High-precision x-ray reflectivity study of ultrathin SiO2 on Si

We have developed a high-precision difference-x-ray-reflectivity technique using intense synchrotron radiation and applied this to evaluate native oxides and ultrathin thermal oxides on Si(100). We have successfully evaluated the density of native oxides. Native oxides formed by HCl and NH4OH solutions have a low density, in contrast to the oxides formed by H2SO4 solution and UV/O3 whose densities are close to those of thermal oxides. By carefully analyzing ultrathin thermally grown oxides with thicknesses of 40 and 70 Å grown at 800–1000 °C, we have revealed the existence of a dense (∼2.4 g/cm3), thin (∼10 Å) layer at the SiO2/Si interface. Oxides grown in O3 or HCl/O2 have a thinner interfacial layer compared to those grown in O2. We have evaluated the effects of ambient and temperature at oxidation on the interfacial layer and the SiO2 layer.