Microstructural and Geometrical Effects on the Deformation Behavior of Sub-micron Scale Nanocrystalline Copper Pillars

The effects of microstructure, sample dimensions, and cross-sectional geometry on the deformation characteristics of electroplated nanocrystalline copper sub-micron pillars are investigated. Nanocrystalline copper pillars were produced with four types of geometry—solid core, hollow, c-shaped, and x-shaped—with outer diameters of ~1000 or 220 nm and three different average grain sizes (between 5.1 and 49.3 nm). Flow stress results from uniaxial compression tests of 1000- and 220-nm-outer-diameter pillars, with average grain sizes in the range between ~32 and 50 nm, revealed there are no observable strength dependences with the pillar cross-sectional geometries. This suggests that they behave with bulk-like character: mechanical properties independent of size and sample geometry. All of the pillar specimens examined exhibit an increase in mechanical strength with reduction of grain sizes, but soften as the crystalline dimensions are smaller than 10 to 20 nm threshold limits. Interestingly, pillars with outer diameters of 220 nm are distinctively softer than the 1000-nm-diameter samples when their grain size is at and below this threshold limit. These results indicate a strength specimen size effect exists for such fine grain copper pillars.