Biomedical Engineering Reference
In-Depth Information
Overlay friendly layout
Overlay hostile layout
Figure 12.3
(See companion CD for color igure.)
.Examples.of.layouts.with.different.extremes.of.overlay.error.tolerance.
they. tend. to. be. in. the. form. of. hard. equipment. fails. that. result. in. yield. loss. rather. than.
defective.products.
Errors.in.EBL.systems.tend.to.be.of.a.different.nature.than.those.occurring.in.optical.
lithography..It.is.not.unheard.of.for.a.terabyte.or.more.of.data.transfer.to.be.needed.to.expose.
a.leading.edge.mask..Fortunately,.with.the.large.volumes.of.digital.data.transmitted.daily.
over.the.Internet,.the.technology.to.assure.data.integrity.in.EBL.has.become.largely.a.non-
issue..Of.more.concern.is.what.happens.at.the.point.where.the.data.takes.an.analog.form.
needed. to. drive. the. voltages. and. currents. to. steer. the. beam.. At. some. point,. the. circuits.
become.open.loop,.and.spurious.events.go.undetected.and.uncorrected..One.approach.to.
address.this.relies.on.direct.feedback.of.the.beam.location.from.the.substrate.for.a.real-
time,.closed.feedback.loop.(Hastings.et.al.,.2003)..Due.to.the.additional.process.complexity,.
this. technique. has. not. been. adopted. commercially.. The. most. common. approach. in.
commercial.systems.is.to.rely.on.ultrastable.(∼1.ppm).electronics.and.extensive.shielding.
from.electromagnetic.interference..When.an.EBL.system.is.being.used.as.a.resource.in.a.
chip.fab.environment,.care.must.be.taken.to.ensure.that.cross.contamination.does.not.take.
place..There.are.many.materials.that.can.disrupt.the.performance.of.nanoscale.transistors.
even.in.trace.amounts..Protocols.must.be.established.to.ensure.that.the.EBL.system.does.
not.become.a.source.of.defects.elsewhere.
12.4 Conclusions and Perspectives
In.many.ways,.the.status.of.lithography.in.the.nanobiological.sector.resembles.the.early.
days. of. the. semiconductor. industry.. While. lithography. has. enabled. numerous. R&D.
activities. for. biological. applications,. there. are. no. multibillion. dollar. fabs. dedicated. to.
the. lithographic. manufacturing. of. chips. with. direct. biological. applications.. One. reason.
for. this. is. that. today's. fabs. are. the. result. of. decades. of. incremental. progress. that. led. to.
the. hundreds. of. interlocking. steps. necessary. to. fabricate. the. modern. chip.. This. low. is.
not.readily.modiied.to.accommodate.the.new.materials.necessary.to.integrate.biological.
