U.S. patent application number 13/784386 was filed with the patent office on 2013-10-24 for plasma etch resistant, highly oriented yttria films, coated substrates and related methods.
The applicant listed for this patent is GREENE, TWEED OF DELAWARE, INC.. Invention is credited to Mohammed Mahbubul Aheem, William Brock Alexander, Sang-ho Lee, Thomas Mercer, Vasil Vorsa.
Application Number | 20130277332 13/784386 |
Document ID | / |
Family ID | 45995342 |
Filed Date | 2013-10-24 |
United States Patent
Application |
20130277332 |
Kind Code |
A1 |
Aheem; Mohammed Mahbubul ;
et al. |
October 24, 2013 |
Plasma Etch Resistant, Highly Oriented Yttria Films, Coated
Substrates and Related Methods
Abstract
Included within the scope of the invention are plasma
etch-resistant films for substrates. The films include a yttria
material and a at least a portion of the yttria material is in a
crystal phase having an orientation defined by a Miller Index
notation {111}. Also included are methods of manufacturing plasma
etch-resistant films on a substrate. Such methods include applying
a yttria material-containing composition onto at least a portion of
a surface of a substrate to form a film. The film includes a yttria
material and at least a portion of the yttria material is in a
crystal phase having an orientation defined by a Miller Index
notation {111}.
Inventors: |
Aheem; Mohammed Mahbubul;
(Souderton, PA) ; Alexander; William Brock; (St.
George, UT) ; Lee; Sang-ho; (North Wales, PA)
; Mercer; Thomas; (Annapolis, MD) ; Vorsa;
Vasil; (Coopersburg, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GREENE, TWEED OF DELAWARE, INC.; |
|
|
US |
|
|
Family ID: |
45995342 |
Appl. No.: |
13/784386 |
Filed: |
March 4, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13280129 |
Oct 24, 2011 |
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13784386 |
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61406445 |
Oct 25, 2010 |
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Current U.S.
Class: |
216/41 ;
156/345.3 |
Current CPC
Class: |
C23C 14/083 20130101;
C23F 11/00 20130101; C23C 4/11 20160101; C23C 16/4404 20130101;
C23C 4/134 20160101 |
Class at
Publication: |
216/41 ;
156/345.3 |
International
Class: |
C23F 11/00 20060101
C23F011/00 |
Claims
1. A plasma etch-resistant film for a substrate comprising a yttria
material wherein at least a portion of the yttria material is in a
crystal phase having an orientation defined by a Miller Index
notation {111}.
2. The film of claim 1, wherein 50% or more of the yttria material
is in a crystal phase having an orientation defined by a Miller
Index notation {111}.
3. The film of claim 1, wherein 90% or more of the yttria material
is in a crystal phase having an orientation defined by a Miller
Index notation {111}.
4. The film of claim 1, wherein 95% or more of the yttria material
is in a crystal phase having an orientation defined by a Miller
Index notation {111}.
5. The film of claim 1, wherein 98% or more or more of the yttria
material is in a crystal phase having an orientation defined by a
Miller Index notation {111}.
6. The film of claim 1, wherein the substrate is chosen from
silica, fused silica, quartz, fused quartz, alumina, sapphire,
silicon, aluminum, anodized aluminum, zirconium oxide, and aluminum
alloy.
7. The film of claim 6, wherein the substrate is a semiconductor
processing apparatus component.
8. The film of claim 7, wherein semiconductor processing apparatus
component is selected from a chamber wall, a chamber floor, a
screw, a wafer boat, a fastener, a window, a dispersion disc, a
shower head, a focus ring, an inner ring, an outer ring, a capture
ring, an insert ring, a gas transfer tube, and a heater block.
9. The film of claim 1, wherein the film has a thickness of about
0.5 microns to about 30 microns.
10. The film of claim 1, wherein the film has a thickness of about
5 microns to about 20 microns.
11. The film of claim 1, wherein the film has a thickness of about
10 microns to about 17 microns.
12. The film of claim 1, wherein the yttria material is yttria.
13. The film of claim 1, wherein the yttria material is a
yttria-derived composite.
14. The film of claim 13, wherein the yttria-derived composite is
selected from yttrium aluminum garnet and yttrium aluminum
perovskite.
15. The film of claim 1, wherein the film is formed using a process
selected from electron beam vapor deposition, electron beam
evaporation, sputtering, plasma spraying, and chemical vapor
deposition (CVD).
16. The film of claim 15, wherein the process is carried out when
the substrate has a temperature of about 21.degree. C. to about
500.degree. C.
17. The film of claim 15, wherein the process is carried out when
the substrate has a temperature of about 100.degree. C. to about
500.degree. C.
18. The film of claim 15, wherein the process is carried out when
the substrate has a temperature of about 400.degree. C. to about
500.degree. C.
19. The film of claim 1, wherein upon exposure to a
fluorine-containing environment, a crack or a fissure present in
the film is self-repaired.
20. A method of manufacturing a plasma etch-resistant film on a
substrate comprising depositing a yttria material-containing
composition onto at least a portion of a surface of a substrate to
form a film, wherein the film comprises a yttria material and at
least a portion of the yttria material is in a crystal phase having
an orientation defined by a Miller Index notation {111}.
21.-75. (canceled)
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit under 35 U.S.C.
.sctn.119(e) of U.S. Provisional Patent Application No. 61/406,445,
filed Oct. 25, 2010, the entire disclosure of which is incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] Resistance to plasmas is a desirable property for components
used in processing chambers where corrosive environments are
present. Process chambers and component apparatus present within or
used in conjunction with processing chambers which are used in the
fabrication of electronic devices and MEMS are frequently
constructed from various substrates such as sapphire, silica, fused
silica, quartz, fused quartz, alumina, sapphire, silicon, aluminum,
anodized aluminum, zirconium oxide, and an aluminum alloy, as these
materials are known to have a level of plasma resistance.
[0003] These materials, however, may be easily eroded during
routine processing conditions whether chemically, physically,
and/or thermally. Typically, the most severe environments are
presented to the substrates during plasma etch processes, whether
as part of etch processing or chamber cleaning. To ameliorate the
erosion or degradation of the substrates, attempts have been made
to protect and preserve them by application of shielding or film
layers. The aim of such shielding or film layers is to act to
reduce exposure to various plasmas (NF.sub.3, Cl.sub.2, CHF.sub.3,
CH.sub.2F.sub.2, SF.sub.6 and HBr) and thereby prevent or reduce
weight loss and/or to reduce particulation during dry etching
processes where particles may be dislodged from the chamber walls
and various components inside the processing chamber.
[0004] Conventional films and methods have been used in an attempt
to develop a suitably shielding or protective layer. For example,
films that contain various ceramic materials such as alumina,
aluminum nitride, and zirconia that are known to be chemically
stable in plasma etching conditions have been prepared. Although
these films often exhibit improved plasma resistance in the form of
reduced weight loss, they still frequently generate unwanted
particulates. Particulates liberated in the processing chamber
result in damaged or flawed wafers, which must then be discarded,
greatly increasing the cost of production and reducing production
line efficiency.
[0005] As an example, alumina-coated silica or alumina-coated
quartz are known to exhibit a reduced etch rate, as compared to
bare silica or quartz. However, in a fluoride-containing etch
environment, one finds that alumina from the film is oxidized,
forming aluminum fluoride, a highly stable and non-volatile
compound that builds on chamber walls. Subsequently, the aluminum
fluoride particulates shed off the chamber walls and contaminate
the wafers.
[0006] Several prior attempts have been made to reduce
particulation by coating quartz substrates with yttria. These
attempts have mostly been with very thick (typically>50 micron)
thermal-spray yttria. Thermal-sprayed yttria films, however, are
porous and generate unwanted particulates.
[0007] There remains a need in the art for a film that can be
applied to substrates that is resistant to degradation upon
exposure to plasma and exhibits reduced particulation.
BRIEF SUMMARY OF THE INVENTION
[0008] Included within the scope of the invention are plasma
etch-resistant films for substrates. The films include a yttria
material and a at least a portion of the yttria material is in a
crystal phase having an orientation defined by a Miller Index
notation {111}. Also included are methods of manufacturing plasma
etch-resistant films on a substrate. Such methods include applying
a yttria material-containing composition onto at least a portion of
a surface of a substrate to form a film. The film includes a yttria
material and at least a portion of the yttria material is in a
crystal phase having an orientation defined by a Miller Index
notation {111}.
[0009] Contemplated with the scope of the invention are
semiconductor processing apparatus components that include a
substrate and a plasma etch-resistant film. The film includes a
yttria material and at least a portion of the yttria material is in
a crystal phase having an orientation defined by a Miller Index
notation {111} or have crystal planes that are substantially
parallel to the surface of the substrate.
[0010] Methods of increasing the plasma resistance of substrate are
also included. These methods include depositing a yttria
material-containing composition onto at least a portion of a
surface of a substrate to form a film. The film comprises a yttria
material and at least a portion of the yttria material is in a
crystal phase having an orientation defined by a Miller Index
notation {111}. The portion of the substrate bearing the film
exhibits an increased resistance to degradation upon exposure to
plasma.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0011] The foregoing summary, as well as the following detailed
description of embodiments of the invention, may be better
understood when read in conjunction with the appended drawings.
However, it should be understood, that the invention is not limited
to the precise arrangements and instrumentalities shown. In the
drawings:
[0012] FIG. 1 is a schematic representation of a polycrystalline
film consisting of a randomly oriented grain structure (top) and
polycrystalline film with the orientation of the invention
(bottom);
[0013] FIG. 2 is an X-ray diffraction spectra of electron-beam
deposited yttrium oxide films on fused quartz showing a film having
a crystal orientation defined by Miller Index notation {111} and a
film having an orientation defined by Miller Index notation
{100};
[0014] FIG. 3A is an optical photograph of an yttrium oxide film
with a crystal orientation defined by Miller Index notation {111}
on fused quartz after the film was subjected to NF.sub.3+O.sub.2
plasmas for 4 hours;
[0015] FIG. 3B is an optical photograph of an yttrium oxide film
with a crystal orientation defined by Miller Index notation [100]
on fused quartz after the film was subjected to NF.sub.3+O.sub.2
plasmas for 4 hours. Significant etching can seen;
[0016] FIG. 4A is an optical microscope image (100.times.) of an
yttrium oxide film with a crystal orientation defined by Miller
Index notation {111} on fused quartz prior to any exposure to
plasmas;
[0017] FIG. 4B is an optical microscope image (100.times.) of an
yttrium oxide film with a crystal orientation defined by Miller
Index notation {111} on fused quartz after the film was subjected
to NF.sub.3+O.sub.2 plasmas for 4 hours;
[0018] FIG. 5A is an optical microscope image (100.times.) of an
yttrium oxide film with a crystal orientation defined by Miller
Index notation {100} on fused quartz prior to any exposure to
plasmas;
[0019] FIG. 5B is an optical microscope image (100.times.) of an
yttrium oxide film with a crystal orientation defined by Miller
Index notation {100} on fused quartz after the film was subjected
to NF.sub.3+O.sub.2 plasmas for 4 hours;
[0020] FIG. 6A is a scanning electron micrograph of a portion of a
film of the invention showing cracks and fissures in its
surface;
[0021] FIG. 6B is scanning electron micrograph of the substantially
identical portion of the film shown in FIG. 6A after exposure to a
fluorine-containing environment for 2 hours;
[0022] FIG. 7A is scanning electron micrograph of a portion of a
film of the invention showing cracks and fissures in its surface;
and
[0023] FIG. 7B is scanning electron micrograph of the substantially
identical portion of the film shown in FIG. 7A after exposure to a
fluorine-containing environment for 2 hours.
DETAILED DESCRIPTION OF THE INVENTION
[0024] It has been found that that by forming a film having the
crystallographic texture described herein, the film's resistance to
degradation upon exposure to gas plasma is improved as are several
other desirable properties. The invention includes a plasma
etch-resistant film for use on various substrates; methods of
preparing the film (and the film and the substrate combination);
various substrates; including those forming portions of
semiconductor processing apparatus components, bearing the film and
methods of increasing the plasma resistance by deposition or
application of the films of the invention to a substrate. In some
embodiments, if the film exhibits one or more desirable properties,
including reduced rate of plasma etching (under exposure to
corrosive chemicals or plasmas), reduced particulation during use
in a semiconductor process, and/or the ability to self-repair
cracks, fissures and other degradation under exposure to gas
plasmas, such as those containing fluorine.
[0025] The invention includes a plasma etch-resistant film for use
on various substrates. By "improved plasma resistance", it is meant
that the film of the invention, upon exposure to corrosive
chemicals, such as gas plasmas (and particularly fluorine plasmas)
is less degraded than is a conventional yttria film. Degradation of
the films may be evaluated using any means commonly accepted in the
art including visual means such as optical or scanning electron
microscopy, wherein areas of cracks, fissures, and undercutting are
assessed; by evaluation of the adhesion of the film to the
substrate, where greater adhesion corresponds to less degradation
or by spectral reflectance.
[0026] The film is formed by deposition or application of yttria
material onto a substrate. The yttria material may be any
yttria-containing or yttria-derived material that exhibits a level
of plasma resistance and/or reduced particulation when exposed to a
plasma containing environment, particularly, for example an
environment containing a fluorine-based plasma. Exemplary yttria
materials include without limitation yttria, yttrium aluminum
garnet, yttrium aluminum perovskite, yttria containing one or more
dopant or other additives, or combinations of these materials.
[0027] The film is deposited on the substrate such that at least a
portion of the yttria material is present in a highly oriented
crystallographic texture. Yttria may exist in a polycrystalline
form and such crystals are commonly understood to have a structure
represented by a cube. As in known in the art, the orientation of
the specific planes of a cubic crystal are represented by a
mathematical description referred to as the Miller Indices (or may
be described using "Miller Index notation"). The Miller Indices are
a notation system to express planes and directions in crystal
lattices, such as those formed by yttria and yttria materials. In
the crystal lattices, a family of lattice planes is determined by
three integers l, m, and n, (these are collectively the Miller
indices). Conventional notation writes these Miller indices as
"(hkl)". Each index denotes a plane orthogonal to a direction (h,
k, l) in the basis of the reciprocal lattice vectors. By
convention, negative integers are written with a bar, as in 3 for
-3. The integers are usually written in lowest terms, i.e., their
greatest common divisor should be 1. For example, in simple cubic
crystals, Miller index (100) represents a plane orthogonal to
direction l; index (010) represents a plane orthogonal to direction
m, and index (001) represents a plane orthogonal to n. When the
Miller indices are notated using the bracket symbol "{hkl}", the
set of all directions that are equivalent to [lmn] by symmetry is
denoted.
[0028] In the film of the invention, the crystals present
predominantly have an orientation described as {111} using Miller
index notation. It is preferred that the yttria material in the
film exists predominantly in the {111} orientation. For clarity, by
having an {111} orientation it is meant that the planes of the
crystals are orientated to as to be substantially parallel to the
surface of the film.
[0029] It is not necessary that all the film's yttria material is
present in the {111} orientation, only that a portion is oriented
{111}. Some material may be present in alternative crystal
orientations and/or may be amorphous (both circumstances
collectively referred herein as "non-parallel orientation").
Specifically, in some conditions, it may be preferred to that about
50%, about 60%, about 70%, about 80%, about 90%, about 95%, about
98% or about 99% or more or more of the yttria material is in a
crystal phase having an orientation defined by a Miller Index
notation {111}.
[0030] In some embodiments, it may be preferred that the portion of
the yttria material having a non-parallel orientation (or a
fraction of that portion) has the alterative orientation described
by the Miller Index notation {101}.
[0031] The film may have any average crystallite size or grain size
and the grain size may vary as a function of the thickness of the
film. However, in some embodiments, it may be preferred that the
average crystal size of the crystallites that are present in the
film have about 100 .ANG. to about 600 .ANG. or about 225 .ANG. to
about 350 .ANG., as measured by X-ray diffraction.
[0032] The film may be any thickness desired and be continuously
applied along the surface of the substrate or discontinuously
applied (that is, the film may be present on only a portion or
portions of the substrate). Thickness and continuity of the film
will necessarily vary depending on the contemplated end application
for the film-coated substrate. In some embodiments, it may be
preferable that the film has a thickness of about 0.1 to about 30
microns, about 0.5 to about 10 microns, about 5 microns to about 20
microns, and/or about 10 microns to about 17 microns.
[0033] In addition to exhibiting a reduced rate of etching and
reduced particulation during use in a semiconductor process as
described above, the film is capable of self repair under specific
conditions, including under exposure to fluorine gas plasma, such
as those used in semiconductor processing. FIGS. 6A and 7A each are
micrographs of an yttrium oxide film of the invention on a quartz
substrate. In each micrograph numerous thermally-induced cracks and
fissures on the surfaces of the films are plainly visible, in
proximity to the marker (designated).
[0034] FIGS. 6B and 7B are micrographs of the substantially
identical location on each of the film surfaces shown in FIGS. 6A
and 6B respectively (note the location of the marker) after each
film was subjected to a fluorine gas plasma-containing environment
for 2 hours. The micrographs clearly show that the cracks and
fissures visible prior to the films' exposure to a fluorine gas
plasma-containing environment have repaired or, in some cases, have
completely disappeared.
[0035] The substrates to which the films of the invention are
applied may be any known in the art, particularly those used in
semiconductor processing. In some circumstances, it may be
preferable that the substrate is a material that, independent of
the film, has one or more high performance properties, such as
resistance to corrosive chemicals, resistance to high temperatures
and/or pressures, resistance to gas plasmas, mechanical strength,
hardness, etc. Exemplary substrates may include polymers, metals,
silica, fused quartz, quartz, alumina, sapphire, silicon, aluminum,
anodized aluminum, and or zirconium oxide.
[0036] In some embodiments, the substrate is a semiconductor
processing apparatus component or a portion of a semiconductor
processing apparatus component. Such components include any known
or developed in the art. Exemplary components may include, without
limitation, a chamber wall, a chamber floor, a screw, a wafer boat
or other tool or device used to position the wafer(s), a fastener,
a window, a dispersion disc, a shower head, a focus ring, an inner
ring, an outer ring, a capture ring, an insert ring, a gas transfer
tube, and a heater block.
[0037] Methods of manufacturing a plasma etch-resistant film on a
substrate are also included within the scope of the invention. Such
methods include depositing or applying a yttria material-containing
composition onto at least a portion of a surface of a substrate to
form the film described above. The composition applied or deposited
may be substantially pure yttria material or it may be yttria
material combined with other coating materials. Depending on the
application or deposition methods used, a carrier (gas or liquid)
may be included.
[0038] The film may be deposited using any suitable methods known
or developed in the art. Exemplary methods may include, without
limitation, aerosol deposition, electron beam evaporation,
sputtering, plasma spraying, atomic layer deposition (ALD), and
chemical vapor deposition (CVD).
[0039] The specific parameters under which the film is
applied/deposited may vary depending on the method of application
or deposition used, although such minor variations within the
ordinary skill of one in the art familiar with such processes.
[0040] An example of a deposition process using a quartz substrate
and an electron beam process may include: precleaning of the bare
substrate using a solvent, such as, for example, an organic solvent
like isopropyl alcohol and pre heating of the electron beam chamber
to a target temperature in range of about 25.degree. C. to about
600.degree. C. Typically the time necessary to achieve preheating
of the substrate is about 1 to about 5 hours, depending on the
substrate mass; optional in situ precleaning of substrate using an
ion beam. If this precleaning step is undertaken, the gases used
may be argon (most typical), oxygen, oxygen/argon blend, or other
noble gases such as xenon. An exemplary process may use granular
Y.sub.2O.sub.3 having a high purity, such as 90% or greater,
preferable 98% or greater purity. The Y.sub.2O.sub.3 is premelted
in a single step or in multiple steps prior to deposition and may
be deposited onto the substrate at a rate of about 1 to about 10
micrometers per hour. During deposition, oxygen gas may be
introduced into the chamber in a partial pressure range of about
5.times.10.sup.-6 to 1.times.10.sup.-3 torr. In some circumstances,
gas introduction may result in improved film quality.
[0041] In some embodiments, ion beam assisted deposition (IBAD) may
be used to carryout the deposition. Typically, gases used in IBAD
include: argon (most typical), oxygen, oxygen/argon blend, or other
noble gases such as xenon. An exemplary process is described in,
for example, Park, S. and Morton, D. P. (2006) Ion beam assisted
texturing of polycrystalline Y.sub.2O.sub.3 films deposited via
electron beam evaporation", Thin Solid Films 510: 142-147.
[0042] After deposition, film-coated substrate is cooled back to
room temperature in a controlled manner, for example at a rate of
about 10.degree. C. to about 200.degree. C. per hour.
[0043] Regardless of the processes selected, it may be desirable
that the yttria material is applied to the substrate to form a film
when the substrate is about room temperature (21.degree. C.) to
about 500.degree. C., about 100.degree. C. to about 500.degree. C.,
and/or about 400.degree. C. to about 500.degree. C.
[0044] In any of the process described herein the yttria material
may be deposited or applied directly on to the surface of the
substrate (that is, the film is formed directly against the surface
of the substrate). Alternatively, the substrate may be coated with
other materials (forming one or more intervening layers of films)
prior to the deposition of the yttria material. In addition or
alternatively, the film of the invention, once formed, may be
coated with additional layer(s), for example an extra sacrificial
layer of alumina, to further enhance overall plasma resistance.
[0045] Also contemplated within the scope of the invention are
methods of increasing the plasma resistance of substrate comprising
depositing a yttria material-containing composition on to at least
a portion of a surface of a substrate to form a film as described
above. The portion of the substrate bearing the film exhibits an
increased resistance to degradation upon exposure to plasma and/or
generates a reduced quantity of contaminating particulates, as
compared to the identical substrate bearing an yttria film that is
formed of a yttria material that is not oriented in the crystal
microstructure described above.
EXAMPLE I
[0046] Two sets of yttrium oxide films were grown on fused quartz
coupons (dimensions: 1 inch.times.1 inch; 1/8 inch thick) by
electron beam evaporation. Each coupon was installed in the
electron beam film chamber and the chamber was vacuum purged
overnight. The film chamber vacuum level was maintained at
2.4.times.10.sup.-5 ton and preheated for 12 hours to ensure
temperature equilibrium was reached.
[0047] High purity (>99.99%) Y.sub.2O.sub.3 target was
evaporated by electron beam and each coupon was coated for 4 hours
to reach target thickness of about 4 microns. During the film
process, temperature of the substrate was maintained between about
150.degree. C. to about 350.degree. C.
[0048] One set of films was grown to predominantly produce a
crystalline structure having an orientation described by Miller
Index notation {100}. The second set was grown to produce
predominantly a crystalline structure having an orientation
described by Miller Index notation {111}.
[0049] FIG. 2 shows x-ray diffraction (XRD) measurements of the two
films. It was found that {111} oriented films exhibited improved
plasma resistance (e.g., resistant to degradation by plasma) as
compared to the films grown with a predominant {100}
orientation.
[0050] FIGS. 2A and 2B show the result of {111} and {100} oriented
films following 4 hours of NF.sub.3/O.sub.2 plasma etch. FIG. 3A
shows that the integrity of the {111} oriented film is much greater
than the {100} oriented film shown in FIG. 3B. The film in 3A shows
some delamination at film edges where the film boundary is.
However, the center of the film in FIG. 3A is intact whereas the
film in FIG. 3B shows plasma attack of the film as manifested by
the horizontal lines seen in the figure. FIGS. 4A and 4B show
microscope images of the {111} film shown in FIG. 3A taken at a
magnification of 100 times.
[0051] FIG. 4A shows the {111} oriented film prior to exposing it
to 4 hours of NF.sub.3/O.sub.2 plasma etch. FIG. 4B shows the same
film following 4 hours of exposure to plasma etch. As can be seen
from FIG. 4B there is no significant change in the film.
[0052] FIGS. 5A and 5B show the {100} oriented film before and
after 4 hours exposure to plasma etch, respectively. It is apparent
from the figures that the {100} oriented film did not withstand the
plasma environment as well as the {111} oriented film. Indeed, the
{100} oriented film performed so poorly that the substrate under
the film layer was exposed to the plasma, leading to the
undercutting of the substrate and eventual delamination of the
film. Moreover, the {111} oriented film exhibits a reduced etch
rate, as compared to the {100} oriented film.
[0053] It will be appreciated by those skilled in the art that
changes could be made to the embodiments described above without
departing from the broad inventive concept thereof. It is
understood, therefore, that this invention is not limited to the
particular embodiments disclosed, but it is intended to cover
modifications within the spirit and scope of the present invention
as defined by the appended claims.
* * * * *