Cyclic depth-sensing indentation of gold wire

Conventional depth-sending indentation and novel cyclic indentation were performed to study the mechanical properties and strengthening mechanism of pure gold and Ca-doped gold of up to 90 ppm. The system was chosen because it is popular materials for microelectronics interconnection, yet the strengthening mechanism is not well-understood. Conventional depth-sensing indentation showed an increase in hardness and modulus of gold with increasing Ca content of up to 90 ppm, which remained significantly higher after annealing. Cyclic depth sensing indentation, where the specimen is loaded to a specific value, unloaded and immediately reloaded, produced a cyclic loop between unloading and reloading curves. The path of the unloading and reloading curves tell the story of the different response of the dislocations generated under the indentation, in different materials, pure gold and Ca-doped gold at different level. Thus their dislocation movement and deformation behavior could be studied by cyclic depth-sensing indentation method. This paper reports cyclic indentation study performed in calcium doped gold widely used in wire bonding and shows that a few ppm level of calcium could result in dislocations-solute interactions, leading to significant strengthening of the materials. TEM micrographs of the material shows that it consists of elongated grains parallel to the drawing direction about 200 nanometers in diameter and a few micrometers in length. It is concluded that cyclic nanoindentation could be used as an extended technique to extract sensitive material information that are not reflected in the conventional test.