Temperature stability improvement of a QVGA uncooled infrared radiation FPA

We have developed a low-cost uncooled infrared radiation focal plane array (FPA) requiring no thermoelectric cooler (TEC), which has 320 x 240 detection pixels with 22 um pitch. The silicon single-crystal series p-n junction diodes and the low-noise readout circuit on the same SOI wafer fabricated by 0.13 um CMOS technology were utilized for infrared (IR) detection. The temperature dependence in the readout circuit was eliminated by correlated double sampling (CDS) operation with reference pixel that was insensitive to infrared radiation. In order to reduce the temperature dependence, we improved the reference pixel and the readout circuit. Although the reference pixels should be completely insensitive to IR radiation, prior reference pixels showed measurable sensitivity. The improved reference pixel was formed by partially releasing with bulk-micromachining and was verified to be insensitive to IR radiation by an object of 400°C. The readout circuit had a differential amplifier instead of a singletransistor amplifier and an analog-to-digital converter (ADC). In each portion, CDS was applied to reduce temperature dependence. The first CDS operation was used for eliminating the pixel output variation and the second operation was used for canceling the variation of the differential amplifier. The output variation referred to input was reduced to 1/30 compared with that of the prior circuit. Moreover, the residual variation of output voltage was reduced by CDS operation in ADC and stable output data was obtained with ambient temperature variation. With these improvements, the sensitivity variation of the FPA was improved to 10% in the range of -30 degrees to 80 degrees and noise equivalent temperature difference (NETD) of 40 mK was achieved.