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The preparation of YBCO thin films by a four ion beam co-deposition system
Journal of the Less-Common
Metals, 151 (1989) 419 - 427
419
THE PREPARATION OF YBCO THIN FILMS BY A FOUR ION BEAM CO-DEPOSITION SYSTEM* S. MICHELt,
J. H. JAMES,
B. DWIR, M. AFFRONTE,
Institute of Micro- and Optoelectronics, Department of Technology, CH-1015, Lausanne (Switzerland) (Received
November
B. KELLETT
and D. PAVUNA
of Physics, Swiss Federal Znstitute
8, 1988)
Summary We present preliminary results for thin films grown with a recently commissioned ion beam co-deposition system designed for in situ growth and etching of YBCO films. Our apparatus consists of a high-vacuum chamber ( lop7 Torr) equipped with four Kaufmann ion beam sources. Three of the sources are directed at targets for ion beam sputter deposition of materials; the fourth ion source is directed at the substrate for substrate precleaning, oxygen ion beam assisted deposition and ion beam etching. We have determined individual sputter deposition rates from Y,O,, BaF, and copper targets as a function of ion beam currents on unheated substrates in argon background, normal background, oxygen background, and oxygen ion assist conditions. These deposition rates were then used to define beam currents for ion beam co-deposition of YBCO films at a rate of 2 A set ‘. Films were studied by X-ray diffraction (XRD), Auger electron spectroscopy (AES), resistivity and Rutherford back-scattering analysis (RBS).
1. Introduction Since the discovery of the high-temperature superconductors, there has been an ever-increasing number of publications on the deposition of thin films by a variety of techniques [l]. The motivation for this research is the widely held belief that the first commercial applications will be in the form of thin films for metallization and opto-microelectronic devices. Although d.c. magnetron sputtering [2] has been used to deposit YBCO films, this technique cannot provide independent control of either ion energy or flux of the
*Paper presented at the Symposium on High Temperature Superconductors - Preparation and Applications, at the E-MRS Fall Meeting, Strasbourg, November 8 - 10, 1988. TPresent address: Commonwealth Scientific Corp., 500 Pendleton St., Alexandria, VA 22314, U.S.A. 0022-5088/89/$3.50
0 Elsevier
Sequoia/Printed
in The Netherlands
420
sputtered material, and, probably more importantly, it is difficult to use non-conducting targets, which limits processing flexibility and greatly complicates in situ oxidation. Consequently, for YBCO films produced by sputtering [3,4] only a few groups have attempted to produce films using an in situ growth technique [5] to avoid a post-deposition annealing stage. In particular there has been, to our knowledge, no attempt to grow YBCO films by using a wholly ion beam co-deposition technique, which has the additional advantage of easily depositing Al,O,, S&N, or InSnO films needed for more ambitious device technology. In this paper we present preliminary results, showing the techniques we used to achieve stoichiometric control of individual yttrium, barium and copper cations by ion beam co-deposition. In addition, we address the problem of oxidation during film growth of single-component films by comparing the effect of inert and oxidizing environments, and oxygen ion assist on film deposition.
2. Ion beam system The system used for this study is shown in Fig. 1. It has been designed to ion beam deposit up to three materials simultaneously, or eight materials sequentially. It consists of a cylindrical vacuum chamber evacuated by a 10 in cryo-pump. The base pressure of the system without bake-out is 2 x lop7 Torr. It is equipped with a quadrupole mass spectrometer for residual gas analysis. The partial pressures of the process gases are maintained by four independently controlled mass flow controllers. Within the
Fig. 1. Commonwealth
ion beam system chamber.
421
Fig. 2. Schematic diagram of a four ion beam co-deposition system, with three sources used for sputter deposition and the fourth used for substrate precleaning and ion assist deposition.
chamber are four Kaufman-type ion beam sources, three multi-sided targetholders for targets of 4 in. diameter, and a substrate-holder which can be rotated and heated to 850 “C. Each ion source has a beam of 3 cm diameter and can produce ions of energy up to 1500 eV with beam currents up to 100 mA. The system is shown in schematic form in Fig. 2. The components are arranged to provide both symmetry and versatility, and to facilitate easy changing of target materials. The three deposition ion sources, equipped with graphite focusing grids, are positioned in a triangular arrangement approximately 15 cm from their corresponding sputter targets. Each target is inclined at 45” to the incident beam. The fourth ion source, equipped with dished defocusing molybdenum grids for precleaning and ion assist, is positioned symmetrically within this triangle and facing the other three ion beam sources. The ion assist source is equipped with an iridium cathode filament and an IrO,-coated anode [6]. The substrate-holder is positioned 26 cm coaxially from the ion assist source and 20 cm from each of the three targets. Three stainless steel shields are inserted between the three targets to minimize cross-contamination of the targets during the codeposition runs. As an additional precaution against any sputtered contaminants, all of the exposed stainless steel surfaces in the vacuum chamber were clad in 0.5 mm thick copper sheeting, although later AES analysis showed no evidence of iron or chrominum contamination.
3. Deposition
of single component
films
As the system was not equipped with deposition monitors/controllers at the time of this study, the individual yttrium, barium and copper cation fluxes were determined from timed deposition rate calibration of the corresponding Y,O,, BaF,, and copper sputter targets. We chose Y,O, as our yttrium source for two reasons: firstly, to provide an additional source of oxygen for the growing YBCO film, and secondly, to avoid the sputter yield variation of pure yttrium with oxygen partial pressure as a result of target oxidation [7]. We
422
chose BaF, as a target rather than the pure metal or BaO because of its stability, lower toxicity and ease of handling [8]. We were also interested in measuring any change in stoichiometry of BaF, with respect to the target during ion beam deposition. Figures 3(a) - 3(c) provide illustrated data on the variation of the deposition rate of the three single-component films with increasing beam current at a constant ion energy of 1000 eV. These tests were all carried out at ambient substrate temperatures and are accurate to _+5 Aminl. To provide information on the effect of the presence of neutral or ionized oxygen the deposition rate calibration was repeated for normal deposition conditions, in oxygen background, and with an oxygen ion assist. The oxygen partial pressure was maintained at 10e4 Torr for both the oxygen background and oxygen ion assist conditions. The ion assist beam current density at 200 eV ion energy, was maintained at 0.10 mA cm-’ (corresponding to an ion dosage of 6.2 x 1014cm-’ s- ‘) as determined by beam profile measurements with a Faraday cup at the substrate. As one would expect deposition rates to be proportional to the beam impinging on the target, the concave upward nature of our curves indicates that as the total beam current increases, the beam is becoming more effectively focused and contained on the target. In comparing the deposition rates of Figs. 3(a) - 3(c) it is evident that copper was the easiest while Y,O, was the most difficult to sputter deposit, as one would expect from their relative bonding energies. The effects we see on the deposition rate of oxygen or argon background can be described as a result of either target or film oxidation. For example, for
(a)
(4 Fig. 3. Deposition rates on (a) copper, (b) BaF, and (c) Y,O, with argon or oxygen background, and oxygen ion assist (a and b only).
the case of copper with oxygen background, the deposition rate is only somewhat lower at intermediate beam currents, suggesting that target oxidation might be occurring. The oxide which forms on the target changes the sputter yield, resulting in lower deposition rates. Although the degree of target oxidation depends on material, as we see in comparing Figs. 3(a) and 3(b) for copper and BaF, respectively, the effect is not as strong if the beam current is sufficiently high to remove material faster than the oxygen adsorption rate. This is shown by comparing copper deposition rates in oxygen and inert backgrounds, for which at higher beam currents the two plots converge. The effect of film oxidation is more evident under oxygen ion assist conditions as the activated oxygen ions are much more reactive than a neutral background gas. The addition of oxygen to the film results in an increased film thickness, reflecting an increased deposition rate. However, this effect can be countered at lower deposition rates by the additional effect of film resputtering. These effects are shown by the convergence of the three plots at lower beam currents in Fig. 3(a). On comparing deposition rates of the three target materials under different conditions we can see that target oxidation appears to be more prevalent for BaF, than for copper. This is apparent from the similarity between the oxygen background and oxygen ion assist, where both curves are below that found with an inert background. Copper, on the other hand, has the highest deposition rates for the oxygen ion assist. Y,O, shows trends similar to those for BaF,. To study film oxidation further, Auger electron spectroscopy (AES) and X-ray diffraction (XRD) were used to verify the composition and phase of each single cation film deposited under its respective condition. Although AES was performed without standards, the measurements could still be expected to provide atomic percentage values to an accuracy of + 20%. Figures 4(a) and 4(b) show composition depth profiles of copper deposited under oxygen background and oxygen ion assist conditions and Figs. 5(a) - 5(c) show composition depth profiles for BaF, deposited under inert and oxygen backgrounds, and in oxygen ion assist conditions respectively. Comparison of Figs. 4(a) and 4(b) shows a significant difference in the copper to oxygen ratio when oxygen is introduced as a neutral or ionized species. This was jointly analysed by XRD, which showed an increased intensity in Cu,O peaks with respect to copper. This occurrence is also evident when comparing the ratio of barium + oxygen in Figs. 5(b) and 5(c). It should be noted that these films are thinner, so determination of the composition is more ambiguous. Figures 5(a) - 5(c) also indicate a barium to fluorine composition ratio of 4:l. This shows that the fluorine has dissociated under ion beam bombardment and has not fully recombined at the substrate. However, XRD did detect some evidence of BaF,. To make the deposition rate information more useful in modelling codeposition, the values must be converted from A min- ’ to equivalent cation flux. This step requires knowledge of the film composition or density.
424
0 (a)
(b)
Fig. 4. AES depth profiles of copper ion beam sputtered films under (a) oxygen ion assist and (b) oxygen background conditions.
Unfortunately, the films deposited during the calibration runs were too thin for accurate determination of film density. From the film compositions evaluated by AES and XRD we determined the major film constituents, which we then used to estimate a film density. For a first approximation we chose Y,O,, BaO, and Cu,O to represent the major constituent for the as-deposited films with oxygen ion assist. From this basis we then calculated a cation flux using the calibrated deposition rate, the atomic density as converted from bulk values, and the cation stoichiometric ratio in the
(4
(b)
Fig. 5. AES depth profiles of BaF, ion beam sputtered oxygen and (c) argon backgrounds of 10P4 Torr.
films under (a) oxygen
ion assist, (b)
425
respective oxide form. With the deposition rate calibration data changed to the form of cation flux as a function of total beam current, we now have an indirect method of predicting the operating conditions necessary to achieve a desired film stoichiometry.
4. Preliminary
co-deposition
Using the calculated cation flux rates of the previous section, we attempted to co-deposit a YBCO film on SrTiO, substrates at ambient temperatures with an oxygen ion beam assist. The as-deposited films, analysed by AES and RBS, had the composition shown in Table 1.
TABLE
1
.Elernent
RBS
AES
Y Ba cu 0
1 2 7.3 7.8
1 3 3 I
As the AES was derived without standards and RBS is based on curve fitting, the difference between the two measurements should not be surprising, and illustrates both the necessity of standards and the desirability of using more than one measurement technique. The principal contaminants were found to be titanium, fluorine and molybdenum. The titanium substrate-holder is believed to be the source of the titanium contamination as this was also detected with other substrates. This source of contamination can be avoided by pre-coating the substrate-holder with YBCO film before each co-deposition run. The as-deposited YBCO films were partially crystalline, insulating and showed no detectable morphology under high-resolution scanning electron microscopy (SEM). Annealing at 950 “C under flowing oxygen induced textured growth, as shown in the XRD spectra of Fig. 6. As Terashima et al. [lo] found, we find that all peaks are of type (OOE),with the c axis aligned normal to the SrTiO, surface. Scanning electron micrographs (see Fig. 7) show a tweed-like microstructure with needles oriented with respect, to the (010)and (001)direction of the SrTiO,. Microprobe analysis indicated that these needles were yttrium deficient, as was most of the film. Sufficient quantities of all three cations were detected at the roundish particles. The film is believed to be tetragonal as it was found to be non-superconducting. We have not re-annealed this specimen to check this hypothesis.
426
Fig. 6. XRD of as-deposited
Fig. 7. Scanning annealing.
electron
and annealed (950 “C in flowing oxygen) thin film showing te xture.
micrograph
of textured tweed-like
microstr ,ucture found after o wwn
5. Conclusions The deposition rates and qualitative analysis of single-component Calms, which are the constituents of YBCO films, have been presented and show the advantages of using oxygen ion beam assist to oxidize films in situ. Based on these calibrations, we have begun to co-deposit oxidized YBCO films in situ; these form textured 123 crystals when annealed at 950 “C. We have also shown evidence that ion beam deposited films from a BaF, target are fluorine deficient with ratios of barium to fluorine on the order of 4:l in the asdeposited films. Continued evaluation of ion beam co-deposition of YBCO is under way, including studies at elevated temperatures.
421
Acknowledgments This work was carried out as part of a collaborative research effort with the Swiss PTT in Bern. We are indebted to Mr. C. Faure for the substrate preparation and for invaluable assistance in operating and maintaining the ion beam system during the calibration runs. We also thank Drs. H. Mathieu and A. Vogel and Mr. N. Xanthopoulos for the AES analysis and useful discussions. Discussions with Prof. F. K. Reinhart and Ms. M. Doss, and contributions of W. Baer and Dr. J. Weber (U. Neuchatel), are also greatly appreciated.
References 1 R. H. Hammond, Thin films aspects of high-Tc superconductors, Phys. Today. 41 (1) (1988) S69 - 570. 2 R. E. Somekh, M. G. Blamire, Z. H. Barber, K. Butler, J. H. James, G. W. Morris, E. J. Tomlinson, A. P. Sehwarzenberger, W. M. Stobbs and J. E. Evetts, High superconducting transition temperatures in sputter-deposited YBaCuO thin films, Nature, 326’ (1987) 857. 3 P. Berdahl and R. B. Buchius, Sputtering of High Temperature Superconductors: A Bibliography, National Technical Information Service, U.S. Department of Commerce. 4 D. Pavuna, W. Baer, H. Berger, V. Gasparov, M. Schmidt, F. Vasey and F. K. Reinhart, Ion beam sputtering and properties of superconducting YBaCuO thin films, Mater. Res. Soe. Symp. Proc., 99 (1988) 681. 5 R. Hammond, Standford University, personal communication, 6 C. R. Guarnieri, K. V. Ramanathan, D. S. Yee and J. J. Cuomo, Improved ion source for use with oxygen ion beams, J. Vuc. Sci. Technol. A, 6 (4) (1988) 2582. 7 A. F. Hebard, R. H. Eick, A. T. Fiory, A. E. White and K. T. Short, Reactive ion beam sputter deposition of high-T, oxide thin films, TMS Proc. 4th Annual 1988 Northeast Regional Meeting, Processing and Applicut~o~ of High-T, Sa~rconductors, The Metallurgical Society, AIME, Warrendale, PA, to be published. 8 P. M. Mankiewich, J. H. Scofield, W. J. Skocpol, R. E. Howard, A. H. Dayem and E. Good, Reproducible technique for fabrication of thin films of high transition temperature superconductors, Appl. Phys. Lett., 51 (21) (1987) 1753. 9 H. R. Kaufman and R. S. Robinson, Operation of Broad-Beam Sources, Commonwealth Scientific Corp., Alexandria, VA, 1967 p. 161. , 10 T. Terashima, K. Iijima, K. Yamamoto, Y. Bando and H. Mazaki, Single crystal YBa,Cu,O, thin films by activated
reactive
evaporation,
Jpn. J. Appl. Phys., 27 (1) (1988) LQl.
Metals, 151 (1989) 419 - 427
419
THE PREPARATION OF YBCO THIN FILMS BY A FOUR ION BEAM CO-DEPOSITION SYSTEM* S. MICHELt,
J. H. JAMES,
B. DWIR, M. AFFRONTE,
Institute of Micro- and Optoelectronics, Department of Technology, CH-1015, Lausanne (Switzerland) (Received
November
B. KELLETT
and D. PAVUNA
of Physics, Swiss Federal Znstitute
8, 1988)
Summary We present preliminary results for thin films grown with a recently commissioned ion beam co-deposition system designed for in situ growth and etching of YBCO films. Our apparatus consists of a high-vacuum chamber ( lop7 Torr) equipped with four Kaufmann ion beam sources. Three of the sources are directed at targets for ion beam sputter deposition of materials; the fourth ion source is directed at the substrate for substrate precleaning, oxygen ion beam assisted deposition and ion beam etching. We have determined individual sputter deposition rates from Y,O,, BaF, and copper targets as a function of ion beam currents on unheated substrates in argon background, normal background, oxygen background, and oxygen ion assist conditions. These deposition rates were then used to define beam currents for ion beam co-deposition of YBCO films at a rate of 2 A set ‘. Films were studied by X-ray diffraction (XRD), Auger electron spectroscopy (AES), resistivity and Rutherford back-scattering analysis (RBS).
1. Introduction Since the discovery of the high-temperature superconductors, there has been an ever-increasing number of publications on the deposition of thin films by a variety of techniques [l]. The motivation for this research is the widely held belief that the first commercial applications will be in the form of thin films for metallization and opto-microelectronic devices. Although d.c. magnetron sputtering [2] has been used to deposit YBCO films, this technique cannot provide independent control of either ion energy or flux of the
*Paper presented at the Symposium on High Temperature Superconductors - Preparation and Applications, at the E-MRS Fall Meeting, Strasbourg, November 8 - 10, 1988. TPresent address: Commonwealth Scientific Corp., 500 Pendleton St., Alexandria, VA 22314, U.S.A. 0022-5088/89/$3.50
0 Elsevier
Sequoia/Printed
in The Netherlands
420
sputtered material, and, probably more importantly, it is difficult to use non-conducting targets, which limits processing flexibility and greatly complicates in situ oxidation. Consequently, for YBCO films produced by sputtering [3,4] only a few groups have attempted to produce films using an in situ growth technique [5] to avoid a post-deposition annealing stage. In particular there has been, to our knowledge, no attempt to grow YBCO films by using a wholly ion beam co-deposition technique, which has the additional advantage of easily depositing Al,O,, S&N, or InSnO films needed for more ambitious device technology. In this paper we present preliminary results, showing the techniques we used to achieve stoichiometric control of individual yttrium, barium and copper cations by ion beam co-deposition. In addition, we address the problem of oxidation during film growth of single-component films by comparing the effect of inert and oxidizing environments, and oxygen ion assist on film deposition.
2. Ion beam system The system used for this study is shown in Fig. 1. It has been designed to ion beam deposit up to three materials simultaneously, or eight materials sequentially. It consists of a cylindrical vacuum chamber evacuated by a 10 in cryo-pump. The base pressure of the system without bake-out is 2 x lop7 Torr. It is equipped with a quadrupole mass spectrometer for residual gas analysis. The partial pressures of the process gases are maintained by four independently controlled mass flow controllers. Within the
Fig. 1. Commonwealth
ion beam system chamber.
421
Fig. 2. Schematic diagram of a four ion beam co-deposition system, with three sources used for sputter deposition and the fourth used for substrate precleaning and ion assist deposition.
chamber are four Kaufman-type ion beam sources, three multi-sided targetholders for targets of 4 in. diameter, and a substrate-holder which can be rotated and heated to 850 “C. Each ion source has a beam of 3 cm diameter and can produce ions of energy up to 1500 eV with beam currents up to 100 mA. The system is shown in schematic form in Fig. 2. The components are arranged to provide both symmetry and versatility, and to facilitate easy changing of target materials. The three deposition ion sources, equipped with graphite focusing grids, are positioned in a triangular arrangement approximately 15 cm from their corresponding sputter targets. Each target is inclined at 45” to the incident beam. The fourth ion source, equipped with dished defocusing molybdenum grids for precleaning and ion assist, is positioned symmetrically within this triangle and facing the other three ion beam sources. The ion assist source is equipped with an iridium cathode filament and an IrO,-coated anode [6]. The substrate-holder is positioned 26 cm coaxially from the ion assist source and 20 cm from each of the three targets. Three stainless steel shields are inserted between the three targets to minimize cross-contamination of the targets during the codeposition runs. As an additional precaution against any sputtered contaminants, all of the exposed stainless steel surfaces in the vacuum chamber were clad in 0.5 mm thick copper sheeting, although later AES analysis showed no evidence of iron or chrominum contamination.
3. Deposition
of single component
films
As the system was not equipped with deposition monitors/controllers at the time of this study, the individual yttrium, barium and copper cation fluxes were determined from timed deposition rate calibration of the corresponding Y,O,, BaF,, and copper sputter targets. We chose Y,O, as our yttrium source for two reasons: firstly, to provide an additional source of oxygen for the growing YBCO film, and secondly, to avoid the sputter yield variation of pure yttrium with oxygen partial pressure as a result of target oxidation [7]. We
422
chose BaF, as a target rather than the pure metal or BaO because of its stability, lower toxicity and ease of handling [8]. We were also interested in measuring any change in stoichiometry of BaF, with respect to the target during ion beam deposition. Figures 3(a) - 3(c) provide illustrated data on the variation of the deposition rate of the three single-component films with increasing beam current at a constant ion energy of 1000 eV. These tests were all carried out at ambient substrate temperatures and are accurate to _+5 Aminl. To provide information on the effect of the presence of neutral or ionized oxygen the deposition rate calibration was repeated for normal deposition conditions, in oxygen background, and with an oxygen ion assist. The oxygen partial pressure was maintained at 10e4 Torr for both the oxygen background and oxygen ion assist conditions. The ion assist beam current density at 200 eV ion energy, was maintained at 0.10 mA cm-’ (corresponding to an ion dosage of 6.2 x 1014cm-’ s- ‘) as determined by beam profile measurements with a Faraday cup at the substrate. As one would expect deposition rates to be proportional to the beam impinging on the target, the concave upward nature of our curves indicates that as the total beam current increases, the beam is becoming more effectively focused and contained on the target. In comparing the deposition rates of Figs. 3(a) - 3(c) it is evident that copper was the easiest while Y,O, was the most difficult to sputter deposit, as one would expect from their relative bonding energies. The effects we see on the deposition rate of oxygen or argon background can be described as a result of either target or film oxidation. For example, for
(a)
(4 Fig. 3. Deposition rates on (a) copper, (b) BaF, and (c) Y,O, with argon or oxygen background, and oxygen ion assist (a and b only).
the case of copper with oxygen background, the deposition rate is only somewhat lower at intermediate beam currents, suggesting that target oxidation might be occurring. The oxide which forms on the target changes the sputter yield, resulting in lower deposition rates. Although the degree of target oxidation depends on material, as we see in comparing Figs. 3(a) and 3(b) for copper and BaF, respectively, the effect is not as strong if the beam current is sufficiently high to remove material faster than the oxygen adsorption rate. This is shown by comparing copper deposition rates in oxygen and inert backgrounds, for which at higher beam currents the two plots converge. The effect of film oxidation is more evident under oxygen ion assist conditions as the activated oxygen ions are much more reactive than a neutral background gas. The addition of oxygen to the film results in an increased film thickness, reflecting an increased deposition rate. However, this effect can be countered at lower deposition rates by the additional effect of film resputtering. These effects are shown by the convergence of the three plots at lower beam currents in Fig. 3(a). On comparing deposition rates of the three target materials under different conditions we can see that target oxidation appears to be more prevalent for BaF, than for copper. This is apparent from the similarity between the oxygen background and oxygen ion assist, where both curves are below that found with an inert background. Copper, on the other hand, has the highest deposition rates for the oxygen ion assist. Y,O, shows trends similar to those for BaF,. To study film oxidation further, Auger electron spectroscopy (AES) and X-ray diffraction (XRD) were used to verify the composition and phase of each single cation film deposited under its respective condition. Although AES was performed without standards, the measurements could still be expected to provide atomic percentage values to an accuracy of + 20%. Figures 4(a) and 4(b) show composition depth profiles of copper deposited under oxygen background and oxygen ion assist conditions and Figs. 5(a) - 5(c) show composition depth profiles for BaF, deposited under inert and oxygen backgrounds, and in oxygen ion assist conditions respectively. Comparison of Figs. 4(a) and 4(b) shows a significant difference in the copper to oxygen ratio when oxygen is introduced as a neutral or ionized species. This was jointly analysed by XRD, which showed an increased intensity in Cu,O peaks with respect to copper. This occurrence is also evident when comparing the ratio of barium + oxygen in Figs. 5(b) and 5(c). It should be noted that these films are thinner, so determination of the composition is more ambiguous. Figures 5(a) - 5(c) also indicate a barium to fluorine composition ratio of 4:l. This shows that the fluorine has dissociated under ion beam bombardment and has not fully recombined at the substrate. However, XRD did detect some evidence of BaF,. To make the deposition rate information more useful in modelling codeposition, the values must be converted from A min- ’ to equivalent cation flux. This step requires knowledge of the film composition or density.
424
0 (a)
(b)
Fig. 4. AES depth profiles of copper ion beam sputtered films under (a) oxygen ion assist and (b) oxygen background conditions.
Unfortunately, the films deposited during the calibration runs were too thin for accurate determination of film density. From the film compositions evaluated by AES and XRD we determined the major film constituents, which we then used to estimate a film density. For a first approximation we chose Y,O,, BaO, and Cu,O to represent the major constituent for the as-deposited films with oxygen ion assist. From this basis we then calculated a cation flux using the calibrated deposition rate, the atomic density as converted from bulk values, and the cation stoichiometric ratio in the
(4
(b)
Fig. 5. AES depth profiles of BaF, ion beam sputtered oxygen and (c) argon backgrounds of 10P4 Torr.
films under (a) oxygen
ion assist, (b)
425
respective oxide form. With the deposition rate calibration data changed to the form of cation flux as a function of total beam current, we now have an indirect method of predicting the operating conditions necessary to achieve a desired film stoichiometry.
4. Preliminary
co-deposition
Using the calculated cation flux rates of the previous section, we attempted to co-deposit a YBCO film on SrTiO, substrates at ambient temperatures with an oxygen ion beam assist. The as-deposited films, analysed by AES and RBS, had the composition shown in Table 1.
TABLE
1
.Elernent
RBS
AES
Y Ba cu 0
1 2 7.3 7.8
1 3 3 I
As the AES was derived without standards and RBS is based on curve fitting, the difference between the two measurements should not be surprising, and illustrates both the necessity of standards and the desirability of using more than one measurement technique. The principal contaminants were found to be titanium, fluorine and molybdenum. The titanium substrate-holder is believed to be the source of the titanium contamination as this was also detected with other substrates. This source of contamination can be avoided by pre-coating the substrate-holder with YBCO film before each co-deposition run. The as-deposited YBCO films were partially crystalline, insulating and showed no detectable morphology under high-resolution scanning electron microscopy (SEM). Annealing at 950 “C under flowing oxygen induced textured growth, as shown in the XRD spectra of Fig. 6. As Terashima et al. [lo] found, we find that all peaks are of type (OOE),with the c axis aligned normal to the SrTiO, surface. Scanning electron micrographs (see Fig. 7) show a tweed-like microstructure with needles oriented with respect, to the (010)and (001)direction of the SrTiO,. Microprobe analysis indicated that these needles were yttrium deficient, as was most of the film. Sufficient quantities of all three cations were detected at the roundish particles. The film is believed to be tetragonal as it was found to be non-superconducting. We have not re-annealed this specimen to check this hypothesis.
426
Fig. 6. XRD of as-deposited
Fig. 7. Scanning annealing.
electron
and annealed (950 “C in flowing oxygen) thin film showing te xture.
micrograph
of textured tweed-like
microstr ,ucture found after o wwn
5. Conclusions The deposition rates and qualitative analysis of single-component Calms, which are the constituents of YBCO films, have been presented and show the advantages of using oxygen ion beam assist to oxidize films in situ. Based on these calibrations, we have begun to co-deposit oxidized YBCO films in situ; these form textured 123 crystals when annealed at 950 “C. We have also shown evidence that ion beam deposited films from a BaF, target are fluorine deficient with ratios of barium to fluorine on the order of 4:l in the asdeposited films. Continued evaluation of ion beam co-deposition of YBCO is under way, including studies at elevated temperatures.
421
Acknowledgments This work was carried out as part of a collaborative research effort with the Swiss PTT in Bern. We are indebted to Mr. C. Faure for the substrate preparation and for invaluable assistance in operating and maintaining the ion beam system during the calibration runs. We also thank Drs. H. Mathieu and A. Vogel and Mr. N. Xanthopoulos for the AES analysis and useful discussions. Discussions with Prof. F. K. Reinhart and Ms. M. Doss, and contributions of W. Baer and Dr. J. Weber (U. Neuchatel), are also greatly appreciated.
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