Fig. 1 Two electropolishing methods to prepare FIM tips.
In order to obtain an electric field high enough to field-evaporate atoms, the specimen for FIM observation and atom probe analysis must be a very sharp needle. These sharpened specimens are called FIM tips. Typical diameter of FIM tips at a typical operating voltage of 10 kV is approximately 100 nm. Most metallic materials can be fabricated to this shape by the standard electropolishing technique as shown in Fig. 1. The starting specimen before electropolishing should be either wire or a rod with a square cross section. By electropolishing these either with DC or AC in a simple setting as shown in Fig. 1 (a), most of metallic specimens can be finished to a FIM tip. Most of the cases, these tips are covered by oxides, particularly aluminum alloys, but these oxides can be removed by field evaporating surface layers in FIM. Some specimens cannot be finished with this simple polishing method or some ruptured specimens need to be re-polished for repeated use. For this purpose, a microelectropolishing technique as shown in Fig 1 (b) is used [1]. An electrolyte is put in a small loop of Pt wire, and preliminary polished thin specimens are polished using this drop of the electrolyte. By this method, local polishing is possible while monitoring the tip shape with an optical microscope, and ruptured tips can be re-sharpened for repeated use. In order to polish away a few micron distance, few millisecond pulse can be applied to the Pt electrode [2].
Recent applications of atom probe are extended to unconventional metallic materials such as melt-spun alloy ribbons and metallic thin films. Specimen of ribbon can be readily prepared by micropolishing thin square rods with a cross section of about 20 x 20 micron^2. These square rods are prepared by mechanically grinding the ribbon sandwiched with two transparent plastic plates as shown in Fig. 2. The specimen must be grounded so that the cross section of the finished rods have square shape; otherwise they cannot be prepared to sharp tips by the subsequent microelectropolishing. Typical thickness of melt-spun ribbons is about 20 micron, thus the tape must be grounded to about 20 micron width from the both side rims of the tape as shown in Fig. 2 (b).
In order to prepare tips from thin films, similar but much finer pieces of square rods must be prepared by the photolithography method as shown in Fig. 3 [3]. In order to remove specimen from the substrate after the lithography process, a buffer layer must be deposited between the substrate and the film specimen. The buffer layer must be chosen so that it dissolves with the chemical solution which is inert to the specimen. After putting photoresist on the film, pattern as shown in Fig. 3 is exposed to the resist. After developing, the specimen film that is not covered by the resist is removed by ion milling from the top. Then the resist is removed, and finally the buffer layer is chemically etched away. Now the small pieces of specimen are free from the substrate, and these are lifted with a thin tungsten wire using silver paste. Subsequently, the specimen is microelectropolished as shown in Fig. 4. Once the fine square rod is lifted with a tungsten wire, sharpening it with the microelectropolishing technique is very easy. Only several pulsing of current is sufficient to finish the specimen to a FIM tip. Although this method made it possible to prepare FIM specimens from thin films of a few mm thick, it was not possible to prepare FIM tips in the perpendicular direction to the film. Such demand was strong, because there have been much interest in analyzing multilayer thin films in the perpendicular direction to the multilayer stacking.
Recently, Larson et al. [4,5,6] applied the focused ion beam (FIB) technique to prepare FIM specimens from multilayer thin films. By fabricating pre-thinned blank specimen using annular ion beam, they succeeded in preparing FIM tips from multi-layer thin films in the perpendicular direction to the film. By this method, they succeeded in analyzing chemical compositional change at the interface of multi-layers with an atomic layer resolution [4,5,6]. The FIB technique can be applied not only to thin films but also to the standard metallic specimens. Thus, if the tip preparation process using FIB is established as a routine process, it will become possible to prepare tips even from the samples that used to be difficult to prepare by the standard electropolishing technique. This should make a significant contribution to widening the application of the atom probe technique. Fig. 5 shows a secondary electron image of an FIM tip of a rapidly solidified SmFeCoZr alloy prepared by the FIB technique. This specimen is crystallized from an amorphous precursor, and is extremely brittle to handle with the conventional method. By the application of the FIB technique, FIM tips suitable for atom probe analysis was successfully prepared.
