Helium ion microscopy imaging.

The helium ion microscopy images were acquired with a Zeiss Orion helium ion microscope. The ion source consisted of a sharpened needle held at high positive voltage and low temperature in the presence of helium gas. The ion beam was transmitted through a two-lens electrostatic ion optical column onto the sample surface. The beam energy was 25 to 35 keV, and the beam current was 0.1 to 10 pA. The image presented here was collected with secondary electrons at 20-nm spatial resolution.