A method of forming an integrated circuit device comprises forming a metal pillar over a semiconductor substrate. The method also comprises forming a solder layer over the metal pillar. The method further comprises forming a metallization layer comprising a cobalt (Co) element, the metallization layer covering the metal pillar and the solder layer. The method additionally comprises thermally reflowing the solder layer to form a solder bump, driving the Co element of the metallization layer into the solder bump. The method also comprises oxidizing the metallization layer to form a metal oxide layer on a sidewall surface of the metal pillar.