Transcript Document 7188824

PLASMA MATERIALS PROCESSING:
CREATING HIGH VALUE
Mark J. Kushner
Iowa State University
104 Marston Hall
Ames, IA 50011
[email protected]
http://uigelz.ece.iastate.edu
January 2005
ISU_0105_01
ACKNOWLEDGEMENTS





Dr. Alex V. Vasenkov (now at CFD Research Corp.)
Dr. Gottlieb Oherlein (U of Maryland)
Dr. Arvind Sankaran (now at Novellus Systems)
Dr. Pramod Subramonium (now at Novellus Systems)
Dr. Rajesh Dorai (now at Varian Semiconductor Equipment)
 Mr. Ananth Bhoj
 Mr. Ramesh Arakoni
 Funding Agencies:





ISU_0105_02
3M Corporation
Semiconductor Research Corporation
National Science Foundation
SEMATECH
CFDRC Inc.
Iowa State University
Optical and Discharge Physics
AGENDA
 Introduction to Plasmas
 Extremes in Physics and Applications
 Plasmas for functionalization of polymers ($0.05/m2)
 Polymers for selectivity in plasma etching ($1000/cm2)
 Challenges for adapting commodity processes for high value
materials.
 Concluding Remarks
ISU_0105_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
ISU_0105_04
Iowa State University
Optical and Discharge Physics
PARTIALLY IONIZED PLASMAS
 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.
ISU_0105_05
Iowa State University
Optical and Discharge Physics
PARTIALLY IONIZED PLASMAS
 The resulting partially
ionized gas is not neutral
on a microscopic scale,
but is neutral on a global
scale.
 An air plasma: N2, O2, N2+,
O2+, O-, e where [e] <<
Neutrals
 A plasma that ionizes itself at the same rate electrons and ions
neutralize is “self sustaining”.
ISU_0105_06
Iowa State University
Optical and Discharge Physics
TECHNOLOGICAL PLASMAS
 Technological plasmas have electron temperatures of a few eV and
electron densities of 1010-1014 cm-3. The gas usually remains cool.
ISU_0105_07
Iowa State University
Optical and Discharge Physics
ELECTRONS AS AN ENERGY TRANSFER MEDIUM
 Electrons transfer power from the "wall plug" to internal modes of
atoms / molecules; dissociating and exciting them, very much like
combustion.
 These activated species can them be used to “make a product.
ISU_0105_08
Iowa State University
Optical and Discharge Physics
COLLISIONAL LOW
TEMPERATURE PLASMAS
 Thrusters
 Lighting
 Spray Coatings
 Materials
Processing
ISU_0105_09
 Displays
PLASMAS FOR MODIFICATION OF SURFACES
 Plasmas are ideal for producing reactive species (radicals, ions) for
modifying surface properties.
 Two of the most technologically (and commercially) important uses
of plasmas add or remove molecules from surfaces to selectively
achieve desired functionality (mechanical or chemical):
 Functionalization of
polymers (high pressure)
 Etching for
microelectronics
fabrication (low pressure)
.
ISU_0105_10
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
LOW COST, COMMODITY
FUNCTIONALIZATION OF POLYMERS
ISU_0105_13
SURFACE ENERGY AND
FUNCTIONALITY OF POLYMERS
 Most polymers, having low surface energy, are hydrophobic.
POLYMER
ISU_0105_14
0
Waterbased inks
10
UV inks
20
Coatings
30
Waterbased adhesives
40
UV adhesives
Polystyrene
Polyethylene
PTFE
20
Polypropylene
30
50
Printing inks
40
0
60
-1
50
10
SURFACE TENSION (mN m )
60
-1
SURFACE ENERGY (mN m )
 For good adhesion and wettability, the surface energy of the
polymer should exceed of the overlayer by 2-10 mN m-1.
LIQUID
Iowa State University
Optical and Discharge Physics
PLASMA SURFACE MODIFICATION OF POLYMERS
Untreated PP
 To improve wetting and adhesion of
polymers atmospheric plasmas are
used to generate gas-phase radicals
to functionalize their surfaces.
 Polypropylene (PP)
Plasma Treated PP
He/O2/N2 Plasma
 Massines et al. J. Phys. D 31,
3411 (1998).
 M. Strobel, 3M
ISU_0105_15
Iowa State University
Optical and Discharge Physics
POLYMER TREATMENT APPARATUS

TYPICAL PROCESS CONDITIONS:
Web speed
Residence time
Energy deposition
Applied voltage
Gas gap
: 10 - 200 m/min
: a few s
: 0.1 - 1.0 J cm-2
: 10-20 kV at a few 10s kHz
: a few mm
FEED ROLL
GROUNDED
ELECTRODE
PLASMA
POWERED
SHOE
ELECTRODE
 Filamentary Plasma
10s – 200 mm
ISU_0105_17
COLLECTOR
ROLL
~
HIGH-VOLTAGE
POWER SUPPLY
Iowa State University
Optical and Discharge Physics
COMMERCIAL CORONA PLASMA EQUIPMENT
 Sherman Treaters
 Tantec, Inc.
ISU_0105_18
Iowa State University
Optical and Discharge Physics
FUNCTIONALIZATION OF THE PP SURFACE
 Untreated PP is hydrophobic.
 Increases in surface energy by plasma treatment are attributed
to the functionalization of the surface with hydrophilic groups.
 Carbonyl (-C=O)
 Alcohols (C-OH)
 Peroxy (-C-O-O)
 Acids ((OH)C=O)
 The degree of functionalization depends on process
parameters such as gas mix, energy deposition and relative
humidity (RH).
ISU_0105_19
Iowa State University
Optical and Discharge Physics
PLASMA PRODUCED WETTABILITY
 Increases in wettability with plasma treatment result from
formation of surface hydrophilic groups such as C-O-O
(peroxy), C=O (carbonyl).
Hydrophobic
Hydrophilic
 Polyethylene, Humid-air
 Akishev, Plasmas Polym. 7, 261 (2002).
ISU_0105_20
 Polypropylene, Air corona
• Boyd, Macromol., 30, 5429 (1997).
Iowa State University
Optical and Discharge Physics
REACTION PATHWAY
e
e
e
HUMID-AIR PLASMA
e
H
e
N2
N
O2
H2O
O
O O2
OH
BOUNDARY LAYER
OH, O
OH
C
C
C
O2
NO
O2
OH
H
C
N
NO
O3
OH, H2O
O
e
C
O2
O
||
C
C
C
C
LAYER 1
C
C
C
C
LAYER 2
C
LAYER 3
H
POLYPROPYLENE
C
C
C
C
C
C
OH
C
ISU_0105_21
C
C
C
C
C
C
C
C
C
Iowa State University
Optical and Discharge Physics
GLOBAL_KIN AND SURFACE KINETICS
 Reaction mechanisms in pulsed atmospheric air plasma
treatment of polymers have been investigated with global
kinetics and surface models.
 GLOBAL_KIN
 2-Zone homogeneous plasma
BULK PLASMA
chemistry (bulk plasma,
CO2
boundary layer)
O
OH
 Plug flow
 Multilayer surface site BOUNDARY  DIFFUSION REGIME
LAYER
balance model
 Circuit module
POLYPROPYLENE
 Boltzmann derived f()
ISU_0105_22
~ mfp
Iowa State University
Optical and Discharge Physics
REACTION MECHANISM FOR HUMID-AIR PLASMA
OH
HNO2
HNO3
OH
OH
NO2
O3, HO2
O
 Initiating radicals are O, N,
OH, H
OH
N
NO
N2
O2 ,
NO3
OH
N2(A)
N
N2O5
O2
HO2
e
N2
OH
O2 e
O2
O
O2(1)
O3
H2O
e
H
O2
H2O2
ISU_0105_23
HO2
HO2
OH
 Gas phase products include
O3, N2O, N2O5, HNO2, HNO3.
Iowa State University
Optical and Discharge Physics
POLYPROPYLENE (PP) POLYMER STRUCTURE
CH3 H
C
H
2
1
CH3 H CH3 H
C
C
C
H
H
H
3
1 - Primary C
C
C
2 - Secondary C
H
H
3 - Tertiary C
 Three types of carbon atoms in a PP chain:
 Primary –
bonded to 1 C atom
 Secondary – bonded to 2 C atoms
 Tertiary –
bonded to 3 C atoms
 The reactivity of an H-atom depends on the type of C bonding.
Reactivity scales as:
HTERTIARY > HSECONDARY > HPRIMARY
ISU_0105_24
Iowa State University
Optical and Discharge Physics
PP SURFACE REACTION MECHANISM: INITIATION
 The surface reaction mechanism has initiation,
propagation and termination reactions.
 INITIATION: O and OH abstract H from PP to produce alkyl
radicals; and gas phase OH and H2O.
(POLYPROPYLENE)
H
~CH2
C
CH2~
(ALKYL RADICAL)
O(g)
OH(g)

~CH2
CH3
C
CH2~
CH3
OH(g), H2O(g)
ISU_0105_25
Iowa State University
Optical and Discharge Physics
PP SURFACE REACTION MECHANISM: PROPAGATION
(ALKOXY RADICAL)
(ALKYL RADICAL)
O(g), O3 (g)

~CH2
C
CH2~
CH3
O
C
~CH2
C
CH2~
CH3
O2 (g)
~CH2
O
O
 PROPAGATION: Abundant O2 reacts
with alkyl groups to produce “stable”
peroxy radicals. O3 and O react to
form unstable alkoxy radicals.
CH2~
CH3
(PEROXY RADICAL)
ISU_0105_26
Iowa State University
Optical and Discharge Physics
PP SURFACE REACTIONS: PROPAGATION / AGING
 PROPAGATION / AGING: Peroxy
radicals abstract H from the PP chain,
resulting in hydroperoxide, processes
which take seconds to 10s minutes.
ISU_0105_27
Iowa State University
Optical and Discharge Physics
PP SURFACE REACTION MECHANISM: TERMINATION
 TERMINATION: Alkoxy radicals react with the PP
backbone to produce alcohols and carbonyls. Further
reactions with O eventually erodes the film.
(ALKOXY RADICAL)
(ALCOHOLS)
O
OH
~CH2
C
CH2~
H(s)
~CH2
CH3

CH2~ + ~CH 2
C
C
CH2~
CH3
CH3 O(g)

CO2 (g)
O
(CARBONYL)
ISU_0105_28
Iowa State University
Optical and Discharge Physics
BASE CASE: ne, Te
 Ionization is dominantly of N2 and O2,
 e + N2  N2+ + e + e,
 e + O2  O2+ + e + e.
 After a few ns current pulse, electrons
decay by attachment (primarily to O2).
 Dynamics plasma-dielectric interaction
enable more efficient ionization on
later pulses.
 N2/O2/H2O = 79/20/1, 300 K
 15 kV, 9.6 kHz, 0.8 J-cm-2
 Web speed = 250 cm/s (460 pulses)
ISU_0105_29
Iowa State University
Optical and Discharge Physics
GAS-PHASE RADICALS: O, OH
 Electron impact dissociation of O2 and H2O produces O and
OH. O is consumed primarily to form O3; OH is consumed by
both bulk and surface processes.
 After 100s of pulses, radicals attain a periodic steady state.
 N
ISU_0105_30
 O
 OH
Iowa State University
Optical and Discharge Physics
PP SURFACE GROUPS vs ENERGY DEPOSITION
 Surface concentrations of alcohols, peroxy radicals are
near steady state with a few J-cm-2.
 Alcohol densities decrease at higher J-cm-2 energy due to
decomposition by O and OH to regenerate alkoxy radicals.
 Air, 300 K, 1 atm, 30% RH
 Ref: L-A. Ohare et al.,
Surf. Interface Anal. 33, 335 (2002).
ISU_0105_31
Iowa State University
Optical and Discharge Physics
HUMIDITY: PP FUNCTIONALIZATION BY OH
 Increasing RH produces OH which react with PP to form alkyl
radicals, which are rapidly converted to peroxy radicals by O2.
PP-H + OH(g)  PP + H2O(g)
PP + O2(g)  PP-O2
 Alcohol and carbonyl densities decrease due to increased
consumption by OH to form alkoxy radicals and acids.
PP-OH+ OH(g)PP-O + H2O(g) , PP=O + OH(g)  (OH)PP=O
ISU_0105_32
Iowa State University
Optical and Discharge Physics
COMMODITY TO HIGH VALUE
 As the material value increases (cents to dollars /cm2?) higher
process refinement is justified to customize functionalization.
 Control of O to O3 ratio using He/O2 mixtures can be used to
customize surface functionalization.
 1 atm, He/O2, 15 kV, 3 mm, 9.6 kHz, 920 pulses.
ISU_0105_33
Iowa State University
Optical and Discharge Physics
COMMODITY TO HIGH VALUE
 Additional “tuning” of functionalization can be achieved with
sub-mTorr control of water content.
 Small water addition
“tuning” of functionalization
can be achieved with submTorr control of water
content.
 H and OH reduce O3 while
promoting acid formation.
 1 atm, He/O2/ H2O, 15 kV, 3 mm,
9.6 kHz, 920 pulses.
ISU_0105_34
Iowa State University
Optical and Discharge Physics
HIGH COST, UTILIZATION OF
POLYMERS IN MICROELECTRONICS
FABRICATION
ISU_0105_35
PLASMAS IN MICROELECTRONICS FABRICATION
 Plasmas play a dual role in microelectronics fabrication.
 First, electron impact on otherwise unreactive gases produces
neutral radicals and ions.
 These species then drift or diffuse to surfaces where they add,
remove or modify materials.
ISU_0105_36
Iowa State University
Optical and Discharge Physics
PLASMAS IN MICROELECTRONICS FABRICATION
 Second, ions deliver directed activation energy to surfaces
fabricating fine having extreme and reproducible tolerances.
 0.25 mm Feature
(C. Cui, AMAT)
ISU_0105_37
Iowa State University
Optical and Discharge Physics
APPLIED MATERIALS DECOUPLED PLASMA SOURCES (DPS)
ISU_0105_38
Iowa State University
Optical and Discharge Physics
rf BIASED INDUCTIVELY
COUPLED PLASMAS
 Inductively Coupled Plasmas (ICPs)
with rf biasing are used here.
 < 10s mTorr, 10s MHz, 100s W – kW,
electron densities of 1011-1012 cm-3.
ISU_0105_39
Iowa State University
Optical and Discharge Physics
SELECTIVITY IN MICROELECTRONICS FABRICATION
 Fabricating complex microelectronic structures made of
different materials requires extreme selectivity in, for example,
etching Si with respect to SiO2.
50 nm
 Ref: G. Timp
 AMD 90 nm Athlon 64
 Complex features are fabricated by selectively removing one
material but not another with near monolayer resolution.
ISU_0105_40
Iowa State University
Optical and Discharge Physics
FLUOROCARBON PLASMA ETCHING: SELECTIVITY
 Selectivity in fluorocarbon etching relies on polymer deposition
from dissociation of feedstock gases.
e + Ar/C4F8
CFn, M+
CFx
CFn, M+
SiFn
COFn, SiFn
Polymer
SiO2
CFn, M+
CFx
Polymer
Si
 Compound dielectrics contain oxidants which consume the
polymer, producing thinner polymer layers.
 Thicker polymer on non-dielectrics restrict delivery of ion energy
(lower etching rates).
Iowa State University
ISU_010541
 G. Oerhlein, et al., JVSTA 17, 26 (1999)
Optical and Discharge Physics
MODELING OF FLUOROCARBON PLASMA ETCHING
 Our research group has developed an integrated reactor and
feature scale modeling hierarchy to model plasma processing
systems.
 HPEM (Hybrid Plasma
Equipment Model)
 Reactor scale
 2- and 3-dimensional
 ICP, CCP, MERIE, ECR
 Surface chemistry
 First principles
ISU_0105_42
 MCFPM (Monte Carlo
Feature Profile Model)
 Feature scale
 2- and 3-dimensional
 Fluxes from HPEM
 First principles
Iowa State University
Optical and Discharge Physics
ELECTROMAGNETICS AND ELECTRON KINETICS
 The wave equation is solved in the frequency domain using tensor
conductivities and sparse matrix techniques:
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 .
ISU_0105_43
Iowa State University
Optical and Discharge Physics
PLASMA CHEMISTRY, TRANSPORT AND ELECTROSTATICS
 Continuity, momentum and energy equations are solved for each species
(with jump conditions at boundaries).

 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
 Implicit solution of Poisson’s equation:


 

   t  t   -  s   qi N i - t   q i  i 
i
i


ISU_0105_44
Iowa State University
Optical and Discharge Physics
ION/NEUTRAL ENERGY/ANGULAR DISTRIBUTIONS
 MC methods are used to
obtain energy and
angular distributions of
particles striking
surfaces.
 Ar/C4F8, 40 mTorr, 10b MHz, MERIE
ISU_0105_45
Iowa State University
Optical and Discharge Physics
MONTE CARLO FEATURE PROFILE MODEL (MCFPM)
 The MCFPM predicts profiles using energy
and angularly resolved neutral and ion 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.
ISU_0105_46
Iowa State University
Optical and Discharge Physics
GAS PHASE REACTION MECHANISM: Ar/c-C4F8/O2/CO
 To achieve selectivity, gas mixtures are complex: Ar/c-C4F8/O2/CO.
e  C4 F8  radicals, ions
Refs:
 G. I. Font, J. Appl. Phys 91, 3530 (2002).
 C. Q. Jiao, Chem. Phys. Lett. 297, 121 (1998).
ISU_0105_48
e  C2 F4  radicals, ions
Iowa State University
Optical and Discharge Physics
SURFACE KINETICS: FLUOROCARBON PLASMA ETCHING Si/SiO2
 CxFy passivation regulates delivery of precursors and activation energy.
 Chemisorption of CFx produces a complex at the oxide-polymer interface.
 2-step ion activated (through polymer layer) etching of the complex
consumes the polymer. Activation scales inversely with polymer thickness.
 Etch precursors and products diffuse through the polymer layer.
F
Plasma
I+
CxFy
CFn
CF4
CxFy
Passivation
Layer
CFx
SiFn ,
CO2
I+, F
I+, F
CO2
 In Si etching, CFx
is not consumed,
resulting in
thicker polymer
layers.
CFn
CFn
SiO2
ISU_0105_54
SiO2
SiO2
SiFxCO2
SiFn
SiFx
Iowa State University
Optical and Discharge Physics
PLASMA PROPERTIES: ICPs IN Ar/c-C4F8/CO/O2
 In mixtures typically used
for dielectric etch c-C4F8 has
a low mole fraction.
 Ions are dominated by Ar+
having temperatures near 1
eV in presheaths.
 Te has large gradients due to
collisional nature of plasma.
 Ar/c-C4F8/CO/O2 = 60/5/25/10,
10 mTorr, 600 W, 13.56 MHz,
20 sccm.
ISU_0105_50
Iowa State University
Optical and Discharge Physics
PLASMA PROPERTIES: ICPs IN Ar/c-C4F8/CO/O2
 M- are dominated by F- due to
charge exchange with CFn- .
 Densities of CmFn+ are
commensurate with CFn+.
 Ratios critically depend on
power, wall reactions and
charge exchange with Ar+.
 Ar/c-C4F8/CO/O2 = 60/5/25/10,
10 mTorr, 600 W, 13.56 MHz,
20 sccm.
ISU_0105_51
Iowa State University
Optical and Discharge Physics
MIXTURES: Ar/c-C4F8, O2/c-C4F8
 Flux of ions to surface can be
tuned by choice of additives.
 Addition of Ar to C4F8 produces
more ions that are on average less
chemically reactive.
 Addition of O2 adds oxidizing ions
without changing fluorocarbon
reactivity.
 Te decreases with Ar and O2 as
ionization is more efficient.
 10 mTorr, 600 W, 13.56 MHz, 40 sccm.
ISU_0105_52
Iowa State University
Optical and Discharge Physics
TEL-DRM Ar / C4F8 / O2
IEADs FOR 2000 W
 Complex gas mixtures produce a
large variety of ion energy and
angular distributions.
 Ar/C4F8/O2 = 200/10/5 sccm,
40 mTorr, 2000 W, 100 G
ISU_0105_53
Iowa State University
Optical and Discharge Physics
ETCH RATES: C2F6, C4F8, CHF3
 Due to small differences in the composition of the flux of
radicals and polymer, and ion energy distributions, etching
characteristics differ with feedstock gases.
 Experiments: Schaepkens et al J. Vac. Sci.
Technol. A 17, 26 (1999): Oehrlein et al private
communications
ISU_0105_55
Iowa State University
Optical and Discharge Physics
POLYMERIZATION PRODUCES SELECTIVITY
 Highly optimized processes produce nearly infinite selectivity.
The underlying Si does not consume the polymer, and so
etches slower.
 10 mTorr, C2F6.
ISU_0105_56
Iowa State University
Optical and Discharge Physics
Etch Rate (nm/min)
500
C4F8/Ar and C4F8/O2
400 SiO2 - M
 Larger ionization rates result in larger
ion fluxes in Ar/C4F8 mixtures which
reduces polymer thickness.
SiO2 - E
300
200
100
C4F8/Ar
0
0
20
40
60
80
100
Ar Content (%)
Etch Rate (nm/min)
300
 O2 etches polymer and reduces its
thickness. Rate has a maximum with
O2, similar to Ar addition.
SiO2 - M
SiO2 - E
200
 With high Ar, the polymer thins to
sub-monolayer (less deposition, more
sputtering). Etch rates decrease.
100
 40 sccm, 600 W ICP, 20
mTorr, -125 V self-bias
C4F8/O2
0
0
20
40
60
80
100
O2 Content (%)
ISU_0105_57
Li et al, J. Vac. Sci. Technol. A 20, 2052,
2002.
Iowa State University
Optical and Discharge Physics
POROUS Si FOR LOW-K DIELECTRIC
 As feature sizes decrease and device count increases, the
diameter of interconnect wires shrinks and path length increases.
 Larger RC-delay limits performance.
 Low-dielectric constant
materials reduce RC.
 Porous SiO2 (xerogels)
have low-k properties due
to their lower mass density
resulting from (vacuum)
pores.
 Porosities: 30-70%
 Pore sizes: 2-20 nm
 Ref: S. Rossnagel, IBM
ISU_0105_58
Iowa State University
Optical and Discharge Physics
PROCESSING OF NANOSTRUCTURED MATERIALS
 The “opening” of pores during etching of porous SiO2 results in
the filling of the voids with polymer, creating thicker layers.
 Ions which would have otherwise hit at grazing or normal angle
now intersect with more optimum angle.
 An important parameter is
L/a (polymer thickness / pore
radius).
 Adapted: Standaert, JVSTA 18, 2742 (2000)
ISU_0105_59
Iowa State University
Optical and Discharge Physics
PORE FILLING BY POLYMER
 Polymer deposition is activated by
low energy ions.
 Polymer removal is activated by
high energy ions.
 Low energy fluxes penetrate into
pores activating polymer deposition
 Pore filling locally increases depth
of polymer.
 Etching proceeds through a series
of “break throughs”, pore-filling,
and polymer removal.
 15 nm, 60% porosity
 ICP CHF3, 6 mTorr, 1400 W
ISU_0105_60
Iowa State University
Optical and Discharge Physics
EFFECT OF PORE RADIUS ON HAR TRENCHES
4 nm
10 nm
16 nm
 With increase in radius, thicker polymer layers are produced in
the pores causing a decrease in etch rates.
ISU_0105_62
Iowa State University
Optical and Discharge Physics
HAR PROFILES: INTERCONNECTED PORES
0%
60%
Interconnectivity
100%
 Higher porosities, larger pores and higher interconnectivity, filling
of pores produces thicker polymer layers and lower etch rates.
ISU_0105_64
Iowa State University
Optical and Discharge Physics
EFFECT OF PORE RADIUS ON CLEANING
 Larger pores have poor view
angles to ions and thicker
polymer layers.
 Lower rate of cleaning results.
ISU_0105_65
ANIMATION SLIDE
4 nm
16 nm
 Ar/O2=99/1, 40 sccm,
600 W, 4 mTorr
Iowa State University
Optical and Discharge Physics
THE CHALLENGE: COMMODITY
PROCESSING FOR HIGH VALUE
MATERIALS
ISU_0105_66
THE ROLE OF PLASMAS IN BIOSCIENCE
 Plasmas, to date, have played
important but limited roles in
bioscience.
 Plasma sterilization
 Plasma source ion
implantation for hardening
hip and knee replacements.
 Modification of surfaces for
biocompatibility (in vitro and
in vivo)
 Artificial skin
 The potential for commodity use of
plasmas for biocompatibility is
untapped.
ISU_0105_67
 Low pressure rf H2O2 plasma
(www.sterrad.com)
Iowa State University
Optical and Discharge Physics
“HIGH VALUE” PROCESSING - CELL MICROPATTERNING
 PEO - polyethyleneoxide
 pdAA – plasma deposited acrylic acid
 Low pressure “microelectronics-like” plasmas are used to pattern
selective substrate regions with functional groups for cell adhesion.
 These processes have costs commensurate with microelectronics:
high value, high cost.
1Andreas
ISU_0105_68
Ohl, Summer School, Germany (2004).
Iowa State University
Optical and Discharge Physics
ATMOSPHERIC PRESSURE PLASMAS: THE CHALLENGE
 10,000 square miles of polymer sheets are functionalized
annually with atmospheric pressure plasmas. Cost: < $0.05 /m2
 Low pressure plasma processing technologies produce
biocompatible polymers having similar functionalities. Cost: up
to $100’s /cm2.
 Microelectronic plasma processing produces functionality at the
nanoscale.
 Can these knowledge bases be combined?
 Can commodity, atmospheric pressure processing technology
be leveraged to produce high value biocompatible films at low
cost? The impact on health care would be large.
$0.05/m2
ISU_0105_69
?
$1000/cm2
Iowa State University
Optical and Discharge Physics
DESCRIPTION OF nonPDPSIM:
CHARGED PARTICLE, SOURCES
 Continuity (sources from electron and heavy particle collisions,
surface chemistry, photo-ionization, secondary emission), fluxes
by modified Sharfetter-Gummel with advective flow field.
 
N i
     Si
t
 Poisson’s Equation for Electric Potential:
     V   S
 Electron energy equation:

 ne   
5
 
 j  E  ne  Ni i       Te , j  qe
t
2

i
 Photoionization, electric field and secondary emission:
 
  r  r  3


d r 
 N i (r ) ij N j (r ) exp 
 
 

S Pi (r )  
 2
r
4

r

Iowa State University

ISU_0105_71
Optical and Discharge Physics
DESCRIPTION OF nonPDPSIM:
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



ISU_0105_72
Iowa State University
Optical and Discharge Physics
CAN COMMODITY PROCESSES
PRODUCE HIGH VALUE MATERIALS?
 Tantec, Inc.
 van Veldhuizen et al
ISU_0105_73
 2 mm gap, 15 kV pulse, N2/O2/H2O
=79.5 / 19.5 / 1, 1 atm
Iowa State University
Optical and Discharge Physics
POTENTIAL, ELECTRIC FIELD, CHARGE
Animation Slide
 Pulse is initiated with electron emission from tip of cathode.
 Development of plasma streamer deforms potential producing
large electric field. Pulse is terminated with dielectric charging.
 Potential
ISU_0105_74
 E/N
 N2/O2/H2O =79.5 / 19.5 / 1, 1 atm,
15 kV, 0-15 ns
 Charge
MIN
MAX
ELECTRON TEMPERATURE, SOURCES
Animation Slide
 Electric field at head of streamer elevates electron temperature,
producing a transitory wave of ionization. 2-body attachment
occurs in high Te regions; 3-body attachment in low Te.
 Te
 N2/O2/H2O =79.5 / 19.5 / 1, 1 atm,
15 kV, 0-15 ns
MIN
ISU_0105_75
 Net
Ionization
MAX
 Net
Attachment
Iowa State University
Optical and Discharge Physics
ELECTRON AND ION DENSITIES
Animation Slide
 Electrons are consumed by 3-body attachment at the end of the
pulse.
 [e]
 N2/O2/H2O =79.5 / 19.5 / 1, 1 atm,
15 kV, 0-15 ns
MIN
ISU_0105_76
 [M+]
MAX
 [M-]
Iowa State University
Optical and Discharge Physics
POST PULSE RADICAL DENSITIES
 Radical and ion densities at end of pulse are as high as 10s ppm.
Temperature rise is nominal due to short pulse duration.
 N2(A)
 O
 N2/O2/H2O =79.5 / 19.5 / 1, 1 atm,
15 kV, 0-15 ns
MIN
ISU_0105_77
 O2(1)
MAX
 H, OH
Iowa State University
Optical and Discharge Physics
SURFACE INTERACTIONS: ELECTRON DENSITY
2x109- 2x1011
2x1010- 2x1012
 Electrons penetrate surface
features on the polymer to a limited
extent due to surface charging.
 -15 kV, 760 Torr,
N2/O2/H2O=79.5/19.5/1
[e] cm-3
1.35 ns
1.45 ns
1.40 ns
1.5 ns
MIN (log scale) MAX
1.65 ns
2x1011- 2x1013
ISU_0105_79
10 mm
1x1011- 5x1013
Iowa State University
Optical and Discharge Physics
SURFACE INTERACTIONS: [O] DENSITY
1x109- 1x1012
5x1010- 5x1013
 Radicals striking the surface
penetrate into the features by
diffusion.
 Unlike ions, the transport of
radicals into features is unimpeded
by surface charging
1.4 ns
1.65 ns
 -15 kV, 760 Torr,
1.5 ns
4.0 ns
N2/O2/H2O=79.5/19.5/1
7.0 ns
[O] cm-3
10 mm
ISU_0105_80
MIN (log scale) MAX
1x1011- 1x1014
Iowa State University
Optical and Discharge Physics
FUNCTIONAL GROUP DENSITIES ON POLYPROPYLENE
ISU_0105_81
 1 atm, N2/O2/H2O=79.5/19.5/1,
1.5 ms, 10 kHz.
Iowa State University
Optical and Discharge Physics
FUNCTIONALIZATION OF SCAFFOLDING
 Functionalization of scaffolding-
like surfaces for cell adhesion.
 Can uniformity of functionalization
be maintained over microscopic
and macroscopic scale lengths.
 Use 1 atm, He/O2/H2O mixtures to
optimize.
ISU_0105_82
Iowa State University
Optical and Discharge Physics
FUNCTIONALIZING PP SCAFFOLDING: HIGH O2 (He/O2/H2O = 69/30/1)
 High O2 produces O3 and rapid
alkoxy formation.
 Reactivity of O3 limits transport and
produces long- and short-scale
nonuniformities.
 1 atm, He/O2/H2O = 69/30/1
ISU_0105_83
Iowa State University
Optical and Discharge Physics
FUNCTIONALIZING PP SCAFFOLDING: LOW O2 (He/O2/H2O = 89/10/1)
 Lower O2 produces less O3 and
limits alkoxy formation.
 Overall uniformity becomes
reaction limited, producing
smoother functionalization.
 1 atm, He/O2/H2O = 89/10/1
ISU_0105_84
Iowa State University
Optical and Discharge Physics
CONCLUDING REMARKS
 Plasmas materials processing spans a vast range in “value
added,” from extremely high value microelectronics to
commodity polymer functionalization.
 Development of nanostructured and bio-compatible materials
using plasmas and their broad implementation are largely
limited by cost.
 The social benefit to reducing cost of these plasma processes
would be large .
 The key to creating high value using commodity processing is
leveraging knowledge bases developed in different contexts.
That is, working at the interfaces between fields.
 Rapid improvement in these knowledge bases will enable high
values materials (plastic microelectronics?) to be produced
using commodity processes during this decade.
ISU_0105_85
Iowa State University
Optical and Discharge Physics