Abstract Summary
One of the advantages of Helium Ion Microscopy (HIM) is the ability to image insulating materials by using an electron flood gun for charge compensation. This paper examines the key parameters for successful imaging of a range of samples.
Introduction
Helium ion microscopes are becoming more widespread as tools for imaging and fabrication at the nano-scale. As in SEM, issues arise when imaging insulating samples (beam deflection, image distortion, brightness artifacts). As the sample charging is always positive (positive ions in; negative electrons out) the problems is simply solved, in principle, with an electron flood gun.
Stokes et al (1) developed a technique for dual-beam gallium FIBs where the electron beam is defocussed to the size of the milled pattern, and the current adjusted in proportion to the ion beam. While Fox et al (2), Joens et al (3) and Bidlack et al (4) demonstrate the effectiveness of the flood gun in imaging polymer, cell, and tooth samples respectively, little detail of the parameters is given. In practice, careful control of the imaging and flood gun parameters is required if imaging is to be successful at resolutions approaching the performance limit of the instrument.
Results
Investigating a range of charging samples some general parameter settings could be arrived at, along with a process for optimizing these parameters. While the flood gun output (~3 microamps) easily swamps the ion beam current (picoamperes) at lower magnifications, moving to fields of view of 10um or less requires careful optimisation of the beam and flood-gun parameters.