Channeling in the helium ion microscope

Abstract

The helium ion microscope (HIM) has become a unique tool for modern materials science and the life sciences due to its high lateral resolution for imaging, ability to treat insulating materials, nano-scale elemental identification, and materials modification. These capabilities are based on the body of science known as “particle-solid interactions”-the interaction of an energetic ion beam with a solid. For crystalline materials the incident charged particle beam may undergo channeling, which strongly modifies all relevant particle-solid interactions including damage creation, ion backscattering probabilities, electron emission, etc. We have used a HIM 30 keV He+ beam on a well-characterized W(1 1 1) single crystal to quantify these channeling effects in a backscattering geometry. The experiments used a novel time of flight (HIM/TOF) detector developed at Rutgers University. Measurements of the minimum backscattering yield (χmin), surface peak (SP), and critical angle, (ψC) are compared to several theoretical estimates. The intensity modifications are far greater for backscattered ions than for secondary electrons, which may be very useful for characterization of a large number of materials. This case of “ideal” channeling with the HIM now provides a basis for investigations of more complex materials such as polycrystalline materials and textured structures.