Patent GB-1432121-A (GB)
Title: Method and system for operation of conductor-insulator-semi conductor capacitor memory having increased storage capability
IPC: H01J29/44
Filed: 1973-03-14
Abstract: 1432121 Semi-conductive stores MICRO BIT CORP 14 March 1973 12364/73 Heading G4C [Also in Division H1] In operating a memory of the M1S type based on P type semi-conductor, prior to writing in the data to be stored, positive charge is uniformly distributed through the entire area of the insulating layer to form an inversion layer and underlying depletion layer in the underlying semi-conductor by directing a defocused or raster-scanning beam of penetrating electrons at the metal layer while it is positively biased relative to the semi-conductor. Binary "0s" are then written by using the same technique but with the bias reversed to remove the charge from selected storage bit sites constituted by areas of the insulation the radius of which is larger by at least one electron diffusion length than that of the electron beam used in reading out information and binary ones by topping up the charge at selected bit sites. In reading out the metal is negatively or zero biased and the electron beam centred on the bit site to be interrogated. Beneath the insulation, except at binary "0" bit sites, the highly conductive inversion layer and associated depletion layer remain. Electronhole pairs are generated beneath the electron beam and if the beam is centred on a binary -1 site are separated by the depletion layer, the electrons reaching and flowing along the inversion layer to charge up the device capacitance. When the beam is centred on a binary "0" site separation does not occur and most generated electrons recombine before reaching the inversion layer around the site so that no charging current flows. Similar results can be achieved when using N type semi-conductor by initially clearing the insulation of charge, using reversed polarities for writing binary "0s" and "1s" and applying a positive bias during read out large enough to cause inversion beneath the uncharged areas of insulation. As described a plurality of MIS elements are disposed in an octagonal, hexagonal or rectangular array in a vacuum tube comprising an electron beam source and a fly's eye lens, with each element disposed beneath a respective lenslet, there being a common coarse deflection system for directing the beam to one of the lenslets and separate fine deflection systems associated with each lenslet. Writing and reading is controlled by a computer issuing instructions to circuitry controlling the actuation and deflection of the electron beam and biasing of the MIS elements. In writing the beam is deflected to a particular site and then either partially defoctised or caused to trace a circular path so that the bombarded area is greater than the normal beam area. Undesired charging or discharging of the element as a whole or sites adjacent that being addressed, due to electron scatter is minimized by optimizing the metal layer thickness and treating parts of the tube to reduce multiple reflections and back-scattering, and compensated for by reversing the polarity of bias after writing information at a site while the electron beam is incident on the centre of the site for a time related to the writing time. A similar step is performed after reading each bit of formation. Each bit may be rewritten directly after reading. In addition after a predetermined number of cycles the memory is scanned to read and rewrite each bit of information and periodically the information in the memory is read out, stored in a peripheral memory and then completely rewritten starting from the uniform charging step. Suitable materials for the target are Si, Ge, InSb, InAs and GaAs as semi-conductor; SiO or SiO 2 , or ferroelectric BaTiO 3  as insulator, and Al, Ag, Ni or Au as metal though preferably the structure consists of a 500 Š layer of Al evaporated on a layer of SiO 2  thermally grown to a thickness of 700 Š on a À2-20 ohm cm. P type silicon wafer, the electron diffusion length being reduced by diffusing in gold and/or annealing in oxygen. Storage sites may be 10 Á diameter with a centre to centre spacing of 20 Á when a 5 Á diameter 10 KV. electron beam is used.