Low Temperature Direct Metal Bonding by Self Assembled Monolayers
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Low Temperature Direct Metal Bonding by Self Assembled Monolayers Xiaofang Ang1, Li Cheong Chin1, Guo Ge Zhang1, Jun Wei2, Zhong Chen1, and Chee Cheong Wong1 1 Division of Materials Technology, School of Materials Science & Engineering, Nanyang Technological University, 50 Nanyang Ave, Academic Complex North N4.1, Singapore, 639798, Singapore 2 Micro-Joining and Substrate Technology, Singapore Institute of Manufacturing Technology, Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore, 638075, Singapore ABSTRACT Elevated bonding temperature for interconnection deteriorates the reliability of both the device and the interconnect; hence the imperative for developing low temperature bonding methods. This study investigates the feasibility of using self-assembled monolayers (SAMs) to assist direct gold-gold (Au-Au) thermocompression bonding. This involves a simple molecular self-assembly process whereby a monolayer of alkyl chains with a sulfur end group is attached to the gold surface prior to bonding. Using this method, we have achieved Au-Au bonding at a bonding temperature below 100∞C, a significant reduction compared to the conventional bonding temperatures of above 150 ∞C. We attribute this temperature reduction to two properties of SAMs - (1) surface passivation of the Au surface that precludes adsorption of surface contaminants, and (2) the easy displacement of SAMs through thermal desorption just before bonding occurs. This SAMs-assisted bonding mechanism is supported by X-ray photoelectron spectroscopy (XPS) and surface plasmon resonance (SPR) results.
INTRODUCTION With exacerbating demands for technological advances, three-dimensional (3-D) integration is essential to achieve highly densified and functionalized integration without incurring a nonlinear rise in wiring length seen in 2D integration. While there are other key factors determining the success of complete implementation in 3D technology over monolithic integration, it is believed that the true bottleneck arises from poor thermal management during the formation of interconnects. During high pressure and temperature bonding, poor thermal dissipation and huge residual thermomechanical stresses damage the chips and limit the progress of 3-D technology. Since 3D integration is so delicate and requires the gentlest possible condition, one way to manage this is to lower the bonding temperature and pressure used in forming the interconnections between the thinned stacking chips. One of the most common bonding methods used is direct metal thermocompression bonding which involves simultaneous application of pressure and temperature between two similar metal surfaces such as gold or copper over a period of a given period of time, usually tenths of seconds. While the method remains advantageous for its strong metallurgical joints formed and superior electrical conductivity, relatively high temperature is still necessary to form such joints due to the presence of an inherent layer of contaminants on exposed surfaces. For bon
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