KrF-excimer-laser-induced native oxide removal from Si(100) surfaces studied by Auger electron spectroscopy

The mechanism of KrF-excimer-laser cleaning of Si(100) surfaces was studied by Auger Electron Spectroscopy (AES) and Low-Energy Electron Diffraction (LEED) spectroscopy. The dependence of the cleaning efficiency on the laser fluence was investigated by using a mildly focused laser beam and carefully measuring the energy density distribution of the laser spot impinging on the sample. These values were compared with the AES spectra measured in different points of the irradiated area and with the morphology observed by optical microscopy. Samples as received from the manufacturer were first investigated. It was found that desorption of weakly bonded organic adsorbates occurs at energy densities as low as 0.3 J/cm2, whereas significant oxide removal takes place only at an energy density above 0.8 J/cm2, which produces damaged surface morphologies. The experimental findings, in agreement with the temperatures calculated for the laser-induced Si heating, indicated that a large fraction of the oxide film is dissolved in the molten silicon, leading to oxygen concentration below the AES detection limit only when the melted depth was of the order of several hundred nanometers. Atomically clean, damage-free Si(100) surfaces were obtained after irradiation of samples pre-etched for 1 min in a HF: H2O (5%) solution, which had only a thin SiOx(x < 2) layer and F, C and O containing adsorbed species. Complete contaminant elimination was achieved in this case with 15 pulses at 0.8 J/cm2 without any damaging of the surface.