U.S. patent application number 12/353766 was filed with the patent office on 2009-07-16 for metal gate device with reduced oxidation of a high-k gate dielectric.
Invention is credited to Justin K. Brask, Robert S. Chau, Suman Datta, Mark L. Doczy, Brian S. Doyle, Jack Kavalieros, Matthew V. Mertz.
Application Number | 20090179282 12/353766 |
Document ID | / |
Family ID | 37572579 |
Filed Date | 2009-07-16 |
United States Patent
Application |
20090179282 |
Kind Code |
A1 |
Doyle; Brian S. ; et
al. |
July 16, 2009 |
METAL GATE DEVICE WITH REDUCED OXIDATION OF A HIGH-K GATE
DIELECTRIC
Abstract
Embodiments of the invention provide a device with a metal gate,
a high-k gate dielectric layer and reduced oxidation of a substrate
beneath the high-k gate dielectric layer. An oxygen barrier, or
capping, layer on the high-k gate dielectric layer and metal gate
may prevent such oxidation during processes such as spacer
formation and annealing of ion implanted regions.
Inventors: |
Doyle; Brian S.; (Portland,
OR) ; Kavalieros; Jack; (Portland, OR) ;
Brask; Justin K.; (Portland, OR) ; Mertz; Matthew
V.; (Portland, OR) ; Doczy; Mark L.;
(Beaverton, OR) ; Datta; Suman; (Beaverton,
OR) ; Chau; Robert S.; (Beaverton, OR) |
Correspondence
Address: |
INTEL/BSTZ;BLAKELY SOKOLOFF TAYLOR & ZAFMAN LLP
1279 OAKMEAD PARKWAY
SUNNYVALE
CA
94085-4040
US
|
Family ID: |
37572579 |
Appl. No.: |
12/353766 |
Filed: |
January 14, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11158621 |
Jun 21, 2005 |
7501336 |
|
|
12353766 |
|
|
|
|
Current U.S.
Class: |
257/410 ;
257/E29.255 |
Current CPC
Class: |
H01L 21/2652 20130101;
H01L 29/513 20130101; H01L 29/6656 20130101; H01L 21/28247
20130101; H01L 29/4983 20130101; H01L 21/26513 20130101; H01L
29/66628 20130101; H01L 29/517 20130101 |
Class at
Publication: |
257/410 ;
257/E29.255 |
International
Class: |
H01L 29/78 20060101
H01L029/78 |
Claims
1. A semiconductor device, comprising: a substrate; a high-k gate
dielectric layer on the substrate; a metal gate electrode on the
high-k gate dielectric layer; and a capping layer that is
substantially free of oxygen and substantially seals a region
between the high-k gate dielectric layer and the substrate from
structures that comprise oxygen.
2. The device of claim 1, further comprising: a first set of
spacers on either side of the metal gate electrode; a second set of
spacers on either side of the first set of spacers; and wherein the
capping layer is between the first set of spacers and the second
set of spacers.
3. The device of claim 2, wherein the second set of spacers have a
bottom surface and the capping layer extends beneath the bottom
surface of the second set of spacers.
4. The device of claim 1, further comprising a thin oxide layer
between the high-k gate dielectric layer and the substrate and a
first set of spacers on either side of the metal gate electrode,
wherein the first set of spacers have a bottom surface and the thin
oxide layer extends beneath the bottom surface of the first set of
spacers.
5. The device of claim 1, wherein the capping layer comprises a
nitride material.
6. The device of claim 5, wherein the capping layer has a thickness
below about 75 angstroms.
7. The device of claim 5, wherein the capping layer comprises a
material selected from the group consisting of 8-12% carbon-doped
silicon nitride, stoichiometric silicon nitride and silicon
carbide.
8. The device of claim 2, wherein the second set of spacers have a
bottom surface and the capping layer extends beneath the bottom
surface of the second set of spacers.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/158,621, filed Jun. 21, 2005, entitled "METAL GATE
DEVICE WITH REDUCED OXIDATION OF A HIGH-K GATE DIELECTRIC" the
entire contents of which are hereby incorporated by reference
herein.
BACKGROUND
[0002] 1. Background of the Invention
[0003] MOS field-effect transistors with very thin silicon dioxide
based gate dielectrics may experience unacceptable gate leakage
currents. Forming the gate dielectric from certain high-k
dielectric materials, instead of silicon dioxide, can reduce gate
leakage. When conventional processes are used to form such
transistors, a silicon dioxide transition layer may form between
the high-k dielectric and the substrate. The presence of that
transition layer may unfavorably contribute to the overall
electrical thickness of the gate dielectric stack.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 is a cross sectional side view that illustrates the
semiconductor device of one embodiment of the present
invention.
[0005] FIG. 2 is a cross sectional side view that illustrates
additional regions added to the substrate in some embodiments.
[0006] FIG. 3 is a cross sectional side view that illustrates the
capping layer deposited on the top surface of the gate stack, the
first set of spacers and the substrate.
[0007] FIG. 4 is a cross sectional side view that illustrates a
second set of spacers formed on either side of the gate
electrode.
[0008] FIGS. 5a and 5b are cross sectional side views that
illustrate the formation of source/drain implant regions.
[0009] FIG. 6 is a cross sectional side view that illustrates the
device of FIG. 5a after annealing of the source/drain implanted
regions.
[0010] FIG. 7 is a cross sectional side view that illustrates the
device after removal of portions of the capping layer.
[0011] FIG. 8 is a flow chart that summarizes a method according to
an embodiment of the present invention.
[0012] FIG. 9 illustrates a system in accordance with one
embodiment of the present invention.
DETAILED DESCRIPTION
[0013] In various embodiments, an apparatus and method relating to
the formation of a substrate are described. In the following
description, various embodiments will be described. However, one
skilled in the relevant art will recognize that the various
embodiments may be practiced without one or more of the specific
details, or with other replacement and/or additional methods,
materials, or components. In other instances, well-known
structures, materials, or operations are not shown or described in
detail to avoid obscuring aspects of various embodiments of the
invention. Similarly, for purposes of explanation, specific
numbers, materials, and configurations are set forth in order to
provide a thorough understanding of the invention. Nevertheless,
the invention may be practiced without specific details.
Furthermore, it is understood that the various embodiments shown in
the figures are illustrative representations and are not
necessarily drawn to scale.
[0014] Reference throughout this specification to "one embodiment"
or "an Embodiment" means that a particular feature, structure,
material, or characteristic described in connection with the
embodiment is included in at least one embodiment of the invention,
but do not denote that they are present in every embodiment. Thus,
the appearances of the phrases "in one embodiment" or "in an
embodiment" in various places throughout this specification are not
necessarily referring to the same embodiment of the invention.
Furthermore, the particular features, structures, materials, or
characteristics may be combined in any suitable manner in one or
more embodiments. Various additional layers and/or structures may
be included and/or described features may be omitted in other
embodiments.
[0015] Various operations will be described as multiple discrete
operations in turn, in a manner that is most helpful in
understanding the invention. However, the order of description
should not be construed as to imply that these operations are
necessarily order dependent. In particular, these operations need
not be performed in the order of presentation. Operations described
may be performed in a different order than the described
embodiment. Various additional operations may be performed and/or
described operations may be omitted in additional embodiments.
[0016] FIG. 1 is a cross sectional side view that illustrates the
semiconductor device of one embodiment of the present invention. In
that semiconductor device, a high-k gate dielectric layer 102 may
be formed on substrate 100, and a metal gate electrode 104 may be
formed on the high-k gate dielectric layer 102. In the illustrated
embodiment, there is a conductive gate layer 108, which may
comprise doped polysilicon, on the metal gate electrode 104,
although in other embodiments, the metal gate electrode 104 may
extend higher and the device may lack a doped polysilicon or other
conductive gate layer 108 on the metal gate electrode.
[0017] Substrate 100 may comprise any material that may serve as a
foundation upon which a semiconductor device may be built. In this
embodiment, substrate 100 is a silicon containing substrate. The
substrate 100 may be a bulk substrate 100, such as a wafer of
single crystal silicon, a silicon-on-insulator (SOI) substrate 100,
such as a layer of silicon on a layer of insulating material on
another layer of silicon, or another type of substrate 100. The
device formed on the substrate 100 may be a transistor in some
embodiments. The device may be a planar transistor on a bulk
substrate 100, a planar transistor on an SOI substrate 100, a
FIN-FET transistor on a bulk substrate 100, a FIN-FET transistor on
an SOI substrate 100, a tri-gate transistor on a bulk substrate
100, a tri-gate transistor on an SOI substrate, or another type of
transistor or other device.
[0018] The high-k gate dielectric layer 102 may comprise, for
example, hafnium oxide, hafnium silicon oxide, lanthanum oxide,
lanthanum aluminum oxide, zirconium oxide, zirconium silicon oxide,
titanium oxide, tantalum oxide, barium strontium titanium oxide,
barium titanium oxide, strontium titanium oxide, yttrium oxide,
aluminum oxide, lead scandium tantalum oxide, and lead zinc
niobate. Although a few examples of materials that may be used to
form the high-k gate dielectric layer 102 are described here, the
high-k gate dielectric layer 102 may be made from other materials
that serve to reduce gate leakage in other embodiments.
[0019] In one embodiment of the present invention, high-k gate
dielectric layer 102 may be formed on the substrate 100 by an
atomic layer chemical vapor deposition ("ALCVD") process. In an
ALCVD process, a growth cycle may be repeated until a high-k gate
dielectric layer 102 of a desired thickness is created. Such a
growth cycle may comprise the following sequence in an embodiment.
Steam is introduced into a CVD reactor for a selected pulse time,
followed by a purging gas. A precursor (e.g., an organometallic
compound, a metal chloride or other metal halide) is then pulsed
into the reactor, followed by a second purge pulse. (A carrier gas
that comprises nitrogen or another inert gas may be injected into
the reactor at the same time.)
[0020] While operating the reactor at a selected pressure and
maintaining the substrate at a selected temperature, steam, the
purging gas, and the precursor are, in turn, fed at selected flow
rates into the reactor. By repeating this growth cycle--steam,
purging gas, precursor, and purging gas--multiple times, one may
create a high-k gate dielectric layer 102 of a desired thickness on
the substrate 100. The pressure at which the reactor is operated,
the gases' flow rates, and the temperature at which the substrate
is maintained may be varied depending upon the application and the
precursor that is used. The CVD reactor may be operated long enough
to form the high-k gate dielectric layer 102 with the desired
thickness. In some embodiments, the high-k gate dielectric layer
102 may be less than about 40 angstroms thick. In other
embodiments, the high-k gate dielectric layer 102 may be between
about 5 angstroms and about 20 angstroms thick.
[0021] The high-k gate dielectric layer 102 may have a k-value
higher than about 7.5 in some embodiments. In other embodiments,
the high-k gate dielectric layer 102 may have a k-value higher than
about 10. In other embodiments, the high-k gate dielectric layer
102 may comprise a material such as Al.sub.2O.sub.3 with a k-value
of about 12, or may comprise a material with a higher k-value than
that. In other embodiments, the high-k gate dielectric layer 102
may have a k-value between about 15 and about 25, e.g. HfO.sub.2.
In yet other embodiments, the high-k gate dielectric layer 102 may
have a k-value even higher, such as 35, 80 or even higher.
[0022] After forming the high-k gate dielectric layer 102 on the
substrate 100, the metal gate electrode 104 may be formed on the
high-k gate dielectric layer 102. Metal gate electrode 104 may be
formed using conventional metal deposition processes, e.g. CVD or
PVD processes, by using ALCVD, or another suitable method, and may
comprise any conductive material from which metal gate electrodes
may be derived. Materials that may be used to form n-type metal
gate electrodes include: hafnium, zirconium, titanium, tantalum,
aluminum, their alloys (e.g., metal carbides that include these
elements, i.e., hafnium carbide, zirconium carbide, titanium
carbide, tantalum carbide, and aluminum carbide), and aluminides
(e.g., an aluminide that comprises hafnium, zirconium, titanium,
tantalum, or tungsten). Materials for forming p-type metal gate
electrodes include: ruthenium, palladium, platinum, cobalt, nickel,
and conductive metal oxides, e.g., ruthenium oxide. Alternatively,
a mid-gap metal gate material, e.g. stoichiometric titanium nitride
or tantalum nitride, may be used in some embodiments.
[0023] In some embodiments, metal NMOS gate electrodes may have a
workfunction that is between about 3.9 eV and about 4.2 eV. In some
embodiments, metal PMOS gate electrodes may have a workfunction
that is between about 4.9 eV and about 5.2 eV. A metal gate
electrode 104 that is formed on a high-k gate dielectric layer 102
may consist essentially of a homogeneous metal layer.
Alternatively, relatively thin n-type or p-type metal layers (like
those listed above) may generate the lower part of the metal gate
electrode, with the remainder of the metal gate electrode
comprising another metal or metals, e.g., a metal that may be
easily polished like tungsten, aluminum, titanium, or titanium
nitride. Although a few examples of materials for forming a metal
gate electrode are identified here, such a component may be made
from many other materials, as will be apparent to those skilled in
the art.
[0024] Additionally, while device may be an NMOS or PMOS device,
other types of devices may be made within the scope of the present
invention as well. For example, a silicon on insulator (SOI) or
other type of device may be made with mid-gap gate electrode
materials, e.g. stoichiometric titanium nitride or tantalum
nitride, among other materials, rather than NMOS or PMOS gate
electrode materials. In some embodiments, the material of the
mid-gap gate electrode 104 may have a workfunction between the
workfunctions of NMOS and PMOS gate electrode materials
[0025] There may be a set of first spacers 106 formed on either
side of the gate electrode 104, high-k gate dielectric layer 102,
and conductive gate layer 108. The spacers 106 may be formed of a
material that is substantially free of oxygen in some embodiments.
For example, in an embodiment the set of first spacers 106 may
comprise a carbon doped nitride, with 8-12% carbon and silicon
nitride. In other embodiments, the set of first spacers 106 may
comprise other materials.
[0026] In an embodiment, the device may be a transistor. There may
be a thin oxide layer 110 on the substrate 100 beneath the gate
stack 102, 104, 108 and first set of spacers 106 in some
embodiments. This thin oxide layer 110 may be as thin as a
monolayer of oxide in some embodiments. The thin oxide layer 110
may provide a potential path for oxygen to travel from an outer
edge of the first set of spacers 106 furthest from the gate stack
102, 104, 108 to a region beneath the gate stack 102, 104, 108. If
oxygen reaches that region it may react with the substrate 100 to
form an unwanted thicker oxide beneath the gate stack 102, 104, 108
and reduce the performance of the transistor.
[0027] FIG. 2 is a cross sectional side view that illustrates
additional regions 202 added to the substrate 100 in some
embodiments. The additional regions 202 may be added in some
embodiments but omitted in other embodiments. For example, when the
device is a Fin-FET transistor or tri-gate transistor, there may be
only a small amount of substrate on either side of the gate stack
102, 104, 108. In such an embodiment, it may be beneficial to add
material to the substrate 100 by forming the additional regions
202. In some embodiments, the additional regions 202 may be added
by epitaxy. In an embodiment, the additional regions 202 may
comprise the same material as the original substrate 100. The
additional regions 202 may be considered portions of the substrate
100 after formation of the additional regions 202. As shown in FIG.
2, the additional regions 202 may have a height above the original
substrate 100 a distance away from the first set of spacers 106,
but the thickness of the additional regions 202 may decrease closer
to the first set of spacers 106. In some embodiments, such as when
the device is a planar transistor, the additional regions 202 may
be omitted.
[0028] FIG. 3 is a cross sectional side view that illustrates
capping layer 302 deposited on the top surface of the gate stack
102, 104, 108, the first set of spacers 106 and the substrate,
according to one embodiment of the present invention. In an
embodiment, the capping layer 302 may be an oxygen barrier layer
302 that at least partially, if not completely, prevents oxygen
from reaching a region 304 beneath the gate stack 102, 104, 108.
The capping layer 302 may seal the thin oxide layer 110 from
oxygen-containing structures and/or ambient oxygen in further
process steps, so may prevent the transport of oxygen by the thin
oxide layer 110 into the region 304 beneath the gate stack 102,
104, 108. This prevention of oxygen transport beneath the gate
stack 102, 104, 108 may at least partially prevent oxidation of the
substrate 100, which could result in the formation of undesired
oxide, such as silicon oxide, beneath the gate stack 102, 104, 108.
Such an undesired oxide could be of a thickness enough to degrade
the performance of the device if its formation is not prevented. In
an embodiment, the capping layer 302 may be less than about 75
angstroms thick. In another embodiment, the capping layer 302 may
be about 50 angstroms thick or less. In an embodiment, the capping
or oxygen barrier layer 302 may comprise a nitride material, such
as a carbon doped nitride, a stoichiometric silicon nitride
deposited in a low O.sub.2-push fashion or a silicon carbide,
although other materials may be used in other embodiments. The
capping layer 302 may be substantially free from oxygen, so as not
to serve as a source of oxygen that could be transported by the
thin oxide layer to the region 304 beneath the gate stack 102, 104,
108. In an embodiment, the capping layer 302 may be a conformal
layer that covers the exposed surfaces of the device. In an
embodiment, the capping or oxygen barrier layer 302 may be
deposited by chemical or physical vapor deposition, although atomic
layer deposition or other methods may be used as appropriate. The
formation of the capping layer 302 may be performed in an ambient
atmosphere with little or no oxygen.
[0029] FIG. 4 is a cross sectional side view that illustrates a
second set of spacers 402 formed on either side of the gate
electrode 104, according to one embodiment. In an embodiment, the
second set of spacers 402 may be formed by depositing a thick layer
of material over the device, then etching portions of the layer
away to form the second set of spacers 402. In an embodiment, the
second set of spacers 402 may comprise an oxygen-containing
material such as a Bis(tert-butylamino)silane-based silicon oxide,
a silicon oxynitride, or another material, depending on the need to
subsequently recess said second spacers following the self-aligned
source/drain implant described below. As the capping layer 302 may
seal the thin layer of oxide 110 away from the layer of material
used to make the second set of spacers 402, the material used to
make the second set of spacers 402 may contain oxygen; the capping
layer 302 may prevent transport of oxygen from the material of the
second set of spacers 402 to the region 304 beneath the gate stack
102, 104, 108, and therefore prevent formation of a thicker oxide
layer 110.
[0030] FIG. 5a is a cross sectional side view that illustrates the
formation of source/drain implant regions 504 by implantation of
ions 502, as is known in the art. In the illustrated embodiment,
the ions 502 are implanted into the substrate 100 to form the
source/drain implanted regions 504 through the capping layer 302 on
the substrate 100.
[0031] FIG. 5b is a cross sectional side view that illustrates
another embodiment of the formation of source/drain implant regions
504 by implantation of ions 502, as is known in the art. In the
illustrated embodiment, portions of the capping layer 302 beyond
the second set of spacers 402 have been removed prior to ion
implantation. The ions 502 are implanted into the substrate 100 to
form the source/drain implanted regions 504 without being implanted
through the capping layer 302 on the substrate 100. The remaining
portions of the capping layer 302 may be sufficient to seal the
thin oxide layer 110 from oxygen present in structures of the
device and present in the ambient atmosphere during further
processing. Thus, as shown in FIGS. 5a and 5b, portions of the
capping layer 302 that do not function to seal the thin oxide layer
from sources of oxygen may be removed at various times during
formation of the device in various embodiments.
[0032] FIG. 6 is a cross sectional side view that illustrates the
device of FIG. 5a after annealing of the source/drain implanted
regions 504 have been performed to form source and drain regions in
the substrate 100, as is known in the art. The annealing process
may be a high temperature annealing process. During the annealing
process, the capping layer 302 may help prevent formation of an
oxide under the gate electrode 104. Absent the capping layer 302,
the high temperature of the annealing process may cause rapid
formation of a thick layer of oxide beneath the gate stack 102,
104, 108, reducing the performance of the device.
[0033] FIG. 7 is a cross sectional side view that illustrates the
device after removal of portions of the capping layer 302 that are
exposed and not covered by the second set of spacers 402. The
removal may be done by a wet etching process in one embodiment,
although any suitable process may be used to remove the exposed
portions of the capping layer 302. As stated above, this removal of
portions of the capping layer 302 may be done at other times during
processing of the device rather than following anneal. Following
anneal and removal of portions of the capping layer 302, additional
steps such as silicidation may be performed to finish fabricating
the device.
[0034] FIG. 8 is a flow chart 800 that summarizes a method
according to an embodiment of the present invention. A gate stack
of a device, such as gate stack 102, 104, 108 shown in FIG. 1, may
be formed 802. A capping layer, such as capping layer 302 of FIG.
3, may be formed 804, sealing a region under the gate stack from
oxygen. For example, region 304 of FIG. 3 is sealed by capping
layer 302 to prevent oxygen from being transported by the thin
oxide layer 110 under the gate stack 102, 104, 108 and forming a
thick oxide layer that would degrade device performance. Processes
at high temperature or processes that involve oxygen (as part of an
ambient atmosphere or part of the material of a structure) may then
be performed 806. Since the under gate region is sealed by the
capping layer 302, oxygen may be mostly or entirely prevented from
being transported to the under gate region during these processes.
Thus, reaction of oxygen with the substrate beneath the gate and
formation of a thick oxide layer beneath the gate may be avoided,
which may prevent degradation of the performance of the device.
[0035] FIG. 9 illustrates a system 900 in accordance with one
embodiment of the present invention. One or more devices formed
with the capping layer 302 as described above may be included in
the system 900 of FIG. 9. As illustrated, for the embodiment,
system 900 includes a computing device 902 for processing data.
Computing device 902 may include a motherboard 904. Coupled to or
part of the motherboard 904 may be in particular a processor 906,
and a networking interface 908 coupled to a bus 910. A chipset may
form part or all of the bus 910. The processor 906, chipset, and/or
other parts of the system 900 may include one or more devices with
the capping layer 302.
[0036] Depending on the applications, system 900 may include other
components, including but are not limited to volatile and
non-volatile memory 912, a graphics processor (integrated with the
motherboard 904 or connected to the motherboard as a separate
removable component such as an AGP or PCI-E graphics processor), a
digital signal processor, a crypto processor, mass storage 914
(such as hard disk, compact disk (CD), digital versatile disk (DVD)
and so forth), input and/or output devices 916, and so forth.
[0037] In various embodiments, system 900 may be a personal digital
assistant (PDA), a mobile phone, a tablet computing device, a
laptop computing device, a desktop computing device, a set-top box,
an entertainment control unit, a digital camera, a digital video
recorder, a CD player, a DVD player, or other digital device of the
like.
[0038] Any of one or more of the components 906, 914, etc. in FIG.
9 may include one or more devices with the capping layer 302 as
described herein. For example, a transistor formed with the capping
layer 302 may be part of the CPU 906, motherboard 904, graphics
processor, digital signal processor, or other devices.
[0039] In an embodiment, the device may be a semiconductor device
including a substrate, a thin oxide layer on the substrate, a
high-k gate dielectric layer on the thin oxide layer, a metal gate
electrode on the high-k gate dielectric layer, and a capping layer
that is substantially free of oxygen and substantially seals the
thin oxide layer from structures that comprise oxygen. The device
may also have a first set of spacers on either side of the metal
gate electrode and a second set of spacers on either side of the
first set of spacers, wherein the capping layer is between the
first set of spacers and the second set of spacers. The second set
of spacers may have a bottom surface and the capping layer may
extend beneath the bottom surface of the second set of spacers. The
first set of spacers may have a bottom surface and the thin oxide
layer may extend beneath the bottom surface of the first set of
spacers. The capping layer may be on the sides of the first set of
spacers and may be on the substrate extending away from the sides
of the first set of spacers for a distance.
[0040] The foregoing description of the embodiments of the
invention has been presented for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise forms disclosed. This description and the
claims following include terms, such as left, right, top, bottom,
over, under, upper, lower, first, second, etc. that are used for
descriptive purposes only and are not to be construed as limiting.
For example, terms designating relative vertical position refer to
a situation where a device side (or active surface) of a substrate
or integrated circuit is the "top" surface of that substrate; the
substrate may actually be in any orientation so that a "top" side
of a substrate may be lower than the "bottom" side in a standard
terrestrial frame of reference and still fall within the meaning of
the term "top." The term "on" as used herein (including in the
claims) does not indicate that a first layer "on" a second layer is
directly on and in immediate contact with the second layer unless
such is specifically stated; there may be a third layer or other
structure between the first layer and the second layer on the first
layer. The embodiments of a device or article described herein can
be manufactured, used, or shipped in a number of positions and
orientations. Persons skilled in the relevant art can appreciate
that many modifications and variations are possible in light of the
above teaching. Persons skilled in the art will recognize various
equivalent combinations and substitutions for various components
shown in the Figures. It is therefore intended that the scope of
the invention be limited not by this detailed description, but
rather by the claims appended hereto.
* * * * *