Negative ion beam bombardment of a protic ionic liquid: Alleviating surface charging and damage and analyzing the surface of organic insulating materials

Abstract

Positive ion beams are widely used in surface processing and analysis; however, serious surface charging can occur in the case of insulating materials. To address this issue, we investigate bombardment effects of ionic liquid negative ions emitted from the tip of a sharp needle wetted with the protic ionic liquid, diethylmethylammonium trifluoromethanesulfonate. Experimental results show that the potential of an electrically floating metal target bombarded with the ionic liquid negative ions is slightly higher (about 1 V) than that of a front electrode, indicating that the target potential can be controlled by adjusting the potential of a nearby electrode. We also investigate the application of the negative ion bombardment in secondary ion mass spectrometry. Two types of insulating materials, polytetrafluoroethylene and polyethylene glycol, are analyzed. Experimental results show that the negative ion bombardment allows one to analyze organic insulating materials by adjusting sample bias potential, without charge neutralization such as electron flooding. Results obtained show that the ionic liquid negative ion beam is a useful tool for alleviating sample charging and damage because tens of negatively charged low-energy constituent atoms hit a surface locally and simultaneously. The ionic liquid negative ion bombardment is shown to have the advantages of both negative and polyatomic ion bombardment.