Low Transmission Loss Cu Wirings with Smooth Seed Layer and High Adhesion against Prepregs

Semiconductor packages for high performance devices with printed circuit boards having multi wiring layers such as FC-BGA have been attracting the attention in order to realize ultra-reliable and low latency communications in 5G networking. Cu wirings for the package are usually fabricated by semi-additive process (SAP) with desmear process and/or modified semi-additive process (MSAP) by using Cu foil with large surface roughness. Though a desmear process and Cu foil can obtain enough adhesion between dielectric and Cu seed layer by anchoring effect to secure reliabilities, the interface between dielectric and Cu seed layer should be smooth to achieve low attenuation of electric signals at high frequencies. Here, instead of that processes, we applied an UV modification for the surface of our developed prepreg in order to realize a smooth and high adhesive seed layer against the dielectric. We also conducted chemical modification for the surface of Cu foil to achieve low attenuation of transmission loss and high adhesion against prepreg. We successfully assembled Cu wirings with L/S = 6/6 μm on prepregs by SAP. High peel strength between Cu foil and prepreg was obtained due to chemical modification for the surface of Cu foil. The normalized transmission loss of Cu wiring assembled through MSAP was improved as compared to Cu foil with rough surface.