Patent GB-1350696-A (GB)
Title: Photo-sensitive target for an electron tube which employs a scanning electron beam in operation
IPC: H01L23/485
Filed: 1971-05-13
Abstract: 1350696 Semi-conductor photosensitive targets; image pick-up tubes PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 13 May 1971 [16 May 1970] 14695/71 Headings HlK and HID A photosensitive target plate, e.g. for a television camera tube wherein a mosaic of rectifying junction regions of a semi-conductor plate is scanned by a coil deflected electron beam and a scan is projected on to the opposite face of the semi-conductor plate (Fig. 1, not shown), comprises a N-type Si substrate 12 on which a mosaic of P-type zones 15 are provided on one side to form rectifying PN junctions separated by inter-zonal grooves 20. An insulant layer 13 having windows 14 overlying the P- type zones 15 is covered with a mosaic 25 of electrical conductors connected to the P-type zones through the windows and gaps 17 lying above the grooves separating the parts of the insulant and conductive layers. The gaps are narrower than the grooves, over which layers 13, 25 project, and zones 15 form a mosaic of rectifying regions with the substrate 12. Operationally, the substrate 12 is positively biased relatively to the electron beam cathode, and the beam changes in turn the respective regions 25, 15 to cathode potential so that the rectifying junctions are successively reverse biased, and the regions 25, 15 are subsequently discharged in dependence on the radiation intensity incident on the other side of the target proximate to the relevant region, which is again charged on arrival of the electron beam in course of scan. A current in a load in the substrate supply is dependent on the radiation intensity discharging successive regions during the scanning cycle. The gaps are insulant coated and provided with a conductive grid in the bottom thereof to remove stray electrons penetrating the gaps so as to reduce output perburtrations. In manufacture, a monocrystal N-type Si plate 12 is oxidized to form an insulant layer 13 in which a mosaic of windows 14 is formed for 13 indiffusion to establish P-type zones 15 providing rectifying PN junctions with the substrate. Layers of borosilicate glass are formed in the windows and a network of gaps 17 is etched in the insulant over photoresist using NH 4 F + HF. Grooves 20 are similarly etched in the substrate using HF +HNO 3  to be wider than gaps 17, and an oxide coating 21 is formed thermally on the groove walls. Thereafter the target is etched from the opposite side to the required thickness, and P is in-diffused, while the borosilicate glass is removed from the windows over a photoresist mask. An Al or Au over Ti conductor is vapour deposited to provide contact regions 25 and conductor grid 26 in the grooves, and may be thickened by electroplating. Polycrystalline acceptor doped vapour deposited Si is an alternative to metal, (Fig. 5). In a modification (Figs. 6 to 9, not shown), boron is diffused into a N-type monocrystal Si substrate to form a P-layer, masked with Si 3 N 4  and SiO 2 , in which a network of gaps is etched over photoresist by HF and H 3 PO 4 . Windows are then etched into the SiO 2  layer of the discrete rectangular sections by HF over photoresist. Grooves are then etched in the semi-conductor substrate through the gap network to be wider than the gaps using HF + HNO 3 , and a channel blocking n + zone is formed by diffusing Ag in to the groove walls, after which a SiO 2  layer is formed by steam oxidation. The opposite side of the plate is etched to the required thickness and Si 3 N 4  is removed by etching with H 3 PO 4  to expose the surfaces of the P-type zones, leaving overhanging SiO 2  layers. Conductive layers of metal or polycrystal Si pre vapour deposited on the latter layers and on the windows to form a scannable mosaic of rectifying junction regions, while a conductive grid is formed in the bottom of the grooves. Alternatively the PN junctions may be formed epitaxially and the conductive layers may be alloys or may be deposited by electroplating. The PN junctions may be transposed with reversal of polarity. Si may be replaced by Ge or A<SP>III</SP> B<SP>V</SP> or A<SP>II</SP> B<SP>VI</SP> compounds. Photosensitive heterojunctions may alternatively be used.