Patent Application
20050016829
1. Field of the Invention
The present invention relates to a distillation process for reducing the concentration of the impurities dinitrogen difluoride and dinitrogen tetrafluoride in nitrogen trifluoride. The process involves distilling the nitrogen trifluoride in the presence of a compound with a higher normal boiling point than nitrogen trifluoride and removing as a distillation column overhead stream nitrogen trifluoride substantially free of said impurities, removing as a distillation column sidedraw a concentrated stream comprising said impurities, and removing as a distillation bottom stream said compound with a higher normal boiling point than nitrogen trifluoride.
2. Description of Related Art
Various gaseous fluorine-containing compounds are utilized in manufacturing processes that plasma-etch silicon-type materials in order to fabricate semiconductor devices. A major use of nitrogen trifluoride (NF3) is as a “chemical vapor deposition” (CVD) chamber cleaning gas in semiconductor device manufacture. CVD chamber cleaning gases are used to form plasmas which interact with the internal surfaces of semiconductor fabrication equipment to remove the various undesirable deposits that accumulate over time.
Perfluorinated chemicals such as NF3 that are used in semiconductor manufacturing applications as cleaning gases are more commonly referred to as “electronic gases”. Electronic gases having high purity are critical for such applications. It is known that even very small amounts of impurities in these gases that enter semiconductor device manufacturing tools can result in wide line width and thus less information per device. Moreover, the presence of these impurities, including but not limited to particulates, metals, moisture, and other halocarbons in the plasma etchant or cleaning gases, even when only present in the part-per-million level, increases the defect rate in the production of these high-density integrated circuits. As a result, there has been increasing demand for higher purity etchant and cleaning gases, and an increasing market value for the materials having the required purity. Identification of impurities and methods for their removal consequently represents a significant aspect of preparing the fluorine-containing compounds for these applications.
NF3 prepared by a variety of methods, such as the method disclosed in U.S. Pat. No. 3,235,474, may contain relatively large amounts of impurities, such as nitrous oxide (N2O), carbon dioxide (CO2), dinitrogen difluoride (N2F2-cis and/or N2F2-trans) and dinitrogen tetrafluoride (N2F4). Dinitrogen difluoride and dinitrogen tetrafluoride are particularly undesirable impurities in a nitrogen trifluoride electronic gas product. Under certain conditions and at relatively low concentration, these compounds can form unstable and even explosive compositions. Therefore, in order to obtain high-purity nitrogen trifluoride that is free from dinitrogen difluoride and dinitrogen tetrafluoride for use as an electronic gas, methods allowing for complete removal of such impurities are necessary.
There are a variety of known methods that may be used to reduce N2F2, N2F4 and other impurities in a NF3 product, ranging from distillation, chemical and thermal treatments, adsorption on zeolites, silica gel, and activated alumina. Silica gel and activated alumina are known as both adsorbents for removing impurities at low temperature, and as reactants for removing impurities at higher temperatures.
The present invention provides distillation processes for reduring fluorinated impurities from NF3 so as to produce a purified NF3 product. More specifically, the present invention comprises a process for reducing the concentration of at least one impurity selected from the group consisting of dinitrogen difluoride and dinitrogen tetrafluoride in a first mixture comprising nitrogen trifluoride and said at least one impurity, said process comprising: (a) distilling said first mixture in a distillation column in the presence of a compound having a higher normal boiling point than nitrogen trifluoride; (b) removing a second mixture comprising said at least one impurity from said distillation column at a point between the top and the bottom of said distillation column; (c) removing a third mixture comprising said compound having a higher normal boiling point than nitrogen trifluoride from the bottom of said distillation column; and (d) removing a nitrogen trifluoride product having reduced concentration of said at least one impurity from the top of said distillation column.
NF3, in its separate and pure state, exhibits properties that are valued for integrated circuit manufacturing and is typically used in related manufacturing steps. The desire for greater precision and consistency of the effect it has during integrated circuit manufacture has made extremely high purities critical for such applications. The presence of any other compounds in the NF3 is objectionable for most of its intended uses. For example, even a 1 part-per-million-molar concentration of another compound would be considered an impurity in NF3 where NF3 is to be sold as a chemical vapor deposition (CVD) chamber cleaning gas. Processes that allow for manufacture of NF3 products having purities that approach 99.999 molar percent purity are desirable, but processes that provide at least 99.9999 molar percent purity for electronic gases applications are preferred.
Analytical methods for gauging such low concentrations of impurities in NF3 are available. For example, methods suitable for analyzing low concentrations of other compounds in NF3 are disclosed in the 1995 SEMI standards, pages 149-153, SEMI C3.39.91-Standard for Nitrogen Trifluoride, herein incorporated by reference.
Conventional processes for manufacturing NF3 often produce at least one of N2F2 and N2F4 as impurities in the NF3 product stream. By N2F2 is meant either the cis isomer (N2F2-cis, herein also referred to as N2F2-c) or the trans isomer (N2F2-trans, herein also referred to as N2F2-t). The presence of such compounds in the NF3 product is objectionable for most of its intended uses. The ability to separate and recover a NF3 product that is substantially-free of such impurities is of considerable value. Herein, “substantially-free” means that at least one of the impurities N2F2 and N2F4 are present in NF3 in less than 10 ppm-molar concentration, preferably less than 1 ppm-molar, most preferably less than 0.1 ppm-molar.
The physical properties of mixtures comprising NF3, N2F2 and N2F4 are non-ideal and are difficult to model by conventional modeling techniques. The present invention derives from the finding that when mixtures comprising NF3 and these impurities are distilled in the presence of another compound having a higher normal boiling point than NF3, the N2F2 and N2F4 are effectively concentrated within the lower distillation sections of a distillation column. That is to say, when mixtures comprising NF3 and these impurities are distilled in the presence of another compound having a higher normal boiling point than NF3, the N2F2 and N2F4 are effectively concentrated within the bottom 50% of the total number of theoretical stages in a distillation column, and more preferably, within the bottom 25% of the total number of theoretical stages in a distillation column.
By another compound having a higher normal boiling point than NF3 is meant a compound that has a normal boiling point of from about −90° C. to about −20° C. Further, it is preferred that the compound having a higher normal boiling point than NF3 does not react with NF3 or the fluorinated impurities under the present process conditions. Suitable compounds having a higher normal boiling point than NF3 of the present process include compounds from the classes: hydrocarbons, hydrofluorocarbons, hydrochlorofluorocarbons, hydrochlorocarbons, perfluorocarbons and organic oxides and inorganic oxides. Representative hydrocarbons include ethane, propane, and propylene. Representative hydrofluorocarbons include methyl fluoride (HFC-41), difluoromethane (HFC-32), 1,1,1-trifluoroethane (HFC-143a), pentafluoroethane (HFC-125), and fluoroethane (HFC-161). Representative hydrochlorofluorocarbons include chlorodifluoromethane (HCFC-22). Representative hydrochlorocarbons include methyl chloride (HCC-40). Representative perfluorocarbons include hexafluoroethane (PFC-116). Representative oxides include nitrous oxide (N2O), carbon dioxide (CO2), carbonyl fluoride (COF2), and perfluoroacetyl fluoride (CF3COF). Preferred compounds having a higher normal boiling point than NF3 of the present process include nitrous oxide (N2O), chlorodifluoromethane (HCFC-22), difluoromethane (HFC-32), fluoroethane (HFC-161) and methyl fluoride (HFC-41). These compounds may be used alone or in combination with each other as the high boiling compound in the present separation. For example, it is known that N2O forms azeotropic or azeotrope-like compositions with HFC-23. The respective N2O/HFC-23 azeotropic or azeotrope-like compositions may be employed as the compound having a higher normal boiling point than NF3 in the present separation.
More specifically, when NF3 and said impurities are distilled in the presence of the present compounds having a higher normal boiling point than NF3, the impurities N2F2 and N2F4 form a maximum concentration at a point within the distillation columns packed or trayed sections rather than in either the distillation column's bottoms or overhead streams. As such, withdrawing a sidedraw from a point between the top and bottom of the distillation column provides a stream higher in concentration of N2F2 and N2F4 than in the distillation bottoms or overhead, which facilitates removal of said at least one of N2F2 and N2F4 from the NF3 product.
From the standpoint of reducing said impurities from an NF3 product, it is more economical to reduce said impurities from a stream containing a “higher” concentration rather than from a stream having said impurities at a “lower” concentration. By removing and treating a sidestream in which these impurities have been concentrated, known thermal and adsorptive purification processes for separating N2F2 and N2F4 from NF3 can be applied to smaller volumes of process material and the associated equipment can in turn be smaller and will operate more efficiently. In addition, larger amounts of NF3 substantially free of impurities may be obtained as a product of the distillation.
Such a sidedraw also provides a method of limiting or reducing the maximum concentration of these impurities developed within the distillation column. N2F2 and N2F4 are thought to be more reactive than NF3, and under certain conditions and at relatively low concentration, N2F2 and N2F4 compounds can form unstable and even explosive compositions and it may be desirable to limit their intracolumn concentration for purposes of materials compatibility or process safety. For example, it is generally preferred that the concentration of these compounds be limited to below 2,000 ppm-molar, more preferably to less than 1,000 ppm-molar intracolumn concentration each of N2F2 and N2F4.
Several of the compounds that are produced by processes that produce NF3 are higher boiling than NF3. Examples of such compounds include hexafluoroethane (C2F6 or PFC-116), sulfur hexafluoride (SF6), carbon dioxide (CO2). In addition, compounds higher boiling than NF3 may be added as extractants in the distillation of a stream comprising NF3 and CF4 to enable the separation of the NF3 and CF4. Examples of such higher boiling compounds suitable as extractants are disclosed in U.S. Pat. No. 6,458,249, herein incorporated by reference, and include chlorodifluoromethane (CHClF2 or HCFC-22), difluoromethane (CH2F2 or HFC-32), ethane (C2H6), fluoroethane (C2H5F or HFC-161), methylfluoride (CH3F or HFC-41), pentafluoroethane (C2HF5 or HFC-125) and nitrous oxide (N2O).
Said invention may be practiced in a distillation column run under a variety of temperatures and pressures. The temperatures at the top of the distillation column should be sufficient to allow condensation of the NF3 at the pressures the column is operated, but it is preferred that the lowest temperature in the column be above the freezing point of said compounds higher boiler than NF3. For example, temperatures at the top of the column ranging from −90° C. to −50° C. are preferred, but lower temperatures at the top of the column may be employed dependent on the higher boiling compounds that may be present.
The present invention is conveniently practiced in combination with the process disclosed in U.S. Pat. No. 6,458,249 for removing CF4 from NF3 by use of an extractant agent in a distillation process. In said process a stream comprising NF3, CF4 and at least one of N2F2 and N2F4 can be fed to a first column where said stream is distilled in the presence of an extractant agent. In the presence of said extractant agent, the CF4 is separated from the NF3, with the CF4 removed in the column distillate. A bottoms streams comprising NF3, extractant agent, and at least one of N2F2 and N2F4 is then sent to a second distillation column. In the second distillation column, in the presence of said extractant agent, said at least one of N2F2 and N2F4 will exhibit a concentration maxiumum at an intermediate point within that column and can be removed from the distillation column in a sidedraw stream while NF3 is recovered as an overhead product. This sidedraw stream may be optionally treated to remove said at least one of N2F2 and N2F4 and the treated stream may then be optionally returned to the distillation train. The extractant is removed from the second column as a bottoms stream, and is optionally recycled back to the extraction column. If other compounds are present that exit the second column bottoms with the extractant agent, some portion of this bottoms stream may either be purged or optionally treated to remove these other high boilers, with the treated bottoms stream then optionally return to the distillation train.
A stream comprising HCFC-22, NF3 substantially-free of PFC-14, and said at least one of N2F2 and N2F4 is then removed from the column 11 bottoms via conduit 17 and sent to cooler 18 and from there fed to distillation column 19. The distillate from column 19 may be fed via conduit 20 to condenser 21. From condenser 21, some amount of condensed distillate may be returned to the column 19 as reflux via conduit 22, while the remainder recovered as product, e.g., as NF3 substantially-free of PFC-14, extractive entraining agent, and at least one of N2F2 and N2F4 via conduit 23. A stream comprising at least one of N2F2 and N2F4 is removed from the column 19 as a sidedraw 24 from the column. Extractive entraining agent, e.g., HCFC-22, with the concentration of non-HCFC-22 compounds reduced compared to their concentrations in stream 17 is obtained as the distillation column bottoms 25. Stream 25 may optionally be fed to cooler 26 and then returned to distillation column 11 as extractant feed, fed to the column at a feed point higher up the column than that feed point of the first mixture to be separated, e.g., PFC-14 and NF3.
The present invention is optionally practiced in combination with known processes for removing N2F2 from a stream comprising NF3. For example, a side-draw stream comprising N2F2 can be treated by passing the side-draw stream through a heated zone consisting of particulate metal, or packed with solid fluorides to decompose and thus remove said N2F2, methods such as those disclosed in U.S. Pat. No. 4,193,976 and U.S. Pat. No. 5,183,647. The treated side-draw stream may then be optionally returned to the distillation train. The current invention provides an advantage over the prior art, in that the N2F2 is concentrated into a small stream which increases the efficiency and economy of the previously disclosed processes.
The present invention may be practiced on a stream comprising N2F2 and N2F4 where said impurities occur over a range of initial concentrations. For example, said invention may be practiced on a first mixture where N2F2 and N2F4 are present in concentrations as high as 10,000 ppm-molar. However, it is preferred that the N2F2 and N2F4 concentrations in the first mixture be less than 5,000 ppm-molar, more preferably less than 2,000 ppm-molar. Where the initial concentrations of N2F2 are higher, said invention is conveniently optionally practiced in combination with known processes for removing N2F2 from a stream comprising NF3. For example, a first mixture comprising NF3 and N2F2 can be treated by passing the side-draw stream through a heated zone consisting of particulate metal, or packed with solid fluorides to decompose and thus reduce the concentration of said N2F2 prior to practicing the present invention.
To determine the relative volatility of any given two compounds in a distillation, the PTx Method is preferred. In this procedure, the total absolute pressure in a cell of known volume is measured at a constant temperature for various compositions of the two compounds. Use of the PTx Method is described in greater detail in “Phase Equilibrium in Process Design”, Wiley-Interscience Publisher, 1970, written by Harold R. Null, on pages 124 to 126; the entire disclosure of which is hereby incorporated by reference.
These measurements can be converted into equilibrium vapor and liquid compositions in the PTx cell by using an activity coefficient equation model, such as the Non-Random, Two-Liquid (NRTL) equation, to represent liquid phase nonidealities. Use of an activity coefficient equation, such as the NRTL equation is described in greater detail in “The Properties of Gases and Liquids,” 4th edition, published McGraw Hill, written by Reid, Prausnitz and Poling, on pages 241 and 387, and in “Phase Equilibria in Chemical Engineering,” published by Butterworth Publishers, 1985, written by Stanley M. Walas, pages 165 to 244; the entire disclosure of each of the previously identified references are hereby incorporated by reference.
Without wishing to be bound by any theory or explanation, it is believed that the NRTL equation, together with the PTx cell data, can sufficiently predict whether or not combinations of compounds described herein behave in an ideal manner, and can sufficiently predict the relative volatilities of the components in such mixtures during a distillation.
The following examples are provided to illustrate certain aspects of the present invention, and are not intended to limit the scope of the invention. The following examples employ the NRTL equations identified above. In the following examples, each stage is based upon a 100% operational or performance efficiency. The total stages include condenser and reboiler, with the condenser counted as stage No. 1, and the reboiler as the highest numbered stage. In the following examples, flow rates are given in pounds(weight)-per-hour(pph); temperatures are expressed in degrees Celsius (° C.); pressures are expressed in pound-per-square-inch-absolute (psia); stream concentrations are expressed in mole percent (mole %) or parts-per-billion-by-moles (ppbm or ppb-molar).
In this comparative example, a crude NF3 feed stream comprising NF3 and N2F2-c, N2F2-t, and N2F4 as impurities is fed to a distillation column operated under the conditions shown in Table 1. The concentrations of the N2F2-c, N2F2-t and N2F4 in the feed stream are 1,000 ppm-molar each. The column in this comparative example is operated to remove the NF3 from the column as an overhead distillate, while recovering the impurities as column bottoms. The results of this distillation are shown in Table 1.
Under the conditions shown in Table 1, the point of maximum concentration of each of the N2F2-cis, N2F2-trans, and N2F4 are at around stages 41 and 42, or more specifically at around the reboiler or bottoms draw of the distillation column. Increasing the bottoms takeoff rate and changing the reboiler temperatures does not change this point of maximum concentration.
In this comparative example, a crude NF3 feed stream comprising NF3 and hexafluoroethane (PFC-116), and N2F2-c, N2F2-t, and N2F4 as impurities, is fed to a distillation column operated under the conditions shown in Table 2. The concentrations of the N2F2-c, N2F2-t and N2F4 in the feed stream are 1,000 ppm-molar each. The column in this comparative example is operated to remove the NF3 from the column as an overhead distillate, while recovering the impurities as column bottoms. In this comparative example, there is no material removed as a sidedraw stream. The results of this distillation are shown in Table 2.
In contrast to comparative example 1, the presence of PFC-116 in this comparative example causes the point of maximum concentration for each of N2F2-c, N2F2-t and N2F4 to occur at higher points in the distillation column. Without a sidedraw stream, the column concentration of each increase significantly, up to 42.1 mole % in one case, before these impurities appear in the bottom stream and can be removed from the column.
This example is identical to comparative example 1, with the exception that a 4 pph sidedraw is removed from the distillation column at stage 35. The results of this distillation are shown in Table 3.
This example shows how a sidedraw stream may be used to remove N2F2-t, N2F2-c and N2F4 from the column, and so reduce the concentration of each developed. The points of maximum concentration of each is still at a higher point than in comparative example 1, but the maximum concentrations of N2F2-c and N2F2-t are considerably reduced compared to comparative example 2.
In this comparative example, a crude NF3 feed stream comprising NF3 and hexafluoroethane (PFC-116) and N2F2-c, N2F2-t, and N2F4 as impurities is fed to a distillation column operated under the conditions shown in Table 4. The concentrations of the N2F2-c, N2F2-t and N2F4 in the feed stream are 1000 ppm-molar each. The feed rate of PFC-116 is 5000 pph. The column in this example is operated to remove the NF3 from the column as an overhead distillate, while recovering the impurities as column bottoms. In this comparative example, there is no material removed as a sidedraw stream. The results of this distillation are shown in Table 4.
In this comparative example, the feed rate of PFC-116 relative to that of NF3 in the crude feed stream is comparable to what would appear in a stripper column having no side draw, said stripper column following an extractive distillation column using 116 as the extractant. The increase in PFC-116 fed versus comparative example 2 causes the points of maximum concentration of each of the N2F2-c, N2F2-t and N2F4 to shift even further up the distillation column, and causes a substantial increase in the intracolumn concentration maximum of N2F2-c and N2F2-t versus comparative example 2. Combined, these cause a significant increase in the concentration of each of N2F2-c, N2F2-t and N2F4 in the NF3 product in the column distillate.
This Example is identical to comparative example 3, except that there is now a 10 pph sided raw and the number of stages has been increased from 42 to 62. The results of this distillation may be seen in Table 5.
This example shows how a sidedraw stream may be used to remove N2F2-t, N2F2-c and N2F4 from the column, and so reduce the concentration of each developed. The combination of increased column stages and sidedraw also allow NF3 substantially free of impurities to be obtained as column distillate.
In this comparative example, a crude NF3 feed stream comprising NF3 and N2F2-c, N2F2-t, and N2F4 as impurities is fed to a distillation column operated under the conditions shown in Table 6. The concentrations of the N2F2-c, N2F2-t and N2F4 in the feed stream are 1,000 ppm-molar each. In case number 15, no additional high boiling compound is fed. In case numbers 16, 17, 18, 19 and 20 the additional high boilers PFC-116, HCFC-22, HFC-23, HFC-32 and HFC-125 respectively are also fed to the column in the crude NF3 feed steam. The column in this comparative example is operated to remove the NF3 from the column as an overhead distillate, while recovering the impurities as column bottoms. In this comparative example, there is no material removed as a sidedraw stream. The results of this distillation are shown in Table 6.
In contrast to case number 15 of table 6, where no additional high boiler is fed, the presence of the high boilers is case numbers 16 through 20 of table 6 cause the point of maximum concentration for each of N2F2-c, N2F2-t and N2F4 to occur at higher points in the distillation column.
This example is identical to case numbers 16 through 20 of comparative example 4, except that there is now a 4 pph sidedraw. The results of this distillation may be seen in Table 7.
This example shows how a sidedraw stream may be used to remove N2F2-t, N2F2-c and N2F4 from the column, and so reduce the maximum concentration of each developed. The points of maximum concentration of each impurity in cases 16 through 25 where a high boiler is added is still at a higher point than in case number 15 where no high boiler is added, but the maximum concentrations of N2F2-c and N2F2-t are considerably reduced compared to each high boilers respective case in comparative example 4.
This application claims the priority benefit of U.S. provisional patent application 60/462,756, filed Apr. 14, 2003.
| Number | Date | Country | |
|---|---|---|---|
| 60462756 | Apr 2003 | US |
