Wafer-level Cu-Sn micro-joints with high mechanical strength and low Sn overflow

In this paper, we report wafer-level bonding using solid-liquid inter-diffusion (SLID) processes for fabricating micro-joints Cu-Sn at low temperature (270 degrees C). The evolution of multilayer Cu/ Sn to micro-joint alloys has been characterized by optical microscopy and mechanical die-shear testing. The Cu-Sn joints with line width from 80 to 200 mu m prove to be reliable packaging materials for bonding vacuum micro-cavities with controllable Sn overflow, as well as high mechanical strength (> 70 MPa). A thermodynamic model has been performed to further understand the formation of Cu-Sn intermetallic alloys. There are two important findings for this work: 1) Using a two-step temperature profile may significantly reduce the amount of Sn overflow; 2) for packaging, a bond frame width greater than 80 mu m will result in high yield.