Plasma cleaning and nitridation of sapphire (alpha-Al2O3) surfaces: New evidence from in situ real time ellipsometry

The interaction of alpha-Al2O3 (0001) surfaces with H atoms and N atoms from remote rf plasmas used for the cleaning and nitridation processes, respectively, is investigated at temperatures in the range of 200-600 degrees C. The chemistry and kinetics of the above processes are monitored in real time by in situ spectroscopic ellipsometry. Also, the chemistry of the nitrided sapphire surfaces is verified by x-ray photoelectron spectroscopy (XPS) analysis. Specifically, H atoms treatments are effective in removing carbon contaminants from the sapphire surface at temperatures of 200-400 degrees C. Real time ellipsometry is suitable to detect the cleaning end point and to verify the onset of the H-atom diffusion into the sapphire substrate. Remote N-2 plasma nitridation at 200 degrees C is found to yield homogeneous and smooth AlN layers of about 5 Angstrom, after approximately 25 min of nitridation, whereas high nitridation temperatures result in a damaged sapphire surface with AlN protrusions. Both ellipsometric and XPS data show that the sapphire nitridation can be interpreted in the framework of a chemical model, where the formation of NO competes with AlN formation. The chemisorption equilibrium of NO, which strongly depends on surface temperature, is the key factor controlling the nitridation chemistry and kinetics