Abstract:
To provide a microwave plasma-treating apparatus which is capable of generating plasma having a high degree of uniformity of density and a high density for executing large-quantity and high-speed processing, capable of generating plasma of a large area, and capable of preventing dielectric windows from being thermally broken despite the apparatus being operated with large electric power for extended periods of time. The microwave plasma-treating apparatus includes a waveguide arranged to feed microwave electric power, a plurality of microwave coupling holes formed in the waveguide in the axial direction thereof, a dielectric member of a piece of plate capable of transmitting microwaves arranged in the waveguide in the axial direction thereof under the microwave coupling holes, a gap formed between the plurality of microwave coupling holes and the dielectric member, and a cooling member for cooling the dielectric member. Desirably, the microwave coupling holes have an annular shape.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to a microwave plasma-treating apparatus for treatment of a substrate, such as chemical vapor deposition (CVD), etching or asking (resist-asking treatment) using plasma generated by microwave electric power. 
       BACKGROUND ART 
       [0002]    In the processes for producing semiconductors and micro machines, plasma has been widely used that is generated when energy is supplied from an external source to a reaction gas. In particular in recent years, it is necessary to develop an apparatus capable of treating substrates of large areas with the plasma to meet the demands for mass-production of wafers of a large diameter. 
         [0003]    Under such circumstances, attention has now been given to a plasma-treating apparatus that generates and sustains plasma by using surface waves generated by introducing microwave electric power into a discharge chamber on account of the following characteristics (1)-(3). The surface wave is an electromagnetic wave propagating along interfaces between two media with different permittivities. In surface waves propagating along the interface between a dielectric and a overdense plasma with densities higher than the cut-off density (7.4×10 10  cm −3  for 2.45 GHz), the energy of microwaves is concentrated in a region near the surface of the plasma and attenuates in a direction toward the interior of the plasma. 
         [0000]    (1) Overdense plasma with densities higher than the cut of density is possible without the application of an external magnetic field.
 
(2) Microwaves (2.45 GHZ) can be utilized, magnetron is inexpensive, impedance matching is easy, no electrode is necessary for generating plasma, and electrodeless operation is possible.
 
(3) Because the plasma is generated by the surface waves, i.e., surface wave plasma (SWP), a large-area plasma with the high uniformity and highly density (˜10 12  cm −3 ) can be obtained even under low pressure (&lt;1 Torr (1 Torr=133 Pa)).
 
         [0004]      FIGS. 14 and 15  illustrate a conventional microwave surface-wave plasma-treating apparatus developed for large-area processing utilizing the above-mentioned characteristics. 
         [0005]      FIG. 14  is a view schematically illustrating a plasma-treating apparatus cut along the axial direction of the waveguide, and  FIG. 15  is a sectional view of the same plasma-treating apparatus cut along A-A in  FIG. 14 . 
         [0006]    The microwave plasma-treating apparatus  60  shown in  FIGS. 14 and 15  has three waveguides  20 ,  30  and  40  arranged in parallel on the upper surface of a plasma-treating chamber  16 , and each of the waveguides  20 ,  30  and  40  has three coupling holes along the axial direction thereof. 
         [0007]    The coupling holes are formed as circular radiation holes for radiating the microwave electric power into the plasma-treating chamber  16 . Referring to  FIG. 14 , three coupling holes  21   a ,  21   b  and  21   c  are formed in the waveguide  20  maintaining an equal distance in the axial direction thereof. The coupling holes  21   a ,  21   b  and  21   c  are circular holes of which the diameters are gradually increasing from the side of feeding the microwave electric power toward the front end side of the waveguide  20 . 
         [0008]    Though  FIG. 14  shows the waveguide  20  and its coupling holes  21   a ,  21   b  and  21   c  only, the waveguide  30  is similarly provided with coupling holes  31   a ,  31   b  and  31   c  (coupling holes  31   a ,  31   c  are not shown) and the waveguide  40  is provided with coupling holes  41   a ,  41   b  and  41   c  (coupling holes  41   a ,  41   c  are not shown). 
         [0009]    On the other hand, circular holes or square holes are formed in the upper wall of the plasma-generating chamber  16  so as to be communicated with each of the coupling holes. Dielectric windows (quartz plates, alumina plates, etc.) are fitted to the hole edges of each of the circular holes to constitute dielectric windows  50  that shut off the plasma-generating chamber  16  from the atmosphere. 
         [0010]    In a metallic rectangular discharge chamber  12 , there are arranged a substrate stage  10  on which a substrate  13  is placed, a gas exhaust port  15  for setting the interior of the treating chamber  16  to a required degree of vacuum, and a gas introduction port  14  for feeding a required reaction gas. 
         [0011]    [Patent document 1] JP-A-2002-280196 
         [0012]    [Patent document 2] JP-A-10-60657 
       DISCLOSURE OF THE INVENTION 
       [0013]    To realize a high-speed processing, a plasma-treating apparatus is necessary that is capable of providing a high plasma density and a high radical density. For this purpose, it is desired that the plasma apparatus can be operated with large electric power. However, according to the conventional microwave plasma-treating apparatus, an increase in the microwave electric power was accompanied by a problem in that the dielectric windows  50  were broken for the following reasons. 
         [0000]    (Reason 1) Heat of the dielectric windows  50  radiates due to natural cooling. If the microwave electric power becomes strong, the temperature of the dielectric windows  50  increases due to heat of the plasma generated in the plasma-treating chamber  16 , and the dielectric windows  50  are thermally broken.
 
(Reason 2) Conventional coupling holes have a circular or square shape, and the microwave electric power intensely concentrate just under the coupling holes causing the dielectric windows  50  to be locally heated and thermally broken.
 
(Reason 3) As the microwave electric power becomes intense, abnormal discharge (arc discharge) occurs in the coupling holes locally causing thermal damage to the dielectric windows  50  fitted to the edges of the coupling holes and, eventually, causing the dielectric windows  50  to be broken.
 
(Reason 4) Heat of the waveguides  20 ,  30  and  40  and heat of the rectangular discharge chamber  12  are removed by cooling water, etc. In the dielectric windows  50 , therefore, the portions where the waveguides  20 ,  30 ,  40  are contacted to the rectangular discharge chamber  12  radiate heat well and assume a low temperature. However, at portions of the dielectrics just under the coupling holes, the microwaves are concentrated and the temperature is high. Accordingly, the dielectric windows  50  develop a large temperature gradient and are thermally broken.
 
         [0014]    Patent document 2, on the other hand, discloses a microwave plasma-treating apparatus in which a gap is formed between the dielectric windows and the microwave coupling holes, and the air is flown through the gap; i.e., even when microwaves of large power is fed to conduct a high-speed processing, damage to the dielectric windows is prevented owing to the supply of a small amount of cooling gas. 
         [0015]    In this plasma apparatus, the air is introduced to effect the cooling by the air itself, and heat is transferred in increased amounts from the dielectric windows to the metal walls surrounding the gap due to the flow of air into the gap of a small area. Therefore, the dielectric windows heated by the generation of plasma can be effectively cooled, and the above problems (reason 1) and (reason 4) can be solved to some extent. 
         [0016]    However, in conducting the operation with large electric power for extended periods of time, even if the dielectric windows are cooled, the temperature gradient increases in the dielectric windows due to local concentrations of the microwave electric power and heat, and the dielectric windows can be cracked. 
         [0017]    The present invention was accomplished in view of the above-mentioned circumstances, and its object is to provide a microwave plasma-treating apparatus which is capable of generating plasma having a high degree of uniformity of density and a high density for executing a large-quantity and high-speed processing, capable of generating plasma of a large area, and is still capable of preventing the dielectric windows from being thermally broken despite the apparatus is operated with large electric power for extended periods of time. 
       MEANS FOR SOLVING THE PROBLEMS 
       [0018]    In order to achieve the above object, the present inventors have studied the causes of breakage to the dielectric windows and have learned the following. 
         [0019]    When the microwave plasma-treating apparatus is operated with large electric power for extended periods of time, the dielectric windows are quickly and markedly heated at positions near the portions where plasma intensely generated by the microwave electric power. Therefore, the conventional dielectric windows provided for every microwave coupling hole as shown in  FIG. 16  are not only entirely heated at high temperatures but are also locally heated at very high temperatures. As a result, the temperature gradient increases in the dielectric windows, thermal stress generates, and the dielectric windows are broken. 
         [0020]    From another point of view, another cause of damage to the dielectric windows stems from that in the plasma-treating apparatus, the outer walls of the waveguides and the discharge chamber are radiating heat due to natural air-cooling or water-cooling. In the conventional circular or square coupling holes formed in the lower surfaces of the waveguides, on the other hand, the metal plate around the coupling holes is locally very highly heated due to the concentration of microwaves. Therefore, if the dielectric windows are in contact with the metal portions that are surrounding them, vicinities of the coupling holes are locally very highly heated while other portions radiate the heat. As a result, the temperature gradient becomes great in the dielectrics, and thermal stress generates causing the dielectric windows to be broken. 
         [0021]    From another point of view, a further cause of damage to the dielectric windows stems from that when microwaves of large electric power is fed, microwaves are intensely concentrated in the conventional coupling holes of the circular shape or the square shape, abnormal discharge (arc discharge) occurs in the coupling holes, and the dielectric windows fitted to the edges of the coupling holes are locally damaged by heat. As a result, the dielectric windows air-tightly sealing the interior of the discharge chamber to be vacuum can no longer withstand the pressure of the atmosphere and can be broken. 
         [0022]    The present inventors, after a great deal of consideration based on the above knowledge, discovered that the heat gradient can be decreased by using dielectric windows of a piece of plate capable of transmitting microwaves arranged in the waveguide in the axial direction thereof under the microwave coupling holes. In order to spatially and widely radiate the microwaves to relax the local concentration of microwaves, further, the inventors have discovered that the microwave coupling holes are better formed in an annular shape. The inventors have, further, discovered that it is better to provide a gap so that the dielectric windows do not come in contact with the metallic wall, and to provide a cooling member for flowing a cooling gas by suitably forming a gas feed port and an exhaust port in the wall of the waveguide or the metallic wall surrounding the dielectric windows. 
         [0023]    The present invention was completed based on the above knowledge, and provides a plasma-treating apparatus which generates plasma in a treating chamber by feeding microwave electric power thereto, which, according to the invention of claim  1 , includes 
         [0024]    a waveguide arranged to feed microwave electric power, 
         [0025]    a plurality of microwave coupling holes formed in the waveguide in the axial direction thereof, 
         [0026]    a dielectric member of a piece of plate capable of transmitting microwaves arranged in the waveguide in the axial direction thereof under the microwave coupling holes, and 
         [0027]    a cooling member for cooling the dielectric member. 
         [0028]    The invention of claim  2  includes: 
         [0029]    a plurality of waveguides arranged in parallel maintaining a predetermined distance to feed microwave electric power, 
         [0030]    a plurality of microwave coupling holes formed in the waveguides in the axial direction thereof, 
         [0031]    dielectric members of a piece of plate capable of transmitting microwaves arranged in the waveguides in the axial direction thereof under the plurality of microwave coupling holes and having a width nearly equal to the width for introducing microwaves of the waveguides, and 
         [0032]    a cooling member for cooling the dielectric members. 
         [0033]    The invention of claim  3  relates to the microwave plasma-treating apparatus of claim  1  or  2 , wherein the plurality of microwave coupling holes have an annular shape. 
         [0034]    The invention of claim  4  relates to the microwave plasma-treating apparatus of claim  3 , wherein the plurality of microwave coupling holes are so adjusted that their areas and/or their distance of arrangement work to control the spatial distribution of plasma density near the dielectric members to thereby relax the temperature gradient of the dielectric members in the axial direction of the waveguides and to increase the degree of uniformity of plasma density in the treating apparatus. 
         [0035]    The invention of claim  5  relates to the microwave plasma-treating apparatus of claim  3  or  4 , wherein the plurality of microwave coupling holes are arranged maintaining a distance of a half wavelength of standing waves of the microwaves formed in the waveguides. 
         [0036]    In the invention of claim  6 , the cooling member flows a cooling gas into a gap formed between the plurality of microwave coupling holes and the dielectric members. 
         [0037]    The invention of claim  7  relates to the microwave plasma-treating apparatus of claim  1  or  2 , wherein a gap is formed surrounding the dielectric members, and a metal mesh is provided in the gap. 
       EFFECTS OF THE INVENTION 
       [0038]    The present invention greatly suppresses the occurrence of problems that, so far, caused dielectric windows to be thermally broken during the operation with large electric power, i.e., greatly suppresses the occurrence of problems such as local and abnormal electric discharge, overheating of the dielectric windows and large temperature gradient in the dielectric windows, and makes it possible to provide a microwave plasma apparatus that can conduct the operation with large electric power. Therefore, there is provided a plasma-treating apparatus featuring high degree of uniformity of density, high density and large area to realize a large-quantity and high-speed processing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]      FIG. 1  is a view illustrating the constitution of a plasma-treating apparatus according to an embodiment of the invention. 
           [0040]      FIG. 2  is a view of when the plasma-treating apparatus of  FIG. 1  is viewed from the upper side. 
           [0041]      FIG. 3  is a view schematically illustrating coupling holes of annular shapes. 
           [0042]      FIG. 4  is a view showing the shape of a coupling hole used in the embodiment of the invention. 
           [0043]      FIG. 5  is a view showing an embodiment of a combination type I of coupling holes of the invention. 
           [0044]      FIG. 6  is a view showing an embodiment of a combination type II of coupling holes of the invention. 
           [0045]      FIG. 7  is a view showing an embodiment of a combination type III of coupling holes of the invention. 
           [0046]      FIG. 8  is a view showing an embodiment of a combination type IV of coupling holes of the invention. 
           [0047]      FIG. 9  is a view showing an embodiment of a combination type V of coupling holes of the invention. 
           [0048]      FIG. 10  is a view showing an embodiment of a combination type VI of coupling holes of the invention. 
           [0049]      FIG. 11  is a view illustrating the constitution of the plasma-treating apparatus according to another embodiment of the invention. 
           [0050]      FIG. 12  is a view of the constitution of when the plasma-treating apparatus of  FIG. 11  is viewed from the upper side. 
           [0051]      FIG. 13  is a diagram showing the experimental results examining changes in the plasma density depending upon the types of arrangement five ring-shaped coupling holes having different sizes (outer diameters and inner diameters). 
           [0052]      FIG. 14  is a view illustrating the constitution of a conventional microwave plasma-treating apparatus. 
           [0053]      FIG. 15  is a sectional view cut along the line A-A in  FIG. 1 . 
           [0054]      FIG. 16  is a view of when a conventional plasma-treating apparatus having dielectric windows for each of the microwave coupling holes is viewed from the upper side. 
       
    
    
       [0000]    
       
           10  substrate stage 
           12  rectangular discharge chamber 
           13  substrate 
           14  gas introduction port 
           15  gas exhaust port 
           16  treating chamber 
           20 ,  20 ,  40  waveguides 
           21   a ,  21   b ,  21   c ,  31   b ,  41   b  coupling holes 
           22  cylindrical discharge chamber 
           50  dielectric windows 
           71   a ,  71   b ,  71   c ,  71   d ,  71   e  coupling hoes 
           72  rectangular dielectric windows 
           73  fan motor 
           75  cooling gas feed holes 
           76  cooling gas exhaust holes 
           77 ,  78  gaps 
           81   a ,  81   b ,  81   c ,  81   d  coupling holes 
           82  circular dielectric windows 
           88  dielectric window cover 
       
     
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0074]    Embodiments of the invention will now be described with reference to the drawings. 
         [0075]      FIG. 1  schematically illustrates an embodiment of a microwave plasma-treating apparatus of the invention, and is a sectional view of the apparatus cut along the axial direction of the waveguide.  FIG. 2  is a view showing the arrangement of the waveguides and the positions of the coupling holes of when the microwave plasma-treating apparatus  70  is viewed from the upper side. In the drawings, reference numeral  10  denotes a substrate stage,  12  denotes a rectangular discharge chamber,  13  denotes a substrate,  14  denotes a gas introduction port,  15  denotes a gas exhaust port,  16  denotes a treating chamber,  20  denotes a waveguide,  71   a  to  71   e  denote microwave coupling holes,  72  denotes rectangular dielectric windows,  73  denotes a fan motor,  75  denotes cooling gas feed holes,  76  denotes cooling gas exhaust holes, and  77  denotes a gap. 
         [0076]    The apparatus shown in  FIGS. 1 and 2  and its functions will now be described in detail. 
       (Formation of Plasma) 
       [0077]    Microwaves introduced from a power source into a waveguide  20  are reflected by an end of the waveguide  20  and form standing waves in the waveguide  20 . The microwaves are introduced into a discharge chamber  12  passing through the microwave coupling holes  71   a ,  71   b ,  71   c ,  71   d ,  71   e  formed in the lower surface of the waveguide  20 , through the gap  77  and through the dielectric windows  72 , and forms plasma near the dielectric windows  72 . The formed plasma spreads over the whole surfaces of the dielectric windows  72  as the surface waves propagate. Based on this principle, the plasma having a uniform distribution of density, forms on each of the dielectric windows. 
         [0078]    The plasma generated near the dielectric windows diffuses and spreads over the whole treating chamber  16 . Therefore, the plasma having a high degree of uniformity of density, high density and large area is fed onto the upper surface of the substrate  13 . 
       (Dielectric Windows and Gap) 
       [0079]    One of the features of the invention is to use a piece of dielectric windows  72  for a plurality of coupling holes formed in the waveguide in the axial direction thereof. 
         [0080]    This makes it possible to relax the temperature gradient in the axial direction of the waveguide  20 , i.e., in the axial direction of the dielectric windows  72 . With this structure, further, a cooling gas flows well into the gap  77  formed between the dielectric windows  72  and the coupling holes  71  enabling the heat of the dielectric windows to be easily radiated and the cooling efficiency to be improved. As a result, it is successful in greatly decreasing the thermal breakage to the dielectric windows  72 . 
         [0081]    Moreover, it is possible to improve the problem of non-uniformity in the plasma density caused by the dielectric windows in the waveguide in the axial direction thereof and by the metal frame relative to the dielectric windows inherent in the prior art (patent document 1). That is, the uniformity of plasma density is improved as the surface waves spread over the whole surfaces of the dielectric windows along the boundaries of the dielectric windows in the plasma. 
       (Coupling Holes) 
       [0082]    Another feature of the present invention is use of coupling holes of an annular shape as the microwave coupling holes. 
         [0083]      FIG. 3  is a view schematically illustrating annular shapes in the invention. In the invention as shown, there is no particular limitation on the “annular” shape which, therefore, may be, for example, a circular shape, an elliptic shape, a square shape or a rectangular shape. In effect, the coupling holes should have any island-shaped shield at the central portions. 
         [0084]    It is difficult to form a completely annular shape or, in other words, island. Therefore, in practice the shape of the coupling hole is produced being divided into a plurality of holes of not less than two. As a concrete example,  FIG. 4  shows a microwave coupling hole constituted being divided into four holes. 
         [0085]    In the present invention, the microwave coupling holes are formed in an annular shape making it possible to uniformly radiate the microwave electric field spatially in all directions as compared to the circular or square coupling holes in the conventional microwave-treating apparatus. As a result, it is made possible to prevent the microwave electric field from being locally concentrated even in the operation with large electric power, and the microwave coupling holes and the dielectric windows are not damaged by abnormal discharge (arc discharge). 
         [0086]    Further, the plurality of microwave coupling holes are so adjusted that their distance of arrangement work to control the spatial distribution of plasma density near the dielectric members to thereby relax the temperature gradient of the dielectric members in the axial direction of the waveguides and, therefore, to increase the degree of uniformity of plasma density in the treating apparatus. 
         [0087]    For example, the ring-shaped coupling holes  71   a ,  71   b ,  71   c ,  71   d  and  71   e  are arranged maintaining a distance of a half wavelength of the standing waves of the microwaves formed in the waveguide  20 . Namely, the center of ring of each coupling hole is arranged at a position where the electric field of the standing wave becomes a peak so that the ring-shaped coupling hole cuts the surface current flowing into the lower surface of the waveguide in a vertical direction. 
         [0088]    Further, the areas of the plurality of coupling holes  71   a ,  71   b ,  71   c ,  71   d  and  71   e  formed in the waveguide in the axial direction thereof may all be the same but may be varied depending on the operating conditions (kind of gas, pressure, microwave electric power, etc.) of the plasma-treating apparatus to vary the coupling coefficients between the microwave electric power and the plasma in the coupling holes and to control the spatial distribution of plasma density near the dielectric windows  72 . This makes it possible to improve the uniformity of plasma density in the treating chamber  16 . The areas of the coupling holes can be easily controlled by controlling the outer diameters or the inner diameters (or both diameters) of the rings. 
         [0089]    Table 1 shows examples of combinations of the coupling holes. For the microwave frequency of 2.45 GHz, the half wavelength is 80 mm in a rectangular waveguide of which the interior is a cavity (specific inductivity is assumed to be 1) having a width of 96 mm and a height of 27 mm in cross section. Therefore, the outer diameters and inner diameters of the coupling holes shown in Table have been determined by taking these values into consideration. 
         [0090]    Combination types I to VI of the microwave coupling holes shown in Table 1 are schematically shown in  FIGS. 5 to 10 . 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Coupling hole 
                 Coupling hole 
                 Coupling hole 
                 Coupling hole 
                 Coupling hole 
               
               
                   
                 71a (outer 
                 71b (outer 
                 71c (outer 
                 71d (outer 
                 71e (outer 
               
               
                 Combination 
                 diameter/inner 
                 diameter/inner 
                 diameter/inner 
                 diameter/inner 
                 diameter/inner 
               
               
                 type 
                 diameter) 
                 diameter) 
                 diameter) 
                 diameter) 
                 diameter) 
               
               
                   
               
             
             
               
                 I 
                 60/40 
                 56/36 
                 52/32 
                 56/36 
                 60/40 
               
               
                 II 
                 60/40 
                 64/44 
                 68/48 
                 64/44 
                 60/40 
               
               
                 III 
                 60/40 
                 64/44 
                 56/36 
                 64/44 
                 60/40 
               
               
                 IV 
                 60/40 
                 56/36 
                 60/42 
                 56/36 
                 60/40 
               
               
                 V 
                 60/40 
                 65/45 
                 70/50 
                 75/55 
                 80/60 
               
               
                 VI 
                 70/50 
                 70/50 
                 70/50 
                 70/50 
                 70/50 
               
               
                   
               
               
                 (unit: mm) 
               
             
          
         
       
     
       (Cooling Gas) 
       [0091]    The cooling gas is fed by the fan motor  73  into the waveguide  20  passing through the cooling gas feed hole  75 , and partly flows in the waveguide  20 , partly introduced into the gap  77  through the coupling holes and flows therein in the axial direction of the waveguide  20 , and is exhausted to the exterior by the exhaust fan motor passing through the cooling gas exhaust hole  76 . 
         [0092]    Therefore, the interior of the waveguide  20  and the heat of the dielectric windows  72  can be forcibly cooled preventing the dielectric windows from being thermally broken even during the operation with large electric power. The air, nitrogen or neon can be used as the cooling gas. As a mechanism for blowing the cooling gas into the waveguide, further, there can be used an air compressor or the like instead of the fan motor. 
         [0093]    In the present invention, the gap  77  formed between the dielectric windows  72  and the coupling holes  71  also plays the role of a heat-insulating layer between the waveguide  20  and the dielectric windows  72  in addition to the role of the cooling gas flow passage. Further, a gap of 2 mm is provided on the side surfaces of the dielectric windows  72  so will not to come in direct contact with the discharge chamber. This relaxes the temperature gradient of the dielectric windows  72  and, therefore, greatly suppresses the thermal breakage to the dielectric windows. 
         [0094]    In the following embodiment, the gap  77  has a height of 2 mm which, however, is better suitably determined by taking the flow of the cooling gas and the mode of electric field of microwaves into consideration. 
         [0095]      FIGS. 1 and 2  show rectangular dielectric windows. In this embodiment, however, the dielectric windows, the gap and the vacuum chamber can be formed in shapes other than the rectangular shape. This can be further applied to the microwave plasma-treating apparatus that uses only one waveguide. The position for providing the cooling gas feed port or the exhaust port is not limited to the waveguide but may be at such a position where the cooling gas can be effectively flown onto the surfaces of the dielectric windows depending on the structure of the apparatus. 
         [0096]      FIGS. 11 and 12  illustrate a microwave plasma-treating apparatus  80  using circular dielectric windows and a cylindrical discharge chamber as another embodiment of the invention. 
         [0097]      FIG. 11  is a sectional view of the microwave plasma-treating apparatus  80  of when it is cut along the axial direction of the waveguide, and  FIG. 12  is a view showing the arrangement of the waveguides and the positions of the coupling holes of when the microwave plasma-treating apparatus  80  shown in  FIG. 11  is viewed from the upper side. 
         [0098]    In the drawings, reference numeral  10  denotes a substrate stage,  13  denotes a substrate,  14  denotes a gas introduction port,  15  denotes a gas exhaust port,  16  denotes a treating chamber,  20  denotes a waveguide,  22  denotes a cylindrical discharge chamber,  73  denotes a fan motor,  75  denotes a cooling gas feed hole,  76  denotes a cooling gas exhaust hole,  87  denotes a gap,  81   a  to  81   d  denote coupling holes,  82  denotes circular dielectric windows and  88  denotes dielectric window covers. 
         [0099]    The invention will be more concretely described by experiments using the apparatus shown in  FIGS. 1 and 2  to which only, however, the invention is in no way limited. 
       (Constitution) 
       [0100]    The microwave plasma-treating apparatus  70  includes a metallic rectangular discharge chamber  12  with its upper end open, four rectangular dielectric windows (e.g., quartz, alumina, etc.)  72  capable of transmitting microwaves being air-tightly attached to the upper end of the rectangular discharge chamber  12  and arranged in parallel maintaining a predetermined distance, four rectangular waveguides  20  mounted thereon being arranged in parallel maintaining a predetermined distance serving as a microwave launcher, five ring-shaped coupling holes  71   a ,  71   b ,  71   c ,  71   d  and  71   e  capable of transmitting microwaves formed in the lower surfaces of each of the rectangular waveguides  20 , and a gap  77  formed between the dielectric windows  72  and the ring-shaped coupling holes  71 ,  71   b ,  71   c ,  71   d  and  71   e . The four waveguides are arranged so that the microwave introduction ports thereof are headed in the same direction but may also be arranged in a zigzag manner as shown in  FIG. 2 . 
         [0101]    On the upper surface of each rectangular waveguide  20 , there are provided a cooling gas feed port  75  mounted on the coupling hole provided at one end portion and constituted by a plurality of small holes, a cooling gas exhaust hole  76  mounted on the coupling hole provided at the other end portion and constituted by a plurality of small holes, a fan motor  73  mounted on the upper part of the cooling gas feed hole  75  and serving as a mechanism for blowing a cooling gas into the waveguide, and a fan motor  73  mounted on the upper part of the cooling gas exhaust hole  76  and serving as a mechanism for blowing out the cooling gas from the waveguide. 
         [0102]    The rectangular discharge chamber  12  has a gas exhaust port  15  for setting the interior of the rectangular discharge chamber  12  to a desired degree of vacuum, a gas introduction port  14  for introducing a required reaction gas, a substrate  13 , and a substrate stage  10  for placing the substrate  13  thereon. The substrate stage  10  is reciprocally moved in a direction perpendicular to the axial direction of the waveguide. This improves the uniformity for treating the substrate with plasma. 
         [0000]    Experiment 1: Experiment for examining changes in the plasma density depending upon the arrangement types of five ring-shaped coupling holes having different sizes (outer diameters and inner diameters). 
       (1) Conditions Concerning the Constitution of the Apparatus. 
       [0103]    Spatial distributions of electron density of plasma were measured by using the combination types I, II, III and IV of coupling holes shown in Table 1. The electron density was measured using a double probe method. Measured were the horizontal distributions of electron density relative to the axial direction of the waveguide at a position 10 mm away from the dielectric window. 
       (2) Conditions of Experiment. 
       [0104]    (a) Gas: hydrogen 
       (b) Pressure: 8 Pa 
       [0105]    (c) Only one microwave launcher was operated. 
       (3) Results of Experiment. 
       [0106]      FIG. 13  shows the measured results from which it was actually proved that the spatial distribution of plasma density can be controlled by varying the sizes of inner diameters and outer diameters of the coupling holes. 
         [0000]    Experiment 2: Experiment for synthesizing nanocrystalline diamond films by using the plasma-treating apparatus of the invention with large electric power.
 
(1) Conditions concerning the constitution of the apparatus.
 
         [0107]    The plasma-treating apparatus was constituted by applying the present invention. 
         [0108]    Where, the gap between the dielectric windows and the lower surfaces of the waveguides was 2 mm. The coupling holes of the combination type IV were employed. 
       (2) Conditions of Experiment. 
       [0109]    (a) Gas: hydrogen (90%)/methane (5%)/carbon dioxide gas (5%) 
       (b) Pressure: 9 Pa 
       [0110]    (c) Substrate: silicon wafer
 
(d) Position where the substrate is placed: 100 mm away from the dielectric windows
 
       (3) Results of Experiment. 
       [0111]    In this experiment, the operation was continued for an extended period of time (10 hours or longer) by feeding the microwave electric power of 5 kW to each waveguide, i.e., by feeding the microwave electric power of a total of 20 kW. However, no dielectric window was thermally broken. As a result of experiment, a thin uniform film of nanocrystalline diamond was successfully synthesized on the surface of the substrate of an area of 300 mm×600 mm. When a thin film was synthesized with the microwave electric power of 3 kW in a customary manner, the rate of growth of the thin film was 20 nm/hour which according to the invention was markedly increased to 300 nm/hour.