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Research Papers

40 μm Copper–Silver Composite Flip-Chip Interconnect Technology Using Solid-State Bonding

[+] Author and Article Information
Chu-Hsuan Sha

Intel Corporation, Hillsboro, OR 97124

Wen P. Lin, Chin C. Lee

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

J. Electron. Packag 134(2), 021004 (Jun 11, 2012) (4 pages) doi:10.1115/1.4006705 History: Received August 05, 2011; Revised February 12, 2012; Published June 11, 2012; Online June 11, 2012

Copper–silver (Cu–Ag) composite flip-chip interconnect between silicon (Si) chips and Cu substrates is demonstrated. Array of Cu–Ag columns, each 28 μm in height and 40 μm in diameter, is electroplated on 2-in. Si wafers coated with chromium (Cr)/gold (Au) dual layer. The Si wafers are diced into 6 mm × 6 mm chips, each containing 50 × 50 Cu–Ag columns. The Si chip with Cu–Ag columns is bonded to Cu substrates at 260 °C in 80 mTorr vacuum. A bonding force of only 1.8 kg is applied, corresponding to 0.71 g per Cu–Ag column. During bonding, Ag atoms in Cu–Ag columns deform and their surfaces conform to and mate with the surface of Cu substrate. Solid-state bonding incurs when Ag atoms in Cu–Ag columns and Cu atoms in Cu substrates are brought within atomic distance so that they share conduction electrons. The Cu–Ag columns are indeed bonded to the Cu. No molten phase is involved in the bonding. The joint consists of 60% Cu section and 40% Ag section. The ductile Ag is able to accommodate the thermal expansion mismatch between Si and Cu. The Cu–Ag joints do not contain any intermetallic compound (IMC). This interconnect technology avoids all reliability issues associated with IMC growth in conventional soldering processes. Compared to tin-based lead-free solder joints, Cu–Ag composite joints have superior electrical and thermal properties.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

Assembly scheme of Si chip with 40 μm Cu–Ag composite flip-chip columns bonded to Cu substrate

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Figure 2

Backscattered SEM images of Cu–Ag columns electroplated on Si/Cr/Au wafers viewed at 45 deg angle: (a) low magnification (×200), (b) medium magnification ( ×1000, and (c) high magnification (×2000). Of the composite columns, the lower portion is Cu and the upper portion is Ag.

Grahic Jump Location
Figure 3

Backscattered SEM images on the cross section of a bonded sample: (a) low magnification (×500) showing Cu–Ag joints in one row of the 50 × 50 array; (b) medium magnification (×2000), exhibiting a single Cu–Ag joint; (c) high magnification (×10,000), displaying the interface between Si and the Cu portion of the Cu/Ag joint; (d) high magnification (×10,000), presenting Cu–Ag interface in the composite joint; and (e) high magnification (×10,000), revealing the bonding interface of Ag on Cu substrate

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