Environmental stability and cryogenic thermal cycling of low-temperature plasma-deposited silicon nitride thin films

Stress in low-temperature plasma-enhanced chemical vapor deposited silicon nitride (SiNx) thin films subject to cryogenic thermal cycling (100-323 K) has been measured. It is observed that the SiNx deposition temperature strongly influences the thin film characteristics. For films deposited between 200 and 300 degrees C, the thermal expansion coefficient is similar to that of silicon over the 180-323 K temperature range. The room temperature thermal expansion coefficient of SiNx films is found to decrease sublinearly from 5.2x10(-6) to 2.6x10(-6) K-1 as the temperature of the deposition process is increased from 50 to 300 degrees C. The negative correlation between deposition temperature and thin film thermal expansion coefficient, and the positive correlation between deposition temperature and the thin film Young's modulus inferred from nanoindentation are postulated to be associated with the local bonding environment within the thin film. The stress state of SiNx films deposited above 150 degrees C is stable under atmospheric conditions, in contrast to SiNx films deposited below 100 degrees C, which under atmospheric storage conditions become more tensile with time due to oxidation. In addition, SiNx thin films deposited below 100 degrees C exhibit higher tensile stress values in vacuum than at atmospheric pressure, and vacuum annealing at 50 degrees C of films deposited below 100 degrees C introduces further tensile stress changes. These stress changes have been shown to be fully reversible upon reexposure to high purity nitrogen, helium, argon, oxygen, or laboratory atmosphere, and are likely to be associated with thin film porosity.