fig1 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 fig3 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 fig2 . 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 fig3 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 ) 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 fig4 ) 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 . fig5 shows an example of a second embodiment of the plasma cvd device of the invention . the plasma cvd device 41 shown in fig5 is identical to the plasma cvd device 1 shown in fig1 , 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 fig5 other than the components relating to the potential control plate 9 are the same as those forming the plasma cvd device 1 shown in fig1 . therefore , in the plasma cvd device 41 shown in fig5 , the same components as those of the plasma cvd device 1 shown in fig1 are given the same symbols as those used in fig1 . fig6 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 fig5 in the plasma cvd device 41 shown in fig5 , 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 . 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 fig1 . 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 . 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 . 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 fig5 . 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 . 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 fig7 . 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 . 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 .