Effect of frequency on fatigue crack growth along interface between copper film and silicon substrate

The mechanical crack growth along the interface between a submicron film (Cu) and a substrate (Si) under fatigue is experimentally investigated under two cyclic frequencies of 0.1 Hz and 1 Hz in a laboratory air (45 ± 5% R.H.). A modified four-point bend specimen, which has only one interface crack to facilitate the control of crack growth, is proposed for the tests. The results reveal that the clear interface crack between Cu and Si grows under the cyclic load. The crack growth rate, da/dN, is governed by the stress intensity factor range, ΔK_i, and the sigmoidal relationship consisting of three stages are observed in the da/dN - ΔK_i curve; the threshold, the stable growth and the critical growth. The region of subcritical crack growth is narrow due to the high yield stress of the thin film and the constraint by the substrate. The crack growth rate accelerates as the frequency decreases in the stable growth region. This is due to the effect of stress corrosion by the humidity in air.