A method of tungsten dopant deposition for dual-dielectric charge-storage cells

Electron-beam evaporation of small single-crystal ingots of tungsten has been employed as a laboratory-scale method for introducing the tungsten interfacial dopant in dual-dielectric charge-storage cells. Several other tungsten-evaporation methods, which are potentially more suitable for large-scale manufacturing operations, are evaluated. They are: 1) evaporation from resistively heated tungsten; 2) evaporation of tungsten trioxide powder from a resistively heated crucible; and 3) reactive evaporation of tungsten trioxide from resistively heated tungsten in a low-pressure ambient of oxygen. The latter method, which seemed the most attractive, was tested and was found to be a practical alternative to the electron-beam method. It possesses the advantages of low operating temperatures, control of small deposition rates to produce tungsten trioxide deposits in the submonolayer range of coverage, pure deposits, and requires a minimum of operator attention. Furthermore, sources can have a long operating life.