Evaluation of Silicon Diaphragms for Hermetic Packaging of Microbolometer Arrays

Hermetic packaging is a critical requirement for microbolometers to maintain long-term reliability. A thinner diaphragm is desirable for vacuum packaging of microbolometers to obtain higher infrared light transmission. However, a thinner diaphragm results in a larger deflection due to the pressure difference from atmosphere, which may influence the IR signal focus and possibly cause mechanical failure. In this paper, the trade-offs of using thin single crystal silicon diaphragms as encapsulation for hermetic packaging of microbolometer arrays have been investigated in terms of the mechanical stability and optical performance using COMSOL and Zemax. The optical simulations show that the bending of the thin diaphragm has negligible effect on the infrared light focus with wavelengths from 8 to 14 µm. The mechanical simulations reveal that a thin diaphragm (thickness < 70 µm) with 10×10 mm2 area and a diaphragm (thickness < 90 µm) with 12×12 mm2 area will cause mechanical failure, and the designed diaphragm thickness must incorporate margins to these values.