Simulated and measured data-line parasitic capacitance of amorphous silicon large-area image sensor arrays

Abstract

Calculations and measurements of parasitic capacitance in active matrix sensor arrays used for light and x-ray imaging are presented. We focus on arrays with continuous sensor layers and base the calculations on actual structures used for x-ray and light imaging. Different cross sections of the pixel allow the various components of the capacitance from the thin film transistor, the sensor, and metal crossovers to be determined by numerical two-dimensional solution of Poisson’s equation. The calculations give the total and the individual components of the parasitic capacitance in the data line, and allow us to evaluate their effect on electronic noise and imager sensitivity. The theoretical values are compared to measurements performed on arrays with 75 μm pixel pitch, showing agreement within 10%–20%. The numerical simulations are used to determine the optimized array configuration that can reduce the parasitic capacitance to ∼6 fF/pixel, which is only 15% of the present values. The capacitance is compared for direct and indirect x-ray detection imagers, using PbI2 and a-Si:H sensor layers, respectively.