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The secret to the amazing resolving power of the helium ion beam starts with the source tip. A finely sharpened needle is made even sharper through a proprietary process that took years to develop. Individual atoms are stripped away from the source until an atomic pyramid is created with just three atoms at the very end of the source tip – a configuration called the "trimer". This repeatable process can be accomplished in-situ.
Once the trimer is formed, the tip is maintained under high vacuum and cryogenic temperatures with helium gas flowing over it. A high voltage is applied to the needle to produce an extremely high electric field at its apex. The helium gas is attracted to the energized tip where it is ionized. With ionization happening in the vicinity of a single atom, the resulting ion beam appears to be emanating from a region that is less than an angstrom in size. This produces an extremely bright beam that can be focused to an extraordinarily small probe size.
 | | Image of the atoms at the end of the source tip emitting helium ions. Since each atom can be individually seen, the virtual source size must be much smaller. |
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This ALIS source is mated with an advanced electrostatic
ion column that focuses the beam with sub-nanometer
precision. Much like a SEM, the beam is rastered across
the sample pixel by pixel.
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| The helium ion beam has a DeBroglie wavelength that is approximately 300 times smaller than an electron beam resulting in much less diffraction. |
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The number of detected secondary electrons is used to determine the gray level of that particular pixel. Since the number of detected secondary electrons varies with material composition and shape, the images provide excellent topographic and compositional information.
Helium ions are about 8000 times heavier than electrons. Because of this, a helium ion beam exhibits very little diffraction when passed through an aperture or across an edge. Diffraction is a significant problem for a SEM where the diffraction effect limits its ultimate spot size. Since the helium ion beam is not affected by diffraction, it can be focused to a much finer spot size, enabling
sub-nanometer resolution.
The helium ion source has a very long lifetime due to the fact that the source tip is always kept at a positive potential. The only things attracted to the tip are electrons, which do not cause any ill effects. Positively charged ions are repelled from the tip and other gasses will be ionized before they have a chance to strike the source tip. For this reason, the source lifetime for the helium ion source is well over 1000 hours.
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