Effect of deposition conditions on cation composition during reactive magnetron sputtering of high-Tc superconductors

Thin Solid Films, 181 (1989) 157 163

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EFFECT OF DEPOSITION CONDITIONS ON CATION COMPOSITION DURING REACTIVE MAGNETRON SPUTTERING OF HIGH-T~ SUPERCONDUCTORS S. I. SHAH E. L du Pont de Nemours & Co., Experimental Station, PO Box 80356, Wilmington, DE 19880-0356

(U.S.A.) (Received March 22, 1989)

Superconducting Bi-Sr-Ca-Cu-O thin films were deposited by reactive magnetron sputtering from a single stoichiometric target on (100)MgO substrates. The effect of deposition conditions on the chemistry and superconducting properties of deposited films have been studied. Films undergo extensive resputtering during deposition which makes control of the chemistry very difficult. The two most important parameters which require close control in order to minimize resputtering are the sputtering pressure and the oxygen partial pressure in the sputtering gas. A low total sputtering pressure and a high oxygen content in the sputtering gas cause the films to be sub-stoichiometric and affect the superconducting properties. Films of correct stoichiometry can only be deposited at relatively higher sputtering pressures and lower oxygen partial pressures. Films under these sputtering conditions were reproducibly grown which have a To(onset)= 115 K, Tc(R = 0) = 87 K, and a critical current density of 5 x l0 s Acm -2 at 4.2 K.

1. INTRODUCTION

Magnetron sputtering has been extensively used for the deposition of high-T~ thin films 1-4. It offers several advantages over other thin film growth techniques in that large area controlled deposition is possible at a very high deposition rate and stoichiometric films can be obtained from a single target provided that the deposition parameters are carefully controlled. There are also some inherent problems associated with the magnetron sputtering of ceramic oxides, e.g. resputtering, secondary electron heating, etc., but all these problems are controllable and stoichiometrically good quality film deposition is possible, as obvious from the number of papers recently published on the deposition of high-T~ Y-Ba-Cu-O (123), Bi-Sr-Cu-O (2212), etc. Nevertheless, reproducibility of the films is guaranteed only when the effect of deposition conditions on the chemistry and structure of the growing film is well understood. We report here, a detailed analysis of the effect on the chemical composition of the films due to variations in deposition parameters, e.g. sputtering atmosphere, sputtering pressure, and substrate bias. We have also investigated the effect of stoichiometric variations due to changes in 0040-6090/89/$3.50

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deposition conditions on the superconducting properties of these films. The stoichiometric variations are explainable by taking into account the preferential sputtering, sticking probabilities, angular distribution of the sputtered species, etc. We have concerned ourselves with the cations only since a post-deposition annealing makes oxygen chemistry controllable at a later stage. The role of oxygen during deposition, however, is important and, therefore, is also considered in our investigation. 2.

EXPERIMENTAL DETAILS

All the deposition experiments were carried out in a r.f. planar magnetron system with a base pressure of 5 x 10-7 Torr. A sputtering atmosphere of argon and oxygen at pressures ranging from 1 to 100 mTorr was used. The oxygen fraction in the gas was varied from 0 to 0.5. A single 5 cm diameter, 0.5 cm thick stoichiometric Bi2SrzCaCu20 x disk was used as a target. Fabrication of the target is described in detail elsewhere 5. The target power density was kept constant at 5 W cm -2. The target was sputter-etched for 6 h before the first film deposition experiment was carried out and for each subsequent deposition at least an hour of sputter cleaning was used prior to the commencement of deposition to assure that the target surface composition had reached steady state. The target-substrate separation ranged between 4 and 10 cm. Films, typically 1 Ixm thick, were grown mainly on (100)MgO substrates, although a few 200-300/~ thick films were also grown in graphite substrates for Rutherford Backscattering Spectrometry (RBS). For deposition rate and composition measurements, substrates were placed along the four orthogonal radial directions on the substrate platform parallel to the target. Substrates were placed immediately underneath the center of the target and up to 10 cm from the center in all four directions. Compositions and deposition rates reported here are an average from the four directions. Films were deposited on water-cooled substrates. As-grown films were, therefore, amorphous and insulating requiring a post-deposition anneal. The annealing procedure is described in detail in our previous paper 5. Film compositions were determined by a J O E L electron beam microprobe and by RBS. Energy dispersive X-ray analysis was used in the microprobe. Pure elemental standards were employed and matrix corrections for X-ray fluorescence, absorption, and atomic number were carried out using the Magic V computer program 6. Reported compositions are accurate to within + 1.0%. RBS was used for complementary composition confirmation. A 2.5 MeV Van de Graaff accelerator was employed. RBS spectra were obtained using 2.0 M e V H e ÷ ions from films deposited on graphite :mbstrates. 3. RESULTS AND DISCUSSION

Figures l(a)-l(d) show the deposition rate at different radial positions underneath the target for A r + 1 0 % O 2 sputtering gas at various sputtering pressures. For a sputtering pressure of 3 mTorr, there are two noticeably different regions (Fig. l(a)). Region I starts from directly underneath the center of the target and extends up to the diameter of the target and region II begins outside the

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diameter of the target. The deposition rate in region I is almost half the maximum deposition rate in region II. This is the resputtered region. The total amount of resputtering decreases as the total sputtering pressure increases (Figs. l(a)-l(d)). Above 10 mTorr the deposition rates in region I approach the deposition rate in region II, as can be seen in Figs. l(c) and l(d). The same effect was observed if the target-substrate distance is increased from 4 cm to about 10 cm, although overall deposition decreases simultaneously. Resputtering has been known to occur during high-T~ thin film deposition because of energetic particle bombardment 7-9. Increasing the pressure or target-substrate distance increases the number of collisions sputtered or reflected energetic particles undergo during the transit from target to the substrate. This effectively reduces the particle energy, thereby reducing the total resputtering effect. Figure 2 is a plot of the variation of cation composition with the radial distance underneath the target for a film grown at 5 mTorr o f A r + 10~o 02 5 cm away from the target. The calcium concentration under the center of the target is almost four times the target calcium concentration and decreases, first gradually and then very rapidly, away from the target center. Calcium concentrations close to that of the target are achieved only outside the diameter of the target. The least abundant element under the target is bismuth but all three, bismuth, strontium and copper, are in sub-stoichiometric proportions. Correct stoichiometry is obtained only beyond the target edge. A bismuth deficiency is expected in the irradiated area because it has the lowest melting point. The irradiated area temperature could become as high as

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200°C during deposition due to energetic particle bombardment and secondary electron impingement as observed by softening of the glass substrates during initial deposition experiments. This substrate temperature is close to the melting point of bismuth (271 °C). Group IB elements exhibit the highest sputter yield, hence a preferential sputtering of copper is also expected. Amongst the two alkaline-earth metals, strontium and calcium, strontium is heavier but has a lower heat of vaporization 1° causing the strontium fraction to be lower than the target composition due to preferential sputtering. Off-stoichiometric deposition can, however, be controlled by either increasing the presure, up to 10 mTorr, and/or by increasing the target-substrate separation. Figure 3 is a plot of the calcium/copper ratio as a function of the total sputtering pressure of Ar+10~oO2 gas. For low pressures, the difference between the stoichiometric ratio (0.5) and the measured ratio is very large, but as the pressure increases the calcium/copper ratio in the films decreases, approaching the value 0.5 at and above 8 mTorr. Higher sputtering pressure does not change the ratio any further. This is again a manifestation of the increase in the number of collisions of the energetic particles with an increase in the gas pressure or target-substrate distance. Preferential sputtering decreases as total resputtering diminishes. The total oxygen concentration in the reactive plasma is also very critical to the cation ratios. Figure 4 shows variation of the calcium/copper ratio with the increase in the fraction of oxygen, f, in the sputtering gas. Stoichiometric calcium/copper ratios can be obtained for oxygen concentrations up to 15~ of the sputtering gas. Above 15~o, the ratio increases to a maximum of 0.75 with f = 0.5. Induced substrate bias changes from slightly positive for f = 0 to - 15 V for f = 0.5 with a simultaneous decrease in the substrate temperature from about 200°C to about 130 °C. The decrease in the substrate temperature can be attributed to the repulsion of secondary electrons as the substrate becomes more and more negatively biased with the increase in f. The increase in calcium/copper ratio suggest that oxygen plays an important role in the erosion or resputtering of the film. The nature of the particle causing resputtering is still not certain. Application of a small negative

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Fig. 4. [Ca]/[Cu] ratio in as-deposited filmsas a function of oxygenfraction,f, in the sputtering gas. substrate bias does not have any significant effect on net deposition rate or the film chemistry. This bias, however, may not have been sufficient to stop substrate impingement if the negative ions are being generated at the target surface thereby feeling the full cathode fall or if they get neutralized during transport to the substrate. Increasing substrate bias complicates the issue as not only effect due to oxygen anions needs to be considered, the effect due to Ar ÷ ions also become significant. The two effects, deceleration of oxygen anions and acceleration of Ar + ions with the increase in the negative bias, become hard to separate. Through control of the deposition parameters, nevertheless, it is possible to minimize the resputtering effect in order to achieve the correct stoichiometry. Deposition of films with compositions very close to the target composition is

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possible without having to move the substrate out of the plasma. Figure 5 is an RBS spectrum of an unannealed film grown directly underneath the target at 10 mTorr sputtering pressure of Ar + 10~ 02. Target substrate separation was 5 cm. Analysis of the spectrum yielded a composition of Bil.9Sr2.oCal.3Cu2AO6. 5. Numbers for calcium concentration may have been affected by the overlap of the argon peak onto the calcium peak. The oxygen concentration is surprisingly high and does not vary with the oxygen partial pressure in the sputtering gas above f = 0.05. However, changing f does affect the T~ in the annealed films. T~(R = 0) decreases from a maximum of 87 K for f up to 0.1 to 60 K for f = 0.5. As f does not affect the oxygen concentration in the film and the films are annealed in a free-flowing oxygen atmosphere after deposition, Tc variation is probably due to the effect of oxygen partial pressure on the cation concentration during deposition. This effect has been illustrated in Fig. 4 and discussed in the last section. This effect, however, again suggests the importance of the role of oxygen in resputtering. Formation of oxygen anions and resputtering due to oxygen anion are known 11,12 and could be a major contributor to the total resputtering. 20O

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4. CONCLUSION The occurrence of resputtering during the sputter deposition of high T¢ thin films has been reported by various groups, although the exact cause is not completely understood. There are several deposition system designs reported to avoid resputtering by energetic particles. Our experiments suggest that it is possible to do conventional planar magnetron deposition without any design alteration provided that the deposition parameters are properly controlled. The most important parameters which need to be considered are the sputtering pressure and the total oxygen content of the sputtering gas. A low-pressure deposition causes severe resputtering during growth resulting in a compositionally unsuitable film. Similarly, a high oxygen content in the sputtering gas causes the fluctuation in stoichiometry, perhaps also due to oxygen-related resputtering, which affects the composition and therefore superconducting properties of the film.

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ACKNOWLEDGMENTS T h e a u t h o r w i s h e s to a c k n o w l e d g e M. A. S u b r a m a n i a n for p r o v i d i n g m a t e r i a l for the target, A. D. M e i n h a l d t a n d B. D o e l e for h e l p in t h i n film d e p o s i t i o n , a n d C. P. S w a n n for R B S analysis o f the films. REFERENCES 1 S.I. Shah and P. F. Carcia, AlP Conf. Proc., 165 (1988) 50. 2 S.I. Shah, C. R. Fincher, M. W. Duch, D. A. Beames, K. M. Unruh and C. P. Swann, Thin Solid Films, 166 (1988) 171. 3 M. Levinson, S. S. P. Shah and N. Naito, Appl. Phys. Lett., 53 (1988) 922. 4 Y. Ichikawa, H. Adachi, K. Setsune, S. Hatta, K. Hirochi and K. Wasa, Appl. Phys. Left., 53 (1988) 919. 5 S.I. Shah, G. A. Jones and M. A. Subramanian, Appl. Phys. Lett., 53 (1988) 429. 6 J. Colby, in G. Mallett, M. Fay and M. M. Mueller (eds.), Advances in X-ray Analysis, Plenum, New York, 1968. 7 S.I. Shah and P. F. Carcia, Appl. Phys. Lett., 51 (1987) 2146. 8 S.M. Rossnagal and J. J. Cuomo, AlP Conf. Proc., 165 (1988) 106. 9 W.Y. Lee, J. Salem, V. Lee, C. T. Rettner, G. Gorman, R. Savoy, V. Deline, T. Huang and D. W. Chung, Thin Solid Films, 166 (1988) 181. 10 H.L. Anderson, Physics Vade Mecum, AlP, New York, 1981, pp. 322-332. 11 R.B. Honig, J. AppL Phys., 29 (1958) 549. 12 J.J. Hanak, Le Vide, 175 (1975) 11.