SULFURIC ACID ELECTROLYSIS METHOD AND SULFURIC ACID ELECTROLYSIS APPARATUS

An electrolysis apparatus comprising: an electrolytic cell in which a sulfuric acid solution is fed and discharged; a conductive anode and cathode electrode of diamond composition; a feeding unit for feeding the sulfuric acid solution to the electrolytic cell; a power supply unit for applying a voltage between the anode and cathode electrodes; and a power control unit for controlling the power supply unit such that a forward voltage is applied between the anode and cathode during normal electrolysis with the polarity applied between the anode and cathode inverted under predetermined conditions during intervals between normal operation to dissolve precipitates of sulfur generated in the electrolytic cell for stabilizing the electrolysis operation.

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Description
TECHNICAL FILED

The present invention relates to an electrolysis method and an electrolysis apparatus of electrolyzing sulfuric acid to generate persulfuric acid.

BACKGROUND ART

Methods of electrolyzing sulfuric acid solution to generate peroxydisulfuric acid and peroxymonosulfuric acid (hereinafter, collectively referred to as persulfuric acid) and using the persulfuric acid for cleaning a semiconductor material are known.

In one of the methods of electrolyzing sulfuric acid to generate persulfuric acid, while passing sulfuric acid solution between electrodes in an electrolytic cell, electrolysis is performed by applying a DC voltage between an anode and a cathode of the electrodes. As structures of the electrolytic cells, there are a single-polarity cell (a pair of anode and cathode is used) and a plural-polarity cell using a bipolar electrode. In these structures of the electrolytic cells, a relationship between the anode and cathode as one pair is the same. A spacer is usually used in order to maintain a constant distance between the electrodes, and a sealing member such as an O-ring is used in order to seal the electrolyte. For example, the inventor of the invention proposed an electrolytic cell having the above-described structure (refer to Patent Document 1).

FIG. 7A schematically illustrates an electrolytic cell. A spacer 22 is disposed between an anode 20 and a cathode 21 to secure a passage 23. In the spacer 22, an inlet hole 22a is formed at inlet side of the spacer 22 and an outlet hole 22b is formed at outlet side of the spacer 22. The inlet hole 22a and the outlet hole 22b are configured as passages which are relatively narrower than the passage 23. In addition, O-rings 24 as sealing members are disposed between the spacer 22 and the electrodes to ensure sealing of the passage 23.

CITATION LIST Patent Document

Patent Document 1: Japanese Unexamined Patent Application Publication No. 2007-262531

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

As generally known, if sulfuric acid is electrolyzed, H2 (gas) is generated at the cathode, and if sulfuric acid is exposed to a reducing atmosphere, S (solid sulfur) or H2S (gas) is generated.

Therefore, if operation is continuously performed, sulfur or chemical species associated with generation of the sulfur is generated on the electrode surface, particularly, in a peripheral end portion of the electrode or in a shadow portion of an O-ring. Fine S particles that are attached and grown on the electrode surface are peeled off from the electrode and moved along the flow of the electrolyte to be attached and accumulated in narrow portions of an outlet hole of the electrolytic cell or an inlet hole of the electrolytic cell, so that there is a problem in that the inlet hole or the outlet hole is clogged in due course. It is understood that the problem easily occurs in the case where a concentration of sulfuric acid is high, the case where a current density is high, and the case where a voltage between the electrodes is high.

A state where sulfur is accumulated in the O-ring portion or in the vicinity of the outlet hole is illustrated in FIG. 7B. Presumptively, in the figure, sulfur is precipitated as indicated by “A”, and the precipitate is peeled off and moved finally, so that the cell outlet or the cell inlet is clogged as indicated by “B” in due course.

The invention is to provide an electrolysis method and an electrolysis apparatus capable of preventing accumulation of sulfur precipitated through electrolysis of sulfuric acid solution and preventing the precipitates of sulfur from clogging inner portions of the system.

Means For Solving Problem

According to a first aspect of the invention, there is provided a sulfuric acid electrolysis method of generating persulfuric acid by performing electrolysis in an electrolytic cell while flowing sulfuric acid solution of 70 mass % or more between an anode and an cathode among a plurality of electrodes which include at least the anode and cathode, each of which at least liquid contact surface is constructed with a conductive diamond, the method including: performing a normal operation of performing the electrolysis by applying a forward voltage between the anode and the cathode of the electrodes; performing a polarity inversion operation of inverting the voltage applied between the anode and the cathode in an interval between the normal operations; and dissolving precipitates of sulfur generated in the electrolytic cell during the normal operation into the sulfuric acid solution during the polarity inversion operation.

According to a second aspect of the invention, in the sulfuric acid electrolysis method according to the first aspect of the invention, the electrolysis is performed while introducing the sulfuric acid solution from an outside of the electrolytic cell into an inside of the electrolytic cell, and the electrolyzed sulfuric acid solution is discharged to the outside of the electrolytic cell.

According to a third aspect of the invention, in the sulfuric acid electrolysis method according to the first or second aspect of the invention, a retention portion where the flowing sulfuric acid solution is retained is included in the electrolytic cell.

According to a fourth aspect of the invention, in the sulfuric acid electrolysis method according to any one of the first to third aspects of the invention, a narrow passage portion where the sulfuric acid solution flows is included in the electrolytic cell.

According to a fifth aspect of the invention, in the sulfuric acid electrolysis method according to the third aspect of the invention, a spacer of securing a passage of the sulfuric acid solution is disposed between the electrodes, and the retention portion is formed with the spacer or with the spacer and other members.

According to a sixth aspect of the invention, in the sulfuric acid electrolysis method according to the fifth aspect of the invention, a sealing member is installed between the spacer and the electrode, and the retention portion is formed with at least the sealing member.

According to a seventh aspect of the invention, in the sulfuric acid electrolysis method according to the fifth or sixth aspect of the invention, an outlet hole as a narrow passage portion through which the sulfuric acid solution passes is formed in the spacer.

According to an eighth aspect of the invention, in the sulfuric acid electrolysis method according to any one of the fifth to seventh aspects of the invention, an inlet hole as a narrow passage portion through which the sulfuric acid solution passes is formed in the spacer.

According to a ninth aspect of the invention, in the sulfuric acid electrolysis method according to any one of the first to eighth aspects of the invention, a circulation line is installed to connect an outlet of the electrolytic cell and an inlet of the electrolytic cell, and a narrow passage portion is included in the circulation line and/or an upstream side of a retention portion of the electrolytic cell.

According to a tenth aspect of the invention, in the sulfuric acid electrolysis method according to any one of the first to ninth aspects of the invention, the polarity inversion operation is performed after continuously performing the normal operation for a predetermined time.

According to an eleventh aspect of the invention, in the sulfuric acid electrolysis method according to any one of the first to tenth aspects of the invention, the polarity inversion operation is performed based on a result of determination of a precipitated state of the sulfur.

According to a twelfth aspect of the invention, in the sulfuric acid electrolysis method according to any one of the first to eleventh aspects of the invention, at least one of the following conditions (a) to (c) is satisfied:

(a) a concentration of sulfuric acid in the electrolytic cell is 85 mass % or more;

(b) a temperature of sulfuric acid at the inlet of the electrolytic cell is 70° C. or more; and (c) a current density in the electrolysis is 50 A/dm2 or more.

According to a thirteenth aspect of the invention, there is provided a sulfuric acid electrolysis apparatus including:

an electrolytic cell which sulfuric acid solution can be fed to and discharged from;

a plurality of electrodes including at least an anode and a cathode in the electrolytic cell, which are disposed with a gap so that the sulfuric acid solution flows between the anode and cathode and each of which at least liquid contact surface is constructed with a conductive diamond;

a spacer which secures the gap of the electrodes;

a narrow passage portion formed in the spacer where the sulfuric acid solution flows;

a feeding unit which feeds the sulfuric acid solution to the electrolytic cell;

a power supply unit which applies a voltage between the anode and the cathode of the electrodes; and

a power control unit which controls the power supply unit to apply a forward voltage between the anode and the cathode during normal electrolysis and to perform polarity inversion of inverting the voltage applied between the anode and the cathode in a predefined condition.

According to a fourteenth aspect of the invention, there is provided a sulfuric acid electrolysis apparatus including:

an electrolytic cell which sulfuric acid solution can be fed to and discharged from;

a plurality of electrodes including at least an anode and a cathode in the electrolytic cell, which are disposed with a gap so that the sulfuric acid solution flows between the anode and cathode and each of which at least liquid contact surface is constructed with a conductive diamond;

a spacer which secures the gap of the electrodes;

a retention portion formed with the spacer or with the spacer and other members, where the sulfuric acid solution is retained;

a power supply unit which applies a voltage between the anode and the cathode of the electrodes; and

a power control unit which controls the power supply unit to perform polarity inversion of inverting a voltage applied between the anode and the cathode during normal electrolysis.

According to the invention, before the solid sulfur or precursors thereof are accumulated on the electrode surface or the shadow portion of the O-ring by allowing the sulfuric acid solution to flow between the electrodes while applying a voltage between the electrodes, the polarity inversion is performed to allow the electrode surface to have an oxidizing property, and the electrolysis is performed for a predetermined time or more, so that the solid sulfur or precursors thereof can be effectively returned to the sulfuric acid or the sulfate ions. The polarity inversion operation can be continuously performed for about 10 to 100 hours.

The above operation is repeated at a predetermined interval, so that the accumulation of sulfur and the clogging of the cell can be prevented. As a method of determining the interval, the interval can be determined according to a certain operation time based on experience or the number of electronic material boards processed by cleaning or the like. However, if sulfur is accumulated, a voltage (voltage between the electrodes) required to flow a predetermined current is increased. Therefore, when the conduction is formed through current control, the voltage can be always monitored. When the voltage is increased up to a predetermined value, the polarity inversion operation may be configured to be started. In addition, even when the polarity inversion is performed, the voltage can be monitored. When the voltage is increased down to a predetermined value, the polarity inversion operation may be configured to be stopped.

Hereinafter, forms of reaction during the electrolysis are described.

In the electrolyte, the sulfuric acid or water molecules are dissociated as follows, so that ions of SO42−, HSO4−, H+, and the like exist.


H2SO4HSO4−+H+


HSO4−SO42−+H+


H2OOH−+H+

The concentrations of H+ (same as H3O+) and HSO4− have peaks at the concentration of sulfuric acid of 70 mass % to 80 mass % and are decreased at the higher concentration side. On the other hand, the concentration of undissociated sulfuric acid molecules H2SO4 (aq) is drastically increased at the higher concentration side. In addition, since a high concentration sulfuric acid solution is a strong acid, the concentration of OH− is low.

At the cathode, H+ ions are attracted, and thus, the H+ ions receive electrons as expressed in the following reaction formula, so that the hydrogen gas H2 is generated.


2H++2e−→H2

At the anode, HSO4− or SO42− ions are attracted, and thus, the HSO4− or SO42− ions release electrons as expressed in the following reaction formula, so that the persulfuric acid H2S2O3 is generated.


2HSO4−→S2O82−+2H++2e−


2SO42−→S2O82−+2e−

In addition, at the anode, water is also electrolyzed as expressed in the following reaction formula, so that oxygen gas O2 is generated.


2OH−→O2+2H++4e−

With respect to oxidization and reduction potentials in the electrode reaction related to the sulfuric acid and the water, Pourbaix diagrams illustrated in FIGS. 4 and 5 are known.

If the potentials of the sulfuric acid and the water under the conditions of the concentration of sulfuric acid=92 mass % and the temperature=60° C. are illustrated based on the Pourbaix diagram and juxtaposed, the potentials are as shown in FIG. 6. In the figure, the numbers of the formulas are based on the formulas on the diagrams illustrated in FIGS. 4 and 5.

Since the sulfuric acid used in the electrolytic cell has a high concentration, pH is almost −2. At the anode side, O2, O3, H2O2, and H2S2O8 are generated with the potential order of O2>O3>H2O2>H2S2O8. Actually, the largest amount of O2 is generated, and the second largest amount of H2S2O8 is generated. The potential order at the cathode is S>H2>H2S. In an actual cell, although the mainly generated material is H2, S can be sufficiently obtained in terms of potential. If the generated S is not retained on the electrode surface or the like, the generated S reacts with oxidizing substance in the solution to return to sulfuric acid. If there is a retention portion, S is accumulated on the retention portion.

With respect to efficiency of the generation of persulfuric acid, actually, 80 to 90% of electrons passing through the anode are associated with the generation of O2, and remaining 10 to 20% is contributed to the generation of persulfuric acid. The O2 is discharged to the outside of the system, and the persulfuric acid is used for oxidation reaction to become sulfuric acid so as to return to the cell. If the solution is used in the above-described circulation manner, water is consumed in the electrolytic cell where the sulfuric acid is electrolyzed, so that the concentration of sulfuric acid is increased.

As described above, although it is difficult in principle that the generation of sulfur at the cathode is completely prevented, the sulfur is allowed to return to the sulfuric acid by oxidizing the sulfur by using the oxidizing substance (persulfuric acid or the like) generated at the anode. In the case where the sulfur is not well oxidized due to the retention of the solution, it is preferable that the polarity inversion be performed to generate persulfuric acid in the vicinity of the retention portion so that the sulfur in the retention portion is removed.

The invention is appropriate for the case where a retention portion in which sulfuric acid solution is retained is formed in the electrolytic cell. The retention portion does not denote a specific position in the electrolytic cell, and the retention portion may be formed at different positions according to the structure of the electrolytic cell. The retention portion is easily formed at the position where the flow of the sulfuric acid solution is disturbed.

The retention portion is easily formed in a concave portion such as a corner of a member where the angle of the surface is rapidly changed. The retention portion may be formed in an intersection portion between the electrode and the spacer and an intersection portion between the sealing member and the electrode or the spacer.

In the case where the electrolytic cell has a narrow passage portion of which passage cross section is relatively smaller than other portions, since the passage is easily clogged by precipitates, the invention is particularly useful for the electrolytic cell having the narrow passage portion. In the case where the narrow passage portion is disposed at the downstream side of the electrolytic cell, the problem easily occurs, for example, at the outlet hole or like. In addition, the precipitates accumulated in the retention portion are peeled off and flowed together with the sulfuric acid solution, so that there may be a problem in that the precipitates are attached and accumulated in the inlet hole of the electrolytic cell to prevent the flow of the sulfuric acid solution by clogging the inlet hole in due course.

According to results of previous research of the inventors, in the case where the concentration of sulfuric acid is 85 mass % or more and the current density is 50 A/dm2 or more, sulfur can be easily precipitated and accumulated. In the case where the concentration of sulfuric acid is higher than the above value, the current density needs to be set to be lower than the above value. In addition, the operation is performed so that the temperature of the inlet of the cell is in a range of 40 to 70° C. However, it may be understood that, if conduction resistance is increased due to the precipitation of sulfur, inner resistance of the cell is increased, so that the temperature of the cell is increased. Therefore, the precipitation of sulfur is accelerated. Quantitatively, it may be understood that this is because vaporization of water is accelerated and, thus, the concentration is remarkably increased, particularly, at the retention portion or the like illustrated in FIG. 7. Particularly, it is not preferable that the operation be performed at the temperature of the inlet of the cell exceeding 70° C.

It can be understood from the above description that the problem of precipitates of sulfur easily occurs when any one of the following three conditions is satisfied. Therefore, it is preferable that the invention be performed in the case where a system is under the following conditions.

(a) The concentration of sulfuric acid in the electrolytic cell is 85 mass % or more.

(b) The temperature of sulfuric acid at the inlet of the electrolytic cell is 70° C. or more.

(c) The current density is 50 A/dm2 or more.

The concentration of TOC after cleaning in an electronic material cleaning by using sulfuric acid solution is in a range of 0 to 10 mg/l, and organic materials do not almost exist.

Effect of the Invention

As described above, according to the invention, when electrolyzing sulfuric acid solution in an electrolytic cell, the problem of accumulation of precipitates of sulfur or precursors thereof is avoided, so that it is possible to obtain an effect in that stable electrolysis can be continuously performed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a single-wafer-type cleaning system including an electrolytic cell according to an embodiment of the invention.

FIG. 2 is a diagram illustrating a batch-type cleaning system including an electrolytic cell according to an embodiment of the invention.

FIG. 3 is a diagram illustrating a batch-type cleaning system using an electrolytic cell without a polarity inversion function.

FIG. 4 is a diagram illustrating a Pourbaix diagram.

FIG. 5 is a diagram illustrating a Pourbaix diagram.

FIG. 6 is a graph illustrating potential of sulfuric acid having a concentration of 92 mass % and a temperature of 60° C. and potential of water based on Pourbaix diagram.

FIG. 7 is a diagram illustrating a structure of an electrolytic cell and an accumulated state of sulfur.

 MODE FOR CARRYING OUT THE INVENTION

(First Embodiment)

Hereinafter, a cleaning system including an electrolysis apparatus according to an embodiment of the invention will be described with reference to the attached drawings.

As illustrated in FIG. 1, an electrolysis apparatus 1 is configured to include an electrolytic cell 2. The electrolytic cell 2 is of a non-diaphragm type, and an anode and a cathode which are configured with diamond electrodes are installed within the electrolytic cell 2 without separation by a diaphragm. As a power supply unit, a DC power supply unit 3 is connected to the two electrodes. A power control unit 4 which controls the direction of voltage applied to the anode and the cathode is connected to the DC power supply unit 3. The power control unit 4 may be configured to include a switching element which switches a path of the voltage applied from the DC power supply unit 3 to the anode and the cathode, for example.

As illustrated in FIG. 7A, the electrolytic cell 2 is configured to include an anode 20 and a cathode 21 which are configured with plate-shaped diamond electrodes. A spacer 22 is disposed between the anode 20 and the cathode 21 to secure a passage 23 between the anode 20 and cathode 21. As the diamond electrode, a diamond electrode having a conductivity which is obtained by forming a diamond thin film in a substrate shape and doping boron in a range of, preferably, 50 to 20,000 ppm with respect to carbon amount of the diamond thin film may be appropriately used.

In the description of the embodiment, the electrolytic cell is configured to include an anode and a cathode as the electrodes. However, besides the anode and the cathode as the electrodes, the electrolytic cell can be configured to include a bipolar electrode. In addition, in the electrolytic cell, multiple layers of electrodes are installed so that gaps are formed between the electrodes, and electrolysis is performed by allowing the sulfuric acid solution to pass between the electrodes.

In the spacer 22, an inlet hole 22a is formed at inlet side of the spacer 22 and an outlet hole 22b is formed at outlet side of the spacer 22. The inlet hole 22a and the outlet hole 22b are configured as passages which are relatively narrower than the passage 23. The inlet hole 22a and the outlet hole 22b correspond to narrow passage portions in the invention. In addition, O-rings 24 as sealing members are disposed between the spacer 22 and the anode 20 and between the spacer 22 and the cathode 21 to ensure sealing of the passage 23. As a material constituting the spacer 22, a material (for example, polytetrafluoroethylene) having an insulating property and corrosion resistance is preferred.

In the electrolytic cell 2, the inner surfaces of the electrodes 20 and 21, the corner portions of the spacer 22, or the inner surface side of the O-ring 24 become retention portions 25 which disturb the flow of the sulfuric acid solution. In order to form an upward flow, the electrolytic cell 2 is arranged so that the inlet side becomes the lower side and the outlet side becomes the upper side.

An electrolyte reservoir 10 is connected to the electrolytic cell 2 through a first circulation line 5 so that the sulfuric acid solution can be circulated and flowed between the electrolytic cell 2 and the electrolyte reservoir 10. Namely, the feeding side of the first circulation line 5 is connected to the electrolytic cell 2 so as to communicate with the inlet side of the electrolytic cell 2, and the returning side of the first circulation line 5 is connected to the electrolytic cell 2 so as to communicate with the outlet side of the electrolytic cell 2.

A gas liquid separation tank 6 is installed at the returning side of the first circulation line 5. The gas liquid separation tank 6 receives sulfuric acid solution containing gas and separates the gas from the sulfuric acid solution to discharge the gas to the outside of the system. As the gas liquid separation tank 6, any known component may be used. In the invention, if separation of gas from solution is available, the configuration is not particularly limited.

In addition, a circulation pump 7 which circulates the sulfuric acid solution and a cooler 8 which cools the sulfuric acid solution are installed at the feeding side of the first circulation line 5. The first circulation line 5 and the circulation pump 7 correspond to a feeding unit in the invention. The cooler 8 cools the sulfuric acid solution to adjust the temperature thereof to be appropriate for the electrolysis, for example, a temperature in a range of 40 to 70° C. The invention is not particularly limited to the above-described configuration. The electrolysis apparatus according to the invention is configured with the electrolytic cell 2, the DC power supply unit 3, the power control unit 4, the first circulation line 5, the gas liquid separation tank 6, the circulation pump 7, and the cooler 8.

The feeding side of a second circulation line 11 is connected to the electrolyte reservoir 10 through a feeding pump 12.

A heater 13 is installed in the feeding direction of the second circulation line 11. The front end side of the second circulation line 11 in the feeding direction at the downstream side of the heater 13 is connected to the single-wafer-type cleaning apparatus 15.

The heater 13 is configured to include a quartz pipe to heat the sulfuric acid solution in a one pass manner, for example, by a near infrared heater. As a result, the sulfuric acid solution is rapidly heated so that the temperature of the sulfuric acid solution in the single-wafer-type cleaning apparatus 15 is in a range of 150 to 220° C.

In the above-described single-wafer-type cleaning apparatus 15, for example, a electronic material substrate 100 is fixed to be mounted on a rotation table, and a process of allowing the sulfuric acid solution containing persulfuric acid to flow down from a nozzle to the semiconductor material is performed.

In the description of the embodiment, the cleaning apparatus is of a single-wafer-type cleaning apparatus. However, the type of cleaning apparatus according to the invention is not limited to the above type, but a batch-type cleaning apparatus may be used.

The returning side of the second circulation line 11 is connected to the single-wafer-type cleaning apparatus 15. At the returning side of the second circulation line 11, along the returning direction, a pump 16, a reaction tank 17, a feeding pump 18, and a cooler 19 are sequentially installed, and the front end side of the second circulation line 11 in the returning direction is connected to the electrolyte reservoir 10.

Next, operations of the cleaning system having the above-described configuration will be described.

Sulfuric acid solution having a sulfuric acid concentration of 85 to 96 mass % and a temperature of 50 to 80° C. is stored in the electrolyte reservoir 10. The sulfuric acid solution is fed to the first circulation line 5 by the circulation pump 7. The cooler 8 adjusts the temperature of the sulfuric acid solution to be appropriate for the electrolysis (40 to 70° C.), and the sulfuric acid solution is introduced to the inlet side of the electrolytic cell 2 to be flowed from the inlet hole 22a into the passage 23.

In the electrolytic cell 2, a forward voltage is applied between the anode and the cathode by the DC power supply unit 3, so that the sulfuric acid solution introduced into the electrolytic cell 2 is electrolyzed. Due to the electrolysis, in the electrolytic cell 2, an oxidizing substance including the persulfuric acid and oxygen gas are generated at the anode side, and hydrogen gas is generated at the cathode side. The oxidizing substance and the gases are flowed through the passage 23 in the state where the oxidizing substance and the gases are mixed with the sulfuric acid solution. The sulfuric acid solution flowing through the passage 23 is fed to the first circulation line 5 through the outlet hole 22b. The sulfuric acid solution discharged from the outlet hole 22b is fed to the gas liquid separation tank 6 through the first circulation line 5, so that the gases are separated from the sulfuric acid solution. The gases are discharged to the outside of the system to be processed safely by a catalytic apparatus (not illustrated) or the like.

The sulfuric acid solution from which gas is separated by the gas liquid separation tank 6 contains persulfuric acid, and the sulfuric acid solution is allowed to return to the electrolyte reservoir 10 through the returning side of the first circulation line 5. After that, the sulfuric acid solution is repetitively fed to the electrolytic cell 2, and thus, the concentration of persulfuric acid can be increased by electrolysis. If the concentration of persulfuric acid reaches an appropriate concentration, a portion of the sulfuric acid solution in the electrolyte reservoir 10 is fed to the heater 13 through the feeding side of the second circulation line 11 by the feeding pump 12.

In the heater 13, while passing through the passage, the sulfuric acid solution containing persulfuric acid is heated by a near infrared heater. It is preferable that, during the feeding of the sulfuric acid solution, the flow rate of the sulfuric acid solution be adjusted such that the passing time from the entrance of the heater 13 until the sulfuric acid solution is used in the single-wafer-type cleaning apparatus 15 is less than 1 minute, preferably, less than 20 seconds, more preferably, less than 10 seconds. In the single-wafer-type cleaning apparatus 15, a flow rate of 500 to 2000 mL/minute is considered to be an appropriate amount, and thus, a passage length and passage cross-section area of the heater 13 and a line length and a passage cross-section area of the second circulation line 11 at the downstream side of the heater 13 and the like are set so that the above-described passing time is less than 1 minute at the flow rate. In the single-wafer-type cleaning apparatus 15, when the sulfuric acid solution is supplied to the electronic material substrate 100, the temperature of the liquid is in a range of 150° C. to 220° C.

A cleaning object of the single-wafer-type cleaning apparatus 15 is a semiconductor material such a silicon wafer where resist implanted with ions of, for example, 1×1012 to 1×1016 atoms/cm2 is formed.

Contaminants such as resist on the electronic material substrate 100 is effectively peeled off and removed by flowing and dropping a small amount of the high-temperature sulfuric acid solution containing persulfuric acid from a nozzle (not illustrated) to be in contact with the electronic material substrate 100 while rotating the electronic material substrate 100 on a rotation table (not illustrated).

The sulfuric acid solution used for the cleaning is discharged from the single-wafer-type cleaning apparatus 15 and is fed to the reaction tank 17 through the returning side of the second circulation line 11 by the pump 16 to be stored in the reaction tank 17. The sulfuric acid solution stored in the reaction tank 17 contains residual organic materials such as resist cleaned by the single-wafer-type cleaning apparatus 15, and during the storage of the sulfuric acid solution in the reaction tank 17, the residual organic materials are oxidatively decomposed by oxidizing substance contained in the sulfuric acid solution. The storage time of the sulfuric acid solution in the reaction tank 17 can be arbitrarily adjusted according to residual organic material content or the like. At this time, since the high-temperature sulfuric acid solution containing persulfuric acid is continuously supplied from the single-wafer-type cleaning apparatus 15, the reaction tank 17 is maintained at an appropriate temperature.

In the reaction tank 17, the sulfuric acid solution where the contained residual organic material is oxidatively decomposed is circulated to the electrolyte reservoir 10 through the cooler 19 installed in the second circulation line 11 by the feeding pump 18.

In addition, if the high-temperature sulfuric acid solution is circulated to the electrolyte reservoir 10, the decomposition of the persulfuric acid in the sulfuric acid solution stored in the electrolyte reservoir 10 is accelerated. Therefore, after the sulfuric acid solution is cooled down to an appropriate temperature of about 50 to 80° C. by the cooler 19, the sulfuric acid solution is introduced into the electrolyte reservoir 10. The sulfuric acid solution introduced into the electrolyte reservoir 10 is fed to the electrolytic cell 2 through the feeding side of the first circulation line 5, and persulfuric acid is generated by electrolysis. The sulfuric acid with the persulfuric acid is fed to the electrolyte reservoir 10 through the returning side of the first circulation line 5. The circulation is repeated, so that the persulfuric acid is continuously generated.

According to the above-described operation, the sulfuric acid solution containing persulfuric acid is fed to be circulated, so that the high-temperature cleaning solution containing the high-concentration persulfuric acid can be continuously supplied to the single-wafer-type cleaning apparatus 15 as a using side.

Although not described above, a discharging line can be connected to the second circulation line 11 at the upstream side of the reaction tank 17 to be branched from the second circulation line 11, so that the sulfuric acid solution is not fed to the reaction tank 17 but discharged to the outside of the system at an appropriate time.

By discharging a small amount of the sulfuric acid solution from the discharging line as needed, it is possible to prevent resist doping elements or other materials which are not oxidatively decomposed accumulated in the solution in the system from being accumulated at a high concentration. The above operation may be performed by controlling opening and closing of an opening/closing valve installed in the circulation line or the discharging line.

By continuously maintaining the operation state in the cleaning system, as described above, in the electrolytic cell 2, sulfur or chemical species associated with generation of the sulfur is generated in the retention portion 25 on the electrode surface, particularly, in the peripheral end portion of the electrode or in the shadow portion of the O-ring. If the generation of the sulfur or the chemical species is not prevented, as described above, the sulfur or the chemical species are gradually grown to cause the above-described peeling or the clogging of the narrow passage.

In the invention, in a case where the continuation time of the electrolysis reaches a predetermined time, in a case where the number of processed electronic material substrates 100 reaches a predetermined number, or in a case where the precipitation of the sulfur component reaches some degrees, the polarity inversion operation of inverting the voltage applied between the anode 20 and the cathode 21 from the DC power supply unit 3 is performed under the control of the power control unit 4, so that the electrolysis is performed. Therefore, the precipitates of sulfur precipitated in the vicinity of the cathode 21 or the like is dissolved by an oxidizing substance which is generated in the vicinity of the cathode 21 functioning as an anode due to the polarity inversion, so that the dissolved sulfur is moved together with the sulfuric acid solution. By maintaining the polarity inversion operation to some extent, the precipitates of sulfur are removed or decreased, so that the stable electrolysis can be continuously performed. After the precipitates of sulfur are removed or decreased, the operation is continuously performed. Before the precipitates of sulfur or precursors thereof are accumulated on the polarity-inverted cathode, the polarity inversion is performed again to invert the voltage applied between the anode 20 and the cathode 21 in the backward direction, so that the electrolysis is performed by applying the voltage in the forward direction.

By repeating the above-described operation, the stable electrolysis can be continuously performed for a long time.

In general, in the polarity inversion as a measure of coping with adhesion of organic materials, the polarity inversion time is limited since the cathode and the anode are different from each other. However, in the embodiment, since all of the electrodes are diamond electrodes, normal operation can be performed for a long time (10 to 100 hours) in the state in which the polarity inversion is performed.

In addition, the continuation time of the operation after the polarity inversion can be set so that the time of the normal electrolysis reaches a predetermined time. The predetermined time may be determined by considering an accumulated current amount and a concentration, temperature, a flow rate, or the like of sulfuric acid solution, and the predetermined time may be obtained through experiments. In addition, in a case where the number of cleaned electronic material substrates 100 is a predetermined number, the polarity inversion operation may also be performed.

Although the continuation time of the operation after the polarity inversion may be set not to be constant, in a case where diamond layers with uniform thickness are laminated on the two surfaces of the diamond electrode, it is preferable that the continuation time is set to be constant in order to equalize abrasion of the diamond according to the operation.

In addition, as a different timing of performing the polarity inversion operation, a result of estimation of a degree of precipitation of sulfur in the electrolytic cell can be used. Namely, in a case where an estimated degree of precipitation of sulfur reaches a predefined degree, the polarity inversion operation is performed. The degree of precipitation of sulfur can be determined by an electrolysis voltage rising when the electrolysis is performed at a constant current as described above. Namely, when the voltage reaches a predefined electrolysis voltage, the precipitation of sulfur proceeds, so that the polarity inversion operation is performed.

(Second Embodiment)

Next, a second embodiment where the above-described electrolysis apparatus 1 is applied to a batch-type cleaning tank 30 will be described with reference to FIG. 2. In the second embodiment, the same components as those of the first embodiment are denoted by the same reference numerals, and the description thereof will not be made or will be simply made.

An electrolyte reservoir 10 is connected to an electrolytic cell 2 through a first circulation line 5. A gas liquid separation tank 6 is installed at the returning side of the first circulation line 5, and a circulation pump 7 and a cooler 8 are sequentially installed at the feeding side of the first circulation line 5.

In the batch-type cleaning tank 30, an outlet side and an inlet side are connected to a second circulation line 31, and a feeding pump 32 and a heater 33 are installed at the returning side of the second circulation line 31. An electronic material substrate 100 is immersed in the sulfuric acid solution in the batch-type cleaning tank 30, so that resist or the like attached on the electronic material substrate 100 is peeled off and cleaned. At this time, while the batch-type cleaning tank 30 is controlled by a heating unit (not illustrated) such as a heater or a heat exchanger so that the temperature of the batch-type cleaning tank 30 is in a range of 120˜190° C., the sulfuric acid solution is circulated.

A returning third circulation line 35 is connected to the second circulation line 31 at the downstream side of the feeding pump 32 and the upstream side of the heater 33 to be branched from the second circulation line 31, and a feeding end side of the returning third circulation line 35 is connected to the electrolyte reservoir 10 through a cooler 37.

A feeding third circulation line 34 is connected to the electrolyte reservoir 10 through a feeding pump 36. The feeding third circulation line 34 is connected to the second circulation line 31 at the downstream side of the heater 33 to merge with the second circulation line 31.

The heater 33 may have the same configuration as that of the above-described heater 13.

Next, operations of the cleaning system having the above-described configuration will be described.

Sulfuric acid solution having a concentration of 85 to 96 mass % and a temperature of 50 to 90° C. is stored in the electrolyte reservoir 10. The sulfuric acid solution is fed to the first circulation line 5 by the circulation pump 7. The cooler 8 adjusts the temperature of the sulfuric acid solution to be appropriate for the electrolysis (40 to 80° C.), and the sulfuric acid solution is introduced from the inlet hole 22a of the electrolytic cell 2 to the passage 23.

In the electrolytic cell 2, a forward voltage is applied between the anode and the cathode by the DC power supply unit 3, so that the sulfuric acid solution introduced into the electrolytic cell 2 is electrolyzed. The electrolyzed sulfuric acid solution is fed to the first circulation line 5 through the outlet hole 22b, so that gas is separated from the gas liquid separation tank 6.

The sulfuric acid solution from which gas is separated by the gas liquid separation tank 6 is allowed to return to the electrolyte reservoir 10 through the returning side of the first circulation line 5. After that, the sulfuric acid solution is repetitively fed to the electrolytic cell 2, so that the concentration of persulfuric acid can be increased by electrolysis. If the concentration of persulfuric acid reaches an appropriate concentration, a portion of the sulfuric acid solution in the electrolyte reservoir 10 is fed to the second circulation line 31 at the downstream side of the heater 33 through the feeding third circulation line 34 by the feeding pump 36 to merge with the sulfuric acid solution of the second circulation line 31. The merged sulfuric acid solution is introduced into the batch-type cleaning tank 30.

In addition, the sulfuric acid solution in the batch-type cleaning tank 30 is circulated through the second circulation line 31 by the feeding pump 32. At this time, the sulfuric acid solution which is heated by the heater 33 is introduced into the batch-type cleaning tank 30.

In the heater 33, while the sulfuric acid solution containing persulfuric acid passes through the passage, the sulfuric acid solution is heated by the heater. At this time, the sulfuric acid solution is heated so that the sulfuric acid solution is mixed with the sulfuric acid solution fed through the feeding third circulation line 34 the temperature thereof is in a range of 120° C. to 190° C. when the sulfuric acid solution is supplied to the batch-type cleaning tank 30.

The electronic material substrate 100 is cleaned in the batch-type cleaning tank 30. While a portion of the sulfuric acid solution used for the cleaning is circulated through the second circulation line 31, the portion of the sulfuric acid solution is heated by the heater 33 to be fed to the batch-type cleaning tank 30, and the remaining portion of the sulfuric acid solution is allowed to return to the electrolyte reservoir 10 through the returning third circulation line 35. At this time, the sulfuric acid solution is cooled by the cooler 37 down to the temperature thereof which is appropriate for the electrolysis, for example, in a range of 40 to 70° C.

In the electrolyte reservoir 10, the sulfuric acid solution is fed to the electrolytic cell 2 through the first circulation line 5 by the circulation pump 7, so that the persulfuric acid is generated and the sulfuric acid solution is returned to the electrolyte reservoir 10.

By repeating the circulation of the sulfuric acid solution, the cleaning of the electronic material substrate 100 can be performed in the state where the concentration of persulfuric acid is stable.

By continuously maintaining the operation state in the cleaning system, as described above, sulfur or chemical species associated with generation of the sulfur is generated in the retention portion 25 in the electrolytic cell 2. In the embodiment, the polarity inversion operation of inverting the voltage applied between the anode 20 and the cathode 21 from the DC power supply unit 3 is performed under the control of the power control unit 4 at a predetermined timing, so that the electrolysis is continuously performed. Therefore, the precipitates of sulfur precipitated in the vicinity of the cathode or the like is dissolved. By maintaining the polarity inversion operation to some extent, the precipitate portion of sulfur are removed or decreased, so that the stable electrolysis can be continuously performed.

(Comparative Example)

This comparative example has the same configuration as that of the second embodiment except that the power control unit 4 of the second embodiment is not included. The comparative example will be described with reference to FIG. 3. In the electrolytic cell 2, a forward voltage is always applied to the anode side and the cathode side by the DC power supply unit 3, and the electrolysis of the sulfuric acid solution can be performed.

In this comparative example, the cleaning object such as a semiconductor substrate can be effectively cleaned by electrolyzing the sulfuric acid solution. However, as time elapses, the precipitates of sulfur is generated in the electrolytic cell, and the peeled precipitates of sulfur clog the narrow passage portions of the electrolytic cell 2, so that cleaning performance may be decreased or the cleaning may become difficult to be performed. If the peeled precipitates of sulfur reach the narrow passage such as the inlet hole of the electrolytic cell, there occurs the problem in that the passage is clogged or the flow becomes worse.

Hereinbefore, the invention is described based on the embodiments. However, the invention is not limited to the embodiments, but it may be appropriately changed and modified without departing from the spirit of the invention.

EXAMPLE Example 1

An example using the single-wafer-type cleaning system illustrated in FIG. 1 will be described. Operation was continuously performed under the conditions of concentration of sulfuric acid=92 mass %, temperature of the liquid at the inlet of the electrolytic cell=60° C., and current density=35 A/dm2. Every time when electrolysis was continuously performed for 50 hours, polarity inversion was performed. After the continuous electrolysis for 50 hours and the polarity inversion was repeated continuously 10 times for 50 hours, the cell was opened, and internal check was performed. Attachment of sulfur was not observed at all.

Example 2

An example using the batch-type cleaning system illustrated in FIG. 2 will be described. Operation was continuously performed under the conditions of concentration of sulfuric acid=85 mass %, temperature of the liquid at the inlet of the electrolytic cell=50° C., and current density=50 A/dm2. 50 wafers were processed in one batch, and polarity inversion was performed every 40 batches. After the polarity inversion was repeated eight times, the cell was opened, and internal check was performed. Attachment of sulfur was not observed at all.

(Comparative Example)

The batch-type cleaning system having no polarity inversion function illustrated in FIG. 3 was used. Operation was continuously performed under the conditions of concentration of sulfuric acid=85 mass %, temperature of the liquid at the inlet of the electrolytic cell=50° C., and current density=50 A/dm2. When 100 batches were processed, voltage started to be increased, and the flow rate was gradually decreased. When the flow rate per cell was decreased by half, the cell was opened and checked. Clogging of the passage in the cell outlet portion by sulfur was observed.

Description of the Reference Numeral

  • 1 electrolysis apparatus
  • 2 electrolytic cell
  • 20 anode
  • 21 cathode
  • 22 spacer
  • 22a inlet hole
  • 22b outlet hole
  • 23 passage
  • 3 DC power supply unit
  • 4 power control unit
  • 5 first circulation line
  • 7 circulation pump
  • 8 cooler
  • 10 electrolyte reservoir
  • 15 single-wafer-type cleaning apparatus
  • 30 batch-type cleaning tank

Claims

1. A sulfuric acid electrolysis method of generating persulfuric acid by performing electrolysis in an electrolytic cell while flowing sulfuric acid solution of 70 mass % or more between an anode and an cathode among a plurality of electrodes which include at least the anode and cathode, each of which at least liquid contact surface is constructed with a conductive diamond, the method comprising:

performing a normal operation of performing the electrolysis by applying a forward voltage between the anode and the cathode of the electrodes;
performing a polarity inversion operation of inverting the voltage applied between the anode and the cathode in an interval between the normal operations; and
dissolving precipitates of sulfur generated in the electrolytic cell during the normal operation into the sulfuric acid solution during the polarity inversion operation.

2. The sulfuric acid electrolysis method according to claim 1, wherein the electrolysis is performed while introducing the sulfuric acid solution from an outside of the electrolytic cell into an inside of the electrolytic cell, and the electrolyzed sulfuric acid solution is discharged to the outside of the electrolytic cell.

3. The sulfuric acid electrolysis method according to claim 1, wherein a retention portion where the flowing sulfuric acid solution is retained is included in the electrolytic cell.

4. The sulfuric acid electrolysis method according to claim 1, wherein the electrolytic cell includes a narrow passage portion through which the flow of the sulfuric acid solution is confined.

5. The sulfuric acid electrolysis method according to claim 3, wherein a spacer of securing a passage of the sulfuric acid solution is disposed between the electrodes, and the retention portion is formed with the spacer or with the spacer and other members.

6. The sulfuric acid electrolysis method according to claim 5, wherein a sealing member is installed between the spacer and the electrode, and the retention portion is formed with at least the sealing member.

7. The sulfuric acid electrolysis method according to claim 5, wherein the electrolytic cell has an outlet hole forming a narrow passage portion in the spacer through which the sulfuric acid solution passes.

8. The sulfuric acid electrolysis method according to claim 5, wherein the electrolytic cell has an inlet hole forming a narrow passage portion in the spacer through which the sulfuric acid solution passes.

9. The sulfuric acid electrolysis method according to claim 1, wherein a circulation line is installed to connect an outlet of the electrolytic cell and an inlet of the electrolytic cell, and a narrow passage portion is included in the circulation line and/or an upstream side of a retention portion of the electrolytic cell.

10. The sulfuric acid electrolysis method according to claim 1, wherein the polarity inversion operation is performed after continuously performing the normal operation for a predetermined time.

11. The sulfuric acid electrolysis method according to claim 1, wherein the polarity inversion operation is performed based on a result of determination of a precipitated state of the sulfur.

12. The sulfuric acid electrolysis method according to claim 1, wherein at least one of the following conditions (a) to (c) is satisfied:

(a) a concentration of sulfuric acid in the electrolytic cell is 85 mass % or more;
(b) a temperature of sulfuric acid at the inlet of the electrolytic cell is 70° C. or more; and
(c) a current density in the electrolysis is 50 A/dm2 or more.

13. A sulfuric acid electrolysis apparatus comprising:

an electrolytic cell which sulfuric acid solution can be fed to and discharged from;
a plurality of electrodes including at least an anode and a cathode in the electrolytic cell, which are disposed with a gap between the anode and cathode so that the sulfuric acid solution flows in the gap and each of which at least liquid contact surface is constructed with a conductive diamond;
a spacer which secures the gap of the electrodes;
a narrow passage portion formed in the spacer where the sulfuric acid solution flows;
a feeding unit which feeds the sulfuric acid solution to the electrolytic cell;
a power supply unit which applies a voltage between the anode and the cathode of the electrodes; and
a power control unit which controls the power supply unit to apply a forward voltage between the anode and the cathode during normal electrolysis and to perform polarity inversion of inverting the voltage applied between the anode and the cathode in a predefined condition.

14. A sulfuric acid electrolysis apparatus comprising:

an electrolytic cell which sulfuric acid solution can be fed to and discharged from;
a plurality of electrodes including at least an anode and a cathode in the electrolytic cell, which are disposed with a gap between the anode and cathode so that the sulfuric acid solution flows in the gap and each of which at least liquid contact surface is constructed with a conductive diamond;
a spacer which secures the gap of the electrodes;
a retention portion formed with the spacer or with the spacer and other members, where the sulfuric acid solution is retained;
a power supply unit which applies a voltage between the anode and the cathode of the electrodes; and
a power control unit which controls the power supply unit to perform polarity inversion of inverting a voltage applied between the anode and the cathode during normal electrolysis.

15. The sulfuric acid electrolysis method according to claim 2, wherein the electrolytic cell includes a narrow passage portion through which the flow of the sulfuric acid solution is confined.

16. The sulfuric acid electrolysis method according to claim 3, wherein the electrolytic cell includes a narrow passage portion through which the flow of the sulfuric acid solution is confined.

17. The sulfuric acid electrolysis method according to claim 6, wherein the electrolytic cell has an outlet hole forming a narrow passage portion in the spacer through which the sulfuric acid solution passes.

Patent History
Publication number: 20130319875
Type: Application
Filed: Feb 23, 2012
Publication Date: Dec 5, 2013
Applicant: KURITA WATER INDUSTRIES LTD. (Tokyo)
Inventor: MINORU UCHIDA (Tokyo)
Application Number: 14/000,239
Classifications
Current U.S. Class: Involving Measuring, Analyzing, Or Testing During Synthesis (205/335); Utilizing Specified Electrode (205/472); With Current, Voltage, Or Power Control Means (204/230.2)
International Classification: C25B 1/28 (20060101); C25B 15/02 (20060101); C25B 11/12 (20060101);