NEW%20OPPORTUNITIES%20IN%20PLASMA-SURFACE%20INTERACTIONS%20FOR%20FUNCTIONALIZATION%20OF%20SURFACES*

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NEW OPPORTUNITIES IN PLASMA-SURFACE INTERACTIONS
FOR FUNCTIONALIZATION OF SURFACES Ananth Bhoj,
Natalie Babaeva, Rajesh Dorai and Mark J.
Kushner Iowa State University 104 Marston
Hall Ames, IA 50011 mjk_at_iastate.edu http//uigelz.
ece.iastate.edu May 2005 Work supported by
National Science Foundation, 3M Inc.
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AGENDA

• Plasmas for modification of surfaces
• Functionalization of polymers
• Challenges for adapting commodity processes for
  high value materials.
• Opportunities for AMO
• Concluding Remarks

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PLASMAS FOR MODIFICATION OF SURFACES

• Plasmas are ideal for producing reactive species
  (radicals, ions) for modifying surface properties
  to achieve desired mechanical or chemical
  functionality.
• Plasma processing that adds or remove molecules
  from surfaces to achieve this functionality span
  orders of magnitude in conditions
• .

• Etching for micro-electronics fabrication (lt100s
  mTorr). Peter Ventzekprior talk.
• Functionalization of polymers (atmospheric
  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?
• What are the opportunities for AMO physics to
  build the knowledge base to meet this challenge?

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Physics
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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).

Iowa State University Optical and Discharge
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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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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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REACTION PATHWAY
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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 as gas
  mix, energy deposition and relative humidity
  (RH).

• Boyd, Macromol., 30, 5429 (1997).

• Polypropylene, Air corona

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POLYPROPYLENE (PP) POLYMER STRUCTURE

• The surface energy of polypropylene C2H3(CH3)n
  is increased by hydrogen abstraction (ions,
  radicals photons) followed by passivation by O
  atoms, in this case forming peroxy groups.

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SITE SPECIFIC REACTIVITY

• 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.

Iowa State University Optical and Discharge
Physics
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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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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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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 of charging of the dielectrics produce
  later pulses with effectively larger voltages
  residual preionization and metastables also
  persist.
• 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).

Iowa State University Optical and Discharge
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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.

Iowa State University Optical and Discharge
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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.

Iowa State University Optical and Discharge
Physics
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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
  microlectronics high value, high cost.

1Andreas Ohl, Summer School, Germany (2004).
Iowa State University Optical and Discharge
Physics
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ATMOSPHERIC PRESSURE PLASMASTHE CHALLENGE

• Controlling functional groups on polymers through
  fundamental understanding of plasma-solid
  interactions will enable engineering large area
  biocompatible surfaces.
• 10,000 square miles of polymer sheets are treated
  annually with atmospheric pressure plasmas to
  achieve specific functionality. Cost lt 0.05 /m2
• Low pressure plasma processing technologies
  produce biocompatible polymers having similar
  functionalities. Cost up to 100s /cm2
  (1000s/cm2 for artificial skin)
• Can commodity, atmospheric pressure processing
  technology be leveraged to produce high value
  biocompatible films at low cost? The impact on
  health care would be immeasurable.

Iowa State University Optical and Discharge
Physics
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POLYMER PROCESSING BY CORONA DBDs

• The surface modification of polymers (such as PP)
  by atmospheric pressure corona DBDs is a
  geometrically complex but cheap process.
• The plasma is filamentary non-uniformly producing
  reactants
• The surface is at best rough and at worst a mesh
  of strands.
• Can these surfaces be functionalized to meet high
  value standards?

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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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Physics
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CAN COMMODITY PROCESSES PRODUCE HIGH VALUE
MATERIALS

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

• Tantec, Inc.

Iowa State University Optical and Discharge
Physics
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E/N, Te, SOURCES, ELECTRON DENSITY
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.

• Te

• Net Ionization

• Te

• e

• E/N

• 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.

• O

• O2(1?)

• N2(A)

• H, OH

• 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
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10 mm
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SURFACE INTERACTIONS O DENSITY
1x109- 1x1012
5x1010- 5x1013
? Radicals striking the surface penetrate into
the features by diffusion. ? Unlike charged
species, with time, the density of radicals such
as O, increases inside these features.
? -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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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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FUNCTIONALIZATION OF SCAFFOLDING

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

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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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REMINDER LOCAL STRUCTURE MATTERS

• The reactivity of C-H to gas phase species
  depends and with other surface species on their
  local bonding and orientation on surface.

• Experimental evidence suggest reactivity scales
  as
• HTERTIARY gt HSECONDARY gt HPRIMARY

? 1 atm, N2/O2/H2O 79.5/19.5/1
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COVERAGE OF PEROXY C-O-O? BY BONDING AT 10 ms

• Primary and secondary sites with large view
  angles are rapidly functionalized to peroxy.
• Alkyl tertiary sites lag and are susceptible to
  OH, O3 passivation

Iowa State University Optical and Discharge
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? 1 atm, N2/O2/H2O 79.5/19.5/1
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COVERAGE OF PEROXY C-O-O? BY BONDING AT 140 ms

• Long term production of O3 and reactions between
  surface species favor secondary and tertiary
  sites.
• Uniformity improves (mostly).

Iowa State University Optical and Discharge
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? 1 atm, N2/O2/H2O 79.5/19.5/1
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PROCESSING COMPLEX SHAPES

• Functionalization of parts with complex shapes
  with dimensions larger than reaction length of
  radicals requires plasma to penetrate into
  structure.
• Demonstration case grooved disk with 30 ?m slots.

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PROCESSING COMPLEX SHAPES PLASMA PENETRATION

• Plasma penetrates through grooves but shadow some
  surfaces.
• Charging of surface steers plasma
• Electron density (max 1014 cm-3)

Animation Slide-GIF
? 1 atm, N2/O2/H2O 79.5/19.5/1, 2 ns
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PROCESSING COMPLEX SHAPES O ATOM DENSITY

• Plasma penetrates through grooves but shadow some
  surfaces.
• Charging of surface steers plasma
• O atom density (max 1015 cm-3)

Animation Slide-GIF
? 1 atm, N2/O2/H2O 79.5/19.5/1, 2 ns
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COMMENTS PHOTONS AND CHARGING

• Unlike neutral radicals that eventually diffuse
  into nooks-and-crannies, shadowing (photons) and
  local electric fields (surface charging) produce
  highly non-uniform profiles.
• What affect does UV illumination and charging
  have on reactivity?

? 1 atm, N2/O2/H2O 79.5/19.5/1, 2 ns
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THE CHALLENGE

• Can established AMO theory and measurement
  techniques developed for gas phase species be
  extended to produce reaction probabilities on the
  surfaces of solid polymers?
• Can scaling laws be developed for going from
  molecules to surfaces?
• For example, how different are..

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OPPORTUNITIES AND CONCLUDING REMARKS

• The interaction of plasma produced species with
  polymer surfaces is an exceedingly rich field of
  study.
• The are very (very very) few fundamental
  studies capable of producing reaction
  probabilities of even simple systems such as O
  atoms on polypropylene or polyethylene.
• Probabilities for reactions between surface
  species are only now becoming quantified.
  (Session C1 Interaction of Slow Electrons with
  Biomolecules)
• Photon and charging effects on ratesunknown.
• Improving our fundamental understanding and
  predictive capability (and leveraging commodity
  techniques) will revolutionize fields such as
  health products.

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