To use a scanning mirror for laser display, the size of the mirror must be larger than that of the laser beam and the laser beam must be scanned linearly with analog operation to produce an undistorted image. To control a scanning mirror of such a large size, a large driving force is also required. As a method of satisfying such requirements for laser display, a 1500μm × 1200 μm silicon scanning mirror having vertical comb fingers has been designed, fabricated and characterized.
It is a vertically driven electrostatic actuator and an electrostatic force is generated by the interdigitated comb fingers. It is composed of two structures having vertical comb fingers. The upper structure is composed of a scanning mirror plate, two torsion bars, a supporting frame and vertical comb fingers (moving electrodes). The lower structure is composed of vertical comb fingers (fixed electrodes), a supporting frame and gold signal lines and pads on the Pyrex glass substrate.
The fabrication processes of a scanning mirror are mainly composed of three steps : fabrication of the upper structure, fabrication of the lower structure and assembly of the upper and lower structures. The vertical comb fingers of the upper and lower structures were fabricated by deep inductively coupled plasma reactive ion etching (ICPRIE). By flip chip bonding, the upper and lower structures having vertical comb fingers were assembled. In this assembling process, electroplated AuSn was used as an eutectic bonding material with a melting temperature of 278℃. To improve the reliability of the scanning mirror, a silicon-on-insulator (SOI) wafer was used and the uniform thickness of torsion bars was obtained by using a buried oxide layer as an etch stop.
The open loop responses were measured using a laser doppler vibrometer (LDV) according to the input signal wave forms and the deflection angle was measured according to the control voltage with the dc bias voltages. As a result, the scanning mirror was controlled perfectly by the ramp wave form of 60 Hz with the duty cycle of 90 % and showed a very linear actuating performance. We obtained the resonant frequency of 1. 353 kHz, and the scan angle of 12° (optical) when driven by the 28 V ac control voltage of a 60 Hz sine wave with the 35 V dc bias voltages.
Using two scanning mirrors, a two-dimensional laser vector graphic video image, which was drawn from point to point, was obtained successfully. Vector image signals from the graphic board were applied to two scanning mirrors, and a SHG green laser was directly modulated to shape independent graphic images.
In conclusion, this scanning mirror can be used as a galvanometric vertical scanner to make the laser display system compact and effective.
