Nanolithography Using Bow-tie Nanoantennas

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Nanolithography UsingBow-tie Nanoantennas

• Rouin Farshchi
• EE235
• 4/18/07

Sundaramurthy et. al., Nano Letters, 6 355-360
(2006)
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Outline

• Near-field optics and Nanoantennas
• Nanolithography
• Bow-tie nanoantennas
• - lithography
• - FDTD modeling
• Summary

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Near-field Optics
Sanchez, PRL 82, 4014 (1999)
Near-field immediate vicinity of light source
with dimensions lt l. Near-field Probes Sharp
tips (ANSOM), coated tapered optical fibers
(NSOM) Nanoantennas plasmon resonance
coupling Nanopartice arrays, Pairs of
nanoparticles
Hecht, JPC 112, 7761 (2000)
Produce greatly enhanced fields upon laser
excitation (up to 103), confined to regions
20nm, significantly defeating diffraction
limits ?microscopy, SERS,
lithography
300 nm
Rechberger, Opt. Comm. 220 (2003) 137141
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Near-field optical lithography

• Achieve l / 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.

Yin et. al., Appl. Phys. Lett. 81 3663 (2002)
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Bow-tie Nanoantennas
103 field enhancement to lt30 nm regions 10
efficiency (define) vs 10-5 for NSOM

• Effects
• Plasmon resonance in each triangle
• Coupling across gap

Au triangles on ITO (fabrication in 1)
Finite difference time domain (FDTD) for
computation of 3 -
intensity enhancement -
scattering efficiency -
resonant wavelengths
- 103 enhancement of incident intensity -
confined to 650 nm2 region
1 Schuck et al., Phys. Rev. Lett. 94, 017402
(2005)
2 Fromm et al., Nano Lett. 4, 957 (2004)
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3 Sundaramurthy et al., Physical Rev. B, 72
165409 (2005)
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Bow-tie Fabrication
Define with e-beam lithography
Measured with TPPL Schuck
Sundaramurthy et al., Nano Letters, 6 355-360
(2006)
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Schuck et al., Phys. Rev. Lett. 94, 017402 (2005)
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Exposure of SU-8 on bowties
Exposure powers 27mW 10 mW
Focus beam to diffraction-limited spot With 1.3
NA 100x obvective lens
polarizer, beam-splitter
Excitation source Tisapphire laser 120 fs, f
75 MHz l 800 nm
Measured with TIR microscopy
Sundaramurthy et al., Nano Letters, 6 355-360
(2006)
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Sundaramurthy et al., Physical Rev. B, 72 165409
(2005)
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AFM / SEM of exposed SU-8
Nano-lithography - Exposure develop, bow-tie
nanoantennas covered with SU-8
AFM At high exposure powers, SU-8 ablation at
bow-ties SU-8 TPP away from bow-ties
TPP only at bow-tie gap
Blanket TPP
No TPP
TPP in vicinity of bow-tie
Sundaramurthy et al., Nano Letters, 6 355-360
(2006)
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AFM of exposed SU-8
Nano-lithography - Exposure develop, bow-tie
nanoantennas covered with SU-8

• 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 mW

Sundaramurthy et al., Nano Letters, 6 355-360
(2006)
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Theory- FDTD
displacement current in gap
16nm gap
current in metal region
0.13 mA peak
frequency dependant (RIT)
far-field radiation power
500nm gap
0.05 mA peak
scattering cross-section
incident power
scattering efficiency
Sundaramurthy et al., Physical Rev. B, 72 165409
(2005)
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Theory- FDTD
The FDTD simulations predict an intensity
enhancement of 107 at 4 nm above each of the
triangle tips exposed at 27 mW, in good agreement
with experimental value of 150 from experiment.
FDTD Calculated enhancement peaks occur within 4
nm of SU-8 peak locations from AFM measurement.
Sundaramurthy et al., Nano Letters, 6 355-360
(2006)
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Conclusion
- large electric-field enhancement in highly
confined regions at tips of gold bow-tie
nanoantennas
- Allows for local exposure of SU-8 resist to
record low dimensions (lt30nm) using record low
power (27 mW)
- Intensity enhancement thought to be due to
coupling of plasmon resonance at tips of
triangles, as suggested by theoretical modeling.
Thank you!
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