PLASMA%20MATERIALS%20PROCESSING:

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PLASMA MATERIALS PROCESSING CREATING HIGH
VALUE Mark J. Kushner Iowa State University 104
Marston Hall Ames, IA 50011 mjk_at_iastate.edu http/
/uigelz.ece.iastate.edu January 2005
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ACKNOWLEDGEMENTS

• Dr. Alex V. Vasenkov (now at CFD Research Corp.)
• Dr. Gottlieb Oherlein (U of Maryland)
• Dr. Arvind Sankaran (now at Novellus Systems)
• Dr. Pramod Subramonium (now at Novellus Systems)
• Dr. Rajesh Dorai (now at Varian Semiconductor
  Equipment)
• Mr. Ananth Bhoj
• Mr. Ramesh Arakoni
• Funding Agencies
• 3M Corporation
• Semiconductor Research Corporation
• National Science Foundation
• SEMATECH
• CFDRC Inc.

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AGENDA

• Introduction to Plasmas
• Extremes in Physics and Applications
• Plasmas for functionalization of polymers
  (0.05/m2)
• Polymers for selectivity in plasma etching
  (1000/cm2)
• Challenges for adapting commodity processes for
  high value materials.
• Concluding Remarks

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PLASMAS 101 INTRODUCTION

• Plasmas (ionized gases) are often called the
  fourth state of matter.
• Plasmas account for gt 99.9 of the mass of the
  known universe (dark matter aside).

• X-ray view of the sun, a plasma.

http//www.plasmas.org/basics.htm
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PARTIALLY IONIZED PLASMAS

• A gas (collection of atoms or molecules) is
  neutral on a local and global basis.

• An energetic free electron collides with an atom,
  creating a positive ion and another free electron.

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PARTIALLY IONIZED PLASMAS

• The resulting partially ionized gas is not
  neutral on a microscopic scale, but is neutral
  on a global scale.
• An air plasma N2, O2, N2, O2, O-, e where e
  ltlt Neutrals

• A plasma that ionizes itself at the same rate
  electrons and ions neutralize is self
  sustaining.

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TECHNOLOGICAL PLASMAS

• Technological plasmas have electron temperatures
  of a few eV and electron densities of 1010-1014
  cm-3. The gas usually remains cool.

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ELECTRONS AS AN ENERGY TRANSFER MEDIUM

• Electrons transfer power from the "wall plug" to
  internal modes of atoms / molecules dissociating
  and exciting them, very much like combustion.
• These activated species can them be used to make
  a product.

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COLLISIONAL LOW TEMPERATURE PLASMAS
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PLASMAS FOR MODIFICATION OF SURFACES

• Plasmas are ideal for producing reactive species
  (radicals, ions) for modifying surface
  properties.
• Two of the most technologically (and
  commercially) important uses of plasmas add or
  remove molecules from surfaces to selectively
  achieve desired functionality (mechanical or
  chemical)
• .

• Functionalization of polymers (high pressure)
• Etching for microelectronics fabrication (low
  pressure)

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EXTREMES IN CONDITIONS, VALUES, APPLICATIONS
Web Treatment of Films
Microelectronics

• High pressure
• High throughput
• Low precision
• Modify cheap materials
• Commodity

• Low pressure
• Low throughput
• High precision
• Grow expensive materials
• High tech

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CREATING HIGH VALUE COMMODITY PROCESSES

• Can commodity processes be used to fabricate high
  value materials?
• Where will, ultimately, biocompatible polymeric
  films fit on this scale? Artificial skin for
  0.05/cm2 or 1000/cm2?

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LOW COST, COMMODITY FUNCTIONALIZATION OF POLYMERS
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SURFACE ENERGY AND FUNCTIONALITY OF POLYMERS

• Most polymers, having low surface energy, are
  hydrophobic.
• For good adhesion and wettability, the surface
  energy of the polymer should exceed of the
  overlayer by ?2-10 mN m-1.

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PLASMA SURFACE MODIFICATION OF POLYMERS

• To improve wetting and adhesion of polymers
  atmospheric plasmas are used to generate
  gas-phase radicals to functionalize their
  surfaces.

• Massines et al. J. Phys. D 31, 3411 (1998).

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POLYMER TREATMENT APPARATUS

• Filamentary Plasma 10s 200 mm

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COMMERCIAL CORONA PLASMA EQUIPMENT

• Sherman Treaters

• Tantec, Inc.

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FUNCTIONALIZATION OF THE PP SURFACE

• Untreated PP is hydrophobic.
• Increases in surface energy by plasma treatment
  are attributed to the functionalization of the
  surface with hydrophilic groups.
• Carbonyl (-CO) ? Alcohols (C-OH)
• Peroxy (-C-O-O) ? Acids ((OH)CO)
• The degree of functionalization depends on
  process parameters such as gas mix, energy
  deposition and relative humidity (RH).

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PLASMA PRODUCED WETTABILITY

• Increases in wettability with plasma treatment
  result from formation of surface hydrophilic
  groups such as C-O-O (peroxy), CO (carbonyl).

• Polyethylene, Humid-air

• Polypropylene, Air corona

• Akishev, Plasmas Polym. 7, 261 (2002).

• Boyd, Macromol., 30, 5429 (1997).

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REACTION PATHWAY
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GLOBAL_KIN AND SURFACE KINETICS

• Reaction mechanisms in pulsed atmospheric air
  plasma treatment of polymers have been
  investigated with global kinetics and surface
  models.

• GLOBAL_KIN
• 2-Zone homogeneous plasma chemistry (bulk plasma,
  boundary layer)
• Plug flow
• Multilayer surface site balance model
• Circuit module
• Boltzmann derived f(?)

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REACTION MECHANISM FOR HUMID-AIR PLASMA

• Initiating radicals are O, N, OH, H

• Gas phase products include O3, N2O, N2O5, HNO2,
  HNO3.

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POLYPROPYLENE (PP) POLYMER STRUCTURE

• Three types of carbon atoms in a PP chain
• Primary bonded to 1 C atom
• Secondary bonded to 2 C atoms
• Tertiary bonded to 3 C atoms
• The reactivity of an H-atom depends on the type
  of C bonding. Reactivity scales as
• HTERTIARY gt HSECONDARY gt HPRIMARY

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PP SURFACE REACTION MECHANISM INITIATION

• The surface reaction mechanism has initiation,
  propagation and termination reactions.
• INITIATION O and OH abstract H from PP to
  produce alkyl radicals and gas phase OH and H2O.

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PP SURFACE REACTION MECHANISM PROPAGATION

• PROPAGATION Abundant O2 reacts with alkyl groups
  to produce stable peroxy radicals. O3 and O
  react to form unstable alkoxy radicals.

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PP SURFACE REACTIONS PROPAGATION / AGING

• PROPAGATION / AGING Peroxy radicals abstract H
  from the PP chain, resulting in hydroperoxide,
  processes which take seconds to 10s minutes.

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PP SURFACE REACTION MECHANISM TERMINATION

• TERMINATION Alkoxy radicals react with the PP
  backbone to produce alcohols and carbonyls.
  Further reactions with O eventually erodes the
  film.

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BASE CASE ne, Te

• Ionization is dominantly of N2 and O2,
• e N2 ? N2 e e,
• e O2 ? O2 e e.
• After a few ns current pulse, electrons decay by
  attachment (primarily to O2).
• Dynamics plasma-dielectric interaction enable
  more efficient ionization on later pulses.
• N2/O2/H2O 79/20/1, 300 K
• 15 kV, 9.6 kHz, 0.8 J-cm-2
• Web speed 250 cm/s (460 pulses)

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GAS-PHASE RADICALS O, OH

• Electron impact dissociation of O2 and H2O
  produces O and OH. O is consumed primarily to
  form O3 OH is consumed by both bulk and surface
  processes.
• After 100s of pulses, radicals attain a periodic
  steady state.

• O

• OH

• N

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PP SURFACE GROUPS vs ENERGY DEPOSITION

• Surface concentrations of alcohols, peroxy
  radicals are near steady state with a few J-cm-2.
• Alcohol densities decrease at higher J-cm-2
  energy due to decomposition by O and OH to
  regenerate alkoxy radicals.

• Air, 300 K, 1 atm, 30 RH

• Ref L-A. Ohare et al.,
• Surf. Interface Anal. 33, 335 (2002).

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HUMIDITY PP FUNCTIONALIZATION BY OH

• Increasing RH produces OH which react with PP to
  form alkyl radicals, which are rapidly converted
  to peroxy radicals by O2.
• PP-H OH(g) ? PP? H2O(g) PP? O2(g) ?
  PP-O2?
• Alcohol and carbonyl densities decrease due to
  increased consumption by OH to form alkoxy
  radicals and acids.

PP-OH OH(g)?PP-O? H2O(g) , PPO? OH(g)
? (OH)PPO
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COMMODITY TO HIGH VALUE

• As the material value increases (cents to dollars
  /cm2?) higher process refinement is justified to
  customize functionalization.

• Control of O to O3 ratio using He/O2 mixtures can
  be used to customize surface functionalization.
• 1 atm, He/O2, 15 kV, 3 mm, 9.6 kHz, 920 pulses.

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COMMODITY TO HIGH VALUE

• Additional tuning of functionalization can be
  achieved with sub-mTorr control of water content.

• Small water addition tuning of
  functionalization can be achieved with sub-mTorr
  control of water content.
• H and OH reduce O3 while promoting acid
  formation.

• 1 atm, He/O2/ H2O, 15 kV, 3 mm,
• 9.6 kHz, 920 pulses.

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HIGH COST, UTILIZATION OF POLYMERS IN
MICROELECTRONICS FABRICATION
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PLASMAS IN MICROELECTRONICS FABRICATION

• Plasmas play a dual role in microelectronics
  fabrication.
• First, electron impact on otherwise unreactive
  gases produces neutral radicals and ions.
• These species then drift or diffuse to surfaces
  where they add, remove or modify materials.

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PLASMAS IN MICROELECTRONICS FABRICATION

• Second, ions deliver directed activation energy
  to surfaces fabricating fine having extreme and
  reproducible tolerances.

• 0.25 ?m Feature
• (C. Cui, AMAT)

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APPLIED MATERIALS DECOUPLED PLASMA SOURCES (DPS)
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rf BIASED INDUCTIVELY COUPLED PLASMAS

• Inductively Coupled Plasmas (ICPs) with rf
  biasing are used here.
• lt 10s mTorr, 10s MHz, 100s W kW, electron
  densities of 1011-1012 cm-3.

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SELECTIVITY IN MICROELECTRONICS FABRICATION

• Fabricating complex microelectronic structures
  made of different materials requires extreme
  selectivity in, for example, etching Si with
  respect to SiO2.
• Complex features are fabricated by selectively
  removing one material but not another with near
  monolayer resolution.

• Ref G. Timp

• AMD 90 nm Athlon 64

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FLUOROCARBON PLASMA ETCHING SELECTIVITY

• Selectivity in fluorocarbon etching relies on
  polymer deposition from dissociation of feedstock
  gases.

• Compound dielectrics contain oxidants which
  consume the polymer, producing thinner polymer
  layers.
• Thicker polymer on non-dielectrics restrict
  delivery of ion energy (lower etching rates).

Iowa State University Optical and Discharge
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• G. Oerhlein, et al., JVSTA 17, 26 (1999)

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MODELING OF FLUOROCARBON PLASMA ETCHING

• Our research group has developed an integrated
  reactor and feature scale modeling hierarchy to
  model plasma processing systems.

• MCFPM (Monte Carlo Feature Profile Model)
• Feature scale
• 2- and 3-dimensional
• Fluxes from HPEM
• First principles

• HPEM (Hybrid Plasma Equipment Model)
• Reactor scale
• 2- and 3-dimensional
• ICP, CCP, MERIE, ECR
• Surface chemistry
• First principles

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ELECTROMAGNETICS AND ELECTRON KINETICS

• The wave equation is solved in the frequency
  domain using tensor conductivities and sparse
  matrix techniques

• Electron energy transport Continuum and Kinetics
  
• where S(Te) Power deposition from electric
  fields L(Te) Electron power loss due to
  collisions ? Electron flux
• ?(Te) Electron thermal conductivity tensor
• SEB Power source source from beam electrons
• Kinetic MCS is used to derive
  including e-e collisions using electromagnetic
  and electrostatic fields .

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PLASMA CHEMISTRY, TRANSPORT AND ELECTROSTATICS

• Continuity, momentum and energy equations are
  solved for each species (with jump conditions at
  boundaries).

• Implicit solution of Poissons equation

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ION/NEUTRAL ENERGY/ANGULAR DISTRIBUTIONS

• MC methods are used to obtain energy and angular
  distributions of particles striking surfaces.

• Ar/C4F8, 40 mTorr, 10b MHz, MERIE

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MONTE CARLO FEATURE PROFILE MODEL (MCFPM)

• The MCFPM predicts profiles using energy and
  angularly resolved neutral and ion fluxes
  obtained from equipment scale models.
• Arbitrary reaction mechanisms may be implemented
  (thermal and ion assisted, sputtering, deposition
  and surface diffusion).

• Mesh centered identify of materials allows
  burial, overlayers and transmission of energy
  through materials.

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GAS PHASE REACTION MECHANISM Ar/c-C4F8/O2/CO

• To achieve selectivity, gas mixtures are complex
  Ar/c-C4F8/O2/CO.

• Refs
• G. I. Font, J. Appl. Phys 91, 3530 (2002).
• C. Q. Jiao, Chem. Phys. Lett. 297, 121 (1998).

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SURFACE KINETICS FLUOROCARBON PLASMA ETCHING
Si/SiO2

• CxFy passivation regulates delivery of precursors
  and activation energy.
• Chemisorption of CFx produces a complex at the
  oxide-polymer interface.
• 2-step ion activated (through polymer layer)
  etching of the complex consumes the polymer.
  Activation scales inversely with polymer
  thickness.
• Etch precursors and products diffuse through the
  polymer layer.

• In Si etching, CFx is not consumed, resulting in
  thicker polymer layers.

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PLASMA PROPERTIES ICPs IN Ar/c-C4F8/CO/O2

• In mixtures typically used for dielectric etch
  c-C4F8 has a low mole fraction.
• Ions are dominated by Ar having temperatures
  near 1 eV in presheaths.
• Te has large gradients due to collisional nature
  of plasma.

• Ar/c-C4F8/CO/O2 60/5/25/10, 10 mTorr, 600 W,
  13.56 MHz, 20 sccm.

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PLASMA PROPERTIES ICPs IN Ar/c-C4F8/CO/O2

• M- are dominated by F- due to charge exchange
  with CFn- .
• Densities of CmFn are commensurate with CFn.
• Ratios critically depend on power, wall reactions
  and charge exchange with Ar.

• Ar/c-C4F8/CO/O2 60/5/25/10, 10 mTorr, 600 W,
  13.56 MHz, 20 sccm.

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MIXTURES Ar/c-C4F8, O2/c-C4F8

• Flux of ions to surface can be tuned by choice of
  additives.
• Addition of Ar to C4F8 produces more ions that
  are on average less chemically reactive.
• Addition of O2 adds oxidizing ions without
  changing fluorocarbon reactivity.
• Te decreases with Ar and O2 as ionization is more
  efficient.

• 10 mTorr, 600 W, 13.56 MHz, 40 sccm.

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TEL-DRM Ar / C4F8 / O2 IEADs FOR 2000 W

• Complex gas mixtures produce a large variety of
  ion energy and angular distributions.

• Ar/C4F8/O2 200/10/5 sccm, 40 mTorr, 2000 W, 100
  G

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ETCH RATES C2F6, C4F8, CHF3

• Due to small differences in the composition of
  the flux of radicals and polymer, and ion energy
  distributions, etching characteristics differ
  with feedstock gases.

• Experiments Schaepkens et al J. Vac. Sci.
  Technol. A 17, 26 (1999) Oehrlein et al private
  communications

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POLYMERIZATION PRODUCES SELECTIVITY

• Highly optimized processes produce nearly
  infinite selectivity. The underlying Si does not
  consume the polymer, and so etches slower.

• 10 mTorr, C2F6.

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C4F8/Ar and C4F8/O2

• Larger ionization rates result in larger ion
  fluxes in Ar/C4F8 mixtures which reduces polymer
  thickness.
• With high Ar, the polymer thins to sub-monolayer
  (less deposition, more sputtering). Etch rates
  decrease.
• O2 etches polymer and reduces its thickness. Rate
  has a maximum with O2, similar to Ar addition.

• 40 sccm, 600 W ICP, 20 mTorr, -125 V self-bias

Iowa State University Optical and Discharge
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Li et al, J. Vac. Sci. Technol. A 20, 2052, 2002.
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POROUS Si FOR LOW-K DIELECTRIC

• As feature sizes decrease and device count
  increases, the diameter of interconnect wires
  shrinks and path length increases.
• Larger RC-delay limits performance.

• Low-dielectric constant materials reduce RC.
• Porous SiO2 (xerogels) have low-k properties due
  to their lower mass density resulting from
  (vacuum) pores.
• Porosities 30-70
• Pore sizes 2-20 nm

• Ref S. Rossnagel, IBM

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PROCESSING OF NANOSTRUCTURED MATERIALS

• The opening of pores during etching of porous
  SiO2 results in the filling of the voids with
  polymer, creating thicker layers.
• Ions which would have otherwise hit at grazing or
  normal angle now intersect with more optimum
  angle.

• An important parameter is
• L/a (polymer thickness / pore radius).

• Adapted Standaert, JVSTA 18, 2742 (2000)

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PORE FILLING BY POLYMER

• Polymer deposition is activated by low energy
  ions.
• Polymer removal is activated by high energy ions.
• Low energy fluxes penetrate into pores activating
  polymer deposition
• Pore filling locally increases depth of polymer.
• Etching proceeds through a series of break
  throughs, pore-filling, and polymer removal.
• 15 nm, 60 porosity
• ICP CHF3, 6 mTorr, 1400 W

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EFFECT OF PORE RADIUS ON HAR TRENCHES
4 nm
16 nm
10 nm

• With increase in radius, thicker polymer layers
  are produced in the pores causing a decrease in
  etch rates.

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HAR PROFILES INTERCONNECTED PORES
100
60
0
Interconnectivity

• Higher porosities, larger pores and higher
  interconnectivity, filling of pores produces
  thicker polymer layers and lower etch rates.

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EFFECT OF PORE RADIUS ON CLEANING

• Larger pores have poor view angles to ions and
  thicker polymer layers.
• Lower rate of cleaning results.

4 nm
16 nm

• Ar/O299/1, 40 sccm, 600 W, 4 mTorr

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ANIMATION SLIDE
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THE CHALLENGE COMMODITY PROCESSING FOR HIGH
VALUE MATERIALS
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THE ROLE OF PLASMAS IN BIOSCIENCE

• Plasmas, to date, have played important but
  limited roles in bioscience.
• Plasma sterilization
• Plasma source ion implantation for hardening hip
  and knee replacements.
• Modification of surfaces for biocompatibility (in
  vitro and in vivo)
• Artificial skin
• The potential for commodity use of plasmas for
  biocompatibility is untapped.

• Low pressure rf H2O2 plasma (www.sterrad.com)

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HIGH VALUE PROCESSING - CELL MICROPATTERNING

• PEO - polyethyleneoxide
• pdAA plasma deposited acrylic acid

• Low pressure microelectronics-like plasmas are
  used to pattern selective substrate regions with
  functional groups for cell adhesion.
• These processes have costs commensurate with
  microelectronics high value, high cost.

1Andreas Ohl, Summer School, Germany (2004).
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ATMOSPHERIC PRESSURE PLASMAS THE CHALLENGE

• 10,000 square miles of polymer sheets are
  functionalized annually with atmospheric pressure
  plasmas. Cost lt 0.05 /m2
• Low pressure plasma processing technologies
  produce biocompatible polymers having similar
  functionalities. Cost up to 100s /cm2.
• Microelectronic plasma processing produces
  functionality at the nanoscale.
• Can these knowledge bases be combined?
• Can commodity, atmospheric pressure processing
  technology be leveraged to produce high value
  biocompatible films at low cost? The impact on
  health care would be large.

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DESCRIPTION OF nonPDPSIM CHARGED PARTICLE,
SOURCES

• Continuity (sources from electron and heavy
  particle collisions, surface chemistry,
  photo-ionization, secondary emission), fluxes by
  modified Sharfetter-Gummel with advective flow
  field.
• Poissons Equation for Electric Potential
• Electron energy equation
• Photoionization, electric field and secondary
  emission

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DESCRIPTION OF nonPDPSIM NEUTRAL PARTICLE
TRANSPORT

• Fluid averaged values of mass density, mass
  momentum and thermal energy density obtained in
  using unsteady algorithms.

• Individual fluid species diffuse in the bulk
  fluid.

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CAN COMMODITY PROCESSES PRODUCE HIGH VALUE
MATERIALS?

• 2 mm gap, 15 kV pulse, N2/O2/H2O 79.5 / 19.5 /
  1, 1 atm

• Tantec, Inc.

• van Veldhuizen et al

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POTENTIAL, ELECTRIC FIELD, CHARGE
Animation Slide

• Pulse is initiated with electron emission from
  tip of cathode.
• Development of plasma streamer deforms potential
  producing large electric field. Pulse is
  terminated with dielectric charging.

• E/N

• Potential

• Charge

• N2/O2/H2O 79.5 / 19.5 / 1, 1 atm,
• 15 kV, 0-15 ns

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ELECTRON TEMPERATURE, SOURCES
Animation Slide

• Electric field at head of streamer elevates
  electron temperature, producing a transitory wave
  of ionization. 2-body attachment occurs in high
  Te regions 3-body attachment in low Te.

• Net Attachment

• Net Ionization

• Te

• N2/O2/H2O 79.5 / 19.5 / 1, 1 atm,
• 15 kV, 0-15 ns

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ELECTRON AND ION DENSITIES
Animation Slide

• Electrons are consumed by 3-body attachment at
  the end of the pulse.

• e

• M

• M-

• N2/O2/H2O 79.5 / 19.5 / 1, 1 atm,
• 15 kV, 0-15 ns

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POST PULSE RADICAL DENSITIES

• Radical and ion densities at end of pulse are as
  high as 10s ppm. Temperature rise is nominal due
  to short pulse duration.

• H, OH

• O

• O2(1?)

• N2(A)

• N2/O2/H2O 79.5 / 19.5 / 1, 1 atm,
• 15 kV, 0-15 ns

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SURFACE INTERACTIONS ELECTRON DENSITY
2x109- 2x1011
2x1010- 2x1012
? Electrons penetrate surface features on the
polymer to a limited extent due to surface
charging.
? -15 kV, 760 Torr, N2/O2/H2O79.5/19.5/1
e cm-3
MIN (log scale) MAX
1.45 ns
1.5 ns
1.65 ns

2x1011- 2x1013
1x1011- 5x1013
Iowa State University Optical and Discharge
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SURFACE INTERACTIONS O DENSITY
1x109- 1x1012
5x1010- 5x1013
? Radicals striking the surface penetrate into
the features by diffusion. ? Unlike ions, the
transport of radicals into features is unimpeded
by surface charging
? -15 kV, 760 Torr, N2/O2/H2O79.5/19.5/1
1.5 ns
1.4 ns
4.0 ns
1.65 ns
7.0 ns

O cm-3
MIN (log scale) MAX
10 mm
1x1011- 1x1014
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Physics
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74
FUNCTIONAL GROUP DENSITIES ON POLYPROPYLENE

• 1 atm, N2/O2/H2O79.5/19.5/1, 1.5 ms, 10 kHz.

Iowa State University Optical and Discharge
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75
FUNCTIONALIZATION OF SCAFFOLDING

• ? Functionalization of scaffolding-like surfaces
  for cell adhesion.
• Can uniformity of functionalization be maintained
  over microscopic and macroscopic scale lengths.
• Use 1 atm, He/O2/H2O mixtures to optimize.

Iowa State University Optical and Discharge
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ISU_0105_82
76
FUNCTIONALIZING PP SCAFFOLDING HIGH O2
(He/O2/H2O 69/30/1)

• High O2 produces O3 and rapid alkoxy formation.
• Reactivity of O3 limits transport and produces
  long- and short-scale nonuniformities.

? 1 atm, He/O2/H2O 69/30/1
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FUNCTIONALIZING PP SCAFFOLDING LOW O2
(He/O2/H2O 89/10/1)

• Lower O2 produces less O3 and limits alkoxy
  formation.
• Overall uniformity becomes reaction limited,
  producing smoother functionalization.

? 1 atm, He/O2/H2O 89/10/1
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CONCLUDING REMARKS

• Plasmas materials processing spans a vast range
  in value added, from extremely high value
  microelectronics to commodity polymer
  functionalization.
• Development of nanostructured and bio-compatible
  materials using plasmas and their broad
  implementation are largely limited by cost.
• The social benefit to reducing cost of these
  plasma processes would be large .
• The key to creating high value using commodity
  processing is leveraging knowledge bases
  developed in different contexts. That is, working
  at the interfaces between fields.
• Rapid improvement in these knowledge bases will
  enable high values materials (plastic
  microelectronics?) to be produced using commodity
  processes during this decade.

Iowa State University Optical and Discharge
Physics
ISU_0105_85