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Nanolithography UsingBow-tie Nanoantennas
Rouin FarshchiEE2354/18/07
Sundaramurthy et. al., Nano Letters, 6 355-360 (2006)
Outline
• Near-field optics and Nanoantennas• Nanolithography • Bow-tie nanoantennas - lithography - FDTD modeling• Summary
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Near-field Optics
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Sanchez, PRL 82, 4014 (1999)
Near-field: immediate vicinity of “light source” with dimensions < .
Near-field Probes:Sharp tips (ANSOM), coated tapered optical fibers (NSOM)
Nanoantennas: plasmon resonance couplingNanopartice arrays, Pairs of nanoparticles
Rechberger, Opt. Comm. 220 (2003) 137–141
Produce greatly enhanced fields upon laser excitation(up to 103), confined to regions ~20nm, significantlydefeating diffraction limits: microscopy, SERS, lithography
~ 300 nm
Hecht, JPC 112, 7761 (2000)
Near-field optical lithography
4Yin et. al., Appl. Phys. Lett. 81 3663 (2002)
• Achieve ~ / 10 resolution by focusing femtosecond laser beam onto Au coated AFM tip in close proximity to SU-8.
• Two-photon polymerization occurs in SU-8 over confined regions due to local enhancement of electric field by surface plasmons on AFM tip.
Bow-tie Nanoantennas
[3] Sundaramurthy et al., Physical Rev. B, 72 165409 (2005)
[1] Schuck et al., Phys. Rev. Lett. 94, 017402 (2005) 5[2] Fromm et al., Nano Lett. 4, 957 (2004)
103 field enhancement to <30 nm regions10% efficiency (define) vs ~10-5 for NSOM
- ~103 enhancement of incident intensity- confined to 650 nm2 region
Au triangles on ITO (fabrication in [1])
Effects:-Plasmon resonance in each triangle-Coupling across gap
Finite difference time domain (FDTD) for computation of [3]: - intensity enhancement - scattering efficiency - resonant wavelengths
Bow-tie Fabrication
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ITO substrate
Ti sticking layer~4 nm
~20 nm~75 nm AuSU-8
16 - 40 nm~80 nm Au
Define with e-beam lithography
Sundaramurthy et al., Nano Letters, 6 355-360 (2006) Schuck et al., Phys. Rev. Lett. 94, 017402 (2005)
Measured with TPPL [Schuck]
Exposure of SU-8 on bowties
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SU-8
Excitation source:Ti:sapphire laser120 fs, f = 75 MHz = 800 nm
Focus beam to diffraction-limited spotWith 1.3 NA 100x obvective lens
Exposure powers:27W – 10 mW
Sundaramurthy et al., Nano Letters, 6 355-360 (2006) Sundaramurthy et al., Physical Rev. B, 72 165409 (2005)
Measured withTIR microscopy
polarizer,beam-splitter
AFM / SEM of exposed SU-8
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AFM:At high exposure powers,SU-8 ablation at bow-tiesSU-8 TPP away from bow-ties
Blanket TPP
TPP onlyat bow-tie gap
No TPP
Nano-lithography:
- Exposure + develop, bow-tie nanoantennas covered with SU-8
Sundaramurthy et al., Nano Letters, 6 355-360 (2006)
TPP in vicinityof bow-tie
AFM of exposed SU-8Nano-lithography:
- Exposure + develop, bow-tie nanoantennas covered with SU-8
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• Au bow-ties “capture” energy of diffraction limited spot and concentrate it at two small areas near the gap, exceeding exposure threshold.
• record 30 nm features with near-field lithography using record low power of 27 W
Sundaramurthy et al., Nano Letters, 6 355-360 (2006)
Theory- FDTDEjJD
EjJ rm 0)1( 2)( iknr
22
20 )(48
kdJZ
Wrad
IW
C radscat
AreaC
Q scatscat
displacement current in gap
current in metal region
frequency dependant (RIT)
far-field radiation power
scattering cross-section
incident power
scattering efficiency
0
2
2ZEI inc
16nm gap
500nm gap
0.13 A peak
0.05 A peak
10Sundaramurthy et al., Physical Rev. B, 72 165409 (2005)
Theory- FDTD
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The FDTD simulations predict an intensity enhancement of 107 at 4 nm above eachof the triangle tips exposed at 27 W, in good agreement with experimental valueof 150 from experiment.
FDTD Calculated enhancement peaks occurwithin 4 nm of SU-8 peak locations from AFM measurement.
Sundaramurthy et al., Nano Letters, 6 355-360 (2006)
Conclusion
- large electric-field enhancement in highly confined regions at tips of gold bow-tie nanoantennas
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- Allows for local exposure of SU-8 resist to record low dimensions (<30nm) using record low power (~27 W)
- Intensity enhancement thought to be due to coupling of plasmon resonance at tips of triangles, as suggested by theoretical modeling.
Thank you!