Fill for Shallow Trench Isolation CMP

Shallow trench isolation (STI) is the mainstream CMOS isolation technology. It uses chemical mechanical polishing (CMP) to remove excess of deposited oxide and attain a planar surface for successive process steps. Despite advances in STI CMP technology, pattern dependencies cause large post-CMP topography variation that can result in functional and parametric yield loss. Fill insertion is used to reduce pattern variation and consequently decrease post-CMP topography variation. Traditional fill insertion is rule-based and is used with reverse etchback to attain desired planarization quality. Due to extra costs associated with reverse etchback, "single-step" STI CMP in which fill insertion suffices is desirable. To alleviate the failures caused by imperfect CMP, we focus on two objectives for fill insertion: oxide density variation minimization and nitride density maximization. A linear programming based optimization is used to calculate oxide densities that minimize oxide density variation. Next a fill insertion methodology is presented that attains the calculated oxide density while maximizing the nitride density. Averaged over the two large testcases, the oxide density variation is reduced by 63% and minimum nitride density increased by 79% compared to tiling-based fill insertion. To assess post-CMP planarization, we run CMP simulation on the layout filled with our approach and find the planarization window (time window in which polishing can be stopped) to increase by 17% and maximum final step height (maximum difference in post-CMP oxide thickness) to decrease by 9%