Interface and reliability analysis of Au passivated Cu-Cu fine pitch thermocompression bonding for 3D IC applications

Bonam, Satish and Panigrahi, Asisa Kumar and Kumar, C Hemanth and Vanjari, Siva Rama Krishna and Singh, Shiv Govind (2019) Interface and reliability analysis of Au passivated Cu-Cu fine pitch thermocompression bonding for 3D IC applications. IEEE Transactions on Components, Packaging and Manufacturing Technology. p. 1. ISSN 2156-3950

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Abstract

Optimally engineered ultra-thin gold (Au) layer as an effective surface passivation for low temperature, low pressure fine pitch Cu-Cu bonding is demonstrated in this work. 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. Additionally, 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 0.3MPa 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 (>200MPa) and a very low and stable specific contact resistance (~1.43 × 10-8 Ω-cm2) at robust conditions, which is significantly better than the values previously reported in literature.

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