Biomedical Engineering Reference
In-Depth Information
8.6 Microelectromechanical System Deformable Mirrors
he.use.of.MEMS.DMs.in.astronomy.and.vision.science.has.been.reviewed.by.Olivier.et.al..(2005)..he.
application.of.AO.in.biological.imaging.requires.high-performance.wavefront.correctors.at.low.cost..
hese.requirements.favor.MEMS.DMs.over.the.piezoelectric.DMs.that.have.historically.been.used.in.
astronomy..hese.mirrors.are.hand.assembled.from.arrays.of.piezoelectric.actuators,.which.limits.the.
pitch.to.several.millimeters.and.the.cost.to.approximately.$1000.per.actuator..MEMS.technology.uses.
semiconductor.batch.fabrication.of.silicon.wafers,.or.similar.substrates.and.processing,.to.lower.the.
cost.to.approximately.$150.per.actuator.and.to.increase.the.actuator.number,.currently.as.high.as.4000.
actuators,.and.density,.currently.on.the.order.of.6.actuators.per.mm
2
..A.typical.semiconductor.process.
is.shown.in.Figure.8.18..he.process.is.cyclic..First,.a.thin.ilm.is.deposited.on.the.wafer.surface.using.
thin.ilm.deposition.techniques..A.uniform.photosensitive.polymer.(photoresist).is.then.deposited.and.
exposed.to.light.from.a.mask.that.contains.the.pattern.that.is.desired.on.the.thin.ilm..he.photoresist.
is. developed. to. obtain. the. desired. pattern.. he. pattern. in. the. photoresist. is. then. transferred. to. the.
thin.ilm.using.an.etching.technique.for.removal.of.the.unwanted.material.and.then.the.photoresist.is.
removed..he.diference.between.semiconductor.processing.and.MEMS.processing.is.that.MEMS.pro-
cesses.usually.require.fewer.processing.cycles.and.use.thicker.ilms.and.deeper.etches.for.patterning.
since.mechanical.rather.than.electrical.components.are.being.fabricated..Another.diference.is.that.in.
some.MEMS.processes,.such.as.surface.micromachining.as.described.below,.the.inal.processing.step.
involves.the.removal.of.an.underlying.“sacriicial”.material,.such.as.an.oxide.layer,.to.release.mechani-
cal.structures.that.have.been.formed.in.the.overlying.structural.layers,.such.as.polysilicon..Ater.the.
fabrication.processing.cycles,.the.wafers.are.sectioned.into.individual.die.and.assembled.into.a.pack-
age..Electrical.connections.between.the.die.and.the.package.are.made.using.wire.bonding. Finally,.the.
package. is. hermetically. sealed. to. protect. the. device. from. the. external. environment. (e.g.,. humidity,.
oxidation).
icker
lms
deeper etches
fewer steps
Removal of underlying
materials to release
mechanical structures
Multiple processing cycles
Deposition
of material
Pattern
transfer
Removal
of material
Probe
testing
Sectioning
Individual
die
Assembly
into package
Package
seal
Final
test
FIGuRE 8.18
Semiconductor.fabrication.cycles.for.microelectronics.and.micromechanics..A.material.is.depos-
ited.onto.a.wafer.using.thin-ilm.deposition.techniques..For.microelectronic.processing,.only.the.electrical.proper-
ties.of.the.material.need.to.be.optimized..For.micromechanical.processing,.both.the.electrical.and.the.mechanical.
properties.need.to.be.optimized..Micromechanical.fabrication.can.also.include.an.underlying.sacriicial.material.
that.is.removed.to.release.mechanical.structures.so.that.they.can.move..A.typical.micromechanical.process.might.
involve.eight.deposition.and.patterning.cycles,.whereas.a.micromechanical.process.could.require.28.cycles..Ater.
deposition.and.patterning.of.all.the.layers,.the.wafer.is.tested,.diced.(sectioned),.and.packaged..he.sacriicial.layers.
can.be.removed.at.either.the.wafer.or.die.level..(Reprinted.from.Karen.W..Markus.and.Kaigham.J..Gabriel,.IEEE.
Trans..Electron.Devices,.MEMS,.he.Systems.Function.Revolution,.©1967.IEEE..With.permission.)
