TEM-Based Analysis of Defects Induced by AC Thermomechanical versus Microtensile Deformation in Aluminum Thin Films

摘要

Thin films of sputtered aluminum were deformed by two different experimental techniques. One experiment comprised passing high electrical AC current density through patterned Al interconnect lines deposited on SiO_2/Si substrates. The other consisted of uniaxial mechanical tensile deformation of a 1μm thick by 5μm wide free standing Al line. In the electrical tests approximately 2 x 10~7 W/cm~2 was dissipated at 200 Hz resulting in cyclic Joule heating, which developed a total thermomechanical strain of about 0.3% per cycle. The tension test showed a gauge length fracture strain of only 0.5% but did display ductile chisel point fracture. In both experiments, certain grains exhibited large, > 30°, rotation away from an initial < 111 > normal orientation toward < 001 >, based on electron backscatter diffraction (EBSD) measurements in the scanning electron microscope (SEM). Transmission electron microscopy (TEM) analysis of specimens from both experiments showed an unusually high density of prismatic dislocation loops. In the mechanically-tested samples, a high density of loops was seen in the chisel point fracture zone. In cross sections of highly deformed regions of the electrical test specimens, very high densities, > 10~(15)/cm~3, of small, < 10 nm diameter, prismatic loops were observed. In both cases the presence of a high density of prismatic loops shows that a very high density of vacancies was created in the deformation. On the other hand, in both cases the density of dislocations in the deformed areas was relatively low. These results suggest very high incidence of intersecting dislocations creating jogs and subsequently vacancies before exiting the sample.