Plasma etching of virtually stress-free stacked silicon nitride films.

Stacked silicon nitride films for use in manufg. of surface micromachined membranes were deposited using custom made plasma-enhanced chem. vapor deposition instrument with silane (SiH4) and ammonia (NH3) gas mixt. as deposition precursor. Deposition conditions were adjusted by varying substrate temp. and SiH4:NH3 flow ratio and temp. to obtain the required stress related and elec. properties of the membranes. Transmission FTIR spectroscopy and SEM were used to investigate the chem. compn. and morphol. of the stacked film components. An increase in the SiH4:NH3 flow ratio and a decrease in temp. resulted in a silicon-rich silicon nitride film, as well as an increased silicon oxide concn. To avoid under-etch and sidewall defects, the plasma power d. 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 under-etch. [on SciFinder(R)]