Focused ion beam (FIB) etching has been shown to be an effective method for TEM sample prep. The system uses a finely focused beam of gallium ions to remove surface material from selected areas. When used in conjunction with a metal-organic gas source, a conductive layer can be deposited in specific areas.

In a production atmosphere, it is not always possible to use the FIB equipment for the length of time required to completely prepare a cross-sectional sample. Instead, the FIB has been used to indicate specific areas, such as blown fuses or shorts by “marking” the area so that it is easily optically located. Unconventional TEM sample prep techniques were developed to prepare the cross-sectional samples from specific target areas.

We have developed an experimental apparatus for heating samples in the temperature range 50–400°C during ion milling. We find that heating to temperatures > 250°C during milling greatly reduces ion sputtering amorphization of the sample surfaces, due to dynamic reannealing of point defects. This allows thinner regions of crystalline sample to be imaged, without the signal being swamped by noise due to milling amorphization. At temperatures ≥300°C, however implantation defects am formed. We discuss the advantages of this technique for high resolution imaging.

While the thickness of ultramicrotomed sections is much more uniform than that of wedge-shaped foils, its variation is seen to be dependent on such factors as the type of material, use of embedding media, water bath interaction (anodic dissolution) and quality of the diamond knife edge. The latter also affects the thickness of surface oxides formed during sectioning. More deleterious surface layers are produced by partial breakdown and redeposition of embedding media under the electron beam. These artefacts indicate that ultramicrotomy is not the panacea for elemental X-ray mapping that might, at first thought, appear to be the case. For the example of a metal matrix composite, useable maps are obtainable, but even the best microtomed sections are limited in terms of mapping by factors such as the lengthy times for map acquisition caused by the low X-ray count rates resulting from thin specimens, particularly where low mass fractions are involved.

Ultramicrotomy has been used to prepare TEM specimens from nickel sulfide inclusions in plate glass and from calcium aluminate / polyvinyl alcohol composite materials. Microstructural studies showed that the inclusion probably contained at least two different phases, namely Ni7S6 and Ni3S2. Although microtomy-induced cracking and chatter were observed, important microstructural features such as twins and Moire patterns were preserved. The microchemistry of the polymer phase in the ceramic / polymer composite was precisely determined.

A technique for preshaping large particles of reacted glass for ultramicrotomy is described. This technique preserves delicate surface layers and results in more sections per block extending deeper into the sample than other techniques.