U.S. patent application number 11/704332 was filed with the patent office on 2008-02-07 for method of manufacturing nanostructures.
This patent application is currently assigned to FUJIFILM Corporation. Invention is credited to Shinya Suzuki, Tadabumi Tomita.
Application Number | 20080029399 11/704332 |
Document ID | / |
Family ID | 38552230 |
Filed Date | 2008-02-07 |
United States Patent
Application |
20080029399 |
Kind Code |
A1 |
Tomita; Tadabumi ; et
al. |
February 7, 2008 |
Method of manufacturing nanostructures
Abstract
A method of manufacturing structures includes a stripping step
in which an aluminum member that includes an aluminum substrate and
an anodized layer present on a surface of the aluminum substrate
and that serves as a cathode is electrolyzed to strip the anodized
layer from the aluminum substrate to obtain a structure composed of
the anodized layer. Electrolysis in the stripping step is carried
out in such a way that a current passes over a surface of the
anodized layer. Structures having a well-ordered array of pits can
be obtained in a short time without the use of substances such as
chromic acid that are deleterious to the environment.
Inventors: |
Tomita; Tadabumi; (Shizuoka,
JP) ; Suzuki; Shinya; (Shizuoka, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
FUJIFILM Corporation
Tokyo
JP
|
Family ID: |
38552230 |
Appl. No.: |
11/704332 |
Filed: |
February 9, 2007 |
Current U.S.
Class: |
205/112 ;
205/640; 205/668 |
Current CPC
Class: |
C25D 11/24 20130101;
C25D 11/00 20130101; B81B 2201/0214 20130101; B81C 99/008 20130101;
C25D 11/20 20130101 |
Class at
Publication: |
205/112 ;
205/640; 205/668 |
International
Class: |
C25D 5/00 20060101
C25D005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 28, 2006 |
JP |
2006-052860 |
Claims
1. A method of manufacturing a structure comprising: a stripping
step in which an aluminum member that includes an aluminum
substrate and an anodized layer present on a surface of the
aluminum substrate and that serves as a cathode is electrolyzed to
strip the anodized layer from the aluminum substrate to thereby
obtain a structure composed of the anodized layer, wherein
electrolysis in the stripping step is carried out in such a way
that a current passes over a surface of the anodized layer.
2. The method of manufacturing the structure according to claim 1,
wherein the electrolysis in the stripping step is carried out in
such a way that the current passes only over the surface of the
anodized layer.
3. The method of manufacturing the structure according to claim 2,
wherein the electrolysis in the stripping step is carried out in a
state where an electrolytic solution is in contact with the surface
of the anodized layer but is in contact with neither the aluminum
substrate nor edges of the anodized layer.
4. The method of manufacturing the structure according to any one
of claims 1 to 3, wherein the electrolysis in the stripping step
reaches completion when the current falls to a value of 0.1
A/dm.sup.2 or below.
5. A structure obtained by the method according to any one of
claims 1 to 4.
6. A method of manufacturing a structure comprising: a stripping
step in which an aluminum member that includes an aluminum
substrate and an anodized layer present on a surface of the
aluminum substrate and that serves as a cathode is electrolyzed to
strip the anodized layer from the aluminum substrate to thereby
obtain a structure composed of the aluminum substrate having pits
formed therein; and an anodizing step in which the aluminum
substrate having the pits formed therein are anodized to obtain the
structure composed of the aluminum substrate having on a surface
thereof a micropore-bearing anodized layer, wherein electrolysis in
the stripping step is carried out in such a way that a current
passes over a surface of the anodized layer.
7. The method of manufacturing the structure according to claim 6,
wherein the electrolysis in the stripping step is carried out in
such a way that the current passes only over the surface of the
anodized layer.
8. The method of manufacturing the structure according to claim 7,
wherein the electrolysis in the stripping step is carried out in a
state where an electrolytic solution is in contact with the surface
of the anodized layer but is in contact with neither the aluminum
substrate nor edges of the anodized layer.
9. The method of manufacturing the structure according to any one
of claims 6 to 8, wherein the electrolysis in the stripping step
reaches completion when the current falls to a value of 0.1
A/dm.sup.2 or below.
10. The method of manufacturing the structure according to any one
of claim 6 to 9, further comprising a chemical dissolution
treatment step which follows the anodizing step and in which the
structure composed of the aluminum substrate having on the surface
thereof the micropore-bearing anodized layer is subjected to a
chemical dissolution treatment.
11. A structure obtained by the method according to any one of
claims 6 to 10.
Description
[0001] The entire contents of all documents cited in this
specification are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a nanostructure and its
manufacturing method.
[0003] In the technical field of metal and semiconductor thin
films, wires and dots, it is known that the movement of free
electrons becomes confined at sizes smaller than some
characteristic length, as a result of which singular electrical,
optical and chemical phenomena become observable. Such phenomena
are called "quantum mechanical size effects" or simply "quantum
size effects." Functional materials which employ such singular
phenomena are under active research and development. Specifically,
materials having structures smaller than several hundred nanometers
in size, typically called microstructures or nanostructures, are
the subject of current efforts in material development.
[0004] Methods for manufacturing such microstructures include
processes in which a nanostructure is directly manufactured by
semiconductor fabrication technology, including micropatterning
technology such as photolithography, electron beam lithography, or
x-ray lithography.
[0005] Of particular note is the considerable amount of research
being conducted today on processes for manufacturing nanostructures
having an ordered microstructure.
[0006] One method of forming an ordered structure in a
self-regulating manner is illustrated by an anodized alumina layer
(anodized layer) obtained by subjecting aluminum to anodizing
treatment in an electrolytic solution. It is known that a plurality
of micropores having diameters of about several nanometers to about
several hundreds of nanometers are formed in a regular arrangement
within the anodized layer. It is also known that when a completely
ordered arrangement is obtained by the self-ordering treatment of
this anodized layer, hexagonal columnar cells will be theoretically
formed, each cell having a base in the shape of a regular hexagon
centered on a micropore, and that the lines connecting neighboring
micropores will form equilateral triangles.
[0007] For example, H. Masuda et al. (Jpn. J. Appl. Phys., Vol. 37,
Part 2, No. 11A, pp. L1340-1342 (Nov. 1, 1998), FIG. 2) describes
an anodized layer having micropores whose pore size dispersion is
3% or less. In another related publication (Hyomen Gijutsu Binran
[Handbook of Surface Technology], edited by The Surface Finishing
Society of Japan (Nikkan Kogyo Shimbun Co., Ltd., 1998), pp.
490-553), it is described that micropores are naturally formed in
an anodized layer as oxidation proceeds. Moreover, H. Masuda
("Highly ordered metal nanohole array based on anodized alumina",
Kotai Butsuri [Solid State Physics], Vol. 31, No. 5, pp. 493-499
(1996)) has proposed the formation of a gold dot array on a silicon
substrate using a porous anodized layer as the mask.
[0008] A plurality of micropores take on a honeycomb-like structure
in which the micropores are formed parallel in a direction
substantially vertical to the substrate surface, and at
substantially equal intervals. This point is deemed to be the most
distinctive characteristic of anodized layers in terms of material.
Another remarkable feature of anodized layers, thought to be absent
in other materials, is the ability to relatively freely control the
pore diameter, pore spacing and pore depth (see Masuda, 1996).
[0009] Known examples of applications for anodized layers include
various types of devices, such as nanodevices, magnetic devices,
and luminescent devices. For example, JP 2000-31462 A mentions a
number of applications, including magnetic devices in which the
micropores are filled with the magnetic metal cobalt or nickel,
luminescent devices in which the micropores are filled with the
luminescent material ZnO, and biosensors in which the micropores
are filled with enzymes/antibodies.
[0010] In addition, in the field of biosensing, JP 2003-268592 A
describes an example in which a structure obtained by filling the
interior of micropores in an anodized layer with a metal is used as
a sample holder for Raman spectroscopy.
[0011] Raman scattering is the effect where, when incident light
(photons) strikes particles and scatters, inelastic collisions with
the particles arise, causing a change in energy. Raman scattering
is used as a technique for spectroscopic analysis, but a current
challenge is how to enhance the intensity of the scattered light
used in measurement so as to improve the sensitivity and accuracy
of analysis.
[0012] A phenomenon that enhances Raman scattered light is known as
the surface-enhanced resonance Raman scattering (SERRS) effect.
This effect is one where the scattering of certain kinds of
molecules absorbed onto the surface of, for example, a metal
electrode, a sol, a crystal, a vapor-deposited film or a
semiconductor, is enhanced relative to within a solution. A
remarkable enhancement effect of from 10.sup.11 to 10.sup.14 times
is seen particularly with gold and silver. The mechanism underlying
the SERRS effect is not yet fully understood, although the surface
plasmon resonance described below is believed to play a role. Use
of the plasmon resonance principle as a means for enhancing the
Raman scattering intensity is a stated object in JP 2003-268592 A
as well.
[0013] Plasmon resonance is the effect where, when the surface of a
noble metal such as gold or silver is irradiated with light so that
the metal surface is placed in an excited state, plasmon
waves--which are localized electron density waves, interact with
electromagnetic waves (resonance excitation) to form a resonance
state. Surface plasmon resonance (SPR) is a type of plasmon
resonance in which, when the metal surface is irradiated with
light, free electrons at the metal surface acquire an excited state
and collectively oscillate, generating a surface plasmon wave which
in turn generates a strong electric field.
[0014] In the near-surface region where plasmon resonance arises,
that is, in the region within about 200 nm from the surface, an
electric field enhancement of several decades (e.g., 10.sup.8 to
10.sup.10 times) can be seen, and a distinct rise is observed in
various optical effects. For example, when light is directed at a
prism having thereon a vapor-deposited thin film of a suitable
metal such as gold at an angle larger than the critical angle,
changes in the dielectric constant of the thin-film surface can be
detected to a high sensitivity as changes in the intensity of the
reflected light due to the surface plasmon resonance effect.
[0015] Specifically, using a SPR sensor which employs the surface
plasmon resonance effect, quantitative measurement of reactions and
bonds between biomolecules and kinetic analysis can be carried out
without labeling and in real time. SPR sensors are used in research
on immune response, signal transduction, and interactions between
various substances such as proteins and nucleic acids. Recently, a
paper was even published on analyzing trace dioxins using an SPR
sensor (Karube, et al., Analytica Chimica Acta 434, No. 2, 223-230
(2001)).
[0016] Various methods are being studied for increasing plasmon
resonance, including techniques that involve localizing plasmons by
using the metal in the form of discrete particles rather than as a
thin film. For example, JP 2003-268592 A describes a technique in
which localization is induced by providing metal particles on
well-ordered pores in an anodized layer.
[0017] According to a research article, when localized plasmon
resonance with metal particles is used, if the metal particles are
present in close proximity to each other, the electric field
strength is enhanced in the gaps between the metal particles,
thereby achieving a state that makes it easier to generate a
plasmon resonance (see T. Okamoto: "A study on metal nanoparticle
interactions and biosensors", found in an Internet search on Nov.
27, 2003 at http://www.plasmon.jp/reports/okamoto.pdf).
[0018] In processes which use the self-ordering treatment of an
anodized layer to fabricate an anodized layer having a well-ordered
arrangement of micropores thereon, it has hitherto been customary
to carry out a self-ordering step in which electrolysis is carried
out for an extended period of time under specific electrolytic
conditions so as to promote the orderly formation of micropores,
then to carry out a layer removal step in which the anodized layer
obtained in the self-ordering step is dissolved in a mixed aqueous
solution of chromic acid and phosphoric acid so that the bottom
portion of the micropores where the pores are the most regularly
arrayed is revealed at the surface.
[0019] JP 61-88495 A describes a process for obtaining a porous
layer by performing anodizing treatment on an aluminum member or an
aluminum alloy so as to form a porous layer, then using reverse
electrolysis means to strip just the porous layer from the parent
material.
[0020] An article in Aruminium Kenkyukaishi (Vol. 201, No. 7, pp.
7-8 (1985)) describes a process in which the barrier layer is
thinned by an electric current recovery technique, following which
reverse electrolysis means is used to strip the anodized layer from
the aluminum member.
SUMMARY OF THE INVENTION
[0021] However, in spite of differences due to the thickness of the
anodized layer, it is generally necessary for the layer removal
step which uses a mixed aqueous solution of chromic acid and
phosphoric acid to be carried out over a long period of time
ranging from several hours to well over ten hours. Also, the
anodized layer is dissolved, making effective use of this layer
impossible. Moreover, such a process has required the use of
chromic acid, which is a substance that is bad for the
environment.
[0022] When use is made of a process that involves reducing the
film thickness by an electric current recovery method, then
employing reverse electrolysis means to strip the anodized layer
from the aluminum member, as described in JP 61-88495 A and
Aruminium Kenkyukaishi (Vol. 201, No. 7, pp. 7-8 (1985)), it was
found that the electric current recovery method leads to the
formation of very small branched pores at the bottom of the
micropores, resulting in the disruption of regularly arrayed pits
at the surface of the aluminum member obtained by delamination.
Therefore, even when an anodized layer is formed by additionally
performing anodizing treatment on the resulting aluminum member,
use in applications such as a sample holder for Raman spectroscopy
has not been possible.
[0023] It is therefore an object of the invention to provide a
manufacturing method from which structures having a well-ordered
array of pits can be obtained in a short time without the use of
substances such as chromic acid that are deleterious to the
environment. Another object of the invention is to provide
structures obtained by such manufacturing method.
[0024] The inventors have made intensive studies to achieve the
above objects and found that by performing electrolysis using as
the cathode an aluminum member having an anodized layer so that the
current passes only over the surface of the anodized layer, a
structure having a well-ordered array of pits can be obtained in a
short period of time.
[0025] Accordingly, the invention provides the following (1) to
(11).
(1) A method of manufacturing a structure comprising:
[0026] a stripping step in which an aluminum member that includes
an aluminum substrate and an anodized layer present on a surface of
the aluminum substrate and that serves as a cathode is electrolyzed
to strip the anodized layer from the aluminum substrate to thereby
obtain a structure composed of the anodized layer,
[0027] wherein electrolysis in the stripping step is carried out in
such a way that a current passes over a surface of the anodized
layer.
(2) The method of manufacturing the structure according to (1)
above, wherein the electrolysis in the stripping step is carried
out in such a way that the current passes only over the surface of
the anodized layer.
[0028] (3) The method of manufacturing the structure according to
(2) above, wherein the electrolysis in the stripping step is
carried out in a state where an electrolytic solution is in contact
with the surface of the anodized layer but is in contact with
neither the aluminum substrate nor edges of the anodized layer.
(4) The method of manufacturing the structure according to any one
of (1) to (3) above, wherein the electrolysis in the stripping step
reaches completion when the current falls to a value of 0.1
A/dm.sup.2 or below.
(5) A structure obtained by the method according to any one of (1)
to (4) above.
(6) A method of manufacturing a structure comprising:
[0029] a stripping step in which an aluminum member that includes
an aluminum substrate and an anodized layer present on a surface of
the aluminum substrate and that serves as a cathode is electrolyzed
to strip the anodized layer from the aluminum substrate to thereby
obtain a structure composed of the aluminum substrate having pits
formed therein; and
[0030] an anodizing step in which the aluminum substrate having the
pits formed therein are anodized to obtain the structure composed
of the aluminum substrate having on a surface thereof a
micropore-bearing anodized layer,
[0031] wherein electrolysis in the stripping step is carried out in
such a way that a current passes over a surface of the anodized
layer.
(7) The method of manufacturing the structure according to (6)
above, wherein the electrolysis in the stripping step is carried
out in such a way that the current passes only over the surface of
the anodized layer.
[0032] (8) The method of manufacturing the structure according to
(7) above, wherein the electrolysis in the stripping step is
carried out in a state where an electrolytic solution is in contact
with the surface of the anodized layer but is in contact with
neither the aluminum substrate nor edges of the anodized layer.
(9) The method of manufacturing the structure according to any one
of (6) to (8) above, wherein the electrolysis in the stripping step
reaches completion when the current falls to a value of 0.1
A/dm.sup.2 or below.
[0033] (10) The method of manufacturing the structure according to
any one of (6) to (9), further comprising a chemical dissolution
treatment step which follows the anodizing step and in which the
structure composed of the aluminum substrate having on the surface
thereof the micropore-bearing anodized layer is subjected to a
chemical dissolution treatment.
(11) A structure obtained by the method according to any one of (6)
to (10) above.
[0034] The manufacturing method of the invention enables structures
having well-ordered arrays of pits to be obtained in a short period
of time.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] In the accompanying drawings:
[0036] FIG. 1 is a graph of the changes in electrical current over
time when electrolysis is performed, using an aluminum member
having an aluminum substrate and an anodized layer as the cathode,
in such a way that the current passes only over the surface of the
anodized layer;
[0037] FIGS. 2A to 2D show diagrams illustrating the inventive
method of manufacturing structures;
[0038] FIG. 3 shows a schematic diagram of a special jig that may
be used in reverse electrolysis; and
[0039] FIGS. 4A and 4B show diagrams illustrating a method for
computing the degree of ordering of pores.
DETAILED DESCRIPTION OF THE INVENTION
[0040] The invention is described more fully below.
[0041] The first aspect of the invention provides a method of
manufacturing a structure including a stripping step in which an
aluminum member that includes an aluminum substrate and an anodized
layer present on a surface of the aluminum substrate and that
serves as a cathode is electrolyzed to strip the anodized layer
from the aluminum substrate to thereby obtain a structure composed
of the anodized layer. In the stripping step, electrolysis is
carried out in such a way that a current passes only over a surface
of the anodized layer.
<Aluminum Member>
[0042] The aluminum member used in the invention has the aluminum
substrate and the anodized layer present on the surface of the
aluminum substrate. Such an aluminum member may be obtained by
performing anodizing treatment on the surface of the aluminum
substrate.
<Aluminum Substrate>
[0043] The aluminum substrate is not subject to any particular
limitation. Illustrative examples include commercial aluminum
substrates; substrates made of low-purity aluminum (e.g., recycled
material) on which high-purity aluminum has been vapor-deposited;
substrates such as silicon wafers, quartz or glass whose surface
has been covered with high-purity aluminum by a process such as
vapor deposition or sputtering; and resin substrates on which
aluminum has been laminated.
[0044] Of the aluminum substrate, the surface on which an anodized
layer is provided by anodizing treatment has an aluminum purity of
preferably at least 99.5 wt %, and more preferably at least 99.80
wt %, but preferably less than 99.99 wt %, and more preferably
99.95 wt % or less. At an aluminum purity of 99.5 wt % or more, the
pore arrangement will be sufficiently well-ordered, and at an
aluminum purity of less than 99.99 wt %, inexpensive production is
possible.
[0045] It is preferable for the surface of the aluminum substrate
to be subjected beforehand to degreasing and mirror-like finishing
treatment.
<Degreasing>
[0046] Degreasing is carried out with a suitable substance such as
an acid, alkali or organic solvent so as to dissolve and remove
organic substances (primarily oils) adhering to the surface. Known
degreasers may be used in degreasing treatment.
[0047] For example, degreasing may be carried out using any of
various commercially available degreasers by the prescribed
method.
[0048] Degreasing may be carried out by, for example, immersing the
aluminum substrate for a length of time during which only a small
amount of air bubbles evolve from the aluminum surface in an
aqueous solution of sodium hydroxide having a pH of 10 to 13 and a
temperature of about 30.degree. C. to about 50.degree. C. or in an
aqueous sulfuric acid solution having a pH of 1 to 4 and a
temperature of about 40.degree. C. to about 70.degree. C.
[0049] Preferred degreasing treatment is exemplified by washing the
aluminum substrate with acetone, then immersing the substrate in
sulfuric acid having a pH of 4 and a temperature of 50.degree. C.
This method is advantageous because it removes oils on the aluminum
surface without substantially any dissolution of the aluminum.
<Mirror-Like Finishing>
[0050] Mirror-like finishing is carried out to eliminate surface
asperities on the aluminum substrate and improve the uniformity and
reproducibility of sealing treatment by a process such as
electrodeposition.
[0051] In the practice of the invention, mirror-like finishing is
not subject to any particular limitation, and may be carried out
using any suitable method known in the art. Illustrative examples
of suitable methods include polishing with various commercial
abrasive cloths, methods that combine the use of various commercial
abrasives (e.g., diamond, alumina) with buffing, electrolytic
polishing and chemical polishing. These methods may be used in
appropriate combinations.
[0052] Examples of electrolytic polishing and chemical polishing
methods include various methods mentioned in the 6.sup.th edition
of Aluminum Handbook (Japan Aluminum Association, 2001), pp.
164-165.
[0053] Mirror-like finishing is preferably performed by a method in
which polishing is performed using abrasives while changing over
time the abrasive used from one having coarser particles to one
having finer particles and thereafter electrolytic polishing is
performed. In such a case, the final abrasive used is preferably
one having a grit size of 1500. This method is capable of removing
rolling streaks that may be formed during rolling when the aluminum
substrate has been produced by a process including rolling.
[0054] Mirror-like finishing enables a surface having, for example,
an average surface roughness R.sub.a of 0.03 .mu.m or less and a
glossiness of at least 70% to be obtained. The average surface
roughness R.sub.a is preferably 0.02 .mu.m or less. The glossiness
is preferably at least 80%.
[0055] The glossiness is the specular reflectance which can be
determined in accordance with JIS Z8741-1997 (Method 3: 60.degree.
Specular Gloss) in a direction perpendicular to the rolling
direction.
<Anodizing Treatment>
[0056] Any conventionally known method can be used for anodizing
treatment. More specifically, a self-ordering method to be
described below is preferably used.
[0057] The self-ordering method is a method which enhances the
orderliness by using the regularly arranging nature of micropores
in the anodized layer and eliminating factors that may disturb an
orderly arrangement. Specifically, an anodized layer is formed on
high-purity aluminum at a voltage appropriate for the type of
electrolytic solution and at a low speed over an extended period of
time (e.g., from several hours to well over ten hours).
[0058] Typical examples of self-ordering methods include those
described in J. Electrochem. Soc. Vol. 144, No. 5, p. L128 (May
1997); Jpn. J. Appl. Phys. Vol. 35, Part 2, No. 1B, p. L126 (1996);
Appl. Phys. Lett. Vol. 71, No. 19, p. 2771 (Nov. 10, 1997), and in
the above-referenced article by Masuda (1998).
[0059] Use of high-purity materials and treatment performed at a
relatively low temperature for a long period of time at a specified
voltage determined according to the electrolytic solution are the
technical features of the methods described in these well-known
articles. More specifically, these methods each use a material
having an aluminum purity of at least 99.99 wt % to carry out the
self-ordering method under the conditions indicated below. [0060]
(1) 0.3 mol/L sulfuric acid, 0.degree. C., 27 V, 450 minutes (J.
Electrochem. Soc., 1997) [0061] (2) 0.3 mol/L sulfuric acid,
10.degree. C., 25 V, 750 minutes (J. Electrochem. Soc., 1997)
[0062] (3) 0.3 mol/L oxalic acid, 17.degree. C., 40 to 60 V, 600
minutes (Jpn. J. Appl. Phys., 1996) [0063] (4) 0.04 mol/L oxalic
acid, 3.degree. C., 80 V, layer thickness, 3 .mu.m (Appl. Phys.
Lett., 1997) [0064] (5) 0.3 mol/L phosphoric acid, 0.degree. C.,
195 V, 960 minutes (Appl. Phys. Lett., 1997).
[0065] The self-ordering anodizing treatment used in this invention
may be carried out by, for example, a method that involves passing
an electrical current through the aluminum substrate as the anode
in a solution having an acid concentration of 1 to 10 wt %.
Solutions that may used in anodizing treatment include any one or
combinations of two or more of the following: oxalic acid, sulfuric
acid, citric acid, malonic acid, tartaric acid and phosphoric
acid.
[0066] The conditions of the self-ordering anodizing treatment vary
depending on the electrolytic solution used, and thus cannot be
strictly specified. However, it is generally suitable for the
electrolyte concentration to be 0.01 to 10 mol/L, the temperature
of the solution to be 0 to 20.degree. C., the current density to be
0.1 to 10 A/dm.sup.2, the voltage to be 15 to 240 V, the amount of
electricity to be 3 to 10,000 C/dm.sup.2, and the period of
electrolysis to be 30 to 1,000 minutes.
[0067] As for the electrolysis, potentiostatic electrolysis is
preferably performed.
[0068] The anodized layer has the following properties.
[0069] The thickness, including the barrier layer, is preferably at
least 0.1 .mu.m, and more preferably at least 1 .mu.m. Within this
range, the micropores are even more highly ordered.
[0070] Moreover, the thickness, including the barrier layer, is
preferably not more than 100 .mu.m. Within this range, stripping
from the aluminum substrate in the subsequently described stripping
step is easy.
[0071] The barrier layer has a thickness of preferably not more
than 600 nm, more preferably from 5 to 400 nm, and even more
preferably from 10 to 80 nm. Within this range, the strippability
in the subsequently described stripping step is excellent.
[0072] The pore diameter is from 10 to 500 nm, preferably from 15
to 100 nm, and more preferably from 20 to 80 nm. Within this range,
when the micropores are filled with metal, the micropores are more
uniformly filled with the metal.
[0073] The pore diameter has a coefficient of variation which,
while not subject to any particular limitation, is preferably less
than 30%, and more preferably from 5 to 20%.
[0074] The coefficient of variation (CV) of the pore diameter is an
indicator of the variation in the pore size. It is defined by the
following equation. Coefficient of Variation of Pore
Diameter=(standard deviation of pore diameter)/(average pore
diameter)
[0075] The micropores have a period of preferably from 20 to 700
nm, more preferably from 25 to 600 nm, and even more preferably
from 25 to 150 nm.
[0076] The period of the micropores has a coefficient of variation
which, while not subject to any particular limitation, is
preferably less than 30%, and more preferably at least 5% but less
than 20%.
[0077] The area ratio occupied by the micropores is preferably from
10 to 70%.
<Pore Widening Treatment>
[0078] In the practice of the invention, the anodized layer of the
above-described aluminum member may be subjected to pore widening
treatment.
[0079] Pore widening treatment, which is carried out after
anodizing treatment, is performed by immersing the aluminum
substrate in an aqueous solution of an acid or an alkali so as to
dissolve the anodized layer and enlarge the diameter of the
micropores. This makes it easy to control the regularity of the
micropore array.
[0080] When pore widening treatment is carried out with an aqueous
acid solution, it is preferable to use an aqueous solution of an
inorganic acid such as sulfuric acid, phosphoric acid, nitric acid
or hydrochloric acid, or a mixture thereof. It is desirable for the
aqueous acid solution to have a concentration of 1 to 10 wt % and a
temperature of 25 to 40.degree. C.
[0081] When pore widening treatment is carried out with an aqueous
alkali solution, it is preferable to use an aqueous solution of at
least one alkali selected from the group consisting of sodium
hydroxide, potassium hydroxide and lithium hydroxide. It is
preferable for the aqueous alkali solution to have a concentration
of 0.1 to 5 wt % and a temperature of 20 to 35.degree. C.
[0082] Specific examples of preferred solutions include a
40.degree. C. aqueous solution containing 50 g/L of phosphoric
acid, a 30.degree. C. aqueous solution containing 0.5 g/L of sodium
hydroxide, and a 30.degree. C. aqueous solution containing 0.5 g/L
of potassium hydroxide.
[0083] The immersion time in the aqueous acid solution or aqueous
alkali solution is preferably 8 to 60 minutes, more preferably 10
to 50 minutes, and even more preferably 15 to 30 minutes.
<Barrier Layer Thinning Treatment>
[0084] In one preferred embodiment of the invention, the
above-described aluminum member is an aluminum member in which the
barrier layer of the anodized layer has been thinned. When the
barrier layer has been thinned, the strippability in the
subsequently described stripping step is excellent.
[0085] The inventors have found that, by using a method which
gradually lowers rather than suddenly changing the voltage, i.e., a
method which lowers the voltage after anodizing treatment without
generating a current recovery period while maintaining a state of
constant current flow, the barrier layer of the anodized layer can
be thinned without a loss in the regularity of the arrangement of
micropores on the anodized layer. This is presumably because fine
branching does not arise owing to the fact that a current recovery
period is not generated.
[0086] Specifically, when the voltage of anodizing treatment is,
for example, 100 V or more, the voltage drop rate is preferably set
to 20 V/min or less, more preferably 10 V/min or less, and even
more preferably 5V/min or less.
[0087] The higher the current maintained, the better. Specifically,
the current is preferably at least 10 .mu.A/cm.sup.2, more
preferably at least 30 .mu.A/cm.sup.2, and even more preferably at
least 50 .mu.A/cm.sup.2.
[0088] If the current is too low, the regularity of the micropore
array is disrupted. Therefore, when the current falls to below 10
.mu.A/cm.sup.2 at the above-indicated rate, it is preferable to
stop the voltage drop and await a current flow of at least 10
.mu.A/cm.sup.2 before continuing the voltage drop.
<Other Treatment>
[0089] Other treatments may be performed as needed.
[0090] For example, when the structure of the invention is to be
used as a sample holder on which an aqueous solution will be
deposited to form a film, hydrophilizing treatment may be performed
to reduce the contact angle with water. Such hydrophilizing
treatment may be performed by a method known in the art.
[0091] Alternatively, when the inventive structure is to be used as
a sample holder for protein that will be denatured or decomposed
with acid, neutralizing treatment may be performed to neutralize
the acids that are used in anodizing treatment and remain as
residues on the aluminum surface. Such neutralizing treatment may
be performed by a method known in the art.
<Stripping Step>
[0092] The stripping step is an operation in which the anodized
layer and the aluminum substrate are separated from each other by
electrolysis using the above-described aluminum member as the
cathode to give a structure composed of the anodized layer. In the
stripping step, electrolysis is carried out using the aluminum
member as the cathode. Because this is the reverse of electrolysis
in anodizing treatment using the aluminum member as the anode, it
is referred to below as "reverse electrolysis."
[0093] In the stripping step, such reverse electrolysis generates
hydrogen at the boundary between the anodized layer and the
aluminum substrate in the aluminum member. Apparently, part of that
portion of the barrier layer, which belongs to the anodized layer
and lies at the boundary between the anodized layer and the
aluminum substrate, in contact with the aluminum substrate is
reduced and dissolved by the hydrogen, becoming aluminum ions,
causing the anodized layer and the aluminum substrate to separate
at the boundary therebetween.
[0094] In reverse electrolysis, electrolysis is performed by using
the above-described aluminum member as the cathode and passing
current along the surface of the anodized layer. This facilitates
stripping.
[0095] In particular, it is preferable to perform electrolysis so
that the electrical current passes only over the surface of the
anodized layer. That is, it is preferable to perform electrolysis
so that the current passes over the surface of the anodized layer
and does not pass through the aluminum substrate of the aluminum
member.
[0096] Specifically, electrolysis is performed in a state where,
for example, the electrolytic solution is in contact with the
surface of the anodized layer, but is in contact with neither the
edges of the anodized layer nor the aluminum substrate. The method
for achieving this state is not subject to any particular
limitation. Illustrative examples include methods in which only the
surface of the anodized layer is left exposed by masking or the use
of a special jig, following which the aluminum member is immersed
in an electrolytic solution; and methods in which the electrolytic
solution is supplied only to the surface of the anodized layer.
[0097] Methods that involve masking are carried out by using an
insulating material to cover those portions of the aluminum member
other than the surface of the anodized layer which is brought into
contact with the electrolytic solution.
[0098] The insulating material, while not subject to any particular
limitation, is preferably a material having a volume resistivity at
20.degree. C. of at least 10.sup.16 .OMEGA.m. Illustrative examples
include resins (natural resins and synthetic resins), rubbers,
ceramics (e.g., glass), metal oxides and mica.
[0099] Of these, a flexible synthetic resin is preferred. Examples
of such synthetic resins include polyvinyl chloride, polycarbonate,
acrylic resins, PET, epoxy resins, polyimides, polypropylene,
polyesters, polyethylene, saran and polyvinylidene chloride.
[0100] Preferred use can also be made of insulating tape composed
of a backing that is made of such a synthetic resin and is coated
with a pressure-sensitive adhesive (such tape is referred to below
as "adhesive tape"). Examples of adhesive tapes include epoxy films
(e.g., Super 10, produced by the 3M Company), polyimide films
(e.g., 1205, produced by the 3M Company), PTFE films (e.g., 62,
produced by the 3M Company), polyester films (e.g., 56, produced by
the 3M Company), plastic film tapes (e.g., Scotch electroplating
tape 470, produced by the 3M Company), polyester films (e.g., ELEP
masking tape N-300, produced by Nitto Denko Corporation), and
polypropylene tapes (e.g., DANPRON, produced by Nitto Denko
Corporation).
[0101] No particular limitation is imposed on the method of
covering with the insulating material. Illustrative examples
include methods that involve coating a liquid resin (e.g., an
adhesive) and methods that involve affixing adhesive tape. For
example, depending on the areas to be covered, these methods may be
used in combination, such as by affixing adhesive tape to the back
of the aluminum substrate, then coating the aluminum substrate and
the edges of the anodized layer with a liquid resin.
[0102] Of these, the method of affixing a resin tape to which a
pressure-sensitive adhesive has been applied is preferable from the
standpoint of the insulating properties within the electrolytic
solution and the ready availability of such tape.
[0103] The thickness of the insulating material in the covered
areas is preferably at least 1 .mu.m from the standpoint of
electrical insulating properties, and preferably from 10 to 200
.mu.m from the standpoint of handleability.
[0104] When a pressure-sensitive adhesive or an adhesive is applied
to the insulating material, for good coating uniformity and to
prevent deformation after bonding, it is preferable that the
coating thickness be from 10 to 100 .mu.m.
[0105] As mentioned above, masking is carried out by covering with
an insulating material the areas of the aluminum member other than
the surface of the anodized layer to be brought into contact with
the electrolytic solution. Specifically, the edges of the anodized
layer and the entire aluminum substrate are covered with the
insulating material.
[0106] It is also possible to cover just part of the anodized
layer. In such a case, it is preferable that a portion of the
anodized layer be covered, and that the remaining open area be
circular, elliptical or in a rectangular shape with rounded
corners, as this facilitates stripping. A circular shape is
especially preferred because reverse electrolysis can be carried
out uniformly in all parts of the region where it is carried
out.
[0107] Methods that involve the use of a special jig are not
subject to any particular limitation, provided the method is one
which uses a jig that allows only the surface of the anodized layer
to be exposed.
[0108] Anodes which may be used in reverse electrolysis are not
subject to any particular limitation. Illustrative examples include
platinum-plated titanium electrodes, platinum electrodes, and
carbon electrodes.
[0109] The electrolytic solution used in reverse electrolysis,
while not subject to any particular limitation, is preferably an
aqueous solution of an acid.
[0110] The aqueous acid solution has a pH of preferably 1 to 7,
more preferably 2 to 6, and even more preferably 2.5 to 5.5. The
aqueous acid solution has an electrical conductivity of preferably
from 0.01 to 100 mS/cm, and more preferably from 0.1 to 50
mS/cm.
[0111] When the aqueous acid solution has a pH and an electrical
conductivity in the above ranges, good stripping is achieved
without corrosion of the aluminum substrate or incomplete removal
of the anodized layer.
[0112] If the electrical conductivity of the aqueous acid solution
is too low, very small current values may not arise. In such a
case, it is preferable to make the ionic concentration in the
aqueous acid solution high so as to allow very small current values
to arise. On the other hand, if the ionic concentration in the
aqueous acid solution is too high, very small current values do
arise, but electrolysis ends in a short time, after which the
current value rises rapidly, making control difficult. Moreover, on
exceeding the time in which a very small current value is reached,
corrosion occurs.
[0113] Preferred examples of acids that may be used in the aqueous
acid solution include oxalic acid, sulfuric acid and phosphoric
acid.
[0114] Alternatively, use can be made of, for example, metallic
salt compounds which exhibit acidity when dissolved in water, and
organic compounds which exhibit acidity when dissolved in
water.
[0115] Illustrative examples of metallic salt compounds which
exhibit acidity when dissolved in water include aluminum oxalate,
aluminum sulfate, aluminum lactate, aluminum fluoride and aluminum
borate.
[0116] Preferred organic compounds which exhibit acidity when
dissolved in water are carboxylic acids. Suitable examples include
saturated aliphatic dicarboxylic acids such as adipic acid,
unsaturated aliphatic dicarboxylic acids such as maleic acid,
aromatic monocarboxylic acids such as benzoic acid, aromatic
dicarboxylic acids such as phthalic acid, and aromatic
oxycarboxylic acids such as salicylic acid.
[0117] Alternatively, use can be made of salts which exhibit
neutrality when dissolved in water, i.e., neutral salts. Suitable
examples of neutral salts include carbonates such as ammonium
carbonate and borates such as ammonium borate.
[0118] In cases where a neutral salt is used, one preferred
embodiment is to prepare a mixed bath which also includes an
additive such as a fluoride, a carbonic acid derivative or an acid
amide. The fluoride is exemplified by ammonium fluoride. The
carbonic acid derivative is exemplified by guanidine carbonate,
urea and formaldehyde. The acid amide is exemplified by
acetamide.
[0119] Of these, the use of oxalic acid, aluminum oxalate, sulfuric
acid, aluminum sulfate or a mixture thereof is preferred. Aluminum
sulfate and sulfuric acid are especially preferred from the
standpoint of availability and wastewater treatability.
[0120] A preferred embodiment involves the use of an electrolytic
solution of the same type as that used in the above-described
anodizing treatment.
[0121] The reverse electrolysis conditions vary depending on the
electrolytic solution used, and thus cannot be strictly
specified.
[0122] When the electrolytic solution is an aqueous solution of
oxalic acid, the concentration is preferably from 0.4 to 10 wt %;
when it is an aqueous solution of sulfuric acid, the concentration
is preferably from 2 to 20 wt %; and when it is an aqueous solution
of phosphoric acid, the concentration is preferably from 0.4 to 5
wt %.
[0123] The electrolytic solution generally has a temperature of
from 0 to 50.degree. C., and preferably from 10 to 35.degree.
C.
[0124] The current density is preferably from 1 to 400 mA/dm.sup.2,
more preferably from 5 to 400 mA/dm.sup.2, and even more preferably
from 10 to 300 mA/dm.sup.2. In the above range, stripping can be
carried out more uniformly.
[0125] The voltage is preferably from 5 to 350 V, more preferably
from 8 to 300 V, and even more preferably from 10 to 240 V. The
voltage is preferably lower than the electrolysis voltage applied
when forming the anodized layer.
[0126] In one preferred embodiment, the voltage is held constant.
In another preferred embodiment, the voltage is increased over
time.
[0127] In the practice of the invention, it is preferable to end
electrolysis after the current value has fallen to 0.1 A/dm.sup.2
or below.
[0128] FIG. 1 is a graph of the changes in electrical current over
time when electrolysis is performed, using an aluminum member
having an aluminum substrate and an anodized layer as the cathode,
in such a way that the current passes only over the surface of the
anodized layer.
[0129] From immediately after the start of electrolysis (T.sub.1)
until T.sub.3, there is a time (T.sub.2) at which the current value
reaches a maximum. Because gas evolution is observed in the
interval between T.sub.1 and T.sub.3, the hydrogen ions H.sup.+
generated at the boundary between the barrier layer of the anodized
layer and the aluminum substrate presumably become hydrogen
molecules as a result of electrochemical reactions.
[0130] The current subsequently undergoes an abrupt drop at time
T.sub.4. At this time, it is thought that separation between the
anodized layer and the aluminum substrate has been completed. The
current value then remains low until T.sub.5. During the interval
between T.sub.4 and T.sub.5, it is assumed that hydrogen gas
accumulates between the anodized layer and the aluminum
substrate.
[0131] When reverse electrolysis is continued further, the current
value reaches a peak at T.sub.6. This is thought to be due to the
formation of cracks in the anodized layer.
[0132] Next, after T.sub.7, the electrolytic solution presumably
penetrates between the anodized layer and the aluminum substrate
via the cracks in the anodized layer, causing corrosion to
proceed.
[0133] In the practice of the invention, it is preferable to end
electrolysis during the interval between T.sub.4 and T.sub.5. In
this way, corrosion due to cracking of the anodized layer and
penetration of the electrolytic solution can be prevented. To this
end, it is desirable to monitor the current value and to end
electrolysis once the current value falls to 0.1 A/dm.sup.2 or
below.
[0134] The current value in the T.sub.4T.sub.5 interval is
typically not more than 30%, and generally not more than 10%, of
the maximum value (at time T.sub.2). Therefore, in one preferred
embodiment, the current value is monitored and electrolysis is
brought to completion once the current value falls to not more than
30%, or to not more than 10%, of the maximum current value.
[0135] Because metallic aluminum is exposed on the back and edges
of the aluminum plate, if masking or the like is not carried out,
the current will concentrate in the metallic aluminum and will not
readily pass over the anodized layer, making stripping difficult to
carry out and thus resulting in a poor stripping uniformity.
[0136] Moreover, in the absence of masking or the like, most of the
current passes through the metallic aluminum, making it difficult
to gauge the stripped state of the anodized layer from changes in
the current value. As noted above, by covering the edges of the
anodized layer and the entire aluminum substrate with an insulating
material, the stripped state of the anodized layer can be
understood from changes in the current values.
[0137] Following the end of electrolysis, the stripped anodized
layer can be separated from the aluminum substrate by, for example,
cutting along the boundary between areas where current was passed
and areas where current was not passed. To prevent the anodized
layer from breaking up due to stress during such cutting, the
anodized layer can be removed after being secured to adhesive tape
or the like.
[0138] There will be times where the aluminum substrate obtained
from the stripping step has remaining thereon a remnant of the
anodized layer in a thickness of up to 0.2 .mu.m over up to 10% of
the stripped surface. To make use of such an aluminum substrate, it
is desirable that it be free of any remnants of the anodized
layer.
[0139] Accordingly, in such a case, it is preferable to remove
remnants of the anodized layer by carrying out chemical treatment
following reverse electrolysis. Specifically, removal can be
effected by chemical treatment (chemical polishing treatment) using
a method which involves bringing any of various acidic or alkaline
aqueous solutions into contact with the anodized layer. The
chemical treatment (chemical polishing treatment) method is not
subject to any particular limitation, and may be carried out by a
method known in the art.
[0140] Examples of acidic aqueous solutions include aqueous
solutions of phosphoric acid, aqueous solutions of sulfuric acid,
aqueous solutions of nitric acid, aqueous solutions of oxalic acid,
and mixed aqueous solutions of chromic acid and phosphoric acid. Of
these, mixed aqueous solutions of chromic acid and phosphoric acid
are preferred.
[0141] The acidic aqueous solution has a pH of preferably -0.3 to
6, more preferably 0 to 4, and even more preferably 2 to 4.
[0142] The temperature of the acidic aqueous solution is preferably
from 20 to 60.degree. C., and more preferably from 30 to 50.degree.
C.
[0143] The treatment time is preferably from 1 second to 6 hours,
more preferably from 5 seconds to 3 hours, and even more preferably
from 10 seconds to 1 hour.
[0144] Examples of alkaline aqueous solutions include aqueous
solutions of sodium hydroxide, aqueous solutions of sodium
carbonate and aqueous solutions of potassium hydroxide.
[0145] The alkaline aqueous solution has a pH of preferably 10 to
13.5, and more preferably 11 to 13.
[0146] The temperature of the alkaline aqueous solution is
preferably from 10 to 50.degree. C., and more preferably from 20 to
40.degree. C.
[0147] The treatment time is preferably from 1 second to 10
minutes, more preferably from 2 seconds to 1 minute, and more
preferably from 3 to 30 seconds.
[0148] It is also possible to use these methods in combination. One
such example is a method that involves alkali treatment in which
the surface of the aluminum substrate is minimally dissolved with
an alkaline aqueous solution to remove remnants of the anodized
layer, following which desmutting treatment is carried out in which
neutralization product that has formed as a result of such alkali
treatment is dissolved and removed with an acidic aqueous
solution.
[0149] A specific illustration would be a process that involves
alkali treatment in which the aluminum substrate is brought into
contact with an aqueous solution containing 5 wt % of sodium
hydroxide (temperature, 70.degree. C.) for 10 seconds, followed by
desmutting treatment in which the aluminum substrate is brought
into contact with an aqueous solution containing 30 wt % of
sulfuric acid (temperature, 50.degree. C.) for 60 seconds.
[0150] Additional examples are various methods described for
chemical pretreatment in Aruminiumu Gijutsu Binran [Handbook of
Aluminum Technology], edited by the Light Metal Association (Kallos
Publishing Co., 1996), pp. 926-929. Of these, preferred examples
include alkali degreasing, acid degreasing, electrolytic
degreasing, and combinations thereof, which have an aluminum
substrate surface layer dissolving action; and alkali etching
treatment, acid etching treatment, as well as combinations thereof,
which have a strong aluminum substrate surface layer dissolving
action.
[0151] Moreover, if a portion of the anodized layer remains behind
even after the stripping step has been carried out, the residual
anodized layer can be completely removed by alternately carrying
out the anodizing treatment and the stripping step a number of
times.
[0152] When the anodized layer is stripped by this method, because
current recovery is not carried out, the orderliness of the array
of pits on the aluminum substrate side is not disturbed. Hence,
this method is advantageous in the subsequently described second
embodiment of the invention.
[0153] Examples of preferred conditions for reverse electrolysis
are given below.
<Preferred Conditions 1>
[0154] Cathode: Anodized layer obtained by anodization with aqueous
solution of oxalic acid (concentration, 0.3 mol/L; temperature,
17.degree. C.) at a voltage of 40 V for a treatment time of 60
minutes; layer thickness, 60 .mu.m. [0155] Anode: Carbon electrode.
[0156] Electrolytic solution: Aqueous solution of aluminum sulfate
having a concentration of 0.04 g/L (aluminum ion basis), a pH of
3.8, an electrical conductivity of 0.6 mS/cm, and a temperature of
33.degree. C. [0157] Voltage: 40 V (voltage setting).
[0158] In the first aspect of the invention, the stripping step
treatment time is very short compared with the time required in a
conventional film removal step involving dissolution with a mixed
aqueous solution of chromic acid and phosphoric acid. Therefore,
structures can be efficiently produced by the method in the first
aspect of the invention.
[0159] Moreover, in a film removal step, when the aluminum oxide
content (as Al.sub.2O.sub.3) of a mixed aqueous solution of chromic
acid and phosphoric acid exceeds 15 g/L, the solvency abruptly
deteriorates, making it necessary to replace the solution with
fresh treatment solution. Because the anodized layer used in the
invention generally has a large thickness, the amount of aluminum
oxide which dissolves out in a single treatment is large, resulting
in rapid degradation of the treatment solution.
[0160] By contrast, in the present invention, because the anodized
layer is stripped off in a solid state at the boundary with the
aluminum substrate, it can easily be separated off with a filter or
the like. Hence, the aqueous acid solution used in reverse
electrolysis does not deteriorate.
[0161] Therefore, the treatment time and the amount of aqueous acid
solution consumed in the stripping step carried out in the
invention are respectively much shorter and much smaller than the
treatment time and the amount of treatment solution consumed in a
conventional film removal step carried out with a mixed aqueous
solution of chromic acid and phosphoric acid.
[0162] In the stripping step, the barrier layer is dissolved by the
above-described reverse electrolysis, giving a structure composed
of the anodized layer.
[0163] At the same time, the aluminum substrate from which the
anodized layer has been stripped becomes an aluminum substrate
having a plurality of pits. This aluminum substrate having a
plurality of pits may be used in the subsequently described second
aspect of the invention. This is explained in detail below in
conjunction with the accompanying diagrams.
[0164] FIGS. 2A to 2D show diagrams illustrating the inventive
method of manufacturing structures.
[0165] FIG. 2A is a schematic cross-sectional view of an aluminum
member prior to the stripping step. As shown in FIG. 2A, an
aluminum member 10 has an aluminum substrate 12 and an anodized
layer 14 present on the surface of the aluminum substrate 12.
Micropores 16 are present within the anodized layer 14, and a
barrier layer 18 is situated below the micropores 16.
[0166] FIGS. 2B and 2C are respectively schematic cross-sectional
views of a structure and an aluminum substrate obtained by the
stripping step.
[0167] A structure 20 shown in FIG. 2B is obtained by dissolving
the barrier layer 18 of the anodized layer 14 in the aluminum
member 10 shown in FIG. 2A, and is composed of the anodized layer
having pits 22.
[0168] An aluminum substrate 24 shown in FIG. 2C is obtained by
dissolving the barrier layer 18 of the anodized layer 14 in the
aluminum member 10 shown in FIG. 2A, and has pits 26.
[0169] The second aspect of the invention provides a method of
manufacturing a structure including a stripping step in which an
aluminum member that includes an aluminum substrate and an anodized
layer present on a surface of the aluminum substrate and that
serves as a cathode is electrolyzed to strip the anodized layer
from the aluminum substrate to thereby obtain a structure composed
of the aluminum substrate having pits formed therein, and an
anodizing step in which the aluminum substrate having the pits
formed therein are anodized to obtain the structure composed of the
aluminum substrate having on a surface thereof a micropore-bearing
anodized layer. Electrolysis in the stripping step is carried out
in such a way that a current passes only over a surface of the
anodized layer.
[0170] The stripping step in the second aspect of the invention is
carried out in the same way as the stripping step in the first
aspect of the invention.
<Anodizing Treatment Step>
[0171] In the second aspect of the invention, an anodizing
treatment step is carried out following the stripping step.
[0172] In the anodizing treatment step, anodizing treatment is
performed on the aluminum substrate having pits obtained from the
stripping step, thereby giving a structure composed of the aluminum
substrate having on the surface a micropore-bearing anodized
layer.
[0173] In the aluminum substrate having pits obtained from the
stripping step, the shape at the bottom of the barrier layer where
the micropores are the most highly ordered is in the form of
surface pits (see FIG. 2C). These surface pits are thus
substantially semi-spherical and regularly arranged like the
micropores.
[0174] The anodizing treatment step may be carried out using a
method known in the art. Specifically, this step is carried out in
the same way as the anodizing treatment used for obtaining the
above-described aluminum member.
[0175] It is preferable to use the same type of electrolytic
solution as that used in the above-described reverse electrolysis.
In this way, reverse electrolysis and the anodizing treatment step
can be carried out in the same electrolytic bath. Moreover, even
when these are carried out in separate electrolytic baths, there
are no adverse effects from the carryover of solution into the
anodizing treatment bath.
[0176] In this anodizing treatment step, the regularly arrayed pits
on the surface of the aluminum substrate serve as the starting
points for anodizing treatment, leading to the formation of an
anodized layer having an orderly array of micropores.
[0177] Therefore, the anodizing treatment step provides a structure
composed of an aluminum substrate having on the surface thereof an
anodized layer with an orderly array of micropores.
[0178] FIG. 2D is a schematic cross-sectional view of a structure
obtained by the anodized treatment step. A structure 28 shown in
FIG. 2D is obtained by subjecting the aluminum substrate 24 shown
in FIG. 2C to anodizing treatment so as to form an anodized layer
30. During anodizing treatment, micropores 32 are formed with the
pits 26 in the aluminum substrate 24 serving as the starting
points. Therefore, the structure 28 is composed of an aluminum
substrate 34 having on the surface the anodized layer 30 with the
micropores 32.
[0179] In the second aspect of the invention, the treatment time
for the stripping step is extremely short compared with the time
required for a conventional film removal step involving dissolution
with a mixed aqueous solution of chromic acid and phosphoric acid.
Hence, structures can be efficiently manufactured by the method
according to the second aspect of the invention.
<Chemical Dissolution Treatment Step>
[0180] In one preferred embodiment of the second aspect of the
invention, following the above-described anodizing treatment step,
a chemical dissolution treatment step is carried out in which
chemical dissolution treatment is performed on the structure
composed of an aluminum substrate having a micropore-bearing
anodized layer on its surface. By carrying out the chemical
dissolution treatment step, the diameter of the pores becomes more
uniform.
[0181] Chemical dissolution treatment can be carried out in the
same manner as in the above-described pore widening treatment.
<Structure>
[0182] The structure composed of an anodized layer obtained in the
first aspect of the invention and the structure composed of an
aluminum substrate having a micropore-bearing anodized layer on the
surface obtained in the second aspect of the invention both have
regularly arrayed pits or micropores, and can therefore be employed
in various applications.
[0183] For example, by carrying out sealing treatment to fill the
pits or micropores with a metal, the structure can be used as a
sample holder for Raman spectroscopy.
[0184] Alternatively, the structure can be used as a nanoimprint
mold.
[0185] In addition, structures composed of an anodized layer
obtained according to the first aspect of the invention can be used
as separation filters.
<Sealing Treatment>
[0186] The metal used in sealing treatment is not subject to any
particular limitation, so long as it is an element having metal
bonds that include free electrons. However, a metal in which
plasmon resonance has been recognized is preferred. Of these, it is
known that gold, silver, copper, nickel and platinum are known to
readily give rise to plasmon resonance (Gendai Kagaku (Contemporary
Chemistry), pp. 20-27 (September 2003)), and are thus preferred.
Gold and silver are especially preferred because of the ease of
electrodeposition and colloidal particle formation.
[0187] Sealing may be carried out using any suitable known
technique without particular limitation.
[0188] Examples of preferred techniques include electrodeposition,
and a method which involves coating the structure of the present
invention with a dispersion of metal colloidal particles, then
drying. The metal is preferably in the form of single particles or
agglomerates.
[0189] An electrodeposition method known in the art may be used.
For example, in the case of gold electrodeposition, use may be made
of a process in which the aluminum member is immersed in a
30.degree. C. dispersion containing 1 g/L of HAuCl.sub.4 and 7 g/L
of H.sub.2SO.sub.4 and electrodeposition is carried out at a
constant voltage of 11 V (regulated with an autotransformer such as
SLIDAC) for 5 to 6 minutes.
[0190] An example of the electrodeposition method which employs
copper, tin and nickel is described in detail in Gendai Kagaku
(Contemporary Chemistry), pp. 51-54 (January 1997)). Use can be
made of this method as well.
[0191] The dispersions employed in methods which use metal
colloidal particles can be obtained by a conventionally known
method. Illustrative examples include methods of preparing fine
particles by low-vacuum vapor deposition and methods of preparing
metal colloids by reducing an aqueous solution of a metal salt.
[0192] The metal colloidal particles have an average particle size
of preferably 1 to 200 nm, more preferably 1 to 100 nm, and even
more preferably 2 to 80 nm.
[0193] Preferred use can be made of water as the dispersion medium
employed in the dispersion. Use can also be made of a mixed solvent
composed of water and a solvent that is miscible with water, such
as an alcohol, illustrative examples of which include ethyl
alcohol, n-propyl alcohol, i-propyl alcohol, 1-butyl alcohol,
2-butyl alcohol, t-butyl alcohol, methyl cellosolve and butyl
cellosolve.
[0194] No particular limitation is imposed on the technique used
for coating the anodized layer with the dispersion of metal
colloidal particles. Suitable examples of such techniques include
bar coating, spin coating, spray coating, curtain coating, dip
coating, air knife coating, blade coating and roll coating.
[0195] Preferred examples of dispersions that may be employed in
methods which use metal colloidal particles include dispersions of
gold colloidal particles and dispersions of silver colloidal
particles.
[0196] Dispersions of gold colloidal particles that may be used
include those described in JP 2001-89140 A and JP 11-80647 A. Use
can also be made of commercial products.
[0197] Dispersions of silver colloidal particles preferably contain
particles of silver-palladium alloys because these are not affected
by the acids which leach out of the anodized layer. The palladium
content in such a case is preferably from 5 to 30 wt %.
[0198] Application of the dispersion is followed by cleaning that
may be appropriately performed using a solvent such as water. As a
result of such cleaning, only the particles filled into the
micropores remain whereas particles that have not been filled into
the micropores are removed.
[0199] The amount of metal deposited after sealing is preferably
100 to 500 mg/m.sup.2.
[0200] The surface porosity after sealing treatment is preferably
not more than 20%. The surface porosity after sealing treatment is
defined as the total surface area of the openings in unsealed pits
or micropores relative to the surface area of the structure
surface. When the surface porosity is in the above range, a
stronger localized plasmon resonance can be obtained.
[0201] At a pore diameter of 50 nm or more, it is preferable to use
a sealing method that employs metal colloidal particles. At a pore
diameter of less than 50 nm, the use of an electrodeposition
process is preferred. Suitable use can also be made of a
combination of both.
[0202] In the structure that has been subjected to sealing
treatment, metal seals the pits or micropores and is present on the
surface of the structure as particles.
[0203] It is generally preferable for the intervals between these
metal particles to be short so as to increase Raman enhancement.
The optimal interval is affected by the size and shape of the metal
particles. Depending on the viscosity of the liquid or the
molecular weight of the substance serving as the Raman spectroscopy
sample, problems such as the inability of the sample to fully enter
between the metal particles may arise.
[0204] Accordingly, the interval between the metal particles cannot
be strictly specified, although it is generally preferable for the
interval to be in a range of 1 to 400 nm, more preferably 5 to 300
nm, and even more preferably 10 to 200 nm. Within the above range,
Raman enhancement increases and the substance serving as the sample
is generally able to enter between the metal particles.
[0205] As used herein, "metal particle interval" refers to the
shortest distance between the surfaces of neighboring
particles.
<Raman Enhancement Owing to Localized Plasmon Resonance>
[0206] Raman enhancement refers to an effect in which the Raman
scattering intensity of molecules adsorbed onto the metal is
enhanced by a factor of about 10.sup.5 to 10.sup.6, and is called
"surface-enhanced Raman scattering" (SERS). The above-referenced
publication Gendai Kagaku No. 9, pp. 20-27 (2003) states that Raman
enhancement can be obtained by localized plasmon resonance using
particles of metals such as gold, silver, copper, platinum and
nickel.
[0207] Compared with the conventional technique, the structure that
has been subjected to sealing treatment can generate a
high-intensity localized plasmon resonance and thus, when used in
Raman spectroscopy, enables a stronger Raman enhancement effect to
be achieved. This demonstrates the utility of sample holders for
Raman spectroscopy obtained from such a sealed structure.
[0208] Sample holders for Raman spectroscopy obtained from the
sealed structure are used in much the same way as conventional
sample holders for Raman spectroscopy. Specifically, by irradiating
with light the Raman spectroscopy sample holder obtained from the
sealed structure and measuring the Raman scattering intensity of
the reflected or transmitted light, the properties of a substance
which is held on the sample holder and is near the metal are
detected.
<Nanoprinting>
[0209] The inventive structure may be used as a nanoimprint mold.
Specifically, by casting a resin or the like into the pits or
micropores in the inventive structure and curing the resin, a
substrate having projections can be obtained. This substrate having
the projections can be used as, for example, an optical device.
EXAMPLES
[0210] Examples are given below by way of illustration and should
not be construed as limiting the invention.
1. Fabrication of Structure
Examples 1 to 15, and Comparative Example 1
[0211] The substrate was subjected to, in order, mirror-like
finishing treatment, self-ordering anodizing treatment, masking and
reverse electrolysis, thereby obtaining a structure composed of an
anodized layer, and an aluminum substrate. The aluminum substrate
thus obtained was successively subjected to main anodizing
treatment and pore widening treatment, giving a structure composed
of the aluminum substrate.
[0212] Each treatment step is described in detail below.
(1) Substrate
[0213] The structure was manufactured using Substrate 1 below.
[0214] Substrate 1: High-purity aluminum. Produced by Wako Pure
Chemical Industries, Ltd. Purity, 99.99 wt %; thickness, 0.4 mm.
(2) Mirror-Like Finishing Treatment
[0215] The above substrate was subjected to the following
mirror-like finishing treatment.
<Mirror-Like Finishing Treatment>
[0216] Mirror-like finishing treatment was performed by
electrolytic polishing. Electrolytic polishing was carried out for
5 minutes using an electrolytic solution of the composition
indicated below (temperature, 65.degree. C.), using the substrate
as the anode and a carbon electrode as the cathode, and at a
constant current of 12.5 A/dm.sup.2. TABLE-US-00001
<Electrolytic Solution Composition> 85 wt % Phosphoric acid
(Wako Pure Chemical 1,320 mL Industries, Ltd.) Aqueous solution of
sulfuric acid (50 wt %) 600 mL Pure water 20 mL
(3) Self-Ordering Anodizing Treatment (Formation of Pits)
[0217] The surface of the mirror-like finished substrate was
subjected to self-ordering anodizing treatment under either of the
following sets of conditions A and B, thereby forming pits. These
pits served as the starting points for micropore formation in the
subsequently described main anodizing treatment.
<Self-Ordering Anodizing Treatment>
<Condition A>
[0218] An aqueous solution of sulfuric acid having a concentration
of 0.3 mol/L and a temperature of 16.degree. C. was prepared using
sulfuric acid (a reagent produced by Kanto Chemical Co., Inc.). The
substrate (area of treatment, 5 cm.times.10 cm) was immersed in
this aqueous solution of sulfuric acid, and self-ordering anodizing
treatment was carried out for 7 hours under constant voltage
conditions at a voltage of 25 V, thereby forming on the substrate
an anodized layer having a film thickness of 90 .mu.m.
[0219] In self-ordering anodizing treatment, use was made of a
SUS304 electrode as the cathode, NeoCool BD36 (Yamato Scientific
Co., Ltd.) as the cooling system, Pairstirrer PS-100 (Tokyo
Rikakikai Co., Ltd.) as the stirring and warming unit, and a
GP0650-2R unit (Takasago, Ltd.) as the power supply. PET tape
(DANPRON Tape, produced by Nitto Denko Corporation) was affixed
beforehand to the side of the substrate not facing the electrode
surface so as to keep it from being anodized.
<Condition B>
[0220] An aqueous solution of oxalic acid having a concentration of
0.5 mol/L and a temperature of 16.degree. C. was prepared using
oxalic acid dihydrate (a reagent produced by Kanto Chemical Co.,
Inc.). The substrate (area of treatment, 5 cm.times.10 cm) was
immersed in this aqueous solution of oxalic acid, and self-ordering
anodizing treatment was carried out for 5 hours under constant
voltage conditions at a voltage of 40 V, thereby forming on the
substrate an anodized layer having a film thickness of 45
.mu.m.
[0221] In self-ordering anodizing treatment, use was made of a
SUS304 electrode as the cathode, NeoCool BD36 (Yamato Scientific
Co., Ltd.) as the cooling system, Pairstirrer PS-100 (Tokyo
Rikakikai Co., Ltd.) as the stirring and warming unit, and a
GP0650-2R unit (Takasago, Ltd.) as the power supply. PET tape
(DANPRON Tape, produced by Nitto Denko Corporation) was affixed
beforehand to the side of the substrate not facing the electrode
surface so as to keep it from being anodized.
<Film Thickness Measurement Method>
[0222] The substrate on which an anodized layer had been formed was
bent, the edge (fracture plane) in a portion of the specimen where
cracking occurred was examined with an ultrahigh-resolution
scanning electron microscope (Hitachi S-900, manufactured by
Hitachi, Ltd.) at an acceleration voltage of 20 V and a
magnification of 200.times., and the film thickness was measured.
Ten spots were randomly selected on each specimen, and the average
value of the measurements was used as the film thickness. The film
thickness value at each of the ten spots was within a range of the
average value .+-.10%.
(4) Masking
[0223] Masking was carried out on the substrate that had been
subjected to self-ordering anodizing treatment. The masking method
is shown in Table 1. Notations used in Table 1 are explained below.
[0224] "None": After self-ordering anodizing treatment, the PET
tape was peeled off and the substrate was subjected to reverse
electrolysis without carrying out masking. [0225] "Back": After
self-ordering anodizing treatment, the substrate was subjected to
reverse electrolysis with the PET tape left in place. [0226]
"Back+Edges": After self-ordering anodizing treatment, the PET tape
was left in place. In addition, the edges of the substrate were
coated with a two-component mixed epoxy resin (Araldite, available
from Nichiban Co., Ltd.) and the substrate was left to stand for
one day to allow the resin to cure, then was subjected to reverse
electrolysis. [0227] "Back+Edges+Surface A": After the
above-described "Back+Edges" treatment, PET tape (DANPRON Tape,
produced by Nitto Denko Corporation) having a circular opening of
1.7 cm radius was affixed to the surface on the anodized layer side
of the substrate. The PET tape was affixed so that the opening was
positioned at substantially the center of the substrate and the
cured resin on the edges of the substrate was also covered. [0228]
"Back+Edges+Surface B": After the above-described "Back+Edges"
treatment, PET tape (DANPRON Tape, produced by Nitto Denko
Corporation) having an elliptical opening with a major axis of 2.5
cm and a minor axis of 1.4 cm was affixed to the surface on the
anodized layer side of the substrate. The PET tape was affixed so
that the opening was positioned at substantially the center of the
substrate and the cured resin on the edges of the substrate was
also covered. [0229] "Back+Edges+Surface C": After the
above-described "Back+Edges" treatment, PET tape (DANPRON Tape,
produced by Nitto Denko Corporation) having an opening in the form
of a square measuring 3 cm on a side with corners that are rounded
to a radius of 5 mm was affixed to the surface on the anodized
layer side of the substrate. The PET tape was affixed so that the
opening was positioned at substantially the center of the substrate
and the cured resin on the edges of the substrate was also covered.
[0230] "Back+Edges+Surface D": After the above-described
"Back+Edges" treatment, PET tape (DANPRON Tape, produced by Nitto
Denko Corporation) having an opening in the form of a square
measuring 3 cm on a side was affixed to the surface on the anodized
layer side of the substrate. The PET tape was affixed so that the
opening was positioned at substantially the center of the substrate
and the cured resin on the edges of the substrate was also covered.
[0231] "Special Jig": A special jig was positioned as shown in FIG.
3 on the anodized layer 14 side of the aluminum substrate 12
(aluminum member 10) on which the anodized layer 14 had been
formed, and the substrate was subjected to reverse electrolysis.
The special jig had a cathode 40, a packing 42 with an inner space
that accommodates the cathode 40, and an electrolytic solution
inlet 44 and an electrolytic solution outlet 46, both provided in
the packing 42. The cathode 40 had a diameter of 3 cm. The inner
space of the packing 42 had a diameter of 3 cm and a depth of 2 cm.
A check valve (not shown) which also served as a pressure valve was
provided in the electrolytic solution outlet 46. During reverse
electrolysis, the electrolytic solution (indicated by hatching in
FIG. 3) flowed through the electrolytic solution inlet 44 into the
inner space of the packing 42, and flowed out from the inner space
of the packing 42 through the electrolytic solution outlet 46.
Moreover, the packing 42 was pressed against the anodized layer 14
so that the edges of the packing 42 were in a state of close
contact with the anodized layer 14 to prevent the electrolytic
solution from leaking. FIG. 3 is a schematic view illustrating a
special jig that may be employed in reverse electrolysis. Details
such as the size of the micropores 16 relative to the size of the
special jig differ from reality. (5) Reverse Electrolysis
[0232] Following masking, reverse electrolysis was carried out,
using the above-described substrate on which an anodized layer had
been formed as the cathode and a platinum electrode as the anode,
in an aqueous solution of aluminum sulfate having a concentration
of 4.5 g/L and a temperature of 33.degree. C. and under constant
voltage conditions at a voltage of 16 V. The substrate on which the
anodized layer had been formed was placed in an aqueous solution of
aluminum sulfate with the planar direction of the substrate
oriented vertically. The anodized layer was thus stripped from the
aluminum substrate.
[0233] In reverse electrolysis, monitoring of the current value was
carried out. Table 1 shows the maximum current value, the current
value at the end of reverse electrolysis, and the current ratio
(current value at the end of reverse electrolysis/maximum current
value).
[0234] In Example 15, instead of constant voltage conditions,
reverse electrolysis was carried out by gradually increasing the
voltage from 0 V to 17.8 V at a rate of 2 V/min in a linear manner
(in Table 1, a maximum current value and a current ratio are not
shown for this example).
[0235] Following the completion of reverse electrolysis, PET tape
(DANPRON Tape, produced by Nitto Denko Corporation) was affixed to
the surface of the anodized layer side. The boundary between the
region which came into contact with the electrolytic solution and
the region which did not come into contact with the electrolytic
solution was then scored with a cutter, and the anodized layer was
separated from the aluminum substrate.
(6) Main Anodizing Treatment
[0236] The aluminum substrate obtained by stripping off the
anodized layer in reverse electrolysis was subjected to a main
anodizing treatment. Aside from setting the treatment time to 2
minutes, the main anodizing treatment was carried out under the
same set of conditions A or B as were used in self-ordering
anodizing treatment.
(7) Pore Widening Treatment
[0237] The aluminum substrate obtained following the main anodizing
treatment was subjected to pore widening treatment to enhance the
uniformity of the subsequently described sealing treatment. Pore
widening treatment was carried out by immersing the aluminum
substrate for 15 minutes in an aqueous solution of phosphoric acid
having a concentration of 5 wt % (temperature, 30.degree. C.).
2. Evaluation of Stripped State
[0238] The state of separation between the anodized layer and the
aluminum substrate following reverse electrolysis was evaluated in
each of the above examples.
[0239] That is, the stripped surface of the aluminum substrate
following reverse electrolysis was visually examined. By inking in
with a pen any white, darkened areas that lacked specular gloss and
carrying out image analysis, the area ratio of the region having
specular gloss was computed, based on which the uniformity of
stripping was assessed.
[0240] The results are shown in Table 1. In the table, the
"Stripping uniformity" was rated as A when the area ratio of
regions having a specular gloss was more than 95% and up to 100%,
as B when the area ratio of regions having a specular gloss was
more than 90% and up to 95%, as C when the area ratio of regions
having a specular gloss was more than 85% and up to 90%, as D when
the area ratio of regions having a specular gloss was more than 80%
and up to 85%, as E when the area ratio of regions having a
specular gloss was more than 70% and up to 80%, and as F when the
area ratio of regions having a specular gloss was up to 70%.
3. Evaluation of Structure Composed of Aluminum Substrate
[0241] The degree of ordering, which is an indicator of the
regularity of the micropores, was determined for the aluminum
substrate following the main anodizing treatment. The results are
shown in Table 1.
[0242] The degree of ordering is defined by the following formula
(1). Degree of Ordering(%)=B/A.times.100 (1)
[0243] In Formula (1), A represents the total number of micropores
in a measurement region; and B represents the number of specific
micropores in the measurement region for which, when a circle is
drawn so as to be centered on the center (of gravity) of a specific
micropore and so as to be of the smallest radius that is internally
tangent to the edge of another micropore, the circle includes the
centers (of gravity) of six micropores other than the specific
micropore.
[0244] FIGS. 4A and 4B show diagrams illustrating the method for
computing the degree of ordering of the pores. Formula (I) is
explained more fully below in conjunction with FIGS. 4A and 4B.
[0245] With regard to a micropore 1 shown in FIG. 4A, when a circle
3 is drawn so as to be centered on the center (of gravity) of the
micropore 1 and so as to be of the smallest radius that is
internally tangent to the edge of another micropore (inscribed in a
micropore 2), the interior of the circle 3 includes the centers (of
gravity) of six micropores other than the micropore 1. Therefore,
the micropore 1 is included in B.
[0246] With regard to a micropore 4 shown in FIG. 4B, when a circle
6 is drawn so as to be centered on the center (of gravity) of the
micropore 4 and so as to be of the smallest radius that is
internally tangent to the edge of another micropore (inscribed in a
micropore 5), the interior of the circle 6 includes the centers (of
gravity) of five micropores other than the micropore 4. Therefore,
the micropore 4 is not included in B. With regard to a micropore 7
shown in FIG. 4B, when a circle 9 is drawn so as to be centered on
the center (of gravity) of the micropore 7 and so as to be of the
smallest radius that is internally tangent to the edge of another
micropore (inscribed in a micropore 8), the interior of the circle
9 includes the centers (of gravity) of seven micropores other than
the micropore 7. Therefore, the micropore 7 is not included in B.
TABLE-US-00002 TABLE 1 Current Reverse Maximum value at end
Self-ordering electrolysis current of reverse Current Degree of
anodizing voltage value electrolysis ratio Stripping ordering
condition Masking method (V) (mA/dm.sup.2) (mA/dm.sup.2) (%)
uniformity (%) Comp. Ex. 1 A None 25 500 500 100 F -- Example 1 A
Back 25 300 200 67 E 50 Example 2 A Back + Edges 25 250 60 24 C 50
Example 3 A Back + Edges 25 250 100 40 D 50 Example 4 A Back +
Edges 25 250 150 60 E 50 Example 5 A Back + Edges + Surface A 25
250 60 24 A 70 Example 6 A Back + Edges + Surface B 25 250 60 24 A
60 Example 7 A Back + Edges + Surface C 25 250 60 24 B 50 Example 8
A Back + Edges + Surface D 25 250 60 24 C 50 Example 9 A Back +
Edges + Surface A 20 250 20 8 C 50 Example 10 A Back + Edges +
Surface A 18 250 20 8 B 60 Example 11 A Back + Edges + Surface A 15
250 30 12 A 70 Example 12 A Back + Edges + Surface A 13 250 50 20 E
60 Example 13 B Back + Edges + Surface A 40 250 60 24 A 70 Example
14 A Special Jig 25 250 60 24 A 70 Example 15 A Back + Edges +
Surface A 0 .fwdarw. 17.8 -- 30 -- A 50
[0247] As is apparent from Table 1, the inventive methods of
manufacturing structures (Examples 1 to 15) provided in each case
an excellent stripping uniformity, and the structures composed of
an aluminum substrate thus obtained all had well-ordered arrays of
pores.
[0248] By contrast, when reverse electrolysis was carried out in
such a way that the current passed through the aluminum substrate
(Comparative Example 1), the stripping uniformity was poor.
* * * * *
References