Gas separation: processes – With control responsive to sensed condition – Concentration sensed
Reexamination Certificate
1998-11-17
2001-08-21
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
With control responsive to sensed condition
Concentration sensed
C095S095000, C095S131000, C095S233000, C096S111000, C096S136000, C096S144000
Reexamination Certificate
active
06277173
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to reuse of a gas discharged from a vacuum pump. More particularly, the invention relates to reuse of a gas discharged from a vacuum pump which operates using a gas for purposes of shaft sealing, improvement of a degree of vacuum, prevention of formation of reaction by-products, inhibition of corrosion, elongation of the life of device, and the like.
2. Description of the Related Art
As an example, in a dry etching step and a chamber cleaning step for a CVD apparatus for a process for manufacture of semiconductor devices, perfluorocompounds (PFCs), such as methane tetrafluoride (CF
4
), ethane hexafluoride (C
2
F
6
), and nitrogen trifluoride (NF
3
), are used to etch polysilicon (poly-Si), silicon oxide (SiO
2
) and the like.
In such steps, a chamber is evacuated by a vacuum pump to a reduced pressure of, for example, the order of 13.3 to 66.5 pascals, and subsequently, a PFC gas is introduced to the chamber to produce a plasma and dry-etch SiO
2
or the like. The details of the mechanism of an etching process are not known, but it is presumed that in a chamber, a radical molecule generated from a PFC excited by a high-frequency field reacts with SiO
2
or the like present on the surface of a treated substrate or deposited on walls and the like inside the chamber, to thereby proceed with etching, which produces gas components such as SiF
4
.
In such an etching step, all of the PFC introduced to a chamber is not decomposed and the undecomposed gas is discharged from the chamber by a vacuum pump. As the vacuum pump, such a pump as a dry pump, a mechanical booster pump, or a turbo molecular pump is used, in general. Nitrogen gas is introduced to the vacuum pump, as an inert gas separate from the gas emitted from the chamber, for purposes of shaft sealing, improvement in the degree of vacuum, prevention of formation of reaction by-products (in the case where the gas from the chamber is reactive, products resulted from reactions can be deposited inside the vacuum pump), inhibition of corrosion, elongation of the life of the device, and the like, and is discharged from the vacuum pump along with the gas emitted from the chamber.
Exhaust discharged from a vacuum pump contains gases such as SiF
4
formed by the reaction of radical molecules generated from a PFC with SiO
2
or the like, and gases formed by decomposition of the PFC by a high-frequency field. Since these gas components include those having a high corrosiveness and high toxicity, the exhaust from a vacuum pump is, in general, modified to have no toxicity by a detoxificating unit prior to being emitted to the atmosphere. As the detoxificating unit, a device which is filled with an adsorbent such as an activated carbon, activated alumina or molecular sieve, or a chemically reactive agent produced by loading an alkaline agent or the like on activated carbon or alumina, is generally used.
Nevertheless, it has recently been deemed to be important to prevent global warming, and the exhaust of gases such as PFCs, having a high global warming potential (GWP), tends to be restricted. To cope with such situation, techniques in which gases having a high GWP in the exhaust from a vacuum pump are decomposed and detoxificated prior to being emitted to the atmosphere, as referred to above, or the exhaust is compressed and gaseous impurities are separated from the compressed exhaust to recover PFCs, have been developed.
As the techniques for decomposing PFCs in an exhaust, methods such as thermal cracking, catalytic thermal cracking, and combustion cracking have been developed. In these processes, PFCs having more carbon atoms are gradually decomposed to PFCs having fewer carbon atoms. For example, when plasma discharge was established in a chamber under the condition of C
2
F
6
flow rate of 0.1 liter per minute, and the exhaust from the chamber was discharged by a vacuum pump to which N
2
was introduced at a flow rate of 28 liters per minute, it was observed that the exhaust at the outlet of the vacuum pump contained 0.01% CF
4
, 0.12% C
2
F
6
, 0.01% CH
2
F
2
, and in addition, trace amounts of SiF
4
, HF, F
2
and so forth. When the exhaust was further treated in a PFC decomposition unit of thermal cracking type, it was observed that the discharged N
2
contained 0.06% CF
4
0.09% CO, and 0.04% CO
2
, which reveals that the decomposition in the PFC decomposition unit resulted in formation of CF
4
.
However, since CF
4
has a very large GWP 100 of 6,500, it is required to further decompose CF
4
into carbon dioxide and fluorine, and change the resultant fluorine to a fixed form prior to removal.
To decompose CF
4
in an exhaust, an unreasonable amount of energy is required, which may be unfavorable in the prevention of global warming, in terms of the carbon dioxide generated during the creation of the energy.
Further, in prior gas discharging or evacuating systems, a large amount of nitrogen is introduced to a vacuum pump, and is incorporated in gas emitted from a chamber, as referred to above. In this case, a decomposing unit consumes energy also for the diluted emitted gas, resulting in an increase in energy loss.
Methods of recovering and reusing exhaust containing PFCs have been developed to prevent global warming. In these methods, since there is dilution of the exhaust by nitrogen introduced into a vacuum pump, complicated processes and much energy are required to separate the PFCs and nitrogen in the exhaust.
SUMMARY OF THE INVENTION
Thus, it is an object of the invention to provide a system for discharging gas emitted from a gas emitting source using a vacuum pump to which a gas such as nitrogen is introduced for purposes of shaft sealing, improvement of degree of vacuum, prevention of formation of reaction by-products, inhibition of corrosion, elongated life of the device, and the like, the system not only reducing or eliminating emission to the atmosphere of global warming gases such as PFCs discharged from the vacuum pump but also making energy-saving possible.
It is also an object of the invention to provide a method of reducing or eliminating emission to the atmosphere of global warming gases such as PFCs discharged from such a vacuum pump and also making energy-saving possible.
According to the invention, the system for evacuating gas from an apparatus as a gas emitting source has a vacuum pump which sucks the gas discharged from the apparatus while being supplied with a gas other than the sucked gas, the system comprising a unit for recovering at least part of the gas discharged from the vacuum pump, and recirculating the recovered gas to the vacuum pump as said gas other than the sucked gas.
The system may comprise means for recirculating part of the recovered gas to the gas emitting source from which the gas to be discharged is emitted.
The invention further provides a method for evacuating gas from an apparatus as a gas emitting source using a vacuum pump which sucks the gas discharged from the apparatus while being supplied with a gas other than the sucked gas, the method comprising recovering at least part of the gas discharged from the vacuum pump, and recirculating the recovered gas to the vacuum pump as said gas other than sucked gas.
The recovered gas from the vacuum pump may also be recirculated to the gas emitting source from which the gas to be discharged is emitted.
In the invention, it can be envisaged that gases emitted from an apparatus or facility and sucked by a vacuum pump include gases such as PFCs responsible for global warming, and gases containing PFCs and the like, and the invention will be discussed referring to these gases hereinafter. However, it is, of course, apparent that the invention may be applied to discharge of any other gas which is not necessarily responsible for global warming. Also, although the following discussion is presented referring to an apparatus for manufacturing semiconductor devices as an apparatus emitting gas to be discharged, it is apparent that the apparatus in the invention are not limite
Ozaki Katsuhiro
Sadakata Takayuki
Yoshinaga Hiroshi
Armstrong Westerman Hattori McLeland & Naughton LLP
Fujitsu Limited
Spitzer Robert H.
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