TEM-Based Analysis of Defects Induced by AC Thermomechanical versus Microtensile Deformation in Aluminum Thin Films
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TEM-Based Analysis of Defects Induced by AC Thermomechanical versus Microtensile Deformation in Aluminum Thin Films R.H. Geiss, R.R. Keller, D.T. Read and Y.-W. Cheng Materials Reliability Division, National Institute of Standards and Technology 325 Broadway Boulder, CO 80305-3328, USA
ABSTRACT 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 SiO2/Si substrates. The other consisted of uniaxial mechanical tensile deformation of a 1 µm thick by 5 µm wide free standing Al line. In the electrical tests approximately 2 x 107 W/cm2 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 normal orientation toward , 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, > 1015/cm3, 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. INTRODUCTION As we transition into a world of smaller dimensions – the nanoworld – it becomes increasingly more difficult to reliably measure the mechanical properties of materials with nano-dimensions (< 100 nm). New measurement tools are needed in the rapidly growing field of nanomaterials. In particular, information about mechanical properties such as elastic modulus, ultimate tensile strength, fatigue life, maximum strain, adhesion and the relation of defect structures to these properties is critical to successful development of new materials. Such information is also needed to assess integrity or reliability in many applications; for example, multilayer electronic interconnects and solder joints. The difficulty of fabricating complex systems requires the use of predictive modeling in order to achieve cost and time savings. However, modeling can correctly predict system performance only if the property data used as input are accurate at the relevant length scales. Furthermore, in heterogeneous systems it is often the localized
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