Abstract:
An object of the present invention is to provide an apparatus for producing stable plasma. Another object of the present invention is to provide an apparatus having a long-lasting cathode electrode which is superior in field emission characteristic since the plasma density has to be raised in order to increase the throughput. The structure of the plasma producing apparatus of the present invention relates to a plasma producing apparatus with a plasma chamber surrounded by walls to make material gas into plasma, characterized in the plasma chamber has a cathode electrode, an anode electrode, means for introducing the material gas, and exhaust means, and that a carbon nano tube is formed on a surface of the cathode electrode and the anode electrode is formed on the surface of the cathode electrode.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an apparatus for producing plasma of high density, and to apparatus that uses the same. For example, the plasma producing apparatus is employed in a doping apparatus, etching apparatus, sputtering apparatus, and film forming apparatus.  
         [0003]     2. Description of the Related Art  
         [0004]     In recent years, manufacture of a transistor from a thin film (thin film transistor: TFT) has been widely researched. In manufacture of a TFT, film formation, etching, and ion introduction are some of indispensable processes and plasma is often utilized in these steps. To be specific, highly reactive ions or radicals are utilized in etching and film formation, particles having large kinetic energy are utilized in sputtering, and charged particles in plasma are utilized in etching and ion implantation. A TFT is manufactured through these steps using sputtering, plasma CVD, etching, ion doping, and the like.  
         [0005]     There are several methods to produce plasma. One is to apply a uniform electric field between two planar electrodes that are set in parallel to each other ( FIG. 4A , the electrode denoted by  51  is a cathode electrode and one denoted by  52  is an anode electrode). Another method is to use a directly-heated thermionic cathode electrode  53  and heat a high-melting point metal such as tungsten up to about 2500° C. to make the metal emit thermoelectrons ( FIG. 4B ). Still another method is indirect heating from the back using a substance having a small work function for a surface of a cathode electrode ( FIG. 4C ). Yet still another method is to produce plasma of high density using as a cathode electrode a cylindrical hollow electrode that is open at one end open and closed on the other end (hollow cathode discharge  54 )(FIG.  4 D).  FIG. 4E  shows a method of producing plasma by placing many permanent magnets along a wall of a plasma chamber and utilizing magnetic fields present on some parts of the wall surface (surface magnetic fields or multipolar magnetic fields).  FIG. 4F  shows a plasma producing method called magnetron discharge in which a magnetic field is applied in parallel to a planar cathode electrode face for electric discharge. These are examples of using direct current discharge but plasma can be produced also by alternating current discharge.  
         [0006]     Major examples of producing plasma by alternating current are shown in  FIGS. 5A  to  5 F.  FIG. 5A  shows a method called capacitively coupled plasma in which high frequency wave or microwave is applied to parallel planar electrodes.  FIGS. 5B and 5C  each show a method called inductively coupled plasma in which a high frequency current is let flow in a helical coil or a spiral coil.  FIG. 5D  shows a method called surface wave plasma in which surface wave is excited by irradiating electromagnetic wave onto high density plasma and is brought against intense microwave.  FIG. 5E  shows a method called ECR (electron cyclotron resonance) plasma which utilizes electron cyclotron resonance in a magnetic field.  FIG. 5F  shows a method called helicon wave plasma in which a high frequency current whose frequency is sufficiently lower than the electron cyclotron frequency is let flow in an antenna.  
         [0007]     An ion doping apparatus is used mainly in manufacture of a thin film transistor (TFT) on a large-area glass substrate. In doping for forming a source region or a drain region, the dose necessary is on the order of 10 15  ions/cm 2 . In channel doping and doping for forming an LDD (lightly doped drain) region, the dose necessary is 1×10 12  to 1×10 14  ions/cm 2 . Since the necessary dose in one type of regions is different from the dose needed in the other type of regions by three orders of magnitude, an ability to control high current density as well as low current density is required for an ion doping apparatus. For instance, an ion doping apparatus with a DC ion source using a filament is popular because the apparatus can handle a wide range of current density.  
         [0008]     A problem of the ion doping apparatus with a DC ion source using a filament is that a filament has a short lifespan. Replacing a filament lowers the availability of the apparatus and reduces the throughput. In addition, it is desirable to replace a filament as less frequently as possible because many ion doping apparatuses use phosphine, diborane, and other harmful gas for material gas and it can contaminate a clean room and adversly affect human health. However, the lifespan of a filament is even shorter in the high dose treatment for forming a source region or a drain region because the ion coulomb density is increased in this treatment in order to raise the throughput. In addition, a filament that is long in use is gradually degraded and a film is adhered to the filament surface. These changes in a filament cause a change with time in field emission characteristic from the filament surface and a change in ion species ratio in plasma, thereby making stable dose control impossible. Stabilizing plasma is the most important objective not only in an ion doping apparatus but in every apparatus that deals with plasma.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention has been made in view of the above, and an object of the present invention is therefore to provide an apparatus for producing stable plasma. Another object of the present invention is to provide an apparatus having a long-lasting cathode electrode which is superior in field emission characteristic since the plasma density has to be raised in order to increase the throughput.  
         [0010]     The present invention is characterized in that carbon, in particular, a carbon nano tube (CNT) is used for a cathode electrode of a plasma producing apparatus, typically, a doping apparatus, etching apparatus, and film forming apparatus. A carbon nano tube is a carbon based material having a tubular graphite structure of 1 nm to several tens nm in diameter. There are various known carbon nano tube structures, including a single wall carbon nano tube (SWNT), a multi wall carbon nano tube (MWNT), a VGCF (vapor growth carbon fiber), a nano horn, and a nano glass fiber. Hereinafter, these are referred to by the generic term of carbon nano tube.  
         [0011]     The shape of a carbon nano tube is characteristically thin and long. Despite being only several nm in diameter, a carbon nano tube can be as long as several thousands nm. Carbon nano tubes are extremely chemically stable since carbon atoms are bonded to one another very solidly. Some carbon nano tubes have conductivity and a large amount of current can flow in them because of the strong bond between carbon atoms. Furthermore, carbon nano tubes have excellent heat resistance.  
         [0012]     If one end of a carbon nano tube is charged in advance, electrons are emitted from the tip of the carbon nano tube with a surprising frequency. This is supposedly due to the elongated shape of a carbon nano tube, in other words, its high aspect ratio. Accordingly, this field emission takes place at a lower voltage than when a usual metal is used, and causes almost no damage, which helps to prolong the lifespan.  
         [0013]     Carbon nano tubes thus have excellent characteristics. The present invention is characterized in that the field emission characteristic of a carbon nano tube is used in a plasma producing source of an apparatus which utilizes plasma. By using a carbon nano tube as a cathode electrode that is set in an ion source, electrons can be supplied to an arc chamber with high efficiency. Because of electrons emitted from tips of a large number of carbon nano tubes, the plasma density in the ion source is increased. In addition, stable plasma can be obtained since carbon nano tubes are superior in heat resistance and current transporting characteristic.  
         [0014]     The structure of the plasma producing apparatus of the present invention relates to a plasma producing apparatus with a plasma chamber surrounded by walls to make material gas into plasma, characterized in that the plasma chamber has a cathode electrode, an anode electrode, means for introducing the material gas, and exhaust means, and that a carbon nano tube is formed on a surface of the cathode electrode and the anode electrode is formed on the surface of the cathode electrode.  
         [0015]     The above structure is characterized in that the carbon nano tube is one or more kinds selected from the group consisting of a single wall carbon nano tube, a multi wall carbon nano tube, a VGCF, a carbon nano horn, and nano glass fiber.  
         [0016]     The above structure is characterized in that the carbon nano tube is manufactured by arc discharge or laser irradiation and bonded to a metal member with a conductive paste. Alternatively, the above structure is characterized in that the carbon nano tube is manufactured through deposition on a metal member by a vapor phase method, preferably, plasma CVD, using as introduction gas methane, ethane, ethylene, acetylene, or a mixture thereof, namely, hydrocarbon-based gas and hydrogen gas.  
         [0017]     When plasma is thus produced using the cathode electrode where carbon nano tubes are formed, high density plasma can be obtained efficiently. In addition, superior heat resistance and current transporting characteristic of carbon nano tubes make it possible to produce stable plasma in an ion source. Then, an ion doping apparatus, etching apparatus, sputter apparatus, and film forming apparatus each having such a cathode electrode can be used to perform a variety of stable processes on a semiconductor film. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     In the accompanying drawings:  
         [0019]      FIG. 1  is a diagram showing an example of the present invention;  
         [0020]      FIGS. 2A and 2B  are diagrams each showing an example of the present invention;  
         [0021]      FIG. 3  is a diagram showing an example of an ion source that uses the present invention;  
         [0022]      FIGS. 4A  to  4 F are diagrams showing examples of direct current discharge; and  
         [0023]      FIGS. 5A  to  5 F are diagrams showing examples of alternating current discharge. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     Embodiment Mode 1  
       [0024]     This embodiment describes with reference to  FIG. 1 a  plasma producing apparatus having a cathode electrode and an anode electrode.  
         [0025]      FIG. 1  shows an example of a plasma producing apparatus of the present invention. In  FIG. 1 , a chamber is equipped with a plasma chamber  101 , a processing chamber  102 , a load lock chamber  103 , vacuum exhaust means  106 , a gas supplying system  111 , a cathode electrode  116  for producing plasma, and an anode electrode  117 . The structure in  FIG. 1  conforms to the capacitively coupled high frequency electric discharge format. Instead, a filament type electrode may be used.  
         [0026]     A carbon nano tube  118  to serve as an electron emission source is formed on a metal member of the cathode electrode  116 . The carbon nano tube  118  is manufactured by arc discharge or laser irradiation to graphite and refined before being bonded to the metal member with a conductive paste. In particular, a carbon nano tube obtained by arc discharge has excellent crystallinity and few defects.  
         [0027]     Alternatively, a carbon nano tube may be formed by direct deposition on the metal member using a vapor phase method. Plasma CVD, preferably, electric field-applied plasma CVD, is employed and methane, ethane, ethylene, acetylene, or a mixture thereof, namely, hydro carbon-based gas and hydrogen gas, are used as introduction gas to let a carbon nano tube grow directly on the metal member. An auxiliary agent is desirably used in forming a carbon nano tube by deposition, and the agent is, for example, Ni, Fe, or Co, or an alloy of two or more chosen from Ni, Fe, and Co. The auxiliary agent is applied by sputtering, plating, baking after application of an organometallic compound, or other methods.  
         [0028]     With such vapor phase methods, the direction in which a carbon nano tube grows can be set perpendicular to the metal member. This directs the tip of the carbon nano tube upward and therefore the field emission characteristic thereof is improved. Furthermore, by directing the growth of the carbon nano tube, the carbon nano tube density per area can be increased and therefore high density plasma is obtained.  
         [0029]     The anode electrode  117  is a grid electrode, which is placed not far from the cathode electrode  116  in order to apply a voltage effectively. Electrons emitted from the carbon nano tube  118  pass through grid holes of the anode electrode  117 , collide with material gas in the chamber, and ionize the material gas. When the grid holes have a small diameter, ions and neutral molecules in the chamber are prevented from reaching the cathode electrode  116 . Desirably, the pressure in the chamber upon introduction of material gas is 1×10 −1  to 1×10 −3  Pa.  
         [0030]     Plasma produced by such a plasma producing apparatus that has the cathode electrode  116  and the anode electrode  117  has high density and is stable.  
       Embodiment Mode 2  
       [0031]     This embodiment describes with reference to  FIGS. 2A and 2B  a cathode electrode structure different from the one in Embodiment Mode 1.  FIG. 2A  is a bird&#39;s-eye view and  FIG. 2B  shows a section perpendicular to a magnetic field that is formed in parallel to a cathode electrode.  
         [0032]     A cathode electrode  31  is composed of a metal member and a carbon nano tube formed on a surface of the metal member. Similar to Embodiment Mode 1, the carbon nano tube is manufactured by arc discharge or laser irradiation and refined before being bonded to the metal member with a conductive paste. Alternatively, the carbon nano tube is obtained through vapor phase growth directly on the metal member using plasma CVD.  
         [0033]     A plate-like or rod-like magnet  32  is placed on each side of the cathode electrode  31  in a manner that makes the north pole and the south pole face each other. A plurality of blocking plates  33  are placed between the magnet  32  and the other magnet  32 . The blocking plates  33  are each formed of a metal material or an insulating material, and are slanted with respect to a sheath electric field that is formed between the cathode electrode and plasma. The inclination is determined by the voltage between the cathode electrode  31  and an anode electrode, the magnetic force of the magnet  32 , the distance between one magnet  32  and the other magnet  32 , and the field emission characteristic of the carbon nano tube.  
         [0034]     When a voltage is applied between the cathode electrode  31  and the anode electrode, electrons are emitted from the carbon nano tube and the orbits of the electrons are bent by a magnetic field created by one magnet  32  and the other magnet  32 . The blocking plates  33  are slanted at an angle that can avoid collision between electrons whose orbits have been bent and the blocking plates  33 . The shape of the blocking plates  33  desirably possesses a curvature. However, the blocking plates  33  may instead be planar.  
         [0035]     Positive ions in plasma are accelerated by the sheath electric field formed between the cathode electrode  31  and plasma. Of these positive ions, ones having a large Larmor radius collide against the blocking plates  33  because their orbits are not bent by the magnetic field. The positive ions are thus prevented from colliding with the carbon nano tube releasing high energy. If points of collision between the positive ions and the blocking plates  33  are formed of BaO, LaB6, or others that readily emit secondary electrons, plasma can effectively be replenished with electrons.  
         [0036]     Plasma produced by such a plasma producing apparatus that has the cathode electrode  31  and the anode electrode  32  has high density and is stable.  
         [0037]     This embodiment mode can be combined with Embodiment Mode 1.  
       Embodiment 1  
       [0038]     This embodiment describes the present invention by taking a doping apparatus as an example and referring to  FIG. 1 .  
         [0039]     Major components of the doping apparatus of  FIG. 1  are an ion source  101 , a processing chamber  102 , a load lock chamber  103 , and vacuum exhaust means  106 . The processing chamber  102  is a place where a substrate is held and ions are implanted, and the chamber is provided with a substrate stage  104  and a clamper  105 . A pin  108  for moving a substrate up and down on the substrate stage is used when a substrate is put on the substrate stage  104  by transporting means  109  after the transporting means brings the substrate from the load lock chamber  103 . The clamper  105  is moved up and down by an arm  107  as the substrate is brought in and out.  
         [0040]     With the clamper  105  lifted above the substrate stage by the arm  107 , a substrate is brought in by the transporting means  109 . At this point, the pin  108  is jetting out from the substrate stage  104  and a substrate  100  is placed on the pin  108 . Thereafter, the pin  108  is lowered and then the arm  107  is lowered to enable the clamper  105  to fix the substrate  100  to the substrate stage.  
         [0041]     The exhaust means  106  is a combination of a dry pump, a mechanical booster pump, a turbomolecular pump, and the like.  
         [0042]     The ion source  101  has a gas supplying system  111  for supplying material gas, a cathode electrode  116  for producing plasma, and an anode electrode  117 . The structure in  FIG. 1  conforms to the capacitively coupled high frequency electric discharge format. Other electrodes may be used instead.  
         [0043]     A carbon nano tube  118  to serve as an electron emission source is formed on a metal member of the cathode electrode  116 . The carbon nano tube  118  is manufactured by arc discharge or laser irradiation to graphite and refined before being bonded to the metal member with a conductive paste. In particular, a carbon nano tube obtained by arc discharge has excellent crystallinity and few defects.  
         [0044]     When a voltage is applied between the cathode electrode  116  and the anode electrode  117 , electrons are emitted from the carbon nano tube  118  formed on the surface of the cathode electrode and collide with molecules of material gas supplied from the gas supplying system  111 . Plasma is thus produced.  
         [0045]     Plasma produced by such a plasma producing apparatus that has the cathode electrode  116  and the anode electrode  117  has high density and is stable.  
         [0046]     The apparatus also has as a lead-out electrode system a lead-out electrode  112 , an acceleration electrode  113 , a suppression electrode  114 , and a ground electrode  115 . These electrodes have many openings through which ions pass. Ions are accelerated by the lead-out electrode  112  to which a lead-out voltage Vex is applied and by the acceleration electrode  113  to which an acceleration voltage Vac is applied. The suppression electrode  114  catches and gathers scattering ions in order to increase the directivity of the ion flow. 10 kV is applied as the lead-out voltage Vex to change the acceleration voltage Vac. In this way, ions are accelerated with an energy of 50 to 100 keV.  
         [0047]     The material gas used is PH 3 , B 2 H 6 , or the like, and is diluted down to 0.1 to 30% with hydrogen or inert gas. When PH 3  is used, PH x   + , P2H x   + , H x   + , and the like are generated as ion species and, if there is no mass separation, these ions are accelerated by the lead-out electrode system and led out to the processing chamber where the substrate is set. As indicated by the arrow in  FIG. 1 , the ions are led out mostly linearly by the four electrodes and are thrown to the substrate.  
         [0048]     In this embodiment, plasma produced is stable and therefore ions can be introduced to the substrate  100  evenly. In addition, CNTs are formed on the cathode electrode  116  to prolong the lifespan of the cathode  116 .  
       Embodiment 2  
       [0049]     This embodiment describes the present invention by taking an ion source as an example and referring to  FIG. 3 .  
         [0050]     An ion source has a wall  11  and an arc chamber (plasma chamber)  12  constituted of the wall  11 . Semiconductor material gas such as phosphine or diborane is introduced from an inlet  13 . A cathode electrode  14  and an anode electrode  15  are set in the arc chamber  12 . A carbon nano structure such as a single wall carbon nano tube (SWNT), a multi wall carbon nano tube (MWNT), a VGCF (vapor growth carbon fiber), a nano horn, or a nano glass fiber is used for the cathode electrode  14 .  
         [0051]     A voltage is applied between the cathode electrode  14  and the anode electrode  15  by an external power source. As a result, high energy electrons are emitted from the cathode electrode  14  and collide with molecules of the material gas introduced through the inlet  13  to produce high density plasma. In an ion doping apparatus, ions are led out of this plasma by a lead-out electrode. Then an acceleration electrode accelerates the ions until an arbitrary energy is reached and the ions are implanted to a given semiconductor substrate.  
         [0052]     The north poles and the south poles of rod-like magnets are placed alternately along the periphery of the wall  11  to form a magnetic field called line cusp, thereby raising the plasma density at the center of the arc chamber  12  and uniformizing the plasma density distribution. In the case of a large capacity ion source for a large-area substrates it is desirable to place more than one cathode electrode  14  in the arc chamber  12 .  
         [0053]     When plasma is thus produced using the cathode electrode  14  where carbon nano tubes are formed, high density plasma can be obtained efficiently. In addition, superior heat resistance and current transporting characteristic of carbon nano tubes make it possible to produce stable plasma in an ion source. If such an ion doping apparatus having the cathode electrode is used to introduce impurity elements into a semiconductor film, the treatment of the film is stable and therefore the yield of a semiconductor device can be improved.  
         [0054]     This embodiment may be combined with Embodiment 1.  
         [0055]     By employing the structure of the present invention, the lifespan of a cathode electrode can be prolonged. Accordingly, the number of times to replace cathode electrodes is greatly reduced. This lowers the possibilities of contaminating a clean room and adversly affecting human health as well as cost.  
         [0056]     The present invention also makes it possible to produce high density plasma that is stable. This means that steps that use plasma, such as film formation, etching, and ion introduction, can be stabilized. Moreover, a semiconductor device manufactured through these steps can be improved in yield.