Optimizing Au and In micro-bumping for 3D chip stacking

3D chip stacking technology requires repeated stacking of additional layers without remelting the bumps at lower levels of the stack. This can be achieved by transient liquid phase (TLP) bonding during which all solder is transformed into intermetallic compounds that have higher melting points than the solder itself. In this paper, we study Au/In reaction at different temperatures in order to develop a robust low temperature Au/In TLP bonding process. It is shown that the kinetics of intermetallic compound formation is diffusion-controlled, and the activation energy of Au/In reaction is temperature-dependent: 0.46 and 0.23 eV for temperatures above and below 150degC, respectively. Moreover, a thin Ti layer between Au and In is found to be an effective diffusion barrier at low temperature, while it does not inhibit intermetallic joint formation at elevated temperatures during flip-chip bonding. This allows us to control the intermetallic formation during the distinct stages of TLP bonding process. In addition, a minimal In thickness of 0.5 mum is required in order to enable TLP bonding. Finally, Au/In TLP joints of phi40 to phi60 mum are successfully fabricated at 180degC and with very small solder volume (1 mum thick). The shear strength of our phi40-60 mum joints is in the range of 6-20 MPa, and the electrical connection yield is 100% for the daisy chain containing 1380 bumps with a diameter of 60 mum.