Maintaining uniform CMP hard mask thickness

Method of forming an interconnect structure comprising the steps of:

• depositing a dielectric layer of ultra low-k material;
• forming a hard mask over the dielectric layer, wherein the hard mask comprises:
• a layer of silicon carbide (SiC) material including (SiCOH) material overlying the dielectric layer;
• and a layer of oxide material overlying the silicon carbide material;
• etching trenches in the dielectric layer;
• depositing a liner over the hard mask and within the trenches;
• and overfilling the trenches with a conductive material;
• perfoming a chemical mechanical polishing process to remove the conductive material which is atop the liner, thereby exposing the liner;
• removing the portion of the liner which is atop the hard mask using a Gas Cluster Ion Beam (GCIB), leaving the conductive material protruding from the trenches;
• and removing the layer of oxide material using a wet etch process, leaving the layer of silicon carbide material, the conductive material and the liner material above the layer of silicon carbide material protruding from the trenches;
• and performing a touch-up polishing process to remove the conductive material and the liner protruding from the trenches.