TY - JOUR
AU1 - Yao,, Bo
AU2 - Coffey, Kevin, R.
AB - Abstract Cross-sectional transmission electron microscopy (XTEM) is a very useful technique to study the interfacial diffusion and reactions and the grain growth of thin films. However, the preparation of XTEM samples of thin films is tedious and challenging. Difficulties may include the delamination of films from the substrate, fracture of brittle substrates and differential milling rates of the substrate and the film. This paper describes an improved technique using a combination of tripod polishing and focused ion beam milling to prepare XTEM samples of thin films. The technique can be widely used for high-throughput production of samples having varying film and substrate properties. Two different geometries are introduced. The first one is suitable for XTEM sample preparation of most films at a high yield rate, but with a limited view area. The other geometry is able to give a larger view area and is more suitable for thicker films. The technique is illustrated by an example of the sample preparation of Fe/Pt multilayer films on SiO2/Si substrates. XTEM sample preparation, thin films, FIB, tripod polishing, FePt Introduction Cross-sectional transmission electron microscopy (XTEM) is a very useful technique to study the interface diffusion and reactions and the grain growth of thin films. However, the preparation of XTEM samples of thin films is usually tedious and challenging. Compared to bulk samples, thin films generally have much smaller feature sizes, usually at the nanometer scale. Consequently, it is desirable to prepare the TEM sample membrane as thin as possible. A very thin membrane is beneficial for high-resolution TEM and scanning transmission electron microscopy (STEM) studies, and is also helpful to minimize the overlap of sample features along the TEM sample thickness direction. Metallic multilayer thin film samples are especially challenging, and XTEM membranes of only 10-nm thickness are recommended for their study [1]. This thickness is especially difficult to reliably achieve due to the related sample preparation difficulties of the delamination of the films from the substrate, the facture of brittle substrates (i.e. GaAs, MgO, Al2O3) and the differential milling rates of the substrate and the film. XTEM samples can be prepared by a variety of means. Conventionally, the XTEM sample is prepared by dimple grinding to thin the cross-sectional sample followed by low-angle ion milling. This technique is difficult to apply for films having a different ion-milling rate from that of the substrate. An illustrative example is the case of a Fe/Pt multilayer film on an oxidized Si substrate. It was found that when the SiO2/Si substrate has already been milled through, the Pt layers are still sufficiently thick as to be opaque to the electron beam. Tripod polishing has also been used to prepare XTEM samples of thin films successfully, especially those on Si-based substrates [2]. The basic principle of this technique is to prepare a wedge-shaped sample. At the tip of the wedge, the sample is intended to be sufficiently electron transparent for TEM examination. The final polishing process to make the tip electron transparent is the most critical step to this technique because the fragile tip can be easily damaged before it is sufficiently thinned. The damage can be caused by scratches or residue on the lapping surface. Successful utilization of this technique generally requires that the substrates must be mechanically stable (i.e. Si), and the films must be bonded well with the substrate to avoid delamination during the final polishing. The first requirement prevents the sample preparation of thin films on brittle substrates, such as GaN, MgO or Al2O3. The second requirement prevents the sample preparation of films on substrates without strong chemical bonding (i.e. the Fe/Pt multilayer films on SiO2/Si substrates studied in this paper), which delaminate readily during the final polishing. Another limitation of this technique is that the final membrane thickness (below 200 nm) cannot be readily monitored and controlled. Focused ion beam (FIB) milling is versatile and suitable for XTEM sample preparation for a wide range of materials [3], with the ex situ and in situ lift-out (also called ‘microsampling’) techniques. The in situ lift-out technique is more commonly used since the final FIB thinning is more convenient. Commonly used procedures of this technique generally include the preparation of a thick membrane (>1 μm) through milling, bonding the membrane on an omniprobe and cutting it from the bulk, landing the membrane on the half-cut Cu grid, releasing it from the omniprobe and thinning the membrane to electron transparency. It has been reported that a TEM sample can be prepared within 1.5 h [4] with this technique. However, in our experience a typical time to prepare a TEM membrane following these procedures is >4 or 5 h, with the last step (final FIB thinning) taking about 1 h. The FIB technique following these procedures is especially useful for site-specific XTEM sample preparation of patterned thin films and devices. For the study of thin films with a uniform area (‘blanket’), these procedures are time consuming and result in a low sample throughput. The method described in this paper was investigated originally to prepare XTEM samples of [Fe/Pt]n multilayer films on SiO2/Si substrates, but has proven itself to be of more general utility. The technique does not require the strong film–substrate adhesion needed for tripod polishing nor are similar ion milling rates of the substrate and film layers required. A similar versatility to FIB techniques is obtained, but at much higher sample throughput rates. As will be described, this method is based on a combination of tripod polishing and FIB milling, and can be used as a general high-throughput approach for the XTEM sample preparation of blanket films for a wide range of film and substrate properties. Two different geometries are introduced, the first one is suitable for XTEM sample preparation of most films at a high yield rate, but with a limited view area, and the other geometry is able to give a larger view area, and is more suitable for thick films or films having surfaces not sensitive to ion beam damage. Methods The [Fe22 nmPt28 nm]6 multilayer films were sputter deposited onto Si (100) substrates having a surface layer of 100 nm of thermally grown SiO2. The deposition was performed in 4 mTorr of Ar + 3% H2. A chamber pressure in the 10−8 Torr range was obtained prior to film deposition. The film samples were annealed in a modified tube furnace in one atmosphere of flowing Ar + 5% H2 process gas at 350°C for 10 and 20 min. The heat treatment of Fe/Pt multilayer films is a very promising approach to form the ordered L10 FePt phase at a reduced processing temperature. This product phase is of interest to magnetic recording and permanent magnet applications due to its high magnetocrystalline anisotropy energy density. A minimum processing temperature is highly desirable for these applications and the XTEM study of Fe/Pt multilayers is helpful to understand the mechanism of the Fe/Pt interface diffusion and reactions. The microscopy was performed under a Tecnai F30 microscope with a field emission gun operating at 300 kV (provided by FEI company, Hillsboro, OR, USA). The technique introduced in this paper is modified from the previously described ‘H-bar technique’ using FIB for XTEM sample preparation [5–7]. In that approach, the sample of interest is cut out from a wafer using a dicing saw and further cut to form a sample ∼50 μm in width. The sample size is ∼0.5 mm wide and 1.5–2.5 mm long, which is small enough for a TEM grid or a direct-mount holder. Next, the sample is transferred into a FIB working chamber for milling. Two trenches are milled, leaving a narrow strip (∼0.1 μm in width), which is the TEM section of interest. The schematic of such prepared sample is shown in Fig. 1. One problem for this approach lies in the difficulty to prepare the sample strip 50 μm or less in width. Another problem is that the 50-μm-wide sample strip is still larger than desired for final FIB thinning, which requires a long milling time. Also, during TEM examination, the 50-μm initial sample width can significantly shadow the film regions of interest. Fig. 1 Open in new tabDownload slide A schematic of traditional FIB technique for XTEM sample preparation. Fig. 1 Open in new tabDownload slide A schematic of traditional FIB technique for XTEM sample preparation. The technique used in this paper takes advantage of both the merits of tripod polishing and the merits of FIB milling, with two different geometries. The first one is suitable for XTEM sample preparation of most films and coatings with a high yield rate, but with a limited view area. The procedures will be described in detail as below. First, several sections of the samples are glued together using an epoxy bond (Gatan G1, Gatan Inc.) to provide a total thickness of ∼2 mm. To polymerize the epoxy bond, the sample is kept at 120°C in a furnace for 30 min. The epoxy bond layer between the sample pieces is kept as thin as possible in order to improve bond strength. The sandwiched sample is then sliced using a diamond saw into one or more small blocks ∼2 mm in length and 2 mm in width. The thickness of the block is equal to the wafer thickness (∼480 μm in this paper) times the number of the sample pieces used in the initial gluing step (also ∼2 mm). Figure 2 shows a schematic of such block. The orientation of the block is defined in the figure to aid the following descriptions of the sample preparation process. Fig. 2 Open in new tabDownload slide A schematic view of the cut block with the defined coordinate. Fig. 2 Open in new tabDownload slide A schematic view of the cut block with the defined coordinate. As will be described in detail below, three stages of tripod polishing are used to quickly thin the sample at points of interest to below 10 μm. For those unfamiliar with the basics of tripod polishing, a detailed description and procedure can be found elsewhere [2]. The first stage of the tripod polishing process used in this paper is to thin the block along the Z direction to obtain a smooth surface in the X–Y plane (coordinate axis as defined in Fig. 2) with the wedge angle set to zero. A high-temperature wax (Mounting wax #71-10040, provided by Allied High Tech Products, Inc.) is used to mount the sample on the Pyrex stage with the X–Y plane against the Pyrex stage surface. A rough polishing (i.e. 9-μm lapping films) and fine polishing (i.e. 3- and 1-μm lapping films) are used sequentially. Further polishing with finer lapping films is unnecessary. A mirror-like, relatively smooth surface here, instead of the jagged surface resulting from the diamond saw cut, makes the final FIB thinning possible. This polishing stage typically reduces the Z dimension from 2 mm to ∼1.5 mm. When a mirror-like surface is achieved, a small piece (∼2.5 mm × 1 mm × 0.5 mm) of the protective Si wafer (without film) is glued (Loctite 460, provided by Henkel Corporation) with the wafer's glossy side on the polished smooth surface of the sample block. The orientation of this protective Si wafer piece is illustrated in Fig. 3. This Si wafer will be used to monitor the final thickness and to protect the sample during subsequent mechanical polishing, and is especially important for brittle substrates or readily delaminated films. Loctite 460 is stable at high temperatures but soluble in acetone. This adhesive allows the protective Si to stay with the sample block during the polishing process, but allows its subsequent removal with the acetone solvent. Fig. 3 Open in new tabDownload slide A schematic view of the block with Si wafer bonded on its polished smooth surface. Fig. 3 Open in new tabDownload slide A schematic view of the block with Si wafer bonded on its polished smooth surface. The second polishing stage is to thin and polish the Si-protected block along the X direction. The change of the polishing orientation is accomplished on the hot plate, and the high-temperature wax is used to re-mount the block on the Pyrex stub. Acetone should not be used for the reason mentioned above. Similarly, the wedge angle is also set to zero. The polishing of the sample is started with a 24- or 9- μm-lapping film. The rough polishing is stopped when the protective Si piece shares the same polish surface as the sample block, in order to use the Si piece to protect the sample region and to monitor the thickness during the final polishing process. Fine polishing with 3- and 1-μm lapping films is recommended to provide a glossy, mirror-like surface. Further polishing with 0.5-μm lapping film or colloidal silica is unnecessary. After the second polishing stage, the sample is put on the hot plate again and turned over (180°) so that it can be further thinned and polished from the X direction. The same wax is used to mount the sample. At this step, a non-zero polishing angle is set at such an orientation that the protective Si piece forms the tip of the wedge sample. The polishing geometry is shown in Fig. 4a. A polishing angle of 2° is recommended for Si substrates while steeper angles may be needed for more brittle substrate materials. The thinning of the sample is started with a 24- or 9-μm lapping film and is continued to the point at which the wedge is fully formed. Subsequent polishing with abrasive sizes of 3 μm and 1 μm are then used to improve the surface quality. The color of the protective Si piece changes to red as it thins to 10 μm, and the polishing process is usually stopped when the protective Si wafer is partially destroyed and appears red under strong backlight for ∼50% of its width, as shown in Fig. 4b. Further thinning, to where the red region reaches the leading edge of the sample block, can be used to reduce FIB milling time, but at a risk of sample damage. As apparent in Fig. 4b, the jagged shape of the leading edge of the Si protective piece illustrates the difficulty in tripod polishing to electron transparency. However, this is not a problem for the combined polishing and FIB milling technique described here. Excessive thinning may cause the undesirable jags to extend to the XTEM sample surface and make further FIB thinning difficult. At this point, the significance of using the Si piece to protect the sample and to monitor the final thickness is clear. Because the sample polishing can be stopped with a readily observable endpoint, the whole polishing process can be accomplished quickly, usually within 30 min. Fig. 4 Open in new tabDownload slide (a) A schematic of polishing geometry. (b) A top view of the polished sample under backlight. The appearance of the red color of protective Si under backlight indicates that it is <10 μm. The jagged side of the protective Si indicates the tip side of the wedged sample. Fig. 4 Open in new tabDownload slide (a) A schematic of polishing geometry. (b) A top view of the polished sample under backlight. The appearance of the red color of protective Si under backlight indicates that it is <10 μm. The jagged side of the protective Si indicates the tip side of the wedged sample. The last procedure is to mount the wedge sample on the half-cut Cu grid for the final thinning of the sample with FIB milling (Fig. 5). To do so, the polished wedged sample along with the Pyrex stub is put in acetone to dissolve both the glue and the wax. When the sample is released from the stub, it is mounted on the Cu grid with M-bond (M-Bond 610, provided by Measurements Group, Inc.). After the curing of M-bond, the sample is transferred to the FIB work chamber for final thinning from near 10 μm to electron transparency. This final milling process usually takes ∼1 h for each interfacial site on the original sample block to be examined. If the samples are initially bonded with two different thin films face-to-face, then two XTEM samples will result from each FIB milling site. Therefore, the average FIB milling time for each sample is ∼30 min, and the total operator's time (including mechanical polishing and FIB thinning) is <1 h. Such time is given as a typical time applicable to most operators and most sample materials. As mentioned earlier, XTEM samples of thin metallic (high atomic number) films usually need to be as thin as possible (i.e. 10 nm). Such a thin membrane is especially desirable for the Fe/Pt sample described here because the penetration ability of the electron beam in Pt is very weak. However, a uniform membrane at such thickness is mechanically unstable. To obtain a high-quality TEM sample, the final milling is accomplished with tilt angles of ± 1.5° at both sides of the membrane. As a result, the two sides meet below the surface and a hole appears. Near the edge of the hole, the sample thickness requirement is readily satisfied, as indicated by the circle in Fig. 5d. Fig. 5 Open in new tabDownload slide (a) A schematic of the geometry and dimensions of film, substrate and Cu grid. (b) The mounting of polished sample on Cu grid for FIB thinning. (c) A schematic of FIB-milled sample, where the enlarged trench is for TEM examination. (d) A low-magnification, bright-field TEM image of the prepared sample. The bright rectangular region is the FIB-milled trench, and the two dark lines near the center of the rectangular region are the Fe/Pt multilayers bonded together with epoxy. The bright region at the bottom on the rectangular region is a hole formed during the FIB milling. The dotted ellipse indicates the sample region with suitable thin film membranes ideal for TEM examination. Fig. 5 Open in new tabDownload slide (a) A schematic of the geometry and dimensions of film, substrate and Cu grid. (b) The mounting of polished sample on Cu grid for FIB thinning. (c) A schematic of FIB-milled sample, where the enlarged trench is for TEM examination. (d) A low-magnification, bright-field TEM image of the prepared sample. The bright rectangular region is the FIB-milled trench, and the two dark lines near the center of the rectangular region are the Fe/Pt multilayers bonded together with epoxy. The bright region at the bottom on the rectangular region is a hole formed during the FIB milling. The dotted ellipse indicates the sample region with suitable thin film membranes ideal for TEM examination. The procedures described above generally have the advantage of high throughput, but the view for TEM examination is limited (usually about 10 μm long). Another benefit of this approach is that the influence of ion beam damage on the sample region of interest is reduced by elimination of the damage from the protective layer deposition and minimization of damage from FIB observation [8]. In the usual FIB milling process for TEM sample preparation, a commonly useful procedure to protect samples from ion beam damage is to deposit a protective layer (i.e. Pt or W) before ion milling. The deposition of the protective layer, unfortunately, is also conducted at a high ion beam voltage (i.e. 30 kV) which can damage 20–30 nm of the sample surface. In the technique introduced in this paper, this damage can be minimized since the thin area for TEM examination can be far from the deposited Pt layer, as indicated by Fig. 5. Another factor reducing the beam damage to the sample region is that FIB observation, is reduced. FIB observation is performed to examine the sample quality and is conducted at a small beam current. However, because such observation is always conducted at a high ion incidence angle, it may yet cause significant beam damage. The reduced FIB milling time used in the newly introduced technique allows correspondingly less FIB observations and hence less beam damage to the sample region. A second geometry that provides a larger TEM viewing area and is more suitable for thicker films is described below, as a variation of the above procedures. First, a small piece of sample (∼2 mm long and 1 mm wide) is cut and mounted to a similar-size Si wafer piece with the film side to the glossy side of Si wafer piece. The Loctite 460 glue is used to bond them together. The Si-protected sample is thinned by polishing along the width direction. Two stages of polishing are involved. The first stage is to achieve a smooth surface at the cross section of the sample and protective Si piece with the procedures described above. The second stage is to mount the smooth surface on the Pyrex stub and to polish the other side. During the second polishing process, the protective Si is oriented to form the tip of a wedged sample, as shown in Fig. 6. A wedge angle of 2° (for Si substrate) is set. The polishing is stopped when the Si appears red, indicating that the thickness is below 10 μm. When the polishing is complete, the sample is released in acetone and mounted on the Cu grid for FIB thinning, similar to Fig. 1. One improvement over the conventional FIB technique is that the sample width is only ∼10 μm, instead of 50 μm, which makes the final FIB milling much easier. In this geometry, the Si protective piece is absent during ion milling, and the damage from the high-energy ion beam should be considered since the film is directly facing the ion beam and the Pt layer is directly deposited on the sample surface. Therefore, if the film is very thin or the surface structure is the point of interest, the film should be sputter coated with a layer (∼100 nm thick) of protective material such as SiO2, Al2O3, etc., prior to the sample preparation. The sputter coating is well known for a minimal effect on sample surface structure. Fig. 6 Open in new tabDownload slide A schematic of the modified geometry to prepare XTEM samples with a large view. Fig. 6 Open in new tabDownload slide A schematic of the modified geometry to prepare XTEM samples with a large view. In case the film has a similar milling rate to that of the substrate, the final thinning can also be accomplished with low-angle ion milling. This approach, unfortunately, is unsuitable for the Fe/Pt films on SiO2/Si substrate as studied in this paper. If the low-angle ion milling is selected as the final thinning approach, the protective Si can be mounted on the sample with epoxy, so that it can be further used to monitor the thickness during the final milling process. Under the white backlight, the Si material appears red when it is <10 μm. Upon further thinning, the red Si will be brighten, turn to yellow and finally become transparent. Results and discussion The Fe/Pt multilayer films were selected as one example in this paper. Figure 7a–c shows the bright-field TEM images of as-deposited, 10-min and 20-min annealed samples, respectively. The bright stripes in the figures indicate the Fe layers and the dark ones show the Pt layers. Figure 7d shows a hollow-cone dark-field TEM image formed using the (001) and (110) super-lattice reflection rings of L10 FePt phase, which is a product of the reaction of Fe and Pt. The white spots circled out in the figure indicate the L10 FePt phase. Fig. 7 Open in new tabDownload slide The BF XTEM images of sample [Fe22nmPt28nm]6 (a) as-deposited, (b) annealed at 350°C for 10 min and (c) annealed at 350°C for 20 min, and (d) hollow-cone dark-field TEM image from L10 001 and 110 superlattice reflections of sample annealed at 350°C for 20 min. The illuminated grains indicate the L10 FePt grains with 001 or 110 orientation, and were circled out to be differentiated from the white and black pixel noise. The images (c) and (d) are from the same region. Fig. 7 Open in new tabDownload slide The BF XTEM images of sample [Fe22nmPt28nm]6 (a) as-deposited, (b) annealed at 350°C for 10 min and (c) annealed at 350°C for 20 min, and (d) hollow-cone dark-field TEM image from L10 001 and 110 superlattice reflections of sample annealed at 350°C for 20 min. The illuminated grains indicate the L10 FePt grains with 001 or 110 orientation, and were circled out to be differentiated from the white and black pixel noise. The images (c) and (d) are from the same region. This TEM study of Fe/Pt multilayers clearly shows that the Pt layers grow at the expense of Fe layers during the interface diffusion. The product L10 FePt phase, however, does not form as a distinct layer, and is not found only at the Fe/Pt interfaces. Instead, L10 FePt is nucleated mainly at interfaces throughout the matrix. This result indicates that formation of L10 FePt is nucleation controlled. While some Fe atoms react with the Pt to form the L10 FePt phase, other Fe atoms diffuse into the Pt to form a solid solution of Pt, which is also called a disordered fcc FePt phase. Consequently, a mixture of L10 FePt and fcc FePt is produced, consistent with a previous magnetic investigation by Reddy et al. [9] Conclusions XTEM is a very useful technique to study the interfacial diffusion and reactions and the grain growth of thin films. However, the preparation of XTEM samples of thin films is tedious and challenging. Difficulties may include the delamination of films from the substrate, fracture of brittle substrates and differential milling rates of the substrate and the film. This paper describes an improved technique using a combination of tripod polishing and FIB milling to prepare XTEM samples of thin films. The technique can be widely used for high-throughput production of samples having varying film and substrate properties. Two different geometries are introduced. The first one is suitable for XTEM sample preparation of most films at a high yield rate, but with a limited viewing area. The other geometry is able to give a larger viewing area and is more suitable for thick films. The technique is illustrated by an example of the preparation of samples of Fe/Pt multilayer films on SiO2/Si substrates. References 1 Han K , Zhang Y . Transmission electron microscopy study of metallic multilayers , Scripta Materialia , 2004 , vol. 50 (pg. 781 - 786 ) Google Scholar Crossref Search ADS WorldCat 2 Benedict J P , Anderson R M , Klepeis S J . Cross sectioning materials for TEM analysis using the tripod polisher , Microstruct. Sci. , 1996 , vol. 23 (pg. 277 - 284 ) OpenURL Placeholder Text WorldCat 3 Giannuzzi L A , Drown J L , Brown S R , Irwin R B , Stevie A . Applications of the FIB lift-out technique for TEM specimen preparation , Microsc. Res. Tech. , 1998 , vol. 41 (pg. 285 - 290 ) Google Scholar Crossref Search ADS PubMed WorldCat 4 Ohnishi T , Koike H , Ishitani T , Tomimatsu S , Umemura K , Kamino T. . , 1999 Proceeding of the 25th International Symposium for Testing and Failure Analysis, pp. 449–453 (Santa Clara, CA) 5 Ishitani T , Yaguchi T . Cross-sectional sample preparation by focused ion beam: a review of ion-sample interaction , Microscopy Research and Technique , 1996 , vol. 35 (pg. 320 - 333 ) Google Scholar Crossref Search ADS PubMed WorldCat 6 Ishitani T , Tsuboi H , Yaguchi T , Koike H . Transmission electron microscope sample preparation using a focused ion beam , J. Electron Microsc. (Tokyo) , 1994 , vol. 43 (pg. 322 - 326 ) OpenURL Placeholder Text WorldCat 7 Yabuuchi Y . New TEM specimen preparation method using a focused ion beam , Electron Microsc. , 1993 , vol. 28 (pg. 119 - 121 ) OpenURL Placeholder Text WorldCat 8 Kato N I . Reducing focused ion beam damage to transmission electron microscopy samples , J. Electron Microsc. , 2004 , vol. 53 (pg. 451 - 458 ) Google Scholar Crossref Search ADS WorldCat 9 Raghavendra Reddy V , Kavita S , Gupta A . Fe Mssbauer study of L10 ordering in Fe/Pt multilayers , J. Appl. Phys. , 2006 , vol. 99 pg. 113906 Google Scholar Crossref Search ADS WorldCat © The Author 2008. Published by Oxford University Press on behalf of Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org Oxford University Press
TI - A high-throughput approach for cross-sectional transmission electron microscopy sample preparation of thin films
JF - Journal of Electron Microscopy
DO - 10.1093/jmicro/dfn021
DA - 2008-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/a-high-throughput-approach-for-cross-sectional-transmission-electron-vhN1wp4wIP
SP - 189
EP - 194
VL - 57
IS - 6
DP - DeepDyve
ER -