Back-end assembly solution to bare copper bond pad wafers

Introducing a thin ceramic/glass coating directly on top of the copper surface has been proposed to prevent copper from further oxidation during wirebonding. The thickness and integrity of the added layer must be such that the coated wafers can be processed using existing assembly equipment. This paper discusses the results of probing and wirebonding on this type of glass coated copper wafers. Probing was performed on both coated and uncoated copper bond pads with epoxy-ring-cantilever-needle probe cards. Probe feasibility was determined with contact resistance measurements and optical inspection of probe marks. Gold wirebonding was evaluated on both probed and non-probed glass-coated copper bond pads using presently available commercial wirebonders. The targeted bonded ball diameter was selected for 63 mum fine pitch bonding. Ball shear, wire pull and Au-Cu intermetallic coverage were the criteria used to evaluate the wirebondability of the glass-coated copper bond pad surfaces. The feasibility of integrating this protective layer process into the current copper-wafer fab and assembly process flow is discussed.