Development of high transmittance, back-illuminated, silicon-on-sapphire substrates for high quantum efficiency and high resolution avalanche photodiode imaging arrays

There is a growing need in medical, scientific and industrial applications for dual-mode, passive and active 2D and 3D LADAR imaging methods. To fill this need, solid state, single-photon sensitive silicon avalanche photodiode (APD) detector arrays offer high sensitivity and the possibility to operate with wide dynamic range in dual linear and Geiger-mode for passive and active imaging. Although silicon avalanche photodiodes offer a promising solution, large scale, high quantum efficiency and high resolution arrays have not been developed yet, primarily due to the increased fabrication complexity of such detector devices and arrays compared to the more common, non-avalanching detectors such as CCDs and CMOS-APS devices. One major fabrication challenge for avalanche type detectors is the requirement of providing effective direct and indirect optical crosstalk isolation between detectors in an array since the avalanche gain process produces photons that could create false detection events in pixels nearby and at a distance thereby increasing the noise. While it is common for CCD arrays to have a pixel pitch between 12-30 μm and for CMOS-APS devices to have pixel pitch below 10 μm, it is more challenging to architect arrays of avalanche photodiodes for example, having such a small pitch due to optical crosstalk. The second major fabrication challenge for linear mode avalanche type detectors, especially critical in arrays is the detector gain uniformity. Detector gain uniformity is a critical performance parameter since an increase in gain excess noise will make the detector arrays unsuitable for precision metrology applications. As solid-state avalanche detectors are made smaller, it becomes more difficult to control the gain excess noise due to smaller area multiplication regions where the effects from slight variations in doping profiles and electric fields produce greater gain variability compared to larger area detectors.