Impurity sources and incorporation pathways during sputter deposition of Mg and Al thin films

Minimizing the impurity concentration in magnetron sputter deposited thin films is desired for the exploration of composition – structure – property relations. Thus, the integral impurity incorporation during growth into Mg and Al sputter deposited thin films is studied by systematic variations of the base pressure, the deposition rate, and the working gas purity. The deposition rate and the base pressure, which was measured before each deposition, were varied simultaneously, resulting in a variation of the impurity-to-metal flux ratios of factor > 40. During growth, Ar gas purities of 99.999 and 99.9999 % were employed. Surprisingly, these systematic growth condition variations did not significantly alter the impurity concentration incorporated into Mg and Al thin films. While the modified parameters clearly are relevant for impurity incorporation, there appears to be at least one additional mechanism affecting the impurity incorporation during thin film growth operational. Mass spectrometry data revealed that residual gases desorb from surfaces within the vacuum chamber as soon as they are heated. Based on the measured thin film composition data, it is inferred that the magnitude of thermally desorbed residual gas appears to affect the impurity concentration to a larger extent than the systematic deposition parameter variations studied here. Therefore, it is reasonable to assume that the here investigated impurity incorporation is governed by the incorporation of thermally desorbed residual gases during thin film growth. Future impurity incorporation studies should, in addition to the base pressure before deposition, also quantify the residual gas partial pressures present during deposition.