Acceleration Of Ions In Beam Plasma Discharge At Low Magnetic Field. Coordination Of Energy Distributions Of Electrons And Ions

N. V. Isaev and E. G. Shustin
Institute of Radio Engineering and Electronics of RAS (Fryazino branch), Moscow reg., 141190, Fryazino,
Vvedensky Pl., 1, Russia
ABSTRACT. In [1] a phenomenon of stable acceleration of ions in beam plasma discharge at a low magnetic field up to
energies, by the order exceeding a thermal energy of electrons is revealed experimentally. The radial current of highenergy
ion component makes an essential part of a total current of ions escaping the region of discharge. In the report [2]
the results of computer simulation of beam instability development at parameters of a system, close in those in [1] are
represented. The effect of formation of paraxial area of plasma with highly heated electrons is detected. Its high
electrostatic potential determines acceleration of ions from this area to peripherals of discharge. For verification of
conclusions of numerical experiment the measurements of a velocity distribution function of electrons escaping area of
discharge to its collector, together with energy distribution of ions which are running out from discharge on a normal
from an axis are carried out. The effect of essential heating of electrons of plasma in paraxial area is detected in those
regimes, when the acceleration of ions is observed. The temporal structure and longitudinal distribution of high frequency
oscillations (in range ω~ωpe) excited in discharge is researched. The effect of accumulation of a field of regular
oscillations in the region of injection of a beam as well as their stochastisation in process of propagation along the axis of
system are detected. The results of physical experiments qualitatively correlate with the data of computer simulation.
Keywords: low temperature plasma; electron beams; beam plasma discharge; ion flow; experiment.
PACS: 52.40.Mj
In order to optimize the treatment of materials in plasma processing reactors operating at low gas pressures, it is
very important to control the parameters of ions bombarding the processed material. Thus, in devices for the ion
etching of semiconductor materials in an rf discharge plasma, the ion energy distribution function (IEDF) and the
angular distribution of the ions bombarding the material’s surface critically affect the rate of etching and the degree
of its anisotropy. Methods for controlling the shape of the IEDF have been mainly studied for plasma processing
reactors based on rf discharges. It has been shown that it can be controlled, e.g., by applying an RF bias voltage
directly to the substrate or by using an auxiliary electron source (either an additional discharge or a thermal-cathode
gun) to inject electrons into the discharge. It was shown in [1] that a beam–plasma discharge (BPD) in a lowpressure
gas can serve as a source of an ion flow escaping the discharge region on normal to its axis. We have shown
that, by changing the external parameters of a beam plasma discharge in the equipotential interaction chamber and
by using electron beams pre-modulated in velocity, the energy of the ions bombarding the surface of a sample placed
near the side wall of the chamber can be varied within a range of 8–50 eV. Note that it is precisely this ion energy
range that is characteristic of plasma processing reactors for surface treatment (such as deposition of thin films and
etching) of materials for semiconductor electronics and acoustoelectronics.
At report [2] is shown by computer simulation that at non-linear development of a beam instability in a bounded
volume of plasma in area occupied by the beam the strongly non-equilibrium plasma with mean energy of electrons
reaching hundreds electron-volts is built up; electrons of this area create an additional current of electrons from
plasma to end plates; the increase of the electron current results in growth of a potential of plasma in area occupied
by the beam; the potential gradient between area occupied by a beam and peripheral area of plasma determines
acceleration of a flow of ions on a normal to an axis of the system.
Results of computer experiment boosted measurements of a spatial distribution of high-frequency fields excited
in the system and analysis of cumulative distribution function of electrons (EDF) on an exit from the interaction
region.
The installation diagram is shown in a fig. 1. The plasma is created in an evacuated chamber – cylinder of
2R0=0,5 m in diameter and same length. The longitudinal magnetic field with induction up to 5 mTl in the chamber
is created with Helmholtz coils. An axial beam originates from Pierce type diode gun with a flat cathode of LaB6
placed at the separate chamber, which incorporates with the main chamber by a pressure differential pipe.
Parameters of the beam on an input to the plasma chamber: accelerating voltage Ub=1-3 kV, current Ib is up to 500
mA, diameter 1÷1,5 cm. Power source of the gun provides its pulsed operation with pulse duration τb =10 ÷ 200 ms.
FIG 1. DIAGRAM OF EXPERIMENT. 1 – Pierce type electron gun; 2 – beam focusing coils; 3 – plasma vessel; 4 – Helmholtz
coils; 5 – ion energy analyzer; 6 – HF dipole probe; 7 – HF frequency analyzer; 8 – collector.
At an opposite wall of the plasma chamber the collector of electrons combined with an energy analyzer of
electrons (an electrostatic grid analyzer with a decelerating field) is placed.
For diagnostics of oscillations in plasma the symmetrical dipole probe loaded with an input of a spectrum
analyzer through a resistance transformer is used. The probe is installed on a mobile rod ensuring movement of the
probe along an axis and on radius of the chamber. The shape and orientation of the probe provide the least influence
of the beam to its characteristics.
As the receiver of an ion flow the electrostatic analyzer with a flat deflecting mirror movable along a side of the
plasma chamber is used. The collimator of ions is oriented perpendicularly to axis of the chamber. Parameters of an
analyzer: range of energies -0÷100 eV, sensitivity ~0,5·10-9 A/cm2, resolution on energies – ΔW/W0 =0.12.
Synchronization of processes and registration of time functions of the analyzer collector current, of spectrum of
a signal from the probe, and also of parameters of experiment regime (beam current and voltage, pressure and
magnetic field in the chamber) are performed with a system of automation of experiment.
In a fig. 2 the power spectra of Ez components in band ω≈ωpe, registered by the probe on different distances L
from a collector are shown.
From this figure it is visible that in the beginning of an interaction area the spectrum is rather narrow-band, then
spectrum widens to more high frequencies. In a fig. 3 the longitudinal distributions of the oscillation intensity in
frequency band appropriate to peak of the spectrum in the beginning of area (an integral of a spectral curve on a
band 610±10 Mc/s) – (curve 1), and in full band of generated oscillations (curve 2) are shown. Effect of
accumulation of regular (near-monochromatic) oscillations near to a point of beam injection is obviously seen here,
which follows more or less smoothly varying growth of intensity of stochastic oscillations to a collector. The
analysis of transversal distribution of a field strength displays, that the oscillations in range of Langmuir frequencies
are obviously localized on radius in limits ~Rbeam.
In a fig. 4 the curves of velocity distribution function (EDF) of electrons arriving at collector are represented at
different chamber pressures (so, at the different relations nb/Np). (For visualization curves in figs. 4 and 6
sequentially are biased on a vertical). The distribution function of electrons on longitudinal velocity on a collector in
pre-discharge regime (p=0,05 mTorr) represents peak of electrons of the beam, spreading to smaller velocities due to

Pump out Pump out
twist of the beam in a nonuniform magnetic field at input to the plasma chamber. At a pressure buildup there is an
avalanche ignition of beam plasma discharge. The EDF of beam is jump-like spread to smaller speeds, running up to
a shape of a plateau at this interaction length. In process of gas pressure growth plasma density increases
(accordingly, the relation nb/Np decreases), and the diffusion of EDF of the beam decreases. Simultaneously with
diffusion of the beam on velocities the part of EDF appropriate to electrons of plasma is obviously dilated also: the
mean energy of a longitudinal motion of plasma electrons in the area occupied by the beam reaches 100-120 eV. A
characteristic feature of EDF is the presence of an intensive bunch of electrons in an energy range of 200-1000 eV.

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