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Application of the ionless tripod polisher to the preparation of YBCO superconducting multilayer and bulk ceramics thin films
ultramicroscoov ELSEVIER
Ultramicroscopy
60 (1995) 195-206
Application of the ionless tripod polisher to the preparation of YBCO superconducting multilayer and bulk ceramics thin films J. Ayache ay*, P.H. AlbarSde b a CSNSM, Britimen! 108, F-91405 Orsay Campus, France b IBM France, Semite 1807-31 U, BP58, F-91105 Corbeil Essonnes, France Received
2 December
1994; in final form 15 May 1995
Abstract
The standard method of polishing using ion milling for cross-section preparation induces atomic diffusion which can be avoided by using an ionless polisher. The present paper gives the details of the method as applied to superconducting YBa,Cu,O, melt-textured ceramics and multilayer thin films. The structure and chemistry of interfaces in this complex system are known to be influenced by changes in the chemical composition. Because of non-stoichiometry related growth of secondary phases, many secondary phases form. We present a technique, which allows preparation of electron-transparent TEM samples, that can be used for HREM and chemical characterizations.
1. Introduction YBa,Cu,O, ceramics are very sensitive to thermodynamic and kinetic process conditions because of their incongruent behavior at high temperature. For example, a same process involving high-temperature heat treatment can lead to two ceramics having different physical properties because of different microstructures and microchemistry. The improvement of the process for industrial applications as thin films for electronic devices, or bulk-textured ceramics for high current transport, is the way to achieve reproducibility in the microstructure and physical properties.
* Corresponding
author.
0304-3991/95/$09.50 0 1995 Elsevier SSDIO304-3991(95)00073-9
Science
A study of grain boundaries and interfaces is required as they are directly involved. YBa,Cu,O, textured ceramics form through a peritectic reaction which leads to the precipitation of Y,BaCuO, inclusions (called 211 precipitate) from the liquid phase. The crystallisation of the liquid gives a YBa,Cu,O, textured matrix called the 123 phase. The superconducting properties CT,), susceptibility (X,), transport properties CR,), and the critical current density (.I,) depend on the presence of defects and particularly on their size and orientation. Defects with dimensioa in the order of theocoherence length (from 3 A along c axis and 20 A in the superconducting ab planes) are most efficient. Because of the dimension of the interface plane, interfaces behave as the vortex pinning centers. The microstructure of interfaces depends on the structure and parameters of the matrix and precipi-
B.V. All rights reserved
196
J. Ayache, P.H. Albarsde / Ultramicroscopy 60 (1995) 195-206
tates and also on their structural crystallographic orientation. We have recently shown the different modes of interfacial strain relaxation [l]. The strain around an inclusion depends on its size and curvature radius. The knowledge of the interface structure between de superconducting YBa,Cu,O, matrix and the insulating precipitate Y,BaCuO, is critical for the understanding of the role of defect with respect to superconducting properties. The study of these interfacial defects requires that the whole interface around a 211 precipitate can be observed. Because of the sputtering behavior difference between the matrix and precipitate, the standard TEM preparation leads to the interfaces that present only parts of the interface around a 211 particle, and variable thickness in the particle and matrix. In the case of epitaxial thin films, the relative crystallographic orientation of the substrate and matrix is fixed and the defects are directly related to the growth mode and process conditions. The thin layers of the 123 matrix make pulverisation more sensitive and the standard TEM preparation more critical. In addition, for both bulk ceramics and thin films ion-beam bombardment during ion milling enhances the formation of defects. In the case of complex non-stoichiometric oxides of superconducting samples, ion milling induces atomic diffusion, even with the use of a cold holder, leading to the formation of a superficial layer of either amorphous or secondary phase material. This atomic diffusion effects is recognizable at the onset of the TEM sample observation. The contrast of a YBCO sample prepared by ion milling changes under the incident electron beam until the defect contrast is clearly stabilized. This can be explained by the heating of the sample in the incident beam. The chemical and structural defects formed at this stage do not belong to the raw material but they are rather a property of the metastable secondary phases. The aim of this work is to describe a technique that avoids the diffusion effects and allows the structure and chemistry of un-stable interfaces as they exist in the raw sample to be observed. Applications to the bulk ceramics, and multilayer thin films of the complex YBCO system will be presented.
2. Experimental 2.1. General The polishing method is based upon the tool developed in East Fishkill IBM (NY) [2]. This tool known as the tripod polisher, defines the polishing plane with three points. One point is the specimen being polished, and the two others are teflon legs moved by micrometers. The accuracy of this mounting is better than 0.2 mrad in angle and 0.2 pm in position. The main body of the tripod is affixed with an L-bracket on which the specimen can be glued in two perpendicular directions. This allows cross-section or plan-view specimens to be prepared. The preparation of the TEM parallel foil requires two polishing steps. The first step corresponds to the first side of a bulk specimen or a cross-section thin film while the second step manufactures the second face of the TEM foil and ensures a thickness suitable for electron transparency. The polishing sequence chains successive grinding with diamond plastic films on an ordinary rotating polisher kept under a water wash. The successive grain sizes are 30, 15, 6, 3, 1 and 0.5 pm. Final scratchless polishing is achieved upon polishing by colloidal silica (20 nm grains) on a felt wheel. This final polishing presents the required optical quality. An important condition of mechanical polishing is the use of decreasing speed from 30 to 0.5 ,um. Down to 6 pm grain size, the grinder can be operated at the same speed. From 6 to 3 pm the speed has to be steeply decreased. From 3 to 1 pm a speed of
PLANAR
CROSS-SECTION
VIEW
Fig. 1. Scheme of sample perpendicular cross-section.
preparation
for planar
view and
J. Ayache, P.H. Albaride / Ultramicroscopy 60 (1995) 195-206
less than 5-6 rpm has to be used while from 1 pm the speed must be kept under 3 rpm. in order to be in hydrodynamic Iubrification conditions, the final polishing stage with colloidal silica requires a faster rotation speed of the grinder. At that stage, polishing is performed using a rocking motion with respect to the polishing direction. Attention has to be paid to the polishing attack direction to protect the thinnest part for the TEM observation. Typically, a TEM sample section is prepared by this technique in about 10 h. 2.1.1. First step The sample is cut and glued either parallel to the glass of the L-bracket of the tripod for planar view or perpendicular for cross-section (Fig. 11. The sample is regularly monitored under the optical microscope until all scratches have been removed. Such continuous optical control at the different polishing steps has already been used for chemical thinning [3]. 2.1.2. Second step At around 300 km thickness, the specimen is turned upside down and glued on the tripod polisher again. In order to handle the specimen after polishing and to monitor progressive thinning from the edges, the second face is cut at an angle of about 50 mrad with respect to the first face. The thinnest region is observed with the TEM equipment.
197
The second polishing face requires a specific preparation of the tripod. After removing the specimen from the glass support of the L-bracket, the glass of the tripod has to be reconditioned in order to be strictly parallel to the polishing plate before applying the angle to the polishing direction. This preparation is carried out on the complete tripod with the L-bracket and without sample. This is obtained by polishing the glass support with 6 pm grain size down to total planarity which serves as a reference for zero tilt. Then, the angle is obtained by increasing the height of the two leg micrometers opposite to the specimen. Finally, the specimen is glued on the glass and ready for the second step of polishing. Polishing the second face uses the same sequence of operations. As an additional requirement, the grinder rotation speeds must be adapted to take the brittleness of ultrathin slices into account. The smaller the grain size, the slower the speed to be used. The thickness is regularly controlled by defocusing the section under the optical microscope in transmitted light. The occurrence of light lattice-fringe interferences on the edges of the samples is the key parameter for controlling the transparency of the sample to electrons. For example, silicon begins to transmit light in a progression of colors when thinned down to less than lo-15 pm (001 direction), and eventually exhibits transmitted and reflected fringes. The best estimation for the thickness can be as good as 50 nm.
Fig. 2. Bright-field TEM micrograph of a YBa2Cu307/SrTi03 multilayer prepared by the classical dimpling and ion milling method on a cold holder. Note the formation of superficial amorphous phase visible on the edges of the sample. Note that the ion milling is not homogeneous along the film. The blurred contrast of the 123 layers in the multilayers is due to atomic diffusion through the interfaces.
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Because the specimen is fragile, any direct contact with the specimen after thinning must be avoided. The section is unglued from the Lbracket in acetone under its own weight and collected on filter paper. A suitable microscopy grid (hole, slot, copper, nickel, gold, ...I is then glued onto the specimen under a binocular optical microscope. 2.2. Application to thin films We have applied the technique presented above to both the thin films and multilayers. The interfaces 123 film/SrTiO, substrate and 123/SrTiO, multilayers are examined in order to characterize the structure and chemistry of intrinsic growth defects of the film produced during elaboration process and extrinsic defects induced by the substrate itself. When classical dimpling followed by ion milling with or without a cold holder is used, the final foil contains only small areas of complete film (Fig. 21. Such a method makes it possible to characterize only some parts of the sample with size not being representative of the whole film. On the contrary, when the section is polished with the tripod down to 1.5Km thickness with a subsequent ion milling time [4] the area of the cross-section of the sample can be extended to a few micrometers. In addition, because of the ion mill bombardment, the surface of the foil is amorphized. When the specimen is to be prepared for TEM, the thin film is first cut from the back side of its surface and gently cleaned before being glued face to face with epoxy or M Bond glue (reference: M bond 610, Viskay Micromeasures Company). After one and half hour drying on a low temperature heater plate, the first face of the cross-section is ready for polishing with the tripod. The specimen is glued and polished perpendicularly to the face of the tripod L-bracket. The polishing sequence is identical to that described above up to the step of final colloidal silicon polishing. Particular attention has to be paid to the polishing direction. Even a small deviation from perpendicularity will generate scratch lines which will spread from the interface and deteriorate the film. The use of too fast a polishing
speed will do the same. In addition, it is very important never to move the sample twice over the same track on the plastic disk, especially for smaller than 3 pm granulation size. The tripod polisher should spiral inward to the center of the plastic disk. Then the plastic diamond film has to be cleaned often for large grain size and every complete spiral from 3 pm granulation. The quality of the polished surface is regularly inspected by reflection optical microscopy. The interface of the film cross-section has to be linear and scratch-free before the second face.
Fig. 3. Optical transmission micrograph of the second polished face of the cross-section preparation at the end of the second polishing. Note the interference fringes at the edge of the cross-section and those in the substrate: the brighter parts present electron transparency.
J. Ayache, P. H. Albart?de / Ultramicroscopy 60 (1995) 195-206
The second face of the cross-section is prepared on a 300 pm thick after the tripod is reconditioned. Then, the polished face has to be cleaned and the foil is glued on the glass support of the L-bracket with ethylenglycol glue. The glue must be liquid as to wet the whole surface of the specimen and to avoid any porosity which can remain and ruin the preparation during the final steps. SrTiO, is very fragile under 30 pm thickness especially when the second face is to be polished. Polishing with the 6 pm grain size requires a low grinder speed avoiding pressure on the specimen. Upon polishing the second face of a silicon specimen the operator should switch to the polishing disk of 3 and 1 pm before thickness is reduced to 30 pm. Inspection of the thickness is helped first by reflection optical microscopy down to 10 pm thickness and by transmission light upon appearance of the fringes on the edge of the specimen. At this level, the thickness of the specimen should be checked regularly. If the thickness is not the same, adjusting the height of the tripod legs will help to recover a large thin zone. Fringes become visible during the 1 pm granulation size polishing. Subsequently, the polishing with 0.5 pm grain size and then with colloidal silica improves fringe visibility. Wider fringes indicates that the thickness decreases. Measuring of the thickness using the microscope defocus is not possible when the fringes are clearly visible as we can see on Fig. 3. Fig. 3a shows the bright fringes on the edge of the cross-section and Fig. 3b the fringes in the substrate. Note the wide fringes on the edge. The brighter fringe corresponds to the thinnest part of the section which is transparent to electrons and suitable for
199
TEM and HREM characterizations. Polishing is complete when the area to be characterized contains wide bright fringes. The cross-section is now ready for TEM observations. The micrograph of Fig. 4 shows the large area cross-section available for TEM and HREM characterization. The micrograph of Fig. 5 represents a bright-field image of a regular ten-period YBa,Cu,O, and SrTiO, multilayer prepared on SrTiO, substrate using a buffer layer [51. This multilayer is clearly visible on the dark-field image of Fig. 6 where the dislocation network at the interface with the substrate can be seen. Some extended defects can also be observed in the multilayers. A higher magnification of the top of the same stack is shown on Fig. 7. On this micrograph, the uppermost layer (SrTiO,) is neither amorphized nor contaminated by the TEM preparation. It was confirmed by using EELS plasmon measurements that the thickness of the specimen is homogeneous across the stack and substrate. In the case of a planar view, the thin film is glued directly on the glass (Fig. 1) in order to protect its surface from the mechanical polishing. The rest of the procedure is essentially the same as for the preparation of cross-section. 2.3. Application to bulk melt-textured ceramics For the melted ceramics we need to characterize the structure and chemistry of the complete 123 matrix/211 precipitate interfaces which are formed during the process and which are responsible for the pinning properties. Usually, with standard dimpling followed by ion milling with or
Fig. 4. TEM bright-field image of the cross-section at low magnification. Note the large area observable for TEM investigations.
200
J. Ayache, P. H. Albaride / Ultramicroscopy 60 (I 995) 195-206
without a cold holder, the thin foil contains only some part of the 123/211 interfaces. Sputtering properties differ between the matrix and 211 precipitate. Consequently, the precipitates are thinned faster than the matrix. Only precipitates smaller than 1 pm are present and can be thinned
on their entire surface. Even in such a case the thickness difference between the matrix and precipitate does not allow the whole interface to be characterized. Fig. 8 shows an example of a 211 precipitate where it is not possible to characterize the structural defects and chemical change be-
Fig. 5. TEM bright-field image of the cross-section showing a good contrast of the substrate and the multilayers. Note the regular stacking of YBCO (dark contrast) and SrTiO, (bright contrast) and the absence of any contamination outer layer.
J. Ayache, P.H. Albakde / Ultramicroscopy 60 (1995) X95-206
201
cause of the thickness difference between the 123 matrix and 211 precipitate. Mechanical polishing avoids differential sputtering between the two phases. The high quality of the polished surface of the first face is crucial for the preservation of the final polished sample. Because of the thinness of the sample, any residual scratch produced on the first face will induce propagation of cracks. If track density is not negligible, the bulk sample can be ruined. Attention has also to be paid to the polishing direction. For dense samples, polishing is easy and any scratch formed disappears progressively. For porous samples, polishing can produce fan-shaped lines. The linear structures in melt-textured ceramics, which are called “high temperature growth induced defects” [l], also favor these spread lines during polishing. Best result are obtained when the sample is polished perpendicularly to the growth direction.
Fig. 7. TEM lattice fringe image of the upper part of the multilayers showing the ab superconducting planes Cd = 11.68 A) in the YBCO layer. Note the rough stacking and the absence of superficial contamination layer.
Fig. 6. TEM dark-field image using SrTiO, (h/d) diffracted spots. At the substrate interface, the dislocation network due to the mismatch between the substrate and the PrBaCuO buffer layer is visible. The regular defects in the multilayers are induced by the growth process.
Sample thickness is commonly reduced down to about 40 pm. The polishing sequence is the same as described before up to the final stage. When the green 211 precipitates begin to be visible in transmitted light, their thickness range from 2 to 5 pm. At this stage, grain size is switched to 1 and 0.5 pm and speed decreased until fringes become visible on the edge of the specimen. The 211 precipitates are clearly visible in transmitted light and all scratches have disappeared. Polishing is completed with colloidal silica in order to extend the fringes to the inner part. The fringes in superconducting ceramics can be easily seen in polarized reflected light; the brighter the fringes, the thinner the specimen. The thinnest part is transparent to electrons and suitable for HREM characterization. The soft mechanical polishing preserves the complete matrix/precipitate interfaces.
202
J. Ayache, P.H. AlbarGde/ Ultramicroscopy 60 (1995) 195-206
Fig. 9 shows a polarized reflected image of the second face at the final stage. Fringes can be seen at a low magnification on the edges of the ceramic (Fig. 9a) and at a higher magnification in the 211 precipitate (Fig. 9b). Fig. 10 shows a large twinned area across a faceted grain boundary of YBa,Cu,O, melt-textured superconducting ceramic located next to a growth induced-defect. Fig. 11 represents a TEM bright field image of a 211 precipitate surrounded by the 123 superconducting matrix. The 123 matrix is twinned and contains planar defects across the twin walls. The 211 precipitate shows dislocations all around its interface with the matrix. The dislocations lay in the interface plane and are interrupted in the
Fig. 9. Optical transmission micrograph of the second polished face of a melt-textured ceramic preparation at the end of the second polishing step. Note the interference fringes at the edges of the cross-section at low (a) and higher (b) magnification: the brighter parts along the edges present electron transparency. Interference fringes in the 211 precipitates can also be observed.
Fig. 8. TEM bright-field image of a Y,BaCuO, precipitate in YBaaCu,O, superconducting matrix formed in a melt-textured ceramic which is prepared by the classical dimpling and ion milling method. The complete interface is not clearly visible because of the differences in the ion milling of the matrix and the precipitate. The darker interfacial zone is too thick for proper HREM characterization and quantitative chemical profile.
matrix next to the twin walls. The inner part of the precipitate can also be seen. Nanocrystalline inclusions of a second phase produced in the texturation process [l] can be observed. Fig. 12 shows a slanted (110) twin wall plane. The orientation makes the (103), (010) and (113) atomic planes of the matrix visible.
J. Ayache, P.H. AlbaGde / Ultramicroscopy 60 (1995) 195-206
3. 1Xscussion 3.1. Comparison merrhod
A definite
of ion milling with the tripod
improvement
characterizes the by mechanical thinnin g. At the beginning of TEM observation, when the sample receives the incident electron beam, no contrast change is observed as is commonly YE ;CO thin foils prepared
203
the case with the classical ion thinning preparation. The contrast in the twinned domains remains extremely stable. The explanation lies in the atomic diffusion, that takes place during ion milling. When the sample is in the microscope, the electron beam stabilizes the metastable phases issued from the ion milling leading to a contrasted zone which is not representative of the raw sample. The 211 precipitate of Fig. 8 corresponds to a
Fig. 10. TEM bright-field image of a faceted grain boundary in the YBCO melt-textured ceramic. Note the net contrast of the twin walls. Small remaining residues of the syton are left after the final step of polishing.
204
J. Ayache, P.H. Albargde / Ultramicroscopy 60 (1995) 195-206
sample from a superconducting ceramic (T, = 92 K) prepared using mechanical dimpling and ion milling in a cold holder. The thickness difference in the matrix and in the precipitate does not permit the interfacial defects around the particle to be observed. It also prevents quantitative
chemical analysis across the interface. The 211 precipitate visible on Fig. 11 is taken from the same sample and was prepared using the tripod method. In this case, the contrast is constant through the interface, in the precipitate and matrix. Small inclusions in the 211 precipitate are
Fig. 11. TEM bright-field image of a YzBaCuOs precipitate in YBa,Cu,O, superconducting matrix formed in the same melt-textured ceramic prepared by the polishing method. The complete interface is clearly visible. Interfacial dislocations in the interface plane and inclusion formed in the 211 precipitate can be distinguished. Note in the matrix, the planar defects crossing the (110) twin walls. A same thickness of the matrix and the precipitate makes HREM characterization and quantitative chemical analysis profile possible.
.I. Ayache, P.H. Alban?de / Ultramicroscopy 60 (1995) 195-206
now visible. Comparison of Figs. 8 and 11, shows that the image contrast between the matrix and precipitate has been improved. In addition, quantitative chemical analysis and chemical profiles can be obtained thanks to identical thickness of the matrix and 211 precipitate. Convergent beam diffraction and EELS plasmons measurements can also be successfully carried out. An interesting feature is that the Holz lines, naturally visible for a specific thickness range, are visible in both the matrix and precipitate. Characterization of the stress associated with the interface is therefore possible. The TEM sample was polished using water as solvent. We have also polished using ethylic alcohol as solvent. Water and alcohol produce identical results. This is probably because the surface of the sample in contact with water is continuously removed by the polishing. Repeated drying for the purpose of observation also seems to prevent reaction between water and the specimen.
205
3.2. Further applications in materials science This polishing method can be applied to other materials with results depending on density and hardness. If the material is hard enough, the diamond will produce a regular polish and allow a smooth thinning. Soft materials will be smeared before they become transparent to electrons. Colloidal alumina polishing compound is useful for polishing soft metals. Typically ceramics and minerals are good candidates as has been proven in this paper. It is sometimes necessary to adapt the criteria of measurement since only some materials are transparent to light. The polishing direction also seems to be important in the case of single crystal and thin films. Thin section in metals are more difficult to obtain. We suggest to thin them down to 5 pm and to finish with an ion mill step. Doing so will save ion milling time and improve the quality of the preparation (surface roughness drastically increases with ion milling time).
Fig. 12. HREM image of a twin wall present in superconducting matrix.
206
J. Ayache, P.H. AlbaGde / Ultramicroscopy 60 (1995) 195-206
4. Conclusion We have discussed how the tripod polisher method helps thin YBCO ceramics. It is demonstrated that a complete mechanical thinning down to electron transparency is routinely obtainable on such materials without resorting to ion milling. The method is efficient on either bulk material or multilayers. It is shown that this method produces better specimens than any other method, especially when chemical stability is required. Polishing superconducting materials with water gives quality specimens provided the material is dense enough.
References [l] J. Ayache, N. Pellerin and P. Odier, Supercond. Sci. Technol. 7 (1994) 65. [Z] J. Benedict, R. Anderson, S. Klepeis and M. Chaker, Mater. Res. Sot. Symp. Proc. 199 (1990) 189. [3] Myrtle B. Ellington, Mater. Res. Sot. Symp. Proc. 11.5 (1988) 265. [4] C. Traeholt, J.G. Svetcnikov, A. Delsing and H.W. Zandhergen, Physica C 206 (1993) 318. [.5] J.P. Contour, C. Sant, D. Ravelosona, C. Dolin, J. Perriire, L. Ranno, P. Auvray and J. Caulet, Appl. Surf. Sci. 75 (1994) 252.
Ultramicroscopy
60 (1995) 195-206
Application of the ionless tripod polisher to the preparation of YBCO superconducting multilayer and bulk ceramics thin films J. Ayache ay*, P.H. AlbarSde b a CSNSM, Britimen! 108, F-91405 Orsay Campus, France b IBM France, Semite 1807-31 U, BP58, F-91105 Corbeil Essonnes, France Received
2 December
1994; in final form 15 May 1995
Abstract
The standard method of polishing using ion milling for cross-section preparation induces atomic diffusion which can be avoided by using an ionless polisher. The present paper gives the details of the method as applied to superconducting YBa,Cu,O, melt-textured ceramics and multilayer thin films. The structure and chemistry of interfaces in this complex system are known to be influenced by changes in the chemical composition. Because of non-stoichiometry related growth of secondary phases, many secondary phases form. We present a technique, which allows preparation of electron-transparent TEM samples, that can be used for HREM and chemical characterizations.
1. Introduction YBa,Cu,O, ceramics are very sensitive to thermodynamic and kinetic process conditions because of their incongruent behavior at high temperature. For example, a same process involving high-temperature heat treatment can lead to two ceramics having different physical properties because of different microstructures and microchemistry. The improvement of the process for industrial applications as thin films for electronic devices, or bulk-textured ceramics for high current transport, is the way to achieve reproducibility in the microstructure and physical properties.
* Corresponding
author.
0304-3991/95/$09.50 0 1995 Elsevier SSDIO304-3991(95)00073-9
Science
A study of grain boundaries and interfaces is required as they are directly involved. YBa,Cu,O, textured ceramics form through a peritectic reaction which leads to the precipitation of Y,BaCuO, inclusions (called 211 precipitate) from the liquid phase. The crystallisation of the liquid gives a YBa,Cu,O, textured matrix called the 123 phase. The superconducting properties CT,), susceptibility (X,), transport properties CR,), and the critical current density (.I,) depend on the presence of defects and particularly on their size and orientation. Defects with dimensioa in the order of theocoherence length (from 3 A along c axis and 20 A in the superconducting ab planes) are most efficient. Because of the dimension of the interface plane, interfaces behave as the vortex pinning centers. The microstructure of interfaces depends on the structure and parameters of the matrix and precipi-
B.V. All rights reserved
196
J. Ayache, P.H. Albarsde / Ultramicroscopy 60 (1995) 195-206
tates and also on their structural crystallographic orientation. We have recently shown the different modes of interfacial strain relaxation [l]. The strain around an inclusion depends on its size and curvature radius. The knowledge of the interface structure between de superconducting YBa,Cu,O, matrix and the insulating precipitate Y,BaCuO, is critical for the understanding of the role of defect with respect to superconducting properties. The study of these interfacial defects requires that the whole interface around a 211 precipitate can be observed. Because of the sputtering behavior difference between the matrix and precipitate, the standard TEM preparation leads to the interfaces that present only parts of the interface around a 211 particle, and variable thickness in the particle and matrix. In the case of epitaxial thin films, the relative crystallographic orientation of the substrate and matrix is fixed and the defects are directly related to the growth mode and process conditions. The thin layers of the 123 matrix make pulverisation more sensitive and the standard TEM preparation more critical. In addition, for both bulk ceramics and thin films ion-beam bombardment during ion milling enhances the formation of defects. In the case of complex non-stoichiometric oxides of superconducting samples, ion milling induces atomic diffusion, even with the use of a cold holder, leading to the formation of a superficial layer of either amorphous or secondary phase material. This atomic diffusion effects is recognizable at the onset of the TEM sample observation. The contrast of a YBCO sample prepared by ion milling changes under the incident electron beam until the defect contrast is clearly stabilized. This can be explained by the heating of the sample in the incident beam. The chemical and structural defects formed at this stage do not belong to the raw material but they are rather a property of the metastable secondary phases. The aim of this work is to describe a technique that avoids the diffusion effects and allows the structure and chemistry of un-stable interfaces as they exist in the raw sample to be observed. Applications to the bulk ceramics, and multilayer thin films of the complex YBCO system will be presented.
2. Experimental 2.1. General The polishing method is based upon the tool developed in East Fishkill IBM (NY) [2]. This tool known as the tripod polisher, defines the polishing plane with three points. One point is the specimen being polished, and the two others are teflon legs moved by micrometers. The accuracy of this mounting is better than 0.2 mrad in angle and 0.2 pm in position. The main body of the tripod is affixed with an L-bracket on which the specimen can be glued in two perpendicular directions. This allows cross-section or plan-view specimens to be prepared. The preparation of the TEM parallel foil requires two polishing steps. The first step corresponds to the first side of a bulk specimen or a cross-section thin film while the second step manufactures the second face of the TEM foil and ensures a thickness suitable for electron transparency. The polishing sequence chains successive grinding with diamond plastic films on an ordinary rotating polisher kept under a water wash. The successive grain sizes are 30, 15, 6, 3, 1 and 0.5 pm. Final scratchless polishing is achieved upon polishing by colloidal silica (20 nm grains) on a felt wheel. This final polishing presents the required optical quality. An important condition of mechanical polishing is the use of decreasing speed from 30 to 0.5 ,um. Down to 6 pm grain size, the grinder can be operated at the same speed. From 6 to 3 pm the speed has to be steeply decreased. From 3 to 1 pm a speed of
PLANAR
CROSS-SECTION
VIEW
Fig. 1. Scheme of sample perpendicular cross-section.
preparation
for planar
view and
J. Ayache, P.H. Albaride / Ultramicroscopy 60 (1995) 195-206
less than 5-6 rpm has to be used while from 1 pm the speed must be kept under 3 rpm. in order to be in hydrodynamic Iubrification conditions, the final polishing stage with colloidal silica requires a faster rotation speed of the grinder. At that stage, polishing is performed using a rocking motion with respect to the polishing direction. Attention has to be paid to the polishing attack direction to protect the thinnest part for the TEM observation. Typically, a TEM sample section is prepared by this technique in about 10 h. 2.1.1. First step The sample is cut and glued either parallel to the glass of the L-bracket of the tripod for planar view or perpendicular for cross-section (Fig. 11. The sample is regularly monitored under the optical microscope until all scratches have been removed. Such continuous optical control at the different polishing steps has already been used for chemical thinning [3]. 2.1.2. Second step At around 300 km thickness, the specimen is turned upside down and glued on the tripod polisher again. In order to handle the specimen after polishing and to monitor progressive thinning from the edges, the second face is cut at an angle of about 50 mrad with respect to the first face. The thinnest region is observed with the TEM equipment.
197
The second polishing face requires a specific preparation of the tripod. After removing the specimen from the glass support of the L-bracket, the glass of the tripod has to be reconditioned in order to be strictly parallel to the polishing plate before applying the angle to the polishing direction. This preparation is carried out on the complete tripod with the L-bracket and without sample. This is obtained by polishing the glass support with 6 pm grain size down to total planarity which serves as a reference for zero tilt. Then, the angle is obtained by increasing the height of the two leg micrometers opposite to the specimen. Finally, the specimen is glued on the glass and ready for the second step of polishing. Polishing the second face uses the same sequence of operations. As an additional requirement, the grinder rotation speeds must be adapted to take the brittleness of ultrathin slices into account. The smaller the grain size, the slower the speed to be used. The thickness is regularly controlled by defocusing the section under the optical microscope in transmitted light. The occurrence of light lattice-fringe interferences on the edges of the samples is the key parameter for controlling the transparency of the sample to electrons. For example, silicon begins to transmit light in a progression of colors when thinned down to less than lo-15 pm (001 direction), and eventually exhibits transmitted and reflected fringes. The best estimation for the thickness can be as good as 50 nm.
Fig. 2. Bright-field TEM micrograph of a YBa2Cu307/SrTi03 multilayer prepared by the classical dimpling and ion milling method on a cold holder. Note the formation of superficial amorphous phase visible on the edges of the sample. Note that the ion milling is not homogeneous along the film. The blurred contrast of the 123 layers in the multilayers is due to atomic diffusion through the interfaces.
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Because the specimen is fragile, any direct contact with the specimen after thinning must be avoided. The section is unglued from the Lbracket in acetone under its own weight and collected on filter paper. A suitable microscopy grid (hole, slot, copper, nickel, gold, ...I is then glued onto the specimen under a binocular optical microscope. 2.2. Application to thin films We have applied the technique presented above to both the thin films and multilayers. The interfaces 123 film/SrTiO, substrate and 123/SrTiO, multilayers are examined in order to characterize the structure and chemistry of intrinsic growth defects of the film produced during elaboration process and extrinsic defects induced by the substrate itself. When classical dimpling followed by ion milling with or without a cold holder is used, the final foil contains only small areas of complete film (Fig. 21. Such a method makes it possible to characterize only some parts of the sample with size not being representative of the whole film. On the contrary, when the section is polished with the tripod down to 1.5Km thickness with a subsequent ion milling time [4] the area of the cross-section of the sample can be extended to a few micrometers. In addition, because of the ion mill bombardment, the surface of the foil is amorphized. When the specimen is to be prepared for TEM, the thin film is first cut from the back side of its surface and gently cleaned before being glued face to face with epoxy or M Bond glue (reference: M bond 610, Viskay Micromeasures Company). After one and half hour drying on a low temperature heater plate, the first face of the cross-section is ready for polishing with the tripod. The specimen is glued and polished perpendicularly to the face of the tripod L-bracket. The polishing sequence is identical to that described above up to the step of final colloidal silicon polishing. Particular attention has to be paid to the polishing direction. Even a small deviation from perpendicularity will generate scratch lines which will spread from the interface and deteriorate the film. The use of too fast a polishing
speed will do the same. In addition, it is very important never to move the sample twice over the same track on the plastic disk, especially for smaller than 3 pm granulation size. The tripod polisher should spiral inward to the center of the plastic disk. Then the plastic diamond film has to be cleaned often for large grain size and every complete spiral from 3 pm granulation. The quality of the polished surface is regularly inspected by reflection optical microscopy. The interface of the film cross-section has to be linear and scratch-free before the second face.
Fig. 3. Optical transmission micrograph of the second polished face of the cross-section preparation at the end of the second polishing. Note the interference fringes at the edge of the cross-section and those in the substrate: the brighter parts present electron transparency.
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The second face of the cross-section is prepared on a 300 pm thick after the tripod is reconditioned. Then, the polished face has to be cleaned and the foil is glued on the glass support of the L-bracket with ethylenglycol glue. The glue must be liquid as to wet the whole surface of the specimen and to avoid any porosity which can remain and ruin the preparation during the final steps. SrTiO, is very fragile under 30 pm thickness especially when the second face is to be polished. Polishing with the 6 pm grain size requires a low grinder speed avoiding pressure on the specimen. Upon polishing the second face of a silicon specimen the operator should switch to the polishing disk of 3 and 1 pm before thickness is reduced to 30 pm. Inspection of the thickness is helped first by reflection optical microscopy down to 10 pm thickness and by transmission light upon appearance of the fringes on the edge of the specimen. At this level, the thickness of the specimen should be checked regularly. If the thickness is not the same, adjusting the height of the tripod legs will help to recover a large thin zone. Fringes become visible during the 1 pm granulation size polishing. Subsequently, the polishing with 0.5 pm grain size and then with colloidal silica improves fringe visibility. Wider fringes indicates that the thickness decreases. Measuring of the thickness using the microscope defocus is not possible when the fringes are clearly visible as we can see on Fig. 3. Fig. 3a shows the bright fringes on the edge of the cross-section and Fig. 3b the fringes in the substrate. Note the wide fringes on the edge. The brighter fringe corresponds to the thinnest part of the section which is transparent to electrons and suitable for
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TEM and HREM characterizations. Polishing is complete when the area to be characterized contains wide bright fringes. The cross-section is now ready for TEM observations. The micrograph of Fig. 4 shows the large area cross-section available for TEM and HREM characterization. The micrograph of Fig. 5 represents a bright-field image of a regular ten-period YBa,Cu,O, and SrTiO, multilayer prepared on SrTiO, substrate using a buffer layer [51. This multilayer is clearly visible on the dark-field image of Fig. 6 where the dislocation network at the interface with the substrate can be seen. Some extended defects can also be observed in the multilayers. A higher magnification of the top of the same stack is shown on Fig. 7. On this micrograph, the uppermost layer (SrTiO,) is neither amorphized nor contaminated by the TEM preparation. It was confirmed by using EELS plasmon measurements that the thickness of the specimen is homogeneous across the stack and substrate. In the case of a planar view, the thin film is glued directly on the glass (Fig. 1) in order to protect its surface from the mechanical polishing. The rest of the procedure is essentially the same as for the preparation of cross-section. 2.3. Application to bulk melt-textured ceramics For the melted ceramics we need to characterize the structure and chemistry of the complete 123 matrix/211 precipitate interfaces which are formed during the process and which are responsible for the pinning properties. Usually, with standard dimpling followed by ion milling with or
Fig. 4. TEM bright-field image of the cross-section at low magnification. Note the large area observable for TEM investigations.
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without a cold holder, the thin foil contains only some part of the 123/211 interfaces. Sputtering properties differ between the matrix and 211 precipitate. Consequently, the precipitates are thinned faster than the matrix. Only precipitates smaller than 1 pm are present and can be thinned
on their entire surface. Even in such a case the thickness difference between the matrix and precipitate does not allow the whole interface to be characterized. Fig. 8 shows an example of a 211 precipitate where it is not possible to characterize the structural defects and chemical change be-
Fig. 5. TEM bright-field image of the cross-section showing a good contrast of the substrate and the multilayers. Note the regular stacking of YBCO (dark contrast) and SrTiO, (bright contrast) and the absence of any contamination outer layer.
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cause of the thickness difference between the 123 matrix and 211 precipitate. Mechanical polishing avoids differential sputtering between the two phases. The high quality of the polished surface of the first face is crucial for the preservation of the final polished sample. Because of the thinness of the sample, any residual scratch produced on the first face will induce propagation of cracks. If track density is not negligible, the bulk sample can be ruined. Attention has also to be paid to the polishing direction. For dense samples, polishing is easy and any scratch formed disappears progressively. For porous samples, polishing can produce fan-shaped lines. The linear structures in melt-textured ceramics, which are called “high temperature growth induced defects” [l], also favor these spread lines during polishing. Best result are obtained when the sample is polished perpendicularly to the growth direction.
Fig. 7. TEM lattice fringe image of the upper part of the multilayers showing the ab superconducting planes Cd = 11.68 A) in the YBCO layer. Note the rough stacking and the absence of superficial contamination layer.
Fig. 6. TEM dark-field image using SrTiO, (h/d) diffracted spots. At the substrate interface, the dislocation network due to the mismatch between the substrate and the PrBaCuO buffer layer is visible. The regular defects in the multilayers are induced by the growth process.
Sample thickness is commonly reduced down to about 40 pm. The polishing sequence is the same as described before up to the final stage. When the green 211 precipitates begin to be visible in transmitted light, their thickness range from 2 to 5 pm. At this stage, grain size is switched to 1 and 0.5 pm and speed decreased until fringes become visible on the edge of the specimen. The 211 precipitates are clearly visible in transmitted light and all scratches have disappeared. Polishing is completed with colloidal silica in order to extend the fringes to the inner part. The fringes in superconducting ceramics can be easily seen in polarized reflected light; the brighter the fringes, the thinner the specimen. The thinnest part is transparent to electrons and suitable for HREM characterization. The soft mechanical polishing preserves the complete matrix/precipitate interfaces.
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Fig. 9 shows a polarized reflected image of the second face at the final stage. Fringes can be seen at a low magnification on the edges of the ceramic (Fig. 9a) and at a higher magnification in the 211 precipitate (Fig. 9b). Fig. 10 shows a large twinned area across a faceted grain boundary of YBa,Cu,O, melt-textured superconducting ceramic located next to a growth induced-defect. Fig. 11 represents a TEM bright field image of a 211 precipitate surrounded by the 123 superconducting matrix. The 123 matrix is twinned and contains planar defects across the twin walls. The 211 precipitate shows dislocations all around its interface with the matrix. The dislocations lay in the interface plane and are interrupted in the
Fig. 9. Optical transmission micrograph of the second polished face of a melt-textured ceramic preparation at the end of the second polishing step. Note the interference fringes at the edges of the cross-section at low (a) and higher (b) magnification: the brighter parts along the edges present electron transparency. Interference fringes in the 211 precipitates can also be observed.
Fig. 8. TEM bright-field image of a Y,BaCuO, precipitate in YBaaCu,O, superconducting matrix formed in a melt-textured ceramic which is prepared by the classical dimpling and ion milling method. The complete interface is not clearly visible because of the differences in the ion milling of the matrix and the precipitate. The darker interfacial zone is too thick for proper HREM characterization and quantitative chemical profile.
matrix next to the twin walls. The inner part of the precipitate can also be seen. Nanocrystalline inclusions of a second phase produced in the texturation process [l] can be observed. Fig. 12 shows a slanted (110) twin wall plane. The orientation makes the (103), (010) and (113) atomic planes of the matrix visible.
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3. 1Xscussion 3.1. Comparison merrhod
A definite
of ion milling with the tripod
improvement
characterizes the by mechanical thinnin g. At the beginning of TEM observation, when the sample receives the incident electron beam, no contrast change is observed as is commonly YE ;CO thin foils prepared
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the case with the classical ion thinning preparation. The contrast in the twinned domains remains extremely stable. The explanation lies in the atomic diffusion, that takes place during ion milling. When the sample is in the microscope, the electron beam stabilizes the metastable phases issued from the ion milling leading to a contrasted zone which is not representative of the raw sample. The 211 precipitate of Fig. 8 corresponds to a
Fig. 10. TEM bright-field image of a faceted grain boundary in the YBCO melt-textured ceramic. Note the net contrast of the twin walls. Small remaining residues of the syton are left after the final step of polishing.
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sample from a superconducting ceramic (T, = 92 K) prepared using mechanical dimpling and ion milling in a cold holder. The thickness difference in the matrix and in the precipitate does not permit the interfacial defects around the particle to be observed. It also prevents quantitative
chemical analysis across the interface. The 211 precipitate visible on Fig. 11 is taken from the same sample and was prepared using the tripod method. In this case, the contrast is constant through the interface, in the precipitate and matrix. Small inclusions in the 211 precipitate are
Fig. 11. TEM bright-field image of a YzBaCuOs precipitate in YBa,Cu,O, superconducting matrix formed in the same melt-textured ceramic prepared by the polishing method. The complete interface is clearly visible. Interfacial dislocations in the interface plane and inclusion formed in the 211 precipitate can be distinguished. Note in the matrix, the planar defects crossing the (110) twin walls. A same thickness of the matrix and the precipitate makes HREM characterization and quantitative chemical analysis profile possible.
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now visible. Comparison of Figs. 8 and 11, shows that the image contrast between the matrix and precipitate has been improved. In addition, quantitative chemical analysis and chemical profiles can be obtained thanks to identical thickness of the matrix and 211 precipitate. Convergent beam diffraction and EELS plasmons measurements can also be successfully carried out. An interesting feature is that the Holz lines, naturally visible for a specific thickness range, are visible in both the matrix and precipitate. Characterization of the stress associated with the interface is therefore possible. The TEM sample was polished using water as solvent. We have also polished using ethylic alcohol as solvent. Water and alcohol produce identical results. This is probably because the surface of the sample in contact with water is continuously removed by the polishing. Repeated drying for the purpose of observation also seems to prevent reaction between water and the specimen.
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3.2. Further applications in materials science This polishing method can be applied to other materials with results depending on density and hardness. If the material is hard enough, the diamond will produce a regular polish and allow a smooth thinning. Soft materials will be smeared before they become transparent to electrons. Colloidal alumina polishing compound is useful for polishing soft metals. Typically ceramics and minerals are good candidates as has been proven in this paper. It is sometimes necessary to adapt the criteria of measurement since only some materials are transparent to light. The polishing direction also seems to be important in the case of single crystal and thin films. Thin section in metals are more difficult to obtain. We suggest to thin them down to 5 pm and to finish with an ion mill step. Doing so will save ion milling time and improve the quality of the preparation (surface roughness drastically increases with ion milling time).
Fig. 12. HREM image of a twin wall present in superconducting matrix.
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4. Conclusion We have discussed how the tripod polisher method helps thin YBCO ceramics. It is demonstrated that a complete mechanical thinning down to electron transparency is routinely obtainable on such materials without resorting to ion milling. The method is efficient on either bulk material or multilayers. It is shown that this method produces better specimens than any other method, especially when chemical stability is required. Polishing superconducting materials with water gives quality specimens provided the material is dense enough.
References [l] J. Ayache, N. Pellerin and P. Odier, Supercond. Sci. Technol. 7 (1994) 65. [Z] J. Benedict, R. Anderson, S. Klepeis and M. Chaker, Mater. Res. Sot. Symp. Proc. 199 (1990) 189. [3] Myrtle B. Ellington, Mater. Res. Sot. Symp. Proc. 11.5 (1988) 265. [4] C. Traeholt, J.G. Svetcnikov, A. Delsing and H.W. Zandhergen, Physica C 206 (1993) 318. [.5] J.P. Contour, C. Sant, D. Ravelosona, C. Dolin, J. Perriire, L. Ranno, P. Auvray and J. Caulet, Appl. Surf. Sci. 75 (1994) 252.