Abstract:
A semiconductor device manufacturing method comprises a process of forming a film on each of multiple substrates arrayed in a processing chamber by a thermal CVD method by supplying a film forming gas into the processing chamber while heating the interior of the processing chamber, wherein in the film forming process, a cycle is performed one time or multiple times with one cycle including a step of flowing the film forming gas from one end towards the other end along the substrate array direction, and a step of flowing the film forming gas from the other end towards the one end along the substrate array direction, without forming temperature gradient along the substrate array direction in the processing chamber.

Description:
BACKGROUND ART 
     1. Technical Field 
     The present invention relates to a substrate processing apparatus and a semiconductor device manufacturing method. The present invention is for example effectively utilized in oxidization devices, diffusion devices, annealing devices or CVD apparatus for forming an insulating film, metallic film, or semiconductor film on a semiconductor wafer (hereinafter called “wafer”) on which semiconductor integrated circuits (hereinafter called “IC”) are formed. 
     2. Description of Related Art 
     The batch type vertical hot wall CVD apparatus (hereinafter called “CVD apparatus”) is widely used in the IC manufacturing field for forming silicon nitride (Si 3 N 4 ) films on multiple wafers. 
     Conventional CVD apparatus of this type contain the following type of processing furnace. 
     Namely, the processing furnace includes an inner tube that forms the processing chamber, and an outer tube is installed concentrically on the outer side of the inner tube. A manifold is installed on the bottom end of the outer tube. A nozzle for supplying gas into the bottom end of the process chamber, and an exhaust pipe connecting to the tubular space between the inner tube and the outer tube, are connected to the manifold. A heater is installed on the outer side of the outer tube. The heater heats the interior of the processing chamber to a specified temperature distribution. 
     During forming of a film in the processing furnace by this CVD apparatus, a boat holding multiple wafers is loaded into the processing chamber from the bottom end opening of the manifold, and a seal cap seals the bottom end opening of the manifold air-tight, and a nozzle supplies a process gas such as chlorosilane gas and ammonia gas as well as other gases into the bottom end section within the processing chamber. 
     The process gas supplied into the bottom end within the processing chamber, makes contact with the wafer group from below, and rises within the processing chamber while forming a film by the thermal CVO reaction, flows from the top end opening of the inner tube into the tubular space and is exhausted through an exhaust pipe connected to the bottom end section of the tubular space. 
     The film forming rate is dependent on the gas flow at this time, so that the film on the wafer placed at the bottom side of the boat tends to be thicker than the film on the wafer placed on the top side of the boat. 
     In the prior art, the control is performed so that the heater forms a temperature gradient within the processing chamber in order to eliminate this difference in film thickness among the wafers so that the film thickness among the wafers is uniform along the entire length of the boat. 
     SUMMARY OF INVENTION 
     However, in film forming where a temperature gradient is formed within the processing chamber, differences in film quality such as in refractive index and etching rate occur between the film formed on wafers positioned on the top side of the boat and the film formed on wafers positioned on the bottom side of the boat. 
     An object of the present invention is to provide a substrate processing apparatus and a semiconductor device manufacturing method capable of preventing the occurrence of differences in film thickness among substrates for processing, while also preventing the occurrence of differences in film quality such as in refractive index and etching rate among substrates for processing. 
     Representative aspects of the present invention for resolving the aforementioned problems are described next. 
     (1) A semiconductor device manufacturing method comprising a process of forming a film on each of multiple substrates arrayed in a processing chamber by a thermal CVD method by supplying a film forming gas into the processing chamber while heating the interior of the processing chamber, wherein in the film forming process, a cycle is performed one time or multiple times with one cycle including a step of flowing the film forming gas from one end towards the other end along the substrate array direction, and a step of flowing the film forming gas from the other end towards the one end along the substrate array direction, without forming temperature gradient along the substrate array direction in the processing chamber. 
     (2) A semiconductor device manufacturing method comprising a process of forming a film on each of multiple substrates arrayed in a processing chamber by a thermal CVD method by supplying a film forming gas into the processing chamber while heating the interior of the processing chamber, wherein in the film forming process, a cycle is performed one time or multiple times with one cycle including a film forming step where the concentration of the film forming gas or pressure in the processing chamber becomes lower from one end towards the other end along the substrate array direction, and a film forming step where the concentration of the film forming gas or pressure in the processing chamber becomes lower from the other end towards the one end along the substrate array direction. 
     (3) A semiconductor device manufacturing method comprising a process of forming a film on each of multiple substrates arrayed in a processing chamber by a thermal CVD method by supplying a film forming gas into the processing chamber while heating the interior of the processing chamber, wherein in the film forming process, a cycle is performed one time or multiple times with one cycle including a film forming step where the thicknesses of the films formed on the substrates become thinner from one end towards the other end along the substrate array direction, and a film forming step where the thicknesses of the films formed on the substrates become thinner from the other end towards the one end along the substrate array direction. 
     (4) A substrate processing apparatus comprising: 
     a processing chamber for processing substrates, 
     a support jig for holding the multiple substrates in the processing chamber, 
     a heater for heating the interior of the processing chamber, 
     a first gas supply section for supplying a film forming gas from one end side along the substrate array direction in the processing chamber, 
     a first gas exhaust section for exhausting the interior of the processing chamber from the other end side along the substrate array direction in the processing chamber, 
     a second gas supply section for supplying the film forming gas from the other end side along the substrate array direction in the processing chamber, 
     a second gas exhaust section for exhausting the interior of the processing chamber from the one end side along the substrate array direction in the processing chamber, and 
     a controller for controlling to perform a cycle one time or multiple times with one cycle including flowing the film forming gas from the one end side towards the other end side along the substrate array direction, and flowing the film forming gas from the other end side towards the one end side along the substrate array direction, without forming temperature gradient along the substrate array direction in the processing chamber. 
     (5) The semiconductor device manufacturing method according to the above first (1) aspect, comprising a purge step of purging the interior of the processing chamber between the step of flowing the film forming gas from the one end towards the other end along the substrate array direction, and the step of flowing the film forming gas from the other end towards the one end along the substrate array direction, wherein the film forming gas contains dichlorosilane gas and ammonia gas, and the purge step is performed under an ammonia gas atmosphere. 
     (6) The semiconductor device manufacturing method according to the above first (1) aspect, comprising a purge step of purging the interior of the processing chamber between the step of flowing the film forming gas from the one end towards the other end along the substrate array direction, and the step of flowing the film forming gas from the other end towards the one end along the substrate array direction, wherein the film forming gas contains dichlorosilane gas and ammonia gas, and the purge step is performed utilizing ammonia gas. 
     (7) The semiconductor device manufacturing method according to the above first (1) aspect, wherein the film forming gas contains dichlorosilane gas and ammonia gas, and ammonia gas is supplied continuously without stopping, while switching the flow of the film forming gas from the one end towards the other end along the substrate array direction, to the flow from the other end towards the one end along the substrate array direction, or switching the flow of the film forming gas from the other end towards the one end along the substrate array direction, to the flow from the one end towards the other end along the substrate array direction. 
     The above aspects are capable of improving both the film quality uniformity (refractive index of film, etching rate) among the substrates and the film thickness uniformity among the substrates. 
     The above aspects are capable of forming one continuous film by controlling so that no interface is formed at the switching time of the gas flow. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a front cross sectional view showing the CVD apparatus of an embodiment of the present invention; 
         FIG. 2  is a drawing for showing the processing furnace; 
         FIG. 3  is a timing chart for showing the gas supply sequence; 
         FIG. 4A  is a drawing for describing the phenomenon for switching the gas flow; 
         FIG. 4B  is a drawing for describing the phenomenon for switching the gas flow; 
         FIG. 5  is a graph for showing the equalizing effect among the wafers for the refractive index of the film. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     An embodiment of the present invention is described next while referring to the drawings. 
     In the present embodiment, the substrate processing apparatus of the present invention is a CVD apparatus (batch type vertical hot wall decompression CVD apparatus) for implementing the film forming process in an IC production method. 
     A CVD apparatus  10  of this embodiment as shown in  FIG. 1  includes a case  11  forming a standby chamber  12  serving as a pre-processing chamber. The case  11  is formed in a rectangular parallelpiped box shape. 
     A boat elevator  13  is installed inside the standby chamber  12 . An arm  14  for raising and lowering the boat elevator  13  horizontally supports a seal cap  15 . 
     A swivel mechanism  16  is installed on the lower surface of the seal cap  15 . A swivel shaft  17  of the swivel mechanism  16  passes through the seal cap  15 . A boat  20  is installed on the upper edge of the swivel shaft  17  by way of a heat insulating plate holder  19  holding a heat insulating plate  18 . 
     The heat insulating plate  18  is formed in a disk shape using a heat-resistant material such as quartz or silicon carbide. The heat insulating plate  18  prevents heat from the heater (described later) from easily propagating to the lower manifold (described later) side. 
     The boat  20  serving as the support jig is formed in long, cylindrical shape as viewed overall and made using multiple support rods  23  and end plates  21 ,  22  at top and bottom made from a heat-resistant material such as quartz or silicon carbide. Many slots (holding grooves)  24  (See  FIG. 2 ) are arrayed at equidistant spaces (perpendicularly) along the length of the support rods  23 . 
     By simultaneously inserting the edges of the wafer  1  into the multiple slots  24  on one level, the boat  20  can support the multiple wafers  1  at multiple levels, horizontally and with their centers aligned. 
     A drive control unit  25  connects electrically by way of an electrical wire  26  to the swivel mechanism  16  and the boat elevator  13 . The drive control unit  25  controls the swivel mechanism  16  and the boat elevator  13  to perform the desired operation at the desired timing. 
     A processing furnace  30  is installed on the case  11  as shown in  FIG. 1 . 
     The processing furnace  30  contains a heater  32  serving as the heater mechanism. The heater  32  is a tubular shape and is installed perpendicularly and supported by a heater base  31  serving as the support plate. 
     A process tube  33  as a reaction tube is installed concentrically with the heater  32  on the inner side of the heater  32 . A heat resistant material such as quartz (SiO 2 ) or silicon carbide (SiC) is utilized as the process tube  33  material. The process tube  33  is formed in a tubular shape open at the top end and the bottom end. 
     A processing chamber  34  is formed by a hollow space inside the process tube  33 . This processing chamber  34  is structured to store the boat  20  holding the wafers  1  serving as the substrate horizontally in multiple vertical stages. 
     As shown in  FIG. 2 , a lower manifold  35  is installed concentrically with the process tube  33  on the lower side of the process tube  33 . The lower manifold  35  is made from stainless steel or quartz, and is formed in a tubular shape open at the top end and the bottom end. 
     The lower manifold  35  engages with the process tube  33 . The lower manifold  35  is structured to support the process tube  33 . 
     An O-ring  36  is installed as a sealing member between the lower manifold  35  and the process tube  33 . The process tube  33  is installed perpendicularly since the lower manifold  35  is supported by the heater base  31 . 
     An upper manifold  37  is installed concentrically with the process tube  33  on the upper side of the process tube  33 . The upper manifold  37  is made from quartz or stainless steel, and is formed in a tubular shape open on the bottom end and closed on the top end. 
     The upper manifold  37  engages with the process tube  33 , an is supported by the process tube  33 . 
     An O-ring  38  is installed as a sealing member between the process tube  33  and the upper manifold  37 . 
     A reaction container  39  is formed from the process tube  33 , the lower manifold  35  and the upper manifold  37 . 
     A first lower nozzle  40  and a second lower nozzle  50  serving as lower gas feed units connect to the lower manifold  35 , and each connects to the interior of the processing chamber  34 . A first gas supply pipe  41  and a second gas supply pipe  51  respectively connect to the first lower nozzle  40  and the second lower nozzle  50 . 
     A first process gas supply source  45  for supplying a first process gas is connected to the upper flow side which is opposite the side connecting to the first lower nozzle  40  of the first gas supply pipe  41  by way of a valve  42 , an MFC (mass flow controller)  43  serving as the gas flow rate controller, and a valve  44 . 
     An inert gas supply pipe  41 A connects to the lower flow side of the valve  42  of the first gas supply pipe  41 . An inert gas supply source  49  is connected to supply inert gas to the inert gas supply pipe  41 A by way of a valve  46 , a MFC  47 , and a valve  48 . 
     A second process gas supply source  55  for supplying a second process gas connects by way of a valve  52 , a MFC  53  and a valve  54 , to the upper flow side which is opposite the side connecting to the second lower nozzle  50  of the second gas supply pipe  51 . 
     An inert gas supply pipe  51 A connects to the lower flow side of the valve  52  of the second gas supply pipe  51 . The inert gas supply pipe  51 A connects by way of a valve  56 , a MFC  57  and a valve  58 , to the inert gas supply source  49  for supplying inert gas. 
     A first process gas supply line is made up of the above described first lower nozzle  40 , first gas supply pipe  41 , valve  42 , MFC  43 , valve  44 , and the first process gas supply source  45 , etc. A second process gas supply line is made up of the second lower nozzle  50 , second gas supply pipe  51 , valve  52 , MFC  53 , valve  54 , and the second process gas supply source  55 , etc. 
     A first gas supply section  59  is made up of the first gas supply pipe  41 , valve  42 , MFC  43 , valve  44  and the first process gas supply source  45  of the first process gas supply line; and the second gas supply pipe  51 , valve  52 , MFC  53 , valve  54  and the second process gas supply source  55  of the second process gas supply line. 
     A first upper nozzle  60  and a second upper nozzle  70  serving as the upper gas feed units connect to the upper manifold  37 , and each connects to the interior of the processing chamber  34 . A first gas supply pipe  61  and a second gas supply pipe  71  respectively connect to the first upper nozzle  60  and the second upper nozzle  70 . 
     A first process gas supply source  65  for supplying a first process gas connects by way of a valve  62 , a MFC  63 , and a valve  64 , to the upper flow side which is opposite the side connecting with the first upper nozzle  60  of the first gas supply pipe  61 . 
     An inert gas supply pipe  61 A is connected to the lower flow side of the valve  62  of the first gas supply pipe  61 . An inert gas supply source  69  for supplying inert gas connects by way of a valve  66 , a MFC  67  and a valve  68  to the inert gas supply pipe  61 A. 
     A second process gas supply source  75  for supplying a second process gas connects by way of a valve  72 , a MFC  73 , and a valve  74 , to the upper flow side which is opposite the side connecting with the second upper nozzle  70  of the second gas supply pipe  71 . 
     An inert gas supply pipe  71 A connects to the lower flow side of the valve  72  of the second gas supply pipe  71 . The inert gas supply pipe  71 A connects by way of a valve  76 , a MFC  77  and a valve  78  to the inert gas supply source  69  for supplying inert gas. 
     A first process gas supply line is made up of the above described first upper nozzle  60 , first gas supply pipe  61 , valve  62 , MFC  63 , valve  64  and the first process gas supply source  65 , etc. A second process gas supply line is made up of the second upper nozzle  70 , second gas supply pipe  71 , valve  72 , MFC  73 , valve  74 , and the second process gas supply source  75 , etc. 
     A second gas supply section  79  is made up of the first gas supply pipe  61 , valve  62 , MFC  63 , valve  64 , and the first process gas supply source  65  of the first process gas supply line; and the second gas supply pipe  71 , valve  72 , MFC  73 , valve  74 , and the second process gas supply source  75  of the second process gas supply line. 
     A chlorosilane gas such as dichlorosilane gas (SiH 2 Cl 2 , hereinafter called “DCS”) is used as the first process gas. 
     An ammonia (NH 3 ) gas for example is used as the second process gas. 
     As shown in  FIG. 1 , a gas flow rate control unit  80  connects electrically by way of an electrical wire  81  to the first gas supply section  59  (more specifically, the MFC  43 , MFC  53 ) and the second gas supply section  79  (more specifically, the MFC  63 , MFC  73 ). 
     The gas flow rate control unit  80  controls the supplied gas (first process gas, second process gas) at a desired timing to flow at the desired rate. 
     The gas flow rate control unit  80  also connects electrically by way of the electrical wire  81  to the MFC  47 ,  57  installed on the inert gas line in the first gas supply section  59  for controlling the flow rate of inert gas, and to the MFC  67 ,  77  installed on the inert gas line in the second gas supply section  79  for controlling the flow rate of inert gas. 
     The gas flow rate control unit  80  controls the supplied inert gas at a desired timing to flow at the desired rate. 
     The gas flow rate control unit  80  also electrically connects to the valve  42 , valve  44 , valve  52 , and the valve  54  of the first gas supply section  59 , and to the valve  62 , valve  64 , valve  72 , and the valve  74  of the second gas supply section  79 . 
     The gas flow rate control unit  80  controls the timing for opening and closing each of the valves, or in other words, the timing for starting/stopping the flow of the gas (first process gas, second process gas). 
     The gas flow rate control unit  80  also electrically connects to the valve  46 , valve  56 , valve  48 , and the valve  58  installed on the inert gas supply line in the first gas supply section  59 , and to the valve  66 , valve  76 , valve  68 , and the valve  78  installed on the inert gas supply line in the second gas supply section  79 . 
     The gas flow rate control unit  80  controls the timing for opening and closing each of the valves, or in other words, the timing for starting/stopping the flow of the inert gas. 
     As shown in  FIG. 1  and  FIG. 2 , a first exhaust port  90  for exhausting the atmosphere within the processing chamber  34  is installed on the upper manifold  37 . A first exhaust pipe  91  connects to the first exhaust port  90 . 
     A pressure sensor  92  functioning as a pressure detector, an exhaust valve  93 , and a conductance valve  94  serving as a pressure adjuster valve are installed on the lower flow side which is opposite the side connecting to the first exhaust port  90  of the upper manifold  37  of the first exhaust pipe  91 . 
     A second exhaust port  95  for exhausting the atmosphere within the processing chamber  34  is installed on the lower manifold  35  in the same way. A second exhaust pipe  96  connects to the second exhaust port  95 . 
     A pressure sensor  97  functioning as a pressure detector, an exhaust valve  98 , and a conductance valve  99  serving as a pressure adjuster valve are installed on the lower flow side which is opposite the side connecting to the second exhaust port  95  of the lower manifold  35  of the second exhaust pipe  96 . 
     The first exhaust pipe  91  and the second exhaust pipe  96  merge into a single piece farther downstream than the conductance valve  94  and the conductance valve  99 . A vacuum pump  101  is connected to a single piece third exhaust pipe  100 . 
     The vacuum pump  101  exhausts the processing chamber  34  to a specified pressure (vacuum intensity) by way of the first exhaust pipe  91 , second exhaust pipe  96  and the third exhaust pipe  100 . 
     A pressure control unit  102  electrically connects by way of an electrical wire  103  as shown in  FIG. 1 , to the conductance valves  94 ,  99  and the pressure sensors  92 ,  97 . 
     The pressure control unit  102  adjusts the conductance valves  94 ,  99  at a desired timing, based on the pressure detected by the pressure sensors  92 ,  97  so that the pressure inside the processing chamber  34  reaches a desired pressure. 
     The pressure control unit  102  for example, adjusts the conductance valve  94  based on the pressure detected by the pressure sensor  92  in a state where the exhaust valve  98  of the second exhaust pipe  96  is closed and the exhaust valve  93  of the first exhaust pipe  91  is open, to control the pressure inside the processing chamber  34  at a desired timing to reach a desired pressure. 
     Moreover, the pressure control unit  102  adjusts the conductance valve  99  based on the pressure detected by the pressure sensor  97  in a state where the exhaust valve  93  of the first exhaust pipe  91  is closed and the exhaust valve  98  of the second exhaust pipe  96  is open, to control the pressure inside the processing chamber  34  at a desired timing to reach a desired pressure. 
     A temperature sensor functioning as a temperature detector not shown in the drawing is installed inside the process tube  33 . A temperature control unit  82  is electrically connected by an electrical wire  83  to the temperature sensor and to the heater  32 . This temperature control unit  82  adjusts the electrical supply to the heater  32  based on the temperature information detected by the temperature sensor, to control the temperature inside the processing chamber  34  at a desired timing to reach a desired temperature distribution. 
     The drive control unit  25 , the gas flow rate control unit  80 , the temperature control unit  82 , and the pressure control unit  102  also make up an operating unit, and input/output unit, and electrically connect to a main control unit  104  for controlling the entire CVD apparatus. A controller  105  is made up of the drive control unit  25 , the gas flow rate control unit  80 , the temperature control unit  82 , the pressure control unit  102  and the main control unit  104 . 
     The film-forming process of the IC production method of an embodiment of the present invention is described next, for the case where forming a thin film on the wafer by the CVD method using the CVD apparatus as described above. 
     In the following description, the controller  105  controls the operation of each unit making up the CVD apparatus. 
     After charging multiple wafers  1  into the boat  20 , the boat elevator  13  raises the boat  20  holding the multiple wafers  1  as shown in  FIG. 2  and the boat  20  is loaded into the processing chamber  34 . 
     In this state, the seal cap  15  seals the bottom end of the lower manifold  35  by way of the O-ring  15   a.    
     Next, the exhaust valve  98  of the second exhaust pipe  96  is closed, the exhaust valve  93  of the first exhaust pipe  91  is opened, and the exhaust pump  101  evacuates the interior of the processing chamber  34  by way of the first exhaust pipe  91  to reach a specified pressure (vacuum intensity). 
     The pressure sensor  92  of the first exhaust pipe  91  at this time measures the pressure inside the processing chamber  34 , and the conductance valve  94  of the first exhaust pipe  91  is feedback-controlled based on the pressure measured by the pressure sensor  92 . 
     At this time, the exhaust valve  93  of the first exhaust pipe  91  may be closed, and the exhaust valve  98  of the second exhaust pipe  96  opened, and the exhaust may be performed by way of the second exhaust pipe  96 . 
     The pressure sensor  97  of the second exhaust pipe  96  at this time measures the pressure inside the processing chamber  34 , and the conductance valve  99  of the second exhaust pipe  96  is feedback-controlled based on the measured pressure. 
     Both the exhaust valve  93  of the first exhaust pipe  91  and the exhaust valve  98  of the second exhaust pipe  96  may be opened, and the exhaust may be performed by way of both the first exhaust pipe  91  and the second exhaust pipe  96 . 
     At this time at least one of the pressure sensor  92  of the first exhaust pipe  91  and the pressure sensor  97  of the second exhaust pipe  96  measures the pressure inside the processing chamber  34 . At least one of the conductance valve  94  of the first exhaust pipe  91  and the conductance valve  99  of the second exhaust pipe  96  is feedback-controlled based on the measured pressure. 
     The pressure adjustment within the processing chamber  34  is carried out while at least one of the first gas supply section  59  and the second gas supply section  79  supplies inert gas into the processing chamber  34 . 
     The heater  32  heats the interior of the processing chamber  34  to reach the desired temperature. Feedback control by way of electrical power conductance is applied to the heater  32  based on the temperature information detected by the temperature sensor so that no temperature gradient forms, or in other words so that the temperature gradient reaches a flat state in at least the wafer group array area (wafer loading area of the boat) within the processing chamber  34 . 
     Thus, the wafers arrayed perpendicularly at equidistant spaces in a horizontal state within the processing chamber  34  are heated to the same temperature across the entire length of the boat  20 . 
     Then, the swivel mechanism  16  rotates the boat  20  to rotate the wafers  1 . 
     Next, the second gas supply section  79  (more specifically, the valves  62 ,  64 ,  72 , and  74 ), and the exhaust valve  98  of the second exhaust pipe  96  are closed according to the sequence shown in  FIG. 3 . 
     In this state, the first gas supply section  59  (more specifically, the valves  42 ,  44 ,  52 , and  54 ), and the exhaust valve  93  of the first exhaust pipe  91  are opened. 
     Thus, the first process gas supply source  45  of the first gas supply section  59  supplies the first process gas (DCS gas), and the second process gas supply source  55  supplies the second process gas (ammonia gas). The first process gas and the second process gas controlled by the MFC  43 ,  53  to the desired flow rate, flow through the first gas supply pipe  41  and the second gas supply pipe  51 , and are supplied from the first lower nozzle  40  and the second lower nozzle  50  into the processing chamber  34 . 
     The supplied gas rises within the processing chamber  34  from inside the lower manifold  35  and is exhausted from the first exhaust port  90  and the first exhaust pipe  91  installed in the upper manifold  37  (first flow). 
     The valves  66 ,  68 ,  76 , and  78  of the inert gas supply pipes  61 A,  71 A of the second gas supply section  79  are opened at this time, and the second gas supply section  79  supplies the inert gas. This supplied inert gas then flows through the gas supply pipes  61 ,  71 , and is supplied from the first upper nozzle  60  and the second upper nozzle  70  into the processing chamber  34 . 
     While passing through the interior of the processing chamber  34 , the first process gas and the second process gas make contact with the surface of the wafer  1 , and a thin film at this time is formed on the surface of the wafer  1  due to the thermal CVD reaction. 
     The pressure sensor  92  installed in the first exhaust pipe  91  at this time measures the pressure inside the processing chamber  34 . The conductance valve  94  installed in the first exhaust pipe  91  is feedback-controlled based on the measured pressure to adjust the pressure inside the processing chamber  34 . 
     The gas flow is switched as shown in  FIG. 3 , when a preset processing time elapses. 
     Namely, the supply of the first process gas and the second process gas from the first gas supply section  59  is stopped by shutting off the first gas supply section  59  (more specifically, the valves  42 ,  44 ,  52 , and  54 ), and closing the exhaust valve  93  of the first exhaust pipe  91 . 
     The second gas supply section  79  (more specifically, the valves  62 ,  64 ,  72 , and  74 ) is opened, and the exhaust valve  98  of the second exhaust pipe  96  is opened while in this state. 
     Thus, the first process gas supply source  65  of the second gas supply section  79  supplies the first process gas, and the second process gas supply source  75  supplies the second process gas. The first process gas and the second process gas whose flow is controlled by the MFC  63 ,  73  to the desired flow rate, flow through the first gas supply pipe  61  and the second gas supply pipe  71 , and are supplied from the first upper nozzle  60  and the second upper nozzle  70  into the processing chamber  34 . 
     The supplied gas descends within the processing chamber  34  from inside the upper manifold  37 , and is exhausted from the second exhaust pipe  96  and the exhaust port  95  installed in the lower manifold  35  (second flow). 
     The valves  46 ,  48 ,  56 , and  58  of the inert gas supply pipes  41 A,  51 A of the first gas supply section  59  are opened at this time and the first gas supply section  59  supplies the inert gas. This supplied inert gas flows through the first gas supply pipe  41  and the second gas supply pipe  51  and is fed into the processing chamber  34  from the first lower nozzle  40  and the second lower nozzle  50 . 
     While passing through the interior of the processing chamber  34 , the first process gas and the second process gas make contact with the surface of the wafer  1 , and a thin film at this time is formed on the surface of the wafer  1  due to the thermal CVD reaction. 
     The pressure sensor  97  installed in the second exhaust pipe  96  at this time measures the pressure inside the processing chamber  34 . The conductance valve  99  installed in the second exhaust pipe  96  is feedback-controlled based on the measured pressure to adjust the pressure inside the processing chamber  34 . 
     One cycle or multiple cycles are performed to form the film having a desired thickness with the one cycle including: a film forming utilizing a first flow from the lower section towards the upper section within the processing chamber  34 , and a film forming utilizing a second flow from the upper section towards the lower section within the processing chamber  34 . 
     This cycle is preferably repeated multiple times in view of the need to improve film thickness uniformity among wafers. 
     After the above described cycle is performed for the preset number of times and a thin film with the desired thickness is formed, the first gas supply section  59  (more specifically, the valves  42 ,  44 ,  52 , and  54 ) and the second gas supply section  79  (more specifically, the valves  62 ,  64 ,  72 , and  74 ) are closed. 
     Moreover, inert gas is supplied from the inert gas supply line of at least one among the first gas supply section  59  and the second gas supply section  79  and preferably is supplied from the inert gas supply lines of both, replacing the atmosphere in the processing chamber  34  with inert gas and returning the pressure in the processing chamber  34  to normal pressure. 
     The boat elevator  13  then lowers the seal cap  15 , and along with opening the bottom end of the lower manifold  35 , the boat  20  holding the processed wafers  1  is unloaded from the bottom end of the lower manifold  35  to outside the process tube  33 . 
     The processed wafers are next discharged from the boat  20 . 
     Conditions for processing the wafers  1  in the processing furnace of this embodiment to form a Si 3 N 4  film are for example a processing temperature of 700 to 800 degrees C., a processing pressure of 10 to 100 Pa, a DCS gas flow rate of 5 to 500 sccm, a NH 3  gas flow rate of 5 to 2000 sccm, and a number of cycles from 1 to 50. The wafers are processed while maintaining each of the processing conditions at a fixed value within each specified range. 
     Alternating the first flow and the second flow of film forming gas (DSC gas and ammonia gas) without forming a temperature gradient within the processing chamber  34  as described above causes the phenomenon shown in  FIG. 4  in the processing chamber  34 . 
     As shown in  FIG. 4A , a first flow of film forming gas from the bottom side (hereinafter called “BTM”) of the processing chamber  34  by way of the center section (hereinafter called “CNT”) of the processing chamber  34 , towards the top side (hereinafter called “TOP”) of the processing chamber  34  forms a film where: 
     the BTM side pressure becomes larger than the TOP side pressure, 
     the BTM side gas concentration becomes larger than the TOP side gas concentration, and 
     the BTM side film thickness is larger than the TOP side film thickness. 
     For example, when a film with a film thickness t 1  is formed on the wafer  1  positioned on the TOP side, 
     a film with a film thickness t 2  is formed on the wafer  1  positioned on the CNT side, and 
     a film with a film thickness t 3  is formed on the wafer  1  positioned on the BTM side, then
 
t1&lt;t2&lt;t3.
 
     Then as shown in  FIG. 4B , a second flow of film forming gas from the TOP side by way of the CNT side towards the BTM side forms a film where: 
     the TOP side pressure becomes larger than the BTM side pressure, 
     the TOP side gas concentration becomes larger than the BTM side gas concentration, and 
     the TOP side film thickness is larger than the BTM side film thickness. 
     Namely, a film with a film thickness t 3  is formed on the film with a film thickness t 1  formed on the wafer  1  on the TOP side; 
     A film with a film thickness t 2  is formed on the film with a film thickness t 2  formed on the wafer  1  on the CNT side, 
     a film with a film thickness t 1  is formed on the film with a film thickness t 3  formed on the wafer  1  on the BTM side (t 1 &lt;t 2 &lt;t 3 ). 
     The total film thickness t formed on the wafer  1  on the TOP side in this way becomes t 1 +t 3 ; 
     the total film thickness t formed on the wafer  1  on the CNT side becomes t 2 +t 2 ; 
     the total film thickness t formed on the wafer  1  on the BTM side becomes t 3 +t 1 , and the total film thickness t for film formed on each wafer  1  is then equivalent at each position. So that t=t 1 +t 3  (TOP side)=t 2 +t 2  (CNT side)=t 3 +t 1  (BTM side). 
     As clearly shown from the total film thickness for each wafer  1  at each position shown in  FIG. 4B , if the film forming utilizing the first flow and the film forming utilizing the second flow set as one cycle and if this cycle is performed at least one time or more, then the difference between the TOP side wafer film thickness and the BTM side wafer film thickness caused by film forming from the first flow, can be canceled out by the film forming from the second flow so that a uniform film thickness can be achieved among the wafers. 
     No temperature gradient is formed in the above described cycle film-forming within the processing chamber  34  so that there is no difference in film quality between the BTM side wafer film quality and the CNT side wafer film quality and the TOP side wafer film quality. In other words, film quality or in other words, the film refractive index and the etching rate among the wafers can be made uniform. 
     An interface however might be possibly formed at the switching time of the above gas flow. 
     However, the forming of an interface between the thin film formed by the first flow and the thin film formed by the second flow can be prevented, by continually supplying ammonia gas to flow into the processing chamber with no stoppages when switching the flow of the film forming gas from the first flow to the second flow, or switching the flow from the second flow to the first flow. In other words, one continuous thin film can be formed. 
     The purge performed between the first flow and the second flow or between the second flow and the first flow is in other words preferably performed under an ammonia gas atmosphere. 
     Inert gas and ammonia gas may be utilized for the purge, or just ammonia gas may be used. 
     In this embodiment, when the first process gas is DCS gas and the second process gas is ammonia gas, ammonia gas continuously flows into the processing chamber  34  with no stoppages when switching the flow of gas from the first flow to the second flow, by closing the valves  42 ,  44  of the first gas supply pipe  41  in the first gas supply section  59 , but not closing the valves  52 ,  54  of the second gas supply pipe  51 . 
     Conversely when switching the flow of gas from the second flow to the first flow, ammonia gas continuously flows into the processing chamber  34  with no stoppages, by closing the valves  62 ,  64  of the first gas supply pipe  61  in the second gas supply section  79  but not closing the valves  72 ,  74  of the second gas supply pipe  71 . 
     During the film forming, ammonia gas is preferably continuously supplied from either of the second gas supply pipes  51 ,  71  of the first gas supply section  59  and the second gas supply section  79 . 
       FIG. 5  is a graph showing the equalizing effect on the film refractive index among the wafers when forming film by the method of this invention, and when forming film by the conventional method. 
     In  FIG. 5 , the refractive index is shown on the vertical axis, and the positions of the slots holding the wafers in the boat are shown in the horizontal axis. 
     The broken line curve shows the conventional example and the solid line curve shows the case of this embodiment. 
     The refractive index is for a silicon nitride film formed on a wafer, using dichlorosilane gas and ammonia gas. 
     As clearly shown in  FIG. 5 , compared to the conventional art, the present embodiment is capable of improving the refractive index uniformity among the wafers. 
     The present invention is not limited to the above described embodiments and all manner of variations or adaptations not departing from the scope or spirit of the present invention are permitted. 
     The present invention for example is not limited to forming an Si 3 N 4  film using DCS (SiH 2 Cl 2 ) gas and NH 3  gas, but can also apply to when forming an Si 3 N 4  film using chlorosilane gas such as TCS (SiHCl 3 ), HCD (Si 2 Cl 6 ); or when forming an Si 3 N 4  film using a hydrazine gas such as N 2 H 4 , etc. 
     Besides the forming of Si 3 N 4  film, the present invention can also apply to general cases when forming films by the CVD method such as when forming HTO (SiO 2 ) film. 
     The CVD apparatus of this invention is not limited to a batch type vertical hot wall CVD apparatus, and may also apply to other CVD apparatus such as horizontal type hot wall CVD apparatus, etc. 
     The case where processing wafers was described in the embodiment but the present invention may also apply to cases where the substrates for processing are liquid crystal panels, photo masks, printed wiring circuit boards, compact disks and magnetic disks, etc.