Long-term environmental stability of residual stress of SiNx, SiOx, and Ge thin films prepared at low temperatures

Optical measurements of thin-film-stress-induced substrate bending have been employed in a characterization of long-term environmental stability of stress of low-temperature (<125 °C) plasma enhanced vapor deposited (PECVD) SiNx, as well as thermally evaporated SiOx, and Ge thin films for applications in micro-electro-mechanical systems (MEMS) fabricated on temperature sensitive, non-standard substrates. It has been found that in comparison to their stress values measured at atmospheric conditions, PECVD SiNx layers prepared below ∼100 °C as well as layers of thermally evaporated Ge exhibit significantly more tensile (less compressive) stress values when measured in vacuum, which are reversible upon re-exposure to an atmospheric, dry nitrogen, helium, argon, or oxygen ambient. Raising the deposition temperature above ∼100 °C results in PECVD SiNx stress being stable in vacuum and dry nitrogen storage, which is complemented by stress stability in laboratory atmosphere for films deposited above ∼125 °C. Stress of thermally evaporated SiOx layers is stable in vacuum and undergoes compressive stress development in either dry nitrogen or laboratory air.