of Technology, Seoul 137-724, Korea (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). Digital Object Identifier 10.1109/JMEMS.2007.892897

including electromagnetic [1]–[3], [7], [8], electrostatic [5], [6], [9]–[15], thermal[16], and piezoelectric actuation[4], [17]. Despite the small power consumption, the electrostatic actuators suffer from relatively small displacement and high actuation voltage, which can be overcome by dual vertical combs [13], structural magnification mechanism [14], resonant mode actuation [9], [10], [13]–[15], and vacuum packaging [15]. However, realization of two-dimensional scan, alignment process for vertical comb drive, and forced linear actuation are still major issues for high-performance electrostatic scanners. To overcome these problems, a wide variety of electrical isolation techniques for two-dimensional scanners and self-alignment process for vertical combs have been developed. A large force density at low actuation voltage can be obtained by thermal and piezoelectric actuation. However, unwanted curvature of the reflective surface and reliability issue of the actuator part, due to the heat conduction through structural layer, should be taken into consideration for thermal actuators [16]. Scanners using thin-film piezoelectric materials suffer from complex fabrication process, linearity of actuation, and small displacement [17]. Electromagnetic actuation provides relatively large torque at small voltage, at the cost of increased volume due to permanent magnet integration and larger driving current [1]–[3], [7]. To overcome the limitations of the conventional electromagnetic actuation method, many alternative approaches have been made recently. Instead of integrating the coil to the micromirror and utilizing an external magnetic field, Torashima et al. assembled cylindrical magnets into the bulk micromachined groove formed on the bottom side of the micromirror [8]. In general, at least two electrically isolated metal layers for the coil and optimization of lateral magnetic field direction are required for a two-dimensional magnetic scanner [1], [2]. Yalcinkaya et al. developed a two-dimensional scanner using single current loop and magnetic field oriented 45 to each scan axis [2]. In this paper, design, fabrication, and measurement results of an electromagnetic two-dimensional scanning micromirror are presented. A two-dimensional actuation mechanism using single turn coil and radial magnetic field from concentric magnet assembly is proposed and verified. As the single turn coil does not form a current loop, the coil can be fabricated with a single layer of metal, resulting in a much simple fabrication process. The concentric magnet assembly, composed of cylindrical and annular magnets and an iron yoke, is designed to provide larger radial magnetic field compared to that of the con-

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Fig. 1. A schematic view of the scanning micromirror. (a) Front-side and (b) backside.

ventional magnets [18]. As the magnet assembly is positioned under the micromirror, a relatively compact package, which is a crucial part in portable applications, can be fabricated. The single turn coil fabricated with a single layer of electroplated copper is formed on the gimbal and outer frame of the mirror to generate separated torque to each scan direction, which are large enough to avoid vacuum packaging. II. DESIGN Fig. 1 shows the schematics of the designed two-dimensional scanner. A circular mirror plate is supported by an elliptical reinforcement rim connected to the gimbal. The copper coil is deposited on the front side of the device, while a bulk etched bottom side is used as the reflective surface. The reinforcement rim is designed to relieve the dynamic deformation of the mirror plate during the actuation and to serve as the actuator for the horizontal scan. The micromirror is actuated by interaction between the radial magnetic field and axis-symmetric current flow through the coil [see Fig. 2(a)]. For an axis-symmetric current flow of through a circular path having radius of , the generated torque can be expressed as

(1)

are the magnetic flux, radial component where , , , and of the magnetic flux, permeability of air, and radial magnetic field intensity, respectively.

Fig. 2. Operation principle. (a) Schematics of the coil and concentric magnet assembly (M: magnetization, F: force, T: torque, H: magnetic field intensity, I: current). (b) Schematics of the current path and induced torque for two-di: vertical torque, T : horizontal torque, I : mensional scan (T input current for vertical drive). (c) Top view of the current path, magnetic field distribution, and induced Lorentz force exerted on the mirror and gimbal. The mean circumferences of the reinforcement rim and the gimbal are ellipses. The major and minor axes of the reinforcement rim are 1980 and 1680 m, and those of the gimbal are 3950 and 3110 m, respectively. (d) Simulated normal and radial magnetic field intensity distribution along radial direction.

To obtain a two-dimensional scan having two orthogonal scan axes, an additional current path oriented at right angles to the one shown in Fig. 2(a) is required. Instead of using two different metal layers for each current path, two orthogonally oriented current paths are connected serially by adding redundant return paths at the center [see Fig. 2(b)]. The refined routing of the current path through the gimbal and reinforcement rim enables the generation of orthogonal torque on gimbal and mirror, respectively. The mirror and the gimbal

JI et al.: ELECTROMAGNETIC TWO-DIMENSIONAL SCANNER USING RADIAL MAGNETIC FIELD

can be controlled independently by the separation of actuation mode and frequency. The magnet assembly comprises oppositely magnetized cylindrical and annular neodymium-iron-boron (Nd-Fe-B) magnets and an iron yoke that serves as a return path for the magnetic flux. The radial field intensity of the conventional cylindrical magnet peaks at the edges, whereas that of the concentric magnet assembly is highest at the gap between the two magnets. For concentric magnet assembly, the radius of the inner cylindrical magnet is 1 mm, and inner and outer radii of the outer concentric magnet are 1.25 and 5 mm, respectively. The thickness of the magnets is 1 mm. The radius and thickness of the iron plate are 5 and 1 mm, respectively. As shown in Fig. 2(d), concentric magnet assembly is designed to maximize mm mm . the radial field in the coil region At 0.5 mm above the magnet surface, the maximum radial magnetic field intensity of the designed magnet assembly is 263 kA/m at radius of 1.14 mm, whereas that of the typical cylindrical magnet having radius of 1 mm and thickness of 1 mm is 110 kA/m at radius of 1 mm. In the simulation, Nd-Fe-B magnets with an energy product of 40 MGOe are used. The scanner is actuated biaxially by applying an input current, which is a combination of the two signals with frequencies of the vertical and horizontal scan to the single turn coil. While the mirror is actuated at the resonant frequency for fast horizontal scan, slow vertical scan is achieved by forced actuation of the gimbal at 60 Hz, which is well below the torsional resonance mode of the gimbal. Fig. 3 shows the modal analysis result of the designed micromirror. The first mode is the vertical scan mode at 594 Hz, where all the structures rotate in phase around the torsion beam supporting the gimbal. For horizontal scan with large scan angle at high frequency up to 20 kHz, the sixth mode can be used. At the sixth mode, the micromirror and the reinforcement rim rotate out of phase with the gimbal around the axis, whereas the micromirror rotates in phase with the gimbal at the fifth mode. Although the fifth mode is also a horizontal scanning mode, torsion beams for the gimbal have to undergo a larger bending compared to the sixth mode. Moreover, the quality factor is higher at the sixth mode. The horizontal scan frequency of 19 923 Hz at the sixth mode is high enough for a super video graphics array (SVGA)-grade raster scanning laser display system. The micromirror and the gimbal are designed to respond only to their respective frequency component of the mixed input current. During the large deflection angle scan at high frequency, the mirror plate undergoes dynamic deformation. The reinforcement rim, which is an elliptical rim structure formed at the outer circumference of the micromirror, suppresses the dynamic deformation by constraining the tip deformation of the micromirror. The rim is connected to the micromirror at the torsion beam side and at both tips of the micromirror. Fig. 4 shows the simulation results of the micromirror having optical scan angle of 25.88 at 19.657 kHz. The dynamic deformation of 51 nm was obtained. Instead of increasing the dynamic deformation of the micromirror by adding to the moment of inertia of the micromirror, the rim functions as a separate structure interrupting the free deformation of the micromirror.

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Fig. 3. Finite-element modal analysis result using ANSYS.

III. FABRICATION The fabrication process is a simple combination of the bulk micromachining process and metallization, without any substrate bonding process. Except for the 7- m-thick plated copper coil, all the features are fabricated monolithically with 120- m -thick silicon device layer of the silicon-on-insulator wafer. The thicknesses of the buried oxide layer and handle wafer are 1 and 300 m, respectively. The fabrication process starts with the bulk etch of 300- m-thick handle wafer with 40 wt% potassium hydroxide (KOH) solution at 85 C [see Fig. 5(a)]. A 0.5- m-thick silicon-rich low-stress nitride is used as the etch mask for the bulk etch process and as an insulation layer between the coil and the single-crystal-silicon layer. Subsequent timed bulk etch of the device layer defines the thickness of the torsion beam sustaining the gimbal. The bottom side of the device layer is partly etched with 40 wt% KOH solution at 74 C, to form a 20- m-thick torsion beam [see Fig. 5(c)]. Buried oxide layer, patterned with spray coated negative photoresist, is used as the etch mask for the second bulk etch process. Copper is electroplated on the device layer, using 15.3- m-thick photoresist mold and seed layer formed with chromium and gold [see Fig. 5(d)]. In the final step of the fabrication process, the mobile part of the device is released by silicon deep etch process using another 15.3- m-thick photoresist mask. During the silicon deep etch process, metal layers should be fully covered by the masking material. As the electroplated copper layer has substantial thickness, a thicker photoresist is required to provide sufficient step coverage. Moreover, the thick photoresist, having more

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Fig. 4. Simulated dynamic deformation of the micromirror. (a) Top view and (b) perspective view.

Fig. 5. Fabrication process (cross-section along line AB in Fig. 1); (a) low-stress nitride patterning (backside), (b) first bulk wet etch, (c) buried oxide patterning and second bulk wet etch, (d) copper electroplating (front side), (e) silicon deep etch (front side).

than twice the thickness of the plated copper, can be coated by a simple spin-coating process. Spray-coated photoresist can also be used as the mask material. Fig. 6 shows the scanning electron microscope (SEM) image of the fabricated scanner. The static deformation of the reflective surface was measured using white-light interferometry. A flat surface was obtained with maximum peak-to-valley deformation of 20 and 48 nm for X, Y lines on Fig. 7, respectively. The surface flatness of the fab-

ricated micromirror is affected by the flatness of the connected structures, such as the reinforcement rim and the gimbal. Distortions in these parts can be ascribed to the stress in electroplated copper. Due to the existence of the redundant copper lines across the micromirror, the surface flatness in Y direction is much inferior to that in X direction. Thus, the flatness of the reflective surface can be further improved by the optimization of the electroplating condition and modification of the current routing.


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Fig. 7. Profile of the reflective surface: (upper right) X profile and (lower right) Y profile.

was 1.2 . The relative horizontal position and the gap between the micromirror and the magnet were determined passively by the slots formed inside the housing. The concentric magnet assembly was fabricated with cylindrical and annular Nd-Fe-B magnets attached to a circular plate of pure iron. The gap between the inner and outer magnets was secured by a plastic o-ring, and the total size of the package was 11 11 3.8 mm [see Fig. 8]. The frequency response of the micromirror was measured with a laser Doppler vibrometer (LDV) and dynamic signal analyzer. The velocity of the out-of-plane motion at the tip of the micromirror was measured with LDV. The measurement point was located at the perimeter of the micromirror near the rotational axis of the gimbal [see Fig. 9(a)]. The bode plot in Fig. 9(b) clearly shows the resonant peaks at modes 1, 5, and 6. Mode 1 was also visible, as the measurement point was slightly off the vertical scan axis. Measured resonant frequencies for the vertical and horizontal scan modes were 540 Hz and 19.7 kHz, compared to the designed value of 594 Hz and 19.9 kHz, which was predicted by finite-element analysis using ANSYS. To analyze the discrepancies between the experimental results and the simulation, a new set of simulations was performed using measured parameters of the fabricated micromirror. The second bulk etch depth, which determines the thickness of the torsion beams for gimbal, was 101 m, compared to the designed value of 100 m [see Fig. 5(c)]. Whereas the bulk etch depth could be measured precisely, the actual thickness of the device layer could not be measured with the fabricated device. By assuming a device layer thickness of 119 m, a good agreement between the experimental and simulation results could be obtained [see Table I]. Fig. 6. SEM image of the fabricated scanning micromirror. (a) Backside. (b) Front side. (c) Closeup view of the torsion beam.

IV. RESULTS AND DISCUSSION A. Modal Analysis A fabricated micromirror was wire-bonded to the printed circuit board attached to the plastic cover of the package, assembled with magnet assembly inside the plastic housing, and tested. Measured resistance of the scanner after packaging

B. Scan Angle Measurement The scan angle of the micromirror was obtained by measuring the length of the scanned beam trajectory using He-Ne laser source. The scan angle was calculated with scanned beam length and distance between the micromirror and the screen. The input beam was normally incident on the reflective surface of the micromirror. The estimated step resolution was 0.044 , and accuracy was 0.057 . As shown in Fig. 10(b), a linear relationship between the input current and the scan angle was observed for the vertical scan, and reduction of the gap between the mirror

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TABLE I FINITE-ELEMENT MODAL ANALYSIS RESULT WITH DIFFERENT PARAMETERS

Fig. 8. Packaged two-dimensional scanner; the insert shows the concentric magnet assembly, which is placed under the scanner inside the package.

during the measurement. For a stable resonant mode actuation, a feedback control system with angular deflection sensors is desirable. The gap between the mirror plate and the magnet assembly was 200 m. The horizontal resonant scan frequency was slightly smaller due to the smaller torsion beam thickness of the sample used in the scan angle measurement. While the horizontal scan frequency is compatible with an SVGA-grade raster scanning display system, the scan angle has to be increased in both scan directions. The scan angle for both scan directions can be further increased by reducing the gap between the mirror and the magnet assembly, increasing the actuation current, and further optimizing the current path. As the driving torque of the micromirror is exerted on the coil formed on the reinforcement rim, the horizontal scan angle can be magnified by increasing the size of the rim and optimizing the shape. The maximum torque radius is much larger than current reinforcement rim size. Due to the radial symmetry of the magnetic field distribution, a circular coil with optimized radius can generate larger torque compared to the elliptical coil. An angular deflection magnifying mechanism can be also employed to further increase the scan angle. Compared to the maximum applied current of 131 mA, experimentally determined maximum current was approximately 200 mA. Assuming that the scan angle magnification can be completed by aforementioned measures, the power consumption for a single axis drive can be less than 50 mW, which is a small enough value for a portable projection system. C. Two-Dimensional Scan Pattern Generation

Fig. 9. Frequency response of the mirror plate measured with LDV: (a) measurement point and (b) frequency response.

plate and the magnet resulted in the scan angle increase. Maximum optical scan angle of 8.3 at 118 mA of 60 Hz sinusoidal input was obtained for the vertical scan, and 8.8 at 131 mA of 19.1 kHz was observed for the horizontal scan, at atmospheric pressure. The measurement was carried out at a constant driving frequency of 19.1 kHz. Abrupt change in the scan angle with increasing driving current is due to the resonant frequency drift

Fig. 11 shows the scan patterns of the fabricated device. Onedimensional scan patterns were achieved by selectively applying a sawtooth input of 60 Hz or a sinusoidal input of 19.1 kHz. A two-dimensional scan pattern was obtained by applying a single input signal with a sinusoidal input of 19.1 kHz superimposed on a sawtooth input of 60 Hz. Although two-dimensional scanning with a forced actuation in vertical direction and a resonant actuation in horizontal direction can be readily applied to raster scanning display applications, the designed scanner cannot be used in applications where two degree-of-freedom (DOF) forced actuation of the micromirror is required. To overcome the limitation, two electrically separated current loops oriented at right angles to one another are required. Ahn et al. realized a complete two DOF micromirror actuated by radial magnetic field, using three separated conductor layers including a compensation current path [18]. Another limitation of the fabricated scanner is the volume of the magnet assembly. Whereas the radial dimensions are optimized by simulation, the thickness of the magnets and the


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iron yoke can be further reduced without substantial decrease in magnetic field intensity. V. CONCLUSIONS We have successfully designed, fabricated, and tested a twodimensional electromagnetic scanning micromirror. A two-dimensional actuation mechanism utilizing a single turn coil and radial magnetic field is proposed and verified. A maximum optical scan angle of 8.3 for the 60 Hz vertical scan and 8.8 for the lateral scan frequency of 19.1 kHz at atmospheric pressure are reported. The optimization of the coil layout and the fabrication process are the next steps to further improve the scan angle and the flatness of the reflective surface. ACKNOWLEDGMENT The authors would like to thank Dr. S.-H. Ahn and Dr. H.-K. Yoon for the thoughtful discussion and for their support in the design and characterization. REFERENCES

Fig. 10. Scan angle measurement. (a) Schematic diagram of the measurement setup. (b) Optical scan angle versus applied current (sinusoidal input of 60 Hz and 19.1 kHz are applied, respectively, for the vertical and horizontal scan).

Fig. 11. Scanned beam image. (a) Initial state. (b) Sinusoidal input of 19.1 kHz applied. (c) Sawtooth input of 60 Hz applied. (d) Sinusoidal input of 19.1 kHz superimposed on sawtooth input of 60 Hz.

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Chang-Hyeon Ji (M’06) received the B.S. and M.S. degrees in electrical engineering and THE Ph.D. degree in electrical engineering and computer science from the Seoul National University, Seoul, Korea, in 1995, 1997, and 2001, respectively. His doctoral dissertation concerned the design, fabrication, and testing of electromagnetic micromirrors for microphotonic applications. He joined the LG Electronics Institute of Technology, Seoul, Korea, in 2001. He is currently a Postdoctoral Fellow at Georgia Institute of Technology, Atlanta. His current research interests include surface and bulk micromachining, microfabricated magnetic components, optical MEMS devices, and micro fluidic devices.

Moongoo Choi received the B.S., M.S., and Ph.D. degrees in physics, from Yonsei University, Seoul, Korea, in 1994, 1996, and 2003, respectively. He worked on the development of small form factor optical pickup at Central Laboratory, Samsung Electro-Mechanics, Korea, in 2003. He has been working for Microsystem Group as a Senior Research Engineer since 2004 and currently is a Chief Research Engineer at Integrated Optics Group in Devices and Materials Laboratory at LG Electronics Institute of Technology, Seoul, Korea. He is currently working on the research and development of LED BLU system for LCD display and LED illumination system for consumer electronics. His research interests are mainly in the areas of optical design for imaging and non imaging optical system, spectroscopic characterization of low-dimensional quantum structures like quantum wells or quantum dots, and practical application of microscopic optical phenomena such as photonic crystal and surface plasmon.

Sang-Cheon Kim received the B.S., M.S., and Ph.D. degrees in physics, from Yonsei University, Seoul, Korea, in 1992, 1994, and 2000, respectively. He joined the LG Electronics Institute of Technology, Seoul, Korea, in 2000, where he is currently a Chief Research Engineer. His research interests include the optical design and development of microoptical elements and devices.

Ki-Chang Song received the B.S. and M.S. degree in metallurgical engineering from Hanyang University, Seoul, Korea, in 1988 and 1990, respectively. He joined the LG Electronics Institute of Technology, Seoul, Korea, in 1991, where he is currently a Chief Research Engineer at Integrated Optics Group. He is working on microoptical devices based on MEMS technology. His research interests include silicon micromachining, electronic and optical device packaging, and LED applications for display systems and illumination.

Jong-Uk Bu (M’04) received the Ph.D. degree in metallurgical engineering from Korea University, Seoul, Korea, in 1992. He is the founder and CEO of Senplus Inc. and management consultant at LG electronics, Korea. He has been with LG Electronics Institute of Technology, Seoul, Korea, from 1984 to 2006, where he worked on the area of silicon micromachining and micro sensors. From 1995 to 1996, he has been with the Center for Integrated Sensors and Circuits, Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, as a Visiting Scholar. His research interests include development of microfabrication and micromachining technologies for microsystem; micro sensors, optical communication components, RF-MEMS, and MEMS embedded high-density data storage systems. He has served as a general Co-Chairman of the IEEE MEMS 2007 conference.

Hyo-Jin Nam received the M.S. and Ph.D. degrees in material science from Korea Advanced Institute of Science and Technology (KAIST) in 1988 and 1998, respectively. He joined the LG Electronics Institute of Technology, Seoul, Korea, in 1988, where he has worked on silicon solar cell, FeRAM and microsensors. He is currently the group leader of Microsystem group at LG Electronics Institute of Technology. His research interests include microsensors, optical MEMS, RF-MEMS, NEMS, and probe-based data storage.