Effects of thermal cycling in a reducing atmosphere on metal/polyimide interfaces
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Effects of thermal cycling in a reducing (deuterium) atmosphere on the structure and chemistry of Cr/Cu/Cr/polyimide (PI)/Si and Au/Cu/Ti/polyimide (PI)/Si multilayer systems have been studied. In the Cr/PI system the interface between the Cr and PI was sharp and distinct in the as-deposited state, and after the anneal in the reducing atmosphere. Tensile cracks through the Cr/Cu/Cr layers were found after annealing and are the result of thermal stresses. No evidence for significant diffusion of Cr into the PI was found. In the Ti/PI system, the interface between the Ti and PI was sharp in the as-deposited state. After annealing in vacuum and in the reducing environment, regions of the interface between the Ti layer and the PI were converted to an oxide, T15O9. Annealing in the deuterium environment also caused delamination of the Ti film from the PI and blistering of the metal in the sample interior. No significant diffusion of the Ti into the PI was detected. In both systems, the metal in contact with the PI acted as a barrier to the diffusion of Cu into the PI.
I. INTRODUCTION Polyimides (PI) find wide application as insulators in the microelectronics industry. They are used at the device and packaging level as they have a low dielectric constant and can be patterned with features in the micrometer range. This leads to the presence of numerous metal/PI interfaces in a microelectronic package. For the overall integrity of the package, adhesion at these interfaces must be strong, tolerant of cyclic thermal stresses, and impervious to gaseous environments. Since mechanical failures often result from thermal cycling and from environmental effects, it is critical to understand the changes in the chemistry and structure of the metal/PI interfaces that are induced by thermal treatments and by the ambient. Numerous studies of the chemistry of metal/PI interfaces and the mechanisms of interface formation have been reported, ^ 8 but only a few address the stability of these interfaces during thermal cycling.9~u Previous in situ XPS studies indicate that bonding at the metal/PI interface is due to interactions between the metal atoms and the carbonyl groups ( C = O ) of the pyromellitic dianhydride (PMDA) part of the PI.1-8 It has been observed that during the initial stages of film growth Cr, Ti, Ni, and Al interact strongly with the carbonyl oxygen forming metal-O-PI bonds, and toward the later stages of film growth, carbide-like metal-C bonds are formed. In
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J. Mater. Res., Vol. 9, No. 2, Feb 1994
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the Cu/PI system, no strong interaction between Cu and the carbonyl group of the PI occurs, and consequently adhesion at the Cu/PI interface is poor. It has been found that for certain combinations of deposition rate and temperature copper diffuses into PI and forms copper clusters.1 The object of this investigation was to understand the effects of thermal cycling in a
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