Evaluation of charge state of gas cluster ions by means of individual crater observations

In this study, charge states of gas cluster ions were evaluated from observations of individual impact craters. Since the diameter of craters formed by gas cluster ion impact increases in proportion to the cubic root of the acceleration energy, and is not dependent on cluster size when energy is greater than 10 eV/atom, the charge state of a cluster ion can be evaluated from the diameter of its individual crater. From AFM images of SiO2 irradiated with Ar cluster ions produced using low electron ionization voltages (Ve), craters with diameters of about 4 nm were observed. However, larger diameter craters started to appear with increasing Ve. From the distribution of charge states at Ve of 200 V, it was shown that there were some Ar cluster ions with six charges. There was no apparent dependence of charge state of Ar-GCIB on electron ionization current (Ie). This result indicates that ionization electron bias is the key parameter to control charge states of gas cluster ions.