We have set up a LPCVD system enabling us to deposit ultra low stress single layer silicon-rich nitride film at high temperature with fast deposition rate for micro-structures fabrication. Silicon-rich nitride films deposited at high temperature have ultra low stress and are relatively independent of silicon and nitrogen containing gas-flow ratio during deposition. Deposition process parameters were optimized employing Taguchi method and no post deposition process is required to obtain low stress films. Detailed study of the effects of deposition parameters on film properties is also presented. The high temperature deposited ultra low stress silicon rich nitride film is resistant to all commonly used silicon anisotropic etchants, an ideal material for various micro-structures fabrication.
In this paper, a new method which simplifies the design process of micromachined deformable mirrors is presented. By varying the widths of an array of constant-pitched electrodes, the electrostatic-force profile needed to shape the mirror can be precisely controlled using only one voltage input. In the past, either several independent voltages were necessary or, if only one voltage was available, numerical schemes were required to search for the optimal sizes and locations of a few electrodes. A mirror is formed by a thin membrane micromachined from a silicon wafer and is coated with a thin metallic film. The electrodes are deposited on a ground plane over which the membrane is suspended. Viewing the mirror as a surface composed of many small patches with the same pitch, we can calculate the average force of each patch from the deformed shape using basic elasticity formulae. Using conformal mapping method, we can solve the analytical solution of the electrostatic field between the mirror and the electrode in one pitch. The relationship between the force and the width of the electrode is established. Finally, the widths of all the electrodes are obtained, and this new method applies equally well to the designs of both membrane electrostatic actuators and capacitive sensors.