MAKING PLASMAS DO SMALL THINGS: FUNCTIONALIZING … · Iowa State University Optical and Discharge Physics •Tissue engineering requires “scaffolding”; substrates with nooks

MAKING PLASMAS DO SMALL THINGS: FUNCTIONALIZING NOOKS-AND-

CRANNIES IN POLYMERS AT LOW AND HIGH PRESSURE

Mark J. KushnerIowa State University

Department of Electrical and Computer EngineeringDepartment of Chemical and Biological Engineering

104 Marston HallAmes, IA 50011

[email protected] http://uigelz.ece.iastate.edu

April 2006

UCLA_0406_01

Iowa State UniversityOptical and Discharge Physics

ACKNOWLEDGEMENTS

• Dr. Rajesh Dorai (now at Varian Semiconductor Equipment)• Dr. Natalie Babeva• Mr. Ananth Bhoj

• Funding Agencies:• 3M Corporation• Semiconductor Research Corporation• National Science Foundation• SEMATECH• CFDRC Inc.

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AGENDA

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• Introduction to Plasma Processing

• Plasma surface functionalization

• Description of the models

• High Pressure:

• Plasma dynamics in He/NH3/H2O and humid air mixtures

• Functionalization of rough and porous surfaces

• Low Pressure: Ions and Shadowing

• Concluding remarks

• Work supported by National Science Foundation, 3M Inc and Semiconductor Research Corp.


PLASMAS 101: INTRODUCTION

• Plasmas (ionized gases) are often called the “fourth state of matter.”

• Plasmas account for > 99.9% of the mass of the known universe (dark matter aside).

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http://www.plasmas.org/basics.htm

• X-ray view of the sun, a plasma.


TECHNOLOGICAL PLASMAS:PARTIALLY IONIZED GASES

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

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


TECHNOLOGICAL PLASMAS:PARTIALLY IONIZED GASES

• Air plasma: N2, O2, N2+, O2

+, O-, e where [e] << [M].

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• The resulting partially ionized gas (N+/N < 10-2-10-6) is not neutral on a microscopic scale, but is neutral on a global scale.

• Partially ionized plasmas contain neutral atoms and molecules, electrons, positive ions and negative ions.


TECHNOLOGICAL PLASMAS:REACTIVE SPECIES

• Electron impact collisions on atoms and molecules produce reactive species.

• These species emit photons, modify surfaces and create new materials.

• These plasmas are called “collisional” because electrons impart energy to neutrals by physical impact.

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)(:3

34

*

*

carbonlikediamondCHasurfaceCH

eHCHCHe

hXeXeeXeXee

−−→→

++→+

+→

+→+

ν

• These systems are the plasmas of every day technology.

• Electrons transfer power from the "wall plug" to internal modes of atoms / molecules to "make a product”, very much like combustion.

• The electrons are “hot” (several eV or 10-30,000 K) while the gas and ions are cool, creating“non-equilibrium” plasmas.

WALL PLUG

POWER CONDITIONING

ELECTRIC FIELDS

ENERGETIC ELECTRONS

COLLISIONS WITHATOMS/MOLECULES

EXCITATION, IONIZATION, DISSOCIAITON (CHEMISTRY)

LAMPS LASERS ETCHING DEPOSITIONE

eA

PHOTONS RADICALS

IONS


COLLISIONAL LOW TEMPERATURE PLASMAS

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•Displays

•Materials Processing

COLLISIONAL LOW TEMPERATURE PLASMAS

• Lighting

• Thrusters

• Spray Coatings

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MULTISCALE MODELING OF PLASMAS AND PLASMA-SURFACE INTERACTIONS

• Our research group develops multi-scale, integrated reactor and feature scale modeling hierarchies to simulate plasma processingsystems.

• Fundamental plasma hydrodynamics transport• Plasma chemistry• Radiation transport• Plasma surface interactions• Materials modification and surface kinetics

• We are very interested in the science of plasmas…but also interested in how plasmas can be used to optimally produce unique materials, properties and structures.

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PLASMA FUNCTIONALIZATION OF SURFACES

• To modify wetting, adhesion and reactivity of surfaces, such as polymers, plasmas are used to generate gas-phase radicals to functionalize their surfaces.

• Example: atm plasma treatment of PP

Untreated PP

Plasma Treated PP

• M. Strobel, 3M

• Polypropylene (PP)

He/O2/N2 Plasma

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

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FUNCTIONALIZATION OF POLYMERS USING PLASMAS

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• Functionalization of surfaces such as polymers occurs by their chemical interaction with plasma produced species - ions, radicals and photons.

• Example: H abstraction in an oxygen containing plasma enables affixing O atoms as a peroxy site.

• Functionalization usually only affects the surface layer.

• Pulsed atmospheric filamentary discharges (coronas) are routinely used to web treat commodity polymers like poly-propylene (PP) and polyethylene (PE).

• Due to the low value of these materials, the costs of the processes must by low, < $0.05/m2.


SURFACE MODIFICATION OF POLYMERS

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• Filamentary Plasma 10s – 200 µm


COMMERCIAL CORONA PLASMA EQUIPMENT

• Tantec, Inc.

• Sherman Treaters

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• Tissue engineering requires “scaffolding”; substrates with nooks and crannies 10s -1000s µm in which cells adhere and grow.

• Scaffolding is chemically treated (functionalized) to enhance cell adhesion or prevent unwanted cells from adhering.

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• E. Sachlos, European Cells and Materials v5, 29 (2003)

• Tien-Min Gabriel Chuhttp://www.engr.iupui.edu/~tgchu

PLASMAS FOR MODIFICATION OFBIOCOMPATIBLE SURFACES: TISSUE ENGINEERING

• Low pressure plasmas (< 1 Torr) are typically “glows” and not streamers.

• Technology used to fabricate microelectronics devices to functionalize features to a few nm.

• Diffusive transport and long mean-free-paths provide inherently better uniformity.

• Energy of ions is typically larger (many eV) and controllable (100’s eV)

• GEC Reference Cell, 100 mTorr Ar

• Ref: G. Hebner


LOW PRESSURE GLOW DISCHARGES

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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 PhysicsISU_0105_11

•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?


CREATING HIGH VALUE: COMMODITY PROCESSES

$0.05/m2 $1000/cm2?

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FUNCTIONALIZING SMALL FEATURES

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• Using atmospheric pressure plasmas (APPs) to functionalize small features is ideal due to their low cost.

• Low pressures plasmas (LPPs), though more costly, likely provide higher uniformity.

•Can APPs provide the needed uniformity and penetration into small features?

•Are LPPs necessarily the plasma of choice for small feature functionalization.

• In this talk, the functionalization of small features using APPs and LPPs will be discussed using results from computer simulations.

•NH3 plasmas for =NHx functionality for cell adhesion.

•O2 plasmas for =O functionality for improved wettability.


ELECTROMAGNETICS AND ELECTRON KINETICS• The wave equation is solved using tensor conductivities:

( ) ( )t

JEtEEE

∂+⋅∂

+∂

∂=⎟⎟

⎠

⎞⎜⎜⎝

⎛∇⋅∇+⎟⎟

⎠

⎞⎜⎜⎝

⎛⋅∇∇−

σεµµ

1 12

2

• Electron energy transport: Continuum and Kinetics

where S(Te) = Power deposition from electric fieldsL(Te) = Electron power loss due to collisionsΦ = Electron fluxκ(Te) = Electron thermal conductivity tensorSEB = Power source source from beam electrons

• Kinetic: MCS is used to derive including e-e collisions using electromagnetic and electrostatic fields .

( ) ( ) ( ) EBeeeeeee STTkTTLTStkTn +⎟⎠⎞

⎜⎝⎛ ∇⋅−Φ⋅∇−−=∂⎟

⎠⎞

⎜⎝⎛∂ κ

25/

23

( )trf ,, rε

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LOW PRESSURE: PLASMA CHEMISTRY, TRANSPORT ELECTROSTATICS

• Continuity, momentum and energy equations are solved for each species.

• Semi-implicit solution of Poisson’s equation:

( ) ( )⎟⎠

⎞⎜⎝

⎛⋅∇⋅∆+=∆+Φ∇⋅∇ ∑∑

iiqt-- i

iiis Nqtt φρε

r

iiii SN

tN

+⋅−∇= )v( r

∂∂

( ) ( ) ( ) ( ) iii

iiiiiii

i

ii BvEmNqvvNTkN

mtvN µ

∂∂

⋅∇−×++⋅∇−∇=rrrrr

r 1

( ) ijjijj

imm

j vvNNm

ji

νrr−−∑

+

( ) 222

2

)()U(UQ E

mqNNP

tN

ii

iiiiiiiii

ii

ωννε

∂ε∂

+=⋅∇+⋅∇+⋅∇+

∑∑ ±−+

++j

jBijjij

ijBijjiji

ijs

ii

ii TkRNNTTkRNNmm

mE

mqN 3)(32

2

ν

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• Continuity: electron collisions, volume and surface chemistry, photo-ionization, secondary emission, Sharfetter-Gummel fluxes.

• Optically thick photoionization sources (important for streamers)

• Fully implicit solution of Poisson’s equation.

• Unstructured mesh.Iowa State University

Optical and Discharge Physics

HIGH PRESSURE: CHARGED PARTICLE TRANSPORT ELECTROSTATICS

( ) ⎟⎠

⎞⎜⎝

⎛+=Φ∇⋅∇ ∑

iiis tNqt-t )()(ρε

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ii S

tN

+⋅∇−=∂∂ φ

rv

⎮⎮⎮

⌡

⌠

−′

′⎟⎟⎠

⎞⎜⎜⎝

⎛ −′−′

= 2

3

4

exp)()()(

rr

rdrr

rNrNrS

jiji

Pi vv

vvv

vv

v

π

λσ


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

)pumps,inlets()v(t

+⋅−∇=rρ

∂ρ∂

( ) ( ) ( ) ∑+⋅∇−⋅∇−∇=i

iii ENqvvNkTtv vrrr

µρ∂ρ∂

( ) ( ) ∑ ∑ ⋅+−⋅∇++∇−−∇=i i

iiifipp EjHRvPTcvTtTc rrr ∆ρκ

∂ρ∂

( ) ( ) ( )SV

T

iTifii SS

NttNNDvtNttN ++⎟

⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛ +∇−⋅∇−=+

∆∆ r

• Individual fluid species diffuse in the bulk fluid.

HIGH PRESSURE: NEUTRAL PARTICLE TRANSPORT

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NANOSCALE EVOLUTION OF SURFACE PROPERTIES

• The Monte Carlo Feature Profile Model (MCFPM) predicts evolution of features using energy and angularly 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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CELL MICROPATTERNING: MODIFICATION OF POLYMERS

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• Modification of polymer surfaces for specified functionality canbe used to create cell adhering or cell repulsing regions.

1Andreas Ohl, Summer School, Germany (2004).

• PEO - polyethyleneoxide

• pdAA – plasma deposited acrylic acid


FUNCTIONALIZATION FOR BIOCOMPATIBILITY

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(1K. Schroeder et al, Plasmas and Polymers, 7, 103, 2002)

• Ammonia plasma treatment affixes amine (C-NH2) groups on surfaces for applications such as cell adhesion, protein immobilization and tissue engineering.

Micropatterned cell growth on NH3 plasma treated PEEK1


GAS PHASE CHEMISTRY - He/NH3/H2O MIXTURES

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• Electron impact reactions initiate dissociate NH3 and H2O into radicals that functionalize surface.

• H, NH2, NH, O and OH are major radicals for surface reactions.

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SURFACE REACTION MECHANISM

• Gas phase H, O and OH abstract H atoms from the PP surface producing reactive surface alkyl (R-•) radical sites.

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SURFACE REACTION MECHANISM

• Gas phase NH2 and NH radicals react with surface alkyl sites creating amine (R-NH2) groups and imine(R-NH•) sites.


TREATMENT OF POROUS POLYMER BEADS

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• Functionalized Porous Bead for Protein Binding sites (www.ciphergen.com)

• Biodegradable porous beads are used for drug delivery and gene therapy.

• Macroporous beads are 10s µm in diameter with pore sizes < 10 µm.

• External and internal surfaces are functionalized for polymer supported catalysts and protein immobilization.

• Penetration of reactive species into pores is critical to functionalization.


DBD TREATMENT OF POROUS POLYMER BEAD

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• Corona treatment of porous polymer beads for drug delivery.

• How well are the internal surfaces of pores accessible to the plasma?

• What is the extent of functionalization on internal surfaces?

• Bead size ~ 10s µm • Pore diameter ~ 2-10 µm

• - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1• PRF – 10 kHz


ELECTRON TEMPERATURE, SOURCE

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• - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1, 0-3.5 ns

• Electron Temperature • Electron SourceAnimation Slide-GIF

MIN MAX


ELECTRON DENSITY

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• Electron density of 1013-1014 cm-3 is produced.

• Electron impact dissociation generates radicals that functionalize surfaces.

• - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1, 0-3.5 ns

Animation Slide-GIF

MIN MAX


POST-PULSE RADICAL DENSITIES

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• - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1

• NH • NH2 • OH

MIN MAX


ELECTRON DENSITY IN AND AROUND BEAD

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• Corona treating a porous polymer bead placed on the lower dielectric.

• - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1, 0-3 ns.

• In negative corona discharge, electrons lead the avalanche frontand initially penetrate into pores. Charging of surfaces limit further electron penetration.

• Electrons (3.7 x 1013 cm-3)50 µm

Animation Slide-GIF

MIN MAX

Iowa State UniversityOptical and Discharge PhysicsUCLA_0406_35

• - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1, 0-3 ns

TOTAL POSITIVE ION DENSITY IN AND AROUND BEAD

• Ions lag electrons arriving at bead but persist at surfaces due to negative charging that makes the surfaces cathode like.

• Lower surface (anode) is ion repelling.

• Ions (3.7 x 1013 cm-3) 50 µm

Animation Slide-GIF

MIN MAX


[NH2] INSIDE PORES

UCLA_0406_36 (log scale)

• - 5 kV, He/NH3/H2O=90/10/0.1, pore dia=4.5 µm, 1 atm

90 µm Bead

2x1010- 2x1013

MIN MAX

3 ns

3 ns 80 µs

9.1x1012- 9.3x1012

7.5x1012- 8.5x1012

• Since electrons poorly penetrate into most pores, little NH2 is initially produced inside bead.

• NH2 later diffuses into pores from outside.

30 µm Bead 80 µs

2x1010- 2x1013

• [NH2] within pores increases with pore diameter during the pulse and in the interpulse period.


[NH2] INSIDE PORES : PORE DIAMETER8.5 µm 4.5 µm 3 µm

(log scale)MIN MAX

[NH2] cm- 3

• - 5 kV, He/NH3/H2O=90/10/0.1, bead dia=90 µm, 1 atm

• t = 3 ns2x1010- 2x1013

• t = 80 µs7.5x1012-8.7x1012


FUNCTIONALIZATION OF POROUS BEAD SURFACES

UCLA_0406_38

[ALKYL]=C•

MIN MAX

1.25x1010 –1.25x1011

1012 – 1013

log scale, cm- 2

[AMINE]=C-NH2

• - 5 kV, 1 atm, 10 kHz, He/NH3/H2O=90/10/0.1, Bead size=90 µm, Pore dia= 4.5 µm, t=0.1 s

A B

C D

EF

G

H I

JK

AB

C

DE

FG

HI

JK

Letters indicate position along the surface.


AMINE SURFACE COVERAGE: SIZE OF BEAD

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• - 5 kV, 1 atm, 10 kHz, He/NH3/H2O=90/10/0.1, t=1 s

• Outer surfaces have significantly higher amine coverage than interior pores.

• Smaller beads pores have more uniform coverage due to shorter diffusion length into pores.

• Beads sitting on electrode shadow portions of surface.

Pore dia = 4.5 µm


BEADS IN DISCHARGE: ELECTRON DENSITY

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• Uniformity may be improved by dropping beads through discharge instead of placing on a surface.

• He/O2/H2O = 89/10/1, 1 atm

• Electrons produce a wake beyond the particle.

• Electron Density (1.6 x 1014 cm-3), 0-2.5 ns

Animation Slide-GIF

MIN MAX


ELECTRON DENSITY AND SOURCE

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• Ionization occurs around particle during initial avalanche and restrike.

• Sheath forms above particle, wake forms below particle.

• He/O2/H2O = 89/10/1, 1 atm

• 0-2.6 ns

• Electron Density (6 x 1013 cm-3)

• Electron Source (1023 cm-3s-1)

Animation Slide-GIF

MIN MAX


POST-PULSE O and OH DENSITIES

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• Directly after the pulse, radicals have a similar wake below the particles.

• He/O2/H2O = 89/10/1, 1 atm

• 0-2.6 ns

• [O] (8 x 1014 cm-3) • [OH] (5 x 1013 cm-3)

MIN MAX


BEADS IN DISCHARGE: SURFACE COVERAGE

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• Uniformity of functionalization, locally poor, is improved around the particle.

• He/O2/H2O = 89/10/1,

• 1 atm

• Alkoxy (=C-O) and Peroxy (=C-OO) Coverage


DBD TREATMENT OF PP SURFACE WITH MICROSTRUCTURE

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• Corona functionalization of rough polymer resembling tissue scaffold.

• 1 atm, He/NH3/H2O, 10 kHz

• Polypropylene.

• E. Sachlos, et al.

MIN MAX Iowa State UniversityOptical and Discharge Physics

PENETRATION INTO SURFACE FEATURES – [e], [IONS]

UCLA_0406_45

[e] cm- 3

t = 2.7 ns t = 4 ns

1010 – 1013

[Positive ions] cm- 3

1010 – 1013

[Surface (-ve) Charge] µC10-1 – 103

• - 5 kV, 1 atm, He/NH3/H2O=98.9/1.0/0.1

log scale


NH2 DENSITY: EARLY AND LATE[NH3]=10%

• - 5 kV, 1 atm

3x1012 - 3x1014

1.8x1012 – 1.9x1012, t =90 µs 2.25x1012 – 2.35x1012, t =90 µs

• NH2 is initially not produced inside the roughness, but later diffuses into the interior.

MIN MAX

[NH3]=30%

t =3 ns

[NH2] cm- 3


SURFACE COVERAGE OF ALKYL RADICALS (=C•)

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• - 5 kV, 1 atm, 10 kHz, He/NH3/H2O=90/10/0.1

• Alkyl sites are formed by the abstraction reactions

OH + PP PP• + H2O H + PP PP• + H2

• Large scale and small scale uniformity improves with treatment.


SURFACE COVERAGE OF AMINE GROUPS [=C-NH2]

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• - 5 kV, 1 atm, 10 kHz, He/NH3/H2O=90/10/0.1, t = 0.1 s

• Amine groups are created by addition of NH2 to alkyl sites.

NH2 + PP• PP-NH2

• Points with large view angles are highly treated.


OPTIMIZE CHEMISTRY, UNIFORMITY WITH GAS MIXTURE

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• Balance of peroxy (PP-OO), alkoxy (PP-O) and alcohol (PP-OH) groups can be controlled by composition of fluxes.

• Example: He/O2/H2O

e + O2 → O + O + e e + H2O → H + OH + e O + O2 + M → O3 + M

• Large f(O2), small f(H2O): Small OH fluxes, large O3 fluxes Small f(O2), large f(H2O): Large OH fluxes, small O3 fluxes

• Impact on polypropylene surface chemistryPP + O → PP• + OH (slow rate)PP + OH → PP• + H2O (fast rate)PP• + O2 → PP-OO (slow rate but a lot of O2)PP• + O3 → PP-O + O2 (fast rate)PP• + OH → PP-OH (fast rate)


CONTROLLING FLUX OF OZONE TO SURFACE

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• Pulsed corona discharge, 10 kHz

• He/O2/H2O = 99-X /X/1

• After short discharge pulse, flux of O atoms is large.

• At end of interpulse period, flux of O atoms is negligible as most O has been converted to O3.

• Flux of O3 increases by nearly 100 with increasing f(O2).

• Non-uniform O3 fluxes results from reaction limited transport into microstructure.


CONTROLLING FLUX OF OZONE TO SURFACE

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• O2 fluxes at any finite mole fraction; peroxy PP-OO formation dominates.

• Large O2 produces large O3 fluxes which favors alkoxy PP-O.

• Small O2 increases OH fluxes by H2O dissociation and so alcohol PP-OH fractions increase.

• Small scale uniformity is dominated by reactivity of O3 and in ability to penetrate deep into crevices.

• Low O3 but moderate OH optimizes uniformity.

• He/O2/H2O = 99-X /X/1


CAN FLOW BE USED TO YOUR ADVANTAGE?

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• Forced flow through the gap will redistribute radicals across the polymer.

• Can this redistribution be used to customize functionalization?

• - 5 kV, 1 atm, He/O2/H2O=89/10/1Inter-electrode gap = 2 mmReactor depth = 1 m


RADICAL DENSITIES FOLLOWING A SINGLE PULSE – 10 slpm

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• - 5 kV, 1 atm, He/O2/H2O=89/10/1, 10 slpm, 0 - 0.01 s

• OH

• O3

• O

• Radicals are produced in 5-10 ns pulse.

• Flow advects radicals downstream; and diffuse upstream.

• Radicals undergo gas phase reactions whoeflowing.

1016 cm-3

1014 cm-3

1014 cm-3

Animation Slide-GIF


RADICAL FLUXES AT POLYMER SURFACE

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• -5 kV, 1 atm, He/O2/H2O=89/10/1, 0.01 s

1013

1015

1017

1016

1018

1020

• 10 slpm

Flux

(cm

-2s-

1)Fl

ux (c

m-2

s-1)

• OH

• O3

Position along Surface

1013

1015

1017

1016

1018

1020

• 1 slpm

• OH

• O3

Position along Surface

Flux

(cm

-2s-

1)Fl

ux (c

m-2

s-1)

Animation Slide-GIF

TIME EVOLUTION OF SURFACE GROUPS ON PP– 10 slpm

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• - 5 kV, 1 atm, He/O2/H2O=89/10/1,0- 0.09 s• 10 slpm

108

1010

1012

Position along the surface


Surf

ace

Cov

erag

e (c

m-2

) • Alkoxy PP-O • Peroxy PP-OO • Alcohol PP-OH

• Alkoxy coverage initially increases as they are formed from alkyl sites and then decreases as they are react to form alcohol groups.

• Peroxy sites monotonically increase as terminal species.

Animation Slide-GIF


SURFACE COVERAGE OF FUNCTIONAL GROUPS

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• - 5 kV, 1 atm, He/O2/H2O=89/10/1, 0.09 s

• 1 slpm • 10 slpm

Position along the surface

109

1011

1013

Surf

ace

Cov

erag

e (c

m-2

)

Surf

ace

Cov

erag

e (c

m-2

)Position along the surface

• Ratio of functional groups can be controlled by transport.

109

1011

1013

PP-OO*

PP-O*

PP-OH

PP-OO*

PP-O*

PP-OH


“LAB ON A CHIP”

• “Lab on a Chip” typically has microfluidicchannels 10s -100s µm wide and reservoirs for testing or processing small amounts of fluid (e.g., blood)

• Internal surfaces of channels and reservoirs must be treated (i.e., functionalized) to control wetting and reactions.

•Desire for mass produced disposable units require cheap process.

•Ref: Calipers Life Sciences, Inc. http://www.caliperls.com

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PLASMA PENETRATION INTO DEEP 50 µm SLOTS: ELECTRONS

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• Slow penetration through dielectric results from surface charging.

• Rapid “restrike”through conductive and precharged slot.

MIN MAX

Animation Slide-GIF• 100 µm • 500 µm • 1000 µm

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

2 mm


• Electron impact ionization in deep slots is augmented by photoionization.

• Fully charging top surface reduces electric penetration.

MIN MAX

Animation Slide-GIF• 100 µm • 500 µm • 1000 µm

PLASMA PENETRATION INTO DEEP 50 µm SLOTS: IONIZATION

• -15 kV, 760 Torr, N2/O2/H2O=79.5/19.5/1


PLASMA PENETRATION IN 10 µm SLOT

• High impedance of small slot slows penetration and limits “restrike”through slot.

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MIN MAX

Animation Slide-GIFS-e e

e

• -15 kV, 760 Torr, N2/O2/H2O=79.5/19.5/1


PLASMA PENETRATIONINTO DEEPER 10 µm SLOT

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• Removal of charge from streamer to charge walls weakens ionization front and stalls streamer.

• Charging of top dielectric shields voltage from penetrating.

MIN MAX

Animation Slide-GIF• 500 µm Thick

• [e] • Ionization

• -15 kV, 760 Torr, N2/O2/H2O=79.5/19.5/1


SHAPES OF SLOTS MATTER: ELECTRONS

• Charging of internal surfaces of slots produce opposing electric fields that limit penetration.

• Restrike fills smaller slot with plasma.

UCLA_0406_62

MIN MAX

Animation Slide-GIF

• 20 and 30 µm slots

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


SHAPES OF SLOTS MATTER: ELECTRONS

• Charging of surfaces and topology of slot determine plasma penetration.

• Here plasma is unable to penetrate through structure.

• Direction of applied electric field and charge induced fields are in the opposite direction of required penetration.

UCLA_0406_63

MIN MAX Animation Slide-GIF

• 20 and 30 µm slots

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


SHOULDN’T LOW PRESSURE BE BETTER?

UCLA_0406_64

• Low pressure discharges with more uniform fluxes, longer mean free paths should be better for functionalization of small features.

• Results from HPEM.

• ICP without bias, He/O2=75/25, 15 mTorr 300 W


ACTIVATION OF SURFACE SITES AND SPUTTERING

UCLA_0406_65

• Large fluxes of O atoms in low pressure systems increase likelihood of alkoxy formation (=C-O)

• Low energy ion activation of surface sites increases rate of reaction direct peroxy (=C-OO formation)

• High energy ions sputter the polymer.

001.01.0

001.0

2 =⋅−=→⋅=+=⋅−=→⋅=+=+⋅=→−=+

pOOCCOpOCCO

pOHCHCO

1.0][][

*2

*

=⋅−=→⋅=+

+⋅=→⋅=++

pOOCCOMCCM

• Strands flex with age. Bottom surfaces may eventually be exposed.

• Top surfaces subject to low energy ion fluxes have activated sites and larger peroxy coverage.

• Results from Monte Carlo Feature Profile Model (MCFPM).

DIRECTIONALITY OF ION FLUXES IS A PROBLEM


• PolypropyleneM. Strobel, 3M

• ICP without bias, He/O2=75/25, 15 mTorr 300 W

FUNCTIONALIZATION:TOP vs BOTTOM OF

STRANDS


• Alkoxy =C-O

• Peroxy =C-OO

• Undersides of strands are mostly alkoxy.

• Topsides, which receive low energy ion activation, are mostly peroxy.

• ICP without bias• He/O2=75/25, 15 mTorr

300 W

UCLA_0406_67

• Even with moderate 35V bias, sputter begins and activation is lessened. Surfaces are almost exclusively alkoxy (=C-O).


• ICP, 35v rf bias, He/O2=75/25, 15 mTorr 300 W

MODERATE BIAS: SPUTTERING, LOW ACTIVATION

• With 85V bias, sputtering is significant and redeposition of sputtered polymer reshapes surface. Functionalized surfaces arealmost exclusively alkoxy (=C-O).


• ICP, 85v rf bias, He/O2=75/25, 15 mTorr 300 W

HIGH BIAS: SPUTTERING, REDEPOSITION


CONCLUDING REMARKS

UCLA_0406_70

• Functionalization of complex surfaces will have challenges at both high and low pressure.

• High Pressure:

• Penetration of plasma into small spaces is problematic.

• Must rely on slower diffusion of neutral radicals.

• 3-body reactions deplete radicals

• Low Pressure:

• Directionality of activation energy, an advantage in microelectronics processing, leads to uneven functionalization.

• Difficult to treat soft materials.

• Developing high pressure processes will result in much reduced cost.

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