6 May 2015 Enhancing the fill-factor of CMOS SPAD arrays using microlens integration
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Arrays of single-photon avalanche diode (SPAD) detectors were fabricated, using a 0.35 μm CMOS technology process, for use in applications such as time-of-flight 3D ranging and microscopy. Each 150 x 150 μm pixel comprises a 30 μm 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 fnumber, and a maximum of ~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.
© (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
G. Intermite, G. Intermite, R. E. Warburton, R. E. Warburton, A. McCarthy, A. McCarthy, X. Ren, X. Ren, F. Villa, F. Villa, A. J. Waddie, A. J. Waddie, M. R. Taghizadeh, M. R. Taghizadeh, Y. Zou, Y. Zou, Franco Zappa, Franco Zappa, Alberto Tosi, Alberto Tosi, Gerald S. Buller, Gerald S. Buller, "Enhancing the fill-factor of CMOS SPAD arrays using microlens integration", Proc. SPIE 9504, Photon Counting Applications 2015, 95040J (6 May 2015); doi: 10.1117/12.2178950; https://doi.org/10.1117/12.2178950

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