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Design Innovation

Direct Silver to Copper Bonding Process

[+] Author and Article Information
Pin J. Wang, Jong S. Kim, Chin C. Lee

Department of Electrical Engineering and Computer Science, and Materials and Manufacturing Technology, University of California, Irvine, CA 92697-2660

J. Electron. Packag 130(4), 045001 (Nov 14, 2008) (4 pages) doi:10.1115/1.2993144 History: Received December 18, 2007; Revised March 17, 2008; Published November 14, 2008

A novel process of bonding silver (Ag) foils to copper (Cu) substrates has been developed. This direct bonding method does not use any intermediate layer in between. An important application of this process is electronic packaging where semiconductor device chips are bonded to Cu substrates or Cu electrodes fabricated on substrates. Cu is chosen as the major material for substrates and electrodes due to its high electrical and thermal conductivities, high strength, adequate rigidity, easiness in forging and machining, and low cost. On the other hand, Cu has a large mismatch in the coefficient of thermal expansion with most semiconductors, particularly with silicon. This makes it very difficult to bond large device chips to Cu substrates with a metallic joint. We thus design the Ag-cladded Cu structure to overcome this difficulty. Ag is quite soft and ductile. It can function as a strain buffer between the semiconductor chip and the Cu substrate. Ag also has superior physical properties. It has the highest electrical and thermal conductivities among all metals. In the beginning, we used an electroplating process to produce Ag-cladded Cu substrates. However, it is time consuming and costly to electroplate thick Ag layers. To obtain thick Ag layers (more than 200μm), this new laminating process is developed. The Ag foil is laminated to the Cu substrate directly with a static load of 1000psi at 250°C in a 50mtorr vacuum to suppress oxidation. No bonding medium is used. Scanning electron microscopy images on cross sections of bonded samples exhibit a perfect Ag–Cu bond.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 7

Cross-section SEM image of the joint after the annealing step. Only pure Ag and solid solution phase (Ag) remain. The melting temperature exceeds 695°C

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Figure 6

Cross-section SEM images of a resulting joint that consists of three layers: Ag, Ag2In, and Ag. (a) Low magnification and (b) high magnification.

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Figure 5

Design of the layer structure for producing a high temperature joint using the In–Ag system: (a) before bonding and (b) after fluxless bonding at 205°C

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Figure 4

Cross-section SEM image of sample D laminated at 250°C with a load of 1000psi. The bond is nearly perfect.

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Figure 3

Cross-section SEM image of sample C laminated at 400°C with a load of 1000psi

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Figure 2

Cross-section SEM image of sample B laminated at 400°C with a load of 600psi

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Figure 1

Cross-section SEM image of sample A laminated at 400°C with a load of 60psi

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