Focused Ion Beam instruments (FIB) are used for the preparation of electron microscopy specimens and for the fabrication of nano and micro components. Using polycrystalline Cu as an example, the influence of the crystallographic orientation, as obtained by EBSD, on the milling result is demonstrated. Different milling rates are due to the channeling effect. With some orientations a topography with characteristic features, like ripples, is generated, which were quantified using AFM images.

FIB Milling: Applications in Materials Science

Focused Ion Beam instruments or microscopes (FIB) [1] are more and more becoming a standard tool for nano-structuring and electron microscope sample preparation [2, 3]. Moreover, an imaging contrast based on the crystallographic orientation can be used, which is not available with other microscopes. A FIB has functional components similar to a SEM, including vacuum system and specimen chamber, except that a Ga+ ion beam instead of an electron beam is generated using a liquid Ga source. The Ga⁺ ions are accelerated with voltages between about 1 and 30 kV and focused to a beam with a diameter of about 5 nm. The beam can be scanned over selected areas or used to write any pattern. Nowadays most commercial instruments are sold as dual systems fitted with an ion source and an electron gun in order to allow the online observation of the ion milling process by SEM imaging. More­over, SEM based methods, including Energy Dispersive X-ray Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD) [4] can be applied after FIB milling using the same instrument.

Two main effects of the ion beam interaction with the specimen are exploited. The milling or etching effect is caused by the sputtering of ions from the specimen surface. The other effect is the deposition of metals or other chemical elements onto the specimen surface. In this case a suitable compound (e. g. a metalorganic compound) is fed by a gas injection system to the surface region of interest. The decomposition of the compound results in a deposition of the metal where the ion beam hit the surface. Thus small structural elements with dimensions of some nm to some ten µm can be „drawn".

The main application of FIB`s in Materials Science is the preparation of TEM and SEM specimens.

The commonly used procedure to prepare a TEM lamella with a thickness of less than about 200 nm is to mill trenches (f. e. 20 x 15 x 15 µm) on either side of the region of interest. A nano-manipulator is then fixed to the lamella by Pt deposition and the lamella is transfered onto a TEM grid after cutting free the remaining sides.
For SEM preparation a cross section is obtained by milling a trench (f. e. 20 x 10 x 5 µm) at any interesting site of a larger specimen (so called target preparation) and the subsurface region is imaged after tilting the specimen, preferably by 45°.

Ion induced electrons (IIE) and secondary ions (SI) are used as signals for imaging. Both give a topographic as well as an atomic number contrast. Moreover, the imaging contrast and the milling result are influenced by the local crystallographic orientation at the specimen surface. A special contribution is due to the ion channeling effect (ICE) (Fig. 1). Strong channeling occurs if the incident angle of the ion beam is smaller than the critical incident angle [5]. The critical angle can be calculated from the atomic numbers of Ga and the specimen atoms, the ion energy, and the distance of the atoms in the lattice direction parallel to the ion beam [6]. The result of ICE is a great interaction depth and a small ion induced electron and secondary ion emission. The grain will be dark in the image. If the grain has a no-channeling crystallographic orientation it will appear bright. This effect is well demonstrated by imaging a polished Cu specimen at different tilting angles (Fig. 2). In many cases the crystal orientation contrast is brilliant for the inspection of microstructures even without chemical or physical etching of the specimen.
Ion channeling is understood as a direct result of the packing density of atoms parallel to the incident ion beam. The change of packing densities can be demonstrated by the projection of the atomic arrangement onto a plane perpendicular to different viewing directions of a crystal (Fig. 3).

Milling Effects in Polycrystalline Cu

The effect of the crystallographic orientation on the milling rate and on the resulting topography has to be studied with regard to the preparation of electron microscopy specimens and the fabrication of nano and micro components.
Results obtained for polycrystalline Cu are given here as an example for the relationship between crystallographic orientation and milling effect [7].
The crystallographic orientation of the grains was obtained by EBSD in a SEM. The specimen has been prepared very carefully by mechanical and subsequent electrochemical polishing to obtain a surface without a residual plastic deformation. This is essential because the information depth of