Feature Density-Dependent Planarization
One of the most difficult to reduce artifacts generated using CMP is varying step height. This is due to variations in feature density. CMP is quite good at planarizing within a die, as long as the features are of uniform spacing. If, however, some features are tightly packed (dense) while others in the die are widely separated (isolated or iso), then the thickness of the material above the different density material will vary. This variance can approach several hundred nanometers. Adjusting to accommodate this is costly.