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Nano-Photolithography and an Introduction to Fabrication and Characterization of Plasmonic

Waveguides

Hamid Nejati

ELEC 527

Professor Tour, Professor Zhong

03/29/2007

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Outline

Motivation and Introduction

Nano-photolithography

Excitation

Fabrication

Characterization

Conclusion

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Motivation

Photolithography: Diffraction limit of light:

resolution >100nm Higher speed and

repeatable Lift off process

E-beam lithography: Higher resolution: around

20nm Lower speed and non

repeatable

Nano-imprint Industrial

Light

Photomask substrate

Laser light

Aqueous development

Chromium etch

Photoresist strip

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Motivation

Photolithography (Mercury lamp) Diffraction limit of light

Plasmonics Sub wavelength operation

Y. Xia et al., “Soft lithography”, Angewandte Chemie International Edition, Vol. 37, pp. 550–575, 1998

~50 nm

Idea: Use near-field coupling between closely spaced metal nanoparticles or planar plasmonic waveguides for information propagation below the diffraction limit of light

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Plasmonic Review

Plasmonics

Surface plasmon polaritons (SPPs)

Plasmonic waveguides

J. Dionne et al., Nanoletters, Vol. 6, Issue 9, pp. 1928-1932, 2006.Maier et al., Physical review B, Vol. 67, p. 205402, 2003.

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Comparing the plasmon printing to photolithography

Projection lithographysmallest feature size ~

Plasmon printingsmallest feature size ~0.1

PHOTORESIST

SUBSTRATE

MASK

410 nm

EXPOSE

DEVELOP

P. Kik et al., Mat. Res. Soc. Symp. Proc. Vol. 705, 2002

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Field enhancement

Plasmon nano-sphere field profile

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Plasmon Printing

P. Kik et al., Mat. Res. Soc. Symp. Proc. Vol. 705, 2002

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Final resolution

P. Kik et al., Mat. Res. Soc. Symp. Proc. Vol. 705, 2002

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Plasmon Excitation

Plasmonic waves have bigger K vector in comparison to the light waves

Sources: He-Ne Laser (632.8 nm) Excimer laser 248nm Laser diode 1550nm

DielectricMetal

DielectricMetalSP KK

0

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Excitation ideas

Trick1: excitation from a high index medium for surface plasmon at air/metal interface (prism)

Trick2: K vector in periodic structures due to Bloch theorem is not single (periodic with reciprocal lattice vector) (grating)

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Excitation ideas

Trick3: excitation with dots

Trick4: array of dots

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Methods of excitation

Methods: Prism coupling (Kretschmann technique) => MI,IMI Grating or roughness coupling => MI,IMI, MIM End-fire excitation technique => MIM, IMI

Incident

light

Evanescent

wave

dp

Prism coupling

Grating coupling

End fire coupling

Otto method

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Prism coupling

Excitation of MI, IMI setups, and Asymmetric modeExcitation angle is dependent on the thickness of the filmCons:

Adjustment of gap separation & beam position, poor stability Expensive high-index prism and adjustment system

Pros: High efficiency and Selecting any guide mode (K-matching) Adjusting in experiment (detachable prism)

DielectricMetal

DielectricMetalKnSinK

Re00

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Inherent Roughness of the Metal Film

Inherent roughness of the metal film acts as a statistically determined distribution of inelastic scatter centers for an SPP => scattered light method

Cons: Low intensity Inhomogeneous distribution

Sharp tip of microscope: tip scatters part of the local optical near field into the fiber and then to the photo detector.

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End-Fire Coupling

Coupling with the direct excitation from the end of waveguide

Butt coupling of Polarization maintained fiber to the input and a single mode fiber at output

Optical index matched gel (OCF446 Nye optical)

Lens system and IR camera

Thermoelectric cooler

Tunable laser or EE-LED diode (1.55μm)+ polarization controller

Optical spectrum analyzer

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Grating Coupling

Pros: High efficiency with optimum design Any guided mode can be excited Compact, stable, and inexpensive for Integration in waveguides

Cons: Complex theoretical calculation and advanced fabrication technique Device parameters can’t be adjusted after fabrication

Other Coupling methods Tapered coupling Prism-grating coupling Holographic coupling

Inherent roughness of filmWaveguide

Source

photodetector

grating grating

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Grating coupling

Grating structure: coupling and shape

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Corrugated surface

Perforated or imperforated surfaces can invert plasmon to light and light to plasmon (even the incident light from mercury lamp in photolithography)

Periodic structures like gratings have different values of K vector (As Bloch waves), which helps the coupling of light to plasmon.

Surface plasmon wavelength is proportional to the periodicity of the lattice

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Interference pattern

Luo et al., Appl. Phys. Let., Vol. 84, No. 23, 2004.

Luo et al., Optics Express, Vol. 12, No. 14, 3055, 2004.

Resolution: 50nm

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Superlens

Snell’s law

Negative refractive index (left handed material LH)

Subwavelength resolution

Source Image

Superlens

n=-1n=1 n=1

RH RHLH

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Transmission

Un/Perforated surface

Bonod et al., Optics Express, Vol. 11, No. 5, 482, 2003.

0.08 μm thickness

Jiao et al., Progress In Electromagnetics Research Symp. 2005.

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Interference patterns

Luo et al., Optics Express, Vol. 12, No. 14, 2004.

Incident light: λ=436nm; E=2843mev

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Periodic Corrugated Setup

Luo et al., Appl. Phys. Let., Vol. 84, No. 23, 2004.

Resolution: 25nm

G-line 436nm

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Plasmonic structures

Planar structures Simple MI waveguide IMI waveguide MIM waveguide Bragg grating

Non-planar structures Nano-particle Nano-shell

I M M M MM I I I

Bragg grating

M I I I I II

Bragg gratingMIM

IMI

IM

Nano-shell Nano-sphereL. Hirish et al, PNAS, Vol. 100, No. 23, p. 13549, 2003.

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Material choice

Metal: Negative permittivity Low loss in desired frequency

Gold: low loss in 1.55μm, Negative permittivity Silver: low loss in He-Ne laser range, Negative permittivity

Insulator: Positive permittivity Compatibility to substrate, or function as photoresist

Sio2 (PECVD) BCB (photo resist for photolithography) PMMA (photo resist for e-beam lithography)

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Substrate choice

Silicon substrate SOI substrate Oxidation Photoresist

Silica substrate Quartz Fused silica glass ITO: special usage for e-beam lithography, need

low conductance (Indium tin oxide coating)

ITO coated glass

Silicon substrate

Fused silica glass

                                                                                 

Quartz substrate

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Fabrication

Photoresist spinning (PMMA) for E-beam lithography BCB, … for photolithography

Exposure an incident beam

Beam Electron with microscope (SEM) X-ray

Light UV Visible

Development Aqueous developer

Metal deposition E- beam evaporation

Lift off

E-beam

PMMA

PMMA

PMMA

Sub

Sub

Sub

Sub

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IMI fabrication

Sputtering 20μm Sio2 on Si Wafer.Molybdenum or Ti adhesive layer (e-beam evaporated, vacuum < Micro Torr) Gold e-beam evaporation under vacuum (24.4nm).Lift off (still & ultrasonic bath)Sputtering 20μm Sio2 on Au.Replacing Sio2 with 15μm BCB.Can be done on Sio2 substrate.

Charbonneau et al., J. of lightwave Tech., Vol.24, No.1, 2006.

Nikolajsen et al., Appl. Phys. Let., Vol. 82, No. 5, 2003.

Si

Sio2 Mo/Ti

Au

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IMI FabricationCyclotene (20μm)Adhesion promoterSi wafer

UV light

Mask

+ photo resist (S1813)Cyclotene (20μm)Si wafer

undercut

+ photo resist (S1813)Cyclotene (20μm)

Si wafer

Gold+ photo resist (S1813)Cyclotene (20μm)Si wafer

Gold

Cyclotene (20μm)

Si wafer

Cyclotene

Gold

Cyclotene

Si wafer

10-20nm

•Spin coat adhesion promoter (AP3000) and cyclotene @ 1000rpm(10s) and 2000rpm(30s)

•Soft bake @ 210 (10min) + hard bake @ 250 (10min)

•Spin coat shipley1813 photoresist @ 4000rpm(30s)

•Pre bake @ 115 (10min) expose (karl-suss MJB3 mask aligner) soak sample in chlorobenzene (10min)

•Develop microposit MF-319 (2min) => undercut => prevent the metal to be coated on side walls=> increase the lift off quality

•Deposit 20nm gold with e-beam evaporation

•Acetone rinsing+ lift off+ isopropanol rinsing

•Spin coat cyclotene + pre and post bake

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Substrate: oxidized Si or SiO2

Insulator: cyclotene or SiO2

Insulator + adhesion metal if needed

Adhesion promoter if needed

UV light

Mask

+ photo resist

Substrate

Undercut

+ photo resist Insulator

Substrate

Gold

Substrate

Insulator+ photo resist

Substrate

Substrate

Substrate

Substrate

Substrate

Insulator

Insulator

Insulator

Insulator

Insulator

Gold

Gold

Gold

Gold

Gold

Insulator

Insulator

InsulatorGold

+ photo resist

+ photo resist

Undercut

MIM Fabrication

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Nano-particle chain fabrication

PMMAITO

Glass

developer

acetone

E-beam

Spin coating

exposure

Lift off

Metal evaporation

development

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MIM fabrication

Au deposition on fused silica substrate by magnetron sputtering (150nm)

Sio2 PECVD (3.3nm, 56nm, 14nm)

No annealing + cleaving

Au deposition on fused silica substrate by magnetron sputtering (150nm)

Covered by Sio2 or air

Miyazaki et al., Phys. Rev. Let., PRL 96, 097401, 2006

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Bragg grating

Photoresist spin coating Pre baka Post bake

Patterning lithography

Etching KOH Enchants

Au deposition E-beam evaporation

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Fabrication of Plasmon Waveguides

Use e-beam lithography with liftoff to fabricate 50 nm Au nanoparticles on ITO coated glass

• Particles are almost spherical in shape

• Good control over size and inter-particle spacing

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Characterization of Metallic Nanoparticle Waveguide and Fabrication of Nanoshell

NSOM excitationFar field detectionFabrication methods:

Silica sphere: stober method, std < 5%

Surface modification: APTMS Attachment of gold nanoparticle (1-

2nm) Reductive growth of thin gold shell

NSOM excitation

NSOM characterization

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Characterization

Experimental Setup

Thorlabs MDT693

Piezo stage controller

Hamatasu infraredCamera controller

HP8164A lightwaveMeasurement mainframe

HP81680ATunable laser

HP81610Ainterface

HP 81624ADetector

HamatasuC5332IR camera

MirrorPolarizer

Microscope field of view

Obj. stageDevice stageFiber stage

Plasmo1n waveguide

Objective lensOptical fiber

Filter

•Optical table

•Butt coupling

•NSOM, Photon-STM

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Methods of characterization

Near field probing

Fluorescence imaging

Light scattering from surface roughness

Fourier domain observation of scattering in a grating array

Optical spectroscopy

NSOM

PSTM

Leakage radiation microscopy

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Transmission loss

Cut-back method

Prism sliding method

Scattering detection method

Lens Lens

Photodetector

cutting

Waveguide

Prism Prism

Matching liquidWaveguide

Photodetector

Sliding <------

Prism

waveguide

Scanning

Fiber probe

PhotodetectorLock-in amplifier

recorder

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NSOM

Aperture near field scanning optical microscopy

Aperture less NSOM

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NSOM

Multi-application 4 probe NSOM, AFM,SPM,…

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NSOM

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Conclusion

Plasmon assisted Nanophotolithography was reviewed

Fabrication, Excitation, and characterization of a plasmonic waveguide is reviewed

Special methods for characterization of MI, MIM, IMI, and Grating setups understood

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Thank you