Refinement of aluminum and the radio-disequilibrium of uranium and thorium isotopes contained in the purified aluminum

In LSI devices, aluminum has widely been used as interconnect material; such aluminum film is usually formed by sputtering. As the film quality reflects the purity of the sputtering target, demands for target having high purity (both metallic and gaseous) and well-controlled microstructure are very severe. To supply LSI manufacturers with high quality aluminum targets, dry refining of raw materials, making ingot under clean vacuum melting, plastic deformation and heat treatment are standard, and the purity level is mainly determined by the results of dry refining process. Among characteristics of highly purified aluminum, low resistivity and low contents of α-emitters are essential in fabricating LSI devices. In this report, we described the radio-disequilibrium enrichment of daughter nuclides in natural α-decay series. Similar enrichment was also found out in high purity fused silica. These findings strongly suggest that the disequilibrium enrichment is not exceptional in modern high purity materials. In order to estimate the soft-error probability, the sophisticated determination of all long life α-emitters in each decay series. (author)