Characterisation of regional variations in a stitched CMOS active pixel sensor

Stitched, large area, complementary metal-oxide-semiconductor (CMOS), active pixel sensors (APS) show promises for X-ray imaging applications. In this paper we present an investigation of the effects of stitching on uniformity of sensor response for an experimental APS. The sensor, known as LAS (large area sensor), was made by reticular stitching onto a single silicon wafer of a 5×5 array of regions consisting of 270×270 pixels with 40 μm pixel pitch, to yield 1350×1350 pixels and an imaging area of 54×54 mm. Data acquired from two different sensors of the same type were filtered to remove spiking pixels and electromagnetic interference (EMI). The non-linear compensation (NLC) technique for CMOS sensor analysis was used to determine the variation in gain, read noise, full well capacity and dynamic range between stitched regions. Variations across stitched regions were analysed using profiles, analysis of pixel variations at stitch boundaries and using a measurement of non-uniformity within a stitched region. The results showed that non-uniformity variations were present, which increased with signal (1.5–3.5% at dark signal, rising to 3–8%). However, these were found to be smaller than variations caused by differences in readout electronics, particularly at low signal levels. The results suggest these variations should be correctable using standard calibration methods.