2-DOF Actuated Micromirror Designed for Large DC Deflection

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1 2-DOF Actuated Micromirror Designed for Large DC Deflection Matthew Last, KSJ Pister Berkeley Sensor and Actuator Center 497 Cory Hall, UC Berkeley, Berkeley CA ABSTRACT A 2 degree-of-freedom micromirror has been designed and fabricated in MCNC s Multi-User MEMS Process (MUMPS). The structure uses thermal actuators to actuate both degrees of freedom. We have demonstrated actuator-controlled DC deflection of up to 28 degrees (56 degrees optical) in the axis of rotation that lies parallel to the substrate, and up to 11 degrees (22 degrees optical) in the axis of rotation perpendicular to the substrate. The force-redirecting linkage has been shown to be able to deflect the mirror more than 45 degrees (90 degrees optical) using a probe tip to provide actuation. Keywords: Agile, Beamsteering, Micromirror, 2 Degrees of Freedom, Scanning 1.INTRODUCTION For applications where line of sight is available between a transmitter and a receiver, free-space optical communications systems can present tremendous power savings over their RF counterparts due to their tightly focused beams. This results in extremely high transmitter antenna gain even in small-aperture transmitters such as a MEMS laser transmitter, allowing typical gains of over 66dB above an isotropic antenna. With this kind of transmitter gain, microwatt signals can be sent over multi-kilometer distances with a strong SNR. However, these power limitations are replaced by the necessity of accurate beam alignment with the receiver station. Current beam steering systems, such as those used in building-tobuilding links or satellite crosslinks are bulky, costly, and slow. MEMS agile beamsteerers offer the potential for high levels of integration in small packages. Other large angle beam steering strategies have been suggested. They fall into two categories: scanning mirrors, and diffraction-mode beam steering. Scanning micromirrors have been fabricated, such as those demonstrated by Wu [7] or Conant [3], that achieve large angle deflection of the laser beam up to 28 degrees optical. However, these mirrors operate at resonance, and therefore cannot provide the same magnitude of deflection at other frequencies. Diffraction-mode devices use a micromirror array where each element can translate perpendicular to the plane of the array in order to shift the phase of the incoming light. This has been shown to be capable of steering a HeNe laser beam over up to six diffractive orders for a total deflection of +/-2.5 degrees optical [1]. We take the approach of a single mirror, which has the advantage of greater optical efficiency than is possible using diffractive-mode optics. It also offers a much greater dwell time than is possible using a mirror scanned at resonance since the laser beam is continuously pointing towards the receiver. Although slower than either of the other two alternatives due to its higher mass, it allows for steady pointing over large beam steering angles. Single degree-of-freedom mirrors have been built to provide beamsteering either in the plane of the substrate or out of it; 2 degree-of-freedom mirrors have been built [7] but are less common in the literature.

2.DESIGN DESCRIPTION Mirror 2 nd DOF Torsion Support Structure Substrate Force-redirecting Linkage 1 st DOF Fig. 2.1: SEM of the back side of 2 degree-of-freedom mirror.

It consists of two large plates (500µm x 500µm x 4.25µm) constructed by sandwiching a.75µm oxide layer between the 2µm thick poly1 layer and the 1.5µm thick poly2 layer. MUMPS gold (0.

The right-hand panel is a support structure, which rotates on substrate hinges on an axis in the plane of the substrate.

2 2.DESIGN DESCRIPTION Mirror 2 nd DOF Torsion Support Structure Substrate Force-redirecting Linkage 1 st DOF Fig. 2.1: SEM of the back side of 2 degree-of-freedom mirror. A 2 degree-of-freedom micromirror was designed and fabricated in Cronos Integrated Microsystem s Multi-User MEMS Process (MUMPS) [5]. Fig. 2.1 shows a scanning electron micrograph of the mirror. It consists of two large plates (500µm x 500µm x 4.25µm) constructed by sandwiching a.75µm oxide layer between the 2µm thick poly1 layer and the 1.5µm thick poly2 layer. MUMPS gold (0.5µm thick) was not used as a reflective coating due to its high residual tensile stress; it was decided that a thinner coating could be applied at a later date if necessary. The right-hand panel is a support structure, which rotates on substrate hinges on an axis in the plane of the substrate. The left-hand panel is the actual mirror, and is suspended from the support structure by two U- shaped torsion springs. The suspended structure is actuated through the linkage shown in Fig This linkage is used to redirect the force applied by the actuator 90 degrees and to provide a longer range of motion than what the actuators could normally provide. The force-redirecting linkage has been shown to be able to deflect the mirror more than 45 degrees using a probe tip to provide actuation. Each axis is actuated using 16 parallel Comtois-style [2] thermal actuators. These actuators use the difference in resistance between a thin polysilicon arm and a thick arm to cause differential heating, Suspension Substrate Redirected Force Force from Actuator Fig : Close-up of torsion hinge suspension and forceredirecting linkage. This linkage provides roughly 5 times the range of motion of the unmodified thermal actuators. Fig : Demonstration of actuated micromirror under static optical DC deflection of 22 degrees. Thermal actuators required 4V. From left to right: Incoming beam from HeNe Laser; reflection off support structure (see figure 1); reflection off mirror.

3 resulting in motion due to the different thermal expansion of the two parallel beams. Actuation of the array required up to 65mA at 5V. A test structure consisting of only one thermal actuator was used to determine the voltage at which plastic deformation and eventual device destruction occurs. Plastic deformation was observed starting at 7.98 V (~5mA per actuator), and device destruction occurred at around 9 volts. Neither linearity nor repeatability of the actuators was tested. Using these actuators, we have been able to achieve 28 degrees (56 degrees optical) in the first degree of freedom, and 11 degrees (22 degrees optical) deflection in the second degree of freedom. The thermal actuators were chosen due to their high reliability and the large static forces that they can generate. However, there are several drawbacks to the thermal actuators. They consume huge amounts of power, up to 325 mw, and they have a range of motion of less than 10 µm. The range of motion issue was resolved by using a force-redirecting linkage (described below) to trade force for travel length. The power issues will be dealt with in a future design iteration by replacing the thermal actuators with electrostatic actuators. They are also the bandwidth limit in this system due to their slow rate of heat dissipation. In qualitative measurements, the actuators were able to drive the mirror to more than 200Hz without observable decrease in range of motion. 3.ANALYSIS AND RESULTS In order to rotate the 2 nd degree of freedom 11 degrees mechanically, a displacement of 4.5µm is required from the force-redirecting linkage. This displacement results in an opposing torque of ~2.5e-10 N*m (10µN with a 25µm moment) from the two 200µm long serpentine torsion springs that suspend the mirror plate. Unfortunately, the thermal actuators have a range of motion of less than 10µm, resulting in a maximum deflection of approximately 23 degrees. This would be sufficient assuming that the thermal actuators could supply enough force over their entire range of motion. However, due to the parasitic internal spring constant of the thermal actuators, a marked decrease in the amount of output force is observed at full deflection. It would be beneficial to perform the majority of the work in deflecting the mirror while the displacement of the actuators is still small. A force redirecting linkage was designed to perform this function. Analysis of the design of this linkage is given in the following paragraphs. L 1 F in L spring L 2 F out Fig. 3.1: Point Force Model of force-redirecting linkage shown in Fig An input force F in results in a scaled output force F out. An input force applied to the force-redirecting linkage causes a torque to be exerted at the tip of the minimum-width (2.0µm wide) poly 1 (2.0µm thick) beam of length L spring. The equation for the angle of the tip of the beam with a given torque at the free end is: Θ=L spring *(F in *L 1 )/(E*I) [6] where E is the Young s modulus of the beam material: 150e11 for MUMPS Polysilicon [4]; and I is a geometry factor for the beam, given by: I=w 3 *t/12 Where w is the width of the beam and t is the thickness of the polysilicon layer. Both are 2.0µm in this case. The displacement of the tip of the spring under a pure torque experienced at the tip is given by: Y s =L s 2 *(F in *L 1 )/(2*E*I)

4 Choosing the length L 2 controls the gain of the linkage. The displacement of the end of the beam of length L 2 is given by: Y L2 =L 2 *sinθ The small angle motion gain, A 21, is calculated using: A 21 = (Y s + Y L2 ) / Y in = (Y s + Y L2 )/(L 1 * Θ) In this design, L spring = 125µm, L 1 = 40µm, and L 2 = 140µm, resulting in a calculated motion gain of 4.9. Although the actuators are not intended to be the focus of this paper, a chart of electrical power vs. mechanical deflection angle is useful for visualizing the response of the mirror to input force. The mirror support structure (1 st degree of freedom) has a fair amount of slop due to play in the substrate hinges. This can be seen in Fig when a sudden jump in power is required around 25 degrees mechanical deflection. As power is applied, a sudden unpredictable jump can occur as the base of the mirror shifts in its hinges. Newer designs solve this problem by using torsion hinges in conjunction with staples to the substrate to create a predictable, repeatable motion (see Fig 3.3.2). actuator power (mw) degrees 1st DOF 2nd DOF mw degrees 1st DOF Fig 3.3.1: Power vs Mechanical Deflection Angle Fig 3.3.2: Power vs Mechanical Deflection Angle: 1st DOF, torsion mirror 4.Discussion In this paper, a 2 degree-of-freedom micromirror fabricated in the Cronos Integrated Microsystems MUMPs process is described. This micromirror is capable of large DC deflections (up to 45 degrees), limited only by the MUMPs actuators. Actuated deflections of up to 28 degrees in the axis parallel to the substrate and 11 degrees in the axis perpendicular to the substrate have been demonstrated using comtoisstyle thermal actuators. The end goal of this line of investigation is to develop a system capable of pointing a laser beam anywhere within a steradian volume. Currently, this system can steer a laser beam over a solid angle of steradians. If powerful enough actuators were available, this structure would be able to cover over 1.75 steradians. The diffraction-limited spot size of this mirror is under 1.6mrad. In order to be able to point the laser at any spot in a full radian in each axis, we need 10 bits of resolution. This sets an upper bound on the noise we can allow in our actuators. Current work focuses on a) extending the range of motion the micro-mirror is capable of, b) integrating a laser diode and collimating optics with the micromirror, and c) investigation of assembled structures to combine optimized structures on a single substrate. An example of this is the recent demonstration of the transfer of an ultra-flat electroplated mirror onto a MUMPS polysilicon hinged mirror by gold-bump bonding [8]. By extending this work, we hope to be able to transfer DRIE-fabricated actuators onto MUMPS chips in order to obtain high-quality electrostatic actuators for this two degree-of-freedom mirror.

5 5.BIBLIOGRAPHY 1. Burns, D., Bright, V., Gustafson, S. et al., Optical Beam Steering using Surface Micromachined Gratings and Optical Phased Arrays, presented at Proc. SPIE, San Diego, CA Comtois, J.H., Bright, V.M., Thermal microactuators for surface-micromachining processes, in Proc. SPIE, vol. 2642, pp , Conant, R., Hagelin, P, Krishnamoorthy, U, Solgaard, O, Lau, KY., Muller, R., A Raster-Scanning Full- Motion Video Display Using Polysilicon Micromachined Mirrors, in Transducers 99 Digest of Technical Papers, Vol. 1, pp., MEMS Material Properties Database, MEMS Clearinghouse, 5. Cronos Integrated Microsystems, Inc, Research Triangle Park, North Carolina 6. Young, Warren C., Roark s Formulas for Stress and Strain, 6 th ed., p.105. McGraw-Hill, W. Piyawattanametha, L. Fan, S. S. Lee, G. D. Su, and M. C. Wu, "MEMS Technology for Optical Crosslinks for Micro/Nano Satellites," International Conference on Integrated Nano/Microtechnology for Space Applications, Houston, Texas, November 1-6, Maharbiz, M.M., Howe, RT., Pister, KSJ, Batch Transfer Assembly of Micro-Components Onto Surface and SOI MEMS, in Transducers 99 Digest of Technical Papers, Vol.2, pp

Optic fiber cores. mirror. fiber radius. centerline FMF. FMF distance. mirror tilt vs. necessary angular precision (fiber radius = 2 microns) 1.

Optic fiber cores. mirror. fiber radius. centerline FMF. FMF distance. mirror tilt vs. necessary angular precision (fiber radius = 2 microns) 1. Comparative Study of 2-DOF Micromirrors for Precision Light Manipulation Johanna I. Young and Andrei M. Shkel Microsystems Laboratory, Department of Mechanical and Aerospace Engineering, University of