WO2004033765A1

WO2004033765A1 - Electrode for electrolytic processing

Info

Publication number: WO2004033765A1
Application number: PCT/JP2003/012650
Authority: WO
Inventors: Yuzo Mori; Hidekazu Goto; Yasushi Toma
Original assignee: Ebara Corporation
Priority date: 2002-10-08
Filing date: 2003-10-02
Publication date: 2004-04-22
Also published as: TW200406276A; TWI288681B

Abstract

An electrode (1, 2) for electrolytic processing has a conductive material (1a, 2a) and an organic compound (1b, 2b) having an ion exchange group. The organic compound (1b, 2b) is chemically bonded to a surface of the conductive material (1a, 2a). The organic compound (1b, 2b) comprises thiol or disulfide. The ion exchange group comprises at least one of a sulfonic acid group, a carboxyl group, a quaternary ammonium group, and an amino group. The conductive material (1a, 2a) includes at least one of gold, silver, platinum, copper, gallium arsenide, cadmium sulfide, and indium oxide (III).

Description

DESCRIPTION

ELECTRODE FOR ELECTROLYTIC PROCESSING

Technical Field The present invention relates to an electrode for electrolyticprocessing, andmore particularly to an electrode to be used as a processing electrode to process a substrate and/or a feeding electrode to feed the substrate during an electrolytic process using a fluid, particularly pure water . The present invention also relates to an electrolytic processing apparatus and method using such an electrode . The present invention also relates to a method of promoting to dissociate liquid into ions, andmore particularly to a method of promoting dissociation of liquid into ions during an electrolytic process.

Background Art

In recent years, there has been a growing tendency to replace aluminum or aluminum alloy as a metallic material for forming interconnection circuits on a substrate such as a semiconductor wafer with copper (Cu) having a low electric resistivity and a high electromigration resistance. Copper interconnections are generally formed by filling copper into fine recesses formed in a surface of a substrate. As methods for forming copper interconnections, there have been employed chemical vapor deposition (CVD) , sputtering, and plating. In any of the methods, after a copper film is formed on substantially the entire surface of a substrate, unnecessary copper is removed by chemical mechanical polishing (CMP) . FIGS .1A through 1C show an example of a process of forming a copper interconnection in a substrate W. As shown in FIG. 1A, an insulating film 2, such as an oxide film of Si0₂ or a film of low-k material, is deposited on a conductive layer la on a semiconductor base 1 on which semiconductor devices have been formed. A contact hole 3 and an interconnection groove 4 are formed in the insulating film 2 by lithography etching technology. Then, a barrier layer 5 made of TaN or the like is formed on the insulating film 2, and a seed layer 7, which is used as a feeding layer for electrolytic plating, is formed on the barrier layer 5 by sputtering, CVD, or the like .

Subsequently, as shown in FIG. IB, a surface of the substrate W is plated with copper to fill the contact hole 3 and the interconnection groove 4 with copper and to form a copper film 6 on the insulating film 2. Thereafter, the surface of the substrate W is polished by chemical mechanical polishing (CMP) to remove the copper film 6 on the insulating film 2 so that the surface of the copper film 6 filled in the contact hole 3 and the interconnection groove 4 is made substantially even with the surface of the insulating film 2. Thus, as shown in FIG. 1C, an interconnection comprising the copper film 6 is formed in the insulating layer 2. Recently, components in various types of equipment have become finer and have required higher accuracy. As submicronic manufacturing technology has commonly been used, the properties of the materials are greatly influenced by the machining method. Under these circumstances, in a conventional mechanical machining method in which a desired portion in a workpiece is physically destroyed and removed from a surface thereof by a tool, a large number of defects may be produced by the machining, thus deteriorating the properties of the workpiece. Therefore, it is important to perform machining without deteriorating the properties of materials .

Some processing methods, such as chemical polishing, electrochemical machining, and electrolytic polishing, have been developed in order to solve the above problem. In contrast to the conventional physical machiningmethods , these methods perform removal processing or the like through a chemical dissolution reaction. Therefore, these methods do not suffer from defects such as formation of an altered layer and dislocation due to plastic deformation, so that processing can be performed without deteriorating the properties of the materials .

In an electrochemical machining process, particularly in an electrochemical machining process using pure water or ultrapure water, an ion exchanger such as an ion exchange membrane or an ion exchange fiber is employed to increase the processing rate. Pure water refers to water having a resistivity of 0.1 MΩ-cm or more at 25 °C, and ultrapure water refers to water having a resistivity of 10 MΩ-cm or more at 25 °C . Ion exchangers generally comprise an ion exchange resin or an ion exchange membrane in which an ion exchange group, such as a sulfonic acid group, a carboxyl group, a quaternary ammonium group (=N⁺=) , or a tertiary or lower amino group, is bonded to a base material, such as a copolymer of styrene and divinylbenzene, or a fluororesin. Further, there has been known an ion exchange fiber in which an ion exchange group is introduced into nonwoven fabric by graft polymerization.

FIG. 2 is a schematic diagram showing an electrolytic processing apparatus using conventional ion exchangers. As shown in FIG. 2, the electrolytic processing apparatus has a power supply 800, an anode (electrode) 810 connected to the power supply 800, and a cathode (electrode) 820 connected to the power supply 800. The anode 810 has an ion exchanger 830 attached to a surface thereof, and the cathode 820 has an ion exchanger 840 attached to a surface thereof. A fluid 860 such as pure water or ultrapure water is supplied between the electrodes 810, 820 and a workpiece 850 (e.g., a copper film) . Then, the workpiece 850 is brought into contact with or close to the ion exchangers 830, 840 attached to the surfaces of the electrodes 810, 820. A voltage is applied between the anode 810 and the cathode 820 by the power supply 800. Water molecules in the fluid 860 are dissociated into hydroxide ions and hydrogen ions by the ion exchangers 830, 840. For example, the produced hydroxide ions are supplied to a surface of the workpiece 850. The concentration of the hydroxide ions is thus increased near the workpiece 850, and atoms in the workpiece 850 and the hydroxide ions are reacted with each other to perform removal of a surface layer of the workpiece 850. Thus, the ion exchangers 830, 840 are considered to have catalysis for decomposing water molecules in the fluid 860 into hydroxide ions and hydrogen ions . However, with respect to the conventional ion exchange resin or ion exchange fiber, when the electrodes 810 and 820 have a small size (i.e., a small diameter) , the ion exchangers 830 and 840 cannot be disposed separately on the surfaces of these electrodes 810 and 820. Therefore, the anode 810 and the cathode 820 have to be covered with an ion exchanger extending over both of the anode 810 and the cathode 820.

In such a case, if the distance Li between the anode

810 and the cathode 820 is smaller than the distance L₂ between the electrodes 810, 820 and metal (e.g., copper) as the workpiece 850, then an electric current flows between the electrodes 810 and 820 more than between the electrodes 810, 820 and the workpiece 850. Therefore, the distance Li between the electrodes 810 and 820 should be set to be larger than the distance L₂ between the electrodes 810, 820 and the workpiece 850.

However, the thicknesses of the ion exchangers 830, 840 prevent the distance L₂ between the electrodes 810, 820 and the workpiece 850 from being sufficiently reduced. Accordingly, the anode 810 and the cathode 820 cannot be disposed as close to each other as would be preferred. As a result, the anode 810 and the cathode 820 have limitations in their shapes or the like. Further, a conventional ion exchange fiber is problematic in that fibers maybe removed fromthe ion exchanger during an electrolytic process so that the removed fibers cause variations of processing properties according to time elapsed. It has been feared that seams of the fibers may have an influence on the surface roughness of the workpiece. From this point of view, in order to flatten the entire surface of a workpiece, attempts have been made to wind a meshed ion exchange fiber around nonwoven fabric and attach it to a cylindrical electrode. However, when an ion exchanger has an uneven thickness, the flatness of the surface of the workpiece may be influenced by the uneven thickness of the ion exchanger.

Disclosure of Invention The present invention has been made in view of the above drawbacks. It is, therefore, a first object of the present invention to provide an electrode for electrolytic processing which can achieve stable processing performance and can flexibly cope with small electrodes and various shapes of electrodes.

A second object of the present invention is to provide an electrolytic processing apparatus and method using such an electrode .

A third object of the present invention is to provide a method of promoting to dissociate liquid into ions which can achieve stable processing performance.

In order to attain the first object, according to a first aspect of the present invention, there is provided an electrode for electrolytic processing. The electrode has a conductive material and an organic compound having an ion exchange group . The organic compound is chemically bonded to a surface of the conductive material. According to the present invention, an ion exchange material having an ion exchange function can be bonded directly to a conductive material . Thus, the conductive material with the ion exchange material can be used as an electrode for electrolytic processing. With such an arrangement, it is possible to reduce the distance between an electrode and a workpiece and hence the distance between an electrode serving as an anode and an electrode serving as a cathode. Therefore, the electrode according to the present invention can flexibly cope with small electrodes and various shapes of electrodes. Furthermore, because ion exchange materials can be bonded separately to a conductive material serving as an anode and a conductive material serving as a cathode, a leakage current can be prevented from being produced between the anode and the cathode . The organic compound may comprise thiol or disulfide. The ion exchange group may comprise at least one of a sulfonic acid group, a carboxyl group, a quaternary ammonium group, and an amino group. The conductive material may include at least one of gold, silver, platinum, copper, galliumarsenide, cadmium sulfide, and indium oxide (III) .

The conductive material should preferably have meshes because such meshes can allow water to pass therethrough to decompose water efficiently. When a workpiece is brought into contact with the electrode during an electrolytic process, scratches may be produced on a surface of the workpiece . From this point of view, it is desirable that a workpiece is not brought into contact with the electrode during an electrolytic process . According to a second aspect of the present invention, there is provided an electrode for electrolytic processing. The electrode has a conductive carbon material and an ionic dissociation functional group. A surface of the conductive carbon material is chemically modified by the ionic dissociation functional group.

With such an electrode, the surface of the electrode has catalysis for decomposing water molecules into ions. Therefore, it is possible to reduce the distance between an electrode and a workpiece and hence the distance between an electrode serving as an anode and an electrode serving as a cathode . Therefore, the electrode according to the present invention can flexibly cope with small electrodes and various shapes of electrodes. Furthermore, because each of an electrode serving as an anode and an electrode serving as a cathode canhave catalysis, a leakage current can be prevented from being produced between the anode and the cathode.

The ionic dissociation functional group may comprise a carboxyl group. The ionic dissociation functional group may comprise at least one of a quaternary ammonium group, and a tertiary or lower amino group. The conductive carbon material may comprise glassy carbon, fullerene, or carbon nanotubes .

According to a third aspect of the present invention, there is provided an electrode for electrolytic processing. The electrode has a graphite intercalation compound containing alkali metal.

With such an electrode, water molecules are considered to be decomposed into ions by alkali metal intercalatedbetween layers of the graphite. Therefore, it is possible to reduce the distance between an electrode and a workpiece and hence the distance between an electrode serving as an anode and an electrode serving as a cathode. Therefore, the electrode according to the present invention can flexibly cope with small electrodes and various shapes of electrodes. Furthermore, because each of an electrode serving as an anode and an electrode serving as a cathode can have catalysis, a leakage current can be prevented from being produced between the anode and the cathode .

In order to attain the second object of the present invention, according to a fourth aspect of the present invention, there is provided an electrolytic processing apparatus having a processing electrode and a feeding electrode to feed a workpiece. The electrolytic processing apparatus also has a workpiece holder for holding the workpiece and bringing the workpiece into contact with or close to the processing electrode . The electrolytic processing apparatus includes a power supply for applying a voltage between the processing electrode and the feeding electrode, and a fluid supply unit for supplying a fluid between the workpiece and the processing electrode. At least one of the processing electrode and the feeding electrode employs any one of the aforementioned electrodes.

According to a fifth aspect of the present invention, there is provided an electrolytic processing method. A workpiece is fed through a feeding electrode. A voltage is applied between the feeding electrode and a processing electrode . A fluid is supplied between the workpiece and the processing electrode. The workpiece is brought into contact with or close to the processing electrode. At least one of the processing electrode and the feeding electrode employs any one of the aforementioned electrodes. In order to attain the third object of the present invention, according to a sixth aspect of the present invention, there is provided a method of promoting dissociation of liquid into ions by a conductive material to which an organic compound having an ion exchange group is chemically bonded.

The organic compound may comprise thiol or disulfide. The ion exchange group may comprise at least one of a sulfonic acid group, a carboxyl group, a quaternary ammonium group, and an amino group. The conductive material may include at least one of gold, silver, platinum, copper, galliumarsenide, cadmium sulfide, and indium oxide (III) .

The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.

Brief Description of Drawings FIGS. 1A through 1C are diagrams showing an example of a process of forming a copper interconnection in a substrate; FIG. 2 is a schematic diagram showing an electrolytic processing apparatus using conventional ion exchangers;

FIG. 3 is a schematic diagram showing an electrolytic processing apparatus using an electrode according to the present invention;

FIGS. 4A and 4B are graphs showing current-voltage properties when an electrolysis process was performed with use of an electrode having a conductive material and an organic compound having an ion exchange group chemically bonded to the conductive material;

FIG. 5 is a schematic diagram showing an example of an electrolytic processing apparatus using an electrode according to the present invention; FIG. 6 is a vertical cross-sectional view showing a main frame in the electrolytic processing apparatus shown in FIG. 5;

FIG. 7 is a perspective view showing a holding portion and a processing electrode in the electrolytic processing apparatus shown in FIG. 6;

FIG. 8 is perspective view showing a holding portion and a processing electrode in another electrolytic processing apparatus according to the present invention;

FIG. 9 is a schematic diagram showing an example of an electrolytic processing apparatus schematic another type of electrode according to the present invention;

FIG.10 is a graph showing the current-voltage properties of the electrode shown in FIG. 9;

FIG.11 is a graph showing the current-voltage properties of the electrode shown in FIG. 9;

FIG. 12 is a schematic diagram showing an example of an electrolytic processing apparatus having another type of electrode according to the present invention;

FIG. 13 is a schematic diagram showing an experimental device used to measure the current-voltage properties of an electrode according to the present invention; and

FIG.14 is a graph showing the current-voltage properties of the electrode shown in FIG. 12, which are measured by the experimental device shown in FIG. 13.

Best Mode for Carrying Out the Invention

An electrode and an electrolytic processing apparatus using an electrode according to embodiments of the present invention will be described below with reference to the accompanying drawings. In the following embodiments, a substrate is used as a workpiece and processed by an electrolytic processing apparatus. However, the present invention is applicable to any workpiece other than the substrate .

FIG. 3 is a schematic diagram showing an example of an electrolytic processing apparatus using an electrode according to the present invention. As shown in FIG. 3, the electrolytic processing apparatus has a pair of electrodes

1 and 2 for electrolytic processing. The electrodes 1 and

2 have conductive materials la and 2a as base materials, respectively. The conductive materials la and 2a are connected to an anode and a cathode of a power supply 3, respectively. An organic compound having an ion exchange group is chemically bonded to a surface of the conductive material la to form an ion exchange material lb on the surface of the conductive material la, and an organic compound having an ion exchange group is chemically bonded to a surface of the conductive material 2a to form an ion exchange material 2b on the surface of the conductive material 2a. A fluid 5 such as pure water or ultrapure water is supplied between the electrodes 1, 2 and a workpiece 4 (e.g., a copper film formed on a substrate) . Then, the workpiece 4 is brought into contact with or close to the ion exchange materials lb and 2b. A voltage is applied between the conductive carbon materials la and 2a in the electrodes 1, 2 by the power supply 3. Water molecules in the fluid 5 are dissociated into hydroxide ions and hydrogen ions by the ion exchange materials lb and 2b. For example, the produced hydroxide ions are supplied to a surface of the workpiece 4. The concentration of the hydroxide ions is thus increased near the workpiece 4, and atoms in the workpiece 4 and the hydroxide ions are reacted with each other to perform removal of a surface layer of the workpiece 4.

According to the present invention, an ion exchange material having an ion exchange function can be bonded directly to a conductive material as a base material. Thus, the conductive material with the ion exchange material can be used as an electrode for electrolytic processing, as shown in FIG.3. With such an arrangement, it is possible to reduce the distance between the electrodes 1, 2 and the workpiece (substrate) 4 and hence the distance between the electrode 1 serving as an anode and the electrode 2 serving as a cathode . Therefore, the electrolyticprocessing apparatus can flexibly cope with small electrodes and various shapes of electrodes. Furthermore, because ion exchange materials can be bonded separately to the conductive material la serving as an anode and the conductive material 2a serving as a cathode, a leakage current can be prevented from being produced between the anode and the cathode, i.e., between the electrodes 1 and 2.

As described above, each of the electrodes has the ion exchange material in which an organic compound having an ion exchange group is chemically bonded to the electrode. The term "bond" means that a material having an ion exchange group is bonded to a conductive material by chemical bond, not by an adhesive or the like. In a usual ion exchange resin, a material having an ion exchange group is "bonded" to an organic matter included in the resin.

It is desirable that the conductive material to which an organic compound is bonded has meshes, e.g., a lattice pattern or a form of a punching metal, because such meshes can allow water to pass therethrough to decompose water efficiently.

Such an electrode can be produced as follows. There will be described an example in which sodium l-propanethiol-3-sulfonate (HSC₃H₆-S0₃Na) was used as an organic compound having an ion exchange group and was bonded directly to a platinum (Pt) substrate to produce an electrode . A sodium salt of sulfonic acid group is substituted at the 3-end of 1-ρropanethiol to form sodium l-propanethiol-3-sulfonate (thiol) .

First, a flat platinum substrate, for example, having a length of 34 mm, a width of 12.5 mm, and a thickness of 0.5 mm, was prepared. An organic matter on a surface of the platinum substrate was removed by a sulfuric acid and hydrogen peroxide aqueous solution. Then, the platinum substrate was immersed in an aqueous solution of sodium l-propanethiol-3-sulfonate, which had a concentration of several milimoles/liter, for about 12 hours. Sodium l-propanethiol-3-sulfonate has hydrophilicity under the influence of a sulfonic acid group as a functional group. Therefore, while the surface of the platinum substrate was hydrophobic before the immersion, the surface of the platinum substrate became hydrophilic after the immersion so that thiol is bonded to the surface of the platinum substrate. Thus, a flat platinum electrode (Pt-SC₃H₆-S0₃Na) , which has a catalyst (an ion dissociation function) , could be produced. The catalysis in dissolution reactions of water molecules was measured on the platinum electrode modified by sodium l-propanethiol-3-sulfonate, which is hereinafter referred to as a thiol platinum electrode. Specifically, a thiol platinum electrode produced as described above was installed into an experimental device having parallel plate electrodes, and electrolysis was performed with ultrapure water. The current-voltage properties were measured for the following cases. Further, the current-voltage properties were measured for a comparative experiment in which normal platinum electrodes were used as an anode and a cathode.

(1) A thiol platinum electrode was used as an anode, and a normal platinum electrode was used as a cathode.

(2) A normal platinum electrode was used as an anode, and a thiol platinum electrode was used as a cathode. A fluororesin sheet was disposedbetween the electrodes . Areas of the electrodes facing each other were set to be about 0.4 cm². The distance between the electrodes was adjusted by the thickness of the fluororesin sheet. Measurements were conducted under two conditions in which the distance between the electrodes was 50 μm and 12 μm.

FIG. 4A is a graph showing results of an experiment in which the distance between the electrodes was 12 μm, and FIG. 4B is a graph showing results of an experiment in which the distance between the electrodes was 50 μm. It can be shown from FIGS. 4A and 4B that when a thiol platinum electrode was used as an anode or a cathode, the electrolytic current was increased by several times to several tens of times (50 times at maximum) as compared to a case where normal platinum electrodes were used as an anode and a cathode. Thus, the thiol platinum electrode served as a catalyst for dissociating water into ions. A liquid in which the dissociation is promoted is not limited to water. It can be seen from FIGS. 4A and 4B that an increase of electrolytic current was larger as the distance between the electrodes was smaller. Specifically, when the distance between the electrodes was 12 μm, the electrolytic current value was about 50 times as large as that in the case of using the normal platinum electrodes (see FIG. 4A) . However, when the distance between the electrodes was 50 μm, the electrolytic current value was about 5 times as large as that the case of using the normal platinum electrodes (see FIG. 4B) .

In the above example, platinumwas used as the conductive material to which the organic compound was bonded. However, the conductive material is not limited to platinum. For example, metal such as gold, silver, or copper may be used as the conductive material. Alternatively, the conductive material may comprise a glass substrate having an Au film, or GaAs (gallium arsenide), CdS (cadmium sulfide), ln₂0₃

(indium oxide (III)), carbon (graphite), or the like.

According to another experiment, it has been confirmed that current-voltage properties similar to the above could be achieved in the case of using a glass substrate having an Au film. Further, an organic conductive material such as a polyaniline based material or carbon nanotubes may be used as the conductive material. Specifically, an organic compound having an ion exchange group may be bonded directly to an organic conductive material.

In the above example, thiol was used as the organic compound to be bonded to the conductive material. However, the organic compound is not limited to thiol. For example, disulfide or an organic conductive material such as a polyaniline based material or carbon nanotubes may be used as the organic compound. Further, the ion exchange group is not limited to a sulfonic acid group as described above. For example, a carboxyl group, a quaternary ammonium group, or an amino group maybe used as the ion exchange group . According to an experiment, it has been confirmed that effects similar to those described above could be achieved when a carboxyl group was used as an ion exchange group of thiol.

An electrode according to the present invention is applicable to an electrolytic processing apparatus as shown in FIGS. 5 through 7. FIG. 5 is a schematic diagram showing an example of an electrolytic processing apparatus using an electrode according to the present invention, FIG. 6 is a vertical cross-sectional view showing a main frame 14 in the electrolytic processing apparatus shown in FIG. 5, and FIG. 7 is a perspective view showing a main part of the main frame 14 shown in FIG. 6.

As showninFIG.5, the electrolyticprocessing apparatus mainly comprises a main frame 14 having a machining chamber 12 for holding ultrapure water 10 therein, an ultrapure water circulation/purification device 22, and a high-pressure ultrapure water supply unit 28. The ultrapure water circulation/purification device 22 has a waste water tank 16, an ultrapure water circulation/purification section 18, and a high-pressure pump 20. The high-pressure ultrapure water supply unit 28 has a plunger pump 24 and a pressure transmitter 26. For example, the machining chamber 12 is made of stainless steel.

As shown in FIG. 5, the main frame 14 has a workpiece holder (holding table) 30 for detachably holding a workpiece W such as a semiconductor wafer in a horizontal direction by suction. The workpiece holder 30 is disposed within the machining chamber 12 and has three degrees XYθ of freedom. Specifically, the workpiece W held by the workpiece holder 30 is horizontally movable in the directions of X and Y (see FIG. 7) and is rotatable about the θ axis (Z axis) on the horizontal plane while the workpiece W is immersed in the ultrapure water 10. The workpiece holder 30 serves to hold the workpiece W and to supply an electric current to the workpiece W. For example, the workpiece holder 30 is made of titanium and has a platinum plated surface of 1 μm in thickness. The workpiece holder 30 is supported in radial and thrust directions by a hydrostatic bearing 32 using ultrapure water (see FIG. 5) .

As shown in FIG.7, a columnar or cylindrical processing electrode 34 is disposed above the workpiece holder 30. The processing electrode 34 has a shaft center 0-0 extending horizontally. The processing electrode 34 is formed by an electrode according to the present invention. Specifically, an ion exchange material 34b having an ion exchange function is formed on a columnar or cylindrical conductive material 34a to which an organic compound having an ion exchange group is chemically bonded. The conductive material 34a in the processing electrode 34 is connected to a power supply 38. The ion exchange material 34b serves as a catalyst for dissociating water molecules in the ultrapure water between the processing electrode 34 and the workpiece W into hydrogen ions and hydroxide ions .

The processing electrode 34 is coupled to a vertically movable rotating shaft 36 extending along the shaft center 0-0. Thus, the processing electrode 34 can be rotated about the shaft center 0-0 in accordance with the rotation of the rotating shaft 36. The processing electrode 34 can be moved vertically so as to adjust the distance between the processing electrode 34 and the workpiece W held by the workpiece holder 30. As with the workpiece holder 30, the rotating shaft 36 is supported in the radial and thrust directions by a hydrostatic bearing (not shown) using ultrapure water . When an electrolytic process is performed, the processing electrode 34 is lowered so as to bring a lower end portion of the ion exchange material 34b into contact with or close to a surface of the workpiece held by the workpiece holder 30.

The electrolytic processing apparatus has the power supply 38 for applying a voltage between the processing electrode 34 and the workpiece W held by the workpiece holder 30. In this embodiment, for example, in order to perform an electrolytic process of copper as the workpiece, the processing electrode 34 is connected to a cathode of the power supply 38, and workpiece (copper) W is connected to an anode of the power supply 38. However, depending upon the types of workpieces, the processing electrode 34 may be connected to the anode of the power supply 38, and the workpiece W may be connected to the cathode of the power supply 38.

As described above, the workpiece holder 30 is rotatable about the vertical axis, and the processing electrode 34 is rotatable about the horizontal axis. The workpiece holder 30 and the processing electrode 34 are respectively rotated in directions such that the ultrapure water 10 is revolved. An ultrapure water supply nozzle 40 for supplying ultrapure water at a high pressure between the workpiece W held by the workpiece holder 30 and the processing electrode 34 is disposed at the upstream side of the directions of the rotation. While at least one of the processing electrode 34 and the workpiece W is being rotated, the ultrapure water 10 is supplied between the processing electrode 34 and the workpiece W from the upstream side of the directions of the rotation to effectively remove bubbles or machining products which would remain between the processing electrode 34 and the workpiece W. As shown in FIG. 5, ultrapure water is purified by the ultrapure water circulation/purification section 18 in the ultrapure water circulation/purification device 22, pressurized by the pressure transmitter 26 in the high-pressure ultrapure water supply unit 28, and supplied into the ultrapure water supply nozzle 40 by the plunger pump 24. Further, as shown in FIG. 5, the ultrapure water 10 held in the machining chamber 12 overflows so as to be stored in the waste water tank 16. Then, the ultrapure water is purified in the ultrapure water circulation/purification device 22 and returned to the machining chamber 12 by the high-pressure pump 20. A portion of the ultrapure water 10 is supplied to the hydrostatic bearing 32.

With the electrolytic processing apparatus thus constructed, the workpiece W is held by the workpiece holder 30, and the processing electrode 34 is lowered so as to bring the ion exchange material 34b of the processing electrode 34 into line contact with or close to a surface of the workpiece W. In this state, ultrapure water 10 within the machining chamber 12 is purified by the ultrapure water circulation/purification device 22 and circulated. The processing electrode 34 is connected to the cathode of the power supply 38, and the^'workpiece W is connected to the anode of the power supply 38, so that a voltage is applied between the processing electrode 34 and the workpiece W. At that time, the workpiece holder 30 and the processing electrode 34 are simultaneously rotated in directions such that the ultrapure water 10 is revolved. The ultrapure water is supplied between the processing electrode 34 and the workpiece W at a high pressure from the ultrapure water supply nozzle 40 disposed at the upstream side of the directions of the rotation. Hydrogen ions and hydroxide ions are produced by a chemical reaction occurring on a solid surface of the ion exchange material (catalyst) 34b to perform removal of the surface of the workpiece W. In this case, a flow of the ultrapure water 10 is formed in the machining chamber 12 and passed through the ion exchange material 34b to produce a large amount of hydrogen ions and hydroxide ions. Thus, a large amount of hydrogen ions or hydroxide ions are supplied onto the surface of the workpiece W to efficiently perform the electrolytic process .

As described above, the workpiece holder 30 and the processing electrode 34 are simultaneously rotated in directions such that the ultrapure water 10 is revolved. The ultrapure water is supplied between the processing electrode 34 and the workpiece W at a high pressure from the ultrapure water supply nozzle 40 disposed at the upstream side of the directions of the rotation. Accordingly, the ultrapure water 10 present between the workpiece W and the processing electrode 34 can effectively be replaced with new ultrapure water, so that gas and machining products produced upon the electrolytic process can efficiently be removed to realize a stable electrolytic process. An electrode according to the present invention is also applicable to an electrolytic processing apparatus as shown in FIG. 8. The electrolytic processing apparatus shown in FIG. 8 differs from the electrolytic processing apparatus shown in FIG.7 in that aprocessing electrode 134 is ellipsoidal or spherical. When the processing electrode 134 is lowered, a lower end portion of an ion exchange material 134b on an outer surface of a conductive material 134a is brought into point contact with the workpiece W held by the workpiece holder 30. In this state, the processing electrode 134 and the workpiece holder 30 are simultaneously rotated. The other structures are the same as the structures of the electrolytic processing apparatus shown in FIG. 7. With this electrolytic processing apparatus, since the area of the processing portion is reduced, the ultrapure water 10 can easily be supplied to a portion around the processing portion. Accordingly, the electrolytic process can be performed under stable conditions. Ultrapure water may not be necessarily ejected from the ultrapure water supply nozzle 40. For example, instead of ejecting ultrapure water, the electrolytic processing apparatus may employ a tank which holds ultrapure water therein and immerses an electrode and a workpiece in the ultrapure water. Since any chemical material, including abrasive particles and dense chemical liquids, other than ultrapure water is not used in the electrolytic processing apparatus, the machining chamber 14 may be contaminated only by reaction products produced during electrochemical processing. Thus, it is possible to simplify or dispense with a cleaning process of a substrate after the electrolytic process. Circulation of the ultrapure water can reduce the amount of wastewater. Further, since it is not necessary to treat any chemical liquids, operating cost can remarkably be reduced. In the above embodiments, an organic compound having an ion exchange group is chemically bonded to a surface of an electrode to form an ion exchanger on the surface of the electrode. Specifically, gold, silver, platinum, copper, indium oxide, or the like is used as an electrode material (conductive material) , and thiol, disulfide, or the like is used as an organic compound having an ion exchange group. Such an organic compound is chemically bonded to the electrode material to introduce the ion exchange group into the electrode material. Instead of using such an electrode, a surface of a conductive carbon material may be chemically modified by an ionic dissociation functional group. Specifically, a conductive carbon material is used as an electrode material, and an ionic dissociation functional group is effectively introduced directly into a surface of the carbon of the conductive carbon material by inorganic reactions. In such a case, there are no carbon chains due to an organic compound between the electrode material and the ionic dissociation functional group (or an ion exchange group) . Therefore, the thickness of the chemical modification layer can be reduced, and the durability (or the resistance to removal) and the conductivity of the ionic dissociation functional group can be improved. FIG. 9 is a schematic diagram showing an electrolytic processing apparatus using such an electrode. As shown in FIG. 9, the electrolytic processing apparatus has a pair of electrodes 201 and 202. The electrodes 201 and 202 have conductive carbon materials 201a and 202a connected to an anode and a cathode of a power supply 203, respectively. A surface of the conductive carbon material 201a is chemically modified with an ionic dissociation functional group 201b, and a surface of the conductive carbon material 202a is chemically modified with an ionic dissociation functional group 202b. A fluid 205 such as pure water or ultrapure water is supplied between the electrodes 201, 202 and a workpiece 204 (e.g., a copper film formed on a substrate) . Then, the workpiece 204 is brought close to the ionic dissociation functional groups 201b, 202b in the electrodes 201, 202. A voltage is applied between the conductive carbon materials 201a and 202a in the electrodes 201/ 202 by the power supply 203. Water molecules in the fluid 205 are dissociated into hydroxide ions and hydrogen ions by the ionic dissociation functional groups 201b, 202b. For example, the produced hydroxide ions are supplied to a surface of the workpiece 204. The concentration of the hydroxide ions is thus increased near the workpiece 204, and atoms in the workpiece 204 and the hydroxide ions are reacted with each other to perform removal of a surface layer of the workpiece 204.

Thus, it is possible to reduce the distance between the electrodes 201, 202 and the workpiece (substrate) 204 and hence the distance between the electrode 201 serving as an anode and the electrode 202 serving as a cathode. Therefore, the electrolytic processing apparatus can flexibly cope with small electrodes and various shapes of electrodes . Furthermore, because the conductive carbon material 201a serving as an anode and the conductive carbon material 202a serving as a cathode are separately bonded to (or chemically modified by) the ionic dissociation functional groups 201b, 202b, a leakage current can be prevented from being produced between the cathode and the anode, i.e., between the electrodes 201 and 202. Such an electrode, whichhas a conductive carbonmaterial and an ionic dissociation functional group chemically modifying a surface of the conductive carbon material, can be used in an electrolytic processing apparatus of the above embodiments shown in FIGS .5 through 7 , instead of an electrode in which an organic compound is chemically bonded to a surface of a conductive material.

The ionic dissociation functional group, which chemically modifies the surface of the conductive carbon material, comprises abasic group such as a quaternary ammonium group or a tertiary or lower amino group, or an acidic group such as a carboxyl group.

When the electrode is to be used to process a relatively large area of about 1 cm² ormore, the conductive carbonmaterial should preferably comprise a carbon material that has a flat and smooth surface and can be processed in shape with high accuracy, such as glassy carbon. When the electrode is to be used to perform fine processing at a level of 1 μm or less than 1 μm, fullerene or carbon nanotubes should preferably be used as the conductive carbon material. It is desirable that the conductive carbon material has meshes because such meshes can allow water to pass therethrough to decompose water efficiently. Methods of chemically modifying a conductive carbon material with an ionic dissociation functional group such as an ion exchange group include immersing a conductive carbon material in a chemical liquid, electrical discharge processing a conductive carbonmaterial in a gaseous phase, and anodizing a conductive carbon material in an electrolytic solution.

For example, as a method of immersing a conductive carbon material in a chemical liquid, a conductive carbon material is immersed in an oxidizing solution such as a nitric acid.

With this method, a surface of the conductive carbonmaterial can be readily chemically modified by an ionic dissociation functional group such as a carboxyl group.

For example, as a method of electrical discharge processing a conductive carbon material in a gaseous phase, plasma is formed in a gas containing oxygen by RF electrical discharge (13.25 MHz), and a conductive carbon material is exposed to the plasma. With this method, a surface of the conductive carbon material can be chemically modified by an ionic dissociation functional group such as a carboxyl group. Plasma may be formed in a nitrogen atmosphere by electrical discharge, and a conductive carbon material may be exposed to the plasma. In such a case, an ionic dissociation functional group having basicity can be introduced into the conductive carbon material. These methods can suitably be used to chemically modify a conductive carbon material by an ionic dissociation functional group. See S. S. Wong, A. T. Woolley, E. Joselevich, C. M. Leiber, Chem. Phys . Lett., 306 (1999) 219. In a method of anodizing a conductive carbon material in an electrolytic solution, a conductive carbon material is usually used as an anode. Metal such as platinum (Pt) , gold (Au) , lead (Pb), and zinc (Zn) , and any carbon material can be used as a cathode. See J. H. Wandass, J. A. Gardella, N. L. Weinberg, M. E. Bolster, L. Salvati, J. Electrochem. Soc, 134 (1987) 2734. The electrolytic solution may contain nitric acid, sulfuric acid, phosphoric acid, hydrochloric acid, hydrobromic acid, or salts having ions contained in these acids. Such salts include salts of alkali metal such as lithium, sodium, and potassium, salts of alkaline-earth metal such as magnesium, calcium, and barium, ammonium salt, sulfonium salt, phosphonium salt, and salts of Fe, Cu, and lanthanoide metal. Practically, a single electrolytic solution or a mixture of these kinds of electrolytic solutions is used. Although it is desirable that the electrolytic current density is in a range of from about 1 to about 100 mA/cm², the method is not limited to these conditions. With this method, a surface of a carbon material is chemically modified by a carboxyl group. According to the method of electrical discharge processing a conductive carbon material in a gaseous phase, an electrode in which a carboxyl group was introduced into a conductive carbon material was produced as follows. Two rod-like electrodes, which were moistened with water, were spaced at about 3 cm. An alternating voltage of 100 V was applied between the electrodes. A carbon rod (conductive carbonmaterial), which was moistened with water, was inserted into between electrodes. Arc discharge was caused in an atmosphere to treat a surface of the carbon rod by the arc discharge so as to introduce a carboxyl group into the surface of the carbon rod (conductive carbon material) . The carbon rod was made of graphite having a diameter of 6 mm. Each end of the carbon rod was rounded. The water used was ultrapure water, which had a resistivity of 18.2 MΩ-cm.

The current-voltage properties were measured in an experimental device in which the carbon rod thus treated was used as an anode, and a platinum plate was used as a cathode. The experimental device had an acrylic container holding ultrapure water therein, which has a resistivity of 18.2 MΩ- cm. The carbon rod and the platinum plate faced each other in the container. After the distance between the carbon rod and the platinum plate was adjusted by a micrometer, a voltage was applied between the carbon rod and the platinum plate while ultrapure water was supplied between the carbon rod and the platinum plate . At that time, a flowing current was measured . The distance between the carbon rod and the platinum plate was set to be 15 μm. Further, the current-voltage properties were measured in a manner similar to the above for a comparative experiment in which a carbon rod before the surface treatment by the arc discharge was used as an anode, and a platinum plate was used as a cathode. FIG. 10 shows results of the above experiments. It can be seen from FIG.10 that the carbon rod into which an carboxyl group was introduced by the surface treatment with the arc discharge had increased current at 60 V by ten or more times as compared to the carbon rod into which the carboxyl group was not introduced.

According to the method of anodizing a conductive carbon material in an electrolytic solution, an electrode in which a carboxyl group was introduced into a conductive carbon material was produced as follows. A carbon rod (conductive carbon material) was used as an anode and anodized in an H₂S0 solution of 20 weight % at a current density of 12.5 mA/cm² for 30 minutes. A platinum plate (Pt) was used as a facing electrode. The carbon rod was made of graphite having a diameter of 6 mm. Each end of the carbon rod was rounded. The current-voltage properties of the anodized carbon rod were measured under conditions similar to the above example . The distance between the carbon rod and the platinum plate was set to be 15 μm.

Further, the current-voltage properties were measured in a manner similar to the above example for a comparative experiment in which a carbon rod before the surface treatment by anodization was used as an anode, and a platinum plate was used as a cathode.

FIG. 11 shows results of the above experiments. It can be seen from FIG. 11 that the carbon rod into which a carboxyl group was introduced by anodization had increased current by ten or more times as compared to the carbon rod into which the carboxyl group was not introduced.

The carbon rod into which a carboxyl group was introduced by anodization was used as a processing electrode to perform an electrolytic process of a copper film formed on a silicon substrate. The electrolytic process was conducted at a voltage of 60 V and a current of 1.07 mA for 10 seconds while the distance between electrodes was 25 μm. As a result of the electrolytic process, the maximum processed depth was 144 nm. At that time, the current efficiency was about 48 %. The current efficiency refers to a ratio of the quantity of electricity used to process the copper film to the entire quantity of electricity passed. The current efficiency was calculated on the assumption that copper was eluted as bivalent ions or bivalent ionic compounds.

The carbon rod into which a carboxyl group was not introduced by anodization was used as a processing electrode to perform an electrolytic process of a copper film formed on a silicon substrate. The electrolytic process was conducted at a voltage of 60 V and a current of 0.043 mA for 60 seconds. As a result of the electrolytic process, the maximum processed depth was 12 nm. At that time, the current efficiency was about 3.3 %.

Thus, it can be seen that the carbon rod into which a carboxyl group was introduced by anodization had increased current during the electrolytic process and increased current efficiency as compared to the carbon rod into which the carboxyl group was not introduced.

Instead of using an electrode in which a surface of a conductive carbon material is chemically modified by an ion dissociation functional group, a graphite intercalation compound containing alkali metal may be used as an electrode. It is generally desirable that high orientated pyrolytic graphite (HOPG) is used as graphite (carbon material) in the graphite intercalation compound. However, when sodium is intercalated as alkali metal between layers of the graphite, it is desirable that low orientated graphite is used as the graphite in the graphite intercalation compound. The graphite intercalation compound should preferably have meshes because such meshes can allow water to pass therethrough to decompose water efficiently.

FIG. 12 is a schematic diagram showing an electrolytic processing apparatus using such an electrode. As shown in FIG. 12, the electrolytic processing apparatus has a pair of electrodes 301 and 302 connected to an anode and a cathode of a power supply 303. The electrodes 301 and 302 are made of a graphite intercalation compound containing alkali metal . A fluid 305 such as pure water or ultrapure water is supplied between the electrodes (graphite intercalation compounds) 301, 302 and a workpiece 304 (e.g., a copper film formed on a substrate) . Then, the workpiece 304 is brought close to the electrodes 301, 302. A voltage is applied between the electrodes 301 and 302 by the power supply 303. Water molecules in the fluid 305 are dissociated into hydroxide ions and hydrogen ions by the electrodes 301 and 302 made of a graphite intercalation compound. For example, the produced hydroxide ions are supplied to a surface of the workpiece 304. The concentration of the hydroxide ions is thus increased near the workpiece 304, and atoms in the workpiece 304 and the hydroxide ions are reacted with each other to perform removal of a surface layer of the workpiece 304. Thus, it is possible to reduce the distance between the electrodes 301, 302 and the workpiece (substrate) 304 and hence the distance between the electrode 301 serving as an anode and the electrode 302 serving as a cathode. Therefore, the electrolytic processing apparatus can flexibly cope with small electrodes and various shapes of electrodes. Furthermore, because the electrode 301 serving as an anode and the electrode 302 serving as a cathode have catalysis, a leakage current can be prevented from being produced between the cathode and the anode, i.e., between the electrodes 301 and 302.

Such an electrode, which includes a graphite intercalation compound containing alkali metal, can be used in an electrolytic processing apparatus of the above embodiments shown in FIGS .5 through 7 , instead of an electrode in which an organic compound is chemically bonded to a surface of a conductive material.

Methods of synthesizing a graphite intercalation compound include a gaseous phase constant-pressure reaction method, a liquid phase contact reaction method, a solid phase pressurizing method, and a solvent method. The gaseous phase constant-pressure reaction method comprises disposing alkali metal and graphite at different positions in a glass tube, sealing the glass tube under a vacuum, and heating the graphite and the alkali metal while controlling the temperatures thereof. The positions into which the alkali metal is intercalated and the amount of the alkali metal intercalated can be adjusted by controlling the temperatures of the alkali metal and the graphite. For example, when potassium is intercalated into HOPG, the temperatures are set at about 250 °C. The liquid phase contact reaction method comprises directly contacting a compound containing alkali in a liquid phase with graphite to react with each other. The solid phase pressurizing method comprises contacting alkali metal with graphite, pressurizing the graphite to about 5 to about 20 atmospheres (about 0.5 to about 2 MPa) , andheating the graphite to about 200 °C. The solvent method comprises dissolving alkali metal in a solvent such as an ammonium solvent, and immersing graphite in the solvent.

According to the liquid phase contact reaction method, an electrode made of a graphite intercalation compound containing alkali metal was produced (synthesized) as follows . Sodium nitrate, which has a melting point of 308 °C, was heated an_d melted in a crucible by a burner . A graphite plate, which had a length of 12.5 mm, a width of 34 mm, and a thickness of 0.5 mm, was immersed in the melted sodium nitrate and heated therein for 2 to 3 minutes . Then, the graphite plate was removed from the crucible and cooled in air. Thus, an electrode made of a graphite intercalation compound having sodium intercalated between layers of the graphite was produced. Then, the current-voltage properties weremeasured in an experimental device as shown in FIG. 13. The experimental device had an acrylic container 320, and a pair of parallel plate electrodes 321 and 322. The electrode made of a graphite intercalation compound was used as the electrode 321, and a platinumplate was used as the electrode 322. These electrodes 321 and 322 were connected to an anode and a cathode of a power supply 323, respectively. The current-voltage properties were measured in ultrapure water 325, which had a resistivity of 18.2 MΩ-cm. At that time, the distance between the electrodes 321 and 322 was set to be 12 μm, and areas of the electrodes 321 and 322 facing each other were set to be about 0.4 cm².

Further, the current-voltage properties were measured in a manner similar to the above for a comparative experiment in which a graphite plate in which sodium was not intercalated between layers of the graphite was used as the electrode.

FIG. 14 shows results of the above experiments. It can be seen from FIG. 14 that the electrode made of a graphite intercalation compound having sodium intercalated between layers of the graphite supplied a current slight lower than 50 mA (a current density of 125 mA-m²) at 150 V and thus had increased current by about 50 times as compared to the graphite plate in which sodium was not intercalated between layers of the graphite. Therefore, the graphite intercalation compound having sodium intercalated between layers of the graphite is considered to be capable of promoting to dissociate ultrapure water into hydrogen ions or hydroxide ions .

In the above example, graphite was immersed in a liquid in which sodium nitrate was heated and melted. However, the graphite may be immersed in any salts containing alkali metal, such as potassium nitrate.

A dilute chemical liquid may be added as an additive to pure water. For example, 2-propanol (IPA) may be added to pure water to adjust the polarity of the pure water.

Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.

Industrial Applicability

The present invention is applicable to an electrode to be used as a processing electrode to process a substrate and/or a feeding electrode to feed the substrate during an electrolytic process using a fluid, particularly pure water.

Claims

1. An electrode for electrolytic processing, said electrode comprising: a conductive material; and an organic compound having an ion exchange group, said organic compound being chemically bonded to a surface of said conductive material.

2. The electrode according to claim 1, wherein said organic compound comprises an organic compound selected from the group consisting of thiol and disulfide.

3. The electrode according to claim 1, wherein said ion exchange group comprises at least one of ion exchange groups selected from the group consisting of a sulfonic acid group, a carboxyl group, a quaternary ammonium group, and an amino group .

4. The electrode according to claim 1, wherein said conductive material includes at least one of gold, silver, platinum, copper, gallium arsenide, cadmium sulfide, and indium oxide (III) .

5. An electrode for electrolytic processing, said electrode comprising: a conductive carbon material; and an ionic dissociation functional group chemically modifying a surface of said conductive carbon material.

6. The electrode according to claim 5, wherein said ionic dissociation functional group comprises a carboxyl group. ^•

7. The electrode according to claim5, wherein said ionic dissociation functional group comprises at least one of ion exchange groups selected from the group consisting of a quaternary ammonium group, and a tertiary or lower amino group .

8. The electrode according to claim 5, wherein said conductive carbon material comprises a conductive carbon material selected from the group consisting of glassy carbon, fullerene, and carbon nanotubes.

9. An electrode for electrolytic processing, said electrode comprising a graphite intercalation compound containing alkali metal.

10. An electrolytic processing apparatus comprising: a processing electrode; a feeding electrode to feed a workpiece; a workpiece holder for holding the workpiece andbringing the workpiece into contact with or close to said processing electrode; a power supply for applying a voltage between said processing electrode and said feeding electrode; and a fluid supply unit for supplying a fluid between the workpiece and said processing electrode, wherein at least one of said processing electrode and said feeding electrode comprises the electrode according to any one of claims 1 through 9.

11. An electrolytic processing method, comprising: feeding a workpiece through a feeding electrode; applying a voltage between the feeding electrode and a processing electrode; supplying a fluid between the workpiece and the processing electrode; and bringing the workpiece into contact with or close to the processing electrode, wherein at least one of the processing electrode and the feeding electrode comprises the electrode according to any one of claims 1 through 9.

12. A method of promoting dissociation of liquid into ions by a conductive material to which an organic compound having an ion exchange group is chemically bonded.

13. The method according to claim 12 , wherein the organic compound comprises an organic compound selected from the group consisting of thiol and disulfide.

14. The method according to claim 12, wherein the ion exchange group comprises at least one of ion exchange groups selected from the group consisting of a sulfonic acid group, a carboxyl group, a quaternary ammonium group, and an amino group .

15. The method according to claim 12, wherein the conductive material comprises a material including at least one of gold, silver, platinum, copper, gallium arsenide, cadmium sulfide, and indium oxide (III) .