Perfluorocompound separation and purification method and system

Provided is a novel method and system for separating and purifying perfluorocompounds (PFCs). The method comprises the steps of: (a) introducing a perfluorocompound-containing gas stream into a first distillation column; (b) removing a light product from the first column, and removing a heavy product from the first column; (c) introducing the first column light product into a second distillation column; (d) removing a light product from the second column, and removing a heavy product from the second column; (e) introducing the second column light product into a third distillation column; and (f) removing a light product from the third column, and removing a heavy product from the third column. The method and system can be advantageously used in the treatment of exhaust gases from semiconductor processing tools, and results in highly purified PFCs which can be recycled, thereby avoiding the release of PFCs into the atmosphere.

CROSS-REFERENCE TO RELATED APPLICATIONS 
This application is related to assignee's copending application Ser. No. 
08/783,941, attorney docket no. Serie 4030-CIP, filed on even date 
herewith, which is herein incorporated by reference. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for separating and purifying 
perfluorocompounds. The invention also relates to a system for separating 
and purifying perfluorocompounds. The inventive method and system have 
particular applicability in semiconductor manufacturing, for example, in 
the treatment of an exhaust gas from a semiconductor processing tool. 
2. Description of the Related Art 
In the semiconductor manufacturing industry, extensive use is made of 
perfluorocompounds (PFCs). For example, PFCs are required in various 
etching processes, such as oxide, metal and dielectric etching steps. In 
such processes, a gas or a plasma atmosphere selectively removes portions 
of a layer deposited on the substrate. Perfluorocompounds are also 
employed in deposition processes, such as silicon chemical vapor 
deposition (CVD), as well as in the cleaning of semiconductor processing 
chambers. 
Perfluorocompound gases used in the above-mentioned processes include, for 
example, carbon tetrafluoride (CF.sub.4), hexafluoroethane (C.sub.2 
F.sub.6), perafluoropropane (C.sub.3 F.sub.8), trifluoromethane 
(CHF.sub.3), sulfur hexafluoride (SF.sub.6), nitrogen trifluoride 
(NF.sub.3) and carbonyl fluoride (COF.sub.2). Such gases can be used in 
either a pure or diluted form. Common carrier gases include air and inert 
gases, such as N.sub.2, Ar, He and mixtures thereof. Perfluorocompounds 
can also be used in a mixture with other PFC gases. 
When used in etching and cleaning processes, the PFCs generally do not 
completely react. As a result, unreacted PFCs may be present in the 
exhaust from the processing tool. 
In addition to the substantial cost associated with the purchase of PFCS, 
it is well known and documented that PFCs are environmentally detrimental 
upon release into the atmosphere. In the Global Warming Symposium, Jun. 
7-8, 1994, Dallas, Tex., CF.sub.4, C.sub.2 F.sub.6, NF.sub.3 and SF.sub.6 
were identified as being greenhouse gases of particular concern in the 
semiconductor manufacturing industry. 
In addition to replacing PFCs with other, less damaging materials, several 
methods for reducing the extent of PFC release into the atmosphere are 
known or are under development. For example, chemical-thermal 
decomposition of PFCs using various activated metals has been proposed. 
However, the spent bed materials must be disposed of, which itself can 
prove to be environmentally hazardous. 
In the combustion-based decomposition process, i.e., chemical-thermal 
process, a flame supplies both the thermal energy and the reactants for 
decomposition of the PFCs. There are, however, some safety issues 
associated with the use of H.sub.2 and natural gas fuels. Furthermore, 
assuming a sufficiently high temperature, all of the PFCs treated by this 
process will produce hydrofluoric acid (HF) as a combustion product. The 
emissions of HF are also of great concern and must themselves be treated. 
Furthermore, combustion processes undesirably produce NO.sub.x, and 
CO.sub.2. 
Plasma-based decomposition has also been proposed as a method for treating 
PFCs. This process involves the generation of a plasma by, for example, an 
RF coupled system to partially decompose C.sub.2 F.sub.6. While 90% 
decomposition of C.sub.2 F.sub.6 is attainable, such systems are not yet 
commercially proven. Moreover, this decomposition process results in the 
generation of HF. 
Methods in which PFCs are recovered, as opposed to being destroyed, are 
considered to be the most environmentally sound, since the PFCs can be 
reused. Such methods, therefore, are of great interest. 
Perfluorocompound recovery based on combinations of adsorption or low 
temperature trapping has been proposed. These adsorption processes pose 
several problems, such as dealing with large amounts of N.sub.2 associated 
with vacuum pump operation, the closeness in boiling points of CF.sub.4 
and NF.sub.3, the mixing of various process streams, and the potential for 
reaction between the PFCs and the adsorbents. 
In the article PFC Concentration and Recycle, presented at the Global 
Warming Symposium, the advantages of recovery processes which avoid the 
production of CO.sub.2, NO.sub.x and HF are acknowledged. A process is 
disclosed which uses a dual bed adsorber with activated carbon. The PFCs 
are adsorbed on the carbon sieves while the "carrier" gas, e.g., N.sub.2, 
H.sub.2, is not adsorbed. One of the issues not yet resolved with such a 
system is that CF.sub.4, which is non-polar, is not readily adsorbed by 
the carbon sieve. Moreover, a PFC purity higher than that achieved with 
such an adsorption unit is desired for reuse. 
To meet the requirements of the semiconductor manufacturing industry and to 
overcome the disadvantages of the related art, it is an object of the 
present invention to provide a novel method for separating and purifying a 
mixture of perfluorocompounds, and in particular for treating an exhaust 
stream from a semiconductor processing tool. The product purities achieved 
according to the inventive process are such that the PFC products can be 
recycled. Consequently, the release of PFCs into the atmosphere and the 
environmental damage associated therewith can be avoided. Furthermore, the 
recovered PFCs can be recycled to the processing tool, which can result in 
substantial savings since lesser volumes of new materials would be 
required. The purified product can also be recycled to the purification 
system itself, which allows for control of the incoming gas composition as 
well as facilitating stable and reliable operation. 
It is a further object of the present invention to provide a system for 
practicing the inventive method for separating and purifying 
perfluorocompounds, and in particular for treating an exhaust stream from 
a semiconductor processing tool. 
Other objects and aspects of the present invention will become apparent to 
one of ordinary skill in the art upon review of the specification, 
drawings and claims appended hereto. 
SUMMARY OF THE INVENTION 
The foregoing objectives are met by the method and system of the present 
invention. According to a first aspect of the present invention, a novel 
method for recovering and purifying perfluorocompounds is provided. The 
method comprises the steps of: 
(a) introducing a perfluorocompound-containing gas stream into a first 
distillation column; 
(b) removing a light product from the first column, and removing a heavy 
product from the first column; 
(c) introducing the first column light product into a second distillation 
column; 
(d) removing a light product from the second column, and removing a heavy 
product from the second column; 
(e) introducing the second column light product into a third distillation 
column; and 
(f) removing a light product from the third column, and removing a heavy 
product from the third column. 
According to a second aspect of the invention, a system for recovering and 
purifying perfluorocompounds is provided. The system comprises: 
(a) a first distillation column and a line connected to the first column 
for introducing a perfluorocompound-containing stream thereto, and a line 
for removing a heavy product from the first column; 
(b) a second distillation column and a line connecting the first column 
with the second column for conveying a light product from the first column 
to the second column, and a line for removing a heavy product from the 
second column; and 
(c) a third distillation column and a line connecting the second column 
with the third column for conveying a light product from the second column 
to the third column, a line for removing a heavy product from the third 
column, and a line for removing a light product from the third column.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
It has now surprisingly and unexpectedly been determined that 
perfluorocompounds (PFCs) present in an effluent gas stream, for example, 
from one or more semiconductor processing tools, can be recovered and 
purified in an effective manner. In the perfluorocompound purification 
process according to the invention, the exhaust from a semiconductor 
processing tool is separated into various components and purified. 
According to a preferred purification process, the end products include a 
pure CF.sub.4 stream (less than about 10 ppm impurities), pure C.sub.2 
F.sub.6 (less than about 10 ppm impurities), a pure N.sub.2 offgas stream 
in the ppm or sub-ppm range, and an SF.sub.6 waste stream. 
As used herein, the terms "perfluorocompound" and "PFC" are used 
interchangeably, and are defined as compounds comprising C, S and/or N 
atoms wherein all or all but one hydrogen have been replaced by fluorine. 
The most common PFCs include, but are not limited to, any of the following 
compounds: fully fluorinated hydrocarbons such as CF.sub.4, 
C.sub.2,F.sub.6, C.sub.3 F.sub.8, C.sub.4 F.sub.10, and other fluorinated 
compounds such as CHF.sub.3, SF.sub.6, and NF.sub.3. PFCs may also include 
BF.sub.3, COF.sub.2, F.sub.2, HF, SiF.sub.4, WF.sub.6, and WOF.sub.4. Perf 
luorocompounds, however, do not include chlorofluorocarbons, or compounds 
comprising two hydrogen substituents or more. 
Also as used herein, the term "heavy product" refers to a stream removed 
from a portion of the distillation column below a feed stage, which is not 
returned to the column. The heavy product can be in a gaseous and/or a 
liquid state, and is preferably removed from the bottom of the column. 
Also as used herein, the term "light product" refers to a stream removed 
from a portion of the distillation column above a feed stage, which is not 
returned to the column as reflux. The light product can be in a gaseous 
and/or a liquid state, and is preferably removed from the top of the 
column. 
The method and system of the invention will now be described generally with 
reference to FIG. 1, which illustrates a process flow according to a first 
embodiment of the invention. 
With reference to FIG. 1, the process begins with an 10 exhaust gas mixture 
from a semiconductor processing tool, which may be any type of tool which 
uses or generates PFCs. The exhaust gas mixture, containing PFCs, carrier 
gases and any other process gases, is removed from the processing tool 
through an exhaust line. Prior to being introduced into the gas 
purification system, the gas mixture is preferably passed through a 
filter, and then compressed in a compressor. The compressed gas mixture is 
then optionally routed to a cooler or a heater to provide a desired 
temperature for the compressed gas mixture. The gas mixture is next 
preferably introduced into a dry scrubber and/or a wet scrubber to remove 
silicon hydrides, e.g., NH.sub.3 and AsH.sub.3, tetraethoxysilane (TEOS), 
halogens and halides. The exhaust stream can next be filtered to remove 
dust, particles, droplets, and the like, having sizes greater than, for 
example, 20 .mu.m. Additionally, particles and dust may be removed in a 
filter upstream from the scrubber. 
The exhaust stream is preferably passed through one or more systems to 
recover a majority of the PFCs while rejecting a majority of the carrier 
gases. Examples of suitable PFC recovery units are described in copending 
application Ser. No. 08/783,941, attorney docket no. Serie 4030-CIP, filed 
on even date herewith. As described in the copending application, the 
exhaust gas can be sent to a membrane unit through which the carrier gases 
of the mixture permeate, and are recovered or vented as a waste gas which 
can be purified and/or recycled according to known means. A concentrated 
PFC feed stream flows from the non-permeate side of the membrane unit. 
This concentrated PFC feed stream is then introduced into a purification 
system, which produces purified product streams and waste streams. The PFC 
feed stream can be introduced to the purification system directly from the 
semiconductor processing tool or the recovery unit, or from a gas storage 
medium such as a cylinder, a bulk storage tank, or a tube trailer. 
The inventive method and system are not limited in any way by the existence 
of any specific type of upstream system. Nor are the method and system 
limited to the treatment of any specific PFCs or PFC mixture. For purposes 
of discussion, a breakdown of the PFCs present in a typical PFC feed 
stream is as follows: 
______________________________________ 
C.sub.2 F.sub.6 
61.0 mol % 
CF.sub.4 30.0 mol % 
SF.sub.6 2.0 mol % 
NF.sub.3 1.5 mol % 
CHF.sub.3 0.5 mol % 
N.sub.2 5.0 mol % 
______________________________________ 
With reference to FIG. 2, which shows the purification system in greater 
detail, concentrated PFC feed stream 1 from the recovery unit is 
compressed to a pressure lower than about 30 bar, preferably in the range 
of from about 5 to 15 bar, and more preferably from about 7 to 12 bar, and 
is cooled to a temperature in the range of from about -120.degree. to 
-30.degree. C., preferably from about -30.degree. to -60.degree. C., by, 
for example, a heat exchanger 24. 
PFC feed stream 1 is then fed to one or more cold adsorption units 2 in 
which any existing impurities in the form of, for example, CHF.sub.3, 
C.sub.2 F.sub.4, and NF.sub.3, are removed through cold adsorption. The 
non-adsorbed gas species in the effluent 3 from cold adsorption units 2 
include, for example, SF.sub.6, C.sub.2 F.sub.6, CF.sub.4 and N.sub.2, and 
may include trace amounts of the aforementioned impurities, i.e., 
CHF.sub.3, C.sub.2 F.sub.4, and NF.sub.3. 
Suitable cold adsorption units 2 are known in the art, and are described, 
for example, in Perry's Chemical Engineers' Handbook. Suitable sorbent 
materials include, but are not limited to, 13X, 10X, 5A, 4A, 3A, Dowrex, 
PCB, and other ion exchanged zeolite adsorbents. 
Adsorption unit effluent 3 is next fed to first cold distillation column 4, 
where effluent 3 is fractionated into light product 5 and heavy product 6. 
The C.sub.2 F.sub.6, CF.sub.4 and N.sub.2 are removed in the light product 
5, which ideally contains no more than 5 ppm of SF.sub.6. Substantially 
all of the SF.sub.6 introduced into first distillation column 4 is removed 
in heavy product 6. Heavy product 6 also includes those components which 
are heavier (i.e., higher-boiling) than SF.sub.6 and may include some 
lighter (i.e., lower-boiling) components. 
First column 4 operates at a pressure in the range of from about 5 to 15 
bar, and a temperature in the range of from about 0.degree. to -90.degree. 
C., preferably from about -10.degree. to -45.degree. C. Control of the 
pressure and temperature inside distillation columns is commonly 
understood by those skilled in the art. 
The cooling duty for condenser 7 of first column 4 is provided by a 
refrigeration unit 8. The operational pressure of first column 4 is such 
that conventional refrigerants can be used in condenser 7. Suitable 
refrigerants are known to those skilled in the art, and include, for 
example, freons such as freon 22. 
Means for providing the heat duty for the reboiler 9 of first column 4 are 
known in the art. For example, the heat duty can be provided by a heat 
source 10, such as an electric heater, an ambient vaporizer, or a heating 
medium stream, for example, a water stream. 
Light product 5 from first distillation column 4 is fed to second 
distillation column 11, which is fractionated into heavy product 12 
containing purified C.sub.2 F.sub.6, and possibly containing impurities 
such as CHF.sub.3, and light product 13 which includes CF.sub.4 and 
N.sub.2. Light product 13 may contain additional impurities, such as 
NF.sub.3. 
Second column 11 preferably operates at a pressure in the range of from 
about 5 to 12 bar and a temperature in the range of from about 0.degree. 
to -120.degree. C., and more preferably from about -25.degree. to 
-100.degree. C. 
Second column light product 13 is next fed to third distillation column 14, 
which is fractionated into a light product 15 and a heavy product 16. 
Light product 15 is N.sub.2 gas, which may contain impurities, such as 
other air impurities. Purified CF.sub.4 is removed as heavy product 16. 
This product may include impurities such as NF.sub.3. 
Third column 14 preferably operates at a pressure in the range of from 
about 1 to 10 bar and a temperature in the range of from about -50.degree. 
to -200.degree. C., more preferably from about -90.degree. to -180.degree. 
C. 
Second distillation column 11 is preferably thermally linked with third 
distillation column 14 by a common reboiler/condenser arrangement. This 
thermal linkage utilizes the heating and/or cooling capacity of one or 
more streams or stream portions from one distillation column to provide 
reboiling and/or condensing duties, respectively, to another column. 
As shown in FIG. 3, the thermal linkage of columns 11 and 14 can be 
accomplished by physically stacking one column on top of the other column. 
The two columns can be contained in a single shell. 
In this unique configuration, condenser 17 of second column 11 is at least 
partially immersed in the liquid at the bottom of third column 14. The 
vapor at the top of second column 11 is conveyed into third column 14 
through condenser 17 via line 18. This vapor provides reboiling duty to 
third column 14 for vaporizing at least a portion of the liquid in the 
bottom of third column 14. 
In the process of vaporizing the liquid in third column 14, heat is removed 
from the vapor in line 18 by the liquid in third column 14, resulting in 
at least partial condensation of the vapor in that line. From the 
condenser outlet, the condensed portion of the stream is returned to 
second column 11 as reflux. The vapor portion is introduced into an 
intermediate portion of the third column. 
The thermal linkage can also be achieved by transporting, e.g., by pumping, 
either the liquid to be vaporized to the reboiler or the liquid reflux 
resulting from the condensation back to the column where the condensing 
vapor is originated. For example, it is additionally or alternatively 
possible for columns 11 and 14 to be located adjacent to each other, 
rather than being stacked. 
In such a configuration, condenser 17 can be located external to the 
column, and the heavy liquid from column 14 can be conveyed to condenser 
17, where it is partially vaporized by the warmer vapor from the top of 
second column 11. The resulting condensed portion of this stream is 
conveyed using a pump or other suitable mechanism back to column 11 as 
reflux. The vapor portion is introduced into the third column as in the 
previously described embodiment. 
The pressures in second column 11 and third column 14 are controlled such 
that there is ample temperature driving force for the colder CF.sub.4 
containing liquid in the bottom of third column 14 to condense the light 
vapor of second column 11. Consequently, the need for an external source 
of refrigeration for the second column condenser 17 can be eliminated. 
Liquid N.sub.2 or another suitable cryogenic source 19 provides the 
refrigeration in the condenser 20 of third column 14, and is thus the only 
external source of refrigeration required by second and third columns 11 
and 14 when such a thermally linked stacked column configuration is used. 
In providing cooling duty to the third column condenser 20, the liquid 
N.sub.2 or suitable cryogen is vaporized in the process. The resulting 
cryogenic vapor stream 21 can be used to provide at least a portion of the 
cooling requirement in heat exchanger 22 for PFC feed stream 1. A liquid 
N.sub.2 stream can also be injected as reflux liquid to the column thus 
economizing the reflux condenser. 
The means for providing the heat duty for the reboiler of second column 11 
can be the same as those specified above with reference to the first 
column, e.g., heat sources, such as an electric heater, an ambient 
vaporizer, or a heating medium stream, for example, a water stream. 
Second and third column heavy product streams 12 and 16 can each be fed 
into a separate storage tank 23 and 24, respectively. A portion of the 
product is vaporized in each of tanks 23 and 24 as purified C.sub.2 
F.sub.6 and CF.sub.4 vapor streams 25 and 26, respectively. At least 
portions of vapor streams 25 and 26 and first column heavy product 6, as 
well as any other product streams, can be recycled and combined with the 
PFC feed stream to control composition, and to dampen out any large 
fluctuations in the composition or flow of the feed. This pure product 
recycle is particularly advantageous to the process. 
According to another embodiment of the invention, shown in FIG. 4, the 
first, second and third distillation columns 4, 11 and 14 can be stacked 
on top of and thermally linked with each other, in a manner similar to 
that described above with reference to the two-column structure. The 
columns can alternatively be disposed adjacent to each other while being 
thermally linked, as described above. 
In this three column, stacked structure, first column 4 is preferably 
disposed on the bottom and third column 14 on top. Given this arrangement, 
the process can be controlled such that the condenser 7 of first column 4 
provides reboil duty to second column 11, and the condenser 17 of second 
column 11 provides reboil duty to third column 14. This embodiment is 
particularly advantageous, since the only required source of refrigeration 
is liquid N.sub.2, or some other suitable cryogenic source. 
According to a further embodiment of the invention, a fourth distillation 
column can be provided to further purify heavy product 12, i.e., the 
C.sub.2 F.sub.6 product, from second distillation column 11. In 
particular, use of a fourth distillation column allows for the removal of 
the remaining impurities, such as CHF.sub.3, from second column heavy 
stream 12. 
In yet another embodiment of the invention, one or more cold adsorption 
units can be added to remove remaining impurities such as NF.sub.3 from 
the CF.sub.4 heavy product of third column 14. Advantages of this 
embodiment include eliminating the possibility of co-adsorption and 
subsequent loss of a desired product, e.g., C.sub.2 F.sub.6, with the 
impurities. 
In another embodiment of the invention, cold adsorption units 2 can be 
moved to a position immediately downstream of first distillation column 4. 
In this case, light product 5 from the first distillation column is 
introduced to the adsorption units 2, with the resulting effluent stream 
being fed to second distillation column 11. 
This configuration makes possible the elimination of the PFC-containing 
stream pre-cooling step prior to introduction into the first column. In 
such a case, the cryogenic source may be used elsewhere. Further 
advantages associated with this embodiment include a decrease in 
adsorption unit size due to the removal of heavy components such as 
SF.sub.6 in first column 4 prior to adsorption. Additionally or 
alternatively, the adsorption can be performed at colder temperatures due 
to the elimination of such heavy components, which freeze at warmer 
temperatures. 
Because the gas feed to the purification system can include recovered 
exhausts from multiple semiconductor processing tools and from multiple 
manufacturing sites, wide variations in feed gas composition are possible. 
By recycling the purified products to the purification system gas feed, an 
exceptional method for controlling the composition and flow rate of the 
feed stream is provided. This facilitates a stable and reliable operation 
of the purification system. 
Additionally or alternatively, at least portions of one or more of the 
product streams can be recycled directly to the semiconductor processing 
tool, or packaged in suitable fashion for recycle and reuse in such tools. 
Considerable savings can result since the volume of fresh materials which 
must be purchased can be significantly reduced. 
While the invention has been described in detail with reference to specific 
embodiments thereof, it will be apparent to those skilled in the art that 
various changes and modifications can be made, and equivalents employed, 
without departing from the scope of the appended claims.