Improvement of Inductively Coupled Plasma Source

Abstract

Disclosed herein is an apparatus for processing substrates using an inductively coupled plasma source. The inductively coupled plasma source utilizes a power source, a shielding member, and a coil coupled to the power source. In certain embodiments, the coil is arranged with a horizontal spiral group and a vertically extending spiral group. According to certain embodiments, the shielding member utilizes a grounded member to function as a Faraday shield. Embodiments herein reduce parasitic losses and instabilities in the plasma generated by the inductively coupled plasma in a substrate processing system.

Description

[0001] This application is a divisional application of the invention patent application with the application date of July 10, 2018, application number "201880055802.9", and invention name "Improvement of Inductively Coupled Plasma Source". Technical Field

[0002] Embodiments of the present disclosure generally relate to inductively coupled plasma sources for processing substrates. Background Art

[0003] Inductively coupled plasma (ICP) process chambers are common in microelectronics manufacturing. In such reactors, plasma is formed in the reactor's gas by using an electric field within the gas, which tends to separate electrons from the gas atoms therein, producing a plasma of ions, free electrons, neutral species, and free radicals. In the ICP method of forming plasma, the electric field is generated by an AC current (e.g., radio frequency (RF)) that flows through one or more coils located outside the processing chamber, thereby inducing a current in the gas within the adjacent chamber gas volume. Power transfer efficiency, that is, the amount of power effectively transferred from the coils to the gas to form the plasma, is a key factor in the ICP process. The ICP source is typically capacitively and inductively coupled to the gas in the chamber, but capacitive coupling has a low coupling efficiency, and for this and other reasons, capacitive coupling is undesirable after the plasma is started or "ignited." Capacitive coupling allows the plasma to be ignited and operated at low power settings, thereby producing a low-density plasma (E-mode plasma). Once the plasma is established, inductive coupling transitions the plasma to a higher density (H-mode plasma), minimizing the effects of capacitive coupling.

[0004] One problem faced with ICP sources as described above is that E-mode plasmas and H-mode plasmas have different load impedances, which the RF matching network must adjust for. RF matching is used between the power source and the coil to "match" the effective coil circuit impedance to the plasma impedance.

[0005] Plasma instabilities can develop when the system switches between two plasma modes. Additionally, low-amplitude oscillations in impedance can cause instabilities with certain gas chemistries and wafer types. The sudden change in impedance presented to the power source and coil circuits by the plasma impedance change causes a change in the output power to the plasma. This, in turn, causes a change in the plasma impedance, which creates a positive feedback loop that leads to instabilities within the plasma coil and power source power coupling circuits. Summary of the Invention

[0006] The present disclosure generally relates to an apparatus for processing a substrate. In one embodiment, the apparatus includes a remote-field generator coupled to a coil and a shielding member. The shielding member includes a plurality of radial spokes extending outward from a center and a plurality of slots between the plurality of radial spokes. The coil can be configured to include an inner coil and an outer coil, wherein the inner coil and the outer coil each have a substantially horizontal portion.

[0007] In a further embodiment, the apparatus includes a remote field generator coupled to the coil and a shielding member. The shielding member includes a plurality of radial spokes of varying lengths extending inwardly from an outer diameter in a repeating pattern. The coil may be configured to include an inner coil and an outer coil, wherein the inner coil and the outer coil each have a substantially horizontal portion.

[0008] In a further embodiment, the apparatus includes a remote-field generator coupled to the coil and a shielding member. The shielding member includes a plurality of semicircular arc-shaped members extending from the radial member in an alternating clockwise-counterclockwise arrangement. The coil may be configured to include an inner coil and an outer coil, wherein the inner coil and the outer coil each have a substantially horizontal portion. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In order that the above-mentioned features of the present disclosure may be understood in detail, a more particular description of the disclosure, briefly summarized above, may be obtained by reference to the embodiments, some of which are illustrated in the accompanying drawings. It should be noted, however, that the drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, as the disclosure may admit to other equally effective embodiments.

[0010] Figure 1 is a schematic cross-sectional view of an exemplary cross-section of a prior art ICP system.

[0011] Figure 2 is a perspective schematic diagram of an inductively coupled source according to one embodiment.

[0012] Figure 3 yes Figure 2 Schematic cross-sectional view of an inductively coupled source.

[0013] Figure 4 is a cross-sectional view of a coil arrangement of one embodiment.

[0014] Figure 5 is a cross-sectional view of a coil arrangement of one embodiment.

[0015] Figure 6 is a plan view of a shield according to an embodiment.

[0016] Figure 7is a plan view of a shield according to an embodiment.

[0017] Figure 8 is a plan view of a shield according to an embodiment.

[0018] Figure 9 It is the circuit diagram of the inductively coupled source.

[0019] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. DETAILED DESCRIPTION

[0020] The present disclosure generally relates to apparatus for processing substrates using an inductively coupled plasma source. The inductively coupled plasma source utilizes a power source, a shielding member, and coils coupled to the power source. In certain embodiments, the coils are arranged in a horizontal spiral group and a vertically extending helical group. According to certain embodiments, the shielding member utilizes a grounded member to function as a Faraday shield. Embodiments herein reduce parasitic losses and instabilities in the plasma of a substrate processing system.

[0021] Figure 1 1 is a schematic cross-sectional view of an exemplary process chamber 100. It should be understood that embodiments may be implemented in conjunction with any process system that utilizes plasma to process substrates. Chamber 100 includes a main body 102 and a dielectric lid 104. A substrate 106 may be positioned on a substrate support 108 within main body 102. The substrate support 108 includes an electrode 110 disposed therein, which is coupled to a power source 112 via an RF matching network 114. A gas source 128 provides process gases into process chamber 100 via an inlet port 130. A vacuum pump 134 and a valve 136 control the pressure and outward flow of gases entering and exiting the process chamber 100.

[0022] An inductively coupled source 116 is disposed adjacent to and above the lid 104. The inductively coupled source 116 includes coils 118 and 120 coupled to an RF power source 122. The RF power source 122 is further coupled to an RF matching network 124. The coils 118 and 120 may be separate coils that are powered separately, or they may be related coils connected in a series arrangement. The inductively coupled source 116 generates a far field within a processing region of the body 102 that ionizes a process gas introduced therein to form a plasma 132 therein. A resonant circuit 126 is disposed between the RF power source 122 and the coils 118 and 120 to selectively deliver power to each coil to enhance control over the formation and maintenance of the plasma 132 and to control the relative intensity of the plasma on surfaces of the substrate 106 exposed to the plasma 132.

[0023] Shield 138 is disposed between inductively coupled source 116 and lid 104. A resistive heating element (not shown) is disposed within shield 138 and coupled to its power source 140. The resistive heating element provides heat to control the temperature of shield 138 and lid 104. Shield 138 is configured as a Faraday shield to increase control of the RF field generated by inductively coupled source 116.

[0024] Figure 2 is a schematic perspective view of an inductively coupled plasma source 200 according to one embodiment. Figure 2 In the figure, for ease of understanding, the source 200 is shown upside down with respect to its orientation when used to form a plasma in a chamber. The inductively coupled plasma source 200 has a top plate 202, which may be similar to Figure 1 The inductively coupled plasma source 116 is provided with a top plate 202. The top plate 202 may be made of a metal, such as aluminum. Other materials, such as quartz, or dielectric materials, such as dielectric ceramics, may also be used as the material for the top plate 202. The top plate 202 includes a plurality of vent holes (not shown) extending through the top plate 202. The vent holes may be configured to allow air from a fan unit (not shown) to flow through the coil and then through the openings to create a flow path for the air to cool the inductively coupled plasma source 200. The top plate 202 serves as a mounting surface for components of the inductively coupled plasma source 200 and includes inner and outer supports 212 and 214 to support the coil in a relatively fixed position spaced apart from the lid 104 of the process chamber 100.

[0025] A plurality of coils 206 (here, four coils) are coupled to the top plate 202 via inner legs 212 and outer legs 214. Each coil 206 includes an inner coil portion 208 (inner coil portions 208a-208d) and an outer coil portion 210 (outer coil portions 210a-210d), which are connected together via a coil shunt portion 211 (coil shunt portions 211a-211d). The inner coil portions 208a-208d are supported by a plurality of (here, four) inner legs 212, and the outer coil portions 210a-210d are supported by a plurality of (here, eight) outer legs 214. Each inner leg 212 and outer leg 214 is configured as an electrical insulator, such as a polymer, for example. As further described herein, the supports 212, 214 support and secure the positions of the coil sections 206, 208 relative to the top plate 202. Note also that Figure 2In the configuration, each inner coil section 208a-208d is supported by all four inner struts 212, but each outer coil section 210a-210d is supported by only seven of the eight outer struts 214, with each outer strut supporting at least three outer coil sections 210a-210d, and in some cases, four outer coil sections 210a-210d. Four freestanding guides 216 are also provided, with one of the four freestanding guides 216 positioned between every other adjacent outer strut 214, which are positioned along a generally circumferential path. The freestanding guides 216 include an upper member 218 and a lower member 220. The upper and lower members 218, 220 are coupled together by fasteners, such as threaded fasteners 213, to form the freestanding guides 216, although any suitable means for coupling the members 218, 220 may be used. The upper member 218 has a generally comb-shaped profile facing the top plate 202 and includes four inwardly extending grooves 222a-222d on its lower surface. Each groove 222 receives a separate outer coil portion 210a-210d therein. The grooves 222a-222d cooperate with the lower member to generally fix the radial position of each outer coil portion 210 and electrically isolate the different outer coil portions 210 from each other. Furthermore, each freestanding guide 216 is circumferentially positioned between two outer stations 214 and located at the location where the coil shunt portion 211 (coil shunt portions 211a-211d) extends from the inner coil portion 208 of each coil 206 to the outer coil portion 210. Thus, the freestanding guide 216 provides fixed vertical support to vertically align the outer coil portions 210a-210d and the coil shunt portions 211a-211d at the point where they intersect. Here, the freestanding guides 216 are separated by 90 degrees around the circumferential path of the outer station 214, with each coil shunt portion 211a-211d extending between its respective inner coil portion 208a-208d and outer coil portion 210a-210d. In some embodiments, the grooves 222a-222d may be formed in the lower member 220 or in both the upper member 218 and the lower member 220. The freestanding guides 216 and the struts 212, 210 are configured of a non-conductive material, such as a polymer or ceramic.

[0026] Figure 3 It is along Figure 2FIG2 is a cross-section of inductively coupled plasma source 200 as viewed along plane AA, wherein portions of the coils and inner and outer legs 208, 210 behind plane AA are not shown for ease of illustration. Top plate 202 is shown at the lower end of the figure. Outer coil portion 210 is supported by outer legs 214. Inner coil portion 208 is supported by inner legs 212. It should be understood that any number and configuration of coils 206, and corresponding inner coil portions 208, outer coil portions 210, and coil shunt portions 211, may be used.

[0027] The retainer 312 is coupled to each outer leg 214 by a threaded fastener 213. However, any means of coupling the legs 214 and the retainer 312 may be used, such as brazing or bonding. The retainer 312 includes one or more grooves 314, which are configured to cover a portion of the winding of the outer coil 210. A single outer coil portion 210 extends inward from each groove 314. The retainer 312 cooperates with the legs 214 to vertically secure the position of each outer coil portion and electrically isolate different outer coil portions 210 from each other. In further embodiments, the grooves 314 may be formed in the legs 214, or in both the retainer 312 and the legs 214. The retainer 312 includes a non-conductive material, such as a non-conductive polymer or a non-conductive ceramic.

[0028] Retainer 316 is similarly coupled to each inner support post 212 via threaded fasteners 213 (shown in shading). However, any device for coupling retainer 316 to support post 212 may be used. A groove 318 is formed within retainer 316 and is configured to receive an inner coil portion 208a-208d therein each time the inner coil portion 208a-208d completes a revolution. Retainer 316 cooperates with support post 212 to vertically secure the inner coil portions 208a-208d and electrically isolate them from each other. In certain embodiments, retainer 316 may include more than one member, such as a horizontal member and a vertical member. In further embodiments, groove 318 may be formed in support post 212 or in both support post 212 and retainer 316.

[0029] Rings 320, 322 are coupled to the inner and outer struts 212, 214, respectively. The inner ring 320 is connected to the inner stations 212 such that its outer diameter abuts the side surface of each inner station 212 and thus helps set the diameter of the circumference around which the inner struts 212 are positioned. The outer ring 322 is connected to the outer stations 214 by fasteners (e.g., threaded fasteners 326) that are placed along the bolt circle and thus the connection of the outer stations 214 to the outer ring 322 helps set the diameter of the circumference around which the outer struts 214 are positioned. The inner ring 320 and the outer ring 322 can be configured as a polymer, such as or other insulating materials, or a combination thereof. Any material suitable for forming an annular ring may be used. Inner ring 320 and outer ring 322 help support the posts 212, 214 to which they are coupled. Rings 320, 322 are coupled to posts 212, 214 by any suitable means, such as bolting, brazing, or bonding.

[0030] Figure 4 is a schematic diagram of a coil arrangement of an inductively coupled source according to one embodiment. Figure 2 The coil 206) includes an inner coil portion 402 and an outer coil portion 404. The outer coil portion 402 is configured as a concentric spiral winding along a substantially horizontal plane. The inner coil portions 402 are configured together in two planes. The first portion of each inner coil portion 402 extends as a nested spiral in a substantially horizontal plane. The second portion of each inner coil portion 402 is configured to be wound from the first portion in a nested manner along a substantially right cylindrical plane in the direction of the top plate 202. Figure 4 , the outer coil 404 includes four outer coil sections. The inner coil 402 includes four inner coil sections. It should be understood that other numbers of coil sections wound in this manner may be used. It should also be understood that the groups of coil windings (sections) may be multiple single continuous windings that include both inner and outer sections as a single length of conductor. In certain embodiments, the inner coil sections 402 and the outer coil sections 404 may be coupled to the same or different RF generators, such as Figure 1 Similarly, a capacitor (such as a tunable capacitor) may be provided at the junction of the inner coil portion 402 and the outer coil portion 404, with the inner coil portion and the outer coil portion connected thereto in parallel.

[0031] Figure 5 5 is an additional schematic representation of a coil arrangement for an inductively coupled source according to an embodiment. This coil 500 includes an inner coil portion 504 and an outer coil portion 502. The outer coil portion 502 is configured as nested concentric spiral windings in a substantially horizontal plane. The inner coil portion 504 is configured along two different geometric layouts. The first portion of the inner coil portion 504 is spirally nested along a substantially horizontal plane. The second portion of the inner coil portion 504 extends along an imaginary frustoconical surface around the axis in the direction of the top plate 202. As the distance D along the axis moves further away from the horizontal first portion of the inner coil portion 504, the turning radius R of the second portion of the inner coil portion 504 decreases. Therefore, the truncated cone of the inner coil portion 504 is formed at the center of the inner coil 504.

[0032] It will be appreciated that other numbers of such wound coil sections may be used. It will also be appreciated that a group of coil windings (sections) may be a plurality of single continuous windings comprising both the inner and outer sections as a single length of conductor. In certain embodiments, the inner coil section 504 and the outer coil section 502 may be coupled to the same or different RF generators, such as Figure 1 Similarly, a capacitor (such as a tunable capacitor) may be provided at the junction of the inner coil portion 504 and the outer coil portion 502, with the inner coil portion and the outer coil portion connected thereto in parallel.

[0033] Figure 4 as well as Figure 5 Embodiments may be provided on a process chamber lid (such as Figure 1 In such a configuration, the air gap between the cover or the shield disposed on the cover is minimized, thereby increasing the inductive coupling between the inductively coupled source and the plasma within the processing chamber. Figure 4 as well as Figure 5 The horizontal coils of the embodiments increase the flux area through which power is transferred from the coil to the plasma. The increased flux area provides increased plasma uniformity within the processing volume due to greater power distribution and increased inductive coupling. By increasing the inductive coupling between the inductively coupled source and the plasma, parasitic power losses are reduced, and plasma uniformity is further increased by reducing impedance variations in the RF matching network.

[0034] Figures 6 to 8 are plan views of various shields according to certain embodiments. Figure 1 A shield 138 is provided between the inductively coupled source and the processing chamber. The shield influences the RF field to modify the plasma within the processing chamber. The shield is typically made of a metal material, such as aluminum. In certain embodiments, the shield may be electrically grounded to form a Faraday shield. The shield may include a heating element coupled to the power source, thereby simultaneously serving as a lid heater. The heating element may be used to control the temperature of the shield and the lid of the processing chamber. The shield may include segments, such as four or more segments, that are electrically connected to form the shield. The configuration of the shield, such as the size and width of the slots or gaps through the shield, the thickness of the shield, or the number of segments, may be adjusted based on the desired level of RF influence.

[0035] Figure 6FIG6 is a plan view of a shield according to an embodiment. Shield 600 includes a discus-shaped body formed from four identical segments 602a, 602b, 602c, and 602d. A hole 604 is formed in the center of shield 600. A gap 606 is formed at the interface between adjacent shield segments 602a-602d and extends radially from hole 604 across the width of shield 600. Spoke-shaped slots 608 extend radially outward from hole 604 partially across the width of shield 600. Slots 610 extend through segments 602a-602d and are disposed between adjacent spoke-shaped slots 608 and between spoke-shaped slots 608 and adjacent gaps 606. The slots 610 are contained within a chevron-shaped portion formed by the spoke-shaped slots 608 and the gaps 606 , ie, opposite ends of radially staggered heaves within the shield segments 602 a - 602 d .

[0036] Figure 7 FIG2 is a plan view of a shield according to an embodiment. Shield 700 includes a generally flat, circular body 702 defining an outer circumference of shield 700. A plurality of spokes 704a, 704b, 704c, and 704d of varying lengths extend radially inward from the outer circumference of body 702 of shield 700 in a repeating pattern of first, second, third, and fourth lengths. Spoke 704a has a first length measured from the outer circumference toward the center of shield 700. Spoke 704b has a length measured from the outer circumference toward the center that is less than the length of spoke 704a. Spoke 704c has a length measured from the outer circumference toward the center of shield body 702 that is less than the length of spoke 704b. Spoke 704d has a length measured from the outer circumference toward the center of shield body 702 that is less than the length of spoke 704c.

[0037] Figure 8 FIG6 is a plan view of another embodiment of a shield. Shield 800 includes radial members 804 and semicircular arcuate members 802a and 802b. Radial members 804 extend radially from a center 806 of shield 600. Arcuate members 802a extend from radial members 804 in a clockwise direction with varying radii of curvature. Arcuate members 802b extend from radial members 804 in a counterclockwise direction with varying radii of curvature. Arcuate members 802a and 802b are configured to interweave or nest in an alternating pattern from center 806 outward.

[0038] Figure 9 is a circuit diagram of an exemplary matching network and source circuit, similar to Figure 1 The matching network 124 and the inductively coupled source 116. Figure 9In FIG, a power source 112 is coupled to a matching circuit 114. The matching circuit 114 includes a variable capacitor 902 and a variable capacitor 904 having two legs coupled at a node 920. The capacitor 904 including the legs is coupled to a source circuit 900. The source circuit 900 includes a first inductor 910 and a second inductor 912. The first inductor 910 may be part of an external coil, for example, Figure 2 as well as Figure 3 The second inductor 912 is coupled in series to a variable capacitor 914. A fixed capacitor 918 is coupled to the circuit between the first inductor 910 and the second inductor 912. The inductor 912 may be an inner coil portion, such as Figure 2 as well as Figure 3 208a-208d. Inductors 910 and 912 inductively couple power to process chamber 928 through chamber lid 104 and are in electromagnetic communication with a plasma disposed within the process chamber. The plasma has a first inductor 922 coupled to first inductor 910, a second inductor 926 coupled to second inductor 912, and a resistor 924.

[0039] Figure 9 The circuit is used to "match" the source power impedance to the impedance of the load in order to maximize the power transfer between the source and the load. Further, by utilizing variable capacitors 902, 904, and 914, the power can be independently controlled for the inner coil section (inductor 912) and the outer coil section (inductor 910). Controlling the power to the independent coil sections increases the uniformity of the plasma formed within the process chamber. In some embodiments, the power to the inner coil and the outer coil can be in-phase. In other embodiments, the power to the inner coil and the outer coil can be out-of-phase. The above embodiments can be used to practice Figure 9 circuit. We also understand that Figure 9 The circuit can be practiced with any coil source where increased control over the power of the inductor is advantageous.

[0040] The embodiments described herein advantageously produce a more uniform plasma field in a plasma processing system. Increasing inductive coupling and reducing capacitive coupling reduces variations in impedance matched by the matching network, which in turn reduces variations in power delivered to the plasma. Furthermore, power transfer efficiency is improved by reducing parasitic losses in the system.

[0041] 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, and the scope of the disclosure is determined by the following claims.

Claims

1. An apparatus for processing a substrate, comprising: roof; A coil, comprising an inner coil and an outer coil, wherein the outer coil is arranged in a first plane, and the inner coil comprises: a first section of the inner coil in the first plane and coupled to the outer coil; and a second segment coupled in series to the first segment, wherein the second segment is configured in a perpendicular arrangement that extends the first segment out of the first plane and terminates at an inner diameter of the inner coil, and the second segment is separated from the outer coil by the first segment; and Inner and outer legs are coupled to the top plate, wherein the inner coil is supported by the inner legs and the outer coil is supported by the outer legs, wherein the inner and outer legs extend from the top plate to support the coils away from the top plate.

2. The apparatus of claim 1, wherein the first section is configured in a horizontal planar arrangement.

3. The apparatus of claim 2, wherein the vertical arrangement is a vertical spiral arrangement with a decreasing turning diameter.

4. The apparatus of claim 3, wherein the coil comprises one or more conductors, and the inner diameter of the inner coil defines the inner diameter of the one or more conductors.

5. The apparatus of claim 3, wherein the first section includes a first set of coil portions, and the second section includes a second set of coil portions, each of the second set of coil portions having a turning diameter that decreases with increasing distance in a direction away from the outer coil.

6. The apparatus of claim 2, wherein the vertical arrangement is a vertical cylindrical arrangement having a constant turning diameter. The apparatus of claim 1 , wherein the top plate is made of aluminum material.

8. The apparatus of claim 1 , further comprising one or more freestanding guides, each of the one or more freestanding guides being disposed between every other adjacent outer struts, wherein each of the one or more freestanding guides supports the outer coil in a first plane.

9. The apparatus of claim 1, wherein the inner coil is connected to the outer coil using a coil shunt.

10. The apparatus of claim 1, wherein the inner and outer legs are constructed using an electrically insulating material.

11. An apparatus for processing a substrate, comprising: One or more conductors, each of the one or more conductors comprising: an inner coil, wherein the inner coil comprises a first segment coupled in series to a second segment; an outer coil coupled in series to the inner coil, the outer coil disposed in a first plane, wherein the outer coil is coplanar with the first segment and is arranged in a second concentric planar arrangement radially outward from the first segment; and a shunt extending from the inner coil of the one or more conductors to the outer coil, wherein the second segment is configured in a perpendicular arrangement extending from the first segment at an angle relative to the first plane and terminating at an inner diameter of the inner coil defining an inner diameter of the one or more conductors, and wherein the second section is separated from the outer coil by the first section; a plurality of inner legs supporting the inner coil of each of the one or more conductors; and A plurality of outer legs support the outer coil of each of the one or more conductors.

12. The apparatus of claim 11, wherein: The plurality of inner struts includes four inner struts, and The plurality of outer struts includes eight outer struts.

13. The apparatus of claim 11, wherein the one or more conductors include four conductors.

14. The apparatus of claim 11, wherein the vertical arrangement is a vertical spiral arrangement with a decreasing turn diameter.

15. The apparatus of claim 11, wherein the vertical arrangement is a vertical cylindrical arrangement having a constant turning diameter.

16. An apparatus for processing a substrate, comprising: One or more conductors, each of the one or more conductors comprising an inner coil coupled in series to an outer coil by a shunt extending from a respective inner coil to a respective outer coil of the one or more conductors, wherein: The outer coil and the shunt are arranged in a first plane; the inner coil comprising a first segment coupled in series to a second segment, wherein the first segment is configured in a first concentric planar arrangement within the first plane; The second section extends from the first section below and non-parallel to the first plane and is separated from the outer coil by the first section; and The second segments are configured in a vertical spiral arrangement or a vertical cylindrical arrangement, and the second segments terminate at an inner diameter of the inner coil that defines an inner diameter of the one or more conductors; one or more inner legs, the inner coil being coupled to the one or more inner legs; and One or more outer legs, the outer coil being coupled to the one or more outer legs.

17. The apparatus of claim 16, wherein the first section comprises at least two concentric coils disposed in the first plane.

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