Biomedical Engineering Reference
In-Depth Information
micro-Coulombs. per. cm
2
. (μC/cm
2
).. A. useful. number. to. know. is. that. 1. electron/nm
2
.
corresponds.to.a.sensitivity.of.16.μC/cm
2
..For.resist.sensitivity,.100.μC/cm
2
.is.considered.
fast.. The. sensitivity. of. a. resist. material. is. not. a. material. constant. but. varies. linearly.
with.the.incident.beam.energy..When.examining.a.manufacture's.data.sheet.for.a.resist,.
it. is. important. to. determine. the. beam. energy. used. to. deine. the. sensitivity.. There. is.
no. standard. for. beam. energy.. At. the. low. end,. add-on. packages. for. scanning. electron.
microscopes.(SEM).can.provide.basic.lithographic.capability.with.beam.energies.ranging.
from.a.few.keV.to.several.tens.of.keV..Commercial.tools.designed.for.EBL.typically.operate.
between.50.and.125.keV..Beyond.that,.some.groups.have.added.pattern.generator.packages.
to.transmission.electron.microscopes.(TEM).and.explored.patterning.up.to.350.keV.(Broers.
et.al.,.1989)..A.resist.that.exhibits.a.sensitivity.of.160.μC/cm
2
.at.100.keV.beam.energy.will.
have.a.sensitivity.of.16.μC/cm
2
.at.10.keV..With.an.incident.dose.of.1.electron/nm
2
,.statistical.
luctuations. in. the. distribution. of. the. electrons. reaching. the. substrate. begin. to. play. an.
important.role.in.deining.pattern.idelity.
12.3.2 Cost
The.expression.“time.is.money”.is.directly.applicable.to.EBL..Because.of.the.serial.nature.
of.the.patterning.done.with.e-beam.writers,.there.can.be.a.large.variation.for.the.time.
it.takes.an.e-beam.writer.to.complete.an.exposure..For.this.reason,.the.cost.of.a.job.is.
almost.always.measured.by.time.on.the.tool..Minimizing.the.time.needed.to.complete.
a. job. should. always. be. a. consideration,. even. if. the. time. is. provided. without. charge..
Keeping.the.exposure.time.to.a.minimum.means.that.more.exposures.can.be.performed,.
increasing. the. overall. productivity. of. the. asset.. Minimizing. the. exposure. time. is. also.
an. asset. to. the. researcher. as. it. improves. turnaround. time. and. productivity.. A. precise.
estimate.of.the.time.required.to.expose.a.pattern.generally.requires.an.in-depth.knowl-
edge.of.both.the.pattern.and.the.system.architecture.to.account.accurately.for.all.of.the.
machine.overheads..It.is.fairly.easy,.in.most.cases,.to.determine.a.lower.bound.on.the.
exposure.time:
S A
I
×
t
=
.
where
S
.is.the.resist.sensitivity.(μC/cm
2
)
A
.is.the.area.to.be.exposed.(cm
2
)
I
.is.the.beam.current.at.the.substrate.(μA)
As.an.example,.the.minimum.time.to.expose.1.cm
2
.of.a.100.μC/cm
2
.resist.with.a.10.nA.beam.
is.100/0.01.=.10,000.s.or.∼3.h..This.is.the.portion.of.the.exposure.time.that.is.required.for.the.
beam.to.expose.the.resist..Other.factors.can.signiicantly.increase.the.actual.time.required..
As.a.further.example,.suppose.that.the.1.cm
2
.area.consists.of.100.nm
2
.spots..1.cm
2
.=.10
14
.nm
2
.
so.it.takes.10
12
.individual.lashes.to.expose.that.1.cm
2
.area..Each.nanosecond.of.overhead.
associated.with.setting.the.tool.up.to.expose.the.next.lash.will.add.1000.s.to.the.exposure.
time..A.100.ns.shape-to-shape.overhead.in.this.example.will.add.10
5
.s.to.the.job.time.and.
easily.swamp.the.exposure..At.the.other.extreme,.if.the.centimeter.to.be.exposed.consists.
of.a.single.large.block,.the.beam.can.be.left.on.continuously.while.the.system.paints.the.
area.. In. that. case,. the. overhead. mostly. consists. of. the. time. associated. with. moving. the.
stage.and.would.typically.add.only.a.few.seconds.to.the.exposure.
