Patent Publication Number: US-2012024817-A1

Title: Apparatus and method for plasma surface treatment

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus and a method for plasma surface treatment, more particularly, to an apparatus and a method for treating a surface of an electrically conductive object with ions from plasma. 
     BACKGROUND ART 
     Thermal treatment of metal material such as a metal sheet or a metal wire without removing contaminants of the surface of the material causes surface contamination and defects. The contaminants such as rolling oils used in the process of cold rolling, dusts form a process are carbonized in the process of thermal treatment at the temperature of 1050-1100° C., and become the cause of surface contamination and defects. Also, when a material being deposited or applied or joined on a surface of an electrically conductive material such as a metal sheet or a metal wire, surface treatments of the electrically conductive material such as cleaning or reforming of a surface of the metal sheet may enhance the adhesion. 
     Therefore, a method for treating a surface of an electrically conductive material is needed. Some surface treatment methods such as using various chemicals or plasma are known. 
     Methods using chemicals remove organic materials by dipping the objects into organic solvents such as alkali, TCE. These methods are very complex processes needing alkali tank, electrolytic cleaning tank, warm water cleaning tank, and drying equipment. Also these methods have to maintain proper temperature and density to keep moderate reactivity. Therefore, these methods consume large amount of energy. Also, these methods use chemicals for treatments for a long time, and contaminants gathered on the surface of the chemicals degrade the quality of cleaning and fail to clean small gap. Also these methods are not eco-friendly. 
     Methods using plasma remove contaminants by impacting ions or radicals from plasma to the surface of a material. A method mainly using ions from plasma comprises positioning an object for surface treatment into a chamber filled with plasma of ions, applying high negative voltage to the object to accelerate the ions in a transition plasma sheath generated around surface of the object. And the accelerated ions impact on the surface of the object. This method reveals good effects in cleaning and reforming the surface of the object. Also this method is speedy. Also this method has a merit that it can remove inorganic contaminants including a metal oxide. 
     DISCLOSURE 
     Technical Problem 
     Though the surface treatment method using ions from plasma has many merits, the method has some difficulties in applying to the following conditions. In case it is necessary to treat a portion of a surface of an electrically conductive material, the method fails to treat the wanted portion of the surface, because high negative voltage applied to the object spread on all the surface of the object material. 
     Also, in case it is necessary to treat a surface of an electrically conductive object continuously supplied and grounded such as metal sheet coil or a wound metal wire, the method fails to treat the surface of such object because high negative voltage can not be applied to the object and ions can not be accelerated. 
     The present invention is devised to solve the problems mentioned above. It is an object of the present invention to provide an apparatus and a method for treating a portion of a surface of an electrically conductive material with ions from plasma. Another object of the present invention is to provide an apparatus and a method for treating a surface of an electrically conductive object continuously supplied such as a metal sheet or a metal wire. 
     Technical Solution 
     According to the present invention, an apparatus for plasma surface treatment which treats a surface of a treatment portion of an electrically conductive object using ions from plasma is provided. The apparatus comprises: a connector electrically connected to the treatment portion for applying negative voltage pulses to the treatment portion; a pulse voltage generating unit electrically connected to the connector; and magnetic cores disposed at the boundary of the treatment portion for preventing electric current caused by the negative voltage pulses applied to the treatment portion from flowing across the boundary of the treatment portion. 
     Also, according to the present invention, a method for plasma surface treatment is provided. The method comprises: disposing magnetic cores at a boundary of a treatment portion of an electrically conductive object for preventing electric current from flowing across the boundary of the treatment portion; supplying a process gas; generating plasma of ions surrounding the electrically conductive object; and applying negative voltage pulses to the treatment portion of the electrically conductive object to accelerate the ions from the plasma toward the electrically conductive object to impact the electrically conductive object with the ions. 
     Also, an apparatus for plasma surface treatment which treats a surface of a treatment portion of an electrically conductive object using ions from plasma is provided. The apparatus comprises: at least a pair of pulse transformer cores disposed at the boundary of the treatment portion and configured to induce negative voltage pulses within the treatment portion; first winding wires wound around each of the pulse transformer cores; and a pulse voltage generating unit electrically connected to the first winding wires. 
     Also, according to the present invention, an apparatus for plasma surface treatment which treats a surface of a treatment portion of an electrically conductive object using ions from plasma is provided. The apparatus comprises: a pulse transformer core disposed at the boundary of the treatment portion for inducing negative voltage pulses in the treatment portion; a first winding wire wound around the pulse transformer core; a pulse voltage generating unit electrically connected to the first winding wire; and a magnetic core disposed at the boundary of the treatment portion for preventing electric current caused by the negative voltage pulses in the treatment portion from flowing across the boundary of the treatment portion. 
     Also, according to the present invention, a method for plasma surface treatment is provided. The method comprises: disposing at least a pair of pulse transformer cores configured to induce negative voltage pulses within a treatment portion of an electrically conductive object at the boundary of the treatment portion; supplying a process gas; generating plasma of ions surrounding the electrically conductive object; and accelerating the ions from the plasma toward the electrically conductive object to impact the electrically conductive object with the ions by applying negative voltage pulses to the pair of pulse transformer cores. 
     Also, according to the present invention, a method for plasma surface treatment is provided. The method comprises: disposing a pulse transformer core configured to induce negative voltage pulses in a treatment portion of an electrically conductive object at the boundary of the treatment portion; disposing a magnetic core at the boundary of the treatment portion of the electrically conductive object for preventing electric current from flowing across the boundary of the treatment portion; supplying a process gas; generating plasma of ions surrounding the electrically conductive object; and accelerating the ions from the plasma toward the electrically conductive object to impact the electrically conductive object with the ions by applying negative voltage pulses to the pair of pulse transformer cores. 
     Advantageous Effects 
     The apparatus and method for plasma surface treatment according to the present invention have advantageous effects as below. 
     First, the apparatus and method for plasma surface treatment according to the present invention can confine the treatment portion by using negative high voltage pulses and magnetic cores. Also, the apparatus and method can apply negative high voltage pulse to the treatment portion of an electrically grounded object such as a metal sheet coil and a metal wire coil. 
     Second, the apparatus and method for plasma surface treatment according to the present invention use plasma ions accelerated by negative high voltage pulses. Thus, surface treatment speed of the present invention is faster than the conventional methods. Therefore, the apparatus and method can be applied to object supplied continuously at high speed. 
     Third, the apparatus and method for plasma surface treatment according to the present invention use plasma ions. Thus, the apparatus and method can be used reliably without restrictions on size or shape of objects. 
     Fourth, the apparatus and method for plasma surface treatment according to the present invention do not use any chemicals. Therefore, there are no issues of waste chemicals treatment and pollution. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a first embodiment of an apparatus for plasma surface treatment according to the present invention; 
         FIG. 2  illustrates the shape of high negative voltage pulses in the present invention; 
         FIG. 3  is a perspective view of a magnetic cores installed in the apparatus for plasma surface treatment illustrated in  FIG. 1 ; 
         FIG. 4  is an electrical equivalent circuit of the portion illustrated in  FIG. 3 ; 
         FIG. 5  is a schematic diagram of a second embodiment of an apparatus for plasma surface treatment according to the present invention; 
         FIG. 6  shows a placement of a pulse transformer installed in the apparatus for plasma surface treatment illustrated in  FIG. 5 ; 
         FIG. 7  is an electric potential distribution diagram induced on a surface of a metal sheet by the pulse transformer illustrated in  FIG. 6 ; 
         FIG. 8  shows a placement of a pulse transformer installed in a third embodiment of an apparatus for plasma surface treatment according to the present invention; 
         FIG. 9  is a schematic diagram of a forth embodiment of an apparatus for plasma surface treatment according to the present invention; 
         FIG. 10  shows a placement of a pulse transformer and magnetic cores of the apparatus for plasma surface treatment illustrated in  FIG. 9 ; 
         FIG. 11  an electrical equivalent circuit of the portion illustrated in  FIG. 10 ; 
     
    
    
     BRIEF DESCRIPTIONS OF REFERENCE NUMERALS 
     
         
         
           
               110 ,  210 ,  310 : vacuum chamber  120 ,  220 ,  320 : vacuum pump 
               130 ,  230 ,  330 : gas providing unit  140 ,  240 ,  340 : plasma generating unit 
               150 ,  250 ,  350 : pulse voltage generating unit  160 : electric roller 
               170 ,  370 : magnetic core  180 ,  280 ,  380 : un-winder 
               190 ,  290 ,  390 : winder  260 ,  270 : pulse transformer 
           
         
       
    
     EMBODIMENTS 
     Hereinafter, with reference to the figures attached, embodiments according to the present invention will be described. 
     The First Embodiment 
       FIG. 1  is a schematic diagram of a first embodiment of an apparatus for plasma surface treatment according to the present invention. The first embodiment is an apparatus for plasma surface treatment of a metal sheet coil supplied continuously. With reference to  FIG. 1 , the present embodiment comprises a vacuum chamber  110 , a vacuum pump  120 , a gas supplying unit  130 , a plasma generating unit  140 , a pulse voltage generating unit  150 , an electric roller  160 , and magnetic cores  170 . 
     The vacuum chamber  110 , for providing a vacuum atmosphere for a treatment portion A of a metal sheet coil W, includes an inlet  112  formed at the upstream of the vacuum chamber  110  and an outlet  114  formed at the downstream of the vacuum chamber  110 . The metal sheet coil W is provided through the inlet  112  of the chamber into the inside of the vacuum chamber  110 , and goes throughout the outlet  114  after plasma surface treatment. Vacuum maintenance means  116  are installed respectively at the inlet  112  and at the outlet  114 . The vacuum maintenance means  116  maintain vacuum by preventing outside air from flowing into the vacuum chamber  110 , when the metal sheet W is provided into the inside of the vacuum chamber  110  and discharged from the vacuum chamber  110 . The present embodiment also includes an un-winder  180  for providing the metal sheet coil W continuously into the inside of the vacuum chamber  110  and a winder for winding metal sheet W discharged from the vacuum chamber continuously after plasma surface treatment. 
     The vacuum pump  120  discharges gases out of the vacuum chamber  110  and produces a vacuum inside the vacuum chamber  110 . A vacuum valve  122  is installed between the vacuum pump  120  and the vacuum chamber  110 . 
     The gas providing unit  130  provides a gas for generating plasma into the vacuum chamber  110 . The gas providing unit  130  can include a gas providing valve, and the gas providing valve is installed between the gas providing unit  130  and the vacuum chamber  110 . The gas provided by the gas providing unit may be nitrogen (N 2 ), hydrogen (H 2 ), oxygen (O 2 ), neon (Ne), argon (Ar), and/or may be any combinations of the gases enumerated. 
     The plasma generating unit  140  generates plasma atmosphere inside the vacuum chamber  110 . The plasma generating unit  140  includes a plasma electrode  142  for generating plasma and a plasma electric source  144  for providing electric power to the plasma electrode  142 . The plasma electric source  144  provides electric power to the plasma electrode  142  through feed through. When electric power being applied to the plasma electrode  142 , molecules of a gas around the plasma electrode  142  are ionized, and a plasma state in which positive ions and negative electrons are mixed is developed inside the vacuum chamber  110 . 
     The pulse voltage generating unit  150  generates negative voltage pulses which are applied to the treatment portion A of the metal sheet coil W. When the negative voltage pulses being applied to the treatment portion A of the metal sheet coil W, a transition plasma sheath is formed around the surface of the treatment portion A of the metal sheet coil W, and ions inside the transition plasma sheath are accelerated toward the surface of the treatment portion A of the metal sheet coil W and impact on the surface of the treatment portion A of the metal sheet coil W. Therefore, plasma surface treatment of the metal sheet is accomplished. With reference to  FIG. 2 , the high voltage pulse generated by the pulse voltage generating unit  150  has pulse duration Tp ranging from 50 nS (nano-second) to 500 mS (milli-second) and pulse magnitude Vp ranging from −100 kV to −10 V. And it is desirable that the negative high voltage pulse has the ratio of Tp to Tm ranging from 1:5 to 1:50,000, wherein Tp stands for pulse duration and Tm stands for time duration between pulse stop time t 1  and next pulse start time t 2 . 
     The electric roller  160  is installed rotatably inside the vacuum chamber  110 . The electric roller  160  is in contact with the lower surface of the treatment portion A of the metal sheet coil W. And the electric roller  160 , being rotated on the surface of metal sheet W, supports the metal sheet W and simultaneously guides the metal sheet W. Also, the electric roller  160  applies negative high voltage pulses generated from the pulse voltage generating unit  150  to the treatment portion A of the metal sheet coil W. The pulse voltage generating unit  150  provides electric roller placed inside vacuum chamber  110  with electric power through feed through  152 . Since the electric roller  160  is installed rotatably, the friction between the electric roller  160  and the metal sheet W can be reduced. Also the enhanced contact between the electric roller  160  and the metal sheet W secures the electric contact between the electric roller  160  and the metal sheet coil W. 
     One magnetic core  170  is installed at the upstream side of the treatment portion A of the metal sheet coil W and the other magnetic core  170  is installed at the downstream side of the treatment portion A of the metal sheet coil W. The magnetic cores  170  prevent electric current, which is caused by the application of the negative voltage pulse to the treatment portion A of the metal sheet coil W, from flowing across the boundary of the treatment portion A of metal sheet coil W.  FIG. 3  is a perspective view of magnetic cores  170  installed in the apparatus  100  for plasma surface treatment illustrated in  FIG. 1  and  FIG. 4  is an electrical equivalent circuit of the portion illustrated in  FIG. 3 . With reference to  FIG. 3 , magnetic cores  170  are in the form of closed loop encompassing a portion of the metal sheet W inside the vacuum chamber  110 . The metal sheet supplied continuously passes through the opening of the magnetic core  170  which is in the form of loop. The surface of the opening of the magnetic core  170  is spaced predetermined distance apart from the metal sheet W. 
     With reference to  FIG. 3  and  FIG. 4 , hereinafter it is described that how the magnetic cores  170  prevent the electric current generated by the application of the negative voltage pulses from flowing across the boundary of the treatment portion A of the metal sheet W. The magnetic cores  170  made of a material having high relative permeability have high inductance L 1 . The outside portion B of the metal sheet W from the boundary of the treatment portion A to the grounded portion of the metal sheet has inductance Lq. Since the metal sheet W is wound on both the un-winder  180  and the winder  190 , the metal sheet W is connected with the un-winder  180  and the winder  190  physically and electrically. Since the un-winder  180  and the winder  190  are grounded, the metal sheet W is grounded. 
     The pulse voltage applied to the treatment portion A equals the sum of the voltage drop at magnetic core  170  and the voltage drop at the outside portion B of the metal sheet W. Though there is a voltage drop caused by the resistance of the metal sheet W, since high frequency pulses are applied to the treatment portion A, the reactance generated by the inductance of the magnetic cores  170  and outside portion B is much higher than the resistance of the metal sheet W. Therefore, the voltage drop caused by the resistance of the metal sheet W is negligible, and the equivalent circuit shown in  FIG. 4  can be configured only with the inductance. The voltage drop at magnetic cores  170  is proportional to L 1  and the voltage drop of the treatment portion A is proportional to Lq. If the inductance L 1  is much higher than the inductance Lq, the voltage drop at the magnetic cores  170  is dominant. It is necessary that the inductance L 1  of magnetic cores  170  is much higher than that of the outside portion B in order to apply the negative high voltage pulses to the treatment portion A without loss. In order to increase the inductance of the magnetic cores  170 , it is desirable to use material having higher relative permeability. 
     With reference to the figures, the functioning of the apparatus  100  for plasma surface treatment will be described below. 
     First, vacuum state is produced inside the vacuum chamber  110  by the working of the vacuum pump  120 , and then a gas for generating plasma is supplied into the vacuum chamber  110 . The gas can be selected from the group of nitrogen (N 2 ), hydrogen (H 2 ), oxygen (O 2 ), neon (Ne), argon (Ar), and/or may be any combinations of the gases enumerated according to the purpose of surface treatment. If the condition of the vacuum chamber  110  becomes a proper condition for plasma generation, the plasma generation unit  140  is operated to produce a plasma state inside the vacuum chamber  110 . The method for producing plasma is well known to the person skilled in the technical fields of plasma. 
     Next, the metal sheet coil W wound at the un-winder is supplied into the vacuum chamber  110  through the inlet  112 . Since the lip seal  116  is mounted at the inlet  112 , it is possible to provide the metal sheet W into the vacuum chamber  110  maintaining vacuum state. The metal sheet W passes through the magnetic core  170  installed between the inlet  112  and the electric roller  160 . In sequence the metal sheet W passes through the magnetic core  170  installed between the electric roller  160  and the outlet  114 , and the metal sheet W is discharged outside the vacuum chamber  110 . Since another lip seal  116  is mounted at the outlet  114 , it is possible to discharge the metal sheet W outside the vacuum chamber  110  maintaining vacuum state. In the process of providing and discharging the metal sheet W, the electric roller  160  keeps contacting with the lower surface of the metal sheet W. 
     When the treatment portion A of the metal sheet W passes through the upstream side magnetic core  170 , negative high voltage pulses are generated from the pulse voltage generating unit  150 . The negative high voltage pulses are applied to the treatment portion A of the metal sheet W through the electric roller  160  connected with the pulse voltage generating unit  150 . The electric current caused by the negative high voltage pulses is blocked by the magnetic cores  170  mounted both at the upstream boundary of the treatment portion A and at the downstream boundary of the treatment portion A. Therefore, the surface treatment portion is confined to the portion A located between the upstream magnetic core  170  and the downstream magnetic core  170 . If the treatment portion is not confined by the magnetic cores  170 , it is impossible to apply negative voltage on the metal sheet W inside vacuum chamber. The electric current caused by the negative high voltage pulses will flow through the metal sheet W to the ground without the confinement of the magnetic cores  170 . 
     Next, a plasma sheath is generated around the treatment portion A by the negative high voltage pulses, and positive ions in the plasma sheath are accelerated toward the treatment portion A, and impact on the surface of the treatment portion A. Contaminants on the surface of the treatment portion are removed by the collision of the ions. The metal sheet W of which surface was treated discharged through the outlet  114  of the vacuum chamber  110  and is wound by the winder  190  as described above. By these processes described above it is possible to accomplish the surface treatment of an electrically conductive metal object continuously. 
     The Second Embodiment 
       FIG. 5  is a schematic diagram of a second embodiment of an apparatus for plasma surface treatment according to the present invention. The apparatus for plasma surface treatment of this embodiment is an apparatus for a metal sheet coil provided continuously. Referring to  FIG. 5 , the apparatus for plasma surface treatment of this embodiment comprises a vacuum chamber  210 , a vacuum pump  220 , a gas providing unit  230 , a plasma generating unit  240 , a pulse voltage generating unit  250 , pulse transformers  260 ,  270 . In this embodiment, the apparatus for plasma surface treatment uses the pulse transformers  260 ,  270  instead of the electric roller  160  and the magnetic cores  170  of the first embodiment. The pulse transformers  260 ,  270  induce negative voltage pulse and prevent the electric current from flowing across the treatment portion A of the metal sheet W. 
     The pulse transformers  260 ,  270  are disposed at the upstream and downstream of treatment portion A of the metal sheet W respectively. The pulse transformers  260 ,  270  include transformer cores  262 ,  272  and first winding wires  264 ,  274  and the metal sheet W takes a role of a second winding wire of the pulse transformers  260 ,  270 . The pulse transformers  260 ,  270  can induce negative voltage pulse to the metal sheet W, because the first winding wires  264 ,  274  are connected with pulse voltage generating unit  250  and the metal sheet W acts as a second winding wire of the pulse transformers  260 ,  270 . 
       FIG. 6  shows a placement of a pulse transformer installed in the apparatus for plasma surface treatment illustrated in  FIG. 5 . Referring to  FIG. 6 , the transformer cores  262 ,  272  are closed-loops encompassing the metal sheet W at a predetermined distance. The metal sheet W passes through the openings of closed-loops. The material, magnetic flux density, and shape parameters etc., of the transformer cores  262 ,  272  are determined by considering magnitude of voltage pulses applied to the first winding wires  264 ,  274 , and magnitude and frequency of the voltage pulse induced to the treatment portion A. 
     The first winding wires  264 ,  274  are made of metal such as copper. The first winding wires  264 ,  274  are electrically connected with the pulse voltage generating unit  250 , so that voltage pulses generated by the pulse voltage generating unit  250  is applied to the pulse transformers  260 ,  270 . The pulse voltage generating unit  250  supplies electric power through feed through  252  to the pulse transformers  260 ,  270  in the vacuum chamber  210 . When the voltage pulses are applied to the first winding wires  264 ,  274 , a magnetic field is induced and magnetic lines of force of the magnetic field pass through the transformer cores  262 ,  272 . The magnetic lines of force passing through the transformer cores  262 ,  272  induce a voltage pulse to the metal sheet W acting as the second winding wire by Faraday&#39;s law of induction. In brief, the voltage pulses generated by the pulse voltage generating unit  250  induce the voltage pulses to the treatment portion A of a metal sheet W through the pulse transformers  260 ,  270 . 
     The pulse transformers  260 ,  270  are disposed at the upstream and downstream of the treatment portion A of the metal sheet W respectively. Each of the first winding wires  264 ,  274  of the pulse transformers  260 ,  270  is wound in opposite direction. Each of the first winding wires  264 ,  274  wound in opposite direction induces negative voltage pulses to the treatment portion A of the wound metal sheet W. Each of the voltages induced by pulse transformer  260 ,  270  has the same magnitude in opposite direction. As a result, the resultant voltage induced to the metal sheet W by the pulse transformers  260 ,  270  is applied only within the treatment portion A and offset outside the treatment portion A. 
       FIG. 7  is an electric potential distribution diagram induced on a surface of a metal sheet by the pulse transformer illustrated in  FIG. 6 . Referring to  FIG. 7 , it will be described in more detail how the voltages induced to the metal sheet W outside the treatment portion A mutually offset. 
     When a negative voltage pulse is applied to the first winding wire  264  of the pulse transformer  260  at the upstream of the treatment portion A, a magnetic field is induced by the first winding wire  264  and a voltage pulse is induced to metal sheet W acting as the second winding wire, and there is a voltage drop at the portion B of the metal sheet W where pulse transformer  260  is disposed. The frequency of the negative voltage pulse is so high that a voltage drop at the treatment portion A of metal sheet W due to the resistance of the metal sheet W can be ignored in compare with the voltage drop at the portion B of the metal sheet W. When the negative voltage pulse is applied simultaneously to the first winding wire  274  of pulse transformer  270  at the downstream of the treatment portion A, a voltage pulse is induced to metal sheet W, and the first winding wire  274  of the pulse transformer  270  is wound in opposite direction of the first winding wire  264  of the pulse transformer  260 . Thus, there is a voltage rise at the portion C of the metal sheet W where the pulse transformer  270  is disposed. The magnitude of the voltage rise at the portion C is similar to the magnitude of the voltage drop at the portion B. As a result, voltage induced to the metal sheet W is mutually offset outside the treatment portion A. 
     With reference to the figures, the functioning of the apparatus  200  for plasma surface treatment will be described below. 
     First, vacuum state is produced inside the vacuum chamber  210  by the working of the vacuum pump  220 , and then a gas for generating plasma is supplied into the vacuum chamber  210 . The gas can be selected from the group of nitrogen (N 2 ), hydrogen (H 2 ), oxygen (O 2 ), neon (Ne), argon (Ar), and/or may be any combinations of the gases enumerated according to the purpose of surface treatment. When the condition of the vacuum chamber  210  becomes a proper condition for plasma generation, the plasma generation unit  240  is operated to produce a plasma state inside the vacuum chamber  210 . The method for producing plasma is well known to the person skilled in the technical fields of plasma. 
     Next, the metal sheet W wound at the un-winder in roll type is supplied into the vacuum chamber  210  through the inlet  212 . Since a lip seal  216  for vacuum is mounted at the inlet  212 , it is possible to provide the metal sheet W into the vacuum chamber  210  maintaining vacuum state. The metal sheet W passes through the pulse transformer  260  installed near the inlet  212 . In sequence the metal sheet W passes through the pulse transformer  270  installed near the outlet  214 , and the metal sheet W is discharged outside the vacuum chamber  210 . Since another lip seal  216  is mounted at the outlet  214 , it is possible to discharge the metal sheet W outside the vacuum chamber  210  maintaining vacuum state. 
     When the treatment portion A of the metal sheet W passed through the upstream side pulse transformer  260 , negative high voltage pulse is generated from the pulse voltage generating unit  250 . The negative high voltage pulse is induced to the treatment portion A of the metal sheet W through the pulse transformers  260 ,  270 . The upstream side pulse transformer  260  and the downstream side pulse transformer  270  have the same property and the first winding wires  264 ,  274  of the pulse transformers  260 ,  270  are wound in opposite directions. Thus, the voltage induced to the metal sheet W by pulse transformers  260 ,  270  is offset outside the boundary of the treatment portion A. Therefore the surface treatment portion is confined to the portion located between the upstream pulse transformer  260  and the downstream pulse transformer  270 . 
     Next, a plasma sheath is generated around the treatment portion A by the negative high voltage pulse, and positive ions in the plasma sheath are accelerated toward the treatment portion A, and impact against the surface of the treatment portion A. Contaminants on the surface of the treatment portion are removed by the collision of the ions. The metal sheet W of which surface was treated discharged through the outlet  214  of the vacuum chamber  210  and is wound by the winder  290  as described above. By these processes described above it is possible to accomplish the surface treatment of an electrically conductive metal object continuously. 
     The Third Embodiment 
       FIG. 8  shows a placement of a pulse transformer installed in a third embodiment of an apparatus for plasma surface treatment according to the present invention. In this embodiment, the apparatus for plasma surface treatment uses multiple sets of pulse transformers serially disposed at the upstream and the downstream of the treatment portion A to increase the voltage induced to the treatment portion A of the wound metal sheet W. 
     The Fourth Embodiment 
       FIG. 9  is a schematic diagram of a forth embodiment of an apparatus for plasma surface treatment according to the present invention. In this embodiment, the apparatus for plasma surface treatment includes a magnetic core  370  instead of the pulse transformer  270  of the second embodiment at the downstream of the treatment portion A. 
     The magnetic core  370  is disposed at the downstream of the treatment portion A. The magnetic core  370  prevents electric current caused by negative voltage pulses induced to the treatment portion A from flowing across the boundary of the treatment portion A.  FIG. 10  shows a placement of a pulse transformer and magnetic cores of the apparatus for plasma surface treatment illustrated in  FIG. 9 . Referring to  FIG. 10 , magnetic core  370  is closed-loop encompassing the metal sheet W at a predetermined distance. The metal sheet W passes through the opening of closed-loop. 
     Referring to  FIG. 10  and  FIG. 11  an electrical equivalent circuit of the portion illustrated in  FIG. 10 , it will be described that how the magnetic cores  370  prevent the electric current generated by the application of the negative voltage pulses from flowing across the boundary of the treatment portion A of the metal sheet W. The magnetic core  370  is made of high relative permeability materials, so that the magnetic core  370  has high inductance L 1 . A portion D of the metal sheet W has inductance Lq. The metal sheet W is wound around the winder  380  and the un-winder  390 , so the metal sheet W is electrically connected with the winder  380  and the un-winder  390  and the winder  380  and the un-winder  390  are grounded. As a result, the metal sheet W is grounded too. 
     The purse voltage induced by the pulse transformer disposed at the upstream of the treatment portion A is the sum of the voltage drop by magnetic core  370  and the voltage drop at portion D. The frequency of the negative voltage pulses is so high that a voltage drop due to the resistance of the metal sheet W can be ignored in compare with the voltage drop at the magnetic core  370  and the portion D of the metal sheet W. Thus, an electrical equivalent circuit of the portion of  FIG. 10  can be used. The voltage drop at the magnetic core  370  is proportional to the inductance L 1  of the magnetic core  370  and the voltage drop at the portion D is proportional to the inductance Lq. So, if the inductance L 1  is much higher than the inductance Lq, most of the voltage drop is occurred at the magnetic core  370 . To put it differently, to reduce the voltage loss at the region D, it is required that the inductance L 1  is much higher than the inductance Lq. The inductance L 1  of the magnetic core  370  can be increased by increasing the turns of winding wire and using the high relative permeability materials. 
     While certain embodiments of the present invention have been described hereinabove, the present invention shall not be limited thereto. It will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention defined in the claims. 
     As one modified embodiment, the surface treatment can be processed in atmospheric pressure. 
     Also, in the first embodiment, the connector other than electric roller can be used. For example, a carbon brush or carbon current collector can be used as the connector. 
     Also, the means to continuously provide the metal sheet other than the un-winder and the winder can be used. 
     Also, though only the metal sheet is described as an object in embodiments, it will be apparent that a metal wire can be treated and electrically conductive materials other than metal can be treated. 
     As another modified embodiment, pulse transformer can be installed outside the vacuum chamber.