Chemical mechanical polishing method for fabricating copper damascene structure

A fabrication method for a copper damascene, applicable to a substrate having a dielectric layer formed thereon, the method comprising:

• forming a damascene opening in the dielectric layer;
• forming a barrier layer conformal to a profile of the damascene opening over the substrate;
• forming a copper seeding layer on the barrier layer;
• forming a copper layer on the copper seeding layer, wherein the copper layer is thick enough so that the damascene opening is filled;
• forming a protective layer on the copper layer;
• performing a first chemical mechanical polishing (CMP) step, so as to remove the protective layer and a portion of the copper layer outside the damascene opening, wherein a polishing rate for the copper layer is a first polishing rate, a polishing rate for the protective layer is a second polishing rate, and the first polishing rate is faster than the second polishing rate;
• and
• performing a second CMP step, so as to remove portions of the copper layer, the copper seeding layer, and the barrier layer outside the damascene opening, whereby a copper damascene is formed, wherein a polishing rate for the copper layer is a third polishing rate, a polishing rate for the barrier layer is a fourth polishing rate, the third polishing rate being slower than the first polishing rate, and the third polishing rate being the same as the fourth polishing rate.