-
! " # $
%An inclined Micromirror with Rotating Motion& '(NSC 88-2218-E-009-008 )*+,87.08.01~88.07.31 -./012 345678#9:;<= >.? @1AB ! "#$%&'()*)+,- ./ "#012345 67 8 9::;<= >?@ ABC./D1EFGH I0JK ;ABL M;NOP QRST :U/ D1EF GH I MVWXYZ [\ <0J]^ _U/`abc defgfh ijk 0J`a lmnopqrJs< t0 ujv w xyz C1AB
In this project, an inclined micromirror with rotating motion was designed and fabricated. Firstly, polyimide was spun on the plate used as the micromirror. After being heated, the polyimide shrunk and tilted the micromirror. The micromirror was constructed on a hollow disk, and the hollow was actuated by a laterally driven mircoactuator. If the hollow disk was
actuated, the inclined micromirror rotated up to 360 degrees. The used microactuator was improved from the old design. Through the use of numerical finite-element program ANSIS53, the microactuator was analyzed. From the simulation, that the improved microactuator had a displacement larger than the old design by 0.6 time was observed.
. DE%Introduction& Micromirrors attract lots of attentions in
Micro-Electro-Mechanical Systems (MEMS) for possible application in minimizing optical device. Different types of micromirrors have different applications. For example, a stationary micromirror can be used as a light or wave guide in optical instruments by reflecting light or wave to the desired direction. Moreover, a movable micromirror can act as an optical switch. For a moving micromirror with high frequency, it can be used as an optical signal modulator. When lots of torsional
mircromirrors are set in an array, they can be applied to a printing or projection display device.
A stationary micromirror usually contains a reflecting surface which can be easily made by anisotropic wet etching of silicon. For a movable micromirror, it needs some actuation mechanisms. Selecting proper actuation mechanisms depends on its application. Here we are interested in designing a micromirror with large rotating angle to reflect light into a wide range at low frequency.
In this project, a micro mechanism for a micromirror to rotate a full circle (up to 360 degrees) at a low frequency is proposed. The rotating axis of the mirror is inclined to the mirror surface. When the micromirror rotates, the light injected along the rotating axis to the micromirror can be reflected to the periphery of the micromirror. The micromirror can be used in the endoscope or other optical scanning applications.
The micromirror is originally constructed in the same layer with a hollow disk. By applying the techniques for bending 3-D structures out of silicon wafer plane, the micromirror is then set to be inclined to the hollow disk. An improved thermally driven actuator is employed to rotate the hollow disk. Four masks will be used and no additional assembling process is needed in the fabrication of the micromirror.
F. GH
Concept Design
The schematic diagram of the proposed micromorror is shown in fig.1. As the diagram shown, the inclined micromirror with rotating motion is based on the vibrimotor structure. A thermally driven actuator with lateral motion is employed to rotate the hollow disk. As the hollow disk rotates, so the inclined micromirror on the disk.
Fig.1.illustration of the inclined micromirror with rotating motion
In order to incline the micromirror, the technology of bend 3D structure out of silicon wafer plane is adopted. The polyimide is employed in our design. The ployimide is put on the plate between the micromirror and the hollow disk. When the polyimide is heated, it shrinks to tilt the micromirror. No additional hand assembly is needed and batch fabrication is possible. The schematic diagram shown below illustrates the principle.
Fig.2.The polyimide is heated and tiles the beam used for micromirror up
An improved thermally driven microactuator with lateral motion
For activating the micromirror proposed in this project, the thermally driven microactuator is employed because of its large output force. In past years, two types of thermal laterally driven microactuators had been presented. One type of the microactuator which could produce motion via the asymmetrical thermal expansion of a microstucture with variable cross sections was proposed by Guckel et al in 1992 [11]. The other type of the microactuator due to the asymmetrical thermal expansion of the microstructure with different lengths of two beams was presented by Pan and Hsu in 1997 [12]. Combining the operation principles of two microactuators mentioned above, an improved thermally driven microacutator is proposed here.
(a)by Guckel (b)by Pan (c)improved design Fig.3.The models for FEM analysis
Fig.4.The analysis of comparing three kinds of microactuators
I.JKLM
Sticking is serious problem in the surface micromaching. After removing the sacrificial layer, the microactuator sticks with the silicon substrate easily. But only a few voltages are applied, the microactuator is actuated and released from the substrate. The microactuator was tested in air by applying an ac triangular voltage. The frequency of the input voltage was between 1~10 Hz. The measured results is shown in table.1.
Table.1.The displacement of the microactuator with difference voltages
Voltage 4 5 8 10 12 Displacement
(um)
2 4 6 10 18
In order to set an inclined micromirror, how the polyimide tilts the beam used for micromirror should be investigated. For the reason, several beams with spun-on polyimide were made up. A sample of micrograph of the inclined beams was shown in figure.5. The Detailed results are shown in table.2.
Fig.5. The micrograph of inclined beams
0. 00 5. 00 10. 00 15. 00 20. 00 5 1 0 1 5 20 25 V olta ge (V ) Displacement (um)
Table.2.The inclined degrees of the test beam (unit degrees) Polyimide (7.5um thick) Az4620 (3.2um thick)
2grooves 1grooves 2grooves 1grooves
45 23 32 33 39 39
38 24 30 32 51 51
42 36 32 40 40 41
44 30 32 31 42 50
N.OPQR
[1] K.E Petersen, “Silicon torsional scanning micromirror”, IBM J Res. Dev., 24 (1980), 631-637
[2] V.P. Jaecklin and C. Linder,
“Line-addressable torsional micromirrors for light modulator
arrays”, Sensors and Actuactors, A 41-42 (1994) 324-329
[3] Jong-Woo Shin, “Design and fabrication of micromirror array supported by vertical Springs”, Sensors and Actuators, A66 (1998) 144-149 [4] W.-H Juan. “High-Aspect-Ration Si
Vertical Micromirror Arrays for Optical Switching”, J. MEMS. Vol. 7, No. 2, (1998) 207-213
[5] Meng-Hsiung Kiang, “Electrostatic Combdrive-Actuated Micromirrors for Laser-Beam Scanning and Positioning”, J. MEMS, Vol. 7, No. 1 (1998) 27-37 [6] R. A. Buser, “Biaxial scanning mirror
activated by bimorph structures for medical Applications”, Sensors and Actuators A. 31 (1992) 29-34
[7] J. Bühler, “Thermally actuated CMOS micromirrors”, Sensors and Actuators
A 46-47, (1995) 572-575
[8] Abraham P. Lee, Per B. Ljung, and Albert P. Pisano, “Polysilicon Micro Vibromotors”, Micro Electro Mechanical Systems’ 92, February 4-7, 1992.
[9] John H. Comtois, “Applications for surface-micromachined polysilicon thermal actuators and arrays”, Sensors and Actuators A58. (1997) 19-25
[10] Kenji Suski, “Insect-Model Based Microrobot with Elastic Hinge”, J. MEMS. Vol. 3, No. 1 (1994) 4-9
[11] Guckel H, “Thermo-magnetic metal flexure actuators”, Technical Digest. IEEE Solid State Sensors and Actuator Workshop (Hilton Head Island, SC) pp 73-5
[12] Chi Shiang Pan, “An
electro-thermally and laterally driven polysilicon microactuator”, J. Micromech. Microeng. 7 (1997) 7-13 [13] K. S. Pister, M. W. Judy, S. R. Burgett,
“Microfabricated hinges”, Sensors and Actuators A. 33 (1992) 249-256
[14] Kazuo Hosokawa, Isao Shimoyama, Hirofumi Miura. “Two dimensional mirco-self-assembly using the surface tension of water”, Sensors and Actuators A. 57, (1996) 117-125.
[15] R. R. A. Syms, “Equilibrium of hinged and hingeless structures rotated using surface tension forces”, Journal of microelectromechanical systems. Vol. 4, No. 4 December 1995
[16] Kenji Suski, Insect-Model Based Microrobot with Elastic Hinge, J. MEMS. Vol. 3, No. 1 (1994) 4-9
