Aerial imaging study of the mask-induced line-width...

Aerial imaging study of the mask-induced line-width roughness of EUV lithography masks

Antoine Wojdyla1,*, Alexander Donoghue1, Markus P. Benk1, Patrick P. Naulleau1

and Kenneth A. Goldberg1

1Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 (USA)

ABSTRACT

EUV lithography uses reflective photomasks to print features on a wafer through the formation of an aerial image. The aerial image is influenced by the mask’s substrate and pattern roughness and by photon shot noise, which collectively affect the line-width on wafer prints, with an impact on local critical dimension uniformity (LCDU). We have used SHARP, an actinic mask-imaging microscope, to study line-width roughness (LWR) in aerial images at sub-nanometer resolution. We studied the impact of photon density and the illumination partial coherence on recorded images, and found that at low coherence settings, the line-width roughness is dominated by photon noise, while at high coherence setting, the effect of speckle becomes more prominent, dominating photon noise for exposure levels of 4 photons/nm2 at threshold on the mask size. Keywords: EUV lithography, LER, LWR, actinic inspection, LCDU, X-Ray Microscopy, photon noise, partial coherence, power-spectral density, speckle, metrology, real-space imaging

1. INTRODUCTION

As extreme ultraviolet (EUV) lithography pushes lithographic resolution closer to the atomic scale, the specifications on critical dimension uniformity (CDU) are becoming more stringent. That is especially true for local CD uniformity (LCDU), which is a direct manifestation of the resist line-edge roughness (LER).

Though often considered within the context of the well-known resolution, LER and sensitivity (RLS) trade-off for photoresists, the final LER in printed features (Figure 1, right) arises from chemical processes in the resist, and from imperfections of the mask, transferred to the aerial image. These include pattern roughness (Figure 1, left) and the influence of speckle (i.e. phase roughness). Phase effects come from mask and multilayer roughness, and can only be seen in the actinic imaging. (Figure 1, center).

Extreme Ultraviolet (EUV) Lithography VII, edited by Eric M. Panning, Proc. of SPIE Vol. 9776,97760H · © 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2219513

Proc. of SPIE Vol. 9776 97760H-1

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2. DATA

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Figure 8. Effect of partial coherence on line-width roughness. (left) overlaid cross-sections for a line from a single measurement acquired with a .33 4xNA lens, 40 s exposure time and partial coherence σ = 0.3, σ = 0.6 and σ = 0.8; (right) overlaid line-width measurements for many measurements of the same line, for partial coherence σ = 0.3, σ = 0.6 and σ = 0.8. Lower coherence decreased speckle but increased sensitivity to photon noise.



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There is however a slight advantage to incoherent illumination in that the roughness no longer has definite coherence length. In that respect, engineered partial coherence can be used [11] to reduce the sensitivity to some kind of defects such as bridging, that would be caused by mask roughness (attributable to a bright speck, akin to a slight phase defect.) Another way to influence the line width is to change the intensity threshold. Since the interpolation is somewhat aggressive, with measured line-width variations smaller than the pixel size, changing the threshold by a small amount does not necessarily involve different image pixels, resulting in a linear scaling. However, the image is oversampled by a factor of 2.7 relative to the Nyquist limit, and the effective pixel size in our experiment is small enough to allow using pair-wise distinct interpolation points (figure 9, left.) Our experiment shows that there is no appreciable change in the LWR or sensitivity to photon noise while changing the threshold within ten percent: CD and CDU do not affect LWR or LCDU measurement (figure 9, right.)

Figure 9. Aerial image line-width for various thresholds, with σ = 0.3 and 40 s exposure time. (left) cross-section of the line and thresholds; (right) corresponding lines-width along the line. The local roughness is not affected by more than the photon noise.

6. POWER SPECTRAL DENSITY Comparing the line-width roughness of various lines in the same field allows us to estimate the power spectral density of the lines and their dependence on the imaging conditions (figure 10.)

Figure 10. Power spectral density of the LWR (averaged over 14 contiguous lines), for various exposure times at σ = 0.3 (left) and for three different σ values at a long exposure time (right.) All lines are statistically independent and identically distributed (i.i.d.)



For longer exposure times and higher partial coherence settings (σ = 0.3), where the photon-noise contribution is small compared with the intrinsic LWR, it is possible to discern a sharp roll-off in the PSD at approximately 6 cycles/µm. We expect the limited numerical aperture (NA) of the lens to cause a spatial frequency cutoff near asin(4xNA/4)/ λ = 6.11 cycles/µm. Partial coherence (high σ) has the effect of the decreasing the steepness of the roll-off at higher spatial frequencies since there is less optical filtering, while the line width becomes more sensitive to photon-noise (which is not band-limited) at higher frequency.

7. CONCLUSION

Though resist line-edge roughness is generally governed by chemical processes (chemical shot noise, acidic diffusion), the roughness embedded in the aerial image, either as a replication of the mask pattern roughness, replicated surface roughness or photon noise, will continue to play a more significant role as pattern dimensions shrink. We have shown that it is possible to study the intrinsic aerial image LWR from actinic photo-mask imaging with sub-nanometer precision. In the examples presented, the conjunction of the illumination coherence and the intrinsic roughness of the mask creates speckle that cause LWR on the order of 5.8 nm on the mask side when studying a nominal CD of 160 nm, which can be measured using photon densities in excess of 4 photons/nm2, at the threshold intensity level. In addition, we found that although incoherent illumination (i.e. higher σ values) reduces speckle amplitude, a smaller NILS makes measurements more sensitive to photon shot noise. The (repeatable) effect of the reproduced mask roughness is replaced by the (statistical) influence of the photon noise, and results seems to indicate that increased exposure time does not affect the minimum LWR set by speckle formation, reducing the impact of speckle mitigation [11]. We recognize that pattern size, threshold level, illumination and NA play a role in the final measured LWR, and require further investigation.

ACKNOWLEDGEMENT

We would like to thank Intel Corporation for the manufacturing of the mask used in these experiments, Suchit Bhattarai and Valeriy Yashchuk for fruitful discussions, and David Johnson for his participation in the data collection. The LBNL EUV program is supported by Eureka, and initial funding for SHARP was from SEMATECH. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

REFERENCES

[1] George, S. A., Naulleau, P. P., Mochi, I., Salmassi, F., Gullikson, E. M., Goldberg K. A., Anderson, E. H., “Extreme ultraviolet mask substrate surface roughness effects on lithographic patterning,” J. Vac. Sci. Technol., B 28(6), 23-30 (2010). [2] Naulleau, P. P., “Relevance of mask-roughness-induced printed line-edge roughness in recent and future extreme-ultraviolet lithography tests,” Appl. Opt. 43(20), 4025-4032 (2004). [3] Naulleau, P. P. and Gallatin, G., “Spatial scaling metrics of mask-induced line-edge roughness,” J. Vac. Sci. Technol., B 26(6), 1903-1910 (2008). [4] Naulleau, P. P., George, S. A., “Implications of image plane line-edge roughness requirements on extreme ultraviolet mask specifications,” Proc. SPIE 7379, 73790O1-11 (2009). [5] Zweber, A. E., Gallagher, E., Sanchez, M., Senna, T., Negishi, Y., Konishi, T., McGuire, A., Bozano, L., Brock. P., Truong, H., “EUV Mask Line Edge Roughness,” Proc. SPIE 8322, 83220O1-10 (2012). [6] Pret, A. V., Gronheid, R., Engelen, J., Yan, P.-Y., Leeson M. J., Younkin, T. R. “Evidence of speckle in extreme-UV lithography,” Appl. Opt. 20(23) 25970-25978 (2012). [7] Yang, P.-Y., Zhang G., Gullikson, E. M., Goldberg, K. A., Benk, M. P., “Understanding EUV mask blank surface roughness induced LWR and associated roughness requirement,” Proc. SPIE 9422, 94220J1-12 (2015).



[8] Goldberg, K. A., Mochi, I., Benk, M. P., Lin, C., Allezy, A., Dickinson, M., Cork, C. W., Macdougall, J. B., Anderson, E,. H., Chao, W., Salmassi, F., Gullikson, E., M., Zehm, D., Vytla, V., Cork, W., DePonte, J., Picchi, G., Pekedis, A., Katayanagi, T., Jones, M. G., Martin, E., Naulleau P. P., Rekawa, S. B., “The SEMATECH high-NA actinic reticle review project (SHARP) EUV mask-imaging microscope, ” Proc. SPIE 8880, 88800T1-9 (2013). [9] Mangat, P., Verduijn, E., Wood, O. R., Benk M. P., Wojdyla, A., Goldberg, K. A., “Mask blank defect printability comparison using optical and SEM mask and wafer inspection and bright field actinic mask imaging,” Proc. SPIE 9658, 96580E1-8 (2015). [10] Wintz, D. T., Goldberg, K. A., Mochi, I., Huh, S., “Photon flux requirements for extreme ultraviolet reticle imaging in the 22- and 16-nm nodes,” J. Micro/Nanolith. MEMS MOEMS 9(4), 041205-1-8 (2010) [11] McClinton, B. M. and Naulleau, P. P., “Mask roughness induced LER control and mitigation: aberrations sensitivity study and alternate illumination scheme,” Proc. SPIE 7969, 79691Z1-11 (2011)



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