US20060263541A1

US20060263541A1 - Photoreduction method for metal complex ions

Info

Publication number: US20060263541A1
Application number: US11/431,546
Authority: US
Inventors: Takuo Tanaka; Atsushi Ishikawa; Satoshi Kawata
Original assignee: RIKEN Institute of Physical and Chemical Research
Current assignee: RIKEN Institute of Physical and Chemical Research
Priority date: 2005-05-12
Filing date: 2006-05-11
Publication date: 2006-11-23
Also published as: JP2006316311A

Abstract

The present invention provides a photoreduction method for metal complex ions by which strict controllability is not required in the control of its exposure amount, and a size of the metallic structure to be produced can be controlled, besides there is no fear of reducing spatial resolution of the size of the metallic structure to be produced. The photoreduction method for metal complex ions wherein a laser beam is beam-irradiated on a metal complex ion dispersion element dispersed in a material to photoreduce the metal complex ions thereby fabricating a metallic structure, includes the steps of adding a predetermined coloring matter to the material in which the metal complex ion dispersion element is dispersed, and beam-irradiating the laser beam to the material to which the predetermined coloring matter has been added.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a photoreduction method for metal complex ions, and more particularly to a photoreduction method for metal complex ions suitable for use in case of fabricating a metallic structure by irradiating a laser beam to photoreduce the metal complex ions.
2. Description of the Related Art
In recent years, ultramicrofabrication technology such as optical lithography technology, and optical disk manufacturing technology wherein a light is applied is widely utilized, and such technology has been studied in a variety of fields.
For instance, the most widely applied ultramicrofabrication technology at present wherein a light is applied is the above-described optical lithography technology. The optical lithography technology is a backbone technology for manufacturing a variety of electronic devices such as a semiconductor chip. The technology relates to a massive copying technology applying a photo-transferring technology in principle wherein a metal in a specified region is dissolved, deposited, or removed finally in a chemical manner, whereby a desired metallic pattern is fabricated as a metallic structure.
On one hand, a manner for fabricating a metallic pattern by irradiating directly a laser beam on a specified material is known as a technology for fabricating a desired metallic pattern as a metallic structure other than the above-described optical lithography. More specifically, there have been proposed a manner wherein a laser beam is beam-irradiated on a dispersion element made of metallic nanoparticles, whereby the metallic nanoparticles are molten and bound at the intermediate focus point of the laser beam applied, so that a metallic pattern is fabricated as a metallic structure; a manner wherein a laser beam is beam-irradiated on metal complex ions, so that the metal complex ions are photoreduced to deposit a metallic body, whereby a metallic pattern is fabricated as a metallic structure, and the like manners.
In the above-described manner wherein the metallic body is deposited by photoreducing the metal complex ions, when the laser beam is scanned while beam-irradiating the metal complex ions, it is possible to fabricate an arbitrary metallic pattern in response to the locus scanned as a metallic structure. Accordingly, its applicable range is extremely wide, so that studies and developments are made upon the manner in a variety of fields in recent years.
The inventor of this application has made on a study as to a manner wherein a multiphoton absorption process using an optical system in which a femtosecond ultrashort pulse laser is applied among lasers as a light source is utilized, whereby metal complex ions are photoreduced, so that the metal complex ions are photoreduced at only the intermediate focus point of the laser beam in a three-dimensional space, and a three-dimensional metallic structure is directly fabricated.
In the meantime, a technology wherein a laser beam is beam-irradiated on metal complex ions to photoreduce the metal complex ions thereby to fabricate a metallic structure involves such a problem that a control of the light exposure is difficult.
One of the major causes of a difficulty in controlling the light exposure is in that a metal deposited as a result of photoreduction of the metal complex ions causes the absorption spectrum or the absorption sectional area of the material itself to change, so that characteristic properties of the material which is subjected to laser beam irradiation change momentarily as a result of the laser beam irradiation. In other words, in case of irradiating a light on a material, a constant exposure pattern cannot be maintained so far as such control that optical intensity of the light to be irradiated following to the precedent irradiation is allowed to change timely in response to an amount of the light which has been exposed until then is executed strictly. However, it has been extremely difficult usually to execute such strict control of the optical intensity.
Furthermore, in the case when a laser beam is beam-irradiated to photoreduce metal complex ions to fabricate a metallic structure, an absorption factor of light increases usually with deposition of a metallic structure. In these circumstances, there are many cases where the reaction proceeds explosively just at the moment when an increased amount of the metallic structure exceeds a certain threshold value. Thus, there is such a problem that it is difficult to control a size of the metallic structure produced.
Particularly, when the above-described explosive reaction begins, the photoreduction of metal complex ions existing in the vicinities of the intermediate focus point of a laser beam proceeds at the same time. Hence, there is also such a problem that a metallic structure produced becomes extremely large as compared with a size of the intermediate focus point of a laser beam, whereby its spatial resolution decreases.

OBJECT AND SUMMARY OF THE INVENTION

The present invention has been made in view of the above-described various problems involved in the prior art, and an object of the invention is to provide a photoreduction method for metal complex ions by which no strict controllability is required in control of an exposure amount, besides a size of a metallic structure produced can be controlled, and further there is no fear of decreasing spatial resolution of the size of the metallic structure produced.
In order to achieve the above-described object, a photoreduction method for metal complex ions according to the present invention is the one wherein a laser beam is beam-irradiated on a metal complex ion dispersion element dispersed in a material such as a liquid, vapor, and a solid to photoreduce the metal complex ions thereby fabricating a metallic structure, including the step of adding a predetermined coloring matter to the material in which the metal complex ion dispersion element is dispersed, whereby photoreduction of the metal complex ions is controlled to improve a process tolerance in case of fabricating the metallic structure. As a result, for example, it becomes possible to directly manufacture a three-dimensional metallic structure having a nanomicron size.
More specifically, in the photoreduction method for the metal complex ions according to the present invention, a specified coloring matter is added to a metal complex ion dispersion element, whereby an absorption spectrum and an absorption sectional area of a non-processed material are maintained at constant, and it is prevented to propagate energy of the laser beam to an area other than the intermediate focus point of the laser beam, besides photoreduction efficiency at the intermediate focus point of the laser beam is improved.
Namely, the present invention may be a photoreduction method for metal complex ions wherein a laser beam is beam-irradiated on a metal complex ion dispersion element dispersed in a material to photoreduce the metal complex ions thereby fabricating a metallic structure, comprises the steps of adding a predetermined coloring matter to the material in which the metal complex ion dispersion element is dispersed; and beam-irradiating the laser beam to the material to which the predetermined coloring matter has been added.
Furthermore, the present invention may be the photoreduction method for metal complex ions wherein the coloring matter has the peak of absorption wavelength in the vicinity of about half of the wavelength of the laser beam which is beam-irradiated to the material.
Moreover, the present invention may be the photoreduction method for metal complex ions wherein the functional groups of the coloring matter have not a reducing ability with respect to the metal complex ion dispersion element dispersed into the material.
Still further, the present invention may be the photoreduction method for metal complex ions wherein the material is Au⁺ aqueous solution; the coloring matter is any of P-Quaterphenyl, Stilbene 420, Coumarin 440, Coumarin 481, Coumarin 485, Coumarin 500, or Coumarin 515; and a solution prepared by dissolving the coloring matter into a dimethylformamide solvent is added to the Au⁺ aqueous solution.
Yet further, the present invention may be the photoreduction method for metal complex ions wherein a concentration of the coloring matter is 0.1 wt % with respect to the dimethylformamide solvent.
Besides, the present invention may be the photoreduction method for metal complex ions wherein the material is Ag⁺ aqueous solution; the coloring matter is any of Stilbene 420, Coumarin 440, Coumarin 504, or Coumarin 515; and a solution prepared by dissolving the coloring matter into an ethanol solvent is added to the Ag⁺ aqueous solution.
In addition, the present invention may be the photoreduction method for metal complex ions wherein a concentration of the coloring matter is set out at the amount of saturation with respect to the ethanol solvent.
Since the present invention is constituted as mentioned above, it may provide such excellent advantageous effects that strict controllability is not required in the control of its exposure amount, and a size of the metallic structure to be produced may be controlled, besides there is no fear of reducing spatial resolution of the size of the metallic structure to be produced.
The method of the present invention as described above may be applied to optical memory technology, optical processing technology, UV light molding technology, or optical lithography technology. Accordingly, the present invention may be applied to manufacturing of optical disks, laser processing equipment, optical molding equipment or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:
FIG. 1 is a schematic constructional explanatory view showing an optical system used for experiments by the inventor of this application;
FIG. 2 is an electron micrograph showing experimental results made by the inventor of this application;
FIG. 3 is an electron micrograph showing experimental results made by the inventor of this application;
FIG. 4 is an optical micrograph showing experimental results made by the inventor of this application; and
FIG. 5 is an optical micrograph showing experimental results made by the inventor of this application.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, an example of a manner of practice of the photoreduction method for metal complex ions according to the present invention will be described in detail by referring to the accompanying drawings.
The photoreduction method for metal complex ions according to the present invention is executed in such that a coloring matter having a predetermined absorption wavelength and absorption sectional area which is in such form that the coloring matter is dissolved in a predetermined solvent (for example, water or an organic solvent) is added to a material in which a metal complex ion dispersion element has been dispersed, and then a laser beam is beam-irradiated to the resulting product.
As an effective coloring matter in case of photoreducing metal complex ions by the use of two-photon absorption process by means of irradiation of a laser beam such as a femtosecond ultrashort pulse laser, the coloring matter the absorption wavelength of which has the peak in the vicinities of about half of the laser beam to be beam-irradiated to a material; and the absorption end on the long wavelength side expands up to substantially red zone, in other words, there is no absorption in near-infrared zone is preferred. For example, when a wavelength of a laser beam to be beam-irradiated to a material has a wavelength of around 800 nm, a coloring matter the absorption wavelength of which has the peak in the vicinities of a wavelength of 350 to 450 nm; and the absorption end on the long wavelength side expands up to substantially red zone, in other words, there is no absorption in near-infrared zone is preferred.
Furthermore, concerning luminous efficacy of a coloring matter, when the coloring matter having high luminous efficacy is added to a material, an action for suppressing absorption characteristics of the material can be obtained, while a coloring matter having low luminous efficacy is added to a material, an action for elevating absorption characteristics of the material may be obtained. This is because the coloring matter having high luminous efficacy absorbs once incident radiation energy, and then, the most of the incident radiation energy is consumed as fluorescence, so that photoreduction of metal complex ions is obstructed. On the other hand, the coloring matter having low luminous efficacy allows the incident radiation energy absorbed to transit directly to metal ions, whereby either metal complex ions are caused to be reduced, or discharged as heat, and then the heat energy is again absorbed by metal ions. As a result, the metal complex ions are reduced. Accordingly, when a coloring matter of low luminous efficacy is added to a material, a reduction efficiency of the metal complex ions is improved by a quantity corresponding to that wherein an absorption amount of light increases as a result of adding a coloring matter.
Moreover, when a coloring matter is selected, such coloring matter wherein a functional group itself in an coloring matter molecule has no reducing ability with respect to metal complex ions is to be selected. This is because when the coloring matter itself reduces the metal complex ions, the metal complex ions in the material are reduced, so that an intended metallic structure cannot be fabricated.
As a result of studying experimentally coloring matters, it has been found that a gold structure can be manufactured by beam-irradiating a laser beam on an Au⁺ aqueous solution being a material prepared by adding the following coloring matters to dimethylformamide solvent without requiring strict controllability in the control of an exposure amount while controlling a size of the resulting gold structure without decreasing the spatial resolution. The Au⁺ aqueous solution is prepared by adding a coloring matter such as p-Quaterphenyl, Stilbene 420, Coumarin 440, Coumarin 481, Coumarin 485, Coumarin 500, or Coumarin 515 which has been dissolved into the above-described dimethylformamide (DMF) solvent to the Au⁺ aqueous solution (for example, an aqueous solution of HAuCl₄) as a material containing a metal complex ion element in the case where gold ions are photoreduced as the metal complex ions.
Furthermore, it has been found that a silver structure can be manufactured by beam-irradiating a laser beam on an Ag⁺ aqueous solution being a material prepared by adding the following coloring matters to ethanol solvent without requiring strict controllability in the control of an exposure amount while controlling a size of the resulting silver structure without decreasing the spatial resolution. The Ag⁺ aqueous solution is prepared by adding a coloring matter such as Stilbene 420, Coumarin 440, Coumarin 504, or Coumarin 515 which has been dissolved into the above-described ethanol solvent to the Ag⁺ aqueous solution (for example, an aqueous solution of AgNO₃) as a material containing a metal complex ion element in the case where silver ions are photoreduced as the metal complex ions.
Concerning a concentration of a coloring matter, when dimethylformamide is used as a solvent and an amount of a coloring matter is made to be 0.1 wt % or less (a concentration of the coloring matter with respect to the dimethylformamide solvent), reduction of gold ions due to addition of the coloring matter is not observed. Moreover, when the invention is compared with the prior art, improvements are observed in resolution or a surface condition of the gold structure fabricated in the photoreduction by means of a laser beam.
On one hand, in the case where silver ions are photoreduced as the above-described metal complex ions, ethanol may be used as a solvent; and a concentration of a coloring matter may be set out to the amount of saturation. The amount of saturation changes dependent upon a coloring matter. For instance, it is 0.01 wt % (the concentration of the coloring matter with respect to an ethanol solvent) in case of Stilbene 420, it is 0.02 wt % (the concentration of the coloring matter with respect to an ethanol solvent) in case of Coumarin 440, itis 0.08 wt % (the concentration of the coloring matter with respect to an ethanol solvent) in case of Coumarin 504, and it is 0.02 wt % (the concentration of the coloring matter with respect to an ethanol solvent) in case of Coumarin 515. When a concentration of a coloring matter is set out to its amount of saturation, improvements are observed in resolution and a surface condition of the silver structure fabricated in photoreduction by means of a laser beam in the invention as compared with the prior art. On the other hand, no reduction of silver ions due to addition of a coloring matter is observed.
The experiments by which the above-described results are obtained and which are made by the inventor of this application will be described hereinbelow as examples 1 to 2.
In the experiments shown as examples 1 to 2, an optical system 10 shown in FIG. 1 is used. The optical system 10 is composed of a titanium-sapphire laser 12 as a femtosecond ultrashort pulse laser, a condenser lens 14 for condensing the laser beam output from the titanium-sapphire laser 12, and an XYZ stage 18 for supporting a transparent glass substrate 16 with respect to the laser beam output from the titanium-sapphire laser 12 and further being transferable freely in an X-axis, a Y-axis, and a Z-axis directions (see the reference drawing corresponding to FIG. 1 showing the XYZ-orthogonal coordinate system).
It is to be noted that the titanium-sapphire laser 12 has 800 nm central wavelength λ, 80 fs pulse width Δt, and 80 MHz repetition frequency f, respectively.
Moreover, on the substrate 16, a material containing a metal complex ion dispersion element, and a material containing a metal complex ion dispersion element added in such form that a coloring matter is dissolved in a solvent are placed as a sample S.
In the construction as described above, the substrate 16 on the top of which the sample S is placed is attached to the XYZ stage 18, and when the XYZ stage 18 is driven in an arbitrary direction along the X-axis direction, the Y-axis direction and the Z-axis direction and further the intermediate focus point A of the laser beam output from the titanium-sapphire laser 12 by means of the condenser lens 14 is arbitrarily transferred to the Z-axis direction in the sample S, metallic structures M are fabricated on a locus of the above-described intermediate focus point in a three-dimensional space.

EXAMPLE 1

FIG. 2 is an electron micrograph showing the results of fabricating a silver structure in the case where a AgNO₃aqueous solution is used as the sample S, and the sample S is relatively scanned with the laser beam output from the titanium-sapphire laser 12 at a scanning speed 50 μm/s by means of the above-described optical system 10 wherein an illuminating radiation power is 78.5 mW with respect to the sample S.
On one hand, FIG. 3 is an electron micrograph showing the results of fabricating a silver structure in the case where a mixture prepared by dissolving Coumarin 440 into an ethanol solvent to a AgNO₃aqueous solution is used as the sample S, and the sample S is relatively scanned with the laser beam output from the titanium-sapphire laser 12 at a scanning speed 50 μm/s by means of the above-described optical system 10 wherein a concentration of Coumarin 440 is 0.02 wt % with respect to the ethanol solvent, while an illuminating radiation power is 14.3 mW with respect to the sample S.
The locus of the laser beam output from the titanium-sapphire laser 12 is transferred so as to fabricate an inverted C-shaped silver structure outside a C-shaped structure in both the experiments shown in FIGS. 2 and 3.
As is apparent from the comparison of FIG. 2 with FIG. 3, it is found that sizes of both the silver structures are controlled at high precision in the control of the exposure amount in the experimental results shown in FIG. 3, and further its spatial resolution is remarkably improved in spite of the fact that no strict control is made in the control of the exposure amount.

EXAMPLE 2

FIG. 4 is an optical micrograph showing the results of fabricating a gold structure in the case where a HAuCl₄aqueous solution is used as the sample S, and the sample S is relatively scanned with the laser beam output from the titanium-sapphire laser 12 at a scanning speed 50 μm/s by means of the above-described optical system 10 wherein an illuminating radiation power is 142.9 mW with respect to the sample S.
On one hand, FIG. 5 is an optical micrograph showing the results of fabricating a gold structure in the case where a mixture prepared by dissolving Coumarin 481 into a dimethylformamide solvent to a HAuCl₄aqueous solution is used as the sample S, and the sample S is relatively scanned with the laser beam output from the titanium-sapphire laser 12 at a scanning speed 50 μm/s by means of the above-described optical system 10.
In this case, a concentration of Coumarin 481 is 0.1 wt % with respect to the dimethylformamide solvent, while an illuminating radiation power is 39.3 mW with respect to the sample S.
The locus of the laser beam output from the titanium-sapphire laser 12 is transferred so as to fabricate an inverted C-shaped gold structure outside a C-shaped structure in both the experiments shown in FIGS. 4 and 5.
As is apparent from the comparison of FIG. 4 with FIG. 5, it is found that sizes of both the gold structures are controlled at high precision in the control of the exposure amount in the experimental results shown in FIG. 5, and further its spatial resolution is remarkably improved in spite of the fact that no strict control is made in the control of the exposure amount.
It is to be noted that a dye-sensitization method utilized in a photo conductor such as a photographic film which has been heretofore known is a method for intending primarily to increase an absorption sectional area of the photo conductor thereby improving sensitivity and specifying wavelengths (cyan, magenta, yellow and the like layers in case of color film).
On the other hand, the photoreduction method for metal complex ions according to the present invention is quite different from a conventional dye-sensitization method in that changes in the absorption spectrum of a material with exposure to light is reduced and an extent over which photoreduction effect due to the light irradiated locally extends is limited in addition to that light absorption characteristics of a material are allowed to change as described above, whereby spatial resolution of the metal structure may be improved. As a result, the following functions and advantageous effects are obtained according to the present invention.
(1) Changes in absorption wavelength and absorption sectional area due to metal fine particles produced by exposure to light may be suppressed by adding a coloring matter.
(2) In the present invention, a femtosecond ultrashort pulse laser is used as a light source for taking place a photoreduction reaction, so that the invention is particularly effective in a system wherein a multiphoton process is used for absorption. When such system as described is applied, it becomes possible that an extent over which influences of a laser beam being condensed at the intermediate focus point extend may be more spatially restricted, and as a result, a metallic structure may be fabricated in finer than a spot diameter of a laser beam decided by diffraction limit of a light.
(3) When a femtosecond ultrashort pulse laser is used as a light source thereby utilizing nonlinearity in a two-photon absorption process, spatial resolution can be given to an irradiation direction of the laser beam, and as a result, a three-dimensional metallic structure can be easily fabricated.
(4) Since a coloring matter may be selected suitably so as to accommodate to a wavelength or a desired absorption wavelength of a light source, a tolerance in case of fabricating a metallic structure becomes high.
(5) As a result of adding a coloring matter, energy conversion efficiency of a light source can be improved, and as a result, even when a laser beam is scanned at high speed, it becomes possible to produce a metallic structure, so that the throughput can be improved.
Furthermore, the above-described manner of practice and examples may be modified as enumerated in the following paragraphs (1) through (4).
(1) In the above-described manner of practice and examples, although the invention is described with respect to the case where a femtosecond ultrashort pulse laser is used as a light source, it is not limited to the femtosecond ultrashort pulse laser as a matter of course, but a variety of pulse lasers and continuous lasers are applicable.
(2) In the above-described manner of practice and examples, although a variety of coloring matters are shown, they are merely exemplifications, and the other coloring matters may also be used as a matter of course.
(3) In the above-described manner of practice and examples, although the gold ions and the silver ions are described for fabricating metallic structures, the metal complex ions to which the present invention is applicable are not limited to the gold ions and the silver ions as a matter of course, but the invention may be applied to a variety of metal complex ions.
(4) The above-described manner of practice may be suitably combined with the modifications described in the above-described paragraphs (1) through (3), respectively.
It will be appreciated by those of ordinary skill in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof.
The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.
The entire disclosure of Japanese Patent Application No. 2005-139329 filed on May 12, 2005 including specification, claims, drawings and summary are incorporated herein by reference in its entirety.

Claims

1. A photoreduction method for metal complex ions wherein a laser beam is beam-irradiated on a metal complex ion dispersion element dispersed in a material to photoreduce the metal complex ions thereby fabricating a metallic structure, comprising:

adding a predetermined coloring matter to the material in which the metal complex ion dispersion element is dispersed; and

beam-irradiating the laser beam to the material to which the predetermined coloring matter has been added.

2. The photoreduction method for metal complex ions as claimed in claim 1, wherein:

the coloring matter has the peak of absorption wavelength in the vicinity of about half of the wavelength of the laser beam which is beam-irradiated to the material.

3. The photoreduction method for metal complex ions as claimed in claim 1, wherein:

the functional groups of the coloring matter have not a reducing ability with respect to the metal complex ion dispersion element dispersed into the material.

4. The photoreduction method for metal complex ions as claimed in claim 1, wherein:

the material is Au⁺ aqueous solution;

the coloring matter is any of P-Quaterphenyl, Stilbene 420, Coumarin 440, Coumarin 481, Coumarin 485, Coumarin 500, or Coumarin 515; and

a solution prepared by dissolving the coloring matter into a dimethylformamide solvent is added to the Au⁺ aqueous solution.

5. The photoreduction method for metal complex ions as claimed in claim 4, wherein:

a concentration of the coloring matter is 0.1 wt % with respect to the dimethylformamide solvent.

6. The photoreduction method for metal complex ions as claimed in claim 1, wherein:

the material is Ag⁺ aqueous solution;

the coloring matter is any of Stilbene 420, Coumarin 440, Coumarin 504, or Coumarin 515; and

a solution prepared by dissolving the coloring matter into an ethanol solvent is added to the Ag⁺ aqueous solution.

7. The photoreduction method for metal complex ions as claimed in claim 6, wherein:

a concentration of the coloring matter is set out at the amount of saturation with respect to the ethanol solvent.