Comparison of the curvature homogeneity and dynamic behaviour of framed and frameless electrostatic X/Y scanning micromirrors

Experimental investigations on the influence of a frame structure around an electrostatic comb-drive actuated micromirror are presented. The comparison between two 800 mm diameter mirrors, one framed and the other frameless, fabricated with a multi-user silicon-on-insulator process was carried out in relation to the dynamic movement behaviour and the static and dynamic curvature of the mirror surfaces. Both mirror types used in the study were carefully chosen to have a similar diameter and torsional resonant frequency of the order of 7 kHz and are actuated with 10 pairs of comb-fingers. The inclusion of the frame structure is shown to increase the homogeneity of the curvature of the mirror, with a difference of radius of curvature between the main axes of 1% compared to a 10% difference for the frameless mirror. The frame also increases the achievable maximum resonant tilt angle around the second main axis by a factor of three, at the same time avoiding nonlinear frequency responses (such as hysteresis) in the tilt angle against frequency curve.