Abstract:
A plasma processing method according to an aspect includes: preparing a plasma processing apparatus including: a chamber; a lower electrode; an upper electrode; a focus ring surrounding a peripheral edge of the lower electrode; and an annular coil disposed on an upper portion of the upper electrode at a more outer position than the peripheral edge of the lower electrode; placing a substrate on the lower electrode, with a peripheral edge of the substrate surrounded by the focus ring; introducing process gas into the chamber; generating plasma of the process gas by applying high-frequency power across the upper electrode and the lower electrode; and leveling an interface of a plasma sheath on an upper portion of the substrate with that on an upper portion of the focus ring by generating a magnetic field by supplying a current to the annular coil.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of prior International Application No. PCT/JP2015/001913 filed on Apr. 6, 2015 which is based upon and claims the benefit of priority from Japanese Patent Application No. 2014-080039 filed on Apr. 9, 2014; the entire contents of all of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    Embodiments described herein relate generally to a plasma processing method and a plasma processing apparatus. 
       BACKGROUND ART 
       [0003]    A plasma processing apparatus has been used in a semiconductor device manufacturing process for etching a substrate (for example, a semiconductor wafer) with plasmatized gas acting on the substrate. Such a plasma processing apparatus includes a capacitive coupling plasma processing apparatus which has an upper electrode and a lower electrode facing each other in a process chamber and generates plasma by applying high-frequency power across these electrodes. Another plasma processing apparatus has the above-described structure and controls the plasma density with a magnetic field. 
         [0004]    The above-described plasma processing apparatus can generate the plasma sheath which has different thicknesses between an area above the substrate and an area above and outside the substrate because a peripheral area and an outside area of the substrate are different in structure and material. The thickness difference in the plasma sheath causes an interface of the plasma sheath (interface between the plasma sheath and the plasma) to have a level difference to have a slanting portion at the boundary between the lower and higher-level portions. Ions are obliquely incident on the slanting portion, resulting in tilting which is a tilting etching state. A measure to flatten the interface of the plasma sheath is to surround the outer periphery of the substrate by a focus ring made of, for example, Si or SiC. Another measure to flatten the interface of the plasma sheath to prevent the tilting is to change the thickness and material of the focus ring or apply a high-frequency bias to the focus ring itself. 
       SUMMARY 
       [0005]    As described above, the plasma processing apparatus has the focus ring surrounds the outside of the substrate to prevent the tilting at the outer peripheral portion of the substrate. In this case, however, a thickness change of the focus ring due to its wear causes the tilting, necessitating frequent replacement of the focus ring. The increased maintenance time decreases production efficiency and increases the production cost. 
         [0006]    Embodiments of the present invention were made to cope with the above-described circumstances and provides a plasma processing method and a plasma processing apparatus that achieve higher production efficiency and lower production cost than conventionally. 
         [0007]    A plasma processing method according to an aspect includes: preparing a plasma processing apparatus, the plasma processing apparatus including: a chamber; a lower electrode disposed in the chamber; an upper electrode disposed in the chamber and facing the lower electrode; a focus ring disposed in the chamber and surrounding a peripheral edge of the lower electrode; and an annular coil disposed on an upper portion of the upper electrode at a more outer position than the peripheral edge of the lower electrode; placing a substrate on the lower electrode, with a peripheral edge of the substrate surrounded by the focus ring; introducing process gas into the chamber; generating plasma of the process gas by applying high-frequency power across the upper electrode and the lower electrode; and leveling an interface of a plasma sheath on an upper portion of the substrate with the interface of the plasma sheath on an upper portion of the focus ring by generating a magnetic field by supplying a current to the annular coil. 
         [0008]    A plasma processing apparatus according to an aspect includes: a chamber; a lower electrode disposed in the chamber to place a substrate on; an upper electrode disposed in the chamber and facing the lower electrode; a focus ring disposed in the chamber and surrounding a peripheral edge of the substrate; an annular coil disposed on an upper portion of the upper electrode at a more outer position than the peripheral edge of the substrate; an introducing part which introduces process gas into the chamber; a high-frequency power source which applies high-frequency power across the upper electrode and the lower electrode to generate plasma of the process gas; and a current source which supplies a current to the annular coil to generate a magnetic field to level an interface of a plasma sheath on an upper portion of the substrate with the interface of the plasma sheath on an upper portion of the focus ring. 
         [0009]    Embodiments of the present invention achieve higher production efficiency and lower manufacturing cost than conventionally. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a view schematically illustrating the rough structure of a plasma etching apparatus according to an embodiment of the present invention. 
           [0011]      FIG. 2  is a view schematically illustrating the rough structure of an essential part of the plasma etching apparatus. 
           [0012]      FIG. 3  are charts illustrating examples of a magnetic field formed by an electromagnet. 
           [0013]      FIG. 4  is an explanatory view of a generation state of tilting. 
           [0014]      FIG. 5  is a view schematically illustrating a relation of a tilt angle, the shape of an interface of a plasma sheath, and an incident direction of ions. 
           [0015]      FIG. 6  is a chart illustrating measurement values of the tilt angle when a 1 G magnetic field is generated. 
           [0016]      FIG. 7  is a chart illustrating measurement values of the tilt angle when an 8 G magnetic field is generated. 
           [0017]      FIG. 8  is a chart illustrating measurement values of the tilt angle when a 15 G magnetic field is generated. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    An embodiment of the present invention will be hereinafter described with reference to the drawings.  FIG. 1  is a view schematically illustrating the rough sectional structure of a plasma processing apparatus according to the embodiment. The plasma processing apparatus  10  illustrated in  FIG. 1  includes an airtight cylindrical process chamber  12  for housing a semiconductor wafer W having a diameter of, for example, 300 mm. 
         [0019]    A disk-shaped mounting table  14  for placing the semiconductor wafer W is in a lower portion of the process chamber (chamber)  12 . The mounting table  14  includes a base  14   a  (lower electrode) and an electrostatic chuck  14   b.  The base  14   a  is a conductive member of, for example, aluminum. 
         [0020]    An annular focus ring  26  is on a peripheral edge region of the upper surface of the base  14   a  to surround the periphery of the semiconductor wafer W. The electrostatic chuck  14   b  is on a center region of the upper surface of the base  14   a.  The electrostatic chuck  14   b  has a disk shape and has an electrode film on an inner side of an insulating film. A DC power source (not illustrated) supplies a DC voltage to the electrode film of the electrostatic chuck  14   b  to generate an electrostatic force, so that the electrostatic chuck  14   b  sucks the semiconductor wafer W as a substrate. 
         [0021]    The center axis Z passing the center of the semiconductor wafer W mounted on the electrostatic chuck  14   b  in the up and down direction substantially coincides with the center axes of the base  14   a  and the electrostatic chuck  14   b.    
         [0022]    The base  14   a  serves as the lower electrode. A first high-frequency power source  18  generates high-frequency power for plasma generation and connects with the base  14   a  via a first matching device  22 . The first high-frequency power source  18  generates the high-frequency power with, for example, a 100 MHz frequency. The first matching device  22  has a circuit for matching an output impedance of the first matching device  22  with a load-side (lower electrode-side) input impedance. 
         [0023]    In this embodiment, the first high-frequency power source  18  applies the high-frequency power for plasma generation in a pulsed form with a desired frequency (for example, 50 kHz) and a desired duty ratio (for example, 20%). This creates a plasma generation period and a plasma non-generation period to enable to reduce the accumulation of electric charges on a specific portion on the semiconductor wafer W. Specifically, during the plasma generation period, the electric charges accumulate on a portion with a high electron density due to unevenness of the electron density in the plasma, but in the plasma non-generation period, the accumulated electric charges can be dispersed to a surrounding area to eliminate the accumulation of the electric charges. This prevents a breakage of the insulating film. 
         [0024]    A second high-frequency power source  20  generates high-frequency bias power for ion attraction and connects with the base  14   a  via a second matching device  24 . The second high-frequency power source  20  generates the high-frequency power having a lower frequency (for example, 3.2 MHz frequency) than that generated by the first high-frequency power source  18 . The second matching device  24  has a circuit for matching an output impedance of the second matching device  24  with a load-side (lower electrode-side) input impedance. 
         [0025]    An upper electrode  16  is above and faces the mounting table (lower electrode)  14 , with a process space S therebetween. The upper electrode  16  has a disk shape and defines the process space S from above. The center axis of the upper electrode  16  substantially coincides with the center axis of the mounting table  14 . 
         [0026]    The upper electrode  16  also functions as a shower head which introduces predetermined process gas in a shower form into the process space S (an introducing part which introduces process gas into the chamber). In this embodiment, the upper electrode  16  has a buffer chamber  16   a,  a gas line  16   b,  and a plurality of gas holes  16   c.  One end of the gas line  16   b  connects with the buffer chamber  16   a.  The gas holes  16   c  connect with the buffer chamber  16   a  and extend downward to open toward the process space S. In this embodiment, the upper electrode  16  and the shower head (introducing part) are integrated, but they may be separate structures. 
         [0027]    An exhaust mechanism (not illustrated) such as Turbo Molecular Pump (TMP) or Dry Pump (DP) connects with the bottom of the process chamber  12  and keeps the inside of the process chamber  12  at a pressure-reduced atmosphere. 
         [0028]    An electromagnet  30  is on an upper portion of the upper electrode  16 . The electromagnet  30  includes a core member  50  and coils (annular coils)  61  to  64 . The core member  50  includes a pillar-shaped portion  51 , a plurality of cylindrical portions  52  to  55 , and a base portion  56  which are integrally formed. The electromagnet  30  is made of a magnetic material. The base portion  56  has a substantially disk shape, and its center axis coincides with the center axis Z. The pillar-shaped portion  51  and the cylindrical potions  52  to  55  protrude downward from the lower surface of the base portion  56 . The pillar-shaped portion  51  has a substantially columnar shape and its center axis coincides with the center axis Z. The pillar-shaped portion  51  has a radius L 1  (refer to  FIG. 2 ) of, for example, 30 mm. 
         [0029]    The cylindrical portions  52  to  55  each have a cylindrical shape extending in the axis Z direction. As illustrated in  FIG. 2 , the cylindrical portions  52  to  55  are respectively along a plurality of concentric circles C 2  to C 5  whose centers are the center axis Z. Specifically, the cylindrical portion  52  is along the concentric circle C 2  with a radius L 2  larger than the radius L 1 . The cylindrical portion  53  is along the concentric circle C 3  with a radius L 3  larger than the radius L 2 . The cylindrical portion  54  is along the concentric circle C 4  with a radius L 4  larger than the radius L 3 . The cylindrical portion  55  is along the concentric circle C 5  with a radius L 5  larger than the radius L 4 . 
         [0030]    For example, the radii L 2 , L 3 , L 4 , L 5  are 76 mm, 127 mm, 178 mm, and 229 mm respectively. The middle circumferences of the coils  61 ,  62 ,  63 ,  64  are distant from the center axis Z by about 50 mm, 100 mm, 150 mm, and 200 mm respectively. 
         [0031]    A groove is between the pillar-shaped portion  51  and the cylindrical portion  52 . As illustrated in  FIG. 1 , this groove houses the coil  61  wound along the outer peripheral surface of the pillar-shaped portion  51 . A groove is also between the cylindrical portion  52  and the cylindrical portion  53  to house the coil  62  wound along the outer peripheral surface of the cylindrical portion  52 . A groove is also between the cylindrical portion  53  and the cylindrical portion  54  to house the coil  63  wound along the outer peripheral surface of the cylindrical portion  53 . A groove is also between the cylindrical portion  54  and the cylindrical portion  55  to house the coil  64  wound along the outer peripheral surface of the cylindrical portion  54 . Both ends of the coils  61  to  64  connect with power sources (current sources)  31  to  34 . A controller Cnt controls current supply and stop of the current supply to the coils  61  to  64  from the power sources  31  to  34  and values of the currents by its control signals. 
         [0032]    In the electromagnet  30 , the radii L 4 , L 5  are larger than the 150 mm radius of the semiconductor wafer W, as described above. Accordingly, as illustrated in  FIG. 1 , the coil  64  is on a more outer side than the outer peripheral edge of the semiconductor wafer W, with at least part thereof being above the focus ring  26 . The coils  61 ,  62  are above the semiconductor wafer W, in particular, on a more inner side than the outer peripheral edge of the semiconductor wafer W. The coil  63  spans a region from an inner side to an outer side of the outer peripheral edge of the semiconductor wafer W. 
         [0033]    The boundary B illustrated in  FIG. 1  demarcates the inner side and the outer side of the outer peripheral edge of the semiconductor wafer W. The boundary B is along the peripheral edge of the semiconductor wafer W and has a substantially cylindrical plane shape having rotational symmetry with respect to the center axis Z. 
         [0034]    The electromagnet  30  as structured above is capable of forming a magnetic field B in the process space S when one or more coils out of the coils  61  to  64  are supplied with the current from the power sources  31  to  34 . The magnetic field B has horizontal magnetic field components BH along the radial direction from the center axis Z.  FIG. 3  illustrate examples of the magnetic field formed by the electromagnet  30 . 
         [0035]      FIG. 3( a )  illustrates a cross section of the electromagnet  30  in a half plane with respect to the center axis Z and the magnetic field B when the current is supplied to the coil  62 .  FIG. 3( b )  illustrates intensity distribution of the horizontal magnetic field components BH when the current is supplied to the coil  62 . 
         [0036]      FIG. 3( c )  illustrates the cross section of the electromagnet  30  in the half plane with respect to the center axis Z and the magnetic field B when the current is supplied to the coil  64 .  FIG. 3( d )  illustrates intensity distribution of the horizontal magnetic field components BH when the current is supplied to the coil  64 . In the graphs illustrated in  FIG. 3( b )  and  FIG. 3( d ) , the horizontal axis represents the radial-direction position from the center axis Z whose position is 0 mm, and the vertical axis represents the intensity (magnetic flux density) of the horizontal magnetic field components BH (horizontal components of the magnetic field). 
         [0037]    The supply of the current to the coil  62  of the electromagnet  30  results in the formation of the magnetic field B illustrated in  FIG. 3( a ) . Specifically, the formed magnetic field B extends from process space S-side end portions of the pillar-shaped portion  51  and the cylindrical portion  52  toward process space S-side end portions of the cylindrical portions  53  to  55 . The radial-direction intensity distribution of the horizontal magnetic field components BH of such a magnetic field B has a peak under the middle circumference of the coil  62  as illustrated in  FIG. 3( b ) . For example, the position of the middle circumference of the coil  62  is about 100 mm distant from the axis Z, and when the wafer W with a 300 mm diameter is processed, this position is a radial-direction middle position between the center and the edge of the wafer W. 
         [0038]    The supply of the current to the coil  64  of the electromagnet  30  results in the formation of the magnetic field B illustrated in  FIG. 3( c ) . Specifically, the formed magnetic field B extends from process space S-side end portions of the pillar-shaped portion  51  and the cylindrical portions  52  to  54  toward a process spaces S-side end portion of the cylindrical portion  55 . The radial-direction intensity distribution of the horizontal magnetic field components BH of such a magnetic field B has a peak under the middle circumference of the coil  64  as illustrated in  FIG. 3( d ) . For example, the position of the middle circumference of the coil  64  is about 200 mm distant from the axis Z, and when the wafer W with a 300 mm diameter (150 mm radius) is processed, this position is radially on an outer side of the edge (boundary B) of the wafer W, that is, is the position of the focus ring  26 . 
         [0039]    In the plasma processing apparatus  10 , the process gas from a gas supply system is supplied to the process space S through the upper electrode  16  constituting the shower head and the high-frequency power from the first high-frequency power source  18  is supplied to the mounting table  14  as the lower electrode, whereby the high-frequency electric field is generated between the upper electrode  16  and the mounting table  14 . This causes the generation of the plasma of the process gas in the process space S. Then, the semiconductor wafer W can be processed by active species of molecules or atoms of the process gas dissociated in the plasma. Further, adjusting the high-frequency bias power to be supplied to the mounting table  14  as the lower electrode from the second high-frequency power source  20  enables the adjustment of an ion attraction degree. 
         [0040]    The plasma processing apparatus  10  includes the controller Cnt. The controller Cnt includes a programmable computer device (a processor, a memory, an input device, and a display). The controller Cnt controls the high-frequency power to be generated by the first high-frequency power source  18 , the high-frequency power to be generated by the second high-frequency power source  20 , an exhaust amount of the exhaust device, the gas to be supplied from the gas supply system, a flow rate of the gas, and values and directions of the currents to be supplied from the power sources  31  to  34  to the coils  61  to  64  of the electromagnet  30 . For this purpose, the controller Cnt sends the control signals to the first high-frequency power source  18 , the second high-frequency power source  20 , the exhaust device, constituent elements of the gas supply system, and the power sources  31  to  34  (the current sources connecting with the electromagnet  30 ), in accordance with recipes (stored in the memory or input from the input device). 
         [0041]    In this embodiment, at the time of the plasma processing, the power source  34  supplies the current to the coil  64  to generate the magnetic field under the control by the controller Cnt. As a result, the interface of the plasma sheath (interface between the plasma sheath and the plasma) above the semiconductor wafer W is leveled with the interface above the focus ring  26  so that the interface of the plasma sheath has a flat plane horizontal to the surface of the semiconductor wafer W. 
         [0042]    The plasma processing apparatus  10  has the focus ring  26  surrounding the periphery of the semiconductor wafer W, thus the state of the plasma on the outer periphery of the semiconductor wafer W becomes uniform with that on the upper portion of the semiconductor wafer W, resulting in more uniform processing in the surface of the semiconductor wafer W. However, if the interface of the plasma sheath has a level difference between its portions above the semiconductor wafer W and above the focus ring  26 , an angle of incidence of the ions becomes oblique at a slanting portion of the interface of the plasma sheath, causing tilting which is a tilting etching state. 
         [0043]      FIG. 4  schematically illustrates an example of an enlarged cross section along the radial direction when holes and line shapes are formed in the semiconductor wafer W by plasma etching. In  FIG. 4 , the left side is a center direction of the semiconductor wafer W and the right side is a direction of the focus ring (peripheral edge of the semiconductor wafer W). 
         [0044]    As illustrated in  FIG. 4 , when the holes and the line shapes are formed by the plasma etching, the oblique incidence of the ions on the semiconductor wafer W makes the etching state tilted relatively to the semiconductor wafer W. This results in the radial-direction deviation between the center position of a space on an upper end of the formed hole or line (space center on the upper end) and the center position of the space on its bottom (space center on the bottom), so that the line A 1  connecting these centers tilts relatively to the vertical line A 2  extending down from the space center on the upper end. 
         [0045]    This is the tilting occurrence state, and the tilting occurrence state can be quantitatively evaluated based on the measurement of an angle made by the line A 1  and the vertical line A 2  (hereinafter, a tilt angle). Such tilting is a problem in the progress of miniaturization and the progress of multilayering in, for example, a three-dimensional NAND. The tilt angle is preferably less than about ±1.0°. 
         [0046]      FIG. 5  schematically illustrates a relation of the aforesaid tilt angle, the shape of the interface of the plasma sheath, and an incident direction of ions.  FIG. 5  shows results of the measurement of the tilt angle at the positions 0 mm, 30 mm, 60 mm, 80 mm, 100 m, 125 mm, and 145 mm distant from the center of the semiconductor wafer W when the current is supplied to the coil  64  to form a 15 G magnetic field. The tilt angles at these positions are 0.0, 0.1, 0.1, −0.5, −0.1, −0.6, and −1.8 degrees (deg) respectively.  FIG. 5  illustrates, in its upper part, the shape of the sheath interface expected in this case and the angle of incidence of the ions. 
         [0047]    As illustrated in  FIG. 5 , the supply of the current to the coil  64  to form the magnetic field makes it possible to control the shape of the interface of the plasma sheath. In this case, at the portion where the magnetic field is formed, electrons stay long to increase the plasma density. Consequently, the plasma sheath becomes thin and the position of the interface of the plasma sheath becomes low. Further, if the interface of the plasma sheath has the level difference, the angle of incidence of the ions (indicated by the arrow in  FIG. 5 ) becomes oblique at a slanting portion of the interface of the plasma sheath and the tilting occurs at this portion. 
         [0048]    As is seen from the results illustrated in  FIG. 5 , in the absence of the magnetic field, the interface of the plasma sheath on the center portion of the semiconductor wafer W is normally a flat surface parallel to the surface of the semiconductor wafer W. This highly possibly causes the interface of the plasma sheath to have the level difference on the peripheral edge portion of the semiconductor wafer W. That is, a difference in material and structure between the semiconductor wafer and the structure therearound affects the state of the plasma formed on their upper portions, which is likely to cause the level difference in the interface of the plasma sheath. To reduce such a phenomenon, the focus ring  26  surrounds the semiconductor wafer W. A thickness change of the focus ring  26  due to its wear, however, also causes a height change of the interface of the plasma sheath on the upper portion of the focus ring  26 . 
         [0049]    As a countermeasure, as illustrated in  FIG. 5 , the magnetic field is formed by the current supply only to the coil  64  located on a more outer side than the outer periphery of the semiconductor wafer W with its part being above the focus ring  26 . This as a result can change the thickness of the plasma sheath, in particular on the upper portion of the focus ring  26  to change the height of the interface of the plasma sheath. 
         [0050]    In this case, as illustrated in  FIG. 3( d ) , the radial-direction intensity distribution of the horizontal magnetic field components BH of the magnetic field formed by the supply of the current only to the coil  64  has the peak (highest value) under the middle circumference of the coil  64  (on the more outer side than the peripheral edge (boundary B) of the semiconductor wafer W). 
         [0051]    In this embodiment, the supply of the current only to the coil  64  to form the magnetic field makes it possible to reduce the occurrence of the tilting at the peripheral edge portion of the semiconductor wafer W. 
         [0052]    Further, the thickness change of the focus ring  26  due to its wear changes the state of the plasma on the focus ring  26 , leading to the height change of the interface of the plasma sheath. To prevent this, this embodiment varies the intensity of the magnetic field by adjusting the current that is to be supplied to the coil  64 . As a result, even if the focus ring  26  wears away, it is possible to flatten the interface of the plasma sheath into a plane parallel to the surface of the semiconductor wafer W to reduce the occurrence of the tilting. 
         [0053]      FIG. 6  to  FIG. 8  illustrate results when a layer to be etched of the semiconductor wafer W is plasma-etched while the current is supplied only to the coil  64  to form the magnetic field. Specifically, the tilt angle of a pattern formed by the etching is measured at the positions 0 mm, 30 mm, 60 mm, 80 mm, 100 mm, 125 mm, and 145 mm distant from the center of the semiconductor wafer W.  FIG. 6  illustrates the result when a 1 G magnetic field is formed.  FIG. 7  illustrates the result when an 8 G magnetic field is formed.  FIG. 8  illustrates the result when a 15 G magnetic field is formed. 
         [0054]    In the plasma etching, gas containing “C 4 F 8 +C 4 F 6 +Ar+O 2 ” was used as the process gas, and its condition was as follows, for instance.
       pressure inside the process chamber: 5.32 Pa (40 mTorr)   first high-frequency: 100 MHz frequency, 300 W   second high-frequency: 3.2 MHz frequency, 10000 W   processing time: one minute       
 
         [0059]    The plasma etching condition is not limited to the above, and the pressure can be set to 1.33 to 5.32 Pa (10 to 40 mTorr) by selecting the flow rates of the process gas from values within the following ranges: C 4 F 8 +C 4 F 6 =10 to 300 sccm, Ar=50 to 800 sccm, and O 2 =5 to 150 sccm. 
         [0060]    As illustrated in  FIG. 6 , when the current is supplied to the coil  64  to form the 1 G magnetic field, the tilt angle at the 125 mm position from the center of the semiconductor wafer W is +1.1°, and thus there is a portion where the tilt angle is 1.0° or more. 
         [0061]    As illustrated in  FIG. 7 , when the current is supplied to the coil  64  to form the 8 G magnetic field, the tilt angles at all the positions are less than ±1.0°, and thus there is no portion where the tilt angle is 1.0° or more. 
         [0062]    As illustrated in  FIG. 8 , when the current is supplied to the coil  64  to form the 15 G magnetic field, the tilt angle at the 145 mm position from the center of the semiconductor wafer W is −1.8°, and thus there is a portion where the tilt angle is 1.0° or more. 
         [0063]    As is seen from the above  FIG. 6  to  FIG. 8 , the state of the tilting at the peripheral edge portion of the semiconductor wafer W can be controlled by supplying the current only to the coil  64  located on a more outer side than the peripheral edge (boundary B) of the semiconductor wafer W to form the magnetic field and by varying the magnitude of the current to adjust the intensity of the magnetic field. As a result, the tilt angle at each portion has a favorable value of less than +1.0°. 
         [0064]    The results illustrated in  FIG. 6  show that the tilt angles at the 125 mm position and the  145  position from the center of the semiconductor wafer W are +1.1° and +0.6° respectively. The results illustrated in  FIG. 8  show that the tilt angles at the 125 mm position and the 145 mm position from the center of the semiconductor wafer W are −0.6° and −1.8° respectively. At this time, an increase in the intensity of the magnetic field can reduce the thickness of the plasma sheath to lower the height of the interface of the plasma sheath. This causes to invert the angle of incidence of the ions to change the tilt angle from a plus direction to a minus direction. 
         [0065]    In the above, the current is supplied only to the coil  64 , but the current can be supplied to at least one (or all) of the coils  61  to  63  in addition to the coil  64 . By supplying the current to at least one of the coils  61  to  63  and to the coil  64  and adjusting the current to be supplied to the coil  64 , it is possible to control the tilting state near the peripheral edge of the semiconductor wafer W. It is only necessary at this time that the magnetic fields from the coils  61  to  63  do not impair the thickness uniformity of the plasma sheath. This is possible by, for example, making the magnetic fields from the coils  61  to  63  smaller than the magnetic field from the coil  64 . 
         [0066]    The thickness reduction of the focus ring  26  due to its wear causes the interface of the plasma sheath formed on the upper portion of the focus ring  26  to undergo a height change to cause the level difference in the interface of the plasma sheath, resulting in the tilting. To cope with this, the replacement of the focus ring  26  has conventionally been required. 
         [0067]    In this embodiment, on the other hand, by controlling the intensity of the magnetic field by adjusting the current to be supplied to the coil  64  according to the worn state of the focus ring  26 , the interface of the plasma sheath can keep flat to reduce the occurrence of the tilting. This enables to put off the replacement time of the focus ring  26 , increase the life of the focus ring  26 , and elongate the maintenance cycle, enabling an improvement of productivity and a reduction of the manufacturing cost. 
         [0068]    The present invention is not limited to the embodiments and the examples described above, and various modifications can of course be made therein,