At 65nm and below, Rapid Thermal Annealing (RTA) makes a significant contribution to manufacturing process variations, degrading the parametric yield. RTA-induced variability strongly depends on circuit layout patterns, particularly the distribution of the density of the Shallow Trench Isolation (STI) regions. In this work, we investigate a two-step approach to reduce the impact of RTA-induced variations.We first solve a floorplanning problem that aims to reduce the RTA variations by evening out the STI density distribution. Next, we insert dummy polysilicon fills to further improve the uniformity of the STI density. Experimental results show that our floorplanner can reduce the global RTA variations by 39% and the local variations by 29% on average with low overhead compared to a traditional floorplanner, and the proposed dummy fill algorithm can further reduce the RTA variations to negligible amounts. Moreover, when inserting dummy fills, for the layouts obtained by our floorplanner, on average, 24% fewer dummy polysilicon fills are inserted, as compared to the results from a traditional floorplanner.