Thick Film Photoresist Process for Copper Pillar Bumps on Surface Acoustic Wave - Wafer Level Packages

SAW-filters (Surface Acoustic Wave) are used for mobile data transfer. Further miniaturization was achieved by replacing the standard interconnect technology via solder balls through CPBs (Copper Pillar Bumps) using a thick film photolithography. CPBs on SAW-filters with a height of approximately 80 μm after reflow, a diameter of 67 μm, almost vertical sidewalls, and excellent adhesion to the seedlayer were fabricated.This study describes in detail the spin coat recipe and the impact of lithography parameters. CPBs were structured with a positive tone photoresist, with a thickness of at least 80 μm (including pre-wet) and almost vertical sidewall resist opening shapes on TFAP™ (Thin Film Acoustic Packaging) packages. The sidewall shape of the resist openings strongly depends on the softbake duration. Exposure dose and post exposure bake duration do influence the resist opening diameters but not the resist sidewall shape if softbake duration is set correctly. The development process had no significant influence.Different tools and processes for plasma cleaning after resist development were investigated to optimize the adhesion of electroplated CPBs. A plasma tool with backside cooling and electrostatic chucking of the lithium niobate wafers allowed an appropriate plasma cleaning descum. The adhesion was determined via shear testing of copper pillars and a subsequent failure mode analysis with optical microscopy.The positive tone resist does not show any damage or peeling from the substrate after the optimized plasma cleaning process or after electroplating of copper, nickel and tin silver alloy. The CPBs withstand all further process steps including DBG (Dicing Before Grinding).