A Novel Polishing Mechanism Used in Manufacturing Ultra-High Uniformity Gold Solder Bump

Abstract

The plating-based gold solder bumps with ultra-high average uniformity across the entire 4 inch wafer by using an unprecedented polishing process are fabricated in this paper. This sophisticated polisher uses the same two polishing mechanisms, mechanical and chemical forces, as those traditionally applied to polish wafer surface with the chemical mechanical polishing (CMP) equipment, but has relatively far less cost than it. Not only can we acquire uniform surface among the solder bumps after plating, but we are able to largely improve the bumps' surface roughness, both of which are two key elements for enhancing assembly yield rate and solving the difficulty in stripping thick photoresist as well. Meanwhile, we employ some compatible measuring instruments such as a surface profile instrument (alpha-Step), an optical microscope (OM) with convertible magnification and a scanning electron microscope (SEM) to identify and evaluate the solder bumps' thickness and surface morphology after each procedure for the sake of strictly controlling their quality. The final results indicate that the poor solder bumps with approximately 9.24% in deviation after plating is narrowed down as accurate as within 3.98% across the entire wafer after polishing, representing the uniformity significantly improved to 2.5 times in comparison with those without using any treatment after electroplating. Even on some local area (die), the uniformity can be accurately controlled up to 1.5%. The surface roughness, on the other hand, has been advanced from the original Ra 0.6 after electroplating to near Ra 0.03 after polishing