1063-7397/05/3405- © 2005 MAIK “Nauka /Interperiodica”
0295
Russian Microelectronics, Vol. 34, No. 5, 2005, pp. 295–308. Translated from Mikroelektronika, Vol. 34, No. 5, 2005, pp. 352–366.
Original Russian Text Copyright © 2005 by Petrin.
1. INTRODUCTION
Low-pressure microwave plasma reactors employ-
ing electron cyclotron resonance (ECR) are widely
used for etching and thin-film deposition in industrial
production [1, 2]. Their main advantage is that they can
sustain high-density plasmas with pressures in the mil-
litorr range or lower; this is important because the qual-
ity of etching or deposition tends to rise with decreasing
process pressure [3]. Magnetic traps of different
designs are widely used in ECR reactors in order to
control the escape of electrons from the magnetized
plasma by cross-field diffusion due to collisions with
neutrals and ions. The diffusion becomes less intense as
the pressure is reduced if the plasma density is fixed
[4, 5]. Magnetic traps often employ electromagnets
because these allow one to easily adjust the magnetic
field and trap parameters [6–8]. On the other hand, such
designs are rather cumbersome, requiring a high-power
current source and, usually, water cooling. An alterna-
tive approach is based on high-field permanent mag-
nets, which have also proven to be useful for creating
multipolar magnetic fields [9, 10]. Permanent-magnet
traps are preferable in many applications as no current
source is required. Moreover, multipolar arrangements
of magnets allow one to build distributed plasma
sources for highly uniform processing of large areas
[11, 12].
There is a wide variety of theoretical methods for
analyzing the ECR interaction between an electromag-
netic wave and a plasma; for details, see paper [13] and
the references therein. However, we have revealed that
local equations of the interaction do not apply at pres-
sures of about 1 mtorr or lower, because the electric
field at a point will govern the currents over a long dis-
tance relative to the width of the ECR layer and the
wavelength in the plasma: the wave–plasma interaction
will become nonlocal [14].
For the above reason, our recent study [15] involved
development of the methods of monograph [16]. A
computer simulation was conducted of electron motion
in a magnetic trap, neglecting collisions with the other
plasma species. The emphasis was on the action of a
microwave on the electrons in an ECR layer. The simu-
lation served to illuminate the mechanism of ECR heat-
ing, and against this background, basic requirements
were set out concerning the design of the microwave
generator and magnets. It has been shown that the con-
figurations of the magnetic system, shields, and feeder
should be such as to ensure the confinement of high-
energy electrons for a time as long as possible. Fur-
thermore, it has been concluded that injecting micro-
wave power into the ECR regions of the magnetic trap
is sufficient to heat the electrons of the whole plasma;
in fact, the heating can be effected by exciting one
ECR region.
In this paper the methods of study [15] are devel-
oped and applied to a comparative evaluation of plasma
reactors with a multipolar magnetic system in respect
of sustaining a low-pressure ECR plasma.
2. MICROWAVE HEATING OF ELECTRONS
IN A TWO-DIPOLE MAGNETIC TRAP
The statistical ensemble of electrons in a multi-
polar magnetic field is simulated by numerical solu-
Sustaining Mechanisms of the ECR Heating of Electrons
in Low-Pressure Microwave Plasmas
under Magnetic Multipole Confinement
A. B. Petrin
Institute for High Energy Density, Russian Academy of Sciences, Moscow, Russia
e-mail: a_petrin@mail.ru
Received December 16, 2004
Abstract
—A theoretical study is conducted on the sustaining of low-pressure electron-cyclotron-resonance
(ECR) plasmas for which ionization is provided by high-energy electrons trapped by a multipolar permanent-
magnet system. A particle simulation is carried out for the electron motion in magnetic systems of different con-
figurations on assumptions suggested by previous experimental results. On this basis, the high-energy tail of
the electron energy distribution function is examined. The escape of high-energy electrons (including that to the
walls) is identified as the major factor behind the loss of electrons in the limit of a collisionless plasma. A com-
parative evaluation of different multipolar magnetic systems is made. Basic requirements are set out concerning
the optimal design of ECR plasma reactors. New approaches to ECR electron heating are proposed.
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