Affiliations: Department of Nanomechanics, Graduate School of Engineering, Tohoku University, Sendai, Japan | Fuji Electric Holdings, Tokyo, Japan
Note:  Address for correspondence: S. Ri, Department of Nanomechanics, Graduate School of Engineering, Tohoku University, Aoba 6-6-01, Aramaki, Aoba-ku, Sendai 980-8579, Japan. E -mail: firstname.lastname@example.org.
Abstract: In modern industrial electronic devices, aluminum (Al) thin films experience repetitive ON/OFF currents and sharp temperature fluctuations during use, which is equivalent to thermal cycling caused by Joule heating. In this research, the thermal fatigues of high-purity Al thin films with a polycrystalline structure that were exposed to high-cycle thermal testing were studied. Two types of thermal cycle testing were performed in which the total holding times at the high temperature were the same. One was high-cycle testing, i.e., a 12-second thermal cycle moving from 70 to 220°C and repeated for 10,000 cycles. The other was ultra-low-cycle or creep testing, i.e., holding the sample at 220°C for 20,000 s. The experimental results show that thermal fatigue is quite different than creep damage. The number of hillocks and voids formed by high-cycle testing was much larger than that formed by ultra-low-cycle testing, and the deterioration of the surface appears suddenly. Thermal cycling accelerated damage growth in Al thin films. The process of surface deterioration in our experiments can be divided into three stages: (I) incubation stage, (II) development stage and (III) equilibrium stage. Within 10,000 thermal testing cycles, the dominant deformation mechanism changed from dislocation glide to grain-boundary sliding and atomic diffusion.