The effects of low‐energy plasma bombardment have been quantified for the first time. The plasma effects on thin copper films (∼50 nm) have been studied by exposing these films to magnetically enhanced Ar plasmas. The Cu films deposited by different techniques were exposed to plasmas by systematically varying the exposure time and the rf power. The microstructural changes (grain size) in the films were studied using transmission electron microscopy. Grain growth is observed in thin Cu films when the films are exposed to low‐energy (63–114 eV) Ar plasmas. rf power is shown to have a strong influence on the microstructure of Cu films. The microstructural changes in sputtered and evaporated films are quite significant whereas the plasma bombardment has less effect on chemical vapor deposited films. These changes occur very rapidly and cannot be attributed solely to the thermal effects especially at low rf powers (300–500 W). The effect of temperature, however, becomes significant at 700 W. The initial microstructure of the film also plays a role in grain growth during plasma exposure.