Biomedical Engineering Reference
In-Depth Information
Commercial.EBL.platforms.generally.include.some.sort.of.system.emulator.that.is.capa-
ble. of. examining. the. exposure. ile. and. making. a. reasonably. accurate. prediction. of. the.
exposure.time..Using.the.simple.methods.described.earlier,.a.potential.EBL.project.can.be.
quickly.scoped.in.size.and.an.early.go/no-go.decision.made.on.a.conceptual.design.
There.are.a.number.of.things.that.can.be.done.during.the.pattern.layout.step.to.improve.
the. overall. exposure. eficiency.. Picking. a. beam. current. is. generally. tied. to. the. smallest.
feature.that.one.needs.to.draw..The.smaller.the.feature.size,.the.less.the.beam.current.that.
can.be.used..Nanometer-sized.features.will.typically.require.beam.currents.in.the.low.single.
digits.of.nano-amps,.while.micron-sized.features.can.be.exposed.at.the.systems.upper.limit.
on.current.delivery,.typically.50-100.nA..If.a.pattern.has.a.large.variation.in.feature.size,.
the.designer.can.elect.to.put.large.features.on.one.level.and.small.features.on.another..As.
the.design.ile.is.converted.to.the.appropriate.EBL.format,.two.separate.control.iles.may.
be. generated,. one. for. each. level.. Considerable. savings. in. exposure. time. may. be. realized.
by.exposing.the.large.areas.with.the.highest.possible.beam.current.and.the.small.features.
with.the.required.small.current..Another.example:.Divide.our.1.cm
2
.pad.into.two.pieces,.
connected.by.a.single.10.nm.line..To.expose.the.10.nm.line,.we.might.require.a.1.nA.beam.
current..Using.that.current.for.both.the.line.and.the.pads,.we.ind.that.our.exposure.time.
increases.to.∼30.h..If.instead,.we.only.use.1.nA.to.expose.the.line,.the.time.is.∼0..Increase.
the. beam. current. to. 100.nA. and. the. time. to. expose. the. pads. is. about. 20.min.. Adopting.
this.simple.strategy.reduced.the.exposure.time.by.nearly.100×!.Even.if.changing.the.beam.
current.is.a.cumbersome.process.taking.several.hours,.the.effort.is.well.worth.it.in.this.case.
12.3.3 imaging
Imaging. is. the. cornerstone. of. lithography.. If. the. images. do. not. accurately. relect. the.
designer's. intent,. the. project. will. not. likely. succeed.. In. the. following. sections,. we. dis-
cuss.the.selection.of.the.resist.material.and.the.appropriate.EBL.system.conigurations.to.
accomplish.a.given.task.
12.3.3.1 Tone Selection
Early.in.the.process.of.planning.the.worklow.for.your.project,.the.question.of.what.needs.
to.be.exposed.must.be.addressed..While.lithographically.patterned.resist.can.be.the.inal.
objective.(e.g.,.channels.for.micro/nanoluidic.applications),.it.is.most.commonly.used.as.
a.masking.layer.for.a.subsequent.process.step..Depending.on.the.subsequent.processing,.
either.the.pattern.or.the.background.may.require.protection..The.decision.that.needs.to.
be.made.is.whether.to.use.a.positive.or.negative.resist.and.whether.to.expose.the.pattern.
or.the.background..The.motivation.for.exposing.the.background.stems.from.the.desire.to.
minimize. the. exposure. time. required.. Table. 12.1. shows. a. simple. matrix. that. guides. the.
decision..An.analogous.matrix.can.be.followed.for.functional.materials.
Once.the.process.tone.has.been.selected,.the.next.decision.point.concerns.the.selection.
of.the.resist..In.the.interest.of.productivity,.a.typical.decision.is.to.pick.the.fastest.resist.
that. will. satisfy. the. imaging. requirements.. In. the. case. of. biological. applications. where.
patterning. may. be. for. material. functionalization,. chemistry,. rather. than. lithographic.
performance,.may.dictate.the.material.choice.and.tone..If.the.lithographic.properties.are.not.
established.from.previous.work,.some.effort.to.characterize.the.lithographic.performance.
envelope.is.advisable.prior.to.the.actual.device.work.to.establish.some.ground.rules.for.
the.pattern.layout..A.good.example.of.characterization.of.functional.ilms.may.be.found.
in.Glezos.et.al..(2002).
