Biomedical Engineering Reference
In-Depth Information
be. required.. Etch. chemistries. may. be. either. wet. or. dry. (plasma).. In. the. semiconductor.
industry,. most. etch. processes. are. dry. due. to. lower. defect. rates.. A. survey. of. dry. etch.
resistance. characteristics. for. a. variety. of. materials. may. be. found. in. Gokan. et. al.. (1983)..
For.speciic.materials,.refer.to.the.manufacture's.data.sheets..In.many.cases,.variations.in.
equipment.require.testing.to.optimize.performance.for.a.particular.site.
12.3.4 image Placement and Overlay
For. many. applications. of. biological. interest,. image. placement. and. overlay. is. not. a.
signiicant. concern.. We. begin. with. a. brief. description. of. image. placement. and. overlay.
and.the.difference.between.them..Image.placement.is.absolute.in.the.sense.that.what.is.
being.measured.is.the.deviation.of.a.pattern.relative.to.an.ideal.Cartesian.grid..The.details.
of. how. this. ideal. grid. is. established. forms. an. important. subset. of. the. metrology. sector.
of. the. semiconductor. industry. (Beyer. et. al.,. 2011).. Image. placement. to. an. absolute. grid.
is. most. important. in. mask. patterning. where. some. standard. is. necessary. to. ensure. that.
all. of. the. different. levels. in. a. device. are. aligned.. For. most. applications. involving. only.
a. single. exposed. layer,. image. placement. is. not. critical.. One. area. where. it. is. important,.
however,. is. the. fabrication. of. large. area. optical. gratings.. In. order. to. maintain. the. long-
range.spatial.coherence.necessary.for.the.fabrication.of.a.quality.grating,.tight.control.over.
image.placement.is.essential.
Overlay,. on. the. other. hand,. is. a. measure. of. the. relative. error. between. levels. in. a.
multilevel. process.. In. the. semiconductor. space,. sub-10.nm. overlay. is. common. and. 5.nm.
is. on. the. horizon. (Felix. et. al.,. 2011).. An. example. in. a. biological. application. where. tight.
overlay. may. be. required. is. the. fabrication. of. a. channel. to. constrain. DNA,. followed. by.
the. fabrication. of. electrodes. to. measure. charge. for. reading. the. DNA. (Kim. et. al.,. 2004)..
The. most. common. method. of. achieving. good. overlay. is. to. use. a. technique. known. as.
registered.write..This.involves.printing.an.initial.set.of.alignment.marks.on.the.substrate.
and.either.etching.or.metalizing.them..The.etching.or.metallization.step.is.necessary.for.
the. subsequent. registration. where. the. EBL. system. scans. the. area. containing. the. marks.
to.determine.their.location.precisely.within.the.EBL.system.address.space..Etch.provides.
topographic. contrast. while. metallization. provides. a. materials. contrast.. Either. one. may.
be.used.to.enhance.the.mark.detection..The.hardware.then.prints.the.pattern.relative.to.
the. known. mark. locations.. Accuracy. varies. by. equipment. quality. but. sub-10.nm. can. be.
achieved.. Figure. 12.3. shows. examples. of. layouts. that. accomplish. similar. objectives. but.
would.have.radically.different.tolerance.for.overlay.error.
12.3.5 Defects
Defects.in.lithography.can.arise.from.a.variety.of.sources..A.common.but.easily.controlled.
source.of.defects.is.the.environment..Dust.particles.and.chemical.contaminants.can.both.
be.readily.controlled.through.the.use.of.air.iltration.of.the.type.typically.found.in.clean-
rooms..For.lithographically.induced.defects,.optical.methods.have.the.distinct.advantage..
By.making.many.copies.from.a.single.physical.master.(the.mask),.it.is.only.necessary.to.
guarantee.that.the.master.is.error.free.to.avoid.errors.at.the.wafer..For.this.reason,.EBL.
systems.in.use.for.mask.manufacturing.place.a.great.deal.of.emphasis.on.data.integrity.
to. achieve. masks. with. low. error. rates. (Wakimoto. et. al.,. 2009).. Sophisticated. inspection.
(Han.et.al.,.2011).and.repair.methods.(Aramaki.et.al.,.2011).are.then.employed.to.yield.an.
error-free. product.. Equally. sophisticated. methods. of. handling,. transport,. and. cleaning.
are.used.to.preserve.the.error-free.condition..When.errors.do.occur.in.production.tools,.
