U.S. patent application number 11/119388 was filed with the patent office on 2005-11-17 for control of gas flow and delivery to suppress the formation of particles in an mocvd/ald system.
Invention is credited to Bhat, Sanjay, Choi, Kenric, Deaton, Paul, Kher, Shreyas, Muthukrishnan, Shankar, Narwankar, Pravin K., Nguyen, Son T., Sangam, Kedarnath, Schwartz, Miriam, Sharangapani, Rahul.
Application Number | 20050252449 11/119388 |
Document ID | / |
Family ID | 34969846 |
Filed Date | 2005-11-17 |
United States Patent
Application |
20050252449 |
Kind Code |
A1 |
Nguyen, Son T. ; et
al. |
November 17, 2005 |
Control of gas flow and delivery to suppress the formation of
particles in an MOCVD/ALD system
Abstract
The embodiments of the invention describe a process chamber,
such as an ALD chamber, that has gas delivery conduits with
gradually increasing diameters to reduce Joule-Thompson effect
during gas delivery, a ring-shaped gas liner leveled with the
substrate support to sustain gas temperature and to reduce gas flow
to the substrate support backside, and a gas reservoir to allow
controlled delivery of process gas. The gas conduits with gradually
increasing diameters, the ring-shaped gas liner, and the gas
reservoir help keep the gas temperature stable and reduce the
creation of particles.
Inventors: |
Nguyen, Son T.; (San Jose,
CA) ; Sangam, Kedarnath; (Sunnyvale, CA) ;
Schwartz, Miriam; (Los Gatos, CA) ; Choi, Kenric;
(Santa Clara, CA) ; Bhat, Sanjay; (Bangalore,
IN) ; Narwankar, Pravin K.; (Sunnyvale, CA) ;
Kher, Shreyas; (Campbell, CA) ; Sharangapani,
Rahul; (Fremont, CA) ; Muthukrishnan, Shankar;
(San Jose, CA) ; Deaton, Paul; (San Jose,
CA) |
Correspondence
Address: |
MOSER, PATTERSON & SHERIDAN, LLP
APPLIED MATERIALS, INC.
3040 POST OAK BOULEVARD, SUITE 1500
HOUSTON
TX
77056
US
|
Family ID: |
34969846 |
Appl. No.: |
11/119388 |
Filed: |
April 29, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60570173 |
May 12, 2004 |
|
|
|
Current U.S.
Class: |
118/715 ;
427/248.1 |
Current CPC
Class: |
C23C 16/45582 20130101;
C23C 16/45544 20130101; C23C 16/40 20130101; Y10T 137/0396
20150401; C23C 16/0272 20130101; C23C 16/56 20130101; Y10T 137/0357
20150401; Y02T 50/60 20130101; C23C 16/45529 20130101; C23C 16/405
20130101; C23C 16/4412 20130101; C23C 16/45531 20130101; C23C
16/401 20130101; C23C 16/4488 20130101; Y10T 137/2087 20150401 |
Class at
Publication: |
118/715 ;
427/248.1 |
International
Class: |
C23C 016/00 |
Claims
1. A gas delivery assembly, comprising: a covering member
comprising an expanding channel at a central portion of the
covering member which comprises a bottom surface extending from the
expanding channel to a peripheral portion of the covering member;
and at least one gas conduit having a first end, with a first
diameter, connected to a gas inlet of the expanding channel, and a
second end, with a second diameter, connected to a valve, wherein
the second diameter is greater than the first diameter and the
diameter of the at least one gas conduit gradually and continuously
increases from the second diameter to the first diameter, and the
at least one gas conduit is positioned at an angle from a center of
the expanding channel.
2. The gas delivery assembly of claim 1, wherein the at least one
gas conduit is disposed normal to a longitudinal axis of the
expanding channel.
3. The gas delivery assembly of claim 1, wherein the at least one
gas conduit is disposed at an angle to a longitudinal axis of the
expanding channel.
4. The gas delivery assembly of claim 1, wherein the at least one
gas conduit is disposed at the same length around the expanding
channel.
5. The gas delivery assembly of claim 1, wherein the expanding
channel comprises a tapered surface extending from the central
portion of the covering member.
6. The gas delivery assembly of claim 1, wherein there are four gas
conduits connected to four gas inlets of the process chamber.
7. An ALD process chamber, comprising: a ring-shaped gas liner
placed between the substrate support and between the chamber wall,
wherein the top surface of the ring-shaped liner is at the same
level as the substrate support during exhaust gas being pumped out
the process chamber.
8. The ALD process chamber of claim 7, wherein the liner is made of
aluminum, quartz, or pyrolitic boron nitride.
9. An ALD process chamber, comprising: at least one reservoir to
store one process gas, wherein the first end of the at least one
reservoir is coupled to a gas valve that connects to a gas conduit
with a length between about 3 cm to about 10 cm connecting a gas
inlet of the process chamber and the second end of the at least one
reservoir couples to a gas source, and the diameter of the first
end of the at least one reservoir gradually and continuously
reduces to the diameter of an inlet of the gas valve and the
diameter of the second end of the at least one reservoir gradually
and continuously reduces to a diameter of a gas line that connects
with the gas source.
10. The ALD process chamber of claim 9, wherein there are four
reservoirs coupled to the ALD process chamber.
11. An ALD process chamber, comprising: a covering member
comprising an expanding channel at a central portion of the
covering member which comprises a bottom surface extending from the
expanding channel to a peripheral portion of the covering member;
at least one gas conduit having a first end, with a first diameter,
connected to a gas inlet of the expanding channel, and a second
end, with a second diameter, connected to a gas valve, wherein the
second diameter is greater than the first diameter and the diameter
of the at least one gas conduit gradually and continuously
increases from the second diameter to the first diameter, and the
at least one gas conduit is positioned at an angle from a center of
the expanding channel; and at least one reservoir to store one
process gas, wherein the first end of the at least one reservoir is
coupled to the gas valve that connects to the at least one gas
conduit and the second end of the at least one reservoir couples to
a gas source, and the diameter of the first end of the at least one
reservoir gradually and continuously reduces to a third diameter of
an inlet of the gas valve and the diameter of the second end of the
at least one reservoir gradually and continuously reduces to a
fourth diameter of a gas line that connects with the gas
source.
12. The ALD process chamber of claim 11, wherein the at least one
gas conduit is disposed normal to a longitudinal axis of the
expanding channel.
13. The ALD process chamber of claim 11, wherein the at least one
gas conduit is disposed at the same length around the expanding
channel.
14. The ALD process chamber of claim 11, wherein the expanding
channel comprises a tapered surface extending from the central
portion of the covering member.
15. The process chamber of claim 11, wherein there are four
reservoirs connected to four gas valves and the four gas valves
connects to four gas conduits respectively.
16. The ALD process chamber of claim 15, wherein the four gas
conduits are equally spaced out around a perimeter of the expanding
channel.
17. The ALD process chamber of claim 15, wherein the four gas
conduits are positioned around the same circular direction.
18. A method of delivering gases to a substrate in a substrate
processing chamber, comprising: providing at least one gas into the
substrate processing chamber from a reservoir wherein the first end
of the reservoir is coupled to a gas valve that connects to a gas
conduit, wherein the gas conduit having a first end, with a first
diameter, connected to a gas inlet of a expanding channel of the
substrate processing chamber, and a second end, with a second
diameter, connected to the gas valve, wherein the second diameter
is greater than the first diameter and the diameter of the gas
conduit gradually and continuously increases from the second
diameter to the first diameter, and the gas conduit is positioned
at an angle from a center of the expanding channel, and the second
end of the reservoir couples to a gas source, and the diameter of
the first end of the reservoir gradually and continuously reduces
to a third diameter of an inlet of the gas valve and the diameter
of the second end of the reservoir gradually and continuously
reduces to a fourth diameter of a gas line that connects with the
gas source; and providing the gases to a central portion of the
substrate.
19. The method of claim 18, wherein providing at least one gas into
the chamber comprises directing the gases in an initial circular
direction over a central portion of the substrate.
20. The method of claim 18, wherein the temperature of the at least
one gas is maintained at above 150.degree. C. throughout the gas
delivery process.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of U.S. provisional patent
application Ser. No. 60/570,173, filed on May 12, 2004, and U.S.
patent application Ser. No. 10/032,284, filed on Dec. 21, 2001,
which are herein incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] Field of the Invention
[0003] Embodiments of the present invention generally relate to an
apparatus and a method to deposit materials on substrates, and more
specifically, to an apparatus and a method for depositing
hafnium-containing compounds, such as hafnium oxides or hafnium
silicates using atomic layer deposition processes.
[0004] In the field of semiconductor processing, flat-panel display
processing or other electronic device processing, chemical vapor
deposition has played an important role in forming films on
substrates. As the geometries of electronic devices continue to
shrink and the density of devices continues to increase, the size
and aspect ratio of the features are becoming more aggressive,
e.g., feature sizes of 0.07 microns and aspect ratios of 10 or
greater are being considered. Accordingly, conformal deposition of
materials to form these devices is becoming increasingly
important.
[0005] While conventional chemical vapor deposition has proved
successful for device geometries and aspect ratios down to 0.15
microns, the more aggressive device geometries require new,
innovative deposition techniques. One technique that is receiving
considerable attention is atomic layer deposition (ALD). In the
scheme, reactants are sequentially introduced into a processing
chamber where each reactant chemisorbs onto the substrate surface
and a reaction occurs. A purge step is typically carried out
between the deliveries of each reactant gas. The purge step may be
a continuous purge with the carrier gas or a pulse purge between
the deliveries of the reactant gases.
[0006] Controlled and repeatable reactive gas delivery and particle
suppression are challenges for advanced ALD processing to deposit
films, especially for depositing hafnium-containing compounds.
Therefore, there is a need for an ALD apparatus to deposit
materials, such as hafnium oxides and hafnium silicates, that are
repeatable and under control with adequate particle
suppression.
SUMMARY OF THE INVENTION
[0007] The embodiments of the invention describe a process chamber
that has gas conduits with gradually increasing diameters to reduce
Joule-Thompson effect during gas delivery, a gas liner leveled with
the substrate support to sustain gas temperature and to reduce gas
flow to the substrate support backside, and a gas reservoir to
allow controlled delivery of process gas. In one embodiment, a gas
delivery assembly comprises a covering member comprising an
expanding channel at a central portion of the covering member which
comprises a bottom surface extending from the expanding channel to
a peripheral portion of the covering member, and at least one gas
conduit having a first end, with a first diameter, connected to a
gas inlet of the expanding channel, and a second end, with a second
diameter, connected to a valve, wherein the second diameter is
greater than the first diameter and the diameter of the at least
one gas conduit gradually and continuously increases from the
second diameter to the first diameter, and the at least one gas
conduit is positioned at an angle from a center of the expanding
channel.
[0008] In another embodiment, an ALD process chamber comprises a
ring-shaped gas liner placed between the substrate support and
between the chamber wall, wherein the top surface of the
ring-shaped liner is at the same level as the substrate support
during exhaust gas being pumped out the process chamber.
[0009] In another embodiment, an ALD process chamber comprises at
least one reservoir to store one process gas, wherein the first end
of the at least one reservoir is coupled to a gas valve that
connects to a gas conduit with a length between about 3 cm to about
10 cm connecting a gas inlet of the process chamber and the second
end of the at least one reservoir couples to a gas source, and the
diameter of the first end of the at least one reservoir gradually
and continuously reduces to the diameter of an inlet of the gas
valve and the diameter of the second end of the at least one
reservoir gradually and continuously reduces to a diameter of a gas
line that connects with the gas source.
[0010] In another embodiment, an ALD process chamber comprises a
covering member comprising an expanding channel at a central
portion of the covering member which comprises a bottom surface
extending from the expanding channel to a peripheral portion of the
covering member, at least one gas conduit having a first end, with
a first diameter, connected to a gas inlet of the expanding
channel, and a second end, with a second diameter, connected to a
gas valve, wherein the second diameter is greater than the first
diameter and the diameter of the at least one gas conduit gradually
and continuously increases from the second diameter to the first
diameter, and the at least one gas conduit is positioned at an
angle from a center of the expanding channel, and at least one
reservoir to store one process gas, wherein the first end of the at
least one reservoir is coupled to the gas valve that connects to
the at least one gas conduit and the second end of the at least one
reservoir couples to a gas source, and the diameter of the first
end of the at least one reservoir gradually and continuously
reduces to a third diameter of an inlet of the gas valve and the
diameter of the second end of the at least one reservoir gradually
and continuously reduces to a fourth diameter of a gas line that
connects with the gas source.
[0011] In yet another embodiment, a method of delivering gases to a
substrate in a substrate processing chamber comprises providing at
least one gas into the substrate processing chamber from a
reservoir wherein the first end of the reservoir is coupled to a
gas valve that connects to a gas conduit, wherein the gas conduit
having a first end, with a first diameter, connected to a gas inlet
of a expanding channel of the substrate processing chamber, and a
second end, with a second diameter, connected to the gas valve,
wherein the second diameter is greater than the first diameter and
the diameter of the gas conduit gradually and continuously
increases from the second diameter to the first diameter, and the
gas conduit is positioned at an angle from a center of the
expanding channel, and the second end of the reservoir couples to a
gas source, and the diameter of the first end of the reservoir
gradually and continuously reduces to a third diameter of an inlet
of the gas valve and the diameter of the second end of the
reservoir gradually and continuously reduces to a fourth diameter
of a gas line that connects with the gas source, and providing the
gases to a central portion of the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] 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
the invention and are therefore not to be considered limiting of
its scope, for the invention may admit to other equally effective
embodiments.
[0013] FIG. 1 depicts a schematic cross-sectional view of one
embodiment of an ALD process chamber of the current invention.
[0014] FIG. 2A shows the simulation result of gas temperature along
a conventional gas conduit.
[0015] FIG. 2B shows the vapor pressures of hafnium precursors as a
function of temperature.
[0016] FIG. 3A depicts a schematic drawing of one embodiment of gas
conduits of current invention for delivering process gas(es) into
the process chamber.
[0017] FIG. 3B shows examples of various profiles of gas conduits
of the current invention.
[0018] FIG. 3C shows the simulation result of gas temperature along
one embodiment of a gas conduit of the current invention.
[0019] FIG. 4A depicts a schematic top cross-sectional view of one
embodiment of the expanding channel of the chamber of FIG. 1.
[0020] FIG. 4B depicts a schematic cross-sectional view of the
expanding channel of the chamber lid of FIG. 1.
[0021] FIG. 5A illustrates a schematic view of the process chamber
with a liner between the substrate support and the chamber
sidewall.
[0022] FIG. 5B shows the simulation results of temperature along
line "L" in the process chamber of FIG. 6 with and without the
liner.
[0023] FIG. 5C shows the flow dynamic simulation of gas flow of
process exhaust gas and purge gas in the process chamber.
[0024] FIG. 6A depicts a schematic drawing of two examples of gas
reservoirs of the current invention.
[0025] FIG. 6B depicts a schematic drawing of exemplary gas
reservoirs connected to the expanding gas conduits of the current
invention.
[0026] FIG. 7 illustrates a process sequence for a
hafnium-containing compound using an ALD technique.
DETAILED DESCRIPTION
[0027] The present invention describes embodiments of an apparatus
and a method for depositing a thin film by processes such as atomic
layer deposition. More specifically, the present invention
describes embodiment of an ALD apparatus for preparing
hafnium-containing compounds used in a variety of applications,
including high-k dielectric materials.
[0028] "Atomic layer deposition" (ALD) or "cyclical deposition" as
used herein refers to the sequential introduction of two or more
reactive compounds to deposit a layer of material on a substrate
surface. The two, three or more reactive compounds may
alternatively be introduced into a reaction zone of a processing
chamber. Usually, each reactive compound is separated by a time
delay to allow each compound to adhere and/or react on the
substrate surface. In one aspect, a first precursor or compound A,
such as a hafnium precursor, is pulsed into the reaction zone
followed by a first time delay. Next, a second precursor or
compound B, such as an oxidizing gas, is pulsed into the reaction
zone followed by a second delay. The oxidizing gas may include
several oxidizing agent, such as in-situ water and oxygen. During
each time delay a purge gas, such as nitrogen, is introduced into
the processing chamber to purge the reaction zone or otherwise
remove any residual reactive compound or by-products from the
reaction zone. Alternatively, the purge gas may flow continuously
throughout the deposition process so that only the purge gas flows
during the time delay between pulses of reactive compounds. The
reactive compounds are alternatively pulsed until a desired film or
film thickness is formed on the substrate surface. In either
scenario, the ALD process of pulsing compound A, purge gas, pulsing
compound B and purge gas is a cycle. A cycle can start with either
compound A or compound B and continue the respective order of the
cycle until achieving a film with the desired thickness.
[0029] FIG. 1 is a schematic cross-sectional view of an exemplary
process chamber 680 including a gas delivery apparatus 730 adapted
for cyclic deposition, such as atomic layer deposition or rapid
chemical vapor deposition. The terms atomic layer deposition (ALD)
and rapid chemical vapor deposition as used herein refer to the
sequential introduction of reactants to deposit a thin layer over a
substrate structure. The sequential introduction of reactants may
be repeated to deposit a plurality of thin layers to form a
conformal layer to a desired thickness. The process chamber 680 may
also be adapted for other deposition techniques.
[0030] The process chamber 680 comprises a chamber body 682 having
sidewalls 684 and a bottom 686. A slit valve 688 in the process
chamber 680 provides access for a robot (not shown) to deliver and
retrieve a substrate 690, such as a semiconductor wafer with a
diameter of 200 mm or 300 mm or a glass substrate, from the process
chamber 680. The process chamber 680 could be various types of ALD
chambers. The details of exemplary process chamber 680 are
described in commonly assigned U.S. Patent Application Publication
Nu. 60/570,173, filed on May 12, 2004, entitled "Atomic Layer
Deposition of Hafnium-containing High-k Materials, U.S. Patent
Application Publication No. 20030079686, filed on Dec. 21, 2001,
entitled "Gas Delivery Apparatus and Method For Atomic Layer
Deposition", which are both incorporated herein in their entirety
by references.
[0031] A substrate support 692 supports the substrate 690 on a
substrate receiving surface 691 in the process chamber 680. The
substrate support (or pedestal) 692 is mounted to a lift motor 714
to raise and lower the substrate support 692 and a substrate 90
disposed thereon. A lift plate 716 connected to a lift motor 718 is
mounted in the process chamber 680 and raises and lowers pins 720
movably disposed through the substrate support 692. The pins 720
raise and lower the substrate 690 over the surface of the substrate
support 692. The substrate support 692 may include a vacuum chuck,
an electrostatic chuck, or a clamp ring for securing the substrate
690 to the substrate support 692 during processing.
[0032] The substrate support 692 may be heated to increase the
temperature of a substrate 690 disposed thereon. For example, the
substrate support 692 may be heated using an embedded heating
element, such as a resistive heater, or may be heated using radiant
heat, such as heating lamps disposed above the substrate support
692. A purge ring 722 may be disposed on the substrate support 692
to define a purge channel 724 which provides a purge gas to a
peripheral portion of the substrate 690 to prevent deposition
thereon.
[0033] A gas delivery apparatus 730 is disposed at an upper portion
of the chamber body 682 to provide a gas, such as a process gas
and/or a purge gas, to the process chamber 680. A vacuum system 778
is in communication with a pumping channel 779 to evacuate any
desired gases from the process chamber 680 and to help maintain a
desired pressure or a desired pressure range inside a pumping zone
766 of the process chamber 680.
[0034] In one embodiment, the chamber depicted by FIG. 1 permits
the process gas and/or purge gas to enter the process chamber 680
normal (i.e., 90.degree.) with respect to the plane of the
substrate 690 via the gas delivery apparatus 730. Therefore, the
surface of substrate 690 is symmetrically exposed to gases that
allow uniform film formation on substrates. The process gas may
include a hafnium-containing compound (e.g., TDEAH or HfCl.sub.4)
during one pulse and includes an oxidizing gas (e.g., water vapor)
in another pulse. Process chamber 680 may dose a hafnium-containing
compound for about 20 seconds or less, preferably process chamber
680 may dose the hafnium-containing compound for about 10 seconds
or less, more preferably for about 5 second or less.
[0035] The process chamber 680 may be adapted to receive three or
four gas flows through three or four gas inlets from three gas
conduits. Each conduit is coupled to a single or plurality of
valves. Further disclosure of process chamber 680 adapted to flow
three process gas flows is described in paragraph 66 of commonly
assigned U.S. Patent Application Publication No. 20030079686, which
is both incorporated herein by reference. The three gas flows may
be a hafnium precursor, a silicon precursor and an oxidizing gas,
for example, the first flow includes HfCl.sub.4, the second flow
includes (Me.sub.2N).sub.3SiH and the third flow includes water
vapor from a WVG system. The four gas flows may be a hafnium
precursor, such as HfCl.sub.4, another hafnium precursor, such as
TDEAH, a silicon precursor, such as (Me.sub.2N).sub.3SiH, and an
oxidizing gas, such as a water vapor from a WVG system.
[0036] The gas delivery apparatus 730 comprises a chamber lid 732.
The chamber lid 732 includes an expanding channel 734 extending
from a central portion of the chamber lid 732 and a bottom surface
760 extending from the expanding channel 734 to a peripheral
portion of the chamber lid 732. The bottom surface 760 is sized and
shaped to substantially cover a substrate 690 disposed on the
substrate support 692. The chamber lid 732 may have a choke 762 at
a peripheral portion of the chamber lid 732 adjacent the periphery
of the substrate 690. The cap portion 772 includes a portion of the
expanding channel 734 and gas inlets 736A, 736B, 736C, 736D. The
expanding channel 734 has gas inlets 736A, 736B, 736C, 736D to
provide gas flows from two similar valves 742A, 742B, 742C, 742D.
The gas flows from the valves 742A, 742B, 742C, 742D may be
provided together and/or separately.
[0037] In one embodiment, valves 742A, 742B, 742C, and 742D are
coupled to separate reactant gas sources but are preferably coupled
to the same purge gas source. For example, valve 742A is coupled to
reactant gas source 738A and valve 742B is coupled to reactant gas
source 738B, and both valves 742A, 742B are coupled to purge gas
source 740. Each valve 742A, 742B, 742C, 742D includes a delivery
line 743A, 743B, 743C 743D. The delivery line 743A, 743B, 743C,
743D is in communication with the reactant gas source 738A, 738B,
738C, 738D and is in communication with the gas inlet 736A, 736B,
736C, 736D of the expanding channel 734 through gas conduits 750A,
750B, 750C, 750D. Additional reactant gas sources, delivery lines,
gas inlets and valves may be added to the gas delivery apparatus
730 in one embodiment (not shown). The purge lines, 745A, 745B,
745C, and 745D, are in communication with the purge gas source 740,
and the flows of the purge lines, 745A, 745B, 745C, and 745D, are
controlled by valves, 746A, 746B, 746C, and 746D, respectively. The
purge lines, 745A, 745B, 745C, and 745D, intersect the delivery
line 743A, 743B, 743C, 743D at the valves, 742A, 742B, 742C, and
742D. If a carrier gas is used to deliver reactant gases from the
reactant gas source 738A, 738B, 738C, 738D, preferably the same gas
is used as a carrier gas and a purge gas (e.g., nitrogen used as a
carrier gas and a purge gas). The valves, 742A, 742B, 742C, and
742D, comprise diaphragms. The diaphragms may be biased open or
closed and may be actuated closed or open respectively. The
diaphragms may be pneumatically actuated or may be electrically
actuated. Examples of pneumatically actuated valves include
pneumatically actuated valves available from Swagelock of Solon,
Ohio. Pneumatically actuated valves may provide pulses of gases in
time periods as low as about 0.020 second. Electrically actuated
valves may provide pulses of gases in time periods as low as about
0.005 second. An electrically actuated valve typically requires the
use of a driver coupled between the valve and the programmable
logic controller, such as 748A, 748B.
[0038] Each valve 742A, 742B, 742C, 742D may be adapted to provide
a combined gas flow and/or separate gas flows of the reactant gas
738A, 738B, 738C, 738D and the purge gas 740. In reference to valve
742A, one example of a combined gas flow of the reactant gas 738A
and the purge gas 740 provided by valve 742A comprises a continuous
flow of a purge gas from the purge gas source 740 through purge
line 745A and pulses of a reactant gas from the reactant gas source
738A through delivery line 743A.
[0039] The delivery lines, 743A, 743B, 743C, and 743D of the
valves, 742A, 742B, 742C, and 742D, may be coupled to the gas
inlets, 736A, 736B, 736C, and 736D, through gas conduits, 750A,
750B, 750C, and 750D. The gas conduits, 750A, 750B, 750C, and 750D,
may be integrated or may be separate from the valves, 742A, 742B,
742C, and 742D. In one aspect, the valves 742A, 742B, 742C, 742D
are coupled in close proximity to the expanding channel 734 to
reduce any unnecessary volume of the delivery line 743A, 743B,
743C, 743D and the gas conduits 750A, 750B, 750C, 750D between the
valves 742A, 742B, 742C, 742D and the gas inlets 736A, 736B, 736C,
736D.
[0040] The gas inlets 736A, 736B, 736C, 736D are located adjacent
the upper portion 737 of the expanding channel 734. In other
embodiments, one or more gas inlets may be located along the length
of the expanding channel 734 between the upper portion 737 and the
lower portion 735.
[0041] As described in the process example above, during film
deposition, the hafnium precursor, such as HfCl.sub.4, is
maintained in a precursor bubbler at a temperature from about
150.degree. C. to about 200.degree. C. and is carried into the one
of the gas inlets, such as 736A or 736B. When the hafnium precursor
is introduced through the gas line into the process chamber 734,
due to the pressure within the delivery line is considerably higher
than the pressure in the process chamber, the gas delivered to the
process chamber expand rapidly and the temperature of the gases
drops. This is the "Joule-Thompson effect". This is also true when
the water vapor is introduced into the process chamber.
[0042] For certain wafer processing steps, this temperature drop
can have unwanted consequences. For example, consider the case of a
gas delivering a low vapor pressure reactant. If this gas undergoes
rapid expansion (the accompanying rapid cooling) as it leaves the
manifold and enters the process chamber, the reactant may condense
from vapor phase and precipitate into fine particles. Similarly,
when the temperature drops below 100.degree. C., water vapor also
condenses into liquid.
[0043] FIG. 2A shows the simulated temperature drop of N.sub.2 gas
along an about 5 cm gas conduit with constant diameter. For
temperature simulation, computation fluid dynamics (CFD) software
CFD-ACE+ by ESI group of France is used. CFD-ACE+ is a general,
partial differential equation (PDE) solver for a broad range of
physics disciplines including: flow, heat transfer,
stress/deformation, chemical kinetics, electrochemistry, and
others. It solves them in multidimensional (0D to 3D), steady and
transient form. CFD-ACE+ is used for complex multi-physics and
multidisciplinary applications. The temperature drops from
200.degree. C. to 108.degree. C. FIG. 2B shows the vapor pressure
of several hafnium precursors as function of temperature. FIG. 2B
shows that the vapor pressure of these hafnium precursors drops
quickly with lowering of temperature between 200.degree. C. to
100.degree. C. For HfCl.sub.4, which is in solid form at room
temperature, when the temperature goes below 150.degree. C., the
vaporized HfCl.sub.4 precursor precipitates into solid. For TDEAH,
which is in liquid form at room temperature, when the temperature
goes below 110.degree. C., the vaporized TDEAH condenses into
liquid, which easily and undesirably decomposes if the surrounding
temperature is greater than 150.degree. C. The decomposed TDEAH
could then react and form particles before it reaches the substrate
surface.
[0044] To avoid this undesirable situation, a gradually and
continuously expanding gas conduit, according to embodiments of the
present invention is believed to reduce the Joule-Thompson effect
of gas expansion. An example of gradual expanding gas conduits,
750A, 750B, 750C, and 750D, is shown in FIG. 1 and detailed
illustration of the gradual expanding gas conduits, 750A, 750B,
750C, and 750D, are shown in FIG. 3A. The disclosed gas conduit
design prevents large temperature drops by allowing the gases to
expand gradually and continuously. This is accomplished by
gradually and continuously increasing or tapering the flow channel
cross-section. In one embodiment, the flow channel transitions from
the cross-sections of delivery gas lines with internal diameter of
between about 3 mm to about 15 mm to a larger chamber inlet with
diameter between about 10 mm to about 20 mm over a distance between
about 30 mm to about 100 mm. This gradual increase in flow channel
cross-section allows the expanding gases to be in near equilibrium
and prevent a rapid temperature drop. The gradually and
continuously expanding channel may comprise one or more tapered
inner surfaces (shown in FIG. 3B), such as a tapered straight
surface, a concave surface, a convex surface, or combinations
thereof or may comprise sections of one or more tapered inner
surfaces (i.e., a portion tapered and a portion non-tapered). The
shapes and sizes of the gas conduits, such as 750A, 750B, 750C, and
750D, do not have to be the same for a process chamber.
[0045] FIG. 3C shows simulated results of the temperature drop
along the about 5 cm tapered gas conduits, 750A, 750B, 750C, and
750D, of FIGS. 1 and 3A. The temperature drops only slightly from
190.degree. C. to 183.degree. C., in contrast to large temperature
drop of 200.degree. C. to 108.degree. C. of the conventional design
as shown in FIG. 4. Gas conduit temperature maintaining above
180.degree. C. helps to keep the hafnium precursor in vapor form.
As evidenced by computer simulations data, the gas flow in the gas
conduit design with tapered flow channels experiences a smaller
temperature drop.
[0046] FIG. 4A is a top cross-sectional view of one embodiment of
the expanding section 734 of the chamber lid 732 of FIG. 1. Each
gas conduit, such as 750A, 750B, may be positioned at an angle
.alpha. from the center line 702 of the gas conduit, such as 750A,
750B, and from a radius line 704 from the center of the expanding
channel 734. Entry of a gas through the gas conduit 750A, 750B
preferably positioned at an angle .alpha. (i.e., when
.alpha.>0.degree.) causes the gas to flow in a circular
direction as shown by arrow 710A (or 710B). Providing gas at an
angle .alpha. as opposed to directly straight-on to the walls of
the expanding channel (i.e. when .alpha.=0.degree.) helps to
provide a more laminar flow through the expanding channel 734
rather than a turbulent flow. It is believed that a laminar flow
through the expanding channel 734 results in an improved purging of
the inner surface of the expanding channel 734 and other surfaces
of the chamber lid 732. In comparison, a turbulent flow may not
uniformly flow across the inner surface of the expanding channel
734 and other surfaces and may contain dead spots or stagnant spots
in which there is no gas flow. In one aspect, the gas conduits,
such as 750A, 750B, and the corresponding gas inlets 736A, 736B are
spaced out from each other and direct a flow in the same circular
direction (i.e., clockwise or counter-clockwise). Gas conduits,
750C and 750D, can be placed below gas conduits, 750A and 750B,
respectively along the expanding channel 734, or be placed next to
gas conduits 750A, 750B and be on the plane level as the gas
conduits 750A, 750B.
[0047] Not wishing to be bound by theory, FIG. 4B is a
cross-sectional view of the expanding channel 734 of a chamber lid
732 showing simplified representations of two gas flows
therethrough. Although the exact flow pattern through the expanding
channel 734 is not known, it is believed that the circular flow 710
(FIG. 4B) may travel as a "vortex," "helix," or "spiral" flow 902A,
902B through the expanding channel 734 as shown by arrows 902A,
902B. As shown in FIG. 3C, the circular flow may be provided in a
"processing region" as opposed to in a compartment separated from
the substrate 690. In one aspect, the vortex flow may help to
establish a more efficient purge of the expanding channel 734 due
to the sweeping action of the vortex flow pattern across the inner
surface of the expanding channel 734.
[0048] In one embodiment, the distance 710A between the gas inlets
736A, 736B and the substrate 690 is made far enough that the
"vortex" flow 902 dissipates to a downwardly flow as shown by
arrows 904 as a spiral flow across the surface of the substrate 690
may not be desirable. It is believed that the "vortex" flow 902 and
the downwardly flow 904 proceeds in a laminar manner efficiently
purging the chamber lid 732 and the substrate 690. In one specific
embodiment the distance 710A, 710B between the upper portion 737 of
the expanding channel 734 and the substrate 690 is about 1.0 inches
or more, more preferably about 2.0 inches or more. In one specific
embodiment, the upper limit of the distance 710A, 710B is dictated
by practical limitations. For example, if the distance 710A, 710B
is very long, then the residence time of a gas traveling though the
expanding channel 734 would be long, then the time for a gas to
deposit onto the substrate would be long, and then throughput would
be low. In addition, if distance 710A, 710B is very long,
manufacturing of the expanding channel 734 would be difficult. In
general, the upper limit of distance 710A, 710B may be 3 inches or
more for a chamber adapted to process 200 mm diameter substrates or
5 inches or more for a chamber adapted to process 300 mm diameter
substrates.
[0049] Referring to FIG. 1, at least a portion of the bottom
surface 760 of the chamber lid 732 may be tapered from the
expanding channel 734 to a peripheral portion of the chamber lid
732 to help provide an improved velocity profile of a gas flow from
the expanding channel 734 across the surface of the substrate 690
(i.e., from the center of the substrate to the edge of the
substrate). The bottom surface 760 may comprise one or more tapered
surfaces, such as a straight surface, a concave surface, a convex
surface, or combinations thereof. In one embodiment, the bottom
surface 760 is tapered in the shape of a funnel.
[0050] In the existing ALD reactor design, when the process exhaust
gas exits the expanding channel 734, it comes in contact of the
reactor inner sidewall 684 and also can escape to the region below
the substrate support 692. When the process exhaust gas, such as
gas containing hafnium precursor and gas containing water vapor,
comes in contact with the reactor inner sidewall 684 and the region
below the substrate support 692, it could result in H.sub.2O vapor
condensation due to lower surface temperature of these areas. The
condensed H.sub.2O reacts with hafnium precursors to form particles
and causes serious particle problems. In addition, once the process
exhaust gas escapes to the region below the substrate support 692,
it is difficult and very time consuming to pump on the exhaust
gas.
[0051] One way to resolve these issues is to provide a gas liner
that is leveled at exhausting ports level or above the wafer
processing plane so that the process exhaust gas does not
experience a lower surface temperature until it is ready to exit
the reactor 680 and also the exiting process gas does not escape to
the backside of the substrate support 692. FIG. 5A shows a
schematic drawing of an ALD chamber with a gas liner 888. The gas
liner 888 is close to the pedestal 692 to take more heat from the
pedestal. This would keep the gas liner 888 at elevated
temperature, preferably above 100.degree. C., to prevent water
vapor from condensing into liquid form at the liner. The liner is
leveled with the substrate support during process exhaust gas being
pumped out and also fill most of the space between the substrate
support 692 and the chamber sidewall 684; therefore, the gas liner
888 prevents the process exhaust gas from escaping to the region
below the substrate support 692 and prevents process exhaust gas
from create back side deposition on the pedestal 692.
[0052] The liner 888 is ring-shaped and it fits between the
substrate support and the chamber wall. The liner's inside wall 887
should be very close to the pedestal 692 to take heat from the
pedestal heater via convention, conduction and radiation heat
transfer. This would make the temperature of the liner to be at
desired temperature of about 100.degree. C. In one embodiment, the
distance between the liner's inside wall 887 to the pedestal (or
substrate support) 692 is between about 0.1 inch (or 0.25 cm) to
about 0.5 inch (or 1.27 cm). When the liner 888 is at this elevated
temperature of about 100.degree. C., the water vapor will not
condense on the liner's wall. The liner's outside wall 886 should
also be very close to the chamber inner wall 684 to prevent process
exhaust gas from escaping to the backside of the pedestal 692 (or
812). In one embodiment, the distance between the liner's outside
wall 886 to the chamber inner wall 684 is between about 0.1 inch
(or 0.25 cm) to about 0.5 inch (or 1.27 cm).
[0053] FIG. 8 shows the temperature simulation results of along
line "L" of reactors between the substrate support 692 and the part
of chamber wall 684 of FIG. 5A. Curve 801 shows the simulated
temperature with the liner 888, while curve 802 shows the simulated
temperature without the liner 888. The temperature of chamber wall
684 is about 85.degree. C. with or without liner. However, liner
maintains the temperature at above 105.degree. C., until it reaches
the chamber wall 684. This helps to keep the water vapor in gas
form. FIG. 5B shows a flow modeling on the design to predict the
effectiveness of the design. The flow simulation that gas(es) would
be pumped out before reaching the back of the pedestal heater.
CFD-ACE+ computation fluid dynamics software is used to perform the
flow simulation. The flow simulation shows that the process gas
mainly is exhausted without escaping to the backside of the
substrate support 692. The flow simulation also shows that bottom
purge gas circulates in the region below the backside of the
substrate support 692 before being pumped out. The bottom purge gas
creates a relative high pressure region to prevent process gas from
reaching the backside of the substrate support 692, or the
heater.
[0054] The materials for the liner 888 depends on the nature of the
process gases. The liner 888 can be made of materials such as
aluminum, if the process gas is non-corrosive, such as TDEAH. The
liner 888 can also be made of corrosion-resistant materials, such
as quartz or pyrolitic boron nitride, if the process gas is
corrosive, such as HfCl.sub.4.
[0055] The existing design of the gas delivery has limitation on
how much reactive precursor can be delivered to the process chamber
in a short amount of time. Advanced ALD process requires the
precursor to be delivered to the process chamber in a short time,
such as between about 50 ms to about 3 seconds to ensure high
substrate processing throughput, and under stable and repeatable
temperature to minimize temperature fluctuation and to ensure low
particle counts. For the existing gas delivery, when the gas valve,
such as 742A, 742B, 742C and 742D, is first opened, the process gas
would burst into the process chamber and cause the gas pressure in
the gas conduit, such as 750A, 750B, 750C, and 750D, to drop
quickly. It takes time for the gas conduit, such as 750A, 750B,
750C, and 750D, to replenish process gas and to recover pressure in
the gas conduit. For ALD processing, the precise control of
pressure in the gas conduit(s) and amount of process gas delivered
is very important. Since the pulsing of the process gas, such as
hafnium precursor gas, could only take 2 seconds or below, the time
it take to recover pressure in the gas conduit makes the precise
control of advanced ALD processing impossible.
[0056] A process gas reservoir located close to the point of use
that allows for a higher concentration of precursor to be delivered
to the chamber in a shorter amount of time and helps to reduce the
pressure drop when the process gas is introduced into the chamber
can be used. FIG. 6A shows the two exemplary designs (A and B) of a
reservoir 889B, which is coupled to the valves 742B and gas source
738B. In one embodiment, the reservoir 889 has a large volume
between about 80 cc to about 200 cc to store reactive precursor
gas, which could be introduced at a higher amount during process.
The gas reservoir 889 is also designed to have gradual increased
diameters at two ends to reduce the Joule-Thompson effect mentioned
above. The gradual increased diameters at the two ends of the
reservoir allows for even temperature distribution across the
reservoir at all time. The reservoir 889B was designed to allow for
a higher volume of the precursor closer to the point of use. The
outlet of the gas reservoir 889B, or the end that is connected a
valve 742B that couples to a gas conduit 750B (not shown here) to
the process chamber, should be at the same level as the gas conduit
750B to avoid needing to bend the gas line. Bending the gas line at
an angle, such as 90 degree, could cause the gas velocity to drop
and could result in change of gas temperature. The gas reservoirs
889A, 889B, 889C, 889D can be coupled to one of the gas conduits
750A, 750B, 750C, 750D which is attached to gas inlets 736A, 736B,
736C, 736D to reduce the Joule-Thompson effect of gas expansion
when the process gas is introduced into the expanding channel 734,
as shown in FIG. 6B.
[0057] In one embodiment, the reservoir is made by drilling out the
desired shape out of an aluminum bulk on the lid of the chamber to
allow for even thermal distribution. Heating materials can be
buried in the aluminum bulk to keep the temperature of the gas
reservoir constant. The reservoir can be made of other types of
conductive materials to allow sufficient heat transfer to maintain
the gas temperature. The reservoir can also be made of sheet of
conductive material, such as aluminum sheet, and be wrapped with
heating medium to control temperature in the reservoir.
[0058] In FIG. 1, a control unit 780, such as a programmed personal
computer, work station computer, or the like, may be coupled to the
process chamber 680 to control processing conditions. For example,
the control unit 780 may be configured to control flow of various
process gases and purge gases from gas sources 738A, 738B, 738C,
738D, 740 through the valves 742A, 742B, 742C, 742D, 746A, 746B,
746C, 746D during different stages of a substrate process sequence.
Illustratively, the control unit 780 comprises a central processing
unit (CPU) 782, support circuitry 784, and memory 786 containing
associated control software 783.
[0059] FIG. 7 illustrates an exemplary process sequence 100 for
forming a hafnium-containing material, such as hafnium oxide,
according to one embodiment of the present invention. A substrate
to be processed is first loaded into a process chamber capable of
performing cyclical deposition and the process conditions are
adjusted (step 110). Process conditions may include temperature,
pressure and flow rate of carrier gas. The substrate is then
exposed to pulse of a hafnium precursor that is introduced into the
process chamber for a time period in a range from about 0.1 second
to about 5 seconds (step 120). A pulse of purge gas is then pulsed
into the processing chamber (step 130) to purge or otherwise remove
any residual hafnium precursor or by-products. Next, a pulse of
oxidizing gas is introduced into the processing chamber (step 140).
The oxidizing gas may include several oxidizing agents, such as
in-situ water and oxygen. A pulse of purge gas is then introduced
into the processing chamber (step 150) to purge or otherwise remove
any residual oxidizing gas or by-products. Suitable carrier gases
or purge gases may include helium, argon, nitrogen, hydrogen,
forming gas, oxygen and combinations thereof. A "pulse" as used
herein is intended to refer to a quantity of a particular compound
that is intermittently or non-continuously introduced into a
reaction zone of a processing chamber.
[0060] Referring to step 160, after each deposition cycle (steps
120 through 150), a hafnium-containing compound, such as hafnium
oxide, having a particular thickness will be deposited on the
substrate surface. Usually, each deposition cycle forms a layer
with a thickness in the range from about 1 .ANG. to about 10 .ANG..
Depending on specific device requirements, subsequent deposition
cycles may be needed to deposit hafnium-containing compound having
a desired thickness. As such, a deposition cycle (steps 120 through
150) can be repeated until the desired thickness for the
hafnium-containing compound is achieved. Thereafter, the process is
stopped as indicated by step 170 when the desired thickness is
achieved. Hafnium oxide deposited by an ALD process has the
empirical chemical formula HfO.sub.x. Hafnium oxide has the
molecular chemical formula HfO.sub.2, but by varying process
conditions (e.g., timing, temperature, precursors), hafnium oxide
may not be fully oxidized, such as HfO.sub.1.8. Preferably, hafnium
oxide is deposited by the processes herein with the molecular
chemical formula of about HfO.sub.2 or less.
[0061] The cyclical deposition process or ALD process of FIG. 1
typically occurs at a pressure in the range from about 1 Torr to
about 100 Torr, preferably in the range from about 1 Torr to about
20 Torr, for example from about 1 Torr to about 10 Torr. The
temperature of the substrate is usually in the range from about
70.degree. C. to about 1,000.degree. C., preferably from about
100.degree. C. to about 650.degree. C., more preferably from about
250.degree. C. to about 500.degree. C.
[0062] In step 120, the hafnium precursor is introduced to the
process chamber at a rate in the range from about 5 mg/m to about
200 mg/m. The hafnium precursor is usually introduced with a
carrier gas, such as nitrogen, with a total flow rate in the range
from about 50 sccm to about 2,000 sccm. In conventional ALD
processes, the hafnium precursor is pulsed into the process chamber
at a duration from about 1 second to about 10 seconds, depending on
the particular process and desired hafnium-containing compound. In
advanced ALD processes, the hafnium precursor is pulsed into the
process chamber at a shorter duration from about 50 ms to about 3
seconds. In one embodiment, the hafnium precursor is preferably
hafnium tetrachloride (HfCl.sub.4). In another embodiment, the
hafnium precursor is preferably tetrakis(diethylamine)hafnium
((Et.sub.2N).sub.4Hf or TDEAH).
[0063] The hafnium precursor is generally dispensed to the process
chamber 180 by introducing carrier gas into a bubbler containing
the hafnium precursor. Suitable bubblers, such as PROE-VAP.TM., are
available from Advanced Technology Materials, Inc., locate in
Danbury, Conn. The temperature of the bubbler 182 is maintained at
a temperature depending on the hafnium precursor within, such as
from about 100.degree. C. to about 300.degree. C. For example, the
bubbler may contain HfCl.sub.4 at a temperature from about
150.degree. C. to about 200.degree. C.
[0064] In step 140, the oxidizing gas is introduced to the process
chamber 180 at a rate in the range from about 10 sccm to about
1,000 sccm, preferably in the range from about 30 sccm to about 200
sccm. For conventional ALD processes, the oxidizing gas is pulsed
into the process chamber 180 at a rate from about 0.1 second to
about 10 seconds, depending on the particular process and desired
hafnium-containing compound. In advanced ALD processes, the
oxidizing gas is pulsed into the process chamber at a shorter
duration from about 50 ms to about 3 seconds.
[0065] In one embodiment, the oxidizing gas is produced from a
water vapor generating (WVG) system 186 that is in fluid
communication to the process chamber 180 by a line 187. The WVG
system 186 generates ultra-high purity water vapor by means of a
catalytic reaction of O.sub.2 and H.sub.2. The WVG system has a
catalyst-lined reactor or a catalyst cartridge in which water vapor
is generated by means of a chemical reaction, unlike pyrogenic
generators that produce water vapor as a result of ignition.
Regulating the flow of H.sub.2 and O.sub.2 allows the concentration
to be precisely controlled at any point from 1% to 100%
concentrations. The water vapor may contain water, H.sub.2, O.sub.2
and combinations thereof. Suitable WVG systems are commercially
available, such as the WVG by Fujikin of America, Inc., located in
Santa Clara, Calif. and the CSGS (Catalyst Steam Generator System)
by Ultra Clean Technology, located in Menlo Park, Calif.
[0066] The pulses of a purge gas, preferably argon or nitrogen, at
steps 130 and 150, are typically introduced at a rate between about
1 slm to about 20 sim, preferably at a rate between about 2 slm to
about 6 slm. Each processing cycle (steps 120 through 150) lasts
from about 0.01 second to about 20 seconds. For example, in one
embodiment, the processing cycle is about 10 seconds, while in
another embodiment, the processing cycle is about 2 seconds. Longer
processing steps lasting about 10 seconds deposit excellent
hafnium-containing films, but the throughput is reduced. The
specific pressures and times are obtained through
experimentation.
[0067] Many precursors are within the scope of the invention. One
important precursor characteristic is to have a favorable vapor
pressure. Precursors at ambient temperature and pressure may be
plasma, gas, liquid or solid. However, within the ALD chamber,
volatilized precursors are utilized. Organometallic compounds or
complexes include any chemical containing a metal and at least one
organic group, such as amides, alkyls, alkoxyls, alkylamidos and
anilides. Precursors comprise of organometallic, inorganic and
halide compounds.
[0068] An exemplary ALD process is a hafnium oxide film grown by
sequentially pulsing a hafnium precursor with in-situ steam formed
from a water generator. A substrate surface is exposed to a
pretreatment to form hydroxyl groups. The hafnium precursor,
HfCl.sub.4, is maintained in a precursor bubbler at a temperature
from about 150.degree. C. to about 200.degree. C. Carrier gas, such
as nitrogen, is directed into the bubbler with a flow rate of about
400 sccm. The hafnium precursor saturates the carrier gas and is
pulsed into the chamber for 3 seconds. A purge gas of nitrogen is
pulsed into the chamber for 3 seconds to remove any unbound hafnium
precursor. Hydrogen gas and oxygen gas with the flow rate of 120
sccm and 60 sccm respectively, are supplied to a water vapor
generator (WVG) system. The in-situ steam exits from the WVG with
approximately 60 sccm of water vapor. The in-situ steam is pulsed
into the chamber for 1.7 seconds. The purge gas of nitrogen is
pulsed into the chamber for 4 seconds to remove any unbound or
non-reacted reagents, such as byproducts, hafnium precursor, oxygen
and/or water or any by-products such as HCl. The temperature of the
substrate is maintained at a temperature between about 400.degree.
C. to about 600.degree. C. Each ALD cycle forms about 0.8 .ANG. of
a hafnium oxide film.
[0069] Although the embodiments of the invention are described to
deposit hafnium-containing compounds, a variety of metal oxides
and/or metal silicates may be formed outside of the
hafnium-containing compounds by alternately pulsing metal
precursors with oxidizing gas derived from a WVG system, such as a
fluid of water vapor and O.sub.2. The ALD processes disclosed above
may be altered by substituting the hafnium and/or silicon
precursors with other metal precursors to form materials, such as
hafnium aluminates, titanium silicates, zirconium oxides, zirconium
silicates, zirconium aluminates, tantalum oxides, tantalum
silicates, titanium oxides, titanium silicates, silicon oxides,
aluminum oxides, aluminum silicates, lanthanum oxides, lanthanum
silicates, lanthanum aluminates, nitrides thereof and combinations
thereof.
[0070] 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.
* * * * *