Rapid Chemical Vapor Deposition of Yba 2 cu 3 o x Coatings
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RAPID CHEMICAL VAPOR DEPOSITION OF YBa 2 Cu 3 0x COATINGS W.J. M.J. K.R.
Lackey*, W.B. Carter*, D.N. Hill*, E.K. Barefield*, J.A. Hanigofsky*, Shapiro*, T.S. Moss III*, A.J. Green*, D.F. O'Brien*, R.A. Jake** and Efferson**
*Georgia Institute of Technology, American Magnetics,
Inc.,
Atlanta, Georgia 30332
Oak Ridge,
Tennessee 37831
ABSTRACT A rapid, controllable process for CVD of superconducting YBa 2 Cu 3 Ox has been developed. The new process relies on slowly feeding and pneumatically transporting powdered solid reagents directly into the CVD furnace; vaporizers are not used. Deposition rates two orders of magnitude greater than those achievable by reagent sublimation have been obtained using a powder feed mixture of Y, Ba, and Cu tmhd complexes (tmhd is tetramethylheptanedionate). Potentially, the process is applicable to thermally unstable, less volatile, lower cost reagents such as the acetylacetonates.
INTRODUCTION Chemical vapor deposition (CVD) is extensively used commercially for preparation of numerous electronic, optical, tribological, and chemically protective coatings. Compared to other coating processes CVD is often faster, yields higher quality more adherent films, and can be used to coat multiple irregularly shaped substrates. Over a dozen investigators have reported on the CVD of superconducting YBa 2 Cu 3 Ox. Some of these films 2 have very high critical current densities (-105 A/cm ) even in strong magnetic fields[l]. Typical reagents have been metal complexes of various Wdiketonate ligands. These are solids at room temperature which slowly sublime when heated to -i00-300OC; the vapor is then swept into the CVD system by a carrier gas. Without exception, however, the very low vapor pressure of the Y, Ba, and Cu precursor reagents has restricted deposition rates to low values, e.g., 1 /t m/h. Furthermore, the vapor pressures of the precursors are strongly dependent on temperature and are subject to change as a result of thermal-environmental induced degradation during sub-limation. Hence, process control and repeatability are unusually poor. Because of the advantages which CVD offers for large scale commercial processing (e.g., economical large scale fabrication of electronic devices, uniform coating of fiber tows to produce wire, and coating the inside surface of cavities) it is important to develop the process to its full potential.
EXPERIMENTAL PROCEDURE Thin films of YBa 2 Cu3 Ox were deposited using a low pressure, hot walled CVD reactor as shown in Figure 1. The precursors were the fl-diketonate complexes Y(tmhd) 3 , Ba(tmhd) 2 and Cu(tmhd) 2 where tmhd denotes 2,2,6,6-tetramethyl-3,5-heptanedionate, [(CH3 ) 3 CC(O)CHC(O) C(CH 3 ) 3 )). The solid reagents were premixed, ground in air, and screened below 44 /Lm. A modified vibratory feeder was used to slowly feed the powder reagent mixture. The powder was pneumatically transported into the furnace using Argon (99.999Z purity) where it vaporized and then reacted and deposited on Mat. Res. Soc. Symp. Proc. Vol. 169. t 1990 Materials R
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