Patent Publication Number: US-2011056912-A1

Title: Plasma processing apparatus and plasma processing method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Japanese Patent Application No. 2009-206890 filed on Sep. 8, 2009 and U.S. Provisional Application Ser. No. 61/252,196 filed on Oct. 16, 2009, the entire disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates to a plasma processing apparatus and a plasma processing method that perform a process on a substrate such as a semiconductor wafer, a FPD (Flat Panel Display) substrate, a solar cell substrate by generating plasma in a processing chamber. 
     BACKGROUND OF THE INVENTION 
     When a plasma process such as sputtering, etching, and film formation is performed on a substrate such as a semiconductor wafer (hereinafter, simply referred to as “wafer”), there has been used a plasma processing apparatus which generates a cusp magnetic field surrounding plasma in a processing chamber in order to perform a uniform process on a process surface of the wafer. 
     In this plasma processing apparatus, a so-called multi-pole ring magnet in which magnets having different polarities are alternately arranged in a circumferential direction is positioned around the processing chamber, thereby generating the cusp magnetic field. Since the plasma can be confined by this cusp magnetic field, uniformity in the plasma process on the wafer can be improved. 
     Conventionally, it has been known that in order to improve uniformity in a process at a central portion and an edge portion of a wafer, two multi-pole ring magnets are vertically arranged and a gap therebetween is controlled or these multi-pole ring magnets are rotated (see, for example, Patent Documents 1 and 2). 
     Patent Document 1: Japanese Patent Laid-open Publication No. 2003-234331 
     Patent Document 2: Japanese Patent Laid-open Publication No. 2000-306845 
     Patent Document 3: Japanese Patent Laid-open Publication No. 2004-111334 
     However, as described in Patent Documents 1 and 2, in a plasma processing apparatus in which ring magnets are vertically arranged, depending on vertical arrangement of polarities, magnetic force lines generating a cusp magnetic field may have a region where a magnetic field perpendicular to a sidewall of the processing chamber is greater than a magnetic field parallel thereto. In this case, since a diffusion coefficient of plasma in a diametric direction (in a direction crossing the magnetic field parallel to the sidewall) cannot be reduced sufficiently, the plasma cannot be confined sufficiently. Accordingly, process uniformity in a central portion and an edge portion of a wafer may be decreased and damage to the sidewall may be caused. 
     Further, in Patent Document 3, it is described that two ring magnets are rotated relative to each other, but they are dipole ring magnets. In this dipole ring magnet, multiple anisotropic segment magnets are arranged in a ring shape around a processing chamber while slightly changing their magnetization directions and a uniform horizontal magnetic field is formed on the entire wafer. Here, a high frequency electric field orthogonal to a process surface of the wafer is applied and a drift motion of electrons at this time is used to perform a plasma process such as etching with very high efficiency. 
     In case of using the dipole ring magnet, process uniformity is highly influenced by a direction of a magnetic field formed on a wafer. Therefore, circumstances are very different from the multi-pole ring magnet in which a magnetic field is hardly formed on a wafer. For this reason, conception of the dipole ring magnet cannot be applied to the multi-pole ring magnet. 
     Accordingly, the present invention has been conceived in view of the foregoing problem and the present invention provides a plasma processing apparatus and a plasma processing method capable of improving uniformity in a plasma process by increasing a plasma confining effect by a cusp magnetic field in a circumferential direction. 
     BRIEF SUMMARY OF THE INVENTION 
     In order to solve the above-mentioned problem, in accordance with one aspect of the present disclosure, there is provided a plasma processing apparatus which performs a process on a substrate by generating plasma of a processing gas in a depressurized processing chamber. The apparatus includes a mounting table provided in the processing chamber and mounting the substrate thereon; a processing gas inlet unit that introduces the processing gas into the processing chamber; a gas exhaust unit that exhausts and depressurizes an inside of the processing chamber; and a magnetic field generation unit including two magnet rings vertically spaced from each other and arranged along a circumferential direction of the processing chamber. Each of the magnet rings includes multiple segments of which magnetic poles are alternately reversed one by one or group by group along a circumferential direction of an inner surface of the magnet ring. Arrangement of upper and lower magnetic poles is changed by rotating one magnet ring in a circumferential direction with respect to the other magnet ring. Here, by way of example, the segments may be composed of permanent magnet segments or magnetic pole segments of electromagnets. 
     In this case, if the number of consecutively arranged segments having a same polarity is m, the one magnet ring may be rotated by 1 segment to (2m−1) segments in a circumferential direction and a plasma process may be performed on the substrate for each rotation. Then, the number of the segments in a case where the best result of the process on the substrate is obtained may be stored in a storage unit as a rotation amount. Further, before a plasma process may be performed on the substrate, the one magnet ring may be rotated as much as the number of the segments as the rotation amount in the circumferential direction with respect to the other magnet ring. 
     Further, the apparatus may further include a ring rotation amount adjusting mechanism that rotates the one magnet ring in the circumferential direction with respect to the other magnet ring; and a controller that controls the ring rotation amount adjusting mechanism. Here, a rotation amount may be obtained for each of processing conditions of the plasma process and the rotation amount may be stored in the storage unit in relation with each of the processing conditions. Before the plasma process is performed on the substrate based on the processing condition, the controller may read the rotation amount related to the processing condition and control the ring rotation amount adjusting mechanism based on the read rotation amount so as to adjust a rotation amount of the one magnet ring in the circumferential direction. 
     In this case, the apparatus may further include a ring gap adjusting mechanism that adjusts a gap between the magnet rings in a vertical direction. The storage unit may store a gap adjustment amount together with the processing condition and the rotation amount. Before the plasma process is performed on the substrate based on the processing condition, the controller may read the gap adjustment amount related to the processing condition and control the ring gap adjusting mechanism based on the read gap adjustment amount so as to adjust a gap in the vertical direction. 
     In order to solve the above-mentioned problem, in accordance with another aspect of the present disclosure, there is provided a plasma processing method of a plasma processing apparatus which performs a process on a substrate by generating plasma of a processing gas in a depressurized processing chamber. The plasma processing apparatus includes a mounting table provided in the processing chamber and mounting the substrate thereon; a processing gas inlet unit that introduces the processing gas into the processing chamber; a gas exhaust unit that exhausts and depressurizes an inside of the processing chamber; and a magnetic field generation unit including two magnet rings vertically spaced from each other and arranged along a circumferential direction of the processing chamber, each of the magnet rings includes multiple segments of which magnetic poles are alternately reversed one by one or group by group along a circumferential direction of an inner surface of the magnet ring; a ring rotation amount adjusting mechanism that rotates one magnet ring in a circumferential direction with respect to the other magnet ring; and a storage unit that stores a rotation amount in relation with each of processing conditions, the rotation amount being obtained for each of the processing conditions of the plasma process. The method includes before the plasma process is performed on the substrate based on each of the processing conditions, reading a rotation amount related to the processing condition; and controlling the ring rotation amount adjusting mechanism based on the read rotation amount so as to adjust a rotation amount of the one magnet ring in the circumferential direction, thereby rotating upper and lower magnetic poles as much as the rotation amount. Here, by way of example, the segments may be composed of permanent magnet segments or magnetic pole segments of electromagnets. 
     In this case, if the number of consecutively arranged segments having a same polarity is m, the one magnet ring may be rotated by 1 segment to (2m−1) segments in a circumferential direction and a plasma process may be performed on the substrate for each rotation. The rotation amount related to each of the processing condition may be the number of the segments in a case where the best result of the process on the substrate is obtained. 
     Further, the plasma processing apparatus may further include a ring gap adjusting mechanism for adjusting a gap between the magnet rings in a vertical direction. The method may further include storing a gap adjustment amount together with the processing condition and the rotation amount in the storage unit; and reading the gap adjustment amount related to the processing condition before the plasma process is performed on the substrate based on the processing condition and controlling the ring gap adjusting mechanism based on the read gap adjustment amount so as to adjust a gap in the vertical direction. 
     In accordance with the present disclosure, by rotating magnetic poles of lower magnet ring with respect to the upper magnet ring, a magnetic field perpendicular to a sidewall of a processing chamber can be decreased and a magnetic field parallel to the sidewall can be increased. Accordingly, it is possible to suppress diffusion of plasma over the whole circumference, and, thus, a plasma confining effect by a cusp magnetic field can be increased and uniformity in a substrate process can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may best be understood by reference to the following description taken in conjunction with the following figures: 
         FIG. 1  is a cross sectional view showing a configuration example of a plasma processing apparatus in accordance with an embodiment of the present invention; 
         FIG. 2A  is a perspective view showing a schematic configuration of a magnet ring in accordance with this embodiment in which there is no rotation amount in a circumferential direction; 
         FIG. 2B  is a perspective view showing a schematic configuration of the magnet ring in this embodiment in which there is a rotation amount in a circumferential direction; 
         FIG. 3A  is a cross sectional view for explaining a case in which a ring gap in a vertical direction is increased by a ring gap adjusting mechanism in this embodiment; 
         FIG. 3B  is a cross sectional view for explaining a case in which a ring gap in a vertical direction is decreased by the ring gap adjusting mechanism in this embodiment; 
         FIG. 4  is a concept view for explaining a magnetic field formed by the magnet ring in the present embodiment; 
         FIG. 5  is a perspective view for explaining magnetic force lines formed by the magnet ring in the present embodiment; 
         FIG. 6A  shows a case in which a magnetic field perpendicular to a sidewall is strong; 
         FIG. 6B  shows a case in which a magnetic field parallel to the sidewall is strong; 
         FIG. 7  shows a relationship between a rotation amount and vertical arrangement of polarities; 
         FIG. 8  shows a relationship between a distance in a diametric direction and a magnitude |B| of a magnetic field and magnitudes |B r |, |B θ |, and |B Z | of its perpendicular directional components; 
         FIG. 9  shows a relationship between an incident angle of magnetic force lines to a sidewall of a processing chamber and a magnetic flux density; 
         FIG. 10  is a concept view for explaining a suppression effect of plasma diffusion by a cusp magnetic field in the present embodiment; 
         FIG. 11  shows a result of measuring an etching rate when a plasma etching process is performed by changing a rotation amount of the magnet ring in the present embodiment; 
         FIG. 12  shows a configuration example in which a magnet ring is composed of an electromagnet in the present embodiment; and 
         FIG. 13  shows a result of measuring an etching rate when a plasma etching process is performed by changing a rotation amount of the magnet ring illustrated in  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be explained in detail with reference to accompanying drawings. Through the present specification and drawings, parts having substantially same function and configuration will be assigned same reference numerals, and redundant description will be omitted. 
     (Configuration Example of a Plasma Processing Apparatus) 
     Above all, a schematic configuration of a plasma processing apparatus in accordance with an embodiment of the present invention will be explained with reference to the drawings.  FIG. 1  is a cross sectional view showing a schematic configuration of a plasma processing apparatus in accordance with the present embodiment. Herein, there will be explained a plasma processing apparatus  100  configured as a capacitively coupled (parallel plate type) plasma etching apparatus in which two different high frequencies are applied to a lower electrode (a susceptor). 
     The plasma processing apparatus  100  includes a processing chamber  102  having a cylinder-shaped processing vessel made of metal such as aluminum or stainless steel of which a surface is anodically oxidized (alumite treated). The processing chamber  102  is grounded. In the processing chamber  102 , there are provided a circular plate-shaped lower electrode (a susceptor)  110  also serving as a mounting table for mounting a substrate such as a semiconductor wafer W (hereinafter, simply referred to as “wafer”) and an upper electrode  120  also serving as a shower head configured to face the lower electrode  110  and supply a processing gas or a purge gas. 
     The lower electrode  110  is made of, for example, aluminum. The lower electrode  110  is held on an insulating cylindrical holder  106  on a cylindrical member  104  extended in a vertically upward direction from a bottom of the processing chamber  102 . On a top surface of the lower electrode  110 , an electrostatic chuck  112  for holding the wafer W by an electrostatic attracting force is installed. The electrostatic chuck  112  includes an electrostatic chuck electrode  114  made of, for example, a conductive film embedded in an insulating film. The electrostatic chuck electrode  114  is electrically connected with a DC power supply  115 . With this configuration of the electrostatic chuck  112 , the wafer W can be attracted to and held on the electrostatic chuck  112  by a Coulomb force caused by a DC voltage from the DC power supply  115 . 
     Installed within the lower electrode  110  is a cooling unit. By way of example, this cooling unit is configured to circulate and supply a coolant (for example, cooling water) at a predetermined temperature to a cooling reservoir  116  extended in a circumferential direction in the lower electrode  110  from a non-illustrated chiller unit through a coolant line. A processing temperature of the wafer W on the electrostatic chuck  112  can be controlled by the coolant. 
     In the lower electrode  110  and the electrostatic chuck  112 , a heat transfer gas supply line  118  is provided toward a rear surface of the wafer W. A heat transfer gas (a backgas) such as a He gas is introduced through the heat transfer gas supply line  118  and supplied between a top surface of the electrostatic chuck  112  and the rear surface of the wafer W. Accordingly, a heat transfer between the lower electrode  110  and the wafer W is accelerated. A focus ring  119  is installed so as to surround the wafer W mounted on the lower electrode  110 . The focus ring  119  is made of, for example, quartz or silicon and installed on a top surface of the cylindrical holder  106 . 
     The upper electrode  120  is provided at a ceiling of the processing chamber  102 . The upper electrode  120  is grounded. The upper electrode  120  is connected with a processing gas supply unit  122  which supplies a gas required for a process in the processing chamber  102  via a gas line  123 . By way of example, the processing gas supply unit  122  includes a gas supply source which supplies a processing gas or a purge gas required for a process performed on a wafer or a cleaning process in the processing chamber  102 , a valve and a mass flow controller which control introduction of a gas from the gas supply source. 
     The upper electrode  120  includes an electrode plate  124  having a plurality of gas vent holes  125  at a bottom surface and an electrode support  126  which supports the electrode plate  124  detachably attached thereto. Provided within the electrode support  126  is a buffer room  127 . A gas inlet  128  of this buffer room  127  is connected with the gas line  123  of the processing gas supply unit  122 . 
     Formed between a sidewall of the processing chamber  102  and the cylindrical member  104  is a gas exhaust path  130 . A ring-shaped baffle plate  132  is positioned at an entrance of the gas exhaust path  130  or on its way, and a gas exhaust port  134  is provided at a bottom portion of the gas exhaust line  130 . The gas exhaust port  134  is connected with a gas exhaust device  136  via a gas exhaust pipe. The gas exhaust device  136  includes, for example, a vacuum pump and is configured to depressurize the inside of the processing chamber  102  to a certain vacuum level. Further, installed at the sidewall of the processing chamber  102  is a gate valve  108  which opens and closes a loading/unloading port for the wafer W. 
     The lower electrode  110  is connected with a power supply device  140  which supplies dual frequency powers thereto. The power supply device  140  includes a first high frequency power supply unit  142  which supplies a first high frequency power (high frequency power for generating plasma) of a first frequency and a second high frequency power supply unit  152  which supplies a second high frequency power (high frequency power for generating a bias voltage) of a second frequency lower than the first frequency. 
     The first high frequency power supply unit  142  includes a first filter  144 , a first matcher  146 , and a first power supply  148  connected to the lower electrode  110  in sequence. The first filter  144  prevents the second frequency power from entering into the first matcher  146 . The first matcher  146  matches the first high frequency power. 
     The second high frequency power supply unit  152  includes a second filter  154 , a second matcher  156 , and a second power supply  158  connected to the lower electrode  110  in sequence. The second filter  154  prevents the first frequency power from entering into the second matcher  156 . The second matcher  156  matches the second high frequency power. 
     A magnetic field generation unit  200  is provided so as to surround the processing chamber  102 . The magnetic field generation unit  200  includes an upper magnet ring and a lower magnet ring vertically spaced from each other and arranged along a circumference of the processing chamber  102 . The magnetic field generation unit  200  generates a cusp magnetic field which surrounds a plasma processing space in the processing chamber  102 . One of the magnet rings  210  and  220  are configured to be rotated in a circumferential direction with respect to the other magnet ring and a vertical directional gap therebetween can be adjusted. 
     Herein, there will be described a case where the lower magnet ring  220  is configured to be rotatable with respect to the upper magnet ring  210  and each of the magnet rings  210  and  220  is configured to be vertically moved from a process surface of a wafer. A detailed configuration of each of the magnet rings  210  and  220  and an effect thereof will be described later. Driving mechanisms of the respective magnet rings  210  and  220  are not limited to examples to be described herein. By way of example, the upper magnet ring  210  may be configured to be rotatable with respect to the lower magnet ring  220 . 
     The plasma processing apparatus  100  is connected with a controller (an overall control device)  160 , and each component of the plasma processing apparatus  100  is controlled by this controller  160 . Further, the controller  160  is connected with a manipulation unit  162  including a keyboard through which an operator inputs commands to manage the plasma processing apparatus  100  or a display which visually displays an operation status of the plasma processing apparatus  100 . 
     Furthermore, the controller  160  is connected with a storage unit  164  that stores therein: programs for implementing various processes (e.g., a plasma process on the wafer W) performed in the plasma processing apparatus  100  under the control of the controller  160 ; and processing conditions (recipes) required for executing the programs. 
     By way of example, the storage unit  164  stores a plurality of processing conditions (recipes). Further, the storage unit  164  may store a rotation amount of each of the magnet rings  210  and  220 , which will be described later, related to each of the processing conditions. Each processing condition includes a plurality of parameter values such as control parameters controlling each component of the plasma processing apparatus  100  and setting parameters. By way of example, each processing condition may include parameter values such as a flow rate ratio of processing gases, a pressure in a processing chamber, and a high frequency power value. 
     Moreover, the programs or processing conditions may be stored in a hard disc or a semiconductor memory, or may be set in a predetermined area of the storage unit  164  in the form of a storage medium readable by a portable computer such as a CD-ROM or a DVD. 
     The controller  160  reads out a program and processing condition from the storage unit  164  in response to an instruction from the manipulation unit  162  and controls each component, thereby carrying out a desired process in the plasma processing apparatus  100 . Further, the processing condition can be edited by the manipulation unit  162 . 
     (Configuration Example of a Magnet Ring) 
     Hereinafter, a configuration example of each of the magnet rings  210  and  220  will be explained with reference to the drawings.  FIGS. 2A and 2B  are perspective views each showing a configuration example of the magnet rings  210  and  220 .  FIG. 2A  shows an example where there is no rotation amount of the magnet ring  220  in a circumferential direction with respect to the magnet ring  210  and  FIG. 2B  shows an example where the lower magnet ring  220  is rotated by one segment in a circumferential direction with respect to the upper magnet ring  210 . 
       FIGS. 3A and 3B  are cross sectional views for explaining a ring gap adjusting mechanism  232 .  FIG. 3A  shows an example where a ring gap in a vertical direction is increased and  FIG. 3B  shows an example where a ring gap in a vertical direction is decreased. A configuration of the processing chamber  102  in  FIGS. 3A and 3B  is the same as that illustrated in  FIG. 1 , but in these drawings, the illustration of the processing chamber  102  is simplified for easy understanding of the ring gap adjusting mechanism  232 . 
     As depicted in  FIG. 2A , multiple segments  212  and  222  are arranged such that magnetic poles of each of the magnet rings  210  and  220  are placed in a ring shape (a concentric circular shape) in a circumferential direction of an inner surface (a surface facing an outer surface of a sidewall of the processing chamber  102 ). By way of example, each of the segments  212  and  222  may be a permanent magnet. A material of magnets constituting the segments  212  and  222  is not particularly limited and a publicly-known magnet material such as a rare earth based magnet, a ferrite magnet, and an Alnico (registered trademark) magnet may be used. A cross sectional shape of the segments  212  and  222  is not limited to a rectangular shape and may be of any shape such as a circular shape, a square shape, and a trapezoidal shape. 
     Hereinafter, a specific arrangement example of the segments  212  and  222  will be described in detail with reference to  FIG. 2A . The segments  212  and  222  of the respective magnet rings  210  and  220  are arranged in the same manner, and, thus, there will be explained only arrangement of the upper magnet ring  210  as a representative example. 
     The segments  212  of the upper magnet ring  210  illustrated in  FIG. 2A  are arranged in a multi-pole state. That is, a plurality of segments  212  is arranged along a circumferential direction of the upper magnet ring  210  such that magnetic poles (an N-pole and an S-pole) of the segments  212  are alternately reversed group-by-group (for example, two by two). In this example, as shown in  FIG. 4 , eighteen poles of the segment magnets are arranged two by two. 
     Further, the number or arrangement of the segments  212  and  222  are not limited to the examples shown in  FIGS. 2A and 4 . By way of example, the number of the consecutively arranged segments  212  and  222  having the same polarity is not limited to two and may be three or more. Furthermore, the segments  212  and  222  each having the opposite polarity may be alternately arranged one by one. 
     As shown in  FIG. 1 , the magnetic field generation unit  200  includes a ring rotation amount adjusting mechanism (for example, a motor)  230  which rotates the lower magnet ring  220  by a predetermined rotation amount in a circumferential direction with respect to the upper magnet ring  210 . The rotation amount may be set by a rotation angle, but herein, it is set by the number n of the rotated segments  212 . By way of example, if the lower magnet ring  220  is rotated by one segment from a position illustrated in  FIG. 2A , it is positioned as shown in  FIG. 2B . 
     Further, as shown in  FIG. 1 , the magnetic field generation unit  200  includes a ring gap adjusting mechanism (for example, a motor)  232  which drives each of the magnet rings  210  and  220  in a vertical direction. A gap between the magnet rings  210  and  220  is decreased from a gap as shown in  FIG. 3A  to a gap as shown in  FIG. 3B , so that a cusp magnetic field generated by the respective magnet rings  210  and  220  may become larger. 
     In this case, desirably, the respective magnet rings  210  and  220  are vertically equi-spaced from a surface of the wafer W. Herein, as illustrated in  FIG. 3A , if a height of the process surface of the wafer W is defined as a reference height (0 mm), each of a distance d mm between the reference height and the upper magnet ring  210  and a distance −d mm between the reference height and the lower magnet ring  220  is a ring gap adjustment amount. 
     Hereinafter, effects of the respective magnet rings  210  and  220  and an operation of the plasma processing apparatus  100  will be explained with reference to the drawings.  FIGS. 4 and 5  are concept views for explaining a magnetic field formed by each of the magnet rings  210  and  220 .  FIG. 4  provides a view of the magnet rings  210  and  220  when viewed from the top.  FIG. 5  is a perspective view for explaining magnetic force lines formed in part of the respective rings  210  and  220 .  FIGS. 4 and 5  show a case where there is no rotation amount of the lower magnet ring  220  in a circumferential direction with respect to the upper magnet ring  210 . Further, in  FIG. 4 , the segments  212  and  222  are illustrated such that two segments of the same polarity are arranged to be spaced from each other for easy understanding of the generated magnetic force lines. 
     When a process such as an etching process is performed on the wafer W, for example, in the processing chamber  102  by the plasma processing apparatus  100  in accordance with the present embodiment, a processing gas is supplied into the processing chamber  102  by the processing gas supply unit  122  and the processing chamber  102  is depressurized to a predetermined vacuum level by evacuating the inside by means of the gas exhaust device  136 . 
     In this state, a first high frequency power of about 10 MHz or higher, for example, about 100 MHz is supplied to the lower electrode  110  from the first power supply  148  and a second high frequency power ranging from about 2 MHz to about 10 MHz, for example, about 3 MHz is supplied to the lower electrode  110  from the second power supply  158 . Accordingly, plasma of the processing gas is generated between the lower electrode  110  and the upper electrode  120  by the first high frequency power and a self bias potential is generated in the lower electrode  110  by the second high frequency power, and, thus, a plasma process such as reactive ion etching can be performed on the wafer W. In this way, by supplying the first high frequency power and the second high frequency power to the lower electrode  110 , plasma can be appropriately controlled and a satisfactory etching process can be performed. 
     At this time, by an operation of the respective magnet rings  210  and  220  of the magnetic field generation unit  200 , as illustrated in  FIG. 4 , a cusp magnetic field  202  is generated at a periphery of the plasma processing space which is the inside from the sidewall of the processing chamber  102  so as to surround the plasma processing space above the wafer W. At this time, at two upper segments  212  each having the opposite polarity and two lower segments  222  each having the opposite polarity in a portion indicated by a dotted line A-A′ in  FIG. 2A , magnetic force lines as shown in  FIG. 5  are generated. 
     Between the segment  212  of an N-pole and the segment  212  of an S-pole arranged adjacently to each other, a magnetic force line  202  starting from the N-pole to the S-pole is generated. Further, between the segment  222  of an N-pole and the segment  222  of an S-pole arranged adjacently to each other, a magnetic force line  203  starting from the N-pole to the S-pole is also generated. 
     In each of the magnet rings  210  and  220 , as shown in  FIG. 2A , since two N-poles and two S-poles are alternately arranged, each of the magnetic force lines  202  and  203  is generated between them. Further, as shown in  FIG. 4 , the cusp magnetic field is generated at a periphery of the plasma processing space which is the inside from the sidewall of the processing chamber  102  so as to surround the plasma processing space above the wafer W. 
     At this time, by way of example, the cusp magnetic field ranging from about 0.02 T to about 0.2 T (i.e., from about 200 Gauss to about 2000 Gauss), desirably, from about 0.03 T to about 0.045 T (i.e., from about 300 Gauss to about 450 Gauss) is generated at the periphery of the plasma processing space, so that a substantially non-magnetic field state is formed on the wafer W. The reason why the magnitude of the magnetic field is set as stated above is that if the magnetic field is too strong, a non-magnetic field state cannot be formed on the wafer W and if the magnetic field is too weak, a plasma confining effect cannot be obtained. Here, an appropriate magnitude of the magnetic field may depend on a configuration of the apparatus, and, thus, its range may vary depending on the apparatus. 
     Herein, “the substantially non-magnetic field state” includes not only a state in which any magnetic field does not exist but also a state in which a magnetic field capable of affecting an etching process is not formed on the wafer W, that is, a magnetic field which substantially cannot affect a process on the wafer W exists. By way of example, desirably, a magnitude of the magnetic field on the wafer W is set in the range from about 0 T to about 0.001 T (i.e., about 10 Gauss) in order to prevent a charge-up damage to the wafer W. 
     As described above, by forming the cusp magnetic field at the periphery of the plasma processing space, plasma can be confined, and, thus, uniformity in an etching rate at a central portion and an edge portion of the wafer W can be improved. 
     However, when the cusp magnetic field is generated by the magnet rings  210  and  220  in a multi-pole state, if the vertically arranged segments have the same polarity (i.e., there is no rotation of the lower magnet ring  220  with respect to the upper magnet ring  220  in a circumferential direction) as depicted in  FIG. 5 , near the sidewall of the processing chamber  102 , there may be a region where a diffusion coefficient of plasma in a diametric direction cannot be reduced. Here, diffusion of plasma describes a phenomenon where particles in the plasma—are spatially diffused from regions of higher density to regions of lower density to reduce non-uniformity in density and thus a group of the particles becomes easy to flow. The particles in the plasma may be active species such as electrons, ions, or radicals. Hereinafter, explanation of electrons will be provided because electrons have low mass among charged particles influenced by a magnetic field. 
     Generally, a diffusion coefficient D v  of plasma perpendicular to a magnetic field can be expressed by the following equation (1). In the following equation (1), D denotes a diffusion coefficient of plasma parallel to a magnetic field or a non-magnetic field, ω c  denotes a cyclotron angular frequency, and Vm denotes a collision frequency. 
         D   v   =D /(1+(ω c   /Vm ) 2 )  (1)
 
     In this case, if a magnetic field is parallel to the sidewall of the processing chamber  102 , the cyclotron angular frequency ω c  is proportional to a magnitude of the magnetic field. Therefore, according to the equation (1), as the magnitude of the magnetic field parallel to the sidewall of the processing chamber  102  is low, the diffusion coefficient of plasma perpendicular to the magnetic field becomes closer to a diffusion coefficient in a non-magnetic field state, and as the magnitude of the magnetic field parallel to the sidewall of the processing chamber  102  is high, the diffusion coefficient of plasma perpendicular to the magnetic field becomes decreased. 
     Hereinafter, there will be explained a relationship between a magnitude of a magnetic field in each direction component and movements of electrons near the sidewall of the processing chamber  102 .  FIGS. 6A and 6B  are explanatory diagrams conceptionally showing movements of electrons near the sidewall of the processing chamber  102 .  FIG. 6A  shows a case where a magnetic field perpendicular to the sidewall is strong, and  FIG. 6B  shows a case where a magnetic field parallel to the sidewall is strong. 
     By way of example, in the vicinity of an S-pole where a magnetic force line  202 &#39;s component Br perpendicular to the sidewall of the processing chamber  102  is strong and magnetic force line  202 &#39;s components B θ  and B Z  parallel to the sidewall are weak, electrons of plasma become easy to be attracted toward the sidewall as depicted in  FIG. 6A , and, thus, a diffusion coefficient D v  of plasma in a diametric direction (in a direction crossing a magnetic field parallel to the sidewall) is not decreased. Meanwhile, a diffusion coefficient D of plasma parallel to the magnetic field does not depend on a magnitude of the magnetic field. 
     When the magnet rings  210  and  220  are vertically arranged as described in the present embodiment, a magnetic force line  204  may be generated between the segment  212  and the segment  222  if there exists an opposite polarity nearby. In this case, as depicted in  FIG. 5 , if the vertically arranged segments have the same polarity, a Z-directional component B Z  of the magnetic force line  204  may be offset but a component B r  perpendicular to the sidewall of the processing chamber  102  and a θ-directional component B θ  remain. At this time, in a region where these components B r  and B θ  are weak, the diffusion coefficient of plasma in the diametric direction (in the direction crossing the magnetic field parallel to the sidewall) is not decreased. 
     If the diffusion coefficients of plasma in the diametric direction are strong over the whole area, there is a problem in that uniformity in an etching rate at a central portion and an edge portion of the wafer W may be decreased or an area facing a magnetic pole at the sidewall of the processing chamber  102  becomes easy to be eroded. 
     Therefore, as an examination result obtained by the present inventor, it has been found that the above-described problem can be solved by slightly rotating the lower magnet ring  220  with respect to the upper magnet ring  210  in a circumferential direction. That is, as depicted in  FIG. 2B , it has been found that by changing the arrangement of the polarities of the vertically arranged segments, in the magnetic force lines generated at the segments  212  and  222 , the component B r  perpendicular to the sidewall of the processing chamber  102  becomes weak and the components B Z  and B θ  parallel to the sidewall become strong. 
     According to this result, the diffusion coefficient of plasma in the diametric direction (in the direction crossing the magnetic field parallel to the sidewall) can be decreased. That is, as depicted in  FIG. 6B , the electrons in plasma become difficult to be attracted toward the sidewall, and, thus, diffusion of the plasma in the diametric direction can be suppressed. Accordingly, the uniformity in the etching rate at the central portion and the edge portion of the wafer W can be improved. Further, it may be possible to suppress erosion of the area facing the magnetic pole at the sidewall of the processing chamber  102 . 
     Hereinafter, referring to the drawings, there will be explained a result of an experiment for checking that if a rotation amount of the magnet ring  220  with respect to the magnet ring  210  is changed, a characteristic of magnetic force lines generated between the segments  212  and  222  is changed.  FIG. 7  shows a relationship between a rotation amount of the magnet ring  220  with respect to the magnet ring  210  used in the experiment and arrangement of the segments  212  and  222 . 
     Herein, the rotation amount of the magnet ring  220  with respect to the magnet ring  210  is expressed by the number n of segments. In a case (a) where the rotation amount is 0 (n=0), a case (b) where there is a rotation by one segment (n=1), a case (c) where there is a rotation by two segments (n=2), and a case (d) where there is a rotation by three segments (n=3), polarities of the segments  212  and  222  are arranged as shown in  FIG. 7 . 
       FIG. 8  shows a magnitude |B| of a cusp magnetic field and magnitudes |B r |, |B θ |, and |B Z | of its perpendicular directional components when the rotation amount of the magnet ring  220  with respect to the magnet ring  210  corresponds to each of the cases (a) to (c). In  FIG. 8 , a diameter of the wafer W is about 300 mm, and, thus, in each graph, a dotted line at a position about 150 mm away from the center of the wafer W corresponds to an edge portion of the wafer W. Since an inner diameter of the processing chamber  102  used in the experiment is about 540 mm, a dotted line at a position about 270 mm away from the center of the wafer W corresponds to an inner surface of the sidewall of the processing chamber  102 . In the present embodiment, it is desirable to generate a cusp magnetic field |B| between the edge portion of the wafer W and the sidewall. 
     According to the experiment result in  FIG. 8 , it can be seen that as the rotation amount of the magnet rings  210  and  220  is increased as shown in the case where there is a rotation by one segment (n=1) and in the case where there is a rotation by two segments (n=2), the component B r  perpendicular to the sidewall of the processing chamber  102  becomes decreased and the components B θ  and B Z  parallel thereto become increased in comparison with the case where the rotation amount is 0 (n=0). 
     Further,  FIG. 9  shows an incident angle of magnetic force lines to the sidewall of the processing chamber  102  when the rotation amount of the magnet ring  220  with respect to the magnet ring  210  corresponds to each of the cases (a) to (c). According to the experiment result in  FIG. 9 , it can be seen that as the rotation amount of the magnet ring  220  with respect to the magnet ring  210  is increased as shown in the case where there is a rotation by one segment (n=1) and in the case where there is a rotation by two segments (n=2), the number of magnetic force lines having an incident angle nearly perpendicular to the sidewall of the processing chamber  102  becomes decreased and the number of magnetic force lines having an incident angle nearly parallel thereto becomes increased in comparison with the case where the rotation amount is 0 (n=0). 
     Since the diffusion coefficient of plasma in a diametric direction can be reduced by the operation of the magnet rings  210  and  220  as described above, it is possible to suppress diffusion of the plasma in the diametric direction near the sidewall of the processing chamber  102 . Accordingly, a decrease in a plasma density on the edge portion of the wafer W can be suppressed, and, thus, uniformity in a process at the central portion and the edge portion of the wafer W can be improved. 
     There will be given a detailed explanation thereof with reference to the drawings. A graph in  FIG. 10  conceptionally shows a relationship between a distance in a diametric direction in the processing chamber  102  and a plasma density. In  FIG. 10 , a solid line graph represents a plasma density when there is no rotation amount of the magnet ring  220  with respect to the magnet ring  210  and a dotted line graph represents a plasma density when there is a rotation amount. As depicted in  FIG. 10 , if diffusion of the plasma in the diametric direction near the sidewall of the processing chamber  102  is suppressed by rotating the magnet ring  220  with respect to the magnet ring  210 , the plasma density is changed from the solid line graph to the dotted line graph, and, thus, a decrease in the plasma density on the edge portion of the wafer W can be suppressed. 
     Hereinafter, referring to the drawings, there will be explained a result of an experiment in which the magnet rings  210  and  220  were rotated in a circumferential direction and an etching rate was actually measured.  FIG. 11  shows a graph obtained by measuring an etching rate of a SiO 2  film when the SiO 2  film formed on the wafer W having a diameter of about 300 mm was etched in each of the cases (a) to (d) shown in  FIG. 7 . 
     As a processing condition, a pressure in the processing chamber was about 30 mTorr, a flow rate ratio of processing gases including a N 2  gas:a CH 4  gas:an O 2  gas was 60 sccm:30 sccm:10 sccm, a frequency and power of a first high frequency power were about 100 MHz and about 2400 W, respectively, and a frequency and power of a second high frequency power were about 3.2 MHz and about 200 W, respectively. Further, in order to conduct an experiment after changing a magnitude of a magnetic field, a gap between the magnet rings  210  and  220  was varied by setting d and −d indicated in  FIG. 3A  to be about 47 mm and about −47 mm, respectively (a magnetic field magnitude A) and to be about 35 mm and about −35 mm, respectively (a magnetic field magnitude B). In  FIG. 11 , the etching rate of the SiO 2  film was measured on each point of the wafer W in each of the cases (a) to (d) and plotted. Here, as the gap between the magnet rings  210  and  220  is decreased, the magnitude of the magnetic field becomes increased. 
     According to the experiment result as shown in  FIG. 11 , in case of the magnetic field magnitude A, averages of etching rates and uniformity in the surface are about 192.5 nm/min±20.9%, about 221.8 nm/min±12.3%, about 259.8 nm/min±7.7%, and about 232.2 nm/min±11.4% in the respective cases (a) to (d). In case of the magnetic field magnitude B, averages of etching rates and uniformity in the surface are about 187.8 nm/min±19.1%, about 206.6 nm/min±16.5%, about 249.2 nm/min±8.2%, and about 217.8 nm/min±14.2% in the respective cases (a) to (d). 
     According to this experiment result, it can be seen that in both cases of the magnetic field magnitude A and the magnetic field magnitude B, the uniformity of the etching rate in the surface is improved in the cases (b), (c), and (d) where there is a rotation amount as compared with the case (a) where a rotation amount is 0, and in the case (c) where there is a rotation by two segments (n=2), the highest uniformity in the surface can be obtained. Further, the etching rate is also improved. It is deemed as a consequence of suppression of the diffusion of plasma in the diametric direction near the sidewall of the processing chamber  102 . 
     Moreover, in the present embodiment, there has been explained the case where the segments  212  and  222  of the respective magnet rings  210  and  220  are composed of the permanent magnets, but the present invention is not limited thereto. For example, they may be composed of magnetic pole segments of electromagnets. 
     Hereinafter, there will be explained a case where the respective magnet rings  210  and  220  are composed of electromagnets with reference to  FIG. 12 . The magnet rings  210  and  220  in  FIG. 12  are configured by winding coils  216  and  226  around ring-shaped cores  218  and  228  respectively, and covering the cores  218  and  228  with a casing. In this case, the segments  212  and  222  are composed of magnetic segments (teeth members) provided on inner surfaces of the ring-shaped cores  218  and  228 . 
     The ring-shaped cores  218  and  228  are made of a magnetic material such as a metal-based magnet, a ferrite-based magnet, and a ceramic-based magnet. Herein, there is explained a case where the ring-shaped cores  218  and  228  are composed of ring-shaped iron cores. Further, the casing is made of, for example, ceramic or quartz so that magnetic force lines generated at the inner surfaces of the ring-shaped cores  218  and  228  can penetrate the casing. The material of the casing is not limited thereto. By way of example, only a bottom surface of the casing may be made of ceramic or quartz and the other parts thereof may be made of stainless steel. An inner surface of the casing may be opened along a circumferential direction. 
     The segments (teeth members)  212  and  222  are spaced apart from each other at the inner surfaces of the ring-shaped cores  218  and  228  in a circumferential direction. Formed between the respective segments  212  and  222  are groove portions, and the coils  216  and  226  are inserted into the groove portions to pass therethrough and wound around the respective segments  212  and  222 . 
     The coils  216  and  226  are wound around the respective segments  212  and  222  along a circumferential direction of the magnet rings  210  and  220  such that magnetic poles (an N-pole and an S-pole) of the segments  212  and  222  are alternately reversed group-by-group (for example, two by two). Herein, there is explained a case where sixteen poles of the segments are arranged two by two. The coils  216  and  226  are connected with power supplies  240  and  242 , respectively, for supplying currents thereto. These power supplies  240  and  242  are configured to be controlled by the controller  160 . 
     The number or arrangement of the segments  212  and  222  are not limited to this example. By way of example, eighteen poles of the segments may be arranged as illustrated in  FIG. 4 . Further, the number of the consecutively arranged segments  212  and  222  having the same polarity is not limited to two and may be three or more. Furthermore, the segments  212  and  222  each having the opposite polarity may be alternately arranged one by one. 
     Hereinafter, there will be explained a result of an experiment in which upper and lower magnetic poles were rotated in the plasma processing apparatus  100  including the segments  212  and  222  composed of electromagnets and an etching rate was actually measured.  FIG. 13  shows a graph obtained by measuring an etching rate of a SiO 2  film when the SiO 2  film formed on the wafer W having a diameter of about 300 mm was etched in each of the cases (a) to (d) shown in  FIG. 7 . Further, a rotation amount of the magnet rings  210  and  220  and arrangement of the segments  212  and  222  are the same as shown in  FIG. 7 . 
     As a processing condition, a pressure in the processing chamber was about 30 mTorr, a flow rate of a processing gas including a CH 4  gas was about 150 sccm, frequency and power of a first high frequency power were about 100 MHz and about 800 W, respectively, and frequency and power of a second high frequency power were about 13.56 MHz and about 200 W, respectively. Further, in order to conduct experiments while changing a magnitude of a magnetic field to be applied to the magnet rings  210  and  220 , experiments under the conditions of (a) and (b) were conducted in case that currents supplied to the coils are about 0 AT (no magnetic field), about 1500 AT (a magnetic field magnitude A), about 2500 AT (a magnetic field magnitude B), and about 3000 AT (a magnetic field magnitude C). Furthermore, experiments under the conditions of (c) and (d) were conducted in case that currents are about 0 AT (no magnetic field) and about 3000 AT. This is because a tendency can be somewhat predicted by the result of the experiments under the conditions of (a) and (b). 
     According to the experiment result as shown in  FIG. 13 , in case of about 1500 AT (the magnetic field magnitude A), averages of etching rates and uniformity in the surface are about 226.8 nm/min±19.4% and about 226.8 nm/min±19.0% in the respective cases (a) and (b). In case of about 2500 AT (the magnetic field magnitude B), averages of etching rates and uniformity in the surface are about 199.9 nm/min±13.7% and about 174.0 nm/min±7.8% in the respective cases (a) and (b). Further, in case of about 3000 AT (the magnetic field magnitude C), averages of etching rates and uniformity in the surface are about 178.3 nm/min±8.9%, about 165.2 nm/min±7.2%, about 181.0 nm/min±20.6%, and about 165.2 nm/min±7.3% in the respective cases (a) to (d). Furthermore, in case of about 0 AT (no magnetic field), average of etching rates and uniformity in the surface is about 234.4 nm/min±20.6% in the cases (a) to (d). 
     According to this experiment result, results obtained from the cases (the magnetic field magnitudes A, B, and C) where there is a magnetic field are improved as compared to the case where there is no magnetic field. Further, it can be seen that in case of the magnetic field magnitude C, the uniformity of the etching rate in the surface is improved in the cases (b), (c), and (d) where there is a rotation amount as compared to the case (a) where a rotation amount is 0, and in the case (b) where there is a rotation by one segment (n=1), the highest uniformity in the surface can be obtained. Furthermore, the etching rate is also improved. Even in case of the magnetic field magnitudes A and B, the uniformity of the etching rate in the surface is improved in the case (b) where there is a rotation amount as compared to the case (a) where a rotation adjustment amount is 0. 
     According to the experiment result as shown in  FIG. 11 , in the case (c) where there is a rotation by two segments (n=2), the highest uniformity in the surface can be obtained. Meanwhile, according to the experiment result as shown in  FIG. 13 , in the case (b) where there is a rotation by one segment (n=1), the highest uniformity in the surface can be obtained. Thus, the optimum rotation amount may vary depending on a configuration of the apparatus and a processing condition. For this reason, it is desirable to determine the optimum rotation amount depending on a configuration of the apparatus and a processing condition. In this case, the optimum rotation amount depending on a processing condition may be stored in advance in the storage unit  164  in relation with a processing condition and before a plasma process is performed, the controller  160  may read the rotation amount related to this processing condition from the storage unit  164  so as to control relative positions of the magnet rings  210  and  220 . 
     If the segments  212  and  222  are composed of electromagnets, by switching magnetic poles of segments of one magnet ring, the magnet rings  210  and  220  may be virtually moved relative to each other. Accordingly, the upper and lower magnetic poles can be changed without rotating the magnet rings. 
     Hereinafter, there will be explained a control method of the respective magnet rings  210  and  220  by the controller  160 . Herein, as a rotation amount (the number n of segments), the optimum value pre-obtained from the experiment is used. In this case, if the number of the consecutively arranged segments  212  and  222  having the same polarity is m, there are (2m−1) ways for rotating polarities of the upper and lower segments  212  and  222 . By way of example, in  FIG. 7 , m is 2, and, thus, the number of ways for rotating polarities of the upper and lower segments  212  and  222  is 3 ((b), (c), and (d) shown in  FIG. 7 ). Thus, one of the magnet rings is rotated by n segments from 1 to (2m−1) in a circumferential direction and a plasma process is performed on the wafer W in each case. Then, it is desirable to store the number n of the rotated segments in the case where the best result of the process on the wafer W can be obtained in the storage unit  164  as a rotation amount. If there are multiple processing conditions, a rotation amount n is stored in relation with each processing condition. At this time, a ring gap adjustment amount (±d) is also stored in advance in the storage unit  164  in relation with each processing condition. 
     Before a plasma process is performed on the wafer W based on each processing condition, the controller  160  reads a rotation amount n and a ring gap adjustment amount (±d) related to the processing condition from the storage unit  164 . Then, the ring gap adjusting mechanism  232  drives the magnet rings  210  and  220  vertically, thereby adjusting a gap therebetween and the ring rotation amount adjusting mechanism  230  rotates the lower magnet ring  220  so as to rotate the lower magnet ring  220  as much as the number n of segments with respect to the upper magnet ring  210 . Accordingly, a rotation amount n and a ring gap adjustment amount (±d) can be automatically adjusted to have the optimum value depending on a processing condition. 
     Further, a rotation amount n and a ring gap adjustment amount (±d) can be flexibly preset by the operator through the operation unit  162 , and the preset values are stored in the storage unit  164 . Furthermore, the ring rotation amount adjusting mechanism  230  may not be provided. In this case, when the lower magnet ring  220  is positioned with respect to the upper magnet ring  210 , the lower magnet ring  220  is rotated as much as a rotation amount n. 
     There have been explained embodiments of the present invention with reference to the accompanying drawings, but the present invention is not limited to the above-described embodiments. It would be understood by those skilled in the art that various changes and modifications may be made within the scope of the claims and their equivalents are included in the scope of the present invention. 
     By way of example, in the above-described embodiments, there has been explained a case where two different high frequency powers are applied only to the lower electrode  110  but the present invention is not limited thereto. The present invention can be applied to a case where high frequency powers are applied to the upper electrode  120  and the lower electrode  110  and a case where a high frequency power is applied only to the upper electrode  120 . Further, there has been explained a case where the wafer W is used as a substrate and an etching process is performed thereon but the present invention is not limited thereto, and other substrates such as a FPD substrate and a solar cell substrate can be used. Furthermore, a plasma process is not limited to an etching process and other processes such as sputtering and CVD can be employed. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be applied to a plasma processing apparatus and a plasma processing method capable of performing a process on a substrate by generating plasma in a processing chamber.