Focus Monitor Using Asymmetric Diffraction Rays by Off-Axis Monopole Illumination

Some methods of focus monitoring for lithography tools use nontelecentric imaging delivered from phase shift masks and require double exposure. A newly proposed focus monitoring method requires neither a special mask nor double exposure. A mask pattern of 1:1 duty ratio is illuminated by an off-axis monopole source. The positive first-order diffraction ray from the pattern is designed to advance in the direction of the optical axis. Then, the second-order diffraction ray is not generated. Only zeroth-order and the first-order diffraction rays contribute to imaging. At defocus, the pattern shifts sideways. On the other hand, a double-pitch pattern does not shift at defocus because rays are generated at symmetrical positions against the optical axis. The focal value is obtained by measuring their relative position. For an ArF immersion exposure tool of 1.3 numerical aperture (NA), pattern sizes of 86 nm half pitch and 43 nm half pitch were used. The focus position was measured from the relative position between the two patterns.