Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
  • C23C16/44—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating
  • C23C16/455—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating characterised by the method used for introducing gases into reaction chamber or for modifying gas flows in reaction chamber
  • C23C16/45563—Gas nozzles
  • C23C16/45565—Shower nozzles
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
  • C23C16/22—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the deposition of inorganic material, other than metallic material
  • C23C16/30—Deposition of compounds, mixtures or solid solutions, e.g. borides, carbides, nitrides
  • C23C16/34—Nitrides
  • C23C16/345—Silicon nitride
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
  • C23C16/44—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating
  • C23C16/455—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating characterised by the method used for introducing gases into reaction chamber or for modifying gas flows in reaction chamber
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
  • C23C16/44—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating
  • C23C16/50—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating using electric discharges
  • C23C16/505—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating using electric discharges using radio frequency discharges
  • C23C16/509—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating using electric discharges using radio frequency discharges using internal electrodes
  • C23C16/5096—Flat-bed apparatus

Definitions

• Embodiments of the invention generally relate to a gas distribution plate assembly and method for distributing gas in a processing chamber.
• PECVD Plasma enhanced chemical vapor deposition
• 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.
• RF radio frequency
• 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 x 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.
• 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.
• Figure 1 illustrates a cross-sectional schematic view 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 Figure 1.
• BCE back channel etch
• the BCE process is preferred, because the gate dielectric (silicon nitride), 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.
• 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.
• the gate electrode layer 102 there may be an optional insulating material, for example, such as silicon dioxide (Si ⁇ 2 ) or silicon nitride (SiN), which may also be formed using an embodiment of a PECVD system described herein.
• 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 (S1O 2 ), silicon oxynitride (SiON), or 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 A to about 6000 A.
• 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 herein or other conventional methods known to the art.
• Bulk semiconductor layer 104 may be deposited to a thickness in the range of about 100 A to about 3000 A.
• 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 herein or other conventional methods known to the art. Doped semiconductor layer 105 may be deposited to a thickness within a range of about 100 A to about 3000 A. 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, SiO 2 or 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 A to about 5000 A.
• 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), SiO 2 , SiON and SiN films used in liquid crystal displays (or flat panels) may all be deposited using an embodiment of a plasma enhanced chemical vapor deposition (PECVD) system described in this invention.
• PECVD plasma enhanced chemical vapor deposition
• the TFT structure described here is merely used as an example.
• a gas distribution plate assembly for a plasma processing chamber comprises a diffuser plate having an upstream side and a downstream side, gas passages passing between the upstream and downstream sides of the diffuser plate and hollow cathode cavities at the downstream side of the gas passages.
• the downstream side of the diffuser plate has a curvature to improve the thickness uniformity and film property uniformity of thin films deposited by PECVD.
• either the hollow cathode cavity volume density, hollow cathode cavity surface area density, or the cathode cavity density of the diffuser increases from the center of the diffuser to the edge of the diffuser.
• downstream side of the gas diffuser plate is divided into a number of concentric zones, wherein the gas passages in each zone are identical and the density, volume, or surface area of hollow cathode cavities of gas passages in each zone gradually increases from the center to the edge of the diffuser plate.
• a plasma processing chamber comprises a diffuser plate having an upstream side and a downstream side, gas passages passing between the upstream and downstream sides of the diffuser plate and hollow cathode cavities at the downstream side of the gas passages.
• the down ⁇ stream side of the diffuser plate has a curvature to improve the thickness uniformity and film property uniformity of thin films deposited by PECVD.
• either the hollow cathode cavity volume density, hollow cathode cavity surface area density, or the cathode cavity density of the diffuser increases from the center of the diffuser to the edge of the diffuser.
• a method of making a gas diffuser plate for a plasma processing chamber comprises softening a diffuser plate by heating, bending the diffuser plate to a curvature with a curvature annealing fixture and machining gas passages into the diffuser plate.
• a method of making a diffuser plate for a plasma processing chamber comprises machining a curvature into a substantially flat diffuser plate and machining gas passages into the diffuser plate.
• a method of depositing a thin film on a substrate comprises placing a substrate in a process chamber with a gas diffuser plate having a curvature, an upstream side and a downstream side, gas passages passing between the upstream and downstream sides of the diffuser plate and hollow cathode cavities at the downstream side of the 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.
• either the hollow cathode cavity volume density, or the hollow cathode cavity surface area density, or the hollow cathode cavity density of the gas passages at the center of the diffuser plate are less than the same parameter of the gas passages at the edge of the diffuser plate.
• Figure 1 illustrates a cross-sectional schematic view of a thin film transistor structure.
• Figure 2 shows thickness profiles of an amorphous silicon film across a 2200 mm wide glass substrate.
• Figure 3 shows thickness profiles of another amorphous silicon film across a 2200 mm wide glass substrate.
• Figure 4 shows thickness profiles of another amorphous silicon film across a 2200 mm wide glass substrate.
• Figure 5 is a schematic cross-sectional view of one embodiment of a plasma enhanced chemical vapor deposition system.
• Figure 6A schematically shows an RF hollow cathode.
• Figures 6B-6G illustrate various designs of hollow cathode cavities.
• Figure 8 depicts a cross-sectional schematic view of a gas diffuser plate.
• Figure 8A 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.
• Figure 9A shows a diffuser plate with diffuser holes in multiple zones.
• Figure 9B shows a diffuser plate with mixed hollow cathode cavity diameters and the inner region hollow cathode cavity volume and/or surface area density is lower than the outer region hollow cathode cavity volume and/or surface area density.
• Figure 9C 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.
• Figure 9D shows the downstream side view of a diffuser plate with varying diffuser hole densities.
• Figure 10 illustrates a schematic cross-section of one embodiment of a gas diffuser plate with a curvature.
• Figure 1OA illustrates a schematic cross-section of one embodiment of a gas diffuser plate with a curvature.
• Figure 1OB illustrates a schematic cross-section of one embodiment of a gas diffuser plate with a curvature.
• Figure 11 shows thickness profiles of an amorphous silicon film across a glass substrate using a diffuser plate with a curvature.
• Figure 12 illustrates a flow chart for the diffuser-annealing process for bending a diffuser plate to the desired curvature.
• Figure 13 illustrates an exemplary weight lay-out for annealing an aluminum diffuser plate that is 1.4 inches thick.
• 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, California.
• PECVD plasma enhanced chemical vapor deposition
• the center thick uniformity problem has been solved by varying the size and/or distribution of cathode cavities on the downstream surface of a PECVD gas diffuser plate.
• the cathode cavities enhance plasma ionization in the PECVD chamber.
• varying the depths, diameters, surface area and/or density of hollow cathode cavities across the surface of a diffuser plate can eliminate the center thick uniformity problem for large substrates.
• This technique is known as the hollow cathode gradient, or HCG, method and is described in more detail below in conjunction with Figures 6A, 8 and 8.
• a complete description of the HCG method is provided in previously referenced United States Patent Application Serial Number 10/889,683, entitled "Plasma Uniformity Control By Gas Diffuser Hole Design," filed July 12, 2004 by Choi, et al.
• Substrate size and “diffuser plate size” as used herein refer to the nominal surface area, or footprint, of a substrate or diffuser plate and not to the wetted surface area, i.e., the total surface area of all sides and surfaces combined.
• a 1 ,000 mm x 1 ,000 mm diffuser plate has a nominal size of 1 ,000,000 mm 2 , but a much higher wetted surface area, which includes the top and bottom surfaces, side edges, and all features machined into the surface of the diffuser.
• Figure 2 shows thickness profiles of an amorphous silicon film across a 2200 mm wide glass substrate.
• the abscissa represents the position, in millimeters, of each thickness measurement along the profile of the 2200 mm long substrate.
• the ordinate represents the deposition rate, in A/min, of the amorphous silicon film deposited on the substrate.
• Two data sets are illustrated in Figure 2, data set 201 by squares, data set 202 by diamonds.
• Data sets 201 and 202 represent the deposition rate profile measured along each diagonal of the substrate. As can be seen in Figure 2, there is no significant difference between the two profiles, so it is assumed that the center thick profile exhibited by data sets 201 and 202 is relatively constant across the length of the diffuser.
• a gas diffuser plate incorporating HCG was used to deposit the ⁇ -Si film measured for data sets 201 and 202.
• the electrode spacing i.e., the distance between the gas diffuser plate and the substrate support in the PECVD chamber, was 0.800 inches while depositing the film.
• Process conditions during deposition of the film were: 10,000 seem SiH 4 gas flow rate, 40,000 seem H 2 gas flow rate, 11 ,000 W RF plasma power, chamber pressure of 2.7 Torr and substrate temperature of 340 C (inner substrate heater) and 360 C (outer substrate heater).
• the components of a PECVD chamber, including the gas diffuser plate, substrate support, and electrode spacing, are described in greater detail in conjunction with Figure 5.
• SiH 4 silicon-containing gases
• Si 2 H 6 silicon-containing gases
• film thickness uniformity of the amorphous silicon film still suffers from the center thick effect — with poor uniformity and film properties at the edge of the substrate.
• the substrate center region 203 of the film uniformity profile indicates acceptable film properties and uniformity while edge regions 204 and 205 show poor uniformity and film properties. It has been shown that HCG does have some effect on
• FIGS 3 and 4 show thickness profiles of an amorphous silicon film across a 2200 mm wide glass substrate with electrode spacings of 0.650 and 0.550 inches, respectively.
• film thickness profiles 301 and 302 show deterioration in uniformity in substrate center region 303 and a slight improvement in thickness uniformity in edge regions 304 and 305.
• the ⁇ -Si film measured for Figure 3 was deposited in an identical PECVD chamber and under the same process conditions as the ⁇ -Si film measured for Figure 2.
• Figure 4 illustrates film thickness profiles 401 and 402 for an ⁇ -Si film deposited under identical process conditions as the films of Figures 2 and 3, except with an electrode spacing of 0.550 inches.
• Film thickness profiles 401 and 402 show further uniformity deterioration in center region 403 and greatly improved thickness uniformity in edge regions 404 and 405.
• the data shown in Figure 2, 3 and 4 indicate that electrode spacing more strongly effects ⁇ -Si film uniformity than the hollow cathode gradient effect.
• a process window is the range of variation for all process parameters, such as substrate temperature or gas flow rate, that still produces acceptable results.
• process parameters such as substrate temperature or gas flow rate
• FIG. 5 is a schematic cross-sectional view of a plasma enhanced chemical vapor deposition system 500 that may be adapted to benefit from the invention.
• PECVD system 500 is available from AKT, a division of Applied Materials, Inc., Santa Clara, California.
• the system 500 generally includes a processing chamber 502 coupled to a gas source 504.
• the processing chamber 502 has walls 506 and a bottom 508 that partially define a process volume 512.
• the process volume 512 is typically accessed through a port (not shown) in the walls 506 that facilitate movement of a substrate 540 into and out of the processing chamber 502.
• the walls 506 and bottom 508 may be fabricated from a unitary block of aluminum or other material compatible with processing.
• a temperature controlled substrate support assembly 538 is centrally disposed within the processing chamber 502.
• the support assembly 538 supports a glass substrate 540 during processing.
• the substrate support assembly 538 comprises an aluminum body 524 that encapsulates at least one embedded heater 532.
• the heater 532 such as a resistive element, disposed in the support assembly 538, is coupled to an optional power source 574 and controllably heats the support assembly 538 and the glass substrate 540 positioned thereon to a predetermined temperature.
• the heater 532 maintains the glass substrate 540 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.
• the support assembly 538 has a lower side 526 and an upper side 534.
• the upper side 534 supports the glass substrate 540.
• the lower side 526 has a stem 542 coupled thereto.
• the stem 542 couples the support assembly 538 to a lift system (not shown) that moves the support assembly 538 between an elevated processing position (as shown) and a lowered position that facilitates substrate transfer to and from the processing chamber 502.
• the stem 542 additionally provides a conduit for electrical and thermocouple leads between the support assembly 538 and other components of the system 500.
• a bellows 546 is coupled between support assembly 538 (or the stem 542) and the bottom 508 of the processing chamber 502.
• the bellows 546 provides a vacuum seal between the process volume 512 and the atmosphere outside the processing chamber 502 while facilitating vertical movement of the support assembly 538.
• the support assembly 538 generally is grounded such that radio frequency (RF) power supplied by a power source 522 to a gas distribution plate assembly 518 positioned between the lid assembly 510 and substrate support assembly 538 (or other electrode positioned within or near the lid assembly of the chamber) may excite gases present in the process volume 512 between the support assembly 538 and the distribution plate assembly 518.
• RF radio frequency
• the RF power from the power source 522 is generally selected commensurate with the size of the substrate to drive the chemical vapor deposition process.
• the lid assembly 510 provides an upper boundary to the process volume 512.
• the lid assembly 510 is fabricated from aluminum (Al).
• the lid assembly 510 includes a pumping plenum 514 formed therein coupled to an external pumping system (not shown).
• the pumping plenum 514 is utilized to channel gases and processing by-products uniformly from the process volume 512 and out of the processing chamber 502.
• the lid assembly 510 typically includes an entry port 580 through which process gases provided by the gas source 504 are introduced into the processing chamber 502.
• the entry port 580 is also coupled to a cleaning source 582.
• the cleaning source 582 typically provides a cleaning agent, such as dissociated fluorine, that is introduced into the processing chamber 502 to remove deposition by-products and films from processing chamber hardware, including the gas distribution plate assembly 518.
• the gas distribution plate assembly 518 is coupled to an interior side 520 of the lid assembly 510.
• the shape of gas distribution plate assembly 518 is typically configured to substantially conform to the perimeter of the glass substrate 540, for example, polygonal for large area flat panel substrates and circular for wafers.
• the gas distribution plate assembly 518 includes a perforated area 516 through which process and other gases supplied from the gas source 504 are delivered to the process volume 512.
• the perforated area 516 of the gas distribution plate assembly 518 is configured to provide uniform distribution of gases passing through the gas distribution plate assembly 518 into the processing chamber 502.
• Gas distribution plates that may be adapted to benefit from the invention are described in commonly assigned United States Patent Application Serial Number 09/922,219, filed August 8, 2001 by Keller et al., United States Patent Application Serial Number 10/140,324, filed May 6, 2002 by Yim et al., and 10/337,483, filed January 7, 2003 by Blonigan et al., United States Patent Number 6,477,980, issued November 12, 2002 to White et al., United States Patent Application Serial Number 10/417,592, filed April 16, 2003 by Choi et al., and United States Patent Application Number 10/823,347, filed on April 12, 2004 by Choi et al., which are hereby incorporated by reference in their entireties.
• the gas distribution plate assembly 518 typically includes a diffuser plate (or distribution plate) 558 suspended from a hanger plate 560.
• the diffuser plate 558 and hanger plate 560 may alternatively comprise a single unitary member.
• a plurality of gas passages 562 are formed through the diffuser plate 558 to allow a predetermined distribution of gas to pass through the gas distribution plate assembly 518 and into the process volume 512.
• a plenum 564 is formed between hanger plate 560, diffuser plate 558 and the interior surface 520 of the lid assembly 510. The plenum 564 allows gases flowing through the lid assembly 510 to uniformly distribute across the width of the diffuser plate 558 so that gas is provided uniformly above the center perforated area 516 and flows with a uniform distribution through the gas passages 562.
• the diffuser plate 558 is typically fabricated from stainless steel, aluminum (Al), nickel (Ni) or other RF conductive material.
• the diffuser plate 558 could be cast, brazed, forged, hot iso-statically pressed or sintered.
• the diffuser plate is fabricated from bare, non-anodized aluminum.
• a non-anodized aluminum surface for diffuser plate 558 has been shown to reduce the formation of particles thereon that may subsequently contaminate substrates processed in PECVD system 500. Additionally, the manufacturing cost of diffuser plate 558 is reduced when it is not anodized.
• a suitable bare aluminum surface for diffuser plate 558 is generally free from scratches and burrs, chemically cleaned before use to eliminate unwanted contamination and may be mechanically polished or electro- polished.
• a non-anodized aluminum diffuser plate that may be adapted to benefit from the invention is described commonly assigned United States Patent Number 6,182,603, entitled “Surface-Treated shower Head For Use In a Substrate Processing Chamber,” filed July 13, 1998 by Shang et al.
• the thickness of the diffuser plate 558 is between about 0.8 inch to about 2.0 inches.
• the diffuser plate 558 could be circular for semiconductor wafer manufacturing or polygonal, such as rectangular, for flat panel display manufacturing.
• diffuser plate 558 It has been standard practice in the art for diffuser plate 558 to be configured substantially flat and parallel to substrate 540 and for the distribution of identical gas passages 562 to be substantially uniform across the surface of diffuser plate 558.
• Such a configuration of diffuser 558 has provided adequate gas flow and plasma density uniformity in process volume 512 for depositing films on substrates smaller than 1 ,200,000 mm 2 .
• thickness uniformity and film property uniformity can be achieved on deposited films solely by changing process parameters, such as process gas flow rates, plasma power, substrate temperature, and process chamber pressure.
• a flat diffuser plate 558 with a uniform distribution of gas passages 562 of uniform size and shape is generally not able to deposit films with acceptable thickness and film property uniformity onto large area substrates.
• HCG hollow cathode gradient
• a diffuser plate 558 that is configured with HCG improves the uniformity of SiN film thickness and film properties by altering the plasma distribution in process volume 512. This is because deposition of films by PECVD depends substantially on the source of the active plasma. Hence, non-uniform plasma distribution in process volume 512 can lead to poor film uniformity on the substrate 540.
• Dense chemically reactive plasma can be generated in process volume 512 of PECVD system 500 due to the hollow cathode effect, described here in conjunction with Figure 6A.
• the driving force in the RF generation of a hollow cathode discharge of a negatively charged RF electrode 601 is the frequency modulated DC voltage V s , known as the self-bias voltage, across space charge sheath, or wall sheath, 602a or 602b at the RF electrode 601.
• Figure 6A schematically shows an RF hollow cathode and the oscillatory movement of electrons, "e”, between repelling electric fields, 603a and 603b, of the opposing space charge sheaths 602a and 602b, respectively.
• the thickness of space charge sheaths 602a and 602b is equal to thickness " ⁇ ".
• Electron “e” is emitted from the cathode wall, in this case RF electrode 601 , which could be the walls of a gas passage 562 that is close to the process volume 512. Gas passage 562 and process volume 512 are shown in Figures 5 and 8. Referring again to Figure 6A, electron “e” is accelerated by the electric field 603a across the space charge sheath 602a. Electron “e” oscillates along path 605 across the inner space between walls of the RF electrode 601 owing to the repelling fields of opposing space charge sheath 602a and 602b. Electron “e” loses energy by collisions with the process gas and creates more ions.
• the created ions can be accelerated to the RF electrode 601 , thereby enhancing emissions of secondary electrons, which could create additional ions.
• the cavities between the cathode walls enhance the electron emission and ionization of the gas.
• Cone frustum-shaped features in the cathode walls such as when the gas passages formed in the diffuser plate with a gas inlet diameter smaller than the gas outlet diameter, are more efficient in ionizing the gas than cylindrical walls.
• An example of a cone frustum-shaped cathode cavity is described in more detail below in conjunction with Figure 8.
• the potential Ez is created due to the difference in ionization efficiency between the gas inlet and gas outlet.
• the hollow cathode cavities are located on the downstream ends of gas passages 562 and are close to the process volume 512. It has been shown that by changing the design of the walls of the cathode cavities of gas passages 562 and the arrangement or density of the hollow cathode cavities, the gas ionization may be modified to control plasma density and, hence, the film thickness and property uniformity of a deposited SiN film. The methods and results that prove this are described in previously referenced United States Patent Application Serial Number 10/889,683, entitled "Plasma Uniformity Control By Gas Diffuser Hole Design.” An example of hollow cathode cavities that are close to the process volume 512 is the second bore 812 of Figure 8.
• the hollow cathode effect mainly occurs in the cone frustum-shaped region of second bore 812 that faces the process volume 512.
• the Figure 8 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 are not limited to, the designs shown in Figures 6B-6G.
• FIG 8 is a partial sectional view of an exemplary diffuser plate 558 that may be adapted to benefit from the invention and is described in commonly assigned United States Patent Application Serial No. 10/417,592, titled “Gas Distribution Plate Assembly for Large Area Plasma Enhanced Chemical Vapor Deposition", filed on April 16, 2003, which is hereby incorporated by reference in its entirety to the extent not inconsistent with the claimed invention.
• the diffuser plate 558 includes a first or upstream side 802 facing the lid assembly 510 and an opposing second or downstream side 804 that faces the support assembly 538.
• Each gas passage 562 is defined by a first bore 810 coupled by an orifice hole 814 to a second bore 812 that combine to form a fluid path through the gas distribution plate 558.
• the first bore 810 extends a first depth 830 from the upstream side 802 of the gas distribution plate 558 to a bottom 818.
• the bottom 818 of the first bore 810 may be tapered, beveled, chamfered or rounded to minimize the flow restriction as gases flow from the first bore into the orifice hole 814.
• the first bore 810 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 812 is formed in the diffuser plate 558 and extends from the downstream side (or end) 804 to a depth 832 of about 0.10 inch to about 2.0 inches. Preferably, the depth 832 is between about 0.1 inch to about 1.0 inch.
• the opening diameter 836 of the second bore 812 is generally about 0.1 inch to about 1.0 inch and may be flared at an angle 816 of about 10 degrees to about 50 degrees.
• the opening diameter 836 is between about 0.1 inch to about 0.5 inch and the flaring angle 816 is between 20 degrees to about 40 degrees.
• the surface area of the second bore 812 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 812 refers to the diameter intersecting the downstream surface 804.
• An example of a diffuser plate, used to process 1870 mm by 2200 mm substrates, has second bores 812 at a diameter of 0.302 inch and at a flare angle 816 of about 22 degrees.
• the distances 880 between rims 882 of adjacent second bores 812 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 810 is usually, but not limited to, being at least equal to or smaller than the diameter of the second bore 812.
• a bottom 820 of the second bore 812 may be tapered, beveled, chamfered or rounded to minimize the pressure loss of gases flowing out from the orifice hole 814 and into the second bore 812.
• the proximity of the orifice hole 814 to the downstream side 804 serves to minimize the exposed surface area of the second bore 812 and the downstream side 804 that face the substrate, the downstream area of the diffuser plate 558 exposed to fjuorine provided during chamber cleaning is reduced, thereby reducing the occurrence of fluorine contamination of deposited films.
• the orifice hole 814 generally couples the bottom 818 of the first hole 810 and the bottom 820 of the second bore 812.
• the orifice hole 814 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 834 of about 0.02 inch to about 1.0 inch, preferably about 0.02 inch to about 0.5 inch.
• the length 834 and diameter (or other geometric attribute) of the orifice hole 814 is the primary source of back pressure in the plenum 564 which promotes even distribution of gas across the upstream side 802 of the gas distribution plate 558.
• the orifice hole 814 is typically configured uniformly among the plurality of gas passages 562; however, the restriction through the orifice hole 814 may be configured differently among the gas passages 562 to promote more gas flow through one area of the gas distribution plate 558 relative to another area.
• the orifice hole 814 may have a larger diameter and/or a shorter length 834 in those gas passages 562, of the gas distribution plate 558, closer to the wall 506 of the processing chamber 502 so that more gas flows through the edges of the perforated area 516 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.
• the volume of second bore (or hollow cathode cavity) 812 can be changed by varying the diameter "D" (or opening diameter 836 in Figure 8), the depth “d” (or length 832 in Figure 8) and the flaring angle " ⁇ " (or flaring angle 816 of Figure 8), as shown in Figure 8A. Changing the diameter, depth and/or the flaring angle would also change the surface area of the second bore 812. It is believed that higher plasma density is likely the cause of the higher deposition rate at the center of substrate 540 (shown in Figure 5).
• the plasma density can be reduced in the center region of the substrate to improve the uniformity of film thickness and film properties.
• changes to hollow cathode cavity design on a diffuser plate must be gradual across the face of the diffuser plate to avoid stepped changes in film thickness on the substrate.
• 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.
• sufficiently gradual change to the hollow cathode cavity design on a diffuser plate 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.
• Figure 9A 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 inner and 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. Zones may be square, rectangular or circular. From zone 1 to zone N, the hollow cathode cavities 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 second bores 812, 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.
• some second bores 812 could be kept the same throughout the diffuser plate, while the rest of the second bores 812 have a gradual increase in the sizes (volumes and/or surface areas) of the hollow cathode cavities.
• the diffuser second bores 812 have a gradual increase in sizes (volumes and/or surface areas) of the hollow cathode cavities, while there are also some small hollow cathode cavities C1 at the edge of the diffuser plate to further increase the overall hollow cathode cavity volume and/or surface area per downstream diffuser plate surface area.
• Figure 9B is a schematic plot of a bottom view of a diffuser plate.
• most of the hollow cathode cavities are uniform across the diffuser plate, while there are a few larger hollow cathode cavities C2 towards the edge of the diffuser plate, as shown in a schematic plot of a diffuser bottom in Figure 9C.
• Plasma and process uniformities can be improved by gradually increasing either the volume or surface area or a combination of both of the hollow cathode cavities from the center region of the diffuser plate to the edge region of the diffuser plate.
• Another way to change the film deposition thickness and property uniformity is by changing the diffuser hole 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 812 intersecting the downstream side 804 by the total surface of downstream side 804 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%.
• the diffuser holes density should be lowered in the central region, compared to the outer region, to reduce the plasma density in the inner region.
• FIG. 9D 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 for SiN films.
• the arrangement of the diffuser holes in Figure 9D 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.
• the spacing of hollow cathode cavities at the downstream end could exceed 0.6 inch.
• electrode spacing is defined as the distance between downstream side 804 of diffuser plate 558 and substrate 540.
• plasma density increases near the center of a PECVD chamber for wider electrode spacings, thus altering the film thickness and film property profile.
• a diffuser plate is provided that combines the benefits of both spacings. This is done by incorporating the wider and narrower spacings into the electrode itself, i.e., the electrode is adapted to provide a wider electrode spacing over the center region of the substrate and a narrower electrode spacing over the edge of the substrate.
• the electrode is adapted to provide a wider electrode spacing over the center region of the substrate and a narrower electrode spacing over the edge of the substrate.
• substrates larger than about 1 ,200,000 mm 2 may be deposited with amorphous silicon films that have acceptable thickness and film property uniformity across the entire substrate.
• FIG. 10 illustrates a schematic cross-section of one embodiment of a gas diffuser plate 1001 with a curvature that may be adapted for use in a PECVD chamber. Gas passages 562 are not shown for clarity. Downstream side 804 of diffuser plate 1001 has a curvature and in this embodiment upstream side 802 of diffuser plate 1001 is substantially flat.
• upstream side 802 of diffuser plate 1001 may also have a curvature, for example when diffuser 1001 is formed by a method using a curvature annealing fixture, described below in conjunction with Figures 12 and 13.
• the maximum displacement 1004 between the curved surface of the downstream side 804 and the surface of a fictitious flat downstream side 804a is also shown.
• the hollow cathode gradient is necessary for the hollow cathode gradient to implement gradual change of hollow cathode cavity volume density, hollow cathode surface area density and/or hollow cavity density across the surface of the gas diffuser. This avoids non-uniformities of the SiN film due to abrupt changes in plasma density in the process volume caused by a hollow cathode gradient that is too great. It is believed that the same principle holds true for improving the film thickness and film property uniformity of amorphous silicon films via an electrode/diffuser plate that is adapted to have a varying electrode spacing over a substrate. Hence, the transition from the narrow spacing region above the edge of the substrate to the slightly wider spacing region above the center of the substrate is preferably smooth and gradual. Therefore, the downstream side 804 of diffuser 1001 is preferably substantially concave, i.e., relatively closer to the substrate around the edges and smoothly transitioning to a high point, or apex 1005, over the center of the substrate.
• the curvature of downstream side 804 is generally an arc with an apex 1005 located approximately over the center point of the substrate.
• the apex 1005 defines the maximum displacement 1004 between the curved surface of the downstream side 804 and the surface of the fictitious flat downstream side 804a, as shown in Figure 10.
• the arc has the curvature corresponding to a segment of a circle or ellipse, as shown in Figure 10. This ensures a smooth transition in electrode spacing from the edge to the center of the diffuser and allows the shape to be easily quantified.
• different methods of describing the curved downstream side 804 may be used.
• a line segment, as shown in Figure 10A may describe the arc.
• the apex 1005 of diffuser 1002 is still located substantially over the center point of the substrate and the electrode spacing increases from the edge of the diffuser to the center.
• the arc may be described by a segment of other mathematical functions besides lines, circles or ellipses, such as exponential, second-order, third-order, sinusoidal, hyperbolic or other geometric functions.
• apex 1005 is located approximately over the center point of the substrate and the electrode spacing increases from the edge of the diffuser to the center.
• downstream side 804 of diffuser 1003 includes a substantially flat region 1007 at the edge of diffuser 1003.
• Curved segment 1007a of downstream side 804 may be described by a segment of a line, circle, ellipse or other mathematical function as described above for other configurations of the invention.
• apex 1005 is located approximately over the center point of the substrate and the electrode spacing increases from the edge of the diffuser to the center.
• maximum displacement 1004 between the surface of the curved downstream side 804 and the surface of a fictitious flat downstream side 804a is small relative to the size of diffuser plate 1001.
• maximum displacement 1004 is no more than about 3% of the characteristic length of the diffuser, preferably from about 0.01% to about 0.30%.
• the characteristic length is considered to be the "equivalent radius".
• the equivalent radius is equal to the radius of the diffuser.
• the equivalent radius is one half the diagonal.
• the advantage of using an electrode with a curved downstream side is that it greatly increases the process window for ⁇ - Si film properties, making the formation of high quality amorphous silicon films on large substrates easier and more reliable for mass production. In some cases, an electrode having curvature is necessary to make the formation of acceptable ⁇ -Si films even possible.
• the wider electrode spacing in the center region of the diffuser is achieved through a curvature of the substrate support.
• the diffuser plate 1010 has a substantially flat downstream side 804 and substrate support 1011 has a curvature with a maximum displacement 1004.
• maximum displacement 1004 is defined as the distance between the substrate support curved surface 1012 of the surface of a fictitious flat substrate support surface 1012a, as shown in Figure 10C.
• both the diffuser plate and the substrate support may each have a curvature, wherein the curvatures are adapted so that the desired wide center region spacing and narrow edge region spacing is achieved.
• This aspect is shown schematically in Figure 10D.
• the curved downstream surface 1016 of diffuser plate 1013 has a more pronounced curvature than the substrate support curved surface 1015 of substrate support 1014. Because of this, center region electrode spacing 1017 is larger than edge region electrode spacing 1018. Hence, the desired wide center region spacing and narrow edge region spacing may be achieved when a diffuser plate and substrate support both have a curvature.
• Figure 11 shows thickness profiles 1 101 and 1 102 of an amorphous silicon film across a 2200 mm wide glass substrate using a diffuser plate having a curvature with a maximum displacement 1004 of 0.100 inches.
• the electrode spacing was 0.650 inches while depositing the film.
• Process conditions during deposition of the film were: 10,000 seem SiH4 gas flow rate, 36,000 seem H 2 gas flow rate, 10,000 W RF plasma power, chamber pressure of 2.5 Torr and substrate temperature of 340 0 C (inner substrate heater) to 360 0 C (outer substrate heater).
• the abscissa represents the position, in millimeters, of each thickness measurement along the profile of the 2200 mm long substrate.
• the ordinate represents the deposition rate, in A/min, of the amorphous silicon film deposited on the substrate.
• Two data sets are illustrated in Figure 11 , data set 1101 by squares, data set 1102 by diamonds.
• Data sets 1101 and 1102 represent the deposition rate profile measured along each diagonal of the substrate. The difference between the two profiles is negligible, implying a constant thickness profile across the length of the diffuser.
• Table 1 Measurement of thickness uniformity of ⁇ -Si film deposited on substrate.
• a PECVD gas diffuser is adapted with a curved downstream side and no hollow cathode gradient. This diffuser improves the film thickness uniformity and film property uniformity of ⁇ -Si films deposited on substrates larger than about 1 ,200,000 mm 2 .
• a PECVD gas diffuser is adapted with a curved downstream side and a hollow cathode gradient. Diffusers so adapted may be used for the processing of either SiN or ⁇ -Si films. This reduces production costs of PECVD chambers and increases chamber flexibility, i.e., a chamber may be used for deposition of either SiN or ⁇ -Si films without changing gas diffuser plates.
• Diffuser plates for processing substrates larger than about 1000 mm x 1200 mm may be difficult to manufacture repeatably. There may be significant variation from the desired shape and/or from diffuser-to-diffuser. This is particularly true for diffuser plates that are not substantially flat, such as diffusers with a curved downstream surface. Because film thickness uniformity and film property uniformity are strongly dependent on electrode spacing for some thin films, such as ⁇ -Si, it is important to minimize variations that may occur between the final curvature of a diffuser after manufacturing and the intended shape. It is also important to minimize variations that may occur between different — but nominally identical — chambers. Methods are provided to allow the manufacturing of curved diffusers for a PECVD chamber in a repeatable and cost-effective manner.
• the desired curvature of the downstream side of the gas diffuser plate is formed by a thermal process in which the diffuser plate is bent to conform to the shape of a curvature annealing fixture.
• the curvature annealing fixture is a metal plate machined to the desired curvature and is used for bending a large number of diffusers.
• Figure 12 illustrates a flow chart for the diffuser-annealing process 1200 for bending a diffuser plate to the desired curvature using a curvature annealing fixture.
• Step 1201 The diffuser plate is aligned with and placed on the curvature annealing fixture. The downstream side of the diffuser should be in contact with the annealing fixture.
• Step 1202 The surface of the diffuser plate is covered by a protective material to prevent damage and contamination from the annealing weights.
• the protective material must be clean, relatively flexible and heat-resistant.
• One example of protective covering that may be used is an anodized aluminum sheet.
• Step 1203 The diffuser plate is loaded with the appropriate weight required to plastically deform the diffuser during the annealing process.
• the weight must be distributed across the diffuser plate such that during the annealing process the diffuser plate completely conforms to the shape of the curvature annealing fixture. In general, weight should first be applied to the center point of the diffuser, then distributed along the diagonals and periphery.
• Figure 13 illustrates an exemplary lay-out for weights "W" for annealing a 2200 mm x 1870 mm aluminum diffuser plate "D" that is 1.44 inches thick.
• the amount and distribution of weight used is variable, being dependent on the size, thickness and composition of the diffuser plate, the curvature of the curvature annealing fixture, and the duration and temperature of the anneal process. However, one skilled in the art can easily determine these factors.
• Step 1204 The temperature of the diffuser plate is increased to the desired anneal temperature at a rate slow enough to prevent warping.
• the temperature ramp rate and anneal temperature are variable since they depend on the size, thickness and composition of the diffuser plate, the curvature of the curvature annealing fixture, and the duration and temperature of the anneal process. However, one skilled in the art can easily determine these factors. In the example of the 2200 mm x 1870 mm aluminum diffuser above, the appropriate temperature ramp rate is no more than about 40 0 C per hour and the annealing temperature about 410 0 C.
• Step 1205 The diffuser plate is annealed, i.e., held at the anneal temperature for the time necessary for the diffuser plate to plastically deform and conform exactly to the shape of the curvature annealing fixture.
• desired anneal time is variable depending on a number of factors. This is easily determined by one skilled in the art. In the exemplary aluminum diffuser above, the anneal time is no less than about 4 hours.
• Step 1206 The temperature of the diffuser plate is decreased to the room temperature at a rate slow enough to prevent warping. As stated above, this is highly variable for different diffuser plates.
• the cooling rate is no more than about 25 0 C per hour.
• Step 1207 After the diffuser plate reaches room temperature, the weights are removed.
• the diffuser plate is not adapted with the hollow cathode gradient and the gas passages and hollow cathode cavities are substantially identical.
• the diffuser plate is adapted with both a curved downstream surface and the hollow cathode gradient.
• machining of the gas passages which is greatly simplified with a substantially flat surface, is preferably performed prior to the annealing process.
• the machining of the gas passages may also be performed after the annealing/bending process. Machining of the gas passages may be manual or numerically controlled (NC), but due to the large number of gas passages on large diffuser plates, NC machining is generally preferred.
• the desired curvature of the downstream side of the gas diffuser plate is formed by machining out the necessary material on the downstream side of the diffuser, using milling or lathe-type metal removal processes which are well known in the art.
• machining of the gas passages is performed prior to formation of the curved surface.
• machining of the gas passages is performed after formation of the curved surface. Machining of the gas passages may be manual or numerically controlled (NC), but due to the large number of gas passages on large diffuser plates, NC machining is generally preferred.
• gas passages are first machined into the gas diffuser plate, then a first curvature is machined into the downstream side of the gas diffuser plate, and lastly the diffuser plate is annealed into a final curvature.
• This embodiment provides a cost effective method for manufacturing a gas diffuser plate that includes both a hollow cathode gradient for uniformly depositing SiN and a substantially concave curvature for uniformly depositing ⁇ -Si.
• the typically identical gas passages are machined into a substantially flat surface. This is much more cost effective and repeatably manufactured than machining gas passages of variable depth and diameter into a curved surface.
• the first curvature is then machined into the downstream side of the gas diffuser plate using milling or lathe-type metal removal process which are well known in the art in order to create the desired hollow cathode cavity gradient across the surface of the diffuser; as more material is removed near the center of the diffuser plate, the resultant hollow cathode cavity size of the initially identical gas passages is reduced accordingly.
• the gas diffuser plate is then formed into the final desired curvature via the annealing/bending process described above. This final step is necessary because the curvature required to create a desired hollow cathode gradient is rarely the same curvature desired for uniformly depositing ⁇ -Si.

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