Low-cost uncooled infrared detectors in CMOS process

Abstract

Abstract This paper reports the implementation and comparison of two low-cost uncooled infrared microbolometer detectors that can be implemented using standard n-well CMOS processes. One type is based on a suspended n-well resistor, which is implemented in a 0.8 μm CMOS process and has a pixel size of 80 μm ×80 μm with a fill factor of 13%; and the other type is based on a suspended p+-active/n-well diode, which is implemented in a 0.35 μm CMOS process and has a pixel size of 40 μm ×40 μm with a fill factor of 44%. These detectors can be obtained with simple bulk-micromachining processes after the CMOS fabrication, without the need for any complicated lithography or deposition steps. The diode type detector has a measured dc responsivity ( R ) of 4970 V/W at 20 μA bias and a thermal time constant of 35.8 ms at 80 mTorr vacuum level, and it has a measured rms noise of 0.52 μV for a 4 kHz bandwidth, resulting in a detectivity (D∗) of 9.7×108 cm Hz1/2/W. The resistive n-well detector can provide the same dc responsivity at 1.68 V detector bias voltage, with about 10 times more self-heating as compared to that of the diode type detector. This detector has a measured rms noise of 0.81 μV for a 4 kHz bandwidth, resulting in a detectivity (D∗) of 8.9×108 cm Hz1/2/W, which can be improved further with higher detector bias voltages at the expense of increased self-heating. The diode type detector is better for low-cost large format infrared detector arrays, since it has a superior response even at reduced pixel sizes and lower biasing levels.