Infrared-absorption spectroscopy of Si(100) and Si(111) surfaces after chemomechanical polishing

Abstract

The mechanism of silicon stock removal in chemomechanical polishing (CMP) is studied by characterizing surface chemical species with infrared-absorption measurements and the corresponding degree of hydrophobicity with contact angle measurements immediately after CMP. Surface properties and stock removal rates are found to depend strongly on the pH of the silica slurry used in this ‘‘syton polishing’’ technique. At the peak of the removal rate [pH∼11 for both Si(100) and Si(111)], the surfaces have the highest hydrophobicity and the highest hydrogen coverage. Si(111) has an ideal monohydride termination, while Si(100) is characterized by a variety of hydrides (mono-, di-, and trihydrides), suggesting different morphologies for the surfaces: atomically flat domains on Si(111) and rougher areas on Si(100). Away from the optimum slurry pH (at lower stock removal rates), a higher concentration of hydroxyl groups is observed, increasing the surface hydrophilicity. At all pH, some oxidation occurs beneath the H-terminated Si surface, as evidenced by a characteristic frequency shift of oxygen-backbonded hydrides. The mechanisms of stock removal are considered in view of these observations for the different ranges of slurry pH. In particular, at the highest removal rates, an interplay of surface oxidation, removal of oxidized silicon, and subsequent H termination is suggested. Based on the spectroscopic characterization of surface morphologies, the relevance of CMP to prepare atomically smooth silicon surfaces is discussed.