Aluminum to copper thermal compression bonding for heterogeneous integration of legacy dielets

Advanced packaging aims to integrate multiple heterogeneous chiplets/dielets at fine (≤ 10 μm) bonding pitches, irrespective of their size, material and function. Metal-to-metal thermal compression bonding (TCB) using copper-copper (Cu), gold-gold (Au), copper-gold interconnects is a versatile technique to reduce die-to-wafer bonding pitches up to 5 μm. However, a scalable metal-to-aluminum (Al) thermal compression bonding process does not currently exist, and all aluminum terminated dielets from the foundry are integrated using conventional solder-based processes, which limits their pitch scaling to 20μm. In this work, we develop an aluminum-to-copper thermal compression bonding process based on a two-step bonding approach, accompanied by hydrofluoric acid and in-situ formic acid vapor treatments to enable solderless Al-Cu bonding. An optimized AlCu bonding process will enable integration of Al-terminated legacy dielets and Cu-terminated advanced dielets on the same Si-based packaging substrate such as the silicon interconnect fabric (Si-IF), eliminating the need for solder bumping for legacy Al-terminated dielets. In this paper, bonding challenges to aluminum, the impact of surface treatment and annealing on Al-Cu interface, and results on shear strength and electrical daisy chain measurements are included. The calculated specific contact resistance of Al-Cu contacts is found to be 0.838 Ω•μm
²
. Furthermore, we have also implemented, for the first time, a heterogeneous assembly consisting of Al-Cu TCB, Cu-Cu TCB and Au-Cu TCB on the same Si-IF silicon substrate.