DESIGN OF PHASE-SHIFTING MASKS FOR ENHANCING RESOLUTION OF IMAGES IN OPTICAL LITHOGRAPHY

The problem of optimal design of phase-shifting masks for enhancing re solution of images in optical lithography is examined based on the gen eral theory of amplitude-phase retrieval in coherent optical system. W e propose a practical algorithm used for the design of phase-shifting masks. To illustrate the new approach the numerical simulation designs of the phase-shifting mask are carried out for several mode images, f or instance, one-dimensional (1-D) equal-spacing lines and two-dimensi onal (2-D) model patterns. We find that for 1-D model images, the opti mal phase distributions are automatically taken binary values, 0 and p i, between the consecutive lines. For the 2-D model images, the compli cated contructions contained in the object patterns can be resolved ve ry well by using the designed phase-shifting mask. In the new approach the phase modulation in the masks is required to encode only the clea r regions of object patterns. Therefore, the new approach may provide an effective scheme for implementing systematic optimal design of phas e-shifting masks used for real integrated circuit lithography.