new opportunities in plasma-surface interactions for functionalization of surfaces* ananth bhoj,...

NEW OPPORTUNITIES IN PLASMA-SURFACE INTERACTIONS FOR FUNCTIONALIZATION OF

SURFACES*

Ananth Bhoj, Natalie Babaeva, Rajesh Dorai and Mark J. Kushner

Iowa State University104 Marston HallAmes, IA [email protected]

http://uigelz.ece.iastate.edu

May 2005

*Work supported by National Science Foundation, 3M Inc.

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mailto:[email protected]

http://uigelz.ece.iastate.edu/

Iowa State University

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

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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 (<100’s mTorr)…. Peter Ventzek…prior talk.

Functionalization of polymers (atmospheric pressure)

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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

Iowa State UniversityOptical and Discharge PhysicsDAMOP_0505_04

Can commodity processes be used to fabricate high value materials?

Iowa State UniversityOptical and Discharge Physics

CREATING HIGH VALUE: COMMODITY PROCESSES

$0.05/m2 $1000/cm2?

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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

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SURFACE ENERGY ANDFUNCTIONALITY 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

PT

FE

Pol

ypro

pyle

ne

Pol

yeth

ylen

e

Pol

ysty

rene

SU

RF

AC

E E

NE

RG

Y (

mN

m-1

)

POLYMER0

10

20

30

40

50

60

Prin

ting

inks

UV

adh

esiv

es

Coa

tings

Wat

erba

sed

adhe

sive

s

UV

inks

Wat

erba

sed

inks

SU

RF

AC

E T

EN

SIO

N (

mN

m-1

)

LIQUID

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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.

Untreated PP

Plasma Treated PP

M. Strobel, 3M

Polypropylene (PP)

He/O2/N2 Plasma

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

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Hydrophilic

Hydrophobic



POLYMER TREATMENT APPARATUS

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HIGH-VOLTAGEPOWER SUPPLY

FEED ROLLGROUNDEDELECTRODE

COLLECTORROLL

PLASMA

~

SHOEELECTRODE

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-2Residence time : a few sWeb speed : 10 - 200 m/min

Filamentary Plasma 10s – 200 m



COMMERCIAL CORONA PLASMA EQUIPMENT

Tantec, Inc.

Sherman Treaters

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N2

O2

H2O

N N2(A)

e

e O

H OH

e

O2

NO

O2,

OH

HNO2

OH

NO2O3, HO2

OHNO3

OHN2

N

O3

HO2

OH

OH

O2

O2(1)

HO2

O2HO2

H2O2

N2O5

NO3OH


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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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

OHO2

O2

HUMID-AIR PLASMA

BOUNDARY LAYER

POLYPROPYLENE

H2O

e e

H OH

N2

e e

N NO2

O O

e e

O2O3

O2NO

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)

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• 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.

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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 > HSECONDARY > HPRIMARY

C C C C C C

H H H H H H

H H HCH3 CH3CH31

2 31 - Primary C

2 - Secondary C

3 - Tertiary C

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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.

~CH2 C CH2~

CH3

~CH2 C CH2~

CH3

H O(g)

OH(g), H2O(g)

(ALKYL RADICAL)(POLYPROPYLENE)

OH(g)

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~CH2 C CH2~

CH3

(ALKYL RADICAL)

O(g), O3 (g)

(ALKOXY RADICAL)

O2 (g)

O

~CH2 C CH2~

CH3

~CH2 C CH2~

CH3

OO

(PEROXY RADICAL)Iowa State University


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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~CH2 C CH2

CH3

OO

C CH2~

CH3

H

~CH2 C CH2

CH3

OO

C CH2~

CH3

H

+ O2 (g)

~CH2 C CH2

CH3

OO

C CH2~

CH3

HO

O


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.

CH2~

O

~CH2 CCH3

+

(ALKOXY RADICAL)

O

~CH2 C CH2~

CH3

(CARBONYL)

OH

~CH2 C CH2~

CH3

H(s)

(ALCOHOLS)

O(g) CO2 (g)

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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.

POLYPROPYLENE

BOUNDARYLAYER ~ 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()

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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.

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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).

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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

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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.

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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.

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THE CHALLENGE: COMMODITY PROCESSING FOR HIGH VALUE

MATERIALS

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Optical and Discharge PhysicsDAMOP_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


Optical and Discharge PhysicsDAMOP_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

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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:

ii St

N

SV

DESCRIPTION OF nonPDPSIM: CHARGED PARTICLE, SOURCES

2

3

4

exp)()(

)(rr

rdrr

rNrN

rSjiji

Pi

DAMOP_0505_31

e

ieiie

e qj,T2

5NnEj

t

n



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) MAX1.35 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 mDAMOP_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 ns1.4 ns

10 m

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 HCH3 CH3CH31

2 31 - 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 HCH3 CH3CH31

2 31 - 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 HCH3 CH3CH31

2 31 - 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 m 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

new opportunities in plasma-surface interactions for functionalization of surfaces* ananth bhoj,...

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