Investigation of Pillar–Concave Structure for Low-Temperature Cu–Cu Direct Bonding in 3-D/2.5-D Heterogeneous Integration

With the advantage of high surface-roughness tolerance, the pillar–concave Cu–Cu direct bonding without chemical-mechanical planarization (CMP) is investigated in detail, including the mechanism of thermal compensation, analysis of roughness, bonding strength, and bonding reliability. With the special design of Cu bond structure, excellent bonding results can be achieved under low thermal budget (150 °C for 1 min, atmosphere) with even high roughness of bonding surface. In addition, the pillar–concave scheme applied to fan-out panel-level package (FOPLP) has been demonstrated, showing the high feasibility of this scheme to realize applications in heterogeneous integration.