Dynamics of Dust Particles in Radio Frequency Discharges

Kravchenko O.Yu., Vakulenko A.V., Lisitchenko T.E., Levada G.I.
Taras Shevchenko National Kyiv University, Volodymirs’ka str. 64, 01033 Kyiv, Ukraine
e-mail: kay@univ.kiev.ua
Abstract. In this paper we are carried out the computer simulation of the dust cloud dynamics in the radio frequency
discharges at the microgravity conditions using PIC/MCC method for electrons and ions and hydrodynamics model for
dust particles. In our model we take into account elastic collisions of electrons and ions with atoms, an ionization and
excitation of atoms by electrons, the charge exchange between ions and atoms, coulomb collisions of electrons and ions
with dust particles, as well as the electron and ion collection and scattering by dust particles. The moving of dust particles
is governed by the electrostatic force, ion and neutral drag forces, which are averaged over period of RF discharge. The
calculations were performed under various gas pressures, densities and radii of dust particles. The obtained results show
that dust particles form layer with sharp boundaries in the discharge chamber. Locations of the layer boundaries don’t
coincide with the edges of the rf-sheathes and depend on size of dust particles and ion density in discharge. The dust
density distribution is almost uniform in the central part of the layer. At the edges of the cloud significant peaks of dust
density are formed.
Keywords: Dusty plasma, computer simulation, radio frequency discharge.
PACS: 52.27.Lw
INTRODUCTION
Physics of dusty plasma are considered now as one of the important branches of science and technology. First of
all an active research on particle formation and behavior has been induced by contamination phenomena of industrial
plasma reactors used for etching, sputtering, and plasma enhanced chemical-vapor deposition. Many experimental
efforts have deal with the detection and dynamics dust particles in capacitively coupled radio frequency discharges,
since they widely used as sources of reactive species for surface treatment. Such particles are trapped in the plasma
bulk, forming dust clouds and voids [1, 2], and significantly affect the discharge behavior. In particular, dust
particles change electron and ion velocity distribution functions and influence on ionization processes and flows of
plasma particles toward the electrodes. The numerical simulations of the dust particles dynamics in RF discharges
was carried out earlier in the frame of hydrodynamic model [3, 4], which is not correct for discharges of the low
pressure. However, many modern plasma reactors operate at low gas pressure and the mean free path of charged
particles becomes comparable with or exceeds the thickness of electrode sheaths. The influence of dust particles on
radio frequency discharge in the kinetic approach was investigated only for the case of static dust particles [5, 6]. In
this paper we are carried out the computer simulation of the dust cloud dynamics in the radio frequency discharges at
the microgravity conditions using PIC/MC method for electrons and ions and hydrodynamics model for dust
particles.
MODEL
We use hydrodynamics approach to describe the dust particles motion.

Figure 3a shows dust density spatial distributions at three values of a neutral gas pressure and rd = 3μm.
Values of gas pressure in figure are indicated in torrs. Note, that the width of dust layer is a monotonically increasing
function of the pressure. This result is a consequence of more thin sheaths at a big pressure and the balance of forces
acting on a dust particle is reached closer to electrodes. Profiles of time-averaged ion current density are depictured
in figure 3b for cases with different gas pressures. It is seen that the ion current is settled nearly zero in the central
part of discharge gap at the increasing p . Peaks of ion current are formed at the locations dust density peaks. It is
significant that ion current value at the electrodes depends on pressure non-monotonically. At the gas pressure
p = 0.2torr the biggest dust density peaks is formed at dust layer boundaries. As a result big potential jumps are
appeared at these locations which accelerate ions toward electrodes.
To conclude, the dust cloud forming have been numerically studied in radio frequency discharge under
microgravity. It was shown that the dust particles compression is happen in some locations of the discharge chamber
due to join action of an electrostatic, an ion drag and a neutral drag forces. It was studied the influence of a dust
particle radius and a neutral gas pressure on plasma parameters distributions.
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