Thin gold to gold bonding for flip chip applications

We have demonstrated a solderless flip chip bonding process that utilizes electroless nickel/palladium, immersion gold pad metallization. This mask-less process enables higher interconnect densities than can be achieved with standard solder bump reflow. The thin (100nm) immersion gold surfaces were coated with dodecanethiol self-assembled monolayers. Strong gold to gold bonds were formed at 185°C with shear strengths that exceed Mil-Std 883 requirements. Gold stud bumps are also promising for flip chip applications, and can be bonded at 150°C when the gold surfaces are properly pre-treated - dilute piranha solution, argon plasma, and dodecanethiol SAM treatments work equally well.