The Helium Ion Microscope (HIM), a new form of sub-nanometer microscopy, has several advantages over
traditional electron based forms of imaging, such as the ability to image insulating samples without a
conducting overlayer. HIM images are generated by rastering a helium ion beam across a sample and collecting
the emitted secondary electrons. The number of secondary electrons corresponds to different grayscale values,
creating a black and white image.
Rutherford Backscattering physics presents a method for determining the elemental composition of the atoms
in samples: The energy of a backscattered helium ion depends on the mass of the sample atom it has scattered
from. By measuring the flight time of the backscattered ion, we determine this energy. Therefore, by detecting
the backscattered helium ions, we can create images of samples that convey this elemental information and
how those elements are distributed.
In order to create accurate images, it is important to understand how the sample is damaged by the incident
ions. We therefore exposed a 1.3 nm thin platinum film on silicon to varying ion beam doses and observed
how the composition of the surface changed with dose. From this we calculated the sputtering coefficient for
platinum. In a separate experiment, using X-ray photoelectron spectroscopy, we verified the thickness of the
platinum film. Our results show that elemental identification is possible on the 500 nanometer scale using this
“time of flight” configuration but that there is a limit to our ability to sensitively identify elements as a result
of beam deterioration. We found a sputtering coefficient that is smaller than the accepted value in the literature
of ion beam sputtering; possible systematic errors to explain this difficulty will be discussed.
This project has been supported by funding from National Science Foundation grant PHY-1560077.