Abstract:
A showerhead electrode, a gasket set and an assembly thereof in plasma reaction chamber for etching semiconductor substrates are provided with improved a gas injection hole pattern, positioning accuracy and reduced warping, which leads to enhanced uniformity of plasma processing rate. A method of assembling the inner electrode and gasket set to a supporting member includes simultaneous engagement of cam locks.

Description:
BACKGROUND 
       [0001]    Disclosed herein is a showerhead electrode of a plasma processing chamber in which semiconductor components can be manufactured. The fabrication of an integrated circuit chip typically begins with a thin, polished slice of high-purity, single crystal semiconductor material substrate (such as silicon or germanium) called a “substrate.” Each substrate is subjected to a sequence of physical and chemical processing steps that form the various circuit structures on the substrate. During the fabrication process, various types of thin films may be deposited on the substrate using various techniques such as thermal oxidation to produce silicon dioxide films, chemical vapor deposition to produce silicon, silicon dioxide, and silicon nitride films, and sputtering or other techniques to produce other metal films. 
         [0002]    After depositing a film on the semiconductor substrate, the unique electrical properties of semiconductors are produced by substituting selected impurities into the semiconductor crystal lattice using a process called doping. The doped silicon substrate may then be uniformly coated with a thin layer of photosensitive, or radiation sensitive material, called a “resist.” Small geometric patterns defining the electron paths in the circuit may then be transferred onto the resist using a process known as lithography. During the lithographic process, the integrated circuit pattern may be drawn on a glass plate called a “mask” and then optically reduced, projected, and transferred onto the photosensitive coating. 
         [0003]    The lithographed resist pattern is then transferred onto the underlying crystalline surface of the semiconductor material through a process known as plasma etching. Vacuum processing chambers are generally used for etching and chemical vapor deposition (CVD) of materials on substrates by supplying an etching or deposition gas to the vacuum chamber and application of a radio frequency (RF) field to the gas to energize the gas into a plasma state. 
       SUMMARY 
       [0004]    Described herein is a showerhead electrode for a showerhead electrode assembly in a capacitively coupled plasma processing chamber, the showerhead electrode assembly comprising a backing plate having gas injection holes extending between upper and lower faces thereof, a plurality of stud/socket assemblies and cam shafts, an alignment ring, and a plurality of alignment pins; the showerhead electrode comprising: a plasma exposed surface on a lower face thereof; a mounting surface on an upper face thereof; a plurality of gas injection holes extending between the plasma exposed surface and the mounting surface thereof and arranged in a pattern matching the gas injection holes in the backing plate; wherein the gas injection holes have a diameter less than or equal to 0.04 inch and are arranged in a pattern with one center gas injection hole at a center of the electrode and eight concentric rows of gas injection holes, the first row having seven gas injection holes located at a radial distance of about 0.6-0.7 inch from the center of the electrode; the second row having seventeen gas injection holes located at a radial distance of about 1.3-1.4 inches from the center of the electrode; the third row having twenty-eight gas injection holes located at a radial distance of about 2.1-2.2 inches from the center of the electrode; the fourth row having forty gas injection holes located at a radial distance of about 2.8-3.0 inches from the center of the electrode; the fifth row having forty-eight gas injection holes located at a radial distance of about 3.6-3.7 inches from the center of the electrode; the sixth row having fifty-six gas injection holes located at a radial distance of about 4.4-4.5 inches from the center of the electrode; the seventh row having sixty-four gas injection holes located at a radial distance of about 5.0-5.1 inches from the center of the electrode; the eighth row having seventy-two gas injection holes located at a radial distance of about 5.7-5.8 inches from the center of the electrode; the gas injection holes in each row are azimuthally equally spaced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1A  shows a partial cross-sectional view of a showerhead electrode assembly along a diameter for a capacitively coupled plasma reaction chamber, according to one embodiment. 
           [0006]      FIG. 1B  shows a partial cross-sectional view of the showerhead electrode assembly of  FIG. 1A  along another diameter. 
           [0007]      FIG. 1C  shows a showerhead electrode with a preferred gas hole pattern. 
           [0008]      FIG. 2A  is a three-dimensional representation of an exemplary cam lock for attaching an outer electrode, an inner electrode and an annular shroud in the showerhead electrode assembly shown in  FIGS. 1A and 1B . 
           [0009]      FIG. 2B  is a partial cross-sectional view of the exemplary cam lock of  FIG. 2A . 
           [0010]      FIG. 3  shows side-elevation and assembly drawings of an exemplary stud used in the cam lock of  FIGS. 2A-2B . 
           [0011]      FIG. 4A  shows a side-elevation view of an exemplary cam shaft used in the cam lock of  FIGS. 2A and 2B . 
           [0012]      FIG. 4B  shows a side view of the cam shaft of  FIG. 4A . 
           [0013]      FIG. 4C  shows an end view of the cam shaft of  FIG. 4A . 
           [0014]      FIG. 4D  shows a cross-sectional view of an exemplary cutter-path edge of a portion of the cam shaft of  FIG. 4B . 
           [0015]      FIG. 4E  shows a partial perspective view of the cam shaft in  FIG. 4A , mounted in a bore in a backing plate. 
           [0016]      FIG. 5A  is a bottom view of an inner electrode in the showerhead electrode assembly in  FIGS. 1A-1B , showing a plasma exposed surface. 
           [0017]      FIG. 5B  is a cross-sectional view of the inner electrode in  FIG. 5A . 
           [0018]      FIG. 5C  is an enlarged view of the area A in  FIG. 5B . 
           [0019]      FIG. 5D  is a top view of the inner electrode in  FIG. 5A , showing a mounting surface. 
           [0020]      FIG. 5E  is a partial cross-sectional view of the inner electrode in  FIG. 5D  across an annular groove  550 . 
           [0021]      FIG. 5F  is a partial cross-sectional view of the inner electrode in  FIG. 5D  across a hole  540   a  or  540   b  in  FIG. 5D . 
           [0022]      FIG. 5G  is a partial cross-sectional view of the inner electrode in  FIG. 5D  across a hole  530   a ,  530   b  or  530   c.    
           [0023]      FIG. 6A  is a top view of an inner gasket, a first annular gasket and a second annular gasket. 
           [0024]      FIG. 6B  is an enlarged view of the inner gasket in  FIG. 6A . 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    A parallel plate capacitively coupled plasma reaction chamber typically consists of a vacuum chamber with an upper electrode assembly and a lower electrode assembly positioned therein. A substrate (usually a semiconductor) to be processed is covered by a suitable mask and placed directly on the lower electrode assembly. A process gas such as CF 4 , CHF 3 , CClF 3 , HBr, Cl 2 , SF 6  or mixtures thereof is introduced into the chamber with gases such as O 2 , N 2 , He, Ar or mixtures thereof. The chamber is maintained at a pressure typically in the millitorr range. The upper electrode assembly includes a showerhead electrode with gas injection hole(s), which permit the gas to be uniformly dispersed through the upper electrode assembly into the chamber. One or more radio-frequency (RF) power supplies transmit RF power into the vacuum chamber and dissociate neutral process gas molecules into a plasma. Highly reactive radicals in the plasma are forced towards the substrate surface by an electrical field between the upper and lower electrodes. The surface of the substrate is etched or deposited on by chemical reaction with the radicals. The upper electrode assembly can include a single (monolithic) electrode or inner and outer electrodes, the monolithic electrode and inner electrode attached to a backing plate made of a different material. The monolithic/inner electrode is heated by the plasma and/or a heater arrangement during operation and may warp, which can adversely affect uniformity of processing rate across the substrate. In addition, differential thermal expansion of the monolithic/inner electrode and the backing plate can lead to rubbing therebetween during repeated thermal cycles. Rubbing can produce particulate contaminants that degrade the device yield from the substrate. 
         [0026]    To reduce warping of the monolithic/inner electrode, described herein is a showerhead electrode assembly including a plurality of cam locks engaged with the interior of a mounting surface of the monolithic/inner electrode. The monolithic/inner electrode is not edge clamped with a clamp ring around the outer edge thereof. Instead, attachment to the backing plate is achieved solely by cam locks which fasten the monolithic/inner electrode to the backing plate at a plurality of positions distributed across the electrode. 
         [0027]      FIG. 1A  shows a partial cross-sectional view of a portion of a showerhead electrode assembly  100  of a plasma reaction chamber for etching semiconductor substrates. As shown in  FIG. 1A , the showerhead electrode assembly  100  includes an upper electrode  110 , and a backing plate  140 . The assembly  100  can also include a thermal control plate (not shown), a temperature controlled upper plate (top plate) (not shown) having liquid flow channels therein. The upper electrode  110  preferably includes an inner electrode  120 , and an outer electrode  130 . The upper electrode  110  can also be a monolithic showerhead electrode. The upper electrode  110  may be made of a conductive high purity material such as single crystal silicon, polycrystalline silicon, silicon carbide or other suitable material. The inner electrode  120  is a consumable part which must be replaced periodically. An annular shroud  190  with a C-shaped cross section surrounds the upper electrode  110 . Details of the annular shroud  190  are described in commonly owned U.S. Provisional Patent Application Ser. Nos. 61/238,656, 61/238,665, 61/238,670, all filed on Aug. 31, 2009, the disclosures of which are hereby incorporated by reference. The backing plate  140  is mechanically secured to the inner electrode  120 , the outer electrode  130  and the shroud  190  with cam locks described below. The cross section in  FIG. 1A  is along a cam shaft  150  shared by two cam locks  151  and  152  engaged on the inner electrode  120 . 
         [0028]    The showerhead electrode assembly  100  as shown in  FIG. 1A  is typically used with an electrostatic chuck (not shown) forming part of a flat lower electrode assembly on which a substrate is supported spaced 1 to 5 cm below the upper electrode  110 . An example of a parallel plate type reactor is the Exelan™ dielectric etch reactor, made by Lam Research Corporation of Fremont, Calif. Such chucking arrangements provide temperature control of the substrate by supplying backside helium (He) pressure, which controls the rate of heat transfer between the substrate and the chuck. 
         [0029]    During use, process gas from a gas source is supplied to the upper electrode  110  through one or more passages in the backing plate which permit process gas to be supplied to a single zone or multiple zones above the substrate. 
         [0030]    The inner electrode  120  is preferably a planar disk or plate. The inner electrode  120  can have a diameter smaller than, equal to, or larger than a substrate to be processed, e.g., up to 300 mm, if the plate is made of single crystal silicon, which is the diameter of currently available single crystal silicon material used for 300 mm substrates. For processing 300 mm substrates, the outer electrode  130  is adapted to expand the diameter of the inner electrode  120  from about 12 inches to about 17 inches (as used herein, “about” refers to ±10%). The outer electrode  130  can be a continuous member (e.g., a single crystal silicon, polycrystalline silicon, silicon carbide or other suitable material in the form of a ring) or a segmented member (e.g., 2-6 separate segments arranged in a ring configuration, such as segments of single crystal silicon, polycrystalline silicon, silicon carbide or other material). To supply process gas to the gap between the substrate and the upper electrode  110 , the inner electrode  120  is provided with a plurality of gas injection holes (not shown), which are of a size and distribution suitable for supplying a process gas, which is energized into a plasma in a reaction zone beneath the upper electrode  110 . 
         [0031]    Details of the gas injection hole pattern can be critical to some plasma processes. Preferably, the diameter of the gas injection holes  106  is less than or equal to 0.04 inch; more preferably, the diameter of the gas injection holes  106  is between 0.01 and 0.03 inch; most preferably, the diameter of the gas injection holes  106  is 0.02 inch. A preferred gas injection hole pattern is shown in  FIG. 1C  which can be used on a (monolithic) single piece electrode (such as the electrode as described in commonly assigned U.S. Published Patent Application No. 2010/0003829, which is hereby incorporated by reference) or inner electrode of an assembly having an inner electrode and outer annular electrode surrounding the inner electrode (such as the inner electrode as described in commonly assigned U.S. Published Patent Application No. 2010/0003824, which is hereby incorporated by reference), one gas injection hole is located at the center of the electrode  120 ; the other gas injection holes are arranged in eight concentric rows with 7 gas injection holes in the first row located about 0.6-0.7 (e.g. 0.68) inch from the center of the electrode, 17 gas injection holes in the second row located about 1.3-1.4 (e.g. 1.34) inch from the center, 28 gas injection holes in the third row located about 2.1-2.2 (e.g. 2.12) inches from the center, 40 gas injection holes in the fourth row located about 2.8-3.0 (e.g. 2.90) inches from the center, 48 gas injection holes in the fifth row located about 3.6-3.7 (e.g. 3.67) inches from the center, 56 gas injection holes in the sixth row located about 4.4-4.5 (e.g. 4.45) inches from the center, 64 gas injection holes in the seventh row located about 5.0-5.1 (e.g. 5.09) inches from the center, and 72 gas injection holes in the eighth row located about 5.7-5.8 (e.g. 5.73) inches from the center. The gas injection holes in each of these rows are azimuthally evenly spaced. 
         [0032]    Single crystal silicon is a preferred material for plasma exposed surfaces of the upper electrode  110 . High-purity, single crystal silicon minimizes contamination of substrates during plasma processing as it introduces only a minimal amount of undesirable elements into the reaction chamber, and also wears smoothly during plasma processing, thereby minimizing particles. Alternative materials including composites of materials that can be used for plasma-exposed surfaces of the upper electrode  110  include polycrystalline silicon, Y 2 O 3 , SiC, Si 3 N 4 , and AlN, for example. 
         [0033]    In an embodiment, the showerhead electrode assembly  100  is large enough for processing large substrates, such as semiconductor substrates having a diameter of 300 mm. For 300 mm substrates, the inner electrode  120  is at least 300 mm in diameter. However, the showerhead electrode assembly  100  can be sized to process other substrate sizes. 
         [0034]    The backing plate  140  is preferably made of a material that is chemically compatible with process gases used for processing semiconductor substrates in the plasma processing chamber, has a coefficient of thermal expansion closely matching that of the electrode material, and/or is electrically and thermally conductive. Preferred materials that can be used to make the backing plate  140  include, but are not limited to, graphite, SIC, aluminum (Al), or other suitable materials. 
         [0035]    The backing plate  140  is preferably attached to the thermal control plate with suitable mechanical fasteners, which can be threaded bolts, screws, or the like. For example, bolts can be inserted in holes in the thermal control plate and screwed into threaded openings in the backing plate  140 . The thermal control plate is preferably made of a machined metallic material, such as aluminum, an aluminum alloy or the like. The upper temperature controlled plate is preferably made of aluminum or an aluminum alloy. 
         [0036]    The outer electrode  130  and the annular shroud  190  can be mechanically attached to the backing plate  140  by cam locks.  FIG. 1B  shows a cross section of the showerhead electrode assembly  100  along another cam shaft  160  shared by two cam locks  161  and  162  engaged on the annular shroud  190  and the outer electrode  130 , respectively. 
         [0037]    The cam locks shown in  FIGS. 1A and 1B  can be the cam locks as described in commonly-assigned WO2009/114175 (published on Sep. 17, 2009) and/or U.S. Patent Application Publication No. 2010/0003829, the disclosures of which are hereby incorporated by reference. 
         [0038]    With reference to  FIG. 2A , a three-dimensional view of an exemplary cam lock includes portions of the outer electrode  130  or the inner electrode  120  or the annular shroud  190 , and the backing plate  140 . The cam lock is capable of quickly, cleanly, and accurately attaching the outer electrode  130 , inner electrode  1210  or the annular shroud  190  to the backing plate  140 . 
         [0039]    The cam lock includes a stud (locking pin)  205  mounted into a socket  213 . The stud may be surrounded by a disc spring stack  215 , such, for example, stainless steel Belleville washers. The stud  205  and disc spring stack  215  may then be press-fit or otherwise fastened into the socket  213  through the use of adhesives or mechanical fasteners. The stud  205  and the disc spring stack  215  are arranged into the socket  213  such that a limited amount of lateral movement is possible between the outer electrode  130  or the inner electrode  120  or the annular shroud  190 , and the backing plate  140 . Limiting the amount of lateral movement allows for a tight fit between the outer electrode  130  or the inner electrode  120  or the annular shroud  190 , and the backing plate  140 , thus ensuring good thermal contact, while still providing some movement to account for differences in thermal expansion between the two parts. Additional details on the limited lateral movement feature are discussed in more detail, below. 
         [0040]    In a specific exemplary embodiment, the socket  213  is fabricated from high strength Torlon®. Alternatively, the socket  213  may be fabricated from other materials possessing certain mechanical characteristics such as good strength and impact resistance, creep resistance, dimensional stability, radiation resistance, and chemical resistance may be readily employed. Various materials such as polyamide-imide, acetals, and ultra-high molecular weight polyethylene materials may all be suitable. High temperature-specific plastics and other related materials are not required for forming the socket  213  as 230° C. is a typical maximum temperature encountered in applications such as etch chambers. Generally, a typical operating temperature is closer to 130° C. 
         [0041]    The cam shaft  160  or  150  is mounted into a bore machined into the backing plate  140 . In a typical application for an etch chamber designed for 300 mm semiconductor substrates, eight or more cam shafts may be spaced around the periphery of the backing plate  140 . 
         [0042]    The stud  205  and cam shaft  160  or  150  may be machined from stainless steel (e.g., 316, 316L, 17-7, NITRONIC-60, etc.) or any other material providing good strength and corrosion resistance. 
         [0043]    Referring now to  FIG. 2B , a cross-sectional view of the cam lock further exemplifies how the cam lock operates by pulling the outer electrode  130 , the inner electrode  120  or the annular shroud  190  in close proximity to the backing plate  140 . The stud  205 /disc spring stack  215 /socket  213  assembly is mounted into the outer electrode  130 , the inner electrode  120  or the annular shroud  190 . As shown, the assembly may be screwed, by means of external threads on the socket  213  into a threaded socket in the outer electrode  130 , the inner electrode  120  or the annular shroud  190 . 
         [0044]    In  FIG. 3 , an elevation and assembly view  300  of the stud  205  having an enlarged head, disc spring stack  215 , and socket  213  provides additional detail into an exemplary design of the cam lock. In a specific exemplary embodiment, a stud/disc spring assembly  301  is press fit into the socket  213 . The socket  213  has an external thread and a hexagonal top member allowing for easy insertion into the outer electrode  130 , the inner electrode  120  or the annular shroud  190  (see  FIGS. 2A and 2B ) with light torque (e.g., in a specific exemplary embodiment, about 20 inch-pounds). As indicated above, the socket  213  may be machined from various types of plastics. Using plastics minimizes particle generation and allows for a gall-free installation of the socket  213  into a mating socket on the outer electrode  130 , the inner electrode  120  or the annular shroud  190 . 
         [0045]    The stud/socket assembly  303  illustrates an inside diameter in an upper portion of the socket  213  being larger than an outside diameter of a mid-section portion of the stud  205 . The difference in diameters between the two portions allows for the limited lateral movement in the assembled cam lock as discussed above. The stud/disc spring assembly  301  is maintained in rigid contact with the socket  213  at a base portion of the socket  213  while the difference in diameters allows for some lateral movement. (See also,  FIG. 2B .) 
         [0046]    With reference to  FIG. 4A , a perspective view  400  of the cam shaft  160  or  150  also indicates a keying stud  402  and a hex opening  403  on one end of the cam shaft  160  or  150 . 
         [0047]    For example, with continued reference to  FIGS. 4A ,  2 A and  2 B, the cam lock is assembled by inserting the cam shaft  160  or  150  into a backing plate bore  211 . The keying stud  402  limits rotational travel of the cam shaft  160  or  150  in the backing plate bore  211  by interfacing with a step on an entrance of the bore  211  as shown in  FIG. 4E . The cam shaft  160  or  150  has two internal eccentric cutouts. In the cam shaft  160 , one cutout engages an enlarged head of a stud  205  on the outer electrode  130  and the other cutout engages an enlarged head of a stud  205  on the annular shroud  190 . In the cam shaft  150 , each of the two cutouts engages an enlarged head of a stud  205  on the inner electrode  120 . The cam shaft  160  or  150  may first be turned in one direction through use of the hex opening  403 , for example, counter-clockwise, to allow entry of the studs  205  into the cam shaft  160  or  150 , and then turned clockwise to fully engage and lock the studs  205 . The clamp force required to hold the outer electrode  130 , the inner electrode  120  or the annular shroud  190  to the backing plate  140  is supplied by compressing the disc spring stacks  215  beyond their free stack height. As the disc spring stacks  215  compress, the clamp force is transmitted from individual springs in the disc spring stacks  215  to the sockets  213  and through the outer electrode  130 , the inner electrode  120  or the annular shroud  190  to the backing plate  140 . 
         [0048]    In an exemplary mode of operation, the cam shaft  160  or  150  is inserted into the backing plate bore  211 . The cam shaft  160  or  150  is rotated counterclockwise to its full rotational travel. The stud/socket assemblies  303  ( FIG. 3 ) lightly torqued into the outer electrode  130 , the inner electrode  120  and/or the annular shroud  190  are then inserted into vertically extending through holes below the horizontally extending backing plate bore  211  such that the heads of the studs  205  engage in the eccentric cutouts in the cam shaft  160  or  150 . The outer electrode  130 , the inner electrode  120  or the annular shroud  190  is held against the backing plate  140  and the cam shaft  160  or  150  is rotated clockwise until the keying pin is limited by the step on the entrance of the bore  211 . The exemplary mode of operation may be reversed to dismount the outer electrode  130 , the inner electrode  120  or the annular shroud  190  from the backing plate  140 . 
         [0049]    With reference to  FIG. 4D , a sectional view A-A of the side-elevation view  420  of the cam shaft  160  or  150  of  FIG. 4A  indicates a cutter path edge  440  by which the head of the stud  205  is fully secured. 
         [0050]      FIGS. 5A-G  show details of the inner electrode  120 . The inner electrode  120  is preferably a plate of high purity (less than 10 ppm impurities) low resistivity (0.005 to 0.02 ohm-cm) single crystal silicon. 
         [0051]      FIG. 5A  is a bottom view of the inner electrode  120 , showing the plasma exposed surface  120   a . Gas injection holes  106  of suitable diameter and/or configuration extend from the mounting surface  120   b  to the plasma exposed surface  120   a  ( FIG. 5B ) and can be arranged in any suitable pattern. Preferably, the gas injection holes  106  are arranged in the pattern as shown in  FIG. 1C . 
         [0052]      FIG. 5B  is a cross-sectional view of the inner electrode  120  along a diameter thereof. The outer circumferential surface includes a single annular step  532 .  FIG. 5C  is an enlarged view of the area A in  FIG. 5B . The step  532  extends completely around the inner electrode  120 . In a preferred embodiment, the inner electrode  120  has a thickness of about 0.40 inch and an outer diameter of about 12.5 inches; the step  532  has an inner diameter of about 12.0 inches and an outer diameter of about 12.5 inches. The step  532  has a vertical surface  532   a  about 0.20 inch long and a horizontal surface  532   b  about 0.25 inch long. An interior corner between the surfaces  532   a  and  532   b  has a fillet with a radius of about 0.06 inch. 
         [0053]      FIG. 5D  is a top view of the inner electrode  120 , showing the mounting surface  120   b . The mounting surface  120   b  includes an annular groove  550  (details shown in  FIG. 5E ) concentric with the inner electrode  120 , the annular groove  550  having an inner diameter of about 0.24 inch, an outer diameter of about 0.44 inch, a depth of at least 0.1 inch, 45° chamfers of about 0.02 inch wide on entrance edges, and a fillet of a radius between 0.015 and 0.03 inch on the bottom corners. 
         [0054]    The mounting surface  120   b  also includes two smooth (unthreaded) blind holes  540   a  and  540   b  configured to receive alignment pins (details shown in  FIG. 5F ) located at a radius between 1.72 and 1.73 inches from the center of the inner electrode  120 . The blind hole  540   b  is offset by about 175° clockwise from the blind hole  540   a . The blind holes  540   a  and  540   b  have a diameter of about 0.11 inch, a depth of at least 0.2 inch, a 45° chamfer of about 0.02 inch wide on an entrance edge, and a fillet with a radius of at most 0.02 inch on a bottom corner. 
         [0055]    The mounting surface  120   b  also includes threaded sockets arranged in a first circular row and a second circular row which divide the mounting surface  120   b  into a central portion, a middle portion and an outer portion. The first circular row is preferably located on a radius of ¼ to ½ the radius of the inner electrode  120 , further preferably at a radial distance of about 2.4-2.6 inches from the center of the inner electrode  120 ; the second circular row is preferably located on a radius greater than ½ the radius of the inner electrode  120 , further preferably at a radial distance of about 5.3-5.5 inches from the center of the inner electrode  120 . In a preferred embodiment, a first row of eight 7/16-28 (Unified Thread Standard) threaded sockets  520   a , each of which configured to receive a stud/socket assembly  303 , are circumferentially spaced apart on a radius between 2.49 and 2.51 inches from the center of the inner electrode  120  and azimuthally offset by about 45° between each pair of adjacent threaded sockets  520   a . Each of the threaded sockets  520   a  has a total depth of about 0.2 inch, a threaded depth of at least 0.163 inch from the entrance edge, and a 45° chamfer of about 0.03 inch wide on an entrance edge. One of the threaded sockets  520   a  is azimuthally aligned with the blind hole  540   a . A second row of eight 7/16-28 (Unified Thread Standard) threaded sockets  520   b , each of which configured to receive a stud/socket assembly  303 , are circumferentially spaced apart on a radius between 5.40 and 5.42 inches from the center of the inner electrode  120  and azimuthally offset by about 45° between each pair of adjacent threaded holes  520   b . Each of the threaded sockets  520   b  and  520   a  has a total depth of about 0.2 inch, a threaded depth of at least 0.163 inch from the entrance edge, and a 45° chamfer of about 0.03 inch wide on an entrance edge. One of the holes  520   b  is azimuthally aligned with the blind hole  540   a.    
         [0056]    The mounting surface  120   b  further includes first, second and third smooth (unthreaded) blind holes configured to receive receipt of alignment pins ( 530   a ,  530   b  and  530   c , respectively, or  530  collectively) (details shown in  FIG. 5G ) radially aligned at a radius between 6.02 and 6.03 inches from the center of the inner electrode  120 . “Radially aligned” means the distances to the center are equal. The holes  530   a  have a diameter between 0.11 and 0.12 inch, a depth of at least 0.1 inch, a 45° chamfer of about 0.02 inch wide on an entrance edge, and a fillet with a radius of at most 0.02 inch on a bottom corner. The first hole  530   a  is offset by about 10° clockwise azimuthally from the blind holes  540   a ; the second hole  530   b  is offset by about 92.5° counterclockwise azimuthally from the first hole  530   a ; the third hole  530   c  is offset by about 190° counterclockwise azimuthally from the first hole  530   a.    
         [0057]    Referring to  FIG. 1A , the inner electrode  120  is fastened to the backing plate  140  by a plurality of (e.g. eight) cam locks  152  engaging the threaded sockets  520   a  and by a plurality of (e.g. eight) cam locks  151  engaging the threaded sockets  520   b  in the upper surface  120   b.    
         [0058]    The cam locks  151  and  152  provide points of mechanical support, improve thermal contact with the backing plate  140 , reduce warping of the inner electrode  120 , and hence reduce processing rate non-uniformity and thermal non-uniformity. 
         [0059]      FIG. 6A  shows a top view of a thermally and electrically conductive gasket set. This gasket set comprises an inner gasket  6100  comprising a plurality of concentric rings connected by a plurality of spokes, a first annular gasket  6200  with a plurality of holes and one cutout, and a second annular gasket  6300  with a plurality of cutouts. The gaskets are preferably electrically and thermally conductive and made of a material without excessive outgas in a vacuum environment, e.g., about 10 to 200 mTorr, having low particle generation, being compliant to accommodate shear at contact points, and free of metallic components that are lifetime killers in semiconductor substrates such as Ag, Ni, Cu and the like. The gaskets can be a silicone-aluminum foil sandwich gasket structure or an elastomer-stainless steel sandwich gasket structure. The gaskets can be an aluminum sheet coated on upper and lower sides with a thermally and electrically conductive rubber compatible in a vacuum environment used in semiconductor manufacturing wherein steps such as plasma etching are carried out. The gaskets are preferably compliant such that they can be compressed when the electrode and backing plate are mechanically clamped together but prevent opposed surfaces of the electrode and backing plate from rubbing against each other during temperature cycling of the showerhead electrode. The gaskets can be manufactured of a suitable material such as “Q-PAD II” available from the Bergquist Company. The thickness of the gaskets is preferably about 0.006 inch. The various features of the gaskets can be knife-cut, stamped, punched, or preferably laser-cut from a continuous sheet. The gasket set is mounted between the inner electrode  120 , outer electrodes  130  and annular shroud  190 , and the backing plate  140  to provide electrical and thermal contact therebetween. 
         [0060]      FIG. 6B  shows the details of the inner gasket  6100 . The inner gasket  6100  preferably comprises nine concentric rings interconnected by radial spokes. A first ring  6101  has an inner diameter of at least 0.44 inch (e.g. between 0.60 and 0.65 inch) and an outer diameter of at most 1.35 inches (e.g. between 0.95 and 1.00 inch). The first ring  6101  is connected to a second ring  6102  by seven radially extending and azimuthally evenly spaced spokes  6112 . Each spoke  6112  has a width of about 0.125 inch. 
         [0061]    The second ring  6102  has an inner diameter of at least 1.35 inches (e.g. between 1.72 and 1.78 inches) and an outer diameter of at most 2.68 inches (e.g. between 2.25 and 2.35 inches). The second ring  6102  is connected to a third ring  6103  by three radially extending and azimuthally evenly spaced spokes  6123   a ,  6123   b  and  6123   c , each of which has a width of about 0.125 inch. One spoke  6123   a  is offset azimuthally from one of the spokes  6112  by about 180°. 
         [0062]    The third ring  6103  has an inner diameter of at least 2.68 inches (e.g. between 3.15 and 3.20 inches) and an outer diameter of at most 4.23 inches (e.g. between 3.70 and 3.75 inches). The third ring is connected to a fourth ring  6104  by four radially extending and azimuthally evenly spaced spokes  6134 . Each spoke has a width of about 0.125 inch. One of the spokes  6134  is offset azimuthally by about 22.5° counterclockwise from the spoke  6123   a . The third ring  6103  also includes two round holes  6103   x  and  6103   y  located at a radial distance between 1.70 and 1.75 inches from the center of the inner gasket  6100 . The round holes  6103   x  and  6103   y  have a diameter of about 0.125 inch. The round hole  6103   x  is offset azimuthally by about 5° counterclockwise from the spoke  6123   a . The round hole  6103   y  is offset azimuthally by about 180° from the spoke  6123   a . The round holes  6103   x  and  6103   y  are configured to receive alignment pins. 
         [0063]    The fourth ring  6104  has an inner diameter of at least 4.23 inches (e.g. between 4.68 and 4.73 inches) and an outer diameter of at most 5.79 inches (e.g. between 5.27 and 5.32 inches). The fourth ring  6104  is connected to a fifth ring  6105  by a set of 8 radially extending and azimuthally evenly spaced spokes  6145   a  and another set of 8 radially extending and azimuthally evenly spaced spokes  6145   b . One of the spokes  6145   b  is offset azimuthally by about 8.5° counterclockwise from the spoke  6123   a . One of the spokes  6145   a  is offset azimuthally by about 8.5° clockwise from the spoke  6123   a . Each spoke  6145   a  and  6145   b  has a width of about 0.125 inch. The spokes  6145   a  and  6145   b  extend inward radially and separate the fourth ring  6104  into eight arcuate sections each of which has a central angle of about 28°. 
         [0064]    The fifth ring  6105  has an inner diameter of at least 5.79 inches (e.g. between 6.33 and 6.38 inches) and an outer diameter of at most 7.34 inches (e.g. between 6.71 and 6.76 inches). The fifth ring  6105  is connected to a sixth ring  6106  by four radially extending and azimuthally evenly spaced spokes  6156 . One of the spokes  6156  is offset azimuthally by about 90° from the spoke  6123   a . Each the spokes  6156  has a width of about 0.125 inch. 
         [0065]    The sixth ring  6106  has an inner diameter of at least 7.34 inches (e.g. between 7.90 and 7.95 inches) and an outer diameter of at most 8.89 inches (e.g. between 8.23 and 8.28 inches). The sixth ring  6106  is connected to a seventh ring  6107  by a set of four radially extending and azimuthally evenly spaced spokes  6167   a  and another set of four radially extending and azimuthally evenly spaced spokes  6167   b . One of the spokes  6167   b  is offset azimuthally by about 6.4° counterclockwise from the spoke  6123   a . One of the spokes  6167   a  is offset azimuthally by about 6.4° clockwise from the spoke  6123   a . Each spoke  6167   a  and  6167   b  has a width of about 0.125 inch. 
         [0066]    The seventh ring  6107  has an inner diameter of at least 8.89 inches (e.g. between 9.32 and 9.37 inches) and an outer diameter of at most 10.18 inches (e.g. between 9.65 and 9.70 inches). The seventh ring  6107  is connected to an eighth ring  6108  by a set of eight radially extending and azimuthally evenly spaced spokes  6178   a  and another set of eight radially extending and azimuthally evenly spaced spokes  6178   b . One of the spokes  6178   b  is offset azimuthally by about 5° counterclockwise from the spoke  6123   a . One of the spokes  6167   a  is offset azimuthally by about 5° clockwise from the spoke  6123   a . Each spoke  6167   a  and  6167   b  has a width of about 0.125 inch. 
         [0067]    The eighth ring  6108  has an inner diameter of at least 10.18 inches (e.g. between 10.59 and 10.64 inches) and an outer diameter of at most 11.46 inches (e.g. between 10.95 and 11.00 inches). The eighth ring  6108  is connected to a ninth ring  6109  by a set of eight radially extending and azimuthally evenly spaced spokes  6189   a  and another set of eight radially extending and azimuthally evenly spaced spokes  6189   b . One of the spokes  6189   b  is offset azimuthally by about 5° counterclockwise from the spoke  6123   a . One of the spokes  6189   a  is offset azimuthally by about 5° clockwise from the spoke  6123   a . Each spoke  6167   a  and  6167   b  has a width of about 0.125 inch. Eight arcuate cutouts  6108   h  with a central angle of about 6° inch separate the eighth ring  6108  into eight sections. The cutouts  6108   h  are azimuthally equally spaced. One of the cutout  6108   h  is azimuthally aligned with the spoke  6123   a.    
         [0068]    The ninth ring  6109  has an inner diameter between 11.92 and 11.97 inches and an outer diameter between 12.45 and 12.50 inches. The ninth ring  6109  has three small-diameter cutouts  6109   a ,  6109   b  and  6109   c  on its inner perimeter. The cutouts  6109   b  and  6109   c  are azimuthally offset from the cutout  6109   a  by about 92.5° counterclockwise and about 190° counterclockwise, respectively. The cutout  6109   c  is azimuthally aligned with the spoke  6123   a . The centers of the cutouts  6109   a ,  6109   b  and  6109   c  are located at a radial distance of about 6.02 inches from the center of the inner gasket  6100 . The cutouts  6109   a ,  6109   b  and  6109   c  face inward and include a semi-circular outer periphery with a diameter of about 0.125 inch and include an inner opening with straight radial edges. The ninth ring  6109  also has three large-diameter round and outwardly facing cutouts  6109   x ,  6109   y  and  6109   z  on its outer perimeter. The cutouts  6109   x ,  6109   y  and  6109   z  are azimuthally equally spaced and have a diameter of about 0.72 inch. Their centers are located at a radial distance of about 6.48 inches from the center of the inner gasket  6100 . The cutout  6109   z  is azimuthally offset from the spoke  6123   a  by about 37.5° clockwise. 
         [0069]    The first annular gasket  6200  has an inner diameter of about 14.06 inches and an outer diameter of about 16.75 inches. The first annular gasket  6200  has eight circular holes  6209   a  equally spaced azimuthally. The centers of the holes  6209   a  are located at a radial distance of about 7.61 inches from the center of the first annular gasket  6200 . The holes  6209   a  have a diameter of about 0.55 inch. When installed in the showerhead electrode assembly  100  (as described in details hereinbelow), one of the holes  6209   a  is azimuthally aligned with spoke  6123   a  of the inner gasket  6100 . The first annular gasket  6200  also has one round inwardly facing cutout  6209   b  on the inner perimeter of the first annular gasket  6200 . The center of this cutout  6209   b  is located at a distance of about 6.98 inches from the center of the first annular gasket  6200 . The cutout  6209   b  has a diameter of about 0.92 inch. When installed in the showerhead electrode assembly  100  (as described in details hereinbelow), the cutout  6209   b  is azimuthally offset from the spoke  6123   a  by about 202.5° counterclockwise. The first annular gasket  6200  further has three circular holes  6210 ,  6220  and  6230  configured to allow tool access. These holes are located at a radial distance of about 7.93 inches and have a diameter of about 0.14 inch. The holes  6210 ,  6220  and  6230  are offset azimuthally by about 7.5°, about 127.5° and about 252.5° respectively clockwise from the cutout  6209   b.    
         [0070]    The second annular gasket  6300  has an inner diameter of about 17.29 inches and an outer diameter of about 18.69 inches. The second annular gasket  6300  has eight round outwardly facing cutouts  6301  equally spaced azimuthally on the outer perimeter. The centers of the cutouts  6301  are located at a radial distance of about 9.30 inches from the center of the third annular gasket  6300 . The cutouts  6301  have a diameter of about 0.53 inch. 
         [0071]    When the inner electrode  120  is installed in the chamber  100 , an alignment ring, two inner alignment pins and three outer alignment pins are first inserted into the annular groove  550 , holes  540   a  and  540   b  and holes  530 , respectively. The inner gasket  6100  is then mounted to the inner electrode  120 . The holes  6103   x  and  6103   y  correspond to the inner alignment pins; and the center hole of the inner gasket  6100  corresponds to the alignment ring and the center gas injection hole in the inner electrode  120 . Openings between the nine rings and in the spokes in the inner gasket  6100  correspond to the first row through the eighth row of gas injection holes in the inner electrode  120 . The cutouts  6109   a ,  6109   b  and  6109   c  on the ninth ring correspond to the holes  530   a ,  530   b  and  530   c , respectively. Eight stud/socket assemblies  303  are threaded into the eight threaded sockets  520   a  and eight stud/socket assemblies  303  are threaded into the eight threaded sockets  520   b  to fasten the inner electrode  120  to the backing plate  140 , with the inner gasket  6100  sandwiched therebetween. The stud/socket assemblies  303  support the inner electrode  120  at a location between the center and outer edge, improve thermal contact with the backing plate  140  and reduce warping of the inner electrode  120  caused by temperature cycling during processing of substrates. The inner electrode  120  is fastened against the backing plate  140  by rotating the cam shafts  150 . Eight stud/socket assemblies  303  are threaded into eight threaded sockets in the outer electrode  130 . The first annular gasket  6200  is placed on the outer electrode  130 . Eight stud/socket assemblies  303  are threaded into eight threaded sockets in the annular shroud  190 . The second annular gasket  6300  is placed on the annular shroud  190 . The outer electrode  130  and the annular shroud  190  are fastened to the backing plate  140  by rotating the cam shafts  160 . The eight holes  6209   a  correspond to the eight stud/socket assemblies  303  threaded on the outer electrode  130 . The cutouts  6301  correspond to the eight stud/socket assemblies  303  threaded on the shroud  190 . 
         [0072]    The rings  6101 - 6109  and the spokes in the inner gasket  6100  may be arranged in any suitable pattern as long as they do not obstruct the gas injection holes  106 , the cam locks  151  and  152 , alignment ring, or alignment pins in the inner electrode  120 . 
         [0073]    While the showerhead electrode assembly, showerhead electrode, outer electrode, gasket set and gas hole pattern have been described in detail with reference to specific embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications can be made, and equivalents employed, without departing from the scope of the appended claims.