Growth of particle-free YBa2Cu3O7 films by off-axis sputtering

Physica C 203 ( 1992) 235-239 North-Holland

Growth of particle-free YBa2Cu307 films by off-axis sputtering B. Wuyts, Z.X. Gao, S. Libbrecht, M. Maenboudt, E. Osquiguil and Y. Bruynseraede Laboratorium voor Vbste Stof-Fysika en Magnetisme, Katholieke Universiteit Leuven, Celest~nenlaan 200 D, B-3001, Leuven, Belgium Received 30 July 1992 Revised manuscript received 11October 1992

The preparation of good YBasCusO, thin films is often hindered by the production of non-stoichiometric particles which degrade the superconducting properties of the layer. A systematic study of films prepared by off-axis single target magnetron sputtering shows that the presence of copper-rich particles dramatically decreases the critical current density but does not affect the critical temperature. The fonuation of these particles can be avoided by carefully adjusting the position of the substrate with respect to the target. Films without off-stoichiometric particles have excellent superconducting properties.

1. Intrutluction One of the successful techniques used for the deposition of superconducting oxide films is the 90” off-axis magnetron sputtering method using a single stoichiometric target [ 11. By positioning the surface of the substrate at an angle of 90” with respect to the target, the resputtering due to energetic particles moving perpendicular to the target surface is avoided [2,3]. However, as similarly reported for other deposition methods [ 4-101, films prepared with this technique often contain off-stoichiometric particles, easily observable on the film surface [ 11,121. In general, the presence of these particles produces a deterioration of the superconducting properties [ 41, in particular a decrease of the critical current density. Moreover, these inclusions increase the surface roughness of the films to such an extent that the fabrication of highquality multilayers or junction structures becomes very difficult. Therefore, it is of great importance to better understand the origin of these apparently method-independent particles and to improve the existing technologies in order to avoid their formation. The particle formation which is mentioned here should not be confused with the formation of stoichiometric particles in the laser ablation process, caused by non-optimal laser parameters [ 13,141.

Recently, Eidelloth et al. [ lo] reported a polishing technique making use of an aequous HF solution, which enables the removal of particles from the film surface, substantially increasing its smoothness. However, a modification of the deposition technique in order to avoid the particle formation during growth would still be preferable. Using e-beam evaporation, cosputtering or metalorganic chemical vapour deposition (MOCVD), it has been shown that the growth of off-stoichiometric particles in YBazCusO, (YBCO) films is caused by deviations from the 1: 2: 3 composition [ 4,6,8]. Thus, in principle,particle-free films can be obtained by slightly varying the cation composition. However, the single target sputtering method is not flexible for making small composition corrections. Strictly speaking, using the latter technique, the composition can only be modified by replacing the 1: 2 : 3 target by a non-stoichiometric, generally insulating, Y-Ba-Cu-0 target. This additionally requires the availability of RF sputtering equipment. In this paper we will show that,using off-axis single target DC sputtering, particle formation during the growth of YBCO films can be avoided by adjusting the position of the substrate with respect to the plasma. Films prepared at the optimum position have smooth surfaces and excellent superconducting properties. The results indicate that the cation dis-

0921-4534/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

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2. Experimental techniques The YBCO lilms are prepared in a MICROSCIENCE R- 10 1 sputtering system with a base pressure of 3 X 1O-* Torr. A heatable substrate holder can be inserted into the main chamber via a load-lock chamber in which pre- and post- heat treatments in oxygen are possible. The MO heater block temperature (max T= 900°C) is controlled by a thermocouple which fits in a hole in the middle of the block. The substrate temperature Ts is calibrated using a thermocouple in contact with the substrate, the measured values being confirmed by optical pyrometry. Polished Zr(Y)O, (random), MgO (100) and SrTiO, ( 100) are used as substrate materials. The substrate is mounted onto the heater block with silver paste, heated up to 350°C inside the load-lock chamber in order to dry the silver paste, and transferred to the main chamber. The disk-shaped 3.8 cm diameter stoichiometric YBCO targets vertically mounted on an Ion Tech 3 14 sputtering cathode, are home made using a classical solid state reaction routine. The off-axis configuration used for the present study, as well as all relevant dimensions, are shown in fig. 1. During the film deposition the substrate temperature T, = 74O”C, and a mixed Ar/Oz (80/20)

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sputtering gas is used at a total pressure of 400 mTorr. These conditions correspond to a point near the stability line (oxygen content x=6.0) in the oxygen pressure Po,-temperature phase diagram for YBCO [ 15-j. A DC voltage of 120 V and current of 200 mA, are applied, yielding a deposition rate of about 20 A/ min. After deposition the sample holder is transferred to the load-lock chamber, where the film is annealed during 15-30 min at T,=500" C and Po, = 1 atm, in order to allow full oxidation. The thicknesses of the films used for the present study vary between 50 nm and 500 nm. Structural, electrical and magnetic characterization is used to study the as-prepared thin films. For structural investigations, X-ray diffraction measurements in the Bragg-Brentano geometry are performed on a Rigaku 12 kW rotating anode diffractometer making use of Cu Ku radiation. A scanning electron microscope (SEM) equipped with an electron microprobe analyser is used for complementary microstructure and composition investigations. Electrical properties (resistivity, critical temperature, critical current density) are examined in a He flow cryostat using the standard four-probe electrical transport measurement technique. For transport critical current measurements, the films are patterned using photolithography and wet etching (HN03) techniques. Most critical current characterisations, however, are performed by magnetisation measurements using a SQUID (MPMS - Quantum Design ) .

3. Results Figure 2 (a) shows a SEM photograph of the surface of an off-axis sputtered thin film on MgO (film thickness t N 200 nm), where the vertical substrate position z,= 38 mm (see fig. 1) was chosen arbitrarily, but the substrate was kept outside the visible plasma. Clearly many pm-sized particles are present on the surface. Their white colour probably originates from electrical charging due to the electron beam, indicating that the particles are insulating. Using the microprobe analysis facility inside the SEM system, typically a cation composition of 97W Cu, 2% Ba, and 1% Y is obtained when the electron beam (spot size= 50 nm) is located on a particle, from

38mm,f z

SUBSTRATE HOLDER Fig. 1. Schematic representation of the off-axis sputtering con& uration with the appropriate dimensions.

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237

morphologies are obtained by changing the total sputtering pressure P ( 10 mTorrsPI 600 mTorr) or the Oz/Ar ratio (0, concentration between 1% and 50%), sometimes resulting in film surfaces showing pits besides the particles. Keeping the substrate temperature and the sputtering pressure constant, but changing the vertical position of the substrate produces drastic improvements in the surface quality. A SEM picture of a particle-free film grown at a vertical substrate position .z= 23 mm is shown in fig. 2 (b ). Additional analysis using a scanning tunneling microscope (STM - Nanoscope II) with a much higher resolution confirms that, for films grown at the optimal vertical position, no particles are observed on the surface. The same STM analysis revealed that the surfaces of the particle-free films contain a high density of screw dislocations [ 16 1, as is reported by other groups [ 17 1. Figure 3 shows the particle density nr, versus vertical position z (z measures the distance from the target centre as shown in fig. 1 ), obtained for films grown on 5 x 5 mm* MgO substrates and T,- 740°C PO,=80 mTorr and P,=320 mTorr. An optimum in the z-position is observed for 20 mm 5 ZI 30 mm, where no particles are present on the film surface within the resolution of the microscope ( N 100 nm). As claimed by several authors [ 4-6,8], the origin

Fig. 2. SEM pictures of (a) an YBa2Cu907 thin film (prepared at position z= 35.5 mm) containing many pm sized copper oxide particles, and (b) an YBalCuS07 thin film (prepared at position .z=23 mm) where no copper oxide particles can be observed. The marker in both photographs corresponds to a length of 100 pm.

which it may be concluded that they are copper oxide. Similar results are obtained for films grown on other substrates, indicating that the growth of the particles is not caused by a film-substrate interaction. Changing various process parameters but keeping the vertical position z. constant does not improve the surface quality. In the temperature range 600 oC I TsI 750” C, a higher substrate temperature during the film growth corresponds to a lower density of particles but with a higher average size (increases from 0.5 to 2 pm), possibly due to an increased atom mobility. Different surface

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2 (mm) Fig. 3. Plot of the particle density np vs. the vertical position z of the substrate.

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of the &-rich particles is related to an off-stoichiometric composition. The present results indicate that for the off-axis sputtering configuration, the film composition depends delicately on the substrate position. Both Rutherford backscattering spectroscopy and electron microprobe analysis measurements were carried out, yielding composition values near 1: 2 : 3 for all films, without any systematic difference between the films with and without particles. However, the intrinsic error of both techniques on this type of film is at least 1O%,so that small composition changes which could be relevant for the present study cannot be detected. One effect which is known to cause off-stoichiometric films and is position, dependent, is the so called resputtering. This effect cannot, however, explain our results,since in order to improve the film composition it was necessary to move the substrate closer towards the plasma, while exactly the opposite is expected in the case of resputtering. Moreover, a clear dependence of the particle density on the total sputtering pressure was not observed. Therefore, a more plausible explanation might be found in a spatially varying distribution of the Y, Ba and Cu atoms inside the cloud of sputtered particles. The exact 1: 2: 3 composition may then be obtained only in a limited spatial region, leading to a position dependence of the particle density as shown in fig. 3. However, we would like to stress that both the size and the position of the optimum region are not absolutely determined, but may depend among others on the sputtering pressure, the sputtering power or the target characteristics. Experiments with larger targets are currently being carried out. So far, only microstructural arguments have been used to define the optimal substrate position for the thin film growth. At this point it is interesting to relate the superconducting and structural properties with the film microstructure discussed above. Concerning the superconducting properties, it was found that the critical temperature T, (measured resistively) does not depend on the presence of Cu-rich particles. Films prepared at the optimal position (20 mm $ z,< 30 mm), as well as those prepared at a position z= 35.5 mm, have T, (R = 0) values varying between 84 and 89 K, with a transition width 0.5 N AT,= 2 K. On the other hand, the critical current density J, depends strongly on the substrate po-

sition, as shown in fig. 4. A clear maximum in .7,, determined at 5 K using magnetization data, coincides with the minimum in particle density, shown in fig. 3. At the optimal substrate position (z-23 mm), .I, is higher by three orders of magnitude compared with J, of films prepared at positions a few millimeters higher or lower. Transport measurements of J, on patterned films confirm this observation. A similar relation between superconducting properties and microstructure was reported for films prepared by e-beam evaporation [4]. In addition to a maximum in J,, a minimum in the c-axis lattice parameter co, determined by X-ray diffraction (XRD ) , was observed. This variation in co was absent in our films where co= 11.71 fO.O1 A for all films. The structural quality of the c-axis oriented films is also not affected by the particles, since the rocking curve width A\o (005) in all cases lies between 0.3” and 0.4”. In fig. 5, the XRD 8-28 spectrum of the particlecontaining film corresponding to fig. 2 (a) is shown together with the spectrum obtained from the particle-free film corresponding to fig. 2(b). The diffraction lines originating from the 1: 2 : 3 film and from the substrate are indicated. Only a few, very small peaks remain unidentified, but they probably

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2 (mm) Fig. 4. Critical current J, (determined at 5 K from the magnetization loops) vs. the vertical position z of the substrate.

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Acknowledgements This work is supported by the Belgian High Temperature Superconductivity Incentive (M.M., E.O. ) and Concerted Action Programs. B.W. is a research fellow of the Belgian F.K.F.O., Z.X.G. is supported by A.B.O.S., and S.L. is a research fellow of the Belgian I.W.O.N.L.

References 0

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20(“1 Fig 5. XRD 628 spectra for the films corresponding to the SEM picturesinIig.2(a) (lowercurve)andinfg2(b) (uppercurve). The diffraction lines originating from the MgG substrate (“s”) and from the (001) planes of the film are indicated in the lower curve.

do not originate from the particles since they appear with comparable intensities in both films. The apparent transparancy of the Cu-rich particles for Xray diffraction, which is also reported by Chew et al. [ 41, might indicate that they are amorphous in structure. It is worthwhile to remark that the spectra shown in fig. 5 do not contain a-axis diffraction lines. This indicates that the films do not suffer from the presence of a-axis grains or outgrowths which would contribute to the surface roughness. In conclusion, we have shown that Cu-rich particles present on the surface of single target off-axis DC sputtered YBCO thin films do not show up in XRD and have no influence on the critical temperature, but severely influence the critical current density. They can be avoided by carefully adjusting the substrate position with respect to the target. If the particles originate from a slightly off-stoichiometric composition - as is now generally agreed upon - then the present results seem to indicate that the composition inside the cloud of sputtered cations is spatially varying.

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