Innovative Advances in Copper Electroplating for IC Substrate Manufacturing

Low cost electroplating of copper to create Fine Line Space (FLS) features and Blind Micro Via's (BMV) is a key technology element of high density substrates for integrated circuit packaging. There is a critical need to provide uniform copper metal deposit across the entire surface of the high density package while also super filling the blind micro via's without any void entrapment. In this paper we report the results of a novel reverse pulse plating process to show both super fill in microvia's and uniform copper deposition on fine trace and pad features. A test coupon was designed and fabricated to evaluate plating uniformity of finely patterned regions as well as gap fill capability on LASER drilled microvia's in an underlying dielectric film. The film stack was defined by BMV feature dimensions with via opening of 25μm on the bottom layer and via opening of 60μm or larger on the ensuing top metal layers. Aspect ratio of these different via geometries varied anywhere from 0.5 to 1.0 depending on the via stack layer. Early into the study, there were significant process challenges including trapped voids inside the gap filled via regions causing poor electrical and yield performance. Such challenges were overcome with the introduction of a novel reverse pulse plating waveform to achieve void free bottom up fill. Limitations of operating under diffusion limited regime were then identified and addressed. Introduction of large reverse pulse current led to highly non-uniform plating thickness with "Ski sloped surfaces" along with poor metallurgical compatibility of the plated copper leading to lifted traces. A balanced approach with the identification of an optimized reverse pulse waveform led to void free gap fill and uniform deposit in fine patterned surfaces.