Transcript Document

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
[email protected]
http://uigelz.ece.iastate.edu
May 2005
*Work supported by National Science Foundation, 3M Inc.
DAMOP_0505_01
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
Iowa State University
Optical and Discharge Physics
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 microelectronics fabrication
(<100’s mTorr)….
Peter
Ventzek…prior talk.
.
DAMOP_0505_03
 Functionalization of
polymers (atmospheric
pressure)
Iowa State University
Optical and Discharge Physics
EXTREMES IN CONDITIONS, VALUES, APPLICATIONS
Web Treatment of Films
 High pressure
 High throughput
 Low precision
 Modify cheap
materials
 Commodity
$0.05/m2
DAMOP_0505_04
Microelectronics
 Low pressure
 Low throughput
 High precision
 Grow expensive
materials
 High tech
$1000/cm2
Iowa State University
Optical and Discharge Physics
CREATING HIGH VALUE: COMMODITY PROCESSES
 Can commodity processes
be used to fabricate high
value materials?
$0.05/m2
?
$1000/cm2
 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?
DAMOP_0505_05
Iowa State University
Optical and Discharge Physics
LOW COST, COMMODITY
FUNCTIONALIZATION OF POLYMERS
DAMOP_0505_06
SURFACE ENERGY AND
FUNCTIONALITY OF POLYMERS
 Most polymers, having low surface energy, are hydrophobic.
POLYMER
DAMOP_0505_07
0
Waterbased inks
10
UV inks
20
Coatings
30
Waterbased adhesives
40
UV adhesives
Polystyrene
Polyethylene
PTFE
20
Polypropylene
30
50
Printing inks
40
0
60
-1
50
10
SURFACE TENSION (mN m )
60
-1
SURFACE ENERGY (mN m )
 For good adhesion and wettability, the surface energy of the
polymer should exceed of the overlayer by 2-10 mN m-1.
LIQUID
Iowa State University
Optical and Discharge Physics
PLASMA SURFACE MODIFICATION OF POLYMERS
Untreated PP
 To improve wetting and adhesion of
polymers atmospheric plasmas are
used to generate gas-phase radicals
to functionalize their surfaces.
 Polypropylene (PP)
Hydrophilic
Plasma Treated PP
Hydrophobic
He/O2/N2 Plasma
 Massines et al. J. Phys. D 31,
3411 (1998).
 M. Strobel, 3M
DAMOP_0505_08
Iowa State University
Optical and Discharge Physics
POLYMER TREATMENT APPARATUS

TYPICAL PROCESS CONDITIONS:
Web speed
Residence time
Energy deposition
Applied voltage
Gas gap
: 10 - 200 m/min
: a few s
: 0.1 - 1.0 J cm -2
: 10-20 kV at a few 10s kHz
: a few mm
FEED ROLL
GROUNDED
ELECTRODE
PLASMA
POWERED
SHOE
ELECTRODE
 Filamentary Plasma
10s – 200 mm
DAMOP_0505_09
COLLECTOR
ROLL
~
HIGH-VOLTAGE
POWER SUPPLY
Iowa State University
Optical and Discharge Physics
COMMERCIAL CORONA PLASMA EQUIPMENT
 Sherman Treaters
 Tantec, Inc.
DAMOP_0505_10
Iowa State University
Optical and Discharge Physics
REACTION MECHANISM FOR HUMID-AIR PLASMA
OH
HNO 2
HNO 3
OH
OH
NO 2
O3, HO2
O
 Initiating radicals are O, N,
OH, H
OH
N
NO
N2
O2 ,
NO3
OH
N2(A)
N
N2O5
O2
HO 2
e
N2
OH
O2 e
O2
O
O2(1)
O3
H 2O
e
H
O2
H2O2
DAMOP_0505_11
HO2
HO 2
OH
 Gas phase products include
O3, N2O, N2O5, HNO2, HNO3.
Iowa State University
Optical and Discharge Physics
REACTION PATHWAY
e
e
e
HUMID-AIR PLASMA
e
H
e
N2
N
O2
H2O
O
O O2
OH
BOUNDARY LAYER
OH, O
OH
C
C
C
O2
NO
O2
OH
H
C
N
NO
O3
OH, H 2O
O
e
C
O2
O
||
C
C
C
C
LAYER 1
C
C
C
C
LAYER 2
C
LAYER 3
H
POLYPROPYLENE
C
C
C
C
C
C
OH
C
DAMOP_0505_12
C
C
C
C
C
C
C
C
C
Iowa State University
Optical and Discharge Physics
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)
 The degree of
functionalization depends
as gas mix, energy
deposition and relative
humidity (RH).
DAMOP_0505_13
• Boyd, Macromol., 30, 5429 (1997).
 Polypropylene, Air corona
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
SITE SPECIFIC REACTIVITY
CH3 H
C
H
2
1
CH3 H CH3 H
C
C
C
H
H
H
3
1 - Primary C
C
C
2 - Secondary C
H
H
3 - Tertiary C
 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
DAMOP_0505_14
Iowa State University
Optical and Discharge Physics
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.
(POLYPROPYLENE)
H
~CH2
C
CH2~
(ALKYL RADICAL)
O(g)
OH(g)

~CH2
CH3
C
CH2~
CH3
OH(g), H 2O(g)
DAMOP_0505_15
Iowa State University
Optical and Discharge Physics
PP SURFACE REACTION MECHANISM: PROPAGATION
(ALKOXY RADICAL)
(ALKYL RADICAL)
O(g), O3 (g)

~CH2
C
CH2~
CH3
O
C
~CH2
C
CH2~
CH3
O2 (g)
~CH2
O
O
 PROPAGATION: Abundant O2 reacts
with alkyl groups to produce “stable”
peroxy radicals. O3 and O react to
form unstable alkoxy radicals.
CH2~
CH3
(PEROXY RADICAL)
DAMOP_0505_16
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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.
(ALKOXY RADICAL)
(ALCOHOLS)
O
OH
~CH2
C
CH2~
H(s)
~CH2
CH3

CH2~ + ~CH2
C
C
CH2~
CH3
CH3 O(g)

CO2 (g)
O
(CARBONYL)
DAMOP_0505_18
Iowa State University
Optical and Discharge Physics
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
BULK PLASMA
chemistry (bulk plasma,
CO2
boundary layer)
O
OH
 Plug flow
 Multilayer surface site BOUNDARY
 DIFFUSION REGIME
LAYER
balance model
 Circuit module
POLYPROPYLENE
 Boltzmann derived f()
DAMOP_0505_19
~ mfp
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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.
 N
DAMOP_0505_21
 O
 OH
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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) , PP=O + OH(g)  (OH)PP=O
DAMOP_0505_23
Iowa State University
Optical and Discharge Physics
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.
DAMOP_0505_24
Iowa State University
Optical and Discharge Physics
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 submTorr 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.
DAMOP_0505_25
Iowa State University
Optical and Discharge Physics
THE CHALLENGE: COMMODITY
PROCESSING FOR HIGH VALUE
MATERIALS
DAMOP_0505_26
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.
DAMOP_0505_27
 Low pressure rf H2O2 plasma
(www.sterrad.com)
Iowa State University
Optical and Discharge Physics
“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
DAMOP_0505_28
Ohl, Summer School, Germany (2004).
Iowa State University
Optical and Discharge Physics
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
DAMOP_0505_29
?
$1000/cm2
Iowa State University
Optical and Discharge Physics
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?
DAMOP_0505_30
Iowa State University
Optical and Discharge Physics
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.
 
N i
     Si
t
 Poisson’s Equation for Electric Potential:
     V  S
 Electron energy equation:

 ne   
5
 
 j  E  ne  Ni i       Te , j  qe
t
2

i
 Photoionization, electric field and secondary emission:
 
  r  r  3


d r 
 N i (r ) ij N j (r ) exp
 
 

S Pi (r )  
 2
r
4

r

Iowa State University

DAMOP_0505_31
Optical and Discharge Physics
CAN COMMODITY PROCESSES
PRODUCE HIGH VALUE MATERIALS
 Tantec, Inc.
DAMOP_0505_32
 Demonstration: corona-rod, 2 mm
gap, 15 kV pulse, N2/O2/H2O =79.5
/ 19.5 / 1, 1 atm
Iowa State University
Optical and Discharge Physics
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.
 E/N
DAMOP_0505_33
 Te
 Te
 N2/O2/H2O =79.5 / 19.5 / 1, 1 atm,
-15 kV, 0-15 ns
 Net
Ionization
MIN
 [e]
MAX
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.
 N2(A)
 O
 N2/O2/H2O =79.5 / 19.5 / 1, 1 atm,
15 kV, 0-15 ns
MIN
DAMOP_0505_34
 O2(1)
MAX
 H, OH
Iowa State University
Optical and Discharge Physics
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/H2O=79.5/19.5/1
[e] cm-3
1.35 ns
1.45 ns
1.40 ns
1.5 ns
MIN (log scale) MAX
1.65 ns
2x1011- 2x1013
DAMOP_0505_35
10 mm
1x1011- 5x1013
Iowa State University
Optical and Discharge Physics
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.
1.4 ns
1.65 ns
 -15 kV, 760 Torr,
1.5 ns
4.0 ns
N2/O2/H2O=79.5/19.5/1
7.0 ns
[O] cm-3
10 mm
DAMOP_0505_36
MIN (log scale) MAX
1x1011- 1x1014
Iowa State University
Optical and Discharge Physics
FUNCTIONAL GROUP DENSITIES ON POLYPROPYLENE
DAMOP_0505_37
 1 atm, N2/O2/H2O=79.5/19.5/1,
1.5 ms, 10 kHz.
Iowa State University
Optical and Discharge Physics
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.
DAMOP_0505_38
Iowa State University
Optical and Discharge Physics
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
DAMOP_0505_39
Iowa State University
Optical and Discharge Physics
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
DAMOP_0505_40
Iowa State University
Optical and Discharge Physics
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.
CH3 H
C
H
2
1
CH3 H CH3 H
C
C
C
H
H
H
3
1 - Primary C
C
C
2 - Secondary C
H
H
3 - Tertiary C
 Experimental evidence suggest reactivity scales as:
HTERTIARY > HSECONDARY > HPRIMARY
 1 atm, N2/O2/H2O = 79.5/19.5/1
DAMOP_0505_41
Iowa State University
Optical and Discharge Physics
COVERAGE OF PEROXY [=C-O-O] BY BONDING AT 10 ms
CH3 H
C
H
2
1
CH3 H CH3 H
C
C
C
H
H
H
3
1 - Primary C
C
C
2 - Secondary C
H
H
3 - Tertiary C
 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
DAMOP_0505_42
Iowa State University
Optical and Discharge Physics
COVERAGE OF PEROXY [=C-O-O] BY BONDING AT 140 ms
CH3 H
C
H
2
1
CH3 H CH3 H
C
C
C
H
H
H
3
1 - Primary C
C
C
2 - Secondary C
H
H
3 - Tertiary C
 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
DAMOP_0505_43
Iowa State University
Optical and Discharge Physics
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 mm slots.
DAMOP_0505_44
Iowa State University
Optical and Discharge Physics
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
DAMOP_0505_45
MIN
MAX
Iowa State University
Optical and Discharge Physics
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
DAMOP_0505_46
MIN
MAX
Iowa State University
Optical and Discharge Physics
COMMENTS: PHOTONS AND CHARGING
 Unlike neutral radicals that eventually diffuse into nooks-andcrannies, 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
DAMOP_0505_47
MIN
MAX
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics