High Density RDL build-up on FO-WLP using RDL-first Approach

FO-WLP based on RDL-first integration flow with 8 metal layers in a single side was proposed and demonstrated to meet advanced, high density applications for SiP. 7 layers of Cu RDL and 1 layer of UBM were fabricated using semi-additive process and low temperature cured photo-definable dielectrics were used as the passivation. The minimum Cu trace L/S of $2 \mu \mathrm{m}$ and contact-via openings of $5\ \mu \mathrm{m}$ were implemented in the test vehicle. Electrical test structures were used to characterize the Cu RDL electrical performance. Electrical continuity was achieved across the daisy chain structure connected to each metal layers. Meander line comb structures shows pico-ampere range leakage current for both intra- and inter-layer Cu traces. I-V curve of $2\ \mu \mathrm{m}$ LW Cu wire shows ohmic behavior with slight increase in resistance with increasing current, due to increasing current density associated with small LW wiring. S-parameters were characterized on GSSG transmission line test structure with $2\ \ \mu \mathrm{m}$ LW Cu traces and 8 mm wire length. Insertion loss was greater 2 dB, due to the small cross-sectional area and long wire length of the signal line. Maximum cross-talk between adjacent signal lines with $4\ \mu \mathrm{m}$ spacing is −20.5 dB.