Nanolithography UsingBow-tie Nanoantennas

Rouin Farshchi

EE235

4/18/07

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

Outline

• Near-field optics and Nanoantennas

• Nanolithography

• Bow-tie nanoantennas

- lithography

- FDTD modeling

• Summary

2

Near-field Optics

3

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 Au

SU-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- FDTD

EjJD

EjJ rm 0)1( 2)( iknr

22

20 )(48

kdJZ

Wrad

I

WC radscat

Area

CQ 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

2Z

EI

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!