Modeling the stress enhancement of plasma enhanced chemical vapor deposited silicon nitride films by UV post treatment – impact of the film density

Abstract

Plasma Enhanced Chemical Vapour Deposited (PECVD) tensile nitride liners have been introduced in the 90 nm CMOS technology node to generate mechanical strain within the transistor silicon channel. With strain, because of the silicon piezoresistance effect, the electron mobility is enhanced within the silicon channel, so the nMOS transistor performance. Since that time, significant work has been carried out to develop silicon nitride films with enhanced tensile stress values to increase the gain provided by these process induced stressors along the introduction of the new technologies. For the 45 nm node, an additional UV post treatment has been introduced to achieve highly tensile residual stress. This study determines the relevant as deposited PECVD nitride film properties that modulate the final stress achievable after UV cure. Thanks to a simple stress model, it is demonstrated that as deposited nitride films must present a nitrogen rich composition and an intrinsic low density to become highly tensile after post UV treatment. With these design rules, nitride films with final stress of 1.6 GPa have been synthesized.