Patent Publication Number: US-8536071-B2

Title: Gasket with positioning feature for clamped monolithic showerhead electrode

Description:
This application is a divisional of U.S. patent application Ser. No. 12/421,845, entitled GASKET WITH POSITIONING FEATURE FOR CLAMPED MONOLITHIC SHOWERHEAD ELECTRODE, filed Apr. 10, 2009 now U.S. Pat. No. 8,272,346 which is herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a showerhead electrode assembly of a plasma processing chamber in which semiconductor components can be manufactured. 
     SUMMARY 
     According to one embodiment, a gasket is provided for a showerhead electrode assembly in which a monolithic stepped electrode is clamped to a backing plate and the showerhead electrode assembly comprises an upper electrode of a capacitively coupled plasma processing chamber. The stepped electrode is a circular plate having a plasma exposed surface on a lower face thereof and a mounting surface on an upper face thereof. The mounting surface includes a plurality of alignment pin recesses configured to receive alignment pins arranged in a pattern matching alignment pin holes in a backing plate against which the plate is held by cam locks and the plate includes process gas outlets arranged in a pattern matching gas supply holes in the backing plate. The upper face includes a plurality of recesses which receive alignment features on the gasket. A plurality of circumferentially spaced apart pockets in an outer region of the mounting surface are configured to receive locking pins therein adapted to cooperate with cam locks to clamp the stepped electrode to the backing plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-sectional view of a showerhead electrode assembly forming an upper electrode of a capacitively coupled plasma reactor for etching substrates having a guard ring. 
         FIG. 2A  is a three-dimensional representation of an exemplary cam lock for clamping a stepped electrode in the reactor shown in  FIG. 1 . 
         FIG. 2B  is a cross-sectional view of the exemplary cam lock electrode clamp of  FIG. 2A . 
         FIG. 3  shows side-elevation and assembly drawings of an exemplary locking pin used in the cam lock clamp of  FIGS. 2A and 2B . 
         FIG. 4A  shows side-elevation and assembly drawings of an exemplary cam shaft used in the cam lock clamp of  FIGS. 2A and 2B . 
         FIG. 4B  shows a cross-sectional view of an exemplary cutter-path edge of a portion of the cam shaft of  FIG. 4A . 
         FIG. 5A  shows a showerhead electrode assembly with a stepped electrode, backing plate, thermal control plate, guard ring and top plate. 
         FIG. 5B  shows a perspective view of the upper face of a modified showerhead electrode and  FIG. 5C  shows a perspective view of the lower face of a modified backing plate. 
         FIGS. 6A and 6B  are perspective views of the stepped electrode of  FIG. 5A . 
         FIG. 7  is a perspective view of a backing plate of  FIG. 5A . 
         FIG. 8  is a perspective view of the showerhead electrode assembly of  FIG. 5A  without the guard ring. 
         FIG. 9  is a bottom view of a gasket according to a preferred embodiment. 
         FIG. 10  is a side view of the gasket shown in  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION 
     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 “wafer.” Each wafer is subjected to a sequence of physical and chemical processing steps that form the various circuit structures on the wafer. During the fabrication process, various types of thin films may be deposited on the wafer 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. 
     After depositing a film on the semiconductor wafer, 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 wafer 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. 
     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. 
     A reactive ion etching system typically consists of an etching chamber with an upper electrode or anode and a lower electrode or cathode positioned therein. The cathode is negatively biased with respect to the anode and the container walls. The wafer to be etched is covered by a suitable mask and placed directly on the cathode. A chemically reactive gas such as CF 4 , CHF 3 , CClF 3 , HBr, Cl 2  and SF 6  or mixtures thereof with O 2 , N 2 , He or Ar is introduced into the etching chamber and maintained at a pressure which is typically in the millitorr range. The upper electrode is provided with gas hole(s), which permit the gas to be uniformly dispersed through the electrode into the chamber. The electric field established between the anode and the cathode will dissociate the reactive gas forming plasma. The surface of the wafer is etched by chemical interaction with the active ions and by momentum transfer of the ions striking the surface of the wafer. The electric field created by the electrodes will attract the ions to the cathode, causing the ions to strike the surface in a predominantly vertical direction so that the process produces well-defined vertically etched sidewalls. The etching reactor electrodes may often be fabricated by bonding two or more dissimilar members with mechanically compliant and/or thermally conductive adhesives, allowing for a multiplicity of function. 
       FIG. 1  shows a cross-sectional view of a portion of a showerhead electrode assembly  100  of a plasma processing system for etching substrates. As shown in  FIG. 1 , the showerhead electrode assembly  100  includes a stepped electrode  110 , a backing plate  140 , and a guard ring (or outer ring)  170 . The showerhead electrode assembly  100  also includes a plasma confinement assembly (or wafer area pressure (WAP) assembly)  180 , which surrounds the outer periphery of the upper electrode  110  and the backing plate  140 . 
     The assembly  100  also includes a thermal control plate  102 , and an upper (top) plate  104  having liquid flow channels therein and forming a temperature controlled wall of the chamber. The stepped electrode  110  is preferably a cylindrical plate and may be made of a conductive high purity material such as single crystal silicon, polycrystalline silicon, silicon carbide or other suitable material (such as aluminum or alloy thereof, anodized aluminum, yttria coated aluminum). The backing plate  140  is mechanically secured to the electrode  110  with mechanical fasteners described below. The guard ring  170  surrounds the backing plate  140  and provides access to cam locking members as described below. 
     The showerhead electrode assembly  100  as shown in  FIG. 1  is typically used with an electrostatic chuck (not shown) incorporating a flat lower electrode on which a wafer is supported at a distance of about 1 to 2 cm below the upper electrode  110 . An example of such a plasma processing system 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 wafer by supplying backside helium (He) pressure, which controls the rate of heat transfer between the wafer and the chuck. 
     The upper electrode  110  is a consumable part which must be replaced periodically. To supply process gas to the gap between the wafer and the upper electrode, the upper electrode  110  is provided with a gas discharge passages  106 , which are of a size and distribution suitable for supplying a process gas, which is energized by the electrode and forms plasma in a reaction zone beneath the upper electrode  110 . 
     The showerhead electrode assembly  100  also includes a plasma confinement assembly (or wafer area plasma (WAP) assembly)  180 , which surrounds the outer periphery of the upper electrode  110  and the backing plate  140 . The plasma confinement assembly  180  is preferably comprised of a stack or plurality of spaced-apart quartz rings  190 , which surrounds the outer periphery of upper electrode  110  and the backing plate  140 . During processing, the plasma confinement assembly  180  causes a pressure differential in the reaction zone and increases the electrical resistance between the reaction chamber walls and the plasma thereby confining the plasma between the upper electrode  110  and the lower electrode (not shown). 
     During use, the confinement rings  190  confine the plasma to the chamber volume and controls the pressure of the plasma within the reaction chamber. The confinement of the plasma to the reaction chamber is a function of many factors including the spacing between the confinement rings  190 , the pressure in the reaction chamber outside of the confinement rings and in the plasma, the type and flow rate of the gas, as well as the level and frequency of RF power. Confinement of the plasma is more easily accomplished if the spacing between the confinement rings  190  is very small. Typically, a spacing of 0.15 inches or less is required for confinement. However, the spacing of the confinement rings  190  also determines the pressure of the plasma, and it is desirable that the spacing can be adjusted to achieve the pressure required for optimal process performance while maintaining plasma. Process gas from a gas supply is supplied to electrode  110  through one or more passages in the upper plate  104  which permit process gas to be supplied to a single zone or multiple zones above the wafer. 
     The electrode  110  is preferably a planar disk or plate having a uniform thickness from center (not shown) to an area of increased thickness forming a step on the plasma exposed surface extending inwardly from an outer edge. The electrode  110  preferably has a diameter larger than a wafer to be processed, e.g., over 300 mm. The diameter of the upper electrode  110  can be from about 15 inches to about 17 inches for processing 300 mm wafers. The upper electrode  110  preferably includes multiple gas passages  106  for injecting a process gas into a space in a plasma reaction chamber below the upper electrode  110 . 
     Single crystal silicon and polycrystalline silicon are preferred materials for plasma exposed surfaces of the electrode  110 . High-purity, single crystal or polycrystalline 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 aluminum (as used herein “aluminum” refers to pure Al and alloys thereof), yttria coated aluminum, SiC, SiN, and AlN, for example. 
     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. 
     The upper electrode  110  is attached mechanically to the backing plate  140  without any adhesive bonding between the electrode and backing plate, i.e., a thermally and electrically conductive elastomeric bonding material is not used to attach the electrode to the backing plate. 
     The backing plate  140  is preferably attached to the thermal control plate  102  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  and screwed into threaded openings in the backing plate  140 . The thermal control plate  102  includes a flexure portion  184  and is preferably made of a machined metallic material, such as aluminum, an aluminum alloy or the like. The upper temperature controlled plate  104  is preferably made of aluminum or an aluminum alloy. The plasma confinement assembly (or wafer area plasma assembly (WAP))  180  is positioned outwardly of the showerhead electrode assembly  100 . A suitable plasma confinement assembly  180  including a plurality of vertically adjustable plasma confinement rings  190  is described in commonly owned U.S. Pat. No. 5,534,751, which is incorporated herein by reference in its entirety. 
     The upper electrode can be mechanically attached to the backing plate by a cam lock mechanism as described in commonly-owned U.S. application Ser. No. 61/036,862, filed Mar. 14, 2008, the disclosure of which is hereby incorporated by reference. With reference to  FIG. 2A , a three-dimensional view of an exemplary cam lock electrode clamp includes portions of an electrode  201  and a backing plate  203 . The electrode clamp is capable of quickly, cleanly, and accurately attaching a consumable electrode  201  to a backing plate in a variety of fab-related tools, such as the plasma etch chamber shown in  FIG. 1 . 
     The electrode clamp 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 electrode  201  and the backing plate  203 . Limiting the amount of lateral movement allows for a tight fit between the electrode  201  and the backing plate  203 , 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. 
     In a specific exemplary embodiment, the socket  213  is fabricated from bearing-grade 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 polyamides, polyimides, 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. 
     Other portions of the electrode clamp are comprised of a camshaft  207  surrounded at each end by a pair of camshaft bearings  209 . The camshaft  207  and camshaft bearing assembly is mounted into a backing plate bore  211  machined into the backing plate  203 . In a typical application for an etch chamber designed for 300 mm semiconductor wafers, eight or more of the electrode clamps may be spaced around the periphery of the electrode  201 /backing plate  203  combination. 
     The camshaft 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 camshaft  207  may be machined from stainless steel (e.g., 316, 316L, 17-7, etc.) or any other material providing good strength and corrosion resistance. 
     Referring now to  FIG. 2B , a cross-sectional view of the electrode cam clamp further exemplifies how the cam clamp operates by pulling the electrode  201  in close proximity to the backing plate  203 . The stud  205 /disc spring stack  215 /socket  213  assembly is mounted into the electrode  201 . As shown, the assembly may be screwed, by means of external threads on the socket  213  into a threaded pocket in the electrode  201 . However, the socket may be mounted by adhesives or other types of mechanical fasteners as well. 
     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 electrode clamp. 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 electrode  201  (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 pocket on the electrode  201 . 
     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 electrode clamp 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 .) 
     With reference to  FIG. 4A , an exploded view  400  of the camshaft  207  and camshaft bearings  209  also indicates a keying pin  401 . The end of the camshaft  207  having the keying pin  401  is first inserted into the backing plate bore  211  (see  FIG. 2B ). A pair of small mating holes (not shown) at a far end of the backing plate bore  211  provide proper alignment of the camshaft  207  into the backing plate bore  211 . A side-elevation view  420  of the camshaft  207  clearly indicates a possible placement of a hex opening  403  on one end of the camshaft  207  and the keying pin  401  on the opposite end. 
     For example, with continued reference to  FIGS. 4A and 2B , the electrode cam clamp is assembled by inserting the camshaft  207  into the backing plate bore  211 . The keying pin  401  limits rotational travel of the camshaft  207  in the backing plate bore  211  by interfacing with one of the pair of small mating holes. The camshaft may first be turned in one direction though use of the hex opening  403 , for example, counter-clockwise, to allow entry of the stud  205  into the camshaft  207 , and then turned clockwise to fully engage and lock the stud  205 . The clamp force required to hold the electrode  201  to the backing plate  203  is supplied by compressing the disc spring stack  215  beyond their free stack height. The camshaft  207  has an internal eccentric internal cutout which engages the enlarged head of the shaft  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 electrode  201  to the backing plate  203 . 
     In an exemplary mode of operation, once the camshaft bearings are attached to the camshaft  207  and inserted into the backing plate bore  211 , the camshaft  207  is rotated counterclockwise to its full rotational travel. The stud/socket assembly  303  ( FIG. 3 ) is then lightly torqued into the electrode  201 . The head of the stud  205  is then inserted into the vertically extending through hole below the horizontally extending backing plate bore  211 . The electrode  201  is held against the backing plate  203  and the camshaft  207  is rotated clockwise until either the keying pin drops into the second of the two small mating holes (not shown) or an audible click is heard (discussed in detail, below). The exemplary mode of operation may be reversed to dismount the electrode  201  from the backing plate  203 . However, features such as the audible click are optional in the cam lock arrangement. 
     With reference to  FIG. 4B , a sectional view A-A of the side-elevation view  420  of the camshaft  207  of  FIG. 4A  indicates a cutter path edge  440  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 optional audible clicking noise described above to indicate when the stud  205  is fully secured. 
       FIG. 5A  illustrates an upper electrode assembly  500  for a capacitively coupled plasma chamber which includes the following features: (a) a cam-locked non-bonded electrode  502 ; (b) a backing plate  506 ; and (c) a guard ring  508  which allows access to cam locks holding the electrode to the backing plate  506 . 
     The electrode assembly  500  includes a thermal control plate  510  bolted from outside the chamber to a temperature controlled top wall  512  of the chamber. The electrode  502  is releasably attached to the backing plate from inside the chamber by cam-lock mechanisms  514  described earlier with reference to  FIGS. 2-4 . 
     In a preferred embodiment, the electrode  502  of the electrode assembly  500  can be disassembled by (a) rotating the guard ring  508  to a first position aligning four holes in the guard ring with four cam locks  514  located at spaced positions in the outer portion of the backing plate; (b) inserting a tool such as an allen wrench through each hole in the guard ring and rotating each cam lock to release a vertically extending locking pin of each respective cam lock; (c) rotating the guard ring 90° to a second position aligning the four holes in the guard ring with another four cam locks; and (d) inserting a tool such as an allen wrench through each hole in the guard ring and rotating each respective cam lock to release a locking pin of each respective cam lock; whereby the electrode  502  can be lowered and removed from the plasma chamber. 
       FIG. 5A  also shows a cross-sectional view of one of the cam lock arrangements wherein a rotatable cam lock  514  is located in a horizontally extending bore  560  in an outer portion of the backing plate  506 . The cylindrical cam lock  514  is rotatable by a tool such as an alien wrench to (a) a lock position at which an enlarged end of a locking pin  562  is engaged by a cam surface of the cam lock  514  which lifts the enlarged head of the locking pin or (b) a release position at which the locking pin  562  is not engaged by the cam lock  514 . The backing plate includes vertically extending bores in its lower face through which the locking pins are inserted to engage the cam locks. 
     In the embodiment shown in  FIG. 5A , an outer step in the backing plate  506  mates with an annular recessed mounting surface on the upper face of the showerhead electrode  502 . In an alternative arrangement, the step and recess can be omitted such that the lower face of the backing plate and the upper face of the showerhead electrode are planar surfaces.  FIG. 5B  shows a cross-section of a modified showerhead electrode  502 A having a flat upper surface  522 A, five alignment pin holes  520 A, eight pockets  550 A, gas holes  528 A, and two recesses  520 B for mating with projections of a gasket located between the third and fourth row of gas holes.  FIG. 5C  shows a modified backing plate  506 A having a planar lower surface  522 B, five alignment pin holes  520 C, eight cam locks  514 B, and annular gasket receiving surfaces G 1  and G 2 . 
       FIGS. 6A-B  show details of the electrode  502 . The electrode  502  is preferably a plate of high purity (less than 10 ppm impurities) low resistivity (0.005 to 0.02 ohm-cm) single crystal silicon with alignment pin holes  520  in an upper face (mounting surface)  522  which receive alignment pins  524 . Gas holes  528  extend from the upper face to the lower face (plasma exposed surface)  530  and can be arranged in any suitable pattern. In the embodiment shown, the gas holes are arranged in 13 circumferentially extending rows with 3 gas holes in the first row located about 0.5 inch from the center of the electrode, 13 gas holes in the second row located about 1.4 inches from the center, 23 gas holes in the third row located about 2.5 inches from the center, 25 gas holes in the fourth row located about 3.9 inches from the center, 29 gas holes in the fifth row located about 4.6 inches from the center, 34 gas holes in the sixth row located about 5.4 inches from the center, 39 gas holes in the seventh row located about 6 inches from the center, 50 gas holes in the eighth row located about 7.5 inches from the center, 52 gas holes in the ninth row located about 8.2 inches from the center, 53 gas holes in the tenth row located about 9 inches from the center, 57 gas holes in the eleventh row located about 10.3 inches from the center, 59 gas holes in the twelfth row located about 10.9 inches from the center and 63 holes in the thirteenth row located about 11.4 inches from the center. 
     In an alternative arrangement, 562 gas holes can be arranged with 4 holes in the first row located 0.25 inch from the center, 10 holes in a second row located about 0.72 inch from the center, 20 holes in a third row about 1.25 inches from the center, 26 holes in a fourth row about 1.93 inches from the center, 30 holes in a fifth row about 2.3 inches from the center, 36 holes in a sixth row about 2.67 inches from the center, 40 holes in a seventh row about 3.0 inches from the center, 52 holes in an eighth row about 3.73 inches from the center, 58 holes in a ninth row about 4.1 inches from the center, 62 holes in a tenth row about 4.48 inches from the center, 70 holes in an eleventh row about 5.17 inches from the center, 74 holes in a twelfth row about 5.44 inches from the center and 80 holes in a thirteenth row about 5.71 inches from the center. 
     In the embodiment shown in  FIG. 5A , the upper face of the electrode includes 9 alignment pin holes with 3 pin holes near the center, 3 pin holes inward of the annular recess and 3 pin holes in the annular recess near the outer edge of the electrode. The 3 central pin holes are radially aligned and include a pin hole at the center of the inner electrode and 2 pin holes between the third and fourth rows of gas holes. The intermediate pin holes near the annular recess include one pin hole radially aligned with the central pin hole and two other pin holes spaced 120° apart. The outer 3 pin holes are spaced 120° apart at locations between adjacent pockets. 
       FIG. 6A  is a front perspective view showing the plasma exposed surface  530  of the electrode  502  with the 13 rows of gas holes.  FIG. 6B  shows a perspective view of the upper face with the 13 rows of gas holes. 
     The electrode  502  includes an outer step (ledge)  536  which supports the guard ring  508 , the upper face (mounting surface)  522  which engages a lower surface of the backing plate  506 , the lower face (plasma exposed stepped surface)  530  which includes inner tapered surface  544 , a horizontal surface  546 , and an outer tapered surface  548  and  8  pockets  550  in upper face  540  in which the locking pins are mounted. 
       FIG. 7  is a perspective view of backing plate  506 . The backing plate includes 13 rows of gas passages  584  which align with the passages  528  in the showerhead electrode  502 . The upper face  586  of the backing plate includes three annular regions  588   a ,  588   b ,  588   c  which contact annular projections of the thermal control plate  510 . The thermal control plate can be attached to the top wall of the plasma chamber by fasteners extending through the top wall into the thermal control plate as disclosed in commonly-assigned U.S. Patent Publication Nos. 2005/0133160, 2007/0068629, 2007/0187038, 2008/0087641 and 2008/0090417, the disclosures of which are hereby incorporated in their entirety. Threaded openings  590  are located in an outer periphery of the upper face  586  and the annular regions  588   a ,  588   b ,  588   c  to receive fasteners extending through openings in the top plate  512  and thermal control plate  510  to hold the backing plate  506  in contract with the thermal control plate  510 . See, for example, commonly-assigned U.S. Patent Publication No. 2008/0087641 for a description of fasteners which can accommodate thermal cycling. A groove  592  in the upper face  586  receives an O-ring which provides a gas seal between the backing plate  506  and the thermal control plate  510 . Alignment pin bores  594  in the upper face  586  receive alignment pins which fit into alignment pin bores in the thermal control plate. Horizontally extending threaded openings  561  at positions between bores  560  receive dielectric fasteners used to prevent the guard ring from rotating and plug the access bores in the guard ring after assembly of the showerhead electrode. 
       FIG. 8  is a perspective view of the showerhead electrode assembly  500  with the guard ring removed. As explained earlier, the guard ring can be rotated to one or more assembly positions at which the cam locks can be engaged and rotated to a lock position at which dielectric fasteners can be inserted into openings  561  to maintain the guard ring out of contact with the outer periphery of the backing plate and thus allow for thermal expansion of the backing plate. The thermal control plate includes a flange  595  with openings  596  through which actuators support the plasma confinement rings. Details of the mounting arrangement of plasma confinement ring assemblies can be found in commonly-assigned U.S. Patent Publication No. 2006/0207502 and 2006/0283552, the disclosures of which are hereby incorporated in their entirety. 
     The mounting surface  522  of the electrode abuts an opposed surface of the backing plate  506  as a result of the clamping force exerted by the 8 locking pins held by the 8 cam locks in the backing plate. The guard ring  508  covers the mounting holes in the backing plate  506  and the access openings in the guard ring are filled with removable inserts made of plasma resistant polymer material such as 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. 
     With reference to  FIG. 5A , electrical contact between the backing plate  506  and electrode  502  is provided by one or more gaskets  556  such as annular sections of a suitable material such as “Q-PAD II” available from the Bergquist Company. Such gaskets are located at the outer periphery of the electrode and at one or more locations between the central alignment pin and the outer gasket. For example, annular gaskets having diameters of about 4 and 12 inches can be used. Commonly-owned U.S. application Ser. No. 11/896,375, filed Aug. 31, 2007, includes details of gaskets made of Q-PAD material, the disclosure of which is hereby incorporated by reference. To provide different process gas mixtures and/or flow rates, one or more optional gas partition seals can be provided between the center alignment pin and the outer gasket. For example, a single O-ring can be provided between the electrode  502  and the backing plate  506  at a location between the inner and outer gaskets to separate an inner gas distribution zone from an outer gas distribution zone. An O-ring  558  located between the electrode  502  and the backing plate  506  along the inner periphery of the outer gasket can provide a gas and particle seal between the electrode and backing plate. 
       FIG. 9  shows a bottom view of a preferred gasket  900  having a plurality of alignment features in the form of projections  902  on a lower surface  904  thereof. The electrode  502 A includes a plurality of recesses ( 520 B in  FIG. 5B ) sized to receive the projections on the gasket  900 . In the embodiment shown, two projections  902  are located 180° apart and the projections have identical cylindrical shapes which fit within round recesses  520 B in the electrode  502 A located between the third and fourth circumferential rows of gas passages  528 A. The projections are preferably sized to be frictionally engaged in the recesses  520 B in the electrode  502 A. While cylindrical projections having diameters greater than half the width of the gasket are shown in  FIG. 9 , the projections can have any desired shape and size and the number of projections can be 3, 4, 5, 6, 7, 8 or more, if desired. For example, the gasket can be a flat ring of uniform thickness of under 0.01 inch and the projections can be at least 2, 3, 4 or 5 times thicker than the thickness of the flat ring. Although the projections could be formed by molding integral projections or deforming portions of the flat ring into projections, it is preferred to form the projections from a different material having a greater thickness than the flat ring and attaching the projections to the flat ring with adhesive compatible in a vacuum environment of a plasma processing chamber. 
     The gasket is preferably electrically and thermally conductive and made of a material which preferably does not outgas in a high-vacuum environment, e.g., about 10 to 200 mTorr, has low particulate generation performance; is compliant to accommodate shear at contact points; is free of metallic components that are lifetime killers in semiconductor substrates such as Ag, Ni, Cu and the like. The gasket can be a silicone-aluminum foil sandwich gasket structure or an elastomer-stainless steel sandwich gasket structure. Preferably, the gasket is an aluminum sheet coated on upper and lower sides with a thermally and electrically conductive rubber compatible in a high vacuum environment used in semiconductor manufacturing wherein steps such as plasma etching are carried out. The gasket is 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 gasket  900  shown in  FIG. 9  is preferably a laminate of electrically and thermally conductive material (such as “Q-PAD” foil material available from The Bergquist Company). The gasket  900  for the G1 location in  FIG. 5C  preferably has an inner diameter of about 2.93 inches, an outer diameter of about 3.43 inches and a thickness of about 0.006 inch. This gasket has two projections  902  which comprise a cylindrical piece of sheet material such as silicone rubber with a diameter of about 0.185 inch and height of about 0.026 to 0.034 inch. The projections  902  are preferably adhesively bonded to one side of the gasket  900  by suitable adhesive such as a silicone elastomer adhesive, e.g. RTV 3140 silicone adhesive available from Dow Corning. The gasket can be made by cutting or stamping a ring out of a sheet of gasket material. Likewise, the projections can be cut or stamped out of the sheet of the same or different material such as a resilient material which may or may not be thermally and/or electrically conductive. For example, the projections can be of a rubbery material such as black silicone rubber which elastically deforms and frictionally engages the recesses in the showerhead electrode. The gasket  900  can thus be mounted on the showerhead electrode without adhesive to allow easy removal of the gasket during cleaning or replacement of the showerhead electrode. 
     While the invention has 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.