CCBDI Readout Circuit For YbaCuO Micromachined Microbolometers
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Semiconducting YBaCuO (Yttrium-Barium-Copper-Oxide) as a new uncooled microbolometer material is evaluated for CMOS compatibility. By not requiring cryogenic cooling, the self-supporting semiconducting YBaCuO micromachined microbolometers have found an attractive application for thermal detection with substantial reduction in system cost, size, weight and power consumption as compared to cooled photon detectors. The lower thermal conductance of the microbolometers to the substrate provides good sensitivity for infrared detection. The CCBDI (Constant Current Buffered Direct Injection) readout circuit is designed to work with a traditional frame rate of 30Hz and as a novel approach improves the performance of uncooled IR cameras by a higher frame rate of 200Hz for a 640x480 array for faster thermal imaging in commercial, military and biomedical applications. The 18-µm-long electrode arm geometry of YBaCuO microbolometer provides a higher frame rate of 200Hz and hence meets the design compatibility requirement for the faster 200Hz frame rate by the CCBDI readout circuit. The 75-µm-long electrode arm geometry is designed to achieve a traditional 30Hz frame rate. YBaCuO microbolometers based on self-supporting structure have been designed, fabricated, on-chip integrated and characterized as a novelty with the CCBDI readout circuit in AMI 1.5 µm double-poly-double-metal n-well 2.5V CMOS technology. The CCBDI readout circuit offers advantages of high linearity and uniformity, low offset error, provides maximum sensitivity at a bias current and has a great potential in the application of large uncooled bolometer focal plane arrays. The CCBDI readout is a better performance circuit as compared to other designs due to increase in the injection efficiency of the CCBDI amplifier and the stable detector bias control. The merits of YBaCuO include relatively high temperature coefficient of resistance (TCR), relatively low 1/f-noise, ease of fabrication by rf magnetron sputtering, room temperature operation and post-CMOS compatibility. The measured TCR is -3.16 %/K at 295K and measured value of thermal conductance is 1.01x10-7 W/K for a 30Hz frame rate. The maximum responsivity of 1.62x105 V/W and maximum detectivity of 3.51x107 cmHz1/2/W are measured of a single YBaCuO microbolometer pixel from the 4x4 array CCBDI readout circuit for a 200Hz frame rate, and hence confirm higher sensitivity.