Evaluating the Dominant Factor for Electromigration in High Purities Al Film Deposited by Sputtering

A way of testing the electromigration (EM) resistance of different high purities Al based on atomic flux divergence (AFD) method has been proposed in our previous work. With the theoretical analysis, the EM resistance has been clarified to depend on various parameters which are related with the material properties such as diffusion coefficient and grain size. The present research compares the EM resistance of different high purities Al films with the same thickness of 5μm. For these films, the grain size, which is realized as a factor affecting the resistance, was changed compared with the 1μm thick samples, which have been used in the previous work. A narrow section was shaped in the film by introducing a double-L shaped slit by using a focused-ion-beam (FIB) system. The samples so fabricated were positioned on a ceramic heater with constant temperature, and a high current density was achieved in the narrow section to cause EM. After experiencing the same time for current supply and confirming having the same local temperature by using chemical reagents having known melting points, the different purities samples were observed with a magnified field emission scanning electron microscopy (FE-SEM). In addition, the comparison of the total hillock volume appeared in the sample with the time for current supply is achieved for the samples. It is known that for a given time, a smaller hillock volume corresponds to a higher resistance of the material against EM. Finally, according with the analysis by the synthesis of the obtained EM ranking for the 1 and 5μm thick films, the evaluation of the dominant factor, diffusion coefficient, for EM in high purities Al film deposited by sputtering was approached.