Vacuum operation of comb-drive micro display mirrors

In this study, we investigate operation characteristics of a comb-drive torsion micro-mirror in vacuum. Three types of the micro-mirror of diameter 420 µm have been designed and fabricated for resonant operation at a high frequency suitable for laser scanning display. The first type (type I) is a conventional comb-drive micro-mirror with the comb fingers installed on both sides of torsion bars. The second type (type II) is a micro-mirror with slanted comb-drive structure. The third type (type III) is the same as type I except for additional comb fingers installed on a mirror plate. In the fabrication process, three masks are used in double-side lithography of a silicon-on-insulator (SOI) wafer. The fabricated micro-mirrors have the torsion resonant frequencies of 18.4 kHz, 24.5 kHz and 14.5 kHz, respectively. The optical scan angle of 5.14° is achieved for type II at a 24.5 kHz resonant frequency and 160 V actuation voltage in atmospheric air. On the other hand, due to the operation in vacuum, the rotation angle of type II increases 52 times more than that in atmospheric air, which corresponds to the decrease of operation voltage by a factor greater than 7 at the 24.5 kHz resonant frequency. The quality factor of 9735 is obtained at a pressure below 7 Pa for type II. The effects of the modified combs of the three types of mirror have been experimentally investigated by changing the environmental pressure.