various preferred embodiments of the present invention will be described below with reference to the accompanying drawings . in the first embodiment , a high - frequency plasma cvd apparatus 1 a shown in fig8 is used . this apparatus 1 a includes a reaction vessel 2 , a ladder electrode ( a discharge electrode ) 303 , a ground electrode 3 , two high - frequency power supplies 5 a and 5 b , a gas supply unit 71 , an exhaust unit 72 , and a controller 73 . the reaction vessel 2 is made airtight , and a gas supply pipe 17 and an exhaust pipe 18 open in appropriate portions of this vessel 2 . the gas supply pipe 17 communicates with the gas supply unit 71 . the gas supply unit 71 incorporates a gas supply source and a mass - flow controller . the gas supply source contains process gases which contain film formation components . the process gases are , e . g ., an si - containing gas such as monosilane or disilane and an additive component gas such as hydrogen gas . the power switch of the mass - flow controller is connected to an output portion of the controller 73 . the exhaust pipe 18 communicates with the exhaust unit 72 . the exhaust unit 72 houses a vacuum pump and a device for making toxic substances harmless . the power switch of the vacuum pump is connected to the output portion of the controller 73 . this vacuum pump can reduce the internal pressure of the reaction vessel 2 to less than about 1 × 10 − 6 torr . the ground electrode 3 and the ladder electrode 303 are opposed with a clearance of , e . g ., 20 mm between them . the ground electrode 3 includes a mechanism ( not shown ) for holding a glass substrate g to be processed and contains a heater ( not shown ) for heating this substrate g . when the dimensions of the substrate g are 2 . 0 m × 1 . 4 m , the dimensions of the ground electrode 3 are 2 . 2 m × 1 . 6 m . the gas supply pipe 17 preferably has a plurality of gas inlet ports . these gas inlet ports are desirably formed behind the ladder electrode 303 so as to form laminar flows of the process gases . the ladder electrode 303 is formed by assembling a plurality of ( e . g ., 17 ) parallel longitudinal rods 304 and one or more pairs of parallel lateral rods 305 into the form of a lattice . this ladder electrode 303 is opposed parallel to the substrate g which is held by the ground electrode 3 . the longitudinal rods 304 are arrayed at equal pitches in the y - axis direction , and the lateral rods 305 are arrayed at equal pitches in the x - axis direction . the length in the x - axis direction of the ladder electrode 303 is about 2 . 2 m , and its length in the y - axis direction is about 1 . 6 m . the ladder electrode 303 has eight first feeding points 9 a and eight second feeding points 9 b 1 . the first feeding points 9 a are formed at nodes between the lateral rods 305 and the longitudinal rods 304 on one side . the second feeding points 9 b 1 are formed at nodes between the lateral rods 305 and the longitudinal rods 304 on the other side . these first and second feeding points 9 a and 9 b 1 are formed at substantially equal pitches . the first feeding points 9 a are connected to a first distributor 26 a by coaxial cables 8 b . to this first distributor 26 a , an impedance matching circuit 7 a , a power meter 6 a , and the first power supply 5 a are connected in this order by a coaxial cable 8 a . the first power supply 5 a contains a high - frequency oscillator whose frequency is set to 60 mhz , and supplies vhf power to the ladder electrode 303 via the eight first feeding points 9 a . the second feeding points 9 b 1 are connected to a second distributor 26 b via coaxial cables 8 b . to this second distributor 26 b , an impedance matching circuit 7 b , a power meter 6 b , and the second power supply 5 b are connected in this order . the second power supply 5 b is independent of the first power supply 5 a and houses a high - frequency oscillator whose frequency is also set to 60 mhz . this second power supply 5 b supplies vhf power to the ladder electrode 303 via the eight second feeding points 9 b 1 . when the actual frequencies of these two power supplies were measured with a high - accuracy spectrum analyzer , the power supplies 5 a and 5 b output high frequencies of 59 . 8 mhz and 60 . 3 mhz , respectively . the actual oscillation frequency usually thus deviates from the set frequency , and this is because different oscillators have different characteristics . accordingly , different vhfs of 59 . 8 mhz and 60 . 3 hz are supplied to the ladder electrode 303 from the two power supplies 5 a and 5 b via the first and second feeding points 9 a and 9 b 1 facing each other . in the present invention , the difference ( 0 . 5 mhz ) between the two vhfs is important . in this embodiment , the first and second feeding points 9 a and 9 b 1 were arranged symmetrically with respect to the central line of the ladder 303 , thereby generating a one - dimensional voltage distribution on each of the longitudinal rod 304 . this caused a phenomenon ( to be described later ) which moved an envelope at high speed . consequently , it was possible to uniformize the longitudinal plasma generation distribution on the longitudinal rods 304 and the plasma generation distribution ( a plasma distribution in the x - axis direction ) between the longitudinal rods 304 . in this embodiment , the number of each set of feeding points is eight ( sixteen in total ). however , the plasma generation distribution in the x - axis direction ( the longitudinal direction of the electrode ) can be further uniformized by increasing the number of feeding points to twelve ( twenty four in total ) or to sixteen ( thirty two in total ). the effects of the present invention are of course found even when the number of feeding points is decreased to six ( twelve in total ), five ( ten in total ), four ( eight in total ), three ( six in total ), two ( four in total ), and one ( two in total ). also , in this embodiment power components are fed to the different feeding points from the two power supplies . however , the discharge plasma density can be made more uniform by increasing the number of power supplies . for example , the uniformity of the discharge plasma density can be improved by supplying high - frequency power components having different frequencies from four power supplies to different feeding points . fig9 is a graph showing a characteristic curve e obtained by measuring , with a ccd camera , the emission intensity of a plasma generated by using the apparatus 1 a of this embodiment . in fig9 a position index ( an arbitrary value ) on the ladder electrode is plotted on the abscissa , and an emission intensity index ( an arbitrary value ) is plotted on the ordinate . referring to fig9 three low - value portions inside the substrate size are behind structures of the apparatus , so the plasma in these portions cannot be seen . therefore , these portions have no connection with the actual emission intensity distribution . this characteristic curve e shows that in the generation of a discharge plasma using the apparatus 1 a , a uniformity of ± 7 % ( a maximum value of 127 / a minimum value of 111 ) of an emission distribution , i . e ., a plasma distribution can be achieved in a very large region of 2 m × 1 . 4 m . the discharge plasma is thus uniformized because “ hum ( beat )” resulting from 0 . 5 mhz as the difference between the first vhf ( 59 . 8 mhz ) and the second vhf ( 60 . 3 mhz ) suppresses the generation of a standing wave a 3 on the electrode . that is , as explained in item ( a ) in “ brief summary of the invention ”, an envelope is changed 0 . 5 million times for 1 sec , i . e ., moved at high speed over the distance of 500 , 000 wavelengths . therefore , although the envelope has a waveform similar to the standing wave a 3 for a very short time period , the amplitude of a voltage can be regarded as having a uniform distribution on time average . so , a plasma generated by this voltage has a substantially uniform distribution . the amplitude of a voltage is the maximum value of the voltage at a certain point . this can also be considered as follows . two high - frequency waves supplied to the electrode , i . e ., a high - frequency wave ( to be referred to as a high - frequency wave a hereinafter ) supplied from the power supply 5 a via the eight first feeding points 9 a and a high - frequency wave ( to be referred to as a high - frequency wave b hereinafter ) supplied from the power supply 5 b via the eight second feeding points 9 b 1 do not interfere with each other because they have different frequencies . therefore , the amplitude distribution of the voltage of the high - frequency wave a and the amplitude distribution of the voltage of the high - frequency wave b are independent of each other . the actual voltage distribution is the superposition of the two . assuming that the attenuation of a high - frequency wave can be neglected and the reflection at the electrode end is negligibly small , the high - frequency waves a and b uniformly progress with no attenuation from the two ends of the longitudinal rods 304 , so the superposition of the two high - frequency waves forms a uniform distribution . this is the gist of this embodiment . in practice , however , the phenomenon is slightly complicated because attenuation and reflection do occur , so these attenuation and reflection will be described below . first , attenuation will be explained . when attenuation occurs , the amplitude distribution of the voltage of the high - frequency wave a is indicated by r 1 shown in fig1 a . referring to fig1 a , the direction of the lateral rods 305 is the y axis , the direction of the longitudinal rods 304 is the x axis , and the amplitude of the voltage is the z axis . in the direction ( the y axis ) of the lateral rods 305 , the voltage amplitude is substantially constant because the eight feeding points 9 a are closely arranged . on the other hand , in the direction ( the x axis ) of the longitudinal rods 304 , the voltage amplitude attenuates from the feeding side , and this produces a distribution . as indicated by r 2 shown in fig1 b , the amplitude of the voltage of the high - frequency wave b also attenuates from the opposite feeding side . as indicated by r 3 in fig1 c , the distribution of the amplitude of the voltage formed by synthesizing the high - frequency waves a and b has high uniformity . accordingly , a plasma having a substantially uniform distribution density is generated . since reflection at the electrode end cannot be neglected in practice , this reflection at the electrode end will also be explained in addition to the above attenuation . referring to fig1 a , the distance ( cm ) from the feeding points in the direction of the longitudinal rods 304 is plotted on the abscissa , and the amplitude distribution of the voltage of the high - frequency wave a is plotted on the ordinate . fig1 a does not show the distribution in the direction of the lateral rods 305 because this distribution is substantially uniform as indicated by r 1 in fig1 a , and shows only one - dimensional distribution in the y - axis direction of the longitudinal rods 304 . a progressive wave g 1 of the high - frequency wave a progresses toward the electrode end while attenuating from the feeding points . since reflection occurs at the electrode end , the high - frequency wave a is reflected at the electrode end to form a reflected wave h , and this reflected wave h progresses to the feeding points . fig1 b shows the amplitude distribution of a voltage generated by the synthesis of the progressive wave g 1 and the reflected wave h . since the progressive wave g 1 and the reflected wave h have the same frequency and interfere with each other , addition taking account of the phase is necessary . a synthetic wave g 2 does not monotonously attenuate and has a standing - wave - like distribution . fig1 b shows that a minimal point is generated near 120 cm . the high - frequency wave b also forms a similar distribution . since the high - frequency waves a and b have different frequencies and hence do not interfere with each other , they can be simply synthesized by addition , and a substantially uniform distribution as shown in fig1 c is obtained . in this embodiment , a reflected wave h of a synthetic wave g 3 was small , resulting in a slight standing - wave - like distribution . therefore , a substantially uniform voltage distribution was finally obtained , so a uniform plasma was generated . as described above , in this embodiment a substantially uniform distribution was obtained because the reflected wave h was small and had little influence . however , if a high - frequency wave ( e . g ., the high - frequency wave a ) fed from one end does not attenuate before reaching the other end , a reflected wave generated by the reflection of this high - frequency wave interferes with the progressive wave . since this makes the standing wave appreciably large , no uniformity can be obtained any longer . hence , a high - frequency wave preferably sufficiently attenuates before arriving at the other end . more specifically , when at the other end of the electrode the voltage of a progressive wave attenuates to about 50 % of that at the feeding points , the reflected wave h generated there becomes about 25 % (= 50 %× ½ ) as shown in fig1 a , so a substantially uniform voltage distribution is obtained . if the voltage of a progressive wave attenuates to about 75 %, the film thickness distribution is not substantially affected although a slight voltage distribution is formed . referring to fig1 , the abscissa indicates the vhf frequency ( mhz ), and the ordinate indicates the distance ( cm ) from the feeding points . a characteristic curve j shows the distance the progressive - wave voltage attenuates to about 50 %, which is approximated by equation l = 4 × 10 3 / f . a characteristic curve k shows the distance the progressive - wave voltage attenuates to about 75 %, which is approximated by equation l = 2 × 10 3 / f . accordingly , the length l of the electrode from one end as a feeder distribution center ( incident point ) to the other end as a reflection point satisfies preferably inequality l ≧ 2 × 10 3 / f , and more preferably , l ≧ 4 × 10 3 / f . within the range of equation l & gt ; 20 × 10 3 / f , attenuation is too large , and no uniformization is possible . therefore , l ≦ 20 × 10 3 / f is preferably met . the frequency difference between the high - frequency waves a and b will be described below by returning to the concept of high - speed movement of an envelope . the larger this frequency difference , the higher the speed at which an envelope is moved . however , to obtain a desired film formation rate and film formation quality , it is undesirable to use a high - frequency wave b with a frequency having a difference of 20 % or more from that of the high - frequency wave a . also , to allow an impedance matching circuit to function which is used to prevent incidence of vhf power to a power supply , the difference between the two vhfs is more preferably 1 % or less . in this embodiment , the difference between the two vhfs was about 0 . 8 %. therefore , the film formation rate and quality were high , and incidence of a high - frequency wave to the power supply could be suppressed to a low value , about 100 w . the above apparatus 1 a was used to form a thin si semiconductor film ( an a - si film or a nanocrystalline si film ) for a solar cell on a glass substrate following the procedures described below . first , the temperature of the ground electrode 3 was set to 200 ° c . by heating by the heater . 2 m × 1 . 4 m substrate g was placed on this ground electrode 3 . sih 4 gas was supplied at a flow rate of 2 , 000 sccm from the gas supply unit 71 into the reaction vessel 2 . in the case of nanocrystalline si film formation , hydrogen gas was supplied at , e . g ., 50 , 000 sccm in addition to the sih 4 gas . the operation of the exhaust unit 72 was controlled by the controller 3 to adjust the internal pressure of the reaction vessel 2 to 200 mtorr . while the first and second impedance matching circuits 7 a and 7 b were so adjusted as to allow efficient supply of vhf power to a plasma , vhf power at a frequency of 59 . 8 mhz was supplied from the first power supply 5 a , and vhf power at a frequency of 60 . 3 mhz was supplied from the second power supply 5 b . these vhf power components were supplied to the ladder electrode 303 such that the total power from the two power supplies 5 a and 5 b was , e . g ., 3 , 000 w , thereby generating a discharge plasma between the substrate g and the ladder electrode 303 . in this discharge plasma , sih 4 decomposed to form an a - si film or a nanocrystalline si film on the surface of the substrate g . that is , a thin film having a desired film thickness was formed by performing film formation in this state for , e . g ., about 10 min . the film thickness distribution of the formed film sample was measured , and the feeder distribution center positions were finely adjusted so that an optimum distribution was obtained . the film formation rate was as high as 1 . 0 nm / sec in the formation of a nanocrystalline film , and the uniformity was slightly over ± 10 %. that is , the uniformity necessary for an si thin - film semiconductor of a solar cell was achieved . in this arrangement , different vhfs are unintentionally supplied by using different power supplies . however , intentionally setting different frequencies is of course similarly effective . fig1 is a graph showing the correlation between the frequency difference ( mhz ) between two high - frequency waves and the film thickness distribution (%) of a film formed on a substrate , by plotting the frequency difference on the abscissa and the film thickness distribution on the ordinate . while the frequency of the high - frequency wave a was fixed to 60 . 0 mhz ( a fundamental frequency ), the frequency of the high - frequency wave b was changed to form a - si films on 2 . 0 m × 1 . 4 m rectangular glass substrates . as is apparent from a characteristics curve f , the film thickness distribution was 10 % or less when the frequency difference between the two high - frequency waves a and b was 1 . 5 to 6 . 0 mhz . also , the film thickness distribution was lower than 15 % when the frequency difference was 1 . 0 to 10 . 0 mhz and lower than 20 % when the frequency difference was 0 . 5 to 10 . 0 mhz . furthermore , even when the frequency difference was only 0 . 1 mhz , the film thickness distribution exceeded 20 % only slightly . furthermore , when the film thickness of a thin silicon film formed on a large - area substrate g ( 2 m × 1 . 4 m ) was measured by raman spectroscopy , the peak ratio of a raman spectrum exceeded 9 : 1 . this proves that the film had high quality as a thin film for a solar cell . also , the refractive index , spectral characteristics , and defect density of the thin film were measured . consequently , the measured values were substantially equal to values measured when a film was formed on a small - area sample substrate ( 5 cm × 5 cm ) by using the same vhf ( 60 mhz ). a high - quality thin film is formed on the large - area substrate g as described above because plasma on / off caused by the high - speed movement of an envelope is much shorter than the life of active species ( radicals ). that is , in this embodiment the difference between the two vhfs is 0 . 5 mhz . therefore , on / off is repeated 500 , 000 × 2 times per second , and one off time is 1 × 10 − 6 sec . this off time is much shorter than the extinction life (( τ =( 2 ( cm )) 2 /( 2 × 2 . 5 × 10 3 ( cm 2 / sec ))= 8 × 10 − 4 sec ) of an sih 3 active molecule , and is much shorter than the extinction life ( 1 . 1 × 10 − 4 sec ) of a hydrogen atom radical . accordingly , on / off of the plasma is substantially negligible in the actual film formation . note that another effect obtained by this embodiment is the very small amount of particles produced during the film formation . this is presumably because , as described in “ silane gas decomposition by high - frequency modulation discharge ”, discharge research no . 138 , pp . 27 to 36 , 1992 ( to be referred to as reference 7 hereinafter ), substantially no particles are produced when the on / off frequency is in the range of 0 . 5 hz ( inclusive ) to 100 khz ( inclusive ), i . e ., the on time is in the range of 0 . 01 msec ( inclusive ) to 1 sec ( inclusive ), and preferably when the on / off frequency is 10 khz or less , i . e ., the on time is 0 . 1 msec or more . that is , since the growth time of particles is about 1 sec , no particles increase for an on time of 1 sec or less . however , when the on / off frequency is 100 khz or more , i . e ., when the off time is 0 . 01 msec or less , a state in which plasma is not regarded to be off is set . that is , in this embodiment almost no particles were produced when the on / off frequency was set to 10 khz , i . e ., when the on time was set to 0 . 1 msec . furthermore , it is also possible to prevent an increase in particles by discharging particles over a sufficient time by prolonging the off time . that is , when a substrate area s is 200 × 200 cm 2 , a distance δx from the discharge electrode to the substrate is 2 cm , and a volume flow rate q is 4 × 10 5 cm 3 / sec , a discharge region dwell time t of a source gas is calculated to be 0 . 2 sec by substituting these values into equation ( 10 ) below . therefore , by setting the off time to be longer than the time t , i . e ., 0 . 2 sec or more , preferably twice the time t , i . e ., 0 . 4 sec or more , it was possible to discharge particles from the plasma generation region and suppress an increase in particles in the reaction vessel . in this embodiment , a ladder electrode was used as the electrode . when a mesh electrode reported in the reference 4 cited previously was used instead of the ladder electrode , a film thickness uniformity of 10 % or less could be obtained . this electrode is a kind of a ladder electrode and has a large number of lateral electrode rods 305 according to this embodiment . also , this embodiment has been explained by taking the use of vhfs around 60 mhz as an example . however , analogous effects were obtained even when vhfs around 20 , 200 , and 700 mhz were used . the second embodiment of the present invention will be described below with reference to fig1 . an apparatus 1 b of this embodiment is obtained by partially changing the vhf feeder circuit of the apparatus 1 a of the first embodiment described above . this change in the feeder circuit permits the apparatus 1 b to have an operation condition range wider than that of the apparatus 1 a . also in this embodiment , the apparatus is used to form a uniform film on a 2 m × 1 . 4 m substrate by using vhfs , and the arrangements of a reaction vessel and the like except for a power - supply system are the same as in the first embodiment . so , an explanation of points common to the two embodiments will be omitted . the apparatus 1 b of the second embodiment differs from the apparatus 1 a of the first embodiment in four points { circle around ( 1 )} to { circle around ( 4 )} below . { circle around ( 1 )} the oscillation frequencies of high - frequency power supplies . in the apparatus 1 a , two different frequencies are generated by using the uncertainty of the built - in quartz oscillators of the high - frequency power supplies 5 a and 5 b . in the apparatus 1 b , a two - wave signal generator 20 b controls the frequency difference to a constant value . in the former apparatus ( the apparatus 1 a ), no arbitrary frequency difference can be selected . accordingly , if the two oscillators have a frequency difference of only 10 hz , the envelope distribution moves only 10 hz , so a plasma turns on / off at this cycle to adversely affect film formation . also , the oscillation frequency difference does not stabilize with time . this may result in low reproducibility . in contrast , the latter apparatus ( the apparatus 1 b ) can be operated with a fixed optimum frequency difference . { circle around ( 2 )} in the apparatus 1 a , protection circuits ( not shown ) of the two high - frequency power supplies 5 a and 5 b are also independent of each other . however , the apparatus 1 b has only one protection circuit 22 . input power components to power supplies 5 a and 5 b are measured by power meters 6 a 1 and 6 b 2 and , if one of the two magnitudes exceeds a limiting value , the outputs from both the power supplies are restricted . in the former apparatus ( the apparatus 1 a ), if the input power ( the sum of the reflected power and the input power from the second power supply 5 b ) to the first power supply 5 a exceeds the tolerance of the first power supply 5 a for some reason , the protection circuit of this first power supply suppresses the output from the first power supply 5 a . however , the excess input from the second power supply 5 b is not suppressed at all and kept supplied . hence , in the worst case the first power supply 5 a is damaged . in contrast , if the same even occurs in the latter apparatus ( the apparatus 1 b ), the excess input to the first power supply 5 a activates the protection circuit 22 to suppress the outputs from both the first and second power supplies 5 a and 5 b . since the input power to the first power supply 5 a is suppressed , this first power supply 5 a is not damaged . while the input power is thus suppressed , an impedance matching circuit 7 b is adjusted to remove the cause of the excess input from the second power supply 5 b . this makes it possible to again increase the outputs from the two power supplies 5 a and 5 b and supply desired power . in the apparatus 1 b , isolators 24 a and 24 b are inserted into first and second feeder circuits to prevent excess input power components to the power supplies 5 a and 5 b , respectively . hence , no protection circuit is particularly necessary . however , if the reflected power exceeds the allowable power of the isolators 24 a and 24 b to make these isolators 24 a and 24 b inoperable , the protecting operation by this protection circuit 22 is required . { circle around ( 3 )} in the apparatus 1 a , the impedance matching circuit 7 a is the only means for suppressing the input power ( the sum of the reflected power and the input power from the second power supply 5 b ) to the first power supply 5 a . in the apparatus 1 b , however , the isolators 24 a and 24 b each comprising a circulator and load are inserted to eliminate the input power from the electrode 303 to the power supplies 5 a and 5 b . in the former apparatus ( the apparatus 1 a ), even when the power generated by reflecting the output from the first power supply 5 a by the electrode 303 can be completely reduced to zero by the impedance matching circuit 7 a , the input power from the other power supply , i . e ., the second power supply 5 b via the electrode 303 and the impedance matching circuit 7 a cannot be simultaneously reduced to zero , because the phases and frequencies are different . therefore , if this power is large ( the power transmitted through the electrode 303 varies , i . e ., increases or decreases , in accordance with the generated state of a plasma ), large power is input to the first power supply 5 a to make the state of this first power supply 5 a unstable . in the worst case , an excess input may destroy the first power supply 5 a . this situation readily occurs especially when there is no plasma load before plasma generation . in contrast , in the latter apparatus ( the apparatus 1 b ) the isolators 24 a and 24 b are inserted into the feeder circuits . accordingly , inputs to the power supplies 5 a and 5 b can be completely absorbed by the loads , and this prevents destruction of the power supplies 5 a and 5 b by an excess input . the frequency band width of an isolator , especially the isolators 24 a and 24 b having kw - class high - frequency power as a rated value as used in this embodiment , is very narrow . that is , the frequency band width at a high - frequency power of 1 kw or less is about 4 % of the frequency used , and that at a high - frequency power of about 2 kw is about 1 % of the frequency used . hence , the frequency difference between the first and second power supplies 5 a and 5 b must be decreased to these values . in this embodiment , the oscillation frequency of the first power supply 5 a is set at 60 . 2 mhz , and the oscillation frequency of the second power supply 5 b is set at 59 . 8 mhz . consequently , the oscillation frequency difference between the two power supplies 5 a and 5 b is set to 0 . 6 mhz or less which corresponds to a frequency band width of 1 % at a rated power of 2 kw . { circle around ( 4 )} in the apparatus 1 a , each system includes one of the two power meters 6 a and 6 b ( two power meters in total ). in the apparatus 1 b , one system includes the two power meters 6 a 1 and 6 a 2 , and another system also includes two power meters 6 b 1 and 6 b 2 ( four power meters in total ). since the isolators 24 a and 24 b are inserted into the feeder circuits , input power components to the power supplies 5 a and 5 b , i . e ., the reflected power components in the power meters 6 a 1 and 6 a 2 are normally zero regardless of the impedance matching state . to optimize the impedance matching state , therefore , it is necessary to arrange the power meters 6 a 2 and 6 b 2 closer to the impedance matching circuits than the isolators 24 a and 24 b and measure the return power components from the impedance matching circuits 7 a and 7 b . although the above improvements complicated the apparatus , a total input power of 4 kw was obtained . hence , a high film formation rate of 1 . 5 nm / sec was obtained in , e . g ., nanocrystalline film formation . the film thickness uniformity was ± 10 %. this clears the film thickness uniformity requirement necessary for an si thin - film semiconductor of a solar cell . the third embodiment of the present invention will be described below with reference to fig1 to 20 . as shown in fig1 , a plasma cvd apparatus 1 c comprises a vacuum chamber 2 c for surrounding a substrate g , a ladder electrode 303 opposed to the substrate g in this vacuum chamber 2 c , and a feeder circuit 8 a for feeding power to this ladder electrode 303 . the vacuum chamber 2 c communicates with a vacuum pump via an exhaust passage ( not shown ) and is evacuated to about an internal pressure of 1 × 10 − 3 to 1 × 10 − 7 torr . also , a gas supply pipe ( not shown ) is placed behind the electrode 303 in the vacuum chamber 2 c . when silane or disilane is supplied as a film formation process gas from a gas supply source to the gas supply pipe , this process gas is supplied from a large number of inlet ports to a space between the electrode 303 and the substrate g . the substrate g is held by a ground electrode 3 and heated to a predetermined temperature region by a built - in heater ( not shown ). as this substrate g , a transparent glass substrate 1 mm thick , 460 mm wide , and 460 mm long was used . the ladder electrode 303 as a discharge electrode is formed by assembling round rod electrode members at equal pitches into the form of a lattice . this ladder electrode 303 is connected to a feeder ( central conductor ) 6 of the circuit 8 a by four feeding points 9 a and 9 b . the two feeding points 9 a are formed in positions which divide the round rod electrode member placed at one end into three substantially equal parts . the other two feeding points 9 b are formed in positions which divide the round rod electrode member placed at the other end into three substantially equal parts . the ladder electrode 303 has dimensions of , e . g ., 520 mm × 520 mm , and the round rod electrode member has a diameter of , e . g ., 6 mm . the feeder circuit 8 a includes a high - frequency oscillator 20 , a distributor 26 , a phase shifter 42 , a pair of amplifiers 41 a and 41 b , and a pair of impedance matching circuits 7 a and 7 b . the high - frequency oscillator 20 is connected to the amplifiers 41 a and 41 b via the distributor 26 . the amplifiers 41 a and 41 b are connected to the impedance matching circuits 7 a and 7 b which are connected to the feeding points 9 a and 9 b , respectively . an output circuit of the distributor 26 branches into two parts , and one of these branched circuits 8 a includes the phase shifter 42 driven and controlled by a computer 43 . each branched circuit 8 a further branches into two circuits 8 a and 8 b . these branched circuits 8 a and 8 b are connected to the electrode 303 via the feeding points 9 a and 9 b , respectively . a quartz oscillator having a rated frequency of 60 mhz was used as the high - frequency oscillator 20 . a full solid - state element type analog voltage - controlled phase shifter was used as the phase shifter 42 . the high - frequency oscillator 20 oscillates a very high frequency ( vhf ), and this vhf is distributed by the distributor 26 and fed to the electrode 303 via the amplifiers 41 a and 41 b , the impedance matching circuits 7 a and 7 b , and the feeding points 9 a and 9 b . on the basis of a predetermined processing data signal transmitted from the computer 43 , the phase shifter 42 performs high - speed phase modulation for one distributed vhf at a repetition rate of a maximum of 100 khz . as shown in fig1 and 17 , the feeder ( central conductor ) 6 is passed through a coaxial cable 61 and connected to the electrode 303 by the feeding points 9 a and 9 b . an insulator 63 is mounted on the end portion of the coaxial cable 61 by a coupling 62 , thereby insulating the electrode 303 from the sheath of the cable 61 . a plurality of holes 64 are formed in side portions of the insulator 63 . a gas passage 66 in the coaxial cable 61 communicates with a gas supply source 68 and the holes 64 of the insulator . the gas supply source 68 contains industrially pure hydrogen gas h 2 . this hydrogen gas is supplied from the gas supply source 68 , passed through the gas passage 66 in the cable 61 , and blown off from the side holes 64 and an upper - end hole 65 , thereby blowing away a silane plasma existing at the feeding points 9 a and 9 b and its vicinity . simultaneously , a plasma having a high hydrogen concentration etches unnecessary film formation components deposited on the insulator 63 and the like near the feeder distribution center . the blow of this hydrogen gas effectively prevents the deposition of unnecessary film formation components and the generation of particles near the feeding points 9 a and 9 b as an electric field concentrated portion . in the above embodiment , the deposition of film formation components and the generation of particles are prevented by blowing pure hydrogen gas against each feeder distribution center . however , the present invention is not limited to this embodiment . that is , a similar effect can be obtained by blowing another gas such as argon gas against each feeder distribution center . furthermore , the method of the present invention is applicable not only to feeding points but also to parts where the deposition of unnecessary film formation components and the generation of particles are undesirable . for example , if a silicon film is deposited on an insulator pillar for supporting an electrode , the creeping insulation strength of the pillar lowers . in the worst case , a line - to - ground fault occurs . when a hydrogen local supply method analogous to the one described above was performed for this pillar , deposition was suppressed and no line - to - ground fault occurred . the formation of a nanocrystalline si film on a glass substrate using the plasma cvd film forming apparatus will be described below . the film formation conditions were substrate dimensions of 46 cm × 46 cm , a substrate temperature of 200 ° c ., a reaction vessel internal pressure of 150 mtorr , a silane gas supply amount of 50 sccm , and a hydrogen gas supply amount of 1 , 500 sccm . when the frequency of high - frequency power was set at 60 mhz and the distance between electrode and substrate was set at 10 mm in this apparatus , a standing wave was generated by the conventional feeding method , and no uniform film formation was possible . fig1 and 19 are three - dimensional graphs showing film formation rate distributions when the phase difference is set to constant values of 0 ° and 90 °, respectively , by a phase modulator . referring to fig1 and 19 , the positions on a substrate with respect to the discharge electrode are plotted on the x and y axes , and the deposition rate ( nm / sec ) is plotted on the ordinate . as shown in fig1 and 19 , with the fixed constant phase difference an envelope remains still as a standing wave to produce a deposition rate distribution . fig1 and 19 also reveal that the deposition rate distribution can be changed by changing the phase difference . this is principally because the change in the phase difference changes the envelope distribution ( varies the nodes and antinodes of the standing wave distribution ). in this embodiment , therefore , high - frequency power components are supplied from two feeding systems , and the phase difference between them is changed with time , thereby moving the envelope and obtaining a uniform film thickness distribution by time quadrature . this is equivalent to case ( b ) ( the method of supplying high - frequency waves having the same frequency while changing the phase difference with time ) in “ brief summary of the invention ”. if the above phase difference is slowly changed , active molecules or atoms in the plasma increase or decrease accordingly . therefore , the arrival flux of these molecules or atoms with respect to the substrate varies with time , and this raises or lowers the deposition rate . in the worst case , a state equivalent to plasma off by which no film is formed appears . additionally , the quality of deposited films temporally changes , and this change sometimes influences the quality of the film finally formed . this is a problem in a nanocrystalline type silicon thin - film solar cell in which delicate film quality is important . this makes the formation of a uniform , high - quality film on a large - area substrate difficult . also , if a mechanical phase modulator is used instead of a phase shifter , frequent maintenance is necessary because sliding portions wear . additionally , high - speed phase modulation is impossible . in the method of the present invention , however , the computer 43 rapidly and accurately controls the phase shifter 42 , and the high - speed phase modulation by this phase shifter 42 shifts the phase of high - frequency power supplied to one end of the electrode 303 from the phase of high - frequency power supplied to the other end . hence , in a certain instant an envelope distribution is generated on the electrode 303 to produce local variations in the voltage amplitude distribution . in the whole processing time , however , the voltage amplitude distribution on the electrode 303 is averaged , and the density of a plasma generated between the electrode 303 and the substrate g is uniformized . when this phase modulation circuit 8 a is used to rapidly modulate a high frequency at a maximum of 100 khz , the envelope generated on the electrode moves at the same frequency . this movement is much shorter than the life of sih 3 active molecules or h active atoms dominating film deposition . therefore , the flux of these active molecules or atoms arriving at the substrate hardly changes with time . this allows the deposition of a film having a uniform film thickness and uniform film quality . furthermore , deposition rate distributions with several phase differences were previously measured as shown in fig1 and 19 and input to the computer 43 . on the basis of this data , deposition time ratios corresponding to the individual phase differences necessary to achieve a uniform distribution were calculated to form phase difference change patterns m 1 , m 2 , and m 3 as shown in fig2 . when the phase shifter 42 was controlled by using these phase difference change patterns m 1 , m 2 , and m 3 , film formation was possible by which both the film thickness distribution and the film quality fell within the ranges of ± 5 % of their respective target values . when the apparatus was operated by setting the maximum value of high - frequency power to about 800 w which is limited by the rating of the maximum power of the high - frequency amplifier , an average deposition rate of about 2 nm / sec was obtained . also , as described above , the film thickness distribution of the deposited films cleared ± 5 % or less necessary for solar cells and thin - film tfts . the substrate dimensions used in this embodiment were 46 cm × 46 cm . however , even when substrates having the same dimensions of 2 . 0 × 1 . 4 m as in the first embodiment were used , a uniformity of 10 % or less could be obtained by a similar method . the fourth embodiment of the present invention will be described below with reference to fig2 . an apparatus 1 d of this embodiment is obtained by partially changing the vhf feeder circuit of the apparatus 1 b of the second embodiment described earlier . a feeder circuit of this apparatus 1 d includes two independent high - frequency power supplies 5 a and 5 b , an oscillator 20 d , phase detectors 30 a and 30 b , a phase - shifting device 33 , and a function generator 34 . the two power supplies 5 a and 5 b incorporate amplifiers and independently feed very - high - frequency ( vhf ) power components having the same frequency of 60 mhz . the phase - shifting device 33 is inserted between the oscillator 20 d and the second power supply 5 b and shifts the phase of the high frequency fed from the second power supply 5 b . consequently , the high frequency fed from the second power supply 5 b to an electrode 303 is so controlled as to have a certain phase difference from the high frequency fed from the first power supply 5 a to the electrode 303 . the function generator 34 transmits an arbitrary waveform signal to the phase - shifting device 33 and controls a temporal change in the phase difference . when the oscillator 20 d oscillates a vhf wave having a frequency of 60 mhz , one system ( a signal s 1 ) of the oscillation signal is directly amplified by the first power supply 5 a and input to the first phase detector 30 a via a power meter 6 a 1 , an isolator 24 a , a power meter 6 a 2 , and an impedance matching circuit 7 a . the phase detector 30 a detects the phase of the signal s 1 , and this signal s 1 is supplied to the electrode 303 via a distributor 26 a . the phase of the other system ( a signal s 2 ) of the oscillation signal is shifted by the phase - shifting device 33 . after that , the signal s 2 is amplified by the second power supply 5 b and input to the second phase detector 30 b via a power meter 6 b 1 , an isolator 24 b , a power meter 6 b 2 , and an impedance matching circuit 7 b . the second phase detector 30 b detects the phase of the signal s 2 , and this signal s 2 is supplied to the electrode 303 via a distributor 26 b . the phase - shifting device 33 performs feedback control such that the difference between the phase of the signal s 1 detected by the first phase detector and the phase of the signal s 2 detected by the second phase detector temporally changes in accordance with the phase difference control signal generated by the function generator 34 . that is , a temporal change in the phase difference is controlled by the phase - shifting device 33 in accordance with the arbitrary waveform signal generated by the function generator 34 . the phase detectors 30 a and 30 b detect the phase difference between the signals s 1 and s 2 immediately before the distributors 26 a and 26 b , respectively . this phase detection signal is supplied to the phase - shifting device 33 to control the phase - shifting operation by feedback . in this embodiment , if the apparatus is operated with a fixed constant phase difference , an envelope spatially stops to generate a standing wave , and this makes a plasma nonuniform . in contrast , by moving the envelope by changing the phase difference with time , it is possible to suppress a standing wave and obtain a uniform plasma and a uniform film thickness distribution by time average within the film formation time . note that the isolators 24 a and 24 b and a protection circuit 22 contribute to stabilization of the power supplies 5 a and 5 b when they are operated , as in the apparatus 1 b described previously . if the phase difference is modulated too rapidly , the frequency band of a vhf widens to exceed the frequency band width of the isolators 24 a and 24 b . this may damage these isolators 24 a and 24 b . therefore , a spectrum analyzer ( not shown ) was connected to the phase detectors 30 a and 30 b to determine the modulation rate such that the band width was 1 % or less of the rated frequency . in this embodiment , the band width did not exceed 1 % when modulation was performed by setting the frequency of the phase control signal from the function generator 34 to 10 khz . in the present embodiment , the uniformity of deposition rate was within ± 8 % when the function generator adjusted the modulation width was adjusted to ± 110 % while the modulation frequency of the phase shifting device was set at 10 khz . the fifth embodiment will be described below with reference to fig2 . in this embodiment , a description of the same portions as in the above embodiment will be omitted to avoid duplication . a high - frequency power supply 5 of a plasma cvd film forming apparatus 1 e is connected to a feeder circuit 8 b via an impedance matching circuit 44 . this feeder circuit 8 b branches into two circuits . one branched circuit 8 b further branches into two branched circuits 8 a which are connected to a ladder electrode 303 via two feeding points 9 a . the other branched circuit 8 b has a phase modulation device 45 . this branched circuit 8 b also further branches into two branched circuits 8 b which are connected to the ladder electrode 303 via two feeding points 9 b . a mechanically driven device having a vacuum capacitor was used as the phase modulation device 45 . the same device as in the third embodiment was used as the high - frequency power supply 5 . in this embodiment , feeding points of a multi - point feeding system are divided into two groups . a difference is produced between the phases of high - frequency power components supplied to feeding points of each group , and the high - frequency power components are applied to the ladder electrode 303 via these feeding points . in this manner , the position of an envelope distribution can be controlled . the apparatus 1 e of this embodiment was used to form a - si films on glass substrates g . the film formation conditions of this embodiment were as follows . even under the above conditions , film formation rate distributions analogous to those shown in fig1 and 19 were obtained with phase differences of 0 ° and 90 °. however , the absolute value of the film formation rate was about ⅕ . films were formed by setting the total film formation time to 6 min and manually controlling the phase difference at 0 ° for the first three minutes and 90 ° for the last three minutes . in this embodiment , an average deposition rate of about 0 . 4 nm / sec was obtained when the apparatus was operated by setting the high - frequency power to about 200 w that was limited by the rating of the phase modulation device . also , the obtained film thickness distribution was ± 15 %. these results show that the method of this embodiment can be used in the manufacture of low - cost versatile solar cells . the sixth embodiment of the present invention will be described below with reference to fig2 . an apparatus 1 f of this embodiment is obtained by partially changing the vhf feeder circuit of the apparatus 1 a of the first embodiment . a feeder circuit of this apparatus 1 f comprises two independent power supplies 5 a and 5 b , two independent power meters 6 a and 6 b , a mixer 40 , an impedance matching circuit 7 , and a distributor 26 . the independent power supplies 5 a and 5 b output vhf power components having different frequencies . the mixer 40 mixes these vhf power components , and the mixed vhf power is supplied to a ladder electrode 303 via the impedance matching circuit 7 and the distributor . in this embodiment , a film thickness uniformity of ± 10 % or less could be obtained regardless of the simplicity of the feeder circuit . note that in this embodiment the maximum value of power is limited to 2 kw by the rating of the mixer 40 . the seventh embodiment of the present invention will be described below with reference to fig2 . an apparatus 1 g of this embodiment is obtained by partially changing the vhf feeder circuit of the apparatus 1 f of the above sixth embodiment . a feeder circuit of this apparatus 1 g comprises two independent high - frequency generators 20 g 1 and 20 g 2 , a mixer 40 , a high - frequency amplifier 41 , a power meter 6 , an impedance matching circuit 7 , and a distributor 26 . the independent high - frequency oscillators 20 g 1 and 20 g 2 oscillate different vhfs . these vhfs are mixed by the mixer 40 , amplified by the amplifier 41 , and supplied to a ladder electrode 303 via the power meter 6 , the impedance matching circuit 7 , and the distributor 26 . also in this embodiment , a film thickness uniformity of ± 10 % or less could be obtained regardless of the simplicity of the feeder circuit . the arrangement of feeding points in this embodiment could be the same as in the sixth embodiment . on the other hand , in this embodiment the maximum value of power was not restricted by the rating of the mixer 40 , and film deposition was possible at 4 kw . the eighth embodiment of the present invention will be described below with reference to fig2 . an apparatus 1 h of this embodiment is obtained by partially changing the vhf feeder circuit of the apparatus 1 g of the above seventh embodiment . in the seventh embodiment , the mixer 40 mixes two signals having different frequencies oscillated by the two high - frequency oscillators , thereby generating a multiple high frequency which is a mixture of plurality of frequencies . in this embodiment , the same effect can be obtained by using one arbitrary waveform generator 20 h . fig2 a is a graph of a frequency spectral waveform showing a multiple high frequency in a frequency band of 59 to 61 mhz . in fig2 a , the frequency ( mhz ) is plotted on the abscissa , and the amplitude is plotted on the ordinate . first , this frequency spectrum was converted into a temporally changing waveform by performing inverse fourier transform by using a computer ( not shown ). the result is shown in fig2 b . that is , fig2 b is a graph showing the waveform obtained by performing inverse fourier transform for the multiple high frequency shown in fig2 a . in fig2 b , the time elapsed (× 10 − 6 sec ) from the beginning of vhf oscillation is plotted on the abscissa , and the voltage ( v ) on the electrode is plotted on the ordinate . fig2 b shows an envelope which contains a wave of about 60 mhz , although details of the waveform are not shown . that is , this waveform contains waves at different frequencies of 59 to 61 mhz . this waveform is stored in the arbitrary waveform generator 20 h and repetitively generated from it . consequently , a multiple high - frequency wave containing frequencies of 59 to 61 mhz is continuously supplied to a high - frequency amplifier 41 . this high - frequency wave is then supplied to a ladder electrode 303 via a power meter 6 , an impedance matching circuit 7 , and a distributor 26 . also in this embodiment , a film thickness uniformity of ± 10 % or less could be obtained as in the seventh embodiment described above . the ninth embodiment of the present invention will be described below with reference to fig2 . an apparatus 1 j of this embodiment includes a circular parallel plate electrode 313 in a vacuum chamber 2 j . six vhf feeder circuits are connected to this parallel plate electrode 313 via six feeding points 9 a . the six feeding points 9 a are connected to the circular parallel plate electrode 313 by point symmetry . the six vhf feeder circuits are independent of one another . the first vhf feeder circuit includes a high - frequency power supply 5 a , a power meter 6 a , and an impedance matching circuit 7 a . analogously , the second to sixth vhf feeder circuits include high - frequency power supplies 5 b to 5 f , power meters 6 b to 6 f , and impedance matching circuits 7 b to 7 f , respectively . in this embodiment , a plasma on the parallel plate electrode 313 could be uniformized by supplying high - frequency waves having different frequencies from the six independent feeder circuits . a film thickness uniformity of ± 10 % or less could be obtained by adjusting the vhf power and the electrode spacing . however , the range of uniformization conditions of the circular parallel plate electrode 313 is narrower than that of the ladder electrode 303 . in this embodiment , the aforementioned uniformity could be obtained only at a power of 2 . 0 to 2 . 2 kw and an electrode spacing of 25 to 30 mm . the 10th embodiment of the present invention will be described below with reference to fig2 , 29 , and 30 . fig2 is a plan block diagram showing an apparatus of this embodiment . fig3 is a sectional block diagram showing the apparatus of this embodiment . fig3 is a perspective block diagram showing the external appearance of the apparatus of this embodiment . an apparatus 1 k of this embodiment includes a shower type parallel plate electrode 323 , instead of the ladder electrode 303 of the apparatus 1 a of the first embodiment . a gas supply pipe 17 is connected to the upper surface of this electrode 323 . this gas supply pipe 17 communicates with a hollow portion 324 in the electrode via a port 321 . the hollow portion 324 communicates with a discharge plasma generation space 330 via a plurality of pores 325 . the electrode 323 is held on the side walls of a chamber 2 k by insulating members 336 and 338 . coaxial cables 8 b of two feeder circuits 341 and 342 are introduced into the chamber 2 k and connected to the electrode 323 via a plurality of feeding points 9 a and a plurality of feeding points 9 b 1 , respectively . insulating members 340 insulate portions where the cables 8 b extend through the walls of the chamber 2 k . the feeder circuit 341 includes a high - frequency power supply 5 a , a power meter 6 a , an impedance matching circuit 7 a , and a distributor 26 a . the distributor 26 a feeds power to the feeding points 9 a via the eight branched cables 8 b . the feeder circuit 342 includes a high - frequency power supply 5 b , a power meter 6 b , an impedance matching circuit 7 b , and a distributor 26 b . the distributor 26 b feeds power to the feeding points 9 b 1 via the eight branched cables 8 b . a ground electrode 3 is supported by a plurality of columns 331 so as to oppose parallel to the shower electrode 323 . a pair of gate valves 334 are formed in the side surfaces of the chamber 2 k to allow a substrate g to be loaded / unloaded into / from the chamber 2 k . when the gate valves 334 are opened , a loader ( not shown ) loads the substrate g into the chamber 2 k and places the substrate g on the ground electrode 3 . a heater 3 a connected to a power supply 332 is embedded in the ground electrode 3 and heats the substrate g . an exhaust pipe 18 communicates with an appropriate portion of the chamber 2 k . an evacuation device 72 evacuates the chamber 2 k to a high vacuum degree . the apparatus 1 k of this embodiment was used to form films on the substrates g under the same conditions as in the first embodiment . consequently , in this embodiment a film thickness uniformity of ± 12 % or less could be obtained . in the apparatus 1 k of this embodiment , when the arrangement of the feeding points was changed , the film thickness uniformity largely changed accordingly . therefore , it was necessary to optimize the feeder distribution center arrangement . the 11th embodiment of the present invention will be described below with reference to fig3 . an apparatus 1 l of this embodiment is obtained by partially changing the very - high - frequency ( vhf ) feeder circuit of the apparatus 1 a of the first embodiment shown in fig8 . this apparatus 1 l comprises an am modulation oscillator 50 , an amplifier 41 , a power meter 6 , an impedance matching circuit 7 , and a distributor 26 . a high - frequency wave from the am modulation oscillator 50 was amplified by the amplifier 41 to generate an am - modulated high - frequency wave having a carrier frequency of 60 mhz , a modulation frequency of 30 mhz , and a modulation amplitude range from 100 % to 20 %. this am - modulated high - frequency wave was fed to a ladder electrode 303 via the power meter 6 , the impedance matching circuit 7 , and the distributor 26 . in this embodiment , a film thickness distribution of ± 15 % could be obtained regardless of the simplicity of the circuit . the 12th embodiment will be described below with reference to fig3 . in this embodiment , a description of the same portions as in the above embodiments will be omitted to avoid duplication . a plasma cvd apparatus 1 m has a high - frequency oscillator 20 m with an fm modulation function as a power supply . an arbitrary waveform generator was used as this high - frequency oscillator 20 m . an amplifier 41 and an impedance matching circuit 7 are connected in series to the high - frequency oscillator 20 m . an output circuit of the impedance matching circuit 7 branches into two circuits , and each branched circuit 8 m further branches into two circuits . these branched circuits 8 a and 8 b are connected to an electrode 303 via a pair of feeding points 9 a and a pair of feeding points 9 b , respectively . in this embodiment , the use of fm modulation made it possible to simplify the feeder circuit and uniformize the film thickness as in the above embodiments . that is , a film thickness uniformity of ± 10 % or less was obtained on a 10 cm × 10 cm square substrate by setting the oscillation frequency to 200 mhz , the modulation frequency to 1 mhz , and the maximum deviation amount to 20 %. this film thickness uniformity was much superior to a film thickness uniformity of ± 50 % obtained by the conventional method . similar effects were also obtained by frequency chirp . that is , the film thickness uniformity was ± 12 % when the chirp frequency was 10 mhz and the maximum deviation amount was 20 %. a thin semiconductor film , thin metal film , or insulating film of a semiconductor element can be etched by generating a plasma by using a halogen - based gas , e . g ., a chlorine - based gas , thereby generating negative chlorine ions ( c 1 − ). in this case , as described in s . samukawa , “ role of negative ions in high - performance etching using pulse - time - modulated plasma ”, extended abstract of 4th international conference on reactive plasma , sr 1 . 04 , p . 415 , 1998 ( to be referred to as reference 8 hereinafter ), a plasma is conventionally generated and extinguished by turning on / off electric power generated from a high - frequency power supply , thereby generating a large amount of negative chlorine ions by the electron adhesion effect when the plasma disappears . in addition to this effect , the rate and quality of etching are increased by using the effect of disappearance of the wall electric charge generated on the substrate surface . in this embodiment , a plasma is turned on / off by moving the nodes and antinodes of an envelope . the apparatus 1 b of the second embodiment shown in fig1 was used . two different frequencies of 750 . 000 and 750 . 004 mhz were used to set the frequency difference at 4 khz , and the electrode dimensions were set to 33 cm × 33 cm . in addition , a halogen - based gas , e . g ., a chlorine - based gas was used to generate a plasma to generate negative chlorine ions ( c 1 − ), thereby etching a thin au film on the substrate surface . in this state , the plasma was on in the antinodes of an envelope distribution and off in its nodes . therefore , etching could be performed at high rate by efficiently and simply generating a large amount of negative chlorine ions by rapidly moving the envelope . since the frequency difference was 4 khz , the changing cycle of the envelope was set to about 250 μsec so that the cycle was longer than , i . e ., twice to four times , a negative chlorine ion generation time of about 100 μsec described in reference 10 . that is , the plasma turns on / off at a cycle equivalent to the reciprocal of the frequency difference between a plurality of high frequencies . accordingly , the plasma off time was about 125 μsec , so sufficient negative ions are generated . since 750 mhz was used as the frequency of a high - frequency wave , the plasma density became higher than when 13 . 56 mhz of the conventional method was used , and simultaneously the thickness of the plasma sheath decreased . consequently , a large amount of negative chlorine ions generated in the plasma efficiently flowed into the substrate surface , and this further raised the etching rate . in this embodiment , it was possible to obtain an etching rate about 40 times that obtained when a conventional single frequency of 13 . 56 mhz was used . the method of this embodiment is also applicable to so - called self - cleaning which removes foreign matter ( thin silicon - based films ) sticking to the chamber inner walls of a pcvd apparatus by using a discharge plasma . the effect of this embodiment is obtained by moving an envelope by using the frequency difference between two frequencies . however , the same effect can naturally be obtained by another envelope moving method , e . g ., a phase difference change , am modulation , or fm modulation . the 14th embodiment of the present invention will be described below with reference to fig3 . an apparatus 1 n of this embodiment is obtained by changing the feeding points and high frequencies of the apparatus 1 a of the first embodiment . this apparatus 1 n includes first and second high - frequency power supplies 5 a and 5 b , two feeding points 9 a attached to a ladder electrode 303 , and two feeding points 9 b 2 attached to a ground electrode 3 . the first power supply 5 a supplies high - frequency power at a frequency of 60 . 00 mhz to the ladder electrode 303 via the feeding points 9 a . the second power supply 5 b supplies high - frequency power at a frequency of 13 . 56 mhz to the ground electrode 3 via the feeding points 9 b 2 . this embodiment was applied to silicon film etching using halogen - based gas nf 3 . consequently , a high etching rate ( about 10 nm / sec ) was uniformly obtained in a large area of 1 m × 1 m by the high density resulting from a very high frequency of 60 mhz , by the substrate bias effect obtained by 13 . 56 mhz , and by the effect of suppressing a standing wave obtained by the difference between the two frequencies . this embodiment can also be applied to a surface treatment method such as plasma cleaning of a reaction vessel used in the formation of a thin silicon film , i . e ., so - called self - cleaning . the 15th embodiment of the present invention will be described below with reference to fig3 . an apparatus 1 q of this embodiment includes upper and lower discharge electrodes 323 a and 323 b in place of the ground electrode 3 of the apparatus 1 k of the 10th embodiment described earlier . these upper and lower electrodes 323 a and 323 b are flat plates having substantially the same size and arranged parallel to each other . four first feeding points 9 a are formed in a side portion of the upper electrode 323 a . an impedance matching circuit 7 a , a power meter 6 a , and a high - frequency power supply 5 a are connected to these four first feeding points 9 a via a coaxial cable 8 a and a t - shaped branched connector 27 a . four second feeding points 9 b are formed in a side portion of the lower electrode 323 b . an impedance matching circuit 7 b , a power meter 6 b , and a high - frequency power supply 5 b are connected to these four second feeding points 9 a via a coaxial cable 8 b and a t - shaped branched connector 27 b . the four first feeding points 9 a are attached at substantially equal intervals to a short edge on one side of the upper electrode 323 a . the four second feeding points 9 b are attached at substantially equal intervals to a short edge on the other side of the lower electrode 323 b . accordingly , the four second feeding points 9 b are symmetrically positioned on the side far away from the four first feeding points 9 a . in the apparatus 1 q of this embodiment , when a vhf is fed from the power supply 5 b to the lower electrode 323 b via the four second feeding points 9 b , the lower electrode 323 b generates a voltage similar to the voltage distribution r 1 shown in fig1 a in the first embodiment . when a vhf is fed from the power supply 5 a to the upper electrode 323 a via the four first feeding points 9 a , the upper electrode 323 a generates a voltage similar to the voltage distribution r 2 shown in fig1 b . therefore , on a substrate g a voltage distribution analogous to the synthetic voltage distribution r 3 of the two voltage distributions r 1 and r 2 is generated . this synthetic voltage distribution r 3 is highly uniform and generates a plasma having a substantially uniform distribution density , so the uniformity of the film thickness improves . a high film formation rate of 0 . 9 nm / sec was obtained in nanocrystalline film formation , and the uniformity was about ± 10 %. that is , the uniformity necessary for s 1 thin - film semiconductors for solar cells was achieved . comparative examples in which amorphous silicon films ( a - si films ) were formed on substrates by using conventional feeding methods will be described below . in comparative example 1 , a parallel plate plasma cvd apparatus 400 shown in fig3 was used . this apparatus 400 is known by the reference 1 cited previously . upper and lower electrodes 402 and 404 are opposed in a vacuum vessel 401 . the upper electrode 402 is mounted on the upper portion of the vacuum vessel 401 via an insulator 405 . this upper electrode 402 is connected to an impedance matching circuit 406 and a high - frequency power supply 407 via a coaxial cable 408 . the power - supply frequency is 13 . 56 mhz . the cable 408 is connected to the upper electrode 402 by a feeder distribution center 409 . this feeder distribution center 409 is formed on the surface ( outer surface ) exposed to the atmosphere of the upper electrode 402 so as not to be exposed in a plasma generation space 410 . the lower electrode 404 houses a heater 403 for heating a substrate g . a heater power supply is controlled by a controller ( not shown ). the substrate g is placed on the lower electrode 404 and heated to a predetermined temperature by the heater 403 . note that the lower electrode 404 is grounded . a gas supply pipe 411 is open in the upper portion of the vacuum vessel 401 . this gas supply pipe 411 communicates with a gas supply source ( not shown ) via a flow rate control valve 416 . the gas supply source contains monosilane ( sih 4 ). an exhaust pipe 412 is open in the lower portion of the vacuum vessel 401 . this exhaust pipe 412 communicates with a vacuum pump 413 . the above apparatus 400 was used to form a - si films on glass substrates . the film formation conditions of comparative example 1 were as follows . as a consequence , a - si films having a variation of ± 18 % with respect to the target film thickness were obtained . the average film formation rate was 0 . 13 nm / sec . in comparative example 2 , a parallel plate plasma cvd apparatus 500 shown in fig3 and 37 was used . this apparatus 500 is known by the reference 1 cited previously . as comparative example 2 , the second apparatus described in reference 1 will be explained with reference to fig3 and 37 . in this comparative example 2 , a description of the same portions as in comparative example 1 described above will be omitted to avoid duplication . reference numeral 521 denotes a gate valve formed in a side wall of a vacuum vessel 501 . a substrate g is loaded / unloaded through this gate valve 521 . a hollow non - grounded electrode 502 is placed inside an insulator 505 which is positioned in the upper portion of the vacuum vessel 501 . a large number of gas supply holes 502 b about 0 . 5 mm in diameter are formed in a lower surface ( front surface ) 502 a of this non - grounded electrode 502 at intervals of 10 to 15 mm to face a plasma generation space 510 . an opening 502 c for a discharge gas is formed in the upper surface of the non - grounded electrode 502 . a discharge gas supply pipe 511 b is connected to this opening 502 c of the non - grounded electrode 502 via a connecting member 523 . this discharge gas supply pipe 511 b supplies , e . g ., monosilane gas ( sih 4 ) into the non - grounded electrode 502 . a power distributor 525 for branching the output from a high - frequency power supply into a plurality of portions is placed on the non - grounded electrode 502 . a glow discharge plasma is generated in the vacuum vessel 501 by the non - grounded electrode 502 and a grounded electrode 504 . the non - grounded electrode 502 is a rectangular or square plate - like member ( about 500 mm × 500 mm to about 1 , 000 mm × 1 , 000 mm and 60 mm in thickness ) and made of stainless steel . this non - grounded electrode 502 is supplied with necessary power by a power supply system composed of a high - frequency power supply 507 , an impedance matching circuit 506 , and the power distributor 525 described above . as shown in fig3 , the power distributor 525 is composed of a columnar connection port 526 placed in the center , four band - like branched ports 527 radially extending from the connection port 526 , and four columnar members 528 a , 528 b , 528 c , and 528 d . reference numerals 509 a , 509 b , 509 c , and 509 d in fig3 denote power supply portions . a gas in the vacuum vessel 501 is exhausted through the exhaust pipe 512 by the vacuum pump 513 . a substrate 514 is placed on the grounded electrode 504 by opening the gate valve 521 and heated to a predetermined temperature by a substrate heater 503 and a substrate heater power supply 515 . the above apparatus 500 was used to form a - si films on glass substrates . the film formation conditions of comparative example 2 were as follows . as a consequence , a - si films having a variation of ± 50 % with respect to the target film thickness were obtained . first , the gate valve 521 was opened to place the substrate g on the grounded electrode 504 , and the gate valve 521 was closed . the vacuum pump 513 was driven to evacuate the vacuum vessel 501 to an internal pressure of about 1 × 10 − 7 torr . while the power supply amount from the power supply 515 was controlled , the substrate g was heated to a predetermined target temperature by the heater 503 . a predetermined amount of monosilane gas was supplied through the discharge gas supply pipes 511 a and 511 b , and the internal pressure of the vacuum vessel was held at 0 . 05 to 0 . 5 torr . a high - frequency voltage was applied to the non - grounded electrode 502 by the power supply 507 , thereby generating a glow discharge plasma between the non - grounded electrode 502 and the grounded electrode 504 . when the monosilane gas was formed into a plasma , radicals such as sih 3 , sih 2 , and sih existing in the plasma diffused by a diffusion phenomenon . these radicals were adsorbed in and deposited on the surface of the substrate g to form an a - si film . in comparative example 3 , a parallel plate plasma cvd apparatus 600 shown in fig3 and 39 was used . this apparatus 600 is known by reference 2 cited previously . as comparative example 3 , the apparatus described in reference 2 will be explained below with reference to fig3 and 39 . a high - frequency oscillator 631 is connected to an impedance matching circuit 606 via a high - frequency power amplifier 632 . the high - frequency oscillator 631 and the high - frequency amplifier 632 constitute a high - frequency power supply . a vacuum current supply terminal 633 is formed on a wall surface 634 of a vacuum vessel 601 . a coaxial cable 608 extending through this supply terminal 633 connects a non - grounded electrode 602 and the impedance matching circuit 606 . the wall surface 634 of the vacuum vessel 601 is grounded . a substrate g is placed on the wall surface 634 . when a gas is supplied into the vacuum vessel 601 and a high frequency is applied between the electrode 502 and the wall surface 634 , a glow discharge plasma is generated between them . the non - grounded electrode is supplied with high - frequency power from the power supply 631 via the vacuum current supply terminal 633 , the coaxial cable 608 , and the impedance matching circuit 606 . the frequency of the high - frequency wave oscillated by the power supply 631 is 70 mhz . as shown in fig3 , an h - shaped feeder 635 is formed on the rear surface ( the surface not in contact with the plasma generation space ) of the electrode 602 . feeding points 609 a , 609 b , 609 c , and 609 d are formed at the four corners of this feeder 635 . the above apparatus 600 was used to form a - si films on glass substrates . the film formation conditions of comparative example 3 were as follows . as a consequence , a - si films having a variation of ± 18 % with respect to the target film thickness were obtained . additional advantages and modifications will readily occur to those skilled in the art . therefore , the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein . accordingly , various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents .