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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 12 — Jun. 9, 2008
  • pp: 9106–9111
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4X reduction extreme ultraviolet interferometric lithography

Artak Isoyan, A. Wüest, John Wallace, Fan Jiang, and Franco Cerrina  »View Author Affiliations


Optics Express, Vol. 16, Issue 12, pp. 9106-9111 (2008)
http://dx.doi.org/10.1364/OE.16.009106


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Abstract

We report the initial results from a 4X reduction interferometric lithography technique using extreme ultraviolet (EUV) radiation from a new undulator on the Aladdin storage ring at the Synchrotron Radiation Center of the University of Wisconsin-Madison. We have extended traditional interferometric lithography by using 2nd diffraction orders instead of 1st orders. This change considerably simplifies mask fabrication by reducing the requirements for mask resolution. Interferometric fringes reduced by 4X (from 70 nm half-period grating to 17.5 nm) have been recorded in a 50 nm thick hydrogen silsesquioxane photoresist using 13.4 nm wavelength EUV radiation.

© 2008 Optical Society of America

1. Introduction

Interferometric lithography is usually used in the 1st order, generating interference fringes displaying a period equal to half the period of the original mask gratings. Although beneficial, a 2X reduction still poses fabrication challenges for nanopatterning. A larger reduction factor (e.g., 4X instead of 2×) would be highly desirable. This change is not trivial, because of the properties of the imaging interferometric system, as discussed below. Careful optimization of the gratings design, fabrication process, and illumination is necessary to achieve high resolution fringes from large period gratings.

2. EUV Interferometric Lithography

Extreme ultraviolet lithography (EUVL) research involves a broad range of topics, including tools, the source, projection optics, resists, and masks. Exposure systems in the EUV region around 13.4 nm are needed for the development of imaging materials, and advanced photoresists are needed for upcoming lithography nodes at 20 nm and below. Typically, high-resolution patterning at or less than 32 nm period lines and spaces requires optical systems that are complex and expensive. Today, only a few such EUV exposure tools exist [1

1. Burn J. Lin, “Sober view on extreme ultraviolet lithography,” J. Microlith., Microfab., Microsyst. 5, 033005 (2006), and the references there in. [CrossRef]

]. Over the course of the last decade, we have developed a new lithographic technique, EUV interference lithography (EUV-IL), to support research and development efforts in many areas that use periodic patterns for research and production [2–6

2. E. H. Anderson, K. Komatsu, and H. I. Smith, “Achromatic holographic lithography in the deep ultraviolet,” J. Vac. Sci. Technol. B 6, 216 (1988) [CrossRef]

]. As we have shown, a natural extension of EUV-IL is EUV holographic lithography (EUV-HL), in which a computer-generated hologram patterned on a suitable carrier is used to form the image of an arbitrary pattern on a given substrate [7

7. Y.-C. Cheng, A. Isoyan, J. Wallace, M. Khan, and F. Cerrina, “Extreme Ultraviolet Holographic Lithography: Initial Results,” Appl. Phys. Lett. 90, 023116 (2007). [CrossRef]

]. The EUV-IL setup is simple, requiring only a bright and small emittance EUV source (ideally an undulator on a synchrotron [8

8. A. Isoyan, Y.-C. Cheng, F. Jiang, J. Wallace, F. Cerrina, and S. Bollepalli, “Progress in extreme ultraviolet interferometric and holographic lithography,” J. Vac. Sci. Technol. B 25, 2145 (2007). [CrossRef]

]) and the interferometer itself. No complex optics are needed, the system is simple and robust, and the pattern is formed over depths of hundred of microns. The high power allows for fast exposures, so that the throughput is high, while the small beam emittance reduces image blurring [8

8. A. Isoyan, Y.-C. Cheng, F. Jiang, J. Wallace, F. Cerrina, and S. Bollepalli, “Progress in extreme ultraviolet interferometric and holographic lithography,” J. Vac. Sci. Technol. B 25, 2145 (2007). [CrossRef]

].

3. Second Order Diffraction Transmission Gratings

Various interferometric lithography techniques based on diffraction gratings have been described and used in the UV/deep-UV regions [5

5. M. L. Schattenburg, C. Chen, P. N. Everett, J. Ferrera, P. Konkola, and H. I. Smith, “Sub-100 nm metrology using interferometrically produced fiducials,” J. Vac. Sci. Technol. B 17, 2692–2697 (1999). [CrossRef]

]. Recently, we have extended this technique to EUV at 13.4 nm [6

6. H. H. Solak, D. He, W. Li, S. Singh-Gasson, F. Cerrina, B. H. Sohn, X. M. Yang, and P. Nealey, “Exposure of 38 nm period grating patterns with extreme ultraviolet interferometric lithography,” Appl. Phys. Lett. 75, 2328 (1999). [CrossRef]

, 9

9. H. H. Solak, C. David, J. Gobrecht, V. Golovkina, F. Cerrina, S. O. Kim, and P. F. Nealey, “Sub-50 nm period patterns with EUV interference lithography,” Microelectron. Eng. 67–68, 56 (2003). [CrossRef]

]. A conceptual design is shown in Fig. 1(a). All of these techniques use transmission grating’s 1st diffraction order to produce an interferometric fringe pattern, which means the period of the produced gratings is reduced by a factor of 2. Here, we report the extension of the interferometric lithography technique to produce gratings using the 2nd diffraction order of the transmission gratings, which gives a reduction factor of 4X. Thus, the period of the interference fringe is 4X smaller than transmission mask gratings period, considerably simplifying the fabrication of the original gratings.

The interference of two plane waves creates a standing wave pattern with a period p given by p=λ/2sinθ, where θ is equal to half of the angle between the propagation directions of the two beams. The fringe pattern formed is independent of the wavelength of the illumination; hence, the name of “achromatic interferometric lithography.” The ultimate resolution limit achievable in interference imaging is equal to one-quarter of the wavelength, corresponding to θ=π/2. When higher order (m) diffracted beams from two gratings overlap in the central area at a certain distance from the gratings, they form additional interference patterns, with the period pm=pgr/2m, where pgr is the period of the original grating and m is the diffraction order. These fringe patterns are achromatic as well. We stress that the period of imaged fringes is reduced by a factor m, or equivalently the spatial frequency is multiplied by a factor of 2m.

Fig. 1. (a). Sample figure EUV-IL detail showing the regions where the beams overlap for the synthesis of 1st and 2nd order. Right, (b) SEM images of the mask grating structure, (c) 1st and (d) 2nd order diffraction exposure interference fringes recorded in PMMA resist. Notice the relative period of the images. The 1st order working distance (WD) is 400 µm and the 2nd order optimal WD is 132 µm. The scale bar indicates 200 nm.

With an interferometer, we must carefully consider the superposition of the orders in the propagation region. The geometry of the 4X reduction exposure system is shown in Fig. 1(a). The 1st order diffracted beams overlap together starting from a distance of wp1(λp)22λ , where w is the grating width, assumed equal to the separation. The optimal working distance is wp1(λp)2λ when the width of overlap region reaches its maximum, and there is no double overlap between 1st and 2nd orders. This gives us an opportunity to use a mask designed for 2nd order to image 1st order interference fringes as well, with reduced efficiency. On the other hand, the 2nd order working distance at wp1(2λp)22λ overlaps with the 1st order. It can be easily derived that to avoid overlap between the 1st and 2nd orders the optimal working distance for the 2nd order should be: wp(1(2λp)2)(1(λp)2)λ1(2λp)2+21(λp)2 .

Fig. 2. SEM image of 17.5nm half-period 4X reduced 2nd order EUV-IL interference fringes recorded in a 50 nm thick hydrogen silsesquioxane (HSQ) resist.

The fabrication process is based on a standard process developed at the Center for NanoTechnology for creating X-ray diffractive optics [7

7. Y.-C. Cheng, A. Isoyan, J. Wallace, M. Khan, and F. Cerrina, “Extreme Ultraviolet Holographic Lithography: Initial Results,” Appl. Phys. Lett. 90, 023116 (2007). [CrossRef]

, 14

14. Artak Isoyan, Yang-Chun Cheng, Fan Jiang, John Wallace, Mikhail Efremov, Paul Nealey, and Franco Cerrina, “Progress in extreme ultraviolet interferometric lithography at the University of Wisconsin,” Proc. SPIE 6921, 6921R (2008).

]. A low-stress 200 nm Si3N4 is chosen as the thin membrane. A~7 nm Cr and a~15 nm Au layers are deposited on the front-side of the wafer. A JEOL JBX-5DII e-beam lithography tool with 50 keV beam energy is used to pattern the EUV-IL mask structure in 150 nm thick negative tone NEB22A resist. After development, an O2 plasma etch/5 sec is done to completely clean any residue. The EUV-IL mask is then plated for 14 min at 5 mA current using TechnoGOLD 25ES plating solution. The Au is ~100 nm thick. Finally, the EUV-IL mask is etched by O2 plasma for 5 min to remove the NEB22A resist. In this case, an additional stop layer is not needed, due to the high absorption in Au. The fabricated EUV-IL mask contains five different period gratings, starting from 110 nm half-period down to 70 nm half-period at a 400 µm 1st order working distance. The scanning electron microscope (SEM) image of the 110 nm half-period grating is shown on Fig. 1(b). Since the grating spacing bar equals p/4 or 3p/4, the fabrication processes for such masks are sometimes easier.

4. Exposure Results

Exposures were performed at the Center for Nanotechnology’s EUV-IL exposure tool at the Synchrotron Radiation Center of the University of Wisconsin-Madison [15

15. John Wallacea, Yang-Chun Cheng, Artak Isoyan, Quinn Leonard, Mike Fisher, Mike Green, Joseph Bisognano, Paul Nealey, and Franco Cerrina, “A novel EUV exposure station for nanotechnology studies,” Nucl. Instrum. Methods Phys. Res. A 582, 254–257, (2007). [CrossRef]

]. Figures 1(c) and 1(d) show the 55 nm half-period 2X reduced 1st order and 27.5 nm half-period 4X reduced 2nd order EUV-IL printed interference fringes on 55 nm thick PMMA resist. Figure 2 shows 17.5nm half-period 4X reduced 2nd order EUV-IL interference fringes recorded in a 50 nm thick hydrogen silsesquioxane (HSQ) resist. Due to a difference in diffraction efficiency, the dose required to print the 2nd order IL image is 4X greater than that for the 1st order. The exposure time is 40 sec for PMMA, which is acceptable. With the existing mask, we have an exposure area of 80um×10um (H×L). By modifying the distance between the gratings this area can be increased, with the size being ultimately limited by the spatial coherence of the source. In the specific case of a synchrotron source like ours [8

8. A. Isoyan, Y.-C. Cheng, F. Jiang, J. Wallace, F. Cerrina, and S. Bollepalli, “Progress in extreme ultraviolet interferometric and holographic lithography,” J. Vac. Sci. Technol. B 25, 2145 (2007). [CrossRef]

] the vertical spatial coherence allows the exposure of several hundred microns.

It is important to clarify the role of “leakage” through the unpatterned regions, particularly in the area separating the gratings [8

8. A. Isoyan, Y.-C. Cheng, F. Jiang, J. Wallace, F. Cerrina, and S. Bollepalli, “Progress in extreme ultraviolet interferometric and holographic lithography,” J. Vac. Sci. Technol. B 25, 2145 (2007). [CrossRef]

]. Because of the geometry of the system, any radiation transmitted through this region will illuminate the interference area. This leakage will introduce a coherent background in the image, reducing the image modulation. A mask plated with 100 nm of Au has ~1% transmission; we note that ~1% leakage is comparable to the diffraction efficiency in the 2nd order. The uniform background introduces an alternating modulation in the fringes. This effect is clearly visible in Fig. 3(a), where the linewidth is modulated. The right panel in Fig. 3(c), shows the simulated image intensity of the fringes taking leakage into account. The leakage can be reduced by increasing the thickness of the absorbing material (Au) in the separation region between the gratings, while keeping the thickness of the absorber in the grating region to its optimal value.

Fig. 3. SEM image of 27.5 nm half-period grating in PMMA resist: (a) “Leakage” caused distortion of the peaks in the interference pattern; notice the alternating narrower and wider lines. (b) Intensity of the SEM scans; (c) Predicted fringe intensity, explaining the corresponding linewidth modulation. The lines have been added to aid the eye.

5. Conclusion

We have presented the initial results of a 2nd order interferometric lithography technique using the EUV-IL exposure system at the University of Wisconsin-Madison Synchrotron Radiation Center. The techniques we have developed allow patterning with a 4X reduction factor. We have successfully recorded 17.5 nm half-period directly from a 70 nm half-period transmission mask in a 50 nm thick HSQ resist using 13.4 nm wavelength EUV radiation from an undulator source. This technique will allow us to print 10 nm half-period interference fringes from 40 nm half-period gratings on the mask.

The analysis presented here is based on a simple binary grating approach. With increasingly finer gratings this hypothesis becomes inadequate, and phase effects must be considered. However, a more accurate analysis will not change the main conclusions. We also note that the calculations presented here assume a scalar diffraction model. A more sophisticated vector model will likely be needed to analyze the larger diffraction angles formed in the 2nd order and at smaller periods, or p/λ value [16

16. M. Goldstein, Sematech (private communication).

]. We plan to address these issues in a later publication.

Acknowledgments

This work was supported in part by Sematech, Albany. It was also supported in part by the Semiconductor Research Corporation under Contract No. 2005-OC-985. It was also funded by the Nanoscale Science and Engineering Center (NSF DMR-0425880), and by the Synchrotron Radiation Center (NSF DMR-0084402), both at the University of Wisconsin-Madison. The fabrication was performed at the Wisconsin Center for Applied Microelectronics (WCAM) and UW Center for Nanotechnology.

References and Links

1.

Burn J. Lin, “Sober view on extreme ultraviolet lithography,” J. Microlith., Microfab., Microsyst. 5, 033005 (2006), and the references there in. [CrossRef]

2.

E. H. Anderson, K. Komatsu, and H. I. Smith, “Achromatic holographic lithography in the deep ultraviolet,” J. Vac. Sci. Technol. B 6, 216 (1988) [CrossRef]

3.

A. Yen, E. H. Anderson, R. A. Ghanbari, M. L. Schattenburg, and H. I. Smith , “Achromatic holographic configuration for 100-nm-period lithography,” Appl. Opt. 31, 4540 (1992). [CrossRef] [PubMed]

4.

M. Wei, D.T. Attwood, T.K. Gustafson, and E.H. Anderson, “Patterning a 50-nm Period Grating using Soft Xray Spatial Frequency Multiplication”, J. Vac. Sci. and Tech. , 12/6, 3648–3652, (1994).

5.

M. L. Schattenburg, C. Chen, P. N. Everett, J. Ferrera, P. Konkola, and H. I. Smith, “Sub-100 nm metrology using interferometrically produced fiducials,” J. Vac. Sci. Technol. B 17, 2692–2697 (1999). [CrossRef]

6.

H. H. Solak, D. He, W. Li, S. Singh-Gasson, F. Cerrina, B. H. Sohn, X. M. Yang, and P. Nealey, “Exposure of 38 nm period grating patterns with extreme ultraviolet interferometric lithography,” Appl. Phys. Lett. 75, 2328 (1999). [CrossRef]

7.

Y.-C. Cheng, A. Isoyan, J. Wallace, M. Khan, and F. Cerrina, “Extreme Ultraviolet Holographic Lithography: Initial Results,” Appl. Phys. Lett. 90, 023116 (2007). [CrossRef]

8.

A. Isoyan, Y.-C. Cheng, F. Jiang, J. Wallace, F. Cerrina, and S. Bollepalli, “Progress in extreme ultraviolet interferometric and holographic lithography,” J. Vac. Sci. Technol. B 25, 2145 (2007). [CrossRef]

9.

H. H. Solak, C. David, J. Gobrecht, V. Golovkina, F. Cerrina, S. O. Kim, and P. F. Nealey, “Sub-50 nm period patterns with EUV interference lithography,” Microelectron. Eng. 67–68, 56 (2003). [CrossRef]

10.

K. Eidmann, M. Kuhne, P. Muller, and G. D. Tsakiris, “Characterization of pinhole transmission gratings,” J. X-Ray Sci. Technol. 2, 259–273 (1990). [CrossRef]

11.

H. W. Schnopper, L. P. Van Speybroeck, J. P. Delvaille, A. Epstein, E. Kallne, R. Z. Bachrach, J. Dijkstra, and L. Lantward, “Diffraction grating transmission efficiencies for XUV and soft x rays,”Appl. Opt. 16, 1088 (1977). [PubMed]

12.

M. Born and E. Wolf, “Principles of Optics,” Macmillan, New York, (1964), 7th Ed., Chap. 8, p. 412.

13.

L. E. Ruggles, M. E. Cuneo, J. L. Porter, D. F. Wenger, and W. W. Simpson, “Measurement of the efficiency of gold transmission gratings in the 100 to 5000 eV photon energy range.,” Rev. Sci. Instrum. 72, 1218 (2001). [CrossRef]

14.

Artak Isoyan, Yang-Chun Cheng, Fan Jiang, John Wallace, Mikhail Efremov, Paul Nealey, and Franco Cerrina, “Progress in extreme ultraviolet interferometric lithography at the University of Wisconsin,” Proc. SPIE 6921, 6921R (2008).

15.

John Wallacea, Yang-Chun Cheng, Artak Isoyan, Quinn Leonard, Mike Fisher, Mike Green, Joseph Bisognano, Paul Nealey, and Franco Cerrina, “A novel EUV exposure station for nanotechnology studies,” Nucl. Instrum. Methods Phys. Res. A 582, 254–257, (2007). [CrossRef]

16.

M. Goldstein, Sematech (private communication).

OCIS Codes
(040.7480) Detectors : X-rays, soft x-rays, extreme ultraviolet (EUV)
(050.0050) Diffraction and gratings : Diffraction and gratings
(110.3175) Imaging systems : Interferometric imaging
(110.4235) Imaging systems : Nanolithography
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
Imaging Systems

History
Original Manuscript: April 14, 2008
Revised Manuscript: May 21, 2008
Manuscript Accepted: May 23, 2008
Published: June 4, 2008

Citation
Artak Isoyan, A. Wüest, John Wallace, Fan Jiang, and Franco Cerrina, "4X reduction extreme ultraviolet interferometric lithography," Opt. Express 16, 9106-9111 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-12-9106


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References

  1. B. J. Lin, "Sober view on extreme ultraviolet lithography," J. Microlith. Microfab. Microsyst. 5, 033005 (2006), and the references there in. [CrossRef]
  2. E. H. Anderson, K. Komatsu, and H. I. Smith, "Achromatic holographic lithography in the deep ultraviolet," J. Vac. Sci. Technol. B 6, 216 (1988) [CrossRef]
  3. A. Yen, E. H. Anderson, R. A. Ghanbari, M. L. Schattenburg, and H. I. Smith, "Achromatic holographic configuration for 100-nm-period lithography," Appl. Opt. 31, 4540 (1992). [CrossRef] [PubMed]
  4. M. Wei, D. T. Attwood, T. K. Gustafson, and E. H. Anderson, "Patterning a 50-nm Period Grating using Soft Xray Spatial Frequency Multiplication," J. Vac. Sci. Tech.,  12/6, 3648-3652 (1994).
  5. M. L. Schattenburg, C. Chen, P. N. Everett, J. Ferrera, P. Konkola, and H. I. Smith, "Sub-100 nm metrology using interferometrically produced fiducials," J. Vac. Sci. Technol. B 17, 2692-2697 (1999). [CrossRef]
  6. H. H. Solak, D. He, W. Li, S. Singh-Gasson, F. Cerrina, B. H. Sohn, X. M. Yang, and P. Nealey, "Exposure of 38 nm period grating patterns with extreme ultraviolet interferometric lithography," Appl. Phys. Lett. 75, 2328 (1999). [CrossRef]
  7. Y.-C. Cheng, A. Isoyan, J. Wallace, M. Khan, and F. Cerrina, "Extreme Ultraviolet Holographic Lithography: Initial Results," Appl. Phys. Lett. 90, 023116 (2007). [CrossRef]
  8. A. Isoyan, Y.-C. Cheng, F. Jiang, J. Wallace, F. Cerrina, and S. Bollepalli, "Progress in extreme ultraviolet interferometric and holographic lithography," J. Vac. Sci. Technol. B 25, 2145 (2007). [CrossRef]
  9. H. H. Solak, C. David, J. Gobrecht, V. Golovkina, F. Cerrina, S. O. Kim, and P. F. Nealey, "Sub-50 nm period patterns with EUV interference lithography," Microelectron. Eng. 67-68, 56 (2003). [CrossRef]
  10. K. Eidmann, M. Kuhne, P. Muller, and G. D. Tsakiris, "Characterization of pinhole transmission gratings," J. X-Ray Sci. Technol. 2, 259-273 (1990) [CrossRef]
  11. H. W. Schnopper, L. P. Van Speybroeck, J. P. Delvaille, A. Epstein, E. Kallne, R. Z. Bachrach, J. Dijkstra, and L. Lantward, "Diffraction grating transmission efficiencies for XUV and soft x rays," Appl. Opt. 16, 1088 (1977). [PubMed]
  12. M. Born and E. Wolf, Principles of Optics, 7th Edition (Macmillan, New York, 1964), Chap. 8, p. 412.
  13. L. E. Ruggles, M. E. Cuneo, J. L. Porter, D. F. Wenger, and W. W. Simpson, "Measurement of the efficiency of gold transmission gratings in the 100 to 5000 eV photon energy range," Rev. Sci. Instrum. 72, 1218 (2001). [CrossRef]
  14. A. Isoyan, Y.-C. Cheng, F. Jiang, J. Wallace, M. Efremov, P. Nealey, and F. Cerrina, ???Progress in extreme ultraviolet interferometric lithography at the University of Wisconsin," Proc. SPIE 6921, 6921R (2008).
  15. J. Wallacea, Y.-C. Cheng, A. Isoyan, Q. Leonard, M. Fisher, M. Green, J. Bisognano, P. Nealey, and F. Cerrina, "A novel EUV exposure station for nanotechnology studies," Nucl. Instrum. Methods Phys. Res. A 582, 254-257, (2007). [CrossRef]
  16. M. Goldstein, Sematech (private communication).

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