Abstract:
A substrate processing apparatus comprises a heating member, a reaction tube body provided in the heating member and having a first gas introducing section and a gas exhausting section, a substrate holder disposed in the reaction tube body for horizontally holding a substrate within the reaction tube body between the first gas introducing section and the gas exhausting section, a gas heating tube provided in the heating member along the reaction tube body, and having a second gas introducing section and a gas discharging section which is in communication with the first gas introducing section of the reaction tube body, the gas heating tube being arranged such that a gas flowing in the gas heating tube first flows form the first gas introducing section side toward the gas exhausting section side, and then returns to flow from the gas exhausting section side toward the first gas introducing section side.

Description:
This application is a divisional of co-pending Application No. 08/881,147, filed on Jun. 24, 1997, U.S. Pat. No. 6,139,641 the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a substrate processing apparatus, and more particularly, to a substrate processing apparatus for subjecting a semiconductor wafer to a process such as a film formation in a single substrate-processing manner or a small number of substrates-processing manner at a time. 
     2. Description of the Related Art 
     Conventionally, a reaction tube used with a substrate processing apparatus of a type described above has the structure shown in. FIG. 15, which is a plan view of a conventional substrate processing apparatus. 
     The substrate processing apparatus  200  is provided with a heater  70  and a reaction tube  80  disposed therein. The reaction tube  80  is provided with a reaction tube body  81 , a reaction gas introducing tube  85  and a reaction tube flange  83 . A reaction gas introducing hole  82  is provided at a central portion of an upstream of the reaction tube body  81 . The reaction gas introducing tube  85  is provided in communication with an interior of the reaction tube body  81  through the reaction gas introducing hole  82 . The reaction tube body  81  is provided at its downstream with the reaction tube flange  83 . The reaction tube flange  83  is provided with a wafer transfer hole  84 . In a state where a semiconductor wafer  90  is held in the reaction tube body  81 , heating is effected by the heater  70  and a reaction gas is introduced from the reaction gas introducing hole  82  into the reaction tube body  81 , and is exhausted from the wafer transfer hole  84  of the reaction tube flange  83 , thereby processing the semiconductor wafer  90  such as to achieve film formation. 
     According to the conventional reaction tube  80 , however, the reaction gas is introduced into the reaction tube  80  without being sufficiently heated and therefore, a temperature of the reaction gas upstream of the semiconductor wafer  90  is lowered, and there is a problem that a thickness of a film formed on the semiconductor wafer  90  becomes nonuniform. 
     SUMMARY OF THE INVENTION 
     Therefore, a main object of the present invention is to provide a substrate processing apparatus capable of conducting a uniform substrate processing such as obtaining an excellent thickness distribution of a formed film. 
     According to a first aspect of the present invention, there is provided a substrate processing apparatus, including: 
     a heater; 
     a reaction tube body provided in the heater, and having a first gas introducing section and a gas exhausting section separated at a predetermined distance from each other in a predetermined first direction; 
     a substrate holder disposed in the reaction tube body and being capable of holding a substrate within the reaction tube body between the first gas introducing section and the gas exhausting section in a state where a main face of the substrate is substantially parallel to a first plane which includes the first direction and a second direction perpendicular to the first direction; and 
     a gas heating tube provided in the heater along the reaction tube body, and having a second gas introducing section and a gas discharging section which is in communication with the first gas introducing section of the reaction tube body, the gas heating tube having a structure wherein a gas flowing in the gas heating tube first flows from the side of the first gas introducing section toward the side of the gas exhausting section and then, returns to flow from the gas exhausting section side toward the first gas introducing section side. 
     According to a second aspect of the present invention, there is provided a hot-wall type substrate processing apparatus, including: 
     a heater; 
     a reaction tube body provided in the heater, the reaction tube body including: a first gas introducing section and a gas exhausting section separated at a predetermined distance from each other in a predetermined first direction; a first side plate substantially perpendicular to the first direction and having the first gas introducing section; a ceiling plate and a bottom plate substantially parallel to a first plane including the first direction and a second direction substantially perpendicular to the first direction; and second and third side plates which is substantially parallel to the first direction and is substantially perpendicular to the first plane; 
     a substrate holder disposed in the reaction tube body and being capable of holding a substrate within the reaction tube body between the first gas introducing section and the gas exhausting section in a state where a main face of the substrate is substantially parallel to the first plane; and 
     a gas heating tube provided in the heater along one of or both of the second side plate and the third side plate, and having a second gas introducing section and a gas discharging section which is in communication with the gas introducing section of the reaction tube body. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further objects, features and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a plan view, partially broken away, for explaining a substrate processing apparatus according to first and second embodiments of the present invention; 
     FIG. 2 is a sectional view taken along the line X 2 —X 2  in FIG. 1; 
     FIG. 3 is an enlarged view of a portion in FIG. 1 surrounded by the circle A; 
     FIG. 4 is a sectional view taken along the line X 4 —X 4  in FIG. 3; 
     FIG. 5 is a sectional view taken along the line X 5 —X 5  in FIG. 3; 
     FIG. 6 is a perspective view for explaining a reaction tube used in the substrate processing apparatus of the first embodiment; 
     FIG. 7 is a perspective view for explaining the reaction tube used in the substrate processing apparatus of the first embodiment; 
     FIG. 8 is an enlarged view of the portion in FIG. 1 surrounded by the circle A for explaining a reaction tube used in a substrate processing apparatus according to the second embodiment; 
     FIG. 9 is a sectional view taken along the line X 9 —X 9  in FIG. 8; 
     FIG. 10 is a sectional view taken along the line X 10 —X 10  in FIG. 8; 
     FIG. 11 is a plan view for explaining a substrate processing apparatus according to a third embodiment, in which a semiconductor wafer is mounted on a wafer mounting plate; 
     FIG. 12 is a sectional view taken along the line X 12 —X 12  in FIG. 11, showing a state before mounting the semiconductor wafer on the wafer mounting plate; 
     FIG. 13 is a perspective view for explaining a reaction tube used in the substrate processing apparatus of the third embodiment; 
     FIG. 14 is a perspective view for explaining the reaction tube used in the substrate processing apparatus of the third embodiment; and 
     FIG. 15 is a plan view for explaining a reaction tube used in a conventional substrate processing apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Referring to FIGS. 1 to  7 , there is shown a substrate processing apparatus  100  of a single substrate-processing type according to a first embodiment of the present invention. The substrate processing apparatus  100  is provided with a heater  70 , a reaction tube  10  provided in the heater  70 , and a thermal insulator  72 . Each of the heater  70  and the reaction tube  10  are covered at their upper, lower, left and right portions with the thermal insulator  72 , and are formed into a so-called hot-wall type structure. The reaction tube  10  is provided with a reaction tube body  20 , a gas heating tube  40  and a reaction tube flange  26 . A wafer mounting plate  120  is provided in the reaction tube body  20 . The wafer mounting plate  120  is formed with a space  124  having a diameter larger than that of a Si-semiconductor wafer  90 . Three wafer supporting claws  122  are projectingly provided in the space  124 . The reaction tube body  20  and the gas heating tube  40  are made of quartz. 
     The reaction tube body  20  is substantially formed into a rectangular parallelepiped shape, and includes a ceiling plate  21 , a bottom plate  22  and side plates  23 ,  24  and  25 . The ceiling plate  21  and the bottom plate  22  are parallel to each other, and the side plates  23  and  24  are parallel to each other. The side plate  25  is perpendicular to the ceiling plate  21 , the bottom plate  22  and side plates  23  and  24 . The wafer mounting plate  120  is disposed, in the reaction tube body  20 , parallel to the ceiling plate  21  and the bottom plate  22 . One sheet of the Si-semiconductor wafer  90  is supported by the wafer supporting claws  122  of the wafer mounting plate  120  and is disposed in the space  124 . A surface of the Si-semiconductor wafer  90  and an upper face of the wafer mounting plate  120  are within the same plane. The Si-semiconductor wafer  90  is held in parallel to the ceiling plate  21  and the bottom plate  22 . 
     The side plate  25  is provided, at a height substantially equal to the semiconductor wafer  90 , with a rectangular opening  28  which is parallel to a surface of the semiconductor wafer  90  and which is long in a lateral direction. The opening  28  extends from the vicinity of a corner portion  31  between the side plates  25  and  24  to the vicinity of a corner portion  32  between the side plates  25  and  23 . The reaction tube flange  26  is provided at a downstream of the reaction tube body  20  at the opposite side of the side plate  25 . The reaction tube flange  26  is provided with a wafer transfer hole  27  which is rectangular in shape. The wafer transfer hole  27  has a size substantially equal to a phantom opening of the reaction tube body  20  obtained by cutting the reaction tube body  20  in a direction intersecting a direction of a gas flow at a right angle. 
     The gas heating tube  40  includes a lower gas heating tube  42  and an upper gas heating tube  43 . The lower gas heating tube  42  includes substantially straight tubes  45 ,  46  and  47 . The upper gas heating tube  43  includes substantially straight tubes  48 ,  49  and  50 . The tubes  45  and  50  are provided in parallel to a surface of the semiconductor wafer  90  along the outer side of the side plate  25 , the tubes  46  and  48  are provided in parallel to the surface of the semiconductor wafer  90  along the outer side of the side plate  23 , and the tubes  47  and  49  are provided in parallel to the surface of the semiconductor wafer  90  along the outer side of the side plate  24 . 
     One end of the tube  44  is in communication with a central portion of the tube  45 , and the other end of the tube  44  is formed with a gas supply hole  41 . One end of the tube  45  is in communication with one end of the tube  46 , the other end of the tube  46  is in communication with one end of the tube  48 , and the other end of the tube  48  is in communication with one end of the tube  50 . The other end of the tube  45  is in communication with one end of the tube  47 , the other end of the tube  47  is in communication with one end of the tube  49 , and the other end of the tube  49  is in communication with the other end of the tube  50 . 
     The tube  50  is provide with a plurality of gas discharging holes  60  forming a line parallel to a surface of the semiconductor wafer  90  at a height substantially equal to the semiconductor wafer  90 . These gas discharging holes  60  are arranged from the vicinity of the corner portion  31  between the side plates  25  and  24  to the vicinity of the corner portion  32  between the side plates  25  and  23 . The opening  28  formed in the side plate  25  is designed such as to expose all of the plurality of gas discharging holes  60  provided in the tube  50 , and these gas discharging holes  60  are in communication with the opening  28  of the side plate  25 . 
     All of the reaction tube body  20 , the gas heating tube  40 , the reaction tube flange  26 , the wafer mounting plate  120 , the semiconductor wafer  90 , the tube  44 , the opening  28 , the plurality of gas discharging holes  60  and the wafer transfer hole  27  are formed symmetrically. 
     The gas heating tube  40  is welded to the reaction tube body  20 . 
     In a state where a sheet of the semiconductor wafer  90  is held in the reaction tube body  20 , a reaction gas is supplied from the gas supply hole  41  while being heated by the heater  70  to conduct a process such as a film formation. 
     The reaction gas supplied from the gas supply hole  41  is supplied to the central portion of the tube  45  through the tube  44 . Thereafter, the reaction gas diverges in left and right directions within the tube  45 , and respectively flow into the lower tubes  46  and  47  which are formed symmetrically. The reaction gas, after passing through the tubes  46  and  47 , returns to flow into the upper tubes  48  and  49 , respectively, and flows therethrough into opposite ends of the tube  50 . Thereafter, the reaction gas is introduced into the reaction tube body  20  through the gas discharging holes  60  provided in the tube  50  forming a line laterally as well as through the opening  28  provided in the side plate  25 . The gas after reaction is exhausted through the wafer transfer hole  27  of the flange  26 . 
     In the present embodiment, the reaction gas is supplied into the reaction tube body  20  after passing through the gas heating tube  40  which is heated by the heater  70 . Therefore, the reaction gas is preheated and introduced into the reaction tube body  20 . As a result, an upstream portion of the semiconductor wafer  90  is restrained from being cooled by the reaction gas, which enhances the uniformity of distribution of temperature over the entire surface of the semiconductor wafer  90 , and also enhances the uniformity of a thickness of a film over the entire surface thereof which is formed on a surface of the semiconductor wafer  90 , especially the uniformity of the thickness of the film in a direction of a flow of the reaction gas. Further, depending on a kind or type of the gas, it is possible to sufficiently decompose the reaction gas within the gas heating tube  40 , and as a result, a quality of the film is enhanced. 
     Further, because the gas heating tube  40  is disposed along the side plates  23 ,  24  and  25  of the reaction tube body  20 , the substrate processing apparatus  100  can be made small in size. 
     All of the reaction tube body  20 , the gas heating tube  40 , the reaction tube flange  26 , the wafer mounting plate  120 , the semiconductor wafer  90 , the tube  44 , the opening  28 , and the plurality of gas discharging holes  60  are formed symmetrically and therefore, it is possible to keep the balance in temperature between left and right sides, and to equalize flow speed of the reaction gas introduced into the reaction tube body  20  from left and right sides. As a result, it is possible to enhance the uniformity of a thickness of a film over the entire surface which is formed on a surface of the semiconductor wafer  90 , especially the uniformity of the thickness of the film in a lateral direction with respect to a direction of a flow of the gas. 
     Further, the tube  50  is provide with the plurality of gas discharging holes  60  forming a line in parallel to a surface of the semiconductor wafer  90 , and the side plate  25  is provided the opening  28  such as to expose the gas discharging holes  60 . Therefore, the reaction gas is introduced like a shower, and the reaction gas flow above the surface of the semiconductor wafer  90  becomes a laminar flow, which further enhances the uniformity of a thickness of the film. In contrast, in the reaction tube  80  of the conventional structure as is shown in FIG. 15, because only one gas introducing hole  82  is provided, the gas flow above the semiconductor wafer  90  does not become a laminar flow and thus, a thickness of the film formed on the semiconductor wafer  90  is prone to be nonuniform. 
     The plurality of gas discharging holes  60  are provided from the vicinity of the corner portion  31  between the side plates  25  and  24  to the vicinity of the corner portion  32  between the side plates  25  and  23 . Therefore, turbulence zones of the reaction gas introduced into the reaction tube body  20  can be decreased and as a result, the gas flow can be formed into a laminar flow, and a replacement efficiency of the gas can be enhanced. 
     Further, in such a hot-wall type substrate processing apparatus, because the entire reaction tube body  20  is kept at a predetermined temperature, even if the gas heating tube  40  is disposed along the side plates  23 ,  24  and  25 , it is possible to sufficiently preheat the reaction gas. Furthermore, because the gas heating tube  40  is disposed along the side plates  23 ,  24  and  25 , and the gas heating tube  40  does not face the ceiling plate  21  and the bottom plate  22 , the heater  70  which faces the ceiling plate  21  and the bottom plate  22  of the reaction tube body  20  is not blocked or interrupted by the gas heating tube  40 , and as a result, the uniformity of the distribution of temperature over the entire surface of the semiconductor wafer  90  is enhanced. 
     Also, by disposing the gas heating tube  40  along the side plates  23 ,  24  and  25  of the reaction tube body  20  as described above, it is possible to restrain the substrate processing apparatus  100  from increasing its height, and it is easy to uniform the distribution of temperature over the entire surface of the semiconductor wafer  90 . In contrast, if the gas heating tube  40  is disposed along the ceiling plate  21  or the bottom plate  22  of the reaction tube body  20 , the height of the substrate processing apparatus  100  is increased by such amount. In order to process a large number of semiconductor wafers  90  in a small floor space, piling up a plurality of reaction tube bodies  20  in a vertical direction seems to be an effective way. In such a case, however, if each of the gas heating tubes  40  is disposed along the ceiling plate  21  or the bottom plate  22  of the reaction tube body  20 , the number of the reaction tube bodies  20  which can vertically be piled up is reduced for that and thus, the number of semiconductor wafers  90  per unit area is reduced. 
     If the gas heating tube  40  is disposed along the ceiling plate  21  or the bottom plate  22  of the reaction tube body  20 , because the ceiling plate  21  and the bottom plate  22  are provided in parallel to a major surface of the semiconductor wafer  90 , it is difficult to uniform the distribution of temperature over the entire surface of the semiconductor wafer  90  unless the gas heating tube  40  is disposed uniformly along the ceiling plate  21  or the bottom plate  22  of the reaction tube body  20 . Further, such a structure itself in which the gas heating tube  40  is uniformly disposed along the ceiling plate  21  or the bottom plate  22  of the reaction tube body  20  is difficult, which increases a manufacturing cost. In contrast, if the gas heating tube  40  is disposed along the side plates  23 ,  24  and  25  of the reaction tube body  20 , it is possible to easily uniform the distribution of temperature over the entire surface of the semiconductor wafer  90  with a simple structure. 
     In the present embodiment, taking into consideration the fact that the amount of gas diverged into the left and right gas heating tube may not always be equal to each other, the left and right portions of the gas heating tube are joined to each other at their end portions, so as to moderate a difference in the amount of gas between the left and right sides of the gas heating tube. Further, in order to avoid disturbing the distribution of temperature in the vicinity of the semiconductor wafer  90 , the gas heating tube  40  is disposed at a location away from the both sides of the semiconductor wafer. 
     Second Embodiment 
     In the first embodiment described above, the tube  50  of the gas heating tube  40  is provided with the plurality of gas discharging holes  60 , and the side plate  25  of the reaction tube body  20  is provided with the opening  28  which exposes the plurality of gas discharging holes  60 . However, as is shown in FIGS. 1,  2  and  8  to  10 , the second embodiment differs from the first embodiment in that the side plate  25  of the reaction body  20  is provided with a plurality of gas introducing holes  128  such as to form a line in parallel to the semiconductor wafer  90  at a height substantially equal to the semiconductor wafer  90 , and the tube  50  is provided with an opening  160  which is long in a lateral direction and which is in communication with the plurality of gas introducing holes  128 . Other structures are the same as those of the first embodiment. 
     Also, in the present embodiment, the reaction gas is supplied into the reaction tube body  20  after passing through the gas heating tube  40  which is heated by the heater  70 . Therefore, the reaction gas is preheated and introduced into the reaction tube body  20 . As a result, an upstream portion of the semiconductor wafer  90  is restrained from being cooled by the reaction gas, which enhances the uniformity of distribution of temperature over the entire surface of the semiconductor wafer  90 , and also enhances the uniformity of a thickness of a film over the entire surface thereof which is formed on a surface of the semiconductor wafer  90 , especially the uniformity of the thickness of the film in a direction of a flow of the reaction gas. Further, depending on a kind or type of the gas, it is possible to sufficiently decompose the reaction gas within the gas heating tube  40 , and as a result, a quality of the film is enhanced. 
     Further, because the gas heating tube  40  is disposed along the side plates  23 ,  24  and  25  of the reaction tube body  20 , the substrate processing apparatus  100  can be made small in size. 
     These gas introducing holes  128  are arranged from the vicinity of the corner portion  31  between the side plates  24  and  25  to the vicinity of the corner portion  32  between the side plates  23  and  25 . All of the reaction tube body  20 , the gas heating tube  40 , the reaction tube flange  26 , the wafer mounting plate  120 , the semiconductor wafer  90 , the tube  44 , the opening  160 , the plurality of gas introducing holes  128  and the wafer transfer hole  27  are formed symmetrically. 
     All of the reaction tube body  20 , the gas heating tube  40 , the reaction tube flange  26 , the wafer mounting plate  120 , the semiconductor wafer  90 , the tube  44 , the opening  160 , and the plurality of gas introducing holes  128  are formed symmetrically and therefore, it is possible to keep the balance in temperature between left and right sides, and to equalize flow speed of the reaction gas introduced into the reaction tube body  20  from left and right sides. As a result, it is possible to enhance the uniformity of a thickness of a film over the entire surface which is formed on a surface of the semiconductor wafer  90 , especially the uniformity of the thickness of the film in a lateral direction with respect to a direction of a flow of the gas. 
     Further, the side plate  25  is provide with the plurality of gas introducing holes  128  forming a line in parallel to a surface of the semiconductor wafer  90 , and the tube  50  is provided the opening  160  such as to communicate with the plurality of gas introducing holes  128 . Therefore, the reaction gas is introduced like a shower, and the reaction gas flow above the surface of the semiconductor wafer  90  become a laminar flow, which further enhances the uniformity of a thickness of the film. 
     The plurality of gas introducing holes  128  are provided from the vicinity of the corner portion  31  between the side plates  25  and  24  to the vicinity of the corner portion  32  between the side plates  25  and  23 . Therefore, turbulence zones of the reaction gas introduced into the reaction tube body  20  can be decreased and as a result, the gas flow can be formed into a laminar flow, and a replacement efficiency of the gas can be enhanced. 
     Third Embodiment 
     In the above described first embodiment, a tip end  401  of the has heating tube  40  extends beyond the semiconductor wafer  90  and the wafer mounting plate  120  and reaches the vicinity of the reaction tube flange  26 , and the gas heating tube  40  is provided along the side plates  23  and  24  of the reaction tube body  20 . However, as is shown in FIGS. 11 to  14 , the third embodiment differs from the first embodiment in that the gas heating tube  40  is provided along the side plates  23  and  24  of the reaction tube body  20  such that the gas heating tube  40  is turned down or bent back before the center of the semiconductor wafer  90 . Other structures are the same as those of the first embodiment. 
     The semiconductor wafer  90  is placed on a tweezer  130  and transferred into the reaction tube body  20 , and is placed on the wafer supporting claws  122  of the wafer mounting plate  120  and disposed in a space  124 . Further, the semiconductor wafer  90  on the wafer mounting plate  120  is placed on the tweezer  130  and carried out from the reaction tube  10 . At those times, in order to determine a position of the tweezer  130  which transfers the semiconductor wafer  90  and an operating range of the tweezer  130 , a teaching process is conducted. When the teaching process is conducted, the thermal insulator  72  and the heater  70  are detached. Clearances in all horizontal directions of the semiconductor wafer  90  and the wafer mounting plate  120  are checked by vertically seeing from above the reaction tube  10  through the ceiling plate  21 . Clearances in vertical direction is also checked by horizontal observation through the side plate  23  and/or the side plate  24 . Points of such checking of the clearances in vertical direction are: to check a height of the wafer mounting plate  120  so that the wafer mounting plate  120  and the semiconductor wafer  90  do not interfere with each other when the tweezer  130  on which the semiconductor wafer  90  is carried passes above the wafer mounting plate  120 ; and to check a vertical stroke motion of the tweezer  130  conducted when the semiconductor wafer  90  placed on the tweezer  130  is lowered to be placed on the wafer mounting plate  120 , and the semiconductor wafer  90  placed on the wafer mounting plate  120  is picked up by the tweezer  130 . 
     According to the third embodiment, when the reaction tube body  20  is viewed from the direction of the side plate  23  or  24 , more than half portion of the semiconductor wafer  90  can be observed, and the wafer mounting plate  120  can also be observed from its one end  121  to a region beyond the central portion of the space  124  in which the semiconductor wafer  90  is to be provided. Therefore, when the teaching process is conducted, because the semiconductor wafer  90  and the wafer mounting plate  120  can sufficiently be observed, the teaching process can easily be conducted. 
     For conducting the teaching process, when the reaction tube body  20  is viewed from the direction of the side plate  23  or  24 , it is preferable that at least a region from the one end  121  of the wafer mounting plate  120  to one end  91  of the semiconductor wafer  90  can be observed. Further, when the reaction tube  10  is viewed from the direction of the side plate  23  or  24 , it is more preferable that at least a half of the semiconductor wafer  90 , as well as a region of the wafer mounting plate  120  from its one end  121  to at least the central portion of the space  124  in which the semiconductor wafer  90  is to be provided. Therefore, it is preferable to provide the gas heating tube  40  along the side plates  23  and  24  of the reaction tube body  20  such that the gas heating tube  40  is bent back before the one end  91  of the semiconductor wafer  90 , and it is more preferable to provide the gas heating tube  40  along the side plates  23  and  24  of the reaction tube body  20  such that the gas heating tube  40  is bent back before the center or the vicinity of the center of the semiconductor wafer  90 . 
     In the third embodiment, a length of the gas heating tube  40  is shorter than that of the first embodiment. However, a length from the central portion  451  of the tube  45  through the tubes  46  and  48  to the corner portion  52  is set 240 mm or longer, and a length from the central portion  451  of the tube  45  through the tubes  47  and  49  to the corner portion  51  is set 240 mm or longer. Therefore, the reaction gas introduced from the gas discharging hole  60  provided in the vicinity of the corner portions  52  ( 32 ) and  51  ( 31 ) is sufficiently heated up to the same temperature as that in the reaction tube body  20 . Therefore, according to the third embodiment also, the reaction gas is sufficiently preheated by the gas heating tube  40  and is introduced into the reaction tube body  20 . As a result, an upstream portion of the semiconductor wafer  90  is restrained from being cooled by the reaction gas, which enhances the uniformity of distribution of temperature over the entire surface of the semiconductor wafer  90 , and also enhances the uniformity of a thickness of a film over the entire surface which is formed on a surface of the semiconductor wafer  90 , especially the uniformity of the thickness of the film in a direction of a flow of the reaction gas. Further, depending on a kind or type of the gas, it is possible to sufficiently decompose the reaction gas within the gas heating tube  40 , and as a result, a quality of the film is enhanced. 
     Further, because the gas heating tube  40  is bent back, it is possible to shorten the length of that region of the gas heating tube  40  which extends along the side plates  23  and  24  of the reaction tube body  20  while keeping a sufficient length of a course of the reaction gas flowing through the gas heating tube  40 . Therefore, it is possible to sufficiently preheat the reaction gas by the gas heating tube  40 , and to increase that region of the reaction tube body  20  which can be observed through the side plates  23  and  24  of the reaction tube body  20  without being interrupted by the gas heating tube  40 . 
     Also, because the gas heating tube  40  is disposed along the side plates  23  and  24 , not along the ceiling plate  21  and the bottom plate  22 , the semiconductor wafer  90  can be observed over its entire range through the ceiling plate  21  or the bottom plate  22 , and the space  124  of the wafer mounting plate  120  and the wafer supporting claws  122  can also be observed over their entire ranges. Therefore, it is possible to easily and reliably conduct the teaching process in all directions on a horizontal plane for a process in which the semiconductor wafer  90  is placed on or taken out from the wafer mounting plate  120 . In this way, the teaching process in all horizontal directions can be conducted by observing through the ceiling plate  23  or the bottom plate  22 . Therefore, a teaching process through the side plate  23  or  24  can be conducted only for a vertical direction, and such teaching is possible only if a portion, rather than the entire, of the semiconductor wafer  90  can be observed. At that time, if about half or more of the semiconductor wafer  90 , or about half or more of the space  124  of the wafer mounting plate  120  can be observed, the teaching process for a process in which the semiconductor wafer  90  is placed on or taken out from the wafer mounting plate  120  can be conducted easier and more reliably. 
     The gas heating tube  40  is disposed along the side plates  23 ,  24  and  25  of the reaction tube body  20 . The gas heating tube  40  is welded to the reaction tube body  20 . The tube  50  is provide with a plurality of gas discharging holes  60  forming a line parallel to a surface of the semiconductor wafer  90 . These gas discharging holes  60  are arranged from the vicinity of the corner portion  31  ( 51 ) between the side plates  25  and  24  to the vicinity of the corner portion  32  ( 52 ) between the side plates  25  and  23 . These gas discharging holes  60  are in communication with the opening  28  formed in the side plate  25 . All of the reaction tube body  20 , the gas heating tube  40 , the reaction tube flange  26 , the wafer mounting plate  120 , the semiconductor wafer  90 , the tube  44 , the opening  28 , the plurality of gas discharging holes  60  and the wafer transfer hole  27  are formed symmetrically. 
     In the first and third embodiments, the tube  50  of the gas heating tube  40  is provided with the plurality of gas discharging holes  60 , and the side plate  25  of the reaction tube body  20  is provided with the opening  28  which is in communication with these plurality of gas discharging holes  60  and which exposes the gas discharging holes  60 . In the second embodiment, the tube  50  of the gas heating tube  40  is provided with the plurality of gas introducing holes  128 , and the tube  50  of the gas heating tube  40  is provided with the opening  160  which is in communication with these plurality of gas introducing holes  128 . Alternatively, the tube  50  of the gas heating tube  40  may be provided with a plurality of gas discharging holes forming a line in parallel to a surface of the semiconductor wafer  90  at a height substantially equal to the semiconductor wafer  90 , and the side plate  25  of the reaction tube body  20  may also be provided with a plurality of gas introducing holes which respectively correspond to these gas discharging holes. In such a case also, all of the reaction tube body  20 , the gas heating tube  40 , the reaction tube flange  26 , the wafer mounting plate  120 , the semiconductor wafer  90 , the tube  44 , the plurality of gas discharging holes, the plurality of gas introducing holes and the wafer transfer hole  27  are formed symmetrically. 
     In the first to third embodiments, the substrate processing apparatus is of a single substrate-processing type in which a sheet of the semiconductor wafer  90  is held in the reaction tube body  20 . But it should be noted that the present invention can preferably be applied to a substrate processing apparatus in which a small number of substrates, preferably, two sheets of the semiconductor wafers  90  are held in the reaction tube body  20 .