Significant decrease in surface charging of electrically isolated ionic liquid by cluster ion bombardment

Abstract

Surface charging caused by the bombardment of samples with positive ion beams is a significant problem in material processing and surface analysis. The charging potential of an electrically isolated sample is commonly believed to increase with the acceleration voltage of a positive ion beam in the absence of charge compensation. Contrary to the common belief, however, this paper reports that the charging potential of an electrically isolated ionic liquid target decreases with increasing acceleration voltage of a positive cluster ion beam. A typical ionic liquid, 1-ethyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)amide (EMI-TFSA), was used as the target. It was placed on a metal plate that was electrically isolated from the ground, and its charging potential during cluster ion bombardment was measured with a high-impedance electrometer. For comparison, an electrically isolated metal plate was used. This study demonstrates that the charging potential varies significantly depending on cluster ion species and target materials. The charging potential of the metal plate increased monotonically with the irradiation time, whereas that of the ionic liquid target saturated at a lower voltage. The charging potential of the ionic liquid target decreased with increasing acceleration voltage of the ion beam even though the beam current increased. Larger cluster ions (m/z 502) caused less charge buildup than lighter cluster ions (m/z 111). The results obtained are explained by considering secondary ion emission from the ionic liquid target, which reduce the incoming net charge into the target, resulting in reduced surface charging.