Tolerance analysis for comb-drive actuator using DRIE fabrication

Abstract

Deep reactive ion etching (DRIE) process is specially invented for bulk micromachining fabrication with the objective of realizing high aspect ratio microstructures. However, various tolerances, such as slanted etched profile, uneven deep beams and undercut, cannot be avoided during the fabrication process. In this paper, the origins of various fabrication tolerances together with its effects on the performances of lateral comb-drive actuator, in terms of electrostatic force, mechanical stiffness, stability and displacement, are discussed. It shows that comb finger with positive slope generates larger electrostatic force. The mechanical stiffness along lateral direction increases when the folded beam slants negatively. The displacement is 4.832 times larger if the comb finger and folded beam are tapered to +1° and −1°, respectively. The uneven deep fingers generate an abrupt force and displacement when the motion distance reaches the initial overlap length. The undercut reduces both the driving force and the mechanical stiffness of the lateral comb-drive actuator. The fabricated comb-drive actuator, with comb finger of +1° profile and 0.025 μm undercut, and folded beam of −1° slope and 0.075 μm undercut, is measured and compared with the models where both show consistent results. These analytical results can be used to compensate the fabrication tolerances at design stage and allow the actuators to provide more predictable performance.