JP2006278689A - Sulfuric-acid recycling cleaning system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable to efficiently and thoroughly clean the resist of a semiconductor wafer. <P>SOLUTION: The cleaning system has with a cleaning device for cleaning a work 1 to be cleaned using a persulfuric acid solution as a cleaning liquid; electrolytic reaction devices 17, 27 and 37 for generating persulfuric ions from sulfate ions by an electrolytic reaction to regenerate a persulfuric acid solution; circulation lines 13, 23, 33, 15, 25 and 35 for circulating the solution between the cleaning device and the electrolytic reaction devices; and liquid transferring pumps 14, 24 and 34. The cleaning device has a plurality stages of cleaning tanks 10, 20 and 30 for successively cleaning the work to be cleaned, and is set so that the concentration of sulfuric acid of the cleaning liquid in the cleaning tank is low as the lower stage. Preferably, the concentration of sulfuric acid of the first stage is 17 M or more, and the concentration of sulfuric acid of the second stage and lower is in a range of 8 M-15 M. Contamination such as resist is efficiently eliminated with concentrated sulfuric acid having high cleaning effect in the cleaning tank in a front stage side and for detailed portions, and contamination is efficiently eliminated with sulfuric acid having low viscosity in a rear stage. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a sulfuric acid recycle type cleaning system that regenerates persulfuric acid and uses it for cleaning while repeatedly using a sulfuric acid solution when cleaning and peeling a contaminant attached to a silicon wafer or the like with a persulfuric acid solution having a high peeling effect. Is.

Wafer cleaning technology in the VLSI manufacturing process is a process for stripping and cleaning resist residues, fine particles, metals and natural oxide films, and ozone gas is blown into concentrated sulfuric acid and hydrogen peroxide mixed solution (SPM) or concentrated sulfuric acid. A solution (SOM) is frequently used. When hydrogen peroxide or ozone is added to high-concentration sulfuric acid, the sulfuric acid is oxidized to produce persulfuric acid. It is known that persulfuric acid has a high cleaning ability because it generates a strong oxidizing power when self-decomposing, and is useful for cleaning the wafer and the like.
As a method for producing persulfuric acid, in addition to the above method, there is also known a method in which an aqueous solution containing sulfate ions is electrolyzed in an electrolytic bath to obtain persulfuric acid-dissolved water and used for washing (Patent Documents 1 and 2) reference).

JP 2001-192874 A Special table 2003-511555 gazette

  By the way, in SPM, it is necessary to repeat the replenishment of the hydrogen peroxide solution in order to compensate for the decomposition when the persulfuric acid generated by the hydrogen peroxide solution is self-decomposed and the oxidizing power is reduced. When the sulfuric acid concentration falls below a certain concentration, it is exchanged with a new high concentration sulfuric acid. However, in the above method, since the persulfuric acid solution is diluted with water in hydrogen peroxide water, it is difficult to maintain a constant liquid composition. Further, the liquid is discarded and renewed every predetermined time or every processing batch. It is necessary to. For this reason, there are problems that the cleaning effect is not constant and a large amount of chemicals must be stored. On the other hand, in the SOM, the liquid is not diluted, and although the liquid renewal cycle can be generally longer than that of the SPM, the cleaning effect is inferior to that of the SPM. In these methods, there is a limit to the concentration of persulfuric acid produced, which leads to the limit of the cleaning effect.

  In addition, in SPM, the amount of persulfuric acid that can be generated by adding high-concentration sulfuric acid that has filled the washing tank once and hydrogen peroxide water several times is small and has a limit. Further, in the SOM method, the generation efficiency of persulfuric acid with respect to the ozone blowing amount is very low. Therefore, in these methods, there is a problem that the concentration of persulfuric acid produced is limited and the cleaning effect is also limited. For this reason, in the cleaning using the above-described SPM method or SOM method, as a pretreatment step of the SPM method or SOM method, dry etching or an ashing process is used to oxidize and ash the organic resist in advance. A process is incorporated. There is a problem that pre-processing such as dry etching or ashing process damages the wafer or increases the cost of the apparatus.

  The present invention has been made against the background of the above circumstances, and by reusing persulfate ions from an aqueous solution of sulfuric acid by electrochemical action while repeatedly using sulfuric acid, the persulfate ions are recycled to greatly increase the amount of sulfuric acid used. It is an object of the present invention to provide a sulfuric acid recycle type cleaning system capable of continuously obtaining a good cleaning action by reducing it to a low level.

  That is, among the sulfuric acid recycling type cleaning systems of the present invention, the first invention generates a persulfate ion from a sulfate ion contained in the solution by a cleaning apparatus for cleaning a material to be cleaned using a persulfuric acid solution as a cleaning liquid. An electrolytic reaction device for regenerating the persulfuric acid solution, and a circulation line for circulating the solution between the cleaning device and the electrolytic reaction device, and the cleaning device sequentially cleans the material to be cleaned. In addition to having a tank, a part or all of the cleaning tank is set to have different sulfuric acid concentrations in the cleaning liquid.

The sulfuric acid recycling type cleaning system according to the second aspect of the invention is such that, in the first aspect of the invention, the sulfuric acid concentration of the cleaning liquid in the k-th (k = 1, 2,... It is characterized by being set to.
C1 ≧ ... ≧ Ck ≧ ... ≧ Cn (1)
Where n is 2 or more

  The sulfuric acid recycle type cleaning system according to a third aspect of the present invention is the first or second aspect of the invention, wherein the first-stage cleaning tank has a sulfuric acid concentration of 17 M or more in the cleaning liquid and the second and subsequent stages. The washing tank is characterized in that the sulfuric acid concentration is in the range of 8M to 15M.

  A sulfuric acid recycle type cleaning system according to a fourth aspect of the present invention is the electrolytic reaction tank according to any one of the first to third aspects, wherein the electrolytic reaction device regenerates the persulfuric acid solution by circulating the solution between each of the cleaning tanks. Characterized by comprising

The sulfuric acid recycle type cleaning system of the fifth invention is the sulfuric acid recycling type cleaning system according to the fourth invention, wherein the total anode area of the electrolytic reaction tank corresponding to the k-th stage (k = 1, 2,... N) cleaning tank is Sk. The sulfuric acid recycle type cleaning system according to claim 4, wherein:
S1 ≧ ... ≧ Sk ≧ ... ≧ Sn (2)
Where n is 2 or more

  The sulfuric acid recycle type cleaning system according to a sixth aspect of the present invention is the first to fifth aspects of the invention, wherein a conductive diamond electrode is used as an anode of the electrolytic reaction device. Sulfuric acid recycling type cleaning system.

  According to the present invention, a persulfate cleaning solution containing sulfate ions and persulfate ions is sent from the electrolytic reaction device to the cleaning device, and in the cleaning device, the persulfate ions in the cleaning solution self-decompose and emit oxidizing power. In addition to the cleaning action by sulfuric acid, the contaminants of the cleaning material are effectively peeled and cleaned by the oxidizing power of persulfuric acid. For example, when cleaning a wafer, the resist adhering to the wafer is peeled off by the cleaning action and dissolved in the cleaning liquid. At this time, the dissolution rate of the resist in the persulfuric acid solution increases as the sulfuric acid concentration increases. The sulfuric acid concentration in the persulfuric acid solution as the cleaning liquid is preferably 8M or higher, more preferably 12M or higher. In the present invention, the cleaning apparatus has a plurality of stages of cleaning tanks for sequentially cleaning the material to be cleaned, and these cleaning tanks are set so that a part or all of them have different sulfuric acid concentrations in the cleaning liquid. Accordingly, it is possible to perform cleaning effectively by selecting these cleaning tanks and sequentially cleaning the materials to be cleaned with cleaning liquids having different sulfuric acid concentrations, that is, cleaning performance.

  In each cleaning tank, if the sulfuric acid concentration of the cleaning liquid in the cleaning tank in the k-th stage (k = 1, 2,... N) is set to Ck, the condition of C1 ≧ ... ≧ Ck ≧ ... ≧ Cn is set. The sulfuric acid concentration tends to be lower in the washing tanks of the washing order. As a result, the material to be cleaned can be initially cleaned with a cleaning solution having a high sulfuric acid concentration and sequentially with a cleaning solution having a lower sulfuric acid concentration. In a state where the cleaning order is early, more contaminants remain in the material to be cleaned, and a higher sulfuric acid concentration is better. For this reason, it is desirable to set the sulfuric acid concentration to a high concentration of 17M or higher in the cleaning tank in the first cleaning order. In this washing tank, almost all the contaminants are peeled off and dissolved in the persulfuric acid solution. The cleaning material that has been cleaned in the first-stage cleaning tank is preferably sent to the second-stage cleaning tank having a sulfuric acid concentration of 8M to 15M and cannot be removed in the first-stage cleaning tank. Remove and remove. Further, when it is desired to remove contaminants in details such as the pattern shape formed on the wafer, a 3rd and subsequent stage cleaning tank is installed, and a sulfuric acid solution having a low sulfuric acid concentration and a low viscosity of 8M to 15M. The contaminant can be peeled and removed by allowing the solution to enter the finer region. The sulfuric acid concentration is lowered in the latter washing tank.

  Also, the dissolution rate of the resist in the persulfuric acid solution increases as the temperature of the persulfuric acid solution increases. However, if it exceeds 160 ° C., the self-decomposition rate of persulfuric acid becomes extremely high. Therefore, it is desirable that the temperature of the persulfuric acid solution as the cleaning liquid is 120 to 150 ° C. Examples of means for heating the cleaning liquid to the above temperature include a heater, a heater using heat exchange with hot water, steam, and the like, but the present invention is not limited to a specific one. The resist dissolved in the cleaning solution is further effectively decomposed by the oxidizing action by persulfate ions.

  A cleaning solution that is used in the cleaning device and has a reduced persulfuric acid concentration by autolysis is sent to the electrolytic reaction device. In the cleaning liquid, the persulfate concentration gradually decreases due to self-decomposition of persulfate ions in the solution in the cleaning device. In an electrolytic reactor, an anode and a cathode are immersed in a solution containing sulfate ions in an electrolytic reaction tank, and an electric current is passed between the electrodes to electrolyze the sulfate ions to regenerate persulfate ions, thereby increasing the concentration of persulfate. It is done. The regenerated persulfuric acid solution is returned again to the cleaning device through the circulation line and is used for cleaning the material to be cleaned in the same manner as described above. In this way, effective cleaning can be continued while maintaining the persulfate composition by circulating the cleaning solution while regenerating the persulfate ions.

Note that, unlike the cleaning device, the electrolytic reaction device has a better persulfuric acid generation efficiency and lower electrode wear as the solution temperature is lower. In the present invention, since the cleaning device and the electrolytic reaction device are separated, the temperature of the solution electrolyzed in the electrolytic reaction device can be kept lower than the temperature of the cleaning liquid in the cleaning tank. In addition, the efficiency in the electrolytic reaction apparatus can be increased. For this reason, the solution electrolyzed by the electrolytic reaction apparatus can be cooled to an appropriate temperature by an appropriate cooling means. Examples of the cooling means include air coolers and water coolers. The appropriate temperature as the solution to be electrolyzed can be in the range of 10 to 90 ° C. When the temperature range is exceeded, the electrolysis efficiency decreases and the wear of the electrode also increases. On the other hand, when the temperature is lower than the above temperature, the heat energy for heating the cleaning tank to a temperature of 130 ° C. becomes enormous, and the piping path for heat exchange becomes significantly longer, which is not practical. For the same reason, it is more desirable to set the lower limit to 40 ° C. and the upper limit to 80 ° C.
The heating means and cooling means described above may be attached to a cleaning device or an electrolytic reaction device, or may be provided in a circulation line. Further, another line may be provided in the cleaning device or the electrolytic reaction device to heat or cool the solution. Further, heat exchange can be performed between the feed side and the return side of the circulation line so as to raise the temperature of the cleaning liquid and lower the temperature of the electrolytic solution.

  In the electrolytic reaction apparatus, electrolysis is performed with an anode and a cathode paired in an electrolytic reaction tank. The material of these electrodes is not limited to a specific one in the present invention. However, when platinum, which is widely used as an electrode, is used as an anode, there is a problem that platinum is eluted. On the other hand, the diamond electrode can efficiently generate persulfate ions and has little electrode wear. Therefore, among the electrodes of the electrolytic reaction apparatus, it is desirable that at least the anode that generates sulfate ions is composed of a diamond electrode, and it is more desirable that both the anode and the cathode are composed of diamond electrodes.

The conductive diamond electrode is based on a semiconductor material such as a silicon wafer, and after synthesizing a conductive diamond thin film on the surface of the wafer, the wafer is melted or deposited and synthesized in a plate shape without using the base. Mention may be made of self-standing conductive polycrystalline diamond. Production of persulfate ions from sulfate ions using a conductive diamond electrode has been reported for a current density of about 0.2 A / cm ² (Ch. Cominellis et al., Electrochemical and Solid- State Letters, Vol. 3 (2) 77-79 (2000), Patent Document 2), an electrode having a diamond thin film supported on a metal substrate has a problem that the diamond film is peeled off and the effect disappears in a short period of time. is there. Therefore, a self-supporting conductive diamond electrode in which the substrate is removed after being deposited on the substrate is desirable.
The conductive diamond thin film is a conductive thin film that is doped with a predetermined amount of boron or nitrogen during the synthesis of the diamond thin film, and is generally boron-doped. If the doping amount is too small, technical significance does not occur. If the doping amount is too large, the doping effect is saturated. Therefore, a doping amount in the range of 50 to 20,000 ppm with respect to the carbon amount of the diamond thin film is suitable. .
In the present invention, the conductive diamond electrode is usually a plate-like one, but a network structure having a plate-like shape can also be used.
The electrolytic treatment in this electrolytic reaction apparatus is carried out by setting the current density on the surface of the conductive diamond electrode to 10 to 100,000 A / m ² , the sulfuric acid solution in the direction parallel to the diamond electrode surface, and the liquid flow rate of 10 to 10,000 m / m ^2. It is desirable to perform the contact treatment with h.

  In addition, in order to appropriately set the sulfuric acid concentration in the plurality of washing tanks as described above, it is provided with an electrolytic reaction tank for regenerating the persulfuric acid solution by circulating the solution individually between the washing tanks. desirable. In each electrolytic reaction tank, the organic substance load is lower as it corresponds to the subsequent washing tank, and the current efficiency is higher and the persulfuric acid generation efficiency is higher as the sulfuric acid concentration is lower. This can reduce equipment costs and running costs. Therefore, in each electrolytic reaction tank, the total anode area of the electrolytic reaction tank corresponding to the k-th (k = 1, 2,... As Sk, it is desirable to satisfy the condition of S1 ≧ ... ≧ Sk ≧ ... ≧ Sn.

In the cleaning system of the present invention, cleaning processing can be performed on various materials to be cleaned, but cleaning processing is performed on electronic material substrates such as silicon wafers, glass substrates for liquid crystals, and photomask substrates. It is suitable for the use to do. More specifically, it can be used for a peeling process of an organic compound such as a resist residue attached on a semiconductor substrate. Further, it can be used for a foreign matter removing process such as fine particles and metal adhering to the semiconductor substrate.
Conventionally, prior to a cleaning process, a process for processing a semiconductor substrate usually includes a step of pre-oxidizing and ashing an organic resist using a dry etching or ashing process as a pre-processing step. Yes. This process has a problem of increasing the apparatus cost and the processing cost. By the way, in the system of the present invention, an excellent cleaning effect can be obtained. Therefore, even when a cleaning process is performed without incorporating a pretreatment process such as the above-described dry etching or ashing process, the resist is sufficiently removed. Is obtained. That is, the present invention makes it possible to establish a process in which these pretreatment steps are omitted.

  As described above, according to the sulfuric acid recycling type cleaning system of the present invention, a persulfate solution is used as a cleaning solution to clean a material to be cleaned, and persulfate ions are generated from sulfate ions contained in the solution by an electrolytic reaction. An electrolytic reaction device for regenerating the persulfuric acid solution, and a circulation line for circulating the solution between the cleaning device and the electrolytic reaction device, and the cleaning device sequentially cleans the material to be cleaned. Since there is a tank and the sulfuric acid concentration of the cleaning liquid is set to be different in part or all of the cleaning tank, the sulfuric acid solution is repeatedly used and the persulfuric acid solution for enhancing the peeling effect is regenerated by the electrolytic reactor. And can be used for cleaning. In addition, efficient cleaning can be continued without requiring addition of a chemical solution such as hydrogen peroxide or ozone from the outside. Furthermore, it becomes possible to perform cleaning effectively by selecting the cleaning tank and sequentially cleaning the materials to be cleaned with cleaning liquids having different sulfuric acid concentrations, that is, cleaning performance.

  In particular, if the sulfuric acid concentration of the cleaning liquid in the k-th stage (k = 1, 2,... N) cleaning liquid is Ck, the condition of C1 ≧ ... ≧ Ck ≧ ... ≧ Cn is satisfied. Contaminants such as a resist can be effectively removed with concentrated sulfuric acid having a high cleaning effect, and for details, contaminants can be effectively removed with sulfuric acid having a low viscosity at a later stage.

  Further, if the total anode area of the electrolytic reaction tank corresponding to the k-th stage (k = 1, 2,... N) cleaning tank is Sk, the equipment satisfies the condition of S1 ≧ ... ≧ Sk ≧ ... ≧ Sn. Costs can be reduced and more efficient electrolysis can be performed.

(Embodiment 1)
Below, one Embodiment of this invention is described based on FIG.
The cleaning apparatus of the present invention has three stages of cleaning tanks (first cleaning tank 10, second cleaning tank 20, and third cleaning tank 30) independently for sequentially cleaning the material to be cleaned. In addition, the electrolytic reaction tanks 17, 27, and 37 that are the electrolytic reaction apparatus of the present invention are connected by the feed pipes 13, 23, and 33 and the return pipes 15, 25, and 35. Each of the feed pipes 13, 23, 33 and the return pipes 15, 25, 35 has at least an inner surface made of tetrafluoroethylene, and the feed pipes 13, 23, 33 are each fed with a solution for feeding a solution. Liquid pumps 14, 24, and 34 are interposed. Further, between the feed pipe 13 and the return pipe 15, between the feed pipe 23 and the return pipe 25, and between the feed pipe 33 and the return pipe 35, the heat exchanger 16 as a heat exchange means of the present invention, 26, 36 are interposed, and the heat exchangers 16, 26, 36 can exchange heat between the solution flowing through the feed pipes 13, 23, 33 and the solution flowing through the return pipes 15, 25, 35. . In addition, it is desirable that at least the inner surface of the flow paths (not shown) in the heat exchangers 16, 26, and 36 be composed of tetrafluoroethylene. The above-described feed pipes 13, 23, 33, liquid feed pumps 14, 24, 34, and return pipes 15, 25, 35 constitute a circulation line of the present invention.
The cleaning tanks 10, 20, and 30 are respectively heated by ultrapure water lines 11, 21, and 31 for supplying ultrapure water and heaters 12, 22, and 32 for heating the cleaning liquid in the tank. It is provided as a device.

For example, in the electrolytic reaction tank 17, an anode 17a and a cathode 17b are disposed, and a bipolar electrode 17c is disposed at a predetermined interval between the anode 17a and the cathode 17b. The present invention is not bipolar, and may be provided with only an anode and a cathode as electrodes. A DC power source 18 is connected to the anode 17a and the cathode 17b, thereby enabling DC electrolysis in the electrolytic reaction tank 17. The electrolytic reaction tank 27 also includes an anode 27a, a cathode 27b, and a bipolar electrode 27c, and a DC power source 28 is connected to the anode 27a and the cathode 27b. 37c, and a DC power source 38 is connected to the anode 37a and the cathode 37b.
In this embodiment, the number of bipolar electrodes in the electrolytic reaction tanks 17, 27, and 37 is different, so that the total anode area S including the anode surface generated by the polarization of the bipolar electrode is the total of the electrolytic reaction tank 17. Anode area S1> total anode area S2 of electrolytic reaction tank 27> total anode area S3 of electrolytic reaction tank 37.

  In this embodiment, the anode, cathode, and bipolar electrode in each of the electrolytic reaction tanks are composed of diamond electrodes. The diamond electrode is produced by forming a diamond thin film on a substrate and doping boron in a range of 50 to 20,000 ppm, preferably with respect to the carbon content of the diamond thin film. Is a self-supporting type in which the substrate is removed after thin film formation.

Next, the operation of the sulfuric acid recycling type cleaning system having the above configuration will be described.
A sulfuric acid solution having a sulfuric acid concentration C1 of 17M in the first cleaning tank 10, a sulfuric acid solution having a sulfuric acid concentration C2 of 8-15M in the second cleaning tank 20, and a sulfuric acid concentration C3 of 8-15M in the third cleaning tank 30. Contains sulfuric acid solution. The sulfuric acid concentration can be adjusted by supplying ultrapure water from the ultrapure water lines 11, 21, and 31. The sulfuric acid concentration C2 in the second-stage cleaning tank 27 is set so that the sulfuric acid concentration C3 in the third-stage cleaning tank 37 is satisfied. In each washing tank, the solution is fed to the electrolytic reaction tanks 17, 27, and 37 by the liquid feeding pumps 14, 24, and 34 accommodated in the tank.

In the electrolytic reaction tank 17, when the anode 17a and the cathode 17a are energized by the DC power source 18, the bipolar electrode 17c is polarized, and the anode and the cathode appear at predetermined intervals. The solution sent to the electrolytic reaction tank 17 is passed between these electrodes. At this time, it is desirable to set the output of the liquid feed pump 14 so that the liquid flow rate is 1 to 10,000 m / hr. In the above energization, it is desirable to control the energization so that the current density on the diamond electrode surface is 10 to 100,000 A / m ² . Also in the electrolytic reaction tanks 27 and 37, the bipolar electrodes are polarized by energization in the same manner as described above, and water is passed between the electrodes. The same range as above is desirable for the flow velocity and current density.

When the solution is energized in the electrolytic reaction tanks 17, 27, and 37, the sulfate ions in the solution undergo an oxidation reaction to generate persulfate ions to obtain a persulfate solution. This persulfuric acid solution is fed from the return pipes 15, 25, and 35 toward the first cleaning tank 10, the second cleaning tank 20, and the third cleaning tank 30, respectively. At this time, heat is exchanged through the heat exchangers 16, 26, and 36 with the solution fed from the respective washing tanks through the feed pipes 13, 23 and 33, and the temperature is raised.
Thereby, a high-concentration persulfuric acid solution is obtained in each of the first cleaning tank 10, the second cleaning tank 20, and the third cleaning tank 30. In the 1st washing tank 10, the 2nd washing tank 20, and the 3rd washing tank 30, although a persulfate ion density | concentration reduces gradually by autolysis, a solution circulates between the electrolytic reaction tanks 17, 27, and 37, and each electrolysis Since persulfate ions are generated by electrolysis in the reaction tank, the concentration of persulfate ions is maintained.

Moreover, the persulfuric acid solution in the 1st washing tank 10, the 2nd washing tank 20, and the 3rd washing tank 30 is heated by the heater 12, 22, 32, and is heated and maintained at 120-150 degreeC.
First, cleaning of the semiconductor wafer 1 as a material to be cleaned is started in the first cleaning tank 10 in the first stage. The semiconductor wafer 1 is immersed in the first cleaning tank 10. In the first cleaning tank 10, a high oxidizing action is obtained by a cleaning action by concentrated sulfuric acid of high concentration and a self-decomposition of persulfate ions, and most of the resist which is a contaminant on the semiconductor wafer 1 is effectively obtained. It is stripped off, transferred into a persulfuric acid solution, and decomposed by persulfuric acid. The solution in the first cleaning tank 10 is sent to the electrolytic reaction tank 17 by the feed pipe 13 and the liquid feed pump 14. The resist dissolved in the first cleaning tank 10 is also decomposed when the feed tube 13 is moved. When liquid is fed through the feed pipe 13, heat is exchanged with the cleaning liquid fed from the electrolytic reaction tank 17 through the return pipe 15 through the heat exchanger 16 and the temperature is lowered.

In the electrolytic reaction tank 17, persulfate ions are generated from sulfate ions by energization through an electrode having a sufficient total anode area, and the persulfate concentration reduced by autolysis is increased to regenerate the persulfate solution. The gas is returned to the first cleaning tank 10 through the return pipe 15.
The semiconductor wafer 1 immersed in the first cleaning tank 10 for a predetermined time is transferred to the second cleaning tank 20 in the second stage. Further, the subsequent semiconductor wafer can be continuously immersed in the first cleaning tank 10 for cleaning.

Next, in the second cleaning tank 20, the resist remaining on the semiconductor wafer 1 is further stripped and removed with a cleaning liquid having a sulfuric acid concentration lower than that in the first cleaning tank 10. The resist dissolved in the solution is decomposed by persulfuric acid. At this time, the dissolved amount of the resist is smaller than that of the first cleaning tank 10, and the decomposition burden due to persulfuric acid is reduced. Also in the second cleaning tank 20, the persulfuric acid solution whose persulfate ion concentration is reduced by autolysis is sent to the electrolytic reaction tank 27 through the feed pipe 23 by the liquid feed pump 24. At this time, heat is exchanged with the solution moving through the return pipe 25 by the heat exchanger 26, and the temperature is lowered. In the electrolytic reaction tank 27, the persulfuric acid solution is regenerated by electrolysis as described above. Although the total anode area in the electrolytic reaction tank 27 is smaller than that in the electrolytic reaction tank 17, the necessary persulfate ion concentration is set low (because the resist dissolution amount is small), the excess to be regenerated. Electrolysis is efficiently performed at an optimum current density according to the sulfate ion concentration. The regenerated persulfuric acid solution is returned to the second washing tank 20 through the return pipe 25 to maintain the persulfuric acid concentration in the tank.
The semiconductor wafer 1 immersed in the second cleaning tank 20 for a predetermined time is transferred to the third cleaning tank 30.

Next, in the third cleaning tank 30 in the third stage, the semiconductor wafer 1 is cleaned with sulfuric acid having a lower concentration than the second cleaning tank 20. Sulfuric acid having a low concentration has a low viscosity and can strip and remove a small amount of resist remaining in the details of the semiconductor wafer 1, for example, the pattern shape. The resist solution dissolved in the solution is decomposed by persulfuric acid. The dissolution amount of the resist is further smaller than that of the second cleaning tank 20, and the decomposition burden due to persulfuric acid is further reduced. In the third washing tank 30, the persulfuric acid solution having a reduced persulfate ion concentration is sent to the electrolytic reaction tank 37 by the liquid feed pump 34 through the feed pipe 33. At this time, heat is exchanged with the solution moving through the return pipe 35 by the heat exchanger 36, and the temperature is lowered. In the electrolytic reaction tank 37, the persulfuric acid solution is regenerated by electrolysis. Although the total anode area in the electrolytic reaction tank 37 is smaller than that in the electrolytic reaction tanks 17 and 27, the necessary persulfate ion concentration is set to be lower, and the efficiency is improved at the optimum current density according to the persulfate ion concentration to be regenerated. Electrolysis is often performed. The regenerated persulfuric acid solution is returned to the third washing tank 30 through the return pipe 35 to maintain the persulfuric acid concentration in the tank.
The semiconductor wafer 1 that has been sequentially cleaned by the cleaning tanks 10 to 30 can be rinsed with ultrapure water or the like as in the conventional case.

By cleaning the semiconductor wafer with the sulfuric acid recycling type cleaning system, it is possible to regenerate the persulfuric acid solution by repeatedly using the persulfuric acid solution without requiring the addition of hydrogen peroxide or ozone. Effective cleaning can be continued.
In the above-described embodiment, the system having the cleaning tank up to the third stage has been described. However, the present invention may be configured by using two or more cleaning tanks.
Although the present invention has been described based on the above embodiment, the present invention is not limited to the description of the above embodiment, and can naturally be changed without departing from the scope of the present invention. .

Next, an example using the system of the above-described embodiment will be described.
In this example, 50 liters of 98% concentrated sulfuric acid (sulfuric acid concentration 18.5 M) was put in the first washing tank 10 and heated to 130 ° C. In the electrolytic reaction tank 17, ten conductive diamond electrodes doped with boron on a Si substrate having a diameter of 15 cm and a thickness of 1 mm were incorporated. Effective anode area for electrolysis is 15dm ^2, by setting the current density 60A / dm ^2, and electrolysis at 40 ° C..
In the second washing tank 20, a high-concentration sulfuric acid solution (sulfuric acid concentration: 14.8M) adjusted at a ratio of 40 liters of 98% concentrated sulfuric acid and 10 liters of ultrapure water was prepared and heated to 130 ° C. In the electrolytic reaction tank 27, five conductive diamond electrodes doped with boron on a Si substrate having a diameter of 15 cm and a thickness of 1 mm were incorporated. Effective anode area for electrolysis is 7dm ^2, by setting the current density 60A / dm ^2, and electrolysis at 40 ° C..
Further, a high concentration sulfuric acid solution (sulfuric acid concentration of about 9.3 M) prepared at a ratio of 25 liters of 98% concentrated sulfuric acid and 25 liters of ultrapure water was prepared in the third washing tank 30 and heated and maintained at 130 ° C. In the electrolytic reaction tank 37, three conductive diamond electrodes doped with boron on a Si substrate having a diameter of 15 cm and a thickness of 1 mm were incorporated. Effective anode area for electrolysis is 3.5dm ^2, by setting the current density 60A / dm ^2, and electrolysis at 40 ° C..

  In the cleaning tank, a 5-inch silicon wafer with a 100 nm patterned resist that has not been subjected to ashing as a pretreatment was immersed in the order of the first cleaning tank 10, the second cleaning tank 20, and the third cleaning tank 30. . The resist was dissolved by dipping 50 sheets / cycle with an immersion cycle of 10 minutes. This solution was circulated at a flow rate of 2 L / min by a liquid feed pump between the electrolytic reaction tank and any of the washing tanks.

In the first cleaning tank 10, the resist peeling effect is good, the solution in the first cleaning tank 10 was initially colored brown, and the TOC concentration was 30 mg / l. The solution in the first cleaning tank 10 was colorless and transparent, and the TOC concentration was below the detection limit. Subsequently, the organic component derived from the resist adhering to the wafer in the second cleaning tank 20 can be completely removed by the persulfuric acid solution generated in the electrolytic reaction tank 27, and the third cleaning is performed in a state where the cleanness of the wafer surface is higher. It was conveyed to the tank 30. In the third cleaning tank 30, the organic component derived from the resist attached around the pattern could be completely removed by the persulfuric acid solution generated in the electrolytic reaction tank 37. Finally, it was rinsed with ultrapure water and spin-dried. When such a cleaned wafer was brought into a wafer analyzer and heated to 400 ° C., and an organic residue was measured with a mass spectrometer, the residue was about ²⁰⁰ to 300 pg / cm ^2. Confirmed that it can be obtained. Furthermore, when the periphery of the pattern was observed with a scanning electron microscope with a low acceleration voltage, no resist residue was observed. Such wafer cleaning was continued for 8 hours (2,400 wafers were cleaned), but the resist stripping effect of the high-concentration sulfuric acid solution was good, and the TOC concentration was below the detection limit in any cleaning tank. . Therefore, it continued for another 32 hours (9,600 cleaning wafers, 12,000 total processing wafers), but the resist stripping effect of the high-concentration sulfuric acid solution is good, and the TOC concentration is high in any cleaning tank. It was below the limit.

(Comparative Example 1)
Next, 40 liters of 98% concentrated sulfuric acid was put in a washing tank, which will be described for a comparative example, and a solution to which 10 liters of 35% hydrogen peroxide solution was added was heated and maintained at 130 ° C. In this solution, a resist-coated wafer having a patterned pattern of 100 nm subjected to ashing treatment as a pretreatment was immersed in the same immersion cycle as in Example 1 to perform resist dissolution. Up to the first 6 cycles (300 wafers to be cleaned), the resist stripping effect on the wafer is good, and the solution is colored brown immediately after immersion, but becomes colorless and transparent in less than 10 minutes, and the detection limit for the TOC concentration It became. However, with respect to the next 50 sheets, the resist remained on the wafer even after 10 minutes had passed immediately after the immersion, and the solution remained brown, and a TOC concentration of 30 mg / l was observed. Therefore, the 50 wafers were extracted as having a poor processing effect. Next, 10 liters of the solution in the washing tank was drawn out, 10 liters of hydrogen peroxide solution was added, and the solution was heated and maintained at 130 ° C. The wafer immersion was continued again. Up to the first 2 cycles (100 wafers to be cleaned), the resist removal effect on the wafer is good, and the solution is colored brown immediately after immersion, but becomes colorless and transparent in less than 10 minutes, and the TOC concentration is also a detection limit. It became. However, with respect to the next 50 sheets, the resist remained on the wafer even after 10 minutes had passed immediately after the immersion, and the solution remained brown, and a TOC concentration of 30 mg / l was observed. Therefore, the 50 wafers were extracted as having a poor processing effect. Again, 10 liters of solution in the washing tank was withdrawn, and 10 liters of hydrogen peroxide water was added. Although the wafer immersion was continued, when 50 wafers were immersed, the resist peeling and dissolving effect was poor, and the resist remained on the wafer even after 10 minutes. When the total number of processed wafers was 400, the entire solution had to be replaced. Moreover, when the wafer subjected to such cleaning is brought into a wafer analyzer and heated to 400 ° C. and an organic residue is measured by a mass spectrometer, the residue is about 500 to 1000 pg / cm ² and has a high cleanliness. A wafer could not be obtained. Furthermore, when the periphery of the pattern was observed with a scanning electron microscope with a low acceleration voltage, it was confirmed that a large amount of resist residue was adhered.

It is the schematic which shows the system in one Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 1st washing tank 20 2nd washing tank 30 3rd washing tank 11, 21, 31 Ultrapure water line 12, 22, 32 Heater 13, 23, 33 Feed pipe 14, 24, 34 Liquid feed pump 15, 25, 35 Return pipe 16, 26, 36 Heat exchanger 17, 27, 37 Electrolytic reaction tank 17a, 27a, 27a Anode 17b, 27b, 27b Cathode 17c, 27c, 37c Bipolar electrode

Claims (6)

  A cleaning device that cleans a material to be cleaned using a persulfuric acid solution as a cleaning solution, an electrolytic reaction device that generates persulfate ions from sulfate ions contained in the solution by electrolytic reaction, and regenerates the persulfuric acid solution, and the cleaning device and the electrolytic reaction A circulation line that circulates the solution between the apparatus and the cleaning apparatus has a plurality of stages of cleaning tanks for sequentially cleaning the material to be cleaned, and the sulfuric acid concentration of the cleaning liquid in a part or all of the cleaning tanks The sulfuric acid recycle type cleaning system is characterized by being set differently. 2. The sulfuric acid recycling type according to claim 1, wherein the sulfuric acid concentration of the cleaning liquid in the k-th stage (k = 1, 2,... n) is set to satisfy the following formula (1), where Ck is the sulfuric acid concentration. Cleaning system.
C1 ≧ ... ≧ Ck ≧ ... ≧ Cn (1)
Where n is 2 or more   Among the cleaning tanks, the first stage cleaning tank has a sulfuric acid concentration of 17 M or more in the cleaning liquid, and the second and subsequent cleaning tanks have a sulfuric acid concentration in the range of 8M to 15M. The sulfuric acid recycling type cleaning system according to claim 1 or 2.   The sulfuric acid recycle according to any one of claims 1 to 3, wherein each of the electrolytic reaction apparatuses includes an electrolytic reaction tank that regenerates the persulfuric acid solution by circulating the solution individually between the cleaning tanks. Mold cleaning system. The sulfuric acid recycle type cleaning according to claim 4, wherein the following equation (2) is satisfied, where Sk is the total anode area of the electrolytic reaction tank corresponding to the k-th stage (k = 1, 2, ... n) cleaning tank. system.
S1 ≧ ... ≧ Sk ≧ ... ≧ Sn (2)
Where n is 2 or more   6. The sulfuric acid recycle type cleaning system according to claim 1, wherein a conductive diamond electrode is used for the anode of the electrolytic reaction apparatus.

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