13. Precision Machinery Using MEMS Technology

Some examples of optical microelectromechanical systems are presented with their fabrication processes. The MEMS-specific driving and display principles and experimental evaluations are also shown. Two types of electrostatic-driven optical MEMS devices: An interferometric display device and an evanescent coupling switching device are shown, in which no liquid crystal or organic electroluminescence materials are required except for a silicon dioxide film with conductive layers.

The interferometric display device consists of a Si reflection mirror at the bottom of an etch pit. On the pit, a SiO₂ film with a metal layer is suspended by two folded leaf springs at both sides, which works as both a half mirror of an interferometer and electrode for positioning. Applying a high voltage between the pit and the half mirror, an electrostatic force acts on two electrodes generating an attractive force to the half mirror. Therefore, the half mirror is drawn to the bottom balancing the repellent force from the leaf springs, which means the distance between the half mirror and the bottom can be controlled by the voltage. Thus, various colors can be generated at the micropit of the interferometer, enabling a microcolor display device. The fabrication process based on the conventional MEMS technology is also described, in which anisotropic etching of the Si is the key technology for forming the flat etch pit of the interferometer.

Another device is based on the evanescent switching between two SiO₂ films. A cantilever,working as a display pixel and an optical pickup from the substrate, is fabricated on an optical waveguide sheet with a separator. The cantilever has a transparent electrode film and it can be attracted and touch down on the substrate when a high voltage is applied between the substrate and the cantilever. Because of the surface roughness of the substrate and cantilever, the two cannot contact perfectly; however, fairly close contact within optical wavelengths can be realized. Therefore, the evanescent light generated by the waveguide sheet can be transferred to the cantilever, resulting in light emission on the cantilever surface as long as it has a high enough roughness for light scattering. The fabrication process using MEMS technology and evaluation of the device are described.