High density interconnection using photosensitive polyimide and electroplated copper conductor lines

Multilayer, high-density, thin-film interconnect structures with a wiring density of up to 1000 lines per inch per signal layer, fabricated using a photosensitive polyimide as the dielectric and electroplated copper conductor lines, are discussed. Lithographic considerations associated with the patterning of high-aspect-ratio (>1.0) polyimide precursor features and their distortions during high-temperature annealing were investigated. An anisotropic shrinkage causing inward skewing of the feature profiles was observed. The shrinkage strongly depended on the ultraviolet exposure conditions and appears to be related to nonuniformity in the precursor crosslink density. The viability of electroplating techniques for fabrication of small-geometry copper features was examined. In contrast to the conventional approach, electroplating of copper features in patterned polyimide trenches provides excellent planarization in interconnect structures containing high-aspect-ratio features. Transmission characteristics of high-frequency signals in these lossy interconnect lines were measured. Minimal degradation of the pulse shape and attenuation of the pulse amplitude was observed