Cu/Cu diffusion bonding is characterised by high electrical and thermal conductivity, as well as the mechanical strength of the interconnects. But despite a number of advantages, Cu oxidises readily upon exposure to air. To break through the adsorbed oxide-layer high temperature and pressure, long bonding time and inert gas atmosphere are required during the bonding process. This paper aims to present the implementation of an organic self-assembled monolayer (SAM) as a temporary protective coating that inhibits Cu oxidation.
Information concerning elemental composition of the Cu surface has been yielded by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy. Two types of substrates (electroplated and sputtered Cu) are prepared for thermocompression bonding in two different ways. In the first case, Cu is cleaned with dilute sulphuric acid to remove native copper oxide. In the second case, passivation with SAM followed the cleaning step with dilute sulphuric acid. Shear strength, fracture surface, microstructure of the received Cu/Cu interconnects are investigated after the bonding procedure.
The XPS method revealed that SAM can retard Cu from oxidation on air for at least 12 h. SAM passivation on the substrates with sputtered Cu appears to have better quality than on the electroplated ones. This derives from the results of the shear strength tests and scanning electron microscopy (SEM) imaging of Cu/Cu interconnects cross sections. SAM passivation improved the bonding quality of the interconnects with sputtered Cu in comparison to the cleaned samples without passivation.
The Cu/Cu bonding procedure was optimised by a novel preparation method using SAMs which enables storage and bonding of Si-dies with Cu microbumps at air conditions while remaining a good-quality interconnect. The passivation revealed to be advantageous for the smooth surfaces. SEM and shear strength tests showed improved bonding quality for the passivated bottom dies with sputtered Cu in comparison to the samples without SAM.
This study was funded by Graduate Academy of TU Dresden (Scholarship Program for the Promotion of Early-Career Female Scientists of TU Dresden (without Faculty of Medicine). The authors would like to acknowledge Fraunhofer Institute for Reliability and Microintegration (IZM), All Silicon System Integration Dresden (ASSID), for the wafer production. Fruitful discussions with Jörg Meyer, Steffen Bickel, Maik Müller, Martin Schubert, Karsten Meyer, Laura Wambera, as well as the support of the stuff of the Institute of Electronic Packaging Technology (IAVT) and the Center of Microtechnical Manufacturing (ZMP) at TU Dresden, are gratefully acknowledged. The authors thankfully acknowledge Markus Franke, René Körbitz, Martin Elstner for providing access to the glove box system, as well as Christian Wenzel, Volker Neumann and other colleagues from the Institute of Semiconductors and Microsystems at TU Dresden for the interest, support and helpful discussions.
Lykova, M., Panchenko, I., Künzelmann, U., Reif, J., Geidel, M., Wolf, M. and Lang, K. (2018), "Characterisation of Cu/Cu bonding using self-assembled monolayer", Soldering & Surface Mount Technology, Vol. 30 No. 2, pp. 106-111. https://doi.org/10.1108/SSMT-10-2017-0033Download as .RIS
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