Mechanism of the variation of Young's modulus and hardness of gold thin films based on ordering of atomic arrangement

<p>A flip chip packaging structure has been widely used in various electronic devices instead of the wire bonding structure. This structure can stack heterogeneous chips, shorten wire length and increase the number of terminals, which can improve their performance. Electroplated gold thin film has been used as micro bumps, however, mechanical properties of gold thin films have wide variation because of their strong crystallographic anisotropy. Therefore, it is important to clarify the mechanism of the variation of mechanical properties in order to assure the reliability of the micro bumps in electronic devices. In this study, gold thin films electroplated by using non-cyan plating solution which fabricated by various electroplating processes such as current density during the electroplating, annealing temperature were evaluated by XRD (X-ray diffraction) analysis in order to clarify the crystal orientation and crystal quality. Also, nano-indentation test was performed for gold thin films to measure mechanical properties such as Young's modulus and hardness. From these experiments, the relationship between the mechanical properties and the crystal orientation, crystal quality of the electroplated gold thin films was investigated. It was found that Young's modulus of the 5-μm thick electroplated gold thin films varied from 65 GPa to 110 GPa, depending on the crystallographic orientation of the films. Also, the hardness of the films changed by about twice mainly depending on their average grain size. The crystallographic orientation did not change the hardness so much. Therefore, it is very important to control the micro texture such as crystallographic orientation and grain size of the micro bumps to minimize the distribution or variation of their mechanical properties.</p>