Presence in cathode area primary electrons, emitted by the cathode which, moving along a field, collide with atoms of gas, results in its ionization and the subsequent bombardment of the cathode - target by positively charged ions. Result of this bombardment is dispersion of the a cathode surface and secondary electrons emission, which also participate in ionization of working gas. Both primary, and secondary electrons can carry out ionization of atoms of working gas within the limits of Debye lengths, i.e. till the moment of recombination with positive ions, colliding with neutral atoms (ν - frequency of electronic collisions). Efficiency of ionization of gas α is determined by expression [1]:
α=Ne/(Ni+NA), (4.1)
And the critical degree of ionization is determined by expression:
αК=1,73·1012·σеА·Те2, (4.2)
where σеА – quantity of electron-nuclear collisions on unit of the area at average electron energy. Thus, to increase the efficiency of ionization it is necessary to increase in electron concentration, or increase the electron path length L up to size L>> λД, that considerably raises probability of their collision with atoms of gas.
The mechanism of magnetron sputtering is characterized by the complex electron movement in the discharge zone, which generally can be divided into four components [2]:
-helio-movement along lines of
the magnetic field which grows out their rotations around the vector
and simultaneous moving
alongthe field;
Reflection from the cathode (« mirror effect ») owing to presence of negative potential and a gradient of a magnetic field on it. As a result of this effect electrons are reflected in one and other sides between points where lines of a field pass through the surface of the cathode;
-
electron drift, caused by action of
Lorentz's forces in the cathode space (fields
and
are perpendicular to each
other) which forces electrons
to move in
parallel with the cathode around lines of the magnetic field;
The anode drift representing electron movement from one line of the field to another in the anode direction which is finally a collector of low-energy electrons.
Movement of
the charged particle with a charge е, weight m and speed v in
the electric field and the magnetic field
(fig. 1.1) is described by
the equation [54]:
. (4.3)
In the homogeneous magnetic field (see fig. 1.1á) electron drifts along the line of the field with a speed vИ, not dependent on size of the field, and rotates around its lines with cyclotron frequency:
ωС=eB/me=1,76·107 В [rad./s], (4.4)
 |
Fig. 4.1 Mechanism of the ionizing electron movement in the homogeneous magnetic field (а), in crossedЕхВ fields(б) and trajectories of electron movement in the cathode area in the crossed fields [54].
Thus electron movement occurs on helio- trajectories to radius rд [54]:
rd=me·v/e·B=3,37·W1/2/B (4.5)
where 
— intensity, Tl; W - the energy connected to electron
movement in the direction perpendicular to
field, eV.
In case of
homogeneous and parallel fields electrons
are freely
accelerated and the step of their spiral trajectory continuously grows. When
the electric field 
is perpendicular to the
magnetic field, the common direction of electron drift is perpendicular as
, and
(fig. 4.1, b), and speed of its
movementv
in this direction
is determined by expression:
vE=108·E/B (4.6)
Radius of its spiral trajectory rд is determined from expression (4.3), where v=vE (fig. 4.1, c). Thus, primary electrons, emitted by the cathode with initial energy ~5 eV, are accelerated, passing dark cathode space. Thus their trajectories do not appear strictly cycloidal in connection with heterogeneity of the electric field in the zone of dark cathode space.
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