Patent Publication Number: US-2005121142-A1

Title: Thermal processing apparatus and a thermal processing method

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a thermal processing apparatus and a thermal processing method for processing substrates, such as semiconductor wafers, by a thermal process, such as a CVD process.  
      2. Description of the Related Art  
      A vertical thermal processing apparatus processes semiconductor wafers (hereinafter referred to simply as “wafers”) in lots by a batch thermal process, such as a batch CVD process (batch chemical vapor deposition process) for depositing a film on wafers, a batch oxidation process or a batch diffusion process. This thermal processing apparatus includes a heating furnace provided with a vertical reaction tube having an open lower end. A wafer boat holding a plurality wafers in a stack is mounted on a lid for closing the open lower end of the reaction tube, the wafer boat is loaded into the reaction tube by raising the lid, and processes the wafers by a predetermined thermal process.  
      A wafer carrier holding wafers is delivered to process stations in a factory or to a stocker. Wafers held in the wafer carrier are contaminated with particles and organic matters and are oxidized by natural oxidation by the oxidizing effect of the atmosphere while the wafer carrier is transported. Therefore, it is a usual procedure to clean wafers by a cleaning apparatus by successive cleaning processes using some chemical solutions including a hydrofluoric acid solution, to put the cleaned wafers in a wafer carrier, and the wafer carrier is carried to the thermal processing apparatus.  
      When the cleaning apparatus is installed in an area separated from an area in which the thermal processing apparatus is installed, the cleaned wafers are exposed to the atmosphere and undergo natural oxidation after cleaning, and an oxide film is formed on the wafers before the wafers are loaded into the reaction vessel of the thermal processing apparatus. The adverse effect of even a little oxide film formed by natural oxidation on the characteristics of devices becomes more serious as the thickness of thin films forming devices decreases progressively. Either organic or inorganic impurities affect the characteristics of devices adversely. For example, there is a tendency for the desired thickness of a silicon oxide film as a gate oxide film for CMOS devices to decrease to thicknesses below 10 nm. Under such circumstances, it is required to reduce the amount of impurities carried into the reaction vessel to the least possible extent; that is it is required to keep the surfaces of wafers as clean as possible.  
      FIGS.  8 ( a ) and  8 ( b ) show a thermal processing apparatus such as proposed in Patent document 1 to meet such a requirement. This prior art thermal processing apparatus has a wafer carrier handling block B 1  for receiving and sending out wafer carriers holding wafers, a cleaning chamber B 2  in which wafers taken out of the wafer carrier are cleaned, and a film forming chamber B 3  in which a film is formed on the cleaned wafers. A wafer boat  11  for holding wafers W in a stack is placed in the cleaning chamber B 2 , wafers W are transferred successively from a wafer carrier to the wafer boat  11 , the cleaning chamber B 2  is sealed hermetically, and then a chemical solution is jetted through vertically arranged nozzles  12  onto the wafers W. Then, pure water is jetted through other nozzles  12  onto the wafers W, IPA (isopropyl alcohol) is jetted through nozzles onto the wafers W, and the wafer boat  11  is rotated by a motor M to remove water drops remaining on the wafers W by centrifugal force. Subsequently, the cleaning chamber B 2  is filled up with nitrogen gas, and then the wafer boat  11  is transferred to an elevator disposed below the film forming chamber B 3 .  
      Patent document 1: JP-A 8-203852 (Paragraph 0028, II. 7 to 9, FIGS. 1, 7 and 9).  
      When the wafers W are processed by this prior art thermal processing apparatus, the wafer W is exposed to the ambient atmosphere while the cleaned wafers W are transferred from the cleaning chamber B 2  to the film forming chamber B 3 . Consequently, for example, an oxide film is formed on the surface of the wafers W by natural oxidation or impurities floating in the ambient atmosphere, such as organic substances including hydrocarbons, and moisture, adhere to the surfaces of the wafers W. If the wafers W thus contaminated are subjected to, for example, a thermal process for forming a silicon dioxide film, parts of a film corresponding to the oxide film formed by natural oxidation are formed in a thickness greater than that of other parts, a film having an irregular thickness is formed, and it is possible that a low-quality silicon dioxide film containing impurities is formed and such a low-quality silicon dioxide film affects the characteristics of devices formed on the wafers W adversely.  
      Since the thermal processing apparatus has the cleaning chamber specially for the cleaning process and needs a wafer boat handling mechanism, the thermal processing apparatus is inevitably large and needs a large space for installation. There is a tendency for the vertical thermal processing apparatus to use a reaction vessel having a low height and capable of batch-processing an increased number of wafers. Consequently, the pitches of wafers on a wafer boat are progressively decreased. When the cleaning liquid is jetted through the nozzles as mentioned in connection with  FIG. 8 ( b ), the cleaning liquid cannot be satisfactorily jetted onto the surfaces of the wafers arranged at short pitches. Consequently, the wafers cannot be satisfactorily cleaned and it is possible that the oxide film formed by natural oxidation remains on the surface of the cleaned wafers.  
     SUMMARY OF THE INVENTION  
      The present invention has been made in view of those problems and it is therefore an object of the present invention to provide a thermal processing apparatus and a thermal processing method capable of keeping the surfaces of substrates in a highly clean condition in processing the substrates by a thermal process in a heating furnace.  
      A thermal processing apparatus in a first aspect of the present invention for processing substrates by a predetermined thermal process that heats the substrates by a heating means includes: a reaction vessel provided with a drain port; a cleaning liquid supply means for supplying cleaning liquids into the reaction vessel to clean the substrates after the substrates have been loaded into the reaction vessel; and a process gas supply means for supplying a process gas into the reaction vessel to process the substrates by the thermal process after the cleaning liquids have been drained away from the reaction vessel.  
      A thermal processing apparatus in a second aspect of the present invention for processing substrates by a predetermined thermal process that heats the substrates by an external heating means includes: a reaction vessel provided with a drain port; a lid for closing an inlet opening of the reaction vessel; a substrate holding device mounted on the lid to hold the substrates; a carrying means for carrying the substrate holding device into and carrying the same out from the processing vessel; a cleaning liquid supply means for supplying cleaning liquids into the reaction vessel to clean the substrates after the substrate holding device holding the substrates has been loaded into the reaction vessel; and a process gas supply means for supplying a process gas into the reaction vessel to process the substrates by the thermal process after the cleaning liquids have been drained away through the drain port.  
      The substrate holding device may hold a plurality of substrates in a vertical position at horizontal intervals. The cleaning liquid supply means may be connected to the lid and/or the drain port may be formed in the lid. The cleaning liquid supply means may be used for filling up the reaction vessel with the cleaning liquid, and the drain port may be closed while the cleaning liquid is supplied into the reaction vessel.  
      A thermal processing method in a third aspect of the present invention includes the steps of: carrying substrates into a reaction vessel; cleaning the substrates by supplying cleaning liquids into the reaction vessel; draining away the cleaning liquids from the reaction vessel; processing the substrates by a thermal process by supplying process gases into the reaction vessel and heating the interior of the reaction vessel after the cleaning liquids have been drained away.  
      A thermal processing method in a fourth aspect of the present invention that processes substrates by a predetermined thermal process in a reaction vessel by heating the substrates with an external heating means includes the steps of: loading a substrate holding device with the substrates; loading the substrate holding device into the reaction vessel and hermetically closing an loading opening of the reaction vessel by a lid; cleaning the substrate by supplying cleaning liquids into the reaction vessel; draining away the cleaning liquids from the reaction vessel; and processing the substrates by the thermal process by supplying a process gas into the reaction vessel and heating the interior of the reaction vessel.  
      The step of loading the substrate holding device with the substrates may load the substrates onto the substrate holding device such that the substrates are held in a vertical position at horizontal intervals. The cleaning liquids may be supplied through a discharge port formed in the lid into the reaction vessel. The cleaning liquids may be drained away through a drain port formed in the lid. The step of supplying the cleaning liquids into the reaction vessel may fill up the reaction vessel with the cleaning liquid.  
      The thermal processing apparatus of the present invention processes the substrates in the reaction vessel by the thermal process after cleaning the substrates with the cleaning liquids in the reaction vessel. Thus, the cleaned substrates are not exposed to the ambient atmosphere, the substrate having the clean surfaces cleaned by the cleaning process can be subjected to the thermal process. Consequently, the substrates can be satisfactorily processed by the thermal process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a plan view of a thermal processing apparatus in a preferred embodiment according to the present invention;  
       FIG. 2  is a longitudinal sectional view of the thermal processing apparatus embodying the present invention;  
       FIG. 3  is a longitudinal sectional view of a heating furnace included in the thermal processing apparatus;  
       FIG. 4  is a plan view of the heating furnace of the thermal processing apparatus;  
       FIG. 5  is a perspective view of a substrate holding device included in the heating furnace;  
       FIG. 6  is a perspective view of a position changing device included in the thermal processing apparatus;  
       FIG. 7  is a view of assistance in explaining a method of supplying a cleaning liquid into a reaction vessel included in the thermal processing apparatus; and  
       FIG. 8  is a view of assistance in explaining a prior art thermal processing apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The general configuration of a thermal processing apparatus in a preferred embodiment according to the present invention will be described with reference to  FIGS. 1 and 2  prior to the description of the thermal processing apparatus. The thermal processing apparatus has a transfer port A 1  for receiving and sending out a wafer carrier C holding, for example, fifteen wafers W, namely, substrates, in a stack, and a loading area A 2  where wafers W are loaded into a heating furnace for carrying out a thermal process to subject the wafers W to the thermal process. The transfer port A 1  and the loading area A 2  are separated by a partition wall  20  so as to isolate the spaces in the transfer port A 1  and the loading area A 2  from each other.  
      Installed in the transfer port A 1  are a first table  21  for supporting a wafer carrier C received from an external apparatus, a second table  22  for supporting a wafer carrier C while wafers W contained in the wafer carrier C are carried into the loading area A 2 , and a carrier carrying mechanism  23  for carrying a wafer carrier C between the tables  21  and  22 . The wafer carrier C is provided with a lid. The wafer carrier C is sealed hermetically to prevent exposing the wafers W held therein to the atmosphere when the wafers W are carried from the preceding process to the thermal process and from the thermal process to the succeeding process. A lid operating mechanism combined with a door  24  closing an opening formed in the partition wall  20  removes the lid to expose the interior of the wafer carrier C to an atmosphere in the loading area A 2 .  
      A side flow of clean air is produced in the loading area A 2 . Installed in the loading area A 2  are a first carrying device  3  and a second carrying device  4  for carrying a wafer W along predetermined paths, respectively, a heating furnace  5  in which wafers W are subjected to the thermal process, and a position changing deice  6  for changing the position of a wafer between a horizontal position and a vertical position.  
      The first carrying device  3  has a carrier base  32 , an arm unit  31  including a plurality of arms arranged at predetermined vertical intervals and supported on the carrier base  32  so as to be horizontally movable to support a peripheral part of each of wafers W from below the wafer, and a driving system  33  for moving the carrier base  32  horizontally, i.e., in directions along the width of the thermal processing apparatus, moving the carrier base  32  vertically and turning the carrier base  32  about a vertical axis. The first carrying device  3  carries wafers W between a wafer carrier C and a position changing device  6 .  
      The second carrying device  4  has a carrier base  42 , an arm unit  41  including a plurality of longitudinally arranged pairs of arms supported for longitudinal movement on the carrier base  42  to support wafers W set in a vertical position by the position changing device  6  by their peripheral parts, i.e., parts outside their device forming regions, and a driving system  43  for moving the carrier base  42  horizontally, i.e., in directions along the width of the thermal processing apparatus, and moving the carrier base  42  vertically. The second carrying device  4  carries wafers W between the position changing device  6  and the heating furnace  5 .  
      The heating furnace  5  will be described with reference to  FIGS. 3 and 4 . Referring to  FIGS. 4 and 5 , a reaction vessel  51  is placed in a furnace body  50 . The reaction vessel  51  has, for example, a cylindrical shape, is formed of a nonmetallic material, such as quartz or a ceramic material, and defines a heating space in which wafers W are placed for the thermal process. The reaction vessel  51  has an open lower end  53  defining an opening through which wafers W are carried into and out of the reaction vessel  51 . A lid  7  is disposed under the open lower end  53 . The lid  7  is vertically movable to close and open the open lower end  53  of the reaction vessel  51 . At least the surface of the lid d 7  is formed of a nonmetallic material, such as quartz or a ceramic material. A wafer boat  71  is supported by a shaft  72  on the lid  7 . The wafer boat  71  is capable of holding a plurality of wafers W, for example, twenty-five to fifty wafers W, in a vertical position in s horizontal arrangement. The lid  7  is raised by a boat elevator  73  included in an elevator unit to load wafers W held on the wafer boat  71  into the reaction vessel  51  and to close the open lower end  53  of the reaction vessel  51  so as to seal the reaction vessel  51  hermetically. A flange  51   a  is formed on the lower end of the reaction vessel  51 , and an O ring  51   b , i.e., a sealing member formed of a resin, is placed on the lid  7 . When the lid  7  supporting the wafer boat  71  is raised and brought into contact with the flange  51   a , the gap between the lid  7  and the open lower end  53  is sealed hermetically by the O ring  51   b.    
      More specifically, as shown in  FIG. 5 , the wafer boat  71  includes a horizontal plate  74 , a pair of vertical end plates  75  set on the horizontal plate  74  so as to be opposed to each other, for example, three support rods  76 , i.e., substrate support members, are extended between the opposite end plates  75 . The support rods  76  are provided with grooves  76   a  formed at longitudinal intervals. The second carrying device  4  brings wafers W from above the wafer boat  71  and inserts peripheral parts of the wafers W in the grooves  76   a  to hold the wafers W by the wafer boat  71 . If necessary, the horizontal plate  74  and the end plates  75  may be provided with openings  77  to enable process gases and cleaning liquids to flow across the horizontal plate  74  and the end plates  75 . The openings  77  enable the process gases or the cleaning liquids to flow smoothly when wafers W are processed. Consequently, the process gases and the cleaning liquids can be uniformly supplied into spaces between the wafers W.  
      Referring to  FIGS. 3 and 4  again, a heater  54 , such as a carbon wire heater, i.e., heating means, is disposed, for example, in a space between the furnace body  50  and the reaction vessel  51 . A carbon wire heater is preferable because the carbon wire heater is capable of heating a process atmosphere in the reaction vessel  51  at a high heating rate of, for example, 100° C./min. The carbon wire heater may be such a heater formed by sealing a carbon wire formed by twisting a plurality of carbon fiber strands in a ceramic case, such as a transparent quartz tube having an outside diameter of ten-odd millimeters. The carbon wire heater is formed vertically inside the furnace body  50 . The heating element of the heater is not limited to the carbon wire and may be a metal wire, such as an iron/nickel/chromium alloy wire.  
      A process gas supply pipe  8 , i.e., process gas supply means, is connected to a lower part of the side wall of the reaction vessel  51  so as to open downward. The process gas supply pipe  8  is connected by a gas supply line  81 , such as gas supply pipes, to process gas sources, i.e., an oxygen source  82  and a steam source  83  in this embodiment. Indicated at V 1  to V 3  are valves (gas supply valves. Nitrogen gas supply pipes  84  for carrying dry nitrogen gas, i.e., inert gas supply pipes for carrying a dry inert gas, are connected to a lower part of the side wall of the reaction vessel  51 . The nitrogen gas supply pipes  84  rise from a lower part to a middle part of the interior of the reaction vessel  51 , and extend horizontally along the direction of arrangement of the wafers W held on the wafer boat  71  respectively on the opposite sides of the arrangement of the wafers W. Outer ends of the nitrogen gas supply pipes  84  are connected to a valve V 4  connected to a nitrogen gas source  85 . The nitrogen gas supply pipes  84  are provided with blowing holes  84   a . Nitrogen gas is blown through the blowing holes  84   a  against the wafers W held on the wafer boat  71  after cleaning the wafers W with cleaning liquids to dry the wafers W. A plurality of nitrogen gas supply pipes  84  may be arranged vertically in a range corresponding to the height of the wafers W. When the nitrogen gas supply pipes  84  are arranged so, nitrogen gas can be more surely blown against the surfaces of the wafers W. The nitrogen gas supply pipes  84  may be vertically movable. A discharge line  86 , such as a discharge pipe, is connected to a discharge port  52  formed in an upper part of the reaction vessel  51 . The discharge line  86  is connected to a valve V 5  connected to a discharge device  87  to discharge gases contained in the reaction vessel  51  from the reaction vessel  51  by the discharge device  87 . A branch line  84   a  branched from the nitrogen gas supply line  84  is connected through a valve V 6  to the discharge line  86 . The valve V 5  is closed and the valve V 6  is opened to supply nitrogen gas from an upper part of the reaction vessel  51  into the reaction vessel  51 .  
      A cleaning liquid supply nozzle  9 , namely, a cleaning liquid supply means, is set, for example, vertically so as to project from the upper surface of the lid  7 . The cleaning liquid supply nozzle  9  is connected to a hydrofluoric acid solution source  92 , a pure water source  93  and an IPA (isopropyl alcohol) source  94  by a cleaning liquid supply line  91 , such as a cleaning liquid supply pipe, extended in the boar elevator  73 . The hydrofluoric acid solution source  92  supplies, for example, a 10% by weight hydrofluoric acid solution. Indicated at V 7  to V 9  are valves (cleaning liquid supply valves), and P 1  to P 3  are pumps. The lid  7  is provided with a drain port  95 . The drain port  95  is connected to a drain tank, not shown, by a drain line  96 , namely, drain pipe. A valve V 10  (drain valve) is placed in the drain line  96  to open and close the drain port  95 .  
      The position changing device  6  for changing the position of wafers W mentioned in connection with  FIGS. 1 and 2  will be described with reference to  FIG. 6 . A swivel box  61  has an open front side. Support rails  62  for supporting wafers W thereon are arranged vertically at intervals corresponding to the thickness of the wafers W on the inner surfaces of the side walls of the swivel box  61 . The wafers W are inserted in spaces between the support rails  62  so as to be supported on the support rails  62  in the swivel box  61 . A horizontal shaft  64  is attached to lower back parts of the side walls of the swivel box  61 . The shaft  64  has a base end connected to a turning mechanism  65 . The turning mechanism  65  turns the shaft  64  in opposite directions through  900  about a horizontal axis to swivel the swivel box  61  forward and backward so that the wafers W contained in the swivel box  61  are turned between a horizontal position and a vertical position.  
      The swivel box  61  has an open back side  61   a . A lifting device  66  provided with a lifting table  67  is disposed such that the lifting table  67  is opposite to the open back side  61   a  of the swivel box  61  when the swivel box  61  is set in a horizontal position in which the wafers W held in the swivel box  61  are set in a vertical position. The lifting table  67  moves through the open back side  61   a  into the swivel box  61  when the lifting table  67  of the lifting device  66  is raised. As shown in  FIG. 6 ( b ), the lifting table  67  is provided in its upper surface with grooves  67   a  for receiving peripheral parts of the wafers W. The edges of the grooves  67   a  are chamfered. The lifting table  67  is connected by a shaft  69  to a lifting mechanism  68 . The lifting mechanism  68  raises the lifting table  67  to lift up the wafers W contained in the swivel box  61 . Then, the second carrying device  4  holds the lifted wafers W to receive the wafers W from the lifting table  67 . Thus the wafers W can be transferred from the lifting device  66  to the second carrying device  66  and vice versa.  
      The thermal process for processing wafers W by the thermal processing apparatus will be described. An oxidation process, as a thermal process, for forming a silicon dioxide film on surfaces of wafers W will be described by way of example. Referring to  FIGS. 1 and 2 , a waver carrier C is delivered to the first table  21  of the transfer port A 1  by an automatic carrying robot. The carrier carrying mechanism  23  carries the wafer carrier C to, for example, a carrier storage unit, not shown, for temporary storage. The carrier carrying mechanism  23  carries the wafer carrier C from the carrier storage unit to the second table  22 . The lid of the wafer carrier C is removed after the wafer carrier C has been pressed against the partition wall  20 , and then the door  24  is opened.  
      Subsequently, the arm unit  31  of the first carrying device  3  advances into the wafer carrier C, takes out a plurality wafers W, for example, five wafers W, simultaneously from the wafer carrier C and carries the wafers W into the swivel box  61  set in a vertical position. The arm unit  31  repeats this operation to transfer all the wafers W contained in the wafer carrier C to the swivel box  61 . Then, the turning mechanism  65  turns the shaft  64  to swivel the swivel box  61  through 90° to set the wafers W held in the swivel box  61  in a vertical position. Then, the arm unit  41  of the second carrying device  4  is moved to a position above the swivel box  61 , holds all the wafers W, for example, fifteen wafers W, lifted up from the swivel box  61  by the lifting device  66 , and carries the wafers W to loads the same on the wafer boat  71 . The arm unit  41  repeats this operation to load the wafer boat  71  with a predetermined number of wafers W. After the wafer boat  71  has been thus fully loaded with the wafers W, the second carrying device  4  is retracted.  
      Subsequently, the boat elevator  73  is raised to load the wafer boat  71  holding the wafers W into the reaction vessel  51 , and the open lower end  53  of the reaction vessel  51  is closed by lid  7  to seal the reaction vessel  51  hermetically. Then, the valves V 5  and V 7  are opened, and the hydrofluoric acid solution, namely, a cleaning liquid, is supplied through the cleaning liquid supply nozzle  9  into the reaction vessel  51  to fill up the reaction vessel  51  with the hydrofluoric acid solution as shown in  FIG. 7 ( a ). Then, the hydrofluoric acid solution contained in the reaction vessel  51  is heated by the heater  54  at a temperature, such as 80° C., at which the hydrofluoric acid solution will not boil and interaction between the hydrofluoric acid solution and oxide films formed by natural oxidation on the wafers W is promoted. A state where the reaction vessel  51  is filled up with the cleaning liquid is a state where the level of the cleaning liquid is higher than that of the upper ends of the wafers W. The oxide films formed by natural oxidation on the surfaces of the wafers W and impurities adhering to the surfaces of the wafers W can be removed by immersing the wafers W in the hydrofluoric acid solution for a predetermined time. Then, the valves V 7  and V 5  are closed, and the valves V 6  and V 10  are opened to drain the hydrofluoric acid solution from the reaction vessel  51  and to supply nitrogen gas into the reaction vessel  51 . After the hydrofluoric acid solution has been completely drained away, the valves V 6  and V 10  are closed, and the valves V 8  and V 5  are opened to fill up the reaction vessel  51  with pure water, namely, cleaning liquid, by supplying pure water through the cleaning liquid supply nozzle  9  into the reaction vessel  51 . Thus the hydrofluoric acid solution remaining on the wafers W is rinsed away. Then, the valves V 8  and V 5  are closed and the valves V 6  and V 10  are opened to drain the pure water from the reaction vessel  51 . Then, the valves V 6  and V 10  are closed, and the valves V 5  and V 9  are opened to fill up the reaction vessel  51  with IPA, namely, cleaning liquid, by supplying IPA trough the cleaning liquid supply nozzle  9  into the reaction vessel  51 . Then, the valves V 9  and V 5  are closed, and the valves V 6  and V 10  are opened to drain the IPA away from the reaction vessel  51 . The IPA reduces the surface tension of water droplets (droplets of the pure water) adhering to the surfaces of the wafers W. Consequently, the water droplets flow along the surfaces of the wafers W held in a vertical position and drop away from the wafers W.  
      The valves V 5  to V 10  are opened and closed on the basis of a sequence program stored in a controller, not shown, to supply hydrofluoric acid solution, pure water and IPA sequentially into the reaction vessel  51  and to supply and to stop supplying nitrogen gas into the reaction vessel  51  for the cleaning process. The valves V 1  to V 3  and V 5  may be opened and closed to supply and to stop supplying the process gases and a purge gas for the thermal process, which will be described later, on the basis of a sequence program stored in the controller. IPA may be supplied in either a liquid or a vapor. An IPA vapor supply line may be connected to the lid  7  when IPA is supplied in a vapor.  
      Then, the valve V 5  is opened, the valves V 6  to V 9  are closed, and the valve V 4  is opened to supply nitrogen gas, namely, a dry gas, through the nitrogen gas supply pipes  84  into the reaction vessel  51  for a drying operation. As mentioned above, the nitrogen gas supply pipes  84  are provided with the blowing holes  84   a  corresponding to the wafers W. Nitrogen gas is blown against the wafers W to dry the wafers W quickly and to purge the reaction vessel  51  with the nitrogen gas. After the drying operation has been continued for a predetermined time, the valve V 4  is closed to stop supplying nitrogen gas. The valve V 5  in the discharge line  86  is kept open while nitrogen gas is supplied into the reaction vessel  51  and is closed when nitrogen gas supply is stopped.  
      Then, the interior of the reaction vessel  51  is heated by the heater  54  at a process temperature of, for example, 1000° C.  
      A process gas, a mixed gas containing, for example, oxygen gas and steam, is supplied through the gas supply pipe and the process gas supply pipe  8  into the reaction vessel, while the discharge device  87  discharges gases contained in the reaction vessel  51  from the reaction vessel  51  so as to keep the atmosphere in the reaction vessel  51  at a predetermined pressure, such as a slightly reduced pressure. Silicon in the surface layers of the wafers W is oxidized to form silicon diocese films on the wafers W, respectively, by the thermal process.  
      After continuing the thermal process for a predetermined time, the valves V 1  to V 3  are closed to stop supplying the process gas. The valve V 4  is closed after purging the reaction vessel  51  with nitrogen gas, the boat elevator  73  is lowered to a predetermined lower position, and the lid  7  is opened to release the reaction vessel  51  from an airtight state. Then, the wafer boat  71  holding the wafers W is unloaded from the reaction vessel  51 . Then the foregoing operation for transferring the wafers W from the wafer carrier C to the wafer boat  71  is reversed to return the wafers W to the wafer carrier C; that is, the second carrying deice  4  receives the wafers W from the wafer boat  71  and carries the wafers W into the swivel box  61 , the swivel box  61  is turned forward to set the swivel box  61  in a vertical position so that the wafers W are held in a horizontal position in the swivel box  61 , and then the first carrying device  3  transfers the wafers W from the swivel box  61  to the wafer carrier C. Thus the thermal process is accomplished.  
      The thermal processing apparatus in this embodiment cleans the wafers W by supplying the cleaning liquids into the heating furnace  5  by the cleaning liquid supply system, and supplies the process gas into the heating furnace  5  to process the wafers W by the thermal process. Therefore, the cleaned wafers W are not exposed to the ambient atmosphere and silicon dioxide films can be formed on the clean surfaces of the cleaned wafers W by the thermal process. Therefore, the silicon dioxide films formed on the surfaces of the wafers by the thermal process are of very high quality not containing any oxide film formed by natural oxidation at all or scarcely containing such an oxide film. Consequently, high-quality thin gate oxide films can be formed and satisfactory semiconductor devices can be fabricated on those wafers W. Nitrogen gas supplied into the reaction vessel  51  works for both drying the cleaned wafers W and purging the reaction vessel  51  to keep the surfaces of the wafers W clean. Therefore, the thermal processing apparatus is simple in construction and is able to operate at a low operation cost because a nitrogen gas atmosphere does not need to be created in the loading area A 2 .  
      Although the oxidation process has been described by way of example, the thermal processing apparatus may be used for carrying out a CVD process. If films deposited on the inner surfaces of the reaction vessel  51  and the surfaces of the wafer boat  71  and the lid  7  during the CVD process are those of substances that can be removed by the cleaning liquid, both cleaning the interior of the reaction vessel  51  and cleaning the wafers W can be achieved simultaneously. An ammonium chloride film is a film that may be possibly deposited. An ammonium chloride film may be deposited as a by-product of process, such as a silicon nitride film forming process that forms a silicon nitride film (Si 3 N 4  film) through the interaction of a silane gas, such as a dichlorosilane gas, and ammonia gas.  
      In the foregoing embodiment, the wafers W held in a vertical position on the wafer boat  71  are subjected to the cleaning process and the thermal process. Therefore, the cleaning liquid adheres scarcely to the vertical surfaces by surface tension, the cleaning liquid drips off the wafers W by gravity and the cleaning liquid can be quickly removed from the surfaces of the wafers W to drain the cleaning liquid from the reaction vessel  51 . Consequently, formation of water marks with the cleaning liquid, particularly with IPA, can be suppressed when the wafers W are dried by using nitrogen gas. The drying gas is not limited to nitrogen gas. For example, high-temperature, high-pressure, dry air having a low oxygen concentration may be used as the drying gas. The use of such dry air is more effective than nitrogen gas in suppressing the formation of water marks.  
      When the wafers W held in a vertical position are subjected to the thermal process, the spaces between the wafers W extend vertically in which the process gas flows from the bottom toward the top of the reaction vessel  51  and hence the process gas is able to flow smoothly upward through the spaces between the wafers W. Since the wafers W are held in a vertical position that facilitate the flow of the process gas in the reaction vessel  51 , the surfaces of the wafers W can be uniformly exposed to the process gas. According to the present invention, wafers W may be processed by the thermal process in either a batch processing mode as mentioned above or a single-wafer processing mode. Although the effect available when wafers W are held in a vertical position, wafers W held in a horizontal position may be processed.  
      According to the present invention, the thermal processing apparatus does not need necessarily provided with a heater capable of heating the interior of the reaction vessel  51  at a high heating rate of 100° C./min as the heater  54 . However, when the cleaning process and the thermal process are carried out in the same chamber, namely, the processing chamber in the heating furnace  5 , it is desirable to keep a low temperature so that the cleaning liquid may not boil during the cleaning process and to keep a high temperature such as 1000° C., to promote the reaction of the process gas during the thermal process. Since the difference between those temperatures respectively for the cleaning process and the thermal process is large, the number of cycles of the thermal process can be increased and the throughput of the thermal processing apparatus can be increased by employing a high-capacity heater as the heater  54  to curtail time necessary for raising the temperature.  
      Moreover, since the present invention subjects the wafers to the thermal process after cleaning, an open wafer carrier may be used instead of the closed wafer carrier C. A wafer carrier C that holds wafers W in a vertical position may be used.