Characterization of a thick copper pillar bump process

As pin counts and interconnection densities increase there is growing interest in copper pillar bumps for flip chip and wafer-level packaging. This trend is driven by the need to increase interconnect performance as well as reduce interconnect cost. Copper pillars retain their shape during solder reflow, allowing finer interconnect pitches with predictable standoff heights. The fabrication of copper pillar bumps requires the use of a very thick photoresist layer for copper and nickel electroplating. This photoresist material must be capable of coating, exposing, developing, electroplating and stripping with conventional track equipment and standard ancillary process chemicals. For the electroplating process the photoresist profile, plating durability and stripability are important considerations. This study will characterize a photoresist for a single coat, 55 μm thick copper process. The lithographic performance of the thick positive photoresist will be optimized using a broad band, low numerical aperture stepper. Results will show good adhesion to copper with no surface treatment and no photoresist cracking during plating. Cross sectional SEM analysis, process latitude, and copper-nickel electroplating performance are used to establish the lithographic capabilities.