Copper to gold thermal compression bonding in heterogenous wafer-scale systems

Abstract

Direct metal-metal thermocompression bonding (TCB) process is necessary to scale the bump pitches to $\leq 10\mu\mathrm{m}$ as previously demonstrated using Copper(Cu) to Copper TCB on Silicon Interconnect Fabric (Si-IF). However, to extend the bonding process to a wafer level assembly of dielets, passivation of exposed Cu is necessary throughout the bonding process. In this work, we explore Copper-to-Gold TCB process to address the problem of Cu oxidation on Si-IF substrate prior to bonding. Any exposed copper the Si-IF is capped with Titanium (Ti) and Gold (Au) of thickness 20 nm and 200 nm respectively. This gold coating prevents any oxidation of copper on the substrate irrespective of the TCB temperature. First, the Cu-Au phase diagram was analysed to get an estimate of the parameters necessary for diffusion between Au and Cu during TCB. Next, bonding tests were performed, in which $2\mathrm{X}2 \text{mm}^{2}$ dielets with copper pads were bonded to blanket Au capped Cu samples. We found Cu to Au thermocompression bonding to be feasible, even when the Si-IF substrate was exposed to high temperatures (≈100°C) for several hours. With optimization of bonding force and time, the shear force required to remove die from the substrate was found to be ≈ 110 N. In addition, the reliability of the bonded structures were investigated through high temperature storage testing. Post high temperature storage at 125 °C for two weeks, the shear strength was found to vary within 10% of its pre-storage value. Using the optimized process, daisy chain dielets terminated with Cu pads were bonded to Au capped Cu pillars on the Si-IF. We observed 100% connectivity across all the testable daisy chains. The average specific contact resistance of each pillar was found to be $0.762 \Omega-\mu\mathrm{m}^{2}$ which is close to that of pure Cu-Cu TCB based daisy chains. The gold capping eliminates the problem of Cu oxidation on Si-IF substrate and therefore this bonding mechanism can be extended to achieve full wafer dielet population even on large 300 mm Si-IF substrates.