Abstract:
In a plasma film-forming apparatus which includes a film-forming chamber in which a substrate is arranged, a film-forming gas introducing pipe connected to a supply source of a film-forming gas at its first end, a shower plate through numerous holes of which a second end of said film-forming gas introducing pipe communicate with said film-forming chamber, film-gas exciting means for exciting film-forming gas introduced through said shower plate into said film-forming chamber, to form a film on the surface of said substrate with the chemical reaction, radicals-producing means which excites said cleaning gas and produces radicals, and cleaning-gas introducing means which introduces said cleaning gas containing said radicals into said film-forming chamber, the improvement in which said cleaning-gas introducing means communicate directly with said film-forming chamber.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a plasma film-forming apparatus and a cleaning method for cleaning the plasma film-forming apparatus.  
           [0003]    2. Description of the Prior Art  
           [0004]    [0004]FIG. 1 shows a plasma film-forming apparatus  1  of the prior art. It is an apparatus to form a film on a substrate  9  by a plasma CVD (Chemical Vapor Deposition) method. A cathode electrode  4  is arranged on the upper wall of a vacuum tank  2 . An anode electrode  3  is arranged opposite to the cathode electrode  4 , in a film-forming chamber  10  of the vacuum tank  2 . The cathode electrode  4  is connected to a high frequency electric power source  8 . The anode electrode  3  is connected to the earth. It functions also as a supporter for substrate. The substrate  9  is mounted on the anode electrode  3 .  
           [0005]    The cathode electrode  4  is dish-shaped. A gas-introducing pipe  13  is connected to a central hole of the upper wall of the cathode electrode  4 . A shower plate  5  is fixed to a lower end of the cathode electrode  4 . Numerous small holes are made in the shower plate  5  which is facing to the substrate  9 .  
           [0006]    One end of a film-forming gas introducing pipe  6  is connected to the gas-introducing pipe  13 . Another end of the film-forming gas-introducing pipe  16  is connected to a not-shown film-forming gas supply source. A radicals producing source  11  is connected to one end of a gas-introducing pipe  12 . Another end of the gas-introducing pipe  12  is connected to a not-shown cleaning gas sipply source. The radicals producing source  11  is further connected to the pipe  13 .  
           [0007]    Next, operations of the above described plasma film-forming apparatus  1  will be described.  
           [0008]    For example, there will be described a case of forming a film of SiNx on the substrate  9 . First, the film-forming chamber  10  is evacuated through an exhaust port  7  and so is put under the lower pressure. For example, SiH 4  gas and NH 3  gas are introduced onto the shower plate  5  through the film-forming gas introducing pipe  6  and the gas introducing pipe  13 . They are ejected through the numerous holes of the shower plate  5  uniformly into the film-forming chamber  10  and toward the substrate  9 .  
           [0009]    Next, a high frequency electric power is supplied to the cathode electrode  4  form the high frequency power source  8 , to decompose and make the introduced gases to react on each other gases in the film-forming chamber  10 . Thus, a film of SiNx is formed on the substrate  9 .  
           [0010]    The above film-forming operations are repeated, and so SiNx films are adhered and piled onto the shower plate  5 , anode electrode  3 , cathode electrode  4  and inner walls of the vacuum tank  2  besides the substrate  9 . The SiNx films on the above portions besides the substrate  9  should be removed (cleaned).  
           [0011]    Next, there will be described cleaning operations of the interior of the film-forming chamber  10 .  
           [0012]    As on the film-forming operation, the film-forming chamber  10  is evacuated through the exhaust port  7  and so put under the lower pressure. For example, NF 3  gas is supplied into the radicals producing source  11 . Microwave is applied to the NF 3  gas there, so that fluorine free radicals are produced there. NF 3  gas including fluorine free radicals are introduced into the film-forming chamber  10  through the gas-introducing pipe  13  and the shower plate  5 .  
           [0013]    Then, fluorine radicals react chemically on the materials (SiNx film) to be cleaned. The SiNx films piled on the inner wall of the vacuum tank  2  are removed. The removed SiNx materials are discharged through the exhaust port  7  together with the cleaning gas.  
           [0014]    The method that the radicals for cleaning are thus previously produced and then introduced into the film-forming chamber  10 , has the advantage that the plasma damage of the shower plate  5  is decreased, in comparison with the method that free radicals for cleaning are produced in the film-forming chamber  10  by the high frequency electric power applied to the cathode electrode  4  from the high frequency power source  8 , as on the film-forming operation. introduced into the film-forming chamber  5 , most of the radicals are dissipated, since the passing rate of the shower plate  5  having numerous small holes is low. Thus there is the problem that the cleaning rate is lowered.  
           [0015]    Further, in consideration of the problem that most of the radicals are dissipated through in the shower plate  5 , a very high frequency microwave such as 2.45 GHz is applied to the radicals-producing source  11  to produce more radicals, in some cases. However, such method requires high cost.  
         SUMMARY OF THE INVENTION  
         [0016]    Accordingly, it is an object of this invention to provide a plasma film-forming apparatus and the cleaning method that the dissipation of the radicals to be introduced into the film-forming chamber can be prevented.  
           [0017]    Another object of this invention is to provide a plasma film-forming apparatus and the cleaning method that the radicals as the cleaning gas produced outside the film-forming chamber, can be effectively used for cleaning the film-forming chamber.  
           [0018]    In accordance with one aspect of the invention, in a plasma film-forming apparatus which includes a film-forming chamber in which a substrate is arranged, a film-forming gas introducing pipe connected to a supply source of a film-forming gas at its first end, a shower plate through numerous holes of which a second end of said film-forming gas introducing pipe communicate with said film-forming chamber, film-gas exciting means for exciting film-forming gas introduced through said shower plate into said film-forming chamber, to form a film on the surface of said substrate with the chemical reaction, radicals-producing means which excites said cleaning gas and produces radicals, and cleaning-gas introducing means which introduces said cleaning gas containing said radicals into said film-forming chamber, the improvement in which said cleaning-gas introducing means communicate directly with said film-forming chamber. film-forming chamber.  
           [0019]    In accordance with another aspect of the invention, in a cleaning method of a plasma film-forming apparatus which, in the film-forming operation, introduces a film-forming gas through a shower plate having numerous holes into a film-forming chamber, excites the introduced gas and forms a film, with the chemical reaction, on a surface of substrate arranged in said film-forming chamber, and in the cleaning operation, introduces a cleaning-gas containing radicals produced by exciting of said cleaning-gas, into said film-forming chamber and cleans said film-forming chamber by chemical reaction of said radicals and removes materials to be cleaned, the improvement in which said cleaning gas containing said radicals is introduced directly into said film-forming chamber. 
       
    
    
     BRIFE DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a vertical cross-sectional view of a plasma film-forming apparatus of the prior art;  
         [0021]    [0021]FIG. 2 is a vertical cross-sectional view of a plasma film-forming apparatus according to first and second embodiment of this invention;  
         [0022]    [0022]FIG. 3 is vertical cross-sectional view of a plasma film-forming apparatus according to a third embodiment of this invention;  
         [0023]    [0023]FIG. 4 is a cross-sectional view taken along the line iv-iv in FIG. 3;  
         [0024]    [0024]FIG. 5 is a graph for showing the comparisons of the cleaning rates of SiNx films between the prior art and the first embodiment of this invention; and  
         [0025]    [0025]FIG. 6 is a graph for showing the cleaning rates of SiOx film by the second embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    Next, embodiments of this invention will be described with reference to the drawings. The parts corresponding to the parts of the above prior art are denoted by the same reference numerals, the detailed description of which will be omitted.  
         [0027]    [0027]FIG. 2 shows a plasma (CVD) film-forming apparatus  20  according to a first embodiment of this invention. A cathode electrode  4  connected to a high frequency electric power source  8  is arranged in the upper wall of a vacuum tank  2 . An anode electrode  3  supporting a substrate  9  and connected to the earth is arranged opposite to the cathode electrode  4  in the film-forming chamber  10 .  
         [0028]    A film-forming gas introducing pipe  15  is connected to a central hole of the upper wall of the cathode electrode  4 . A shower plate  5  having numerous small holes is fixed to the lower end of the cathode electrode  4 , opposite to the substrate  9 .  
         [0029]    A radicals-producing means  21  is arranged outside the vacuum tank  2 . An input side of the radicals-producing means  21  is connected through a conduit  22  to a not-shown cleaning gas supply source. The radicals-producing means  21  consists of a chamber for a cleaning gas introduced from the conduit  22  and a high frequency electric power source applying a high frequency electric power to the contained cleaning gas in the chamber for producing radicals.  
         [0030]    An output side of the radicals-producing means  21  is connected through a valve  24  to one end of a pipe  23  for introducing a cleaning gas. Another end of the pipe  23  is connected to a hole made in the side wall of the vacuum tank  2 , positioning between the shower plate  5  and the anode electrode  3 . Thus, the pipe  23  for introducing the cleaning gas directly communicates with the inside of the film-forming chamber  10 .  
         [0031]    In the film-forming operation, the film-forming chamber  10  is evacuated through the exhaust port  7  and is put under the lower pressure, as in that the prior art. A film-forming gas (SiH 4  gas, NH 3  gas) is supplied through the film-forming gas introducing pipe  15  onto the shower plate  5 . It is ejected into the film-forming chamber  10  from the numerous small holes of the shower plate  5 . A high frequency electric power is supplied to the cathode electrode  4  by the high frequency electric power source  8  to decompose and make the introduced film-forming gas reacting. Thus, a film of SiNx is formed on the substrate  9 .  
         [0032]    In the cleaning operation of the film-forming chamber  10 , the film-forming chamber  10  is evacuated through the exhaust port  7  and put under the lower pressure. Then, the cleaning gas such as NF 3  gas is supplied to the radicals producing source  21  to which a high frequency electric power (400 kHz) is supplied. Fluorine radicals are produced in the radicals-producing source  21 . The valve  24  is opened to introduce directly the NF 3  gas containing the fluorine radicals into the film-forming chamber  10  through the gas-introducing pipe  23  as the means for introducing the cleaning gas. The fluorine radicals react on the SiNx film to be cleaned. Thus, the interior of the film-forming chamber  10  is cleaned. Thus, in this embodiment, the radicals pass not through the shower plate  5 , but directly introduced into the film-forming chamber  10  to be cleaned. Thus, most of the radicals can be prevented from dissipating before introduced into the film-forming chamber  10 . The film-forming chamber  10  can be effectively cleaned. As shown in FIG. 5, the cleaning rate of the SiNx according to this embodiment is higher about twenty times than the prior art method in which the radicals pass through the shower plate  5 .  
         [0033]    Further, the micro-wave generator of a high frequency such as 2.45 GHz was used for producing radicals in the radicals-producing means of the prior art. It is very expensive. In the embodiment of this invention, it is not necessary to use such as an expensive high-frequency electric power source. A high frequency electric power source of 400 HKz, which takes lower cost, can be used to produce radicals. The experimental results as shown in FIG. 3 were obtained with the electric power source of 400 KHz. The frequency is not limited to 400 KHz. Similar effects can be obtained within the range of 100 to 1000 KHz. A high frequency electric power source of lower frequency than 1000 KHz takes low cost. Accordingly, a plasma film-forming apparatus using such a high frequency electric power source takes lower cost, in comparison with the prior art plasma film-forming apparatus.  
         [0034]    Further, in the embodiment of this invention, polyfluoro ethylene (trade name-Tefron) is coated on the inner surface of the cleaning gas introducing pipe  23 . Accordingly, the radicals can be transported through the cleaning gas introducing pipe  23  without the dissipation. Thus, the life of the produced radicals can be longer.  
         [0035]    Sufficient cleaning rate can be obtained for SiNx film, even only by radicals. However, radicals are very directional. Accordingly, there is the possibility that the films are not removed around the shower plate  5  and anode electrode  3 , when only the radicals are used for cleaning. Accordingly, in the cleaning operation, Argon gas as inert gas for sputter cleaning is introduced into the film-forming chamber  10  besides NF 3  gas including fluorine radicals. A high frequency electric power of 27.12 MHz frequency and 0.15 W/cm 2  electric power density is applied to the introduced gases from the high frequency electric power source  8  which is used also for film-forming.  
         [0036]    Thus, the argon gas is electrically devided into Ar ions (Ar + ) and electrons. The film-forming chamber  10  is cleaned both with the chemical reaction by radicals and with Ar ions sputtering. It can be more uniformly cleaned, and the cleaning efficiency can be improved. The Ar gas is introduced into the film-forming chamber  10  through the cleaning gas introducing pipe  23  or through the film-forming gas introducing gas  15 . Insteads, it may be introduced through a special pipe for spluttering gas.  
         [0037]    Next, there will be described a second embodiment of this invention. SiO 2  film is formed in the same plasma film-forming apparatus  20  as in the first embodiment. For example, SiH 4  gas and N 2 O gas are used as a film-forming gas. The SiO 2  film is formed on the substrate  9  in the same manner as the first embodiment.  
         [0038]    In the cleaning operation of the film-forming chamber  10 , NF 3  gas containing fluorine radicals is directly introduced into the film-forming chamber  10  from the gas introducing pipe  23 . The fluorine radicals reacts chemically with the SiO 2  film to be cleaned. Thus, the film-forming chamber  10  is cleaned.  
         [0039]    Although the radicals are effectively introduced into the film-forming chamber  10 , a sufficient cleaning rate cannot be obtained for SiO 2  film. Accordingly, Ar gas is introduced into the film-forming chamber  10 . The high frequency electric power is applied to the Ar gas from the cathode electrode  4  by the high frequency electric power source  8 . Ar ions are produced. The film-forming chamber  10  is cleaned also by the Ar ion sputtering.  
         [0040]    [0040]FIG. 6 shows the comparison results of the cleaning of the SiO 2  films among the cleaning only by the radicals (fluorine radicals), the cleaning only by the ions (Ar + ) and the cleaning by the ions (Ar + ) and radicals. When the film-forming chamber  10  was cleaned only by the ions, the high frequency electric power was applied to the cathode electrode  4  at the frequency of 27.12 MHz and the power density of 0.67W/cm 2 . When the film-forming chamber  10  was cleaned by the radicals and ions, the high frequency electric power was applied to the cathode electrode  4  at the same frequency as that of the cleaning only by the ions, and at the half power density of that of the cleaning only by the ions.  
         [0041]    The cleaning rate of the cleaning operation only by the radicals are low. However, that of the cleaning operation by combination of the radicals and ions is substantially equal to that of the cleaning operation only by the ions. The required power of high frequency in the cleaning operation by combination of the radicals and ions is about half of that in the cleaning operation only by the ions. Accordingly, the plasma damage to the shower plate  5  can be reduced, and so the shower plate  5  can be prevented from being deteriorated.  
         [0042]    Next, there will be described a third embodiment of this invention. Parts in this embodiment which correspond to those in the first and the second embodiments, are denoted by the same reference numerals, the detailed description of which will be omitted.  
         [0043]    [0043]FIG. 3 shows a vertical cross-sectional view of a plasma film-forming apparatus  30  according to this embodiment. FIG. 4 shows a cross-sectional view taken along the lines IV-IV in FIG. 3. It is used for a large-sized substrate.  
         [0044]    In the first and the second embodiments as shown in FIG. 2, the radicals are introduced laterally into the film-forming chamber  10 . Accordingly, portions nearer to the outlet of the gas-introducing pipe  23  are sooner cleaned. When the size of the substrate  9  is about 400 mm×500 mm, there is no problem. However, when the size of the substrate is large as 730 mm×920 mm, the cleaning rate is generally lowered. The film-forming chamber  10  is large-sized for a large substrate. The cleaning rates are considerably different between portions near to the outlet of the gas-introducing pipe  23  and portions farther from that. Totally, the cleaning rate is lowered.  
         [0045]    In this embodiment, a first cleaning-gas introducing pipe  33   a  is connected to one side wall  2   a  of the film-forming chamber  10 , and another cleaning-gas introducing pipe  33   b  is connected to another side wall  33   b  of the film-forming chamber  10 , which is facing to the one side wall  2   b.    
         [0046]    The cleaning-gas is introduced into the film-forming chamber  10  from the two outlets. As shown in FIG. 3, the first and second cleaning-gas introducing pipes  33   a  and  33   b  are shifted from the centers of the walls in opposite directions. The cleaning-gas is more uniformly introduced into the film-forming chamber  10  than in the case that the cleaning-gas introducing pipes  33   a  and  33   b  are connected to the walls, facing to each other. Of course, they may be connected to the walls, facing to each other.  
         [0047]    The cleaning rate of the large film-forming chamber  10  with the arrangement of FIG. 4 is about three times as high as that in the case that only one cleaning-gas introducing pipe  22  is connected to the one side wall as in FIG. 2. The high frequency electric power source of about 100 to 1000 KHz for producing radicals, is simple in constructions and small-sized in comparison with the micro-wave generator.  
         [0048]    The price of the former is one third as low as that of the latter. Accordingly, plural radical-producing sources can be easily arranged, and the manufacturing cost is not so high.  
         [0049]    While the preferred embodiments have been described, variations thereto will occur to those skilled in the art within the scope of the present inventive concepts which are delineated by the following claims.  
         [0050]    For example, in the above embodiments, NF 3  is used as the cleaning-gas. However, it is not limited to NF 3 , but CF 4 , C 2 F 6 , C 3 F 3 , CHF 3 , SF 6  etc. may be used as the cleaning-gas. Inert gas for sputtering cleaning is not limited to Ar. Further, the film to be formed in the substrate or to be cleaned, is not limited to SiNx and SiO 2 . Further, the high frequency power to be applied to the cathode electrode  4 , is not limited to the above frequency and to the above electric power density. Frequency between 10 to 100 MHZ may be adjusted.  
         [0051]    Electric power density between 0.03 to 0.7 W/cm 2  may be adjusted.  
         [0052]    In the third embodiment, two cleaning-gas introducing pipes are connected to the film-forming chamber  10 . The number of the connected pipes is not limited to two, and more than two. The wall connecting the cleaning-gas introducing pipe, is not limited to the side wall, and may be upper wall or bottom wall, of the vacuum tank  2 .