Design and implementation of perforation on high fill factor structures for uncooled infrared sensors

2016
Çetin, Ramazan
This thesis presents the design and implementation of the perforation on high fill factor structures; e.g., umbrella structures, for uncooled infrared detectors. The study is mostly focused on the effect of the perforation on the absorption performance of the sensor pixels. The introduction of the perforation to the umbrella structure is expected to improve the absorption performance which can be degraded due to a trending issue nowadays, the pixel size reduction. In literature, perforation is also proposed to decrease the thermal time constant, to relieve the low vacuum requirements and to ease the etching of the underneath layers. Within the scope of this study, a canonical topology including only the perforated umbrella structure, but still resembling to the full pixel case, is designed, optimized and fabricated successfully. The canonical topology is used for the optimization of the perforated structures to have a high enough, broadband absorption rate in the LWIR (Long Wave Infrared, 8μm-12μm) range using the Cascaded Transmission Line (CTL) technique and the commercial High Frequency Structural Simulator (HFSS) program. The LWIR is the optimized infrared range to get images of the objects having a body temperature around 300 Kelvin. Through these optimizations, a perforation ratio of 50%; i.e., the ratio of the perforation size to the period size is equal to 0.5, is advised to be the optimized value both to have a good absorption performance and also to benefit from other improvements that the perforation offers. The Frequency Transform Infrared (FTIR) spectroscopy results of a successful implementation of perforation on high performance pixels is presented, an average of 90% absorption rate is obtained in the LWIR range. Additionally, a contactless and non-destructive characterization of the absorbing Nichrome (NiCr) layer is presented. Also an improvement by more than 10% with the thermal time constant with the introduction of the perforation is proved through thermal simulations.  

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Citation Formats
R. Çetin, “Design and implementation of perforation on high fill factor structures for uncooled infrared sensors,” M.S. - Master of Science, Middle East Technical University, 2016.