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Thin solid film sample preparation by a small-angle cleavage for transmission electron microscopy
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Thin Solid Films 304 (1997) I57-159
Thin solid film sample preparation by a small-angle cleavage for transmission electron microscopy Suli Suder *, C.A. Faunce, S.E. Donnelly Joule Physics Laboratory Science Research Institute, University of Salford, Salford M5 4WT, UK
Received 14 October 1996; accepted 16 January 1997
Abstract
Thin solid film samples have been prepared by a small-angle cleavage technique using hand tools. Cleaved wedges from the same material are mounted both as plan-view and cross-sectional samples on the same transmission electron microscopy (TEM) specimen grid allowing convenient examination in both views. The samples of Si3N4, Zr and Co films deposited on Si prepared by this technique are shown to be suitable for analysis in TEM. © 1997 Published by Elsevier Science S.A. Ke;~words: Thin solid films; Cleavage;Transmissioneieclron microscopy
Analytical high-resolution transmission electron microscopy (TEM) is widely used in materials research and yields information on the microstructure and composition of materials. However, the preparation of thin sections of material suitable for TEM, particularly cross-sections of deposited thin films, is generally a time-consuming and laborious process. In addition, the commonly-used ionmilling technique can result in artifacts resulting from the interaction of the energetic ions with the material unless extreme care is taken to use very low ion energies and very shallow angles of incidence. The small-angle cleavage technique (SACT), reported by McCaffrey [1,2], was developed for the preparation of TEM cross-sectional samples of semiconductors and related materials. Compared with the ion milling method with its reliance on relatively complex equipment, the SACT uses only hand tools such as a scriber and tweezers and readily available laboratory equipment such as a sample grinder, a low-power stereo light microscope, a hot plate and a low-temperature oven. In addition, only a small piece of material (5 nma × 5 mm) is required to produce cross-sectional specimens. The procedure is simple, generally consisting of three steps: back
" Correspondingauthor. Tel: +44 16i 745 5000 ext. 3264; fax: +44 161 745 5119; e-mail: [email protected]
thinning of the bulk sample, cleaving, and wedge mounting. It is also rapid, taking about 1 h to produce a cross-sectional sample. It is, of course, free of ion milling artifacts. In the present work, the SACT has been extended to prepare both plan-view and cross-sectional samples of both amorphous and polycrystalline thin films on < 001 > and < 111 > silicon substrates. Essentially the crack that propagates through the Si by cleavage continues in the thin film even though there may be no well defined cleavage planes in this material. The small-angle wedges of material for the two views are cleaved from a piece of silicon measuring only about 3 m m × 2 mm. The wedges are mounted both as plan-view and cross-sectional samples on the same TEM grid allowing examination of both views without changing specimen, therefore greatly facilitating characterization of film samples by TEM. Si3N4, Zr and Co films on Si substrates have been prepared by SACT and examined by TEM. The results show that all the samples prepared by this technique are suitable for TEM analysis. To prepare a pair of specimens of a film deposited on < 001 > Si, a small piece typically measuring 3 mm X 2 mm of the substrate and film is cleaved from the wafer ready for use. Only the key details of the microcleavage procedure will be given here since it has been described thoroughly elsewhere [2]. Firstly, the small piece of the
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material is mounted polished surface down on a specially designed polishing jig with low temperature wax, and carefully polished by hand with 600 grit abrasive paper to about 100 ~xm thickness with the direction of abrasion along the {120} planes. The specimen is then scribed on the back and then cleaved along {120} planes to make two or three pieces. A second cleavage is conducted on the front surface along {110} planes, forming pieces with a wedge angle of 18.43 °. Wedges with sharp tips are selected using a low-power optical microscope and are then mounted with the appropriate orientation on a 600 I~m hole TEM grid using a vacuum compatible conductive epoxy. Fig. l(a) shows that geometrical shape of the tip of a cleaved < 001 > wedge which is a three-face pyramid intersected by (001), (120) and (111) planes. Lines OA and OB are defined as the cross-sectional axis and plan-view axis respectively. Besides mounting the wedges as crosssectional samples, for samples whose deposited film thickness renders them electron transparent in plan view, by mounting the (001) plane of the wedges parallel to the surface of the grid, plan-view samples can also be produced on the same grid with the cross-sectional samples of same material. The success rate in producing suitably
(a)
o
~O°
,i¢ (m)
90°
c
Fig. I. Schematic drawing of the tip of a small-angle cleaved < 001 > crystal wedge. It is a three-face pyramid which is intersected by (00i), (120) and (111) planes. For ideally cleaved wedges, a and y should be
18.43° and 54.75° respectively. /3 is determined by o~ and y and is 24.08 °. OA and OB are defined as the cross-sectional axis and plan-view axis respectively. (b) SEM photograph of a typical pair of "two-view" TEM samples mounted with two plan-view and two cross-sectional
samples.
-.Eg-i .~]
Fig. 2. TEM images of a sample of Si3N 4 film deposited on < 001 > Si. (a) Cross-sectional BF micrograph of the sample taken 18.4° away from the < 1 i0 > zone axis. Inset: plan-view diffraction pattern of the sample shows that the film is amorphous. (b) Lattice image of the interface area of the sample.
electron transparent wedges is approximately 50%. Two of each kind of sample are generally mounted on the same grid as shown in Fig. l(b), giving a reasonable probability that both views of the film will be available without changing samples in TEM. After the epoxy has cured, the samples are ready for TEM examination. For simplicity m manipulating the specimens in a double-tilt holder in the microscope, they are generally mounted with each wedge axis approximately aligned along either the X- or Y-tilt axis. All samples were examined using a JEOL 3010 transmission electron microscope equipped with parallel electron energy toss spectroscopy (PEELS) and X-ray energy dispersive spectroscopy (EDS). Fig. 2(a) shows that the sample of amorphous S i 3 N 4 film on <001 > Si. The image was taken 18.4 ° away from the < 110> zone axis in order to show the 3D
S. Suder et al. / Thin Solid Films 304 (1997) 157-159
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Fig. 5. Plan-view BF micrograph of Co(/3 ) film deposited on < 111 > Si. Sample is prepared by the SACT. Inset: Plan-view diffraction pattern of the film.
Fig. 3. TEM images of Zr film sample prepared by the small-angle cleavage technique. (a) BF image of the cross-sectional sample. (b) Cross-sectional lattice image of the polycrystalline film area of the sample.
picture of the film. The shape of the substrate tip is generally a three-face pyramid but the film is a four-sided wedge. Fig. 3 shows that the results of a cleaved sample with an about 150 nm thick polycrystalline Zr film. Even though the sample tip was broken, the cross-section is still electron transparent. Fig. 4 shows that a cleaved sample from an approximately 500 nm thick amorphous Si3N 4 film; the tip of the sample was broken where the specimen was fairly thick indicating that there is a deposited film thickness limit for the film sample preparation by the SACT. Fig. 5 is an example of < 111 > Si with a polycrystalline Co film sample prepared by the SACT. We have shown that thin solid film samples can be prepared by the small-angle cleavage technique. Besides cross-sectional samples, plan-view samples can be produced for those samples whose deposited film thickness renders them electron transparent in plan view. TEM results have shown that the small-angle cleavage technique is suitable to prepare good quality thin solid film samples.
Acknowledgements The authors acknowledge support from the EPSRC (grant No. GR/H65320) for the Salford University High Resolution Microscopy Facility. One of us (SS) thanks the University of Salford for providing of a studentship. We acknowledge M. Lambrinos, I. Tashlykov and J. Fallon for providing their bulk samples.
References Fig. 4. Cross-sectional micrograph of an amorphous Si3 N4 film deposited on < 001 > Si. Sample is cleaved by the SACT.
[1] J.P. McCaffrey, Ultramicroscopy, 38 (1991) 149-157. [2] J.P. McCaffrey, Mater. Res. Soc. Symp. Proc., 254 (i992) i09-120.
Thin Solid Films 304 (1997) I57-159
Thin solid film sample preparation by a small-angle cleavage for transmission electron microscopy Suli Suder *, C.A. Faunce, S.E. Donnelly Joule Physics Laboratory Science Research Institute, University of Salford, Salford M5 4WT, UK
Received 14 October 1996; accepted 16 January 1997
Abstract
Thin solid film samples have been prepared by a small-angle cleavage technique using hand tools. Cleaved wedges from the same material are mounted both as plan-view and cross-sectional samples on the same transmission electron microscopy (TEM) specimen grid allowing convenient examination in both views. The samples of Si3N4, Zr and Co films deposited on Si prepared by this technique are shown to be suitable for analysis in TEM. © 1997 Published by Elsevier Science S.A. Ke;~words: Thin solid films; Cleavage;Transmissioneieclron microscopy
Analytical high-resolution transmission electron microscopy (TEM) is widely used in materials research and yields information on the microstructure and composition of materials. However, the preparation of thin sections of material suitable for TEM, particularly cross-sections of deposited thin films, is generally a time-consuming and laborious process. In addition, the commonly-used ionmilling technique can result in artifacts resulting from the interaction of the energetic ions with the material unless extreme care is taken to use very low ion energies and very shallow angles of incidence. The small-angle cleavage technique (SACT), reported by McCaffrey [1,2], was developed for the preparation of TEM cross-sectional samples of semiconductors and related materials. Compared with the ion milling method with its reliance on relatively complex equipment, the SACT uses only hand tools such as a scriber and tweezers and readily available laboratory equipment such as a sample grinder, a low-power stereo light microscope, a hot plate and a low-temperature oven. In addition, only a small piece of material (5 nma × 5 mm) is required to produce cross-sectional specimens. The procedure is simple, generally consisting of three steps: back
" Correspondingauthor. Tel: +44 16i 745 5000 ext. 3264; fax: +44 161 745 5119; e-mail: [email protected]
thinning of the bulk sample, cleaving, and wedge mounting. It is also rapid, taking about 1 h to produce a cross-sectional sample. It is, of course, free of ion milling artifacts. In the present work, the SACT has been extended to prepare both plan-view and cross-sectional samples of both amorphous and polycrystalline thin films on < 001 > and < 111 > silicon substrates. Essentially the crack that propagates through the Si by cleavage continues in the thin film even though there may be no well defined cleavage planes in this material. The small-angle wedges of material for the two views are cleaved from a piece of silicon measuring only about 3 m m × 2 mm. The wedges are mounted both as plan-view and cross-sectional samples on the same TEM grid allowing examination of both views without changing specimen, therefore greatly facilitating characterization of film samples by TEM. Si3N4, Zr and Co films on Si substrates have been prepared by SACT and examined by TEM. The results show that all the samples prepared by this technique are suitable for TEM analysis. To prepare a pair of specimens of a film deposited on < 001 > Si, a small piece typically measuring 3 mm X 2 mm of the substrate and film is cleaved from the wafer ready for use. Only the key details of the microcleavage procedure will be given here since it has been described thoroughly elsewhere [2]. Firstly, the small piece of the
0040-6090/97/$17.00 © 1997 Published by Elsevier Science S.A. All rights reserved. PII S0040-6090(97)00042-4
158
S. Suder ez al. / Thin Solid Films 304 (1997) 157-159
material is mounted polished surface down on a specially designed polishing jig with low temperature wax, and carefully polished by hand with 600 grit abrasive paper to about 100 ~xm thickness with the direction of abrasion along the {120} planes. The specimen is then scribed on the back and then cleaved along {120} planes to make two or three pieces. A second cleavage is conducted on the front surface along {110} planes, forming pieces with a wedge angle of 18.43 °. Wedges with sharp tips are selected using a low-power optical microscope and are then mounted with the appropriate orientation on a 600 I~m hole TEM grid using a vacuum compatible conductive epoxy. Fig. l(a) shows that geometrical shape of the tip of a cleaved < 001 > wedge which is a three-face pyramid intersected by (001), (120) and (111) planes. Lines OA and OB are defined as the cross-sectional axis and plan-view axis respectively. Besides mounting the wedges as crosssectional samples, for samples whose deposited film thickness renders them electron transparent in plan view, by mounting the (001) plane of the wedges parallel to the surface of the grid, plan-view samples can also be produced on the same grid with the cross-sectional samples of same material. The success rate in producing suitably
(a)
o
~O°
,i¢ (m)
90°
c
Fig. I. Schematic drawing of the tip of a small-angle cleaved < 001 > crystal wedge. It is a three-face pyramid which is intersected by (00i), (120) and (111) planes. For ideally cleaved wedges, a and y should be
18.43° and 54.75° respectively. /3 is determined by o~ and y and is 24.08 °. OA and OB are defined as the cross-sectional axis and plan-view axis respectively. (b) SEM photograph of a typical pair of "two-view" TEM samples mounted with two plan-view and two cross-sectional
samples.
-.Eg-i .~]
Fig. 2. TEM images of a sample of Si3N 4 film deposited on < 001 > Si. (a) Cross-sectional BF micrograph of the sample taken 18.4° away from the < 1 i0 > zone axis. Inset: plan-view diffraction pattern of the sample shows that the film is amorphous. (b) Lattice image of the interface area of the sample.
electron transparent wedges is approximately 50%. Two of each kind of sample are generally mounted on the same grid as shown in Fig. l(b), giving a reasonable probability that both views of the film will be available without changing samples in TEM. After the epoxy has cured, the samples are ready for TEM examination. For simplicity m manipulating the specimens in a double-tilt holder in the microscope, they are generally mounted with each wedge axis approximately aligned along either the X- or Y-tilt axis. All samples were examined using a JEOL 3010 transmission electron microscope equipped with parallel electron energy toss spectroscopy (PEELS) and X-ray energy dispersive spectroscopy (EDS). Fig. 2(a) shows that the sample of amorphous S i 3 N 4 film on <001 > Si. The image was taken 18.4 ° away from the < 110> zone axis in order to show the 3D
S. Suder et al. / Thin Solid Films 304 (1997) 157-159
159
Fig. 5. Plan-view BF micrograph of Co(/3 ) film deposited on < 111 > Si. Sample is prepared by the SACT. Inset: Plan-view diffraction pattern of the film.
Fig. 3. TEM images of Zr film sample prepared by the small-angle cleavage technique. (a) BF image of the cross-sectional sample. (b) Cross-sectional lattice image of the polycrystalline film area of the sample.
picture of the film. The shape of the substrate tip is generally a three-face pyramid but the film is a four-sided wedge. Fig. 3 shows that the results of a cleaved sample with an about 150 nm thick polycrystalline Zr film. Even though the sample tip was broken, the cross-section is still electron transparent. Fig. 4 shows that a cleaved sample from an approximately 500 nm thick amorphous Si3N 4 film; the tip of the sample was broken where the specimen was fairly thick indicating that there is a deposited film thickness limit for the film sample preparation by the SACT. Fig. 5 is an example of < 111 > Si with a polycrystalline Co film sample prepared by the SACT. We have shown that thin solid film samples can be prepared by the small-angle cleavage technique. Besides cross-sectional samples, plan-view samples can be produced for those samples whose deposited film thickness renders them electron transparent in plan view. TEM results have shown that the small-angle cleavage technique is suitable to prepare good quality thin solid film samples.
Acknowledgements The authors acknowledge support from the EPSRC (grant No. GR/H65320) for the Salford University High Resolution Microscopy Facility. One of us (SS) thanks the University of Salford for providing of a studentship. We acknowledge M. Lambrinos, I. Tashlykov and J. Fallon for providing their bulk samples.
References Fig. 4. Cross-sectional micrograph of an amorphous Si3 N4 film deposited on < 001 > Si. Sample is cleaved by the SACT.
[1] J.P. McCaffrey, Ultramicroscopy, 38 (1991) 149-157. [2] J.P. McCaffrey, Mater. Res. Soc. Symp. Proc., 254 (i992) i09-120.