Hydrogen diffusion and pressure control of encapsulated MEMS resonators

We have investigated the diffusion of hydrogen through the oxide and silicon of our single-wafer vacuum package. We have encapsulated micromechanical silicon resonators in vacuum beneath a 20 μm polysilicon layer. While we have not been able to measure any change in pressure of parts at room temperature over a period of nine months, we have been able to accelerate the diffusion of hydrogen through the encapsulation using elevated temperatures. While placing encapsulated resonators in elevated temperature, we select the furnace gas condition to diffuse hydrogen gas in or out. This is an enabling step toward forecasting long-term hermiticity of the encapsulation, and it provides the ability to set the pressure inside the encapsulation with a simple set of furnace processes. Also, the ability of the encapsulation to withstand more than twenty of these temperature cycles between room temperature and 300°C-400°C is evidence of the robustness of the package.