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 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
Iowa State University
Optical and Discharge Physics
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.
Iowa State University
Optical and Discharge Physics
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.
Iowa State University
Optical and Discharge Physics
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.
Iowa State University
Optical and Discharge Physics
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.
Iowa State University
Optical and Discharge Physics
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
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COLLISIONAL LOW TEMPERATURE PLASMAS
UCLA_0406_08
Displays
Materials Processing
COLLISIONAL LOW TEMPERATURE PLASMAS
Lighting
Thrusters
Spray Coatings
UCLA_0406_09
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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.
Iowa State UniversityOptical and Discharge Physics
SURFACE MODIFICATION OF POLYMERS
UCLA_0406_13
Filamentary Plasma 10s – 200 m
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Optical and Discharge Physics
COMMERCIAL CORONA PLASMA EQUIPMENT
Tantec, Inc.
Sherman Treaters
UCLA_0406_14
Iowa State University
Optical and Discharge Physics
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
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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?
Iowa State UniversityOptical and Discharge Physics
CREATING HIGH VALUE: COMMODITY PROCESSES
$0.05/m2 $1000/cm2?
ISU_0105_12
Iowa State University
Optical and Discharge Physics
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.
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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
Optical and Discharge Physics
HIGH PRESSURE: CHARGED PARTICLE TRANSPORT ELECTROSTATICS
iiis tNqt-t )()(
UCLA_0406_21
ii St
N
2
3
4
exp)()(
)(rr
rdrr
rNrN
rSjiji
Pi
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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
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Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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
Iowa State UniversityOptical and Discharge Physics
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
Iowa State UniversityOptical and Discharge Physics
SURFACE REACTION MECHANISM
Gas phase NH2 and NH radicals react with surface alkyl sites creating amine (R-NH2) groups and imine (R-NH) sites.
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Optical and Discharge Physics
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.
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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
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Optical and Discharge Physics
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
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Optical and Discharge Physics
POST-PULSE RADICAL DENSITIES
UCLA_0406_33
- 5 kV, 1 atm, He/NH3/H2O=90/10/0.1
NH NH2 OH
MIN MAX
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Optical and Discharge Physics
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
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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
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Optical and Discharge Physics
[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
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Optical and Discharge Physics
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.
Iowa State University
Optical and Discharge Physics
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
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Optical and Discharge Physics
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
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Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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
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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
Iowa State University
Optical and Discharge Physics
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
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Optical and Discharge Physics
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.
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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.
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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)
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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.
Iowa State University
Optical and Discharge Physics
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
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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
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Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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
Iowa State UniversityOptical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
“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
Iowa State University
Optical and Discharge Physics
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
Iowa State University
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
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Optical and Discharge Physics
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
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Optical and Discharge Physics
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
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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
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Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
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
Iowa State UniversityOptical and Discharge PhysicsUCLA_0406_66
PolypropyleneM. Strobel, 3M
ICP without bias, He/O2=75/25, 15 mTorr 300 W
FUNCTIONALIZATION:TOP vs BOTTOM OF
STRANDS
Iowa State UniversityOptical and Discharge Physics
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).
Iowa State UniversityOptical and Discharge PhysicsUCLA_0406_68
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).
Iowa State UniversityOptical and Discharge PhysicsUCLA_0406_69
ICP, 85v rf bias, He/O2=75/25, 15 mTorr 300 W
HIGH BIAS: SPUTTERING, REDEPOSITION
Iowa State University
Optical and Discharge Physics
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.