Plasma etching of virtually stress-free stacked silicon nitride films

Stacked silicon nitride films for use in manufacturing of surface micromachined membranes were deposited using custom made plasma-enhanced chemical vapor deposition instrument with silane (SiH4) and ammonia (NH3) gas mixture as deposition precursor. Deposition conditions were adjusted by varying substrate temperature and SiH4 to NH3 flow ratio and temperature to obtain the required stress related and electrical properties of the membranes. Transmission Fourier transformed infrared spectroscopy and scanning electron microscopy were used to investigate the chemical composition and morphology of the stacked film components. An increase in the SiH4 to NH3 flow ratio and a decrease in temperature resulted in a silicon-rich silicon nitride film, as well as an increased silicon oxide concentration. To avoid underetch and sidewall defects, the plasma power density during the plasma etching was changed from 0.5 W/cm2 during the etching of both top and bottom layers in a stacked film, to 1.0 W/cm2 during the etching of the middle both silicon and silicon oxide rich film. This resulted in an improved overall stacked film sidewall quality and reduced the unwanted underetch.