Reduction in overlay error from mark asymmetry using simulation and alignment models

Three methods to minimize the impact of alignment mark asymmetry on overlay variation are demonstrated. These methods are measurement based optimal color weighting (OCW), simulation based optimal color weighting, and wafer alignment model mapping (WAMM). Combination of WAMM and OCW methods delivers the highest reduction in overlay variation of 1.3nm (X direction) and 1.2nm (Y direction) as compared to best single color recipe. Simulation based OCW produces a similar reduction in overlay variation as compared to measurement based OCW, and simulation based OCW has the advantage that the scanner alignment recipe with optimize weights can be determined before the mark asymmetry excursion has occurred. Finally, WAMM is capable of reducing the contribution of mark asymmetry on overlay by using a more optimal high order wafer alignment recipe. Capabilities of WAMM can also be combined with OCW solutions.