Three-Dimensional MOS Process Development

A novel MOS technology for three-dimensional integration of electronic circuits on silicon substrates was developed. Selective epitaxial growth and epitaxial lateral overgrowth of monocrystalline silicon over oxidized silicon were employed to create locally restricted silicon-on-insulator device islands. Thin gate oxides were discovered to deteriorate in ambients typically used for selective epitaxial growth. Conditions of general applicability to silicon epitaxy systems were determined under which this deterioration was greatly reduced. Selective epitaxial growth needed to be carried out at low temperatures. However, crystalline defects increase as deposition temperatures are decreased. An exact dependence between the residual moisture content in epitaxial growth ambients, deposition pressure, and deposition temperature was determined which is also generally applicable to silicon epitaxy systems. The dependences of growth rates and growth rate uniformity on loading, temperature, flow rates, gas composition, and masking oxide thickness were investigated for a pancake type epitaxy reactor. A conceptual model was discussed attempting to describe the effects peculiar to selective epitaxial growth. The newly developed processing steps were assembled to fabricate three dimensional silicon-on-insulator capacitors. These capacitors were electrically evaluated. Surface state densities were in the order of 1O11cm-2 eV-1 and therefore within the range of applicability for a practical CMOS process. Oxidized polysilicon gates were overgrown with silicon by epitaxial lateral overgrowth. The epitaxial silicon was planarized and source and drain regions were formed above the polysilicon gates in Silicon-on-insulator material. The modulation of the source-drain current by bias changes of the buried gate was demonstrated