Abstract:
A showerhead assembly includes a front plate having a front surface, a back surface and a plurality of first through holes connecting the front surface and the back surface, a back plate having a front surface, a back surface and a plurality of second through holes connecting the front surface and the back surface, and an adhesive layer joining the back surface of the front plate and the front surface of the back plate. The plurality of first through holes are aligned with the plurality of second through holes, and the front plate and the back plate are formed from dissimilar materials.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 61/888,995, filed on Oct. 9, 2013, which herein is incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    Embodiments of the present disclosure generally relate to apparatus and methods for semiconductor processing. More particularly, embodiments of the present disclosure relate to apparatus and methods for delivering one or more processing gas in disassociated phase to a processing chamber. 
         [0004]    2. Description of the Related Art 
         [0005]    Disassociated processing gas is commonly used in semiconductor processing. Hollow cathode discharge plasma and electron beam sources are one of the common plasma sources for generating disassociated processing gas. Generally, a hollow cathode plasma source includes a cathode having an inner volume and a ground anode disposed apart from the cathode facing the inner volume. During operation, molecular gases are introduced to the inner volume of the cathode while a RF power is applied between the cathode and the ground electrode to create radicals from the molecular gas within the inner volume. After generation, the disassociated processing gas may be delivered to a processing chamber through a showerhead assembly attached to the hollow cathode plasma source. 
         [0006]    Active species in the disassociated processing gas may recombine and form a deposition on the surfaces of the flow path, such as on surfaces of the showerhead, while being delivered to the processing chamber. The unintended deposition on the surfaces of showerhead may cause various problems, such as lowered efficiency and particle contamination. Conventionally, a showerhead may be cleaned using a cleaning chemistry in a regular basis to remove any unintended deposition. Showerheads, usually formed from metallic material, such as aluminum or stainless steel, are conventionally coated with a compatible coating to prevent any damages to the showerhead during cleaning. However, as demands for uniformity increases, openings in a showerhead become too small and/or too high aspect ratio to receive a coating and subject the showerhead either to damage from disassociated cleaning gas or unintended deposition from the disassociated processing gas. 
         [0007]    Therefore, there is a need for improved showerhead for delivering processing gas in disassociated phase to a processing chamber. 
       SUMMARY 
       [0008]    Embodiments of the present disclosure generally relate to apparatus and methods for delivering disassociated processing gas to a processing chamber. 
         [0009]    One embodiment of the present disclosure provides a showerhead assembly. The showerhead assembly includes a front plate having a front surface, a back surface and a plurality of first through holes connecting the front surface and the back surface and a back plate having a front surface, a back surface and a plurality of second through holes connecting the front surface and the back surface. The plurality of first through holes are smaller in diameter than the plurality of second through holes. The showerhead assembly further includes a protective coating formed on surfaces of the back plate, and an adhesive layer joining the back surface of the front plate and the front surface of the back plate. The plurality of first through holes are aligned with the plurality of second through holes, and the front plate and the back plate are formed from dissimilar materials. 
         [0010]    Another embodiment of the present disclosure provides an apparatus for processing semiconductor substrate. The apparatus includes a chamber body defining an inner volume, a substrate support assembly disposed in the inner volume, a hollow cathode plasma source configured to provide one or more processing gas in disassociated phase to the inner volume, and a showerhead assembly disposed between the hollow cathode plasma source and the substrate support assembly to deliver disassociated processing gas from the hollow cathode plasma source to the inner volume. The showerhead assembly includes a front plate having a front surface, a back surface and a plurality of first through holes connecting the front surface and the back surface, wherein the front surface of the front plate faces the inner volume, a back plate having a front surface, a back surface and a plurality of second through holes connecting the front surface and the back surface, wherein the back surface of the back plate faces the hollow cathode plasma source, and an adhesive layer joining the back surface of the front plate and the front surface of the back plate, wherein the plurality of first through holes are aligned with the plurality of second through holes, and the front plate and the back plate are formed from dissimilar materials. 
         [0011]    Another embodiment of the present disclosure provides a method for delivering processing gas in disassociated phase. The method includes generating disassociated processing gas within a plasma cavity of a hollow cathode plasma source, flowing the disassociated processing gas through a plurality of through holes in a back plate of a showerhead assembly, and flowing the disassociated processing gas through a plurality of through holes in a front plate of the showerhead assembly. A back surface of the front plate and a front surface of the back plate are joined by an adhesive layer, the plurality of through holes of the back plate are aligned with the plurality of through holes of the front plate, and the front plate and the back plate are formed from dissimilar materials. 
     
    
     
       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 typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments. 
           [0013]      FIG. 1  is a schematic sectional view of a plasma processing chamber. 
           [0014]      FIG. 2A  is a schematic partial sectional view of a showerhead assembly. 
           [0015]      FIG. 2B  is a schematic partial top view of a gas distribution plate of the showerhead assembly of  FIG. 2A . 
           [0016]      FIG. 2C  is an enlarged sectional view of a joint of a showerhead assembly of  FIG. 2A . 
           [0017]      FIG. 2D  is a schematic top view of a fastener cap. 
       
    
    
       [0018]    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 disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation. 
       DETAILED DESCRIPTION 
       [0019]    Embodiments of the present disclosure generally relate to apparatus and methods for delivering processing gas in disassociated phase to a processing environment. One embodiment provides a showerhead assembly having a front plate and a back plate joined together by an adhesive layer. The front plate and back plate may be formed from different materials. The front plate may be formed from a material compatible with cleaning gas in disassociated state and has a plurality of through holes small enough to satisfy processing requirements. The back plate may be formed from metallic material and has a plurality through holes large enough to receive a coating compatible with the processing gas. In one embedment, a clamping mechanism may be applied to the front plate and the back plate to ensure uniform contact between the front plate and back plate. The conductive back plate of the showerhead assembly may be coupled to an RF ground of the plasma source. The front plate may face the processing environment of the processing chamber and delivers disassociated processing gas through the plurality of small through holes. The showerhead assembly of the present disclosure is compatible with cleaning chemistry and including small through holes for gas delivery. 
         [0020]      FIG. 1  is a schematic sectional view of a plasma processing chamber  100  according to one embodiment of the present disclosure. The plasma processing chamber  100  includes a showerhead assembly  160  according to embodiments of the present disclosure. 
         [0021]    The plasma processing chamber  100  generally includes a chamber body  102  and a liner  103  disposed in inside the chamber body  102 . The liner  103  defines a chamber volume  104  for substrate processing. A substrate support assembly  106  is disposed in the chamber volume  104  to support a substrate  108  to be processed. An exhaust port  107  may be formed through the chamber body  102  and connected with a vacuum pump  105  to maintain a low pressured process environment during operation. 
         [0022]    The hollow cathode plasma source  110  is disposed above the chamber body  102  for supplying processing gas in disassociated phase and/or molecular phase to the chamber volume  104 . The hollow cathode plasma source  110  includes a hollow cathode  112 , a ground electrode  128  and an isolator  126  disposed between the hollow cathode  112  and the ground electrode  128 . 
         [0023]    The hollow cathode  112  may be formed from a RF conductive material and have an inner volume  114  which serves as plasma cavity during plasma generation. The hollow cathode  112  and the inner volume  114  may be symmetric about a central axis  118 . A central rod  116 , formed from a RF conductive material, is coupled to the hollow cathode  112 . A lower end  120  of the central rod  116  extends along the central axis  118  out of the inner volume  114  towards the ground electrode  128 . By extending out of the inner volume  114 , the lower end  120  of the central rod  116  is the closest to the ground electrode  128  compared with other portions of the hollow cathode  112 , as a result, plasma always ignites at the lower end of the central rod  116 . 
         [0024]    A gas channel  122  is formed in the hollow cathode  112  for delivering one or more processing gas from a gas source  156  to the inner volume  114  of the hollow cathode  112 . The gas channel  122  includes a plurality of outlets  124  formed around the upper end  119  of the central rod  116 . The plurality of outlets  124  may be evenly distributed about the central axis  118  and the central rod  116 . 
         [0025]    The ground electrode  128  may be a conductive plate having a recess  132  along the central axis  118  for receiving the isolator  126  and the hollow cathode  112 . A plurality of through holes  130  may be formed through the ground electrode  128  to allow the plasma formed in the inner volume  114  to enter the chamber volume  104  for processing. 
         [0026]    A RF connector  134  may be attached to the hollow cathode  112  on the upper side  112   a . The RF connector  134  may be disposed along the central axis  118  to provide electrical symmetry with a central RF feed configuration. The RF connector  134  may be coupled to a RF output  136   a  of a RF power source  136  so that the hollow cathode  112  is RF hot during operation. 
         [0027]    The plasma processing chamber  100  further includes a RF ground shield assembly  140  that encloses the hollow cathode  112  and the RF connector  134 . During operation, a RF ground  136   b  of the RF power source  136  may be connected to the RF ground shield assembly  140 . 
         [0028]    An outer shell  158  may be disposed over the chamber body  102  to shield the hollow cathode plasma source  110  from any external noises, such as magnetic noises. In one embodiment, the outer shell  158  may be formed from a material having high magnetic permeability, such as mu-metal. 
         [0029]    The showerhead assembly  160  is disposed between the hollow cathode plasma source  110  and the substrate support assembly  106 . The showerhead assembly  160  may be used to uniformly distribute processing gas from the hollow cathode plasma source  110  to the chamber volume  104 . 
         [0030]    The showerhead assembly  160  includes a front plate  162  facing the chamber volume  104  and a back plate  166  attached to the front plate  162 . The back plate  166  faces the hollow cathode plasma source  110  in a space apart relation. In one embodiment, the front plate  162  and the back plate  166  may be joined by an adhesive layer  170 . A plurality of flow paths  172  may be formed through the front plate  162 . In one embodiment, the plurality of flow paths  172  may be formed from a plurality of through holes  164  in the front plate  162  and the plurality of through holes  168  in the back plate  166 . Each of the plurality of through holes  164  may align with a corresponding one of the plurality of through holes  168  to form one flow paths  172 . 
         [0031]    The plurality of through holes  164  on the front plate  162  may be smaller in diameter than the plurality of through holes  168  in the back plate  166  to create a restriction in each flow path  172 . The restriction created by the smaller through holes  164  provides a back pressure for the processing gas passing through that facilitates even pressure distribution in a plenum  111  defined between the hollow cathode plasma source  110  and the showerhead assembly  160  which provides uniform gas flow towards and across the width of the substrate  108 . 
         [0032]    The adhesive layer  170  may be a perforated adhesive sheet having a plurality of openings  171  aligns with the through holes  164  and  168 . The plurality of openings  171  are formed to allow precise alignment between the corresponding through holes  164  and  168 , thus keep the through holes  164 ,  168  from clogging. Alternatively, the adhesive layer  170  may include a plurality of adhesive beads or adhesive rings. The adhesive layer  170  may be formed from an adhesive material having thermal conductive additives to ensure thermal exchange between the front plate  162  and the back plate  166 . In one embodiment, the thermal conductive additive may be a metal powder. 
         [0033]    In one embodiment, the back plate  166  includes a temperature control element  173  to main a target temperature in the showerhead assembly  160  during operation. The temperature control element  173  may include an embedded heating element and/or cooling channel for circulating a temperature control fluid therein. The temperature control element  173  may be used to maintain a desired temperature of the back plate  166  and the front plate  162  during processing. Formed from a thermal conductive material, the back plate  166  may respond to the temperature control element  173  quickly and reach uniform temperature in a timely manner. In one embodiment, the back plate  166  may have enough mass to act as a heat sink for the front plate  162  to maintain a desired temperature in the front plate  162 . 
         [0034]    The showerhead assembly  160  may also include a clamp ring  182  attached to the back plate  166 . The clamp ring  182  may be disposed radially outside the front plate  162 . In one embodiment, the clamp ring  182  and the front plate  162  may overlap to secure the front plate  162  on the back plate  166 . The clamp ring  182  functions to facilitate secure and uniform contact between the front plate  162  and the back plate  166  across the showerhead assembly  160 . By clamping the front plate  162  and the back plate  166  together, the clamp ring  182  enables consistent thermal conduct between the front plate  162  and the back plate  166  to prevent thermal non-uniformity in the front plate  162  caused by deterioration of the adhesive layer  170 . The clamp ring  182  may be formed from a material compatible with the processing chemistry. In one embodiment, the clamp ring  182  may be formed from the same material as the liner  103 , such as silicon carbide. 
         [0035]    The showerhead assembly  160  may also include a blocker plate  174  disposed upstream to the back plate  166 . The blocker plate  174  may be secured against a plurality of posts  176  extending from the back plate  166 . A gas redistributing volume  178  is formed between the back plate  166  and the blocker plate  174 . The blocker plate  174  may include a plurality of through holes  180  for directing processing gas from the hollow cathode plasma source  110  to the gas redistributing volume  178 . 
         [0036]      FIG. 2A  is a schematic partial enlarged sectional view of the showerhead assembly  160  according to one embodiment of the present disclosure. The clamp ring  182  may be coupled to the back plate  166  by a plurality of fasteners  252  securing an edge  254  of the front plate  162  therebetween. The plurality of fasteners  252  may be evenly distributed along the clamping ring  182 . 
         [0037]    As discussed above, the back plate  166  may be formed from a thermal and/or electrical conductive material, such as a metal, so that the back plate  166  may be connected to the RF ground or RF source output and to facilitate temperature control during operation. In one embodiment, the back plate  166  is formed form aluminum. A protective coating  250  may be formed on surfaces, including interior surfaces of the plurality of through holes  168 , to protect the metallic body of the back plate  166  from cleaning chemistry and/or processing chemistry. The plurality of through holes  168  may be large enough so that traditional coating formation methods, such as spray coating, may be used to form the protective coating  250  over the entire interior surfaces of the plurality of through holes  168 . Similarly, a protective coating  256  of the same type may be formed on surfaces on the blocker plate  174 . In one embodiment, the protective coatings  250 ,  256  may be formed from nickel, aluminum oxide, Yttria based coating or the like. In one embodiment, the protective coating may be a nickel film formed by electroless plating. 
         [0038]    The front plate  162  may be formed from a material that is compatible to the processing chemistry and/or cleaning chemistry without using a protective coating. The front plate  162  may be formed from ceramic or semiconductor material without any coatings on surfaces. Particularly, no coatings are applied on surfaces defining the plurality of through holes  164 . In one embodiment, the front plate  162  may be formed from silicon to be compatible with etch/clean chemistry. For example, the processing chemistry may be a processing gas including NH 3  and NF 3  for performing etch or chamber cleaning. During processing, the processing gas including NH 3  and NF 3  may be disassociated in the hollow cathode plasma source  110  and the disassociated processing gas passes through the showerhead assembly  160  to remove native oxide from surfaces of the chamber components or from a substrate being processed. 
         [0039]    The material choice of the front plate  162  enables the through holes  164  to be small in diameter to achieve desired flow conductance. The through holes  164  of the front plate  162  may have a diameter between about 0.020 inch and about 0.040 inch or a length/diameter aspect ratio between about 21.7 and about 10.85. Prior to applying the protective coating  250 , the through holes  168  of the back plate  166  may have a diameter between about 0.089 inch and about 0.099 inch or a length/diameter aspect ratio between about 1.90 and about 2.11. Thickness of the protective coatings  250 ,  256  may determine the lifetime of the back plate  166  and the blocker plate  174 . The thicker the coating, the longer the lifetime. The showerhead assembly  160  with smaller through holes  164  in the front plate  162  allows the back plate  166  and the blocker plate  174  to have through holes components, thus thicker coatings which lead to longer lifetime. In one embodiment, the protective coatings  250 ,  256  may have a thickness between about 0.0010 inch and about 0.0012 inch. 
         [0040]    The blocker plate  174  may be attached to one or more of the plurality of posts  176  by a plurality of fasteners  202 . The blocker plate  174  may include a plurality of recess  206  for receiving the plurality of fasteners  202 . A cap  204  may cover each of the plurality of recesses  206 . In one embodiment, the protective coating  256  may be applied over the cap  204 . 
         [0041]    The posts  176  may extend from the back plate  166  through rounded corners  205  to enhance fluid flow in the gas redistributing volume  178 . In one embodiment, the posts  176  may include rounded corners  258  for to avoid plasma arcing, avoid gas/particle traps, and reduce mechanical stress. The concentration and distribution of the posts  176  may be arranged to provide uniform thermal exchange between the blocker plate  174  and the back plate  166 .  FIG. 2B  is a schematic partial top view of the back plate  166  showing the distribution of the posts  176  according to one embodiment of the present disclosure. In the embodiment where the hollow cathode plasma source  110  includes multiple cathodes, concentric walls  212  (shown in ghost lines in  FIG. 2A ) may extend from the back plate  166  to the blocker plate  174  to form concentrically isolated zones around one or more hollow cathode plasma sources  110  in the gas redistributing volume  178 . 
         [0042]      FIG. 2C  is an enlarged sectional view of the fastener  202  and cap  204 . The cap  204  may be secured to the blocker plate  174  by threads. A notch  208  may be formed on the cap  204  to enable fastening and loosening. A vent hole  210  may be formed through the cap  204  to allow equal pressure on opposite sides of the cap  204 .  FIG. 2D  is a schematic top view of the cap  204  showing the notch  208  and the vent hole  210 . 
         [0043]    The showerhead assembly of the present disclosure may be used to any suitable processes. In one embodiment, the showerhead assembly may be used to perform a dry etch process for removing silicon oxide using an ammonia (NH 3 ) and nitrogen trifluoride (NF 3 ) gas mixture. 
         [0044]    While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.