Abstract:
An apparatus for plasma processing includes a first plasma source, a first planar electrode, a gas distribution device, a plasma blocking screen and a workpiece chuck. The first plasma source produces first plasma products that pass, away from the first plasma source, through first apertures in the first planar electrode. The first plasma products continue through second apertures in the gas distribution device. The plasma blocking screen includes a third plate with fourth apertures, and faces the gas distribution device such that the first plasma products pass through the plurality of fourth apertures. The workpiece chuck faces the second side of the plasma blocking screen, defining a process chamber between the plasma blocking screen and the workpiece chuck. The fourth apertures are of a sufficiently small size to block a plasma generated in the process chamber from reaching the gas distribution device.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to plasma processing systems. 
       BACKGROUND 
       [0002]    In plasma processing, plasmas create ionized and/or energetically excited species for interaction with workpieces that may be, for example, semiconductor wafers. To create and/or maintain a plasma, one or more radio frequency (RF) and/or microwave generators typically generate oscillating electric and/or magnetic fields. In some wafer processing systems, a plasma is generated in the same location as one or more wafers being processed; in other cases, a plasma is generated in one location and moves to another location where the wafer(s) are processed. The plasmas produced often contain highly energetic and/or corrosive species and/or highly energetic electrons, such that the equipment that produces them sometimes degrades from contact with the energetic species and/or electrons. For example, materials that are exposed to highly energetic species and/or electrons may be etched and/or sputtered, generating etched and/or sputtered material that can move about, and can react or deposit on various surfaces. 
       SUMMARY 
       [0003]    In an embodiment, an apparatus for plasma processing includes a first plasma source, a first planar electrode, a gas distribution device, a plasma blocking screen and a workpiece chuck. The first plasma source produces first plasma products. The first planar electrode includes a first plate that defines a plurality of first apertures therethrough, a first side of the first planar electrode being disposed relative to the first plasma source such that the first plasma products pass away from the first plasma source through the plurality of first apertures to a second side of the first planar electrode. The gas distribution device includes a second plate that defines a plurality of second apertures therethrough, a first side of the gas distribution device being disposed facing the second side of the first planar electrode, such that the first plasma products continue through the plurality of second apertures to a second side of the gas distribution device. The plasma blocking screen includes a third plate that defines a plurality of fourth apertures therethrough, a first side of the plasma blocking screen being disposed facing the second side of the gas distribution device such that the first plasma products pass through the plurality of fourth apertures to a second side of the plasma blocking screen. The workpiece chuck faces the second side of the plasma blocking screen, such that a process chamber is defined between the plasma blocking screen and the workpiece chuck. The fourth apertures are of a sufficiently small size to block a plasma generated in the process chamber from reaching the gas distribution device. 
         [0004]    In an embodiment, a plasma processing chamber includes a workpiece holder and a planar electrode. The planar electrode defines parallel and opposing first and second planar surfaces, separated by a thickness, over a central region thereof. The second planar surface is disposed facing the workpiece holder. The planar electrode defines a plurality of apertures therethrough. Each of the apertures is characterized by a first aperture section and a second aperture section. The first aperture section defines an aperture axis and a first aperture minor lateral dimension perpendicular to the aperture axis, the first aperture section extending from the first planar surface through at least half of the thickness. The second aperture section defines a second aperture minor lateral dimension that is less than the first aperture minor lateral dimension, and extends from the second planar surface through less than half the thickness. The first and aperture sections adjoin axially to form a continuous one of the apertures from the first planar surface to the second planar surface. 
         [0005]    In an embodiment, an apparatus for plasma processing includes a gas source, a first planar electrode, a second planar electrode, a first power supply, a plasma blocking screen, a workpiece chuck and a second power supply. The first planar electrode includes a first plate that defines a plurality of first apertures therethrough, a first side of the first planar electrode being disposed relative to the gas source such that gases from the gas source pass through the plurality of first apertures to a second side of the first planar electrode. The second planar electrode includes a second plate that defines a plurality of second apertures therethrough, a first side of the second planar electrode being disposed facing the second side of the first planar electrode. The first power supply couples radio frequency (RF) power across the first planar electrode and the second planar electrode. A first plasma is generated, from the gases, between the first planar electrode and the second planar electrode, and first plasma products from the first plasma pass through the plurality of second apertures to a second side of the second planar electrode. The plasma blocking screen includes a third plate that defines a plurality of third apertures therethrough, a first side of the plasma blocking screen being disposed facing the second side of the second planar electrode such that the first plasma products pass through the plurality of third apertures to a second side of the plasma blocking screen. The workpiece chuck faces the second side of the plasma blocking screen, defining a process chamber between the plasma blocking screen. The second power supply couples radio frequency (RF) power across the plasma blocking screen and the workpiece chuck, generating a second plasma, from the gases, between the plasma blocking screen and the workpiece chuck. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The present disclosure may be understood by reference to the following detailed description taken in conjunction with the drawings briefly described below, wherein like reference numerals are used throughout the several drawings to refer to similar components. It is noted that, for purposes of illustrative clarity, certain elements in the drawings may not be drawn to scale. Specific instances of an item may be referred to by use of a numeral followed by a second numeral within parentheses (e.g., plasma blocking screens  270 ( 1 ),  270 ( 2 ) etc.) while numerals not followed by a dash refer to any such item (e.g., plasma blocking screens  270 ). In instances where multiple instances of an item are shown, only some of the instances may be labeled, for clarity of illustration. 
           [0007]      FIG. 1  schematically illustrates major elements of a plasma processing system, according to an embodiment. 
           [0008]      FIG. 2  schematically illustrates major elements of a plasma processing system, in a cross-sectional view, according to an embodiment. 
           [0009]      FIG. 3  illustrates a portion of a plasma blocking screen that is part of the plasma processing system of  FIG. 2 , according to an embodiment. 
           [0010]      FIG. 4  illustrates a portion of another plasma blocking screen, according to an embodiment. 
           [0011]      FIG. 5  is a graph of modeling data related to choice of a minor lateral dimension of second aperture sections of a plasma blocking screen, according to an embodiment. 
           [0012]      FIG. 6  schematically illustrates a region noted in  FIG. 2  in an enlarged view. 
           [0013]      FIG. 7  schematically illustrates major elements of another plasma processing system, in a cross-sectional view, according to an embodiment. 
       
    
    
       [0014]    Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention. The features and advantages of the invention may be realized and attained by means of the instrumentalities, combinations, and methods described in the specification. 
       DETAILED DESCRIPTION 
       [0015]      FIG. 1  schematically illustrates major elements of a plasma processing system  100 , according to an embodiment. System  100  is depicted as a single wafer, semiconductor wafer plasma processing system, but it will be apparent to one skilled in the art that the techniques and principles herein are applicable to plasma generation systems of any type (e.g., systems that do not necessarily process wafers or semiconductors). Processing system  100  includes a housing  110  for a wafer interface  115 , a user interface  120 , a plasma processing unit  130 , a controller  140 , one or more power supplies  150  and one or more radio frequency (RF) generators  165 . Processing system  100  is supported by various utilities that may include gas(es)  155 , external power  170 , vacuum  160  and optionally others. Internal plumbing and electrical connections within processing system  100  are not shown, for clarity of illustration. 
         [0016]    Processing system  100  is shown as a so-called indirect plasma processing system that generates a plasma in a first location and directs the plasma and/or plasma products (e.g., ions, molecular fragments, energized species and the like) to a second location where processing occurs. Thus, in  FIG. 1 , plasma processing unit  130  includes a plasma source  132  that supplies plasma and/or plasma products for a process chamber  134 . Process chamber  134  includes one or more workpiece holders  135 , upon which wafer interface  115  places a workpiece  50  (e.g., a semiconductor wafer, but could be a different type of workpiece) for processing. In operation, gas(es)  155  are introduced into plasma source  132 , and at least one of the RF generators  165  supplies power to ignite a first plasma within plasma source  132 . There may be multiple regions within plasma source  132  at which RF power is applied and plasmas are generated. Plasma and/or plasma products pass from plasma source  132  through a diffuser plate  137  to process chamber  134 . Additional gases may be added to the plasma and/or plasma products in process chamber  134 , RF power may also be provided within process chamber  134  to generate another plasma. Workpiece  50  is processed in process chamber  134 . 
         [0017]    Therefore, generally, plasmas may be ignited at one, two or more locations within a plasma processing system, and the techniques disclosed herein may be adapted to plasma processing systems that ignite and/or use plasmas formed at single or multiple locations. Certain electronics manufacturers may prefer systems with the flexibility of igniting and/or using plasmas in a variety of configurations, so that each system can be adapted for a corresponding variety of processing needs. 
         [0018]      FIG. 2  schematically illustrates major elements of a plasma processing system  200 , in a cross-sectional view, according to an embodiment. Plasma processing system  200  is an example of plasma processing unit  130 ,  FIG. 1 . Plasma processing system  200  includes a plasma source  210  and a process chamber  205  that may also generate a plasma, as discussed below. In the orientation of  FIG. 2 , a general direction of gas and/or plasma product flow is downwards, and this direction may be referred to as “downstream” herein, while an opposing direction upwards in the orientation of  FIG. 2 , may be referred to as “upstream.” Also, significant portions of the apparatus shown in  FIG. 2  may be cylindrically symmetric about a central axis  201 , with associated directions being defined as a radial direction  202  and an azimuthal direction  203 . This convention of directions may be used herein, although one skilled in the art will understand that many of the principles described herein are not limited to cylindrically symmetric systems. 
         [0019]    As shown in  FIG. 2 , plasma source  210  may introduce gases, and/or gases that are ionized by an upstream remote plasma source, as plasma source gases  212 , through an RF electrode  215 . RF electrode  215  is electrically tied to a first gas diffuser  220  and a face plate  225  that serve to redirect flow of the source gases so that gas flow is uniform across plasma source  210  (uniform from left to right in the view of  FIG. 2 ). It should be noted that all of the diffusers or screens herein may be characterized as electrodes, as any such diffusers or screens may be tied to a particular electrical potential. An insulator  230  electrically insulates RF electrode  215 , including face plate  225 , from a diffuser  235  that is held at electrical ground. Diffuser  235  serves as a second electrode counterfacing face plate  225  of RF electrode  215 . Surfaces of face plate  225 , diffuser  235  and insulator  230  define a first plasma generation cavity where a first plasma  245  may be created when plasma source gases  212  are present and RF energy is provided at face plate  225  through RF electrode  215 . RF electrode  215 , face plate  225  and diffuser  235  may be formed of any conductor, and in embodiments are formed of aluminum (or an aluminum alloy, such as the known “6061” alloy type). Surfaces of face plate  225  and diffuser  235  that face plasma  245  directly may be coated with ceramic layers of, for example, yttria (Y 2 O 3 ) or alumina (Al 2 O 3 ) for resistance to bombardment by energetic plasma products generated in plasma  245 . Other surfaces of face plate  225  and diffuser  235  that are not necessarily exposed directly to plasma, but are exposed to reactive gases and/or radicals generated by plasmas, may be coated either with ceramic layers (e.g., yttria, alumina) or with a suitable passivating layer (e.g., an anodized layer, or a chemically generated alumina layer) for chemical resistance. Insulator  230  may be any insulator, and in embodiments is formed of ceramic. 
         [0020]    Plasma products generated in plasma  245  pass through diffuser  235  that again helps to promote the uniform distribution of plasma products, and may assist in electron temperature control. Upon passing through diffuser  235 , the plasma products pass through an optional gas distribution device  260  that promotes uniformity. (Certain embodiments do not include gas distribution device  260 ; see, e.g.,  FIG. 7 ) Optional gas distribution device  260  is also held at electrical ground. Apertures that pass completely through optional gas distribution device  260  are of a diameter at least three times a diameter of apertures within diffuser  235 . Also, gas distribution device  260  includes further gas channels  250  that may be used to introduce one or more further gases  155 ( 2 ) to the plasma products as they enter process chamber  205  (that is, gases  155 ( 2 ) emerge only from a side of gas distribution device  260  that is distal to diffuser  235 ). Optional gas distribution device  260  may also be made of aluminum or aluminum alloy, and like face plate  225  and diffuser  235  discussed above, may be at least coated with a passivating layer for chemical resistance, or may be coated with a ceramic layer. 
         [0021]    Gases  155 ( 1 ),  155 ( 2 ) and/or plasma products from plasma  245  enter a plenum cavity  265 , then pass through a plasma blocking screen  270 ( 1 ) to process chamber  205 . Plasma blocking screen  270 ( 1 ) may form a thickness in the range of 0.15 to 1.0 inches, and forms many small apertures that are configured to allow gases and plasma products from upstream sources pass through into process chamber  205 , while substantially blocking downstream plasmas and plasma products from upstream components, as discussed in detail below. In embodiments, plasma blocking screens  270  may advantageously form at least ten apertures per square inch in a central region thereof, and in certain embodiments may form thirty or more apertures per square inch. Like optional gas distribution device  260 , plasma blocking screen  270 ( 1 ) is also held at electrical ground. Like face plate  225  and diffuser  235  discussed above, surfaces of plasma blocking screen  270 ( 1 ) that are exposed directly to plasma are advantageously coated with ceramic (e.g., alumina or yttria) while surfaces that are not exposed directly to plasma may also be coated with ceramic, and are advantageously at least coated with a passivating layer for chemical resistance to reactive gases and activated species. All of the gases and/or plasma products, generated as described above, interact with workpiece  50  within process chamber  205 , and a further plasma  275  may be generated may be generated within process chamber  205 . When a plasma is desired within process chamber  205 , because diffuser  235  is held at electrical ground (and, when present, optional gas distribution device  260 ), RF power to create plasma  275  is applied to workpiece holder  135 . A DC bias may also be applied to workpiece holder  135  to steer ions generated in plasma  275  to facilitate directional (anisotropic) etching of workpiece  50 , as discussed below. Workpiece holder  135  may be switchably connected with RF and/or DC bias sources, so as to generate a plasma within process chamber  205  at selected times and not at other times. Workpiece holder  135  may be connected with the same RF power supply as is used to create plasma  245  between face plate  225  and diffuser  235 , or may be connected with a different RF power supply. 
         [0022]    The use of plasma blocking screen  270 ( 1 ), the ability to choose whether to generate a plasma by providing RF power and/or DC bias to workpiece holder  135 , or not to generate such plasma, and other features described herein, provide application flexibility to processing system  200 . For example, at a first time, processing system  200  may be operated in a mode wherein plasma is not generated within process chamber  205 . At the first time, the gases and/or plasma products provided by upstream portions of processing system  200  may provide isotropic etching, and workpiece holder  135  may be held at DC ground (although a DC offset may be provided across spatial portions of workpiece holder  135 , to provide electrostatic wafer chucking). At a second time, processing system  200  may be operated in a mode wherein plasma is generated within process chamber  205 , and plasma products thereof may be steered by DC bias between plasma blocking screen and workpiece holder  135 . At the second time, the plasma products steered by the DC bias may provide anisotropic etching, for example to remove broad surface depositions on a workpiece while leaving sidewalls, or to clear materials within deep trenches in a workpiece. Features of plasma blocking screen  270 ( 1 ) are illustrated in further detail in  FIGS. 3 and 4 , while an enlarged view of a portion noted as A in  FIG. 2  is illustrated in detail in  FIG. 6 . 
         [0023]      FIG. 3  illustrates a portion of a plasma blocking screen  270 , such as plasma blocking screen  270 ( 1 ),  FIG. 2 . Plasma blocking screen  270  defines parallel and opposing planar surfaces  268  and  269 , and defines a plurality of apertures  271  extending through plasma blocking screen  270 , as shown in  FIG. 2 . First planar surface  268  and second planar surface  269  characterize a central portion of plasma blocking screen  270 ; however, outside the central portion, plasma blocking screen may be thicker or thinner to provide attachment points for other structures of plasma processing system  200 . Apertures  271  are specialized to allow significant passage of plasma products therethrough with minimal wall effects, but to block plasma from plasma  275  from other parts of plasma processing system  200 , where the plasma can damage other components.  FIG. 3  illustrates features of a single aperture  271  that is characterized by a first aperture section  272  and a second aperture section  273 , as shown. First aperture sections  272  may be cylindrical, which may facilitate fabrication, but may also define other shapes, for example of rectangular or hexagonal cross-section. Any such cross-section is maintained relatively constant through a vertical depth H 1  of first aperture section  272  (given the orientation of plasma blocking screen  270 ( 1 ) as shown in  FIGS. 2 and 3 ) and is considered to define an aperture axis (e.g., vertical, in  FIGS. 2 and 3 ). A minor lateral dimension of first aperture section  272  in a lateral direction (e.g., perpendicular to the aperture axis) is shown as W 1 . “Minor lateral dimension” herein means the largest of any lateral dimensions characterizing an aperture having vertical sides (e.g., if first aperture section  272  is cylindrical, minor lateral dimension W 1  is a diameter of the cylinder, if first aperture section  272  is rectangular, minor lateral dimension W 1  is the smaller of the rectangle sides, and the like). Similarly, a vertical depth of second aperture section  273  is shown as H 2 , while a minor lateral dimension of second aperture section  273  in a lateral direction is shown as W 2 . The sum of H 1  and H 2  is the thickness of plasma blocking screen  270  between planar surfaces  268  and  269 . 
         [0024]    Referring now to both  FIG. 2  and  FIG. 3 , dimensions H 1 , W 1 , H 2  and W 2  are selected to minimize wall effects as plasma products from above (in the orientation shown in  FIG. 2 ) pass through aperture  271 , but also to block products of plasma  275  from reaching optional gas distribution device  260  (when present) and/or other upstream components of plasma processing system  200 . Research has shown (see  FIG. 5 ) that even when aperture  271  is provided with a second aperture section  273  that is very short in height, an appropriate width W 2  will significantly reduce electron density within first aperture section  272  and above. The electron density can be used to estimate an extent to which any plasma product will pass through aperture  271  and affect upstream components. In embodiments herein, width W 2  is less than 0.050″, while height H 2  is between 0.050″ and 0.100″. For mechanical integrity, H 1  is typically larger than H 2 , and in embodiments herein, H 1  is typically 0.10″ or greater. W 1  is less critical than W 2 , but a larger W 1  reduces wall effects (e.g., recombination of ions, deactivation of activated species and the like) to maintain activity of plasma products passing through aperture  271 . W 1  may be, for example, 0.02″ to 0.25″. 
         [0025]      FIG. 4  illustrates a portion of plasma blocking screen  270 ′, which is a modified case of plasma blocking screen  270 ,  FIG. 3 . Plasma blocking screen  270 ′ defines a plurality of apertures  271 ′ that, like apertures  271 , are specialized to allow significant passage of plasma products therethrough with minimal wall effects, but to block plasma from plasma  275  from other parts of plasma processing system  200 .  FIG. 4  illustrates features of a single aperture  271 ′ that is characterized by a first aperture section  272  and a second aperture section  273 ′, as shown. First aperture sections  272  are identical in character to the same feature illustrated in  FIG. 3 , and similarly, and thus may be cylindrical, which may facilitate fabrication, but first aperture sections  272  may also define other shapes, for example rectangular or hexagonal cross-sections. Second aperture section  273 ′ defines a cylindrical upper portion  276  that adjoins first aperture section  272 , and a lower portion  277  that flares outwardly from upper portion  276  toward second planar surface  269 . Lower portion  277  may in fact be conical, as shown in  FIG. 4 , or may be simply curved outwards from the vertical. An angle θ from the aperture axis (e.g., vertical, in  FIG. 4 ) of 15 to 65 degrees may be advantageous, as discussed below. Thus, upper portion  276  may be characterized as having a diameter, and a diameter of lower portion  277  is greater where lower portion  277  adjoins planar surface  269 , than the diameter of upper portion  276 . Upper portion  276  and lower portion  277  adjoin axially to complete aperture  271 ′ through plasma blocking screen  270 ′. 
         [0026]    Forming apertures  271 ′ to include conical lower portions  277  has been found advantageous for fabricating plasma blocking screen  270  with a high quality, continuous coating of alumina or yttria on surfaces (such as second planar surface  269  and side surfaces of second aperture sections  273 ,  273 ′) that face active plasma. First planar surface  268  and side walls of first aperture section  272  are also, optionally, coated with alumina or yttria. As is known to those skilled in the art, certain deposition techniques such as sputtering tend to be highly directional, that is, the substance being deposited tends to travel in a straight line from a source and stick to the first thing that the substance encounters. This makes it difficult to provide films of uniform thickness and high density within narrow apertures (such as second aperture section  273 ,  FIG. 3 ). Providing a conical or otherwise tapered edge section such as lower portion  277 ,  FIG. 4 , not only allows for improved deposition on the tapered edge, but also further back into upper portion  276 , because each point of the interior wall of upper portion  276  “sees” a broader range of angles from which material may be deposited. For this reason, it is also appreciated that when lower portion  277  is present, certain aspect ratios of upper portion  276  (e.g., depth H 3  of upper portion  276  relative to width W 2  of upper portion  276 , as shown in  FIG. 4 ) and the corresponding angle θ thus created may be advantageous. When lower portion  277  provides relatively open access to sputtering sources, an aspect ratio within the range of 0.5 to 4.0 may advantageously provide a broad range of angles to each point on surfaces of upper portion  276 . 
         [0027]      FIG. 5  shows a graph  300  of modeling data that supports the choice of the minor lateral dimension of second aperture sections  273 ,  273 ′,  FIGS. 3 and 4 . In graph  300 , the horizontal axis models position, with conductors (e.g., workpiece holder  135  and wafer  50 ) assumed to be present out to about 4 centimeters, a structural void (e.g., process chamber  205  with plasma  275  therein) from about four to almost 7 centimeters (noted in graph  300  as a broken line), and plasma blocking screen  270  assumed to extend from about 7 centimeters to the end of graph  300 . The vertical axis indicates modeled electron density within a plasma region between the solid conductor and the workpiece holder. The modeling assumed that a plasma was ignited with He gas at a pressure of 1 Torr and a power of 200 W. Graph  300  shows modeled electron density data for plasma blocking screen  270  having several minor lateral dimensions of second aperture section  273 . Electron densities in the range of about 10 2  to low 10 3  are regarded as showing regions that are essentially free of plasma, that is, a number of electrons and/or other plasma products is negligibly low. Electron density is high within the plasma region, and falls off at plasma blocking screen  270 . Using the notation shown in  FIG. 3 , all of the plasma blocking screens  270  modeled assumed H 2  in the range of 0.050″ to 0.100″, W 1  in the range of 0.100″ to 0.150″ and H 2  in the range of 0.300″ to 0.400″. Open circle data  310  is for second aperture section  273  having W 2  in the range of 0.100″ to 0.150″, solid triangle data  320  is for second aperture section  273  having W 2  in the range of 0.050″ to 0.100″, and solid rectangle data  330  is for second aperture section  273  having W 2  in the range of 0.020″ to 0.050″. 
         [0028]    Data  310  and  320  show that plasma blocking screens  270  having second apertures with the respective W 2  noted will decrease electron density transmitted to an adjacent region (e.g., a region extending to the right hand side of graph  300 , or above plasma blocking screen  270 ( 1 ) in  FIG. 2 ), but will not effectively block it. Data  330  shows that a plasma blocking screen  270  having W 2  in the range of 0.020″ to 0.050″ will block plasma almost completely from the adjacent upstream region. Although effective at blocking active plasma from the upstream region, plasma blocking screen  270  does not block plasma products from upstream plasmas, and/or gases, from passing in the downstream direction. 
         [0029]      FIG. 6  schematically illustrates region A noted in  FIG. 2 . As in  FIG. 2 , a workpiece  50  is shown on a workpiece holder  135  within process chamber  205 . Gases  155  and/or previously formed plasma products flow through plasma blocking screen  270 ( 1 ) into process chamber  205 , where a further plasma  275  is formed. As noted above, plasma blocking screen  270 ( 1 ) is held at electrical ground. RF energy, and a DC bias, are applied to workpiece holder  135  to provide energy for plasma  275 . Due to the presence of both reactive species and ion bombardment sources within process chamber  205 , interior surfaces of process chamber  205  are provided with materials (generally, but not limited to, ceramics) capable of resisting attack from such sources. Materials may also be chosen to manage electric field distributions, both in a DC sense and an AC sense, to maximize RF power delivery into plasma  275 . For example, workpiece holder  135  may be coated with alumina or aluminum nitride, and plasma blocking screen  270 ( 1 ) may be coated with alumina or yttria. An optional ceramic spacer  350  and/or an optional ceramic pumping liner  370  may be used to reduce lateral electric fields at the edge of workpiece holder  135 . Ceramic spacer  350  and ceramic pumping liner  370  are ring shaped such that they extend about a periphery of process chamber  205 , but not across the central region of process chamber  205 , and are advantageously fabricated from low loss tangent materials such as high purity alumina, silicon nitride and/or silicon carbide. Materials having loss tangents within the range of 0.1 to 0.0001 provide useful results, while materials having loss tangents within the range of 0.005 to 0.001 represent a range of high performance at reasonable cost. Portions of both plasma blocking screen  270 ( 1 ) and ceramic spacer  350  are disposed atop a portion of a grounded lift plate  390 , as shown, and obtain mechanical support therefrom. Lift plate  390  is mechanically connected with plasma blocking screen  270 ( 1 ), ceramic spacer  350  and other overlying structures so as to enable lifting of all such structures from the vicinity of workpiece holder  135  for assembly and/or maintenance purposes. Plasma blocking screen  270 ( 1 ) is electrically grounded through contact with lift plate  390 . A thickness of ceramic spacer  350  is controlled to leave a gap  360  between plasma blocking screen  270 ( 1 ) and ceramic spacer  350 , to ensure that ceramic spacer  350  does not interrupt continuous contact of plasma blocking screen  270 ( 1 ) with lift plate  390  in the azimuthal direction, about a periphery of process chamber  205 . 
         [0030]    Forming ceramic spacer  350  and ceramic pumping liner  370  of low loss tangent dielectric materials is comparatively expensive (as compared to, for example, fabricating such items from aluminum with a ceramic coating) but reduces electric field effects at the edges of workpiece holder  135 , and reduces reflected RF power when plasma  275  is generated within process chamber  205 . Substituting ceramic spacer  350  and ceramic pumping liner  370  also reduces ion bombardment related contamination as compared with equivalent aluminum parts used in the same locations. Use of ceramic spacer  350  and ceramic pumping liner  370  thus promotes plasma and process stability, and reduces contamination. Electric fields are schematically illustrated using dotted arrows in  FIG. 6 ; the primary electric field is between workpiece holder  135 /workpiece  50  and plasma blocking screen  270 ( 1 ). It is advantageous for the electric fields between workpiece holder  135 /workpiece  50  and plasma blocking screen  270 ( 1 ) be strong and uniform in direction, since the electric fields steer ions involved with anisotropic etching. That is, to clear material at the bottoms of vertical trenches, the electric fields steering the ions need to be correspondingly vertical. Weaker fields exist between workpiece holder  135  and grounded lift plate  390 , through ceramic spacer  350  and ceramic pumping liner  370 . These electric fields are weakened by the dielectric materials of ceramic spacer  350  and ceramic pumping liner  370  being interposed between workpiece holder  135  and lift plate  390 . Weakening the sideways electric fields at edges of workpiece holder  135  has two benefits; (1) electric field directionality, and thus etch directionality, is maintained out to edges of workpiece  50 , and (2) the weaker fields generate less sputtering damage than higher fields. 
         [0031]      FIG. 7  schematically illustrates major elements of a plasma processing system  400 , in a cross-sectional view, according to an embodiment. Plasma processing system  400  is an example of plasma processing unit  130 ,  FIG. 1 . Like plasma processing system  200 ,  FIG. 2 , plasma processing system  400  enables creation of a plasma  275  adjacent to workpiece  50 . Plasma processing system  400  includes many components identical in structure and function to those found in plasma processing system  200 ,  FIG. 2 , but lacks optional gas distribution device  260  shown in plasma processing system  200 . Also, plasma processing system  400  includes a plasma blocking screen  270 ( 2 ) configured differently from the equivalent item shown in  FIG. 2 , and an insulator  280  between diffuser  235  and plasma blocking screen  270 ( 2 ). 
         [0032]    Similarly to plasma processing system  200 , an insulator  230  electrically insulates RF electrode  215 , including face plate  225 , from a diffuser  235  that is held at electrical ground. Diffuser  235  serves as a second electrode counterfacing face plate  225  of RF electrode  215 . Surfaces of face plate  225 , diffuser  235  and insulator  230  define a first plasma generation cavity where a first plasma  245  may be created when plasma source gases  212  are present and RF energy is provided at face plate  225  through RF electrode  215 . Plasma blocking screen  270 ( 2 ) is also held at electrical ground, but insulator  280  allows plasma blocking screen  270 ( 2 ) to be isolated from diffuser  235 , providing an independent RF ground return path. Apertures within plasma blocking screen  270 ( 2 ) are configured like those of plasma blocking screen  270 ( 1 ), that is, they form shapes like those illustrated in  FIGS. 3 and 4 , with similar dimensions, so that upstream plasma products may pass through, but active plasma  275  is blocked from diffuser  235  and other upstream components. 
         [0033]    Embodiments herein may be rearranged and may form a variety of shapes. For example, many components shown in  FIG. 2  and  FIG. 7 , such as RF electrode  215 , diffusers  220  and  235 , gas distribution device  260 , face plate  225 , insulator  230 , plasma blocking screens  270  and others may be substantially radially symmetric about a central axis, for processing a circular semiconductor wafer as workpiece  50 . However, such features may be of any shape that is consistent with use as a plasma source. An exact number and placement of features for introducing and distributing gases and/or plasma products, such as diffusers, face plates and the like, may also vary. Moreover, in a similar manner to gas distribution device  260  including gas channels  250  to add gas  155 ( 2 ) to plasma products from plasma  245  as they enter process chamber  205 , other components of plasma processing system  200  may be configured to add or mix gases  155  with other gases and/or plasma products as they pass through the system to process chamber  205 . As also suggested by  FIG. 2 ,  FIG. 6  and  FIG. 7 , many system components will form perforate planar shapes within central regions of a plasma processing unit so as to provide uniform process conditions to a planar workpiece, but may form different shapes such as flanges, thickness changes, solid imperforate surfaces, and the like at edges of the plasma processing unit for structural purposes. Extents of the central regions may vary, in embodiments, to accommodate different workpiece sizes, especially but not limited to, diameters of wafers as workpieces. When the workpiece is a wafer, the central region will generally encompass a central region that extends at least to one-half of a radius of each such system component. 
         [0034]    Specific details are given in the above description to provide a thorough understanding of the embodiments. However, it is understood that the embodiments may be practiced without these specific details. For example, well-known processes, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments. While the principles of the disclosure have been described above in connection with specific apparatuses and methods, it is to be clearly understood that this description is made only by way of example and not as limitation on the scope of the disclosure. 
         [0035]    It is appreciated that the arrangements shown are exemplary only; other embodiments may differ greatly in configuration, including how source gases are introduced, how electrodes and insulators are arranged, how plasma and/or plasma products are handled after generation, and how grooves are formed in insulators. It is contemplated that the techniques and apparatus disclosed herein are applicable to these and other arrangements wherein conductive material builds up during use and thereby creates leakage and/or discharge paths. 
         [0036]    As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a process” includes a plurality of such processes and reference to “the electrode” includes reference to one or more electrodes and equivalents thereof known to those skilled in the art, and so forth. Also, the words “comprise,” “comprising,” “include,” “including,” and “includes” when used in this specification and in the following claims are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, acts, or groups.