The focused ion beam can be used for both material removal by sputtering as well as deposition with gaseous precursors via beam induced deposition.1 The combination of these two techniques enables the creation of one, two, and three dimensional structures. The FIB-SEM is unique in the ability to make high fidelity 3D nanostructures of any arbitrary shape. The major drawback of FIB nanofabrication is that it can be significantly more time consuming than other techniques like photolithography and self-assembly.
FIB Etching/Milling #
Many 3D nanostructures are made through the use of sputtering to remove material from a substrate and leave behind the object of interest. Many normal structures used in routine FIB analysis employ this technique, like TEM lamella, micropillars, and APT tips.
FIB accelerating voltages are key to achieving optimal results during milling. Higher accelerating voltages result in larger sputter rates and increase the amount of material removed in a set time. The higher accelerating voltages can also implant ions into the substrate further, which can lead to increased stresses in the material as well as changes in conductivity and crystallinity. Larger sputter yields are also observed with higher atomic number ion sources. The sputter yield of Ga is two orders of magnitude larger than He at 30 kV. Beam shape also has a large role in the FIB milling results; larger beam tails from a Xe plasma FIB may prevent sharp sidewalls in a vertical cut, but a Ga FIB may allow better wall angles. Decreasing beam currents (spot size) will also enable milling of finer structures.
Another challenge for FIB milling is re-deposition of the substrate material. Re-deposition can be minimized through the use of advanced beam raster patterns that re-mill the same area multiple times, with control of the spot dwell times, and use of reactive gases to enhance the etching.
FIB Deposition #
Using beam induced deposition, one can make a variety of additive three-dimensional structures in a FIB-SEM. The type of material deposited is dependent on the precursor gas used in the gas injection system. Deposits of metals (W, Au, Pt, Al) and C have been widely demonstrated and are commercially available on most FIB instruments. Additionally, deposits of Co, Cu, Rh, and Fe have been reported in the literature and well characterized. Deposition of oxides like SiO2 are less widely reported.
The physical properties like deposit purity, conductivity, tensile strength, etc. are highly dependent on both the incident ion beam identity as well as the ligands used to volatilize the precursor metals.
