An uncooled infrared focal plane array for low-cost applications fabricated with standard CMOS technology

This paper reports the design, fabrication and assessment of a low-cost uncooled infrared imager that has been conceived as a general purpose system to be used in a wide range of infrared applications. The imager has been fabricated using the AMS 0.8 μm CYE CMOS process together with a compatible front-side bulk micromachining post-process provided by the CMP service of the TIMA laboratory. The adopted fabrication approach does not involve any lithography step, material deposition or particular etch-stop technique after the CMOS process, so that the imager cost is almost equal to the CMOS chip cost. The infrared imager is composed of a focal plane array (FPA) with 16 × 16 thermopile pixels, which are monolithically integrated with the addressing and readout electronics. Each pixel consists of a thermally isolated micromachined membrane suspended by two arms that contain the polysilicon/aluminium thermocouples of the embedded thermopile sensor. The pixel membrane also includes a heating resistor intended to implement a self-test function that allows an electrical test of the FPA without need of specific infrared equipment. Since the voltage levels generated by the thermopile pixels are in the range of a few μV the readout channel consists of a low-noise voltage amplifier with a high variable gain that can be tuned for different operation conditions. The readout circuit makes use of the chopper principle and the correlated double sampling technique to reduce the noise floor and the amplifier offset levels. Optical measurements performed with the fabricated prototypes have shown a pixel responsivity of 15.0 V/W, a noise equivalent power of 1.37 nW and a normalized detectivity of 1.05 × 107 cm Hz1/2 W−1, values that are in line with current state of the art. The readout channel features a maximum gain of 85 dB with a 4.3 kHz bandwidth and an equivalent input noise of 22 nV/Hz1/2. An infrared imager based on the FPA has been build and thermal imaging has been demonstrated.