Interpixel capacitance in nondestructive focal plane arrays

Abstract

Inter-pixel capacitive coupling can exist in a non-destructive detector array if the detector nodes change voltage as they integrate charge and the design of the device allows for an electric field to exist between adjacent collection nodes. Small amounts of inter-pixel capacitance can cause large errors in the measurement of poissonian noise versus signal, and all subsequently derived measurements such as nodal capacitance and quantum efficiency. Crosstalk and MTF can also be significantly influenced by interpixel capacitance. Two 1k by 1k Raytheon SB226-based hybridized silicon PIN arrays were tested for nodal capacitance and MTF. Initial results indicated unexpected and unexplainably large nodal capacitance, poor MTF, and odd edge spread. It was hypothesized that inter-pixel capacitive coupling was responsible for these discrepancies. A stochastic method of measuring the coupling using 2D autocorrelation and Fourier Transform techniques was devised and implemented. Autocorrelation of the shot noise in the images revealed a correlation consistent with 3.2% interpixel capacitive coupling. When the effects of the measured interpixel capacitance were taken into account, the initially measured nodal capacitance of 56 fF was found to be 31% higher than the corrected nodal capacitance measurement of 43 fF. Large discrepancies between the theoretical and observed edge spread response were also greatly reduced. A simulation of the electric field in the PIN detector intrinsic region predicted an interpixel coupling very close to the observed coupling. Interpixel capacitance was also observed in a 2k by 2k Raytheon SB304-based InSb detector array, but was not strongly evident in a bare Raytheon SB226 multiplexer.