FR3146696A1 - Installation for chemical vapor deposition assisted by a microwave plasma stabilized in a resonant cavity - Google Patents

Abstract

The invention relates to an installation for a chemical vapor deposition assisted by a microwave plasma (2), at a given source frequency (F), of layers of materials on substrates (81) arranged inside a plasma chamber (7). The installation comprises at least one microwave generator (3) which generates electromagnetic waves at this source frequency, which are directed inside the plasma chamber to allow the ignition of the plasma. The installation is remarkable in that it comprises at least one monitoring device (10) for monitoring a position of the plasma inside the plasma chamber, in that the microwave generator is a solid-state and variable-frequency generator, and in that it comprises a control unit (11) connected to the at least one monitoring device and to said microwave generator to vary the source frequency to control the position of the plasma. Abstract figure: Figure 1

Description

Installation for chemical vapor deposition assisted by a stabilized microwave plasma in a resonant cavity

The invention relates to an installation for chemical vapor deposition assisted by microwave plasma, as well as to an associated chemical deposition method.

The invention thus relates to the field of microwave plasma-assisted chemical vapor deposition, also called MPACVD for “Microwave Plasma Assisted Chemical Vapor Deposition” in English.

The invention finds a preferred application in the field of diamond synthesis, and can also be applied more widely in the synthesis of carbon materials such as carbon nanotubes and graphene, or other materials, in particular in the semiconductor sector for the deposition of thin films/layers of material.

As is known, microwave plasma-assisted chemical vapor deposition is implemented within a plasma reactor, comprising a resonant cavity which contains a substrate on which a layer of material (such as a diamond layer) whose chemical elements are present in the gaseous state in a reaction gas, inside the resonant cavity, must be synthesized.

The chemical elements are deposited on the substrate from a plasma generated in this reaction gas by applying an electrical discharge produced in the resonant cavity by a microwave source.

Generally, the cavities of these plasma reactors are designed to maintain a volume of plasma, often in the shape of a half-sphere, at a given distance (often of the order of a millimeter) above the substrate. Thus, the dimensions of the resonant cavity are related to the frequency (generally 915 MHz or 2450 MHz) at which the microwaves causing the discharge in the reaction gas are generated, making it possible to obtain a very localized microwave electric field and thereby create a plasma above and sufficiently close to the substrate. The plasma thus allows an optimal transfer of the elements (radicals, ions, etc.) which are produced inside it and which are necessary for the growth of the layer of material, without the plasma being in contact with this cavity to avoid its overheating.

However, disturbances can occur in the resonant cavity and cause the microwave resonance to change, thus causing a variation in the position of the plasma, similar to a plasma jump. In some cases, the plasma, following the jump, may come into contact with non-metallic parts of the plasma reactor, for example a microwave-transparent window (usually made of quartz) separating the resonant cavity from a coupling system used to establish electromagnetic waves after their generation by the microwave source. The layer of material deposited on the substrate can then potentially be contaminated by elements from these non-metallic parts, such as silicon atoms. Furthermore, a change in the position of the plasma can also possibly lead to its extinction, and therefore a stoppage of the chemical deposition process.

These drawbacks can therefore significantly impact the performance of the plasma reactor and the efficiency of the process, leading to additional costs due to a sudden shutdown and/or contamination of the deposit.

Also, the short distance between the plasma and the substrate can be problematic in the case of a long-term thin layer deposition. Indeed, in this application context, the thickening and increase in the thickness of the layer leads after a long growth period to the entry of the layer into the plasma, suddenly increasing the temperature in this layer, and thereby modifying the growth conditions.

An existing solution is to equip the resonant cavity with a substrate holder that can move vertically in translation, in order to be able to lower the substrate holder as the thickness of the material layer increases so that this layer is always located under the plasma at the desired distance. However, this solution has two drawbacks: the translation distance is generally limited, and a translation of the substrate holder is accompanied by a change in the resonance of the cavity which can in the worst case lead to the extinction of the plasma. Furthermore, these substrate holders are generally expensive due to their mechanical precision, since they must perform vertical translation movements over distances of the order of a millimeter or even less.

The invention aims, for plasma reactors comprising a coupling system and a resonant cavity, to resolve the aforementioned drawbacks, by proposing an installation which makes it possible to control the position of the plasma within the resonant cavity, and in particular to return the plasma to its initial position in the event of a plasma jump.

To this end, the invention provides an installation for microwave plasma-assisted chemical vapor deposition, the installation comprising:
- a microwave generator generating electromagnetic waves at a given source frequency within at least one frequency interval of a microwave frequency range,
- an electromagnetic wave coupling system connected to the microwave generator, and which is configured to transfer the electromagnetic waves from the microwave guide to a resonant cavity,
which resonant cavity comprising at least:
- a plasma chamber in which is arranged a substrate holder on which at least one substrate can be placed, said plasma chamber being provided with an inlet for a reaction gas, and a pumping system for establishing a reduced pressure inside the plasma chamber,
- at least one dielectric, transparent to microwaves, separating the plasma chamber from the electromagnetic wave coupling system for propagation of the electromagnetic waves inside the plasma chamber and through said dielectric,

the installation being remarkable: in that it comprises at least one monitoring device for measuring a physical parameter dependent on a position of the plasma inside the plasma chamber, in that the microwave generator is a solid-state and variable-frequency generator, and in that it comprises a control unit connected to the at least one monitoring device and to said microwave generator for varying the source frequency as a function of a measurement of the physical parameter for controlling the position of the plasma.

In other words, the installation advantageously proposes to resolve the problem of plasma jump, more precisely of a plasma volume, by controlling and automatically repositioning the position of the plasma inside the plasma chamber (also called process chamber or deposition chamber) of a resonant cavity.

For this, the position of the plasma is controlled by a control unit which communicates with at least one monitoring device, and a solid-state, variable-frequency microwave generator generating electromagnetic waves at a source frequency for the formation of the plasma inside the resonant cavity, which source frequency is included in at least one frequency interval contained in a microwave frequency range, with for example the microwave frequency range between 300 MHz and 10 GHz.

The at least one monitoring device is configured to measure a physical parameter representative of and dependent on the position of the plasma, meaning that a change in the position of the plasma results in a variation in the measured physical parameter.

The control unit is configured to detect the source frequency at which the plasma is generated, from the measurements it receives from the at least one monitoring device, whether the physical parameter varies or not.

If so, the control unit then commands the microwave generator to vary the source frequency in the at least one frequency interval until the distribution of the electromagnetic field in the plasma chamber allows the plasma to be readjusted to a more suitable position, for example the position it initially occupied.

The at least one monitoring device is configured to measure and communicate the value of the physical parameter measured continuously throughout the duration of the chemical deposition.

According to a characteristic of the invention, the source frequency is initially at an initial frequency, and the control unit is configured to determine a change in position of the plasma generated at this initial frequency from a first position to a second position distinct from the first position, from a variation in the physical parameter measured by the at least one monitoring device, and to vary the source frequency from the initial frequency until reaching a correction frequency bringing the plasma from the second position to substantially the first position.

According to a characteristic of the invention, the control unit is configured to determine the change in position of the plasma when the physical parameter varies from a first value to a second value, and to vary the source frequency from the initial frequency to the correction frequency until the physical parameter varies inversely from the second value to substantially the first value.

In other words, and in connection with the previous point, at a source frequency called the initial frequency, the plasma is located in a first position in the resonant cavity, with the physical parameter having a first value representative of this first position. The control unit is configured to detect at this initial frequency a variation of the physical parameter from the first value to a second value, i.e. a change in position of the plasma from the first position to a second position.

The second position can be anywhere in the plasma chamber. It can be below or above the first position, and for example it can be as close as possible to its walls or even close to the dielectric.

If the control unit detects a variation in the physical parameter, it then controls the microwave generator so that it varies the source frequency until it reaches a correction frequency for which the plasma is returned either to its original position, i.e. the first position or a position substantially close to the first position (with the physical parameter having varied from the second value to then be equal to the first value or a value substantially equal to the first value).

Since the source frequency can take on different frequency values before the correction frequency is identified and reached, the plasma can possibly occupy between the second position and its return to the first position or a substantially close position different intermediate positions in the plasma chamber, called parasitic positions.

In practice, the first position corresponds to a position allowing optimal deposition of the layer of material on the at least one substrate, which first position is generally located between 0.5 and 3 mm, and generally approximately 1 mm, above the at least one substrate.

Advantageously, the installation makes it possible to maintain/reposition the plasma in the first position, or in a substantially close position, for the best possible resonant chamber efficiency/chemical deposition growth.

According to a characteristic of the invention, the control unit:

- following detection of the change in position of the plasma, is configured to send a control signal to the microwave generator which, upon receipt of said control signal, varies the source frequency in the at least one frequency interval;
- following a return of the plasma to the first position, is shaped to send a stop signal to the microwave generator which, upon receipt of said stop signal, ceases to vary the source frequency in the at least one frequency interval, the source frequency then being equal to the correction frequency.

In other words, when the control unit detects a variation of the physical parameter from the first value to a second value, it controls the microwave generator by transmitting a control signal to it so that it varies the source frequency from the initial frequency in the at least one frequency interval. When it detects that the physical parameter varies from the second value to the first value or a value substantially close to the first value, it sends a stop signal to the microwave generator so that it stops the variation in frequency; the frequency at which the generator stopped being considered as the correction frequency.

Advantageously, according to this operating principle, and as indicated above, the control unit autonomously and automatically manages the positioning of the plasma in the resonant cavity, in particular its maintenance/stabilization in the first position.

According to one embodiment of the invention, once the plasma has been repositioned in the first position following the variation of the source frequency, the control unit is configured to vary the source frequency of the microwave generator from the correction frequency until returning to the initial frequency.

In other words, once the plasma is positioned in the first position or a position close to it at the correction frequency, the control unit is configured to drive the microwave generator so as to return the source frequency from the correction frequency to the initial frequency. This return to the initial frequency can optionally be implemented in the case where, compared to the initial frequency, the plasma would be less stable at the first position for the correction frequency.

In the event that a new change in the position of the plasma is detected by the control unit following the return of the source frequency to the initial frequency, the control unit is configured to control the microwave generator again from the initial frequency to a new correction frequency identical to or substantially close to the previous correction frequency for which the plasma is again brought back to the first position or a position sufficiently close to it.

In an alternative embodiment of the invention, the control unit can again carry out a readjustment of the source frequency from the new correction frequency to the initial frequency.

In another embodiment of the invention, the control unit only performs one readjustment. Indeed, if following the first readjustment a new change in plasma is detected at the initial frequency, it can be considered that the probability of occurrence of a new change in position of the plasma at the initial frequency is sufficiently high not to implement a new readjustment.

According to a feature of the invention, when the source frequency is initially at the initial frequency, the control unit is configured to vary the source frequency of the microwave generator from the initial frequency until reaching a growth frequency included in the at least one frequency interval, in response to a variation of the physical parameter measured by the at least one monitoring device such that said physical parameter no longer complies with a given growth threshold value, said growth frequency being established by said control unit so that said physical parameter again complies with the growth threshold value for this growth frequency.

As explained above, as the chemical deposition progresses, the thickness of the material layer increases. Since the plasma is positioned at a short distance above the at least one substrate or substrate holder for efficient deposition, it is possible, for long-term chemical deposition, for the thickness of the material layer to come into contact with the plasma or partially penetrate inside, modifying the conditions of the chemical deposition and, as a result, altering the properties of the material layer.

Thus, the control unit is also configured to detect whether, at the initial frequency, the value of the physical parameter recorded by the at least one monitoring device is also in compliance with a growth threshold value representative of a favorable deposition condition, and which is predetermined.

In the case where the value of the physical parameter is no longer in conformity with the growth threshold value, that is to say that the favorable deposition condition is no longer encountered (with the thickness of the material layer in interaction with the plasma), the control unit is configured to control the microwave generator to vary the source frequency from the initial frequency until reaching a growth frequency for which the value of the physical parameter becomes again in conformity with the growth threshold value.

In practice, the frequency change aims to reposition the plasma from a position where growth was no longer effective to a new position where it becomes so again.

According to a feature of the invention, the growth position becomes the first position, and the growth frequency becomes the initial frequency.

Thus, the invention advantageously addresses the problem relating to the growth of long-term layers of material on the at least one substrate, by automatically adjusting the frequency at which the electromagnetic waves are generated during chemical deposition in order to shift the resonance and move the plasma away from the layer of material when it begins to interact with it; the plasma being shifted a sufficient distance from the at least one substrate for the chemical deposition to continue.

Furthermore, thanks to the invention, the substrate holder of the resonant cavity does not need to be removable, making it possible to overcome the drawbacks of such a substrate holder (for example, the limited vertical translation of the substrate holder, which can possibly induce a change in the resonance of the cavity and produce an unwanted plasma jump).

According to a characteristic of the invention, the control unit is configured to:
- in response to the variation of the physical parameter such that said physical parameter no longer conforms to the growth threshold value, sending a start signal to the microwave generator which, upon receipt of said start signal, varies the source frequency from the initial frequency in the at least one frequency interval; and
- in response to the measurement of the physical parameter such that said physical parameter becomes compliant with the growth threshold value again, sending an end signal to the microwave generator which, upon receipt of said end signal, ceases to vary the source frequency which has reached the growth frequency.

In other words, when the control unit detects that the value of the physical parameter is no longer in conformity with the initial frequency with the growth threshold value, it transmits to the microwave generator a start signal so that it varies the source frequency from the initial frequency in the at least one frequency interval. At the moment when it detects that the value of the physical parameter becomes conformable again with the growth threshold value, it sends an end signal to the microwave generator so that it stops the variation in frequency; the frequency at which the generator stopped being considered as the growth frequency.

Advantageously, according to this operating principle, and as indicated above, the control unit autonomously and automatically manages the positioning of the plasma in the resonance cavity for efficient or even optimal chemical deposition of a layer of material.

According to one embodiment of the invention, the at least one monitoring device comprises a brightness detector comprising an optical fiber oriented towards a point in the plasma chamber where it captures a light intensity of the plasma which thus constitutes the physical parameter associated with said brightness detector, such that said brightness detector detects a variation in the light intensity at said point in the case where the plasma moves.

According to one embodiment of the invention, the optical fiber is oriented towards the first position, the change in position of the plasma at the initial frequency being detected by the control unit following the variation in light intensity detected by the brightness detector.

In other words, in a first variant embodiment of the invention, the at least one monitoring device comprises a brightness detector comprising an optical fiber in the direction of a point in the plasma chamber, more precisely in the direction of the first position. The at least one monitoring device transmits as a physical parameter to the control unit a light intensity representative of the brightness at the first position. Depending on whether or not the plasma is positioned in the first position, the measured brightness intensity is different. Thus, depending on the values/variations in brightness intensity recorded, the control unit is able to determine: whether at the initial frequency, the plasma has jumped from the first position to reach a second position; and whether, when controlling the microwave generator for the variation of the source frequency following the plasma jump, the plasma has jumped from the second position to return to the first position or reach a position sufficiently close to it.

According to one embodiment of the invention, the at least one monitoring device comprises a microwave power measuring device configured to measure a power reflected by the resonant cavity, which reflected power thus constituting the physical parameter associated with said microwave power measuring device.

According to one embodiment of the invention, the control unit detects at the initial frequency the change in position of the plasma when the microwave power measuring device measures an increase in the reflected power.

In other words, in a second variant embodiment of the invention, the at least one monitoring device comprises a microwave power measuring device. The at least one monitoring device transmits as a physical parameter to the control unit a power reflected by the resonant cavity. The jump of the plasma from the first position to a second position results in a more or less significant increase in the reflected power, the resonant cavity being, before the jump of the plasma, adapted. The return of the plasma to the first position at the correction frequency results in the vast majority of cases in a decrease in the reflected power, the resonant cavity being adapted again.

Thus, the control unit commands the microwave generator and makes it start or stop the variation of the source frequency depending on whether it detects this increase or decrease in the power reflected by the resonant cavity.

According to one embodiment of the invention, the at least one monitoring device comprises a temperature measuring device configured to measure a target temperature inside the plasma chamber, at the level of the chemical deposition on the at least one substrate, which target temperature thus constituting the physical parameter associated with said temperature measuring device.

According to one embodiment of the invention, the control unit detects at the initial frequency the change in position of the plasma when the temperature measuring device measures a decrease in the target temperature.

In other words, in a third embodiment of the invention, the at least one monitoring device comprises a device shaped to measure a temperature that can correspond to the temperature of the layer of material/chemical deposit, or of the at least one substrate, or of the substrate holder. This temperature therefore corresponds to the physical parameter that is transmitted to the control unit. When the plasma is close to the chemical deposit (for example, 1 mm above it) in order to grow it, the chemical deposit is at a certain temperature. When the plasma jumps to position itself in the second position, the temperature of the chemical deposit decreases. It is also conceivable that a decrease in temperature can be measured at the level of the at least one substrate or of the substrate holder if they are not manufactured with a temperature-sensitive material. The control unit thus sends the control signal to the microwave generator for a variation of the source frequency when it detects this decrease in temperature from the measurements transmitted by the at least one monitoring device. Conversely, it sends the stop signal to the microwave generator that stops the frequency sweep when it detects a rise in temperature at the chemical deposition, or at least one substrate, or the substrate holder, meaning that the plasma has jumped from the second position to reach the first position or a position sufficiently close to it.

According to one embodiment of the invention, the growth threshold value corresponds to a critical growth temperature of the target temperature measured by the temperature measuring device, such that the physical parameter no longer conforms to the growth threshold value when the target temperature is greater than or equal to said critical growth temperature, and the physical parameter again conforms to the growth threshold value when the target temperature becomes lower than the critical growth temperature.

According to one embodiment of the invention, the critical growth temperature is equal to 1000°C, plus or minus 100°C.

In other words, when the at least one monitoring device comprises a temperature measuring device, the growth threshold value corresponds to a critical growth temperature of the chemical deposit, or of the at least one substrate or of the substrate holder if they are manufactured from a temperature-sensitive material, above which the temperature will alter the good growth conditions, or even deteriorate the chemical deposit. Generally, this critical temperature is equal to 1000°C, plus or minus 100°C. In the case where the growth is of long duration, if the plasma, the at least one substrate and the substrate holder are maintained in their position, there is a risk that the chemical deposit will be in contact with the plasma or enter it, resulting in a drastic increase in its temperature. By heat transfer from the chemical deposit, the at least one substrate and the substrate holder will also increase in temperature. Thus, the control unit is configured to detect from the measurements transmitted by the at least one monitoring device whether or not the abrupt temperature rise exceeds the critical growth temperature. If so, it sends the start signal to the microwave generator so that it varies the source frequency. It then transmits the end signal to it to stop the frequency sweep as soon as it detects that the temperature of the chemical deposition, or of the at least one substrate or of the substrate holder, falls below the critical growth temperature, meaning that the chemical deposition is no longer in contact with or inside the plasma.

The control unit is also configured to check whether this temperature decrease does not fall below a minimum growth temperature for which any temperature lower than it does not allow chemical deposition. Thus, the control unit is able to determine whether the temperature decrease results in a jump of the plasma to a position sufficiently close to the chemical deposition for its growth to continue, or on the contrary to a second position preventing the deposition from continuing. In the second case, the control unit is configured to send a control signal to the microwave generator so that it varies the source frequency and then a stop signal when it detects that the temperature of the chemical deposition, of the at least one substrate or of the substrate holder is between the minimum growth temperature and the critical growth temperature, meaning that the plasma is then positioned at a height suitable for the chemical deposition for its growth.

According to a characteristic of the invention, the frequency range of the microwaves is between 300 MHz and 10 GHz.

According to a characteristic of the invention, the at least one frequency interval is between 902 MHz and 928 MHz, or between 2400 MHz and 2500 MHz.

Generally, microwave plasma-assisted chemical vapor depositions are performed for an excitation frequency equal to 915 MHz or 2.45 GHz, with the geometric dimensions of the resonant cavity adapted to produce the plasma initiation at this frequency. Advantageously, the proposed installation makes it possible to perform chemical depositions for these two frequencies. Thus, the initial frequency for which the plasma is in the first position is substantially equal to one of these two frequencies. When the plasma has moved to the second position, the control unit, via the microwave generator, varies the source frequency until determining the correction frequency in the at least one frequency interval, which corresponds to an interval of frequencies authorized for the emission of the waves. In the case where the initial frequency is substantially equal to 915 MHz, the at least one frequency interval is between 902 MHz and 928 MHz. If it is substantially equal to 2.45 GHz, then the at least one frequency interval is between 2.4 GHz and 2.5 GHz.

Note that the variation of the frequency, whether it is a decrease or an increase in frequency, is carried out progressively and not abruptly around the initial frequency since too strong a variation could not allow the correction frequency to be detected or cause other jumps in the plasma, or even its extinction.

According to a characteristic of the invention, the electromagnetic wave coupling system comprises an antenna.

Thus, the antenna is used to propagate the electromagnetic waves inside the resonant cavity.

According to one embodiment of the invention, the resonant cavity comprises a precavity, having a pressure higher than that of the plasma chamber, and interfaced with the plasma chamber by means of the at least one dielectric.

Advantageously, the precavity allows the establishment of electromagnetic waves before they propagate in the plasma chamber.

The invention also relates to a method for microwave plasma-assisted chemical vapor deposition, the method being implemented by an installation conforming to the installation described above, and comprising at least: - a monitoring step during which the at least one monitoring device measures the physical parameter dependent on the position of the plasma inside the plasma chamber;
- a transmission step during which the at least one monitoring device transmits the measurement of the physical parameter to the control unit;
- a control step during which the control unit varies the source frequency according to the measurement of the physical parameter to control the position of the plasma.

Advantageously, the method for controlling the position of the plasma is an automatic method, with the control unit which controls the microwave generator so that it varies or not the source frequency in order to control the position of the plasma in the resonant cavity according to the analysis of the measurements of the physical parameter which it receives from the at least one monitoring device.

The monitoring step and the transmission step are carried out continuously throughout the duration of the chemical deposition.

According to a characteristic of the invention, the control step comprises the following steps:
- a detection step during which the control unit determines, at the initial frequency, a change in position of the plasma from a first position to a second position, from a variation in the physical parameter measured by the at least one monitoring device;
- a scanning step taking place after the detection step, during which the control unit sends a control signal to the microwave generator so that it varies the source frequency from the initial frequency in the at least one frequency interval;
- a step of determining the correction frequency during which the control unit determines the return of the plasma from the second position to substantially the first position, then sends a stop signal to the microwave generator so that it stops varying the source frequency, the correction frequency then now being considered as the source frequency.

According to a characteristic of the invention, during the detection step, the control unit is configured to determine the change in position of the plasma when the physical parameter varies from a first value to a second value.

According to a characteristic of the invention, during the step of determining the correction frequency, the control unit is configured to determine the return of the plasma from the second position to substantially the first position when the physical parameter varies from the second value to substantially the first value.

According to a characteristic of the invention, the method comprises a step of returning to the initial frequency which takes place after the step of determining the correction frequency, and during which the control unit, after having determined the return of the plasma to the first position, is configured to send a return signal to the microwave generator so that it varies the source frequency from the correction frequency to the initial frequency.

According to a characteristic of the invention, the method comprises a stability verification step which takes place after the step of returning to the initial frequency, during which the control unit determines, when the source frequency is again equal to the initial frequency, whether the plasma again makes a change of position from the first position to the second position; the method then implementing again, if the change of position is determined, the scanning step and the step of determining the correction frequency.

According to a characteristic of the invention, the method comprises:
- a growth limit determination step, during which the control unit is configured to determine whether the physical parameter measured by the at least one monitoring device no longer complies with a given growth threshold value;
- a second scanning step occurring after the growth limit determination step, during which the control unit sends a start signal to the microwave generator so that it varies the source frequency from the initial frequency in the at least one frequency interval;
- a step of determining a growth frequency, during which the control unit determines that the physical parameter becomes compliant with the growth threshold value again and that the plasma is located in a growth position such that the latter is a given growth distance above the first position, then sends an end signal to the microwave generator so that it stops varying the source frequency which is then equal to the growth frequency established by said control unit so that said physical parameter is again compliant with the growth threshold value for this growth frequency.

Advantageously, the method for controlling/managing the growth of the chemical deposit is an automatic method, with the control unit successively and automatically implementing the step of determining the growth limit, the second scanning step, and the step of determining a new growth frequency as a function of the comparison between the value of the physical parameter that it receives from the at least one monitoring device and the growth threshold value.

Other characteristics and advantages of the present invention will appear on reading the detailed description below, of a non-limiting example of implementation, made with reference to the appended figures in which:

is a schematic view of an installation within the meaning of the invention according to a first design mode;

is a schematic view of the installation according to a second design mode;

is a schematic view of the installation according to a third design mode;

is a schematic view of the installation according to a fourth design mode;

is a schematic view of the installation according to a fifth design mode;

is a schematic illustration, with reference to the first embodiment of the , of a change in position of the plasma in the plasma chamber, for a frequency delivered by the microwave generator corresponding to the initial frequency, from a first position (a), located here above the substrate holder, to a second position (b), located here below the dielectric; then of a return of the plasma from the second position to a position substantially comparable to the first position (and considered here as being the first position) following a variation of the source frequency from the initial frequency until reaching the correction frequency;

is a three-dimensional modeling of a resonant cavity of a plasma reactor for chemical vapor deposition (CVD) modeled in simulation software, with the plasma represented as a half-sphere, the resonant cavity is connected to a coupling system, and comprises a plasma chamber and a precavity which are interfaced to each other by means of a microwave transparent dielectric, similar to the resonant cavity as designed in the illustrated embodiment of the invention ;

illustrated by means of three-dimensional modeling of the plasma reactor of the a first configuration for which no plasma is generated/contained inside the plasma chamber; which illustration comprises two measurement points corresponding respectively to the first and second positions where electric field intensities E1, E2 are measured in the plasma chamber as a function of the source frequency;

illustrates, for the first configuration presented , the evolution of the ratio R between the electric field intensities E1, E2 measured respectively at the first and second positions, such that R = E1/E2 for different source frequency values;

illustrates, for the first configuration presented And , the electric field profile in the resonant cavity when the source frequency is equal to 2.40 GHz (a), 2.45 GHz (b) and 2.50 GHz (c);

illustrated by means of three-dimensional modeling of the plasma reactor of the a second configuration for which a plasma has been modeled in the plasma chamber, and such that it is located above the substrate holder; which illustration also comprises the first position and the second position where the intensities of the electric field E1, E2 in the plasma chamber are measured as a function of the source frequency, as for the application context of the ;

illustrates, for the second configuration presented , the evolution of the ratio R between the electric field intensities E1, E2 measured respectively at the first and second positions, such that R = E1/E2 for different source frequency values;

illustrates an evolution of the power reflected by the resonant cavity of the installation within the framework of the second configuration shown in Figures 11 and 12;

illustrates, for the second configuration presented in Figures 11 to , the electric field profile in the resonant cavity when the source frequency is equal to 2.40 GHz (a), 2.45 GHz (b) and 2.50 GHz (c);

illustrated by means of three-dimensional modeling of the plasma reactor of the a third configuration in which the plasma is positioned/stuck at the microwave-transparent dielectric; which illustration also includes the first and second positions where the electric field intensities E1, E2 in the plasma chamber are measured as a function of the source frequency, as for the application contexts of Figures 8 and 11;

illustrates, for the third configuration presented , the evolution of the ratio R between the electric field intensities E1, E2 measured at the first and second positions, such that R = E1/E2 for different source frequency values;

illustrates an evolution of the power reflected by the resonant cavity of the installation within the framework of the third configuration shown in Figures 15 and 16;

illustrates, for the third configuration presented in Figures 15 to , the electric field profile in the resonant cavity when the source frequency is equal to 2.40 GHz (a), 2.473 GHz (b) and 2.5 GHz (c);

is a schematic illustration, with reference to the first embodiment of the , of an application context of a long-term deposition of a layer of material on a substrate at the initial frequency, with the thickness of the chemical deposit which ends up being such that it is in contact with or penetrates the plasma ((a) and (b)); the installation then being configured so that the source frequency at which the microwave generator generates the electromagnetic waves varies from the initial frequency until reaching a growth frequency for which the plasma is moved from its initial position, in which it is therefore in contact with the chemical deposit, to a new position so that the deposition can continue efficiently and without altering or damaging the chemical deposit (c); the detection that the chemical deposit interacts with the plasma and the determination of the growth frequency based respectively on an excess of the temperature of the chemical deposit above a temperature called the critical growth temperature for which the favorable deposition conditions are no longer encountered, and a decrease in the temperature of the chemical deposit below the critical growth temperature;

is equivalent to the , and illustrates an application context for which, following the interaction between the plasma and the chemical deposition (a), the installation varies the source frequency until reaching a frequency called the intermediate frequency for which the temperature of the chemical deposition is below the critical growth temperature but also below a minimum growth temperature, meaning that the plasma is not positioned in a position favorable to chemical deposition on the substrate (b); the installation then being configured to again vary the source frequency of the microwave generator until a growth frequency verifying that the temperature of the chemical deposition is indeed between the minimum growth temperature and the critical growth temperature (c);

presents a flowchart illustrating the automatic method for microwave plasma-assisted chemical vapor deposition implemented by the installation of the invention, in an embodiment for which the control unit is configured to vary the source frequency of the microwave generator in the case where, at the initial frequency: the plasma changes position in the plasma chamber, the source frequency variation having the purpose of returning the plasma to a position substantially equivalent to its initial position; the conditions for efficient deposition of the layer of material on a substrate are no longer met, the source frequency variation then having the purpose of determining a frequency called the growth frequency such that said conditions are again respected.

[Detailed description of one or more embodiments of the invention]

The invention is implemented by means of an installation 1 which can be designed according to different design modes. Non-limitingly, five different designs of the installation 1 are presented with reference to the has (such that each figure presents one of the five conceptions).

Each of the five installations 1 presented comprises a solid-state, variable-frequency microwave generator 3 shaped to generate electromagnetic waves at a source frequency F in the microwave frequency range, this in a frequency interval comprised, according to two embodiments of the invention, between 902 MHz and 928 MHz, or between 2.4 GHz and 2.5 GHz; the two frequency intervals corresponding to an interval of frequencies authorized for the emission of electromagnetic waves by the Radiofrequency standards. These two frequency intervals comprise the two excitation frequencies for which the plasmas 2 are generated and the chemical vapor depositions assisted by a microwave plasma 2 are carried out; and which are respectively equal to 915 MHz and 2.45 GHz.

A microwave guide 4 is connected at the output of the microwave generator 3 to guide the electromagnetic waves to the input of an electromagnetic wave coupling system 5. In another design mode, the microwave guide 4 is replaced by a coaxial cable to guide the electromagnetic waves from the microwave generator 3 to the electromagnetic wave coupling system 5.

This electromagnetic wave coupling system 5 is designed to transfer the electromagnetic waves from the microwave guide 4 to a resonant cavity 6 which integrates a plasma chamber 7 inside which a microwave plasma 2 will be generated.

According to one possibility, the electromagnetic wave coupling system 5 comprises:
- an antenna 12 (also called a probe) propagating/distributing the electromagnetic waves inside the resonant cavity, and/or
- a short circuit (not shown) sliding in the plane normal to the plane of the microwave guide 4 and/or another short circuit sliding in the same plane as the plane of the microwave guide but on the side opposite to the side where the microwave generator 3 is connected and where the electromagnetic waves arrive.

The electromagnetic wave coupling system 5 can form a coaxial applicator, with the antenna 12 which forms a central conductive core, surrounded by an external conductive shield having a peripheral wall surrounding said antenna 12, and with a medium for propagating microwave energy located between the antenna 12 and the shield.

With reference to the installations 1 of the design modes presented in Figures 1 to 5, the resonant cavity 6 connected to the output of the electromagnetic wave coupling system 5 comprises:
- the plasma chamber 7, at reduced pressure (or depression) relative to atmospheric pressure, in which a substrate holder 8 is arranged on which at least one substrate 81 can be placed. The plasma chamber 7 is provided with an inlet for a reaction gas 70, and a pumping system 71 for obtaining the reduced pressure inside the plasma chamber 7 (the inlet for the reaction gas 70 and the pumping system 71 are shown schematically only on the ),

- at least one dielectric 9, transparent to microwaves, separating the plasma chamber 7 from the electromagnetic wave coupling system 5 for propagation of the electromagnetic waves inside the plasma chamber 7 and through the at least one dielectric 9.

In one embodiment of the invention, the at least one dielectric 9 is a window made of quartz or another dielectric material, which can be mounted or fixed on a free end of the antenna 12.

In the , , And , the electromagnetic wave coupling system 5 is connected to an upper face of the resonant cavity 6, that is to say above the substrate holder 8.

In the And , the output of the electromagnetic wave coupling system 5 is interfaced to the upper face of the resonant cavity 6. In the , the plasma chamber 7 occupies the entire internal volume of the resonant cavity 6; with a dielectric 9 located at the interface between the upper face of the resonant cavity 6 and the output of the electromagnetic wave coupling system 5 to separate the plasma chamber 7 from the electromagnetic wave coupling system 5.

In the design mode of the , the design of the antenna 12 is such that its end extends inside the resonant cavity 6 and overhangs the plasma chamber 7 and the substrate holder 8. The plasma chamber 7 is separated from the rest of the internal volume of the resonant cavity 6 by a dielectric 9 surrounding the lower face of the end of the antenna 12 opposite the substrate holder 8. This embodiment allows a more targeted/oriented propagation of the electromagnetic waves towards the substrate holder 8 and this in a more restricted space.

In the design mode of the , the output of the electromagnetic wave coupling system 5 directly enters the interior of the resonant cavity 6. It is in the form of a T-shaped conduit with the end of the antenna 12 passing through the central part of the conduit, and the two lateral and opposite input/outputs of the conduit each oriented towards a side wall of the resonant cavity 6. Just as in the design mode of the , the plasma chamber 7 occupies the entire internal volume of the resonant cavity 6. Also, a dielectric 9 is affixed to each of the inlets/outlets of the conduit for the separation between the plasma chamber 7 and the electromagnetic wave coupling system 5.

In the design mode of the , the output of the electromagnetic wave coupling system 5 is connected to the resonant cavity 6 by its lower face with one of the ends of the antenna 12 penetrating inside the resonant cavity 6 below the substrate holder 8. The end of the antenna 12 is separated from the plasma chamber 7 by a dielectric 9 surrounding the lower face of the substrate holder 8, and which is also shaped to support/maintain/stabilize the substrate holder 8 inside the resonant cavity 6. Thus, in this design mode, the antenna distributes the electromagnetic waves under the substrate holder 8.

In the presented design mode , the resonant cavity 6 may comprise a precavity 13, interfacing with the output of the electromagnetic wave coupling system 5 and into which the antenna 12 penetrates. The precavity 13 has a pressure higher than that of the plasma chamber 7, and is interfaced with the latter by means of the dielectric 9. Advantageously, the precavity 13 allows the establishment of the electromagnetic waves before they propagate in the plasma chamber 7.

In the following description, the elements provided concern installation 1 regardless of its design method.

Depending on the application context, and as indicated above, plasma 2 is generated either at the excitation frequency (otherwise, the source frequency F) of 915 MHz, or at the excitation frequency of 2.45 GHz.

In reference to the (which for illustrative and explanatory purposes, shows the resonant cavity of the installation 1 according to the first design mode), the invention is remarkable in that it allows, first of all, to detect whether the plasma 2, which is positioned in a first position P1 located at a distance of between 0.5 mm and 3 mm (generally 1 mm) above the substrate holder 8 so that the deposition of a layer of material at the excitation frequency/source frequency F is carried out effectively, performs or does not perform a jump/change position. It should be noted that the position of the plasma 2 is assimilated to the distance or height of the plasma 2 with respect to the substrate holder 8.

In the remainder of the description, the source frequency F such that the plasma is in position P1 is called the initial frequency Fi. The plasma 2, if it performs a jump, can position itself in a second position P2 throughout the volume of the resonant cavity 6. In the illustrative example given, the position P2 can be located as close as possible to the dielectric 9, or under the substrate holder 8, or at the level of the internal side walls of the resonant cavity 6, etc.

The invention is then remarkable in that it implements a method 100, illustrated , making it possible to control during a control step S3 the position of the plasma 2 in the plasma chamber 7. In other words, if the plasma 2 has made a jump from the first position P1 to the second position P2, the latter is shaped to vary the source frequency F at which the microwave generator 3 generates the electromagnetic waves, i.e. the initial frequency Fi, until reaching a frequency called the correction frequency Fc for which the plasma 2 positions itself in a third position P3 which is either substantially close to the first position P1, or which corresponds to the first position P1.

In the application context where the initial frequency Fi is equal to 915 MHz (respectively 2.45 GHz), the correction frequency Fc is sought in the frequency interval between 902 MHz and 928 MHz (respectively between 2.4 GHz and 2.5 GHz).

Note that since the source frequency F can take different frequency values before the correction frequency Fc is identified and reached from the initial frequency Fi, the plasma 2 can possibly occupy, between the second position P2 and its return to the first position P1 or to the third position P3, different intermediate positions in the plasma chamber 7, called parasitic positions.

The control of the microwave generator 3 for the variation of the source frequency F in the event of a change in the position of the plasma 2 is ensured by a control unit 11 physically connected and/or in communication with the microwave generator 3; this control unit 11 may comprise a processor and/or a controller and/or an electronic card, as well as at least one memory in which a program is loaded allowing the implementation of the method.

The control unit 11 determines during a detection step Q1 whether or not the plasma 2 changes position in the plasma chamber 7 as a function of the variation in the value of a physical parameter Phy representative of the position of the plasma 2.

In other words, with reference to the , when plasma 2 is in the first position P1 at the initial frequency Fi, the value of the physical parameter Phy is equal to a first value val1. When plasma 2 changes position to position itself in the second position P2, the value of the physical parameter Phy also changes, going from the first val1 to a second value val2.

If the control unit 11 determines a jump of the plasma 2 to the initial frequency Fi, then the control step S3 is implemented which comprises:
- a scanning step S4 during which the control unit 11 controls the microwave generator 3 by sending it a control signal so that it varies the source frequency F and the initial frequency Fi in the frequency interval considered for the operation of the invention (i.e., between 902 MHz and 928 MHz, or between 2.4 GHz and 2.5 GHz), and
- a step S5 of determining the correction frequency Fc, during which the control unit 11 determines that the plasma 2 has returned to its first position P1 or to a third position P3 substantially close to the first position P1 when it detects that the value of the physical parameter Phy becomes substantially equal to val1 again. Following this detection, it sends a stop signal to the microwave generator 3 so that it stops the frequency sweep; the source frequency on which the microwave generator having stopped corresponds to the correction frequency Fc.

Note that the variation of the source frequency F, whether it is a decrease or an increase in frequency, is carried out progressively and not abruptly around the initial frequency Fi since too strong a variation could not allow the correction frequency Fc to be detected; or cause other jumps of the plasma 2, or even its extinction.

The values taken by the physical parameter Phy are measured, during a monitoring step S1, and transmitted, during a transmission step S2, continuously to the control unit 11 by a monitoring device 10, which is therefore physically connected and/or in communication with the latter, and which can be partly contained inside the resonant cavity 6 and/or the plasma chamber 7 to carry out the measurements.

In a first embodiment of the invention, the monitoring device 10 comprises a brightness detector comprising an optical fiber in the direction of a point in the plasma chamber 7, more precisely in the direction of the first position P1 located at a predetermined distance above the substrate holder 8. The monitoring device 10 transmits as a physical parameter Phy to the control unit 11 a light intensity representative of the brightness at the first position P1. Depending on whether or not the plasma 2 is positioned in the first position P1, the measured brightness intensity is different. Thus, depending on the values/variations in brightness intensity recorded, the control unit 11 is able to determine: whether at the initial frequency Fi, the plasma 2 has jumped from the first position P1 to reach a second position P2; and if when controlling the microwave generator 3 for the variation of the source frequency F following the jump of the plasma 2, the plasma has jumped from the second position P2 to return to the first position P1 or reach a third position P3 substantially close to the first position P1.

In a second embodiment of the invention, the monitoring device 10 comprises a microwave power measuring device, and transmits as a physical parameter Phy to the control unit 11 a power reflected Pr by the resonant cavity 6. The jump of the plasma 2 from the first position P1 to a second position P2 results in a more or less significant increase in the reflected power Pr, the resonant cavity 6 being, before the jump of the plasma 2, adapted. The return of the plasma 2 to the first position P1, or to a third position P3 substantially close to the first position P1, at the correction frequency Fc results in the vast majority of cases in a decrease in the reflected power Pr, the resonant cavity 6 being adapted again. Thus, the control unit 11 controls the microwave generator 3 and causes it to initiate or stop the frequency sweep depending on whether it detects an increase or a decrease in the reflected power Pr by the resonant cavity 6.

In a third embodiment, the monitoring device 10 comprises a device shaped to measure a temperature that can correspond to the target temperature Tm of the material layer/chemical deposit, or of a substrate 81, or of the substrate holder 8, which it transmits as a physical parameter Phy to the control unit 11. When the plasma 2 is, in its first position P1, close to the chemical deposit (for example, 1 mm above it) with a view to growing it, the chemical deposit is at a certain target temperature Tm. When the plasma 2 jumps to position itself in the second position P2, the target temperature Tm of the chemical deposit decreases. It is also conceivable that a decrease in the target temperature Tm can be measured at the substrate 81 or the substrate holder 8 if they are not manufactured with a temperature-sensitive material. The control unit 11 thus sends the control signal to the microwave generator 3 for a variation of the source frequency F when it detects a decrease in the target temperature Tm from the measurements transmitted by the monitoring device 10. Conversely, it sends the stop signal to the microwave generator 3 that it stops the frequency sweep when it detects an increase in the target temperature Tm at the chemical deposition, or at least one substrate 81, or the substrate holder 8, signifying that the plasma 2 has jumped from the second position P2 to reach the first position P1 or a third position P3 sufficiently close.

With reference to Figures 7 to 18, the plasma jump phenomenon 2 in a resonant cavity 6 is illustrated by means of simulations carried out under design/simulation software, from a three-dimensional modeling of a resonant cavity 6 of a plasma reactor. The effect of the variation of the source frequency F on the resonance in the resonant cavity 6 is also illustrated.

In reference to the , the three-dimensional modeling of the resonant cavity 6 includes:
- a plasma chamber 7 maintained under vacuum for the generation of plasma 2 and maintaining a reaction volume sealed against impurities;
- a substrate holder 8;
- a precavity 13 at a pressure higher than the plasma chamber 7 for establishing the electromagnetic waves before their arrival in the plasma chamber 7;
- a dielectric 9 (a quartz window) separating the precavity 13 from the plasma chamber 7;
- an electromagnetic wave coupling system 5 comprising an antenna 12 for routing the electromagnetic waves into the precavity 13.

Plasma 2 is considered to be a fictitious charge with modeled dielectric properties (permittivity, losses).

Three configurations are considered:
- the first configuration, illustrated in Figures 8 to 10, corresponds to the start-up of the installation 1 before a plasma 2 is generated in the plasma chamber 7;
- the second configuration, illustrated in Figures 11 to 14, corresponds to the application context for which the plasma 2 is located in the first position P1, 1 mm above the substrate holder 8; and
- the third configuration, illustrated in Figures 15 to 18, corresponds to the application context in which the plasma 2 has made a jump to position itself in the second position P2 which is here a position in which the plasma 2 is almost stuck or pressed under the dielectric 9; with the dielectric 9 positioned at a distance of 120 mm above the substrate holder 8, and the plasma 2 having a thickness of approximately 10 mm.

For each of the configurations ( , And ) :
- is traced, respectively in the , And , for different values of source frequency F between 2.4 GHz and 2.5 GHz, the ratio R between: the intensity E1 of the electric field measured at the first position P1 which is located 1 mm above the substrate holder 8 along the x axis; and the intensity E2 of the electric field measured at the second position P2 which is located 115 mm above the substrate holder 8 along the x axis and therefore 5 mm below the dielectric 9;
- is illustrated, respectively in the and the , the distribution of the electromagnetic field in the resonant cavity 6 when the source frequency F is equal to 2.4 GHz, 2.45 GHz and 2.5 GHz; and in the when the frequency F is equal to 2.4 GHz, 2.473 GHz and 2.5 GHz.

For the second and third configuration, the evolution of the reflected power Pr in the resonant cavity 6 for different values of source frequency F between 2.4 GHz and 2.5 GHz is also illustrated, respectively in Figures 13 and 17.

In the first configuration, before the ignition of plasma 2, the variation of the source frequency F causes a displacement of the field maxima in the cavity ( ). The ratio R of the intensities E1 and E2 of the electric fields measured at the substrate holder 8 at the first position P1 and below the dielectric 9 at the second position P2, increases from 2 for F = 2.4 GHz to 2.75 for F = 2.45 GHz and 3.6 for F = 2.5 GHz. This means that the probability of creating a plasma at the substrate holder 8 increases with the increase in the source frequency F.

In the second configuration, the distribution of the electric field in the resonant cavity 6 varies little as a function of the frequency ( ). In reference to the , the ratio R also varies little, being equal to 2 for F = 2.4 GHz, 2.15 for F = 2.45 GHz and 2.4 for F = 2.5 GHz ( ).

In reference to the , the reflected power Pr is about 20% at the source frequency F of 2.45 GHz. In practice, in plasma reactors, the reflected power Pr is reduced by an impedance matcher (typically a 3-stub matcher and possibly a short-circuit piston, located upstream of the electromagnetic wave coupling system 5). The three-dimensional model of the resonant cavity 6 presented is therefore similar to a system without an impedance matcher, with the reflected power Pr varying from 30% to 2% when the source frequency F varies between 2.4 GHz and 2.5 GHz. Thus, in one embodiment of the installation, the frequency variation can be considered as a solution to perform impedance matching, thus making it possible to do without a mechanical matcher.

In the third configuration, when plasma 2 is located under dielectric 9, the source frequency variation F, as illustrated in the , has a strong influence on the distribution of the electromagnetic field in the cavity. Indeed, as shown , the ratio R of the intensities E1 and E2 of the electric fields varies strongly with the variation of the source frequency F, reaching a maximum of 298 for F = 2.473 GHz. This value is about 25 times higher than that obtained for F = 2.5 GHz, about 50 times higher than its value for F = 2.4 GHz, and about 17 times higher than its value for F = 2.45 GHz. In other words, at F = 2.473 GHz, the electric field above the substrate holder 8 is about 300 times higher than that absorbed by the plasma 2.

Note that the ratio R of the intensities E1 and E2 of the electric fields at the source frequency F of 2.45 GHz increases from approximately 2.15 to 17 when the plasma moves from its initial position / first position P1 (illustrated by the second configuration) to its second position P2; meaning that it is possible that plasma 2 can reposition itself in another position. Generally speaking, the probability that plasma 2 jumps from the first position P1 to the second position P2 is higher than the probability of jumping from the second position P2 to the first position P1, especially when the plasma positions itself as here as close as possible to the arrival of the electromagnetic waves. However, in the example illustrated, the difference between the ratios R of the electric fields is such (the ratio R at F = 2.473 GHz is 50 times higher than that for the frequency F = 2.45 GHz) that the probability of jumping from the second position P2 to the first position P1 is high.

The three-dimensional modeling of the resonant cavity 6 thus shows that the electric field is particularly sensitive to the source frequency F when the plasma 2 is in a second position P2, and that a frequency variation can lead, following the strong changes in the ratio R of the electric field at the substrate holder 8 to the electric field in the plasma 2 in its second position P2, to a repositioning of the plasma at the substrate holder 8 (i.e. in its first position P1).

In reference to the , the reflected power Pr varies slightly for the different values of source frequency F but remains higher than when plasma 2 is in first position P1 (see ), showing that an abrupt variation of the reflected power Pr can indicate a plasma jump 2; especially since in the first position P1, the resonant cavity 6 is then adapted, causing the reflected power to then vary from almost 0 to several percent or even tens of percent. This is why the reflected power is considered, in one of the embodiments of the invention, as being the physical parameter Phy used to control the position of the plasma 2.

Optionally, with reference to the , the method 100 implemented for controlling the position of the plasma 2, after the latter has returned to the first position P1 or to a third position P3 sufficiently close to the correction frequency Fc, may comprise a step S6 of returning to the initial frequency Fi. During this step, the control unit 11 sends a return signal to the microwave generator 3 so that it varies the source frequency F from the correction frequency Fc to the initial frequency Fi.

This return to the initial frequency Fi can be implemented in the case where, compared to the initial frequency Fi, the plasma 2 would be less stable at the correction frequency Fc at the first position P1 or in the third position P3 nearby.

Following the return step S6 to the initial frequency Fi, the control unit is configured, during a stability verification step Q3, to check whether the plasma 2, following this return to the initial frequency Fi, again makes a jump or not from the first position P1 (or the third position P3 close by) to a second position P2 (which may correspond to the same second position P2 as previously, or to another position in the resonant cavity 6 or the plasma chamber 7).

If no jump is detected, then the frequency F remains at the initial frequency Fi. If, on the contrary, a new jump is detected by the control unit 11, the scanning step S4 and the step S5 of determining the correction frequency Fc are implemented again.

On the other hand, once the step S5 of determining the correction frequency has been carried out, the source frequency F remains at the correction frequency Fc, given that a return to the initial frequency Fi has previously caused a jump in plasma 2.

This is why the method implements, in the embodiment presented, following the step S5 of determining the correction frequency Fc, an adjustment verification step Q2 during which the control unit checks whether or not it has already controlled the microwave generator for a return or not to the initial frequency Fi. If this is the case, the source frequency F remains at the value of the correction frequency Fc. If not, the step S6 of returning to the initial frequency Fi is carried out.

A second application of the installation 1 is to allow the position of the plasma 2 to be controlled for efficient chemical deposition of a material layer on the substrates 81. As previously indicated, the plasma 2 is initially positioned in a first position P1 sufficiently close to the substrates and/or the substrate holder 8. However, as the chemical deposition progresses, the thickness of the material layer increases. Also, for long-term chemical deposition, it is possible for the thickness of the material layer to come into contact with the plasma 2 or partially penetrate inside, modifying the conditions of the chemical deposition and, as a result, altering the properties of the material layer.

Thus, during a growth limit determination step Q4, the control unit 11 detects whether at the initial frequency Fi, the value of the physical parameter Phy recorded by the monitoring device 10 is also in conformity with a growth threshold value representative of a favorable deposition condition, and which is predetermined.

In the case where the value of the physical parameter Phy is no longer in conformity with the growth threshold value, that is to say that the favorable deposition condition is no longer encountered (with the thickness of the material layer in interaction with the plasma 2), the control unit 11 initiates a second scanning step during which it sends a start signal to the microwave generator 3 so that it carries out a frequency scan from the initial frequency Fi. As a reminder, if the initial frequency Fi is equal to 915 MHz (respectively 2.45 GHz), the frequency scan is carried out between 902 MHz and 928 MHz (respectively between 2.4 GHz and 2.5 GHz).

The frequency sweep here aims to identify a source frequency F such that the plasma 2 performs a jump and repositions itself in a position sufficiently distant from the substrates 81 and/or the substrate holder 8 so that the deposition of the material layer is efficient again. As explained previously, the advantage of such an operating principle is to no longer have to use a removable substrate holder 8 for the deposition of the material layer, and to overcome its drawbacks; for example: a limited vertical translation of the substrate holder 8 may possibly induce a change in the resonance of the resonant cavity 6 and produce an undesired jump of the plasma 2).

Following the second scanning step S7, the control unit 11, during a step S8 of determining a growth frequency Fg, detects the value of the physical parameter Phy becomes consistent with the growth threshold value again. It then sends an end signal to the microwave generator 3 so that it stops the frequency variation. The source frequency F is then equal to a growth frequency Fg for which the deposition of the layer of material on the substrates 81 continues.

In connection with the description of the set of steps allowing the control of the position of plasma 2 when it makes an unwanted jump, the growth frequency Fg can be seen as a new initial frequency Fi, while the position in which plasma 2 is located at this frequency as a new first position P1.

When the growth is of long duration, the contact between the chemical deposit and the plasma 2 results in a drastic increase in the target temperature Tm. By heat transfer from the chemical deposit, the substrates 81 and the substrate holder 8 will also increase in temperature if they are sensitive to it.

When the target temperature Tm exceeds a critical growth temperature Tcrit, which is generally of the order of 1000°C, the growth conditions of the material layer deposition are altered. In the worst case, a degradation/deterioration of the properties of the chemical deposit is observed.

Thus, when the physical parameter Phy measured by the monitoring device corresponds to the target temperature Tm of the material layer, or of a substrate 81, or of the substrate holder 8, the control unit 11 is configured to detect during the growth limit determination step Q4, from the temperature measurements transmitted by the monitoring device 10 during the transmission step S2, whether or not the measured target temperature Tm exceeds the critical growth temperature Tcrit (which therefore corresponds in this embodiment to the growth threshold value). If this is the case, the second scanning step S7 and the step S8 of determining the growth frequency Fg are implemented. During the determination step S8, the control unit determines that the source frequency F corresponds to the growth frequency Fg when:
- the target temperature Tm measured by the monitoring device 10 falls below the critical growth temperature Tcrit, but also
- that the measured target temperature Tm does not fall below a minimum growth temperature (Tmin) for which any target temperature Tm below it does not allow chemical deposition.

In other words, the control unit 11 detects that the growth frequency Fg is reached when the measured target temperature Tm is between the critical growth temperature Tcrit and the minimum growth temperature (Tmin). It also manages to determine whether the decrease in target temperature Tm results in a jump in plasma 2:
- in a position sufficiently close to the chemical deposition for the chemical deposition to continue efficiently (i.e. a target temperature Tm measured above the minimum growth temperature Tmin), or on the contrary
- in a second position preventing further deposition (i.e. a target temperature Tm measured below the minimum growth temperature Tmin).

In the second case, the control unit 11 is configured to control the microwave generator 3 so that it varies the source frequency F, which then corresponds to an intermediate frequency Fx, until reaching a new frequency for which the measured target temperature Tm becomes higher than the minimum growth temperature Tmin while remaining lower than the critical growth temperature Tcrit, meaning that the plasma 2 is positioned at a suitable height of the chemical deposition for its growth. This new frequency is then considered to be the growth frequency Fg.

Claims (26)

Installation (1) for chemical vapor deposition assisted by a microwave plasma (2), the installation (1) comprising:
- a microwave generator (3) generating electromagnetic waves at a given source frequency (F) included in at least one frequency interval of a microwave frequency range,
- an electromagnetic wave coupling system (5) connected to the microwave generator (3), and which is configured to transfer the electromagnetic waves to a resonant cavity (6),
which resonant cavity (6) comprises at least:
- a plasma chamber (7) in which is arranged a substrate holder (8) on which at least one substrate (81) can be placed, said plasma chamber (7) being provided with an inlet for a reaction gas, and with a pumping system (71) for establishing a reduced pressure inside the plasma chamber (7),
- at least one dielectric (9), transparent to microwaves, separating the plasma chamber (7) from the electromagnetic wave coupling system (5) for propagation of the electromagnetic waves inside the plasma chamber (7) and through said dielectric (9),
the installation (1) being characterized in that it comprises at least one monitoring device (10) for measuring a physical parameter (Phy) dependent on a position of the plasma (2) inside the plasma chamber (7), in that the microwave generator (3) is a solid-state and variable-frequency generator, and in that it comprises a control unit (11) connected to the at least one monitoring device (10) and to said microwave generator (3) for varying the source frequency (F) as a function of a measurement of the physical parameter (Phy) for controlling the position of the plasma (2). Installation (1) according to claim 1, in which the source frequency (F) is initially at an initial frequency (Fi), and the control unit (11) is configured to determine a change in position of the plasma (2) generated at this initial frequency (Fi) from a first position (P1) to a second position (P2) distinct from the first position (P1), from a variation of the physical parameter (Phy) measured by the at least one monitoring device (10), and to vary the source frequency (F) from the initial frequency (Fi) until reaching a correction frequency (Fc) bringing the plasma (2) from the second position (P2) to substantially the first position (P1). Installation (1) according to claim 2, in which the control unit (11) is configured to determine the change in position of the plasma (2) when the physical parameter (Phy) varies from a first value (val1) to a second value (val2), and to vary the source frequency (F) from the initial frequency (Fi) to the correction frequency (Fc) until the physical parameter (Phy) varies inversely from the second value (val2) to substantially the first value (val1). Installation (1) according to claim 2 or 3, in which the control unit (11):
- following detection of the change in position of the plasma (2), is configured to send a control signal to the microwave generator (3) which, upon receipt of said control signal, varies the source frequency (F) in the at least one frequency interval;
- following a return of the plasma (2) to the first position (P1), is shaped to send a stop signal to the microwave generator (3) which, upon receipt of said stop signal, ceases to vary the source frequency (F) in the at least one frequency interval, the source frequency (F) then being equal to the correction frequency (Fc). Installation (1) according to any one of claims 2 to 4, in which once the plasma (2) has been repositioned in the first position (P1) following the variation of the source frequency (F), the control unit (11) is configured to vary the source frequency (F) of the microwave generator (3) from the correction frequency (Fc) until returning to the initial frequency (Fi). Installation (1) according to any one of the preceding claims, in which the source frequency (F) is initially at the initial frequency (Fi), and the control unit (11) is configured to vary the source frequency (F) of the microwave generator (3) from the initial frequency (Fi) until reaching a growth frequency (Fg) included in the at least one frequency interval, in response to a variation of the physical parameter (Phy) measured by the at least one monitoring device (10) such that said physical parameter (Phy) is no longer in conformity with a given growth threshold value, said growth frequency (Fg) being established by said control unit (11) so that said physical parameter (Phy) is again in conformity with the growth threshold value for this growth frequency (Fg). Installation (1) according to claim 6, in which the control unit (11) is configured to:
- in response to the variation of the physical parameter (Phy) such that said physical parameter (Phy) is no longer in conformity with the growth threshold value, sending a start signal to the microwave generator (3) which, upon receipt of said start signal, varies the source frequency (F) from the initial frequency (Fi) in the at least one frequency interval; and
- in response to the measurement of the physical parameter (Phy) such that said physical parameter (Phy) becomes compliant with the growth threshold value, send an end signal to the microwave generator (3) which, upon receipt of said end signal, stops varying the source frequency (F) which has reached the growth frequency (Fg). Installation (1) according to any one of the preceding claims, in which the at least one monitoring device (10) comprises a brightness detector comprising an optical fibre oriented towards a point in the plasma chamber (7) where it captures a light intensity of the plasma (2) which thus constitutes the physical parameter (Phy) associated with said brightness detector, such that said brightness detector detects a variation in the light intensity at said point in the case where the plasma (2) moves. Installation (1) according to claims 2 and 8, in which the optical fiber is oriented towards the first position (P1), the change in position of the plasma (2) at the initial frequency (Fi) being detected by the control unit (11) following the variation in the light intensity detected by the brightness detector. Installation (1) according to any one of the preceding claims, in which the at least one monitoring device (10) comprises a microwave power measuring device shaped to measure a power reflected (Pr) by the resonant cavity (6), which reflected power thus constituting the physical parameter (Phy) associated with said microwave power measuring device. Installation (1) according to claims 2 and 10, in which the control unit (11) detects at the initial frequency (Fi) the change in position of the plasma (2) when the microwave power measuring device measures an increase in the reflected power (Pr). Installation (1) according to any one of the preceding claims, in which the at least one monitoring device (10) comprises a temperature measuring device shaped to measure a target temperature (Tm) inside the plasma chamber (7), at the level of the chemical deposition on the at least one substrate (81), which target temperature (Tm) thus constituting the physical parameter (Phy) associated with said temperature measuring device. Installation (1) according to claims 2 and 12, in which the control unit (11) detects at the initial frequency (Fi) the change in position of the plasma (2) when the temperature measuring device measures a decrease in the target temperature (Tm). Installation (1) according to claims 6 and 12, in which the growth threshold value corresponds to a critical growth temperature (Tcrit) of the target temperature (Tm) measured by the temperature measuring device, such that the physical parameter (Phy) no longer complies with the growth threshold value when the target temperature (Tm) is greater than or equal to said critical growth temperature (Tcrit), and the physical parameter (Phy) again complies with the growth threshold value when the target temperature (Tm) becomes lower than the critical growth temperature (Tcrit). Installation (1) according to claim 14, in which the critical growth temperature (Tcrit) is equal to 1000°C, plus or minus 100 degrees. Installation (1) according to any one of the preceding claims, in which the frequency range of the microwaves is between 300 MHz and 10 GHz. Installation (1) according to claim 16, in which the at least one frequency interval is between 902 MHz and 928 MHz, or between 2400 MHz and 2500 MHz. Installation (1) according to any one of the preceding claims, in which the electromagnetic wave coupling system (5) comprises an antenna (12). Installation (1) according to any one of the preceding claims, in which the resonant cavity (6) comprises a precavity (13), having a pressure higher than that of the plasma chamber (7), and interfaced with the plasma chamber (7) by means of the at least one dielectric (9). Method (100) for chemical vapor deposition assisted by a microwave plasma (2), the method (100) being implemented by an installation (1) according to any one of the preceding claims, and comprising at least:
- a monitoring step (S1) during which the at least one monitoring device (10) performs a measurement of the physical parameter (Phy) dependent on the position of the plasma (2) inside the plasma chamber (7);
- a transmission step (S2) during which the at least one monitoring device (10) transmits the measurement of the physical parameter (Phy) to the control unit (11);
- a control step (S3) during which the control unit (11) varies the source frequency (F) as a function of the measurement of the physical parameter (Phy) to control the position of the plasma (2). Method (100) according to claim 20, wherein the controlling step (S3) comprises the following steps:
- a detection step (Q1) during which the control unit (11) determines, at the initial frequency (Fi), a change in position of the plasma (2) from a first position (P1) to a second position (P2), from a variation in the physical parameter (Phy) measured by the at least one monitoring device (10);
- a scanning step (S4) taking place after the detection step (Q1), during which the control unit (11) sends to the microwave generator (3) a control signal so that it varies the source frequency (F) from the initial frequency (Fi) in the at least one frequency interval;
- a step of determining (S5) the correction frequency (Fc) during which the control unit (11) determines the return of the plasma (2) from the second position (P2) to substantially the first position (P1), then sends a stop signal to the microwave generator (3) so that it stops varying the source frequency (F), the correction frequency (Fc) then now being considered as the source frequency (F). Method (100) according to claim 21, wherein, during the detection step (Q1), the control unit (11) is configured to determine the change in position of the plasma (2) when the physical parameter (Phy) varies from a first value (val1) to a second value (val2). Method (100) according to claim 21 or 22, for which, during the step of determining (S5) the correction frequency (Fc), the control unit (11) is configured to determine the return of the plasma (2) from the second position (P2) to substantially the first position (P1) when the physical parameter (Phy) varies from the second value (val2) to substantially the first value (val1). Method (100) according to any one of claims 21 to 23, wherein the method (100) comprises a step of returning (S6) to the initial frequency (Fi) which takes place after the step of determining (S5) the correction frequency (Fc), and during which the control unit (11), after having determined the return of the plasma (2) to the first position (P1), is shaped to send a return signal to the microwave generator (3) so that it varies the source frequency (F) from the correction frequency (Fc) to the initial frequency (Fi). Method (100) according to claim 24, wherein the method (100) comprises a stability verification step (Q3) which takes place after the step of returning (S6) to the initial frequency (Fi), during which the control unit (11) determines, when the source frequency (F) is again equal to the initial frequency (Fi), whether the plasma (2) again makes a change of position from the first position (P1) to the second position (P2); the method (100) then implementing again, if the change of position is determined, the scanning step (S4) and the step of determining (S5) the correction frequency (Fc). The method (100) of any one of claims 20 to 25, wherein said method (100) comprises:
- a growth limit determination step (Q4), during which the control unit (11) is configured to determine whether the physical parameter (Phy) measured by the at least one monitoring device (10) no longer complies with a given growth threshold value;
- a second scanning step (S7) taking place after the growth limit determination step (Q4), during which the control unit (11) sends to the microwave generator (3) a start signal so that it varies the source frequency (F) from the initial frequency (Fi) in the at least one frequency interval;
- a step (S8) of determining a growth frequency (Fg), during which the control unit (11) determines that the physical parameter (Phy) becomes again compliant with the growth threshold value and that the plasma (2) is located in a growth position such that the latter is distant from a given growth distance above the first position (P1), then sends an end signal to the microwave generator (3) so that it stops varying the source frequency (F) which is then equal to the growth frequency (Fg) established by said control unit (11) so that said physical parameter (Phy) is again compliant with the growth threshold value for this growth frequency (Fg).