>K9m 2014, TOM 146, Bbin. 2 (8), rap. 368 372
© 2014
EFFECT OF COLD ELECTRON EMISSION ON DIFFUSION PLASMA PARAMETERS AND THE SHEATH STRUCTURE IN A DOUBLE PLASMA DEVICE
M. K. Mishraa* A. Phukanb, M. Chakraborty€
" Department of Physics, Baosi Banikanta Kakati College, Nagaon, Barpeta Assam 781311, India
b Department of Physics, Madhabdev College, Narayanpur, Lakhimpur Assam 784164, India
" Centre of Plasma Physics-Institute for Plasma Research, Tepesia, Sonapur Assam 782402, India
Received December 24, 2013
It is observed experimentally that by injecting cold electrons in the discharge region of a double plasma device, the plasma parameters and sheath structure can be controlled in the other region, which is devoid of any electrical discharge. The main discharge region is separated from the region under investigation by a grounded mesh grid. Both cold and hot ionizing electrons are emitted from separate sets of filaments in the discharge region. With an increase in the cold electron emission current, the plasma parameters in the discharge region get changed, which in turn alter the plasma parameters in the other region. Two important effects caused by cold electrons in the diffusion region are the increase in the plasma density and decrease in the plasma potential. The increase in the plasma density and decrease in the sheath potential drop therefore cause the contraction of the sheath.
DOI: 10.7868/S0044451014080136
1. INTRODUCTION
The plasma produced by a dc discharge has wide applications in basic studies as well as in technology. Normally, the plasma parameters such as the electron temperature and the plasma potential determine the processing performance. The plasma potential determines the bombarding energy of plasma ions in film deposition on a substrate. The decrease in electron temperature is very important in the plasma etching process.
The study of sheath phenomena is very important in understanding the process between plasma and a solid surface because of its practical applications in material processing. The plasma sheath problem in a low-pressure discharge was first presented by Tonks and Langmuir [1]. The explicit formula and a clear interpretation of sheath formation is due to Bohm, who introduced the idea of a pre-sheath, a weak electric field
E-mail: mishra.mrinal'fflrediffmail.com
that exists between plasma and the sheath edge [2]. The pre-sheath accelerates the ions to a sufficient velocity known as the Bohm velocity, such that the ion density exceeds the electron density everywhere within the sheath.
Actually, the thickness (width) of the sheath is a concept that can be either determined experimentally or defined theoretically. The thickness of an ion sheath formed at a wall depends on the sheath potential drop and the amount of the ion flux reaching it. The sheath potential drop is the difference between the plasma potential and the applied wall potential. Therefore, any change in plasma parameters such as the plasma potential and density plays a crucial role in the formation of the sheath structure.
The double plasma (DP) device, where weakly ionized plasma is produced by the filament discharge process, has often been used to study the effect of both ion and electron beams on plasma parameters and the sheath structure. An ion or an electron beam injected into the plasma is supposed to alter the electron energy probability function (EEPF) [3,4].
>K3TO, TOM 146, Bbin.2(8), 2014
Effect of cold electron emission
In a DP device, both ion and electron beams can be produced by creating a suitable potential difference between the two plasma regions, a source and a target. The effect of a low-energy electron beam on the plasma parameters and the sheath structure was observed in [3]. The authors of [3] observed a contraction of the ion sheath with an increase in the electron beam energy, and also observed that the ion sheath expands in the presence of an ion beam [4].
The changes in plasma parameters and sheath structure due to extraction of charged particles in a DP device were experimentally observed in [5].
In this paper, we inject cold electrons into the discharge region of a DP device to control the plasma parameters and sheath structure in the other region, which is devoid of any electrical discharge. The two regions are separated by a grounded mesh grid of high transparency. The injection voltage or bias voltage applied to the cold electrons is lower than the ionization potential of the gas used.
Previously, many researchers have used cold electrons for many purposes. In [6], the electron temperature was successfully decreased by replacing high-cncr-gy electrons with cold electrons emitted from an auxiliary hot electrode. On the other hand, in [7], the electron temperature was increased by injecting cold electrons in a multi-dipole plasma. Fast ions were confined by a negative electrostatic potential produced by-cold electron injection in a multi-dipole plasma [8]. Recently, Mislira and Phukan controlled discharge plasma parameters by injecting cold electrons in the diffusion plasma region of a DP device [9].
Here, we successfully controlled the plasma potential and density in a region where no electrical discharge occurs. The cold electrons injected in the discharge region alter the plasma parameters, which in turn affects plasma parameters in the nearby region separated by a grounded mesh grid.
The change in the density and the plasma potential affects the collection current of the plate, and the sheath structure changes as a result. The technique adopted here is free from any geometrical perturbation or any perturbation caused by the external biasing.
2. EXPERIMENTAL SETUP
The experiment is carried out in a DP device consisting of two identical cylindrical multi-dipole cage structures 35 cm in length and 25 cm in diameter. These two cages, the source and the target, are electrically isolated from each other. A stainless steel mesh
VF VD Vcf VIN
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Pump - -
Permanent, magnet.
Field lines
Fig. 1. a) A sketch of the experimental setup. G is the mesh grid between the source and the target section: F is the filament used for emission of ionizing electrons in the source section and Fc is the filament used for the cold electron injection, Vf and Vd are the ionizing electron emitting filament voltage and the discharge voltage in the source, Vcf and Vin are the cold electron emitting filament voltage and the cold electron injection voltage, L is the Langmuir probe, Vb is the probe biasing voltage, P is the stainless steel plate, and Vp is the plate biasing voltage, b) The schematic diagram of the surface magnetic cusp field produced by multi-dipole magnets and the location of the filaments inside the magnetic cage
grid 24 cm in diameter is placed between the two cages. To place the grid, magnetic rows are removed from one end of each magnetic cage. Charged particles can move from one plasma region to the other through this grid.
The magnetic cages of both the source and the target are grounded. The separation grid of the device is also grounded because the negative floating potential of the grid would provide a potential barrier to electrons, which may reduce the flow of electrons from the source to the target region. The wires of the grid collect some of the electrons, but due to the high transparency (20 lines per cm) of the grid used in the experiment, most electrons are able to escape through the spacing between the grid wires.
The schematic diagram of the experimental setup is shown in Fig. la.
A multi-dipole cage is a set of alternating rows
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of north and south polo permanent magnets placed around the discharge surface in a cylindrical shape. The alternating rows of magnets generate a line cusp magnetic configuration in which the magnetic field strength is maximum near the magnets and decays with the distance to the chamber. Hence, the bulk plasma volume is virtually free of the magnetic field, but a strong field of about 1 kG exists near the chamber wall, inhibiting plasma loss and leading to an increase in plasma density and uniformity. The schematic diagram of the surface magnetic field produced by the multi-dipolo magnets and the location of the filaments inside the cage are shown in Fig. 16.
The base pressure of the chamber is 4 • 10-6 mbar. Plasma is solely produced in the source region by electron bombardment of a neutral Ar gas at 5 • 10-4 mbar by applying a dc voltage between the hot filament (cathode) and the magnetic cage (anode). There are four filaments (F) that emit ionizing electrons and three other filaments (Fc) that emits cold electrons in the source region of the device. The electrons emitted from the hot filaments (cathode) ionize the background gas on their way to the anode (magnetic cage).
A negative voltage (V/a< = —12 V) is applied to the filaments in order to inject cold electrons to the plasma. The applied injection voltage is less than the ionization potential of the gas used (15.8 oV for an argon atom). The emission of cold electron can be increased by increasing the injection current Ic- The cold electron emission current Ic is controlled by changing the filament heating voltage Vcf-
In the source region, the discharge voltage (Vd) and the discharge current (Id) are respectively fixed at 50 V and CO mA.
A plane Langmuir probe L of diameter 4111111 is used to measure the plasma parameters in the target region. The plasma potential is determined as the probe voltage at which the first derivative of the probe characteristics has a maximum [10].
In order to measure the thickness of the ion sheath, a stainless steel plate 5 cm in diameter is inserted in the target region. The plate is placed at the distance of 10 cm from the separation grid. The backside of the plate is covered by an insulating material, such that plate collects charged particles only from the front side. The plate is bia
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