No abstract

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 µm), low native fill factor (as low as ~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 at wafer reticle level on different designs in the same technology and on single large SPAD arrays with very thin residual layers (~10 µm), as needed for better efficiency at higher numerical aperture (NA > 0.25). In general, concentration factors within 15-20% of the theoretical maximum were obtained for the smaller arrays achieving an effective fill factor of 75.6-83.2%. A concentration factor up to 4.2 was measured on large arrays with a pitch of 16.38 m 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.

Silicon-based single-photon avalanche diodes (SPADs) implemented in front-side illuminated arrays and imagers have often suffered from fill factor limitations. The corresponding reduced sensitivity can be sometimes traded off with longer acquisition times thanks to SPAD's noiseless read-out. The use of SPADs can however be critically affected in many applications, especially when photon-starved, or when several photons need to be detected in coincidence. The fill factor loss can be recovered by employing microlens arrays, which are difficult to build with relatively large pitch (> 10 µm) and low native SPAD fill factor (as low as 10%). To address these challenges, we have developed several generations of refractive microlenses by photoresist reflow used to fabricate molds. These structures were used to imprint UV-curable hybrid polymer microlenses on SPAD arrays. Replications were successfully carried out on large SPAD arrays with very thin residual layers (~10 µm), as required for higher numerical aperture (NA > 0.25). Replications were also carried out for the first time in a multi-chip operation regime at the wafer reticle level. By optimizing the lens sag and residual layer thickness, concentration factors (CFs) within 15-20% of the theoretical maxima were obtained for the smaller arrays (32×32 and 512×1). The spectral response was flat above 400 nm. CF values up to 4.2 with good uniformity were measured on large 512×512 arrays with 16 µm pixel pitch and a native fill factor of 10.5%. This result was confirmed by simulations when using the actual measured lens shape. We thus demonstrated good spectral and spatial uniformity and high CF, while moving to higher NAs and larger sensor sizes with respect to previous work.

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 µm), low native fill factor (as low as ∼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 (∼10 µm), 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 (32×32 and 512×1), achieving for example an effective fill factor of 75.6-83.2% for a 28.5 µm pixel pitch with a native fill factor of 28%. A concentration factor up to 4.2 was measured on large 512×512 arrays with a pixel pitch of 16.38 µm 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.

We present an analog silicon photomultiplier (SiPM) based on a standard 55 nm Bipolar-CMOS-DMOS (BCD) technology. The SiPM is composed of 16×16 single-photon avalanche diodes (SPADs) and measures 0.29×0.32 mm 2 . Each SPAD cell is passively quenched by a monolithically integrated 3.3 V thick oxide transistor. The measured gain is 3.4× 10 5 at 5 V excess bias voltage. The single-photon timing resolution (SPTR) is 185 ps and the multiple-photon timing resolution (MPTR) is 120 ps at 3.3 V excess bias voltage. We integrate the SiPM into a coaxial light detection and ranging (LiDAR) system with a timecorrelated single-photon counting (TCSPC) module in FPGA. The depth measurement up to 25 m achieves an accuracy of 2 cm and precision of 2 mm under the room ambient light condition. With co-axial scanning, the intensity and depth images of complex scenes with resolutions of 128×256 and 256×512 are demonstrated. The presented SiPM enables the development of cost-effective LiDAR system-on-chip (SoC) in the advanced technology.