Residual stress in silicon nitride films

Abstract

The mechanical stress caused by Si₃N₄ films on (111) oriented Si wafers was studied as a function of the Si₃N₄ film thickness, deposition rate, deposition temperature and film composition. The Si₃N₄ films were prepared by the reaction of gaseous SiH₄ and NH₃ in the temperature range 700–1000°C. The curvature of the Si substrates caused by the Si₃N₄. films is related to the film stress; the substrate curvature was measured by an optical interference technique. The measured Si₃N₄. film stress was found to be highly tensile with a magnitude of about 10¹⁰ dynes/cm². For the thickness range of 2000–5000Å, there was no change in the measured stress. The total film stress was observed to decrease for decreasing deposition rate and increasing deposition temperature. A large change in film stress was observed for films containing excess Si; the stress decreased with increasing Si content. Based on published values for the thermal expansion coefficients for Si and Si₃N₄, a published value for Young’s Modulus for Si₃N₄, and the measured total stress values, a consistent argument is developed in which the total stress consists of a compressive component due to thermal expansion coefficient mismatch and a larger tensile intrinsic stress component. Both the thermal and intrinsic stress components vary with film deposition temperature in directions which decrease the total room temperature stress for higher deposition temperatures.