DTIC ADA531703: Projecting Deep-Subwavelength Patterns from Diffraction-Limited Masks Using Metal-Dielectric Multilayers

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APPLIEDPHYSICSLETTERS93,111116(cid:1)2008(cid:2)
Projecting deep-subwavelength patterns from diffraction-limited masks
using metal-dielectric multilayers
Yi Xiong,1 Zhaowei Liu,1 and Xiang Zhang1,2,a(cid:1)
1NSFNanoscaleScienceandEngineeringCenter(NSEC),UniversityofCalifornia,Berkeley,California
94720,USA
2MaterialsSciencesDivision,LawrenceBerkeleyNationalLaboratory,1CyclotronRoad,Berkeley,
California94720,USA
(cid:1)Received 19August 2008; accepted 28August 2008; published online 19 September 2008(cid:2)
We utilize a metal-dielectric multilayer structure to generate deep-subwavelength one-dimensional
andtwo-dimensionalperiodicpatternswithdiffraction-limitedmasks.Theworkingwavelengthand
the pattern are set by the flexible design of the multilayer structure. This scheme is suitable to be
applied to deep-subwavelength photolithography. As an example, we numerically demonstrate
pattern periods down to 50 nm under 405 nm light illumination. © 2008 American Institute of
Physics. (cid:3)DOI:10.1063/1.2985898(cid:4)
Therapidprogressinthenanoscalescienceandtechnol- cal transfer function of the multilayer structure, we can ad-
ogy has increased the demand for fabrication of nanoscale justtheperiodratiobetweenthediffraction-limitedmaskand
patterns. Photolithography is the most widely used micro- the deep-subwavelength lithographic pattern.
fabrication technique as it is a parallel, cost effective, and For simplicity, we consider a 1D periodic mask with
highthroughputprocess.However,theconventionalphotoli- period (cid:1) (cid:1)(cid:1) is larger than the diffraction limit(cid:2) to illustrate
thography techniques have a resolution barrier due to the the principle, which can be extended to the case of 2D
diffraction limit of light. To improve the photolithography periodicmaskseasily.Letusdefinexasthegratingdirection
resolution, one straightforward approach is to reduce the of the 1D periodic mask and z as the direction perpendicu-
wavelength of the illumination light into deep UV,1 extreme lar to the grating plane (cid:3)see Fig. 2(cid:1)b(cid:2)(cid:4). The waves emer-
UV,2 or even x-ray3 wavelengths. The main drawbacks of ging from the mask are the sum of a series of diffracted
these approaches, however, are the drastically increased planewaves,(cid:5)i(cid:2)=−(cid:2)T(cid:1)i(cid:2)exp(cid:3)j(cid:1)kx(cid:1)i(cid:2)x+kz(cid:1)i(cid:2)z(cid:2)(cid:4),where kx(cid:1)i(cid:2)=i2(cid:3)/(cid:1),
instrument complexity and the corresponding cost. Several (cid:3)k(cid:1)i(cid:2)(cid:4)2=(cid:1)n2(cid:3)/(cid:4)(cid:2)2−(cid:3)k(cid:1)i(cid:2)(cid:4)2,wherei isanintegerdiffractionor-
z x
other techniques are also available to achieve nanoscale der, T(cid:1)i(cid:2) is the amplitude of ith order plane wave, (cid:4) is
feature sizes: electron-beam lithography,4 focused ion- the wavelength of the incident light in free space, and n is
beam lithography,5 dip-pen lithography,6,7 and imprint the refractive index of the medium on the transmission
lithography,8,9justtonameafew.Althoughthesetechniques side of the grating. Depending on whether (cid:3)k(cid:1)i(cid:2)(cid:4)2 is larger or
z
have been widely used, each of them has to face its own smaller than zero, the diffracted plane wave is propagating
disadvantages. or evanescent, respectively. The evanescent waves carry
Recently, plasmonic nanolithography10–17 was demon- subdiffraction-limited information of the mask but decay
strated to improve the photolithography resolution by utiliz- exponentially with the distance away from the mask. There-
ing surface plasmons. The wavelength of surface plasmons foreinconventionalprojectionphotolithography,theevanes-
is smaller than that of light in free space at the same fre- cent waves play no role, and the resolution is diffraction-
quency.18 Hence, photolithography assisted with surface limited. By including the evanescent waves, near field
plasmons can generate patterns with subwavelength feature contact lithography forms subdiffraction-limited patterns in
sizes.Furthermore,plasmonicnanolithographyiscompletely the near field of the mask but requires subdiffraction-limited
compatible with conventional photolithography as they both masks.
share the same optical frequency and process flow. In our approach of photolithography, we generate
Near field contact lithography is another promising subdiffraction-limited patterns from a diffraction-limited
subdiffraction-limited photolithography method, which mask,usingaspecialslabofmaterialthatonlyallowswaves
forms patterns with subdiffraction-limited resolution in the
near field of the mask.19–21 However, the patterns formed by
the near field contact lithography are identical to the masks.
Therefore, subdiffraction-limited masks are always required
to form subdiffraction-limited patterns.
In this paper, we propose a deep-subwavelength photo-
lithography method which uses a designed metal-dielectric
multilayertogeneratethesubwavelengthfeaturesfromatra-
ditional one-dimensional (cid:1)1D(cid:2) or two-dimensional (cid:1)2D(cid:2)
diffraction-limited mask (cid:1)see Fig. 1(cid:2). By designing the opti-
λ
a(cid:2)Author to whom correspondence should be addressed. Electronic mail: FIG.1. (cid:1)Coloronline(cid:2)Schematicofdeep-subwavelengthphotolithography
[email protected]. usingmetal-dielectricmultilayer.
0003-6951/2008/93(cid:2)11(cid:1)/111116/3/$23.00 93,111116-1 ©2008AmericanInstituteofPhysics
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Projecting deep-subwavelength patterns from diffraction-limited masks
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using metal-dielectric multilayers
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Prescribed by ANSI Std Z39-18
111116-2 Xiong,Liu,andZhang Appl.Phys.Lett.93,111116(cid:2)2008(cid:1)
(a) (b) ky/k0
-6 0.07
x (a)
-4
z
-2
0
k /k
x 0
2
(c)
4
|H| 6 0
0.1 -6 -4 -2 0 2 4 6
k /k
x 0
y(nm) |E|
0
0 400nm (b) 200 0.013
Cr Ag SiO
2
FIG.2. (cid:1)Coloronline(cid:2)(cid:1)a(cid:2)Thetransmissionvsthetangentialwavevectorfor x y
a10pairsof40nmAgand35nmSiO multilayer.ThepolarizationisTM
2
polarization.Workingwavelengthis405nm.(cid:1)b(cid:2)Configurationforphotoli-
thographyandthesimulated(cid:6)H(cid:6)fieldafterthemultilayer.Onlyoneperiod
(cid:1)400nm(cid:2)ofthegratingmaskisshown.(cid:1)c(cid:2)The(cid:6)E(cid:6)fieldinthephotoresistat
themultilayer-photoresistinterface(cid:1)blue(cid:2),10nm(cid:1)green(cid:2),and20nm (cid:1)red(cid:2)
awayfromtheinterface.
0 0.009
0 200
with tangential wave vector larger than nk (cid:1)k =2(cid:3)/(cid:4)(cid:2) to x(nm)
0 0
passthrough.Furthermore,thewidthofthespatialfrequency
FIG. 3. (cid:1)Color online(cid:2) (cid:1)a(cid:2) The transmission vs tangential wavevector k
pass band is narrow enough to allow one diffraction order x
and k (cid:1)2D transfer function(cid:2) for a 12 pairs of 35nm Ag and 21nm
(cid:1)(cid:5)mth order(cid:2) from the 1D mask only. When we place this SiO myultilayer at a wavelength of 405nm. (cid:1)b(cid:2)The simulated (cid:6)E(cid:6) field at
2
slab material immediately after a diffraction-limited mask, a theplane3nmaftertheAgandSiO multilayer.Theleftinsetisthecon-
2
subdiffraction-limited pattern with period (cid:1)/2m is formed figurationforphotolithography.Onlyoneperiod(cid:1)200nm(cid:2)isshown.
ontheothersideoftheslabmaterialasaresultofthesuper-
position of the (cid:5)mth order diffraction waves. TMpolarizationatawavelengthof405 nm.Thepermittivity
The abovementioned special slab material can be real- of Ag is −4.67+0.22i.33 The permittivity of SiO is 2.16.34
2
ized by a metamaterial, an artificial fabricated structure that Thepermittivityofthephotoresistafterthemultilayeris2.89
has received significant attention recently because of its (cid:1)negativephotoresistNFR105 GfromJSRMicro(cid:2).Fromthe
properties that are not observed in nature.22–24 A metal- transferfunctionofthemultilayer,onlyabandofwaveswith
dielectric multilayer structure, one of the simplest metama- tangential wavevector around 3k can pass through this
0
terials,hasbeenproposedforvariousapplications.25–29Asan structure. If a grating mask with 400 nm period is added in
example, we shall show a designed slab of multilayer front of the multilayer structure and illuminated by a light
metamaterial can realize the deep-subwavelength photoli- at 405 nm, only the (cid:5)3 order diffraction waves from the
thography. A metal-dielectric multilayer has those special maskcangothroughthemultilayersandformapatternwith
propertiesbecauseofthesplitsurfaceplasmonmodesonthe period six times smaller compared with the mask. This
metal-dielectricmultilayer.Surfaceplasmonmodessplitona spatialfrequencysextuplingeffectisevidentlyshownbythe
metal thin film due to the interaction of modes on two metal simulated (cid:6)H(cid:6) field distribution (cid:1)COMSOL MULTIPHYSICS
surfaces.30 As the number of the metal thin films increases, 3.4(cid:2) after the multilayer in Fig. 2(cid:1)b(cid:2). The thickness of the
the number of the split surface plasmon modes increases Cr mask is 50 nm, and the opening width is 100 nm. For
accordingly.31 With proper design, the split surface plasmon photolithography purpose, the (cid:6)E(cid:6) field distribution is
modes can be highly compacted, and form a band. Subse- also shown in Fig. 2(cid:1)c(cid:2) at planes 0, 10, and 20 nm after
quently, the transmission (cid:1)through the multilayer(cid:2) of a range the Ag and SiO multilayer. The intensity contrast is
2
of tangential wavevector band corresponding to the surface (cid:1)(cid:6)E(cid:6)2 −(cid:6)E(cid:6)2 (cid:2)/(cid:1)(cid:6)E(cid:6)2 +(cid:6)E(cid:6)2 (cid:2)(cid:7)0.2, which satisfies the
max min max min
plasmon mode band is large and the transmission of the rest minimum contrast required for common negative
tangential wavevector band is negligible. Therefore, the de- photoresists.35
signed multilayer can act as an extraordinary spatial filter, Theabovemethodcanbeeasilyextendedto2Dpatterns
i.e.,onlyallowsabandofwaveswithtangentialwavevector generation, as illustrated by the following example: Addi-
larger than nk to go through it. The location and the band- tionally, we redesign the Ag–SiO multilayer to show that
0 2
width of the pass band can be tuned by changing the thick- different period ratio between the mask and the lithographic
nesses of the metal and dielectric layers.32 pattern is achievable by adjusting the configuration of the
We firstly show one example where a metal-dielectric multilayer. Figure 3(cid:1)a(cid:2) is the 2D transfer function (cid:1)the trans-
multilayer can generate a deep-subwavelength 1D pattern mission versus tangential wavevector k and k (cid:2) for a 12
x y
fromadiffraction-limitedmask.Figure2(cid:1)a(cid:2)showsthetrans- pairs of 35 nm Ag and 21 nm SiO multilayer at a wave-
2
mission versus the tangential wavevector (cid:1)transfer function(cid:2) length of 405 nm. Each set of k and k can define an inci-
x y
for a 10 pairs of 40 nm Ag and 35 nm SiO multilayer for dentplane.The2Dtransferfunctioniscalculatedintheway
2
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111116-3 Xiong,Liu,andZhang Appl.Phys.Lett.93,111116(cid:2)2008(cid:1)
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