JOURNAL OF ELECTRON MICROSCOPY TECHNIQUE 16:249-253 (1990)
The Preparation of Cross-Sectional Transmission Electron Microscopy Specimens of Nb/AI Multilayer Thin Films on Sapphire Substrates KATAYUN BARMAK, DAVID A. RUDMAN, AND SIMON FONER Department of Materials Science and Engineering (K.B., D.A.R.), and Francis Bitter National Magnet Laboratory and Department of Physics (S.F.), Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
KEY WORDS
Nb/Al, Thin films, Refractory materials, Brittle ceramic substrates
ABSTRACT We have developed a technique for preparation of cross-sectional transmission electron microscopy samples of reacted and unreacted NbiAl multilayer thin films on sapphire substrates. The choice of substrate was found to be extremely important. Sapphire sputters more slowly than Nb and Nb-compounds and therefore makes it possible to obtain the electron transparent regions in the thin films rather than in the substrate. However, the brittle nature of the sapphire restricts the types of thinning- techniques that can be used, requiring extensive ion thinning as a final stage. INTRODUCTION Abrahams and Buiocchi (1974) gave the first description of cross-sectional transmission electron microscopy (XTEM) sample preparation. Since then the technique has been modified and developed further and applied to many materials. In multilayer thin films it is possible to examine many or all the layers at the same time. XTEM samples not only provide a wealth of microstructural and morphological information, but also allow a detailed study of the phase formation sequence and location in multilayer thin films. In this paper we discuss the technique we developed, based on the approach described by Bravman and Sinclair (1984) and Newcomb et al. (19851, for preparation of XTEM samples from NbiAl multilayer films on sapphire substrates. The samples were used as part of a study of the phase formation sequence in the reaction of Nb/A1 in thin films, the details of which will be reported by Barmak et al. (1990). Samples of deposited thin films usually comprise several different materials in cross section. The goal of sample preparation is to thin the samples to electron transparency in all regions. It is the presence of different materials that makes the preparation difficult. In general, the first thinning steps are mechanical grinding and polishing to remove most of the sample relatively quickly. These mechanical steps are usually followed by chemical polishing or ion beam thinning. In chemical etching (jet electropolishing), the variation in etch rate for different materials makes it possible to prepare XTEM samples only for specific multilayer film systems, such as 111-V compound semiconductors, where appropriate etches exist (Chu and Sheng, 1984). The thinning of multilayer XTEM samples is therefore generally carried out by ion beam thinning. Even though the variation in removal rate for different materials is less in ion thinning than in chemical etching, it is often still too high, producing samples that are not thinned in the regions of interest or are not thinned
0 1990 WILEY-LISS, INC.
uniformly. These problems are particularly evident for the system of interest for this study. Nb/Al multilayers on sapphire substrates, where mechanical thinning is severely restricted by the brittle nature of the sapphire substrate, appropriate chemical etches do not exist, and the constituents ion beam thin a t different rates. Although the general principles for the preparation of XTEM samples were already known, it was necessary to modify the existing techniques in order to produce the large number of samples necessary to determine the reaction sequence in the NbiAl system. In addition, the limited quantity of films available for this study required that the probability of yielding an appropriate XTEM sample from each film be high. The methods we developed to meet the above requirements are presented in detail in this paper. Only those micrographs necessary for illustrating the sample preparation technique are presented.
MATERIALS AND METHODS The Nb/Al multilayer films are deposited onto (1102) single crystal sapphire substrates by electron beam evaporation. The films are 1 pm thick and have various layer thicknesses. The substrates are 12 x 6 x 0.5 mm in size. After removal from the evaporation system, each substrate is cut to yield two 6 x 6 x 0.5 mm pieces by scribing with a diamond tip on the film side of the substrate and tapping the sample. The films are then annealed to the desired stage of the reaction. Bulk analyses, such as x-ray diffraction and superconducting measurements, are carried out before XTEM preparation and will be reported by Barmak et a]. (1990) elsewhere. The choice of substrates for this study
Received September 19, 1989 accepted in revised form December 10, 1990. Address reprint requests to Dr. Katayun Barmak, 300-40E IBM East Fishkill Facility, Hopewell Junction, NY 12533-0999.
250
K. BARMAK ET AL.
a. EPOXY
\
SaDDhire
1
Sapphire
I
I
.
I
I
b. k L.......................... ................I] Film BraSS2Tubeyd /
c.
1 Fig. 1. Schematic diagram of cross-sectionaltransmission electron microscopy sample preparation technique.
was restricted by the reactivity of Nb at the typical annealing temperatures ( 1,OOO”C).Many common substrates such as Si and SiO, are not appropriate due to filmisubstrate interactions. Sapphire, however, is well known as a nonreactive substrate for use with Nb films. The reacted samples are mounted onto brass slabs using a strong wax (“CRYSTALBOND”)and are sliced into two 6 x 2 x 0.5 mm strips (Fig. la) with a diamond saw. The samples are then thoroughly cleaned and degreased by immersing them in 1,1,1 trichloroethane and then in acetone. The strips are now cemented together. The epoxy adhesive we have judged best is “VARIAN TORR SEAL” (available from Varian Associates Inc., Vacuum Products Division, Lexington, MA 02173). This epoxy has been used before for preparation of XTEM samples (e.g., Vanhellemont et al., 1988). It has a high viscosity, so that the chance of relative rotation of the substrates is minimized, and is impervious to attack by solvents once it is set. More importantly for this application, the epoxy ion mills a t
about the same rate as Nb. This allows the use of an optical stereo zoom microscope to determine when the film is electron transparent, a crucial step for reducing sample preparation time and increasing the yield. However the epoxy does require overnight curing at room temperature. The epoxy is spread on the film side of the sapphire substrates, and the two pieces mounted with the film sides towards each other. The composite is sandwiched carefully between Teflon sheets and held together in a C-clamp while the epoxy is allowed to cure. The composite strip is easily separated from the Teflon pieces and ground down from both sides on 240 Sic paper to a total thickness of 0.5 mm. Any excess epoxy on the edges of the strip is also ground away. The strip is then cleaned in distilled water and acetone to remove any grinding debris, and glued with epoxy into a 0.6 mm x 2.35 mm x 37.5 mm slot in a molybdenum rod. The use of slotted metal rods for mounting of samples is similar to that reported by Newcomb et al. (1985). Molybdenum is chosen for its slow ion milling rate. This structure is in turn epoxied inside a 25 mm long brass tube of 2.5 mm i.d. and 3 mm 0.d.. The epoxy is again allowed to cure overnight at room temperature. The final epoxied structure is shown in Figure lb. The composite rod structure is sliced into thin disks, using a diamond wafering saw. A thickness of 0.75 mm was found to be convenient. The top view of a slice is shown in Figure lc. At this point, the process of thinning the sample in cross section to electron transparency begins. Much of the mechanical thinning is done by hand. The thickness of each slice is measured and is then mounted with a strong wax on the central stainless block of a “GATAN PRECISION GRINDER’ (available from Gatan Inc., Warrendale, PA 15086). A glass slide is used to press the sample during mounting to ensure that it is mounted flat. The Precision Grinder must be zeroed with the stainless block before the sample is mounted. The slice is ground on 240, 320, and 600 grit Sic paper, changing the orientation by 90” every time, down to a thickness of 250 pm. This should be done with plenty of distilled water as the lubricant. The sample is then polished on 0.3 pm alumina slurry for 3 minutes followed by a 1 minute polish on 0.05 pm alumina. If the grinding is done properly, the molybdenum will have a mirror finish. The sample is removed from the block by heating the wax to about 80°C and removing the excess wax in acetone. The sample is remounted with the unpolished side up, again using a glass slide to mount the sample flatly onto the stainless steel block as before. The same grinding procedure described above is used to thin the sample to about 50 pm. This requires great care and attention since it is very easy to grind away the sample completely while trying to reduce the thickness to the final 50 pm. The sample is then polished on alumina as described above. The reason for hand grinding of the entire sample to 50 pm is that the brittle nature of the sapphire did not allow dimple grinding of the central region of the sample. However, it may be possible to successfully use a dimple grinder whose vibrations are damped.
NIOBIUM-ALUMINUM MULTILAYERS ON SAPPHIRE
At this stage the sample is very fragile, and should be removed from the block only by immersion in acetone. The wax will take approximately 1 hour to be dissolved in acetone. Ultrasonic or even hand agitation of the acetone bath can cause the sample to disintegrate. Once the wax is dissolved, the sample is carefully removed from the bath and mounted onto a 3 mm molybdenum ring with 1mm diameter central hole. A very thin coating of clear nail varnish, which is easy to apply and dries quickly, is spread all around the molybdenum ring and the sample carefully lifted with vacuum tweezers and put down across the central hole. The sample is now ready for ion milling. We used a "GATAN' model 600 dual gun ion mill with argon as the plasma gas. Each gun was operated a t 5 kV and 0.5 mA and the sample was rotated. For unreacted samples or those reacted to less than 300"C,the sample must be liquid nitrogen cooled during ion milling to prevent sample heating. Sputtering yield is highly sensitive to angle of incidence of the beam. Typically, samples are ion milled a t relatively large angles (e.g., 220") a t the early stages to take advantage of the fast milling rate. As the sample thins this angle is reduced to 512" to ensure large thinned regions. However, for our samples we have no simple means of knowing when to reduce the angle. We therefore kept the angle fixed for the entire duration of ion milling. Leaving the samples a t 220" to perforation lead to poor quality samples with no electron transparent regions. Fixing the angle at 512" lead to unacceptably long milling times. By trial and error we found the best angle for ion milling to be 15". Depending on the exact operating parameters for each gun, the ion milling can take anywhere from 5 to 50 hours. It is therefore necessary to check the sample optically every 1-2 hours. Since the specific epoxy chosen has an ion milling rate similar to the sample it becomes possible to use the optical microscope for checking the progress of the thinning, greatly reducing sample preparation time. The epoxy is observed to develop a hole in its center between the thin films. This hole is allowed to expand during further ion milling until it appears to reach the sapphire (the multilayer film itself can not be resolved). At this point the sample should have electron transparent areas, and must be examined in the TEM. The ion milling can be continued in case the multilayer is not sufficiently thin. Great care must be taken in handling the sample at this point since it is extremely fragile. The sample should only be picked up with vacuum tweezers by the molybdenum outer ring. We also attempted to use silicon as a substrate for this study, with a sputtered layer of alumina between the silicon and the multilayer film as a barrier against substrate reactions. Silicon has the advantage that it can be dimple ground, simplifying the mechanical thinning portion of the process. Im and Morris (1988) have used Si as a substrate and were able to dimple to perforation before ion milling. However, we found that due to its fast milling rate, silicon would thin to perforation before the NWA1 had thinned to electron transparency. Further ion milling of such a sample would rarely yield a suitably thinned region due to the unfavorable geom-
25 1
Fig. 2. Cross-sectional transmission electron micrograph of a Nbi A1 multilayer film deposited onto a sapphire substrate a t nominally room temperature. The A1 layer is insular and has been partially milled away.
etry of the film relative to the ion beam. The slower milling rate of sapphire on the other hand ensures that the multilayer is thinned before the substrate is entirely removed. The samples were all examined in a "JEOL 2OOCX" microscope. It was necessary t o operate the microscope at 200 keV, since the Nb rich portions of the samples were not electron transparent at lower voltages.
RESULTS Figure 2 shows a NbiAl multilayer with an A1 layer thickness of 34 nm that was deposited on sapphire at nominally room temperature. The A1 layer is insular. For the studies of the Nb/Al reaction, reported by Barmak et al. (1989) and Coffey (1990), it was important that the A1 layers be contiguous. The deposition process was therefore modified to include liquid nitrogen cooling of the substrates during deposition. Figure 3 shows that for the same nominal A1 thickness as in Figure 2, the layer is now contiguous. The grain size of the as-deposited A1 layer is of the order of the layer thickness, as seen in Figure 4,and that of the Nb is < 30 nm independent of the layer thickness. Figures 2 and 4 show the difficulty in obtaining thin regions simultaneously in both Nb and Al. In Figure 2 it is the Nb regions that are thin, while the A1 regions have been removed in ion milling. In Figure 4 it is the A1 region that is thin, while the Nb regions are barely transparent. Occasionally, samples are found where the Nb and A1 are both appropriately thinned (Fig. 3). In preparation of unreacted samples such as shown in Figures 2-4 we found it necessary to cool the samples to liquid nitrogen temperatures during ion milling.
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Fig. 3. Cross-sectional transmission electron micrograph of a Nbi A1 multilayer film deposited onto a liquid nitrogen-cooled sapphire substrate. The A1 layer is contiguous.
Samples that were not cooled reacted in the ion mill to form NbAl,, as seen in Figure 5a. The white regions are NbAI, instead of Al. This was confirmed by electron diffraction (Fig. 5b) and darkfield microscopy. NbAI, is the first phase to form in the reaction of thin NWA1 films, as reported by Bormann et al. (19861, Im and Morris (19881, and Coffey et al. (1989). Since this
Fig. 4. Cross-sectional transmission electron micrograph of a Nbi A1 multilayer film showing that the grain size of the as-deposited A1 layer is of the order of layer size. The sample also shows the difficulty of obtaining electron transparent regions simultaneously in both Nb andA1.
phase only forms a t temperatures above approximately 250°C, the temperature of the sample in the ion mill must have risen to at least this value. This evidence for
Fig. 5. a: Cross-sectional transmission electron micrograph of a NbiAl multilayer film reacted in the ion mill to form NbAl,, suggesting local heating in excess of 250°C occurred during milling. b: Electron diffraction pattern of the sample in Figure 5a. The inner ring marked with a n arrow is from NbA1, only.
NIOBIUM-ALUMINUM MULTILAYERS ON SAPPHIRE
high local heating during ion milling can have important implications for sample preparation in other heat sensitive systems, and has also been reported by Kim and Carpenter (1987). As evidence of the applicability of our technique to similar thin film structures, we have also prepared XTEM samples of reactively sputtered NbN and (Nb,Ta)N in the manner described above. These samples were extremely important to Juang et al. (1989) in explaining the large increase in superconducting critical current density by the introduction of Ta into Nb.
DISCUSSION In this paper we have described in detail a reliable and reproducible technique for the preparation of electron transparent cross-sectional samples of reacted and unreacted multilayer thin films of Nb/Al. The technique has also been applied to reactively sputtered thin film samples of NbN and (Nb,Ta)N. The substrate in both cases was a brittle single crystal sapphire. The brittle nature of the substrate required careful mechanical thinning followed by extensive ion beam etching. The slow etch rate of the sapphire ensured that the films were thinned before complete removal of the substrate. It was discovered that temperatures in excess of 250°C can occur during ion thinning, necessitating the use of liquid nitrogen cooling to avoid undesired reactions during ion milling. The technique developed here should have general applicability to thin films deposited on brittle ceramic substrates. ACKNOWLEDGMENTS The provision of ion milling facilities by Prof. L.W. Hobbs and Prof. Y.M. Chiang is gratefully acknowledged. We are indebted to Kevin Coffey for assistance
253
with the deposition of the films and to John Paul Clarke for assistance with sample preparation. Financial support was provided by the Center for Materials Science and Engineering, NSF DMR-8802613, by DOE Basic Energy Sciences Grant DE-FG02-85ER45206 and AT&T Foundation Fellowship (K. Barmak). REFERENCES Abrahams, M.S., and Buiocchi, C.J. (1974) Cross-sectional specimens for transmission electron microscopy. J. Appl. Phys., 453315-3316. Barmak, K., Rudman, D.A., Coffey, K.R., and Foner, S. (1990) Phase formation sequence for the reaction of multilayer thin films of niobiumialuminum, unpublished. Bormann, R., Krebs, H.U., and Kent, A.O. (1986) The formation of metastable phase Nb,Al by a solid state reaction. Adv. Cryog. Eng., 32:1041-1045. Bravman, J.C., and Sinclair, R. (1984) The preparation of cross-section specimens for transmission electron microscopy. J . Electron Microsc. Technique, 153-61. Chu, S.N.G., and Sheng, T.T. (1984) TEM cross section sample preparation for 111-V compound semiconductor device materials by chemical thinning. J. Electrochem. SOC.,131:2663-2667. Coffey, K.R., Ph.D. Thesis (1989) Phase formation in the reaction of multilayer thin films of niobiumialuminum, Massachusetts Institute of Technology, Cambridge. Coffey, K.R., Clevenger, L.A., Barmak, K., Rudman, D.A., and Thompson, C.V. (1989) Experimental evidence for nucleation during thin film reactions. Appl. Phys. Lett., 552352-854. Im, Y., and Morris, J.W., Jr. (1988) Sequence of phase formation in Nb/Al multilayerd samples. J . Appl. Phys., 64(7):3487-3491. Juang, J.Y., Barmak, K., Rudman, D.A., and van Dover, R.B. (1989) Enhancement of the critical current by grain size refinement in Ta co-sputtered NbN thin films. J. Appl. Phys., 663136-3143. Kim, M.J., and Carpenter, R.W. (1987) TEM specimen preparation during ion beam thinning: Microstructural instability. Ultramicroscopy, 21:327-334. Newcomb, S.B., Boothroyd, C.B., and Stobbs, W.M. (1985) Specimen preparation methods for the examination of surfaces and interfaces in the transmission electron microscope. J . Microsc., 141(2):195207. Vanhellemont, J., Bender, H., and Roussou, L. (1988) A rapid specimen preparation technique for cross-section TEM investigation of semiconductors and metals. Mat. Res. Symp. Proc., 115247-252.
The Preparation of Cross-Sectional Transmission Electron Microscopy Specimens of Nb/AI Multilayer Thin Films on Sapphire Substrates KATAYUN BARMAK, DAVID A. RUDMAN, AND SIMON FONER Department of Materials Science and Engineering (K.B., D.A.R.), and Francis Bitter National Magnet Laboratory and Department of Physics (S.F.), Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
KEY WORDS
Nb/Al, Thin films, Refractory materials, Brittle ceramic substrates
ABSTRACT We have developed a technique for preparation of cross-sectional transmission electron microscopy samples of reacted and unreacted NbiAl multilayer thin films on sapphire substrates. The choice of substrate was found to be extremely important. Sapphire sputters more slowly than Nb and Nb-compounds and therefore makes it possible to obtain the electron transparent regions in the thin films rather than in the substrate. However, the brittle nature of the sapphire restricts the types of thinning- techniques that can be used, requiring extensive ion thinning as a final stage. INTRODUCTION Abrahams and Buiocchi (1974) gave the first description of cross-sectional transmission electron microscopy (XTEM) sample preparation. Since then the technique has been modified and developed further and applied to many materials. In multilayer thin films it is possible to examine many or all the layers at the same time. XTEM samples not only provide a wealth of microstructural and morphological information, but also allow a detailed study of the phase formation sequence and location in multilayer thin films. In this paper we discuss the technique we developed, based on the approach described by Bravman and Sinclair (1984) and Newcomb et al. (19851, for preparation of XTEM samples from NbiAl multilayer films on sapphire substrates. The samples were used as part of a study of the phase formation sequence in the reaction of Nb/A1 in thin films, the details of which will be reported by Barmak et al. (1990). Samples of deposited thin films usually comprise several different materials in cross section. The goal of sample preparation is to thin the samples to electron transparency in all regions. It is the presence of different materials that makes the preparation difficult. In general, the first thinning steps are mechanical grinding and polishing to remove most of the sample relatively quickly. These mechanical steps are usually followed by chemical polishing or ion beam thinning. In chemical etching (jet electropolishing), the variation in etch rate for different materials makes it possible to prepare XTEM samples only for specific multilayer film systems, such as 111-V compound semiconductors, where appropriate etches exist (Chu and Sheng, 1984). The thinning of multilayer XTEM samples is therefore generally carried out by ion beam thinning. Even though the variation in removal rate for different materials is less in ion thinning than in chemical etching, it is often still too high, producing samples that are not thinned in the regions of interest or are not thinned
0 1990 WILEY-LISS, INC.
uniformly. These problems are particularly evident for the system of interest for this study. Nb/Al multilayers on sapphire substrates, where mechanical thinning is severely restricted by the brittle nature of the sapphire substrate, appropriate chemical etches do not exist, and the constituents ion beam thin a t different rates. Although the general principles for the preparation of XTEM samples were already known, it was necessary to modify the existing techniques in order to produce the large number of samples necessary to determine the reaction sequence in the NbiAl system. In addition, the limited quantity of films available for this study required that the probability of yielding an appropriate XTEM sample from each film be high. The methods we developed to meet the above requirements are presented in detail in this paper. Only those micrographs necessary for illustrating the sample preparation technique are presented.
MATERIALS AND METHODS The Nb/Al multilayer films are deposited onto (1102) single crystal sapphire substrates by electron beam evaporation. The films are 1 pm thick and have various layer thicknesses. The substrates are 12 x 6 x 0.5 mm in size. After removal from the evaporation system, each substrate is cut to yield two 6 x 6 x 0.5 mm pieces by scribing with a diamond tip on the film side of the substrate and tapping the sample. The films are then annealed to the desired stage of the reaction. Bulk analyses, such as x-ray diffraction and superconducting measurements, are carried out before XTEM preparation and will be reported by Barmak et a]. (1990) elsewhere. The choice of substrates for this study
Received September 19, 1989 accepted in revised form December 10, 1990. Address reprint requests to Dr. Katayun Barmak, 300-40E IBM East Fishkill Facility, Hopewell Junction, NY 12533-0999.
250
K. BARMAK ET AL.
a. EPOXY
\
SaDDhire
1
Sapphire
I
I
.
I
I
b. k L.......................... ................I] Film BraSS2Tubeyd /
c.
1 Fig. 1. Schematic diagram of cross-sectionaltransmission electron microscopy sample preparation technique.
was restricted by the reactivity of Nb at the typical annealing temperatures ( 1,OOO”C).Many common substrates such as Si and SiO, are not appropriate due to filmisubstrate interactions. Sapphire, however, is well known as a nonreactive substrate for use with Nb films. The reacted samples are mounted onto brass slabs using a strong wax (“CRYSTALBOND”)and are sliced into two 6 x 2 x 0.5 mm strips (Fig. la) with a diamond saw. The samples are then thoroughly cleaned and degreased by immersing them in 1,1,1 trichloroethane and then in acetone. The strips are now cemented together. The epoxy adhesive we have judged best is “VARIAN TORR SEAL” (available from Varian Associates Inc., Vacuum Products Division, Lexington, MA 02173). This epoxy has been used before for preparation of XTEM samples (e.g., Vanhellemont et al., 1988). It has a high viscosity, so that the chance of relative rotation of the substrates is minimized, and is impervious to attack by solvents once it is set. More importantly for this application, the epoxy ion mills a t
about the same rate as Nb. This allows the use of an optical stereo zoom microscope to determine when the film is electron transparent, a crucial step for reducing sample preparation time and increasing the yield. However the epoxy does require overnight curing at room temperature. The epoxy is spread on the film side of the sapphire substrates, and the two pieces mounted with the film sides towards each other. The composite is sandwiched carefully between Teflon sheets and held together in a C-clamp while the epoxy is allowed to cure. The composite strip is easily separated from the Teflon pieces and ground down from both sides on 240 Sic paper to a total thickness of 0.5 mm. Any excess epoxy on the edges of the strip is also ground away. The strip is then cleaned in distilled water and acetone to remove any grinding debris, and glued with epoxy into a 0.6 mm x 2.35 mm x 37.5 mm slot in a molybdenum rod. The use of slotted metal rods for mounting of samples is similar to that reported by Newcomb et al. (1985). Molybdenum is chosen for its slow ion milling rate. This structure is in turn epoxied inside a 25 mm long brass tube of 2.5 mm i.d. and 3 mm 0.d.. The epoxy is again allowed to cure overnight at room temperature. The final epoxied structure is shown in Figure lb. The composite rod structure is sliced into thin disks, using a diamond wafering saw. A thickness of 0.75 mm was found to be convenient. The top view of a slice is shown in Figure lc. At this point, the process of thinning the sample in cross section to electron transparency begins. Much of the mechanical thinning is done by hand. The thickness of each slice is measured and is then mounted with a strong wax on the central stainless block of a “GATAN PRECISION GRINDER’ (available from Gatan Inc., Warrendale, PA 15086). A glass slide is used to press the sample during mounting to ensure that it is mounted flat. The Precision Grinder must be zeroed with the stainless block before the sample is mounted. The slice is ground on 240, 320, and 600 grit Sic paper, changing the orientation by 90” every time, down to a thickness of 250 pm. This should be done with plenty of distilled water as the lubricant. The sample is then polished on 0.3 pm alumina slurry for 3 minutes followed by a 1 minute polish on 0.05 pm alumina. If the grinding is done properly, the molybdenum will have a mirror finish. The sample is removed from the block by heating the wax to about 80°C and removing the excess wax in acetone. The sample is remounted with the unpolished side up, again using a glass slide to mount the sample flatly onto the stainless steel block as before. The same grinding procedure described above is used to thin the sample to about 50 pm. This requires great care and attention since it is very easy to grind away the sample completely while trying to reduce the thickness to the final 50 pm. The sample is then polished on alumina as described above. The reason for hand grinding of the entire sample to 50 pm is that the brittle nature of the sapphire did not allow dimple grinding of the central region of the sample. However, it may be possible to successfully use a dimple grinder whose vibrations are damped.
NIOBIUM-ALUMINUM MULTILAYERS ON SAPPHIRE
At this stage the sample is very fragile, and should be removed from the block only by immersion in acetone. The wax will take approximately 1 hour to be dissolved in acetone. Ultrasonic or even hand agitation of the acetone bath can cause the sample to disintegrate. Once the wax is dissolved, the sample is carefully removed from the bath and mounted onto a 3 mm molybdenum ring with 1mm diameter central hole. A very thin coating of clear nail varnish, which is easy to apply and dries quickly, is spread all around the molybdenum ring and the sample carefully lifted with vacuum tweezers and put down across the central hole. The sample is now ready for ion milling. We used a "GATAN' model 600 dual gun ion mill with argon as the plasma gas. Each gun was operated a t 5 kV and 0.5 mA and the sample was rotated. For unreacted samples or those reacted to less than 300"C,the sample must be liquid nitrogen cooled during ion milling to prevent sample heating. Sputtering yield is highly sensitive to angle of incidence of the beam. Typically, samples are ion milled a t relatively large angles (e.g., 220") a t the early stages to take advantage of the fast milling rate. As the sample thins this angle is reduced to 512" to ensure large thinned regions. However, for our samples we have no simple means of knowing when to reduce the angle. We therefore kept the angle fixed for the entire duration of ion milling. Leaving the samples a t 220" to perforation lead to poor quality samples with no electron transparent regions. Fixing the angle at 512" lead to unacceptably long milling times. By trial and error we found the best angle for ion milling to be 15". Depending on the exact operating parameters for each gun, the ion milling can take anywhere from 5 to 50 hours. It is therefore necessary to check the sample optically every 1-2 hours. Since the specific epoxy chosen has an ion milling rate similar to the sample it becomes possible to use the optical microscope for checking the progress of the thinning, greatly reducing sample preparation time. The epoxy is observed to develop a hole in its center between the thin films. This hole is allowed to expand during further ion milling until it appears to reach the sapphire (the multilayer film itself can not be resolved). At this point the sample should have electron transparent areas, and must be examined in the TEM. The ion milling can be continued in case the multilayer is not sufficiently thin. Great care must be taken in handling the sample at this point since it is extremely fragile. The sample should only be picked up with vacuum tweezers by the molybdenum outer ring. We also attempted to use silicon as a substrate for this study, with a sputtered layer of alumina between the silicon and the multilayer film as a barrier against substrate reactions. Silicon has the advantage that it can be dimple ground, simplifying the mechanical thinning portion of the process. Im and Morris (1988) have used Si as a substrate and were able to dimple to perforation before ion milling. However, we found that due to its fast milling rate, silicon would thin to perforation before the NWA1 had thinned to electron transparency. Further ion milling of such a sample would rarely yield a suitably thinned region due to the unfavorable geom-
25 1
Fig. 2. Cross-sectional transmission electron micrograph of a Nbi A1 multilayer film deposited onto a sapphire substrate a t nominally room temperature. The A1 layer is insular and has been partially milled away.
etry of the film relative to the ion beam. The slower milling rate of sapphire on the other hand ensures that the multilayer is thinned before the substrate is entirely removed. The samples were all examined in a "JEOL 2OOCX" microscope. It was necessary t o operate the microscope at 200 keV, since the Nb rich portions of the samples were not electron transparent at lower voltages.
RESULTS Figure 2 shows a NbiAl multilayer with an A1 layer thickness of 34 nm that was deposited on sapphire at nominally room temperature. The A1 layer is insular. For the studies of the Nb/Al reaction, reported by Barmak et al. (1989) and Coffey (1990), it was important that the A1 layers be contiguous. The deposition process was therefore modified to include liquid nitrogen cooling of the substrates during deposition. Figure 3 shows that for the same nominal A1 thickness as in Figure 2, the layer is now contiguous. The grain size of the as-deposited A1 layer is of the order of the layer thickness, as seen in Figure 4,and that of the Nb is < 30 nm independent of the layer thickness. Figures 2 and 4 show the difficulty in obtaining thin regions simultaneously in both Nb and Al. In Figure 2 it is the Nb regions that are thin, while the A1 regions have been removed in ion milling. In Figure 4 it is the A1 region that is thin, while the Nb regions are barely transparent. Occasionally, samples are found where the Nb and A1 are both appropriately thinned (Fig. 3). In preparation of unreacted samples such as shown in Figures 2-4 we found it necessary to cool the samples to liquid nitrogen temperatures during ion milling.
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Fig. 3. Cross-sectional transmission electron micrograph of a Nbi A1 multilayer film deposited onto a liquid nitrogen-cooled sapphire substrate. The A1 layer is contiguous.
Samples that were not cooled reacted in the ion mill to form NbAl,, as seen in Figure 5a. The white regions are NbAI, instead of Al. This was confirmed by electron diffraction (Fig. 5b) and darkfield microscopy. NbAI, is the first phase to form in the reaction of thin NWA1 films, as reported by Bormann et al. (19861, Im and Morris (19881, and Coffey et al. (1989). Since this
Fig. 4. Cross-sectional transmission electron micrograph of a Nbi A1 multilayer film showing that the grain size of the as-deposited A1 layer is of the order of layer size. The sample also shows the difficulty of obtaining electron transparent regions simultaneously in both Nb andA1.
phase only forms a t temperatures above approximately 250°C, the temperature of the sample in the ion mill must have risen to at least this value. This evidence for
Fig. 5. a: Cross-sectional transmission electron micrograph of a NbiAl multilayer film reacted in the ion mill to form NbAl,, suggesting local heating in excess of 250°C occurred during milling. b: Electron diffraction pattern of the sample in Figure 5a. The inner ring marked with a n arrow is from NbA1, only.
NIOBIUM-ALUMINUM MULTILAYERS ON SAPPHIRE
high local heating during ion milling can have important implications for sample preparation in other heat sensitive systems, and has also been reported by Kim and Carpenter (1987). As evidence of the applicability of our technique to similar thin film structures, we have also prepared XTEM samples of reactively sputtered NbN and (Nb,Ta)N in the manner described above. These samples were extremely important to Juang et al. (1989) in explaining the large increase in superconducting critical current density by the introduction of Ta into Nb.
DISCUSSION In this paper we have described in detail a reliable and reproducible technique for the preparation of electron transparent cross-sectional samples of reacted and unreacted multilayer thin films of Nb/Al. The technique has also been applied to reactively sputtered thin film samples of NbN and (Nb,Ta)N. The substrate in both cases was a brittle single crystal sapphire. The brittle nature of the substrate required careful mechanical thinning followed by extensive ion beam etching. The slow etch rate of the sapphire ensured that the films were thinned before complete removal of the substrate. It was discovered that temperatures in excess of 250°C can occur during ion thinning, necessitating the use of liquid nitrogen cooling to avoid undesired reactions during ion milling. The technique developed here should have general applicability to thin films deposited on brittle ceramic substrates. ACKNOWLEDGMENTS The provision of ion milling facilities by Prof. L.W. Hobbs and Prof. Y.M. Chiang is gratefully acknowledged. We are indebted to Kevin Coffey for assistance
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with the deposition of the films and to John Paul Clarke for assistance with sample preparation. Financial support was provided by the Center for Materials Science and Engineering, NSF DMR-8802613, by DOE Basic Energy Sciences Grant DE-FG02-85ER45206 and AT&T Foundation Fellowship (K. Barmak). REFERENCES Abrahams, M.S., and Buiocchi, C.J. (1974) Cross-sectional specimens for transmission electron microscopy. J. Appl. Phys., 453315-3316. Barmak, K., Rudman, D.A., Coffey, K.R., and Foner, S. (1990) Phase formation sequence for the reaction of multilayer thin films of niobiumialuminum, unpublished. Bormann, R., Krebs, H.U., and Kent, A.O. (1986) The formation of metastable phase Nb,Al by a solid state reaction. Adv. Cryog. Eng., 32:1041-1045. Bravman, J.C., and Sinclair, R. (1984) The preparation of cross-section specimens for transmission electron microscopy. J . Electron Microsc. Technique, 153-61. Chu, S.N.G., and Sheng, T.T. (1984) TEM cross section sample preparation for 111-V compound semiconductor device materials by chemical thinning. J. Electrochem. SOC.,131:2663-2667. Coffey, K.R., Ph.D. Thesis (1989) Phase formation in the reaction of multilayer thin films of niobiumialuminum, Massachusetts Institute of Technology, Cambridge. Coffey, K.R., Clevenger, L.A., Barmak, K., Rudman, D.A., and Thompson, C.V. (1989) Experimental evidence for nucleation during thin film reactions. Appl. Phys. Lett., 552352-854. Im, Y., and Morris, J.W., Jr. (1988) Sequence of phase formation in Nb/Al multilayerd samples. J . Appl. Phys., 64(7):3487-3491. Juang, J.Y., Barmak, K., Rudman, D.A., and van Dover, R.B. (1989) Enhancement of the critical current by grain size refinement in Ta co-sputtered NbN thin films. J. Appl. Phys., 663136-3143. Kim, M.J., and Carpenter, R.W. (1987) TEM specimen preparation during ion beam thinning: Microstructural instability. Ultramicroscopy, 21:327-334. Newcomb, S.B., Boothroyd, C.B., and Stobbs, W.M. (1985) Specimen preparation methods for the examination of surfaces and interfaces in the transmission electron microscope. J . Microsc., 141(2):195207. Vanhellemont, J., Bender, H., and Roussou, L. (1988) A rapid specimen preparation technique for cross-section TEM investigation of semiconductors and metals. Mat. Res. Symp. Proc., 115247-252.
