Ion-beam thinning

Specimens for AEM should be on the order of 20-100nm thick and should accurately represent the features which are to be analyzed. In general, these requirements are often difficult to achieve simultaneously, and various specimen preparation methods must be used to approach the ideal specimen. For catalyst specimens, three main specimen preparation methods can be used depending on the catalyst material, the form of the catalyst, and the information desired. These are grinding and dispersing, microtomy, and ion-beam thinning. 

Ion-beam thinning is usually used for dense bulk specimens where particular regions must be analyzed. It can be useful in AEM for thinning the same single crystals used in surface analysis to make direct comparisons with results from AES, XPS, etc. Ion-beam thinning can also be useful in analysis of interfaces and defects within bulk metallic catalysts such as Pt and Pd and their alloys. 

Ion-beam thinning Universally applicable good for two-phase materials and chemically resistant materials large thin areas reproducible Slow, ion-beam damage and structural alterations often possible... 

Fig. 5.4 (A.B) Types I and II C-S-H, respectively (SEM of fracture surfaces courtesy K. L. Scrivener). (C,D) SEM/STEM pair of ion beam thinned section, showing Type III C-S-H (top, right) and Type IV C-S-H (top, left and bottom, right Jennings et al. (HO)). (A) is of an ordinary Portland cement paste, w/c = 0.5, aged 10 h. (B) is of a paste of an oil well cement, w/c = 0.44, with 2.4% of CaClj on the weight of cement, aged 1 day. (C) and (D) are of a CjS paste, w c = 0.47, aged 330 days.

Figure 3. TEM images of ion beam-thinned porous anodic alumina films formed in phosphoric acid electrolyte (a) 0.3M acid at MOV (b) 1.2 M acid at a constant current of 5 mA/cm. ...

The structure and chemical composition of the layers was studied by XRD and TEM-EDX in combination with ion beam thinning of the layers. The XRD results indicate the presence of randomly distributed silicalite-1. 

For valence band excitations, however, a different method has to be used to study the anisotropic dielectric properties due to the smaller q ( rv- 0.02 A 1 for 20 eV energy loss with 100 kV incoming beam energy), which means that the momentum transfer in the low loss region is mostly due to q. This requires the sample to be oriented in a certain direction relative to the direction of the electron beam to study the dielectric response in a particular orientation. Because of the small beam size available in cFEG TEM, a specific orientation of a thin section with millimeter size area can be easily obtained by ion beam thinning techniques to directly measure the anisotropic properties of the material. 

Specimen preparation methods have also improved enormously in the last decades and a wealth of documentation is now available. Specimen preparation methods such as ion beam thinning enable one to prepare thin transverse sections of an interface that can be imaged at atomic resolution in a 300 kV TEM. With so many techniques available, it is not... 

There is a need to make a thin sample that represents the bulk without introducing defects into the thin sample. The sample is ideally less than 100 nm thick, of uniform thickness, and both mechanically stable and electrically conductive under the electron beam. Methods to be considered are electropolishing, ion beam thinning, and ultramicrotomy. 

The choice of method depends on the sample if it is conducting, e.g., a metal foil, then electropolishing will normally be the answer since this is the fastest method available. However, if the sample is nonconducting, or is multiphase with one phase thinning faster than the others, then ion beam thinning will be preferable. If the sample is a powder or a thin wire, then the ultramicrotome may provide a solution depending on the hardness of sample. 

Ion beam thinning The idea of this technique is to bombard the sample with a beam of argon ions to remove surface atoms, thus thinning the specimen. The ion guns are designed to produce a fine beam of ions and/or neutral atoms. In its simplest form, an... 

Samples of aluminum foil were prepared by each of the above techniques and, as expected, they produced essentially the same thinned specimen with the exception that the grain boundaries were less obvious in the sectioned sample (probably owing to the lack of any preferential thinning). The ion beam-thinned sample was more irregular since the initial surface was not perfectly flat. Since the sample is readily thinned by electropolishing, this would be the normal method selected. 

FIB and FIB/SEM systems are routinely used to prepare TEM lamellae (both cross-section and plan views) and SEM specimens, which have been used for secondary ion mass spectrometry, electron backscat-ter diffraction, and Auger analysis. These systems offer numerous advantages relative to the other techniques described in the previous sections. The main advantage is that the specimens can be prepared to within 50 nm of a feature of interest (FOI). Other advantages are (1) the lamella can be prepared with near parallel sidewalls enabling quantitative chemical analysis to be performed over the lamella (2) the ion beam is perpendicular to the surface of the sample so that less preferential milling occurs than with ion beam thinning and (3) the sample preparation time can be as short as 2 h. 

Ceramics such as silicon and a-Al203 have been examined in recent studies because of their application as semiconductor and substrate materials and the fact that ion beam thinning and surface cleaning is often used. Ion beams consisting of and Nj have been used to change the surface properties. In such an operation the sequence shown in Figure 4.5 occurs as the positive ion content builds up. 

Thompson G.E., FumeauxR.C. and Wood G.C. (1978), Electron microscopy of ion beam thinned porous anodic films formed on aluminium , Corros. Sci., 18,481-98. 

Y.G. Shen, D.J. Oconnor, H. Vanzee, K. Wandelt, R.J. Macdonald, The growth of thin Cu films on an O-precovered Ru(OOOl) surface studied by low-energy ion-beams. Thin Solid Films 263(1), 72-78 (1995)... 

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