Study On Fabrication Process Of Self-aligned Vertical Comb Actuator Using Silicon Plastic Deformation

Electrostatic comb actuator is usually used to accomplish in-plane drive, but it is difficult to accomplish the vertical drive. Some foreign scholars fabricate the vertical actuator through double-side aligning and selected etching several times, but the process is very complex and unpractical, because it needs DRIE and CVD several times and it is difficult to accomplish precise aligning.In this paper, the plastic-deformation method is used to design and fabricate a self-aligned vertical comb actuator. A suspension-beam electrostatic comb structure is designed, and there is vertical offset between the movable comb and the fixed comb. It will be driven 12.7um in the vertical direction under voltage 30V in theory. The suspension-structure is fabricated by KOH etching back cavity and Si-Si bonding, and the movable comb and the fixed comb are etched by DRIE. This method accomplishes the isolation between the electrodes, and reduces the fabrication cost a lot due to using non-SOI wafer. And the movable comb and the fixed comb are self-aligned because they are etched through photolithography and DRIE once. The mirror is pushed down by the cover wafer with stages which is etched by KOH, and the beams will be plastic deformed when its maximum stress is larger than its yield stress under high temperature. The movable comb connected with mirror is lower than the fixed comb because of plastic deformed beams, and the vertical offset comb structure is fabricated. In order to improve the aligning precision between two wafers, the stages on the cover wafer which are fabricated by KOH anisotropic etching are used in the precise aligning structure. The silicon wafer with holes is used to fabricate the electrode pad as mask because the electrode pad must be fabricated after releasing structure. Finally, with the process, a vertical offset comb actuator is fabricated successfully, and the value of vertical offset between the movable comb and the fixed comb is 22.1um.The vertical comb actuator using silicon plastic deformation in this paper reduces the cost of fabrication a lot because of using non-SOI wafer, and accomplishes long vertical driving distance. It will be widely used in many MEMS devices in future, such as optical tunable attenuator, optical tunable filter, tunable focus les and so on.