Vaporized water clusters were produced by an adiabatic expansion phenomenon, and various substrates such as Si(100), SiO2, polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), and polycarbonate (PC) were irradiated by water cluster ion beams. The sputtered depth increased with increasing acceleration voltage, and the sputtering rate was much larger than that obtained using Ar monomer ion irradiation. The sputtering yield for PMMA was approximately 200 molecules per ion, at an acceleration voltage of 9 kV. X-ray photoelectron spectroscopy (XPS) measurements showed that high-rate sputtering for the PMMA surface can be ascribed to the surface erosion by the water cluster ion irradiation. Furthermore, the micropatterning was demonstrated on the PMMA substrate. Thus, the surface irradiation by water cluster ion beams exhibited a chemical reaction based on OH radicals, as well as excited hydrogen atoms, which resulted in a high sputtering rate and low irradiation damage of the substrate surfaces.

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We have investigated impact-process of water cluster ions on solid surfaces. Various kinds of substrates such as Si(100), SiO2 and PMMA substrates were irradiated by adjusting the acceleration voltage and the ion dose. The sputtered depth of these substrates increased with increase of the acceleration voltage, and it was 37 nm for Si, 49 nm for SiO2, and 2.9 µm for PMMA substrates, respectively at an acceleration voltage of 9 kV and an ion dose of 1.0×1016 ions/cm 2 . The sputtering yield calculated was 19 atoms per ion for Si and 13 molecules per ion for SiO2, which was approximately ten times larger than that for Ar monomer ion irradiation. The XPS measurement showed that the Si substrate surfaces sputtered at an ion dose of 1.0×1016 ions/cm 2 had an oxide layer such as SiOx. The oxide layer thickness increased with increase of the acceleration voltage, and it was approximately 10 nm at an acceleration voltage of 6 kV. On the other hand, at lower ion doses such as 1.0×1014 ions/cm 2 , the ratio of the sputtered depth of Si surface to SiO2 surface became approximately 10. This indicated that the chemical erosion such as silicon hydride occurred, resulting in enhancement of chemical sputtering of Si substrates. Furthermore, for the case of PMMA substrates, the chemical erosion of the substrate surfaces occurred probably through the exchange between CH3 and H radicals, and the chemical sputtering by the ejection of methacrylic acid molecule was enhanced. The surface roughness of PMMA substrates irradiated was less than 2 nm, and the smooth surface at the nano-level was obtained. In addition, micro-patterning was demonstrated with the water cluster ion beams on the PMMA substrates.

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