Size analysis of water cluster ions and their irradiation effects on solid surfaces

This article reports the development of a liquid cluster ion source. When water vapor was injected through a nozzle into a high-vacuum region, water clusters were produced at vapor pressures greater than 1 atm. The size of the water cluster ions was measured by the time-of-flight (TOF) method, and it was distributed between a few hundreds and a few tens of thousands of molecules. The intensity of the water cluster ions increased with increase of the vapor pressure, and the peak size was about 2500 molecules per cluster. The effects of irradiation by water cluster ions on solid surfaces such as Si(100) and SiO₂ surfaces were investigated. The sputtered depth of the Si and SiO₂ surfaces increased with increase of the acceleration voltage, and the SiO₂ surfaces were sputtered more than the Si surfaces. The sputtering yields at an acceleration voltage of 9 kV were 17.8 atoms per ion for Si and 17.1 molecules per ion for SiO₂, which were approximately ten times larger than that for Ar monomer ion irradiation. Rutherford backscattering spectroscopy (RBS) measurement showed that the number of displacement atoms for irradiation by water cluster ions on the Si surfaces at ion doses larger than 1 × 10¹⁵ ions/cm² was less than that for Ar monomer ion irradiation at the same acceleration voltage. In addition, the ellipsometry and X-ray photoelectron spectroscopy (XPS) measurements showed that the thickness of the oxide layer formed on the Si surface increased with increase of the acceleration voltage, and it was between 10 nm and 15 nm at an acceleration voltage of 9 kV.