J. Phys. D: Appl. Phys. 42 (2009) 135202 G P Canal et al
some of the radiation is reabsorbed in the plasma or reflected
at its boundary, one must solve radiative transfer equations. In
the absence of the self-absorption term in the rate equations, the
analysis becomes considerably simplified. This is often the
case for the optical emission from laboratory plasmas with
low to moderate temperatures, except in the case of resonant
transitions. In the case of local thermodynamic equilibrium
(LTE) [2], the density of specific quantum states can be
determined for a system that has the same total mass density,
temperature and chemical composition as the actual system.
The relevant temperature corresponds to the distribution
function of the species dominating the reaction rates. In that
case electrons and ions will both have nearly Maxwellian
velocity distributions, even if the two kinetic temperatures
may be quite different. From the Saha equation, a relation
between the total densities of two subsequent ionization stages
can be derived. When the equilibrium condition is not
fulfilled, the ion and electron velocity distributions are no
longer Maxwellian and a modified version of the Saha equation
has to be implemented.
The Saha equations can be solved either by a kinetics
theory method [3] or from the thermodynamics for equilibrium
systems [4, 5]. When equilibrium thermodynamics is used,
two different results are obtained for a two temperature plasma.
Morro and Romero [4] proposed a modified Saha equation that
depends on the electron and the ion temperatures [6, 7]. Later,
Van de Sanden et al [5] and Chen and Han [8] showed that
the modified Saha equation obtained by Morro is not valid for
a two temperature plasma, because its derivation is based on
equilibrium assumptions (from the minimization of the Gibbs
free energy). On the other hand, the equation obtained by Chen
for a two temperature plasma has the same mathematical form
of the standard Saha equation, only changing T by T
e
.
In this work we derive a modified version of the Saha
equation from the kinetic solution by assuming a Druyvesteyn
velocity distribution function for the electron and the ion
species. We apply the modified Saha equation to a low pressure
argon plasma produced by an inductive RF discharge to obtain
electron temperatures, which are consistent with the result
obtained with an electrostatic Langmuir probe. The obtained
temperatures are almost an order of magnitude higher than
those predicted by the standard Saha equation.
2. Experimental apparatus
The experimental apparatus consists basically of an inductively
coupled plasma produced by a three loop antenna placed inside
a cylindrical stainless steel (316L) chamber. The RF power
supply is based on a push–pull oscillator designed with a
variable output power ranging from 10 to 500 W, operating
at 13.56 MHz. The chamber is pumped to a base pressure
of 10
−7
mbar; during operation it is filled with argon and the
working pressure is kept constant (5 × 10
−2
). The chamber
has a quartz window for optical emission spectroscopy (OES)
and the target holder and the Langmuir probe are placed on
two separated retractile manipulators.
Figure 1. Calibrated optical emission spectrum measured for a
pressure of 5 ×10
−2
mbar and 120 W of RF power.
2.1. Optical emission spectroscopy
For OES measurements, an optical system consisting of
a set of collimating lenses is used to focus the plasma
light onto the entrance of an optical fibre coupled to a
Czerny–Turner spectrometer (HR4000 model from Ocean
Optics). The spectrometer is equipped with a holographic
grating (Composite™) of 300 lines mm
−1
with a linear CCD
array of 3468 pixels, yielding a resolution of 0.5 nm in the
spectral range from 200 to 1100 nm. Emission intensities
are corrected according to the wavelength dependence of the
spectral sensitivity of the CCD and of the grating transmission
efficiency. Our measured spectra were further corrected
with a NIST-traceable calibrated tungsten halogen light
source (300–1050 nm) from Ocean Optics (model LS-1-CAL).
A typical calibrated optical emission spectrum is shown in
figure 1 for a pressure of 5× 10
−2
mbar and 120 W of RF
power.
2.2. Electrostatic probe measurement
A single Langmuir probe was constructed with a tungsten tip
of 0.5 mm diameter brazed to a glass tube head. A low band
pass filter is placed inside the tube and close to the probe tip
to reduce RF distortion. The glass head, glued to a stainless
steel tube, is inserted along the axis of the vacuum chamber
and can be rotated and displaced to allow a radial sweep. In
the measurements, the probe was biased from −70 to 70 V
and the voltage applied together with the current output were
simultaneously measured by an ADC (USB6008, National
Instruments). In figure 2 we have a typical I –V curve for
a pressure of 5× 10
−2
mbar and 120 W of RF power.
The EEDF F
e
(E) was obtained following the standard
second derivative analysis of the I –V curve [9],
I
e
=
1
4
2e
3
m
e
1/2
A
∞
V
E
1/2
F
e
(E)
1 −
V
E
dE, (1)
where A is the area of the collecting probe surface, m
e
and e are
the electron mass and charge, respectively, V = φ
p
−V
b
is the
2