Enhancing the fill-factor of CMOS SPAD arrays using microlens integration

G. Intermite*, R. E. Warburton, A. McCarthy, X. Ren, F. Villa, A. J. Waddie, M. R. Taghizadeh, Y. Zou, F. Zappa, A. Tosi, G. S. Buller

*Corresponding author for this work

Research output: Chapter in Book/Report/Conference proceedingConference contribution

6 Citations (Scopus)


Arrays of single-photon avalanche diode (SPAD) detectors were fabricated, using a 0.35 mu m CMOS technology process, for use in applications such as time-of-flight 3D ranging and microscopy. Each 150 x 150 mu m pixel comprises a 30 mu m active area diameter SPAD and its associated circuitry for counting, timing and quenching, resulting in a fill-factor of 3.14%. This paper reports how a higher effective fill-factor was achieved as a result of integrating microlens arrays on top of the 32 x 32 SPAD arrays. Diffractive and refractive microlens arrays were designed to concentrate the incoming light onto the active area of each pixel. A telecentric imaging system was used to measure the improvement factor (IF) resulting from microlens integration, whilst varying the f-number of incident light from f/2 to f/22 in one-stop increments across a spectral range of 500-900 nm. These measurements have demonstrated an increasing IF with f-number, and a maximum of similar to 16 at the peak wavelength, showing a good agreement with theoretical values. An IF of 16 represents the highest value reported in the literature for microlenses integrated onto a SPAD detector array. The results from statistical analysis indicated the variation of detector efficiency was between 3-10% across the whole f-number range, demonstrating excellent uniformity across the detector plane with and without microlenses.

Original languageEnglish
Title of host publicationPhoton Counting Applications 2015
Editors Prochazka, R Sobolewski, RB James
Place of PublicationBellingham
Number of pages12
ISBN (Print)9781628416251
Publication statusPublished - 2015
EventSPIE Conference on Photon Counting Applications 2015 - Prague, Czech Republic
Duration: 13 Apr 201515 Apr 2015

Publication series

NameProceedings of SPIE
ISSN (Print)0277-786X


ConferenceSPIE Conference on Photon Counting Applications 2015
Country/TerritoryCzech Republic


  • CMOS SPAD array
  • SPAD imager
  • diffractive microlens
  • NM

ASJC Scopus subject areas

  • Applied Mathematics
  • Computer Science Applications
  • Electrical and Electronic Engineering
  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics


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