Abstract:
A thermal processing unit of the present invention includes: a reaction container which an object to be processed is conveyed into and from; a process-gas introducing part for introducing a process gas into the reaction container; a replacement-gas introducing part for introducing a replacement gas into the reaction container, the replacement-gas introducing part being independent of the process-gas introducing part; and a discharging part for discharging a gas in the reaction container. A controlling part is connected to the process-gas introducing part, the replacement-gas introducing part and the discharging part. The controlling part is adapted to control the discharging part so as to lower a pressure in the reaction container with respect to at a thermal process, then control the process-gas introducing part and the replacement-gas introducing part so as to stop introducing the process gas and introduce the replacement gas into the reaction container as well as control the discharging part so as to raise the pressure in the reaction container with respect to at the thermal process, and then control the discharging part so as to lower the pressure in the reaction container with respect to at the thermal process.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a thermal processing unit and a thermal processing method for carrying out a thermal process while supplying a process gas to an object to be processed.  
           [0003]    2. Description of the Related Art  
           [0004]    There is a process to form a film on an object to be processed by a so-called reduced-pressure CVD (Chemical vapor deposition) method as one of manufacturing processes of a semiconductor device. A vertical thermal processing unit, as shown in FIG. 9, for example, is used as a unit for carrying out such a film-forming process. This vertical thermal processing unit conducts a thermal process to objects to be processed in a batch manner. Specifically, the vertical thermal processing unit is provided with a cylindrical reaction tube  10  consisting of quartz double tubes, an inner tube  11  and an outer tube  12 . A wafer-boat  13  holding many semiconductor wafers (to be referred to as a wafer hereinafter) W, which are the objects to be processed, is conveyed from a lower side of the reaction tube  10  into the reaction tube  10 . Inside of the reaction tube  10  is evacuated by a not-shown vacuum pump via a discharging tube  14  so as to be a reduced-pressure atmosphere. On the other hand, a process gas is introduced into the reaction tube  10 . A not-shown heater surrounds a side circumference of the reaction tube  10 . The film-forming process of the wafers W is carried out by heat generated by the heater.  
           [0005]    When the film-forming process is a film-forming process of, for example, a silicon nitride film, ammonium (NH 3 ) gas and dichlorosilane (SiH 2 Cl 2 ) gas, for example, are used as process gases. A gas-supplying system in this case will be briefly described. The ammonium gas is supplied from a gas supplying source  15   a  through a gas tube  16   a  and the dichlorosilane gas is supplied from a gas supplying source  15   b  through a gas tube  16   b , into the reaction tube  10  respectively. Incidentally, in order to make a maintenance cycle of the outer tube  12  longer, nitrogen gas as a purge gas is adapted to be supplied from a gas supplying source  15   c  through a gas tube  16   c  into a room between the inner tube  11  and the outer tube  12 . Vc 1  and Vc 2  indicate valves and Mc indicates a flow-rate adjusting part. In the gas tube  16   a , a valve Va 1 , a flow-rate adjusting part Ma, and a valve Va 2  are provided in this order from an upstream side thereof. In the gas tube  16   b , a valve Vb 1 , a flow-rate adjusting part Mb and a valve Vb 2  are provided as well.  
           [0006]    By the way, the aforementioned process gases are poisonous. Therefore, when the processed wafers W are taken out from the reaction tube  10  immediately after the completion of the film-forming process, the poisonous process gases, which remain in the reaction tube  10  and the gas tubes  16   a ,  16   b  communicated thereto for supplying the process gases, may be flown to the outside. Therefore, the nitrogen gas is flown from the gas supplying source  15   c  to the gas tubes  16   a ,  16   b  as a replacement gas (purge gas) after the completion of the film-forming process, so that the remaining process gases can be replaced with the nitrogen gas.  
           [0007]    Specifically, the gas tube  16   c  branches into four tubes, i.e. bypass ways  17   a ,  17   b ,  18   a ,  18   b  at an upstream portion with respect to the flow-rate adjusting part Mc. The bypass ways  17   a  and  18   a  are connected to an upstream side and a downstream side with respect to the flow-rate adjusting part Ma of the gas tube  16   a , and the bypass ways  17   b  and  18   b  are connected to an upstream side and a downstream side with respect to the flow-rate adjusting part Mb of the gas tube  16   b . A valve Va 3  is provided with the bypass way  17   a , a valve Vb 3  is provided with the bypass way  17   b , a flow-rate adjusting part Md and a valve Vd 4  are provided with the bypass way  18   a  in this order from an upstream side thereof, and a flow-rate adjusting part Me and a valve Vb 4  are provided with the bypass way  18   b  in this order from an upstream side thereof, respectively.  
           [0008]    As described above, the nitrogen gas supply is conducted by two lines to each of the gas tubes  16   a ,  16   b  that are for supplying the process gases. This is because a flow-rate adjusting range of each of the process gases is narrow and a maximum flow rate of each of the flow-rate adjusting parts Ma, Mb is small. In other words, the bypass ways  18   a ,  18   b  are provided in order to ensure a flow rate of the nitrogen gas.  
         SUMMARY OF THE INVENTION  
         [0009]    The inventor has been studying a method, for example, illustrated in FIG. 10 as a method for removing a process gas. First of all, at a time t1 when a film-forming process is completed, all the gas supplies into the gas tubes  16   a  and  16   b  are stopped. Then, the process gas is discharged toward the discharging tube  14  so that the previous process pressure of 13.3 Pa (0.1 Torr) is reduced to 0.133 Pa. However, the rate for the process gas to be discharged gradually becomes slower. Therefore, in order to enhance a probability of collision of nitrogen gas molecules and process gas molecules by raising the pressure in the reaction tube  10  up to the previous process pressure once, the nitrogen gas supply to the gas tubes  16   a  and  16   b  are started (a time t2). This leads a dilution ratio of the process gases remaining in the reaction tube  10  to be lowered to 1.0×10 −2 . Thereafter, the process gases are discharged toward the discharging tube  14  so as to reduce the pressure at a blast (a time t3). Then, when the nitrogen gas supply to the gas tubes  16   a  and  16   b  is started again (a time t4), the concentration of the remaining process gases becomes about 1.0×10 −4 . By repeating such pressure-raising/lowering steps, the dilution ratio of the process gases in the reaction tube  10  is lowered to be not more than a safety standard value of, for example, 1.0×10 −14.    
           [0010]    However, such a dilution process takes a long time, for example, about 30 minutes. This is because a power usage system is arranged at a position away from the thermal processing unit and a gas supplying unit is provided therein. In other words, since a crossover from the gas supplying unit to the thermal processing unit is long, it takes a long time to put out the process gases remaining in this part. Thereby, a period from the completion of the film-forming process of the wafers W to conveyance of the wafers W is long, which is one of causes of the low throughput.  
           [0011]    This invention is based on the above issues and the object thereof is to provide a technique capable of shortening a required time from conveying-in of an object to be processed to conveying-out thereof in a thermal processing unit and a thermal processing method, in which a process gas is supplied to the object to be processed so as to conduct a thermal process.  
           [0012]    The present invention is a thermal processing unit comprising: a reaction container which an object to be processed is conveyed into and from; a process-gas introducing part for introducing a process gas into the reaction container; a replacement-gas introducing part for introducing a replacement gas into the reaction container, the replacement-gas introducing part being independent of the process-gas introducing part; a discharging part for discharging a gas in the reaction container; and a controlling part connected to the process-gas introducing part, the replacement-gas introducing part and the discharging part, the controlling part being adapted to: control the discharging part so as to lower a pressure in the reaction container with respect to at a thermal process, then control the process-gas introducing part and the replacement-gas introducing part so as to stop introducing the process gas and introduce the replacement gas into the reaction container as well as control the discharging part so as to raise the pressure in the reaction container with respect to at the thermal process, and then control the discharging part so as to lower the pressure in the reaction container with respect to at the thermal process.  
           [0013]    According to the feature, it is possible to complete a gas replacement process of the reaction container within a short time, so that it is possible to immediately proceed to a conveying-out step of the object to be processed.  
           [0014]    Preferably, the process-gas introducing part has a process-gas way for introducing the process gas into the reaction container and a first open-close unit arranged in a vicinity of the reaction container, the first open-close unit opening and closing the process-gas way, and the controlling part is adapted to control the first open-close unit.  
           [0015]    In addition, preferably, the discharging part has a discharging way for discharging the gas in the reaction container and a pressure-adjusting unit arranged in the discharging way, the pressure-adjusting unit adjusting to open and close the discharging way so as to adjust the pressure in the reaction container, and the controlling part is adapted to control the pressure-adjusting unit.  
           [0016]    In this case, it is preferable that the thermal processing unit further comprises a bypass way connected between an upstream portion with respect to the first open-close unit in the process-gas way and the discharging way, the bypass way bypassing the reaction container, and a second open-close unit that opens and closes the bypass way.  
           [0017]    In this case, it is further preferable that an assistant replacement-gas introducing part for introducing the replacement gas into the processing-gas way is provided at an upstream portion with respect to a position connecting to the bypass way in the process-gas way.  
           [0018]    Further preferably, the assistant replacement-gas introducing part has an assistant replacement-gas way for introducing the replacement gas into the process-gas way and a third open-close unit that opens and closes the assistant replacement-gas way.  
           [0019]    Further preferably, the controlling part is connected to the second open-close unit and the third open-close unit, is adapted to control the first open-close unit and the third open-close unit to stop introducing the process gas and to introduce the replacement gas into the process-gas way so as to generate a pressure-raised state in the process-gas way, and then is adapted to control the second open-close unit so as to discharge the gas in the process-gas way through the bypass way.  
           [0020]    According to the features, it is possible to carry out a gas replacement process in the process-gas way, for example while the object to be processed is conveyed into or from the reaction container. Therefore, the throughput is enhanced.  
           [0021]    Moreover, the present invention is a thermal processing method for conducting a thermal process to an object to be processed by using a thermal processing unit comprising: a reaction container which an object to be processed is conveyed into and from; a process-gas introducing part for introducing a process gas into the reaction container; a replacement-gas introducing part for introducing a replacement gas into the reaction container, the replacement-gas introducing part being independent of the process-gas introducing part; and a discharging part for discharging a gas in the reaction container; the method comprising: a first pressure-lowering step of controlling the discharging part so as to lower a pressure in the reaction container with respect to at the thermal process, the first pressure-lowering step being conducted after completing the thermal process; a pressure-raising step of controlling the process-gas introducing part and the replacement-gas introducing part so as to stop introducing the process gas and introduce the replacement gas into the reaction container as well as controlling the discharging part so as to raise the pressure in the reaction container with respect to at the thermal process, the pressure-raising step being conducted after the first pressure-lowering step; and a second pressure-lowering step of controlling the discharging part so as to lower the pressure in the reaction container with respect to at the thermal process, the second pressure-lowering step being conducted after the pressure-raising step.  
           [0022]    Furthermore, the present invention is a thermal processing method for conducting a thermal process to an object to be processed by using a thermal processing unit comprising: a reaction container which an object to be processed is conveyed into and from; a process-gas introducing part for introducing a process gas into the reaction container; a replacement-gas introducing part for introducing a replacement gas into the reaction container, the replacement-gas introducing part being independent of the process-gas introducing part; and a discharging part for discharging a gas in the reaction container; the process-gas introducing part having a process-gas way for introducing the process gas into the reaction container and a first open-close unit arranged in a vicinity of the reaction container, the first open-close unit opening and closing the process-gas way, the controlling part being adapted to control the first open-close unit, the discharging part having a discharging way for discharging the gas in the reaction container and a pressure-adjusting unit arranged in the discharging way, the pressure-adjusting unit adjusting to open and close the discharging way so as to adjust the pressure in the reaction container, the controlling part being adapted to control the pressure-adjusting unit, the thermal processing unit further comprising: a bypass way connected between an upstream portion with respect to the first open-close unit in the process-gas way and the discharging way, the bypass way bypassing the reaction container; and a second open-close unit that opens and closes the bypass way, an assistant replacement-gas introducing part for introducing the replacement gas into the processing-gas way being provided at an upstream portion with respect to a position connecting to the bypass way in the process-gas way, and the assistant replacement-gas introducing part having an assistant replacement-gas way for introducing the replacement gas into the process-gas way and a third open-close unit that opens and closes the assistant replacement-gas way, the method comprising: a pressure-raising step of controlling the first open-close unit and the third open-close unit so as to stop introducing the process gas and introduce the replacement gas into the process-gas way so as to generate a pressure-raised state in the process-gas way, and a step of controlling the second open-close unit to discharge the gas in the process-gas way through the bypass way, the step being conducted after the pressure-raising step.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a schematic block diagram showing one embodiment of a thermal processing unit according to the present invention;  
         [0024]    [0024]FIG. 2 is a vertical cross sectional view of a reaction container in FIG. 1;  
         [0025]    [0025]FIG. 3 is a transversal cross sectional view of the reaction container in FIG. 1;  
         [0026]    [0026]FIG. 4 is a vertical cross sectional view illustrating a tip portion of a gas tube for introducing nitrogen gas;  
         [0027]    [0027]FIG. 5 is an illustrative view for illustrating an operation of the thermal processing unit in FIG. 1;  
         [0028]    [0028]FIG. 6 is a specific chart for illustrating a process to discharge a process gas from the reaction container in the thermal processing unit in FIG. 1;  
         [0029]    [0029]FIG. 7 is an illustrative view for illustrating an operation of the thermal processing unit in FIG. 1;  
         [0030]    [0030]FIG. 8 is an illustrative view for illustrating an operation of the thermal processing unit in FIG. 1;  
         [0031]    [0031]FIG. 9 is a schematic block diagram showing a thermal processing unit according to a conventional art; and  
         [0032]    [0032]FIG. 10 is a specific chart for illustrating a process to discharge a process gas from a reaction container in the prior art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0033]    [0033]FIG. 1 is a whole schematic view showing one embodiment of a thermal processing unit according to the present invention. A vertical thermal processing unit of this embodiment is provided with a reaction container  2 . FIG. 2 is a vertical cross sectional view showing the reaction container  2 . A reaction tube  21  shown in FIG. 2 is made of, for example, quartz. The reaction tube  21  has, for example, a double-tube structure consisting of an inner tube  22  and an outer tube  23 . The outer tube  23  is provided coaxially with respect to the inner tube  22  so as to form an appropriate space. An upper end of the outer tube  23  is closed, and a lower end thereof is air-tightly connected to a cylindrical metallic manifold  24 . On the other hand, an upper end of the inner tube  22  is opened. The inner tube  22  is supported by a supporting ring  25  formed to protrude from an inner peripheral face of the manifold  24 .  
         [0034]    Wafers W which are objects to be processed are placed on a wafer-boat (supporting member)  27  in a tier-like manner. A boat-elevator  26  is adapted to convey the wafer-boat (supporting member)  27  through an opening portion at a lower side of the manifold  24  into the inner tube  22 . The inner tube  22  forms a thermal processing atmosphere for the wafers W, which are objects to be processed. The boat-elevator  26  is provided with a lid  28  capable of closing the opening portion at the lower side of the manifold  24 . Additionally, a thermal insulator  29   a  is arranged so as to surround the circumference of the reaction tube  21 . A heater  29   b  composed of, for example, a resistive heating element is provided on an inner wall face of the thermal insulator  29   a . The thermal insulator  29   a  and the heater  29   b  form a heating furnace  29 .  
         [0035]    A plurality of gas introducing tubes (for convenience, only one of them is shown in the figure) for introducing a process gas, a replacement gas and the like into the reaction container  2  pierce through a lower part of the manifold  24  with respect to the supporting ring  25 . In order to supply the gases along an inner face of the inner tube  22 , tip portions of the gas introducing tubes are bent upward. In this embodiment, the gas introducing tubes correspond to gas tubes  5 ,  6  which are process gas ways and a gas tube  7  which is a replacement gas way. On the other hand, a discharging port  30  is formed at a circumferential face of an upper part of the manifold  24  with respect to the supporting ring  25 . In addition, in order to prevent adhesion of a thin film onto the inner tube  22  and the outer tube  23 , a gas tube  4  for introducing nitrogen gas pierces through an upper part of the manifold  24  with respect to the supporting ring  25 .  
         [0036]    A discharging tube  3  serving as a discharging way is hermetically connected to the discharging port  30 . A vacuum pump  32  is connected to the discharging tube  3  through a pressure-adjusting unit  31 . The pressure-adjusting unit  31  may be composed of an appropriate unit which adjusts an opening degree of the discharging tube  3 . A controlling part  100  is adapted to control the pressure-adjusting unit  31 , i.e. to adjust the opening degree of the discharging tube  3 , depending on a measured pressure obtained by a pressure gauge  101 , which measures the pressure inside the reaction container  2 . Note that the controlling part  100  conducts not only control of the pressure-adjusting unit  31  but also control of all valves used in this embodiment.  
         [0037]    Next, the plurality of gas introducing tubes for introducing the gases into the inner tube  22  will be described with reference to FIG. 3. For example, three gas introducing tubes are provided as shown in FIG. 3. Two of them are gas tubes  5 ,  6  serving as process gas ways, and one of them is a gas tube  7  serving as a replacement gas way. The gas tubes  5 ,  6  communicate with their tip portions  50 ,  60  via valve units  51 ,  61  provided in a vicinity of the reaction container  2 , respectively. Each of the tip portions  50 ,  60  protrudes into the inside of the inner tube  22 . The valve unit  51  is a combination of a valve  51   a  which serves as a first open-close unit and a valve  51   b  which serves as a second open-close unit, as shown in FIG. 1. As well, the valve unit  61  is also a combination of a valve  61   a  which serves as a first open-close unit and a valve  61   b  which serves as a second open-close unit.  
         [0038]    The valves  51   a ,  61   a  are to open and close the gas tubes  5 ,  6 , respectively. The valves  51   b ,  61   b  are respectively adapted to open and close bypass ways  52 ,  62 , which are branched out from upstream portions with respect to the valves  51   a ,  61   a  of the gas tubes  5 ,  6  and are joined with (connected to) the discharging tube  3 . Incidentally, the two bypass ways  52 ,  62  are joined together on their way (before being connected to the discharging tube  3 ), and are connected at a connection point P 1  of the discharging tube  3 .  
         [0039]    Here, the gas tube  7  will be described with reference to FIG. 3. The gas tube  7  communicates with its tip portion  70  through a valve  71  provided in a vicinity of the reaction container  2 . The tip portion  70  protrudes into the inside of the inner tube  22 .  
         [0040]    The valve units  51 ,  61  and the valve  71  are fixed to, for example, a common fixing member  20  provided in a vicinity of the reaction container  2 . Besides the aforementioned gas tubes  4  to  7 , the reaction container  2  may be provided with a cleaning-gas introducing tube for removing an adhesion which has adhered on the inner wall, and the like, which may also protrude into the inside of the inner tube  22 , although they are not shown here.  
         [0041]    By the way, there is a probability that particles in the reaction container  2  are stirred up by a pressure of the nitrogen gas when a large amount of the nitrogen gas is introduced from the gas tube  7  into the reaction container  2 , for example after completion of a film-forming process. Accordingly, it is preferable that the tip of the gas tube  7  is covered with, for example, a ceramic porous layer, which is described in Japanese Patent Laid-Open Publication (Kokai) 2000-58530. The ceramic porous layer is, for example, a silica porous layer  9  as shown in FIG. 4. As shown in FIG. 4, this silica porous layer  9  has a cylindrical structure whose upper end is closed, and is fixed to the tip of the gas tube  7  by welding. A thickness of the silica porous layer  9  is, for example, 10 to 50 mm.  
         [0042]    Next, with reference to FIG. 1, a structure at a base side of the aforementioned tube system i.e. the gas tubes  4 ,  5 ,  6 ,  7  is explained. The gas tube  4  and the gas tube  7  are both provided for supplying the nitrogen gas into the reaction container  2 . Therefore, the base sides of the gas tubes  4  and  7  are commonly connected to a nitrogen gas supplying source  4   b  through a valve  4   a . A valve  42  and a flow-rate adjusting part  43  are further provided between the valve  4   a  and the valve  41  in the gas tube  4 . In addition, a valve  72  and a flow-rate adjusting part  73  are also provided between the valve  4   a  and the valve  71  in the gas tube  7 .  
         [0043]    A gas tube  5  is to supply, for example, ammonium (NH 3 ) gas and a gas tube  6  is to supply dichlorosilane (SiH 2 Cl 2 ) gas. A base side of the gas tube  5  with respect to the valve unit  51  is connected to a gas supplying source  56  of the ammonium gas thorough a valve  53 , a flow-rate adjusting part  54 , and a valve  55 . A base side of the gas tube  6  with respect to the valve unit  61  is connected to a gas supplying source  66  of the dichlorosilane gas thorough a valve  63 , a flow-rate adjusting part  64 , and a valve  65 . Furthermore, bypass ways  8   a  and  8   b  branched out from between the valve  4   a  and the flow-rate adjusting part  43  of the gas tube  4 , which serve as assistant replacement-gas ways, are respectively connected to the gas tubes  5  and  6 . Accordingly, the nitrogen gas may be introduced from the nitrogen gas supplying source  4   b  to the gas tubes  5  and  6 .  
         [0044]    A tip of the bypass way  8   a  is connected to a point P 2  between the flow-rate adjusting part  54  and the valve  55  of the gas tube  5 . A tip of the bypass way  8   b  is connected to a point P 3  between the flow-rate adjusting part  64  and the valve  65  of the gas tube  6 . Additionally, a valve  81  which corresponds to a third open-close unit is provided in the bypass way  8   a . Additionally, a valve  82  which corresponds to a third open-close unit is also provided in the bypass way  8   b   1 . These bypass ways  8   a  and  8   b  are both to replace the process gas remaining in the gas tubes  5 ,  6  with the nitrogen gas, for example after the thermal process is completed and the valves  55 ,  65  are closed.  
         [0045]    Next, an operation by the aforementioned unit will be described, taking a case as an example, in which a silicon nitride film is formed. For example, 150 sheets of wafers W are placed on the wafer-boat  27  in a tier-like manner. This wafer-boat  27  is conveyed by the boat-elevator  26  into the inner tube  22  from the opening portion provided at the lower part of the manifold  24 . This opening portion is hermetically closed by the lid  28 . Next, the inside of the reaction tube  21  is heated by the heater  29  to be, for example, about 760° C. Then, a pressure in the reaction tube  21  is adjusted to be, for example, 133×10 −1  Pa (1.0×10 −1  Torr) by adjusting a discharging (exhaust) volume. On the other hand, the ammonium gas and the dichlorosilane gas, which are process gases, are respectively supplied through the gas tube  5  and the gas tube  6 , so that a film-forming process is carried out on the wafers W.  
         [0046]    A gas flow in the film-forming process is shown in FIG. 5. In FIG. 5, a flowing state is indicated with a solid line and a non-flowing state is indicated with a dotted line. As shown in FIG. 5, the process gases flow in the gas tubes  5  and  6 . In addition, as described above, the nitrogen gas flows in the gas tube  4  in order to prevent reaction products generated by a reaction of the process gases from adhering to an inner wall of the outer tube  23 . On the other hand, the nitrogen gas does not flow in the gas tube  7  as shown in FIG. 5 because the valve  72  is closed. Furthermore, the nitrogen gas also does not flow at downstream portions of the respective valves  81 ,  82  of the bypass ways  8   a  and  8   b  because the valves  81  and  82  are also closed. Moreover, the valves  51   b ,  61   b  on the side of the bypass ways in the valve units  51 ,  61  are closed during the film-forming process for both of the gas tubes  5 ,  6 , in which the gas flows. Accordingly, a sucking force of the vacuum pump  32  reaches only the side of the reaction tube  21 . At that time, the controlling part  100  controls the pressure-adjusting unit  31  in order to maintain the pressure in the reaction container  2  at the aforementioned processing pressure of, for example, 133×10 −1  Pa (1.0×10 −3  Torr).  
         [0047]    Next, a step to discharge the process gases remaining in the reaction container  2  and in the gas tubes  5  and  6  after the completion of the film-forming process is explained with reference to FIGS.  6  to  8 . FIG. 6 is a specific chart showing a pressure change in the reaction container  2  as time passes and a concentration of the remaining process gases (dilution ratio) as well. As shown in FIG. 6, when the film-forming process is completed at the time t1, the valves  55  and  65  are closed so that flow of the process gases into the gas tubes  5 ,  6  is stopped. In addition, the valves  51   a  and  61   a  are closed, and the pressure-adjusting unit  31  are fully opened (refer to FIG. 7). Thereby, the inside of the reaction container  2  is quickly shifted to a pressure-reduced state. At that time, the pressure inside the reaction container  2  is maintained to be, for example, 133×10 −3  Pa (1.0×10 −3  Torr).  
         [0048]    Next, at the time t2, the pressure-adjusting unit  31  is once closed and the valve  71  is opened, so that the nitrogen gas introduction from the gas tube  7  is started. Thereafter, while the process gas(es) remaining in the reaction container  2  is replaced with the replacement gas (the discharge from the reaction container  2  is continued), the pressure in the reaction container  2  is raised to be not less than the pressure at the film-forming process. This pressure-raising step is carried out aiming that the pressure in the reaction container  2  is raised by introducing the nitrogen gas into the reaction container  2  so that a probability of collision between remaining process gas molecules and nitrogen gas molecules is enhanced. This allows more process gases to be discharged during a pressure-lowering step which is carried out later. In the pressure-raising step, the nitrogen gas from the gas tube  7  flows into the reaction container  2  at a blast. Here, if the silica porous layer  9  is provided at a gas outlet of the gas tube  7 , the nitrogen gas diffuses uniformly and is introduced at a large flow rate without stirring up particles in the reaction container  2 . The inside pressure of the reaction container  2  is raised to, for example, 133×10 2  Pa, and a dilution ratio of the process gases is, for example, 1.0×10 −2 .  
         [0049]    After the aforementioned pressure-raised state is maintained for a time during which enough amount of the nitrogen gas to replace the process gas(es) is introduced, for example, for about 5 minutes, the pressure-adjusting unit  31  is unfastened at the time t3. Thereby, the inside pressure of the reaction container  2  is lowered to, for example, 133×10 −3  Pa, which is the same as the pressure in the first pressure-lowering step carried out previously. Accordingly, the process gas(es) is discharged together with the nitrogen gas. By repeating such a pressure-raising/lowering steps, the dilution ratio of the process gas(es) in the reaction container  2  is lowered to be not more than a safety dilution ratio which permits the reaction container  2  to be opened, for example, to be about 1.0×10 −14 . Thereafter, the nitrogen gas is introduced from the gas tube  7  to the reaction container  2  with the pressure-adjusting unit  31  being closed, so that the inside of the reaction container  2  is caused back to be at the atmospheric pressure. Thereafter, the wafer-boat  27  is lowered. Incidentally, a time to maintain this pressure-reduced (lowered) state is, for example, about 5 minutes.  
         [0050]    After the dilution ratio of the process gases remaining in the reaction container  2  is lowered to be not more than the safety dilution ratio, for example, 1.0×10 −14 , and when the inside of the reaction container  2  starts to be caused to get back to the atmospheric pressure, an operation to replace with the nitrogen gas the process gases remaining in the gas tubes  5 ,  6  for supplying the process gases is started in parallel to that. In other words, in the reaction container  2 , for example while the wafer-boat  27  is lowered and/or the wafers W are conveyed out from the wafer-boat  27 , the replacement process of the process gases in the gas tubes  5 ,  6  is carried out concurrently.  
         [0051]    Specifically, as shown in FIG. 8, the valve  71  is closed from the state shown in FIG. 7, and the supply of the nitrogen gas to the reaction container  2  is stopped. On the other hand, the valve  81  and the valve  82  are opened, and the nitrogen gas is introduced from the bypass ways  8   a ,  8   b  to the gas tubes  5 ,  6 . At that time, the valves  51   a ,  51   b  and the valves  61   a ,  61   b  are both closed in the valve units  51 ,  61 , and the bypass ways  52 ,  62  and the gas tubes  5 ,  6  are not connected (communicated) therebetween. Therefore, pressures in the gas tube  5  and in the gas tube  6  are raised at a blast by the pressure of the nitrogen gas supplied from the upstream side. Then, when the pressures inside the tubes are raised to be, for example, 2 atmosphere, the valves  51   b ,  61   b  are opened, so that the gas tubes  5 ,  6  and the bypass ways  52 ,  62  are communicated with each other. Thereby, the process gases remaining in the tubes are flown out to the discharging tube  3  together with the nitrogen gas. This pressure-raising/lowering steps in the gas tubes  5 ,  6 , in which the pressure-raising operation by the introduction of the nitrogen gas and the discharging operation are repeated, are to be repeated until the concentration of the process gases is lowered to be a certain value.  
         [0052]    According to the above-described embodiment, the valves  51   a ,  61   a  that are the first open-close units are provided respectively, in the vicinities of the reaction container  2  of the gas tubes  5 ,  6  for supplying the process gases. Therefore, a room, from which the process gases therein should be discharged, is separated after the completion of the film-forming process, the inside of the reaction container  2  is firstly caused to be in the pressure-reduced state, then the nitrogen gas is introduced from the gas tube  7  into the reaction container  2  so that the inside of the reaction container  2  is caused to be at a pressure higher than that during the thermal process, and thereafter the inside of the reaction container  2  is again caused to be in the pressure-reduced state. This enables the gas replacement process in the reaction container  2  to be completed within a short time, so that it is possible to immediately proceed to a step for conveying out the wafers W. A time for making the concentration of the process gases in the reaction container not more than the safety standard value after the completion of the thermal process is about 25 minutes in a sequence using the unit shown in FIG. 9 described in the prior art, but about 15 minutes in a sequence in this embodiment.  
         [0053]    Furthermore, according to this embodiment, the gas replacement process in the gas tubes  5 ,  6 , which are the process gas ways, is carried out while the wafers W are transferred. Therefore, the time of the gas replacement process in the gas tubes  5 ,  6  does not influence the throughput. Incidentally, it is useful to remove the gases remaining in the gas tubes  5 ,  6  for carrying out a next thermal process excellently.  
         [0054]    Moreover, as for the gas replacement process of the separated process-gas way, a large amount of the replacement gas is introduced into an area to be gas-replaced so that the pressure in the area is raised, the probability of molecular collision among the process gases and the nitrogen gas within a short time is enhanced, and thereafter this pressure-raised state is shifted to the pressure-lowered state quickly. Accordingly, it is possible to complete the replacement of the process gas within a shorter time compared with a usual evacuating case. Furthermore, it is possible to significantly enhance the effect by repeating such pressure-raising/lowering steps.  
         [0055]    Furthermore, a step may be carried out, in which evacuation is firstly carried out with the valves  51   a ,  61   a  being opened after the thermal process, then the valves  51   a ,  61   a  are closed and the replacement gas is introduced into the reaction container  2  so that the pressure is raised higher than at the thermal process.  
         [0056]    Note that the double tube structure is used for the reaction container  2  in the above embodiment, but the present invention is also applicable to a unit using a reaction container composed of, for example, a single tube.