Characteristics of a Novel Compliant Bump for 3-D Stacking With High-Density Inter-Chip Connections

This paper reports on the detailed characteristics of a novel compliant bump for 3-D stacking with high-density inter-chip connections. The novel compliant bump is a cone-shaped bump made of Au. It is fabricated using electroplating of the Au into undercut holes formed in a photoresist. Because the cone bump is easily deformed under a pressing load, it possesses superior properties for inter-chip connection. First, it suppresses a bonding failure by compensating for the bump-height deviation and the nonuniform bonding pressure, and consequently, offers high-density inter-chip connections of which number is at least 30 600 with 20 μm pitch. Second, it reduces the change in the transconductance g_m of the metal-oxide-semiconductor field effect transistor (MOSFET) after chip stacking. In other words, it reduces the strain generation at the Si device level. Third, room-temperature bonding is achieved by mechanical caulking between the cone bumps and the doughnut-shaped electrodes.