Nanolithography Using Bow-tie NanoantennasOutlineNear-field OpticsNear-field optical lithographyBow-tie NanoantennasBow-tie FabricationExposure of SU-8 on bowtiesAFM / SEM of exposed SU-8AFM of exposed SU-8Theory- FDTDSlide 11ConclusionNanolithography UsingBow-tie NanoantennasRouin FarshchiEE2354/18/07Sundaramurthy et. al., Nano Letters, 6 355-360 (2006)Outline•Near-field optics and Nanoantennas•Nanolithography •Bow-tie nanoantennas - lithography - FDTD modeling• Summary2Near-field Optics3Sanchez, 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 nanoparticlesRechberger, Opt. Comm. 220 (2003) 137–141Produce greatly enhanced fields upon laser excitation(up to 103), confined to regions ~20nm, significantlydefeating diffraction limits: microscopy, SERS, lithography~ 300 nmHecht, JPC 112, 7761 (2000)Near-field optical lithography4Yin 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 regionAu 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 wavelengthsBow-tie Fabrication6ITO substrateTi sticking layer~4 nm~20 nm~75 nmAuSU-816 - 40 nm~80 nmAuDefine with e-beam lithographySundaramurthy 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 bowties7SU-8Excitation source:Ti:sapphire laser120 fs, f = 75 MHz = 800 nmFocus beam to diffraction-limited spotWith 1.3 NA 100x obvective lensExposure powers:27W – 10 mWSundaramurthy et al., Nano Letters, 6 355-360 (2006) Sundaramurthy et al., Physical Rev. B, 72 165409 (2005) Measured withTIR microscopypolarizer,beam-splitterAFM / SEM of exposed SU-88AFM:At high exposure powers,SU-8 ablation at bow-tiesSU-8 TPP away from bow-ties Blanket TPPTPP 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-tieAFM of exposed SU-8Nano-lithography:- Exposure + develop, bow-tie nanoantennas covered with SU-8 9• 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 WSundaramurthy et al., Nano Letters, 6 355-360 (2006)Theory- FDTDEjJDEjJrm 0)1(2)( iknr2220)(48kdJZWradIWCradscatAreaCQscatscatdisplacement current in gapcurrent in metal regionfrequency dependant (RIT)far-field radiation powerscattering cross-sectionincident powerscattering efficiency022ZEIinc16nm gap500nm gap0.13 A peak0.05 A peak10Sundaramurthy et al., Physical Rev. B, 72 165409 (2005)Theory- FDTD11The 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 12- 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
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