Method of forming a solder bump on a substrate

A method of forming a solder bump on a substrate, comprising the steps of:

• forming at least one conductive layer on the substrate, wherein the substrate has a surface on which an electrode pad is prepared;
• forming a resist layer on the at least one conductive layer, the resist layer having an opening over the electrode pad;
• forming a metal pillar in the opening of the resist layer, wherein the metal pillar comprises a first conductive material;
• forming a space between a sidewall of the resist layer and a sidewall of the metal pillar, wherein the step of forming the space between the sidewall of the resist layer and the sidewall of the metal pillar includes exposing the resist layer using light to shrink the resist layer;
• forming a metal barrier layer in the space and on a top surface of the metal pillar, wherein the metal barrier layer comprises a second conductive material which is different from the first conductive material of the metal pillar;
• forming a solder layer on the metal barrier layer over the top surface of the metal pillar;
• removing the resist layer;
• removing the at least one conductive layer;
• and
• forming the solder bump by reflowing the solder layer.