Channeling in the helium ion microscope

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 (χ_min), surface peak (SP), and critical angle, (ψ_C) 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.