U.S. patent application number 14/380987 was filed with the patent office on 2015-01-15 for self-aligned tunable metamaterials.
The applicant listed for this patent is Arturo A. Ayon, Ramakrishna Kotha, Diana Strickland. Invention is credited to Arturo A. Ayon, Ramakrishna Kotha, Diana Strickland.
Application Number | 20150017466 14/380987 |
Document ID | / |
Family ID | 49117430 |
Filed Date | 2015-01-15 |
United States Patent
Application |
20150017466 |
Kind Code |
A1 |
Ayon; Arturo A. ; et
al. |
January 15, 2015 |
SELF-ALIGNED TUNABLE METAMATERIALS
Abstract
A self-aligned tunable metamaterial is formed as a wire mesh.
Self-aligned channel grids are formed in layers in a silicon
substrate using deep trench formation and a high-temperature
anneal. Vertical wells at the channels may also be etched. This may
result in a three-dimensional mesh grid of metal and other
material. In another embodiment, metallic beads are deposited at
each intersection of the mesh grid, the grid is encased in a rigid
medium, and the mesh grid is removed to form an artificial
nanocrystal.
Inventors: |
Ayon; Arturo A.; (San
Antonio, TX) ; Kotha; Ramakrishna; (Austin, TX)
; Strickland; Diana; (San Antonio, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ayon; Arturo A.
Kotha; Ramakrishna
Strickland; Diana |
San Antonio
Austin
San Antonio |
TX
TX
TX |
US
US
US |
|
|
Family ID: |
49117430 |
Appl. No.: |
14/380987 |
Filed: |
March 11, 2013 |
PCT Filed: |
March 11, 2013 |
PCT NO: |
PCT/US2013/030268 |
371 Date: |
August 26, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61609109 |
Mar 9, 2012 |
|
|
|
Current U.S.
Class: |
428/596 ;
117/105; 117/106; 117/56; 117/58; 204/192.1; 205/50; 205/76;
428/137 |
Current CPC
Class: |
B32B 15/018 20130101;
C30B 29/66 20130101; C30B 29/58 20130101; C30B 23/02 20130101; H01Q
1/24 20130101; C09K 13/00 20130101; C30B 19/06 20130101; H01L
21/3065 20130101; C30B 29/52 20130101; C30B 33/12 20130101; C30B
29/607 20130101; Y10T 428/12361 20150115; C30B 29/68 20130101; C30B
19/103 20130101; B82Y 40/00 20130101; C30B 23/025 20130101; C30B
19/12 20130101; H01Q 15/0086 20130101; C30B 29/06 20130101; C30B
30/02 20130101; H01P 11/00 20130101; C23F 1/02 20130101; C23F 1/20
20130101; B82Y 20/00 20130101; H01L 21/3083 20130101; C30B 29/10
20130101; C30B 33/10 20130101; B32B 15/017 20130101; H01Q 3/01
20130101; Y10T 428/24322 20150115; C23F 1/44 20130101; C30B 23/08
20130101 |
Class at
Publication: |
428/596 ; 205/76;
205/50; 117/106; 117/58; 204/192.1; 117/105; 117/56; 428/137 |
International
Class: |
C30B 19/12 20060101
C30B019/12; C30B 23/02 20060101 C30B023/02; C30B 19/06 20060101
C30B019/06; C30B 30/02 20060101 C30B030/02; C30B 29/66 20060101
C30B029/66; C30B 29/68 20060101 C30B029/68; C30B 29/58 20060101
C30B029/58; C30B 29/52 20060101 C30B029/52; C30B 19/10 20060101
C30B019/10; C30B 23/08 20060101 C30B023/08 |
Claims
1. A method of manufacturing a metamaterial comprising: etching a
two-dimensional horizontal grid pattern into a substrate block, the
two-dimensional grid pattern and depth of the etch being selected
for a desired metamaterial property; annealing the substrate to
form a plurality of channel grids, each channel grid being
characterized by the two-dimensional grid pattern and each channel
grid being substantially identical to and vertically aligned with
each other channel grid; etching a plurality of vertical wells in
the substrate block, each vertical well substantially orthogonally
intersecting at least two channel grids; and depositing a first
material in the channel grids to form a multi-layer wire mesh.
2. The method of claim 1 further comprising depositing the first
material in the vertical wells.
3. The method of claim 1 further comprising depositing a second
material in the vertical wells.
4. The method of claim 1 further comprising removing the substrate
block.
5. The method of claim 1 wherein depositing the first material
comprises depositing a metal on the edges of the channels whereby
hollow metallic tubes are formed.
6. The method of claim 1 wherein depositing the first material
comprises filling the channels with the first material whereby
solid wires are formed.
7. The method of claim 1 wherein the grid pattern is
non-rectilinear.
8. A wire mesh metamaterial constructed according to the process of
claim 1.
9. The method of claim 1 wherein the operation of depositing a
first material in the channel grids precedes the step of etching a
plurality of vertical wells and further comprising: depositing a
second material on a plurality of intersecting lines of the first
material exposed by the vertical wells; depositing a third material
in the vertical wells; removing the substrate block; encasing the
multi-layer wire mesh in a rigid medium; removing the first
material; and back-filling voids in the rigid medium left by the
removal of the first material.
10. An artificial nanocrystal constructed according to the method
of claim 9.
11. The method of claim 9 wherein depositing the second material
comprises electrochemical deposition.
12. The method of claim 9 wherein the first material and the third
material are the same.
13. The method of claim 9 wherein the first material is aluminum
and the second material is gold.
14. The method of claim 9 wherein etching the vertical wells
comprises etching with a fluorechemical process.
15. An artificial self-aligned nanocrystal comprising a plurality
of nanoparticles suspended in a rigid substrate, the nanoparticles
arranged to impart to the nanocrystal a characteristic resonant
frequency.
16. The artificial nanocrystal of claim 15 wherein the
nanoparticles are gold beads.
17. The artificial nanocrystal of claim 15 wherein the rigid
substrate is a polymer.
18. A method of manufacturing an artificial nanocrystal comprising:
deeply etching a two-dimensional horizontal grid pattern into a
substrate block, the two-dimensional grid pattern and depth of the
etch being compatible with an array of beads having a programmed
resonant frequency; annealing the substrate to form a plurality of
channel grids, each channel grid being characterized by the
two-dimensional grid pattern and each channel grid being
substantially identical to and vertically aligned with each other
channel grid; metallizing the channel grids to form a plurality of
two-dimensional wire grids, each wire grid having a plurality of
intersections; etching a plurality of vertical wells in the
substrate block to expose the intersections, the vertical wells
being etched orthogonal to the wire grid and each vertical well
being etched at a depth selected to expose at least two
intersections; depositing a conductive metal bead on each exposed
intersection; filling the vertical wells with a rigid structural
medium; and chemically removing the two-dimensional wire grids.
19. The method of claim 18 further comprising: chemically removing
the substrate block to leave a three-dimensional mesh structure;
encasing the three-dimensional mesh structure in the rigid
structural medium;
20. The method of claim 18 further comprising filling voids left by
removal of the wire grid with a rigid structural medium.
21. The method of claim 18 wherein the two-dimensional grid pattern
and the depth of the vertical wells are selected to place the metal
beads substantially equidistant from each other throughout the
artificial nanocrystal.
22. An artificial nanocrystal manufactured according to the method
of claim 18.
23. A method of manufacturing a metamaterial comprising: etching a
two-dimensional horizontal grid pattern into a substrate block, the
two-dimensional grid pattern and depth of the etch being selected
for a desired metamaterial property; annealing the substrate to
form a channel grid, the channel grid being characterized by the
two-dimensional grid pattern; and depositing a material in the
channel grid to form a single-layer wire mesh.
Description
BACKGROUND
[0001] 1. Field of the Disclosure
[0002] This disclosure relates to the field of tunable
metamaterials, and more particularly to self-aligned arrays of
nanomaterials.
[0003] 2. Description of the Related Art
[0004] Metamaterials are synthetic, composite materials with
periodic structures, known for their ability to create
electromagnetic or acoustic properties that are not found in nature
and that may determine how the material interacts with various
types of radiation. Metamaterials may direct radiation either due
to the external shape of a metamaterial structure or by spatially
indexing the metamaterial. Conventional methods of forming
metamaterial periodic and spatially indexed arrays in the nanoscale
range, such as ion beam methods, are limited by processing options,
materials, and ultimately, economic feasibility, and may be poorly
suited to gain widespread industrial applicability. Therefore,
there is a need in the art for an industrial method of producing
desired tunable nanowire arrays in sufficient quantities and at
relatively low cost.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a prior art representation of an
empty-space-in-silicon process;
[0006] FIG. 2A is a block diagram of selected elements of an
embodiment of a structure for forming a self-aligned
metamaterial;
[0007] FIG. 2B is a block diagram of selected elements of an
embodiment of a structure for forming a self-aligned
metamaterial;
[0008] FIG. 2C is a block diagram of selected elements of an
embodiment of a structure for forming a self-aligned
metamaterial;
[0009] FIG. 3 is a block diagram of selected elements of an
embodiment of a self-aligned metamaterial;
[0010] FIG. 4 is a flow chart of an exemplary method of producing a
self-aligned metamaterial; and
[0011] FIG. 5 is a perspective view of an artificial
nanocrystal.
DESCRIPTION OF THE EMBODIMENT(S)
[0012] Various types of metamaterials are known that possess bulk
electromagnetic properties different from materials observed in
nature. These properties may create specific dispersion
characteristics within the metamaterial, or they may control the
way the metamaterial reflects, refracts, absorbs, scatters or
transmits radiation.
[0013] Metamaterials are also known that direct electromagnetic
radiation. The ability to direct radiation can result from an outer
form or shape of a given material, for example, as in a
conventional lens. Another way to control the path of radiation can
result from the internal structure of a material. Spatial indexing,
as used herein, refers to a patterned structure of a material that
enables tuning of the electromagnetic properties of the material in
space. For example, the geometry of an array structure and/or the
constituent material composition may be varied in space. In one
embodiment, a smooth increase of an array periodicity in a given
direction may result in a gradual alteration of a metamaterial's
permittivity in the given direction. Such spatially indexed
properties of a metamaterial may be realized in 2-D and/or 3-D and
may encompass tailoring the metamaterial for different kinds of
properties and/or combinations of properties varying in space, as
desired. One example of a spatially indexed metamaterial may
exhibit a smooth increase in array periodicity in a single given
direction. In other examples, the smooth increase in array
periodicity may be present in two or three directions.
[0014] Control of bulk properties may be accomplished by
positioning particles in periodic or non-periodic arrays with
dimensions much smaller than an operational wavelength of
electromagnetic radiation. The shape, placement and/or orientation
of the particles, as well as constituent materials of the particles
and/or a host material in which the particles reside, may determine
the bulk properties of the array structure. The bulk properties may
be tuned for a desired interaction in a particular range of
wavelengths (or frequencies) of electromagnetic radiation. For
example, for long wave infrared frequencies and higher, the array
structure may be formed in nanoscale dimensions. Below infrared
frequencies, the array structure may also be of nanoscale
dimensions. The array structures may be formed as grids, meshes, or
crystal-like lattices in nanoscale dimensions that may be 2-D
and/or 3-D in scope.
[0015] The novel and non-obvious fabrication processes, as
described in further detail herein, provide various means for
forming the array structure, as well as placing particles composed
of desired constituent materials at desired locations in the array
structure. Various dimensions, features, and compositions may be
employed for tuning the array structure to interact with radiation
of a desired frequency and/or frequency range(s). The control of
bulk properties realized using the methods described herein may
enable applications such as absorbers, waveguides, sensors,
reflectors, phase control devices, radiation concentrators,
cloaking devices, imaging devices, and electromagnetic pulse
protectors, among others.
[0016] One example structure for realizing a spatially indexed
metamaterial is an array structure, such as a grid, mesh, or
lattice, that may be implemented as a 3-D framework. Such an array
structure may be realized in one embodiment using nanowires to form
the framework. The tunability of such a 3-D nanowire array may be
achieved by modulating dimensional properties, such as wire
thickness, array spacing, etc., as well as through selection of the
material(s) for the nanowire array. Voids and/or empty space may
also be used to tune the properties of the metamaterial, as will be
discussed in further detail below.
[0017] While conventional semiconductor processing methods may be
useful to produce array structures on a 2-D surface of a flat
substrate, such methods may be excessively costly and laborious in
the nanoscale range. Furthermore, many conventional processing
methods do not generally scale up to a 3-D approach. Thus,
producing 2-D and/or 3-D nanoscale array structures with specific
tunable properties using conventional methods, for example by
forming layer upon layer in a successive manner, may not be
economically feasible and may be unsuitable for widespread
industrial exploitation. As will be described herein, a novel and
patentably distinct method for forming 2-D and 3-D nanoscaled
arrays is disclosed that relies on self-alignment through inherent
thermodynamic properties of a silicon substrate.
[0018] In the following description, details are set forth by way
of example to facilitate discussion of the disclosed subject
matter. It should be apparent to a person of ordinary skill in the
field, however, that the disclosed embodiments are exemplary and
not exhaustive of all possible embodiments.
[0019] Throughout this disclosure, a hyphenated form of a reference
numeral refers to a specific instance of an element and the
un-hyphenated form of the reference numeral refers to the element
generically or collectively. Thus, for example, widget 12-1 refers
to an instance of a widget class, which may be referred to
collectively as widgets 12 and any one of which may be referred to
generically as a widget 12.
[0020] Turning now to the figures, FIG. 1, is a prior art
representation of empty-space-in-silicon process 100, sometimes
also known as silicon-on-nothing [see T. Sato, et al., Electrochem.
Soc. Proc. 539, 2000-17 (2000)]. The process 100 is an example of a
method of producing self-aligned structures in a silicon substrate.
In operation 101, deep reactive etching (also known as DRIE) is
performed on an etched single crystal silicon substrate to produce
a deep channel of desired dimensions (i.e., width and depth). In
operation 102, several intermediate states of a single annealing
step are shown that may result in self-organization (i.e.,
self-alignment) of empty bubbles in the bulk silicon that become
buried upon healing of the top silicon surface. The annealing may
be performed under hydrogen at relatively high temperatures (e.g.,
at about 1100.degree. C.) and at ambient pressures of about 10
Torr. Such processing conditions promote silicon migration and
formation of voids or channels, depending upon the etch
geometry.
[0021] Referring now to FIG. 2A, a block diagram of selected
elements of an embodiment of structure 200-1 for forming a
self-aligned metamaterial is shown. Structure 200-1 is shown as a
cut-away view and may be representative for structures of various
dimensions and/or may represent a repeating element in a larger
super-structure (not shown). Structure 200-1 shows a pattern formed
in photoresist 202 at the surface of a crystalline silicon
substrate 204 that has been etched to produce deep channels 206 of
a desired geometry. The dimensions of the geometry, including width
208 and depth 210 may be determined at this step for tuning final
dimensions of a 3-D nanowire array, as will be subsequently
demonstrated. For example, width 208 may be determined by
patterning photoresist 202 accordingly, while depth 210 may be
determined by process etch parameters. A number of deep channels
206 as well as spacing between deep channels 206 may also be
determined by patterning photoresist 202. In this manner, tuning of
a 3-D nanowire array may be performed using 2-D patterning and
etching techniques.
[0022] FIG. 2A is a block diagram of a prior-art embodiment of
structure 200-2 for forming a self-aligned metamaterial. Structure
200-2 is similar to structure 200-1 (see FIG. 2A) but is shown
after an annealing step that has transformed deep channels 206 into
channel grids 216, which are self-aligned along the original mask
pattern in photoresist 202. The annealing step may be varied to
control a number and arrangement of linear channels 212, in various
embodiments. It is noted that in certain embodiments, linear
channels 212 may be formed in a single horizontal layer (not shown)
buried beneath a surface of structure 200-2, which may result in a
2-D array structure (not shown). It is further noted that the
dimensions of linear channels 212 also correspond to those of deep
channels 206 and of the mask pattern, accordingly. In some
embodiments, crystalline silicon substrate 222 itself may be used
as a metamaterial with anisotropic bulk permittivity that may have
a value of one (1) or a low value. In various embodiments,
crystalline silicon substrate 222 may exhibit a spatially indexed
permittivity that may vary in a regular or irregular manner in
space.
[0023] FIG. 2C is a block diagram of selected elements of an
embodiment of structure 200-3 for forming a self-aligned
metamaterial is shown. Structure 200-3 is similar to structure
200-2 (see FIG. 2B) but is shown with an orthogonal pattern of
channels forming a grid pattern in photoresist 214, which has
resulted in a stack of channel grids 216. In certain embodiments,
substrate 218 having channel grids 216 may serve as a metamaterial
with desired bulk properties and/or as a spatially indexed
metamaterial.
[0024] Also formed in structure 200-3 are vertical wells 220, which
may also be formed using a patterning/etch technique or another
method to remove material at a particular location. Vertical wells
220 may penetrate substrate 218 vertically to a desired depth
(obscured from view in structure 200-3) to intersect one or more
channel grids 216. Vertical channels (not shown) may also be formed
that do not intersect channel grids 216. Either with or without
intersections, substrate 218 may be used in various embodiments to
form metamaterials with isotropic and/or anisotropic bulk
permittivity at may have a value of one (1) or a low value. In
different embodiments, substrate 218 may exhibit a spatially
indexed permittivity that may vary in a regular or irregular manner
in space. As shown in FIG. 2C, vertical wells 220 are open at a top
surface of substrate 218, which may extend empty space within
channel grids 216 in 3-D. In certain embodiments, vertical wells
220 may provide a route for subsequent deposition steps that fill
the array. In other embodiments, vertical wells are formed in a
subsequent processing step after channel grids 216 have been formed
and/or filled with another material, as will be described below in
more in detail. In yet other embodiments, vertical wells 220 may be
etched at angles other than normal to channel grids 216, for
example by rotating substrate block 218. This process may be used,
for example, to create a Bragg diffraction grating.
[0025] After channel grids 216 and/or vertical wells 220 have been
formed, substrate 218 may be sectioned vertically to reveal ends of
channel grids 216, exposing an inner surface of channel grids 216
to an external atmosphere. In this state, structure 200-3 may be
subject to a deposition process (e.g., metallic vaporization,
evaporation, sputtering, electroless deposition, and/or
electroplating) to form a solid structure within channel grids 216
and/or vertical wells 220. Different materials may be deposited
within channel grids 216 from those deposited within vertical wells
220 to form a nanoscale composite material. In certain embodiments,
a partial deposition may result in a tubular (i.e., hollow) lattice
structure being deposited within channel grids 216 and/or vertical
wells 220. In one embodiment, a relatively thin deposited metal
layer creates a material with permittivity tailored in three
dimensions that is substantially zero or near zero (also referred
to as epsilon near zero or ENZ). In other instances, a full
deposition may result in a solid structure being formed in an
interior volume defined within channel grids 216 and/or vertical
wells 220.
[0026] Mask 202 is depicted in structure 200 with lines removed for
etching linear trenches. In other embodiments (not shown), the mask
pattern and subsequent trenches may be curved. One specific
embodiment may include curved patterns to form broken or split
rings that allow magnetic and/or magnetoelectric structures to be
formed. Arrays of these structures formed as planes of broken
toroidal voids and subsequently metallized may represent omega
structures (e.g., omega-particle metamaterials), which may display
negative permittivity and/or magnetoelectric properties. In
combination with metallized vertical channels, such omega
structures may exhibit negative index of refraction. Other magnetic
structures, such as so-called Swiss rolls and/or oriented helix
arrays resulting from tapered walls on deep channels 206 may also
be formed in various embodiments. Such magnetic and/or
magnetoelectric structures may be used to tune the permeability or
magnetoelectric bulk or spatially indexed parameters of the
metamaterial.
[0027] Turning now to FIG. 3, a block diagram of selected elements
of an embodiment of self-aligned metamaterial 300 is shown. The
bulk or spatially indexed properties of self-aligned metamaterial
300 may be achieved by virtue of the dimensions and arrangement of
an array structure. In certain embodiments, self-aligned
metamaterial 300 is a nanowire array formed by depositing a desired
material, as described previously, within channel grids 216 and/or
vertical wells 220 (see FIG. 2C). Accordingly, vertical elements
320 may correspond to vertical wells 220, while horizontal elements
316 may correspond to channel grids 216. Prior to metallization,
various process steps may be undertaken, such as additional etching
and/or surface treatments. After metallization, the silicon
substrate matrix may be etched away or otherwise removed to result
in self-aligned metamaterial 300 as shown in FIG. 3. In certain
embodiments, a subsequent deposition step may be performed to add a
matrix, a surface layer, and/or a desired coating to the structure
depicted in FIG. 3. For example, a ferroelectric coating may
provide dynamic tunability to self-aligned metamaterial 300.
[0028] Vertical elements 320 may be formed from the same material
as horizontal elements 316 and may be selected to provide
structural support and rigidity to self-aligned metamaterial 300.
In given embodiments (not shown), vertical elements 320 may be
formed at every intersection of horizontal elements 316. Since
vertical elements 320 may be formed by different processing
operations than horizontal elements 316, it is noted that vertical
elements 320 may also be formed from a different material than
horizontal elements 316 for a variety of purposes and applications.
For example, vertical elements 320 may be formed as beads or
connectors that are insulators (such as SU8) or semiconductors, for
example when horizontal elements 316 are conductors and may result
in a semiconductor device formed within self-aligned metamaterial
300. Vertical elements 320 may be flexible to provide desired
elasticity or resonance, for example, when horizontal elements 316
are relatively stiff. Vertical elements 320 may also be made of
metals, dielectrics, bi-metallics, ferroelectrics, and/or
ferromagnetics in order to alter bulk properties. In particular
embodiments, vertical elements 320 are formed to achieve
deformable, reconfigurable, and/or dynamically controllable mobile
structures. In other embodiments, active and/or dynamic materials
may be formed as 2-D or 3-D arrays of antennas driven through
vertical elements 320 that are metallized. Individual elements in
such arrays may be individually driven (or driven in groups) to
create shaped radiation transmission/reception patterns, or may be
phased to create steerable radiation beams. In other embodiments,
horizontal elements 316 may be formed at least in part as
ferroelectric dots that provide controllable features. For example,
the ferroelectric dots may be individually voltage biased (or
biased in groups) to form arrays that are dynamically tunable and
may be spatially indexed along one or more directions or axes. In
certain embodiments, horizontal elements 316 may be formed at least
in part as magnetic dots and may form a material with permanent
magnetization that is also tunable in one or more directions. When
vertical elements 220 are etched at certain angles with respect to
a substrate surface and subsequently metallized, a material with a
very high index of refraction and special dispersion properties may
be formed. Self-aligned metamaterial 300 may allow for economical
and industrial scale production of a wide variety of novel 3-D
array structures that have previously been inaccessible.
[0029] FIG. 4 is a flow chart of an exemplary method of
manufacturing a metamaterial according to the disclosure of the
present disclosure. Elements depicted in method FIG. 4 may be
omitted or rearranged, as desired in different embodiments. FIG. 4
provides an illustrative and non-limiting example of formation of
self-aligned metamaterials, as previously discussed herein.
[0030] Manufacturing begins with a masked substrate block 200, with
mask 202. In step 410, a two-dimensional grid pattern is laid out
on the mask. The grid pattern, including the length, width, and
intersections of trace liens, is selected for a desired
metamaterial property, such as a specific desired permittivity or
resonant frequency. In some embodiments, the grid pattern is
selected to be periodic. Further in some embodiments, the grid
pattern may be a substantially rectilinear grid, as shown in FIG.
2C. In other embodiments, curved or other non-rectilinear grid
patterns may be used, including wavy lines, circles, magnetic field
lines, spirals, or randomized lines. In some embodiments,
non-rectilinear patterns may be used to create artificial
magnets.
[0031] The patter of step 410 may involve various geometries and
designs, and is not limited to the linear examples presented herein
for descriptive clarity (see FIGS. 2A-C, 3). Various types of
patterning and etch processes may be employed to create deep
channels in the silicon substrate. The pattern features and
dimensions may be selected for a desired configuration of the
self-aligned metamaterial, as mentioned previously.
[0032] Once the two-dimensional grid pattern is laid out, in step
420 block 200 is etched using for example deep reactive etching.
The etching depth is selected for a desired metamaterial property,
and in particular may be selected to provide a desired number of
grid layers. In an exemplary embodiment, deep reactive etching is
carried out to a uniform depth throughout the two-dimensional grid
pattern. However, those having skill in the art will recognize that
in some embodiments, the etching depth can be selectively varied to
create non-uniform channel depths, which may control desired
properties of a metamaterial.
[0033] In step 430, substrate block 200 is annealed to create a
plurality of layered channel grids, each layer following the
pattern of the two-dimensional grid pattern. The annealing process
is known in the prior art, and may involve hydrogen annealing at
relatively high temperature. Annealing creates bubbles that "heal"
in layers into the desired pattern. Each channel grid thus formed
is analogous to a two-dimensional network of tunnels, and each
occupies a single vertical position in the substrate block. In an
exemplary embodiment, each channel grid is substantially identical
to each other channel grid because the deep reactive etch of step
420 is performed to a uniform depth throughout the block. The
channel grids collectively are all vertically self-aligned with one
another. At step 430, the channel grids are not interconnected with
each other, but rather lie in a parallel stack of several layers.
Those with skill in the art will recognize that substances other
than silicon may be used as a substrate, in which case, another
type of anneal or thermodynamically self-aligned void array
formation process may be used.
[0034] Etching step 440 is a common to a plurality of variations of
the method disclosed herein. However, in some embodiments,
intervening step 460 may be performed. In general, the method that
follows the step-440 branch, wherein etching step 440 immediately
follows annealing step 430 is suitable for embodiments wherein the
target metamaterial is a mesh structure. The step-460 branch,
wherein annealing step 430 is followed by a filling step 460 is
suitable for embodiments wherein the target metamaterial is an
artificial nanocrystal, or wherein vertical well portions of the
metamaterial are to be constructed of material different from the
material of the two-dimensional grid. Those having skill in the art
will appreciate that many combinations of the basic steps disclosed
are possible.
[0035] Following the step-440 branch, at step 440, vertical wells
220 are etched substantially orthogonally to the channel wells. For
example, in some embodiment, a vertical well is etched at every
intersection formed by the two-dimensional grid pattern. In other
embodiments, vertical wells may be selectively placed only at some
intersections, or may be placed at non-intersecting points along
trace lines. The arrangement of vertical wells is selected to
impart the desired metamaterial property.
[0036] In some embodiments, where a complete wire mesh is desired,
vertical wells 220 may be etched at every intersection, and each
vertical well will pass through each layer, so that a network of
vertical wells joins every intersection of each layer to the
corresponding intersections of each other layer. In other
embodiments, a plurality of vertical wells are provided, at least
some of which will pass through more than one layer, so that
individual wells join two or more layers to one another. The result
is a three-dimensional network containing a plurality of
substantially identical channel grids joined to one another by a
plurality of vertical wells, called herein a 3-D cavity mesh.
[0037] In step 444, the 3-D cavity mesh is filled with a material.
This step may include, for example, deposition of a metal along the
walls of each channel so that a network of hollow "tubes" is
formed. In other embodiments, the 3-D channel mesh may be
completely filled with metal, resulting in a solid wire mesh
metamaterial such as self-aligned metamaterial 300. This exemplary
metamaterial is composed entirely of one substance, and
metamaterial properties are affected by the substance itself as
well as the final arrangement of the mesh.
[0038] The filling step may involve exposing vertical edges of the
channel grid so that they can be metallized. The horizontal grid
pattern may include vertical intersections. In certain embodiments,
metallization in may be replaced with another desired deposition
process and type of material.
[0039] In step 450, the substrate 200 is removed from around
metamaterial 300 only if it is desirable or beneficial to do so.
For example, if metamaterial 300 is a wire mesh to be used as an
antenna at infrared frequencies, it may be beneficial to leave
substrate 200 in place. However, in antennas at other frequencies,
it may be beneficial or necessary to chemically remove substrate
200. In step 454, metamaterial 300 may optionally be encased for
example in a resin. The desirability of encasing in resin will
depend on the metamaterial and the desired properties. In some
embodiments, encasing metamaterial 300 in resin may defeat some or
all of the desired metamaterial properties.
[0040] Similar methods may be used to make mixed-material
metamaterials. For example, the process as described above may be
suitable for making a metamaterial antenna. The process can be
modified slightly to make a metamaterial filter for example by
making metamaterial 300 in three separate blocks, two of which are
made of conductive metal and one of which is made of a dielectric.
By joining three separate blocks in the pattern
conductor-dielectric-conductor, a high-frequency passband can be
produced and used to filter unwanted frequencies.
[0041] Returning to the step-460 branch of FIG. 4, at step 460, the
two-dimensional channel grids are filled with a first material.
This may be done in the same manner as described in step 444.
[0042] In step 464, vertical wells are etched. This method is
similar as to that described in step 440. In step 464, selection of
an appropriate etchant and process is necessary because an existing
wire mesh is already in place. For example, if a chlorinated
chemical process is used, the existing wire pieces may vertically
block the etch, so that "walls" of substrate material are left
between layers of wire mesh. To avoid formation of walls, a
fluorinated chemical etch process may be used instead to ensure a
clean etch around each individual wire and between wires. The
result is a line of exposed wire intersections running down the
well.
[0043] In step 470, a second material may deposited on exposed
intersections. For example, if the wire mesh is made of aluminum, a
gold beat may be deposited on each exposed junction. This can be
accomplished for example by electrically or chemically "growing"
the beads on the junctions according to methods known in the
art.
[0044] In step 474, after beads of the second material are
deposited or "grown" on the exposed junctions, the remainder of
each well may be filled with a desired fill material, such as
resin. Those with skill in the art will recognize that other
combinations are possible to select for desired metamaterial
properties. For example, vertical wells may be filled with the
first material after beads of the second material are deposited on
the junctions.
[0045] In step 480, once the vertical wells are filled in, the wire
mesh has sufficient three-dimensional structure that substrate 200
can be removed. The result is a wire mesh similar to metamaterial
300, with a plurality of beads of a second material deposited on
intersections throughout, and vertical structure provided by a
third material such as resin.
[0046] In step 484, the new wire mesh may be encased in a material
such as resin, which may be the same resin as the resin of the
third material in step 474. The resin encasing provides mechanical
structure.
[0047] In step 490, the first material of step 460 is removed. For
example, in an exemplary embodiment, the first material is
aluminum, the second material is gold, and the third material is
resin. Thus, before step 490, a wire mesh of aluminum is encased
within a resin body. At each intersection in the wire mesh there is
a gold bead. In step 490, terminal ends of the aluminum wires may
be exposed to a reactive chemical such as nitric acid, fluoric
acid, or an aluminum etchant. In this case, it is important to
select a second material that is not reactive with or is less
reactive with the chemical agent, such as gold. The chemical agent
dissolves the aluminum wire mesh, but does not destroy the gold
beads. Any spaces left by the dissolved aluminum may then be
back-filled with additional resin.
[0048] The result of step 490 is an artificial resin crystal having
a plurality of gold beads dispersed throughout the resin in a
pattern selected for a desired metamaterial property. For example,
the gold beads may impart a desired resonant frequency.
[0049] Although a series of steps have been disclosed in FIG. 4 in
an exemplary order, those with skill in the art will recognize that
the order of some steps may be varied, and that some steps may be
optional to certain embodiments. For example, a first exemplary
process may include, in order, steps
410.fwdarw.420.fwdarw.430.fwdarw.440.fwdarw.444 and optionally
either or both of 450 and 454. The result of this exemplary method
is a uniform wire mesh that is optionally left inside the substrate
or optionally encased in a polymer resin or other casing
material.
[0050] A second exemplary process may include, in order, steps
410.fwdarw.420.fwdarw.430.fwdarw.460.fwdarw.464.fwdarw.470.fwdarw.474
and optionally steps 480, 484, and 490. The result of this process
may be an exemplary artificial nanocrystal (FIG. 5).
[0051] A third exemplary process may include, in order, steps
410.fwdarw.420.fwdarw.430.fwdarw.460.fwdarw.464.fwdarw.474 and
optionally step 480 and 484. The result of this process is a
three-dimensional mesh wherein the each mesh grid is upheld by a
material different from the grid material, for example a wire mesh
with structural resin providing vertical support between
layers.
[0052] A fourth exemplary process includes following the method of
FIG. 4, but etching the substrate deep enough to form one or more
two-dimensional wire meshes after the annealing and metalizing
operations. In this example, steps 440 and 464 (etching vertical
wells) are unnecessary. The wire meshes may remain buried in the
substrate or may be exposed to the surface of the. This process is
useful for buried in-plane propagation, for example in
electro-optical circuits, and also may be used as a method of
creating integrated or printed circuits. In this exemplary
embodiment, the substrate remains substantially in place.
Advantageously, wires thus formed will have circular cross sections
rather than rectangular cross sections, which may impart desired
properties including isotropy in a wave's behavior. Furthermore, in
an exemplary electro-optical circuit, a wire mesh could be used in
optical transmission lines to affect the propagation phase. This is
important when using nanoantennas to transmit optical signals
because such antennas, when arrayed, must be fed with the correct
phase by transmission lines to achieve the desired radiation
pattern. In electro-optical circuits (or any on-chip circuits),
space is at a premium. The ability to array antennas in whatever
space and direction is available, limited only by the desired
pattern, is therefore helpful.
[0053] A fifth exemplary process includes etching and annealing
vertical wells without constructing the grid pattern. In this case,
a plurality of wells may be etched in a desired pattern, and the
wells then annealed, so that a three-dimensional matrix of bubbles
is formed in the substrate. The bubbles may be useful in tuning
permittivity or spatial indexing. They can also create broadband
negative refraction materials. In some cases, creation of a bubble
matrix may be combined with other exemplary methods to further
select a desired metamaterial property.
[0054] FIG. 5 is a perspective view of an exemplary artificial
nanocrystal 500 manufactured according to the process of FIG. 4.
Nanocrystal 500 is formed with a structural body of resin 510, and
has placed therein a plurality of beads 520. The beads may be
placed for example in an ordered or periodic pattern selected for a
particular resonance. In some embodiments, an artificial
nanocrystal 500 will have a much sharper resonance curve than a
natural crystal. If nanocrystal 500 is an electromagnetic of
photonics crystal, the useful frequencies will be set by the
wavelengths equivalent to repeated unit cell size. Electromagnetic
or photonics crystals are often used to create a bandgap so that
radiation at a selected frequency will not propagate. Defects may
also be introduced throughout the material, for example by chemical
etching additional vertical wells, which may serve as "light
tunnels."
[0055] The novel and patentably distinct methods of producing
metamaterials described herein may also be used for producing other
complex electromagnetic media, such as photonic crystals and/or
electromagnetic crystals. The methods described herein may be
applicable to produce materials having repeated structural
dimensions that are smaller than and/or about the same size as an
operational wavelength of radiation. Other applications include
electromagnetic filters configured to block specific wavelengths of
radiation; for example, a filter could be configured to block
wavelengths in common communication-band frequencies but pass
visible light.
[0056] While the subject of this specification has been described
in connection with one or more exemplary embodiments, it is not
intended to limit the claims to the particular forms set forth. On
the contrary, the appended claims are intended to cover such
alternatives, modifications and equivalents as may be included
within their spirit and scope.
* * * * *