making plasmas do small things:

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

mailto:[email protected]

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

Iowa State University

Optical 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.

UCLA_0406_02



AGENDA

UCLA_0406_03

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).

UCLA_0406_04

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.

UCLA_0406_05

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].

UCLA_0406_06

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.

UCLA_0406_07

)(

:3

34

*

*

carbonlikediamond

CHasurfaceCH

eHCHCHe

hXeXe

eXeXee

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

Iowa State UniversityOptical and Discharge Physics

COLLISIONAL LOW TEMPERATURE PLASMAS

UCLA_0406_08

Displays

Materials Processing

COLLISIONAL LOW TEMPERATURE PLASMAS

Lighting

Thrusters

Spray Coatings

UCLA_0406_09



MULTISCALE MODELING OF PLASMAS AND PLASMA-SURFACE INTERACTIONS

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

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.

UCLA_0406_10



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).

UCLA_0406_11



FUNCTIONALIZATION OF POLYMERS USING PLASMAS

UCLA_0406_12

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

UCLA_0406_13

Filamentary Plasma 10s – 200 m



COMMERCIAL CORONA PLASMA EQUIPMENT

Tantec, Inc.

Sherman Treaters

UCLA_0406_14



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

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

UCLA_0406_17

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

Tien-Min Gabriel Chu

http://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

UCLA_0406_16

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?

ISU_0105_12



FUNCTIONALIZING SMALL FEATURES

UCLA_0406_18

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

JE

t

EEE

1

1

2

2

Electron energy transport: Continuum and Kinetics

where S(Te) = Power deposition from electric fieldsL(Te) = Electron power loss due to collisions = Electron flux(Te) = Electron thermal conductivity tensor

SEB = Power source source from beam electrons

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

EBeeeeeee STTkTTLTStkTn

2

5/

2

3

trf ,,

UCLA_0406_19



LOW PRESSURE: PLASMA CHEMISTRY, TRANSPORT ELECTROSTATICS

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

Semi-implicit solution of Poisson’s equation:

iiqt-- i

iiis Nqtt

iiii SNt

N )v(

iii

iiiiiii

i

ii BvEm

NqvvNTkN

mt

vN

1

ijjijj

imm

j vvNNm

ji

222

2

)()U(UQ E

m

qNNP

t

N

ii

iiiiiiiii

ii

j

jBijjij

ijBijjiji

ijs

ii

ii TkRNNTTkRNNmm

mE

m

qN3)(32

2

UCLA_0406_20

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


HIGH PRESSURE: CHARGED PARTICLE TRANSPORT ELECTROSTATICS

iiis tNqt-t )()(

UCLA_0406_21

ii St

N

2

3

4

exp)()(

)(rr

rdrr

rNrN

rSjiji

Pi



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

)pumps,inlets()v(t

i

iii ENqvvNkTt

v

i i

iiifipp EjHRvPTcvTt

Tc

SV

T

iTifii SS

N

ttNNDvtNttN

Individual fluid species diffuse in the bulk fluid.

HIGH PRESSURE: NEUTRAL PARTICLE TRANSPORT

UCLA_0406_22



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.

UCLA_0406_23



CELL MICROPATTERNING: MODIFICATION OF POLYMERS

UCLA_0406_24

Modification of polymer surfaces for specified functionality can be 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

UCLA_0406_25

(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

UCLA_0406_26

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.

UCLA_0406_27


SURFACE REACTION MECHANISM

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

UCLA_0406_28


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

UCLA_0406_29

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

UCLA_0406_30

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 m Pore diameter ~ 2-10 m

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

PRF – 10 kHz



ELECTRON TEMPERATURE, SOURCE

UCLA_0406_31

- 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

UCLA_0406_32

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

UCLA_0406_33

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

NH NH2 OH

MIN MAX



ELECTRON DENSITY IN AND AROUND BEAD

UCLA_0406_34

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 front and initially penetrate into pores. Charging of surfaces limit further electron penetration.

Electrons (3.7 x 1013 cm-3)50 m

Animation Slide-GIF

MIN MAX


Optical 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 m

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 m, 1 atm

90 m Bead

2x1010- 2x1013

MIN MAX

3 ns

3 ns 80 s

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 m Bead 80 s

2x1010- 2x1013

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

Iowa State UniversityOptical and Discharge PhysicsUCLA_0406_37

[NH2] INSIDE PORES : PORE DIAMETER

8.5 m 4.5 m 3 m

(log scale)MIN MAX

[NH2] cm- 3

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

t = 3 ns 2x1010- 2x1013

t = 80 s 7.5x1012-8.7x1012



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 m, Pore dia= 4.5 m, t=0.1 s

A B

C D

EF

G

HI

J

K

AB

C

DE

FG

H

I

J

K

Letters indicate position along the surface.



AMINE SURFACE COVERAGE: SIZE OF BEAD

UCLA_0406_39

- 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 m



BEADS IN DISCHARGE: ELECTRON DENSITY

UCLA_0406_40

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

UCLA_0406_41

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

UCLA_0406_42

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

UCLA_0406_43

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

UCLA_0406_44

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] C

10-1 – 103

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

log scale



UCLA_0406_46

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

- 5 kV, 1 atm

3x1012 - 3x1014

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

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)

UCLA_0406_47

- 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]

UCLA_0406_48

- 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

UCLA_0406_49

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 chemistry

PP + 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

UCLA_0406_50

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

UCLA_0406_51

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?

UCLA_0406_52

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=89/10/1 Inter-electrode gap = 2 mm Reactor depth = 1 m



RADICAL DENSITIES FOLLOWING A SINGLE PULSE – 10 slpm

UCLA_0406_53

- 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 whoe flowing.

1016 cm-3

1014 cm-3

1014 cm-3

Animation Slide-GIF



RADICAL FLUXES AT POLYMER SURFACE

UCLA_0406_54

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

1013

1015

1017

1016

1018

1020

10 slpm

Flu

x (c

m-2 s

-1)

Flu

x (c

m-2 s

-1)

OH

O3

Position along Surface

1013

1015

1017

1016

1018

1020

1 slpm

OH

O3

Position along Surface

Flu

x (c

m-2 s

-1)

Flu

x (c

m-2 s

-1)

Animation Slide-GIF

TIME EVOLUTION OF SURFACE GROUPS ON PP– 10 slpm

UCLA_0406_55

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

10 slpm

108

1010

1012

Position along the surface


Su

rfac

e C

ove

rag

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

UCLA_0406_56

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

1 slpm 10 slpm

Position along the surface

109

1011

1013

Su

rfac

e C

ove

rag

e (c

m-2

)

Su

rfac

e C

ove

rag

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 microfluidic channels 10s -100s m 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

UCLA_0406_57



PLASMA PENETRATION INTO DEEP 50 m SLOTS: ELECTRONS

UCLA_0406_58

Slow penetration through dielectric results from surface charging.

Rapid “restrike” through conductive and precharged slot.

MIN MAX

Animation Slide-GIF 100 m 500 m 1000 m

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

2 mm


Optical and Discharge PhysicsUCLA_0406_59

Electron impact ionization in deep slots is augmented by photoionization.

Fully charging top surface reduces electric penetration.

MIN MAX

Animation Slide-GIF 100 m 500 m 1000 m

PLASMA PENETRATION INTO DEEP 50 m SLOTS: IONIZATION

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



PLASMA PENETRATION IN 10 m SLOT

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

UCLA_0406_60

MIN MAX

Animation Slide-GIFS-e e

e

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



PLASMA PENETRATIONINTO DEEPER 10 m SLOT

UCLA_0406_61

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 m 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 m 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 m 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.

001.0

1.0

001.0

2

pOOCCO

pOCCO

pOHCHCO

1.0][

][*

2

*

pOOCCO

MCCM

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 are almost 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.

making plasmas do small things:

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