Pass number dependence of through-thickness microstructure homogeneity in tantalum sheets under the change of strain path

Abstract

Microstructure and crystallographic texture are the key factors that determine the properties of sputtering target used in integrated circuit fabrication. Multi-pass clock-rolling has been used recently to produce Ta sputtering targets. However, the effect of strain per pass, or the number of passes to achieve desired thickness, in clock-rolling on the homogeneity of final microstructure has not been investigated yet. In this study, deformation and recrystallization behaviors of Ta sheets during 8-pass and 16-pass clock-rolling as well as subsequent annealing are investigated. X-ray diffraction reveal that 16-pass sample has relatively homogeneous {111} ⟨uvw⟩ {〈111〉//normal direction (ND)} and {100} ⟨uvw⟩ {〈100〉//ND} fibers along its thickness, whereas the distribution of texture in 8-pass sample is uneven. X-ray line profile analysis show that the distribution of stored energy is also more homogeneous in the 16-pass sample. Electron backscatter diffraction and transmission electron microscopy reveal numerous microshear bands and well-arranged microbands predominantly occurring in {111} deformed matrix. These prevail in the center of the 8-pass sample, whereas grain subdivision within {100} and {111} matrix in the 16-pass sample is more homogeneous through thickness. Average grain orientation spread (GOSaverage) within the {111} matrix in the center of the 8-pass sample is significantly higher than that at the surface and quarter-thickness, indicating enhanced grain subdivision. Schmid factor and Taylor model analysis demonstrate that a greater probability of activation of the primary slip system only in the 8-pass sample compared to larger number of active slip systems in the 16-pass sample. Upon annealing, uniform nucleation combined with a slower grain growth rate results in a finer and more uniform grain size in the 16-pass sample. By contrast, severe gradient in through-thickness recrystallization microstructure forms in the 8-pass sample. This is attributed to higher stored energy and preferential nucleation sites in the center leading to faster recrystallization compared to the surface. Overall, the increase of pass number improves the homogeneity of through-thickness recrystallization microstructure in Ta sheets under the change of strain path.