Vertical-actuated electrostatic comb drive with in situ capacitive position correction for application in phase shifting diffraction interferometry

This research utilizes the levitation effect of electrostatic comb fingers to design vertical-to-the-substrate actuation for optical phase shifting interferometry applications. For typical polysilicon comb drives with 2 μm gaps between the stationary and moving fingers, as well as between the microstructures and the substrate, the equilibrium position is nominally 1-2 μm above the stationary comb fingers. This distance is ideal for most phase shifting interferometric applications. A parallel plate capacitor between the suspended mass and the substrate provides in situ position sensing to control the vertical movement, providing a total feedback-controlled system. The travel range of the designed vertical microactuator is 1.2 μm. Since the levitation force is not linear to the input voltage, a lock-in amplifier capacitive sensing circuit combined with a digital signal processor enables a linearized travel trajectory with 1.5 nm position control accuracy. A completely packaged micro phase shifter is described in this paper. One application for this microactuator is to provide linear phase shifting in the phase shifting diffraction interferometer (PSDI) developed at LLNL which can perform optical metrology down to 2 Å accuracy.