Abstract:
A turbo-molecular pump has a casing, a stator fixedly mounted in the casing, and a rotor supported in the casing for rotation relatively to the stator. A turbine blade pumping assembly and a thread groove pumping assembly for discharging gas molecules are disposed between the stator and the rotor. The rotor comprises at least two components constituting the turbine blade pumping assembly and the thread groove pumping assembly. The components are separable from each other at a predetermined position, and joined to each other to form the rotor.

Description:
This is a continuation of application Ser. No. 09/531,894 filed Mar. 21, 2000 now U.S. Pat. No. 6,343,910. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a turbo-molecular pump for evacuating gas in a chamber used in a semiconductor fabrication process or the like, and more particularly to a turbo-molecular pump which is compact and has a high evacuating capability. 
     2. Description of the Related Art 
     Processes of fabricating high-performance semiconductor devices employ a turbo-molecular pump for developing high vacuum or ultrahigh vacuum. The turbo-molecular pump comprises a rotor rotatably supported in a cylindrical casing and having a plurality of rotor blades, the cylindrical casing having a plurality of stator blades projecting from an inner surface thereof between the rotor blades. The interdigitating rotor and stator blades make up a turbine blade pumping assembly. When the rotor is rotated at a high speed, gas molecules move from an inlet of the cylindrical casing to an outlet thereof to develop a high vacuum in a space that is connected to the inlet. 
     In order to achieve a high vacuum, it is necessary for the pump to provide a large compression ratio for the gas. Conventional efforts to meet such a requirement include providing the rotor and stator blades in a multistage manner or incorporating a thread groove pumping assembly downstream of the turbine blade pumping assembly. The rotor and a main shaft supporting the rotor are supported by magnetic bearings for easy maintenance and high cleanliness. 
     Recently, semiconductor fabrication apparatuses tend to use a larger amount of gas as wafers are larger in diameter. Therefore, a turbo-molecular pump used to evacuate gas in a chamber in such a semiconductor fabrication apparatus is required to evacuate gas in the chamber at a high rate, keep the chamber under a predetermined pressure or less, and have a high compression capability. 
     However, the turbo-molecular pump capable of evacuating gas in the chamber at a high rate and having a high compression capability has a large number of stages, a large axial length, and a large weight, and is expensive to manufacture. In addition, the turbo-molecular pump takes up a large space around the chamber in a clean room. Such space needs a large construction cost and maintenance cost. Another problem is that when the rotor is broken under abnormal conditions, the turbo-molecular pump produces a large destructive torque, and hence cannot satisfy desired safety requirements. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a turbo-molecular pump which is axially compact and has a sufficient evacuation and compression capability. 
     In order to achieve the above object, according to the present invention, there is provided a turbo-molecular pump comprising: a casing; a stator fixedly mounted in the casing; a rotor supported in the casing and being rotatable at a high speed; and a turbine blade pumping assembly and a thread groove pumping assembly which are disposed between the stator and the rotor; the rotor being formed by joining at least two components which are separable from each other at a predetermined position. The rotor comprises at least two components that are axially separate from each other. 
     The components of the rotor can individually be manufactured by machining, for example. The rotor can easily be manufactured under less strict machining limitations so as to have a shape suitable for a high evacuation and compression capability. Therefore, the turbo-molecular pump can evacuate gas at a high rate and has high compression capability. 
     The thread groove pumping assembly may comprise at least one of a spiral thread groove pumping assembly for discharging gas molecules radially and a cylindrical thread groove pumping assembly for discharging gas molecules axially. A plurality of cylindrical thread groove pumping assemblies may be radially superposed to provide a passage of increased length for discharging gas molecules. 
     The components of the rotor can be joined by shrink fitting or bolts. If the components of the rotor have interfitting recess and projection, then the components can easily be positioned with respect to each other and firmly be fixed to each other. The position where the components of the rotor are separable from each other is determined in consideration of simplicity for manufacturing the rotor and the mechanical strength of the rotor. For example, the components of the rotor may be separate from each other between the turbine blade pumping assembly, and the spiral thread groove pumping assembly or the cylindrical thread groove pumping assembly. 
     The spiral thread groove pumping assembly is usually disposed downstream of the turbine blade pumping assembly, and has evacuating passages for discharging gas molecules in a radial direction. Therefore, the spiral thread groove pumping assembly has an increased evacuation and compression capability without involving an increase in the axial dimension thereof. Although the rotor with the spiral thread groove pumping assembly is complex in shape, the rotor can be manufactured with relative ease because it is composed of at least two components which are separable from each other. 
     The cylindrical thread groove pumping assembly is usually disposed downstream of the turbine blade pumping assembly, and provides a cylindrical space between the rotor and the stator. The cylindrical thread groove pumping assembly may be arranged to provide two or more radially superposed passages for discharging gas molecules. The cylindrical thread groove pumping assembly having the above structure provides a long passage for discharging gas molecules, and has an increased evacuation and compression capability without involving an increase in the axial dimension thereof. Although the rotor with the cylindrical thread groove pumping assembly is complex in shape, the rotor can be manufactured with relative ease because it is composed of at least two components which are separable from each other. 
     The components of the rotor may be made of one material or different materials. Blades of the stator and rotor may be made of an aluminum alloy. However, when the turbo-molecular pump operates under a higher back pressure than the conventional one, the components made of the aluminum alloy tend to suffer strains caused by forces or pressures applied to the rotor or creep caused by increase of temperature, resulting in adverse effects on the stability and service life of the pump. In addition, the rotor may rotate unstably because the components of the aluminum alloy are liable to be expanded at higher temperatures. According to the present invention, some or all of the components of the rotor may be made of a titanium alloy which has a high mechanical strength at high temperatures or ceramics which have a high specific strength and a small coefficient of thermal expansion. The components made of the titanium alloy or ceramics are prevented from being unduly deformed or thermally expanded to reduce adverse effects on the service life of the pump and to operate the pump stably. These materials are also advantageous in that they are highly resistant to corrosion. Furthermore, because the rotor is composed of at least two components, the rotor may be made of one or more of different materials depending on the functional or manufacturing requirements for the pump. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an axial cross-sectional view of a turbo-molecular pump according to a first embodiment of the present invention; 
     FIG. 2A is a plan view of a rotor blade of a thread groove pumping assembly in the turbo-molecular pump shown in FIG. 1; 
     FIG. 2B is a cross-sectional view of a rotor blade of the thread groove pumping assembly in the turbo-molecular pump shown in FIG. 1; 
     FIG. 3 is an axial cross-sectional view of a turbo-molecular pump according to a second embodiment of the present invention; 
     FIG. 4 is an axial cross-sectional view of a turbo-molecular pump according to a third embodiment of the present invention; 
     FIG. 5 is an axial cross-sectional view of a pump according to a fourth embodiment of the present invention; 
     FIG. 6 is an axial cross-sectional view of a turbo-molecular pump according to a fifth embodiment of the present invention; 
     FIG. 7 is an axial cross-sectional view of a pump according to a sixth embodiment of the present invention; 
     FIG. 8 is an axial cross-sectional view of a pump according to a seventh embodiment of the present invention; 
     FIG. 9 is an axial cross-sectional view of a turbo-molecular pump according to an eighth embodiment of the present invention; 
     FIG. 10 is an axial cross-sectional view of a turbo-molecular pump according to a ninth embodiment of the present invention; and 
     FIG. 11 is an axial cross-sectional view of a turbo-molecular pump according to a tenth embodiment of the present invention. 
     FIG. 12 is a detailed axial cross-sectional view of the turbo-molecular pump according to the first embodiment. 
     FIG. 13 is a detailed axial cross-sectional view of the turbo-molecular pump according to the first embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Like or corresponding parts are denoted by like or corresponding reference numerals throughout views. 
     FIGS. 1,  2 A and  2 B show a turbo-molecular pump according to a first embodiment of the present invention. As shown in FIG. 1, the turbo-molecular pump according to the first embodiment has a cylindrical pump casing  10  housing a rotor R and a stator S therein, and a turbine blade pumping assembly L 1  and a thread groove pumping assembly L 2  provided between the rotor R and the stator S. The pump casing  10  has flanges  12   a ,  12   b  on respective upper and lower ends thereof. An apparatus or a pipe to be evacuated is connected to the upper flange  12   a  which defines an inlet port therein. In this embodiment, the thread groove pumping assembly L 2  comprises a spiral thread groove pumping assembly. 
     The stator S comprises abase  14  joined to the lower flange  12   b  in covering relationship to a lower opening of the pump casing  10 , a cylindrical sleeve  16  extending vertically from the central portion of the base  14 , and stationary components of the turbine blade pumping assembly L 1  and the thread groove pumping assembly L 2 . The base  14  has an outlet port  18  defined therein for discharging the gas delivered from the apparatus or the pipe to be evacuated. 
     The rotor R comprises a main shaft  20  inserted coaxially in the sleeve  16 , and a rotor body  22  mounted on the main shaft  20  and disposed around the sleeve  16 . The rotor body  22  comprises a component  22   a  of the turbine blade pumping assembly L 1  and a component  22   b  of the thread groove pumping assembly L 2 . The components  22   a  and  22   b  are composed of discrete members. The component  22   b  is positioned downstream of the component  22   a , but is axially joined to the component  22   a.    
     Between an outer circumferential surface of the main shaft  20  and an inner circumferential surface of the sleeve  16 , there are provided a motor  24  for rotating the rotor R, an upper radial magnetic bearing  26 , a lower radial magnetic bearing  28 , and an axial magnetic bearing  30  which support the rotor R out of contact with the stator s. The axial bearing  30  has a target disk  30   a  mounted on the lower end of the main shaft  20 , and upper and lower electromagnets  30   b  provided on the stator side. By this magnetic bearing system, the rotor R can be rotated at a high speed by the motor  24  under 5-axis active control. The sleeve  16  supports touch-down bearings  32   a ,  32   b  on its upper and lower portions for holding the main shaft  20  in a contact manner. 
     The rotor R also includes a plurality of axially spaced disk-shaped rotor blades  34  integrally projecting radially outwardly from an outer circumferential surface of the component  22   a  of the rotor body  22 . The stator S includes a plurality of axially spaced stator blades  36  integrally projecting radially inwardly from an inner circumferential surface of the pump casing  10 . The rotor blades  34  and the stator blades  36  are alternately disposed in an axial direction. The stator blades  36  have radially outer edges vertically held in position by stator blade spacers  38 . The rotor blades  34  have inclined blades (not shown) radially extending between an inner circumferential hub and an outer circumferential frame for imparting an axial impact to gas molecules to discharge the gas upon rotation of the rotor Rat a high speed. 
     The thread groove pumping assembly L 2  is disposed downstream, i.e., downwardly, of the turbine blade pumping assembly L 1 . The rotor R further includes a plurality of axially spaced disk-shaped rotor blades  40  integrally projecting radially outwardly from an outer circumferential surface of the component  22   b  of the rotor body  22 . The stator S further includes a plurality of axially spaced stator blades  42  integrally projecting radially inwardly from an inner circumferential surface of the pump casing  10 . The rotor blades  40  and the stator blades  42  are alternately disposed in an axial direction. The stator blades  42  have radially outer edges vertically held in position by stator blade spacers  44 . 
     As shown in FIGS. 2A and 2B, each of the rotor blades  40  has spiral ridges  46  on its upper and lower surfaces, with spiral thread grooves  48  defined between the spiral ridges  46 . The spiral thread grooves  48  on the upper surface of each of the rotor blades  40  are shaped such that gas molecules flow radially outwardly in the direction indicated by the solid-line arrow B in FIG. 2A when the rotor blades  40  rotate in the direction indicated by the arrow A. The spiral thread grooves  48  on the lower surface of each of the rotor blades  40  are shaped such that gas molecules flow radially inwardly in the direction indicated by the broken-line arrow C in FIG. 2A when the rotor blades  40  rotate in the direction indicated by the arrow A. 
     As described above, the rotor body  22  has such a structure that the component  22   a  of the turbine blade pumping assembly L 1  and the component  22   b  of the thread groove pumping assembly L 2  which are separately formed are joined to each other. The component  22   a  includes the rotor blades  34  and a boss  23  fitted over the main shaft  20 , the rotor blades  34  and the boss  23  being integrally formed by machining. The component  22   b  includes the rotor blades  40  with the spiral thread grooves, and are formed by machining or the like. The components  22   a ,  22   b  have annular steps  25   a ,  25   b  on their mating ends which are held in interfitting engagement with each other. The components  22   a ,  22   b  may be joined to each other by shrink fitting or bolts. 
     The thread groove pumping assembly L 2  provides a long zigzag discharge passage extending downwardly in a relatively short axial range between the stator blades  42  and the rotor blades  40 . The rotor R of the above structure can easily be manufactured under less strict machining limitations, but is of a shape suitable for a high evacuation and compression capability. Therefore, the turbo-molecular pump can evacuate gas at a high rate, and has high compression capability. 
     If the rotor body  22  which has the rotor blades  34  of the turbine blade pumping assembly L 1  and the rotor blades  40  of the thread groove pumping assembly L 2  are to be machined as an integral body, then a highly complex and costly machining process need to be performed over along period of time because the spiral thread grooves  48  of the rotor blades  40  are complex in shape. It may even be impossible to carry out such a machining process depending on the shape of the spiral thread grooves  48 . According to the illustrated embodiment, however, since the component  22   a  of the turbine blade pumping assembly L 1  and the component  22   b  of the thread groove pumping assembly L 2  are manufactured separately from each other, the rotor body  22  can be machined much more easily at a highly reduced cost. 
     In the first embodiment, the component  22   b  of the thread groove pumping assembly L 2  may comprise a single component. However, the component  22   b  of the thread groove pumping assembly L 2  may comprise a vertical stack of joined hollow disk-shaped members divided into a plurality of stages (see FIG.  1 ). Those hollow disk-shaped members may be joined together by shrink fitting or bolts, shown in FIGS. 12 and 13, respectively. It is preferable to construct the component  22   b  by a plurality of members in case that the spiral thread grooves are complex in shape and are impossible to be machined practically. 
     In the illustrated embodiment, the rotor blades  40  has the spiral thread grooves  48  in the thread groove pumping assembly L 2 . However, the stator blades  42  may have the spiral thread grooves  48 . Such a modification is also applicable to other embodiments of the present invention which will be described below. 
     FIG. 3 shows a turbo-molecular pump according to a second embodiment of the present invention. As shown in FIG. 3, the turbo-molecular pump according to the second embodiment includes a rotor body  22  which has a thread groove pumping assembly L 2  comprising a spiral thread groove pumping assembly L 21  and a cylindrical thread groove pumping assembly L 22  disposed upstream of the spiral thread groove pumping assembly L 21 . The cylindrical thread groove pumping assembly L 22  has cylindrical thread grooves  50  defined in an outer circumferential surface of a component  22   b  of the thread groove pumping assembly L 2 . The cylindrical thread groove pumping assembly L 22  also has a spacer  52  in the stator S which is positioned radially outwardly of the cylindrical thread grooves  50 . When the rotor R rotates at a high speed, gas molecules are dragged and discharged along the cylindrical thread grooves  50  of the cylindrical thread groove pumping assembly L 22 . 
     FIG. 4 shows a turbo-molecular pump according to a third embodiment of the present invention. As shown in FIG. 4, the turbo-molecular pump according to the third embodiment includes a rotor body  22  which has a thread groove pumping assembly L 2  comprising a spiral thread groove pumping assembly L 21  and a cylindrical thread groove pumping assembly L 22  disposed downstream of the spiral thread groove pumping assembly L 21 . 
     FIG. 5 shows a turbo-molecular pump according to a fourth embodiment of the present invention. As shown in FIG. 5, the turbo-molecular pump according to the fourth embodiment includes a rotor body  22  which has a thread groove pumping assembly L 2  comprising a cylindrical thread groove pumping assembly only. Specifically, the thread groove pumping assembly L 2  has a substantially cylindrical component  22   b  having cylindrical thread grooves  50  defined in an outer circumferential surface thereof. The thread groove pumping assembly L 2  also has a spacer  52 ˜ n  the stator S which is positioned radially outwardly of the cylindrical thread grooves  50 . When the rotor R rotates at a high speed, gas molecules are dragged and discharged along the cylindrical thread grooves  50  of the thread groove pumping assembly L 2 . 
     FIG. 6 shows a turbo-molecular pump according to a fifth embodiment of the present invention. As shown in FIG. 6, the turbo-molecular pump according to the fifth embodiment has a thread groove pumping assembly L 2  comprising a spiral thread groove pumping assembly L 21 , a cylindrical thread groove pumping assembly L 22  positioned downstream of the spiral thread groove pumping assembly L 21 , and a dual cylindrical thread groove pumping assembly L 23  positioned within the cylindrical thread groove pumping assembly L 22 . Specifically, the thread groove pumping assembly L 2  has a component  22   b  having a recess  54  formed in the lower end thereof, and the dual cylindrical thread groove pumping assembly L 23  has a sleeve  56  disposed in the recess  54 . The sleeve  56  has cylindrical thread grooves  58  defined in inner and outer circumferential surfaces thereof. 
     In operation, the cylindrical thread grooves  58  formed in the outer circumferential surface of the sleeve  56  discharge gas molecules downwardly due to a dragging action produced by rotation of the rotor R, and the cylindrical thread grooves  58  formed in the inner circumferential surface of the sleeve  56  discharge gas molecules upwardly due to a dragging action produced by rotation of the rotor R. Therefore, a discharge passage extending from the cylindrical thread groove pumping assembly L 22  through the dual cylindrical thread groove pumping assembly L 23  to the outlet port  18  is formed. Since the dual cylindrical thread groove pumping assembly L 23  is disposed in the cylindrical thread groove pumping assembly L 22 , the turbo-molecular pump shown in FIG. 6 has a relatively small axial length, and has a higher evacuation and compression capability. 
     FIG. 7 shows a turbo-molecular pump according to a sixth embodiment of the present invention. As shown in FIG. 7, the turbo-molecular pump according to the sixth embodiment has a thread groove pumping assembly L 2  comprising a cylindrical thread groove pumping assembly similar to the cylindrical thread groove pumping assembly shown in FIG. 5, and a dual cylindrical thread groove pumping assembly L 23  positioned within the cylindrical thread groove pumping assembly L 22 . Specifically, the thread groove pumping assembly L 2  of the rotor body  22  has a component  22   b  with a recess  54  defined therein and extending in substantially the full axial length thereof. The dual cylindrical thread groove pumping assembly L 23  has a sleeve  56  disposed in the recess  54 . The sleeve  56  has cylindrical thread grooves  58  defined in inner and outer circumferential surfaces thereof. 
     FIG. 8 shows a turbo-molecular pump according to a seventh embodiment of the present invention. As shown in FIG. 8, the turbo-molecular pump according to the seventh embodiment has a thread groove pumping assembly L 2  comprising, in addition to the spiral thread groove pumping assembly shown in FIGS. 1,  2 A and  2 B, an inner cylindrical thread groove pumping assembly L 24  disposed within the thread groove pumping assembly L 2 . Specifically, the component  22   b  of the thread groove pumping assembly L 2  of the rotor body  22  has a recess  60  defined therein around the cylindrical sleeve  16  to provide a space between the inner circumferential surface of the component  22   b  and the outer inner circumferential surface of the cylindrical sleeve  16 . A sleeve  56  having cylindrical thread grooves  58  formed in an outer circumferential surface thereof is inserted in the space. 
     Therefore, in this embodiment, a discharge passage extending from the lowermost end of the spiral thread groove pumping assembly upwardly between the rotor body  22  and the sleeve  56  and then downwardly between the sleeve  56  and the cylindrical sleeve  16  to the outlet port  18  is formed. 
     FIG. 9 shows a turbo-molecular pump according to an eighth embodiment of the present invention. As shown in FIG. 9, the turbo-molecular pump according to the eighth embodiment has a thread groove pumping assembly L 2  comprising, in addition to the spiral thread groove pumping assembly L 21  and the cylindrical thread groove pumping assembly L 22  disposed upstream of the spiral thread groove pumping assembly L 21  shown in FIG. 4, an inner cylindrical thread groove pumping assembly L 24  disposed within the spiral thread groove pumping assembly L 21  and the cylindrical thread groove pumping assembly L 22 . 
     FIG. 10 shows a turbo-molecular pump according to a ninth embodiment of the present invention. As shown in FIG. 10, the turbo-molecular pump according to the ninth embodiment has a thread groove pumping assembly L 2  comprising, in addition to the spiral thread groove pumping assembly L 21  and the cylindrical thread groove pumping assembly L 22  disposed downstream of the spiral thread groove pumping assembly L 21  shown in FIG. 3, an inner cylindrical thread groove pumping assembly L 24  disposed within the spiral thread groove pumping assembly L 21  and the cylindrical thread groove pumping assembly L 22 . 
     FIG. 11 shows a turbo-molecular pump according to a tenth embodiment of the present invention. As shown in FIG. 11, the turbo-molecular pump according to the tenth embodiment has a thread groove pumping assembly L 2  comprising, in addition to the cylindrical thread groove pumping assembly shown in FIG. 5, an inner cylindrical thread groove pumping assembly L 24  disposed within the cylindrical thread groove pumping assembly L 2 . 
     In the embodiments shown in FIGS. 6 through 11, the thread groove pumping assembly provides dual passages that are radially superposed for discharging gas molecules. However, the thread groove pumping assembly may provide three or more radially superposed passages for discharging gas molecules. 
     In the above embodiments, the stator blades and/or the rotor blades may be made of aluminum or its alloys. However, the stator blades and/or the rotor blades may be made of an alloy of titanium or ceramics. with the stator blades and/or the rotor blades being made of an alloy of titanium or ceramics, the turbo-molecular pump has a high mechanical strength, a high corrosion resistance, and a high heat resistance. Alloys of titanium have a high mechanical strength at high temperatures, can reduce the effect of creeping on the service life of the turbo-molecular pump, and are highly resistant to corrosion. Since ceramics has a very small coefficient of linear expansion and is thermally deformable to a smaller extent than the aluminum alloys, the rotor blades made of ceramics can rotate highly stably at high temperatures. Inasmuch as titanium and ceramics have a high specific strength than aluminum, the rotor made of titanium or ceramics can be increased in diameter for a greater evacuating capability. 
     The rotor blades, the stator blades, and the components with the spiral thread grooves and the multiple cylindrical thread grooves defined therein may be constructed as members of different materials, e.g., aluminum, titanium, and ceramics, that are individually formed and subsequently joined together. For example, the rotor blades may be made of aluminum, and the components with the spiral thread grooves may be made of titanium. Of course, the rotor blades, the stator blades, and the components with the spiral and cylindrical thread grooves defined therein may be composed of one material. 
     According to the present invention, as described above, the rotor can easily be manufactured in a shape suitable for a high evacuation and compression capability. Therefore, the turbo-molecular pump can evacuate gas in the desired apparatus or pipe at a high rate and has high compression capability. Consequently, the turbo-molecular pump can effectively be incorporated in a facility where the available space is expensive, such as a clean room in which a semiconductor fabrication apparatus is accommodated therein, for reducing the costs of equipment and operation. 
     Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.