Publication
The overall sensitivity of frontside-illuminated, silicon single-photon avalanche diode (SPAD) arrays has often suffered from fill factor limitations. The fill factor loss can however be recovered by employing microlenses, whereby the challenges specific to SPAD arrays are represented by large pixel pitch (> 10 pm), low native fill factor (as low as similar to 10%), and large size (up to 10 mm). In this work we report on the implementation of refractive microlenses by means of photoresist masters, used to fabricate molds for imprints of UV curable hybrid polymers deposited on SPAD arrays. Replications were successfully carried out for the first time, to the best of our knowledge, at wafer reticle level on different designs in the same technology and on single large SPAD arrays with very thin residual layers (similar to 10 pm), as needed for better efficiency at higher numerical aperture (NA > 0.25). In general, concentration factors within 15-20% of the simulation results were obtained for the smaller arrays (32x32 and 512x1), achieving for example an effective fill factor of 75.6-83.2% for a 28.5 pm pixel pitch with a native fill factor of 28%. A concentration factor up to 4.2 was measured on large 512x512 arrays with a pixel pitch of 16.38 pm and a native fill factor of 10.5%, whereas improved simulation tools could give a better estimate of the actual concentration factor. Spectral measurements were also carried out, resulting in good and uniform transmission in the visible and NIR. (c) 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement