Abstract:
A thin film of a high temperature superconductive oxide of rare earth metal-alkali earth metal-copper-oxygen system or group VA metal-alkali earth metal-copper-oxygen system, which has an excellent crystallinity, particularly a single crystalline structure, is formed on a substrate by a CVD method, in which halides of the metals and an oxygen source gas are separately flowed over a substrate and caused to react with each other over the substrate, to deposit a desired superconducting oxide film.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for chemical vapor deposition (CVD) of a superconductive oxide, more specifically a process for CVD of a superconductive oxide of (a rare earth metal or a metal of the VA group of the periodic table)-(alkali earth metal)-(copper)-(oxide) system on a substrate. 
     2. Description of the Related Art 
     The speed of computers has been remarkably increased, and a multiplication of processors, increase of the switching speed of devices, and a high density packaging of such devices for shortening the length of wiring are carried out to cope with this increase of the speed of computers. A high density wiring or interconnection necessitates fine wiring or interconnection patterns, which decreases the sectional areas of conductors used for the wiring or interconnection but increases the electrical resistance of the wiring or interconnection. This lowers the speed of an electrical signal transmission and distorts the wave-shape thereof. 
     If a superconductive material can be used as a material for wiring instead of a normal conductor such as copper, the above-mentioned problems will be quickly solved, and if a Josephson element and the like are formed with a superconductive material and integrated, the high speed and low electric power consumption thereof, in combination with a fine packaging art, will allow the realization of a super high speed computer system. 
     Conventional superconductive materials need a low temperature for transition to a superconductive state and, therefore, must be cooled by liquid helium or liquid hydrogen. Since these cooling mediums are difficult to handle and are expensive, it is practically difficult to use these superconductive materials. 
     Nevertheless, high temperature superconductive materials, represented by Y-Ba-Cu-O system oxide or ceramics, have been recently developed, and this has opened up new possibilities in the utilization of superconductive materials. 
     Since oxide superconductors exhibit a superconducting behavior at a relatively high temperature, i.e., higher than the boiling point of liquid nitrogen (77K), oxide superconductors can be widely utilized in, for example, semiconductor devices such as IC&#39;s, as parts of various devices, and as wiring in devices a strong demand has arisen for such superconductors. To satisfy this demand, it is necessary to efficiently form a high quality thin film. For example, a semiconductor integrated circuit is composed completely of thin film elements, including a Josephson element, and as a result, the characteristics of a thin film, which depend on the crystallinity thereof, such as crystal size and crystal orientation of the thin film, and the uniformity and reproducibility of the thin film, are important factors determining the yield and reliability of elements and an integrated circuit. 
     Conventional methods of forming a thin film of a semiconductor material include sputtering and evaporation. In the sputtering process, a target having a composition similar to that of a material to be deposited is used and is vaporized by ion sputtering to be deposited on a substrate. In the evaporation process, a material (source) for forming a thin film is heated until evaporation occurs and is deposited on a substrate. 
     These conventional thin film forming methods may be applied to a high temperature superconductive material but it is difficult to provide a good crystallinity thin film, particularly a single crystalline thin film, thereby. The sputtering method is suitable for forming a thin film of a single element (Si or a metal) or a simple compound which is not easily decomposed (SiO 2 , Al 2  O 3 , etc.), but is difficult to form a thin film of a complex compound by sputtering, since such a compound is decomposed by the sputtering, and thus control of a composition is very difficult. In the evaporation process, if a compound composed of multi-elements is used, it is difficult to form a film having a uniform composition, since an element which is easily evaporated is first evaporated and a material which is not easily evaporated remains. Particularly, it is difficult to evaporate a compound such as an oxide and deposit it in a uniform manner. In this regard, a method has been proposed of depositing a film of metals having a ratio between the metals required for a desired metal oxide, followed by oxidizing the metal film. In this method, however, the volume of the film is changed by oxidation, which causes roughness of the surface of the film, peeling of the film, or a nonuniform film quality, and it is difficult to obtain a dense film thereby. 
     In either the sputtering or the evaporation process, it is difficult to form a single crystalline film having a complex composition, since a deposition of discrete metals, alloy and metal oxides or compounds on a substrate occurs. For example, as seen in FIG. 1, when a superconductive oxide of the Y-Ba-Cu-O system mentioned above is formed, discrete metals such as Y, Ba and Cu, various discrete oxides or compounds such as Y 2  O 3 , BaO 2 , Y-Ba-O, CuO, Cu 2  O, Y-Cu-O, Y-O, Ba-Cu-O and Ba-O, or alloys, are deposited on a substrate, and as a result, it is difficult to obtain a film of a compound having a desired composition or a good crystallinity, and such a film has a disadvantageously decreased current density and reduced boundary characteristics when made into a fine pattern, and the like, and thus it is not practical for application to a semiconductor integrated circuit. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a method of forming a thin film of a high temperature superconductive oxide having an excellent crystallinity, which is applicable to thin film devices such as a semiconductor integrated circuit, etc. 
     This and other objects, features, and advantages of the present invention are attained by providing a process for chemical vapor deposition of an oxide superconductive film on a substrate. In this process, the substrate is held in a reaction chamber and heated to a first temperature; a first flow of vapors of a rare earth metal, an alkali earth metal or a halide of an alkali earth metal and a halide of copper having a second temperature equal to or lower than the first temperature is introduced at a position over the substrate in the reaction chamber; a second flow of an oxygen source gas is introduced into the reaction chamber at a position over the substrate; the first and second flows being separated from each other until approaching the substrate, at which point the flows come into contact with each other, and are heated to the first temperature, and react with each other to deposit a superconductive oxide of rare earth metal-alkali earth metal-copper-oxygen system on the substrate. The typical superconductive oxide formed in the above process is represented by the formula X 1  Z 2  CuO x  where X stands for at least one rare earth metal, Z stands for at least one alkali earth metal, and x has a value different to the stoichiometric value. 
     In accordance with the present invention, there is also provided a process for chemical vapor deposition of an oxide superconductive film on a substrate, wherein the substrate is kept in a reaction chamber and heated to a first temperature; a first flow of vapors of a halide of a group metal VA of the elemental periodic table, halides of two alkali earth metals, and a halide of copper having a second temperature lower than the first temperature is introduced to a position over to the substrate in the reaction chamber; a second flow of an oxygen source gas is introduced to a position over the substrate in the reaction chamber; the first and second flows being separated from each other until approaching the substrate, at which point the flows come in contact with each other, are heated to the first temperature, and react with each other to deposit a superconductive oxide of a group VA metal of the periodic table - an alkali earth metal - an alkali earth metal-copper-oxide system on the substrate. The typical superconductive oxide formed in this latter process is represented by the formula QZ 1  Z 2  Cu y  O x , where Q stands for a metal of the V group of the periodic table, Z 1  and Z 2  stand for an alkali earth metal, y has a value of about 2 or about 1.5, x has a value different from the stoichiometric value. 
     In the above two chemical formulae, the ratios of metals may be deviated from that in the formulae; typically, within 10% from the formulae. 
     The first flow is usually produced by flowing a carrier gas such as helium or argon. 
     The oxygen source gas may be oxygen, air (oxygen-containing gas), water vapor, etc. 
     Preferably hydrogen is used, as it has an effect of accelerating the reaction for the deposition a superconductive oxide film. 
     The main feature of the process of the present invention is that (1) source gases or vapors of metals or metal halides and (2) an oxygen source gas are separated until approaching near to a substrate, and that a temperature of the source gases or vapors of metals or metal halides before approaching the substrate is lower than a temperature of the substrate at the point at which the source gases or vapors react with an oxygen source gas. 
     If the source gases or vapors of metals or metal halides and an oxygen source gas are not separated until nearing a substrate, i.e., come into contact with each other before nearing the substrate, they react with each other to form an oxide there and control of the composition of the source gases or vapors near the substrate, i.e., where a desired reaction should occur becomes impossible. 
     If a temperature of the source gases or vapors of metals or metal halides is higher than a temperature of the substrate, there must be a portion having a temperature lower than the temperature of the source gases or vapors before the substrate where the source gases or vapors are deposited, making it impossible to control the composition of the source gases or vapors to a desired one near the substrate. 
     For the same reasons as above, the source gas or vapor should not be cooled before reaching the substrate after it is evaporated from a source thereof at a required temperature, to provide a desired concentration. This is the second important feature of the process of the present invention. 
     The third important feature of the process of the present invention is that the appropriate temperature ranges of the various source gases or vapors of metals or metal halides, are as follows: 
     
         ______________________________________Source      General range  Preferred range______________________________________BiCl.sub.3  150-250° C.                      170-200° C.CuCl        300-500° C.                      350-400° C.CuBr.sub.2  350-500° C.                      400-450° C.CuI         400-600° C.                      450-550° C.YCl.sub.3   650-750° C.                      675-725° C.CaI.sub.2   700-900° C.                      750-850° C.SrI.sub.2   750-950° C.                      775-850° C.BaI.sub.2   850-1050° C.                      900-1000° C.BaCl.sub.2  950-1100° C.                      1000-1050° C.MgCl.sub.2  700-850° C.                      750-800° C.Ba          600-800° C.                      650-750° C.______________________________________ 
    
     According to the process of the present invention, a film of a single crystalline high Tc (transition temperature) superconductive oxide can be obtained, whereby a film of a high Tc superconductive oxide having an excellent film quality, film uniformity and reproducibility, due to a high crystallinity such as crystal size and crystal orientation, can be deposited, and a result, by applying such a high quality superconductive oxide film to IC&#39;s, to various parts of devices, and to wiring, the yield and reliability thereof can be increased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates the deposition of a Y-Ba-Cu-O system film by sputtering; 
     FIG. 2 is a sectional view of an apparatus for CVD; 
     FIG. 3 is an X-ray diffraction pattern of a YBa 2  Cu 3  O 7-x  film formed in Example 1; 
     FIG. 4 is a photograph of the electron diffraction pattern of film shown in FIG. 3; 
     FIG. 4 schematically illustrates the determination of an electrical resistance of a film; 
     FIG. 5 is a an graph of an electrical resistance of the film formed in Example 1, with respect to temperature; 
     FIG. 6 schematically illustrates the deposition of a Y-Ba-Cu-O system film deposited by CVD; 
     FIG. 7 is a sectional view of another CVD apparatus; 
     FIG. 8 is a photograph of the electron diffraction pattern of the film shown in FIG. 6; 
     FIG. 9 is an X-ray diffraction pattern of the BiSrCaCuO x  film formed in Example 6; 
     FIG. 10 is a graph of an electrical resistance of the film shown in FIG. 9, with respect to temperature; and, 
     FIGS. 11 and 12 are CVD apparatuses which may be used in the process of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is described below with reference to the drawings. 
     EXAMPLE 1 
     FIG. 2 illustrates an apparatus for CVD used to obtain this example. In FIG. 2, a cylindrical reaction tube 1 is made of quartz and is heat resistant. The reaction tube 1 is surrounded by four resistance heaters 2a-2d, which generate heat by passing an electrical current therethrough to heat respective portions of the reaction tube 1. In the reaction tube 1, a source chamber 3 is arranged in which three source boats 4a-4c are placed. The source boats 4a-4c contain BaCl 2 , YCl 3 , and CuCl, respectively, and release BaCl 2  gas, YCl 3  gas, and CuCl gas, respectively, when heated by the heaters 2b-2d. 
     The source chamber 3 has a small-diameter gas inlet port 3a at one end of the chamber 3, through which helium gas as a carrier gas is introduced. The other end of the source chamber 3 is open, and a substrate supporter 5 is placed near that other end at a predetermined distance therefrom. The substrate supporter 5 is made of quartz or ceramics, and substrates 6 on which a superconductive oxide is to be deposited are mounted on the substrate supporter 5. The reaction tube 1 also has a small diameter gas inlet port 1a at one end of the reaction tube 1, through which helium gas as a carrier gas, as well as carbon dioxide gas and hydrogen gas serving as a reducing agent, are introduced into the reaction chamber 1. The reaction chamber 1 has an outlet port 1b at the other end thereof for evacuating the gas in the reaction tube 1. 
     In the operation to obtain this example, first, substrates 6 are mounted on the supporter 5 and BaCl 2 , YCl 3 , and CuCl are placed in the source boats 4a to 4c, respectively. Then, the reaction tube 1 is heated by the resistance heaters 2a to 2d to release BaCl 2  gas, YCl 3  gas, and CuCl gas while a carrier gas (He) is introduced through the gas inlet port 3a into the source chamber 3 to carry the released gases over the substrates 6. Also, a carrier gas (He) as well as CO 2  and H 2  gases are introduced through the gas inlet port 1a into the reaction tube 1 outside the source chamber 3 and passed over the substrates 6 while the substrates 6 are heated by the resistance heater 2a, and as a result, oxidation and reduction reactions occur over or near the substrates and a high temperature superconductive oxide film of YBa 2  Cu 3  O 7-x  is deposited on the substrates 6 by the following chemical reaction. 
     
         YCl.sub.3 +2BaCl.sub.2 +3CuCl+7CO.sub.2 +5H.sub.2 →YBa.sub.2 Cu.sub.3 O.sub.7-x +7CO+10HCl 
    
     The particular conditions for deposition are as follows. 
     Temperature of substrates (T sub ): 950°-1200° C. 
     Temperature of BaCl 2  (T Ba ): 950°-1100° C. 
     Temperature of YCl 3  (T y ): 650°-750° C. 
     Temperature of CuCl (T Cu ): 300°-500° C. 
     CO 2  concentration: 0.01-10% of He concentration 
     H 2  concentration: 0.01-10% of He concentration 
     Flow rate of He carrying CO 2  and H 2  : 5-20 l/min 
     Flow rate of He carrying BaCl 2 , etc.: 5-20 l/min 
     Pressure: 760 mmHg 
     Substrate: (1102) sapphire, (100)MgO, (100)SrTiO 3 , (100)MgO.Al 2  O 3 , and MgO on MgO.Al 2  O 3  on Si 
     Thickness of deposited film: 0.2-5 μm 
     In this operation, the heating temperatures of the CuCl, YCl 3 , BaCl 2  and the substrates are selected such that these temperatures increase from T Cu  to T y  to T Ba  to T sub . Namely, the arrangement of CuCl, YCl 3 , and BaCl 2  in the source boats 4c to 4a should be such that the heating temperatures of the heaters 4a to 4c are increased from 4c to 4b to 4a, in that order. 
     The obtained YBa 2  Cu 3  O 7-x  film is annealed in an oxygen atmosphere in the reaction tube 1 with the heater 2a maintained at 850° C. for 8 hours, and then gradually cooled. FIG. 3 shows an X-ray diffraction pattern of the thus-obtained film on a (100)MgO substrate with Cu, Kα ray. This pattern clearly demonstrates a formation of (001)YBa 2  Cu 3  O 7-x  on (100)MgO. 
     Next, as shown in FIG. 4, the substrate 6 on which the YBa 2  Cu 3  O 7-x  film 7 is formed is cut into pieces measuring 5 mm×10 mm, after annealing. 
     Four probes 8 are formed by silver paste on this film 7, and wirings 9a to 9d with a constant electric current source 10 and a voltage meter 11 are connected to the probes 8. 
     The electrical resistance of the film 7 is then determined with respect to the temperature, and as shown in FIG. 5 the electrical resistance rapidly decreases at about 90K and reaches a zero electrical resistance at about 87K, demonstrating the existence of a superconductive state. The conditions of forming this film are T sub  =1000° C., T Ba  =1000° C., T y  =670° C., and T Cu  =350° C. The superconductive behavior was also confirmed for other films formed under other conditions. 
     Accordingly, an excellent single crystalline high temperature superconductive oxide film can be easily formed in accordance with the present invention. 
     FIG. 6 schematically illustrates the process of this deposition, wherein gases of YCl 3 , CuCl, HCl, BaCl 2 , H 2 , CO, CO 2 , etc., chemically react with each often over the substrate to uniformly deposit a film of a compound of a Y-Ba-Cu-O system on the substrate. 
     EXAMPLE 2 
     In this example, O 2  and H 2  are used as the oxidizing and reducing agents instead of the CO 2  and H 2   used in Example 1. The operation of forming a superconductive oxide film is carried out in the same way as in Example 1, except that the following conditions prevail. 
     T sub  : 900° C. 
     T Ba  : 900° C. 
     T y  : 630° C. 
     T Cu  : 320° C. 
     O 2  concentration: 0-30% of He 
     Bubbling temperature: 20° C. 
     Substrate: (1102)sapphire, (100)MgO, (100)SrTiO 3 , (100)MgO.Al 2  O 3 , and MgO on MgO.Al 2  O 3  on Si 
     Flow rate of He carrying O 2  and H 2  O: 15 l/min 
     Flow rate of He carrying BaCl 2  etc.: 15 l/min 
     Thickness of deposited film: 0.2-5 μm 
    
     After the deposition of a Y-Ba-Cu-O system film, the film is annealed in an oxygen atmosphere as in Example 1, and as a result, a single crystalline film of YBa 2  Cu 3  O 7-x  exhibiting superconductive behavior was obtained. 
     Note that the CO 2  and H 2  used as oxdizing and reducing agents in Example 1 must be selected with great care and the conditions of the operation must be strictly controlled, since O 2  has a weak oxidizing effect and H 2  has a strong reducing effect, and thus the deposited film can be reduced from an oxide to a metal. In comparison with this, the use of O 2  and H 2  permits a less careful selection and easier control of the conditions of operation. 
     EXAMPLE 3 
     The deposition of a YBa 2  Cu 3  O 7-x  film was carried out as in Examples 1 and 2, but BaBr 2  or BaI 2  was used instead of BaCl 2 , YF 3  or YBr 3  instead of YCl 3 , and CuF, CuF 2 , CuCl 2 , CuBr, CuBr 2  or CuI instead of CuCl. 
     The resultant films also proved to be excellent single crystalline superconductive oxide films. 
     EXAMPLE 4 
     The deposition of a YBa 2  Cu 3  O 7-x  film was carried out as in Example 2, but Ba was used instead of BaCl 2 . The conditions of operation were as follows: 
     T sub  : 900° C. 
     Temperature of Ba (T Ba ): 700° C. 
     T y  : 630° C. 
     T Cu  : 320° C. 
     Flow rate of He carrying Ba etc.: 15 l/min 
     O 2  concentration: 0-30% of He 
     Bubbling temperature: 0°-100° C. 
     Flow rate of He carrying O 2  and H 2  : 15 l/min 
     Substrate: (1102)sapphire, (100)MgO, (100)SrTiO 3 , (100)MgO.Al 2  O 3 , and MgO on MgO.Al 2  O 3  on Si 
     Thickness of deposited film: 0.2-5 μm 
     After the deposited film was annealed in an oxygen atmosphere, a single crystalline YBa 2  Cu 3  O 7-x  film exhibiting superconductive behavior was obtained. 
     EXAMPLE 5 
     Using the same procedures as in the above Examples, a film of a compound having a composition of LnBa 2  Cu 3  O 7-x  where Ln is a lanthanide element was deposited on a substrate by using a halide of a lanthanide element instead of a halide of yttrium. The lanthanide element included Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. Two or more lanthanide elements could be used in combination. The halide of the lanthanide element includes the chloride, bromide, and iodide of the element. 
     The thus obtained LnBa 2  Cu 3  O 7-x  film was also a single crystalline superconductive oxide film exhibiting superconductive behavior at about 90K. 
     EXAMPLE 6 
     FIG. 7 illustrates the apparatus for CVD used in this Example for forming a film of Bi-Sr-Ca-Cu-O system superconductive oxide. 
     The structure of this apparatus is very similar to that of the apparatus in FIG. 2 except that, in this Example, four source boats and five resistance heaters are used. 
     In FIG. 7, reference numeral 21 denotes a reaction tube, 21a a gas inlet port, 21b a gas outlet port, 22a to 22e resistance heaters, 23 a source chamber, 23a a gas inlet port, 24a to 24d source boats, 25 a substrate supporter, and 26 substrates. 
     SrI 2 , CaI 2 , CuI and BiCl 3  were placed in the source boats 24a to 24d, respectively, and a carrier gas of He was introduced through the gas inlet port 23a into the source chamber 23. The substrates 26 were of (100)MgO. Through the gas inlet port 21a, another carrier gas of He in combination with O 2  and H 2  as the oxidizing and reducing agents was introduced into the reaction tube 21 outside the source chamber 23. The reaction gas was evacuated from the outlet port 21b. 
     The conditions of operation used for depositing a BiSrCaCuO x  on a substrate were as follows: 
     Temperature of substrates (T sub ): 750°-950° C. 
     Temperature of SrI 2  source (T Sr ): 750°-950° C. 
     Temperature of CaI 2  source (T Ca ): 700°-900° C. 
     Temperature of CuI source (T Cu ): 400°-600° C. 
     Temperature of BiCl 3  source (T Bi ): 150°-250° C. 
     Flow rate of He carrying BiCl 3  etc: 10-20 l/min 
     Flow rate of He carrying O 2  and H 2  : 10-20 l/min 
     Flow rate of O 2  gas: 10-5000 cc/min 
     Bubbling temperature of H 2  O: 23° C. 
     Flow rate of He bubbling gas: 10-1000 cc/min 
     Deposition rate: 3-30 nm/min 
     Thickness of deposited film: 0.1-10 μm 
     Oxygen annealing temperature: 400°-850° C. 
     Oxygen annealing time: 30-60 minutes 
     Substrate: (1102)sapphire, (100)MgO, (100)SrTiO 3 , (100)MgO.Al 2  O 3 , and MgO on MgO.Al 2  O 3  on Si 
     FIG. 8 is a photograph of the transmission electron diffraction pattern of (100)BiSrCaCuOx on (100)MgO, which clearly demonstrates that the BiSrCaCuOx film definitely has a single crystalline form. This film was obtained under the conditions of T sub  =825° C., T Bi  =170° C., T Sr  =825° C., T Cu  =450° C., and T ca  =800° C. 
     FIG. 9 shows an X ray diffraction pattern obtained from a film deposited according to the above operation, with CuKα ray. In FIG. 9, in addition to the diffraction peaks of (100)MgO, diffraction peaks (008), (0010) and (0012), which stem from particular diffraction planes oriented to (100) of MgO were observed, and thus the formation of a single crystal of BiSrCaCuO x  was confirmed. The particular conditions of operation for this film of FIG. 8 were; 
     T sub  =825° C., T Bi  =170° C., T sr  =825° C., T Ca  =800° C., T Cu  =450° C. 
     The electrical resistance of the film obtained above was determined with respect to the temperature, in the same manner as described before with reference to FIG. 4. The result is shown in FIG. 10, in which the electrical resistance rapidly decreases at about 90K and reaches a zero electrical resistance at about 77K, thus exhibiting a superconductive state. 
     EXAMPLE 7 
     In the same way as in Example 6, other QZ 1  Z 2  CuO x  films were deposited, where Q stands for a group VA metal of the periodic table, and Z 1  and Z 2  stand for a alkali earth metal. The group VA metal may be Sb or Bi and the alkali earth metal may be Ba, Mg, Be, etc. The obtained QZ 1  Z 2  CuO x  films were single crystalline and exhibited superconductive behavior. 
     Note that a high temperature superconductive film of BiSrCaCuO x  can be deposited at about 800° C., which is about 100°-200° C. lower than that for a film of YBa 2  Cu 3  O 7-x , and as a result, during the deposition of a BiSrCaCuO x  film, a mutual reaction between the substrate and the deposited film is prevented, giving a superconductive film having an abrupt interface. This advantageously enhances the performance of elements for which the interface characteristics thereof are important, such as a Josephson element and a superconductive transistor, etc. 
     Although only one source chamber 3 or 23 was used in the apparatus for CVD in FIG. 2 and 7, a plurality of source chambers may be used for each source, for example, as shown in FIG. 11. 
     The CVD apparatus shown in FIG. 11 is the same as that in FIG. 2 except that three source chambers 3--1 to 3--3 are provided in which sources 4a to 4c are arranged, respectively. In this apparatus in FIG. 11, the temperature control of the three sources 4a to 4c is similar to that for the apparatus of FIG. 2, but the flow rates of the respective source gases can be separately controlled, which allows a more precise control of the amount or flow rate of sources gases. 
     FIG. 12 schematically illustrates another CVD apparatus which may be used for the process of the present invention. In this apparatus, source gases are separately formed and fed to a reactor 31. The source feeding lines 34a to 34c are separated and separately heated, so that each source gas can be fed to the reactor 31 at a desired temperature and a desired flow rate. In this configuration, the temperature of a source gas can be selected regardless of the temperatures of the other source gases.