Hermeticity of SI1-XGEX Diaphragms for the Fabrication of a Capacitive Post-Cmos Pressure Sensor

In this work, the hermeticity of diaphragm structures is investigated and optimized. The diaphragms are developed for the monolithic post-CMOS integration of capacitive pressure sensors. Si_1-XGe_X is used as diaphragm material and was deposited at temperatures below 400 °C.The hermeticity of the diaphragms was evaluated at a He pressure of 1800 hPa and in a temperature range from 50 °C to about 100 °C. The diffusion coefficients were determined by measuring the changes of diaphragm deflections due to He-diffusion inside the cavity.In the CVD process of Si_1-XGe_X cover layer on a polycrystalline p⁺Si_1-XGe_X diaphragm for closing the etch access holes, a variation of the SiH₄ and GeH₄ gas flows at a substrate temperature of about 380 °C was investigated regarding the selectivity of the layer growth on different surfaces (p⁺Si_1-XGe_X, Si, and SiO₂). The selectivity of the layer growth against Si and SiO₂ increases with the GeH₄ ratio in the process gas flow. With a pure GeH₄ gas flow, an optimisation of the parameters selectivity, He-diffusion and intrinsic stress of the Si_1-XGe_X cover layer was found.