Heterogenous Bump Metallurgy Through a Sequential Plating Based Process

A novel, heterogeneous solder bump structure and means to achieve is proposed. By exploiting a sequential plating process for bump fabrication, a diffusion barrier is placed between solder structures comprising different Ag contents. By controlling the nature and shape of the barrier, a specific structure was derived that successfully maintained a low Ag, ductile solder region proximal to the fragile BEOL of the chip during all solder reflow processes that would occur prior to a reinforcing underfill. A subsequent long dwell reflow step was demonstrated to be capable of breaking the barrier to allow Ag diffusion from the high Ag content region, thus creating a homogeneous interconnect structure with sufficiently high Ag content to encourage high electromigration resistance. The derived temporary structure has the added advantage of ensuring a high aspect ratio, pillar-like solder structure that enables a high-density interconnect design without the use of a stiff Cu structure.