Disclosed is a turbo-molecular pump, which comprises a rotor including a first cylindrical body formed with a part of a plurality of rotating blades arranged in multistage, a second cylindrical body integrally connected to an outer peripheral region of a downstream end of the first cylindrical body and formed with the remaining rotating blades, and a stress-releasing protrusion extending from the downstream end of the first cylindrical body along a direction of a rotation axis of the rotor. The turbo-molecular pump of the present invention can reduce a stress in the downstream end of the first cylindrical body.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a turbo-molecular pump.

2. Description of the Related Art

Heretofore, a turbo-molecular pump has been used as a pump for evacuating an inside of a process chamber to perform a given treatment in a high vacuum atmosphere during a semiconductor manufacturing process. The turbo-molecular pump comprises a rotor which has a plurality of rotating blades formed in an outer peripheral surface of a bell-shaped cylindrical body thereof, in a multistage arrangement. In view of a need for each of the rotating blades to achieve a higher compression ratio as it is located on a more downstream side, a downstream cylindrical body provided with a part of the rotating blades located in a downstream region (i.e., downstream rotating blades) is designed to have an outer diameter greater than that of an upstream cylindrical body provided with the remaining rotating blades located in an upstream region (i.e., upstream rotating blades), in order to provide a higher peripheral speed to a root portion of each of the downstream rotating blades (see, for example, JP 2006-090231A).

In a portion of the upstream cylindrical body integrally connected to the downstream cylindrical body, the downstream cylindrical body is applied thereto as an additional mass to cause an increase in stress in a downstream end of the upstream cylindrical body. Moreover, there is a problem that, when it is attempted to increase a rotation speed of the rotor so as to obtain enhanced evacuation performance, an upper limit of the rotor rotation speed will be undesirably restricted by the stress in the downstream end.

SUMMARY OF THE INVENTION

In view of the above circumstances, it is an object of the present invention to provide a turbo-molecular pump comprising upstream and downstream cylindrical bodies provided with a plurality of rotating blades in a multistage arrangement, capable of reducing a stress in a downstream end of the upstream cylindrical body.

In order to achieve this object, the present invention provides a turbo-molecular pump which comprises a rotor formed with a plurality of rotating blades in a multistage arrangement and adapted to be rotated at a high speed so as to perform an evacuation operation. The rotor includes: a first cylindrical body formed with a part of the plurality of rotating blades; a second cylindrical body integrally connected to an outer peripheral region of a downstream end of the first cylindrical body, and formed with the remaining rotating blades; and a stress-releasing protrusion extending from the downstream end of the first cylindrical body along a direction of a rotation axis of the rotor.

The first cylindrical body may be formed in a conical shape.

The turbo-molecular pump may further include a screw stator which extends to have a distal edge located between the second cylindrical body and the protrusion, so that an outer peripheral surface of the screw stator forms thread groove pumping means in cooperation with an inner peripheral surface of the second cylindrical body.

The screw stator may have an upstream end formed with a convex portion protruding toward the protrusion, and the inner peripheral surface of the second cylindrical body may be formed with a concave portion in opposed relation to the convex portion.

As above, in the present invention, the downstream end of the first cylindrical body integrally connected with the second cylindrical body is provided with the stress-releasing protrusion extending along a direction of a rotation axis of the rotor. This makes it possible to reduce a stress in the downstream end of the first cylindrical body.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

With reference to the drawings, the present invention will be specifically described based on an exemplary embodiment thereof.FIG. 1is a sectional view schematically showing the structure of a turbo-molecular pump according to one embodiment of the present invention. As shown inFIG. 1, the turbo-molecular pump comprises: a rotor3formed with a plurality of rotating blades1a,1bin a multistage arrangement and a cylindrical-shaped screw rotor portion2; a plurality of stationary blades4a,4b; and two screw stators5a,5b; and a pump casing10housing the above components. In the turbo-molecular pump illustrated inFIG. 1, a combination of the rotating blades1a,1band the stationary blades4a,4bmakes up a turbo-molecular pumping section on a high-vacuum side, and a combination of the screw rotor portion2and the screw stators5a,5bmakes up a thread groove pumping section on a low-vacuum side.

Each of the rotating blades1aand the stationary blades4adisposed on an upstream side of the turbo-molecular pumping section has a relatively long blade length, and each of the rotating blades1band the stationary blades4bdisposed on a downstream side of the turbo-molecular pumping section has a relatively short blade length. In the following description, the rotating blades1aand the stationary blades4aon the upstream side will be referred to respectively as “upstream rotating blades1a” and “upstream stationary blades4a”, and the rotating blades1band the stationary blades4bon the downstream side will be referred to respectively as “downstream rotating blades1b” and “downstream stationary blades4b”. Each of the upstream stationary blades4aand the downstream stationary blades4bis positioned and fixed by a plurality of spacer rings7stacked on a base member6.

In the thread groove pumping section, the screw stator5ais disposed in opposed relation to an outer peripheral surface of the screw rotor portion2with a small gap therebetween, and the screw stator5bis disposed in opposed relation to an inner peripheral surface of the screw rotor portion2with a small gap therebetween. The screw stators5a,5bare fixed to the base member6by a bolt or the like.

The rotor3is attached to a rotary shaft8. The rotary shaft8is adapted to be supported by a plurality of electromagnets9a,9b,9cfor a magnetic bearing system, and drivenly rotated by a motor11. When the magnetic bearing system is not activated, the rotary shaft8is supported by a mechanical bearing12. The pump casing10and the base member6are integrated together by a bolt or the like. The pump casing10has a flange10aadapted to be fastened to a flange of a target apparatus13by a bolt15so as to allow the turbo-molecular pump to be mounted to the target apparatus13.

When the rotary shaft8and the rotor3are rotated at a high speed by the motor11, an evacuation function is produced in each of the turbo-molecular pumping section and the thread groove pumping section. Thus, gas on the side of an inlet port11bis evacuated in a direction indicated by the arrows. The gas pumped out from the turbo-molecular pumping section to the thread groove pumping section is pumped out downwardly (inFIG. 1) by an action of the screw rotor portion3and the screw stator5a, and then directed upwardly (inFIG. 1) through the gap between the screw rotor section2and the screw stator5b. The base member6is formed with a discharge port14fluidically connected to a low-vacuum pump. Thus, the gas pumped out of the thread groove pumping section is discharged to an outside of the pump through the low-vacuum pump.

FIG. 2is a sectional view showing the rotor3. As shown inFIG. 2, the rotor3has a bell-shaped cylindrical body having the rotating blades1a,1band the screw rotor portion2formed thereon. Each of the rotating blades1a,1bprovided in a multistage arrangement along a direction of a rotation axis of the rotor3is designed to achieve a higher compression ratio as it is located on a more downstream side. For this purpose, the length of each of the downstream rotating blades1bis set to be less than that of each of the upstream rotating blades1a. Thus, each of the downstream rotating blades1bcan have a higher peripheral speed in a root portion thereof, as compared with the upstream rotating blades1a, to provide enhanced evacuation performance.

The bell-shaped cylindrical body of the rotor3comprises an upstream cylindrical body30having the upstream rotating blades1aformed thereon, and a downstream cylindrical body31having the downstream rotating blades1band the screw rotor portion2formed thereon. The upstream cylindrical body30is formed in a conical shape having a diameter which gradually increases in a downstream direction. The downstream cylindrical body31is formed to have a diameter greater than that in a downstream end of the upstream cylindrical body30. A bottom of the downstream (inFIG. 3, lower) end of the upstream cylindrical body30has a protrusion32(shaded area) formed to protrude in the downstream (inFIG. 3, downward) direction. Specifically, the protrusion32is formed to extend from the downstream end of the upstream cylindrical body30downwardly, and have a radial thickness which gradually decreases in the downstream direction.

The downstream cylindrical body31with a larger diameter than that of the upstream cylindrical body30receives a larger centrifugal force according to rotation of the rotor. In addition, the downstream cylindrical body31acts as an additional mass which leads to a larger centrifugal force around the downstream end of the upstream cylindrical body30. Consequently, in conventional turbo-molecular pumps, a stress in the downstream end of the upstream cylindrical body30is increased to impose restrictions on an upper limit of rotation speed of the rotor.

In this embodiment, the protrusion32is formed to extend from the bottom of the downstream end of the upstream cylindrical body30in the downstream direction along a rotation axis (i.e., the rotary shaft) of the rotor. Thus, the upstream cylindrical body30and the protrusion32may be considered as a single piece of cylindrical body. This makes it possible to distribute a stress in the downstream end to the protrusion32so as to reduce the stress in the downstream end. In addition, the upstream cylindrical body30formed in a conical shape can have a uniform stress distribution.

Therefore, the upper limit of the rotor rotation speed can be increased to achieve enhanced evacuation performance of the turbo-molecular pump. Additionally, a distal edge (inFIG. 3, upper end) of the screw stator5bis extended into a space A defined between the protrusion32and the downstream cylindrical body31. Thus, in addition to the inner peripheral surface of the screw rotor portion2, an inner peripheral surface of the downstream cylindrical body31corresponding to a region having the downstream rotating blades1bformed thereon can be used as a part of the thread groove pumping section. This makes it possible to increase a length of a pumping path of the thread groove pumping section so as to achieve higher compression ratio and higher pumping speed during high flow pumping. If the evacuation performance is set at conventional level, an axial dimension of the screw rotor portion2can be reduced to facilitate reduction in axial size of the pump.

Furthermore, in the event of breakage of the rotor, the protrusion32collides with a region of the screw stator5bextended into the space A, so that the screw stator5bcan be deformed to absorb a part of energy during breakage of the rotor. This makes it possible to reduce a shock to be transferred to the pump casing10so as to reduce shack energy to be applied to the pump-fastening bolt15to prevent breakage of the bolt15.

In the above embodiment, the protrusion32having a triangular shape in section is formed in the downstream end of the upstream cylindrical body30. Alternatively, as shown inFIG. 3, a protrusion33having a rectangular shape in section may be formed. This protrusion33can achieve the same advantages as those in the protrusion32illustrated inFIG. 2.

FIG. 4is a sectional view showing one example of a modification of the aforementioned turbo-molecular pump. This modification is intended to provide enhanced rotor-restraining function in the event of breakage of the pump, based on the screw stator5b. In the event of breakage of the rotor, resulting broken rotor pieces move in a slightly upward direction, instead of a horizontally outward direction. Thus, in a configuration of the protrusion32and the screw stator5billustrated inFIG. 1, an upper portion of the screw stator5bis deformed toward the pump casing to have a mortar-like shape, due to collision with the broken rotor pieces, and thereby the protrusion32on the side of the rotor is likely to be disengaged from the screw stator5b.

In the modification illustrated inFIG. 4, a convex portion50is formed on an inner peripheral surface of the region of the screw stator5bextended into the space A, and a concave portion32ais formed in an outer surface of the protrusion32, in opposed relation to the convex portion50. Thus, in the event of breakage of the rotor, even if the screw stator5bis deformed, this configuration can reduce a risk of release of an engagement between the concave portion32aon the side of the rotor and the convex portion50of the screw stator5b. This makes it possible to prevent broken rotor pieces from colliding with the pump casing10.

The above embodiment has been described by taking the turbo-molecular pump having the turbo-molecular pumping section and the thread groove pumping section, as one example. Alternatively, the present invention may be applied to a turbo-molecular pump having only a turbo-molecular pumping section comprising a combination of the rotating blades1a,1band the stationary blades4a,4b. Furthermore, the present invention may be applied to a turbo-molecular pump other than the magnetic bearing type. It is understood that the present invention is not limited to the above embodiment, but various changes and modifications may be made therein without departing from the spirit and scope of the present invention as set forth in appended claims.

In a correspondence between the above embodiment and elements of the appended claims, the upstream cylindrical body30serves as the first cylindrical body, and the downstream cylindrical body31serves as the second cylindrical body. This correspondence between the above embodiment and elements of the appended claims is described only by way of example, and this description is not meant to be construed in a limiting sense.