Stoichiometry control of magnetron sputtered Bi2Sr2Ca1−xYxCu2Oy (0 ⩽ x ⩽ 0.5) thin film, composition spread libraries: Substrate bias and gas density factors

A magnetron sputtering method for the production of thin-film libraries with a spatially varying composition, x, in Bi2Sr2Ca1−xYxCu2Oy (0 ⩽ x ⩽ 0.5) has been developed. Two targets with a composition of Bi2Sr2YCu2O8.5+δ and Bi2Sr2CaCu2O8+δ are co-sputtered with appropriate masks. The target masks produce a linear variation in opposite, but co-linear radial direction, and the rotation speed of the substrate table is sufficient to intimately mix the atoms. EDS/WDS composition studies of the films show a depletion of Sr and Bi that is due to oxygen anion resputtering. The depletion is most pronounced at the centre of the film (i.e. on-axis with the target) and falls off symmetrically to either side of the 75 mm substrate. At either edge of the film the stoichiometry matches the desired ratios. Using a 12 mTorr process gas of argon and oxygen in a 2:1 ratio, the strontium depletion is corrected. The bismuth depletion is eliminated by employing a rotating carbon brush apparatus which supplies a −20 V DC bias to the sample substrate. The negative substrate bias has been used successfully with an increased chamber pressure to eliminate the resputtering effect across the film. The result is a thin film composition spread library with the desired stoichiometry.