Fine pitch Cu pillar wafer process development and seed layer etching characterization

This paper will revise the traditional wafer fabrication process flow to accommodate the new material used to achieve 40 um fine pitch Cu pillar with minimize seed layer undercut. New photo-resist material is introduced to attain a single coating of 40um thickness and it has demonstrated the capability of attaining an aspect ratio of 2. The wafer fabrication process ended using a combination of seed layer (Ti/Cu) wet and dry etching. It has shown the potential of achieving almost zero undercut which is very critical for a 20um via. Cross-sectional SEM will be carried out to verify the side wall profile and the footing of the photo-resist. FIB cross-section is done to identify Ti and Cu undercut. Bump shear test will be performed after the seed layer etching to quantify the failure mode of a bump with and without seed layer undercuts.