Development of a low stress, silicon-rich silicon nitride film for micromachined sensor applications

Wet etch processes are important for the production of MEMS devices. Sacrificial oxides are often used to help define polysilicon structures, and these films are often etched using solutions containing hydrofluoric acid. One important consideration is the use of a etch stop which is resistant to HF. In this study, a silicon-rich silicon nitride film was developed for this purpose. Process parameters such as DCS:NH3 ratio, pressure and temperature have been varied in order to create a film that has a low wet etch rate, good cross-wafer and cross-load uniformity, and low conductivity for good isolation resistance. The film is also designed to have a low tensile stress, which should minimize etch rate and also minimize:wafer curvature, which is beneficial for subsequent photo steps. Finally, film characterization using the index of refraction as the primary process control is discussed.