Sputtered Ti-Cu as a superior barrier and seed layer for panel-based high-density RDL wiring structures

This paper demonstrates that sputtered Ti-Cu is a superior barrier and seed layer on glass and organic panel substrates, over traditional electroless seeding, for the fabrication of ultra-fine copper traces (2-5μm) on dry film polymer dielectrics for high-density 2.5D interposers. The current semi-additive processes using electroless Cu seed face several challenges in scaling the copper trace widths below 5μm due to two main reasons: high-roughness of dielectric and high-thickness of copper seed. In this paper, both the above limitations are addressed by an advanced Physical Vapor Deposition (PVD) process that can be scaled to large panels with high throughputs. The PVD process developed in this study is capable of depositing Ti-Cu barrier and seed layer on 500 mm size panels at a low enough temperature for dry film polymer dielectrics of glass transition temperatures (T_g) of 150-160°C. The superiority of sputtered Ti-Cu over the conventional electroless Cu seeding for achieving good and reliable adhesion between Cu and dry film polymer dielectrics was investigated by peel strength measurements after highly-accelerated stress tests (HAST). The results indicate that sputtered process results in higher peel strengths and without adhesive failures at the Ti-Cu-polymer interfaces. Adhesive failures, however, were observed with the traditional electroless seed processes. In addition, the PVD processes resulted in small 2-5μm Cu traces on smooth dielectric films like ZS-100, requiring no desmear treatment. Such a process promises to be scalable to large panels leading to low-cost fabrication of high-density 2.5D interposers.