Development and characterization of low-cost uncooled infrared sensors for commercial applications

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2013
Tankut, Firat
This thesis reports the study on the development and characterization of low-cost uncooled microbolometer type infrared detectors, which are fabricated using standard CMOS and MEMS processes. Characterization of the detectors is the first step of developing infrared sensors with better performance. The characterized pixel has a 70 μm pitch and includes 4 serially connected diodes as the detector circuit. Thermal conductance (Gth), temperature sensitivity (TC) and, optical absorption are measured in scope of the characterization tests. The optical absorption of the detectors is measured by Fourier Transform Infrared Spectroscopy (FTIR) method and average absorption is found as 78 % for 7-14 μm wavelength range. The noise of the detector is characterized in detail. Test results show that, the predominant noise mechanism in the characterized low-cost infrared sensors is the Random Telegraph Signal (RTS) noise. After characterizing the pixel, the readout electronics of the 70 μm pixel pitch QCIF (160x120) resolution Focal Plane Array (FPA) are improved for better performance. Three main revisions on the current design are made, and the revised readout circuits are verified with detailed tests in the scope of this thesis. In the first revision, power dissipation of the chip is decreased by changing the architecture of various blocks of the readout circuit. After this revision, the Noise Equivalent Temperature Difference (NETD) value is improved to 280 mK. The second design revision decreased the impact of the column noise that was caused by the RTS mechanism. Lastly, a third revision is made in order to improve the temperature stability and noise rejection. These revised sensors are used in developing miniature thermal camera systems in the scope of another work, and the resulting miniature cameras are among the smallest thermal camera cores in the world. As the final step, second generation low-cost uncooled microbolometer pixels are developed, where the pixel pitch is reduced to 50 μm while preserving the same performance level as the 70 μm pixels. In order to alleviate the performance degradation caused by the reduction of the area that absorbs radiation, more serially connected detector diodes and thinner support arms are used. Thermal parameters are simulated using Finite Element Method (FEM). The expected thermal conductance and the thermal time constant are calculated as 222 nW/K and 48.8 ms respectively from the simulations. Optical absorption is also simulated and the average optical absorption is calculated to be 77 % for 7-14 μm wavelength region from the simulations. The FPA chip utilizing second generation pixels is tested, and average pixel noise is measured as 9.8 μVRMS including the readout circuit noise.

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Citation Formats
F. Tankut, “Development and characterization of low-cost uncooled infrared sensors for commercial applications,” M.S. - Master of Science, Middle East Technical University, 2013.