Abstract:
A method and electrode assembly for treating a substrate with a non-equilibrium plasma in which the electrode assembly has two or more spaced barrier electrodes and a ground electrode spaced apart from the two spaced barrier electrodes for passage of a substrate to be treated. Plasma fluid medium is introduced between the barrier electrodes and is biased to provide a greater flow to an inlet region of the electrode assembly to help inhibit the ingress of air. Each of the barrier electrodes can be provided with central and leg sections having passages for introducing a cooling fluid into one of the leg sections and discharging said cooling fluid from the other of the leg sections. The central section can be provided with a transverse cross-sectional area less than that of the leg sections to increase velocity in the central section.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a divisional of prior U.S. application Ser. No. 10/832,376, filed Apr. 27, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention provides an electrode assembly for use in treating a substrate with a non-equilibrium plasma in which a plasma medium is injected between barrier electrodes to prevent the ingress of air during treatment of the substrate.  
       BACKGROUND OF THE INVENTION  
       [0003]     Non-equilibrium plasma, produced by a uniform glow discharge, is utilized for the surface treatment of polymer films, fabrics, wool, metal, and paper to improve the physical and optical properties of the surface. Such properties include printability, wetability, durability, and adhesion of coatings.  
         [0004]     The non-equilibrium plasma is generated within a thin gap between two electrodes. The gap is generally less than about two millimeters. A high voltage is applied to an active electrode. The active electrode is encased within a dielectric barrier that can be a ceramic or glass to ensure uniformity of the discharge. A grounded, counter electrode is positioned opposite to the active electrode and can be in the form of a rotating drum or a flat plate. A plasma medium which can be helium is injected into the region between the two electrodes to generate the non-equilibrium plasma. The substrate, which can be in sheet form, is passed between the active and counter electrodes to be treated by the non-equilibrium plasma. At high processing speeds, difficulties have arisen in treating the substrate due to a laminar flow barrier created by air entrainment. The entrained air flow mixes with the gas that is used as a plasma medium to alter the composition of the plasma, as well as its chemical kinetics and stability.  
         [0005]     It is known to inject the plasma medium gas between electrodes and toward the substrate. For instance, in U.S. Pat. No. 6,361,748 B1 a barrier electrode arrangement is disclosed in which a process gas or plasma medium, that is also used for cooling purposes, is injected between two electrodes and towards the substrate surface to be treated. U.S. Pat. No. 6,429,595 discloses two air cooled electrodes in which the plasma medium gas is injected between the electrodes through a porous ceramic that acts as a diffuser. In both of these patents, at high processing speeds, air would tend to be drawn into the plasma medium to alter its composition.  
         [0006]     As will be discussed, the present invention solves this problem by utilizing plasma medium in such a manner as to inhibit air ingress into the electrode assembly.  
       SUMMARY OF THE INVENTION  
       [0007]     In one aspect, the present invention provides a method of treating a substrate with a non-equilibrium plasma. In accordance with the method, the substrate is passed within an electrode assembly for generating the non-equilibrium plasma such that the substrate moves from an inlet region of the electrode assembly to an outlet region of the electrode assembly. The motion of the substrate tends to entrain air into the electrode assembly from the inlet region thereof by virtue of motion of the substrate. The electrode assembly has at least two spaced barrier electrodes and a ground electrode spaced apart from the at least two spaced barrier electrodes for passage of the substrate therebetween. Each of the at least two barrier electrodes have an elongated configuration and a transverse orientation with respect to a direction of motion of the substrate.  
         [0008]     The plasma medium is introduced between the at least two barrier electrodes toward the substrate so that the plasma medium flows toward the substrate and spreads out along the substrate towards the inlet region and the outlet region of the electrode assembly. The flow of the plasma medium is biased toward the inlet region of the electrode assembly, thereby to inhibit ingress of the air into the electrode assembly.  
         [0009]     Each of the barrier electrodes can be formed of a dielectric material and can be provided with a central section containing a high voltage conductor and two leg sections angled away from the central section. The plasma medium is passed into a chamber located between and connected to the two barrier electrodes. A cooling fluid can be passed into cooling fluid passages located within said central and leg sections of said barrier electrodes by introducing said cooling fluid into one of said leg sections and discharging said cooling fluid from the other of the leg sections. The central section has a transverse cross-sectional area less than that of the leg sections so that the cooling fluid has a higher velocity in the central section than said leg sections. Since the high voltage conductor is in the central section and heat is generated from such conductor, the presence of higher flow velocity helps to increase the heat transfer in such central section of the electrode.  
         [0010]     The cooling fluid is preferably made up of the plasma medium.  
         [0011]     The ground electrode can be of flat, plate-like configuration. In such case, first and second sets of the at least two spaced barrier electrodes and chamber can be separated by the ground electrode. This allows two of the substrates to be passed into the electrode assembly between the first of the sets of the at least two spaced barrier electrodes and the ground electrode and between the second of the two sets of the at least two spaced barrier electrodes and plasma medium inlets and the ground electrode.  
         [0012]     The present invention can also be effectuated in connection with a ground electrode in the form of a rotating cylinder rotating in the direction of motion of the substrate.  
         [0013]     In embodiments of the present invention having a chamber, a plate-like baffle can extend from the chamber towards the ground electrode. The plasma medium can be biased by introducing a greater flow rate of the plasma medium along one side of the plate-like baffle than the other side thereof.  
         [0014]     Another aspect of the present invention involves the provision of an electrode assembly for treatment of a substrate by a non-equilibrium plasma. In accordance with such aspect, at least two spaced barrier electrodes and a ground electrode are used. The ground electrode is spaced apart from the two at least two spaced barrier electrodes for passage of the substrate therebetween.  
         [0015]     A chamber can be located between and connected to the at least two spaced barrier electrodes. The chamber has openings for introducing the plasma medium between the at least two barrier electrodes towards the substrate so that the plasma medium flows toward the substrate and spreads out along the substrate towards inlet and outlet regions of the electrode assembly. A plate-like baffle extends from the chamber towards the ground electrode. The openings to the chamber are located on opposite sides of said plate-like baffle to allow the flow of the plasma medium to be biased toward the inlet regions of the electrode assembly at which the substrate first enters the electrode assembly during treatment and thereby, to prevent ingress of air thereto.  
         [0016]     Each of the at least two barrier electrodes can be formed of a dielectric material and has an elongated configuration and a transverse orientation with respect to a direction of motion of the substrate. A central section contains a high voltage conductor and two leg sections are angled away from the central section. The central and leg sections of said barrier electrodes have passages for introducing a cooling fluid into one of the leg sections and discharging the cooling fluid from the other of the leg sections. A high voltage conductor is located within the central section. The central section has a transverse cross-sectional area less than that of the leg sections so that the cooling fluid has a higher velocity in the central section than the leg sections. A chamber can be located between and connected to the at least two spaced barrier electrodes. The chamber is provided with openings for introducing the plasma medium between the at least two barrier electrodes towards the substrate so that the plasma medium flows toward the substrate and spreads out along the substrate towards inlet and outlet regions of the electrode assembly.  
         [0017]     This aspect of the present invention allows an electrode to be constructed that in which the heat transfer capability of the heat transfer fluid is increase where needed, namely, the high voltage electrode.  
         [0018]     The ground electrode can be of flat, plate-like configuration. In such case, the electrode assembly can further comprise first and second sets of the at least two spaced barrier electrodes and chamber separated by the ground electrode. This allows two of the substrates to pass into the electrode assembly between the first of the sets of the at least two spaced barrier electrodes and the ground electrode and between the second of the sets of the at least two spaced barrier electrodes and the ground electrode to simultaneously treat the two of the substrates.  
         [0019]     Alternatively, the ground electrode can be a rotating cylinder rotating in the direction of motion of the substrate.  
         [0020]     In any embodiment of the present invention, the high voltage conductor can be brazed to the central section. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     While the specification concludes with claims distinctly pointing at the subject matter that Applicant regards as his invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which:  
         [0022]      FIG. 1  is a schematic sectional view of an electrode assembly for carrying out a method in accordance with the present invention; and  
         [0023]      FIG. 2  is a sectional, schematic view of an alternative embodiment of an electrode assembly for carrying out a method in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0024]     With reference to  FIG. 1 , an electrode assembly  1  is illustrated for treating substrates  2  and  3  by generation of a non-equilibrium plasma.  
         [0025]     Electrode assembly  1  is provided with a first set of pairs of barrier electrodes  12  and  14 . Pair  12  consists of two barrier electrodes  16  and  18  and pair  14  consists of two barrier electrodes  20  and  22 . A second set of pairs of barrier electrodes  24  and  26  can be provided. Pair  24  consists of two barrier electrodes  28  and  30  and pair  26  consists of two barrier electrodes  32  and  34 .  
         [0026]     Each of the barrier electrodes  16 ,  18 ,  20 ,  22 ,  28 ,  30 ,  32  and  34  are of elongated configuration and are oriented transversely to the direction of travel of the substrates  2  and  3 . Further each of the barrier electrodes  16 ,  18 ,  20 ,  22 ,  28 ,  30 ,  32 , and  34  are formed of a dielectric material, for instance glass or a ceramic that enclose a high voltage conductor  36 .  
         [0027]     With reference to barrier electrode  16  (although the discussion has equal applicability to each of the other barrier electrodes  18 ,  20 ,  22 ,  28 ,  30 ,  32  and  34  ), a high voltage conductor is located within a central section  38 . Two leg sections  39  and  40  that are angled away from central section  38 . Central section  38  and leg sections  39  and  40  are hollow to provide flow passages located therein. A coolant, that can be the plasma medium, is introduced into one leg section  39  and is discharged from the other leg section  40  after having passed through central section  38 . Central section  38  has a lower transverse cross-sectional flow area than those of leg sections  39  and  40  so that the velocity of the flow is greater in central section  38  than leg section  39  and therefore, the heat transfer rate. This is advantageous in that a strategic cooling can be achieved using the generated high speed cooling jet towards the high voltage conductor  36  where the heat is generated.  
         [0028]     The high voltage conductor  36  is strip-like and is connected to central section  38  by such means as adhesives and brazing. In this regard to obtain excellent hermetic properties and reduce problems related to voids and thermal expansion, the high voltage conductor  24  and dielectric barrier surfaces are assembled with the necessary brazing assembly materials. The brazing solder materials can be pre-applied to the individual piece in the quantities required for selected metal and dielectric materials. Typical materials used for an electrode assembly in accordance with the present invention and brazing solder combinations are listed in the table below.  
                               TABLE                                   High voltage       Dielectric           conductor 24   Brazing-solder   Material                           Cu   AgCu 28%   SiO2           Fe/Ni   AgCu 15%   Si3N4           Kov   AgGe 13%   Al2O3           Fe/N142   AgSn 20%   TiO2, Ta2O5                      
 
         [0029]     For compatibility with highly diversified substrates during thermal expansion for thin electrodes, the high voltage conductors can be deposited directly on the dielectric surface using metal pastes such as Cu paste, Ag/Cu paste, and Ag/Pt paste etc. Selected powders used in the pastes can produce remarkably thick and dense film on the dielectric surfaces.  
         [0030]     A counter or ground electrode  52  is provided between the sets of barrier electrodes  16 ,  18 ,  20 ,  22 ,  28 ,  30 ,  32  and  34  with clearance for substrates  2  and  3 . The aforesaid arrangement of barrier electrodes  16 - 34  provide an inlet region  54  and an outlet region  56  for the electrode assembly  1 . Substrates  2  and  3  enter electrode assembly  1  through inlet region  54  and after treatment pass out of electrode assembly  1  from outlet region  56 . The motion of substrates  2  and  3  tends to entrain air into the electrode assembly.  
         [0031]     A plasma medium, for instance, helium, is obtained from a source  58 , which may be a tank of helium. The plasma medium is introduced into a plasma/cooling medium plenum  60 . Plasma/cooling medium plenum  60  is a pipe having cooling fins and a draft fan to circulate draft air past the cooling fins for cooling purposes.  
         [0032]     Plasma/cooling medium plenum  60  is connected by way of a conduit  62  to a feed manifold  64 . Feed manifold  64  is in turn connected by conduits  66  and  68  to chambers  70  and  72  of barrier electrode pairs  16 ,  18  and  20 ,  22 , respectively. Additionally, feed manifold  64  is similarly connected to chambers  74  and  76  associated with barrier electrode pairs  28 ,  30  and  32 ,  34 , respectively, by a conduit  78 .  
         [0033]     Plasma medium passes through openings provided for in chambers  70 ,  72 ,  74  and  76  and is directed towards substrates  2  and  3 , respectively. As such each of the chambers  70 ,  72 ,  74  and  76  is open to allow the plasma medium to escape toward substrates  2  and  3  and is elongated to distribute the plasma medium along the lengths of the electrode pairs. As will be discussed, the plasma medium enters chambers  70 ,  72 ,  74  and  76  through openings that will be discussed hereinafter. When the plasma medium reaches substrates  2  and  3 , it spreads out toward the inlet and outlet regions  54  and  56  of the electrode assembly  1 .  
         [0034]     A glow discharge generated by a high voltage applied to high voltage conductors  36  and ground electrode  52  produces a non-equilibrium plasma to treat the surfaces of substrates  2  and  3 .  
         [0035]     Each of the chambers  70 ,  72 ,  74  and  76  is divided by an elongated, plate-like baffle  80  produce two open chambers  82 ,  84  for each of pairs of barrier electrodes,  12 ,  14  and  24 ,  26 . Openings  85  and  86  are provided in chamber  70  on either side of plate-like baffle  80  with openings  85  being closer to inlet  84 . In this regard, openings  85  or openings  86  would be an array of openings along the length of chamber  70  or any other chamber illustrated herein. The flow to chamber  82  is greater than the flow to chamber  84  to bias the flow. This can be accomplished by providing openings  85  with a high cross-sectional area than openings  86  or by providing the plasma medium to openings  85  at a higher pressure than openings  86 . This creates a greater flow in chambers  82  than in chambers  84 . Since chambers  82  are closer to inlet region  54 , the flow of plasma fluid is greater in directions of arrow A as opposed to arrowheads B. Alternatively, the baffles  80  could be positioned closer to outlet region  56  to provide a similar effect. A still further possibility would be to shape electrode pairs, for instance, the side  86  of electrode  18  to be closer to vertical than the side  88  of electrode  16 , thereby urging the flow of plasma medium toward region  54 . Still another means to bias the flow would be to provide a greater flow to electrode pairs to  16 ,  18  and  28 ,  30  as opposed to electrode pairs  20 ,  22  and  32 ,  34 .  
         [0036]     As mentioned above, each of the barrier electrodes  16 ,  18 ,  20 ,  22 ,  28 ,  30 ,  32  and  34  is hollow to allow for the passage of a cooling fluid. The cooling fluid can be the same as the plasma medium, for instance, helium. As illustrated, conduit  88  is connected to feed manifold  64  and is provided with branches  90 ,  92 ,  94  and  96  in case of barrier electrode pairs  16 ,  18  and  20 ,  22  and branches  88 ,  100 ,  102  and  104  from conduit  78  previously discussed with respect to feeding plasma fluid medium to plasma fluid medium inlets  74  and  76 . After having been heated, the barrier fluid is returned to a return manifold  106  by way of return conduits  108 , branch  110  joining conduit  108  and return conduits  110  and  112 . Return conduit branches  114 ,  116 ,  118  and  120  feed into return conduit  122  to return the heated cooling fluid to return manifold  106 . A pump  108  is connected to return manifold  106  to pump the heated cooling fluid to pump the heated cooling fluid back to plasma/cooling medium plenum  60  which as stated previously is provided with cooling fins and a draft fan to cool the heated fluid plasma medium.  
         [0037]     As may be appreciated, an embodiment of present invention could be provided with only a single pair of barrier electrodes, for example, barrier electrodes  16  and  18 . Similarly, a single set of barrier of electrodes could be provided, for instance, barrier electrodes  16 ,  18 ,  20  and  22 . In such case, barrier electrodes  28 ,  30 ,  32  and  34  would be omitted. Such device would only be capable of treating a single substrate at any one time, for instance, substrate  2 .  
         [0038]     With reference to  FIG. 2  an alternative electrode assembly  2  of the present invention is illustrated. In this embodiment, two barrier electrodes  130  and  132  are provided and a rotating cylindrical ground electrode  134  is situated beneath barrier electrodes  130  and  132 . Each of the barrier electrodes  130  and  132  has a body formed of a dielectric and is provided with elongated, high voltage conductors  136  connected in place in the manner described above with respect to high voltage conductors  36 .  
         [0039]     Each of the barrier electrodes  130  and  132  are of similar design to the barrier electrodes discussed in reference to  FIG. 1  in that each has a central section  135  containing the high voltage conductor  136  and two leg sections  138  and  140  angled away from central section  134 . Each barrier electrode  130  and  132  is of elongated configuration and is oriented transversely to the direction of travel of the substrate. High voltage conductor is in the form of a conductive strip.  
         [0040]     Leg sections  138  of barrier electrodes  130  and  132  are connected by a chamber  142  which would be of elongated configuration and open at the bottom (as viewed in the illustration). Chamber  142  has arrays of openings  144  and  146 , extending along the length of chamber  142 , that are separated by an elongated plate-like baffle  148 .  
         [0041]     A substrate to be treated enters an inlet region  150  and is discharged from an outlet region  152  defined between leg sections  140  and ground electrode  134  which would rotate in a counter clockwise direction. The motion of the substrate to be treated and the rotation of ground electrode  134  tends to cause air to enter inlet region  150  and mix with the plasma medium. In order to combat this, In the same manner as discussed with respect to chambers  70 ,  72  and etc., the flow may be biased towards inlet region  150  by increasing the flow, shown again by arrowhead “A” through openings  146 .  
         [0042]     As indicated above, each of the barrier electrodes  130  and  132  is hollow to provide cooling fluid passages. The cooling fluid is introduced into leg section  138  in the direction of arrowhead “C” and discharged from leg section  140  in the direction of arrowhead “D”. Central section  135  has a smaller, transverse cross-sectional flow are to increase the velocity of the cooling fluid and hence, also increase the heat transfer in the area of high voltage conductor  136  where heat is generated. It is to be noted that a similar arrangement of distribution manifolds and conduits to that shown in connection with  FIG. 1  could be used to circulate cooling fluid and plasma medium which could have the same make-up, for instance, helium.  
         [0043]     While the present invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions may be made without departing from the spirit and scope of the present invention.