Spatial distortions in solid-state semiconductor imagers in x-ray region (abstract)

Abstract

When detecting x-ray images by solid-state semiconductor imagers (charge-coupled devices, photodiode arrays) there arise specific distortions caused by the diffusion of minority photogenerated carriers in the semiconductor substrate. The mode and scale of the diffusion and, accordingly, of distortions which arise are determined by properties of both the detector and the image. In this paper, some aspects of the diffusion are experimentally studied and the results obtained are interpreted on the basis of a theoretical model obtained in definite approximations. The result of the theoretical model is presented as the Fourier transfer function of the time and spatial frequencies, normalized over the quantum efficiency. This function incorporates the following parameters: the diffusion length and diffusion factor for minority carriers, the depth of the depletion regions of sensitive cells, the thickness of the epitaxial layer (or the single-crystal substrate), the linear absorption coefficient of radiation for a semiconductor material, and the angle of incidence of the radiation on the device surface. For the corresponding point-spread function, among the properties illustrated experimentally are the peculiarity providing a good geometrical resolution, the formation of a ‘‘stepped’’ profile at the grazing incidence, the formation of the ‘‘halo’’ of the scattered charge. The results of the x-ray and optic measurements have also shown that diffusion from the passive (peripheral) regions, which determines, to a considerable extent, the applicability of linear imagers (lines) in high-resolution systems, contributes appreciably to the distortions. The authors have considered some ways of diminishing the distortions which are accessible to experimenters: diaphragming and oblique incidence of the radiation, and an increase of the depths of the depletion regions of the cells. In addition, the authors dwell briefly upon the influence of the diffusion on the quantum noise and the effect of spectra superimposition, the contribution of x-ray fluorescence and the photoelectron range to the distortions. As an illustration, the possibility of utilizing solid-state imaging detectors to x-ray tomography systems has been evaluated.