Abstract:
Showerheads for semiconductor processing equipment are disclosed that include various features designed to minimize or eliminate non-uniform gas delivery across the surface of a wafer due to gas flow transients within the showerhead.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 62/166,612, filed on May 26, 2015, and titled “ANTI-TRANSIENT SHOWERHEAD,” which is hereby incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    Semiconductor processing tools often include components designed to distribute process gases in a relatively even manner across a semiconductor substrate or wafer. Such components are commonly referred to in the industry as “showerheads.” Showerheads typically include a faceplate that fronts a semiconductor processing volume in which semiconductor substrates or wafers may be processed. The faceplate may include a plurality of gas distribution ports that allow gas in the plenum volume to flow through the faceplate and into a reaction space between the substrate and the faceplate (or between a wafer support supporting the wafer and the faceplate). In some instances, a showerhead may be configured to distribute two different gases across a semiconductor substrate or wafer in a simultaneous fashion while isolating the gases from each other within the showerhead. The gas distribution ports are typically arranged such that the gas distribution across the wafer results in substantially uniform substrate processing. 
       SUMMARY 
       [0003]    One aspect of the disclosure pertains to an apparatus having: a first gas inlet, a first surface, a plurality of first gas distribution ports, a second surface, a third surface interposed between the first surface and the second surface, a fourth surface interposed between the third surface and the second surface, and a plurality of first gas flow passages interposed between the first surface and the third surface. In such an apparatus, the first gas inlet may be configured to deliver a first process gas through the first surface and the first gas distribution ports may be configured to deliver the first process gas through the second surface. 
         [0004]    The apparatus may have a first inlet plenum volume that is fluidically connected with the first gas inlet, the first inlet plenum volume being at least partially defined by the first surface and the third surface. The apparatus may further have a first gas distribution plenum volume that is fluidically connected with the first gas distribution ports, the first gas distribution plenum volume being at least partially defined by the second surface and the fourth surface. 
         [0005]    The first gas flow passages may each have a first end that fluidically connects that first gas flow passage with the first inlet plenum volume and a second end that fluidically connects that first gas flow passage with the first gas distribution plenum volume. Each first gas flow passage may be substantially the same overall length, extend away from the first inlet plenum volume at the first end, and include between 140° and 200° of bends between the first end and the second end such that the second end of that first gas flow passage is oriented towards the first inlet plenum volume. 
         [0006]    In some embodiments, the second end of each first gas flow passage may be fluidically connected with the first gas distribution plenum volume by a corresponding first hole passing through the fourth surface; each first hole may have a nominal hole diameter. In some embodiments, a plurality of first raised bosses may extend up from the second surface towards the fourth surface, each first raised boss centered on one of the first holes and having a nominal boss diameter. In some such embodiments, each first raised boss may be offset from the fourth surface by a distance of between 0.025 mm and 1.2 mm. In other or additional such embodiments, each first raised boss may be offset from the fourth surface by a distance of between 1/11th and 1/13th of the nominal diameter. In yet other additional or alternative such embodiments, each first raised boss may be offset from the fourth surface by a distance that is less than twice the difference between the nominal boss diameter and the nominal hole diameter and is greater than 0.2 times the difference between the nominal boss diameter and the nominal hole diameter. 
         [0007]    In some embodiments, a different subset of first gas distribution ports may be adjacent to each of the first raised bosses and each first raised boss may be centered between the first gas distribution ports in the plurality of first gas distribution ports adjacent to that first raised boss. 
         [0008]    In some embodiments, a plurality of first support columns may span between the second surface and the fourth surface. 
         [0009]    In certain embodiments, the first holes may have diameters between 1.5 mm and 3 mm, and in certain alternative or additional embodiments, the first bosses may have diameters that are between 5 mm and 8 mm. 
         [0010]    In some embodiments, the apparatus may also include a plurality of first peninsulas. Each first peninsula may protrude into the first inlet plenum volume, and the second end of one or more of the first gas flow passages may extend into each of the first peninsulas. In such an embodiment, the second end of the first gas flow passages in the first peninsulas may be closer to the first center point of the first inlet plenum volume than the first ends of such first gas flow passages. 
         [0011]    In some embodiments, the first gas flow passages may include between 150° and 190° of bends between the first end and the second end. In some embodiments, each of the first gas flow passages may have a length within ±30%, ±20, ±10%, or ±5% of the other first gas flow passages. 
         [0012]    In some embodiments, each of the first gas flow passages may have a constant cross-sectional area along its length. In some embodiments, the first end of each of the first gas flow passages may be equidistant from a first axis of the apparatus. In some embodiments, the apparatus may include between 20 and 100 first gas flow passages. 
         [0013]    In some embodiments, the apparatus may also include: a second gas inlet, a fifth surface, a plurality of second gas distribution ports, a sixth surface, a seventh surface interposed between the fifth surface and the sixth surface, an eighth surface interposed between the sixth surface and the seventh surface, and a plurality of second gas flow passages interposed between the fifth surface and the seventh surface. In such embodiments, the second gas inlet may be configured to deliver a second process gas through the fifth surface and the second gas distribution ports may be configured to deliver the second process gas through the sixth surface. 
         [0014]    In some embodiments, the apparatus may have a second inlet plenum volume that is fluidically connected with the second gas inlet. The second inlet plum volume may be at least partially defined by the fifth surface and the seventh surface. The apparatus may further have a second gas distribution plenum volume that is fluidically connected with the second gas distribution ports and the second gas distribution plenum volume may be at least partially defined by the sixth surface and the eighth surface. 
         [0015]    In some embodiments, the second gas flow passages may each have a first end that fluidically connects that second gas flow passage with the second inlet plenum volume and a second end that fluidically connects that second gas flow passage with the second gas distribution plenum volume. Each second gas flow passage may be substantially the same overall length, extend away from the second inlet plenum volume at the first end, and include between 140° and 200° of bends between the first end and the second end such that the second end of that second gas flow passage is oriented towards the first inlet plenum volume. 
         [0016]    In some embodiments, the second end of each first gas flow passage of such an apparatus may be fluidically connected with the first gas distribution plenum volume by a corresponding first hole passing through the fourth surface; each first hole may have a nominal hole diameter. In some cases, a plurality of first raised bosses may extend up from the second surface towards the fourth surface, and each first raised boss may be centered on one of the first holes and may have a nominal boss diameter. In such an embodiment, the second end of each second gas flow passage may also be fluidically connected with the second gas distribution plenum volume by a corresponding second hole passing through the eighth surface; each second hole may have a nominal hole diameter. In some cases, a plurality of second raised bosses extend up from the sixth surface towards the eighth surface, where each second raised boss is centered on one of the second holes and may have a nominal boss diameter. 
         [0017]    In some cases, each first raised boss may be offset from the fourth surface and/or each second raised boss may be offset from the eighth surface by a distance of between 0.025 mm and 1.2 mm. In other or additional cases, each first raised boss may be offset from the fourth surface and/or each second raised boss may be offset from the eighth surface by a distance of between 1/11th and 1/13th of the respective nominal diameter of each raised boss. In yet other cases, each first raised boss may be offset from the fourth surface and/or each second raised boss may be offset from the eighth surface by a distance that is less than twice the difference between the nominal boss diameter and the respective nominal hole diameter and is greater than 0.2 times the difference between the nominal boss diameter and the respective nominal hole diameter. 
         [0018]    In certain embodiments, the apparatus may have one or more additional first gas inlets, and the first inlet plenum volume may be partitioned into multiple first inlet plenum sub-volumes which are each fed by a different one of the first gas inlets. 
         [0019]    In certain embodiments, the first inlet plenum volume and the first gas distribution plenum volume may be interposed between the second inlet plenum volume and the second gas distribution plenum volume. In other embodiments, the first inlet plenum volume and the second gas distribution plenum volume may be interposed between the second inlet plenum volume and the first gas distribution plenum volume. 
         [0020]    In some embodiments, a different subset of first gas distribution ports in the apparatus are adjacent to each of the first raised bosses and each first raised boss is centered between the first gas distribution ports adjacent to that first raised boss. 
         [0021]    In some embodiments, a different subset of second gas distribution ports in the modified apparatus are adjacent to each of the second raised bosses and each second raised boss is centered between the second gas distribution ports adjacent to that second raised boss. 
         [0022]    In certain embodiments, the apparatus may also contain a plurality of first support columns that span between the second surface and the fourth surface and a plurality of second support columns that span between the sixth surface and the eighth surface. 
         [0023]    In some embodiments, the apparatus may also include a plurality of first peninsulas, each first peninsula protruding into the first inlet plenum volume and the second end of one or more of the first gas flow passages extending into each of the first peninsulas. In such an embodiment, the second end of the first gas flow passages in the first peninsulas may be closer to the first center point of the first inlet plenum volume than the first ends of those first gas flow passages. 
         [0024]    In some embodiments, the apparatus may also have a plurality of second peninsulas, each second peninsula protruding into the second inlet plenum volume and the second end of one or more of the second gas flow passages extending into each of the second peninsulas. In such an embodiment, the second end of the second gas flow passages in the second peninsulas may be closer to the second center point of the second inlet plenum volume than the first ends of those second gas flow passages. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  depicts an isometric exploded view of an example anti-transient showerhead. 
           [0026]      FIG. 2  depicts a plan view of a first partition plate of the example anti-transient showerhead of  FIG. 1 . 
           [0027]      FIG. 3  depicts a plan view of a faceplate of the example anti-transient showerhead of  FIG. 1 . 
           [0028]      FIG. 4  depicts an isometric cutaway view of the example anti-transient showerhead of  FIG. 1 . 
           [0029]      FIG. 5  depicts a section view of the example anti-transient showerhead of  FIG. 1 . 
           [0030]      FIG. 6  depicts a detail view of a portion of  FIG. 5 . 
           [0031]      FIG. 7  depicts an isometric exploded view of an example anti-transient, dual-plenum showerhead. 
           [0032]      FIG. 8  depicts a plan view of a first partition plate of the example anti-transient, dual-plenum showerhead of  FIG. 7 . 
           [0033]      FIG. 9  depicts a plan view of a baffle plate of the example anti-transient, dual-plenum showerhead of  FIG. 7 . 
           [0034]      FIG. 10  depicts a plan view of a second partition plate of the example anti-transient, dual-plenum showerhead of  FIG. 7 . 
           [0035]      FIG. 11  depicts a plan view of a faceplate of the example anti-transient, dual-plenum showerhead of  FIG. 7 . 
           [0036]      FIG. 12  depicts an isometric cutaway view of the example anti-transient, dual-plenum showerhead of  FIG. 7 . 
       
    
    
       [0037]      FIGS. 1 through 12  are drawn to scale within each Figure, although the scale from Figure to Figure may vary. 
       DETAILED DESCRIPTION 
       [0038]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the presented concepts. The presented concepts may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail so as to not unnecessarily obscure the described concepts. While some concepts will be described in conjunction with the specific embodiments, it will be understood that these embodiments are not intended to be limiting. 
         [0039]    In this application, the terms “semiconductor wafer,” “wafer,” “substrate,” “wafer substrate,” and the like are used interchangeably. A wafer or substrate used in the semiconductor device industry typically has a diameter of 200 mm, 300 mm, or 450 mm, but may also be non-circular and of other dimensions. In addition to semiconductor wafers, other work pieces that may take advantage of this invention include various articles such as printed circuit boards, magnetic recording media, magnetic recording sensors, mirrors, optical elements, micro-mechanical devices and the like. 
         [0040]    Several conventions may have been adopted in some of the drawings and discussions in this disclosure. For example, reference is made at various points to “volumes,” e.g., “plenum volumes.” These volumes may be generally indicated in various Figures, but it is understood that the Figures and the accompanying numerical identifiers represent an approximation of such volumes, and that the actual volumes may extend, for example, to various solid surfaces that bound the volumes. Various smaller volumes, e.g., gas inlets or other holes leading up to a boundary surface of a plenum volume, may be fluidly connected to those plenum volumes. 
         [0041]    It is to be understood that the use of relative terms such as “above,” “on top,” “below,” “underneath,” etc. are to be understood to refer to spatial relationships of components with respect to the orientations of those components during normal use of a showerhead or with respect to the orientation of the drawings on the page. In normal use, showerheads are typically oriented so as to distribute gases downwards towards a substrate during substrate processing operations. 
         [0042]    Semiconductor fabrication often requires that process gases, such as deposition and etch gases, be flowed in a uniform or controlled manner over a semiconductor wafer or substrate undergoing processing. To that end, a “showerhead,” also referred to herein as a gas distribution manifold and sometimes also referred to as a gas distributor, may be used to distribute gases across the surface of a wafer. When gas is initially flowed into a showerhead, it may take varying amounts of time for the initial gas flow to reach each of the gas distribution ports arranged across the faceplate of the showerhead, which may result in a non-uniform gas distribution across the face of the showerhead. After the gas flow through the showerhead has stabilized, e.g., after the pressure environment within the plenum volume(s) of the showerhead has stabilized, the gas flow may be much more uniform. During the initial transient period, however, the pressure within the plenum volumes may fluctuate, and this may result in unbalanced flow characteristics across the faceplate. Due to the unpredictability of such transient flow, the transient flow period is typically “lost” time during a semiconductor process. 
         [0043]    During long-duration semiconductor processes, e.g., processes having cycle times of hundreds of seconds or longer, the transient period, which may be a few seconds, may constitute a relatively small portion of the overall cycle duration, and thus the “lost” time may constitute a relatively small fraction of the overall cycle time. In short duration semiconductor processes, however, such as atomic layer deposition (ALD), the transient period may have a much more pronounced effect. For example, in ALD, gas delivery times on the order of seconds or tenths of a second are common—if each cycle must also accommodate the time lost due to transients, then it is easy to see how transient loss may dramatically lengthen the overall process time. 
         [0044]    The anti-transient showerheads discussed herein provide a new mechanism for minimizing or reducing transient gas flow response, or even eliminating it entirely for the relevant cycle time, from semiconductor processing systems. 
         [0045]    Anti-transient showerheads, generally speaking, may be configured with at least two plenums—a gas inlet plenum and a gas distribution plenum. Each of these plenums may define a separate plenum volume. Such showerheads may also include a multitude of gas flow passages that are fluidically connected with the gas inlet plenum volume at a first end and with the gas distribution plenum volume at the second end. In many cases, a partition plate may separate the gas inlet plenum from the gas distribution plenum, and the gas flow passages may be machined into one face of the partition plate; holes located at the second end of the gas flow passages may allow gas that flows from the inlet plenum volume and into the gas flow passages to then pass through the partition plate and into the gas distribution plenum volume. The purpose of the gas flow passages is to deliver substantially equal proportions of gas from the inlet plenum volume to distributed locations with the gas distribution plenum volume. For example, the second ends of the gas flow passages may be arranged in a plurality of concentric or near-concentric, e.g., having center points within a few millimeters of each other, circular patterns so as to deliver gas into the gas distribution plenum volume at various distributed locations. Thus, some second ends may be located near the periphery of the gas distribution plenum volume, some towards the center of the gas distribution plenum volume, and some in between those two locations. 
         [0046]    Each gas flow passage may have substantially the same length, e.g., having ±5% variation in length, and may maintain a similar cross-sectional profile or area along its length, e.g., each gas flow passage may have a constant cross-sectional area along its length. Each gas flow passage may also include one or more bends that cause the gas flow passage to ultimately change direction by ±X degrees from some common angle, e.g., 170°±15° or ±20° between the first end and the second end. These bends may include, by way of example, a single bend of 170°, two bends of 100° and 70°, three bends of 50°, 40°, and 80°, etc. The number of bends in each gas flow passage may be the same, or may vary from passage to passage—regardless of how many bends are in each passage, however, the total bend angle for each passage may be within the limits stated above. It is to be understood that the “total bend angle” is the total of the absolute values of the bend angles for a given gas flow passage. Thus, if a gas flow passage undergoes a bend of 90° to the left and then 90° to the right, the total bend angle would be 180°, not 0°. By including the same nominal total bend angle, cross-sectional area profile, and passage length in each gas flow passage, the gas flow passages may be caused to exhibit substantially similar flow resistance, which may cause gas that is flowed through the gas flow passages to flow at the same rates through all of the passages, even during transient flow. In some implementations, the total bend angle may be between, but not limited to, 140° to 200° degrees, i.e., more relaxed or more bent than the 170°±15° discussed above. 
         [0047]    Further performance increases may be obtained by including a plurality of raised bosses that protrude up from the faceplate towards the holes that deliver the gas from the gas flow passages to the gas distribution plenum volume. Each of these raised bosses may be centered underneath a corresponding one of the holes such that gas that exits the hole impinges on the center of the raised boss, causing the gas to undergo a change of flow direction of approximately 90°, e.g., the gas flow changes from flowing along the hole axis to flowing in a direction generally parallel to the faceplate. The raised boss thus acts as a “mini-baffle” that serves to further distribute the gas in a more even manner throughout the gas distribution plenum volume. 
         [0048]      FIG. 1  depicts an isometric exploded view of an example anti-transient showerhead. As can be seen, an anti-transient showerhead  100  is shown. The showerhead  100  includes a stem  180  that may be used to deliver a first process gas to the showerhead  100 ; the stem may provide gas to a first gas inlet  112  (alternatively, the stem  180  also may be thought of as the first gas inlet  112 ). The stem  180  may connect with a backplate  102 , e.g., through a brazed, diffusion bonded, welded, or bolted connection. The backplate  102  may, in turn, be mated with a first partition plate  108 . The first partition plate  108  may include various features that are machined or otherwise formed into it that define a first inlet plenum volume  142  and a plurality of first gas flow passages  138 . The first gas flow passages  138  may have first ends that are fluidically connected with the first inlet plenum volume  142  and that are arranged along an outer perimeter of the first inlet plenum volume  142 ; the first gas flow passages  138  may generally radiate outwards from this outer perimeter of the first inlet plenum volume  142  before substantially reversing their direction by virtue of the above-mentioned bends in each first gas flow passage  138 . 
         [0049]    The showerhead  100  may also include a faceplate  104  that includes a plurality of first gas distribution ports  134  arranged in a pattern across the faceplate  104 . The faceplate  104  may be mated to the first partition plate  108  such that a first gas distribution plenum volume  146  is formed. The first gas distribution plenum volume  146  may be fluidically connected with the first inlet plenum volume  142  by the plurality of first gas flow passages  138 . 
         [0050]    Generally speaking, the first inlet plenum volume  142  and the first gas distribution plenum volume  146  may be bounded, at least in part, various major surfaces. For example, the backplate  102  may provide a first surface  116  through which process gas may be introduced from the first gas inlet  112  and into the first inlet plenum volume  142 ; the first surface  116  may thus act as one boundary for the first inlet plenum volume  142 . Similarly, the faceplate  104  may provide a second surface  118  through which the process gas may be flowed from the first gas distribution plenum volume  146  by way of the first gas distribution ports  134 ; the second surface  118  may thus act as one boundary for the first gas distribution plenum volume  146 . Similarly, the first partition plate  108  may have a third surface  120  and a fourth surface  122 , which may serve as further boundaries for the first inlet plenum volume  142  and the first gas distribution plenum volume  146 , respectively. 
         [0051]    It is to be understood that these surfaces need not necessarily be provided by the exact components depicted. In fact, in some implementations, there may not even be discrete faceplates, backplates, or partition plates. For example, the showerhead  100  may be manufactured as a monolithic structure, e.g., by using additive manufacturing techniques such as direct laser metal sintering or, if a ceramic showerhead is desired, a ceramic sintering process. In implementations where multiple plate structures are used, such as in the depicted example, it may be desirable to include an indexing pin  106  or other similar feature to ensure that the various plates are lined up correctly. It is to be understood that if a multiple-plate structure is used, the various plates that form the overall showerhead structure may be brazed or diffusion bonded together along their mating surfaces to prevent gas flow between the contacting surfaces of the plates. 
         [0052]    Also visible in  FIG. 1  are a plurality of first raised bosses  160 , each of which is positioned beneath the second end of the one of the first gas flow passages  138 . In addition to the first raised bosses, a number of first support columns  164  may be optionally included. Unlike the first raised bosses  160 , which do not contact the fourth surface  122 , the first support columns  164  may provide structural support and a thermally conductive path to the faceplate  104 , and thus may extend to and contact the fourth surface  122  (and may be brazed or diffusion bonded to it for structural support). 
         [0053]    From a practical perspective, it may be desirable to include a large number of gas flow passages in an anti-transient showerhead. However, as the number of gas flow passages included in an anti-transient showerhead increases, the size of the corresponding inlet plenum volume must also increase to accommodate the increased number of junctions between each gas flow passage and the inlet plenum volume along the perimeter of the inlet plenum volume. At some point, as the number of gas flow passages is increased, the size of the inlet plenum volume may expand to a large enough extent that it may be desirable to place some of the holes that feed gas from the gas flow passages to the gas distribution plenum volume within the perimeter of the gas inlet plenum volume. In order to do so while maintaining fluidic isolation between each gas flow passage, a number of peninsulas may be included. Each peninsula may protrude into the inlet plenum volume from the nominal outermost perimeter of the inlet plenum volume. Each peninsula may include one or more gas flow passages that may be used to deliver gas to such locations. 
         [0054]      FIG. 2  depicts a plan view of the first partition plate  108  of the example anti-transient showerhead of  FIG. 1 . As can be seen in more detail in this Figure, each first gas flow passage  138  has a bend  154  that occurs at some point along the passage&#39;s length. Moreover, each first gas flow passage  138  has substantially the same length as the other first gas flow passages  138 , in this case, ˜170 mm±5%. For purposes of illustration, the first gas flow passages  138  in this example are approximately 2 mm wide. Each first gas flow passage  138  is fluidically connected with the first inlet plenum volume  142  at a first end  150  and is fluidically connected to the first gas distribution plenum volume  146  at a second end  152  by way of a first hole  156 . As can be seen, six of the first holes  156  are located within the outermost circumference of the first inlet plenum volume  142 ; the first gas flow passages  138  that provide gas to these first holes  156  are partitioned off from the first inlet plenum volume  142  by peninsulas  168 , which extend into the first inlet plenum volume  142 . As can be seen in the showerhead  100 , there are sixty first gas flow passages  138  radiating out from the first inlet plenum volume  142 . Such a high number of first gas flow passages  138  would not be able to fit (at the indicated passage width) along the outer perimeter of the first inlet plenum volume  142  if the first inlet plenum volume was sized so as to be within the innermost pattern of first holes  156  (the first holes  156  that are located within the peninsulas  168 ). 
         [0055]      FIG. 3  depicts a plan view of the faceplate  104  of the example anti-transient showerhead of  FIG. 1 . As can be seen, the first raised bosses  160  are arranged in a relatively distributed manner throughout the first gas distribution plenum volume  146   
         [0056]      FIG. 4  depicts an isometric cutaway view of the example anti-transient showerhead of  FIG. 1 . As can be seen, each first hole  156  is positioned directly above a corresponding first raised boss  160 . Gas that flows into the first inlet plenum volume  142  through the first gas inlet  112  may generally reach the first ends  150  of all of the first gas flow passages  138  at the same time and may, by virtue of the first gas flow passages  138  being nominally the same length and having nominally the same total bend angle and cross-sections along their length, reach the second ends  152  of the first gas flow passages  138  at generally the same time. This has the result of introducing the gas into the first gas distribution plenum volume at a plurality of points, e.g., each first hole  156 , more or less simultaneously. The first raised bosses  160  may the act to further distribute the gas throughout the first gas distribution plenum volume  146  such that the gas flows through the first gas distribution ports  134  in a generally uniform manner, even when the gas flow within the showerhead  100  has not yet reached steady state. 
         [0057]      FIG. 5  depicts a section view of the example anti-transient showerhead of  FIG. 1 . The various plates, e.g., the faceplate  104 , the backplate  102 , and the first partition plate  108 , are depicted, as well as the first surface  116 , the second surface  118 , the third surface  120 , and the fourth surface  122 .  FIG. 5  also indicates a circled area that is show in more detail in  FIG. 6 . 
         [0058]      FIG. 6  depicts a detail view of a portion of  FIG. 5 . This detail view shows the second end  152  of one of the first gas flow passages  138 , as well as the first hole  156  that fluidically connects the first gas flow passage  138  to the first gas distribution plenum volume  146 . Also depicted in  FIG. 6  is a first support column  164 , which spans between the second surface  118  and the fourth surface  122 . Two first raised bosses  160  are also visible, including one that is directly beneath the first hole  156 . As is evident, the first raised boss  160  below the first hole  156  is centered underneath the first hole  156 . Moreover, a first gap  176  exists between the first raised boss  160  and the fourth surface  122 . In the depicted example, the first hole  156  has a diameter of 2 mm, the first raised boss  160  a diameter of 6.5 mm, and the first gap  176  is 0.5 mm. In several implementations, the first gap  176  may be a function of the nominal diameter of the first raised boss  160  and may, in some such implementations, range from 1/11th of the raised boss nominal diameter to 1/13th of the raised boss nominal diameter, e.g., approximately 1/12th of the raised boss diameter. In other implementations, the first gap  176  may be a function of the nominal diameter of the first raised boss  160  and the diameter of the first hole  156 , e.g., the first gap  176  may be selected such that the first gap  176  divided by twice the difference between the first raised boss  160  diameter and the first hole  156  diameter is between 0.1 and 1. 
         [0059]    The previous example was directed at a showerhead  100  that only supports flow of a single process gas. As discussed, the concepts discussed herein may be applied to multi-flow or multi-plenum showerheads as well. This concept is discussed in more detail below with respect to a showerhead configured to flow two process gases simultaneously. Many of the structures in this dual-flow example correspond with structures discussed previously with respect to the single-flow showerhead  100 . To avoid prolixity, these components may not be described again below; in such cases, the previous discussion of similar structures in the showerhead  100  may be referred to for a description. Components that are similar between the showerhead  100  and the dual-flow showerhead discussed below may share the last two digits of their drawing reference numbers in common. 
         [0060]      FIG. 7  depicts an isometric exploded view of an example anti-transient, dual-plenum showerhead. As can be seen, a stem  780  is provided that allows for two separate gases to be supplied to a showerhead  700 ; the stem may be connected to a backplate  702 . The stem  780  may include two sets of passages, one that includes a passage that runs along the center of the stem  780 , and the other that includes a circular array of passages that is interposed between that center passage and an outer sleeve (the lower portion of the stem  780  shown). In this example, the circular array of gas flow passages in the stem provides gas for a first gas inlet  712  (six holes arrayed about the center hole of the backplate  702 ), and the center gas flow passage provides gas for the second gas inlet  714  (the center hole in the backplate  702 ). The showerhead  700  may also include a first partition plate  708 , a second partition plate  710 , a baffle plate  778 , and a faceplate  704 . 
         [0061]      FIG. 8  depicts a plan view of the first partition plate of the example anti-transient, dual-plenum showerhead of  FIG. 7 . The first partition plate  708  is very similar to the first partition plate  108 , but with at least two differences. For example, there may be a raised center boss  782  that is located at the center of the first inlet plenum volume  742 ; this raised center boss  782  may serve to decrease the size of the first inlet plenum volume  742  to decrease the amount of time it takes to flow the first process gas through the first inlet plenum volume, and may also, in some implementations, help even out the gas flow from the six ports that serve as the first gas inlet  712 . In some implementations, the first inlet plenum volume may be partitioned into multiple first inlet plenum sub-volumes, each fed by a different one of the first gas inlets. The other difference is that there are a number of second holes  758  through the first partition plate  708 . 
         [0062]      FIG. 9  depicts a plan view of the baffle plate of the example anti-transient, dual-plenum showerhead of  FIG. 7 . In this implementation, the baffle plate  778  is very similar to the faceplate  104 , except that the first support columns  764  are more numerous and are arranged differently from the first support columns  164 . As can be seen from  FIG. 7 , each first support column  764  corresponds in location to one of the second holes  758  in the first partition plate  708 , and the corresponding second hole  758  continues through the first support column  764 . Thus, the first support columns  764  not only provide structural support and a thermally conductive pathway between the first partition plate  708  and the baffle plate  778 , but also provide an avenue for gas flow through the baffle plate that keeps such gas isolated from the gas that is within the first gas distribution plenum volume  746 . As with the faceplate  104 , the baffle plate  778  may include a plurality of first raised bosses  760  that are each positioned beneath a first hole  756  (shown later) that fluidically connects one of the first gas flow passages  738  with the first gas distribution plenum volume  746 . As with the faceplate  104 , the baffle plate  778  may include a plurality of first gas distribution ports  734  that may supply gas from the first gas distribution plenum volume  746  to either the second gas distribution plenum volume immediately beneath the first gas distribution plenum or to a wafer processing area beneath the showerhead  700 . 
         [0063]      FIG. 10  depicts a plan view of the second partition plate of the example anti-transient, dual-plenum showerhead of  FIG. 7 . The second partition plate  710  may serve a function similar to the first partition plate  708 , but with respect to the second gas inlet  714 . As can be seen, a plurality of substantially equal-length second gas flow passages  740  fluidly connects a second inlet plenum volume  744  with a second gas distribution plenum volume  748  (see  FIG. 11 ) via second holes  758 ; the second gas distribution plenum volume  748 , in this example, is a plenum volume formed between the baffle plate  778  and the faceplate  704 . The second gas flow passages  740 , in this case, and as with the first gas flow passages  738 , may have first ends  750  that connect with the second inlet plenum volume  744  and second ends  752  that are fluidically connected with the second holes  758 . As can be seen, each second gas flow passage  740  may also include a bend  754  which may be similar to the bends  754  in the first gas flow passages  738 , although, as can be seen, the bend angles may be more relaxed. Similar to the first peninsulas  768  in the first inlet plenum volume  742 , the second inlet plenum volume  744  may also include a plurality of second peninsulas  770  that allow some of the second holes  758  to be located within the outer periphery of the second inlet plenum volume  744 . 
         [0064]      FIG. 11  depicts a plan view of the faceplate of the example anti-transient, dual-plenum showerhead of  FIG. 7 . The faceplate  704  may include a plurality of second gas distribution ports  736  and a plurality of first gas distribution ports  734 . The second gas distribution plenum volume  748  may be formed between the faceplate  704  and the baffle plate  77 , and may include a pattern of second raised bosses  762  that serve as mini-baffles for the second holes  758 , much as the first raised bosses  760  do for the first holes  756 . 
         [0065]    In some implementations, each first gas distribution port  734  in the baffle plate  778  may be fluidically connected to the corresponding first gas distribution port  734  in the faceplate  704  by a tubular structure  784  that isolates the gas flowing through the first gas distribution ports  734  from the gas flowing through the second gas distribution plenum volume  748  within the showerhead  700 . 
         [0066]      FIG. 12  depicts an isometric cutaway view of the example anti-transient, dual-plenum showerhead of  FIG. 7 , and may provide more insight into the structure of the showerhead  700 . 
         [0067]    Similar to the first inlet plenum volume  742  and the first gas distribution plenum volume  746 , the second inlet plenum volume  744  and the second gas distribution plenum volume  748  may also be bounded by various surfaces. These surfaces are indicated in  FIG. 7 . For example, the second inlet plenum volume  744  may be bounded, in part, by a fifth surface  724  and a seventh surface  728 , whereas the second gas distribution plenum volume  748  may be bounded, in part, by a sixth surface  726  and an eighth surface  730 . 
         [0068]    In multi-plenum showerheads, the positioning of the inlet plenums with respect to the gas distribution plenums may be re-ordered as needed for any particular design—they need not be in the arrangement depicted. For example, in the depicted implementations, the first inlet plenum volume and the first gas distribution plenum volume are bracketed between the second inlet plenum volume and the second gas distribution plenum volume. In other implementations, however, this ordering may be altered. By way of non-limiting example, any of the following orders may also be used in various implementations of this concept: 
         [0000]                                        Implementation   Implementation   Implementation   Implementation       1   2   3   4                   First inlet   Second inlet   First inlet   Second inlet       plenum volume   plenum volume   plenum volume   plenum volume       Second inlet   First inlet   Second inlet   First inlet       plenum volume   plenum volume   plenum volume   plenum volume       Second gas   Second gas   Second gas   First gas       distribution   distribution   distribution   distribution       plenum volume   plenum volume   plenum volume   plenum volume       First gas   First gas   First gas   Second gas       distribution   distribution   distribution   distribution       plenum volume   plenum volume   plenum volume   plenum volume                    
In such cases, plenum volumes for a first gas that have plenum volumes for a second gas interposed between them may be fluidically connected by causing the holes, e.g., the first holes, that fluidically connect the plenums for the first gas to pass between the gas flow passages for the second gas or through support columns within the plenum(s) for the second gas.