Research On Grain Control And Grain Boundary Character Distribution Of Ultra High Purity Copper

With the development of ULSI, the chip size trend to be submicron and nanometer, the RC delay and EM are the main factors decreasing the reliability properties of chips. While traditional interconnect material, aluminum and aluminum alloy, can’t meet the development of integrated circuit technology, copper has been the main interconnect material replacing aluminum used in IC manufacturing of 130nm wafer and below because of the lower electrical resistivity and higher electromigration resistance. As a critical source material used to deposit thin film called seed layer in copper process, ultra-high purity copper target (UHPCT) must be strictly required to attain homogeneous film and superior step coverage in high power sputtering, so purity (≥99.9999% 6N), average grain size (≤50nm), homogeneous microstructure and weak texture should be general demanded in target. The impurity of raw ultra-high purity (6N) copper is less than 1ppm, extremely lacking in solvent, grain boundary is metastable for the lack in the pinning effect on the grain boundaries during the deformation. When copper is under applied stress and heating, grain boundary diffusion coefficient increases sharply leading to the acceleration of grain boundary migration, the grain growth becomes rapidly, and abnormal grain growth is beginning. Therefore, the study of ultra high purity copper sputtering target on grain size controlling and suppressing the generation of abnormal grain boundaries by improving grain doundary distribution, has great significance.To control the size of recrystallized grains and suppress abnormal grains, the deformation and annealing process of ultra-high purity copper have been investigated. The evolution of grain size and texture during deforming and annealing were analyzed. Cold rolling copper was annealed at different temperature, and its mechanical property, microstructure and change rule of grain size after annealing has been analyzed to affirm the recrystallization temperature at different deformation and the annealing conditions that recrystallization get into steady state. The change features of texture during deformation and recrystallization also can be achieved by EBSD.Result shows that while annealing 60min with the deforamiton varies from 40 to 90%, the initial recrystallization temperature of cold rolling high purity copper varies from 130℃ to 180℃, which is less than 0.2 copper melting point. Further optimal process shows that the ideal annealing process for ultr high purity sputtering target is annealing at 300℃ after 80% cold rolling, with average grain size less than 20μm can be acquired. The typical rolling textures after deformation is not strong and a large number of low-angle grain boundaries are found. It is mainly caused by a large amount of energy releasing during the multi-pass rolling process, which leads to quickly increasing of the temperature in the billets’ surface, recovery and recrystallization occur in ultra-high purity copper at this temperature. After annealing treatment, the rolling texture and recrystallization texture comes to co-existence. The texture distribution is uniform without strong grain orientation and high-angle boundaries demonstrate in the microstructure. Compared to normal longitudinal rolling, cross rolling is more homogenous and isometric, which means the grain size difference is fine and uniform. As the rolling direction is changing, the grain orientation is evenly distributed in each direction with a weak texture, the anisotropy can be lighten or eliminated in copper.To improve the stability of the grain boundary, the idea and process of grain boundary engineering(GBE) has been used for reference. Especially in the process of strain-recrystallization, the inhomogeneous preprocessed grain was under a multiple rolling and annealing treatment to refine grain. According to the meaning of GBE, the process of strain-recrystallization can increase the content of special grain boundary to increase the stability of the grain boundary, which is conducive to suppress abnormal grains.Experimental results show that, when compared to the conventional deformation and annealing process, the three kinds process of grain boundary engineering used in this study, a significant effect on controlling grain size and improving the stability of the grain boundary. Especially the process of strain-recrystallization, the GBCD majorization was completed under a multiple rolling and annealing treatment, as the whole deformation was divided into several parts. In a single recrystallization process, because of its shorten annealing time, there’s not enough time for recrystallized grain to grow. In the process of strain-recrystallization, homogeneous grain with an average grain size less than 10μm and a standard deviation of 2.8μm can be acquired, while the average grain size of the conventional deformation and annealing process is 15.7μm with a standard deviation of 4.5μm. The grain size is effective refined and the uniformity of the grain size distribution improved. During the processing route of multiple cycles of strain-recrystallization, with the increase in the number of multiple cycles, the content of S3 and coherent twin boundary increased significantly. During iterative GBE processing, the fraction of special boundaries ∑3 varies from 78.9% to 86%,with the fraction of coherent twin boundary in ∑3 varies from 71.2%to 95%. During single-step recrystallization process, since the annealing time is short and the deformation is small, grain can not be fully recrystallized, There remains deformed grains and thecontent of low-angle grain boundaries is reach up to 15.6%.In a small region, the average grain size is 11.3μm with a standard deviation of 2.3μm. During the strain-annealing, since the low annealing temperature and the small deformation, recrystallization driving force is weak, leading to the final grain distribution is extremely nonuniform, the average grain size is 17.5μm with a standard deviation of 4.5μm.