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Beam Scattering
Imaging with charged particle beams – either electron or helium ion beams – is subject to a beam scattering effect. When the beam's primary particles strike the sample surface, they interact with the surrounding material, causing the emission of secondary electrons from an area that is somewhat larger than the size of the beam itself. The larger the area of surface interaction, the lower is the ultimate imaging resolution. Conversely, the smaller the area of surface interaction, the higher will be the ultimate image resolution.
In the case of a SEM, the beam's electrons are scattered rapidly in the sample, resulting in secondary electrons being emitted from an area many times larger than the beam itself. Fortunately, when the helium ion beam strikes the sample with its larger and heavier particles, the particles do not scatter near the surface. This translates into a smaller area of surface interaction and much higher resolution images for the helium ion microscope.
Material Contrast
Although the area of surface interaction for the helium ion beam is relatively small – compared to a typical SEM, the total number of secondary electrons produced is greater. This larger secondary electron yield, and the large difference in yield between different materials, provides higher contrast imaging, making it easier to differentiate between materials with the helium ion microscope.
In addition, the helium ion microscope can collect backscattered
helium ions, much like in Rutherford Backscattered Spectroscopy. With proper selection and positioning of the detector, the total backscattered ion yield is directly proportional to the mass of the sample atoms. Using this imaging mode, it is possible to easily differentiate between sample materials.
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