Micromachined bolometers on polyimide

This paper presents design, fabrication and measured performance parameters of micromachined infrared detectors on polyimide substrates. The detectors are of bolometer type made of semiconducting yttrium–barium–copper oxide (YBCO) as the thermometer material and are built on a 40–50 μm-thick layer of polyimide (PI5878G) that serves as a flexible substrate. A thin film of Si3N4 supports the detectors. Surface micromachining is used to isolate the detectors and the supporting nitride layer from the substrate. Despite being supported by a layer of Si3N4, an average thermal conductivity of 5.61 × 10−7 W/K, comparable to self-supporting devices on flexible substrates has been measured. This opens the possibility of placing such flexible sensors on micromachined membranes, integrated with other functions such as pressure sensing to build “Smart Skin.” Room-temperature responsivity of 7.4 × 103 V/W and detectivity of 6.6 × 105 cm Hz1/2/W were demonstrated on 40 μm × 40 μm microbolometers. The effect of substrate heating was also investigated and was found not to cause cross talk between the pixels. If these sensors are to be used as part of multi-functional, flexible sensor systems, device-level vacuum packaging is necessary. This was investigated using computer simulations for structural analysis and found to be feasible.