Basic imaging characteristics of phase edge lithography and impact of lens aberration on these

Imaging characteristics of lithography using phase edge type PSM are investigated intensively based on optical image calculations. Even in the absence of aberration, imaging characteristics are very complicated for high coherent illumination. Shape of CD-Focus curve of isolated line varies from concave to convex with increasing dark line, i.e., Cr line width on mask. Hence, in the case that DOF is optimized for smallest Cr width on mask, DOF decreases with increasing image CD. Moreover, CD-Focus characteristics varies with shifter width on mask. As a result, fair CD-Focus characteristics can be obtained for some specific patterns. In contrast with this, under medium coherent illumination, variation of CD-Focus characteristics with changing mask pattern is less significant than that for high coherent illumination. Large DOF can be obtained for wide range of image CD and pattern pitch. However, finest CD with iso-focal characteristics, which is given by zero Cr width mask, is larger for lower coherent illumination. In the presence of lens aberration, CD-Focus characteristics are significantly degraded in general. Iso-focal CD characteristics for fine line, which motivate the development of this technology, is disappeared by spherical aberration due to iso-focal tilt. It is revealed that very low aberration with RMS-OPD of 0.01 wave may disrupt good characteristic which would be obtained under ideal optics even for medium coherent illumination. However, it is found that the degraded focus characteristics can be almost completely recovered by adjusting primary spherical aberration to optimum one while higher order aberration is not changed. Moreover, after adjustment of aberration, better DOF can be obtained for some specific pattern with RMS-OPD larger than that in current stepper. As a conclusion, by the optimization of primary spherical aberration, phase edge lithography can become applicable to formation of device pattern under some restriction of pattern layout which depends on partial coherence of illumination.