Evaluation of Top, Angle, and Side Cleaned FIB Samples for TEM Analysis

Transcription

1 MICROSCOPY RESEARCH AND TECHNIQUE 70: (2007) Evaluation of Top, Angle, and Side Cleaned FIB Samples for TEM Analysis EDUARDO MONTOYA,* SARA BALS, MARTA D. ROSSELL, DOMINIQUE SCHRYVERS, AND GUSTAAF VAN TENDELOO EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium KEY WORDS FIB; TEM sample preparation; sputter coating; LaAlO 3 /SrTiO 3 ; HRTEM/ HAADF-STEM; double cross sectional TEM ABSTRACT TEM specimens of a LaAlO 3 /SrTiO 3 multilayer are prepared by FIB with internal lift out. Using a Ga 11 beam of 5 kv, a final cleaning step yielding top, top-angle, side, and bottomangle cleaning is performed. Different cleaning procedures, which can be easily implemented in a dual beam FIB system, are described and compared; all cleaning types produce thin lamellae, useful for HRTEM and HAADF-STEM work up to atomic resolution. However, the top cleaned lamellae are strongly affected by the curtain effect. Top-angle cleaned specimens show an amorphous layer of around 5 nm at the specimen surfaces, due to damage and redeposition. Furthermore, it is observed that the LaAlO 3 layers are preferentially destroyed and transformed into amorphous material, during the thinning process. Microsc. Res. Tech. 70: , VC 2007 Wiley-Liss, Inc. INTRODUCTION LaAlO 3 /SrTiO 3 (LAO/STO) multilayers are important heterostructures because of their electronically coupled interfaces, which can be either high mobility electron conductors or insulating, depending on the atomic stacking sequences (Huijben et al., 2006; Ohtomo and Hwang, 2004). The determination of the atomic stacking sequences is performed by advanced transmission electron microscopy; in the transmission mode (HRTEM) as well as in the scanning transmission mode (HAADF-STEM). Also electron energy loss spectroscopy (EELS) and energy filtered microscopy (EFTEM) are used to obtain extra information. For all these techniques the quality of the samples is extremely important. Particularly for quantitative work a constant thickness is preferred; this is difficult to fulfil using conventional preparation techniques such as electro-polishing or broad ion milling. Another problem of TEM sample preparation methods is the preferential milling at interfaces with a different chemical composition. Focused ion beam (FIB) techniques are increasingly used for the preparation of high quality TEM specimens, and several advantages of the technique have been recently reviewed (Langford, 2006). Using FIB, TEM lamellae can be prepared to within 50 nm of a feature of interest and it has been claimed that the technique has a lower preferential sputtering rate than any other conventional TEM sampling method (Kamino et al., 2004). On the other hand, the use of high energy Ga 11 ions for milling leads to the formation of damaged and amorphous layers at the faces of the sample. The formation of these layers and its minimization have been investigated by several authors (Cairney and Munroe, 2003; Hata et al., 2006; Kamino et al., 2004; Kato, 2004; Ko et al., 2007; Reiner et al., 2004; Rubanov and Munroe, 2004; Wang et al., 2005; Yabuuchi et al., 2004; Yu et al., 2006). It is now well established that these amorphous layers are produced by direct damage due to the ion beam and redeposition of sputtered material. Another important issue of preparing FIB samples is known as the curtain effect. This effect consists of the formation of striations across the lamella when the sample has a surface with uneven topography or chemical composition (Langford, 2006; Ishitani et al., 2004). An immediate consequence of the curtain effect is that the sample will no longer have a constant thickness. To avoid this problem, bottom and side cleaning procedures (Langford, 2006; Wang et al., 2004) have been proposed. In this article, the preparation of TEM samples of LaAlO 3 /SrTiO 3 multilayers is investigated using the micro sampling technique for the milling and lift out steps, followed by top, top angle, side, and bottom angle cleaning. Special attention is paid to the eventual presence of the curtain effect and preferential milling. EXPERIMENTAL PART Preparation The LaAlO 3 /SrTiO 3 multilayers are grown by pulsed laser deposition on a (100)-oriented single crystal of SrTiO 3 (Huijben et al., 2006). Prior to FIB preparation, a piece of the multilayer sample was glued onto a FIB holder and covered, by sputtering, with a 60-nm layer of fine grained Au particles (diameter 2 10 nm). On top of this first layer, a second layer with a thickness of about 500 nm, of coarse grained gold was deposited. The sputtering of gold was performed using a Balzers Union SCD 004 sputtering machine. The source target *Correspondence to: Eduardo Montoya, EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium. Eduardo.montoyarossi@ua.ac.be Received 23 January 2007; accepted in revised form 29 June 2007 DOI /jemt Published online 23 August 2007 in Wiley InterScience ( wiley.com). VC 2007 WILEY-LISS, INC.

2 EVALUATION OF FIB SAMPLES FOR TEM ANALYSIS 1061 Fig. 1. (A) FEG-SEM image of a FIB prepared cross-section of the studied material, showing the fine-grained and coarse-grained Au protective layers, on top of the LaAlO 3 /SrTiO 3 multilayer system. The arrows indicate examples of the discontinuities present in the coarse-grained gold layer. The question marks indicate an unexpected thin layer of coarse grained gold. (B,C) HRTEM pictures showing the general structure of the multilayer system, being (C) a magnified view of the area enclosed by the white rectangle in (B). The observed thickness of the LAO layers, labeled as L6 and L8 are of 2- and 1-U cells. TABLE 1. Experimental conditions used for the preparation of the TEM specimens Step I (na) Milling box type and relevant data Tilting Time (min) Pt deposition 0.3 Rectangle: x 5 (10 20 lm), 528 (sample surface 8 to12 y lm, z 5 2 lm \ to the ion beam) Bulk milling 20 Regular cross-section to 40 Bottom cutting 20 Rectangle to5 Rough cleaning 3 Rectangle and cleaning 548 and 508 (grazing 10 to 30 cross-section (in sequence) angles of 628) Lift out (internal) 0.1 (Pt deposition), 1.0 Rectangle (releasing lamella from bulk) Welding lamella to grid 0.3 Rectangle and releasing from the extraction needle Thinning (final 0.3 and 0.1 Rectangle and cleaning 548 and 508 (grazing 20 to 30 thickness 100 nm) cross-section (in sequence) angles of 628) Cleaning and (E ion beam 5 5 kv) Rectangle (nominal z nm) 608 and 448 (grazing angles of 688) 30 to 60 (in steps of <60 s each one) distance and the argon pressure were kept at (40 6 2) mm and ( ) mbar, respectively. Intensity currents of ( ) ma and (15 6 2) ma were used for the deposition of fine grained and coarse grained gold, respectively. An overview of the cross-sectional structure of the studied system is illustrated in Figure 1. A number of 15 layers of LAO and STO have been observed and labelled from L1 to L15, starting at the side of the substrate. The layers have a different thickness, ranging from 2- or 1-U cells (LAO layers L6 and L8) up to 14-U cells. The FIB work was performed using a FEI NANO- LAB 200 TM dual beam FIB system, equipped with a Pt deposition needle, which is inserted from the left-back side, and an OMNIPROBE TM extraction needle, which is inserted from the left-front side. The experimental conditions for the preparation of the specimens are shown in Table 1. An ion beam of E 5 30 kv was used for all steps, except for the final cleaning, where an energy E 5 5kV was used. The dwell time was kept at 1 ls for milling and 0.2 ls for Pt deposition. The TEM grid on which the lamella is being glued can be mounted in the grid-holder of the FIB system using angles of about 458 as shown in Figure 2. This can be combined with the degrees of freedom of the FIB sample stage, which allows reaching the angles necessary for the different cleaning schedules. The rotation of the grid changes the azimuth angle (908-b in Fig. 2D) between the beam and the lamella, whereas the tilting degree of freedom of the stage is used for adjusting the elevation angle between the beam and the lamella (see Fig. 2D). OMNIPROBE GRD copper TEM grids were used in the present work. Figure 3 illustrates the approximation of a lamella (attached to the extraction needle) to a grid mounted

3 1062 E. MONTOYA ET AL. Fig. 2. The grid can be mounted on the grid-holder of the FIB system in a non-rotated way (A) or rotated in different ways (B,C). This can be combined with the degrees of freedom of the sample stage of the FIB system, allowing the application of different cleaning schedules. In (D), the lamella is drawn in the plane XY and the ion beam on the plane XZ. The angle b is complementary to the azimuth between the ion beam and the lamella. The rotation of the grid changes the angle b while the tilting degree of freedom of the stage is used for adjusting the elevation angle between the beam and the lamella. nonrotated (Fig. 3A), to a grid mounted rotated by about 458 (Fig. 3B), and to a grid still mounted as illustrated in Figure 3B, but rotated by about 708 instead of 458. Mounting the grid using different rotations, before and after welding the lamella to it will produce top cleaned, top-angle cleaned, side cleaned, and bottomangle cleaned specimens, as illustrated in Figure 4. To change the rotation of a mounted grid, after welding a lamella to it, the sample chamber of the FIB system was opened, and the grid remounted with the desired rotation. All specimens were plasma cleaned before TEM observation; in order to minimize the growing of carbon contamination. A Fischione plasma cleaner model 1400 was used with a mixture of 75% Ar to 25% O. Preparation of Double Cross-Sections The thickness of the amorphous layers, still present at the faces of the FIB prepared specimen, after the final cleaning step, were studied by preparing and examining double cross-sectional lamellae ( cross-sections of the cross-section ) (Boxtleitner et al., 2001; Langford, 2006; Rubanov and Munroe, 2004). The two faces of a grid, containing a FIB prepared and examined lamella, were covered by sputtered gold to protect the lamella. Then the grid was horizontally mounted, as shown in Figure 5A and inserted in the FIB chamber in order to deposit a platinum layer of about 2 lm, onto one face of the gold covered lamella. The grid was taken out of the FIB system, flipped over, remounted, and inserted again into the FIB chamber for depositing platinum on the other face. This leads to a gold covered lamella embedded into a platinum brick. Finally, this platinum brick was sliced, using a Ga 11 ion beam of 30 kv and a current of 1 na, to prepare cross-sectional lamellae, which, after internal lift out and welding to new grids, were thinned and cleaned at the conditions shown in the two last rows of Table 1. A cross-sectional, FEG-SEM, image of the platinum brick with an embedded gold covered lamella is shown in Figure 5B. Figures 5C and 5D are also FEG-SEM images showing general and detailed views, of a FIB milled double cross-sectional slice, ready for the step of internal lift-out. Analysis The FEG-SEM (with detection of secondary electrons) and FIB (ion beam induced secondary electron micrographs) pictures were taken with the FEI-NANO- LAB 200 dual beam FIB system. HRTEM images were

4 Fig. 3. FEG-SEM images illustrating the approximation of a lamella (attached to the extraction needle) to a grid mounted non rotated (A), to a grid mounted rotated by about 458 (B), and to a grid still mounted as illustrated in Figure 2B, but rotated by about 708 instead of 458 (C). taken with a JEOL4000EX electron microscope. HAADF STEM and EFTEM chemical maps were obtained with a JEOL3000F electron microscope. RESULTS AND DISCUSSION Preparation In a similar multilayer system, it was found that Pt deposition in the FIB system, prior to milling with the Ga beam induces damage, even if the deposition is done by the electron beam (see Fig. 6A). Therefore, following (Rubanov and Munroe, 2003), a protective Au film was sputtered on top of the sample. However, one of the problems of using Au sputter-coated films is that the discontinuities present in the Au layer (see Fig. 1A) lead to the curtain effect. As described above, a thick layer of coarse grained gold is deposited, which EVALUATION OF FIB SAMPLES FOR TEM ANALYSIS 1063 allows to study the presence/absence of the curtain effect for the different cleaning procedures that are evaluated. Figure 6B is a HRTEM picture of a bottom-angle cleaned specimen, illustrating that the use of the sputtered gold layer is indeed effective to avoid the damage that could have been caused during the Pt deposition step. It is shown that the LAO layer (L15) and even the capping STO layer on top of it have been successfully preserved. The coarse grained Au can also be observed in Figure 1A, as a thin layer (indicated by the question marks) between the fine grained gold and the LAO/ STO multilayer. It could be produced by a partial recrystallization of the fine grained Au during the FIB processing, or produced during the first few seconds of sputtering, which are unavoidably carried out at a current of 6 ma when the plasma is started. The preparation of different kinds of cleaned lamellae is illustrated in Figure 4. The first column shows the conventional way of working: when the grid is mounted without rotation before and after attaching a lamella to it, a top cleaned specimen is obtained. The other columns show a practical way of obtaining top-angle, side, and bottom-angle cleaned lamellae. Of course, if the rotation of the mounted grid is not changed after gluing a lamella to it, conventional topcleaned specimens would be produced in all cases. All cleaning types can be easily implemented for any dual beam FIB system and the process of attaching a lamella to a rotated-mounted grid is straightforward and reliable, despite that the views presented by the last two pictures of Figure 3 look somewhat unconventional. Nevertheless, two practical remarks, holding at least for the configuration of the FIB system used here have to be taken in account: (1) The use of grid rotation angles larger than 708 will complicate the approximation of the lamella to the grid due to a blocking effect of the left side arm of the grid, which will also block the ion beam when tilting to 528 for the final thinning and cleaning steps. (2) The use of a grid rotated as it is shown in Figure 2C will not allow welding the lamella to the grid because the right side arm of the grid will block the Pt deposition needle. In the FIB system used, it was not possible to work with the used OMNIPROBE grids mounted at rotation angles of Hence the preparation of pure bottom-cleaned lamellae was found being not practical and so was not tried. Analysis All specimens, obtained using the different cleaning procedures, as well as the double cross-sectional lamellae, were investigated by TEM and HAADF STEM. The presence of the curtain effect is illustrated, in a comparative way, in the HAADF STEM images of Figure 7. The presence of the curtain effect in the top and the top-angle cleaned specimens is indicated by small vertical arrows. It can be seen that a strong curtain effect is present in the top cleaned specimen and only a weak curtain effect is observable for the top-angle cleaned specimen. The smooth surface of the side cleaned specimen, observed in Figure 7C, indicates not only the absence of any apparent curtain effect but also

5 1064 E. MONTOYA ET AL. Fig. 4. Using grids mounted-rotated to produce different types of cleaned specimens. The angles in (A D) and (E H) indicate the nominal rotation of the grid, before and after welding the lamella. The white lined black rectangles representing the attached lamellae are exaggerated in size, for better visibility. The white lines indicate the location and orientation of the multilayer with respect to the ion beam. In (A D) note that the lamellae are always attached to the grids with the same orientation of the multilayer. The column A, E, I illustrates the conventional way of working. The way of producing top-angle cleaned, side cleaned and bottom-angle cleaned specimens is shown in columns B, F, J, C,G,K, and D, H, L. The magnification and the direction of the component of the ion beam in the plane of the lamellae are the same for the FEG-SEM images (I L). a high quality of FIB milling. The case of the bottomangle cleaned specimen, Figures 7D and 7E, is more complicated. In fact, in Figure 7D the surface of the bottom-angle cleaned specimen does not look smooth in the region of the substrate. This effect does not affect the region of the multilayer and is not caused by a curtain effect like in the case of the top and top-angle cleaned specimens (Figs. 7A and 7B). The bottom-angle cleaned specimens reflect a general uneven milling of the SrTiO 3 substrate, which was used as its own sacrificial layer. For the other cleaning treatments; this role is taken up by the Pt protective layer. Figure 7E, corresponding to a slightly thicker region as Figure 7D, shows a smooth milling of substrate and multilayer, but also a weak curtain effect that starts to develop at the Au and Pt regions (see black arrows), however it does not affect the region of interest. Another important feature shown in Figure 7 is the fact of that the thin regions, suitable for high resolution TEM, are relatively small for the side and bottomangle cleaned specimens, in comparison with the top and top-angle cleaned specimens. Hence, the top-angle cleaning schedule seems to be the best choice when extended observation areas are required. Figure 8 shows that all cleaning procedures yield specimens of high quality suitable for HRTEM. In every picture of Figure 8, atomic resolution is obtained and even the thinnest LAO layers (L6, L8), with thickness as small as 2- or 1-U cells can be clearly distinguished. In addition, the whole image contrast is fairly uniform, indicating that the thickness of the sample is homogeneous over the field of view. This is also shown in Figures 1 and 6, corresponding to side cleaned and bottom-angle cleaned specimens. Figures 9 and 10 confirm that for all cleaning procedures the specimens are also suitable for HAADF- STEM, up to atomic resolution. The atomic columns are clearly observed in the chemical sensitivity Z-contrast imaging mode, even for the L6 and L8 LAO layers. For thin regions, the intensity of a HAADF-STEM image increases with the projected Z number, as well as with the thickness of the sample. Because of this, it is illustrative to consider the intensity scanplots (see Figs. 9A1, 9A2, 9B1, 9B2, 10A1, 10A2, 10B1, and 10B2), taken from the marked areas in the direction indicated by the thin arrows. The scanplots in the x direction (parallel to the LAO layers) run along areas of the same chemical composition and therefore the slow changing variations in intensity reflect the variations of the specimen thickness. By visual comparison of the intensity profiles in the

6 EVALUATION OF FIB SAMPLES FOR TEM ANALYSIS 1065 Fig. 6. (A) TEM image of a SrMnO 3 /LaMnO 3 multilayer, consisting of eight bi-layer blocks, on a STO substrate. The image shows a damaged layer of four blocks caused by the e-beam induced Pt deposition. (B) HRTEM image illustrating the effect of the sputtered gold to avoid damage during the Pt deposition. The LAO layers are labeled according Figure 1. The LAO layer (L15) and even the capping STO layer on top of it have been successfully preserved. The coarsegrained Au observed in (A) is the one indicated by question marks in Figure 1. Fig. 5. FEG-SEM images illustrating selected steps of the doublecross-section experiment. (A) how to mount a grid in a horizontal way, with the purpose of covering the grid with sputtered gold, as well as the steps of forming the platinum brick and FIB milling for the double cross-sectional slices. (B) Cross-sectional view of a FIB prepared TEM specimen, gold covered and embedded in a platinum brick. (C,D) General and detailed views of the platinum brick, with a FIB milled slice ready for the internal lift-out. x direction with the corresponding images, it can be seen that the observed variations are caused by small dark regions. These dark regions were also observed by classical argon ion milling; their origin is still under debate. (Scott et al., 2006) indicated that conventional (2 4 kv) argon ion milling and FIB methods tend to leave mottling visible on thin specimen areas. It has been proposed that the cause could be the damage and amorphisation produced by the ion beam. The small dark regions observed here could therefore correspond to the mottling mentioned by (Scott et al., 2006). In the intensity scan-plots along the y direction, a variation of the amplitude of the intensity oscillations of the lattice (high frequency component) is observed, as the scan proceeds across the alternating STO and LAO layers. The signal of the different LAO layers has been marked in the intensity scan-plots and every LAO layer can be detected in a reliable way. It is observed however that the ratio R between the amplitudes of the intensity oscillations, corresponding to the La and Sr columns is not strictly constant (Fig. 9C). The varia-

7 1066 E. MONTOYA ET AL. Fig. 8. HRTEM images of top (A), top-angle (B), side (C) and bottom-angle (D) cleaned lamellae, showing that all used cleaning schedules yield specimens useful for HRTEM. The LAO layers are labelled according to Figure 1. The whole image contrast of the HRTEM pictures is fairly uniform. The arrows indicate the direction of the component of the ion beam in the plane of the lamellae during the final thinning and cleaning steps. Fig. 7. Low magnification HAADF-STEM images of the four types of prepared specimens. Suitable regions for high resolution are indicated by white parallelepipeds. Intense and weak curtain effects (see thin arrows) are observed for the top and top-angle cleaned specimens (A,B). No curtain effect is observed for the side-cleaned or bottomangle cleaned specimens (C E), although D presents uneven milling. The thick arrows indicate the direction of the component of the ion beam in the plane of the lamellae. tions of R can be partially attributed to the fact that the rows and columns of dots, which indicate the positions of the atomic columns, present some scan distortion. This is a common problem of STEM imaging related to the drift of the sample during the scanning procedure, as well as due to external electromagnetic fields and hysteresis effects during the fly back between scan lines (Sanchez et al., 2005). Another reason for the variation could be that the thickness of the crystalline part is not perfectly constant over the area. In Figures 9 and 10, no indication of carbon contamination growth is visible. It is well known that this problem tends to occur under the high electron dose conditions normally used in STEM. This indicates that the plasma cleaning was successful. The use of plasma cleaning for reducing the thickness of the amorphous layers and for the removal of the Ga contamination introduced by the FIB milling has been proposed in the literature (Hata et al., 2006; Ko et al., 2007). In the present work, the used Ar O plasma has a mean energy of about ev/ion, which is not enough for producing sputtering. Hence a further removal of the damaged/amorphous layers, produced by the FIB process, can not be expected. In the HRTEM images in Figures 11 and 12, in some cases the LAO layers are destroyed faster than the STO layers which means that preferential milling, or at least preferential amorphisation occurred. When there is no vacuum between two edges of a discontinuity in the multilayer, the preferential destruction of the LAO layers is not observed for top cleaned specimens (see Fig. 11A). On the contrary, when one edge of the thinned part is in contact with the vacuum (see Fig. 11B) the LAO layers are destroyed faster than the STO ones. The preferential destruction of the LAO layers is also evident in Figures 11C (top-angle cleaning) and 12A (side cleaning), but it is not observed in Figure 12B (bottom-angle cleaned specimen). The preferential destruction of the LAO layers is not compatible with the measured etching rate of pure LAO and STO (upper part of Fig. 13). The measurements were done by practicing identical FIB milled boxes (x 5 20 lm, y lm), on samples of STO (100) oriented and LAO oriented along a pseudo-cubic (100) direction. The millings were performed under the same conditions (E 5 30 kv, I 5 1 na, milling

8 EVALUATION OF FIB SAMPLES FOR TEM ANALYSIS 1067 Fig. 9. HAADF-STEM images of top (A), top-angle (B) cleaned lamellae and their corresponding intensity scan-plots in directions parallel (x) and perpendicular (y) with respect to the LAO/STO multilayer. The scan-plots are taken from the regions inside the rectangles, following the directions indicated by the thin arrows. The intensity signals from the LAO layers are labelled. The thick arrows indicate time s, grazing angle 5 108). Both samples were mounted on the same holder and milled in sequence, under the same alignment of the ion beam. The milled boxes were filled with platinum and FIB milled again the direction of the component of the ion beam in the plane of the lamellae during the final cleaning and thinning steps. (C) Zoomofthe marked area in (B2) illustrating the ratio of amplitudes for the intensity oscillations, corresponding to the atomic columns of La and Sr. Note that R = R 0. to expose their cross-sections in order to measure the milling depths. A value of z lm was found for both boxes (Figs. 13A and 13B), indicating that to a good approximation, both pure materials were milled

9 1068 E. MONTOYA ET AL. Fig. 10. HAADF-STEM images of side (A) and bottom-angle (B) cleaned lamellae and their corresponding intensity scan-plots in directions parallel (x) and perpendicular (y) with respect to the LAO/ STO multilayer. The scan-plots are taken from the regions inside the rectangles, following the directions indicated by the thin arrows. The intensity signals from the LAO layers are labelled. The thick arrows indicate the direction of the component of the ion beam in the plane of the lamellae during the final cleaning and thinning steps. at the same speed. Also the Monte Carlo simulations, performed using the SRIM program (Ziegler, 2003), predict a similar milling behaviour for both LAO and STO pure materials (Figs. 13C and 13D). The reason for the preferential destruction of the LAO layers is therefore to be found in the particular nature of multilayer. For a LAO/STO multilayer, the rombohedral LAO with a pseudo cubic lattice parameter a nm, has to grow on a cubic STO with lattice parameter a nm, the LAO layers will be in tension which could explain their faster destruction. It is also observed that for the thinner LAO layers, L6 and L8, the preferential destruction of the LAO layers is less, in agreement with the fact that for the thinnest LAO layers the built up strain will be less. Figure 14 shows two HRTEM images, taken on partially overlapping regions of a double cross-section, of a top-angle cleaned specimen. The effect of preferential milling (or at least preferential amorphisation) of the LaAlO 3 layers and the relatively uniform milling of the SrTiO 3 substrate is evident. Also the presence of an amorphous layer with a thickness of about 5 nm, is observed. It is accepted the FIB technique produces a lower rate of preferential sputtering compared to the conventional sample preparation techniques (Kamino et al., 2004). However, our results show that the LaAlO 3 layers tend to be preferentially destroyed in a systematic way and this effect definitely has to be considered for quantitative HRTEM or HAADF-STEM interpretation. The preferential milling/amorphisation observed in Figure 14 can reach a depth of about 3 nm from each face of the specimen and this will strongly influence e.g. the HAADF-STEM Z contrast differences between the LaAlO 3 and SrTiO 3 layers. In principle, the preferential destruction of the LAO layers can be attenuated by the reduction of the grazing

10 EVALUATION OF FIB SAMPLES FOR TEM ANALYSIS 1069 Fig. 11. HRTEM images of top (A,B) and top-angle (C) cleaned specimens, showing that the LAO layers are destroyed faster than the STO ones. The effect is not observed in A and it is less intense for the thinnest LAO layers (L6, L8) in B and C. The arrows have the same meaning as in Figure 10. Fig. 13. Cross-sectional FEG-SEM images of identical FIB milled boxes (x 5 20 lm, y lm) of (A) STO (100) oriented and (B) LAO oriented along a pseudo cubic (100) direction. The millings were performed under the same conditions (E 5 30 kv, I 5 1 na, milling time s, grazing angle 5 108). Both samples were mounted on the same holder and milled in sequence, under the same alignment for the ion beam. The boxes were filled with platinum, before the last milling step, made to expose the cross-sections. (C,D) Monte Carlo simulations of the trajectories of 100 Ga 11 ions bombarded on STO and LAO respectively (operating voltage of 5 kv and a grazing angle of 88). Fig. 14. HRTEM images of a double cross-section of an angle cleaned specimen. The effect of preferential milling (or at least preferential amorphisation) of the LAO layers and the relatively uniform milling of the STO substrate, as well as the presence of an amorphous layer with a thickness of about 5 nm, are shown. The observed thickness of the amorphous layer is consistent with the Monte Carlo simulations shown in Figure 13. Fig. 12. HRTEM images of side (A) and bottom - angle (B) specimens, showing that the LAO layers can be destroyed faster than the STO layers. The effect is less intense for the thinnest LAO layers (L6, L8) in A, and it is not observed in B. The arrows have the same meaning as in Figure 10. angle of the 5 kv ion beam during the final cleaning step. However in practice this is not so simple. At low kv settings it becomes difficult to set the FIB area precisely (Yabuuchi et al., 2004), and then tilting the sidewalls by a few degrees and scanning the Ga 11 ions over them has to be used for doing the FIB milling at 5 kv. On top of that, the low penetration depth of the Ga 11 ions at this low E setting makes it difficult to ensure a good interaction between the ions and the surfaces of the specimen to be cleaned, if the grazing angles are too low.

11 1070 E. MONTOYA ET AL. Applied cleaning schedule Relative size of thin (HRTEM) area TABLE 2. Comparative merit of the different kinds of prepared specimens HRTEM Specimen s performance/feature HAADF STEM Curtain effect Preferential milling/amorphisation of LAO Top Extended Good Good Strong Observed Top-angle Extended Good Good Weak Observed Side Small Good Good Not observed Observed Bottom-angle Small Good Good Not observed but uneven milling Not observed The thickness of the amorphous layer observed in Figure 14, is consistent with the penetration of the ion beam calculated by Monte Carlo simulations (Fig. 13). Although the thickness of the amorphous layer was only measured for cross-sections of a top-angle cleaned specimen, it can be expected from the SRIM calculations that the thickness of the damaged/amorphous layer should be similar for the other cleaning procedures. EFTEM mapping of a double cross-section (not shown here) indicates that the La is spread inside the amorphous layers, strongly suggesting a re-deposition effect (Montoya et al., 2006). The comparative performance/quality of the four different kinds of cleaned specimens studied in the present work is summarized in Table 2. Although all types of prepared specimens are able to give good HRTEM and HAADF-STEM pictures, the bottom-angle cleaning schedule is favoured because of the absence of a curtain effect and because of the absence of a preferential disintegration of the LAO layers. Bottom-angle cleaned specimens present only a small region thin enough for high resolution work and also exhibit uneven milling. From Table 2, it can be inferred that the ideal sample preparation, exhibiting extended thin areas, the absence of a curtain effect and no preferential disintegration of the LAO layers has not yet been obtained. From a practical point of view, the preparation of top-angle cleaned specimens is relatively easy. Hence the use of smaller grazing angles for the 5 kv cleaning step of topangle cleaned specimens, with the purpose of reducing the preferential disintegration of the LAO layers will be further elaborated. CONCLUSIONS All four 5 kv FIB cleaning procedures yield lamellae useful for high resolution TEM as well as for atomic-resolution HAADF-STEM. The presence of the curtain effect is intense for the top cleaned specimens and weak for the top-angle cleaned ones. No curtain effect is observed for the side cleaned or bottom-angle cleaned specimens. The top cleaned and top-angle cleaned specimens present extended areas, thin enough for high resolution work, while those areas are comparatively small for the side cleaned and bottomangle cleaned specimens. The bottom-angle cleaned specimens present an uneven milling. Evidence of preferential destruction of the LAO layers, is found for the top, top-angle and side cleaned specimens, but not for those bottom-angle cleaned. The double cross-section experiment revealed that the amorphous layers remaining onto the faces of a top-angle cleaned specimen have an average thickness of about 5 nm; these layers are formed (at least partially) by redeposition. ACKNOWLEDGMENTS This work has been performed under the Interuniversity Attraction Poles programme, Belgian State, Belgian Science Policy. The authors are grateful to M. Huijben and G. Rijnders of the NESA+ group at the University of Twente (NI) for the growth of the multilayers. The authors acknowledge Jan Eysermans for the design and machining of the device shown in Figure 5A. The authors wish to thank Dr. Rolf Erni for the helpful discussions. S. Bals is grateful to the Fund for Scientific Research Flanders. 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