Biomedical Engineering Reference
In-Depth Information
LSPR.or.SERS.that.can.meet.these.requirements:.electron.beam.lithography,.nanoimprint.
lithography.(NIL),.template-based.methods,.nanosphere.lithography.(NSL),.and.an.oblique.
angle.vapor.deposition.(OAD).or.glancing.angle.deposition.(GLAD).method.
5.3.2.1 Electron Beam Lithography
Electron.beam.lithography.(EBL).is.a.direct-write.nanopattern.transfer.method.using.a.vir-
tual.mask.for.sequential.feature.exposure.on.a.substrate..High-resolution.features.on.the.
order.of.sub-10.nm.are.obtainable.
42
.In.addition.to.nanoscale.feature.resolution,.additional.
advantages. include. control. over. beam. energy. and. dose,. elimination. of. physical. masks,.
high. accuracy. over. small. regions. on. a. wafer,. reduction. in. defect. densities,. and. ability.
for.large.depth.of.focus.on.changing.topographies.
42
.Although.EBL.provides.an.effective.
method.for.fabricating.reproducible.nanoscale.features,.including.nanodots,
43
.
nanowells,
44
.
and.nanoring.antennaes,
45
.the.long.writing.times.and.high.costs.are.the.main.disadvan-
tages.of.using.EBL.for.high-throughput,.low-cost.LSPR.applications.
5.3.2.2 Nanoimprint Lithography
Nanoimprint.lithography.(NIL).has.the.potential.to.be.a.low-cost,.mass.production.method.
for.fabricating.LSPR.nanostructures..The.process.is.twofold..First,.a.hard.mask,.typically.
made.of.metal,.dielectrics,.or.semiconductor.material,.is.pressed.into.a.thin.layer.of.poly-
mer. heated. above. its. glass. transition. temperature.
46
. When. pressed. together,. the. viscous.
polymer.conforms.to.the.mold.topography,.creating.thickness.variations.in.the.substrate.
upon. removal. of. the. mold.
47
. A. reactive. ion. etch. (RIE). is. then. used. to. complete. the. pat-
tern. transfer.. NIL. processes. have. been. used. to. create. gold. rectangular,. cylindrical,. and.
diamond-shaped.nanoblocks.based.on.grating.mold.orientation,
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.lat,.grated,.and.pillared.
silver. nanostructures. for. SERS,
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. and. gold. nanodisks.
50
. A. potential. disadvantage. of. the.
NIL.process.is.the.embedded.cost.of.fabricating.the.original.mold,.which.requires.access.
to.high-resolution.lithographic.tools,.such.as.EBL..The.mold-making.process.involves.plac-
ing.photoresist.on.the.mold.substrate,.nanopattern.exposure,.hard.mask.(metal).deposi-
tion.on.the.template,.followed.by.RIE.for.selective.etching.of.the.mold.
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.Once.the.mold.is.
fabricated,.though,.the.reuse.of.the.NIL.mold.for.multiple.transfers.provides.a.high.level.
of.repeatability.for.nanoscale.features.
5.3.2.3 Template-Based Methods
Template-based. methods. typically. utilize. the. regular. array. of. nanometer-scale. holes.
in. a. hard. material. such. as. anodic. aluminum. oxide. (AAO). for. electrochemical. deposi-
tion. of. metals,. semiconductors,. or. polymers.
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. For. AAO. templates,. the. pore. diameter. is.
controlled.by.changes.in.oxidation.conditions.and.feature.sizes.can.range.from.5.to.500.
nm.
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.Following.material.deposition,.which.can.include.electrochemical.deposition.ther-
mal. vacuum. evaporation,. or. RF. sputtering,. the. AAO. template. is. removed. by. chemical.
dissolution.
53,54
. The. advantages. of. AAO. templates. include. large. working. areas. (>1. cm
2
),.
compatibility.with.different.materials,.and.tunable.size.properties.
53
.AAO-templated.nano-
structures. for. plasmonic. applications. include. nanorods,
54,55
. nanopores,
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. nanopillars,
57
.
nanowires,
52b
.and.nanocrescents.
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.Depending.on.the.composition.of.the.electrochemical.
