<title>Effect of variable sigma aperture on lens distortion and its pattern size dependence</title>

Abstract

While the modified illumination with various ifiumination apertures is the promising method to get fmer patterns, there are many problems in putting it into practical use. In particular, the different apertures give different effects on the lens distortion and they cause the pattern displacement. In this paper, we studied the image placement error of fme patterns under 1 im and its pattern pitch dependence using various illumination apertures. The 1st layer of O.8prn lines and spaces is patterned using conventional aperture(NA=O.57, a=O.6) and the 2nd layer is exposed on the etched patterns with various apertures such as conventional, annular and small sigma. The patterns of the 2nd layer are O.4jim lines with various spaces. Pattern displacements of the 2nd layer are measured within the lens field along the X axis. The results show that the displacements are affected by the illumination status. In addition, the displacement varies with the pattern pitch and the placements of the fine patterns are different from those of large overlay patterns such as box-in-box. These results are confirmed by the optical simulator. The simulated results correspond to the experimental results and they point out the 3rd order coma aberration most effects the pattern displacement. The pattern size and pitch dependence of the image displacement reveals that the measurement results of the conventional overlay patterns such as box-in-box with the sizes of 1O-2Ojim doesn't represent the overlay of the fine patterns of actual devices.

Level-specific lithography optimization for 1-Gb DRAM

“…Attention must also be paid to the impact of resolution enhancement techniques on overlay and logic circuitry connecting the DRAM arrays. Image placement error depends on both illumination condition and pattern size, and overlay measurements must be adjusted accordingly [51]. Moreover, the use of annular illumination, despite improving the process latitude of the dense wordline grating pattern, degrades integrity of peripheral patterns unless proper biasing is applied or the etch process is changed [18].…”

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

“…The aberration function with coma of Zernike's polynomials is given by w(R,e) = z7(3R2 2)RcosO + Z8(3R2 2)RsinO , (2) where Z7 and 4 are coefficients of primary coma aberration. If W(R,O=O) and W(R,O=ic/2) are defined to be Wx(R) and W(R), we can rewrite (2) as w(R) = z7(3R2 2)R, w(R) = z8(3R2 2)R .…”