Characterization of interfacial morphology of low temperature, low pressure Au-Au thermocompression bonding

Au-Au thermocompression bonding is a versatile technique of high interest for a variety of applications. We have investigated Au-Au bonding using sputter deposited Au films under conditions of low temperature (150-250 °C) and low bonding pressure (∼3 MPa) for short times (15 min). The combination of low temperature and short times is important for applications involving both hermetic sealing and packaging scaling. The initial surface roughness of the Au film was in the 3-5 nm range with peak-to-valley heights of 20-30 nm and a lateral correlation length of ∼400 nm. For samples bonded at 150 °C, the void morphology at the bonded interface was related to the initial surface roughness. The void morphology was different when bonding at the higher temperatures: the void length (along the bonded interface) decreased significantly but the void height (perpendicular to the interface) increased. These results can be understood in terms of a combination of increased surface Au diffusivity and decreased yield stress and elastic modulus with increased bonding temperature.