Patent Publication Number: US-2023141298-A1

Title: Etch uniformity improvement for single turn internal coil pvd chamber

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. provisional patent application Ser. No. 63/276,493, filed Nov. 5, 2021, which is herein incorporated by reference in its entirety. 
    
    
     FIELD 
     Embodiments of the present disclosure generally relate to substrate processing equipment. 
     BACKGROUND 
     The manufacture of the sub-half micron and smaller features in the semiconductor industry rely upon a variety of processing equipment, such as process chambers, for example, physical vapor deposition (PVD) chambers, chemical vapor deposition (CVD) chambers, atomic layer deposition (ALD) chambers, and the like. The process chambers may use coils disposed between a target and a substrate support of the process chamber to maintain a plasma in the process chamber. However, the inventors have observed PVD chambers having a single turn internal coil may lead to asymmetrical material etch on a substrate being processed in the process chamber when operating at low bias power and high pressure 10 mTorr). 
     Therefore, the inventors have provided improved PVD process chambers to help improve process uniformity. 
     SUMMARY 
     Methods and apparatus for generating a magnetic field external to a physical vapor deposition (PVD) chamber to improve etch or deposition uniformity on a substrate disposed inside of the PVD chamber are provided herein. In some embodiments, a process chamber, includes a chamber body defining an interior volume therein; a pedestal disposed in the interior volume for supporting a substrate; a coil disposed in the interior volume above the pedestal; and an external magnet assembly, comprising: a housing coupled to the chamber body; and a plurality of magnets disposed external to the chamber body coupled to the housing and arranged asymmetrically about the chamber body. 
     In some embodiments, an external magnet assembly for use with a process chamber includes: a housing having a plate and an inner lip extending upward from the plate to define a central opening of the housing; a plurality of arcuate base plates coupled to the housing; and a plurality of magnets coupled to each of the plurality of arcuate base plates. 
     In some embodiments, a process chamber includes: a chamber body coupled to a lid to define an interior volume therein; a pedestal disposed in the interior volume for supporting a substrate; a coil disposed in the interior volume above the pedestal; and an external magnet assembly, comprising: a housing coupled to the chamber body, two arcuate base plates coupled to the housing; and a plurality of magnets disposed external to the chamber body coupled to the housing via the two arcuate base plates and arranged asymmetrically about the chamber body, wherein the plurality of magnets are arranged in a plurality of magnet sets. 
     Other and further embodiments of the present disclosure are described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present disclosure, briefly summarized above and discussed in greater detail below, can be understood by reference to the illustrative embodiments of the disclosure depicted in the appended drawings. However, the appended drawings illustrate only typical embodiments of the disclosure and are therefore not to be considered limiting of scope, for the disclosure may admit to other equally effective embodiments. 
         FIG.  1 A  depicts a schematic cross-sectional view of a process chamber in accordance with at least some embodiments of the present disclosure. 
         FIG.  1 B  depicts a close-up cross-sectional view of an interface between a coil and the inner shield in accordance with at least some embodiments of the present disclosure. 
         FIG.  2    is a top schematic view of a plurality of magnets placed about the chamber body in accordance with at least some embodiments of the present disclosure. 
         FIG.  3    is a top schematic view of a plurality of magnets placed about the chamber body in accordance with at least some embodiments of the present disclosure. 
         FIG.  4    is an isometric exploded view of a portion of a process chamber and an external magnetic assembly in accordance with at least some embodiments of the present disclosure. 
         FIG.  5    is a top schematic view of a plurality of magnets placed about the chamber body in accordance with at least some embodiments of the present disclosure. 
         FIG.  6    is an isometric exploded view of a portion of an external magnetic assembly in accordance with at least some embodiments of the present disclosure. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The figures are not drawn to scale and may be simplified for clarity. Elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     Methods and apparatus for generating a magnetic field external to a physical vapor deposition (PVD) chamber to improve etch uniformity of a substrate disposed inside of the PVD chamber are provided herein. One or more external magnets may be disposed about the PVD chamber to advantageously improve plasma density near the external magnets inside of the PVD chamber during operation. 
     The PVD may use coils disposed between a target and a substrate support of the process chamber to maintain a plasma in the process chamber. The amount of capacitive coupling from the coil to the plasma and the discrete termination impedance, or capacitance, to the coil, affects the uniformity of plasma across the chamber. Finite capacitive coupling from the coil reduces the conduction current along the path of the coil, thereby reducing the magnitude of the local inductive electric field and affecting the ion distribution. The effect on the ion distribution leads to asymmetries, or non-uniformity, of etch rates on a substrate being processed. The inventors have observed that the asymmetries are correlated with azimuthal variations in input and pumping of gases, circuit issues related to transmission line matching to the coil, and particulars of the reactor configuration. The one or more external magnets may advantageously be disposed vertically above the coils of the PVD chamber and proximate the target of the PVD chamber. In some embodiments, the external magnet assembly is disposed vertically between the coil and the target. The one or more external magnets are generally disposed radially outward of the target. 
       FIG.  1 A  depicts a schematic cross-sectional view of a process chamber  101  in accordance with at least some embodiments of the present disclosure. The process chamber  101  may be a PVD chamber or any other suitable deposition or etch chamber. The process chamber  101  has a chamber body  105  that includes sidewalls  102  and a bottom  103 . A lid  104  is disposed on the chamber body  105  to define an interior volume  106  therein. A substrate support having a pedestal  108  is disposed in the interior volume  106  of the process chamber  101 . A substrate transfer port  109  is formed in the sidewalls  102  for transferring substrates into and out of the interior volume  106 . 
     An adapter  115  may be disposed between the chamber body  105  and the lid  104 . An external magnet assembly  184  having one or more external magnets (e.g., the one or more magnets  210 ) may be disposed between the sidewalls  102  and the lid  104  or the sidewalls  102  and the adapter  115 . The lid  104  may support a sputtering source, such as a target  114 . The target  114  generally provides a source of material which will be deposited in the substrate  118 . The target  114  consists essentially of a metal, such as titanium (Ti), tantalum (Ta), tungsten (W), cobalt (Co), nickel (Ni), copper (Cu), aluminum (Al), ruthenium (Ru), niobium (Nb), alloys thereof, combinations thereof, or the like. In some embodiments, the target  114  is at least about 99.9% of a metal, such as titanium (Ti), tantalum (Ta), tungsten (W), cobalt (Co), nickel (Ni), copper (Cu), aluminum (Al), ruthenium (Ru), or niobium (Nb). 
     The target  114  may be coupled to a DC source power assembly  116 . A magnetron  119  may be coupled adjacent to the target  114 . Examples of the magnetron  119  assembly include an electromagnetic linear magnetron, a serpentine magnetron, a spiral magnetron, a double-digitated magnetron, a rectangularized spiral magnetron, or the like. Alternately, powerful magnets may be placed adjacent to the target  114 . The magnets may be rare earth magnets such as neodymium or other suitable materials for creating a strong magnetic field. The magnetron  119  may be configured to confine the plasma as well as distribute the concentration of plasma along the target  114 . 
     A gas source  113  is coupled to the process chamber  101  to supply process gases into the interior volume  106 . In some embodiments, process gases may include one or more inert gases or reactive gases. Examples of process gases that may be provided by the gas source  113  include, but not limited to, argon (Ar), helium (He), neon (Ne), nitrogen (N 2 ), oxygen (O 2 ), chlorine (Cl), water vapor (H 2 O), or the like. 
     A pumping device  112  is coupled to the process chamber  101  in communication with the interior volume  106  to control the pressure of the interior volume  106 . In some embodiments, the pressure of the process chamber  101  may be maintained at about 1 Torr or less. In some embodiments, the pressure within the process chamber  101  may be maintained at about 500 millitorr or less. In other embodiments, the pressure within the process chamber  101  may be maintained between about 1 millitorr and about 300 millitorr. 
     In some embodiments, a controller  131  is coupled to the process chamber  101 . The controller  131  includes a central processing unit (CPU)  160 , a memory  168 , and support circuits  162 . The controller  131  is utilized to control the process sequence, regulating the gas flows from the gas source  113  into the process chamber  101  and controlling ion bombardment of the target  114 . The CPU  160  may be of any form of a general-purpose computer processor that can be used in an industrial setting. The software routines can be stored in the memory  168 , such as random-access memory, read only memory, floppy or hard disk drive, or other form of digital storage. The support circuits  162  are conventionally coupled to the CPU  160  and may comprise cache, clock circuits, input/output subsystems, power supplies, and the like. The software routines, when executed by the CPU  160 , transform the CPU  160  into a computer (controller  131 ) that controls the process chamber  101  such that the processes are performed in accordance with the present disclosure. The software routines may also be stored and/or executed by a second controller (not shown) that is located remotely from the process chamber  101 . 
     An additional RF power source  181  may also be coupled to the process chamber  101  through the pedestal  108  to provide a bias power between the target  114  and the pedestal  108 , as needed. In some embodiments, the RF power source  181  may provide power to the pedestal  108  to bias the substrate  118  at a frequency between about 1 MHz and about 100 MHz, such as about 13.56 MHz. 
     The pedestal  108  may be moveable between a raised position and a lowered position, as shown by arrow  182 . In the lowered position, a top surface  111  of the pedestal  108  may be aligned with or just below the substrate transfer port  109  to facilitate entry and removal of the substrate  118  from the process chamber  101 . The top surface  111  may have an edge deposition ring  136  sized to receive the substrate  118  thereon while protecting the pedestal  108  from plasma and deposited material. The pedestal  108  may be moved to the raised position closer to the target  114  for processing the substrate  118  in the process chamber  101 . A cover ring  126  may engage the edge deposition ring  136  when the pedestal  108  is in the raised position. The cover ring  126  may prevent deposition material from bridging between the substrate  118  and the pedestal  108 . When the pedestal  108  is in the lowered position, the cover ring  126  is suspended above the pedestal  108  and substrate  118  positioned thereon to allow for substrate transfer. 
     During substrate transfer, a robot blade (not shown) having the substrate  118  thereon is extended through the substrate transfer port  109 . Lift pins (not shown) extend through the top surface  111  of the pedestal  108  to lift the substrate  118  from the top surface  111  of the pedestal  108 , thus allowing space for the robot blade to pass between the substrate  118  and pedestal  108 . The robot may then carry the substrate  118  out of the process chamber  101  through the substrate transfer port  109 . Raising and lowering of the pedestal  108  and/or the lift pins may be controlled by the controller  131 . 
     During sputter deposition, the temperature of the substrate  118  may be controlled by utilizing a thermal controller  138  disposed in the pedestal  108 . The substrate  118  may be heated to a desired temperature for processing. After processing, the substrate  118  may be rapidly cooled utilizing the thermal controller  138  disposed in the pedestal  108 . The thermal controller  138  controls the temperature of the substrate  118  and may be utilized to change the temperature of the substrate  118  from a first temperature to a second temperature in a matter of seconds to about a minute. 
     An inner shield  150  may be positioned in the interior volume  106  between the target  114  and the pedestal  108 . The inner shield  150  may be formed of aluminum or stainless steel among other materials. In some embodiments, the inner shield  150  is formed from stainless steel. An outer shield  195  may be formed between the inner shield  150  and the sidewall  102 . The outer shield  195  may be formed from aluminum or stainless steel among other materials. The outer shield  195  may extend past the inner shield  150  and is configured to support the cover ring  126  when the pedestal  108  is in the lowered position. 
     In some embodiments, the inner shield  150  includes a radial flange  123  that includes an inner diameter that is greater than an outer diameter of the inner shield  150 . The radial flange  123  extends from the inner shield  150  at an angle of about ninety degrees or greater relative to the inside diameter surface of the inner shield  150 . The radial flange  123  may be a circular ridge extending from the surface of the inner shield  150  and is generally adapted to mate with a recess formed in the cover ring  126  disposed on the pedestal  108 . The recess may be a circular groove formed in the cover ring  126  which centers the cover ring  126  with respect to the longitudinal axis of the pedestal  108 . 
     The process chamber  101  has a coil  170  disposed in the interior volume  106  between the target  114  and the pedestal  108 . The coil  170  of the process chamber  101  may be just inside the inner shield  150  and positioned above the pedestal  108 . In some embodiments, the coil  170  is positioned nearer to the pedestal  108  than the target  114 . The coil  170  may be formed from a material similar in composition to the target  114 , for example, any of the materials discussed above to act as a secondary sputtering target. The coil  170  may be a single turn coil. 
     In some embodiments, the coil  170  is supported from the inner shield  150  by a plurality of chamber components, such as chamber component  100 , which may comprise or consist of coil spacers  110  (see  FIG.  1 B ). The coil spacers  110  may electrically isolate the coil  170  from the inner shield  150  and other chamber components. The coil  170  may be coupled to a power source  151 . The power source  151  may be an RF power source, a DC power source, or both an RF power source and a DC power source. The power source  151  may have electrical leads which penetrate the sidewall  102  of the process chamber  101 , the outer shield  195 , the inner shield  150  and the coil spacers  110 . The coil  170  includes a plurality of hubs  165  for providing power to the coil  170  and couple the coil  170  to the inner shield  150 , or another chamber component. The electrical leads connect to one or more hubs of the plurality of hubs  165  on the coil  170  for providing power to the coil  170 . One or more of the plurality of hubs  165  may have a plurality of insulated electrical connections for providing power to the coil  170 . Additionally, the plurality of hubs  165  may be configured to interface with the coil spacers  110  and support the coil  170 . In some embodiments, the power source  151  applies current to the coil  170  to induce an RF field within the process chamber  101  and couple power to the plasma for increasing the plasma density, i.e., concentration of reactive ions. 
       FIG.  1 B  depicts a close-up cross-sectional view of an interface between a coil  170  and the inner shield  150  in accordance with at least some embodiments of the present disclosure. The chamber component  100  may include a coil spacer  110 . In some embodiments, the chamber component  100  includes only a coil spacer  110 . The chamber component  100  may optionally include at least one hub receptor  130 . A fastener  135  may be utilized to hold the hub receptor  130  and coil spacer  110  together to form the chamber component  100 . For example, the fastener  135  may extend through the hub receptor  130  and into one of the plurality of hubs  165 . In some embodiments, the fastener  135  may include a central channel  175  extending through the fastener  135  along an elongate axis of the fastener  135  to prevent air pockets between the fastener  135  and plurality of hubs  165 . 
     The coil spacer  110  has a top portion  140  and a bottom portion  145 . The bottom portion  145  may be disposed proximate the inner shield  150 . The coil spacer  110 , the hub receptor  130 , and the fastener  135  may attach together to secure the coil spacer  110  to the inner shield  150 . In some embodiments, the bottom portion  145  of the coil spacer  110  is disposed proximate an opening  155  between the coil  170  and the inner shield  150 . The coil spacer  110  may facilitate maintaining the opening  155  between the coil  170  and the inner shield  150  to electrically isolate the coil  170  from the inner shield  150 . In some embodiments, the inner shield  150  may have a feature (not shown) which inter-fits with a complimentary feature of the coil spacer  110  to locate and/or secure the coil spacer  110  to the inner shield  150 . For example, the coil spacer  110  may have threads, ferrule, taper, or other structure suitable for attaching the coil spacer  110  to the inner shield  150 . 
     The hub receptor  130  may serve as a backing or structural member for attaching the coil spacer  110  to the inner shield  150 . Additionally, the hub receptor  130  or fastener  135  may interface with one of the plurality of hubs  165  of the coil  170 . The hub receptor  130  may have receiving features  185  for forming a joint or connection with respective complimentary hub features  180  on the one of the plurality of hubs  165 . In some embodiments, the hub features  180  and the receiving features  185  engage to form a structural connection between the one of the plurality of hubs  165  and the coil spacer  110  for supporting the coil  170 . The receiving features  185  and the hub features  180  may be finger joints, tapered joint, or other suitable structure for forming a union between the plurality of hubs  165  and each of the coil spacers  110  suitable for supporting the coil  170 . In some embodiments, the receiving features  185  may form part of an electrical connection. 
     One or more of the coil spacers  110  may have an electrical pathway (not shown in  FIG.  1 B ) extending there through. The electrical pathway may be configured to provide an electrical connection between the plurality of hubs  165  on the coil  170  and the power source  151  for energizing the coil  170 . Alternately, the coil spacers  110  may not provide an electrical pathway and the power for energizing the coil  170  is provided in another manner without passing through one of the coil spacers  110 . The electrical pathway may be a conductive path for transmitting an electrical signal. Alternately, the electrical pathway may be a void or space which provides accessibility of electrical connections between the power source  151  and one or more of the plurality of hubs  165  of the coil  170 . 
     The coil spacer  110  may be formed from a metal, such as stainless steel. In some embodiments, stainless steel powder having a size of 35-45 micrometers is a suitable precursor material as described further below. The coil spacer  110  may electrically isolate the coil  170  from the inner shield  150 . The coil spacer  110  may have an opening  190 . The opening  190  may be configured to accept one of the plurality of hubs  165 . The opening  190  may be disposed in the top portion  140  and extend towards the bottom portion  145 . In some embodiments, the opening  190  has a circular profile and is configured to accept one of the plurality of hubs  165  having a round shape. In another embodiment, the opening  190  is shaped to receive one of the plurality of hubs  165  having a complimentary inter-fitting shape. 
     In some embodiments, the coil spacer  110  includes a base plane  198  in alignment with an axis  197  and the bottom portion  145 . The base plane  198  generally extends across bottom portion  145 .  FIG.  1 B  also shows the outer shield  195  adjacent the chamber component  100 . While not connected with the chamber component  100 , the outer shield  195  is shown aligned in parallel with the axis  197 , the bottom portion  145 , and the base plane  198 . 
     In some embodiments, one or more of the coil spacer  110  or the coil  170  may have surfaces that are texturized to promote adhesion and minimize flaking of deposited material during operation of the process chamber  101 . For example, although not visible in  FIG.  1   , the coil  170  may have an inner sidewall that is texturized. 
       FIG.  2    is a top schematic view of a plurality of magnets  210  placed about the chamber body  105 . In some embodiments, the external magnetic assembly  184  includes a housing  208 . In some embodiments, the plurality of magnets  210  are coupled to the housing  208 . In some embodiments, the housing  208  is disposed between the chamber body  105  and the lid  104 . In some embodiments, the housing  208  is coupled atop the chamber body  105 . The housing  208  includes a central opening  206 . 
     The plurality of magnets  210  may be disposed at any suitable location, have any suitable polarity, and have any suitable strength. In some embodiments, the plurality of magnets  210  are permanent magnets. In some embodiments, the plurality of magnets  210  have a field strength of about 1500 to about 3000 gauss. In some embodiments, the plurality of magnets  210  have a diameter of about 0.3 to about 1.0 inches. The plurality of magnets  210  may be oriented in different directions, for example, north side up or south side up, as desired to facilitate a more uniform etch rate or deposition rate. In some embodiments, a coil RF side  230  of the process chamber  101  corresponding with a location of the power source  151  may be disposed opposite the substrate transfer port  109 . 
     In some embodiments, the plurality of magnets  210  are arranged in a plurality of magnet sets. In some embodiments, each magnet of the plurality of magnets  210  are the same size and strength. In some embodiments, the magnets sets comprise a single magnet or multiple magnets stacked on top of each other. In some embodiments, as shown in  FIG.  2   , the plurality of magnets  210  are arranged in four magnet sets. In some embodiments, the plurality of magnets  210  are disposed in an asymmetrical arrangement about the housing  208 . In some embodiments, some of the plurality of magnets  210  are oriented south side up and others of the plurality of magnets  210  are oriented north side up. In some embodiments, a first magnet set  210 A of the plurality of magnets  210  may include a single magnet oriented south side up. The first magnet set  210 A may be disposed on the coil RF side  230 . In some embodiments, a second magnet set  210 B of the plurality of magnets  210  may include one or more magnets, for example two magnets, stacked on top of each other so that the magnetic field strength of the second magnet set  210 B is greater than the magnetic field strength of the first magnet set  210 A. In some embodiments, the second magnet set  210 B is oriented south side up. In some embodiments, a third magnet set  210 C may include a single magnet oriented north side up. In some embodiments, a fourth magnet set  210 D may include one or more magnets stacked on top of each other. In some embodiments, the fourth magnet set  210 D may comprises two magnets oriented north side up. 
     In some embodiments, at least two magnet sets of the plurality of magnets  210  are disposed about 180 degrees from each other about a central axis  248  of the central opening  206 . For example, the second magnet set  210 B and the fourth magnet set  210 D may be disposed about 180 degrees from each other about the central axis  248 . In some embodiments, the first magnet set  210 A and the second magnet set  2106  are disposed at an angle  254  about a central axis  248  of the central opening  206 . In some embodiments, the angle  254  is about 50 to about 75 degrees. In some embodiments, the second magnet set  210 B and the third magnet set  210 C are disposed at an angle  256  about the central axis  248 . In some embodiments, the angle  256  is about 105 to about 125 degrees. In some embodiments, the third magnet set  210 C and the fourth magnet set  210 D are disposed at an angle  252  about the central axis  248 . In some embodiments, the angle  252  is about 80 to about 100 degrees. In some embodiments, the third magnet set  210 C is proximate the substrate transfer port  109 . 
       FIG.  3    is a top schematic view of the external magnetic assembly  184  in accordance with some embodiments of the present disclosure. In some embodiments, one or more base plates  320  are coupled to the housing  208 , and the plurality of magnets  210  are coupled to the one or more base plates  320 . The one or more base plates  320  may be arcuate shaped. For example, the one or more base plates  320  may comprise two base plates, each base plate coupled to a plurality of magnet sets of the plurality of magnets  210 . For example, the first magnet set  210 A and the second magnet set  210 B may be coupled to a first of the one or more base plates  320 , while the third magnet set  210 C and the fourth magnet set  210 D may be coupled to a second of the one or more base plates  320 . 
       FIG.  4    is an isometric exploded view of a portion of the process chamber and the external magnetic assembly  184 . In some embodiments, the one or more base plates  320  are coupled to the housing  208  via fasteners  402  extending through openings  404  in the one or more base plates  320 . In some embodiments, the external magnetic assembly  184  is coupled to the chamber body  105  via fasteners  412  extending through openings  410  in the housing  208 . In some embodiments, the housing  208  includes a plate  432  and an inner lip  436  extending upward from the plate  432  to define the central opening  206 . 
       FIG.  5    is a top schematic view of a plurality of magnets placed about the chamber body in accordance with at least some embodiments of the present disclosure.  FIG.  6    is an isometric exploded view of a portion of an external magnetic assembly  184  in accordance with at least some embodiments of the present disclosure. In some embodiments, as shown in  FIGS.  5  and  6   , the plurality of magnets  210  are arranged in six magnet sets ( 210 A through  210 F). In some embodiments, the plurality of magnets  210  are disposed within or embedded in the one or more base plates  320 . For example, the one or more base plates  320  may include openings to receive all or a portion of each of the plurality of magnets  210 . In some embodiments, a fastener plate  610  may be coupled to the one or more base plates  320  via one or more fasteners  620  to retain each magnet of the plurality of magnets  210  in the respective openings of the one or more base plates  320 . 
     In some embodiments, the first magnet set  210 A, the second magnet set  210 B, and the third magnet set  210 C may be coupled to a first base  320 A of the one or more base plates  320  with the second magnet set  210 B disposed between the first magnet set  210 A and the third magnet set  210 C. In some embodiments, the fourth magnet set  210 D, the fifth magnet set  210 E, and the sixth magnet set  210 F may be coupled to a second base  320 B of the one or more base plates  320  with the fifth magnet set  210 E disposed between the fourth magnet set  210 D and the sixth magnet set  210 F. In some embodiments, the first magnet set  210 A, the fifth magnet set  210 E, and the sixth magnet set  210 F are oriented north side up. In some embodiments, the second magnet set  2106 , the third magnet set  210 C, and the fourth magnet set  210 D are oriented north side down. 
     In some embodiments, each of the plurality of magnet sets are diametrically opposed to another set of the plurality of magnet sets. In some embodiments, the third magnet set  210 C is closer to the second magnet set  210 B than the first magnet set  210 A. In some embodiments, the sixth magnet set  210 F is closer to the fifth magnet set  210 E than the fourth magnet set  210 D. In some embodiments, the first magnet set  210 A and the second magnet set  210 B are disposed at an angle  510  from each other about the central axis  248 . In some embodiments, the angle  510  is about 70 to about 90 degrees. In some embodiments, the second magnet set  210 B and the third magnet set  210 C are disposed at an angle  520  from each other about the central axis  248 . In some embodiments, the angle  520  is about 40 to about 60 degrees. In some embodiments, the fourth magnet set  210 D and the third magnet set  210 C are disposed at an angle  530  from each other about the central axis  248 . In some embodiments, the angle  530  is about 40 to about 60 degrees. 
     While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof.