Dielectric thin films for MEMS-based optical sensors

Nanoindentation and optical measurements have been employed in order to investigate the mechanical properties of low-temperature (50–330 °C) plasma-enhanced chemical vapour 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. The temperature of the SiNx deposition process is found to strongly influence Young’s modulus, hardness, and stress, with a critical deposition temperature in the 100 °C to 150 °C range which depends on the details of other deposition conditions such as chamber pressure and RF-power. The properties of PECVD SiNx films deposited above this critical temperature are found to be suitable for MEMS applications, whereas films deposited at lower temperatures exhibit low Young’s modulus and hardness, as well as environment-induced stress instabilities. The investigated thin films have been incorporated into a monolithic integrated technology comprising low-temperature (∼125 °C) MEMS and HgCdTe IR detectors, in order to realize successful prototypes of tuneable IR microspectrometers.