High-efficiency fill factor recovery using refractive microlens arrays imprinted on 0.5-256 kpixel front-side illuminated SPAD imagers

C. Bruschini; M. Antolovic; F. Zanella; A. C. Ulku; S. Lindner; A. Kalayanov; T. Milanese; E. Bernasconi; V. Pešić; E. Charbon

doi:10.1117/12.2652962

15 March 2023 High-efficiency fill factor recovery using refractive microlens arrays imprinted on 0.5-256 kpixel front-side illuminated SPAD imagers

C. Bruschini, M. Antolovic, F. Zanella, A. C. Ulku, S. Lindner, A. Kalayanov, T. Milanese, E. Bernasconi, V. Pešić, E. Charbon

Author Affiliations +

Proceedings Volume 12433, Advanced Fabrication Technologies for Micro/Nano Optics and Photonics XVI; 124330K (2023) https://doi.org/10.1117/12.2652962
Event: SPIE OPTO, 2023, San Francisco, California, United States

Conference Poster

Abstract

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.

Citation Download Citation

C. Bruschini, M. Antolovic, F. Zanella, A. C. Ulku, S. Lindner, A. Kalayanov, T. Milanese, E. Bernasconi, V. Pešić, and E. Charbon "High-efficiency fill factor recovery using refractive microlens arrays imprinted on 0.5-256 kpixel front-side illuminated SPAD imagers", Proc. SPIE 12433, Advanced Fabrication Technologies for Micro/Nano Optics and Photonics XVI, 124330K (15 March 2023); https://doi.org/10.1117/12.2652962

ACCESS THE FULL ARTICLE

INSTITUTIONAL
Select your institution to access the SPIE Digital Library.

SELECT YOUR INSTITUTION

PERSONAL
Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.

PERSONAL SIGN IN

No SPIE Account? Create one

PURCHASE THIS CONTENT

SUBSCRIBE TO DIGITAL LIBRARY

50 downloads per 1-year subscription

Members: $195

Non-members: $335 ADD TO CART

25 downloads per 1 - year subscription

Members: $145

Non-members: $250 ADD TO CART

PURCHASE SINGLE ARTICLE

Includes PDF, HTML & Video, when available