Abstract:
Disclosed is a method of forming a thin film on a substrate surface by a CVD method, including the steps of arranging a substrate such that one main surface of the substrate is exposed to a closed space, and decomposing by heating a raw material gas filling the closed space so as to form a thin film containing at least one element constituting the raw material gas on the main surface of the substrate, the raw material gas containing a gas component generated by heating a material, which is solid or liquid at room temperature, arranged within the closed space.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method and apparatus for forming a thin film by a CVD method and a method of manufacturing a semiconductor device comprising the step of forming a thin film by a CVD method. 
     With progress in the degree of integration of a DRAM, it is of high importance to decrease the thickness of a capacitor insulating film. In the case of using the conventional material for forming a capacitor insulating film, the capacitor insulating film must be formed very thin in order to obtain a sufficient electrostatic capacitance. Therefore, it is proposed in recent years to use a material having a high dielectric constant such as BaSrTi oxide (hereinafter referred to as “BSTO”) for forming a capacitor insulating film. 
     In forming a BSTO thin film on a semiconductor substrate by a CVD method, it is ideal to use as a Ba source and a Sr source materials having a high vapor pressure and gaseous at room temperature. However, such a Ba source and a Sr source are unknown. Therefore, materials solid at room temperature and sublimated to form a gaseous phase when heated to a predetermined temperature are used as the Ba source and Sr source, said materials including, for example, bis(2,2,6,6-tetramethyl-3,5-heptanedionate)barium (hereinafter referred to as “Ba(THD) 2 ”), and bis(2,2,6,6-tetramethyl-3,5-heptanedionate)strontium (hereinafter referred to as “Sr(THD) 2 ”). Also, bis(2,2,6,6-tetramethyl-3,5-heptanedionate)titanium oxide (hereinafter referred to as “TiO(THD) 2 ”) was used as a Ti source. Incidentally, TiO(THD) 2  is solid at room temperature and sublimated when heated. 
     FIG. 1 schematically shows a conventional CVD apparatus used for forming a BSTO thin film. In the conventional method, a solid Ba(THD) 2    16  housed in a container  7  is heated by a heater 25 to 215° C. so as to be sublimated to generate a Ba(THD) 2  gas. A nitrogen gas is supplied as a carrier gas from a nitrogen gas supply source  14  into the container  7 . The flow rate of the nitrogen gas into the container  7  is controlled by a mass flow controller  22 . It follows that the Ba(THD) 2  gas and the nitrogen gas within the container  7  are supplied together into a chamber  1  through a valve  6 . 
     A Sr(THD) 2  gas and a TiO(THD) 2  gas are also supplied similarly into the chamber  1 . To be more specific, a solid Sr(THD) 2    17  housed in a container  20  and a solid TiO(THD) 2    18  housed in a container  19  are heated to 215° C. and 130° C., respectively, so as to generate a Sr(THD) 2  gas and a TiO(THD) 2  gas. Also, a nitrogen gas is supplied from a nitrogen gas source  14  into the containers  20  and  19  through mass flow controllers  21 ,  23 , respectively. Naturally, the Sr(THD) 2  gas and the TiO(THD) 2  gas are supplied together with the nitrogen gas from the containers  20  and  19  into the chamber  1  through valves. Further, an oxygen gas is supplied from an oxygen gas supply source  15  into the chamber  1  through a mass flow controller  24 . 
     The Ba(THD) 2  gas, Sr(THD) 2  gas, TiO(THD) 2  gas, oxygen gas and nitrogen gas are mixed within the chamber  1  to form a gas stream  5  within the chamber  1 . The gas pressure within the chamber  1  is monitored by a pressure gauge  13  and controlled at about 10 Torr by a conductance valve  12  for pressure control. 
     A wafer  2  and a susceptor  8  within the chamber  1  are heated to about 600° C. by the light emitted from a lamp  3  and transmitted through the quartz wall of the chamber  1 . As a result, the mixed gas within the chamber  1  is partially decomposed, and the decomposed materials carry out reactions. The reaction product is deposited on the wafer  2  to form a BSTO thin film. 
     A stagnant layer  4  through which a gas does not flow is formed in the vicinity of the surface of the wafer  2  during formation of the BSTO thin film. The mixed gas forming the gas stream  5  is partly supplied into the stagnant layer  4 . The mixed gas supplied into the stagnant layer  4  is diffused within the stagnant layer  4  so as to reach the wafer surface. As a result, the raw material gas components contained in the mixed gas are decomposed so as to bring about deposition of BSTO. 
     As described above, the stagnant layer  4  contributes to the deposition of BSTO. Therefore, in order to form a BSTO thin film of a uniform thickness, it is necessary to control highly accurately the thickness, etc. of the stagnant layer  4  and, thus, to make the deposition rate uniform. 
     However, the thickness of the stagnant layer  4  tends to be affected by the gas stream  5 . Also, it is very difficult to keep the gas stream  5  constant and uniform, leading to a non-uniform supply of the raw material gas components onto the wafer surface and to non-uniform deposition rate. It follows that it is difficult to form a BSTO thin film of a uniform thickness. 
     It should also be noted that the amount of the raw material gas components supplied from the gas stream  5  into the stagnant layer  4  is dependent in general on the partial pressure of the raw material gas components contained in the mixed gas forming the gas stream  5 . Under the conditions described above, the amount of the raw material gas components supplied into the stagnant layer  4  is only several percent of the raw material gas components contained in the mixed gas forming the gas stream  5 . In other words, the amount of the raw material gas components which are decomposed and consumed for the formation of the BSTO thin film is only several percent of all the raw material gas components supplied to the chamber  1 . Naturally, a major portion of the raw material gas components supplied to the chamber  1  is not decomposed so as to be discharged to the outside of the apparatus through a main valve  9 , a conductance valve  12  for the pressure control, a pipe  11  and a pump  10 , leading to a markedly high manufacturing cost of a semiconductor device including a BSTO thin film. 
     It should also be noted that the Ba(THD) 2  gas, Sr(THD) 2  gas and TiO(THD) 2  gas used in the conventional method are prepared by sublimation of the solid raw materials  16  to  18 . This makes it necessary to heat the pipe, valve, etc. connected to the chamber  1  so as to prevent the Ba(THD) 2  gas, etc. from being solidified. However, the valve, etc. used in the CVD apparatus tends to bring about deterioration of the driving section when the valve is exposed to high temperatures. It follows that the valve, etc. must be renewed frequently. 
     In order to suppress the deterioration, it is necessary to set the heating temperature of the pipe, valve, etc. at a low level. If the heating temperature is lowered, however, the flow rate of the raw material gas components must be maintained at a low level in order to prevent the raw material gas components from being solidified. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method and apparatus for forming a thin film and a method of manufacturing a semiconductor device wherein a film of a uniform thickness can be formed. 
     Another object is to provide a method and apparatus for forming a thin film and a method of manufacturing a semiconductor device wherein a thin film can be formed at a low cost. 
     Still another object of the present invention is to provide a method and apparatus for forming a thin film and a method of manufacturing a semiconductor device wherein a thin film can be formed under higher temperature conditions without renewing frequently the constituent members of the apparatus. 
     According to an aspect of the present invention, there is provided a method of forming a thin film on a substrate surface by a CVD method, comprising the steps of arranging a substrate such that one main surface of the substrate is exposed to a closed space, and decomposing by heating a raw material gas filling the closed space so as to form a thin film containing at least one element constituting the raw material gas on the main surface of the substrate, the raw material gas containing a gas component generated by heating a material, which is solid or liquid at room temperature, arranged within the closed space. 
     According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising the step of forming a thin film on a semiconductor substrate by a CVD method, the step including the sub-steps of arranging a semiconductor substrate such that one main surface of the substrate is exposed to a closed space, and decomposing by heating a raw material gas filling the closed space so as to form a thin film containing at least one element constituting the raw material gas on the main surface of the semiconductor substrate, the raw material gas containing a gas component generated by heating a material, which is solid or liquid at room temperature, arranged within the closed space. 
     Further, according to still another aspect of the present invention, there is provided an apparatus for forming a thin film on a substrate surface by a CVD method, comprising a substrate holding means for holding a substrate, a reaction vessel forming a closed space to which one main surface of the substrate is exposed, a material holding means arranged inside the reaction vessel for holding a solid or liquid material, and a heating means mounted on the side of the other main surface of the substrate for heating the substrate. 
     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 t e invention. 
     FIG. 1 schematically shows a conventional CVD apparatus; 
     FIG. 2 schematically shows a thin film forming apparatus according to first and third embodiments of the present invention; and 
     FIGS. 3A and 3B schematically show thin film forming apparatuses according to second and fourth embodiments, respectively, of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Let us describe the present invention more in detail with reference to the accompanying drawings. 
     Specifically, FIG. 2 schematically shows a thin film forming apparatus according to first and third embodiments of the present invention. 
     In the first embodiment of the present invention, the apparatus shown in FIG. 2 is used for film formation in which a part of the raw material gas components is generated from a solid material. As shown in the drawing, the apparatus includes a reaction furnace  31 . A front chamber  39  is connected to the reaction furnace  31  via a gate valve  38 . A substrate  32  such as a semiconductor wafer is supplied from the front chamber  39  into the reaction furnace  31 . 
     Arranged within the reaction furnace  31  are a susceptor  36 , a heater  37  and a reaction vessel  33 . The wafer  32  transferred from the front chamber  39  into the reaction furnace  31  is supported by the susceptor  36 , and the wafer  32  is heated by the heater  37  via the susceptor  36 . The reaction vessel  33  is movable relative to the susceptor  36 . Where, for example, the wafer  32  is disposed on the susceptor  36 , the reaction vessel  33  is moved to a position away from the susceptor  36 . On the other hand, the reaction vessel  33  is moved for the film forming operation such that the peripheral portion of the reaction vessel  33  is brought into contact with the susceptor  36  so as to form a closed space  51 . 
     A container  50  acting as a material holding means for holding a material generating a reactant gas, i.e., raw gas component is arranged inside the reaction vessel  33 . The material holding means is not particularly limited as far as the material holding means is capable of holding the material generating the reactant gas and is shaped to permit the generated reactant gas to be diffused within the closed space  51 . The reactant gas generating material used in this embodiment is solid at room temperature and generates a gas when heated to a predetermined temperature. It follows that a plate-like body can be used in this embodiment as the material holding means. 
     An oxygen gas supply source  52  and a nitrogen gas supply source  53  are connected to the reaction furnace  31 . A mass flow controller  34  and a valve  35  are mounted to the pipe connecting the oxygen gas supply source  52  to the reaction furnace  31 . Likewise, a mass flow controller  46  and a valve  45  are mounted to the pipe connecting the nitrogen gas supply source  53  to the reaction furnace  31 . 
     A pipe  41  is connected at one end to the reaction furnace  31  and at the other end to a pump  40 . Further, a pressure gauge  44 , a valve  43  and a conductance valve  42  are mounted to the pipe  44  in the order mentioned when viewed from the reaction furnace  31 . 
     The apparatus shown in FIG. 2 is used for forming, for example, a BSTO thin film. In forming a BSTO thin film, the pressure within the front chamber  39  housing the semiconductor wafer  32  is reduced to 10 −2  Torr or less. Then, the gate valve  38  is opened to transfer the wafer  32  from the front chamber  39  into the reaction furnace  31  having the inner pressure reduced in advance to 10 −2  Torr or less. The wafer  32  transferred into the reaction furnace  31  is disposed on the susceptor  36 . In this step, the reaction vessel  33  is positioned away from the susceptor  36 , and the temperature within the reaction furnace  31  is set at 300° C. or less. Also, the pump  40  is driven, and the valve  43  is opened so as to reduce in advance the inner pressure of the reaction furnace  31  to 10 −2  Torr or less, as described above. 
     Then, the gate valve  38  is closed, and the valve  35  is opened to introduce an oxygen gas into the reaction furnace  31 . In this step, the discharge power is controlled by the conductance valve  42  on the basis of the indication of the pressure gauge  44  so as to control the pressure within the reaction furnace  31  at 10 Torr. 
     Further, the reaction vessel  33  is moved to bring the periphery of the reaction vessel  33  into contact with the susceptor  36  so as to form the closed space  51 . Under this condition, a nitrogen gas, which is an inert gas, is introduced into the reaction furnace  31  in place of the oxygen gas. To be more specific, the flow rate of the oxygen gas is lowered by using the mass flow controller  34  and, at the same time, the valve  45  is opened so as to supply a nitrogen gas from the nitrogen gas supply source  53  into the reaction furnace  31 . The pressure within the reaction furnace  31  is maintained at 10 Torr by increasing the flow rate of the nitrogen gas by using the mass flow controller  46  in accordance with decrease in the oxygen gas flow rate. By the particular operation, the oxygen gas in the space within the reaction furnace  31  and outside the reaction chamber  33  is replaced by the nitrogen gas. On the other hand, the closed space  51  remains to be filled with the oxygen gas. 
     In the next step, the susceptor  36  is heated by the heater  37  to elevate the wafer temperature to about 600° C. In this step, solid materials  47  to  49  consisting of, for example, TiO(THD) 2 , Ba(THD) 2  and Sr(THD) 2 , respectively, which act as raw material gas sources, are arranged within the container  50 . When the wafer  32  is heated, these solid materials  47  to  49  are heated to 300° C. or more by, for example, the heat radiation from the wafer  32 , with the result that a TiO(THD) 2  gas, a Ba(THD) 2  gas and a Sr(THD) 2  gas are generated from these solid materials  47 ,  48  and  49 , respectively. 
     These TiO(THD) 2  gas, Ba(THD) 2  gas and Sr(THD) 2  gas scarcely form a gas stream and are diffused uniformly within the closed space  51 . It follows that a region producing a function equal to that produced by the stagnant layer formed in the conventional method is formed over the entire region of the closed space  51 . The TiO(THD) 2  gas, Ba(THD) 2  gas, Sr(THD) 2  gas and oxygen gas within the closed space  51  are uniformly mixed and, then, decomposed on the wafer  32  so as to deposit BSTO on the wafer  32 . 
     The wafer  32  was kept heated for one minute, followed by lowering the wafer temperature. At the same time, the reaction vessel  33  was moved away from the susceptor  36  so as to purge the gas within the closed space  51 . 
     The BSTO thin film thus formed was found to have a uniform thickness of ±2%. In the first embodiment of the present invention, the raw material gas components do not form a gas stream and are diffused uniformly within the closed space  51 , leading to the high uniformity in the thickness of the resultant BSTO thin film. In other words, if the raw material gas stream is non-uniform, the deposition rate is rendered nonuniform, resulting in failure to form a thin film of a uniform thickness. 
     It should also be noted that, in the first embodiment of the present invention, the raw material gas components are supplied to the stagnant layer at a high efficiency, compared with the conventional method. As a result, such a high film forming rate as about 200 Å/min was obtained in the first embodiment of the present invention. 
     Also, in the first embodiment of the present invention, all the raw material gas components were utilized for forming a stagnant layer, making it possible to form a BSTO thin film at a low cost, compared with the conventional method. 
     Also, in the first embodiment of the present invention, the solid materials  47  to  49  providing the sources of the raw material gas components are sublimated within the closed space  51 . In other words, the sublimation is carried out within the stagnant layer, making it unnecessary to heat the pipe, valve, etc. unlike the case where the evaporation is carried out at a position remote from a position where the stagnant layer is formed. Naturally, the valve, etc. are prevented from being deteriorated. 
     In the embodiment described above, a BSTO thin film was formed under the conditions of 10 Torr and 600° C. However, the BSTO thin film formation can be carried out under the pressure of 1 mTorr to 200 Torr and the temperature of 300° C. to 700° C. Also, the thin film formed by the method of the present invention is not limited to a BSTO film. Specifically, the method described above is applicable to formation of a thin film using a material which is solid in the vicinity of room temperature as a raw material gas source. 
     Let us describe a second embodiment of the present invention with reference to FIGS. 3A and 3B. A thin film forming apparatus differing from that used in the first embodiment is used in the second embodiment. 
     As shown in FIG. 3A, the thin film forming apparatus shown in FIG. 3A comprises a reaction chamber  55 . A waiting chamber  56  is positioned adjacent to the reaction chamber  55  with a shutter  57  interposed therebetween. If the shutter  57  is opened, the reaction chamber  55  is allowed to communicate with the waiting chamber  56 , as shown in FIG. 3B. A front chamber  39  is connected to the waiting chamber  56  via a gate valve  38 . In the apparatus shown in FIG. 3A, a substrate  32  such as a semiconductor wafer is transferred from the front chamber  39  into the waiting chamber  56  and, then, into the reaction chamber  55 . 
     A reaction vessel  33  supported by a shaft  58  is arranged within the waiting chamber  56 . In the second embodiment, the inner diameter of the reaction vessel  33  is smaller than the diameter of the wafer  32  such that a closed space  51  is defined by the reaction vessel  33  and the wafer  32 . Also, the reaction vessel  33  can be moved into the reaction chamber  55  by opening the shutter  57  and moving the shaft  58  upward, as shown in FIG.  3 B. 
     An oxygen gas supply source  52  is connected to the waiting chamber  56 . A mass flow controller  34  and a valve  35  are mounted to the pipe connecting the oxygen gas supply source  52  to the reaction chamber  31 . Also, a pipe  41 - 1  is connected at one end to the waiting chamber  56  and to a pump  40 - 1  at the other end. Further, a pressure gauge  44 - 1 , a valve  43 - 1  and a conductance valve  42 - 1  are mounted to the pipe  41 - 1  in the order mentioned as viewed from the waiting chamber  56 . 
     A plurality of containers  50  are mounted within the reaction vessel  33  as holding means for holding the reactant gas generating material. The holding means is not particularly limited as far as the holding means is shaped to hold the reactant gas generating material and to diffuse the generated gas into the reaction vessel  33 . The reactant gas generating material used in this embodiment is not particularly limited as far as the material is solid at room temperature and generates a gas when heated to temperatures higher than a predetermined temperature. Naturally, the reactant gas generating means used in this embodiment may be in the form of a plate. 
     Each of the containers  50  is mounted to a shaft  61 . It is possible for the shaft  61  to be capable of moving, for example, each of these containers  50  in a vertical direction. 
     A nitrogen gas supply source  53  is connected to the reaction chamber  55 . A mass flow controller  46  and a valve  45  are mounted to the pipe connecting the nitrogen gas supply source  53  to the reaction chamber  55 . Also, a pipe  41 - 2  is connected at one end to the reaction chamber  55  and to a pump  40 - 2  at the other end. Further, a pressure gauge  44 - 2 , a valve  43 - 2  and a conductance valve  42 - 2  are mounted to the pipe  41 - 2  in the order mentioned as viewed from the reaction chamber  55 . It should be noted that a ceiling plate  60  of the reaction chamber  55  is formed of a material having a relatively high heat conductivity, with the result that the heat generated from a heater  37  can be conducted into the reaction chamber  55 . 
     Let us describe how to form, for example, a BSTO thin film by the film forming method using the apparatus shown in FIG.  3 A. In the first step, the front chamber  39  housing the semiconductor wafer  32  is evacuated by a pump (not shown) to a vacuum of 10 −2  Torr or less. In this step, the pump  40 - 1  is driven and the valve  43 - 1  is opened so as to maintain the pressure within the waiting chamber  56  at a level equal to the inner pressure of the front chamber  39 . Then, the gate valve  38  is opened so as to transfer the wafer  32  from the front chamber  39  into the waiting chamber  56 . After transfer of the wafer  32  into the waiting chamber  56 , the gate valve  38  is closed while maintaining the wafer  32  at a position remote from the reaction chamber  33  by a mechanism (not shown). 
     In the next step, the valve  35  is opened so as to introduce an oxygen gas into the waiting chamber  56 . In this step, the pressure within the waiting chamber  56  is adjusted at 10 Torr by operating the conductance valve  42 - 1  based on the indication of the pressure gauge  44 - 1  so as to control the exhausting power. Then, the wafer  32  is disposed on the reaction vessel  33  so as to form the closed chamber  51 . It should be noted that solid materials  47  to  49  of TiO(THD) 2 , Ba(THD) 2  and Sr(THD) 2  are housed in advance in the containers  50  arranged within the reaction vessel  33 . 
     Then, a nitrogen gas is supplied into the reaction chamber  55  at a flow rate of 1 slm, and the pressure within the reaction chamber  55  is set at 10 Torr by operating the conductance valve  42 - 2  based on the indication of the pressure gauge  44 - 2  so as to control the exhausting power. After the valves  35  and  43 - 1  are closed, the shutter  57  is opened as shown in FIG.  3 B. As a result, a gas in the reaction chamber  55  and the waiting chamber  56  is substituted by a nitrogen gas. It should be noted that the closed space  51  is left filled with an oxygen gas in this step. 
     In the next step, the reaction vessel  33  is moved upward by the shaft  58  so as to permit the wafer  32  to approach the ceiling plate  60 . The ceiling plate  60  is heated in advance to 900° C. by the heater  37 . Therefore, the wafer  32  is heated by the heat radiation from the ceiling plate  60 . It should also be noted that the solid materials  47  to  49  of TiO(THD) 2 , Ba(THD) 2  and Sr(THD) 2  are also heated in accordance with the temperature elevation of the wafer  32 . As a result, a TiO(THD) 2  gas, a Ba(THD) 2  gas and a Sr(THD) 2  gas are generated from these solid materials  47  to  49 , respectively. 
     The temperatures of the wafer  32  and the solid materials  47  to  49  are dependent on the distances from the heat source. Naturally, the temperatures of the solid materials  47  to  49  are somewhat lower than the temperature of the wafer  32 . In this embodiment, the distance between the ceiling plate  60  and the wafer  32  is controlled to set the wafer temperature at about 600° C. It should be noted that the solid material  47  generates a raw material gas component at a temperature lower than the temperatures at which the solid materials  48  and  49  generate raw material gas components. The distances of the solid materials  47  to  49  from the ceiling plate  60  are determined in view of the raw material gas generating temperatures noted above. To be more specific, the solid materials  48  and  49  are positioned closer to the heat source than the solid material  47  so as to set the temperature of the solid materials  48  and  49  at about 300° C. On the other hand, the solid material  47  is positioned to set the temperature thereof at about 200° C. 
     The TiO(THD) 2  gas, the Ba(THD) 2  gas and the Sr(THD) 2  gas thus generated scarcely form a gas stream and is diffused uniformly within the closed space  51 . In other words, a region performing the function equal to that performed by the stagnant layer in the conventional method is formed in the entire region of the closed space  51 . 
     The TiO(THD) 2  gas, the Ba(THD) 2  gas, the Sr(THD) 2  gas, and the oxygen gas within the closed space are mixed uniformly and, then, decomposed on the wafer  32  so as to deposit a BSTO film on the wafer  32 . 
     The BSTO deposition was continued for one minute, followed by moving the reaction vessel  33  downward so as to lower the wafer temperature. Then, the shutter  57  was closed, and the wafer  32  was arranged at a position away from the reaction vessel  33  by using a mechanism (not shown). At the same time, the pump  40 - 1  was driven so as to purge the gas within the closed space  51 , followed by taking the wafer  32  out of the apparatus. 
     The second embodiment described above also produces effects similar to those described previously in conjunction with the first embodiment. What should also be noted is that, in the second embodiment, the concentrations of the raw material gas components within the closed space can be controlled by controlling the distances between the solid materials  47  to  49  and the heater  37 , making it possible to control highly accurately the composition of the BSTO thin film. 
     Incidentally, the nitrogen gas can be supplied into the closed space  51  through an axial bore formed in the shaft  58 . Also, the film forming conditions can be changed in various fashions as in the first embodiment described previously. 
     Let us describe a third embodiment of the present invention. Third embodiment is substantially equal to the first embodiment, except that TEOS, which is liquid at room temperature, was used in place of the solid materials  47  to  49  and the wafer  32  was heated to 700° C. in the film forming step. In the third embodiment, a silicon oxide film having a uniformity of ±2% was formed at such a high film forming rate as about 1000 Å/min. 
     Let us describe a fourth embodiment of the present invention. The fourth embodiment is substantially equal to the second embodiment, except that TEOS, which is liquid at room temperature, was used in place of the solid materials  47  to  49 , and the wafer  32  was heated to 700° C. in the film forming step. In the fourth embodiment, a silicon oxide film having a uniformity of ±2% was formed at such a high film forming rate as about 1000 Å/min. Also, the fourth embodiment was advantageous over the third embodiment in that TEOS was poured easily into the container  50 . 
     In each of the first to fourth embodiments described above, the film thickness was controlled by controlling the film forming time. However, it is also possible to control the film thickness by controlling the supply amounts of the raw material gas components. Specifically, the amounts of the raw materials such as the oxygen gas, the solid materials  47  to  49 , etc. required for forming a thin film of a desired thickness are calculated in advance. Also, the amounts of the raw materials such as the oxygen gas, the solid materials  47  to  49 , etc. within the closed space  51  are made equal to the calculated amounts so as to permit these raw materials to be consumed completely in a single film formation. Under the particular condition, the film thickness reaches saturation in a certain time, making it possible to form a thin film of a desired thickness without relying on the film forming time. 
     As described above, a thin film is formed within a closed space in the present invention, and the raw material gas components are diffused uniformly within the closed space without forming a gas stream. It follows that the present invention makes it possible to form a thin film of a uniform thickness. What should also be noted is that the present invention permits supplying the raw material gas components to the stagnant layer at a higher efficiency than in the conventional method, leading to an improved film forming rate. Further, in the present invention, all the raw material gas components are utilized for formation of a stagnant layer. It follows that the present invention makes it possible to form a thin film at a low cost, compared with the conventional method. Still further, the sources of the raw material gas components are evaporated within a closed space in the present invention. To be more specific, the evaporation is carried out within the stagnant layer in the present invention. This makes it unnecessary to heat the pipe, valve, etc. in the present invention, unlike the prior art in which the evaporating position is apart from the forming position of the stagnant layer. Naturally, deterioration of the valve, etc. can be prevented in the present invention. 
     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.