Patent Publication Number: US-2009220865-A1

Title: Method and apparatus for source field shaping in a plasma etch reactor

Description:
BACKGROUND 
     1. Field 
     Embodiments of the present invention generally relate to methods and apparatus for plasma etching and, in particular, to a method and apparatus which provide improved etch control in a plasma etch reactor. 
     2. Description of the Related Art 
     Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two year/half-size rule (often called Moore&#39;s Law), which means that the number of devices on a chip doubles every two years. Today&#39;s fabrication plants are routinely producing devices having 0.15 μm and even 0.13 μm feature sizes, and tomorrow&#39;s plants soon will be producing devices having even smaller geometries. 
     The increasing circuit densities have placed additional demands on processes used to fabricate semiconductor devices. For example, as circuit densities increase, the widths of vias, contacts and other features, as well as the dielectric materials between them, decrease to sub-micron dimensions, whereas the thickness of the dielectric layers remains substantially constant, with the result that the aspect ratios for the features, i.e., their height divided by width, increases. Reliable formation of high aspect ratio features is important to the success of sub-micron technology and to the continued effort to increase circuit density and quality of individual substrates. 
     High aspect ratio features are conventionally formed by patterning a surface of a substrate to define the dimensions of the features and then etching the substrate to remove material and define the features. To form high aspect ratio features with a desired ratio of height to width, the dimensions of the features are required to be formed within certain parameters that are typically defined as the critical dimensions of the features. Consequently, reliable formation of high aspect ratio features with desired critical dimensions requires precise patterning and subsequent etching of the substrate. 
     Photolithography is a technique used to form precise patterns on the substrate surface, and then the patterned substrate surface is etched to form the desired device or features. Photolithography techniques use light patterns and resist materials deposited on a substrate surface to develop precise patterns on the substrate surface prior to the etching process. In conventional photolithographic processes, a resist is applied on the layer to be etched, and the features to be etched in the layer, such as contacts, vias, or interconnects, are defined by exposing the resist to a pattern of light through a photolithographic reticle having a photomask layer disposed thereon. The photomask layer corresponds to the desired configuration of features. A light source emitting ultraviolet (UV) light or low X-ray light, for example, may be used to expose the resist in order to alter the composition of the resist. Generally, the exposed resist material is removed by a chemical process to expose the underlying substrate material. The exposed underlying substrate material is then etched to form the features in the substrate surface while the retained resist material remains as a protective coating for the unexposed underlying substrate material. 
     Binary photolithographic reticles typically include a substrate made of an optically transparent silicon-based material, such as quartz (i.e., silicon dioxide, SiO 2 ), having an opaque light-shielding layer of metal, or photomask, typically chromium, disposed on the surface of the substrate. The light-shielding layer is patterned to correspond to the features to be transferred to the substrate. Binary photolithographic reticles are fabricated by first depositing a thin metal layer on a substrate comprising an optically transparent silicon-based material, and then depositing a resist layer on the thin metal layer. The resist is then patterned using conventional laser or electron beam patterning equipment to define the critical dimensions to be transferred to the metal layer. The metal layer is then etched to remove the metal material not protected by the patterned resist; thereby exposing the underlying optically transparent material and forming a patterned photomask layer. Photomask layers allow light to pass therethrough in a precise pattern onto the substrate surface. The terms “mask”, “photomask” or “reticle” will be used interchangeably to denote generally a substrate containing a precise pattern used for patterning other substrates. 
     Conventional etching processes, such as wet etching, tend to etch isotropically, which can result in an undercut phenomenon in the metal layer below the patterned resist. The undercut phenomenon can produce patterned features on the photomask that are not uniformly spaced and do not have desired straight, vertical sidewalls, thereby losing the critical dimensions of the features. Additionally, the isotropic etching of the features may over-etch the sidewalls of features in high aspect ratios, resulting in the loss of the critical dimensions of the features. Features formed without the desired critical dimensions in the metal layer can detrimentally affect light passing therethrough and result in less than desirable patterning by the photomask in subsequent photolithographic processes. 
     Plasma etch processing, known as dry etch processing or dry etching, provides a more anisotropic etch than wet etching processes. The dry etching process has been shown to produce less undercutting and to improve the retention of the critical dimensions of the photomask features with straighter sidewalls and flatter bottoms. However, dry etching may over-etch or imprecisely etch the sidewalls of the openings or pattern formed in the resist material used to define the critical dimensions of the metal layer, and imprecise etching may result from a lack of uniformity in the etching process across the photomask surface. Excess side removal of the resist material results in a loss of the critical dimensions of the patterned resist features, which may translate to a loss of critical dimensions of the features formed in the metal layer defined by the patterned resist layer. Further, imprecise etching may not sufficiently etch the features to provide the necessary critical dimensions. Failure to sufficiently etch the features to the critical dimensions is referred to as a “gain” of critical dimensions. The degree of loss or gain of the critical dimensions in the metal layer is referred to as “etching bias” or “CD bias”. The etching bias can be as large as 120 nm in photomask patterns used to form 0.14 μm features on substrate surfaces. 
     The loss or gain of critical dimensions of the pattern formed in the metal layer can detrimentally affect the light passing therethrough and produce numerous patterning defects and subsequent etching defects in the substrate patterned by the photolithographic reticle. The loss or gain of critical dimensions of the photomask can result in insufficient photolithographic performance for etching high aspect ratios of sub-micron features and, if the loss or gain of critical dimensions is severe enough, the failure of the photolithographic reticle or subsequently etched device. 
     With ever-decreasing device dimensions, the design and fabrication of photomasks for advanced technology becomes increasingly complex, and control of critical dimensions and etching uniformity becomes increasingly more important. Therefore, there is an ongoing need for improved plasma etching process control in photomask fabrication. 
     SUMMARY 
     Embodiments of the present invention generally provide improved methods and apparatus for plasma etching of various substrates, such as photomasks, semiconductor wafers, or other types of substrates. 
     One embodiment provides an apparatus for plasma etching which includes a process chamber defining a processing volume, a substrate pedestal disposed in the process chamber and configured to support a substrate thereon, one or more induction coils disposed outside the chamber above the substrate pedestal, and a field-shaping magnet assembly having a field-shaping magnet which is disposed adjacent to an induction coil. 
     In another embodiment, a method is disclosed for improved etching using a processing chamber having a substrate pedestal and induction coils disposed outside the chamber and above the substrate pedestal, and at least one field-shaping magnet disposed adjacent to an induction coil and above said pedestal, the method includes generating an electric field within a substrate processing volume using the induction coils, and selectively modifying the electric field using at least one field-shaping magnet. 
     In another embodiment, an apparatus for improved plasma etching is provided. The apparatus generally includes a process chamber defining a processing volume, a substrate pedestal disposed in the process chamber and configured to support a substrate thereon, one or more induction coils disposed outside the chamber and above the substrate pedestal, and a field-shaping magnet assembly disposed within a boundary defined by one of the induction coils, the field-shaping magnet assembly including a field-shaping magnet having a magnet and mounting base and a support member having a support column and support base. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a schematic cross-sectional view of an etch reactor. 
         FIG. 2  is a schematic view of an electric field produced by the induction coils shown in  FIG. 1 . 
         FIG. 3A  is a schematic cross-sectional detail view of the chamber shown in  FIG. 1 . 
         FIG. 3B  is a top view of the inductively coupled electric field shown in  FIG. 3A . 
         FIG. 3C  shows a plasma-produced ion flux for the chamber shown in  FIG. 3A . 
         FIG. 4  is a schematic cross-sectional detail view of an etching chamber according to an embodiment of the invention that may be implemented in the etch reactor of  FIG. 1 . 
         FIG. 5  is a top view of the inductively coupled electric field shown in  FIG. 4 . 
         FIG. 6  shows a plasma-produced ion flux for the chamber shown in  FIG. 4 . 
         FIG. 7A  is a top view of a field-shaping magnet shown in  FIG. 4  according to one embodiment of the invention. 
         FIG. 7B  is a top view of a field-shaping magnet shown in  FIG. 4  according to a second embodiment of the invention. 
         FIG. 7C  is a top view of the field-shaping magnet shown in  FIG. 4  according to a third embodiment the present invention. 
         FIG. 8  is a schematic cross-sectional view of another embodiment of the chamber shown in  FIG. 6  according to the present invention. 
         FIG. 9  is a schematic cross-sectional view of another embodiment of the chamber shown in  FIG. 8  according to the present invention. 
     
    
    
     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 
     Embodiments of the present invention generally provide a method and apparatus for improving the uniformity of plasma etching across the surface of a substrate, such as a photomask. Aspects of the invention will be described below in reference to an inductively coupled plasma etch chamber. Suitable inductively coupled plasma etch chambers include the Tetra™ photomask etch chamber and the Decoupled Plasma Source (DPS™) processing chamber available from Applied Materials, Inc., of Santa Clara, Calif. 
     Other process chambers may be used to perform the processes of the invention, including, for example, magnetically enhanced ion etch chambers as well as inductively coupled plasma etch chambers of different designs. Although the processes are advantageously performed with the Tetra™ photomask etch chamber, the description of the processing chamber is illustrative, and should not be construed or interpreted to limit the scope of any aspect of the invention. It is also contemplated that the invention may be beneficially practiced in other processing chambers, including those from other manufacturers. 
       FIG. 1  is a schematic cross-sectional view of an exemplary etch reactor  100  generally comprising a processing chamber  102  having a substrate pedestal  124 , processing volume  101 , and a controller  146 . The processing chamber  102  includes a chamber body  104  having conductive walls that support a substantially flat dielectric ceiling  108  which is transparent to radio frequency (RF) radiation. Other embodiments of the processing chamber  102  may have other types of ceilings, e.g., a dome-shaped ceiling. Induction coils  130  which are co-axially aligned and function as an antenna are disposed above the dielectric ceiling  108  and directly above the substrate pedestal  124  and processing volume  101 . The induction coils  130  comprise an inner coil  110 A and an outer coil  110 B that are co-axial and may be selectively controlled. The induction coils  130  are coupled through a first matching network  114  to a plasma power source  112 . The plasma power source  112  is typically capable of producing up to about 3000 Watts (W) at a tunable frequency in a range from about 50 kHz to about 60 MHz, with a typical operating frequency of about 13.56 MHz. The processing chamber  102  may also include a plasma screen  192  which is utilized to confine the plasma. 
     The substrate pedestal  124  (which acts as a cathode) supports a substrate “S” and is coupled through a second matching network  142  to a biasing power source  140 . The biasing power source  140  provides between about zero to about 600 W at a tunable pulse frequency in the range of about 1 to about 10 kHz. The biasing power source  140  produces pulsed RF power output. Alternatively, the biasing power source  140  may produce pulsed DC power output. It is contemplated that the biasing power source  140  may also provide a constant DC and/or RF power output. 
     A gas panel  120  is coupled to the processing chamber  102  to provide process and/or other gases to the interior of the processing chamber  102 . In the embodiment depicted in  FIG. 1 , the gas panel  120  is coupled to one or more gas inlets  116  formed in an annular gas channel  118  located within the sidewall of chamber body  104 . It is contemplated that the one or more gas inlets  116  may be provided in other locations, for example, in the dielectric ceiling  108  of the processing chamber  102 . 
     The pressure in the processing chamber  102  is controlled using a throttle valve  162  and a vacuum pump  164 . The vacuum pump  164  and throttle valve  162  are capable of maintaining chamber pressures in the range of about 1 to about 30 mTorr. 
     The temperature of the chamber body  104  may be controlled using liquid-containing conduits (not shown) that run through the walls of the chamber body  104 . Wall temperature is generally maintained at about 65 degrees Celsius. Typically, the chamber body  104  is formed from a metal (e.g., aluminum, stainless steel, and the like) and is coupled to an electrical ground  106 . The etch reactor  100  also comprises conventional systems for process control, internal diagnostic, end point detection, and the like. Such systems are collectively shown as support systems  154 . 
     The substrate pedestal  124  has a central protruding portion having a shape and dimensions that substantially match those of a typical substrate, e.g., a square shaped substrate, such as a photomask. A cover ring  175  and a capture ring  180  are disposed above the substrate pedestal  124 . An annular insulator  190  is provided between an outer portion of the substrate pedestal  124  and the cover ring  175 . 
     The capture ring  180  is designed to be moved between two positions by a lift mechanism  138  which comprises a plurality of lift pins  131  (one lift pin is shown) that travel through respective guide holes  136 . In a first position, the capture ring  180  is lowered beneath the top surface of the substrate pedestal  124 , leaving the substrate “S” supported by the substrate pedestal  124  for processing. In this first position, the capture ring  180  essentially couples with the protruding portions (not shown) of the cover ring  175  to form a complete annular ring such that the top surfaces of the capture ring  180  and the cover ring  175  are substantially in the same horizontal plane. At least certain portions of the capture ring  180  and the cover ring  175  are complementarily shaped, in certain embodiments. After substrate processing is completed, the capture ring  180  is lifted upwards to its second position, supporting the substrate “S” for transfer out of the processing chamber  102 , and is ready for receiving another substrate for processing. 
     The controller  146  comprises a central processing unit (CPU)  150 , a memory  148 , and support circuits  152  for the CPU  150  and facilitates control of the components of the etch reactor  100  and, as such, of the etching process. The inventive method is generally stored in the memory  148  or other computer-readable medium accessible to the CPU  150  as a software routine. Alternatively, such software routine may also be stored and/or executed by a second CPU (not shown) that is remotely located from the hardware being controlled by the CPU  150 . 
       FIG. 2  is a schematic view of an electric field produced by the induction coils  130  shown in  FIG. 1 . A single winding  203  of the induction coil  130  is used to represent multiple windings of either the inner coil  110 A or outer coil  110 B. The plasma power source  112  creates a time-varying current in the windings  203  which creates a time-varying magnetic field “B” represented by magnetic field lines  201  having the direction shown (indicated by arrows) at a moment in time. The changing magnetic field produces a changing magnetic flux which creates an electric field “E” perpendicular to the magnetic field as indicated by the circular electric field lines  202 . In the instant shown, the magnetic flux is increasing in magnitude and the electric field lines  202  have the direction shown (indicated by arrows, counterclockwise). As the magnetic field “B” changes, so does the magnitude and direction (clockwise or counterclockwise) of the electric field “E”. Since the processing chamber  102  has a dielectric ceiling  108  which is transparent to radio frequency (RF) radiation, the electric field “E” is inductively coupled to the processing volume  101 . 
       FIG. 3A  is a schematic cross-sectional detail view of the chamber shown in  FIG. 1 . The electric field produced by the inner coil  110 A and outer coil  110 B is represented by an edge view of circular electric field lines  202  in processing volume  101 . Process gas is provided to the processing volume  101  through gas inlets  116 . The process gas may include reactive gases and inert gases (e.g., argon) to enable etching of the substrate “S.” The electric field ionizes the process gas to create a plasma  301  which is inductively coupled to the processing chamber  102  through induction coils  130 . The ions created by the plasma  301  drift towards substrate “S” and are attracted to the surface of the substrate “S” by a bias electric field represented by electric field lines  302  which have the direction shown (arrows) at an instant in time (for a pulsed or alternating power source). The ions are accelerated along the direction of the bias electric field and strike the surface of the substrate “S” and etch the surface through physical (sputter etch) and/or chemical means (reactive etch). The bias electric field is provided by the biasing power source  140  which is coupled to the substrate pedestal  124 . 
       FIG. 3B  is a top view of the inductively coupled electric field shown in  FIG. 3A . In one embodiment, the substrate “S” is a photomask and may have a rectangular or square shape. In another embodiment, the substrate “S” is circular in shape (e.g., semiconductor wafer). The electric field which produces plasma  301  is represented by circular electric field lines  202  which are concentric and have the direction shown (arrows) at one moment in time. 
       FIG. 3C  shows a plasma-produced ion flux for the chamber shown in  FIG. 3A . The ions produced by plasma  301  may tend to drift towards substrate “S” and then accelerate under the bias electric field as the ions near the substrate. The motion of the ions from the plasma  301  to the substrate “S” and surrounding areas defines an ion flux which is represented by ion flux lines  305  with the direction shown (arrows). The actual trajectory of any individual ion may not be parallel to the direction of the ion flux lines  305 , but the net motion of many ions which move from the plasma  301  to the substrate “S” may be represented by the ion flux lines  305  as shown. 
     The number of ions in the ion flux may be determined in part by the number of ions produced by plasma  301 , which in turn may be determined by various processing parameters, such as gas pressure and the strength of the plasma-producing electric field. The etching rate along the surface of the substrate depends in part upon the ion flux at the substrate surface, and so it may be desirable to control the number of ions and the distribution of ions at the substrate surface. For example, edge effects may cause an edge-fast etch condition along the edges of the substrate so that the substrate is etched faster along the edges than at interior regions of the substrate, and so a reduction in the number of ions striking the edges of the substrate may allow a slowing of the etch rate at the edges to provide more uniform etching across the entire substrate surface. In another example, the chamber geometry may tend to cause a center-fast etch condition at a central region of the substrate, and it may be desirable to reduce the ion flux near the center of the substrate. 
       FIG. 4  is a schematic cross-sectional detail view of an etching chamber according to an embodiment of the invention that may be implemented in the etch reactor of  FIG. 1 . A field-shaping magnet assembly  400  is located between inner coil  110 A and outer coil  110 B and above dielectric ceiling  108 . In another embodiment, the processing chamber  102  may have more than two induction coils  130  and the field-shaping magnet assembly  400  may be disposed within a boundary defined by one of the induction coils  130 . The field-shaping magnet assembly  400  comprises a field-shaping magnet  406  and a support member  420 . The field-shaping magnet  406  comprises one or more magnets  401  and a mounting base  402 . 
     The mounting base  402  is a ring-shaped plate which encircles inner coil  110 A. In another embodiment, the mounting base  402  may be a square or rectangular plate with a central opening large enough to allow the inner coil  110 A to pass through, but the mounting base  402  may also have other shapes. In one embodiment, the mounting base  402  and the one or more magnets  401  are made of the same or similar materials. In another embodiment, the mounting base  402  is made of a material which is different than the material used for the one or more magnets  401 . 
     In one embodiment, the mounting base  402  comprises a material which is transparent to the RF radiation produced by the induction coils  130  and which can withstand heating during the operation of the induction coils  130 . In one example, the mounting base  402  comprises a temperatures resistant plastic, such as polyetheretherketone (e.g., PEEK), for example, although other suitable materials, other than plastics, may be used. In another embodiment, the mounting base  402  may comprise one or more magnetizable materials, which may include but are not limited to ferromagnetic materials, rare earth alloys, or alnico, for example. In yet another embodiment, the mounting base  402  may comprise a combination of magnetizable and non-magnetizable materials. 
     The magnet  401  comprises a ring-shaped permanent magnet coupled to the mounting base  402  and has a north pole “NP” and a south pole “SP” with the orientation shown in  FIG. 4A . In another embodiment, the polarity may be reversed so that the south pole “SP” is on “top” and further from dielectric ceiling  108  and the north pole “NP” is on “bottom” and closer to the dielectric ceiling  108 . In another embodiment, the magnet  401  comprises an electromagnet. The field-shaping magnet  406  may comprise one or more magnets  401 , and may include a combination of permanent magnets and electromagnets. The one or more magnets  401  may comprise one or more magnetizable materials, which may include but are not limited to ferromagnetic materials, rare earth alloys, or alnico, for example. 
     The mounting base  402  is coupled to the support member  420  which supports the mounting base  402  and magnet  401 . The support member  420  comprises a support column  422  attached to a support base  421 . The support member  420  is suitably adapted so that the mounting base  402  may be moved up or down the support column  422  so that the distance of the field-shaping magnet  406  from the dielectric ceiling  108  and substrate pedestal  124  may be adjusted. In another embodiment, the distance of the field-shaping magnet  406  from the dielectric ceiling  108  is fixed. In yet another aspect of the invention, the support member  420  may be adapted to include an actuator (e.g., electric motor, air cylinder) which can raise or lower the field-shaping magnet  406  upon command from controller  146 . In one embodiment, more than one support member  420  maybe used to support the field-shaping magnet  406 . The support member  420  may be made of the same or similar materials as the mounting base  402  so that the support member  420  is transparent to the RF radiation produced by the induction coils  130  and can withstand heating during the operation of the induction coils  130 . In another embodiment, the support member  420  comprises materials which are different from the materials used for the mounting base  402 . 
     Referring to  FIG. 4 , the field-shaping magnet  406  may be aligned approximately co-axially to induction coils  130  and have a ring-shaped magnet  401  located at a distance “R M ” from an axis  451  of the field-shaping magnet  400 . The field-shaping magnet  406  produces a static magnetic field which may interact with the changing magnetic field created by the induction coils  130 . The interaction of the magnetic fields may result in a decrease in the rate of change of the magnetic flux within regions of the processing volume directly beneath the field-shaping magnet  406 . According to Faraday&#39;s law, a decrease in the rate of change of the magnetic flux results in a decrease in the electric field strength for the electric field created by the changing magnetic field. This reduction in electric field strength is indicated in  FIG. 4  by a depletion zone  450  shown as dashed lines for electric field lines  202 . The depletion zone  450  is located approximately beneath the magnet  401  of field-shaping magnet  406  and at a radial distance “R” relative to the center of substrate “S”, and the radial distance “R” may be adjusted by adjusting the radial distance “R M .” In this way, the radial profile of the plasma-producing electric field may be adjusted along the surface of substrate “S” by changing the diameter or size of the field-shaping magnet  406 . 
       FIG. 5  is a top view of the inductively coupled electric field shown in  FIG. 4 . The depletion zone  450  is approximately located at radial distance “R” and near substrate edge  122 . The reduced electric field strength is indicated by dashed lines for electric field lines  202 . The depletion zone  450  “shadows” the ring shaped field-shaping magnet  406 . 
       FIG. 6  shows a plasma-produced ion flux for the chamber shown in  FIG. 4 . The reduced electric field strength within depletion zone  450  will tend to produce a reduction in plasma density within plasma  301  with a corresponding reduction in ion flux as indicated by the absence of ion flux lines  305  in the depletion zone  450 . It is to be understood that the absence of ion flux lines  305  merely indicates a reduction in the plasma density and ion flux and does not necessarily imply an absence of plasma  301  or ion flux within the depletion zone  450 . The reduction in plasma density and ion flux will tend to reduce the etch rate at those regions of the substrate surface which fall within the depletion zone  450 . In the example shown in  FIG. 6 , the depletion zone  450  is located over substrate edge  122  and so the etch rate may be reduced along the substrate edge  122 . Such a reduction in etch rate at the substrate edge  122  may be desirable when an edge-fast etching condition would otherwise normally exist. 
     As described herein, changing the diameter or size of the field-shaping magnet  406  and, thus, the location of depletion zone  450  relative to the substrate surface provides some adjustability in the radial profile of the plasma-producing electric field. The etch rate may also be controlled along the surface of substrate “S,” for instance, by changing the diameter or size of the field-shaping magnet  406 . For a given location of the depletion zone  450 , the etch rate may also be controlled by moving the field-shaping magnet  406  in a vertical direction relative to the substrate “S.” For example, if the field-shaping magnet  406  is moved closer to the dielectric ceiling  108  and substrate “S”, the reduction in electric field strength and plasma density may be increased resulting in a greater reduction in etch rate within the depletion zone  450 . The increased proximity of the field-shaping magnet  406  to the plasma  301  may have the effect of increasing the strength of magnet  401  relative to the induced magnetic field within processing volume  101  and thereby further reduce the strength of the plasma-producing electric field. On the other hand, if the field-shaping magnet  406  is moved further away from the dielectric ceiling  108  and substrate “S”, the effect on electric field strength and plasma density may be reduced to produce a much smaller or minimal reduction in the etch rate. As the previous examples suggest, the effect of the field-shaping magnet  406  may also be enhanced by increasing the field strength of magnets  401 . 
       FIG. 7A  is a top view of a field-shaping magnet  406  shown in  FIG. 4  according to one embodiment of the invention. The field-shaping magnet  406  comprises a single, ring-shaped, permanent magnet  401  having a radius or radial distance “R M ” from axis  451  of the field-shaping magnet  406 . The field-shaping magnet  406  is located between inner coil  110 A and outer coil  110 B. In another embodiment, the magnet  401  comprises an electromagnet which allows adjustability of the magnetic field strength by adjusting the current flow through the electromagnet. 
       FIG. 7B  is a top view of a field-shaping magnet  406  shown in  FIG. 4  according to a second embodiment of the invention. The field-shaping magnet  406  comprises two or more magnets  401  which are coupled to mounting base  402 . The mounting base  402  is a ring-shaped plate which encircles inner coil  110 A and has magnets  401  located at a radial distance “R M ” from axis  451  of the field-shaping magnet  406 . The magnets  401  may be permanent magnets and/or electromagnets. The electromagnets may be adapted so that the current flowing through the electromagnets may be adjusted to increase or decrease the magnetic field strength simultaneously for all electromagnets. In another embodiment, the current and magnetic field strength may be separately adjusted for each electromagnet. In yet another embodiment, the current and magnetic field strength may be separately adjusted for different groups of electromagnets of the field-shaping magnet  406 . 
       FIG. 7C  is a top view of the field-shaping magnet  406  shown in  FIG. 4  according to a third embodiment of the present invention. The mounting base  402  is square or rectangular in shape with a central opening or aperture  405  large enough to encircle inner coil  110 A. Multiple magnets  401  are coupled to the mounting base  402 . The locations of the magnets  401  may be indicated by one or more radial distances “R M ” which are measured from the magnet  401  to axis  451  of the field-shaping magnet  406 . In another embodiment, a single, permanent magnet  401  may be coupled to mounting base  402 . 
       FIG. 8  is a schematic cross-sectional view of another embodiment of the chamber shown in  FIG. 6  according to the present invention. A second field-shaping magnet assembly  500  is used with a first field-shaping magnet assembly  400 . The field-shaping magnet assembly  500  comprises a field-shaping magnet  506  and a support member  420 . The field-shaping magnet  506  comprises a magnet  501  and a mounting base  502  The magnet  501  and mounting base  502  are suitably sized to encircle outer coil  110 B, and the second field-shaping magnet  506  may be used to further enhance etch rate reduction near outer areas of substrate “S”, such as areas near substrate edge  122 . In another embodiment, the field-shaping magnet assembly  400  and field-shaping magnet assembly  500  are suitably adapted so that both are disposed between inner coil  110 A and outer coil  110 B. In a further aspect of the invention, three or more field-shaping magnets  406  and associated support members  420  may be disposed above dielectric ceiling  108  and around induction coils  130 . 
       FIG. 9  is a schematic cross-sectional view of another embodiment of the chamber shown in  FIG. 8  according to the present invention. A central field-shaping magnet assembly  600  is disposed within inner coil  110 A. The central field-shaping magnet assembly  600  comprises a field-shaping magnet  606  and a support member  420 . The field-shaping magnet  606  comprises a magnet  601  and a mounting base  602 . The magnet  601  comprises a single permanent magnet located above a central area of the substrate “S.” In another aspect of the invention, the magnet  601  comprises a single electromagnet. In another embodiment, the field-shaping magnet  606  is adapted to include multiple magnets  601  which may be disposed on mounting base  602  as shown in  FIGS. 7B and 7C . The field-shaping magnet  606  produces a depletion zone  450  located over a central area of substrate “S” and may be useful in reducing the etch rate near the substrate center. The embodiments shown in  FIGS. 4 ,  7 A- 7 C,  8  and  9  and described herein may be combined and used with other embodiments described herein for the field-shaping magnet assembly  400 . 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.