Feasibility of Micro-Grass on Silicon Cap as an Alternative Antireflective Layer in Vacuum Packaging of Microbolometer Arrays

Micro-grass has been introduced as an antireflective layer on silicon (Si) caps used for vacuum packaging of microbolometer arrays (MBA), acting as infrared (IR) detectors in thermal imaging cameras. Compared to conventional antireflective coatings (ARCs), the Si grass structure, also known as black Si, offers a much better thermal budget available for the vacuum packaging process of MBA. The fabrication of Si grass is based on reactive ion etching process, and could therefore be combined with the formation of (deep) cavity in the Si caps, which enlarges the volume, and hence prolongs the lifetime of vacuum sealed MBA packages. Fabricated samples with Si grass were characterized by means of IR transmittance, determined by Fourier transmission infrared spectrometer, and scanning electron microscopy. Si grass exhibited competitive antireflection performance compared to conventional ARCs in the long-wave IR (8-14μm), as well as promising performance in the mid-wave IR (3–5 μm). Test results showed no significant impacts of the vacuum sealing process, including long-term storage, high-temperature bonding, and cap deflection on vacuum sealed packages, as well as mechanical vibration on the antireflection performance of Si grass. These confirm the feasibility of using micro-grass as antireflective layers on Si caps for MBA vacuum packages.