CHARGE EFFECTS IN DOPED SILICON DIOXIDE

1493954 Semi-conductor devices WESTERN ELECTRIC CO Inc 9 Dec 1974 [10 Dec 1973] 53087/74 Heading H1K A silicon dioxide layer formed on a semiconductor body contains one or more of the metals tantalum, niobium and uranium in an at least partially oxidized state, the total number of the metal ions per sq. cm. ranging from 1013 to 1À5 Î 1015 and being distributed through a region at least 50 Š thick. The ions provide non-annealable fast surface states, getter sodium ions present in the layer, or provide a stable negative charge in the layer. As described, tantalum is introduced into the silicon dioxide layer forming the gate insulation of a silicon MOSFET by ion implantation into the layer itself or into the surface silicon which is oxidized to provide the layer, or is deposited on the oxide by beam evaporation or chemical vapour deposition, or shallowly implanted and diffused in by heat treatment, the preferred method depending on where in the layer the tantalum is required. To create surface states it should be adjacent the silicon surface, for permanent storage of negative charge as close to that surface as is consistent with the avoidance of tunnelling of charges to and from the surface, e.g. 100 Š from it, while for gettering alone it should be remote from the surface to avoid unwanted electrical effects. Thus to produce surface states ions are implanted into the silicon prior to oxidation which concentrates the tantalum at the silicon-oxide interface, and at the surface of the oxide which is then removed chemically or mechanically. Gettering and charge storage are effected by specified heat treatments in helium with an electrical field across the layer.