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Vacuum exhaust apparatus and drive method of vacuum exhaust apparatus

Vacuum exhaust apparatus and drive method of vacuum exhaust apparatus description/claims

This application is a division of U.S. patent application Ser. No. 10/486,189, filed Feb. 5, 2004.

This invention relates to a vacuum exhaust apparatus, for example, for semiconductor manufacturing equipment and more particularly to a vacuum exhaust apparatus of the energy conservation type and a driving method for the vacuum exhaust apparatus to reduce consumed electric power.

Oil-sealed rotary vacuum pumps were used in most cases for early semiconductor manufacturing equipment. Electric power consumed by the oil-sealed rotary vacuum pump is generally little and it has a construction by which low ultimate pressure can be easily obtained. However, the following points should be noted where the oil-sealed rotary pump is used in semiconductor manufacturing equipment:

(1) The gasses used in semiconductor manufacturing equipment are strongly reactant in most cases. When such gas is exhausted, it reacts with vacuum pump oil and so produces reaction materials which sometimes makes the vacuum pump impossible to rotate or the pump oil is caused to deteriorate in lubrication capability.

(2) Vapor of the vacuum pump oil diffuses into the vacuum treatment or processing chamber and contaminates it.

(3) Used vacuum pump oil contains arsenic compound and phosphorus compound as toxic material in most cases. Treatment of such harmful industrial waste requires considerable cost and the number of management procedures is also considerable. For the above reasons a recently developed dry vacuum pump (called also a “dry-sealed vacuum pump” according to the international standard ISO/DIS3529/‡U-1975) which does not use sealing oil, is used instead of the oil-sealed rotary vacuum pump.

The dry vacuum pump can vacuum-pump a chamber from atmosphere and dose not contain seal oil in its compression chamber. It is a so-called mechanical vacuum pump. Positive displacement types such as a Roots type vacuum pump, a claw type vacuum pump and a screw type are used as the dry vacuum pump in most cases. These vacuum pumps all have a construction having two axes to which a pair of rotors is fixed and spaced a little gap from each other. The paired rotors are rotated in opposite directions for the vacuum pumping. The pump life is long because of non-contact portions. It can also exhaust solid components contained in the gas sucked from the semiconductor manufacturing equipment. Further, resistance to corrosive gas can be easily imparted to the positive displacement type.

Thus, a dry vacuum pump has been substituted with the oil-sealed rotary vacuum pump in semiconductor manufacturing equipment. However, there is a problem that the required electric power of the dry vacuum pump is larger than that of the oil-sealed vacuum pump.

It is required to save consumption of energy from an ecological standpoint. It is also required to reduce the cost of manufacture of the semiconductor. From the above requirements, it is desired to reduce the consumed electric power of the dry vacuum pump to less than 50% of the present.

For example, the Roots-type dry vacuum pump is provided with plural rotors fixed to the axes, adjacent each other. The paired rotors are spaced by a small gap from each other and rotated in opposite directions to suck and exhaust gas. The rotors constitute normally three to six pump chambers. The respective pump chambers effect pumping actions in regular sequences. The gas to be exhausted is displaced from the earlier or former stages to the latter stages. The gas pressure rises with the displacement. Accordingly, the throughput of the latter stages may be smaller than that of the earlier stages.

In the multi-stage Roots-type dry vacuum pump, the external form of the rotors are all made to be the same from the viewpoint of simple manufacture and synchronization of the rotations of the rotors. For the above reason, the thickness of the rotors is stepwise smaller towards the outlet side from the inlet side so as to reduce stepwise the throughputs of the pump chambers.

In the Roots-type dry vacuum pump, gas to be exhausted is temporarily confined in a space formed by an inner surface of a casing and a hollow on the surface of the rotor and the space comes to communicate with the outlet side space, with further rotation of the rotor. On that communication, the outlet side gas is reversely flowed into the space. An ultimate pressure of about 1 to 10 Pa can be obtained by the Roots-type dry vacuum pump. A pressure from the ultimate pressure to about 3 kPa is a normal pressure. The outlet pressure is atmospheric pressure. Accordingly, in order to maintain the inlet side at the reduced pressure, the gas reversed into the rotor chamber in the compression process should be pushed back. About 70 to 80 percentage of the total electric power of the pump is consumed to receive the back flow or push the flow gas from the atmosphere in the rotor chamber of the last stage.

In the Roots-type dry vacuum pump, the power for the final stage becomes smaller with a decrease of the amount of gas to be pushed back. Accordingly, as above described, the thickness of the rotor of the latter stage is reduced so as to reduce the throughput. Thus, when the throughput of the rotor chamber of the last stage is so set as to be small, the required power of the pump is reduced in the range of the normal pressure, and therefore energy conservation is obtained.

The exhaust principle of the claw-type dry vacuum pump is quite the same as that of the Roots-type dry vacuum pump, although the shapes of the rotors are different between the Roots-type and the claw type. On the other hand, the gas is transported axially along the space constituted by the two screw grooves, in the screw type. The gas of the outlet portion is flowed into the space formed by the screw grooves, to compress the gas. It is equal to the Roots-type dry vacuum pump.

Since the screw grooves are continuous, the pitches of the screw grooves are continuously reduced towards the last stage. Thus, the throughput is made smaller towards the last stage, although the throughput is arbitrarily reduced towards the last stage in the Roots-type dry vacuum pump and the claw-type dry vacuum pump. However, the pitch changes of the screw grooves are limited to some extent. Accordingly, blocks different in screw pitch are prepared, and so combined with each other as to reduce the throughput of the last stage pump chamber.

Further description about the above follows.

When the rotors are equal in size as shown in FIG. 18, the pumping speed changes with the inlet pressure (Pa) as shown by letter a in FIG. 21. When the two rotors of the former stage are equal in size, those of the middle stage are smaller than the former two, and those of the latter stage are further smaller than those of the middle stage, as shown in FIG. 19, the pumping speed changes with the inlet pressure as shown by the letter b in FIG. 21. When the two rotors of the latter stage are further smaller in comparison with the case of FIG. 19, as shown in FIG. 20, the pumping speed changes with the inlet pressure, as shown in FIG. 21.

FIG. 22 shows the relationships between the consumed electric power and the inlet pressure, in the respective cases of FIG. 18, FIG. 19 and FIG. 20. Under pressure less than 102 Pa, which is a normal pressure for semiconductor manufacturing equipment, the consumed electric power is the smallest in the case of FIG. 20, that is smaller than the case of FIG. 19 and it is the largest in the case of FIG. 18, as shown by letters c′, b′ and a′.

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