Review of dust particle formation, charging, and transport

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					Review of dust particle formation,
     charging, and transport




           John Goree
         The Univ. of Iowa
       Outline

1.   Particle detection
2.   Particle formation
3.   Particle charging
4.   Particle transport
                                 Outline

1. Particle detection                3. Particle charging
       •   video imaging                    •   e & ion collection
       •   electron microscopy              •   electron emission
       •   other                     4. Particle transport
2. Particle formation                       •   Forces:
       •   flaking                               •    Coulomb
       •   gas-phase                             •    ion drag
                                                 •    radiation pressure
                                                 •    gas drag
                                                 •    thermophoretic
                                                 •    gravity
       Particle detection




1. Particle detection
       •   video imaging
       •   electron microscopy
       •   other
                                     Particle detection
   Particles were always                                  Images from Gary Selwyn:
   there, but you didn’t know                             The original discovery that RF
                                                          plasmas (etching/deposition)
   it until you looked for them                            particle growth in the gas
   the right way                                          phase, and particle levitation




                                                                          electron
                                                                          microscopy
                   Laser light scattering


G.S. Selwyn, Plasma Sources Sci. Tehcnol. 3, 340 (1994)
                      Particle detection
Other detection methods,
used in semiconductor
mfg:

In the reactor exhaust:
     – Coulter counter
     – Measure plasma
        impedance (dust makes
        a glow discharge
        impedance more
        resistive, less reactive)   Coulter counter:
                                    particle detection and sizing
           Formation



2. Particle formation
       •   flaking
       •   gas-phase
                                   Formation: flaking
Flakes:
Thin films
 • deposited on
   surfaces by
   sputtering or
   evaporation
 • films
   subsequently
   crumble




    J. Winter, Phys. Plasmas 7, 3862 (2000)
                                Formation: tubules
Tubules:
Reported in
tokamak T-10,

Growth mechanism?




B.N. Kolbasov et al.,
Phys. Letta A 269, 363 (2000)
                                  Formation: gas phase
Gas-phase formation in
 astrophysics:

• Vapor flowing outward
  from a carbon star cools
  & nucleates, resulting in
  dust.
• Dust grains then grow by
  “coagulation”




M16 pillar, Credit: NASA, HST, J. Hester & P. Scowen (ASU)
                                  Formation: gas phase
Gas-phase
 formation




 G. Praburam and J. Goree
 Cosmic dust synthesis by accretion and coagulation
 Astrophys. J 1995
                          Formation: gas phase
Cauliflower particles grow in the gas phase:




intact                               fractured




 Gary Selwyn, IBM, 1989               Ganguly et al., J. Vac. Sci. Technol. 1993
                            Formation: gas phase
Explanation
proposed for
cauliflower shape:

The origin of the
bumpy shape has
been attributed to
columnar growth.

If true, column size will
depend on
temperature



                                          J.A. Thornton, J. Vac. Sci. Technol. A 11, 666 (1974).


                            columnar growth, for thin films on a planar surface, using sputter
                            deposition
                                     Formation: gas phase
Gas-phase formation resulting from sputtering:

• Targets were sputtered by Ar+ in a glow discharge
• Particles grew in the gas phase
 vacuum vessel
 inside wall
                                                    gas inlet

                           ground                               upper elec trode
  laser sheet               shield
                                          powered
  = 488 nm                              electrode



                                          grounded
                                          electrode

                                                                lower electrode
                       grounded plate



                4 cm
                                        turbopump
 D. Samsonov and J. Goree
 Particle growth in a sputtering discharge
 J. Vac. Sci. Technol. A 1999
                                    Formation: gas phase
Gas-phase
                                                                     400                                                      10 10
 formation
 resulting from
 sputtering:



                                       dust particle diameter (nm)
                                                                     300                                                      10 9




                                                                                                                                     dust number density ( -3 )
                                                                                                                                                       cm
                                                                     200                                                      10 8



                                                                     100                                                      10 7



                                                                       0                                                      10 6
                                                                           0       50    100      150       200   250   300
                                                                                               time (sec)


                                                                               Growth of carbon particles, from sputtering
                                                                                graphite in an rf discharge


 D. Samsonov and J. Goree
 Particle growth in a sputtering discharge
 J. Vac. Sci. Technol. A 1999
                                   Formation: gas phase

   Particles grown
   by sputtering tungsten




D. Samsonov and J. Goree
Particle growth in a sputtering discharge
J. Vac. Sci. Technol. A 1999
                                   Formation: gas phase

   Particles grown
   by sputtering graphite




D. Samsonov and J. Goree
Particle growth in a sputtering discharge
J. Vac. Sci. Technol. A 1999
                                   Formation: gas phase

   Particles grown
   by sputtering titanium

   Spherical-shaped primary
   particles that have coagulated
   into aggregates consisting of a
   few spheres.

   The surface of the
   particles appears smoother
   than that of the graphite.




D. Samsonov and J. Goree
Particle growth in a sputtering discharge
J. Vac. Sci. Technol. A 1999
                                   Formation: gas phase

   Particles grown
   by sputtering stainless steel




D. Samsonov and J. Goree
Particle growth in a sputtering discharge
J. Vac. Sci. Technol. A 1999
                                   Formation: gas phase

   Particles grown
   by sputtering aluminum




D. Samsonov and J. Goree
Particle growth in a sputtering discharge
J. Vac. Sci. Technol. A 1999
                                   Formation: gas phase
      Particles grown
      by sputtering copper




D. Samsonov and J. Goree
Particle growth in a sputtering discharge
J. Vac. Sci. Technol. A 1999
                                   Formation: gas phase

Particles grown
by sputtering

Growth rate varies
tremendously,
depending on the
material




D. Samsonov and J. Goree
Particle growth in a sputtering discharge
J. Vac. Sci. Technol. A 1999
           Charging



3. Particle charging
       •   e & ion collection
       •   electron emission
                               Charging: mechanisms

  Ielectron collection                      Ielectron collection
  + Iion collection                         + Iion collection
                                            + Ielectron emission
                            H+                                     H+
              e-                                           e-


                                  _
                                                           e-            +
   Charging by collecting                  Electron emission
   electrons and ions only                    • secondary emission due to e- impact
   Q<0                                       • photoemission
                                              • thermionic
                                            Q>0
Goree, Plasma Sources Sci. Technol. 1994
                                Charging: mechanisms
Particles immersed in a plasma are in charge equilibrium:

Itotal = Ielectron collection + Iion collection + Ielectron emission


Each of these currents depends on the potential V of the particle

Equilibrium: at a “floating potential” V, the currents balance: Itotal = 0

Q = CV

C = 4 p e0 a is capacitance of sphere of radius a

Example: Q = 695 e for a = 1 mm particle with a surface potential V = 1 Volt



 Goree, Plasma Sources Sci. Technol. 1994
                               Charging: mechanisms
Charging by collecting electrons & ions             Ielectron collection
only
                                                    + Iion collection

Consider a particle that is initially uncharged &
is suddenly immersed in a plasma:                                          H+
                                                                e-
 • Initially it collects electrons more rapidly
   than ions, due to higher vte
 • Eventually it reaches equilibrium “floating                                  _
   potential”
 • Popular model: OML (Orbital motion limited
   currents), yields a value for Q accurate to a
   factor of ~2.
 • V = -2.5 kTe (Hydrogen, Te = Te , for an
   isolated particle)



Goree, Plasma Sources Sci. Technol. 1994
                         Charging: OML predictions
                                                     10 4
OML charge




                              - <N> = <Q> / e
                                                            (a)

 • Charge  Te
                                                     10 3
 • Charge  a


                              mean ch arge nu mber   10 2

                                                                                                        Ti / Te ion
                                                                                                           1    Ar+
                                                     10 1                                                  1    H+
                                                                                                         0.05   Ar+
                                                                                                         0.05   H+

                                                     10 0
                                                        1                10                       100                 1000
                                                                              grain radius (nm)

                                                                                         KQ = -1737
                                                             Q / e  K Q amm TeV
                                                                                         for hydrogen, Te = Ti
Goree, Plasma Sources Sci. Technol. 1994
                         Charging: electron depletion
                                                                                         10




                            potential (normalized by electron temperature)
                                                                                     -

Electron
depletion

When the density
N of negatively-                                                                         -   1
charged dust is
high:

 • Dust potential                                                                                 plasma potential   floating potential
   is reduced
                                                                                 -   0.1                                     of particle
 • Dust charge is
   reduced
 • Plasma
   potential is                                                              -   0.01
   altered                                                                          0.01         0.1         1            10               100   1000
                                                                                                  normalized particle number density P

                                                                                                  P  695 TeV amm N cm3 / ncm3
Goree, Plasma Sources Sci. Technol. 1994
                       Charging: secondary emission
Secondary electron
emission (electron impact)

For mono-energetic electrons:

Yield                                                           ( E)  7.4  m ( E / Em ) exp(2 E / Em )
                                                          1.2

Graphite in bulk:                                          1
 m = 1
 Em = 400 eV                               yield  / m   0.8

                                                          0.6
For small particles, yield is bigger
than for bulk, because of bigger
                                                          0.4
solid angles for secondary
electrons to escape particle
                                                          0.2

                                                           0
                                                                0       2        4            6    8          10
Goree, Plasma Sources Sci. Technol. 1994                                             E / Em
                        Charging : secondary emission
Secondary electron emission (electron
                                                  Ielectron collection
impact)
                                                  + Iion collection
For Maxwellian electrons:                         + Ielectron emission

Meyer-Vernet* provides formulae for electron
                                                                         H+
current, result:                                                 e-

 • Polarity of particle’s charge switches from
   negative to positive
 • Occurs for Te in range 1 – 10 eV, depending
   on m                                                         e-           +
Other electron emission processes:
 • photoemission due to UV (very common in
   space)
 • thermionic emission (uncommon?)

*Meyer-Vernet, Astron. Astrophys. 105,98 (1982)
                             Charging: charging time

 Charging time

 A particle’s charge:
  • Can change at a finite rate, as plasma conditions change
  • Fluctuates randomly as individual electrons & ions are collected

 Characteristic time scale is called “charging time, can be defined as:
  • charge / current of one of the two incident species“floating potential V


                   TeV                     Kt = -1510 sec
         t  Kt                            for hydrogen, Te = Ti
                amm ncm3

 Typically t 1 msec for a 1 micron grain in a glow discharge

Goree, Plasma Sources Sci. Technol. 1994
                    Charging: stochastic fluctuations
                                                0
Charge
fluctuations                                          charging time t

                                                -5
Stochastic, due to
                                                                                         discrete

                             charge number N
collection of                                           continuous
individual
electrons and ions                             -10
at random times

Q  0.5 (Q/e)1/2
                                               -15




                                               -20
                                                  0        0.5             1.0     1.5          2.0   2.5   3.0
                                                                                 t (msec)



Goree, Plasma Sources Sci. Technol. 1994
           Transport


4. Particle transport
       •   Forces:
            •    Coulomb
            •    ion drag
            •    radiation pressure
            •    gas drag
            •    thermophoretic
            •    gravity
                         Transport: forces

Forces acting on a particle

  Coulomb          QE           a

  Lorentz          Q v B       a      tiny except in astronomy

  Ion drag                      a2   big for high-density plasmas

  Radiation pressure            a2   if a laser beam hits particle

  Gas drag                       a2    requires gas

  Thermophoretic force           a2    requires gas

  Gravity                        a3    tiny unless a > 0.1 mm
               Transport: ion drag

       Momentum is imparted to the dust particle




         _                                      _

Orbit force:                           Collection force:
Ion orbit is deflected                 Ion strikes particle
                    Transport: ion drag

Voids in particle
suspensions are
experimental evidence of
the ion drag force

                                Glow (image of Ar I spectral line)

 RF parallel-plate discharge,
 imaged from the side, zero-g
 conditions (NASA’s KC-135
 airplane)



                                Particles (image of scattered laser light
                                       Transport: ion drag
Gas-phase
 formation
 resulting from                                                Dust (laser light
                                                               scattering from a
 graphite
                                                               horizontal laser
 sputtering:
                                                               sheet)

“filamentary-mode”
   instability, driven by
                                                               Glow
   ion drag, for nm
   size particles




D. Samsonov and J. Goree
Instabilities in a Dusty Plasma with Ion Drag and Ionization
Physical Review E Vol. 59, 1047-1058, 1999
                                       Transport: ion drag
Gas-phase
 formation
 resulting from                                                Dust (laser light
                                                               scattering from a
 graphite
                                                               horizontal laser
 sputtering:
                                                               sheet)

“great void”
  instability, driven by
                                                               Glow
  ion drag, for nm
  size particles




D. Samsonov and J. Goree
Instabilities in a Dusty Plasma with Ion Drag and Ionization
Physical Review E Vol. 59, 1047-1058, 1999
                                         Transport: ion drag

       Ion drag force



                                                   dusty          void
                                                   plasma
                                                                      ionization
                                                                 & pos. plasma potl.
                                                                      outward
                                                                      ion flow




J. Goree, G. E. Morfill, V. N. Tsytovich and S. V. Vladimirov,
Theory of Dust Voids in Plasmas,
Physical Review E Vol. 59, 7055-7067, 1999
                                          Transport: ion drag

        Ion drag force
 Two contributions:                                                                                                               Collection force




                                                          ion drag force, normalized
                                                                                                                                from OML model
       • Orbit force (this is the                                                                     V
         usual drag force for                                                          10                                                V2
         Coulomb collisions,                                                                                 Orbit force
         except that lnL is                                                                           from Rosenbluth potential
         problematic)
                                                                                                                         V-2
       • Collection force (ions
         actually strike the
         particle)
                                                                                        1
                                                                                        0.01          0.1           1             10           100
 Depends on ion velocity ui
                                                                                                       ion velocity / ion thermal velocity
 Force  ni
                                                                                               Te / Ti = 60, mi = 40 amu, D = 130 mm
                                                                                               Ion drag is normalized by 4 p ni a2 Te / (Ti/Te)0.5

J. Goree, G. E. Morfill, V. N. Tsytovich and S. V. Vladimirov,
Theory of Dust Voids in Plasmas,
Physical Review E Vol. 59, 7055-7067, 1999

E. C. Whipple, Rep. Prog. Phys. 44, 1198 (1981)
                                       Transport: ion drag

     Ion drag force                                                          1000


• Fusion edge plasma




                                                ion drag force, normalized
  parameters:                                                                100



• Te = Ti, deuterium mass                                                      10



                                                                                1



                                                                              0.1
                                                                                0.01          0.1             1                 10
                                                                                          ion velocity / ion thermal velocity

                                                                                    Te / Ti = 1, mi = 2 amu, D = 13 mm
                                                                                    Ion drag is normalized by 4 p ni a2 Te / (Ti/Te)0.5

  Data computed March 2005 using the same code as in
  J. Goree, G. E. Morfill, V. N. Tsytovich and S. V. Vladimirov,
  Theory of Dust Voids in Plasmas,
  Physical Review E Vol. 59, 7055-7067, 1999
                         Transport: ion drag


• Gettering particles:
    – Design your ion flow so that it pushes particles where you want them to go
                         Transport: radiation pressure

Radiation pressure



                            pa I laser
                                  2                                            transparent   incident
   Fradiation  q                                                           pa I
                                                                              2microsphere   laser
                                   c                      F
                                                    momentum imparted
                                                           laser       q        laser
                                                    to microsphere             c

 Radiation pressure force  a2


       q               dimensionless
                       (q = 1 if all photons are absorbed)
       Ilaser          laser intensity
       c               speed of light

 B. Liu, V. Nosenko, J. Goree and L. Boufendi, Phys. Plasmas (2003).
                Transport: radiation pressure

Next:

Experimental demonstration of the force due to radiation pressure
       Transport: radiation pressure

                                             VCR
                 video camera
                     (top view)


scanning                                                     scanning
  mirrors        microspheres                                  mirrors
                                         lower electrode


                       y
                           x



    mod ulator                                         modulator 1
                  RF              video camera
Ar laser                             (side view)              Ar laser
beam 2                                                        beam 1


     two Ar+ laser beams:
        • 0.61 mm width
        • rastered into vertical sheets
Transport: radiation pressure
         Ar+ laser pushes particles
          undisturbed monolayer




   Transport: radiation pressure




  low high power: melting the lattice
 medium power: plastic deformation, flow
      power: slow deformation, rotation
                 Transport: radiation pressure


• Gettering particles:
    – Not very practical to use radiation pressure force; it requires intense cw laser light

    – High-power pulsed lasers (~ 1J YAG lasers) have been used to explode dust
      particles suspended in a plasma. This isn’t the radiation pressure force, but it
      works. Maybe not very practical.
                                    Transport: gas drag

Gas drag

  Gas drag force                             a2
                                                         4p
  Epstein expression:                      Fgas           N gas mgas cgas a 2V
                                                          3


        •   Ngas         gas atom: number density
        •   mgas                      mass
        •   cgas                      mean thermal speed
        •   V            velocity of particle with respect to the gas
        •               dimensionless, ranges from 1.0 to 1.442


  P. Epstein, Phys. Rev. 23, 710 (1924).

  M. J. Baines, I. P. Williams, and A. S. Asebiomo, Mon. Not. R. Astron. Soc. 130, 63 (1965).
                                       Transport: gas drag

       Demonstration of gas drag
                                                                        camera
                                                                      (top view)
•     Track the motion of a single                                      -
      microsphere in a glow
      discharge
                                                                                        shaker
•     Particle is levitated by a
      sheath electric field

•     Sheath is curved  harmonic                   Sheath                             Ar+ laser sheet
      confining potential                            edge
                                                    Copper
                                                             -55 V
•     Apply a laser beam, and then                  ring
      turn the laser
                                                                     Lower electrode
•     Observe damped harmonic
      motion




    B. Liu, V. Nosenko, J. Goree and L. Boufendi,
    Phys. Plasmas (2003).
                                       Transport: gas drag

       Gas drag demonstration:

•     Track the motion of a single
      microsphere in a glow                                               6
                                                                                   laser on          laser off
      discharge
•     Particle is levitated by a                                          4




                                                    velocity x ( mm s )
                                                    -1
      sheath electric field
                                                                          2
•     Sheath is curved  harmonic
      confining potential                                                 0
•     Apply a laser beam to push
      the particle away from the                                          -2
      bottom of the harmonic well,
      and then turn the laser off                                         -4
•     Observe damped harmonic
      motion                                                              -6
                                                                               0          1    2          3      4
                                                                                              time (s)
    B. Liu, V. Nosenko, J. Goree and L. Boufendi,
    Phys. Plasmas (2003).
                        Transport: gas drag
•   Gettering particles:

     – It is practical to use gas flow to
       push particles where you want
       them to go.
     – This is a method of cleaning
       plasma-processing discharges
       before turning the RF power off.
     – Method requires gas & gas
       flow.
                              Transport: thermophoresis
      Thermophoresis
The force that pushes soot in a lantern
 toward a cool glass envelope
Due to a temperature gradient in neutral
 gas

Force  a2 ngas a
Particles are pushed toward cold
 surfaces




G.M. Jellum, J.E. Daugherty, and D. B. Graves
Particle Thermophoresis in low pressure glow discharges
J. Appl. Phys. 69 6923 (1991)
                   Transport: thermophoresis


• Gettering particles:
    – Use an LN2 trap or a Peltier element to cool a surface.
    – This method requires gas.
                    Transport: gravity
To demonstrate the role of gravity, we must eliminate it:
Parabolic flights, NASA KC-135
                   Transport: gravity
Parabolic flights, NASA KC-135
     Transport: gravity



Parabolic flights, NASA KC-135



            video
                         Transport: gravity
• Gettering particles:
    – Gutters are used in
      semiconductor plasma
      processing for semiconductors
    – “Downspout” leads to the
      vacuum pump
    – Gravity is effective when it is
      big compared to other forces
      (especially ion drag, if ne is big)
    – Not effective for particles < 100
      nm
                                 Outline

1. Particle detection                3. Particle charging
       •   imaging                          •   e & ion collection
       •   electron microscopy              •   electron emission
       •   other                     4. Particle transport
2. Particle formation                       •   Forces:
       •   flaking                               •    Coulomb
       •   gas-phase                             •    ion drag
                                                 •    radiation pressure
                                                 •    gas drag
                                                 •    thermophoretic
                                                 •    gravity
                                 Formation: gas phase
Gas-phase
 formation
 resulting from
 graphite
 sputtering:

• Graphite targets
  were sputtered by
  Ar+ in a glow
  discharge
• Particles grew in
  the gas phase
• Particles (white)
  are imaged here
  resting on the
  graphite lower
  electrode
 G. Praburam and J. Goree
 Cosmic Dust Synthesis by Accretion and Coagulation
 Astrophysical Journal Vol. 441, pp. 830-838, 1995
                                 Formation: gas phase
Gas-phase
 formation
 resulting from
 graphite
 sputtering:

• Graphite targets
  were sputtered by
  Ar+ in a glow
  discharge
• Particles grew in
  the gas phase
• Particles (white)
  are imaged here
  resting on the
  graphite lower
  electrode
 G. Praburam and J. Goree
 Cosmic Dust Synthesis by Accretion and Coagulation
 Astrophysical Journal Vol. 441, pp. 830-838, 1995
                                 Formation: gas phase
Gas-phase
 formation
 resulting from
 graphite
 sputtering:

• Graphite targets
  were sputtered by
  Ar+ in a glow
  discharge
• Particles grew in
  the gas phase
• Particles (white)
  are imaged here
  resting on the
  graphite lower
  electrode
 G. Praburam and J. Goree
 Cosmic Dust Synthesis by Accretion and Coagulation
 Astrophysical Journal Vol. 441, pp. 830-838, 1995
                                   Formation: gas phase




D. Samsonov and J. Goree
Particle growth in a sputtering discharge
J. Vac. Sci. Technol. A 1999
                         Log lambda used in code
log_lambda = max([3.,alog(debye_length / max([b_c,b_pi]))]) ; John's ad-hoc Coulomb logarithm
                                          ; corresponds to impact parameters ranging from
                                          ; the one that causes pi/2 scattering or collection on grain
                                          ; whichever is bigger, to the Debye length
                                          ; the outermost max function assures a nearly zero log lambda if the
                                          ; Debye length is shorter than the other length
                                          ; minimum value of 3 is suggested by Tsytovich (private communication)