Thin film formation apparatus

Abstract: An apparatus (10) for forming a thin film on a substrate (S) comprises a reactor vessel (12), and a discharge electrode (20) and grounded electrode (22) opposably disposed within the reactor vessel, the discharge (20) electrode having a hollow space (20c) opened to the grounded electrode (22), a starting gas being introduced into the reactor vessel through that hollow space (22c). A radio frequency voltage is applied between the discharge and grounded electrodes to produce a discharge plasma zone therebetween. A potential of the discharge electrode is biased toward the negative side so that the discharge electrode behaves as a cathode to which a direct current voltage is applied, whereby a high density plasma is obtained in the hollow space (22c) of the discharge electrode.

Description: BACKGROUND OF THE INVENTION 1) Field of the invention The present invention relates to an apparatus for forming a thin film of a semiconductor such as hydrogenated amorphous silicon, an insulator such as hydrogenated amorphous carbon, or the like on a substrate, and in particular, relates to a thin film formation apparatus in which a chemical vapor deposition (CVD) process is utilized for the formation of a thin film. 2) Description of the Related Arts Various types of CVD process are known for the formation of a thin film on a substrate; for example, a thermal decomposition CVD process, an photo-assisted CVD process, and a plasma-assisted CVD process. In the thermal decomposition CVD process, a gas of a starting material (hereinafter referred to as a starting gas) is thermally decomposed and activated within a reactor vessel to produce an active species as decomposition products, and these active species are deposited on a substrate to form a thin film thereon. In general, in this type of process, the substrate is heated at a high temperature so that the decomposition of the starting gas occurs in the vicinity of a surface of the substrate. For this reason, the substrate must be formed of a material able to endure heating at a high temperature. Namely, a substrate material usable in the thermal decomposition CVD process is restricted to a material which cannot be fused or deformed by heating. Also, if the substrate is overheated during the thermal decomposition CVD process, the formation of the thin film on the substrate may be unsatisfactory because a component of the deposited compound may be released therefrom due to the overheating of the substrate. In the photoactivated CVD process, a starting gas is decomposed and activated by an energy of light within a reactor vessel, to produce an active species as decomposition products, and the produced active species are deposited on a substrate to form a thin film thereon. Namely, the starting gas is irradiated by a light having a wavelength in the ultraviolet band, which is liable to be absorbed by the starting gas, so that the active species are decomposed and produced from the starting gas. In general, as a light source for the irradiation of light, a low-pressure mercury-vapor lamp, a deuterium discharge tube, and an ArF excimer laser device may be used. The low-pressure mercury-vapor lamp and the deuterium discharge tube emit a continuous light, but an intensity thereof is small and thus it is difficult to efficiently carry out the decomposition of the starting gas, resulting in a poor rate of growth of the thin film. On the other hand, it is impossible to obtain a continuous light from the ArF excimer laser, since the ArF excimer laser is obtained as a pulse light having a pulse frequency of less than 1 kHz. This means that a life of the produced active species is shorter than a time of one cycle of the pulse frequency, resulting in a deterioration of the characteristics of the deposited thin film. In addition, during the photo-assisted CVD process, the light is introduced into the reactor vessel through a glass window formed therein, so that the decomposition of the starting gas is liable to occur in the vicinity of the glass window, whereby the produced active species may be deposited on the glass window, and thus, as more of the active species is deposited on the glass window, the introduction of the light through the glass window is impeded. In the plasma-assisted CVD process, a starting gas is introduced into a plasma zone generated within a reactor vessel by utilizing one of a direct current (DC) discharge, a radio frequency (RF) discharge and a microwave discharge. The introduced starting gas is made to collide with electrons and ions in the plasma zone so that the starting gas is dissociated to produce active species which deposit on the substrate to form a thin film thereon. When the DC discharge is utilized in the plasma-assisted CVD process, the formation of an excellent thin film conductor can be carried out because an stable DC discharge may be easily maintained during the formation of the thin film conductor, but it is impossible to carry out the formation of an excellent thin film semiconductor or insulator because the DC discharge cannot be stably maintained. Namely, the active species for the formation of a semiconductor or insulator deposit on not only the substrate but also on the electrodes, so that an electric resistance is gradually increased between the electrodes. Accordingly, the DC discharge plasma-assisted CVD process cannot be utilized for the formation of the thin film semiconductor or insulator. The RF discharge can be utilized in the plasma-assisted CVD process to form both the thin film conductor and the thin film semiconductor or insulator. Industrially, the RF discharge plasma-assisted CVD process is most widely used because a starting voltage at which the RF discharge is caused is relatively low, and because a stable plasma can be easily obtained over a wide area. Nevertheless, since a density of the plasma generated by the RF discharge is low (10⁸ to 10¹⁰ cm ⁻³), the growth rate of the thin film is low and the starting gas cannot be effectively consumed. The RF discharge plasma-assisted CVD process is frequently used to form an amorphous thin film semiconductor such as a hydrogenated amorphous silicon (a-Si:H) thin film, and in general, the formation of the amorphous thin film semiconductor is carried out in such a manner that a pressure within a reactor vessel (a pressure of a starting gas) is maintained within a range from 13 to 1300 Pa (0.1 to 10 torr). When the formation of the amorphous thin film semiconductor is carried out within a high pressure range of from about 40 to 1300 Pa (0.3 to 10 torr), the growth rate of the thin film is relatively high, but the amorphous thin film semiconductor may be defective. In particular, under the high pressure range of from 40 to 1300 Pa (0.3 to 10 torr), the active species dissociated from the starting gas collide with each other and the starting gas molecules so that high-molecular compounds are produced as a fine powder within the reactor vessel. For example, when the formation of the hydrogenated amorphous silicon (a-Si:H) thin film is carried out within the high pressure range of from 40 to 1300 Pa (0.3 to 10 torr), the high-molecular powders Si n H m (n, m = natural number) deriving from the starting gas such as SiH₄, Si₂H₆ or the like are produced, and these high-molecular powders may deposit together with the active species on the substrate, resulting in defects in the hydrogenated amorphous silicon thin film. Since the high-molecular powders also may deposit on the discharge electrodes and the inner wall surface of the reactor vessel, the reactor vessel must be cleaned after every formation process, to remove the high-molecular powders. Furthermore, when the CVD reactor vessel is in communication with another reactor vessel through the intermediary of a valve, so that the substrate can be moved from one reactor vessel to the other reactor vessel, the high-molecular powders may deposit on the valve, and thus the valve cannot operate normally. On the other hand, when the formation of the amorphous thin film semiconductor is carried out within a low pressure range of from about 13 to 40 Pa (0.1 to 0.3 torr), it is possible to prevent the production of the high-molecular powder because the probability of collision among the active species and the starting gas molecules is considerably lowered due to the low pressure of the starting gas, whereby the formation of an excellent amorphous thin film semiconductor can be carried out without defects. This, however, means that the growth rate of the thin film is very low, and that the starting gas cannot be effectively consumed. The microwave discharge can be also utilized in the plasma-assisted CVD process to form both the thin film conductor and the thin film semiconductor or insulator. In general, the microwave discharge plasma-assisted CVD process is performed by introducing a microwave into a reactor vessel through a waveguide. This is because, although an antenna for generating a microwave is disposed within a reactor vessel, a discharge plasma can be obtained only at a localized zone in the vicinity of the antenna. The microwave discharge can be stably maintained even under a very low pressure of 1.310⁻² Pa (10⁻⁴ torr) by utilizing a magnetic field (electron cyclotron resonance (ECR)), so that the probability of collision between the starting gas molecules and electrons is considerably enhanced, whereby not only can the rate of the thin film be increased, but also the starting gas can be effectively consumed. In the microwave discharge plasma-assisted CVD process, however, it is impossible to obtain a wide plasma zone because a cross-sectional area of the plasma zone is restricted to that of the waveguide due to a strong directivity of the microwave. By incorporating a cavity resonator into the reactor vessel, the plasma zone is able to be widened to an inner space of the cavity resonator, but the size of the cavity resonator is restricted by a frequency of the microwave used. For this reason, the microwave discharge plasma-assisted CVD process cannot be utilized to form a thin film on a large scale substrate. In addition, in general, a characteristic of a thin film fomed by the RF discharge plasma-assisted CVD process is inferior to that of a thin film obtained by the microwave discharge plasma-assisted CVD process. It is also known from JP-A-62 23871 to trap electrons in a recessed part of the discharge electrode by magnets surrounding the recess, in order to increase the electron density at a region close to the substrate being processed. In US-A-4 633 809, there is disclosed a plasma etching apparatus designed for forming an amorphous silicon film on a substrate and equipped with a conductive mesh structure disposed in a reaction chamber so as to surround the plasma generating region and prevent the plasma from dispersing while allowing the silicon compound compound gas to permeate therethrough. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an apparatus for forming a thin film on a substrate by utilizing a radio frequrency discharging plasma, wherein not only can the growth rate of thin film be facilitated, but also a starting gas can be effectively consumed under a low pressure of the starting gas. Therefore, in accordance with the present invention, there is provided an apparatus for forming a thin film on a substrate, comprising a reactor vessel, a first electrode and a second electrode opposably disposed within the reactor vessel, an introduction means for introducing a starting gas into the reactor vessel through the hollow space of the first electrode. The thin film formation apparatus according to the present invention also comprises a circuit including a radio frequency electric source for applying a radio frequency voltage between the first and second electrodes to provide a discharging plasma zone therebetween for dissociating the starting gas, the circuit being arranged so that the first electrode serves as a discharge electrode and the second electrode is grounded. According to the present invention, the first electrode includes walls defining a hollow space and having an outlet that is open to the second electrode, and the circuit further includes a biasing means for biasing a potential of the first electrode toward the negative side, whereby a high density plasma is produced in the hollow space whereby dissociation of the starting gas introduced therin is facilitated. In the present invention, the biasing means may comprise a blocking capacitor incorporated into the circuit. In this case, the first electrode has a discharge area equal to or preferably smaller than an area of the second electrode opposite to the discharge area of the first electrode. Under these conditions, the first electrode captures a part of the electrons generated at the beginning of the discharge between the first and second electrodes, so that the first electrode self-biases its potential to the negative side until the first electrode is saturated by the electrons. On the other hand, the biasing means may comprise a direct current voltage source incorporated into the circuit. In this case, a potential of the first electrode is forcibly biased to the negative side. BRIEF DESCRIPTION OF THE INVENTION Other objects and advantages of the present invention will be better understood from the following description, with reference to the accompanying drawings, in which: Figure 1 is a schematic view showing a first embodiment of a thin film formation apparatus according to the present invention; Figure 2(a) is a perspective view showing a first electrode used in the first embodiment of Fig. 1; Figures 2(b) to 2(d) are perspective views showing an alternative embodiment of the first electrode of Fig. 2(a); Figure 3 is a schematic view showing a modification of the first embodiment shown in Fig. 1; Figure 4 is a schematic view showing a second embodiment of a thin film formation apparatus according to the present invention; Figure 5 is a schematic view showing a third embodiment of a thin film formation apparatus according to the present invention; Figure 6 is a graph showing a characteristic of a hydrogenated amorphous silicon thin film obtained in a thin film formation apparatus assembled according to the third embodiment of Fig. 5; Figures 7(a) to 7(c) are graphs showing a waveform of a potential of the first electrode used in the thin film formation apparatus assembled according to the third embodiment of Fig. 5; Figure 8(a) is a schematic perspective view showing a fourth embodiment of a thin film formation apparatus according to the present invention; Figure 8(b) is a schematic cross-sectional view taken along a horizontal plane including a longitudinal axis of an inlet pipe forming a part of the thin film formation apparatus of Fig. 8(a); Fig. 9(a) is a schematic view showing a fifth embodiment of a thin film formation apparatus according to the present invention; Figure 9(b) is a schematic perspective view showing discharge electrode elements used in the thin film formation apparatus assembled according to the fifth embodiment of Fig. 9(a); Figure 10 is a graph showing a waveform of a potential of the discharge electrode elements used in the thin film formation apparatus assembled according to the fifth embodiment of Fig. 9(a); Figure 11 is a three-dimensional graph showing a thickness distribution of a thin film formed by the thin film formation apparatus assembled according to the fifth embodiment of Fig. 9(a); Figure 12 is a three-dimensional graph showing a thickness distribution of a thin film formed by a conventional thin film formation apparatus; Figures 13(a) and 13(b) are schematic views showing a sixth embodiment of a thin film formation apparatus according to the present invention, Fig. 13(a) being a horizontal cross-sectional view of Fig. 13(b) and Fig. 13(b) being a vertical cross-sectional view of Fig. 13(a); Figure 14 is a graph showing a waveform of a potential of the discharge electrode elements used in the thin film formation apparatus assembled according to the fifth embodiment of Fig. 13(a) and 13(b); Figure 15 is a graph showing a thickness distribution of a thin film formed by the thin film formation apparatus assembled according to the fifth embodiment of Fig. 13(a) and 13(b); Figure 16 is a schematic view showing a seventh embodiment of a thin film formation apparatus according to the present invention; Figures 17(a) and 17(b) are a schematic view showing a eighth embodiment of a thin film formation apparatus according to the present invention, Fig. 17(a) being a horizontal cross-sectional view of Fig. 17(b) and Fig. 17(b) being a vertical cross-sectional view of Fig. 17(a); Figure 18 is a graph showing a rate of thin film formation with respect to thin films produced by a thin film formation apparatus assembled according to the eighth embodiment of Figs. 17(a) and 17(b); Figure 19 is a schematic view showing a ninth embodiment of a thin film formation apparatus according to the present invention; and Figures 20(a) and 20(b) are schematic views showing a tenth embodiment of a thin film formation apparatus according to the present invention, Fig. 20(a) being a horizontal cross-sectional view of Fig. 20(b) and Fig. 20(b) being vertical cross-sectional view of Fig. 20(a). DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 1, a thin film formation apparatus, generally designated by reference numeral 10, comprises a reactor vessel 12 having an inlet port 14 and an outlet port 16. The inlet port 14 is connected to one end of an inlet pipe 18, the other end of which is connected to a suitable starting gas source (not shown), the outlet port 16 is connected to a suitable vacuum pump (not shown) such as a rotary pump or a mechanical booster pump. The thin film formation apparatus 10 also comprises first and second electrodes 20 and 22 opposably provided within the reactor vessel 12 to define a discharge zone therebetween; the first and second electrodes 20 and 22 being formed of a suitable conductive material such as a stainless steel. In this embodiment, the first electrode 20 is constructed as shown in Figure 2(a). Namely, the first electrode 20 includes a rectangular flat portion 20a and an upright portion 20b integrally projected from the flat portion 20a along the longitudinal axis thereof. The upright portion 20b has a rectangular hollow space 20c formed therein, which is extended in the flat portion 20a and opend at a discharge surface thereof. As seen from Figs. 1 and 2(a), the upright portion 20b has plurality of opening holes 20e formed at the head thereof aligned with each other. The first electrode 20 is suspended from the top wall of the reactor vessel 12. In particular, the head of the upright portion 20b is projected through the top wall of the reactor vessel 12 and is attached to the top wall of the reactor vessel 12 through the intermediary of an insulator 23, and a header element 14a by which the inlet port 14 is defined is mounted on the head top of the upright portion 20b in such a manner that the inlet port 14 is in communication with the hollow space 20c through the opening holes 20e. Note that the header element 14a is coextensive with the upright portion 20b so that all of the opening holes 20e are opened to the inlet port 14, and that the header element 14a is formed of a suitable conductive material to establish an electrical connection between the header element 14a and the upright portion 20b of the first electrode 20. The first electrode 20 is covered by a shield 24 suspended from the top inner wall of the reactor vessel 12. The first electrode 20 is connected to a radio frequency electric source 26 through a impedance matching box 28 and a blocking capacitor 30; the RF source 26 being grounded as in Fig. 1. Further, the second electrode 22 includes a rectangular flat portion 22a and an extended portion 22b integrally and downwardly projected from the flat portion 22a. The second electrode 22 is mounted on the bottom inner wall of the reactor vessel 12, and is grounded as shown in Fig. 1. A substrate S is put on the flat portion 22a at a predetermined location, and is heated by a suitable electric heater 25 incorporated therein. Note that the flat portion 22a of the second electrode 22 is preferably wider than the flat portion 20a of the first electrode 20, for the reasons described hereinafter. In this operation, first air is drawn from the reactor vessel 12 through the outlet port 16 by the vacuum pump until a pressure in the reactor vessel 12 is lower than 0.13 Pa (10⁻³ torr). Then the substrate S is heated to a predetermined temperature by the electric heater 25 while a starting gas is successively introduced into the reactor vessel 12 through the inlet port 14, the opening holes 20e, and the hollow space 20c, so that the pressure therein is maintained at a predetermined level of from about 1.3 Pa (0.01 torr) and about 4.0 Pa (0.3 torr). The vacuum pump is continuously driven. When a predetermined RF voltage is applied between the first and second electrodes 20 and 22 by the RF elecctric source 26 to cause an RF discharge therebetween, the electrons generated by the RF discharge collide with the starting gas molecules so that a plasma zone is formed between the first and second electrodes 20 and 22. At the beginning of the RF discharge, the electrons , which are vibrated in the vicinity of the first electrode 20, are captured by the first electrode 20 due to the existance of the blocking capacitor 30, so that a potential of the first electrode 20 is self-biased toward the negative side. If the flat portion 22a of the second electrode 22 is wider than the flat portion 20a of the first electrode 20, i.e., if the opposed area of the flat portion 20a is smaller than that of the flat portion 22a, the capture of the electrons by the first electrode 20 is facilitated. When the first electrode 20 is saturated with the captured electrons, the first and second electrodes 20 and 22 behave as a cathode and an anode, respectively, as if a direct current voltage were applied therebetween, so that the electrons existing in the hollow space 20c are rapidly vibrated by the electrical repulsion which the electrons receive from the hollow space wall. Namely, the probablity of collision between the electrons and the starting gas molecules in the hollow space 20c is considerably enhanced, so that a high density plasma is generated in a central portion of the discharge zone, which is in the vicinity of the hollow space 20e, as shown by the closed pitch lines in Fig. 1. As a result, the starting gas is directly introduced into the high density plasma and the active species are more efficiently dissociated from the starting gas in comparison with the conventional RF discharge plasma-assisted CVD process whereby not only can a growth rate of a thin film on the substrate S be increased, but also the starting gas can be effectively consumed. In this case, of course, the production of high-molecular powders deriving from the starting gas can be prevented because, although the density of the plasma (electrons) is increased, the pressure of the starting gas is maintained at a low level (1.3 to 4 Pa) (0.01 to 0.3 torr). Further, a low density plasma is generated in the remaining portions of the discharge zone, as shown by the widened pitch lines in Fig. 1. Of course, the dissociation of the starting gas can be also carried out in the low density plasma. Note that no discharge (plasma) is generated in the opening holes 20e because of the small size (about 2mm) thereof. Figures 2(b), 2(c) and 2(d) show modifications of the first electrode 20 which are usable in the thin film formation apparatus 10, as shown in Fig. 1. In Fig. 2(b), the electrode 32 includes a rectangular plate element 32a having a plurality of holes 32b formed therein, and sleeve elements 32c attatched to the plate element 32a at the locations of the holes 32b. The starting gas is introduced into the hollow spaces of the sleeve elements 32c through the respective holes 32b. In Fig. 2(c), the electrode 34 includes a disc element 34a having a hole formed at the center thereof, and a cylindrical element 34b attached to the disc element 34a at the center thereof and having a hollow space or bore 34c formed therein, the bore 34c being extended in the disc element 34a and opened at the center thereof. A hole 34d is formed in the top of the cylindrical element 34b, and the starting gas is introduced into the bore 34c through the hole 34d. In Fig. 2(d), the electrode 36 includes a disc element 36a, a general rectangular parallelepiped-shaped mesh element 36b attached diametrically to the disc element 36a, and a fork-shaped pipe element 36c having branch pipe sections 36c₁ and 36c₂ between which the mesh element 36b is disposed. The mesh element 36b includes a pair of wider side walls 36b₁, 36b₂ and a pair of narrower side walls 36b₃, 36b₄ which are formed of a suitable conductive mesh material such as a stainless steel mesh. The pipe sections 36c₁ and 36c₂ have opening holes (not shown) formed therein, which are oriented to the corresponding wider side wall 36b₁, 36b₂. The starting gas is ejected from the holes of the two-branched pipe sections 36c₁ and 36c₂, and then the ejected gas is introduced into a space defined by the side walls 36b₁, 36b₂, 36b₃ and 36b₄ through the wider side walls 36b₁ and 36b₂. Accordingly, the high density plasma can be obtained in the space defined by the side walls 36b₁, 36b₂, 36b₃ and 36b₄ in the same manner as in the hollow space 20c. Figure 3 shows a modification of the thin film formation apparatus 10 as shown in Fig. 1. In Fig. 3, the features similar to those of Fig. 1 are indicated by the same reference numerals. The modified apparatus is the same as in Fig. 1 except that a DC source 30′ is used in place of the blocking capacitor 30 for biasing the potential of the first electrode 20 toward the negative side. Figure 4 shows a second embodiment of the present invention, which is generally similar to the thin film formation apparatus 10 as shown in Fig. 1. In Fig. 4, the features similar to those of Fig. 1 are indicated by the same reference numerals. In the second embodiment of Fig. 4, instead of forming the hollow space 20c in the upright portion 20b of the first electrode 20, the first electrode 20 includes a cylindrical mesh element 20f formed of a stainless steel mesh and attached to the flat portion 20a so as to be suspended therefrom. The upright portion 20b is attached to the top wall of the reactor vessel 12 in the same manner as in Fig. 1, and is connected to the RF electric source 26 through the impedance matching box 28 and the blocking capacitor 30. The inlet pipe 18 for feeding a starting gas is extended through the side wall of the reactor vessel 12 and is connected to an annular pipe 18a surrounding the cylindrical mesh element 20f. The annular pipe 18a has a plurality of opening holes (not shown) which are formed in the inner pheripheral wall portion thereof so that the starting gas is downwardly ejected from the holes to define an angle of 35° with respect to the horizontal. The ejected starting gas is introduced into an inner space of the cylindrical mesh element 20f through the mesh wall thereof. Accordingly, the high density plasma, as shown by the closed pitch lines in Fig. 4, can be obtained in the space of the cylindrical mesh element 20f in the same manner as in the hollow space 20c. On the other hand, the low density plasma is generated in the outside zone of the cylindrical mesh element 20f, as shown by the widened pitch lines in Fig. 4. To demonstrate an actual formation of a hydrogenated amorphous silicon (a-Si:H) thin film on a suitable substrate, a thin film formation apparatus corresponding to the second embodiment of Fig. 4 was assembled. The assembled apparatus had the following characteristics: (A) Diameter of cylindrical mesh (20f) 50 mm (B) Height of cylindrical mesh (20f) 25 mm (C) Distance between first and second electrodes (20, 22) 35 mm (D) Distance between discharge surface of first electrode (20) and holes for starting gas 15 mm (E) Diameter of second electrode (22) 100 mm The actual thin film formation was carried out under the following conditions: (A) Starting gas Si₂H₆ (B) Flow rate of starting gas 2.0 sccm (C) Pressure of starting gas 8 Pa (0.06 torr) (D) Power of RF source 5.0 W (E) Temperature of substrate 230 °C It has been observed that the growth rate of the thin film was 0.87nm/s, and that the percentage of the starting gas consumption was 18 %. Also, the finished hydrogenated amorphous silicon (a-Si:H) thin film exhibited a resistance of 3.7 x 10⁷ Ω · cm in the dark, and exhibited a resistance of 2.0 x 10¹⁴ Ω · cm under an irradiation of a white lamp (1 mW/cm²). Furthermore, no high-molecular powder (Si n H m ) was generated during the operation. The formation of another hydrogenated amorphous silicon (a-Si:H) thin film was actually carried out under the same conditions, except that the cylindrical mesh element was omitted from the above apparatus. This apparatus corresponds to a conventional RF discharge plasma-assisted CVD apparatus. It was observed that the growth rate of thin film was 1. 0.3nm/s, and that the percentage of starting gas consumption was about 3 %. A comparison of the above results proves that the growth rate of thin film as well as the percentage of starting gas consumption are considerably improved by the present invention. Figure 5 shows a third embodiment of the present invention, which is basically similar to the thin film formation apparatus 10 shown in Fig. 1. In Fig. 5, the features similar to those of Fig. 1 bear the same reference numerals and the features corresponding to those of Fig. 1 bear like reference numerals suffixed with a prime. The third embodiment is adapted to form a thin film on a relatively long substrate S. In the third embodiment, the reactor vessel 12 is constructed in a cylindrical form, and is shown by the phantom line in Fig. 5. The first and second electrodes 20 and 22 are vertically and opposably disposed , and are suitably supported by the reactor vessel 12. The portion 20b projected from the flat portion 20a has the hollow space 20c which is vertically extended along the longitudinal axis thereof. In addition to the hollow space 20c, the projected portion 20b also has an elongated chamber 14′ formed therein, which vertically coextends with the hollow space 20c, and is in communication with the hollow space 20c through the holes 20e formed between the elongated chamber 14′ and the hollow space 20c. The inlet pipe 18 for feeding a starting gas is extended through the cylindrical side wall portion of the reactor vessel 12, and is connected to the projected portion 20b of the first electrode 20 in such a manner that the inlet pipe 18 is in communication with the elongated chamger 14′. Also, the inlet pipe 18 is formed of a suitable conductive material, and is connected to the RF electric source 26 through the impedance matching box 28 and the blocking capacitor 30. An outlet pipe 16′ is connected to the cylindrical side wall portion of the reactor vessel 12 so as to be in communication with the reactor vessel, and the outer end of the outlet pipe is connected to a vacuum pump (not shown). The starting gas fed by the inlet pipe 18 is once introduced into the elongated chamber, and is then ejected from the holes 20e into the hollow space 20c. Accordingly, the high density plasma can be obtained in the hollow space 20c in the same manner as in the first embodiment of Fig. 1. Also, a thin film formation apparatus corresponding to the third embodiment of Fig. 5 was actually assembled, and a formation of a hydrogenated amorphous silicon (a-Si:H) thin film on a suitable substrate was carried out. The assembled apparatus had the following characteristics: (A) Discharge area of first electrode (20) 464 x 136 mm (B) Length l of hollow space (20c) 462 mm (C) Width w of hollow space (20c) 8 mm (D) Depth d of hollow space (20c) 28 mm (E) Pitch of holes (20e) 40 mm (F) Diameter of holes (20e) 2 mm (G) Distance between first and second electrodes (20, 22) 23 mm Note, in the assembled apparatus, the first electrode is covered by a shield except for the discharge surface thereof, so that a discharge occurs only between the first and second electrodes. The actual thin film formation was carried out under the following conditions: (A) Starting gas Si₂H₆ (B) Flow rate of starting gas 50 sccm (C) Pressure within reactor vessel 9.3 Pa (0.07 torr) (D) Power of RF source 100 W (E) Substrate quartz (F) Temperature of substrate 270°C It was observed that the growth rate of thin film was 2.55nm/s. Also, the finished hydrogenated amorphous silicon thin film (a-Si:H) exhibited a resistance of 6.9 x 10¹⁰Ω · cm in the dark, and exhibited a resistance of 8.6 x 10⁶Ω · cm under an irradiation of a white lamp (1 mW/cm²). Furthermore, no high-molecular powder (Si n H m ) was generated during the operation. The formation of another hydrogenated amorphous silicon (a-Si:H) thin film was actually carried out under the same conditions, except that in place of the discharge electrode with the hollow space, a discharge electrode without a hollow space was used. This apparatus corresponds to a conventional RF discharge plasma-assisted CVD apparatus. It was observed that the growth rate of thin film was 1.38nm/s. Also, the finished hydrogenated amorphous silicon (a-Si:H) thin film exhibited a resistance of 3.0 x 10¹⁰Ω · cm in the dark, and exhibited a resistance of 3.2 x 10⁶ Ω · cm under an irradiation of a white lamp (1 mW/cm²). Furthermore, no high-molecular powder (Si n H m ) was generated during the operation. As apparent from a comparison of the above results, the growth rate of thin film was considerably improved by the present invention. Also, the a-Si:H thin film obtained according to the present invention was evaluated with respect to the hydrogen content thereof and the number of silicon-hydrogen bonds by using a Fourier transform infrared spectrometer. The results are shown in Fig. 6, wherein the characteristic represents the absorbance spectra deriving from the stretching mode vibration of Si-H. From the characteristic of Fig. 6, it was found that the hydrogen content was 8.7 at%, and the obtained a-Si:H thin film included Si-H m (m≧2). It, however, was found that the amount of Si-H₃ and (-SiH₂-) n which deteriorates a photoconductivity of the a-Si:H thin film was little. Furthermore, in the thin film formation apparatus according to the third emdodiment (Fig. 5), the relationship between a stability of the plasma and a potential of the discharge electrode was investigated with respect to each of monosilane SiH₄, disilane Si₂H₆, argon Ar, helium He, hydrogen H₂, and mixtures thereof, which were used as the discharge gas. As a result, it was observed that a pressure range of the discharge gas, a power range of the RF electric source, and a flow rate range of the discharge gas for a stabilization of the generated plasma were varied according to the kind of discharge gas used, but in general, the following findings were obtained: 1) When a bright plasma is stably generated over the hollow space, a potential of the discharge electrode is varied as shown in Figures 7(a) and 7(b). These waveforms essentially correspond to the original waveform of the RF electric source. 2) When a bright plasma is partially generated in the hollow space, i.e., when the generated plasma includes a dark area, a potential of the discharge electrode is varied as shown in Figure 7(c). This waveform of Fig. 7(c) is subjected to some disturbance, in comparison with the original waveform of the RF electric source. 3) When a dark area of the generated plasma is varied to be alternately widened and narrowed over a relative long period, the waveform of Fig. 7(b) is shifted upward and downward in response to the alternate variation of the dark area. 4) Although the plasma is unstably generated, the disturbance of the waveform as shown in Fig. 7(c) does not appear, depending on the requirements of the discharge. From the results shown above, it can be understood that, when a potential of the discharge electrode is strongly biased to the negative side, the generation of plasma becomes unstable. Accordingly, during the formation of a thin film, it is necessary to maintain the potential of the discharge electrode as shown in Figs. 7(a) and 7(b). Figures 8(a) and 8(b) show a fourth embodiment of the present invention, which is basically similar to the thin film formation apparatus 10 as shown in Fig. 1. In Fig. 8(a) and 8(b), the features similar to those of Fig. 1 bear the same reference numerals and the features corresponding to those of Fig. 1 bear like reference numerals suffixed with a prime. The fourth embodiment is adapted to produce a photoreceptor drum used in the electrophotographic field. In the fourth embodiment, the reactor vessel 12 is constructed in a cylindrical form, and is shown by the phantom line in Fig. 8(a). The first electrode 20 is concentrically disposed within the cylindrical reactor vessel 12. As shown in Figs.8(a) and 8(b), the electrode 20 has a double-wall construction with an interior annular chamber. Namely, the electrode 12 includes an outer cylindrical wall portion 208a, an inner cylindrical wall portion 208b, and two annular end wall portions 208c, by which the interior annular chamber is defined. The inner cylindrical wall portion 208b has two hollow spaces or elongated grooves 208d formed therein and diametrically opposed to each other, and which are extended along the generatrix thereof. Each of the grooves 208d has a bottom wall section by which a part of each groove is defined and in which a plurality of opening holes 208e are formed, as shown in Fig. 8(b). These opening holes 208e are aligned with each other along the longitudinal axis of each groove 208d. In the fourth embodiment, the inlet pipe 18 for feeding a starting gas is extended through the cylindrical side wall portion of the reactor vessel 12, and is connected to the outer cylindrical side wall portion 208a of the first electrode 20 in such a manner that the inlet pipe 18 is in communication with the interior annular chamber of the first electrode 20. Also, the inlet pipe 18 is formed of a suitable conductive material and is connected to the RF electric source 26 through the impedance matching box 28 and the blocking capacitor 30. An outlet pipe 16′ is connected to the cylindrical side wall portion of the reactor vessel 12 so as to be in communication with the reactor vessel 12, and the outer end of the outlet pipe is connected to a vacuum pump (not shown). In the fourth embodiment, a substrate per se serves as the grounded electrode. For example, the substrate may comprise an aluminium drum blank D concentrically disposed within the cylindrical discharge electrode 20 and which is able to be rotated around the longitudinal axis thereof. In particular, a suitable electric motor 38 is disposed beneath the reactor vessel 12 so that an output shaft 38a of the motor 38 is extended through the bottom of the reactor vessel 12 along the longitudinal axis thereof, and the output shaft 38a of the motor 38 is provided with a sleevelike mount 40 for the drum blank D, a diameter of the mount 40 being the same as that of the drum blank D. The mount 40 is adapted to be cooperated with a sleevelike clamp member 42 so that the drum blank D is fixed on the mount 40 by the clamp member 42, a diameter of the clamp member 42 also being the same as that of the drum blank D. The mount 40 and the clamp member 42 are formed of a suitable conductive material, so that the mount 40 and clamp member 42 also form a part of the grounded electrode. As shown in Fig. 8(a), the mount 40, the drum blank D, and the clamp 42 are grounded through the shaft 38a of the motor 38. Further, each of the mount 40 and the clamp member 42 forms a cylindrical surface portion extended from the drum blank D, whereby an excellent thin film can be formed on the end sides of the drum blank D. The heater 25 for heating the substrate or the drum blank D comprises a spiral resistor element extended through the drum blank D. To bias a potential of the discharge electrode 20 toward the negative side, a discharge area of the first electrode 20, i.e., a peripheral area of the inner cylindrical wall portion 208b, must be larger than a peripheral area of the drum blank D (inclusive of the mount 40 and clamp 21). Accordingly, in the fourth embodiment, a pair of arcuate shield plate elements 44 is partially applied to the peripheral area of the inner cylindrical wall portion 208b of the first electrode 20, each of the shield elements 44 being grounded. As shown in Figs. 8(a) and 8(b), the shield plate elements 44 are diametrically disposed within the discharge electrode 20 so as to be close to the inner wall surface thereof, whereby the dicharge area can be restricted so that the potential of the discharge electrode 20 is biased toward the negative side. In operation, the starting gas fed by the inlet pipe 18 is once introduced into the interior annular chamber of the first electrode 20, and is then ejected from the holes 208e into the hollow space 208d. Accordingly, the high density plasma can be obtained in the hollow space 208d in the same manner as in the first embodiment of Fig. 1. During the operation, the drum blank D is rotated by the motor 38, whereby a thin film such as a photosensitive film can be uniformly formed on the drum blank D. Furthermore, a thin film formation apparatus corresponding to the fourth embodiment of Figs. 8(a) and 8(b) was actually assembled, and a formation of a B(boron)-doped a-Si:H thin film on an aluminum drum blank was carried. The assembled apparatus had the following characteristics: (A) Length of hollow space (208d) 468 mm (B) Width of hollow space (208d) 10 mm (C) Depth of hollow space (208d) 20 mm (D) Arcuate width of shield plate (44) 45 mm Note, in the assembled apparatus, the discharge electrode (20) is provided with four hollow spaces (208d) disposed at regular intervals, the four shield plates (44) are disposed between the four hollow spaces (208d), and the outer surface and the annular end faces are covered by a stainless shield. The actual thin film formation was carried out under the following conditions: (A) Flow rate of starting gas 100 sccm (B) Pressure within reactor vessel 6.7 Pa (0.05 torr) (C) Power of RF source 300 W (D) Temperature of drum blank 250°C Note, as a starting gas, a disilane (Si₂H₆) gas to which a diborane (B₂H₆) gas diluted with a helium (He) gas was added was used, and for an evaluation of a finished B-doped a-Si:H thin film, a sample substrate of quartz was attached to a part of the drum blank from which a part of the blank material was removed. It was observed that the growth rate of the B-doped a-Si:H thin film was 2.9 nm/s, and that the percentage of the starting gas consumption was 19 %. Also, the finished the B-doped a-Si:H thin film exhibited a resistance of 1.3 x 10¹¹Ω · cm in the dark, and exhibited a resistance of 1.3 x 10⁷Ω · cm under an irradiation of a white lamp. Accordingly, the produced drum is usable as a photoreceptor drum in the electrophotographic field. Figures 9(a) and 9(b) show a fifth embodiment of the present invention, which is generally similar to the thin film formation apparatus 10 shown in Fig. 1. In Figs. 9(a) and 9(b), the features similar to those of Fig. 1 are indicated by the same reference numerals and the features correspnding to those of Fig. 1 bear like reference numerals suffixed prime. In the fifth embodiment, the discharging electrode 20 comprises three discharge electrode elements 209a disposed side by side above the grounded electrode 22. As shown in Fig. 9(b), each of the discharge electrode elements 209a has an elongated rectangular parallelepiped shape and has a hollow space or groove 209b formed therein, which is extended along the longitudinal axis thereof. A plurality of opening holes 209c are formed in an elongated bottom of the groove 209b to be longitudinally aligned with each other. Note, the grounded electrode 22 is substantially identical to that of Fig. 1. The inlet pipe 18 for feeding a starting gas is extended through the top wall of the reactor vessel 12, and is connected to a header 14a′ having three elongated opening ports 14′, each of which is substantially coextended with the groove 209b. The discharge electrode elements 209a are attached to the header 14a′ so that the opening ports 14′ are closed by the discharge electrode elements 209a, respectively, as shown in Fig. 9(a), and so that each of the opening ports 14′ is in communication with the groove 209b of the corresponding electrode element 209a through the opening holes 209c thereof. The inlet pipe 18 and the header 14a′ are formed of a suitable conductive material, and are connected to the RF electric source 26 through the impedance matching box 28 and the blocking capacitor 30. The header 14a′ is covered by shield elements 24′ so that a discharge occurs only between the discharge electrode elements 209a and the grounded electrode 22. In operation, the starting gas fed by the inlet pipe 18 is once introduced into the header 14a′, and is then ejected from the opening holes 209c into the hollow spaces or grooves 209b through the opening ports 14′. Accordingly, the high density plasma can be obtained in each of the hollow spaces 209b in the same manner as in the first embodiment of Fig. 1. According to the fifth embodiment of Figs. 9(a) and 9(b), the apparatus is provided with more than one high density plasma zone within the reactor vessel, whereby the formation of a thin film can be uniformly carried out over a wider area. Furthermore, a thin film formation apparatus corresponding to the fifth embodiment of Figs. 9(a) and 9(b) was actually assembled, and the formation of a hydrogenated amorphous silicon (a-Si:H) thin film on a plurality of substrates was carried out. The assembled apparatus had the following characteristics: (A) Length of discharge electrode element (209a) 170 mm (B) Width of discharge electrode element (209a) 50 mm (C) Thickness of discharge electrode element (209a) 35 mm (D) Length of hollow space (209b) 150 mm (E) Width of hollow space (209b) 10 mm (F) Depth of hollow space (209b) 30 mm (G) Diameter of hole (209c) 1.5 mm (H) Pitch of holes (209c) 20 mm (I) Number of holes (209c) 7 (J) Pitch of discharge electrode elements (209a) 30 mm The actual thin film formation was carried out under the following conditions: (A) Starting gas SiH₄ (B) Flow rate of starting gas 20 sccm (C) Pressure within reactor vessel 67 Pa (0.5 torr) (D) Power of RF source 40 W (F) Substrate quartz (G) Size of substrate 40 x 40 mm (H) Number of substrates 16 (I) Temperature of substrate 250 °C (J) Operation time 20 min. Note, during the operation, a potential of the discharge electrode was observed as shown Fig. 10. It was observed that an average thickness of the produced a-Si:H thin film was 2.09 µm, and that a dispersion of the thickness was ± 0.22 µm. Accordingly, the growth rate of thin film is 6.26 µm/h on average, and the dispersion is 8 %. Also, It has been observed that a percentage of starting gas consumption was about 26 %. Note, Fig. 11 shows a thickness distribution of the thin film produced according to the present invention. Another formation of a thin film (a-Si:H) was been actually carried out in a conventional apparatus under the same conditions. It was observed that the growth rate of the produced thin film was 2.56 µm/h on average, that the dispersion was about 50 % on maximum, and that the percentage of starting gas consumption was about 10 %. Note, Fig. 12 shows a thickness distribution of the thin film produced according to the conventional apparatus. A comparison of the above results proves that the growth rate of thin film as well as the percentage of starting gas consumption are considerably improved by the present invention. Also, both the produced thin films were evaluated with respect to a photo- and dark-resistivity ratio (ρ d /ρ p ). Herein: ρ d is a resistance in the dark, and ρ p is a resitance under an irradiation of a white lamp (1 mW/cm²). When these thin film are used as a photoconductor for a solar cell, a photoreceptor drum or the like, the resistivity ratio (ρ d /ρ p ) is one of important factors. The results of the evaluation were as follows: Invention: 4.6 ≦Log(ρ d /ρ p ) ≦4.9 Prior Art: 4.7 ≦Log(ρ d /ρ p ) ≦5.1 Figures 13(a) and 13(b) show a sixth embodiment of the present invention, which is similar to the thin film formation apparatus shown in Figs. 8(a) and 8(b). In Figs. 13(a) and 13(b), the features similar to those of Figs. 8(a) and 8(b) bear the same reference numerals. In the sixth embodiment, the discharge electrode 20 comprises two identical discharge electrode elements 2013a diametrically disposed within the reactor vessel 12. Each of the discharge electrode elements 2013a is a hollow construction having a sector-shaped cross section, as shown in Fig. 13(a), and having a central recess 2013b formed in the cylindrical inner wall portion thereof, which is extended along the longitudinal axis of the reactor vessel 12. The central recess 2013b has a generally T-shaped cross section, the cross bar portion thereof having a shield plate element 2013c disposed therein, whereby each of the electrode elements 2013a is virtually divided into two discharge electrode sections 2013a′. Each of the discharge electrode sections 2013a′ has a hollow space or groove 2013e formed in the cylindrical inner wall portion thereof, which is extended along the longitudinal axis of the reactor vessel 12. As shown in Fig. 13(b), a plurality of opening holes 2013f are formed in the bottom of each groove 2013e and longitudinally aligned with each other. As shown in Fig. 13(a) and 13(b), the side faces of the discharge electrode elements 2013a are covered by shield plate elements 46, and the end faces of the discharge electrode elements 2013a are covered by shield plate elements 48. As shown in Fig. 13(a), each of the discharge electrode elements 2013a has an inlet pipe 18′ projected from the cylindrical outer wall portion thereof and extended through the cylindrical side wall portion of the reactor vessel 12. The inlet pipes 18′ are connected to the RF electric source 26 through the impedance matching box 28 and the blocking capacitor 28. In the sixth embodiment of Figs. 13(a) and 13(b), the grounded electrode may comprise an aluminum drum blank D, as in the fourth embodiment of Figs. 8(a) and 8(b). The drum blank D may be set at the central postion within the reactor vessel 12 in the same manner as in Figs. 8(a) and 8(b). In operation, the starting gas fed by each inlet pipe 18′ is once introduced into the corresponding electrode 2013a, and is then ejected from the opening holes 2013f into the hollow spaces 2013e thereof. Accordingly, the high density plasma can be obtained in each of the hollow spaces 2013e in the same manner as in the first embodiment of Fig. 1. Furthermore, a thin film formation apparatus corresponding to the sixth embodiment of Figs. 9(a) and 9(b) was actually assembled, and a formation of a p-type a-Si:H thin film on an aluminum drum blank was carried out. The assembled apparatus had the following characteristics: (A) Height of sector electrode (2013a) 560 mm (B) Height of central recess (2013b) 560 mm (C) Width of central recess (2013b) 14 mm (D) Depth of central recess (2013b) to shield (2013c) 21.5 mm (E) Length of hollow space (2013e) 400 mm (F) Width of hollow space (2013e) 14 mm (G) Depth of hollow space (2013e) 20 mm (H) Distance between drum blank (D) and sector electrode (2013a) 20 mm The actual thin film formation was carried out under the following conditions: (A) Starting gas Si₂H₆ (B) Flow rate of starting gas 100 sccm (C) Doping gas (helium-diluted: 1000ppm) B₂H₆ (D) Flow rate of starting gas 100 sccm (E) Pressure within reactor vessel 27 Pa (0.2 torr) (F) Power of RF source 250 W (G) Temperature of drum blank 300 °C (H) Operation time 20 min. During the operation, a potential of the discharge electrode was observed as shown in Fig. 14. Also, a thickness of the produced p-type a-Si:H thin fim exhibited a characteristic as shown in Fig. 15. Note, no high-molecular powder was generated during the operation. Figure 16 shows a seventh embodiment of the present invention, which is basically similar to the thin film apparatus 10 of Fig. 1. In Fig. 16, the feature similar to those of Fig. 1 are indicated by the same reference numerals, and the features corresponding to those of Fig. 1 bear like reference numerals suffixed with a prime. In the seventh embodiment, the discharging electrode 20 comprises a general rectangular parallelpiped-shaped electrode element 2016a preferably extended in a direction perpendicular to a plane of the drawing sheet. The discharge electrode element 2016a includes a top wall portion 2016a₁, a bottom wall portion 2616a₂, side wall portions 2016a₃, and end wall portions (not shown), formed of a suitable conductive material such as a stainless steel. The discharge electrode element 2016a has an inlet pipe 18′ integrally projected from the top wall portion 2016a₁ thereof, the inlet pipe 18′ being extended through and joined to a top wall portion 1216a of the reactor vessel 12 (partially illustrated) through the intermediary of an insulator element 23′, so that the discharge electrode 2013a is suspended from the top wall portion 1216 of the reactor vessel 12. As shown in Fig. 16, an O-ring seal 50 is applied to the inlet pipe 18′ at the through hole formed in the top wall portion 1216a for passing through the inlet pipe 18′, resulting in an airtight seal between the inlet pipe 18′ and the through hole therefor. The inlet pipe 18′ is led to a starting gas source (not shown), and is connected to the RF electric source 26 through the impedance matching box 28 and the blocking capacitor 30. The discharge electrode elements 2016a are covered by the shield 24 except for the bottom wall portion 2616a₂ thereof, an outer surface of the bottom wall portion 2616a₂ serving as a discharge area. The shield 24 is suspended from the top wall portion 1216a of the reactor vessel 12. An insulator element 52 is intervened between the shield 24 and the surfaces of the electrode element 2016a to be shielded, to prevent any contact therebetween. The grounded electrode 22, which is substantially the same as in Fig. 1, is installed on the bottom 1216b, and a substrate S is put on the grounded electrode 22. Although not illustrated, a suitable electric heater is incorporated into the grounded electrode 22 for heating the substrate. An outlet pipe 16′ is projected from the bottom 1216b, and is adapted to be connected to a suitable vacuum pump (not shown). The discharge electrode 2016a has a hollow space or groove 2016b formed in the bottom wall portion 2016a₂ thereof. The groove 2016b is extended along the longitudinal axis of the discharge electrode element 2016a, and a pluraliy of opening holes 2016c are formed in a bottom of the groove 2016b so as to be aligned with each other along the longitudinal axis thereof, whereby the groove 2016b is communicated with the interior chamber of the discharge electrode element 2016a. The seventh embodiment of Fig. 16 is characterized in that the outlet opening of the groove 2016b is restricted by opposed projections 2016d extending from opposed edges by which the outlet opening of the groove 2016b is defined. A space a between the opposed projections 2016d is suitably selected in accordance with factors such as the kind of the starting gas used, the pressure within the reactor vessel 12, and the like, so that a discharge can occur between the opposed projections 2016d. Namely, the space a cannot be so narrowed that no discharge occurs therein. In operation, the starting gas is fed into the interior chamber of the discharge electrode element 2016a through the inlet pipe 18′, and is then introduced into the hollow space or groove 2016 through the opening holes 2016c. Accordingly, the high density plasma zone can be obtained in the hollow space 2016b in the same manner as in the first embodiment of Fig. 1. According to the seventh embodiment of Fig. 16, however, the density of plasma can be further enhanced because electrons caused by the dissociation of the starting gas molecules and by collisions between ions and the groove surfaces can be confined in the groove 2016b by the opposed projections 2016d. In particular, as mentioned hereinbefore, the electrons within the hollow space or groove are rapidly vibrated between the groove side surfaces (see arrow A in Fig. 16), so that when the electrons escape from the groove 2016b, they run obliquely toward a plane of the opening outlet of the groove 2016b (see arrow B in Fig. 16). Accordingly, the opposed projections 2016d are able to effectively confine the electrons in the groove. On the other hand, the active species dissociated from the starting gas molecules is very heavy in comparison with the electrons, so that the active species can pass through the opening outlet of the groove 2016b without being subjected to an active electric influence (see arrow C in Fig. 16). In brief, according to the seventh embodiment, the growth rate of thin film and the rate of starting gas consumption can be further facilitated. Figures 17(a) and 17(b) show an eighth embodiment of the present invention, which is similar to the fourth invention of Figs. 8(a) and 8(b) but has the feature of the seventh embodiment, as mentioned above, incorporated therein. In Figs. 17(a) and 17(b), the feature similar to those of Figs. 8(a) and 8(b) are indicated by the same reference numerals. In the eighth embodiment, the discharge electrode 20 comprises a double-wall cylindrical electrode element 2017a concentrically disposed within the cylindrical reactor vessel 12, and including a cylindrical inner wall portion 2017a₁, a cylindrical outer wall portion 2017a₂, and annular end wall portions 2017a₃ (Fig. 17(b). These wall portions define an annular chamber 2017b′, and are formed of a suitable conductive material such as a stainless steel. The discharge electrode element 2017a has an inlet pipe 18′ integrally projected from the outer wall portion 2017a₂ thereof, the inlet pipe 18′ being extended through and joined to the outer wall portion 2017a₂ of the reactor vessel 12 through the intermediary of an insulator element 23′. The inlet pipe 18′ is led to a starting gas source (not shown), and is connected to the RF electric source 26 through the impedance matching box 28 and the blocking capacitor 30. The annular end wall portions 2017a₃ of the discharge electrode element 2017a are covered by two annular shield plates 24′ integrally joined to the cylindrical inner wall surface of the reactor vessel 12. In this embodiment, the cylindrical side wall of the reactor vessel 12 serves as a shield for the cylindrical outer wall portion 2017a₂ of the electrode element 2017a because of a closeness therebetween. The discharge electrode element 2017a has four hollow spaces or grooves 2017b formed in the cylindrical inner wall portion 2017a₁ thereof, which is extended along the longitudinal axis of the reactor vessel 12. As shown in Fig. 17(b), a plurality of opening holes 2017c are formed in the bottom of each groove 2017c to be longitudinally aligned with each other. An elongated opening outlet of each groove 2017c is restricted by opposed projections 2017d extending from opposed edges by which the outlet opening of the groove 2017c is defined, whereby the electrons can be confined in the groove 2017c, as mentioned in the seventh embodiment of Fig. 16. An outlet pipe 16′ is projected from the cylindrical outer wall of the reactor vessel 12, and is adapted to be connected to a suitable vacuum pump (not shown) through an adjustable valve 54. The grounded electrode 22 comprises a drum-like electrode element 2217 concentrically disposed within the double wall cylindrical electrode element 2017a and formed of a suitable conductive material such as aluminum. The grounded drum-like electrode element 2217 may be supported in the same manner as in the fourth embodiment of Figs. 8(a) and 8(b). Namely, the drum-like electrode element 2217 is fixed on the sleeve-like mount 40 by the sleeve-like clamp member 42, and is rotated by a suitable motor (not shown). In this embodiment, a plurality of substrates S are detachably attached to the drum-like electrode element 2217 at predetermined locations where a part of the material thereof is removed. As shown in Fig. 17(b), a pair of small plate-like holders 56 is secured to each of the material-removed regions for attaching the substrates S, so that the pair of plate-like holders 56 partially overlap the corresponding region to form a pair of channels for holding the substrate S. As shown in Fig. 17(a), the four arcuate shield plate elements 44 are partially applied to the cylindrical inner wall surface of the discharge electrode element 2017a so as to be diametrically disposed at regular intervals, whereby the discharge area is restricted so that a potential of the discharge electrode 20 can be biased toward the negative side. In operation, the starting gas is fed into the annular chamber 2017b′ of the discharge electrode element 2017a through the inlet pipe 18′, and is then introduced into the hollow space or groove 2017b through the opening holes 2017c. Accordingly, a higher density plasma zone can be obtained in the hollow spaces 2017b in the same manner as in the seventh embodiment of Fig. 16. A thin film formation apparatus corresponding to the eighth embodiment of Figs. 17(a) and 17(b) was actually assembled, and a formation of an a-Si:H thin film on substrates was carried out. The assembled apparatus had the following characteristics: (A) Width of hollow space (2017b) 7 mm (B) Depth of hollow space (2017b) 30 mm Note, in the assembly apparatus, a projected length of the opposed projections (2017d) was discretely varied: Thin film formations were carried out under the following condition: (A) Power of RF source 300 W (B) Pressure within reactor vessel 6.7 Pa (50 mtorr) Also, as the starting gas, a Si₂H₆ gas and a mixed gas of Si₂H₆ and He (1:1) were used. A flow rate of the Si₂H₆ gas was 100 sccm, and a flow rate of the mixied gas of Si₂H₆ and He was 200 sccm. As a projected length of the opposed projections (2017d), three lengths of 0 mm, 1.0 mm, and 1.5 mm were selected. The produced a-Si:H thin films were evaluated with respect to the growth rate of thin film, and the results are shown in Fig. 18. As apparent from Fig. 18, when the projected length X is 1.5 mm, the rate of thin film formation is maximum, and when the projected length X is 2.0 mm, the growth rate of thin film is remarkably lowered. This means that the opening outlet of the hollow space is too restricted and thus the discharge can occur only with difficulty. Figure 19 shows a ninth embodiment of the present invention, which is constructed substantially in the same manner as the seventh embodiment of Fig. 16, except for a part thereof. In particular, both embodiments differ from each other only in that the opposed projections 2016d are used for confining the electrons in the hollow space 2016b in the seventh embodiment of Fig. 16, whereas magnetic elements 58 are used for the same purpose in this ninth embodiment. Accordingly, in Fig. 19, the same reference numerals as in Fig. 16 are used to indicate the same elements. The magnetic elements 58 are disposed within the interior chamber of the discharge electrode element 2016a to surround the opening outlet of the hollow space 2016b, whereby the electrons within the hollow space 2016b are subjected to the Lorentz force and confined therein. Furthermore, the electrons are subjected to a spiral motion due to the magnetic elements 58, so that the probablity of collision between the electrons and starting gas molecules can be further raised, whereby the growth rate of thin film and the percentage of starting gas consumption can be considerably enhanced. Figure 20(a) and (b) show a tenth embodiment of the present invention, which is constructed substantially in the same manner as the eighth embodiment of Figs. 17(a) and 17(b), except for a part thereof. In particular, both embodiments differ from each other only in that the opposed projections 2017d are used for confining the electrons in the hollow spaces 2017b in the eighth embodiment of Figs. 17(a) and 17(b) whereas magnetic elements 60 are used for the same purpose in this tenth embodiment. Accordingly, in Fig. 20, the same reference numerals as in Figs. 17(a) and 17(b) are used to indicate the same elements. The magnetic elements 60 are disposed within the interior annular chamber 2017b′ of the discharge electrode element 2017a to surround the opening outlet of each hollow spaces 2017b. A thin film formation apparatus corresponding to the eighth embodiment of Figs. 20(a) and 20(b) was actually assembled, and a formation of an a-Si:H thin film on substrates was carried out. The assembled apparatus had the following characteristics: (A) Width of hollow space (2017b) 8 mm (B) Depth of hollow space (2017b) 30 mm A thin film formation was carried out under the following conditions: (A) Power of RF source 800 W (B) Pressure within reactor vessel 6.7 Pa (50 mtorr) Also, as the starting gas, a Si₂H₆ gas and a mixed gas of Si₂H₆ and He (1:1) were used. A flow rate of the Si₂H₆ gas was 100 sccm, and a flow rate of the mixied gas of Si₂H₆ and He was 200 sccm. Formation of another thin film was carried out under the same conditions as mentioned above except that the magnetic elements (60) were not used. The produced a-Si:H thin films were evaluated with respect to the growth rate of thin film, and the results are shown in the following table: magnetic mon-magnetic Si₂H₆ 7.1 µm/h 3.6 µm/h Si₂H₆+He 4.3 µm/h 1.2 µm/h Although the embodiments of the present invention are especially explained in relation to an a-Si:H thin film formation, obviously, the present invention can be applied to a formation of another type of thin film such as a-SiC:H, a-SiGe:H, a-SiN:H, micro-crystallized silicon, a-C:H, Si 1-x N x (0 < x < 1), Si 1-x O x (0 < x < 1), W₂Si₃, BN, B₄C, SiC, and the like.

Claims: An apparatus for forming a thin film on a substrate, comprising: a reactor vessel (12); a first electrode (20; 32; 34) and a second electrode (22) opposably disposed within said reactor vessel; an introduction means (14, 18) for introducing a starting gas into said reactor vessel through a hollow space (20c) of said first electrode (20; 32; 34); a circuit means including a radio frequency electric source (26) for applying a radio frequency voltage between said first electrode (20) and said second electrode (22) to produce a discharge plasma zone therebetween for dissociating the starting gas; said circuit means being arranged so that said first electrode (20; 32; 34) serves as a discharge electrode and said second electrode (22) is grounded; characterized in that said first electrode includes walls defining said hollow space (20c) with an outlet that is open to said second electrode, and said circuit means further includes a biasing means (30; 30') for biasing a potential of said first electrode toward the negative side, whereby a high density plasma is produced in said hollow space (20c) and dissociation of starting gas introduced therein is facilitated. An apparatus as set forth in claim 1, wherein said biasing means comprises a blocking capacitor (30) incorporated into said circuit means and said first electrode (20; 32; 34) has a discharge area smaller than an area of said second electrode (22) opposed to the discharge area of said first electrode. An apparatus as set forth in claim 1, wherein said biasing means comprises a direct current voltage source (30') incorporated into said circuit means. An apparatus as set forth in claim 1, wherein the hollow space (20c) of said first electrode (20) is formed therein in such a manner that the material thereof is removed, said introduction means including at least one opening hole (20e) formed in a bottom of said hollow space (20c) for ejecting the starting gas therethrough. An apparatus as set forth in claim 1, wherein the hollow space of said first electrode (32) is formed by attaching a sleeve element (32c) to said first electrode, said introduction means including an opening hole which is formed in said first electrode for ejecting the starting gas therethrough. An apparatus as set forth in claim 1, wherein the hollow space of said first electrode is defined by a conductive mesh element (36b) attached to said first electrode, said introduction means including a pipe element (36c1, 36c2) which surrounds said conductive mesh element, and which has a plurality of opening holes formed therein for ejecting the starting gas therethrough and into the hollow space. An apparatus as set forth in claim 1, wherein said first electrode (20) has a hollow construction in which an interior chamber is formed as a plenum chamber for the starting gas, the hollow space of said first electrode being formed as a groove (208d) recessed in a wall of said hollow construction thereof, said introduction means including at least one opening hole (208e) formed in a bottom of said groove for intercommunicating the plenum and the groove. An apparatus as set forth in claim 1, wherein said first electrode (20) comprises at least two electrode elements (2013a), each having a sector-shaped hollow construction, said second electrode being formed as a drum-shaped element, and being adapted to be rotated, the sector-shaped electrode elements being circumferentially and concentrically disposed around said drum-shaped element at regular intervals, each of the sector-shaped electrode elements having at least one hollow space formed as a groove (2013e) recessed in an inner wall of the hollow construction thereof, and said introduction means including at least one opening hole formed in a bottom of said groove for ejecting the starting gas therethrough. An apparatus as set forth in claim 1, wherein said first electrode (20) has an annular-shaped hollow construction (2017a1, 2017a2), said second electrode (22) being formed as a drum-shaped element (2217), and being adapted to be rotated, said second electrode being circumferentially and concentrically disposed within the annular-shaped hollow construction of said first electrode, the hollow space of said first electrode being formed as at least one groove (2017b) recessed in an inner wall of the hollow construction thereof, said introduction means including at least one opening hole (2017c) formed in a bottom of said groove for ejecting the staring gas therethrough. An apparatus as set forth in claim 1, wherein said first electrode (20) comprises more than one discharging electrode element (209a), each of said discharge electrode elements having a hollow space (209b) formed therein in such a manner that the material thereof is removed, said introduction means including at least one opening hole formed in a bottom of the hollow space of each discharge electrode element for ejecting the starting gas therein. An apparatus as set forth in claim 1, wherein the opening outlet of the hollow space of said first electrode (20), which is opened to said second electrode (22) is restricted by opposed projections (2016d) formed as a part of said first electrode, whereby the electrons included in the high density plasma produced in said hollow space (2016b) are confined therein. An apparatus as set forth in claim 1, wherein said first electrode (20) includes at least one magnet element (60) disposed to surround the opening outlet of the hollow space thereof, and opened to said second electrode (22), whereby the electrons included in the high density plasma produced in said hollow space not only can be confined therein, but also can be subjected to a spin motion due to the presence of the magnet element.

Inventors: ARAKI SHIN, KAMAJI HIDEKI, ARAKI, SHIN, KAMAJI, HIDEKI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1989
Main classification: C23C16
IPC codes: H01J37, C23C16