Charging of Dust in a Plasma with Negative Ions

11th Workshop on the Physics of Dusty Plasmas Williamsburg, VA June 28 – July 1, 2006 Charging of Dust in a Plasmawith Negative Ions Su-Hyun Kim and Bob Merlino The University of Iowa Supported by DOE

Positive Dust • Photoelectric charging • Secondary emission • Ti > (mi/me) Te • If electrons areattached to heavynegative ions

Dust in plasmas with negative ions • the plasma environment of the nighttime polar mesosphere is dominated by negative and positive ions, few electrons, and nanometer size particles • microelectronic manufacturing devices using reactive gases, such as silane,SiH4, have large concentrations of negative ions and dust particles

Plasma densities in the polar mesosphere M. Rapp et. al, GRL 32, L23821 (2005)

Mesospheric particle detector current The detector currents correspond to positive charge densities, eZN ~ 100 e/cm-3

Hydrogenated amorphous silicon (a-Si:H) semiconductors • for liquid crystal displays and solar cells • made by PECVD in rf discharges using hydrogen-diluted silane • SixHm (x 200) nm size particles also form and incorporate into the depositing film • particles must be neutral or + to get out • in these electronegative discharges ne<<ni • effect of –’ve ions must be considered

STM image of a nanoparticle on a 4.2 nm thick a-Si:H film nanoparticle film substrate Tannenbaum, et. al, APL 68, 1705 (1996)

Charging of dust with positive ions, negative ions and electrons[Mamun & Shukla, PoP 10,1518 (2003)] Vs = Vf  Vp Vs < 0 Vs > 0

Solutions to the charging equations n+ = ne + n– = ne/n+ n– = (1– )n+

negative ion plasma, with ne << n+ dust particles Experiment • Q machine plasma • K+ ions • Te = Ti  0.2 eV • admit SF6 gas to form • negative ions • Disperse hollow glass • microspheres (35 mm) • using rotating cylinder

Negative ion plasmas in a Q machine very effective due to low Te N. Sato, in K Q machine

Langmuir curves before and after dust added P(SF6) =0 7 x10-4 I0 I 4.6 x10-5 I+ I+0

Data analysis • Analysis of the Langmuir probe currents can be used to determine how the charge in the plasma is divided between free +/- ions, free electrons and dust particles. • Charge neutrality in dusty plasma: en+ + Qnd = ene + en– h is determined from the changes in negative and positive probe currents when dust is introduced

Data analysis if e = ne/n+ <<1 (electrons attached to SF6) The sign of R– – R+ determines the sign of the dust charge, Q

Results

Qnd/eno e = ne/n+ Qnd/eno vs. e

Langmuir Probe floating potential (relative tothe plasma potential) vs. P(SF6)

Summary and Conclusions • dust charging in a plasma with negative ions has been studied experimentally • the addition of negative ions reduces the density of electrons leading to a reduction in the (negative) charge on dust • conditions have been established which cause positive charging of dust, in a plasma with light + ions and heavy - ions

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Charging of Dust in a Plasma with Negative Ions