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Report No.

Reduction of charging effects on negative secondary ion TOF mass spectra of PMMA using cluster ion impacts

Hirata, Koichi*; Saito, Yuichi; Chiba, Atsuya; Yamada, Keisuke; Narumi, Kazumasa; Kamiya, Tomihiro

Secondary ion (SI) mass spectrometry, based on the phenomenon that primary ion impacts on a target produce SIs originating from the target, is one of the most powerful tools for surface analysis. One of the difficulties in SI mass measurements of insulating targets is to stabilize the surface potential during the measurements, because ion impacts on insulating targets usually lead to an increase in the potential caused by the difference between the injected and emitted charges. We compared time-sequential variations of negative-SI TOF mass spectra of a thin PMMA film target for monoatomic (C$$_1$$$$^+$$) and cluster ions (C$$_8$$$$^+$$) impact with the same incident energy per atom (0.5 MeV/atom) to study difference in charging effects on N-SI mass spectra of insulating targets. For C$$_1$$$$^+$$ impacts, the peak shift to longer flight time and SI intensity reduction are observed. In contrast to the C$$_1$$$$^+$$ impacts, the N-SI TOF mass spectra for C$$_8$$$$^+$$ do not show the peak shift and SI intensity reduction, which shows that the surface potential is stable during the C$$_8$$$$^+$$ bombardment. The results demonstrate that N-SI TOF mass measurement of the insulating thin film target is available using cluster ions as a primary ion without charge compensation.



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