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Journal of Electron Microscopy 56(2): 43–49 (2007) doi: 10.1093/jmicro/dfm003

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Physical: Full-length

Transmission electron microscopy and atom probe specimen preparation from mechanically alloyed powder using the focused ion-beam lift-out technique Pyuck-Pa Choi1,∗ , Young-Soon Kwon2 , Ji-Soon Kim2 and Tala’at Al-Kassab3 1

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Abstract

The preparation of transmission electron microscopy (TEM) and atom probe-field ion microscopy (AP-FIM) specimens from mechanically alloyed Ti-Cu-Ni-Sn powder has been explored. Applying the focused ion beam (FIB) based in situ lift-out technique, it has been demonstrated that specimen preparation can be carried on single micrometre-sized powder particles without the use of any embedding media. Since the particles did not incorporate any micropores, as revealed by cross-sectioning, the standard procedure known for bulk samples could be simply implemented to the powder material. A sequence of rectangular cuts and annular milling was found to be a highly efficient way of forming a tip-shaped AP-FIM specimen from a square cross-section blank. A Ga level ≤1 at.% was detected if a low beam current of 10 pA was chosen for the final ion-milling stages. Implanted Ga ions were mostly confined to a zone of about 2 nm in thickness and indicated that ion-induced structural transformations were negligible. ................................................................................................................................................................................................ Keywords transmission electron microscopy, field ion microscopy, atom probe, focused ion beam, in-situ lift-out, powder materials ................................................................................................................................................................................................ Received 10 October 2006, accepted 25 January 2007 ................................................................................................................................................................................................

Introduction Mechanicalalloying (MA) by means of ball milling is a wellknown method of circumventing the limitations of conventional alloying and producing novel types of materials. During ball milling, powder particles (typically between 1 and 100 µm in size) are subjected to severe plastic deformation and repeated fracture and cold-welding processes. Metastable materials such as highly supersaturated solid solutions, chemically disordered intermetallics, nanocrystalline and amorphous alloys, etc. can be prepared in this way [1,2]. Owing to their unique microstructure, mechanically alloyed powders have been widely used as feedstock for the fabrication of bulk samples showing outstanding macroscopic properties. For instance, amorphous powders have been consolidated into high strength bulk amorphous alloys, which have great potential for engineering applica-

tions [3–5]. To gain a full understanding of the properties of the compacts, it is often essential to characterize the structure and elemental distribution of the initial powder on a sub-nanometre scale. This can be done applying transmission electron microscopy (TEM) in combination with atom probe–field ion microscopy (AP-FIM). In the case of fine micrometre-sized powder, however, such analyses are often hampered by the difficult specimen preparation. While for TEM investigations thin membranes of less than 1 µm in thickness are required, AP-FIM specimens must have the shape of a fine tip having a radius of curvature smaller than 100 nm. Conventional electropolishing methods can only be applied after the incorporation of powder particles in an embedding medium, which is time consuming and does not always guarantee a high success rate. Therefore, more efficient and generally applicable specimen preparation techniques need to be developed.

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Nano-Materials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea, 2 Research Center for Machine Parts and Materials Processing, University of Ulsan, ¨ Gottingen, ¨ Friedrich-Hund-Platz 1, P.O. Box 18, Ulsan 680-749, Korea, 3 Insitut fur ¨ Materialphysik, Universitat ¨ D-37077 Gottingen, Germany *To whom correspondence should be addressed. E-mail: [email protected]

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Focused ion beam (FIB) milling has received substantially growing attention as a sophisticated instrument for TEM [6–17] and AP-FIM specimen preparation [18–23]. FIB has been of particular interest owing to the development of the lift-out technique, in which a selected part of the specimen is cross-sectioned and lifted out. This method is of particular interest for site-specific TEM [6–12,15,17] and AP-FIM [23] analyses but it has also been explored for the preparation of TEM specimens from micrometre-sized powders [16,17]. Although the lift-out method can be used for AP-FIM specimen preparation from powders as well, there has been no such report to our knowledge. Miller et al. [23] demonstrated that an individual powder particle can be positioned on a support using an ex situ manipulator and subsequently FIB milled to an AP-FIM tip. However, it was reported that this method is rather promising for particles larger than 100 µm.

In the present study, TEM specimen preparation using the FIB lift-out technique has been reproduced for a mechanically alloyed Ti-Cu-Ni-Sn powder. Furthermore, a slight modification of the lift-out technique, which is described in [23], has been implemented for the preparation of the AP-FIM specimens from single powder particles.

Methods A powder mixture having a nominal composition of Ti50 Cu25 Ni20 Sn5 was mechanically alloyed for 20 h in a high-energy planetary ball mill (AGO-2) under protective Ar atmosphere. As-milled powder samples were characterized using X-ray diffraction (XRD). Only a broad halo peak, characteristic of an amorphous structure, was detected. The preparation of TEM and AP-FIM specimens was conducted with a dual beam FIB instrument (FEI, Nova Nanolab 600),

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Fig. 1 FIB-based in situ lift-out method of a TEM lamella from a single powder particle.

P.-P. Choi et al. Atom probe specimen from mechanically alloyed powder

Results and discussion Scanning electron micrographs revealed particle sizes of a few tens of micrometres for the studied powder [compare Fig. 1(a)–(h)]. Thus, the particles were large enough to be cross-sectioned for the application of the in situ lift-out technique. In the first step, a sacrificial Pt layer (of about 2 µm in thickness) was deposited on the centre of a selected particle [see Fig. 1(a)], protecting the particle surface against Ga implantation. Deep sloped trenches were milled on the

front as well as on the back side of this Pt layer [see Fig. 1(b)], leaving behind a lamella of ∼20 µm × 10 µm × 2 µm in dimension. Cross-sectioning did not reveal the presence of any micrometre-sized pores inside the powder particles [see Fig. 1(c)], which may deteriorate the mechanical stability of the specimens.The lamellae were found to exhibit sloped side walls, which has been referred to in the literature as the classic V shape and was ascribed to the redeposition of sputtered material [15,25,26]. The lift-out of the lamella was performed with a sharp needle-probe according to a standard procedure as shown in Fig. 1(d)–(h). Ion-milling of the lamella to an electron transparent membrane was not performed prior to the lift-out, as thin membranes were susceptible to bending during attachment to the needle-probe. Thinning was done after attaching the lamella to a support, as illustrated in Fig. 2(a)–(f). Rectangular ion milling patterns and successively decreasing ion beam currents ranging from 500 to 50 pA were used. The lamella was slightly tilted into the ion beam to an incidence angle of 88◦ (with respect to the surface normal) in order to eliminate the V shape. Ion-milling was continued until the entire membrane was thinner than 200 nm. Regions close to its upper edge were particularly thin and electron transparent. Except for the Pt layer, no crystallites could be observed within the specimen. A diffraction pattern of a region not including the Pt layer only exhibited diffused rings (compare Fig. 3). These observations were in good agreement with previous XRD investigations. The first stages of AP-FIM specimen preparation were performed analogous to the TEM specimen preparation except

Fig. 2 (a)–(c) Attachment of a lamella to a TEM specimen support. (d)–(e) Thinning of the lamella to an electron transparent membrane.

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using 30 keV Ga+ ions. This instrument allowed the specimen to be in situ manipulated with a needle-probe and also incorporated a gas injection system for ion-beam-assisted chemical vapour deposition. Powder samples for FIB were prepared by dusting a small portion onto double-stick carbon tape adhered to a specimen holder. Ion beam currents ranging from 1 to 7 nA were used for cross-sectioning the powder particles, while ion-milling of blanks to TEM and APFIM specimens was conducted at currents between 10 and 500 pA. TEM investigations were performed with a Philips EM 420T at an acceleration voltage of 120 kV. FIM images were obtained at a sample temperature of about 30 K, using neon at a partial pressure of 7 × 10−3 Pa as an imaging gas. Atom probe analyses were carried out with a tomographic atom probe. Details about the setup and operation of this type of apparatus are described elsewhere [24]. A sample temperature of 30 K, a pulse frequency of 1000 Hz and a pulse voltage to base voltage ratio of 20% were found to be appropriate acquisition parameters.

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Fig. 3 TEM image and selected area diffraction pattern of the specimen shown in Fig. 2.

Fig. 4 In situ lift-out of a cross-section blank from a single powder particle for AP-FIM specimen preparation. (a)–(c) Attachment of a blank positioned to a tungsten support. (c)–(f) Sharpening of the blank to a AP-FIM specimen using a sequence of rectangular and annular ion-milling patterns.

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that a square cross section blank was lifted out of a powder particle instead of the entire lamella [see Fig. 4(a)–(b)]. The blank was attached to a W support by depositing Pt on each side of the blank/substrate interface after 90◦ rotation. An efficient way of forming an AP-FIM specimen from the blank was found to be a sequence of rectangular and annular milling patterns. First, a thin post of ∼1 µm in width was prepared from the upper half of the blank [compare Fig. 4(c)–(f)]. Subsequently, the post was sharpened to a fine tip by annular ion-milling, where the inner and outer radius of the milling pattern was gradually decreased. Using this ionmilling sequence, machining of the entire blank was avoided

and specimen preparation time was shortened. Protrusions resulting from undercutting the shank of the tip were usually small in dimension and could be easily removed in a separate step [compare Fig. 4(e) and (f)]. Specimens prepared in the above-described way exhibited substantially higher mechanical stability than specimens prepared by annular milling of the entire blank, as the latter often ruptured at the blank/substrate interface. Filling the annular gap between tip and remaining blank with Pt may be considered as a method of further enhancing the mechanical stability of the specimen. The elemental maps detected by atom probe show homogeneous distributions [see Fig. 5(a)] within the amorphous Ti-Cu-Ni-Sn alloy. Neither clusters nor any other kinds of chemical heterogeneities can be seen. This observation can be quantitatively confirmed by statistical χ2 tests [compare Fig. 5(b)]. The measured concentration frequency distributions do not significantly differ from binomial distributions and χ2 values do not exceed critical χa 2 values corresponding to a significance level of 95%. It should be mentioned that the detected composition value exhibits a deviation from the nominal composition. While the low Ti concentration may be attributed to a compositional shift due to the sticking of powder particles to the milling tools during ball milling, the increased Ni and decreased Cu concentration values are probably artifacts of atom probe analysis. Cu exhibits a lower evaporation field strength than Ni [27].

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Therefore, Cu atoms are expected to be preferentially fieldevaporated and to be detected with a reduced concentration. One ofthe major concerns of FIB milling for TEM and APFIM specimen preparation has been the implantation of Ga ions, which induces lattice defects and can lead to the formation of an amorphous surface phase for high ion doses [15]. Therefore, special care must be taken that specimens are exposed to the lowest possible ion dose during imaging and ion-milling. In these studies, the beam current was reduced from 50 to 10 pA for a tip radius smaller than 200 nm. The mass spectrum and of a specimen sharpened to about 100 nm radius of curvature reveals only a small number of

singly charged detected Ga ions [see Fig. 6(a)], where the total Ga content is about 0.5 at.%. Assuming that annular milling of a tip with a low shank angle occurs at a Ga incidence nearly parallel to the tip surface, this value is comparable to the estimated level of 0.7 at.% for Ga+ ions implanted into a 100 nm thick TEM specimen of Si at 30 keV and an incident angle of 87.5◦ [26]. Figure 6(b) shows the corresponding distribution map of the detected Ga ions. The initial Ga concentration is slightly elevated to ∼1 at.% and decreases to a value of 0.3 at.%. The thickness of the Ga rich surface region is about 2 nm. Taking into consideration that the first few atomic layers are field evaporated prior to atom

Fig. 6 (a) Detected mass spectrum (partly shown). (b) Corresponding Ga distribution map.

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Fig. 5 (a) Elemental distribution maps as detected by a tomographic atom probe. (b) Measured concentration frequency distributions in comparison with binomial distributions.

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Acknowledgements This work was supported by the 2006 Research Fund of the University of Ulsan.

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Fig. 7 FIM image of a specimen prepared by FIB milling.

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Concluding remarks The present studies have shown that high-quality TEM and AP-FIM specimens can be prepared from mechanically alloyed powder materials applying FIB and the in situ lift-out technique. One of the major advantages of this method is that no embedding medium is required and specimens can be prepared from single powder particles. The lack of any micrometre-sized pores inside the powder particles facilitates the specimen preparation. The results of this work are strongly encouraging for a systematic characterization of a variety of mechanically alloyed powder materials using TEM and AP-FIM.

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