Optical-stack optimization for improved SPAD photon detection efficiency

Single-photon avalanche diodes (SPADs) are direct photon-to-digital detectors that enable scalable arrays with Poisson-limited signal-to-noise ratio and picosecond timing resolution. However, SPAD detectors require a guard-ring structure to prevent lateral edge breakdown. The guard ring, in addition to pixel electronics, reduces the sensitive area within the pixel, often below 50%. We present the simulation, design and characterization of microlens structures to increase the effective fill factor and SPAD photon detection efficiency. The main challenges in designing microlenses for SPADs are a relatively large SPAD pitch and a low native fill factor, requiring high microlens efficiency over a wide angular distribution of light. In addition, we addressed the requirements of several designs in the same technology, featuring native fill factors which range from 10.5% to 28%, by carrying out the microlens fabrication at wafer reticle level. The fabrication process starts with creating a photoresist microlens master, used to fabricate a mould for microlens imprints. After dispensing a UV curable hybrid polymer on top of the SPAD array, the mould is used to imprint the microlens array shape, and then cured with UV exposure. By using microlenses, we were able to increase the initial fill factor to more than 84% effective fill factor for a 28.5 μm pixel pitch. We also explore the influence of the passivation layer on the SPAD photon detection efficiency.