Bonding behavior of Cu/CuO thick film on a low-firing ceramic substrate
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Bonding behavior of CuyyCuO thick film on a low-firing ceramic substrate Sang-Jin Lee and Waltraud M. Kriven Department of Materials Science and Engineering, University of Illinois at Urbana – Champaign, Urbana, Illinois 61801
Jeong-Hyun Park Department of Ceramic Engineering, Yonsei University, Seoul, Korea
Young-Soo Yoon Department of Electrical Engineering, University of Minnesota, Minneapolis, Minnesota 55455 (Received 12 July 1996; accepted 18 February 1997)
The adhesion strength between a low-firing substrate consisting of an alumina/glass composite and a copper thick film was affected by the addition of cupric oxide and glass frit to the copper paste in a new co-firing process. An interlayer, 3–4 mm in thickness, was produced in the metal-ceramic interface during the new co-firing process due to the diffusion of copper. At the same time, the adhesion strength was improved by controlling the cupric oxide content. The addition of about 3 wt. % glass frit (softening point 670 ±C, based on the calcium-barium borosilicate glass composition) to the metal paste resulted in highest adhesion strength of 3 kgymm2 with a shift of the debonding site toward the ceramic substrate within the interlayer. The shift of the debonding site could be observed by comparing the ratios of Al2 O3yCu and Ca concentration at the test pad areas on the substrate after debonding. The shift of the debonding site is attributed to the migration of glass frit into the interfacial region. The migration of glass frit occurred easily when the softening point of the glass frit was compatible with the new co-firing process, regardless of how much frit was used.
I. INTRODUCTION
Low-firing ceramic/glass composite substrates with characteristics of low thermal expansion coefficients and low dielectric constants are being studied widely for the purpose of developing new ceramic substrate materials.1–6 The co-firing process of copper, a metal conductor, and the low-firing substrate is also being studied for applications in microelectronic multilayer ceramic packages.7,8 Copper is currently viewed as the most suitable metal conductor for ceramic circuit boards because of its low electrical resistivity and low migration resistance.2,3 However, because of the low co-firing temperature and the necessity to fire in a non-oxygen atmosphere (to prevent copper oxidation during the firing process), there are difficulties in the processing, especially in the binder burn-out process.9–11 This insufficient binder burn-out causes blistering and degradation of the insulating properties of the dielectric layer. Furthermore, the residual carbon lessens sinterability of the dielectrics. Ishida et al.12,13 have studied a new co-firing process lessening the difficulty of binder burn-out by changing the firing atmosphere. Initially, firing was done in an air atmosphere to ease binder burn-out, and then a hydrogen atmosphere was applied to reduce the oxidized copper. Finally the reduced sample was sintered in a J. Mater.
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