Different sputtering methods for the deposition of high-Tc superconducting thin films

Vacuum/volume 44/numbers Printed in Great Britain

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Different sputtering methods for the deposition of high-TC superconducting thin films E SteinbeiR,

H Bruchlos,

Ph ysikalisch - Technisches

T Eick, M Manzel, lnstitut,

Helmholtzweg

W Michalke,

T Schiiler

4, 0- 6900 Jena,

and K Steenbeck,

Germany

The deposition of high-T, superconducting thin films by magnetron sputtering of ceramic and alloy targets is strongly influenced by the bombardment of energetic oxygen particles generated and accelerated at the target surface. In this paper we examine different sputtering methods to overcome the disturbing effects of these energetic particles on the composition of the deposited films.-

1. Introduction The intense world-wide research in the field of high-T, superconducting materials has now reached a level appropriate for the first applications of epitaxial thin films in microelectronic and microwave devices. The high film quality necessary for these applications can be achieved by all technically important deposition methods, such as sputtering, evaporation, LPVD and CVD. The main problem of all these technologies is the precise control of the chemical composition during film growth, because the quality of the HTSC thin films very sensitively depends on small deviations of their stoichiometric composition. With respect to economic considerations, the sputtering technique is favoured, because the target composition is usually transferred one-to-one to the film composition even for complex alloys and compounds. A second advantage of the sputtering technique is the possibility of controlling the film growth and the film properties by the interaction with energetic particles. But precisely as in the case of sputtering of HTSC materials, the one-to-one transfer is strongly disturbed by the bombardment of a special type of highly energetic particles (O--ions) generated and accelerated at the target surface’. This oxygen bombardment induces selective desorption processes at the film surface which increase with increasing oxygen partial pressure and also with increasing substrate temperature and decreasing deposition rate due to a superposition of thermally activated processes. Because of the inhomogeneous distribution of the highly energetic oxygen particles, especially in the case of magnetron sources, the film properties generally show an inhomogeneous lateral distribution’. We have investigated three different sputtering methods to overcome the disturbing interactions with energetic particles. 2. In situ deposition of REBaCuO sputtering at high gas pressure

ensures the stability of the perovskite phase during the film growth3,4. In order to suppress residual desorption effects, the high pressure sputtering method is often used in combination with different off-axis arrangements of the substrates, for example, by the application of cylindrical magnetron sources5. A similar effect can be achieved by using standard planar magnetron sources with increased diameter of the erosion zone and a central substrate position, because the disturbing energetic oxygen particles are strongly concentrated around the erosion zone with a direction normal to the target surface (Figure l), so that an extended region around the magnetron axis is not affected by the oxygen particles. Already magnetron sources with a 3 in. target diameter are sufficient for the deposition of high quality REBaCuO thin films on different single crystal substrates, if optimal parameters for target-substrate distance, gas pressure and substrate temperature are chosen. The size of the homogeneously deposited area is several square centimetres as can be seen from the local dependence of the critical temperature illustrated in Figure 2. The quality of the films is characterized by the measurements shown in Figures 3-5.

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thin films by dc-magnetron

The first method is the so-called in situ deposition of REBaCuO thin films by dc-sputtering of ceramic oxide targets at a high total pressure (50 Pa) and a high oxygen partial pressure (10 Pa) onto single crystal substrates at temperatures around 800°C. Up to now the epitaxial films grown by this method have the best superconducting properties. The high total pressure is responsible for the thermalization of all energetic particles while the high oxygen partial pressure

Figure 1. Propagation directions of sputtered particles (--+) and energetic oxygen particles (---) from the erosion zone of a planar magnetron source. 1113

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thin film deposition

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R(T) curves for epitaxial GdBa,Cu,O,

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Although thin films of very high quality can be prepared, the high pressure sputtering method has several serious disadvantages, especially the high substrate temperature and the low deposition rate. Therefore, the application of this sputtering version is difficult for deposition on both sides of large area substrates and for deposition onto temperature sensitive substrates.

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A second principal way to reduce the bombardment effects of energetic oxygen particles is sputtering at low oxygen partial pressure. An additional advantage can be achieved if the sputtering is performed not only at low oxygen pressure but also at low total pressure. This leads to a remarkable increase of the deposition rate, especially ifmetallic alloy targets are used instead of ceramic oxide targets. In this case, the films are growing outside the stability range of the perovskite phase and they have an oxygen deficiency. In order to induce superconducting properties, a separate annealing process at high temperatures (- SOO’C) and cooling in oxygen is necessary. This ex situ technology is very useful for the deposition on the front and rear sides of large area substrates because during the deposition no substrate heating is necessary and the formation process can be performed e.x situ in standard annealing equipment. We have used the ex situ

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Pg (Pa) Figure 6. Variations of the Ba, Ca and Cu content C, for dc sputtered precursor films as a function of the total pressure (discharge power = 200 W, oxygen partial pressure = 0. I Pa).

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of a Tl-HTSC

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technology especially for the successful preparation of HTSC thin films of the Tl-system6. Because of the problems with the very toxic T&O, we have deposited, in a first step, Tl-less precursor films by reactive sputtering of metallic Ba2Ca,Cu3 alloy targets and then added the T1 ex situ in a second diffusion step. The one-to-one transfer of the target composition could be obtained at a high discharge power and low total pressure of the sputtering gas, Figure 6, which translates into a very high deposition rate (500 A min-‘). The high sputtering rate attainable with alloy targets is necessary to overcome residual selective desorption effects. A further condition for the preparation of high quality TlBaCaCuO thin films is a suitable annealing programme for the precursor films, especially the choice of an optimal annealing temperature which determines the Tl content, the morphology and the crystal structure (Figure 7). The best TlBaCaCuO thin films on SrTiO,, MgO and YAG substrates, which we have prepared up to now, have critical temperatures as high as 114 K (Figure 8) and a homogeneous phase composition, as can be concluded from the steep jump in the x(T)-curves (Figure 9). The films generally have a

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Figure 10. Critical current density J, vs magnetic field for a Tl-HTSC film on SrTiO, at T = 53 K. The magnetic field was oriented parallel /I and perpendicular I to the film surface.

pronounced c-axis texture which follows from X-ray diffraction measurements and from the strongly anisotropic dependence of the critical current density of the magnetic field (Figure 10).

composition of the deposited films on the substrate temperature, which is very sensitive to impact-induced thermally activated desorption processes. As shown in Figure 12, no effects could be observed. Further, we have measured the lateral dependence of the element distribution for the two magnetron types (Figure 13). The observed distribution shows a small residual inhomogeneity, but is nearly independent of the oxygen partial pressure. Therefore, we explain this effect as a consequence of the different angular distribution of the sputtered particles, depending on their mass and binding energy, which can be described and fitted for example by the empirical formula of Rosendaal and Sanders8.

4. In situ deposition of textured REBaCuO thin films at low gas pressure Because of the absence of energetic particles during the phase formation in the sputtering variants discussed above, only thermally activated processes determine the film structure and the film quality. Contrary to these methods, the in situ sputtering technique at low gas pressure (1 Pa) reveals that it is possible to also activate the phase formation by the energy content of the sputtered particles. In this technique, the disturbing high-energy oxygen particles must be excluded by an off-axis arrangement’. We have investigated two special magnetron types, illustrated in Figure 11, a roof-like (type 1) and a target facing magnetron

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(type 2). In order to check the absence of disturbing selective desorption effects, we have investigated the dependence of the chemical

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types for the in situ deposition of REBa CuO, films at low gas pressure, a roof like type 1 and a facing target type 2. Denoted are the E and H fields for the two magnetron types.

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Figure 13. Composition

of sputtered YBaCuO films as a function of the distance r, from the magnetron (type 1) axis at a substrate-target distance of 25 mm (target composition: YBa,Cu,OJ.

E Steinbe/J et a/; High-T,

thin film deposition

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1000: T&l / K) Figure 14. Oxygen partial pressure pO, vs temperature diagram for YBaZ Cu,O,. Optimal preparation conditions for the high pressure and the low pressure sputtering methods are denoted by 1 and 2, respectively.

In spite of this residual deviation, we found an extended region where the transfer of the target composition is nearly one-to-one. A further condition for in situ deposition is a suitable choice of oxygen partial pressure and substrate temperature, which is limited in the low pressure region by the phase boundary (Figure 14). By empirical optimization of these parameters it was possible to prepare highly textured YBa,Cu,O, and GdBa,Cu,O, films on SrTiO, and MgO substrates. The best films prepared up to now show a critical temperature up to 87 K (Figure 15) and a good c-axis orientation perpendicular to the film surface, as can be concluded from the X-ray diffraction pattern (Figure 16).

l-

30

35

40

45

Figure 16. X-ray diffraction pattern (CrK,) and rocking curve of the (005) reflex for a c-axis textured GdBa,Cu,O, film on SrTiO, prepared by the in situ sputtering method at low gas pressure.

5. Summary We have shown that the in situ sputtering technique at high gas pressure even with standard planar magnetron sources is a suitable technology for the preparation of REBa,Cu,O, films of high quality. But the high substrate temperatures and the high gas pressure are serious disadvantages, especially with respect to the solution of the problem of large area and rear side deposition of HTSC thin films. Therefore, we believe that the low pressure technology and the ex situ technology, especially using metallic alloy targets, are important alternatives for future technological developments. Acknowledgements Mrs G Bruchlos for microprobe measureof SEM micrographs and Dr L Illgen, alloy targets. This work was supported No. FKZ 13N5926 and FKZ 13N5927.

References

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Teta (de@

The authors may thank ments and preparation ZFW Dresden, for the by BMFT under Grant

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Figure 15. R (T) curve for a GdBa&u,O, film on SrTi03 Ihe in situ sputtering method at low gas pressure (2 Pa).

prepared

by

’ S M Rossnagel and J J Cuomo, AZP Conf Proc, 165, 106 (1988). * W Gawalek, W Michalke, H Bruchlos, T Eick, R Hergt and G Schmidt, Phys Status Solidi (a), 109, 503 (1988). 3 R Bormann and J Nolting, Appl Phys Left, 54,2148 (1989). 4 H U Krebs, M Kehlenbeck, M Steins and V Kupcik, J Appl Phys, 69, 2405 (1991). ‘G Linker et al, J Less-Common Met, 151,357 (1989). ‘M Manzel, H Bruchlos, G Bruchlos, T Eick, E Steinbeig and L Illgen, Phys Status Solidi (a), 128, 175 (1991). ‘T Hirata and M Naoe, J Appi Phys, 67,5047 (1990). “H E Rosendaal and J B Sanders, Proc Symp on Sputtering, p 302, Perchtoldsdorf, Wien (1980).

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