Numerous studies have demonstrated the outstanding planarization capabilities of Fixed Abrasives in CMP processes. The ability to stop on nitride in direct STI CMP has also been demonstrated in production environments for DRAM wafers. When performing direct STI CMP using selective slurries a potential difficulty is with oxide dishing during overpolish. This is an issue particularly when a wide variety of pattern densities exist, and outstanding endpointing systems are necessary to prevent excessive dishing. While Fixed Abrasives do not generally have oxide dishing effects with overpolish, they may cause erosion in isolated, low density areas. Overpolishing can produce significant erosion in 0-20% density areas while stopping on nitride in the higher density areas (30-70%). Selective chemistries have now been identified that allow Fixed Abrasive polishing to stop on nitride in low density areas while maintaining the characteristic of very low oxide dishing, even after a 100% overpolish. When using this system, MIT 964 mask wafers are shown to fully clear leaving with-in-die nitride ranges of <70 A and trench oxide ranges <250 A from the 10% to 100% densities and with low dishing (<100 A) in all but the largest trench areas (1000 μm pitch or 0% density). Optimal polishing conditions, concentrations of chemicals and pHs are shown to produce nitride to oxide selectivities as high as 200:1 but since there is no mechanism for a Fixed Abrasive to remove trench oxide once the nitride has been cleared, the polish rate is effectively stopped everywhere.
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