Optical emission spectroscopic studies on the growth of YBCO thin films by dc-94.92 MHz hybrid plasma sputtering
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Optical emission spectroscopic studies of dc-94.92 MHz hybrid plasma generated in the newly developed magnetron sputtering system were performed during the growth of YBa2Cu3O7_g (YBCO) thin films. All the detectable species showed uniform spatial distribution along the radial direction of the target more than 8 mm above the target surface. High cathode current conditions in the hybrid plasma were found to make the plasma with high ion density. The high ionizing efficiency was concluded to be essential in obtaining an excellent crystalline film. This result is explained by the ion acceleration mechanism through the ion sheath formed near the substrate surface. Oxygen gas introduced into the growing chamber during deposition was found to be responsible for the oxidation of the target surface.
I. INTRODUCTION Recently, a number of techniques have been developed to prepare high quality YBa2Cu3O7_g (YBCO) thin films. High crystallinity of the superconducting films is technologically important as it might enable one to artificially control the properties of the superconductors by means of doping impurities or introducing defects. We have already reported the preparation method of YBCO films1"4 using the 94.92 MHz magnetron sputtering. We have also further developed a new sputtering technique, i.e., dc-94.92 MHz hybrid (called merely "hybrid" hereafter) plasma magnetron sputtering. The obtained a-axis oriented films using this technique have the best crystallinity,5 a full width at half maximum value of 0.027° in the rocking curve through the (200) peak and a ^ m i n of 2.0% estimated from the barium signal behind the surface peak in the Rutherford backscattering channeling measurement. To prevent or to reduce the resputtering effect by energetic oxygen negative ion and to give depositing species a chance for sufficient migration on the substrate surface are important to achieve an excellent crystallinity. The above films have been prepared at high total pressure (typically, several hundred mTorr) in order to eliminate the effect of the resputtering. Even at such high pressures, the hybrid plasma magnetron sputtering technique can produce films with a reasonable deposition rate,6 which depends strongly on the cathode current. Such high pressures, however, result in reduced energy of the depositing species due to many collisions of species present in the plasma. Therefore, achieving the high crystallinity in the films prepared using the hybrid plasma technique means that an adequate energy for the migration is supplied to the depositing species in some way. One possible 1082
http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 5, May 1994
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explanation is the acceleration mechanism of the ions through the ion sheath formed near the substrate surface. Therefore, the information about the ions existing in the plasma, especially near the substrate surface, is very important. One of the most powerful and simple methods to evaluate the ions in the plasma is the optical emission spectroscopic tech
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