Deposition methods of high- Tc superconductors

Abstract

Depositing high- T c oxide superconductors poses the most demanding challenge for thin film technology. The complexity of the superconductor film formation parallels the complexity of the materials themselves. Making films of these materials requires transporting all the elements in the proper stoichiometry onto the substrates, forming the correct crystal structure and layer stacking sequence, and providing sufficient oxygen to form the superconducting phase. Evaporation of separate metal constituents generally is a low-pressure process. A high oxygen-atom arrival rate at the substrate under this restriction can successfully be provided by oxygen rf-plasmas, microwave-generated atomic oxygen, electron cyclotron resonance (ECR) sources, and ozone-generation techniques. Sputter deposition suffers from negative ion bombardment of the substrate. This can be minimized by geometries where the substrate is held outside the plasma, and by applying high sputtering pressures with attendant short mean free paths. Chemical vapour deposition (CVD) still shows some problems with volatile barium sources. Nevertheless, it represents one of the most promising techniques for large-scale applications. In the pulsed laser deposition (PLD) technique, a high-energy pulse of ultraviolet or visible laser radiation vaporizes and ejects the surface material which impinges on the substrate subsequently. The process runs far off the thermal equilibrium, and therefore produces an extremely good stoichiometric material transfer. Furthermore, parameters can be chosen to allow for a wide dynamic range between molecular beam and heavy sputter conditions. there are practically no restrictions regarding the reactive gas atmosphere in the deposition chamber. Highest quality epitaxial films of the YBa 2Cu 3O 7−δ superconductor made to date have been produced this way. Other approaches include, e.g. thermal spraying, chemical spray pyrolysis, sol-gel spin casting, dipping, and electrodeposition. Nevertheless, traditional vacuum techniques, or the more novel laser deposition processes, are most likely of finding general acceptance for a thin film process.