PHYSICAL CHARACTERIZATION OF ULTRATHIN ANODIC SILICON-OXIDE FILMS

Ultrathin oxides formed on p-type (100) Si using anodic oxidation in dilute aqueous NH4OH solution have been characterized by Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), and x-ray reflectometry. The aim of the work was to optimize the growth and annealing conditions for fabrication of ultrathin gate oxides. Two alternate growth conditions (potentiostatic and galvanostatic) could be used to grow oxides of thickness between 3 and 16 nm. There was very little difference between the two types of oxides; however, the FTIR asymmetric stretch maximum nu(m) was at slightly higher frequencies and this band was slightly narrower for potentiostatic oxides compared to galvanostatic oxides of the same thickness. For both types of films, nu(m) increased with film thickness, while the corresponding full width at half-maximum decreased. As-grown approximately 11-nm-thick films of both types contain 3.8 +/- 0.3% -OH (bound as isolated silanol) 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 density of 2.27-0.03 g cm-3 measured for thermal oxides. Thus, the composition 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 approximately 11-nm-thick films were annealed at temperatures between 300 and 900-degrees-C in forming gas. Two different stages were observed as a function of anneal temperature. At temperatures below 500-degrees-C, water and/or associated silanol was ejected from the films. This resulted in a maximum in the stress and/or disorder in the oxides at anneal temperatures of 500-degrees-C. At temperatures above 500-degrees-C, the remainder of the silanol was removed from the films; some kind of stress relief occurred. The oxides became stoichiometric at temperatures 700-degrees-C and above.