Evacuating apparatus

Details Evacuating apparatus Evacuating apparatus

2000-11-15

2002-04-23

Walberg, Teresa (Department: 3742)

Pumps

Condition responsive control of pump drive motor

By control of electric or magnetic drive motor

C418S104000

Reexamination Certificate

active

06375431

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an evacuating apparatus for use to exhaust a vacuum chamber in the semiconductor manufacturing plant.
In the semiconductor vacuum devices, it is particularly important that an evacuated chamber can attain a degree of vacuum of about 10
−3
Pa, and oil molecules must not enter the evacuated chamber. Thus, as a vacuum pump to meet such demands at a single stage, a screw vacuum pump (JP-B-7-9239) has been proposed which can exhaust the chamber from the atmospheric pressure to about 10
−3
Pa at a single stage (with a high compression ratio and a wide operable pressure range), and is oil free.
However, the screw vacuum pump had the following intrinsic problems.
(1) The screw vacuum pump is small in conductance because a thread groove is used to receive and transfer molecules of gas to be exhausted. Accordingly, the pumping speed is slow in a molecular flow range.
(2) The screw vacuum pump is necessary to have a clearance between mating faces of the male and female screws, and between the outer periphery of a screw and the inner periphery of a housing. Accordingly, the vacuum sealing ability is bad, which has an adverse effect on the ultimate vacuum.
(3) The screw vacuum pump has a bad vacuum sealing ability, as described above, and when used as a roughing vacuum pump, takes a large motive power (power loss) to recompress and discharge a back streaming of air from the atmosphere side. In particular, for the screw vacuum pump having a high pumping speed, the total amount of clearance as defined in (2) becomes large, resulting in a great tendency of motive power loss. Further, when a screw pump is used as a roughing vacuum pump, the screw pump produces a large power loss, which is caused by a difference in pressure between the suction side and the atmosphere side, even though a necessary degree of vacuum has been already reached on the suction side.
For the above-mentioned problems intrinsic to the screw vacuum pump, the following solving means has been conventionally proposed.
(A) First, solving means for a problem of conductance of the item (1) has been proposed in which the screw vacuum pump is used as the roughing vacuum pump that is less problematical with the conductance, and the booster pump is a Roots vacuum pump having large conductance.
In this two-stage pump, however, because the Roots vacuum pump has a small compression ratio, the pumping speed of the screw pump as the roughing vacuum pump can not be made too small. Owing to the fact that the pumping speed of the roughing vacuum pump can not be reduced, it follows that the capacity of the motor for driving this roughing vacuum pump can not be reduced, and each motive power loss of (3) can not be decreased. (A problem of (2) still remains.)
(B1) Solving means of a problem regarding the sealing ability of (2) has been proposed in which a plurality of chambers for transferring the fluid are provided between the suction port and the exhaust port by providing a plural number of turns of screw in the screw pump used at a single stage, to enhance the sealing ability (JP-B-7-9239). However, such solving means has an increased axial length of the screw, so that the devices become larger. Further, the plural number of turns of screw will not simply lead to solving the problem (3).
(B2) Similarly, solving means of the problem regarding the sealing ability of (2) has been proposed in which a screw vacuum pump is used as the booster pump which is less problematical with the sealing ability and a diaphragm pump or oil-sealed rotary vacuum pump having good sealing ability is used as the roughing vacuum pump (JP-A-62-243982). Since the oil-sealed rotary vacuum pump is usually provided with a check valve at a discharge port, it is possible to prevent back streaming of the air from the atmosphere side, so that each motive power loss as in (3) can be reduced.
In such two-stage pump, however, since the diaphragm pump or oil-sealed rotary vacuum pump having good sealing ability is necessary to be used as the roughing vacuum pump, in a case of the diaphragm pump, for example, reaction products (which are produced from a reactive gas flowed through the evacuated chamber) are likely to remain in the inside of the pump. If the reaction products remain, the exhaust performance may degrade remarkably, and it takes a lot of time and cost for overhaul. Also, in a case of the oil-sealed rotary vacuum pump, there is the danger that the evacuated chamber may be contaminated with oil molecules, and there is the problem that the oil may degrade in short time owing to a reactive gas, or must be exchanged frequently.
(C1) Solving means of a problem regarding the motive power loss in (3) has been proposed in which a micro-pump having a very small pumping speed is provided on the exhaust side of the roughing screw vacuum pump (JP-A-7-119666, JP-A-10-184576). The pumping speed of this micro-pump is large enough to suck and exhaust the reactive gas of a minute amount (no more than 50 to 150 cc/min) flowed through the vacuum chamber (the pumping speed is one several hundredths or less that of the roughing vacuum pump). In other words, the pumping speed is set to be very small. Accordingly, since the inverse torque owing to the difference in pressure which acts on the micro-pump becomes also very small, the motive power loss becomes very small.
However, this solution is that the roughing screw vacuum pump exhausts continuously from the atmospheric pressure to a high vacuum state, i.e., from a viscous flow area of the gas to a molecular flow area. Accordingly, in order to improve the sealing ability in the viscous flow area (roughing exhaust), it is required that the number of turns of screw is increased, and the clearance between the screw and the housing is reduced. And in order to satisfy the pumping speed in the molecular flow area, a large gas transfer volume must be provided. Accordingly, the screw vacuum pump becomes large in the radial and axial directions, resulting in the severe problem of clearance variations owing to thermal expansion. Consequently, high precision machining of the screw and its screw accommodating chamber (housing) is necessary, leading to higher costs. Since the screw vacuum pump of large volume exhausts the gas near the atmospheric pressure, a motor for driving the screw vacuum pump must also have a large capacity.
(C2) Similarly, solving means of the problem of motive power loss in (3) has been proposed in which the screw vacuum pump is used at a single stage by having not only a plural number of turns of screw but also a small volume of the transfer chamber on the exhaust side, as shown in
FIGS. 11 and 12
. This conventional example will be described below to facilitate the understanding of this invention.
A rotor accommodating chamber
210
b
formed inside a housing
210
rotatably accommodates a main screw rotor
220
constituted of male and female screw rotors
220
m
and
220
f
having a ratio of teeth of 4 to 5, and a sub-screw rotor
230
constituted of another male and female screw rotors
230
m
and
230
f
having a ratio of teeth of 4 to 5.
If a motor
243
is rotated, the male rotors
230
m,
220
m
connected to this motor
243
are caused to rotate, while at the same time the female rotors
220
f
and
230
f
are caused to rotate via the timing gears
241
and
242
. In this way, if the main and sub rotors
220
and
230
are driven to rotate, the gas within the evacuated chamber is sucked through a suction port
210
a
into the inside of the housing
210
, transferred and compressed, and exhausted to the outside through an exhaust port
210
c.
By the way, the motive power required for a positive displacement vacuum pump
200
at the exhaust operation can be divided into a transfer motive power for transferring a sucked compressed fluid to the exhaust port
210
c,
a volume compression motive power owing to the volume of a transfer chamber of the positive displacement pump
200
being smaller from the suction port
210
a
to the exhaust port
210

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