Sintering of Silver Nanoparticles for the Formation of High Temperature Interconnect Joints
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Sintering of Silver Nanoparticles for the Formation of High Temperature Interconnect Joints A. J. Murray1,2, P. Jaroenapibal3, B. Koene4, and S. Evoy1,2 1 Electrical and Computer Engineering, University of Alberta, 2nd Floor ECERF, 9107 - 116th street, Edmonton, Alberta, T6G 2V4, Canada 2 National Research Council Canada, National Institute for Nanotechnology, Edmonton, Alberta, T6G 2V4, Canada 3 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, 19104.6272 4 Luna Innovations, Blacksburg, Virginia, 24060 ABSTRACT We report the development of a metallic colloid sintering process enabling the creation of bonding layers at moderate temperatures ranging from 150 °C to 300 °C, and pressures lower than 5 MPa. This colloidal n-propyl acetate dispersion of Ag nanoparticles, having an average size distribution of 103 nm, was used in sintered interconnect fabrication. Open air sintering of a 10 um thick film resulted in an average resistivity of 0.20·10-6 Ω.m - 0.30·10-6 Ω.m. Film resistances were found to be as low as 0.18·10-6 Ω.m. Independent tests varying either pressure or temperature were correlated to ultimate shear strength and modulus. The analysis of a 1cm2 bond area resulted in a peak shear strength of 5.83 MPa and shear modulus of 346 MPa which occurred following bonding at 300 °C with a pressure of 4.219 MPa. INTRODUCTION Oil field, automotive, aerospace and military instrumentation operates in extreme environments ranging from 200 °C to over 500 °C. High temperature combined with rapid thermal fluctuations inherent to high power semiconductor devices render conventional lead based soldering techniques inadequate. Alternate bonding processes are required to support advances towards such applications. Ag colloids have previously been used as a die attach at temperatures less than 300 °C and pressures of 40 MPa [1]. Synthesis costs of Ag nanoparticles have recently dropped dramatically enabling their widespread application in the microelectronic industry. Colloids have been investigated as a die attach replacement for several years [1-4]. Recent research has shown that metallic colloidal films are a viable solution to high temperature interconnects. Other new applications have combined current ink jet technology with colloidal inks. Fuller et al. [5] have developed a circuit fabrication method and possible die attach solution that avoids harsh wet chemical or vacuum environments. Sintering processes were studied using 50% by weight n-propyl acetate dispersion of an Ag colloid. The colloidal solution exhibited a bimodal size distribution with peaks at 16.2 nm and 103.7 nm. Sintering was performed at temperatures of 150 °C to 300 °C while being subjected to pressures up to 4 MPa. The Ag thin film joint formation was characterized through the combined use of shear delamination tests, conductivity measurements, and SEM characterization. Significant particle sintering was observed when the Ag nanoparticles were annealed at 300 °C, resulting in initial sh
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