Performance comparison of pH sensors fabricated in a CMOS process

A comparison of performance in three pH-sensitive CHEMFET devices in a standard 1.6 micron CMOS process using aluminum oxide, silicon nitride, and silicon oxide is described. Experimental results compare sensitivity, resolution, and signal-to-noise ratio for the three devices across their usable lifetime. The best signal to noise ratio (19.3 dB) occurs with a silicon oxide pH sensitive layer and the worst occurs with the silicon nitride pH sensitive layer (8.8 dB). The devices with the best resolution are based on silicon nitride (0.44 pH units) and the devices with the worst resolution use silicon oxide. The aluminum oxide sensing layer is the most sensitive (0.22 mV/pH) and the silicon oxide sensing layer is the least sensitive (0.038 mV/pH). Lifetime of these devices is limited to 8 hours or less when exposed to solutions in the pH range of 5.0 to 8.0. Although the CMOS-based fabrication process ensures that these sensors can be manufactured at a few cents per device in a mass production situation, their relatively low resolution and need for individual calibration severely limits use in pH sensing applications.