Surface composition analysis of HF-vapour-cleaned silicon by X-ray photoelectron spectroscopy

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Abstract
X-ray photoelectron spectroscopy (XPS) measurements on silicon surfaces treated by HF gaseous cleaning are described. Various cleaning recipes, which essentially differ by the amount of water present during the reaction, were studied; the composition of the silicon surface was measured in terms of monolayer coverage of oxygen, fluorine and carbon. These gaseous-cleaned surfaces are compared with the silicon samples deglazed by using an aqueous HF bath. The F(1s), O(1s), Si(2p), C(1s) photoelectron lines were monitored, and concentrations determined as usual by integration of the lines after removal of a nonlinear background. The F(1s), C(1s) and Si(2p) lines were decomposed into several components corresponding to different chemical bonds. The results show that the amount of fluorine is directly correlated with the amount of oxygen: fluorine concentration increases slightly with oxygen concentration from 0.2 monolayers (ML) up to 0.7 (ML); most is in a O-Si-F bonding state. For more aggressive etching, leaving less than one monolayer of oxygen, the Si-F bond becomes predominant. The ratio of the SiF to OSiF concentrations is a significant indicator of the deoxidation state of the surface. Hydrophobicity of the water appears in the range of 25% Si-F bonds. With very aggressive etching processes, 67% Si-F bonds and 33% O-Si-F bonds are reached and the total amount of fluorine drops below 0.3 ML. For comparison, only Si-F bonds are observed after wet etching in a dilute HF bath without a rinse with a much lower fluorine concentration. The balance between Si-F and O-Si-F remains stable and seems to be representative of the surface states provided by the etching process.