Spectral Noise Analysis of a CMOS Imager at Low Temperature for Logarithmic Mode

The most common and useful CMOS highdynamic-range imager is the one employing the active pixel sensor (APS) operating in the logarithmic mode. Notwithstanding, temporal noise can compromise the quality of the image generated by the focal-plane array. Most of the image-sensor noise models and results found in the literature are limited to a temperature range higher than that of the freezeout temperature, where the semiconductor doping impurities are all activated. The focus of this paper is to compare the noise spectrum behavior of the logarithmic image sensor at room temperature with that of a temperature near and below the freezeout point. Experimental results carried out in a small bidimensional pixel array fabricated in a standard 4-metal 2-poly 0.35-μm CMOS technology suggests that thermal noise remains quite steady as the temperature reduces due to the increase in the sense-node resistance of the pixel circuitry. On the other hand, “spikes” have been identified with significant intensity in the spectral noise in all pixels when studied individually. These spikes in the spectrum have the same value in frequency when the device is submitted to different light intensities and temperatures. However, they were only observed for temperatures below 150 K and for frequencies below the corner frequency. The nonrandomic presence of these spikes in the noise spectrum indicates the presence of traps related to the defects inside the gate oxide and at the oxide-silicon interface. At room temperature, these spikes are not observed due to the thermal energy being higher than the binding energy of these traps.