OSA's Digital Library

Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 4 — Feb. 25, 2013
  • pp: 5209–5214
« Show journal navigation

Optical patterning of features with spacing below the far-field diffraction limit using absorbance modulation

Farhana Masid, Trisha L. Andrew, and Rajesh Menon  »View Author Affiliations


Optics Express, Vol. 21, Issue 4, pp. 5209-5214 (2013)
http://dx.doi.org/10.1364/OE.21.005209


View Full Text Article

Acrobat PDF (1139 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

Absorbance modulation is an approach that enables the localization of light to deep sub-wavelength dimensions by the use of photochromic materials. In this article, we demonstrate the application of absorbance modulation on a transparent (quartz) substrate, which enables patterning of isolated lines of width 60nm for an exposure wavelength of 325nm. Furthermore, by moving the optical pattern relative to the sample, we demonstrate patterning of closely spaced lines, whose spacing is as small as 119nm.

© 2013 OSA

The smallest focal spot with conventional far-field optics is limited by the diffraction limit to about λ/2, where λ is the illumination wavelength [1

1. E. Abbé, “Beitragezurtheorie des mikroskops und der mikroskopischenwahrnehmung,” Arch. Mikrosk. Anat. Entwichlungsmech 9(1), 413–418 (1873). [CrossRef]

]. This diffraction limit also constraints the resolution of features that can be patterned using optics. There are a number of approaches that have been proposed to circumvent the far-field diffraction limit. These approaches include near-field optical [2

2. E. A. Ash and G. Nicholls, “Super-resolution aperture scanning microscope,” Nature 237(5357), 510–512 (1972). [CrossRef] [PubMed]

4

4. L. Novotny, B. Hecht, and D. Pohl, “Implications of high resolution to near-field optical microscopy,” Ultramicroscopy 71(1-4), 341–344 (1998). [CrossRef]

] or contact lithography [5

5. T. Ito, M. Ogino, T. Yamanaka, Y. Inao, T. Yamaguchi, N. Mizutani, and R. Kuroda, “Fabrication of sub-100nm patterns using near-field mask lithography with ultra-thin resist process,” J. Photopolym. Sci. Technol. 18(3), 435–441 (2005). [CrossRef]

], where in the maskless case, a nanoscale tip or aperture, which serves as the source of photons scans across a photosensitive material. The high spatial frequencies that are normally evanescent and hence, negligible in the far-field, can contribute to the near-field. In this case, the size of the spot is comparable to the size of the tip. Unfortunately, this size is very sensitive to the distance between the tip and the sample, and hence quite difficult to control. Furthermore, this process is serial and very slow. It is difficult to parallelize due to the challenge of maintaining the spacing between the tip and the substrate. Contact photolithography suffers from the requirement of a mask with its concomitant disadvantages. Furthermore, intimate contact across the image field that is required for high resolution is extremely challenging to achieve [6

6. J. Goodberlet, “Patterning 100 nm features using deep-ultraviolet contact photolithography,” Appl. Phys. Lett. 76(6), 667 (2000). [CrossRef]

].

The new process is shown schematically in Figs. 1(e)-1(g). The substrate is comprised of a quartz slide that is transparent to both wavelengths of interest. The quartz substrate is first cleaned by using a mixture of NH4OH:H2O2:H2O (1:1:5) at 80°C for 30mins.HMDS is then spun-cast at 6000rpm for 60s and air-dried for 10minutes. This serves as an adhesion promotion layer. The AML is comprised of 1,2-bis[2-methyl-5-(5′-methyl-2'-thienyl)-3-thienyl]hexafluorocyclopentene (BTE) mixed into a 30 mg/mL solution of PMMA in anisole at a 95 weight-percent loading. Then, the AML isspun-cast at 1000rpm for 3s and 500rpm for 60s. After baking the sample in an oven at 110°C for 60 minutes, the AML forms a layer of thickness 410nm, which we verified with a surface profiler. Then a solution of PVA in water at concentration of 1:4.4 by weight is spun-cast at 3000rpm for 60s.Upon baking the sample in an oven at 80°C for 5 minutes, the PVA forms a layer of thickness 12nm. A second layer of HMDS is spun-cast on the PVA layer at 6000rpm for 60s to improve adhesion to the last photoresist layer. The sample is air-dried for 10 minutes. Finally, a solution of Shipley 1813 photoresist thinned down with type-P thinner to a concentration of 1:11 by weight is spun-cast at 2000rpm for 60s. After baking the sample in an oven at 110°C for 15 minutes, the photoresist forms a layer of thickness 50nm. Exposure is performed through the quartz substrate as illustrated in Fig. 1(f). The exposure system is a modified Lloyd’s-mirror interferometer that utilizes two laser wavelengths as illustrated in Fig. 2(a)
Fig. 2 (a) Illustration of the dual-wavelength Lloyd’s mirror interferometer, where the sample is illuminated by two standing waves. The period of the λ1 standing wave is approximately half that of the λ2 standing wave. (b) Atomic-force micrograph of lines in developed resist after a single exposure. (c) Linewidth as a function of exposure time for single exposures.
. A standing wave of period ~280 nm was formed at a wavelength, λ1 = 325nm and a standing wave of period ~570 nm was formed at a wavelength, λ2 = 647nm. The intensity of the λ1 beam was ~4μW/cm2 and that of the λ2 beam was ~54mW/cm2. After exposure, the sample was developed directly without any intervening process steps as illustrated in Fig. 1(g).

An atomic force micrograph of the developed photoresist surface is shown in Fig. 2(b). Note that lines of width as small as 60nm are clearly resolved. This corresponds to λ1/5.4. The far-field diffraction limit is given by half the period of the standing wave at λ1, i.e., 140nm. It is important to point out that the λ2 photons do not have sufficient energy to expose the photoresist. Nevertheless, the lines are spaced by the period of the λ2 standing wave as expected by the illustration in Fig. 1(f). The linewidth variation across the image field is likely due to variations in the thickness of the AML as well as some high-spatial frequency noise in the laser illumination. Better AML formulation that allows for increased solvation and uniform packing of the BTE molecules within the polymer matrix, as well as improved processing conditions (e.g., faster spin coating) will enhance the AML film quality in the future. In Fig. 2(c), we show how the linewidths change with exposure dose. As expected, the linewidth increases slowly at first and at a faster rate at higher exposure dose. This is opposite to what one would expect with a simple sinusoid intensity distribution of the λ1 illumination. As we have discussed earlier, absorbance modulation increases the normalized image slope (and image contrast) [18

18. R. Menon and H. I. Smith, “Absorbance-modulation optical lithography,” J. Opt. Soc. Am. A 23(9), 2290–2294 (2006). [CrossRef] [PubMed]

], which agrees with the linewidth-dose dependence that we observed here.

By introducing a relative rotation between the multiple exposures, it is possible to create more complex geometries. An example with two exposures is shown in Fig. 4(a)
Fig. 4 (a) Schematic of a 2-step exposure, where the 2nd exposure is rotated with respect to the 1st. (b) and (c) Atomic-force micrographs of two samples that were exposed twice with a small rotation in between. Black-dashed circles show the corresponding regions.
, where two exposures (6 hours each) were performed with a small rotation between the patterns. In this case, we blocked half the UV beam such that the sample was exposed to a uniform beam at λ1 and a standing wave at λ2. The intervening exposure to only the red beam was for 4 hours. For the samples shown in Figs. 4(b) and 4(c), the AML was spun-cast at 500 rpm for 5 minutes and formed a layer of thickness ~700nm. The atomic-force micrographs shown in Figs. 4(b) and 4(c) show exposed regions that are spaced by a distance of 142nm and 119nm, respectively. This corresponds to λ2/4.6 and λ2/5.44, respectively. Note that here λ2 is used for comparison since the λ1 beam is unpatterned and hence, has no spatial frequencies to contribute. These examples indicate that absorbance modulation enables patterns whose spacing can be smaller than the far-field diffraction limit of the optical system. Note that in this case, the limit is defined by the largest spatial frequency in the λ2 beam, which corresponds to a spacing of 570nm/2 = 285nm.

Optics has significant advantages for high-throughput nanomanufacturing as evidenced by the ubiquitous popularity of optical-projection lithography in semiconductor manufacturing. However, the far-field diffraction limit is a fundamental physical barrier that curtails nanomanufacturing. In this article, we described preliminary results that demonstrate the feasibility of absorbance-modulation optical lithography (AMOL) as a means to multiple exposures with no intervening process steps. Further optimization of the photochromic material and the photoresist, when combined with an array of two-dimensional nodes in the λ2 beam can generate nanoscale patterns of complex geometries analogous to super-resolution imaging of complex distribution of fluorophores [20

20. S. Berning, K. I. Willig, H. Steffens, P. Dibaj, and S. W. Hell, “Nanoscopy in a living mouse brain,” Science 335(6068), 551 (2012). [CrossRef] [PubMed]

].

Acknowledgments

We would like to thank Brian Baker and Brian Van Devener of the Utah nanofabrication facility for assistance with characterizing the nanostructures. We also thank Apratim Majumder for assistance with the Lloyd’s-mirror setup. Financial support from DARPA and the Utah Science, Technology and Research (USTAR) Initiative are gratefully acknowledged.

References and links

1.

E. Abbé, “Beitragezurtheorie des mikroskops und der mikroskopischenwahrnehmung,” Arch. Mikrosk. Anat. Entwichlungsmech 9(1), 413–418 (1873). [CrossRef]

2.

E. A. Ash and G. Nicholls, “Super-resolution aperture scanning microscope,” Nature 237(5357), 510–512 (1972). [CrossRef] [PubMed]

3.

E. Betzig, J. K. Trautman, T. D. Harris, J. S. Weiner, and R. L. Kostelak, “Breaking the diffraction barrier - optical microscopy on a nanometric scale,” Science 251(5000), 1468–1470 (1991). [CrossRef] [PubMed]

4.

L. Novotny, B. Hecht, and D. Pohl, “Implications of high resolution to near-field optical microscopy,” Ultramicroscopy 71(1-4), 341–344 (1998). [CrossRef]

5.

T. Ito, M. Ogino, T. Yamanaka, Y. Inao, T. Yamaguchi, N. Mizutani, and R. Kuroda, “Fabrication of sub-100nm patterns using near-field mask lithography with ultra-thin resist process,” J. Photopolym. Sci. Technol. 18(3), 435–441 (2005). [CrossRef]

6.

J. Goodberlet, “Patterning 100 nm features using deep-ultraviolet contact photolithography,” Appl. Phys. Lett. 76(6), 667 (2000). [CrossRef]

7.

S. W. Hell, A. Engler, E. Rittweger, B. Harke, J. Engelhardt, and S. W. Hell, “Far-field optical nanoscopy,” Science 316(5828), 1153–1158 (2007). [CrossRef] [PubMed]

8.

J. Fischer, G. von Freymann, and M. Wegener, “The materials challenge in diffraction-unlimited direct-laserwriting optical lithography,” Adv. Mater. 22(32), 3578–3582 (2010). [CrossRef] [PubMed]

9.

J. T. Fourkas, “Nanoscale photolithography with visible light,” J. Phys. Chem. Lett. 1(8), 1221–1227 (2010). [CrossRef]

10.

L. J. Li, R. R. Gattass, E. Gershgoren, H. Hwang, and J. T. Fourkas, “Achieving lambda/20 resolution by one-color initiation and deactivation of polymerization,” Science 324(5929), 910–913 (2009). [CrossRef] [PubMed]

11.

T. F. Scott, B. A. Kowalski, A. C. Sullivan, C. N. Bowman, and R. R. McLeod, “Two-color single-Photon photoinitiation and photoinhibition for sub-diffraction photolithography,” Science 324(5929), 913–917 (2009). [CrossRef] [PubMed]

12.

T. Tsuujioka, M. Kume, Y. Horikawa, A. Ishikawa, and M. Irie, “Super-resolution disk with a photochromic mask layer,” Jpn. J. Appl. Phys. 36(Part 1, No. 1B), 526–529 (1997). [CrossRef]

13.

T. Tsujioka, M. Kume, and M. Irie, “Theoretical analysis of super-resolution optical disk mastering using a photoreactive dye mask layer,” Opt. Rev. 4(3), 385–389 (1997). [CrossRef]

14.

T. L. Andrew, H.-Y. Tsai, and R. Menon, “Confining light to deep sub-wavelength dimensions to enable optical nanopatterning,” Science 324(5929), 917–921 (2009). [CrossRef] [PubMed]

15.

H.-Y. Tsai, H. I. Smith, and R. Menon, “Reduction of focal-spot size using dichromats in absorbance modulation,” Opt. Lett. 33(24), 2916–2918 (2008). [CrossRef] [PubMed]

16.

H.-Y. Tsai, G. M. Wallraff, and R. Menon, “Spatial-frequency multiplication via absorbance modulation,” Appl. Phys. Lett. 91(9), 094103 (2007). [CrossRef]

17.

R. Menon, H.-Y. Tsai, and S. W. Thomas 3rd, “Far-field generation of localized light fields using absorbance modulation,” Phys. Rev. Lett. 98(4), 043905 (2007). [CrossRef] [PubMed]

18.

R. Menon and H. I. Smith, “Absorbance-modulation optical lithography,” J. Opt. Soc. Am. A 23(9), 2290–2294 (2006). [CrossRef] [PubMed]

19.

R. F. Pease and S. Y. Chou, “Lithography and other patterning techniques for future electronics,” Proc. IEEE 96(2), 248–270 (2008). [CrossRef]

20.

S. Berning, K. I. Willig, H. Steffens, P. Dibaj, and S. W. Hell, “Nanoscopy in a living mouse brain,” Science 335(6068), 551 (2012). [CrossRef] [PubMed]

OCIS Codes
(110.4235) Imaging systems : Nanolithography
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
Imaging Systems

History
Original Manuscript: January 17, 2013
Revised Manuscript: February 14, 2013
Manuscript Accepted: February 16, 2013
Published: February 22, 2013

Citation
Farhana Masid, Trisha L. Andrew, and Rajesh Menon, "Optical patterning of features with spacing below the far-field diffraction limit using absorbance modulation," Opt. Express 21, 5209-5214 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-4-5209


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. E. Abbé, “Beitragezurtheorie des mikroskops und der mikroskopischenwahrnehmung,” Arch. Mikrosk.Anat. Entwichlungsmech9(1), 413–418 (1873). [CrossRef]
  2. E. A. Ash and G. Nicholls, “Super-resolution aperture scanning microscope,” Nature237(5357), 510–512 (1972). [CrossRef] [PubMed]
  3. E. Betzig, J. K. Trautman, T. D. Harris, J. S. Weiner, and R. L. Kostelak, “Breaking the diffraction barrier - optical microscopy on a nanometric scale,” Science251(5000), 1468–1470 (1991). [CrossRef] [PubMed]
  4. L. Novotny, B. Hecht, and D. Pohl, “Implications of high resolution to near-field optical microscopy,” Ultramicroscopy71(1-4), 341–344 (1998). [CrossRef]
  5. T. Ito, M. Ogino, T. Yamanaka, Y. Inao, T. Yamaguchi, N. Mizutani, and R. Kuroda, “Fabrication of sub-100nm patterns using near-field mask lithography with ultra-thin resist process,” J. Photopolym. Sci. Technol.18(3), 435–441 (2005). [CrossRef]
  6. J. Goodberlet, “Patterning 100 nm features using deep-ultraviolet contact photolithography,” Appl. Phys. Lett.76(6), 667 (2000). [CrossRef]
  7. S. W. Hell, A. Engler, E. Rittweger, B. Harke, J. Engelhardt, and S. W. Hell, “Far-field optical nanoscopy,” Science316(5828), 1153–1158 (2007). [CrossRef] [PubMed]
  8. J. Fischer, G. von Freymann, and M. Wegener, “The materials challenge in diffraction-unlimited direct-laserwriting optical lithography,” Adv. Mater.22(32), 3578–3582 (2010). [CrossRef] [PubMed]
  9. J. T. Fourkas, “Nanoscale photolithography with visible light,” J. Phys. Chem. Lett.1(8), 1221–1227 (2010). [CrossRef]
  10. L. J. Li, R. R. Gattass, E. Gershgoren, H. Hwang, and J. T. Fourkas, “Achieving lambda/20 resolution by one-color initiation and deactivation of polymerization,” Science324(5929), 910–913 (2009). [CrossRef] [PubMed]
  11. T. F. Scott, B. A. Kowalski, A. C. Sullivan, C. N. Bowman, and R. R. McLeod, “Two-color single-Photon photoinitiation and photoinhibition for sub-diffraction photolithography,” Science324(5929), 913–917 (2009). [CrossRef] [PubMed]
  12. T. Tsuujioka, M. Kume, Y. Horikawa, A. Ishikawa, and M. Irie, “Super-resolution disk with a photochromic mask layer,” Jpn. J. Appl. Phys.36(Part 1, No. 1B), 526–529 (1997). [CrossRef]
  13. T. Tsujioka, M. Kume, and M. Irie, “Theoretical analysis of super-resolution optical disk mastering using a photoreactive dye mask layer,” Opt. Rev.4(3), 385–389 (1997). [CrossRef]
  14. T. L. Andrew, H.-Y. Tsai, and R. Menon, “Confining light to deep sub-wavelength dimensions to enable optical nanopatterning,” Science324(5929), 917–921 (2009). [CrossRef] [PubMed]
  15. H.-Y. Tsai, H. I. Smith, and R. Menon, “Reduction of focal-spot size using dichromats in absorbance modulation,” Opt. Lett.33(24), 2916–2918 (2008). [CrossRef] [PubMed]
  16. H.-Y. Tsai, G. M. Wallraff, and R. Menon, “Spatial-frequency multiplication via absorbance modulation,” Appl. Phys. Lett.91(9), 094103 (2007). [CrossRef]
  17. R. Menon, H.-Y. Tsai, and S. W. Thomas, “Far-field generation of localized light fields using absorbance modulation,” Phys. Rev. Lett.98(4), 043905 (2007). [CrossRef] [PubMed]
  18. R. Menon and H. I. Smith, “Absorbance-modulation optical lithography,” J. Opt. Soc. Am. A23(9), 2290–2294 (2006). [CrossRef] [PubMed]
  19. R. F. Pease and S. Y. Chou, “Lithography and other patterning techniques for future electronics,” Proc. IEEE96(2), 248–270 (2008). [CrossRef]
  20. S. Berning, K. I. Willig, H. Steffens, P. Dibaj, and S. W. Hell, “Nanoscopy in a living mouse brain,” Science335(6068), 551 (2012). [CrossRef] [PubMed]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Figures

Fig. 1 Fig. 2 Fig. 3
 
Fig. 4
 

« Previous Article

OSA is a member of CrossRef.

CrossCheck Deposited