Novel micromirror design with variable pull-in voltage

Abstract

A novel micromirror is presented having a variable pull-in voltage. This allows analog Pulse Width Modulation (PWM) so that the number of grey levels is not limited by the number of subframes or bitplanes. The levels can be chosen arbitrarily, allowing less severe speed requirements for the electronic layer below the MEMS, less image processing hardware and memory. The presented design of the mirror has two attracting electrodes and one landing electrode at either side of the mirror. One electrode is used as ‘fixed’ electrode, influencing the other attracting electrode’s pull-in voltage. If we apply a general triangular waveform at the former electrode, the ‘fixed’ electrode voltage determines the duty cycle of the mirror. The mirror implements analog PWM, without needing a transistors based comparator at the CMOS level. When each ‘fixed’ electrode is connected to an active matrix cell, an active matrix display can be formed. The mirrors with variable pull-in voltage were fabricated using SiGe as the structural layer. The variable pull-in principle is demonstrated by measurements performed on these SiGe mirrors [1].