Process monitoring and defect characterization of single photon avalanche diodes

Silicon p-n junctions fabricated in a CMOS compatible process can be operated above breakdown in Geiger mode. Geiger mode operation allows for single photon detection. These devices are known as single photon avalanche diodes (SPAD). SPAD quality can be assessed by the dark count present during Geiger mode operation. Higher dark counts have been attributed to defects within the shallow p-n junction. While higher dark counts are detrimental to SPAD operation, the variance in dark count due to defects makes them suitable for monitoring the junction quality of a CMOS process. At present, the dark count of a SPAD is a qualitative measure of the presence of defects within the junction of a diode. The SPAD geometry does not allow the extraction of the necessary terms to fully characterize the SPAD. This prevents quantitative process characterization to be made. Special test structures and test methodologies have been designed to allow for the extraction of the parameters characterizing SPADs. The test characterization and methodology required to extract the SPAD parameters are shown. In addition, it is shown that with suitable test structures, SPADs can be characterized and used as process and device monitors in a CMOS process.