The Effect of Target Purities on Grain Growth in Sputtered Copper Thin Films

Grain growth rates in sputtered Cu thin films were found to be influenced by impurity levels of the sputtering targets. The Cu thin films with thickness of 100 nm or 1mm were deposited on the rigid substrates by a sputter-deposition technique using the Cu target with purity of 99.99% (4N) or 99.9999% (6N), then subsequently annealed at room temperatures. The microstructures of the Cu films were analyzed by scanning-ion microscopy and the sheet resistivities was measured by a four-point probe method. Significant grain growth and reduction of the electrical resistivities was observed during room-temperature storage in these sputtered Cu films. For the Cu films with a thickness of 1 mm, the grain growth rates of the Cu films were not influenced by the impurity levels of the targets. However, for the films with a thickness of 100 nm, the rate of the grain growth in the 6N-Cu films was found to be slower than that in the 4N-Cu films. This was contradictory to the grain growth mechanism of bulk Cu. The grain growth rates of the Cu films at room temperature, which were strongly influenced by the existence of a small amount of impurities in the Cu films, were well explained by the difference of the strain relaxation mechanisms in the films.