Abstract:
An installation having a housing, a substrate support ( 20 ) received in the housing, diffuser ( 42 ) for diffusing an inert gas towards the substrate support, and at least one head ( 30 ) defining an inner volume (V) opened opposite to the top, the head being provided with at least two electrodes ( 8, 8′, 8 ″) for creating an electric discharge and with an injector ( 7, 7′, 7 ″) for injecting a gaseous mixture towards the substrate. The injector has at least one injection tube ( 7, 7′, 7 ″) placed between two adjacent electrodes or between one electrode and a peripheral wall, the tube being provided with injection holes facing the substrate support, for injecting the gaseous mixture on the substrate, whereas diffuser is provided inside the head, the injection tube being placed between the substrate support and the diffuser so that, in use, the gaseous mixture is urged against the substrate by the inert gas.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The invention relates to an industrial installation for the surface treatment of a moving substrate, more precisely an installation for implementing a method whereby the substrate is subjected to a plasma generated in a gaseous mixture, by which the surface is modified and/or a film is deposited onto said moving surface. The invention relates in particular to such an installation operating at a pressure close to atmospheric pressure, and that is suitable for the surface treatment of polymer films in a continuous roll-to-roll process. 
         [0002]    Methods are already known for modifying and improving the properties of the surface of a substrate by means of a plasma. Such relevant properties can be for instance the surface energy and the adhesion properties of this substrate. Substrates can be insulators such as polymer films, or metal films. 
         [0003]    According to these known methods, in order to deposit a thin solid film onto the surface of the substrate, the surface of the substrate is subjected to a plasma created by an electrical discharge in a gas and, at the same time or subsequently, the substrate thus treated is exposed to an active gaseous mixture that contains an active gaseous compound capable of causing the deposition of a thin solid film. 
         [0004]    Methods are known in which a treatment of a substrate with electrical discharge in a gaseous mixture is performed continuously, at substrate speeds ranging from about ten to several hundred meters per minute, usually in a chamber. The chamber usually contains, in addition to the electrodes that are necessary for creating a discharge, a device for injecting the active gaseous mixture, as well as means for extracting the gaseous emissions. 
         [0005]    WO 2004/432175 discloses a device for the treatment of a material in a plasma-assisted process. This device comprises a vacuum chamber, in which is disposed a rotating drum supporting and conveying the substrate to be treated. A plurality of independent magnetron electrodes which have rectangular magnetron fronts are arranged next to the substrate. Gas-feeding means are provided, for delivering a process gas or a process gas mixture into the space between the drum and the magnetron electrodes. 
         [0006]    WO 2005/076918 describes a method for depositing an extremely thin barrier layer on a film, carried on a process roll in a process chamber. This is a roll to roll process carried out under vacuum. The deposition takes place in individual deposition chambers. The vacuum chamber has a common inert gas inlet. 
         [0007]    WO 2008/042310 describes an electrode for an atmospheric corona plasma discharge. The electrode comprises a cavity for diffusing gas through a slot onto the substrate. The gas enters the cavity though a passageway and a porous body. 
         [0008]    FR 2 816 726 A describes an installation of the above described type, comprising first a support roll against a region of which the substrate to be treated is applied. This installation also comprises electrodes housed in a chamber, which is adjacent to the support over at least the width of the substrate and along at least part of this region. 
         [0009]    Moreover, specifically designed inlet and outlet units adjoin the chamber to oppose respectively the ingress of air into the chamber and the exit of gaseous emissions therefrom. Each unit comprises first a nitrogen-injection slit, a nitrogen introduction chamber and, downstream the slit, several grooves located near the slit. This assembly creates a zone of raised pressure in the direction of travel of the substrate, which discharges the nitrogen further upstream by forming a piston for the air stream that is to be prevented from entering. Finally, means are provided for regulating the flow rate of gas drawn out by extraction device, so as to maintain an approximately zero pressure difference between the inside of the chamber and the surrounding atmosphere. 
         [0010]    However, the installation described in FR-A-2 816 726 has some drawbacks. First, it uses a gas knife upstream and downstream the chamber, which implies a high consumption of nitrogen. Moreover, the monitoring of the conditions inside the chamber is rather complicated; in particular, pressure inside the chamber cannot be regulated in a stable and reproducible manner. Finally, the structure of this installation is rather complicated, especially as far as the inlet and outlet units, described above, is concerned. 
         [0011]    The objective of the present invention is to at least partially overcome these disadvantages and to propose an installation of the above type which ensures a reliable roll-to-roll surface treatment of the substrate at a pressure close to atmospheric pressure, which does not require an air-tight chamber, and using a process that significantly reducing the consumption of inert gas with respect to prior art. 
         [0012]    Another objective of the invention is to propose such an installation, which ensures a reliable surface treatment of the substrate, while being relatively easy to monitor. 
         [0013]    Another objective of the invention is to propose such an installation, which has relatively few mechanical components, and the structure of which is simple. 
         [0014]    Another objective of the invention is to propose such an installation, which is modular and can be quickly and easily modified to suit different types of surface treatments of the substrate. 
       SUBJECT MATTER 
       [0015]    According to the invention, the problems are solved by an installation for surface-treating a moving substrate, comprising
       a housing;   a substrate support received in said housing;   a counter electrode;
           diffusion means for diffusing an inert gas, such as nitrogen, towards said substrate support;   at least one head, each head comprising a top and peripheral walls, said head defining an inner volume opened opposite to said top, said head being provided with
               at least one electrode adapted to cooperate with said counter electrode for creating an electric discharge; and with   injection means for injecting a gaseous mixture towards said substrate;   
               
               
 
         [0023]    characterized in that 
         [0024]    said injection means comprise at least one injection tube placed between two adjacent electrodes and/or between one electrode and a peripheral wall, said at least one injection tube being provided with injection holes facing said substrate support, for injecting said gaseous mixture on said substrate, 
         [0025]    and in that 
         [0026]    said diffusion means are provided inside said head, said injection tube being placed between said substrate support and said diffusion means so that, in use, said gaseous mixture is pushed against said substrate by said inert gas. 
         [0027]    The installation according to the invention avoids the use of inlet and outlet units with nitrogen knifes, such as known from FR 2 816 726 A. Therefore, the structure of the installation according to the present invention is far less complicated, compared to this prior art. Moreover, the consumption of inert gas is greatly reduced, in particular due to the suppression of these inlet and outlet units. 
         [0028]    Moreover the Applicant has discovered that, surprisingly, providing the diffusion means for inert gas inside the head induces no substantial loss of gaseous mixture outside the inner volume of the head. In other words, substantially the whole gaseous mixture is deposited on surfaces exposed to plasma. 
         [0029]    Without being bound by theory, this may be due to the fact that the gaseous substance is pushed or pressed against the substrate, under the action of inert gas diffusing from the inside of the head. In addition this inert gas avoids the arrival of air, driven by the moving substrate, into the discharge zone delimited by two adjacent electrodes. 
         [0030]    Advantageously, the housing comprises at least one means, especially at least two means, for a removable fixation of said at least one head. The installation may comprise several heads, at least one of these heads being adapted to cooperate with at least one fixation means. At least two heads, amongst these several heads, may be different. 
         [0031]    Providing the housing with such removable fixation means brings specific advantages. First, in case of a failure of a given head, the latter may be very easily replaced by another similar head. Moreover, this head may be replaced by a different head, in order to change the nature of the treatment to which the substrate is to be submitted. The expression “different heads” means that at least one of the following parameters varies from one head to the other:
       Number of electrodes and/or tubes   Nature of electrodes and/or tubes   Positioning of these electrodes and tubes   Length, i.e. dimension along the axis of the support   Nature of the gaseous mixture connected to the tubes.       
 
         [0037]    If at least two removable fixation means are provided, the installation according to the invention may have further use possibilities. Indeed, if several identical heads are mounted on the housing, a thicker deposit can be obtained without reducing the speed of the substrate. Moreover, if at least two different heads are used, the deposit may include at least two different layers. 
         [0038]    According to one embodiment of the invention, the housing comprises opposite peripheral walls, in particular front and rear walls, and means for a removable fixation extend between said opposite peripheral walls and cooperate with side walls of said head. According to a constructive point of view, said means for a removable fixation may comprise rails extending between said opposite peripheral walls, said walls of said head being clipped between said rails. 
         [0039]    Preferably, the installation further is adapted for maintaining an approximately zero pressure difference between the inside of the housing and the surrounding atmosphere. The expression “approximately zero” according to the present invention is to be understood as meaning a pressure difference (positive or negative) not exceeding a few tens of Pa or even 100 Pa. In particular the installation according to the invention is capable of providing plasma treatment and/or plasma deposition at atmospheric pressure. 
         [0040]    According to other advantageous but non compulsory features of the invention:
       the installation comprises a source of said inert gas, means for flowing said inert gas from said source to each electrode, as well as means for flowing said inert gas from each electrode to said diffusion means.   said diffusion means is placed close to said top whereas electrodes and tubes are placed close to said support.   said head is provided with a filter dividing said head into two parts, diffusion means being placed in a first part whereas electrodes and tubes are placed in a second part.   said tubes are elongated and both ends of each tube are fixed on brackets located close to opposite peripheral walls of the head.   the installation comprises means for an adjustable fixation of said tubes on at least one of said brackets to permit a rotation of said tubes along their main axis.   said means for an adjustable fixation comprise at least one clamp provided on said bracket for clipping said tube.   each tube is provided with at least two rows of holes, two adjacent rows being offset the one towards the other.   said electrodes are elongated and both ends of each electrode are fixed on said brackets.   said electrodes have a square cross-section.   the distance between injection holes of each tube and the substrate, as well as the distance between each electrode and the substrate, are adapted to permit non turbulent flow of gas.   the installation comprises means for controlling the pressure of the inert gas diffused in the head.   said means for controlling the pressure comprises means for controlling the flow rate of the inert gas, connected to means for measuring oxygen concentration in the vicinity of the electrodes.   said head comprises several tubes and several electrodes arranged alternately along the travel of the substrate.   the installation comprises means for a removable fixation of said tubes and/or said electrodes on said head.       
 
         [0055]    The objectives of the invention are also achieved with an installation for surface treatment of the surface treatment of a moving substrate, such as film deposition onto and/or modification of the surface of a moving substrate, said installation comprising
       a housing;   a substrate support received in said housing;   a counter electrode
           at least one head, each head comprising a top and peripheral walls, said head defining an inner volume opened opposite to said top, said head being provided with
               at least one electrode, adapted to cooperate with said counter electrode for creating an electric discharge,   injection means facing said substrate support, for injecting a gaseous mixture on said substrate,   diffusion means, for diffusing an inert gas, such as nitrogen, in the vicinity of said substrate,   
               
               
 
         [0063]    wherein the housing comprises at least one means, especially at least two means, for a removable fixation of said at least one head. 
         [0064]    The immediately above subject matter can be combined with any technically compatible feature recited in the present description, as well as recited in the appended claims. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0065]      FIG. 1  is a front view, showing the installation of the invention. 
           [0066]      FIG. 2  is a perspective view, showing a housing which belongs to the installation of the invention. 
           [0067]      FIG. 3  is a perspective view, showing a head which belongs to the installation of the invention, at a greater scale, the front wall of this head being omitted. 
           [0068]      FIG. 4  is a longitudinal view, showing the head of  FIG. 3 . 
           [0069]      FIG. 5  is a longitudinal view, showing a tube which belongs to the head of  FIG. 3 . 
           [0070]      FIGS. 6 and 7  are views at a larger scale, showing the details VI and VII on  FIG. 5 . 
           [0071]      FIG. 8  is a front view, showing at a still greater scale two electrodes and a tube which belong to the head of  FIG. 3 . 
           [0072]      FIG. 9  is a front view, schematically showing an installation according to a variant of the invention. 
           [0073]      FIGS. 10 and 11  are schematic front views, analogous to  FIG. 9 , showing two different ways of carrying out the installation of  FIG. 9 . 
       
    
    
       [0074]    Following reference numerals will be used throughout the present application: 
         [0000]    
       
         
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 7, 7′, 7″ Tubes 
               
               
                   
                 L7 Length of 7 
               
               
                   
                 D7 Diameter of 7 
               
               
                   
                 R7 Direction of rotation of 7 
               
               
                   
                 d7 Distance between 7 and S 
               
               
                   
                 8, 8′, 8″Electrodes 
               
               
                   
                 L8 Length of 8 
               
               
                   
                 W8 Width of 8 
               
               
                   
                 d8 Distance between 8 and S 
               
               
                   
                  10 Housing 
               
               
                   
                 W10 Width of 10 
               
               
                   
                 L10 Length of 10 
               
               
                   
                  11 Top of 10 
               
               
                   
                  12 Front wall of 10 
               
               
                   
                  13 Rear wall of 10 
               
               
                   
                  14 Side wall of 10 
               
               
                   
                  15 Side wall of 10 
               
               
                   
                  16 Rails 
               
               
                   
                  17 Exhaust duct 
               
               
                   
                  18 Window 
               
               
                   
                  19 Door 
               
               
                   
                  20 Drum 
               
               
                   
                 R20 Rotation of 20 
               
               
                   
                 S Substrate 
               
               
                   
                  22 Nip 
               
               
                   
                  30 Head 
               
               
                   
                 W30 Width of 30 
               
               
                   
                 L30 Length of 30 
               
               
                   
                  31 Top of 30 
               
               
                   
                  32 Front wall of 30 
               
               
                   
                  33 Rear wall of 30 
               
               
                   
                  34 Side wall of 30 
               
               
                   
                  35 Side wall of 30 
               
               
                   
                 V Inner volume of 30 
               
               
                   
                 G Gap between 30 and S 
               
               
                   
                  40 Upper part of 30 
               
               
                   
                  42 Diffusion means 
               
               
                   
                  50 Lower part of 30 
               
               
                   
                  60 Filter 
               
               
                   
                  71 Holes of 7 
               
               
                   
                  72 Holes of 7 
               
               
                   
                 L71 Length of 71 
               
               
                   
                  76 Bracket 
               
               
                   
                  78 Clamp 
               
               
                   
                 d78 Distance between 7 and 8 
               
               
                   
                  90 Source of nitrogen 
               
               
                   
                  91 Upstream line 
               
               
                   
                  92 Downstream line 
               
               
                   
                  94 Sensor 
               
               
                   
                  96 Controller 
               
               
                   
                  97 Line 
               
               
                   
                  98 Line 
               
               
                   
                 110 Housing 
               
               
                   
                 116, 116′, 116″Rails 
               
               
                   
                 120 Drum 
               
               
                   
                 130 1 -130 n Heads   
               
               
                   
                 VI Detail on FIG. 5 
               
               
                   
                 VII Detail on FIG. 5 
               
               
                   
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION 
       [0075]    The installation of the invention first comprises a body or housing  10 , which has a top  11  and peripheral walls, formed by parallel front and rear walls  12  and  13 , as well as parallel side walls  14  and  15 . By way of example, its length L 10 , i.e. the distance between walls  12  and  13 , is between 100 millimeters and 10 meters. By way of example, its width W 10 , i.e. the distance between walls  14  and  15 , is between 100 millimeters and 2000 millimeters. Housing  10  is also provided with a duct  17 , known as such, in view of the exhaust of excess gas out of the inner volume of this housing. 
         [0076]    As schematically shown on  FIG. 2 , illustrating only the housing, rails  16  extend between walls  12  and  13 . They are fixed on these walls with any appropriate means. The function of these rails will be described hereafter. A window  18  and a door  19  are provided on wall  12 , to permit access to the rails  16 . 
         [0077]    The installation also comprises a drum  20  which rotates in use in the direction shown by the arrow R 20 . This drum forms a support for the substrate S to be treated. In the present embodiment, this drum also ensures a further function of counter electrode which cooperates with electrodes which will be described hereafter. However, the counter electrode may be formed by another component of the installation. By way of example, the substrate is a polymer film, whereas its thickness is between 5 micrometers and 50 millimeters. 
         [0078]    In its upstream part, referring to the travel of the substrate, the drum is associated to a nip, or pressing roll  22 , known as such. This secondary roll  22  makes it possible to press the substrate against the drum  20 , so as to avoid the formation of a potential air layer between this substrate and this drum. Any local default of treatment on the substrate is therefore substantially avoided. 
         [0079]    Above the drum  20 , a cap or head  30  is arranged, which holds tubes and electrodes, as will be explained hereafter. The width W 30  of the head  30  is far inferior to that W 10  of the housing. This head covers part of the arc of a circle defined by the drum  20 , approximately centrally. 
         [0080]    The length L 30  of this head may be slightly inferior to that L 10 , in particular if the substrate covers substantially the whole drum length. However, if this substrate covers only a part of this drum, the length L 30  of this head may then be far inferior to L 10 , so that the head does not project longitudinally beyond the substrate. 
         [0081]    Turning to  FIGS. 3 and 4 , the head  30  comprises a top  31  and peripheral walls, formed by parallel front and rear walls  32  and  33 , as well as parallel side walls  34  and  35 . By way of example, it is made of a non conductive material, such as PET (polyethylene terephthalate). For the sake of clarity, front wall  32  has been omitted on  FIG. 3 , whereas it is shown on  FIG. 4 . 
         [0082]    The upper parts of side walls cooperate with above mentioned rails  16 , in view of the fixation of the head. In a preferred way, this fixation makes it possible to mount head  30  on rails  16  in a removable manner, in particular by clipping. 
         [0083]    This head  30  defines an inner volume V that is opened towards the drum  20 . Each free end of these side walls and the drum  20  define two gaps G, the thickness of which may be well-defined in a way known as such. Typically this gap is between 0.5 and 2.5 millimeters. This value may be varied, by moving the head  30  relative to the support  20 . This possibility is shown by arrow T 30  which highlights translation movement of head  30  towards support, as well as by arrow R 30 , which highlights rotation movement of head. 
         [0084]    The head is divided into two parts by a filter  60 , called hereafter upper  40  and lower  50  parts. In its upper part  40 , the head is provided with diffusion means, connected with a source of an inert gas such as nitrogen, as will be explained hereafter. In the illustrated example these diffusion means are formed by several diffusers  42 , of any appropriate type. These diffusers, which have a multiplicity of diffusion holes, are regularly provided over the surface of the head. This filter, which is known per se, has amongst other the function of improving the homogeneity of nitrogen sent into lower part  50  of the head. 
         [0085]    The lower part  50  of the head accommodates several tubes and electrodes. In the illustrated example, there are three tubes  7 ,  7 ′ and  7 ″, as well as three electrodes  8 ,  8 ′ and  8 ″, which extend substantially parallel the one to the other, and are arranged alternately along the direction of rotation of the drum. In other words, upstream tube  7  is disposed between side wall  34  and upstream electrode  8 , whereas tubes  7 ′ and  7 ″ are disposed between two adjacent electrodes, namely  8 ,  8 ′ and  8 ′,  8 ″. 
         [0086]    According to some variants, which are not shown, the invention encompasses other relative arrangements of tubes and electrodes. By way of example, two electrodes may be placed side by side, or one tube may be disposed between downstream electrode  8 ″ and other side wall  35 . Two tubes may also be placed side by side, while being disposed between two electrodes or between a side wall and an electrode. 
         [0087]    By way of example, the tubes are made of metal, or of plastic material, such as a polymeric material, in particular PET. They are connected to a non shown source of an active gaseous mixture, in view of carrying out a chosen treatment of the substrate S. 
         [0088]    The structure of tube  7  will now be described, bearing in mind that other tubes  7 ′ and  7 ″ have the same structure. 
         [0089]    Turning to  FIG. 5 , tube  7  is elongated and has a circular shape, in cross-section. Its length L 7  is slightly inferior to that L 30 . By way of example, it is between 100 millimeters and 10 meters. Its external diameter D 7  is for example between 2 and 40 millimeters, whereas the thickness of its wall is between 0.5 and 10 millimeters. 
         [0090]    Tube  7  is provided with two parallel rows  71  and  72  of injection holes, the manufacture of which is carried out according to any appropriate process. These holes extend over a length L 71 , which is a substantial part of the length of the tubes. Preferably, ratio L 71 /L 7  is superior to 98%. 
         [0091]    In the example, there are two rows of holes, which are offset as shown in  FIG. 6  at a greater scale. This makes it possible to reduce the effects of injection turbulence, as well as to improve the homogeneity of the deposition. This avoids a potential deposition of an undesired film on the electrodes themselves, which would reduce the deposition speed on the substrate and would be harmful in view of the quality of this deposition. 
         [0092]    Both ends of each tube are mounted on brackets  76 , located in the vicinity respectively of front and rear walls. According to an advantageous embodiment, at least one of these ends is clipped on a clamp  78 , supported by the bracket, which has the shape of a part of cylinder (see  FIG. 7 ). 
         [0093]    Therefore, this tube may be moved into rotation around its main longitudinal axis, as indicated by arrow R 7 , which makes it possible to vary the angle of injection of gaseous mixture towards the substrate. This makes it possible to reduce the effects of injection turbulence, as explained above. 
         [0094]    Moreover, each tube is advantageously fixed on the head in a removable manner, either by clamping as above described, or in any analogous manner. Therefore, one tube may be easily replaced by another similar tube, in particular in case of a failure. This tube may also be replaced by a different one. The expression “different tubes” means that at least one of the following parameters varies from one tube to the other:
       Dimension of the whole tube   Dimension of the holes   Positioning of these holes, in particular number of rows   Length L 71  of the perforated zone.       
 
         [0099]    Advantageously, each electrode has a smooth outer surface. This avoids the creation of turbulence in the plasma formation zone. This electrode is preferably made of a ceramic material, which makes it possible to treat an electrically conductive substrate. As an alternative, electrodes may also be made of any other suitable material, such as a metallic material. 
         [0100]    The structure of electrode  8  will now be described, bearing in mind that other electrodes  8 ′ and  8 ″ have the same structure. Turning to  FIGS. 4 and 8 , electrode  8  is elongated and has a square shape, in cross-section. Its length L 8  is substantially equal to that L 7  of tube  7 . Its width W 8  is similar to the diameter D 7  of the tube  7 , in particular between 10 and 20 millimeters. 
         [0101]    Both ends of each electrode are fixed on the above brackets  76  ( FIG. 4 ), in the vicinity of the ends of the tubes. Contrary to the tubes, these electrodes are not mounted on these brackets with a possibility of rotation. The electrodes  8  are connected to a source of very high voltage, which is not shown. This latest aspect is not part of the invention. 
         [0102]    Moreover, each electrode is advantageously fixed on the head in a removable manner, thanks to any appropriate means. Therefore, one electrode may be easily replaced by another similar electrode, in particular in case of a failure. This electrode may also be replaced by a different one. The expression “different electrodes” means that at least one of the following parameters varies from one electrode to the other:
       Dimension of the whole electrode   Material of the electrode   Shape of the electrode.       
 
         [0106]    As shown also on  FIG. 4 , electrodes  8  are connected to a common source of nitrogen, referenced as  90 , via upstream lines  91 . Nitrogen flows along these electrodes, then along downstream lines  92  which lead into each diffuser  42 . Moreover a sensor  94 , of any appropriate known type, makes it possible to measure the oxygen content in the vicinity of the electrodes. As shown in  FIG. 3 , this sensor  94  is positioned close to gap G, upstream in reference of drum rotation. This sensor is connected to a controller  96  ( FIG. 4 ), via a line  97 , in order to control the flow rate of nitrogen via a further line  98 , which leads in source  90 . 
         [0107]    The closest distance between injection holes  71 ,  72  and the substrate S is referenced d 7  ( FIG. 8 ), whereas d 8  denotes the closest distance between each electrode  8  and the substrate. Distance d 8  is for example between 0.5 and 2.5 millimeters, typically between 0.5 and 1.5 millimeters, in particular equal to 1.0 millimeter. Advantageously distance d 7  is slightly superior to that d 8 . The positioning of the head towards the counter electrode  20  may be carried out, as explained above, without any risks of contacting tubes  7  with this counter electrode. Distances d 7  and d 8  are chosen, so that they permit a non turbulent flow of gas in the vicinity of tubes and electrodes. 
         [0108]    Each tube is substantially equidistant from the two electrodes, between which it is located. By way of example, the closest distance d 78  ( FIG. 8 ) between a tube and an electrode is between 1 and 40 millimeters, in particular between 5 and 10 millimeters. If distance d 78  is too little, an electric arc may be created. Otherwise, if d 78  is too high, it may create a substantial dead volume through which gaseous mixture may flow. 
         [0109]    In use, nitrogen is first admitted into volume V via diffusers  42 . The substrate is not moved, until oxygen concentration measured by sensor  94  drops under a given threshold, for example equal to 20 ppm (parts per million). When the value of this concentration is appropriate, the substrate is then moved by the support, whereas active gaseous mixture is injected via tubes  7 , and discharge is created by electrodes  8 . Nitrogen flowing into electrodes, via lines  91 , makes it possible to evacuate heat out of these electrodes. The housing  10  is advantageously maintained at a pressure close to atmospheric pressure, during use. 
         [0110]    Advantageously, oxygen concentration is continuously measured by sensor  94  during the purge phase with inerting gas. When this concentration exceeds the above defined threshold, the controller  90  increases the flow rate of nitrogen, to lower this oxygen concentration. This increase of flow rate induces a corresponding increase of nitrogen overpressure, into inner volume V of the head  30 . 
         [0111]      FIGS. 9 to 11  illustrate an alternate advantageous embodiment of the invention. Mechanical elements of these  FIGS. 9 to 11 , which are similar to those of the first embodiment of  FIGS. 1 to 8 , have the same reference numerals incremented with  100 . 
         [0112]    Housing  110  is provided with several pairs of rails  116 ,  116 ′ and  116 ″, which extend side by side along the travel of the substrate. In other words, there are respective upstream  116 , intermediate  116 ′ and downstream rails  116 ″. Access to these rails is given by non shown windows and doors, as previously described. Moreover the installation comprises several heads  130   1  to  130   n , which may be identical or different, according to the definition given above. 
         [0113]    According to a first non shown possibility, one single head may be positioned on any pair of rails, in particular the intermediate one  116 ′. A first kind of treatment of the substrate may be carried out, in the same way as described above. 
         [0114]      FIG. 10  shows another possibility in which two heads, for example  130   1  and  130   2 , are placed on two pairs of rails, in particular two adjacent pairs  116  and  116 ′. If these heads are identical, a thick deposit can be obtained without reducing the speed of the substrate. Moreover, if these heads are different, i.e. their tubes are connected with sources of different gaseous mixtures, the deposit may include two different layers. 
         [0115]      FIG. 11  shows another further possibility in which three heads, for example  130   1 ,  130   2  and  130   3 , are placed on the three pairs of rails. If these heads are identical, still a thicker deposit can be obtained without reducing the speed of the substrate. Moreover, if two of these heads are the same and the third one is different, the deposit may include two different layers, one of which is thicker than the other. Finally, if these heads are different, the deposit may include three different thin layers. 
         [0116]    The installation according to the invention can be used for surface treatment of various substrates, especially films, such as metal films (for example aluminium foil) or polymer films, such as polyethylene films. 
         [0117]    It allows efficient roll-to-roll surface treatments at industrial speed. Said surface treatment can be in one step or in two or more consecutive steps, each step being carried out with a specific head. Two and more consecutive heads can also be used to carry out the same surface treatment: 
         [0118]    multiplying the number of heads allows for instance to deposit thicker solid films without decreasing the speed of the moving substrate.