Abstract:
An inner electrode of a showerhead electrode assembly useful for plasma etching includes features providing improved positioning accuracy and reduced warping, which leads to enhanced uniformity of plasma processing rate. The assembly can include a thermal gasket set and fasteners such as bolts or cam locks located on a radius of ¼ to ½ the radius of the inner electrode. A method of assembling the inner electrode and gasket set to a supporting member is also provided.

Description:
[0001]    This application claims priority under 35 U.S.C. §119 to U.S. Provisional Application No. 61/251,177 entitled EDGE-CLAMPED AND MECHANICALLY FASTENED INNER ELECTRODE OF SHOWERHEAD ELECTRODE ASSEMBLY, filed Oct. 13, 2009, the entire content of which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    Disclosed herein is a showerhead electrode assembly 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. 
         [0003]    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. 
         [0004]    The lithographed resist pattern is then transferred onto the underlying crystalline surface of the semiconductor material through a process known as 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 
       [0005]    A showerhead electrode assembly for a plasma reaction chamber used in semiconductor substrate processing includes an inner electrode mechanically attached to a backing plate by a clamp ring and threaded fasteners such as a plurality of bolts or cam locks. The threaded fasteners and the clamp ring provide laterally spaced points of support, improve thermal contact with the backing plate and reduce warping of the inner electrode during operation of the plasma reaction chamber. The inner electrode has on its mounting surface a plurality of gas injection holes arranged in at least one concentric row, a plurality of unthreaded blind holes configured to receive alignment pins, an annular groove configured to receive an alignment ring, and a plurality of threaded blind holes configured to receive the threaded fasteners such as bolts or a plurality of threaded sockets that hold spring biased studs engageable with rotatable cam shafts mounted in the backing plate. A set of gaskets is sandwiched between the inner electrode and the backing plate and between an outer electrode and the backing plate to provide thermal and electrical contact and eliminate rubbing contact therebetween. The gaskets have holes and/or cutouts aligned with alignment pins inserted in the inner electrode during assembly. The alignment pins ensure accurate positioning of the gaskets relative to the inner electrode. The gaskets also have holes and/or cutouts aligned with the threaded blind holes, and gas injection holes on the inner electrode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1A  shows a partial cross-sectional view of a showerhead electrode assembly for a capacitively coupled plasma reaction chamber, according to one embodiment. 
           [0007]      FIG. 1B  shows a partial cross-sectional view of a showerhead electrode assembly for a capacitively coupled plasma reaction chamber, according to another embodiment. 
           [0008]      FIG. 1C  shows the details of a compression ring mounted on a clamp ring. 
           [0009]      FIG. 2A  is a three-dimensional representation of an exemplary cam lock for attaching an outer electrode in the showerhead electrode assembly shown in  FIG. 1A . 
           [0010]      FIG. 2B  is a cross-sectional view of the exemplary cam lock of  FIG. 2A . 
           [0011]      FIG. 2C  is a three-dimensional representation of an exemplary cam lock for attaching an outer electrode and an inner electrode in the showerhead electrode assembly shown in  FIG. 1B . 
           [0012]      FIG. 2D  is a cross-sectional view of the exemplary cam lock of  FIG. 2C . 
           [0013]      FIG. 3  shows side-elevation and assembly drawings of an exemplary stud used in the cam lock of  FIGS. 2A-2D . 
           [0014]      FIG. 4A  shows side-elevation and assembly drawings of an exemplary cam shaft used in the cam lock of  FIGS. 2A and 2B . 
           [0015]      FIG. 4B  shows a cross-sectional view of an exemplary cutter-path edge of a portion of the cam shaft of  FIG. 4A  or  FIG. 4C . 
           [0016]      FIG. 4C  shows side-elevation and assembly drawings of an exemplary cam shaft used in the cam lock of  FIGS. 2C and 2D . 
           [0017]      FIG. 4D  shows a partial perspective view of the cam shaft in  FIG. 4C , mounted in a bore in a backing plate. 
           [0018]      FIG. 5A  is a bottom view of an inner electrode in the showerhead electrode assembly in  FIG. 1A , showing a plasma exposed surface. 
           [0019]      FIG. 5B  is a cross-sectional view of the inner electrode in  FIG. 5A . 
           [0020]      FIG. 5C  is an enlarged view of the area A in  FIG. 5B . 
           [0021]      FIG. 5D  is a top view of the inner electrode in  FIG. 5A , showing a mounting surface. 
           [0022]      FIG. 5E  is a partial cross-sectional view of the inner electrode in  FIG. 5D  or  FIG. 5K  across an annular groove  550 A or  550 B. 
           [0023]      FIG. 5F  is a partial cross-sectional view of the inner electrode in  FIG. 5D  or  FIG. 5K  across a hole  540 A in  FIG. 5D  or a hole  540 Ba or  540 Bb in  FIG. 5K . 
           [0024]      FIG. 5G  is a partial cross-sectional view of the inner electrode in  FIG. 5D  across a hole  530   aa ,  530   ab  or  530   ac  in  FIG. 5D . 
           [0025]      FIG. 5H  is a bottom view of an inner electrode in the showerhead electrode assembly in  FIG. 1B , showing a plasma exposed surface. 
           [0026]      FIG. 5I  is a partial cross-sectional view of the inner electrode in  FIG. 5H . 
           [0027]      FIG. 5J  is an enlarged view of the area A in  FIG. 5I . 
           [0028]      FIG. 5K  is a top view of the inner electrode in  FIG. 5H , showing a mounting surface. 
           [0029]      FIG. 5L  is a partial cross-sectional view of the inner electrode in  FIG. 5K  across a hole  530   ba ,  530   bb  or  530   bc  in  FIG. 5K . 
           [0030]      FIG. 6  is an enlarged view of the proximity of a bolt  160 A in  FIG. 1A . 
           [0031]      FIG. 7A  is a top view of an inner gasket, a middle gasket and an outer gasket. 
           [0032]      FIG. 7B  is an enlarged view of the inner gasket  7100  in  FIG. 7A . 
           [0033]      FIG. 7C  is a top view of an inner gasket, a first annular gasket, a second annular gasket and a third annular gasket. 
           [0034]      FIG. 7D  is an enlarged view of the inner gasket  7400  in  FIG. 7C . 
           [0035]      FIG. 7E  is a top view of an inner gasket, a first annular gasket, a second annular gasket and a third annular gasket. 
           [0036]      FIG. 7F  is an enlarged view of the inner gasket  7800  in  FIG. 7E . 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    A 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 is provided 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 an inner electrode attached to a backing plate made of a different material from the inner electrode. The 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 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. 
         [0038]    To reduce warping of the inner electrode, described herein is a showerhead electrode assembly including a plurality of threaded fasteners such as bolts or cam locks engaged with the interior of a mounting surface of the inner electrode and a clamp ring around the edge of the inner electrode. The bolts or cam locks and clamp ring fasten the inner electrode to the backing plate at a plurality of positions distributed across the inner electrode. 
         [0039]      FIG. 1A  shows a partial cross-sectional view of a portion of a showerhead electrode assembly  100 A of a plasma reaction chamber for etching semiconductor substrates. As shown in  FIG. 1A , the showerhead electrode assembly  100 A includes an upper electrode  110 A, and a backing plate  140 A. The assembly  100 A also includes a thermal control plate  102 A, a temperature controlled upper plate (top plate)  104 A having liquid flow channels (not shown) therein. The upper electrode  110 A preferably includes an inner electrode  120 A, and an outer electrode  130 A. The inner electrode  120 A 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 A is a consumable part which must be replaced periodically. The backing plate  140 A is mechanically secured to the inner electrode  120 A and the outer electrode  130 A with mechanical fasteners described below. 
         [0040]    The showerhead electrode assembly  100 A 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 A. An example of such a plasma reaction chamber is a parallel plate type reactor, such as the Exelan™ dielectric etch systems, 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. 
         [0041]    During use, process gas from a gas source is supplied to the inner electrode  120 A through one or more passages in the upper plate  104 A which permit process gas to be supplied to a single zone or multiple zones above the substrate. 
         [0042]    The inner electrode  120 A is preferably a planar disk or plate. The inner electrode  120 A 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 A is adapted to expand the diameter of the inner electrode  120 A from about 12 inches to about 17 inches (as used herein, “about” refers to ±10%). The outer electrode  130 A 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 A, the inner electrode  120 A is provided with a plurality of gas injection holes  106 A, 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 A. 
         [0043]    Single crystal silicon is a preferred material for plasma exposed surfaces of the inner electrode  120 A and the outer electrode  130 A. 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 inner electrode  120 A and the outer electrode  130 A include polycrystalline silicon, Y 2 O 3 , SiC, Si 3 N 4 , and AlN, for example. 
         [0044]    In an embodiment, the showerhead electrode assembly  100 A 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 A is at least 300 mm in diameter. However, the showerhead electrode assembly  100 A can be sized to process other substrate sizes. 
         [0045]    The backing plate  140 A 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 A include, but are not limited to, graphite, SiC, aluminum (Al), or other suitable materials. 
         [0046]    The backing plate  140 A is preferably attached to the thermal control plate  102 A with suitable mechanical fasteners, which can be threaded bolts, screws, or the like. For example, bolts (not shown) can be inserted in holes in the thermal control plate  102 A and screwed into threaded openings in the backing plate  140 A. The thermal control plate  102 A is preferably made of a machined metallic material, such as aluminum, an aluminum alloy or the like. The upper temperature controlled plate  104 A is preferably made of aluminum or an aluminum alloy. 
         [0047]    The outer electrode  130 A can be mechanically attached to the backing plate by a cam lock mechanism as described in commonly-assigned WO2009/114175 (published on Sep. 17, 2009) and U.S. Published Application 2010/0003824, the disclosures of which are hereby incorporated by reference. With reference to  FIG. 2A , a three-dimensional view of an exemplary cam lock includes portions of the outer electrode  130 A and the backing plate  140 A. The cam lock is capable of quickly, cleanly, and accurately attaching the outer electrode  130 A to the backing plate  140 A in a variety of semiconductor fabrication-related tools, such as the plasma etch chamber shown in  FIG. 1A . 
         [0048]    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 A and the backing plate  140 A. Limiting the amount of lateral movement allows for a tight fit between the outer electrode  130 A and the backing plate  140 A, 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. 
         [0049]    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. 
         [0050]    Other portions of the cam lock are comprised of a cam shaft  207 A optionally surrounded at each end by a pair of cam shaft bearings  209 . The cam shaft  207 A and cam shaft bearing assembly is mounted into a backing plate bore  211 A machined into the backing plate  140 A. In a typical application for an etch chamber designed for 300 mm semiconductor substrates, eight or more of the cam locks may be spaced around the periphery of the outer electrode  130 A/backing plate  140 A combination. 
         [0051]    The cam shaft bearings  209  may be machined from a variety of materials including Torlon®, Vespel®, Celcon®, Delrin®, Teflon®, Arlon®, or other materials such as fluoropolymers, acetals, polyamides, polyimides, polytetrafluoroethylenes, and polyetheretherketones (PEEK) having a low coefficient of friction and low particle shedding. The stud  205  and cam shaft  207 A 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. 
         [0052]    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 A in close proximity to the backing plate  140 A. The stud  205 /disc spring stack  215 /socket  213  assembly is mounted into the outer electrode  130 A. As shown, the assembly may be screwed, by means of external threads on the socket  213  into a threaded hole in the outer electrode  130 A. 
         [0053]    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 A (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 A. 
         [0054]    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 .) 
         [0055]    With reference to  FIG. 4A , an exploded view  400 A of the cam shaft  207 A and cam shaft bearings  209  also indicates a keying pin  401 . The end of the cam shaft  207 A having the keying pin  401  is first inserted into the backing plate bore  211 A (see  FIG. 2B ). A pair of small mating holes (not shown) at a far end of the backing plate bore  211 A provide proper alignment of the cam shaft  207 A into the backing plate bore  211 A. A side-elevation view  420 A of the cam shaft  207 A clearly indicates a possible placement of a hex opening  403 A on one end of the cam shaft  207 A and the keying pin  401  on the opposite end. 
         [0056]    For example, with continued reference to  FIGS. 4A and 2B , the cam lock is assembled by inserting the cam shaft  207 A into the backing plate bore  211 A. The keying pin  401  limits rotational travel of the cam shaft  207 A in the backing plate bore  211 A by interfacing with a slot at the bottom of the bore  211 A. The cam shaft  207 A may first be turned in one direction though use of the hex opening  403 A, for example, counter-clockwise, to allow entry of the stud  205  into the cam shaft  207 A, and then turned clockwise to fully engage and lock the stud  205 . The clamp force required to hold the outer electrode  130 A to the backing plate  140 A is supplied by compressing the disc spring stack  215  beyond their free stack height. The cam shaft  207 A has an internal eccentric cutout which engages the enlarged head of the stud  205 . As the disc spring stack  215  compresses, the clamp force is transmitted from individual springs in the disc spring stack  215  to the socket  213  and through the outer electrode  130 A to the backing plate  140 A. 
         [0057]    In an exemplary mode of operation, once the cam shaft bearings  209  are attached to the cam shaft  207 A and inserted into the backing plate bore  211 A, the cam shaft  207 A is rotated counterclockwise to its full rotational travel. The stud/socket assembly  303  ( FIG. 3 ) is then lightly torqued into the outer electrode  130 A. The head of the stud  205  is then inserted into the vertically extending through hole below the horizontally extending backing plate bore  211 A. The outer electrode  130 A is held against the backing plate  140 A and the cam shaft  207 A is rotated clockwise until either the keying pin reaches the end of the slot at the bottom of the bore  211 A or an audible click is heard (discussed in detail, below). The exemplary mode of operation may be reversed to dismount the outer electrode  130 A from the backing plate  140 A. 
         [0058]    With reference to  FIG. 4B , a sectional view A-A of the side-elevation view  420 A of the cam shaft  207 A of  FIG. 4A  indicates a cutter path edge  440 A by which the head of the stud  205  is fully secured. In a specific exemplary embodiment, the two radii R 1  and R 2  are chosen such that the head of the stud  205  makes the audible clicking noise described above to indicate when the stud  205  is fully secured. 
         [0059]      FIGS. 5A-G  show details of the inner electrode  120 A. The inner electrode  120 A is preferably a plate of high purity (less than 10 ppm impurities) low resistivity (0.005 to 0.02 ohm-cm) single crystal silicon. 
         [0060]      FIG. 5A  is a bottom view of the inner electrode  120 A, showing the plasma exposed surface  120   aa.  Gas injection holes  106 A of suitable diameter and/or configuration extend from the mounting surface  120   ab  to the plasma exposed surface  120   aa  ( FIG. 5B ) and can be arranged in any suitable pattern. Preferably, the diameter of the gas injection holes  106 A is less than or equal to 0.04 inch; more preferably, the diameter of the gas injection holes  106 A is between 0.01 and 0.03 inch; further preferably, the diameter of the gas injection holes  106 A is 0.02 inch. In the embodiment shown, one gas injection hole is located at the center of the inner electrode  120 A; the other gas injection holes are arranged in eight concentric rows with 8 gas injection holes in the first row located about 0.6-0.7 (e.g. 0.68) inch from the center of the electrode, 18 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, 38 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, 58 gas injection holes in the sixth row located about 4.4-4.5 (e.g. 4.45) inches from the center, 66 gas injection holes in the seventh row located about 5.0-5.1 (e.g. 5.09) inches from the center, and 74 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. 
         [0061]      FIG. 5B  is a cross-sectional view of the inner electrode  120 A along a diameter thereof. The outer circumferential surface includes two steps.  FIG. 5C  is an enlarged view of the area A in  FIG. 5B . An inner step  532   a  and an outer step  534   a  extend completely around the inner electrode  120 A. In a preferred embodiment, the silicon plate has a thickness of about 0.40 inch and an outer diameter of about 12.5 inches; the inner step  532   a  has an inner diameter of about 12.0 inches and an outer diameter of about 12.1 inches and; the outer step  534   a  has an inner diameter of about 12.1 inches and an outer diameter of about 12.5 inches. The inner step  532   a  has a vertical surface  532   aa  about 0.13 inch long and a horizontal surface  532   ab  about 0.07 inch long and the outer step  534   a  has a vertical surface  534   aa  about 0.11 inch long and a horizontal surface  534   ab  about 0.21 inch long. 
         [0062]      FIG. 5D  is a top view of the inner electrode  120 A, showing the mounting surface  120   ab.  The mounting surface  120   ab  includes an annular groove  550 A (details shown in  FIG. 5E ) concentric with the inner electrode  120 A, the annular groove  550 A 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, a 45° chamfer of about 0.02 inch wide on the entrance edge, and a fillet of a radius between 0.015 and 0.03 inch on the bottom corners. 
         [0063]    The mounting surface  120   ab  also includes two smooth (unthreaded) blind holes  540 A 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 A and offset by about 180° from each other, the blind holes  540 A having a diameter of about 0.11 inch, a depth of at least 0.2 inch, a 45° chamfer of about 0.02 inch on an entrance edge, and a fillet with a radius of at most 0.02 inch on a bottom corner. 
         [0064]    The mounting surface  120   ab  also includes threaded blind holes arranged in an annular mounting hole zone which divides the mounting surface into a central portion and an outer portion. The mounting hole zone is preferably located on a radius of ¼ to ½ the radius of the inner electrode  120 A. In a preferred embodiment, a row of eight ¼-32 (Unified Thread Standard) threaded blind holes  520 A, are located on a radius between 2.4 and 2.6 inches (e.g., 2.5 inches) from the center of the inner electrode  120 A and azimuthally offset by about 45° between each pair of adjacent holes  520 A. Each of the holes  520 A has a total depth of about 0.3 inch, a threaded depth of at least 0.25 inch from the entrance edge, and a 45° chamfer of about 0.05 inch wide on the entrance edge. One of the holes  520 A is azimuthally aligned with another one of the holes  540 A. As used herein, two objects being “azimuthally aligned” means a straight line connecting these two objects passes through the center of a circle or ring, in this embodiment, the center of the inner electrode  120 A. 
         [0065]    The mounting surface  120   ab  further includes first, second and third smooth (unthreaded) blind holes configured to receive alignment pins ( 530   aa , 530   ab  and  530   ac , respectively, or  530   a  collectively) (details shown in  FIG. 5G ) radially aligned at a radius between 6.0 and 6.1, preferably between 6.02 and 6.03 inches from the center of the inner electrode  120 A. “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   aa  is offset by about 10° clockwise azimuthally from one of the unthreaded blind holes  540 A; the second hole  530   ab  is offset by about 92.5° counterclockwise azimuthally from the first hole  530   aa;  the third hole  530   ac  is offset by about 190° counterclockwise azimuthally from the first hole  530   aa.    
         [0066]    Referring to  FIG. 1A , the inner electrode  120 A is clamped to the backing plate  140 A by a clamp ring  150 A engaging the outer step  534   a  on the lower face and a plurality of bolts  160 A engaging the threaded blind holes  520 A in the mounting surface  120   ab.  The clamp ring  150 A includes a series of holes which receive fasteners such as bolts (screws) threaded into threaded openings in an underside of the backing plate  140 A. To avoid contact of the clamp ring  150 A with the step  534   a  on the inner electrode  120 A, a compression ring  170 A of a stiff material such as a hard polyimide material such as CIRLEX® is compressed between opposed surfaces of the inner electrode  120 A and the clamp ring  150 A ( FIG. 1C ). 
         [0067]      FIG. 6  shows an enlarged portion in  FIG. 1A  near one of the bolts  160 A. The bolts  160 A are of 8-32 size. During installation of the inner electrode  120 A, a plastic insert  610 A preferably made of TORLON® 5030 is threaded into each threaded blind hole  520 A. The plastic insert  610 A has an inner thread of 8-32 and an outer thread of ¼-32. An 8-32 bolt  160 A is threaded into each plastic insert  610 A. During operation of the showerhead electrode assembly  100 A, the inner electrode  120 A is heated by a plasma and/or heating arrangement and this heating can cause warping in the inner electrode  120 A and adversely affect the uniformity of the plasma processing rate across the plasma chamber. The bolts  160 A in combination with the clamp ring  150 A provide points of mechanical support, reduce warping of the inner electrode  120 A, and hence reduce processing rate non-uniformity and thermal non-uniformity. 
         [0068]      FIG. 7A  shows a top view of a thermally and electrically conductive gasket set. This gasket set comprises an inner gasket  7100  comprising a plurality of concentric rings connected by a plurality of spokes, an annular middle gasket  7200  with a plurality of cutouts on an outer and an inner perimeter, and an annular outer gasket  7300  with a plurality of cutouts on an outer perimeter and one cutout on an inner perimeter. The gaskets are preferably electrically and thermally conductive and made of a material compatible for semiconductor processing 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 it 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 backing plate  140 A and the inner electrode  120 A and outer electrode  130 A to provide electrical and thermal contact therebetween. 
         [0069]      FIG. 7B  shows the details of the inner gasket  7100 . The inner gasket  7100  preferably comprises seven concentric rings interconnected by radial spokes. A first ring  701  has an inner diameter of at least 0.44 inch (e.g. between 0.62 and 0.65 inch) and an outer diameter of at most 1.35 inches (e.g. between 0.97 and 1.00 inch). The first ring  701  is connected to a second ring  702  by eight radially extending and azimuthally evenly spaced spokes  712 . Each spoke  712  has a width of about 0.125 inch. 
         [0070]    The second ring  702  has an inner diameter of at least 1.35 inches (e.g. between 1.74 and 1.76 inches) and an outer diameter of at most 2.68 inches (e.g. between 2.26 and 2.29 inches). The second ring is connected to a third ring  703  by four radially extending and azimuthally evenly spaced spokes. Two of these four spokes  723   a  and  723   b  oppose each other about the center of the inner gasket  7100  and each has a width of about 0.5 inch and a rounded rectangular opening ( 723   ah  or  723   bh ) of about 0.25 inch by about 0.46 inch. The other two of these four spokes  723   c  and  723   d  oppose each other about the center of the inner gasket  7100  and each has a width of about 0.25 inch. One spoke  723   c  is offset azimuthally from one of the spokes  712  by about 22.5°. 
         [0071]    The third ring  703  has an inner diameter of at least 2.68 inches (e.g. between 3.17 and 3.20 inches) and an outer diameter of at most 4.23 inches (e.g. between 3.71 and 3.74 inches). The third ring is connected to a fourth ring  704  by four radially extending and azimuthally evenly spaced spokes  734 . Each spoke has a width of about 0.18 inch. One of the spokes  734  is offset azimuthally by about 45° from the spoke  723   c.  The third ring  703  also includes two round holes  703   x  and  703   y  azimuthally offset by about 180° from each other and located at a radial distance between 1.72 and 1.74 inches from the center of the inner gasket  7100 . The round holes  703   x  and  703   y  have a diameter of about 0.125 inch. One round hole  703   x  is offset azimuthally by about 90° from the spoke  723   c.  The round holes  703   x  and  703   y  are configured to receive alignment pins. 
         [0072]    The fourth ring  704  has an inner diameter of at least 4.23 inches (e.g. between 4.78 and 4.81 inches) and an outer diameter of at most 5.79 inches (e.g. between 5.19 and 5.22 inches). The fourth ring  704  is connected to a fifth ring  705  by four radially extending and azimuthally evenly spaced spokes. Two of these four spokes  745   a  and  745   b  oppose each other about the center of the inner gasket  7100  and each has a width of about 0.5 inch and a rounded rectangular opening ( 745   ah  or  745   bh ) of about 0.25 inch by about 0.51 inch. The other two of these four spokes  745   c  and  745   d  oppose each other about the center of the inner gasket  7100  and each has a width of about 0.25 inch. One spoke  745   a  is offset azimuthally by about 90° counterclockwise from the spokes  723   c.  The fourth ring  704  also includes eight round holes  704   s,    704   t,    704   u,    704   v,    704   w,    704   x,    704   y  and  704   z  (configured to receive bolts) azimuthally offset by about 45° between each adjacent pair and located at a radial distance between 2.49 and 2.51 inches from the center of the inner gasket  7100 . These round holes  704   s,    704   t,    704   u,    704   v,    704   w,    704   x,    704   y  and  704   z  have a diameter of about 0.18 inch. One round hole  704   s  is offset azimuthally by about 90° counterclockwise from the spoke  723   c.  Around each of the round holes  704   s,    704   u,    704   w  and  704   y,  the fourth ring  704  has a round protrusion on the inner periphery thereof. Around each of the round holes  704   t,    704   v,    704   x  and  704   z,  the fourth ring  704  has a round protrusion on the outer periphery thereof. Each protrusion has an outer diameter of about 0.36 inch. 
         [0073]    The fifth ring  705  has an inner diameter of at least 5.79 inches (e.g. between 6.35 and 6.37 inches) and an outer diameter of at most 7.34 inches (e.g. between 6.73 and 6.75 inches). The fifth ring  705  is connected to a sixth ring  706  by four radially extending and azimuthally evenly spaced spokes  756 . One of the spokes  756  is offset azimuthally by about 45° 0  from the spoke  723   c.  Each the spokes  756  has a width of about 0.5 inch and a rectangular opening  756   h  of about 0.25 inch by about 0.60 inch. 
         [0074]    The sixth ring  706  has an inner diameter of at least 7.34 inches (e.g. between 7.92 and 7.95 inches) and an outer diameter of at most 8.89 inches (e.g. between 8.16 and 8.36 inches). The sixth ring  706  is connected to a seventh ring  707  by four radially extending and azimuthally evenly spaced spokes. Two of these four spokes  767   a  and  767   b  oppose each other about the center of the inner gasket  7100  and each has a width of about 0.5 inch and a rectangular opening ( 767   ah  or  767   bh ) of about 0.25 inch wide. The openings  767   ah  and  767   bh  extend outward radially and separate the seventh ring  707  into two half circles. The other two of these four spokes  767   c  and  767   d  oppose each other about the center of the inner gasket  7100  and each has a width of about 0.25 inch. Spoke  767   d  is offset azimuthally by about 180° from the spoke  723   c.    
         [0075]    The seventh ring  707  has an inner diameter of at least 8.89 inches (e.g. between 9.34 and 9.37 inches) and an outer diameter of at most 10.18 inches (e.g. between 9.66 and 9.69 inches). Each corner at joints between the rings and spokes in the inner gasket  7100  is rounded to a radius of about 0.06 inch. 
         [0076]    The middle gasket  7200  (see  FIG. 7A ) has an inner diameter of about 11.95 inches and an outer diameter of about 12.47 inches. The middle gasket  7200  has three small-diameter cutouts  708   a,    708   b  and  708   c  on its inner perimeter. The cutouts  708   b  and  708   c  are azimuthally offset from the cutout  708   a  by about 92.5° clockwise and about 190° clockwise, respectively. The centers of the cutouts  708   a,    708   b  and  708   c  are located at a radial distance of about 6.02 inches from the center of the middle gasket  7200 . The cutouts  708   a,    708   b  and  708   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 middle gasket  7200  also has three large-diameter round and outwardly facing cutouts  708   x ,  708   y  and  708   z  on its outer perimeter. The cutouts  708   x,    708   y  and  708   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 middle gasket  7200 . The cutout  708   x  is azimuthally offset from the cutout  708   a  by about 37.5° clockwise. When installed in the showerhead electrode assembly (as described in details hereinbelow), the cutout  708   a  is azimuthally aligned with the hole  703   x  on the third ring  703  in the inner gasket  7100 . 
         [0077]    The outer gasket  7300  has an inner diameter of about 13.90 inches and an outer diameter of about 15.31 inches. The outer gasket  7300  has eight semicircular outwardly facing cutouts  709   a  equally spaced azimuthally on its outer perimeter. The centers of the cutouts  709   a  are located at a radial distance of about 7.61 inches from the center of the outer gasket  7300 . The cutouts  709   a  have a diameter of about 0.62 inch. When installed in the showerhead electrode assembly (as described in details hereinbelow), one of the cutouts  709   a  is azimuthally aligned with the hole  703   x  on the third ring  703  in the inner gasket  7100 . The outer gasket  7300  also has one round inwardly facing cutout  709   b  on the inner perimeter thereof. The center of this cutout  709   b  is located at a distance of about 6.98 inches from the center of the outer gasket  7300 . The cutout  709   b  has a diameter of about 0.92 inch. When installed in the showerhead electrode assembly (as described in details hereinbelow), the cutout  709   b  is azimuthally offset from the hole  703   x  by about 22.5° 0  counterclockwise. 
         [0078]    When the inner electrode  120 A is installed in the showerhead electrode assembly  100 A, an alignment ring  108 A ( FIG. 1A ), two inner alignment pins  109 A ( FIG. 1A ) and three outer alignment pins (not shown in  FIG. 1A ) are first inserted into the annular groove  550 A, holes  540 A and holes  530   a  ( FIG. 5D ), respectively. The inner gasket  7100  is then mounted to the inner electrode  120 A. The holes  703   x  and  703   y  ( FIG. 7B ) correspond to the inner alignment pins  109 A; and the center hole of the inner gasket  7100  corresponds to the alignment ring  108 A and the center gas injection hole in the inner electrode  120 A. Rectangular and quarter-circular openings between the seven rings and in the spokes in the inner gasket  7100  correspond to the first row through the sixth row of gas injection holes in the inner electrode  120 A. The middle gasket  7200  is mounted onto the inner electrode  120 A. The cutouts  708   a,    708   b  and  708   c  correspond to the holes  530   ac ,  530   ab  and  530   aa , respectively. The seventh and eighth rows of gas injection holes fall in the opening between the inner gasket  7100  and the middle gasket  7200 . Eight bolts  160 A with their corresponding inserts  610 A are threaded into the eight threaded blind holes  520 A to fasten the inner electrode  120 A to the backing plate  140 A, with the inner gasket  7100  and middle gasket  7200  sandwiched therebetween. The clamp ring  150 A is fastened onto the backing plate  140 A by a plurality of bolts threaded into threaded openings in the underside of the backing plate  140 A. The bolts  160 A and the clamp ring  150 A support the inner electrode  120 A at a location between the center and outer edge and at the outer edge, respectively, in order to reduce warping of the inner electrode  120 A caused by temperature cycling during processing of substrates. The outer gasket  7300  is placed on the outer electrode  130 A. The eight cutouts  709   a  correspond to the eight cam lock mechanisms. The outer electrode  130 A is fastened against the backing plate  140 A by rotating the cam shaft  207 A of each cam lock. 
         [0079]      FIG. 1B  shows a cross-sectional view of a portion of another showerhead electrode assembly  100 B of a plasma reaction chamber for etching semiconductor substrates. As shown in  FIG. 1B , the showerhead electrode assembly  100 B includes an upper electrode  110 B, and a backing plate  140 B. The assembly  100 B also includes a thermal control plate  102 B, and a top plate  104 B having liquid flow channels therein. The upper electrode  110 B preferably includes an inner electrode  120 B, and an outer electrode  130 B. The inner electrode  120 B 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 B is a consumable part which must be replaced periodically. An annular shroud  190  with a C-shaped cross section surrounds the outer electrode  130 B. The backing plate  140 B is mechanically secured to the inner electrode  120 B, the outer electrode  130 B and the shroud  190  with mechanical fasteners described below. 
         [0080]    During use, process gas from a gas source is supplied to the inner electrode  120 B through one or more passages in the upper plate  104 B which permit process gas to be supplied to a single zone or multiple zones above the substrate. 
         [0081]    The inner electrode  120 B is preferably a planar disk or plate. The inner electrode  120 B 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 B is adapted to expand the diameter of the inner electrode  120 B from about 12 inches to about 17 inches. The outer electrode  130 B 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 B, the inner electrode  120 B is provided with a plurality of gas injection holes  106 B, 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 B. 
         [0082]    Single crystal silicon is a preferred material for plasma exposed surfaces of the inner electrode  120 B and the outer electrode  130 B. 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 inner electrode  120 B, the outer electrode  130 B and the annular shroud  190  include polycrystalline silicon, Y 2 O 3 , SiC, Si 3 N 4 , and AlN, for example. 
         [0083]    In an embodiment, the showerhead electrode assembly  100 B 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 B is at least 300 mm in diameter. However, the showerhead electrode assembly  100 B can be sized to process other substrate sizes. 
         [0084]    The backing plate  140 B 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 B include, but are not limited to, graphite, SiC, aluminum (Al), or other suitable materials. 
         [0085]    The backing plate  140 B is preferably attached to the thermal control plate  102 B with suitable mechanical fasteners, which can be threaded bolts, screws, or the like. For example, bolts (not shown) can be inserted in holes in the thermal control plate  102 B and screwed into threaded openings in the backing plate  140 B. The thermal control plate  102 B is preferably made of a machined metallic material, such as aluminum, an aluminum alloy or the like. The temperature controlled top plate  104 B is preferably made of aluminum or an aluminum alloy. 
         [0086]      FIGS. 5H-5L  show details of the inner electrode  120 B. The inner electrode  120 B is preferably a plate of high purity (less than 10 ppm impurities) low resistivity (0.005 to 0.02 ohm-cm) single crystal silicon. 
         [0087]      FIG. 5H  is a bottom view of the inner electrode  120 B, showing the plasma exposed surface  120   ba.  Gas injection holes  106 B of suitable diameter and/or configuration extend from the mounting surface  120   bb  to the plasma exposed surface  120   ba  ( FIG. 5I ) and can be arranged in any suitable pattern. Preferably, the diameter of the gas injection holes  106 B is less than or equal to 0.04 inch; more preferably, the diameter of the gas injection holes  106 B is between 0.01 and 0.03 inch; further preferably, the diameter of the gas injection holes  106 B is 0.02 inch. In the embodiment shown, one gas injection hole is located at the center of the inner electrode  120 B; the other gas injection holes are arranged in eight concentric rows with 8 gas injection holes in the first row located about 0.6-0.7 (e.g. 0.68) inch from the center of the electrode, 18 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. 
         [0088]      FIG. 5I  is a partial cross-sectional view of the inner electrode  120 B along a diameter thereof. The outer circumferential surface includes two steps.  FIG. 5J  is an enlarged view of the area A in  FIG. 5I . An inner step  532   b  and an outer step  534   b  extend completely around the inner electrode  120 B. In a preferred embodiment, the silicon plate has a thickness of about 0.40 inch and an outer diameter of about 12.5 inches; the inner step  532   b  has an inner diameter of about 12.00 inches and an outer diameter of about 12.1 inches and; the outer step  534   b  has an inner diameter of about  12 . 1  inches and an outer diameter of about 12.5 inches. The inner step  532   b  has a vertical surface  532   ba  about 0.13 inch long and a horizontal surface  532   bb  about 0.07 inch long and the outer step  534   b  has a vertical surface  534   ba  about 0.11 inch long and a horizontal surface  534   bb  about 0.21 inch long. The circular line of intersection between the surfaces  534   ba  and  534   bb  is rounded to a radius of about 0.06 inch. 
         [0089]      FIG. 5K  is a top view of the inner electrode  120 B, showing the mounting surface  120   bb . The mounting surface  120   bb  includes an annular groove  550 B (details shown in  FIG. 5E ) concentric with the inner electrode  120 B, the annular groove  550 B 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, a 45° chamfer of about 0.02 inch wide on the entrance edge, and a fillet of a radius between 0.015 and 0.03 inch on the bottom corner. 
         [0090]    The mounting surface  120   bb  also includes two smooth (unthreaded) blind holes  540 Ba and  540 Bb 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 B. The blind hole  540 Bb is offset by about 175° clockwise from the blind hole  540 Ba. The blind holes  540 Ba and  540 Bb have a diameter between 0.11 and 0.12 inch, a depth of at least 0.2 inch, a 45° chamfer of about 0.02 inch on an entrance edge, and a fillet with a radius of at most 0.02 inch on a bottom corner. 
         [0091]    The mounting surface  120   bb  also includes threaded blind holes  520 B arranged in an annular mounting hole zone which divides the mounting surface into a central portion and an outer portion. The mounting hole zone is preferably located on a radius of ¼ to ½ the radius of the inner electrode  120 B. In a preferred embodiment, eight 7/16-28 (Unified Thread Standard) or other suitably sized threaded holes  520 B, 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 B and azimuthally offset by about 45° between each pair of adjacent threaded holes  520 B. Each of the threaded holes  520 B 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 the entrance edge. One of the holes  520 B is azimuthally aligned with the hole  540 Ba. 
         [0092]    The mounting surface  120   bb  further includes first, second and third smooth (unthreaded) blind holes configured to receive alignment pins ( 530   ba ,  530   bb  and  530   bc , respectively, or  530   b  collectively) (details shown in  FIG. 5K ) radially aligned at a radius between 6.02 and 6.03 inches from the center of the inner electrode  120 B. The holes  530   b  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   ba  is offset by about 10° clockwise azimuthally from the unthreaded blind holes  540 Ba; the second hole  530   bb  is offset by about 92.5° counterclockwise azimuthally from the first hole  530   ba;  the third hole  530   bc  is offset by about 190° counterclockwise azimuthally from the first hole  530   ba.    
         [0093]    In the top view of the inner electrode  120 B in  FIG. 5K  (the view of the mounting surface  120   bb ), a gas injection hole in the first row is azimuthally aligned with the hole  530   bc;  a gas injection hole in the second row is azimuthally aligned with the hole  530   bc;  a gas injection hole in the third row is azimuthally offset by about 3.2° counterclockwise from the hole  530   bc;  a gas injection hole in the fourth row is azimuthally offset by about 4.5° counterclockwise from the hole  530   bc ; a gas injection hole in the fifth row is azimuthally offset by about 3.75° counterclockwise from the hole  530   bc;  a gas injection hole in the sixth row is azimuthally offset by about 3.21° counterclockwise from the hole  530   bc;  a gas injection hole in the seventh row is azimuthally offset by about 2.81° counterclockwise from the hole  530   bc;  a gas injection hole in the eighth row is azimuthally offset by about 2.5° counterclockwise from the hole  530   bc.    
         [0094]    Referring to  FIG. 1B , the inner electrode  120 B is clamped to the backing plate  140 B by a clamp ring  150 B engaging the outer step on the lower face and a plurality of cam locks  160 B (such as 4 to 8 cam locks) engaging the threaded holes in the upper surface. The clamp ring  150 B includes a series of holes which receive fasteners such as bolts (screws) threaded into threaded openings in an underside of the backing plate  140 B. To avoid contact of the clamp ring  150 B with the step  534   b  on the inner electrode  120 B, a compression ring  170 B of a stiff material such as a hard polyimide material such as CIRLEX® is compressed between opposed surfaces of the inner electrode  120 B and the clamp ring  150 B ( FIG. 1C ). 
         [0095]    The cam locks  160 B in combination with the clamp ring  150 B provide points of mechanical support, improve thermal contact with the backing plate  140 B, reduce warping of the inner electrode  120 B, and hence reduce processing rate non-uniformity and thermal non-uniformity. 
         [0096]    In the embodiment shown, the outer electrode  130 B is mechanically attached to the backing plate by 8 cam locks and the inner electrode  120 B is mechanically attached to the backing plate by another 8 cam locks.  FIG. 2C  shows a three-dimensional view of an exemplary cam lock including portions of the outer electrode  130 B and the backing plate  140 B. 
         [0097]    The cam locks as shown in  FIGS. 2C and 2D  include a stud/socket assembly  303  comprising a stud (locking pin)  205  mounted into a socket  213 , as described above and shown in  FIG. 3 . 
         [0098]    To allow simultaneous engagement of cam locks on the inner and outer electrodes, eight elongated cam shafts  207 B are mounted into backing plate bores  211 B machined into the backing plate  140 B. Each cam shaft  207 B engages on a stud/socket assembly  303  of one cam lock on the outer electrode  1308  and a stud/socket assembly  303  of one cam lock on the inner electrode  120 B. 
         [0099]    Referring now to  FIG. 2D , a cross-sectional view of the cam lock further exemplifies how the cam lock operates by placing the outer electrode  130 B and the inner electrode  120 B in close proximity to the backing plate  140 B. The stud  205 /disc spring stack  215 /socket  213  assembly is mounted into the outer electrode  130 B and the inner electrode  120 B. As shown, the stud/socket assembly may be screwed, by means of external threads on the socket  213  into a threaded hole in the outer electrode  1308  or the inner electrode  120 B. 
         [0100]    With reference to  FIG. 4C , an exploded view  400 B of the cam shaft  207 B also indicates a keying stud  402  and a hex opening  403 B on one end of the cam shaft  207 B. 
         [0101]    For example, with continued reference to  FIGS. 4C ,  2 C and  2 D, the cam lock is assembled by inserting the cam shaft  207 B into the backing plate bore  211 B. The keying stud  402  limits rotational travel of the cam shaft  207 B in the backing plate bore  211 B by interfacing with a step on an entrance of the bore  211 B as shown in  FIG. 4D . The cam shaft  207 B has two internal eccentric cutouts. One cutout engages an enlarged head of the stud  205  on the outer electrode  1308  and the other cutout engages an enlarged head of the stud  205  on the inner electrode  120 B. The cam shaft  207 B may first be turned in one direction though use of the hex opening  403 B, for example, counter-clockwise, to allow entry of the studs  205  into the cam shaft  207 B, and then turned clockwise to fully engage and lock the studs  205 . The clamp force required to hold the outer electrode  130 B and the inner electrode  120 B to the backing plate  140 B 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 B and the inner electrode  120 B to the backing plate  140 B. 
         [0102]    In an exemplary mode of operation, the cam shaft  207 B is inserted into the backing plate bore  211 B. The cam shaft  207 B is rotated counterclockwise to its full rotational travel. The stud/socket assemblies  303  ( FIG. 3 ) lightly torqued into the outer electrode  130 B and the inner electrode  120 B are then inserted into vertically extending through holes below the horizontally extending backing plate bore  211 B such that the heads of the studs  205  engage in the eccentric cutouts in the cam shaft  207 B. The outer electrode  130 B and the inner electrode  120 B are held against the backing plate  140 B and the cam shaft  207 B is rotated clockwise until the keying pin is limited by the step on the entrance of the bore  211 B. The exemplary mode of operation may be reversed to dismount the outer electrode  130 B and the inner electrode  120 B from the backing plate  140 B. 
         [0103]    With reference to  FIG. 4B , a sectional view A-A of the side-elevation view  420 B of the cam shaft  207 B of  FIG. 4C  indicates a cutter path edge  440 B by which the head of the stud  205  is fully secured. 
         [0104]      FIG. 7C  shows a top view of another gasket set. This gasket set comprises an inner gasket  7400  comprising a plurality of concentric rings connected by a plurality of spokes, a first annular gasket  7500  with a plurality of cutouts on an outer and an inner perimeter, a second annular gasket  7600  with a plurality of holes and one cutout, and a third annular gasket  7700  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 backing plate  140 B and the inner electrode  120 B and outer electrode  130 B to provide electrical and thermal contact therebetween. 
         [0105]      FIG. 7D  shows the details of the inner gasket  7400 . The inner gasket  7400  preferably comprises seven concentric rings interconnected by radial spokes. A first ring  7401  has an inner diameter of at least 0.44 inch (e.g. between 0.62 and 0.65 inch) and an outer diameter of at most 1.35 inches (e.g. between 0.97 and 1.00 inch). The first ring  7401  is connected to a second ring  7402  by eight radially extending and azimuthally evenly spaced spokes  7412 . Each spoke  7412  has a width of about 0.125 inch. 
         [0106]    The second ring  7402  has an inner diameter of at least 1.35 inches (e.g. between 1.74 and 1.76 inches) and an outer diameter of at most 2.68 inches (e.g. between 2.26 and 2.29 inches). The second ring  7402  is connected to a third ring  7403  by four radially extending and azimuthally evenly spaced spokes. Two of these four spokes  7423   a  and  7423   b  oppose each other about the center of the inner gasket  7400  and each has a width of about 0.56 inch and a rounded rectangular opening ( 7423   ah  or  7423   bh ) of about 0.31 inch by about 0.46 inch. The other two of these four spokes  7423   c  and  7423   d  oppose each other about the center of the inner gasket  7400  and each has a width of about 0.125 inch. One spoke  7423   c  is offset azimuthally from one of the spokes  7412  by about 22.5°. 
         [0107]    The third ring  7403  has an inner diameter of at least 2.68 inches (e.g. between 3.17 and 3.20 inches) and an outer diameter of at most 4.23 inches (e.g. between 3.71 and 3.74 inches). The third ring is connected to a fourth ring  7404  by four radially extending and azimuthally evenly spaced spokes  7434 . Each spoke has a width of about 0.125 inch. One of the spokes  7434  is offset azimuthally by about 22.5° counterclockwise from the spoke  7423   c.  The third ring  7403  also includes two round holes  7403   x  and  7403   y  located at a radial distance between 1.72 and 1.74 inches from the center of the inner gasket  7400 . The round holes  7403   x  and  7403   y  have a diameter of about 0.125 inch. The round hole  7403   x  is offset azimuthally by about 95° counterclockwise from the spoke  7423   c.  The round hole  7403   y  is offset azimuthally by about 90° clockwise from the spoke  7423   c.  The round holes  7403   x  and  7403   y  are configured to receive alignment pins. 
         [0108]    The fourth ring  7404  has an inner diameter of at least 4.23 inches (e.g. between 4.78 and 4.81 inches) and an outer diameter of at most 5.79 inches (e.g. between 5.19 and 5.22 inches). The fourth ring  7404  is connected to a fifth ring  7405  by a set of 8 radially extending and azimuthally evenly spaced spokes  7445   a  and another set of 8 radially extending and azimuthally evenly spaced spokes  7445   b.  One of the spokes  7445   b  is offset azimuthally by about 8.5° counterclockwise from the spoke  7423   c.  One of the spokes  7445   a  is offset azimuthally by about 8.5° clockwise from the spoke  7423   c.  Each spoke  7445   a  and  7445   b  has a width of about 0.125 inch. The spokes  7445   a  and  7445   b  extend inward radially and separate the fourth ring  7404  into eight arcs each of which has a central angle of about 28°. 
         [0109]    The fifth ring  7405  has an inner diameter of at least 5.79 inches (e.g. between 6.35 and 6.37 inches) and an outer diameter of at most 7.34 inches (e.g. between 6.73 and 6.75 inches). The fifth ring  7405  is connected to a sixth ring  7406  by four radially extending and azimuthally evenly spaced spokes  7456 . One of the spokes  7456  is offset azimuthally by about 90° from the spoke  7423   c.  Each the spokes  7456  has a width of about 0.125 inch. 
         [0110]    The sixth ring  7406  has an inner diameter of at least 7.34 inches (e.g. between 7.92 and 7.95 inches) and an outer diameter of at most 8.89 inches (e.g. between 8.16 and 8.36 inches). The sixth ring  7406  is connected to a seventh ring  7407  by a set of four radially extending and azimuthally evenly spaced spokes  7467   a  and another set of four radially extending and azimuthally evenly spaced spokes  7467   b.  One of the spokes  7467   b  is offset azimuthally by about 6.4° counterclockwise from the spoke  7423   c.  One of the spokes  7467   a  is offset azimuthally by about 6.4° clockwise from the spoke  7423   c.  Each spoke  7467   a  and  7467   b  has a width of about 0.125 inch. 
         [0111]    The seventh ring  7407  has an inner diameter of at least 8.89 inches (e.g. between 9.34 and 9.37 inches) and an outer diameter of at most 10.18 inches (e.g. between 9.66 and 9.69 inches). Two cutouts  7407   ah  and  7407   bh  with a width of about 0.25 inch separate the seventh ring  7407  into two sections. The cutout  7407   ah  is offset azimuthally by about 90° counterclockwise from the spoke  7423   c.  The cutout  7407   bh  is offset azimuthally by about 90° clockwise from the spoke  7423   c.    
         [0112]    The first annular gasket  7500  (see  FIG. 7C ) has an inner diameter of about 11.95 inches and an outer diameter of about 12.47 inches. The first annular gasket  7500  has three small-diameter cutouts  7508   a,    7508   b  and  7508   c  on its inner perimeter. The cutouts  7508   b  and  7508   c  are azimuthally offset from the cutout  7508   a  by about 92.5° clockwise and about 190° clockwise, respectively. The centers of the cutouts  7508   a,    7508   b  and  7508   c  are located at a radial distance of about 6.02 inches from the center of the first annular gasket  7500 . The cutouts  7508   a,    7508   b  and  7508   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 first annular gasket  7500  also has three large-diameter round and outwardly facing cutouts  7508   x,    7508   y  and  7508   z  on its outer perimeter. The cutouts  7508   x,    7508   y  and  7508   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 first annular gasket  7500 . The cutout  7508   x  is azimuthally offset from the cutout  7508   a  by about 37.5° clockwise. When installed in the showerhead electrode assembly  100 B (as described in details hereinbelow), the cutout  7508   a  is offset azimuthally by about 90° counterclockwise from the spoke  7423   c  in the inner gasket  7400 . 
         [0113]    The second annular gasket  7600  has an inner diameter of about 13.90 inches and an outer diameter of about 16.75 inches. The second annular gasket  7600  has eight circular holes  7609   a  equally spaced azimuthally. The centers of the holes  7609   a  are located at a radial distance of about 7.61 inches from the center of the second annular gasket  7600 . The holes  7609   a  have a diameter of about 0.55 inch. When installed in the showerhead electrode assembly  100 B (as described in details hereinbelow), one of the holes  7609   a  is azimuthally aligned with the hole  7403   y  on the third ring  7403  in the inner gasket  7400 . The second annular gasket  7600  also has one round inwardly facing cutout  7609   b  on the inner perimeter of the outer gasket  7300 . The center of this cutout  7609   b  is located at a distance of about 6.98 inches from the center of the second annular gasket  7600 . The cutout  7609   b  has a diameter of about 0.92 inch. When installed in the showerhead electrode assembly  100 B (as described in details hereinbelow), the cutout  7609   b  is azimuthally offset from the hole  7403   y  by about 202.5° counterclockwise. The second annular gasket  7600  further has three circular holes  7610 ,  7620  and  7630  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  7610 ,  7620  and  7630  are offset azimuthally by about 7.5°, about 127.5° and about 252.5° respectively clockwise from the cutout  7609   b.    
         [0114]    The third annular gasket  7700  has an inner diameter of about 17.29 inches and an outer diameter of about 18.69 inches. The third annular gasket  7700  has eight round outwardly facing cutouts  7701  equally spaced azimuthally on the outer perimeter. The centers of the cutouts  7701  are located at a radial distance of about 9.30 inches from the center of the third annular gasket  7700 . The cutouts  7701  have a diameter of about 0.53 inch. 
         [0115]    When the inner electrode  120 B is installed in the showerhead electrode assembly  100 B, an alignment ring  108 B ( FIG. 1B ), two inner alignment pins  109 B (not shown in  FIG. 1B ) and three outer alignment pins (not shown in  FIG. 1B ) are first inserted into the annular groove  550 B, holes  540 Ba/ 540 Bb and holes  530   b  ( FIG. 5K ), respectively. The inner gasket  7400  is then mounted to the inner electrode  120 B. The holes  7403   x  and  7403   y  ( FIG. 7D ) correspond to the inner alignment pins  109 B; and the center hole of the inner gasket  7400  corresponds to the alignment ring  108 B and the center gas injection hole in the inner electrode  120 B. Openings between the seven rings and in the spokes in the inner gasket  7400  correspond to the first row through the sixth row of gas injection holes in the inner electrode  1208 . The first annular gasket  7500  is mounted onto the inner electrode  120 B. The cutouts  708   a,    708   b  and  708   c  correspond to the holes  530   bc ,  530   bb  and  530   ba , respectively. The seventh and eighth rows of gas injection holes fall in the opening between the inner gasket  7400  and the first annular gasket  7500 . Eight stud/socket assemblies  303  are threaded into the eight threaded holes  520 B to fasten the inner electrode  120 B to the backing plate  140 B, with the inner gasket  7400  and first annular gasket  7500  sandwiched therebetween. The clamp ring  150 B is fastened onto the backing plate  140 B by a plurality of bolts threaded into threaded openings in the underside of the backing plate  140 B. The stud/socket assemblies  303  and the clamp ring  150 B support the inner electrode  120 B at a location between the center and outer edge and at the outer edge, respectively, improve thermal contact with the backing plate  140 B and reduce warping of the inner electrode  120 B caused by temperature cycling during processing of substrates. The second annular gasket  7600  is placed on the outer electrode  130 B. The eight holes  7609   a  correspond to the eight cam locks threaded on the outer electrode  130 B. The outer electrode  130 B and the inner electrode  120 B are fastened against the backing plate  140 B by rotating the cam shafts  207 B. The C-shaped shroud  190  in  FIG. 1B  is fastened to the backing plate  140 B by a plurality of (preferably eight) cam locks. The third annular gasket  7700  is placed between the shroud  190  and the backing plate  140 B. The cutouts  7701  correspond to the cam locks between the shroud  190  and the backing plate  140 B. 
         [0116]    The rings  7401 - 7407  and the spokes in the inner gasket  7400  may be arranged in any suitable pattern as long as they do not obstruct the gas injection holes  106 B, the cam locks  160 B, alignment ring  108 B, or alignment pins  109 B in the inner electrode  120 B. 
         [0117]      FIG. 7E  shows a top view of yet another gasket set. This gasket set comprises an inner gasket  7800  comprising a plurality of concentric rings connected by a plurality of spokes, a first annular gasket  7500  with a plurality of cutouts on an outer and an inner perimeter, a second annular gasket  7600  with a plurality of holes and one cutout, and a third annular gasket  7700  with a plurality of cutouts. This gasket set is identical to the gasket set shown in  FIGS. 7C and 7D , except that the inner gasket  7800  (as shown in  FIG. 7F ) does not have the seventh ring and spokes connecting the sixth and the seventh rings. 
         [0118]    The rings and the spokes in the inner gasket  7800  may be arranged in any suitable pattern as long as they do not obstruct the gas injection holes  106 B, cam locks  160 B, alignment ring  108 B, or alignment pins  109 B in the inner electrode  120 B. 
         [0119]    While the showerhead electrode assemblies, inner electrodes, outer electrodes and gasket sets 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.