<title>Characterization and optimization of deep dry etching for MEMS applications</title>

Abstract

Deep dry etching of silicon has become an increasingly important process for a number of applications, including optical and microinertial MEMS. The Bosch process, with alternate etch and deposition steps, has become a dominant technique. Key responses include etch rate and depth uniformity. Two of the most important factors determining these are aspect ratio dependent etching (ARDE) and loading effects: both global and local. An RSM (response surface methodology) experiment was performed as the basis for subsequent optimization of the etch with respect to ARDE. The wall angle, etch rate and uniformity across the wafer were kept within predefined limits. By sacrificing some etch rate (approximately equals 25%), it was possible to achieve more than a 50% reduction in the difference in etch depth between 2 micrometers and 20 micrometers wide features. Loading effects, dependent on the exposed surface area of silicon, cause local or global variations in the etch rate. To investigate these effects, silicon wafers were patterned with different densities to change the global exposed surface area from 1% - 27%. Local density variations were used to investigate microloading. The etch rate decreased almost linearly with global exposed silicon area. Local variations showed a less pronounced effect.