Design of Silicon Cap for Hermetic Packaging of Microbolometer Focal Plane Arrays

A microbolometer array (MBA) is used as a detector in an infrared (IR) camera. It converts the temperature change induced by the incoming longwave IR (LWIR) light into an electrical signal for obtaining the thermal image information. The MBA must be hermetically packaged to maintain the necessary performance and long lifetime. Cavities are often micromachined on single-crystal silicon (SCS) cap wafers for the encapsulation of the sensor, where an increased cavity volume facilitates maintaining the vacuum level. The resulting thin cap is suitable for high transmission of IR signals. However, a thinner cap causes a larger deflection after hermetic packaging because of the pressure difference between the atmosphere and the vacuum inside the package. Large cap deflection can affect focus of the IR light on the MBA, while thinner caps are also prone to fracture. This article investigates the mechanical behavior and optical performance of silicon (Si) cap as a function of its thickness in the range 60– $300 \mu \text{m}$ , for size 8 mm $\times8$ mm, 10 mm $\times10$ mm, and 12 mm $\times12$ mm using COMSOL and Zemax software. The leak rate necessary to fulfill the lifetime requirements of the MBAs is also discussed. The mechanical simulations indicate that mechanical failure will occur for a cap thinner than $70 \mu \text{m}$ with a chip size of 10 mm $\times10$ mm, and $90 \mu \text{m}$ with a chip size of 12 mm $\times12$ mm. No failure is observed for a chip size of 8 mm $\times8$ mm with a cap thickness in the range of 60– $300 \mu \text{m}$ . The optical simulation results reveal that the increased deflection of the thinner caps has a negligible impact on the IR light focusing.