ABSTRACT
U. SENER,1 A. ENTENBERG,2 B. KAHN,3 F. D. EGITT0,4 L. J. MATIENZ0,4 T. DEBIES5 and G. A. TAKACS*·' 1Department of Chemistry, Center for Materials Science and Engineering, Rochester Institute of Technology, Rochester, NY 14623, USA 2Department of Physics, RIT, Rochester, NY 14623, USA 3Department of Imaging & Photographic Technology, RIT, Rochester, NY 14623, USA 4Endicott Interconnect Technologies, Endicott, NY 13760, USA 5Xerox Corporation, Webster, NY 14580, USA
Polyimides (Pis) have attractive thermal, mechanical and electrical properties and, therefore, are widely used in high-performance military aircraft and spacecraft applications (Ref. [1] and references therein) and in microelectronic devices [2]. In the latter application, Pis are commonly employed as a protective overcoat for integrated circuit chips because they possess high thermal stability, have excellent planarizing characteristics, are good barriers to alpha particles and are low in alpha particle emission. In addition to these attributes, the moderate dielectric constant, low permittivity and flexibility of PI films makes them attractive as dielectric materials in the fabrication of thin-film electronic packages, for example,
Adhesion between two surfaces results from a combination of mechanical, chemical and electrostatic contributions. In conjunction with these contributions, diffusion characteristics at the mating surfaces of the materials must also be considered. Surface micro-roughness can induce mechanical interlocking, as well as produce a greater surface area for chemical interactions between the components of the interface. Chemical interactions include acid-base [4] and dipolar effects. Interfacial characteristics are determined primarily by the strength of chemical bonding between the two surfaces in contact. Numerous investigators have reported that adhesion of Cu coming from the vapor phase to pristine PI is typically quite weak, whereas adhesion of Cr is stronger [5, 6]. As such, Cr is often used as a seed layer (or tie layer) for enhancing adhesion of Cu to PI. Ho and co-workers [7-9] found that for polyimide, the chemical bond strength for Cu, Al and Cr increased in the order Cu<Al<Cr. They concluded that Cu atoms diffused into polyimide to form clusters, Al intermixed with polyimide without cluster formation, and Cr formed metal-organo compounds. almost immediately upon deposition, leading to a uniform interface with little intermixing. Murdey and Stuckless [10] made calorimetry measurements of heats of reaction for Cu and Cr on untreated PI. Adhesion of deposited Cu was poor and the heat of reaction could be accounted for by Cu-Cu bonding without significant contribution from Cu-PI bond formation. Cr caused formation of a reaction layer with irreversible cleavage of PI chemical bonds. Wolany et al. [11] conducted in situ time-of-flight secondary ion mass spectrometry (TOP-SIMS) and X-ray photoelectron spectroscopy (XPS) analyses of Cu on untreated PI and observed binding of Cu to carbonyl groups to form Cu-0-C bonds.
