U.S. patent application number 11/047089 was filed with the patent office on 2006-08-03 for periodic layered structures and methods therefor.
This patent application is currently assigned to Hewlett-Packard Development Company, L.P.. Invention is credited to Pavel Kornilovich, Peter Mardilovich, Sriram Ramamoorthi.
Application Number | 20060169592 11/047089 |
Document ID | / |
Family ID | 36755348 |
Filed Date | 2006-08-03 |
United States Patent
Application |
20060169592 |
Kind Code |
A1 |
Mardilovich; Peter ; et
al. |
August 3, 2006 |
Periodic layered structures and methods therefor
Abstract
A periodic layered structure having physical and chemical
properties varying periodically at least along a direction
perpendicular to its layers is made by providing a substrate,
depositing a quantity of non-porous electrochemically oxidizable
material over the substrate, at least partially anodizing the
non-porous electrochemically oxidizable material, and repeating
similar steps until a layered structure having a desired
periodicity and a desired total structure thickness is
completed.
Inventors: |
Mardilovich; Peter;
(Corvallis, OR) ; Kornilovich; Pavel; (Corvallis,
OR) ; Ramamoorthi; Sriram; (Corvallis, OR) |
Correspondence
Address: |
HEWLETT PACKARD COMPANY
P O BOX 272400, 3404 E. HARMONY ROAD
INTELLECTUAL PROPERTY ADMINISTRATION
FORT COLLINS
CO
80527-2400
US
|
Assignee: |
Hewlett-Packard Development
Company, L.P.
Fort Collins
CO
|
Family ID: |
36755348 |
Appl. No.: |
11/047089 |
Filed: |
January 31, 2005 |
Current U.S.
Class: |
205/324 |
Current CPC
Class: |
C25D 11/02 20130101;
C25D 1/003 20130101 |
Class at
Publication: |
205/324 |
International
Class: |
C25D 11/04 20060101
C25D011/04 |
Claims
1. A method for fabricating a layered structure having physical and
chemical properties varying periodically at least along a direction
perpendicular to its layers, the method comprising the steps of: a)
providing a substrate, b) depositing a quantity of non-porous
electrochemically oxidizable material over the substrate to form a
first layer having a first-layer thickness, c) anodizing the
electrochemically oxidizable material until a second-layer
thickness of oxide is formed, and d) repeating alternately the
steps b) of depositing and c) of anodizing until a layered
structure having a desired periodicity and a desired structure
thickness is completed.
2. The method of claim 1, wherein the electrochemically oxidizable
material comprises a non-porous material selected from the list
consisting of Al, Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg, Sn, Zn, Ti,
Cu, Mo, Hf, Zr, Ti, V, Au, and Cr, and alloys, mixtures, and
combinations thereof.
3. The method of claim 1, wherein the electrochemically oxidizable
material comprises non-porous silicon (Si).
4. The method of claim 1, wherein the periodic layered structure
comprises a superlattice.
5. The method of claim 1, wherein the first-layer thickness of the
first layer is greater than or about equal to two nanometers (2
nm).
6. The method of claim 1, wherein the periodicity is greater than
or about equal to five nanometers (5 nm).
7. The method of claim 1, wherein the substrate is substantially
planar.
8. The method of claim 1, wherein the steps are performed in the
order recited.
9. The method of claim 1, further comprising the step of
planarizing the first layer.
10. The periodic layered structure made by the method of claim
1.
11. A method for fabricating a layered structure having physical
and chemical properties varying periodically at least along a
direction perpendicular to its layers, the method comprising the
steps of: a) providing a substrate, b) depositing a quantity of a
first non-porous electrochemically oxidizable material over the
substrate to form a first layer having a first-layer thickness, c)
anodizing the first layer until a desired thickness of first oxide
is formed, d) depositing a quantity of a second non-porous
electrochemically oxidizable material over the first oxide, e)
anodizing the second non-porous electrochemically oxidizable
material until a desired thickness of second oxide is formed, and
f) repeating the previous four steps b), c), d), and e) a number of
times until a layered structure having a desired periodicity and a
desired structure thickness is completed.
12. The method of claim 11, wherein the first non-porous
electrochemically oxidizable material and the second non-porous
electrochemically oxidizable material comprise different
materials.
13. The method of claim 11, wherein each of the first non-porous
electrochemically oxidizable material and the second non-porous
electrochemically oxidizable material comprises a material selected
from the list consisting of Al, Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg,
Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and Cr, and alloys,
mixtures, and combinations thereof.
14. The method of claim 11, wherein the first non-porous
electrochemically oxidizable material comprises aluminum.
15. The method of claim 11, wherein the first non-porous
electrochemically oxidizable material comprises tantalum.
16. The method of claim 11, wherein the second non-porous
electrochemically oxidizable material comprises aluminum.
17. The method of claim 11, wherein the second non-porous
electrochemically oxidizable material comprises tantalum.
18. The method of claim 11, wherein the first non-porous
electrochemically oxidizable material and the second non-porous
electrochemically oxidizable material comprise the same
material.
19. The method of claim 18, wherein both the first non-porous
electrochemically oxidizable material and the second non-porous
electrochemically oxidizable material comprise the same material
selected from the list consisting of Al, Ta, Nb, W, Bi, Sb, Ag, Cd,
Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and Cr, and
alloys, mixtures, and combinations thereof.
20. The method of claim 11, wherein the periodic layered structure
comprises a superlattice.
21. The method of claim 11, wherein the first-layer thickness of
the first layer is greater than or about equal to two nanometers (2
nm).
22. The method of claim 11, wherein the periodicity is greater than
or about equal to five nanometers (5 nm).
23. The method of claim 11, wherein the substrate is substantially
planar.
24. The method of claim 11, wherein the steps are performed in the
order recited.
25. The method of claim 11, further comprising the step of
planarizing the first layer.
26. The method of claim 11, further comprising the step of
planarizing the second layer.
27. The periodic layered structure made by the method of claim
11.
28. A method for fabricating a layered structure having physical
and chemical properties varying periodically at least along a
direction perpendicular to its layers, the method comprising the
steps of: a) providing a substrate, b) depositing a quantity of a
first non-porous electrochemically oxidizable material over the
substrate to form a first layer having a first-layer thickness, c)
anodizing the first non-porous electrochemically oxidizable
material until a desired thickness of first oxide is formed, and d)
repeating steps b) and c) a number of times respectively for a
number n of non-porous electrochemically oxidizable materials until
a layered structure having a desired periodicity and a desired
structure thickness is completed.
29. The method of claim 28, wherein each of the non-porous
electrochemically oxidizable materials of the number n of
non-porous electrochemically oxidizable materials comprises a
material selected from the list consisting of Al, Ta, Nb, W, Bi,
Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and
Cr, and alloys, mixtures, and combinations thereof.
30. The method of claim 28 wherein the non-porous electrochemically
oxidizable materials of the number n of non-porous
electrochemically oxidizable materials all comprise the same
material.
31. The method of claim 28 wherein the non-porous electrochemically
oxidizable materials of the number n of non-porous
electrochemically oxidizable materials comprise at least two
different materials.
32. The method of claim 28 wherein the non-porous electrochemically
oxidizable materials of the number n of non-porous
electrochemically oxidizable materials comprise a number m of
different materials, wherein m is less than n.
33. The method of claim 28 wherein the non-porous electrochemically
oxidizable materials of the number n of non-porous
electrochemically oxidizable materials comprise n different
materials.
34. The method of claim 28, wherein the periodic layered structure
comprises a superlattice.
35. The method of claim 28, wherein the first-layer thickness of
the first layer is greater than or about equal to two nanometers (2
nm).
36. The method of claim 28, wherein the periodicity is greater than
or about equal to five nanometers (5 nm).
37. The method of claim 28, wherein the substrate is substantially
planar.
38. The method of claim 28, wherein the steps are performed in the
order recited.
39. The method of claim 28, further comprising the step of
planarizing the first layer.
40. The method of claim 28, further comprising the step of
planarizing the m.sup.th layer, wherein m is less than or equal to
n.
41. The periodic layered structure made by the method of claim
28.
42. A method for fabricating a layered structure having physical
and chemical properties varying periodically at least along a
direction perpendicular to its layers, the method comprising the
steps of: a) providing a substrate, b) depositing a quantity of a
first non-porous electrochemically oxidizable material over the
substrate to form a first electrochemically oxidizable layer having
a first-layer thickness, c) anodizing the first electrochemically
oxidizable layer fully until a homogeneous layer of first oxide is
formed, thereby substantially replacing the first electrochemically
oxidizable layer with the first oxide, d) depositing a quantity of
a second material over the first oxide, and e) repeating the
previous three steps b), c) and d) a number of times until a
layered structure having a desired periodicity and a desired
structure thickness is completed.
43. The method of claim 42 wherein each of the first non-porous
electrochemically oxidizable material and second material comprises
a material selected from the list consisting of Al, Ta, Nb, W, Bi,
Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and
Cr, and alloys, mixtures, and combinations thereof.
44. The method of claim 42, wherein the first non-porous
electrochemically oxidizable material and the second material
comprise different materials.
45. The method of claim 42, wherein the second material comprises a
material that is not electrochemically oxidizable.
46. The method of claim 42, wherein the second material comprises a
metal selected from the list consisting of platinum, palladium, and
rhodium.
47. The method of claim 42, wherein the first non-porous
electrochemically oxidizable material comprises aluminum.
48. The method of claim 42, wherein the first non-porous
electrochemically oxidizable material comprises tantalum.
49. The method of claim 42, wherein the second material comprises
aluminum.
50. The method of claim 42, wherein the second material comprises
tantalum.
51. The method of claim 42, wherein the first non-porous
electrochemically oxidizable material and the second material
comprise the same material.
52. The method of claim 42, further comprising the step of
oxidizing the second material.
53. The method of claim 52, further comprising the step of forming
an edge on the second material before oxidizing the second
material.
54. The method of claim 53, wherein the step of oxidizing the
second material is performed by anodizing its edge.
55. The method of claim 42, wherein the periodic layered structure
comprises a superlattice.
56. The method of claim 42, wherein the first-layer thickness of
the first electrochemically oxidizable layer is greater than or
about equal to two nanometers (2 nm).
57. The method of claim 42, wherein the periodicity is greater than
or about equal to five nanometers (5 nm).
58. The method of claim 42, wherein the substrate is substantially
planar.
59. The method of claim 42, wherein the steps are performed in the
order recited.
60. The method of claim 42, further comprising the step of
planarizing the first electrochemically oxidizable layer.
61. The method of claim 42, further comprising the step of
planarizing the first oxide.
62. The method of claim 42, further comprising the step of
planarizing the second material.
63. The periodic layered structure made by the method of claim
42.
64. A method for fabricating a layered structure having physical
and chemical properties varying periodically at least along a
direction perpendicular to its layers, the method comprising the
steps of: a) providing a substrate, b) depositing a quantity of a
first non-porous electrochemically oxidizable material over the
substrate to form a first layer having a first-layer thickness, c)
depositing a quantity of a second non-porous electrochemically
oxidizable material over the first layer to form a second layer
having a second-layer thickness, d) anodizing both the first and
second layers until a composite layer of first and second oxides is
formed, and e) repeating the previous three steps b), c) and d) a
number of times until a periodic layered structure having a desired
periodicity and a desired structure thickness is completed.
65. The method of claim 64 wherein each of the first and second
non-porous electrochemically oxidizable materials comprises a
material selected from the list consisting of Al, Ta, Nb, W, Bi,
Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and
Cr, and alloys, mixtures, and combinations thereof.
66. The method of claim 64, wherein the first non-porous
electrochemically oxidizable material and the second non-porous
electrochemically oxidizable material comprise different
materials.
67. The method of claim 64, wherein the first non-porous
electrochemically oxidizable material comprises aluminum.
68. The method of claim 64, wherein the non-porous
electrochemically oxidizable material comprises tantalum.
69. The method of claim 64, wherein the second non-porous
electrochemically oxidizable material comprises aluminum.
70. The method of claim 64, wherein the second non-porous
electrochemically oxidizable material comprises tantalum.
71. The method of claim 64, wherein the first non-porous
electrochemically oxidizable material and the second non-porous
electrochemically oxidizable material comprise the same
material.
72. The method of claim 64, wherein step d) is performed by
partially anodizing at least the second layer.
73. The method of claim 64, wherein the periodic layered structure
comprises a superlattice.
74. The method of claim 64, wherein the first-layer thickness is
greater than or about equal to two nanometers (2 nm).
75. The method of claim 64, wherein the second-layer thickness is
greater than or about equal to two nanometers (2 nm).
76. The method of claim 64, wherein the periodicity is greater than
or about equal to five nanometers (5 nm).
77. The method of claim 64, wherein the substrate is substantially
planar.
78. The method of claim 64, wherein the steps are performed in the
order recited.
79. The method of claim 64, further comprising the step of
planarizing the first layer.
80. The method of claim 64, further comprising the step of
planarizing the second layer.
81. The periodic layered structure made by the method of claim
64.
82. A method for fabricating an imprinting stamp for lithography,
the method comprising the steps of: a) providing a periodic layered
structure having a periodicity, the periodic layered structure
comprising layers of at least two materials differing in etch rate
to a predetermined etchant, at least one of the layers being formed
by electrochemical oxidation, the layers having edges of the at
least two materials, and b) selectively etching back the edge of at
least one of the at least two materials with the predetermined
etchant to form recesses separated by salient portions, thereby
forming an imprinting stamp having the periodicity of the periodic
layered structure.
83. The imprinting stamp made by the method of claim 82.
84. A method for fabricating an imprinting stamp for lithography,
the method comprising the steps of: a) providing a substrate, b)
depositing a quantity of non-porous electrochemically oxidizable
material over the substrate to form a first layer having a
first-layer thickness and a first-layer edge, c) anodizing the
non-porous electrochemically oxidizable material until a
second-layer thickness of oxide is formed, d) repeating alternately
the steps b) of depositing and c) of anodizing until a periodic
layered structure having a desired periodicity and a desired
structure thickness is completed, and e) selectively etching back
one of the first-layer edge or the second-layer oxide of each layer
to form a recess, whereby an imprinting stamp having the desired
periodicity is formed.
85. The imprinting stamp made by the method of claim 84.
86. A method for fabricating an imprinting stamp for lithography,
the method comprising the steps of: a) providing a substrate, b)
depositing a quantity of a first non-porous electrochemically
oxidizable material over the substrate to form a first layer having
a first-layer thickness, c) anodizing the first layer until a
desired thickness of first oxide is formed, d) depositing a
quantity of a second material over the first oxide, whereby a
second layer having a second-layer thickness is formed, e)
repeating the previous three steps b), c), and d) a number of times
until a layered structure having a desired periodicity and a
desired structure thickness is completed, f) forming a second-layer
edge of at least each second layer, g) anodizing each second-layer
edge until a desired thickness of second oxide is formed, and h)
selectively etching back one of the first oxide and second oxide,
whereby an imprinting stamp having the desired periodicity is
formed.
87. The method of claim 86, wherein the first non-porous
electrochemically oxidizable material and the second material
comprise different materials.
88. The method of claim 86, wherein the first non-porous
electrochemically oxidizable material and the second material
comprise different materials selected form the list consisting of
Al, Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf,
Zr, Ti, V, Au, and Cr, and alloys, mixtures, and combinations
thereof.
89. The method of claim 86, wherein the first non-porous
electrochemically oxidizable material is aluminum and the first
oxide is aluminum oxide.
90. The method of claim 86, wherein the second material is tantalum
and the second oxide is tantalum oxide.
91. The method of claim 86, wherein the periodic layered structure
comprises a superlattice.
92. The method of claim 86, wherein the first-layer thickness is
greater than or about equal to two nanometers (2 nm).
93. The method of claim 86, wherein the second-layer thickness is
greater than or about equal to two nanometers (2 nm).
94. The method of claim 86, wherein the periodicity is greater than
or about equal to five nanometers (5 nm).
95. The method of claim 86, wherein the substrate is substantially
planar.
96. The method of claim 86, wherein the steps are performed in the
order recited.
97. The method of claim 86 further comprising the step of
planarizing the first layer.
98. The method of claim 86 further comprising the step of
planarizing the first oxide.
99. The method of claim 86, further comprising the step of
planarizing the second layer.
100. The imprinting stamp made by the method of claim 86.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is related to co-pending and commonly
assigned application Ser. No. 10/817,729, filed Apr. 2, 2004
(attorney docket no. 200311571), the entire disclosure of which is
incorporated herein by reference, and is related to co-pending and
commonly assigned applications Ser. No. ______ (attorney docket no.
200401845) and Ser. No. ______ (attorney docket no. 200406118).
TECHNICAL FIELD
[0002] This invention relates generally to periodic layered
structures and methods for fabricating and using such periodic
layered structures.
BACKGROUND
[0003] Industrial interest in materials having structural and
functional features with nanoscale dimensions has been growing
rapidly. Nano-structures have been fabricated by semiconductor
processing techniques including patterning techniques such as
photolithography, electron-beam lithography, ion-beam lithography,
X-ray lithography, and the like. Other nano-structures have also
been fabricated utilizing structures formed by self-ordering
processes.
[0004] Some devices for manipulating optical signals have
incorporated nanostructures, often including periodic structures
such as photonic crystals, for example. Fabrication of optical
devices, including macroscopic, microscopic, and nanoscopic
elements, has usually used glasses such as silica glasses,
transparent crystals, or polymeric materials.
[0005] Nanostructures have been applied to display devices,
magnetic recording media, quantum-well devices, molecular and gas
sensors, optical devices, electroluminescent devices, and
electrochromic devices, for example. Periodic layered structures
such as superlattice structures may be used in many such
applications. Efficient, reproducible, low-cost methods for making
periodic layered structures are therefore needed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The features and advantages of the disclosure will readily
be appreciated by persons skilled in the art from the following
detailed description when read in conjunction with the drawings,
wherein:
[0007] FIG. 1A is a flowchart of a first embodiment of a method for
fabricating a periodic layered structure.
[0008] FIG. 1B is a flowchart of a second embodiment of a method
for fabricating a periodic layered structure.
[0009] FIGS. 2A-2F are cross-sectional side elevation views of an
embodiment of a periodic layered structure at various stages of its
fabrication.
[0010] FIGS. 3A-3F are cross-sectional side elevation views of
another embodiment of a periodic layered structure at various
stages of its fabrication.
[0011] FIG. 4 is an exemplary graph showing oxide thickness as a
function of time as observed while practicing a step of an
embodiment of a method for fabricating a periodic layered
structure.
[0012] FIG. 5 is an exemplary graph showing oxide thickness as a
function of anodization voltage as observed while practicing a step
of an embodiment of a method for fabricating a periodic layered
structure.
[0013] FIGS. 6A-6C are cross-sectional side elevation views of
another embodiment of a periodic layered structure at various
stages of fabrication for an embodiment of an imprint lithography
stamp.
[0014] FIGS. 7 and 8 are cross-sectional side elevation views of
two particular embodiments of periodic layered structures.
[0015] FIG. 9 is a top plan view of an embodiment such as the
embodiments shown in FIGS. 7 and 8.
DETAILED DESCRIPTION OF EMBODIMENTS
[0016] For clarity of the description, the drawings are not drawn
to a uniform scale. In particular, vertical and horizontal scales
may differ from each other and may vary from one drawing to
another. In this regard, directional terminology, such as "top,"
"bottom," "front," "back," "leading," "trailing," etc., is used
with reference to the orientation of the drawing figure(s) being
described. Because components of the invention can be positioned in
a number of different orientations, the directional terminology is
used for purposes of illustration and is in no way limiting.
[0017] The term "anodization" is used in this specification and the
appended claims to mean electrochemical oxidation of an oxidizable
material (such as an oxidizable metal) by employing the oxidizable
material as an anode in an electrolytic cell and by operating the
electrolytic cell with voltage and current suitable to partially or
fully oxidize the material of the anode. An "anodic oxide" is the
oxide thus formed. An "anodizable material" is a material that can
be oxidized in that manner. "Partial anodization" refers to
oxidation of less than the entire thickness of a metal layer; i.e.,
some thickness of unoxidized metal remains after partial
anodization, unless full anodization is explicitly specified. "Full
anodization" refers to oxidation of the entire thickness of a metal
layer. References herein to a layer of electrochemically oxidizable
metal are intended to include semiconductor materials such as
silicon which, with respect to their anodization, behave like the
electrochemically oxidizable metals.
[0018] One aspect of the invention provides embodiments of a
periodic layered structure having physical and chemical properties
varying periodically at least along a direction perpendicular to
its layers. The periodicity (pitch) of the layered structure may be
characterized by a nanoscale dimension. Throughout this
specification and the appended claims, the term "nanoscale" refers
to a length of scale corresponding generally to the scale in the
definition of U.S. Patent Class 977, generally less than about 100
nanometers (nm).
[0019] The term "superlattice" has been used to denote structures
such as periodic heterojunctions having a periodicity longer than
that of the characteristic crystal lattice of a base material,
e.g., due to lattice mismatch. The term "superlattice" has also
been used more generally to denote periodic structures constructed
from other crystalline, polycrystalline, or non-crystalline
materials. "Superlattice" is used in this more general sense in the
present specification and claims.
[0020] An embodiment of a periodic layered structure may be made by
providing a substrate, depositing a quantity of non-porous
electrochemically oxidizable material such as a metal over the
substrate, anodizing the non-porous electrochemically oxidizable
material (partially or fully), and repeating similar steps until a
layered structure having a desired periodicity and a desired total
structure thickness is completed. The thickness of each layer
and/or the periodicity of the periodic layered structure may be
nanoscopic. Thus, another aspect of the invention provides methods
for fabricating embodiments of periodic layered structures,
including structures whose layers have nanoscale dimensions and
periodic structures with nanoscale pitch.
[0021] One embodiment of a method for fabricating a periodic
layered structure (having physical and chemical properties varying
periodically at least along a direction perpendicular to its
layers) employs steps of providing a substrate, depositing a
quantity of non-porous electrochemically oxidizable material over
the substrate to form an electrochemically oxidizable layer,
anodizing the non-porous electrochemically oxidizable material
until a layer of oxide is formed, and repeating alternately the
depositing and anodizing steps until a periodic layered structure
having a desired periodicity and a desired total thickness is
completed. The periodic layered structure may be one of the types
known as a superlattice. The non-porous electrochemically
oxidizable material is anodized until a layer of oxide having a
desired thickness is formed. In some cases, that anodization may be
a partial anodization, i.e., less than the entire thickness of the
electrochemically oxidizable material is oxidized.
[0022] Many electrochemically oxidizable materials are known,
including the metals aluminum (Al), tantalum (Ta), niobium (Nb),
tungsten (W), bismuth (Bi), antimony (Sb), silver (Ag), cadmium
(Cd), iron (Fe), magnesium (Mg), tin (Sn), zinc (Zn), titanium
(Ti), copper (Cu), molybdenum (Mo), hafnium (Hf), zirconium (Zr),
titanium (Ti), vanadium (V), gold (Au), and chromium (Cr), along
with their electrochemically oxidizable alloys, mixtures, and
combinations, all of which are suitable for use in this method.
Another suitable material is non-porous silicon (Si), although it
is not classified as a metal, but as a semiconductor. Thus,
references herein to a layer of non-porous electrochemically
oxidizable material or metal are intended to include non-porous
semiconductor materials such as non-porous silicon which, with
respect to their anodization, behave like the non-porous
electrochemically oxidizable metals. To simplify the description
and drawings, embodiments using metals for a non-porous
electrochemically oxidizable material will be described. Those
skilled in the art will understand that any non-porous
electrochemically oxidizable material may be substituted wherever
"metal" is mentioned, except where the metal is explicitly
described as not being electrochemically oxidizable.
[0023] While porous oxides have been formed by anodization, control
of the thickness of oxides thus formed can be problematical.
However, the thickness of dense oxide films (with densities
comparable to theoretical oxide densities) formed by
electrochemical oxidation of non-porous electrochemically
oxidizable material is precisely controllable by controlling the
anodization voltage, as described in more detail hereinbelow.
[0024] Suitable non-porous electrochemically oxidizable materials
include the "valve metals," defined in the review paper by M. M.
Lohrengel, "Thin anodic oxide layers on aluminium and other valve
metals: high field regime," Materials Science and Engineering Vol.
R11 (1993) pp. 243-294, for example. As described in the review
paper by Lohrengel, valve metals may be defined to be in accordance
with i=i.sub.0*exp(.beta.*E), where i is the oxide formation
current, i.sub.0 and .beta. are material-dependent constants, and E
is the electric field strength in the oxide. Lohrengel goes on to
list various properties typical of a valve metal: The surface of an
(electro-) polished electrode is covered with 2-5 nanometers of
oxide from air or electrolyte passivation. This corresponds to an
open circuit potential of about zero V (vs. a hydrogen electrode in
the same solution). The thickness of the oxide layer increases
during anodization. In a galvanostatic experiment the potential
increases almost linearly with time; in a potential sweep
experiment with constant potential sweep the current is almost
constant. These are equivalent and correspond to a constant charge
and, therefore, a constant increase of thickness for a given
potential change. The oxide layer is not reduced by (moderate)
cathodic currents. Further oxide growth is only observed when the
potential exceeds the previous formation potential. The ionic
conductivity is small (steady state conditions or at potentials
smaller than the formation potential). The electronic conductivity
(of undoped oxides) and, hence, oxygen evolution are negligible. An
addition of redox systems to the electrolyte causes no additional
currents. Corrosion is small at moderate pH values. The oxide grows
independently of the composition of the electrolyte. A (possible)
incorporation of anions from the electrolyte, for example, causes
no fundamental changes of the layer properties. The combination of
the low oxide electrode potential (and, therefore, air
passivation), negligible electronic conductivity, and the lack of
oxygen evolution is not accidental, as the oxide electrode
potential depends almost linearly on the band gap. Valve metals are
usually covered by oxide films of the barrier type. An ideal
barrier oxide ". . . is a nonporous, thin oxide layer possessing
electronic and ionic conductivity at high electric field
strength."
[0025] Returning now to the description of a method embodiment for
fabricating a periodic layered structure, the layer of non-porous
electrochemically oxidizable metal (or, in the case of silicon, for
example, non-porous electrochemically oxidizable semiconductor) may
be deposited by any suitable conventional deposition method, such
as evaporation, sputtering, plating, electroplating, atomic layer
deposition (ALD), or chemical vapor deposition (CVD). The metal
layer may have a thickness of about two nanometers (2 nm) or
greater, for example, with essentially no theoretical upper limit,
but limited only by practical considerations such as anodizing
voltage, application requirements, etc.
[0026] In practice, the metal layer may be made thinner on a
smooth, substantially planar substrate than on a substrate which is
not smooth and planar. The substrate may be prepared by polishing
to a smooth planar surface before depositing the metal layer. Also,
the metal layer may be planarized after its deposition, e.g., by
mechanical polishing, chemical polishing, electrochemical
polishing, chemical mechanical polishing (CMP), or other
planarization technique.
[0027] The desired periodicity of structures that are made by such
a method depends on the intended application of the periodic
layered structure. For example, a periodic layered structure to be
used as a photonic crystal may require a periodicity determined by
the wavelength of the electromagnetic radiation that is to be
processed by the photonic crystal. Method embodiments performed in
accordance with the present invention may make periodic layered
structures having periodicities (pitch) of about five nanometers
(nm) or greater.
[0028] FIGS. 1A and 1B are flowcharts illustrating embodiments of
methods for fabricating periodic layered structures. Steps of the
methods are denoted by reference numerals S10, S20, . . . , S160.
FIGS. 2A-2F and 3A-3F are cross-sectional side elevation views of
embodiments of periodic layered structures at various stages of
their fabrication.
[0029] FIG. 1A is a flowchart of a first embodiment of a method for
fabricating a periodic layered structure. As shown in FIG. 1A, a
suitable substrate is provided (step S10). For many applications, a
suitable substrate is a smooth planar silicon wafer as is commonly
used in semiconductor manufacturing. For some applications, a layer
of insulating material such as silicon oxide or silicon nitride may
be formed on the silicon wafer so that the top surface of the
substrate is an insulator. In step S20, a layer 20 of a first metal
is deposited (FIG. 2A). The first metal is a non-porous
electrochemically oxidizable material. The thickness of this layer
of first metal is chosen to provide a suitable amount of metal such
that anodization in the next step (step S30) will contribute
suitably to the desired periodicity (pitch) of the periodic layered
structure.
[0030] When the metal layer is anodized, the total thickness
typically increases. The volume ratio of oxide to consumed metal is
typically greater than one. For example, a five-nanometer-thick
aluminum layer may be converted by anodization to about six and a
half nanometers of aluminum oxide if the full thickness of aluminum
is anodized, and a partially anodized layer (the entire film,
aluminum and alumina together) has a thickness intermediate between
five and about 6.5 nanometers. Similarly, partial anodization of a
five nanometer film of tantalum results in a tantalum oxide film
having a thickness intermediate between zero (or none) and about
11.5 nanometers (the entire film having thickness intermediate
between 5 and 11.5 nanometers).
[0031] FIG. 2B shows the oxide layer 30 formed by partial oxidation
of layer 20, leaving a thinner remaining metal layer 21. In step
S40, a layer 40 of a second metal is deposited (FIG. 2C). The
second metal is also a non-porous electrochemically oxidizable
material. In step S50, the second metal is anodized, forming the
oxide layer 50 shown in FIG. 2D by partial oxidation of layer 40,
leaving a thinner remaining metal layer 41. If the second metal
deposited and anodized in steps S40 and S50 is different from the
metal as deposited and anodized in steps S20 and S30, the
completion of step S50 serves to provide one composite layer, i.e.,
one period, of the periodic layered structure. However, the second
metal deposited and anodized in steps S40 and S50 may be the same
as the metal which was deposited and anodized in steps S20 and S30.
In that case, if the corresponding thicknesses are also made equal,
the completion of step S50 serves to provide two composite periodic
layers, i.e., two periods, of the periodic layered structure. If
the structure is complete, this ends the process. The structure is
determined to be complete if the total structure thickness has
reached the desired thickness and/or if the total number of layers
provided is the desired total number. If the periodic layered
structure is not complete, suitable steps S20, S30, S40, and S50
are repeated until the desired periodic layered structure is
complete, as determined in decision step S60. FIGS. 2E and 2F show
deposition of layer 60 and partial anodization to form layer 70,
for example, leaving a thinner remaining metal layer 61. As shown
by the dashed line in FIG. 1A, steps S40 and S50 may be repeated
separately (for example, if the second metal deposited and anodized
in steps S40 and S50 is the same as the metal deposited and
anodized in steps S20 and S30 and if the corresponding thicknesses
are equal). Those skilled in the art will recognize that, if the
first and second metals deposited in steps S20 and S40 are
different, the periodicity may be different than if they are the
same metal, especially if the deposition thicknesses and
corresponding anodization thicknesses are the same. Also, for
example, the volume ratio (oxide/consumed metal) and/or expansion
coefficients can be different for different metals. FIG. 2F shows a
completed periodic layered structure 80.
[0032] Thus, another aspect of the invention provides embodiments
of a method for fabricating a periodic layered structure by
employing the steps of providing a substrate, depositing a quantity
of a first metal, anodizing the first metal until a desired
thickness of first oxide is formed, depositing a quantity of a
second metal over the first oxide, anodizing the second metal until
a desired thickness of second oxide is formed, and repeating the
preceding four steps a number of times until a layered structure
having a desired periodicity and a desired total structure
thickness is completed.
[0033] The first and second metals may be distinct and different
metals, or they may be the same metal. As in embodiments described
above, each of the metals may be a material selected from among Al,
Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr,
Ti, V, Au, and Cr, and their alloys, mixtures, and combinations.
Some embodiments worth mentioning specifically are those in which
the first metal comprises aluminum or tantalum, and those in which
the second metal comprises aluminum or tantalum. Examples of such
embodiments are described in more detail hereinbelow.
[0034] Those skilled in the art will recognize that the number of
distinct metals and their corresponding oxides may also be more
than two, as illustrated in FIG. 1B. FIG. 1B is a flowchart of a
second embodiment of a method for fabricating a periodic layered
structure. As shown again in FIG. 1B, a suitable substrate is
provided (step S10). In step S100, a layer 20 of a first metal is
deposited (FIG. 3A). The first metal may be a non-porous
electrochemically oxidizable material, but for some applications
this first metal may be a metal, such as platinum, that is not
readily oxidizable electrochemically. In step S110, a layer 35 of a
second metal is deposited (FIG. 3B). The second metal is a
non-porous electrochemically oxidizable material. The total
thickness of these two layers of first and second metals is such
that anodization of the second metal in the next step (step S120)
will contribute suitably to the desired periodicity of the periodic
layered structure. FIG. 3C shows second-metal-oxide layer 45 formed
by anodic oxidation of second-metal layer 35. In step S130, another
layer 55 of the first metal is deposited (FIG. 3D). In step S140, a
layer 65 of a third metal is deposited
[0035] (FIG. 3E). The third metal is also a non-porous
electrochemically oxidizable material. In step S150, the third
metal is anodized. FIG. 3F shows third-metal-oxide layer 75 formed
by anodization of third-metal layer 65.
[0036] If the second metal and third metals deposited in steps S120
and S140 are different from each other and different from the first
metal as deposited in steps S100 and S130, the completion of
anodization step S150 serves to provide one composite layer 95,
i.e., one period, of the periodic layered structure. For some
applications the series of steps S100-S150 is then repeated until
the desired periodic layered structure is complete, as determined
in decision step S160.
[0037] Thus, when the structure is complete after a suitable number
of repetitions, completion of anodization step S150 ends the
process. The structure is determined to be complete if the total
structure thickness has reached the desired thickness and/or if the
total number of layers provided is the desired total number. If the
periodic layered structure is not complete, suitable steps S100,
S110, S120, S130, S140, and S150 are repeated until the desired
periodic layered structure is complete. If the first, second, and
third metals shown in FIG. 1B comprise three different materials,
for example, the suitable set of steps to be repeated may be the
full set of steps S100-S150.
[0038] As shown by the dashed line in FIG. 1B, steps S130, S140 and
S150 may be repeated separately (for example, if the third metal
deposited in step S140 and anodized in step S150 is the same as the
second metal deposited in step S110 and anodized in step S120 and
if the corresponding thicknesses are equal). In that case, if the
corresponding thicknesses are made equal, the completion of step
S150 serves to provide two composite periodic layers, i.e., two
periods, of the periodic layered structure.
[0039] Also, both the second metal deposited and anodized in steps
S110 and S120 and the third metal deposited and anodized in steps
S140 and S150 may be the same as the first metal as deposited in
steps S100 and S130. In that case, if the corresponding thicknesses
are also made equal, the completion of step S150 serves to provide
three composite periodic layers, i.e., three periods of the
periodic layered structure. Again, if the structure is complete as
determined in decision step S160, this ends the process. Otherwise,
suitable steps are repeated until the structure is complete. The
structure is determined to be complete if the total structure
thickness has reached the desired thickness and/or if the total
number of layers provided is the desired total number.
[0040] In another example, an embodiment may be made that is
related to the embodiments illustrated by FIG. 1B. In that
embodiment, adjacent layers of two electrochemically oxidizable
materials may be anodized together. For example, as previously
described, the second and third metals in steps S110 and S140
respectively are non-porous electrochemically oxidizable materials.
If the first metal deposited in steps S100 and S130 is also a
suitable non-porous electrochemically oxidizable material, it can
be anodized when the second metal is anodized in step S120 and/or
when the third metal is anodized in step S150. Those skilled in the
art will recognize that the anodization time, voltage, current
density, composition and concentration of the electrolyte,
temperature and/or other parameter may be suitably adjusted during
this combined anodization step for a second layer to be anodized,
e.g., when anodization of a first material to be anodized is
complete.
[0041] Those skilled in the art will recognize that various
suitable electrolytes and various suitable conditions of
anodization may be used for different non-porous electrochemically
oxidizable materials. For example, dense anodic oxide films on Al
and Ta may be prepared in electrolytes based on citric acid, boric
acid, ammonium tartrate, ammonium borate, and many others. Tungsten
may be oxidized in sulfuric-acid-based electrolyte, for example,
and zinc may be oxidized in NaOH and K.sub.2Cr.sub.2O.sub.7, etc.
In general, electrolytes may also include other surfactants and/or
buffer materials.
[0042] Additional embodiments related to the embodiments
illustrated by FIGS. 1A or FIG. 1B include embodiments in which a
layer of a non-porous electrochemically oxidizable material is
electrochemically oxidized substantially completely, i.e.,
substantially its entire thickness is converted to oxide. For
example, in FIG. 1A, the layer of a first metal may be
electrochemically oxidized completely in step S30 to form a layer
of first-metal oxide, whereby the first metal is substantially
replaced by the first oxide. Then, if the second metal were not
completely electrochemically oxidized in step S50, the periodic
layered structure would comprise repeated instances of a stack
consisting of three sublayers: the oxide of the first metal, the
second metal, and the oxide of the second metal. On the other hand,
if the second metal were completely electrochemically oxidized in
step S50, whereby the second metal is substantially replaced by the
second oxide, then the periodic layered structure would comprise
repeated instances of a stack consisting of two sublayers: the
oxide of the first metal and the oxide of the second metal.
Periodic layered structure embodiments employing many variations of
such sequences may be readily constructed by those skilled in the
art.
[0043] Thus, another aspect of the invention provides a method for
fabricating a periodic layered structure including the steps of
providing a substrate, depositing a quantity of a first metal over
the substrate to form a first metal layer, depositing a quantity of
a second metal over the first metal layer to form a second metal
layer, anodizing both the first and second metal layers until a
composite layer of first and second oxides is formed, and repeating
the three steps (two depositions and one combined anodization) a
number of times until a layered structure having a desired
periodicity and a desired structure thickness is completed.
[0044] In all the embodiments illustrated in FIGS. 1A and 1B and
the variations described above, the method may include one or more
steps of planarization or polishing. The substrate may be
planarized or polished if it is not already sufficiently planar or
smooth. As mentioned hereinabove, the first metal layer may be
planarized after its deposition, e.g., by mechanical polishing,
chemical polishing, electrochemical polishing, chemical mechanical
polishing (CMP), or other planarization technique. Similarly, the
second and/or third metal layers may be planarized after their
depositions if necessary.
[0045] The method embodiments illustrated by FIG. 1B may be
generalized from the three metals enumerated in FIG. 1B to a more
general method employing a number n of metals. Thus, another aspect
of the invention provides embodiments of a method for fabricating a
periodic layered structure, comprising the steps of providing a
substrate, depositing a quantity of a first metal of the n metals
over the substrate to form a first layer, anodizing the first metal
until a desired thickness of first oxide is formed, and repeating
the depositing and anodizing steps a number of times respectively
for n metals until a layered structure having a desired periodicity
and a desired structure thickness is completed. Again, each of the
n metals may be a non-porous electrochemically oxidizable material,
such as Al, Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu,
Mo, Hf, Zr, Ti, V, Au, Cr, or their alloys, mixtures, and
combinations. Such periodic layered structures may be made in which
the n metals all comprise the same material, the n metals comprise
at least two different materials, or, more generally, the n metals
comprise a number m of different materials, where m is less than or
equal to n.
[0046] All of the periodic layered structure embodiments made by
the methods described herein may be one of the types of layered
structure known as a superlattice. The periodicities (pitch) of the
periodic layered structures may be comparable to the thickness of
native oxides commonly formed on metals that are electrochemically
oxidizable, e.g., about 5 nanometers or greater.
[0047] It was mentioned above that a first metal in an embodiment
of a periodic layered structure may comprise a metal, such as
platinum, that is not readily oxidizable electrochemically. More
generally, one or more of the layers of the periodic layered
structure may comprise such a metal, e.g., platinum, palladium, or
rhodium, while one or more other layers comprise a material that is
readily oxidizable electrochemically.
[0048] Of the known electrochemically oxidizable metals, the
anodization processes of tantalum and aluminum, especially, have
been extensively studied. Thus, specific embodiments in which a
first metal comprises aluminum or tantalum, and those in which a
second metal comprises aluminum or tantalum (and the corresponding
oxides formed are aluminum oxide and tantalum oxide respectively)
provide good examples.
[0049] At least some of the method embodiments described herein are
believed to operate in accordance with a common regime for
electrochemical oxidation of metals such as tantalum (Ta), aluminum
(Al), and other metals to produce dense oxides, including two major
stages: galvanostatic and potentiostatic. During the galvanostatic
stage, characterized by constant current density, steady state
oxidation of metal occurs. During the potentiostatic stage,
characterized by constant cell voltage, generally there is no more
metal consumption, but the oxide layer thickness is still
increasing due to diffusion of oxygen ions into the oxide matrix.
However, the invention should not be construed as being limited to
the consequences of any particular theory of operation.
[0050] FIGS. 4 and 5 illustrate quantitative details of the
anodization process for tantalum. Anodization for the measurements
shown in FIGS. 4 and 5 was performed at 20.degree. C., using 0.1%
citric acid in water as the electrolyte with current density of 0.5
milliamperes/square centimeter (mA/cm.sup.2), and using a platinum
(Pt) cathode.
[0051] FIG. 4 is an exemplary graph showing resulting oxide
thickness as a function of time as observed in performing the
second anodization step on a tantalum metal layer while practicing
an embodiment of a method for fabricating a periodic layered
structure. Curve 210 shows how tantalum oxide thickness in
nanometers (nm) shown on the vertical axis varied with time in
minutes, shown on the horizontal axis. The final anodization
voltage was fixed at 70 volts.
[0052] FIG. 5 is an exemplary graph showing resulting oxide
thickness as a function of final anodization voltage as observed in
performing the anodization step on a tantalum metal layer. Line 230
shows how tantalum oxide thickness in nanometers (nm) shown on the
vertical axis varied with final anodization voltage in volts (V),
shown on the horizontal axis. The final anodization voltage (V) is
the constant voltage used in a potentiostatic step. The oxide
thickness can depend on duration of the potentiostatic step if that
duration is less than about 15 minutes, but that variation is
reproducible and predictable, so that the thickness of the oxide
can be controlled precisely. The anodization time at final voltage
V for the anodization illustrated in FIG. 5 was 30 minutes. Line
230 shows that tantalum oxide thickness varied linearly, directly
proportional to final anodization voltage. The anodization
coefficient was 1.9 nanometer/volt (nm/V). Similar results for
aluminum had an anodization coefficient of 1.3 nanometer/volt
(nm/V).
[0053] As illustrated by FIGS. 6A-6C, another aspect of the present
invention provides embodiments of a method for fabricating an
imprinting stamp for lithography (e.g., a nano-imprinting stamp for
nano-lithography). In such an embodiment, a suitable substrate 10
is provided, a quantity of a first metal is deposited over the
substrate to form a first metal layer having a first-metal edge,
and the metal is anodized to convert all of the first metal to its
oxide, forming a first-metal oxide layer 45 having a first-metal
oxide edge.
[0054] A second metal layer is deposited over the first-metal
oxide, forming a second-metal layer 20 having a second-metal edge.
Similarly to the other embodiments described hereinabove, the
first-metal depositing, anodizing, and second-metal depositing
steps are repeated alternately until a periodic layered structure
having a desired periodicity and a desired structure thickness is
completed (as shown in FIG. 6A). For simplicity of illustration,
only a few layers of the periodic layered structure are shown in
FIGS. 6A-6C. While FIG. 6A shows only two and a half periods of the
periodic layered structure, those skilled in the art will recognize
that any number of periods may be made. In FIG. 6A, layer 75 is the
first-metal-oxide layer of the second period, layer 40 is the
second metal of the second period, and layer 85 is the
first-metal-oxide layer of a third (incomplete) period. Then the
second-metal edge of each second-metal layer is selectively
anodized, forming oxide edges 100 (as shown in FIG. 6B). The oxide
edges 100 are selectively etched back (as shown in FIG. 6C),
forming recesses 105 and leaving salient portions at the edges of
the first-metal-oxide layers. The salient portions and intervening
recesses form an imprinting stamp having the desired periodicity
and dimensions.
[0055] Thus, another aspect of the invention provides embodiments
of a method for fabricating an imprinting stamp for lithography
(such as nano-lithography), comprising steps of providing a
substrate, depositing a quantity of a first metal over the
substrate to form a first metal layer having a first-layer
thickness, anodizing the first metal until a desired thickness of
first oxide is formed, depositing a quantity of a second metal over
the first oxide, whereby a second metal layer having a second-metal
layer edge is formed, repeating the previous three steps a number
of times until a layered structure having a desired periodicity and
a desired structure thickness is completed, selectively anodizing
each second-metal layer edge until a desired thickness of second
oxide is formed, and selectively etching back one of the two
different oxides (the first oxide and second oxide), thus forming
an imprinting stamp having the desired periodicity. The imprinting
stamp has salient portions of one of the two oxides, separated by
recesses where the other oxide has been etched back.
[0056] For this method embodiment, the first and second metals
should comprise different metals, at least one of which can be
electrochemically oxidized, such as Al, Ta, Nb, W, Bi, Sb, Ag, Cd,
Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and Cr, or their
alloys, mixtures, or combinations.
[0057] For example, in FIGS. 6A-6C, the first-metal-oxide layers
45, 75, and 85 may be tantalum oxide formed by anodization of
tantalum layers, and second-metal layers 20 and 40 may be aluminum.
Together, the alternating tantalum oxide and aluminum layers form a
periodic layered structure. Edges suitable for the next steps may
be made, for example, by cutting, sawing, cleaving, and/or
polishing the periodic layered structure. After the edges of
aluminum layers 20 and 40 are anodized, aluminum oxide edges 100
are formed. These aluminum oxide edges 100 may be etched back with
a selective etchant comprising a mixture of phosphoric acid
(H.sub.3PO.sub.4), chromium oxide (CrO.sub.3), and water.
[0058] A suitable etchant, for example, has about 5-40 wt. %,of
H.sub.3PO.sub.4, about 2-15 wt. %, of CrO.sub.3, and etching
temperature of 80-100.degree. C. This etchant does not appreciably
etch the aluminum or tantalum oxide. When the aluminum oxide has
been etched out, remaining recesses 105 have reproducible profiles
and dimensions suitable for use as an imprinting stamp. The salient
portions composed of tantalum oxide also have suitable reproducible
profiles and dimensions.
[0059] Another embodiment of an imprinting stamp also has alternate
layers of metal and oxide. To fabricate such an embodiment, a
suitable substrate is provided, a quantity of metal is deposited
over the substrate to form a metal layer having a metal edge, and
the metal is anodized until a second-layer thickness of oxide is
formed. As in other embodiments described hereinabove, the
depositing and anodizing steps are repeated alternately until a
periodic layered structure having a desired periodicity and a
desired structure thickness is completed. Then the metal edge of
each metal layer is selectively etched back to form a recess,
whereby an imprinting stamp having the desired periodicity is
formed. Alternatively, in principle, the edges of the oxide layers
could be selectively etched using a suitably selective etchant,
leaving salient portions of the metal layers. This etching may be
performed for a predetermined time.
[0060] The method embodiments described above for fabricating an
imprinting stamp for lithography are specific examples of a more
general method for fabricating an imprinting stamp. The more
general method includes steps of providing a periodic layered
structure comprising layers of at least two materials differing in
etch rate, and of selectively etching back the edge of at least one
of the at least two materials to form recesses separated by salient
portions, thereby making an imprinting stamp having the periodicity
of the periodic layered structure. In the imprinting stamp
embodiments disclosed herein, the periodic layered structure is
provided by methods using electrochemical oxidation as described
above.
[0061] FIGS. 7 and 8 are cross-sectional side elevation views of
two particular embodiments of periodic layered structures. FIG. 9
shows a top plan view of a test pattern embodiment corresponding to
the embodiments shown in FIGS. 7 and 8. Thus, FIGS. 7 and 8 are
both cross-sectional side elevation views of the structure
illustrated by FIG. 9.
[0062] The periodic layered structures of FIGS. 7 and 8 are formed
on non-planar substrates, resulting in layers 20 and 30 having
edges which are not simply straight lines. The non-planar
substrates of FIGS. 7 and 8 are formed by deposition and patterning
of a dielectric material feature 240 on a base substrate 10 (which
itself may be planar as shown in FIGS. 7 and 8). The dielectric
feature 240 in FIG. 7 has sloped sides, while the dielectric
feature 240 in FIG. 8 has vertical sides. In both cases, periodic
layered structures are formed over the dielectric features 240.
Layers of the metals for the periodic layered structures are
deposited over the dielectric features 240, using conventional
deposition methods (such as atomic layer deposition (ALD), or
chemical vapor deposition (CVD)), including deposition on side
walls of the dielectric features 240. The layers may be deposited
conformally over the dielectric features 240, using conventional
conformal deposition methods. The metals are anodized as described
hereinabove. While, for clarity of illustration, FIGS. 7 and 8 show
only a few layers 20 and 30 in the periodic layered structures,
many more layers may be used. In each case, when the periodic
layered structure is complete, a dielectric material 250 may be
deposited over the periodic layered structure. For many
applications, material 250 may be the same material as dielectric
feature 240. The resulting surface may be planarized, as shown in
FIGS. 7 and 8. (FIGS. 7 and 8 show the structures after this
planarization step.) If the periodic layered structures of FIGS. 7
and 8 are formed in accordance with the embodiments described above
for imprinting stamps for lithography, they may be suitably etched
to form the imprinting stamps, with imprinting features that are
not simply straight lines, but form angles, corners, and curves, as
shown in FIGS. 7-9.
INDUSTRIAL APPLICABILITY
[0063] Methods performed in accordance with the invention are
useful for fabricating imprinting stamps for lithography.
Structures made in accordance with the invention may also be used
for photonic-crystal applications and, more generally, for many
other applications requiring superlattices or other periodic
layered structures.
[0064] Although the foregoing has been a description and
illustration of specific embodiments of the invention, various
modifications and changes thereto can be made by persons skilled in
the art without departing from the scope and spirit of the
invention as defined by the following claims. For example, the
steps of the various method embodiments may be performed in the
order recited, or the order of steps may be varied somewhat.
Functionally equivalent materials may be substituted for materials
described in this specification and the claims. It is not intended
that the methods and the resulting structures described should
exclude from the periodic layered structures incorporation of
layers that are not anodized. Thus, for example, an insulating
layer that is not formed by electrochemical oxidation may be
included in the stack of layers of the periodic layered structure.
For specific examples of this, a metal or semiconductor layer may
be thermally oxidized, or a layer of silicon oxide, silicon
nitride, or diamond may be periodically deposited as one or more
sublayers of the stack if desired for a particular application.
* * * * *