Patent Publication Number: US-2023140263-A1

Title: Showerheads with high solidity plenums

Description:
FIELD 
     The present disclosure relates generally to substrate processing systems and more particularly to showerheads with high solidity plenums. 
     BACKGROUND 
     The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named Applicants, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     A substrate processing system typically comprises a plurality of stations (also called processing chambers or process modules) in which to perform deposition, etching, and other treatments on substrates such as semiconductor wafers. Examples of processes that may be performed on a substrate include a chemical vapor deposition (CVD) process, a chemically enhanced plasma vapor deposition (CEPVD) process, a plasma enhanced chemical vapor deposition (PECVD) process, a sputtering physical vapor deposition (PVD) process, atomic layer deposition (ALD), and plasma enhanced ALD (PEALD). Additional examples of processes that may be performed on a substrate include, but are not limited to, etching (e.g., chemical etching, plasma etching, reactive ion etching, atomic layer etching (ALE), plasma enhanced ALE (PEALE), etc.) and cleaning processes. 
     During processing, a substrate is arranged on a substrate support such as a pedestal in a station. During deposition, gas mixtures including one or more precursors are introduced into the station, and plasma may be optionally struck to activate chemical reactions. During etching, gas mixtures including etch gases are introduced into the station, and plasma may be optionally struck to activate chemical reactions. A computer-controlled robot typically transfers substrates from one station to another in a sequence in which the substrates are to be processed. 
     Atomic Layer Deposition (ALD) is a thin-film deposition method that sequentially performs a gaseous chemical process to deposit a thin film on a surface of a material (e.g., a surface of a substrate such as a semiconductor wafer). Most ALD reactions use at least two chemicals called precursors (reactants) that react with the surface of the material one precursor at a time in a sequential, self-limiting manner. Through repeated exposure to separate precursors, a thin film is gradually deposited on the surface of the material. Thermal ALD (T-ALD) is carried out in a heated processing chamber. The processing chamber can be maintained at a sub-atmospheric pressure using a vacuum pump and a controlled flow of an inert gas. The substrate to be coated with an ALD film is placed in the processing chamber and is allowed to equilibrate with the temperature of the processing chamber before starting the ALD process. Atomic layer etching comprises a sequence alternating between self-limiting chemical modification steps that affect only top atomic layers of a substrate and etching steps that remove only the chemically-modified areas from the substrate. The sequence allows removal of individual atomic layers from the substrate. 
     SUMMARY 
     A showerhead comprises a base portion and a backplate. The backplate has a different shape than the base portion and extends from the base portion. The showerhead comprises a plurality of pillars is arranged in a plenum defined between an upper region of the base portion and a lower region of the backplate within sidewalls of the base portion and the lower region of the backplate. The pillars extend vertically between the base portion and the lower region of the backplate. 
     In additional features, the base portion is cylindrical. The backplate comprises a cylindrical base and a conical portion. The cylindrical base is attached to the base portion. The conical portion extends from the cylindrical base. 
     In additional features, the backplate comprises a recess in a bottom region abutting the base portion. The base portion comprises the pillars that extend through the recess and contact the cylindrical base. 
     In additional features, the base portion comprises a recess in the upper region abutting the cylindrical base. The cylindrical base comprises the pillars that extend through the recess and contact the base portion. 
     In additional features, the showerhead further comprises a stem portion attached to the conical portion of the backplate. The stem portion comprises a gas inlet. The conical portion comprises a plurality of bores in fluid communication with the gas inlet. The bores extend towards the base portion and connecting to the plenum. 
     In additional features, the showerhead further comprises a stem portion attached to the conical portion of the backplate. The backplate comprises a plurality of bores extending from the stem portion towards the base portion for receiving a plurality of heaters, respectively. 
     In additional features, the showerhead further comprises a stem portion attached to the conical portion of the backplate. The backplate comprises one or more bores extending from the stem portion towards the base portion for receiving one or more temperature sensors, respectively. 
     In additional features, the showerhead further comprises a stem portion attached to the conical portion of the backplate. The stem portion comprises a gas inlet. The backplate comprises a first plurality of bores in fluid communication with the gas inlet. The first plurality of bores extends towards the base portion and connecting to the plenum. The backplate comprises a second plurality of bores extending from the stem portion towards the base portion for receiving a plurality of heaters, respectively. The backplate comprises one or more bores extending from the stem portion towards the base portion for receiving one or more temperature sensors, respectively. The first and second plurality of bores and the one or more bores are interstitial to each additional. The base portion comprises a plurality of through holes extending vertically from a bottom surface of the base portion to the plenum. The through holes are arranged interstitially with the pillars. 
     In additional features, the pillars are arranged in a first pattern. Each of the pillars is surrounded by a set of the through holes arranged in a second pattern. 
     In additional features, the first and second patterns are hexagonal. 
     In additional features, the base portion comprises a plurality of through holes extending vertically from a bottom surface of the base portion to the plenum. The through holes are arranged interstitially with the pillars. 
     In additional features, diameters of the base portion and the cylindrical base are equal. 
     In still other features, a showerhead comprises a base portion and a backplate. The backplate has a different shape than the base portion and extends from the base portion. The backplate and the base portion are monolithic. The showerhead comprises a plurality of pillars is arranged in a plenum defined within sidewalls of the base portion. The pillars extend vertically towards the backplate. 
     In additional features, the base portion is cylindrical. The backplate comprises a conical portion extending from the base portion. The conical portion and the base portion are monolithic. 
     In additional features, the base portion comprises a plurality of sets of bores extending across the base portion. The sets of bores intersect each additional. Intersections of the sets of bores define the pillars. 
     In additional features, the sets of bores have first openings on the sidewalls of the base portion. The showerhead further comprises a stem portion extending from the conical portion. The stem portion comprises a gas inlet. The conical portion comprises a plurality of bores in fluid communication with the gas inlet. The plurality of bores extend towards the base portion and have second openings on the sidewalls of the base portion above the first openings. The showerhead further comprises an annular sealing member is attached to the base portion below the first openings and to the conical portion above the second openings defining an annular volume in fluid communication with the plenum. The base portion comprises a plurality of through holes extending vertically from a bottom surface of the base portion to the plenum. The through holes are arranged interstitially with the pillars. 
     In additional features, the showerhead further comprises a stem portion extending from the conical portion. The conical portion comprises a plurality bores extending from the stem portion towards the base portion for receiving a plurality of heaters, respectively. 
     In additional features, the showerhead further comprises a stem portion extending from the conical portion. The conical portion comprises one or more bores extending from the stem portion towards the base portion for receiving one or more temperature sensors, respectively. 
     In additional features, the showerhead further comprises a stem portion extending from the conical portion. The stem portion comprises a gas inlet. The conical portion comprises a first plurality of bores in fluid communication with the gas inlet. The first plurality of bores extends towards the base portion and connects to the plenum. The conical portion comprises a second plurality of bores extending from the stem portion towards the base portion for receiving a plurality of heaters, respectively. The conical portion comprises one or more bores extending from the stem portion towards the base portion for receiving one or more temperature sensors, respectively. The first and second plurality of bores and the one or more bores are interstitial to each additional. The base portion comprises a plurality of through holes extending vertically from a bottom surface of the base portion to the plenum. The through holes are arranged interstitially with the pillars. 
     In additional features, the sets of bores have first openings on the sidewalls of the base portion, and the plurality of bores have second openings on the sidewalls of the base portion above the first openings. The showerhead further comprises an annular sealing member attached to the base portion below the first openings and to the conical portion above the second openings defining an annular volume in fluid communication with the plenum. The base portion comprises a plurality of through holes extending vertically from a bottom surface of the base portion to the plenum. The through holes are arranged interstitially with the pillars. 
     In additional features, the conical portion comprises a second plurality of bores extending from the stem portion towards the base portion for receiving a plurality of heaters, respectively. The conical portion comprises one or more bores extending from the stem portion towards the base portion for receiving one or more temperature sensors, respectively. The plurality of bores, the second plurality of bores, and the one or more bores are interstitial to each additional. The base portion comprises a plurality of through holes extending vertically from a bottom surface of the base portion to the plenum. The through holes are arranged interstitially with the pillars. 
     In additional features, the base portion comprises a plurality of through holes extending vertically from a bottom surface of the base portion to the plenum. The through holes are arranged interstitially with the pillars. 
     In additional features, the pillars are arranged in a first pattern. Each of the pillars is surrounded by a set of the through holes arranged in a second pattern. 
     In additional features, the first and second patterns are square patterns. 
     In additional features, a first set of the through holes is arranged in a first pattern in a first region of the base portion. A second set of the through holes is arranged in a second pattern in a second region of the base portion. 
     In additional features, the first and second regions are concentric. 
     In still other features, a showerhead comprises a base portion and a backplate having a different shape than the base portion extending from the base portion. The backplate and the base portion are monolithic. The showerhead comprises a first plurality of pillars arranged in a first plenum defined within sidewalls of the base portion. The first plurality of pillars extends vertically towards the backplate. The showerhead comprises a second plurality of pillars arranged in a second plenum defined within the sidewalls of the base portion above the first plenum. The second plurality of pillars extends vertically towards the backplate. 
     In additional features, the base portion is cylindrical. The backplate comprises a conical portion extending from the base portion. The conical portion and the base portion are monolithic. 
     In additional features, the second plurality of pillars are interstitial to the first plurality of pillars. 
     In additional features, the first and second plenums are disjoint. 
     In additional features, the base portion comprises first sets of bores extending across the base portion. The first sets of bores intersect each additional defining the first plurality of pillars at first intersections of the first sets of bores. The base portion comprises second sets of bores extending across the base portion above the first sets of bores. The second sets of bores intersect each additional defining the second plurality of pillars at second intersections of the second sets of bores. 
     In additional features, the first sets of bores have first openings on the sidewalls of the base portion. The second sets of bores have second openings on the sidewalls of the base portion above the first openings. The showerhead further comprises a stem portion extending from the conical portion. The stem portion comprises a first gas inlet and a second gas inlet. The conical portion comprises a first bore in fluid communication with the second gas inlet. The first bore extends from the stem portion through the conical portion into the second plenum. The conical portion comprises a second bore in fluid communication with the first gas inlet. The second bore extends from the stem portion into the conical portion. The conical portion comprises a plurality of bores extending from a distal end of the second bore towards the base portion and having third openings on the sidewalls of the base portion. The third openings are above the first and second openings. The showerhead further comprises a first annular sealing member attached to the base portion below the first and second openings and to the conical portion above the third openings defining an annular volume in fluid communication with the first plenum. The showerhead further comprises a second annular sealing member attached to the sidewalls of the base portion closing the first and second openings and separating the second plenum from the first plenum. The base portion comprises a first plurality of through holes extending vertically from a bottom surface of the base portion to the first plenum. The first plurality of through holes is arranged interstitially with the first plurality of pillars. The base portion comprises a second plurality of through holes extending vertically from the bottom surface of the base portion through the first plurality of pillars to the second plenum. The second plurality of through holes is arranged interstitially with the second plurality of pillars. 
     In additional features, the showerhead further comprises a stem portion extending from the conical portion. The conical portion comprises a plurality of bores extending from the stem portion towards the base portion for receiving a plurality of heaters, respectively. 
     In additional features, the showerhead further comprises a stem portion extending from the conical portion. The conical portion comprises one or more bores extending from the stem portion towards the base portion for receiving one or more temperature sensors, respectively. 
     In additional features, the conical portion comprises a second plurality of bores extending from the stem portion towards the base portion for receiving a plurality of heaters, respectively. The conical portion comprises one or more bores extending from the stem portion towards the base portion for receiving one or more temperature sensors, respectively. The plurality of bores, the second plurality of bores, and the one or more bores are interstitial to each additional. The base portion comprises a first plurality of through holes extending vertically from a bottom surface of the base portion to the first plenum. The first plurality of through holes is arranged interstitially with the first plurality of pillars. The base portion comprises a second plurality of through holes extending vertically from the bottom surface of the base portion through the first plurality of pillars to the second plenum. The second plurality of through holes is arranged interstitially with the second plurality of pillars. 
     In additional features, the base portion comprises a first plurality of through holes extending vertically from a bottom surface of the base portion to the first plenum. The first plurality of through holes is arranged interstitially with the first plurality of pillars. The base portion comprises a second plurality of through holes extending vertically from the bottom surface of the base portion through the first plurality of pillars to the second plenum. The second plurality of through holes is arranged interstitially with the second plurality of pillars. 
     In additional features, the first plurality of pillars is arranged in a first pattern. Each of the first plurality of pillars is surrounded by the first plurality of through holes arranged in a second pattern. 
     In additional features, the first and second patterns are square patterns. 
     In additional features, the second plurality of pillars and the second plurality of through holes are arranged in a square pattern. 
     In additional features, a first set of the second plurality of through holes is arranged in a first pattern in a first region of the base portion. A second set of the second plurality of through holes is arranged in a second pattern in a second region of the base portion. 
     In additional features, the first and second regions are concentric. 
     In additional features, the first and second regions lie in different quadrants. 
     In additional features, the quadrants are adjacent. 
     In additional features, the quadrants are diagonally opposite to each additional. 
     In still other features, a showerhead comprises a base portion and a backplate having a different shape than the base portion extending from the base portion. The showerhead comprises a first plurality of pillars arranged in a first plenum defined between the base portion and the backplate within sidewalls of the base portion and the backplate. The first plurality of pillars extends vertically between the base portion and the backplate. The showerhead comprises a second plurality of pillars arranged in a second plenum defined above the first plenum within the sidewalls of the base portion and the backplate. The second plurality of pillars extends vertically towards the backplate. 
     In additional features, the base portion is cylindrical. The backplate comprises a cylindrical base and a conical portion. The cylindrical base is attached to the base portion. The conical portion extends from the cylindrical base. The first plenum is defined between an upper region of the base portion and a lower region of the cylindrical base within the sidewalls of the base portion and the cylindrical base. The first plurality of pillars extends vertically between the base portion and the cylindrical base. The second plenum is defined within the sidewalls of the base portion and the cylindrical base. The second plurality of pillars extend vertically towards the conical portion. 
     In additional features, the second plurality of pillars are interstitial to the first plurality of pillars. 
     In additional features, the first and second plenums are disjoint. 
     In additional features, the showerhead further comprises a metal plate sealingly attached to the upper region of the base portion and a bottom region of the cylindrical base. The metal plate separates the second plenum from the first plenum. The first and second plurality of pillars respectively contact bottom and upper surfaces of the metal plate. 
     In additional features, the base portion comprises a first recess in the upper region abutting the cylindrical base and comprises the first plurality of pillars that extend through the first recess towards the cylindrical base. The cylindrical base comprises a second recess in bottom region abutting the base portion and comprises the second plurality of pillars that extend through the second recess towards the base portion. The showerhead further comprises a metal plate sealingly attached to the upper region of the base portion and the bottom region of the cylindrical base. The metal plate contacts the first and second plurality of pillars. 
     In additional features, the base portion comprises a first recess in the upper region abutting the cylindrical base and comprises the first plurality of pillars that extend through the first recess towards the cylindrical base. The cylindrical base comprises a second recess in bottom region abutting the base portion. The showerhead further comprises a metal plate. The metal plate comprises the second plurality of pillars arranged on an upper surface of the metal plate. The metal plate sealingly attached to the upper region of the base portion and the bottom region of the cylindrical base. A bottom surface of the metal plate contacts the first plurality of pillars. The second plurality of pillars extends through the second recess and contacting the cylindrical base. 
     In additional features, the base portion comprises a first recess in the upper region abutting the cylindrical base. The cylindrical base comprises a second recess in bottom region abutting the base portion. The showerhead further comprises a metal plate sealingly attached to the upper region of the base portion and the bottom region of the cylindrical base. The metal plate comprises the first and second plurality of pillars respectively arranged on bottom and top surfaces of the metal plate. The first plurality of pillars extends through the first recess towards the base portion and contacts the base portion. The second plurality of pillars extends through the second recess towards the cylindrical base and contacts the cylindrical base. 
     In additional features, the showerhead further comprises a stem portion attached to the conical portion of the backplate. The stem portion comprises a first gas inlet and a second gas inlet. The backplate comprises a first bore in fluid communication with the second gas inlet, the first bore extending from the stem portion through the conical portion into the second plenum. The backplate comprises a second bore in fluid communication with the first gas inlet, the second bore extending from the stem portion into the conical portion. The backplate comprises a plurality of bores extending from a distal end of the second bore towards the base portion and connecting to the first plenum. The base portion comprises a first plurality of through holes extending vertically from a bottom surface of the base portion to the first plenum. The first plurality of through holes is arranged interstitially with the first plurality of pillars. The base portion comprises a second plurality of through holes extending vertically from the bottom surface of the base portion through the first plurality of pillars to the second plenum. The second plurality of through holes is arranged interstitially with the second plurality of pillars. 
     In additional features, the showerhead further comprises a stem portion attached to the conical portion of the backplate. The backplate comprises a plurality of bores extending from the stem portion towards the base portion for receiving a plurality of heaters, respectively. 
     In additional features, the showerhead further comprises a stem portion attached to the conical portion of the backplate. The backplate comprises one or more bores extending from the stem portion towards the base portion for receiving one or more temperature sensors, respectively. 
     In additional features, the backplate comprises a second plurality of bores extending from the stem portion towards the base portion for receiving a plurality of heaters, respectively. The backplate comprises one or more bores extending from the stem portion towards the base portion for receiving one or more temperature sensors, respectively. The plurality of bores, the second plurality of bores, and the one or more bores are interstitial to each additional. The base portion comprises a first plurality of through holes extending vertically from a bottom surface of the base portion to the first plenum. The first plurality of through holes is arranged interstitially with the first plurality of pillars. The base portion comprises a second plurality of through holes extending vertically from the bottom surface of the base portion through the first plurality of pillars to the second plenum. The second plurality of through holes is arranged interstitially with the second plurality of pillars. 
     In additional features, the base portion comprises a first plurality of through holes extending vertically from a bottom surface of the base portion to the first plenum. The first plurality of through holes is arranged interstitially with the first plurality of pillars. The base portion comprises a second plurality of through holes extending vertically from the bottom surface of the base portion through the first plurality of pillars to the second plenum. The second plurality of through holes is arranged interstitially with the second plurality of pillars. 
     In additional features, the first plurality of pillars is arranged in a first pattern. Each of the first plurality of pillars is surrounded by the first plurality of through holes arranged in a second pattern. 
     In additional features, the first and second patterns are hexagonal. 
     In additional features, the second plurality of pillars and the second plurality of through holes are arranged in a hexagonal pattern. 
     In additional features, the base portion comprises a first plurality of through holes extending vertically from a bottom surface of the base portion to the first plenum. The first plurality of through holes is arranged interstitially with the first plurality of pillars. The base portion comprises a second plurality of through holes extending vertically from the bottom surface of the base portion through the first plurality of pillars and the metal plate to the second plenum. The second plurality of through holes is arranged interstitially with the second plurality of pillars. 
     In additional features, diameters of the base portion and the cylindrical base are equal. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG.  1 A  shows an example of a substrate processing system that uses a single plenum showerhead in a processing chamber; 
         FIG.  1 B  shows an example of a substrate processing system that uses a dual plenum showerhead in a processing chamber; 
         FIG.  2    shows a side view of a first single plenum showerhead used in the processing chamber of  FIG.  1 A ; 
         FIG.  3    shows a top view of the first showerhead of  FIG.  2   ; 
         FIG.  4    shows a cross-sectional view of a base portion of the first showerhead of  FIG.  2    showing the plenum of the first showerhead of  FIG.  2   ; 
         FIG.  5    shows an example of a pattern of pillars and through holes in the plenum of the first showerhead of  FIG.  2   ; 
         FIG.  6    shows a cross-sectional view of the first showerhead of  FIG.  2    showing heater bores of the first showerhead of  FIG.  2   ; 
         FIG.  7 A  shows a cross-sectional view of the first showerhead of  FIG.  2    showing bores for flowing process gases through the plenum of the first showerhead of  FIG.  2   ; 
         FIG.  7 B  shows a side view of the base portion and cylindrical base of the backplate of the first showerhead of  FIG.  2    showing an alternate way to form the plenum of the first showerhead of  FIG.  2   ; 
         FIG.  8    shows a cross-sectional view of the first showerhead of  FIG.  2    showing a bore for a temperature sensor used in the first showerhead of  FIG.  2   ; 
         FIG.  9    shows a side view of a second single plenum showerhead used in the processing chamber of  FIG.  1 A ; 
         FIG.  10    shows a top view of the second showerhead of  FIG.  9   ; 
         FIG.  11    shows a cross-sectional view of the second showerhead of  FIG.  9    showing the plenum and bores for flowing process gases through the second showerhead of  FIG.  9   ; 
         FIG.  12    shows a cross-sectional view of the second showerhead of  FIG.  9    showing a bore for a temperature sensor used in the second showerhead of  FIG.  9   ; 
         FIG.  13    shows a cross-sectional view of the second showerhead of  FIG.  9    showing heater bores of the second showerhead of  FIG.  9   ; 
         FIG.  14 A  shows a cross-sectional view of a base portion of the second showerhead of  FIG.  9    showing the plenum of the second showerhead of  FIG.  9   ; 
         FIG.  14 B  shows an example of a pattern of pillars and through holes in the plenum of the second showerhead of  FIG.  9   ; 
         FIG.  15    shows a cross-sectional view of the base portion of the second showerhead of  FIG.  9    showing a section of top of the plenum of the second showerhead of  FIG.  9   ; 
         FIG.  16    shows a bottom view of the second showerhead of  FIG.  9    showing an example of a pattern of through holes of the second showerhead of  FIG.  9   ; 
         FIGS.  17  and  18    shows additional examples of patterns in which the through holes of the second showerhead of  FIG.  9    can be arranged; 
         FIG.  19    shows a top view of a ring attached to the cylindrical base and the base portion of the second showerhead of  FIG.  9   ; 
         FIG.  20    shows a cross-sectional view of the ring attached to the cylindrical base and the base portion of the second showerhead of  FIG.  9   ; 
         FIG.  21    shows a cross-sectional view of the second showerhead of  FIG.  9    similar to  FIG.  11    showing the bores for flowing process gases and the ring of  FIG.  20   ; 
         FIG.  22    shows a side view of a third showerhead, which is a dual plenum showerhead, used in the processing chamber of  FIG.  1 B ; 
         FIG.  23    shows a top view of the third showerhead of  FIG.  22   ; 
         FIG.  24 A  shows a cross-sectional view of the third showerhead of  FIG.  22    showing the dual plenums and bores for flowing process gases through the third showerhead of  FIG.  22   ; 
         FIG.  24 B  shows a side view of a base portion of the third showerhead of  FIG.  22    showing the dual plenums, specifically a second plenum stacked on a first plenum in the base portion; 
         FIG.  24 C  shows the side view of the base portion of the third showerhead of  FIG.  22    showing a ring around the second plenum; 
         FIG.  24 D  shows a cross-sectional view of the base portion of the third showerhead of  FIG.  22    showing the second plenum; 
         FIG.  24 E  shows an example of a pattern of pillars and through holes in the second plenum of the third showerhead of  FIG.  22   ; 
         FIG.  25    shows a cross-sectional view of the third showerhead of  FIG.  22    showing a bore for a temperature sensor used in the third showerhead of  FIG.  22   ; 
         FIG.  26    shows a cross-sectional view of the third showerhead of  FIG.  22    showing heater bores of the third showerhead of  FIG.  22   ; 
         FIG.  27 A  shows a cross-sectional view of the base portion of the third showerhead of  FIG.  22    showing a first example of the first plenum of the third showerhead of  FIG.  22   ; 
         FIGS.  27 B and  27 C  show an example of a pattern of pillars and through holes in the first example of first plenum of the third showerhead of  FIG.  22   ; 
         FIG.  28 A  shows a cross-sectional view of the base portion of the third showerhead of  FIG.  22    showing a second example of the first plenum of the third showerhead of  FIG.  22   ; 
         FIGS.  28 B and  28 C  show an example of a pattern of pillars and through holes in the second example of first plenum of the third showerhead of  FIG.  22   ; 
         FIG.  29    shows a top cross-sectional view of an example of a layout of the through holes of the second plenum relative to the pillars of the first plenum; 
         FIGS.  30 - 37    show various examples of patterns in which the through holes of the second plenum can be arranged; 
         FIGS.  38  and  39    show examples of patterns in which the through holes of the first plenum can be arranged; 
         FIG.  40    shows a cross-sectional view of the third showerhead of  FIG.  9    similar to  FIG.  24 A  showing the bores for flowing process gases and a ring around the first plenum similar to that shown in  FIG.  21   ; 
         FIG.  41    shows a side view of a fourth dual plenum showerhead, which is a dual plenum showerhead, used in the processing chamber of  FIG.  1 B ; 
         FIG.  42    shows a top view of the fourth showerhead of  FIG.  41   ; 
         FIG.  43 A  shows a cross-sectional view of the fourth showerhead of  FIG.  41    showing the dual plenums and bores for flowing process gases through the fourth showerhead of  FIG.  41   ; 
         FIG.  43 B  shows a side view of a cylindrical base of a backplate of the fourth showerhead of  FIG.  41    showing a first example of forming the second plenum in the cylindrical base of the backplate of the fourth showerhead of  FIG.  41   ; 
         FIG.  43 C  shows a side view of the cylindrical base of the backplate of the fourth showerhead of  FIG.  41    showing a second example of forming the second plenum in the cylindrical base of the backplate of the fourth showerhead of  FIG.  41   ; 
         FIG.  43 D  shows a side view of the base portion and cylindrical base of the backplate of the fourth showerhead of  FIG.  41    showing an alternate way to form the dual plenums of the fourth showerhead of  FIG.  41   ; 
         FIG.  44    shows a cross-sectional view of a base portion of the fourth showerhead of  FIG.  41    showing the first plenum of the fourth showerhead of  FIG.  41   ; 
         FIG.  45    shows an example of a pattern of pillars and through holes in the first plenum of the fourth showerhead of  FIG.  41   ; 
         FIG.  46    shows a cross-sectional view of the second plenum of the fourth showerhead of  FIG.  41    showing a layout of pillars and through holes of the second plenum; 
         FIG.  47    shows an example of a pattern of the pillars and the through holes in the second plenum of the fourth showerhead of  FIG.  41   ; 
         FIG.  48    shows a cross-sectional view of the fourth showerhead of  FIG.  41    showing heater bores of the fourth showerhead of  FIG.  41   ; and 
         FIG.  49    shows a cross-sectional view of the fourth showerhead of  FIG.  41    showing a bore for a temperature sensor used in the fourth showerhead of  FIG.  41   . 
     
    
    
     In the drawings, reference numbers may be reused to identify similar and/or identical elements. 
     DETAILED DESCRIPTION 
     Demand for improving film properties and deposition rates in plasma enhanced chemical vapor deposition (PECVD) and atomic layer deposition (ALD) equipment is increasing. The increasing demand is driving requirements for higher radio frequency (RF) power and faceplate temperatures for showerheads. The higher RF power and faceplate temperatures for the showerheads create two interrelated problems. First, a higher heat flow through the showerhead increases temperature gradients within the showerhead. The temperature gradients increase thermomechanical stresses in the showerhead due to differential thermal expansion of the showerhead. Second, the higher showerhead faceplate temperatures reduce yield strength and viscoelastic creep modulus of structural materials of the showerhead. Thermo-elastic stresses cause higher plastic strains and higher creep rates in the showerhead. In combination, these effects lead to progressive deformation of the showerhead with each thermal cycle. The deformation locally alters a process gap (i.e., a gap between the faceplate and the substrate), which perturbs the deposition process. 
     Both of these effects are amplified by the typical construction of a PECVD/ALD showerhead. In the typical showerhead, a disc-shaped faceplate is welded along its rim to a disc-shaped backplate above the disc-shaped faceplate. A cylindrical gas plenum is enclosed within the faceplate and the backplate. The gas plenum distributes process gases to an array of gas orifices in the faceplate. A heat sink is connected to an upper central (stem) region of the backplate. The gas plenum has negligible thermal conductivity. Therefore, the heat incident on the faceplate due to thermal radiation and/or interaction with an RF-generated plasma must flow radially outward within the faceplate. Then the heat must flow radially inward within the backplate to reach the heat sink. These radial heat flows create large and opposing radial temperature gradients in the faceplate and the backplate. The temperature gradients generate high thermomechanical stresses that may exceed a local yield strength. As a result, when used for a high-rate deposition of advanced hard mask films, these showerheads undergo rapid progressive plastic deformation. The deformation leads to process failures. The deformation also shortens the life of these showerheads. 
     The present disclosure provides various showerhead designs that alleviate the above problems. Specifically, the showerheads of the present disclosure include a high-solidity plenum. The high-solidity plenum is a region between the faceplate and the backplate. The region combines high gas conductance in horizontal (i.e., radial) directions with high heat conduction in the vertical (i.e., axial) direction through a dense array of vertical pillars. The array of pillars solves the problem of temperature gradients in two ways. First, a high heat conduction in the vertical direction through the pillars causes the radial temperature gradients above and below the plenum to be closely similar rather than opposing. The closely similar radial temperature gradients largely eliminates the source of deformational thermomechanical stresses. Second, any remaining deformational loads are distributed across the many pillars. The distribution of the deformational loads further reduces the stresses. In addition, each of the showerheads of the present disclosure comprisesa conical backplate made of solid metal. The stem of the showerhead is connected to the apex of the conical backplate. The conical backplate further helps in conducting heat from the faceplate to the heat sink located at the stem of the showerhead. The following is a brief summary of various showerhead designs according to the present disclosure. The showerhead designs are described in detail with references to  FIGS.  2 - 49   . 
     In an example, shown and described in detail with reference to  FIGS.  2 - 8   , the showerhead is manufactured from a stack of two or more metal plates joined together by diffusion welding. The pillars are defined by removing material from (e.g., by machining) one or both plates bounding a plenum volume. The pillars are arranged so as to not interfere with gas orifices in the lowermost plate. For example, the pillars are arranged in a pattern that is interstitial to a gas orifice pattern. Alternatively, the plates are joined by diffusion brazing or another furnace brazing process rather than by diffusion welding. A showerhead manufactured in this manner may include more than one such plenum. For example, a stack of three plates can be used to bound two plenums arranged one above the other. An upper plenum distributes gas to channels passing vertically through the pillars in a lower plenum to gas orifices arranged interstitially to those fed by the lower plenum. An example of such as dual plenum showerhead is shown and described with reference to  FIGS.  41 - 49   . The same principle can be used to provide three or more plenums arranged vertically. Any plenum bounded by two such plates may be subdivided by vertical walls into multiple coplanar plenums, for example, in concentric radial zones, azimuthal zones, or a combination of these. In some examples, one or more of the plates can be formed to net shape by processes such as forging, die casting, upsetting, thixo-molding, or metal injection molding. In other examples, one or more of the plates can be manufactured by an additive manufacturing process such as powder bed fusion, transfer welding, direct energy deposition, or electron-beam freeform fabrication. 
     In another example, shown and described in detail with reference to  FIGS.  9 - 21   , the plenum comprises intersecting linear arrays of bores lying in a horizontal plane (i.e., parallel to the faceplate). The pillars comprise the material remaining between bores, which are machined cylindrical features having a large length-to-diameter ratio such as in deep/gun drillings. The bores may lie along two orthogonal directions, two non-orthogonal directions, or three directions approximately 120° apart. The gas orifices are arrayed so as to intersect the axes of these bores. Each gas orifice may intersect a single bore, or lie within an intersection of two or three bores. A showerhead manufactured as described above may comprise more than one such intersecting-bore plenum. For example, two networks of bores may be arranged one above the other. An upper network of bores distributes gas to channels passing vertically through the pillars in a lower plenum to gas orifices. The gas orifices are arranged interstitially to those fed by a lower network of bores. An example of such as dual plenum showerhead is shown and described with reference to  FIGS.  22 - 40   . The same principle can be used to provide three or more plenums arranged vertically. Alternatively, the two or more networks may feed orifices in different concentric radial zones, azimuthal zones, or a combination of these. 
     The fabrication processes used to manufacture an intersecting-bore plenum, such as deep-hole drilling cause the bores to extend to the edges of the workpiece. The resulting open ends of the bores can be enclosed to prevent process gases from escaping. For example, a ring can be attached to the circumference of the workpiece to form a gas-tight (i.e., sealing) boundary. Such a ring may be fabricated using any of several processes including milling, lathe turning, forging, stamping, drawing, spinning, die casting, thixo-molding, metal injection molding, powder bed fusion, transfer welding, or electron-beam freeform fabrication. The ring may be attached to the circumference of the workpiece using a process such as welding, brazing, threading, upsetting, swaging, interference fitting, or shrink fitting. Alternatively, each open bore end may be closed by an individual plug. Such plugs may be fabricated by any of the previously mentioned fabrication processes, for example by drawing or by turning on a screw machine. The plugs can be attached using any of the previously mentioned attachment processes, for example by threading, upsetting, interference fitting, or shrink fitting. 
     The showerheads designed according to the present disclosure provide the following advantages over prior art showerheads. The showerheads have a lower rate of progressive thermomechanical deformation, which translates into a longer lifetime before process failure can occur. The showerheads can tolerate a higher faceplate heat flux or temperature, which allows higher deposition rates, higher wafer throughput, and/or provide better properties to the deposited film. The showerheads have a smaller radial temperature gradient in the faceplate, which reduces the radial non-uniformity of properties in the deposited film. The radial temperature gradient is edge-hot (rather than center-hot), which reduces the radial non-uniformity of properties in the deposited film. The reduction occurs because the edge-hot radial temperature gradient helps to offset the undesirably higher radiative heat loss from the near-edge region of the wafer to sidewalls of the processing chamber. These and other features of the showerheads are described below in detail. 
     The present disclosure is organized as follows. Before describing the showerhead designs, examples of substrate processing systems in which the showerheads can be used are shown and described with reference to  FIGS.  1 A and  1 B . Thereafter the first, second, third, and fourth showerheads are shown and described with reference to  FIGS.  2 - 8   ,  FIGS.  9 - 21   ,  FIGS.  22 - 40   , and  FIGS.  41 - 49   , respectively. 
     Examples of Substrate Processing Systems 
       FIG.  1 A  shows an example of a substrate processing system  100  comprising a processing chamber  102  configured to process a substrate using processes such as PECVD or thermal ALD (T-ALD). The processing chamber  102  encloses other components of the substrate processing system  100 . The processing chamber  102  comprises a substrate support (e.g., a pedestal)  104 . During processing, a substrate  106  is arranged on the pedestal  104 . 
     One or more heaters  108  (e.g., a heater array) may be disposed in a ceramic plate arranged on a metallic baseplate of the pedestal  104  to heat the substrate  106  during processing. One or more additional heaters called zone heaters or primary heaters (not shown) may be arranged in the ceramic plate above or below the heaters  108 . Additionally, while not shown, a cooling system comprising cooling channels through which a coolant can flow to cool the pedestal  104  may be disposed in the baseplate of the pedestal  104 . Additionally, while not shown, one or more temperature sensors may be disposed in the pedestal  104  to sense the temperature of the pedestal  104 . 
     The processing chamber  102  comprises a gas distribution device  110  such as a showerhead to introduce and distribute process gases into the processing chamber  102 . The gas distribution device (hereinafter showerhead)  110  is made of a metal such as aluminum or an alloy and is a chandelier style showerhead. The showerhead  110  can include any of the showerheads shown in  FIGS.  2 - 21    and is described in further detail with reference to  FIGS.  2 - 21   . 
     Briefly, the showerhead  110  comprises a base portion  114  and a backplate  115 . The base portion  114  is cylindrical. A substrate-facing surface of the base portion  114  is called a faceplate (shown in subsequent figures). The faceplate comprises a plurality of outlets or features (e.g., slots or through holes collectively called orifices) through which precursors flow into the processing chamber  102 . The backplate  115  is conical, made of solid metal, and extends upwards from the base portion  114 . The backplate  115  comprises heaters, temperature sensors, and bores for supplying process gases, all of which are shown in subsequent figures. 
     The showerhead  110  further comprises a cylindrical stem portion  112 . A first end of the stem portion  112  is connected to an apex of the conus of the backplate  115 . A second end of the stem portion  112  is connected to a top plate of the processing chamber  102 . A heat sink  113  is connected to the second end of the stem portion  112 . For example, the heat sink  113  may include cooling channels through which a coolant (e.g., water) is circulated. The backplate  115  conducts heat from the base portion  114 . The heat sink  113  removes heat from the backplate  115 . 
     If plasma is used, the substrate processing system  100  may include an RF generating system (or an RF source)  120  that generates and outputs an RF voltage. The RF voltage may be applied to the showerhead  110 , and the pedestal  104  can be DC grounded, AC grounded, or floating as shown. Alternatively, while not shown, the RF voltage can be applied to the pedestal  104 , and the showerhead  110  may be DC grounded, AC grounded, or floating. For example, the RF generating system  120  may include an RF generator  122  that generates RF power. The RF power is fed by a matching and distribution network  124  to the showerhead  110  or the pedestal  104 . In other examples, while not shown, the plasma may be generated inductively or remotely and then supplied to the processing chamber  102 . 
     A gas delivery system  130  comprises gas sources  132 - 1 ,  132 - 2 , ..., and  132 -N (collectively, the gas sources  132 ), where N is a positive integer. The gas delivery system  130  comprises valves  134 - 1 ,  134 - 2 , ..., and  134 -N (collectively, the valves  134 ). The gas delivery system  130  comprises mass flow controllers  136 - 1 ,  136 - 2 , ..., and  136 -N (collectively, the mass flow controllers  136 ). The gas sources  132  are connected by the valves  134  and the mass flow controllers  136  to a manifold  138 . In some processes, a vapor delivery system  137  supplies vaporized precursors to the manifold  138 . An output of the manifold  138  is fed to the showerhead  110 . The gas sources  132  may supply process gases, cleaning gases, purge gases, inert gases, and so on to the processing chamber  102 . 
     A fluid delivery system  140  supplies a coolant to the cooling system in the pedestal  104  and to the heat sink  113  of the showerhead  110 . A temperature controller  150  may be connected to the heaters  108 , the zone heaters, the cooling system, and the temperature sensors in the pedestal  104 . The temperature controller  150  may also be connected to the heat sink  113  and to the heaters and the temperature sensors in the showerhead  110 . The temperature controller  150  may control power supplied to the heaters  108 , the zone heaters, and coolant flow through the cooling system in the pedestal  104  to control the temperature of the pedestal  104  and the substrate  106 . The temperature controller  150  may also control power supplied to the heaters disposed in the showerhead  110  and coolant flow through the heat sink  113  of the showerhead  110  to control the temperature of the showerhead  110 . 
     A vacuum pump  158  can maintain sub-atmospheric pressure inside the processing chamber  102  during substrate processing. A valve  156  is connected to an exhaust port of the processing chamber  102 . The valve  156  and the vacuum pump  158  are used to control pressure in the processing chamber  102 . The valve  156  and the vacuum pump  158  are used to evacuate reactants from the processing chamber  102  via the valve  156 . A system controller  160  controls the components of the substrate processing system  100 . 
       FIG.  1 B  shows an example of a substrate processing system  101  that is identical to the substrate processing system  100  except for the following differences. The substrate processing system  101  comprises a dual plenum showerhead  111 . The dual plenum showerhead  111  is also a chandelier type showerhead. The dual plenum showerhead  111  can include any of the showerheads shown in  FIGS.  22 - 49   . The dual plenum showerhead  111  is described in further detail with reference to  FIGS.  22 - 49   . One set of the gas sources  132 , valves  134 , and MFCs  136  supplies a second gas to a second plenum of the dual plenum showerhead  111 . The description of other elements of the substrate processing system  101  having identical reference numbers as those shown in  FIG.  1 A  is not repeated for brevity. 
     The showerheads according to the present disclosure are now described in detail. Initially, two types of showerheads are disclosed: First, a showerhead having two distinct (i.e., separate) metallic elements - a faceplate and a backplate (shown in  FIGS.  2 - 8   ); and second, a monolithic showerhead with the faceplate and the backplate manufactured from a single piece of metal (shown in  FIGS.  9 - 21   ). Subsequently, two dual plenum showerheads are shown and described with reference to  FIGS.  22 - 49   . 
     Each showerhead is now described with reference to respective sets of figures, which show various views of the respective showerheads. During the description of each showerhead, while each view is described with reference to a particular figure, in the description of each figure, other figures from the set of figures for that showerhead and other showerheads are referenced as needed to aid the discussion. This is because an element being described with reference to a figure may be better seen in another referenced figure. 
     Each showerhead described below can be made of a solid metallic material (i.e., the showerheads are not hollow except for the bores and plenums described below). While the base portions of the showerheads are shown and described as being cylindrical in shape, the base portions can be in the form of a conical frustum instead, with the backplate extending from the top of the conical frustum. Accordingly, the base portions can be considered as being substantially cylindrical in shape. Similarly, the cylindrical bases of the backplates are shown and described as being cylindrical in shape. However, the cylindrical bases of the backplates can also be in the form of a conical frustum instead, with the conical portion of the backplate extending from the top of the conical frustum. Accordingly, the cylindrical bases of the backplates can also be considered as being substantially cylindrical in shape. Further, the backplates of the showerheads are shown and described as being conical in shape. However, the conical shape of the backplates can include compound curvature. Accordingly, the backplates can be considered as being substantially conical in shape. 
     First Showerhead (Single Plenum, Non-Monolithic) 
       FIGS.  2 - 8    show various views of a first showerhead  200 .  FIG.  2    shows a side view of the showerhead  200 .  FIG.  3    shows a top view of the showerhead  200 .  FIGS.  4 - 8    show various cross-sectional views of the showerhead  200 . Each cross-sectional view shows different features of the showerhead  200 . 
     In  FIG.  2   , the showerhead  200  comprises a base portion  202 , a backplate  204 , and a stem portion  206 . The base portion  202  is cylindrical. The base portion  202  is described in further detail with reference to  FIGS.  4  and  5   . The backplate  204  has a cylindrical base  207  that is attached (i.e., joined) to the base portion  202  using one or more manufacturing processes described above. 
     The backplate  204  has a conical portion  209 . The conical portion  209  extends upwards from the cylindrical base  207 . The conical portion  209  attaches to the stem portion  206 . The backplate  204  is a separate element than the base portion  202 . The cylindrical base  207  and the conical portion  209  of the backplate  204  are integral (i.e., the backplate  204  is a single piece). The backplate  204  and the stem portion  206  can be separate pieces joined together or can also be integral (i.e., a single piece). 
     The backplate  204  is solid (i.e., is not hollow). The backplate  204  helps conduct heat from the base portion  202  to the heat sink  113  shown in  FIG.  1 A . The conical portion  209  slants relative to the cylindrical base  207  at an angle α. The angle α determines the volume of the conical portion  209 . The angle α determines the heat conduction through the backplate  204 . The angle α determines the weight of the showerhead  200 . Greater volume of the conical portion  209  is desirable to conduct more heat. However, the angle α is selected so as to balance the heat conduction through the backplate  204  and the weight of the showerhead  200 . For example, the angle α is about 45° but can be between 10° and 60°. The backplate  204  is described in further detail with reference to  FIGS.  6 - 9   . 
     A gas inlet  208  is provided in the center of the stem portion  206 . The gas inlet  208  receives process gases from the gas delivery system  130  shown in  FIG.  1 A . The internal structure of the showerhead  200  including a plenum and bores for heaters, gas supply, and temperature sensors is shown and described in detail with reference to  FIGS.  4 - 9   . 
       FIG.  3    shows a top view of the showerhead  200 . The stem portion  206  comprises bores  210 - 1 ,  210 - 2 ,  210 - 3 , and  210 - 4  (collectively, the bores  210 ). The bores  210  receive fasteners (not shown) used to attach the showerhead  200  to the top plate of the processing chamber  102  shown in  FIG.  1 A . The stem portion  206  comprises bores  212 - 1  and  212 - 2  (collectively, the bores  212 ). Heaters are disposed through the bores  212  into the backplate  204  as shown in  FIG.  6   . The stem portion  206  comprises a bore  214   through which a temperature sensor (e.g., a thermocouple) is disposed into the backplate  204  as shown in  FIG.  8   . 
     Various cross-sections of the showerhead  200  are identified in  FIGS.  2  and  3   . These cross-sections are shown in  FIGS.  4 - 9    and are used to describe the internal structure of the showerhead  200  including the plenum and the bores for heaters, gas supply, and temperature sensors in further detail. 
       FIG.  4    shows a top view of a cross-section of the base portion  202  of the showerhead  200  taken along lines A-A shown in  FIG.  2   . The cross-section A-A shows a plenum  224  formed in the base portion  202  in detail. Another example of the plenum  224  is shown and described with reference to  FIG.  7 B . 
     In  FIG.  4   , the base portion  202  comprises a plurality of pillars  220 - 1 ,  220 - 2 ,  220 - 3 , ..., and  220 -N (collectively, the pillars  220 ), where N is a positive integer. The pillars  220  are formed by removing (e.g., machining) material from a top surface  205  of the base portion  202 , which abuts a bottom surface  211  of the cylindrical base  207  of the backplate  204 . The material is removed from the top surface  205  of the base portion  202  from the center of the base portion  202  to an inner diameter (ID) of a rim  203  of the base portion  202 . The pillars  220  are solid (i.e., are not hollow). The pillars  220  extend vertically upwards towards the bottom surface  211  of the cylindrical base  207  of the backplate  204 . The pillars  220  contact the bottom surface  211  of the cylindrical base  207  of the backplate  204 . 
     The pillars  220  extend along an axis perpendicular to a plane in which the base portion  202  lies (hereinafter called the vertical axis of the showerhead  200 ). The vertical axis is perpendicular to the diameter of the base portion  202 . The plane in which the base portion  202  lies is parallel to a plane in which the substrate  106  and a top surface of the pedestal  104  on which the substrate  106  is arranged. Accordingly, the vertical axis is also perpendicular to the plane of the substrate  106  and the top surface of the pedestal  104 . 
     The pillars  220  are shown as being circular in shape for example only. Alternatively, the pillars  220  can be of any other polygonal or non-polygonal shape. Further, all of the pillars  220  need not have the same shape and/or size. The pillars  220  can have different shapes. For example, some of the pillars  220  can be circular while others can be hexagonal. The pillars  220  can have different diameters. 
     The pillars  220  are distributed from the center of the base portion  202  to the ID of the rim  203  of the base portion  202 . The pillars  220  lie in a plane parallel to the diameter of the base portion  202  and perpendicular to the vertical axis of the shower head  200 . The pillars  220  are distributed along first and second axes  221  and  223 . The first and second axes  221  and  223  are perpendicular to each other. The first and second axes  221  and  223  are parallel to the diameter of the base portion  202 . The pillars  220  conduct heat from a bottom surface  213  of the base portion  202  to the bottom surface  211  of the cylindrical base  207  of the backplate  204 . The backplate  204  conducts the heat from the base portion  202  to the heat sink  113  shown in  FIG.  1 A . The pillars  220  conduct the heat along the vertical axis, which reduces radial temperature gradients across the base portion  202  and the backplate  204 . 
     The top surface  205  of the base portion  202  is attached (i.e., joined) to the bottom surface  211  of the cylindrical base  207  of the backplate  204  at the peripheries of the base portion  202  and the backplate  204 . Specifically, the rim  203  of the base portion  202  is attached (i.e., joined) to the rim of the bottom surface  211  of the cylindrical base  207  of the backplate  204 . The top surface  205  of the base portion  202 , the pillars  220 , and the bottom surface  211  of the cylindrical base  207  of the backplate  204  define the plenum  224  of the showerhead  200 . The plenum  224  is cylindrical. The plenum  224  extends from the center of the base portion to the ID of the rim  203  of the base portion  202 . The plenum  224  has the same diameter as the ID of the rim  203  of the base portion  202 . 
     The pillars  220  extend vertically upwards from the base portion  202  through the plenum  224 . The pillars  220  contact the bottom surface  211  of the cylindrical base  207  of the backplate  204 . The pillars  220  reduce the volume (i.e., cavity or hollowness) of the plenum  224 . In other words, the pillars  220  provide solidity to the plenum  224 . For example, the pillars  220  fill about 10% of the volume of the plenum  224 . The amount of heat conducted by the pillars  220  from the bottom surface  213  of the base portion  202  to the bottom surface  211  of the backplate  204  is directly proportional to a density of the pillars  220 . The density of the pillars  220  is the number of pillars  220  per unit area of the base portion  202  in the plenum  224 . In other words, the amount of heat conducted by the pillars  220  is directly proportional to the number of pillars  220 . The density of the pillars  220  is specified by requirements of processes performed on the substrate  106 . 
     The base portion  202  comprises through holes (also called orifices)  222 - 1 ,  222 - 2 ,  222 - 3 , ..., and  222 -M (collectively, through holes  222 ), where M is an integer greater than N. The through holes  222  are drilled between the top surface  205  and the bottom surface  213  of the base portion  202  facing the substrate  106  shown in  FIG.  1 A . The through holes  222  are distributed around the pillars  220  from the center of the base portion  202  to the ID of the rim  203  of the base portion  202 . The process gases received through the gas inlet  208  flow through a plurality of bores (shown in  FIG.  7 A ) drilled in the backplate  204 . The process gases flow through the plurality of bores into the plenum  224 . The process gases exit the plenum  224  via the through holes  222  into the processing chamber  102  towards the substrate  106  shown in  FIG.  1 A . 
       FIG.  5    shows an example of a pattern in which the pillars  220  and the through holes  222  are arranged in the base portion  202 . The pillars  220  are arranged interstitially relative to the through holes  222 . The through holes  222  are arranged around the pillars  220 . For example, the pillars  220  are arranged on vertices of a first hexagon  230 . One pillar  220  lies at the center of the hexagon  230 . For example, the through holes  222  are arranged around each pillar  220  on vertices of a second hexagon  232 . The pattern of the pillars  220  and the through holes  222  extends from the center of the base portion  202  to the ID of the rim  203  of the base portion  202 . 
     The pillars  220  and the through holes  222  can be arranged in other symmetric or asymmetric patterns. The patterns may depend on the requirements of processes performed on the substrate  106 . The patterns are designed while maintaining a specified density of the pillars  220  in the plenum  224 . The density of the pillars  220  in the plenum  224  determines the solidity of the plenum  224 . The solidity of the plenum  224  determines the heat conduction through the base plate  202  to the backplate  204 . The density of the pillars  220  is constrained by the requirements of the through holes  222  specified for the processes. 
       FIG.  6    shows a cross-section of the showerhead  200  taken along lines B-B shown in  FIG.  3   . The cross-section B-B shows a bore  250  that extends from the gas inlet  208  vertically downwards towards the base portion  202  through the stem portion  206 . The bore  250  extends into the conical portion  209  of the backplate  204 . The bore  250  extends through the center of the stem portion  206  and through the center of the conical portion  209  of the backplate  204  along the vertical axis of the showerhead  200 . A distal end  251  of the bore  250  extends approximately half-way through the conical portion  209  of the backplate  204  as shown. Alternatively, the distal end  251  of the bore  250  can extend further up or down in the conical portion  209  of the backplate  204 . From the distal end  251  of the bore  250 , a plurality of bores (shown in  FIG.  7 A ) extend laterally outwards and downwards through the conical portion  209  of the backplate  204 . These bores connect to the plenum  224  in the base portion  202  at  252 - 1  and  252 - 2 . These bores supply process gases from the gas inlet  208  to the plenum  224  as shown and described with reference to  FIG.  7 A . 
     The cross-section B-B shows the bores  212  for the heaters in detail. The bores  212  extend vertically downwards through the stem portion  206  and the conical portion  209  of the backplate  204 . The bores  212  extend towards the base portion  202  along the vertical axis of the showerhead  200 . The bores  212  extend into the cylindrical base  207  of the backplate  204  but do not extend to the bottom surface  211  of the cylindrical base  207 . While only two bores  212  are shown, additional bores  212  for additional heaters can be similarly arranged. As explained with reference to  FIGS.  7  and  8   , the bores  212  are arranged so as to not interfere with the plurality of bores (shown in  FIG.  7 A ) that supply process gases from the gas inlet  208  to the plenum  224 . The bores  212  also do not interfere with the bore  214  for the temperature sensor (shown in  FIG.  8   ). 
       FIG.  7 A  shows a cross-section of the showerhead  200  taken along lines C-C shown in  FIG.  3   . The cross-section C-C shows a plurality of bores  254 - 1 ,  254 - 2  (hereinafter the bores  254 ) that extend laterally outwards and downwards towards the base portion  202 . The bores  254  extend through the conical portion  209  of the backplate  204  from the distal end  251  of the bore  250 . The bores  254  descend from the distal end  251  of the bore  250  at an acute angle relative to the vertical axis of the showerhead  200 . The bores  254  can be but need not be parallel to the walls of the conical portion  209  of the backplate  204 . When the bores  254  are parallel to the walls of the conical portion  209 , the angle between each of the bores  254  and the plane (or diameter) of the base portion  202  is α. The bores  254  extend down to the bottom surface  211  of the cylindrical base  207 . When the base portion  202  with the plenum  224  is attached to the cylindrical base  207 , the bores  254  connect to the plenum  224  in the base portion  202 . The bores  254  connect to the plenum  224  near the ID of the rim  203  at  252 - 1  and  252 - 2 . The bores  254  are in fluid communication with the plenum  224 . 
     While only two bores  254  are shown, additional bores  254  can be similarly arranged. The bores  254  are arranged so as to not interfere with the bores  212  for the heaters (shown in  FIG.  6   ). The bores  254  are arranged so as to not interfere with the bore  214  for the temperature sensor (shown in  FIG.  8   ). The process gases from the gas delivery system  130  shown in  FIG.  1 A  flow through the gas inlet  208 , the bore  250 , the bores  254 , the plenum  224 , and the through holes  222  into the processing chamber  102  shown in  FIG.  1 A . 
       FIG.  7 B  shows an alternate way to form the plenum  224 . Instead of forming the pillars  220  in the base portion  202 , the pillars  220  can be formed at the bottom surface  211  of the cylindrical base  207  of the backplate  204 . 
     The pillars  220  can be formed in a bottom center region  534  of the cylindrical base  207  of the backplate  204 . The bottom center region  534  of the cylindrical base  207   lies between an upper region  506  of the cylindrical base  207  and the bottom surface  211  of the cylindrical base  207 . That is, the bottom center region  534  of the cylindrical base  207  lies between the upper region  506  of the cylindrical base  207  and the top surface  205  of the base portion  202 . The bottom center region  534  is concentric with the cylindrical base  207 . The bottom center region  534  has a smaller diameter than the cylindrical base  207 . The bottom center region  534  has a smaller diameter than the ID of the rim  203  of the base portion  202 . The bottom center region  534  lies between distal ends of the bores  254  that connect to the plenum  224  at  252 - 1  and  252 - 2 . 
     The bottom center region  534  can be machined to form the pillars  220  in a recess  535 . The recess  535  is cylindrical. The recess  535  is concentric with the cylindrical base  207 . The recess  535  is has a smaller diameter than the bottom center region  534 . The recess  535  is has a depth h1. The diameter of the slot  535  is less than or equal to the ID of the rim  203  of the base portion  202 . The recess  535  extends radially in the bottom center region  534 . The bores  254  connect to the recess  535  at the periphery or an OD of the recess  535  at  252 - 1 ,  252 - 2 . Accordingly, when the base portion  202  is sealingly attached to the cylindrical base  207 , the bores  254  are in fluid communication with the recess  535 . The pillars  220  extend vertically downwards from the upper region  506  of the cylindrical base  207  through the recess  535 . The pillars  220  extend towards the bottom surface  211  of the cylindrical base  207  parallel to the vertical axis of the showerhead  200 . The pillars  220  contact the bottom surface  211  of the cylindrical base  207 . The pillars  220  are distributed across the recess  535 . A height h2 of the pillars  220  is equal to the depth h1 of the recess  535  to allow the process gases to flow in the plenum  224 . 
     The plenum  224  is defined by the bottom center region  534 , the recess  535 , and the pillars  220 . The pillars  220  are arranged across the recess  535  in the pattern described above with reference to  FIGS.  4  and  5    (i.e., in the same manner as the pillars  220  can be alternatively arranged in the base portion  202  as described above). The pillars  220  provide solidity to the plenum  224  as described above. The through holes  222  are drilled from the bottom surface  213  of the base portion  202  into the recess  536 . The pillars  220  are interstitial with the through holes  222 . The process gases flow through the bores  254 , the plenum  224 , and exit the through holes  222  into the processing chamber  102  shown in  FIG.  1 A . 
       FIG.  8    shows a cross-section of the showerhead  200  taken along lines D-D shown in  FIG.  3   . The cross-section D-D shows the bore  214  for the temperature sensor. The bore  214  extends vertically downwards through the stem portion  206  into the conical portion  209  of the backplate  204 . The bore  214  extends towards the base portion  202  along the vertical axis of the showerhead  200 . The bore  214  extends into the conical portion  209  of the backplate  204  approximately up to the top of the cylindrical base  207  of the backplate  204 . The bore  214  does not extend to the bottom surface  211  of the cylindrical base  207 . While only one bore  214  is shown, additional bores  214  for additional temperature sensors can be similarly arranged. The bore (or bores)  214  is arranged so as to not interfere with the bores  212  for the heaters (shown in  FIG.  6   ). The bore (or bores)  214  is arranged so as to not interfere with the bores  254  for the process gases (shown in  FIG.  7 A ). 
     Second Showerhead (Single Plenum, Monolithic) 
       FIGS.  9 - 21    show various views of a second showerhead  300 .  FIG.  9    shows a side view of the showerhead  300 .  FIG.  10    shows a top view of the showerhead  300 .  FIGS.  11 - 21    show various cross-sectional views of the showerhead  300 . Each cross-sectional view shows different features of the showerhead  300 . 
     Unlike the showerhead  200 , the showerhead  300  is monolithic (i.e., made of a single piece of metal). Specifically, the base portion  202  and the backplate  204  of the showerhead  200  are two separate pieces. In contrast, the base portion and the backplate of the showerhead  300  are made from a single metal piece. Therefore, the showerhead  300  is described as comprising various portions or elements for the purpose of describing the features of the showerhead  300 . However, these portions or elements are not separate or distinct from each other and are not joined or attached to each other (except for a ring shown as element  317  in  FIGS.  19 - 21   ). Rather, these portions or elements are made from a single piece of metal. 
     In  FIG.  9   , the showerhead  300  comprises a base portion  302 , a backplate  304 , and a stem portion  306 . The base portion  302  is cylindrical. The bottom of the base portion  302  faces the substrate  106  shown in  FIG.  1 A . The bottom of the base portion  302  comprises a flange  303  that extends radially outwards. The ring  317  shown in  FIGS.  19 - 21    is sealingly attached to the flange  303  and to the backplate  304  as shown and described with reference to  FIGS.  19 - 21   . The base portion  302  is described in further detail with reference to  FIGS.  14 - 18   . 
     The backplate  304  comprises a cylindrical base  307  and a conical portion  309 . The cylindrical base  307  extends vertically upwards from the base portion  302 . An outer diameter (OD) of the cylindrical base  307  is greater than an OD of the base portion  302 . The OD of the cylindrical base  307  is less than an OD of the flange  303 . The conical portion  309  extends vertically upwards from the cylindrical base  307  to the stem portion  306 . Since the showerhead  300  is monolithic, the cylindrical base  307  and the conical portion  309  of the backplate  304  are also monolithic. Further, the base portion  302 , the backplate  304 , and the stem portion  306  are monolithic. 
     The backplate  304  is solid (i.e., is not hollow). The backplate  304  helps conduct heat from the base portion  302  to the heat sink  113  shown in  FIG.  1 A . The conical portion  309  slants relative to the cylindrical base  307  at an angle α. The angle α determines (in direct proportion) the volume of the conical portion  309 . The angle α determines the heat conduction through the backplate  304 . The angle α determines the weight of the showerhead  300 . Greater volume of the conical portion  309  is desirable to conduct more heat. However, the angle α is selected so as to balance the heat conduction through the backplate  304  and the weight of the showerhead  300 . For example, the angle α is about 45° but can be between 10° and 60°. The backplate  304  is described in further detail with reference to  FIGS.  11 - 13   . 
     A gas inlet  308  is provided in the center of the stem portion  306  to receive process gases from the gas delivery system  130  shown in  FIG.  1 A . The internal structure of the showerhead  300  including a plenum and various bores for heaters, gas supply, and temperature sensors is shown and described in detail with reference to  FIGS.  11 - 21   . 
       FIG.  10    shows a top view of the showerhead  300 . The stem portion  306  comprises bores  310 - 1 ,  310 - 2 ,  310 - 3 , and  310 - 4  (collectively, the bores  310 ). The bores  310  receive fasteners (not shown) used to attach the showerhead  300  to the top plate of the processing chamber  102  shown in  FIG.  1 A . The stem portion  306  comprises bores  312 - 1  and  312 - 2  (collectively, the bores  312 ). The stem portion  306  comprises a bore  314  through which a temperature sensor (e.g., a thermocouple) is disposed into the backplate  304  as shown in  FIG.  12   . Heaters are disposed through the bores  312  as shown in  FIG.  13   . 
     Various cross-sections of the showerhead  300  are identified in  FIGS.  9  and  10   . These cross-sections are shown in  FIGS.  11 - 21    and are used to describe the internal structure of the showerhead  300  including the plenum and the bores for heaters, gas supply, and temperature sensors in further detail. 
       FIG.  11    shows a cross-section of the showerhead  300  taken along lines A-A shown in  FIG.  10   . The cross-section A-A shows a bore  350  that extends from the gas inlet  308  vertically downwards towards the base portion  302  through the stem portion  306 . The bore  350  extends into the conical portion  309  of the backplate  304  along a vertical axis of the showerhead  300 . The vertical axis of the showerhead  300  is similar to that of the showerhead  200  shown in  FIGS.  1 - 8    and is therefore not redefined for brevity. The bore  350  extends through the center of the stem portion  306  and through the center of the conical portion  309  of the backplate  304 . A distal end  351  of the bore  350  extends approximately half-way through the conical portion  309  of the backplate  304  as shown. Alternatively, the distal end  351  of the bore  350  can extend further up or down in the conical portion  309  of the backplate  304 . 
     The cross-section A-A shows a plurality of bores  354 - 1 ,  354 - 2  (hereinafter the bores  354 ) that extend laterally outwards and downwards towards the base portion  302 . The bores  354  extend through the conical portion  309  and the cylindrical base  307  of the backplate  304  from the distal end  351  of the bore  350 . The bores  354  open where a bottom end of the cylindrical base  307  and a top end of the base portion  302  meet. Specifically, the bores  354  have openings  355 - 1 ,  355 - 2  (collectively the openings  355 ) at the bottom end of the cylindrical base  307  and at the top end of the base portion  302 . The openings  355  of the bores  354  are flush (i.e., level) with the OD of the base portion  302 . 
     The bores  354  descend from the distal end  351  of the bore  350  at an acute angle relative to the vertical axis of the showerhead  300 . The bores  354  can be but need not be parallel to the walls of the conical portion  309  of the backplate  304 . When the bores  354  are parallel to the walls of the conical portion  309 , the angle between each of the bores  354  and the base portion  302  is α. While only two bores  354  are shown, additional bores  354  can be similarly arranged. The bores  354  are arranged so as to not interfere with the bore  314  for the temperature sensor (shown in  FIG.  12   ). The bores  354  are arranged so as to not interfere with the bores  312  for the heaters (shown in  FIG.  13   ). 
     The base portion  302  comprises a plenum  360  defined by a plurality of bores (shown in  FIGS.  14 A and  14 B ) drilled horizontally through the base portion  302 . The plenum  360  and the bores are shown and described in detail with reference to  FIGS.  14  and  15   . Briefly, at least two sets of bores are cross-drilled through the base portion  302  forming vertical pillars at the intersections of the cross-drilled bores. A plurality of through holes (also called orifices)  322 - 1 ,  322 - 2 ,  322 - 3 , ..., and  322 -M (collectively, through holes  322 ), where M is a positive integer, are drilled around the pillars from a bottom surface  313  of the base portion  302 . The through holes  322  extend from the bottom surface  313  of the base portion  302  into the plenum  360  defined by the cross-drilled bores. The through holes  322  and the plenum  360  are shown in further detail in  FIGS.  14 - 18   . 
     The process gases from the gas delivery system  130  shown in  FIG.  1 A  flow through the gas inlet  308 , the bore  350 , and the bores  354 . The ring (element  317  shown in  FIGS.  19 - 21   ) is sealingly attached to the flange  303  of the base portion  302  and to the OD of the cylindrical base  307  of the backplate  304  defining an annular plenum  362  (shown in  FIG.  21   ). The annular plenum  362  is defined between the ring  317 , the OD of the cylindrical base  307  of the backplate  304 , and the OD of the base portion  302 . The annular plenum  362  is in fluid communication with the plenum  360  defined by the cross-drilled bores as shown and described in detail with reference to  FIGS.  20 - 21   . Accordingly, the annular plenum  362  and the plenum  360  form a single plenum enclosed by the ring  317  and are hereinafter collectively called the plenum  360 . The plenum  360  is in fluid communication with the through holes  322 . The process gases flow through the openings  355  of the bores  354  at the OD the base portion  302  and through the plenum  360  in the base portion  302  (shown in detail in  FIGS.  14  and  15   ). The process gases exit via the through holes  322  into the processing chamber  102  shown in  FIG.  1 A . 
       FIG.  12    shows a cross-section of the showerhead  300  taken along lines B-B shown in  FIG.  10   . The cross-section B-B shows the bore  314  for the temperature senor. The bore  314  extends vertically downwards through the stem portion  306  into the conical portion  309  of the backplate  304 . The bore  314  extends towards the base portion  302  along the vertical axis of the showerhead  300 . The bore  314  extends into the cylindrical base  307  of the backplate  304 . The bore  314  does not extend to the base portion  302 . While only one bore  314  is shown, additional bores  314  for additional temperature sensors can be similarly arranged. The bore (or bores)  314  is arranged so as to not interfere with the bores  354  for the process gases (shown in  FIG.  11   ). The bore (or bores)  314  is arranged so as to not interfere with the bores  312  for the heaters (shown in  FIG.  13   ). 
       FIG.  13    shows a cross-section of the showerhead  300  taken along lines C-C shown in  FIG.  10   . The cross-section C-C shows the bores  312  for the heaters in detail. The bores  312  extend vertically downwards through the stem portion  306  towards the base portion  302  along the vertical axis of the showerhead  300 . The bores  312  extend into the cylindrical base  307  of the backplate  304 . While only two bores  312  are shown, additional bores  312  for additional heaters can be similarly arranged. The bores  312  are arranged so as to not interfere with the bores  354  for process gases (shown in  FIG.  11   ). The bores  312  are arranged so as to not interfere with the bore  314  for the temperature sensor (shown in  FIG.  12   ). 
       FIGS.  14 A and  14 B  show a cross-section of the showerhead  300  taken along lines D-D shown in  FIG.  9   . In  FIG.  14 A , the cross-section D-D shows the cross-drilled bores and the plenum  360 . Accordingly, the plenum  360  is called a cross-bored plenum  360 . In the example shown, two sets of bores are cross-drilled through the base portion  302  orthogonally (i.e., perpendicular to each other). Specifically, a first set of bores  380 - 1 ,  380 - 2 ,  380 - 3 , ...,  380 -N (collectively, the first set of bores  380 ), where N is a positive integer, is drilled horizontally through the base portion  302 . The first set of bores  380  is drilled along a first axis  382  (i.e., along chords of the base portion  302  parallel to the first axis  382 ). A second set of bores  390 - 1 ,  390 - 2 ,  390 - 3 , ...,  390 -N (collectively, the second set of bores  390 ), where N is a positive integer, is drilled horizontally through the base portion  302 . The second set of bores  390  is drilled along a second axis  392  (i.e., along chords of the base portion  302  parallel to the second axis  392 ). The first axis  382  is perpendicular to the second axis  392 . 
     The first and second sets of bores  380 ,  390  create pillars  370 - 1 ,  370 - 2 ,  370 - 3 , ..., and  370 -M (collectively, the pillars  370 ), where M is a positive integer greater than N, at the intersections of the first and second sets of bores  380 ,  390 . Specifically, since the first and second sets of bores  380 ,  390  are drilled perpendicularly to each other, the pillars  370  are rectangular in shape. More specifically, in the example shown, the bores in the first and second sets of bores  380 ,  390  are of equal diameter and are equidistant from each other. Consequently, the pillars  370  are square in shape. 
     The pillars  370  are distributed from the center of the base portion  302  to the OD of the base portion  302 . The pillars  370  help conduct heat from the bottom surface  313  of the base portion  302  to the cylindrical base  307  of the backplate  304 . The backplate  304  conducts the heat to the heat sink  113  shown in  FIG.  1 A . The pillars  370  conduct the heat along the vertical axis of the showerhead  300 . The heat conduction reduces radial temperature gradients across the base portion  302  and the backplate  304 . 
     The first and second sets of bores  380 ,  390  define the plenum  360  within the base portion  302 . The pillars  370  reduce the volume (i.e., cavity or hollowness) of the plenum  360 . In other words, the pillars  370  provide solidity to the plenum  360 . For example, the pillars  370  fill about 10% of the volume of the plenum  360 . The amount of heat conducted by the pillars  370  from the bottom surface  313  of the base portion  302  to the cylindrical base  307  of the backplate  304  is directly proportional to a density of the pillars  370 . The density of the pillars  370  is the number of pillars  370  per unit area of the base portion  302  in the plenum  360 . In other words, the amount of heat conducted by the pillars  370  is directly proportional to the number of pillars  370 . The density of the pillars  370  is specified by requirements of processes performed on the substrate  106 . 
     The through holes  322  are distributed radially from the center of the base portion  302  to the OD of the base portion  302 . Specifically, the through holes  322  are drilled around each pillar  370  as shown. Some of the through holes  322  are not visible in  FIG.  14 A  and are shown in detail in  FIG.  14 B . In the example shown, as shown in  FIG.  14 B , when the bores in the first and second sets of bores  380 ,  390  are of equal diameter and are equidistant from each other, four pillars  370  lie on vertices of a square  371 . The four pillars  370  include two pillars  370  along the first axis  382  and two pillars  370  along the second axis  392 . One pillar  370  lies at the center of the square  371  (i.e., at the intersection of the diagonals of the square  371 ). One through hole  322  lies between each successive pillar  370  along the first and second axes  382 ,  392 . Thus, two through holes  370  lie on each diagonal of the square  371 . Additionally, one through hole  322  lies at the center of each side of the square  371 . Accordingly, each pillar  370  is surrounded by eight through holes  322 . The eight through holes  322  are arranged as follows. 
     Of the eight through holes  322 , a first set of four through holes  322  lie on vertices of a square  396 . The vertices of the square  396  lie at the centers of the four sides of the square  371 . A second set four through holes  322  lie at the centers of the four sides of the square  396 . The pillar  370  that lies at the center of the square  371  also lies at the center of the square  396 . The second set of four through holes  322  that lie at the centers of the four sides of the square  396  lie on the diagonals of the square  371 . 
     In some examples, the spacing between the bores in the first set of bores  380  may be different than the spacing in the second set of bores  390 . For example, the bores in the first set of bores  380  may be separated from each other by a first distance. The bores in the second set of bores  390  may be separated from each other by a second distance. In other examples, the bores in the first set of bores  380  and/or in the second set of bores  390  may be spaced (i.e., separated from each other) by gradually varying distances. For example, the distance between the bores in the first set of bores  380  and/or in the second set of bores  390  may increase from the center of the base portion  302  towards the circumference of the base portion  302 . In some examples, the distance between the bores in the first set of bores  380  and/or in the second set of bores  390  may decrease from the center of the base portion  302  towards the circumference of the base portion  302 . 
     In still other examples, the number of bores in the first set of bores  380  and the second set of bores  390  may be equal. In further examples, some of the bores in the first set of bores  380  and/or in the second set of bores  390  may be omitted. In some examples, the diameters of the bores in the first set of bores  380  and/or in the second set of bores  390  may be varied similar to the spacing variations described above. In still other examples, the bores in the first set of bores  380  and/or in the second set of bores  390  may be arranged in groups. In these still other examples, the spacing (i.e., distance) between the bores and/or the diameters of the bores in the groups may be varied as described above. 
     Further, the two sets of bores  380 ,  390  are shown for example only. In some examples additional sets of bores can be drilled creating pillars of different shapes. The variations in quantity (i.e., the number of bores in a set) and/or diameter, the variations in spacing and grouping of the bores described above can be added to these additional sets of bores creating different patterns of pillars. The arrangement of the bores may be dictated by the pattern of the through holes  322  specified by the processes performed on the substrate  106 . 
       FIG.  15    shows a cross-section of the showerhead  300  taken along lines E-E shown in  FIG.  9   . The cross-section E-E shows a section  315  of the base portion  302  that lies above the plenum  360 . The section  315  is taken along a horizontal plane parallel to the diameter of the base portion  302  and perpendicular to the vertical axis of the showerhead  300 . The first and second sets of bores  380 ,  390  and the plenum  360  lie under the section  315  of the base portion  302 . The first and second sets of bores  380 ,  390  and the plenum  360  are parallel to the section  315  of the base portion  302 . The cylindrical base  307  of the backplate  304  lies on top of the section  315  of the base portion  302 . The cylindrical base  307  of the backplate  304  is parallel to the section  315  of the base portion  302 . 
       FIG.  16    shows a bottom view of the showerhead  300  taken along lines F-F shown in  FIG.  9   . The bottom view shows the through holes  322  arranged on the bottom surface  313  of the base portion  302 . The through holes  322  are arranged on the bottom surface  313  of the base portion  302  in the pattern described above with reference to  FIGS.  14 A and  14 B . Additional alternative patterns in which the through holes  322  can be arranged on the bottom surface  313  of the base portion  302  are shown in  FIGS.  17  and  18   . 
     In the example shown in  FIG.  17   , the through holes  322  are arranged on the bottom surface  313  of the base portion  302  in a square pattern or a diamond shaped pattern. In the example shown in  FIG.  18   , the through holes  322  are arranged using a combination of the patterns shown in  FIGS.  16  and  17   . The combination pattern is also called a zoned pattern. As shown, a first portion of the through holes  322  is arranged in the pattern shown in  FIG.  17    in a center region (also called a first zone or an inner zone) of the base portion  302 . The center region extends from the center of the base portion  302  to a predetermined portion of the radius of the base portion  302 . A second portion of the through holes  322  is arranged in the pattern shown in  FIG.  16    in a second region (also called a second zone or an outer zone) of the base portion  302 . The second region extends from the periphery or an OD of the center region to the OD of the base portion  302 . The center and second regions are concentric. 
     Alternatively, while not shown, the patterns shown in  FIG.  18    can be reversed. That is, in the reversed pattern, the first portion of the through holes  322  is arranged in the pattern shown in  FIG.  16    in the second region. Further, in the reversed pattern, the second portion of the through holes  322  is arranged in the pattern shown in  FIG.  17    in center region. While only two concentric regions are shown, additional concentric regions may be used. Various patterns may be used to arrange the through holes  322  in the additional concentric regions. Further, while not shown, the through holes  322  may be arranged in pie shaped regions or zones. Furthermore, while not shown, the through holes  322  may be arranged in a combination of concentric and pie shaped regions or zones. 
       FIGS.  19 - 21    show the ring  317  (also called an annular sealing member) used to cover the openings  355  of the bores  354  described above.  FIG.  19    shows a top view of the ring  317  that is sealingly attached to the cylindrical base  307  and the base portion  302  of the showerhead  300 . The ring  317  has a bottom cylindrical portion  317 - 1  and an upper annular portion  317 - 2 . The bottom cylindrical portion  317 - 1  has an outer diameter equal to the OD of the flange  303  of the base portion  302 . The upper annular portion  317 - 2  initially extends vertically upwards from the bottom cylindrical portion  317 - 1 . Then the upper annular portion  317 - 2  extends radially inwards towards the cylindrical base  307  of the backplate  304 . An inner diameter of the upper annular portion  317 - 2  is equal to the OD of the cylindrical base  307  of the backplate  304 . The ring  317  is monolithic. A distal end of the upper annular portion  317 - 2  is sealingly attached to cylindrical base  307  of the backplate  304 . A distal end of the bottom cylindrical portion  317 - 1  is sealingly attached to the flange  303  of the base portion  302 . 
       FIG.  20    shows a cross-section of the ring  317  taken along lines G-G shown in  FIG.  19   .  FIG.  21    shows the cross-section A-A of the showerhead  300  shown in  FIG.  11    with the addition of the ring  317 .  FIG.  21    shows the ring  317  that is sealingly attached to the flange  303  and to the periphery (OD) of the cylindrical base  307  of the backplate  304  as described above. The ring  317  prevents the process gases from the bores  354  from escaping (i.e., exiting) the showerhead  300 . Instead, the ring  317  directs or routes the process gases from the bores  354  into the plenum  360 . The base portion  302 , the cylindrical base  307  of the backplate  304 , the ring  317 , the first and second sets of bores  380 ,  390  define the plenum  360  described above. 
     Third Showerhead (Dual Plenum, Monolithic) 
       FIGS.  22 - 40    show various views of a third showerhead  400 .  FIG.  22    shows a side view of the showerhead  400 .  FIG.  23    shows a top view of the showerhead  400 .  FIG.  24 A -40 show various cross-sectional views of the showerhead  400 . Each cross-sectional view shows different features of the showerhead  400 . 
     The showerhead  400  differs from the showerhead  300  in that unlike the showerhead  300 , the showerhead  400  is a dual plenum showerhead. Accordingly, unlike the showerhead  300 , the showerhead  400  allows supplying two different process gases into the processing chamber  102  as shown in  FIG.  1 B . Specifically, as shown in  FIG.  24 A -40 and as described below in further detail, the showerhead  400  defines two separate plenums. The two separate plenums are not in fluid communication with each other. The showerhead  400  comprises two separate gas inlets. The two separate gas inlets receive two separate process gases from the gas delivery system  130  shown in  FIG.  1 B . The two separate process gases are respectively supplied to the two separate plenums. Since the inlets and the plenums of the showerhead  400  are disjoint, the two separate process gases do not mix in the showerhead  400 . While not shown, the design of the showerhead  400  can be extended to include additional disjoint inlets and plenums to supply additional process gases separately into the showerhead  400 . 
     Additional differences between the showerhead  300  and the showerhead  400  are shown and described below with reference to  FIGS.  22 - 40   . Except for these differences, the showerhead  400  is similar to the showerhead  300 . Therefore, identical reference numerals from the showerhead  300  are used to identify elements and features of the showerhead  400  that are similar to the respective elements and features of the showerhead  300 , and their description is not repeated for brevity. 
       FIG.  22    shows that the showerhead  400  has the gas inlet  308  (hereinafter the first gas inlet  308 ) and a second gas inlet  311 . The first and second gas inlets  308 ,  311  are coaxial. In addition to the plenum  360  (hereinafter the first plenum  360 ), the showerhead  400  further comprises a second plenum  402  in the base portion  302 . The second plenum  402  extends radially across the base portion  302  as shown and described in detail with reference to  FIG.  24 A -26 and  FIG.  40   . Briefly, the second plenum  402  is located directly above the first plenum  360 . The second plenum  402  is not in fluid communication with the first plenum  360 . Instead, as shown and described with reference to  FIGS.  24 A- 28 C , top ends of the pillars  370  in the first plenum  360  abut the bottom of the second plenum  402 . The top ends of the pillars  370  include through holes that extend from the bottom of the second plenum  402 , through the pillars  370 , and through the bottom surface  313  of the base portion  302 . Accordingly, the through holes in the pillars  370  are in fluid communication with the second plenum  402  but are not in fluid communication with the first plenum  360 . 
     A first gas supplied by the gas delivery system  130  shown in  FIG.  1 B  flows through the first gas inlet  308 . Specifically, the first gas flows through an annular volume between an outer wall of the second gas inlet  311  and an inner wall of the first gas inlet  308 . A second gas supplied by the gas delivery system  130  shown in  FIG.  1 B  flows through the second gas inlet  311 . As shown and described in further detail with reference to  FIGS.  24 A- 40   , the first and second gases flow through bores extending from the first and second gas inlets  308 ,  311  into the first and second plenums  360 ,  402 , respectively. 
       FIG.  23    is identical to  FIG.  10    except that in addition to showing all elements shown in  FIG.  10   ,  FIG.  23    shows the additional second gas inlet  311  described above. 
       FIG.  24 A  shows a cross-section of the showerhead  400  taken along lines A-A shown in  FIG.  23   .  FIG.  24 A  is identical to  FIG.  11    except for the following additions. Hereinafter, the bore  350  is called the first bore  350 . A second bore  404  extends from the second gas inlet  311  vertically downwards towards the base portion  302  through the stem portion  306 . The second bore  404  extends into the conical portion  309  of the backplate  304  along a vertical axis of the showerhead  400 . The vertical axis of the showerhead  400  is similar to that of the showerhead  300  and is therefore not redefined for brevity. The bore  404  extends through the center of the stem portion  306  and through the center of the backplate  304  into an upper region  406  of the base portion  302  of the showerhead  400 . A distal end  405  of the bore  404  is connected to the second plenum  402  at the center of the second plenum  402 . The second plenum  402  is shown and described below in further detail with reference to  FIGS.  24 B- 24 E . 
       FIGS.  24 B and  24 C  show side views of the base portion  302  showing the second plenum  402  in further detail. In  FIG.  24 B , the second plenum  402  is formed in the upper region  406  of the base portion  302 . The upper region  406  of the base portion  302  lies directly above an upper surface  410  of the first plenum  360 . The second plenum  402  is formed by removing material from the upper region  406 . The material is removed from the upper region  406  by cross-drilling bores through a plurality of openings  432 - 1 ,  432 - 2 ,  432 - 3 , ..., and  432 -N (collectively, the openings  432 ), where N is a positive integer. The openings  432  are formed on sidewalls  408  of the base portion  302 . The bores are cross-drilled through the upper region  406  along the first and second axes  382 ,  392  (i.e., along chords of the base portion  302  parallel to the first and second axes  382 ,  392 ). The bores are cross-drilled through the upper region  406  similar to the manner in which the first and second sets of bores  380 ,  390  are drilled to form the first plenum  360 . 
     The cross-drilled bores in the upper region  406  create a plurality of pillars  433 - 1 ,  433 - 2 ,  433 - 3 , ..., and  433 -M (collectively the pillars  433 ), where M is a positive integer. The cross-drilled bores in the upper region  406  create the pillars  433  in the upper region  406  (i.e., in the second plenum  402 ) directly above the upper surface  410  of the first plenum  360 . The bores are cross-drilled through the upper region  406  parallel to and interstitially relative to the first and second sets of bores  380 ,  390  of the first plenum  360 . The first and second sets of bores  380 ,  390  of the first plenum  360  are drilled directly below the second plenum  402  (i.e., directly below the upper surface  410  of the first plenum  360 ). The bores are cross-drilled through the upper region  406  such that the pillars  433  in the second plenum  402  are interstitial relative to the pillars  370  of the first plenum  360 . The pillars  370  in the first plenum  360  are level (flush) with the upper surface  410  of the first plenum  360  and abut the bottom of the second plenum  402 . 
     In  FIG.  24 C , a ring  434  (also called an annular sealing member) is sealingly attached to the upper region  406  of the base portion  302 . The ring  434  covers the openings  432  on the sidewalls  408  of the base portion  302  to form the second plenum  402 . In some examples, while not shown, plugs can be inserted into the openings  432  to close the openings  432  instead of attaching the ring  434  to the base portion  302  to form the second plenum  402 . When the ring  434  is sealingly attached to the upper region  406  of the base portion  302  (or plugs are used to close the openings  432 ), the ring  434  (or the plugs) prevents process gases from the first and second plenums  360 ,  402  from mixing with each other. 
     Thus, the second plenum  402  is defined by the upper region  406  of the base portion  302 , the upper surface  410  of the first plenum  360 , the portions of the sidewalls  408  of the base portion  302  between the openings  432 , and the ring  434  (or plugs) covering the openings  432  and the portions of the sidewalls  408  of the base portion  302 . The second plenum  402  extends radially across the base portion  302  and lies in a plane perpendicular to the vertical axis of the showerhead  400 . 
     In  FIG.  24 A , the second plenum  402  lies between the bores  354 . Specifically, the second plenum  402  lies directly below a plane in which the openings  355  of the bores  354  lie. A plurality of through holes  420 - 1 ,  420 - 2 ,  420 - 3 , ..., and  420 -M (collectively the through holes  420 ), where M is a positive integer, are drilled at the bottom of the second plenum  402 . The through holes  420  are drilled through the bottom surface  313  of the base portion  302  and through the center of the pillars  370  (one through hole  420  per pillar  370 ). The through holes  420  are in fluid communication with the second plenum  402  but are not in fluid communication with the first plenum  360 . As shown and described in further detail with reference to  FIGS.  24 D,  24 E, and  27 A- 28 C , the through holes  420  of the second plenum  402  are arranged interstitially with the through holes  322  of the first plenum  360 . The pillars  433  of the second plenum  402  are arranged interstitially with the pillars  370  of the first plenum  360 . 
     The first gas flows through the first gas inlet  308 , through the bores  350  and  354 , the first plenum  360 , and the through holes  322  into the processing chamber  102  shown in  FIG.  1 B . The second gas flows through the second gas inlet  311 , through the bore  404 , the second plenum  402 , and the through holes  420  into the processing chamber  102  shown in  FIG.  1 B . The remaining features shown in  FIG.  24 A  are shown and described with reference to  FIG.  11   , and their description is omitted for brevity. 
       FIGS.  24 D and  24 E  show a cross-section of the second plenum  402  taken along lines P-P shown in  FIG.  24 B . The cross-section shows the layout of the pillars  433  and the through holes  420  in the second plenum  402 . In  FIG.  24 D , the layout of the pillars  433  is similar to the layout of the pillars  370  shown in  FIGS.  14 A and  14 B  and is therefore not described again for brevity. The pillars  433  are structurally and functionally similar to the pillars  370 . Therefore, all the structural and functional details of the pillars  370  described above with reference to the showerhead  300  apply equally to the pillars  433  and are therefore not repeated for brevity. The layout of the through holes  420  is described below with reference to  FIG.  24 E . 
     In  FIG.  24 E , the through holes  420  are arranged between each of the pillars  433 . Specifically, when the bores drilled through the openings  432  are of equal diameter and are equidistant from each other, four pillars  433  lie on vertices of the square  371 . The four pillars  433  include two pillars  433  along the first axis  382  and two pillars  433  along the second axis  392 . One pillar  433  lies at the center of the square  371  (i.e., at the intersection of the diagonals of the square  371 ). One through hole  420  lies between each successive pillar  433  along the first and second axes  382 ,  392 . Thus, two through holes  420  lie on each diagonal of the square  371 . The four through holes  420  that lie on the diagonals of the square  371  also lie on vertices of the square  396 . The vertices of the square  396  lie at the centers of the four sides of the square  371 . Consequently, the pillar  370  that lies at the center of the square  371  also lies at the center of the square  396 . 
     Accordingly, each pillar  433  is surrounded by four through holes  420  in a square pattern described above. The arrangement of the through holes  420  relative to the pillars  370  of the first plenum  360  is shown and described below with reference to  FIG.  29   . These features (e.g., the pillars  433 , the bores, the openings  432 , and the ring  434 ) of the second plenum  402  that are shown in  FIGS.  24 B- 24 E  are omitted from  FIGS.  25 ,  26 , and  40    (but are presumed present therein). These features are omitted from  FIGS.  25 ,  26 , and  40    to simplify illustration of the other additional features of the showerhead  400  shown in these figures but are presumed present therein. 
       FIG.  25    shows a cross-section of the showerhead  400  taken along lines B-B shown in  FIG.  23   .  FIG.  25    is identical to  FIG.  12    except that in addition to showing all elements shown in  FIG.  12   ,  FIG.  25    shows the second gas inlet  311 , the bore  404 , the second plenum  402 , the ring  434 , and the through holes  420  described above. The arrangement of the pillars  370  and the through holes  322  shown in  FIG.  25    is also different than the arrangement of the pillars  370  and the through holes  322  shown in  FIG.  12   . The different arrangement of the pillars  370  and the through holes  322  in  FIG.  25    is shown and described in further detail with reference to  FIGS.  27 A- 28 C . The remaining features shown in  FIG.  25    are shown and described with reference to  FIG.  12    and their description is therefore omitted for brevity. 
       FIG.  26    shows a cross-section of the showerhead  400  taken along lines C-C shown in  FIG.  23   .  FIG.  26    is identical to  FIG.  13    except that in addition to showing all elements shown in  FIG.  13   ,  FIG.  26    shows the second gas inlet  311 , the bore  404 , the second plenum  402 , the ring  434 , and the through holes  420  described above. The arrangement of the pillars  370  and the through holes  322  shown in  FIG.  26    is also different than the arrangement of the pillars  370  and the through holes  322  shown in  FIG.  13   . The different arrangement of the pillars  370  and the through holes  322  in  FIG.  26    is shown and described in further detail with reference to  FIGS.  27 A- 28 C . The remaining features shown in  FIG.  26    are shown and described with reference to  FIG.  13    and their description is therefore omitted for brevity. 
       FIGS.  27 A- 28 C  show cross-sections of the base  302   302  taken along lines D-D shown in  FIG.  22    showing the first plenum  360  in detail.  FIGS.  27 A- 27 C  show a first pattern in which the pillars  370  and the through holes  322 ,  420  are arranged.  FIGS.  28 A- 28 C  show a second pattern in which the pillars  370  and the through holes  322 ,  420  are arranged. The second pattern differs from the first pattern in that the second pattern comprises additional through holes  322  than the first pattern as explained below in detail. Some of the through holes  322  not visible in  FIGS.  27 A and  27 B  are shown in detail in  FIGS.  27 B,  27 C,  28 B, and  28 C . 
     The first and second patterns differ from the pattern shown in  FIGS.  14 A and  14 B  as follows. In  FIGS.  14 A and  14 B , the first and second sets of bores  380 ,  390  are drilled such that the center of the base portion  302  has a pillar  370 . In contrast, in  FIGS.  27 A- 28 C , the first and second sets of bores  380 ,  390  are drilled such that the center of the base portion  302  has a through hole  322  instead of a pillar  370 . In  FIGS.  14 A and  14 B , the bores in the first and second sets of bores  380 ,  390  do not intersect at the center of the base portion  302 . In contrast, in  FIGS.  27 A- 28 C , the bores in the first and second sets of bores  380 ,  390  intersect at the center of the base portion  302 . 
     In addition, in  FIGS.  14 A and  14 B , the pillars  370  do not include the through holes  420  since the showerhead  300  does not include the second plenum  402 . In contrast, since the showerhead  400  comprises the second plenum  402 , the pillars  370  in the first and second patterns shown in  FIGS.  27 A- 28 C  include the through holes  420  as shown in  FIGS.  27 A- 28 C . In  FIGS.  27 B and  28 B , when the bores in the first and second sets of bores  380 ,  390  are of equal diameter and are equidistant from each other, the through holes  420  lie at the vertices and the center of the square  371 . 
     In  FIGS.  27 A and  27 B , in the remaining portions of the first and second sets of bores  380 ,  390  (i.e., in the region of the base portion  302  that is radially outward from the center of the base portion  302 ), the first pattern of the pillars  370  and the through holes  322  is identical to that shown in  FIGS.  14 A and  14 B  except for the additional through holes  420 , which are absent in  FIGS.  14 A and  14 B , and two other differences. First, the first pattern shown in  FIGS.  27 A and  27 B  comprises the additional through holes  420  that are absent in the pattern shown in  FIGS.  14 A and  14 B . Second, the first pattern shown in  FIGS.  27 A and  27 B  does not include the through holes  322  that lie at the centers of the four sides of the square  396  shown in  FIGS.  14 B and  28 B . 
     In  FIGS.  28 A and  28 B , in the remaining portions of the first and second sets of bores  380 ,  390  (i.e., in the region of the base portion  302  that is radially outward from the center of the base portion  302 ), the second pattern of the pillars  370  and the through holes  322  is identical to that shown in  FIGS.  14 A and  14 B  except for the additional through holes  420 , which are absent in  FIGS.  14 A and  14 B . 
     In the first and second patterns, as shown in  FIGS.  27 A,  27 C,  28 A, and  28 C , the center of the base portion  302  has a through hole  322 . When the bores in the first and second sets of bores  380 ,  390  are of equal diameter and are equidistant from each other, the centers of the pillars  370  immediately adjacent to the through hole  322  at the center of the base portion  302  lie on vertices of a square  450 . Consequently, the through holes  420  in the pillars  370 , which are at the center of the respective pillars  370 , lie at the vertices of the square  450 . The through hole  322  at the center of the base portion  302  lies at the center of the square  450 . That is, the through hole  322  at the center of the base portion  302  lies at the intersection of the diagonals of the square  450 . The side of the squares  450  and  371  are equal. 
     In  FIG.  27 A , subsequent to the pattern shown in  FIG.  27 C , in the region of the base portion  302  that is radially outward from the center of the base portion  302 , the pattern of the pillars  370 , the through holes  420 , and the through holes  322  shown in  FIG.  27 B , extends (i.e., replicates) along the first and second axes  382 ,  392 . 
     In  FIG.  28 C , in the second pattern, as shown in  FIGS.  28 A and  28 C , the pattern of the pillars  370 , the through holes  420 , and the through holes  322  is identical to the pattern shown in  FIG.  27 C  except that four additional through holes  322  lie at the centers of the four sides of the square  450 . 
     The pattern shown in  FIG.  28 B  is identical to the pattern shown in  FIG.  14 B  except for the addition of the through holes  420  at the centers of the pillars  370  as shown in  FIG.  28 B . In  FIG.  28 A , subsequent to the pattern shown in  FIG.  28 C , in the region of the base portion  302  that is radially outward from the center of the base portion  302 , the pattern of the pillars  370 , the through holes  420 , and the through holes  322  shown in  FIG.  28 B , extends (i.e., replicates) along the first and second axes  382 ,  392 . 
     In  FIGS.  27 A- 28 C , the variations described with reference to  FIGS.  14 A and  14 B  regarding the arrangements and geometries of the first and second sets of bores  380 ,  390  can be employed while maintaining the patterns shown in  FIGS.  27 C and  28 C  at the center of the base portion  302 . The variations are not described again for brevity. Further, corresponding variations can also be employed in drilling the bores used to form the second plenum  402 . 
       FIGS.  29 - 37    show cross-sections of the base portion  302  taken along lines E-E shown in  FIG.  22   . The pillars  433  are omitted (but are presumed present) to simplify the illustrations of the through holes  322  and their alignment with the pillars  370 . Each of the  FIGS.  29 - 37    shows a different pattern of the through holes  420  that can be used in the showerhead  400  depending on different process requirements. Each of the  FIGS.  29 - 37    shows a top view of the second plenum  402  and the through holes  420  that are at the center of the pillars  370 . The pillars  370  are not visible but are shown in dotted lines to illustrate the alignment of the through holes  420  with the center of the pillars  370 . 
     For example,  FIG.  29    shows the pillars  370  and the through holes  420  arranged in the base portion  302  as shown in  FIGS.  27 A and  28 A . In some examples, the through holes  420  can be arranged in zones (i.e., one or more regions of the base portion  302 ) instead of being arranged throughout the base portion  302  as shown in  FIGS.  27 A and  28 A . For example, the zones can be radial, azimuthal, or a combination thereof. Various examples of zoned arrangements of the through holes  420  are shown in  FIGS.  30 - 37   . 
     For example,  FIG.  30    shows the through holes  420  arranged in an outer radial zone  460  in the base portion  302 . The outer radial zone  460  extends from a predetermined distance from the center of the base portion  302  to the OD of the base portion  302 .  FIG.  31    shows the through holes  420  arranged in an inner radial zone  470  in the base portion  302 . The inner radial zone  470  extends from the center of the base portion  302  to a predetermined distance from the center of the base portion  302 . 
     In other examples,  FIG.  32    shows the through holes  420  arranged in concentric radial zones  480  and  490  in the base portion  302 . The radial zone  480  is an inner radial zone, and the radial zone  490  is an outer radial zone arranged concentrically with the radial zone  480 . The radial zone extends from the center of the base portion  302  to a first predetermined distance from the center of the base portion  302 . The radial zone  490  extends from a second predetermined distance from the center of the base portion  302  to the OD of the base portion  302 . The second predetermined distance is greater than the first predetermined distance. 
     In further examples,  FIGS.  33  and  34    show the through holes  420  arranged in azimuthal zones. For example,  FIG.  33    shows the through holes  420  arranged in second, third, and fourth quadrants (as in co-ordinate geometry) of the base portion  302 . Alternatively, while not shown, the through holes  420  can be arranged in any one of the four quadrants of the base portion  302 . In another example,  FIG.  34    shows the through holes  420  arranged in second and fourth quadrants of the base portion  302 . Alternatively, while not shown, the through holes  420  can be arranged in first and second quadrants, first and fourth quadrants, second and third quadrants, or third and fourth quadrants of the base portion  302 . In still other examples,  FIGS.  35 - 37    show examples of the through holes  420  arranged in various combinations of radial and azimuthal zones in the base portion  302 . Various other arrangements based on requirements of processing the substrate  106  are contemplated. 
       FIG.  38    shows a bottom view of the showerhead  400  taken along lines F-F shown in  FIG.  22    and shows the through holes  322 ,  420  arranged on the bottom surface  313  of the base portion  302 . The through holes  322 ,  420  are arranged in the pattern described above with reference to  FIGS.  27 A- 27 C . 
       FIG.  39    shows an example of an alternative pattern in which the through holes  322 ,  420  can be arranged on the bottom surface  313  of the base portion  302 . In this example, the through holes  322 ,  420  are arranged in the pattern described above with reference to  FIGS.  28 A- 28 C . 
       FIG.  40    is identical to  FIG.  21    except that in addition to showing all elements shown in  FIG.  21   ,  FIG.  40    shows the second gas inlet  311 , the bore  404 , the second plenum  402 , the ring  434 , and the through holes  420  described above. 
     Fourth Showerhead (Dual Plenum, Non-Monolithic) 
       FIGS.  41 - 49    show various views of a fourth showerhead  500 .  FIG.  41    shows a side view of the showerhead  500 .  FIG.  42    shows a top view of the showerhead  500 .  FIGS.  43 - 49    show various cross-sectional views of the showerhead  500 . Each cross-sectional view shows different features of the showerhead  500 . 
     The showerhead  500  differs from the showerhead  200  in that unlike the showerhead  200 , the showerhead  500  is a dual plenum showerhead. Accordingly, unlike the showerhead  200 , the showerhead  500  allows supplying two different process gases into the processing chamber  102  as shown in  FIG.  1 B . Specifically, as shown in  FIGS.  41 - 49    and as described below in further detail, the showerhead  500  defines two separate plenums that are disjoint and are not in fluid communication with each other. The showerhead  500  comprises two separate gas inlets that receive two separate process gases from the gas delivery system  130  shown in  FIG.  1 B . The two separate process gases are respectively supplied to the two separate plenums. Since the inlets and the plenums of the showerhead  500  are disjoint, the two separate process gases do not mix in the showerhead  500 . While not shown, the design of the showerhead  500  can be extended to include additional disjoint inlets and plenums to supply additional process gases separately into the showerhead  500 . 
     Additional differences between the showerhead  200  and the showerhead  500  are shown and described below with reference to  FIGS.  41 - 49   . Except for these differences, the showerhead  500  is similar to the showerhead  200 . Therefore, identical reference numerals from the showerhead  200  are used to identify elements and features of the showerhead  500  that are similar to the respective elements and features of the showerhead  200 , and their description is not repeated for brevity. 
       FIG.  41    shows that the showerhead  500  has the gas inlet  208  (hereinafter the first gas inlet  208 ) and a second gas inlet  508 . The first and second gas inlets  208 ,  508  are coaxial. The second gas inlet  508  surrounds the first gas inlet  208  and is not in fluid communication with the first gas inlet  208 . In addition to the plenum  224  (hereinafter the first plenum  224 ), the showerhead  500  further comprises a second plenum  502  arranged above the base portion  202  in the bottom center region  534  of the cylindrical base  207  of the backplate  204 . The second plenum  502  abuts the top surface  205  of the base portion  202  and the bottom surface  211  of the cylindrical base  207  of the backplate  204 . The second plenum  502  extends radially across the bottom center region  534  of the cylindrical base  207  as shown and described in detail with reference to  FIGS.  43 A- 49   . 
     Briefly, the second plenum  502  is located directly above the first plenum  224  and is not in fluid communication with the first plenum  224 . Instead, as shown and described with reference to  FIGS.  43 A- 49   , top ends of the pillars  220  in the first plenum  224  abut the bottom of the second plenum  502 . The top ends of the pillars  220  include through holes (shown in subsequent figures) that extend from the bottom of the second plenum  502 , through the pillars  220 , and through the bottom surface  213  of the base portion  202 . Accordingly, the through holes in the pillars  220  are in fluid communication with the second plenum  502  but are not in fluid communication with the first plenum  224 . In addition, as shown and described with reference to  FIGS.  43 A- 49   , the second plenum  502  comprises pillars similar to the pillars  220  in the first plenum  224  to increase the solidity and hence the heat conduction through the second plenum  502  as described below in further detail. 
     A first gas supplied by the gas delivery system  130  shown in  FIG.  1 B  flows through the first gas inlet  208 . Specifically, the first gas flows through an annular volume between an outer wall of the second gas inlet  508  and an inner wall of the first gas inlet  208 . A second gas supplied by the gas delivery system  130  shown in  FIG.  1 B  flows through the second gas inlet  508 . As shown and described in further detail with reference to  FIGS.  43 A- 49   , the first and second gases flow through bores extending from the first and second gas inlets  208 ,  508  into the first and second plenums  224 ,  502 , respectively. 
       FIG.  42    is identical to  FIG.  3    except that in addition to showing all elements shown in  FIG.  3   ,  FIG.  42    shows the additional second gas inlet  508  described above. 
       FIG.  43 A  shows cross-sectional view of the showerhead  500  taken along lines A-A shown in  FIG.  42   .  FIG.  43 A  is identical to  FIG.  7 A  except for the following additions. Hereinafter, the bore  250  is called the first bore  250 . A second bore  504  extends from the second gas inlet  508  vertically downwards towards the base portion  202 . The second bore  504  extends through the stem portion  206  into the conical portion  209  of the backplate  204  along a vertical axis of the showerhead  500 . The vertical axis of the showerhead  500  is similar to that of the showerhead  200  and is therefore not redefined for brevity. The second bore  504  extends through the center of the stem portion  206  and through the center of the backplate  204 . The second bore  504  extends towards the bottom surface  211  of the cylindrical base  207  of the backplate  204 . A distal end  505  of the bore  504  is connected to the second plenum  502  at the center of the second plenum  502 . The second plenum  502  is shown and described below in further detail with reference to  FIGS.  43 B,  43 C,  46 , and  47   . 
       FIG.  43 B  shows a side view of the bottom center region  534  of the cylindrical base  207  of the backplate  204  showing the second plenum  502  in further detail. The bottom center region  534  of the cylindrical base  207  lies between the upper region  506  of the cylindrical base  207  and the bottom surface  211  of the cylindrical base  207 . That is, the bottom center region  534  of the cylindrical base  207  lies between the upper region  506  of the cylindrical base  207  and the top surface  205  of the base portion  202 . The bottom center region  534  is concentric with the cylindrical base  207 . The bottom center region  534  has a smaller diameter than the cylindrical base  207 . The bottom center region  534  has a smaller diameter than the ID of the rim  203  of the base portion  202 . The bottom center region  534  lies directly above the first plenum  224  in the base portion  202 . The bottom center region  534  lies between distal ends of the bores  254  that connect to the first plenum  224  in the base portion  202  at  252 - 1  and  252 - 2 . 
     The second plenum  502  is formed in the bottom center region  534  by removing (by machining) material from the bottom center region  534  to form a recess  535  in the bottom center region  534 . The recess  535  is cylindrical. The recess  535  has a smaller diameter than the bottom center region  534 . The recess  535  and has a depth h1. A metal plate  550  having a slightly smaller diameter and less height than the bottom center region  534  is machined to form pillars  520 ,  520 - 2 ,  520 - 3 , ..., and  520 -N (collectively the pillars  520 ), where N is a positive integer. The pillars  520  extend vertically upwards into the recess  535  from the metal plate  550  parallel to the vertical axis of the showerhead  500 . A combined height h2 of the metal plate  550  and the pillars  520  is equal to the depth h1 of the recess  535 . Accordingly, when the metal plate  550  is inserted into the recess  535 , the pillars  520  contact the upper edge of the recess  535  (i.e., the pillars  520  contact the bottom center region  534 ). The pillars  520  are distributed from the center of the bottom center region  534  towards an OD of the recess  535  in the bottom center region  534 . The pillars  520  are interstitial but otherwise structurally and functionally similar to the pillars  220  in the first plenum  224  as described above with reference to the first showerhead  200  and as explained in further detail with reference to  FIGS.  46  and  47   . 
     The metal plate  550  is inserted into the recess  535  and is sealingly attached to the bottom center region  534  such that the bottom of the metal plate  550  is level (flush) with the bottom surface  211  of the cylindrical base  207 . The second plenum  502  is defined by the bottom center region  534 , the metal plate  550 , the recess  535 , and the pillars  520 . The metal plate  550  separates and seals the second plenum  502  from the first plenum  224 . When the metal plate  550  is sealingly attached to the bottom center region  534 , the metal plate  550  prevents process gases from the two plenums from mixing with each other. Thus, the bottom center region  534  and the metal plate  550  define the second plenum  502 . 
     The second plenum  502  extends radially across the bottom center region  534  of the cylindrical base  207 . The second plenum  502  lies in a plane perpendicular to the vertical axis of the showerhead  500 . The pillars  520  increase the solidity and hence the heat conduction through the second plenum  502  in the same manner as the pillars  220 . That is, all of the characteristics (e.g., design features and constraints) of the pillars  220   described above with reference to the showerhead  200  apply equally to the pillars  520  and are therefore not repeated for brevity. 
     The second plenum  502  lies directly above the first plenum  224  in the base portion  202  along the vertical axis of the showerhead  500 . Accordingly, when the base portion  202  with the first plenum  224  is attached to the backplate  204  with the intervening metal plate  550  between the first and second plenums  224 ,  502 , the pillars  220  in the first plenum  224  abut the bottom of the second plenum  502 . The pillars  220  in the first plenum  224  are interstitial to the pillars  520  in the second plenum  502  in the backplate  204 . 
       FIG.  43 C  shows another way to define the second plenum  502 . Instead of machining the metal plate  550  to form the pillars  520 , the bottom center region  534  of the cylindrical base  207  can be machine to form the pillars  520  in the recess  535 . The pillars  520  extend from the upper region  506  of the cylindrical base  207  through the recess  535 . The pillars  520  extend towards the bottom surface  211  of the cylindrical base  207  parallel to the vertical axis of the showerhead  500 . The height of the pillars  520  is equal to the depth h1 of the recess  535 . The pillars  520  formed on the bottom center region  534  are otherwise identical to the pillars  520  formed on the metal plate  550  (shown in  FIG.  43 B ). A metal plate  553  without the pillars  520  is sealingly attached to the bottom center region  534  in the same manner as the metal plate  550  is attached to the bottom center region  534  as described above to define the second plenum  502 . The pillars  520  contact the metal plate  553 . The base portion  202  with the first plenum  224  is attached to the backplate  204  with the metal plate  553  interposed between the first and second plenums  224 ,  502 . The second plenum  502  is defined by the bottom center region  534 , the metal plate  553 , the recess  535 , and the pillars  520 . 
     The pillars  520  formed in the bottom center region  534  of the cylindrical base  207  increase the solidity and hence the heat conduction through the second plenum  502  in the same manner as the pillars  220 . That is, all of the characteristics (e.g., design features and constraints) of the pillars  220  described above with reference to the showerhead  200  apply equally to the pillars  520  and are therefore not repeated for brevity. 
       FIG.  43 D  shows an alternate way to form both the first and second plenums  224 ,  502 . For example, the recess  535  can be machined in bottom center region  534  of the cylindrical base  207 . A recess  536  can be machined in the base portion  202  by removing material from the top surface  205  of the base portion  202 . The recess  536  is also cylindrical and concentric with the base portion  202 . A diameter of the recess  536  is equal to the ID of the rim  203  of the base portion  202 . The diameter of the recess  536  is greater than the diameter of the recess  535 . The recess  536  has a depth h3. The recess  536  extends radially in the base portion  202  up to  252 - 1 ,  252 - 2  to where the bores  254  connect to the base portion  202 . Accordingly, when the base portion  202  is attached to the cylindrical base  207 , the bores  254  are in fluid communication with the recess  536 . 
     A metal plate  551  having a slightly smaller diameter than the bottom center region  534  can be machined to form the pillars  520 ,  220  on top and bottom surfaces of the metal plate  551 . The pillars  520  extend vertically upwards into the recess  535  from the metal plate  551  parallel to the vertical axis of the showerhead  500 . When the metal plate  551  is attached to the bottom center region  534  of the cylindrical base  207 , the pillars  520  contact the upper edge of the recess  535  (i.e., the pillars  520  contact the bottom center region  534 ). The pillars  220  extend vertically downwards into the recess  536  from the metal plate  551  parallel to the vertical axis of the showerhead  500 . When the metal plate  551  is attached to the bottom center region  534  of the cylindrical base  207  and the base portion  202  is attached to the cylindrical base  207 , the pillars  220  contact the lower edge of the recess  536  (i.e., the pillars  220  contact the base portion  202 ). 
     When the metal plate  551  is sealingly attached to the bottom center region  534  of the cylindrical base  207  as described above, the second plenum  502  is defined by the bottom center region  534 , the top surface of the metal plate  551 , the recess  535 , and the pillars  520 . Thereafter, when the base portion  202  is sealingly attached to the cylindrical base  207 , the first plenum  224  is defined by the base portion  202 , the bottom surface of the metal plate  551 , the recess  536 , and the pillars  220 . When the metal plate  551  is sealingly attached to the bottom center region  534  of the cylindrical base  207 , the bottom surface of the metal plate  551  is level (flush) with the bottom surface  211  of the cylindrical base  207 . Accordingly, when the base portion  202  is sealingly attached to the cylindrical base  207 , the pillars  220  extend below the bottom surface  211  of the cylindrical base  207  into the first plenum  224  and abut the bottom of the recess  536  in the base portion  202 . Accordingly, the through holes  522  can be drilled from the bottom surface  213  of the base portion  202  through the pillars  520  into the second plenum  502 . 
     The recess  535  has a depth h1 as described above. The recess  536  has a depth h3. A height h2 from the bottom surface of the metal plate  550  and to the top of the pillars  520  is equal to the depth h1 of the recess  535 . A height h4 of the pillars  220  is equal to the depth h3 of the recess  536 . The combined thickness of the metal plate  551  measured from the top of the pillars  520  to the bottom of the pillars  220  is h2+h3. The pillars  520  are distributed from the center of the bottom center region  534  towards an OD of the recess  535  in the bottom center region  534 . The pillars  220  are distributed from the center of the base portion  202  towards an OD of the recess  536  in the base portion  202 . The pillars  520  are interstitial with the pillars  220  in the first plenum  224  and align with through holes  522  (described below). The through holes  222  are drilled from the bottom surface  213  of the base portion  202  into the recess  536  (i.e., into the first plenum  224 ). The through holes  522  are drilled from the bottom surface  213  of the base portion  202 , through the metal plate  551  (i.e., through the pillars  520 ) into the recess  535  (i.e., into the second plenum  502 ). Thus, the first and second plenums  224 ,  502  are disjoint (i.e., not in fluid communication with each other). 
     The pillars  520  and  230  formed on the metal plate  551  increase the solidity and hence the heat conduction through the first and second plenums  224 ,  502  in the same manner as the pillars  220 . That is, all of the characteristics (e.g., design features and constraints) of the pillars  220  described above with reference to the showerhead  200  apply equally to the pillars  520  and  230  formed on the metal plate  551  and are therefore not repeated for brevity. 
     In  FIG.  43 A , a plurality of through holes  522 - 1 ,  522 - 2 ,  522 - 3 , ..., and  522 -M (collectively the through holes  522 ), where M is a positive integer, are drilled through the bottom surface  213  of the base portion  202 . The through holes  522  are drilled through the center of the pillars  220  (one through hole  522  per pillar  220 ) and through the metal plate  550  shown in  FIGS.  43 B and  43 C . The through holes  522  are in fluid communication with the second plenum  502  but are not in fluid communication with the first plenum  224 . As shown and described in further detail with reference to  FIGS.  44 - 47   , the pillars  520  of the second plenum  502  are arranged interstitially with the pillars  220 . The through holes  522  of the second plenum  502  are arranged interstitially with the through holes  222 . 
     The first gas flows through the first gas inlet  208 , through the bores  250  and  254 , the first plenum  224 , and the through holes  222  into the processing chamber  102  shown in  FIG.  1 B . The second gas flows through the second gas inlet  508 , through the bore  504 , the second plenum  502 , and the through holes  522  in the pillars  220  into the processing chamber  102  shown in  FIG.  1 B . The remaining features shown in  FIG.  43 A  are shown and described with reference to  FIG.  7 A  and their description is therefore omitted for brevity. 
       FIGS.  44  and  45    show a top view of a cross-section of the base portion  202  taken along lines D-D shown in  FIG.  41    showing the first plenum  224  in detail.  FIG.  45    shows the pattern of elements  220 ,  222 ,  520 , and  522  in greater detail than in  FIG.  44   . The top view of the cross-section of the base portion  202  is described with reference to both  FIGS.  44  and  45   .  FIGS.  44  and  45    are identical to  FIGS.  4  and  5    except that unlike in  FIGS.  4  and  5   , in  FIGS.  44  and  45   , the pillars  220  of the first plenum  224  additionally include the through holes  522  (one through hole  522  per pillar  220 ). The through holes  522  are drilled through the bottom surface  213  of the base portion  202 , the pillars  220  of the first plenum  224 , the top surface  205  of the base portion  202 , and the metal plate  550 . Thus, the through holes  522  are in fluid communication with the second plenum  502  but are not in fluid communication with the first plenum  224 . The through holes  522  are distributed from the center of the bottom center region  534  towards the OD of the bottom center region  534 . The through holes  522  follow the geometrical arrangement of the pillars  220 , which is described with reference to  FIGS.  4  and  5    and is therefore not repeated for brevity. 
       FIGS.  46  and  47    show a top view of a cross-section of the bottom center region  534  of the cylindrical base  207  of the backplate  204  taken along lines E-E shown in  FIG.   41    showing the second plenum  502  in detail.  FIG.  46    shows the arrangement of the pillars  520  and the through holes  522  of the second plenum  502 .  FIG.  47    shows the pattern of the pillars  520  and the through holes  522  in detail. 
     In  FIG.  46   , the pillars  520  and the through holes  522  are arranged in the second plenum  502  along the first and second axes  221 ,  223 . The pillars  520  and the through holes  522  are arranged such that one through hole  522  lies between two pillars  520  along the first and second axes  221 ,  223 . 
     In  FIG.  47   , the through holes  522  are arranged on the vertices of the hexagon  230 . One through hole  522  lies at the center of the hexagon  230 . Consequently, since the pillars  220  of the first plenum  224 , which lies directly below the second plenum  502 , are also arranged on the vertices and at the center of the hexagon  230 , each through hole  522  of the second plenum  502  aligns with a pillar  220  of the first plenum  224  along the vertical axis of the showerhead  500 . 
     Additionally, in each hexagon  230 , one pillar  520  lies between two through holes  522  along the first and second axes  221 ,  223 . Accordingly, each pillar  520  of the second plenum  502  lies above two through holes  222  of the first plenum  224  that lie between two pillars  220  arranged in the hexagon  230  in the first plenum  224  as shown in  FIG.  45   . Thus, the pillars  520  in the second plenum  502  are interstitial to the pillars  220  in the first plenum  224 . The through holes  522  in the second plenum  502  not only align with the pillars  220  in the first plenum  224  but are also interstitial to the through holes  222  in the first plenum  224 . 
       FIG.  48    shows a cross-section of the showerhead  500  taken along lines B-B shown in  FIG.  42   .  FIG.  48    is identical to  FIG.  6    except that in addition to showing all of the elements shown in  FIG.  6   ,  FIG.  48    shows the additional second gas inlet  508 , the bore  504 , and the second plenum  502  with the pillars  520  and the through holes  522  described above. 
       FIG.  49    shows a cross-section of the showerhead  200  taken along lines C-C shown in  FIG.  42   .  FIG.  48    is identical to  FIG.  6    except that in addition to showing all of the elements shown in  FIG.  6   ,  FIG.  48    shows the additional second gas inlet  508 , the bore  504 , and the second plenum  502  with the pillars  520  and the through holes  522  described above. 
     Throughout the present disclosure, references are made to hexagons and hexagonal patterns of the pillars and through holes. As used herein, a hexagon comprises a regular-hexagon. Alternatively, a hexagonal pattern can also be viewed as including patterns arranged in equilateral triangles. Accordingly, in the hexagonal patterns of the pillars and through holes described above, a hexagonal unit cell of the pillars and through holes can include a regular-hexagon-shaped unit cell or an equilateral-triangular-shaped unit cell. 
     Further, throughout present disclosure, the gas inlets of the dual plenum showerheads are shown and described as being coaxial. Instead, the gas inlets and corresponding bores can be arranged side-by-side (i.e., adjacent to each other). Alternatively, the inlets and the respective bores can be arranged in other ways. 
     The foregoing description is merely illustrative in nature and is not intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure comprises particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure. 
     Further, although each of the embodiments is described above as having certain features, any one or more of those features described with respect to any embodiment of the disclosure can be implemented in and/or combined with features of any of the other embodiments, even if that combination is not explicitly described. In other words, the described embodiments are not mutually exclusive, and permutations of one or more embodiments with one another remain within the scope of this disclosure. 
     Spatial and functional relationships between elements (for example, between modules, circuit elements, semiconductor layers, etc.) are described using various terms, including “connected,” “engaged,” “coupled,” “adjacent,” “next to,” “on top of,” “above,” “below,” and “disposed.” Unless explicitly described as being “direct,” when a relationship between first and second elements is described in the above disclosure, that relationship can be a direct relationship where no other intervening elements are present between the first and second elements, but can also be an indirect relationship where one or more intervening elements are present (either spatially or functionally) between the first and second elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR, and should not be construed to mean “at least one of A, at least one of B, and at least one of C.” 
     In some implementations, a controller is part of a system, which may be part of the above-described examples. Such systems can comprise semiconductor processing equipment, including a processing tool or tools, chamber or chambers, a platform or platforms for processing, and/or specific processing components (a wafer pedestal, a gas flow system, etc.). These systems may be integrated with electronics for controlling their operation before, during, and after processing of a semiconductor wafer or substrate. The electronics may be referred to as the “controller,” which may control various components or subparts of the system or systems. 
     The controller, depending on the processing requirements and/or the type of system, may be programmed to control any of the processes disclosed herein, including the delivery of processing gases, temperature settings (e.g., heating and/or cooling), pressure settings, vacuum settings, power settings, radio frequency (RF) generator settings, RF matching circuit settings, frequency settings, flow rate settings, fluid delivery settings, positional and operation settings, wafer transfers into and out of a tool and other transfer tools and/or load locks connected to or interfaced with a specific system. 
     Broadly speaking, the controller may be defined as electronics having various integrated circuits, logic, memory, and/or software that receive instructions, issue instructions, control operation, enable cleaning operations, enable endpoint measurements, and the like. The integrated circuits may include chips in the form of firmware that store program instructions, digital signal processors (DSPs), chips defined as application specific integrated circuits (ASICs), and/or one or more microprocessors, or microcontrollers that execute program instructions (e.g., software). 
     Program instructions may be instructions communicated to the controller in the form of various individual settings (or program files), defining operational parameters for carrying out a particular process on or for a semiconductor wafer or to a system. The operational parameters may, in some embodiments, be part of a recipe defined by process engineers to accomplish one or more processing steps during the fabrication of one or more layers, materials, metals, oxides, silicon, silicon dioxide, surfaces, circuits, and/or dies of a wafer. 
     The controller, in some implementations, may be a part of or coupled to a computer that is integrated with the system, coupled to the system, otherwise networked to the system, or a combination thereof. For example, the controller may be in the “cloud” or all or a part of a fab host computer system, which can allow for remote access of the wafer processing. The computer may enable remote access to the system to monitor current progress of fabrication operations, examine a history of past fabrication operations, examine trends or performance metrics from a plurality of fabrication operations, to change parameters of current processing, to set processing steps to follow a current processing, or to start a new process. 
     In some examples, a remote computer (e.g. a server) can provide process recipes to a system over a network, which may include a local network or the Internet. The remote computer may include a user interface that enables entry or programming of parameters and/or settings, which are then communicated to the system from the remote computer. In some examples, the controller receives instructions in the form of data, which specify parameters for each of the processing steps to be performed during one or more operations. It should be understood that the parameters may be specific to the type of process to be performed and the type of tool that the controller is configured to interface with or control. 
     Thus, as described above, the controller may be distributed, such as by comprising one or more discrete controllers that are networked together and working towards a common purpose, such as the processes and controls described herein. An example of a distributed controller for such purposes would be one or more integrated circuits on a chamber in communication with one or more integrated circuits located remotely (such as at the platform level or as part of a remote computer) that combine to control a process on the chamber. 
     Without limitation, example systems may include a plasma etch chamber or module, a deposition chamber or module, a spin-rinse chamber or module, a metal plating chamber or module, a clean chamber or module, a bevel edge etch chamber or module, a physical vapor deposition (PVD) chamber or module, a chemical vapor deposition (CVD) chamber or module, an atomic layer deposition (ALD) chamber or module, an atomic layer etch (ALE) chamber or module, an ion implantation chamber or module, a track chamber or module, and any other semiconductor processing systems that may be associated or used in the fabrication and/or manufacturing of semiconductor wafers. 
     As noted above, depending on the process step or steps to be performed by the tool, the controller might communicate with one or more of other tool circuits or modules, other tool components, cluster tools, other tool interfaces, adjacent tools, neighboring tools, tools located throughout a factory, a main computer, another controller, or tools used in material transport that bring containers of wafers to and from tool locations and/or load ports in a semiconductor manufacturing factory.