PHYSICAL CHARACTERIZATION OF ULTRATHIN ANODIC SILICON-OXIDE FILMS

Ultrathin oxides formed on p-type (100) Si using anodic oxidation in d ilute aqueous NH4OH solution have been characterized by Fourier transf orm infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XP S), and x-ray reflectometry. The aim of the work was to optimize the g rowth and annealing conditions for fabrication of ultrathin gate oxide s. Two alternate growth conditions (potentiostatic and galvanostatic) could be used to grow oxides of thickness between 3 and 16 nm. There w as very little difference between the two types of oxides; however, th e FTIR asymmetric stretch maximum nu(m) was at slightly higher frequen cies and this band was slightly narrower for potentiostatic oxides com pared to galvanostatic oxides of the same thickness. For both types of films, nu(m) increased with film thickness, while the corresponding f ull width at half-maximum decreased. As-grown approximately 11-nm-thic k films of both types contain 3.8 +/- 0.3% -OH (bound as isolated sila nol) and 5.0 +/- 0.4% -OH (bound as H2O and/or associated silanol) by mass, and have a density of 2.05 +/- 0.03 g cm-3 compared with a densi ty of 2.27-0.03 g cm-3 measured for thermal oxides. Thus, the composit ion of the as-grown anodic oxides can be written as SiO1.93(OH)0.14.0. 18H2O. Discounting the H content, this converts to an O/Si ratio of 2. 25 +/- 0.02, which can be compared to the O/Si ratio of 2.27 +/- 0.06 measured for as-grown films by XPS. Potentiostatically grown approxima tely 11-nm-thick films were annealed at temperatures between 300 and 9 00-degrees-C in forming gas. Two different stages were observed as a f unction of anneal temperature. At temperatures below 500-degrees-C, wa ter and/or associated silanol was ejected from the films. This resulte d in a maximum in the stress and/or disorder in the oxides at anneal t emperatures of 500-degrees-C. At temperatures above 500-degrees-C, the remainder of the silanol was removed from the films; some kind of str ess relief occurred. The oxides became stoichiometric at temperatures 700-degrees-C and above.