U.S. patent application number 11/202121 was filed with the patent office on 2005-12-08 for method and apparatus for treating a waste gas containing fluorine-containing compounds.
Invention is credited to Kyotani, Takashi, Mori, Yoichi, Shinohara, Toyoji.
Application Number | 20050271568 11/202121 |
Document ID | / |
Family ID | 18209974 |
Filed Date | 2005-12-08 |
United States Patent
Application |
20050271568 |
Kind Code |
A1 |
Mori, Yoichi ; et
al. |
December 8, 2005 |
Method and apparatus for treating a waste gas containing
fluorine-containing compounds
Abstract
An apparatus for treatment of a waste gas, containing
fluorine-containing compounds, comprises: a solids treating device
for separating solids from the waste gas; an addition device for
adding H.sub.2 and/or H.sub.2O, or H.sub.2 and/or H.sub.2O and
O.sub.2, as a decomposition assist gas to the waste gas leaving the
solids treating device; a thermal decomposition device that is
packed with .gamma.-alumina heated at 600-900.degree. C., and which
thermally decomposes the waste gas to which the decomposition
assist gas has been added; an acidic gas treating device for
removing acidic gases from the thermally decomposed waste gas; and
channels or lines for connecting these devices in sequence. The
apparatus preferably includes an air ejector which is capable of
adjusting an internal pressure of the apparatus.
Inventors: |
Mori, Yoichi; (Kanagawa-ken,
JP) ; Kyotani, Takashi; (Kanagawa-ken, JP) ;
Shinohara, Toyoji; (Tokyo, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
18209974 |
Appl. No.: |
11/202121 |
Filed: |
August 12, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11202121 |
Aug 12, 2005 |
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09714220 |
Nov 17, 2000 |
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6949225 |
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Current U.S.
Class: |
423/240S ;
422/171; 422/172; 422/173 |
Current CPC
Class: |
B01D 2257/2066 20130101;
B01D 53/685 20130101; B01D 53/8662 20130101; B01D 53/8659 20130101;
Y02C 20/30 20130101; B01D 53/70 20130101; B01D 2257/204
20130101 |
Class at
Publication: |
423/240.00S ;
422/171; 422/172; 422/173 |
International
Class: |
B01D 053/68 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 18, 1999 |
JP |
328411/1999 |
Claims
1-7. (canceled)
8. A method for treating waste gas, comprising: separating solids
from a waste gas containing fluorine-containing compounds; adding
to said waste gas H.sub.2 and/or H.sub.2O or H.sub.2 and/or
H.sub.2O and O.sub.2 as a decomposition assist gas; thermally
decomposing said waste gas by contacting said waste gas with
.gamma.-alumina at a temperature of from 500.degree. C. to
1000.degree. C., thereby providing a decomposed waste gas; and
removing acidic gases from said decomposed waste gas.
9. The method according to claim 8, wherein thermally decomposing
said waste gas by contacting said waste gas with .gamma.-alumina at
a temperature of from 500.degree. C. to 1000.degree. C. comprises
thermally decomposing said waste gas by contacting said waste gas
with .gamma.-alumina at a temperature of from 600.degree. C. to
900.degree. C.
10. The method according to claim 8, wherein separating solids from
a waste gas containing fluorine-containing compounds comprises
separating solids from a waste gas containing perfluoro-carbons and
fluorinated hydrocarbons as well as oxidizing gases, acidic gases
and CO.
11. The method according to claim 10, wherein separating solids
from a waste gas containing perfluoro-carbons and fluorinated
hydrocarbons as well as oxidizing gases, acidic gases and CO
comprises separating solids from a waste gas supplied from a
semiconductor fabrication process.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to treatment of a waste gas
containing fluorine-containing compounds. More particularly, it
relates to a method and an apparatus for efficient treatment of
emissions from semiconductor fabrication plants, particularly from
steps of dry cleaning an inner surface of a fabrication apparatus
and etching various kinds of deposited films with perfluorocarbons
(PFCs) and halogenated hydrocarbons such as C.sub.2F.sub.6,
C.sub.3F.sub.8, CHF.sub.3, SF.sub.6 and NF.sub.3. The waste gases
contain not only PFCs but also oxidizing gases such as F.sub.2, C
12 and Br.sub.2, acidic gases such as HF, HCl, HBr, SiF.sub.4,
SiCl.sub.4, SiBr.sub.4 and COF.sub.2, as well as CO.
[0002] Semiconductor fabrication plants use many kinds of harmful
gases that can potentially pollute the environment. PFCs contained
in waste gases that typically result from etching and CVD steps are
suspected of causing global warming, and it is urgently needed to
establish an effective system for their removal.
[0003] Various breaking and recovery techniques have heretofore
been proposed for PFC removal. Catalytic thermal decomposition is
one of the breaking techniques and uses versatile compounds such as
Pt catalyst, zeolite-based catalysts, activated charcoal, activated
alumina, alkali metals, alkaline earth metals and metal oxides.
However, none of these catalytic compounds have proved completely
satisfactory.
[0004] Waste gases discharged from a semiconductor fabrication
process contain not only PFCs, but also oxidizing gases such as
F.sub.2, Cl.sub.2 and Br.sub.2, acidic gases such as HF, HCl, HBr,
SiF.sub.4, SiCl.sub.4, SiBr.sub.4 and COF.sub.2, as well as CO;
however, no method has yet been established that can realize a
thorough and effective treatment of these harmful gases.
[0005] If one wants to treat oxidizing gases such as F.sub.2,
Cl.sub.2 and Br.sub.2 by a wet method, thorough treatment cannot be
achieved by use of water alone. If alkali agents or reducing agents
are also used, not only process control but also a treatment
apparatus becomes complicated and, in addition, cost of treatment
increases.
[0006] To remove CO, it has to be decomposed with oxidizers such as
those based on Cu or Mn. As for PFCs, it has been proposed to use
alumina as an agent for removing them (Japanese Patent Public
Disclosure No. 286434/1998), and this method is characterized by
contacting C.sub.2F.sub.6 with molecular oxygen. However, the
lifetime of alumina is very short and throughput or an amount of
C.sub.2F.sub.6 that can be treated for 100% decomposition is only
4.8 L/L. Additionally, no effective way has been proposed to deal
with CO that occurs as a by-product of C.sub.2F.sub.6
decomposition, and oxidizing gases and acidic gases that occur
concomitantly with PFCs.
SUMMARY OF THE INVENTION
[0007] The present invention has been accomplished under these
circumstances and has as an object providing a method for treating
waste gases containing fluorine-containing compounds, which method
achieves high percent decomposition of PFCs. The method proves to
be effective for a prolonged time and simultaneously realizes
effective removal of oxidizing gases, acidic gases and CO that are
contained in the waste gases.
[0008] Another object of the invention is to provide an apparatus
for implementing this method.
[0009] A first object of the invention can be attained by a method
for treatment of a waste gas containing fluorine-containing
compounds, which method comprises: separating solids from the waste
gas; adding H.sub.2 and/or H.sub.2O, or H.sub.2 and/or H.sub.2O and
O.sub.2, as a decomposition assist gas; thermally decomposing the
waste gas by contact with .gamma.-alumina usually at
500-1000.degree. C., preferably at 600-900.degree. C., and more
preferably at 700-900.degree. C.; and removing acidic gases from
the decomposed waste gas.
[0010] In this method, the waste gas containing fluorine-containing
compounds may be a waste gas from a semiconductor fabrication
process that contains not only perfluorocarbons and fluorinated
hydrocarbons but also oxidizing gases, acidic gases and CO.
[0011] A second object of the invention can be attained by an
apparatus for treatment of a waste gas containing
fluorine-containing compounds, which apparatus comprises a solids
treating device for separating solids from a waste gas containing
fluorine-containing compounds; an addition device for adding
H.sub.2 and/or H.sub.2O, or H.sub.2 and/or H.sub.2O and O.sub.2, as
a decomposition assist gas to the waste gas leaving the solids
treating device; a thermal decomposition device that is packed with
.gamma.-alumina heated at 600-900.degree. C., and which thermally
decomposes the waste gas to which the decomposition assist gas has
been added; an acidic gas treating device for removing acidic gases
from the thermally decomposed waste gas; and channels or lines for
connecting these device in sequence.
[0012] In this treatment apparatus, a water scrubber may be used as
the solids treating device or the acidic gas treating device. This
treatment apparatus may have not only an air ejector capable of
adjusting pressure in the apparatus through which the waste gas
passes, but also an FT-IR analyzer for controlling emission density
of treated gas.
[0013] A first step in the method of the invention for treating a
waste gas containing fluorine-containing compounds is passing the
waste gas through a solids treating device such as a water
scrubber. Exit gas is passed through a thermal decomposition device
packed with .gamma.-alumina heated at 500-1000.degree. C.,
preferably 600-900.degree. C., and more preferably 700-900.degree.
C., with H.sub.2 and/or H.sub.2O, or H.sub.2 and/or H.sub.2O and
O.sub.2, being added as a decomposition assist gas, so that PFCs,
oxidizing gases and CO are completely decomposed into acidic gases
and CO.sub.2. Generated acidic gases are removed with an acidic gas
treating device such as a water scrubber.
[0014] The method may also employ not only an air ejector capable
of adjusting pressure in the apparatus through which the waste gas
passes, but also an FT-IR analyzer for controlling emission density
of treated gas.
BRIEF DESCRIPTION OF THE DRAWING
[0015] FIG. 1 is a flowchart for a waste gas treatment apparatus of
the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0016] The present invention is described below in detail. In a
first step, a waste gas containing PFCs, oxidizing gases, acidic
gases and CO is passed through a solids treating device such as a
water scrubber in order to remove not only solids such as SiO.sub.2
in the waste gas but also Si compounds such as SiF.sub.4,
SiCl.sub.4 and SiBr.sub.4 that may potentially solidify in a
thermal decomposition device of a next stage. If the waste gas is
directly introduced into the thermal decomposition device without
being passed through the solids treating device, clogging or other
blocking problems will occur, thereby potentially preventing the
waste gas from smoothly flowing through a packed .gamma.-alumina
layer. Performance of .gamma.-alumina may also deteriorate. By
passing the waste gas through the solids treating device, solids
and acidic gases containing Si compounds are removed, whereas part
of oxidizing gases such as F.sub.2, Cl.sub.2 and Br.sub.2 as well
as all volumes of PFCs and CO are discharged.
[0017] The waste gas emerging from the solids treating device is
then introduced into the thermal decomposition device so that the
waste gas is decomposed through contact with .gamma.-alumina heated
at 500-1000.degree. C., preferably 600-900.degree. C., more
preferably 700-900.degree. C. On this occasion, H.sub.2 and/or
H.sub.2O; or H.sub.2 and/or H.sub.2O and O.sub.2 are added to the
waste gas as a decomposition assist gas so that components of the
waste gas are decomposed into acidic gases and CO.sub.2 according
to the following reaction schemes:
CF.sub.4+2H.sub.2+O.sub.2.fwdarw.CO.sub.2+4HF
CF.sub.4+2H.sub.2O.fwdarw.CO.sub.2+4HF
F.sub.2+H.sub.2.fwdarw.2HF
2F.sub.2+2H.sub.2O.fwdarw.4HF+O.sub.2
2CO+O.sub.2.fwdarw.2CO.sub.2
[0018] Thus, PFC reacts with H.sub.2 and O.sub.2 or H.sub.2O to be
decomposed into CO.sub.2 and HF. Acidic gases such as F.sub.2 react
with H.sub.2 or H.sub.2O to be decomposed to another acidic gas HF.
Carbon monoxide (CO) is oxidized to CO.sub.2.
[0019] As for PFC, H.sub.2 or H.sub.2O is added in moles at least
equal to the moles necessary for F atoms in the PFC to be converted
into HF, and O.sub.2 is added in moles at least equal to the moles
necessary for C atoms in the PFC to be converted into CO.sub.2.
Preferably, O.sub.2 is added in moles which are at least equal to
the sum of one mole and the above-defined minimum number of moles.
As for oxidizing gases, H.sub.2 is introduced in moles at least
equal to the moles necessary for halogen atoms (X) in an oxidizing
gas to be converted into an acidic gas (HX).
[0020] The waste gas leaving the thermal decomposition device only
contains acidic gases (HX) and CO.sub.2, and by subsequent
treatment with an acidic gas treating device such as a water
scrubber, these acidic gases are completely removed.
[0021] Alumina to be used in the invention may have a
.gamma.-crystalline structure without a uniform pore distribution.
While the shape of the alumina is not limited in any particular
way, spheres are easy to handle and, hence, preferred. To an extent
that will not unduly increase resistance to the passage of the
waste gas, a particle size of .gamma.-alumina should be as small as
possible, preferably between 0.8 mm and 2.6 mm. The .gamma.-alumina
may be held at between 500.degree. C. and 1000.degree. C.,
preferably 600.degree. C. and 900.degree. C. and more preferably
700.degree. C. and 900.degree. C. during the passage of the waste
gas.
[0022] The solids treating device and the acidic gas treating
device are preferably a packed column or a spray column, on
condition that they are adapted to spray water. The thermal
decomposition device should be adapted to permit introduction of
H.sub.2 and/or H.sub.2O, or H.sub.2 and/or H.sub.2O and O.sub.2, as
a decomposition assist gas.
[0023] FIG. 1 is a flowchart for a waste gas treatment apparatus of
the invention. The apparatus generally comprises a solids treating
device 1, an .gamma.-alumina packed layer 2, a thermal
decomposition device 3, a cleaning water circulating pump 4, an
acidic gas treating device 5, a Fourier Transform Infrared
Spectroscopy analyzer 6 (hereinafter referred to as an FT-IR
analyzer), an air ejector 7 and a bypass valve 8.
[0024] A waste gas 9 containing PFCs, oxidizing gases, acidic gases
and CO is first passed through a spray column (solids treating
device) 1 so as to remove solids and Si compounds. The waste gas is
then passed through the thermal decomposition device 3, which is
also supplied with H.sub.2, O.sub.2 and H.sub.2O to decompose the
PFCs, oxidizing gases and CO into acidic gases and CO.sub.2. The
acidic gases are removed by passage through a subsequent spray
column (acidic gas treating device) 5, from which treated gas 10
emerges.
[0025] The air ejector 7 is installed to control pressure in each
of the treating devices 1, 3 and 5, and the FT-IR analyzer 6 is
provided to monitor the treated gas.
[0026] Spray water 11 is introduced into the acidic gas treating
device 5, and spent water is forced to the solids treating device 1
via the cleaning water circulating pump 4. This water is used for
spraying in the solids treating device 1, and is then discharged as
wastewater 12.
[0027] The following examples are provided for the purpose of
further illustrating the present invention but are in no way to be
taken as limiting.
EXAMPLE 1
[0028] An experiment was conducted using a quartz column of 25
mm.sup..phi., which was packed with .gamma.-alumina to a height of
100 mm. The .gamma.-alumina was a commercial product of Mizusawa
Kagaku K.K. (NEOBEAD GB-08) having a particle size of 0.8 mm. The
quartz column was installed in a ceramic electric furnace and the
.gamma.-alumina was heated at 800.degree. C.
[0029] In addition to CF.sub.4 diluted with N.sub.2 gas, H.sub.2
and O.sub.2 were supplied as decomposition assist gases, with the
amount of H.sub.2 being such that the number of H atoms was at
least equal to the number of F atoms in CF.sub.4, and the amount of
O.sub.2 being at least equimolar to the amount of H.sub.2 supplied.
These gases were flowed into the column at a total rate of 408 sccm
and their entrance concentrations were 1.0% (CF.sub.4), 3.0%
(H.sub.2) and 5.7% (O.sub.2).
[0030] In order to evaluate performance of the treatment system,
exit gas was analyzed periodically and passage of the CF.sub.4, gas
was stopped when removal of CF.sub.4 dropped below 98%. Throughput
was determined from the amount of CF.sub.4 that had been passed
through the system. The analysis of CF.sub.4 and other gases was
conducted with a gas chromatographic apparatus equipped with a mass
detector.
[0031] As it turned out, the removal of CF.sub.4 dropped to 98%
when its passage continued for 920 min. At this point in time, the
throughput as determined from the quantity of the supplied CF.sub.4
was 77 L/L. Throughout the experiment, concentration of CO emission
was below a tolerable level (25 ppm).
COMPARATIVE EXAMPLE 1
[0032] An experiment was conducted using the same equipment as in
Example 1, which was packed with the same .gamma.-alumina in the
same amount and heated to the same temperature as that of Example
1. Total gas flow rate was 408 sccm; feed gas was a mixture of
N.sub.2-diluted CF.sub.4 and SiF.sub.4; in addition, H.sub.2 and
O.sub.2 were supplied as decomposition assist gases, with the
amount of H.sub.2 being such that the number of H atoms was at
least equal to the total number of F atoms in CF.sub.4 and
SiF.sub.4, and the amount of O.sub.2 being at least equimolar to
the amount of H.sub.2 supplied. These gases were flowed into the
column at respective concentrations of 0.95% (CF.sub.4), 0.97%
(SiF.sub.4), 5.3% (H.sub.2) and 6.0% (O.sub.2)
[0033] As it turned out, removal of CF.sub.4 dropped below 98% when
passage of the CF.sub.4/SiF.sub.4 gas continued for 510 minutes. At
this point in time, throughput was 40 L/L, which was nearly one
half the throughput for the case where only CF.sub.4 gas was
supplied. Throughout the experiment, concentration of CO was below
a tolerable level.
EXAMPLE 2
[0034] An experiment was conducted using the same equipment as in
Example 1, which was packed with the same .gamma.-alumina in the
same amount and heated to the same temperature as that of Example
1. Total gas flow rate was 408 sccm; feed gas was a mixture of
N.sub.2-diluted CF.sub.4 and F.sub.2; in addition, H.sub.2 and
O.sub.2 were supplied as decomposition assist gases, with the
amount of H.sub.2 being such that the number of H atoms was at
least equal to the total number of F atoms in CF.sub.4 and F.sub.2,
and the amount of O.sub.2 being at least equimolar to the amount of
H.sub.2 supplied. These gases were flowed into the column at
respective concentrations of 0.92% (CF.sub.4), 1.1% (F.sub.2), 5.0%
(H.sub.2) and 6.0% (O.sub.2).
[0035] As it turned out, the removal of CF.sub.4 dropped below 98%
when passage of the CF.sub.4/F.sub.2 gas continued for 25 hours. At
this point in time, throughput was 115 L/L, which was 1.51 times
higher than the throughput for the case where only CF.sub.4 gas was
supplied. Throughout the experiment, concentrations of CO and
F.sub.2 were below tolerable levels (1 ppm for F.sub.2), provided
that F.sub.2 had been decomposed into HF.
REFERENCE EXAMPLE 1
[0036] An experiment was conducted using the same equipment as in
Example 1, which was packed with the same .gamma.-alumina in the
same amount and heated to the same temperature as that of Example
1. The total gas flow rate was 408 sccm; in addition to
N.sub.2-diluted CO, O.sub.2 was supplied in moles at least equal to
the moles necessary for CO to be converted into CO.sub.2, and their
respective entrance concentrations were 1.4% (CO) and 5.7%
(O.sub.2). Throughout passage of a feed gas for 30 minutes,
concentration of CO was below the detection limit (2 ppm), and all
of CO had been oxidized into CO.sub.2.
COMPARATIVE EXAMPLE 2
[0037] An experiment was conducted using the same equipment as in
Example 1, which was packed with the same .gamma.-alumina in the
same amount and heated to the same temperature as that of Example
1. Total gas flow rate was 408 sccm. In addition to N.sub.2-diluted
CO, H.sub.2O was supplied at a rate of 0.090 mL/min, which was 22
times as much as CO, and an entrance concentration of CO was
1.3%.
[0038] As it turned out, 1000 ppm of CO leaked as a result of a 15
minute passage of a feed gas. Obviously, concentration of CO could
not be reduced to below a tolerable level (25 ppm) by the sole
addition of H.sub.2O.
REFERENCE EXAMPLE 2
[0039] An experiment was conducted using the same equipment as in
Example 1, which was packed with the same .gamma.-alumina in the
same amount and heated to the same temperature as that of Example
1. Total gas flow rate was 408 sccm. In addition to N.sub.2-diluted
CO, H.sub.2O was supplied at a rate of 0.090 mL/min, which was 18
times as much as CO, and O.sub.2 was supplied in moles at least
equal to the moles necessary for CO to be converted into CO.sub.2.
Entrance concentrations of CO and O.sub.2 were 1.5% and 3.4%,
respectively.
[0040] As it turned out, concentration of CO had been reduced to
below a detection limit (2 ppm) after passage of a feed gas for 3
hours. Obviously, CO was oxidized to CO.sub.2 by addition of
O.sub.2.
EXAMPLE 3
[0041] An experiment was conducted using the same equipment as in
Example 1, which was packed with the same .gamma.-alumina in the
same amount as that of Example 1 and heated to 700.degree. C. Total
gas flow rate was 408 sccm. In addition to N.sub.2-diluted
CF.sub.4, H.sub.2O was supplied at a rate of 0.040 mL/min, which
was 14 times as much as CF.sub.4, and O.sub.2 was supplied in moles
at least equal to the moles necessary for the C atom in CF.sub.4 to
be converted into CO.sub.2. Entrance concentrations of CF.sub.4 and
O.sub.2 were 0.89% and 3.0%, respectively.
[0042] As it turned out, removal of CF.sub.4 dropped below 98% when
passage of a feed gas continued for 23 hours. At this point in
time, throughput was 110 L/L, which was 1.4 times higher than the
throughput of CF.sub.4 treatment in the presence of added H.sub.2
and O.sub.2. Throughout the experiment, the concentration of CO was
below a tolerable level.
COMPARATIVE EXAMPLE 3
[0043] In order to evaluate effectiveness of a wet process in
treating oxidizing gases and acidic gases, a water cleaning column
(210 mm.sup..phi..times.430 mm.sup.H with a Raschig ring packed to
a height of 170 mm) was supplied with a waste gas at a total rate
of 60 L/min, and with spray water at a rate of 3.5 L/min. The waste
gas was prepared from F.sub.2, SiF.sub.4 and Cl.sub.2, which had
entrance concentrations of 1100 ppm, 1600 ppm and 5100 ppm,
respectively. At an exit of the column, F.sub.2, SiF.sub.4 and
Cl.sub.2 were detected at concentrations of 11 ppm, <1 ppm, and
3300 ppm, respectively. Obviously, SiF.sub.4 was effectively
treated but F.sub.2 and Cl.sub.2 leaked out.
EXAMPLE 4
[0044] A water cleaning column (210 mm.sup..phi..times.430 mm.sup.H
with a Raschig ring packed to a height of 170 mm) was used as a
solids treating device. This column was combined with a thermal
decomposition device comprising a preheating chamber and a catalyst
packed chamber, and an acidic gas treating device which was the
same as the water cleaning column. Exit gas leaving the acidic gas
treating device was monitored with an FT-IR analyzer (Infinity 6000
of MATTSON), and pressure in the experimental system was controlled
with an air ejector of Daito Seisakusho K.K. The solids treating
device and the acidic gas treating device were supplied with
cleaning water at respective flow rates of 2 L/min and 4 L/min. The
thermal decomposition device was supplied with air and pure water
at respective flow rates of 10 L/min and 2.4 mL/min. A catalyst was
15 L of .gamma.-alumina (NEOBEAD GB-08 of Misusawa Kagaku K.K.)
[0045] A gas dryer (MD-70-72P of PERMAPUR) was installed ahead of
the FT-IR analyzer for removing moisture in the waste gas. Air was
supplied into an air ejector at a rate of 30 L/min so that pressure
in the system was kept at a negative value of -0.5 kPa. A waste gas
was introduced at a flow rate of 60 L/min, and it was prepared from
a N.sub.2 base containing CF.sub.4, SiF.sub.4, F.sub.2 and CO at
respective concentrations of 0.5%, 0.3%, 0.3% and 0.3%. The waste
gas was first passed through the solids treating device, then
passed through the thermal decomposition device in the presence of
added water and O.sub.2, with the catalyst being heated at
700.degree. C. The waste gas was subsequently passed through the
acidic gas treating device, and treated gas was continuously
analyzed by FT-IR. After passage of the waste gas for 10 hours,
CO.sub.2, was detected in an amount of 6900 ppm, but each of
CF.sub.4, SiF.sub.4, HF and CO had been treated to below 1 ppm. No
F.sub.2 was detected by ion chromatographic analysis.
EXAMPLE 5
[0046] A waste gas treatment was conducted with the same
experimental setup under the same conditions as in Example 4,
except that CF.sub.4 was replaced by C.sub.2F.sub.6, and that waste
gas was prepared from a N.sub.2 base containing C.sub.2F.sub.6,
SiF.sub.4, F.sub.2 and CO at respective concentrations of 0.5%,
0.3%, 0.3% and 0.3%. The waste gas was passed through the solids
treating device, the thermal decomposition device and the acidic
gas treating device.
[0047] Treated gas emerging from the acidic gas treating device was
continuously analyzed by FT-IR. After the passage of the waste gas
for 10 hours, CO.sub.2 was detected in an amount of 11000 ppm, but
each of C.sub.2F.sub.6, SiF.sub.4, HF and CO had been treated to
below 1 ppm. No F.sub.2 was detected by ion chromatographic
analysis.
[0048] According to the invention, harmful waste gases, from a
semiconductor fabrication process, that contain PFCs, oxidizing
gases, acidic gases and CO, and which are a potential accelerator
of global warming, can be treated in such a way that high percent
decomposition is maintained for a prolonged time.
* * * * *