SiO2 Uncooled MEMS IR and THz Detectors

Optically-probed uncooled microelectromechanical system infrared (MEMS IR) detectors have many advantages over conventional uncooled (bolometric) detectors. These advantages include fabrication simplicity, robustness, high yield and low cost. We have implemented several designs in order to bridge the performance gap between MEMS-based and conventional uncooled thermal imagers. We used silicon dioxide (SiO2) as a structural material, chosen for its low thermal conductivity and high mismatch of its thermal expansion coefficient compared to Al (used for metallization) [1]. We removed silicon substrate beneath each detector, allowing unobstructed path for IR radiation [2]. The absence of a Si substrate along with increased detector area also allowed potential THz imaging. Summary of Research: We have implemented two designs of MEMS detector arrays. The first design involved three photolithographic steps and featured thermal isolation regions (SiO2) and bimaterial regions (SiO2-Al). The second design involved four photolithographic steps and featured a region with an embedded optical cavity between SiO2 and Al layers in addition to thermal-isolation and bimaterial regions. The structures of the first type were tiled densely across the 4-inch wafer in order to investigate the structural integrity of the wafer when most of the substrate is removed and the remaining Si forms a see-through mesh (20 μm walls surrounding 200 μm holes) structure (Figure 1). The second design was represented by 120 × 120 arrays to match our IR imaging system [3]. We have created structures with both thermally grown and plasma enhanced chemical vapor deposited (PECVD) SiO2. Both types of films were approximately 1000 nm thick. The optical cavity was created by a 1100 nm thick PECVD-grown layer of amorphous Si. E-gun evaporation was used for liftoff metallization of a 200 nm thick Al film. The suspended structure and the optical cavity have been patterned using reactive ion etching. To form wells underneath each detector, anisotropic Figure 1: 20 μm thick walls approximately 200 μm apart formed a mesh sufficient to preserve the structural integrity of a 300 μm Si wafer. Light has been placed behind the wafer to show that it appears transparent.