Patent Publication Number: US-2009229755-A1

Title: Plasma processing apparatus

Description:
This is a division of application Ser. No. 10/493,946, filed Apr. 28, 2004, which claims the benefit of PCT International Application No. PCT/JP2003/008491 filed Jul. 3, 2003, which claims the benefit of Japanese Patent Application No. 2002-197227 filed Jul. 5, 2002, all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to plasma processing apparatuses and more particularly to a microwave plasma processing apparatus. 
     Plasma processing and plasma processing apparatus are indispensable technology for the fabrication of ultrafine semiconductor devices of recent years called deep submicron devices or deep sub-quarter micron devices having a gate length near 0.1 μm or less or for the fabrication of high-resolution flat panel display devices including a liquid crystal display device. 
     Various methods are used conventionally for exciting plasma in plasma processing apparatuses for use for production of semiconductor devices and liquid crystal display devices. Among others, a parallel-plate type processing apparatus using high-frequency excited plasma or induction-coupled type plasma processing apparatus are used generally. However, these conventional plasma processing apparatuses have a drawback in that plasma formation is not uniform and the region of high electron density is limited. Thus, with such a conventional plasma processing apparatus, it is difficult to achieve a uniform processing over the entire surface of the substrate to be processed with large processing rate and hence with large throughput. It should be noted that this problem becomes particularly serious when processing a large diameter substrate. Further, these conventional plasma processing apparatuses suffer from some inherent problems, associated with its high electron temperature, in that damages are tend to be caused in the semiconductor devices formed on the substrate to be processed. Further, there is caused severe metal contamination caused by sputtering of the processing vessel wall. Thus, with such conventional plasma processing apparatuses, it is becoming more difficult to satisfy the stringent demands for further miniaturization of the semiconductor devices or liquid crystal display devices and further improvement of productivity. 
     Meanwhile, there has been proposed conventionally a microwave plasma processing apparatus that uses high-density plasma excited by a microwave electric field without using a d.c. magnetic field. For example, there is proposed a plasma processing apparatus having a construction in which a microwave is emitted into a processing vessel from a planar antenna (radial-line slot antenna) having a number of slots arranged to produce a uniform microwave. According to this plasma processing apparatus, plasma is excited as a result of the microwave electric field causing ionization in the gas inside the evacuated vessel. Reference should be made to Japanese Laid-Open Patent Application 9-63793. In the microwave plasma excited according to such a process, it becomes possible to realize a high plasma density over a wide area underneath the antenna, and it becomes possible to conduct uniform plasma processing in short time. Further, because the electron density is low in the microwave plasma thus excited due to the use of microwave for the excitation of the plasma, it becomes possible to avoid damaging or metal contamination of the substrate to be processed. Further, because it is possible to excite uniform plasma over a large area substrate, the foregoing technology can easily attend to the fabrication of semiconductor devices that uses a large-diameter semiconductor substrate or production of large liquid crystal display devices. 
     BACKGROUND ART 
       FIGS. 1A and 1B  show the construction of a conventional microwave plasma processing apparatus  100  that uses such a radial line slot antenna, wherein  FIG. 1A  is a cross-sectional view of the microwave plasma processing apparatus  100 , while  FIG. 1B  shows the construction of the radial line slot antenna. 
     Referring to  FIG. 1A , the microwave plasma processing apparatus  100  has a processing chamber  101  evacuated from plural evacuation ports  116 , and a stage  115  that holds a substrate  114  to be processed is formed inside the processing chamber  101 . In order to realize uniform evacuation of the processing chamber  101 , there is formed a ring-shaped space  101 A around the stage  115 , wherein the processing chamber  101  can be evacuated uniformly via the space  101 A and the evacuation ports  116  by forming the plural evacuation ports  116  in communication with the space  101 A with equal interval, in other words, in axial symmetry with regard to the substrate to be processed. 
     On the processing chamber  101 , there is formed a shower plate  103  of low-loss dielectric material with a plate-like form, wherein the shower plate  103  has a number of apertures  107  and is provided via a seal ring  109  as a part of the outer wall of the processing vessel  101  at a location corresponding to the substrate  114  to be processed on the stage  115 . Further, a cover plate  102  also of a low loss dielectric material is provided outside the shower plate  103  via another seal ring  108 . 
     On the shower plate  103 , there is formed a passage  104  of a plasma gas on the top surface thereof, and each of the plural apertures  107  is formed in communication with the plasma gas passage  104 . Further, there is formed a supply passage  106  of the plasma gas inside the shower plate  103  in communication with a plasma gas supplying port  105  provided on the outer wall of the processing vessel  101 . Thereby, a plasma gas such as Ar or Kr is supplied to the plasma gas supplying port  105 , wherein the plasma gas thus supplied is further supplied to the apertures  107  from the supply passage  108  via the passage  104 . The plasma gas is then released into a space  101 B right underneath the shower plate  103  inside the processing vessel  101  from the apertures  107  with substantially uniform concentration. 
     On the processing vessel  101 , there is provided a radial line slot antenna  110  having a radiation surface shown in  FIG. 1B  at the outer side of the cover plate  102  with a separation of 4-5 mm from the cover plate  102 . The radial line slot antenna  110  is connected to an external microwave source (not shown) via a coaxial waveguide  110 A, and the microwave from the microwave source causes excitation of the plasma gas released into the foregoing spate  101 B. Further, the gap between the cover plate  102  and the radiation surface of the radial line slot antenna  110  is filled with the air. 
     The radial line slot antenna  110  is formed of a flat, disk-like antenna body  110 B connected to an external waveguide forming the coaxial waveguide  110 A, and a radiation plate  110 C is formed at the mouth of the antenna body  110 B, wherein the radiation plate  110 C is formed with a number of slots  110   a  and a number of slots  110   b  perpendicular to the slots  110   a.  Further, there is interposed a retardation plate  110 D of a dielectric plate having a uniform thickness between the antenna body  110 B and the radiation plate  110 C. 
     In the radial line slot antenna  110  of such a construction, the microwave supplied from the coaxial waveguide  110 A spreads as it travels between the disk-like antenna body  110 B and the radiation plate  110 C in the radial direction, wherein the retardation plate  110 D functions to compress the wavelength thereof. Thus, by forming the slots  110   a  and  110   b  concentrically in correspondence to the wavelength of the microwave traveling in the radial direction in a mutually perpendicular relationship, it becomes possible to emit a plane wave having a circular polarization in the direction substantially perpendicular to the radiation plate  110 C. 
     By using such a radial line slot antenna  110 , uniform high-density plasma is formed in the space  101 B right underneath the shower plate  103 . The high-density plasma thus formed has the feature of low electron temperature, and thus, there is caused no damaging in the substrate  114  to be processed. Further, there is caused no metal contamination originating from the sputtering of the chamber wall of the processing vessel  101 . 
     In the plasma processing apparatus  100  of  FIG. 1 , there is further formed with a conductive structure  111  inside the processing vessel  101  between the shower plate  103  and the substrate  114  to be processed, wherein the conductive structure  111  is formed with a large number of nozzles  113  supplied with a processing gas from an external processing gas source (not shown) via a processing gas passage  112  formed in the processing vessel  101 , wherein each of the nozzles  113  releases the supplied processing gas to a space  101 C between the conductive structure  111  and the substrate  114  to be processed. Thus, the conductive structure  111  functions as a processing gas supplying part. The conductive structure  111  thus constituting the processing gas supplying part is formed with apertures between adjacent nozzles  113  and  113  with a size allowing efficient passage of the plasma formed in the space  101 B into the space  101 C as a result of diffusion. 
     Thus, in the case the processing gas is released into the space  101 C from the processing gas supplying part  111  via the nozzles  113 , the released processing gas undergoes excitation in the processing space  101 B by the high-density plasma and there is conducted a uniform plasma processing on the substrate  114  to be processed, efficiently and at high speed, without damaging the substrate and the device structure on the substrate and without contaminating the substrate. On the other hand, the microwave emitted from the radial line slot antenna  110  is blocked by the process gas supplying part  111  formed of a conductor, and thus, there is no risk that the substrate  114  to be processed is damaged. 
     In the foregoing plasma processing apparatus  100  explained with reference to  FIGS. 1A and 1B , it is important to excite the plasma uniformly with high density in the space  101 B right underneath the shower plate  103 . For this to be achieved, it is important to ensure that there occurs no plasma excitation in the spaces other than the space  101 B, in which the plasma excitation occurs easily, such as the plasma gas passage  104  where the microwave electric field is strong and plasma is tend to be excited or in the foregoing apertures  107 . 
     However, in the case the plasma is actually excited in the present apparatus  100 , there is a possibility that the plasma is also excited in the plasma gas passage  104  and further in the apertures  107  depending on the condition of the substrate processing. Once the plasma is excited in the plasma passage  104  or the apertures  107 , the microwave power is consumed and the plasma density in the space  101 B is decreased. Further, there appears a difference in the plasma density between the region right underneath the apertures  107  and the region far from the apertures  107 . Thereby, there arises the problem of non-uniformity in the plasma density over the entire space  101 B, which serves for the plasma excitation space. 
     DISCLOSURE OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a novel and useful plasma processing apparatus wherein the foregoing problems are eliminated. 
     Another and more specific object of the present invention is to excite high-density plasma in a desired space with excellent uniformity, without causing plasma excitation in a space in the path for introducing a plasma gas. 
     Another object of the present invention is to provide a plasma processing apparatus, comprising: 
     a processing vessel defined by an outer wall and provided with a stage for holding a substrate to be processed; 
     an evacuation system coupled to said processing vessel; 
     a microwave window provided on said processing vessel as a part of said outer wall so as to face said substrate to be processed on said stage; 
     a plasma gas supplying part supplying a plasma gas into said processing vessel; and 
     a microwave antenna provided on said processing vessel in correspondence to said microwave, 
     said plasma gas supplying part including a porous medium, said plasma gas supplying part supplying said plasma gas to said processing vessel via said porous medium. 
     According to the present invention, following measures have been taken in the plasma processing apparatus processing a substrate in the purpose of preventing excitation of plasma except for the plasma excitation space used for plasma excitation. In the plasma gas passage, more specifically, the plasma excitation is prevented by using a plasma gas pressure condition set such that there is caused no plasma excitation. For the shower plate from which the plasma gas is radiated, on the other hand, a mechanism that supplies the plasma gas via pores of a porous medium is employed. When the plasma gas is thus supplied via the narrow space of the pores, the electrons accelerated by the microwave collide with the inner wall of the space defining the pore, and the acceleration necessary for causing plasma excitation is not attained for the electrons. With this, the plasma excitation is prevented. As a result, it becomes possible to cause high-density and uniform plasma excitation in a desired plasma excitation space. 
     Other objects and further features of the present invention will become apparent from the following detailed description of the present invention made hereinafter with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIGS. 1A and 1B  are diagrams showing the construction of a conventional microwave plasma processing apparatus that uses a radial line slot antenna; 
         FIGS. 2A and 2B  are diagrams showing the construction of a microwave plasma processing apparatus according to a first embodiment of the present invention; 
         FIG. 3  is a diagram showing the condition for causing excitation of microwave plasma with regard to the microwave electric field and the pressure of Ar used for the plasma gas; 
         FIGS. 4A and 4B  are diagrams showing the construction of a processing gas supplying structure according to a second embodiment of the present invention; 
         FIGS. 5A and 5B  are diagrams showing the construction of a plasma processing apparatus according to a third embodiment of the present invention; 
         FIGS. 6A and 6B  are diagrams showing the construction of a plasma processing apparatus according to a fourth embodiment of the present invention; 
         FIGS. 7A and 7B  are diagrams showing the construction of a plasma processing apparatus according to a fifth embodiment of the present invention; 
         FIGS. 8A and 8B  are diagrams showing the construction of a plasma processing apparatus according to a sixth embodiment of the present invention. 
     
    
    
     BEST MODE FOR IMPLEMENTING THE INVENTION 
     First Embodiment  
       FIGS. 2A and 2B  show the construction of a microwave plasma processing apparatus  200  according to a first embodiment of the present invention, wherein those parts explained previously are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to  FIG. 2A , the shower plate  103  of the foregoing microwave plasma processing apparatus  100  is replaced with a disk-like shower plate  201  of a porous medium such as a porous ceramic material formed for example by Al 2 O 3  sintered at an ordinary pressure. The shower plate  201  is formed with a passage  202  of the plasma gas on the top surface thereof, wherein the plasma gas of Ar or Kr supplied to the plasma gas supplying port  105  is passed through the plasma gas passage  202  and supplied to the space  101 B right underneath the shower plate uniformly through the pores in the porous medium that constitutes the shower plate  201 . 
     As noted before, there is induced a strong microwave electric field in the plasma gas passage  202 , and thus, there is a tendency that plasma is excited in such a plasma gas passage  202 . Thus, it is necessary to set the pressure of the plasma gas passage  202  to a pressure in which there occurs no excitation of the microwave plasma. 
       FIG. 3  shows the region in which excitation of microwave plasma occurs for the case in which the strength of the microwave electric field and the pressure of Ar used for the plasma excitation gas are changed. In the illustrated example, the frequency of the microwave is set to 2.45 GHz. In the drawing, it should be noted that the region designated as region A is the region in which plasma excitation takes place. Thus, excitation of the microwave plasma takes place at the microwave electric field strength and the Ar pressure of the region A. 
     Referring to  FIG. 3 , there occurs ignition of microwave plasma at the microwave strength of about 0.3 W/cm 2  in the case the pressure is set to about 1 Torr. In this case, the microwave plasma is excited with a near-minimum microwave strength. When the pressure is increased or decreased from 1 Torr, on the other hand, the microwave strength necessary for causing plasma excitation increases, and there appears a condition in which plasma is less easily excited. In the present apparatus, plasma excitation in the plasma gas passage  202  is prevented by setting the pressure of the plasma gas passage to about 6.67 kPa-13.3 kPa (about 50 Torr-100 Torr). 
     Further, it should be noted that the space  101 B used for the plasma excitation space and the plasma gas passage  202 , which serves for the plasma gas feeding path, are isolated form each other by the shower plate  201  formed of the porous medium. Thus, the plasma gas is supplied from the plasma gas passage  202  to the foregoing space  101 B through the pores of the porous medium forming the shower plate  201 . As there exist no sufficiently large space in the pores for causing plasma excitation, there occurs no excitation of plasma in such pores. More specifically, even when there is caused acceleration of electrons in the pores by the microwave, the electrons collide with the wall of the pores before it is accelerated to the degree for causing plasma excitation. 
     Thus, in the present apparatus  200 , there is caused no plasma excitation inside the shower plate  201 , which serves for the plasma gas inlet continuous to the space  101 B, and it becomes possible to excite high-density plasma uniformly in the space  101 B. 
     Second Embodiment  
       FIGS. 4A and 4B  show the construction of a microwave plasma processing apparatus  200 A according to a second embodiment of the present invention, wherein those parts explained previously are designated with the same reference numerals and the description thereof will be omitted. 
     Referring to  FIG. 4A , it can be seen that the lower shower plate  111  is removed in the microwave plasma processing apparatus  200 A of the present embodiment. Because the lower shower plate  111  is omitted, the apparatus cannot carry out film formation process or etching process by supplying a processing gas separately to the plasma gas. On the other hand, the apparatus can form an oxide film, nitride film or oxynitride film on the surface of the substrate to be processed, by supplying an oxidation gas or nitridation gas from the shower plate  201  together with a plasma gas. 
     In the present embodiment, too, there occurs no plasma excitation inside the shower plate  201 , and it becomes possible to excite high-density and uniform plasma in the space right underneath the shower plate. 
     Third Embodiment  
       FIGS. 5A and 5B  show the construction of a microwave plasma processing apparatus  10  according to a third embodiment of the present invention. 
     Referring to  FIG. 5A , the microwave plasma processing apparatus  10  includes a processing vessel  11  and a stage  13  provided in the processing vessel  11 , wherein the stage  13  is formed by hot isotropic pressing process of AlN or Al 2 O 3  and holds the substrate  12  to be processed by an electrostatic chuck. In the processing vessel  11 , there are formed at least two, preferably three or more evacuation ports  11   a  in a space  11 A surrounding the stage  13  with a uniform interval, and hence in axial symmetry with regard to the substrate  12  to be processed on the stage  13 . Thereby, the processing vessel  11  is evacuated by a vacuum pump via such evacuation ports  11   a  for reducing the pressure therein. 
     Preferably, the processing vessel  11  is formed of an austenite stainless steel containing Al and a passivation film of aluminum oxide is formed on the inner wall surface thereof by an oxidation processing. Further, there is formed a disk-like shower plate  14  of a porous medium, such as Al 2 O 3  sintered at ordinary pressure in the form of porous ceramic material, in a part of the outer wall of the processing vessel corresponding to the substrate  12  to be processed. 
     The shower plate  14  is mounted on the processing vessel  11 , wherein there is provided a cover plate  15  of dense Al 2 O 3  formed by HIP processing on the shower plate  14 . The Al 2 O 3  cover plate  15  thus formed by the HIP process is formed by using Y 2 O 3  as a sintering additive and has the porosity of 0.03% or less. This means that the Al 2 O 3  cover plate  15  is substantially free from pores or pinholes. Further, the Al 2 O 3  cover plate  15  has a very large thermal conductivity for a ceramic, which reaches the value of 30 W/mK. Further, as noted before, sealing of the processing vessel  11  to the environment is achieved by urging the seal ring  11   s  to the cover plate  15 , and thus, there is applied no load to the porous and fragile shower plate  14  in such a structure. The shower plate  14  is formed, in the side thereof that makes contact with the cover plate  15 , with a depressed plasma gas passage  14 A for causing to flow the plasma gas, wherein the foregoing plasma gas passage  14 A is connected to a plasma gas inlet  21 A formed in the upper part of the shower plate as will be described. 
     The shower plate  14  is supported by projections  11   b  formed on the inner wall of the processing vessel  11 , wherein the part of the projection  11   b  supporting the shower plate  14  is formed to have a rounded surface for suppressing anomalous electric discharge. 
     Thus, the plasma gas such as Ar or Kr supplied to the plasma gas inlet  21 A is supplied to the space  11 B right underneath the shower plate uniformly through the pores of the porous medium forming the shower plate  14 , after passing through the plasma gas passage  14 A inside the shower plate  14 . Further, there is inserted a seal ring  15   s  in the part where the plasma gas inlet  21 A and the cover plate  15  engage with each other for confinement of the plasma gas. 
     Further, a radial line slot antenna  20  is provided on the cover plate  15 , wherein the radial line stop antenna  20  includes a disk-shaped slot plate  16  contacting with the cover plate  15  and formed with numerous slots  16   a  and  16   b  shown in  FIG. 5B , a disk-like antenna body  17  holding the slot plate  16 , and a retardation plate  18  of a low-loss dielectric material such as Al 2 O 3 , Si 3 N 4 , SiON, SiO 2  or the like sandwiched between the slot plate  16  and the antenna body  17 . Further, a plasma gas/microwave inlet part  21  is formed on the upper part of the radial line slot antenna  20 . It should be noted that the foregoing plasma gas/microwave inlet  21  part includes a part  21 C connected to the antenna body  17  with circular or rectangular cross-section and forming therein a microwave inlet passage, a microwave inlet part  21 B of rectangular or circular cross-section, and a plasma gas inlet passage  21 A having a generally cylindrical form. Thereby, a plasma gas such as Ar or Kr is supplied to the plasma gas inlet passage  21 A. The radial line slot antenna  20  is mounted on the processing vessel  11  via a seal ring  11   u , and a microwave of 2.45 GHz or 8.3 GHz frequency is supplied to the radial line slot antenna from an external microwave source (not shown) connected to the microwave inlet part  21 B of the plasma gas/microwave inlet part  21 . The microwave thus supplied is emitted into the processing vessel  11  through the cover plate  15  and the shower plate  14  after emitted from the slots  16   a  and  16   b  on the slot plate  16  and excites plasma in the plasma gas supplied from the shower plate  14  in the space  11 B right underneath the shower plate  14 . Thereby, it should be noted that the cover plate  15  and the shower plate  14  are formed of Al 2 O 3  and serves for an efficient microwave window. Thereby, the pressure of the plasma gas is maintained to about 6.67 kPa-13.3 kPa (about 50-100 Torr) in the plasma gas passage  14 A for avoiding plasma excitation in the plasma gas passage  14 A. 
     In this case, it should be noted that the foregoing space  11 B serving for the plasma excitation space is isolated from the plasma gas passage  14 A acting as the passage for supplying the plasma gas, by the shower plate  14  of the porous medium. As noted before, the plasma gas is supplied from the plasma gas passage to the space  11 B through the pores in the shower plate  14 . Because there is no sufficient space for plasm a excitation in the pores, there is caused no plasma excitation. 
     Because there is caused no plasma excitation in the shower plate  14  serving for the plasma gas inlet passage to the space  11 B also in the apparatus  10  of the present embodiment, it becomes possible to excite high-density and uniform plasma in the space  11 B. In order to improve intimacy of the radial line slot antenna  20  to the cover plate  15 , there is formed a ring-shaped groove  11   g  on a part of the top surface of the processing vessel  11  that engages with the slot plate  16  in the microwave plasma processing apparatus  10  of the present embodiment. Thus, by evacuating such a groove  11   g  via an evacuation port  11 G communicating therewith, the pressure in the gap formed between the slot plate  16  and the cover plate  15  is reduced. With this, the radial line slot antenna  20  is urged firmly against the cover plate  15  by the atmospheric pressure. It should be noted that such a gap includes not only the slots  16   a  and  16   b  formed in the slot plate  16  but also other gaps formed by various reasons. It should be noted that such a gap is sealed by a seal ring  11   u  provided between the radial line slot antenna  20  and the processing vessel  11 . 
     Further, by filling the gap between the slot plate  16  and the cover plate  15  with an inert gas of low molecular weight via the evacuation port  11 G and the groove  11   g , it is possible to facilitate heat transfer from the cover plate  15  to the slot plate  16 . For such an inert gas, it is preferable to use He having a large thermal conductivity and large ionization energy. In the case of filling the gap with He, it is preferable to set the pressure to about 0.8 atmosphere. In the construction of  FIG. 3 , a valve  11 V is connected to the evacuation port  11 G for evacuation of the groove  11   g  and for filling the groove  11   g  with the inert gas. 
     The waveguide  21 C of the gas/plasma inlet  21  is connected to the disk-shaped antenna body  17 , and the plasma gas inlet  21 A extends through the opening  18 A formed in the retardation plate  18  and the opening  16   c  formed in the slot plate  16  and is connected to the cover plate opening  15 A. Thus, the microwave supplied to the microwave inlet part  21 B is emitted from the slots  16   a  and  16   b  as it is propagating in the radial direction between the antenna body  17  and the slot plate  16  after passing through the waveguide  21 C. 
       FIG. 5B  shows the slots  16   a  and  16   b  formed on the slot plate  16 . 
     Referring to  FIG. 5B , the slots  16   a  are arranged in a concentric relationship, and in correspondence to each of the slots  16   a,  there is formed a slot  16   b  perpendicularly thereto, such that the slots  16   b  are formed also in a concentric relationship. The slots  16   a  and  16   b  are formed with an interval corresponding to the wavelength of the microwave compressed by the retardation plate  18  in the radial direction of the slot plate  16 , and as a result, the microwave is emitted from the slot plate  16  generally in the form of plane wave. Because the slots  16   a  and  16   b  are formed in a mutually perpendicular relationship, the microwave thus emitted form a circular polarization containing two, mutually perpendicular polarization components. 
     At the center of the slot plate  16 , there is formed an opening  16   c  for insertion of the plasma gas passage  21 A. 
     Further, in the plasma processing apparatus  10  of  FIG. 5A , a cooling block  19  formed with a cooling water passage  19 A is formed on the antenna body  17 . Thus, by cooling the cooling block  19  by the cooling water inside the cooling water passage  19 A, the heat accumulated in the shower plate  14  is absorbed via the radial line slot antenna  20 . The cooling water passage  19 A is formed in a spiral form on the cooling block  19 , and cooling water, preferably the one in which oxidation-reduction potential is controlled by eliminating dissolved oxygen by means of bubbling of an H 2  gas, is passed through the cooling water passage  19 A. 
     Further, in the microwave plasma processing apparatus  10  of  FIG. 5A , there is provided a process gas supplying structure  31  having a lattice-shaped process gas passage in the processing vessel  11  between the shower plate  14  and the substrate  12  to be processed on the stage  13 , wherein the process gas supplying structure  31  is supplied with a processing gas from a processing gas inlet port  11   r  provided on the outer wall of the processing vessel and releases the same from a number of processing gas nozzle apertures  31 A. Thereby, a desired uniform substrate processing is achieved in the space  11 C between the processing gas supplying structure  31  and the substrate  12  to be processed. It should be noted that such a substrate processing includes plasma oxidation processing, plasma nitridation processing, plasma oxynitridation processing, plasma CVD processing, and the like. Further, by supplying a fluorocarbon gas such as C 4 F 8 , C 5 F 8 , C 4 F 6 , and the like or an etching gas containing F or Cl from the processing gas supplying structure  31  and further by applying a high-frequency voltage to the stage  13 A from a high-frequency source  13 A, it becomes possible to conduct a reactive ion etching process on the substrate  12  to be processed. 
     According to the microwave plasma processing apparatus  10  of the present embodiment, deposition of reaction byproducts on the inner wall surface of the processing vessel is avoided by heating the outer wall of the processing vessel  11  to the temperature of about 150° C., and continuous and stable operation becomes possible by conducting a dry cleaning process once in a day or so. 
     Fourth Embodiment  
       FIGS. 6A and 6B  show an example of a microwave plasma processing apparatus  10 A according to a fourth embodiment of the present invention, wherein those parts explained previously are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to  FIG. 6A , there is provided a shower plate  40  of dense Al 2 O 3  formed by an HIP process and having at least one or more apertures  40 B in place of the shower plate  14  of porous medium used for the microwave plasma processing apparatus  10 . Further, at the side of the shower plate  40  contacting the cover plate  15 , there is formed a plasma gas passage  40 A in the form of a depression as the passage of the plasma gas, such that the plasma gas passage  40 A communicates each of the apertures  40 B. Further, each of the apertures  40 B is inserted with a plasma gas inlet component  41  of a porous medium such as a porous ceramic of Al 2 O 3  sintered at ordinary pressure. Thereby, the plasma gas of Ar or Kr is supplied to the foregoing space  11 B generally uniformly via the pores of the porous medium in the plasma gas inlet component  41 , after passing through the plasma gas passage  40 A. 
     In this case, too, there is caused no plasma excitation in the plasma gas passage  40 A or plasma gas inlet component  41 , similarly to the case of the microwave plasma processing apparatus  10 . Thus, it becomes possible to excite high-density and uniform plasma in the space  11 B. 
     Fifth Embodiment  
     Next, an example of the microwave plasma processing apparatus  10 B according to a fifth embodiment of the present invention is shown in  FIGS. 7A and 7B , wherein those parts in the drawings corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to  FIG. 7A , it will be noted that the lower shower plate  31  is removed in the microwave plasma processing apparatus lob of the present embodiment. Further, the entire surface of the projections  11   b  supporting the shower plate  14  is provided with a rounded surface. 
     Although the plasma processing apparatus  10 B of such a construction cannot achieve film formation or etching by supplying a processing gas separately to the plasma gas because of elimination of the lower shower plate  31 , it is possible to form an oxide film, a nitride film or an oxynitride film on the surface of the substrate to be processed by supplying an oxidizing gas or nitriding gas from the shower plate  14  together with the plasma gas. 
     In the present embodiment, too, there is caused no plasma excitation in the plasma gas passage  14 A and inside the shower plate  14 , and thus, it becomes possible to excite high-density and uniform plasma in the space right underneath the shower plate. 
     Sixth Embodiment  
       FIGS. 8A and 8B  show an example of a microwave plasma processing apparatus  10 C according to a sixth embodiment of the present invention, wherein those parts in the drawings corresponding to the parts explained previously are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to  FIG. 8A , a plasma gas of Ar or Kr is supplied to the processing vessel  11  with the microwave plasma processing apparatus  10 C of the present embodiment by way of the shower plate  40  of dense Al 2 O 3  formed by a HIP process, the shower plate  40  being formed with at least one aperture  40 B, and the plasma gas inlet component  41  of a porous medium inserted into the aperture  40 B such as a porous ceramic material of sintered Al 2 O 3 , similarly to the case of the microwave plasma processing apparatus  10 C explained previously. 
     Further, the lower shower plate  31  is eliminated similarly to the case of foregoing apparatus  10 B, and the entire surface of the projection  11   b  holding the shower plate  14  is formed with a rounded surface. 
     Although the plasma processing apparatus  10 B of such a construction cannot achieve film formation or etching by supplying a processing gas separately to the plasma gas because of elimination of the lower shower plate  31 , it is possible to form an oxide film, a nitride film or an oxynitride film on the surface of the substrate to be processed by supplying an oxidizing gas or nitriding gas from the shower plate  14  together with the plasma gas. 
     In the present embodiment, too, there is caused no plasma excitation in the plasma gas passage  40 A or in the plasma gas inlet component  41 , and thus, it becomes possible to excite high-density and uniform plasma in the space  11 B. 
     Further, while the embodiments heretofore have been explained for the porous ceramic material of Al 2 O 3  sintered at ordinary pressure as an example of the porous medium, it should be noted that the present invention is not limited to this material. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, it becomes possible to excite high-density and uniform plasma in a desired plasma excitation space while suppressing plasma excitation in a plasma gas inlet passage, by separating the space for plasma excitation and the plasma gas inlet passage by a porous medium such as a porous ceramic material in a plasma processing apparatus for processing a substrate.