Subwavelength photolithography based on

surface-plasmon polariton resonance

Xiangang Luo and Teruya Ishihara
Frontier Research System, RIKEN, 2-1 Hirosawa, Wako 351-0198, Japan
lxg@postman.riken.go.jp

Abstract: The use of surface-plasmon polariton (SPP) resonance in the

optical near field of a metallic mask to produce fine patterns with a
resolution of subwavelength scale is proposed. Preliminary numerical
simulations indicate that the critical resolution is determined mainly by the
thickness of the metallic mask. The surface of the metallic mask on the
illuminated side collects light through SPP coupling, and the interference of
SPPs on the exit side of the metallic mask results in enhanced optical
intensity with high spatial resolution, which can facilitate nanolithography
efficiently by use of conventional photoresist with simple visible or
ultraviolet light sources. Several schemes for sub-half-wavelength
lithography based on SPPs are described. Inasmuch as the technique is not
diffraction limited, nanostructures can be reproduced photolithographically.
 2004 Optical Society of America
OCIS codes: (350.4600) Optical engineering; (220.3740) Lithography;
(220.4000) Microstructure fabrication; (240.6680) Surface plasmons; (260) Physical optics;
(260.2110) Electromagnetic theory; (290.0290) Scattering; (350.3950) Micro-optics.

References and links

1. M. D. Levenson, “Extending the lifetime of optical lithography technologies with wavefront engineering,”
Jpn. J. Appl. Phys. 33, 6765-6773 (1994).
2. M. V. Klein, Optics (Wiley, New York, 1970).
3. S.Okazaki, “Resolution limits of optical lithography,” J. Vac. Sci. Technol. B 9, 2829-2833 (1991).
4. M. Rothschild, T. M. Bloomstein, J. E. Curtin, D. K. Downs, T. H. Fedynyshyn, D. E. Hardy, R. R. Kunz,
V. Liberman, J. H. C. Sedlacek, R. S. Uttaro, A. K. Bates, and C. Van Peski, “157 nm: deepest deep-
ultraviolet yet,” J. Vac. Sci. Technol. B 17, 3262-3266 (1999).
5. C. W. Gwyn, R. Stulen, D. Sweeney, and D. Attwood, “Extreme ultraviolet lithography,” J. Vac. Sci.
Technol. B 16, 3142-3149 (1998).
6. J. P. Silverman, “Challenges and progress in x-ray lithography,” J. Vac. Sci. Technol. B 16, 3137-3141
(1998).
7. G. Timp, R. E. Behringer, D. M. Tennant, J. E. Cunningham, M. Prentiss, and K. K. Berggren, “Using light
as a lens for submicron, neutral-atom lithography,” Phys. Rev. Lett. 69, 1636-1639 (1992).
8. K. K. Berggren, A. Bard, J. L. Wilbur, J. D. Gillaspy, A. G. Heig, J. J. McClelland, S. L. Rolston, W. D.
Phillips, M. Prentiss, and G. M. Whitesides, “Microlithography using neutral metastable atoms and self-
assembled monolayers,” Science 269, 1255-1257 (1995).
9. K. S. Johnson, J. H. Thywissen, N. H. Dekker, K. K. Berggren, A. P. Chu, R.Younkin, and M. Prentiss,
“Formation and detection of atomic wavepackets localized to the Heisenberg uncertainty limit: a new
nanolithographic technique,” Science 280, 1583- 1586 (1998).
10. A. N. Boto, P. Kok, D. S. Abrams, S. L. Braunstein, C. P. Williams, and J. P. Dowling, “Quantum
interferometric optical lithography: exploiting entanglement to beat the diffraction limit,” Phys. Rev. Lett.
85, 2733-2736 (2000).
11. M. A. McCord, “Electron beam lithography for 0.13 µ m manufacturing,” J. Vac. Sci. Technol. B 15, 2125
–2129 (1997).
12. J. Melngailis, “Focused ion beam lithography,” Nucl. Instrum. Methods Phys. Res. B 80, 1271-1280
(1993).
13. M. A. McCord and R. F. W. Pease, “Lithography with the scanning tunneling microscope,” J. Vac. Sci.
Technol. B 4, 86-88 (1986).
14. E. Betzig and J. K. Trautman, “Near-field optics: microscopy, spectroscopy, and surface modification
beyond the diffraction limit,” Science 257, 189-195 (1992).

#4258 - $15.00 US Received 23 April 2004; revised 15 June 2004; accepted 16 June 2004
(C) 2004 OSA 12 July 2004 / Vol. 12, No. 14 / OPTICS EXPRESS 3055
 15. S. Y Chou, P. R. Krauss, and P. J. Renstrom, “Nanoimprint lithography,” J. Vac. Sci. Technol. B 14, 4129-
4133 (1998).
16. H. Tan, A. Gilbertson, and S. Y. Chou, “Roller nanoimprint lithography,” J. Vac. Sci. Technol. B 16, 3926
–3928 (1998).
17. H. Schmid, H. Biebuyck, B. Michel, and O. J. F. Martin, “Light-coupling masks for lensless,
subwavelength optical lithography,” Appl. Phys. Lett. 72, 2379-2381 (1998).
18. J. G. Goodberlet, “Patterning 100 nm features using deep-ultraviolet contact photolithography,” Appl. Phys.
Lett. 76, 667-669 (2000).
19. J. G. Goodberlet and H. Kavak, “Patterning sub-50 nm features with near-field embedded-amplitude
masks,” Appl. Phys. Lett. 81, 1315-1317 (2002).
20. M. M. Alkaisi, R. J. Blaikie, S. J. McNab, R. Cheung, and D. R. S. Cumming, “Sub-diffraction-limited
patterning using evanescent near-field optical lithography,” Appl. Phys. Lett. 75, 3560-3562 (1999).
21. P. G. Kik, A. L. Martin, S. A. Maier, and H. A. Atwater, “Metal nanoparticle arrays for near field optical
lithography,” in Properties of Metal Nanostructures, N. J. Halas, ed., Proc. SPIE 4810, 7-13 (2002).
22. O. J. Martin, N. B. Piller, H. Schmid, H. Biebuyck, and B. Michel, "Energy flow in light-coupling masks
for lensless optical lithography," Opt. Express 3, 280-285 (1998),
http://www.opticsexpress.org/abstract.cfm?URI=OPEX-3-7-280
23. H. A. Bethe, “Theory of diffraction by small holes,” Phys. Rev. 6, 163-182 (1944).
24. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer-Verlag,
Heidelberg, 1988), Chap. 2, pp. 4-39.
25. G. G. Nenninger, P. Tobiška, J. Homola, and S. S. Yee, “Long-range surface plasmons for high-resolution
surface plasmon resonance sensors,” Sensors Actuators 74, 145-151 (2001).
26. W. L. Barnes, S. C. Kitson, T. W. Preist, and J. R. Sambles," Photonic surfaces for surface plasmon-
polaritons," J. Opt. Soc. Am. A 14, 1654-1661 (1997).
27. F. Yang, J. R. G. Sambles, and W. Bradberry, “ Long-range surface modes supported by thin films,” Phys.
Rev. B. 44, 5855- 5872 (1991).
28. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6, 4370-4379
(1972).
29. T. W. Ebbesen, H. L. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, “Extraordinary optical transmission
through sub-wavelength hole arrays,” Nature (London) 391, 667- 669 (1998).
30. H. F. Ghaemi, T. Thio, D. E. Grupp, T. W. Ebbesen, and H. J. Lezec, “Surface plasmons enhance optical
transmission through subwavelength holes,” Phys Rev. B 58, 6779-6782 (1998).
31. L. Salomon, F. Grillot, A. Zayats, and F. De Fornel, “Near-field distribution of optical transmission of
periodic subwavelength holes in a metal film,” Phys. Rev. Lett. 86, 1110-1113 (2001).
32. E. Popov, M. Neviere, S. Enoch, and R. Reinisch, “Theory of light transmission through subwavelength
periodic hole arrays,” Phys. Rev. B 62, 16,100-16,108 (2000).
33. U. Schroter and D. Heitmann, “Surface-plasmon-enhanced transmission through metallic gratings,” Phys.
Rev. B 58, 15,419 –15,421(1998).
34. L. Martin-Moreno, F. J. Garcia-Vidal, H. J. Lezec, K. M. Pellerin, T. Thio, J. B. Pendry, and T. W.
Ebbesen, “Theory of extraordinary optical transmission through subwavelength hole arrays,” Phys. Rev.
Lett. 86, 1114-1117 (2001).
35. J. A. Porto, F. J. Garcia-Vidal, and J. B. Pendry, “Transmission resonances on metallic gratings with very
narrow slits,” Phys. Rev. Lett. 83, 2845-2848 (1999).
36. W. L. Barnes, W. A. Murray, J. Dintinger, E. Devaux, and T. W. Ebbesen, “Surface plasmon polaritons and
their role in the enhanced transmission of light through periodic arrays of subwavelength holes in a metal
film,” Phys. Rev. Lett. 92, 107401 (2004)
37. M. G. Moharam and T. K. Gaylord, “Rigorous coupled-wave analysis of metallic surface-relief gratings,” J.
Opt. Soc. Am. A 3, 1780-1787 (1986).
38. S. C. Hohng, Y. C. Yoon, D. S. Kim, V. Malyarchuk, R. Muller, C. Lienau, J. W. Park, K. H. Yoo, J. Kim,
H. V. Ryu, and Q. H. Park, “Light emission from the shadows: surface plasmon nano-optics at near and far
fields,” Appl. Phys. Lett. 81, 3239-3241 (2002).
39. X. Luo and T. Ishihara, “Resonant light transmission in metallic photonic crystal slabs,” Int. J. Nanosci. 1,
657-661 (2002).
40. J. B. Pendry, “Negative refraction makes a perfect lens,” Phys. Rev. Lett. 85, 3966-3969 (2000).

1. Introduction
Combined with wave-front engineering technologies, photolithography has remained a useful
microfabrication technology because of its ease of repetition and suitability for large-area
fabrication [1]. The diffraction limit [2], however, restricts the fabrication scale of
conventional photolithography [3]. Potential solutions that have actually been pursued require
increasingly shorter illumination wavelengths for replicating increasingly smaller structures.

#4258 - $15.00 US Received 23 April 2004; revised 15 June 2004; accepted 16 June 2004
(C) 2004 OSA 12 July 2004 / Vol. 12, No. 14 / OPTICS EXPRESS 3056
However, it is becoming more difficult and complicated to use the short optical wavelengths
to reach the desired feature sizes. For example, the use of wavelengths in the deep-ultraviolet
[4], the extreme-ultraviolet [5], or the x-ray regime [6] or in atomic lithography [7-9] requires
significantly difficult adjustments of the lithographic process, including the development of
new light sources, photoresists, and optics. Although the introduction of light sources with
shorter-wavelength and wave-front engineering techniques should help to maintain the current
rate of device miniaturization for several more years, it seems that a point will be reached at
which the traditional approaches of shortening the exposure wavelength will no longer be able
to produce the required feature sizes, so large-scale efforts to develop other approaches to
nanolithography have been made. For example, quantum lithography has been explored in
recent years, although it is far from application [10].
A relatively well-established method for the production of high-resolution patterns is the
use of sequential techniques. One such technique is the use of focused particle beams, e.g.,
focused electron beams [11] or ion beams [12], which expose a resist layer as it is scanned
across the substrate. Another involves the use of a local probe such as the tip of an atomic-
force microscope [13] or the tip of the near-field scanning optical microscope [14]. Although
it produces high-resolution patterns, the sequential nature of this technique results in long
writing times. Several other lithographic techniques under development, such as
nanoimprinting [15,16], have the capability to overcome these limits, but any shift to a
nonoptical lithographic technique will require the introduction of a new infrastructure of tools,
materials, and processing technologies, the research and development costs of which will be
huge.
Recently, however, optical contact or near-field lithography, with which it has been
shown that sub-100-nm features can be produced with one-to-one amplitude masks brought
into contact with a resist-coated substrate, has been developed experimentally as a promising
parallel route to overcoming the diffraction limit. Several schemes that do not require short-
wavelength light, such as contact printing through a transmission mask or a binary phase-shift
mask [17,18], evanescent near-field lithography [19, 20], plasmon printing, [21] and the use
of light-coupling masks [22], have been successfully demonstrated for performing near-field
photolithography. Near-field contact lithography uses comparably simpler illumination
sources without sophisticated imaging optics while maintaining large fields of exposure. The
light coming from the apertures on the mask exposes the photoresist. However, according to
standard diffraction theory, as the feature size on the mask shrinks much below the exposed
wavelength, the amount of transmitted light is small, and light is diffracted into directions
with large divergent angles [23]. Especially on a nanoscale, the transmission of visible or
ultraviolet light through two-dimensional apertures on a metallic mask is extremely low, thus
limiting the exposure efficiency. This process suffers from a long exposure time and low
contrast. The low transmission and low contrast are considered fundamental constraints on the
achievable resolution and depth of exposure for technological purposes.
In this paper we propose a new technique named surface-plasmon polaritonic lithography
(SPPL) that has the potential to produce subwavelength structures with high efficiency by the
coherent illumination of a standard photoresist with ultraviolet or visible light. The key idea is
to use surface-plasmon polaritons (SPPs) instead of photons as an exposure source. The
wavelength of a SPP is much shorter than that of illumination light, so the spatial resolution is
much enhanced. On thin metallic films, the wavelength of a SPP is determined mainly by the
coupling of SPPs on both sides of the films. The smaller the thickness of the metallic film is,
the shorter the wavelength of the SPP is. The materials close to the metallic interface with a
large dielectric constant can also contribute a large resolution. In this sense, SPPL is similar to
optical lithography for emersion in a high-index liquid or for a solid-emersion lens, but this
effect is limited by the index of material that is used. The resolution of SPPL not only is
related to the refractive index but also is determined by the thickness of the metallic film. The
basic method of operation of SPPL employed in the study described in this paper is described
in Section 2. Results of numerical simulations are detailed in Section 3. The discussion is
given in Section 4.

#4258 - $15.00 US Received 23 April 2004; revised 15 June 2004; accepted 16 June 2004
(C) 2004 OSA 12 July 2004 / Vol. 12, No. 14 / OPTICS EXPRESS 3057
2. Method
A schematic of the SSPL process is shown in Fig. 1(a). The mask consists of a thin metallic
film with apertures much smaller than the wavelength of the illumination light. In a traditional
situation this is a typical diffraction-limited exposure, so a resolution of less than
approximately λ/2 cannot be achieved. In SPPL, however, the excitation of SPPs on the
metallic surfaces gives us a way to circumvent this limit (Fig. 1(b)). The resultant enhanced
optical field close to the metal surface can cause increased exposure of the resist layer with
enough contrast directly below the mask patterns with high resolution.
When a metallic mask is placed in an optical field, it can exhibit collective electron
oscillations known as SPPs, which are charge density waves that are characterized by intense
electromagnetic fields confined to the metallic surface. For a weak corrugation upon a semi-
infinite silver surface, the dispersion of SPPs can be approximately replaced by that of a
smooth metal film [24]:

Fig. 1. a, Schematic representation of SPPL. b, Schematic representation of resolution

enhancement by SPPL.

ksp = ω(εdεm/(εd + εm))1/2/c , (1)

where ω is the excitation frequency, ksp is the wave vector of SPP, εd is the dielectric constant
of the interface medium, and εm is the dielectric constant of the metal. The dispersion of SPPs
on semi-infinite silver is shown in Fig. 2 (red curve). If the metal layer is sufficiently thin,
plasma waves at both metal interfaces are coupled, resulting in symmetric and antisymmetric
SPPs [25, 26]. For a thin metallic film with thickness d specified by ε2 surrounded by media
specified by ε1 and ε3, the coupling equations are as follows [27]:
tanh(α2d) = -ε2α2(ε1α3+ε3α1)/( ε1ε3α22+ε22α1α3)
α2j = k2-k20ε j, j = 1, 2, 3 , (2)
k = kr-iki
where k0 is the value of the wave vector of illumination light, k is the value of the wave vector

#4258 - $15.00 US Received 23 April 2004; revised 15 June 2004; accepted 16 June 2004
(C) 2004 OSA 12 July 2004 / Vol. 12, No. 14 / OPTICS EXPRESS 3058
of SPP, kr and ki are real part and imaginary part of SPP wave vector. Solving the coupling
equations in a silver film yields the dispersion curves of SPPs for silver masks with different
thickness that are shown in Fig. 2. The frequency-dependent dielectric constant of silver is
incorporated by use of a second-order polynomial fitted to tabulated data over the 350–900-
nm range [28]. The surrounding refractive index is 1.7, which is within the range of refractive
indices of photoresists. For a semi-infinite silver film the wave vector of a SPP is larger than
that of the light line when the optical frequencies are below 3250 meV. However, for the thin
films (100, 60, and 50 nm) the wave vectors of SPPs are always larger than that of the light
line. In particular, when the illumination frequency is close to 3250 meV, the wave vectors of
SPPs are much larger than that of an illumination wave, so the resolution can be enhanced
greatly compared with that of traditional kinds of photolithography, for which the resolution is
diffraction limited.

4000

3500

3000
Energy (meV)

2500

2000 Light line

semi-infinite Ag
100 nm Ag
1500 60 nm Ag
50 nm Ag
1000

0.00 0.04 0.08 0.12 0.16 0.20 0.24

-1
Wave vector (nm )
Fig. 2. Dispersion of SSPs on several masks.

Figure 3 shows the critical resolution of SPPL obtained by the interference of SPPs on
metallic films. When there is no SPP effect, the resolution limit is just half a wavelength of
illumination light. However, with SPP excitation the critical resolution is much smaller than
the illumination wavelength. For a g-line (one spectrum from mercury light), which has an
optical energy of 2843 meV that corresponds to 436 nm in wavelength, the resolution of SPPL
is ~50 nm (dashed line). However, the critical resolution of traditional g-line photolithography
without SPP excitation is just 218 nm (dashed–dotted line). The smaller the thickness of the
metallic mask is, the higher the resolution enhancement that is obtained. For example, the
maximum resolution of a mask with a thickness of 50 nm can reach 20 nm when the energy
approaches 3250 meV (dotted line).
For a periodic mask, one can excite the SPP by coupling the illumination light with the
grating momentum [24]:
ksp= kx± mxGx ± myGy , (3)
where ksp is the surface-plasmon wave vector, kx is the in-plane component of the incident
wave vector, Gx and Gy are the reciprocal lattice vectors in the x and y directions, and mx and
my are integers. With the help of reciprocal lattice vectors, input light with long wavelength
can excite SPPs. For normal incidence, ksp = ± mxGx ± myGy, which means that ksp = ± mxGx
and ksp = ± myGy may be excited simultaneously. That is to say, the SPP waves on the mask
have opposite propagating directions along the metallic films. The interference of the SPP

#4258 - $15.00 US Received 23 April 2004; revised 15 June 2004; accepted 16 June 2004
(C) 2004 OSA 12 July 2004 / Vol. 12, No. 14 / OPTICS EXPRESS 3059
waves results in standing waves along the metallic film. For one-dimensional patterns, the
resonant wavelength λ at normal incidence is
λ =2π (εdεm/(εd + εm))1/2/mxGx (4)
The positions that correspond to selected values of mx agree well with the far-field
transmission peaks, as has been shown theoretically [29-36]. One can tune the wavelength at
which resonance occurs by changing the pitch, the incident angle, or the refractive index of
the surrounding material in the mask. For example, for a given photoresist (the refractive
index is 1.7), when the pitch is 400 nm, exposure sources with a wavelength of 436 nm can be
used. When the pitch is changed to 300 nm, the resonant wavelength is shifted to 365 nm.
If the parameters of the patterns on the exit side of the metallic mask are much smaller
than the illumination wavelength, the oscillating charges produce an oscillating dipole field
about the structured surface, which produces a localized distribution of the electromagnetic
field about the metal part. If the resonance frequency falls within the sensitivity range of a
photoresist, the field enhancement can be used to locally expose a thin layer of resist. For
example, using standard g-line or i-line photoresist requires a resonance wavelength of 300–
460 nm.

4000

3500

3000
Energy (meV)

Without SPP
2500 Semi-infinite Ag
100 Ag
60 Ag
2000
50 Ag

1500

1000

0 200 400 600 800 1000 1200

Resolution (nm)
Fig. 3. Resolution of surface-plasmon polaritonic interference.

3. Electric field distribution

In what follows, we analyze the behavior of the electric field distribution on a metallic mask
based on rigorous coupled-wave theoretical models [37] that provide a qualitative, idealized
illustration of some of the quantitative support of this technique. Several conditions are
defined in our description. The numerical values of refractive indices for the substrate of the
mask and the resist used in simulation are 1.46 and 1.7, respectively. The dielectric constant
of the silver is taken from the experimental measurement [28]. The perfect boundary
condition is used for both top and bottom metal interfaces. For a deep corrugation the resonant
condition of light leaving a structured mask in contact with a substrate varies with the
parameters of the configuration and the details of the implementation. The wafer layer is not
included in this simulation because of the small decay length of the near field. Here we give
the electric field distributions under SPP excitation for several different plasmon masks. For

#4258 - $15.00 US Received 23 April 2004; revised 15 June 2004; accepted 16 June 2004
(C) 2004 OSA 12 July 2004 / Vol. 12, No. 14 / OPTICS EXPRESS 3060
simplicity, two-dimensional simulation is performed in the study reported in this paper;
however, the simulation can be extended to three dimensions.
3.1 Symmetric perforated plasmon mask
Figure 4 shows the electric field distribution on a mask consisting of an array of metallic holes
(we refer it as a perforated plasmon mask). The thickness of the mask is 50 nm. The line/space
ratio on the masks is fixed to 1:10. The wavelength of the illumination light is 436 nm, which
corresponds to 2843 meV in energy. The electric field distribution represents the case in
which a photoresist layer in the near field of the mask is indexed matched to its supporting
substrate, which is experimentally possible with appropriate choices of materials.

Fig. 4. Electric field distributions for four masks.

In Figs. 4(a), 4(b), 4(c), and 4d the periodicities of the masks are 450, 600, 900, and 1200
nm, respectively. The corresponding pattern numbers of the interference of SPPs between the
adjacent slits are 4, 6, 8, and 12, which means that for different periodicities the number of
high-intensity ranges near the mask is different. Close to the metallic surface, a high-intensity
range with a spatial resolution of ~50 nm can be observed, which we attribute to the
interference of SPPs with large wave vectors. Because the illumination wavelength and the
thickness of the mask are not changed, the obtainable resolution does not change obviously
when the width of the metal between the apertures on the mask is modified. The emission
from the holes’ locations decays rapidly with increasing distance from the bottom surface of
the mask. The field that originates from the metal parts decays slowly, which clearly indicates
that a large portion of the field emanates from the metal surfaces. So most of the light
transmitted is found not at the locations of the holes but on the metal surfaces. These
phenomena are similar to the observations studied by Hohng et al [38]. The field distribution
shows that the interference of SPPs causes a highly directional high intensity range, which
results in a finite depth of the electric field in the surrounding materials. In this sense,
conformal contact is not needed during exposure. In experiment, a layer of refractive-index-

#4258 - $15.00 US Received 23 April 2004; revised 15 June 2004; accepted 16 June 2004
(C) 2004 OSA 12 July 2004 / Vol. 12, No. 14 / OPTICS EXPRESS 3061
matched material is necessary to be added on the bottom side of the mask as a protection layer.
The width of the exposed pattern can be controlled by changing the thickness of the protection
layer.
Qualitatively similar results are obtained when the mask and the substrate layers are not
index matched. We believe that the presence of these phenomena is an intrinsic feature caused
by the interference of SPP waves. The result suggests that, in this type of geometry, it is
possible to fabricate resist pattern groups with different numbers of features according to
requirements by changing the periodicity of the mask.
3.2 Symmetric unperforated plasmon mask
In this subsection we propose another approach to achieving SPPL. The distinguishable
physical characteristic of the mask is that all the holes or slits are sealed with some metal. It is
generally thought that either an increase in thickness or the presence of additional metal layers
can drastically reduce the transmission of visible light owing to absorption and reflection.
However, our recent study has shown that light can be transmitted through unperforated
metallic structures to the far field [39], which provides us a hint for doing lithography with
unperforated metallic plasmon masks. Without loss of generality, we studied the electric field
distribution under SPPs excited by visible light.
The mask that we studied had an array of periodic grooves corrugated on both surfaces
with a periodicity of 900 nm, a groove width of 180 nm, and a center layer with a thickness of
10 nm. On the illumination side, one collects photons by exciting SPP modes and then by
tunneling the plasmon through the grooves to establish the SPP modes on the other side.
Finally, the interference of the SPPs on the exit side results in high spatial resolution. Figure 5
shows the electric field distribution at the resonant condition achieved by excitation of the
SPP on the interface between the mask and the photoresist with a wavelength of 932 nm,
which corresponds to 1328 meV in energy. Because of the coupling between the modes on
opposite sides, the SPP on the illuminated side is magnified through a metallic slab [40] and
reaches a maximum on the exit side with a resolution of ~180 nm, which is approximately one
fifth of the illumination wavelength. The field is located mainly in the photoresist. Another
significant new feature is the appearance of strong bright spatial intensity distribution with a
size of 20 nm at the aperture corner. The influence of the edge effects can be removed by
addition of a thin layer of index-matching materials with a thickness of 5–10 nm on the
bottom side of the metallic mask. The results show that the near-field distribution of SPPs has
high field intensity and high spatial resolution with enough contrast. It is good for
subwavelength lithography.

Fig. 5. Electric field distribution of SPPs for a symmetric unperforated plasmon mask.

#4258 - $15.00 US Received 23 April 2004; revised 15 June 2004; accepted 16 June 2004
(C) 2004 OSA 12 July 2004 / Vol. 12, No. 14 / OPTICS EXPRESS 3062
3.3 Asymmetric unperforated plasmon mask
The mask with symmetric grooves on the opposite faces shows a strongly radiative response.
How about the mask with asymmetric grooves on both sides? As another example of SPPL,
we designed an asymmetric unperforated silver mask that has separate arrays of periodic
grooves corrugated on the illuminated side and on the resist side with 180o phase shift. The
parameters are as follows: The groove depth is 30 nm, the thickness of the center layer is 30
nm, the groove width is 140 nm, the periodicity is 700 nm, and the illumination wavelength is
539 nm, which corresponds to 2300 meV in energy. Figure 6 shows that at the resonant
condition the grooves on the illuminated (upper) face collect light by improving the coupling-
in efficiency, whereas the grooves on the lower face redistribute the field distribution with a
spatial resolution of 140 nm, which is almost one fourth of the illumination wavelength. The
density of the interferential patterns increases to four times compared with that of the pattern
features on the mask.

Fig. 6. Electric field distribution of SPPs in an asymmetric unperforated plasmon mask.

From the discussion above, one can see how to control the density of the resist patterns
by changing the parameters of the mask. Inasmuch as the electric field strength drops off
gradually with increasing distance from the bottom surface of the mask, the intensity and the
width of the high-intensity features gradually become smaller when the distance between the
exposure location and the bottom side of the metallic mask increases (Figs. 3–6), so the size
of the exposed area and the exposure time will depend on the intensity and the duration of the
exposure.
From the electric field distribution discussed above, our type of lithography is different
from previously used contact lithography. In our case the exposure field comes from the metal
surface, whereas in previous methods the exposure field came from the location of the holes
in the mask [20]. It should be noted that the effect of resist absorption on the plasmon
resonance has not been taken into account in these calculations. For a highly absorbing resist
layer, the absorption may reduce the magnitude of the field enhancement.
4. Discussion
We have demonstrated a method for obtaining subwavelength resolution that is based on the
interference of SPPs in the near field of a metallic mask. The spatial patterns are determined
by the coherent interference of scattered SPP waves. For the same resolution, the electric field
distribution has different spatial configurations when the apertures on the mask are different.
The interference of the SPPs causes a highly directional intensity range with high intensity in

#4258 - $15.00 US Received 23 April 2004; revised 15 June 2004; accepted 16 June 2004
(C) 2004 OSA 12 July 2004 / Vol. 12, No. 14 / OPTICS EXPRESS 3063
a finite depth of electric field, which is good for noncontact exposure. From the field
distribution in this paper, we can see the electric-field intensity at the edge of the apertures on
the mask is much higher than that from the flat part, so the contact-exposed result will not be
uniform. However, the high intensity from the edges decay very fast with increasing distance
from the bottom surface of the mask, while the field from the flat part decays slowly. By
utilizing the decay properties, the edge effect can be removed by adding a layer of refractive-
index-matched material. For example, photoresist is one good candidate to be attached on the
bottom surface of the plasmon mask as a protection layer to get rid of edge effect. The detail
information of the protection layer will be analysised elsewhere. In this paper our purpose for
introducing periodicity is to excite SPPs. Once the SPPs are excited, the interference of SPPs
can result in high spatial resolution, so the number of periods is not critical. For example, a
metallic film mask can be designed in this way: Periodic structures are introduced on both
ends of a flat metallic film. Then light is shone on one of the periodic structures to excite a
SPP. The SPP propagates along the flat metallic film. It will be reflected by the periodic
structure on the other end of the film when it meets the structure on the other end. The
reflected SPP interferes with the original SPP to form standing SPP waves along the metallic
film. One possible way to achieve much higher resolution is to decrease the thickness of the
metallic mask. Figure 7 shows the resolution of thin metallic masks of different thicknesses.
The smaller the thickness of the metallic film is, the higher the resolution that can be obtained.
Another way is achieve higher resolution is to excite SPPs by tuning the illumination energy
to 3250 meV. When the illumination frequency is smaller than 3250 meV, the resolution of
SPPL is much larger than the critical resolution of traditional optical lithographies. For a
metallic film of 20 or 10 nm the resolution can be several nanometers, which can be
comparable with the resolution obtained by x-ray lithography, although the illumination
frequencies are just within the optical frequency range. In this case, although the wavelength
of SPP is very short, the lithographic scheme has to be changed because the film has already
become transparent, which causes worse contrast. For example, a metallic film with a
thickness of 10 nm can be used for excitation of a SPP that has a short wavelength, and other
metallic tips can be introduced to guide the SPP to expose photoresist. For arbitrary features
we can use an unperforated mask to obtain isolated or arbitrary patterns by modifying the
metallic patterns on the exit side of the mask or use metallic tips.
4000

3500

3000
Energy (meV)

Without SPP
Semi-infinite Ag
2500 10 nm
20 nm
30 nm
2000
40 nm

1500

1000

0 200 400 600 800 1000 1200 1400

Resolution (nm)
Fig. 7. Resolution of SPPL for a thin metallic film.

#4258 - $15.00 US Received 23 April 2004; revised 15 June 2004; accepted 16 June 2004
(C) 2004 OSA 12 July 2004 / Vol. 12, No. 14 / OPTICS EXPRESS 3064
5. Conclusions
In conclusion, we have demonstrated that subwavelength photolithography beyond the
diffraction limit can be attained with SPPs excited by optical frequency. Several schemes for
fabricating resist patterns were proposed. The effective wavelength of the exposure is that of
the SPP, which is much smaller than that of the illumination light because of the coupling
between photon and fluctuating electrons, so the resolution is improved. One can produce
arbitrary patterns by optimizing the configuration on the exit side of the unperforated metallic
mask. SPPL satisfies the requirements of many single-step lithographic applications.
However, the difference between the patterns on the mask and the patterns in the resist
remains open to subsequent investigation. The simplicity, utility, and economy of the
approach nevertheless make high-resolution, high-density optical lithography a practical
reality in nanotechnology.
Acknowledgments
We thank L. Jay Guo of the University of Michigan and S. Tikhodeev and N. Gippius of the
Russian Academy of Sciences for valuable discussions and suggestions.

#4258 - $15.00 US Received 23 April 2004; revised 15 June 2004; accepted 16 June 2004
(C) 2004 OSA 12 July 2004 / Vol. 12, No. 14 / OPTICS EXPRESS 3065