Quantitative Measurements of Subcritical Debonding of Cu Films from Glass Substrates
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QUANTITATIVE MEASUREMENTS OF SUBCRITICAL DEBONDING OF CU FILMS FROM GLASS SUBSTRATES M. Pang and S.P. Baker Cornell University, Department of Materials Science and Engineering Bard Hall, Ithaca, NY, 14853 ABSTRACT Subcritical debonding of Cu films from silicate glass substrates has been studied using a driver film method. In this method, a highly stressed Cr overlayer is imposed to debond the target Cu film from the substrate. Substrates included a commercial flat panel display glass and glasses having composition (2SiO2)x(CaO·Al2O3)1-x. A range of strain energy densities for driving interfacial crack growth was provided by depositing Cr driver layers with varying thickness. The film debond velocity was obtained experimentally. The stresses in Cu and Cr were measured and used to obtain the strain energy release rate. The effect of oxygen concentration and substrate composition on subcritical debonding was studied. A kinetic crack model previously developed for bulk ceramics was modified and applied. Differences in adhesion could be attributed to differences in density of strong Cu-O bonds across the interface. INTRODUCTION Increasing demand for “smaller, faster, cheaper, and better” devices pushes the microelectronic, optical, and data storage industries to rely heavily on thin films and structures made from thin films. Due to the constraint of the substrate to which they are typically attached, such thin films often show very high stresses, which may lead to device failure. Of a number of stress-related reliability issues, subcritical interfacial delamination is one of the most vexing. For example, in a typical microelectronic device there are many thin films stacked on the silicon substrate, with numerous interfaces between chemically dissimilar layers. Of particular interest here are the interfaces between Cu interconnects and surrounding oxide layers. The absence of strong chemical reactions between Cu and typical oxides (or nitrides) leads to weakly bonded interfaces, which may be the weakest link in the multilevel thin film structure. Since high temperature is normally encountered by such a device during fabrication, one might expect failures to occur due to critical (fast) fracture during or immediately after manufacture. However, even if the film stack survives such processing, it may be still vulnerable to subcritical (slow) de-adhesion associated with either thermomechanical fatigue or environmentally assisted stress corrosion cracking in service. The later is the focus of this study. A driver film method, in which a highly stressed overlayer is deposited onto a target layer to elevate the total strain energy and induce debonding, was used in this work. This method has a number of advantages compared to other techniques which have been developed to study subcritical adhesion [1]. First, most other methods require bonding the film either to another layer or to a load transfer device. If this bonding process involves high temperature, or chemical processes, the microstructure and chemistry of the ori
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