U.S. patent application number 15/380121 was filed with the patent office on 2018-06-21 for pellicle structures and methods of fabricating thereof.
The applicant listed for this patent is Taiwan Semiconductor Manufacturing Co., Ltd.. Invention is credited to Hsuan-Chen CHEN, Pei-Cheng HSU, Hsin-Chang LEE, Yun-Yue LIN, Hsuan-I WANG, Anthony YEN.
Application Number | 20180173093 15/380121 |
Document ID | / |
Family ID | 62554660 |
Filed Date | 2018-06-21 |
United States Patent
Application |
20180173093 |
Kind Code |
A1 |
HSU; Pei-Cheng ; et
al. |
June 21, 2018 |
PELLICLE STRUCTURES AND METHODS OF FABRICATING THEREOF
Abstract
A structure including an EUV mask and a pellicle attached to the
EUV mask. The pellicle includes a pellicle frame and a plurality of
pellicle membrane layers attached to the pellicle frame. The
plurality of pellicle membrane layers include at least one core
pellicle membrane layer and an additional pellicle membrane layer
is disposed on the at least one core pellicle membrane layer. In
some embodiments, the additional pellicle membrane layer is a
material having a thermal emissivity greater than 0.2, a
transmittance greater than 80%, and a refractive index (n) for 13.5
nanometer source of greater than 0.9.
Inventors: |
HSU; Pei-Cheng; (Taipei,
TW) ; LEE; Hsin-Chang; (Hsinchu County, TW) ;
LIN; Yun-Yue; (Hsinchu City, TW) ; CHEN;
Hsuan-Chen; (Tainan City, TW) ; WANG; Hsuan-I;
(New Taipei City, TW) ; YEN; Anthony; (Hsinchu,
TW) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Taiwan Semiconductor Manufacturing Co., Ltd. |
Hsin-Chu |
|
TW |
|
|
Family ID: |
62554660 |
Appl. No.: |
15/380121 |
Filed: |
December 15, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03F 1/62 20130101; G03F
1/64 20130101; G03F 1/22 20130101; G03F 7/2004 20130101 |
International
Class: |
G03F 1/64 20060101
G03F001/64; G03F 1/22 20060101 G03F001/22; G03F 7/20 20060101
G03F007/20 |
Claims
1. A structure, comprising: an EUV mask; a pellicle attached to the
EUV mask, wherein the pellicle includes: a pellicle frame; a
plurality of pellicle membrane layers attached to the pellicle
frame, wherein the plurality of pellicle membrane layers include:
at least one core pellicle membrane layer; and an additional
pellicle membrane layer disposed on the at least one core pellicle
membrane layer and being a nearest adjacent layer of the plurality
of pellicle membrane layers to the EUV mask, wherein the additional
pellicle membrane layer is silicon carbide (SiC).
2. The structure of claim 1, wherein the SiC has a thickness of
between 10 nm and 25 nm.
3. The structure of claim 1, wherein the at least one core pellicle
membrane layer includes a layer of ruthenium.
4. The structure of claim 3, wherein the layer of ruthenium defines
a top surface of the pellicle and the additional pellicle material
layer directly interfacing the layer of ruthenium.
5. The structure of claim 1, wherein the at least one core pellicle
membrane layer includes a first layer of silicon nitride, a second
layer of polysilicon and a third layer of silicon nitride.
6. The structure of claim 5, wherein the SiC directly interfaces
the third layer of silicon nitride.
7. The structure of claim 6, wherein the first layer of silicon
nitride provides a top surface of the pellicle.
8. A structure, comprising: an EUV mask; a pellicle attached to the
EUV mask, wherein the pellicle includes: a pellicle frame; a
plurality of pellicle membrane layers attached to the pellicle
frame, wherein the plurality of pellicle membrane layers include:
at least one core pellicle membrane layer; and an additional
pellicle membrane layer disposed on the at least one core pellicle
membrane layer and being a nearest adjacent layer of the plurality
of pellicle membrane layers to the EUV mask, wherein the additional
pellicle membrane layer is a material having a thermal emissivity
greater than 0.2, a transmittance greater than 80%, and a
refractive index (n) for 13.5 nanometer source of greater than
0.9.
9. The structure of claim 8, wherein the additional pellicle
material layer is SiC.
10. The structure of claim 8, wherein a thickness of the additional
pellicle material layer is between approximately 10 and 25
nanometers (nm).
11. The structure of claim 8, wherein the at least one core
pellicle membrane layer is a single layer of ruthenium.
12. The structure of claim 8, wherein the at least one core
pellicle membrane layer is a stack of layers including a first
silicon nitride layer and second silicon nitride layer interposed
by a polysilicon layer.
13. The structure of claim 8, wherein the additional pellicle
material layer is at least one of Ir, Y, Zr, Mo, Nb and their
alloy, B.sub.4C, Si, SiC, SiN, their carbonitride, and their
oxynitide.
14. The structure of claim 8, wherein the pellicle frame has a
first physical interface with the at least one core pellicle
membrane and a second physical interface with the additional
pellicle material layer.
15. The structure of claim 14, wherein the second physical
interface is a sidewall wall of the second physical interface and
wherein the first physical interface is a bottom surface of the at
least one core pellicle membrane.
16. A method, comprising: providing an EUV lithographic mask
including a patterned surface; and forming at least one pellicle
membrane layer; attaching a pellicle frame to the at least one
pellicle membrane layer; and after the attaching, forming another
membrane layer on the at least one pellicle membrane layer, wherein
the another membrane layer is a material having a property of
having a thermal emissivity greater than 0.2, a transmittance
greater than 80%, and a refractive index (n) for 13.5 nanometer
source of greater than 0.9; and mounting a pellicle including the
pellicle frame, the at least one pellicle membrane layer, and the
another membrane layer to the EUV lithographic mask.
17. The method of claim 16, further comprising: exposing the
mounted pellicle and the EUV lithographic mask to an EUV
radiation.
18. The method of claim 16, wherein the forming the another
membrane layer is performed by at least one of a chemical vapor
deposition process (CVD) and a physical vapor deposition process
(PVD).
19. The method of claim 18, wherein the another membrane layer is
performed by CVD selected from the group consisting of atmospheric
pressure CVD process (APCVD); a low pressure CVD process (LPCVD); a
laser-enhanced CVD process (LECVD); and a plasma enhanced CVD
process (PECVD).
20. The method of claim 18, wherein the another membrane layer is
performed by PVD processes selected from the group consisting of
using electrically heated evaporation sources, pulsed laser
deposition, electron-beam evaporation, molecular beam epitaxy, ion
beam assisted evaporation, and discharge based deposition methods.
Description
BACKGROUND
[0001] The electronics industry has experienced an ever increasing
demand for smaller and faster electronic devices which are
simultaneously able to support a greater number of increasingly
complex and sophisticated functions. Accordingly, there is a
continuing trend in the semiconductor industry to manufacture
low-cost, high-performance, and low-power integrated circuits
(ICs). Thus far these goals have been achieved in large part by
scaling down semiconductor IC dimensions (e.g., minimum feature
size) and thereby improving production efficiency and lowering
associated costs. However, such scaling has also introduced
increased complexity to the semiconductor manufacturing process.
Thus, the realization of continued advances in semiconductor ICs
and devices calls for similar advances in semiconductor
manufacturing processes and technology.
[0002] As merely one example, semiconductor lithography processes
may use lithographic templates (e.g., photomasks or reticles) to
optically transfer patterns onto a substrate. Such a process may be
accomplished, for example, by projection of a radiation source,
through an intervening photomask or reticle, onto the substrate
having a photosensitive material (e.g., photoresist) coating. The
minimum feature size that may be patterned by way of such a
lithography process is limited by the wavelength of the projected
radiation source. In view of this, extreme ultraviolet (EUV)
radiation sources and lithographic processes have been introduced.
However, EUV systems, which utilize reflective rather than
conventional refractive optics, can be sensitive to contamination
issues. In one example, particle contamination introduced onto a
reflective EUV mask can result in significant degradation of the
lithographically transferred pattern. As such, it is necessary to
provide a pellicle membrane over an EUV mask, to serve as a
protective cover which protects the EUV mask from damage and/or
contaminant particles. While study has provided pellicle membranes
of different configurations and/or materials have been satisfactory
in some respects, further improvement in their performance and use
in volume production may be desired.
BRIEF DESCRIPTION OF THE DRAWINGS
[0003] Aspects of the present disclosure are best understood from
the following detailed description when read with the accompanying
figures. It is noted that, in accordance with the standard practice
in the industry, various features are not drawn to scale. In fact,
the dimensions of the various features may be arbitrarily increased
or reduced for clarity of discussion.
[0004] FIG. 1 is a schematic view of a lithography system, in
accordance with some embodiments;
[0005] FIG. 2 is a cross-sectional view of an EUV mask, in
accordance with some embodiments;
[0006] FIG. 3A is a top-view and FIG. 3B is a cross-sectional view
of a mask and pellicle, according to some embodiments;
[0007] FIG. 4 is a cross-sectional view of an embodiment of a
pellicle, according to one or more aspects of the present
disclosure;
[0008] FIG. 5 is a cross-sectional view of another embodiment of a
pellicle, according to one or more aspects of the present
disclosure;
[0009] FIG. 6 is a flow chart illustrating an embodiment of a
method of lithography, according to one or more aspects of the
present disclosure; and
[0010] FIG. 7 illustrates a graphical representation of some
experimental data associated with some embodiments according to one
or more aspects of the present disclosure.
DETAILED DESCRIPTION
[0011] The following disclosure provides many different
embodiments, or examples, for implementing different features of
the provided subject matter. Specific examples of components and
arrangements are described below to simplify the present
disclosure. These are, of course, merely examples and are not
intended to be limiting. For example, the formation of a first
feature over or on a second feature in the description that follows
may include embodiments in which the first and second features are
formed in direct contact, and may also include embodiments in which
additional features may be formed between the first and second
features, such that the first and second features may not be in
direct contact. In addition, the present disclosure may repeat
reference numerals and/or letters in the various examples. This
repetition is for the purpose of simplicity and clarity and does
not in itself dictate a relationship between the various
embodiments and/or configurations discussed.
[0012] Further, spatially relative terms, such as "beneath,"
"below," "lower," "above," "upper" and the like, may be used herein
for ease of description to describe one element or feature's
relationship to another element(s) or feature(s) as illustrated in
the figures. The spatially relative terms are intended to encompass
different orientations of the device in use or operation in
addition to the orientation depicted in the figures. The apparatus
may be otherwise oriented (rotated 90 degrees or at other
orientations) and the spatially relative descriptors used herein
may likewise be interpreted accordingly. Additionally, throughout
the present disclosure, the terms "mask", "photomask", and
"reticle" may be used interchangeably to refer to a lithographic
template, such as an EUV mask.
[0013] It is noted that the present disclosure discusses the
application of various embodiments of pellicles to EUV lithography
processes or EUV masks. However, it will be appreciated that
pellicles may now provide benefits in other processes now existing
or later developed and aspects of the present disclosure apply to
those systems as well.
[0014] Illustrated in FIG. 1 is a schematic view of a lithography
system 100, in accordance with some embodiments. In various
embodiments, the pellicle membrane described herein may be coupled
to an EUV mask utilized within the lithography system 100. The
lithography system 100 may also be generically referred to as a
scanner that is operable to perform lithographic processes
including exposure with a respective radiation source and in a
particular exposure mode. In at least some of the present
embodiments, the lithography system 100 includes an EUV lithography
system designed to expose a resist layer disposed on a target
substrate by EUV light. Inasmuch, in various embodiments, the
resist layer includes a material sensitive to the EUV light (e.g.,
an EUV resist). In an embodiment, the EUV light is provided at an
EUV wavelength such as 13.5 nm. At least some embodiments described
herein may include pellicle membranes coupled to a mask utilized
within an optical lithography system, such as an optical
lithography system using a deep UV (DUV) light source to expose a
resist layer sensitive to the DUV light source. By way of example,
some DUV light sources may include a KrF excimer laser (e.g., to
provide a 248 nm light source), an ArF excimer laser (e.g., to
provide a 193 nm light source), or an F.sub.2 eximer laser (e.g.,
to provide a 157 nm light source).
[0015] With reference to FIG. 1, the lithography system 100
illustrated includes a plurality of subsystems such as a radiation
source 102, an illuminator 104, a mask stage 106 configured to
receive a mask 108, projection optics 110, and a substrate stage
118 configured to receive a target substrate, such as semiconductor
substrate 116. A general description of the operation of the
lithography system 100 may be given as follows: EUV light from the
radiation source 102 is directed toward the illuminator 104 (which
includes a set of reflective mirrors) and projected onto the mask
108. A reflected mask image is directed toward the projection
optics 110, which focuses the light and projects the light onto the
semiconductor substrate 116 to expose an EUV resist layer deposited
thereupon. Additionally, in various examples, each subsystem of the
lithography system 100 may be housed in, and thus operate within, a
high-vacuum environment, for example, to reduce atmospheric
absorption of EUV light.
[0016] In the embodiments described herein, the radiation source
102 may be used to generate the light (e.g., EUV light). In some
embodiments, the radiation source 102 includes a plasma source,
such as for example, a discharge produced plasma (DPP) or a laser
produced plasma (LPP). In some examples, the light may include
light having a wavelength ranging from about 1 nanometer (nm) to
about 100 nm. In one particular example, the radiation source 102
generates EUV light with a wavelength centered at about 13.5 nm.
Accordingly, the radiation source 102 may also be referred to as an
EUV radiation source 102. In some embodiments, the radiation source
102 also includes a collector, which may be used to collect EUV
light generated from the plasma source and to direct the EUV light
toward imaging optics such as the illuminator 104.
[0017] As described above, light from the radiation source 102 is
directed toward the illuminator 104. In some embodiments, the
illuminator 104 may include reflective optics (e.g., for the EUV
lithography system 100), such as a single mirror or a mirror system
having multiple mirrors in order to direct light from the radiation
source 102 onto the mask stage 106, and particularly to the mask
108 secured on the mask stage 106. In some examples, the
illuminator 104 may include a zone plate, for example, to improve
focus of the EUV light. In some embodiments, the illuminator 104
may be configured to shape the EUV light passing therethrough in
accordance with a particular pupil shape, and including for
example, a dipole shape, a quadrapole shape, an annular shape, a
single beam shape, a multiple beam shape, and/or a combination
thereof. In some embodiments, the illuminator 104 is operable to
configure the reflective elements (e.g., mirrors of the illuminator
104) to provide a desired illumination to the mask 108. In one
example, the elements of the illuminator 104 are configurable to
reflect EUV light to different illumination positions. In some
embodiments, a stage prior to the illuminator 104 may additionally
include other configurable mirrors that may be used to direct the
EUV light to different illumination positions within the mirrors of
the illuminator 104. In some embodiments, the illuminator 104 is
configured to provide an on-axis illumination (ONI) to the mask
108. In some embodiments, the illuminator 104 is configured to
provide an off-axis illumination (OAI) to the mask 108. It should
be noted that the optics employed in the EUV lithography system
100, and in particular optics used for the illuminator 104 and the
projection optics 110, may include mirrors having multilayer
thin-film coatings known as Bragg reflectors. By way of example,
such a multilayer thin-film coating may include alternating layers
of Mo and Si, which provides for high reflectivity at EUV
wavelengths (e.g., about 13 nm).
[0018] As discussed above, the lithography system 100 also includes
the mask stage 106 configured to secure the mask 108. The
lithography system 100 may be housed in, and thus operate within, a
high-vacuum environment, and in some embodiments the mask stage 106
may include an electrostatic chuck (e-chuck) to secure the mask
108. As with the optics of the EUV lithography system 100, the mask
108 is also typically reflective. Details of the mask 108 are
discussed in more detail below with reference to the example of
FIG. 2. As illustrated in the example of FIG. 1, light is reflected
from the mask 108 and directed towards the projection optics 110,
which collects the EUV light reflected from the mask 108. By way of
example, the EUV light collected by the projection optics 110
(reflected from the mask 108) carries an image of the pattern
defined by the mask 108. In various embodiments, the projection
optics 110 provides for imaging the pattern of the mask 108 onto
the semiconductor substrate 116 secured on the substrate stage 118
of the lithography system 100. In particular, in various
embodiments, the projection optics 110 focuses the collected EUV
light and projects the EUV light onto the semiconductor substrate
116 to expose a resist layer (e.g., EUV sensitive resist) deposited
on the semiconductor substrate 116. As described above, the
projection optics 110 may include reflective optics, as used in EUV
lithography systems such as the lithography system 100. In some
embodiments, the illuminator 104 and the projection optics 110 are
collectively referred to as an optical module of the lithography
system 100.
[0019] In some embodiments, the lithography system 100 also
includes a pupil phase modulator 112 to modulate an optical phase
of the EUV light directed from the mask 108, such that the light
has a phase distribution along a projection pupil plane 114. In
some embodiments, the pupil phase modulator 112 includes a
mechanism to tune the reflective mirrors of the projection optics
110 for phase modulation. In some embodiments, the pupil phase
modulator 112 utilizes a pupil filter placed on the projection
pupil plane 114.
[0020] As discussed above, the lithography system 100 also includes
the substrate stage 118 to secure the semiconductor substrate 116
to be patterned. In various embodiments, the semiconductor
substrate 116 includes a semiconductor wafer, such as a silicon
wafer, germanium wafer, silicon-germanium wafer, III-V wafer, or
other type of wafer as known in the art. The semiconductor
substrate 116 may be coated with a resist layer (e.g., an EUV
resist layer) sensitive to EUV light.
[0021] In the embodiments described herein, the various subsystems
of the lithography system 100, including those described above, are
integrated and are operable to perform lithography exposing
processes including EUV lithography processes. The lithography
system 100 is exemplary only may further include other modules or
subsystems which may be integrated with (or be coupled to) one or
more of the subsystems or components described herein.
Additionally, the schematic representation of the lithography
system 100 is exemplary only and other configurations for
lithography systems, now known or later developed, are equally
applicable to aspects of the present disclosure.
[0022] An exemplary mask 108 is described in further detail with
respect to the example of FIG. 2. FIG. 2 illustrates an example
cross-section of a mask that may be substantially similar to the
mask 108 described above with reference to FIG. 1. As shown in FIG.
2, the EUV mask 108 may include a substrate 202 having a backside
coating layer 203, a multi-layer structure 204, a capping layer
206, and one or more absorbers 208 having an anti-reflective
coating (ARC) layer 210. It is again noted that the mask 108 is
exemplary only and may take other forms in other embodiments that
may also benefit from aspects of the present disclosure including
those with respect to to the embodiments of pellicles presented
below.
[0023] In some embodiments, the substrate 202 of the mask includes
a low thermal expansion material (LTEM) substrate (e.g., such as
TiO.sub.2 doped SiO.sub.2). In an embodiment, the backside coating
layer 203 includes a chromium nitride (Cr.sub.xN.sub.y) layer. In
some examples, substrate 202 has a thickness of about 6.3 to 6.5
mm. By way of example, the multi-layer structure 204 may include
molybdenum-silicon (Mo--Si) multi-layers deposited on top of the
substrate 202 for example, using an ion deposition technique. In
some embodiments, the multi-layer structure 204 has a thickness of
about 250-350 nm, and in some examples each Mo--Si layer pair has a
thickness of about 3 nm (for the Mo layer) and about 4 nm (for the
Si layer). The desired diffraction characteristics may impact the
thickness and/or number of layers of the multi-layer structure 204.
In various embodiments, the capping layer 206 includes a ruthenium
(Ru) capping layer, a Si capping layer, and/or other suitable
materials that may help to protect the multi-layer structure 204
(e.g., during fabrication or use of the mask 108). In some
embodiments, the absorbers 208 may include for example, a
Ta.sub.xN.sub.y layer or a Ta.sub.xB.sub.yO.sub.zN.sub.u layer,
which may have a thickness of about 50-75 nm and are configured to
absorb EUV light (e.g., with a wavelength of about 13.5 nm). In
some examples, other materials may be used for the absorbers 208,
including, for example, Al, Cr, Ta, W, and/or other suitable
compositions. In some examples, the ARC layer 210 includes at least
one of a Ta.sub.xB.sub.yO.sub.zN.sub.u layer, a Hf.sub.xO.sub.y
layer, or a Si.sub.xO.sub.yN.sub.z layer. While some examples of
materials that may be used for each of the substrate 202, the
backside coating layer 203, the multi-layer structure 204, the
capping layer 206, the absorbers 208, and the ARC layer 210 have
been given, it will be understood that other suitable materials as
known in the art may be equally used without departing from the
scope of the present disclosure. Similarly, other mask
configurations may also be provided as the mask 108.
[0024] Exemplary fabrication methods for the mask 108 are briefly
summarized below. Again, these steps are exemplary and not intended
to be limiting beyond what is specifically provided in the claims
recited below. In some embodiments, the fabrication process
includes two process stages: (1) a mask blank fabrication process,
and (2) a mask patterning process. During the mask blank
fabrication process, the mask blank is formed by depositing
suitable layers (e.g., reflective multiple layers such as Mo--Si
multi-layers) on a suitable substrate (e.g., an LTEM substrate
having a flat, defect free surface). By way of example, a capping
layer (e.g., ruthenium) is formed over the multilayer coated
substrate followed by deposition of an absorber layer. The mask
blank may then be patterned (e.g., the absorber layer is patterned)
to form a desired pattern on the mask 108. In some embodiments, an
ARC layer may be deposited over the absorber layer prior to
patterning the mask blank. The patterned mask 108 may then be used
to transfer circuit and/or device patterns onto a semiconductor
wafer. In various embodiments, the patterns defined by the mask 108
can be transferred over and over onto multiple wafers through
various lithography processes. In addition, a set of masks (such as
the mask 108) may be used to construct a complete integrated
circuit (IC) device and/or circuit.
[0025] In various embodiments, the mask 108 (described above) may
be fabricated to include different structure types such as, for
example, a binary intensity mask (BIM) or a phase-shifting mask
(PSM). An exemplary BIM includes opaque absorbing regions and
reflective regions, where the BIM includes a pattern (e.g., and IC
pattern) to be transferred to the semiconductor substrate 116. The
opaque absorbing regions include an absorber, as described above,
that is configured to absorb incident light (e.g., incident EUV
light). In the reflective regions, the absorber has been removed
(e.g., during the mask patterning process described above) and the
incident light is reflected by the multi-layer. Additionally, in
some embodiments, the mask 108 may include a PSM which utilizes
interference produced by phase differences of light passing
therethrough. Examples of PSMs include an alternating PSM (AltPSM),
an attenuated PSM (AttPSM), and a chromeless PSM (cPSM). For
example, an AltPSM may include phase shifters (of opposing phases)
disposed on either side of each patterned mask feature. In some
examples, an AttPSM may include an absorber layer having a
transmittance greater than zero (e.g., Mo--Si having about a 6%
intensity transmittance). In some cases, a cPSM may be described as
a 100% transmission AltPSM, for example, because the cPSM does not
include phase shifter material or chrome on the mask.
[0026] As described above, the mask 108 includes a patterned image
that may be used to transfer circuit and/or device patterns onto a
semiconductor wafer (e.g., the semiconductor substrate 116) by the
lithography system 100. To achieve a desirably high fidelity
pattern transfer from the patterned mask 108 to the semiconductor
substrate 116, introduction of defects onto the mask 108 may be
desired to be reduced and/or avoided. As shown in FIG. 2, particles
212 may be unintentionally deposited on the surface of the mask
108. The particles 202 can result in degradation of
lithographically transferred patterns if not removed. While the
particles 212 are illustrated as having a circular shape, it will
be understood that other particle shapes and sizes are possible,
and are intended to fall within the scope of the present
disclosure. Particles 212 may be introduced by any of a variety of
fabrication methods, handling methods, and/or use of the mask 108
in the lithography system such as the lithography system 100.
[0027] One method of avoiding and/or removing particle
contamination (e.g., particles 212) of a reflective EUV mask (e.g.,
the mask 108) may include cleaning process such as, a wet chemical
processes. In some examples, such wet cleans may be performed with
the addition of physical force. Cleans of the mask, such as mask
108, may be performed prior to or after mounting a pellicle onto
the mask as discussed below.
[0028] Alternatively or in addition to mask cleaning, a pellicle
assembly (or simply a pellicle) may be used over a mask to serve as
a protective cover. The pellicle assembly can serve to protects the
mask from damage and/or contaminant particles. With reference to
FIGS. 3A and 3B, illustrated therein is a top-view and a
cross-sectional view, respectively, of a mask attached to a
pellicle. In particular, FIGS. 3A/3B illustrate a mask 108, which
may be substantially similar to as discussed above with reference
to FIGS. 1 and 2, and a pellicle assembly 302. The pellicle
assembly 302 or simply pellicle 302 includes a pellicle frame 304,
and a pellicle membrane 306.
[0029] As discussed above, the mask 108 includes a patterned
surface 308 used to pattern an image into a semiconductor substrate
by a lithographic process. By way of example, the pellicle 302 is
mounted such that the pellicle membrane 306 is suspended (e.g., by
the pellicle frame 304) a distance `d1` away from the patterned
surface 308 of the mask 108. In an embodiment, the distance `d1` is
several millimeters. The pellicle membrane 306 is within an optical
path between the patterned surface 308 of the mask 108 and a target
substrate (e.g., wafer) to be patterned. In this manner, any
particle which land on the pellicle membrane 306 is held away from
a focal plane of the projection optics such as the projection
optics 110 discussed above. As such, the particle may not be imaged
onto a target substrate. In contrast, without the use of the
pellicle membrane 306 a particle may instead be incident the
patterned surface 308, thus within the focal plane and more likely
imaged onto the target substrate.
[0030] The design of the pellicle membrane 306 can affect the EUV
process. For example, the pellicle membrane 306 must have a
suitable material and thickness in order to avoid undesired EUV
absorption. Other considerations of the pellicle membrane 306 are
also important. During an exposure process, when EUV light hits the
pellicle membrane 306, the temperature of the pellicle membrane may
increase. Thus, the thermal properties of the pellicle membrane 306
are important. In particular, increases in temperature may cause
excessive stress and lead to the pellicle membrane deformation and
transmission decrease. Additionally, in some embodiments the
pellicle membrane desirably remains stable at a target power for
the EUV lithography process. Pellicle configurations that may
address one or more of these aspects of the use of pellicles are
discussed below.
[0031] In particular, embodiments of the present disclosure offer
advantages over the existing art, though it is understood that
other embodiments may offer different advantages, not all
advantages are necessarily discussed herein, and no particular
advantage is required for all embodiments. For example, embodiments
of the present disclosure provide exemplary pellicle assemblies
which employ advantageous material and configurations as discussed
below.
[0032] Referring now to FIG. 4, illustrated is a pellicle assembly
(or simply, pellicle) 400. The pellicle 400 includes a pellicle
membrane stack 402 and a pellicle frame 404. The pellicle membrane
stack 402 may be substantially similar to the pellicle membrane
306, described above with respect to FIGS. 3A/3B; the pellicle
frame 404 may be substantially similar to the pellicle frame 304,
described above with respect to FIGS. 3A/3B. In some embodiments,
the pellicle 400 is used in the system 100 and/or in conjunction
with the mask 108 described above with reference to FIGS. 1, 2, 3A
and 3B.
[0033] The pellicle frame 404 may include a suitable material such
as aluminum, stainless steel, polyethylene, and/or other suitable
materials. In some embodiments, small holes are disposed in the
pellicle frame 404 to accommodate air pressure equivalence. The
pellicle frame 404 may include various layers including for
example, adhesive coatings, mounting adhesives, and the like in
addition to the frame material itself. In some embodiments,
mounting adhesives secure the pellicle frame 404 to the mask such
as mask 108.
[0034] In an embodiment, the pellicle frame 404 has a height that
extends from the mask 108 to a height H. The height H is provided
such that it is out of the depth-of-focus. For example, the height
H may be between approximately 5 and 10 millimeters, thus out of
the focal plane which may be less than a micron. The height H may
also be provided such that the light intensity is not overly
degraded.
[0035] Attached to an upper portion (e.g., surface) of the pellicle
frame 404 is a pellicle membrane stack (or simply pellicle
membrane) 402. In the embodiment of the system 400 of FIG. 4, a
pellicle membrane stack 402 is provided. The pellicle membrane
stack 402 includes a plurality of layers including a first layer
406, a second layer 408, a third layer 410, and a fourth layer 412.
In an embodiment, the first, second, third, and fourth layers 406,
408, 410 and 412 are configured as illustrated namely with an
interface between the first layer 406 and the second layer 408; an
interface between the second layer 408 and the third layer 410; an
interface between the third layer 410 and the fourth layer 412. In
an embodiment, the fourth layer 412 is a bottom layer, which faces
(e.g., is the nearest layer to) an underlying mask such as mask 108
discussed above. In some embodiments, additional layers may be
included in the pellicle stack 402 including, for example,
interposing layers 406, 408, and/or 410.
[0036] The first layer 406, the second layer 408, and the third
layer 410 may be referred to as the core pellicle material layers.
In an embodiment, the first layer 406 is silicon nitride. In an
embodiment, the second layer 408 is polysilicon (p-Si). In an
embodiment, the third layer 410 is silicon nitride. Thus, in some
embodiments, the pellicle membrane stack 402 includes a core
pellicle material of SiN/p-Si/SiN stack. As illustrated in FIG. 4,
in some embodiments a bottom surface of the bottommost layer of the
core pellicle material layers, in this case, the bottom surface of
the third layer 410, is disposed directly on the pellicle frame
404.
[0037] The fourth layer 412 of the pellicle material stack 402 may
be a material meeting one or more of criteria for thermal
emissivity, transmittance, and/or reflective index. Thermal
emissivity is a measure of a material's effective ability to emit
thermal radiation from its surface and having a maximum value of 1
(unitless). Transmittance is a measurement of the ratio of the
light energy falling on the layer to that transmitted through it.
The refractive index (n) (also referred to as index of refraction)
is a dimensionless number that describes how light propagates
through that medium and is defined as n=c/v where c is the speed of
light in vacuum and v is the phase velocity of light in the medium.
The refractive index determines how much light is bent, or
refracted, when entering a material. In an embodiment, the material
of the fourth layer 412 has a thermal emissivity greater than
approximately 0.2; transmittance greater than approximately 80%,
and/or a refractive index (n) at 13.5 nm, 193 nm of greater than
approximately 0.9.
[0038] In an embodiment, the fourth layer 412 is silicon carbide.
Thus, in an embodiment, the pellicle membrane stack 402 includes a
silicon nitride layer (406), a polysilicon layer (408), a silicon
nitride layer (410), and a silicon carbide layer (412).
[0039] In another embodiment, the fourth layer 412 is at least one
of Ir, Y, Zr, Mo, Nb and their alloy, B4C, Si, SiC, SiN and their
carbonitride or oxynitide. As discussed herein, the emissivity
and/or transmittance of the layer, including fourth layer 412, is
dependent upon its thickness and the thickness of the layer
including fourth layer 412 is selected to provide the desired
properties as discussed herein.
[0040] In an embodiment, the thickness of layer 412 (e.g., SiC) is
between approximately 10 nm and approximately 25 nm. The thickness
of the layer 412 may be determined based on the desired
transmittance. See FIG. 7. It is also noted that a thickness of
layer 412 that is too thin may experience discontinuities that
affect it performance including uniformity of thermal and
transmittance capabilities.
[0041] In an exemplary embodiment, the first layer 406 (e.g., SiN)
is approximately 2.5 nanometers (nm), the second layer 408 (e.g.,
polysilicon) is approximately 4 nm, and/or the third layer 410 is
approximately 2.5 nm. In a further embodiment, the fourth layer 412
is approximately 20 nm.
[0042] In an embodiment, emissivity is equal to about 0.83 for
layer 412 of SiC. In an embodiment, transmittance is equal to about
90% for layer 412 of SiC. In an embodiment, the transmittance of
layer 412 is 91.4% for 20 nm of layer 412 of SiC.
[0043] In an embodiment, EUV reflectivity can be eliminated by
approximately 5% by using the fourth layer 412 having a composition
meeting the description above. For example, in an embodiment the
stack-up includes a Ru layer (as opposed to layer 412), the EUV
reflectivity may be approximately 0.42; whereas providing the layer
412 (e.g., as SiC) can reduce the reflectivity to approximately
0.02.
[0044] The light sources (e.g., EUV sources) can emit light over a
wide spectral range, including out-of-band (OOB) radiation, in
addition to the desired wavelength. The OOB radiation can expose
resists depending on the sensitivity of the material, and this
exposure can result in resist film thickness loss, pattern
reproducibility issues, and/or other issues. In an embodiment, the
layer 412 has properties (e.g., transmittance) configured to reduce
the out-of-band effect. In some embodiments, the out-of-band effect
is reduced by 13.5 to 350 nanometers in comparison with a pellicle
membrane that omits layer 412 (e.g., in favor of a Ru layer).
[0045] In an embodiment, the fourth layer 412 is a bottom layer,
which faces (e.g., nearest layer to) an underlying mask such as
mask 108 discussed above. One advantage of this configuration is an
improvement of the thermal emissivity. That is, upon irradiation of
the pellicle 400 and the associated mask, heat may be generated may
be effectively emitted by a suitable composition of the fourth
layer 412 (e.g., a thermal emissivity of greater than 0.2 (e.g.,
SiC).
[0046] As illustrated in FIG. 4, the fourth layer 412 may be
disposed adjacent the pellicle frame 404 such that a sidewall of
the fourth layer 412 abuts a sidewall of the pellicle frame 404. In
other embodiments, the configuration of the fourth layer 412 may be
different, for example, extending under the length of the third
layer 410.
[0047] In an embodiment, one or more of the layers of the membrane
stack 402 including fourth layer 412 are deposited by chemical
vapor deposition (CVD) process such as, for example, atmospheric
pressure CVD process (APCVD); a low pressure CVD process (LPCVD); a
laser-enhanced CVD process (LECVD); and/or a plasma enhanced CVD
process (PECVD). In an embodiment, one or more of the layers of the
membrane stack 402 including fourth layer 412 are deposited by
physical vapor deposition (PVD) process such as, for example,
electrically heated evaporation sources (Thermal evaporation),
pulsed laser deposition, electron-beam evaporation, molecular beam
epitaxy, ion beam assisted evaporation, and/or discharge based
deposition methods (e.g., sputtering, arc evaporation).
[0048] In another embodiment, the fourth layer 412 is disposed over
the first layer 406. In an embodiment, the bottom surface of the
fourth layer 412 physically interfaces with the top surface of the
first layer 406. The fourth layer 412 may be substantially similar
to as discussed above. In an embodiment where the fourth layer 412
is disposed over the first layer 406, the stack 402 may have an
advantage of providing thermal emissivity for the pellicle membrane
stack 402 and/or pellicle 400 generally.
[0049] It is noted that in an embodiment, a ruthenium (Ru) layer is
not included in the pellicle membrane stack 402 or the pellicle 400
generally. While benefits of Ru layers may increase thermal
emissivity during the irradiation with EUV light, the inclusion of
Ru may provide a stack of the membrane that undesirably reflects
EUV light. This can cause a critical dimension (CD) drop at a field
edge. Thus, in some embodiments, the pellicle 400 and the pellicle
membrane stack 402 do not include Ru and do include the fourth
layer 412 as discussed above.
[0050] Referring now to FIG. 5, illustrated is a pellicle assembly
(or simply, pellicle) 500. The pellicle 500 includes a pellicle
membrane stack 502 and a pellicle frame 404. The pellicle membrane
stack 502 may be substantially similar to the pellicle membrane
306, described above with respect to FIGS. 3A/3B; the pellicle
frame 404 may be substantially similar to the pellicle frame 304,
described above with respect to FIGS. 3A/3B. The pellicle frame 404
may be substantially similar to the similar pellicle frame 404 of
the pellicle assembly 400, described above with reference to FIG.
4. In some embodiments, the pellicle 500 is used in the system 100
and/or in conjunction with the mask 108 described above with
reference to FIGS. 1, 2, 3A, and 3B.
[0051] The pellicle membrane stack 502 includes a first layer 504
and a second layer 506. In some embodiments, the pellicle 500
includes only these two layers as the pellicle membrane. In an
embodiment, the first and second layers 504 and 506 are configured
as illustrated namely with an interface between the first layer 504
and the second layer 506. In an embodiment, the second layer 506 is
a bottom layer, which faces (e.g., nearest layer to) an underlying
mask such as mask 108 discussed above. The second layer 506 may
serve to provide thermal emissivity for the pellicle 500. The first
layer 504 may also be referred to as the core material of the
pellicle membrane stack 502.
[0052] In an embodiment, the first layer 504 is ruthenium (Ru). The
first layer 504 may be between approximately 0.1 and 50 nm in
thickness. In an embodiment, the first layer 504 is approximately 7
nm. The thickness of the first layer 504 may be selected such as to
provide suitable transmission and structural performance.
[0053] The second layer 506 may be a material meeting one or more
of criteria for thermal emissivity, transmittance, and/or
reflective index. In an embodiment, the material of the second
layer 506 has a thermal emissivity greater than approximately 0.2;
transmittance greater than approximately 80% and/or a refractive
index (n) at 13.5 nm, 193 nm of greater than approximately 0.9.
[0054] In an embodiment, the second layer 506 is silicon carbide.
Thus, in an embodiment, the pellicle membrane stack 502 includes a
Ru layer (504) and a silicon carbide layer (506). In some
embodiments, the second layer 506 may be at least one of Ir, Y, Zr,
Mo, Nb and their alloy, B.sub.4C, Si, SiC, SiN and their
carbonitride or oxynitide. In addition to selecting the composition
of the second layer 506, the thickness of the second layer 506 is
also selected as discussed below to provide the relevant emissivity
and transmittance to provide the desired properties (e.g., a
thermal emissivity greater than approximately 0.2; transmittance
greater than approximately 80% and/or a refractive index (n) at
13.5 nm, 193 nm of greater than approximately 0.9).
[0055] In an embodiment, the thickness of layer 506 (e.g., SiC) is
between approximately 10 nm and approximately 25 nm. The thickness
of the layer 506 may be determined based on the desired
transmittance. See FIG. 7. It is also noted that a thickness of
layer 506 that is too thin may experience discontinuities that
affect it performance including uniformity of thermal and
transmittance capabilities.
[0056] In an embodiment, the pellicle membrane stack 502 includes a
first layer 504 of Ru at a thickness of approximately 7 nm and a
second layer 506 of SiC at a thickness of approximately 30 nm. In
other words, a total thickness of the stack 502 may be
approximately 37 nm. The exemplary embodiment can provide an EUV
transmittance of approximately 80%; a thermal emissivity of about
0.4; an EUV radiation of less than approximately 0.04; a
transmission percentage (at 193 nm wavelength source) of 45%;
and/or a transmission percentage (at 248 nm wavelength source) of
greater that approximately 70%. An indication of the mechanical
strength of a material may be given by the Young's modulus value
for the material, where the Young's modulus is a measure of the
stiffness of the material. By way of example, the Young's modulus
of approximately 450 MPa may be provided by the exemplary
embodiment.
[0057] In an embodiment, the second layer 506 is a bottom layer,
which faces (e.g., nearest layer to) an underlying mask such as
mask 108 discussed above. One advantage of this configuration is an
improvement of the thermal emissivity. That is, upon irradiation of
the pellicle 500 and the associated mask, heat may be generated may
be effectively emitted by a suitable composition of the second
layer 506 (e.g., a thermal emissivity of greater than 0.2 (e.g.,
SiC).
[0058] As illustrated in FIG. 5, the second layer 506 may be
disposed adjacent the pellicle frame 404 such that a sidewall of
the second layer 506 abuts a sidewall of the pellicle frame 404. In
other embodiments, the configuration of the second layer 506 may be
different, for example, extending under the length of the first
layer 504.
[0059] In an embodiment, one or more of the layers of the pellicle
membrane stack 502 are deposited by chemical vapor deposition (CVD)
process such as, for example, atmospheric pressure CVD process
(APCVD); a low pressure CVD process (LPCVD); a laser-enhanced CVD
process (LECVD); and/or a plasma enhanced CVD process (PECVD). In
an embodiment, one or more of the layers of the membrane stack 502
are deposited by physical vapor deposition (PVD) process such as,
for example, electrically heated evaporation sources (Thermal
evaporation), pulsed laser deposition, electron-beam evaporation,
molecular beam epitaxy, ion beam assisted evaporation, and/or
discharge based deposition methods (e.g., sputtering, arc
evaporation).
[0060] In another embodiment, the second layer 506 is disposed over
the first layer 504. For example, in some embodiments, the second
layer 506 is disposed over and not under the first layer 504. In an
embodiment, the bottom surface of the second layer 506 physically
interfaces with the top surface of the first layer 504. The second
layer 506 may be substantially similar to as discussed above. In an
embodiment where the second layer 506 is disposed over the first
layer 504, the stack 502 may have an advantage of providing thermal
dissipation and/or desired transmittance.
[0061] Thus, embodiments of the pellicle membranes disclosed herein
have excellent transmission and thermal properties. For example,
because of their improved thermal emissivity of the respective
layers (e.g., layer 506, layer 412 respectively) the pellicle
configurations allow for temperatures of the respective pellicle
membranes to remain lower during the EUV exposure processing. As
discussed above, some embodiments also provide relatively higher
EUV transmittance (e.g., through use of the layer 412, layer 506).
Those skilled in the art will recognize other benefits and
advantages of the methods and structures as described herein, and
the embodiments described are not meant to be limiting beyond what
is specifically recited in the claims that follow.
[0062] Referring now to FIG. 6, illustrated is a method 600 for
forming a pellicle and performing a lithography process. To be
sure, the method steps as to forming the pellicle do not
necessitate performance of the lithography process using said
pellicle. Likewise, performing the lithography process using a
pellicle formed according to one or more aspects of the present
disclosure does not require fabrication of said pellicle being
performed in conjunction with the lithography process.
[0063] The method 600 begins at block 602 where a one or more
layers of pellicle membrane are formed. The pellicle membrane
layers formed in block 602 may be referred to as the core pellicle
membrane layers. In some embodiments, the plurality of pellicle
membrane layers may be substantially similar to the first, second
and third layers 406, 408 and 410 as discussed above with reference
to the pellicle 400 and FIG. 4. In some embodiments, one pellicle
membrane layer may be formed. In a further embodiment, the pellicle
membrane layer formed may be substantially similar to the first
layer 504 as discussed above with reference to the pellicle 500 and
FIG. 5.
[0064] In some embodiments, to form the one or more pellicle
membrane layers a substrate is provided upon which the pellicle
membrane layers are formed. By way of example, the substrate may
include a silicon substrate. In some embodiments, the substrate may
alternatively and/or additionally include germanium,
silicon-germanium, another III-V compound, one or more thin film
layers, glass, dielectric materials, and/or other suitable
substrate material.
[0065] The one or more of the layers formed in block 602 may be
deposited by chemical vapor deposition (CVD) process such as, for
example, atmospheric pressure CVD process (APCVD); a low pressure
CVD process (LPCVD); a laser-enhanced CVD process (LECVD); and/or a
plasma enhanced CVD process (PECVD). In embodiment, the additional
pellicle material layer may be deposited by physical vapor
deposition (PVD) process such as, for example, electrically heated
evaporation sources (Thermal evaporation), pulsed laser deposition,
electron-beam evaporation, molecular beam epitaxy, ion beam
assisted evaporation, and/or discharge based deposition methods
(e.g., sputtering, arc evaporation).
[0066] As indicated above, in some embodiments, pellicle material
layers substantially similar to layers 406, 408, and 410 are formed
on a substrate (e.g., SiN, poly-Si, SiN). In some embodiments, a
pellicle material layer substantially similar to layer 504 (e.g.,
Ru) is formed on a substrate.
[0067] Proceeding to block 604 of the method 600, after formation
of one or more pellicle membrane layers on the substrate, the
method proceeds in some embodiments to provide a pellicle frame
mounted onto the pellicle membrane layer(s) provided in block 604.
The pellicle frame may be mounted to at least one of the pellicle
membrane layers by way of an adhesive (e.g., glue) layer. In other
embodiments of the method 600, block 606 and the formation of the
additional pellicle membrane layer may be performed prior to
attaching the membrane to the pellicle frame.
[0068] The method 600 then proceeds to block 606 where an
additional pellicle membrane layer is formed on the pellicle
membrane layers of block 602. In an embodiment, the additional
pellicle membrane layer may serve to improve the thermal emissivity
of the pellicle membrane stack. In some embodiments, the additional
pellicle membrane layer may be substantially similar to the fourth
layer 412 described above with reference to the pellicle 400 and
FIG. 4. In some embodiments, the additional pellicle membrane layer
may be substantially similar to the second layer 506 described
above with reference to the pellicle 500 and FIG. 5. Thus, in an
embodiment, the additional pellicle layer is SiC.
[0069] By way of example, the additional pellicle membrane layer
may have a thickness between approximately 10 and 25 nanometers as
discussed above. The additional pellicle membrane layer is
deposited by chemical vapor deposition (CVD) process such as, for
example, atmospheric pressure CVD process (APCVD); a low pressure
CVD process (LPCVD); a laser-enhanced CVD process (LECVD); and/or a
plasma enhanced CVD process (PECVD). In embodiment, the additional
pellicle material layer may be deposited by physical vapor
deposition (PVD) process such as, for example, electrically heated
evaporation sources (Thermal evaporation), pulsed laser deposition,
electron-beam evaporation, molecular beam epitaxy, ion beam
assisted evaporation, and/or discharge based deposition methods
(e.g., sputtering, arc evaporation).
[0070] As discussed above, in some embodiments after the formation
of the additional pellicle material layer in block 606, the formed
membrane is attached to the pellicle frame. For example, in some
embodiments, each and every layer of the membrane are formed prior
to attaching the membrane to the pellicle frame.
[0071] The method 600 then proceeds to block 608 where the pellicle
is attached to the mask. The pellicle may be attached using
suitable adhesive material. The figures described above, including
FIGS. 3A/3B provide exemplary mounting of the pellicle onto the
mask.
[0072] The method 600 then proceeds to block 610 where a
lithography process is performed using the pellicle mounted to the
mask. The lithography process may be performed by a system such as
the exemplary system 100, described above with reference to FIG. 1.
A target substrate, such as a wafer, is coated with resist
sensitive to the radiation source (e.g., EUV). The target substrate
is then exposed in the lithography system using the mask having the
pellicle mounted thereto to define a pattern that is imaged onto
the target substrate. As discussed above, the light from the source
passes through the pellicle and is incident the mask attached
thereto. The pellicle may serve to improve the performance of the
fidelity of the imaged pattern due to reduction of particles on the
mask. The method may continue to other steps including developing
the imaged resist pattern.
[0073] Referring to FIG. 7 illustrated is a graphical
representation of a thickness of a pellicle layer relative to the
transmittance. Specifically fitted line 702 illustrates the
thickness of SiC in nanometers related to the transmittance. Fitted
line 704 illustrates transmittance with respect to a Ru/SiN stack.
The Ru/SiN may have a thickness of approximately 4 nm/5 nm. As
indicated by FIG. 7, the SiC transmittance provides advantages to a
pellicle stack substantially as discussed above.
[0074] Thus, provided are embodiments of pellicles and methods of
fabricating pellicles that provide for numerous advantages as
discussed above including, but not limited, improved transmittance,
thermal emissivity, reduction of OOB effect, and/or other
properties. One or more of these improvements may be provided by
the configuration of materials of the core pellicle membrane in
conjunction with additional layers (e.g., thermal emissivity) such
as layers 412 and 506 respectively.
[0075] Thus, embodiments of the present disclosure described a
structure including an EUV mask and a pellicle attached to the EUV
mask. The pellicle includes a pellicle frame and a plurality of
pellicle membrane layers attached to the pellicle frame. The
plurality of pellicle membrane layers include at least one core
pellicle membrane layer. The plurality of pellicle membrane layers
further include an additional pellicle membrane layer disposed on
the at least one core pellicle membrane layer and being a nearest
adjacent layer of the plurality of pellicle membrane layers to the
EUV mask. In some embodiments, the additional pellicle membrane
layer is silicon carbide (SiC).
[0076] In another of the broader structures discussed herein, a
structure including an EUV mask and a pellicle attached to the EUV
mask. The pellicle includes a pellicle frame and a plurality of
pellicle membrane layers attached to the pellicle frame. The
plurality of pellicle membrane layers include at least one core
pellicle membrane layer and an additional pellicle membrane layer
is disposed on the at least one core pellicle membrane layer. The
additional pellicle membrane was the nearest adjacent layer of the
plurality of pellicle membrane layers to the EUV mask. In some
embodiments, the additional pellicle membrane layer is a material
having a thermal emissivity greater than 0.2, a transmittance
greater than 80%, and a refractive index (n) for 13.5 nanometer
source of greater than 0.9.
[0077] Also provided is a method including providing an EUV
lithographic mask including a patterned surface forming at least
one pellicle membrane layer. The method attaching a pellicle frame
to the at least one pellicle membrane layer. After the attaching,
the method includes forming another membrane layer on the at least
one pellicle membrane layer. The another membrane layer is a
material having a property of having a thermal emissivity greater
than 0.2, a transmittance greater than 80%, and a refractive index
(n) for 13.5 nanometer source of greater than 0.9. A pellicle
including the pellicle frame, the at least one pellicle membrane
layer, and the another membrane layer is mounted to the EUV
lithographic mask.
[0078] The foregoing outlines features of several embodiments so
that those skilled in the art may better understand the aspects of
the present disclosure. Those skilled in the art should appreciate
that they may readily use the present disclosure as a basis for
designing or modifying other processes and structures for carrying
out the same purposes and/or achieving the same advantages of the
embodiments introduced herein. Those skilled in the art should also
realize that such equivalent constructions do not depart from the
spirit and scope of the present disclosure, and that they may make
various changes, substitutions, and alterations herein without
departing from the spirit and scope of the present disclosure.
* * * * *