Novel three-layer stacking process with face-to-back CoW 6 μm-pitch hybrid bonding

We developed an innovative chip-on-wafer-on-wafer (CoWoW) process, involving a three-layer vertically stacked structure comprising face-to-back (F2B) chip-on-wafer (CoW) and face-to-face (F2F) wafer-on-wafer (WoW) using 6 μm-pitch Cu-Cu connections. We controlled the bowing of the top chip (17 × 24 mm, t = 0.15 mm) to achieve void-free CoW bonding. Moreover, we simulated the bonding strength of CoW using the elastic strain energy. Consequently, we obtained excellent 6 μm-pitch Cu-Cu connections of F2B CoW both at the center and the edge of the chip, as well as F2F WoW. Additionally, the 6 μm-pitch Cu-Cu connections using CoWoW exhibited high reliability in the stress-induced voiding and electromigration tests. These results demonstrated that a successful electrical connection through three layers could be achieved, proving that a process for three-layer stacked 3D heterogeneous integration could be established.