U.S. patent application number 12/831522 was filed with the patent office on 2011-10-20 for showerhead assembly with metrology port purge.
This patent application is currently assigned to APPLIED MATERIALS, INC.. Invention is credited to Sumedh Acharya, Anzhong Chang, Alexander Tam.
Application Number | 20110253044 12/831522 |
Document ID | / |
Family ID | 44787169 |
Filed Date | 2011-10-20 |
United States Patent
Application |
20110253044 |
Kind Code |
A1 |
Tam; Alexander ; et
al. |
October 20, 2011 |
SHOWERHEAD ASSEMBLY WITH METROLOGY PORT PURGE
Abstract
A method and apparatus that may be utilized for chemical vapor
deposition and/or hydride vapor phase epitaxial (HVPE) deposition
are provided. In one embodiment, the apparatus is a processing
chamber that includes a showerhead with separate inlets and
channels for delivering separate processing gases into a processing
volume of the chamber without mixing the gases prior to entering
the processing volume. In one embodiment, the showerhead includes
metrology ports with purge gas assemblies configured and positioned
to deliver a purge gas to prevent deposition thereon. In one
embodiment, the metrology port is configured to receive a
temperature measurement device, and the purge gas assembly is a
concentric tube configuration configured to prevent deposition on
components of the temperature measurement device. In one
embodiment, the metrology port has a sensor window and is
configured to receive an optical measurement device, and the purge
gas assembly and sensor window are configured to prevent deposition
on the sensor window.
Inventors: |
Tam; Alexander; (Union City,
CA) ; Chang; Anzhong; (San Jose, CA) ;
Acharya; Sumedh; (Pune, IN) |
Assignee: |
APPLIED MATERIALS, INC.
Santa Clara
CA
|
Family ID: |
44787169 |
Appl. No.: |
12/831522 |
Filed: |
July 7, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61324271 |
Apr 14, 2010 |
|
|
|
Current U.S.
Class: |
118/666 ;
118/712 |
Current CPC
Class: |
B05B 1/18 20130101; H01L
21/67115 20130101; C23C 16/45519 20130101; C23C 16/52 20130101;
C30B 25/14 20130101; C23C 16/45565 20130101 |
Class at
Publication: |
118/666 ;
118/712 |
International
Class: |
B05C 11/10 20060101
B05C011/10 |
Claims
1. A showerhead assembly, comprising: a showerhead having: a first
metrology port defining an interior region and extending through
the showerhead; and a second metrology port extending through the
showerhead; a first metrology assembly having an optical element
that is at least partially disposed within the interior region of
the first metrology port; a first purge gas assembly having a first
gas inlet coupled to a purge gas source and configured to direct a
purge gas through the interior region of the first metrology port
to prevent deposition of material on the optical element; a second
metrology assembly having a sensor window disposed adjacent the
second metrology port; and a second purge gas assembly having a gas
inlet coupled to the purge gas source and configured to direct the
purge gas toward the sensor window to prevent deposition of
material thereon.
2. The assembly of claim 1, wherein a sheath is concentrically
disposed about the optical element and within the interior
region.
3. The assembly of claim 2, wherein the sheath has an aperture in
fluid communication with the first gas inlet.
4. The assembly of claim 3, wherein the first purge gas assembly
has a second gas inlet fluidly coupled to the purge gas source and
configured to direct the purge gas through the interior region of
the first metrology port, wherein the first gas inlet is in fluid
communication with a first portion of the interior region between
the optical element and a first surface of the sheath, and wherein
the second gas inlet is in fluid communication sheath.
5. The assembly of claim 1, wherein the second purge gas assembly
further comprises a gas distribution device having an annular
channel in fluid communication with the gas inlet.
6. The assembly of claim 5, wherein the gas distribution device is
configured to direct the purge gas into a vortex adjacent the
sensor window.
7. The assembly of claim 6, wherein the gas distribution device has
a plurality of passages fluidly connecting the annular channel with
a central aperture formed through the gas distribution device.
8. The assembly of claim 1, wherein the showerhead has: a first gas
channel formed in the showerhead; and a second gas channel formed
in the showerhead and isolated from the first gas channel, wherein
the first and second metrology ports extend through the first gas
channel and the second gas channel.
9. The assembly of claim 8, wherein the showerhead has a
temperature control channel formed in the showerhead and isolated
from the first and second gas channels, wherein the first and
second metrology ports extend through the temperature control
channel.
10. The assembly of claim 1, wherein the first and second purge gas
assemblies are each coupled to a cleaning gas source.
11. The assembly of claim 1, wherein the sensor window has a
cross-section in the shape of a wedge.
12. The assembly of claim 1, wherein the sensor window is
positioned at an angle between about 1 degree and about 4 degrees
with respect to the gas inlet of the second purge gas assembly.
13. A showerhead assembly, comprising: a showerhead having a
metrology port defining an interior region and extending through
the showerhead; a metrology assembly having an optical element that
is at least partially disposed within the interior region of the
metrology port; and a purge gas assembly having a first gas inlet
coupled to a purge gas source and configured to direct a purge gas
toward the optical element to prevent deposition of material
thereon, wherein a sheath is concentrically disposed about the
optical element and within the interior region, and wherein the
sheath has an aperture in fluid communication with the first gas
inlet.
14. The assembly of claim 13, wherein the purge gas assembly has a
second gas inlet fluidly coupled to the purge gas source and
configured to direct the purge gas through the first metrology
port.
15. The assembly of claim 13, wherein the showerhead has: a first
gas channel formed in the showerhead; and a second gas channel
formed in the showerhead and isolated from the first gas channel,
wherein the metrology port extends through the first gas channel
and the second gas channel.
16. The assembly of claim 15, wherein the showerhead has a
temperature control channel formed through the showerhead and
isolated from the first and second gas channels, wherein the
metrology port extends through the temperature control channel.
17. A showerhead assembly, comprising: a showerhead having a
metrology port extending through the showerhead; a metrology
assembly having a sensor window disposed adjacent the metrology
port; and a purge gas assembly having a gas inlet coupled to a
purge gas source and a gas distribution device having an annular
channel in fluid communication with the gas inlet, wherein the gas
distribution device is configured to direct the purge gas into a
vortex adjacent the sensor window.
18. The assembly of claim 17, wherein the gas distribution device
has a plurality of passages fluidly connecting the annular channel
with a central aperture formed through the gas distribution
device.
19. The assembly of claim 17, wherein the showerhead has: a first
gas channel formed in the showerhead; and a second gas channel
formed in the showerhead and isolated from the first gas channel,
wherein the metrology port extends through the first and second gas
channels.
20. The assembly of claim 19 wherein the showerhead has a
temperature control channel formed in the showerhead and isolated
from the first and second gas channels, wherein the metrology port
extends through the first and second gas channels.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of U.S. Provisional Patent
Application Ser. No. 61/324,271 (APPM/015324L), filed Apr. 14,
2010, which is herein incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] Embodiments of the present invention generally relate to
methods and apparatus for chemical vapor deposition (CVD) on a
substrate, and, in particular, to a showerhead design for use in
metal organic chemical vapor deposition (MOCVD) and/or hydride
vapor phase epitaxy (HVPE).
[0004] 2. Description of the Related Art
[0005] Group III-V films are finding greater importance in the
development and fabrication of a variety of semiconductor devices,
such as short wavelength light emitting diodes (LEDs), laser diodes
(LDs), and electronic devices including high power, high frequency,
high temperature transistors and integrated circuits. For example,
short wavelength (e.g., blue/green to ultraviolet) LEDs are
fabricated using the Group III-nitride semiconducting material
gallium nitride (GaN). It has been observed that short wavelength
LEDs fabricated using GaN can provide significantly greater
efficiencies and longer operating lifetimes than short wavelength
LEDs fabricated using non-nitride semiconducting materials, such as
Group II-VI materials.
[0006] One method that has been used for depositing Group
III-nitrides, such as GaN, is metal organic chemical vapor
deposition (MOCVD). This chemical vapor deposition method is
generally performed in a reactor having a temperature controlled
environment to assure the stability of a first precursor gas which
contains at least one element from Group III, such as gallium (Ga).
A second precursor gas, such as ammonia (NH.sub.3), provides the
nitrogen needed to form a Group III-nitride. The two precursor
gases are injected into a processing zone within the reactor where
they mix and move towards a heated substrate in the processing
zone. A carrier gas may be used to assist in the transport of the
precursor gases towards the substrate. The precursors react at the
surface of the heated substrate to form a Group III-nitride layer,
such as GaN, on the substrate surface. The quality of the film
depends in part upon deposition uniformity which, in turn, depends
upon uniform mixing of the precursors across the substrate.
[0007] Multiple substrates may be arranged on a substrate carrier
and each substrate may have a diameter ranging from 50 mm to 100 mm
or larger. The uniform mixing of precursors over larger substrates
and/or more substrates and larger deposition areas is desirable in
order to increase yield and throughput. These factors are important
since they directly affect the cost to produce an electronic device
and, thus, a device manufacturer's competitiveness in the
marketplace.
[0008] Interaction of the precursor gases with the hot hardware
components, which are often found in the processing zone of an LED
or LD forming reactor, generally causes the precursor to break-down
and deposit on these hot surfaces. Typically, the hot reactor
surfaces are formed by radiation from the heat sources used to heat
the substrates. The deposition of the precursor materials on the
hot surfaces can be especially problematic when it occurs in or on
the precursor distribution components, such as the showerhead.
Deposition on the precursor distribution components affects the
flow distribution uniformity over time. Additionally, deposition on
metrology ports disposed in the precursor distribution components
affects the accurate measurement and control of conditions within
the processing zone of the reactor. Therefore, there is a need for
a gas distribution apparatus that prevents or reduces the
likelihood that the MOCVD precursors, or HVPE precursors, are
heated to a temperature that causes them to break down and affect
the performance of the gas distribution and metrology
components.
[0009] Also, as the demand for LEDs, LDs, transistors, and
integrated circuits increases, the efficiency of depositing high
quality Group-III nitride films takes on greater importance.
Therefore, there is a need for an improved deposition apparatus and
process that can provide consistent film quality over larger
substrates and larger deposition areas.
SUMMARY OF THE INVENTION
[0010] The present invention generally provides improved methods
and apparatus for depositing Group III-nitride films using MOCVD
and/or HVPE processes.
[0011] One embodiment of the present invention provides a
showerhead assembly comprising a showerhead having a first
metrology port defining an interior region and extending through
the showerhead and a second metrology port extending through the
showerhead. The showerhead assembly further comprises a first
metrology assembly having an optical element that is at least
partially disposed within the interior region of the first
metrology port, a first purge gas assembly having a first gas inlet
coupled to a purge gas source and configured to direct a purge gas
through the interior region of the first metrology port to prevent
deposition of material on the optical element, a second metrology
assembly having a sensor window disposed adjacent the second
metrology port, and a second purge gas assembly having a gas inlet
coupled to the purge gas source and configured to direct the purge
gas toward the sensor window to prevent deposition of material
thereon.
[0012] Another embodiment provides a showerhead assembly comprising
a showerhead having a metrology port defining an interior region
and extending through the showerhead, a metrology assembly having
an optical element that is at least partially disposed within the
interior region of the metrology port, and a purge gas assembly
having a first gas inlet coupled to a purge gas source and
configured to direct a purge gas toward the optical element to
prevent deposition of material thereon, wherein a sheath is
concentrically disposed about the optical element and within the
interior region, and wherein the sheath has an aperture in fluid
communication with the first gas inlet.
[0013] Yet another embodiment of the present invention provides a
showerhead assembly comprising a showerhead having a metrology port
extending through the showerhead, a metrology assembly having a
sensor window disposed adjacent the metrology port, and a purge gas
assembly having a gas inlet coupled to a purge gas source and a gas
distribution device having an annular channel in fluid
communication with the gas inlet, wherein the gas distribution
device is configured to direct the purge gas into a vortex adjacent
the sensor window.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] So that the manner in which the above recited features of
the present invention can be understood in detail, a more
particular description of the invention, briefly summarized above,
may be had by reference to embodiments, some of which are
illustrated in the appended drawings. It is to be noted, however,
that the appended drawings illustrate only typical embodiments of
this invention and are therefore not to be considered limiting of
its scope, for the invention may admit to other equally effective
embodiments.
[0015] FIG. 1 is a schematic plan view illustrating one embodiment
of a processing system for fabricating compound nitride
semiconductor devices according to embodiments described
herein.
[0016] FIG. 2 is a schematic cross-sectional view of a
metal-organic chemical vapor deposition (MOCVD) chamber for
fabricating compound nitride semiconductor devices according to one
embodiment of the present invention.
[0017] FIG. 3 is a schematic, cross-sectional view of a first
metrology assembly attached to the showerhead depicted in FIG. 2
according to one embodiment of the present invention.
[0018] FIG. 4A is a schematic, cross-sectional view of a second
metrology assembly attached to the showerhead depicted in FIG. 2
according to one embodiment of the present invention.
[0019] FIG. 4B is a top view of a gas distribution device depicted
in FIG. 4A according to one embodiment.
[0020] FIG. 5A is a schematic, cross-sectional view of the second
metrology assembly attached to the showerhead depicted in FIG. 2
according to another embodiment of the present invention.
[0021] FIG. 5B is a schematic, cross-sectional view of a sensor
window as it is positioned in the second metrology assembly in FIG.
5A according to one embodiment.
[0022] FIG. 5C is a schematic, cross-sectional view of the sensor
window as it is positioned in the second metrology assembly in FIG.
5A according to another embodiment.
[0023] FIG. 6A is a schematic bottom view of the showerhead in FIG.
2 according to one embodiment.
[0024] FIG. 6B is a schematic bottom view of the showerhead in FIG.
2 according to another embodiment.
[0025] FIG. 6C is an enlarged view of a portion of the surface of
the showerhead shown in FIG. 6A.
[0026] To facilitate understanding, identical reference numerals
have been used, where possible, to designate identical elements
that are common to the figures. It is contemplated that elements
and features of one embodiment may be beneficially incorporated in
other embodiments without further recitation.
DETAILED DESCRIPTION
[0027] Embodiments of the present invention generally provide a
method and apparatus that may be utilized for deposition of Group
III-nitride films using MOCVD and/or HVPE hardware. In one
embodiment, the apparatus is a processing chamber that includes a
showerhead with separate inlets and channels for delivering
separate processing gases into a processing volume of the chamber
without mixing the gases prior to entering the processing volume.
In one embodiment, the showerhead includes metrology ports with
purge gas assemblies configured and positioned to deliver a purge
gas to prevent deposition thereon. In one embodiment, the metrology
port is configured to receive a temperature measurement device, and
the purge gas assembly is a concentric tube configuration
configured to prevent deposition on components of the temperature
measurement device. In one embodiment, the metrology port has a
sensor window and is configured to receive an optical measurement
device that is configured to deliver and receive electromagnetic
energy through the sensor window to measure a property of one or
more substrates disposed in a processing region of the processing
chamber. In certain embodiments of the invention, the purge gas
assembly and sensor window are configured to prevent the
obstruction of the delivery or reception of the electromagnetic
energy, due to the deposition of material on the sensor window.
[0028] FIG. 1 is a schematic plan view illustrating one embodiment
of a processing system 100 that comprises the one or more MOCVD
chambers 102 for fabricating compound nitride semiconductor devices
according to embodiments described herein. In one embodiment, the
processing system 100 is closed to atmosphere. The processing
system 100 comprises a transfer chamber 106, a MOCVD chamber 102
coupled with the transfer chamber 106, a loadlock chamber 108
coupled with the transfer chamber 106, a batch loadlock chamber
109, for storing substrates, coupled with the transfer chamber 106,
and a load station 110, for loading substrates, coupled with the
loadlock chamber 108. The transfer chamber 106 comprises a robot
assembly (not shown) operable to pick up and transfer substrates
between the loadlock chamber 108, the batch loadlock chamber 109,
and the MOCVD chamber 102. Although a single MOCVD chamber 102 is
shown, it should be understood that more than one MOCVD chamber 102
or additionally, combinations of one or more MOCVD chambers 102
with one or more Hydride Vapor Phase Epitaxial (HVPE) chambers may
also be coupled with the transfer chamber 106. It should also be
understood that although a cluster tool is shown, the embodiments
described herein may be performed using linear track systems.
[0029] In one embodiment, the transfer chamber 106 remains under
vacuum during substrate transfer processes. The transfer chamber
vacuum level may be adjusted to match the vacuum level of the MOCVD
chamber 102. For example, when transferring substrates from a
transfer chamber 106 into the MOCVD chamber 102 (or vice versa),
the transfer chamber 106 and the MOCVD chamber 102 may be
maintained at the same vacuum level. Then, when transferring
substrates from the transfer chamber 106 to the load lock chamber
108 (or vice versa) or the batch load lock chamber 109 (or vice
versa), the transfer chamber vacuum level may be adjusted to match
the vacuum level of the loadlock chamber 108 or batch load lock
chamber 109 even through the vacuum level of the loadlock chamber
108 or batch load lock chamber 109 and the MOCVD chamber 102 may be
different. Thus, the vacuum level of the transfer chamber 106 is
adjustable. In certain embodiments, substrates are transferred in a
high purity inert gas environment, such as, a high purity N.sub.2
environment. In one embodiment, substrates transferred in an
environment having greater than 90% N.sub.2. In certain
embodiments, substrates are transferred in a high purity NH.sub.3
environment. In one embodiment, substrates are transferred in an
environment having greater than 90% NH.sub.3. In certain
embodiments, substrates are transferred in a high purity H.sub.2
environment. In one embodiment, substrates are transferred in an
environment having greater than 90% H.sub.2.
[0030] In the processing system 100, the robot assembly (not shown)
transfers a substrate carrier plate 112 loaded with substrates into
the single MOCVD chamber 102 to undergo deposition. In one
embodiment, the substrate carrier plate 112 may have a diameter
ranging from about 200 mm to about 750 mm. The substrate carrier
plate 112 may be formed from a variety of materials, including SiC
or SiC-coated graphite. In one embodiment, the substrate carrier
plate 112 comprises a silicon carbide material. In one embodiment,
the substrate carrier plate 112 has a surface area of about 1,000
cm.sup.2 or more, preferably 2,000 cm.sup.2 or more, and more
preferably 4,000 cm.sup.2 or more. After some or all deposition
steps have been completed, the substrate carrier plate 112 is
transferred from the MOCVD chamber 102 back to the loadlock chamber
108 via the transfer robot. In one embodiment, the substrate
carrier plate 112 is then transferred to the load station 110. In
another embodiment, the substrate carrier plate 112 may be stored
in either the loadlock chamber 108 or the batch load lock chamber
109 prior to further processing in the MOCVD chamber 102. One
exemplary processing system 100 that may be adapted in accordance
with embodiments of the present invention is described in U.S.
patent application Ser. No. 12/023,572, filed Jan. 31, 2008, now
published as US 2009-0194026, entitled PROCESSING SYSTEM FOR
FABRICATING COMPOUND NITRIDE SEMICONDUCTOR DEVICES, which is hereby
incorporated by reference in its entirety.
[0031] In one embodiment, a system controller 160 controls
activities and operating parameters of the processing system 100.
The system controller 160 includes a computer processor and a
computer-readable memory coupled to the processor. The processor
executes system control software, such as a computer program stored
in memory. Exemplary aspects of the processing system 100 and
methods of use adaptable to embodiments of the present invention
are further described in U.S. patent application Ser. No.
11/404,516, filed Apr. 14, 2006, now published as US 2007-024516,
entitled EPITAXIAL GROWTH OF COMPOUND NITRIDE STRUCTURES, which is
hereby incorporated by reference in its entirety.
[0032] FIG. 2 is a schematic cross-sectional view of the MOCVD
chamber 102 according to embodiments of the present invention. The
MOCVD chamber 102 comprises a chamber body 202, a chemical delivery
module 203 for delivering precursor gases, carrier gases, cleaning
gases, and/or purge gases, a remote plasma system 226 with a plasma
source, a susceptor or substrate support 214, and a vacuum system
212. The chamber body 202 encloses a processing volume 208. A
showerhead assembly 201 is disposed at one end of the processing
volume 208, and the substrate carrier plate 112 is disposed at the
other end of the processing volume 208. The substrate carrier plate
112 may be disposed on the substrate support 214. The substrate
support 214 has z-lift capability for moving in a vertical
direction, as shown by arrow 215. In one embodiment, the z-lift
capability may be used to move the substrate support 214 upwardly,
and closer to the showerhead assembly 201, and downwardly, and
further away from the showerhead assembly 201. In one embodiment,
the distance from the surface of the showerhead assembly 201 that
is adjacent the processing volume 208 to the substrate carrier
plate 112 during processing ranges from about 4 mm to about 41 mm.
In certain embodiments, the substrate support 214 comprises a
heating element (e.g., a resistive heating element (not shown)) for
controlling the temperature of the substrate support 214 and
consequently controlling the temperature of the substrate carrier
plate 112 and substrates 240 positioned on the substrate carrier
plate 112 and the substrate support 214.
[0033] In one embodiment, the showerhead assembly 201 includes a
showerhead 204. In one embodiment, the showerhead 204 is a single
plate having a plurality of channels and apertures formed therein.
In another embodiment, the showerhead 204 includes a plurality of
plates machined and attached such that a plurality of channels and
apertures are formed therein. In one embodiment, the showerhead 204
has a first processing gas channel 204A coupled with the chemical
delivery module 203 via a first processing gas inlet 259 for
delivering a first precursor or first process gas mixture to the
processing volume 208. In one embodiment, the chemical delivery
module 203 is configured to deliver a metal organic precursor to
the first processing gas channel 204A. In one example, the metal
organic precursor comprises a suitable gallium (Ga) precursor
(e.g., trimethyl gallium ("TMG"), triethyl gallium (TEG)), a
suitable aluminum precursor (e.g., trimethyl aluminum ("TMA")), or
a suitable indium precursor (e.g., trimethyl indium ("TMI")).
[0034] In one embodiment, a blocker plate 255 is positioned across
the first processing gas channel 204A. The blocker plate 255 has a
plurality of orifices 257 disposed therethrough. In one embodiment,
the blocker plate 255 is positioned between the first processing
gas inlet 259 and the first processing gas channel 204A for
uniformly distributing gas received from the chemical delivery
module 203 into the first processing gas channel 204A.
[0035] In one embodiment, the showerhead 204 has a second
processing gas channel 204B coupled with the chemical delivery
module 203 for delivering a second precursor or second process gas
mixture to the processing volume 208 via a second processing gas
inlet 258. In one embodiment, the chemical delivery module 203 is
configured to deliver a suitable nitrogen containing processing
gas, such as ammonia (NH.sub.3) or other MOCVD or HVPE processing
gas, to the second processing gas channel 204B. In one embodiment,
the second processing gas channel 204B is separated from the first
processing gas channel 204A by a first horizontal wall 276 of the
showerhead 204.
[0036] The showerhead 204 may further include a temperature control
channel 204C coupled with a heat exchanging system 270 for flowing
a heat exchanging fluid through the showerhead 204 to help regulate
the temperature of the showerhead 204. Suitable heat exchanging
fluids include, but are not limited to, water, water-based ethylene
glycol mixtures, a perfluoropolyether (e.g., Galden.RTM. fluid),
oil-based thermal transfer fluids, or similar fluids. In one
embodiment, the second processing gas channel 204B is separated
from the temperature control channel 204C by a second horizontal
wall 277 of the showerhead 204. The temperature control channel
204C may be separated from the processing volume 208 by a third
horizontal wall 278 of the showerhead 204.
[0037] In one embodiment, the first precursor or first processing
gas mixture, such as a metal organic precursor, is delivered from
the first processing gas channel 204A through the second processing
gas channel 204B and the temperature control channel 204C into the
processing volume 208 via a plurality of inner gas conduits 246.
The inner gas conduits 246 may be cylindrical tubes located within
aligned holes disposed through the first horizontal wall 276, the
second horizontal wall 277, and the third horizontal wall 278 of
the showerhead 204. In one embodiment, the inner gas conduits 246
are each attached to the first horizontal wall 276 of the
showerhead 204 by suitable means, such as brazing.
[0038] In one embodiment, the second precursor or second processing
gas mixture, such as a nitrogen precursor, is delivered from the
second processing gas channel 204B through the temperature control
channel 204C and into the processing volume 208 via a plurality of
outer gas conduits 245. The outer gas conduits 245 may be
cylindrical tubes each located concentrically about a respective
inner gas conduit 246. The outer gas conduits 245 are located
within the aligned holes disposed through the second horizontal
wall 277 and the third horizontal wall 278 of the showerhead 204.
In one embodiment, the outer gas conduits 245 are each attached to
the second horizontal wall 277 of the showerhead 204 by suitable
means, such as brazing.
[0039] In certain embodiments of the present invention, a purge gas
(e.g., nitrogen, hydrogen, argon) is delivered into the chamber 102
from the showerhead 204 through one or more purge gas channels 281
coupled to a purge gas source 282. In this embodiment, the purge
gas is distributed through a plurality of orifices 284 about the
periphery of the showerhead 204. The plurality of orifices 284 may
be configured in a circular pattern about the periphery of the
showerhead 204 and positioned distribute the purge gas about the
periphery of the substrate carrier plate 112 to prevent undesirable
deposition on edges of the substrate carrier plate 112, the
showerhead 204, and other components of the chamber 102, which
result in particle formation and, ultimately contamination of the
substrates 240. The purge gas flows downwardly into multiple
exhaust ports 209, which are disposed around an annular exhaust
channel 205. An exhaust conduit 206 connects the annular exhaust
channel 205 to a vacuum system 212, which includes a vacuum pump
207. The pressure of the chamber 102 may be controlled using a
valve system, which controls the rate at which the exhaust gases
are drawn from the annular exhaust channel 205.
[0040] In other embodiments, purge gas tubes 283 are disposed near
the bottom of the chamber 102. In this configuration, the purge gas
enters the lower volume 210 of the chamber 102 and flows upwardly
past the substrate carrier plate 112 and exhaust ring 220 and into
the multiple exhaust ports 209.
[0041] In one embodiment, the showerhead assembly 201 comprises a
first metrology assembly 291 attached to a first metrology port 296
and a second metrology assembly 292 attached to a second metrology
port 297. The first and second metrology ports 296, 297, each
include a tube 298 that is positioned in an aperture formed through
the showerhead 204 and attached to the showerhead 204, such as by
brazing, such that each of the channels (204A, 204B, and 204C) are
separated and sealed from one another. The first and second
metrology assemblies 291, 292 are used to monitor the processes
performed on the surface of the substrates 240 disposed in the
processing volume 208 of the chamber 102. In one embodiment, the
first metrology assembly 291 includes a temperature measurement
device, such as an optical pyrometer.
[0042] In one embodiment, the second metrology assembly 292
includes an optical measurement device, such as an optical stress,
or substrate bow, measurement device. Generally, the optical
measurement device (not shown) includes an optical emitter, such as
a light source, for emitting one or more beams of light through a
sensor window disposed in the second metrology port 297 as
subsequently described with respect to FIGS. 4 and 5. The beams of
light are generally focused through the sensor window onto a
substrate 240 disposed in the processing volume 208 of the chamber
102. The beams of light strike the substrate 240 and are reflected
back through the sensor window and received by an optical detector
within the optical measurement device. The received beams of light
are then compared with the emitted beams of light to determine a
property of the substrate 240, such as the amount of bow of the
substrate 240 (i.e., amount of convex or concave curvature of the
upper surface of the substrate 240).
[0043] In one embodiment, the first metrology assembly 291 and the
second metrology assembly 292 include a first purge gas assembly
291A and a second purge gas assembly 292A, respectively, that are
adapted to deliver and position a purge gas from the purge gas
source 282 to the metrology assemblies 291, 292 so as to prevent
deposition of material on the surface of components within the
assemblies. In one embodiment, the first purge gas assembly 291A
and second purge gas assembly 292A are further connected to a
cleaning gas source (e.g., the chemical delivery module 203) and
are adapted to deliver and position a cleaning gas into the
metrology assemblies 291, 292 to remove any deposited material from
components in the metrology assemblies 291, 292 during a cleaning
process. In one embodiment, the cleaning gas may include gases such
as fluorine (F.sub.2) gas, chlorine (Cl.sub.2) gas, bromine
(Br.sub.2) gas, or iodine (I.sub.2) gas. In another embodiment, the
cleaning gas may include a gas comprising hydrogen iodide (HI),
hydrogen chloride (HCl), hydrogen bromide (HBr), hydrogen fluoride
(HF), nitrogen trifluoride (NF.sub.3), and/or other similar gases.
In one embodiment, diatomic chlorine (Cl.sub.2) gas is used as the
cleaning gas. In another embodiment, diatomic fluorine (F.sub.2)
gas is used as the cleaning gas. In one embodiment, the showerhead
204 has a plurality of first metrology ports 296 and/or a plurality
of second metrology ports 297, and the showerhead assembly 201 has
a respective plurality of first and/or second metrology assemblies
291, 292 and first and/or second purge gas assemblies 291A, 292A
attached thereto, respectively.
[0044] FIG. 3 is a schematic, cross-sectional view of the first
metrology assembly 291 attached to the showerhead 204 at the first
metrology port 296 according to one embodiment of the present
invention. In one embodiment, the first metrology assembly 291
includes a pyrometer assembly 320 attached to an optical element
301 that extends into the tube 298 of the first metrology port 296.
The optical element 301 may be a sapphire rod, a sapphire rod
coated with reflecting material, or a fiber optic cable with a core
and integrated cladding. The optical element 301 and pyrometer
assembly 320 are used to gather valuable processing temperature
data during deposition processes in the chamber 102. The
temperature data may be analyzed and stored for diagnostic purposes
and/or used in feedback temperature control during processing.
However, during processing, precursor gases may migrate from the
processing volume 208 into the first metrology port 296 and into
contact with the transmission surface 301A of the optical element
301 and components of the pyrometer assembly 320 and deposit
material thereon. The transmission surface 301A is generally a
surface through which the IR radiation transmitted from a body
(e.g., substrate), which is disposed in the processing volume 208
of the processing chamber 102, is received before the IR radiation
is transmitted through the optical element 301 to an optical sensor
disposed in the pyrometer assembly 320. As a result of the
undesirable deposition, the temperature data gathered by the
pyrometer assembly 320 is unreliable, which results in unreliable
diagnostic and control data. Thus, it is desirable to prevent
and/or remove such unwanted deposition.
[0045] In one embodiment, in order to prevent and/or remove
unwanted deposition of material on the optical element 301 and
components of the pyrometer assembly 320, the first purge gas
assembly 291A is positioned between the first metrology assembly
291 and the showerhead 204. The purge gas assembly 291A may be
attached to both the showerhead 204 and the pyrometer assembly 320
via suitable fasteners (not shown). The purge gas assembly 291A may
include a sheath 305 positioned concentrically about the optical
element 301. In one embodiment, the sheath 305 is a tube made of a
refractory and/or optically absorbing material, such as silicon
carbide, silicon carbide coated graphite, silicon nitride, or
aluminum nitride. In one embodiment, the sheath 305 is attached to
a coupler 315 and extends into the tube 298 of the first metrology
port 296.
[0046] In one embodiment, the coupler 315 includes a first inlet
316 that fluidly couples the purge gas source 282 with an aperture
317 disposed through a wall of the sheath 305. In one embodiment,
the first inlet 316 is also coupled to a cleaning gas source. Thus,
the purge gas (during deposition operations) or the cleaning gas
(during cleaning operations) flows through the first inlet 316, the
aperture 317, and into an interior region 315A of the sheath 305
surrounding the optical element 301. The gas then flows through the
interior region 315A of the sheath 305, about the optical element
301, and through the showerhead 204 into the processing volume 208
of the chamber 102. During deposition operations, the concentric
flow of purge gas across the optical element 301 prevents the
precursor gases located in the processing volume 208 from migrating
into the sheath 305 and depositing material on the transmission
surface 301A of the optical element 301. During cleaning
operations, the concentric flow of cleaning gas across the optical
element 301 acts to remove deposited material from the transmission
surface 301A of the optical element as well as the interior surface
of the sheath 305.
[0047] In one embodiment, the coupler 315 includes a second inlet
318 that fluidly couples the purge gas source 282 with an outer
interior region 298A of the tube 298 of the first metrology port
296 surrounding the sheath 305. In one embodiment, the second inlet
318 is also coupled to a cleaning gas source. Thus, the purge gas
(during deposition operations) or the cleaning gas (during cleaning
operations) flows through the second inlet 318 and through the
outer interior region 298A of the tube 298 into the processing
volume 208 of the chamber 102. During deposition operations, the
additional concentric flow of purge gas about the sheath 305 and
across the transmission surface 301A of the optical element 301
further prevents undesirable deposition of material on the
transmission surface 301A of the optical element 301, resulting in
the gathering of more reliable temperature information by the
pyrometer assembly 320, by adding an additional "curtain" of gas
that isolates the transmission surface 301A of the optical element
301. The additional gas flow is believed to help surround and
support the gas flow delivered through the interior of the sheath
305 to promote the isolation of the transmission surface 301A of
the optical element 301. During cleaning operations, the flow of
cleaning gas acts to remove any deposited particles formed on the
surfaces of the tube 298, the sheath 05, and the transmission
surface 301A of the optical element 301.
[0048] FIG. 4A is a schematic, cross-sectional view of the second
metrology assembly 292 attached to the second metrology port 297 of
the showerhead 204 according to one embodiment of the present
invention. In one embodiment, the second metrology assembly 292
includes an optical sensor assembly 420, such as an optical stress
or deflection measurement device, attached to a coupler 412, and a
sensor window 410 above the second metrology port 297. The optical
sensor assembly 420 may be used to gather valuable metrology data
during the processing of the substrates 240 in the chamber 102.
However, during processing, processing gases from the processing
volume 208 tend to migrate into the tube 298 of the second
metrology port 297 adjacent the sensor window 410, which results in
undesirable deposition of material on the sensor window 410. Such
undesirable deposition prevents reliable metrology data
gathering.
[0049] In one embodiment, to prevent and/or remove the undesirable
deposition of material on the sensor window 410, the coupler 412
couples the optical sensor assembly 420 to the sensor window 410
via the second purge gas assembly 292A. The second purge gas
assembly 292A may include a gas coupling 405 attached to the second
metrology port 297 of the showerhead 204 and the coupler 412 via
suitable fasteners (not shown). In one embodiment, the second purge
gas assembly 292A further includes a gas distribution device 415
having a central aperture formed therethrough and positioned
between the sensor window 410 and the second metrology port 297 of
the showerhead 204 via the gas coupling 405. The gas coupling 405
may include a gas inlet 418 coupling the purge gas source 282 to
the gas distribution device 415. In one embodiment, the gas inlet
418 is further coupled to a cleaning gas source.
[0050] FIG. 4B is a top view of the gas distribution device 415
depicted in FIG. 4A. In one embodiment, the gas distribution device
415 includes a flow control orifice 416, which fluidly couples an
annular gas channel 417 in the gas distribution device 415 with the
gas inlet 418. The annular gas channel 417 is fluidly coupled with
its central aperture and the interior of the tube 298 of the second
metrology port 297 through a plurality of distribution channels 419
formed in a center ridge 419A of the gas distribution device 415.
In one embodiment, the plurality of distribution channels 419 are
positioned at an angle relative to the center of the sensor window
410 to create a vortex of gas flowing through the gas distribution
device 415 near a lower surface 411 of the sensor window 410. Thus,
a purge gas (during deposition operations) or a cleaning gas
(during cleaning operations) flows, through the inlet 418 in the
gas coupling 405, through the flow control orifice 416, and into
the annular gas channel 417 in the gas distribution device 415. The
gas then flows from the annular gas channel 417, through the
plurality of distribution channels 419 toward the lower surface 411
of the sensor window 410. As previously described, the plurality of
distribution channels 419 are angled such that the gas flowing
therethrough creates a vortex near the lower surface 411 of the
sensor window 410. In the case of a purge gas flowing during
deposition operations, the gas vortex prevents processing gases
from the processing volume 208 of the chamber 102 from depositing
material on the lower surface 411 of the sensor window 410. In the
case of a cleaning gas flowing during cleaning operations, the gas
vortex acts to remove any deposited material from the lower surface
411 of the sensor window 410. In either case, the vortex of gas is
then pushed through the tube 298 of the second metrology port 297
and into the processing volume 208 by the incoming gas delivered
through the distribution channels 419. As a result, the purge gas
assembly 292A directs a purge gas to prevent undesirable deposition
on the sensor window 410 of the second metrology assembly 292,
resulting in more reliable metrology data gathering during
deposition processes. Further, during cleaning operations, the
purge gas assembly 292A directs a cleaning gas to remove any
deposited material from the sensor window 410 of the second
metrology assembly 292, resulting in more reliable metrology data
gathering during deposition processes.
[0051] FIG. 5A is a schematic, cross-sectional view of the second
metrology assembly 292 attached to the second metrology port 297 of
the showerhead 204 according to another embodiment of the present
invention. In one embodiment, the second metrology assembly 292
includes an optical sensor assembly 520, such as an optical stress
or deflection measurement device, attached to a coupler 512, and a
sensor window 510. The optical sensor assembly 520 may be used to
gather valuable metrology data during the processing of the
substrates 240 in the chamber 102. As previously described, during
processing, processing gases from the processing volume 208 tend to
migrate into the tube 298 of the second metrology port 297 into a
region surrounding the sensor window 510, which results in
undesirable deposition of material on the sensor window 510. Such
undesirable deposition prevents reliable metrology data
gathering.
[0052] In one embodiment, to prevent and/or remove undesirable
deposition of material on the sensor window 510, the coupler 512
couples the optical sensor assembly 520 to the sensor window 510
via the second purge gas assembly 292A. The second purge gas
assembly 292A may include a gas coupling 505 attached to the
showerhead 204 and the coupler 512 via suitable fasteners (not
shown). The gas coupling 505 may include a gas inlet 518 disposed
through the gas coupling 505, which couples the purge gas source
282 to a central region near a lower surface 511 of the sensor
window 510 and into the interior of the tube 298 of the second
metrology port 297. In one embodiment, the gas inlet 518 is further
coupled to a cleaning gas source. In one embodiment, the purge gas
(during deposition processes) or the cleaning gas (during cleaning
processes) flows through the gas inlet 518, onto the lower surface
511 of the sensor window 510, through the tube 298, and into the
processing volume 208 of the chamber 102. The resulting flow of
purge gas helps prevent processing gases from the processing volume
208 of the chamber 102 from depositing material on the surface of
the sensor window 510 during deposition processes. During cleaning
operations, the flow of cleaning gas acts to remove any deposited
material from the surface of the sensor window 510.
[0053] In one embodiment, in order to increase the effectiveness of
the purge gas assembly 292A described with respect to FIG. 5A, the
sensor window 510 may be configured such that the gas flowing
through the gas inlet 518 contacts the surface of the sensor window
510 at a desirable angle. FIG. 5B is a schematic, cross-sectional
view of the sensor window 510 as it is positioned in the second
metrology assembly 292 in FIG. 5A according to one embodiment. In
the embodiment, depicted in FIG. 5B, the sensor window 510 is in
the shape of a wedge, such that the lower surface 511 of the sensor
window 510 is situated at an angle A with respect to the gas inlet
518, and the upper surface 513 of the sensor window 510 is situated
at an angle B with respect to the gas inlet 518. In one embodiment,
the angles A and B are between about 1.degree. and about 3.degree..
In one embodiment, the angles A and B are about 2.degree.. FIG. 5C
is a schematic, cross-sectional view of the sensor window 510 as it
is positioned in the second metrology assembly 292 in FIG. 5A
according to another embodiment. In the embodiment, depicted in
FIG. 5C, the sensor window 510 has substantially parallel upper and
lower surfaces 513, 511, but the sensor window 510 is positioned in
the second metrology assembly 292 such that the lower surface 511
is at an angle C with respect to the gas inlet 518. In one
embodiment, the angle C is between about 1.degree. and about
4.degree.. In one embodiment, the angle C is about 2.5.degree..
[0054] In the embodiments of FIGS. 5B and 5C, the sensor window 510
is positioned such that the lower surface 511 is at a desired angle
with respect to the gas inlet 518. In such a configuration, the gas
flowing through the gas inlet 518 in the gas coupler 505 contacts
the lower surface 511 of the sensor window 510 at a desirable angle
to create a desirable distribution and flow of gas across the lower
surface 511 of the sensor window 510. The gas then flows through
the tube 298 of the second metrology port 297 and into the
processing volume 208 of the chamber 102. As a result, during
deposition processes, the purge gas assembly 292A prevents
undesirable deposition on the sensor window 510 of the second
metrology assembly 292, resulting in more reliable metrology data
gathering during deposition processes. Further, during cleaning
operations, the purge gas assembly 292A directs a cleaning gas to
remove any deposited material from the sensor window 510 of the
second metrology assembly 292, resulting in more reliable metrology
data gathering during deposition processes.
[0055] FIG. 6A is a bottom view of the showerhead assembly 201
shown in FIG. 2 according to one embodiment of the invention. In
one embodiment, the showerhead assembly 201 includes a plurality of
first metrology ports 296 arranged in a radial line from the center
of the showerhead 204 to the perimeter of the showerhead 204. In
such an embodiment, the first metrology ports 296 are arranged so
that the first metrology assemblies 291 can detect the temperature
distribution from the center to the perimeter of the processing
volume 208 of the processing chamber 102. In one embodiment, the
substrates 240 are arranged in a circular pattern about the center
point of the carrier plate 112 (FIG. 2), and the carrier plate 112
is rotated during processing. In such an embodiment, the showerhead
assembly 201 may include a plurality of second metrology ports 297
positioned concentrically about the center of the showerhead 204 at
a position such that they are centered over a central portion of
the substrates 240 disposed on the carrier plate 112 as it is
rotated during processing.
[0056] FIG. 6B is a bottom view of the showerhead assembly 201
shown in FIG. 2 according to another embodiment of the invention.
In one embodiment, the showerhead assembly 201 includes one first
metrology port 296 positioned at the center of the showerhead 204
and a plurality of first metrology ports 296 arranged in a
concentric pattern about the center of the showerhead 204. As
described with respect to FIG. 6A, the showerhead assembly 201 may
further include a plurality of second metrology ports 297
positioned concentrically about the center of the showerhead 204 at
a position such that they are centered over a central portion of
the substrates 240 disposed on the carrier plate 112 as it is
rotated during processing.
[0057] FIG. 6C is an enlarged schematic view of a portion of the
bottom surface of the showerhead 204. In one embodiment, the inner
and outer gas conduits 246, 245 are positioned across the surface
of the showerhead 204 in a hexagonal close-packed arrangement as
shown.
[0058] Referring back to FIG. 2, a lower dome 219 may be disposed
at one end of a lower volume 210, and the substrate carrier plate
112 may be disposed at the other end of the lower volume 210. The
substrate carrier plate 112 is shown in an elevated, process
position, but may be moved to a lower position where, for example,
the substrates 240 may be loaded or unloaded. An exhaust ring 220
may be disposed around the periphery of the substrate carrier plate
112 to help prevent deposition from occurring in the lower volume
210 and also help direct exhaust gases from the chamber 102 to
exhaust ports 209. The lower dome 219 may be made of transparent
material, such as high-purity quartz, to allow light to pass
through for radiant heating of the substrates 240. The radiant
heating may be provided by a plurality of inner lamps 221A and
outer lamps 221B disposed below the lower dome 219. Reflectors 266
may be used to help control exposure of the chamber 102 to the
radiant energy provided by the inner and outer lamps 221A, 221B.
Additional rings of lamps (not shown) may also be used for finer
temperature control of the substrates 240.
[0059] The chemical delivery module 203 supplies chemicals to the
MOCVD chamber 102. Reactive gases (e.g., first and second precursor
gases), carrier gases, purge gases, and cleaning gases may be
supplied from the chemical delivery system through supply lines and
into the chamber 102. In one embodiment, the gases are supplied
through supply lines and into a gas mixing box where they are mixed
together and delivered to the showerhead assembly 201. Generally
supply lines for each of the gases include shut-off valves that can
be used to automatically or manually shut-off the flow of the gas
into its associated line, and mass flow controllers or other types
of controllers that measure the flow of gas or liquid through the
supply lines. Supply lines for each of the gases may also include
concentration monitors for monitoring precursor concentrations and
providing real time feedback. Backpressure regulators may be
included to control precursor gas concentrations. Valve switching
control may be used for quick and accurate valve switching
capability. Moisture sensors in the gas lines measure water levels
and can provide feedback to the system software which in turn can
provide warnings/alerts to operators. The gas lines may also be
heated to prevent precursors and cleaning gases from condensing in
the supply lines. Depending upon the process used some of the
sources may be liquid rather than gas. When liquid sources are
used, the chemical delivery module includes a liquid injection
system or other appropriate mechanism (e.g., a bubbler) to vaporize
the liquid. Vapor from the liquids is then usually mixed with a
carrier gas as would be understood by a person of skill in the
art.
[0060] The remote plasma system 226 can produce a plasma for
selected applications, such as chamber cleaning or etching residue
from a process substrate. Plasma species produced in the remote
plasma system 226 from precursors supplied via an input line are
sent via a conduit 204D for dispersion through the showerhead 204
to the MOCVD chamber 102. Precursor gases for a cleaning
application may include chlorine containing gases, fluorine
containing gases, iodine containing gases, bromine containing
gases, nitrogen containing gases, and/or other reactive elements.
The remote plasma system 226 may also be adapted to deposit CVD
layers flowing appropriate deposition precursor gases into remote
plasma system 226 during a layer deposition process. In one
embodiment, the remote plasma system 226 is used to deliver active
chlorine species to the processing volume 208 for cleaning the
interior of the MOCVD chamber 102.
[0061] The temperature of the walls of the MOCVD chamber 102 and
surrounding structures, such as the exhaust passageway, may be
further controlled by circulating a heat-exchange liquid through
channels (not shown) in the walls of the chamber 102. The
heat-exchange liquid can be used to heat or cool the chamber body
202 depending on the desired effect. For example, hot liquid may
help maintain an even thermal gradient during a thermal deposition
process, whereas a cool liquid may be used to remove heat from the
system during an in-situ plasma process, or to limit formation of
deposition products on the walls of the chamber. This heating,
referred to as heating by the "heat exchanger", beneficially
reduces or eliminates condensation of undesirable reactant products
and improves the elimination of volatile products of the process
gases and other contaminants that might contaminate the process if
they were to condense on the walls of cool vacuum passages and
migrate back into the processing chamber during periods of no gas
flow.
[0062] In one embodiment, during processing, a first precursor gas
flows from the first processing gas channel 204A in the showerhead
204 and a second precursor gas flows from the second processing gas
channel 204B formed in the showerhead 204 towards the surface of
the substrates 240. As noted above, the first precursor gas and/or
second precursor gas may comprise one or more precursor gases or
process gasses as well as carrier gases and dopant gases which may
be mixed with the precursor gases. The draw of the exhaust ports
209 may affect gas flow so that the process gases flow
substantially tangential to the substrates 240 and may be uniformly
distributed radially across the substrate deposition surfaces in a
laminar flow. In one embodiment, the processing volume 208 may be
maintained at a pressure of about 760 Torr down to about 80
Torr.
[0063] Exemplary showerheads that may be adapted to practice
embodiments described herein are described in U.S. patent
application Ser. No. 11/873,132, filed Oct. 16, 2007, now published
as US 2009-0098276, entitled MULTI-GAS STRAIGHT CHANNEL SHOWERHEAD,
U.S. patent application Ser. No. 11/873,141, filed Oct. 16, 2007,
now published as US 2009-0095222, entitled MULTI-GAS SPIRAL CHANNEL
SHOWERHEAD, and U.S. patent application Ser. No. 11/873,170, filed
Oct. 16, 2007, now published as US 2009-0095221, entitled MULTI-GAS
CONCENTRIC INJECTION SHOWERHEAD, all of which are incorporated by
reference in their entireties. Other aspects of the MOCVD chamber
102 are described in U.S. patent application Ser. No. 12/023,520,
filed Jan. 31, 2008, published as US 2009-0194024, and titled CVD
APPARATUS, which is herein incorporated by reference in its
entirety.
[0064] In summary, embodiments of the present invention include a
showerhead assembly having separate inlets and channels for
delivering separate processing gases into a processing volume of
the chamber without mixing the gases prior to entering the
processing volume. In one embodiment, the showerhead includes
metrology ports with purge gas assemblies configured and positioned
to deliver a purge gas to prevent deposition thereon, thus
increasing the reliability of data gathered by metrology assemblies
attached to the metrology ports. In one embodiment, the metrology
port is configured to receive a temperature measurement device, and
the purge gas assembly is a concentric tube configuration
configured to prevent deposition on components of the temperature
measurement device, resulting in more reliable temperature data
gathered thereby. In another embodiment, the metrology port has a
sensor window and is configured to receive an optical measurement
device. The purge gas assembly and sensor window are configured to
prevent deposition on the sensor window, resulting in more reliable
data gathered by the optical measurement device.
[0065] While the foregoing is directed to embodiments of the
present invention, other and further embodiments of the invention
may be devised without departing from the basic scope thereof, and
the scope thereof is determined by the claims that follow. For
example, certain embodiments of the showerhead assembly 201 do not
include all of the channels 204A-C. In addition, certain
embodiments of the showerhead assembly 201 do not include any of
the channels 204A-C.
* * * * *