Cleaning compositions for solid surfaces – auxiliary compositions – Cleaning compositions or processes of preparing – Liquid composition
Reexamination Certificate
1997-04-17
2001-04-24
Gupta, Yogendra (Department: 1751)
Cleaning compositions for solid surfaces, auxiliary compositions
Cleaning compositions or processes of preparing
Liquid composition
C203S050000, C203S067000, C423S488000, C570S178000, C510S408000
Reexamination Certificate
active
06221830
ABSTRACT:
FIELD OF THE INVENTION
The instant invention relates to the field of removing impurities from hexafluoroethane (CF
3
CF
3
), also known as Perfluorocarbon 116 (PFC-116) or Fluorocarbon 116 (FC-116), by using azeotropic distillation such that an overhead product consisting essentially of HCl-hexafluoroethane is formed, optionally combined with a phase separation step to break the HCl-hexafluoroethane azeotropic or azeotrope-like composition thereby permitting recovery of substantially pure hexafluoroethane. Unreacted hydrogen fluoride (HF) may be removed from hexafluoroethane during the above azeotropic distillation with HCl or alternatively by an azeotropic distillation wherein an HF-hexafluoroethane azeotropic or azeotrope-like composition exits overhead and substantially pure HF exits in the bottoms stream.
BACKGROUND OF THE INVENTION
Conventional methods for manufacturing hexafluoroethane typically result in undesired impurities. Hexafluoroethane can be manufactured by fluorinating at least one of trichlorotrifluoroethane, dichlorotetrafluoroethane and/or chloropentafluoroethane. This hexafluoroethane manufacturing method often produces a product stream containing significant amounts of fluorocarbon and acid impurities which are difficult to remove by conventional distillation techniques.
Various gaseous fluorine-containing compounds are utilized to plasma etch silicon-type materials in order to fabricate semiconductor devices, e.g., A. J. Woytek, J. Fluor. Chem. 33, 331-334 (1986); the disclosure of which is hereby incorporated by reference. A major use of hexafluoroethane is as a plasma etchant in semiconductor device fabrication. It interacts with the surface of the integrated circuit wafer, modifying it so as to lay down the electrical pathways and providing for the surface functionalities that define the integrated circuit. As manufacturers are continually trying to increase the number of functionalities packed per unit surface area, the increasing fineness of surface detail in turn requires greater precision and consistency of the effect the etchant has on the wafer substrate. Products of high purity are critical for this application. It has been found that even very small amounts of impurities can result in wide line width and thus less information bits per chip. Moreover, the presence of these impurities, including but not limited to particulates, metals, moisture, and other halocarbons in the plasma etchant, even when present only in the part per million level, increases the defect rate in the production of these higher density integrated circuits. As a result there has been continually increasing market demand for higher and higher purity etchants, and an increasing market value for materials having the required purity. Consequently, identification of the offending impurities and their removal represents a significant aspect of preparing the fluorine-containing compounds for these applications.
SUMMARY OF THE INVENTION
The instant invention solves the problems associated with conventional hexafluoroethane (FC-116) manufacturing methods by providing an azeotropic distillation method for purifying FC-116. As a result, the instant invention provides a high purity FC-116 product that is needed as an etchant in the electronics industry.
In one aspect of the invention, we have found that at least one of chlorotrifluoromethane (CFC-13), trifluoromethane (HFC-23), chlorodifluoromethane (HCFC-22), chloropentafluoroethane (CFC-115), pentafluoroethane (HFC-125), difluoromethane (HFC-32), 1,1,1-trifluoroethane (HFC-143a), 1,1-difluoroethane (HFC-152a), and HF, among others, can be removed from hexafluoroethane by being distilled in the presence of anhydrous HCl; usually, in the presence of an amount of HCl that is sufficient to form an azeotropic or azeotrope-like composition with all of the hexafluoroethane. The instant invention provides a process whereby an HCl-hexafluoroethane azeotropic or azeotrope-like composition, which is substantially free of impurities such as chlorotrifluoromethane, trifluoromethane, chlorodifluoromethane, chloropentafluoroethane, pentafluoroethane, difluoromethane, 1,1,1-trifluoroethane, 1,1-difluoroethane and HF, can be removed as the overhead stream from a distillation column. By “substantially free” of impurities or “substantially pure”, it is meant that the stream contains less than about 1.0 wt %, normally less than 0.1 wt %, and most often less than about 10 ppm of undesired impurities, e.g., the instant invention can produce at least about 99.9999 wt % pure PFC-116. These impurities and/or their azeotropes with HCl or HF can be removed from the bottom of the distillation column. This invention can consequently produce PFC-116 that is greater than 99.999% wt. PFC-116 based on the weight of all components contained within, e.g., 99.9999 wt % pure PFC-116 or containing less than about 1 ppm of undesired impurities.
Another aspect of the invention provides a process for breaking the HCl-hexafluoroethane azeotropic or azeotrope-like compositions into their individual components by liquefying and cooling the recovered azeotrope composition, and allowing the cooled composition to separate into HCl-rich and hexafluoroethane-rich layers within a decanter. The latter layer may then be purified by, for example, by azeotropic distillation thereby yielding substantially pure hexafluoroethane. Optionally, the HCl-rich layer may be purified, for example, by azeotropic distillation to produce substantially pure anhydrous HCl.
In another aspect, the invention comprises i) removing a portion of the trifluoromethane (HFC-23) along with the HCl-hexafluoroethane azeotropic or azeotrope-like composition from a first column's overhead stream wherein the remainder of the HFC-23 (with the other impurities) exits in the column's bottom stream, ii), separating the overhead stream within a decanter into HCl-rich and hexafluoroethane-rich layers as previously described, and iii) distilling the hexafluoroethane-rich layer in a second distillation column and recycling a HCl-hexafluoroethane azeotrope composition, which exits from the second distillation column and now contains a portion of the original trifluoromethane, to the first distillation column. The hexafluoroethane exiting from the bottoms of the second distillation column will now be substantially free of trifluoromethane and HCl as well as the other impurities.
In another case, the hexafluoroethane may contain quantities of HCl and optionally at least one of CFC-13, CFC-115, HFC-23, among others. By operating a distillation column under conditions that cause formation of azeotropic or azeotrope-like compositions consisting essentially of HCl and one or more of PFC-116, CFC-13, CFC-115 and HFC-23, such compositions can be removed from the distillation column as an overhead product thereby purifying the hexafluoroethane. The hexafluoroethane can, if desired, be purified further by using the aforementioned methods.
In another case, the hexafluoroethane may contain quantities of HF and optionally one or more of CFC-115, CFC-114, CFC-114a, CFC-113, CFC-113a, CFC-13, HCFC-22, HFC-143a, HFC-125 among others. By operating a distillation column under conditions that cause formation of azeotropic or azeotrope-like composition consisting essentially of HF and one or more of PFC-116, CFC-115, CFC-114, CFC-114a, CFC-113, CFC-113a, CFC-13, HCFC-22, HFC-143a and HFC-125, such azeotropes can be removed thereby purifying the hexafluoroethane. The hexafluoroethane can, if desired, be purified further by using the aforementioned methods.
In still another case, the hexafluoroethane may contain quantities of at least one of HFC-23, CFC-13, among others. By operating a distillation column under conditions that cause formation of azeotropic or azeotrope-like compositions consisting essentially of FC-116 and one or more of HFC-23 and CFC-13, such azeotropes can be removed as an overhead product thereby purifying the hexafluoroethane. The remaining hexafluoroethane can, if desired, be purified by using the aforementioned m
Deschere Mark Richard
Mahler Barry Asher
Miller Ralph Newton
Muthu Olagappan
E. I. Du Pont de Nemours and Company
Gupta Yogendra
Shipley James E.
Webb Gregory E.
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