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Title:
SHOWERHEAD WITH INTERLACED GAS FEED AND REMOVAL AND METHODS OF USE
Document Type and Number:
WIPO Patent Application WO/2019/199620
Kind Code:
A1
Abstract:
Gas distribution modules comprising a housing with an upper plenum and a lower plenum are described. One of the upper plenum and lower plenum is in fluid communication with an inlet and the other is in fluid communication with an outlet. A plurality of upper passages connects the upper plenum to the bottom of the housing to allow a flow of gas to pass through and be isolated from the first plenum.

Inventors:
BERA, Kallol (5058 Garden Way, Fremont, California, 94536, US)
RAUF, Shahid (6167 Corte Padre, Pleasanton, California, 94566, US)
CARDUCCI, James (731 Gail Avenue, Sunnyvale, California, 94086, US)
KNYAZIK, Vladmir (1975 De La Pena Ave, Santa Clara, California, 95050, US)
SUBRAMANI, Anantha K. (4245 Merlot Court, San Jose, California, 95135, US)
Application Number:
US2019/026235
Publication Date:
October 17, 2019
Filing Date:
April 08, 2019
Export Citation:
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Assignee:
APPLIED MATERIALS, INC. (3050 Bowers Avenue, Santa Clara, California, 95054, US)
International Classes:
H01L21/67; H01J37/32; H01L21/02; H05H1/46
Foreign References:
US20120222616A12012-09-06
US20060090850A12006-05-04
US20050014382A12005-01-20
US20110048325A12011-03-03
US20150007771A12015-01-08
Attorney, Agent or Firm:
BLANKMAN, Jeffrey I. (SERVILLA WHITNEY LLC, 33 Wood Avenue SouthSuite 83, Iselin New Jersey, 08830, US)
Download PDF:
Claims:
What is claimed is:

1. A gas distribution module comprising:

a housing having sides, a top, a bottom and a partition between the top and bottom, the sides, partition and top defining an upper plenum, and the sides, partition and bottom defining a lower plenum;

an inlet in fluid communication with one of the upper plenum or the lower plenum;

an outlet in fluid communication with the other of the upper plenum or the lower plenum;

a plurality of upper passages extending from the upper plenum, through the lower plenum and extending through the bottom of the housing to form upper openings in the bottom, the upper passages separated from the lower plenum by an upper passage wall and providing fluid communication between the upper plenum and an outer face of the bottom of the housing; and

a plurality of lower passages extending through the bottom of the housing to form lower openings in the bottom, the lower passages providing fluid communication between the lower plenum and the outer face of the bottom of the housing.

2. The gas distribution module of claim 1 , wherein each of the upper openings in the bottom is surrounded by three lower openings and each of the lower openings in the bottom is surrounded by three upper openings.

3. The gas distribution module of claim 1 , wherein the inlet openings are arranged in circular zones alternating with circular regions of outlet openings.

4. The gas distribution module of claim 1 , wherein there are the same number of upper openings and lower openings.

5. The gas distribution module of claim 1 , wherein the one of the lower openings or upper openings surround the other of the lower openings and upper openings to form ring shaped openings.

6. The gas distribution moduie ot ciaim 5, wherein the openings in fluid communication with the inlet are surrounded by the openings in fluid communication with the outlet.

7. The gas distribution moduie of ciaim 1 , wherein the openings in fluid communication with the inlet have a diffusion plate at the bottom of the housing, the diffusion plate having a plurality of apertures separating a flow of gas in the opening into multiple flows.

8. The gas distribution module of claim 7, wherein the diffusion plate comprises at least one aperture oriented at an angle to at least one aperture to provide non-parallel flows of gases through apertures.

9. The gas distribution moduie of ciaim 1 , further comprising a second inlet in fluid communication with one of the upper plenum or lower plenum.

10. The gas distribution moduie of claim 9, wherein the second inlet is in fluid communication with the same plenum as the inlet.

1 1. The gas distribution moduie of claim 9, wherein the second inlet is in fluid communication with the other plenum than the inlet.

12. The gas distribution module of ciaim 9, wherein the plenum that the second inlet is in fluid communication with further comprises one or more divider to split the plenum into a first plenum and a second plenum.

13. The gas distribution module of claim 1 , wherein the partition is in electrical connection with an RF feed line and the bottom is electrically grounded to generate a plasma in the lower plenum.

14. The gas distribution module of ciaim 1 , wherein the bottom of the housing is in electrical connection with an RF feed line to generate a plasma on an opposite side of the bottom than the lower plenum.

15. A gas distribution moduie comprising: a housing having sides, a top, a bottom and a partition between the top and bottom, the sides, partition and top defining an upper plenum, and the sides, partition and bottom defining a lower plenum;

an inlet in fluid communication with one of the upper plenum or the lower plenum;

an outlet in fluid communication with the other of the upper plenum or the lower plenum;

a plurality of upper passages extending from the upper plenum, through the lower plenum and extending through the bottom of the housing to form upper openings in the bottom, the upper passages separated from the lower plenum by an upper passage wall and providing fluid communication between the upper plenum and an outer face of the bottom of the housing; a plurality of lower passages extending through the bottom of the housing to form lower openings in the bottom, the lower passages providing fluid communication between the lower plenum and the outer face of the bottom of the housing; and

an RF feed line in electrical communication with one of the partition to generate a plasma in the upper plenum or lower plenum and the bottom of the housing to generate a plasma in the lower plenum or on an opposite side of the bottom than the lower plenum.

Description:
SHOWERHEAD WITH INTERLACED GAS FEED AND REMOVAL AND

METHODS OF USE

TECHNICAL FIELD

[0001] The present disclosure generally relate to an apparatus and a method for providing a flow of gas info and out of a processing chamber. More specifically, embodiments of the disclosure are directed to gas distribution modules with interlaced gas feed and removal components.

BACKGROUND

[0002] In the field of semiconductor processing, flat-panel display processing or other electronic device processing, vapor deposition processes have played an important role in depositing materials on substrates With smaller technology node, on-wafer process uniformity during steady state and transient becomes more stringent. Gas flow velocity, pressure and chemical species distributions are important for on-wafer performance. During wafer surface process, by-products are generated that changes the species composition adversely affecting wafer surface process (deposition, etch, etc ). Center feed and edge exhaust based gas distribution apparatus can lead to flow velocity and pressure variations and by product accumulation from center to edge leading on-wafer process non-uniformity.

[0003] Therefore, there is an ongoing need in the art for improved gas distribution apparatuses, including showerheads, to supply uniform flows of gases to a substrate

SUMMARY

[0004] One or more embodiments of the disclosure are directed to gas distribution modules comprising a housing, an inlet and an outlet. The housing has at least one side, a top, a bottom and a partition between the top and bottom. The at least one side, partition and top define an upper plenum. The at least one side, partition and bottom define a lower plenum. The inlet is in fluid communication with one of the upper plenum or the lower plenum. The outlet is in fluid communication with the other of the upper plenum or the lower plenum. A plurality of passages extends from the upper plenum, through the lower plenum and extending through the bottom of the housing to form upper openings in the bottom. The upper passages are separated from the lower plenum by an upper passage wall and provide fluid communication between the upper plenum and an outer face of the bottom of the housing. A plurality of lower passages extends through the bottom of the housing to form lower openings in the bottom. The lower passages provide fluid communication between the lower plenum and the outer face of the bottom of the housing.

[0005] Additional embodiments of the disclosure are directed to gas distribution modules comprising a housing, an inlet and an outlet. The housing has at least one side, a top, a bottom and a partition between the top and bottom. The at least one side, partition and top define an upper plenum. The at least one side, partition and bottom define a lower plenum. The inlet Is in fluid communication the upper plenum and the outlet is in fluid communication with the lower plenum. A plurality of upper passages extends from the upper plenum, through the lower plenum and through the bottom of the housing to form upper openings in the bottom. The upper passages are separated from the lower plenum by an upper passage wall and provide fluid communication between the upper plenum and an outer face of the bottom of the housing. A plurality of lower passages extends through the bottom of the housing to form lower openings in the bottom. The lower passages provide fluid communication between the lower plenum and the outer face of the bottom of the housing. The lower openings are arranged so that there are three or more lower openings around each upper opening.

[0008] Further embodiments of the disclosure are directed to gas distribution modules comprising a housing, an inlet and an outlet. The housing has at least one side, a top, a bottom and a partition between the top and bottom. The at least one side, partition and top define an upper plenum. The at least one side, partition and bottom define a lower plenum. The inlet is in fluid communication with one of the upper plenum or the lower plenum. The outlet is in fluid communication with the other of the upper plenum or the lower plenum. A plurality of upper passages extends from the upper plenum, through the lower plenum and through the bottom of the housing to form upper openings in the bottom. The upper passages are separated from the lower plenum by an upper passage wall and provide fluid communication between the upper plenum and an outer face of the bottom of the housing. A plurality of lower passages extends through the bottom of the housing to form lower openings in the bottom. The lower passages provide fluid communication between the lower plenum and the outer face of the bottom of the housing. An RF feed line is in electrical communication with one of the partition to generate a plasma in the upper plenum or lower plenum, or the bottom of the housing to generate a plasma in the lower plenum or on an opposite side of the bottom than the lower plenum.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] So that the manner in which the above recited features of the disclosure are attained and can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments.

[0008] FIG. 1 shows a view of a gas distribution module with cutaway portion in accordance with one or more embodiments of the disclosure;

[0009] FIG. 2 shows an embodiment of a gas distribution module in accordance with one or more embodiment of the disclosure;

[0010] FIG. 3 shows an embodiment of a gas distribution module in accordance with one or more embodiment of the disclosure;

[0011] FIG. 4 shows an embodiment of a gas distribution module in accordance with one or more embodiment of the disclosure;

[0012] FIG. 5 shows an embodiment of a gas distribution module in accordance with one or more embodiment of the disclosure;

[0013] FIG. 6 shows an expanded view of region VI of FIG. 5;

[0014] FIG. 7 shows a partial cross-sectional schematic of a gas distribution module in accordance with one or more embodiment of the disclosure; [0015] FIG. 8 shows a partial cross-sectional schematic view of a gas distribution module in accordance with one or more embodiment of the disclosure;

[0018] FIG. 9 shows a partial cross-sectional schematic view of a gas distribution module in accordance with one or more embodiment of the disclosure;

[0017] FIG. 10A shows a cross-sectional schematic view of a gas distribution module with an RF powered partition in accordance with one or more embodiment of the disclosure;

[0018] FIG. 10B shows a cross-sectional schematic view of a gas distribution module with an RF powered bottom in accordance with one or more embodiment of the disclosure;

[0019] FIG. 10C shows a cross-sectional schematic view of a gas distribution module with an RF powered top in accordance with one or more embodiment of the disclosure; and

[0020] FIG. 1 1 shows a gas distribution assembly incorporating gas distribution modules according to one or more embodiment of the disclosure.

DETAILED DESCRIPTION

[0021] Embodiments of the disclosure are directed to gas distribution apparatus for use in chemical vapor deposition type processes. One or more embodiments of the disclosure are directed to atomic layer deposition processes and apparatus [also called cyclical deposition) incorporating the gas distribution apparatus described. The gas distribution apparatus described may be referred to as a showerhead or gas distribution plate, but it will be recognized by those skilled In the art that the apparatus does not need to be shaped like a showerhead or plate. The terms “showerhead" and“plate” should not be taken as limiting the scope of the disclosure.

[0022] In one or more embodiments of the disclosure, feed gas carrying chemical species from a recursive feed structure comes to an upper plenum (or lower plenum in a different configuration). The gas flows through multiple tubes (inlets) form the upper plenum across the lower plenum to the process region. The process gas interacts with the wafer surface leading to surface processing [deposition or etch). The process gas with by-products is removed through the exhaust tubes that are interlaced with the inlets. In this manner, by-products (which include reaction products) are removed locally minimizing variation of chemical species across the wafer. The exhaust tubes are connected to the lower plenum (or upper plenum in a different configuration). The lower plenum for exhaust is recursively connected to the pump port. Each part of the wafer“sees" similar steady state and transient response of the variables in terms of chemical species, flow velocity and pressure, improving on-wafer process uniformity and exposure time.

[0023] One or more embodiments of the disclosure advantageously provide gas distribution modules that allow each part of the wafer to see a similar environment in terms of chemical species, flow velocity and pressure. Some embodiments reduce by-products due to chemical reaction (on the wafer surface) by pumping out the by products, locally minimizing variation of chemical species across the wafer. Some embodiments advantageously provide uniform exposure time of chemical species in some embodiments, saturation of chemical species occurs almost simultaneously across the wafer minimizing the need for over-exposure. Some embodiments reduce the overall exposure time to the wafers, increasing overall throughput.

[0024] FIG. 1 illustrates an embodiment of a gas distribution module 100 in accordance with one or more embodiment of the disclosure. A housing 1 10 illustrated with a cutout portion 105 to show a portion of the inside of the housing. The housing 1 10 illustrated is a wedge-shaped component that can be incorporated into a larger gas distribution assembly by combining various wedge-shaped components to form a circular gas distribution system in some embodiments, the housing 1 10 is a circular shaped component that can be used without additional components.

[0025] The housing 1 10 of some embodiments has at least one side, a top and a bottom. For example, a round housing would have one continuous side; whereas a wedge-shaped housing would have four sides, as described with respect to FIG. 1 . The housing 1 10 illustrated has a top 1 1 1 , first side 1 12, second side 1 13 (collectively referred to as sides), an inner peripheral end 1 14, an outer peripheral end 1 15 and a bottom 1 18. Each of the top, bottom and sides has an inner face and an outer face defining a thickness. [0026] The housing has a partition 120 separating the inside of the housing into an upper plenum 130 and a lower plenum 140. The partition has a first side and a second side defining a thickness. The upper plenum 130 is defined by the top 1 1 1 of the housing 1 10 and partition 120. The lower plenum 140 is defined by the partition 120 and the bottom 1 16. The skilled artisan will recognize that the plenums are bounded by more than a top and bottom boundary. In the wedge-shaped embodiment illustrated, the sides 1 12, 1 13 and ends 114, 1 15 form lateral boundaries of the plenums 130, 140. In an embodiment with a circular shape (not shown), the wail is continuous, forming the lateral boundaries of the plenums.

[0027] An inlet 150 is in fluid communication with one of the upper plenum 130 or the lower plenum 140. An outlet 160 is in fluid communication with the other of the upper plenum 130 or lower plenum 140 than the inlet 150. In the illustrated embodiment the inlet 150 is in fluid communication with the upper plenum 130 and the outlet is in fluid communication with the lower plenum 140. In some embodiments, the inlet is in fluid communication with the lower plenum and the outlet is in fluid communication with the upper plenum.

[0028] The inlet 150 is configured to be connected to a gas source to provide a flow of gas into the plenum. The outlet 160 is configured to be connected to a vacuum source to provide a vacuum stream to draw gases out of the plenum connected to the outlet. In some embodiments, the gas distribution module 100 includes one or more of a gas source (not shown) or vacuum (not shown).

[0029] The upper plenum 130 includes a plurality of upper passages 131 extending from the upper plenum 130, through the lower plenum 140 and extending through the bottom 1 16 of the housing to form upper openings 135 in the bottom 1 16. The upper passages 131 are separated from the lower plenum 140 by an upper passage wall 132. The upper passages 131 provide fluid communication between the upper plenum 130 and the outer face 1 17 of the bottom 1 16 of the housing 1 10. The outer face 1 17 is also referred to as the portion of the bottom 1 16 opposite the lower plenum 140.

[0030] The lower plenum 140 includes a plurality of lower passages 141 extending through the bottom 1 16 of the housing 1 10 to form lower openings 145 in the bottom 1 16. The lower passages 141 provide fluid communication between the lower plenum 140 and the outer face 1 17 of the bottom 1 16 of the housing 1 10.

[0031] in use, a gas source (gas cylinder) can be connected to the inlet 150 and a vacuum source (e.g., vacuum pump or foreline) connected to the outlet 160. The gas source provides a flow of gas into the upper plenum 130 (in the illustrated embodiment). The gas pressure in the upper plenum 130 reaches a steady state. The gas in the upper plenum 130 flows through the upper passages 131 out the outer face 1 17 of the housing 1 10 into a process region 102 located on the opposite side of the bottom 1 16 than the lower plenum 140.

[0032] The arrangement of openings 135, 145 in the bottom 1 16 of the module housing 1 10 can have an effect on the overall efficiency of the gas flows in some embodiments, each of the upper openings 135 is surrounded by three lower openings 145 and each of the lower openings 145 is surrounded by three upper openings 135. This can be seen in the embodiment illustrated in FIG. 2, which shows a view of the bottom 1 16 of a gas distribution module 100 with a round housing 1 10. As used in this manner, the term“surrounded by” means that the closest openings to the subject opening are the stated opening. For example, an upper opening 135 is labeled in FIG. 2 and shows that the three closes openings are lower openings 145. The grid lines shown in FIG 2 are for illustrative purposes only to show the arrangement of openings 135, 145.

[0033] in some embodiments, each of the upper openings 135 is surrounded by four, five or six lower openings 145 and each of the lower openings 145 are surrounded by the same number of upper openings 135 For example, FIG. 3 illustrates an embodiment in which the upper opening 135 is surrounded by four lower openings 145 and each lower opening 145 is surrounded by four upper openings 135. in some embodiments, there are an equal number of lower openings 145 as upper openings 135. In some embodiments, there are a different number of lower openings 145 than upper openings 135.

[0034] FIG. 4 illustrates another embodiment of the disclosure in which the upper openings 135 are arranged in circular zones alternating with circular zones of lower openings 145. In some embodiments, each upper opening 135 has a lower opening 145 closer than another upper opening 135. The illustrated embodiment has four zones: innermost zone 170a has one upper opening 135; zone 170b is around innermost zone 170a and has four lower openings 145; zone 170c is around zone 170b and has eight upper openings 135; and zone 170d is around zone 170c and has eight lower openings 145. Although four zones are shown, the skilled artisan will recognize that this is merely representative of one possible arrange and the size, number and spacing the zones can be varied and the number of openings in each zone can be varied.

[0035] In some embodiments, one of the lower openings 145 or upper openings 135 bound the other of the lower openings 145 or upper opening 135. For example, FIG. 5 illustrates an embodiment of a gas distribution module 100 in which each upper openings 135 is bounded by a ring-shaped lower opening 145. Stated differently, in some embodiments, the openings in fluid communication with the inlet 150 (through the intervening plenum and passage) are bounded by the openings in fluid communication with the outlet 180 (through the intervening plenum and passage). Region VI in FIG. 5 is shown expanded in FIG. 8. The lower opening 145 is shown as a ring-shaped opening with two bridges 1 19. The bridges 1 19 may be included to connect the portion of the bottom 1 16 between the circular upper opening 135 and the ring shaped lower opening 145 with the remaining portion of the housing 1 10. The embodiment illustrated has two bridges 1 19; however it will be recognized that there can be more or less bridges 1 19.

[0036] in some embodiments, the openings in fluid communication with the inlet 150 have a diffusion plate 180 at the bottom 1 16 of the housing 1 10. FIG. 7 illustrates a cross-sectional view of an embodiment in which a diffusion plate 180 with apertures 185 are positioned at the bottom 1 16 of the housing 1 10 aligned with the upper openings 135. in some embodiments, the diffusion plate 180 has a plurality of apertures to separate the flow of gas through the opening 135 into multiple flows. The number of apertures can vary. In some embodiments, there are one or more apertures 185 in the diffusion plate 180 for each of the upper openings 135 and/or each of the lower openings 145. in some embodiments, there are in the range of about 1 to about 15 apertures 185 in the diffusion plate 180. in some embodiments, at least one of the apertures 185 is oriented at an angle to at least one aperture 185 to provide non-parallel flows of gases through the apertures. The diffusion plate 180 shown in FIG. 7 has three apertures 185 with the center aperture directing a flow norma! to the diffusion plate 180 with the opening 135 and the outer apertures 185 direct flows outward, relative to the center aperture, so that the flow are non-paral!ei. The angle of the apertures can be any suitable angle in some embodiments, the angle is in the range of about Q Q to about 8Q Q relative to the diffusion plate 180 normal.

[0037] FIG. 9 shows another embodiment of the disclosure in which the module 100 includes a second inlet 151 in fluid communication with one of the upper plenum 130 {as shown) or the lower plenum 140 (not shown). The second inlet 151 can provide a flow of the same gas as the inlet 150 or a different gas. For example, in a GVD type process, the inlet 150 and second inlet 151 may provide different reactive species which can react in the upper plenum 130, flow through the upper openings 135 into the reaction space of the process region 102. In some embodiments, the second inlet 151 provides a flow of diluent gas into the plenum. In some embodiments, as shown in FIG. 9, the second inlet 151 flows into upper plenum 130 which is split by divider 190 into a first plenum 130a and a second plenum 130b. The outlet 160 and lower openings are not illustrated in this Figure for clarity purposes only.

[0038] Some embodiments of the disclosure provide gas distribution modules 100 that can be used in plasma enhanced processes. In FIG. 10A the partition 120 is connected to an RF feed line 210 (e.g., a coaxial feed line) and the bottom 1 16 of the housing 1 10 is electrically grounded (or held at a different potential than the partition). The sides and top of the housing are made of a suitable dielectric material in this embodiment, energizing the partition 120 can result in formation of a plasma 220 in the lower plenum 140. A suitable height for plenum 140 is chosen to generate plasma in lower plenum 140 while a different height is chosen for upper plenum 130 not to generate plasma in upper plenum. The plasma species can flow through the lower openings 145 into the process region 102. Embodiments of FIG. 10A are also referred to as remote plasma sources (RPS) because the plasma is generated remotely from the processing region 102. [0039] In another embodiment, as shown in FIG. 10B, the bottom 1 16 of the housing 1 10 is connected to an RF feed line 210 and the partition 120, top 1 1 1 and ends 1 14, 1 15 are non-conductive. In this embodiment, the substrate 230 or pedestal 235 can act as the ground electrode (or be held at a different potential than the bottom 1 16) so that a plasma 220 is generated in the processing region 102. in some embodiments, the heights of upper plenum 130 and lower plenum 140 are suitably chosen not to generate plasma therein. Stated differently, the plasma 220 is generated on an opposite side of the bottom 1 16 than the lower plenum 140. Embodiments of the type shown in FIG. 10B in which the substrate or pedestal act as an electrode for plasma generation are also referred to as a direct plasma.

[0040] In another embodiment, as shown in FIG. 10C, the top 1 1 1 of the housing 1 10 is connected to an RF feed line 210 and the partition 120 and ends 1 14, 1 15 are non-conductive. in this embodiment, the partition 120 can act as the ground electrode (or can be held at a different potential than the top 1 1 1 ) so that a plasma 220 is generated in the upper plenum 130. A suitable height for plenum 130 is chosen to generate plasma in upper plenum 130 while a different height can be chosen for lower plenum 140 to prevent plasma ignition in the lower plenum 140. The plasma species can flow from the upper plenum 130 into the process region 102.

[0041] The gas distribution module 100 can be used to form one or more layers during a plasma enhanced atomic layer deposition (PEALD) or plasma enhanced chemical vapor deposition (PECVD) process in some processes, the use of plasma provides sufficient energy to promote a species into the excited state where surface reactions become favorable and likely. Introducing the plasma into the process can be continuous or pulsed. The plasma may be generated via any suitable plasma generation process or technique known to those skilled in the art. For example, plasma may be generated by one or more of a microwave (MW) frequency generator or a radio frequency (RF) generator. The frequency of the plasma may be tuned depending on the specific reactive species being used. Suitable frequencies include, but are not limited to, 2 MHz, 13.56 MHz, 40 MHz, 60 MHz, 100 MHz, 121 MHz and 162 MHz. Although plasmas may be used during the deposition processes disclosed herein, if should be noted that plasmas may not be required. [0042] In some embodiments, the gas distribution module 100 is a wedge-shaped component that is combined with other wedge-shaped modules to form a circular or disc-shaped gas distribution assembly. FIG. 11 shows a gas distribution assembly 270 comprising at least one gas distribution module 100 (two are shown) and a plurality of injector units 240 combined to make a circular assembly. A substrate can be rotated along a circular path beneath each of the gas distribution modules 100 and injector units 240 to be exposed to a plurality of different processing regions to deposit or remove a film from the substrate. Some embodiments of the disclosure are directed to processing chambers comprising the gas distribution module 100.

[0043] Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the method and apparatus of the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include modifications and variations that are within the scope of the appended claims and their equivalents.