Introduction: The optical nature of a charged particle beam 13

10 m
Figure 1.6: SEM picture of self-assembled silica spheres, showing
high depth of focus.
A scanning helium ion micrograph is shown in Figure 1.7, obtained
using an Orion SIM available commercially from Carl Zeiss SMT.
A scannng electron micrograph of the same specimen is shown
for comparison, obtained using a Leo SEM also available commercially from Carl Zeiss SMT. The full-scale vertical dimension
is 6 µm, and the beam energy is 20 KeV in both cases. The helium ion micrograph shows striking surface detail. This is due to
the fact that the helium ions are stopped within a few tens of
nanometers of the surface, while the electrons penetrate several
microns into the material. As a result, the material appears more
transparent to electrons than to helium ions. The electron micrograph shows more contrast due to the different materials present.
This is due to the fact that materials with high atomic number and
high mass density preferentially scatter the electrons much more
strongly than low atomic number and low mass density materials.