Advances in embedded traces for 1.5µm RDL on 2.5D glass interposers

This paper presents the first demonstration of 1.5μm ultra-fine copper trace re-distribution layers (RDL) with embedded trace processes for 2.5D glass interposers. Two approaches described in this paper include: (1) Copper Trace Transfer (CTT) and (2) Photo Trench Embedding (PTE), both of which can be fabricated using existing double-side package substrate process tools on large panels. 2.5D Interposers, interconnecting logic and memory devices at high-bandwidth, require ultra-fine I/O pitch below 40μm. In order to escape route bumps at less than 40μm pitch, the interposer requires RDLs with less than 5μm ultra-fine copper wiring. Organic package substrates, using semi-additive processes (SAP), face a number of challenges in achieving less than 5μm line and space. The two embedded trace process methods in this paper are targeted at extending RDL feature sizes beyond these SAP limits. In the CTT approach, each RDL metal layer is pre-fabricated on a removable carrier and then transferred to a polymer dielectric layer laminated on an interposer core substrate. 1.5 to 5μm Copper traces were plated and successfully transferred onto a polymer laminated glass core. In the PTE approach, a high-resolution photosensitive dry film (TMMF-2014, 14μm thick) and liquid photosensitive dielectric (PN-0371D) films were explored for the RDLs. The initial feasibility of fine line photolithography and trench copper filling processes was investigated. Results showed that the film has a 2μm resolution, with an aspect ratio of seven. The feasibility of resolving 1.2μm line and space patterns was demonstrated in 4.2μm thick liquid photosensitive dielectric material. RDL traces down to 3μm using dry film and 1.5μm using liquid-based film were plated with sputtered Ti-Cu seed layers and trench fill electroplating processes. Both embedded trace approaches demonstrate the potential to scale 1.5-3&#- 03BC;m RDL wiring using double-side, large-panel processing for low-cost, high-density interposers.