Interdiffusion in the CuAu thin film system at 25°C to 250°C

Interdiffusion in the Cu-Au system has been studied in the temperature range 25–250°C using conductivity measurements and Auger in-depth composition profiles. Samples were prepared by three different methods: (1) Au was evaporated over evaporated Cu, (2) Au was electroplated over electroplated Cu and (3) Au was electroplated over wrought (bulk) Cu. The conductivity data for both the evaporated and the plated samples could be represented by ΔGG = 0.030{1−exp(−105tk)} − (tk)12 where G is the conductance, t the time and k is an acceleration factor which is a function of temperature and thickness. Interdiffusion coefficients D were obtained from the conductivity data by using the measured acceleration factors. These data can also be used to predict sheet conductance changes for reasonable thicknesses of Cu and Au films. Auger composition profiles were obtained by analysis beginning at the original Au-to-air interface and proceeding through the Au film into the Cu. Three distinct regions were found: (1) Cu oxide growth at the original Au surface, (2) a plateau region of a few atomic per cent Cu in the Au film and (3) a zone around the original Au-Cu interface where the Cu concentration rises to 100%. Interdiffusion coefficients were obtained from the Auger profiles based on the concentration gradient at the Au-Cu interface (region (3)). Interdiffusion coefficients obtained from ΔGG and from the Auger profiles agreed within a factor of 10 for all three methods of specimen preparation. Over the range 50–250°C the interdiffusion coefficient could be represented by D = 2.9 × 10−3 exp(−1.21 eVkT) cm2 s−1, giving 1.3 × 10-19 cm2 s-1 at 100°C and 6.2 × 10-15 cm2 s-1 at 250°C. These values fall within the range of extrapolations of previously published data obtained at higher temperatures. Auger profiles from a Cu ribbon which had been Au plated and stored for 18 years at room temperature prior to analysis were used to obtain an upper limit of 2 × 10-20 cm2 s-1 for the interdiffusion coefficient at 25°C. Subsequent measurements of D on this sample at 150–250°C were in agreement with values obtained for the other two types of CuAu specimens. This agreement leads to the conclusion that the room temperature interdiffusion coefficient in the other two types of samples is also less than 2 × 10-20 cm2 s-1.