Gas separation: processes – With control responsive to sensed condition – Concentration sensed
Reexamination Certificate
1999-07-30
2001-04-17
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
With control responsive to sensed condition
Concentration sensed
C095S019000, C095S127000, C096S111000, C096S113000, C096S144000
Reexamination Certificate
active
06217633
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to a process and an apparatus for recovering a noble gas, more particularly to a process and an apparatus for recovering a noble gas contained in an exhaust gas from a noble gas employing system, for example, a plasma sputtering system, a plasma CVD system and a reactive ion etching system.
In processes for producing semiconductor devices such as semiconductor integrated circuits, active matrix liquid crystal panels, solar cell panels and magnetic discs, employed are systems for generating plasma in a noble gas atmosphere under reduced pressure to carry out various treatments of the semiconductor devices with the plasma, for example, a sputtering system, a plasma CVD system and a reactive ion etching system.
For example, in the sputtering system, a chamber is evacuated by a vacuum pump while a noble gas is introduced into the chamber at a flow rate of about 500 cc/min; and a high-frequency wave (radiofrequency) is applied to electrodes housed in the chamber with the internal pressure of the chamber being maintained to about 1 Pa to generate plasma which is used for sputtering of a solid film-forming material contained in the chamber to deposit it on the wafer surface and form a thin film.
Meanwhile, in the plasma CVD system, plasma is generated while a mixture of a film-forming gas and a noble gas is introduced into a process chamber at a flow irate of about 100 cc/min with the internal pressure of the chamber being maintained to about 100 Pa, and the thus generated plasma is used for decomposition of the film-forming gas to be deposited onto the surface of a wafer heated to about 300° C. and form a thin film.
Further, in the reactive ion etching system, plasma is generated while a mixture of an etching gas and a noble gas is introduced into a process chamber with the internal pressure of the chamber being maintained to about several Pa, and the thus generated plasma is used for excitation of the etching gas and to carry out etching using the excited ion.
In these various kinds of systems, since the treatments are carried out using plasma having high energy, energy, the presence of other gas species than those which contribute to film formation, for example, nitrogen, oxygen and moisture, in the atmosphere used for treatment makes it impossible to form a predetermined thin film or to carry out etching. For example, in the case where formation of metal wiring for semiconductor integrated circuits is carried out using the sputtering system, if moisture, oxygen or the like is present in the atmosphere, the metal thin film is oxidized to increase the wiring resistance. In the case of tantalum (Ta), the crystal structure can be changed. Further, if oxygen, moisture, organic impurity or the like is present in the atmosphere of forming a polycrystalline silicon thin film by means of plasma CVD, there occur various inconveniences: the resulting crystal grains have nonuniform size; and electron mobility is extremely lowered. If such impurities are present when etching is carried out by means of reactive ion etching, the desired selectively etching ratio of materials cannot be designed accurately, resulting in etching failure or damage of wafers. Accordingly, the impurity content in the noble gas to be introduced to a plasma-employing system should be reduced to several ppb or less.
FIG. 4
is a system diagram showing a conventional sputtering system as an example of plasma treating system. Usually, in such a sputtering system, a loading chamber
3
for loading wafers
2
is installed on the upstream side of a process chamber
1
, and the wafers
2
are treated one by one. The inside of the loading chamber
3
is of an atmosphere of a purge gas such as dry air and nitrogen gas supplied from a purge gas source
4
and is also maintained under reduced pressure by a pair of evacuators
6
a
and
6
b
connected to the loading chamber
3
via a gate valve
5
. After evacuation of the loading chamber
3
and process chamber
1
, the untreated wafer
2
supported in the loading chamber
3
is introduced into the process chamber
1
through a gate valve
7
interposed between these chambers
1
and
3
and is mounted on a wafer susceptor
8
in the process chamber.
After the gate valve
7
interposed between the chamber
1
and the chamber
3
is closed, a noble gas having passed a purifier
9
where impurities are removed is introduced from a gas source
10
, through a line
10
a
, into the process chamber
1
. Usually, in order to form a noble gas atmosphere in the process chamber
1
, the cycle of evacuating the process chamber by the evacuators
11
a
and
11
b
and introducing the noble gas from the gas source
10
is repeated once or more by opening and closing the valves in a predetermined order in accordance with the commands from a controller
12
. After formation of noble gas atmosphere in the process chamber, plasma is generated in the process chamber by application of a high-frequency wave from a high-frequency power source
14
via a matching circuit
13
to sputter a solid film-forming material with the plasma generated and deposit a thin film on the wafer. The wafer
2
having a predetermined thin film formed thereon is forwarded from the process chamber
1
through the loading chamber
3
to a subsequent step for another treatment. Loading and unloading of wafers are carried out about 30 times/hour as described above.
By the way, the exhaust gas let out from the sputtering system by the evacuators
11
a
and
11
b
, no matter whether it is a purge gas for the process chamber or is a gas used for film formation, is exhausted out of the system as such through an exhaust passage
15
. Meanwhile, the noble gas supplied from a noble gas tank
16
is of the kind which is present in the atmosphere in a very small amount: xenon content in the atmosphere is 0.086 ppm. Since such noble gases are prepared by cryogenic separation of air to form a concentrated oxygen, followed by fractionation, they are not readily available in large amounts.
Accordingly, the exhaust gas to be exhausted through the exhaust passage
15
to the outside of the system (the exhaust gas at the point of time where the noble gas occupies the greatest part) is recovered to a container or balloon installed separately, and the thus recovered gas is subjected to concentration and fractionation to separate the noble gas therefrom for recirculation. According to this recovery process, since the exhausted noble gas is recovered into a container, a balloon or the like, the concentrated and fractionated noble gases can be utilized in various fields of industries advantageously. However, there occurs another problem that costs for transporting the containers and balloons in which noble gases are recovered increase. Further, atmospheric components readily migrate into the noble gases during attachment and detachment of the containers, noble gases having stable purity cannot be obtained when they are concentrated and fractionated, disadvantageously.
FIG. 5
is a system diagram showing an example where the conventional noble gas recovering apparatus is applied to the sputtering system shown in FIG.
4
. More specifically, there is also proposed, in the conventional noble gas recovering apparatus, a process for recovering a noble gas contained in an exhaust gas (the exhaust gas at the time point where the noble gas occupies the greatest part of it) to be exhausted from a process chamber, through a closed loop, as shown in FIG.
5
. According to this process, a recovery passage
23
connected to a noble gas recovering apparatus
22
is installed on an exhaust passage
15
of a sputtering system
21
having the same constitution as described above, while an outlet passage
24
of the noble gas recovering apparatus
22
is connected to the purifier
9
, and the exhaust gas (noble gas) discharged from a process chamber
1
is designed to be introduced to the noble gas recovering apparatus
22
by opening and closing interchangeably a pair of selector valves
25
a
and
25
b
lo
Ishihara Yoshio
Ohmi Tadahiro
Armstrong Westerman Hattori McLeland & Naughton LLP
Nippon Sanso Corporation
Spitzer Robert H.
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