The FIB/TEM method for the characterisation of rolled Al surfaces

For the characterisation of Al surfaces after thermo mechanical processing of rolled strips TEM is imperative. Especially cross-sectional TEM specimens highlight the multi-layered structures of the surface area. The specimen preparation of cross-sections is a critical step in TEM analysis. A common technique is ultramicrotomy, where a diamond knife is used to cut electron transparent slices. The major disadvantage of the method - especially in the case of aluminium - is the strong mechanical deformation resulting from cutting. Another technique is the conventional ion beam milling. In order to provide views of subsurface regions, the sample surfaces are initially glued together, then cut perpendicular to the sheet surface, mechanically thinned, cut to form disks, dimpled, and ion beam thinned. Here, a new method for preparing cross-sectional TEM specimens of aluminium surfaces is introduced: Focused Ion Beam thinning (FIB), which became a standard method for preparation in the semiconductor industry. This technique has the potential of introducing nearly no chemical and structural modifications in the specimen. TEM lamellae of equal thickness can be produced in less than two hours and the success rate is nearly 100%. As an example for this new preparation method the paper reports about a (S)TEM investigation of a FIB prepared specimen of the surface of an AlMg0.5 hot rolled strip.