Directly deposited optical-blocking filters for single-photon x-ray imaging spectroscopy

Optical blocking performance of CCDs developed for the X-ray Astronomy Satellite XRISM

“…It was possible for the light scattered in the sheet to enter the imaging area. Similar cases are reported by (Bautz et al [16] and Ryu et al [10]) as backside leakage. To block the entering light, we additionally formed an aluminum filter on the backside electrode (under the passivation) layer of the XRISM CCDs with a width of 2 mm from the edge of the silicon substrate.…”

“…When visible light passing through the voids is scattered in the silicon dioxide layer between the OBL and silicon substrate, false signals are generated as a single-or multi-pixel event at each position under the pinholes and its surroundings (within 3 ×3 pixels at a maximum). A similar phenomenon is reported on a ground-base test of CCDs for Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx [9,10]). They indicate that it is possibly due to particles or surface irregularities of the silicon substrate under the aluminum layer.…”

“…They speculate that the light entered through the edge of the MAXI CCD, whereas in our case the boundary around the imaging area of the Hitomi CCD has a dummy pixel region with a width of 250 µm, that is larger than the attenuation length of visible light from the Sun. We thus conclude that the Hitomi CCD has another light path and that a sidewall coating of a silicon layer (e.g., [10]) is less effective for our case.…”