A method for fabricating metal interconnect structure includes the steps of: forming a first metal interconnection in a first inter-metal dielectric (IMD) layer on a substrate; forming a cap layer on the first metal interconnection; forming a second IMD layer on the cap layer; performing a first etching process to remove part of the second IMD layer for forming an opening; performing a plasma treatment process; and performing a second etching process to remove polymers from bottom of the opening.