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

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

Characterization of the Interface in Rubber/Silica Composite Materials

Abstract: in the rubber industry, especially tyre production, the most widely used elastomer is styrene-butadiene rubber (SBR) in which the styrene units generally are present at 25 wt.%. The unit repeats are random distributed along the macromolecular chains and the configuration of the butadiene units is mostly 1,4-trans. The elastomer blends are mixed with fillers, acting as reinforcing agents, and the effect that silica particles have on their physical and mechanical properties is now well established. One of the most important parameters in determining the performance of such composite materials is the degree of adhesion at the rubber/silica interface. In this context, the interface characterization has been performed through a spectroscopic investigation (XPS/x-ray-induced Auger electron spectroscopy) in order to derive information from core-level and Auger line chemical shifts. A series of composite rubbers have been examined by means of a detailed curve-fitting procedure that allows the determination of intrinsic and extrinsic structures connected to each photopeak and the spectroscopic results compared with those of reference compounds. The changes in the C 1s lineshape and shake-up region of the polymers and the reduced binding energies of silicon, oxygen and sulphur core lines have provided clear evidence of interfacial reactions. Moreover, the Auger parameters of silicon show systematic shifts that can be interpreted on a chemical state plot in terms of initial- and final-state contributions and used for theoretical investigation of the local chemical environment. Copyright (C) 2002 John Wiley Sons, Ltd

Optimization of epitaxial growth for thick Ge-on-Si structures used for single photon avalanche diode applications

SPAD structures have been grown by RP-CVD and shown to have: excellent crystallinity, with low TDD; a smooth surface, suitable for device incorporation; and sharp doping profiles required to maximize performance. Device measurements have produced the highest SPDE of any Ge on Si SPAD recorded

Design and performance of a prototype mesa-geometry Ge-on-Si single-photon avalanche diode detector at 1310 nm and 1550 nm wavelengths

Ge-on-Si single-photon detectors are fabricated and characterized at 1310 and 1550 nm. 4 % single-photon detection efficiency is observed at 1310 nm demonstrating the lowest reported noise equivalent power for Ge-on-Si single-photon detectors (1×10-14 WHz-1/2)