damop_2005_gif.ppt

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@iastate.edu
http://uigelz.ece.iastate.edu
May 2005
*Work supported by National Science Foundation, 3M Inc.
DAMOP_0505_01

Optical and Discharge Physics
AGENDA
 Plasmas for modification of surfaces
 Functionalization of polymers
 Challenges for adapting commodity processes for high value
materials.
 Opportunities for AMO
 Concluding Remarks
DAMOP_0505_02

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
(<100’s mTorr)…. Peter
Ventzek…prior talk.
 Functionalization of
polymers (atmospheric
pressure)
DAMOP_0505_03

 Low pressure
 Low throughput
 High precision
 Grow expensive
materials
 High tech
 High pressure
 High throughput
 Low precision
 Modify cheap
materials
 Commodity
Web Treatment of Films
$0.05/m2 $1000/cm2
Microelectronics
EXTREMES IN CONDITIONS, VALUES, APPLICATIONS
DAMOP_0505_04

 Can commodity processes
be used to fabricate high
value materials?
CREATING HIGH VALUE: COMMODITY PROCESSES
$0.05/m2
$1000/cm2
?
DAMOP_0505_05
 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?

LOW COST, COMMODITY
FUNCTIONALIZATION OF POLYMERS
DAMOP_0505_06

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.
0
10
20
30
40
50
60
PTFE
Polypropylene
Polyethylene
Polystyrene
SURFACE
ENERGY
(mN
m
-1
)
POLYMER
0
10
20
30
40
50
60
Printing
inks
UV
adhesives
Coatings
Waterbased
adhesives
UV
inks
Waterbased
inks
SURFACE
TENSION
(mN
m
-1
)
LIQUID
DAMOP_0505_07

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.
Untreated PP
Plasma Treated PP
 M. Strobel, 3M
 Polypropylene (PP)
He/O2/N2 Plasma
 Massines et al. J. Phys. D 31,
3411 (1998).
DAMOP_0505_08
Hydrophilic
Hydrophobic

POLYMER TREATMENT APPARATUS
DAMOP_0505_09
HIGH-VOLTAGE
POWER SUPPLY
FEED ROLL
GROUNDED
ELECTRODE
COLLECTOR
ROLL
PLASMA
~
SHOE
ELECTRODE
POWERED
TYPICAL PROCESS CONDITIONS:

Gas gap : a few mm
Applied voltage : 10-20 kV at a few 10s kHz
Energy deposition : 0.1 - 1.0 J cm-2
Residence time : a few s
Web speed : 10 - 200 m/min
 Filamentary Plasma
10s – 200 mm

COMMERCIAL CORONA PLASMA EQUIPMENT
 Tantec, Inc.
 Sherman Treaters
DAMOP_0505_10

N2
O2
H2O
N N2(A)
e
e
O
H OH
e
O2
NO
O2,
OH
HNO2
OH
NO2
O3, HO2
O
HNO3
OH
N2
N
O3
HO2
OH
OH
O2
O2(1)
HO2
O2
HO2
H2O2
N2O5
NO3
OH
REACTION MECHANISM FOR HUMID-AIR PLASMA
 Initiating radicals are O, N,
OH, H
 Gas phase products include
O3, N2O, N2O5, HNO2, HNO3.
DAMOP_0505_11

REACTION PATHWAY
DAMOP_0505_12
C C C C C C C C C
C C C C C C C C C C
O
H
OH
O
||
OH, O
C C C C C C C C C C C
LAYER 1
LAYER 2
LAYER 3
H
OH, H2O
OH
OH
O2
O2
HUMID-AIR PLASMA
BOUNDARY LAYER
POLYPROPYLENE
H2O
e e
H OH
N2
e e
N N
O2
O O
e e
O2
O3
O2
NO
NO

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 (-C=O)
 Alcohols (C-OH)
 Peroxy (-C-O-O)
 Acids ((OH)C=O)
DAMOP_0505_13
• Boyd, Macromol., 30, 5429 (1997).
 Polypropylene, Air corona
 The degree of
functionalization depends
as gas mix, energy
deposition and relative
humidity (RH).

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.
DAMOP_0505_13A

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 > HSECONDARY > HPRIMARY
C C C C C C
H H H H H H
H H H
CH3 CH3
CH3
1
2 3
1 - Primary C
2 - Secondary C
3 - Tertiary C
DAMOP_0505_14

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.
~CH2 C CH2~
CH3

~CH2 C CH2~
CH3
H O(g)
OH(g), H2O(g)
(ALKYL RADICAL)
(POLYPROPYLENE)
OH(g)
DAMOP_0505_15

~CH2 C CH2~
CH3

(ALKYL RADICAL)
O(g), O3 (g)
(ALKOXY RADICAL)
O2 (g)
O
~CH2 C CH2~
CH3
~CH2 C CH2~
CH3

O
O
(PEROXY RADICAL)
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.
DAMOP_0505_16

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.
DAMOP_0505_17

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.
CH2~

O
~CH2 C
CH3
+
(ALKOXY RADICAL)
O
~CH2 C CH2~
CH3
(CARBONYL)
OH
~CH2 C CH2~
CH3
H(s)
(ALCOHOLS)
O(g)
 CO2 (g)
DAMOP_0505_18

GLOBAL_KIN AND SURFACE KINETICS
 Reaction mechanisms in pulsed atmospheric air plasma
treatment of polymers have been investigated with global
kinetics and surface models.
POLYPROPYLENE
BOUNDARY
LAYER ~ mfp
BULK PLASMA
DIFFUSION REGIME
O OH CO2

 GLOBAL_KIN
 2-Zone homogeneous plasma
chemistry (bulk plasma,
boundary layer)
 Plug flow
 Multilayer surface site
balance model
 Circuit module
 Boltzmann derived f()
DAMOP_0505_19

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)
DAMOP_0505_20

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.
DAMOP_0505_21
 O  OH
 N

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).
DAMOP_0505_22

 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.
HUMIDITY: PP FUNCTIONALIZATION BY OH
PP-OH+ OH(g)PP-O + H2O(g) , PP=O + OH(g)  (OH)PP=O
DAMOP_0505_23

COMMODITY TO HIGH VALUE
 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.
 As the material value increases (cents to dollars /cm2?) higher
process refinement is justified to customize functionalization.
DAMOP_0505_24

COMMODITY TO HIGH VALUE
 1 atm, He/O2/ H2O, 15 kV, 3 mm,
9.6 kHz, 920 pulses.
 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.
DAMOP_0505_25

THE CHALLENGE: COMMODITY
PROCESSING FOR HIGH VALUE
MATERIALS
DAMOP_0505_26

DAMOP_0505_27
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)

“HIGH VALUE” PROCESSING - CELL MICROPATTERNING
DAMOP_0505_28
 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).
 PEO - polyethyleneoxide
 pdAA – plasma deposited acrylic acid

DAMOP_0505_29
ATMOSPHERIC PRESSURE PLASMAS:THE 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: < $0.05 /m2
 Low pressure plasma processing technologies produce
biocompatible polymers having similar functionalities. Cost: up
to $100’s /cm2 ($1000’s/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.
$0.05/m2
$1000/cm2
?

 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?
POLYMER PROCESSING BY CORONA DBDs
DAMOP_0505_30

 Continuity (sources from electron and heavy particle collisions,
surface chemistry, photo-ionization, secondary emission), fluxes
by modified Sharfetter-Gummel with advective flow field.
 Poisson’s Equation for Electric Potential:
 Electron energy equation:
 Photoionization, electric field and secondary emission:
i
i
S
t
N










S
V 


 





DESCRIPTION OF nonPDPSIM: CHARGED PARTICLE, SOURCES













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
 
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
 2
3
4
exp
)
(
)
(
)
(
r
r
r
d
r
r
r
N
r
N
r
S
j
ij
i
Pi 










DAMOP_0505_31
 
e
i
e
i
i
e
e
q
j
,
T
2
5
N
n
E
j
t
n






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

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
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


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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.
DAMOP_0505_32

E/N, Te, SOURCES, ELECTRON DENSITY
 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.
 N2/O2/H2O =79.5 / 19.5 / 1, 1 atm,
-15 kV, 0-15 ns
 E/N
MIN MAX
Animation Slide
DAMOP_0505_33
 Te  Net
Ionization
 Te  [e]

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)
MIN MAX
 N2/O2/H2O =79.5 / 19.5 / 1, 1 atm,
15 kV, 0-15 ns
 N2(A)
DAMOP_0505_34
 H, OH

 Electrons penetrate surface
features on the polymer to a limited
extent due to surface charging.
SURFACE INTERACTIONS: ELECTRON DENSITY
 -15 kV, 760 Torr,
N2/O2/H2O=79.5/19.5/1
MIN (log scale) MAX
1.35 ns 1.40 ns
2x109- 2x1011
2x1011- 2x1013
1.45 ns
2x1010- 2x1012
1.5 ns
1x1011- 5x1013
1.65 ns
[e] cm-3
10 mm
DAMOP_0505_35

 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.
SURFACE INTERACTIONS: [O] DENSITY
 -15 kV, 760 Torr,
N2/O2/H2O=79.5/19.5/1
1x109- 1x1012
1x1011- 1x1014
1.65 ns
5x1010- 5x1013
4.0 ns 7.0 ns
MIN (log scale) MAX
[O] cm-3
1.5 ns
1.4 ns
10 mm
DAMOP_0505_36

FUNCTIONAL GROUP DENSITIES ON POLYPROPYLENE
 1 atm, N2/O2/H2O=79.5/19.5/1,
1.5 ms, 10 kHz.
DAMOP_0505_37

FUNCTIONALIZATION OF SCAFFOLDING
DAMOP_0505_38
 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.

FUNCTIONALIZING PP SCAFFOLDING: HIGH O2 (He/O2/H2O = 69/30/1)
DAMOP_0505_39
 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

FUNCTIONALIZING PP SCAFFOLDING: LOW O2 (He/O2/H2O = 89/10/1)
DAMOP_0505_40
 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

REMINDER: LOCAL STRUCTURE MATTERS
DAMOP_0505_41
 The reactivity of =C-H to gas phase species depends and with
other surface species on their local bonding and orientation on
surface.
 1 atm, N2/O2/H2O = 79.5/19.5/1
C C C C C C
H H H H H H
H H H
CH3 CH3
CH3
1
2 3
1 - Primary C
2 - Secondary C
3 - Tertiary C
 Experimental evidence suggest reactivity scales as:
HTERTIARY > HSECONDARY > HPRIMARY

COVERAGE OF PEROXY [=C-O-O] BY BONDING AT 10 ms
DAMOP_0505_42
 Primary and secondary sites with
large view angles are rapidly
functionalized to peroxy.
 Alkyl tertiary sites lag and are
susceptible to OH, O3 passivation
 1 atm, N2/O2/H2O = 79.5/19.5/1
C C C C C C
H H H H H H
H H H
CH3 CH3
CH3
1
2 3
1 - Primary C
2 - Secondary C
3 - Tertiary C

COVERAGE OF PEROXY [=C-O-O] BY BONDING AT 140 ms
DAMOP_0505_43
 Long term production of O3 and
reactions between surface species
favor secondary and tertiary sites.
 Uniformity improves (mostly).
 1 atm, N2/O2/H2O = 79.5/19.5/1
C C C C C C
H H H H H H
H H H
CH3 CH3
CH3
1
2 3
1 - Primary C
2 - Secondary C
3 - Tertiary C

PROCESSING COMPLEX SHAPES
DAMOP_0505_44
 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 mm slots.

PROCESSING COMPLEX SHAPES: PLASMA PENETRATION
DAMOP_0505_45
 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
MIN MAX

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)
DAMOP_0505_46
 1 atm, N2/O2/H2O = 79.5/19.5/1, 2 ns
MIN MAX
Animation Slide-GIF

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?
DAMOP_0505_47
 1 atm, N2/O2/H2O = 79.5/19.5/1, 2 ns
MIN MAX

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…..
DAMOP_0505_48

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 rates……unknown.
 Improving our fundamental understanding and predictive
capability (and leveraging commodity techniques) will
revolutionize fields such as health products.
DAMOP_0505_49

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