Patent Publication Number: US-2005120955-A1

Title: Film forming apparatus

Description:
This application is a Continuation Application of PCT International Application No. PCT/JP03/08800 filed on Jul. 10, 2003, which designated the United States.  
     FIELD OF THE INVENTION  
      The present invention relates to a semiconductor manufacturing apparatus; and, more particularly, to a semiconductor manufacturing apparatus capable of enhancing a film forming rate during a film forming process employing a low vapor pressure source material.  
     BACKGROUND OF THE INVENTION  
      With the recent increase in the size of semiconductor substrates, the semiconductor manufacturing apparatus tends to perform a single substrate processing, rather than a batch processing used to simultaneously treat a plurality of semiconductor substrates. In order to improve the processing efficiency or throughput of the apparatus which performs the single substrate processing, the processing time per substrate need be shortened. Accordingly, attempts have been made to increase the flow rate of a source gas supplied to a processing vessel of the semiconductor manufacturing apparatus, to thereby increase the film forming rate (deposition rate) and reduce the processing time.  
      Further, in case of such a single substrate processing apparatus, the flow rate of the source gas need be stabilized, before the source gas is supplied to the processing vessel of the semiconductor manufacturing apparatus. Therefore, as shown in  FIG. 5 , a source supply line  30 ′ which supplies a source gas to a processing vessel  120 ′ of a conventional semiconductor manufacturing apparatus is normally provided with a pre-flow line  33 ′ which bypasses the processing vessel  120 ′. In such a semiconductor manufacturing apparatus, the source gas, before being introduced into the processing vessel  120 ′, is fed to the pre-flow line  33 ′ by switching a valve  26 ′; and then, after stabilizing the flow rate thereof, the source gas is supplied to the processing vessel  120 ′ by another switching operation of the valve  26 ′.  
      In order to gasify a solid or a liquid source material and supply the source gas to the semiconductor manufacturing apparatus at room temperature, the liquid or the solid source material is typically heated or, alternatively, the liquid source material itself or the solid source material dissolved in a solvent is supplied to a vaporizer, and then the source material vaporized at the vaporizer is provided as the source gas into the processing vessel.  
      However, in case of a film forming process for the formation of high-k dielectric films or ferroelectric films, e.g., Ru films or W films, recently employed in semiconductor devices, the heating of the source material may not produce a source gas in a sufficient quantity due to its low vapor pressure. In such a case, the source gas is supplied to the processing vessel  120 ′ with the aid of a carrier gas. In order to increase the flow rate of the source gas when employing such a low vapor pressure source material, it may be required to increase the vapor pressure by heating the source material at a higher temperature and facilitate the vaporization of the source material by way of depressurizing the source vessel. As illustrated in  FIG. 5 , therefore, a turbo molecular pump (TMP)  14 ′ and a dry pump (DP)  16 ′ are provided at a gas exhaust line  32 ′ of the conventional semiconductor manufacturing apparatus to depressurize the source vessel  10 ′ and the processing vessel  120 ′.  
      However, as described above, even in case the source vessel  10 ′ and the like are depressurized by using the turbo molecular pump  14 ′ and the like, the capacity to increase the flow rate of the source gas is still restricted in case of using a low vapor pressure source material in addition to the small inner diameter, e.g., ¼ inch, of the piping generally used in the art. Moreover, due to the small piping diameter, the pressure losses at the source supply line  30 ′ may hinder an efficient depressurization of the source vessel  10 ′ and, consequently, an efficient vaporization of the source material.  
      Furthermore, in the prior art equipment, since the pre-flow line  33 ′ bypasses the turbo molecular pump  14 ′ as shown in  FIG. 5  and the piping diameter of the pre-flow line  33 ′ is generally smaller than or equal to that of the source supply line  30 ′, the pressure in the source vessel  10 ′ while the source gas is flowing through the pre-flow line  33 ′ may be different from the pressure in the source vessel  10 ′ when the film forming process is performed. Thus, even in case the source gas is made to flow through the pre-flow line  33 ′ before the film forming process to stabilize the flow rate thereof, there still remains a problem that the flow rate thereof is not actually stabilized.  
     SUMMARY OF THE INVENTION  
      It is, therefore, a primary object of the present invention to provide a film forming apparatus capable of substantially improving the film forming rate by increasing the flow rate of a source gas supplied to a processing vessel of a semiconductor manufacturing device.  
      It is another object of the present invention to provide a film forming apparatus including a pre-flow line capable of substantially stabilizing the flow rate of a source gas before conducting a film forming process.  
      In accordance with a first aspect of the present invention, there is provided a film forming apparatus including: a source vessel for accommodating a source material used to generate a source gas; a film forming chamber wherein a film forming process is performed on a semiconductor substrate; a source supply channel for supplying the source gas from the source vessel to the film forming chamber; and a gas exhaust channel, having a vacuum pump system, for exhausting the film forming chamber, wherein the source supply channel includes a piping with an inner diameter of greater than 6.4 mm.  
      In accordance with a second aspect of the present invention, there is provided a film forming apparatus including: a source vessel for accommodating a source material used to generate a source gas; a film forming chamber wherein a film forming process is performed on a semiconductor substrate; a source supply channel for supplying the source gas from the source vessel to the film forming chamber; a gas exhaust channel, having a vacuum pump system comprised of a turbo molecular pump and a dry pump, for exhausting the film forming chamber; and a pre-flow channel branching off from the source supply channel and joining to the gas exhaust channel, wherein a second turbo molecular pump is provided at the pre-flow channel.  
      In the second aspect of the present invention, alternatively, the pre-flow channel may be made to join the gas exhaust channel at an upstream of the turbo molecular pump. In this case, since the vacuum pump system of the gas exhaust channel can be used while the pre-flow channel is activated, it is possible to reduce the difference between the pressure in the source vessel during the activation of the pre-flow channel and the pressure in the source vessel during the film forming process, without having to provide the second turbo molecular pump at the pre-flow channel.  
      In accordance with a third aspect of the present invention, there is provided a film forming apparatus including: a source vessel for accommodating a source material used to generate a source gas; a film forming chamber wherein a film forming process is performed on a semiconductor substrate; a source supply channel for supplying the source gas from the source vessel to the film forming chamber; a gas exhaust channel, having a vacuum pump system comprised of a turbo molecular pump and a dry pump, for exhausting the film forming chamber; and a pre-flow channel branching off from the source supply channel and joining to the gas exhaust channel, wherein the piping diameter of the pre-flow channel is enlarged to reduce a pressure difference between the pressure in the source vessel during the activation of the pre-flow line and the pressure in the source vessel during the film forming process.  
      In each of the afore-mentioned aspects of the present invention, valves disposed at the pre-flow channel and/or the source supply channel preferably have a conductance Cv which is greater than or equal to 1.5. Especially, each of the valves disposed at the pre-flow channel and the source supply channel preferably has a conductance Cv greater than or equal to 1.5. Further, the source supply channel preferably includes a piping having an inner diameter of greater than 6.4 mm over a range of, at least, 80% of the entire length thereof. The source supply channel is designed to maintain the difference between the pressure in the source vessel and that in the film forming chamber during the film forming process, to be smaller than 2000 Pa. The source supply channel preferably includes a piping having an inner diameter of greater than or equal to about 16 mm. A source gas, generated from a source material having a vapor pressure lower than 133 Pa at a vaporization temperature, may flow through the source supply channel. An exemplary source material thereof is W(CO) 6 . The pressure in the film forming chamber is preferably maintained to be lower than 665 Pa during the film forming process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The objects, features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments with reference to the accompanying drawings, wherein:  
       FIG. 1  shows schematically the configuration of a CVD film forming unit  100 ;  
       FIG. 2  depicts schematically the configuration of a source supply unit  200  in accordance with a first preferred embodiment of the present invention;  
       FIGS. 3A and 3B  provide schematically the configurations of a source supply unit  200  in accordance with a second preferred embodiment of the present invention;  
       FIG. 4  presents a table for comparing the differences between the pressure in a processing vessel and that in a source vessel, while varying a piping diameter; and  
       FIG. 5  represents schematically the configuration of a conventional semiconductor manufacturing apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The preferred embodiments of the present invention will now be described with reference to the accompanying drawings.  
      [A First Preferred Embodiment] 
       FIG. 1  is a sectional view showing schematically the configuration of a CVD film forming unit  100  in accordance with a first preferred embodiment of the present invention.  
      As shown in  FIG. 1 , the CVD film forming unit  100  includes a processing vessel  120  of an airtight structure; a mounting table  130 , disposed at a central portion of the processing vessel  120 , for supporting a semiconductor substrate  101  and burying therein a heating device  132  connected to a power supply; a shower head  110 , so disposed as to face the mounting table  130 , for introducing a gas, which is supplied from a source supply line  30  (to be described later), into the processing vessel  120 ; a gate valve (not shown), disposed at a sidewall of the processing vessel  120 , for loading/unloading the semiconductor substrate  101  into/from the processing vessel  120 ; and a gas exhaust line  32 , having a vacuum pump system, for exhausting the processing vessel  120 .  
       FIG. 2  illustrates the configuration of a source supply unit  200  in accordance with the first preferred embodiment of the present invention.  
      Referring to  FIG. 2 , a carrier gas comprised of a inert gas such as Ar, Kr, N 2  and He is supplied to the source vessel  10  via a mass flow controller (MFC)  12 . The mass flow controller  12  controls the flow rate of the carrier gas supplied to the source vessel  10 . The source vessel  10  accommodates therein a liquid source material or solid source material which is to be used for the film forming process. A source gas is generated by vaporizing the source material by a bubbling process or the like in the source vessel  10  and then transferred to the CVD film forming unit  100  by the carrier gas through the source supply line  30 . Further, near an outlet of the source vessel  10  of the source supply line  30  is provided a pressure gauge  18  for detecting the pressure in the source vessel  10 .  
      Provided at the source supply line  30  is a pre-flow line  33  which bypasses the CVD film forming unit  100  in the downstream of the source vessel  10 . A carrier gas containing the source gas (hereinafter, referred to as “mixed gas”) is supplied from the source supply line  30  to the pre-flow line  33 . The mixed gas is selectively supplied to the pre-flow line  33  or the source supply line  30  which passes through the CVD film forming unit  100 , by opening or closing valves  28  and  27 .  
      Moreover, the pre-flow line  33  serves as a gas flow path for stabilizing the flow rate of the mixed gas supplied to the CVD film forming unit  100  during the film forming process. For such purpose, the mixed gas is supplied to the pre-flow line  33  before a single substrate processing is executed one by one on the semiconductor substrate  101 .  
      Gas lines are connected via valves to the source supply line  30  which extends from a junction node B, at which the pre-flow line  33  diverges, to the CVD film forming unit  100 . The gas lines supply various gases to be required during the film forming process and a cleaning gas for cleaning the processing vessel  120  after executing the film forming process and the like. These gases may be introduced into the processing vessel  120  while the mixed gas is flowing through the pre-flow line  33  (i.e., while the valve  28  is open and the valve  27  is closed).  
      A turbo molecular pump (TMP)  14  is provided at a gas exhaust line  32  for evacuating the reaction gas and the like from the CVD film forming unit  100 . A dry pump (DP)  16  is provided at a downstream of the turbo molecular pump  14 . These pumps  14  and  16  maintain the internal pressure of the processing vessel  120  under a certain vacuum level. The turbo molecular pump  14 , together with the dry pump  16 , is able to set the pressure in the processing vessel  120  to be at a high vacuum level, i.e., smaller than or equal to, e.g., 1 Torr (133 Pa). Thus, the turbo molecular pump  14  and the dry pump  16  are especially needed in case a low vapor pressure source material, such as dimethylaluminumhydride (DMAH), biscyclopentadienyllutenium (RuCp 2 ), hexacarbonyltungsten (W(CO) 6 ) or the like, is used for the film forming process.  
      The pre-flow line  33  joins to the gas exhaust line  32  at an upstream of the dry pump  16 . Therefore, while the mixed gas is flowing through the pre-flow line  33 , the source vessel  10  is depressurized by the dry pump  16 . During the film forming process, however, the source vessel  10  is depressurized by the dry pump  16  and the turbo molecular pump  14 .  
      Meanwhile, in order to improve the film forming rate, there is a need to increase the flow rate of the source gas, which is contained in the mixed gas and supplied to the CVD film forming unit  100 . The flow rate of the source gas can be increased by increasing the flow rate of the carrier gas and the temperature of the source vessel  10 . On the other hand, the flow rate of the source gas decreases, as the pressure in the source vessel  10  increases. Accordingly, in order to increase the flow rate of the source gas, the pressure in the source vessel  10  is required to be as low as possible.  
      As described above, the source vessel  10  is depressurized by the turbo molecular pump  14  and the like via the processing vessel  120  and the source supply line  30 . However, in order to achieve a high efficiency of the depressurization and, at the same time, to increase the flow rate of the source gas, pressure losses should be reduced as much as possible at the flow path which extends from the turbo molecular pump  14  to the source vessel  10 .  
      Meanwhile, since the flow rate of the source gas is in proportion to that of the carrier gas, it is possible to increase the flow rate of the carrier gas in order to increase that of the source gas. However, if the source supply line  30  has a piping diameter of ¼ inch which is generally employed in the art, the conductance of the source supply line  30  becomes so small that the capacity to increase the flow rate of the carrier gas (and that of the source gas) by the above-mentioned depressurization is limited.  
      High-k dielectric films or ferroelectric films, e.g., Ru films or W films, recently employed in semiconductor devices, is formed by employing low vapor pressure source materials. For example, W(CO) 6  that can be used for forming a W film has a vapor pressure of 3.99 Pa (0.03 Torr) at 25 C; 6.65 Pa (0.05 Torr) at 30 C; and 33.25 Pa (0.25 Torr) at 45 C. However, in case such a low vapor pressure source material is used, it is very difficult to increase the flow rate of the source gas.  
      Accordingly, in the first embodiment of the present invention, the source supply line  30  is set to have a piping diameter greater than ¼ inch (about 6.4 mm), e.g., {fraction (1/2 )} inch (about 13 mm) or ¾ inch (about 19 mm), in order to increase the flow rate of the carrier gas (and that of the source gas accompanied by the carrier gas). The source supply line  30  having the piping diameter greater than ¼ inch preferably covers from the source vessel  10  to the processing vessel  120 . In other words, the source supply line  30 , through which the source gas flows, is preferably comprised of a piping having an inner diameter which is constant until it reaches the processing vessel  120 .  
      However, if the length from the source vessel  10  to the processing vessel  120  is short, the source supply line  30  can be comprised of a piping having different inner diameters. For instance, referring to  FIG. 2 , a piping having an inner diameter of ½ inch may be used within a short range from an outlet of the source vessel  10  while a piping having an inner diameter of ¾ inch is used for a range covering the major parts from the source vessel  10  to the processing vessel  120 .  
      Further, from such a point of view as described above, valves  25  and  27  that may be disposed at the source supply line  30  preferably have diameters equal to the inner diameter of the source supply line  30 . However, like as the valve  25  illustrated in  FIG. 2 , the valves  25  and  27  may have the inner diameters of ⅜ inch which is used extensively in case the source supply line  30  has the inner diameter of ½ inch. Furthermore, the entire length of the source supply line  30  may be set as short as possible in order to reduce the energy loss of the mixed gas, to thereby increase the flow rate thereof. For example, the source supply line  30  illustrated in  FIG. 2  is comprised of a piping having the inner diameter of ¾ inch, which has an entire length of 1000 mm, except a portion of piping having the inner diameter of ½ inch.  
      Although the source supply unit  200  in accordance with the first embodiment has one source supply line  30 , a plurality of source supply lines may be provided in case multiple types of source gases are employed. In such case, each of the source supply lines for transferring the low vapor pressure source material may be comprised of a piping having an inner diameter of greater than ¼ inch; however, each of the source supply lines for transferring a relatively high vapor pressure source material may be generally comprised of a piping having an inner diameter of ¼ inch.  
      In accordance with the first embodiment of the present invention, the flow rate of a fluid flowing through a piping increases in proportion to the fourth power of the inner diameter of the piping, so that the flow rate of the source gas introduced into the processing vessel  120  can be drastically increased. Further, since the pressure loss of the mixed gas at the source supply line  30  is reduced as the piping diameter of the source supply line  30  increases, the workload of the turbo molecular pump  14 , which functions to reduce the pressure in the source vessel  10 , can be lowered. Furthermore, in case the pressure loss at the source supply line  30  is small, the flow rate of the source gas introduced into the processing vessel  120  can be further increased.  
      In case the film forming process is performed by using a low vapor pressure source material, such as W(CO) 6 , the pressure in the source vessel  10  may be preferably maintained at a high vacuum level of, e.g., smaller than or equal to 2 Torr (266 Pa) by using the turbo molecular pump  14  in order to increase the flow rate of the source gas.  
      However, while the pre-flow line  33  is activated, it may not be possible to maintain the pressure in the source vessel  10  at such a low pressure level using the dry pump  16  alone. Therefore, even in case the mixed gas flows through the pre-flow line before conducting the film forming process, the pressure in the source vessel  10  may vary by performing a switching of a flow path for the film forming process, resulting in a fluctuation in the flow rate of the source gas during the film forming process.  
      As will be described next, a source supply unit  200  provided in accordance with a second preferred embodiment of the present invention to be described in the following, however, solves the aforementioned drawbacks by improving or modifying the pre-flow line  33  of the source supply unit  200  in accordance with the first embodiment of the present invention.  
      [A Second Preferred Embodiment] 
       FIG. 3A  illustrates the configuration of the source supply unit  200  in accordance with the second preferred embodiment of the present invention. Referring to  FIG. 3A , a second turbo molecular pump  15  is disposed at the pre-flow line  33  of the source supply unit  200  in accordance with the second embodiment of the present invention. Thus, while the mixed gas is flowing through the pre-flow line  33  (while the pre-flow line  33  is activated), the source vessel  10  is depressurized by the dry pump  16  and the second turbo molecular pump  15 . During the film forming process, on the other hand, the source vessel  10  is depressurized by the dry pump  16  and the turbo molecular pump  14 .  
      As a result, the difference between the pressure in the source vessel  10  while the mixed gas is flowing through the pre-flow line  33  and that in the source vessel  10  when the film forming process is performed is reduced. In other words, the pressure in the source vessel  10  can be maintained at a high vacuum level of, e.g., smaller than or equal to 2 Torr (266 Pa) during the film forming process using a low vapor pressure source material, such as W(CO) 6 , and at the same time, the high vacuum level in the source vessel  10  can also be achieved by the second turbo molecular pump  15  while activating the pre-flow line  33 . Accordingly, the pressure variation in the source vessel  10 , which causes a fluctuation in the flow rate of the source gas, can be suppressed, so that the film forming process can be stably performed without the fluctuation in the flow rate of the source gas.  
      Moreover, from such a point of view as described above, the piping diameter of the pre-flow line  33  may be preferably selected to be equal to or greater than that of the source supply line  30  in order to reduce the difference between the pressure in the source vessel  10  when the film forming process is performed and that in the source vessel  10  while the pre-flow line is activated. By adjusting the disposed position of the second turbo molecular pump  15  at the pre-flow line  33 , the pressure in the source vessel  10  while the mixed gas is flowing through the pre-flow line  33  may be made approximately equal to that in the source vessel  10  when the film-forming process is performed. Accordingly, the flow rate of the source gas while the pre-flow line  33  is activated can be approximately equal to that of the source gas when the film forming process is performed.  
      In accordance with the second embodiment of the present invention, it is possible to substantially reduce or eliminate the difference between the flow rate of the source gas while the pre-flow line  33  is activated and that of the source gas introduced into the processing vessel  120 . Therefore, an amount of the fluctuation in the flow rate of the source gas can be kept very small while a flow path is switched from the pre-flow line  33  to the source supply line  30  by a three-way valve  26 , so that the film forming process can be stably performed.  
       FIG. 3B  provides a modified version of the source supply unit  200  provided in accordance with the second embodiment of the present invention. In the configuration depicted in  FIG. 3B , the second turbo molecular pump  15  is not provided at the pre-flow line  33 . Instead, the pre-flow line  33  joins to the gas exhaust line  32  at an upstream of the turbo molecular pump  14 . In such configuration, in case the pre-flow line  33  is activated, the source vessel  10  is depressurized by the dry pump  16  and the turbo molecular pump  14 , like the case of the film forming process.  
      Therefore, in accordance with the modified embodiment, it is possible to substantially reduce the difference between the flow rate of the source gas while the pre-flow line  33  is activated and that of the source gas introduced into the processing vessel  120 . Accordingly, the fluctuation in the flow rate of the source gas is very small while switching a flow path from the pre-flow line  33  to the source supply line  30 , so that the film forming process can be stably performed without the fluctuation in the flow rate of the source gas during the film forming process.  
      Further, in this modified embodiment, in order to minimize the fluctuation in the flow rate of the source gas while a flow path is switched by the three-way valve  26 , the electric power to the turbo molecular pump  14  disposed at the gas exhaust line  32  may be adjusted and controlled. Furthermore, the piping diameter of the pre-flow line  33  may be equal to or greater than that of the source supply line  30  so as to reduce the difference between the pressure in the source vessel  10  when the film forming process is performed and that in the source vessel  10  while the pre-flow line is activated.  
      Moreover, in the second embodiment of the present invention, the valves  28  and  27  provided in the first embodiment may be employed instead of the three-way valve  26 . In addition, both in the first and the second embodiments, each of the valves  25 ,  26  and  27  provided at the source supply line  30  and the pre-flow line  33  (i.e., each valve provided at a flow path extending from the source vessel  10  to the turbo molecular pump) preferably has a conductance Cv of greater than or equal to 1.5. Accordingly, the pressure loss in each valve is reduced so that the aforementioned effects can be further enhanced.  
      Herein, the Cv of a valve is defined to be a value calculated based on the equation of Cv=Qg/406×{Gg(273+t)/(P 1 −P 2 )P 2 } 1/2  which applies in case the absolute pressure P 1 [kgf·cm 3 abs] on a first side (i.e., the side near the source vessel  10 ) is smaller than twice the absolute pressure P 2 [kgf·cm 3 abs] on a second side (i.e., the side near the processing vessel  120 ), i.e., P 1 &lt;2P 2 , and that of Cv=Qg/203 P 1 ×{Gg(273+t)} 1/2  which applies in case P 1  is greater than or equal to 2P 2 , i.e., P 1 ≧2P 2 . Further, in the above-described equations, t[° C.], Qg[Nm 2 /h] and Gg indicate a gas temperature, the flow rate of a gas in the standard state (15° C., 760 mmHgabs) and the specific gravity of a gas in case that of air being set to be 1, respectively.  
     [EXAMPLE 1] 
      The results shown in  FIG. 4  represent the differences between the pressure in the processing vessel  120  and that in the source vessel  10  obtained as a function of the piping diameter in accordance with the first preferred embodiment.  
      As shown in  FIG. 4 , in case a piping having the inner diameter of ¾ inch was employed for the source supply line  30  and the pressure in the processing vessel  120  was set to be 13.3 Pa (0.1 Torr), the pressure in the source vessel  10  was depressurized to 79.8 Pa (0.6 Torr).  
      Therefore, it can be seen that even in case a low vapor pressure source material, such as W(CO) 6  with a vapor pressure of 3.99 Pa (0.03 Torr) at 25° C. and that of 33.25 Pa (0.25 Torr) at 45° C. is employed, the pressure in the processing vessel  120  can be sufficiently depressurized, so that a source gas of a sufficient flow rate can be obtained.  
      In the meantime, in case a piping having the inner diameter of ¼ inch was employed and the pressure in the processing vessel  120  was set to be 66.6 Pa (0.5 Torr), the pressure in the source vessel  10  was measured to be 2660 Pa (20 Torr). In comparison, in case a piping having the inner diameter of ¾ inch was employed and the pressure in the processing vessel  120  was set to be 66.6 Pa (0.5 Torr), the pressure in the source vessel  10  was 372 Pa (2.8 Torr).  
      Moreover, in case a piping having the inner diameter of ½ inch was employed and the pressure in the processing vessel  120  was set to be 133 Pa (1 Torr), the pressure in the source vessel  10  ranged from 1051 to 1596 Pa (7.9 to 12 Torr).  
      From the above test results, it can be seen that in case the source supply line  30  has the inner diameter of ¼ inch, the difference between the pressure in the processing vessel  120  and that in the source vessel  10  is, at least, greater than or equal to 1995 Pa (15 Torr), whereas in case the source supply line  30  has the inner diameter of ½ inch or ¾ inch, the difference is, at most, smaller than or equal to 1995 Pa (15 Torr), so that the pressure loss at the source supply line  30  is reduced.  
      Hereinafter, an exemplary film forming process, that was carried out for the purpose of comparing the film forming rate while varying the piping diameters, will be described.  
      First of all, as a comparative example, a W film was formed from W(CO) 6  utilizing the source supply line  30  comprised of a piping having an inner diameter of ¼ inch and a length of 2 m, by using the thermal CVD method. The temperature of the source vessel  10  was set to be 45° C., and the flow rate of the carrier gas was set to be 300 sccm (1 sccm represents the flow rate of a fluid with the volume of 1 cm 3  at 0° C. and 1 atm). Further, the film forming process was carried out at the pressure (i.e., the pressure in the processing vessel  120 ) of 20.0 Pa (0.15 Torr) and at the substrate temperature of 450° C. As a result, the tungsten film was formed at the film forming speed of 10 Å/min, and the resistivity of the tungsten film obtained was 54 μΩcm.  
      In contrast, in case a piping having the inner diameter of ½ inch and the length of 2 m was employed for the source supply line  30 , the tungsten film was formed at the film forming speed of 40 Å/min, and the resistivity of the tungsten film was 40 μΩcm.  
      Further, in case a piping having the inner diameter of ¾ inch and the length of 1 m was employed for the source supply line  30 , the tungsten film was formed at the film forming speed of 300 Å/min, and the resistivity of the tungsten film was 45 μΩcm.  
      From the above working examples, it was confirmed that in case a piping having the inner diameter of, e.g., greater than or equal to ½ inch is employed for the source supply line  30  extending from the source vessel  10  to the processing vessel  120 , the flow rate of the source gas substantially increases and the film forming rate is greatly improved.  
     [EXAMPLE 2] 
      This Example was performed for the purpose of comparing the second embodiment of the present invention, described above, with the prior art illustrated in  FIG. 5 .  
      In this Example, pressure variances in the source vessel  10 , which caused fluctuations in the flow rate of the source gas, were compared.  
      At first, using the conventional system shown in  FIG. 5  as a comparative example, the mixed gas was made to flow through the pre-flow line  33 ′ before conducting the film forming process and then the pressure in the source vessel  10 ′ was measured by the pressure gauge  18 ′. Thereafter, by switching a flow path using the valve  26 ′, the mixed gas was provided into the source supply line  30 ′ which was connected to the processing vessel  120 ′, and then the pressure in the source vessel  10 ′ was measured by the pressure gauge  18 ′.  
      When the pre-flow line  33 ′ was activated, the pressure in the source vessel  10 ′ was 3990 Pa (30 Torr). However, when the source gas was introduced into the processing vessel  120 ′, the pressure in the source vessel  10 ′ was 1330 Pa (10 Torr), showing a considerable pressure difference therebetween. From these measurements, it was confirmed that, in the conventional system, the flow rate of the source gas fluctuates greatly during the film forming process.  
      On the other hand, when the pre-flow line  33  of the present invention, as shown in  FIG. 3A , was used, it was possible to maintain the pressure in the source vessel  10  at 1330 Pa (10 Torr) both during the activation of the pre-flow line  33  and during the introduction of the source gas into the processing vessel  120 . Accordingly, in accordance with the second embodiment of the present invention, it was possible to conduct the film forming process with a constant level of the source gas, i.e., without a fluctuation in the flow rate of the source gas.  
      As demonstrated above, in accordance with each embodiment of the present invention, due to the increase in the conductance of the source supply channel, it is possible to substantially increase the flow rate of the source gas being introduced into the film forming chamber. Further, since the pressure loss at the source supply channel (i.e., the difference between the pressure in the source vessel and that in the film forming chamber during the film forming process) is reduced due to the use of a larger diameter piping, it is possible to efficiently reduce the pressure in the source vessel during the film forming process. Furthermore, the reduction of the pressure loss at the source supply channel contributes to an increase in the amount of the source gas produced from the source material in the film forming chamber. As a result, the film forming rate is considerably improved, thereby greatly increasing the throughput.  
      In addition, by providing an additional turbo molecular pump at the pre-flow channel, it is possible to greatly reduce the difference between the pressure in the source vessel while the pre-flow channel is activated and that in the source vessel when the film forming process is performed. Accordingly, the flow rate of the source gas is further stabilized during the film forming process, thereby achieving a high-quality film forming process.  
      Moreover, since the pressure in the source vessel is efficiently reduced during the film forming process, even if a low vapor pressure source material is employed, it is possible to obtain a sufficient flow rate of the source gas.  
      While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.