Measurement and modeling of microlenses fabricated on single-photon avalanche diode arrays for fill factor recovery

Abstract

Single-photon avalanche diode (SPAD) imagers typically have a relatively low fill factor, i.e. a low proportion of the pixel's surface is light sensitive, due to in-pixel circuitry. We present a microlens array fabricated on a 128×128 single-photon avalanche diode (SPAD) imager to enhance its sensitivity. The benefits and limitations of these light concentrators are studied for low light imaging applications. We present a new simulation software that can be used to simulate microlenses' performance under different conditions and a new non-destructive contact-less method to estimate the height of the microlenses.Results of experiments and simulations are in good agreement, indicating that a gain >10 can be achieved for this particular sensor. Abstract: Single-photon avalanche diode (SPAD) imagers typically have a relatively low fil factor, i.e. a low proportion of the pixel's surface is light sensitive, due to in-pixel circuitry. We present a microlens array fabricated on a 128x128 single-photon avalanche diode (SPAD) imager to enhance its sensitivity. The benefit and limitations of these light concentrators are studied for low light imaging applications. We present a new simulation software that can be used to simulate microlenses' performance under different conditions and a new non-destructive contact-less method to estimate the height of the microlenses. Results of experiments and simulations are in good agreement, indicating that a gain >10 can be achieved for this particular sensor.

Architecture and applications of a high resolution gated SPAD image sensor

“…Upon fabrication the polymer is UV-cured and stabilized through thermal cycling. In [20], Mata Pavia et al provide details on the fabrication process and lens performance analysis. Another promising solution, to be implemented in future CMOS SPADs, uses 3D fabrication techniques.…”

SPAD imagers for super resolution localization microscopy enable analysis of fast fluorophore blinking

“…the ratio between the photosensitive area and the overall area of the pixel, is limited to 5%. It can, however, be increased by building a microlens array on the imager to concentrate light on the photosensitive area16. By using microlenses, we achieved a concentration factor of 12 for partially collimated light, leading to an effective fill factor of 60% and a peak photon detection efficiency (PDE) of 20% at 450 nm13 for an with a f/# of 16.…”

Smart-aggregation imaging for single molecule localisation with SPAD cameras

“…It was assumed that the effective fill factor will, in time, increase three-fold, with the DCR reducing by a factor of four. It is believed that these are realistic assumptions; there are a number of avenues in which such a fill factor improvement may be realized, including microlensing22, back-side illumination23 or stacking24. Moreover, experimental studies suggest that SPADs with a similar structure to the sensor studied here exhibit a halving of DCR for every 8 °C of cooling until around −10 °C25.…”