Biomedical Engineering Reference
In-Depth Information
process.described.above.can.be.avoided..In.a.HARM.process,.thick.sacriicial.layers.can.be.obtained,.
enabling.larger.gaps.and.longer.mirror.strokes..To.increase.the.vertical.dimensions,.thick.ilm.polymer.
photoresists. are. used. as. molds. for. electrodeposition. of. structural. and. sacriicial. layers.. One. process,.
called.LIGA.(Becker.1986),.a.German.acronym.for. Lithographie, Galvanoformung, Abformung (lithog-
raphy,.electroplating,.and.molding),.uses.synchrotron.radiation.to.expose.a.thick.resist..he.synchro-
tron.radiation.is.highly.collimated.and.forms.vertical.sidewalls.in.polymethyl.methacrylate.(PMMA).
photoresists. that. can. be. hundreds. of. micrometers. thick.. Using. this. process,. feature. sizes. as. small. as.
0.1 μm.can.be.replicated.using.the.developed.PMMA.layer.as.a.mold.for.electrodeposition.of.metals.such.
as.copper,.gold,.and.nickel..Since.there.are.etches.with.high.selectivity.between.these.metals,.one.metal.
can. be. used. as. the. structural. layer. and. one. layer. used. as. the. sacriicial. layer.. An. overview. of. a. LIGA.
process.is.shown.in.Figure.8.36,.where.a.small.metal.gear.has.been.fabricated.
he.LIGA.process.can.also.be.used.to.electroform.two.diferent.metals,.one.for.a.structural.layer.and.
one.for.a.sacriicial.layer,.and.the.process.can.be.repeated.ater.the.planarization.step.to.build.up.more.
complex-layered.structures..An.example.is.the.electrochecmical.fabrication.(EFAB).process.where.one.
metal.layer,.such.as.copper,.is.used.for.the.sacriicial.material.and.a.diferent.metal.layer,.such.as.nickel,.
is. used. for. the. structural. layer. (Cohen. 2001).. A. schematic. diagram. of. the. EFAB. process. is. shown. in.
Figure.8.37 .
A. high-aspect-ratio. micromachining. process. has. been. used. to. fabricate. large-stroke. DMs. for. AO.
(Fernandez.2010)..By.using.a.HARM.fabrication.process,.a.thick.sacriicial.layer.(copper).could.be.elec-
trodeposited.on.top.of.gold.structural.layers.to.enable.a.large.electrostatic.gap.for.high-stroke.actuation..
Synchrotron
radiation
X-ray mask
PMMA
Plating base
Substrate
(1) Expose
(2) Develop
(3) Electroform
(4) Planarize
(5) Remove PMMA
(6) Release
FIGuRE 8.36 Overview. of. the. LIGA. ( Lithographie, Galvanoformung, Abformung ). high-aspect-ratio. microma-
chining.process..(1).A.highly.collimated.synchrotron.radiation.is.used.to.expose.a.thick.polymethyl.methacrylate.
(PMMA).photoresist.on.a.plating.base.through.an.X-ray.mask..(2).he.PMMA.is.then.developed.to.form.a.mold.for.
electroplating..(3).Using.electroforming,.this.mold.is.then.illed.with.a.metal..(4).Using.mechanical.polishing.tech-
niques,.the.PMMA.and.ill.metal.are.then.planarized..(5).he.PMMA.is.then.removed,.leaving.the.electroformed.
metal.part.attached.to.the.plating.base..(6).he.metal.part.is.then.released.from.the.plating.base.
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