Patent Publication Number: US-10312058-B2

Title: Plasma uniformity control by gas diffuser hole design

Description:
CROSS-REFERENCE TO OTHER APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/932,618 (APPM/9162C03), filed Nov. 4, 2015, which is a continuation of U.S. patent application Ser. No. 13/207,227 (APPM/9162C02), filed Aug. 10, 2011, which is a continuation of U.S. patent application Ser. No. 10/889,683 (APPM/9162), filed Jul. 12, 2004, which issued as U.S. Pat. No. 8,083,853 on Dec. 27, 2011, which claims benefit of U.S. Provisional Patent Application Ser. No. 60/570,876 (APPM/9162L), filed May 12, 2004. Each of the aforementioned patent applications is herein incorporated by reference. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     Field of the Invention 
     Embodiments of the invention generally relate to a gas distribution plate assembly and method for distributing gas in a processing chamber. 
     Description of the Background Art 
     Liquid crystal displays or flat panels are commonly used for active matrix displays such as computer and television monitors. Plasma enhanced chemical vapor deposition (PECVD) is generally employed to deposit thin films on a substrate such as a transparent substrate for flat panel display or semiconductor wafer. PECVD is generally accomplished by introducing a precursor gas or gas mixture into a vacuum chamber that contains a substrate. The precursor gas or gas mixture is typically directed downwardly through a distribution plate situated near the top of the chamber. The precursor gas or gas mixture in the chamber is energized (e.g., excited) into a plasma by applying radio frequency (RF) power to the chamber from one or more RF sources coupled to the chamber. The excited gas or gas mixture reacts to form a layer of material on a surface of the substrate that is positioned on a temperature controlled substrate support. Volatile by-products produced during the reaction are pumped from the chamber through an exhaust system. 
     Flat panels processed by PECVD techniques are typically large, often exceeding 370 mm×470 mm. Large area substrates approaching and exceeding 4 square meters are envisioned in the near future. Gas distribution plates (or gas diffuser plates) utilized to provide uniform process gas flow over flat panels are relatively large in size, particularly as compared to gas distribution plates utilized for 200 mm and 300 mm semiconductor wafer processing. 
     As the size of substrates continues to grow in the TFT-LCD industry, film thickness and film property uniformity control for large area plasma-enhanced chemical vapor deposition (PECVD) becomes an issue. TFT is one type of flat panel display. The difference of deposition rate and/or film property, such as film stress, between the center and the edge of the substrate becomes significant. 
     Therefore, there is a need for an improved gas distribution plate assembly that improves the uniformities of film deposition thickness and film properties. 
     SUMMARY OF THE INVENTION 
     Embodiments of a gas distribution plate for distributing gas in a processing chamber are provided. In one embodiment, a gas distribution plate assembly for a plasma processing chamber comprises a diffuser plate having an upstream side and a downstream side, and inner and outer gas passages passing between the upstream and downstream sides of the diffuser plate and comprising hollow cathode cavities at the downstream side, wherein the hollow cathode cavity volume density of the inner gas passages are less than the hollow cathode cavity volume density of the outer gas passages. 
     In another embodiment, a gas distribution plate assembly for a plasma processing chamber comprises a diffuser plate having an upstream side and a downstream side, and inner and outer gas passages passing between the upstream and downstream sides of the diffuser plate and comprising hollow cathode cavities at the downstream side, wherein the hollow cathode cavity surface area density of the inner gas passages are less than the hollow cathode cavity surface area density of the outer gas passages. 
     In another embodiment, a gas distribution plate assembly for a plasma processing chamber comprises a diffuser plate having an upstream side and a down stream side, and a plurality of gas passages passing between the upstream and downstream sides of the diffuser plate, wherein the densities of hollow cathode cavities gradually increase from the center to the edge of the diffuser plate. 
     In another embodiment, a plasma processing chamber comprises a diffuser plate having an upstream side and a downstream side, inner and outer gas passages passing between the upstream and downstream sides of the diffuser plate and comprising hollow cathode cavities at the downstream side, wherein the hollow cathode cavity volume density of the inner gas passages are less than the hollow cathode cavity volume density of the outer gas passages, and a substrate support adjacent the downstream side of the diffuser plate. 
     In another embodiment, a plasma processing chamber comprises a diffuser plate having an upstream side and a downstream side, inner and outer gas passages passing between the upstream and downstream sides of the diffuser plate and comprising hollow cathode cavities at the downstream side, wherein the hollow cathode cavity surface area density of the inner gas passages are less than the hollow cathode cavity surface area density of the outer gas passages, and a substrate support adjacent the downstream side of the diffuser plate. 
     In another embodiment, a plasma processing chamber comprises a diffuser plate having an upstream side and a down stream side, a plurality of gas passages passing between the upstream and downstream sides of the diffuser plate, wherein the densities of hollow cathode cavities gradually increase from the center to the edge of the diffuser plate, and a substrate support adjacent the downstream side of the diffuser plate. 
     In another embodiment, a gas distribution plate assembly for a plasma processing chamber comprises a diffuser plate having an upstream side and a down stream side and the gas diffuser plate are divided into a number of concentric zones, and a plurality of gas passages passing between the upstream and downstream sides of the diffuser plate, wherein the gas passages in each zones are identical and the density, the volume, or surface area of hollow cathode cavities of gas passages in each zone gradually increase from the center to the edge of the diffuser plate. 
     In another embodiment, a method of making a gas diffuser plate for a plasma processing chamber, comprises making a gas diffuser plate to have an upstream side and a down stream side, and a plurality of gas passages passing between the upstream and downstream sides of the diffuser plate, bending the diffuser plate to make it convex smoothly toward downstream side, and machining out the convex surface to flatten the downstream side surface. 
     In another embodiment, a method of making a gas diffuser plate for a plasma processing chamber comprises machining a gas diffuser plate to have an upstream side and a down stream side, and a plurality of gas passages passing between the upstream and downstream sides of the diffuser plate, wherein densities, volumes or surface area of hollow cathode cavities of the diffuser plate gradually increase from the center to the edge of the diffuser plate. 
     In another embodiment, a method of depositing a thin film on a substrate comprises placing a substrate in a process chamber with a gas diffuser plate having an upstream side and inner and outer gas passages passing between the upstream and downstream sides of the diffuser plate and comprising hollow cathode cavities at the downstream side, wherein either the hollow cathode cavity volume density, or the hollow cathode cavity surface area density, or the hollow cathode cavity density of the inner gas passages are less than the same parameter of the outer gas passages, flowing process gas(es) through a diffuser plate toward a substrate supported on a substrate support, creating a plasma between the diffuser plate and the substrate support, and depositing a thin film on the substrate in the process chamber. 
     In another embodiment, a diffuser plate comprises a body having a top surface and a bottom surface, a plurality of gas passages between the top surface the bottom surface, and an outer region and an inner region wherein the body between the top and the bottom of the outer region is thicker than the body between the top and the bottom of the inner region. 
     In another embodiment, a method of making a gas diffuser plate for a plasma processing chamber comprises making a gas diffuser plate to have an upstream side and a down stream side, and a plurality of gas passages passing between the upstream and downstream sides of the diffuser plate, and machining the downstream surface to make the downstream surface concave. 
     In yet another embodiment, a method of making a gas diffuser plate for a plasma processing chamber comprises bending a diffuser plate that have an upstream side and a down stream side to make the downstream surface concave and the upstream surface convex, making a plurality of gas passages passing between the upstream and downstream sides of the diffuser plate by making hollow cathode cavities to the same depth from a fictitious flat downstream surface, and making all gas passages to have the same-size orifice holes which are connected to the hollow cathode cavities. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  depicts a cross-sectional schematic view of a bottom gate thin film transistor. 
         FIG. 2  is a schematic cross-sectional view of an illustrative processing chamber having one embodiment of a gas distribution plate assembly of the present invention. 
         FIG. 3  depicts a cross-sectional schematic view of a gas diffuser plate. 
         FIG. 4A  shows the process flow of depositing a thin film on a substrate in a process chamber with a diffuser plate. 
         FIG. 4B  shows the deposition rate measurement across a 1500 mm by 1800 mm substrate collected from deposition with a diffuser plate with uniform diffuser holes diameters and depths. 
         FIG. 5  shows 2 sides ( 501  and  502 ) of the substrate that are close to the sides with pumping channels closed and the 5 measurement locations on a substrate. 
         FIG. 6A  (Prior Art) illustrates the concept of hollow cathode effect. 
         FIGS. 6B-6G  illustrates various designs of hollow cathode cavities. 
         FIG. 7A  shows the definition of diameter “D”, the depth “d” and the flaring angle “□” of the bore that extends to the downstream end of a gas passage. 
         FIG. 7B  shows the dimensions of a gas passage. 
         FIG. 7C  shows the dimensions of a gas passage. 
         FIG. 7D  shows the dimensions of a gas passage. 
         FIG. 7E  shows the distribution of gas passages across a diffuser plate. 
         FIG. 8  shows the deposition rate measurement across a 1500 mm by 1800 mm substrate collected from deposition with a diffuser plate with a distribution of gas passages across the diffuser plate as shown in  FIG. 7E . 
         FIG. 9A  shows the process flow of making a diffuser plate. 
         FIG. 9B  shows a bent diffuser plate. 
         FIG. 9C  shows a diffuser plate that was previously bent and the side that facing the downstream side was machined to be flat. 
         FIG. 9D  shows the distribution of depths of diffuser bores that extends to the downstream ends of gas passages of a diffuser plate used to process 1500 mm by 1850 mm substrates. 
         FIG. 9E  shows the measurement of deposition rates across a 1500 mm by 1850 mm substrate. 
         FIG. 9F  shows the distribution of depths of diffuser bores that extends to the downstream ends of gas passages of a diffuser plate used to process 1870 mm by 2200 mm substrates. 
         FIG. 9G  shows the measurement of deposition rates across an 1870 mm by 2200 mm substrate. 
         FIG. 10A  shows the process flow of bending the diffuser plate by a thermal process. 
         FIG. 10B  shows the diffuser plate on the supports in the thermal environment that could be used to bend the diffuser plate. 
         FIG. 10C  shows the convex diffuser plate on the supports in the thermal environment. 
         FIG. 11A  shows the process flow of bending the diffuser plate by a vacuum process. 
         FIG. 11B  shows the diffuser plate on the vacuum assembly. 
         FIG. 11C  shows the convex diffuser plate on the vacuum assembly. 
         FIG. 12A  shows the process flow of creating a diffuser plate with varying diameters and depths of bores that extends to the downstream side of the diffuser plate. 
         FIG. 12B  shows the cross section of a diffuser plate with varying diameters and depths of bores that extends to the downstream side of the diffuser plate. 
         FIG. 12C  shows a diffuser plate with substantially identical diffuser holes from center to edge of the diffuser plate. 
         FIG. 12D  shows the diffuser plate of  FIG. 12C  after the bottom surface has been machined into a concave shape. 
         FIG. 12E  shows the diffuser plate of  FIG. 12D  after its bottom surface has been pulled substantially flat. 
         FIG. 12F  shows a diffuser plate, without any diffuser holes, that has been bent into a concave (bottom surface) shape. 
         FIG. 12G  shows the diffuser plate of  FIG. 12F  with diffuser holes. 
         FIG. 12H  shows the diffuser plate of  FIG. 12G  after its bottom surface has been pulled substantially flat. 
         FIG. 12I  shows a diffuser plate with diffuser holes in multiple zones. 
         FIG. 12J  shows a diffuser plate with mixed hollow cathode cavity diameters and the inner region hollow cathode cavity volume and/or surface area density is higher than the outer region hollow cathode cavity volume and/or surface area density. 
         FIG. 12K  shows a diffuser plate with most of the hollow cathode cavities the same, while there are a few larger hollow cathode cavities near the edge of the diffuser plate. 
         FIG. 13  shows the downstream side view of a diffuser plate with varying diffuser hole densities. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     The invention generally provides a gas distribution assembly for providing gas delivery within a processing chamber. The invention is illustratively described below in reference to a plasma enhanced chemical vapor deposition system configured to process large area substrates, such as a plasma enhanced chemical vapor deposition (PECVD) system, available from AKT, a division of Applied Materials, Inc., Santa Clara, Calif. However, it should be understood that the invention has utility in other system configurations such as etch systems, other chemical vapor deposition systems and any other system in which distributing gas within a process chamber is desired, including those systems configured to process round substrates. 
       FIG. 1  illustrates cross-sectional schematic views of a thin film transistor structure. A common TFT structure is the back channel etch (BCE) inverted staggered (or bottom gate) TFT structure shown in  FIG. 1 . The BCE process is preferred, because the gate dielectric (SiN), and the intrinsic as well as n+ doped amorphous silicon films can be deposited in the same PECVD pump-down run. The BCE process shown here involves only 5 patterning masks. The substrate  101  may comprise a material that is essentially optically transparent in the visible spectrum, such as, for example, glass or clear plastic. The substrate may be of varying shapes or dimensions. Typically, for TFT applications, the substrate is a glass substrate with a surface area greater than about 500 mm 2 . A gate electrode layer  102  is formed on the substrate  101 . The gate electrode layer  102  comprises an electrically conductive layer that controls the movement of charge carriers within the TFT. The gate electrode layer  102  may comprise a metal such as, for example, aluminum (Al), tungsten (W), chromium (Cr), tantalum (Ta), or combinations thereof, among others. The gate electrode layer  102  may be formed using conventional deposition, lithography and etching techniques. Between the substrate  101  and the gate electrode layer  102 , there may be an optional insulating material, for example, such as silicon dioxide (SiO 2 ) or silicon nitride (SiN), which may also be formed using an embodiment of a PECVD system described in this invention. The gate electrode layer  102  is then lithographically patterned and etched using conventional techniques to define the gate electrode. 
     A gate dielectric layer  103  is formed on the gate electrode layer  102 . The gate dielectric layer  103  may be silicon dioxide (SiO 2 ), silicon oxynitride (SiON), or silicon nitride (SiN), deposited using an embodiment of a PECVD system described in this invention. The gate dielectric layer  103  may be formed to a thickness in the range of about 100 Å to about 6000 Å. 
     A bulk semiconductor layer  104  is formed on the gate dielectric layer  103 . The bulk semiconductor layer  104  may comprise polycrystalline silicon (polysilicon) or amorphous silicon (□-Si), which could be deposited using an embodiment of a PECVD system described in this invention or other conventional methods known to the art. Bulk semiconductor layer  104  may be deposited to a thickness in the range of about 100 Å to about 3000 Å. A doped semiconductor layer  105  is formed on top of the semiconductor layer  104 . The doped semiconductor layer  105  may comprise n-type (n+) or p-type (p+) doped polycrystalline (polysilicon) or amorphous silicon (□-Si), which could be deposited using an embodiment of a PECVD system described in this invention or other conventional methods known to the art. Doped semiconductor layer  105  may be deposited to a thickness within a range of about 100 Å to about 3000 Å. An example of the doped semiconductor layer  105  is n+ doped □-Si film. The bulk semiconductor layer  104  and the doped semiconductor layer  105  are lithographically patterned and etched using conventional techniques to define a mesa of these two films over the gate dielectric insulator, which also serves as storage capacitor dielectric. The doped semiconductor layer  105  directly contacts portions of the bulk semiconductor layer  104 , forming a semiconductor junction. 
     A conductive layer  106  is then deposited on the exposed surface. The conductive layer  106  may comprise a metal such as, for example, aluminum (Al), tungsten (W), molybdenum (Mo), chromium (Cr), tantalum (Ta), and combinations thereof, among others. The conductive layer  106  may be formed using conventional deposition techniques. Both the conductive layer  106  and the doped semiconductor layer  105  may be lithographically patterned to define source and drain contacts of the TFT. Afterwards, a passivation layer  107  may be deposited. Passivation layer  107  conformably coats exposed surfaces. The passivation layer  107  is generally an insulator and may comprise, for example, silicon dioxide (SiO 2 ) or silicon nitride (SiN). The passivation layer  107  may be formed using, for example, PECVD or other conventional methods known to the art. The passivation layer  107  may be deposited to a thickness in the range of about 1000 Å to about 5000 Å. The passivation layer  107  is then lithographically patterned and etched using conventional techniques to open contact holes in the passivation layer. 
     A transparent conductor layer  108  is then deposited and patterned to make contacts with the conductive layer  106 . The transparent conductor layer  108  comprises a material that is essentially optically transparent in the visible spectrum and is electrically conductive. Transparent conductor layer  108  may comprise, for example, indium tin oxide (ITO) or zinc oxide, among others. Patterning of the transparent conductive layer  108  is accomplished by conventional lithographical and etching techniques. 
     The doped or un-doped (intrinsic) amorphous silicon (□-Si), silicon dioxide (SiO2), silicon oxynitride (SiON) and silicon nitride (SiN) films used in liquid crystal displays (or flat panels) could all be deposited using an embodiment of a plasma enhanced chemical vapor deposition (PECVD) system described in this invention. The TFT structure described here is merely used as an example. The current invention applies to manufacturing any devices that are applicable. 
       FIG. 2  is a schematic cross-sectional view of one embodiment of a plasma enhanced chemical vapor deposition system  200 , available from AKT, a division of Applied Materials, Inc., Santa Clara, Calif. The system  200  generally includes a processing chamber  202  coupled to a gas source  204 . The processing chamber  202  has walls  206  and a bottom  208  that partially define a process volume  212 . The process volume  212  is typically accessed through a port (not shown) in the walls  206  that facilitate movement of a substrate  240  into and out of the processing chamber  202 . The walls  206  and bottom  208  are typically fabricated from a unitary block of aluminum or other material compatible with processing. The walls  206  support a lid assembly  210  that contains a pumping plenum  214  that couples the process volume  212  to an exhaust port (that includes various pumping components, not shown). 
     A temperature controlled substrate support assembly  238  is centrally disposed within the processing chamber  202 . The support assembly  238  supports a glass substrate  240  during processing. In one embodiment, the substrate support assembly  238  comprises an aluminum body  224  that encapsulates at least one embedded heater  232 . The heater  232 , such as a resistive element, disposed in the support assembly  238 , is coupled to an optional power source  274  and controllably heats the support assembly  238  and the glass substrate  240  positioned thereon to a predetermined temperature. Typically, in a CVD process, the heater  232  maintains the glass substrate  240  at a uniform temperature between about 150 to at least about 460 degrees Celsius, depending on the deposition processing parameters for the material being deposited. 
     Generally, the support assembly  238  has a lower side  226  and an upper side  234 . The upper side  234  supports the glass substrate  240 . The lower side  226  has a stem  242  coupled thereto. The stem  242  couples the support assembly  238  to a lift system (not shown) that moves the support assembly  238  between an elevated processing position (as shown) and a lowered position that facilitates substrate transfer to and from the processing chamber  202 . The stem  242  additionally provides a conduit for electrical and thermocouple leads between the support assembly  238  and other components of the system  200 . 
     A bellows  246  is coupled between support assembly  238  (or the stem  242 ) and the bottom  208  of the processing chamber  202 . The bellows  246  provides a vacuum seal between the chamber volume  212  and the atmosphere outside the processing chamber  202  while facilitating vertical movement of the support assembly  238 . 
     The support assembly  238  generally is grounded such that RF power supplied by a power source  222  to a gas distribution plate assembly  218  positioned between the lid assembly  210  and substrate support assembly  238  (or other electrode positioned within or near the lid assembly of the chamber) may excite gases present in the process volume  212  between the support assembly  238  and the distribution plate assembly  218 . The RF power from the power source  222  is generally selected commensurate with the size of the substrate to drive the chemical vapor deposition process. 
     The support assembly  238  additionally supports a circumscribing shadow frame  248 . Generally, the shadow frame  248  prevents deposition at the edge of the glass substrate  240  and support assembly  238  so that the substrate does not stick to the support assembly  238 . The support assembly  238  has a plurality of holes  228  disposed therethrough that accept a plurality of lift pins  250 . The lift pins  250  are typically comprised of ceramic or anodized aluminum. The lift pins  250  may be actuated relative to the support assembly  238  by an optional lift plate  254  to project from the support surface  230 , thereby placing the substrate in a spaced-apart relation to the support assembly  238 . 
     The lid assembly  210  provides an upper boundary to the process volume  212 . The lid assembly  210  typically can be removed or opened to service the processing chamber  202 . In one embodiment, the lid assembly  210  is fabricated from aluminum (Al). The lid assembly  210  includes a pumping plenum  214  formed therein coupled to an external pumping system (not shown). The pumping plenum  214  is utilized to channel gases and processing by-products uniformly from the process volume  212  and out of the processing chamber  202 . 
     The lid assembly  210  typically includes an entry port  280  through which process gases provided by the gas source  204  are introduced into the processing chamber  202 . The entry port  280  is also coupled to a cleaning source  282 . The cleaning source  282  typically provides a cleaning agent, such as dissociated fluorine, that is introduced into the processing chamber  202  to remove deposition by-products and films from processing chamber hardware, including the gas distribution plate assembly  218 . 
     The gas distribution plate assembly  218  is coupled to an interior side  220  of the lid assembly  210 . The gas distribution plate assembly  218  is typically configured to substantially follow the profile of the glass substrate  240 , for example, polygonal for large area flat panel substrates and circular for wafers. The gas distribution plate assembly  218  includes a perforated area  216  through which process and other gases supplied from the gas source  204  are delivered to the process volume  212 . The perforated area  216  of the gas distribution plate assembly  218  is configured to provide uniform distribution of gases passing through the gas distribution plate assembly  218  into the processing chamber  202 . Gas distribution plates that may be adapted to benefit from the invention are described in commonly assigned U.S. patent application Ser. No. 09/922,219, filed Aug. 8, 2001 by Keller et al., U.S. patent application Ser. No. 10/140,324, filed May 6, 2002 by Yim et al., and Ser. No. 10/337,483, filed Jan. 7, 2003 by Blonigan et al., U.S. Pat. No. 6,477,980, issued Nov. 12, 2002 to White et al., U.S. patent application Ser. No. 10/417,592, filed Apr. 16, 2003 by Choi et al., and U.S. patent application Ser. No. 10/823,347, filed on Apr. 12, 2004 by Choi et al., which are hereby incorporated by reference in their entireties. 
     The gas distribution plate assembly  218  typically includes a diffuser plate (or distribution plate)  258  suspended from a hanger plate  260 . The diffuser plate  258  and hanger plate  260  may alternatively comprise a single unitary member. A plurality of gas passages  262  are formed through the diffuser plate  258  to allow a predetermined distribution of gas passing through the gas distribution plate assembly  218  and into the process volume  212 . The hanger plate  260  maintains the diffuser plate  258  and the interior surface  220  of the lid assembly  210  in a spaced-apart relation, thus defining a plenum  264  therebetween. The plenum  264  allows gases flowing through the lid assembly  210  to uniformly distribute across the width of the diffuser plate  258  so that gas is provided uniformly above the center perforated area  216  and flows with a uniform distribution through the gas passages  262 . 
     The diffuser plate  258  is typically fabricated from stainless steel, aluminum (Al), anodized aluminum, nickel (Ni) or other RF conductive material. The diffuser plate  258  could be cast, brazed, forged, hot iso-statically pressed or sintered. The diffuser plate  258  is configured with a thickness that maintains sufficient flatness across the aperture  266  as not to adversely affect substrate processing. The thickness of the diffuser plate  258  is between about 0.8 inch to about 2.0 inches. The diffuser plate  258  could be circular for semiconductor wafer manufacturing or polygonal, such as rectangular, for flat panel display manufacturing. 
       FIG. 3  is a partial sectional view of an exemplary diffuser plate  258  that is described in commonly assigned U.S. patent application Ser. No. 10/417,592, titled “Gas Distribution Plate Assembly for Large Area Plasma Enhanced Chemical Vapor Deposition”, filed on Apr. 16, 2003. The diffuser plate  258  includes a first or upstream side  302  facing the lid assembly  210  and an opposing second or downstream side  304  that faces the support assembly  238 . Each gas passage  262  is defined by a first bore  310  coupled by an orifice hole  314  to a second bore  312  that combine to form a fluid path through the gas distribution plate  258 . The first bore  310  extends a first depth  330  from the upstream side  302  of the gas distribution plate  258  to a bottom  318 . The bottom  318  of the first bore  310  may be tapered, beveled, chamfered or rounded to minimize the flow restriction as gases flow from the first bore into the orifice hole  314 . The first bore  310  generally has a diameter of about 0.093 to about 0.218 inches, and in one embodiment is about 0.156 inches. 
     The second bore  312  is formed in the diffuser plate  258  and extends from the downstream side (or end)  304  to a depth  332  of about 0.10 inch to about 2.0 inches. Preferably, the depth  332  is between about 0.1 inch to about 1.0 inch. The diameter  336  of the second bore  312  is generally about 0.1 inch to about 1.0 inch and may be flared at an angle  316  of about 10 degrees to about 50 degrees. Preferably, the diameter  336  is between about 0.1 inch to about 0.5 inch and the flaring angle  316  is between 20 degrees to about 40 degrees. The surface of the second bore  312  is between about 0.05 inch 2  to about 10 inch 2  and preferably between about 0.05 inch 2  to about 5 inch 2 . The diameter of second bore  312  refers to the diameter intersecting the downstream surface  304 . An example of diffuser plate, used to process 1500 mm by 1850 mm substrates, has second bores  312  at a diameter of 0.250 inch and at a flare angle  316  of about 22 degrees. The distances  380  between rims  382  of adjacent second bores  312  are between about 0 inch to about 0.6 inch, preferably between about 0 inch to about 0.4 inch. The diameter of the first bore  310  is usually, but not limited to, being at least equal to or smaller than the diameter of the second bore  312 . A bottom  320  of the second bore  312  may be tapered, beveled, chamfered or rounded to minimize the pressure loss of gases flowing out from the orifice hole  314  and into the second bore  312 . Moreover, as the proximity of the orifice hole  314  to the downstream side  304  serves to minimize the exposed surface area of the second bore  312  and the downstream side  304  that face the substrate, the downstream area of the diffuser plate  258  exposed to fluorine provided during chamber cleaning is reduced, thereby reducing the occurrence of fluorine contamination of deposited films. 
     The orifice hole  314  generally couples the bottom  318  of the first hole  310  and the bottom  320  of the second bore  312 . The orifice hole  314  generally has a diameter of about 0.01 inch to about 0.3 inch, preferably about 0.01 inch to about 0.1 inch, and typically has a length  334  of about 0.02 inch to about 1.0 inch, preferably about 0.02 inch to about 0.5 inch. The length  334  and diameter (or other geometric attribute) of the orifice hole  314  is the primary source of back pressure in the plenum  264  which promotes even distribution of gas across the upstream side  302  of the gas distribution plate  258 . The orifice hole  314  is typically configured uniformly among the plurality of gas passages  262 ; however, the restriction through the orifice hole  314  may be configured differently among the gas passages  262  to promote more gas flow through one area of the gas distribution plate  258  relative to another area. For example, the orifice hole  314  may have a larger diameter and/or a shorter length  334  in those gas passages  262 , of the gas distribution plate  258 , closer to the wall  206  of the processing chamber  202  so that more gas flows through the edges of the perforated area  216  to increase the deposition rate at the perimeter of the glass substrate. The thickness of the diffuser plate is between about 0.8 inch to about 3.0 inches, preferably between about 0.8 inch to about 2.0 inch. 
     As the size of substrate continues to grow in the TFT-LCD industry, especially, when the substrate size is at least about 1000 mm by about 1200 mm (or about 1,200,000 mm 2 ), film thickness and property uniformity for large area plasma-enhanced chemical vapor deposition (PECVD) becomes more problematic. Examples of noticeable uniformity problems include higher deposition rates and more compressive films in the central area of large substrates for some high deposition rate silicon nitride films. The thickness uniformity across the substrate appears “dome shaped” with film in center region thicker than the edge region. The less compressive film in the edge region has higher Si—H content. The manufacturing requirements for TFT-LCD include low Si—H content, for example &lt;15 atomic %, high deposition rate, for example &gt;1500 Å/min, and low thickness non-uniformity, for example &lt;15%, across the substrate. The Si—H content is calculated from FTIR (Fourier Transform Infra-Red) measurement. The larger substrates have worse “dome shape” uniformity issue. The problem could not be eliminated by process recipe modification to meet all requirements. Therefore, the issue needs to be addressed by modifying the gas and/or plasma distribution. 
     The process of depositing a thin film in a process chamber is shown in  FIG. 4A . The process starts at step  401  by placing a substrate in a process chamber with a diffuser plate. Next at step  402 , flow process gas(es) through a diffuser plate toward a substrate supported on a substrate support. Then at step  403 , create a plasma between the diffuser plate and the substrate support. At step  404 , deposit a thin film on the substrate in the process chamber.  FIG. 4B  shows a thickness profile of a silicon nitride film across a glass substrate. The size of the substrate is 1500 mm by 1800 mm. The diffuser plate has diffuser holes with design shown in  FIG. 3 . The diameter of the first bore  310  is 0.156 inch. The length  330  of the first bore  310  is 1.049 inch. The diameter  336  of the second bore  312  is 0.250 inch. The flaring angle  316  of the second bore  312  is 22 degree. The length  332  of the second bore  312  is 0.243 inch. The diameter of the orifice hole  314  is 0.016 inch and the length  334  of the orifice hole  314  is 0.046 inch. The SiN film is deposited using 2800 sccm SiH 4 , 9600 sccm NH 3  and 28000 sccm N 2 , under 1.5 Torr, and 15000 watts source power. The spacing between the diffuser plate and the support assembly is 1.05 inch. The process temperature is maintained at about 355° C. The deposition rate is averaged to be 2444 Å/min and the thickness uniformity (with 15 mm edge exclusion) is 25.1%, which is higher than the manufacturing specification (&lt;15%). The thickness profile shows a center thick profile, or “dome shape” profile. Table 1 shows the film properties measured from wafers placed on the glass substrate for the above film. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Measurement of thickness and film properties 
               
               
                 on a substrate deposited with SiN film. 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Stress 
                   
                   
               
               
                   
                   
                   
                 (E9 
                 Si-H 
                   
               
               
                 Measurement 
                 Thickness 
                   
                 Dynes/ 
                 (atomic  
                 WER 
               
               
                 location 
                 (Å) 
                 RI 
                 cm 2 ) 
                 %) 
                 (Å/min) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Edge I 
                 5562 
                 1.92 
                 −0.7 
                 12.5 
                 664 
               
               
                 Center 
                 8544 
                 1.90 
                 −6.7 
                 4.2 
                 456 
               
               
                 Edge II 
                 6434 
                 1.91 
                 −1.2 
                 10.8 
                 665 
               
               
                   
               
            
           
         
       
     
     Edge I and Edge II represent two extreme ends of the substrate with width at 1800 mm. The refractive index (RI), film stress, Si—H concentration data and wet etch rate (WER) data show a more compressive film near the center region in comparison to the edge region. The Si—H concentrations at the substrate edges are approaching the manufacturing limit of 15%. Wet etch rate is measured by immersing the samples in a BOE (buffered oxide etch) 6:1 solution. 
     One theory for the cause of the center to edge non-uniformity problem is excess residual gas between diffuser plate and substrate and in the center region of the substrate that could not be pumped away effectively, which may have caused high deposition rate and more compressive film in the center region of the substrate. A simple test has been designed to see if this theory would stand. As shown in  FIG. 5 , a thermo-resistant tape is used to block of the pumping channels  214  (shown in  FIG. 2 ) near side  501  and side  502  of substrate in a PECVD process chamber. The pumping channels  214  near the other two sides are left open. Due to this, an asymmetric gas pumping situation was created. If the cause of the “dome shape” problem is due to excess residual gas that could not be pumped away at the edge of the substrate, the use of thermo-resistant tape near two edges of the substrate should worsen the uniformity issue and cause worse uniformity across the substrate. However, little changes has been observed comparing the deposition results between deposition done with 2 pumping channels blocked and deposition with all pumping channel opened (see Table 2). The diffuser plate used here has the same design and dimensions as the one used for  FIG. 4B  and Table 1. The SiN films in Table 2 are deposited using 3300 sccm SiH 4 , 28000 sccm NH 3  and 18000 sccm N 2 , under 1.3 Torr, and 11000 watts source power. The spacing between the diffuser plate and the support assembly is 0.6 inch. The process temperature is maintained at about 355° C. Film thickness and properties are measured on location 1, 2, 3, 4 and 5 (as shown in  FIG. 5 ) on the substrates. The SiH content shown is Table 2 is measured in atomic %. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 SiN thickness and film properties comparison  
               
               
                 between deposition with all pumping channels 
               
               
                 open and with 2 pumping channels closed. 
               
            
           
           
               
               
               
            
               
                   
                 All pumping channels open 
                 2 pumping channels blocked 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Stress 
                   
                   
                   
                 Stress 
                   
               
               
                   
                 Thick- 
                   
                 (E9  
                   
                 Thick- 
                   
                 (E9 
                   
               
               
                   
                 ness 
                   
                 dynes/ 
                 SiH 
                 ness 
                   
                 dynes/ 
                 SiH 
               
               
                 Position 
                 (Å) 
                 RI 
                 cm 2 ) 
                 (%) 
                 (Å) 
                 RI 
                 cm 2 ) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 6156 
                 1.92 
                 −4.6 
                 11.1 
                 5922 
                 1.93 
                 −3.9 
                 11.5 
               
               
                 2 
                 7108 
                 1.91 
                 −5.1 
                 8.8 
                 7069 
                 1.92 
                 −5.1 
                 9.1 
               
               
                 3 
                 7107 
                 1.91 
                 −5.1 
                 8.5 
                 7107 
                 1.91 
                 −4.8 
                 8.9 
               
               
                 4 
                 7052 
                 1.91 
                 −5.0 
                 8.1 
                 7048 
                 1.91 
                 −4.6 
                 8.5 
               
               
                 5 
                 6173 
                 1.92 
                 −4.2 
                 10.8 
                 6003 
                 1.92 
                 −3.8 
                 11.2 
               
               
                   
               
            
           
         
       
     
     The results in Table 2 show little difference between the deposition done with 2 pumping channels blocked and deposition with all pumping channel opened. In addition, there is little difference between measurement collected at locations 1 and 5, which should be different if residual gas is the cause of the problem. Therefore, the theory of excess residual gas between diffuser and substrate and in the center region of the substrate not being pumped away effectively is ruled out. 
     Another possible cause for the center to edge non-uniformity is plasma non-uniformity. Deposition of films by PECVD depends substantially on the source of the active plasma. Dense chemically reactive plasma can be generated due to hollow cathode effect. The driving force in the RF generation of a hollow cathode discharge is the frequency modulated d.c. voltage Vs (the self-bias voltage) across the space charge sheath at the RF electrode. A RF hollow cathode and oscillation movement of electrons between repelling electric fields, Es, of the opposite sheaths are shown schematically in  FIG. 6A . An electron emitted from the cathode wall, which could be the walls of the reactive gas passages that are close to the process volume  212 , is accelerated by the electric field Es across the wall sheath “□”. The electron oscillates across the inner space between walls of the electrode owing to the repelling fields of the opposite wall sheaths. The electron loses energy by collisions with the gas and creates more ions. The created ions can be accelerated to the cathode walls thereby enhancing emissions of secondary electrons, which could create additional ions. Overall, the cavities between the cathode walls enhance the electron emission and ionization of the gas. Flared-cone shaped cathode walls, with gas inlet diameter smaller than the gas outlet diameter, are more efficient in ionizing the gas than cylindrical walls. The potential Ez is created due to difference in ionization efficiency between the gas inlet and gas outlet. 
     By changing the design of the walls of the hollow cathode cavities, which faces the substrate and are at the downstream ends of the gas diffuser holes (or passages), that are close to the process volume  212  and the arrangement (or density) of the hollow cathode cavities, the gas ionization could be modified to control the film thickness and property uniformity. An example of the walls of the hollow cathode cavities that are close to the process volume  212  is the second bore  312  of  FIG. 3 . The hollow cathode effect mainly occurs in the flared cone  312  that faces the process volume  212 . The  FIG. 3  design is merely used as an example. The invention can be applied to other types of hollow cathode cavity designs. Other examples of hollow cathode cavity design include, but not limited to, the designs shown in  FIGS. 6B-6G . By varying the volume and/or the surface area of the hollow cathode cavity, the plasma ionization rate can be varied. 
     Using the design in  FIG. 3  as an example, the volume of second bore (or hollow cathode cavity)  312  can be changed by varying the diameter “D” (or diameter  336  in  FIG. 3 ), the depth “d” (or length  332  in  FIG. 3 ) and the flaring angle “α” (or flaring angle  316  of  FIG. 3 ), as shown in  FIG. 7A . Changing the diameter, depth and/or the flaring angle would also change the surface area of the bore  312 . Since the center of substrate has higher deposition rate and is more compressive, higher plasma density is likely the cause. By reducing the bore depth, the diameter, the flaring angle, or a combination of these three parameters from edge to center of the diffuser plate, the plasma density could be reduced in the center region of the substrate to improve the film thickness and film property uniformities. Reducing the cone (or bore) depth, cone diameter, flaring angle also reduces the surface area of the bore  312 .  FIGS. 7B, 7C and 7D  show 3 diffuser passage (or diffuser hole) designs that are arranged on a diffuser plate shown in  FIG. 7E .  FIGS. 7B, 7C and 7D  designs have the same cone (or bore) diameter, but the cone (or bore) depth and total cone (bore) surface areas are largest for  FIG. 7B  design and smallest for  FIG. 7D  design. The cone flaring angles have been changed to match the final cone diameter. The cone depth for  FIG. 7B  is 0.7 inch. The cone depth for  FIG. 7C  is 0.5 inch and the cone depth for  FIG. 7D  is 0.325 inch. The smallest rectangle  710  in  FIG. 7E  is 500 mm by 600 mm and the diffuser holes have cone depth 0.325 inch, cone diameter 0.302 inch and flare angel 45° (See  FIG. 7D ). The medium rectangle in  FIG. 7E  is 1000 mm by 1200 mm. The diffuser holes in the area  720  between the medium rectangle and the smallest rectangle have cone depth 0.5 inch, cone diameter 0.302 inch and flare angle 30° (See  FIG. 7C ). The largest rectangle in Figure is 1500 mm by 1800 mm. The diffuser holes in the area  730  between the largest rectangle and the medium rectangle have cone depth 0.7 inch, cone diameter 0.302 inch and flare angle 22° (See  FIG. 7B ) The orifice holes diameters are all 0.03 inch and holes depths are all 0.2 inch for  FIGS. 7B, 7C and 7D . The thickness of the three diffuser plates are all 1.44 inch. The diameters for first bore  310  of  FIGS. 7B, 7C and 7D  are all 0.156 inch and the depth are 0.54 inch ( FIG. 7B ), 0.74 inch ( FIG. 7C ) and 0.915 inch ( FIG. 7C ) respectively. 
       FIG. 8  shows the deposition rate across the substrate. Region I correlates to the area under “0.325 inch depth” cones, while regions II and III correlates to “0.5 inch depth” (region II) and “0.7 inch depth” (region III) respectively. Table 3 shows the measurement of film thickness and properties across the substrate. The SiN film in Table 3 is deposited using 3300 sccm SiH 4 , 28000 sccm NH 3  and 18000 sccm N 2 , under 1.3 Torr, and 11000 watts source power. The spacing between the diffuser plate and the support assembly is 0.6 inch. The process temperature is maintained at about 355° C. The locations 1, 2, 3, 4 and 5 are the same locations indicated in  FIG. 5 . 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 SiN film thickness and property measurement with 
               
               
                 diffuser plate with 3 regions of varying cone depths. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Stress 
                   
               
               
                   
                   
                 Cone  
                   
                   
                 (E9 
                 SiH 
               
               
                   
                   
                 depth 
                 Thickness 
                   
                 dynes/ 
                 (atomic  
               
               
                   
                 Position 
                 (inch) 
                 (Å) 
                 RI 
                 cm 2 ) 
                 %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 1 
                 0.7 
                 6060 
                 1.924 
                 −4.09 
                 9.10 
               
               
                   
                 2 
                 0.5 
                 6631 
                 1.921 
                 −5.49 
                 9.66 
               
               
                   
                 3 
                 0.325 
                 5659 
                 1.915 
                 −2.02 
                 12.34 
               
               
                   
                 4 
                 0.5 
                 6956 
                 1.916 
                 −5.45 
                 9.37 
               
               
                   
                 5 
                 0.7 
                 6634 
                 1.917 
                 −4.14 
                 8.83 
               
               
                   
                   
               
            
           
         
       
     
     The results show that reducing the cone depth and cone surface area reduces the deposition rate. The results also show that reducing the volume and/or surface area of hollow cathode cavity reduces the deposition rate. The reduction of the plasma deposition rate reflects a reduction in plasma ionization rate. Since the change of cone depth and total cone surface area from region I to region II to region III is not smooth, the deposition rates across the substrate shows three regions. Regions I, II and III on the substrate match the diffuser holes regions  710 ,  720  and  730 . This indicates that changing the hollow cathode cavity design can change the plasma ionization rate and also the importance of making the changes smooth and gradual. 
     There are many ways to gradually change hollow cathode cavities from inner regions of the diffuser plate to the outer regions of the diffuser plate to improve plasma uniformity. One way is to first bend the diffuser plate, which has identical gas diffusing passages across the diffuser plate, to a pre-determined curvature and afterwards machine out the curvature to leave the surface flat.  FIG. 9A  shows the process flow of this concept. The process starts by bending the diffuser plate to make it convex at step  901 , followed by machining out the curvature of the convex diffuser plate to make the diffuser plate surface flat at step  902 .  FIG. 9B  shows a schematic drawing of a convex diffuser plate with an exemplary diffuser hole (or gas passage)  911  at the edge (and outer region) and an exemplary diffuser hole  912  in the center (and inner region) as diffuser holes. The diffuser holes  911  and  912  are identical before the bending process and are simplified drawings of diffuser holes as shown in  FIGS. 3 and 7A . However, the invention can be used for any diffuser holes designs. The design in  FIG. 3  is merely used for example. Diffuser plate downstream surface  304  faces the process volume  212 . The gradual changing distance between the  913  surface and the flat  914  surface (dotted due to its non-existence) shows the curvature. The edge diffuser cone  915  and center diffuser cone  916  are identical in size and shape prior to the bending process.  FIG. 9C  shows the schematic drawing of a diffuser plate after the curvature has been machined out. The surface facing the process volume  212  is machined to  914  (a flat surface), leaving center cone  918  significantly shorter than the edge cone  917 . Since the change of the cone size (volume and/or surface area) is created by bending the diffuser plate followed by machining out the curvature, the change of the cone size (volume and/or surface area) from center to edge is gradual. The center cone  918  would have diameter “D” and depth “d” smaller than the edge cone  917 . The definition of cone diameter “D” and cone depth “d” can be found in the description of  FIG. 7A . 
       FIG. 9D  shows the depth “d” of the bores  312  (or cone) that extend to the downstream side of an exemplary diffuser plate, which is used to process 1500 mm by 1850 mm substrates. The diffuser plate has diffuser holes with design shown in  FIG. 7A . The diameter of the first bore  310  is 0.156 inch. The length  330  of the first bore  310  is 1.049 inch. The diameter  336  of the second bore  312  is 0.250 inch. The flaring angle  316  of the second bore  312  is 22 degree. The length  332  of the second bore  312  is 0.243 inch. The diameter of the orifice hole  314  is 0.016 inch and the length  334  of the orifice hole  314  is 0.046 inch. The measurement of depths of the second bores in  FIG. 9D  shows a gradual increasing of bore depth  332  (or “d” in  FIG. 7A ) from center of the diffuser plate to the edge of the diffuser plate. Due to the bending and machining processes, the diameter  336  (or “D” in  FIG. 7A ) of the bore  312  also gradually increases from center of the diffuser plate to the edge of the diffuser plate. 
       FIG. 9E  shows the thickness distribution across a substrate deposited with SiN film under a diffuser plate with a design described in  FIGS. 9B, 9C and 9D . The size of substrate is 1500 mm by 1850 mm, which is only slightly larger than the size of substrate (1500 mm by 1800 mm) in  FIG. 4B  and Table 1. Typically, the diffuser plate sizes scale with the substrate sizes. The diffuser plate used to process 1500 mm by 1850 mm substrates is about 1530 mm by 1860 mm, which is slightly larger than the diffuser plate used to process 1500 mm by 1800 mm substrates (diffuser plate about 1530 mm by 1829 mm). The thickness uniformity is improved to 5.0%, which is much smaller than 25.1% for film in  FIG. 4B . Table 4 shows the film property distribution across the substrate. The diffuser plate has diffuser holes with design shown in  FIG. 7A . The diameter of the first bore  310  is 0.156 inch. The length  330  of the first bore  310  is 1.049 inch. The diameter  336  of the second bore  312  is 0.250 inch. The flaring angle  316  of the second bore  312  is 22 degree. The length  332  of the second bore  312  is 0.243 inch. The diameter of the orifice hole  314  is 0.016 inch and the length  334  of the orifice hole  314  is 0.046 inch. The SiN films in  FIG. 9E  and Table 4 are deposited using 2800 sccm SiH 4 , 9600 sccm NH 3  and 28000 sccm N 2 , under 1.5 Torr, and 15000 watts source power. The spacing between the diffuser plate and the support assembly is 1.05 inch. The process temperature is maintained at about 355° C. Edge I and Edge II represent two extreme ends of the substrate, as described in Table 1 measurement. The film thickness and property data in Table 4 show much smaller center to edge variation compared to the data in Table 1. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 SiN film thickness and property measurement using a 
               
               
                 diffuser plate with gradually varied bore depths and diameters  
               
               
                 from center to edge for a 1500 mm by 1850 mm substrate. 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Stress 
                   
                   
               
               
                   
                   
                   
                 (E9 
                 Si-H 
                   
               
               
                 Measurement 
                 Thickness 
                   
                 Dynes/ 
                 (atomic 
                 WER 
               
               
                 location 
                 (Å) 
                 RI 
                 cm 2 ) 
                 %) 
                 (Å/min) 
               
               
                   
               
               
                 Edge I 
                 6405 
                 1.92 
                 −0.7 
                 13.3 
                 451 
               
               
                 Center 
                 6437 
                 1.91 
                 −1.8 
                 12.7 
                 371 
               
               
                 Edge II 
                 6428 
                 1.92 
                 −1.2 
                 11.9 
                 427 
               
               
                   
               
            
           
         
       
     
     Comparing the data in Table 4 to the data in Table 1, which are collected from deposition with a diffuser plate with the same diameters and depths of bores  312  across the diffuser plate, the variation of thickness, stress, Si—H content and wet etch rate (WER) are all much less for the data in Table 4, which is collected from deposition with a diffuser plate with gradually increasing diameters and depths of bore  312  from the center to the edge of the diffuser plate. The results show that uniformity for thickness and film properties can be greatly improved by gradually increasing the diameters and depths of the bores, which extend to the downstream side of the diffuser plate, from center to edge. The wet etch rates in the tables are measured by immersing the samples in a BOE 6:1 solution. 
       FIG. 9F  shows the depth “d” measurement of the bores  312  across an exemplary diffuser plate, which is used to process 1870 mm by 2200 mm substrates Curve  960  shows an example of an ideal bore depth distribution the diffuser plate. The measurement of depths of the bores in  FIG. 9F  shows a gradual increasing of bore depth from center of the diffuser plate to the edge of the diffuser plate. The downstream bore diameter would also gradually increase from center of the diffuser plate to the edge of the diffuser plate. 
       FIG. 9G  shows the thickness distribution across a substrate deposited with SiN film under a diffuser plate with a design similar to the one described in  FIGS. 9B, 9C and 9F . The size of the substrate is 1870 mm by 2200 mm. Table 5 shows the film property distribution across the substrate. The diffuser plate has diffuser holes with design shown in  FIG. 7A . The diameter of the first bore  310  is 0.156 inch. The length  330  of the first bore  310  is 0.915 inch. The diameter  336  of the second bore  312  is 0.302 inch. The flaring angle  316  of the second bore  312  is 22 degree. The length  332  of the second bore  312  is 0.377 inch. The diameter of the orifice hole  314  is 0.018 inch and the length  334  of the orifice hole  314  is 0.046 inch. The SiN films in Table 5 are deposited using 5550 sccm SiH 4 , 24700 sccm NH 3  and 61700 sccm N 2 , under 1.5 Torr, and 19000 watts source power. The spacing between the diffuser plate and the support assembly is 1.0 inch. The process temperature is maintained at about 350° C. Edge I and Edge II represent two extreme ends of the substrate, as described in Table 1 measurement. The film thickness and property data in Table 5 show much smaller center to edge variation compared to the data in Table 1. The thickness uniformity is 9.9%, which is much better than 25.1% for film in  FIG. 4B . The data shown in  FIG. 4B  and Table 1 are film thickness and property data on smaller substrate (1500 mm by 1800 mm), compared to the substrate (1870 mm by 2200 mm) for data in  FIG. 9G  and Table 5. Thickness and property uniformities are expected to be worse for larger substrate. The uniformity of 9.9% and the improved film property data in Table 5 by the new design show that the new design, with gradual increasing diameters and depths of diffuser bores extended to the downstream side of the diffuser plate, greatly improves the plasma uniformity and process uniformity. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 SiN film thickness and property measurement using a  
               
               
                 diffuser plate with gradually varied bore depths and diameters  
               
               
                 from center to edge for an 1870 mm by 2200 mm substrate. 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Stress 
                   
                   
               
               
                   
                 Thick- 
                   
                 (E9 
                 Si-H 
                   
               
               
                 Measurement 
                 ness 
                   
                 Dynes/ 
                 (atomic  
                 WER 
               
               
                 location 
                 (Å) 
                 RI 
                 cm 2 ) 
                 %) 
                 (Å/min) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Edge I 
                 5814 
                 1.94 
                 −0.3 
                 16.4 
                 509 
               
               
                 Center 
                 5826 
                 1.93 
                 0.8 
                 17.3 
                 716 
               
               
                 Edge II 
                 5914 
                 1.92 
                 −0.6 
                 13.9 
                 644 
               
               
                   
               
            
           
         
       
     
     Although the exemplary diffuser plate described here is rectangular, the invention applies to diffuser plate of other shapes and sizes. One thing to note is that the convex downstream surface does not have to be machined to be completely flat across the entire surface. As long as the diameters and depths of the bores are increased gradually from center to edge of the diffuser plate, the edge of the diffuser plate could be left un-flattened. 
     There are also many ways to create curvature of the diffuser plate. One way is to thermally treat the diffuser plate at a temperature that the diffuser plate softens, such as a &gt;400° C. temperature for aluminum, for a period of time by supporter only the edge of the diffuser plate. When the metal diffuser plate softens under the high temperature treatment, the gravity would pull center of the diffuser plate down and the diffuser plate would become curved.  FIG. 10A  shows the process flow  1000  of such thermal treatment. First, at step  1001  place the diffuser plate, which already has diffuser holes in it, in an environment  1005  or chamber that could be thermally controlled and place the diffuser plate  1010  on a support  1020  that only support the edge of the diffuser plate (See  FIG. 10B ). The diffuser plate facing down is the downstream surface  304  of the diffuser plate. Afterwards at step  1002 , raise the temperature of the environment and treat the diffuser plate at a thermal condition at a temperature that the diffuser plate softens. One embodiment is to keep the thermal environment at a constant treatment temperature (iso-thermal), once the constant treatment temperature has been reached. After the curvature of the diffuser plate has reached the desired curvature, stop the thermal treatment process at step  1003 . Note that in the thermal environment, optional diffuser support  1030  could be placed under diffuser plate  1010  at support height  1035  lower than the support height  1025  of support  1020  and at a support distance  1037  shorter than the support distance  1027  of support  1020 . The optional support  1030  could help determine the diffuser curvature and could be made of elastic materials that could withstand temperature greater than 400° C. (the same temperature as the thermal conditioning temperature) and would not damage the diffuser plate surface.  FIG. 10C  shows that the curved diffuser plate  1010  resting on the diffuser plate supports  1020  and  1030  after the bending process. 
     Another way to create curvature is to use vacuum to smoothly bend the diffuser plate to a convex shape.  FIG. 11A  shows the process flow  1100  of such bending by vacuum process. First, at step  1101  place the diffuser plate, which already has diffuser holes in it and the downstream side  304  facing down, on a vacuum assembly  1105  and seal the upstream end  302  of the diffuser plate with a cover. The material used to cover (or seal) the upstream end of the diffuser plate must be strong enough to keep its integrity under vacuum. The vacuum assembly only supports the diffuser plate at the edge (See  FIG. 11B ) by diffuser plate holder  1120 . The vacuum assembly  1105  is configured to have a pump channel  1150  to pull vacuum in the volume  1115  between the diffuser plate and the vacuum assembly  1105  when the upstream end of the diffuser plate is covered. The pumping channel  1150  in  FIGS. 11B and 11C  are merely used to demonstrate the concept. There could be more than one pumping channels placed at different locations in the vacuum assembly  1105 . Afterwards at step  1102 , pull vacuum in the volume  1115  between the diffuser plate and the diffuser plate holder. When the curvature of the diffuser plate has reached the desired curvature, stop the vacuuming process at step  1103  and restore the pressure of the volume  1115  between the diffuser plate and the vacuum assembly to be equal to the surrounding environment  1140  to allow the diffuser plate to be removed from the vacuum assembly  1105 . Note that in the vacuum assembly, optional diffuser support  1130  could be placed under diffuser plate  1110  at support height  1135  lower than the support height  1125  of the diffuser plate support  1120  and at a support distance  1137  shorter than the support distance  1127  of support  1120 . The optional support could help determine the diffuser curvature and could be made of materials, such as rubber, that would not damage the diffuser plate surface.  FIG. 11C  shows that the curved diffuser plate  1110  resting on the diffuser plate supports  1120  and  1130  after the bending process. 
     Another way to change the downstream cone ( 312  in  FIG. 3 ) depth, cone diameter, cone flaring angle or a combination of these three parameters is by drilling the diffuser holes with varying cone depth, cone diameter or cone flaring angles from center of the diffuser plate to the edge of the diffuser plate. The drilling can be achieved by computer numerically controlled (CNC) machining.  FIG. 12A  shows the process flow of such a process  1200 . The process  1200  starts at step  1230  by creating bores that extend to the downstream side of a diffuser plate with gradually increasing bore depths and/or bore diameters from center to edge of the diffuser plate. The flaring angle can also be varied from center to edge of the diffuser plate. Next at step  1240 , the process is completed by creating the remaining portions of the gas passages of the diffuser plate. The downstream cones can be created by using drill tools. If drill tools with the same flaring angle are used across the diffuser plate, the cone flaring angles would stay constant and cone depth and cone diameter are varied. The cone diameter would be determined by the flaring angle and cone depth. The important thing is to vary the cone depth smoothly and gradually to ensure smooth deposition thickness and film property change across the substrate.  FIG. 12B  shows an example of varying cone depths and cone diameters. Diffuser hole  1201  is near the center of the diffuser plate and has the smallest cone depth  1211  and cone diameter  1221 . Diffuser hole  1202  is between the center and edge of the diffuser plate and has the medium cone depth  1212  and cone diameter  1222 . Diffuser hole  1203  is near the edge of the diffuser plate and has the largest cone depth  1213  and cone diameter  1223 . The cone flaring angle of all diffuser holes are the same for the design in  FIG. 12B . However, it is possible to optimize deposition uniformity by varying the cone design across the diffuser plate by varying both the cone diameters, cone depths and flaring angles. Changing the cone depth, cone diameter and cone flaring angle affects the total cone surface area, which also affects the hollow cathode effect. Smaller cone surface area lowers the plasma ionization efficiency. 
     Yet another way to change the downstream bore ( 312  in  FIG. 3 ) depth (“d”), and bore diameter (“D”) is by drilling identical diffuser holes across the diffuser plate (see  FIG. 12C ). In  FIG. 12C , the gas diffuser hole  1251  at the edge (at outer region) of the diffuser plate is identical to the gas diffuser hole  1252  at the center (at inner region) of the diffuser plate. The downstream bore  1255  is also identical to downstream bore  1256 . The downstream surface  1254  of gas diffuser plate is initially flat. Afterwards, machine downstream side of the diffuser plate to make a concave shape with center thinner than the edge. The machining can be achieved by computer numerically controlled machining or other types of controlled machining to make the machining process repeatable. After machining the downstream surface  1254  to a concave shape (surface  1259 ), the downstream bore  1258  at the center (an inner region) of the diffuser plate has smaller diameter (“D”) and smaller length (“d”) than the downstream bore  1257  at the edge (an outer region) of the diffuser plate. The diffuser plate can be left the way it is as in  FIG. 12D , or downstream surface  1259  can be pulled flat as shown in  FIG. 12E , or to other curvatures (not shown), to be used in a process chamber to achieve desired film results. 
     Yet another way to change the downstream bore ( 312  in  FIG. 3 ) depth (“d”), and bore diameter (“D”) is by bending the diffuser plate without any diffuser hole into concave shape (See  FIG. 12F ). In  FIG. 12F , the downstream surface is surface  1269 . Afterwards, drill the downstream bores to the same depth using the same type of drill from a fictitious flat surface  1264  (See  FIG. 12G ). Although downstream bore  1268  at the center of the diffuser plate is drilled to the same depth from the fictitious surface  1264  as the downstream bore  1267 , the diameter and length of the downstream bore  1268  are smaller than the diameter and length of the downstream bore  1267 . The rest of the diffuser holes, which include orifice holes  1265 , upstream bores  1263 , and connecting bottoms, are machined to complete the diffuser holes. All orifice holes and upstream bores should have identical diameters, although it is not necessary. The diameters and lengths of the orifice holes should be kept the same across the diffuser plate (as shown in  FIG. 12G ). The orifice holes controls the back pressure. By keeping the diameters and the lengths of the orifice holes the same across the diffuser plate, the back pressure, which affects the gas flow, can be kept the same across the diffuser plate. The diffuser plate can be left the way it is as in  FIG. 12G , or downstream surface  1269  can be pulled flat as shown in  FIG. 12H , or to other curvatures (not shown), to be used in a process chamber to achieve desired film results. 
     The changes of diameters and/or lengths of the hollow cathode cavities do not have to be perfectly continuous from center of the diffuser plate to the edge of the diffuser plate, as long the changes are smooth and gradual. It can be accomplished by a number of uniform zones arranged in a concentric pattern as long as the change from zone to zone is sufficiently small. But, there need to be an overall increase of size (volume and/or surface area) of hollow cathode cavity from the center of the diffuser plate to the edge of the diffuser plate.  FIG. 12I  shows a schematic plot of bottom view (looking down at the downstream side) of a diffuser plate. The diffuser plate is divided into N concentric zones. Concentric zones are defined as areas between an inner and an outer boundaries, which both have the same geometric shapes as the overall shape of the diffuser plate. Within each zone, the diffuser holes are identical. From zone 1 to zone N, the hollow cathode cavity gradually increase in size (volume and/or surface area). The increase can be accomplished by increase of hollow cathode cavity diameter, length, flaring angle, or a combination of these parameters. 
     The increase of diameters and/or lengths of the hollow cathode cavities from center to edge of the diffuser plate also do not have to apply to all diffuser holes, as long as there is an overall increase in the size (volume and/or surface area) of hollow cathode cavities per downstream diffuser plate surface area of the hollow cathode cavities. For example, some diffuser holes could be kept the same throughout the diffuser plate, while the rest of the diffuser holes have a gradual increase in the sizes (volumes and/or surface areas) of the hollow cathode cavities. In another example, the diffuser holes have a gradual increase in sizes (volumes and/or surface areas) of the hollow cathode cavities, while there are some small hollow cathode cavities at the edge of the diffuser plate, as shown in  FIG. 12J . Yet in another example, most of the hollow cathode cavities are uniform across the diffuser plate, while there are a few larger hollow cathode cavities towards the edge of the diffuser plate, as shown in  FIG. 12K . 
     We can define the hollow cathode cavity volume density as the volumes of the hollow cathode cavities per downstream diffuser plate surface area of the hollow cathode cavities. Similarly, we can define the hollow cathode cavity surface area density of the hollow cathode cavity as the total surface areas of the hollow cathode cavities per downstream diffuser plate surface area of the hollow cathode cavities. The results above show that plasma and process uniformities can be improved by gradual increase in either the hollow cathode cavity volume density or the hollow cathode cavity surface area density of the hollow cathode cavities from the inner regions to the outer regions of the diffuser plate, or from center to edge of the diffuser plate. 
     Another way to change the film deposition thickness and property uniformity is by changing the diffuser holes density across the diffuser plate, while keeping the diffuser holes identical. The density of diffuser holes is calculated by dividing the total surface of holes of bores  312  intersecting the downstream side  304  by the total surface of downstream side  304  of the diffuser plate in the measured region. The density of diffuser holes can be varied from about 10% to about 100%, and preferably varied from 30% to about 100%. To reduce the “dome shape” problem, the diffuser holes density should be lowered in the inner region, compared to the outer region, to reduce the plasma density in the inner region. The density changes from the inner region to the outer region should be gradual and smooth to ensure uniform and smooth deposition and film property profiles.  FIG. 13  shows the gradual change of diffuser holes density from low in the center (region A) to high at the edge (region B). The lower density of diffuser holes in the center region would reduce the plasma density in the center region and reduce the “dome shape” problem. The arrangement of the diffuser holes in  FIG. 3  is merely used to demonstrate the increasing diffuser holes densities from center to edge. The invention applies to any diffuser holes arrangement and patterns. The density change concept can also be combined with the diffuser hole design change to improve center to edge uniformity. When the density of the gas passages is varied to achieve the plasma uniformity, the spacing of hollow cathode cavities at the down stream end could exceed 0.6 inch. 
     The inventive concept of gradual increase of hollow cathode cavity size (volume and/or surface area) from the center of the diffuser plate to the edge of the diffuser plate can be accomplished by a combination of the one of the hollow cathode cavity size (volume and/or surface area) and shape variation, with or without the diffuser hole density variation, with one of the diffuser plate bending method, and with one of the hollow cathode cavity machining methods applicable. For example, the concept of increasing density of diffuser holes from the center to the edge of the diffuser plate can be used increasing the diameter of the hollow cathode cavity (or downstream bore) from the center to the edge of the diffuser plate. The diffuser plate could be kept flat and the diffuser holes are drilled by CNC method. The combination is numerous. Therefore, the concept is very capable of meeting the film thickness and property uniformity requirements. 
     Up to this point, the various embodiments of the invention are mainly described to increase the diameters and lengths of the hollow cathode cavities from center of the diffuser plate to the edge of the diffuser plate to improve the plasma uniformity across the substrate. There are situations that might require the diameter and the lengths of the hollow cathode cavities to decrease from the center of the diffuser plate to the edge of the diffuser plate. For example, the power source might be lower near the center of the substrate and the hollow cathode cavities need to be larger to compensate for the lower power source. The concept of the invention, therefore, applies to decreasing the sizes (volumes and/or areas) hollow cathode cavities from the center of the diffuser plate to the edge of the diffuser plate. 
     The concept of the invention applies to any design of gas diffuser holes, which includes any design of hollow cathode cavity, and any shapes/sizes of gas diffuser plates. The concept of the invention applies to a diffuser plate that utilizes multiple designs of gas diffuser holes, which include multiple designs of hollow cathode cavities. The concept of the invention applies to diffuser plate of any curvatures and diffuser plate made of any materials, for example, aluminum (Al), tungsten (W), chromium (Cr), tantalum (Ta), or combinations thereof, among others, and by any methods, for example, cast, brazed, forged, hot iso-statically pressed or sintered. The concept of the invention also applies to diffuser plate made of multiple layers of materials that are pressed or glued together. In addition, the concept of the invention can be used in a chamber that could be in a cluster system, a stand-alone system, an in-line system, or any systems that are applicable. 
     Although several preferred embodiments which incorporate the teachings of the present invention have been shown and described in detail, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.