Patent Publication Number: US-2023137818-A1

Title: High-frequency wave applicator, associated coupler and device for producing a plasma

Description:
TECHNICAL FIELD 
     The present invention relates to the field of producing plasma excited by a high-frequency wave. It has a particularly advantageous application in producing high-power plasma (power leading to power densities greater than 10 W/cm 2 ) and in the range of high pressures (pressure greater than 1 Torr, corresponding to around 133 Pa in the international unit system). 
     STATE OF THE ART 
     Numerous deposition techniques are plasma-assisted. For example, the deposition of a polycrystalline silicon or diamond film on a substrate can advantageously be performed by such techniques. It is reminded that a plasma is a conductive gas medium constituted of electrons, ions and neutral particles, and electrically macroscopically neutral. A plasma is in particular obtained by ionisation of a gas by electrons. In this case, plasmas excited by high-frequency electromagnetic waves are interesting, and more specifically, the range of microwaves. Certain applications require a deposition on wide surfaces, and therefore the generation of a uniform plasma in an extended production zone. For this, several technological solutions exist. 
     One of these solutions consists of spatially distributing high-frequency waves, and in particular microwaves, by using a waveguide wherein the waves are propagated and injected, via injection slots, in a chamber where the deposition is performed. However, the waveguides remain bulky and undesirable couplings between the waves injected via different injection slots can limit the stability of the plasma. 
     Another solution consists of distributing couplers independently powered with high-frequency waves. Generally, a coupler for producing a plasma is configured to transfer an electromagnetic wave of a rear end, connected to a wave generator, at a front end, where the coupling of the wave with electrons allows to generate a plasma. 
     To transfer the waves and couple them with the electrons in order to generate a plasma, the coupler comprises a front terminal part, named below as applicator. The applicator comprises a coaxial structure generally open at its front end where an electromagnetic field leads and radiates in the vacuum chamber of a plasma production device. 
     As illustrated by  FIG.  1   , a coupler has a rear end connected to a wave generator  5 , and comprises a wave applicator, generally constituted of two electrical conductors: an inner conductor  11  and an outer conductor  12 , together forming a coaxial structure  10 , the inner conductor  11  and the outer conductor  12  being separate together by a wave dielectric propagation medium  13 . The applicator can be disposed at a wall  300  of the chamber  30  of the device or inserted at least partially in the chamber. 
     The propagation medium  13  is constituted of at least one dielectric transparent to waves. The propagation medium  13  can comprise different dielectric materials disposed by sections. The propagation medium contains at least one wave passage dielectric  130  which has a solid body configured to obtain a vacuum sealing between at least one part of the propagation medium  13 , for example to the atmospheric pressure, and the vacuum chamber  30  of the plasma generation device. The passage dielectric  130  can, for example, be positioned at the front end of the applicator, or, as represented in  FIG.  1   , removed with respect to this end. 
     Moreover, a coupler is known from document WO03103003 A1, aiming to produce a plasma layer on the surface of the wall of the chamber of a plasma generation device. The coupler comprises an inner conductor substantially flush with the wall of the chamber, the inner conductor and the wall of the chamber being separated by a space coaxial to the inner conductor, forming the propagation medium. The coaxial space is filled at the end of the coupler by a wave passage dielectric having a solid body. 
     High-frequency wave applicators can however be subjected to significant energy flows at their front end, in contact with the plasma, and in particular when the coupler operates at high power and at high pressure. These energy flows are conveyed by significant heat quantities, inducing thermomechanical stresses. These constraints can lead to stresses and deformations of the elements constituting the applicator, even leading to their fracture. Thus, the distribution of waves by couplers remains limited to intermediate pressure ranges, not exceeding, generally, 0.5 Torr. This limits their use for deposits demanding, in addition to a high power, high pressures, like for example the deposition of diamond. 
     An aim of the present invention is therefore to propose a high-frequency wave applicator allowing a good power transfer, even a good coupling between an electromagnetic wave and electrons for the production of a plasma, by improving the dissipation of energy flows. 
     Other aims, features and advantages of the present invention will appear upon examining the following description and the accompanying drawings. It is understood that other advantages can be incorporated. 
     SUMMARY 
     To achieve this aim, according to a first aspect, a high-frequency wave applicator for a coupler for producing a plasma is provided, comprising:
         an inner conductor and an outer conductor together forming a coaxial structure extending in a main propagation direction of the wave inside the coaxial structure,   a high-frequency wave propagation medium delimited by an outer surface of the inner conductor and an inner surface of the outer conductor, and comprising a so-called high-frequency wave passage dielectric, the passage dielectric comprising a sealing solid body disposed between the inner conductor and the outer conductor,
 
the inner conductor has, in a transverse direction perpendicular to the main propagation direction, a first outer dimension d 1  taken between two points of its outer surface relatively opposite an axis of the coaxial structure, and the outer conductor has, in the transverse direction, an inner dimension d 2  taken between two points of its inner surface relatively opposite the axis of the coaxial structure.
       

     Advantageously, the first outer dimension d 1  and the inner dimension d 2  are such that: 
     
       
         
           
             0.2 
             &lt; 
             
               
                 
                   d 
                   2 
                 
                 - 
                 
                   d 
                   1 
                 
               
               
                 d 
                 2 
               
             
             &lt; 
             0.55 
           
         
       
     
     This ratio of dimensions of the inner conductor and of the outer conductor allows a good surface distribution of power, while maintaining a good coupling with the plasma and a low level of insertion losses. Thus, the applicator allows to generate a high-power and high-pressure plasma, while improving the dissipation of the energy flows on the surface of the applicator, and in particular on the surface of a front end of the inner conductor. The reliability of the applicator is thus increased, which allows to improve the stability and the reproducibility of the methods wherein the applicator is used. The applicator can thus be used for producing high-power plasma and in the range of high pressures. 
     Moreover, the surface distribution of power allows to extend the power deposition zone, and therefore that of producing plasma. 
     The applicator is particularly adapted to plasma-assisted deposition methods, such as PECVD (Plasma-Enhanced Chemical Vapour Deposition), and more specifically, large surface diamond deposition methods. These methods generally require high concentrations of species in the plasma generated, and preferably on extended surfaces, to accelerate the deposition speed and/or cadence. The applicator such as introduced above allows to respond to this necessity. 
     According to an example, the inner conductor and the outer conductor can together form a cylindrical coaxial structure extending in a main propagation direction. The inner conductor can have, in a transverse direction perpendicular to the main propagation direction, an outer radius r 1  and the outer conductor can have, in the transverse direction, an inner radius r 2 . The outer radius r 1  and the inner radius r 2  can be such that, with r 1  equal to d 1 /2 and r 2  equal to d 2 /2: 
     
       
         
           
             0.2 
             &lt; 
             
               
                 
                   r 
                   2 
                 
                 - 
                 
                   r 
                   1 
                 
               
               
                 r 
                 2 
               
             
             &lt; 
             0.55 
           
         
       
     
     A second aspect relates to a high-frequency wave coupler for producing a plasma, comprising:
         a coaxial structure formed from an inner conductor, and from an outer conductor, configured to be connected to a high-frequency wave generator,   a high-frequency wave applicator according to the first aspect, the coaxial structure of the applicator being disposed in the continuity of the coaxial structure of the coupler.       

     According to an example, the high-frequency wave applicator is configured to be removably fixed to the coaxial structure of the coupler. The applicator can thus be mounted on different coaxial coupler structures. These coaxial structures, of a lesser cost, can be designed to ensure a good coupling with the wave discharge coming from the generator, for different plasma impedances, for one same applicator. The use of a high-frequency wave coupler is therefore made more flexible. Furthermore, if only one from among the applicator and the coaxial structure is damaged, it is not necessary to change all of the coupler. According to an example, the applicator can be configured to be fixed manually by a user to the coaxial structure of the coupler. 
     A third aspect of the invention relates to a device for producing a plasma comprising a chamber and at least one high-frequency wave coupler according to the second aspect. 
     By the features of the coupler, and in particular of the applicator, the plasma production device has several advantages with respect to the current solutions. The reliability of the device is further increased by the improvement of the dissipation of the energy flows on the at least one coupler, while offering a good coupling, even an improved coupling. 
     The applicator could be mounted on different coaxial coupler structures, of a lesser cost, the associated investment costs are reduced, while allowing to use different operating conditions. The device is therefore adapted to different methods, and in particular to high-speed plasma-assisted and/or large deposition cadence methods. 
     According to an example, the device can comprise a plurality of couplers, the couplers being disposed on at least two, even three, walls of the chamber so as to form an at least two-dimensional, even three-dimensional network. The applicator allowing to extend the power deposition zone, and therefore that of producing plasma, a plurality of couplers can be used to obtain uniform plasmas on large dimensions. A species high-density uniform plasma can be obtained, which allows to considerably increase the speed of the methods implementing the device. Furthermore, the number of parts to be treated can be increased by increasing the number of couplers and, therefore, the volume of plasma generated. Thus, the production costs are decreased. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The aims, objectives, as well as the features and advantages of the invention will emerge best from the detailed description of an embodiment of the latter, which is illustrated by the following accompanying drawings, wherein: 
         FIG.  1    represents a view along a longitudinal cross-section of a coupler illustrating the state of the art. 
         FIG.  2    represents a view of the chamber of a plasma production device, according to an embodiment of the invention. 
         FIG.  3    represents a view along a longitudinal cross-section of a coupler, according to an embodiment of the invention. 
         FIG.  4    represents a view along a longitudinal cross-section of an applicator, according to a first embodiment of the invention. 
         FIG.  5    represents a view along a longitudinal cross-section of an applicator, according to a second embodiment of the invention. 
         FIG.  6    represents a view along a longitudinal cross-section of an applicator, according to a third embodiment of the invention. 
         FIG.  7    represents a view along a longitudinal cross-section of an applicator, according to a fourth embodiment of the invention. 
         FIG.  8    is a graph representing the surface distribution of power (in W·cm −2 ) on the front end of the applicator for several radius values of the inner conductor, according to an embodiment of the invention. 
         FIG.  9    is a graph of the insertion losses, in relative values, represented according to the relative dimensions of the applicator according to different embodiments of the invention. 
         FIG.  10    is a graph of the relative variation of the vacuum impedance, i.e. without plasma generation, on the front end of the applicator, standardised at its characteristic impedance, and represented according to the relative dimensions of the applicator according to different embodiments of the invention. 
         FIG.  10 A  is a graph of the variation of the ratio Z N /Z N   min  represented according to the relative dimensions of the applicator according to different embodiments of the invention. 
     
    
    
     The drawings are given as examples and are not limiting of the invention. They constitute principle schematic representations intended to facilitate the understanding of the invention and are not necessarily to the scale of practical applications. In particular, the relative dimensions of the different elements of the applicator are not necessarily representative of reality. 
     DETAILED DESCRIPTION 
     Before starting a detailed review of embodiments of the invention, below are stated optional features which can possibly be used in association or alternatively:
         the high-frequency wave has a frequency greater than 100 MHz. According to an example, the wave is a microwave wave, and in particular the wave has a frequency of between 300 MHz and 10 GHz. According to an example, the frequency can be 352 MHz, 433 MHz, 915 MHz, 2.45 GHz, 5.8 GHz,   the microwave passage dielectric can be in a thin window configuration. More specifically, the passage dielectric can be disposed at a front end of the propagation medium, and extend, in the main propagation direction, over a length substantially equal to a multiple of a tenth of a quarter of the wavelength of the wave and strictly less than a quarter of the wavelength of the wave. The passage dielectric is thus in a so-called thin window configuration. According to an example, the wavelength of the wave is its wavelength in the passage dielectric,   the coaxial structure can have a rotational symmetry about its axis,   the inner conductor can have, on a portion extending from a front end of the inner conductor, a narrowing so as to have, in the transverse direction, from the portion and to its rear end, a second outer dimension d 1′ , between two points of its outer surface relatively opposite the axis of the coaxial structure, the first outer dimension d 1  being greater than the second outer dimension d 1′ ,   the applicator can comprise a so-called overlay dielectric having a solid body and covering at least one front end of the inner conductor,   the passage dielectric can be disposed at a front end of the propagation medium, and the overlay dielectric can further cover a front end of the outer conductor and the passage dielectric,   the passage dielectric and the overlay dielectric can form an assembly having a common body without discontinuity,   the assembly formed by the passage dielectric and the overlay dielectric can have, in the main propagation direction and at the propagation medium, a length substantially equal to a multiple of a tenth of a quarter of the wavelength of the wave and strictly less than a quarter of the wavelength of the wave. According to an example, the wavelength of the wave is its wavelength in the passage dielectric,   the applicator can further comprise a cooling module disposed in the inner conductor, the cooling module comprising a cooling chamber delimited by a front end of the inner conductor. The inner conductor can have a reduced thickness at the level of the cooling chamber,   the thickness e 112  of the inner conductor at the cooling chamber can be less than or equal to       

     
       
         
           
             
               e 
               11 
             
             × 
             
               
                 k 
                 11 
               
               
                 k 
                 14 
               
             
           
         
       
         
         
           
             where k 11  and k 14  represent respectively the thermal conductivities of the inner conductor and of the overlay dielectric and e 11  the thickness of the inner conductor, 
             the applicator can comprise an overlay dielectric having a solid body and covering at least one front end of the inner conductor, and a ceramic junction disposed in contact between at least the overlay dielectric and the inner conductor, and preferably in contact between the inner conductor and the overlay dielectric and in contact between the inner conductor and the passage dielectric, 
             the passage dielectric, the ceramic junction and the inner conductor can be formed of materials, the ratio of which between them of their thermal expansion coefficients is between 0.5 and 1.5, 
             the applicator can further comprise a solder bead disposed between the passage dielectric and the outer conductor, 
             the passage dielectric, the solder bead and the outer conductor can be formed of materials, the ratio of which between them of their thermal expansion coefficients is between 0.5 and 1.5. 
           
         
       
    
     Below in the description, use will be made of terms such as “longitudinal”, “transverse”, “front” and “rear”. These terms must be interpreted relatively, relative to the normal position of use of the high-frequency wave applicator or of the coupler in the plasma production device. For example, by “front” end, this means the end of the applicator or of the coupler rotated towards the chamber of the plasma production device. The “rear” end means the end of the applicator or of the coupler rotated opposite, i.e. towards the outside of the plasma production device. “Longitudinal” means, with respect to the main extension direction of the applicator or of the coupler, parallel to the main propagation direction of the waves. 
     “Inner” means the elements or the faces rotated towards the inside of the applicator or of the coupler, and “outer” means the elements or the faces rotated towards the outside of the applicator or of the coupler. According to an example, the coaxial structure of the applicator and of the coupler having a central axis A, “inner” means the elements or the faces rotated towards this axis, and “outer” means the elements or the faces rotated opposite this central axis. 
     By a parameter “substantially equal to/greater than/less than” a given value, this means that this parameter is equal to/greater than/less than the given value, within 10% or less, even within 5% or less, of this value. 
     By a material of an element of the applicator or of the coupler with the basis of a compound A, this means an element comprising this compound A and possibly other materials, even the material is mainly formed of this compound A. 
     The thickness of an element or of a wall is measured, for at least one considered portion, at each point of the surface of the element or of the wall for the at least one considered portion, in a direction perpendicular to the tangent at this point. 
     The plasma production device  3  is described in reference to  FIG.  2   . The device comprises a chamber  30  having several walls  300 . At least one high-frequency wave coupler  2  is disposed on a wall  300  of the chamber  30 . The coupler  2  aims to ensure the propagation of an electromagnetic wave from a microwave generator to the inside of the chamber  30  with a minimum power loss. The coupler  2  further allows to couple an electromagnetic wave  4 , preferably high-frequency, transmitted by the coupler to the electrons. This coupling allows to ionise a gas or a gas mixture present in the chamber  30  to generate a plasma. The frequency of the wave can be greater than 100 MHz. More specifically, the frequency of the wave can be in the range of microwaves, and for example between 300 MHz and 10 GHz. Below, the non-limiting example is referred to, wherein the wave is a microwave wave. 
     For this, the device  3  can comprise gas introduction modules configured to supply gas or the gas mixture into the chamber  30 , as well as pumping modules, not represented in  FIG.  3    and known to a person skilled in the art. The gas introduction modules and the pumping modules allow to maintain the pressure of the gas to be ionised at a desired value, chosen in particular according to the nature of the gas, and the desired density of species in the plasma generated. 
     Typically, the pressure of the gas or of the gas mixture can be between a few milliTorr to a few tens of Torr (corresponding to around a few tenths of Pa to a few thousands of Pa, in the international unit system). More specifically, the plasma production device  3  is configured to operate in the range of high pressures, i.e. at a pressure greater than 1 Torr, corresponding to 133 Pa. Furthermore, the device  3  can be configured to operate at a high microwave power leading to high power densities, for example, at a power density greater than 10 W/cm 2 . 
     Indeed, the plasma production device  3  comprises a coupler  2 , configured to support the application of high powers and high pressures. Thus, the device  3  is adapted to the production of plasmas of very high densities of species, for example in the methods for treating high-speed plasma and/or high production cadence. As a non-limiting example, in particular plasma-enhanced chemical vapour deposition (PECVD) methods, such as deposition of diamond, deposition of polycrystalline silicon, deposition of anticorrosive film, resin removal are considered. 
     The coupler  2  can be disposed at a wall  300  of the chamber  30  so as to be flush with this wall  300  according to the example illustrated in  FIG.  2   , or inserted at least partially into the chamber  30 . Preferably, the coupler  2  is disposed so as to be flush with the wall of the chamber, to increase the uniformity of the plasma. 
     The device  3  can comprise a plurality of couplers  2 , in order to form a network extending over at least one wall  300  of the chamber  30 . By increasing the number of couplers, the volume of the plasma generated can be extended. Thus, the surface treated by the plasma and/or the number of parts to be treated can be increased, leading to a decrease in the production costs and allowing the implementation of methods for treating large surfaces. According to an example, a plurality of couplers  2  is disposed on at least two walls  300 , in order to form a two-dimensional network. According to the example illustrated in  FIG.  2   , a plurality of couplers  2  is disposed on three walls  300 , in order to form a three-dimensional network. Thus, the surface treated by the plasma and/or the number of parts to be treated can be further increased. Furthermore, it is possible to treat an object having a complex surface, for example the surface of the object extends into the three dimensions of the space. 
     The microwave coupler  2  is now described in reference to  FIG.  3   . The coupler  2  comprises a rear part and a front terminal part, referenced below by the term microwave applicator  1 . The rear part of the coupler  2  and the applicator  1  comprise an inner conductor  11 ,  21 , also referenced in the field by the term “central core”, and an outer conductor  12 ,  22 , also referenced in the field by the term “shielding”. The inner conductor  11 ,  21 , and the outer conductor  12 ,  22  are electrically conductive structures. The inner conductor  11 ,  21  extends in a main direction x between a front end  112 ,  212  intended to be directed towards the inside of the chamber  30  of the device  3 , and a rear end  113 ,  213 . The outer conductor  12 ,  22  extends in a main direction x between a front end  122 ,  222  intended to be directed towards the inside of the chamber  30  of the device  3 , and a rear end  123 ,  223 . For the rear part of the coupler  2  and the applicator  1 , the outer conductor  12 ,  22  surrounds the inner conductor  11 ,  21 , at least partially in a main direction x, and form a coaxial structure  10 ,  20  having a central axis A parallel to the main direction x. According to an example, each coaxial structure  10 ,  20  has a rotational symmetry about the central axis A called rotational axis. For example, the inner conductor  11 ,  21  and the outer conductor  12 ,  22  are cylindrical. Below, equivalently, the rear part of the coupler  2  is referenced by coaxial structure  20 . 
     In order to transmit the microwaves of the rear end of the coupler  2  to the front end of the applicator  1  where the plasma is produced, a propagation medium  13 ,  23  is delimited by the outer surface  111 ,  211  of the inner conductor  11 ,  21  and the inner surface  120 ,  220  of the outer conductor  12 ,  22 . The propagation medium  13 ,  23  is a dielectric medium, and therefore transparent to microwaves. This medium extends in a main propagation direction of the microwaves, parallel, even combined, to the direction x. The propagation medium  13 ,  23  can be formed of one from among several dielectric materials, such as air, quartz, and alumina. As illustrated in  FIG.  3   , the coaxial structure  13  of the applicator  1  and the coaxial structure  20  of the coupler  2  can be disposed in the continuity of one another. 
     The coupler  2  can be connected to a microwave generator  5  and be configured to inject microwaves into the propagation medium  13 ,  23 . For this, the inner conductor  21  has, at its rear end  213 , a bottom  2130  located at a distance d 7  from the connector for injecting microwaves  50  in the direction x, and delimiting the propagation medium  23  at the rear ends  213 ,  223  of the inner  21  and outer  22  conductors. This distance d 7  is generally chosen as a quarter of a wave λ/4, with λ the wavelength of the microwaves. It is noted that this distance d 7  can be different according to the design of the coupler  2 , and in particular, of its coaxial structure  20 . 
     The microwave applicator  1  can be arranged at a wall  300  of the chamber  30  according to the example illustrated in  FIG.  2   . For thus, the applicator  1  can comprise an abutment module  124 , for example with a fixed abutment  124  disposed on the perimeter of the outer conductor  12 . It is understood that according to the arrangement of the abutment module  124 , the applicator  1  can be flush with the wall  300  of the chamber  30 , or be inserted at least partially into the chamber  30 . 
     According to an example, the applicator  1  and the rear part of the coupler  2  can form one single and same part. Alternatively, the microwave applicator  1  can be configured to be removably fixed to the coaxial structure  20  of the coupler  2 . The microwave applicator  1  can thus be mounted on any coaxial structure  20  of the coupler  2  configured such that the coupler transmits the microwaves from one end to another of the coupler  2 . It is known to a person skilled in the art that the coaxial structures  20 , of a lesser cost, can be designed to ensure a good coupling with the discharge of microwaves coming from the generator  5 . For example, different coaxial structures  20  can be used for different plasma impedances, i.e. different pressure and power windows of use, for one same applicator  1 . The use of the microwave coupler  2  is therefore made more flexible. The investment cost associated with the device  3  is further reduced, since it is possible to use the applicator  1  for different operating conditions and, therefore different treatment methods. Furthermore, if only one from among the applicator  1  and the coaxial structure  20  of the coupler  2  is damaged, it is not necessary to change the assembly of the coupler  2 . 
     The applicator  1  can be fixed to the coaxial structure  20  of the coupler  2  by means of tools, or preferably manually by a user. For this, the applicator  1  can comprise a complementary fixing module  123 ′ of a fixing module  222 ′ of the coaxial structure  20  of the coupler  2 , and configured to secure the applicator  1  to the coaxial structure  20 . For example, these fixing modules have complementary threads. According to another example, these fixing modules have complementary reliefs specific to being clipped. According to the example illustrated in  FIG.  2   , the fixing module  123 ′ can be disposed at the rear end  123  of the outer conductor  12  of the applicator  1 , and the fixing module  222 ′ can be disposed at the front end  222  of the outer conductor  22 . Furthermore, the inner conductor  11  of the applicator can have a profile complementary to the front end  212  of the inner conductor  21  of the coaxial structure  2 , at its rear end  113 . In order to perform a vacuum sealing between the inner conductor  11 ,  21 , a seal, and for example, an O-ring  115 , can be disposed at their interface. Furthermore, the contact surface between the inner conductors  11 ,  21  extends in the direction  11  to improve the thermal transfer along inner conductors  11 ,  21 , as well as ensuring a good mechanical guiding when the applicator  1  is mounted on the coaxial structure  20 . 
     Moreover, the outer conductor  22  of the coaxial structure  20 , even the outer conductor  12  of the applicator has a nominal diameter compatible with the standards, such as the standards ND40 (40 mm), ND25 (25 mm), ND16 (16 mm). 
     The microwave applicator is now described in detail in reference to  FIGS.  4  to  7   . When a coupler  2  operates a high power and at high pressure, the applicator  1  in contact with the plasma is exposed to significant energy flows, which conveys an exposure to significant heat quantities. The applicator  1  is, in this case, configured for these significant energy flows, by an effective distribution of heat and its dissipation. Thus, the thermomechanical constraints leading to stresses and deformations, even a mechanical fracture of the elements of the applicator  1  are reduced, even avoided. 
     The applicator  1  has, more specifically, a configuration of the inner  11  and outer  12  conductors, as well as an assembly of different materials allowing its operation without damage, in particular when the energy flow to which the coupler  2  is exposed becomes significant. 
     For this, in a transverse direction y, perpendicular to the main propagation direction x, the inner conductor  11  has a first outer dimension d 1  between two points of its outer surface  111  relatively opposite the axis of the coaxial structure  10 , and the outer conductor  12  has an inner dimension d 2  between two points of its inner surface  120  relatively opposite the axis of the coaxial structure  10 , the first outer dimension d 1  and the inner dimension d 2  being relatively chosen so as to allow a good surface distribution of power, while maintaining a good coupling with the plasma and a low level of insertion losses. 
     The increase or the difference of the dimension d 1  of the inner conductor  11  with respect to the dimension d 2  of the outer conductor  12  allows to considerably improve the surface distribution of power. However, this increase or difference can induce, on the one hand, a quasi-exponential increase in insertion losses (α c ), which decreases the power transmitted to the plasma and, on the other hand, an increase in the impedance in the outlet plane of the vacuum-radiating applicator Z V  standardised to the characteristic impedance Z 0 , referenced Z N  below, with Z N =Z v /Z 0 , which degrades the coupling. 
     In this case, the dimension d 1  of the inner conductor  11  with respect to the dimension d 2  of the outer conductor  12  can be limited according to the insertion losses (α c ) in the conductors  11 ,  12  of the applicator and according to the standardised impedance Z N . For example, at constant dimension d 2 , according to a standardised diameter of the outer conductor, and at a fixed frequency, the dimension d 1  is chosen such that:
         Δα c /α cmin  is less than 180%, where Δα c  corresponds to the difference between α c  and α cmin , α cmin  corresponding to the insertion loss coefficient in the inner  11  and outer  12  conductors;   Z N /Z Nmin  is less than 1.65, where Z Nmin  is equal to (Z V /Z 0 ) min , Z Nmin  corresponding to an impedance in the outlet plane of the vacuum-radiating applicator close to the characteristic impedance Z 0 , Z Nmin  pushing towards 1.       

     Starting with the conditions above, the decrease in the dimension d 1  of the inner conductor  11  with respect to the dimension d 2  of the outer conductor  12  allows to minimise, both the insertion losses, and the standardised impedance. However, this decrease reduces the distribution surface of the power. In this case, the maximum decrease of the dimension d 1  of the inner conductor  11  can be limited by the minimum values of the insertion losses (α c ) and of the standardised impedance Z N . Beyond these minimum values, not only the power is disadvantageously distributed over a very small surface, but also the insertion losses and the standardised impedance Z N  increase drastically again. 
     During the development of the invention, a ratio of the dimensions d 1  and d 2  of the respectively inner  11  and outer  12  conductors has been highlighted in order to obtain the extension of the power distribution zone, while maintaining a low level of insertion losses and of the ratio of standardised impedances Z N /Z Nmin , i.e. impedances in the outlet plane of the vacuum-radiating applicator quite close to the characteristic impedance of the applicator (Z V /Z 0 &lt;10). 
     The first outer dimension d 1  and the inner dimension d 2  are such that 
     
       
         
           
             0.2 
             &lt; 
             
               
                 
                   d 
                   2 
                 
                 - 
                 
                   d 
                   1 
                 
               
               
                 d 
                 2 
               
             
             &lt; 
             0.55 
           
         
       
     
     According to the example wherein the inner  11  and outer  12  conductors are cylindrical, with d=2r, the following ratio is obtained. 
     
       
         
           
             0.2 
             &lt; 
             
               
                 
                   r 
                   2 
                 
                 - 
                 
                   r 
                   1 
                 
               
               
                 r 
                 2 
               
             
             &lt; 
             0.55 
           
         
       
     
     Preferably, the decrease of the dimension d 1  of the inner conductor  11  is limited by Δα c /α cmin &gt;15% and Z N /Z Nmin &gt;1.01 to have a sufficiently extended power distribution surface. Thus, the ratio of the dimensions presented in the two abovementioned ratios can be between 0.2 and 0.55. The ratio of the dimensions presented in the two abovementioned ratios can be more limited and between 0.2 and 0.4. 
     Thus, the surface power is distributed on the surface of the inner conductor  11 , while minimising the insertion losses and the standardised impedance, and more specifically by keeping the insertion losses low of between 15 and 180*α cmin  and of the impedances of 1.01 to 1.65*Z Nmin , i.e. relative differences of 0.01 to 0.65*Z Nmin . 
     Below, it is considered, in a non-limiting manner, that the inner  11  and outer  12  conductors are cylindrical, and therefore form a cylindrical coaxial structure  10 . 
     As an example,  FIG.  8    illustrates the surface distribution of the microwave power (in W·cm −2 ) on the front end of the applicator  1  for several radius values of the inner conductor  11 , for an argon discharge at a pressure of substantially 1 Torr by a couple of nominal diameter 25 mm when it is powered by 30 W of microwave power at 915 MHz. It is observed that the more the radius r 1  of the inner conductor increases from 3.1 to 9.5 mm, the more the microwave power is distributed along the inner conductor  11  in a direction transverse to the rotational axis A. 
       FIG.  9    illustrates a graph of the insertion losses, in relative values, calculated according to the ratio (r 2 −r 1 )/r 2  for a coaxial waveguide made of aluminium of different nominal diameters (abbreviated to ND) given in mm, with an air propagation medium  13  at a microwave frequency of 915 MHz or of 2.45 GHz. According to the example illustrated in  FIG.  9   , a loss minimum of α cmin =3.10 −3  m −1  can be obtained for a radius r 1  of 3 to 4 mm (that is a relative loss of Δα c /α cmin =(α c −α cmin )/α cmin ≈0), but the power deposited in the plasma, as illustrated by  FIG.  8   , remains localised on a radius zone of around the radius r 1  of the inner conductor, which is broadly less than the radius r 2  of the outer conductor  12 , of 12.5 mm according to this example. For a radius r 1  of between 7 mm and 9.5 mm, the relative insertion losses Δα c /α cmin  are less than 180% while allowing a better surface expansion of power, according to  FIG.  8   . 
       FIG.  10 A  is proposed as a reduced version of  FIG.  10    so as to make reading it easier by a person skilled in the art. In this regard,  FIG.  10 A  proposes a direct representation of the ratio Z N /Z N   min  instead of the representation of the relative values (Z N /Z N   min −1)×100 in % that  FIG.  10    gives. In addition,  FIG.  10 A  illustrates the range of values of the relative dimensions of the applicator such as introduced above. 
     For the three values considered as an example in  FIG.  8    for an applicator ND25 or radius r 2  of 12.5 mm,  FIG.  10    shows that the best coupling by having Z N /Z Nmin ≈1.03&gt;1.01, corresponds to the radius r 1  of 3.1 mm, but the ratio (d 2 −d 1 )/d 2 =0.75 does not satisfy the criterion (d 2 −d 1 )/d 2 &lt;0.55. In addition, according to  FIG.  8   , the power is concentrated on a narrow distribution zone, of radius comparable to the radius r 1  which leads to very high power densities (2 kW/cm 2  for a power of 600 W supplied to the applicator). The maximum value (d 2 −d 1 )/d 2 =0.55 is reached for a radius r 1  of 5.6 mm. For one same radius r 2 , the coupling corresponding to the best surface distribution of power (0.2 kW/cm 2  for a power of 600 W) is obtained for the radius of 9.5 mm with the ratios (d 2 −d 1 )/d 2 =0.24 and Z N /Z Nmin ≈1.48 included in the range of validity. The maximum value (d 2 −d 1 )/d 2 =0.2 is reached for a radius r 1  of 10 mm. According to this example of an embodiment of an applicator ND25 of radius r 2  of 12.5 mm, this can therefore have a radius r 1  of maximum 10 mm and of minimum 5.6 mm to respond to the desired criteria from the standpoint of insertion losses and plasma coupling. 
     The insertion losses being kept at a low level and the coupling between the microwaves  4  and the electrons being ensured, the applicator  1  allows the production of a high-power plasma with an advantageous distribution of power and therefore the dissipation of energy flows on the surface of the applicator. The thermomechanical strength of the applicator  1  is thus improved. Its reliability is therefore increased, which allows to improve the stability and the reproducibility of the methods wherein the applicator  1  is used. The applicator  1  can thus be used for the high-power production of plasma and in the range of high pressures, for the production of plasma with a high density of species. 
     This configuration of the applicator  1  allows to extend the power deposition zone of the microwaves and, therefore that of generating plasma. This has, as a consequence, the reduction of discontinuity between the generation zones when a plurality of couplers  2  is disposed in a plasma production device  3 . Uniform plasmas over large dimensions can thus be obtained. 
     Synergistically, a uniform plasma with a high density of species can be generated, in particular when couplers are disposed in an at least two-dimensional network. This allows to considerably increase the speed of a treatment method implementing the applicator  1 . Uniform and high-pressure treatment methods can be implemented over large surfaces, which resolves one of the main challenges of high-pressure plasmas of current solutions. Moreover, the maintenance costs are reduced, thanks to the increase of the reliability of the applicator  1 . 
     The propagation medium  13  of the applicator  1  is now described in detail. The propagation medium  13  is constituted of at least one dielectric transparent to microwaves, and for example, air. The propagation medium  13  further comprises a passage dielectric  130  of the microwave  4  have a so-called sealing solid body, based on a dielectric material, and disposed between the inner conductor  11  and the outer conductor  12 . The term “solid” specifies a solid state with respect to a gaseous or liquid state. The passage dielectric  130  is configured so as to allow the passage of the microwave  4  from the propagation medium  13  to the chamber  30 . The passage dielectric is further configured so as to maintain a vacuum sealing between the chamber  30  and the rest of the propagation medium  13 , which is for example, at atmospheric pressure. 
     According to the example illustrated in  FIG.  4   , the passage dielectric can be positioned at the front ends  112 ,  122  of the inner  11  and outer  12  conductor, so as to form a dielectric stopper at the front end  131  of the propagation medium  13 . 
     The passage dielectric  130  of the microwaves can be in a thin window configuration. For this, the passage dielectric  130  has a length L, in the main propagation direction x, substantially equal to a multiple of a tenth of a quarter of the wavelength of the microwave  4  in the passage dielectric  130 . This configuration has several advantages, with respect to the current solutions, wherein the length of the passage dielectric is a multiple of a half and/or a quarter of the wavelength of the microwaves. The length of the passage dielectric  130  can be less than that of current solutions, which facilitates the dissipation of the energy flows in the dielectric, and therefore its cooling. Furthermore, a thin window limits the impedance mismatch between the applicator  1  constructed and that provided by digital simulations. This mismatch is in particular induced by possible differences between the dielectric permittivity of the passage dielectric  130 , indicated by the suppliers, used as input data during the digital design of the couplers  2 , and the actual dielectric permittivity. Thus, the applicator  1  allows to limit, even avoid, a power loss of the microwaves. 
     To guarantee the sealing by the passage dielectric  130  between the dielectric  130  and the outer conductor  12 , a bead  17  is disposed at their interface. A bead  17  corresponds to a metal connection between the dielectric  130  and the outer conductor  12 . The bead is preferably a solder bead  17 , allowing a fusionless connection of the dielectric  130  and of the outer conductor  12 , different from a solder bead. The solder bead  17  allows to replace an O-ring  18  generally used for this function, as illustrated in  FIG.  1   . Yet, during the use of a coupler  2 , an O-ring disposed at the end of the applicator  1  in contact with the plasma, can overheat and be damaged, even destroyed. This can lead to electromagnetic leakages, even coupling instabilities. Furthermore, the solder bead  17  confers a mechanical solidity to the applicator. Preferably, and as described in more detail below, the solder bead is made of metal. 
     Synergically, the thin window configuration facilitates the soldering operation. During this operation, it is easier to control the diffusion of the solder bead over a shorter distance and thus ensure the sealing. 
     The applicator further comprises an overlay dielectric  14 , a part with the basis of a dielectric material configured to cover at least the front end  112  of the inner conductor  11 . According to an example, the overlay dielectric  14  further covers the front end  122  of the outer conductor  12  and the passage dielectric  130  on its front face. The overlay dielectric  14  can thus cover all of the surface of the applicator  1  in contact with the plasma. The overlay of the surface of the applicator  1  allows to form a barrier to the chemical reactions which could be activated by the high temperature of this surface and, therefore, to protect the applicator against a contamination of the method. The reliability of the applicator is thus further increased. 
     The overlay dielectric  14  and the passage dielectric  130  can further be juxtaposed in the direction x without discontinuity. For example, it can be provided that the overlay  14  and passage dielectrics of the microwaves  130  are juxtaposed without forming a common body, these dielectrics being, for example, assembled using a ceramic junction. 
     Preferably, the overlay dielectric  14  can form a common body with the passage dielectric  130 . The overlay dielectric  14  and the passage dielectric  130  can be directly juxtaposed in the direction x without discontinuity and be formed of the same material. Thus, the mechanical adjustment stresses between the passage dielectric  130  and the overlay dielectric  14  are thus avoided. The problems with misalignment of the dielectrics during mounting are further offset. Moreover, the formation of microcavities between the passage dielectric  130  and the overlay dielectric  14  is thus avoided. The formation of micro-plasmas in these microcavities can cause a local overheating and a deterioration of the applicator  1 . The dissipation of energy flows on the surface of the applicator is therefore further improved. 
     The assembly formed by the passage dielectric  130  and the overlay dielectric  14  can be in a thin window configuration. For this, the assembly formed by the passage dielectric  130  and the overlay dielectric  14  can have a length L, in the main propagation direction x and at the propagation medium  13 , substantially equal to a multiple of a tenth of a quarter of the wavelength of the microwave  4  in the passage dielectric  130  and strictly less than a quarter of the wavelength of the wave. 
     The overlay dielectric  14  can be thin, and in particular as thin as possible. The minimum thickness of the overlay dielectric  14  is more specifically imposed by its mechanical strength. For example, the thickness of the overlay dielectric  14  is substantially greater than 100 μm (10 −4  m). 
     To improve the dissipation of heat on the surface of the applicator  1  in contact with the plasma, the applicator comprises a cooling module  15  allowing an effective transfer of the quantity of heat, deposited by the plasma on the applicator  1 . This cooling module  15  is configured to make a cooling liquid  153  circulate, for example, water, to dissipate the heat received by the applicator  1  from the plasma by transferring it to the cooling liquid  153 . 
     As illustrated by  FIG.  4   , the cooling module  15  can be disposed inside the inner conductor  11 . The cooling module  15  can comprise a cooling chamber  150 , configured to engage with an injection element  151  of the cooling liquid  153  disposed on the coaxial structure  20  of the coupler  2 , and a discharge conduit  152  of this liquid. 
     The cooling chamber  150  can be delimited by the front end  112  of the inner conductor  11 , by its inner surface  110 . The injection element  151 , such as a bevelled needle, can lead into the cooling chamber  150 , facing the front of the applicator  1 . 
     The discharge conduit  152  can extend from the cooling chamber  150  in the direction x in the inner conductor  21  of the coaxial structure until crossing the bottom  2130 , so as to discharge the cooling fluid  153  once the heat transfer is performed. The discharge conduit  152  can more specifically be delimited by the inner surface  210  of the inner conductor  21 . 
     According to the example illustrated by  FIG.  4   , the inner radius r 5  of the inner conductor  11  can be greater than the inner radius r 3  of the inner conductor  21 . Thus, the cooling chamber  150  allows to make the cooling liquid in contact circulate with a maximum of the front face  112  and with the inner surface  110  of the outer conductor. 
     The applicator  1  can be configured so as to have no air pocket between the cooling chamber  150  and the overlay dielectric  14 . For this, the applicator  1  can comprise a ceramic junction  16 , a part with the basis of a ceramic material disposed in contact between at least the overlay dielectric  14  and the inner conductor  11 , and preferably also in contact between the inner conductor  11  and the passage dielectric  130 , and configured to establish a junction between these elements. The ceramic junction  16  can be configured so as to establish a direct contact, without film or air pockets, between the inner conductor  11  and the overlay dielectric  14  and passage dielectric  130 . Indeed, the presence of layers or air pockets is damaging from the standpoint of heat dissipation due to the very low thermal conductivity of air, of around 0.5 to 0.6 W·K −1 ·m −1  over a range of 800 to 1000 K, with respect to those of the surrounding materials, described in detail below, and for example alumina (30 W·K −1 ·m −1 ), Kovar (17 W·K −1 ·m −1 ), or also aluminium (238 W·K −1 ·m −1 ). Synergically, with the cooling module  15 , the heat transfer and therefore the dissipation of the energy flows are further improved. 
     According to an example, the inner conductor  11  can have, on a portion  114 , a narrowing  114 ′. More specifically, and as illustrated by  FIGS.  5  to  7   , the inner conductor  11  can have, from its first radius r 1  end, a narrowing  114 ′ to have from the portion  114  and to its rear end  113 , a second radius r 1′ , the first radius r 1  being greater than the second radius r 1′ . Thus, in a direction going from the rear to the front of the applicator  1 , the inner conductor  11  has a portion aligned with the inner conductor  21  of the coaxial structure  20 , then has an extended portion  112 ′ on its front end  112 . According to a projection perpendicular to the direction x, the perimeter of the portion aligned with the inner conductor  21  of the coaxial structure  20  can be completely comprised in the perimeter of the extended portion  112 ′. According to the example illustrated by  FIGS.  5  to  7   , and in a direction going from the front to the rear of the applicator  1 , the narrowing  114 ′ extends from a rear end of the passage dielectric  130 . The wall of the inner conductor  11  at the narrowing  114 ′ can further extend obliquely with respect to the direction x. 
     The outer radius r 1′  of the inner conductor  11  and the outer radius r 4  of the inner conductor  21  can thus be reduced, while preserving the configuration of the end  112  of the inner conductor  11  allowing a compromise between distribution of heat flows and minimisation of insertion losses. The ratio of the radiuses r 1′ /r 2 , and r 4 /r 2  can thus be decreased, to improve the transfer of microwaves, by minimising the phenomena of reflection and/or appearance of stationary waves. Subsequently, the applicator allows to further limit, even avoid, a loss of power of the microwaves. 
     The narrowing  114 ′ moreover allows to increase the inner surface  110  of the inner conductor  11  in contact with the cooling fluid  153  at the cooling chamber  150 . The thermal transfer and therefore the dissipation of the energy flows are further improved. 
     With or without the narrowing  114 ′, the thickness e 112  of at least one part of the front end  112  of the inner conductor  11 , at the cooling chamber  150 , can be minimised. With the thickness of the inner conductor  11  being reduced, the cooling of the front end of the applicator  1  is facilitated. At the connection between the inner conductors  11 ,  21 , the thickness e 11  of the inner conductor  11  can be between e 112  and 2*e 112 . 
     The thickness e 112  of the inner conductor  11  and/or the thickness of the overlay dielectric  14  can more specifically be linked to the thermal resistance of each of the two materials forming these elements. This thermal resistance is preferably low to not induce significant temperature gradients in the materials, which would lead to damaging stresses and deformations, such as fissures in the passage dielectric  130  and/or in the overlay dielectric  14 . 
     The thickness e 112  of the inner conductor  11  at the cooling chamber  150  can be less than or equal to: 
     
       
         
           
             
               e 
               11 
             
             × 
             
               
                 k 
                 11 
               
               
                 k 
                 14 
               
             
           
         
       
     
     where k 11  and k 14  respectively represent the thermal conductivities of the inner conductor  11  and of the overlay dielectric  14  and e 11  the thickness of the inner conductor. 
     According to an example, the thickness of the conductor  21 , defined by the difference between its outer radius r 4  and its inner radius r 3 , is greater than the thickness of the inner conductor  11  to improve the mechanical strength of the coupler  2 . 
     The relative position of the inner  11  and outer  12  conductors is now described in reference to  FIGS.  4  to  7   . More specifically, the conductors can be in one same plane or offset against one another. As illustrated by  FIGS.  4  and  5   , the inner  11  and outer  12  conductors can be aligned such that their front end  112 ,  122  are disposed in one same plane P 1 . Furthermore, the passage dielectric of the microwaves  130  can be aligned on its front face in the same plane. 
     Alternatively, the front end  112  of the inner conductor  11  can be disposed removed from the front end  122  of the outer conductor  12 . According to the example illustrated in  FIG.  6   , the front end  112  of the inner conductor  11  can more specifically be disposed at a distance d 5  of the front end  122  from the outer conductor  12 , d 5  which could preferably be limited such that the thickness of the assembly formed by the overlay dielectric  14  and the passage dielectric of the microwaves  130 , at the front end of the passage medium  13  of the microwaves, that is in the thin window configuration. 
     Alternatively, the front end  112  of the inner conductor  11  can be disposed in front of the front end  122  of the outer conductor  12 . According to the example illustrated in  FIG.  7   , the front end  112  of the inner conductor  11  can more specifically be disposed at a distance d 6  of the front end  122  from the outer conductor  12 , d 6  which could preferably be limited such that the thickness of the assembly formed by the overlay dielectric  14  and the passage dielectric of the microwaves  130 , at the front end of the passage medium  13  of the microwaves, that is in the thin window configuration. 
     It is noted that although the examples illustrated in  FIGS.  6  and  7    have a narrowing  114 ′, the relative different positions of the conductors  11 ,  12  can apply with or without the narrowing  114 ′. Furthermore, according to the relative position of the conductors  11 ,  12 , the dimensions of the assembly formed by the passage dielectric  130  and the overlay dielectric  14  can be adapted, in particular, to respecting the thin window configuration. 
     As stated above, the different constitutive elements of the applicator  1  are formed of materials allowing its operation without damage, in particular when the energy flow to which the coupler  2  is exposed becomes significant. The materials chosen are preferably compatible from the thermal and chemical standpoint, in order to be able to:
         solder between the outer conductor  12  and the passage dielectric  130 , even the overlay dielectric  14 ,   produce the junction between the dielectrics  130 ,  14  and the inner conductor  11 ,   prevent the creation of thermal bridges and the appearance of thermomechanical constraints leading to stresses and deformations, even to the mechanical fracture, of the elements constituting the applicator  1 , even the coupler  2 ,   guarantee the mechanical solidity of the assembly.       

     The materials which meet these criteria are now described. At the interfaces between different elements of the applicator  1 , the materials of the elements are a given interface can have thermal expansion coefficients of these close materials, for example the ratio of which between them, or equivalently, the ratio two-by-two, is between 0.5 and 1.5, and preferably between 0.8 and 1.2. Thus, the deformation risk of these elements against one another is limited during the use of the applicator  1 . This feature relates more specifically to the assembly formed by the overlay dielectric  14 , the passage dielectric  130 , the ceramic junction  15  and the inner conductor  11 , and/or the assembly formed by the passage dielectric  130 , the solder bead  17  and the outer conductor  12 . 
     The overlay dielectric  14  preferably has a good chemical stability at high temperature, and preferably at a temperature greater than 300° C. For example, the overlay dielectric  14  is made of alumina Al 2 O 3 . The overlay dielectric  14  is thus stable with respect to the metal materials generally used to cover the front end of the couplers, such as aluminium or stainless steel. In addition, the metals have a lower melting point T f  (T f-Al =660° C. against T f-Al2O3 =2054° C., at atmospheric pressure), and can induce a contamination of the plasma, and therefore of the method, with metal vapours. 
     The outer conductor  12  can comprise at least two portions formed of separate materials, in order to improve the chemical and physical compatibility with other elements close to the applicator, in particular concerning possible thermal deformations during the operation of the applicator  1 . 
     In order to solder between the outer conductor  12  and the passage dielectric  130 , even the overlay dielectric  14 , the materials of these elements are preferably thermally compatible together and chemically with the material of the solder bead  17 , comprising for example, a copper and silver alloy. The front end  122  of the inner conductor is therefore preferably iron, nickel and cobalt alloy-based with a low thermal dilatation coefficient, such as Kovar©, and the passage dielectric  130 , even the overlay dielectric  14 , made of alumina. An iron, nickel and cobalt alloy with a low thermal dilatation coefficient, such as Kovar©, can in particular be used to seal together the pairs of glass/metal or ceramic/metal materials in a wide temperature range and for multiple applications. It can therefore be used to solder with a dielectric, for example made of alumina Al 2 O 3 . Furthermore, Kovar© and alumina have close thermal expansion coefficients (TEC): TEC Kovar ≈5-6×10 −6  K −1  and TEC Al2O3 ≈8−9×10 −6  K −1 . 
     The outer conductor  22  and the inner conductor  21  of the coaxial structure  20  of the coupler  2  can be with the basis of a metal having a high thermal conductivity, such as silver, copper, aluminium, duralumin, a conductive brass, respectively having a thermal conductivity of 400, 380, 238, 160 and 120 W·K −1 ·m −1 . Indeed, the conductors of the coaxial structure  20  are cooled very effectively by the bottom  2130  of the coupler  2 , illustrated in  FIG.  3   , which increases the dissipation speed of the energy flows. It is noted that the choice of the metal can further be made so as to minimise the insertion losses of the microwaves. Preferably, the outer conductor  22  and the inner conductor  21  are aluminium-based. 
     The assembly formed by the outer conductors  12 ,  22  is preferably vacuum-sealed. For this, the outer conductor  12  can comprise a front portion  125  made of Kovar© and a rear portion  126  made of metal, the front portion  125  and the rear portion  126  being able to be, for example, welded together. In order to allow this welding, the metal of the rear portion  126  preferably has a melting point T f  close to the front portion  125 . For example, the rear portion  126  is made of stainless steel (abbreviated to inox): T f-Kovar =1450° C. and T f-inox ≈1500° C. As stated above, the portion  126  can be assembled to the outer conductor of the coaxial structure  20  by the fixing module  123 ′, for example by a thread. 
     The inner conductor  11  of the applicator is preferably made of iron, nickel and cobalt alloy with a low thermal dilatation coefficient, such as Kovar©. Indeed, aluminium is not very thermally compatible with the alumina of the overlay dielectric  14  and of the passage dielectric  130 , for example in terms of thermal expansion coefficients (TEC Al2O3 ≈8-9×10 −6 K −1 &lt;&lt;TEC Al =23-25×10 −6  K −1 ). 
     The ceramic junction preferably has a good temperature stability and a high thermal conductivity. A ceramic adhesive, or equivalently, an alumina-based ceramic gluing cement can be used, such as 903HP having a melting point T f-903HP  equal to 1790° C., and a thermal conductivity of around 5.6 W·K −1 ·m −1 . 903HP, further has a good chemical compatibility with alumina Al 2 O 3  and Kovar©, as well as a close thermal expansion compatibility (TEC Kovar ≈5-6×10 −6  K −1 , TEC 903HP =7.2×10 −6  K −1 , TEC Al2O3 ≈8-9×10 −6  K −1 ). 
     In view of the description above, it clearly appears that the invention proposes a high-frequency wave applicator allowing a good transfer, even a good coupling between an electromagnetic wave and electrons to produce a plasma, by improving the dissipation of the energy flows. 
     The invention is not limited to the embodiments described above and extends to all the embodiments covered by the claims. 
     In the description above, it is considered that the inner and outer conductors are cylindrical. The conductors can sometimes have the whole geometry allowing to form a coaxial structure and allowing the transfer and the coupling of a high-frequency wave. 
     LIST OF REFERENCES 
     
         
         
           
               1 . Microwave applicator 
               10 . Coaxial structure 
               11 . Inner conductor 
               110 . Inner surface 
               111 . Outer surface 
               112 . Front end 
               112 ′. Extended portion 
               113 . Rear end 
               114 . Portion 
               114 ′. Narrowing 
               115 . O-ring 
               12 . Outer conductor 
               120 . Inner surface 
               121 . Outer surface 
               122 . Front end 
               123 . Rear end 
               123 ′. Fixing module 
               124 . Abutment element 
               125 . Front portion 
               126 . Rear portion 
               13 . Propagation medium 
               130 . Passage dielectric 
               131 . Front end 
               14 . Overlay dielectric 
               140 . Front face 
               141 . Rear face 
               15 . Cooling module 
               150 . Cooling chamber 
               151 . Injection needle 
               152 . Discharge conduit 
               153 . Cooling fluid 
               16 . Ceramic junction 
               17 . Solder bead 
               18 . O-ring 
               2 . Microwave coupler 
               20 . Coaxial structure 
               21 . Inner conductor 
               210 . Inner surface 
               211 . Outer surface 
               212 . Front end 
               213 . Rear end 
               2130 . Bottom 
               22 . Outer conductor 
               220 . Inner surface 
               221 . Outer surface 
               222 . Front end 
               222 ′. Fixing module 
               223 . Rear end 
               23 . Propagation medium 
               3 . Production device 
               30 . Chamber 
               300 . Walls 
               4 . Wave 
               5 . Microwave generator 
               50 . Microwave injection connector