Performance and Spatial Sensitivity Variations of Single-Photon Avalanche Diodes Manufactured in an Image Sensor CMOS Process

In this letter, we present the results from a series of single-photon avalanche diode (SPAD) structures implemented in a commercial 0.18-μm CMOS process intended for CMOS image sensors. Variations without any effect on the performance and variations that produced non-functional devices are described. Devices based on the p+/n-well and deep-n-well/p-doped epitaxial (P-EPI) SPADs' junctions were found to work well in this process. When biased for 10% quantum efficiency, the best 10-μm diameter p+/n-well SPADs exhibited a dark count rate (DCR) of ~1 kHz, whereas the DCR of the deep-n-well/ P-EPI SPADs was only 10 Hz under the same conditions. We also show that the former type exhibited local sensitivity variations within the SPADs ranging from a factor 4 at low excess voltage to 1.2 at an excess voltage of ~0.5 V. No significant sensitivity variations were found for the deep-n-well/P-EPI SPADs, but they were found to exhibit a significant sensitivity outside the central junction, contributing from 8.3% at low excess voltage to ~70% at high excess voltage.