The effect of loading frequency and microstructure on thermal fatigue damage in 200 and 300 nm thick polycrystalline sputtered Cu lines on Si substrates has been investigated. Alternating currents were used to generate temperature cycles (with ranges from 100 to 300 degrees C) and thermal strains (with ranges from 0.14% to 0.42%) in the Cu lines at frequencies of 0.2 and 20 kHz. Fatigue loading caused the development of severe surface roughness that was localized within individual grains. Raising the loading frequency accelerated damage formation and failure. The frequency effect is believed to result from differences in the concentration of defects created by the deformation-induced motion of dislocations to the grain boundaries. (c) 2006 Elsevier B.V. All rights reserved.