Thermal Infrared Radiometer Structure Optimization Design And Nano-electronic Devices

Thermal imaging is one of the major detecting technologies used in national security that plays an important role in the domains of national defence and spaceflight. Among all kinds of thermal imaging technologies, uncooled focal plane array (UFPA) using vanadium dioxide as the thermal sensing material has become a hotspot due to its high performance, light weight and low cost. Whereas, the fabrication of UFPA is a complex micro-electromechanical system (MEMS) engineering which involves IC design, infrared optics, thin films deposition, as well as MEMS technologies.Our work is concentrated on the structure optimization, thin films deposition and the fabrication process. We not only fabricated a 1×10 VOX microbolometer pixel array with silicon nitride (SiNx) microbridge structure but also measured its response to IR radiation.1. In order to optimize the structure design of microbolometer, we calculated the microbolometer’s IR absorption as a function of the height of the vacuum gap based on the optical transfer matrix theory, and found that the absorptivity is maximum when that gap is 2.5μm. Thermal and deformation simulation by ANSYS indicated that low thermal conductivity of the bridge arms will improve the detector’s responsivity.2. Thin films deposition is the critical factor in the microbolometer fabrication. VOX thin films were deposited by reactive electron beam evaporation. After post-deposition thermal annealing, the film showed a thermal coefficient of resistance (TCR) of -3%/K and had a change of resistance by an order of 2 around 55℃. High IR absorptive and low stress SiNx films were obtained by PECVD and pre-deposition stress release trough technique.3. To improve the process and lower the cost, the image-reversal photolithography using AZ5214E photoresist and the wet etch of PI sacrificial layer were studied in detail. Clear patterns of photoresist were produced when the temperature of reversal baking was 96～98℃. Meanwhile, the exposure time and the flood exposure time were 8.1s and 8.4s repectively. Based on the reveral property of AZ5214E, a 2×3×3μm3 aluminum pillar enwrapped by SiO2 wall was fabricated by lift off process. Since polyimide (PI) was soluble in alkalescent solution, PI sacrificial layer was precured under 122℃and then patterned by wet etch.4. Finally, The IR response and the TCR of microbolometer were measured after encapsulation. The test result indicated the microbolometer showed response to IR radiation, but its TCR was smaller compared to VOx thin film grown on Si substrate, and we supposed that films residual stress had a negative impact on TCR.In addition, the main innovation of this paper is as follows:1. Lift off technique based on AZ5214E image-reversal photolithography was used to fabricate the AL/SiO2 micropillar. This technique had advantages over traditional positive photolithography which was not suitable for lifting off patterns with high aspect ratio.2. Since anisotropic dry etch always creates vertical step in PI patterns which may result in the crack of SiNx arms, wet etch was used to pattern the PI sacrificial layer. The smooth step around AL/SiO2 micropillar thus obtained are good for levelling off the microbridge’s surface.