Fabrication of wafer-level thermocompression bonds

Thermocompression bonding of gold is a promising technique for achieving low temperature, wafer-level bonding. The fabrication process for wafer bonding at 300°C via compressing gold under 7 MPa of pressure is described in detail. One of the issues encountered in the process development was e-beam source spitting, which resulted in micrometer diameter sized Au on the surfaces, and made bonding difficult. The problem was solved by inserting a tungsten liner to the graphite crucible. Surface segregation of Si on the Au surface at the bonding temperature was observed. Using Auger spectroscopy, a 1500 Å SiO₂ barrier layer was shown to be sufficient in preventing Si from reaching the surface. Lastly, a four-point bend delamination technique was used to quantify the bond toughness. The associated process steps that were required to prepare the test specimens are described. The critical strain energy release rate for the bonds ranged between 22 to 67 J/m² and was not shown to be strongly associated with the gold bond layer thickness in the thickness range studied (0.23 to 1.4 μm).