FLUORINATION WITH REMOTE INDUCTIVELY COUPLED PLASMAS SUSTAINED IN Ar/F2 AND Ar/NF3 GAS MIXTURES*

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FLUORINATION WITH REMOTE INDUCTIVELY COUPLED
PLASMAS SUSTAINED IN Ar/F2 AND Ar/NF3 GAS
MIXTURES Sang-Heon Songa) and Mark J.
Kushnerb) a)Department of Nuclear Engineering
and Radiological Sciences b)Department of
Electrical Engineering and Computer
Science University of Michigan, Ann Arbor, MI
48109, USA a)ssongs_at_umich.edu,
b)mjkush_at_umich.edu http//uigelz.eecs.umich.edu Ma
rk Strobel and Seth Kirk 3M Company, St. Paul, MN
55144 USA June 2010
Work supported by 3M Company.
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AGENDA

• Fluorination of polymers by plasma treatment
• Description of the reaction mechanism and the
  model
• Typical plasma properties
• Fluorination properties
• Long exposure time
• Short exposure time
• Concluding remarks

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PLASMA SURFACE FLUORINATION OF POLYMERS

• Plasma surface treatment can customize the
  surface energy and adhesion properties of
  polymers by modifying surface resident chemical
  groups.
• Fluorination of polypropylene (PP) decreases the
  surface potential and increases the
  hydrophobicity.

• Hydrophobicity

• In this talk, results from a computational
  investigation of remote, low pressure plasma
  fluorination will be discussed
• F/C ratio and degree of cross-linking
• Limits of fluorination with long and short
  exposure times

http//www.nanotechweb.org
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BASIC FLUORINATION PROCESS

• Structure of polypropylene (PP) polymer

• Sequential reactions
• Abstraction of H by F
• (CH2)(CH)(CH3) Fg ? (CH2)(CH)(CH2)
  HFg
• Followed by passivation
• (CH2)(CH)(CH2) F2g ? (CH2)(CH)(CH2F)
  Fg
• (CH2)(CH)(CH2) Fg ? (CH2)(CH)(CH2F)
• Reactions occur repeatedly until all H is
  substituted by F... However....

Yang et al, Plasma Proc. Polymers 7, 123 (2010).
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CROSS-LINKING

• The free radical sites produced by abstraction of
  H atoms by F atom can be cross-linked.
• Cross-linking consumes C-bonding that might
  otherwise be fluorinated and so potentially
  reduces the F/C ratio from its maximum value of
  2.

Yang et al, Plasma Proc. Polymers 7, 123 (2010).
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REMOTE PLASMA FLUORINATION

• To reduce the likelihood of energetic ion
  modification of the PP, a remote plasma system is
  used.
• An upstream inductively coupled plasma provides
  an effluent of F atoms injected into the
  processing chamber.

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HYBRID PLASMA EQUIPMENT MODEL (HPEM)

• Remote Plasma Source Simulation Inductively
  Coupled Plasma

Fluid-Kinetics Poissons Eq.
Electron Energy Transport Module
Frequency Domain Wave Eq.
Neutral Fluxes

• Fluorination Reactor Simulation Neutrals Only
• Fluid Kinetics Module continuity, momentum,
  energy
• Surface Kinetics Module With surface reaction
  mechanism

Fluxes
Surface Kinetics Module
Fluid-Kinetics
Sticking Coefficients
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ICP AND FLUORINATION REACTOR GEOMETRIES

• Remote Plasma

• Fluorination Reactor

• 2D in cylindrically symmetric for ICP tube.
• 2D in Cartesian coordinates for reactor.
• Ar/F2 60/40, Ar/NF3 60/40
• 600 mTorr, 720 sccm
• 10 MHz, varying coil power from 40 W to 180 W

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REMOTE PLASMA PROPERTIES Ar/F2

• eMax 1.7 x 1012 cm-3
• Telectron 0.2 5 eV
• FMax 4.8 x 1015 cm-3
• Tgas-Max 910 K
• Plasma is concentrated on the wall due to skin
  depth and rapid attachment.
• F atoms are generated by the dissociation of F2
  passes through the plasma zone.
• Some component of thermal dissociation.

• Ar/F2, 600 mTorr, 720 sccm
• 180 W, 10 MHz

Log Scale
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REMOTE PLASMA PROPERTIES Ar/NF3

• eMax 7.7 x 1011 cm-3
• Telectron 0.2 7 eV
• FMax 3.5 x 1015 cm-3
• Tgas-Max 1764 K
• Vibrational excitation and V-T relaxation in NF3
  increases gas temperature more thermal
  dissociation.
• Electron densities are lower as more power goes
  into vibrational excitation.

• Ar/NF3, 600 mTorr, 720 sccm
• 180 W, 10 MHz

MIN
MAX
Log Scale
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FRACTIONAL DISSOCIATION

• Fractional dissociation based on fluxes emerging
  from plasma tube.
• Dissociation scales sub-linearly with power.

• Ar/NF3 60/40

• Ar/F2 60/40

F/(F2F2)
F/(3NF32NF2NF2F2F)

• 600 mTorr, 720 sccm, 10 MHz, 0.1 cm/s

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FLUORINATION REACTOR PROPERTIES

• Effluents from upstream ICP source are injected
  through nozzles in downstream reactor.
• Ar/F2 mixture has more F radicals for a given
  plasma power.
• Factional dissociation in the injected flow

• Ar/F2, 0.4
• Ar/NF3, 0.3

• 600 mTorr, 720 sccm, 180 W, 10 MHz

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FLUORINATIONLONG EXPOSURE TIME
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SATURATION OF F/C RATIO

• Full fluorination of PP

• Relationship between F/C and cross-linking

Hydrogen atoms 6
Cross-linking CL
Carbon atoms 3
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REMOTE PLASMA FLUORINATION POWER

• Move web slowly for long exposure to F atom
  fluxes.
• F/C ratio (2 is full fluorination) saturates at
  about 1.9 at earlier times at higher powers
  though not linearly with power.

• Ar/NF3 60/40

• Ar/F2 60/40

• 600 mTorr, 720 sccm, 10 MHz, 0.1 cm/s

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REMOTE PLASMA FLUORINATION FLOW RATE

• Plasma power to flow rate ratio is constant (0.25
  W/sccm).
• F/C ratio saturates at earlier times at higher
  flow rate.

• Ar/NF3

• Ar/F2

• 600 mTorr, 10 MHz, 0.1 cm/s

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DIRECT PLASMA SOURCE PROPERTIES

• e

• e with a peak value of 2.6 x 1010 cm-3 has a
  maximum near the edge of the powered electrode.
• Te in the bulk plasma is 2.5 eV, peaking near
  powered electrode.
• Fractional dissociation is about 0.2 based on
  reactor averaged F and F2 density.

• Te

• F

• F2

• 700 mTorr, 1500 W, 10 MHz
• Ar/F2 60/40, 600 sccm

Yang et al, Plasma Proc. Polymers 7, 123 (2010).
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DIRECT PLASMA FLUORINATION

• When plasma is in direct contact with polymer,
  ion bombardment sputters surface sites.
• Cross-linked and fluorinated sites are sputtered,
  exposing fresh PP, thereby decreasing F/C.

• CCP, 700 mTorr, 600 sccm, 1500 W, 0.1 cm/s

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REMOTE vs DIRECT FLUORINATION

• Cross-linking consumes carbon-bonding that might
  otherwise be fluorinated.
• With remote plasma F/C is limited only by
  cross-linking.
• In direct plasma treatment, fluorination is
  limited by ion sputtering and cross-linking.
• For direct CCP

• Web speed 0.1 cm/s

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FLUORINATIONSHORT EXPOSURE TIME
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FLUORINATION PROPERTIES POWER

• F/C ratio increases with power, but limited by
  availability of F atoms.
• F/C ratio in Ar/NF3 is lower than in Ar/F2 due to
  lower F atom fluxes.

• Ar/NF3

• Ar/F2

• 600 mTorr, 720 sccm, 10 MHz, 9 cm/s

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FLUORINATION PROPERTIES FLOW RATE

• Fluorination rate increases with flow rate with
  constant power to flow rate ratio.
• Maximum F/C Ar/F2 1.8, Ar/NF3 1.74

• Ar/NF3

• Ar/F2

• 600 mTorr, 10 MHz, 9 cm/s

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CONCLUDING REMARKS

• Fluorination properties of PP are similar for
  remote Ar/F2 and Ar/NF3 plasmas.
• Fluorination properties with long exposure time
• F/C ratio is limited only by cross-linking in
  remote plasma fluorination system.
• In direct plasma fluorination system, F/C ratio
  is limited by the combination of ion sputtering
  and cross-linking.
• Fluorination properties with short exposure time
• F/C ratio is limited by reaction rate.
• Fluorination rate is ultimately a function of
  plasma power and flow rate to increase fluence of
  F atoms.

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BACKUP FOR PROBABILITY

• Sequential reactions
• Abstraction of H by F
• (CH2)(CH)(CH3) Fg ? (CH2)(CH)(CH2)
  HFg
• Probability 5 x 10-5
• Followed by passivation
• (CH2)(CH)(CH2) F2g ? (CH2)(CH)(CH2F)
  Fg
• Probability 0.2 x 10-4
• (CH2)(CH)(CH2) Fg ? (CH2)(CH)(CH2F)
• Probability 1 x 10-4