Separation of CF4 and C2F6 from a perfluorocompound mixture

Details Separation of CF4 and C2F6 from a perfluorocompound mixture Separation of CF4 and C2F6 from a perfluorocompound mixture

1998-04-17

2001-02-13

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

Selective diffusion of gases

Selective diffusion of gases through substantially solid...

C095S048000, C095S050000, C095S131000, C095S135000

Reexamination Certificate

active

06187077

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to a gas separation process. The invention particularly relates to a process for separating CF
4
and C
2
F
6
from a perfluorocompound gas mixture by a hybrid system involving membrane and adsorption separation techniques.
BACKGROUND OF THE INVENTION
Various fluorinated hydrocarbon gases including tetrafluoromethane (CF
4
) and hexafluoroethane (C
2
F
6
) are used in the semiconductor industry to etch silica materials for use in integrated circuits. A major use of C
2
F
6
, for example, is as a plasma etchant in semiconductor device fabrication. Gases of high purity are critical for this application. It has been found that even small amounts of impurities in the etchant gas can increase the defect rate in the production of these integrated circuits. Thus, there has been a continuous effort in the art to provide a relatively simple and economical process for producing etchant gases having minimal amounts of impurities.
One source of such etchant gases, of course, is the exhaust or vent gas from a semiconductor plasma etching process. The exhaust gas often contains unreacted CF
4
and/or C
2
F
6
, and other perfluorocompounds (PFCs) such as SF
6
, NF
3
, and CHF
3
as well as N
2
. The exhaust gas is usually recovered from the plasma etching process and concentrated from a few parts per million to above 90% by volume in a PFC recovery stage. This concentrated exhaust gas is sometimes referred to as a PFC mixture or a PFC soup. The PFC mixture normally contains about 90% by volume of CF
4
and/or C
2
F
6
, and about 10% by volume of N
2
, SF
6
, NF
3
, and CHF
3
.
One way of purifying the PFC mixture to obtain substantially pure CF
4
and/or C
2
F
6
is by cryogenic distillation. However, there are some drawbacks to such a process. Cryogenic distillation often requires special equipment and has high utility costs. In addition, the PFC mixture is difficult to separate by cryogenic distillation due to the physical properties of the gaseous components themselves; e.g., CF
4
and NF
3
, and C
2
F
6
and CHF
3
form an azeotropic mixture with each other.
It is also known in the art to use activated carbon or zeolites to remove chlorotrifluoromethane (CClF
3
) and/or fluoroform (CHF
3
) from C
2
F
6
. See, e.g., U.S. Pat. No. 5,523,499. However, this adsorption process is not said to be able to remove N
2
, SF
6
, and/or NF
3
from a gas mixture such as PFC soup.
Thus, it is an object of the present invention to address this need in the art by providing a process that can separate CF
4
and/or C
2
F
6
from a PFC mixture.
This and other objects of the invention will become apparent in light of the following specification, and the appended drawing and claims.
SUMMARY OF THE INVENTION
The present invention relates to a process for separating at least one of CF
4
and C
2
F
6
from a gas. The process comprises the steps of:
(a) contacting a gas mixture comprising (i) at least one of CF
4
and C
2
F
6
, (ii) at least one of NF
3
, CHF
3
, and N
2
, and (iii) SF
6
with a membrane at conditions effective to obtain a retentate stream rich in SF
6
and at least one of CF
4
and C
2
F
6
, and a permeate stream rich in at least one of NF
3
, CHF
3
, and N
2
; and
(b) contacting the retentate stream with an adsorbent at conditions effective to adsorb SF
6
and produce a product stream rich in at least one of CF
4
and C
2
F
6
.
In a preferred embodiment, the present invention relates to a process for separating both CF
4
and C
2
F
6
from a gas. The process comprises the steps of:
(a) contacting a gas mixture comprising CF
4
, C
2
F
6
, NF
3
, CHF
3
, N
2
, and SF
6
with a membrane at conditions effective to obtain a retentate stream rich in SF
6
, CF
4
, and C
2
F
6
, and a permeate stream rich in NF
3
, CHF
3
, and N
2
; and
(b) contacting the retentate stream with an adsorbent at conditions effective to adsorb SF
6
and produce a product stream rich in CF
4
and C
2
F
6
.


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