Interface and Reliability Analysis of Au-Passivated Cu–Cu Fine-Pitch Thermocompression Bonding for 3-D IC Applications

Abstract

Optimally engineered ultrathin gold (Au) layer as an effective surface passivation for low-temperature, low-pressure fine-pitch Cu–Cu bonding is demonstrated in this paper. The Au passivation layer not only performs the role of protecting the underlying Cu surface from unwanted oxidation but it also helps in reducing the effective surface roughness, which are two major requirements for thermocompression bonding. In addition, Au, being a noble metal, ensures efficient surface passivation of Cu even at elevated temperatures. Furthermore, Au-passivated Cu surfaces show significantly high {111}-oriented surface planes with random grain structures at the bonding interface, which account for enhanced diffusion ability. Herein, an optimized Au passivation layer of 3 nm has resulted in high-quality fine-pitch Cu–Cu thermocompression bonding at 140 °C and 0.3-MPa pressure. The bonded samples have further been subjected to various reliability studies in order to confirm the efficacy of the proposed bonding scheme, along with mathematical modeling to cross check using Fick’s second law of approximation. The bonded samples have attributed to a high bond strength (>200 MPa) and a very low and stable specific contact resistance ( $\sim 1.43 \times 10^{\mathrm {-8}}\Omega $ cm $^{2}$ ) under robust conditions, which is significantly better than the values previously reported in the literature.