Au-Sn Transient Liquid Phase bonding for hermetic sealing and getter activation

Results on a promising high temperature bonding process, Au-Sn Transient Liquid Phase bonding, based on interdiffusion in the Au-Sn system, are presented. The bonds formed with this method offer the same excellent properties as Au-Sn (80% wt. Au/20% wt. Sn) eutectic solder like fluxless bonding, oxidation resistance and excellent mechanical strength, while providing solid joints at a temperature well above the first melting temperature of 278°C. Si-caps with thick Au/ eutectic preform or Au/Sn electroplated rings were bonded to standard ceramic packages. The sealed packages were found hermetic (high resolution helium leak-tester). SEM/EDS analysis of the cross-sections showed uniform bonds consisting of Au and an Au-rich phase identified as the ξ'/ξ-phase, with a melting point up to 519°C. The joint, which originally melted at 278°C, will therefore remain solid during getter activation. The sealed packages were then submitted to annealing treatments up to 380°C for 30 min, corresponding to the temperature and duration of a possible getter activation. No major change of the interface was detected with further cross-sections. In a second step, vacuum sensors and getter parts were sealed in the packages with the same bonding process. The subsequent getter activation led to an improvement of the vacuum level by more than a factor of 10.