WO2009087931A1 - 電解方法 - Google Patents
電解方法 Download PDFInfo
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- WO2009087931A1 WO2009087931A1 PCT/JP2008/073760 JP2008073760W WO2009087931A1 WO 2009087931 A1 WO2009087931 A1 WO 2009087931A1 JP 2008073760 W JP2008073760 W JP 2008073760W WO 2009087931 A1 WO2009087931 A1 WO 2009087931A1
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- Prior art keywords
- sulfuric acid
- solution
- viscosity
- electrolysis
- electrolytic
- Prior art date
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- 238000000034 method Methods 0.000 title claims abstract description 21
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 claims abstract description 136
- 238000005868 electrolysis reaction Methods 0.000 claims abstract description 49
- 229910003460 diamond Inorganic materials 0.000 claims abstract description 20
- 239000010432 diamond Substances 0.000 claims abstract description 20
- 239000000243 solution Substances 0.000 claims description 66
- 239000007788 liquid Substances 0.000 claims description 37
- 150000002500 ions Chemical class 0.000 claims description 30
- 239000003792 electrolyte Substances 0.000 claims description 23
- 239000008151 electrolyte solution Substances 0.000 claims description 21
- 238000010494 dissociation reaction Methods 0.000 claims description 6
- 230000005593 dissociations Effects 0.000 claims description 6
- 150000004968 peroxymonosulfuric acids Chemical class 0.000 description 18
- 238000004140 cleaning Methods 0.000 description 16
- 230000007423 decrease Effects 0.000 description 6
- 238000002474 experimental method Methods 0.000 description 6
- -1 persulfate ions Chemical class 0.000 description 6
- MHAJPDPJQMAIIY-UHFFFAOYSA-N Hydrogen peroxide Chemical compound OO MHAJPDPJQMAIIY-UHFFFAOYSA-N 0.000 description 5
- 238000004519 manufacturing process Methods 0.000 description 5
- 230000004907 flux Effects 0.000 description 4
- 239000007789 gas Substances 0.000 description 4
- 229910052739 hydrogen Inorganic materials 0.000 description 4
- 239000001257 hydrogen Substances 0.000 description 4
- 239000004065 semiconductor Substances 0.000 description 4
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 3
- QAOWNCQODCNURD-UHFFFAOYSA-L Sulfate Chemical compound [O-]S([O-])(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-L 0.000 description 3
- 230000000052 comparative effect Effects 0.000 description 3
- 238000000354 decomposition reaction Methods 0.000 description 3
- 238000009792 diffusion process Methods 0.000 description 3
- 230000001590 oxidative effect Effects 0.000 description 3
- 238000012545 processing Methods 0.000 description 3
- 238000000926 separation method Methods 0.000 description 3
- 239000002699 waste material Substances 0.000 description 3
- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical compound [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 description 2
- 238000004364 calculation method Methods 0.000 description 2
- 238000006243 chemical reaction Methods 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 230000007613 environmental effect Effects 0.000 description 2
- 238000001704 evaporation Methods 0.000 description 2
- 230000008020 evaporation Effects 0.000 description 2
- 239000012528 membrane Substances 0.000 description 2
- 239000011259 mixed solution Substances 0.000 description 2
- 238000002156 mixing Methods 0.000 description 2
- 239000002245 particle Substances 0.000 description 2
- 230000009467 reduction Effects 0.000 description 2
- 238000003860 storage Methods 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- BZSXEZOLBIJVQK-UHFFFAOYSA-N 2-methylsulfonylbenzoic acid Chemical compound CS(=O)(=O)C1=CC=CC=C1C(O)=O BZSXEZOLBIJVQK-UHFFFAOYSA-N 0.000 description 1
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 239000007864 aqueous solution Substances 0.000 description 1
- 239000003125 aqueous solvent Substances 0.000 description 1
- 125000004432 carbon atom Chemical group C* 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 238000001816 cooling Methods 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 239000003814 drug Substances 0.000 description 1
- 229940079593 drug Drugs 0.000 description 1
- 230000005684 electric field Effects 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 150000002431 hydrogen Chemical class 0.000 description 1
- QOSATHPSBFQAML-UHFFFAOYSA-N hydrogen peroxide;hydrate Chemical compound O.OO QOSATHPSBFQAML-UHFFFAOYSA-N 0.000 description 1
- 239000003014 ion exchange membrane Substances 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- JRKICGRDRMAZLK-UHFFFAOYSA-L peroxydisulfate Chemical compound [O-]S(=O)(=O)OOS([O-])(=O)=O JRKICGRDRMAZLK-UHFFFAOYSA-L 0.000 description 1
- JRKICGRDRMAZLK-UHFFFAOYSA-N peroxydisulfuric acid Chemical compound OS(=O)(=O)OOS(O)(=O)=O JRKICGRDRMAZLK-UHFFFAOYSA-N 0.000 description 1
- 235000020030 perry Nutrition 0.000 description 1
- 238000000746 purification Methods 0.000 description 1
- 229910052710 silicon Inorganic materials 0.000 description 1
- 239000010703 silicon Substances 0.000 description 1
- 239000011780 sodium chloride Substances 0.000 description 1
- 239000000758 substrate Substances 0.000 description 1
- 238000012360 testing method Methods 0.000 description 1
- 238000012795 verification Methods 0.000 description 1
- 238000005406 washing Methods 0.000 description 1
Images
Classifications
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B1/00—Electrolytic production of inorganic compounds or non-metals
- C25B1/01—Products
- C25B1/28—Per-compounds
- C25B1/30—Peroxides
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B1/00—Electrolytic production of inorganic compounds or non-metals
- C25B1/01—Products
- C25B1/28—Per-compounds
- C25B1/29—Persulfates
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
- C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
- C25B11/042—Electrodes formed of a single material
- C25B11/043—Carbon, e.g. diamond or graphene
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B15/00—Operating or servicing cells
- C25B15/08—Supplying or removing reactants or electrolytes; Regeneration of electrolytes
-
- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
- G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
- G03F7/26—Processing photosensitive materials; Apparatus therefor
- G03F7/42—Stripping or agents therefor
- G03F7/422—Stripping or agents therefor using liquids only
- G03F7/423—Stripping or agents therefor using liquids only containing mineral acids or salts thereof, containing mineral oxidizing substances, e.g. peroxy compounds
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
- H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
- H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
- H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
- H01L21/302—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to change their surface-physical characteristics or shape, e.g. etching, polishing, cutting
- H01L21/304—Mechanical treatment, e.g. grinding, polishing, cutting
Definitions
- the present invention relates to an electrolysis method suitable for electrolysis using various aqueous solutions, solvents and the like as an electrolyte, particularly when electrolysis is performed at a high current density or when the viscosity of the electrolyte is high.
- an electrolysis method when resist removal from a wafer surface in a semiconductor manufacturing process is performed with a persulfuric acid solution produced by electrolyzing a sulfuric acid solution (hereinafter sometimes referred to as “persulfuric acid method”).
- an SPM method (Surfuric acid and hydrogen Peroxide Mixture: a cleaning method using a mixed solution of concentrated sulfuric acid and hydrogen peroxide solution) has been widely used.
- SPM method hydrogen peroxide water is mixed with the cleaning solution (concentrated sulfuric acid solution) every time it is used, so the sulfuric acid concentration decreases with the mixing of hydrogen peroxide, and when the number of mixings exceeds a certain level, the mixed solution The oxidizing power of is reduced. At that time, the cleaning solution is discarded, and there is a problem of increase in chemical cost and environmental load.
- the persulfuric acid method which compensates the fault of SPM method is proposed (refer patent document 1).
- an aqueous sulfuric acid solution is electrolyzed to generate persulfuric acid (peroxodisulfuric acid), and the strong oxidizing power of persulfuric acid is used. Since persulfuric acid is converted to CO 2 and H 2 O by oxidizing and decomposing the resist, the persulfuric acid returns to sulfuric acid. Therefore, this solution can be recycled by returning it to the electrolysis cell. For this reason, higher merit than the SPM method is expected from the viewpoint of drug cost and environmental load reduction.
- the electrode used for the electrolysis of sulfuric acid is called a “diamond electrode”.
- a diamond crystal is deposited on a silicon substrate to form a diamond layer on the order of several ⁇ m to several tens of ⁇ m, and this is made conductive. Therefore, a boron-implanted (doped) material is used. This is used as the anode and cathode of the electrolysis cell.
- a diamond electrode it is considered that a stable electrolytic treatment can be performed without deterioration of the electrode even at a high current density.
- the persulfuric acid method using a diamond electrode has revealed a phenomenon that the diamond layer on the surface of the diamond electrode is gradually worn with continuous operation. For example, when there is a 50 ⁇ m diamond layer on the anode surface, it may become half or a fraction of a thickness after 100 hours of operation. This phenomenon occurs only on the anode surface and does not occur on the cathode surface. Electrode wear greatly affects the electrode life, but the diamond electrode is expensive, and therefore it is a major problem for the practical use of the persulfuric acid method that it is worn out in a short time.
- the present invention has been made against the background of the above circumstances, and an object thereof is to provide an electrolysis method capable of performing electrolysis with high efficiency while reducing electrode wear in electrolysis.
- diamond electrode wear is caused by handling high-concentration sulfuric acid with high viscosity and low ion dissociation, and having a very high current density. I thought. Specifically, it is as follows. In order to perform electrolysis at a predetermined current density, a corresponding number of ions must move toward the anode or cathode within a unit time. When the sulfuric acid solution is electrolyzed, SO 4 2 ⁇ and HSO 4 ⁇ move toward the anode and react on the anode surface to produce S 2 O 8 2 ⁇ .
- the ion concentration depends on the electrolyte concentration and the degree of ion dissociation, and the degree of ion dissociation depends on the electrolyte concentration and temperature, it can be said that the ion concentration is a function of the electrolyte concentration and temperature. Therefore, wear and tear of the electrode can be avoided if the concentration and temperature of the sulfuric acid solution are properly managed.
- the viscosity becomes small, so that wear can be avoided and the current efficiency (the amount of persulfuric acid produced per unit current) is increased.
- the sulfuric acid concentration is low, the vapor pressure of moisture increases, so that the amount of moisture evaporation increases in the resist stripping treatment tank, and there is a risk that the operation may be hindered. Therefore, it is important to select an appropriate combination of concentration and temperature.
- the temperature of the sulfuric acid solution decreases as the temperature rises, but if it is too high, there is a concern that the electrolytic efficiency will decrease and the self-decomposition of persulfuric acid will be accelerated, so the solution temperature must also be set appropriately.
- the ion concentration is not necessarily higher as the electrolyte concentration is higher, and a maximum value exists at an intermediate concentration. Therefore, it is necessary to set the density appropriately.
- the problem of electrode wear is a problem in general electrolytic treatment, but in normal electrolytic treatment, the electrode life is shortened as the treatment is performed at a high temperature, so it is often preferable to perform the treatment at a low temperature.
- the present inventors have discovered a problem of electrode wear in the persulfuric acid method, and found a new problem that when the temperature of the sulfuric acid solution is too low, the viscosity of the sulfuric acid solution increases and electrode wear proceeds. The invention has been completed.
- the first present invention is such that an electrolytic solution is passed through an electrolytic cell having an anode and a cathode as at least one pair of electrodes, and the electrolyte is electrolyzed by energizing the electrodes.
- the electrolysis is performed by setting the viscosity of the electrolytic solution in a range corresponding to the current density at the time of energization.
- the electrolysis method of the second aspect of the present invention is characterized in that, in the first aspect of the present invention, the electrolytic solution is a sulfuric acid solution.
- the electrolysis method of the third aspect of the present invention is characterized in that, in the second aspect of the present invention, when the current density is 50 A / dm 2 or less, the viscosity of the sulfuric acid solution is 10 cP or less.
- the electrolysis method of the fourth aspect of the present invention is characterized in that, in the second aspect of the present invention, when the current density is more than 50 to 75 A / dm 2 , the viscosity of the sulfuric acid solution is 8 cP or less.
- the electrolysis method of the fifth aspect of the present invention is characterized in that, in the second aspect of the present invention, when the current density is more than 75 to 100 A / dm 2 , the viscosity of the sulfuric acid solution is 6 cP or less.
- the electrolysis method of the sixth aspect of the present invention is characterized in that, in any of the first to fifth aspects of the present invention, the viscosity of the electrolytic solution is controlled by adjusting the electrolyte concentration and the liquid temperature of the electrolytic solution. .
- the electrolysis method according to a seventh aspect of the present invention is the electrolysis method according to any one of the first to sixth aspects of the present invention, wherein the concentration of ions generated by dissociation of the electrolyte of the electrolyte and the viscosity of the electrolyte are expressed by the following formula (1): coefficient P f is calculated so that a 1.2mol / (L ⁇ cP) or more, and adjusting the electrolyte concentration and temperature on the basis of.
- the eighth electrolysis method of the present invention is characterized in that, in any of the first to seventh inventions, the anode and the cathode are diamond electrodes.
- the electrolysis method of the present invention in the electrolysis method of electrolyzing an electrolyte solution by passing an electrolyte solution through an electrolytic cell including an anode and a cathode as at least one pair of electrodes, the electrolyte solution Since the electrolysis is performed in a range corresponding to the current density at the time of energization, electrolysis can be performed with high efficiency while reducing electrode wear during electrolysis. In particular, even when electrolyzing a high-concentration sulfuric acid solution with a diamond electrode at a high current density, it is possible to perform electrolytic treatment with high efficiency while reducing electrode wear.
- FIG. 3 shows a flow of a cleaning system for stripping and cleaning a resist of a semiconductor wafer with a persulfuric acid solution generated by electrolyzing a sulfuric acid solution using a diamond electrode.
- the cleaning system is configured by a combination of a resist stripping apparatus 10 and an electrolytic sulfuric acid unit 20. This will be described in detail below.
- the resist stripping apparatus 10 includes a processing tank 11 that contains a persulfuric acid solution as a cleaning liquid, inserts a semiconductor wafer, and strips the resist.
- a circulation line 12 having a pump 13 is provided in the processing tank 11. It is connected.
- a heater 14 and a filter 15 are sequentially provided on the downstream side of the pump 13.
- the circulation line is branched between the pump 13 and the heater 14, and a liquid supply line 22 a for supplying cleaning waste liquid to the electrolytic sulfuric acid unit 20 is connected.
- a return liquid line 22b for sending a persulfuric acid solution from the electrolytic sulfuric acid unit 20 is connected to the circulation line 12 on the downstream side of the filter 15 and is joined.
- the electrolytic sulfuric acid unit 20 includes an electrolytic cell 21 that electrolyzes a sulfuric acid solution to generate persulfate ions.
- the electrolytic cell 21 includes an anode 21a, a cathode 21b, an anode 21a, and a cathode that are constituted by diamond electrodes. And a bipolar electrode 21c disposed between 21b.
- the bipolar electrode 21c is polarized by energization, and an anode and a cathode appear according to the electrodes facing each other, and functions as the anode and cathode of the present invention.
- the electrolytic cell 21 has the liquid feeding line 22a connected to the liquid inlet side and the return liquid line 22b connected to the liquid outlet side.
- the liquid feed line 22a is provided with a cooler 23 for cooling the solution flowing through the line, and the return liquid line 22b is sequentially provided with a gas-liquid separator 24, a storage tank 26, and a pump 27.
- a hydrogen treatment device 25 is connected to the gas-liquid separator 24 on the separation gas side.
- the treatment tank 11 of the resist stripping apparatus 10 contains a solution having a predetermined sulfuric acid concentration (predetermined electrolyte concentration) and is operated at 120 ° C. to 150 ° C.
- the solution contains persulfate ions during cleaning, but persulfate ions are generated by electrolysis of sulfate ions, and the persulfate ions return to sulfate ions by self-decomposition, so the total amount of ions before and after electrolysis is approximately equal.
- the electrolyte concentration of the solution can be indicated by the sulfuric acid concentration.
- the temperature of the solution is maintained by heating the solution with a heater 14 interposed in the circulation line 12 when the solution is circulated through the circulation line 12.
- a part of the solution in the circulation line 12 is withdrawn through the liquid feed line 22 a and sent to the electrolytic cell 21 through the cooler 23.
- the solution is cooled to the desired temperature of 40 ° C. to 70 ° C. at the outlet of the cooler 23 and reaches the electrolytic cell 21.
- the branch liquid is electrolyzed by energizing the anodes 21a and 21b with a predetermined current density.
- the electrolysis reaction is an exothermic reaction, and usually the outlet temperature of the electrolytic cell 21 rises by about 10 ° C. to 20 ° C. from the inlet temperature. For this reason, the viscosity of the solution in the electrolytic cell 21 can be kept at a desired value or less if the solution is kept at a predetermined viscosity or less at the outlet of the cooler 23 according to the current density. .
- Electrolysis generates persulfate ions from the sulfuric acid solution and drains it from the electrolytic cell 21 through the return line 22b.
- an electrolytic gas is generated by an electrolytic reaction and is sent together with the solution to the return line 22b. Therefore, gas-liquid separation is performed by the gas-liquid separator 24, and the separated gas, particularly hydrogen, is processed by the hydrogen treatment device 25.
- the solution from which the gas has been separated by the gas-liquid separator 24 is stored in a storage tank 26 and returned to the circulation line 12 by a pump 27 as necessary.
- the persulfate ions consumed in the treatment tank 11 can be replenished and the concentration of persulfate in the cleaning liquid can be kept substantially constant.
- the present invention is intended to prevent the sulfuric acid concentration from exceeding a predetermined concentration in order to prevent electrode wear.
- the sulfuric acid concentration is low, the viscosity of the sulfuric acid solution is reduced, so that electrode wear can be avoided.
- the current efficiency is increased, and the production efficiency of persulfuric acid is increased.
- the sulfuric acid concentration is too low, the vapor pressure of the water becomes too high, and the amount of water evaporation increases in the resist stripping treatment tank, which may cause problems such as a problem in operation. Further, a high sulfuric acid concentration is required for wafer cleaning. Therefore, there is a lower limit for the preferred sulfuric acid concentration for the cleaning liquid.
- the present invention is to prevent the temperature of the sulfuric acid solution from becoming lower than a predetermined temperature in order to prevent electrode wear.
- the temperature of the sulfuric acid solution becomes high, the viscosity decreases. There is concern about a reduction in electrolytic efficiency and promotion of self-decomposition of persulfuric acid. Therefore, there is an upper limit for the preferable solution temperature as the electrolytic solution or the cleaning solution.
- the description has been made by focusing on electrolysis using a sulfuric acid solution with a diamond electrode.
- the influence of the viscosity, ion concentration, and current density of the electrolyte Therefore, the present invention is effective because there is a risk of electrode wear.
- the liquid temperature of the electrolytic solution is kept within a predetermined range based on the liquid temperature of the sulfuric acid solution to be electrolyzed, that is, the liquid temperature at the electrolytic cell inlet of the electrolytic solution flowing through the electrolytic cell.
- the liquid temperature of the electrolytic solution can also be maintained based on the liquid temperature of the electrolytic drainage discharged from the cell.
- the temperature of the sulfuric acid solution is 40 ° C.
- the viscosity of the sulfuric acid solution does not become 10 cP or less unless the concentration is maintained at 78 wt% or less according to FIG. Therefore, when the current density is 50 A / dm 2 or less, for example, the temperature of the sulfuric acid solution is maintained at 50 ° C. or higher, or the temperature of the sulfuric acid solution is maintained at 40 to 50 ° C. and the sulfuric acid concentration is maintained at 78 wt% or lower. If so, the risk of electrode wear can be avoided.
- the range of the viscosity of the sulfuric acid solution that can be electrolyzed without damaging the electrode was experimentally examined and found to be 6 cP or less.
- the temperature of the sulfuric acid solution to be electrolyzed is 50 ° C., according to FIG. 1, if the sulfuric acid concentration is 75 wt% or less, the viscosity does not exceed 6 cP, so that the risk of electrode wear is small.
- the sulfuric acid concentration exceeds about 80 wt%, the viscosity hardly changes. Therefore, even if the viscosity satisfies the above conditions, wear can be avoided even if the concentration is increased.
- wear exceeds 85 wt%. This is because, as shown in FIG. 2, when the concentration exceeds 80 wt%, the concentration of SO 4 2 ⁇ or HSO 4 ⁇ rapidly decreases, so that sufficient charges are not carried. Therefore, it is desirable to more appropriately use a parameter that takes into consideration the ion concentration in addition to the viscosity.
- the inventors defined the coefficient P f as this parameter, and based on the results of the electrode durability test, the wear avoidance condition in the case of a current density of 50 A / dm 2 was expressed by the following formula (2).
- the present invention is not limited to the electrolytic treatment of sulfuric acid.
- the electrolytic treatment of alkaline electrolyte for hydrogen production the electrolytic treatment of NaCl electrolytic solution for sodium chlorate production
- the electrolytic treatment of waste liquid for waste liquid purification The same can be applied to electrolytic treatment for other uses.
- Equation (5) if the ion radius (particle radius, r) is considered constant, D is a function of concentration and temperature.
- the situation of wear was not improved even when the flow rate (flow rate) of the liquid passing through the electrolysis cell was increased. Only the second term of (4), the term due to electrical attraction, is considered dominant. Therefore, an index of how much ion flux can be obtained at the same electric field intensity can be taken as the following equation (6).
- a new parameter P f can be defined as in the following equation (1).
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Abstract
Description
例えば、硫酸溶液を電解することによって製造した過硫酸溶液によって半導体製造工程におけるウェハ表面からのレジスト剥離を行う(以下「過硫酸法」と称す場合がある)ときの電解方法が挙げられる。
所定の電流密度で電気分解を行うには、それに応じた数のイオンが単位時間内に陽極あるいは陰極に向かって移動しなければならない。硫酸溶液を電気分解すると、SO4 2-やHSO4 -が陽極に向かって移動し、陽極表面で反応してS2O8 2-が生成する。しかし、電流密度が高い場合には十分な数のSO4 2-やHSO4 -の供給が追いつかず、ダイヤモンド電極表面の炭素原子が引き抜かれて酸化し、CO3 2-が生成してしまう。これが電極の損耗に繋がると考えられる。イオンの移動速度は液の粘度に大きく依存する。粘度が低いと、イオンは液中を容易に移動することができる。粘度は濃度と温度の関数である。また、粘度が同一の場合、イオン濃度が高い方がより多くの電荷を運ぶことができる。イオン濃度は電解質濃度とイオン解離度に依存するものであり、イオン解離度は電解質濃度と温度に依存するので、即ち、イオン濃度は電解質濃度と温度との関数であると言える。従って、硫酸溶液の濃度と温度を適切に管理すれば、電極の損耗を避けることができる。
11 処理槽
12 循環ライン
14 加熱ヒータ
20 電解硫酸ユニット
21 電解セル
21a 陽極
21b 陰極
21c バイポーラ電極
22a 送液ライン
22b 戻り液ライン
該洗浄システムは、レジスト剥離装置10と、電解硫酸ユニット20との組み合わせによって構成されている。以下に、詳細に説明する。
前記循環ラインはポンプ13と加熱ヒータ14との間において分岐し、洗浄排液を電解硫酸ユニット20に送液する送液ライン22aが接続されている。電解硫酸ユニット20から過硫酸溶液を送出する戻り液ライン22bが前記循環ライン12とフィルタ15の下流側において接続され、合流するように構成されている。
レジスト剥離装置10の処理槽11には、所定の硫酸濃度(所定の電解質濃度)を有する溶液が収容され120℃~150℃で運転される。なお、溶液は、洗浄に際しては過硫酸イオンを含むものであるが、硫酸イオンの電解によって過硫酸イオンが生成され、過硫酸イオンは自己分解で硫酸イオンに戻るので、電解前後の全イオン量は概略等しいと見なして、硫酸濃度によって溶液の電解質濃度を示すことができる。
また本発明は、電極損耗を防止するために硫酸溶液の液温が所定温度以下にならないようにするというものであるが、一方、硫酸溶液の温度が高温になると粘性が下がるが、高すぎると電解効率の低下や過硫酸の自己分解の促進が懸念される。よって電解液または洗浄液として、好ましい溶液温度には上限値が存在する。
そこで、この範囲の粘度を得るために必要な硫酸濃度と溶液温度の関係を図1に基づいて検討する。電解する硫酸溶液の液温が50℃であれば、図1によると硫酸濃度が80~95wt%になっても粘度が10cPを超えないので、電極損耗の恐れは小さい。
しかし硫酸溶液の液温が40℃のときは、図1によると濃度78wt%以下に保持しなければ硫酸溶液の粘度が10cP以下にならないので電極損耗の恐れがある。
従って電流密度が50A/dm2以下の場合は、例えば硫酸溶液の液温を50℃以上に保持するか、硫酸溶液の液温を40~50℃に保持すると共に硫酸濃度を78wt%以下に保持すれば、電極損耗のリスクを回避することができる。
電解する硫酸溶液の液温が50℃のときは、図1によると硫酸濃度が75wt%以下であれば粘度が6cPを超えないので、電極損耗の恐れは小さい。
図1において、硫酸濃度が約80wt%を超えると粘度はほとんど変化しない。よって、粘度さえ前記条件を満たせば濃度を濃くしても損耗を避けられることになる。しかし、実際には85wt%を超えると損耗が見られる。これは図2に示すように、80wt%を超えるとSO4 2-やHSO4 -の濃度が急激に低下するため、十分な電荷が運ばれないためである。
そこで、より適切には粘度に加えてイオン濃度も考慮したパラメーターを指標とすることが望ましい。発明者等は、このパラメーターとして上記係数Pfを定義し、電極耐久性試験の結果から、電流密度50A/dm2の場合の損耗回避条件を以下の式(2)で示した。
電流密度を75A/dm2、100A/dm2というように増やした場合は、それに比例してイオンフラックスが取れれば損耗を生じないことになる。50A/dm2の場合の必要条件(限界粘度およびイオン濃度)が分かっているので、その他の電流密度については計算で求めることが可能である。
電解溶液中のイオンの移動速度については、一般に、以下に示すNernst-Planckの式(3)が成り立つ。(文献ii)右辺第1項は濃度差によるイオンの拡散、第2項は電気的引力によるイオンの移動、そして第3項は対流によるイオンの移動を表す。
1.複数種のイオンを代表するイオン半径は濃度・温度が変わっても不変である。
2.Cb>>Cs
文献ii)“Ion-Exchange Membrane Separation Processes”,Membrane Science and Technology Series,9,H.Strathmann,Elsevier,pp.70(2004)
文献iii)“Perry’s Chemical Engineer’s Handbook”,7th Edition,pp.5-50 McGraw-Hill(1997)
[比較例1]
電解処理温度が低温である方が電流効率は良いので、電解セル入口温度30℃で運転した。その他の条件は以下の通りとした。
(電極形状寸法=150mmφ、電流密度=50A/dm2、溶液濃度=86wt%、流量=0.86L/min)
この結果、運転開始後50時間で陽極表面に損耗が見られ、顕微鏡で観察したところ、結晶粒子の変形とダイヤモンド層厚さの減少が見られた。
比較例1に対して、温度と濃度を変えて実験を行った。条件は以下の通りである。
(電極形状寸法=150mmφ、電流密度=50A/dm2、流量=0.86L/min)
この結果、運転開始50時間後の損耗の有無を比較例1のデータを含めて比較したところ、表2のようになった。表2に示す実験結果より、電流密度=50A/dm2の場合には溶液粘度が概ね10cP以下になるように濃度および温度を選定すれば電極の損耗が避けられると判断される。
実施例1において電流密度を75A/dm2、100A/dm2とした場合、上記のNernst-Planckの式とStokes-Einsteinの式を用いて、イオンフラックスが75/50=1.5倍、あるいは100/50=2.0倍になる条件を計算すると表4に示す結果になる。また、より好ましくは、パラメーターPfを用いて、電流密度75、100A/dm2の場合には、それぞれPf≧1.8、Pf≧2.4(mol/(L・cP))を損耗回避条件とすることができる。
即ち、電流密度が高い場合には溶液粘度をより小さくするか、またはイオン濃度をより大きくしなければならないことが明らかである。
Claims (8)
- 陽極と陰極とを少なくとも1対の電極として備える電解セルに電解液を通液し、該電極に通電することによって電解液を電解する電解方法において、前記電解液の粘度を、前記通電の際の電流密度に応じた範囲にして、前記電解を行うことを特徴とする電解方法。
- 前記電解液は硫酸溶液であることを特徴とする請求項1記載の電解方法。
- 前記電流密度を50A/dm2以下とする場合、前記硫酸溶液の粘度を10cP以下とすることを特徴とする請求項2記載の電解方法。
- 前記電流密度を50超~75A/dm2とする場合、前記硫酸溶液の粘度を8cP以下とすることを特徴とする請求項2記載の電解方法。
- 前記電流密度を75超~100A/dm2とする場合、前記硫酸溶液の粘度を6cP以下とすることを特徴とする請求項2記載の電解方法。
- 前記電解液の粘度を、該電解液の電解質濃度及び液温の調整によって制御することを特徴とする請求項1~5のいずれかに記載の電解方法。
- 前記陽極及び陰極がダイヤモンド電極であることを特徴とする請求項1~7のいずれかに記載の電解方法。
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