CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase application of PCT International Application No. PCT/JP2010/068557, filed Oct. 21, 2010, and claims priority to Japanese Patent Application No. 2009-251656, filed Nov. 2, 2009, the disclosures of which PCT and priority applications are incorporated herein by reference in their entireties for all purposes. 
     FIELD OF THE INVENTION 
     The present invention relates to a plasma CVD device (plasma chemical vapor deposition device), and a method of manufacturing a silicon thin film using a plasma CVD method. In particular, the invention relates to a plasma CVD device for forming a silicon thin film used for thin film silicon solar cells, thin film silicon transistors, etc., and a method of manufacturing a silicon thin film by using a plasma CVD method. 
     BACKGROUND OF THE INVENTION 
     Compared with single-crystalline silicon solar cells and multi-crystalline silicon solar cells mainly used as solar cells, thin film silicon solar cells attract attention as solar cells of the next generation since they are advantageous in view of cost without using expensive silicon substrates. 
     As a method of manufacturing an amorphous silicon thin film used for thin film silicon solar cells, a manufacturing method using a parallel-plates type plasma CVD device is known. A conventional parallel-plates type plasma CVD device used in this manufacturing method is shown in  FIG. 7 . 
     The conventional parallel-plates type plasma CVD device  61  shown in  FIG. 7  has a vacuum vessel  62  for performing plasma treatment. The vacuum vessel  62  usually has exhaust ports  62   a  to be connected with a high-vacuum exhaust equipment and a process exhaust equipment. The high-vacuum exhaust equipment is used to obtain the back pressure inside the vacuum vessel  62 , and as the high-vacuum exhaust equipment, usually a turbo molecular pump or the like is used. The process exhaust equipment is used to maintain the pressure required in a plasma treatment process, and in the case of a general CVD process, as the process exhaust equipment, a mechanical booster pump or the like is used, though depending on the process pressure. 
     Inside the vacuum vessel  62 , a discharge electrode plate  63  and an earth electrode plate  610  are installed to face each other with a clearance therebetween. On the upper surface of the earth electrode plate  610 , a substrate  612  is held. The earth electrode plate  610  is internally provided with a heating mechanism  611  for heating the substrate  612 . 
     At the lower face of the discharge electrode plate  63 , a hollow portion  63   a  is provided, and a shower plate  66  is installed at the lower face of the discharge electrode plate  63 , to close the hollow portion  63   a . In the shower plate  66 , numerous gas introduction holes  66   a  are provided therethrough from the upper surface to the lower surface of the shower plate  66 . The vacuum vessel  62  is provided with a raw gas supply pipe  65  extending from a gas supply equipment (not shown in the drawing) installed outside the vacuum vessel  62  and passing through the discharge electrode plate  63 , to reach the hollow portion  63   a.    
     The raw gas supply pipe  65  is electrically insulated from the discharge electrode plate  63  though not shown in the drawing. The vacuum vessel  62  is also electrically insulated from the discharge electrode plate  63  though not shown in the drawing. The vacuum vessel  62  is earthed by a conductor  62   c . Between the vacuum vessel  62  and the earth electrode plate  610 , an insulator  610   a  is provided, and the earth electrode plate  610  is earthed by a conductor  610   c.    
     Raw gas necessary for plasma treatment is supplied from the raw gas supply equipment through the raw gas supply pipe  65  into the hollow portion  63   a . The gas supplied into the hollow portion  63   a  passes through the numerous gas introduction holes  66   a  of the shower plate  66  and is uniformly supplied to the substrate  612  held on the earth electrode plate  610 . 
     A high frequency power supply  614  is connected with the discharge electrode plate  63  via a matching box  613 . The exhaust equipment keeps the inside of the vacuum vessel  62  at a constant pressure, and a high frequency power is applied to the discharge electrode plate  63  by the high frequency power supply  614 , to generate plasma. Generated plasma forms an amorphous silicon thin film on the surface of the substrate  612 . 
     However, it is known that if the amorphous silicon thin film produced by using such a parallel-plates type plasma CVD device is irradiated with light, dangling bonds (defects) increase in the film, to cause light-induced degradation. The problem of light-induced degradation was found as the Staeber-Wronski effect more than 30 years ago, but is not yet solved. 
     The mechanism in which the light-induced degradation is caused is not yet clearly clarified. However, it is known that the light-induced degradation has correlation with a Si—H 2  bond concentration in the film. Further, it is reported that if the Si—H 2  bond concentration in the film is low, the light-induced degradation is also small. It is indicated as a cause of the increase in the Si—H 2  bond concentration that high order silane-related species (Si m H n : m=2 or more) growing during formation of the film are incorporated into the film. It is considered that the high order silane-related species grow due to the successive reaction in which the SiH 2  radicals produced in the plasma are inserted into Si—H bonds, and are mixed into the film, to increase the Si—H 2  bonds, causing initial dangling bonds to be formed in the film. 
     On the other hand, the reactions in the plasma start when electrons having some energy collide with SiH 4  acting as parent molecules, to decompose them into various molecules such as SiH 3  radicals and SiH 2  radicals. In general, the electron temperatures (Te) showing energy of electrons in the plasma have a distribution, and in addition to the SiH 3  radicals as a precursor contributing to creation of the film, SiH 2  radicals are produced without fail. For this reason, in the case where the conventional parallel-plates type plasma CVD device is used to manufacture an amorphous silicon thin film, the power applied is set at a low level in order to decrease the generation of high order silane-related species, for thereby inhibiting the generation of SiH 2  radicals and high order silane-related species. However, because of the low power level, the depositing rate cannot be enhanced (Non Patent Literature 1). 
     On the other hand, as a film deposition method for obtaining an amorphous silicon thin film with a low Si—H 2  bond concentration, a triode deposition system is proposed. A plasma CVD device using the triode deposition system is shown in  FIG. 8 . The plasma CVD device  71  using the triode deposition system shown in  FIG. 8  is identical to the plasma CVD device  61  shown in  FIG. 7  in basic structure. Accordingly the same components as those of  FIG. 7  are indicated by the same symbols in  FIG. 8 . The difference between the device  71  of  FIG. 8  and the device  61  of  FIG. 7  is that a mesh electrode plate  716  is installed between the discharge electrode plate  63  and the earth electrode plate  610 . 
     In  FIG. 8 , a DC variable power supply  715  is connected with the mesh electrode plate  716 . As can be seen from  FIG. 8 , the triode deposition system also uses a parallel-plates type CVD device. The mesh electrode plate  716  is inserted between the discharge electrode plate  63  and the earth electrode plate  610 , and a potential (usually a negative potential) is applied to the mesh electrode plate  716 . Thus, it is considered that the plasma can be contained between the discharge electrode plate  63  and the mesh electrode plate  716 . No plasma is generated between the mesh electrode plate  716  and the earth electrode plate  610 . On the other hand, the radicals contributing to creation of the film are produced between the discharge electrode plate  63  and the mesh electrode plate  716  and diffused by the mesh electrode plate  716 , to reach the substrate  612 . 
     The diffusion distance of radicals is proportional to the square root of the inverse number of the molecular weight. Therefore, it is intended to use that the diffusion distance of high order silane-related radicals is shorter than that of SiH 3  radicals, in order to selectively transport the SiH 3  radicals to the substrate  612 . 
     With this configuration, a very low Si—H 2  bond concentration can be achieved to obtain an amorphous silicon thin film having a low light-induced degradation rate. However, in order to remove high order silane-related radicals in the triode deposition system, it is necessary that the distance between the mesh electrode plate  716  and the earth electrode plate  610  is long enough. For this reason, the triode deposition system has a problem that the depositing rate cannot be enhanced (Non Patent Literature 2). 
     Further, the gas temperature in the plasma is also an important factor. It is known that the successive reaction for growing high order silane-related species (Si m H n : m=2 or more) is a third-body reaction. As a means for inhibiting this reaction, gas heating is considered effective. The high order silane-related species produced by the insertion reaction of SiH 2  radicals into Si—H bonds are stabilized by making a third body (usually SiH 4  acting as parent molecules) absorb extra energy. 
     Consequently in the state where the third body is not in a position to accept energy, that is, in the state where the temperature is high, the third-body reaction does not take place, and high order silane-related species are inhibited (Patent Literature 1). Therefore, in order to heat the space for depositing the film, it is desirable that the plasma near the sheath on the discharge electrode plate side where the high order silane-related species are considered to be most generated is heated from the discharge electrode plate side. However, it is structurally difficult to apply a high frequency to the electrode plate and further to introduce a heater. Usually in order to control the substrate temperature, the earth electrode plate supporting the substrate is heated. This causes also the plasma to be heated via the substrate, but since the place is distant from the sheath on the discharge electrode plate side, the state of effective and positive heating cannot be achieved. Accordingly if the substrate temperature is further raised to higher than the optimum substrate temperature, defects in the film increase. Therefore, there is a problem that the highest heating temperature is limited. 
     Patent Literature 
     Patent Literature 1: JP 08-91987 A 
     Non Patent Literature 
     Non Patent Literature 1: Madoka Takai et al., APPLIED PHYSICS LETTERS 77 (2000) 2828 
     Non Patent Literature 2: Satoshi Shimizu et al., JOURNAL OF APPLIED PHYSICS 101, 064911, (2007) 
     SUMMARY OF THE INVENTION 
     As described above, in order to inhibit the light-induced degradation of thin film silicon solar cells, hitherto, attempts have been made to decrease the ingress of high order silane-related species into the amorphous silicon thin film, for thereby lowering the Si—H 2  bond concentration in the film as fax as possible. However, these attempts do not suit the manufacturing of solar cells due to very low depositing rates and high depositing temperatures. That is, any practical method for depositing a film, which ensures both a high depositing rate and a low Si—H 2  bond concentration in the film, has not been found yet. 
     Embodiments of the invention provide a plasma CVD device capable of manufacturing an amorphous silicon thin film having a low Si—H 2  bond concentration therein at a high depositing rate and at a low depositing temperature, and also provide a method of manufacturing a silicon thin film by using the plasma CVD device. 
     An embodiment of a plasma CVD device of the invention is as follows: 
     A plasma CVD device provided with (comprising or consisting of): 
     (a) a vacuum vessel, 
     (b) an exhaust equipment for keeping the inside of the vacuum vessel under reduced pressure, 
     (c) a discharge electrode plate installed in the vacuum vessel, 
     (d) an earth electrode plate for supporting a substrate for forming a thin film thereon, installed to face the discharge electrode plate with a clearance kept therefrom, 
     (e) a high frequency power supply for applying a high frequency power to the discharge electrode plate, and 
     (f) a raw gas supply equipment for supplying a raw gas for forming the thin film into the vacuum vessel, which comprises 
     (g) a plurality of gas introduction holes formed in the discharge electrode plate, connected on one side with the gas supply equipment and opening on the other side at plural places on one surface of the discharge electrode plate, and a plurality of gas exhaust holes formed in the discharge electrode plate and passing therethrough from the surface where the plurality of gas introduction holes open, to the surface of the other side, 
     (h) a gas exhaust equipment for exhausting the gas exhausted from the plurality of gas exhaust holes to outside the vacuum vessel, 
     (i) an earth cover plate installed between the discharge electrode plate and the earth electrode plate with clearances kept from the respective electrode plates, and 
     (j) a plurality of second gas introduction holes formed through the earth cover plate at the positions in the earth cover plate corresponding to the plurality of gas introduction holes and a plurality of second gas exhaust holes formed through the earth cover plate at the positions in the earth cover plate corresponding to the plurality of exhaust holes. 
     In embodiments of the plasma CVD device of the invention, it is preferred that the diameter of the gas exhaust holes is 2 mm to 100 mm. 
     In embodiments of the plasma CVD device of the invention, it is preferred that the clearance between the discharge electrode plate and the earth cover plate is 0.5 mm to 10 mm. 
     In embodiments of the plasma CVD device of the invention, it is preferred that the diameter of the second gas exhaust holes is 0.5 to 1.5 times the diameter of the gas exhaust holes. 
     In embodiments of the plasma CVD device of the invention, it is preferred that the diameter of the second gas introduction holes is 7 mm or less in so far as the flow of the gas in the second introduction holes is not prevented. 
     In embodiments of the plasma CVD device of the invention, it is preferred that the earth cover plate is earthed. 
     In embodiments of the plasma CVD device of the invention, it is preferred that the earth cover plate is provided with a heating mechanism. 
     In embodiments of the plasma CVD device of the invention, it is preferred that a potential control plate capable of controlling the potential is installed between the earth cover plate and the earth electrode plate with clearances kept from the respective plates, and that a plurality of third gas introduction holes positioned in correspondence to the plurality of second gas introduction holes and a plurality of third gas exhaust holes positioned in correspondence to the plurality of second gas exhaust holes are formed in the potential control plate, to pass through the potential control plate. 
     In embodiments of the plasma CVD device of the invention, it is preferred that the potential applied to the potential control plate is a negative potential. 
     An embodiment of a method of manufacturing a silicon thin film of the invention is as follows. 
     A method of manufacturing a silicon thin film comprises the steps of plasmatizing a raw gas containing a Si compound by using the plasma CVD device of the invention, making the earth electrode plate for supporting a substrate for forming a thin film thereon support the substrate for forming a thin film thereon, and depositing a silicon thin film on the substrate. 
     The invention provides embodiments of a plasma CVD device capable of manufacturing a high quality amorphous silicon thin film having few defects and little ingress of high order silane-related species by controlling flow of a gas introduced into a vacuum vessel and controlling plasma for removing high order silane-related species and further by controlling a gas temperature separately from a substrate temperature, and also provides embodiments of a method of manufacturing a high quality amorphous silicon thin film. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic vertical sectional view showing an embodiment (first embodiment) of the plasma CVD device of the invention. 
         FIG. 2  is a partially enlarged vertical sectional view showing the discharge electrode plate and the earth cover plate of the plasma CVD device shown in  FIG. 1 . 
         FIG. 3  is a plan view taken in the direction of the arrows along the line X-X in  FIG. 1 . 
         FIG. 4  is a plan view taken in the direction of the arrows along the line Y-Y in  FIG. 1 . 
         FIG. 5  is a schematic vertical sectional view showing another embodiment (second embodiment) of the plasma CVD device of the invention. 
         FIG. 6  is a partially enlarged vertical sectional view showing the discharge electrode, the earth cover plate and the potential control plate of the plasma CVD device shown in  FIG. 5 . 
         FIG. 7  is a schematic vertical sectional view showing an example of the conventional plasma CVD device. 
         FIG. 8  is a schematic vertical sectional view showing an example of the conventional CVD device using a triode deposition system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first embodiment is described below. 
       FIGS. 1 to 4  show an example of a first embodiment of the plasma CVD device of the invention. The plasma CVD device  1  has a vacuum vessel  2 . The vacuum vessel  2  is formed by side plate  2   a , a top plate  2   b  closing the opening at the top plane of the side plate  2   a , and a bottom plate  2   c  closing the opening at the bottom plane of the side wall  2   a . The vacuum vessel  2  has interior side plate  2   d  attached to the lower surface of the top plate  2   b . In the space inside the interior side plate  2   d , a gas exhaust cap  2   f  having a hollow portion  2   e  and open at the bottom plane is installed, and a portion (preferably a central portion) of the top plate of the gas exhaust cap  2   f  is guided outward from the vacuum vessel  2 , forming a gas exhaust conduit (gas exhaust equipment)  2   g.    
     At the bottom plane of the gas exhaust cap  2   f , a discharge electrode plate  3  is installed to close the opening of the hollow portion  2   e . The discharge electrode plate  3  has a raw gas supply hole  3   a  therein and numerous gas introduction holes  18  opening at the lower surface of the discharge electrode plate  3 , which branch from the raw gas supply hole  3   a . The discharge electrode plate  3  has numerous gas exhaust holes  17  passing therethrough from the surface where the numerous gas introduction holes  18  open, to the surface of the other side, i.e., from the lower surface to the upper surface. The numerous gas introduction holes  18  and the numerous gas exhaust holes  17  are formed at the positions different from each other. Being numerous in this case is intended to mean that the raw gas can flow almost uniformly in the vacuum vessel  2 . An example of the arrangement of the numerous gas exhaust holes  17  in the discharge electrode plate  3  is shown in  FIGS. 3 and 4  as plan views. 
     The upstream side of the raw gas supply hole  3   a  is connected with a raw gas supply pipe  5  extending outward from the vacuum vessel  2 . The upstream side of the raw gas supply pipe  5  is connected with a raw gas supply source not shown in the drawing. 
     At the bottom plane of the interior side plate  2   d , an earth cover plate  8  is attached to cover the inside space of the interior side plate  2   d . The upper surface of the earth cover  8  faces the lower surface of the discharge electrode plate  3  with a clearance kept therebetween. The earth cover plate  8  has numerous second gas introduction holes  20  formed at the positions corresponding to the gas introduction holes  18  of the discharge electrode plate  3 , and numerous second gas exhaust holes  19  formed at the positions corresponding to the gas exhaust holes  17  of the discharge electrode plate  3 . Both the second gas introduction holes  20  and the second gas exhaust holes  19  pass through the earth cover plate  8  in the thickness direction thereof. An enlarged view showing an example of the positional relation between the gas introduction holes  18  and the gas exhaust holes  17  of the discharge electrode plate  3  on one hand and the second gas introduction holes  20  and the second gas exhaust holes  19  of the earth cover plate  8  on the other hand is shown in  FIG. 2 . 
     Between the inner wall face of the interior side plate  2   d  on one hand and the side wall face of the discharge electrode plate  3  and the side wall face of the gas exhaust cap  2   f  on the other hand, space is formed for preventing the electric conduction between those components. 
     The discharge electrode plate  3  and the gas exhaust cap  2   f  are formed of an electric conductor. The gas exhaust cap  2   f  is connected with a high frequency power supply  14  in the portion of the gas exhaust conduit  2   g . Between the high frequency power supply  14  and the gas exhaust cap  2   f , if necessary, a matching box  13  is provided. Between the gas exhaust cap  2   f  and the top plate  2   b  of the vacuum vessel  2 , electric insulators  2   h  and  2   i  are provided. The vacuum vessel  2  is earthed by a conductor  2   j.    
     The interior side plate  2   a  and the earth cover plate  8  are formed of an electric conductor. The earth cover plate  8  is also earthed by the conductor  2   j  via the interior side plate  2   a , and the top plate  2   b , side plate  2   a  and bottom plate  2   c  of the vacuum vessel  2 . 
     Inside the vacuum vessel  2 , at a portion (preferably at a central portion) on the upper surface of the bottom plate  2   c  of the vacuum vessel  2 , an earth electrode plate  10  is installed via an electric insulator  10   a . A portion (preferably a central portion) of the lower surface of the earth electrode plate  10  is guided outward from the vacuum vessel  2  and earthed by a conductor  10   b . On the upper surface of the earth electrode plate  10 , a substrate  12  for forming a thin film thereon is mounted. Inside the earth electrode plate  10 , if necessary, a substrate heating mechanism  11  is installed. 
     The bottom plate  2   c  of the vacuum vessel  2  is provided with exhaust ports  2   k  for exhausting gas in the vacuum vessel  2 . The exhaust ports  2   k  are connected with a high vacuum exhaust equipment (not shown in the drawing) and a process exhaust equipment (not shown in the drawing). The high vacuum exhaust equipment is provided to obtain the back pressure inside the vacuum vessel  2 , and as the high vacuum exhaust equipment, a turbo molecule pump or the like is used. The process exhaust equipment is provided to maintain the pressure necessary for the plasma treatment process, and as the process exhaust equipment, a mechanical booster pump, turbo molecule pump or the like is used. 
     The frequency of the high frequency power supply  14  can be selected as desired. It is considered that if the frequency is higher, the electron temperature declines. In view of productivity and the uniformity of the produced thin film, it is preferred that the frequency used is 100 kHz to 100 MHz. A more preferred range is 10 MHz to 60 MHz. 
     The substrate  12  for forming a thin film thereon is mounted on the earth electrode plate  10 . For example, the earth electrode plate  10  can be counter-sunk, and the substrate  12  can be placed in the counter-sunk portion. Otherwise, another fixture may also be used to press the substrate  12  onto the earth electrode plate  10  for mounting. 
     The gas exhaust holes  17  formed in the discharge electrode plate  3  function to make gas flow from the lower surface side to the upper surface side of the discharge electrode plate  3 , to ensure that the gas reaching the hollow portion  2   e  may be exhausted from the gas exhaust conduit  2   g  to outside the vacuum vessel  2 . In addition, the gas exhaust holes  17  also function to localize the plasma therein. Consequently it is important to design the diameter of the gas exhaust holes in response to the pressure for depositing the film. 
     That is, in the case where the pressure for depositing the film is low, gas exhaust holes having a large diameter are used, and in the case where the pressure for depositing the film is high, gas exhaust holes having a small diameter are used. On the other hand, if the diameter of the gas exhaust holes  17  is too small, sufficient gas exhaust capability may not be obtained or it may be difficult to manufacture the discharge electrode plate  3  by machining. 
     Further, if the diameter of the gas exhaust holes  17  is too large, there may occur a problem that the film thickness distribution of the thin film formed on the substrate  12  may not be uniform. It is preferred that the diameter of the gas exhaust holes  17  is 2 mm to 100 mm. A more preferred range is 5 mm to 50 mm. As shown in  FIGS. 3 and 4 , it is preferred that the numerous gas exhaust holes  17  are arranged uniformly in the discharge electrode plate  3 . 
     Exhaust of gas in the vacuum vessel  2  while the thin film is formed on the substrate  12  may be performed through the gas exhaust conduit  2   g  from the gas exhaust holes  17  formed in the discharge electrode plate  3 , but in addition can also be performed through the exhaust ports  2   k  formed in the vacuum vessel  2  while the pressure in the vacuum vessel  2  is adjusted. 
     While the flow rate of the raw gas is controlled by a mass flow controller not shown in the drawing, the raw gas is introduced into the chamber of the vacuum vessel  2  through the raw gas supply pipe  5 , the raw gas supply hole  3   a  and the numerous gas introduction holes  18 . 
     As the raw gas, in general, silane (SiH 4 ) is used, but such a gas as a halide or disilane can also be used. The raw gas can also be diluted by hydrogen or an inert gas such as argon. A doping gas can also be introduced into the raw gas. 
     It is preferred that the clearance between the discharge electrode plate  3  and the earth cover plate  8  is set at such a clearance as not to cause discharge between the discharge electrode plate  3  and the earth cover plate  8 . It is preferred that the clearance is 0.5 mm to 10 mm, and a more preferred range is 0.5 mm to 5 mm. 
     The numerous second gas introduction holes  20  and the numerous second gas exhaust holes  19  formed in the earth cover plate  8  and the numerous gas introduction holes  18  and the numerous gas exhaust holes  17  formed in the discharge electrode plate  3  are positioned to face each other in such a state as not to block the openings of the other plate. 
     If the diameter of the second gas exhaust holes  19  of the earth cover plate  8  is too larger than the diameter of the gas exhaust holes  17  of the discharge electrode plate  3 , plasma leak may be caused. On the other hand, if the former is too smaller than the latter, the deposition rate capable of inhibiting the diffusion of active species from the plasma declines. It is preferred that the diameter of the second gas exhaust holes  19  of the earth cover plate  8  is 0.5 to 1.5 times the diameter of the gas exhaust holes  17  of the discharge electrode plate  3 . The form of the second gas exhaust holes  19  of the earth cover plate  8  can also be such a form as to change in diameter in the hole axis direction, for example, a tapered form. 
     It is preferred that the diameter of the second gas introduction holes  20  of the earth cover plate  8  formed to face the gas introduction holes  18  of the discharge electrode plate  3  is such a size as not to prevent flow of gas through the second gas introduction holes  20  from the upper surface side to the lower surface side of the earth cover plate  8  and such a size as not to allow the plasma to enter the second gas introduction holes  20 . It is preferred that the diameter of the second gas introduction holes  20  of the earth cover plate  8  is 7 mm or less. More preferred is 2 mm or less. 
     If the earth cover plate  8  having such second gas introduction holes  20  and such second gas exhaust holes  19  is installed to face the discharge electrode plate  3  with a clearance kept therebetween, the plasma can be confined inside the gas exhaust holes  17  formed in the discharge electrode plate  3  and the second gas exhaust holes  19  formed in the earth cover plate  8 . The earth cover plate  8  can be electrically shielded or can also have a potential applied thereto. However, considering the stability of discharge, it is preferred that the earth cover plate  8  is earthed. 
     If the plasma is confined in the gas exhaust holes  17  and the second gas exhaust holes  19  as described above, the plasma remains to exist in gas flow in these holes. Therefore, the high order silane-related species short in diffusion length are exhausted through these holes by gas flow in these holes, and on the other hand, the SiH 3  radicals long in diffusion length are diffused in the direction toward the substrate  12  by diffusion. 
     Further, if the plasma is confined inside the gas exhaust holes  17  of the discharge electrode plate  3  and the second gas exhaust holes  19  of the earth cover plate  8 , the plasma between the earth cover plate  8  and the substrate  12  is weakened, and few new active species exist in the space between the earth cover plate  8  and the substrate  12 . As a result, a state where only the high order silane-related species, SiH 2  radicals and SiH 3  radicals respectively contributing to deposition of the film are diffused from the earth cover plate  8  toward the substrate  12  is formed. 
     The following reactions are considered to occur in the space between the earth cover plate  8  and the substrate  12 .
 
Si m H 2m+1 +SiH 4  →Si m H 2m+2 +SiH 3    (Formula 1)
 
SiH 2 +SiH 4 →Si 2 H 6    (Formula 2)
 
SiH 3 +SiH 4 →SiH 4 +SiH 3    (Formula 3)
 
     Formula 1 indicates that the high order silane-related radicals diffused in the direction toward the substrate  12  against flow of gas in the exhaust direction react with SiH 4  acting as parent molecules during the diffusion toward the substrate  12 , to produce inactive high order silane-related species, being exhausted without participating in the deposition of the film. Formula 2 indicates that SiH 2  radicals react with SiH 4  acting as parent molecules, to be inactive in the process of growing into high order silane-related species while being diffused, and are exhausted without participating in the deposition of the film. On the other hand, formula 3 indicates that SiH 3  radicals remaining unchanged without reacting with parent molecules reach the substrate  12 , to contribute selectively to the deposition of the thin film, for providing a high quality thin film. 
     On the other hand, it is feared that since the production of SiH 2  radicals continue in the plasma localized inside the second gas exhaust holes  19  of the earth cover plate  8  and inside the gas exhaust holes  17  of the discharge electrode plate  3 , high order silane-related species are produced according to formulae 1 and 2. These reactions are third-body reactions where extra energy is absorbed by the third body (usually SiH 4  acting as parent molecules), to stabilize the product as described above. Consequently, if a heating mechanism  21  (see  FIG. 4 ) is installed in the earth cover plate  8 , to effectively heat the plasma localized inside the gas exhaust holes  17  and the second gas exhaust holes  19  considered to generate the highest order radicals, the progress of the third-body reactions can be inhibited. 
     A second embodiment of the present invention is described below. 
       FIG. 5  shows an example of a second embodiment of the plasma CVD device of the invention. The plasma CVD device  41  shown in  FIG. 5  is identical to the plasma CVD device  1  shown in  FIG. 1 , except that a potential control plate  9  provided with a power supply for keeping the potential constant is installed between the earth cover plate  8  and the earth electrode plate  10 . 
     The components forming the plasma CVD device  41  shown in  FIG. 5  other than the components relating to the potential control plate  9  are the same as those forming the plasma CVD device  1  shown in  FIG. 1 . Therefore, in the plasma CVD device  41  shown in  FIG. 5 , the same components as those of the plasma CVD device  1  shown in  FIG. 1  are given the same symbols as those used in  FIG. 1 .  FIG. 6  is a partially enlarged vertical sectional view showing the discharge electrode plate  3 , the earth cover plate  8  and the potential control plate  9  of the plasma CVD device  41  shown in  FIG. 5   
     In the plasma CVD device  41  shown in  FIG. 5 , the potential control plate  9  that is earthed is installed between the earth cover plate  8  and the earth electrode plate  10 . The potential control plate  9  and the earth cover plate  8  face each other with a clearance kept therebetween. The potential control plate  9  and the earth electrode plate  10  also face each other with a clearance kept therebetween. 
     The potential control plate  9  has numerous third gas introduction holes  23  formed therein at the positions facing the gas introduction holes  18  of the discharge electrode plate  3  and the second gas introduction holes  20  of the earth cover plate. The potential control plate  9  further has numerous third gas exhaust holes  22  formed therein at the positions facing the gas exhaust holes  17  of the discharge electrode plate  3  and the second gas exhaust holes  19  of the earth cover plate. Both the third gas introduction holes  23  and the third gas exhaust holes  22  pass through the potential control plate  9  in the thickness direction thereof. The potential control plate  9  is connected with a power supply  15  capable of controlling the potential applied to the potential control plate  9 . 
     The power supply  15  is able to apply a potential to the potential control plate  9  and to allow the potential to be controlled. It can be a DC variable power supply or an AC power supply of the KHz order in frequency capable of generating self-bias for applying a DC potential or even an AC power supply of KHz level or a high frequency power supply. 
     In the case where there is no potential control plate  9 , the confinement of the plasma in the second gas exhaust holes  19  can be controlled by the thickness of the earth cover plate  8 . That is, the earth area capable of supplying sufficient electrons to the plasma localized in the second gas exhaust holes  19  is preferred. Consequently if it is attempted to confine the plasma in the holes as far as possible, it is preferable to enlarge the area of the inner wall faces of the second gas exhaust holes  19 . As a result, it is preferable to increase the is thickness of the earth cover plate  8 . 
     However, if the thickness of the earth cover plate  8  is increased, the opening rate toward the substrate  12  from the plasma decreases and the radicals released from the second gas exhaust holes  19  decrease to remarkably lower the depositing rate. It was found that this problem can be solved by installing the potential control plate  9  below the earth cover plate  8  and applying a negative potential to the potential control plate  9 . If the potential control plate  9  is installed, the plasma can be confined in the second gas exhaust holes  19  without lowering the depositing rate. 
     EXAMPLE 1  
     A substrate  12  made of single-crystalline silicon for forming a thin film thereon was mounted on the upper surface of the earth electrode plate  10  of the plasma CVD device  1  shown in  FIG. 1 . A high frequency power supply  14  of 60 MHz was connected with the discharge electrode plate  3  via the matching box  13 . 
     The diameter of the gas exhaust holes  17  formed in the discharge electrode plate  3  was set at 10 mm, and the clearance between the discharge electrode plate  3  and the earth cover plate  8  was set at 0.8 mm. Further, the diameter of the second gas exhaust holes  19  formed in the earth cover plate  8  was set at 10 mm, and the diameter of the second gas introduction holes  20  was set at 2 mm. The thickness of the earth cover plate  8  was set at 10 mm. 
     The temperature of the heating mechanism (heater)  11  installed in the earth electrode plate  10  was set at 250° C., and the temperature of the earth cover plate  8  was set at 200° C. by the heating mechanism (heater)  21  installed in the earth cover plate  8 . In this state, the temperature of the surface of the substrate 12 was measured and found to be 217° C. 
     Gas in the vacuum vessel  2  was let out through the exhaust ports  2   k  formed in the bottom surface of the vacuum vessel  2  till the pressure in the vacuum vessel  2  reached 1×10 −4  Pa. 
     Subsequently, SiH 4  gas (raw gas) was introduced into the numerous gas introduction holes  18  formed in the discharge electrode plate  3  through the raw gas supply hole  3   a  from the raw gas supply pipe  5  at a flow rate of 50 sccm by a mass flow controller, and the exhaust route from the gas exhaust conduit  2   g  was used to exhaust gas in the vacuum vessel  2  through the numerous second gas exhaust holes  19  formed in the earth cover plate  8  and the numerous gas exhaust holes  17  formed in the discharge electrode plate  3 , to adjust the pressure in the vacuum vessel  2  to 30 Pa. 
     Then, a power of 30 W was supplied from the high frequency power supply  14  to the discharge electrode plate  3 , to generate plasma, for forming an amorphous silicon thin film on the substrate  12 . 
     EXAMPLE 2 
     An amorphous silicon thin film was formed on the substrate  12  under the same conditions as those of Example 1, except that the flow rate of the raw gas was changed to 100 sccm. 
     EXAMPLE 3 
     A substrate  12  made of single-crystalline silicon for forming a thin film thereon was mounted on the upper surface of the earth electrode plate  10  of the plasma CVD device shown in  FIG. 5 . A high frequency power supply  14  of 60 MHz was connected with the discharge electrode plate  3  via the matching box  13 . Further, a DC power supply  15  was connected with the potential control plate  9  installed with a clearance kept from the earth cover plate  8 . 
     The diameter of the gas exhaust holes  17  formed in the discharge electrode plate  3  was set at 10 mm, and the clearance between the discharge electrode plate  3  and the earth cover plate  8  was set at 0.8 mm. Further, the diameter of the second gas exhaust holes  19  formed in the earth cover plate  8  was set at 10 mm, and the diameter of the second gas introduction holes  20  was set at 2 mm. The diameter of the third gas exhaust holes  22  formed in the potential control plate  9  was set at 10 mm, and the diameter of the third gas introduction holes  23  was set at 2 mm. The thickness of the earth cover plate  8  was set at 10 mm, and the thickness of the potential control plate  9  was set at 1 mm. 
     The temperature of the heating mechanism (heater) 11 installed in the earth electrode plate  10  was set at 250° C., and the temperature of the earth cover plate  8  was set at 200° C. by the heating mechanism (heater)  21  installed in the earth cover plate  8 . 
     Gas in the vacuum vessel  2  was let out through the exhaust ports  2   k  formed in the bottom surface of the vacuum vessel  2  till the pressure in the vacuum vessel  2  reached 1×10 4  Pa. 
     Subsequently SiH 4  gas (raw gas) was introduced into the numerous gas introduction holes  18  formed in the discharge electrode plate  3  through the raw gas supply hole  3   a  from the raw gas supply pipe  5  at a flow rate of  50  sccm by a mass flow controller, and the exhaust route from the gas exhaust conduit  2   g  was used to exhaust gas in the vacuum vessel  2  through the numerous third gas exhaust holes  22  formed in the potential control plate  9 , the numerous second gas exhaust holes  19  formed in the earth cover plate  8  and the numerous gas exhaust holes  17  formed in the discharge electrode plate  3 , to adjust the pressure in the vacuum vessel  2  to 25 Pa. 
     Then, a potential of −15 V was applied to the potential control plate  9  by using the power supply  15 . Further, a power of 30 W was supplied from the high frequency power supply  14  to the discharge electrode plate  3 , to generate plasma, for forming an amorphous silicon thin film on the substrate  12 . 
     COMPARATIVE EXAMPLE 1 
     A substrate  612  made of single-crystalline silicon for forming a thin film thereon was mounted on the upper surface of the earth electrode plate  610  of the conventional plasma CVD device  61  shown in  FIG. 7 . A high frequency power supply  614  of 60 MHz was connected via the matching box  613  with the discharge electrode plate  63 . The temperature of the heating mechanism (heater)  611  of the earth electrode plate  610  was set at 270° C. In this state, the temperature of the surface of the substrate  612  mounted on the earth electrode plate  610  was measured and found to be 230° C. 
     Gas in the vacuum vessel  62  was let out through the exhaust ports  62   a  formed in the bottom surface of the vacuum vessel  62  till the pressure in the vacuum vessel  62  reached 1×10 −4  Pa, to keep the inside of the vacuum vessel  62  substantially in a vacuum state. 
     Subsequently SiH 4  gas (raw gas) was introduced into the vacuum vessel  62  through the numerous gas introduction holes  66   a  formed in the shower plate  66  from the raw gas supply pipe  65  at a flow rate of 50 sccm by a mass flow controller, and gas in the vacuum vessel  62  was let out from the exhaust ports  62   a  formed in the bottom surface of the vacuum vessel  62 , to adjust the pressure in the vacuum vessel  62  to 10 Pa. 
     Then, a power of 30 W was supplied to the discharge electrode plate  63  from the high frequency power supply  614 , to generate plasma, for forming an amorphous silicon thin film on the substrate  612 . 
     The Si—H 2  bond concentrations in the amorphous silicon thin films obtained in Examples 1 to 3 were determined by using a Fourier transform infrared spectrometer (FT/IR-6100 produced by JASCO Corporation), and the results are shown in Table 1. 
     
       
         
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Si—H 2  bond 
               
               
                   
                 concentration (at. %) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Example 1 
                 0.56 
               
               
                   
                 Example 2 
                 0.41 
               
               
                   
                 Example 3 
                 0.22 
               
               
                   
                 Comparative Example 1 
                 3.34 
               
               
                   
                   
               
             
          
         
       
     
     It can be seen that in Example 1, even though the temperature of the substrate  12  was as relatively low as 217° C., the Si—H 2  bond concentration in the film was lower than 1 at %, and that in Example 2, since the gas flow rate was raised to enhance the flow velocity of plasma in the exhaust holes, a low Si—H 2  bond concentration was obtained. 
     It can be seen that in Example 3, a further lower Si—H 2  bond concentration than that of Example 1 was obtained by applying a negative potential to the potential control plate  9 , to confine plasma. 
     It can be seen that, as described above, if an embodiment of the plasma CVD device of the invention is used to form a silicon thin film, the obtained silicon thin film is smaller in the ingress of high order silane-related species, being a high quality thin film having less defects than the silicon thin film formed by the conventional parallel-plates type plasma CVD device. If the high quality amorphous silicon thin film is used for solar cells, solar cells having little light-induced degradation and high conversion efficiency can be manufactured. 
     Embodiments of the plasma CVD device of the invention can be used not only for manufacturing amorphous silicon thin films but also for manufacturing various thin films such as microcrystalline silicon thin films. Further, embodiments of the plasma CVD device of the invention can also be used as an etching device and a plasma surface treatment device. 
     REFERENCE NUMBERS 
       1  plasma CVD device 
       2  vacuum vessel 
       2   a  side plate 
       2   b  top plate 
       2   c  bottom plate 
       2   d  interior side plate 
       2   e  hollow portion 
       2   f  gas exhaust cap 
       2   g  gas exhaust conduit 
       2   h  electric insulator 
       2   i  electric insulator 
       2   j  conductor 
       2   k  exhaust port 
       3  discharge electrode plate 
       3   a  raw gas supply hole 
       5  raw gas supply pipe 
       8  earth cover plate 
       9  potential control plate 
       10  earth electrode plate 
     
       10 
       a electric insulator  
     
     
       10 
       b conductor  
     
       11  substrate heating mechanism 
       12  substrate 
       13  matching box 
       14  high frequency power supply 
       15  power supply 
       17  numerous gas exhaust holes 
       18  numerous gas introduction holes 
       19  numerous second gas exhaust holes 
       20  numerous second gas introduction holes 
       21  heating mechanism 
       22  numerous third gas exhaust holes 
       23  numerous third gas introduction holes 
       41  plasma CVD device 
       61  plasma CVD device 
       62  vacuum vessel 
       62   a  exhaust port 
       63  discharge electrode plate 
       63   a  hollow portion 
       65  raw gas supply pipe 
       66  shower plate 
       66   a  numerous gas introduction holes
A plasma CVD device comprises a vacuum vessel that houses a discharge electrode plate and a ground electrode plate to which is attached a substrate for thin film formation. The plasma CVD device has an earth cover at an interval from and facing the aforementioned discharge electrode plate; the aforementioned discharge electrode plate has gas inlets and exhaust outlets (which expel gas introduced through said gas inlets) that are connected at one end to equipment supplying raw gas for thin film formation and that open at the other end at the bottom face of the aforementioned discharge electrode plate; the aforementioned earth cover has second gas inlets corresponding to the aforementioned gas inlets, and second exhaust outlets corresponding to the aforementioned exhaust outlets. The plasma CVD device has an electric potential control plate disposed at an interval from and facing the aforementioned ground cover.