Abstract:
Apparatus for plasma processing of semiconductor substrates. Aspects of the apparatus include an upper shield with a gas diffuser arranged at a center of the upper shield. The gas diffuser and upper shield admit a process gas to a processing chamber in a laminar manner. A profile of the upper shield promotes radial expansion of the process gas and radial travel of materials etched from a surface of the substrates. Curvatures of the upper shield direct the etched materials to a lower shield with reduced depositing of etched materials on the upper shield. The lower shield also includes curved surfaces that direct the etched materials toward slots that enable the etched materials to exit from the process chamber with reduced depositing on the lower shield.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    Embodiments of the present invention generally relate to plasma cleaning or etching in the process of fabricating integrated circuits. In particular, the invention relates to a process kit for a plasma chamber. 
         [0003]    2. Description of the Related Art 
         [0004]    Plasma chambers are used in integrated circuit manufacturing to remove contaminants from the surface of a substrate and/or to etch surfaces of a substrate. To perform a plasma cleaning or etching process, an integrated circuit is placed in a plasma chamber and a pump removes most of the air from the chamber. A gas, such as argon, can then be injected into the chamber. Electromagnetic energy (e.g., radio frequency) is applied to the injected gas to excite the gas into a plasma state. The plasma releases ions that bombard the surface of the substrate to remove contaminants and/or material from the substrate. Atoms or molecules of the contaminants and/or substrate material are etched from the substrate and are, for the most part, pumped out of the chamber. However, some of the contaminant and/or etched material may be deposited on surfaces of the chamber. 
         [0005]    Some plasma process chambers are designed with liners having walls that form a tortuous flow path for gasses passing through the chamber. The parts of the plasma process chamber that form the liners are referred to as a process kit. The walls of the liners trap the plasma in the chamber while providing a path for the displaced contaminants and/or substrate materials to escape. Inevitably, some of the displaced materials are deposited on the walls of the chamber, especially at corner locations where the displaced materials change direction. Eventually, the parts making up the process kit need to be cleaned or replaced due to the buildup of displaced materials. Otherwise, the buildup of displaced materials could become a particle source that could affect chip yield. 
         [0006]      FIG. 1  is a cross-sectional schematic side view of one embodiment of a conventional plasma processing chamber  100  in which a process kit may be used. The plasma processing chamber  100  includes chamber walls  104  and a lid  102  that define a volume. The lid includes a port  106  through which a process gas, such as argon, can be introduced. The wall  104  includes a port  108  through which contaminants and/or substrate materials removed from a substrate  126  and any plasma that escapes the process region  132  can be removed. For example, the port  108  could be in communication with a pump (e.g., a turbo pump) that pulls such materials out of the volume. The plasma processing chamber  100  includes a pedestal  124  on which a substrate  126  can be mounted for processing. The pedestal  124  and the substrate  126  can be in electrical communication with a radio frequency source  128  (e.g., a dual frequency radio frequency source). The electromagnetic energy transmitted by the radiofrequency source  128  through the pedestal  124  excites the process gas into a plasma  130  above the substrate  126 . The plasma processing chamber  100  also includes a process kit that defines boundaries of the process region  132 . The process kit includes an upper shield  110  and a lower shield  116 . 
         [0007]    The upper shield  110  includes a top portion  115  and a cylindrical liner  114 . The lower shield  116  includes a horizontal portion  118  extending from the pedestal  124  and a vertical portion  120  extending from the horizontal portion  118 . Generally, the lower shield  116  is electrically isolated from the pedestal  124  by an insulating material. The upper shield  110  and the lower shield  116  include and/or define apertures that provide a fluid flow path from the port  106  in the lid to the port  108  in the wall  104 . The top portion  115  of the upper shield  110  includes a plurality of apertures  112  arranged around the circumference of the top surface  115  that allow process gas into the process region  132 . The cylindrical liner  114  of the upper shield  110  and the vertical portion  120  of the lower shield  116  define an annular aperture or apertures  122  there between that enable materials removed from the substrate  126  to escape. Arrows A-F illustrate the flow of process gas into the plasma process chamber and the flow of contaminants and/or substrate material out of the plasma processing chamber  100 . Arrow A illustrates the process gas entering through the port  106  in the lid  102 . The process gas may be provided by a supply line, a pressurized canister, or the like that is connected to the port  106 . As illustrated by arrow B, after entering through the port  106 , the process gas travels radially outward toward the apertures  112  in the upper shield  110 . The process gas then passes through the apertures  112  in the direction of arrow C to enter the process region  132 . In the process chamber, the process gas is ignited to form a plasma by the electromagnetic energy from the radiofrequency source  128 . The electromagnetic energy also generally contains the plasma  130  in a region above the substrate  126 . The ions from the plasma bombard the surface of the substrate  126 , causing contaminants and/or substrate material to be released from the surface of the substrate  126 . The released contaminants and/or substrate material exits the process region  132  through the annular aperture  122  between the cylindrical liner  114  of the upper shield  110  and the vertical portion  120  of the lower shield  116 , as indicated by arrow D. The released contaminants and/or substrate material can then move past the lower shield in the direction of arrow E and can then exit the plasma processing chamber  100  through the port  108  in the wall  104 , as indicated by arrow F. As discussed above, a pump, such as a turbo pump, can provide a vacuum that urges the contaminants and/or substrate material to exit the plasma processing chamber  100 . 
       SUMMARY 
       [0008]    In various instances, a process kit for a plasma processing chamber includes an upper shield. The upper shield includes an interior cylindrical liner defining a cylindrical volume, wherein a bottom of the cylindrical liner defines a plane. The upper shield also includes a circular interior top surface. A center of the circular interior top surface includes a circular aperture. The circular interior top surface also includes a first portion extending radially outward from the aperture in which a distance from the interior top surface to the plane increases as the distance from the aperture increases. The circular interior top surface also includes a second portion extending radially outward from the first portion in which the interior top surface smoothly curves toward and mates with an inward-facing surface of the cylindrical liner. 
         [0009]    In various instances, a process kit for a plasma processing chamber includes a gas diffuser configured to be arranged within a circular aperture on an upper shield for a plasma processing chamber. The gas diffuser includes a cylindrical housing that defines a cylindrical exterior surface. The cylindrical exterior surface defines a diameter that is smaller than a diameter of the circular aperture of the upper shield to form an annular gap when the gas diffuser is arranged within the circular aperture. 
         [0010]    The gas diffuser also includes an interior cylindrical cavity in a side of the housing facing away from the cylindrical volume, wherein the interior cylindrical cavity is adapted for fluid communication with a process gas supply. The gas diffuser also includes a plurality of holes in the housing in communication with the interior cylindrical cavity and the annular gap. 
         [0011]    In various instances, a process kit for a plasma processing chamber includes a lower shield. The lower shield includes an annular ring and an annular channel in a top surface of the annular ring. A radially-outward-facing liner and a bottom liner of the annular ring include a circularly-curved liner portion there between. The annular channel also includes a radially-outward-projecting undercut portion. The lower shield also includes a first plurality of slots between the undercut portion and the top surface of the annular ring, wherein the first plurality of slots are arranged around a circumference of the annular ring. The lower shield also includes a second plurality of slots between the undercut portion and a bottom surface of the annular ring, wherein the second plurality of slots are arranged around a circumference of the annular ring. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, may admit to other equally effective embodiments. 
           [0013]      FIG. 1  is a cross-sectional block diagram of a conventional plasma process chamber; 
           [0014]      FIG. 2A  is a cross-sectional side view of a process kit according to various aspects that includes an upper shield, a diffuser, and a lower shield; 
           [0015]      FIG. 2B  is a cross-sectional view of the process kit of  FIG. 2A  installed in a plasma process chamber; 
           [0016]      FIG. 3A  is a bottom perspective view of the diffuser shown in  FIG. 2A ; 
           [0017]      FIG. 3B  is a cross-sectional side view of the diffuser of  FIG. 2A ; 
           [0018]      FIG. 4  is a cross-sectional side view of the diffuser of  FIG. 2A  installed in an upper shield shown in  FIG. 2A ; 
           [0019]      FIG. 5A  is a bottom perspective view of a first portion of the lower shield shown in  FIG. 2A ; 
           [0020]      FIG. 5B  is a side view of the first portion of the lower shield shown in  FIG. 2A ; 
           [0021]      FIG. 5C  is a cross-sectional side view of the first portion of the lower shield shown in  FIG. 2A ; 
           [0022]      FIG. 6A  is a top perspective view of a second portion of the lower shield shown in  FIG. 2A ; 
           [0023]      FIG. 6B  is a cross-sectional side view of the second portion of the lower shield shown in  FIG. 2A ; and 
           [0024]      FIG. 7  is a cross-sectional side view of a portion of the lower shield and a portion of the upper shield of  FIG. 2A . 
       
    
    
       [0025]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
       DETAILED DESCRIPTION 
       [0026]    Aspects of a process kit described herein can provide more efficient plasma processing while reducing the buildup of contaminants on surfaces of the process kit. Contaminants on surfaces of the process kit could become particle sources that could contaminate substrates in the chamber and affect chip yield. 
         [0027]      FIG. 2A  is an exploded side view of a process kit  200  according to various aspects for use in a plasma processing chamber.  FIG. 2B  shows the process kit  200  arranged in a plasma processing chamber  300 . The process kit  200  includes at least one or more of a diffuser  202 , an upper shield  220 , and a lower shield  250 ,  270 . The diffuser  202  sits within an aperture  222  in the upper shield  220  such that process gas is injected through the diffuser  202  and the upper shield  220  in the middle or center of the upper shield  220  rather than along the perimeter of the upper shield  110  shown in  FIG. 1 . The upper shield  220  and the lower shield  250 ,  270  include rounded corners that help to smoothly direct contaminants and/or substrate material removed by plasma out of the process region  246  and, as a result, to minimize buildup of such contaminants and/or substrate material on surfaces of the upper shield  220  and the lower shield  250 ,  270 . 
         [0028]    The upper shield  220  includes a top  232  and a cylindrical liner  230 . Interior surfaces  234  and  236  of the top  232  and interior surfaces  240  of the cylindrical liner  230  define boundaries of a process region  246 . A bottom  242  of the cylindrical liner  230  defines a plane  244  (indicated by a dashed line). The top  232  includes an aperture  222  located at the center of the top  232 . An inward portion of the interior surface  234  of the top  232  extends radially away from the aperture  222  and diverges from the plane  244  at greater radial distances. Put differently, a distance (indicated by arrow d) from the plane  244  to the inward portion of the interior surface  234  is larger at larger radial distances from the aperture  222 . In various instances, the inward portion of the interior surface  234  could include a conical profile, wherein the distance from the plane  244  to the inward portion of the interior surface  234  increases linearly with respect to the radial distance from the aperture  222 . In various other instances, the inward portion of the interior surface  234  could include a curved profile, wherein the distance from the plane  244  to the inward portion of the interior surface  234  increases nonlinearly with respect to the radial distance from the aperture  222 . For example, as illustrated in  FIG. 2 , the inward portion of the interior surface  234  could have a circular profile. For example, in various aspects, the cylindrical liner  230  of the upper shield  220  has a diameter of approximately 16 inches and the circularly-curved inward portion of the interior surface  234  has a radius of curvature R 2  of approximately 35 inches. As another example, the radius of curvature R 2  of the circularly curved interior surface  234  could be between 30 inches and 40 inches. An outward portion of the interior surface  236  of the top  232  extends radially away from interior surface  234  and curves inwardly to meet the interior surface  240  of the cylindrical liner  230 . The outward portion of the interior surface  236  can define a radius of curvature R 1  of 1.1 inches in some instances. In various other instances, the outward portion of the interior surface  236  can define a radius of curvature R 1  of between 0.9 inches and 2 inches, for example. 
         [0029]    Referring now to  FIGS. 3A and 3B , the diffuser  202  can be configured to fit within the aperture  222  of the upper shield  220 . The diffuser  202  includes a body  204  and a gas injection housing  212 . The body  204  can include threaded holes  210  that can align with holes  228  in the upper shield  220 . Cap screws or other fasteners can be placed through the threaded holes  210  and into the holes  228  in the upper shield to secure the diffuser  202  to the upper shield  220 . The gas injection housing  212  can include a first outer cylindrical wall  215  and a second outer cylindrical wall  214 . The second outer cylindrical wall  214  can be arranged closer to the interior surface  234  of the upper shield  220 . The first outer cylindrical wall  215  can define a larger diameter than a diameter D 1  of the second outer cylindrical wall  214 . For example, in various instances, the first outer cylindrical wall  215  defines a diameter of 1.060 inches and the second outer cylindrical wall  214  defines a diameter D 1  of 1.030 inches. The diffuser  202  and gas injection housing  212  define an interior cylindrical cavity  206  that can be in communication with a process gas supply for a plasma processing chamber. An array of gas injection apertures  208  can be arranged in the gas injection housing  212  from the interior cylindrical cavity  206  to the second outer cylindrical wall  214 . As shown in  FIGS. 3A and 3B , the gas injection apertures  208  can be arranged around a circumference of the interior cylindrical cavity  206  and/or at different longitudinal locations along the interior cylindrical cavity  206 . In the exemplary diffuser  202  shown in  FIGS. 3A and 3B , there are four longitudinal rows of gas injection apertures  208 , and each row of gas injection apertures  208  includes twelve gas injection apertures  208  arranged around a circumference of the interior cylindrical cavity  206 . In various instances, each of the gas injection apertures  208  has a diameter of 0.030 inches. In various other instances, each of the gas injection apertures  208  includes a diameter between 0.020 inches and 0.040 inches. In various other instances, each of the gas injection apertures  208  could have a diameter of between 0.010 inches and 0.050 inches. 
         [0030]      FIG. 4  illustrates the diffuser  202  installed in the upper shield  220 . Referring also to  FIG. 2 , the upper shield  220  includes an aperture  222  that includes a first portion  224  that defines a first diameter and a second portion  226  that defines a second diameter D 2 . When the diffuser  202  is installed in the upper shield  220 , the body  204  of the diffuser  202  sits within the first portion  224  of the aperture  222  and the gas injection housing  212  sits within the second portion  226  of the aperture  222 . The diameter D 2  of the second portion  226  of the aperture  222  is equal to or greater than the diameter of the first outer cylindrical wall  215  of the diffuser  202 . For example, as described above, the diameter of the first outer cylindrical wall  215  of the diffuser  202  could be 1.060 inches in some instances. The diameter D 2  of the second portion  226  of the aperture  222  could be between 1.060 inches and 1.065 inches, for example. As a result, in some instances, the diffuser  202  can snugly fit within the aperture  222  of the upper shield  220 . 
         [0031]    As discussed above, the diameter D 1  of the second outer cylindrical wall  214  of the diffuser  202  is smaller than the diameter of the first outer cylindrical wall  215 . As a result, the diameter D 1  of the second outer cylindrical wall  214  is also smaller than the diameter D 2  of the second portion  226  of the aperture  222 . The smaller diameter D 1  of the second outer cylindrical wall  214  relative to the diameter D 2  of the second portion  226  of the aperture  222  results in an annular gap  232  between the upper shield  220  and the diffuser  202 . For example, as described above, the diameter D 1  of the second outer cylindrical wall  214  could be 1.030 inches. If the diameter D 2  of the second portion  226  of the aperture  222  in the upper shield  220  is 1.060 inches, then the resulting annular gap  232  would have a width of 0.015 inches. The annular gap  232  is in communication with the process region  246  defined by the upper shield  220 . In various instances, a length of the second outer cylindrical wall  214  is approximately 0.60 inches. As a result, an aspect ratio of the annular gap  232  (defined as the length of the annular gap  232  divided by the width of the annular gap  232 ) would be 0.60 inches divided by 0.015 inches, or 30 to 1. In various instances, the aspect ratio of the annular gap  232  can be between 20 to 1 and 40 to 1. The annular gap  232  is also in communication with the interior cylindrical cavity  206  in the diffuser  202  via the gas injection apertures  208 . In various instances, the gas injection apertures  208  can be arranged in the interior cylindrical cavity  206  such that they exit through the second outer cylindrical wall  214  in the first third of a length of the second outer cylindrical wall  214  closest to the first outer cylindrical wall  215 . In such an arrangement, process gas flows into the interior cylindrical cavity  206  of the diffuser  202  in the direction of arrow G. The process gas then flows through the gas injection apertures  208  (as indicated by arrows H) and into the annular gap  232  (as indicated by arrows I). The process gas then moves along the annular gap  232  to enter the process region  246  defined by the upper shield  220  (as indicated by arrows J). The process gas enters the process region  246  in the shape of an annular ring. In various instances, the process gas can enter the process region  246  in a laminar flow arrangement. In the exemplary instance described above in which there are  48  gas injection apertures  208  each with the diameter of 0.030 inches, the total area of the gas injection apertures  208  would be 0.017 in. 2 . Furthermore, for an annular gap  232  having an inner diameter D 1  of 1.030 inches and a gap of 0.015 inches, the total area for the annular gap  232  would be approximately 0.10 inches. Thus, a ratio of the total area for the annular gap  232  to the total area of the gas injection apertures  208  is almost 6 to 1. In various instances the ratio of the total area for the annular gap  232  to the total area of the gas injection apertures  208  can be between 4 to 1 and 15 to 1. A combination of the large aspect ratio (e.g., 30 to 1) and ratio of total areas (e.g., 6 to 1) can result in a significant pressure drop of the process gas as it flows from the interior cylindrical cavity  206  into the process region  246 . This large pressure drop can ensure that the process gas enters the process region  246  in a laminar flow. 
         [0032]    Referring again to  FIGS. 1 and 2 , when the process gas enters the process region  246  in the direction of arrow J, electromagnetic energy ignites the process gas (e.g., argon) into a plasma (e.g., plasma  130 ). The plasma can then spread radially outward in the process region  246  in the direction of arrow L. The diverging interior surface  234  of the upper shield  220  can encourage radially outward movement of the plasma. Most of the plasma is contained in a region above the substrate (e.g., substrate  126  in  FIG. 1 ) by the electromagnetic energy. Contaminants on the substrate and/or substrate material removed by ions from the plasma moves further radially outward in the direction of arrow L and are then deflected in a downward direction, as indicated by arrow M, by the inwardly curved surfaces of the outward portion of the interior surface  236  and the cylindrical liner  230  of the upper shield  220 . The contaminants and/or substrate material are directed toward the lower shield  250 ,  270 . The relatively large radius of curvature of the outward portion of the interior surface  236  can promote movement of the contaminants and/or substrate material compared to a sharp corner or a smaller radius of curvature. Put differently, a sharp corner or a corner with a small radius of curvature may cause the contaminants and/or substrate material flowing in the direction of arrow M to momentarily stagnate at the corner. Such stagnation can allow the contaminants and/or substrate material to accumulate on the interior surfaces  234 ,  236 , and  240  of the upper shield. 
         [0033]    In the exemplary lower shield  250 ,  270  shown in the figures, the lower shield includes a first portion  250  in the second portion  270 .  FIGS. 5A and 5B  illustrate a first portion  250  of the lower shield and  FIGS. 6A and 6B  illustrate a second portion  270  of the lower shield. The first portion  250  includes a ring-shaped body  252  that includes a plurality of fastener holes  254  arranged around a circumference. A radially-inward facing side of the ring-shaped body  252  includes a plurality of outward-facing undercuts  260  that are arranged between the fastener holes  254 . A plurality of upward facing slots  262  can extend from each outward-facing undercut  260  to a top surface  261  of the ring-shaped body  252 . A plurality of downward-facing slots  258  can extend from each outward-facing undercut  260  toward a bottom surface  259  of the ring-shaped body  252 . For example, the upward facing slots  262  can include three slots  262   a,    262   b,  and  262   c  and downward-facing slots  258  can include three slots  258   a,    258   b,  and  258   c.  A set of the three slots  262   a,    262   b,  and  262   c  can be arranged at the same circumferential location on the ring-shaped body  252 , but at different radial locations. In the exemplary ring-shaped body  252  shown in  FIGS. 5A-5C , slot  262   a  is the most radially outboard slot, slot  262   b  is the middle slot, and slot  262   c  is the most radially inboard slot. Similarly, a set of the three slots  258   a,    258   b,  and  258   c  can be arranged at the same circumferential location on the ring-shaped body  252 , but at different radial locations. In the exemplary ring-shaped body  252  shown in  FIGS. 5A-5C , slot  258   a  is the most radially outboard slot, slot  258   b  is the middle slot, and slot  258   c  is the most radially inboard slot. In certain instances, the slots  258 ,  262  can define aspect ratios (defined as a length of the slot from the outward-facing undercut  260  to an exterior surface of the ring-shaped body  252  divided by a width of the slot in a radial direction) of 4.5 to 1. For example, the length of each slot may be 0.45 inches and the width of each slot may be 0.10 inches, resulting in an aspect ratio of 4.5 to 1. In various other instances, the aspect ratio for the slots may be between 3 to 1 and 6 to 1, for example. In various instances, the bottom surface  259  of the ring-shaped body  252  can be arranged at an angle a relative to a horizontal bottom surface  264  such that the bottom surface  259  increases in height at larger radial distances. Such an angle a can provide clearance for the slots  258  from the second portion  270  of the lower shield and from other structures, such as the bracket  312  shown in  FIG. 2B . The outward-facing undercut  260  can include a similarly-angled surface so that the aspect ratios for the slots  258  are maintained. 
         [0034]      FIGS. 6A and 6B  illustrate the second portion  270  of the lower shield. The second portion  270  includes a vertical liner portion  274  and a horizontal liner portion  280  with a curved liner portion  276  there between. The curved liner portion  276  defines a radius of curvature R 3 . In certain instances, the radius of curvature R 3  may be 0.40 inches. In various other instances, the radius of curvature R 3  may be between 0.2 inches and 0.6 inches. The second portion  270  also includes a fastener flange  272  that includes a plurality of threaded holes  278  arranged there around. A transition from the horizontal surface  282  the fastener flange  272  includes a vertical lip  282 . Fasteners, such as cap screws, can pass through the fastener holes  254  in the ring-shaped body  252  of the first portion  250  and engage the threaded holes  278  in the fastener flange  272  to fasten the first portion  250  and the second portion  270  together. 
         [0035]      FIG. 7  illustrates the first portion  250  and second portion  270  of the lower shield assembled together.  FIG. 7  also illustrates a bottom portion of the cylindrical liner  230  of the upper shield  220  engaged with the first portion  250  of the lower shield (e.g., engaged in the plasma processing chamber  300  shown in FIG.  2 B). As shown in  FIG. 7 , the cylindrical liner  230  of the upper shield  220  can be separated from the first portion  250  of the lower shield such that an annular gap  271  is formed between an outward-facing surface  238  of the cylindrical liner  230  and an inward-facing upper surface  268  of the first portion  250 . The annular gap  271  can have a similar aspect ratio to the slots  262 . Such an annular gap could function as a slot in addition to the slots  262  in the first portion  250  of the lower shield  250 ,  270 . Alternatively, the outward-facing surface  238  of the cylindrical liner  230  could lightly contact or be closely-spaced from the inward-facing upper surface  268  of the first portion  250  of the lower shield. When the first portion  250  of the lower shield is assembled with the second portion  270 , a bottom surface  264  of the first portion  250  rests on the fastener flange  272 . Also, a lower inward-facing surface  266  of the first portion  250  is adjacent to the vertical lip  282  of the second portion  270 . In various instances, the inward-facing surface  266  contacts the vertical lip  282 . In various other instances, the inward-facing surface  266  may be separated from the vertical lip  282  by an annular gap. The first portion  250  and the second portion  270  create a flow path for contaminants and/or substrate materials to escape from the process region  246 .  FIG. 7  illustrates arrow M which is the flow of contaminants and/or substrate materials from the process region  246  after the materials have been redirected downwardly by the outward portion of the interior surface  236  and the cylindrical liner  230  of the upper shield  220 . The materials are turned radially outward, as indicated by arrow N, by the vertical liner portion  274 , the curved liner portion  276 , and the horizontal liner portion  280  of the second portion  270  of the lower shield. The materials enter the outward-facing undercuts  260  in the first portion  250  of the lower shield. The materials then flow in the direction of arrow O through the upward-facing slots  262  and in the direction of arrow P to the downward-facing slots  258 . 
         [0036]      FIG. 2B , illustrates the process kit  200  arranged in a plasma processing chamber  300  and the flow of process gas through the plasma processing chamber  300 . The plasma processing chamber  300  includes a lid  302  and chamber walls  304 . The diffuser  202  of the process kit  200  is installed in and attached to the upper shield  220 . The lower shield  250 ,  270  is mounted to a pedestal  310  by a bracket  312 . The pedestal  310  can move up and down (in the directions of arrow Z). The pedestal  310  may be moved downwardly (away from the upper shield  220 ) to position the substrate supported on the pedestal  310  below the upper shield  220  to allow the substrate to be robotically transferred from the pedestal  310 . The pedestal  310  can be moved upwardly (toward the upper shield  220 ) to engage the cylindrical wall  230  of the upper shield  220  with the first portion  250  of the lower shield (as discussed above with reference to  FIG. 7 ) and, as a result, form the process region  246 . 
         [0037]    An edge ring  314  can surround and partially rest on a top surface of the pedestal. The edge ring  314  can ensure that plasma in the process region  246  extends across the entire substrate. The diffuser  202 , upper shield  220 , and lower shield  250 ,  270  are grounded from the pedestal  310  and radiofrequency source. The edge ring  314  may be made of quartz or another electrically insulating material. In various aspects, the bracket  312  that attaches the lower shield  250 ,  270  to the pedestal may also be made of an electrically insulating material, such as a plastic material. In various other aspects, the bracket  312  can be made of metal and an insulative material can be arranged between the bracket  312  and the pedestal  312 . A plurality of grounding rings  318  can be attached to a bracket  316  in the plasma processing chamber  300 . For example, referring again to  FIG. 5A , the grounding rings  318  can be spaced apart circumferentially such that each grounding ring  318  is aligned with one of the fastener holes  254  in the first portion  250  of the lower shield. When the pedestal  310  is raised toward the upper shield  220 , the grounding rings  318  contact in the top surface  261  of the first portion  250  of the lower shield  250 ,  270 , thereby electrically coupling the lower shield  250 ,  270  to the grounded upper shield  220  and lid  302  of the plasma processing chamber  300 . The grounding rings  318  are generally hoop shaped and are elastically deformable in the direction of arrows Z. As a result, the grounding rings  318  can maintain contact with the fasteners in the first portion  250  of the lower shield over a range of positions of the pedestal  310  and the lower shield  250 ,  270  relative to the upper shield  220 . 
         [0038]      FIG. 2B  illustrates the flow of process gas (e.g., argon) into the plasma processing chamber  300 , through the process region  246 , and out of the plasma processing chamber  300 . The process gas enters the plasma processing chamber  300  through a port  308  in the lid  302  (as indicated by arrow G). The port  308  is in communication with the diffuser  202  such that the process gas passes through the diffuser  202  in the center of the upper shield  220  and into the process region  246  (as indicated by arrows J). In the process region  246 , the process gas is ignited into a plasma by electromagnetic energy. The plasma etches contaminants and/or substrate materials from a substrate on the pedestal  310 . The etched contaminants and/or substrate materials (and any plasma that may escape from the electromagnetic field) flow radially outward in the direction of arrows L. The etched materials are then deflected downward by the upper shield  220  toward the lower shield  250 ,  270  in the direction of arrow M. The second portion of the lower shield  270  directs the etched materials radially outward in the direction of arrow N. The etched materials then pass through the slots  262  and  258  in the first portion  250  of the lower shield  250 ,  270  in the directions of arrows O and P, respectively. The etched materials can then pass through the port  306  in the direction of arrow Q to leave the plasma processing chamber  300 . 
         [0039]    Testing of plasma processing chambers using the process kit  200  shown in  FIG. 2A  demonstrate an improvement in plasma processing efficiency. For example, a plasma processing chamber using a process kit like the process kit  200  shown in  FIG. 2A  has shown approximately a four-fold increase in flow of process gas and removed contaminant and/or substrate materials for a given operating pressure (e.g., 0.007 Torr). Additionally, the curved surface  236  on the upper shield  220  and curved surface  276  on the lower shield  250 ,  270  show significant reductions in the accumulation of contaminants and/or substrate materials on the upper shield  220  and lower shield  250 ,  270 , which should result in fewer required maintenance steps for the process kit  200 .