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Introduction to BioMEMS & Medical Microdevices
Sil icon Micro fabr icat ion Part 1
Companion lecture to the textbook: Fundamentals of BioMEMS and Medical Microdevices,
by Dr. Steven S. Saliterman www.tc.umn.edu/~drsteve
R012208
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Steven S. Saliterman, MD, FACP
Micromachining vs. “Soft” Fabrication
Microfabrication is the process for the
production of devices in the submicron tomillimeter range.
Micromachining of silicon and other ceramics
is similar to integrated circuit fabrication. “Soft” fabrication techniques include molding,
embossing, stamping, casting, thick-filmapplication, self-assembled monolayers
(SAMs), and array patterning using polymersand biological substances.
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Steven S. Saliterman, MD, FACP
Micromachining Materials MEMS devices are made from the same materials
used for microelectronics, including: Single crystal silicon wafers.
Deposited layers of polycrystalline silicon (polysilicon) forresistive elements.
Gold, aluminum, copper and titanium for conductors.
Silicon oxide for insulation and as a sacrificial layer (to allowrelease of moving parts).
Silicon nitride and titanium nitride for electrical insulation andpassivation.
The silicon materials have high strength at smallscales which allows higher strain levels and lesssusceptibility to damage and fracture.
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Steven S. Saliterman, MD, FACP
“Soft” Fabrication Materials Polymers
Surface modification for improved functionality. Hydrogels
Environmentally induced changes in shape, sizeand other attributes.
Electroactive Polymers Electrically induced changes in shape, size and
other attributes.
Biological Materials DNA fragments, biotin labeled albumin, and
streptavidin coated polystyrene beads for example.
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Steven S. Saliterman, MD, FACP
Microelectronics Revolution
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Steven S. Saliterman, MD, FACP
From Molten Silicon to IC Chips
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Steven S. Saliterman, MD, FACP
Electronic Grade Silicon (EGS)1. Quartzite is placed in a furnace with carbon releasing materials,
and reacts as shown, forming metallurgic grade silicon (MGS):
2Si0 (s) + 2C(s) Si(s) + 2CO(g)heat
→
2. MGS is then treated with hydrogen chloride to form
trichlorosilane:
3 2Si + 3HCl SiHCl (g) + H (g)→heat
3. Next fractional distillation reduction with hydrogen produceselectronic grade silicon (EGS):
3 2SiHCl (g) + H (g) Si(s) + 3HCl(g)heat
→
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Steven S. Saliterman, MD, FACP
Czochralski Puller
Gardner JW et al, Microsensors, MEMS and Smart Devices, 2001
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Steven S. Saliterman, MD, FACP
Cubic Crystal System
Sze 1985
(a) SC (b) BCC (c) FCC
Crystalline silicon forms a covalently bonded structure and coordinates
itself tetrahedrally (bottom). Silicon (and germanium) crystalize as two
interpenetrating FCC sublattices.
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Miller Indices
Maluf 2005
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Surface Micromachining Steps
Fatikow S & Rembold U, Microsystem Technology and Microrobot ics, 1997
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Silicon Wafer Preparation
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Steven S. Saliterman, MD, FACP
RCA Cleaning Bench
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Thermal Silicon Oxide
SiO2 is a silicon atom surrounded
tetrahedrally by four oxygen atoms.
Structure may be crystalline (quartz) or
amorphous (thermal deposition).
Gardner JW et al, Microsensors, MEMS and Smart Devices, 2001
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Steven S. Saliterman, MD, FACP
The chemical reaction thatoccurs is:
Dry oxidation at 900-1500°C in pure oxygenproduces a better oxide,with higher density thansteam oxidation.
Thermal silicon oxide isamorphous.
900 1200
2 2
2 2 2
Si (solid) + O (gas) SiO (solid)
and
Si (solid) + 2H O (gas) SiO (solid) + 2H (gas)
− °
→
→
C
Madou M, Fundamentals of Microfabrication, 2002
Thermal Silicon Oxide Methods
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Spin-Casting Resist
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Steven S. Saliterman, MD, FACP
Both “positive” and “negative” resists can
be chosen, depending on whether it isdesirable to have the opaque regions ofthe mask protect the resist, and hence the
substrate below, vs. having thetransparent regions protect the resistwhen exposed to UV.
Areas where the resist is removed willultimately be etched. Remember that“positive protects.”
Resist Types
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Steven S. Saliterman, MD, FACP
Positive resists include poly(methyl
methacrylate) (PMMA), and a two partsystem, diazoquinone ester plus phenolicnovolak resin (DQN).
Negative resists include SU-8,bis(aryl)azide rubber and Kodak KTFR.
Critical Dimension - this is the smallestfeature size to be produced.
Resolution – smallest line width to beconsistently patterned.
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Steven S. Saliterman, MD, FACP
Mask Creation
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Steven S. Saliterman, MD, FACP
UV Exposure at 350-500 nm
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Steven S. Saliterman, MD, FACP
Developing the UV Exposed Wafer
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Steven S. Saliterman, MD, FACP
Etching Methods
Subtractive processes:
Dry etching (plasma), Glow discharge methods (diode setups):
Plasma etching (PE),
Reactive ion etching (RIE),
Physical sputtering (PS).
Ion beam methods (triode setups):
Ion beam milling (IBM),
Reactive ion beam etching (RIBE),
Chemical assisted ion beam etching (CAIBE).
Deep Reactive Ion Etching (DRIE).
Wet etching (chemical liquids).
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Steven S. Saliterman, MD, FACP
Etching Profiles
Ziaie B et al, Hard and Soft Micromachining for BioMEMS, 2004
Isotropic Etching Anisotropic Etching
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Steven S. Saliterman, MD, FACP
Plasma Etching occurs at relatively lowerenergy and higher pressure (less vacuum),and is isotropic, selective and less prone tocause damage.
Reactive Ion Etching is more middle ground in
terms of energy and pressure, with betterdirectionality.
Physical Sputtering and Ion Beam Milling relyon physical momentum transfer from higher
excitation energies and very low pressures,and result in poor selectivity with anisotropicetching and increased radiation damage.
Energy, Vacuum & Directionality
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Steven S. Saliterman, MD, FACP
Plasma Etching (PE)
Madou M, Fundamentals of Microfabrication, 2002
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Steven S. Saliterman, MD, FACP Madou M, Fundamentals of Microfabrication, 2002
Reactive Ion Etching (RIE)
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Steven S. Saliterman, MD, FACP
Physical Sputtering Bombarding a surface with inert ions (e.g. argon) has
an effect related to the kinetic energy of the incomingparticles.
At energies < 3 eV (electron volts) particles are simplyreflected or absorbed.
At surface energies between 4-10 eV some surface
sputtering occurs. At surface energies of 10-5000 eV momentum transfer
causes bond breakage and ballistic material ejectionacross the reactor to the collecting surface. A low
pressure and long mean free path are necessary toprevent the material from redepositing.
Implantation (doping) occurs at 10,000-20,000 eV.
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Steven S. Saliterman, MD, FACP
Sputter yield is the number of atoms removed
from the surface per incident ion. Sputter yield depends on the following:
Incident ion energy (max yield 5-50 keV).
Mass of the ion Mass of the substrate atom to be etched away.
Crystallinity and crystal orientation of the substrate.
Temperature of the substrate Partial pressure of oxygen in the residual gas.
Sputter Yield
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Steven S. Saliterman, MD, FACP Madou M, Fundamentals o f Microfabrication, 2002 (left)
Ion Beam Milling (IBM)
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Steven S. Saliterman, MD, FACP Madou M, Fundamentals of Microfabrication, 2002
Reactive Ion Beam Etching and
Chemical Assisted Ion Beam Etching
RIBE CAIBE
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Resist Stripping
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Profilometry
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Profilometer Screen Display
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Profilometry Graph
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Dicing Chips
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Wire Bonding
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Steven S. Saliterman, MD, FACP
Summary
Microfabrication is the process for the
production of devices in the submicron tomillimeter range.
Micromachining of silicon and other ceramics
is similar to integrated circuit fabrication. Crystalline silicon forms a covalently bonded
structure and coordinates itself tetrahedrally
(bottom). Silicon (and germanium) crystalizeas two interpenetrating FCC sublattices.
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Steven S. Saliterman, MD, FACP
Surface micromachining concepts discussed:
Mask creation, Silicon wafer preparation,
Thin-films deposition such as SiO2,
Resist (positive or negative) application, UV exposure and development,
Etching methods (substrative processes),
Resist stripping,
Inspection with profilometer .
Dicing and Wire Bonding