Low Temperature Copper-Nanorod Bonding for 3D Integration
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0970-Y04-07
Low Temperature Copper-Nanorod Bonding for 3D Integration Pei-I Wang1, Tansel Karabacak2, Jian Yu3, Hui-Feng Li4, Gopal G. Pethuraja1, Sang Hwui Lee1, Michael Z. Liu1, J.-Q. Lu1, and T.–M. Lu1 1 Center of Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NY, 12180 2 Now with Department of Applied Science, University of Arkansas at Little Rock, Little Rock, AR, 72204 3 Now with IBM, Fishkill, NY, 12524 4 Now with Freescale Semiconductor, Austin, TX, 78729 ABSTRACT Wafer bonding is an emerging technology for fabrication of complex three-dimensional (3D) structures; particularly it enables monolithic wafer-level 3D integration of high performance, multi-function microelectronic systems. For such a 3D integrated circuits, lowtemperature wafer bonding is required to be compatible with the back-end-of-the-line processing conditions. Recently our investigation on surface melting characteristics of copper nanorod arrays showed that the threshold of the morphological changes of the nano-rod arrays occurs at a temperature significantly below the copper bulk melting point. With this unique property of the copper nanorod arrays, wafer bonding using copper nanorod arrays as a bonding intermediate layer was investigated at low temperatures (400 °C and lower). 200 mm Wafers, each with a copper nanorod array layer, were bonded at 200 – 400 °C and with a bonding down-force of 10 kN in a vacuum chamber. Bonding results were evaluated by razor blade test, mechanical grinding and polishing, and cross-section imaging using a focus ion beam/scanning electron microscope (FIB/SEM). The FIB/SEM images show that the copper nanorod arrays fused together accompanying by a grain growth at a bonding temperature of as low as 200 °C. A dense copper bonding layer was achieved at 400 °C where copper grains grew throughout the copper structure and the original bonding interface was eliminated. The sintering of such nanostructures depends not only on their feature size, but also significantly influenced by the bonding pressure. These two factors both contribute to the mass transport in the nanostructure, leading to the formation of a dense bonding layer. INTRODUCTION Wafer bonding is an emerging and promising technology for the manufacturing of complex three-dimensional (3D) structures. Photolithography, which enables the mass production and batch fabrication of Micro-Electro-Mechanical Systems (MEMS) devices, is subject to the limited manufacturability of 3D structures, resulting in a narrow design space and deterioration in the device performance. In wafer bonding, device wafers are patterned individually to create semi-3D structures by microfabrication and, subsequently, bonded together to form complex 3D structures at the wafer level. Wafer bonding started in MEMS as a process that helped form a part of a device and provided a first-level package.1 From MEMS, wafer bonding is now advancing microelectronics2 and optoelectronics.3,4 The continuing push for smaller and faster electronics has led to tremendous advances in th
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