Negative-cluster ion beam production from the tip of a sharp needle: Suppression of surface charging and surface analysis of an insulated sample

In focused ion beam technology, liquid metal ion sources (LMISs) are widely used; however, they cannot produce negative ion beams. In this work, we demonstrate stable negative-cluster ion beam production from a point source by a method similar to LMISs. A negative-cluster ion beam is stably produced from the tip of a sharp needle wetted with a protic ionic liquid, diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]). The time rate of change in the negative ion beam current is less than 1% per hour. The beam is estimated to contain two kinds of negative-cluster ions: [TfO](-)([dema][TfO]) (m/z 386.02) and [TfO](-)([dema][TfO])(2) (m/z 623.08). These cluster ions contain reactive protons enhancing protonation. We investigate surface charging in an electrically insulated material irradiated by the negative ion beam, showing that charging voltage is as low as approximately 1 V. We report that secondary ion mass spectrometry of an insulated organic sample is possible using the negative ion beam without charge compensation.