Stress response of low temperature PECVD silicon nitride thin films to cryogenic thermal cycling

An investigation is presented of the stress response of low-temperature plasma-enhanced chemical vapour deposited silicon nitride thin films to thermal cycling over the temperature range of 100K-325K. The room temperature thermal expansion coefficient of SiNx films increases in a general parabolic fashion from 2.6 ppm/K to 5.2 ppm/K, as the temperature of the deposition process is lowered from 300 degrees C to 50 degrees C. It is observed that the thermal expansion coefficients of the SiNx. films and silicon substrates show similar temperature dependences over the 100K-325K operating temperature region. Importantly, it has been found that the porous nature of SiNx thin films deposited below 100 degrees C caused higher tensile thin film stress values when measured in vacuum than stress values measured at atmospheric pressure. Annealing at 50 degrees C of films deposited at and below 100 degrees C introduced further tensile stress changes, and resulted in hysterisism of the thermal cycling stress response curves. These stress changes have been shown to be fully reversible upon reexposure to atmosphere or high purity nitrogen, helium, argon, or oxygen.