Creep-dominated fatigue of freestanding gold thin films studied by bulge testing

Although metallic thin films are widely used in microelectronics, their high-cycle fatigue properties are not wellunderstood. Cyclic testing was conducted on 150-nm thick freestanding gold thin films using nominal stress amplitudes between 50 and 150 MPa at 23-100 degrees C. As expected, lifetime decreases with increasing stress amplitude and temperature. The strain at failure hardly exceeds 1.5%, which is in agreement with previous studies on metallic thin films. The hysteresis stress-strain curves suggest that strain accumulates mostly due to cyclic creep. Based on this observation, we calculated the stress exponent n and activation energy Q - which are typical creep parameters - from the apparent steady-state strain rate observed during cyclic testing. Comparison to preexisting creep bulge tests show that the higher the temperature, the more pronounced the influence of creep on the fatigue behavior. Especially at 100 degrees C, creep mechanisms clearly dominate. At lower temperatures (23 degrees C) the data indicate that creep still plays a role, but intrinsic cycling effects become more pronounced. That leads us to postulate that the fatigue lifetime of gold thin films is closely connected to their creep properties.

Automated summary

The high-cycle fatigue properties of metallic thin films, especially their relation to creep behavior, were investigated. Cyclic testing on gold thin films showed that the lifetime decreases with increasing stress amplitude and temperature. Strain accumulation is mainly attributed to cyclic creep, and creep becomes more pronounced at higher temperatures. The study suggests that the fatigue lifetime of gold thin films is closely connected to their creep properties.