Wetting behavior of electrolyte in fine pitch Cu/Sn bumping process by electroplating

Abstract

This work reports on the wetting behavior of electrolyte in fine pitch Cu/Sn bumping process by electroplating. Three methods containing adding complex wetting agent to electrolyte, plasma treatment to photo-resist and ultrasonic vibration were taken to improve the wettability between electrolyte and related materials. Contact angles of electrolyte containing different amount of complex wetting agent with sputtered copper and photo-resist, before and after plasma treatment, were measured respectively. Certain amount of wetting agent can decrease the contact angles of photo-resist with electrolyte by nearly 20deg, and wettability of photo-resist was further enhanced by plasma treatment that makes the contact angles decrease by about 40deg. The contact angle of sputtered copper and electrolyte can be decreased by nearly 10deg. SEM observations of cross sections and top view of bumps revealed that complex wetting agent help electrolyte get to the bottom of patterned figure quickly. In tin bumping process, this is the last step of wetting behavior for the bubbles in patterned figure would be discharged in electroplating process. While in copper bumping process, ultrasonic vibration was used to help remove the bubbles for the bubbles cannot be small enough by surface tension to overcome the adhesion effect of sidewall. Copper and tin bumps with a diameter of 60 mum, height of 60 mum and pitch of 180 mum were fabricated by rational use of methods above.