A contribution from Wei Ma, H. -Y. Chan, Yick Chuen Chan, Chun Cheong Wong, Francis Chee Shuen Lee from ASTRI in HongKong.
This micro-mechanical scanning mirror has drawn wide attention recently as its combination with diode light sources (e.g., laser diodes) provides a promising solution for mini image projectors, which can be integrated in portable electronics. The combination of a large scanning angle at a high resonant frequency but low actuation power is essential for this application.
Resonant 2D scanning mirrors, whose rotation is actuated by vertical electrostatic combs and can enable the out-of-plane rotation both in x- and y- direction by a gimbal structure , are very attractive in the application of mini image projectors due to their simple driving mechanism and fabrication process. An electrostatically (ES) actuated 2D scanning mirror is shown in Figure 1.
Figure 1: An electrostatically actuated 2D scanning mirror. Insert: A SEM (scanning electron microscope) picture of the 2D scanning mirror
Coventor's Architect software is employed to build a 3D model of the device. A simplified schematic of the model is shown in Figure 2. Mirror plate, supported by a pair of ‘Beam' components, was modeled using ‘Rigid Plate' component from the Architect parts library. The electrical driving force was applied to the mirror by the component ‘Comb Stator'. A 2D model was realized by adding another set of ‘Rigid Plate', ‘Beam' and ‘Comb Stator', which represent the gimbal structure.
Figure 2: Schematic of a 2D scanning mirror.
As it is known that the motion of the mirror is highly nonlinear, Architect is used to simulate the transient frequency response instead of solving complicated ordinary differentiate equation. The mirror response due to a down sweeping frequency of an excitation source is shown in Figure 3.
Figure 3: Transient response of one-axis of the 2D scanning mirror. A frequency down sweep is applied.
Indeed, the response of the mirror to the down and up sweeping frequency is different and there exists hysteresis. This could be predicted by Architect and the results, as shown in Figure 4, are compared with the experimental one measured from a fabricated 2D mirror. It is shown that the Architect model is a fantastic tool for device simulation and further design optimization. An animation of the transient response of a 2D scanning mirror is shown in Figure 5.
Figure 4: Comparisons of Architect's results with the experimental one.
Figure 5. An animation of the transient response.
 Wei Ma, H. -Y. Chan, Yick Chuen Chan, Chun Cheong Wong, Francis Chee Shuen Lee, "Design optimization of MEMS 2D scanning mirrors with high resonant frequencies", MEMS 2010, Hong Kong, Jan. 2010, pp. 823-826.