Patent Publication Number: US-9899228-B2

Title: Showerhead electrode assemblies for plasma processing apparatuses

Description:
This application is a divisional of U.S. patent application Ser. No. 12/155,739, entitled SHOWERHEAD ELECTRODE ASSEMBLIES FOR PLASMA PROCESSING APPARATUSES, filed on Jun. 9, 2008, the entire content of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     In the field of semiconductor devices processing, semiconductor material processing apparatuses including vacuum processing chambers, are used to perform various processes, such as etching and deposition of various materials on substrates, and resist stripping. As semiconductor technology evolves, decreasing device sizes, for example, transistor sizes, call for an ever higher degree of accuracy, repeatability and cleanliness in wafer processes and process equipment. Various types of equipment exist for semiconductor processing, including applications that involve the use of plasmas, such as plasma etch, plasma-enhanced chemical vapor deposition (PECVD) and resist strip and the like. The types of equipment required for these processes include components which are disposed within the plasma chamber, and must function in that environment. The environment inside the plasma chamber may include exposure to the plasma, exposure to etchant gasses, and thermal cycling. Materials used for such components must be adapted to withstand the environmental conditions in the chamber, and do so for the processing of many wafers which may include multiple process steps per wafer. To be cost effective, such components must often withstand hundreds or thousands of wafer cycles while retaining their functionality and cleanliness. There is generally extremely low tolerance for components which produce particles, even when those particles are few and no larger than a few tens of nanometers. It is also necessary for components selected for use inside plasma processing chambers to meet these requirements in the most cost-effective manner. 
     SUMMARY 
     An embodiment of a showerhead electrode assembly comprises a showerhead electrode adapted to be mounted in an interior of a vacuum chamber and powered by radio frequency (RF) energy; a backing plate attached to the showerhead electrode; a thermal control plate attached to the backing plate via a plurality of fasteners at multiple contact regions across the backing plate; and interface members separating the backing plate and the thermal control plate at the contact regions, wherein each interface member comprises a thermally and electrically conductive gasket portion bounded on a periphery by a particle mitigating seal portion. 
     An embodiment of a method of controlling plasma etching in a plasma etching chamber comprises supplying process gas to the plasma etching chamber through the showerhead electrode assembly, the process gas flowing into a gap between the showerhead electrode and a bottom electrode on which a semiconductor substrate is supported; and etching a semiconductor substrate in the plasma etching chamber by applying RF power to the showerhead electrode and energizing the process gas into a plasma state, wherein the temperature of the showerhead electrode is controlled by the thermal control plate via enhanced thermal conduction through the thermally and electrically conductive gasket portion of the interface members. In the method, the above-described embodiment of the showerhead electrode assembly can be used. 
     Another embodiment of a showerhead electrode assembly comprises a showerhead electrode adapted to be mounted in an interior of a vacuum chamber; a thermal control plate attached to the showerhead electrode at multiple contact regions across the showerhead electrode with plenums between the thermal control plate and the showerhead electrode located between the contact regions; and an interface member separating the showerhead electrode and the thermal control plate, at each of the contact regions, wherein the interface member comprises a thermally and electrically conductive gasket portion bounded on a periphery by a particle mitigating seal portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary embodiment of a showerhead electrode assembly of a semiconductor material plasma processing apparatus. 
         FIG. 2  is an enlarged view of a portion of the showerhead electrode assembly shown in  FIG. 1  including an embodiment of an interface member. 
         FIG. 3  is a schematic diagram of an embodiment of an interface member. 
         FIG. 4  is a plan view of an embodiment of an interface member. 
         FIGS. 5A-5C  illustrate an embodiment of an interface member. 
         FIG. 6  is an enlarged view of a portion of a showerhead electrode assembly shown in  FIG. 1  including another embodiment of an interface member. 
         FIG. 7  illustrates a portion of a showerhead electrode assembly according to another embodiment including an embodiment of an interface member. 
         FIG. 8  is an enlarged view of a portion of a showerhead electrode assembly according to the other embodiment including another embodiment of an interface member. 
     
    
    
     DETAILED DESCRIPTION 
     Plasma processing apparatuses for semiconductor substrates, such as silicon wafers, include plasma etch chambers which are used in semiconductor device manufacturing processes to etch such materials as semiconductors, metals and dielectrics. For example, a dielectric etch chamber might be used to etch materials such as silicon dioxide or silicon nitride. During the etch process, components within the etch chamber heat up and cool down and experience thermal stresses as a result. For actively heated components of a heated showerhead assembly, this temperature cycling can result in increased particle generation. 
     A showerhead electrode assembly having a heater to prevent the showerhead electrode from falling below a minimum temperature is described in commonly-owned U.S. Patent Publication No. 2005/0133160A1, the disclosure of which is hereby incorporated by reference in its entirety. The heater cooperates with a thermal control plate in heat transfer with a temperature controlled top plate which forms a top wall of a plasma etch chamber. 
       FIG. 1  depicts one-half of a showerhead assembly  100  of a parallel plate capacitively-coupled plasma chamber (vacuum chamber) comprising a top electrode  103  and an optional backing member  102  secured to the top electrode  103 , a thermal control plate  101 , and a top plate  111 . Thermal chokes  112  can be provided on the upper surface of the thermal control plate  101 . The top electrode  103  is positioned above a substrate support  160  supporting a semiconductor substrate  162 , e.g., semiconductor wafer. 
     The top plate  111  can form a removable top wall of the plasma processing apparatus, such as a plasma etch chamber. As shown, the top electrode  103  can include an inner electrode member  105 , and an optional outer electrode member  107 . The inner electrode member  105  is typically made of single crystal silicon. If desired, the inner and outer electrodes  105 ,  107  can be made of a single piece of material such as CVD silicon carbide, single crystal silicon or other suitable material. 
     The inner electrode member  105  can have a diameter smaller than, equal to, or larger than a wafer to be processed, e.g., up to 200 mm. For processing larger semiconductor substrates such as 300 mm wafers, the outer electrode member  107  is adapted to expand the diameter of the top electrode  103  from about 12 inches to about 19 inches, such as about 15 inches to about 17 inches. The outer electrode member  107  can be a continuous member (e.g., a poly-silicon or silicon carbide member, such as a ring), or a segmented member (e.g., 2-6 separate segments arranged in a ring configuration, such as segments of single crystal silicon). In embodiments in which the top electrode  103  includes a multiple-segment outer electrode member  107 , the segments preferably have edges which overlap each other to protect an underlying bonding material from exposure to plasma. The inner electrode member  105  preferably includes multiple gas passages  104  for injecting a process gas into a space in a plasma reaction chamber below the top electrode  103 . Optionally, the outer electrode member  107  can include multiple gas passages (not shown). The outer electrode  107  preferably forms a raised step at the periphery of the electrode  103  which does not include gas passages. Further details of a stepped electrode can be found in commonly-owned U.S. Pat. No. 6,824,627, the entire disclosure of which is hereby incorporated by reference. 
     Single crystal silicon is a preferred material for plasma exposed surfaces of the inner electrode member  105  and the outer electrode member  107 . 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. 
     The showerhead electrode assembly  100  can be sized for processing large substrates, such as semiconductor wafers having a diameter of 300 mm. For 300 mm wafers, the top electrode  103  is at least 300 mm in diameter. However, the showerhead electrode assembly can be sized to process other wafer sizes or substrates having a non-circular configuration such as substrates for flat panel displays. 
     The backing member  102  includes a backing plate  106  and optionally a backing ring  108 . In such configurations, the inner electrode member  105  is co-extensive with the backing plate  106 , and the outer electrode member  107  is co-extensive with the surrounding backing ring  108 . However, the backing plate  106  can extend beyond the inner electrode member such that a single backing plate can be used to support the inner electrode member and the segmented outer electrode member or support a single piece inner electrode and outer electrode. The inner electrode member  105  and the outer electrode member  107  are preferably attached to the backing member  102  by a bonding material, such as an elastomeric bonding material. The backing plate  106  includes gas passages  113  aligned with the gas passages  104  in the inner electrode member  105  to provide gas flow into the plasma processing chamber. If the outer electrode  107  includes gas passages, the backing ring  108  includes gas passages aligned with such optional gas passages in the outer electrode  107  (not shown). The gas passages  113  can typically have a diameter of about 0.04 inch, and the gas passages  104  can typically have a diameter of about 0.025 inch. 
     In the embodiment, the backing plate  106  and backing ring  108  are made of an aluminum material, which is typically an aluminum alloy material such as 6061 or other alloy suitable for use in semiconductor processing. The backing plate  106  and backing ring  108  can be made of bare aluminum, i.e., aluminum that has a surface native oxide (and is not anodized). 
     The top electrode  103  can be attached to the backing plate  106  and backing ring  108  with a thermally and electrically conductive elastomer bonding material that accommodates thermal stresses, and transfers heat and electrical energy between the top electrode  103  and the backing plate  106  and backing ring  108 . Alternatively, the elastomer can be thermally conductive, but not electrically conductive. The use of elastomers for bonding together surfaces of an electrode assembly is described, for example, in commonly-owned U.S. Pat. No. 6,073,577, which is incorporated herein by reference in its entirety. 
     The backing plate  106  and the backing ring  108  are preferably attached to the thermal control plate  101  with suitable 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  101  and screwed into threaded openings in the backing member  102 . The thermal control plate  101  is in heat transfer relationship with an actively controlled heater. See, for example,  FIGS. 1 and 2  and the description thereof in commonly-owned U.S. Published Application No. 2005/0133160A1, the entire disclosure of which is hereby incorporated by reference. The holes in the thermal control plate  101  can be oversized to allow for movement due to differences in thermal expansion that accommodates thermal stresses. The thermal control plate  101  includes a flexure portion  109  and is preferably made of a machined metallic material, such as aluminum, an aluminum alloy such as aluminum alloy 6061 or other alloy suitable for use in semiconductor processing. The thermal control plate  101  can be made of bare aluminum, i.e., aluminum that has a surface native oxide (and is not anodized). The top plate  111  is preferably made of aluminum or an aluminum alloy such as aluminum alloy 6061. A plasma confinement assembly  110  is shown outwardly of the showerhead electrode assembly  100 . A suitable plasma confinement assembly including a vertically-adjustable, plasma confinement ring assembly is described in commonly-owned U.S. Pat. No. 6,433,484, which is incorporated herein by reference in its entirety. 
     The thermal control plate  101  preferably includes at least one heater operable to cooperate with the temperature-controlled top plate  111  to control the temperature of the top electrode  103 . For example, in a preferred embodiment, the heater is provided on the upper surface of the thermal control plate  101  and includes a first heater zone surrounded by a first projection  61 , a second heater zone between the first projection  61  and a second projection  63 , and a third heater zone between the second projection  63  and the flexure portion  109 . The number of heater zones can be varied; for example, in other embodiments the heater can include a single heater zone, two heater zones, or more than three heater zones. The heater can alternatively be provided on a bottom surface of the thermal control plate. 
     The heater preferably comprises a laminate including a resistively heated material disposed between opposed layers of a polymeric material that can withstand the operating temperatures reached by the heater. An exemplary polymeric material that can be used is a polyimide sold under the trademark Kapton®, which is commercially available from E.I. du Pont de Nemours and Company. Alternatively, the heater can be a resistive heater embedded in the thermal control plate (e.g., a heating element in a cast thermal control plate or a heating element located in a channel formed in the thermal control plate). Another embodiment of the heater includes a resistive heating element mounted on the upper and/or lower surface of the thermal control plate. Heating of the thermal control plate can be achieved via conduction and/or radiation. 
     The heater material can have any suitable pattern that provides for thermally uniform heating of the first heater zone, second heater zone, and third heater zone. For example, the laminate heater can have a regular or non-regular pattern of resistive heating lines such as a zig-zag, serpentine, or concentric pattern. By heating the thermal control plate with the heater, in cooperation with operation of the temperature-controlled top plate, a desirable temperature distribution can be provided across the top electrode during operation of the showerhead electrode assembly. 
     The heater sections located in the first heater zone, second heater zone, and third heater zone can be secured to the thermal control plate by any suitable technique, e.g., the application of heat and pressure, adhesive, fasteners, or the like. 
     The top electrode can be electrically grounded, or alternatively can be powered, preferably by a radio-frequency (RF) current source  170 . The output power of the RF current source  170  powering the top electrode can have a frequency ranging from 50 to 80 MHz, preferably a frequency of 60 MHz, or a similar frequency. In such an alternative embodiment, the bottom electrode can be coupled to the ground potential and the top electrode coupled to the RF source  170 . The RF source  170  can have a voltage of between about 100 volts and about 2000 volts. In a preferred embodiment, the top electrode is grounded, and power at one or more frequencies is applied to the bottom electrode to generate plasma in the plasma processing chamber. The RF source  170  powering the bottom electrode can have a frequency of between about 400 kHz and about 60 MHz. For example, the bottom electrode can be powered at frequencies of 2 MHz and 27 MHz by two independently controlled radio frequency power sources. 
     After a substrate has been processed (e.g., a semiconductor substrate has been plasma etched), the supply of power to the bottom electrode is shut off to terminate plasma generation. The processed substrate is removed from the plasma processing chamber, and another substrate is placed on the substrate support for plasma processing. In a preferred embodiment, the heater is activated to heat the thermal control plate  101  and, in turn, the top electrode  103 , when power to the bottom electrode is shut off. As a result, the top electrode  103  temperature is preferably prevented from decreasing below a desired minimum temperature. For etching dielectric materials, the top electrode temperature is preferably maintained at approximately a constant temperature such as 150 to 250° C. between successive substrate processing runs so that substrates are processed more uniformly, thereby improving process yields. The power supply preferably is controllable to supply power at a desired level and rate to the heater based on the actual temperature and the desired temperature of the top electrode. 
     In exemplary embodiments, the top electrode  103  can be heated to a temperature of at least about 80° C., such as heating and maintaining at least a portion of the showerhead electrode at a temperature of at least 100° C., at least about 150° C., or at least 180° C. The top electrode  103  can be heated before etching of a semiconductor substrate. The etching can comprise etching openings in an oxide layer on the semiconductor substrate, where the openings are defined by a patterned photoresist. 
     The plasma chamber can also include, for example, a temperature controller; a power supply adapted to supply power to a heater which heats the thermal control plate in thermal response to the temperature controller; a fluid control adapted to supply fluid to a temperature controlled top wall of the chamber in response to the temperature controller; and a temperature sensor arrangement adapted to measure the temperature of one or more portions of the showerhead electrode and supply information to the temperature controller. 
     The illustrated embodiment of the showerhead electrode assembly also comprises an aluminum baffle ring arrangement  120  used to distribute process gasses in a plasma chamber. The aluminum baffle ring arrangement  120  in  FIG. 1  includes six rings made from aluminum or an aluminum alloy, such as 6061 aluminum, which comprises by weight from about 96 to about 98% Al, about 0.8 to about 1.2% Mg, about 0.4 to about 0.8% Si, about 0.15 to 0.4% Cu, about 0.04 to 0.35% Cr, and optionally Fe, Mn, Zn and/or Ti. The baffle rings  120  can have an anodized outer surface. The six concentric L-shaped rings are located within the plenums above the backing member  102  and below the thermal control plate  101 . For example, a central plenum can include a single ring, the adjacent plenum can include two rings separated by a ½ to 1 inch gap, the next adjacent plenum can include two rings separated by a ½ to 1 inch gap and an outer plenum can include a single ring. The rings are mounted to the thermal control plate  101  with screws. For example, each ring can include circumferentially spaced apart stand-offs or bosses with through holes for receiving the screws, e.g., three bosses arranged apart can be used. Each ring can have a horizontal section of about 0.040 inch thickness and a vertical flange of about ¼ inch in length. 
     When the top surface  134  of the aluminum backing plate  106  and an annular projection  136  of the thermal control plate  101  come into contact in a contact region  132  during operation of the showerhead electrode assembly  100 , galling can occur between the thermal control plate  101  and the aluminum backing member  102  including the backing plate  106  and backing ring  108  along contact regions located between them. Details of galling are described in commonly-owned co-pending U.S. patent application Ser. No. 11/896,375, the entire contents of which are hereby incorporated by reference. In the thermal control plate  101 , the contact regions  132  can cover about 1% to about 30% of the surface area of the backing plate  102 . 
     This galling can occur on both of the thermal control plate  101  and aluminum backing member  102 , and is caused by relative motion and rubbing occurring between the opposed surfaces of the thermal control plate  101  and aluminum backing member  102  as a result of temperature cycling. This galling is highly undesirable for a number of reasons. First, galling can cause a reduction in thermal transfer and thus a shift in the temperature including, for example, a localized temperature non-uniformity, of the top electrode  103  including the illustrated inner electrode member  105 . This temperature shift can cause a process shift during processing of semiconductor substrates in the plasma processing chamber. 
     Galling of the thermal control plate  101  and aluminum backing member  102  can also cause particle generation, or cause fusing of the thermal control plate  101  and aluminum backing member  102 , which then requires excessive force to separate these components, which can result in damage to these components. 
     Galling of the thermal control plate  101  and aluminum backing member  102  can also increase the difficulty of cleaning the top electrode  103 . 
     Additionally, galling of the thermal control plate  101  and aluminum backing member  102  degrades the cosmetic appearance of these components and reduces their lifetime. 
       FIG. 2  illustrates an exemplary embodiment of the showerhead electrode assembly including a modification that reduces the occurrence of galling of the thermal control plate  101  and aluminum backing plate  106  and backing ring  108  and consequently also reduces problems associated with such galling. Particularly, as shown in  FIG. 2 , an interface member  151  which comprises a thermally and electrically conductive gasket  145  and particle mitigating seal portions  147   a  and  147   b , is located between the bottom surface of the annular projection  136  of the thermal control plate and the top surface  134  of the aluminum backing plate  102 . 
       FIG. 3  shows a cross-section of a portion of an embodiment of an interface member  151 . As depicted, the interface member  151  comprises a thermally and electrically conductive gasket portion  145  bounded on a periphery  149  by a particle mitigating seal portion  147   a . In this embodiment, the gasket portion  145  preferably comprises a laminate of coaxial annular rings such as a central portion  143  sandwiched between upper and lower portions  141   a  and  141   b . For example, the central portion  143  can be a strip of aluminum and the upper and lower portions  141   a / 141   b  can be strips of carbon loaded silicone. Alternatively, the gasket portion  145  is a thermal filler material such as a silicone filled with boron nitride (such as CHO-THERM 1671 manufactured by Chomerics), a graphite (such as eGraf 705 manufactured by Graftech), an indium foil, a sandwich (such as Q-pad II by Bergquist), or a phase change material (PCM) (such as T-pcm HP105 by Thermagon). 
     The thermally and electrically conductive gasket portion  145  can be, for example, a conductive silicone-aluminum foil sandwich gasket structure, or a elastomer-stainless steel sandwich gasket structure. In a preferred embodiment, the gasket  145  is Bergquist Q-Pad II composite materials available from The Bergquist Company, located in Chanhassen, Minn. These materials comprise aluminum coated on both sides with thermally/electrically conductive rubber. The materials are compatible in vacuum environments. The contact surfaces of the thermal control plate and aluminum backing member, e.g., backing plate, each have some degree of roughness caused by processing, e.g., machining. The gasket material is preferably also sufficiently compliant so that it compensates for surface roughness of the contact surface and effectively fills regions (e.g., microvoids) of the contact surfaces to enhance thermal contact between the contact surfaces. Most preferably, the gasket portion is Lambda Gel COH-4000 (available from Geltec). 
     To minimize graphite generation from the gasket material, the gaskets can be cleaned using deionized water, such as by wiping. The gasket material can alternatively be coated with a suitable coating material, such as a fluoroelastomer material. 
     The particle mitigating seal portion  147   a / 147   b  can be an elastomer or a polymer resistant to erosion from radicals in a vacuum environment. Preferably, the seal portion  147   a / 147   b  is an in-situ cured elastomer or polymer resistant to erosion from radicals produced by plasma in a vacuum environment and resistant to degradation at high temperatures such as above 200° C. Polymeric materials which can be used in plasma environments above 160° C. include polyimide, polyketone, polyetherketone, polyether sulfone, polyethylene terephthalate, fluoroethylene propylene copolymers, cellulose, triacetates, silicone, and rubber. 
     More preferably, the seal portion  147   a / 147   b  is an in-situ room temperature vulcanized (RTV) unfilled silicone exhibiting appropriate pre-cure and post-cure properties such as adhesion strength, elastic modulus, erosion rate, temperature resistance and the like. For example, an in-situ curable silicone can be a two-part or one-part curing resin using platinum, peroxide or heat. Preferably, the silicone elastomer material has a Si—O backbone with methyl groups (siloxane). However, carbon or carbon-fluorine backbones can also be used. Most preferably, the silicone material cures in-situ for isolating the thermally and electrically conductive gasket portion  145  from the vacuum environment in the chamber forming an unfilled, cross-linked silicone rubber. An especially preferred elastomer is a polydimethylsiloxane containing elastomer such as a catalyst cured, e.g. Pt-cured, elastomer available from Rhodia as Rhodorsil V217, an elastomer which is stable at temperatures of 250° C. and higher. 
     The thermally and electrically conductive gasket  145  is made of a material that is electrically conductive (to provide an RF path to the electrode) and thermally conductive to provide electrical and thermal conduction between the thermal control plate  101  and the aluminum backing plate  106 . The gasket  145  provides an electrically-conductive thermal interface. The gasket  145  also enhances heat transfer between the top electrode  103  including the inner electrode member  105  and the thermal control plate  101 . The particle mitigating seal portion  147   a / 147   b  can be dip coated, molded, spray coated or the like onto the periphery  149  of the thermally and electrically conductive gasket portion  145 . 
     Preferably, the seal portion  147   a / 147   b  is spray coated onto the periphery  149  of the gasket portion  145 . Spray coating can result in the mitigating seal portion  147   a / 147   b  having various cross-sectional shapes (profiles), for example,  FIG. 3  shows a mitigating seal portion  147   a / 147   b  having a rounded-rectangular cross section. If desired, the gasket portion  145  can be in the shape of an annular ring having an outward perimeter and an inward aperture where the particle mitigating seal portion  147   a  and  147   b , respectively, is bonded to the gasket portion  145 .  FIG. 4  shows a plan view of an embodiment of an interface member  151  comprising the gasket portion  145  shaped as an annular ring with the seal portion  147   a  bonded to the outward perimeter  149  and the seal portion  147   b  bonded to the inner aperture  155 . Bolt holes  157  are also shown in  FIG. 4 , which allow bolts (not shown) to be inserted in holes in the thermal control plate  101  and screwed into threaded openings in the backing plate  106  and backing ring  108  with the interface member  151  located between the bottom surface of the annular projection  136  of the thermal control plate and the top surface  134  of the aluminum backing plate  102 . 
     Also preferably, as shown in  FIG. 5A , the seal portion  147   a / 147   b  can be in the form of uncured elastomeric sheets  147   c / 147   d / 147   e / 147   f  in the shape of annular rings sized to overlap the outward perimeter and the inward aperture of the gasket portion  145 . The uncured elastomeric sheets  147   c / 147   d / 147   e / 147   f  can be located on the gasket portion  145  ( FIG. 5B ) and cured to provide an interface member  151  ( FIG. 5C ). 
     As shown in  FIG. 2 , the particle mitigating seals  147   a / 147   b  can be shaped such as O-rings disposed on the outer and inner periphery  149  of each annular gasket portion  145 . More generally, (with reference to  FIGS. 2 and 4 ) the particle mitigating seal portions  147   a / 147   b  of the interface member  151  can have a curved surface and protrude from the outward perimeter  153  and inward aperture  155  of each thermally and electrically conductive gasket portion  145 . A plurality of annular projections  136  and annular thermally and electrically conductive gasket portions  145  result in a plurality of seals  147   a / 147   b , for example, from 4 to 20 seal portions  147   a / 147   b . The plurality of annular gasket portions  145  require fairly precise placement. Since the interface member  151  provides the particle mitigating seal portions  147   a / 147   b  bonded to the periphery  149  of the gasket portion  145 , installation of the interface member  151  simplifies precise installation of a thermally and electrically conductive gasket portion  145  sealed off from the vacuum chamber by particle mitigating seal portions  147   a / 147   b . Particularly, when the contact surfaces of the annular gaskets  145  in contact regions  132  are small to allow for variations in location during installation. 
     As also shown in  FIG. 2 , shims  146  having about the same thickness as the gasket  145  are located between the aluminum baffle rings  120  and the bottom surface  142  of the thermal control plate  101 . The shims  146  can be of a dielectric material. 
     The thermal control plate  101  includes several annular projections  136  establishing plenums at the backside of the backing plate  106 , e.g., 2 to 10, preferably 4 to 8 projections. An interface member  151  is arranged over the contact surfaces of each annular projection. 
       FIG. 6  shows another embodiment of an interface member  151 ′ located between the bottom surface of the annular projection  136  of the thermal control plate and the top surface  134  of the aluminum backing plate  102 . As depicted, this embodiment of an interface member  151 ′ has a thermally and electrically conductive gasket portion  145 ′ bounded on each periphery by a particle mitigating portion  147   a ′/ 147   b ′. For example, the interface member  151 ′ shown in  FIG. 6  has an annular thermally and electrically conductive gasket portion  145 ′ bounded on an outer perimeter by a first co-planar particle mitigating seal portion  147   a ′ and bounded on an inner aperture by a second co-planar particle mitigating seal portion  147   b ′. Such an embodiment of a showerhead electrode assembly including the interface member  151 ′ has a fewer number of parts than required if installing a plurality of separate thermally and electrically conductive gaskets and a separate outer and inner seal such as an O-ring, for each thermally and electrically conductive gasket. The interface member  151 ′ is easy to install and completely covers contact regions  132 . A plurality of fasteners (such as 3 to 15 bolts) pass through openings  157  ( FIG. 4 ) in each of the annular gasket portions  145 / 145 ′ to secure the thermal control plate  101  to the backing plate  106 . Furthermore, by using an interface member  151 ′, baffles  120  and shims  146  can be omitted if desired. 
     By enhancing thermal transfer through the contact regions  132 , it is possible to reduce temperature differences between the top electrode  103  including the inner electrode member  105  and the thermal control plate  101 , such that “first wafer effects” can also be reduced during consecutive processing of a series of wafers. That is, “first wafer effects” refers to secondary heating of subsequent wafers caused indirectly by the heating of the first-processed wafer. Specifically, upon completion of processing of the first wafer, the heated processed wafer and the process chamber side walls radiate heat toward the upper electrode. The upper electrode then indirectly provides a secondary heating mechanism for subsequent wafers that are processed in the chamber. As a result, the first wafer processed by the system may exhibit a larger than desired critical dimension (CD) variation than subsequent wafers processed by the system since wafer temperature variation can affect CD during etching of high aspect ratio contact vias in semiconductor substrates. Subsequently processed wafers may have different and/or less CD variation than the first processed wafer due to stabilization of temperature in the chamber. 
     Across-wafer and wafer-to-wafer temperature variation can also be preferably reduced by enhancing thermal transfer through the contact regions  132 . Also, chamber-to-chamber temperature matching can be preferably achieved where multiple plasma etching chambers in different processing lines are used for a desired process or throughput, by enhancing thermal transfer through the contact regions  132 . 
     Typically, a one degree Centigrade variation in wafer temperature across-wafer, wafer-to-wafer, or chamber-to-chamber, can cause a CD variation increase at 3σ (3× standard deviation) by about 0.5 to 0.1 nm (e.g., 0.4 nm/° C.-0.2 nm/° C. or 0.35 nm/° C.-0.25 nm/° C.). 
     As mentioned, after the first wafer has been processed, the temperature of subsequently processed wafers can stabilize, such that temperature variation of reference points on subsequently processed wafers is preferably less than about 10° C., more preferably, less than about 5° C., such that, for example, the CD variation can be controlled to within about 5 nm (0.5 nm/° C.×10° C.), more preferably, to within about 3 nm (0.3 nm/° C.×10° C.), most preferably to within about 0.5 nm (0.1 nm/° C.×5° C.) for etching high aspect ratio contact vias in semiconductor substrates. 
     For memory applications the CD variation is desirably less than 4 nm at 3σ. With the enhanced thermal transfer through the contact regions  132  provided by the interface members  151 / 151 ′, the CD variation is preferably, 1 nm or less wafer-to-wafer and 4 nm or less chamber-to-chamber. For logic applications the CD variation is desirably less than 3 nm at 3σ. With the enhanced thermal transfer through the contact regions  132  provided by the interface members  151 / 151 ′, the CD variation is preferably, 2 nm or less wafer-to-wafer and 4 nm or less chamber-to-chamber. 
     Preferably, the interface members  151 / 151 ′ minimize temperature shifts from the center of the electrode to the edge of the electrode by less than 10° C. and minimize azimuthal temperature shifts to 5° C. or less. Electrode temperature variation due to use of new or used aluminum backing members is related to the contact surface condition of the new and used aluminum backing members. The interface members  151 / 151 ′ preferably can minimize electrode temperature shifts caused by new and used aluminum backing members to less than about 5° C. Also, parts may be removed to be cleaned and it is preferred that a part shows the same thermal performance after such cleaning. The interface members  151 / 151 ′ preferably minimize thermal performance shifts between before and after cleaning of the aluminum backing members to less than about 5° C. change in electrode temperature. 
     Preferably, the interface members  151 / 151 ′ can also reduce or prevent fusing or galling of the thermal control plate  101  and aluminum backing member  102 , which allows these components to be separated from each other with minimum force. 
     Preferably, the interface members  151 / 151 ′ are made of a material that preferably: does not outgas in a high-vacuum environment of, e.g., about 10 to about 200 mTorr; has low particulate generation performance; is compliant to accommodate shear at contact regions; is free of metallic components that are lifetime killers in semiconductor substrates, such as Ag, Ni, Cu and the like; and can minimize the generation of particles during cleaning of the aluminum backing member  102 . 
       FIG. 7  illustrates a portion of another embodiment of a showerhead electrode assembly. Referring to  FIGS. 2 and 6 , the embodiment shown in  FIG. 7  does not include a backing member, and the thermal control plate  101  is secured directly to the inner electrode member  105 . 
     When the top surface  160  of the top electrode  103  and an annular projection  136  of the thermal control plate  101  come into contact in a contact region  158  during operation of the showerhead electrode assembly  100 , galling can occur between the thermal control plate  101  and the top electrode  103  including the inner electrode member  105  and the optional outer electrode member  107  along contact regions located between them. 
     This galling can occur on both of the thermal control plate  101  and the top electrode  103 , and is caused by relative motion and rubbing occurring between the opposed surfaces of the thermal control plate  101  and the top electrode  103  as a result of temperature cycling. This galling is undesirable for reasons similar to those described above in relation to the top surface  134  of the aluminum backing plate  106  and an annular projection  136  of the thermal control plate  101  contacting in a contact region during operation of the showerhead electrode assembly  100 . For example, galling can cause a reduction in thermal transfer and thus a shift in the temperature including, for example, a localized temperature non-uniformity, of the top electrode  103  including the illustrated inner electrode member  105 . This temperature shift can cause a process shift during processing such as plasma etching of semiconductor substrates in the plasma processing chamber. 
     Galling of the thermal control plate  101  and the top electrode  103  can also cause particle generation, or cause fusing of the thermal control plate  101  and the top electrode  103 , which then requires excessive force to separate these components, which can result in damage to these components. Galling of the thermal control plate  101  and the top electrode  103  can also increase the difficulty of cleaning the top electrode  103 . Additionally, galling of the thermal control plate  101  and the top electrode  103  degrades the cosmetic appearance of these components and reduces their lifetime. 
     The showerhead electrode assembly shown in  FIG. 7  can also include an optional outer electrode member, such as the outer electrode member  107  shown in  FIG. 1 . The outer electrode member can have a ring configuration comprised of a plurality of segments. The thermal control plate  101  can be secured directly to the inner electrode member  105  and optional outer electrode member  107  in any suitable manner, such as by fasteners and/or adhesive bonding, such as elastomer bonding. As shown in  FIG. 7 , there is a contact region  158  between the top surface  160  of the inner electrode member  105  and the annular projection  136  of the thermal control plate  101 . In the embodiment, the outer surface of the thermal control plate  101  can be anodized except at the surface at the contact region  158 , which is of bare aluminum (non-anodized). Contact region  158  provides a thermal path to remove heat from the inner electrode member  105  and an RF path for RF power passing through the inner electrode member  105 . 
     An interface member  151  such as described above with reference to  FIG. 2  is provided between the top surface  160  of the inner electrode member  105  and the annular projection  136  of the thermal control plate  101 . As described above, thermally and electrically conductive gasket portion  145  provides a thermal path to remove heat from the inner electrode member  105  and an RF path for RF power passing through the inner electrode member  105 . Particle mitigating seal portions  147   a / 147   b  are bonded to the gasket portion  145  and are disposed in offsets  139  between the aluminum baffle rings  120  and the top surface  160  to form a gas-tight seal. The upper ends of the vertical walls of the baffle rings  120  are separated from the bottom surface  142  of the thermal control plate  101  by shims  146 . The shims  146  are typically made of a dielectric material, such as Kapton®. 
       FIG. 8  illustrates an embodiment of an interface member  151 ′ in a showerhead electrode assembly to reduce the occurrence of galling between the thermal control plate  101  and the inner electrode member  105  (and also the optional outer electrode member) along contact regions located between them, and consequently to also reduce problems associated with such galling, such as particle generation. For example, for a silicon electrode member, galling can cause silicon particle generation and aluminum particle generation. Particularly, as shown in  FIG. 8 , the interface member  151 ′ is located between the bottom surface of the annular projection  136  of the thermal control plate  101  and the top surface  160  of the inner electrode member  105 . The interface member  151 ′ separates adjacent ones of the plenums formed in the thermal control plate  101  from each other. 
     The interface member  151 ′ can be made of the same materials as the interface members  151 / 151 ′ described above with respect to the embodiment of the showerhead electrode assembly shown in  FIGS. 6 and 7 . The gasket portion  145 ′ material is electrically and thermally conductive to provide electrical and thermal conduction between the thermal control plate  101  and the inner electrode member  105  (and optional outer electrode member), i.e., the gasket  145 ′ provides an electrically-conductive thermal interface between the contact regions. 
     As also shown in  FIG. 8 , shims  146  having about the same thickness as the interface member  151 ′ are located between the aluminum baffle rings  120  and the bottom surface  142  of the thermal control plate  101 . The shims  146  can be of a dielectric material. The interface member  151 ′ allows the aluminum baffles  120  and shims  146  to be omitted if desired. 
     The modification to the showerhead electrode assemblies shown in  FIGS. 2 and 6-8  reduce the number of parts, simplify installation and completely cover contact regions  132 / 158 , as well as, reduce the occurrence of galling between the thermal control plate  101  and the inner electrode member  105  (and also the optional outer electrode member) along contact regions  132 / 158  located between them, and consequently also reduce problems associated with such galling, such as particle generation. For example, for a silicon electrode member, galling can cause silicon particle generation and aluminum particle generation. Particularly, as shown in  FIG. 8 , an interface member  151 ′ is located between the bottom surface of the annular projection  136  of the thermal control plate  101  and the top surface  160  of the inner electrode member  105 . The interface member  151 ′ separates adjacent ones of the plenums formed in the thermal control plate  101  from each other. 
     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.