Transcript With Animations (12.6 MB Powerpoint)

MAKING PLASMAS DO SMALL THINGS:
FUNCTIONALIZING NOOKS-ANDCRANNIES IN POLYMERS AT LOW AND
HIGH PRESSURE
Mark J. Kushner
Iowa State University
Department of Electrical and Computer Engineering
Department of Chemical and Biological Engineering
104 Marston Hall
Ames, IA 50011
[email protected] http://uigelz.ece.iastate.edu
April 2006
UCLA_0406_01
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
 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.
UCLA_0406_03
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).
 X-ray view of the sun, a plasma.
http://www.plasmas.org/basics.htm
UCLA_0406_04
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.
 An energetic free electron collides with an atom, creating a
positive ion and another free electron.
UCLA_0406_05
Iowa State University
Optical and Discharge Physics
TECHNOLOGICAL PLASMAS:
PARTIALLY IONIZED GASES
 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.
 Air plasma: N2, O2, N2+, O2+, O-, e where [e] << [M].
UCLA_0406_06
Iowa State University
Optical and Discharge Physics
TECHNOLOGICAL PLASMAS:
REACTIVE SPECIES
e  Xe  Xe  e
*
Xe*  Xe  h
 Electron impact collisions on
atoms and molecules
produce reactive species.
e  CH 4  CH 3  H  e
 These species emit photons,
modify surfaces and create
new materials.
CH 3  surface  a : CH
(diamond  like  carbon)
UCLA_0406_07
 These plasmas are called
“collisional” because
electrons impart energy to
neutrals by physical impact.
Iowa State University
Optical and Discharge Physics
COLLISIONAL LOW TEMPERATURE PLASMAS
 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.
UCLA_0406_08
Iowa State University
Optical and Discharge Physics
COLLISIONAL LOW
TEMPERATURE PLASMAS
 Thrusters
 Lighting
 Spray Coatings
 Materials
Processing
UCLA_0406_09
 Displays
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
Untreated PP
 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
 Polypropylene (PP)
Plasma Treated PP
He/O2/N2 Plasma
 M. Strobel, 3M
UCLA_0406_11
 Massines J. Phys. D
31, 3411 (1998).
Iowa State University
Optical and Discharge Physics
FUNCTIONALIZATION OF POLYMERS USING PLASMAS
 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.
UCLA_0406_12
Iowa State University
Optical and Discharge Physics
SURFACE MODIFICATION OF POLYMERS
 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.
 Filamentary Plasma 10s – 200 mm
UCLA_0406_13
Iowa State University
Optical and Discharge Physics
COMMERCIAL CORONA PLASMA EQUIPMENT
 Sherman Treaters
 Tantec, Inc.
UCLA_0406_14
Iowa State University
Optical and Discharge Physics
PLASMAS FOR MODIFICATION OF
BIOCOMPATIBLE SURFACES: TISSUE ENGINEERING
 Tissue engineering requires “scaffolding”; substrates with nooks
and crannies 10s -1000s mm in which cells adhere and grow.
 Scaffolding is chemically treated (functionalized) to enhance cell
adhesion or prevent unwanted cells from adhering.
 E. Sachlos, European Cells and
Materials v5, 29 (2003)
http://www.engr.iupui.edu/~tgchu
 Tien-Min Gabriel Chu
UCLA_0406_17
Iowa State University
Optical and Discharge Physics
LOW PRESSURE GLOW DISCHARGES
 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-freepaths 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
UCLA_0406_16
Iowa State University
Optical and Discharge Physics
EXTREMES IN CONDITIONS, VALUES, APPLICATIONS
Web Treatment of Films
 High pressure
 High throughput
 Low precision
 Modify cheap
materials
 Commodity
$0.05/m2
ISU_0105_11
Microelectronics
 Low pressure
 Low throughput
 High precision
 Grow expensive
materials
 High tech
$1000/cm2
Iowa State University
Optical and Discharge Physics
CREATING HIGH VALUE: COMMODITY PROCESSES
 Can commodity processes
be used to fabricate high
value materials?
$0.05/m2
?
$1000/cm2
 Where will, ultimately,
biocompatible polymeric
films fit on this scale?
Artificial skin for $0.05/cm2
or $1000/cm2?
ISU_0105_12
Iowa State University
Optical and Discharge Physics
FUNCTIONALIZING SMALL FEATURES
 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.
UCLA_0406_18
Iowa State University
Optical and Discharge Physics
ELECTROMAGNETICS AND ELECTRON KINETICS
 The wave equation is solved using tensor conductivities:
1

1
  2 E     E  J 
    E      E  

2
t
t
m

m

 Electron energy transport: Continuum and Kinetics
3

5

 ne kTe  / t  S Te   LTe      kTe   Te   Te   S EB
2

2

where S(Te)
L(Te)

(Te)
SEB
=
=
=
=
=
Power deposition from electric fields
Electron power loss due to collisions
Electron flux
Electron thermal conductivity tensor
Power source source from beam electrons

 Kinetic: MCS is used to derive f  , r , t  including e-e collisions using
electromagnetic and electrostatic fields .
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.

 Ni
   ( N i v i )  S i
t


 N i vi  1
qi N i   
 kNiTi     N i vi vi  
E  vi  B    m i
t
mi
mi
mj
 

N i N j vi  v j  ij


j
mi  m j
 N i i 
Nq
   Qi  Pi   U i    ( N i U i i ) 
E2
t
mi (   )
mij
N i qi2 2

Es   3
N i N j Rij k B (T j  Ti )   3 N i N j Rij k BT j
mi i
mi  m j
j
j
2
i i i
2
2
i
 Semi-implicit solution of Poisson’s equation:


 

   t  t   -  s   qi N i - t   q i  i 
i
i


UCLA_0406_20
Iowa State University
Optical and Discharge Physics
HIGH PRESSURE:
CHARGED PARTICLE TRANSPORT ELECTROSTATICS
 Continuity: electron collisions, volume and surface chemistry,
photo-ionization, secondary emission, Sharfetter-Gummel fluxes.
 
N i
     Si
t
 Optically thick photoionization sources (important for streamers)
 
  r  r  3


d r 
 N i (r ) ij N j (r ) exp 
 
 

S Pi (r )  
 2
r
4

r


 Fully implicit solution of Poisson’s equation.


   t   -  s (t )   qi N i (t ) 
i


 Unstructured mesh.
UCLA_0406_21
Iowa State University
Optical and Discharge Physics
HIGH PRESSURE: NEUTRAL PARTICLE TRANSPORT
 Fluid averaged values of mass density, mass momentum and
thermal energy density obtained in using unsteady algorithms.


   ( v )  ( inlets , pumps )
t



 v 
 NkT     v v     m   qi N i Ei
t
i
 
 c pT 

  T  v c pT   Pi   v f   Ri H i   ji  E
t
i
i
 Individual fluid species diffuse in the bulk fluid.

 N i t  t   
   SV  S S
N i t  t   N i t      v f  Di NT 

N
T



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
 PEO - polyethyleneoxide
 pdAA – plasma deposited acrylic acid
 Modification of polymer surfaces for specified functionality can
be used to create cell adhering or cell repulsing regions.
1Andreas
UCLA_0406_24
Ohl, Summer School, Germany (2004).
Iowa State University
Optical and Discharge Physics
FUNCTIONALIZATION FOR BIOCOMPATIBILITY
Micropatterned cell growth on
NH3 plasma treated PEEK1
 Ammonia plasma treatment affixes amine (C-NH2) groups on
surfaces for applications such as cell adhesion, protein
immobilization and tissue engineering.
(1K. Schroeder et al, Plasmas and Polymers, 7, 103, 2002)
UCLA_0406_25
Iowa State University
Optical and Discharge Physics
GAS PHASE CHEMISTRY - He/NH3/H2O MIXTURES
 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_26
Iowa State University
Optical 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_27
Iowa State University
Optical 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.
UCLA_0406_28
Iowa State University
Optical and Discharge Physics
TREATMENT OF POROUS POLYMER BEADS
 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.
 Functionalized Porous Bead for Protein
Binding sites (www.ciphergen.com)
UCLA_0406_29
Iowa State University
Optical and Discharge Physics
DBD TREATMENT OF POROUS POLYMER BEAD
 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 mm
 Pore diameter ~ 2-10 mm
 - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1
 PRF – 10 kHz
UCLA_0406_30
Iowa State University
Optical and Discharge Physics
ELECTRON TEMPERATURE, SOURCE
 Electron Temperature
 Electron Source
 - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1, 0-3.5 ns
UCLA_0406_31
MIN
MAX
Animation Slide-GIF
Iowa State University
Optical and Discharge Physics
ELECTRON DENSITY
 Electron density of 10131014 cm-3 is produced.
 Electron impact
dissociation generates
radicals that functionalize
surfaces.
Animation Slide-GIF
 - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1, 0-3.5 ns
UCLA_0406_32
MIN
MAX
Iowa State University
Optical and Discharge Physics
POST-PULSE RADICAL DENSITIES
 NH
 NH2
 OH
 - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1
UCLA_0406_33
MIN
MAX
Iowa State University
Optical and Discharge Physics
ELECTRON DENSITY IN AND AROUND BEAD
 Corona treating a
porous polymer bead
placed on the lower
50 mm
dielectric.
 Electrons (3.7 x 1013 cm-3)
 In negative corona discharge, electrons lead the avalanche front
and initially penetrate into pores. Charging of surfaces limit
further electron penetration.
Animation Slide-GIF
 - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1, 0-3 ns.
UCLA_0406_34
MIN
MAX
Iowa State University
Optical and Discharge Physics
TOTAL POSITIVE ION DENSITY IN AND AROUND BEAD
 Ions (3.7 x 1013 cm-3)
50 mm
 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.
 - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1, 0-3 ns
UCLA_0406_35
MIN
MAX
Animation Slide-GIF
Iowa State University
Optical and Discharge Physics
3 ns
90 mm Bead
2x1010- 2x1013
3 ns
80 ms
7.5x1012- 8.5x1012
30 mm Bead
80 ms
[NH2] INSIDE PORES
 Since electrons poorly
penetrate into most
pores, little NH2 is
initially produced inside
bead.
 NH2 later diffuses into
pores from outside.
 - 5 kV, He/NH3/H2O=90/10/0.1,
pore dia=4.5 mm, 1 atm
9.1x1012- 9.3x1012
2x1010- 2x1013
MIN
UCLA_0406_36
(log scale)
MAX
Iowa State University
Optical and Discharge Physics
[NH2] INSIDE PORES : PORE DIAMETER
8.5 mm
4.5 mm
3 mm
 t = 3 ns
2x1010- 2x1013
 t = 80 ms
7.5x1012-8.7x1012
 [NH2] within pores increases with pore diameter
during the pulse and in the interpulse period.
UCLA_0406_37
 - 5 kV, He/NH3/H2O=90/10/0.1, bead
dia=90 mm, 1 atm
[NH2] cm- 3
MIN
(log scale)
MAX
Iowa State University
Optical and Discharge Physics
FUNCTIONALIZATION OF POROUS BEAD SURFACES
[ALKYL]
=C
1.25x1010 –
1.25x1011
C
E
A
E
G
F
C
D
I
H
B
K
Letters indicate position
along the surface.
UCLA_0406_38
MIN
D
F
I
H
K
J
 - 5 kV, 1 atm, 10 kHz,
He/NH3/H2O=90/10/0.1,
Bead size=90 mm,
Pore dia= 4.5 mm, t=0.1 s
1012 – 1013
J
A
B
[AMINE]
=C-NH2
G
log scale, cm- 2
MAX
Iowa State University
Optical and Discharge Physics
AMINE SURFACE COVERAGE: SIZE OF BEAD
 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 mm
 - 5 kV, 1 atm, 10 kHz,
He/NH3/H2O=90/10/0.1, t=1 s
UCLA_0406_39
Iowa State University
Optical and Discharge Physics
BEADS IN DISCHARGE: ELECTRON DENSITY
 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.
UCLA_0406_40
 Electron Density
(1.6 x 1014 cm-3), 0-2.5
ns
MIN
MAX
Animation Slide-GIF
Iowa State University
Optical and Discharge Physics
ELECTRON DENSITY
AND SOURCE
 Electron Source (1023 cm-3s-1)
 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
UCLA_0406_41
MIN
1013
cm-3)
MAX
Animation Slide-GIF
Iowa State University
Optical and Discharge Physics
POST-PULSE O and
OH DENSITIES
 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)
UCLA_0406_42
MIN
 [OH] (5 x 1013 cm-3)
MAX
Iowa State University
Optical and Discharge Physics
BEADS IN DISCHARGE: SURFACE COVERAGE
 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
UCLA_0406_43
Iowa State University
Optical and Discharge Physics
DBD TREATMENT OF PP SURFACE WITH MICROSTRUCTURE
 E. Sachlos, et al.
 Corona functionalization of
rough polymer resembling
tissue scaffold.
 1 atm, He/NH3/H2O, 10 kHz
 Polypropylene.
UCLA_0406_44
Iowa State University
Optical and Discharge Physics
PENETRATION INTO SURFACE FEATURES – [e], [IONS]
t = 2.7 ns
t = 4 ns
[e] cm- 3
1010 – 1013
[Positive
ions] cm- 3
1010 – 1013
[Surface (-ve)
Charge] mC
10-1 – 103
 - 5 kV, 1 atm, He/NH3/H2O=98.9/1.0/0.1
MIN
UCLA_0406_45
MAX
log scale
Iowa State University
Optical and Discharge Physics
NH2 DENSITY: EARLY AND LATE
[NH3]=10%
[NH3]=30%
3x1012 - 3x1014 t =3 ns
1.8x1012 – 1.9x1012, t =90 ms
2.25x1012 – 2.35x1012, t =90 ms
 NH2 is initially not produced inside the roughness, but later
diffuses into the interior.
 - 5 kV, 1 atm
UCLA_0406_46
[NH2] cm- 3
MIN
MAX
Iowa State University
Optical and Discharge Physics
SURFACE COVERAGE OF ALKYL RADICALS (=C)
 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.
 - 5 kV, 1 atm, 10 kHz, He/NH3/H2O=90/10/0.1
UCLA_0406_47
Iowa State University
Optical and Discharge Physics
SURFACE COVERAGE OF AMINE GROUPS [=C-NH2]
 Amine groups are created by addition of NH2 to alkyl sites.
NH2 + PP  PP-NH2
 Points with large view angles are highly treated.
 - 5 kV, 1 atm, 10 kHz,
He/NH3/H2O=90/10/0.1, t = 0.1 s
UCLA_0406_48
Iowa State University
Optical and Discharge Physics
OPTIMIZE CHEMISTRY, UNIFORMITY WITH GAS MIXTURE
 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 + O 2 + 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)
UCLA_0406_49
Iowa State University
Optical and Discharge Physics
CONTROLLING FLUX OF OZONE TO SURFACE
 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
UCLA_0406_50
CONTROLLING FLUX OF OZONE TO SURFACE
 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
Iowa State University
Optical and Discharge Physics
UCLA_0406_51
CAN FLOW BE USED TO YOUR ADVANTAGE?
 Forced flow through the gap will
redistribute radicals across the
polymer.
 Can this redistribution be used to
 - 5 kV, 1 atm, He/O2/H2O=89/10/1
Inter-electrode gap = 2 mm
Reactor depth = 1 m
customize functionalization?
UCLA_0406_52
Iowa State University
Optical and Discharge Physics
RADICAL DENSITIES FOLLOWING A SINGLE PULSE – 10 slpm
1014 cm-3
 OH
1016 cm-3
 Radicals are
produced in 5-10 ns
pulse.
 Flow advects radicals
downstream; and
diffuse upstream.
 Radicals undergo gas
phase reactions whoe
flowing.
 O3
1014 cm-3
 - 5 kV, 1 atm,
He/O2/H2O=89/10/1,
10 slpm, 0 - 0.01 s
 O
Animation Slide-GIF
UCLA_0406_53
Iowa State University
Optical and Discharge Physics
RADICAL FLUXES AT POLYMER SURFACE
 10 slpm
 1 slpm
1017
Flux (cm-2 s-1)
 OH
1015
1013
1020
1020
 O3
1018
 O3
1018
1016
1016
Position along Surface
Position along Surface
 -5 kV, 1 atm, He/O2/H2O=89/10/1, 0.01 s
UCLA_0406_54
 OH
1015
1013
Flux (cm-2 s-1)
Flux (cm-2 s-1)
Flux (cm-2 s-1)
1017
Animation Slide-GIF
Iowa State University
Optical and Discharge Physics
Surface Coverage (cm-2)
TIME EVOLUTION OF SURFACE GROUPS ON PP– 10 slpm
 Alkoxy PP-O
 Alcohol PP-OH
 Peroxy PP-OO
1012
1010
108
Position along the surface
 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.
 - 5 kV, 1 atm, He/O2/H2O=89/10/1,0- 0.09 s
 10 slpm
UCLA_0406_55
Animation Slide-GIF
Iowa State University
Optical and Discharge Physics
SURFACE COVERAGE OF FUNCTIONAL GROUPS
 10 slpm
1013
1013
PP-OO*
1011
PP-OH
PP-O*
109
Surface Coverage (cm-2)
Surface Coverage (cm-2)
 1 slpm
Position along the surface
PP-OO*
1011
PP-OH
PP-O*
109
Position along the surface
 Ratio of functional groups can be controlled by transport.
 - 5 kV, 1 atm, He/O2/H2O=89/10/1, 0.09 s
UCLA_0406_56
Iowa State University
Optical and Discharge Physics
“LAB ON A CHIP”
 “Lab on a Chip” typically has microfluidic
channels 10s -100s mm 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 mm SLOTS: ELECTRONS
 Slow penetration
through dielectric
results from
2 mm surface charging.
 Rapid “restrike”
through
conductive and
precharged slot.
 100 mm
 500 mm
 1000 mm
 -15 kV, 1 atm,
N2/O2/H2O=79.5/19.5/1
UCLA_0406_58
MIN
MAX
Animation Slide-GIF
Iowa State University
Optical and Discharge Physics
PLASMA PENETRATION INTO DEEP 50 mm SLOTS: IONIZATION
 Electron impact
ionization in deep
slots is
augmented by
photoionization.
 Fully charging top
surface reduces
electric
penetration.
 100 mm
 500 mm
 1000 mm
 -15 kV, 760 Torr,
N2/O2/H2O=79.5/19.5/1
UCLA_0406_59
MIN
MAX
Animation Slide-GIF
Iowa State University
Optical and Discharge Physics
PLASMA PENETRATION IN 10 mm SLOT
e
S-e
e
Animation Slide-GIF
 High impedance of small slot slows penetration and limits “restrike”
through slot.
MIN
MAX
UCLA_0406_60
 -15 kV, 760 Torr,
N2/O2/H2O=79.5/19.5/1
Iowa State University
Optical and Discharge Physics
PLASMA PENETRATION
INTO DEEPER 10 mm SLOT
 Removal of charge from
streamer to charge walls
weakens ionization front
and stalls streamer.
 Charging of top dielectric
shields voltage from
penetrating.
 [e]
 Ionization
 500 mm Thick
Animation Slide-GIF
 -15 kV, 760 Torr,
N2/O2/H2O=79.5/19.5/1
UCLA_0406_61
MIN
MAX
Iowa State University
Optical and Discharge Physics
SHAPES OF SLOTS MATTER: ELECTRONS
 Charging of internal
surfaces of slots
produce opposing
electric fields that
limit penetration.
 Restrike fills smaller
slot with plasma.
 20 and 30 mm slots
Animation Slide-GIF
 -15 kV, 1 atm,
N2/O2/H2O=79.5/19.5/1
UCLA_0406_62
MIN
MAX
Iowa State University
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.
 20 and 30 mm slots
 -15 kV, 1 atm,
N2/O2/H2O=79.5/19.5/1
UCLA_0406_63
Animation Slide-GIF
MIN
MAX
Iowa State University
Optical and Discharge Physics
SHOULDN’T LOW PRESSURE BE BETTER?
 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
UCLA_0406_64
Iowa State University
Optical and Discharge Physics
ACTIVATION OF SURFACE SITES AND SPUTTERING
 Large fluxes of O atoms in low pressure systems increase
likelihood of alkoxy formation (=C-O)
O   C  H   C  OH
O  C
  C O
O2   C 
  C  OO 
p  0.001
p  0.1
p  0.001
 Low energy ion activation of surface sites increases rate of
reaction direct peroxy (=C-OO formation)
M    C   [ C]*  M
O2  [ C]*   C  OO  p  0.1
 High energy ions sputter the polymer.
UCLA_0406_65
Iowa State University
Optical and Discharge Physics
DIRECTIONALITY OF ION FLUXES IS A PROBLEM
 Polypropylene
M. Strobel, 3M
 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).
 ICP without bias, He/O2=75/25, 15 mTorr 300 W
UCLA_0406_66
Iowa State University
Optical and Discharge Physics
FUNCTIONALIZATION:
TOP vs BOTTOM OF
STRANDS
 Alkoxy =C-O
 Undersides of strands
are mostly alkoxy.
 Topsides, which
receive low energy ion
activation, are mostly
peroxy.
 Peroxy =C-OO
UCLA_0406_67
 ICP without bias
 He/O2=75/25, 15 mTorr
300 W
Iowa State University
Optical and Discharge Physics
MODERATE BIAS: SPUTTERING, LOW ACTIVATION
 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
UCLA_0406_68
Iowa State University
Optical and Discharge Physics
HIGH BIAS: SPUTTERING, REDEPOSITION
 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
UCLA_0406_69
Iowa State University
Optical and Discharge Physics
CONCLUDING REMARKS
 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.
UCLA_0406_70
Iowa State University
Optical and Discharge Physics