Sensitivity of coma monitors to resist processes

State of the art exposure tools today are expected to operate at very small k1 factors for semiconductor manufacturing, imposing very tight requirements on lens performance. To evaluate the lens quality with respect to coma and other asymmetric aberrations, two types of monitors, 5-line monitor structure and box in box structure, are used. For a 5-line monitor structure lens coma assessment is achieved by measuring the difference of printed CD between the left most outer and the right most outer line. The principle for box in box structure is to utilize the effect of feature size-dependent image placement. Coma assessment can be achieved by measuring overlay shift between a frame of target line width and a square of larger dimension.