Biomedical Engineering Reference
In-Depth Information
12.2 History and Overview of Lithography
Lithography.is.literally.“drawing.on.stone”.and.was.irst.patented.in.England.in.1801.by.
Alois.Senenfelder.(Steendrukmuseum.Nederlands,.n.d.).as.a.low-cost.means.of.printing.
Photolithography. on. silicon. wafers. was. irst. introduced. in. 1955. by. Jules. Andrus. and.
Walter.Bond.at.Bell.Labs.(Computer.History.Museum,.n.d.).and.continues.to.the.present.
day. as. the. only. cost-effective. means. of. large-scale. production. of. integrated. circuits..
Outside. of. the. technical. community,. the. central. role. of. lithography. in. our. twenty-irst.
century.high-tech.culture.is.not.generally.appreciated..A.Google.search.on.“lithography”.
still.lists.the.artistic.applications.of.lithography.as.the.top.hits.
Photolithography.is.capable.of.patterning.over.a.large.array.of.feature.sizes..The.lower.
limit. in. high-volume. manufacturing. as. of. 2012. is. about. 30.nm. (ASML,. 2012;. Benschop,.
2008)..Due.to.the.high.cost.of.ownership.of.photolithographic.equipment.capable.of.pat-
terning.at.the.smallest.feature.sizes,.the.technology.is.only.affordable.for.large.production.
runs..With.larger.minimum.feature.size,.the.cost.of.photolithography.decreases..A.typical.
form. of. low-cost. photolithography. is. found. in. the. “contact. aligner”. in. which. a. mask. in.
contact.with.a.resist-coated.substrate.selectively.exposes.different.regions.to.UV.light..This.
technology.dates.back.to.the.beginning.of.the.integrated.circuit.era.and.continues.to.see.
widespread.use.in.many.noncritical.applications.(VLSI.Research.Inc.,.1992)..Typical.resolu-
tions.achievable.with.this.technology.are.approximately.2.μm.
Below.the.resolution.limits.of.optical.lithography,.there.are.two.commercial.technologies.
available.for.patterning,.each.with.a.different.application.space..Nanoimprint.lithography.
(Chou.et.al.,.1996;.Colburn.et.al.,.1999;.Hoff.et.al.,.2004;.Hua.et.al.,.2004;.McClelland.et.al.,.
2002).is.closest.in.spirit.to.lithography.as.originally.conceived.in.1801..A.high-resolution.
master. stamp. or. template. is. fabricated. using. EBL,. and. the. pattern. is. transferred. to. a.
polymer-coated.wafer.by.a.variety.of.means,.including.heat,.pressure,.and.photocuring..
The.resolution.of.nanoimprint.lithography.appears.to.be.effectively.unlimited,.with.single.
carbon.nanotubes.being.used.as.masters.in.the.imprint.process.and.successfully.replicated.
(Hua. et. al.,. 2004).. Compared. to. high-resolution. photolithography,. nanoimprint. is. at. a.
disadvantage.on.throughput,.overlay,.and.defects;.it.has.the.advantage.for.resolution.and.
parity.with.lexibility.(as.with.photolithography,.nanoimprint.requires.a.new.template.for.
each.design.change)..Imprint.lithography.is.a.candidate.in.situations.that.require.nanoscale.
patterning.but.cannot.tolerate.the.conditions.of.EBL.
The. advantage. of. EBL. lies. primarily. in. lexibility. and. resolution.. Overlay. and. defect.
control.can.be.on.par.with.photolithography,.while.throughput.is.a.distinct.disadvantage..
For. this. reason,. e-beam. lithography. is. most. frequently. conined. to. the. R&D. world.. The.
most. common. commercial. application. of. EBL. is. mask. patterning. for. photolithography.
where. a. single. e-beam. exposure. can. be. optically. repeated. as. much. as. a. million. times..
Most.forms.of.EBL.available.in.2012.are.serial.in.nature,.meaning.that.patterns.are.built.up.
sequentially,. either. pixel-by-pixel. or. shape-by-shape.. Shaped. beam. lithography.(Pfeiffer,.
1978). introduces. a. modest. amount. of. parallelism. into. the. exposure. process. by. group.
exposing.hundreds.to.thousands.of.pixels.into.a.shape.that.is.lashed.as.a.single.exposure..
A. new. generation. of. multibeam. pattern. generators. are. currently. under. development.
that. potentially. can. decrease. the. time. to. form. a. pattern. with. EBL. by. several. orders. of.
magnitude.(Hartley,.2003;.Kampherbeek.et.al.,.2000;.Petric.et.al.,.2009)..Figure.12.1.presents.
a.cartoon.overview.of.the.relative.roles.played.by.electron.beam.and.photolithography..For.
researchers.contemplating.a.high-volume.commercial.target.for.their.research.efforts,.any.
