Abstract:
A radiofrequency plasma reactor with first and second spaced electrodes has a concave surface facing a substrate supporting surface. A process area between the electrodes has a gas inlet for a process gas. A radiofrequency generator for frequencies greater than 13.56 MHz is connected to an electrode for generating a plasma discharge in and a gas outlet evacuates process gas. A dielectric layer has a convex surface engaging the concave electrode surface and an opposite planar surface. The substrate supporting surface receives a substrate of at least 0.7 m and defines a boundary of the process area to be exposed to the plasma. The dielectric layer is electrically in series with the substrate and plasma discharge and has capacitance per unit surface values which are not uniform for a distribution profile to compensate process non-uniformity along the working surface.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a divisional of application Ser. No. 09/824,936 filed Apr. 3, 2001, now abandoned which is a divisional of application Ser. No. 09/401,158 filed Sep. 22, 1999, now U.S. Pat. No. 6,228,438, and which claims priority of Swiss application number 1466/99 filed Aug. 10, 1999. The disclosures of all of these applications are incorporated here by reference in their entireties. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The invention relates to a capacitively coupled radiofrequency (RF) plasma reactor and to a process for treating at least one substrate in such a reactor. Especially, the present invention applies to a large size capacitive capacitively coupled (RF) plasma reactor. 
     Often, such a reactor is known as a “capacitive” RF glow discharge reactor, or planar plasma capacitor or parallel plate RF plasma reactor, or as a combination of the above named. 
     Capacitive RF plasma reactors are typically used for exposing a substrate to the processing action of a glow discharge. Various processes are used to modify the nature of the substrate surface. Depending on the process and in particular the nature of the gas injected in the glow discharge, the substrate properties can be modified (adhesion, wetting), a thin film added (chemical vapor deposition CVD, diode sputtering) or another thin film selectively removed (dry etching). 
     The table shown below gives a simplified summary of the various processes possibly performed in a low pressure capacitive discharge. 
     
       
         
               
               
               
               
             
           
               
                   
               
               
                   
                 Substrate 
                   
                   
               
               
                 Industry 
                 type 
                 Process 
                 Inlet gas nature 
               
               
                   
               
             
             
               
                 Semiconductor 
                 Wafer up to 
                 Surface Cleaning 
                 Ar 
               
               
                   
                 30 cm 
                 PECVD 
                 SiH 4 , . . . 
               
               
                   
                 diameter 
                 Dry Etching 
                 CF 4 , SF 6 , Cl 2 , . . . 
               
               
                   
                   
                 Ashing 
                 O 2   
               
               
                 Disks for 
                 Polymeror 
                 Diode sputtering 
                 Ar + others 
               
               
                 memory 
                 glass up to 
                 PECVD 
                 Organometallics 
               
               
                   
                 30 cm 
                 Surface activation 
                 O 2 , etc. . . . 
               
               
                   
                 diameter 
               
               
                 Flat display 
                 Glass up to 
                 Same as for 
                 Same as for 
               
               
                   
                 1.4 m 
                 semiconductors 
                 semiconductors 
               
               
                   
                 diagonal 
               
               
                 Window pane 
                 Glass up to 
                 Cleaning/activation, 
                 Air, Argon - 
               
               
                 web coater 
                 3 m width, 
                 Nitriding, polymer 
                 Monomer, 
               
               
                   
                 foil, plastic 
                 PECVD 
                 Nitrogen, . . . 
               
               
                   
                 or metal 
               
               
                   
               
             
          
         
       
     
     The standard frequency of the radiofrequency generators mostly used in the industry is 13.56 MHz. Such a frequency is allowed for industrial use by international telecommunication regulations. However, lower and higher frequencies were discussed from the pioneering days of plasma capacitor applications. Nowadays, for example for PECVD applications, (plasma enhanced chemical vapor deposition) there is a trend to shift the RF frequency to values higher than 13.56 MHz, the favorite values being 27.12 MHz and 40.68 MHz harmonics of 13.56 MHz). 
     So, this invention applies to RF frequencies (1 to 100 MHz range), but it is mostly relevant to the case of higher frequencies (above 10 MHz). The invention can even be applied up to the microwave range (several GHz). 
     An important problem was noted especially if the RF frequency is higher than 13.56 MHz and a large size (surface) substrate is used, in such a way that the reactor size is no more negligible relative to the free space wavelength of the RF electromagnetic wave. Then, the plasma intensity along the reactor can no longer be uniform. Physically, the origin of such a limitation should lie in the fact that the RF wave is distributed according to the beginning of a “standing wave” spatial oscillation within the reactor. Other non uniformities can also occur in a reactor, for example non uniformities induced by the reactive gas provided for the plasma process. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to propose a solution for eliminating, or at least notably reducing, an electromagnetic (or a process) non uniformity, in a reactor. Thus, according to an important feature of the invention, an improved capacitively coupled RF plasma reactor should comprise:
         at least two electrically conductive electrodes spaced from each other, each electrode having an external surface,   an internal process space enclosed between the electrodes,   gas providing means for providing the internal process space with a reactive gas,   at least one radiofrequency generator connected to at least one of the electrodes, at a connection location, for generating a plasma discharge in the process space, and potentially an additional RF generator for increasing the ion bombardment on the substrate,   means for evacuating the reactive gas from the reactor, so that said gas circulates within the reactor, at least in the process space thereof,   at least one substrate defining one limit of the internal process space, to be exposed to the processing action of the plasma discharge, said at least one substrate extending along a general surface and being arranged between the electrodes,
 
characterized in that it further comprises at least one dielectric “corrective” layer extending outside the internal process space, as a capacitor electrically in series with said at least one substrate and the plasma, said at least one dielectric layer having capacitance per unit surface values which is not uniform along at least one direction of said general surface, for compensating a process non uniformity in the reactor or to generate a given distribution profile.
       

     In other words, the proposed treating process in the reactor of the invention comprises the steps of:
         locating the at least one substrate between at least two electrodes, the substrate extending along a general surface,   having a reactive gas (or gas mixture) in an internal process space arranged between the electrodes,   having a radiofrequency generator connected to at least one of the electrodes, at a connection location,   having a plasma discharge in at least a zone of the internal process space facing the substrate, in such a way that said substrate is exposed to the processing action of the plasma discharge,   creating an extra-capacitor electrically in series with the substrate and the plasma, said extra-capacitor having a profile, and   defining the profile of the extra-capacitor in such a way that it has location dependent capacitance per unit surface values along at least one direction of the general surface of the substrate.       

     It is to be noted that such a solution is general. It is valid for all plasma processes, but only for a determined RP frequency. 
     The “tailored extra-capacitor” corresponding to the above-mentioned said (substantially) “dielectric layer” acts as a component of a capacitive divider. 
     Advantageously, the capacitive variations will be obtained through a non uniform thickness of the layer. Thus, the extra-capacitor will have a profile having a non planar-shape along a surface. 
     For compensating a non uniform voltage distribution across the process space of the reactor, said thickness will preferably be defined in such a way that:
         the so-called “corrective layer” is the thickest in front of the location in the process space (where the plasma is generated) which is the farthest away from the connection location where the radiofrequency generator is connected to said at least one electrode, the distance being measured by following the electrode external surface,   and said thickness preferably decreases from said process space location, as the distance between the process space location and the connection location on the corresponding electrode decreases.       

     Of course, it is to be understood that the above-mentioned “distance” is the shortest of all possible ways. 
     So, if the electromagnetic traveling waves induced in the process space combine each other near the center of the reactor to form a standing wave having a maximum of voltage in the vicinity of the reactor center, the thickness of the so-called “corrective layer” will be larger in the vicinity of the center thereof, than at its periphery. 
     One solution in the invention for tailoring said “corrective layer” is to shape at least one surface of the layer in such a way that the layer has a non planar-shaped external surface, preferably a curved concave surface facing the internal process space where the plasma is generated. Various ways can be followed for obtaining such a “non planar shaped” surface on the layer. 
     It is a privileged way in the invention to shape at least one of the electrodes, in such a way that said electrode has a non planar-shaped surface facing the substrate, and especially a generally curved concave surface. 
     It is another object of the invention to define the composition or constitution of the so-called “corrective layer”. 
     According to a preferred solution, said layer comprises at least one of a solid dielectric layer and gaseous dielectric layer. 
     If the layer comprises such a gaseous dielectric layer, it will preferably be in gaseous communication with the internal process space where the plasma is generated. 
     A substrate comprising a plate having a non planar-shaped external surface is also a solution for providing the reactor of the invention with the so-called “corrective layer”. 
     Another object of the invention is to define the arrangement of the substrate within the reactor. Therefore, the substrate could comprise (or consist in) a solid member arranged against spacing members located between said solid member and one of the electrodes, the spacing member extending in said “corrective layer” along a main direction and having, each, an elongation along said main direction, the elongations being non uniform along the solid member. 
     So, the invention suggests that the spacing members preferably comprise a solid end adapted to be arranged against the solid member, said solid end having a space therearound. 
     Below, the description only refers to a capacitively coupled RF plasma reactor in which the improvements of the invention notably reduce the electromagnetic non uniformity during the plasma process. 
     First of all, for most processing plasmas, the electromagnetic propagation brings really a limitation in RF plasma processing for substrate sizes of the order, or larger than 0.5 m 2  and especially larger than 1 m 2 , while the frequency of the RF source is higher than 10 MHz. More specifically, what is to be considered is the largest dimension of the substrate exposed to the plasma. If the substrate has a substantially square surface, said “largest dimension” is the diagonal of the square. So, any “largest dimension” higher than substantially 0.7 m is critical. Thus, the substrate for the present invention has a largest dimension of at least 0.7 m. 
     A basic problem, which is solved according to the present invention, is that, due to the propagative aspect of the electromagnetic wave created in the plasma capacitor, the RF voltage across the process space is not uniform. If a RF source is centrally connected to an electrode, the RF voltage decreases slightly from the center to the edges of said electrode. 
     As above-mentioned, one way to recover a (substantially) uniform RF voltage across the plasma itself, is the following:
         a capacitor is introduced between the electrodes, said capacitor being in series with the plasma (and the substrate) in the reactor,   this extra-capacitor acts with the plasma capacitor itself as a voltage divider tailoring the local RF power distribution, to (substantially) compensate a non uniformity of the process due, for example, to gas compositional non uniformity, to edge effects or to temperature gradient.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Below is a more detailed description of various preferred embodiments according to the invention, in reference to drawings in which: 
         FIGS. 1 and 2  are two schematic illustrations of an improved reactor according to the invention ( FIG. 1  is a section of  FIG. 2  along lines I-I). 
         FIGS. 3 ,  4 ,  5 ,  6 ,  7  and  8  show alternative embodiments of the internal configuration of such a reactor. 
         FIGS. 9 ,  10 ,  11 ,  12  and  13  show further schematic embodiments of typical processes corresponding to the invention. 
         FIG. 14  illustrates the “tailoring” concept applied to a variation of thickness. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIGS. 1 and 2 , the reactor is referenced  1 . Reactor  1  encloses two metallic electrodes  3 ,  5  which have an outer surface,  3   a ,  5   a,  respectively. The electrodes are spaced from each other. 
     A gas source  7  provides the reactor with a reactive gas (or a gas mixture) in which the plasma is generated through a radiofrequency discharge (see the above table). Pumping means  8  are further pumping the gas, at another end of the reactor. 
     The radiofrequency discharge is generated by a radiofrequency source  9  connected at a location  9   a  to the upper electrode  3 . The location  9   a  is centrally arranged on the back of the external surface  3   a  of the electrode. 
     These schematic illustrations further show an extra-capacitor  11  electrically in series with the plasma  13  and a substrate  15  located thereon. 
     The plasma  13  can be observed in the internal space (having the same numeral reference) which extends between the electrode  3  and the substrate  15 . 
     The substrate  15  can be a dielectric plate of a uniform thickness  e  which defines the lower limit of the internal process space  13 , so that the substrate  15  is exposed to the processing action of the plasma discharge. The substrate  15  extends along a general surface  15   a  and its thickness  e  is perpendicular to said surface. 
     The extra-capacitor  11  interposed between the substrate  15  and the lower electrode  5  induces a voltage modification in such a way that the RF voltage (V P ) across the plasma (for example along line  17 , between the electrode  3  and the substrate  15 ), is only a fraction of the radiofrequency voltage (V RF ) between the electrodes  3 ,  5 . 
     It is to be noted that the extra-capacitor  11  is materially defined as a dielectric layer (for example a ceramic plate) having a non uniform thickness e 1  along a direction perpendicular to the above-mentioned surface  15   a.    
     Since the location of the RF source on the electrode  3  is central, and because of the arrangement (as illustrated in  FIGS. 1 and 2 ) of the above-mentioned elements disposed in the reactor, the thickness e 1  of the dielectric plate  11  is maximal at the center thereof and progressively decreases from said center to its periphery, in such a way to compensate the electromagnetic non uniformity in the process space  13 . So, the presence of said relatively thick series capacitor  11  reduces the effective voltage across the plasma. Hence, for the compensation of electromagnetic effects in a large surface reactor as illustrated in  FIGS. 1 and 2 , the series capacitor  11  has to be a bit thicker in the center of the reactor and must be thinned down toward the periphery thereof. 
     The schematic illustrations of  FIGS. 3 to 8  show various possible configurations allowing such a compensation of non uniformity in a capacitively coupled radiofrequency plasma reactor, of the type illustrated in the above  FIGS. 1 and 2 . It will be noted that combinations of the basic options illustrated in  FIGS. 3 to 8  are possible. 
     In  FIG. 3 , a flat, planar ceramic plate  21  of a uniform thickness e 2  is attached to the upper electrode  23 . There is a tailored spacing  31  between the metal electrode  23  and the ceramic plate  21 . Above the other electrode  25  is arranged a substrate  35  which can be either dielectric or metallic (or electrically conductive on at least one of its surface). 
     In  FIGS. 3 to 8 , the location of the connection between the power source (such as the RF source  9  of  FIGS. 1 and 2 ) and the corresponding metallic electrode is supposed to be centrally arranged on said electrode, and the general geometry of the reactor is also supposed to be as illustrated, so that, in such conditions, the tailored layer  31  has a back surface  31   a  which is curved with a concave regular profile facing the process space  13 . 
     Thus, the corresponding upper electrode  23  (the internal limit of which, facing the process space  13 , is defined by surface  31   a ) has a variable thickness e 3 . The dimension e 3  is the thinnest at the center of the electrode and the thickest at its periphery. 
     The second opposed electrode  25  is generally parallel to the first electrode  23  and has a uniform thickness e 4 . 
     It will be noted that the connection between the solid dielectric plate  21  and the tailored gap  31  is not a gas-tight connection. So, the reactive gas introduced within the process space  13  can circulate in the gap  31  which will preferably have a thickness adapted for avoiding a plasma discharge therein. Providing the “corrective gap”  31  with complementary means for avoiding said plasma discharge therein is also possible. 
     In  FIG. 4 , the electrode  23  has the same internal profile  31   a  as in  FIG. 3 . 
     But, the “corrective layer” is presently a ceramic plate  41  having a variable thickness e 5 . 
     In  FIGS. 5 to 8 , the substrates  35 ′ are dielectric substrates. 
     In  FIG. 5 , the above electrode  33  is a planar metallic electrode having a uniform thickness e 4 . The lower electrode  45  corresponds to the upper electrode  23  of  FIG. 3 . The electrode  45  has an internal upper surface  51   b  which defines a rear limit for the curved concave gaseous “corrective layer”. Above said layer  51  is arranged a dielectric planar horizontal plate  21 . The ceramic plate  21  of a uniform thickness e 2  is connected at its periphery to the lower electrode  45  (counterelectrode). The substrate  35 ′ is arranged on the ceramic plate  21 . 
     Since the pressure of the reactive gas adapted to be introduced within the reactive space is typically between 10 −1  Pa to 10 3  Pa, the pressure within the gaseous corrective gap can be substantially equal to said injected gas pressure. Typically, the reactive gas pressure within the plasma discharge zone  13  will be comprised between 1 Pa and 30 Pa for an etching process, and will be comprised between 30 Pa and 10 3  Pa for a PECVD process. Accordingly, the pressure within the corrective gap ( 31 ,  51  . . . ) will typically be a low pressure. So, such a gaseous dielectric gap could be called as a “partial vacuum gap”. 
     In  FIG. 6 , the substrate  35 ′ (of a uniform thickness) is laying on a solid dielectric plate (surface  41   a ) which can correspond to the ceramic plate  41  of  FIG. 4  in an inverted position. The front, inner surface  41   a  of the plate  41  is flat, while its back surface  41   b  is convex and directly in contact with the lower metallic electrode  45 , the inner surface of which is presently concave. So, the plate  41  is a sort of “lens”. 
     The electrodes  33 ,  45  illustrated in  FIG. 7  correspond to the electrodes of  FIG. 5 . The substrate  35 ′, which has a uniform thickness, is planar and parallel to the upper metallic electrode  33 . Substrate  35 ′ is laying on small posts  47  which are erected between the electrode  45  and the substrate. The non planar internal upper surface  51   b  of the electrode  45  gives a non uniform thickness e 6  to the gaseous gap  61  between the electrode  45  and the substrate  35 ′. Thus, the space  61  acts as a corrective dielectric layer for compensating the process non uniformity and enables the substrate  35 ′ to be uniformly treated by the plasma discharge. 
     In  FIG. 8 , the two opposed electrodes  25 ,  33  have a uniform thickness, are planar and are parallel from each other. The tailored layer  71  is obtained from a non planar substrate  65  arranged on erected posts  57 . The elevations of such “spacing elements”  57  are calculated for giving the substrate  65  the required non planar profile. 
     The design of  FIG. 8  should be mechanically the most attractive, because both electrodes  33 ,  25  remain flat and the profile of the small gap  71  is defined by the inserts  57 . 
     For any purpose it may serve, it will be noted that the radiofrequency power can be fed either on the electrode on which the substrate is attached, or on the opposite electrode. 
     In the examples of arrangements illustrated in  FIGS. 1 to 8 , it will further be noted that the tailored layer ( 11 ,  31 ,  41 ,  51 ,  61 ,  71 ) will preferably have a thickness calculated as a Gaussian bell-shape for the electrode to electrode distance (on the basis of the above-mentioned “central” arrangement). Then, said tailored layer itself will be deduced from a truncation of the bell-shape, what is left, namely the pedestal of the bell-shape after truncation is the space for the plasma gap (internal process space  13 ), and the substrate. 
       FIGS. 9 to 15  show other embodiments of an improved capacitively coupled radiofrequency plasma reactor, according to the invention. 
       FIG. 9  shows the most straightforward implementation of the invention. The radiofrequency power source  9  is centrally connected to an upper electrode  3  called “shower head electrode” having holes  83  through its lower surface facing the plasma process space  13 , within the inner chamber  81  of the reactor  10 . The counter-electrode  30  is defined by the metallic external wall of the chamber  81 . The admission of the reactive gas is not illustrated. But the pumping of said reactive gas is made through the exhaust duct  85 . 
     It will be noted that all the mechanical (material) elements arranged within the reactor  10  and illustrated in  FIG. 9  are kept flat (electrodes and substrate  135 , notably). However, the substrate  135  (which has a uniform thickness e 7 ) is curved by laying it on series of spacing elements  87  erected between the substrate and the counter-electrode  30 . The spacing supports  87  have variable height. The substrate  135  is curved due to its own flexibility. The average distance between the supports is defined by the substrate thickness and its Young modulus. 
     In this arrangement, there are two layers in the space between the electrodes that are not constant (uniform) in thickness: the plasma process space  13  itself and the “corrective space”  89  behind the substrate. Although this example is not a straightforward solution, this configuration is effective, because the RF power locally generated in the plasma depends far more on the little variation of the thin “gaseous” capacitive layer behind the substrate, than the small relative variation of the thickness e 8  of the plasma process space  13  (along the direction of elongation of electrode  3 ). 
     The “corrective” tailored layer  89  is, in that case, behind the substrate. It is a gaseous (or partial vacuum) tailored layer, such a wording “vacuum” or “gaseous” being just used to stress the fact that this layer has a dielectric constant of 1. The layer can contain gases (the dielectric constant is not affected). 
     There is a danger that the supports  87 , whether they are metallic or dielectric, introduce a local perturbation of the process. 
     Indeed, just at the support level where the series capacitor of the tailored “corrective” layer  89  is not present, the RF field is locally going to be larger. The perturbation, as seen by the plasma, is going to spread over a given distance around the support. This distance scales as the substrate thickness e 7  plus the “plasma sheath thicknesses” (typically 2-4 mm) referenced as  13   a  and  13   b  in  FIG. 9 . 
       FIG. 9   a  shows a potential way to reduce to a bearable level the perturbation due to a support. The solution consists in surrounding each spacing member  89  by a small recess  91 . At the recess level, the capacitive coupling is reduced. By adjusting the recess to make an exact compensation, the local perturbation should be practically eliminated. 
     In relation to the invention, such an arrangement shows that the tailored “corrective” layer proposed in the invention should follow the tailored profile, on the average: very local perturbations on the profile could be accepted as long as the capacitive coupling, remains substantially continuous and properly tailored, when averaged over a scale of a few millimeters. 
     In the arrangement of  FIG. 9 , the substrate  135  is a dielectric member. This is important, since any tailored dielectric layer (such as  89 ) must absolutely be within the space defined by the two extremely opposed metallic layers defining the “process gap”. If a substrate is metallic (electrically conductive), it screens off the effect of any underlying tailored capacity. Then, the substrate must be considered as one of the electrode. 
     In  FIG. 10  is illustrated a rather common design in the process industry. The reactor  20  is fed with two different driving energy sources: a RF high frequency source (higher than 30 MHz) and a RF bias source  93  (lower than 15 MHz). The upper “shower head” electrode  3  is connected to the high frequency source  91  and the low electrode  45  is connected to the RF bias source  93 . 
     One of the sources is meant to provide the plasma (in that case, we assume that it is an RF driving frequency with a rather high frequency, through source  91 ). The other source  93  is presently used as an additive to provide an extra ion bombardment on the substrate  35 . Typically, such an extra input ( 93 ) is plugged on the “susceptor” side and is driven at 13.56 MHz. 
     Such a RF bias feature is often used in etching systems to provide the reactive ion etching mode. It has been used in combination with many types of plasma (such as microwave, or electron cyclotron resonance). 
     In the example of  FIG. 10 , there are two electrodes ( 3 ,  45 ) facing each other. None of them is actually grounded. However, even in that particular configuration, the tailored capacitor of the invention (layer  95  of a non uniform thickness) is appropriate. In the case of  FIG. 10 , the configuration of  FIG. 5  is implemented. An important feature is that the active part of the reactor  20  (plasma process space  13 , substrate  35 , flat planar dielectric plate  21  of a uniform thickness and tailored gaseous gap  95  of a non uniform thickness) is between two metallic plates (electrodes  3 ,  45 ). The fact that one is grounded or not and the fact that one or several RF frequencies are fed on one and/or the other electrode, are irrelevant. The most important fact is that there is an RF voltage difference propagating between the two metallic plates  3 ,  45 . In the example of  FIG. 10 , two RF frequencies are used. The drawing shows two injections (up and down) for the two RF waves. It is arbitrary. They could be injected from the top together, or from the bottom (upper electrode  3  or lower electrode  45 ). What is important here is that there are two different frequencies, one high frequency and one low frequency. Both propagate in the capacitive reactor. 
     If, as proposed, a tailored capacitor such as  95  is introduced to compensate for the high frequency non uniformity, it will make the “low frequency” non uniform. The “low” frequency wave amplitude will then provide a slightly hollow electric power profile due to the extra tailored capacitor in the center. In other words, applying the “tailoring” concept of the invention here makes sense only if the “high” frequency local power uniformity is more important for the process than the “low” frequency power uniformity. 
     In  FIG. 11 , the tailored capacitive layer  105  is a gaseous space between a ceramic liner  105  and the metallic electrode  109  which has been machined to have the smooth and tailored recess (because of its non planar internal surface  109   a ) facing the back part of the ceramic plate  107 . The ceramic liner  107  has many small holes  107   a  which transmit the reactive gas provided by the holes  109   b  in the backing metal electrode  109 . The reactive gas is injected through ducts  111  connected to an external gas source  113  (the pumping means are not illustrated). The RF source  115  is connected to the electrode  109 , as illustrated. 
     The design of the backing electrode  109  could have been a traditional “shower head” as electrode  3  in  FIG. 10 . Another option is the cascaded gas manifold design which is shown in  FIG. 11 . 
     In  FIG. 12 , a microwave capacitive plasma reactor  40  is diagrammatically illustrated. The illustration shows a possible design according to which a rather thick tailored layer generally referenced as  120  (the thickness of which is designated as e 9 ) is used to compensate for the drastic non uniformity due to electromagnetic propagation. The illustrated reactor  40  is a reactor for etching a rather small wafer. The microwave comes from a coaxial wave guide  121  which expands gradually at  122  (“trumpet” shaped) to avoid reflection. Then, the microwave reaches the process zone  13  where the wave should converge to the center of the reactor (which is cylindrical). 
     For the dimensions, the substrate  35  arranged on a flat counter-electrode  126  has a diameter of about 10 cm, and an 1 GHz wave is generated by the microwave generator  123  (30 cm free space wave length). The central thickness of the tailored layer  120  (if made of quartz) should be about the same as the space  13  of the free plasma itself. 
     It is presently proposed that the tailored layer  120  be obtained from three dielectric plates defining three steps (discs  120   a ,  120   b ,  120   c ). The discontinuity of the steps should be averaged out by the plasma. The tailored layer is preferably very thick and it would actually make sense to call it “a lens”. The number of disks used to constitute the lens could be four or higher if the ideally smooth shape of the lens must be reproduced with a better approximation. 
     In said  FIG. 12 , it will be noted that the reactive gas is introduced through the gas inlet  124 , said reactive gas being pumped via a series of slits (preferably radially oriented) through the counter-electrode  126  and ending into a circular groove  125 . The exhaust means for evacuating the reactive gas injected in the reactive space between the electrodes are not illustrated. 
     In  FIG. 13 , the reactor  50  corresponds to the reactor  40  of  FIG. 12 , except that, in this case, the step variation of the “corrective” dielectric layer  130  is not due to a change of thickness, but to a change of material constituting said layer  130  which has a uniform thickness along its surface. In other words, layer  130  is a variable dielectric constant layer having a uniform thickness e 10 . The low dielectric constant layer is the central plate  131  which is concentrically surrounded by a second plate  132  having a medium dielectric constant layer. The third external concentric plate  133  has the highest dielectric constant. 
     Hence, the equivalent thickest part of the tailored layer  130  is made of the lowest dielectric material (quartz for example), whereas the intermediate layer  132  can be made of a material such as silicon nitride, the highest dielectric constant material at the periphery  133  being presently made of aluminum oxide. 
     The example of  FIG. 13  clearly shows that the dielectric layer of the invention having a capacitance per unit surface values which are not uniform along a general surface generally parallel to the substrate can be obtained through a variation of the dielectric constant of said layer, while the thickness thereof remains uniform along its surface. 
     From the above description and the illustration of  FIG. 14  (based on the embodiment of  FIG. 1 ), it must be clear that, in any case in which the thickness of the “corrective layer”, such as  140 , is used to compensate the process non-uniformity, as observed, the corrective layer(s) will be the thickest in front of the location in the process space (or on the facing electrode, such as  3 ) which is the farthest away from the electrode connection ( 9   a ). It is to be noted that the “way” (referenced as  150 ) for calculating said “distance” must follow the external surface (such as  3   a ) of the corresponding electrode. 
     Said thickness will be the lowest at the corresponding location where the above “distance” is the smallest, and the non planar profile of the layer will follow said distance decreasing.