The Formation of Cu 3 Si from Cu/a-Si Multilayers
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ABSTRACT The kinetics of the formation of Cu 3 Si in Cu/a-Si diffusion couples have been investigated by means of differential scanning calorimetry and x-ray diffraction. Multilayered composites of average stoichiometry Cu 3Si were prepared by sputter deposition with individual layer thicknesses varying in different samples between 2 and 100 nm. We observed diffusion limited growth of Cu 3 Si upon annealing these diffusion couples below 500 K. Reaction constants were measured for a temperature range of 455 to 495 K for thicknesses of growing CuSi between 2.6 and 80 nm. The temperature dependence of the reaction constant, k2, was characterized as k 2 =k. exp(- E. 1k, T) with activation energy, E. = 1.0 eV/atom and pre-factor, k. = 1.9x10"3 cm 2/s.
INTRODUCTION Significant effort has been focused on the study of the formation of metal silicides by solid state reaction, generally for metals on crystalline Si [1-12]. Reaction sequences and the kinetics of formation of intermetallic compounds have been characterized by a number of different structural analysis techniques, including x-ray diffraction, Rutherford backscattering and electron microscopy. These are often coupled with other probes, such as measurements of electrical resistivity. In many cases, it is observed at lower temperatures that the most metal rich silicide forms first, while kinetic constraints inhibit the formation of other compounds until higher temperatures or longer times. In some cases, such kinetic constraints initially inhibit the growth of all equilibrium compounds, and an amorphous silicide is observed to grow first. A number of researchers have examined the interdiffusion in Cu/Si diffusion couples. While presently there is a great deal of interest in Cu as an interconnect material, such an application requires the use of a diffusion barrier. It is generally reported that Cu 3Si forms at or near room temperature [112]. In long term anneals in air, Cu3 Si has been observed to be unstable with respect to formation of SiO2 and Cu [2,13]. Efforts have been made to quantify the thermal stability of Cu/Si structures. Measurements of reaction constants for the formation of Cu 3 Si have previously been conducted at temperatures of greater than 475 K, and length scales of 1 jim or greater [5-10]. We have examined the formation of Cu 3Si in multilayered composites of Cu and amorphous Si at temperatures between room temperature and 500 K, and length scales between 2.6 and 80 nm. Using differential scanning calorimetry, the rate of reaction in these planar diffusion couples is directly monitored by measuring the rate of heat flow from the sample as a function of temperature and time. Structural changes are determined using x-ray diffraction analysis of samples cooled to room temperature, with some use of transmission electron microscopy.
EXPERIMENTAL Multilayer Cu/a-Si thin film diffusion couples were prepared by sequential deposition of the two materials by DC (for Cu) and RF (for a-Si) sputtering. The system used for sample preparation was allowed to
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