1. Technical Field
The disclosure relates generally to wafer polishing, and more particularly, to a fixed abrasive chemical mechanical polishing system, sub-pad therefor and method employing different pressures applied through the sub-pad to a rotating pad that contacts the wafer.
2. Background Art
Fixed abrasive chemical mechanical planarization (FA CMP) has been used in production of integrated circuit (IC) chips for several years, and across many technologies starting from 90 nanometer (nm) and continuing to 32 nm and 22 nm nodes. FA CMP includes using a rotating pad including abrasives fixed therein with a slurry that does not include abrasives for polishing (i.e., planarizing) a surface of a wafer.
One issue in implementing FA CMP is that the removal rate varies and decreases as topography is reduced. This situation causes under-polish of the wafer which results in having to scrap affected wafers/lots. Several processes have been tried to increase removal rate. In one approach, ribs were placed on the sub-pad under the abrasive pad to create topography on the abrasive pad. Here, the size and spacing of the ribs is larger than that of the fixed abrasives on the abrasive pad, causing the pad to bend between the ribs so some of the fixed abrasives are not contacting the wafer. To overcome this problem, increased down force is used which causes increased scratching of the wafer. Another approach includes using chemistries that increase the friction on the wafer in order to boost oxide removal rate. This method also increases scratches.