Overlay error due to lens coma and asymmetric illumination dependence on pattern features

Abstract

Coma aberration of projection optics and asymmetric illumination degrade a resist profile symmetricity and enhanced a placement error which depend on pattern size, density and feature. Relative displacements between 2.0 micrometer isolated lines and sub-micron periodic lines are measured to characterize the coma aberration and the illumination telecentricity. A conventional overlay inspection tool and bars-in-bars marks are used for this evaluation. It is difficult to measure the placement error of periodic patterns because their inner patterns have a different placement error from their both ends. The differences in line- width between both ends of periodic lines are caused by the differences in placement errors between their inner and their both ends patterns. The measurement marks for periodic lines are fabricated by the double exposure of two marks. When the resist patterns are imaged by the interference of only three diffracted beams, the coma aberration of Zernike's polynomials can be calculated by the simple expressions from the measurement results. We derived the condition of coherency (sigma) and pattern pitch P which satisfy the imaging of only three diffracted beams. The coma aberration is estimated to be 0.12 wavelength at maximum for the exposure system we applied. In this method, the focus is not so important for the accurate measurement because the measurement data remain constant at any foci. It takes only one hour to estimate the coma aberration completely including a sample making (sample making: 0.5 h, measurement/evaluation: 0.5 h). The relative pattern shifts between large patterns and fine patterns could be usually caused by not only coma aberration but also illumination optics. The telecentricity of the illumination optics is also discussed in this paper.