OPTIMIZATION OF J(C) OF YBCO THIN-FILMS PREPARED BY PHOTO-ASSISTED MOCVD THROUGH STATISTICAL ROBUST DESIGN

Intrinsically, the thin-film preparation technique of metalorganic chemical vapor deposition involves a large number of control variables which are especially critical in work with quaternary or higher high-T-c materials. Thus, effective methods are needed to optimize variable settings in the preparation of high-T-c films. A matrix experimental plan and robust design methodology has been employed for this purpose. The first design phase was based on existing knowledge regarding growth temperature, pressure and annealing temperature for photo-assisted preparation of YBCO thin films on LaAlO3 (100) substrates. A battery of only nine experiments plus three confirmation depositions were then used to optimize the process control parameters of precursor oven temperature, carrier gas (Ar) flow rate, and the flow rates of two oxidizing gases, O-2 and N2O, with respect to critical current density (J(c)) of the films. J(c) (77 K, zero field) values for deposited YBCO films (with thickness in the 0.4-0.6 mu m range) were found to be improved from about 1 to 3 X 10(6) A/cm(2). J(c) for a thinner sample (0.15 mu m) reached 5.0 X 10(6) A/cm(2). It was found that under these processing conditions the most significant process control parameter affecting J(c) is the flow rate of O-2, or more precisely, the oxidizing agent now-rate ratio of O-2 and N2O. Analysis of the variance of the J(c) data indicated that there likely exist interactions among some of the control variables, most obviously between the flow rates of O-2 and N2O.