Pretreatments for photoresist-patterned wafer to improve Cu pillar electrodeposition

Cu micropillars are essential components in the electronics packaging, basically connecting multiple chips. The pillars are fabricated by Cu electrodeposition that fills inside via holes formed through a photoresist (PR) layer. Unlike Damascene and TSV processes, the Cu seed layer is only opened at the bottom of via holes, while the rest of the wafer surface is covered by the PR. Therefore, additional pretreatments are required to improve the Cu electrodeposition process due to air bubbles entrapped inside PR holes, organic residues, and native oxide on Cu seed layers. This research systematically investigated the pretreatments for PR-patterned wafers. A humidifier, hot steam, and compressed water spray were assessed for surface hydration to remove air bubbles from the PR holes. Ethanol- and citric acid-based aqueous solutions were then applied to remove organic residues and native oxide from the surface of Cu seed layers. The advantages of these pretreatments were assessed by analyzing the quality of Cu electrodeposits, and the optimum methods and solutions for Cu pillar electrodeposition on the PR-patterned wafer were suggested.