;The thin CuO flm is acquired by a quick dip of copper in H2O2 solutions at room temperature. The CuO filmq appears smooth and exhibits superhydrophilic nature. The composition change cannot be verifed by X-ray photoelectron spectroscopy but can be manifested by the water contact angle. In the ambient condition, the thickness of the oxidized layer and the surface hydrophobicity grow gradually, while the chemical composition of the overall oxidized film remains essentially unchanged. That is, in the vacuum, the growth rate of the hydrophobicity is significantly elevated, revealing deoxidation on the upmost surface. Our results indicate that growing hydrophobicity on the CuO film is spontaneous and the reversible wettability transition can be observed by H2O2 oxidation and vacuum deoxidation.
The wet cleaning process in semiconductor fabrication often involves the immersion of the copper wafer into etching solutions and thereby its surface properties are significantly altered. The wetting behavior of a copper film deposited on silicon wafer is investigated after a short dip in various etching solutions. The etchants include glacial acetic acid and dilute solutions of nitric acid, hydrofluoric acid, and tetramethylammonium hydroxide. A thin oxide layer always remains on the surface of as-received Cu wafers when they are subject to etching treatments. A pure Cu wafer can be obtained by the glacial acetic acid treatment and its water contact angle (CA) is about 45. As the pure Cu wafer is placed in the ambient condition, the oxide thickness grows rapidly to the range of 10 to 20 Å within 3 hours and the CA on the hydrophilic surface also rises. In the vacuum, it is surprising to find that the CA and surface roughness of the pure Cu wafer can grows significantly. These interesting results may be attributed to the rearrangement of surface Cu atoms to reduce the surface free energy.
