Patent Publication Number: US-2022238311-A1

Title: Substrate processing method and substrate processing apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to a substrate processing method and a substrate processing apparatus. 
     BACKGROUND 
     Patent Document 1 discloses a thin film forming method for forming a film by an ALD (Atomic Layer Deposition) method, particularly PEALD (Plasma Enhanced ALD). In this thin film forming method, the opening degree of a conductance valve provided in an exhaust passage connecting a processing container capable of being depressurized and a vacuum pump is maintained at a reference value during an ALD film forming process. This reference value is identified during a preparatory period prior to the start of the ALD film forming process. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Laid-open Publication No. 2006-45460 
     The present disclosure provides some embodiments of a technique capable of allowing the pressure in a processing container to quickly reach a desired pressure in a step of supplying a processing gas into the processing container to form plasma of the processing gas in a substrate processing process such as an ALD process using plasma or the like. 
     SUMMARY 
     According to one embodiment of the present disclosure, there is provided a substrate processing method for performing a predetermined process on a substrate, including: performing, a plurality of times, a cycle including (a) supplying a first processing gas into a processing container to which an exhaust pipe is connected and which accommodates the substrate and (b) supplying a second processing gas into the processing container, wherein at least one of (a) and (b) includes (c) introducing a ballast gas into the exhaust pipe and forming plasma of the processing gas supplied into the processing container. 
     According to the present disclosure, it is possible to allow the pressure in a processing container to quickly reach a desired pressure in a step of supplying a processing gas into the processing container to form plasma of the processing gas in a substrate processing process such as an ALD process using plasma or the like. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing an example of a wafer to be processed. 
         FIG. 2  is a diagram showing an example in which anisotropic film formation is performed on the wafer shown in  FIG. 1 . 
         FIG. 3  is a diagram showing an example in which isotropic film formation is performed on the wafer shown in  FIG. 1 . 
         FIG. 4  is a vertical cross-sectional side view of a film forming apparatus according to the present embodiment. 
         FIG. 5  is a schematic diagram for explaining a process performed by the film forming apparatus shown in  FIG. 1 . 
         FIG. 6  is a schematic diagram for explaining a process performed by the film forming apparatus shown in  FIG. 1 . 
         FIG. 7  is a schematic view for explaining a process performed by the film forming apparatus shown in  FIG. 1 . 
         FIG. 8  is a schematic diagram for explaining a process performed by the film forming apparatus shown in  FIG. 1 . 
         FIG. 9  is a timing chart showing a change in the amount of gas supplied in the process performed by the film forming apparatus shown in  FIG. 1   
         FIG. 10  is a diagram showing an example in which film formation is performed on a wafer having recesses with a high aspect ratio. 
     
    
    
     DETAILED DESCRIPTION 
     For example, in a manufacturing process of a semiconductor device or the like, various processes such as a film forming process and an etching process are performed on a semiconductor wafer (hereinafter referred to as “wafer”). In addition, ALD may be used as a method for forming a film on a wafer. In this ALD, a cycle that includes an adsorption step of supplying a raw material gas to be adsorbed on a surface of a wafer into a processing container having a vacuum atmosphere and a reaction step of supplying a reaction gas (also referred to as a reducing gas) reacting with the raw material gas into the processing container is performed a plurality of times. As a result, an atomic layer of a reaction product is deposited on the surface of the wafer to form a film. In PEALD, which is a kind of ALD method, plasma of the reaction gas is formed in the reaction step when the film is formed by ALD. 
     By the way, in the adsorption step and the reaction step in PEALD, there are suitable pressures as the pressures in the processing container. For example, in the reaction step in which the plasma of the reaction gas is formed, there is a suitable pressure zone as the pressure zone in the processing container for the following reasons. That is, the ratios of charged particles and neutral particles in the plasma of the gas contained in the processing container differ depending on the pressure in the processing container. Furthermore, the ratio of ions in the charged particles and the ratio of radicals in the neutral particles also vary depending on the pressure in the processing container. As used herein, the term “proportion” refers to a density proportion or a flux proportion on the wafer surface. Further, if the proportions of the charged particles and the neutral particles in the plasma in the reaction step are different, the mode of film formation by PEALD becomes different. For example, as shown in  FIG. 1 , if the proportion of ions as charged particles is large when a film is formed on a wafer W having a recess on its surface, the ions are drawn into the wafer W by a bias voltage applied to the stage on which the wafer W is placed. Therefore, a film F is anisotropically formed as shown in  FIG. 2 . Specifically, a thick film F is formed on the top surface and the bottom surface forming the recess of the wafer W, and a thin film F is formed on the side surface of the recess. On the other hand, when the proportion of neutral particles is large, even if a bias voltage is applied, unlike the case of ions, the force that draws the neutral particles toward the wafer does not act on the neutral particles. Therefore, as shown in  FIG. 3 , a film F is isotropically formed. Specifically, a film F having substantially the same thickness is formed on all of the top surface, the bottom surface, and the side surface forming the recess of the wafer W. Whether isotropic film formation or anisotropic film formation is required depends on the shape or the like of the wafer to be processed. Furthermore, the proportions suitable as the proportion of ions in the charged particles and the proportion of radicals in the neutral particles in the plasma also differ depending on the type or the like of the film to be formed. Therefore, in the reaction step in which the plasma of the reaction gas is formed, there is a suitable pressure zone suitable as the pressure zone in the processing container. 
     However, if the opening degree of the conduction valve provided in the exhaust passage connecting the processing container and the vacuum pump is maintained at a reference value during the PEALD process as in Patent Document 1, for example, it takes time from the start of the reaction step to the time at which the pressure inside the processing container becomes suitable for the reaction step. Therefore, there is room for improvement in terms of throughput. Further, by using a so-called APC (Auto Pressure Control) valve or by moving the stage on which the wafer W is placed up and down in the processing container to increase or decrease the volume of the processing space, it is possible to shorten the time until the above-mentioned suitable pressure is obtained. However, since these methods involve the rotation of the valve body of the APC valve and the movement of the stage, it is not possible to sufficiently shorten the time required to reach the above-mentioned suitable pressure. 
     The above points are also common to an ALE (Atomic Layer Etching) process using plasma. 
     Therefore, the technique according to the present disclosure can allow the pressure inside a process container to quickly reach a desired pressure zone in a step of supplying a processing gas into the processing container to form plasma of the processing gas in an ALD process or an ALE process using plasma. 
     Hereinafter, the substrate processing method and the substrate processing apparatus according to the present embodiment will be described with reference to the drawings. In the subject specification and the drawings, elements having substantially the same functional configuration are designated by like reference numerals, and duplicate description is omitted. 
       FIG. 4  is a vertical cross-sectional diagram schematically showing a film forming apparatus as a substrate processing apparatus according to the present embodiment. The film forming apparatus  1  shown in  FIG. 4  is a single-wafer type apparatus. Further, the film forming apparatus  1  forms a SiO 2  film on a wafer W as a substrate by PEALD. Specifically, the film forming apparatus  1  forms a SiO 2  film on a wafer W by performing, a plurality of times, a cycle that includes an adsorption step of supplying a raw material gas as a first processing gas into a below-described processing container  10  in which the wafer W is accommodated, and a reaction step of supplying a reaction gas as a second processing gas into the processing container  10  to form plasma of the reaction gas. More specifically, the film forming apparatus  1  performs a cycle that includes, in the named order, the above-mentioned adsorption step, a step of supplying a purge gas as a replacement gas into the processing container  10  and discharging the raw material gas in the processing container  10 , the above-mentioned reaction step, and a step of supplying a purge gas into the processing container  10  and discharging the reducing gas in the processing container  10 . As a result, the SiO 2  film is formed on the wafer W. During the film forming process by PEALD, a carrier gas for introducing the raw material gas and the reaction gas into the processing container  10  is continuously supplied into the processing container  10 . 
     The film forming apparatus  1  is configured to be depressurized and includes a processing container  10  for accommodating a wafer W. The processing container  10  includes a container body  11  formed in a bottom-closed cylindrical shape. On the side wall of the container body  11 , an opening  11   a  as a loading/unloading port for the wafer W and a gate valve  12  for opening and closing the opening  11   a  are provided. Further, an exhaust duct  17  to be described later is provided on the container body  11  to form a part of the side wall of the processing container  10 . 
     Further, a stage  20  on which the wafer W is placed is provided in the processing container  10 . The stage  20  constitutes a lower electrode. A heater (not shown) is built in the stage  20  and can heat the wafer W placed on the stage  20  to a predetermined temperature. Radio-frequency power for bias is supplied to the stage  20  from a radio-frequency power source  30  provided outside the processing container  10  via a matcher  30   a.    
     Further, the stage  20  is provided with a cylindrical cover member  21  that surrounds the stage  20 . An upper end of a support column  22  extending in the vertical direction is connected to the central portion of the lower surface of the stage  20 . The lower end of the support column  22  extends to the outside of the processing container  10  through an opening  11   b  provided at the bottom of the processing container  10  and is connected to an elevating mechanism  23 . By driving the elevating mechanism  23 , the stage  20  can be moved up and down between a transfer position indicated by a one-dot chain line and a processing position above the transfer position. The transfer position is a position where the stage  20  waits when the wafer W is delivered between a transfer mechanism (not shown) for the wafer W entering the processing container  10  through the opening  11   a  of the processing container  10  and support pins  26   a  described later. The processing position is a position where the wafer W is processed. 
     A flange  24  is provided on the support column  22  outside the processing container  10 . A bellows  25  is provided between the flange  24  and the portion of the bottom wall of the processing container  10  through which the support column  22  penetrates, to surround the outer peripheral portion of the support column  22 . As a result, an airtight seal of the processing container  10  is maintained. 
     Below the stage  20  in the processing container  10 , a wafer elevating member  26  having a plurality of, for example, three support pins  26   a  is provided. The wafer elevating member  26  can be moved up and down by an elevating mechanism  28 . Further, as the wafer elevating member  26  is moved up and down, the support pins  26   a  protrude and retract from the upper surface of the stage  20  through through-holes  20   a  formed in the stage  20  to deliver the wafer W. 
     An annular insulating support member  13  is provided on the upper side of the exhaust duct  17  in the processing container  10 . A shower head support member  14  made of quartz is provided on the lower surface side of the insulating support member  13 . The shower head support member  14  supports a shower head  15  which is a gas introduction part for introducing a processing gas into the processing container  10  and which constitutes an upper electrode. 
     The shower head  15  has a disk-shaped head body portion  15   a  and a shower plate  15   b  connected to the head body portion  15   a . A gas diffusion space S 1  is formed between the head body portion  15   a  and the shower plate  15   b . The head body portion  15   a  and the shower plate  15   b  are made of metal. Two gas supply paths  15   c  and  15   d  leading to the gas diffusion space S 1  are formed in the head body portion  15   a . A large number of gas discharge holes  15   e  leading from the gas diffusion space S 1  are formed in the shower plate  15   b . Further, radio-frequency power for plasma formation is supplied to the shower head  15  from a radio-frequency power source  31  provided outside the processing container  10  via a matcher  31   a.    
     Inside the processing container  10 , an annular member  16  is provided so that the inner wall of the processing container  10  protrudes above the opening  11   a . The annular member  16  is arranged to come close to the outside of the cover member  21  of the stage  20  at the processing position and to surround the cover member  21 . Further, an exhaust duct  17  formed to be curved in an annular shape is provided on the upper portion of the side wall of the processing container  10 . The inner peripheral surface side of the exhaust duct  17  is opened on the annular member  16  in the circumferential direction. The processing space S 2  can be evacuated through a gap  18  formed between the cover member  21  and the lower peripheral edge portion of the shower plate  15   b.    
     One end of the exhaust pipe  32  is connected to the exhaust duct  17 , and the other end of the exhaust pipe  32  is connected to an evacuation pump  33  as an exhaust device. An APC valve  34  and an on-off valve  35  are provided sequentially from the upstream side between the exhaust duct  17  and the evacuation pump  33  in the exhaust pipe  32 . 
     Further, the downstream ends of gas flow paths  41  and  61  are connected to the gas supply paths  15   c  and  15   d , respectively. 
     The upstream end of the gas flow path  41  is connected to a supply source  44  of a BDEAS (bisdiethylaminosilane) gas, which is a raw material gas, via a valve V 1 , a gas storage tank  42 , and a flow rate adjustment part  43  sequentially from the downstream side. The flow rate adjustment part  43  is composed of a mass flow controller and is configured to adjust the flow rate of the BDEAS gas supplied from the supply source  44  to the downstream side. Other flow rate adjustment parts  47 ,  52 ,  63 ,  67 ,  72  and  82 , which will be described later, are also configured in the same manner as the flow rate adjustment part  43  to adjust the flow rate of the gas supplied to the downstream side of the flow path. 
     The gas storage tank  42  temporarily stores the BDEAS gas supplied from the supply source  44  before supplying the BDEAS gas into the processing container  10 . After the BDEAS gas is stored in this way and the pressure inside the gas storage tank  42  is increased to a desired pressure, the BDEAS gas is supplied from the gas storage tank  42  to the processing container  10 . The supply and cutoff of the BDEAS gas from the gas storage tank  42  to the processing container  10  is performed by opening and closing the valve V 1  described above. By temporarily storing the BDEAS gas in the gas storage tank  42  in this way, the BDEAS gas can be stably supplied to the processing container  10  at a relatively high flow rate. 
     Like the gas storage tank  42 , the gas storage tanks  46 ,  62 ,  66  and  81 , which will be described later, are also gas storage parts that serve to stabilize the flow rates of the respective gases supplied to the processing container  10  or the exhaust pipe  32  by temporarily storing the respective gases supplied from the gas supply sources on the upstream side of the gas flow path. Then, the supply and cutoff of the gases from the gas storage tanks  46 ,  62 ,  66  and  81  from the processing container  10  or the exhaust pipe  32  are performed by opening and closing the valves V 2 , V 4 , V 5  and V 10  provided on the downstream side of the respective gas storage tanks  46 ,  62 ,  66  and  81 . 
     Further, the downstream end of the gas flow path  45  is connected to the gas flow path  41  on the downstream side of the valve V 1 . The upstream end of the gas flow path  45  is connected to the Ar gas supply source  48  via the valve V 2 , the gas storage tank  46 , and the flow rate adjustment part  47  in the named order from the downstream side. 
     Further, the downstream end of a gas flow path  51  is connected to the gas flow path  45  on the downstream side of the valve V 2 . The upstream end of the gas flow path  51  is connected to the Ar gas supply source  53  via a valve V 3  and the flow rate adjustment part  52  in the named order from the downstream side. An orifice  54  is formed in the gas flow path  51  on the downstream side of the valve V 3 . That is, the diameter of the gas flow path  51  on the downstream side of the valve V 3  is smaller than the diameter of the gas flow path  51  on the upstream side of the valve V 3  and the diameters of the gas flow paths  41  and  45 . Gases are supplied to the gas flow paths  41  and  45  by the gas storage tanks  42  and  46  at relatively large flow rates. It is possible for the orifice  54  to prevent the gases supplied to the gas flow paths  41  and  45  from flowing back through the gas flow path  51 . 
     By the way, the Ar gas supplied from the Ar gas supply source  48  to the gas flow path  45  is supplied into the processing container  10  to perform purging. On the other hand, the Ar gas supplied from the Ar gas supply source  53  to the gas flow path  51  is a carrier gas for the BDEAS gas. As described above, the carrier gas is continuously supplied into the processing container  10  during the processing of the wafer W. The carrier gas is also supplied into the processing container  10  when purging is performed. Therefore, the time zone in which the carrier gas is supplied into the processing container  10  overlaps with the time zone in which the Ar gas is supplied from the supply source  48  into the processing container  10  to perform purging. The carrier gas is also used in the purging. However, for the sake of convenience of description, the gas supplied from the supply source  48  to the gas flow path  45  is described as a purge gas, and the gas supplied from the supply source  53  to the gas flow path  51  is described as a carrier gas. The carrier gas is also a backflow-preventing gas for preventing the BDEAS gas from flowing back through the gas flow path  51 . 
     Subsequently, the gas flow path  61  connected to the gas supply path  15   d  of the processing container  10  will be described. The upstream end of the gas flow path  61  is connected to the supply source  64  of the O 2  gas, which is a reaction gas, via the valve V 4 , the gas storage tank  62 , and the flow rate adjustment part  63  in the named order from the downstream side. The gas flow path  61  is a reaction gas flow path and is formed independently of the gas flow path  41 , which is a raw material gas flow path. 
     The downstream end of the gas flow path  65  is connected to the gas flow path  61  on the downstream side of the valve V 4 . The upstream end of the gas flow path  65  is connected to the Ar gas supply source  68  via the valve V 5 , the gas storage tank  66 , and the flow rate adjustment part  67  in the named order from the downstream side. Further, the downstream end of the gas flow path  71  is connected to the gas flow path  65  on the downstream side of the valve V 5 . The upstream end of the gas flow path  71  is connected to the Ar gas supply source  73  via the valve V 6  and the flow rate adjustment part  72  in the named order from the downstream side. An orifice  74  is formed in the gas flow path  71  on the downstream side of the valve V 6 . That is, the diameter of the gas flow path  71  on the downstream side of the valve V 6  is smaller than the diameter of the gas flow path  71  on the upstream side of the valve V 6  and the diameters of the gas flow paths  61  and  65 . Like the orifice  54 , the orifice  74  is formed to prevent the gases supplied to the gas flow paths  61  and  65  at relatively large flow rates from flowing back through the gas flow path  71 . 
     The Ar gas supplied from the supply source  68  to the gas flow path  65  is supplied into the processing container  10  to perform purging. The Ar gas supplied from the supply source  73  to the gas flow path  71  is a carrier gas for the O 2  gas and is also used for purging like the carrier gas for the BDEAS gas. However, for the sake of convenience of description, the gas supplied from the supply source  68  to the gas flow path  65  is described as a purge gas, and the gas supplied from the supply source  73  to the gas flow path  71  is described as a carrier gas. The carrier gas is also a backflow-preventing gas for preventing the O 2  gas from flowing back through the gas flow path  71 . 
     By forming the respective gas flow paths as described above, the gas flow path  51  includes the valve V 3  and the flow rate adjustment part  52  as a carrier gas supply control device. In the gas flow path  45 , the valve V 2  and the flow rate adjustment part  47  are provided as a purge gas supply control device distinguished from the carrier gas supply control device. Further, the gas flow path  71  includes the valve V 6  and the flow rate adjustment part  72  as another carrier gas supply control device. The valve V 5  and the flow rate adjustment part  67  are provided as another purge gas supply control device distinguished from another carrier gas supply control device. 
     By the way, as described above, the purge gas is supplied to the processing container  10  from both of the gas flow paths  45  and  65 . This is to purge not only the BDEAS gas and the  02  gas remaining in the processing container  10 , but also the BDEAS gas remaining in the gas flow path  41  on the downstream side of the valve V 1  and the O 2  gas remaining in the gas flow path  61  on the downstream side of the valve V 4 . That is, two flow paths for the purge gas are formed to purge the BDEAS gas and the O 2  gas more reliably. 
     Further, the downstream end of a gas flow path  80  is connected to the exhaust pipe  32  on the upstream side of the APC valve  34 . The upstream end of the gas flow path  80  is connected to a supply source  83  of an Ar gas as an inert gas, which is a ballast gas, via a valve V 10 , a gas storage tank  81 , and a flow rate adjustment part  82  in the named order from the downstream side. The pressure in the processing space S 2  can be adjusted according to the amount of the ballast gas supplied from the gas flow path  80  to the exhaust pipe  32 , and the like. 
     A high-speed valve, which has a very high responsiveness and is used for the valves V 1  to V 6 , is used as the valve V 10 . In the high-speed valve, the time period from the time at which a control signal is inputted to open the valve to the time at which the valve is actually opened is very short. Further, in the high-speed valve, the time period from the time at which the control signal is inputted to the time at which the valve is actually opened is, for example, about 10 msec. The gas flow path  80 , the valve V 10 , the gas storage tank  81 , and the flow rate adjustment part  82  constitute a ballast gas introduction mechanism  8  for introducing the ballast gas into the exhaust pipe  32 . 
     The film forming apparatus  1  configured as described above is provided with a controller  100 . The controller  100  is composed of, for example, a computer equipped with a CPU, a memory, and the like, and includes a program storage part (not shown). The program storage part stores programs or the like for realizing a below-described wafer processing process in the film forming apparatus  1  by controlling various devices such as a heater (not shown) in the stage  20 , the valves V 1  to V 6 , V 10 , and  34 , and the flow rate adjustment parts  43 ,  47 ,  52 ,  63 ,  67 ,  72  and  82 . The programs may be recorded on a computer-readable storage medium and may be installed on the controller  100  from the storage medium. In addition, a part or all of the programs may be realized by dedicated hardware (circuit board). 
     Subsequently, a film forming process performed in the film forming apparatus  1  will be described with reference to  FIGS. 5 to 8  that show the open/closed states of the respective valves, and the gas flow states in the respective gas flow paths. In  FIGS. 5 to 8 , the valves in the open state are shown in white, the valves in the closed state are shown in black, and the gas flow paths through which gases flow toward the downstream side are shown by thick lines. Further, in  FIGS. 5 to 8 , the processing container  10  and the respective parts in the processing container  10  are shown in a simplified manner as compared with  FIG. 4 . Moreover, in the following description of the film forming process, the timing chart of  FIG. 9  is also referred to as appropriate. In this timing chart, the time zones in which the BDEAS gas, the O 2  gas, the carrier gas, and the purge gas flow are shown by rectangular regions with gray scales having different concentrations. The height of each rectangular region corresponds to the amount of the gas supplied into the processing container  10 . The larger the height of the rectangular region, the larger the amount of gas supplied. 
     (Step S1: Wafer Loading) 
     First, the gate valve  12  is opened while closing the valves V 1  to V 6  and V 10 . A transfer mechanism (not shown) holding a wafer W is inserted into the processing container  10  from a vacuum atmosphere transfer chamber (not shown) adjacent to the processing container  10  through the opening  11   a . For example, a recess as shown in  FIG. 1  is formed on the surface of the wafer W. Next, the wafer W is transferred above the stage  20  located at the above-mentioned transfer position. Then, the wafer W is delivered onto the raised support pins  26   a . Thereafter, the transfer mechanism is pulled out from the processing container  10  and the gate valve  12  is closed. At the same time, the support pins  26   a  are lowered and the stage  20  is raised, whereby the wafer W is placed on the stage  20 . 
     (Step S2: Wafer Heating) 
     Subsequently, the stage  20  is moved to the above-mentioned processing position to form the processing space S 2 , and the wafer W is heated to a desired temperature by the heater (not shown) provided in the stage  20 . During the heating, the pressure in the processing container  10  is regulated to a desired vacuum pressure by the APC valve  34 . 
     (Step S3: Keeping APC Valve Fully Opened) 
     After heating the wafer to the desired temperature, the APC valve  34  is fully opened. Thereafter, the APC valve  34  is kept fully opened until film formation is completed. 
     (Step S4: Supply of Carrier Gas Alone) 
     Further, the valves V 3  and V 6  are opened to supply a carrier gas (Ar gas) at a flow rate of, for example, 500 sccm, from the Ar gas supply sources  53  and  73  to the gas flow paths  51  and  71 , respectively. That is, the carrier gas is supplied into the processing container  10  at a total flow rate of 1,000 sccm. On the other hand, while closing the valves V 1 , V 4  and V 10 , a BDEAS gas, an O 2  gas and an Ar gas are supplied from the supply sources  44 ,  64  and  83  to the gas flow paths  41 ,  61  and  80  at the respective timings. As a result, the BDEAS gas, the O 2  gas and the Ar gas are stored in the gas storage tanks  42 ,  62  and  81 , respectively, and the pressures inside the gas storage tanks  42 ,  62  and  81  are increased. 
     (Step S5: Adsorption) 
     At time t 1  in  FIG. 9  at which a predetermined time has elapsed from the start of supply of the carrier gas, the valves V 1  and V 10  are opened as shown in  FIG. 5 . As a result, the Ar gas stored in the gas storage tank  81  is supplied into the exhaust pipe  32 , and the BDEAS gas stored in the gas storage tank  42  is supplied into the processing container  10 , whereby the BDEAS gas is adsorbed to the surface of the wafer W. In this way, the Ar gas as a ballast gas is introduced into the exhaust pipe  32  when the BDEAS gas is supplied to the processing container  10 . Therefore, the amount of the gas discharged from the processing space S 2  can be quickly changed to a desired value. Accordingly, the pressure in the processing container  10  (specifically, the partial pressure of the BDEAS gas in the processing space S 2 ) can be quickly set within a desired range as compared with the case where the pressure is regulated by the APC valve  34 . Since the pressure fluctuation caused by the introduction of the ballast gas is propagated to the exhaust pipe  32  and the like at the speed of sound, the amount of the gas discharged from the processing space S 2  can be quickly changed to the desired value. In parallel with the supply of the BDEAS gas into the processing container  10 , a purge gas (Ar gas) is supplied from the supply sources  48  and  68  to the gas flow paths  45  and  65 , respectively, while closing the valves V 2  and V 5 . As a result, the purge gas is stored in the gas storage tanks  46  and  66 , and the pressures inside the gas storage tanks  46  and  66  are increased. 
     (Step S6: Discharge of Adsorption Gas) 
     At time t 2  in  FIG. 9 , for example, 0.05 seconds after time t 1 , as shown in  FIG. 6 , the valves V 1  and V 10  are closed and the valves V 2  and V 5  are opened. As a result, the supply of the BDEAS gas into the processing container  10  and the supply of the Ar gas into the exhaust pipe  32  are stopped, and the purge gas stored in the gas storage tanks  46  and  66  is supplied into the processing container  10 , whereby the BDEAS gas is discharged from the processing container  10 . That is, the BDEAS gas atmosphere inside the processing container  10  is replaced with an Ar gas atmosphere. While the inside of the processing container  10  is purged as described above, the valve V 1  is closed so that the BDEAS gas supplied from the supply source  44  to the gas flow path  41  is stored in the gas storage tank  42 , whereby the pressure inside the gas storage tank  42  is increased. Further, the valve V 10  is closed so that the Ar gas supplied from the supply source  83  to the gas flow path  80  is stored in the gas storage tank  81 , whereby the pressure inside the gas storage tank  81  is increased. 
     (Step S7: Reaction) 
     At time t 3  in  FIG. 9 , for example, 0.2 seconds after time t 2 , as shown in  FIG. 7 , the valves V 2  and V 5  are closed and the valves V 4  and V 10  are opened. As a result, the supply of the purge gas into the processing container  10  is stopped, the Ar gas stored in the gas storage tank  81  is supplied into the exhaust pipe  32 , and the O 2  gas stored in the gas storage tank  62  is supplied into the processing container  10 . Further, radio-frequency power is supplied from the radio-frequency power source  31  to the shower head  15 , and radio-frequency power is supplied from the radio-frequency power source  30  to the stage  20 . As a result, O 2  gas plasma P is formed, and active species such as ions and radicals in the plasma P react with the BDEAS gas adsorbed on the surface of the wafer W to form an atomic layer of SiO 2  which is a reaction product. In this step, as described above, when the O 2  gas is supplied to the processing container  10 , the Ar gas as a ballast gas is introduced into the exhaust pipe  32 . Therefore, the amount of the gas discharged from the processing space S 2  can be quickly changed to a desired value. Accordingly, the pressure in the processing container  10  (specifically, the partial pressure of the BDEAS gas in the processing space S 2 ) can be quickly set within a desired range as compared with the case where the pressure is regulated by the APC valve  34 . Meanwhile, as the valves V 2  and V 5  are closed, the purge gases supplied from the supply sources  48  and  68  to the gas flow paths  45  and  65  are stored in the gas storage tanks  46  and  66 , respectively, whereby the pressures inside the gas storage tanks  46  and  66  are increased. 
     (Step S8: Discharge of Reaction Gas) 
     At time t 4  in  FIG. 9 , for example, 0.3 seconds after time t 3 , the supply of the radio-frequency power is stopped. As shown in  FIG. 8 , the valves V 4  and V 10  are closed and the valves V 2  and V 5  are opened. As a result, the plasma P in the processing container  10  disappears. Further, the supply of the O 2  gas into the processing container  10  and the supply of the Ar gas into the exhaust pipe  32  are stopped, the purge gases stored in the gas storage tanks  46  and  66  are supplied into the processing container  10 , and the O 2  gas is discharged from the processing container  10 . That is, the O 2  gas atmosphere inside the processing container  10  is replaced with an Ar gas atmosphere. While the inside of the processing container  10  is purged as described above, the valve V 4  is closed so that the O 2  gas supplied from the supply source  64  to the gas flow path  41  is stored in the gas storage tank  62 , whereby the pressure inside the gas storage tank  62  is increased. Further, the valve V 10  is closed so that the Ar gas supplied from the supply source  83  to the gas flow path  80  is stored in the gas storage tank  81 , whereby the pressure inside the gas storage tank  81  is increased. 
     At time t 5  in  FIG. 9 , for example, 0.3 seconds after time t 4 , the valves V 2  and V 5  are closed and the valves V 1  and V 10  are opened. As a result, the supply of the purge gas into the processing container  10  is stopped, the Ar gas stored in the gas storage tank  81  is supplied into the exhaust pipe  32 , and the BDEAS gas stored in the gas storage tank  42  is supplied into the processing container  10 . That is, the above-described step S 1  is performed again. Therefore, the time t 5  at which the discharge of the reaction gas ends is also the time t 1  at which the adsorption step of the BDEAS gas starts. After step S 5  is performed, the above-described steps S 6  to S 8  are performed, and then steps S 5  to S 8  are further performed. That is, if the above-described steps S 5  to S 8  are regarded as one cycle, this cycle is repeated to deposit atomic layers of SiO 2  on the surface of the wafer W, whereby a SiO 2  film is formed. The flow rate of the ballast gas in the adsorption step of step S 5  is common between cycles, and the flow rate of the ballast gas in the reaction step of step S 7  is common between cycles. 
     (Step S9: Unloading) 
     Then, when the film formation is completed by executing the above-described cycle a predetermined number of times, the wafer W is unloaded from the processing container  10  in a procedure opposite to the procedure of loading the wafer W into the processing container  10 . 
     As described above, in the film forming method according to the present embodiment, a cycle including the adsorption step of supplying the BDEAS gas into the processing container  10  to which the exhaust pipe  32  is connected and which accommodates the wafer W, and the reaction step of supplying the O 2  gas into the processing container  10  is performed a plurality of times. Then, in the reaction step, the ballast gas is introduced into the exhaust pipe  32 , and the plasma of the O 2  gas supplied into the processing container  10  is formed. That is, the ballast gas is introduced into the exhaust pipe  32  in the reaction step in which the plasma is formed. Therefore, in the above reaction step, the pressure inside the processing container  10  can be quickly regulated to a desired pressure. Therefore, since each cycle of the PEALD process can be shortened, it is possible to improve the throughput at the time of forming a film by the PEALD process. Further, since the pressure inside the processing container  10  becomes a desired pressure zone in the reaction step in which the plasma of the O 2  gas is formed, the proportions of the charged particles and the neutral particles in the plasma can be set to desired proportions. Therefore, isotropic film formation and anisotropic film formation can be appropriately performed, and a SiO 2  film having a desired shape can be formed. Furthermore, since the pressure inside the processing container  10  becomes a desired pressure in the reaction step in which the plasma of the O 2  gas is formed, the proportion of ions in the charged particles in the plasma and the proportion of radicals in the neutral particles in the plasma can be set to proportions suitable for forming a SiO 2  film. Therefore, it is possible to form a high quality SiO 2  film. Further, according to the present embodiment, since the use of plasma eliminates the need to raise the temperature of the wafer W to a high temperature, it is possible to form a film having a good film quality. Even if the radio-frequency power conditions (power, etc.) for forming the plasma of the reaction gas such as an O 2  gas or the like are adjusted, the proportions of charged particles to neutral particles in the plasma, the proportion of ions in the charged particles and the proportion of radicals in the neutral particles cannot be changed as in the case of regulating the pressure. 
     By the way, the desired pressure in the processing container  10  in the reaction step may be ½ or less of the desired pressure in the processing container  10  in the adsorption step. For example, the inside of the processing container  10  may have a relatively high pressure of about 5 to 10 Torr in the adsorption step and a low pressure of about 2.5 to 5.0 Torr in the reaction step. The reason that the pressure is kept relatively high in the adsorption step as in the above example is to, for example, allow the adsorption gas to be adsorbed to the inner portion of a recess of the wafer W formed on the surface thereof. Further, the reason that the pressure is kept relatively low in the reaction step as in the above example is to, for example, perform anisotropic film formation using the ions in view of the fact that the proportion of ions to the radicals in the plasma of the O 2  gas is larger in the low pressure. Even when the desired pressure in the processing container  10  in the reaction step is ½ or less of the desired pressure in the adsorption step as described above, according to the present embodiment, the pressure inside the processing container  10  can be quickly changed to a desired pressure in the reaction step. 
     In the above example, the flow rate of the ballast gas in the reaction step using the plasma of the O 2  gas is common between the cycles. In this case, for example, as shown in  FIG. 10 , a SiO 2  film F 1  may continue to be isotropically formed on the wafer W having a recess with a high aspect ratio on the surface thereof, and a void V may be generated. Thus, the flow rate of the ballast gas in the reaction step using the plasma of the O 2  gas may be made different between the cycles. In one cycle, the flow rate of the ballast gas may be increased to increase the pressure in the processing container  10 , and in another cycle, the flow rate of the ballast gas may be lowered to lower the pressure in the processing container  10 . In the O 2  gas plasma, the proportion of radicals increases at a high pressure and the proportion of ions increases at a low pressure. Therefore, in one cycle, the SiO 2  film is anisotropically formed by the ions in the plasma, and in another cycle, the SiO 2  film is isotropically formed by the radicals in the plasma. As a result, it is possible to prevent the generation of the void V. 
     In the present embodiment, the ballast gas is introduced into the exhaust pipe  32  in the adsorption step also. Therefore, the pressure inside the processing container  10  in the adsorption step can be quickly regulated to a desired pressure. Accordingly, each cycle of the PEALD process can be further shortened, which makes it possible to further improve the throughput when performing the film formation by the PEALD process. 
     As described above, when the flow rate of the ballast gas in the reaction step using the plasma of the O 2  gas is made different between the cycles, it is naturally necessary to make variable the flow rate of the ballast gas supplied to the exhaust pipe  32 . Further, when the flow rate of the ballast gas supplied to the exhaust pipe  32  is made common in the adsorption step and the reaction step, if the pressure inside the processing container  10  cannot be brought to a desired pressure at a desired speed in both steps, it is necessary to vary the flow rate of the ballast gas. As a method of varying the flow rate of the ballast gas supplied to the exhaust pipe  32  in this way, there is a method of providing a plurality of ballast gas supply systems. 
     Further, the ballast gas may be introduced into the exhaust pipe  32  only in the reaction step, and not in the adsorption step. This makes it possible to reduce the number of ballast gas supply systems and to prevent the cost and size of the film forming apparatus  1  from increasing. In the adsorption step, the pressure regulation may not be as precise as in the reaction step. Therefore, from this point as well, the introduction of the ballast gas in the adsorption step may be omitted. 
     A mass flow controller may be provided, for example, in the gas flow path  80  on the downstream side of the valve V 10  in order to vary the flow rate of the ballast gas supplied to the exhaust pipe  32 . 
     Further, an optical sensor for receiving light of a specific wavelength among lights generated from the plasma of the reaction gas may be provided, and the flow rate of the ballast gas supplied to the exhaust pipe  32  in the reaction step may be adjusted (e.g., by using the mass flow controller provided on the downstream side of the valve V 10  described above) based on the detection result obtained by the optical sensor. Since the state of the light generated from the plasma varies depending on the proportion of ions and the proportion of radicals in the plasma of the reaction gas, it is possible to bring the proportion of ions and the like to a desired proportion by adjusting the flow rate of the ballast gas as described above. 
     Further, in the above description, the purge gas is supplied into the processing container  10  to discharge the adsorption gas and the reaction gas. However, the adsorption gas and the reaction gas may be discharged by merely using the evacuation pump  33  without supplying the purge gas. Further, in the above description, the ballast gas is introduced into the exhaust pipe  32  on the upstream side of the APC valve  34 . However, the ballast gas may be introduced into the exhaust pipe  32  on the downstream side of the APC valve  34 . The timing of starting the introduction of the ballast gas into the exhaust pipe  32  and the timing of starting the supply of the BDEAS gas or the O 2  gas may be the same or different. Specifically, the timing of opening the valve V 10  for the ballast gas and the timing of opening the valves V 1  and V 4  for the BDEAS gas and the O 2  gas may be the same or different. For example, when the purge gas is not used as the adsorption gas or the reaction gas, the timing of opening the valves V 10  may be earlier than the timing of opening the valves V 1  and V 4 . In addition, the gas supply sources of the carrier gas, the purge gas and the ballast gas may be the same. 
     In the above description, the film is formed by the ALD process using the O 2  gas plasma, i.e., the oxide plasma. However, the technique according to the present disclosure may also be applied to film formation by ALD process using other plasma such as nitride plasma or the like. Further, the technique according to the present disclosure may also be applied to ALE process using plasma. Furthermore, the technique according to the present disclosure may also be applied to not only the case where in the ALE process using plasma, the desired pressure in the processing container in the step of supplying the second processing gas is ½ or less of the desired pressure in the processing container in the step of supplying the first processing gas, but also the case where the desired pressure in the processing container in the step of supplying the second processing gas is twice or more of the desired pressure in the processing container in the step of supplying the first processing gas. Even in these cases, in the step of supplying the second processing gas, the pressure inside the processing container can be quickly brought to a desired pressure. 
     The embodiments disclosed herein should be considered to be exemplary in all respects and not limitative. The above-described embodiments may be omitted, replaced or modified in various forms without departing from the scope of the appended claims and their gist. 
     The following configurations also belong to the technical scope of the present disclosure. (1) A substrate processing method for performing a predetermined process on a substrate, comprising: 
     performing, a plurality of times, a cycle including (a) supplying a first processing gas into a processing container to which an exhaust pipe is connected and which accommodates the substrate and (b) supplying a second processing gas into the processing container, 
     wherein at least one of (a) and (b) includes (c) introducing a ballast gas into the exhaust pipe and forming plasma of the processing gas supplied into the processing container. 
     According to (1), the processing container can quickly reach a desired pressure zone in the process of supplying the processing gas into the processing container and forming the plasma of the processing gas in an ALD process or an ALE process using the plasma. 
     (2) The method of (1), wherein the cycle includes supplying a replacement gas into the processing container to discharge the first processing gas after (a) and before (b), and supplying a replacement gas into the process container to discharge the second processing gas after (b). 
     (3) The method of (2), wherein the predetermined process is an ALD process, the first processing gas is a raw material gas, and the second processing gas is a reaction gas. 
     (4) The method of (3), wherein, of (a) and (b), only (b) of supplying a reducing gas as the second processing gas includes (c). 
     (5) The method of any one of (1) to (4), wherein the pressure in the processing container in (b) is ½ or less or twice or more of the pressure in the processing container in (a). 
     (6) The method of any one of (1) to (5), wherein the flow rate of the ballast gas in (c) is made different between the cycles, the pressure in the processing container in (c) is increased in one of the cycles, and the pressure in the processing container in (c) is lowered in another cycle. 
     According to (6), it is possible to combine anisotropic film formation and isotropic film formation, or to combine anisotropic etching and isotropic etching. 
     (7) A substrate processing apparatus for performing a predetermined process on a substrate, comprising: 
     a processing container configured to be subjected to depressurization and configured to accommodate the substrate; 
     a stage provided in the processing container and configured to place the substrate thereon; 
     a gas introduction part configured to introduce a processing gas into the processing container; 
     a gas supply mechanism configured to supply a first processing gas and a second processing gas into the processing container via the gas introduction part; 
     an exhaust pipe configured to connect an exhaust device for evacuating the processing container to the process container; 
     a ballast gas introduction mechanism configured to introduce a ballast gas into the exhaust pipe; and 
     a controller, 
     wherein at least one of the stage and the gas introduction part is connected to a radio-frequency power source for forming plasma of the processing gas in the processing container, and 
     wherein the controller is configured to control the gas supply mechanism, the ballast gas introduction mechanism, and the radio-frequency power source so that a cycle including (a) supplying the first processing gas into the processing container accommodating the substrate and (b) supplying the second processing gas into the processing container is performed a plurality of times and so that (c) introducing the ballast gas into the exhaust pipe and forming plasma of the processing gas supplied into the processing container is performed in at least one of (a) and (b). 
     EXPLANATION OF REFERENCE NUMERALS 
       1 : film forming apparatus,  10 : processing container,  15 : shower head,  20 : stage,  30 : radio-frequency power source,  31 : radio-frequency power source,  32 : exhaust pipe,  33 : evacuation pump,  100 : controller, P: plasma, W: wafer