This invention relates to the generation of high-density plasma, particularly inductively coupled plasma (ICP), useful in processes such as semiconductor wafer processing. 
     BACKGROUND OF THE INVENTION 
     Gas plasma generation is widely used in a variety of integrated circuit (IC) fabrication processes, including plasma etching, plasma enhanced chemical vapor deposition (PECVD), and plasma sputter deposition applications. Generally, plasmas are produced within a process chamber by introducing a process gas at vacuum pressure into the chamber and then coupling electrical energy into the chamber to create and sustain a plasma in the process gas. The plasma may exist a t various ionization fractions from 10 −6  to a fully ionized plasma. 
     The plasma generally contains positively charged ions of working gas that are used for etching a surface of a substrate, sputtering material from a target for depositing a layer of the material onto such a substrate and ions of vaporized coating material to control the deposition of the material onto the substrate by ionized physical vapor deposition (iPVD). The plasma typically contains electrons equivalent in number to the positive charges in the plasma so that the plasma is macroscopically quasi-neutral. 
     Various ways of producing a plasma within a process chamber are used. Opposed electrodes can be oriented within the chamber to capacitively couple energy to the plasma. Microwave energy and electron cyclotron resonance (ECR) devices are also used. Inductive coupling of energy to the plasma is particularly desirable for producing a high-density plasma, particularly plasmas having a high ionization fraction with a relatively low electron energy or plasma potential. Inductively coupled plasmas (ICP) often use a coil or antenna-shaped and positioned with respect to the processing chamber to inductively couple energy into the processing chamber and thus create and sustain a plasma therein. 
     In some ICP systems, an inductive coil or antenna is positioned proximate the top portion of the chamber to create a plasma within the chamber. The antenna is positioned on one side of a dielectric plate or window at the top of the processing chamber, and electromagnetic energy from the antenna is coupled through the dielectric window and into the plasma. One such design is illustrated in U.S. Pat. No. 5,556,521. In other ICP systems, helical or solenoidal-shaped coils are wound around the outside of a cylindrical dielectric sidewall of the processing chamber to inductively couple energy to the plasma. One suitable dielectric material for a window or chamber sidewall is quartz. 
     The geometry of an ICP system is a factor in determining both plasma density and uniformity, which, in turn, can affect the processing uniformity over the area of the substrate. It is usually desirable to produce a uniform, high-density plasma over a significantly large area so that large substrate sizes can be accommodated. Ultra large-scale integrated (ULSI) circuits, for example, are presently formed on wafer substrates having diameters of 200 mm and 300 mm. 
     In an ICP system, plasma is excited by heating or exciting electrons in the plasma region of the processing chamber. The inductive currents which heat the plasma electrons are der iv ed from oscillating magnetic fields which are produced proximate the inside of the dielectric window or sidewall by RF currents within the inductive antenna or coil. The spatial distribution of those magnetic fields is a function of the sum of the individual magnetic fields produced by each portion or segment of the antenna or coil conductor. Therefore, the geometry of the inductive antenna or coil significantly determines the spatial distribution of the plasma, and particularly the spatial distribution and uniformity of the plasma ion density within the process chamber. Some coil configurations achieve a goal of delivering power linearly over a wide power range within a chamber of a given radius, but it is difficult to scale the process chamber to a larger size for handling larger substrates without significantly increasing the dimensions of the antenna or coil. Replacing an ICP antenna with one of a larger footprint calls for expensive modification to the processing system, and larger antennas and their associated plasmas exhibit greater sensitivity to process parameters within the chamber. For example, with a larger antenna, the plasma process becomes more sensitive to substrate-to-target distance, the target material, the pressure within the process chamber, and the height and width configuration of the chamber. Furthermore, large coils call for large dielectric windows, which must be very thick to withstand the pressure differential across the wall of a high vacuum chamber. 
     Current ICP systems utilizing planar spiral antennas exhibit asymmetry wherein the distribution of the plasma that is not aligned with the central axis of the chamber, which degrades the uniformity of the deposition or etch process over the area of the substrate. Further, planar antennas often exhibit a ring or doughnutshaped plasma for one process and corresponding set of parameters, while creating a centrally peaked plasma for another process and other parameters. Accordingly, the plasma shape and uniformity is not consistent within such ICP systems and will be process dependent. Therefore, the overall IC fabrication process will not be consistent from one plasma process to another plasma process. 
     Another drawback of planar antenna systems using an S-shaped coil is that the outer portions of the coil marginally affect the plasmas created by the central region of the coil and give an effect on the uniformity and density of the plasma that is different along one axis of the plane of the coil than along another axis in the plane of the coil. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to overcome drawbacks in the prior art and provide a plasma processing system, particularly an ICP system, in which a dense, uniform plasma is created. Another objective of the present invention is to provide a uniform plasma that is less dependent upon the size and shape of the process chamber than current plasma processing systems. Still another objective of the invention is to provide a plasma that is symmetrical in the processing chamber. 
     A further objective of the present invention is to provide a uniform, dense plasma over a large area, such as an area sufficient to handle 200 mm and 300 mm wafers, while maintaining a compact and inexpensive design of the inductive coil or antenna. A still further objective of the present invention to provide consistent plasma generation and thereby provide consistent processes that are less dependent upon process parameters such as pressure and chamber geometry or size. 
     According to principles of the present invention, a high-density, inductively coupled plasma (ICP) producing source is provided for coupling RF energy into a vacuum processing chamber. The source includes a window of a dielectric material in the chamber wall and having a surface in contact with a processing gas within a vacuum processing chamber. An RF antenna in the form of a coil is isolated from the processing gas by the dielectric material and has first coil segments, preferably lying in a plane parallel to the window, extending circumferentially in a ring close to the surface of the window. A permanent magnet assembly is configured and positioned to generate a ring-shaped magnetic tunnel in the processing chamber near the surface of the dielectric material in a ring-shaped area opposite the window from the coil segments. 
     In preferred embodiments of the invention, the RF coil is a three-dimensional coil having further second coil segments extending farther from the surface of the window than those extending in the ring. The further segments preferably lie in planes that intersect the dielectric material. Preferably, the half turn segments close to the window include two pairs of segments. The turns of each pair extend around the ring so that current flows in opposite directions, one clockwise and one counterclockwise. 
     The preferred coil is formed of segments of a single hollow conductor having a cooling fluid passage therein, with the segments connected in series across an RF power generator. The order of the segments preferably is: a clockwise extending first segment, then a second segment, then a clockwise extending first segment, then at least one segment, then a counterclockwise extending first segment, then a second segment, then a counterclockwise extending first segment. 
     In certain embodiments, at least three of the second coil segments are provided, with the segments arranged as: a clockwise extending first segment, then a second segment, then a clockwise extending first segment, then at least one second segment, then a counterclockwise extending first segment, then a second segment, then a counterclockwise extending first segment. In some such embodiments, the coil includes only seven segments, including the three curved second segments and two pairs of the first segments, arranged with no other intervening segments. In other such embodiments, other segments may be included. 
     In other embodiments, the coil includes a plurality of the curved second segments, at least two pairs of the first segments and at least one relatively straight intermediate segment lying in t he plane containing the pairs of first segments. Preferably, the segments are arranged as: a clockwise extending first segment, then a second segment, then a clockwise extending first segment, then an intermediate segment, then a counterclockwise extending first segment, then a second segment, then a counterclockwise extending first segment. In some such embodiments, the coil includes only seven segments, including the two curved second segments, two pairs of the first segments and one relatively straight intermediate segment lying in the plane containing the pairs of first segments and oriented such that magnetic field lines encircling the intermediate segment in a plane perpendicular thereto circulate in a direction opposite magnetic field lines encircling the first segments in the same plane, with the segments arranged as: a clockwise extending first segment, then a second segment, then a clockwise extending first segment, then the intermediate segment, then a counterclockwise extending first segment, then a second segment, then a counterclockwise extending first segment. In other such embodiments, other segments may be included. 
     For example, a coil that has a plurality of the curved second segments, at least two pairs of the first segments and at least one relatively straight intermediate segment lying in the plane containing the pairs of first segments, may include a plurality of at least four curved second segments, at least two pairs of the first segments and at least one relatively straight intermediate segment lying in the plane containing the pairs of first segments and oriented such that magnetic field lines encircling the intermediate segment in a plane perpendicular thereto circulate in a same direction as magnetic field lines encircling the first segments in the same plane, with the segments arranged as: a clockwise extending first segment, then a second segment, then a clockwise extending first segment, then a second segment, then an intermediate segment, then a second segment, then a counterclockwise extending first segment, then a second segment, then a counterclockwise extending first segment. In one such other embodiment, the coil includes four curved second segments, two pairs of the first segments and one such relatively straight intermediate segments. 
     In various embodiments, the plasma producing source includes a Faraday shield inside the chamber, closely spaced from, and generally parallel to, the dielectric material. The shield has a plurality of slots therethrough immediately opposite the dielectric material from the first segments. The slots include slots that lie in planes generally perpendicular to the first segments, and preferably that lie immediately opposite the dielectric material from the ring and oriented in a generally radial direction. Where intermediate segments are provided, slots are included immediately opposite the dielectric material from the each intermediate that lie in planes generally perpendicular to the intermediate segments. 
     The permanent magnet assembly preferably includes an inner magnet having a generally circular outer perimeter forming a first pole spaced radially inward of the ring and an annular outer magnet having an inner perimeter forming a second pole, the magnetically opposite of the first pole, spaced radially outward of the ring. The permanent magnet assembly may be located in various positions, one, for example, being adjacent the dielectric material outside of the chamber and another being adjacent, for example, embedded within, the shield. 
     The source is useful in an etching apparatus in combination with the vacuum chamber having a substrate support within the chamber. Alternatively, the source is further useful in an iPVD apparatus in combination with the vacuum chamber having a substrate support within the chamber that is further equipped with a magnetron sputtering cathode for sputtering material into a region within the chamber containing the high-density inductively coupled plasma. 
     RF antennae for inductively coupling energy into a vacuum processing chamber for sustaining a high-density plasma formed of the coils described herein are also provided by the invention. Faraday shields for use with RF antenna are also provided by the invention. Such shields include a conductive metal disc having a plurality of slots therethrough in a concentric annular area and oriented in the annular area in a generally radial direction and may include slots therethrough in a central area within the annular area. For deposition applications, particularly those involving coatings of metal, the slots are preferably angled, such as in a chevron shape or Z-shape, so as to prevent line-of-sight paths for particles of coating material moving from the plasma onto the dielectric material. 
     The plasma source of the invention used in etching coating applications produces a uniformity of plasma in the processing zone, and increased plasma source efficiency. 
     These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified diagram of an ionized physical vapor deposition or alternative etching apparatus embodying principles of the present invention. 
     FIG. 1A is an enlarged perspective view of the area designated  1 A in FIG.  1 . 
     FIG. 2 is a cross-sectional view through one embodiment of an ICP source according to principles of the present invention utilizing external magnets. 
     FIG. 3 is a cross-sectional view through an alternative to the ICP source embodiment of FIG. 2 utilizing internal magnets. 
     FIGS. 4A-4C are perspective views of three-dimensional coils of various embodiments of the ICP sources of FIGS. 2 and 3. 
     FIGS. 5A-5C are instantaneous magnetic field diagrams of the antennae of the respective FIGS. 4A-4C. 
     FIGS. 6A-6E are plan views of five alternative Faraday shield slot patterns for use with the respective coils of FIGS. 4A-4C. 
     FIGS. 7A-C are plan views of the power density distribution deposited into the plasma for each of the respective coils of FIGS. 4A-4C. 
     FIG. 8 is a cross-sectional graph of RF power density deposited into the plasma, with and without a Faraday shield, azimuthally averaged from FIG.  7 B. 
    
    
     DETAILED DESCRIPTION 
     The ICP source of the present invention can be used in various plasma processing systems, such as those for performing sputter etching and deposition processes, plasma-enhanced CVD (PECVD) processes, ionized PVD (iPVD) processes, and reactive ion etching processes (RIE). To facilitate understanding of the ICP source and its implementation, it is described in an iPVD apparatus of the type disclosed in U.S. Pat. No. 6,080,287, owned by the assignee of the current application. 
     FIG. 1 is a diagram of an iPVD apparatus  10  of the type described in U.S. Pat. No. 6,080,287, hereby expressly incorporated by reference herein. The apparatus  10  includes a vacuum chamber  12  bounded by a chamber wall  11  having a central vertical axis  13  and a dielectric window  14  centered on the axis  13  and sealed to an opening  15  at the top end of the chamber  12 . At the bottom of the chamber  12 , centered on the axis  13  and opposite the window  14 , is a vertically adjustable substrate support  16 , which may have a bias circuit  17  connected thereto to apply a controlled DC or RF potential to a substrate  18 , such as a semiconductor wafer, supported on the substrate holder  15  for processing. 
     On the top of the chamber  12  outside of the window  14 , is a threedimensional coil or antenna  20  connected through a matching network (not shown) to an RF energy source  21 . Preferably, but optionally, on the inside of the window  15 , parallel thereto, is a metal Faraday shield  22  having slots  23  therein. The shield  22  facilitates inductive coupling of RF energy from the coil  20  into the processing gas maintained at a vacuum inside of the chamber  12  while retarding capacitive coupling, which has the desirable effect of keeping the plasma potential low. The coil  20  and the shield  22  in combination with the window  14  may be defined to constitute an ICP source  25 . 
     For deposition, a magnetron sputtering cathode assembly  30  is provided that includes a sputtering target  32  supported in the wall  11  of the chamber  12 . The target  30  is preferably annular and surrounds the window  16 . A DC power supply  31  connected between the target  32  and the wall  11 , which is grounded, applies a negative potential to the target  32 , causing the target to discharge electrons into the gas within the chamber  12  to form a sputtering plasma adjacent the surface of the target  30 . Behind the target  32  is a permanent magnet assembly  33  which produces magnetic field lines forming a magnetron magnetic tunnel  34  that confines a sputtering plasma over the surface of the target  32 . 
     For etching systems, the magnetron cathode assembly  30  is absent or inoperative. For etching or for the deposition of electrically non-conductive material, the cross section of the slots  23  in the shield  22  may be rectangular or any other cross section, while with the deposition of metal or other electrically conductive coating materials, the slot cross section should be such as to provide no line-of-sight between the plasma and the dielectric window. Details of such a shield and other components of an iPVD apparatus are set forth in U.S. patent application Ser. No. 09/442,600 hereby expressly incorporated by reference herein. 
     The coil  20  is configured, as more fully explained below, to produce a generally ring-shaped high-density inductively coupled plasma within the chamber  12 . To enhance confinement of the ring-shaped plasma, permanent magnets  40  are provided in a plane parallel to the window  14  in the form of concentric rings  41 ,  42  centered on the chamber axis  13 , radially inside and outside, respectively, of segments  26  of the coil  20  that are proximate the window. The magnet rings  40  concentrate the IC plasma in a region below the antenna conductors  26  facing the plasma. In certain embodiments, the coil  20  may couple energy only inductively into the plasma within the chamber  12 . In other embodiments, by appropriate selection of design parameters, in lieu of, or in addition to, inductive coupling, RF power may be coupled to the plasma by electron plasma wave coupling and/or other resonant coupling of RF power producing electromagnetic waves inside the plasma. 
     FIG. 2 illustrates an embodiment of the ICP source  25  in which the permanent magnet rings  41 ,  42  are outside of the chamber  12  behind the window  14 . The magnet rings  41 ,  42  of the magnets  40  develop magnetic field lines  43  that arch over a region of the chamber inside of the window  14  adjacent the coil segments  26 . The magnet rings  41 ,  42  are shielded from the electromagnetic fields from the coil  20  by conductive shields  46 ,  47 . An insulator cover or cup  48  is provided which holds the coil  20  in place and fills the space between the coil  20  and the window  14  with a dielectric material such as TEFLON™. 
     FIG. 3 illustrates an alternative embodiment of the ICP source  25  in which the permanent magnet rings  41 ,  42  are inside of the chamber  12  embedded in the Faraday shield  22 . The magnet rings  41 ,  42  of the magnets  40 , so located, similarly develop magnetic field lines  43  that arch over a region of the chamber inside of the window  14  adjacent the coil segments  26 . The magnet rings  41 ,  42  are shielded from the electromagnetic fields from the coil  20  by a metallic shield  45  and by the Faraday shield  22  itself. An insulator cup or disc  29  may be provided to hold the coil  20  in place and fill the space between the coil  20  and the window  14  with dielectric material, preferably TEFLON™. 
     In both the embodiments of FIGS. 2 and 3, the magnet rings  41  and  42  may be continuous magnetic rings or may be formed of segmented rings or a series of individual magnets, and may form, for example, a toroidal magnetic field inside the plasma within the chamber  12 . The inner magnet  41  may also be in the shape of a disc. 
     In FIG. 4A, a three-dimensional coil or antenna  20   a  is illustrated, with antenna turns, first and second segments  51 ,  52 , respectively, in two plane angles such that approximately half of each complete turn is in one plane and half in the other. The planes are illustrated as perpendicular to each other, but other angles may also be used and more than two plane angles may be used. The distance and angles of the half turns with respect to the dielectric window affect the RF magnetic field penetration through the window and electrostatic Faraday shield  22 . Consequently, the distribution inside of the plasma of the power coupled from the coil  20  is also affected. The magnetic field line distribution can accordingly be adjusted to either spread or constrict the size of the plasma. 
     Examples of different coil structures are illustrated with the coils  2   b ,  2   c  of FIGS. 4B and 4C. Both of these coils  2   b ,  2   c  include the coil first and second segments  51 ,  52 . The segments  51  are those close to the window  14  for providing the most immediate energy for coupling into a ring-shaped space in the chamber  12  for forming the ring-shaped plasma. In addition, the coils  2   b ,  2   c  include a central coil segment  53 ,  54 , respectively. The current in the segment  53  is in the same direction as the current in segments  51  where the current in segment  54  is in the opposite direction as the current in segments  51 . 
     The coils  20   a ,  20   b ,  20   c  produce instantaneous magnetic fields having the shapes illustrated in FIGS. 5A-C, respectively, when the antennae are energized in the absence of the Faraday shield  22  or the permanent magnets  40 . All of the antennae or coils  20  produce strong RF magnetic field lines  58  around the conductors  51  that are close to the window  14 . These field lines  58  extend into the chamber and energize the ring-shaped plasma. With the Faraday shield  22  designed to effectively inductively couple energy into the plasma, the magnets  40  concentrate and confine the plasma energized in the RF field lines  58  under the field lines  43  from the permanent magnets  40 . The field lines  43  produce a magnetic tunnel for the high-density ICP that is distinct from the magnetron cathode magnetic tunnel  34  over the target  32 . 
     The Faraday shield  22  is a conductive metal plate having the slots  23  formed therethrough in patterns that most effectively facilitate inductive coupling of RF energy from the coil  20  into the plasma while, preferably, avoiding capacitive coupling from the coil  30  into the plasma. FIGS. 6A-6E illustrate alternative shields  22   a - 22   e , each having a different pattern of slots  23   a - 23   e . Slots  23  that will achieve such coupling include slots that are generally perpendicular to the conductors  51  that are immediately adjacent the window  14  and generally perpendicular to conductors  53 ,  54 , where present. Slot pattern  23   a  of the shield  22   a  is acceptable if used with any of the coils  20   a - 2   c  and preferably with the slots  23   a  oriented perpendicular to the segments  52 . Slot pattern  23   c  of the shield  22   c  is a recommended pattern for the coil  20   a , but not coils  2   b  or  2   c , since they do not permit effective inductive coupling from the central segments  53 ,  54  into the chamber  12 . The other slot patterns  23   b ,  23   d ,  23   e  are recommended patterns for coils  20   b ,  20   c , and are effective also with the coil  20   a.    
     FIGS. 7A-C are two-dimensional graphs representing the power density distribution deposited into the plasma in a plane parallel and close to the window  14  inside of the chamber  12 , particularly inside of the Faraday shield  22 . In these figures, relatively brighter areas represent relatively greater plasma density. In FIG. 7A, a ring-shaped plasma, as produced by the coil  20   a , is illustrated having greater intensity at the top and bottom than at the sides. In FIG. 7B, a ring-shaped plasma, as produced by the coil  20   b , is illustrated as having a more distinct ringshaped plasma than the coil  20   a  as well as a lower density plasma in the center. In FIG. 7C, a ring-shaped plasma, as produced by coil  20   c , is less distinct and has a center component due to the reverse direction central coil segment  54 . FIG. 8 is a graph of power density deposited into the plasma azimuthally averaged from data from the coil  20   b  of FIG. 7B, with and without the Faraday shield  22 . 
     While the above description and accompanying drawings set forth various embodiments of the invention, it will be apparent to those skilled in the art that additions and modifications may be made without departing from the principles of the invention.
A processing system for processing a substrate with a plasma is provided with an antenna for producing a ring-shaped inductively coupled plasma in a vacuum processing chamber particularly useful for coating or etching semiconductor wafer substrates. A three-dimensional antenna in the form of a coil provides spacial distribution of plasma parameters in a ring-shaped region inside of the chamber that can be adapted to specific physical and process requirements. An axially symmetric permanent magnet assembly enhances the ring-shaped concentration of a high-density inductively coupled plasma by trapping the plasma in the ring-shaped region near the inside of a dielectric window located in the chamber wall in close proximity to segments of the antenna that lie adjacent the outside of the window.