US20080029399A1

US20080029399A1 - Method of manufacturing nanostructures

Info

Publication number: US20080029399A1
Application number: US11/704,332
Authority: US
Inventors: Tadabumi Tomita; Shinya Suzuki
Original assignee: Fujifilm Corp
Current assignee: Fujifilm Corp
Priority date: 2006-02-28
Filing date: 2007-02-09
Publication date: 2008-02-07
Also published as: CN101074486B; JP4824430B2; CN101074486A; JP2007231340A

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.

Description

The entire contents of all documents cited in this specification are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a nanostructure and its manufacturing method.
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.
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.
Of particular note is the considerable amount of research being conducted today on processes for manufacturing nanostructures having an ordered microstructure.
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.
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.
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).
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.
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.
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.
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 10¹¹to 10¹⁴times 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.
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.
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., 10⁸to 10¹⁰times) 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.
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)).
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.
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).
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.
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.
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

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.
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.
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.
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.
Accordingly, the invention provides the following (1) to (11).
(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 (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.
(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/dm²or 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:
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 (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.
(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/dm²or below.
(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.
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

In the accompanying drawings:
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;
FIGS. 2A to 2D show diagrams illustrating the inventive method of manufacturing structures;
FIG. 3 shows a schematic diagram of a special jig that may be used in reverse electrolysis; and
FIGS. 4A and 4B show diagrams illustrating a method for computing the degree of ordering of pores.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.
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>
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>
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.
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.
It is preferable for the surface of the aluminum substrate to be subjected beforehand to degreasing and mirror-like finishing treatment.
<Degreasing>
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.
For example, degreasing may be carried out using any of various commercially available degreasers by the prescribed method.
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° C. to about 50° C. or in an aqueous sulfuric acid solution having a pH of 1 to 4 and a temperature of about 40° C. to about 70° C.
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° C. This method is advantageous because it removes oils on the aluminum surface without substantially any dissolution of the aluminum.
<Mirror-Like Finishing>
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.
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.
Examples of electrolytic polishing and chemical polishing methods include various methods mentioned in the 6^thedition of Aluminum Handbook (Japan Aluminum Association, 2001), pp. 164-165.
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.
Mirror-like finishing enables a surface having, for example, an average surface roughness R_aof 0.03 μm or less and a glossiness of at least 70% to be obtained. The average surface roughness R_ais preferably 0.02 μm or less. The glossiness is preferably at least 80%.
The glossiness is the specular reflectance which can be determined in accordance with JIS Z8741-1997 (Method 3: 60° Specular Gloss) in a direction perpendicular to the rolling direction.
<Anodizing Treatment>
Any conventionally known method can be used for anodizing treatment. More specifically, a self-ordering method to be described below is preferably used.
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).
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).
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.

(1) 0.3 mol/L sulfuric acid, 0° C., 27 V, 450 minutes (J. Electrochem. Soc., 1997)
(2) 0.3 mol/L sulfuric acid, 10° C., 25 V, 750 minutes (J. Electrochem. Soc., 1997)
(3) 0.3 mol/L oxalic acid, 17° C., 40 to 60 V, 600 minutes (Jpn. J. Appl. Phys., 1996)
(4) 0.04 mol/L oxalic acid, 3° C., 80 V, layer thickness, 3 μm (Appl. Phys. Lett., 1997)
(5) 0.3 mol/L phosphoric acid, 0° C., 195 V, 960 minutes (Appl. Phys. Lett., 1997).

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.
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° C., the current density to be 0.1 to 10 A/dm², the voltage to be 15 to 240 V, the amount of electricity to be 3 to 10,000 C/dm², and the period of electrolysis to be 30 to 1,000 minutes.
As for the electrolysis, potentiostatic electrolysis is preferably performed.
The anodized layer has the following properties.
The thickness, including the barrier layer, is preferably at least 0.1 μm, and more preferably at least 1 μm. Within this range, the micropores are even more highly ordered.
Moreover, the thickness, including the barrier layer, is preferably not more than 100 μm. Within this range, stripping from the aluminum substrate in the subsequently described stripping step is easy.
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.
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.
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%.
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)
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.
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%.
The area ratio occupied by the micropores is preferably from 10 to 70%.
<Pore Widening Treatment>
In the practice of the invention, the anodized layer of the above-described aluminum member may be subjected to pore widening treatment.
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.
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° C.
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° C.
Specific examples of preferred solutions include a 40° C. aqueous solution containing 50 g/L of phosphoric acid, a 30° C. aqueous solution containing 0.5 g/L of sodium hydroxide, and a 30° C. aqueous solution containing 0.5 g/L of potassium hydroxide.
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>
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.
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.
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.
The higher the current maintained, the better. Specifically, the current is preferably at least 10 μA/cm², more preferably at least 30 μA/cm², and even more preferably at least 50 μA/cm².
If the current is too low, the regularity of the micropore array is disrupted. Therefore, when the current falls to below 10 μA/cm²at the above-indicated rate, it is preferable to stop the voltage drop and await a current flow of at least 10 μA/cm²before continuing the voltage drop.
<Other Treatment>
Other treatments may be performed as needed.
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.
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>
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.”
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.
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.
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.
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.
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.
The insulating material, while not subject to any particular limitation, is preferably a material having a volume resistivity at 20° C. of at least 10¹⁶Ω·m. Illustrative examples include resins (natural resins and synthetic resins), rubbers, ceramics (e.g., glass), metal oxides and mica.
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.
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).
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.
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.
The thickness of the insulating material in the covered areas is preferably at least 1 μm from the standpoint of electrical insulating properties, and preferably from 10 to 200 μm from the standpoint of handleability.
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 μm.
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.
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.
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.
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.
The electrolytic solution used in reverse electrolysis, while not subject to any particular limitation, is preferably an aqueous solution of an acid.
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.
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.
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.
Preferred examples of acids that may be used in the aqueous acid solution include oxalic acid, sulfuric acid and phosphoric acid.
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.
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.
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.
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.
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.
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.
A preferred embodiment involves the use of an electrolytic solution of the same type as that used in the above-described anodizing treatment.
The reverse electrolysis conditions vary depending on the electrolytic solution used, and thus cannot be strictly specified.
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 %.
The electrolytic solution generally has a temperature of from 0 to 50° C., and preferably from 10 to 35° C.
The current density is preferably from 1 to 400 mA/dm², more preferably from 5 to 400 mA/dm², and even more preferably from 10 to 300 mA/dm². In the above range, stripping can be carried out more uniformly.
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.
In one preferred embodiment, the voltage is held constant. In another preferred embodiment, the voltage is increased over time.
In the practice of the invention, it is preferable to end electrolysis after the current value has fallen to 0.1 A/dm²or below.
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.
From immediately after the start of electrolysis (T₁) until T₃, there is a time (T₂) at which the current value reaches a maximum. Because gas evolution is observed in the interval between T₁and T₃, the hydrogen ions H⁺ 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.
The current subsequently undergoes an abrupt drop at time T₄. 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 T₅. During the interval between T₄and T₅, it is assumed that hydrogen gas accumulates between the anodized layer and the aluminum substrate.
When reverse electrolysis is continued further, the current value reaches a peak at T₆. This is thought to be due to the formation of cracks in the anodized layer.
Next, after T₇, the electrolytic solution presumably penetrates between the anodized layer and the aluminum substrate via the cracks in the anodized layer, causing corrosion to proceed.
In the practice of the invention, it is preferable to end electrolysis during the interval between T₄and T₅. 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/dm²or below.
The current value in the T₄T₅interval is typically not more than 30%, and generally not more than 10%, of the maximum value (at time T₂). 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.
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.
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.
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.
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 μm 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.
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.
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.
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.
The temperature of the acidic aqueous solution is preferably from 20 to 60° C., and more preferably from 30 to 50° C.
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.
Examples of alkaline aqueous solutions include aqueous solutions of sodium hydroxide, aqueous solutions of sodium carbonate and aqueous solutions of potassium hydroxide.
The alkaline aqueous solution has a pH of preferably 10 to 13.5, and more preferably 11 to 13.
The temperature of the alkaline aqueous solution is preferably from 10 to 50° C., and more preferably from 20 to 40° C.
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.
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.
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° 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° C.) for 60 seconds.
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.
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.
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.
Examples of preferred conditions for reverse electrolysis are given below.
<Preferred Conditions 1>

Cathode: Anodized layer obtained by anodization with aqueous solution of oxalic acid (concentration, 0.3 mol/L; temperature, 17° C.) at a voltage of 40 V for a treatment time of 60 minutes; layer thickness, 60 μm.
Anode: Carbon electrode.
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° C.
Voltage: 40 V (voltage setting).

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.
Moreover, in a film removal step, when the aluminum oxide content (as Al₂O₃) 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.
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.
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.
In the stripping step, the barrier layer is dissolved by the above-described reverse electrolysis, giving a structure composed of the anodized layer.
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.
FIGS. 2A to 2D show diagrams illustrating the inventive method of manufacturing structures.
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.
FIGS. 2B and 2C are respectively schematic cross-sectional views of a structure and an aluminum substrate obtained by the stripping step.
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.
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.
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.
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>
In the second aspect of the invention, an anodizing treatment step is carried out following the stripping step.
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.
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.
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.
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.
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.
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.
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.
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>
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.
Chemical dissolution treatment can be carried out in the same manner as in the above-described pore widening treatment.
<Structure>
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.
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.
Alternatively, the structure can be used as a nanoimprint mold.
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>
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.
Sealing may be carried out using any suitable known technique without particular limitation.
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.
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° C. dispersion containing 1 g/L of HAuCl₄and 7 g/L of H₂SO₄and electrodeposition is carried out at a constant voltage of 11 V (regulated with an autotransformer such as SLIDAC) for 5 to 6 minutes.
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.
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.
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.
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.
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.
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.
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.
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 %.
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.
The amount of metal deposited after sealing is preferably 100 to 500 mg/m².
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.
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.
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.
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.
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.
As used herein, “metal particle interval” refers to the shortest distance between the surfaces of neighboring particles.
<Raman Enhancement Owing to Localized Plasmon Resonance>
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 10⁵to 10⁶, 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.
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.
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>
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

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

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.
Each treatment step is described in detail below.
(1) Substrate
The structure was manufactured using Substrate 1 below.

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

The above substrate was subjected to the following mirror-like finishing treatment.
<Mirror-Like Finishing Treatment>
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° C.), using the substrate as the anode and a carbon electrode as the cathode, and at a constant current of 12.5 A/dm².

<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)
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>
An aqueous solution of sulfuric acid having a concentration of 0.3 mol/L and a temperature of 16° C. was prepared using sulfuric acid (a reagent produced by Kanto Chemical Co., Inc.). The substrate (area of treatment, 5 cm×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 μm.
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>
An aqueous solution of oxalic acid having a concentration of 0.5 mol/L and a temperature of 16° C. was prepared using oxalic acid dihydrate (a reagent produced by Kanto Chemical Co., Inc.). The substrate (area of treatment, 5 cm×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 μm.
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>
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×, 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
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.

“None”: After self-ordering anodizing treatment, the PET tape was peeled off and the substrate was subjected to reverse electrolysis without carrying out masking.
“Back”: After self-ordering anodizing treatment, the substrate was subjected to reverse electrolysis with the PET tape left in place.
“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.
“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.
“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.
“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.
“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.
“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

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° 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.
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).
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).
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
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
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° C.).
2. Evaluation of Stripped State
The state of separation between the anodized layer and the aluminum substrate following reverse electrolysis was evaluated in each of the above examples.
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.
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
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.
The degree of ordering is defined by the following formula (1).
Degree of Ordering(%)=B/A×100 (1)
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.
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.
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.

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 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/dm²)	(mA/dm²)	(%)	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 → 17.8	—	30	—	A	50

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.
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.

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/dm²or 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,

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/dm²or 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.