Silicon cluster formation by molecular ion irradiation — relationship between irradiated ion species and cluster yield

When an ion beam is irradiated to a solid surface at an extremely high flux (∼mA/cm2), it is known that a remarkable formation of clusters is observed during the sputtering process. We have observed strongly enhanced cluster formation at quite low fluxes (∼μA/cm2) by irradiation of molecular ion. In the present study, various molecular ions are irradiated to a silicon surface. The relationship between irradiated ion species [SFn+ (n=1,5), Xe+,Ar+] and the obtained silicon cluster yield is investigated. Observed mass spectra show that relative yield of the Sin cluster Y(Sin) for SF5+ (m/e=127) irradiation is substantially higher than that for SF+ (m/e=51) irradiation. In the case of monatomic Xe+ (m/e=132) and Ar+ (m/e=40) irradiation, which have fairly the same mass compared with SF5+ and SF+, respectively, the cluster yield for Xe+ is also higher than that for Ar+ irradiation. However, the difference of the cluster yield for molecular ions is larger than that for monatomic ions [Y(Sin) (SF5+)⪢Y(Sin) (SF+),Y(Sin) (Xe+)>Y(Sin) (Ar+)]. These behaviors imply that the size of the incident ions greatly affects the cluster yield compared with the mass of the ions.