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The beam current is reduced again, and the membrane is ion polished on both sides to a nominal thickness of
approximately 0.3 micrometres. At this point, the membrane is very close to being finished. The minimal beam
current associated with this step in the procedure leaves the membrane with almost no trace of the ion polish.
However, for most materials, 300 nanometres is still much too thick to be electron transparent. So the membrane
still requires another very careful, very gentle polishing step.
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