JPH0483585A - Purification of ultra-pure water - Google Patents

Abstract

PURPOSE:To remove the heavy metal ion of a ppt level contained in ultra-pure water used in the semiconductor industry to purify said ultra-pure water by filtering ultra-pure water by a specific porous membrane with a specific apparent stagnation time. CONSTITUTION:Ultra-pure water whose electric specific resistance value at 25 deg.C is 17.0mOMEGA.cm or more is filtered by a porous membrane having a functional group capable of bonding a heavy metal ion in a chelated state on the surface thereof and the surfaces of the pores thereof in such a state that the apparent stagnation time of the ultra-pure water in the membrane is set to 0.1-300 sec. A chelate group is pref. bonded to the surface of the porous base membrane and the surfaces of the pores thereof. The base membrane pref. has a three- dimensional reticulated structure formed by a microphase separation method or a mixing extraction method rather than a structure having orthogonal piercing type pores obtained by a stretching method or an etching method. The apparent stagnation time is shown by T/F when filtering speed is set to Fcm/sec and membrane thickness is set to Tcm.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to the semiconductor industry, particularly to the purification of ultrapure water used in manufacturing highly integrated semiconductors.

[Prior Art] Ultrapure water used in semiconductor manufacturing is required to be of extremely high quality.

That is, after treating general industrial water etc. with coagulation sedimentation and sand filtration, gases such as oxygen and carbon dioxide are removed, UV irradiation is performed, then treatment with a reverse osmosis membrane, and then irradiation with low pressure UV oxidation is performed to produce an ion exchange resin. Process with etc. Furthermore, fine particles, pyrogen, etc. are completely removed using an ultrafiltration membrane, and the product is sent to the point of use where it is processed by a final filter (mainly a microfilter) before being used.

Recently, semiconductor manufacturing technology has become more and more sophisticated as the degree of integration has become more precise, such as 16 Mbit, and
Metal ions on the order of 1/2 of that) have also come to be seen as a problem. Furthermore, recent reports have focused more on the presence of heavy metal ions generated from high-pressure pumps used to operate reverse osmosis membranes used in ultrapure water production than on alkali metals. Furthermore, the influence of heavy metal ions is considered to be important in the processing of semiconductor wafers.

The above-mentioned treatment methods have already become unsatisfactory as methods for removing such extremely small amounts of heavy metals, and in particular, ultrafiltration and microfilters used in the terminal parts cannot reach this level. It's difficult.

Furthermore, even with the ion exchange resin (Polylasha) used before the ultrafiltration membrane, it is currently difficult to remove less than the ppt level.

[Problem to be solved by the invention]

An object of the present invention is to provide a method for removing and purifying heavy metal ions below the ppt level contained in ultrapure water used in the semiconductor industry and the like.

[Means to solve the problem]

The ultrapure water purification method of the present invention uses ultrapure water with an electrical resistivity value of 17.0 mΩ·1 or more at 25°C through porous membranes and pores having functional groups capable of chelating with heavy metal ions on the surface of the membrane and pores. The method is characterized in that the ultrapure water is filtered using a transparent membrane with an apparent residence time of 0.1 to 300 seconds inside the membrane.

The method of the present invention is preferably employed as late as possible in the conventional purification process, ie as a replacement for the ultrafiltration membrane used after the polyurethane of the conventional process or for the part of the microfilter used at the point of use. Also,
In order to treat high-purity water having an electrical resistivity value of 17.5 mΩ·1 or more at 25° C., it is preferable to purify the water with high precision as late as possible.

The porous membrane used in the present invention has 1. chelate groups capable of binding heavy metals on the membrane surface and the pore surfaces. Preferably, chelate M is bonded to the membrane surface and the pore surface of the porous base membrane in an amount of 0.3 milliequivalent or more per 1 g of the porous membrane, the average pore diameter is 0901 to 5 μm, and the porosity is 20 to 90%. , a porous membrane having a thickness of 10 μm to 5 μm is used.

As the material of the base film, a film made of polyolefin, a copolymer of an olefin and a halogenated olefin, a polyvinylidene fluoride, or a polysulfone is used, and a film made of a polyolefin or a copolymer of an olefin and a halogenated olefin is preferably used.

Examples of polyolefins include polyethylene, polypropylene, polybutene and mixtures thereof. Examples of copolymers of olefins and halogenated olefins include copolymers of at least one selected from ethylene, propylene, butene, pentene, and hexane and halogenated olefins such as tetrafluoroethylene and chlorotrifluoroethylene. can give.

The structure of the base film is preferably a three-dimensional network structure formed by a microphase separation method, a mixed extraction method, or the like, rather than one having straight through-holes obtained by a stretching method or an etching method. Particularly preferred is the membrane structure shown in Japanese Patent Application Laid-Open No. 55131028.

The shape of the base membrane may be any of a flat membrane (including pleated and spiral shapes), a tubular membrane, and a hollow fiber membrane, but a hollow fiber membrane is preferable.

For bonding the chelate group to the base film, it is preferable to use a chemically bonded chelate group, and the bond may be direct, or a polymer containing a chelate group may be bonded.

In some cases, the chelate group may not be directly bonded to the base membrane, but may be incorporated into the membrane indirectly by means such as coating, direct addition to a surface crosslinked polymer, or the like. Most preferred, however, are those in which the chelating group is chemically bonded to the membrane via a graft chain.

Heavy metal ions that exist in ultrapure water include iron,
These are ions such as nickel, copper, cobalt, and zinc. Therefore, the most preferable example of the chelate group used in the present invention is iminodiacetic acid group (N (CJCOOH)z
) is used as a functional group. The chelate groups are bonded in an amount of 0.3 milliequivalent or more per gram of the porous membrane in a dry state. Preferably, a porous membrane containing chelate groups in an amount of 1.5 to 10 mequivalents can be used.

Outside this range, the ion removal ability or water permeation ability of the membrane may decrease. Milliequivalents here refer to milliequivalents of functional groups; for example, 1 mmole of iminodiacetic acid is 2 milliequivalents.

The amount of chelate groups in the present invention refers to milliequivalents per gram of porous membrane, but 1 gram of membrane here refers to a value based on the fairly macroscopic weight of the membrane, for example, It does not refer to the weight of only a part or part of the inside. In order to bond the chelate group while maintaining the excellent mechanical properties of the membrane, it is preferable to have the chelate group present as uniformly and preferentially on the surface of the membrane and pores as possible. Homogeneity is acceptable. Therefore,
The meaning of 1 g of membrane here indicates a value measured evenly over the entire surface of the membrane, and does not mean the weight from an extremely microscopic viewpoint.

The average pore diameter of the porous membrane is 0.01 to 5 μm, preferably 0.01 to 5 μm.
．． The diameter is selected from the range of 0.01 μm to 1 μm. If it is smaller than this range, the water permeability is not sufficient for practical performance, and if it is larger than this range, ion removal becomes a problem.

There are many methods for measuring the average pore size, but in the present invention, we use the air permeation flow between dry and wet membranes when changing the air pressure, which is usually called the air flow method described in ASTM F-316-70. Comply with the method of measuring from a bundle.

The porosity of the porous membrane is 20% to 90%, preferably 50%
-80% is used.

Here, the porosity is measured by immersing the membrane in a liquid such as water in advance, then drying it, and measuring the weight change before and after that. Porosity values outside the above range are unfavorable in terms of permeation rate and mechanical properties.

A method for introducing a chelate group into the side chain of the polymer constituting the base film, for example, a method for introducing iminodiacetic acid groups into the side chain of polyethylene, is to irradiate the polyethylene film with an electron beam or the like, and then evaporate the styrene. A method of grafting in a phase and then reacting with iminodiacetic acid is adopted.

In addition, after irradiating the polyethylene film with an electron beam, etc.,
Another preferred method is to graft glycidyl methacrylate in the gas phase and then add iminodiacetic acid.

In order to introduce the chelate group into the base membrane, it is possible to introduce it into the polymer before it is formed into a membrane, but after it is formed into a membrane, it can be chemically added and bonded to the inner and outer surfaces of the membrane and the surface of the pores. The method is preferred. It is desirable that the chelate group be bonded to each surface of the membrane as uniformly as possible, but it may be preferentially bonded to the pore surfaces of the membrane in some cases.

The graft chain that binds the chelate group to the membrane is preferably derived from glycidyl methacrylate.

The porous membrane having the chelate group has an incomparably smaller pore diameter than that of an ion exchange resin (the resin has a pore size of several tens of μm).
m to 100 μm, whereas the membrane is less than 5 μm). Therefore, the ion removal performance (rate) of the membrane is incomparably superior to that of ion exchange resins, and when measured by a radioisotope method, it is possible to remove ions of less than 0,00 ppt.

This is because the adsorption behavior of ions to the membrane is very different from that of ion exchange resins, and multiple heavy metal ions are adsorbed onto the membrane over time in a layered manner from the raw water side to the filtered water side, and the chelate group This shows that even if multiple heavy metal ions are contained with considerably different adsorption equilibrium properties, they can be removed equally efficiently.

This fact indicates the superiority of membrane-based purification in removing multiple heavy metal ions. The rate-limiting effect due to the balance of ions and hydrogen in the membrane is considerably smaller than in the case of ion exchange resins.

In the present invention, using the porous membrane having the chelate group, the electrical resistivity value at 25° C. is 17.0 mΩ·011
One or more ultrapure waters are filtered with an apparent residence time of the ultrapure water in the membrane of 0.1 to 300 seconds, preferably 0.2 to 100 seconds.

The apparent residence time is expressed as T/F, where the filtration rate is Fcv/sec and the film thickness is Tcm.

This so-called apparent residence time limiting condition is generally on the overwhelmingly higher permeation rate side than in the case of ion exchange resins.

If the T/F is less than 0.10 seconds, the electrical resistivity value of the water obtained will not reach the desired level. Moreover, if it exceeds 300 seconds, it is not preferable from the economic point of view.

EXAMPLES The present invention will be explained below with reference to examples, but these are not intended to limit the invention.

〔Example〕

Example 1 In fine powder silicic acid, 22.1 parts by weight of Psil VN3LP), 55.0 parts by weight of dibutyl phthalate (DBP), polyethylene resin powder (manufactured by Asahi Kasei ■, 5H-800 grade) 23.
After premixing 0 parts by weight of the composition, it was extruded into a hollow fiber shape using a 30 mm twin-screw extruder, and then immersed in 1°1.1-)U chloroethane [Chlorocene VC, (trade name)] for 60 minutes. , DBP was extracted.

Further, it was immersed in a 40% aqueous solution of caustic soda at a temperature of 60° C. for about 20 minutes to extract fine powder silicic acid, followed by washing with water and drying.

An electron accelerator (pressure voltage 1.5
Meν electron beam current 1 mA) under nitrogen atmosphere
After irradiating with an electron beam using KGy, glycidyl methacrylate was almost completely grafted in the gas phase, followed by washing and drying.

The amount of glycidyl methacrylate added was 1 g (7.0 meq.) per gram of the original base film (according to the gravimetric method).

Next, this graft membrane was immersed in a 0.4 mol/1 aqueous solution of sodium iminodiacetate whose pH was adjusted to 12 with sodium carbonate, and reacted at 80°C for 24 hours. After neutralization, the membrane was washed with water to form porous iminodiacetic acid groups. A porous membrane was obtained in which 0.7 mmol (1.4 milliequivalent) was bound per membrane Ig (measured in dry state). The obtained porous membrane had an inner diameter of 0.7 wu.
n, thickness 0.25mm, porosity 50%, average pore size 20
It was μm. Note that the quantitative determination of iminodiacetic acid groups was calculated using two methods: a gravimetric method and a cobalt ion adsorption equilibrium method.

2,000 of the above hollow fiber porous membranes were housed in a polysulfone outer cylinder having an outer diameter of 3 inches and having two nozzles, and a module with an effective membrane length of 110 cm was assembled. Here, the module refers to a membrane bundle and an outer cylinder end that are liquid-tightly sealed with epoxy resin at both ends of the membrane, and the membrane is open at both end faces and attached to the outer cylinder end via an O-ring. This is a conventionally known filtration device that communicates with a space formed by a cap with a nozzle and communicates with the outside.

As the feed water, ultrapure water having the following water quality was used, which had been pretreated with a reverse osmosis membrane, UV, Porinscher, and an ultrafiltration membrane.

Table 1: Water quality of ultrapure water for supply Electrical specific resistance value at 25℃ 17.5 mΩ・1
Fe ion concentration” (ppt) 45Cu
Ion concentration (ppt) 20Zn ion concentration (ppt) 251) Manufactured by Kurita Industries ■
Measured value by KX-4 2) 1. C, P, mass spectrometer (
This ultrapure water was supplied to the module and filtered under the following conditions. Table 1 shows the water quality after filtration and purification.

Note) Detection limit value: several pp+ (1, C, P, by mass spectrometer) Comparative Example 1 Example 1 was repeated except that the filtration conditions were changed as shown in Table 1. The results are shown in Table 1.

Comparative Example 2 Filtration was performed in the same manner as in Example 1, except that a microfilter manufactured by Millipore, Durapore CVGL, 01 TP3 was used instead of the module. The results are shown in Table 1.

[Effects of the present invention]

The present invention makes it possible to produce ultrapure water that is compatible with future high integration in the semiconductor industry, and is expected to be useful in improving the quality of products in this industry.

Patent applicant: Asahi Kasei Industries, Ltd.

Claims (1)

[Claims] Electrical specific resistance value at 1.25°C is 17.0 mΩ・cm
Using a porous membrane having functional groups capable of chelating with heavy metal ions on the surface of the membrane and pores, the ultrapure water has an apparent residence time of 0. 1
Ultrapure water purification method 2, in which the porous membrane is filtered for ~300 seconds, Claim 1, wherein the porous membrane has iminodiacetic acid groups chemically bonded to the surface of the membrane and the pores in an amount of 0.3 milliequivalent or more per gram of the membrane. How to describe