EUV Image Fading Contributes to the Stochastic Defect Floor

EUV lithography systems differ from other lithography systems in that they are based on reflective optics. This includes the patterned mask itself, which includes a reflective multilayer. Since the light is coming from an off-vertical, off-axis angle, the image is shifted as the reflected light propagates through the system. In actual practice, there is light incident onto the mask from more than one angle. The angles should be balanced around the optical axis so that the image on the wafer doesn’t shift with defocus [1]. For example, 32 nm pitch vertical lines can only receive incident light from certain angles in a 0.33 NA EUV system, which can be plotted on a pupil map (Figure 1).

Figure 1. Pupil map for 32 nm pitch vertical lines for a 0.33 NA EUV system. The x and y coordinates represent 4x the sines of the incident angles on the mask with respect to the optical axis of the system.

The off-center nature of the distribution of angles means that the images formed from the left side will be shifted relative to the images formed from the right side [1,2]. In the case of Figure 1, the images will be shifted by ~2.3 nm to the right and left, respectively [1]. When added together at the wafer, the resulting image has reduced contrast, i.e., reduced difference between maximum and minimum intensity at the wafer.

Figure 2. The superposition of the images from the left and right monopoles (left and right sectors of the dipole, respectively) result in a net image that has reduced contrast (difference between maximum and minimum intensity).

This reduction of contrast is referred to as “image fading” [1] or “contrast fading” [2]. It can be modeled as an effective blur of ~2.1 nm. To quantify this effect, we can average the dose along the contour which is supposed to be minimum dose, and divide that by the average dose along the contour which is supposed to be maximum dose. Figure 3 shows the reduced min/max dose ratio from fading.

Figure 3. Min/max ratio for 32 nm pitch vertical lines, with and without fading. The reduced contrast from fading increases the ratio.

The reduced contrast means greater sensitivity to stochastic dose variations, since the printing threshold is more likely to be within the range of the variations, now that the maximum and minimum are closer together. In particular, this means some portions of the regions that are not supposed to be exposed, will get exposed (Figure 4).

Figure 4. Left: 32 nm pitch vertical line without fading. Right: 32 nm pitch vertical line with fading. The absorbed dose is 12 mJ/cm2. The resist blur is taken to be 3 nm.

For these defects, resist would be removed where they should remain. This will be made worse with the inclusion of non-EUV exposure from the plasma environment in the EUV system, or the secondary electrons backscattering from the substrate [3]. We also have defects from resist remaining in regions that are supposed to be exposed. This is aggravated by low doses as well as blur. Increasing doses or using special etching to get rid of these excess resist defects will unfortunately add more of the first (resist-missing) type of defect. This enforces a “floor” level of defectivity that is the bane of EUV lithography.

The above case used the specific example of vertical 32 nm pitch lines. It should also be noted that different patterns will have different image shifts from different incident angles used. The degree of fading will vary and will need rigorous calculations to be determined. But it may be expected that smaller pitches will have worse fading [1]. As soon as more of that information becomes available, the impact on defectivity should be studied.

References

[1] J-H. Franke, T. A. Brunner, and E. Hendrickx, J. Micro/Nanopattern. Mater. Metrol. 21, 030501 (2022).

[2] C. van Lare et al., J. Micro/Nanopattern. Mater. Metrol. 20, 021006 (2021).

[3] F. Chen, Non-EUV Exposures in EUV Lithography Systems Provide the Floor for Stochastic Defects in EUV Lithography, https://www.linkedin.com/pulse/non-euv-exposures-euv-lithography-systems-provide-floor-chen-jymgc

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