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Low Stress Under Bump Metallizations for Direct Chip Attach

Published online by Cambridge University Press:  10 February 2011

P. Su,
T. M. Korhonen,
S. J. Hong,
M. A. Korhonen and
C. Y. Li
P. Su
Affiliation:
Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, ps44@msc cornell.edu
T. M. Korhonen
Affiliation:
Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, ps44@msc cornell.edu
S. J. Hong
Affiliation:
Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, ps44@msc cornell.edu
M. A. Korhonen
Affiliation:
Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, ps44@msc cornell.edu
C. Y. Li
Affiliation:
Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, ps44@msc cornell.edu
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Abstract

In order to use a flip chip method for bonding the Si chip directly to an organic substrate, compatible under bump metallization (UBM) must be available. Conventional schemes with a copper-based solderable layer are not well compatible with the high-tin solders (such as eutectic Pb-Sn) used with organic substrates. This is due to the rapid reaction between Sn and Cu which depletes the UBM of copper. Ni-based schemes exhibit slower reaction with the solder and have been identified by the semiconductor industry as preferable replacements to Cu-based UBM's. However, Ni-containing metallurgies are often associated with high stresses, which results in poor practical adhesion between the silicon chip and the metallization, leading to interfacial failure during fabrication or service. In this research, several nickel-containing UBM schemes are studied experimentally. Stress measurements are made for each metallization before patterning of UBM pads. An optimal Ni concentration for the UBM is suggested based on the results from this study.

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