P-4 process. A novel high-density, low-cost multilayer planar thin-film PWB technology

The advantages of thin film on PWB wiring substrate (MCM-D/L) include sequential fabrication and blind/buried microvias to enable fine-line, high-density wiring; and PWB-compatible materials and processes to ensure low cost. Currently, the majority of sequential build-up activities focuses on one or two layers of thin film wiring for surface redistribution. The trends in IC technology, however, require high-density, multilayer thin film interconnection in the next few years. A novel plated-post, photo-polymer (P-4) technology is described in this study, which offers a solution to multilayer thin-film buildup of planar structures on PWBs. The key to this P-4 process is the unique microvia formation and metallization processes. Copper posts are electroplated prior to photo-polymer dielectric layer coating. Microvias are formed right on top of the posts with a significantly reduced aspect ratio compared to conventional microvias. The conductor lines and via posts are fabricated by semi-additive copper plating to achieve fine feature size and minimize waste. As a result, excellent planarity was achieved, which enables the stacking of vias in consecutive layers. Key material and process issues relevant to the P-4 process are addressed. Variation and possible extension of this technology are also explored.