Overlay and lens distortion in a modified illumination stepper

Abstract Optical Lithography, when extended by phase shift mask technology and modified illumination techniques,is a promising technology for sub-half-micron devices. Phase shift mask technology uses phase-shifted light to enhance resolution and process latitude by adding a shifter layer on the mask. Modified illumination, which uses circular, rectangular, and cross apertures, can also improve imaging. Modified illumination can improve the resolution limit and depth of focus, but the imaging profile ischanged, with pattern type, direction, and density having an effect on the result. The uniformity of the illumination system also differs according to aperture type. Because lens distortion may be affected by theaerial image and structure of illumination optics, we can expect that a modified illumination system mayaffect lens distortion and overlay accuracy in a real process. A comparison of changes in overlay and lens distortion was done for different illumination conditions. Focus was varied for each combination.As a result, we can observe the variation of overlay error in a modified illumination system relative to theconventional system. The deviation of overlay and distortion error in a field is dependent on the structure ofthe aperture illumination optics. To use modified illumination in subhalf-micron processes, this distortionerror must be reduced.