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A cross-sectional TEM sample preparation method for films deposited on metallic substrates
Materials Characterization 58 (2007) 666 – 669
Short communication
A cross-sectional TEM sample preparation method for films deposited on metallic substrates Yan Liu a,⁎, Ruobin Wang b , Xinqiu Guo b , Jiawei Dai a a
State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai, 200030, China b Instrumental Analysis Center of Shanghai Jiao Tong University Shanghai, 200030, China Received 18 July 2006; accepted 27 July 2006
Abstract This paper concerns the method on how to prepare TEM samples for the films deposited on metallic substrates. This method is described in a step-by-step way and applied to the VN/SiO2 superlattice to testify to its feasibility in the second part. © 2006 Elsevier Inc. All rights reserved. Keywords: Films; Superlattice film; Cross-sectional TEM sample preparation; Metallic substrate
1. Introduction To date, because of their attractive physical characteristics, films of two-dimensional materials have been extensively used for a number of industrial applications, such as conductive or dielectric coatings in IC fabrication or protective coatings in manufacturing. Unfortunately, a lot of general characterization methods for microstructural analysis are difficult to carry out on films, especially those synthesized by PVD or CVD, because their thickness is often limited to a micrometer, or even thinner scale. Nevertheless, transmission electron microscopic (TEM) analysis, with its high spatial resolution down to the atomic scale and good broad analytical ability, has played a critical role in recent microstructural analysis of thin films. As for TEM analysis techniques, plan-view and cross-sectional observations are two commonly used methods. Compared with the plan-view, the cross-sec⁎ Corresponding author. Tel.: +86 21 62932106. E-mail address: [email protected] (Y. Liu). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.07.016
tional TEM observation is more favorable for obtaining microstructural information since films grow coherently on the substrates and often present columnar crystals with preferred orientation. For example, the layered structure of superlattice films can be observed more meaningfully from a cross-sectional view rather than plan-view. Yet, difficulties in sample preparation limit the application of cross-sectional TEM observation. Although several researchers [1,2] have reported techniques for preparing such samples for films deposited on silicon wafers, there is little literature published for films on metallic substrates. As we know, surface modification methods such as coating, nitriding and charring are widely carried out on metallic substrates in industrial applications, and the microstructure of deposited thin films depends significantly on their substrates. Hence, information derived from the study of films deposited on silicon wafers may not be germane to the situation for films on metallic substrates. In this paper, we outline a procedure for preparing cross-sectional TEM samples from films deposited on a metallic substrate.
Y. Liu et al. / Materials Characterization 58 (2007) 666–669
2. Sample preparation procedure For films deposited on metallic substrates, the procedure for the preparation of cross-sectional TEM samples is described in the sequence shown in Fig. 1. 2.1. Sampling Two semicircular columns, 2 mm in diameter and about 10 mm in length, are removed from the metallic substrate where the thin film has been deposited, using the electric spark linear cutting
667
technique (Fig. 1a). The films are on the planar side of the columns. 2.2. Embedding A stainless steel or copper tube (3 mm outer and 2 mm inner diameter) is cut into small pieces, 10 mm in length, the same length as the semicircular columns. After the columns and the metallic tubes are ultrasonically cleaned in acetone and alcohol, the two semicircular bars are bonded together with epoxy, with the film surfaces facing each other, and then inserted into the tube (Fig.
Fig. 1. A schematic drawing for preparing the cross-sectional TEM specimen: (a) specimen removing, (b) embedding, (c) cutting, (d) aperture grid gluing and (e) dimpling.
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Y. Liu et al. / Materials Characterization 58 (2007) 666–669
grid (1.5 mm inner and 3 mm outer diameter) is bonded to each disc with epoxy after preliminary grinding, leaving the hole of the Mo ring uncontaminated by the epoxy as shown in Fig. 1d. After curing, the same thinning procedure is carried out on the disc side until the thickness of the specimen is reduced to less than 0.08 mm, i.e. the disc is less than 0.05 mm. When the specimen is less than 0.1 mm thick, it can be easily damaged. Moreover, the epoxy is more quickly removed than the metal and the ceramic during the ion milling. So it is necessary for the inner diameter of the Mo aperture grid to be a bit smaller than the tube, in this way, the grid provides mechanical support which avoids the separation of the disc along the central gluing line and strengthens all the bonding faces during further thinning and ion milling. 2.5. Dimpling For successful ion milling, preparation of the sample before it goes into the ion mill is critical. Dimpling is the
Fig. 2. Metallurgical microscopic observation of the specimen after the ion milling procedure: (a) low magnification and (b) high magnification.
1b). During the process, it is important to make sure the whole inner surface of the tube and the outsides of the two semicircular bars are smeared with epoxy. Then the specimen is heated or just allowed to sit for hours at room temperature according to the solidification conditions of the epoxy in use. 2.3. Cutting After the epoxy is cured, the specimen is sliced into about 0.3 mm thick discs by a low speed diamond wheel saw (Fig. 1c). Since the damage zone is thicker by electric spark linear cutting, the thickness of the discs should be nearer to 0.5 mm if the latter technique is used. 2.4. Aperture grid gluing and pre-thinning At first, the discs are manually thinned to approximately 0.1 mm from both sides in a disc grinder using 30–10 μm grit paper. Then a 0.03 mm thick Mo aperture
Fig. 3. Cross-sectional TEM SADP (a) and HRTEM (b) images of VN/ SiO2 superlattice.
Y. Liu et al. / Materials Characterization 58 (2007) 666–669
last abrasive pre-thinning step. It is performed from only one side of the specimen, directly on top of the disc (Fig. 1e). The dimpling procedure produces a thin central region in the disc. It is carried out until the central thickness approaches 0.02 mm. The sample must not be too thin since it would be easily broken, or too thick because it will correspondingly extend the subsequent ion milling time. 2.6. Ion milling Ion milling is the last but the most important step of the preparation process. The sample is mounted on a specimen holder and ion beam-polished to generate an electron transparent area. This is accomplished by turning the ion guns on and off during the sample rotation. The left and right ion guns are tilted 10° from the top and the bottom, respectively. As soon as the perforation is detected, the voltage of the ion beams is reduced to a small level and the incident angle changed to 4° to enlarge the transparent area for TEM investigation. In general, the ion milling rate of metal is much lower than that of silicon. Therefore, in preparing TEM samples from films deposited on metallic substrates, it is sometimes observed that the films might be completely removed by ion bombardment before there was a thin area on the metallic substrates. Because of this situation, it is not feasible to specify the ion milling time required for the creation of a thin area in metallic substrates. For each combination of substrate and film, the conditions and time required for the milling must be confirmed by a specific analysis or an elaborate experiment to make sure there is a thin area in the films suitable for TEM observation. 3. TEM observation on a VN/SiO2 superlattice film In order to demonstrate the feasibility of this method, a specimen of a VN/SiO2 superlattice film was prepared according to the process mentioned above. The VN/SiO2 superlattice film was prepared by reactive magnetron sputtering using vanadium and silicon oxide targets. The reactive gas was nitrogen and the sputtering gas was argon. The multilayered structure was prepared by alternately stopping the substrate holder in front of Vand SiO2 targets to receive layers of VN and SiO2. The total film thickness was about 2 μm. The optical microscopic images in Fig. 2 display the appearance of the VN/SiO2 specimen after ion milling. The overall view of the specimen with the aperture is illustrated in Fig. 2a. Magnifying the location of the
669
aperture, Fig. 2b, the thin area and the film located in the aperture are clearly observed and are able to be used for further TEM observation. Fig. 3 exhibits the cross-sectional TEM images of the VN (3.6 nm)/SiO2 (0.9 nm) superlattice film. The dark and light layers correspond to the VN and SiO2 layers, respectively. In Fig. 3a, superlattices present a welldefined structure with planar modulation layers parallel to the surface of the substrate. Additionally, the selected area electron diffraction (SAED) pattern on the top left corner is characterized by a set of FCC polycrystalline rings, indicating that the superlattice exists in a NaCl structure. In Fig. 3b, the highly magnified lattice image shows that the lattice fringes penetrate several modulation layers, which demonstrates that the SiO2 layers have crystallized and have grown epitaxially with the VN layers. Influenced by the “template effects” of the VN layers [3], the as-deposited SiO2 that used to be amorphous is forced to crystallize at this thickness and grows epitaxially with the VN layers. SiO2 layers are more likely to form as pseudocrystals with a similar NaCl structure as VN in this condition [4]. The crystallization of amorphous layers in these superlattices may bring novel changes to their properties [5]. These results serve to demonstrate the utility of the sample preparation procedure described for preparing and examining thin deposited films. Acknowledgement This work was financially supported by the National Natural Chinese Foundation of China (Grant No. 50571062). References [1] Weaver L. Cross-section TEM sample preparation of multilayer and poorly adhering films. Microsc Res Tech 1997;36:368–71. [2] Zhang H. Transmission electron microscopy for the semi-conductor industry. Micron 2002;33(6):515–21. [3] Li Geyang, Lao Jijun, Tian Jiawan, Han Zenghu, Gu Mingyuan. The coherent growth and mechanical properties of AlN/VN multilayers. J Appl Phys 2004;95:92–6. [4] Wei Lun, Mei Fanghua, Shao Nan, Kong Ming, Li Geyang, Li Jianguo. Template-induced crystallization of amorphous SiO2 and its effects on the mechanical properties of TiN/SiO2 nanomultilayers. Appl Phys Lett 2005;86(2) [021919(1-3)]. [5] Mei Fanghua, Shao Nan, Wei Lun, Dong Yunshan, Li Geyang. Coherent epitaxial growth and superhardness effects of c-TiN/hTiB2 nanomultilayers. Appl Phys Lett 2005;87:011906.
Short communication
A cross-sectional TEM sample preparation method for films deposited on metallic substrates Yan Liu a,⁎, Ruobin Wang b , Xinqiu Guo b , Jiawei Dai a a
State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai, 200030, China b Instrumental Analysis Center of Shanghai Jiao Tong University Shanghai, 200030, China Received 18 July 2006; accepted 27 July 2006
Abstract This paper concerns the method on how to prepare TEM samples for the films deposited on metallic substrates. This method is described in a step-by-step way and applied to the VN/SiO2 superlattice to testify to its feasibility in the second part. © 2006 Elsevier Inc. All rights reserved. Keywords: Films; Superlattice film; Cross-sectional TEM sample preparation; Metallic substrate
1. Introduction To date, because of their attractive physical characteristics, films of two-dimensional materials have been extensively used for a number of industrial applications, such as conductive or dielectric coatings in IC fabrication or protective coatings in manufacturing. Unfortunately, a lot of general characterization methods for microstructural analysis are difficult to carry out on films, especially those synthesized by PVD or CVD, because their thickness is often limited to a micrometer, or even thinner scale. Nevertheless, transmission electron microscopic (TEM) analysis, with its high spatial resolution down to the atomic scale and good broad analytical ability, has played a critical role in recent microstructural analysis of thin films. As for TEM analysis techniques, plan-view and cross-sectional observations are two commonly used methods. Compared with the plan-view, the cross-sec⁎ Corresponding author. Tel.: +86 21 62932106. E-mail address: [email protected] (Y. Liu). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.07.016
tional TEM observation is more favorable for obtaining microstructural information since films grow coherently on the substrates and often present columnar crystals with preferred orientation. For example, the layered structure of superlattice films can be observed more meaningfully from a cross-sectional view rather than plan-view. Yet, difficulties in sample preparation limit the application of cross-sectional TEM observation. Although several researchers [1,2] have reported techniques for preparing such samples for films deposited on silicon wafers, there is little literature published for films on metallic substrates. As we know, surface modification methods such as coating, nitriding and charring are widely carried out on metallic substrates in industrial applications, and the microstructure of deposited thin films depends significantly on their substrates. Hence, information derived from the study of films deposited on silicon wafers may not be germane to the situation for films on metallic substrates. In this paper, we outline a procedure for preparing cross-sectional TEM samples from films deposited on a metallic substrate.
Y. Liu et al. / Materials Characterization 58 (2007) 666–669
2. Sample preparation procedure For films deposited on metallic substrates, the procedure for the preparation of cross-sectional TEM samples is described in the sequence shown in Fig. 1. 2.1. Sampling Two semicircular columns, 2 mm in diameter and about 10 mm in length, are removed from the metallic substrate where the thin film has been deposited, using the electric spark linear cutting
667
technique (Fig. 1a). The films are on the planar side of the columns. 2.2. Embedding A stainless steel or copper tube (3 mm outer and 2 mm inner diameter) is cut into small pieces, 10 mm in length, the same length as the semicircular columns. After the columns and the metallic tubes are ultrasonically cleaned in acetone and alcohol, the two semicircular bars are bonded together with epoxy, with the film surfaces facing each other, and then inserted into the tube (Fig.
Fig. 1. A schematic drawing for preparing the cross-sectional TEM specimen: (a) specimen removing, (b) embedding, (c) cutting, (d) aperture grid gluing and (e) dimpling.
668
Y. Liu et al. / Materials Characterization 58 (2007) 666–669
grid (1.5 mm inner and 3 mm outer diameter) is bonded to each disc with epoxy after preliminary grinding, leaving the hole of the Mo ring uncontaminated by the epoxy as shown in Fig. 1d. After curing, the same thinning procedure is carried out on the disc side until the thickness of the specimen is reduced to less than 0.08 mm, i.e. the disc is less than 0.05 mm. When the specimen is less than 0.1 mm thick, it can be easily damaged. Moreover, the epoxy is more quickly removed than the metal and the ceramic during the ion milling. So it is necessary for the inner diameter of the Mo aperture grid to be a bit smaller than the tube, in this way, the grid provides mechanical support which avoids the separation of the disc along the central gluing line and strengthens all the bonding faces during further thinning and ion milling. 2.5. Dimpling For successful ion milling, preparation of the sample before it goes into the ion mill is critical. Dimpling is the
Fig. 2. Metallurgical microscopic observation of the specimen after the ion milling procedure: (a) low magnification and (b) high magnification.
1b). During the process, it is important to make sure the whole inner surface of the tube and the outsides of the two semicircular bars are smeared with epoxy. Then the specimen is heated or just allowed to sit for hours at room temperature according to the solidification conditions of the epoxy in use. 2.3. Cutting After the epoxy is cured, the specimen is sliced into about 0.3 mm thick discs by a low speed diamond wheel saw (Fig. 1c). Since the damage zone is thicker by electric spark linear cutting, the thickness of the discs should be nearer to 0.5 mm if the latter technique is used. 2.4. Aperture grid gluing and pre-thinning At first, the discs are manually thinned to approximately 0.1 mm from both sides in a disc grinder using 30–10 μm grit paper. Then a 0.03 mm thick Mo aperture
Fig. 3. Cross-sectional TEM SADP (a) and HRTEM (b) images of VN/ SiO2 superlattice.
Y. Liu et al. / Materials Characterization 58 (2007) 666–669
last abrasive pre-thinning step. It is performed from only one side of the specimen, directly on top of the disc (Fig. 1e). The dimpling procedure produces a thin central region in the disc. It is carried out until the central thickness approaches 0.02 mm. The sample must not be too thin since it would be easily broken, or too thick because it will correspondingly extend the subsequent ion milling time. 2.6. Ion milling Ion milling is the last but the most important step of the preparation process. The sample is mounted on a specimen holder and ion beam-polished to generate an electron transparent area. This is accomplished by turning the ion guns on and off during the sample rotation. The left and right ion guns are tilted 10° from the top and the bottom, respectively. As soon as the perforation is detected, the voltage of the ion beams is reduced to a small level and the incident angle changed to 4° to enlarge the transparent area for TEM investigation. In general, the ion milling rate of metal is much lower than that of silicon. Therefore, in preparing TEM samples from films deposited on metallic substrates, it is sometimes observed that the films might be completely removed by ion bombardment before there was a thin area on the metallic substrates. Because of this situation, it is not feasible to specify the ion milling time required for the creation of a thin area in metallic substrates. For each combination of substrate and film, the conditions and time required for the milling must be confirmed by a specific analysis or an elaborate experiment to make sure there is a thin area in the films suitable for TEM observation. 3. TEM observation on a VN/SiO2 superlattice film In order to demonstrate the feasibility of this method, a specimen of a VN/SiO2 superlattice film was prepared according to the process mentioned above. The VN/SiO2 superlattice film was prepared by reactive magnetron sputtering using vanadium and silicon oxide targets. The reactive gas was nitrogen and the sputtering gas was argon. The multilayered structure was prepared by alternately stopping the substrate holder in front of Vand SiO2 targets to receive layers of VN and SiO2. The total film thickness was about 2 μm. The optical microscopic images in Fig. 2 display the appearance of the VN/SiO2 specimen after ion milling. The overall view of the specimen with the aperture is illustrated in Fig. 2a. Magnifying the location of the
669
aperture, Fig. 2b, the thin area and the film located in the aperture are clearly observed and are able to be used for further TEM observation. Fig. 3 exhibits the cross-sectional TEM images of the VN (3.6 nm)/SiO2 (0.9 nm) superlattice film. The dark and light layers correspond to the VN and SiO2 layers, respectively. In Fig. 3a, superlattices present a welldefined structure with planar modulation layers parallel to the surface of the substrate. Additionally, the selected area electron diffraction (SAED) pattern on the top left corner is characterized by a set of FCC polycrystalline rings, indicating that the superlattice exists in a NaCl structure. In Fig. 3b, the highly magnified lattice image shows that the lattice fringes penetrate several modulation layers, which demonstrates that the SiO2 layers have crystallized and have grown epitaxially with the VN layers. Influenced by the “template effects” of the VN layers [3], the as-deposited SiO2 that used to be amorphous is forced to crystallize at this thickness and grows epitaxially with the VN layers. SiO2 layers are more likely to form as pseudocrystals with a similar NaCl structure as VN in this condition [4]. The crystallization of amorphous layers in these superlattices may bring novel changes to their properties [5]. These results serve to demonstrate the utility of the sample preparation procedure described for preparing and examining thin deposited films. Acknowledgement This work was financially supported by the National Natural Chinese Foundation of China (Grant No. 50571062). References [1] Weaver L. Cross-section TEM sample preparation of multilayer and poorly adhering films. Microsc Res Tech 1997;36:368–71. [2] Zhang H. Transmission electron microscopy for the semi-conductor industry. Micron 2002;33(6):515–21. [3] Li Geyang, Lao Jijun, Tian Jiawan, Han Zenghu, Gu Mingyuan. The coherent growth and mechanical properties of AlN/VN multilayers. J Appl Phys 2004;95:92–6. [4] Wei Lun, Mei Fanghua, Shao Nan, Kong Ming, Li Geyang, Li Jianguo. Template-induced crystallization of amorphous SiO2 and its effects on the mechanical properties of TiN/SiO2 nanomultilayers. Appl Phys Lett 2005;86(2) [021919(1-3)]. [5] Mei Fanghua, Shao Nan, Wei Lun, Dong Yunshan, Li Geyang. Coherent epitaxial growth and superhardness effects of c-TiN/hTiB2 nanomultilayers. Appl Phys Lett 2005;87:011906.