Biomedical Engineering Reference
In-Depth Information
Silicon nitride
surface material
PSG spacer material
(a)
(b)
Polysilicon
structural material
Anchor area
(c)
(d)
Anchor
Microstructure
Released microstructure
(e)
(f )
FIGuRE 8.20
Polysilicon. surface. micromachining.. (a). An. insulating. surface. layer,. such. as. silicon. nitride,. is.
deposited.on.the.substrate..(b).A.sacriicial.spacer.layer.such.as.phosphosilicate.glass.(PSG),.which.can.be.etched.
quickly,.is.deposited..(c).A.hole.is.cut.through.the.sacriicial.layer.to.the.insulating.layer,.where.the.structure.is.to.
be.anchored.to.the.underlying.surface.material..(d).he.structural.layer,.in.this.example.polysilicon,.is.then.depos-
ited.. he. deposition. is. conformal. and. ills. in. the. hole. that. had. previously. been. cut. through. the. sacriicial. layer..
he.structural.layer.is.(e).patterned.and.(f).then.released..(Reprinted.from.Gary.K..Fedder,.ITC.International.Test.
Conference,.MEMS.Fabrication,.©1967.IEEE..With.permission.)
the.sacriicial.etch..he.structural.material.is.then.patterned.to.deine.a.microstructure,.as.shown.in.
Figure.8.20e..Finally,.the.spacer.material.is.etched.to.realize.a.released.microstructure,.as.shown.in.
Figure.8.20f..his.process.can.be.repeated.a.number.of.times.to.fabricate.a.number.of.structural.lay-
ers..A.cross.section.of.the.process.that.uses.three.structural.layers.and.two.sacriicial.layers.is.shown.
in.
Figure.8.21
.(Cowen.2011).
One. of. the. limitations. of. the. polysilicon. surface. micromachining. process. is. the. thickness. of. the.
sacriicial. space. layer,. as. shown. in.
Figure. 8.22
.
. For. practical. considerations,. it. is. limited. to. several.
micrometers.owing.to.the.thin.ilm.deposition.processing.that.is.used.in.the.fabrication.process..his.
spacer. layer. sets. limits. on. the. maximum. stroke. that. can. be. used. unless. the. structural. layer. is. lited.
out.of.plane,.as.described.below.for.the.Iris.AO.segmented.DM..Alternatively,.bulk.micromachining.
and.wafer.bonding.can.be.used.to.fabricate.a.large.gap,.as.described.below.for.bulk.etched.membrane.
mirrors.
8.6.2 electrostatic Actuation
Electrostatic.actuators.are.commonly.used.in.MEMS.devices.as.they.scale.well.in.the.microdomain,.use.
very.little.power,.and.are.straightforward.to.fabricate.in.a.number.of.diferent.processes..Two.common.
forms. are. parallel. plate. actuators. and. comb. drive. actuators.. he. parallel. plate. actuator. is. a. .capacitor.
