Abstract:
A wafer processing chamber includes a wafer support, a dielectric window and coaxial coils located outside the dielectric window for inducing a plasma within the chamber. A variety of coil/dielectric windows are described together with protocols for their control.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This is a continuation-in-part (C.I.P.) of application Ser. No. 08/938,995, filed Sep. 26, 1997, the entirety of which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to plasma processing apparatus.  
           [0004]    2. Description of the Related Art  
           [0005]    Plasma processing apparatus is used extensively in the fabrication of semiconductor devices and in many other processes in which the deposition of coatings or the etching of surfaces is required. Particularly when these processes are being carried out on a production scale, uniformity between batches becomes extremely important.  
           [0006]    Generally such plasmas are generated in a vacuum chamber and it is preferred that the means for generating the plasma is either located outside the chamber or within a protective wall, in order to reduce the disturbance to the plasma potential when radio frequency power is applied.  
           [0007]    Thus there have been suggestions of surrounding the plasma chamber by a single coil as in U.S. Pat. No. 4,844,775 and a number of Parties have suggested placing a single spiral coil against one end of a processing chamber. An example of this is EP-A-0379828. Usually a dielectric window is provided adjacent the coil. U.S. Pat. No. 5,309,063 describes a variation on this in which the single coil in re-entrant into the plasma chamber within a dielectric cup and proposals have been made in U.S. Pat. Nos. 52,801,154 and 5,397,962 for a plurality of spaced antennae located so as to allow the creation of rotating electro-magnetic fields. U.S. Pat. No. 5,468,296 discloses a plasma generating apparatus formed as a series of interconnected concentric rings and connected to a single power source. EP-A-489407 provides a stack of single turn antennae which are positioned one above the other at the same radial distances from the axis of a plasma generating reactor. U.S. Pat. No. 5,309,063 discloses a plasma generator wherein a single spiral coil is set into a cup-shaped window set into the centre of the top wall of the processing chamber of the device. WO-A-96 18208 illustrates a plasma processor having a planar coil located adjacent to individually supported dielectric windows of a processing chamber.  
         SUMMARY OF THE INVENTION  
         [0008]    The aforementioned proposals suffer to one degree or another from lack of uniformity in the plasma, for example in the vicinity of a workpiece. In some cases a controlled lack of uniformity is desirable. This would be difficult to achieve with the prior art arrangement, but can be provided by at least some embodiments of the invention.  
           [0009]    From one aspect the present invention consists in plasma processing apparatus including a processing chamber having a working volume of regular cross-section such that an axis passes through the centre of symmetry of a pair of opposed faces of the volume, and a plurality of separate Radio Frequency (RF) plasma generating antennae for predominately inducing an electric field, each having a coil portion of geometric shape corresponding to the shape of the cross-section, disposed at or adjacent to a wall of the chamber with the portions being coaxial with each other and set at different distances from said axis.  
           [0010]    The matching of the geometry of the coils with the cross-section of the working volume of the chamber means that the plasma which is struck conforms essentially to the chamber. Further the provision of these independent coils in this array allows independent control of the coils and hence control of the plasma between different parts of the working volume. Thus, in a preferred embodiment, the apparatus further comprises means for varying the effective output of the antennae. For example the means may vary the magnitude, frequency and/or relative phase of the RF power supplied and/or the relative spacing between the coils and the working volume.  
           [0011]    The cross-section, and hence the coil portions, may be circular, square, rectangular, hexagonal or octagonal. The coil portions may surround the chamber, but it is preferred that they are disposed adjacent to or on the end of the chamber. The chamber may include at least one dielectric window adjacent to a coil portion. This window may be re-entrant relative to the chamber and there may be a plurality of discrete windows each extending parallel to and over at least the width of an associated coil portion. Thus, when the coil portions are generally circular, there may be a series of spaced annular windows.  
           [0012]    The apparatus may further include at least one detector for detecting the degree of uniformity of the plasma or of a plasma driven process and feedback means for varying the effective relative outputs of the antennae to improve the uniformity.  
           [0013]    From another aspect the invention consists in plasma processing apparatus including a processing chamber having a working volume, at least one Radio-Frequency (RF) plasma-generating antenna, the or each antenna being disposed in a respective dielectric trough located in a wall of the chamber.  
           [0014]    The or each antenna may be non-planar and may transfer power through the inner and outer wall and the base of the trough, in which case one turn may lie adjacent one wall of the trough, another adjacent the other wall and a third adjacent the base.  
           [0015]    From a still further aspect the invention consists in plasma processing apparatus wherein the antennae and an associated power source are designed to create a plasma of a predetermined skin depth and the antennae are separated by at least twice said predetermined skin depth.  
           [0016]    The invention further includes plasma processing apparatus including a processing chamber having a working volume, a plurality of Radio-Frequency (RF) plasma generating antennae and an associated power source designed to create a plasma of a predetermined skin depth, and said antennae being separated by at least twice said predetermined skin depth.  
           [0017]    Although the invention has been defined above it is to be understood that it includes any inventive combination of the features set out above in the following description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The invention may be performed in various ways and specific embodiments thereof will now be described, by way of example with reference to the following drawings, in which:  
         [0019]    [0019]FIG. 1 is a schematic drawing of plasma process apparatus;  
         [0020]    [0020]FIG. 2 is a cross-sectional view through a coil assembly for use in the apparatus of FIG. 1;  
         [0021]    [0021]FIG. 3 is a view from above of the assembly of FIG. 2;  
         [0022]    [0022]FIG. 4 illustrates an alterative coil assembly;  
         [0023]    [0023]FIGS. 5 and 6 are respective vertical sections and plan views of further coil assembly;  
         [0024]    [0024]FIG. 7 is a vertical section through yet another coil assembly;  
         [0025]    [0025]FIGS. 8 and 9 are a vertical section and a plan view respectively through a further coil assembly; and  
         [0026]    [0026]FIG. 10 is a vertical section through yet another embodiment of the coil assembly; and  
         [0027]    [0027]FIG. 11 is a vertical section through a still further coil assembly;  
         [0028]    [0028]FIGS. 12 and 13 are a plan view and a vertical section respectively of an additional coil assembly;  
         [0029]    [0029]FIGS. 14 and 15 a - 15   d  are a plan view and alternative vertical sectional views, respectively, of another coil assembly;  
         [0030]    [0030]FIG. 16 is a plan view of a further coil assembly whilst  
         [0031]    [0031]FIG. 17 is a schematic 3D representation of the coil. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    [0032]FIG. 1 shows a plasma processing apparatus  10  in schematic vertical cross-section. It includes the wafer processing chamber  11  and the means  12  by which a wafer is supported and moved to the appropriate level for processing. The processing chamber  11  is generally a metal container of circular or rectangular cross-section, although other multi-sided forms are possible. One or more ports will exist to permit gas to be admitted and possibly diagnostics. A part  13  of the chamber  11 , or re-entrant module, is constructed out of a dielectric material and electrical coils  14 , 15  (two of which are shown by way of example) are located outside of this dielectric part  13  and are used to couple radio frequency power into the plasma within the chamber  11 . The chamber contains a cylindrical or rectangular working volume  16  in which the plasma is struck. This volume has an axis  17  which passes through the centres of symmetry of its end faces.  
         [0033]    In order to improve the confinement of the plasma within the chamber and therefore increase the plasma density, magnets (not shown) may be located in particular orientations around the walls and/or top of the chamber  11 . The use of small permanent magnets for this purpose is well known and is applicable to all embodiments described.  
         [0034]    Under normal operating conditions a high vacuum is maintained within the chamber  11 , which necessitates that the part  13  constructed out of dielectric material is appropriately sealed to the main structure of the chamber  11 .  
         [0035]    The coils for coupling radio frequency power into the plasma are located close to the surface of the dielectric section in order to produce an efficient transformer action into the plasma. They may be wound in a number of forms as described later.  
         [0036]    Radio frequency power produced in an appropriate power supply  18 , is fed to each of the coils  14 ,  15  by an impedance matching system  19 . This system present a constant impedance to the power supply, while allowing the RF power fed to each coil  14 ,  17  to be controlled. Alternatively separate power supplies and impedance matching systems may be used for each coil, or other variations of the system. The control of the RF power fed to each coil may be by operator intervention. Alternatively, and of potentially greater application, it may be by control signals received from a system which monitors the plasma or process spatial uniformity, thereby forming a feedback loop. Thus, by way of examples more RF power may be coupled to the plasma by coils  14 ,  15  further from the axis of symmetry  17 , enabling the wall loses of plasma, which usually occur to be compensated for.  
         [0037]    Details of coil configurations are shown in FIG. 2 and following Figures. In all cases multiple coils are used, permitting control of the plasma spatial uniformity by adjustment of the magnitude, frequency or relative phase of the radio frequency power fed to each coil, or by adjustment of the physical position of each coil.  
         [0038]    [0038]FIGS. 2 and 3, shows a design in which each coil  14   a ,  15   a  is of planar, or close to, annular spiral form and is placed behind a corresponding annular ring  20 ,  21  of dielectric material set in the top of the plasma chamber  11 . However, coils of the type shown in FIG. 14 and  15  may advantageously replace the coils  14   a  and  15   a  to enhance the power transfer. In that case the rings  20 ,  21  will be replaced by dielectric troughs. Since the supporting structure  22 ,  23  for the annular dielectric rings is also annular in form, a number of substantial bracing struts  24  are required to provide support against the forces encountered when the chamber is evacuated. Under some circumstances, particularly when the innermost coil is of small diameter, the innermost dielectric ring  20  may be replaced with a circular disc of dielectric material, thereby removing the requirement for the central support.  
         [0039]    As an alternative to the dielectric ring structure, a single disc of dielectric material may be used with the annular spiral coils set behind it. This approach reduces the complexity of the system, but brings the disadvantage that because of its large diameter, the dielectric disc must be constructed from thick material in order to withstand the close to atmospheric pressure differential across it. The efficiency with which radio frequency power can be coupled into the plasma is reduced as the distance between the coils and the plasma is increased.  
         [0040]    When a planar annular spiral coil is located outside of a dielectric window and a plasma is formed in the chamber below the coil, then the RF magnetic field lines will be almost radial for the greater part of the distance that they intersect the plasma. From Faraday&#39;s law, an azimuthal electric field and an associated current are induced within the plasma. The plasma current opposes the direction of the coil current and is confined to a layer near the window having a thickness of the order of the skin depth for penetration of RF into the plasma.  
         [0041]    Provided that separate coil units are sufficiently well spaced apart, the interaction between coil units will be minimal and each may be treated separately.  
         [0042]    While circular coils and cylindrical working volume have been described in connection with this and the following embodiments, this does not preclude the use of planar annular rectangular coils, or hexagonal coils, or coils of any other shape appropriate to the plasma chamber or working volume. The dielectric window, when used in the various embodiments, may then be in one piece to suit the coil shape or in annular sections of appropriate shape.  
         [0043]    The coils in every case may be constructed out of single or multiple strands of conducting material of circular, square, hexagonal or other appropriate cross-sectional form, or particularly for high power operation, the coils may be formed out of tubular conducting material through which water or another suitable coolant can be passed.  
         [0044]    For all embodiments the gas feed to the chamber may be via a plenum feeding a series of small equispaced holes in a ring about the axis of symmetry of the working volume, set in an appropriate support ring between dielectric rings. This does not preclude, however, the use of additional/alternative gas feed points at other positions in the chamber.  
         [0045]    Further, to improve plasma confinement in the chamber, small permanent magnets may be located in suitable slots in some or all of the support rings, in addition to any that are located on the side walls of the chamber.  
         [0046]    [0046]FIG. 4, shows a design in which a number of annular planar, or near planar, coils  14   b ,  15   b  are located inside the plasma chamber. Each coil is electrically insulated from the plasma by means of continuous solid, or flexible insulating material around each individual turn of each coil or around each complete coil unit. Electrical connections to each coil unit are brought into the plasma chamber through suitable vacuum seals in the chamber walls or top.  
         [0047]    The coils, at least as shown in FIGS.  2  to  4 , may be single turn coils and as such neither planar coils nor solenoids. In some instances a mixture of single turn and multiple turn coils may be used.  
         [0048]    [0048]FIGS. 5 and 6, show a design in which a circularly symmetric assembly  25  constructed wholly or partially out of a dielectric material, is re-entrant in to the top of the chamber. The coils  14   d ,  15   d  are solenoidal form and are located on the outside of the assembly, adjacent to the sidewalls  26  of the assembly  25 , which may be vertical or set at an appropriate angle to the axis. The centre of each coil  14   d  and  15   d  falls on the axis of symmetry  17 .  
         [0049]    As has been mentioned above, when a solenoidal coil is located outside of a dielectric window and a plasma is formed in the chamber, either inside or outside of the coil depending on the position of the window, then the RF magnetic field lines will be almost parallel to the coil axis (for a solenoid of constant diameter), for the greater part of the distance that they intersect the plasma. From Faraday&#39;s law, an azimuthal electric field and an associated current are induced within the plasma. The plasma current opposes the direction of the coil current and is confined to a layer near the window having a thickness of the order of the skin depth for penetration of RF into the plasma. Provided that separate coil units are sufficiently well spaced apart, the interaction between coil units will be minimal and each may be treated separately.  
         [0050]    Thus, as can be seen from the derivation in, for example, “Principles of Plasma Discharges and Materials Processing”, M A Lieberman and A. J. Lichtenberg (Wiley 1994), the skin depth δ is characterized as follows:  
       δ   =       (     m       e   2          μ   0          n   3         )       1   2                             
 
         [0051]    where m is the electron mass  
         [0052]    e is the charge on the electron  
         [0053]    μ 0  is the permeability of free space  
         [0054]    And n 3  is the electron density at the plasma edge.  
         [0055]    In inductively coupled plasmas, the electron density is typically of order 1×10 17  m −3 , or greater, giving a skin depth of 17 mm or less.  
         [0056]    Thus if the antennae are separated by at least twice the skin depth, they will couple into separate regions of space. The Applicants will usually space the antennae by around 100 mm and so do not seek to create a uniform plasma immediately in front of the antennae, but rather create local regions of dense plasma which by diffusion form a uniform plasma above a substrate placed at an appropriate distance away from the antennae. This arrangement is particularly suitable for a feed back control system such as described in our co-pending PCT Application filed on Sep. 24, 1997 and entitled “Plasma Processing Apparatus”, which is incorporated herein by reference.  
         [0057]    The design shown in FIGS. 5 and 6 utilises multiple solenoidal coils. At least one coil is adjacent to a section of the dielectric structure  26  which has smaller radius than the coil, and at least one coil is adjacent to a section of the dielectric structure  26  which has larger radius than the coil. Radio frequency power is coupled to those regions of the plasma inside of the chamber which are separated by the dielectric structure from the adjacent coil. The double hatched section  26  may be either of dielectric or conductive material.  
         [0058]    [0058]FIGS. 7, 8 and  9 , shows designs in which a circularly symmetric assembly  27  constructed wholly or partially out of a dielectric material, is re-entrant into the top of the chamber. (The cross-hatched areas can once again be dielectric or conductive). The coils  14   e ,  15   e  and  29  are of solenoidal form and are located on the outside of the assembly in axially symmetric channels  28 . The centre of each coil falls on the axis of symmetry  17 . The coils  14   e ,  15   e  are located in channels  28  of dielectric material, appropriate to their shape.  
         [0059]    In the arrangements shown in FIGS. 8 and 9 each coil can couple to both the region of the plasma which is formed at a smaller radius than the coil, and that region which is at a larger radius than the coil.  
         [0060]    [0060]FIG. 10, shows a design in which a number of axially concentric solenoidal coils  14   f ,  15   f  are located inside the plasma chamber  11 . Each coil  14   f ,  15   f  is electrically insulated from the plasma by means of continuous solid, or flexible insulating material  30  around each individual turn of each coil or around each complete coil unit. Electrical connections to each coil unit are brought into the plasma chamber through suitable vacuum seals in the chamber walls or top.  
         [0061]    As is illustrated in FIG. 11, at least one of the co-axial coils may be constituted by an annular coil  15   g  extending around the wall of the chamber adjacent an annular dielectric window  31 . Once more the number of turns shown is merely exemplary, and it will be understood that appropriate sealing provision will need to be made for the dielectric window  31 . A man skilled in the art would be familiar with the sealing methods required. Further, an external yoke or the like would need to be provided to provide strong mechanical attachment or location between the parts of the chamber which are on opposite sides of the window  31 .  
         [0062]    As has been mentioned above, radio frequency power may be inductively coupled to the plasma within a processing chamber through a dielectric window, with the antenna adjacent to the opposite side of the window to that facing the plasma. The window serves to maintain the pressure differential between the low pressure interior of the chamber where the plasma is formed and the, usually, atmospheric pressure exterior where the antenna is located.  
         [0063]    The dielectric window frequently forms part of one end or part of the side walls of the chamber. When multiple antennas are utilised it may be advantageous to use a number of small separate dielectric windows each dedicated to a particular antenna, in order to balance the need for thin windows for high RF coupling efficiency with the need for windows of adequate thickness to withstand the pressure differential across them. Such arrangements are shown in for example in FIGS. 2, 5,  7  and  8 .  
         [0064]    In particular antenna(s) may be located in trough(s) constructed out of dielectric material set into the end or side of a chamber. For a cylindrical chamber concentric circular troughs may be set into the end of the chamber. The design is not, however limited to cylindrical chambers, and troughs of dielectric material may be formed in other shapes appropriate to a specific chamber.  
         [0065]    Three further designs of antennas to operate with dielectric windows in trough form are described in connection with FIGS.  12  to  17 . The descriptions are with respect to cylindrical geometry for the dielectric trough, but are not intended to preclude the application of similar designs to other geometries. In each case a single antenna of one or more turns is located in each dielectric trough.  
         [0066]    The first design is shown in FIGS. 12 and 13. The antenna  14   h  is a single turn coil with a profile which approximates the profile of its dielectric trough  32 . Variations include the attachment of a cooling fluid pipe to the inner surface or closure of the fourth side to form a rectangular section pipe. The current flowing in the coil  14   h  induces an electric field and associated current in the plasma on the other side of the dielectric trough  32 . Because of the geometrical shape of the dielectric trough  32  and antenna  14   h , power is coupled to plasma below and on both sides of the trough. Thus power is coupled into a relatively large volume of the plasma whilst maintaining a relatively small, structurally strong dielectric window. This non-planar design of antenna may be used in each of a number of concentric dielectric trough windows on a multi antenna plasma chamber.  
         [0067]    In FIGS. 14 and 15 a  a second non-planar design of antenna  14   i  is shown. The antenna  14   i  is a three turn coil which is arranged so that one turn is wound adjacent to one side wall  33  of its dielectric trough  32 , the second turn is wound adjacent to the bottom  34  of the trough and the third turn is wound adjacent to the second side wall  35  of the trough. Each turn of the coil thus couples power into the plasma adjacent to the respective side of the trough  32  and therefore increases the volume of plasma into which power is coupled. The coil is shown as wound with circular section material, but other sections may be used, and the material may be any of multi stranded wire, rod or tube, through which a cooling fluid may be passed. Although a three turn coil is shown, this does not preclude the use of more or less turns e.g. 2 turns one near the bottom of the trough  32  and one near one side, or six turns with two near each side wall and two near the bottom. A further advantage of this design of antenna is that by using more than one turn, the inductance of the coil is increased compared with a single turn coil, which can simplify the matching circuitry required between the RF power supply and the antenna. Antennas of this design may be located in each of a number of concentric dielectric trough windows on multiple antenna systems.  
         [0068]    The design shown in FIGS. 16 and 17 is of a single turn antenna  14   j  which is would in a spiral shape and is then located in a dielectric trough window  37 . The spiral is wound so that sections of the coil are in close proximity to each of the three sides of the trough  37 . Power is coupled into the plasma adjacent to each of the three sides of the dielectric trough. The pitch of the spiral is chosen on the basis of a number of constraints, which must be balanced, for example a tighter pitch will mean the coil passes close to a given side of the trough more often and therefore will be more effective at coupling power into the local plasma, while the length of the wire, rod or pipe used to construct the coil will be longer leading to greater ohmic losses.  
         [0069]    It will be understood that the use of multiple nested coils enables a large scale plasma source to be constructed and hence provides the opportunity of processing workpieces of greater area. Although coils and chambers of regular cross-section have been described, many of the advantages of nested coils can be equally obtained where the cross-section, and hence the coil is not regular, e.g. when it is kidney shaped.  
         [0070]    Described above is the use of multiple antennae located on/in an apparatus, to inductively couple Radio Frequency power into a plasma formed within that apparatus.  
         [0071]    Various arrangements of antennae are described, but one arrangement in particular is equally applicable to an apparatus which has a single antenna, while exhibiting features which are novel when compared with single antenna arrangements described in prior patents and in the open literature.  
         [0072]    The arrangement consists of a dielectric window in the form of an annular trough as shown in the FIGS. 14 and 15 a - 15   d . In a cylindrical chamber it is appropriate for the trough to take a circular form as shown in these figures, but this does not preclude other shapes such as square for use in a chamber with a square cross-section. The cross-section of the trough may form a U-shape with almost square lower corners, or may have corners of larger radii. In the extreme case, the bottom of the U may take the form of a semicircle of a single radius. The trough shaped window is normally located in the lid of the chamber, such that the trough forms part of the lid. Variants are a trough shaped dielectric window inserted into a side wall of a chamber, either completely located in one side wall (for example in a chamber of square cross-section), or forming a structure which is symmetrical around the major axis of the chamber. Under normal circumstances the dielectric trough is inserted into the chamber, such that the inside of the trough, where the antenna is located, is at atmospheric pressure, while the outside of the trough is exposed to the pressure in the chamber. For plasma processing, the base pressure in the chamber will be a good vacuum, typically around 10-9 bar. When the chamber is operational, for processing of silicon wafers or other workpieces, the pressure in the chamber will be typically a few microbar, or tens of microbar of the process gas.  
         [0073]    The antenna is located in the dielectric trough, and may consist of a coil of one or more turns of a suitable conductor, for example copper pipe, which may be plated with another conductor which has lower resistivity, and/or may have a cooling fluid or gas passed through it to remove heat. The antenna coil may be formed of a single turn of conducting material or multiple turns. The cross-section of the coil may be shaped so that it follows the shape of the trough. Alternatively, the cross-section of the coil may be circular or any other shape convenient to construction.  
         [0074]    In a particular form shown in FIGS. 14 and 15, the antenna consists of a coil of more than one turn with individual turns placed adjacent to each side wall of the trough and the bottom. A possible arrangement for a three-turn coil is shown in FIG. 15 a . Some part of antenna will ideally be less than about 20 mm from each wall in order to couple power into each of the three directions. At the very least, power should be conducted in at least the downward direction and one of the side faces (for example a single coil could be located in a corner of the trough, see FIG. 15 b ). There could be two turns to the coil, either one offset from the other (FIG. 15 c ), or one next to the other (FIG. 15 d ). If coupling was over, say, 3 to 4 cm, then the trough width could be made relative to that distance to ensure coupling through the base and one sidewall. It is preferred, though, that coupling should occur both downward and in an inward and outward direction so as to create the greatest area of gas exposed to power in creation of plasma.  
         [0075]    There are two major points of the design that incorporates an antenna located in a dielectric trough window. The antenna couples power into effectively three regions of the plasma, at both smaller and larger radii than the trough, as well as below the trough. This is the case whatever the number of turns of the antenna coil, but the efficiency may be greater when multiple turns are used, with turns adjacent to each of the three interior surfaces of the trough. This increases efficiency and is in contrast to other designs for antennae and corresponding dielectric windows through which RF power is inductively coupled into a plasma, which only couple power into one region of the plasma.  
         [0076]    Specifically other designs described in prior patents or the open literature describe (a) a flat or domed window with essentially planar coil antenna, coupling power to the plasma below the window, (b) a tubular dielectric window with an antenna coiled around the tube, coupling power to the plasma inside, (c) an antenna located in a dielectric pocket, coupling power to the plasma below the pocket and at greater radius than that of the pocket.  
         [0077]    The trough shaped design of the dielectric window ensures significant physical strength while maintaining small wall thickness. The efficiency of power coupling from the antenna to the plasma is reduced if the dielectric material is thick; therefore small wall thickness is a desirable characteristic. However, in order to maintain almost one atmosphere pressure differential between inside and outside of the dielectric trough, the wall thickness must be sufficiently great. The structure of the dielectric window, since parts of the structure are perpendicular to, or near perpendicular to, other parts, provides multi-dimensional rigidity.  
         [0078]    There is a continuing requirement to process ever larger semiconductor wafers or other workpieces. This necessitates the antenna on a process chamber being moved out to an ever greater radius, to achieve a reasonably uniform process (exact radial position is something of a compromise). The trough design has great structural strength, and therefore can be formed with a large radius while maintaining a thin wall. In comparison, a flat or slightly domed disc dielectric window extending out to a comparable radius must be made of much thicker material, which reduces the efficiency for coupling RF power into the plasma.