HK1233321B - Device for forming a quasi-neutral beam of oppositely charged particles - Google Patents

Description

The invention relates to a device for forming a quasi-neutral beam of particles with opposite charges.

Such devices are particularly used for plasma thrusters (applications to satellites for trajectory correction, space probes, etc.), particle deposition devices on a target (vapor phase deposition, for example; field of microelectronics), target etching devices, polymer treatment devices, or target surface activation devices.

Typically, such a device includes a chamber, means for introducing an ionizable gas into the chamber, means for ionizing the gas to form a plasma, and means for extracting and accelerating charged particles of the plasma out of the chamber.

In the field of electric propulsion, there are different techniques to ensure the acceleration of the vehicle equipped with a plasma beam generator device, which is then considered as a plasma thruster.

Thus, for a plasma thruster in which the plasma is composed of positive ions and electrons, it is possible to extract and accelerate only the positive ions out of the chamber, and to ensure the electroneutrality of the positive ion beam after exiting the chamber by injecting electrons downstream of the chamber exit.

Ensuring the beam's electroneutrality at the chamber exit is indeed essential to prevent spacecraft from becoming electrically charged, as the ion beam current is not limited by space charge.

However, this type of plasma thruster has the disadvantage of requiring an additional electron source to ensure electroneutrality, an additional source that is generally the origin of a lack of reliability.

To ensure this electroneutrality by increasing reliability (thus freeing from the auxiliary electron source), several approaches have been considered.

A first approach is to produce a plasma containing positive ions, negative ions, and electrons, and to filter the electrons within the chamber so that only or almost only positive and negative ions emerge from the chamber outlet. The particles of opposite charges in the beam are thus formed by positive and negative ions.

A second approach is to produce a plasma containing positive ions and electrons, and to provide means for extracting and accelerating the positive ions and electrons at the exit of the chamber in order to ensure electroneutrality. The charged particles of the beam are thus composed of positive ions and electrons.

Solutions corresponding to the first approach described above are proposed in documents WO 2007/065915, WO 2010/060887 or WO 2012/042143.

All these solutions must implement an electronegative ionizable gas capable of generating positive ions, negative ions, and electrons, as well as a means for filtering electrons, in order to obtain, at the output of the chamber, only or practically only, positive and negative ions.

In document WO 2007/065915, two grids 3, 4 in contact with the plasma are used as means for extraction and acceleration, and they are located at the outlet of the chamber in the same plane (one on top, the other at the bottom), one being negatively biased and the other positively biased.

Figure 1 is a schematic representation of the device proposed in WO 2007/065915. In this figure, chamber 1 contains a plasma consisting of positive ions A+, negative ions A-, and electrons e-. The electron filtering means is referenced as 2.

Thus, a simultaneous extraction and acceleration of positive and negative ions is achieved, ensuring the electrical neutrality of the ion beam after exiting the chamber.

However, this solution is difficult to implement due to the presence of grids with opposite polarizations. Indeed, the presence of these grids with opposite polarizations can lead to significant beam curvatures coming from each grid.

The document WO 2010/060887 proposes an improved solution compared to that of document WO 2007/065915, which provides for two different gases instead of one gas as in WO 2007/065915. One of these gases is electronegative, while the other can be either electropositive or electronegative.

In document WO 2012/042143, it is proposed to implement a 5th extraction and acceleration grid powered by an alternating positive and negative voltage source, through the voltage source 6. This grid 5 is associated with another grid 7 which is grounded at 8.

When a positive voltage is applied to grid 5, the plasma potential becomes positive, and consequently, the positive ions A+ are accelerated toward the other grid 7, which is at ground potential. Indeed, under these conditions, a positive sheath forms around grids 5 and 7, allowing the acceleration of positive ions. The sheath is an area formed between each grid 5, 7 and the plasma, where the density of positive ions differs from that of negative ions. Under these conditions, the extraction and acceleration of negative ions are blocked.

Then, when a negative voltage is applied to grid 5, the plasma potential becomes negative and the negative ions A− are accelerated towards the other grid 7. More specifically, after applying a positive voltage to grid 5, the positive sheath disappears rapidly (about 1 microsecond) and a negative sheath forms under the effect of the negative polarization of this grid 5. Under these conditions, the extraction and acceleration of positive ions are blocked.

Depending on the polarization of grid 5, it is thus possible to accelerate and extract either positive ions or negative ions.

A representative diagram of the device proposed in document WO 2012/042143 is shown in Figure 2(a). The electron-negative gas is denoted as A2 and the electron filtering means as 2. RF' here denotes the means that allows generating the plasma from the electron-negative gas A2 injected into chamber 1. It is an alternating sinusoidal magnetic field source operating in the radiofrequency range.

Here, there is no disadvantage of having two opposite polarization grids, as in documents WO 2007/065915 and WO 2010/060887. However, since positive ions A+ and negative ions A− are extracted successively, it is proposed to optimize the shape of the voltage signal generated by the alternating voltage source 6 connected to grid 5, in order to best ensure the charge neutrality of the ion beam exiting the chamber. This alternating voltage source 6 can take advantage of measurements from a probe S in the output beam and/or the RF signal' used to generate the plasma.

This optimized signal is shown in Figure 2(b).

With this optimized signal, a good beam electroneutrality is achieved at the output of chamber 1, but only on average.

Indeed, successively extracting positive ions and then negative ions, and vice versa, does not always result in a consistently neutral beam. As a result, the thruster's potential varies over time, depending on the shape of the signal shown in Figure 2(b).

Furthermore, it should be noted that all ion-ion extraction devices use an electronegative gas, which is generally highly reactive (presence of fluorine, chlorine, etc.), thus limiting the device's lifetime.

Moreover, the solutions proposed in documents WO 2007/065915 (Figure 1), WO 2010/060887, and Figures 2(a) and 2(b) (WO 2012/042143) are limited to ion-ion extraction, but cannot be considered for ion-electron extraction.

Another solution corresponding to the second approach described above is proposed in the article by S.V. Dudin and D.V. Rafalskyi, "On the simultaneous extraction of positive ions and electrons from single-grid ICP source," A Letters Journal Exploring the Frontiers of Physics, EPL, 88 (2009) 55002, p1-p4.

This solution consists of implementing an electrode 9 in the center of the chamber 1 (thus within the plasma), the electrode 9 being powered by a radiofrequency voltage source 10 (RF; an alternating sinusoidal voltage source at a frequency within the radiofrequency range) through a capacitor 11, and associating it with a grid 7", located at the outlet of the chamber 1, in contact with the plasma and connected to ground 8.

One can refer to Figure 3, which is a schematic representation of the device. In this Figure 3, RF' represents a radiofrequency source (for example, one or more coils) for ionizing the gas and thus forming a plasma containing positive ions and electrons. The means 12 is a vacuum chamber in which means for characterizing the ion beam coming from chamber 1 are installed; these do not intervene in the extraction and acceleration of the ions.

The operation of the device is as follows.

By design, electrode 9 has a significantly larger surface area than grid 7" located at the output of chamber 1 and connected to ground 8.

Generally, applying an RF voltage to an electrode with a larger surface area than the 7" grid causes an additional potential difference to be generated at the interface between electrode 9 and the plasma, as well as at the interface between the 7" grid and the plasma. This additional potential difference is added to the RF potential difference. The total potential difference is distributed across a sheath. Here, the sheath is a region formed between the 7" grid or electrode 9, on one side, and the plasma, on the other side, where the density of positive ions is higher than that of electrons. This sheath has a variable thickness due to the RF signal applied to the electrode.

In practice, however, most of the effect of applying an RF signal to electrode 9 is located in the grid sheath 7. (The electrode-grid system can be considered as a capacitor with two asymmetric walls; in this case, the potential difference is applied across the part with lower capacitance, hence with smaller surface area.)

With the capacitor 11 in series with the RF source 10, applying the RF signal causes the RF voltage to be converted into a constant DC voltage due to the charging of capacitor 11, mainly at the grid sleeve 7".

This constant DC voltage in the 7" grid housing implies that positive ions are constantly accelerated. Indeed, this DC potential difference causes the plasma potential to become positive. As a result, the positive ions of the plasma are continuously accelerated toward the 7" grid (which is at ground potential) and extracted from chamber 1 through this 7" grid. The energy of the positive ions corresponds to this DC potential difference (average energy).

The variation of the RF voltage, 10, allows the RF + DC potential difference between the plasma and the grid 7" to vary. At the grid sheath 7", this results in a change in the thickness of this sheath. When this thickness becomes smaller than a critical value, which occurs during a time period at regular intervals given by the RF signal frequency, the potential difference between the grid and the plasma approaches zero (thus the plasma potential approaches zero, the grid being at ground), which allows the extraction of electrons.

In practice, the plasma potential below which electrons can be accelerated and extracted (= critical potential) is given by Child's law, which relates this critical potential to the critical thickness of the sheath below which the sheath disappears ("sheath collapse" according to Anglo-Saxon terminology).

As long as the plasma potential is lower than the critical potential, there is simultaneous acceleration and extraction of electrons and ions.

Although electron extraction is only feasible during a certain time period within the applied RF signal to electrode 9, this article demonstrates the possibility of complete compensation of the positive ion charge, and therefore good beam electroneutrality at the output of the plasma chamber.

Furthermore, a simultaneous acceleration and extraction of positive ions and electrons are achieved during one period of the RF signal, in contrast to the solution proposed in WO 2012/042143, whether for ion-ion or ion-electron extraction.

The technique proposed in this article is therefore very different from those proposed in documents WO 2007/065915, WO 2010/060887 and WO 20121042143 (notably that of figure 3, for ion-electron extraction), by implementing a single grid (at ground potential) in contact with the plasma and a capacitor 11, which provides a DC component to the potential difference in the sheath, in series with an RF voltage source 10.

One disadvantage of this technique is that there are many losses of accelerated positive ions, that is, high-energy positive ions that do not pass through the grid openings. This causes faster wear of the grid and consequently limits the lifespan of the grid. In the case of an application to a plasma thruster (satellite, space probe, etc.), this drawback can become critical. In practice, therefore, it is advisable to use ions with energy lower than 300 eV.

Moreover, this technique cannot work for ion-ion extraction and acceleration.

An objective of the invention is to provide a device for forming a beam of positive ions and electrons, which exhibits good electroneutrality and improved extraction efficiency compared to known devices.

Improving efficiency is notably reflected in the device's lifespan, which can be enhanced for a given extraction energy.

Another objective of the invention is to provide such a device capable, moreover, of extracting ions with increased energy compared to known devices.

In order to achieve at least one of these objectives, the invention proposes a device for forming a quasi-neutral ion and electron beam, comprising: a chamber, an assembly of means for forming an ion-electron plasma within this chamber; a means for extracting and accelerating charged particles from the plasma out of the chamber, capable of forming said beam, said extraction and acceleration means including an assembly of at least two grids located at one end of the chamber; a radiofrequency alternating voltage source adapted to generate a signal whose radiofrequency lies between the ion plasma frequency and the electron plasma frequency, said radiofrequency voltage source being connected in series with a capacitor and connected, via one of its outputs and through this capacitor, to at least one of the grids of said assembly of at least two grids, at least one other grid of said assembly of at least two grids being either set to a reference potential or connected to the other output of the radiofrequency voltage source.

This device may further include the following features, either individually or in combination: the set of means for forming the ion-electron plasma includes one or more coil(s) powered by an alternating radiofrequency voltage source; the radiofrequency voltage source powering the coil or each coil is the same as the radiofrequency voltage source connected in series with the capacitor which are connected to at least one of the two grids, the device further including a means for managing the signal provided by said source, on one hand, toward the coil or each coil and on the other hand, toward said at least one grid; the set of means for forming the ion-electron plasma in the chamber includes a reservoir containing at least one electropositive gas; the grids have circular openings.whose diameter is between 0.5 mm and 10 mm, for example between 1 mm and 2 mm; the distance between the two grids is between 0.5 mm and 10 mm, for example between 1 mm and 2 mm; the grids have slit-shaped openings; the beam's electroneutrality of ions and electrons is at least partially achieved by adjusting the duration of application of the positive and/or negative potentials from the radiofrequency alternating voltage source; the beam's electroneutrality of ions and electrons is at least partially achieved by adjusting the amplitude of the positive and/or negative potentials from the radiofrequency alternating voltage source; the radiofrequency alternating voltage source is arranged to produce a rectangular signal; the radiofrequency alternating voltage source is arranged to produce a sinusoidal signal.

A more comprehensive objective of the invention is to obtain a device that further enables the extraction and acceleration of both negative and positive ions, while ensuring good beam electroneutrality.

To achieve this objective, the invention also proposes a device for forming a quasi-neutral particle beam of opposite charges, comprising: a device for forming a quasi-neutral beam of ions and electrons according to the invention; an assembly of means for forming an ion-ion plasma in the chamber, said assembly including an electron filtering means; a low-frequency alternating voltage source adapted to generate a signal whose radiofrequency is less than or equal to the ion plasma frequency; a means capable of connecting one of the grids either to the low-frequency voltage source while activating the electron filtering means to form an ion-ion beam, or to the radiofrequency voltage source in series with the capacitor while deactivating the electron filtering means to form an ion-electron beam.

The system for forming a quasi-neutral particle beam of opposite charges may further include the following features, either alone or in combination: the charge neutrality of the ion beam is at least partially achieved by adjusting the duration of application of the positive and/or negative potentials coming from the low-frequency alternating voltage source; the charge neutrality of the ion beam is at least partially achieved by adjusting the amplitude of the positive and/or negative potentials coming from the low-frequency alternating voltage source; the low-frequency alternating voltage source is arranged to produce a rectangular signal; the set of means for forming an ion plasma in the chamber includes a reservoir containing at least one electron-negative gas.

Finally, it should be noted that the usable gases can be selected, depending on their electropositivity or electronegativity, from argon (Ar), hydrazine (N2H4), xenon (Xe), carbon tetrafluoride (CF4), sulfur hexafluoride (SF6), iodine (I2), nitrogen (N2), or hydrogen (H2).

The invention will be better understood, and other objects, advantages, and features of it will become more clearly apparent upon reading the following description, which refers to the attached figures, wherein: FIG. 4 is a schematic diagram of a first embodiment of the invention, with which it is possible to extract and accelerate positive ions and electrons; FIG. 5 is an equivalent electrical circuit diagram of the device shown in FIG. 4; FIG. 6 shows an RF voltage signal that can be applied to a grid of the device of FIG. 4 through a capacitor connected in series with the RF source,This signal of tension corresponds approximately to the plasma potential; Figure 7 is a variant embodiment of the device proposed in Figure 4; Figure 8 represents another variant embodiment of the device proposed in Figure 4; Figure 9 is a schematic diagram of a second embodiment of the invention, with which it is possible to extract and accelerate either positive and negative ions or positive ions and electrons; Figure 10 is a schematic diagram of a test setup allowing testing of the device according to the invention, which is in accordance with that of Figure 4; Figures 10(a) to 10(c) show some measurement results obtained with the test setup of Figure 10 in the case of an ion-electron regime; Figure 11 provides measurement results obtained with the device in accordance with that of Figure 9.with the measurement methods shown in Figure 10.

A first embodiment of the invention is described below with reference to Figure 4.

The device 100 comprises a chamber 20 into which a gas can be introduced, for example stored in a reservoir 31, capable of forming a plasma containing ions and electrons. This introduction is carried out via the means 30, such as a conduit connected to the reservoir 31 to introduce the gas into the chamber 20. It also includes means 40, 58 for ionizing the gas to form the plasma. For example, the means 40 can consist of coils powered by a radiofrequency source 58. Alternatively, other known means 40, 58 could be provided, such as, without limitation, a microwave source 58 with a resonator 40 or a direct current power source 58 with electrodes.

The device 100 finally includes means 50 for extracting and accelerating positive ions and electrons out of the chamber 20. This extraction/acceleration allows the formation of a beam 60 at the outlet of the chamber.

The 50 extraction and acceleration means include an assembly of at least two grids 51, 54 arranged at the end of the chamber 20. A first grid 51 is connected to an alternating voltage source at a frequency within the radiofrequency range, hereinafter referred to as the radiofrequency source RF, 52, through a capacitor 53. The capacitor 53 is arranged in series with the radiofrequency source RF, 52. A second grid 54 is set to a reference potential 55, for example, ground.

In practice, for certain applications, the reference potential may be ground (mass). However, for other applications, such as in the space domain, the reference potential may be that of the satellite or probe in question.

In the following description regarding this first embodiment, the reference potential will be considered as ground, unless otherwise stated.

The RF source, 52, is set to define an angular frequency ωRF such that is the plasma frequency of electrons and is the plasma oscillation frequency of positive ions; where: e₀, the charge of the electron, ε₀, the permittivity of free space, nₚ, the plasma density, mᵢ, the mass of the ion, and mₑ, the mass of the electron.

It should be noted that ωpi << ωpe due to mi >> me.

In general, the frequency of the signal provided by the RF source, 52, can range from a few MHz to a few hundred MHz, depending on the gas used for plasma generation in the chamber 20, and this is intended to be between the ion plasma frequency and the electron plasma frequency.

The device according to the invention shown in Figure 4 can be, in a simplified way, associated with the equivalent circuit diagram of Figure 5.

On this diagram, one can recognize the RF source 52, the capacitor 53 in series with this source, and the ground 55.

The reference P represents the plasma.

Cint represents the capacity between the two grids 51, 54.

Block B1 represents the effect of the sheath that forms between the plasma and the first grid 51, which can be represented by a diode. in parallel with a capacity Block B2 represents the effect of the sheath that forms between the plasma and the second grid 54, which can be associated with a diode. in parallel with a capacity

The existence of a diode-like function for each of the blocks B1 or B2 is due to the fact that ions cannot follow the instantaneous evolution of the electric field between the grids, imposed by the radiofrequency variation of the signal coming from the radiofrequency source 52, but only the average value of this field, while electrons can follow the instantaneous evolution of the electric field. This arises because the mass of electrons (me) is very small compared to the mass (mi) of positive ions (me << mi), and the frequency of the signal imposed by the source 52 (radiofrequency; angular frequency ωRF) is chosen to be between the ion plasma frequency and the electron plasma frequency, namely, ωpi ≤ ωRF ≤ ωpe.

For this reason, when an RF voltage (VRF) is applied through the source 52, the capacitor 53 becomes charged.

This charging of the capacitor 53 then produces a continuous DC voltage across the capacitor. Finally, a voltage VRF + DC is obtained across the entire assembly consisting of the RF source 52 in series with the capacitor (Figure 5).

The DC constant component of the voltage VRF + DC then allows to define an electric field between the two grids 51, 54, the average value of the only VRF signal being zero. This DC value therefore enables the extraction and acceleration of positive ions through the two grids 51, 54 continuously.

Moreover, the capabilities and They are very different due to the arrangement of grids 51 and 54 in the device. Indeed, from the perspective of the positive ions or electrons present in the plasma, the second grid 54, located downstream relative to the first grid 51, with respect to the direction of propagation of the beam 60, presents an effective surface. Much weaker than the effective surface from the first grid 51 since the second grid 54 is not visible, for the plasma, through the openings of the first grid 51, namely: This therefore results in the inequality Even for identical 51, 54 grids. In practice, the combination of two 51, 54 grids thus allows forming a capacitor with asymmetric surfaces.

For this reason, when a radio frequency voltage (VRF) is applied through the source 52, the voltage VRF + DC across the assembly formed by the RF source 52 in series with the capacitor 53 is expressed as car (English: "car") Where It represents the potential difference in the sheath formed between the plasma and the first grid 51. It represents the potential difference in the sheath formed between the plasma and the second grid 54.

The first grid 51 to which the RF signal 52 is applied via the capacitor 53 is in contact with the plasma and interacts with it. The plasma potential follows the potential applied to the first grid 51, that is, VRF + DC.

As for the second grid 54, at mass 55, it is also in contact with the plasma but only during the brief temporal intervals when electrons are extracted together with the positive ions, that is, when It is lower than a threshold value φcr below which the sheath disappears ("sheath collapse" according to Anglo-Saxon terminology).

This threshold value φcr is defined by Child's law. This law is expressed as follows: where s is the thickness of the sheath at which it becomes smaller than the size of the grid openings; ε0, the vacuum permittivity; e0, the charge of the electron; mi, the mass of an ion; and ji, the ion current density.

The operation of the 100 device is shown in Figure 6.

Figure 6 shows an example of the plasma potential evolution over time, related to the application of an RF voltage through capacitor 53 on the first grid 51.

The dashed line represents the constant DC component, here 550V, which is related to the presence of capacitor 53. This component defines the energy of the positive ions present in the plasma that are continuously extracted and accelerated by the two grids 51, 54.

However, the plasma potential varies between extreme values (+1050V; 50V) around the constant component (550V, here) due to the RF signal provided by the source 52.

When the plasma potential reaches the critical potential from which the sheath disappears, electrons are extracted and accelerated through grids 51, 54 along with positive ions.

Ici, φcr ≅ 200V.

This can be achieved using two identical grids 51 and 54, whose circular openings have a diameter of 1.5 mm (which allows defining the value of s in Eq. 1). The distance between the two grids is comparable to the diameter of a grid opening. The gas used is argon. The ion current density associated with these openings and this gas is 5 mA/cm².

It can be noted in this figure that the plasma frequency is 13.56 MHz, to ensure that ωpi ≤ ωRF ≤ ωpe.

The beam 60's electroneutrality upon exiting chamber 20 is achieved by extracting electrons through the two grids 51, 54 when the sheath disappears at the level of the first grid 51.

Beyond the example associated with Figure 6, it should be noted that the sizing of grids 51, 54 will therefore depend on the gas used and on the ion current density desired, in accordance with Child's law.

Generally, identical grids 51, 54 will be used. Each grid 51, 54 can have circular holes whose diameter ranges between 1 mm and 2 mm. The distance between the two grids 51, 54 is then within the same range of values as the diameter of the holes.

As an alternative, each grid 51, 54 has slots-shaped openings.

It is appropriate to point out several differences between the implementation mode according to the invention (figures 4 and 5) compared to the aforementioned article (figure 3), both in terms of structure and operation.

Unlike the aforementioned article (Figure 3), it is not at the level of grid 54 connected to ground 55 that there is, most often, an interaction with the plasma and the formation of a variable-thickness sheath.

In terms of structure, the device 100 according to the invention differs from the device proposed in the article by S.V. Dudin and D.V. Rafalskyi, in that it implements means for extracting and accelerating positive ions and electrons based on two grids 51, 54 located at the outlet of the chamber, rather than a single grid cooperating with an electrode at the center of the plasma.

In terms of operation, using two grids 51, 54 at the output of the chamber modifies the operation of extraction and acceleration, compared to the aforementioned article (Figure 3).

Indeed, if a sheath forms at the first grid 51, whose thickness varies according to the plasma potential, the voltage difference with the plasma in the sheath is small because the plasma potential follows the voltage applied to the first grid 51.

Therefore, the constant DC potential difference is applied between the two grids 51 and 54, unlike in the aforementioned article where it is applied at the grid connected to ground. The acceleration of the positive ions comes from this DC potential difference that exists between the two grids 51 and 54.

As a result, the trajectory of the positive ions is better controlled, and many fewer positive ions strike the first grid 51. These positive ions no longer strike the wall of the second grid 54, which, from the perspective of these ions, is only visible through the holes of the first grid 51.

Furthermore, when the sheath disappears (plasma potential lower than or equal to the critical potential), electrons pass through the apertures of the first grid 51 and have little tendency to hit the wall of the second grid 54, which is visible to the electrons only through the apertures of the first grid 51. Therefore, the electron trajectory is well controlled.

Therefore, one can consider a device with a significantly improved lifetime or implementing positive ions with higher energy than in the aforementioned article (Figure 3).

The operation of the extraction and acceleration means, consisting of a set of at least two grids 51, 54 according to the invention, also differs from the two-grid means 5', 7' proposed in document WO 2012/042143 (figure 3; positive ion extraction - electrons).

Indeed, the alternating signal printed on the 5' grid is centered around the zero value (absence of a capacitor). Therefore, no constant DC component is present in this device between the two grids 5' and 7', as the potential difference is solely related to the variation of the alternating signal printed on the 5' grid. No constant extraction of positive ions is possible in WO 2012/042143, but only a successive extraction of positive ions and electrons.

Figure 7 shows a variant implementation of the device 100 shown in Figure 4, in which the radiofrequency source 52 is not used. In this case, a radiofrequency source 58' used to activate the means 40, such as coils, is also used to power the grid 51. It is then necessary to provide a means 59 for managing the signal provided by the aforementioned source, both towards the means 40, such as one or more coil(s), and towards the grid 51. This design can be advantageous for space applications because it reduces the risk of failure of the entire device 100.

In figures 4, 5, and 7, the case where grid 51 is connected to the RF source 52 (figures 4 and 5), or 58' (figure 7) in series with capacitor 53 and the other grid 54, which is set to a reference potential, for example ground, is shown. In this case, one of the outputs of the RF source 52 (or 58' in figure 7) is set to a reference potential, for example ground. However, it does not matter which output of the RF source 52 is connected to which grid 51 or 54. In other words, grid 51 could be set to a reference potential while the other grid 54 is connected to the RF source 52 (figures 4 and 5) or 58' (figure 7) in series with capacitor 53.

Figure 8 represents another alternative implementation of the device 100 shown in Figure 4.

In this variant, the RF (radio frequency) source, 52, is connected to both grids 51 and 54. More specifically, the RF source, 52, is connected in series with the capacitor 53 and connected, via one of its outputs and through this capacitor 53, to one of the two grids 51, 54. In other words, one output of the RF source, 52, is connected to the capacitor 53, which in turn is connected to one of the two grids 51, 54. The other output of the RF source, 52, is then connected to the other grid 54 of the two grids 51, 54. On Figure 8, it is the grid 51 that is connected to the capacitor 53, but alternatively, the capacitor 53 could be connected to the grid 54 and the grid 51 connected to the output of the RF source, 52, which is not connected to the capacitor 53.

In addition, it should be noted that such a variant could be provided for the device 100 shown in Figure 7. In this case, the radiofrequency source RF, 58' is connected to both grids 51, 54, according to the description of the previous paragraph.

Therefore, this variant does not involve a reference potential.

In the space field, such a connection ensures the absence of stray currents circulating between, on one hand, the external conductive parts of the satellite or space probe and, on the other hand, the particle extraction device itself for opposite charges.

Finally, the signal applied to the relevant grid can be a signal obtained at least in part by adjusting the duration of application of the positive and/or negative potentials from the radiofrequency (RF) alternating voltage source, 52, 58', thereby improving the charge neutrality of the ion-electron beam. Alternatively or in addition, the signal applied to the relevant grid can be a signal obtained at least in part by adjusting the amplitude of the positive and/or negative potentials from the radiofrequency (RF) alternating voltage source, 52, 58', thereby improving the charge neutrality of the ion-electron beam.

It may be an arbitrary shape signal, for example a rectangular one.

In particular, it may be a rectangular signal such as that shown in Figure 2(b), namely, a rectangular signal is composed of a sequence of positive rectangular pulses (b'+) and negative rectangular pulses (b'-) with variable amplitude (a') and duration (d'). The adjustment is thus performed both on the duration of application of the positive and negative potentials and on the amplitude of these potentials.

Alternatively, it may be a sinusoidal signal.

A second embodiment is described below with reference to Figure 9. This second embodiment can implement two operating modes: one allowing the formation of a quasi-neutral beam of ions and electrons as particles with opposite charges, and the other allowing the formation of a quasi-neutral beam of positive ions and negative ions or ion-ion pairs, as particles with opposite charges.

The 100' system includes all the means implemented in the 100 system according to the first embodiment.

However, the device 100 further comprises a set of means 32, 30, 40, 58, 80 for forming an ion-ion plasma in the chamber 20.

Regarding the means provided in scheme 100 for generating an electron-ion plasma, the means 32, 30, 40, 58, 80 include notably a reservoir 32 containing at least one electron-negative ionizable gas, capable of generating positive and negative ions as well as electrons, and an electron filtering means 80 for the electrons produced by this electron-negative gas. The means 80 preferably produces a constant magnetic field H oriented transversely relative to the direction of movement of the ions and electrons in the chamber 20.

The 100' device also includes an alternating voltage source called low frequency LF, 56, which is capable of being connected to the first grid 51 through a controllable means 57 that can be positioned either on the radiofrequency source RF, 52, or on the low frequency source LF, 56. By low frequency source LF, 56, it is meant a source emitting at a frequency lower than or equal to the ion plasma frequency. It should be noted that the means 57 also allows activating or deactivating the filtering means 80.

The signal coming from this low-frequency alternating voltage source, 56, can be obtained at least in part by adjusting the duration of the positive and/or negative potentials coming from this source, in order to control the ion beam's electroneutrality. Alternatively or in addition, the signal from this low-frequency alternating voltage source, 56, can be obtained at least in part by adjusting the amplitude of the positive and/or negative potentials coming from this source, in order to improve the ion beam's electroneutrality.

It may particularly be a rectangular pulse, such as the one shown in Figure 2(b), that is, a rectangular signal composed of a sequence of positive rectangular pulses b'+ and negative rectangular pulses b'- with amplitude a' and variable duration d'. The adjustment is thus performed both on the duration of application of the positive and negative potentials, and on the amplitude of these potentials.

More generally, a rectangular pulse waveform can be considered.

Therefore, the 100' device has two operating modes.

In the first mode of operation, the means 57 is positioned on the RF source 52, in series with the capacitor 53. A gas capable of generating a plasma containing positive ions and electrons is introduced into the chamber 20 through the reservoir 31 and the conduit 30. The electron filtering means 80 is deactivated, with the means 57 also controlling the activation or deactivation of this electron filtering means 80. The operation of the device 100' is identical to that described for the first embodiment (device 100) for extracting positive ions and electrons.

In the second mode of operation, the device 57 is positioned on the low-frequency (LF) source 56 and also activates the filtering device 80. The LF source 56 emits a signal whose value alternates between positive and negative at a frequency lower than or equal to the ion plasma frequency, thereby sequentially extracting positive and negative ions. A electronegative gas must be introduced into chamber 20. The electron filtering device 80 must be activated to eliminate or nearly eliminate electrons, so that only or almost only positive and negative ions emerge from this device 80.

The charge neutrality of the beam obtained at the output is ensured.

To the best of the applicant's knowledge, no plasma generation device offers such an all-in-one device. In particular, when device 100' is used in ion-ion mode, its use can be limited over time by switching to positive ion-electron mode, in order to avoid early aging problems of the equipment.

Referring to Figure 9, it can be noted that two distinct RF sources 58, 52 are implemented. However, it could also be provided an installation scheme conforming to that shown in Figure 8 for the device 100', namely a single RF source.

Finally, and in general, the gases usable in devices 100 or 100' can be selected, depending on their electropositivity or electronegativity, among argon (Ar), hydrazine (N2H4), xenon (Xe), carbon tetrafluoride (CF4), sulfur hexafluoride (SF6), iodine (I2), nitrogen (N2), or hydrogen (H2).

Tests were conducted to demonstrate the interest of the proposed solution within the scope of the invention.

Figure 10 is a schematic representation of a test installation used with the device 100 according to the invention. On the left side of this Figure 10, the device 100 shown in Figure 4 can be recognized.

The characteristics of the 60 beam are determined in a 200 vacuum chamber, via a 205 pump. The measurement is performed using a 201 target, which is associated with an energy analyzer 203 mounted on the target 201; this analyzer is connected to a processing unit 204. This analyzer is known by the acronym RFEA (Retarding Field Energy Analyzer according to Anglo-Saxon terminology). The target 201 is connected to a means for determining its potential 202.

Some results are shown in figures 10(a) to 10(c), related to ion-electron operation.

The gas used is argon (25 sccm).

The frequency of the RF source, 52, is 4 MHz. The voltage applied to the grid 51 by this source can have an amplitude ranging from 0 to 300 V (i.e., up to 600 V peak-to-peak).

The magnetic filter is inactive.

Figure 10(a) shows the energy distribution functions (EDF; y-axis; arbitrary units, a.u.) for Argon ions (IEDF) and electrons (EEDF) as a function of the energy of these ions/electrons (x-axis). The presence of a peak for both electrons and ions indicates that both types of charged particles are extracted and accelerated. This figure shows that ions are extracted and accelerated to an average energy of 150 eV, and electrons to an average energy of 10 eV. These measurements were obtained with an RF potential of 150 V amplitude (300 V peak-to-peak).

Figure 10(b) shows the evolution of the target potential 201 (y-axis; φfloat) as a function of the average energy of argon ions. It can be noted that, over the range of argon ion energies considered for this measurement (0 to 300 eV), the target potential is lower than 15 V, which is relatively low compared to the ion energy. In other words, this shows that the beam is well compensated in terms of charge (electroneutrality is ensured).

Figure 10(c) shows a potential-current (Upr/Ipr) curve for target 201; for an argon ion energy of 300 eV. The argon ion current and electron current are equal (Ipr = 0) when the target potential is 15 V. It should be noted that φfloat equals Upr when Ipr = 0.

Figure 11 shows results related to the ion-ion operation mode. These results were obtained using a test setup implementing the device 100' shown in Figure 9, with the measurement means for characterizing the beam 60 coming from this device 100', which have been previously described in support of the 10.

The gas used is sulfur hexafluoride (SF6).

The LF frequency of the 56 voltage source is 20kHz. The potential applied to the grid connected to the 56 voltage source ranges from -350V to +350V.

The magnetic filter 80 is active, in order to eliminate or nearly eliminate the electrons produced by gas ionization.

Figure 11 shows more precisely the energy distribution functions (IEDF; y-axis; arbitrary units, a.u.) for positive ions (solid line) and negative ions (dashed line) as a function of the energy of these ions (x-axis). The presence of a peak both for electrons and ions indicates that extraction and acceleration of both types of charged particles are achieved. This Figure 11 shows that positive and negative ions are extracted and accelerated to an average energy higher than 300 eV.

The devices 100, 100' according to the invention can notably be used for: plasma thrusters (applications for satellites for trajectory correction, space probes, etc.), particle deposition devices on a target (vapor phase deposition, for example; microelectronics field), target etching devices, polymer treatment devices, or target surface activation devices.

Claims (17)

1. A device (100) for forming a quasi-neutral beam of ions and electrons, comprising:

   - a chamber (20),

   - a set of means (31, 30, 40, 58) for forming an ion-electron plasma in this chamber (20);

   - a means (50) for extracting and accelerating charged particles of the plasma out of the chamber (20) able to form said beam, said extraction and acceleration means (50) comprising a set of at least two grids (51, 54) located at one end of the chamber;

   - a radiofrequency alternating voltage source (52, 58') adapted for generating a signal, the radiofrequency of which is comprised between the plasma frequency of the ions and the plasma frequency of the electrons, said radiofrequency voltage source (52, 58') being positioned in series with a capacitor (53) and connected, through one of its outlet and via this capacitor (53), to at least one of the grids of said set of at least two grids (51, 54), at least one other grid of said set of at least two grids (51, 54) either being set to a reference potential, or connected to the other one of the outlets of the radiofrequency voltage source (52, 58').
2. The device (100) according to claim 1, wherein the set of means (31, 30, 40, 58) for forming the ion-electron plasma comprises one or several coils powered by a radiofrequency alternating voltage source (58'; 52).
3. The device (100) according to the preceding claim, wherein the radiofrequency voltage source (58') powering said or each coil (40) is the same as the radiofrequency voltage source (52) in series with the capacitor which are connected to at least one of the two grids (51, 54), the device further comprising a means (59) for handling the signal provided by said source (58') towards said or each coil on the one hand and towards said at least one grid on the other hand.
4. The device (100) according to one of the preceding claims, wherein the set of means (31, 30, 40, 58) for forming the ion-electron plasma in the chamber (20) comprises a tank (31) including at least one electropositive gas.
5. The device (100) according to one of the preceding claims, wherein the grids (51, 54) have circular orifices, the diameter of which is comprised between 0.5 mm and 10 mm, for example between 1 mm and 2 mm.
6. The device (100) according to one of the preceding claims, wherein the distance between both grids (51, 54) is comprised between 0.5 mm and 10 mm, for example between 1 mm and 2 mm.
7. The device (100) according to one of claims 1 to 4, wherein the grids (51, 54) have slot-shaped orifices.
8. The device (100) according to one of the preceding claims, wherein the electro-neutrality of the beam of ions and electrons is at least partly obtained by adjusting the period of application of the positive and/or negative potentials stemming from the radiofrequency alternating voltage source (RF, 52, 58').
9. The device (100) according to one of claims 1 to 7, wherein the electro-neutrality of the beam of ions and electrons is obtained at least partly by adjusting the amplitude of the positive and/or negative potentials stemming from the radiofrequency alternating voltage source (RF, 52, 58').
10. The device (100) according to one of the preceding claims, wherein the radiofrequency alternating voltage source (RF, 52, 58') is laid out so as to produce a rectangular signal.
11. The device (100) according to one of claims 1 to 7, wherein the radiofrequency alternating voltage source (RF, 52, 58') is laid out so as to produce a sinusoidal signal.
12. A device for forming a quasi-neutral beam of oppositely charged particles, characterized in that it comprises:

    - a device (100) for forming a quasi-neutral beam of ions and electrons according to one of the preceding claims;

    - a set of means (32, 30, 40, 58, 80) for forming an ion-ion plasma in the chamber (20), said set including a means (80) for filtering out electrons;

    - a so called low frequency alternating voltage source (LF, 56) which is adapted for generating a signal, the radiofrequency of which is less than or equal to the plasma frequency of the ions;

    - a means (57) able to connect one of the grids (51, 54) either to the low frequency voltage source (LF; 56) while activating the means (80) for filtering out electrons in order to form an ion-ion beam, or to the radiofrequency voltage source (52, 58) in series with the capacitor (53) while deactivating the means (80) for filtering out electrons in order to form an ion-electron beam.
13. The device (100') according to the preceding claim, wherein the electro-neutrality of the ion-ion beam is at least partly obtained by adjusting the period of application of the positive and/or negative potentials stemming from the low frequency alternating voltage source (LF, 56).
14. The device (100') according to one of claims 12 or 13, wherein the electro-neutrality of the ion-ion beam is at least partly obtained by adjusting the amplitude of the positive and/or negative potentials stemming from the low frequency alternating voltage source (LF, 56).
15. The device (100') according to the preceding claim, wherein the low frequency alternating voltage source (LF, 56) is laid out so as to produce a rectangular signal.
16. The device (100') according to one of claims 12 to 15, wherein the set of means (32, 30, 40, 58, 80) for forming an ion-ion plasma in the chamber (20) comprises a tank (32) including at least one electronegative gas.
17. The device (100, 100') according to one of the preceding claims, wherein the gases which may be used are selected, according to their electropositivity or electronegativity, from among argon (Ar), hydrazine (N₂H₄), xenon (Xe), carbon tetrafluoride (CF₄), sulphur hexafluoride (SF₆), di-iodine (I₂), dinitrogen (N₂) or dihydrogen (H₂).