- Email: [email protected]
Nuclear microscopy: A new way of analyzing materials
3
MATERIALS CHARA CTERIZA TION 25:3-15 (1990)
Nuclear Microscopy: A New Way of Analyzing Materials
DAVID N. JAMIESON.* LINDA T. ROMAN(),+ GEOFF W. GRIME,* AND FRANK WATT* *University of Oxfi)rd, Department of Nuclear Physics, Keble Rd., Oxfi)rd OXI 3RH, U.K.: and ÷University of Oxford, Department of Metallurgy, Parks Rd., Oxford OXl 3PH, U.K.
A scanned, focused high-energy proton beam (2-3 MeV) allows the techniques of proton induced x-ray emission (PIXE), nuclear elastic backscattering spectrometry (BS), channeling contrast microscopy (CCM), and secondary electron microscopy to be used simultaneously to image the structural and elemental composition of technologically important materials. Quantitative information about the materials may be obtained from the images. These techniques are described, and examples are presented of characterization, at 1 ~m spatial resolution, of mosaic single crystal Y~Ba2Cu307 ~ high temperature superconductors and heteroepitaxial GaAs films grown on (100) Si substrates.
Introduction Over the past decade, ion-beam-based analytical techniques such as proton induced x-ray emission (PIXE) [1], Rutherford backscattering spectrometry (RBS) [2] and ion beam channeling [2, 3] have been done with focused ion beams using nuclear microprobes [4]. MeV PIXE analysis provides optimum sensitivity for detection of trace elements with concentrations as low as parts-per-million. RBS analysis allows standardless analysis of composition and depth profiles of major elements (those > - 1 % concentration) to a depth of a few tens of microns (for MeV proton beams). In a nuclear microprobe, the focused ion beam is scanned over a sample to produce several types of images. Elemental distribution maps may be obtained by collection of the elemental characteristic x-ray intensity as a function of beam position on the target. Simultaneously, maps of buried The present address of David N. Jamieson is School of Physics, University of Melbourne, Parkville, 3052, Australia. © Elsevier Science Publishing Co., Inc., 1990 655 Avenue of the Americas, New York, NY 10010
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D . N . Jamieson et al.
features may be obtained from the yield of backscattered particles from atoms in the buried feature. Maps of crystal quality can be obtained from single crystal material if one of the crystal axes is aligned with the analysis beam. Contrast in this case arises because the backscattered yield from regions of good crystal aligned with the beam is much less than the yield from poor quality or misaligned crystal. This technique, which was first developed by the Melbourne Microprobe Group [5], is known as channeling contrast microscopy (CCM). In addition to the induced x-rays and the backscattered particles, the analysis beam induces secondary electrons that may be used to image surface features of the sample. Since the secondary electron production cross section is very large compared to PIXE or RBS cross sections, secondary electron images can be obtained in real time to allow regions of interest to be positioned rapidly under the analysis beam. During the past year, a specialized nuclear microprobe has been constructed within the Oxford Nuclear Microprobe Analytical Unit that is dedicated to analysis of single-crystal samples. A photograph of the new microprobe system is shown in Fig. 1. The New Oxford nuclear microprobe has the capability of producing simultaneous PIXE, RBS/CCM, and secondary electron maps of samples to a lateral resolution of sub-
FIG. I. The new nuclear microprobe.
Nuclear Microscopy
5
micron. The new microprobe incorporates several unique facilities [6]: 1) a versatile data acquisition system; that includes the online accumulation of up to 35 two-dimensional maps and 5 three-dimensional (E,n,y) data arrays (cubes); 2) a large-area, high-resolution, custom-built Si(Li) x-ray detector; 3) a large-area, high-resolution, custom-built annular backscattered particle detector; 4) a new type of magnetic quadrupole lens system that is practically free from parasitic aberration that would otherwise degrade the resolution of the probe; and 5) a precision eucentric goniometer for sample manipulation. In addition, a channeltron electron detector is used to provide simultaneous imaging of the sample surface features from the secondary electron emission. The precision eucentric goniometer sample manipulator allows the sample to be positioned to an accuracy of 5 ixm over a range of _+1.4 cm and tilted about the x- and y-axes to an accuracy of 0.1 mrad over a range of _+21 °. The eucentric property of the goniometer allows the sample to be tilted without transverse motion relative to the analysis beam. This is essential for channeling ion beam analysis of single crystal samples, as it allows a fresh region of the sample to be selected for final analysis, avoiding regions that may have been damaged by prolonged exposure to the beam during crystal alignment procedures. The present work makes use of a 3 MeV H + analysis beam focused to a diameter of 1 txm. At this energy, the proton nuclear elastic scattering cross section for O16(p,p)O t6 is generally an order of magnitude greater than the Rutherford cross section, which gives much greater sensitivity for detection of O compared to RBS analysis with traditional - 2 MeV He ÷ beams. Therefore, the more general term BS is used to describe the scattering process in the present work, since the cross sections are not necessarily given by the Rutherford law. The BS spectra in the present work have been analyzed with use of empirical cross sections, details of which are given in Jamieson et al. [6].
Mosaic Single Crystal YIBaaCu307-~ Measurements of the crystal quality of Y]Ba2Cu307-5 crystals of size 1-3 mm along the a- and b-axes, with conventional ion channeling techniques using unfocused beams, have been reported in the literature [7, 8]. However, measurement of the crystal quality of crystals much less than I mm in size with unfocused beams is extremely difficult. Collimation of the beam to the size of the crystals in general, reduces the beam current to a point where an unacceptably long time is required to collect data with sufficient statistical accuracy. However, the micron-sized focused beam of the proton microprobe, which has a relatively high-current den-
6
D . N . Jamieson et al.
sity, allows crystals as small as a few tens of microns to be analyzed. Some previous workers [9] used 5 ~m D (deuterium), H, and He probes to measure the composition uniformity of superconducting YBa2Cu306.5+× pellets over 150 x 150 ~m 2 regions. The present work shows how it is also possible to perform CCM analysis of this material. The Oxford microprobe was used to study single crystals of YIBa2Cu3OT_ 6 grown in alumina crucibles by the method of spontaneous nucleation from a nonstoichiometric flux bath [10]. After the flux had cooled, crystals of size 0.4-2 mm 2 across were found embedded in residual flux, or partially embedded in each other. The crystals were mechanically separated from the solidified melt, and electron microprobe xray analysis showed the composition to be Y : Ba: Cu = ! : 2.1 : 3. Transmission electron diffraction indicated a (001), c-axis, growth direction perpendicular to the surface. The surface morphology revealed by optical microscopy showed the crystals to be either mirror smooth or to have spiral-like growth patterns, similar to those reported previously by other workers [I I]. For nuclear microprobe analysis, the crystals were aligned with the analysis beam by scanning a 1 p,m beam over a 100 × 100 ~m 2 region in the center of the crystal, so that the yield of backscattered particles could be minimized as a function of sample tilt about the x- and y-axes. The yield of backscattered particles was taken only from the top 1-2 p,m of the sample, which corresponds to the region of the BS spectrum between the surface energy of Y and the surface energy of Ba. The yield from this region should be most sensitive to sample orientation. When the backscattered yield was minimized, the scan size was increased to cover the entire sample, so that other regions of single crystal with the same orientation could be found. Figure 2 shows some typical maps, obtained from a sample with both mirror smooth regions and spirallike growth pattern regions. The BS maps show a region of crystal aligned with the beam near the center of the sample that is surrounded by regions that are either of poor-quality crystal or are of single-crystal material misaligned with the central region. The reduction in yield from the aligned region is most clearly evident in the BS maps as well as the Ba L and Cu Ka PIXE maps. The Cu K~ x-ray was used to produce the Cu map instead of the more intense Cu Ks x-ray because of interference between the Cu K,~ and Ni K~ contributed from the target backing. The Y PIXE map shows little structure because of the fact that the cross section for the Y K,~ x-ray, from which the Y PIXE map was obtained, is significantly lower than that for the Ba or Cu x-rays; hence, there are relatively few counts in the Y map, and the channeling contrast is not evident.
Nuclear Microscopy
FIG. 2. Simultaneous BS, PIXE, and secondary electron analysis of a polycrystalline high-temperature superconductor crystal with a 1.75 x 1.75 mm z scan. (A) Photomicrograph; (B) Proton-induced secondary electron map showing steps and edges; (C) Surface Ba yield showing reduced yield from aligned crystals near the center of the sample; (D) Deeper BS yield; (E) Ba PIXE yield; (F) Y PIXE yield; (G) Cu PIXE yield; and (H] CI PIXE yield showing mainly Bremstrahlung, but also the higher yield from CI originally present as NaC1 in the flux.
Once a region of good crystal was identified, the crystal quality was measured from a 500 x 500 p~m2 scan over the center of the sample. These maps are shown in Fig. 3, which also shows maps from the same region of the sample randomly aligned with the analysis beam. No structure is visible in the maps obtained for the random alignment, which confirms that the observed contrast for the aligned crystal is due to channeling and not to variations in sample composition. A fit to the BS spectrum from the randomly aligned crystal using the empirical nuclear cross section for 160 gives a composition of Y:Ba:Cu = 1:1.9:3.1:8.5. This agrees well with the result obtained from the electron probe measurement; The small differences could be attributed to actual composition differences between different regions of the crystals. The measured O composition is overestimated by a few percent because of small differences between the present detector geometry and that used to obtain the empirical cross section.
8
D . N . Jamieson et al.
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FIG. 3. A 500 x 500 Izm2 scan covering the region of good crystal that was revealed in the larger area scan of Fig. 2. The square scans were obtained with the sample c-axis aligned with the analysis beam (see text). The rectangular scans were obtained from the right side of the same region with the sample randomly oriented to the analysis beam.
The maps obtained for the aligned orientation may be divided into three different subregions of interest based on variations in crystal quality. Diagonally, to the lower left, is a 50 × 200 txm2 region of best quality crystal where the BS yield is the lowest, designated region A. This region is most readily seen in the BS map from the surface Ba in Fig. 3. The remainder of the lower half of the map shows the second subregion where the crystal quality is of slightly worse quality, region B. Finally, the subregion consisting of the top half of the map consists of poor quality or misaligned crystal, region C, where the backscattered yield is equal to the backscattered yield from the randomly oriented sample. The primary scan used to align the crystal was positioned over the boundary of regions A and B. Comparison of BS spectra extracted from the three subregions allows the crystal quality to be compared. The BS spectra for the subregions were obtained by applying software masks off-line to the data array " c u b e . " The software masks were generated by placing upper and lower thresholds on the Ba BS yield map from Fig. 3. The thresholds are represented in Fig. 4, which shows a projection of the Ba BS yield onto the y-axis. The BS spectra from regions with a Ba BS yield between the thresholds, which also correspond to the three regions of interest, may then be extracted from the " c u b e . " The regions covered by the software masks are shown in Fig. 4, where they are located on a photomicrograph of the sample. The BS spectra from the three regions of interest are shown in Fig. 5. The spectra show that the yield from the region of best quality crystal
N u c l e a r Microscopy '
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FIG. 4. (Lower left) x-y masks generated from thresholds (shown upper left) set on the yield in the BS Ba surface map shown in Fig. 3 (see text) that cover three regions of interest: (A) good crystal, (B) worse quality or slightly misaligned crystal, and (C) region from which the BS yield was the same as the yield from the randomly oriented crystal. Right: The same three regions of interest identified on a photograph of the sample. (region A) has a Xmin for proton channeling of 20.8%. This value for Xmin is high compared to those previously reported [7, 8] for YtBa2Cu3OT_8 single crystals using conventional He channeling. Regions B and C consist of crystal displaying the spiral-like growth patterns separated by a faceted interface. Region B has an average Xmi, of about 53%, showing that the crystal quality is poorer than A. However, the BS yield from region C was equal to the yield from the randomly oriented sample, This indicates a misorientation of region C from A and B by at least 2 °, since the experimentally measured critical angle, q~t/2, from region A was 1° CCM analysis of this sample has revealed that the regions of the crystal that appear featureless and mirror smooth in an optical microscope are the best-quality crystal. The regions that display the spiral-like growth patterns have a poor overall crystal quality.
Heteroepitaxial GaAs on Si
Semiconductor devices that exploit the best properties of Si and GaAs by combining Si and GaAs devices on a single chip require successful
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FiG. 5. BS spectra obtained from the three regions of the sample shown in Fig. 4. The bars labeled Ba Surf and YBC Deep delineate the energy windows used to produce that maps labeled Ba Surface and YBC Deep in Figs. 2 and 3. The smooth curve is a best fit simulation to the spectrum for the randomly oriented sample that was done to determine the nominal sample composition. A 3-MeV proton beam and annular detector with an average scattering angle of 175.4° were used to obtain these BS spectra and the maps shown in Figs. 2 and 3.
integrated growth of these two materials. One major problem in obtaining high-quality epitaxial GaAs layers on single crystal Si substrates is the 4% lattice mismatch. A MBE GaAs growth process has been developed [12] that produces good quality GaAs films on patterned (100)Si substrates. The patterned Si substrate is prepared first by growing a surface oxide layer on a Si wafer, then square trenches are etched through the oxide layer into the Si substrate. The oxide is then in the form of a grid on the surface of the Si substrate. A GaAs film a few micrometers thick is then grown by MBE and an in situ anneal. Epitaxial growth of GaAs occurs in the trenches, with polycrystalline GaAs forming on the oxide. Conventional broad beam ion channeling analysis of the crystal quality of the GaAs film is not possible because of the small scale of the trenches (less than 50 txm across). However, the Oxford microprobe can readily be used to measure the crystal quality of the GaAs layers in the trenches.
Nuclear Microscopy
11
The present work shows how the crystal quality can be measured in a sample with both 50 × 50 ~m z and 25 x 25 Fxm2 trenches. The sample was aligned with the analysis beam as described in the previous section, and the resulting maps are shown in Fig. 6. The secondary electron images were used to position the scan over a junction between the regions of large and small trenches. The maps for the aligned sample clearly show the contrast cause by the significantly lower yield from the epitaxial GaAs in the trenches compared to the polycrystalline GaAs on the oxide layer. Maps obtained from energy windows that correspond to the top 1.7 ~m of the GaAs layer clearly show that the contrast vanishes when the sample is randomly oriented to the analysis beam. Contrast is also visible in the BS maps obtained from the deepest part of the GaAs layer when randomly oriented to the beam. Surprisingly, this result suggests that the polycrystalline GaAs layer over the oxide is thinner than the epitaxial GaAs layer in the trenches. This result is also suggested by the Ga + As PIXE map, where contrast is also observed in
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FIG. 6. Maps of the BS and PIXE yields from GaAs epilayers grown on a patterned (100)Si substrate. The secondary electron map reveals the trenches in the substrate in which epitaxial GaAs was grown, as shown by the contrast seen in the BS maps for the sample aligned with the analysis beam. The BS maps are labeled with the approximate depth that corresponds to the energywindows used to produce the maps.
12
D. N. Jamieson et al.
the map for the randomly oriented sample. The reason why the epitaxial GaAs regions should be thicker than the polycrystalline GaAs regions, given that the entire GaAs layer was deposited simultaneously, is yet to be explained. The crystal quality of the epitaxial GaAs in the trenches was found from BS specta extracted from the " c u b e " as described in the previous section. The spectra, shown in Fig. 7, give a Xmin of approximately 22% for proton channeling. However, the true figure may be smaller, since the "surface peak" and the signal from the layer of best crystal just beneath the surface are not resolved from the signal from layers of poorer quality crystal deeper in toward the interface with the Si substrate. A measurement of the best quality crystal near the surface could be obtained with a MeV He microbeam. The yield from the polycrystalline GaAs above the oxide is about 10% less than the yield from the GaAs in the randomly oriented sample with
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FIc. 7. BS spectra from subregions of the maps in Fig. 6. Histogram: spectrum from the randomly oriented sample; Squares: from the poly-GaAs on the oxide; Triangles: from the epi-GaAs in the trenches. The bars labeled A, B, and C delineate the energy windows used to produce the BS maps shown in Fig. 6. A 2-MeV proton beam and an annular detector at a scattering angle of 174.5° were used to obtain the maps in Fig. 6 and, hence, these spectra. The smooth curve shows a simulation for a 2.7-1~m GaAs layer on a Si substrate, which is the nominal sample structure.
Nuclear Microscopy
13
the same energy width. This implies that the polycrystalline GaAs contains pinholes or interdiffused Si. Further evidence for this is provided by the high-energy tails on the Si signal, which are most clearly evident when compared to the simulated spectrum for the nominal sample structure (see Fig. 7). Indeed, the entire GaAs layer appears to contain pinholes or interdiffused Si, since the high-energy tail on the Si signal is evident in all BS spectra. Finally, the crystal quality as a function of position across a period of the sample may be extracted from the map for the 0-0.8 p.m surface window (map labeled 0-0.8 txm in Fig. 6 and window labeled A in Fig. 7). The resulting Xmin as a function of position, shown in Fig. 8, reveals that the GaAs crystal quality is uniform across a trench to the limit for the spatial resolution. The spatial solution was less than 1.8 ~m, which was obtained by fitting an error function to the rise of the ×rain as a function of position across an oxide region. The actual edge profile must be deconvoluted from this number to obtain the true spatial resolution. Figure 8 shows that the Xminis I00% over the oxide region surrounding the smaller I
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Fio. 8. The Xmin of the GaAs layer as a function of position across the center of a wide and narrow trench from the maps shown in Fig. 6. The bars labeled " O x i d e " represent the position of the SiO2 layer surrounding the trenches in which the epi-GaAs has grown. The GaAs on the oxide is polycrystalline (×min -- 100%) but is epitaxial (Xmin ~ 20%) in the trenches.
14
D. N. Jamieson et al.
trenches, as expected, since the GaAs is assumed to be polycrystalline over these regions; however, its is only about 75% in the GaAs over the oxide surrounding the larger trenches. This suggests that there is some degree of preferred orientation of the polycrystalline GaAs in these regions; however, further work with complementary techniques is required to determine the reason for this result.
Conclusion Nuclear microscopy has been used to image single-crystal samples using contrast from P1XE, BS, CCM, and secondary electrons with a scanned 2- or 3-MeV proton microbeam. The techniques, when combined with a sophisticated computerized data acquisition system, have allowed quantitative ion channeling measurements of the crystal quality of regions as small as a few tens of microns in size. In addition, the images provide information about the structure and composition of the sample.
We acknowledge C. Chen (visiting scholar from the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, P.R. China) of the Clarendon Laboratory, University of Oxford, for growing the Y~Ba2Cu3Ov_~ crystals. L.T.R. acknowledges the support of Cookson PLC.
Glossary of Terms PIXE (proton induced x-ray emission): When a high-energy proton collides with an atom, there is a high probability that an inner-core electron is ejected, and following the subsequent rearrangement of the electronic structure, an x-ray is emitted whose energy is characteristic of the parent atom. Measurement of the energies of these characteristic x-rays provides an analytical technique that is capable of analytical sensitivities at the ppm level for Na and above in the periodic table. RBS (Rutherford backscattering spectrometry): When a high-energy ion (such as a proton of an alpha particle) collides with an atom, there is a small probability of an elastic collision with the nucleus. By measuring the energy of an ion that has suffered such a collision and has backscattered out of the sample, the mass of the target nucleus, as well as the depth of the nucleus in the sample, can be determined. CCM (channeling contrast microscopy): When a high-energy ion microbeam is channeled into crystalline material, then the beam penetrates deeper into the crystal before interactions take place. This is observed
Nuclear Microscopy
15
as a reduction in the PIXE and RBS yield. When the beam is scanned over a crystalline target, any variations in yield can therefore represent structural variations in the sample.
Our thanks to Buehler Ltd. and the International Metallographic Society fi~r assistance in the reproduction of the color illustrations. References 1. S. A. E. Johansson and T. B. Johansson, Analytical application of particle induced xray emission, Nucl. lnstrum. Meth. 137:473-516 (1976). 2. W.-K. Chu, J. W. Mayer, and M.-A. Nicolet, Backscattering Spectrometry, Academic (1978). 3. L. C. Feldman, J. W. Mayer, and S. T. Picraux, Materials Analysis by Ion Channeling, Academic (1982). 4. See, e.g., the review by: F. Watt and G. W. Grime, eds., Principles and Applications of High Energy Ion Microbeams, Adam Hilger, Bristol, UK (1987). 5. J. C. McCallum, R. A. Brown, E. Nygren, J. W. Williams, and G. L. Olson, Channeling contrast microscopy: A powerful tool for examining semiconductor structures, Mat. Res. Soc. Symp. Proc. 69:305-309 (1986). 6. D. N. Jamieson, G. W. Grime, and F. Watt, The New Oxford Scanning Proton Microprobe Analytical Facility, Nucl. lnstrum, and Meth. B40/41:669-674 (1989). 7. N. G. Stoffel, P. A. Morris, W. A. Bonner, and B. J. Wilkins, Ion-channeling study of single-crystal YBa2Cu3Ox, Phys. Rev. B 37(4):2297-2300 (1988). 8. R. P. Sharma, L. E. Rehn, P. M. Baldo, and J. Z. Liu, Ion-channeling investigation of thermal vibration amplitudes across the superconducting transition in YBazCu307 8, Phys. Rev. B 38(13):9287-9290 (1988). 9. G. Demortier, F. Bodart, G. Deconninck, G. Terwagne, Z. Gabelica, and E. G. Derouane, Stoichiometric characterization of Y - B a - C u - O superconductors with nuclear probes, Nucl. lnstrum. Meth. B30:491-496 (1988). 10. B. M. Wanklyn, C. Chen, B. E. Watts, P. Haycock, and F. Pratt, The flux growth of YBa2Cu307-~, Solid State Comm. 66(4):441-443 (1988). 11. K. Hayashi, M. Tokumoto, K. Takahashi, Y. Suzuki, K. Murata, H. Ihara, N. Koshizuka, and Y.-i. Kimura, Growth and domain structures of BazYCu307 _~. single crystals. J. J. Appl. Phys. 27:L1646-1649 (1988). 12. Y. C. Kao et al., MBE growth of GaAs on (100)Si, private communication, 1989. Received August 1989; accepted February 1990.
MATERIALS CHARA CTERIZA TION 25:3-15 (1990)
Nuclear Microscopy: A New Way of Analyzing Materials
DAVID N. JAMIESON.* LINDA T. ROMAN(),+ GEOFF W. GRIME,* AND FRANK WATT* *University of Oxfi)rd, Department of Nuclear Physics, Keble Rd., Oxfi)rd OXI 3RH, U.K.: and ÷University of Oxford, Department of Metallurgy, Parks Rd., Oxford OXl 3PH, U.K.
A scanned, focused high-energy proton beam (2-3 MeV) allows the techniques of proton induced x-ray emission (PIXE), nuclear elastic backscattering spectrometry (BS), channeling contrast microscopy (CCM), and secondary electron microscopy to be used simultaneously to image the structural and elemental composition of technologically important materials. Quantitative information about the materials may be obtained from the images. These techniques are described, and examples are presented of characterization, at 1 ~m spatial resolution, of mosaic single crystal Y~Ba2Cu307 ~ high temperature superconductors and heteroepitaxial GaAs films grown on (100) Si substrates.
Introduction Over the past decade, ion-beam-based analytical techniques such as proton induced x-ray emission (PIXE) [1], Rutherford backscattering spectrometry (RBS) [2] and ion beam channeling [2, 3] have been done with focused ion beams using nuclear microprobes [4]. MeV PIXE analysis provides optimum sensitivity for detection of trace elements with concentrations as low as parts-per-million. RBS analysis allows standardless analysis of composition and depth profiles of major elements (those > - 1 % concentration) to a depth of a few tens of microns (for MeV proton beams). In a nuclear microprobe, the focused ion beam is scanned over a sample to produce several types of images. Elemental distribution maps may be obtained by collection of the elemental characteristic x-ray intensity as a function of beam position on the target. Simultaneously, maps of buried The present address of David N. Jamieson is School of Physics, University of Melbourne, Parkville, 3052, Australia. © Elsevier Science Publishing Co., Inc., 1990 655 Avenue of the Americas, New York, NY 10010
1044-5803/90/3.50
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D . N . Jamieson et al.
features may be obtained from the yield of backscattered particles from atoms in the buried feature. Maps of crystal quality can be obtained from single crystal material if one of the crystal axes is aligned with the analysis beam. Contrast in this case arises because the backscattered yield from regions of good crystal aligned with the beam is much less than the yield from poor quality or misaligned crystal. This technique, which was first developed by the Melbourne Microprobe Group [5], is known as channeling contrast microscopy (CCM). In addition to the induced x-rays and the backscattered particles, the analysis beam induces secondary electrons that may be used to image surface features of the sample. Since the secondary electron production cross section is very large compared to PIXE or RBS cross sections, secondary electron images can be obtained in real time to allow regions of interest to be positioned rapidly under the analysis beam. During the past year, a specialized nuclear microprobe has been constructed within the Oxford Nuclear Microprobe Analytical Unit that is dedicated to analysis of single-crystal samples. A photograph of the new microprobe system is shown in Fig. 1. The New Oxford nuclear microprobe has the capability of producing simultaneous PIXE, RBS/CCM, and secondary electron maps of samples to a lateral resolution of sub-
FIG. I. The new nuclear microprobe.
Nuclear Microscopy
5
micron. The new microprobe incorporates several unique facilities [6]: 1) a versatile data acquisition system; that includes the online accumulation of up to 35 two-dimensional maps and 5 three-dimensional (E,n,y) data arrays (cubes); 2) a large-area, high-resolution, custom-built Si(Li) x-ray detector; 3) a large-area, high-resolution, custom-built annular backscattered particle detector; 4) a new type of magnetic quadrupole lens system that is practically free from parasitic aberration that would otherwise degrade the resolution of the probe; and 5) a precision eucentric goniometer for sample manipulation. In addition, a channeltron electron detector is used to provide simultaneous imaging of the sample surface features from the secondary electron emission. The precision eucentric goniometer sample manipulator allows the sample to be positioned to an accuracy of 5 ixm over a range of _+1.4 cm and tilted about the x- and y-axes to an accuracy of 0.1 mrad over a range of _+21 °. The eucentric property of the goniometer allows the sample to be tilted without transverse motion relative to the analysis beam. This is essential for channeling ion beam analysis of single crystal samples, as it allows a fresh region of the sample to be selected for final analysis, avoiding regions that may have been damaged by prolonged exposure to the beam during crystal alignment procedures. The present work makes use of a 3 MeV H + analysis beam focused to a diameter of 1 txm. At this energy, the proton nuclear elastic scattering cross section for O16(p,p)O t6 is generally an order of magnitude greater than the Rutherford cross section, which gives much greater sensitivity for detection of O compared to RBS analysis with traditional - 2 MeV He ÷ beams. Therefore, the more general term BS is used to describe the scattering process in the present work, since the cross sections are not necessarily given by the Rutherford law. The BS spectra in the present work have been analyzed with use of empirical cross sections, details of which are given in Jamieson et al. [6].
Mosaic Single Crystal YIBaaCu307-~ Measurements of the crystal quality of Y]Ba2Cu307-5 crystals of size 1-3 mm along the a- and b-axes, with conventional ion channeling techniques using unfocused beams, have been reported in the literature [7, 8]. However, measurement of the crystal quality of crystals much less than I mm in size with unfocused beams is extremely difficult. Collimation of the beam to the size of the crystals in general, reduces the beam current to a point where an unacceptably long time is required to collect data with sufficient statistical accuracy. However, the micron-sized focused beam of the proton microprobe, which has a relatively high-current den-
6
D . N . Jamieson et al.
sity, allows crystals as small as a few tens of microns to be analyzed. Some previous workers [9] used 5 ~m D (deuterium), H, and He probes to measure the composition uniformity of superconducting YBa2Cu306.5+× pellets over 150 x 150 ~m 2 regions. The present work shows how it is also possible to perform CCM analysis of this material. The Oxford microprobe was used to study single crystals of YIBa2Cu3OT_ 6 grown in alumina crucibles by the method of spontaneous nucleation from a nonstoichiometric flux bath [10]. After the flux had cooled, crystals of size 0.4-2 mm 2 across were found embedded in residual flux, or partially embedded in each other. The crystals were mechanically separated from the solidified melt, and electron microprobe xray analysis showed the composition to be Y : Ba: Cu = ! : 2.1 : 3. Transmission electron diffraction indicated a (001), c-axis, growth direction perpendicular to the surface. The surface morphology revealed by optical microscopy showed the crystals to be either mirror smooth or to have spiral-like growth patterns, similar to those reported previously by other workers [I I]. For nuclear microprobe analysis, the crystals were aligned with the analysis beam by scanning a 1 p,m beam over a 100 × 100 ~m 2 region in the center of the crystal, so that the yield of backscattered particles could be minimized as a function of sample tilt about the x- and y-axes. The yield of backscattered particles was taken only from the top 1-2 p,m of the sample, which corresponds to the region of the BS spectrum between the surface energy of Y and the surface energy of Ba. The yield from this region should be most sensitive to sample orientation. When the backscattered yield was minimized, the scan size was increased to cover the entire sample, so that other regions of single crystal with the same orientation could be found. Figure 2 shows some typical maps, obtained from a sample with both mirror smooth regions and spirallike growth pattern regions. The BS maps show a region of crystal aligned with the beam near the center of the sample that is surrounded by regions that are either of poor-quality crystal or are of single-crystal material misaligned with the central region. The reduction in yield from the aligned region is most clearly evident in the BS maps as well as the Ba L and Cu Ka PIXE maps. The Cu K~ x-ray was used to produce the Cu map instead of the more intense Cu Ks x-ray because of interference between the Cu K,~ and Ni K~ contributed from the target backing. The Y PIXE map shows little structure because of the fact that the cross section for the Y K,~ x-ray, from which the Y PIXE map was obtained, is significantly lower than that for the Ba or Cu x-rays; hence, there are relatively few counts in the Y map, and the channeling contrast is not evident.
Nuclear Microscopy
FIG. 2. Simultaneous BS, PIXE, and secondary electron analysis of a polycrystalline high-temperature superconductor crystal with a 1.75 x 1.75 mm z scan. (A) Photomicrograph; (B) Proton-induced secondary electron map showing steps and edges; (C) Surface Ba yield showing reduced yield from aligned crystals near the center of the sample; (D) Deeper BS yield; (E) Ba PIXE yield; (F) Y PIXE yield; (G) Cu PIXE yield; and (H] CI PIXE yield showing mainly Bremstrahlung, but also the higher yield from CI originally present as NaC1 in the flux.
Once a region of good crystal was identified, the crystal quality was measured from a 500 x 500 p~m2 scan over the center of the sample. These maps are shown in Fig. 3, which also shows maps from the same region of the sample randomly aligned with the analysis beam. No structure is visible in the maps obtained for the random alignment, which confirms that the observed contrast for the aligned crystal is due to channeling and not to variations in sample composition. A fit to the BS spectrum from the randomly aligned crystal using the empirical nuclear cross section for 160 gives a composition of Y:Ba:Cu = 1:1.9:3.1:8.5. This agrees well with the result obtained from the electron probe measurement; The small differences could be attributed to actual composition differences between different regions of the crystals. The measured O composition is overestimated by a few percent because of small differences between the present detector geometry and that used to obtain the empirical cross section.
8
D . N . Jamieson et al.
Max,
Min
FIG. 3. A 500 x 500 Izm2 scan covering the region of good crystal that was revealed in the larger area scan of Fig. 2. The square scans were obtained with the sample c-axis aligned with the analysis beam (see text). The rectangular scans were obtained from the right side of the same region with the sample randomly oriented to the analysis beam.
The maps obtained for the aligned orientation may be divided into three different subregions of interest based on variations in crystal quality. Diagonally, to the lower left, is a 50 × 200 txm2 region of best quality crystal where the BS yield is the lowest, designated region A. This region is most readily seen in the BS map from the surface Ba in Fig. 3. The remainder of the lower half of the map shows the second subregion where the crystal quality is of slightly worse quality, region B. Finally, the subregion consisting of the top half of the map consists of poor quality or misaligned crystal, region C, where the backscattered yield is equal to the backscattered yield from the randomly oriented sample. The primary scan used to align the crystal was positioned over the boundary of regions A and B. Comparison of BS spectra extracted from the three subregions allows the crystal quality to be compared. The BS spectra for the subregions were obtained by applying software masks off-line to the data array " c u b e . " The software masks were generated by placing upper and lower thresholds on the Ba BS yield map from Fig. 3. The thresholds are represented in Fig. 4, which shows a projection of the Ba BS yield onto the y-axis. The BS spectra from regions with a Ba BS yield between the thresholds, which also correspond to the three regions of interest, may then be extracted from the " c u b e . " The regions covered by the software masks are shown in Fig. 4, where they are located on a photomicrograph of the sample. The BS spectra from the three regions of interest are shown in Fig. 5. The spectra show that the yield from the region of best quality crystal
N u c l e a r Microscopy '
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FIG. 4. (Lower left) x-y masks generated from thresholds (shown upper left) set on the yield in the BS Ba surface map shown in Fig. 3 (see text) that cover three regions of interest: (A) good crystal, (B) worse quality or slightly misaligned crystal, and (C) region from which the BS yield was the same as the yield from the randomly oriented crystal. Right: The same three regions of interest identified on a photograph of the sample. (region A) has a Xmin for proton channeling of 20.8%. This value for Xmin is high compared to those previously reported [7, 8] for YtBa2Cu3OT_8 single crystals using conventional He channeling. Regions B and C consist of crystal displaying the spiral-like growth patterns separated by a faceted interface. Region B has an average Xmi, of about 53%, showing that the crystal quality is poorer than A. However, the BS yield from region C was equal to the yield from the randomly oriented sample, This indicates a misorientation of region C from A and B by at least 2 °, since the experimentally measured critical angle, q~t/2, from region A was 1° CCM analysis of this sample has revealed that the regions of the crystal that appear featureless and mirror smooth in an optical microscope are the best-quality crystal. The regions that display the spiral-like growth patterns have a poor overall crystal quality.
Heteroepitaxial GaAs on Si
Semiconductor devices that exploit the best properties of Si and GaAs by combining Si and GaAs devices on a single chip require successful
I0
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integrated growth of these two materials. One major problem in obtaining high-quality epitaxial GaAs layers on single crystal Si substrates is the 4% lattice mismatch. A MBE GaAs growth process has been developed [12] that produces good quality GaAs films on patterned (100)Si substrates. The patterned Si substrate is prepared first by growing a surface oxide layer on a Si wafer, then square trenches are etched through the oxide layer into the Si substrate. The oxide is then in the form of a grid on the surface of the Si substrate. A GaAs film a few micrometers thick is then grown by MBE and an in situ anneal. Epitaxial growth of GaAs occurs in the trenches, with polycrystalline GaAs forming on the oxide. Conventional broad beam ion channeling analysis of the crystal quality of the GaAs film is not possible because of the small scale of the trenches (less than 50 txm across). However, the Oxford microprobe can readily be used to measure the crystal quality of the GaAs layers in the trenches.
Nuclear Microscopy
11
The present work shows how the crystal quality can be measured in a sample with both 50 × 50 ~m z and 25 x 25 Fxm2 trenches. The sample was aligned with the analysis beam as described in the previous section, and the resulting maps are shown in Fig. 6. The secondary electron images were used to position the scan over a junction between the regions of large and small trenches. The maps for the aligned sample clearly show the contrast cause by the significantly lower yield from the epitaxial GaAs in the trenches compared to the polycrystalline GaAs on the oxide layer. Maps obtained from energy windows that correspond to the top 1.7 ~m of the GaAs layer clearly show that the contrast vanishes when the sample is randomly oriented to the analysis beam. Contrast is also visible in the BS maps obtained from the deepest part of the GaAs layer when randomly oriented to the beam. Surprisingly, this result suggests that the polycrystalline GaAs layer over the oxide is thinner than the epitaxial GaAs layer in the trenches. This result is also suggested by the Ga + As PIXE map, where contrast is also observed in
Migned
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FIG. 6. Maps of the BS and PIXE yields from GaAs epilayers grown on a patterned (100)Si substrate. The secondary electron map reveals the trenches in the substrate in which epitaxial GaAs was grown, as shown by the contrast seen in the BS maps for the sample aligned with the analysis beam. The BS maps are labeled with the approximate depth that corresponds to the energywindows used to produce the maps.
12
D. N. Jamieson et al.
the map for the randomly oriented sample. The reason why the epitaxial GaAs regions should be thicker than the polycrystalline GaAs regions, given that the entire GaAs layer was deposited simultaneously, is yet to be explained. The crystal quality of the epitaxial GaAs in the trenches was found from BS specta extracted from the " c u b e " as described in the previous section. The spectra, shown in Fig. 7, give a Xmin of approximately 22% for proton channeling. However, the true figure may be smaller, since the "surface peak" and the signal from the layer of best crystal just beneath the surface are not resolved from the signal from layers of poorer quality crystal deeper in toward the interface with the Si substrate. A measurement of the best quality crystal near the surface could be obtained with a MeV He microbeam. The yield from the polycrystalline GaAs above the oxide is about 10% less than the yield from the GaAs in the randomly oriented sample with
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FIc. 7. BS spectra from subregions of the maps in Fig. 6. Histogram: spectrum from the randomly oriented sample; Squares: from the poly-GaAs on the oxide; Triangles: from the epi-GaAs in the trenches. The bars labeled A, B, and C delineate the energy windows used to produce the BS maps shown in Fig. 6. A 2-MeV proton beam and an annular detector at a scattering angle of 174.5° were used to obtain the maps in Fig. 6 and, hence, these spectra. The smooth curve shows a simulation for a 2.7-1~m GaAs layer on a Si substrate, which is the nominal sample structure.
Nuclear Microscopy
13
the same energy width. This implies that the polycrystalline GaAs contains pinholes or interdiffused Si. Further evidence for this is provided by the high-energy tails on the Si signal, which are most clearly evident when compared to the simulated spectrum for the nominal sample structure (see Fig. 7). Indeed, the entire GaAs layer appears to contain pinholes or interdiffused Si, since the high-energy tail on the Si signal is evident in all BS spectra. Finally, the crystal quality as a function of position across a period of the sample may be extracted from the map for the 0-0.8 p.m surface window (map labeled 0-0.8 txm in Fig. 6 and window labeled A in Fig. 7). The resulting Xmin as a function of position, shown in Fig. 8, reveals that the GaAs crystal quality is uniform across a trench to the limit for the spatial resolution. The spatial solution was less than 1.8 ~m, which was obtained by fitting an error function to the rise of the ×rain as a function of position across an oxide region. The actual edge profile must be deconvoluted from this number to obtain the true spatial resolution. Figure 8 shows that the Xminis I00% over the oxide region surrounding the smaller I
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14
D. N. Jamieson et al.
trenches, as expected, since the GaAs is assumed to be polycrystalline over these regions; however, its is only about 75% in the GaAs over the oxide surrounding the larger trenches. This suggests that there is some degree of preferred orientation of the polycrystalline GaAs in these regions; however, further work with complementary techniques is required to determine the reason for this result.
Conclusion Nuclear microscopy has been used to image single-crystal samples using contrast from P1XE, BS, CCM, and secondary electrons with a scanned 2- or 3-MeV proton microbeam. The techniques, when combined with a sophisticated computerized data acquisition system, have allowed quantitative ion channeling measurements of the crystal quality of regions as small as a few tens of microns in size. In addition, the images provide information about the structure and composition of the sample.
We acknowledge C. Chen (visiting scholar from the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, P.R. China) of the Clarendon Laboratory, University of Oxford, for growing the Y~Ba2Cu3Ov_~ crystals. L.T.R. acknowledges the support of Cookson PLC.
Glossary of Terms PIXE (proton induced x-ray emission): When a high-energy proton collides with an atom, there is a high probability that an inner-core electron is ejected, and following the subsequent rearrangement of the electronic structure, an x-ray is emitted whose energy is characteristic of the parent atom. Measurement of the energies of these characteristic x-rays provides an analytical technique that is capable of analytical sensitivities at the ppm level for Na and above in the periodic table. RBS (Rutherford backscattering spectrometry): When a high-energy ion (such as a proton of an alpha particle) collides with an atom, there is a small probability of an elastic collision with the nucleus. By measuring the energy of an ion that has suffered such a collision and has backscattered out of the sample, the mass of the target nucleus, as well as the depth of the nucleus in the sample, can be determined. CCM (channeling contrast microscopy): When a high-energy ion microbeam is channeled into crystalline material, then the beam penetrates deeper into the crystal before interactions take place. This is observed
Nuclear Microscopy
15
as a reduction in the PIXE and RBS yield. When the beam is scanned over a crystalline target, any variations in yield can therefore represent structural variations in the sample.
Our thanks to Buehler Ltd. and the International Metallographic Society fi~r assistance in the reproduction of the color illustrations. References 1. S. A. E. Johansson and T. B. Johansson, Analytical application of particle induced xray emission, Nucl. lnstrum. Meth. 137:473-516 (1976). 2. W.-K. Chu, J. W. Mayer, and M.-A. Nicolet, Backscattering Spectrometry, Academic (1978). 3. L. C. Feldman, J. W. Mayer, and S. T. Picraux, Materials Analysis by Ion Channeling, Academic (1982). 4. See, e.g., the review by: F. Watt and G. W. Grime, eds., Principles and Applications of High Energy Ion Microbeams, Adam Hilger, Bristol, UK (1987). 5. J. C. McCallum, R. A. Brown, E. Nygren, J. W. Williams, and G. L. Olson, Channeling contrast microscopy: A powerful tool for examining semiconductor structures, Mat. Res. Soc. Symp. Proc. 69:305-309 (1986). 6. D. N. Jamieson, G. W. Grime, and F. Watt, The New Oxford Scanning Proton Microprobe Analytical Facility, Nucl. lnstrum, and Meth. B40/41:669-674 (1989). 7. N. G. Stoffel, P. A. Morris, W. A. Bonner, and B. J. Wilkins, Ion-channeling study of single-crystal YBa2Cu3Ox, Phys. Rev. B 37(4):2297-2300 (1988). 8. R. P. Sharma, L. E. Rehn, P. M. Baldo, and J. Z. Liu, Ion-channeling investigation of thermal vibration amplitudes across the superconducting transition in YBazCu307 8, Phys. Rev. B 38(13):9287-9290 (1988). 9. G. Demortier, F. Bodart, G. Deconninck, G. Terwagne, Z. Gabelica, and E. G. Derouane, Stoichiometric characterization of Y - B a - C u - O superconductors with nuclear probes, Nucl. lnstrum. Meth. B30:491-496 (1988). 10. B. M. Wanklyn, C. Chen, B. E. Watts, P. Haycock, and F. Pratt, The flux growth of YBa2Cu307-~, Solid State Comm. 66(4):441-443 (1988). 11. K. Hayashi, M. Tokumoto, K. Takahashi, Y. Suzuki, K. Murata, H. Ihara, N. Koshizuka, and Y.-i. Kimura, Growth and domain structures of BazYCu307 _~. single crystals. J. J. Appl. Phys. 27:L1646-1649 (1988). 12. Y. C. Kao et al., MBE growth of GaAs on (100)Si, private communication, 1989. Received August 1989; accepted February 1990.