There is provided a bump structure for a semiconductor device, comprising a metal post formed on and electrically connected to an electrode pad on a substrate, a solder post formed on the top surface of the metal post, said solder post having the same horizontal width as the metal post and the top surface of the solder post being substantially rounded, and an intermetallic compound layer disposed at the interface between the metal post and the solder post. An oxide layer formed on the solder post prevents solder post under reflow from being changed into a spherical shape. An intermetallic compound layer may be formed by an aging process at the interface between the metal post and the solder post. The bump structure can realize fine pitch semiconductor package without a short between neighboring bumps.