TECHNICAL FIELD 
       [0001]    The disclosure relates to a placing table structure used for a thin film forming apparatus to form a thin film on a subject to be processed, such as a semiconductor wafer. 
       BACKGROUND ART 
       [0002]    In general, when a semiconductor integrated circuit is manufactured, various heating processes, such as film forming, etching, annealing, modifying and crystallizing process, are repeatedly performed with respect to the subject to be processed, such as the semiconductor wafer, to form a desired integrated circuit. For instance, in the case of a single type film forming apparatus, which forms a film on semiconductor wafers one by one, a placing table equipped with a resistance heater is installed in an evacuable processing container and a semiconductor wafer is loaded on a top surface of the placing table. In this state, a film forming gas is injected into a processing space so that a thin film is formed on the semiconductor wafer under a predetermined process condition. 
         [0003]    Such a thin film can be formed through the thermal decomposition of raw material gas. For instance, the thin film can be formed through a CVD (chemical vapor deposition) process disclosed in, for example, Japanese Unexamined Patent Publication Nos. 2001-023966 and 2003-007694. 
         [0004]    When the thin film is formed through the above process, the thin film is of course formed on a top surface of the semiconductor wafer. However, since the film forming gas may be introduced into a gap between a back surface of the wafer and the placing table by way of a peripheral portion and a lateral side of the wafer, the thin film may also be formed from the peripheral portion to the entire lateral side of the wafer. In other words, the thin film may be formed on a bevel portion of the wafer to a certain degree as well as a back surface of the peripheral portion of the wafer. 
         [0005]    The thin film unnecessarily deposited on the bevel portion or the back surface of the wafer may be delaminated in the subsequent processes, so the particles are generated or contamination may occur caused by the thin film unnecessarily deposited on the bevel portion or the back surface of the wafer. 
         [0006]    In particular, the critical dimensions of the semiconductor device have recently become finer, so the process conditions tend to be set with high step coverage to ensure the embeddability for various holes or recesses formed in the surface of the wafer. That is, the film forming gas may tend to flow through the bevel portion or the back surface of the wafer, thereby causing the above problems. 
         [0007]    In order to solve the above problems, there is provided a method for preventing the formation of undesired thin films by introducing a purge gas such an inert gas to the peripheral portion of the wafer. However, if the purge gas which is not related to the thin film formation is introduced into the processing space, the thin film may not be formed on a local area of the wafer due to the purge gas, so that thickness uniformity of the thin film may be degraded. 
         [0008]    In particular, a precious metal thin film is recently formed by using metal carbonyl gas as raw material gas to reduce the contact resistance. When the precious metal thin film is formed, the process condition tends to be set with the high step coverage, so it is necessary to solve the above problems to provide the precious metal thin film having high quality. 
       SUMMARY 
       [0009]    The present invention has been made to solve the above problems occurring the in prior art, and an object of the present invention is to provide a placing table structure capable of preventing formation of a thin film on a bevel portion and a back surface of a subject to be processed while improving the thickness uniformity of the thin film by supplying decomposition restraint gas to a peripheral portion of the subject to be processed, while appropriately restraining the thermal decomposition of the raw material gas, when the thin film is formed by using the raw material gas causing a reversible thermal decomposition reaction. 
         [0010]    According to the present invention, a placing table structure is installed in a processing container to place a subject to be processed thereon when a thin film is formed on the subject in the processing chamber by using a raw material gas causing a reversible thermal decomposition reaction. The placing table structure includes a placing table to place the subject on a placing surface, which is a top surface of the placing table, and a decomposition restraint gas feeding unit installed in the placing table to feed a decomposition restraint gas, which restrains the thermal decomposition of the raw material gas, to a peripheral portion of the subject placed on the placing surface of the placing table. 
         [0011]    According to the present invention, when the thin film is formed by using the raw material gas causing the reversible thermal decomposition reaction, the decomposition restraint gas is fed toward the peripheral portion of the subject to be processed from a decomposition restraint gas feeding unit, so that the thermal decomposition of the raw material gas is restrained. Thus, the thin film can be formed on the top surface of the subject to be processed with a uniform thickness while preventing formation of the thin film on the bevel portion and the back side of the subject to be processed. 
         [0012]    According to the exemplary embodiment of the disclosure, the decomposition restraint gas feeding unit includes a gas discharge port formed along a circumference of the placing table corresponding to the peripheral portion of the subject placed on the placing surface of the placing table, a gas path communicated with the gas discharge port, and a decomposition restraint gas source connected to the gas path to store a decomposition restraint gas. 
         [0013]    In this case, the gas discharge port is preferably communicated with the gas path through an annular diffusion chamber formed in the placing table, along the circumference of the placing table. 
         [0014]    For example, the gas discharge port includes an annular slit formed along the circumference of the placing table. 
         [0015]    Alternatively, the gas discharge port includes a plurality of exhaust holes formed along the circumference of the placing table in a predetermined interval. 
         [0016]    Also, a recess is preferably formed in the placing surface to receive the subject to be processed therein and the recess has a depth corresponding to a thickness of the subject to be processed. 
         [0017]    Also, the placing surface is preferably formed in the circumference thereof with an annular groove to define a gas staying space to temporally stay the decomposition restraint gas. 
         [0018]    Also, the placing table is preferably provided with a ring member having a shape of a thin ring plate and positioned at an outer peripheral portion of the subject to be processed. 
         [0019]    In this case, the ring member is preferably movable up and down and serves as a clamp ring having an inner peripheral portion of the ring member making contact with a top surface of the peripheral portion of the subject to be processed to press the subject. 
         [0020]    Alternatively, the ring member preferably serves as a cover ring to prevent the thin film from being formed on a region where the ring member is disposed. 
         [0021]    Also, the placing table is preferably provided therein with a heating unit to heat the subject to be processed. 
         [0022]    Also, the decomposition restraint gas preferably has a composition identical to a composition of a gas generated through a thermal decomposition reaction of the raw material gas. 
         [0023]    Also, the raw material gas preferably includes a metal carbonyl raw material gas. 
         [0024]    For example, the metal carbonyl raw material gas includes at least one selected from the group consisting of Ru 3 (CO) 12 , W(CO) 6 , Ni(CO) 4 , Mo(CO) 6 , Co 2 (CO) 8 , Rh 4 (CO) 12 , Re 2 (CO) 10 , Cr(CO) 6 , Os 3 (CO) 12  and Ta(CO) 5 . 
         [0025]    Also, a thin film forming apparatus is provided to form a thin film on a subject to be processed. The thin film forming apparatus includes a processing container having a gas exhaust function, a placing table structure having one of the above features, and a gas feeding unit to feed a raw material gas causing a reversible thermal decomposition reaction to the processing container. 
         [0026]    Alternatively, the present invention provides a method of forming a thin film on a subject to be processed, which is placed on a placing table in a processing container, by using a raw material gas causing a reversible thermal decomposition reaction. The method includes feeding the raw material gas into the processing container and feeding a decomposition restraint gas toward a peripheral portion of the subject to be processed to restrain the thermal decomposition of the raw material gas. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0027]      FIG. 1  is a cross sectional view showing a thin film forming apparatus employing a placing table structure according to the present invention. 
           [0028]      FIG. 2  is a plan view showing a placing surface of a placing table which is a top surface of the placing table. 
           [0029]      FIG. 3  is a transverse sectional view showing a diffusion chamber of the placing table. 
           [0030]      FIG. 4  is a partially-enlarged sectional view showing a part of the placing table. 
           [0031]      FIG. 5  is a sectional view for explaining an operation of a decomposition restraint gas feeding unit. 
           [0032]      FIG. 6  is a view showing a placing surface formed with a modified gas discharge port. 
           [0033]      FIGS. 7A and 7B  are enlarged sectional views showing the placing table including a modified ring member. 
           [0034]      FIG. 8  is a graph showing the thickness of a thin film deposited on a peripheral portion (bevel portion) of a semiconductor wafer. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Hereinafter, the placing table structure according to the exemplary embodiment of the present invention will be described in detail with reference to accompanying drawings.  FIG. 1  is a cross sectional view showing a thin film forming apparatus employing a placing table structure according to the disclosure.  FIG. 2  is a plan view showing a placing surface of the placing table which is a top surface of the placing table.  FIG. 3  is a transverse sectional view showing a diffusion chamber of the placing table.  FIG. 4  is a partially-enlarged sectional view showing a part of the placing table.  FIG. 5  is a sectional view for explaining an operation of a decomposition restraint gas feeding unit. In the following description, a Ru layer is formed as an example of a metal thin film by feeding raw material gas, such as metal carbonyl gas (Ru 3 (CO) 12 ), together with carrier gas, such as CO gas. 
         [0036]    As shown in  FIG. 2 , a film forming apparatus  2  includes a processing container  4 , which has a substantially circular sectional shape and is made from aluminum or an aluminum alloy. A shower head  6  is provided on the ceiling of processing container  4  to supply a predetermined gas, that is, film forming gas into processing container  4 . In addition, a plurality of gas injection holes  10  are formed at a gas injection surface  8  formed at the bottom surface of shower head  6 , so that the film forming gas is injected into processing space S through gas injection holes  10 . 
         [0037]    A gas diffusion chamber  12  is formed in shower head  6 . The film forming gas introduced into gas diffusion chamber  12  is horizontally diffused and then discharged through gas injection holes  10  communicated with diffusion chamber  12 . Shower head  6  may be formed by using nickel, a nickel alloy, such as HASTELLOY (registered trademark), aluminum or an aluminum alloy. The metal carbonyl gas (Ru 3 (CO) 12 ) is used as the raw material gas to form the thin film. The raw material gas is sublimated and then carried by the carrier gas, such as the CO gas. A seal member  14 , such as an O-ring, is provided at the bonding section between shower head  6  and the upper opening of processing container  4  for the air-tightness of processing container  4 . 
         [0038]    In addition, a loading/unloading opening  16  is formed at the sidewall of processing container  4  to load or unload the subject to be processed, such as a semiconductor wafer W, into or from processing container  4 . A gate valve  18  is installed in loading/unloading opening  16  to open or close loading/unloading opening  16 . 
         [0039]    In addition, an exhaust space  22  is formed in the vicinity of a bottom part  20  of processing container  4 . In detail, an opening  24  having a large size is formed at the center of bottom part  20  of processing container  4  and a cylindrical partition wall  26  having a bottom part  28  may extend downward from opening  24 . Opening  24  and cylindrical partition wall  26  may define exhaust space  22 . In addition, a placing table structure  29  is uprightly installed on bottom part  28  of cylindrical partition wall  26  defining exhaust space  22  such that semiconductor wafer W to be processed can be mounted on placing table structure  29 . In detail, placing table structure  29  may include a hollow cylindrical support  30  and a placing table  32  fixedly bonded to the upper end of hollow cylindrical support  30 . Details of placing table structure  29 , which is the technical feature of the present invention, will be described later. 
         [0040]    Opening  24  of exhaust space  22  has a diameter smaller than the diameter of placing table  32 . Thus, processing gas flowing through the outer peripheral portion of placing table  32  is introduced into the lower portion of placing table  32  and then introduced into opening  24 . Cylindrical partition wall  26  is formed at the lower lateral portion thereof with an exhaust port  34  communicated with exhaust space  22 . Exhaust port  34  is connected to an exhaust system  36 . Exhaust system  36  has an exhaust pipe  38  in which a pressure regulating valve  40  and a vacuum pump  42  are sequentially installed. Therefore, gas is exhausted from processing container  4  and exhaust space  22 , so that the pressure can be adjusted to a predetermined level. 
         [0041]    In addition, as described above, placing table  32  is installed upright at the center of processing container  4  by cylindrical support  30 . For instance, cylindrical support  30  may include a ceramic material, such as aluminum nitride (AlN). In addition, placing table  32  may include a ceramic material, such as aluminum nitride (AlN). A circular recess  44  (see,  FIG. 2 ) is formed on a placing surface  43  (see,  FIG. 4 ) of placing table  32 . Circular recess  44  has a depth corresponding to the thickness of wafer W and a diameter slightly larger than the diameter of wafer W. Wafer W is received in circular recess  44 . 
         [0042]    As shown in  FIG. 4 , a groove  45  having a rectangular sectional shape can be formed at a boundary portion of circular recess  44  by cutting the boundary portion of circular recess  44 . Groove  45  has an annular (ring) shape formed along the circumference of placing table  32 . A gap between groove  45  and the outer peripheral portion of wafer W may serve as a gas stagnation space. For instance, groove  45  has a width of about 4 mm. Thus, the lower surface of the peripheral portion of wafer W narrowly faces an opening of groove  45 . In addition, groove  45  may not be formed and omitted. 
         [0043]    In addition, a heater  46  can be installed in placing table  32  as a heating unit. For instance, heater  46  is buried in placing table  32  in a predetermined pattern shape. In this case, heater  46  can be arranged over a region having a diameter larger than a diameter of a region where semiconductor wafer W is placed. For instance, heater  46  can be arranged over the whole area of the top surface of placing table  32 . Heater  46  is connected to an electric feed bar (not shown) inserted into cylindrical support  30 , and power is applied to heater  46  from an external heat source so that the temperature of heater  46  can be controlled to the desired level. In addition, for instance, heater  46  is electrically divided into an inner zone and an outer zone concentrically surrounding the inner zone in such a manner that the temperature control (power control) can be independently performed for the inner and outer zones. 
         [0044]    In addition, a pin elevating unit  48  is installed on placing table  32  to move up and down wafer W. In detail, a plurality of pin insertion holes  50 , for instance, three pin insertion holes  50  are provided in placing table  32  (only two pin insertion holes are shown in  FIG. 1 ). Pin elevating unit  48  includes push pins  52 , which are bent in an L-shape and movably inserted into pin insertion holes  50 . 
         [0045]    Each push pin  52  is supported by a support rod  54  extending perpendicular to push pin  52  (only two push pins are shown in  FIG. 1 ). A lower end of each support rod  54  is connected to a push ring  56  having a circular ring shape and made from a ceramic material, such as alumina. Push ring  56  is supported by an upper end of an elevating rod  60  extending through bottom part  20  of processing container  4 . Elevating rod  60  can be moved up and down by an actuator  62 . That is, wafer W is moved up and down by moving elevating rod  60  up and down. 
         [0046]    In addition, a flexible bellows  65  is provided between actuator  62  and a predetermined portion of bottom part  20  of processing container  4  where elevating rod  60  extends. Thus, elevating rod  60  can be moved up and down while keeping air-tightness in processing container  4 . 
         [0047]    A ring member  64  prepared as a thin ring plate is placed on the top surface of placing table  32  adjacent to the outer peripheral portion of wafer W. As shown in  FIG. 4 , ring member  64  may serve as a cover ring  66  for preventing the thin film from being deposited on the outer peripheral portion of placing table  32 . In addition, a diameter of an inner peripheral surface of ring member  64  is slightly larger than the diameter of wafer W. Ring member  64  (cover ring  66 ) can be made by using a ceramic material, such as nitride aluminum or alumina (Al 2 O 3 ). In addition, ring member  64  (cover ring  66 ) is fixed to the upper end of each support rod  54 . Therefore, ring member  64  (cover ring  66 ) can move up and down integrally with push pin  52 . 
         [0048]    In addition, a decomposition restraint gas feeding unit  70 , which is the technical feature of the present invention, is installed in placing table  32  to feed decomposition restraint gas for restraining the thermal decomposition of the raw material gas. In detail, as shown in  FIGS. 2 to 5 , decomposition restraint gas feeding unit  70  mainly includes a gas discharge port  72  formed along the circumference of placing surface  43  of placing table  32  corresponding to the outer peripheral portion of wafer W, a gas path  74  communicated with gas discharge port  72 , and a restraint gas source  76  for storing the decomposition restraint gas. Restraint gas source  76  is connected to gas path  74 . The decomposition restraint gas includes CO gas having an identical composition to the gas generated when the raw material gas (Ru 3 (CO) 12 ) is thermally decomposed. 
         [0049]    Gas discharge port  72  is open at a part of placing surface  43 , that is, at the bottom surface of groove  45 . In addition, as shown in  FIG. 2 , gas discharge port  72  is defined by an annular slit  78  formed along the outer circumferential portion of placing table  32 . For instance, slit  78  has a width of about 1 mm. 
         [0050]    Gas path  74  communicated with gas discharge port  72  may include a main gas path  74 A formed through cylindrical support  30  and branch gas paths  74 B formed in placing table  32  while branching from the upper end of main gas path  74 A. Although three branch gas paths  74 B having the same angle are shown in  FIG. 3 , the number of branch gas paths  74 B may not be limited to this number. In addition, a diffusion chamber  80  is formed at an immediate below of gas discharge port  72  along the outer peripheral portion of placing table  32 . The front end of each branch gas path  74 B is communicated with diffusion chamber  80 . 
         [0051]    Thus, gas discharge port  72  is communicated with each branch gas path  74 B through diffusion chamber  80 . Accordingly, the CO gas, which is the decomposition restraint gas flowing into branch gas paths  74 B, may be diffused along the outer peripheral portion of placing table  32  in diffusion chamber  80  so that the CO gas can be uniformly discharged through gas discharge port  72 . 
         [0052]    At this time, the discharged gas from gas discharge port  72  may be directed to the outer peripheral portion of wafer W. Therefore, the thin film may not be deposited on the outer peripheral portion of wafer W due to the decomposition restraint gas. Main gas path  74 A is wider than branch gas path  74 B. A flow rate controller  82 , such as a mass flow controller, is installed in main gas path  74 A, and opening/closing valves  84  are provided at both sides of main gas path  74 A. 
         [0053]    In order to control the operation of thin film forming apparatus  2 , a control unit  86  including a computer may be provided. Control unit  86  controls the start and the end of gas feeding, the flow rate of gas, the process pressure, and the temperature of wafer W. Control unit  86  has a storage medium  88  for storing computer program to perform the control operation as described above. Storage medium  88  may include a flexible disc, a compact disc (CD), a CD-ROM, a hard disc, a flash memory or a DVD. 
         [0054]    Hereinafter, the operation of film forming apparatus  2  having the above structure will be described. 
         [0055]    First, semiconductor wafer W to be processed is loaded into processing container  4  through gate valve  18  and loading/unloading opening  16  by a transfer arm (not shown). Then, wafer W is transferred to push pin  52 , which has been moved up together with ring member  64  of pin elevating unit  48 . After that, as push pin  52  is moved down, wafer W is placed on placing surface  43 , which is the top surface of placing table  32 . 
         [0056]    In this manner, if wafer W has been placed on placing table  32 , a predetermined gas, for instance, the raw material gas for the thin film is supplied into processing space S from shower head  6 . At this time, the flow rate of the raw material gas is controlled. Thus, processing container  4  can be maintained at the predetermined process pressure. For instance, if the Ru layer is formed, the Ru 3 (CO) 12  gas is supplied as the raw material gas together with the CO gas serving as the carrier gas. 
         [0057]    Then, power is applied to the heater installed on the placing table  32  so that wafer W is heated to the predetermined process temperature through placing table  32 . Accordingly, the Ru layer, which is a thin metal layer, is formed on the surface of wafer W through the thermal CVD process under the process conditions of the process pressure of about 13.3 Pa, and the wafer temperature of about 200° C. to about 250° C. In addition, shower head  6  and the sidewall of processing container  4  are also heated by a heater (not shown) to the temperature of about 75° C. to about 80° C. 
         [0058]    In general, when forming the thin film through the above procedure, the raw material gas may be diffused radially outward of processing space S formed above wafer W and then introduced into exhaust space  22  after flowing downward from the outer peripheral portion of placing table  32 . After that, the raw material gas is discharged to exhaust system  36  from exhaust space  22  through exhaust port  34 . At this time, some of the exhaust gas flows into the gap formed between the back surface of wafer W and placing surface  43  by detouring around the peripheral portion (edge portion) of wafer W, so that the thin film may be unnecessarily deposited on the region corresponding to the flowing route of the raw material gas. 
         [0059]    For this reason, according to the conventional thin film forming apparatus of the related art, the thin film is unnecessarily deposited from the outer peripheral portion of wafer W to the entire lateral side of wafer W. Specifically, the thin film is unnecessarily deposited on bevel portion  90  (see,  FIGS. 4 and 5 ) or the back surface of wafer W. In particular, if the process condition is set with the high step coverage in order to ensure the embeddability for the various holes and recesses, the thin film may be formed in a fine gap and the formation of the undesired thin film may significantly occur. 
         [0060]    However, according to the present embodiment of the disclosure, decomposition restraint gas feeding unit  70  is installed in placing table structure  29  to feed the decomposition restraint gas, such as the CO gas for restraining the thermal decomposition of the raw material gas, to the outer peripheral portion of wafer W, so that the thermal decomposition of the raw material gas may be restrained at the outer peripheral portion of wafer W, thereby preventing the formation of the undesired thin film on the outer peripheral portion of wafer W. 
         [0061]    Specifically, as shown in  FIG. 1 , the CO gas is supplied to main gas path  74 A of gas path  74  from a decomposition restraint gas source  76 . At this time, the flow rate of the CO gas is controlled by a flow rate controller  82 . Then, the CO gas reaches placing table  32  and flows through branch gas paths  74 B. 
         [0062]    Then, as shown in  FIGS. 4 and 5 , the CO gas is introduced into diffusion chamber  80  formed below the peripheral portion of wafer W. The CO gas is diffused in diffusion chamber  80  along the outer peripheral portion of placing table  32  and is discharged upward through annular slit  78  of gas discharge port  72 . Thus, the CO gas is discharged toward the peripheral portion of the wafer W as indicated by an arrow  92  (see,  FIG. 5 ). Thus, as mentioned above, the thermal decomposition of the raw material gas may be restrained, so that the thin film may not be deposited on bevel portion  90  or the back surface of the peripheral portion of wafer W. 
         [0063]    In particular, since annular groove  45  is formed in placing table  32  corresponding to the peripheral portion of wafer W, as shown in  FIG. 5 , if wafer W is placed on placing surface  43 , gas staying space  94  is formed between groove  45  and the outer peripheral portion of wafer W, and the CO gas discharged from gas discharge port  72  may be temporally stored in gas staying space  94 . Therefore, the density of CO may be increased in the vicinity of gas staying space  94 , so that the formation of the undesired thin film can be effectively prevented. 
         [0064]    Hereinafter, the decomposition restraining mechanism by the CO gas of the Ru 3 (CO) 12  gas, which is the raw material gas, will be explained. The Ru 3 (CO) 12  gas performs the reversible thermal decomposition reaction according to the following chemical formula. 
         [0000]      Ru 3 (CO) 12           Ru 3 (CO) 12 ↑
 
         [0000]      Ru 3 (CO) 12 ↑         Ru 3 (CO) 12-x ↑+XCO↑
 
         [0000]      Ru 3 (CO) 12-x ↑+Q→3Ru+(12-X)CO↑
 
         [0000]      Ru 3 (Co) 12 ↑+Q→3Ru+12CO↑
   In the above chemical formula, “         ” represents a reversible reaction, “↑” represents a gas phase, and the elements having no “↑” represent a solid phase. “Q” represents applying calorie.   
 
         [0066]    As can be understood from the above chemical formula, according to the second chemical formula, the Ru 3 (CO) 12  gas and the CO gas are reversibly generated through the thermal decomposition reaction. Thus, if the CO gas is supplied from the outside, the forward reaction (→) is restrained and the reverse reaction (←) is performed. As a result, the thermal decomposition of the Ru 3 (CO) 12  gas is restrained so that the formation of the undesired thin film may be restrained. The thermal deposition reaction may include the forward reaction and the reverse reaction, and the thermal decomposition may refer to the forward reaction. 
         [0067]    Since the CO gas, which is the decomposition restraint gas, is an identical gas to the composition of gas generated when the raw material gas is thermally decomposed, the CO gas may not exert great influence upon the formation of the thin film, which is different from the related art using Ar gas as purge gas. Thus, the thickness uniformity of the thin film formed on the top surface of wafer W may not be degraded, but may be improved. 
       Modification of Gas Discharge Port  72   
       [0068]    According to the present embodiment, as shown in  FIG. 2 , annular slot  78  is formed as gas discharge port  72 , but the present invention is not limited thereto. For instance, the gas discharge port can be configured as shown in  FIG. 6 .  FIG. 6  is a view showing the placing surface formed with the modified gas discharge port. In the following description, details of the elements and structures that have been described with reference to  FIG. 2  will be omitted in order to avoid redundancy and the same reference numerals will be designated to the same elements. Referring to  FIG. 6 , a plurality of discharge holes  96  are formed along the circumference of placing table  32 . 
         [0069]    The interval between discharge holes  96  is about 21 mm if exhaust hole  96  has a diameter of 1 mm, and about 31 mm if discharge hole  96  has a diameter of 1.2 mm. Preferably, discharge holes  96  have the same pitch. In this case, the CO gas can be uniformly discharged through discharge holes  96 . The effect obtained from the previous embodiment can be achieved in the embodiment shown in  FIG. 6 . 
       Modification of Ring Member  64   
       [0070]    According to the embodiments described above, as shown in  FIGS. 4 and 5 , the inner peripheral portion of ring member  64  is slightly spaced apart from the edge portion of wafer W in the horizontal direction, but the disclosure is not limited thereto.  FIGS. 7A and 7B  are enlarged sectional views showing the placing table including the modified ring member. In the following description, details of the elements and structures that have been described with reference to  FIGS. 4 and 5  will be omitted in order to avoid redundancy and the same reference numerals will be designated to the same elements. 
         [0071]    Referring to  FIG. 7A , the inner peripheral portion of cover ring  66  serving as ring member  64  may extend inward so that cover ring  66  overlaps with an edge portion of wafer W in the vertical direction by a predetermined length L 1 . That is, cover ring  66  is located above wafer W without making contact with wafer W. 
         [0072]    In this case, an upper portion of gas staying space  94  defined by the outer peripheral surface of bevel portion (edge portion)  90  of wafer W and groove  45  are covered with the inner peripheral portion of cover ring  66 . As a result, the CO gas can stay in gas staying space  94  for a long time, so that the formation of the undesired thin film on bevel portion  90  can be effectively prevented. 
         [0073]    Referring to  FIG. 7B , ring member  64  serving as cover ring  66  may be located slightly lower than ring member  64  shown in  FIG. 7A , and the inner peripheral portion of ring member  64  extends inward to serve as a clamp ring  98 . In detail, a bottom surface of the inner peripheral portion of clamp ring  98  makes contact with the top surface of the edge portion of wafer W so that the top surface of the edge portion of wafer W may be pressed against placing table  32 . To this end, preferably, a taper surface  100  is formed at the inner peripheral portion of clamp ring  98 . 
         [0074]    In this case, the upper portion of gas staying space  94  defined by the outer peripheral surface of bevel portion (edge portion)  90  of wafer W and groove  45  are substantially covered (sealed) with the inner peripheral portion of clamp ring  98 . As a result, the CO gas can stay in gas staying space  94  for a relatively long time as compared with the case shown in  FIG. 7A , so that the formation of the undesired thin film on bevel portion  90  can be effectively prevented. 
       Evaluation Test for the Invention 
       [0075]    Hereinafter, the evaluation test performed with respect to the placing structure of the present invention will be described.  FIG. 8  is a graph showing thickness of the thin film deposited on the peripheral portion (bevel portion) of the semiconductor wafer. The Ru layer is formed by using the placing table structure shown in  FIG. 4  under the process conditions as follows: the temperature of the placing table  32  is 215° C., the flow rate of the carrier gas (CO gas) is 100 sccm, the flow rates of the decomposition restraint gas (CO gas) supplied to the peripheral portion of the wafer are three kinds of 0 sccm, 10 sccm and 100 sccm, thin film forming time is 90 sec, and the diameter of the wafer is 300 mm. 
         [0076]    Referring to the graph shown in  FIG. 8 , in the X-axis (position of the peripheral portion of the wafer), the center of the thickness at the peripheral portion of the wafer is represented as 0, the front side from the center is represented with “+” and the back side from the center is represented with “−”. The position of the wafer is schematically shown in  FIG. 8 . In addition, in the Y-axis (thickness of relative thin film), the thickness of the relative thin film of the Ru layer is represented in an arbitrary unit (a.u.) and the XRF (X-ray fluorescence) is used to measure the thickness of relative thin film. 
         [0077]    As shown in  FIG. 8 , when the flow rate of the decomposition restraint gas (CO gas) is supplied to the peripheral portion of the wafer is 10 sccm, the thickness profile is substantially identical to the thickness profile when the flow rate of the decomposition restraint gas is 0 sccm, and the thickness curves are substantially overlapped with each other. That is, the formation of the undesired Ru layer may be rarely prevented. 
         [0078]    In contrast, if the flow rate of the decomposition restraint gas is 100 sccm, as indicated by an arrow  110 , the formation of the undesired Ru layer is significantly reduced on the peripheral portion of the wafer. In detail, the thickness is reduced by 0.05 [a.u.] at the front side (+) of the peripheral portion of the wafer, and the thickness is reduced by 0.2 [a.u.] in maximum at the back side (−) of the peripheral portion of the wafer. That is, the formation of the undesired Ru layer may be effectively prevented. Therefore, when the flow rate of the decomposition restraint gas is about 1.06 sccm/cm [=100 sccm/(30 cam×π)], the effect of the present invention appears to be exhibited. 
       Raw Material Gas 
       [0079]    Although the Ru 3 (CO) 12  gas, which is a material for metal carbonyl, is used as the raw material gas in the above embodiments, the disclosure is not limited thereto. The metal carbonyl raw material gas may include at least one of the elements selected from the group consisting of Ru 3 (CO) 12 , W(CO) 6 , Ni(CO) 4 , Mo(CO) 6 , Co 2 (CO) 8 , Rh 4 (CO) 12 , Re 2 (CO) 10 , Cr(CO) 6 , Os 3 (CO) 12  and Ta(CO) 5 . 
       Subject to be Processed 
       [0080]    In addition, although the semiconductor wafer is used as the subject to be processed in the above embodiments, the semiconductor wafer may include a silicon substrate or a compound semiconductor substrate such as GaAs, SiC or GaN. Furthermore, the present invention is not limited to the above substrates, but may be applied to a substrate such as a glass substrate or ceramic substrate.
Provided is a placing table structure which is disposed in a processing container and has a subject to be processed thereon so as to form a thin film on the subject in the processing container by using raw material gas which generates thermal decomposition reaction having reversibility. The placing table structure is provided with a placing table for the purpose of placing the subject to be processed on a placing surface, i.e., an upper surface of the placing table structure, and a decomposition suppressing gas supply means which is arranged in the placing table for the purpose of supplying decomposition suppressing gas, which suppresses thermal decomposition of the raw material gas, toward a peripheral section of the subject placed on the placing surface of the placing table.