Electrons, used in scanning electron microscopes and transmission electron microscopes, are not the only charged particles that can be accelerated and focused using electric and magnetic fields. An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The lightest ion has almost 2000 times the mass of an electron and heavier ions can be another 250 times as massive. In a scanning electron microscope, the relatively low-mass electrons interact with a sample non-destructively to generate secondary electrons which, when collected, provide high quality image resolution down to the sub-nanometer range. A focused ion beam (FIB) instrument is almost identical to a SEM, but uses a beam of ions rather than electrons. The focused ion beam can directly modify or "mill" the specimen surface, via the sputtering process, and this milling can be controlled with nanometer precision. By carefully controlling the energy and intensity of the ion beam, it is possible to perform very precise nano-machining to produce minute components or to remove unwanted material. In addition, ion beam assisted chemical vapor deposition can be used to deposit material with a level of precision similar to FIB milling. A small quantity of a specifically selected precursor gas is injected into the vicinity of the beam, where it is decomposed by the beam, depositing the nonvolatile decomposition products on the specimen surface while the volatile products are extracted by the vacuum system.
A FIB becomes even more powerful when it is combined with a SEM as in the FEI DualBeam system. In a DualBeam, the electron and ion beams intersect at a 52° angle at a coincident point near the sample surface, allowing immediate, high resolution SEM imaging of the FIB-milled surface. Such systems combine the benefits of both the SEM and FIB and provide complementary imaging and beam chemistry capabilities.
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