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 maintaining the article in the atmosphere of fluorine-type gas.

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. As prior art there is known JP-A-63-310683 which discloses a method of cleaning an article of fluorine-containing resin, the method comprising the steps of introducing steam into a washing container, maintaining the container at a high temperature, a high pressure and a high humidity, reducing the internal pressure of the container and rapidly cooling the article by showering water over the article. However, the publication sets forth nowhere that fine particles can be removed from the article of fluorine-containing resin to a satisfactory cleaning level by maintaining the article at a high temperature in the atmosphere of inactive gas. J. Colloid and Interface Science 47 (1974) 650 describes a method of elucidating the nature of the surface of articles made of polymers, and states on page 655 a method wherein an etched sample is exposed to 200°C in a circulating air oven in one of post-treatments of the etched sample in determining the difference between the surface of a film sample of PTFE or FEP and the etched surface thereof. However, the publication merely sets out a method wherein an article of a polymer is heated to a high temperature in one of procedures taken in the course of examining the properties of the surface of the polymer article, but teaches nowhere any means of removing fine particles from an article of fluorine-containing resin to a high cleaning level. 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. 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 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 at a temperature of not lower than 150°C to a temperature lower than the melting point of the article in the atmosphere of inactive gas. (2) a method as defined above in item (1) in which the article is cleaned at least once with at least one of ultrapure water,a cleaning agent and an aqueous solution of electrolyte at a high concentration (not lower than 0,5 % by weight) before or after, or both before and after, conducting the procedure as defined in item (1). The method of the invention essentially comprises subjecting an article of fluorine-containing resin to a method as defined above. The procedure will be described below in greater detail. In this procedure, an article of fluorine-containing resin is maintained at a high temperature ranging from 150°C to a temperature lower than the melting point of the article, preferably about 150 to about 260°C, to vaporize or scatter away the particles. The vaporization and scattering of particles can be beneficially accelerated by conducting an additional procedure such as rotation of article, agitation of atmosphere gas, or spray of atmosphere gas. When the article is heated in air, a reaction product of particles and oxygen may deposit on the article at a high temperature, and the fluorine-containing resin may become impaired in properties. To avoid this possibility, the article is placed into a heating oven and heated in the atmosphere of inactive gas. The longer the retention time at a high temperature, the higher the degree of cleaning is but with economical disadvantage. The retention time is not specifically limited and can be suitably determined for example depending on the degree of contamination of article or the required level of cleaning. Usually it is about 10 to about 30 minutes. The procedure may be executed 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, NH 4 F, HCl, HNO 3 , H 2 O 2 , H 2 SO 4 , H 3 PO 4 and NH 3 ; 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 said method. Examples of useful electrolytes are NH 4 Cl, NH 4 F, KCl, KF, NaCl, MgCl 2 , MgCO 3 , ZnCl 2 , ZnSO 4 , FeCl 3 , AlCl 3 and Al 2 (SO 4 ) 3 . The method of the present invention is 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.

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 at a temperature of not lower than 150°C to a temperature lower than the melting point of the article in the atmosphere of inactive gas. Method as defined in claim 1 in which the article is cleaned at least once with at least one of ultrapure water, a cleaning agent and an aqueous solution of electrolyte at a concentration of not lower than 0.5 % by weight, before or after, or both before and after maintaining the article of fluorine-containing resin at a temperature of not lower than 150°C to a temperature lower than the melting point of the article in the atmosphere of inactive 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