Optical Field and Absorbed Photon Density Mapping for CMOS SPADs
Conference
Hicks, S, Nomerotski, A, Shawkat, MSA. (2026). Optical Field and Absorbed Photon Density Mapping for CMOS SPADs
. IEEE SOUTHEASTCON 2015, 10.1109/SoutheastCon63549.2026.11476250
Hicks, S, Nomerotski, A, Shawkat, MSA. (2026). Optical Field and Absorbed Photon Density Mapping for CMOS SPADs
. IEEE SOUTHEASTCON 2015, 10.1109/SoutheastCon63549.2026.11476250
Single-photon avalanche diodes (SPADs) implemented in CMOS technologies are key devices for photon-counting applications, including quantum imaging and sensing applications. In these systems, the wavelength dependence of photon detection performance is strongly influenced by how the multilayer CMOS optical stack and device geometry shape the local optical field and where photons are actually absorbed within silicon. This paper presents an optical field and absorbed photon density mapping study of a CMOS SPAD cross-section using the RSoft FullWAVE finite-difference time-domain (FDTD) solver. We simulate front-side illumination over a set of wavelengths spanning the visible to near-infrared band and compute lateral optical field-intensity profiles at the silicon surface together with two-dimensional absorbed photon density (APD) maps within the device cross-section. The resulting datasets are imported into Sentaurus Visual to generate consistent APD heatmaps that highlight how absorption localization shifts with wavelength. This also illustrates surface-dominated absorption at shorter wavelengths, deeper penetration into the bulk at longer wavelengths, and lateral shadowing from shallow trench isolation and metal interconnects. As an initial scalar metric, we evaluate the fraction of absorbed photons within a nominal multiplication region to illustrate how optical stack and geometry choices influence effective sensitivity. Preliminary results demonstrate that spatially resolved APD heatmaps can be used to provide useful insight into wavelength-dependent tradeoffs among junction depth, optical window placement, and isolation geometry. Overall, this study provides the foundation for a detailed electro-optical analysis of the CMOS SPAD pixels to optimize their performance before fabrication, saving time and cost.
