Measurement and improvement of the adhesion of copper to polyimide
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Measurement and improvement of the adhesion of copper to polyimide M. Menezes, I.M. Robertson, and H.K. Birnbaum Department of Materials Science and Engineering and Materials Research Laboratory, University of Illinois, Urbana, Illinois (Received 3 April 1998; accepted 27 July 1999)
A contact angle measurement technique has been used to obtain an estimate of the interfacial energy and thermodynamic adhesive strength between copper and polyimide [pyromellitic dyanhydride oxydianalyn (PMDA-ODA) and p-phenylene biphenyltetracarboinide (BPDA-PDA)]. Values of the strength of adhesion from these contact angle measurements are in reasonable agreement with values calculated using the Girifalco–Good–Fowkes nonpolar interfacial adhesion theory. Based on the surface energy it was predicted and experimentally observed that small copper clusters would embed into the polymer matrix if heated under ultrahigh vacuum conditions at temperatures near Tg of the polymer. Controlled embedding of nanometer clusters was utilized to produce a textured interface, where the partially embedded clusters acted as “nanonails” to anchor a metal overlayer to the underlying polyimide substrate. These nanonails greatly increased the bonding between the copper overlayer and the polyimide, as demonstrated by mechanical debonding studies.
I. INTRODUCTION
Metal–polymer interfaces play a prominent role in a variety of industrial applications. This is especially true in the microelectronics industry, where polymers are commonly used as insulators between conducting metallized layers to form multilayered structures for chip packaging.1,2 The mechanical integrity of the interfaces in these multilayered structures is a major reliability concern as it can be degraded during fabrication and operation by the stresses generated due to thermal mismatch between the metal and the polymer. Chemical and physical interactions between a substrate and an overlayer govern the fracture toughness of the interface, which in turn controls the adhesion between the overlayer– substrate system. Regardless of how adhesion is defined, the measured value is often much larger than the “intrinsic” or thermodynamic adhesive strength, because the act of obtaining the measurement mechanically almost always causes plastic deformation of the material.3–7 We used a method involving measurement of the contact angle of metal clusters on polymer substrates to obtain an estimate of the intrinsic interfacial adhesion. Polyimides are one class of polymers that have found significant use in the microelectronics industry because of their excellent thermal stability, high glass transition temperature (Tg), low dielectric constant, and excellent planarization capability.8 To be able to increase current densities and reduce device sizes, copper is desired over aluminum because of its superior electromigration and lower electrical resistance.9 One of the drawbacks of usJ. Mater. Res., Vol. 14, No. 10, Oct 1999
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