A novel multiple focus position control method by conjugate twin-shifter phase shift lithography

A novel phase shift lithography method has been developed that allows different IC features on the same mask to be focused simultaneously in different optical phases. This permits the focal position of each feature to be shifted to match the surface topography of the wafer. The direction and magnitude of each focal shift are determined by the design of mask phase shifters. This method is applicable for use with both conventional Cr opaque mask patterns and contrast-enhanced phase shift patterns. The effective depth of focus (DOF) range can be increased by 2.0 mu m or more by permitting more of the available DOF budget to be used for random focus fluctuations rather than for normal surface topography. Characteristics of this method are evaluated both experimentally and theoretically using TCC optical simulation. Typical mask structures are described. Results are shown using positive and negative i-line resist processes on features down to 0.30 mu m lines and isolated features down to 0.35 mu m.<>