Hermetical sealing of pressure sensor diaphragms by CVD of Si1-XGeX with minimized cavity deposition
In this work, the hermeticity of diaphragm structures for later post-CMOS integration is investigated and optimized. The diaphragms were completely made of Si1-XGeX, which is a high quality MEMS material that can be deposited at temperatures below 400 °C. The hermeticity tests of the diaphragms were performed in a helium atmosphere at 1800 hPa and in a temperature range from 50 °C to 100 °C. To calculate the diffusion coefficients, the change of diaphragm deflection caused by He-diffusion through the diaphragm is measured optically. In the fabrication process, a variation of the deposition parameters of a CVD Si1-XGeX sealing layer on a polycrystalline p+Si1-XGeX diaphragm was investigated regarding the selectivity of the layer growth on different surfaces (p+Si1-XGeX, Si, and SiO2). An increasing selectivity of the layer growth against Si and SiO2 was shown, which increases with the GeH4 ratio in the process gas flow. With a GeH4 pure gas flow, the best results were observed with the highest selectivity, a minimal diffusion and a minimal intrinsic stress of the Si1-XGeX sealing layer.