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

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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