Photoresist Removal Requirements for Advanced Wafer Level Packaging Technology

It has been several years since wafer level packaging (WLP) technology has emerged, enabling the packaging industry to successfully fabricate smaller, more reliable modules. The use of this technology is expected to continue to grow in the future. One current WLP trend is to replace high lead solder bumps with lead-free solder bumps. Two of the more promising lead-free bumps are Cu-pillars and bumps. The use of the Cu-pillar/bump approach continues to increase as fabricators drive to increase yields, create more I/O pins per wafer thus resulting in finer bump pitch. The use of liquid photoresist (PR) and/or dry film photoresist (DFR) has become the standard for fine bump pitch formation. The major challenges in using these resists are 1) the effective removal of the resist without residue or redeposit; and 2) the compatibility of the removal chemistry with under bump materials (UBM). This paper will discuss WLP market trends, the technical requirements associated with PR/DFR removal, and conclude with some experimental results of advanced PR/DFR removal.