Process for removing fine particles from articles of fluorine-containing resin

Abstract: The invention provides a method for removing the fine particles deposited on an article of fluorine-­ containing resin, the method comprising conducting at least one of the procedures of: (i) maintaining the article of fluorine-­ containing resin at a high temperature, (ii) maintaining the article in the atmosphere of fluorine-type gas, and (iii) bringing the article into contact with a polar solvent or an aqueous solution of polar solvent.

Description: The present invention relates to a process for removing particulate components deposited as impurities on the molded products of fluorine-containing resin. Currently even fine particles of submicron size must be removed from the molded products of fluorine-containing resin in the field of the so-called high technology. Further, a demand for higher degree of cleaning is expected to increase. For example, containers made of fluorine-containing resin are currently used for holding an agent for wet etching in production of semiconductors, i.e. representative products in the field of the high technology. These containers are usually cleaned with a dilute aqueous solution of a surfactant, strong acid, ultrapure water, or when required, may be cleaned by supersonic treatment or immersion. These methods, however, can not achieve a satisfactory level of cleaning on containers of fluorine-containing resin, compared with the level on containers of polyethylene and therefore may be unempolyable for containers for holding an agent for wet etching. More specifically, the containers of fluorine-containing resin for holding a 50% aqueous solution of HF to be used in the manufacture of semiconductors must be cleaned to such an extent that after cleaning, 10 particles or less are contained in one milliliter of the solution in terms of the particles not smaller than 0.3 µm in particle size, and that after cleaning, one particle or less is contained in one milliliter thereof in terms of the particles not smaller than 0.5 µm in particle size. The cleaning with ultrapure water can not attain the above levels at all. Use of containers of fluorine-containing resin is limited because they remain difficult to clean. Consequently such containers may become substantially out of use when a higher cleaning level is set in future. From JP-A-63 310 683 there is known a method for cleaning articles of fluorine-containing resins, comprising introducing steam into a washing container, maintaining the container at a high temperature, high pressure and high humidity for a specific period of time reducing the internal pressure of the container and showering the article in the container with water to rapidly cool the article. EP-A- 300 385 describes a method for improving the barrier properties of thermoplastic resins such as HDPE, LDPE, PP and PET, comprising bringing the thermoplastic resin into contact with a gaseous reactant steam containing F₂ and O₂. US-A- 4 680 060 discloses a process for extracting contaminants from the surface of plastics articles using alcohol. None of the above mentioned three references discloses that fine particles deposited on articles of fluorine-containing resins can be removed to a high degree of cleaning by maintaining the article of fluorine-containing resins in the atmosphere of fluorine-type gas. It is an object of the present invention to provide a method for removing the particulate components as impurities from the molded products of fluorine-containing resin. Other objects and features of the invention will become apparent from the following description. In view of the aforesaid state of the art, we conducted extensive research and found that the fine particles can be effectively removed from an article of fluorine-containing resin when the article is maintained at a high temperature. Our another finding was that a similar particle-removing effect can be also produced when the article is maintained in the atmosphere of a specific fluorine-type gas. Our additional finding was that a similar particle-removing effect can be also achieved when the article is brought into contact with a specific solvent. The present invention has been accomplished based on these novel findings. According to the the present invention, there are provided: (1) a method for removing the fine particles deposited on an article of fluorine-containing resin, the method comprising maintaining the article of fluorine-containing resin in the atmosphere of fluorine-type gas; (2) a method as defined above in item (1) in which the fluorine-type gas used is a type of gas capable of acting as a fluorinating agent or an oxidizing agent; (3) a method as defined above in item (2) in which the fluorine-type gas used is at least one gas selected from the group consisting of gases of F₂, ClF₅, ClF₃, ClF, BrF₅, BrF₃, BrF, ClO₃F, SF₄, O₂F₂ and NF₃, the gas being one diluted or not diluted with an inactive gas; (4) a method as defined above in item (2) or (3) in which the gas is maintained at a temperature higher than room temperature in the procedure (ii); and (5) a method as defined above in item (1) in which the article is cleaned at least once with ultrapure water and/or with a cleaning agent before or after, or both before and after, conducting said method. The method of the invention essentially comprise subjecting an article of fluorine-containing resin to a method as defined above. The procedure will be described below in greater detail. An article of fluorine-containing resin is maintained in the atmosphere of fluorine-type gas in this procedure. Beneficially the removal of particles can be accelerated by performing an additional procedure such as rotation of article, agitation of atmosphere gas and spray of atmosphere gas. In case the article of fluorine-containing resin is a container, the fluorine-type gas may be filled into the container. Useful fluorine-type gases include gases capable of acting as a fluorinating agent or an oxidizing agent. Specific examples of such gases are gases of F₂, ClF₅, ClF₃, ClF, BrF₅, BrF₃, BrF, ClO₃F, SF₄, O₂F₂ and NF₃. These gases are usable singly or at least two of them can be used in mixture. The gases may be used after dilution with an inactive gas. Examples of useful inactive gases for dilution are nitrogen, argon, and helium. A preferred concentration of the fluorine-type gas diluted with inactive gas is not lower than 0.1% by volume. The temperature of the fluorine-type gas atmosphere may be room temperature and is preferably elevated to accelerate the cleaning. The maximum elevated temperature is lower than the melting point of the article. The longer the retention time in the fluorine-type gas atmosphere, the higher the degree of cleaning is. However, the longer retention time is economically disadvantageous. The retention time, although suitably determinable depending on the degree of contamination of article and the required level of cleaning, is usually not shorter than 1 hour, preferably about 4 to 12 hours, in performing the procedure without heating, but about 10 to about 30 minutes in performing the procedure with heating. The cleaning in the atmosphere of fluorine-type gas may be effected either on a molded product of fluorine-containing resin or during the molding process of fluorine-containing resin. The degree of cleaning on articles of fluorine-containing resin can be also increased by performing an additional cleaning procedure at least once with ultrapure water and/or with a cleaning agent before or after, or both before and after, carrying out said method. Useful cleaning agents include: (a) acids and alkali-type agents such as aqueous solutions containing at least one species selected from HF, NH₄F, HCl, HNO₃, H₂O₂, H₂SO₄, H₃PO₄ and NH₃; and (b) organic solvents such as trichloroethylene, methyl alcohol, ethyl alcohol, isopropyl alcohol and acetone. The foregoing cleaning agents are stored in containers or tanks made of fluorine-containing resin or transported in pipes made of fluorine-containing resin for use in the field of manufacture of semiconductors. The degree of cleaning on articles of fluorine-containing resin can be also increased by conducting additional cleaning at least once with an aqueous solution of electrolyte at a high concentration (not lower than 0.5% by weight) before or after, or both before and after, performing at least one of the procedures (i), (ii) and (iii). Examples of useful electrolytes are NH₄Cl, NH₄F, KCl, KF, NaCl, MgCl₂, MgCO₃, ZnCl₂, ZnSO₄, FeCl₃, AlCl₃ and Al₂(SO₄)₃. The method of the present invention are suitable for removing the fine particles from the fluorine-containing resin article which must be cleaned to a great extent, particularly articles of fluorine-containing resin such as containers, tanks, pipes, tubes or wafer carriers for use in the field of manufacture of semiconductors. The following great advantages can be offered by cleaning an article of fluorine-containing resin according to the present invention. (A) The method of the invention can produce a cleaning effect to an extent heretofore unattainable by conventional techniques, more specifically, to such level that after cleaning, 10 particles or less are contained in one milliliter of the solution held in a container of fluorine-containing resin in terms of the particles not smaller than 0.3 µm in particle size, and that after cleaning, one particle or less is contained in one milliliter thereof in the container in terms of particles not smaller than 0.5 µm in particle size. Therefore the methods of the invention can meet the requirement for higher cleaning levels to be expected in the field of manufacture of semiconductors and other fields. (B) If the method of the invention is carried out during molding of fluorine-containing resin, a cleaning procedure need not be conducted on the molded product. Given below are Examples and Comparison Examples to clarify the features of the present invention in greater detail. Comparison Example 1 Two kinds of molded bottles each made of tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (hereinafter referred to as "PFA bottle") and tetrafluoroethylene-hexafluoropropylene copolymer (hereinafter referred to as "FEP bottle"), respectively were cleaned with ultrapure water (specific resistance [MΩ·cm at 25°C]: at least 18, number of fine particles [number of particles/ml, having a size of not smaller than 0.3 µm]: 1 or less, T.O.C. [ppb]: 10 to 100, ultrapure water of this type will be also used hereinafter) in the order as shown below in Table 1. The bottles were cleaned by pouring an appropriate quantity of ultrapure water into each bottle, shaking the bottle under the conditions of vibrational amplitude of 4 cm and the number of vibration of 160 vibrations/min for 5 minutes and allowing the bottle to stand. The number of fine particles was counted with use of a counter for counting the number of fine particles in liquid (trade name "RION KL-21", manufactured by RION Co., Ltd.**). Table 1 shows the number of fine particles present in ultrapure water and having a size of at least 0.3 µm or at least 0.5 µm after completion of each cleaning procedure. Further, a 40% aqueous solution of NH₄F or a 50% aqueous solution of HF from which fine particles were removed beforehand by filtration using a filter (0.1 µm) was poured into the bottles for cleaning the bottles internally thereof in the same manner as above. Table 1 also shows the number of fine particles counted in this case. In the following Examples and Comparison Examples, cleaning with ultrapure water was conducted in the same manner as in this Comparison Example unless otherwise specified. Further, 40% aqueous solutions NH₄F and 50% aqueous solutions of HF were filtered by the same procedure as above and used in the following cleaning operations. Each value shown in the following tables and obtained using PFA bottles is the average of two data determined by actual count, and that obtained used FEP bottles is the average of three such data. Table 1 shows how the number of fine particles changes through the intermediate stages in the cleaning method using ultrapure water which is one of the conventional cleaning techniques. In the following Examples and Comparison Examples, the degree of the improvement of cleaning effect is evaluated by comparison with the results shown in Table 1. Example 1 A molded 0.5 ℓ-PFA bottle and a molded 1 ℓ-FEP bottle were internally cleaned once with ultrapure water in the same manner as in Comparison Example 1. The bottles were charged with an F₂-containing gas (15% F₂-85% N₂) and were retained under the following conditions. Time for which F₂-containing gas was retained (hr) Heating temperature (°C) Heating time (min) PFA, 0.5 ℓ 12 30 - PFA, 0.5 ℓ 4 250 30 FEP, 1 ℓ 4 200 30 FEP, 1 ℓ 4 200 30 The bottles were further cleaned with ultrapure water four times by the same procedure as in Comparison Example 1. Moreover, a 40% aqueous solution of NH₄F or a 50% aqueous solution of HF was used for cleaning the bottles in the same manner as in Comparison Example 1. Table 2 shows the results. The results as shown in Table 5 clearly indicate that the F₂-containing gas, when introduced into a container made of fluorine-containing resin, exhibits cleaning effect even at ordinary temperature. Example 2 A molded 0.5 ℓ-PEA bottle was cleaned with ultrapure water once by the same procedure as in Comparison Example 1. Thereafter the bottle was charged with an F₂-containing gas (15% F₂-85% N₂) and maintained under the following conditions. Time for which F₂- containing gas was retained (hr) Heating temperature (°C) Heating time (min) 12 30 - 4 75 30 4 100 30 4 125 30 4 150 30 4 200 30 4 250 30 Then the bottle was further cleaned with ultrapure water four times in the same manner as in Comparison Example 1. Moreover, a 40% aqueous solution of NH₄F was used for cleaning the bottle by the same procedure as in Comparison Example 1. Table 3 shows the results. The results shown in Table 3 indicate that a cleaning effect is recognized when the bottle made of fluorine-containing resin is filled with F₂-containing gas regardless of the temperature at which the bottle is maintained. This is because the particulate component present in the bottle reacts with the F₂-containing gas. Consequently, the particulate component is removed presumably by the following phenomena. (a) A fluorine-containing compound is obtained as the reaction product, is lowered in boiling point and thus evaporates off. (b) A reaction product high in solubility in water is obtained, markedly dissolved in ultrapure water and removed. (c) The reaction product is introduced into the polymer of fluorine-containing resin and integrated with the bottle. In Examples 1 and 2, use of a fluorine-containing gas such as ClF₃, NF₃ or the like in place of the F₂-containing gas produces substantially the same outstanding effect.

Claims: A method for removing the fine particles deposited on an article of fluorine-containing resin, the method comprising maintaining the article of fluorine-containing resin in the atmosphere of fluorine-type gas. A method as defined in claim 1 in which the fluorine-type gas is a type of gas capable of acting as a fluorinating agent or an oxidizing agent. A method as defined in claim 2 in which the fluorine-type gas is at least one gas selected from the group consisting of gases of F₂, ClF₅, ClF₃, ClF, BrF₅, BrF₃, BrF, ClO₃F, SF₄, O₂F₂ and NF₃, the gas being one diluted or not diluted with an inactive gas. A method as defined in claim 2 or 3 in which the gas is maintained at a temperature higher than room temperature. A method as defined in claim 1 in which the article is cleaned at least once with superpure water and/or a cleaning agent before or after, or both before and after maintaining the article in the atmosphere of fluorine-type gas.

Inventors: AOMI HIDEKI, ITANO MITSUSHI, SODA TOMIZO, AOMI, HIDEKI, ITANO, MITSUSHI, SODA, TOMIZO
Applicants: DAIKIN IND LTD, DAIKIN INDUSTRIES, LIMITED
Application year: 1990
Main classification: C08J7
IPC codes: C08J7, H01L21, B08B3, C11D7, B29C71, B01J19, B29K27