Abstract:
Method for purging a furnace comprising a closable processing tube defining in its interior a closable, processing space, a purging gas being flowed along the outer surface of the processing tube separated from the processing gas flow inside the processing tube, wherein said processing gas flow comprises a gas being reactive with air at elevated temperature and said purging gas comprises an inert gas. A furnace assembly for realizing this method is proposed comprising a controller for controlling force connected in a conduit connected to a source of reactive and inert gas respectively. This controller is realized such that only after placing the closure of the processing chamber in the closing position the valve in the supply conduit for reactive gas can be opened and said closure can only be removed from said closing position after said valve in supply conduits for reactive gas has been closed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to the operation of a furnace for the treatment of objects (substrates) such as wafers with a reactive gas. Such a reactive gas can comprise hydrogen or another gas that, if it comes into contact with air, can form an explosive mixture.  
           [0002]    In the prior art this problem is avoided by providing an additional tube surrounding the process tube hermetically and purging the gap between the process tube and the outer tube with an inert gas. However, in case an event results in heavy damage of the process tube, and consequently, in escape of reactive gas from the process tube, it is likely that the same event results also in heavy damage of the outer tube because both tubes are closely connected together and both tubes are made of refractory materials like quartz or silicon carbide which are susceptible to breakage upon mechanical impact. Such an event can be an earthquake which are regular phenomena is some of the worlds most important semiconductor manufacturing areas. Also it is possible that, because of the close connection of the tubes, breakage of one tube induces breakage of the other tube.  
         SHORT DESCRIPTION OF THE INVENTION  
         [0003]    It is an object of the invention to provide a method for the treatment of an object in a furnace, wherein it is possible to use a high concentration of reactive gas without the risk of an explosion.  
           [0004]    It is a further object of the present invention to provide a furnace in combination with conduits (system) with which the treatment of a substrate with a reactive gas in a comparatively high concentration is possible.  
           [0005]    Is a further object of the present invention to carry out such a treatment at a comparatively high temperature.  
           [0006]    According to a further object of the present invention it is intended, after the treatment at a comparatively high temperature, to comparatively quickly and uniformly realize cooling of the process space and thus of the substrate.  
           [0007]    According to an aspect of the invention, a method is provided for purging a furnace comprising a closable processing tube defining in its interior a closable processing space, a purging gas being flowed along the outer surface of the processing tube separated from the processing gas flow inside the processing tube, wherein said processing gas flow comprises a gas being reactive with air at elevated temperature and said purging gas comprises at least 90 vol. % of an inert gas.  
           [0008]    According to a further aspect of the invention, the purging gas comprises at least 99% by volume of an inert gas. Nitrogen can be used as the inert gas. Furthermore the purging gas can comprise at least 100 ppm by volume of an oxidizing gas and preferably at least 0.1% by volume of an oxidizing gas. According to a preferred embodiment of the invention, the reactive gas comprises at least 6% by volume of hydrogen.  
           [0009]    The oxidizing gas can comprise oxide, H 2 O, CO 2  or N 2 O alone or in any combination. Preferably oxygen or air (compressed) is used.  
           [0010]    The pressure in the furnace or other treatment room is preferably atmospheric. According to a further aspect of the invention, the purging gas is not only used for purging the furnace as well as the space around it, but the purging gas also acts as a cooling medium.  
           [0011]    Optimal safety is achieved if the treatment space is also purged, this purging preferably taking place after the treatment has finished, but before the treatment room is brought into contact with the environment.  
           [0012]    If the purging gas is used for cooling, according to a special embodiment of the invention, the direction of movement of this gas is periodically reversed so that an even cooling occurs from the inlet to the outlet.  
           [0013]    According to a further aspect, the invention relates to a method for treating a substrate in a furnace at elevated temperature with a processing gas, said processing gas being reactive with air at processing temperature of said substrate, wherein after said substrate is introduced in said furnace, the processing chamber thereof is purged with an inert gas before reactive processing gas is introduced in said processing chamber.  
           [0014]    Moreover, the invention relates to a furnace assembly comprising a furnace having a processing chamber surrounded by heating means, said processing chamber having a removable closure, at least a gas inflow and gas outflow opening, said gas inflow opening being connected to at least two supply conduits each comprising a controller controlled valve and connected to a source of reactive and inert gas respectively, a controller for controlling said valves such that only after placing said closure in the closing position in said processing chamber the valve in the supply conduit for reactive gas can be opened and said closure can only be removed from said closing position after said valve in supply conduits for reactive gas has been closed.  
           [0015]    Preferably the space through which the purge gas is conducted is defined by the heating element and/or the isolation thereof.  
           [0016]    Moreover, safety measures are present. These are implemented such that the continuous purging during a certain period of time is guaranteed before the reactive gas is brought into the treatment space (while it is closed) or before the treatment space is brought into contact with the environment. Likewise (constant) checks are made on the purging gasses for the presence of oxygen to detect any leaks in the system.  
           [0017]    According to a further aspect of the invention there are various valves for the supply of process gas and inert respectively purging gas, implemented such that when the energy supply fails, the supply of the inert/purging gas is still guaranteed.  
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    The invention will be explained below with reference to two embodiments, schematically shown in the attached drawing, wherein the advantages and particular qualities of the invention are elucidated, and wherein:  
         [0019]    [0019]FIG. 1 shows a schematic drawing of a furnace;  
         [0020]    [0020]FIG. 2 shows a first embodiment of the system according to the invention as a schematic diagram; and  
         [0021]    [0021]FIG. 3 a shows second embodiment of the system according to invention, whereby the furnace is provided with quick cooling.  
     
    
       [0022]    In FIG. 2, a first embodiment of the system according to the invention is indicated as a whole by  1 . This consists of a furnace  2 , of which details are shown in FIG. 1. Furnace  2  is built up of processing tube  3 , which preferably consists of quartz or silicon carbide material. In the processing tube a boat  13  is schematically shown, filled with wafers  14 . The furnace is heated with heating elements  4 , around which a thermal insulation  5  is fitted. Processing tube  3  is provided with an inflow opening  6  and an outflow opening  29 . The processing tube is open on the underside and can be closed off by the door plate  7  from the sluice space and carousel space which are situated under the furnace. The space between processing tube  3  and the heating elements  4  and insulation  5 , respectively, is indicated by  8 . This is provided with inlets  9  and outlets  10 . A sluice space  11  is situated under the furnace  2 , under which, for example, a carousel space  12  can be fitted. As an extra precaution, to prevent process gas from getting into the carousel space  12 , the sluice space  11  is purged with nitrogen during the execution of process.  
         [0023]    The aim is to carry out a process within processing tube  3 , wherein a reactive gas such as hydrogen is used. This gas is used in such a quantity, that, at the elevated temperature at which this reaction is carried out, the possible presence of oxygen (air) leads to a real danger of explosion.  
         [0024]    To prevent such risks wherever possible, a number of measures are taken. According to the invention it is proposed, that as long as the above described real danger of explosion exists, space  8  be purged with nitrogen. To that end, the inlets  9  are connected to a conduit  15 , which in turn is connected to a source of nitrogen  16 . A valve  17  is present in conduit  15 . This is connected to controller  18 . Source  16  is also connected to carousel space  12  via conduit  19 . Dosing is controlled by a valve  20 , which is connected to controller  18 . The sluice space  11  can also be provided with nitrogen. This has a higher purity, however, and the corresponding nitrogen source is indicated by  21  and the supply conduit  22 .  
         [0025]    At the outer circumference, space  8  is delimited by the heating elements. When, with a view to the safety, space  8  is purged with a pure inert gas like nitrogen, the lifetime of the heating element will be severely affected. This is caused by the fact that by exposure of the metal heating wire of the heating elements to air at elevated temperatures a film of metal oxide is formed on the wire surface. This metal oxide appears to be very essential for the mechanical stability of the heating wire and serves as a protective film against evaporation of the metal wire at high temperature. During temperature cycling of the furnace, cracks can form in the protective metal oxide film, locally exposing the metal surface and leading to localized aging of the wire. Therefore, continuous presence of a minimum concentration of oxidizing gas in space  8  during high temperature operation of the furnace is essential to ensure a long life time of the heating element. A minimum concentration of 100 ppm by volume, and more preferably 0.1% by volume is required to maintain the metal oxide film in good condition. This minimum concentration of oxidizing gas can be achieved by providing the heating element, which forms the enclosing shell of space  8 , by an amount of leaks that, on the one hand, allows a sufficient amount of indiffusion of air and, on the other hand, allows to maintain a gas atmosphere in space  8  that is predominantly composed of inert gas. As an alternative, a small flow of air can be mixed with the large nitrogen flow that is fed into space  8  in order to achieve the required low concentration of oxidizing gas. For this purpose, a conduit  55  is connected to inlets  9 , which conduit  55  is connected to a source of compressed air  56 . A valve  57  is present in conduit  55  which valve is connected to controller  18 .  
         [0026]    Controller  18  consists of a combination of hardware and software. In general, the aim according to invention is to implement in hardware as many as possible of the controls that are important for the security of the system. In this way, the effect of problems in the software can be avoided, as far as possible. It should be understood that, depending on the development of the software and the inherent security thereof, some parts can be implemented in software. In the controller  18 , at least one timer is present for controlling the time period during which purging is carried out, in the method described below.  
         [0027]    The outlets  10  are connected to the outlet  24  of the system. Connected onto here in parallel are an oxygen analysing apparatus  23  to detect the presence of oxygen in parallel with a special analysing apparatus  43  for analysing the presence of the reactive process gas. Each analyser is connected to controller  18 .  
         [0028]    The outlet  24  is connected to a central extraction system, not shown in FIG. 2 and not belonging to the system as such. The operational safety is increased by providing the drain  24  with dilution by nitrogen gas and with an extra pump action and further dilution by air through the nitrogen, originating from source  16 , to be introduced by means of an inlet  26  working using the venturi principle. Air is let in via conduit  25  and is discharged to conduit  24 . The drain of outlet  10  connects onto conduit  24  directly upstream of venturi-inlet  26 .  
         [0029]    A further oxygen analysing apparatus is indicated by  27 . An amount of gas is continuously pumped around in the carousel space  12  by means of a pump, not shown in the drawing.  
         [0030]    The outlet of sluice space  11  is indicated by  28 . This also ends up in the discharge conduit  24 . When the furnace is working, the spaces around the processing tube are purged with nitrogen. This relates to both the purging of space  8  and the sluice space  11  and carousel space  12 . Clearly, for the purging of space  8 , a comparatively small amount of gas will be used, which does not or barely influences the warming of processing tube  13 . With the help of the above described oxygen analysing apparatus, the presence of oxygen in the various spaces is continuously checked. The outflow opening of processing tube  13  is indicated by  29  (FIG. 1) and ends up in the conduit  30 . This conduit  30  leads on one side to conduit  24 , while part of the gas flowing through there is branched off to oxygen analysing apparatus  31  connected to controller  18 .  
         [0031]    A source of argon is indicated by  32 , and  33  indicates a source of treatment gas such as hydrogen. Valves  34 ,  35  are present to regulate the flow of the argon and nitrogen. The outlets of these valves are connected by the conduit  36 , which is connected to the inlet  6  of the processing tube.  
         [0032]    A door switch is indicated by  38  which passes on to controller  18  whether or not the door plate  7  is open. If one or more wafers in processing tube  13  have to be treated with a reactive gas, such as one originating from source  33 , a boat  13  with wafers  14  is introduced under an inert atmosphere. Such an inert atmosphere can be obtained when valve  34  is open and valve  35  is closed. Purging with nitrogen is possible by operating valve  37 .  
         [0033]    After the introduction of the objects to be treated and closing of the door plate  7 , a signal is sent via door switch  38  to controller  18 . Consequently a timing cycle is started. Purging of the space around the processing tube with nitrogen takes place. During a minimum purging time of 10 minutes with a minimum flow of nitrogen of 5 slm (Standard liters per minute) it is not possible to open the valve  35  of the reactive process gas. Only after the minimal purging requirements are met is the valve  35  again released for use. After completion of the purging cycle, valve  34  is closed and valve  35  is opened by controller  18 .  
         [0034]    Consequently, the process can take place in processing tube  3 . Gas originating from the outflow opening  29  of the processing tube is continuously discharged to outlet  24 . By the introduction of nitrogen via the venturi-inlet and the air carried along by the venturi-action, any unused hydrogen present is diluted to such an extent that there is no longer an explosive mixture. The presence of oxygen is continuously monitored with the help of an analysis apparatus  31 . Should any irregularities arise, then the supply of gas from source  33  is immediately stopped and valve  34  is opened. It has been implemented such that, at the loss of flow, a similar situation occurs.  
         [0035]    After the process with hydrogen is completed, by the continual purging of the space around the processing tube  3  with nitrogen, which took place during the whole treatment, the supply of hydrogen is blocked by closing valve  35 . Then either argon is let into the processing tube, or extremely pure nitrogen.  
         [0036]    During a minimum purging time of  10  minutes with a minimum stream of argon or nitrogen of 5 slm (Standard liters per minute), it is not possible to open door plate  7 . Only after the minimum purging requirements are met can the door plate  7  be opened again.  
         [0037]    For the quick cooling of the processing tube it is possible to move an increased amount of nitrogen from inlet  9  to outlet  10 . This absorbs the heat of the objects situated in processing tube.  
         [0038]    With the apparatus described above it is guaranteed that during the stage in which the reactive gas such as hydrogen is introduced, an atmosphere of nitrogen is continuously present around the treatment space, while likewise the presence of oxygen in the various drains is checked. Moreover, the process can only be started up after a certain period of purging with inert gas has taken place and the same holds for the removal of the wafers from the processing tube.  
         [0039]    In FIG. 3 a variant of the apparatus of FIG. 2 is shown, wherein the furnace is used according to FIG. 1.  
         [0040]    The most important difference is that a particular system is included to be able to cool the processing tube quicker after the reaction with the reactive gas.  
         [0041]    The parts corresponding to the embodiment shown in FIG. 2 are provided with the same reference numbers. The whole system shown in FIG. 3 is indicated by  41 . Space  8  in the shown embodiment is filled with an inert gas originating from source  16  via conduit  15  controlled by valve  17 . However, in contrast to the embodiment described above, no continuous complete discharge of the gas takes place. Only a small amount of the gas is discharged via conduit  42 .  
         [0042]    A special analysing apparatus  43  for detecting the presence of reactive process gas is fitted in conduit  42  and connected to controller  18 . In parallel to special analysing apparatus  43 , an oxygen analysing apparatus, connected to controller  18 , is provided.  
         [0043]    In this embodiment, the inlets  9  and the outlets  10  are connected to each other via conduit  46 , in which a pump  44  is fitted. A heat exchanger  45  is also fitted. This can be a gas-gas heat exchanger, which is connected on the other side to the ambient air. This can also be a gas-liquid heat exchanger. Working of pump  4  is regulated by controller  18 .  
         [0044]    With the system  41  according to FIG. 3, in principle, the same working is possible as in system  1  that is described above. However, for the quick and even cooling of the processing tube  3 , gas in space  8  is moved at increased speed from inlet  9  to outlet  10  by the effects of pump  44 , whereby the heat exchanger  45  removes the absorbed heat. An even cooling is achieved by the periodic reversing of the direction of movement of the gas by means of a valve system (not shown). For details of the method of working reference is made to WO 98/00151.  
         [0045]    Also for this embodiment it is essential that during the process, processing tube  3  is continuously enclosed in a “shell” of a gas that is inert with respect to the reactive gas which is applied in the processing tube  3 .  
         [0046]    While the invention above is described with reference to a preferred embodiment, for the persons skilled in the art it is clear that numerous changes are possible which are clear after reading the description above and are within the scope of the attached claims.