Abstract:
In processes for coating objects, such as semiconductor wafers, with a film of metal, such as titanium metal, a metal-containing compound, such as TiCl 4 , is injected into a chamber containing the object and a portion of the metal-containing compound reacts to provide the film of metal on the object and a gas containing by-products, such as unreacted TiCl 4  and TiCl x  (x&lt;4), which is discharged out of the chamber and passed through a trap mechanism and an eliminator for the removal of the by-products out of the gas. The by-products have relatively high vapor pressures, making them difficult to trap. The Applicants have found that by adding a reagent, such as water, O 2  or NH 3 , into the exhaust gas at a location upstream of the trap mechanism and eliminator, the reagent reacts with the by-product in the gas to produce a compound, such as TiCl 4 .2NH 3 , which has a significantly lower vapor pressure than the by-product and can be removed in the trap mechanism.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-333433, filed Nov. 24, 1999, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an exhaust apparatus for a process gas, which is used in combination with a process apparatus for forming a layer on an object to be processed using the process gas, and relates to a method of removing an impurity gas (unprocessed gas, non-reacted process gas) formed by a process gas. 
     In general, to form an integrated circuit, such as an IC, or a logic device, a step of depositing a desired thin film on the surface of an object to be processed, such as a semiconductor wafer, glass substrate or LCD substrate, and a step of etching the deposited film to a desired pattern are repeatedly carried out. 
     With regard to the film deposition step, for example, a thin silicon film, a thin film of silicon oxide or nitride, a thin metal film, a thin film of metal oxide or nitride, or the like is formed on a to-be-processed object by reacting a predetermined process gas (source gas) in a process vessel. It is known that an excess reaction by-product is produced at the same time as the reaction for film deposition and the reaction by-product, and a non-reacted process gas is discharged together with an exhaust gas. 
     The reaction by-product and non-reacted process gas, if discharged directly in an atmosphere, would cause an environmental pollution or the like. To prevent such pollution, an exhaust apparatus is connected to the process vessel. The exhaust apparatus has a trap mechanism provided in an exhaust gas system that extends from the process vessel to trap and removes a reaction by-product or non-reacted process gas or the like contained in the exhaust gas. 
     Various structures have been proposed for the trap mechanism in accordance with the characteristics of the reaction by-product or the like that should be trapped and removed. To eliminate a reaction by-product which is liquefied or solidified and condensed at an ordinary temperature, for example, a trap mechanism has multiple fins provided in a casing which has an inlet port and an exhaust port for the exhaust gas. The fins are orderly arranged in the flow direction of the exhaust gas and trap a reaction by-product or the like contained in the exhaust gas on their surfaces as the exhaust gas passes between the fins. Such an apparatus typically cools the fins with a cooling fluid or the like in order to improve the trapping efficiency. 
     A description will now be given of the case where a TiCl 4  (titanium tetrachloride) gas of a high-melting point metal halogen compound is used as a source gas to form a Ti metal film on a semiconductor wafer. H 2  gas is used as a source gas in addition to the TiCl 4  gas. The H 2  gas is activated by plasma in the process container under an Ar gas atmosphere and reduced with hydrogen, thus depositing a Ti film on the surface of the semiconductor wafer. At this time, TiCl x  (x&lt;4) is produced as a reaction by-product, and a non-reacted TiCl 4  gas is present in the process vessel. The TiCl x  gas and TiCl 4  gas or the like flow out of the exhaust apparatus in the form of impurities in the exhaust gas. Because the TiCl x  gas and TiCl 4  gas or the like are impurity gases that would cause air pollution or the like, they should be trapped by the aforementioned trap mechanism in the exhaust apparatus. 
     Because the aforementioned impurity gases, such as the TiCl 4  gas or a non-reacted gas and the TiCl x  gas or a reaction by-product, have relatively high vapor pressures, it is very difficult to completely trap and eliminate those gases in the trap mechanism even if the interior of the trap mechanism is cooled as mentioned above. This may result in insufficient trapping. In this respect, an eliminator is provided at the downstream of the trap mechanism to completely eliminate the impurity gas that has passed through the trap mechanism. Such an eliminator is complicated and should be inspected frequently, leading to a higher running cost and a shorter service life. To solve this problem, the eliminator should have a very large capacity, which inevitably enlarges the whole apparatus and increases the cost. Such a shortcoming is common to various process apparatuses which use a high-melting point metal halogen compound gas such as TiCl 4 , WF 6  or (Ta(OE) 5 ) 2  (pentoethoxy tantalum). 
     A method of depositing a TiN film is known as another process method which uses TiCl 4  gas. This method will be explained with reference to the case where TiCl 4  (titanium tetrachloride) gas of a high-melting point metal halogen compound is used as a source gas to form a TiN film. NH 3  gas is used as a source gas in addition to the TiCl 4  gas and both gases are reacted in a reactor to deposit a TiN film on the surface of a semiconductor wafer. At this time, NH 4 Cl and TiCl 4 (NH 3 ) n  (n: a positive integer) are produced as reaction by-products, and non-reacted TiCl 4  gas is also present in the reactor. The gas components flow out of the reactor in the form of impurities in the exhaust gas and are trapped by the aforementioned trap mechanism and/or eliminator. 
     Because an unnecessary film which causes particles sticks on the inner wall of the process chamber of the process apparatus or the surface of a structure inside the vessel as the film deposition is carried out, cleaning is executed as needed, which regularly or irregularly supplies a cleaning gas into the process chamber to eliminate the unnecessary film. In this case, various kinds of fluorohalogen-based gases, such as ClF 3  gas, are used as cleaning gases. The ClF 3  gas removes the unnecessary film stuck on the inner wall or the like of the process chamber and is reacted with a reaction by-product of TiCl 4 (NH 3 ) n , thus yielding another reaction by-product, such as TiF 4 (NH 3 ) n . 
     As NH 4 Cl, TiCl 4 (NH 3 ) n , TiF 4 (NH 3 ) n , etc. are sequentially stored in the trap mechanism as reaction by-products, the trap mechanism is regularly or irregularly detached from a vacuum exhaust system, opening the interior so that the reaction by-products are cleaned out. At the time the trap mechanism is released in an atmosphere, NH 4 Cl hardly makes a problem because it is relatively stable. However, TiCl 4 (NH 3 ) n  or TiF 4 (NH 3 ) n  produce HCl gas, HF gas and NH 3  gas, harmful to human bodies, as indicated by the following formulas (1), (2) if reacting oxygen in the air. Some countermeasures are therefore demanded. 
     
       
         TiCl 4 (NH 3 ) n +O 2 →TiO 2 +HCl+NH 3   (1) 
       
     
     
       
         TiF 4 (NH 3 ) n +O 2 →TiO 2 +HF+NH 2   (2) 
       
     
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an exhaust apparatus for a process apparatus and an impurity-gas removing method, which can sufficiently remove a non-reacted source gas, its reaction by-product or the like. 
     It is another object of this invention to provide an impurity-gas removing method and an exhaust apparatus for a process apparatus, which can stabilize a reaction by-product trapped by a trap mechanism. 
     To achieve the above objects, an exhaust apparatus according to one aspect of this invention comprises an exhaust pipe to be connected to an exhaust port of a process apparatus; a trap mechanism disposed in the exhaust pipe, for removing an impurity gas contained in an exhaust gas, is exhausted from the process apparatus; reaction-gas supply means provided in the trap mechanism and/or in the exhaust pipe at an upstream of the trap mechanism, for feeding a reaction gas which is reacted with the impurity gas in at least one of the trap mechanism and the exhaust pipe to lower a vapor pressure; and exhaust-gas discharging means provided in the exhaust pipe at a downstream of the trap mechanism, for discharging the exhaust gas from the process apparatus outside via the exhaust pipe. 
     As the reaction gas is fed from the reaction-gas supply means into the trap mechanism and/or the exhaust pipe at the upstream of the trap mechanism reach, a reaction by-product whose vapor pressure is lower than that of the impurity gas is formed. It is therefore possible to easily condense and solidify the impurity gas in the trap mechanism and trap the gas there. 
     It is preferable that the reaction-gas supply means should be located in the exhaust pipe near the exhaust port of the process apparatus so that mixed diffusion of the reaction gas is accelerated while the exhaust gas reaches the trap mechanism, an accelerates the reaction, making it possible to trap and eliminate more reliably an impurity gas such as a process. 
     Oxidative-gas supply means for feeding an oxidative gas for reacting with and oxidizing a reaction by-product in the trap mechanism may be provided in the trap mechanism or a portion of the exhaust pipe at the upstream of the trap mechanism. 
     This structure can oxidize and stabilize an unstable reaction by-product by feeding an oxidative gas to the exhaust system before the trap mechanism is detached from the exhaust system. This makes it possible to detach the trap mechanism from the exhaust system and clean the inside of the trap mechanism while safely keeping the trap mechanism open. 
     In this case, the exhaust apparatus may comprise a bypass pipe connected to the process apparatus to bypass the trap mechanism. 
     In this case, it is preferable that, when the oxidative gas is made to contact the reaction by-product in the trap mechanism, the process apparatus is evacuated with a large inverse diffusion coefficient though a bypass pipe provided to bypass the trap mechanism. This prevents the reverse diffusion of the oxidative gas to the deposition apparatus, thus preventing, for example, a precoat film or the like formed on the inner wall or the like of process vessel of the process apparatus from being altered by the oxidative gas. 
     Further, it is preferable that, in the step of stabilizing the reaction by-product, a step of sealing the oxidative gas at a pressure higher than that needed at a time of evacuating the trap mechanism and a step of exhausting the sealed oxidative gas should be sequentially repeated several times. 
     As the oxidative gas is sealed inside the trap mechanism under a pressure higher than the pressure involved at the time of evacuating the trap mechanism, the reaction of the reaction by-product with the oxidative gas is accelerated. This ensures faster stabilization of the reaction by-product. 
     The reaction gas may be at least one of an ammonia gas, oxygen-containing gas, vapor, and an inert gas mixed with at least one of them. 
     Preferably, the process gas may be a high-melting point metal compound gas such as titanium-containing gas (e.g. TiCln), tungsten-containing gas (e.g. WFn), tantalum-containing gas (e.g. TaCln, TaBrn, organic Ta) and silicon-containing gas (e.g. SinH 2 , SiHnCl(2n+2)-SiClm) (n: a positive integer, and m+n=4). 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a view schematically illustrating an exhaust apparatus according to one embodiment of the present invention which is combined with a process apparatus; 
     FIG. 2 is a cross-sectional view of an exhaust pipe, showing the attachment of a reaction-gas injection nozzle; 
     FIG. 3 is a diagram depicting vapor pressure curves of TiCl 4 , TiCl 3  and NH 4 Cl; 
     FIGS. 4A to  4 C are cross-sectional views showing exhaust pipes provided with modified different injection nozzles, respectively; 
     FIG. 5 is a view schematically showing a trap mechanism to which a reaction-gas injection nozzle is attached; 
     FIG. 6 is a view schematically illustrating an exhaust apparatus according to another embodiment of this invention which is combined with a process apparatus; 
     FIG. 7 is a flow illustrating one example of an impurity-gas removing method of this invention; 
     FIG. 8 is a flow illustrating another example of the impurity-gas removing method of this invention; and 
     FIG. 9 is a diagram schematically illustrating an exhaust apparatus according to a further embodiment of this invention which is combined with a process apparatus. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of an exhaust apparatus for a process apparatus such as a film forming apparatus and an impurity-gas removing method according to the present invention will be described below with reference to the accompanying drawings. 
     FIG. 1 shows an exhaust apparatus for a process apparatus according to this invention. FIG. 2 is a cross-sectional view of an exhaust pipe, showing the attachment of a reaction-gas injection. 
     The following description of this embodiment will be given of the case where a Ti film is deposited on the surface of a semiconductor wafer (including the surface of a film or layer formed on the semiconductor wafer), which is an object to be processed by plasma CVD (Chemical Vapor Deposition) using a TiCl 4  gas (i.e., a high-melting point metal halogen compound gas) as a high-melting point metal compound gas. 
     As shown in FIG. 1, the film deposition system comprises mainly a process apparatus  2  which actually deposits a Ti film on a semiconductor wafer W and an exhaust apparatus  4  which vacuums inside the process apparatus  2 . 
     To begin with, the process apparatus  2  will be discussed. The process apparatus  2  has a cylindrical process vessel  6  of, for example, aluminum, which is grounded. Provided in the process vessel  6  is a susceptor  10  which is supported on the bottom by conductive support columns  8  and on which the semiconductor wafer W is to be placed. The susceptor  10  is made of a conductive material, such as Ni, and serves as a lower electrode, or a non-conductive material such as AlN. A resistance heating heater  12  for heating the semiconductor wafer W is embedded in the susceptor  10 . 
     A shower head  14  which feeds as a source or process gas, into the process vessel  6  is provided on the ceiling of the process vessel  6  though an insulator  16 . A common gas supply pipe  20  provided with a supply-side open-close valve  18  is connected to the shower heat  14 , and to gas sources (not shown) for necessary gases, such as a TiCl 4  gas, H 2  gas and Ar gas, via a flow controller and open-close valves. The gases can therefore be supplied into the process vessel  6  with their flow rates controlled. Because such a gas supply mechanism is generally well known, it is not particularly illustrated. Alternatively, gas supply pipes from the several gas sources may be directly connected to the shower head  14  to feed the gases several independently into the process vessel  6 . 
     The shower head  14 , which serves as an upper electrode, is connected to a high-frequency power supply  24  of, for example, 450 kHz via a matching circuit  22 , and so that the high frequency from the supply  24  generates the plasma of the process gas between the susceptor  10  and the shower head  14 . 
     A gate valve  26 , through which the loading and unloading of the wafer w are carried out, is provided in the side wall of the process vessel  6 . A plurality of exhaust ports  28  (one may be feasible) are provided in the peripheral portion of the bottom of the process vessel  6 . 
     The exhaust apparatus  4  that is connected to the deposition apparatus  2  of the above-described structure has an exhaust pipe  30  of stainless steels. The pipe  30  has an inner diameter of about 10 cm and is connected at one end to the exhaust ports  28 . A trap mechanism  32  for removing an impurity gas in the exhaust gas, a vacuum pump  33  which evacuates the process vessel  6 , and an eliminator  34  which completely eliminates the residual impurity gas in the exhaust gas are arranged in the order mentioned, in the exhaust pipe  30  toward the downstream. 
     Disposed at the uppermost stream side of the exhaust pipe  30  (considerably upstream of the trap mechanism  32 ) or in the vicinity of exhaust ports  28  are an exhaust valve  36  for opening and closing the exhaust passage of the exhaust pipe  30  and a pressure control valve  38  for controlling the pressure in the process apparatus or in the exhaust passage. A bypass pipe  40  with a bypass valve  42  disposed therein is connected to the gas supply pipe  20  to connect the gas supply pipe  20  with that portion of the exhaust pipe  30  which is located immediately downstream of the pressure control valve  38  (connection node P 1 ). The bypass pipe  40  can allow the source gas, which is fed at the time of stabilizing the gas flow rate, to be supplied directly to the exhaust pipe  30  without going through process vessel  6 . 
     A reaction-gas supply mechanism or means  44  is connected to that portion of the exhaust pipe  30  which is in the vicinity of the exhaust ports  28 , more specifically, directly downstream of the connection node P 1  of the bypass pipe  40  with respect to the exhaust pipe  30 . Specifically, the reaction-gas supply mechanism  44  comprises a reaction-gas injection nozzle  46  whose distal end is inserted into the exhaust pipe  30  through the peripheral wall thereof, a reaction gas pipe  48  connected to the inlet of the nozzle, and a reaction gas source  50  connected to the inlet of the reaction gas pipe  48 . Sequentially disposed in this reaction gas pipe  48  are a reaction gas valve  52  and a flow controller  54  which controls the flow rate of the reaction gas. NH 3  (ammonia) gas is used as the reaction gas that reacts with an impurity gas TiCl 4  gas in this example contained in the exhaust gas, to produce a compound whose vapor pressure becomes lower than that of the original gas. 
     The trap mechanism  32  has a housing  32 B having an inlet port  32 C in an upper portion of one side wall and an outlet port  32 D in a lower portion of the other side wall. Multiple fins  32 A, which have an ordinary temperature or are cooled by a coolant as in the exemplified case, are provided in the housing at predetermined intervals in the vertical direction. The outlet port of the pipe portion that is a part of the exhaust pipe  30  is connected to the inlet port of the housing whose outlet port is connected to the inlet port of the pipe portion that is the other portion of the exhaust pipe  30 . 
     Tape heaters  55 A,  55 B and  55 C are respectively wound around the gas supply pipe  20 , the bypass pipe  40  and that portion of the exhaust pipe  30  which is the upstream of the trap mechanism  32  as indicated by broken lines. The heaters  55 A,  55 B and  55 C heat the respective pipes at predetermined temperatures, thereby preventing the gas components in the pipes (passages) from being condensed and liquefied. 
     A description will now be given of how to deposit a film by the thus constituted process apparatus and how to remove an impurity gas by the exhaust apparatus. 
     At the time of depositing a Ti film, the semiconductor wafer W is placed on the susceptor  10  in the process vessel  6  and is heated to a predetermined temperature and maintained at that temperature. At the same time, a high-frequency voltage is applied between the susceptor  10  used as the lower electrode and the shower head  14  used as the upper electrode. Predetermined gases, such as TiCl 4  gas, H 2  gas and Ar gas, are fed from the shower head  14  while controlling the flow rate, thus producing the plasma in the processing space to deposit a Ti film onto the wafer W. At the same time, the exhaust apparatus  4  is driven to evacuate the process vessel  6  to keep the internal pressure at a predetermined pressure. 
     With the wafer size being 8 inches, for example, the process conditions are the process pressure of 665 Pa (about 5 Torr), the process temperature of about 650° C., the flow rate of the TiCl 4  gas of about 5 sccm, the flow rate of the H 2  gas of about 2000 sccm and the flow rate of the Ar gas of about 500 sccm. 
     To the deposition of the Ti film generally consumes approximately 10% of the TiCl 4  gas, and the remaining gas (about 90%) as a non-reaction gas and reaction by-products of TiCl 2 , TiCl 3  and HCl are fed into the exhaust pipe  30  from the exhaust ports  28  together with the exhaust gas by the vacuum pump  33 . The exhaust gas further flows down in the order of the trap mechanism  32 , the vacuum pump  33  and the eliminator  34 . In this case, the non-reacting TiCl4 gas, and the reaction by-products, have a relatively high vapor pressure, which are not generally possible to be sufficiently removed by the trap mechanism  32 . 
     In this respect, the NH 3  gas is supplied into the exhaust pipe  30  as a reaction gas from the reaction-gas injection nozzle  46  of the reaction-gas feeding mechanism  44  in this apparatus. The NH 3  gas reacts mainly with the TiCl 4  gas in the exhaust pipe  30  at the upstream of the trap mechanism  32 , forming the complex of TiCl 4 .2NH 3 . This complex has a significantly lower vapor pressure than the TiCl 4  gas, e.g., about 1×10− 4  Pa (not shown) at 21.3° C. in contrast to 1300 Pa for the TiCl 4  gas at 21.3° C. as shown in, for example, FIG.  3 . The HCl gas reacts with the NH 3  gas and becomes an NH 4 Cl gas which also has a low vapor pressure as illustrated. FIG. 3 shows the vapor pressure characteristic of TiCl 3 , too. 
     The non-reacted residual gas is reacted mainly with the NH 3  gas to be converted to a compound having a lower vapor pressure or HCl as a reaction by-product is reacted with the NH 3  gas to be converted to a compound having a lower vapor pressure. The residual gas easily and almost completely condenses, solidifies and traps the gas in the trap mechanism  32 . In this case, the internal temperature and pressure of the trap mechanism  32  only needs to be set to such conditions based on the vapor pressure curves that a compound, such as the complex or NH 4 Cl gas can be condensed and solidified. With an attention given to the complex, for example, if the temperature inside the trap mechanism  32  is 21.3° C., the pressure should be set equal to or higher than 1×10− 4  Pa. The fins  32 A should of course be kept at a low temperature by a coolant or the like and/or have their large contact surfaces, in order to increase the trapping yield. 
     Also, the other reaction by-products, such as TiCl 2 , TiCl 3  and NH 4 Cl, have lower vapor pressures than the TiCl 4  gas. Therefore, those by-products are trapped and removed by the trap mechanism under the aforementioned conditions. To remove the impurity gas reliably, it is preferable to feed the NH 3  gas of, for example, 10 sccm or greater which is twice the supply amount of the TiCl 4  gas of 5 sccm or greater. That is, to convert the entire TiCl 4  gas to the complex of TiCl 4 .2NH 3 , the amount of moles of NH 3  should be twice or more as great as the amount of moles of TiCl 4  as readily understood from the following equation. 
     
       
         TiCl 4 +2NH 3 →TiCl 4 .2NH 3   
       
     
     In other words, it is desirable that the flow rate of the reaction gas with respect to the non-reacted gas be set to such a value as to make the entire non-reacted gas react and become a complex with a low vapor pressure. 
     As the trapping yield of each impurity gas is improved, it becomes possible to reduce the size of the eliminator  34  located at the downstream of the trap mechanism  32 , to decrease the running cost, and to elongate the service life. It is also possible to prevent the impurity gas component from being condensed inside the vacuum pump  33  and stuck there. 
     The reaction-gas injection nozzle  46  is provided way up in the exhaust pipe  30  to make the distance between the nozzle  46  and the trap mechanism  32  longer. The reaction gas is therefore sufficiently diffused in the exhaust gas before the exhaust gas reaches the trap mechanism  32 . Thus, the reaction of the TiCl 4  gas can be accelerated. This can result in further improvement of the trapping yield. The portion of the exhaust pipe at the upstream of the trap mechanism  32  is heated by the tape heater  55 C, to a temperature higher than the condensing temperature of the complex of TiCl 4 .2NH 3  which has the highest vapor pressure in the compounds, e.g., about 170° C. Hence, the complex will not be condensed or clog the pipe. 
     Before the deposition process, the TiCl 4  gas or the like may be directly fed to the exhaust pipe  30  via the bypass pipe  40 , without flowing into the process vessel  6 , in order to stabilize the flow of the TiCl 4  gas or the like. In this case, the TiCl 4  gas can be reliably removed as mentioned earlier by flowing the NH 3  gas as the reaction gas into the exhaust pipe  30 . 
     According to this embodiment, the distal end  46 A of the reaction-gas injection nozzle  46  is positioned slightly inward of the side wall of the exhaust pipe  30  as shown in FIG.  2 . Nonetheless, for example, the distal end  46 A of the reaction-gas injection nozzle  46  may be positioned approximately at the cross-sectional center of the exhaust pipe  30  as shown in FIG. 4A, thereby accelerate the diffusion of the fed reaction gas. 
     This invention is not limited to the pipe-like nozzle shape but a ring-shaped annular pipe  56  may be connected to the distal end of the nozzle  46  with multiple gas injection holes  58  provided in the pipe  56  to supply the reaction gas as shown in FIG.  4 B. Further, straight pipes  60  may be provided in a cross shape to communicate with the annular pipe  56  with gas injection holes  58  provided also in the straight pipes  56  to supply the reaction gas as shown in FIG.  4 C. 
     The structures as shown in FIGS. 4B and 4C can further increase the diffusion of the supplied reaction gas, thereby further improving the trapping yield. 
     The reaction-gas injection nozzle  46  is provided at the relatively upstream side of the exhaust pipe  30  in the apparatus exemplified in FIG.  1 . This arrangement is not restrictive. The nozzle  46  may be provided in the exhaust pipe  30  anywhere as long as the location is upstream of the trap mechanism  32  or may be provided in the trap mechanism  32  itself as shown in FIG.  5 . In the case of providing the reaction-gas injection nozzle  46  in the trap mechanism  32 , it is preferable to provide the nozzle as close as possible to a gas inlet ports  32 C of the housing  32 B of the trap mechanism  32 , in order to increase the trapping efficiency. 
     Although the foregoing description of this embodiment has been given of the case where a Ti film is deposited by plasma CVD using the TiCl 4  gas, H 2  gas and Ar gas (for the plasma), this invention is in no way limited to this particular case. Rather, it may be adapted to the case where a TiN film is deposited by thermal CVD using, for example, the TiCl 4  gas and NH 3  gas. The latter case will be discussed later. Although the NH 3  gas is previously included as a source gas in this case, the TiCl 4  gas alone may flow in the bypass pipe without feeding the NH 3  gas to stabilize the flow rate of the TiCl 4  gas or an insufficient amount of the NH 3  gas in chemical equivalence may flow. In such a case, the residual TiCl 4  gas in the exhaust gas is reacted with the NH 3  gas fed into the exhaust pipe to be surely removed. 
     Although the foregoing description has been given of the case where the TiCl 4  gas is used as a high-melting point metal halogen compound gas, the same can be applied to the case where another compound gas, such as a WF 6 , is used. This invention may be adapted to the case where the WF 6  gas and NH 3  gas are used to form a WN film, the case where the WF 6  gas and SiH 4  gas are used to form a tungsten film, and to the case where the WF 6  gas and SiH 2 Cl 2  gas are used to form a WSi film. 
     This invention can also be adapted to the case where a (Ta(OE) 5 ) 2 , a high-melting point metal organic compound gas, is used as a high-melting point metal compound gas to form a Ta 2 O 5  film from (Ta(OE) 5 ) 2  and O 2 . 
     Although NH 3  gas is used as a reaction gas in this example, O 2 -containing gas (including pure oxygen) or H 2 O (water vapor) may be fed into the exhaust gas instead. In this case, the TiCl 4  gas reacts with the O 2 -containing gas or H 2 O to yield a TiO 2  compound. The TiO 2  compound has a considerably low vapor pressure and is easily condensed and solidified at the heating temperature of 170° C. for the exhaust pipe  30 . It is therefore desirable to directly feed the O 2 -containing gas or water vapor in the trap mechanism  32  in the case of supplying the O 2 -containing gas or water vapor into the exhaust gas. 
     A description will now be given of a method of stabilizing a reaction by-product which is produced at the time of depositing a TiN film. The method may be used at the time of stabilizing a reaction by-product which is produced at the time of depositing the aforementioned Ti film as will be discussed later. 
     FIG. 6 is a structural view illustrating an exhaust apparatus for a process apparatus according to another embodiment of this invention. 
     The following description of this embodiment will be given of the case where a TiN film is deposited on a semiconductor wafer that is an object to be processed by CVD using a TiCl 4  gas (i.e., high-melting point metal halogen compound gases) as high-melting point metal compound gases and NH 3  gas. 
     As shown in FIG. 6, this film deposition system mainly comprises a process apparatus  102  which actually deposits a Ti film on a semiconductor wafer W and an exhaust apparatus  104  which evacuates the inside of the process apparatus  102 . 
     To begin with, the process apparatus  102  will be discussed. This process apparatus  102  has a cylindrical process container  106  of, for example, aluminum. Provided in the process vessel  106  is a susceptor  110  which is supported on the bottom of the vessel by support columns  108  and on which the semiconductor wafer W is to be placed an held. The susceptor  110  is made of ceramics, such as aluminum nitride, and a resistance heating  112  for heating the semiconductor wafer W is embedded in the susceptor  110 . 
     A shower head  114  which feeds a necessary gas, such as a source or process gas, into the process vessel  106  is provided on the ceiling portion of the process vessel  106 . A gas supply pipe  118  which has a supply-side valve  116  disposed therein is connected to the shower head  114  so that necessary gases, such as TiCl 4  gas, NH 3  gas and H 2  gas, or ClF 3  gas used as a cleaning gas can be supplied into the process vessel  106  with their flow rates controlled. The individual gases may be supplied from independent supply pipes. Note that the N 2  gas can be used as a carrier gas for the TiCi 4  gas or can be fed alone as needed. 
     A gate valve  120  through which the loading and unloading of the wafer W are carried out is provided in the side wall of the process vessel  106  and exhaust ports  122  and a bypass exhaust port  124  are provided in the lower portions of the process vessel  106 . 
     The exhaust apparatus  104  that is connected to the process apparatus  102  with the above-described structure has an exhaust pipe  130  of stainless with an inner diameter of about 10 cm, which has one end connected to the exhaust ports  122 . A trap mechanism  132  for removing an impurity gas in the exhaust gas, a vacuum pump  134  which evacuates the process vessel  106  and an eliminator  136  which completely eliminates the residual impurity gas in the exhaust gas are disposed in order in the exhaust pipe  130  in the downstream direction. 
     Disposed at the topmost stream side of the exhaust pipe  130  are a pressure control valve  138  which controls the pressure in process apparatus or in the exhaust passage of the exhaust pipe  130  and a first exhaust valve  140  for opening and closing the exhaust pipe  130 . A bypass pipe  133  containing a bypass valve  131  is connected between the exhaust pipe  130  directly downstream of the first exhaust valve  140  and the gas supply pipe  118 , in order to permit the gas to bypass the process vessel  106  as needed. 
     Valves  142  and  144  which are closed at the time of sealing the trap mechanism  132  airtight are provided directly upstream and downstream of the trap mechanism  132 . An upstream flange joint  146  and downstream flange joint  148 , which are joined at the time of connecting the trap mechanism  132  to the exhaust pipe  130 , are provided directly upstream and downstream of those valves  142  and  144 . 
     Second exhaust valves  150 A,  150 B are provided between the nozzle  162  and the upstream flange joint  146 , and the downstream flange joint  148  and the vacuum pump  134  respectively. An exhaust bypass pipe  152  having a bypass valve  154  disposed therein is provided to communicate the bypass exhaust port  124  of the process container  106  with the portion  130 A or the exhaust pipe  130  directly downstream of the second exhaust valve  150 B. The inner diameter of the exhaust bypass pipe  152  is, for example, 20 mm, significantly smaller than the inner diameter of the exhaust pipe  130  which carries out main exhaust. The process container  106  can be therefore evacuated with a large inverse diffusion coefficient as will be discussed later. 
     Oxidative-gas feeding means  160  is connected to that portion of the exhaust pipe  130  which is located directly downstream of the first exhaust valve  140 . Specifically, this oxidative-gas supply means  160  comprises a gas injection nozzle  162  whose distal end is inserted into the exhaust pipe  130  through the peripheral wall thereof, an oxidative gas pipe  164  connected to the nozzle  162 , and an oxidative gas source  166 . The gas injection nozzle  162  and the exhaust pipe  130  may be those illustrated in FIGS. 4A,  4 B and  4 C may be used. An oxidative gas valve  168  and a flow controller  179 , which controls the flow rate of the oxidative gas are disposed in order in the oxidative gas pipe  164 . Any gas which oxidizes and stabilizes a reaction by-product can be used as the oxidative gas. Although the O 2  gas is used in this embodiment, another gas, such as O 3  (ozone), a gas containing dry-air O 2  or H 2 O (water vapor), can be used as well. The nozzle  162  may be provided in the trap mechanism  132  so as to directly feed the oxidative gas into the trap mechanism  132 . 
     Multiple fins  174  which have an ordinary temperature or are cooled by a coolant as in the illustrated case are provided inside a housing  172  of the trap mechanism  132 . A reaction by-product or the like sticks on those fins  174  and is trapped there. A tape heater  176  is wound around the portion of the exhaust pipe  130  which is located upstream of the trap mechanism  132 , as indicated by the broken lines, to heat the pipe  130  to a predetermined temperature. This prevents the gas components from being condensed and liquefied inside the pipe. Likewise, a tape heater  180  for preventing the liquefaction of the source gas is put around the gas supply pipe  118  and the bypass pipe  133 . 
     Referring to embodiment of FIGS. 7 and 8, a description will now be given of how to deposit a film by the thus constituted process apparatus and how to remove an impurity gas by the exhaust apparatus. 
     At the time of depositing a TiN film, the semiconductor wafer W is placed on the susceptor  110  in the process container  106  and is heated to a predetermined temperature and maintained at that temperature. At the same time, predetermined process gases, such as the TiCl 4  gas, NH 3  gas and N 2  gas are fed from the shower head  114  under the flow-rate control and are reacted with the impurity gas in the processing space, thereby depositing a TiN film. At the same time, the exhaust apparatus  104  is driven to vacuum the interior of the process vessel  106  to keep the internal pressure at a predetermined pressure. 
     For example, in the case of the wafer size being 8 inches, for example, the process conditions here are the process pressure of 39.9 Pa (about 0.3 Torr), the process temperature of about 680° C., the flow rate of the TiCl 4  gas of about 30 sccm, the flow rate of the NH 3  gas of about 400 sccm and the flow rate of the N 2  gas of about 340 sccm. 
     The deposition-reaction of the TiN film causes NH 4 Cl, TiCl 4 (NH 3 ) n  gas and the like to flow as reaction by-products together with the exhaust gas into the exhaust pipe  130  from the exhaust ports  122 . Therefore, the exhaust gas further flows down in the order of the trap mechanism  132 , the vacuum pump  134  and the eliminator  136 . Further, the non-reacted TiCl 4  gas also flows down together with the exhaust gas. Most of the non-reacted gas and reaction by-product gas are cooled to be solidified by the trap mechanism  132  and removed by this trap mechanism  132 . 
     The impurity gas that cannot be removed by the trap mechanism  132  flows downstream to be almost surely removed by the eliminator  136 , making the exhaust gas harmless. 
     When film deposition on a certain number of wafers is completed, undesirable films, which are to produce particles, are deposited on the inner wall of the process vessel  106  and the surface or the like of the susceptor  110 . Cleaning is preferably executed regularly irregularly to remove the undesirable films. In this case, for example, a ClF 3  gas is used as the cleaning gas, the susceptor  110  is heated and kept at, for example, 250° C. and the process container  106  is evacuated while this gas is kept supplied into the process container  106  from the shower head  114  in order to increase the cleaning efficiency. At this time, the ClF 3  gas that flows inside the trap mechanism  132  reacts with TiCl 4 (NH 3 ) n  that is trapped there, thus producing TiF 4 (NH 3 ) as a reaction by-product. 
     As the film deposition and the cleaning process are repeated by the proper number of times this way, reaction by-products are gradually accumulated in the trap mechanism  132  as mentioned earlier. It is therefore necessary to detach the trap mechanism  132  from the exhaust system and clean out the reaction by-products. 
     If the trap mechanism  132  is exposed to the air or atmosphere without doing the necessary processing, the reaction by-product contacts the air, causing the reactions given in the equations (1) and (2). Impurity gases harmful to human bodies are undesirably generated from the reaction by-products. According to this embodiment, therefore, the O 2  gas, for example, is made to flow in the trap mechanism  132  as the oxidative gas to cause the reactions given in the formulas (1) and (2) before the trap mechanism  132  is detached from the exhaust system. The trap mechanism  132  is detached from the exhaust system when the reaction by-products are stabilized. One example of the stabilization of the reaction by-products will be discussed specifically referring to FIG.  7 . 
     When the cleaning process following the deposition process is completed, the supply of the deposition gas and cleaning gas is stopped and evacuation continued, while maintaining the rotation speed of the vacuum pump  134  at an appreciated value (S 1 ). The temperature of the susceptor  110  may be lowered to room temperature. It takes much time to lower the temperature, which would reduce the throughput. To avoid a throughput reduction, N 2  gas is fed into the process vessel  106  at a predetermined rate (S 3 ), while keeping the temperature of the susceptor  110  at the process temperature or while first lowering the temperature to an idling temperature higher than the room temperature and then maintaining it at the idling temperature (S 2 ). The N 2  gas is made to flow this way, in order to expel the unnecessary metal component or gas component emanating from the wall of the process vessel or the susceptor  110  at high temperature. 
     Next, the first exhaust valve  140  of the exhaust pipe  130  is closed, blocking the exhaust pipe  130 , and the bypass valve  154  of the exhaust bypass pipe  152  is opened, opening the exhaust bypass pipe  152  (S 4 ). As a result, the vacuum pump  134  forces the N 2  gas out of the process vessel  106  via the exhaust bypass pipe  152  whose inside diameter is smaller than that of the exhaust pipe  130 . 
     With this state maintained, the oxidative gas valve  168  on the oxidative gas pipe  164  is opened, feeding the oxidative gas or the O 2  gas to the exhaust pipe  130  at a predetermined flow rate (S 5 ). As the O 2  gas further flows down in the trap mechanism  132  by vacuum, it contacts the reaction by-product trapped in the trap mechanism  132 , causing reactions of the formulas (1) and (2) and stabilizing the by-products. That is, TiCl 4 (NH 3 ) n  and TiF 4 (NH 3 ) n  react with O 2  (the formulas (1) and (2)), generating TiO 2 , HCl, HF and NH 3  gases TiO 2  is a stable compound and fixed inside the trap mechanism  132 , whereas HCl, HF and NH 3  flow down in a gaseous form and are made harmless in the eliminator  136 . At this time, the pressure in the trap mechanism  132  is about 665 Pa (about 5 Torr). The reaction by-product is stabilized over a predetermined and sufficient time, e.g., for several hours (S 6 ). Then, the oxidative gas valve  168  is closed to stop feeding the O 2  gas to the trap mechanism  132  (S 7 ). 
     Next, the second exhaust valves  150 A,  150 B at the near the trap mechanism  132  are closed (S 8 ), and both valves  142  and  144  at the upstream of the trap mechanism  132  are closed, sealing the trap mechanism  132  airtight and isolate it (S 9 ). 
     Then, both flange joints  146  and  148 , which connect the trap mechanism  132  to the exhaust system, are unfastened. The trap mechanism  132  is detached from the exhaust pipe  130  (S 10 ). In this state, N 2  gas is supplied into the process vessel  106  and evacuation is carried out via the exhaust bypass pipe  152  as mentioned earlier. Then, the trap mechanism  132  is released at a predetermined location, removing TiO 2  or the like out of the trap mechanism  132  (S 11 ). At this time, the reaction by-product has been oxidized, forming TiO 2  or the like. TiO 2  or the like is stable substance. Thus, impurity gas harmful to human bodies is hardly generated when the trap mechanism  132  is released. 
     As described above, the oxidative gas is supplied into the trap mechanism  132  to oxidize and stabilize the trapped reaction by-product before the trap mechanism  132  is detached from the exhaust pipe  130  in this embodiment. It is therefore possible to safely clean the inside of the trap mechanism  132  detached. 
     If the oxidative gas flows reversely and enters the process vessel  106 , it will affect the precoat film stuck on the wall or the like of the process vessel  106 . In embodiment, however, the process vessel  106  is evacuated at a large inverse diffusion coefficient via the exhaust bypass pipe  152  while the oxidative gas is being supplied. This can prevent the counterflow of the oxidative gas into the process container  106 . The trap mechanism  132  can therefore be removed and cleaned, with the susceptor  110  heated. The throughput is thereby improved. 
     The inverse diffusion coefficient of the exhaust bypass pipe  152  will be discussed. 
     It is empirically proved that if the Peclet number Pe in the exhaust bypass pipe  152  is 10 or more during the aforementioned processing, the oxidative gas is hardly diffused reversely and hardly flows reversely into the process vessel  106 . The Peclet number Pe is a dimensionless value called “inverse diffusion coefficient.” It is given by the following equation. 
     
       
         
           Pe=Vs·Ls/D 
         
       
     
     where Vs is the flow velocity of the gas in the exhaust bypass pipe  152 , Ls is the length of the exhaust bypass pipe  152  and D is a diffusion constant (mutual diffusion). In this embodiment, the length and the inside diameter of the exhaust bypass pipe  152  may be respectively about 2.5 m and about 20 mm. If so, Pe is approximately 170, much greater than “10,” which would reliably inhibit the counterflow of the oxidative gas  106  into the process container  106 . 
     Although stabilization of the reaction by-product is performed by feeding O 2  gas for a predetermined time, e.g., for several hours, under the pressure of, for example, 665 Pa, this case is not restrictive. Also, as shown in FIG. 8, the step of temporarily trapping the oxidative gas in the trap mechanism under pressure, and the step of discharging the gas may be repeatedly performed. The steps S 21  to S 26  illustrated in FIG. 8 replace the steps FIG.  7 . 
     When the step shown in S 4  in FIG. 7 is completed, i.e., when the first exhaust valve  140  is closed and the bypass valve  154  is opened, the second exhaust valve  150  is closed and the oxidative gas valve  168  is opened, supplying the O 2  gas into the trap mechanism  132 . The O 2  conditions are, for example: the outside temperature of 25° C., the supply pressure of 0.1 Mpa, the flow rate of O 2  gas of 251/m and the temperature of O 2  of 5° C. to 30° C. 
     Note that the downstream valve  144  may be closed, instead of closing the second exhaust valve  150 B. 
     The O 2  gas is enclosed in the trap mechanism  132  until the pressure in the mechanism  132  changes to a predetermined pressure, e.g., the atmospheric pressure (S 22 ). When the pressure in the trap mechanism  132  changes to the atmospheric pressure, the oxidative gas valve  168  is closed to stop feeding the O 2  gas and isolate the trap mechanism  132  (S 23 ). The upstream valve  142  may be closed, instead of closing the oxidative gas valve  168 . 
     As the O 2  gas is kept locked in the trap mechanism  132  for a predetermined time under a pressure higher than the pressure needed for evacuation or under the atmospheric pressure, the oxidation stabilization of the reaction by-product in the trap mechanism  132  is accelerated more than that in the case where the O 2  gas is simply fed. 
     When trapping for a predetermined time, e.g., about 20 to 40 minutes, is completed (YES in S 24 ), the second exhaust valves  150 A,  150 B are opened to discharge the impurity gas component produced from the trap mechanism  132  over a predetermined time (about 20 minutes) (S 25 ). At this time, the oxidative gas valve  168  may be opened to let the O 2  gas flow to accelerate the discharging of the impurity gas. The steps of S 21  to S 25  are repeatedly performed until a sequence of steps from A 1  in the flow up to the present point is repeated a predetermined number of times (NO in S 26 ). In each repeated sequence of steps, the O 2  gas may be included des in the trap mechanism  132  for the same time or a different time. For example, the time may be set longer as the number of repetitions increases. When the sequence of the steps is repeated a predetermined number of times, the flow goes to step S 8  in FIG.  7 . 
     The oxidative gas is locked in the trap mechanism  132  under a pressure higher than the one needed for evacuating the trap mechanism or and is discharged from the mechanism  132  several times. The reaction by-product can therefore be stabilized quickly. The O 2  gas remains locked in the trap mechanism  132  for a predetermined time (FIG.  8 ). Nonetheless, this invention is not limited to this particular case. Rather, the O 2  gas may be discharged immediately without waiting for a predetermined time. 
     The reaction by-product is stabilized with the susceptor  110  heated and the trap mechanism is cleaned, in order to improve the throughput in this embodiment. This mode is not restrictive, nevertheless. The aforementioned sequence of operations may be performed after the susceptor  110  is completely cooled down to the room temperature. 
     The trap mechanism  132  can be detached from the exhaust system and cleaned, after the reaction by-product is stabilized. This invention is not limited to this mode. The normal film deposition may be effected immediately, without detaching the trap mechanism  132  after the reaction by-product is stabilized. In this case, the stabilization of the reaction by-product with the supplied O 2  gas can reduce the volume of the contents in the trap mechanism  132 . This increases the detachment and cleaning cycle of the trap mechanism  132 , thus elongating the life of the trap mechanism  132 . 
     A TiN film is deposited in the embodiment described above. This technique may be applied to form a Ti film accommodated by using TiCl 4  gas and H 2  gas. In this case, surface nitriding process is particularly, performed with the supplied NH 3  gas after deposition of the Ti film, so that the same chemical reaction as described above may occur. 
     While description has been made of the case where the TiCl 4  gas is used as a high-melting point metal halogen compound gas, this invention can be applied to the case where another compound gas, such as WF 6 , is used. This invention may be adapted to the case of forming a WN film by using the WF 6  gas and NH 3  gas, the case of forming a tungsten film by using the WF 6  gas and SiH 4  gas, and the case of forming a WSi film by using the WF 6  gas and SiH 2 Cl 2  gas. 
     If a (Ta(OE) 5 ) 2  (pentoethoxy tantalum) or a high-melting point metal organic compound gas is used as a high-melting point metal compound gas, the invention can also be adapted to the case of forming a Ta 2 O 5  film from (Ta(OE) 5 ) 2  and O 2 . 
     As shown in FIG. 9, the oxidative-gas supply means  160 , which comprises the oxidative gas source  166 , oxidative gas pipe  164  and gas injection nozzle  162 , may be connected to the exhaust pipe  30  of the exhaust apparatus  4  or the trap mechanism  32 , both shown in FIG.  1 . (Although the oxidative-gas feeding means  160  is connected to the exhaust pipe  30  in the illustrated example, the gas injection nozzle  162  may be provided directly on the trap mechanism  32  in the manner shown in FIG. 5.) In this case, it is also preferred that the exhaust bypass pipe  152  having the bypass valve  154  disposed therein be connected to the bypass exhaust port  124  formed in the process vessel  6  and a portion directly upstream of the vacuum pump  33 . The description of the opening/closing valves is omitted. This structure can oxidize and stabilize the reaction by-product in the trap mechanism  32 , which has been produced by reaction with the NH 3  gas supplied from the reaction-gas supply means  44 . 
     Although description of the individual embodiments has been made of the case where a semiconductor wafer is used as an object to be processed, the invention is not limited to this case. It may be adapted to the case where the object to be processed is a glass substrate, LCD substrate or the like. 
     As described above, the exhaust apparatus for a process apparatus and the impurity-gas removing method according to this invention have the following advantages. 
     According to one aspect of the invention, as the reaction gas from the reaction-gas supply means is fed into the trap mechanism or the exhaust pipe at the upstream of the trap mechanism, the reaction gas reacts with the impurity gas in the exhaust gas and becomes a reaction by-product whose vapor pressure is lower than that of the reaction gas. It is therefore possible to easily condense and solidification the impurity gas in the trap mechanism and trap the gas there. 
     According to another aspect of the invention, mixed diffusion of the reaction gas is accelerated while the exhaust gas reaches the trap mechanism. This accelerates the reaction accordingly, making it possible to more surely trap and eliminate an impurity gas such as a high-melting point metal compound gas. 
     According to a different aspect of the invention, it is possible to oxidize and stabilize an unstable reaction by-product by feeding an oxidative gas to the exhaust system before the trap mechanism is detached from the exhaust system. This makes it possible to remove the trap mechanism from the exhaust system and clean the inside of the trap mechanism while safely keeping the trap mechanism open. 
     According to an yet further aspect of the invention, it is possible to oxidize and stabilize an unstable reaction by-product by feeding an oxidative gas to the exhaust system before the trap mechanism is detached from the exhaust system. This makes it possible to remove the trap mechanism from the exhaust system and clean the inside of the trap mechanism while safely keeping the trap mechanism open. 
     According to a still further aspect of the invention, it is possible to prevent the reverse diffusion of the oxidative gas to the process apparatus. A precoat film or the like formed on the inner wall or the like of process container of the process apparatus, for example, can therefore be prevented from being altered by the oxidative gas. 
     According to an yet still further aspect of the invention, because the oxidative gas is locked inside the trap mechanism under a pressure higher than the pressure involved at the time of vacuuming the trap mechanism, the reaction of the reaction by-product with the oxidative gas is accelerated, thus ensuring faster stabilization of the reaction by-product. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.