Photodetectors Based on Back-Illuminated Silicon Photodiode Arrays for X-ray Imaging Systems

The properties of ultra-thin, pin photodiode arrays designed for CT and other scintillator-based applications are discussed. The photodiode arrays were built on 75-μm thick single Si dies. The element size varied from over 1.5 mm to 200-μm with the gaps (dead spaces) between adjacent elements as small as 25 μm. The most pronounced features of the arrays described here include very small AC and DC crosstalk very low leakage currents, high shunt resistance, very fast signal rise time, and good photo-response linearity with radiation flux. The crosstalk depended on the dead spaces between pixels but was always better than 0.1% for the arrays with close to 25-μm gaps and better than 0.01% for arrays with over ~50-μm gaps. The photo-sensitivity linearity was found to be better than 0.01% within the spectral range from 450 to 1000 nm and in the range of input light fluxes above ~100 pW/pixel. For lower light fluxes, the non-linearity of the photo-sensitivity was hindered by the noise current of the array pixels and different methods should be applied to provide measurements with an accuracy of better than 0.1%. This work also gives theoretical estimations for the detectivity limits and sensitivity linearity characteristics of the arrays and compares them with those for the conventional arrays used in X-ray imaging systems.