A metal gate structure comprises a metal layer partially filling a trench of the metal gate structure. The metal layer comprises a first metal sidewall, a second metal sidewall and a metal bottom layer. By employing an uneven protection layer during an etching back process, the thickness of the first metal sidewall is less than the thickness of the metal bottom layer and the thickness of the second metal sidewall is less than the thickness of the metal bottom layer. The thin sidewalls allow extra space for subsequent metal-fill processes.