Patent Publication Number: US-2009239383-A1

Title: Manufacturing method of semiconductor device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 200B-0724 90, filed on Mar. 19, 2008; the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a manufacturing method of a semiconductor device which includes a substrate. 
     2. Description of the Related Art 
     Conventionally, plasma processing apparatuses for performing plasma processing such as plasma CVD processing and plasma etching processing on a substrate have been broadly used in manufacturing steps of semiconductor devices. A plasma processing apparatus includes: a lower electrode including a placing face on which a substrate is placed; and an upper electrode facing the lower electrode while being located above the lower electrode. When plasma processing is performed on a substrate placed on the placing face, application of a voltage between the lower and upper electrodes generates plasma in a processing space provided between the lower and upper electrodes. 
     In order to uniformly perform the plasma processing on a substrate by use of the above-described plasma processing apparatus, it is desirable that uniform plasma be generated in the processing space. Accordingly, for the purpose of reducing non-uniformity of plasma in the processing space, there has been a proposal of providing, on a placing table, an annular member surrounding a periphery of a substrate (refer to Japanese Patent Application Laid-open Publication No. 2002-241946). 
     However, since the upper electrode is heated to a high temperature by being directly exposed to plasma, a central portion of the upper electrode is deformed into a shape that is convex downward. Accordingly, a distance between the upper and lower electrodes is changed during plasma processing, and a non-uniform electric field is formed in the processing space. As a result, uniform plasma cannot be generated in the processing space, whereby it has been difficult to uniformly perform plasma processing on a substrate. In particular, this problem has been more severe with a larger substrate. 
     SUMMARY OF THE INVENTION 
     The present invention was made in consideration of the above-described problem, and an object thereof is to provide a manufacturing method of a semiconductor device by which plasma processing can be uniformly performed on a substrate. 
     A manufacturing method of a semiconductor device according to one aspect of the present invention is summarized as a manufacturing method of a semiconductor device including a substrate, the manufacturing method including the step of performing plasma processing on the substrate by using a plasma processing apparatus. In the method, the plasma processing apparatus includes: a first electrode including a placing face on which the substrate is placed; a flat plate-shaped second electrode provided so as to face the first electrode; and an auxiliary electrode provided annularly along a periphery of the first electrode on a lateral side of the first electrode, and, in the step of performing plasma processing: a potential of the first electrode is set lower than a potential of the flat plate-shaped second electrode; and a potential of the auxiliary electrode is set lower than the potential of the flat plate-shaped second electrode. 
     According to the manufacturing method of the semiconductor device of the one aspect of the present invention, even when a central portion of the upper electrode is deformed into a shape convex toward the lower electrode by having the upper electrode heated, an electric field intensity between an edge portion of the upper electrode and the lower electrode can be intensified. Therefore, electric field intensities in a processing space formed between the upper electrode and the lower electrode can be suppressed from becoming non-uniform. As a result, plasma can be uniformly generated in the processing space, whereby plasma processing can be uniformly performed on the substrate. 
     In the one aspect of the present invention, the first electrode and the flat plate-shaped second electrode may overlap each other on a projection plane substantially parallel to the placing table, the auxiliary electrode may be located outside the flat plate-shaped second electrode on the projection plane; and, in the step of performing plasma processing, the potential of the auxiliary electrode may be set lower than the potential of the first electrode. 
     In the one aspect of the present invention, the auxiliary electrode may be located inside the flat plate-shaped second electrode on a projection plane substantially parallel to the placing table; and, in the step of performing plasma processing, the potential of the auxiliary electrode is set higher than the potential of the first electrode. 
     According to the present invention, a semiconductor device manufacturing method by which plasma processing can be uniformly performed on a substrate can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a plasma processing apparatus  100  according to a first embodiment of the present invention. 
         FIG. 2  is a projection view of a lower electrode  10 , an upper electrode  20  and an auxiliary electrode  30  according to the first embodiment of the present invention. 
         FIG. 3  is a schematic view of a processing space I according to the first embodiment of the present invention. 
         FIG. 4  is a schematic view of the processing space I according to the first embodiment of the present invention. 
         FIG. 5  is a cross-sectional view of a plasma processing apparatus  200  according to a second embodiment of the present invention. 
         FIG. 6  is a projection view of the lower electrode  10 , upper electrode  20  and auxiliary electrode  30  according to the second embodiment of the present invention. 
         FIG. 7  is a schematic view of the processing space I according to the second embodiment of the present invention. 
         FIG. 8  is a schematic view of the processing space I according to the second embodiment of the present invention. 
         FIG. 9  is a schematic view showing an electrode configuration of Comparative Example 1. 
         FIGS. 10A and 10B  are view and graph, respectively, showing simulation results of Comparative Example 1. 
         FIG. 11  is a schematic view showing an electrode configuration of Example 1. 
         FIGS. 12A and 12B  are view and graph, respectively, showing a simulation result of Example 1. 
         FIGS. 13A and 13B  are view and graph, respectively, showing a simulation result of Example 2. 
         FIG. 14  is a schematic view showing an electrode configuration of Comparative Example 2. 
         FIGS. 15A and 15B  are view and graph, respectively, showing a simulation result of Comparative Example 2. 
         FIG. 16  is a schematic view showing an electrode configuration of Example 3. 
         FIGS. 17A and 17B  are view and graph, respectively, showing simulation results of Example 3. 
         FIGS. 18A and 18B  are view and graph, respectively, showing simulation results of Example 4. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, embodiments of the present invention will be described by use of the drawings. In the following description of the drawings, the same or corresponding elements are denoted with the same or corresponding reference numerals. However, the drawings are schematic, and it should be noted that proportions between dimensions and the like are not actual. Consequently, specific dimensions and the like should be determined in consideration of the following description. Additionally, it is obvious that the drawings include parts which differ in relations and proportions between the drawings. 
     First Embodiment 
     Configuration of Plasma Processing Apparatus 
     With reference to  FIG. 1 , a configuration of a plasma processing apparatus  100  according to a first embodiment of the present invention will be described.  FIG. 1  is a cross-sectional view of the plasma processing apparatus  100 . 
     In this embodiment, the plasma processing apparatus  100 , which performs deposition processing on a substrate S by using a plasma enhanced chemical vapor deposition (PECVD) method, will be described as one example of plasma processing apparatuses. 
     The plasma processing apparatus  100  includes a vacuum chamber  1 , a lower electrode  10 , an upper electrode  20 , an auxiliary electrode  30 , a gas supply passage  40  and a gas discharge passage  50 . 
     The vacuum chamber  1  is a processing container obtained by molding, for example, aluminum into a cylinder. 
     The lower electrode  10  functions as a placing table including a placing face  10 A on which the substrate S is placed. The lower electrode  10  is supported by a support portion  11  so as to be vertically movable. 
     Additionally, the lower electrode  10  is connected to ground through the support portion  11 , thereby functioning as an anode electrode. In an inside of the lower electrode  10 , a heating mechanism (unillustrated) formed of, for example, molybdenum wire is provided. When plasma processing is performed on the substrate S, the lower electrode  10  is heated by the heating mechanism. The lower electrode  10  is formed of general electrically-conductive material such as carbon, graphite or aluminum. 
     The upper electrode  20  is provided over the lower electrode  10  so as to face the lower electrode  10 . The upper electrode  20  is supported by the support portion  21  so as to be suspended from a ceiling of the vacuum chamber  1 . Multiple gas supply openings  20   a  are formed in the upper electrode  20 . Deposition gas and plasma-forming gas pass through a later-described gas supply passage  40 , and are supplied to the inside of the vacuum chamber  1  from the plurality of gas supply openings  20   a . Consequently, the upper electrode  20  functions as a gas supply mechanism. 
     Additionally, the upper electrode  20  functions as a cathode electrode if a direct-current voltage or a high-frequency voltage is applied, as a bias voltage, to the upper electrode  20  by use of an unillustrated power supply device. Consequently, a potential of the lower electrode  10  is lower than a potential of the upper electrode  20 . Plasma is generated in a processing space I provided between the lower electrode  10  and the upper electrode  20  by applying a bias voltage to the upper electrode  20 . The upper electrode  20  is formed of general electrically-conductive material such as carbon, graphite or aluminum. 
     An auxiliary electrode  30  is provided in a predetermined position relative to the lower electrode  10 , the position being on the lateral side of the lower electrode  10 . Additionally, the auxiliary electrode  30  is provided annularly along a periphery of the lower electrode  10 . 
     Additionally, if a direct-current voltage or a high-frequency voltage is performed on the auxiliary electrode  30  by use of the unillustrated power supply device, a potential of the auxiliary electrode  30  becomes lower than the potential of the upper electrode  20 . The auxiliary electrode  30  is formed of general electrically-conductive material such as carbon, graphite or aluminum. 
       FIG. 2  is a projection view in which the lower electrode  10 , the upper electrode  20  and the auxiliary electrode  30  are projected on a projection plane substantially parallel to the placing face  10 A of the lower electrode  10 . As shown in  FIG. 2 , planer shapes of the lower electrode  10  and the upper electrode  20  have substantially equal dimensions, whereby the lower electrode  10  and the upper electrode  20  overlap each other. The auxiliary electrode  30  surrounds peripheries of the lower electrode  10  and the upper electrode  20 . 
     Note that planar shapes of the lower electrode  10 , the upper electrode  20  and the auxiliary electrode  30  are not limited to rectangles, and may be round shapes or the like. Additionally, a cross-sectional shape of the auxiliary electrode  30  is not limited to a rectangle, and may be a round shape or a void shape. 
     The gas supply passage  40  is a gas supply tube used for supplying a deposition gas and a plasma-forming gas to the inside of the vacuum chamber  1 . Although only one gas supply passage  40  is provided in  FIG. 1 , a gas supply passage supplying a deposition gas and a gas supply passage supplying a plasma-forming gas may be provided separately. 
     A gas discharge passage  50  is a gas discharge tube used for discharging gas in the vacuum chamber  1  so that interior of the vacuum chamber  1  becomes a vacuum. 
     (Electric Fields Formed in Processing Space) 
     Next, electric fields formed in the processing space I will be described with reference to  FIGS. 3 and 4 .  FIGS. 3 and 4  are schematic views of the processing space I. Note that  FIGS. 3 and 4  show a state where a central portion of the upper electrode  20  is deformed into a shape convex toward the lower electrode  10  by having the upper electrode  20  heated to high temperature. 
     1) A Case where a Potential of the Auxiliary Electrode  30  is Lower than that of the Lower Electrode  10 : 
     With reference to  FIG. 3 , a case where a potential of the auxiliary electrode  30  is lower than that of the lower electrode  10  will be described. A relationship among the lower electrode  10 , the upper electrode  20  and the auxiliary electrode  30  in terms of potentials is the upper electrode  20 &gt;the lower electrode  10 &gt;the auxiliary electrode  30 . 
     As shown in  FIG. 3 , when the central portion of the upper electrode  20  is deformed into a shape convex toward the lower electrode  10 , a distance between the central portion of the upper electrode  20  and the lower electrode  10  becomes shorter. Here, a relationship indicated by expression (1) is established among an electric field intensity (E), a distance (L) between the upper electrode  20  and the lower electrode  10 , and a voltage (V). 
       E∝(V/L)  (1) 
     Consequently, an electric field intensity between an edge portion of the upper electrode  20  and the lower electrode  10  becomes relatively weak as compared to that between the central portion of the upper electrode  20  and the lower electrode  10 . 
     If the potential of the auxiliary electrode  30  is set lower than those of the upper electrode  20  and the lower electrode  10  in this condition, a new electric field is formed between the auxiliary electrode  30  and the upper electrode  20 . Between the auxiliary electrode  30  and the upper electrode  20 , a strong electric field extending into space over the lower electrode  10  is formed. Specifically, electric field intensities in a processing space I 2  and processing space I 3  which are shown in  FIG. 3  are intensified. 
     2) A Case where a Potential of the Auxiliary Electrode  30  is Higher than that of the Lower Electrode  10 : 
     With reference to  FIG. 4 , a case where a potential of the auxiliary electrode  30  is higher than that of the lower electrode  10  will be described. A relationship among the lower electrode  10 , the upper electrode  20  and the auxiliary electrode  30  in terms of potentials is the upper electrode  20 &gt;the auxiliary electrode  30 &gt;the lower electrode  10 . 
     As has been described above, when the central portion of the upper electrode  20  is deformed into a shape convex toward the lower electrode  10 , an electric field intensity between the edge portion of the upper electrode  20  and the lower electrode  10  becomes relatively weak as compared to that between the central portion of the upper electrode  20  and the lower electrode  10 . 
     If the potential of the auxiliary electrode  30  is set lower than the upper electrode  20  and higher than the lower electrode  10  in this condition, a new electric field is formed between the auxiliary electrode  30  and the lower electrode  10 . Specifically, electric field intensities in a processing space I 4  and processing space I 5  which are shown in  FIG. 4  are intensified. Note that the electric field newly formed between the auxiliary electrode  30  and the lower electrode  10  concentrates between the auxiliary electrode  30  and an edge portion of the lower electrode  10 . 
     (Functions and Effects) 
     The plasma processing apparatus  100  according to this embodiment includes the auxiliary electrode  30  provided annularly along the periphery of the lower electrode  10  on the lateral side thereof. When plasma processing is performed on the substrate S, a potential of the lower electrode  10  is set lower than a potential of the upper electrode  20 , and a potential of the auxiliary electrode  30  is set lower than the potential of the upper electrode  20 . 
     Consequently, even when the central portion of the upper electrode  20  is deformed in a shape convex toward the lower electrode  10  by having the upper electrode  20  heated, an electric field intensity between the edge portion of the upper electrode  20  and the lower electrode  10  can be intensified. Therefore, electric field intensities in the processing space I can be suppressed from becoming non-uniform. As a result, plasma can be uniformly generated in the processing space I, whereby plasma processing can be uniformly performed on the substrate S. 
     Additionally, the potential of the auxiliary electrode  30  is set lower than the potential of the lower electrode  10  in a case where, the auxiliary electrode  30  is located outside the upper electrode  20  while the lower electrode  10  and the upper electrode  20  overlap each other on the projection plane substantially parallel to the placing face  10 A. 
     In this case, a strong electric field extending into space over the lower electrode  10  can be formed between the auxiliary electrode  30  and the upper electrode  20 . As a result, plasma can be more uniformly generated in the processing space I, whereby plasma processing can be more uniformly performed on the substrate S. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described with reference to  FIGS. 5 and 6 .  FIG. 5  is a cross-sectional view of a plasma processing apparatus  200  according to this embodiment.  FIG. 6  is a projection view in which the lower electrode  10 , the upper electrode  20  and the auxiliary electrode  30  are projected on a projection plane substantially parallel to the placing face  10 A of the lower electrode  10 . 
     A difference between this embodiment and the above-mentioned first embodiment is in that the auxiliary electrode  30  is provided inside the upper electrode  20  on projection plane. Since there are no other differences therebetween, the above difference will be mainly described below. 
     (Configuration of Plasma Processing Apparatus) 
     The lower electrode  10  functions as an anode electrode by being connected to ground through the support portion  11 . The upper electrode  20  functions as a cathode electrode if a bias voltage is performed on the upper electrode  20 . Consequently, a potential of the lower electrode  10  is lower than a potential of the upper electrode  20 . 
     The auxiliary electrode  30  is provided in a predetermined relative to the lower electrode  10 , the position being on the lateral side of the lower electrode  10 . Additionally, the auxiliary electrode  30  is provided annularly along a periphery of the lower electrode  10 . 
     As shown in  FIG. 5 , the lower electrode  10  and the auxiliary electrode  30  are located inside the upper electrode  20  on projection plane. The auxiliary electrode  30  surrounds the periphery of the lower electrode  10 . 
     (Electric Fields Formed in Processing Space) 
     Next, electric fields formed in the processing space I will be described with reference to  FIGS. 7 and 8 .  FIGS. 7 and 8  are schematic views of the processing space I. Note that  FIGS. 7 and 8  show a state where a central portion of the upper electrode  20  is deformed into a shape convex toward the lower electrode  10  by having the upper electrode  20  heated. 
     1) A Case where a Potential of the Auxiliary Electrode  30  is Lower than that of the Lower Electrode  10 : 
     With reference to  FIG. 7 , a case where a potential of the auxiliary electrode  30  is lower than that of the lower electrode  10  will be described. A relationship among the lower electrode  10 , the upper electrode  20  and the auxiliary electrode  30  in terms of potentials is the upper electrode  20 &gt;the lower electrode  10 &gt;the auxiliary electrode  30 . 
     As shown in  FIG. 7 , when the central portion of the upper electrode  20  is deformed into a shape convex toward the lower electrode  10 , an electric field intensity between an edge portion of the upper electrode  20  and the lower electrode  10  becomes relatively weak as compared to that between the central portion of the upper electrode  20  and the lower electrode  10  in accordance with expression (1) mentioned above. 
     If the potential of the auxiliary electrode  30  is set lower than those of the upper electrode  20  and the lower electrode  10  in this condition, a new electric field is formed between the auxiliary electrode  30  and the upper electrode  20 . Accordingly, electric field intensities in a processing space I 6  and processing space I 7  which are shown in  FIG. 7  are intensified. 
     Note that, since an electric field directed from the lower electrode  10  toward the auxiliary electrode  30  is formed between the auxiliary electrode  30  and the lower electrode  10 , electric field intensities in a processing space I 6 ′ and processing space I 7 ′ which are shown in  FIG. 7  tends not to be intensified. 
     2) A Case where a Potential of the Auxiliary-Electrode  30  is Higher than that of the Lower Electrode  10 : 
     With reference to  FIG. 8 , a case where a potential of the auxiliary electrode  30  is higher than that of the lower electrode  10  will be described. A relationship among the lower electrode  10 , the upper electrode  20  and the auxiliary electrode  30  in terms of potentials is the upper electrode  20 &gt;the auxiliary electrode  30 &gt;the lower electrode  10 . 
     As has been described above, when the central portion of the upper electrode  20  is deformed into a shape convex toward the lower electrode  10 , an electric field intensity between the edge portion of the upper electrode  20  and the lower electrode  10  becomes relatively weak as compared to that between the central portion of the upper electrode  20  and the lower electrode  10 . 
     When a potential of the auxiliary electrode  30  is set lower than the upper electrode  20  and higher than the lower electrode  10  in this condition, a new electric field directed from the auxiliary electrode  30  toward the lower electrode  10  is formed. Consequently, electric field intensities in processing space I 8  and processing space I 9  which are shown in  FIG. 7  are intensified. 
     (Functions and Effects) 
     By the plasma processing apparatus  200  according to this embodiment, an electric field intensity between the edge portion of the upper electrode  20  and the lower electrode  10  can be intensified as in the case of the plasma processing apparatus  100  according to the above-mentioned first embodiment. Therefore, electric field intensities in the processing space I can be suppressed from being non-uniform. As a result, plasma can be uniformly generated in the processing space I, whereby plasma processing can be uniformly performed on the substrate S. 
     Additionally, a potential of the auxiliary electrode  30  is set higher than a potential of the lower electrode  10  in a case where the auxiliary electrode  30  is located inside the upper electrode  20  on the projection plane substantially parallel to the placing face  10 A. 
     In this case, an electric field directed from the auxiliary electrode  30  to the lower electrode  10  can be formed. As a result, plasma can be more uniformly generated in the processing space I, whereby plasma processing can be more uniformly performed on the substrate S. 
     Other Embodiments 
     While the present invention has been described through the abovementioned embodiments, discussions and drawings, which constitute parts of this disclosure, should not be understood as limiting this invention. Through this disclosure, various alternative embodiments, examples and operational technologies will be apparent to those skilled in the art. 
     For example, in the above embodiments, while description has been given to the cases where deposition processing is performed on the substrate S by using the plasma processing apparatuses  100  and  200 , other plasma processing such as plasma etching processing may be performed by the plasma processing apparatuses  100  and  200 . 
     Additionally, while a potential of the lower electrode  10  is set to a ground potential in the above-mentioned embodiments by having the lower electrode  10  connected to the ground, it is only necessary that a potential of the lower electrode  10  be lower than that of the upper electrode  20 . 
     Additionally, while levels of the lower electrode  10  and the auxiliary electrode  30  are set equal to each other in the abovementioned first embodiment, levels of the lower electrode  10  and the auxiliary electrode  30  may be different from each other. Likewise, while the auxiliary electrode  30  is provided in a position higher than the lower electrode  10  in the abovementioned second embodiment, the auxiliary electrode  30  may be provided in a position lower than the lower electrode  10 . 
     Additionally, while the auxiliary electrode  30  is fixed to the inside of the vacuum chamber  1  through a support portion  31  in the abovementioned embodiments, the auxiliary electrode  30  may be supported so as to be vertically movable. 
     Additionally, while the lower electrode  10  and the upper electrode  20  have been described as examples of a first electrode and a second electrode of the present invention, respectively, in the abovementioned embodiments, arrangement of the first electrode and the second electrode is not limited to the above arrangement. That is, the first electrode and the second electrode may be set standing substantially vertically to a horizontal face, or the first electrode may be arranged over the second electrode. 
     (Electric Field Intensities Simulations) 
     Next, description will be given of simulative measurement which was carried out with respect to electric field intensities in the processing space I by use of a publicly available two-dimensional electric field simulator (retrieved on Mar. 10, 2008 through URL: http://www.ansoft.com). 
     Comparative Example 1 
     An electrode configuration according to Comparative Example 1 is schematically shown in  FIG. 9 . A simulation result in a case where standardized voltage values of the lower electrode  10  and the upper electrode  20  were set to 0 and +100, respectively, in the configuration shown in  FIG. 9  is shown in  FIGS. 10A and 10B . 
       FIG. 10A  is a schematic view of an electric field formed in the processing space I.  FIG. 10B  is a graph showing a relationship between a standardized electric field intensity value in the processing space I and a distance from the center of the processing space I. 
     Example 1 
     An electrode configuration according to Example 1 is schematically shown in  FIG. 11 . A simulation result in a case where standardized voltage values of the lower electrode  10 , the upper electrode  20  and the auxiliary electrode  30  were set to 0, +100 and −50, respectively, in the configuration shown in  FIG. 11  is shown in  FIGS. 12A and 12B . 
     Example 2 
     An electrode configuration according to Example 2 is the same as one shown in  FIG. 11 . A simulation result in a case where standardized voltage values of the lower electrode  10 , the upper electrode  20  and the auxiliary electrode  30  were set to 0, +100 and +50, respectively, in the configuration shown in  FIG. 11  is shown in  FIGS. 13A and 13B . 
     Comparative Example 2 
     An electrode configuration according to Comparative Example 2 is schematically shown in  FIG. 14 . A simulation result in a case where standardized voltage values of the lower electrode  10  and the upper electrode  20  were set to 0 and +100, respectively, in the configuration shown in  FIG. 14  is shown in  FIGS. 15A and 15B . 
       FIG. 15A  is a schematic view of an electric field formed in the processing space I.  FIG. 15B  is a graph showing a relationship between a standardized electric field intensity value in the processing space I and a distance from the center of the processing space I. 
     Example 3 
     An electrode configuration according to Example 3 is schematically shown in  FIG. 16 . A simulation result in a case where standardized voltage values of the lower electrode  10 , the upper electrode  20  and the auxiliary electrode  30  were set to 0, +100 and −80, respectively, in the configuration shown in  FIG. 16  is shown in  FIGS. 17A and 17B . 
     Example 4 
     An electrode configuration according to Example 4 is the same as one shown in  FIG. 16 . A simulation result in a case where standardized voltage values of the lower electrode  10 , the upper electrode  20  and the auxiliary electrode  30  were set to 0, +100 and +80, respectively, in the configuration shown in  FIG. 16  is shown in  FIGS. 18A and 18B . 
     (Consideration) 
     Examples 1 and 2 were capable of forming more uniform electric fields than Comparative Example 1. This is because each of Examples 1 and 2 was capable of intensifying electric fields in an edge portion of the processing space I by being provided with the auxiliary electrode  30 . 
     Additionally, Example 1 was capable of forming a more uniform electric field than Example 2. This is because, while electric field intensities in the processing space I 2  and the processing space I 3  which are shown in  FIG. 3  were intensified in Example 1, a new electric field was locally formed around an edge portion of the lower electrode  10  in Example 2. Based on these results, it was confirmed that, in a case where the auxiliary electrode  30  is located outside the upper electrode  20  on the projection plane, it is preferable that a potential of the auxiliary electrode  30  be lower than a potential of the lower electrode  10 . 
     On the other hand, Examples 3 and 4 were capable of forming more uniform electric fields than Comparative Example 2. This is because provision of the auxiliary electrode  30  made it possible to intensify an electric field in an edge portion of the processing space I. 
     Additionally, Example 4 was capable of forming a more uniform electric field than Example 3. This is because, while an electric field directed from the auxiliary electrode  30  toward the lower electrode  10  was formed in Example 4, an electric field directed from the lower electrode  10  toward the auxiliary electrode  30  was formed in Example 3. It was therefore hard for electric field intensities in the processing space Id and the processing space I 7 ′ which are shown in FIG.  7  to be intensified in Example 3. Based on these results, it was confirmed that, in a case where the auxiliary electrode  30  is located inside the upper electrode  20  on the projection plane, it is preferable that a potential of the auxiliary electrode  30  be higher than a potential of the lower electrode  10 .