Abstract:
A new chemical vapor reaction system is described. Instead of ECR where electrons can move as independent particles without interaction, a mixed cyclotron resonance is a main exciting principal for chemical vapor reaction. In the new proposed resonance, the resonating space is comparatively large so that a material having a high melting point such as diamond can be deposited in the form of a thin film by this inovative method.

Description:
This is a Divisional application of Ser. No. 07/993,523 filed Dec. 21, 1992; which itself is a continuation of Ser. No. 07/366,364 filed Jun. 15, 1989 now abandoned; which is a continuation of Ser. No. 07/114,203 filed Oct. 29, 1987. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a microwave enhanced method and an apparatus therefor. 
     Recently, electron cyclotron resonance chemical vapor deposition (ECR CVD) has attracted the interests of researchers as a new method of manufacturing thin films, particularly amorphous thin films. For example, Matsuo et al discloses one type of such a ECR CVD apparatus in U.S. Pat. No. 4,401,054. This recent technique utilizes microwaves to energize a reactive gas into a plasma state by virtue of a magnetic field which functions to pinch the plasma gas within the excitation space. With this configuration, the reactive gas can absorb the energy of the microwaves. A substrate to be coated is located distant from the excitation space (resonating space) for preventing the same from being sputtered.) The energized gas is showered on the substrate from the resonating space. In order to establish an electron cyclotron resonance, the pressure in a resonating space is kept at 1×10 −3  to 1×10 −5  Torr at which electrons can be considered as independent particles, and resonate with a microwave in an electron cyclotron resonance on a certain surface on which the magnetic field takes a particular strength required for ECR. The excited plasma is extracted from the resonating space, by means of a divergent magnetic field, to a deposition space which is located distant from the resonating space and in which is disposed a substrate to be coated. 
     In such a prior art method, it is very difficult to form a thin film of a polycrystalline or single-crystalline structure, so that currently available methods are almost limited to processes for manufacturing amourphous films. Also, high energy chemical vapor reaction is difficult to take place in accordance with such a prior art method and therefore a diamond film or other films having high melting points, or uniform films on an even surface having depressions and caves can not be formed. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a microwave enhanced CVD method and an apparatus therefor capable of forming a film, which can not be formed by the prior art method, such as a diamond film, or a film of a material having a high melting point. 
     According to one aspect of the invention, the surface of an object to be coated is located in a region of a resonating space in which the electric field of a microwave takes its maximum value. By this configuration, the deposition can be carried out while the deposited film is being partially sputtered, and therefore, e.g., a diamond film can be formed. 
     According to another aspect of the invention, a new CVD process has been achieved. The new Process utilizes a mixed cyclotron resonance which was derived first by the inventors. In the new type of exciting process, interaction of reactive gas itself must be taken into consideration as a non-negligible perturbation besides the magnetic field and microwave, and therefore charged particles of a reactive gas can be absorbed in a relatively very wide resonating space. For the mixed resonance, the pressure in a reaction chamber is elevated 10 2 −10 5  times as high as that of the prior art method. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section view showing a CVD apparatus in accordance with the present invention. 
     FIG.  2 (A) is a graphical diagram showing the profile of the equipotential surfaces of magnetic field in cross section. 
     FIG.  2 (B) is a graphical diagram showing the strength of electric field. 
     FIGS.  3 (A) and  3 (B) are graphical diagrams showing equipotential surfaces in terms of magnetic field and electric field respectively. 
     FIG. 4 is a cross section view showing another embodiment in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a microwave enhanced plasma CVD apparatus accordance pith the present invention is illustrated. In the figure, the apparatus comprises a reaction chamber in which a plasma generating space  1  and an auxiliary space  2  are defined and can be held at an appropriate pressure, a microwave generator  4 , electro-magnets  5  and  5 ′ in the form of solenoids surrounding the space  1 , a power supply  25  for supplying an electric power to the electro-magnets  5  and  5 ′, and a water cooling system  18 . The plasma generating space  1  has a circular cross section. In the plasma generating space  1 , a substrate holder  10 ′ made of a highly thermal conductive ceramic such as aluminum nitride is provided on which a substrate  10  is mounted. The substrate holder  10 ′ is irradiated and heated to 150-1000° C. with an infrared light  24 , which is emitted from an IR heater  20 , reflected from an IR reflection parabola mirror  21  and focused on the back surface of the holder  10 ′ through a lens  22 . A reference numeral  23  designates a power supply for the IR heater  20 . Provided for evacuating the reaction chamber is an evacuating system comprising a turbo molecular pump  8  and a rotary pump  14  which are connected with the reaction chamber through pressure controlling valves  11 ,  12  and  13 . The substrate temperature may reach a sufficient value only with the plasma gas generated in the reaction chamber. In this case, the heater can be dispensed with. Further, depending on the condition of the plasma, the substrate temperature might elevate too high to undergo a suitable reaction. In the case, cooling means has to be provided. The process with this apparatus is carried out as follows. 
     A substrate  10  is mounted on the substrate holder  10 ′ and heated by infrared light  24  to 500° C. Then, hydrogen gas is introduced from a gas introducing system  6  at 10 SCCM, and a microwave of 1 Kilo Gauss or stronger, e.g., 2.45 GHz is emitted from the microwave generator thorough a microwave introduction window  15  to the plasma generating space  1  which is subjected to a magnetic field of about 2 K Gauss induced by the magnets  5  and  5 ′. The hydrogen is excited in a high density plasma state in the space  1  by the energy of the microwave. The surface of the substrate is cleaned by high energy electrons and hydrogen atoms. In addition to the introduction of the hydrogen gas, C 2 H 2  and CH 4  are inputted through an introduction system  7 , and excited by the energy of microwave at 1-800 Torr in the same manner as carried out with hydrogen explained in the foregoing description. As a result of a chemical vapor reaction, carbon is deposited in the form of a diamond film or an i-carbon (insulated carbon) film. The i-carbon is comprised of a mixture of diamond and amorphous carbon. 
     FIG.  2 (A) is a graphical diagram showing the distribution of magnetic field on the region  30  in FIG.  1 . Curves on the diagram are plotted along equipotential surfaces and marked with the strength values of the magnetic field induced by the magnet  5  having a power of 2000 Gauss. By adjusting the power of the magnets  5  and  5 ′, the strength of the magnetic field can be controlled so that the magnetic field becomes largely uniform over the surface to be coated which is located in the region  100  Where the magnetic field (875±185 Gauss) and the electric field interact. In the diagram, a reference  26  designates the equipotential surface of 875 Gauss at which ECR (electron cyclotron resonance) condition between the magnetic field and the frequency of the microwave is satisfied. Of course, in accordance with the present invention, ECR can not be established due to the high pressure in the reaction chamber, 1-800 Torr, but instead a mixed cyclotron resonance (MCR) takes place in a broad region including the equipotential surface of the ECR condition. FIG.  2 (B) is a graphical diagram corresponding to FIG.  2 (A) and shows the strength of electric field of the microwave in the plasma generating space  1 . The strength of the electric field takes its maximum value in the regions  100  and  100 ′. However, in the region  100 ′, it is difficult to heat the substrate  10 ′ without disturbing the propagation of the microwave. In other regions, a film is not uniformly deposited, but deposited in the form of a doughnut. It is for this reason that the substrate  10  is disposed in the region  100 . The plasma flows in the lateral direction. According to an experiment, a uniform film can be formed on a circular substrate having a diameter of less than 100 mm. Preferably, a film is formed in the chamber on a circular substrate having a diameter of less than 50 mm with a uniform thickness and a uniform quality. When a larger substrate is desired to be coated, the diameter of the space  1  can be doubled by making use of 1.225 GHz as the frequency of the microwave. FIGS.  3 (A) and  3 (B) are graphical diagrams showing the distributions of the magnetic field and the electric field on a cross section of the plasma generating space  1 . The curves plotted in the circles of the figures correspond equipotential surfaces. As shown in FIG.  3 (B), the electric field reaches its maximum value at 25 KV/m. 
     The diffraction images of films formed in accordance with the present invention were obtained. As results, halo patterns were observed together with spots indicating the existence of diamond. The halo patterns, which correspond to an amorphous state, gradually disappear as the substrate temperature, increases, and when the temperature rises beyond 650° C. the film became a diamond. At less than 150° C., i-carbon film could not be formed. For reference, a film formation process was performed in the same manner as in the above but without using a magnetic field. As a result, a graphite film was deposited. 
     In the same manner, polycrystalline silicon carbide films and aluminum nitride films can be respectively formed by making use of methyl silane, and aluminum compound gas and ammonia gas respectively as the reactive gases. Further, other films having high melting points of tungsten, titanium, molybdenum or their silicon compounds can be formed in a similar manner. 
     Referring to FIG. 4, another embodiment of the invention is illustrated. In the figure, the apparatus comprises a reaction chamber in which a plasma generating space  1  and an auxiliary space  2  are defined and can be held at an appropriate pressure, a microwave generator  4 , electro-magnets  5  and  5 ′ which are supplied with electric power from a power supply  25 , and a water cooling system  18 . The plasma generating space  1  has a circular cross section. In the plasma generating space  1 , a hollow cylinder  10 ′ having inward-turned brims is rotatably supported in the space so that a microwave emitted from the microwave generator  4  passes through the cylinder along its axis. The cylinder  10 ′ is made of a stainless steel or quartz and turned by means of a motor  16  through a gear. Provided for evacuating the reaction chamber is an evacuating system comprising a turbo molecular pump  8  and a rotary pump  14  which are connected with the reaction chamber through pressure controlling valves  11 ,  12  and  13 . The process with this apparatus is carried out as follows. 
     Objects  10  to be coated, for example, metallic, plastic, ceramic parts (such as gears, screws, ornament jigs, or micro-particles for grinding) are put in the cylinder  10 ′ and rotated at 0.1-10 rpm during the process. The cylinder  10 ′, although the means is not illustrated in the figure, is shaken vibrated by a micro-vibration of 100 Hz-10 KHz. By the rotation and the vibration, the surfaces of the objects exposed to the surrounding ambient are always changing during the process. The reaction chamber is evacuated by the turbo molecular pump  8  and the rotary pump to 1×10 −6  Torr or lower. Then, argon, helium or hydrogen as a non-productive gas is introduced to the reaction chamber from a gas introducing system  6  at 30 SCCM, and a microwave of 2.45 GHz is emitted from the microwave generator at 500 W through a microwave introduction window  15  to the plasma generating space  1  which is subjected to an magnetic field of about 2 K Gauss induced by the magnets  5  and  5 ′. The pressure of the non-productive gas is 1×10 −4  Torr. A plasma is generated in the space  1  at a high density by the energy of the microwave. The surfaces of the objects  10  is cleaned by high energy electrons and non-productive atoms. In addition to the introduction of nonproductive gas, C 2 H 2 , C 2 H 4  and/or CH 4  are introduced through a introduction system  7  at 1-800 Torr, preferably 3-30 Torr, e.g., 10 Torr, and excited by the energy of microwave in the same manner as carried out with the non-productive gas explained in the foregoing description. As a result of a mixed resonance, carbon is deposited in the form of a diamond film or an i-carbon film on the objects  10 . In this embodiment, a heating means as illustrated in FIG. 1 may be used as in FIG.  1 . 
     The distributions of magnetic field and electric field are same as FIGS.  2 (A),  2 (B),  3 (A) and  3 (B) explained in conjunction with the preceding embodiment and therefore redundant description is dispensed with. 
     The diffraction images of films formed in accordance with the present invention were obtained. As results, halo patterns were observed together with spots indicating the existence of diamond. The halo patterns gradually disappear as the substrate temperature elevates, and when the temperature rises beyond 650° C. the film became a diamond. At less than 150° C., i-carbon film could not be formed. For reference, a film formation process was performed in the same manner as in the above but without using a magnetic field. As a result, a graphite film was deposited. 
     In the same manner, polycrystalline silicon carbide filmse and aluminum nitride films can be respectively formed by making use of methylsilane and an aluminum compound gas and ammonia gas as the reactive gases. Further, other films having high melting points of tungsten, titanium, molybdenum or their silicon compounds can be formed in a similar manner. For example, a BN or BP film could be formed in accordance with the present invention. 
     The pressure in the reaction chamber is chosen at that required for ECR condition, so that a preliminary plasma discharge takes place. While the discharge continues, the pressure is changed to 1 Torr to 3×10 −3 . Torr where a mixed resonance takes place with a plasma of which particles have a mean free path of 0.05 mm to several milimeters, normally not more than 1 mm. 
     The process proposed by this invention is suitable for manufacturing superconductig ceramics including one or more rare earth elements, one or more alkaline earth element (including Be and Mg) and Cu. In this case, a process gas is prepared by bubbling a solution of compounds of the elements with oxygen gas. For example, an organic solution (benzene, or alcohol solution) or a water solution of alkylenes or halides of Y(OC 2 H 5 ) 3 , Ba(OC 2 H 5 ) 3  and CuBr 3  so that the stoichometric ratio among Y, Ba and Cu is 1:2:3. The stoichometric formula of the deposited product is YBa 2 Cu 3 O 6-8 . In place of the bubbling method, compounds of the elements can be inputted to the rection chamber by blowing the compounds in the form of fine powder with a highly pressurized oxygen gas, that is by the so called spraying method. 
     The invention should not be limited to the above particular embodiments and many modifications and variations may occur to those skilled in the art. For example, instead of the hollow cylinder, any hollow member having a cross section of a polygon can be employed. Although the embodiments employ microwave energy alone under the exsitence of magnetic field, photon energy can be applied to the reactive gas excited by the mixed resonance, an a position separated from the mixed resonance.