Abstract:
A reactant vapor distribution assembly for Chemical Vapor Deposition (CVD) apparatus which includes an upper flange which has a plenum disposed on its lower face and vapor injectors for injecting reactant vapors into the plenum. The distribution assembly also includes a lower flange having a peripheral rim surrounding a lower wall and a plenum on its upper face, certain of the vapor injectors are used to inject reactant vapors into this plenum. The lower flange includes fluid channels bored in the lower wall beneath the plenum and a number of gas flow openings bored in the lower wall of the lower flange to permit the precursor gases to flow from the plenum into the deposition chamber. The fluid channels may be used to heat or cool the flange. The lower flange has no welds or joints facing the hostile environment of the deposition chamber and all critical parts of the lower flange may be formed from a single billet of material.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. provisional application Ser. No. 60/920,125 filed Mar. 27, 2007. 
     
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
       [0002]    This application is directed to a showerhead assembly within the preferred embodiment of Chemical Vapor Deposition (CVD) apparatus and more specifically to an improved showerhead design allowing cooling and uniform distribution of the reactant gases in a deposition reactor. 
         [0003]    Chemical Vapor Deposition (CVD) systems are widely used to deposit elemental, alloy and compound films in the manufacture of electronic devices, such as integrated circuits formed by the sequential or simultaneous deposition of compounds upon a heated substrate, which is usually in the form of a wafer that is typically mounted on a “susceptor” which may or may not rotate. A showerhead provides distribution and passage for one or more reactant gases with the deposition chamber. The reactants are transported to the surface of the substrate, in the gas phase, by typically one or more carrier gases. The elements deposit on the wafer surface, forming the desired compound and any undesirable by-products are pumped away in a gaseous form. A heating element (filament) is mounted below the susceptor and heats the wafers. 
         [0004]    In many CVD applications, wherein films are formed at a hot surface by the thermal driven reaction of precursor vapors, the mechanism that heats the surface to drive the surface thermal driven reactions may also radiate sufficient heat to generate gas phase reactions and or heat the vapor inlet mechanism sufficiently to drive thermal reactions at the vapor inlet mechanism. Reactions at the precursor inlet mechanism, commonly called a showerhead, are generally detrimental to the process because such coatings formed by the reactions can disturb or otherwise block desired flow patterns and or such coatings may flake off generating particles and the coatings may also act as a source of an element that may not be desired in a subsequent layer of a multilayer deposition. 
         [0005]    The present invention is directed to a reactant vapor distribution assembly for Chemical Vapor Deposition (CVD) apparatus includes an upper flange which includes a plenum disposed on its lower face and vapor injectors for injecting reactant vapors into the plenum. The distribution assembly also includes a lower flange having a peripheral rim surrounding a lower wall and a plenum on its upper face, certain of the vapor injectors are used inject reactant vapors into this plenum. The lower flange includes fluid channels bored in the lower wall beneath the plenum and a number of gas flow openings drilled through the lower wall of the lower flange to permit the precursor gases to flow from the plenum. The fluid channels may be used to heat or cool the flange. The lower flange has no welds or joints facing the hostile environment of the deposition chamber and all critical parts of the lower flange may be formed from a single billet of material. 
         [0006]    This work builds upon and improves upon our prior work, wherein we disclosed aspects of integrating showerhead cooling mechanisms. Many prior showerhead designs were constructed of a multiplicity of tubes, plates and flanges which had to carefully welded together into a gastight assembly. However every weld is a potential failure point. The present approach is directed to a showerhead design wherein no process side surface is exposed to welds (eliminating the potential of thermal cycling or other stress induced leaks), further it includes a showerhead design wherein the precursor gases may be introduced separately from one another (minimizing prereactions). 
         [0007]    The showerhead also includes a mechanism wherein the precursor concentrations can be varied radially, thus improving uniformity of the deposit; as well as canceling depletion effects of consumed precursors forming in the deposit. The showerhead further provides a uniform carrier gas flow into the deposition chamber which promotes uniform laminar flow without recirculation. By the ordering and assemblage of components in the assembly the showerhead face closest to the heat source is temperature controlled by thermal regulating fluid flow. A large window for optical access to the deposition plane through the showerhead is also provided (thus allowing a multitude of deposition optical monitors and or imagers—such as temperature, deposition rate, bandgap, stress, and so on). 
         [0008]    By adding a top flange fluid channel in this arrangement, we also have the option to heat or cool the entire assembly and thereby set a temperature that eliminates any condensation and mitigates the pre-reaction issue. An additional feature is that an electrode can be inserted in the upper or lower plenums such that at either level, but separate from the process reactor, can generate reactive ionic, excited or non molecular species for subsequent flow into the deposition reactor. 
         [0009]    It should also be noted that this structure can also effectively be modified to allow some gases to heat on the way into the reactor; wherein a diffuser forms the lowest face of the showerhead so that a more contiguous gas flow is achieved across the whole surface. In this case the more thermally sensitive reactants are still distributed through the narrow holes  80  as shown in  FIG. 3   b , but other portions just through this final heated lower diffuser plate. Further, the lower diffuser plate can be made of ceramic to be less thermally conductive. Further, channels can be placed in the lower portion of the showerhead assembly that can in turn be filled with vaporizable material. In this way, additional elements or compounds, in the form of vapors, can be contributed to the growing material. It should also be noted that while the assemblies have been shown downwardly directed, but could equally be inverted for gas flow to be generally upward. 
       PUBLISHED REFERENCES 
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                 EP 0 697 749 
                 Crawley 
                 Dec. 20, 1995 
               
               
                   
                 U.S. Pat. No. 5,595,606 
                 Fujikawa 
                 Jan. 21, 1997 
               
               
                   
                 U.S. Pat. No. 5,624,498 
                 Lee 
                 Apr. 29, 1997 
               
               
                   
                 U.S. Pat. No. 6,533,867 
                 Doppelhammer 
                 Mar. 18, 2003 
               
               
                   
                 US 2006/0021574 
                 Armour 
                 Feb. 02, 2006 
               
               
                   
                 US 2007/0248515 
                 Tompa 
                 Oct. 27, 2007 
               
               
                   
                   
               
             
          
         
       
     
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates a general overview of a Chemical Vapor Deposition (CVD) System; 
           [0012]      FIG. 2   a  is an exploded view, looking downwardly, of the showerhead assembly in accordance with the present invention; 
           [0013]      FIG. 2   b  is an exploded view, looking upwardly, of the showerhead assembly in accordance with the present invention; 
           [0014]      FIG. 3   a  is a perspective view, looking downwardly, of the lower showerhead flange in accordance with the present invention; 
           [0015]      FIG. 3   b  is a perspective view, looking upwardly, of the lower showerhead flange in accordance with the present invention; 
           [0016]      FIG. 3   c  is a sectional view cut along a horizontal plane of lower wall of the lower flange of the showerhead assembly; 
           [0017]      FIG. 3   d  is a sectional view cut along a vertical plane of the lower flange of the showerhead assembly; 
           [0018]      FIG. 4  is an exploded view, looking upwardly, of the showerhead assembly in accordance with a second embodiment of the present invention; 
           [0019]      FIG. 5   a  is an exploded view, looking downwardly, of the lower flange of the showerhead assembly in accordance with a third embodiment of the present invention; and 
           [0020]      FIG. 5   b  is a sectional view cut along a vertical plane of the lower flange of the showerhead assembly of  FIG. 5   a.    
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]      FIG. 1  illustrates schematically a Chemical Vapor Deposition (CVD) System  10  which, in general overview: includes a reactor chamber  14 , sealed to the atmosphere, to which is mounted a vapor distribution housing in the form of a showerhead assembly  12  for the film growth reactant gases which is the portion of the Chemical Vapor Deposition (CVD) System  10  to which the present invention is directed. Showerhead assembly  12  (described in detail in the drawing figures and text below) directs the reactant gases over one or more substrate wafers  16 , mounted, in this example, on a rotatable susceptor  18  which is rotated through a shaft  20  by a motor  22  mounted externally from reactor chamber  14 , and which are heated by a heater unit  24 . The reactant and carrier gases generated by external sources (not shown) are distributed though the distribution housing and flow over heated wafers  16  where the gases will decompose (react at the wafer surface) and deposit their compounds, thereafter an exhaust unit  26  will remove the spent gases from reactor chamber  14 . 
         [0022]      FIG. 2   a  is an exploded view of showerhead assembly  12  looking downwardly, which includes an upper showerhead flange  30 , a lower showerhead flange  32  and a uniform gas flow diffuser  34  located therebetween. As best seen in  FIG. 2   b  located on the underside of upper showerhead flange  30  is an upper plenum  36 . A series of uniform push/carrier gas injectors  38  (within which some precursors can also be supplied) are mounted to upper showerhead flange  30  and extend to upper plenum  36  to deliver gases thereto. Other precursor injectors  40  are mounted to upper showerhead flange  30  extend through upper plenum  36  and gas flow diffuser  34  to deliver gases to lower plenum zones  42  located in lower showerhead flange  32 . Also mounted to upper showerhead flange  30  are viewports  44  which extend through upper showerhead flange  30  and are closed by gastight windows  46  to permit the operators of CVD system  10  to observe the deposition process. Gas is typically flowed over windows  46  to mitigate coating build-up on the window. The window flanges may also be water cooled to minimize effects of window material heat absorption 
         [0023]    Uniform gas flow diffuser  34  is located between and separates upper plenum  36  of upper showerhead flange  30  and lower plenum zones  40  of lower showerhead flange  32 . Gas flow diffuser  34  is constructed of a gas permeable material, such as porous stainless steel, molybdenum, other metals, or ceramics to permit gases from upper plenum  36  of upper showerhead flange  30  to diffuse into lower plenum zones  40  of lower showerhead flange  32 . The porosity of gas flow diffuser  34  is generally sized with the flow to assure that the pressure in the upper plenum is greater than that in the lower plenum. Gas flow diffuser  34  thus mitigates back flow from the lower plenum  42  to upper plenum  36 . Uniform gas flow diffuser  34  also includes openings  48  which are aligned with precursor injectors  40  to permit direct injection of precursor gases into lower plenum zones  40 . Elongated openings  50  in uniform gas flow diffuser  34  align with view ports  44  in upper showerhead flange  30  to permit unobstructed viewing of the deposition process. 
         [0024]    The design of the lowermost portion of a showerhead assembly is of critical importance to the integrity of the CVD system since it is exposed to the environment of deposition chamber  13 . In the present invention all critical components of lower showerhead flange  32  can be preferably machined from a single billet of material, such as stainless steel, without any welds being exposed to the process atmosphere, eliminating the potential of thermal cycling or other stress induced leaks. 
         [0025]      FIG. 3   a  and  3   b  are perspective views, looking downwardly and upwardly respectively, of lower showerhead flange  32  which includes a relatively thick lower wall  60  and precursor injections zones  62  formed by concentrically configured walls  64  within plenum  42  for precursor injection.  FIG. 3   c  is a sectional view cut along a horizontal plane of lower wall  60  and  FIG. 3   d  is a sectional view cut along a vertical plane of lower showerhead flange  32 . Walls  64  within plenum  42  form individual plenums (i.e. injections zones  62 ) for precursor injection from precursor injectors  40  in upper showerhead flange  30 . Injection zones  62  formed by walls  64  permit the precursor gases to be introduced separately from one another thus minimizing pre-reactions. Furthermore, radially extending walls may also be added to plenum  42  to further isolate the precursor vapors from one another. Elongated openings  66  extend through plenum  42  and lower wall  60  of lower showerhead flange  32  to align with view ports  44  in upper showerhead flange  30  to permit viewing of the deposition process. 
         [0026]    As noted above lower wall  60  of lower showerhead flange  32  is relatively thick to permit a series of fluid channels  70  to be “gun drilled” therethrough, as illustrated in  FIGS. 3   c  and  3   d  which are cross sections of lower wall  60  of lower showerhead flange  32 . Each fluid channel  70  may be formed from a first bore  72  in lower wall  60  which intersects a second bore  74  in lower wall  60  at a right angle or other suitable angle. A fluid inlet fitting  76  is joined, such as by way of example welding, to bore  72  and a fluid outlet fitting  78  is joined to bore  74 . Fittings  76  and  78  are connected to an external source of fluid such as water, or other suitable coolant (or heated) liquid. Coolant liquid flowing within channels  70  cools lower showerhead flange  32  and assures that the precursors do not decompose in flange  32 . As noted above rather than a coolant, certain processes may require that the fluid flowing through channels  70  be used to heat the showerhead assembly, the present design readily accommodates this modification. 
         [0027]    A multiplicity of gas flow openings  80  are drilled vertically through lower wall  60  of lower showerhead flange  32  to permit the precursor gases to flow form plenum  42  to the interior of CVD system  10  and thereafter to substrate wafers  16 . It is to be noted that gas flow openings  80  are positioned so that they do not intercept water channels  70  so as to maintain the water tightness of channels  70 . This can be best seen in  FIG. 3   b  wherein the outlines of channels  70  are seen in lower wall  60  of lower showerhead flange  32  without any gas flow openings  80  drilled therein. 
         [0028]    Lower showerhead flange  32  includes a circular rim  82  which includes a series of bores  84  through which rim  82  will be bolted to the upper rim of the deposition chamber of CVD reactor  10  by bolts which also serve to secure upper showerhead flange  30  to lower showerhead flange  32 . As such, fluid inlet fittings  76  and fluid outlet fittings  78  are located outside of deposition chamber  13  of CVD reactor  10 . Thus only the bottom surface of lower wall  60  of lower showerhead flange  32  faces the heated substrates and the flowing coolant assures that the precursors do not decompose in showerhead assembly  12 . The design described herein can maintain the face of the showerhead at less than 100° C. when facing a heat source ranging from room temperature to greater than 1650° C. All of the critical components of lower showerhead flange  32  are preferably machined from the same billet of material as a unitary component by standard CNC equipment which assures a gastight assembly as every weld is a potential failure point. 
         [0029]      FIG. 4  is an exploded view, looking downwardly, of a showerhead assembly  86  in accordance with a second embodiment of the present invention. In this embodiment one of the viewports in upper showerhead flange  30  has been replaced with an opening  88  to permit the insertion of one or more plasma generating electrodes to generate ionic, excited and or elemental gas phase species of the reactant vapors. As shown in the drawing a first electrode  90  has a shorter shaft which may be used to generate a plasma in plenum  36  in upper showerhead flange  30 . A second electrode  92  has a relatively longer shaft which may be used to generate a plasma in plenum  42  in lower showerhead flange  32 . Upper showerhead flange  30  may preferably include fluid channels so as to dissipate the heat caused by the generated plasma. It should be noted that upper showerhead flange  30  could include two openings  88  to permit both electrodes  90  and  92  to be used simultaneously and that an electrode can be used in conjunction with two viewports in upper showerhead flange  30 . 
         [0030]      FIG. 5   a  is an exploded view, looking downwardly, of lower flange  32  of the showerhead assembly in accordance with a third embodiment of the present invention; and  FIG. 5   b  is a sectional view cut along a vertical plane of lower flange  32 . This embodiment includes a series of semicircular, U-shaped in cross-section, troughs  96  which are configured to be positioned within injections zones  62  formed in plenum  42  in lower flange  32 . Troughs  96  are used to hold a material within the showerhead and within the gas flow and at a temperature that creates vapors at a specific vapor pressure and when flow is passed over it is used to carry the vapors into the reactor. A port, not shown, in the showerhead can be used to refill the materials. Most advantageous is a material which melts so it can more easily fill troughs  96  uniformly. 
         [0031]    Also included in this embodiment is a cover plate  98  disposed on the underside of lower flange  32 . Cover plate  98  is used to form a third and lowest level plenum. Cover plate  98  is porous so that it can also pass a flow of gas into the reactor uniformly. Further, the gas flowing through cover plate  98  can be heated as it passes through porous cover plate  98  with heat from radiation from the heated wafers. Preheating some gases over others can help enhance the reaction rate at the surface, but not so much as to create to high a rate of gas phase pre-reactions. Further, the other gases coming through gas flow openings  80  lower flange  32  remain essentially cool. The two gas flows combine to make a uniform flow down to the heated surface. 
         [0032]    The invention has been described with respect to preferred embodiments of apparatus for film deposition on a wafer surface. However, as those skilled in the art will recognize, modifications and variations in the specific details which have been described and illustrated may be resorted to without departing from the spirit and scope of the invention as defined in the appended claims