Degradation of the Polyimide/Copper Interface
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DEGRADATION OF THE POLYIMIDE/COPPER INTERFACE K. K. Chakravorty, V.A. Loebs, and S.A. Chambers Boeing Electronics High Technology Center, P.O. Box 24969, M/S 9Z-80, Seattle, Washington 98124-6269
ABSTRACT A comparative investigation of polyimide/Cu interface degradation has been carried out for ultrathin photosensitive, non-photosensitive, and preimidized polyimide precursor films cured while in contact with a Cu substrate. The role of curing byproducts and environmental conditions on interface degradation has been elucidated by means of X-ray photoelectron spectroscopy. Immediately after curing, we observed some oxidation of the Cu in contact with non-photosensitive and photosensitive polyimide overlayers. On the other hand, only negligible oxidation was observed for the preimidized polyimide/Cu interface. Experiments in which samples were stored in vacuum, air and a humidity chamber show a dependence of the oxidation kinetics on air/moisture exposure. Preimidized and photosensitive polyimide/Cu interfaces, stored in air, became more extensively oxidized with time relative to identical samples stored in vacuum. Moreover, all three polyimide/Cu interfaces exhibited significantly more oxidation after 3 days in a humidity chamber than after 19 days of storage in air. Taken together, these data clearly demonstrate that absorbed water and its interaction with curing byproducts are key factors in the extent of Cu oxidation at the interface. INTRODUCTION Excellent thermal stability, adherence, and favorable dielectric properties have resulted in the widespread use of polyimide for microelectronics applications. Polyimide has been used as an interlevel dielectric [1] and a final passivant [2] for multilevel metal transistors. More recently, polyimide has been used as a dielectric in multilayer thin film interconnect structures such as multi-chip modules [3,4]. Most of these applications involve a substantial number of metal-polyimide interfaces. Therefore the nature of interfacial bonding between the two materials is a key parameter in the overall reliability of the structures. Consequently, a number of studies have been performed in which the chemistry of interface formation for metals evaporated onto cured polyimide films in an ultrahigh vacuum system has been investigated using X-ray photoelectron spectroscopy [5-7]. The first observation of Cu oxidation at the inverted polyimide on Cu interface was recently reported by Chambers and Chakravorty [8]. Most of these studies have focused on a nonphotosensitive, PMDA-ODA precursor type of polyimide. Similar information is lacking for the other types of polyimide used for microelectronics applications, such as the photosensitive precursor-based and preimidized versions. Due to their significantly different precursor composition and curing chemistry, different interfacial interactions are expected. Therefore, our goal in the present work has been to examine polyimide/Cu interfaces for these three different types of polyimide and to bring out any systematics that may exist be
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