Infrared Internal Reflection Spectroscopy of Silicon Dioxide on Silicon

The oxidation of silicon forms the basis of a multibillion dollar electronics industry. With increasing miniaturization required for the next generation of integrated circuits, understanding the structure of thin oxides and the oxide-silicon interface has become increasingly important. In contrast to analytical tools such as X-ray photoelectron spectroscopy that use high energy radiation capable of altering the structure under study, infrared spectroscopy uses benign low energy radiation to probe molecular bonds. I devised a previously unrecognized application of infrared internal reflection spectroscopy to analyze oxides as thin as 10A on silicon. The technique exploits displacement continuity in a thin low refractive index material sandwiched between two high refractive index materials to achieve an unprecedented spectral amplification. Spectra of the thin oxides revealed peaks at 1080 and 1240 cm^{-1}. The 1080-cm^{-1} peak was an asymmetrical Si-O-Si stretching vibration. With the aid of an infrared polarizer, experiments on a variety of oxides identified the 1240-cm^{ -1} peak as a longitudinal optical phonon mode. The experiments also supported a similar interpretation of the 1230-cm^{-1} peak in oxygen-containing silicon that had been spuriously attributed to a variety of origins such as crystobalite or combination modes of interstitial oxygen. One of the more interesting results of this research is that it showed that there was negligible native oxide growth (native oxide defined as SiO_{ rm x} with 1 <=q x <=q 2) within the typical time scale of electronics processing. A native oxide of 10 -15A is generally thought to form instantly on exposure of the clean silicon surface to air at room temperature. This oxide has been considered the cause of a variety of problems in silicon processing. The "native oxide" here is attributed to an ellipsometric artifact of surface roughness and to adsorbed contamination such as water and hydrocarbons. Because adsorbed contamination is less tightly bound to the silicon surface than oxide, this finding has far-ranging processing ramifications.