In this work, we first demonstrate the interface roughness as a function of oxide thickness. For intentionally roughened samples, interface roughness always decreases as oxide thickness increases. On the other hand, samples only treated with a regular preoxidation clean show a different quantitative trend. For such samples, the interface is initially roughened, and then becomes smooth as oxide thickness increases. This implies that the evolution of interface roughness depends on:how samples are prepared. Our results suggest that the most important factor dictating interface morphology is the oxide thickness, rather than thermal history and growth ambient. Instead of using simple statistical descriptions, such as root-mean-square roughness, we also employ power spectral density on atomic force microscopy image analysis. Our results suggest that the roughness components of higher spatial frequencies are greatly reduced during thermal oxidation.