A low-cost small pixel uncooled infrared detector for large focal plane arrays using a standard CMOS process

This paper reports the development of a low-cost, small pixel uncooled infrared detector using a standard CMOS process. The detector is based on a suspended and thermally isolated p(+)-active/n-well diode whose forward voltage changes due to an increase in the pixel temperature with absorbed infrared radiation. The detector is obtained with simple post-CMOS etching steps on dies fabricated using a standard n-well CMOS process. The post-CMOS process steps are achieved without needing any deposition or lithography, therefore, the cost of the detector is almost equal to the cost of the fabricated CMOS chip. Before suspending the pixel using electrochemical etch-stop technique in TMAH, the required etch openings to reach the silicon substrate are created with a simple dry etch process while CMOS metal layers are used as protection mask. Since the etch mask is implemented with available CMOS layers, the etch openings can be reduced significantly, allowing to implement small pixel sizes with reasonable fill factor. This approach is used to implement a 40mumx40mum diode pixel with a fill factor of 44%, suitable for large format FPAs. The p(+)-active/n-well diode has a low 1/f noise, due to its single crystal nature and low bias requirement. Optimum pixel performance is achieved when the pixel is biased at 20muA, where self-heating effect is less than 0.5K. Measurements and calculations show that this new detector has a thermal conductance (G(th)) of 1.4x10(-7)W/K and provides a responsivity (R) of 5800V/W and a detectivity (D) value of 1.9x10(9)cm Hz/W when scanned at 30fps with an electrical bandwidth of 4kHz. If this detector is used to implement a 64x64 or 128x128 FPA with sufficient number of parallel readout channels, these FPAs will provide an NETD value of 195mK considering only the detector noise. When the readout noise is included, these FPAs are expected to provide NETD value below 300mK. Such FPAs are very suitable for ultra low-cost infrared imaging applications.