Investigations on hydrophilic and hydrophobic silicon (100) wafer surfaces by X-ray photoelectron and high-resolution electron energy loss-spectroscopy

Abstract

The paper reports on surface spectroscopy measurements of silicon single-crystal wafers which have been treated in order to obtain hydrophilic and hydrophobic surfaces, respectively. The wafers are characterized in terms of the oxidation behaviour in air (“native oxides”), their surface chemical composition and the chemical bonds involved. It is shown that the oxide on hydrophilic wafers mainly grows in the cleaning agent and consists of hydrated SiO₂ through all stages of the growth. On a hydrophobic surface, however, the oxidation begins with the formation of a lower oxidation state which turns into SiO₂ on storage in air. The thickness of the oxides on both surface types reaches 1.4–1.5 nm. Both the chemical shift in photoelectron spectroscopy and the frequency of the asymmetric Si-O-Si vibration in electron energy loss spectroscopy support the assumption of a reduced bonding angle of the oxygen bridge.

Hydrophilicity is caused by singular and associated OH groups on the surface. Singular groups could be detected up to 700 K. There are hints that OH groups stabilize the oxide during heating. The hydrophobic state is mainly characterized by Si-H and Si-CH _x groups on the surface, whereas Si-F exists only in minor quantities. Si-H groups were stable up to approximately 900 K in UHV. Si-CH _x dehydrogenizes at temperatures between 500 and 700 K leaving SiC on the surface.