EP1794600A1

EP1794600A1 - Probe for measuring characteristics of an excitation current of a plasma, and associated plasma reactor

Info

Publication number: EP1794600A1
Application number: EP05789496A
Authority: EP
Inventors: Sébastien DINE; Jacques Jolly; Jean Bernard Pierre Larour
Original assignee: Centre National de la Recherche Scientifique CNRS; Ecole Polytechnique
Current assignee: Centre National de la Recherche Scientifique CNRS; Ecole Polytechnique
Priority date: 2004-09-16
Filing date: 2005-09-15
Publication date: 2007-06-13
Anticipated expiration: 2025-09-15
Also published as: FR2875304A1; JP2008513940A; JP5209313B2; FR2875304B1; US7615985B2; US20070252580A1; WO2006030024A1; EP1794600B1

Abstract

The invention relates to a probe for measuring electrical characteristics of an excitation current of a plasma, said probe being mounted on a conductive line (20) that comprises an inner conductor (21) and an outer conductor (22), comprising a current sensor (41) and a voltage sensor (42), characterized in that: the current sensor comprises: a groove (410) formed in the ground of one of the conductors (22) in order to form a detour for the current flowing through said conductor, and a point for measuring electric voltage between a ground connected to said conductor and a point of the groove, said current sensor thereby being able to measure a voltage (V1) proportional to the first time derivative of intensity (Iplasma) of the excitation current. The voltage sensor (42) is a shunt sensor capable of measuring a voltage (V2) proportional to the first time derivative of the voltage (Vplasma) of the excitation current. The invention also relates to a plasma reactor comprising a probe of the aforementioned type.

Description

Measuring probe for characteristics of a plasma excitation current, and associated plasma reactor

The present invention relates to a device for measuring current and electrical voltage on the power supply circuit of a plasma (this text will be known as a "probe" measuring device).

And the applications of the invention cover all of the plasma-assisted industrial processes implemented inside a plasma reactor. Such methods include (without this list being limiting):

• plasma etching (used in particular in microelectronics or in the field of nanotechnologies),

Plasma-assisted thin film deposition (used for example for the manufacture of liquid crystal flat screens, etc.),

Applications for which the plasma is used as a light source or as a device for treating gaseous effluents for depollution applications or as a thermonuclear fusion reactor, etc. The invention applies to the measuring current and voltage in a plasma reactor using one or more variable voltage or current electrical sources.

For processes as mentioned above, the invention makes it possible to know, in real time and without disturbing the course of the process, the essential electrical characteristics of the plasma (intensity, voltage, but also phase shift between intensity and voltage, etc.). , and thus makes it possible to modify in real time the characteristics of the electric sources used in these processes, in order to modify the characteristics of the plasma. Such a modification in real time can be exploited in particular to perform a real-time servoing using a non-diagnostic disturbance based on electrical measurements; and thus avoid process drifts.

An application of the invention is indeed the control of these methods thanks to the electrical measurements provided by the probe.

Presentation of a plasma reactor

Prior to the description of embodiments of the invention which follows, a few characteristics of a (non-limiting) example of a plasma reactor that may be implemented in the context of the invention will be set forth below.

Plasma reactors can be used to coat a sample with a thin layer of material, to etch a sample by ion bombardment, or more generally to modify the structure or chemical composition of a surface.

A plasma reactor may also be used as a light source or as a waste gas treatment device for pollution control applications or as a thermonuclear fusion reactor. FIG. 1 represents, schematically and in section, an example of a plasma reactor to which the present invention applies. This is, for example, a so-called radiofrequency excitation (RF) reactor by capacitive or inductive coupling.

Such a reactor comprises a vacuum chamber 53. Near a first wall 54 of this chamber is placed on a substrate holder 55, a sample 56 to be treated.

The sample 56 is generally in the form of a disc whose surface 57 directed towards the inside of the enclosure 53 constitutes the surface to be treated. The enclosure 53 is filled with a gas at low pressure, for example of the order of a few tens to a few hundred millitorrs (a few tens to a few hundred pascals). The gas comes from a source 57 to be injected into the enclosure of the reactor through a gas supply pipe 58, the gas flow rate being regulated by a flowmeter 59.

When a gas mixture is used, several sources, flow meters and supply pipes are used in parallel. The gas is evacuated from the enclosure 53 by a discharge pipe 60 connected to a pumping system 61 consisting of one or more vacuum pumps in series. The pumping volumetric flow rate is adjusted by means of a valve 62.

The pressure in the chamber is controlled with the valve 62 and / or the flowmeter 59.

A plasma reactor can also operate at atmospheric pressure or in a coarse vacuum (gas pressure between one-tenth atmosphere and one atmosphere). The treatment of gaseous effluents for depollution applications is often carried out at these pressures.

This is also the case for the large-area continuous treatment such as the deposition of thin layers on the glazing or the cleaning of the steel plates at the output of the rolling mill.

Several means can be used to generate the plasma 63. For example, in a so-called "capacitive coupling reactive ion etching" configuration, a radiofrequency voltage is applied to the substrate holder. It is also possible, as shown in FIG. 1, to generate the plasma 63 by means of a source 64 independent of the substrate holder 55.

This source 64 may be associated with a generator 65, for example for the following source types:

• electrode powered by a high frequency generator (capacitive source),

• electrode powered by a low frequency generator, Electrode powered by voltage pulses delivered by a pulsed generator,

• coil powered by a radiofrequency generator (inductive source),

• microwave generator. The last two types of sources, inductive and microwave, may possibly be associated with an application of a static magnetic field. In the case of the use of a source independent of the substrate holder, the latter may be biased by a radiofrequency source 66 to establish self-polarization and thus increase the impact energy of the ions on the surface to be treated.

When the plasma source is a radiofrequency source, the latter may optionally be polarized at a higher frequency than that applied to the substrate holder 55 in order to control the electronic density preferentially. When the plasma source is a radiofrequency source (HF, VHF or microwire), an impedance matching circuit 67 (or matching circuit) is arranged between the generator 65 and the plasma source 64. This circuit is connected to the generator 65 by a transmission line 68, generally coaxial with characteristic impedance equal to 50 ohms. An impedance matching circuit is used to prevent the reflection of electromagnetic energy towards the source. This allows on the one hand to protect the source and on the other hand to optimize the transfer of power to the plasma. This circuit modifies the electrical impedance of the plasma source in order to make it equal to the characteristic impedance of the line 68. The transmission line 68 is said to be adapted. The tuning circuit 67 is connected to the plasma source 64 by a coaxial or radial transmission line 69. This line is mismatched because the impedance of the plasma source is not equal to the characteristic impedance of line 69.

When the substrate holder is powered by a radiofrequency source, a tuning circuit 70 is interposed between the substrate holder and the source. The latter is connected to the tuning circuit by a line of adapted coaxial impedance transmitting transmission 71 characteristic generally equal to 50 ohms. The output of the impedance circuit 70 is connected to the substrate holder by a mismatched radial or coaxial transmission line 72. Plasma processes using a radiofrequency source most often use a frequency in the high frequency domain (HF band: 3 MHz - 30 MHz). In this range, the 13.56 MHz frequency is currently the most used.

Plasmas concerned by the present invention include chemically reactive plasmas (and in which chemical reactivity in addition to ion bombardment may be used).

The only reactivity of the gas or gas mixture injected into the chamber is sometimes the only phenomenon exploited. But in general this reactivity is improved or even generated by collisions of electrons on neutral atoms or molecules thus producing radicals, unstable chemical species absent in the gas without the presence of electrons. These radicals as well as the reactive ions are responsible for the deposition or etching. In the case of deposition, this is known as plasma-enhanced chemical vapor deposition. This reactivity initiated by the electrons avoids the need for a significant heating of the gas or substrate holder, which would damage the sample to be treated.

The production rate of radicals produced by electronic collisions is a function of electronic concentration. Similarly, the flow of charged particles (electrons and ions) that arrive and leave the surface to be treated is proportional to the electronic concentration. Chemical reactivity and ion bombardment generally act synergistically in these plasmas.

However, the electron concentration and the ion flux are proportional to the electric current in the plasma. The flow of ions and the energy of the ions bombarding the surface to be treated are proportional to the voltage applied to the substrate holder 55 or the electrode 64 in the case of a capacitively coupled source.

In a process of plasma deposition or etching, it is important to know the characteristics of the plasma to be able to control the implementation of the process and its reproducibility, in particular to control the speed of deposition or etching as a function of the thickness. deposition or depth of the desired etching.

After deposition or etching all surfaces exposed to the plasma (electrodes, walls) are covered with a deposit that must be removed in order to treat a new sample. This cleaning step is often performed using a plasma which is used chemical reactivity and ion bombardment.

The measurement of the current flowing in the plasma or of the voltage applied on the electrodes 55 or 64 is therefore a means of controlling characteristics of the plasma without disturbing it. This measurement is performed during the process or during the cleaning and is preferably located on mismatched transmission lines 69 and 72 in order to be carried out as close as possible to the plasma. The measurement probe may also be located on the appropriate transmission lines 68 and 71 to measure the quality of the impedance matching. And this, to possibly modify the characteristics of the impedance matching circuits 67 and 70 and to improve the level of adaptation on the lines 68 and 71.

The measurement of the current and the voltage can be associated with a device responsible for measuring the temporal phase shift between the current and the voltage in order to deduce the power dissipated in the plasma and the impedance of the plasma. These last two parameters as well as the amplitudes of the voltage and the current are useful to control the smooth running of these processes and the plasma cleaning steps of the reactors. They can optionally control a servo to avoid process drifts. The quality of the control is strongly Dependent on the performance of the probe used to measure current and voltage.

It is specified that the invention applies more particularly to plasmas excited by a source of current or variable voltage, such as a sinusoidal voltage generator or pulse voltage.

And more specifically, the invention will find particularly advantageous applications in such excited plasmas with a sinusoidal radiofrequency voltage of frequency between 1 MHz and 1GHz. The electrical impedance of a plasma depends on the current flowing in the plasma: it is called non-linear. One of the consequences of this non-linearity is that a plasma excited by an alternating voltage source of frequency f generates harmonics of this excitation voltage at the frequencies of f. For example, for a plasma generated by a sinusoidal voltage at 13.56 MHz, sinusoidal components at 27.12 MHz, 40.68 MHz, 54.24 MHz ... appear in the measured voltage and current signals).

During an industrial process such as those mentioned above, the measurement of the temporal evolution of the amplitude of these harmonics, in addition to the amplitude of the fundamental frequency during an industrial process is of a great utility.

Such a measurement can in particular be used to detect the end of the plasma etching of a dielectric layer on a microprocessor during manufacture. It is specified that the amplitudes of these harmonics at 2f, 3f, 4f ... are much smaller than the amplitude of the fundamental component f, and that it is therefore necessary to be able to isolate them from this fundamental component by filtering.

In addition, plasma processes using an RF frequency above 13.56 MHz, and in particular in the very high frequencies (especially VHF band: 30 MHz - 300 MHz) are more and more common.

At such frequencies, the voltage and current probes must operate in a very wide frequency range because the frequency difference between each harmonic of the fundamental frequency component is higher than in the case where the fundamental frequency used is higher. low (13.56 MHz for example).

Most existing probes designed to operate at 13.56 MHz are no longer usable in VHF. And it would be advantageous to have a probe capable of operating in a wide frequency range.

In addition, the size of plasma etching and deposition reactors used in industry also tends to increase in order to process a larger number of devices at one time. These large reactors require the use of higher RF power. The currents and RF voltages to be measured therefore increase too.

However, the risks of overheating, short circuit and electrical breakdown increase for currents and higher voltages. And it would be beneficial to reduce these risks, especially to measure large currents and tensions.

As discussed above, it is often desired to measure the current and voltage on the electrical power supply circuit of the plasma. It is also often desired to determine the temporal phase shift between the current and the voltage in order to deduce the power dissipated in the plasma and the impedance thereof.

The quality of the phase shift measurement is strongly dependent on the performance of the sensor used to measure the current and the voltage. This measurement must be precise because the phase shift variations are often very small.

However, it can be seen with the known voltage and current probes that the phase difference measured between the current and the voltage is tainted by an error (this error being generally greater when the current and voltage sensors of the probe are distant from each other). It would naturally be desirable to overcome this type of error.

The solution would be to bring the current and voltage sensors of a state-of-the-art probe (such as that shown in FIG. 2) to the same level in an attempt to overcome this type of problem. This error would increase the risk of mutual disruption and lead to a deterioration of the frequency response. The range of frequency of use of the probe would be reduced, It is therefore necessary with this type of known probe to find a compromise between the risk of mutual disruption, the degradation of the measured phase shift and the frequency range of use.

As mentioned above, there are already probes for measuring the current and voltage delivered to a plasma. FIG. 2 thus exposes in longitudinal section a probe 10 mounted on an electrically conductive coaxial transmission line 20 which includes an inner conductor 21 and an outer conductor 22 which surrounds the inner conductor

The coaxial line 20 is connected: by its two conductors, to an impedance matching circuit (not shown in the figure) also connected to an alternating RF voltage source (or RF generator) which excites the plasma (connection by the part of the line at the top of the figure), By its internal conductor, to a radiofrequency electrode (RF) 31 in the form of a solid disc - only the section of this disc appearing in the figure (connection by the part of the line which is at the bottom of the figure),

By its external conductor to a conductive cover 32 which is also in the form of a disc and extends facing and away from the electrode 31 to define between the electrode and the cover a space 30. The cover 32 is also electrically driver.

The coaxial line 20 described above corresponds, for example, to line 69 or line 72 of FIG. 1. The radiofrequency electrode 31 corresponds, for example, to the substrate holder 55 or to the plasma source 64 of this same FIG. cover 32 corresponds for example to the enclosure

53 or the wall 54 of the vacuum chamber of this figure.

Between the RF generator and the tuning circuit, the line is said to be adapted. Between the tuning circuit and the plasma, the line is called mismatched.

The space between the inner conductor and the outer conductor is electrically insulating - it can be evacuated or filled with a dielectric material.

The line is traversed by currents moving in opposite directions along the core 21 and the envelope 22. These currents are generated by the AC voltage source which excites the plasma via the RF electrode 31 which is in contact with the plasma.

These currents cancel each other out and change their direction - while remaining in opposite directions to each other - twice per AC voltage cycle.

It is specified that under the skin effect the high frequency currents (the "high frequencies" being included in the context of this text as frequencies greater than 1 MHz) propagate on the surface of the conductive elements that it crosses (soul 21, envelope 22, electrode 31, cover 32 ...) and opposite, that is to say outside the core 21 and outside the envelope 22.

The probe 10 comprises means 11 for measuring the voltage between the current flowing through the line 10 and a ground connected to the outer conductor 22, and means 12 for measuring the intensity of this current.

The voltage measuring means 11 comprise:

A conductive disk 110 disposed near the inner conductor 21 and connected to a conductive cable 111 which passes through the outer conductor 22, and a second conductive cable 112, connected to the outer conductor 22.

The measurement of the voltage V2 between the two cables 111 and 112 thus corresponds normally to the voltage that it is desired to measure.

However, a voltage measured between these two cables has certain limitations: on the one hand, the response of such a voltage probe is restricted in frequency,

On the other hand the operation of the transmission line 20 is disturbed by the proximity of the disc 110 with the inner conductor 21,

And finally the line 20 is partially short-circuited by the conductor 110, which can cause a breakdown, thus restricting the measurable voltage range.

The current measurement means 12 comprise a conductive loop 121 (or several loops in series) disposed near the inner conductor 21, and one end of which is connected to ground (connection to the outer conductor 22).

The inner conductor is traversed by the sinusoidal current I _p ι _aS m _has that is to be measured.

This current induces a sinusoidal and azimuth magnetic field B, which induces a voltage (or electromotive force) between the ends of the This is an indirect technique for measuring current since it uses the magnetic field induced by the current to be measured.

The potential difference V1 measured between the ground and the end 1210 of the loop which is not grounded is in principle proportional to the first derivative of the current I _p ι _aS ma on the line

In practice, however, the loop 121 is also capacitively coupled to the central conductor, which has the consequence of adding to the voltage measured across the loop a voltage which is proportional to the voltage V _p ι _aS ma between the two conductors of the line 20.

This constitutes an additional voltage component which makes the measurement of the current less accurate, and also disturbs the measurement of the temporal phase shift between the current and the voltage.

The loop 121 disturbs the line 20 because it forms a partial short circuit between the two conductors 21 and 22, which can cause a breakdown. In practice, the use of such a loop is thus limited to powers less than 10 kW.

In addition, because of the large size of the loop it is also difficult to have a sensor of the voltage V2 nearby without the current and voltage sensors mutually disturb. It is then necessary to move these two sensors away from each other - which introduces an error in the measurement of the phase difference between the current and the voltage.

It will be added that it is necessary to calibrate very finely such a known probe, taking into account the characteristics (geometry, size, ..) of the loop 121.

It is specified that all the probes currently existing in the targeted field of application use variants of the probe described above. Moreover, these probes all implement a measurement of the indirect current because they use the magnetic field induced by the currents flowing in the line 20.

The various known probe variants can make it possible to overcome one or more of the limitations that have been exposed, without ever overcoming them all. For example, the probes described in the documents US Pat. No. 5,834,931, US 5,808415 and US Pat.

It thus appears that the existing probes and intended to measure in real time the current and the voltage delivered by an RF generator to a plasma have limitations.

An object of the invention is to overcome these limitations.

Another object of the invention is to allow simultaneous and accurate measurement of current and voltage at very close points. Yet another object of the invention is to make it possible to carry out such measurements over a wide range of power.

Yet another object of the invention is to make it possible to perform such measurements over a wide range of frequencies.

In order to achieve these aims, the invention proposes, in a first aspect, a probe for measuring the electrical characteristics of a plasma excitation current, said probe being mounted on a conductive line which comprises an internal conductor and a conductor external device, comprising a current sensor and a voltage sensor, characterized in that: • the current sensor comprises:

a groove formed in the mass of one of the conductors in order to form a detour for the current traversing said conductor, and a point of measurement of electrical voltage between a mass connected to said conductor and a point of said groove, said current sensor being thus able to measure a voltage proportional to the first time derivative of the intensity of said excitation current

• The voltage sensor is a drift sensor, able to measure a voltage proportional to the first time derivative of the voltage of said excitation current.

Preferred but non-limiting aspects of the probe of the invention are the following:

The excitation current is an alternating current RF,

• the throat defines a detour a length of the order of a centimeter,

Said current sensor and said voltage sensor are both located on the outer conductor. Said voltage sensor is a sensor comprising a conical transmission line, terminated by a slightly curved surface capacitively coupled to the conductor other than that on which said voltage sensor is mounted,

The coupling capacitance between said curved surface and said conductor other than that on which said voltage sensor is mounted is of the order of 0.3 pF,

Said current sensor and said voltage sensor are located at the same level on the current path on the surface of said conductor,

Said conductive line is a coaxial cylindrical line, said conductive line is a radial cylindrical line, and

The probe comprises means for measuring the temporal phase shift between the current and the voltage of said excitation current. According to a second aspect, the invention also proposes a plasma reactor comprising an RF generator and a probe as mentioned above.

Preferred but non-limiting aspects of the reactor according to the invention are the following:

The probe is located between an impedance matching circuit connected to said RF generator and a plasma excitation RF electrode, and

The probe is located between said RF generator and a tuning box, on a so-called adapted line. Other aspects, objects and advantages of the invention will appear better on reading the following description, made with reference to the appended drawings, in which, in addition to FIGS. 1 and 2, which have already been commented on with reference to the state of the invention. Technical

FIG. 3 is a block diagram of a current and voltage measuring probe according to the invention,

FIG. 4 is a representation of an electrical diagram equivalent to this probe according to the invention,

FIGS. 5a to 5d are views of a practical embodiment of a probe according to the invention; FIG. 6 illustrates the character proportional to the frequency f of the current and voltage measured by a probe according to the invention;

FIG. 7 illustrates an embodiment of the invention in which a probe according to the invention is implanted in a radial line.

Referring to Figure 3, there is shown schematically a probe according to the invention.

Here again, the probe is mounted between an RF electrode and an impedance matching circuit connected to an RF generator (not shown). As explained above, an impedance matching circuit can indeed be used in plasma processes in particular in order to optimize the transfer to the plasma of the power delivered by the RF generator. It is specified that the elements already commented on the known probe of Figure 2 will be referenced in the same way with reference to this Figure 3 (without being introduced again).

We thus find on this figure:

A coaxial transmission conductor line 20 which comprises an inner conductor 21 and an outer conductor 22,

An RF electrode 31 in the form of a disk, and an associated lid 32.

However, it is specified that the probe according to the invention can be mounted differently - we will return to this aspect.

There is also a current sensor (here 41) and a voltage sensor (here 42). These sensors are specific to the invention.

Note that these two sensors are arranged extremely close to each other.

The probe according to the invention is indeed intended to measure at extremely close points, simultaneously, instantaneous current and voltage, especially in plasmas using electrical power in the field of radio frequency (RF).

This measurement is carried out at a point on the transmission lines responsible for transporting the electric power, delivered by an RF generator, to the enclosure in which the plasma is confined. The invention will in particular be implemented advantageously on so-called mismatched transmission lines. The two sensors 41, 42 are thus inserted in series in a section of the outer conductor 22, being separated only by a distance of the order of 5 millimeters.

Such spacing is considered in the sense of the present invention as negligible, and it will therefore be considered that the two sensors are located at the same level on the path of the current on the surface of the conductor 22. This can also be expressed by saying that the two sensors 41 and 42 are implanted in a plane (Z constant), with the dimension Z defined by the axis A which is parallel to the conductors 21 and 22.

Line 20 may be a cylindrical coaxial line, or any type of coaxial line in which an inner conductor is surrounded by an outer conductor.

The outer conductor 22 is connected to the electrical ground of the system.

An RF voltage V _p ι _aS ma is applied at the output of the tuning circuit, between the internal and external conductor, at the input of this line section (that is to say in its upper part on the representation of FIG. 3).

The resulting alternating RF current passes completely or partially through the plasma (shown under the electrode 31) and returns through the external conductor.

As has been mentioned earlier in this text, in the field of HF and higher frequencies the current flows on the surface of the conductors to a depth of a few micrometers. The current thus flows on the surface of the central conductor and on the inner face of the outer conductor.

We will now detail the structure of the sensors 41 and 42.

With regard first of all to the sensor 41, a groove 410 is hollowed out in the internal face of the outer conductor 22 in order to make RF current of skin an additional path (of the order of one centimeter long). The current path on the walls of this groove is illustrated by arrows.

The groove is symmetrical about the central axis A of the line 20. It follows a geometry of revolution relative to this axis.

Means for measuring a voltage V1 are associated with this groove.

These means measure the potential difference V1 between two points located on the detour formed by the throat. Figure 4 gives the equivalent electrical diagram of the probe.

The detour of the groove 410 behaves like a low-value inductance L _m (of the order of nanohenry, which is not significant) for comparison the simple inductance of the conductors 21 and

22 is typically a few tens of nanohenry per meter) placed in series on the current path.

The presence of this detour does not significantly modify the properties of this line.

In the diagram of FIG. 4, the measurement of the voltage V1 amounts to measuring the voltage across a portion (L _m ) of the total inductance Ltot.

And the voltage across the inductance L _m is equal to the first time derivative of the current I _p ι _a sma which passes through it. As this current is sinusoidal, the amplitude of the measured voltage is therefore proportional

To measure V1, a high frequency coaxial base 411 of the SMA type (50 ohms) is driven from the outside into an orifice in the wall of the conductor 22 which opens into the groove (see FIG. 5d). . This base 411 has a screw connector for connecting a conventional coaxial cable (50 ohms) to carry the measured signal to a display device (oscilloscope ...) or acquisition (analog-to-digital conversion card) ). The current sensor 41 is a sensor called "differentiator". The measured signal (V1 (t)) at the output of this sensor is shifted by + ² / / with respect to the signal (l _p i _s sma (t)) that one seeks to measure.

And the voltage sensor 42 is also a divider, which makes it possible to use the probe for measuring phase-shifts between the current and the voltage: with a voltage sensor 42 measuring a phase-shifted voltage of + ^7% with respect to the voltage V _p ι _a sma, a phase shift is obtained between the measured signals V1 and V2 which is identical to the phase shift between the current (l _p ι _aS ma) and the voltage (V _p ι _aS ma) on the coaxial line. The invention thus preferably uses a voltage sensor 42 comprising a so-called conical transmission line 420 terminated by a slightly convex surface 421 in "capacitive" coupling with the inner conductor 21. The coupling capacitance between the surface 421 and the conductor internal is of the order of 0.3 pF. In practice, the critical dimensions of the elements forming the probe (diameter of the conductors, spacing between inner and outer conductor, spacing between the two sensors of the probe, ...) will be adapted according to the operating parameters of this probe (range of value voltages that we want to measure, accuracy that we want to obtain on the current / voltage phase shift, frequency at which we work, ...). In any case care should be taken to ensure that the internal and external conductors are sufficiently spaced to prevent breakdown. In one embodiment, the dimensions of the conical line are chosen so that its characteristic impedance is equal to 50 ohms - which makes it possible to connect this conical line to a coaxial transmission line constructed from an SMA base identical to that used for the current sensor 41.

And here again, the output of the voltage sensor can be connected with a coaxial cable to a display and acquisition device.

The conical line of the sensor 42 allows:

• to guarantee a derivative operation of the probe in a wide frequency range,

• while keeping the voltage sensor away from the RF high voltage.

The conical line is partially embedded in the conductor 22 which is grounded (see Figure 5c).

It will be noted that if the conical lines are known as such, they have hitherto been used for the measurement of very specific currents (transient currents of several mega-amperes during pulses of a hundred nanoseconds duration) which are quite different from those used in the present invention. In addition, placing the current sensor on the return conductor by mass goes against the normal approach of the skilled person. The external conductor is considered as a simple shield blocking the electromagnetic radiation emitted by the inner conductor, and not as a conductor carrying the return electric current that can be operated.

Thus, in the context of the present invention: • contrary to what is usually used in RF metrology, the current is measured directly. This is measured for the voltage V1 which appears at the terminals of a detour in which the RF current is forced to pass after having completely or partially traversed the plasma, and the voltage measurement is carried out by a capacitively coupled voltage probe. extended by a conical line. The capacitive coupling voltage probe, commonly used in RF metrology, is used here with a tapered line that guarantees a derivative operation of the probe in a wide frequency range while keeping the voltage sensor away from the high RF voltage.

The electrical circuit equivalent to the conical line voltage sensor is shown in Figure 4. Without the use of a conical line, there would be a capacitor in parallel between the sensor and ground. This is the case for conventional voltage sensors. The presence of this additional component alters the frequency response of the sensor. In particular, it reduces the frequency range for which its response is derived.

The advantage of a conical line is that it ensures a continuous transition between the curved sensor and the charged cylindrical coaxial line conveying the voltage to be measured to a display and acquisition device. This is to integrate this parasitic capacitor in those normally present between the two conductors of a coaxial line so that it does not alter the response of the probe. In the embodiment illustrated in FIGS. 5a to 5c, the probe comprises two main tubular elements 4100, 4200 which are intended to be aligned and assembled, each of these two elements being associated with a respective sensor of the probe (sensor 41 for element 4100, sensor 42 for element 4200). And in this embodiment, the element 4200 serves to close the groove of the current probe (see FIG. 5b), with the two sensors 41,

42 located closest to the plane of contact between the two elements 4100,

4200. The two probes are thus arranged as close as possible to one another (see Figure 5a).

The sensor prototype shown in FIGS. 5a to 5c is of generally cylindrical shape.

Its length is five centimeters and a diameter of 4.5 centimeters. It consists essentially of brass. This is a "repositionable" type probe because it has at its ends coaxial screw connectors. These are for example N or HN type to ensure good shielding and carry high power. These connectors are modifiable in order to adapt to the types of connectors (size and type) used on the transmission line on which the electrical measurements are to be made. Figure 5a shows a probe mounted with male coaxial connectors of type HN. Figure 5b shows a disassembled probe with coaxial connectors N of female type.

The invention can also be arranged on a transmission line permanently (without screw connectors) as illustrated by the diagram of FIG. 3.

The transmission line on which the sensor is inserted is not necessarily cylindrical and coaxial. It can be a coaxial line with square or rectangular section. More generally, the line must have two conductors, one of which encloses the other and propagating mainly a type of electromagnetic mode "TEM" (transverse electrical and magnetic).

The line in which the sensor is implanted may also be a radial line such as that constituted by the RF electrode 31 and the cover 32 in the form of a concentric ring. In such a case, current-diverting groove can be made in the wall of the cover which is opposite the RF electrode. An example of implantation of the invention in a radial line is shown in FIG.

Since the sensor does not disturb the line, it can be arranged on a suitable transmission line without any risk of being unadapted, as for example on the lines 68 and 71 of FIG. 1 situated between the RF power generator and the tuning circuit. in impedance.

Prior to any metrological use, the sensor has been calibrated (or characterized). Figure 6 shows an example of the results of this calibration. This figure indeed exposes from the measurements made for VI and V2:

• V1 / Ipiasma (curve 51), and

• V2 / Vpiasma (curve 52).

It can be seen that these two curves plotted as a function of the RF frequency are close to straight lines, which indicates that the sensors are indeed differentiators (linear response with the frequency).

In the example illustrated here, this behavior of linear variation with the frequency is particularly observable for frequencies up to 500 MHz. Since the industrial processes targeted by the invention use a fundamental frequency (frequency of operation of the RF generator) of less than 100 MHz, the probe whose calibration is illustrated in FIG. 6 can therefore be used to measure the amplitude of at least four of the first harmonics of current and voltage in these industrial processes.

The measured voltage (V2) is thus proportional to the voltage to be measured (V _p ι _aS ma _> that can be noted V ₀ ) with a multiplicative factor (fVo) proportional to the frequency of the signal that is to be measured (and it is the same for the current).

It will be noted that the probe according to the invention is particularly simple to construct. The prototype shown in FIGS. 5a to 5d, and whose calibration curves are shown in FIG. 6, only required the machining of four metal parts, the use of twelve screws for assembly and purchase four coaxial connectors.

The machining of the parts has been done without difficulty using standard machine tool machines (a machining tolerance of the order of a tenth of a millimeter is sufficient). Finally the brass used to make the pieces is an inexpensive material.

Another advantage of the probe lies in its simple geometry. This geometry has the advantage of being easily modeled using an analytical calculation. It is thus not necessary to manufacture a large number of prototypes or to resort to complex computer modeling to design and dimension a probe according to the present invention.

The probe according to the invention also has a great compactness (use of compact sensors themselves embedded in a conductor connected to the electrical ground). And it is possible to have these sensors close to each other without disturbing them.

The probe according to the invention is moreover able to operate over large frequency ranges (typically between 1 MHz and 1 GHz), and is thus not subject to the frequency range limitation of known probes. Another advantageous aspect of the invention resides in the fact that, on the one hand, the measurement of current is direct since it does not use the magnetic field induced by the current to be measured, and that on the other hand the throat ensures its own shielding against magnetic fields external variables. Even in the presence of such fields, the output voltage of the current sensor is not parasitized.

The linear frequency response favors the high frequencies on the low frequencies in the signal to be measured. This has two advantages:

• on the one hand, this renders the probe insensitive to the presence of low frequency components (<100 kHz) due to instabilities in the plasma,

• and on the other hand it favors the measurement of harmonics whose amplitude is always weaker than that of the fundamental one: it is a "compensation in frequency".

It should be noted that the connection inversion of the probe does not change the voltage measurement but changes the sign of the measurement of I (phase shift of -π). The invention uses non-intrusive sensors embedded, or partially embedded, in a conductor connected to the electrical earth. This feature greatly reduces the risk of electrical breakdown (short circuit) caused by the presence of sensors.

The probe object of the present invention can therefore measure voltages and currents much higher than conventional devices.

Finally, it will be added that the "direct" current and voltage measurements being proportional to the frequency (modulo l _p ι _a sma and V _p ι _aS ma) make it easier to use the probe according to the invention to high frequencies for reliable measurements - this advantage being reinforced by the fact that plasma processes are currently evolving towards higher and higher frequencies.

Claims

1. A probe for measuring electrical characteristics of a plasma excitation current, said probe being mounted on a conductive line (20) comprising an inner conductor (21) and an outer conductor (22), comprising a sensor current sensor (41) and a voltage sensor (42), characterized in that:

The current sensor comprises:

a groove (410) formed in the mass of one of the conductors (22) to form a detour for the current flowing through said conductor,

and a point of measurement of electrical voltage between a mass connected to said conductor and a point of said groove, said current sensor being thus able to measure a voltage (V1) proportional to the first time derivative of the intensity

(I plasma) of said excitation current

• the voltage sensor (42) is a drift sensor, able to measure a voltage (V2) proportional to the first time derivative of the voltage (V _p ι _aS ma) of said excitation current.

2. Probe according to the preceding claim, characterized in that the excitation current is an alternating current RF.

3. Probe according to one of the preceding claims, characterized in that the groove defines a detour of a length of the order of one centimeter.

4. Probe according to one of the preceding claims, characterized in that said current sensor (41) and said voltage sensor (42) are both implanted on the outer conductor.

5. Probe according to one of the preceding claims, characterized in that said voltage sensor (42) is a sensor comprising a conical transmission line (420) terminated by a slightly curved surface (421) capacitively coupled to the conductor (21). ) other than that on which said voltage sensor is mounted.

6. Probe according to the preceding claim characterized in that the coupling capacitance between said curved surface (421) and said conductor (21) other than that on which said voltage sensor is mounted is of the order of 0.3 pF.

7. Probe according to one of the preceding claims, characterized in that said current sensor (41) and said voltage sensor (42) are located at the same level on the current path to the surface of said conductor.

8. Probe according to one of the preceding claims, characterized in that said conductive line (20) is a coaxial cylindrical line.

9. Probe according to one of claims 1 to 7, characterized in that said conductive line is a radial cylindrical line.

10. Probe according to one of the preceding claims, characterized in that the probe comprises means for measuring the phase shift between the current and the voltage of said excitation current.

11. Plasma reactor comprising an RF generator characterized in that it comprises a probe according to one of the preceding claims.

12. Reactor according to the preceding claim characterized in that the probe is located between an impedance matching circuit connected to said RF generator and a plasma excitation electrode (31) RF.

13. Reactor according to claim 11 characterized in that the probe is located between said RF generator and a tuning box, on a so-called adapted line.