Method for determining absolute plasma parameters

In accordance with the method of the present invention, the radio frequency discharge current generated in a plasma reactor is measured in the form of analog signals at a portion of the reactor acting as an earth electrode. The analog signals are converted into digital signals, and plasma parameters are evaluated from the digital signals by means of a mathematical algorithm.

BACKGROUND OF THE INVENTION 
The invention relates to a method for determining of absolute plasma 
parameters of unsymmetrical radio frequency (RF) low-pressure plasmas. The 
invention further relates to an apparatus particularly for measuring of 
the radio frequency discharge current at a portion of a plasma reactor 
acting as ground electrode or earth electrode. 
In physics, the meaning of the term "plasma" is that of an ionized gas. In 
this process, a gas molecule is ionized, i.e. a gas molecule is split up 
into a free electron and a positive ion (ionized molecule). This process 
often is an electron collision ionization. The feeding of a radio 
frequency alternating electric field leads to an accumulation of energy 
with respect to the electrons for an inelastic collision. The electrons 
impinge on other gas molecules which are again split up as mentioned 
before into free electrons and positive ions. A plasma state is formed in 
the entire reactor by this process. Electrons and ions recombine on the 
wall since the charge carrier, namely the free electrons and the positive 
ions, discharge to the walls of the reactor. Thus, a gas molecule is again 
formed so that an equilibrium is finally established in the reactor. 
The radio frequency alternating electric field is generated in the plasma 
reactor by means of parallel-plates wherein an excited or driven radio 
frequency (RF) electrode is positioned opposite to an earth electrode. The 
RF electrode is also called "hot electrode". In the following, the real 
electrode which electrically forms the earth or ground, which is directly 
opposed to the RF electrode and which also includes portions of the 
reactor wall acting as earth, is called "earth electrode". The electrodes 
of the plasma reactor are normally arranged in a horizontal manner. The RF 
electrode extends close and in parallel to the bottom wall or to the top 
wall of the reactor. 
In the following, a plasma having an excitation frequency between 10 and 
100 MHz and a pressure of 0.1 to 100 Pa, preferably 0.1 to 20 Pa, is 
called RF low-pressure plasma. Therefore, the gas is held under a 
predetermined constant pressure (vacuum) in the reactor. It is also 
possible to continuously feed the gas into the reactor and to continuously 
discharge the gas from the reactor. 
Thus, the plasma processes take place in an evacuated reactor. RF 
low-pressure plasmas are used in the field of fundamental research and 
above all in the field of the semiconductor technology. Methods such as 
plasma etching (PE), reactive ion etching (RIE) and plasma enhanced 
chemical vapor deposition (PECVD) are for example known. 
DESCRIPTION OF THE PRIOR ART 
The following methods of plasma diagnostics are substantially known with 
respect to RF low-pressure plasmas. 
The optical methods are above all characterized by emission spectrometric 
methods wherein the emission may partly be externally excited. This method 
has the advantage that the test object is influenced in a hardly 
observable manner; this method however has the disadvantage that only 
relative measuring results can normally be achieved. Mass spectrometric 
methods enable the determination of the relative intensity or further the 
mass selective determination of the ion energy distribution. The 
determination of absolute values for the particle density or for the 
current density is normally not possible by reason of the small aperture 
angle and the non-constant transmission. The determination of local plasma 
parameters, for example the electron density, the ion density and the 
electron energy, may be achieved by means of Langmuir probes. Langmuir 
probes are however only applicable with respect to reactive or layer 
forming plasmas in a very limited manner. Integral plasma parameters, like 
for example the average density of electrons, may be determined by means 
of microwave interferometry. The frequency of the microwaves is above the 
local electron plasma frequency in order to enable the wave propagation in 
the plasma. This method normally needs a rigid reference line (in the form 
of a hollow conductor). Thus, this method is not very flexible. Moreover, 
the knowledge of the spatial distribution of the density of the electrons 
is necessary for an exact determination of the average density of the 
electrons. The spatial distribution of the density of the electrons is 
additionally to be determined for example by means of a Langmuir probe. 
Integral plasma parameters may however also be determined by means of 
resonance probes. These probes use the capability of forming resonances of 
the plasma including its boundary layers with respect to surrounding 
solids. Depending on the thickness of the boundary layers, the plasma 
resonance frequency which may also be called geometric plasma resonance 
frequency, is always slightly lower than the electron plasma frequency. 
Resonance probes may not be used with respect to RF discharges since the 
discharge itself generates a wide spectrum of harmonic oscillations and 
thus superimposes the response of the external excitation via the 
resonance probe. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a method 
as mentioned above by which absolute plasma parameters may be determined 
in a reliable and reproducible manner. 
It is another object of the present invention to provide an apparatus 
particularly for measuring of a part of the discharge current at a plasma 
reactor. 
These and other objects are solved by a method for determining of absolute 
plasma parameters in unsymmetrical radio frequency (RF) low-pressure 
plasmas characterized by the following steps: 
a) measuring of a part of a radio frequency discharge current generated in 
a plasma reactor in form of analog signals at a part of the reactor acting 
as earth electrode; 
b) converting of the analog signals measured under a) into digital signals; 
and 
c) evaluating of the plasma parameters from the digital signals obtained 
under b). 
The present invention is based on a radio frequency measuring system which 
is insensitive with respect to thin but generally optical dense layers so 
that the method according to the present invention is relatively 
insensitive with respect to dirt accumulations, layers of materials and 
with respect to reactive materials. As a result, the method ensures a high 
stability over a long time and a good reliability and reproducibility of 
the absolute plasma parameters determined by the method according to the 
present invention. 
According to a preferred embodiment of the method according to the present 
invention, at least one resonance frequency of the discharge is determined 
from how the discharge current is timely developing, wherein the discharge 
is self-excited on this resonance frequency by means of harmonic 
oscillations in the discharge current which are generated by virtue of the 
nonlinearity of the space charge sheath in front of the RF electrode. This 
resonance frequency is not the electron plasma frequency (Langmuir 
frequency) which describes the resonance in the plasma itself without 
participation of the boundary layers (space charge sheathes). This 
resonance frequency however indicates a geometric resonance being one 
geometry factor lower than the real plasma frequency. The geometric 
resonance is distinctly marked with respect to a strong unsymmetrical RF 
discharge. It is advantageous that a forced, i.e. external, excitation is 
not necessary because of the self excitation. 
The ion energy distribution in the harmonic oscillation in front of the RF 
electrode is determined from how the discharge current is timely 
developing wherein the measured discharge current forms a portion 
independent from the frequency of the entire discharge current and wherein 
this portion is preferably estimated by means of the area ratios of the 
part of the reactor acting as earth electrode and the RF electrode. It is 
advantageous that the average ion energy of the ions during impinging on 
the surface of the electrodes may be determined from the determined ion 
density distribution if the temporarily averaged potential difference 
between the plasma and the RF electrode is known. The average ion energy 
is an important parameter of the process during treating of a substrate, 
for example a semiconductor plate, which is positioned on the RF 
electrode. 
It is further advantageous that the damping constant of the system which is 
dampedly oscillating because of the collision of the electrons with 
neutral particles of the plasma, is determined from how the discharge 
current is timely developing. An effective electron-collision rate (also 
called effective electron collision frequency) may also be determined from 
the damping constant, said rate being a measure for the collisions of the 
electrons with the neutral particles of the plasma. 
According to a preferred embodiment of the method according to the present 
invention, the potential fed to the RF electrode or at least the dc part 
(direct current part) thereof is additionally required for the 
determination of the absolute value of the average thickness of the space 
charge sheath in front of the RF electrode and/or for the determination of 
the average ion energy. Thus, important parameters which characterize the 
plasma and which enable for example conclusions with respect to the 
etching process, may be determined in a reliable and finally reasonable 
manner. 
The above mentioned objects are further solved by an apparatus particularly 
for measuring of the radio frequency discharge current at a part of a 
plasma reactor acting as earth electrode, characterized in that 
a meter electrode (which may also be called measuring electrode) comprises 
an electrical insulation on its peripheral surface, and the meter 
electrode is positioned in a flange or recess of the reactor wall which 
acts at least as a part of the earth electrode, such that the insulation 
is at least arranged between meter electrode and wall of the reactor and 
that the front face of the meter electrode is approximately aligned to the 
inner wall of the reactor directed to the interior space of the reactor, 
and in that the front face of the meter electrode is dimensioned in such a 
manner that the RF potential in the reactor effected by the meter 
electrode is negligibly small if compared to the potential between plasma 
and earth electrode. 
According to the present invention, the meter electrode enables the 
measurement of the discharge current at the wall of the reactor wherein 
the meter electrode forms a virtual portion of the wall. According to the 
present invention, the RF potential at the meter electrode will be small 
with respect to the potential between plasma and meter electrode. The 
predescribed RF potential is between about 5 and 100 mV, preferably 
between 10 and 20 mV. The RF potential is thus several orders of magnitude 
below the plasma potential and its RF part. 
According to a preferred embodiment of the apparatus according to the 
present invention, the meter electrode is fixed at the reactor by means of 
an electrically insulated vacuum feedthrough. The meter electrode 
comprises a rod shaped electrical conductor which is connected to ground 
outside the reactor. The electrical conductor is connected with a current 
transformer which generates a voltage from the measured current. This 
embodiment represents the first embodiment of the apparatus according to 
the present invention which has a simple construction and by which usual 
commercial vacuum feedthroughs may be used. Because of the possible great 
surface of electrodes, this meter electrode has a relatively high 
sensitivity. 
It is a further advantage that a probe is provided which is fixed at the 
reactor by means of an electrically insulated vacuum feedthrough, which 
probe comprises an inner conductor connected with the meter electrode, an 
insulating body circumferentially surrounding the inner conductor, the 
insulation of the meter electrode and which probe comprises an outer 
conductor connected with the reactor wall. The outer conductor should 
preferably be provided within the flange, but as close as possible to the 
inner side of the reactor wall. This probe represents the second 
embodiment of the apparatus according to the present invention. It is 
further advantageous that such a probe has a high bandwidth with respect 
to the frequency to be measured. This bandwidth starts from the excitation 
frequency of the plasma, for example from 13.56 MHz, and goes further than 
the geometric plasma resonance frequency so that it is possible to measure 
the discharge current with a frequency of more than 300 MHz up to about 
500 MHz. 
It is a further advantage that the vacuum feedthrough of the predescribed 
second embodiment of the apparatus according to the present invention is 
provided at both sides with a coaxial connection Which is connectable to a 
corresponding counterconnection which is connected to the inner conductor, 
the insulating body and the outer conductor. Thus, it is possible to form 
the probe like a coaxial cable even in the reactor up to the actual meter 
electrode in order to achieve the predescribed bandwidth which should be 
as wide as possible. The probe may thus be represented in a simple and a 
reasonable manner by a "cut off" coaxial cable which is directly connected 
with a meter electrode. Any separation of meter electrode and vacuum 
feedthrough is easily possible by means of the coaxial connections so that 
components of the probe may quickly be replaced if desired.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
An apparatus 1 for determining of absolute plasma parameters of 
unsymmetrical radio frequency (RF) low-pressure plasmas is schematically 
shown in FIG. 3. 
An electrode 3 is located in a reactor 2, which is called plasma reactor in 
the following. The electrode 3 horizontally extends in parallel to the top 
wall 4 of the reactor 2 and a certain distance apart from said top wall. 
The electrode 3 is electrically connected to the top wall. The reactor 2 
further comprises side walls 5, 6 which are connected with the top wall 4 
on the one hand and which are connected to the bottom wall 7 on the other 
hand. 
The radio frequency (RF) electrode 10 extends close to the bottom wall 7 
and in parallel thereto in horizontal direction. The electrode 10 is 
electrically connected by a conductor 11 with an accommodating network 12 
and by a further conductor 13 with a generator 14. The generator 14 
generates a voltage having a sinusoidal course. The conductor 11 comprises 
an insulation 15 at least in the region of its leadthrough through the 
bottom wall 7 so that the RF electrode 10 and the connector 11 are not 
electrically connected to the bottom wall 7. As already described above, 
the electrode 3 including the walls 4, 5, 6 and 7 of the reactor 2 is 
designated as earth electrode in the following. In reactor 2, the plasma 
17 is located in the region 16 between the RF electrode 10 and the 
electrode 3. It is emphasized that in FIG. 3 region 16 is shown in the 
form of straight lines only for drawing purposes. 
A probe 20 is mounted on the side wall 5. By means of this probe, a part of 
the radio frequency discharge current, namely the part of the current 
which impinges on meter electrode 21 located in side wall 5, is measured. 
The probe 20 is connected to an analog/digital (A/D) converter 22, for 
example a digital storage oscilloscope. The A/D converter 22 is connected 
to a data processing device 23, for example a personal computer. 
The analog signals measured by the probe 20 reach the A/D converter 22 via 
the conductor 24, which converter 22 converts the analog signals (data) 
into digital signals and which send the digital signals via the conductor 
25 to the data processing device 23. The control of the A/D converter 22 
is effected via conductor 26 by means of the data processing device 23. 
The conductor 11 connecting the accommodating network 12 with the RF 
electrode 10 is connected via a voltage divider 27, preferably a probe, 
with the A/D converter 22 by means of the conductor 30. 
In the following, the method according to the present invention is 
described in detail with respect to FIG. 1 to 3. 
By feeding of a radio frequency alternating electric field the gas 
molecules which are located in the reactor 2 are ionized, i.e. the gas 
molecules are split up into free electrons and positive ions. In the state 
of plasma, streamlines 31 are formed in reactor 2, respectively comprising 
radial and axial components with respect to the axis of symmetry. For 
purposes of a better survey, only two streamlines 31 are shown in FIG. 3. 
The streamlines go from the RF electrode 10 to the earth electrode 
(electrode 3, top wall 4, side walls 5, 6, bottom wall 7). They are 
perpendicularly located on the RF electrode as well as on the respective 
part of the earth electrode. For purposes of simplicity, the streamlines 
31 in FIG. 3 are only shown within region 16. 
A portion of the radio frequency discharge current, namely the portion of 
the discharge current which impinges on the meter electrode 21, is 
measured by means of the meter electrode 21 of probe 20. According to FIG. 
1, this measured portion of the discharge current I.sub.p is shown versus 
the normalized time. The following equation is valid for the normalized 
time: 
EQU .phi.=.omega..sub.0 .multidot.t 
wherein .omega..sub.0 designates the excitation frequency 1/s! and t 
designates the time s!. This figure is valid for a pressure p of 10 Pa 
and for an excitation frequency of 13.56 MHz. 
Moreover, FIG. 1 shows the electrode potential U versus the normalized 
time, said potential being fed to RF electrode 10 and electrode 3. 
FIG. 1 shows a saw-tooth course together with superimposed oscillations of 
the measured discharge current over the normalized time. Such a course has 
the meaning that within the period of one oscillation first of all a steep 
rise and in the following a gradual drop of the measured current takes 
place. This saw-tooth course is the result of nonlinear distortions 
because of the nonlinearity of the space charge sheath 33 in front of the 
RF electrode 10 (in FIG. 3 above the RF electrode 10). By means of the 
nonlinearity of the space charge sheath, harmonic oscillations are 
generated in the discharge current. Since the thickness of the space 
charge sheath is dependent on the potential difference between the plasma 
and the RF electrode the space charge sheath acts as a nonlinear capacity. 
The nonlinear effect of the space charge sheath is considerably affected 
by the ion density distribution in the space charge sheath in front of the 
RF electrode 10. 
The electrode potential U shown in FIG. 1 versus the normalized time 
indicates a course which is nearly a sinusoidal oscillation. For purposes 
of a better understanding, this nearly sinusoidal course is indicated in 
FIG. 1 by dotted lines. 
The discharge consisting of the plasma body and of the space charge 
sheathes represents an oscillating system by reason of the inert mass of 
the electrons and the restoring force with respect to the electrical 
fields in the space charge sheathes between the plasma and the earth 
electrode on the one side and between the plasma and the RF electrode on 
the other side. The natural frequency of the oscillating system is below 
the electron plasma frequency (also called Langmuir frequency). The system 
is damped by the collisions of the electrons with the neutral particles of 
the gas so that finally a damped oscillation is performed. 
By means of the predescribed nonlinearity of the space charge sheath in 
front of the RF electrode and the harmonic oscillations in the discharge 
current resulting therefrom, the plasma is self-excited to oscillations 
which are distinct adjacent to the resonance frequency. In FIG. 2, a 
discrete spectrum of the measured values is shown. As a result, the 
measured discharge current shown in the Fourier spectrum of FIG. 2 
indicates a maximum approximately in the region of the 10th to 11th 
harmonic oscillation. Since in FIG. 2 the Fourier spectrum is shown above 
the normalized frequency .omega./.omega..sub.0 it follows that the 
resonance frequency .omega..sub.r is about 10 times greater than the 
excitation frequency .omega..sub.0. It is emphasized that the predescribed 
resonance frequency does not correspond with the electron plasma frequency 
.omega..sub.pe. 
According to FIG. 1, about 10 superimposed oscillations are performed 
within the period of one oscillation. This result coarsely corresponds 
with the representation in FIG. 2 according to which the resonance 
frequency occurs in the region between the 10th and 11th harmonic 
oscillation. 
The predescribed serf-excitation of the system is distinct in case of a 
strongly unsymmetrical radio frequency discharge. During such a discharge, 
the space charge sheath 33 in front of RF-electrode 10 is the most 
important. Thus, the space charge sheath in front of the earth electrode 
can be neglected. Since the self-excitation extends at least up to the 
10th harmonic oscillation any external excitation of the system is not 
necessary. 
According to the present invention, the radio frequency discharge current 
is measured in the form of analog signals by means of the meter electrode 
21 of the probe 20 at a part of reactor 2 acting as earth electrode. The 
radio frequency discharge current is passed via the conductor 24 to the 
A/D converter 22. Here, the measured analog signals are converted into 
digital signals. The latter are passed via the conductor 25 to the data 
processing device 23. The plasma parameters are evaluated by means of a 
mathematical algorithm from the signals leaving the A/D converter. 
The mathematical algorithm is based on a suitable representation of the 
voltage drop over the space charge sheath in front of the RF electrode 
according to the displacement flux at the surface of the electrode in the 
differential equation describing the system. The latter also takes into 
account the inert mass of the electrons, their collisions with the neutral 
particles of the gas and the ion current to the wall (earth electrode) or 
to the RF electrode. By means of a subsequent Fourier-transform the set of 
parameters comprising the parameters to be determined may be obtained by 
means of algebraic transformations. 
In more detail, the inert mass of the electron can be treated as an 
inductance and the collisions with neutrals, including power absorption in 
the expanding sheath, as a resistance. Finally there is the sheath, which 
acts as a nonlinear capacitance. Therefore the plasma can be regarded as a 
damped serial oscillation circuit. The nonlinear sheath capacitance 
excites the plasma by providing harmonic oscillations to damped 
oscillations close to the geometric resonance frequency, which is below 
the plasma frequency (Langmuir frequency). The relation of the temporal 
derivation of the sheath voltage uV!, sheath width sm! and displacement 
current iA! 
##EQU1## 
indicates the nonlinear properties of the sheath, where A.sub.0 denotes 
the area m.sup.2 ! of the RF electrode. Using a hydrodynamic approach for 
the motion of electrons, the known equation for the permittivity 
.epsilon.As/(Vm)! of the "cold" plasma is 
##EQU2## 
where .epsilon..sub.0 is the permittivity As/(Vm)! of the free space 
(vacuum), .omega..sub.e is the (electron) plasma frequency 1/s!, v is the 
collision frequency 1/s!, e is the elementary charge As!, n is the 
electron plasma density 1/m.sup.3 ! and m.sub.e is the electron mass 
kg!. For vanishing collision frequency .nu. equation (2) is called the 
ECCLES relation. The plasma conductivity can now be written as 
##EQU3## 
and the potential drop of the plasma is 
##EQU4## 
Neglecting the conduction currents of ions and electrons in the sheath, it 
is obtained for the whole discharge driven by the voltage u.sub.rf at the 
RF electrode 
##EQU5## 
which is a nonlinear inhomogeneous differential equation of second order. 
On the right hand side of the above equation, there is a linear 
oscillation term with the geometric resonance frequency 
.omega..sub.p.sup.2 =.omega..sub.c.sup.2 s/1 for a plasma with small 
damping, where 1 denotes the geometric length of the plasma. 
The dependence of the sheath width s on the displacement current involves a 
special approach and usually requires a numerical solution of the 
differential equation (5). 
This differential equation can be interpreted using an equivalent circuit. 
On the one side there is the external excitation--the matchbox including 
the RF generator. The discharge is treated as a damped oscillating circuit 
as suggested by the right hand side of equation (5). 
Finally it rests the nonlinear phenomena, given by the second term on the 
left hand side of equation (5), represented by voltage sources as the 
internal excitation depending on the displacement current and, as a 
further nonlinear effect, the bias voltage. The nonlinearity of the sheath 
given by equation (1) provides harmonic oscillations and the oscillating 
circuit an additional resonance--particularly in the discharge current. 
Using the differential equation and the measured discharge current the 
unknown coefficients can be determined. The electron plasma density, the 
collision frequency, the plasma resistance and the power dissipated in the 
plasma body may be calculated. 
The predescribed at least one resonance frequency may be determined from 
how the measured discharge current is timely developping, the discharge 
being self-excited on said resonance frequency by harmonic oscillations in 
the discharge current which are generated by reason of the nonlinearity of 
the space charge sheath in front of the RF electrode. Several resonance 
frequencies may appear because of two-dimensional components of the 
current conduction (each streamline 31 in FIG. 3 has an axial and a radial 
component) and the nonlinearity of the space charge sheath. Only one 
resonance frequency is obtained at higher pressures. Furthermore, the ion 
density distribution in the harmonic oscillation in front of RF electrode 
may be determined from how the discharge current is timely developping, 
wherein the measured discharge current forms a portion of the entire 
discharge current, said portion being independent of the frequency. This 
frequency-independent portion of the entire discharge current is estimated 
in a first approach by the area ratios of the part of the reactor acting 
as earth electrode and the RF electrode. With respect to the mathematical 
model, his portion may also be estimated by the determination of the 
current dividing factor and the thickness of the electrical space charge 
sheath in front of the RF electrode. Moreover, the damping constant of the 
dampedly oscillating system may be determined from how the discharge 
current is timely developing. Insofar the potential being fed to the RF 
electrode or at least the dc part thereof is additionally measured for 
example at a location 34 outside from reactor 2 in FIG. 3, the absolute 
value of the average thickness of the space charge sheath in front of the 
RF electrode and the average ion energy may additionally be determined. 
By means of the predescribed method, important plasma parameters may be 
determined in absolute values which definitely characterise the operating 
conditions in the reactor. One of the important plasma parameters, the 
electron density averaged over the volume, may easily be determined from 
the electron plasma frequency (also called Langmuir frequency) or directly 
from the geometric resonance frequency. By means of these parameters, 
conclusions may be drawn regarding the state of the plasma and the real 
case of application, for example an etching process of semiconductor 
wafers. Thus, it is possible that unsymmetrical radio frequency 
low-pressure plasmas may reproducibly, i.e. reliably, and quickly be 
characterized. Therefore, an "in-situ" checking of plasma parameters is 
possible by means of the method according to the present invention. Dirt 
accumulations or depositions of layers on the meter electrode below about 
0.1 mm do not cause any worth mentioning errors by reason of the only 
measurement of a radio frequency current. The method according to the 
present invention may also be used in connection with plasmas having a 
combined excitation, i.e. a RF excitation and an excitation by means of 
microwaves since the excitation frequency of the microwaves, about 2.45 
GHz, is normally substantially higher than the predescribed RF excitation. 
In the following, the apparatus according to the present invention is 
described with reference to several preferred embodiments. The apparatus 
according to the present invention is particularly suitable for measuring 
of a radio frequency discharge current at a part of the plasma reactor 
acting as earth electrode. 
An apparatus 35 for measuring of a radio frequency discharge current at 
plasma reactor 2 is for example shown in FIG. 4 to 6 wherein the 
embodiment according to FIG. 6 is also schematically indicated in FIG. 3. 
According to the present invention, the meter electrode 21 comprises an 
electrical insulation 37 on its peripheral surface 36. The insulation 37 
is preferably formed like a ring. The meter electrode 21 is positioned in 
a flange or recess 40 of the reactor wall 5 acting as a part of the earth 
electrode 3 to 7 such that the insulation 37 is at least arranged between 
meter electrode 21 and wall 5 of reactor 2 and that the front face 41 of 
meter electrode 21 is aligned to the inner wall 43 of reactor 2 which 
inner wall is directed to the interior space 42 of the reactor. Thus, 
meter electrode 21 and insulation 37 form a part of the wall of the 
reactor. According to the present invention, the front face 41 of meter 
electrode 21 is dimensioned in such a manner that the RF potential in 
reactor 2 effected by meter electrode 21 is negligible with respect to the 
potential between plasma 17 and earth electrode 3 to 7. 
A first embodiment of apparatus 35 is shown in FIG. 4. Meter electrode 21 
is mounted on wall 5 of reactor 2 by means of an electrically insulated 
vacuum feedthrough 44 which is indicated in FIG. 4 by broken lines. The 
meter electrode 21 comprises a rod-shaped electrical conductor 45 which is 
grounded outside from the reactor 2. The conductor 45 is connected to a 
current transformer 46 which generates a voltage U from the measured 
current I. The current transformer 46 is connected via a conductor 47 
again to ground, i.e. to the wall 5 of the reactor. As described above, 
this voltage U is fed as an input quantity to the A/D converter 22 and 
from there as a digital signal to the data processing device 23 for 
further processing. 
It is an advantage of this embodiment of the present invention that the 
construction is simple and that usual commercial vacuum feedthroughs may 
be used. The apparatus according to the first embodiment comprises a 
relatively high sensitivity because of the great surface of the electrodes 
which are used in this embodiment. The bandwidth of this apparatus is 
limited by the upper limiting frequency of the current transformer and by 
parasitic inductivities of the feed lines as well as by stray capacitances 
to about 150 MHz. 
According to another embodiment (not shown) of the present invention, it is 
further possible to replace the current transformer 46 in FIG. 4 by a 
terminal resistor 50 of preferably 50 Ohm, said resistor being for 
instance shown in FIG. 5. 
It is emphasized that a vacuum is fed to the interior space of the vacuum 
feedthrough 44 and to the interior space 42 of the reactor. These spaces 
are gas tight with respect to the environmental atmosphere. 
A further embodiment of apparatus 35 of the present invention is 
schematically and partly in cross-section shown in FIG. 5 wherein in the 
following identical or similar elements show the same reference numerals 
as in the first embodiment according to FIG. 4. 
Regarding this embodiment, a probe 20 is again provided which is mounted on 
wall 5 of reactor 2 by means of the vacuum feedthrough 44. The probe 20 
comprises an inner conductor 51 connected to the meter electrode 21, an 
insulating body 52 circumferentially surrounding the inner conductor 51, 
the insulation 37 of the meter electrode 21 and an outer conductor 53 
connected to wall 5 of reactor 2. The outer conductor 53 should preferably 
be provided within the flange or recess, but as close as possible to the 
inner side of the reactor wall 5. This probe 20 thus corresponds to a 
coaxial cable having a constant characteristic wave impedance of 
preferably 50 Ohm so that probe 20 has nearly the same construction as the 
above mentioned coaxial cable. The inner conductor 51 passes the current I 
measured by the meter electrode 21 to the real terminal resistor 50. This 
resistor is electrically connected via the conductor 47 to outer conductor 
53 and wall 5 (ground) of the reactor. The current in the inner and outer 
conductor have the same amount. No external magnetic fields thus appear in 
connection with such a probe. 
According to FIG. 5, the probe 20 is connected to a coaxial cable 54. This 
connection is however not shown in detail in FIG. 5 and may for example be 
a so called BNC connector, thus a bushing/connector arrangement 
(male/female connection). It is further possible that the vacuum 
feedthrough 44 only extends up to the outer end of the probe 20 and is 
sealed against the probe. The insulation 37 as well as the insulating body 
52 are preferably made of Teflon (PTFE). 
According to FIG. 3, the probe 20 shown in FIG. 5 and 6 is connected to the 
A/D converter 22 having a thermal resistor 50 of preferably 50 Ohm via the 
coaxial cable 54 comprising the inner conductor 51, the insulating body 52 
and the outer conductor 53. The characteristic wave impedance of said 
coaxial cable corresponds to the characteristic wave impedance of probe 20 
and terminal resistor 50. 
Since apparatus 35 according to the present invention is formed up to the 
actual meter electrode 21 as a coaxial cable (see FIG. 5), a wide 
bandwidth with respect to the frequency of the discharge current to be 
measured may be achieved by this probe. Thus, a transmission of the 
measured signal having a wide bandwidth may be performed free from 
reflections from the meter electrode to for example the entrance of the 
A/D converter 22. If the terminal resistor 50 has for example 50 Ohm, 
which corresponds to the characteristic wave impedance of the cable, the 
input resistance of the probe 20 is small if compared to the reactive 
impedance of the space charge capacity of the meter electrode 21 towards 
the plasma so that the RF potential in the reactor effected by the meter 
electrode is negligibly small if compared to the potential between the 
plasma and the earth electrode. For technical plasmas and excitation 
frequencies used therein below 50 MHz the diameter of the meter electrode 
21 is about 5 mm. 
A further preferred embodiment of apparatus 35 according to the present 
invention is schematically shown partly in cross-section in FIG. 6. 
The vacuum feedthrough 44 shown therein is provided with a cover plate 55 
which is securely connected to a flange-shaped shoulder 60 at the free end 
of the vacuum feedthrough 44 via a sealing ting 57 by means of damps 56 
which are not shown in detail. The cover plate 55 comprises at both sides 
a coaxial connector 61 wherein the one end is preferably formed like a 
connector (male) and the other end is preferably formed like a bushing 
(female). The coaxial connector 61 which is directed to the wall 5 of the 
reactor is connected to a corresponding counter connector 62 which is not 
shown in detail in FIG. 6. The elements of this connection (inner 
conductor 51, insulating body 52, outer conductor 53) correspond to those 
of the embodiment according to FIG. 5. As a result, the inner conductor 51 
is electrically directly connected to the actual meter electrode 21 and 
the outer conductor 53 is electrically connected with the wall 5 of the 
reactor 2 via a wall element 63. The insulting body 52 and the insulation 
37 extend on the one hand between the inner conductor 51 and the meter 
electrode 21 and on the other hand between the outer conductor 53 and the 
wall element 63. The insulating body 52 and the insulation 37 as well as 
the cover plate 55 are preferably made of Teflon. According to FIG. 6, the 
cylindrical portion of the vacuum feedthrough 44 is welded to the side 
wall 5 of the reactor. 
Regarding the last mentioned embodiment of the present invention, the 
discharge current flows from the meter electrode 21 via the inner 
conductor 51 and the conductor 24 shown in FIG. 3 to the A/D converter 22 
and from said converter via a conductor 24 which is also formed as a 
coaxial cable, and the outer conductor 53 as well as the wall element 63 
back to the side wall 5 of the reactor 2 which acts as a part of the earth 
electrode. It is apparent that the sealing ring 57 is formed like a vacuum 
sealing and that an unobjectionable electrical contact exists between the 
circumferential surface of the wall element 63 and the side wall 5 of the 
reactor. 
The excitation in the discharge current may be measured with a wide 
bandwidth up to about 500 MHz by the apparatus according to the last 
mentioned embodiment of the present invention. This measurement is 
relatively independent of the distance between probe 20 and A/D converter 
22.