Patent Publication Number: US-6910850-B2

Title: Vacuum pump

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a vacuum pump used in an apparatus such as a semiconductor manufacturing apparatus, an electron microscope, a surface analysis apparatus, a mass spectrograph, a particle accelerator, and a nuclear fusion experiment apparatus, and, more particularly, to the structure of an inexpensive vacuum pump which has a large pumping capacity and can be handled easily. 
   2. Description of the Related Art 
   In a process such as dry etching, chemical vapor deposition (CVD), or the like performed in a high-vacuum process chamber in a semiconductor manufacturing step, a vacuum pump such as a turbo-molecular pump is used of producing a degree of high vacuum in the process chamber by exhausting gas from the process chamber. 
   As shown in  FIG. 6 , in the conventional turbo-molecular pump P 6 , a plurality of rotor blades  17  are provided on the outer wall of a cylindrical the rotor  16 , a plurality of stator blades  18 , which are positioned and fixed between rotors  17 , are fixed on the inner wall of the pump case  11 , and the rotor  16  is integrally secured to the rotor shaft  15 . 
   The process chamber connected to a gas suction port  12  at the top of the pump case  11  is in a high vacuum state. By driving a drive motor  19  so as to rotate the rotor shaft  15  at high speed, gas taken in from the gas suction port  12  is fed to a thread groove pump mechanism portion as the lower stage of the turbo molecular pump by the interaction between the rotor blades  17 , rotating at high speed together with the rotor shaft  15 , and the stator blades  18 , compressed from an intermediate flow state to a viscous flow stated by the interaction between the cylindrical surface of the outer wall of the rotor  16  and thread grooves  21  on the inner wall of a threaded stator  20 , and then discharged from a gas exhaust port  13  as the final stage of the turbo molecular pump P 6 . During this operation, since the temperature of the rotation body composed of the rotor  16  and the rotor blades  17  is increased by the heat of gas compression, it is necessary to cool the rotation body by dissipating the heat in the rotation body to stationary components in the pump case  11 . 
   Heat radiation and heat transfer are well known as means for dissipating the heat in the rotation body. The former is performed by means (a) which radiates the heat from the rotor blades  17  to the stator blades  18 , and the latter is performed by means (b) which transfers the heat by conduction via gas or means (c) which transfers the heat by conduction via bearings. However, as shown in  FIG. 6 , in the turbo molecular pump P 6  in which the rotor shaft  15  is supported in a non-contacting manner by magnetic bearings composed of radial electromagnets  22  and axial electromagnets  23 , since the rotor shaft  15  does not come into contact with protection ball bearings  24  during normal operation of the turbo molecular pump P 6 , the heat dissipation is not realized by the direct heat transfer via bearings of the means (c) but achieved by the heat radiation and the heat transfer via gas of the means (a) and (b), respectively. Further, when an amount of gas flowing in the pump case  11  is small or a low-thermal-conductivity gas such as Ar gas is pumped, the heat transfer via gas of the means (b) cannot be expected. Thus, only the heat radiation of the means (a) is dependable means for dissipating the heat, thereby resulting in poor heat-dissipation efficiency and accordingly causing the heat of compression of the gas pumped by the turbine to be likely stored in the rotor blades  17 . 
   To solve this problem, as shown in  FIG. 6 , so far the rotating cylindrical body composed of the rotor  16  and the rotor blades  17  has been cooled by feeding a high-thermal-conductivity purging gas such as nitrogen gas (i.e., N 2  gas) into the pump case  11  from the outside. More particularly, as  FIG. 6  shows the flows of the vacuum-pumping gas and the purging gas indicated by the dotted and solid arrow lines, respectively, the purging gas flows along a passage R, which is in communication with the gap between the outer wall of the rotor shaft  15  and the inner wall of a stator column  14  and with the other gap between the outer wall of the stator column  14  and the inner wall of the rotor  16 , and exits from the gas exhaust port  13 , thereby the heat of gas compression stored in the rotor  16  being dissipated from the inner wall of the rotor  16  to the outer wall of the stator column  14 . 
   According to this method, in order to improve the cooling effect, a gap g 1  between the inner wall of the rotor  16  and the outer wall of the stator column  14  is required to be as small as possible. That is because, if the gap g 1  is large, a thermal boundary layer is produced in a viscous flow region, thereby lowering the thermal conductivity of the purge gas between the inner wall of the rotor  16  and the outer wall of the stator column  14 , and also if the gap g 1  becomes larger than an average free path of gas molecules in a molecular flow region, the probability in which the gas molecules released from the surface of the rotor  16  directly reaches the surface of the stator column  14  becomes lower, thereby lowering the thermal conductivity of the purge gas in the same fashion as described above. 
   However, as shown in  FIG. 7 , in a turbo molecular pump P 7  in which, when a rotor  16 - 1  having rotor blades  17 - 1  having a larger diameter L 7  than the rotor blades  17  having a diameter L 6  shown in  FIG. 6  is mounted on the rotor shaft  15  shown in  FIG. 6  so as to pump a larger amount of gas, the rotor  16 - 1  and the stator column  14  have a very large gap g 2  between the inner wall of the rotor  16 - 1  and the outer wall of the stator column  14 , compared to the small gap g 1  shown in  FIG. 6  between the inner wall of the rotor  16  and the outer wall of the stator column  14 . Since such a large gap g 2  causes the purging gas to have a dramatically lowered thermal conductivity as described above, it is required to make the gap g 2  smaller down to the predetermined gap g 1  by forming the outer-wall shape of the stator column  14  based on the inner-wall shape of the rotor  16 - 1  so as to achieve a desired thermal conductivity of the purging gas. 
   As a method for making the gap g 2  smaller, forming the rotor  16 - 1  so as to have a thick lower part when manufacturing is considered. However, the thicker the lower part, the higher the cost of the rotor  16 - 1  becomes. In addition, since the rotor  16 - 1  is a high-speed rotating component during operation of the turbo molecular pump, the thicker lower part leads to the heavier rotor  16 - 1 , and thus the turbo molecular pump requires a larger power for its operation, thereby resulting in a deteriorated compression performance and likely causing the rotation body to rotate in an unbalanced state. 
   As another method for making the gap g 2  smaller, forming the stator column  14  so as to have an outer-wall shape based on the inner-wall shape of the rotor  16 - 1  is considered. However, in this case, several types of the stator columns  14 , having different outer-wall shapes and accommodating expensive electrical components and the like therein, must be prepared and disposed in the pump case  11  depending on the inner-wall shape of the rotor, thereby causing a dramatic cost increase in manufacturing the turbo molecular pump. 
   The present invention has been made in view of the above-described problems. Accordingly, it is an object of the present invention to provide a vacuum pump in which, when a rotor having a large diameter is mounted so as to pump a large amount of gas, a small gap is easily formed, with a small amount of additional cost, between the inner wall of the rotor and the outer wall of a stator column, and which achieves a dramatic cost reduction in manufacturing the vacuum pump compared to the manufacturing cost of the conventional vacuum pump. 
   SUMMARY OF THE INVENTION 
   To achieve the above-described object, a vacuum pump according to the present invention comprises a rotor shaft rotatably supported in a pump case having a gas suction port at the top thereof and a gas exhaust port at a part of the lower side wall thereof; a drive motor for rotating the rotor shaft; a stator column accommodating the rotor shaft and the drive motor and provided in the pump case so as to be erected; a rotor surrounding the stator column and fixed to the rotor shaft; and a spacer having an outer-wall shape based on the inner-wall shape of the rotor-and detachably fixed to the peripheral outer surface of the stator column. 
   In the vacuum pump according to the present invention, the spacer may fill in the gap between the stator column and the rotor so as to provide a predetermined small gap between the outer wall surface of the spacer and the inner wall surface of the rotor. 
   Also, in the vacuum pump according to the present invention, the spacer may be composed of a high-thermal-conductivity metal material. 
   In the vacuum pump according to the present invention, the fixing structure between the stator column and the spacer may be adopted the construction in which a part of the outer wall of the spacer is cut out so as to form a flange and the spacer is fixed to the stator column by clamping the flange. 
   Further, the fixing structure between the stator column and the spacer may be adopted the construction in which the spacer is fixed to the stator column by fastening a setscrew screwed from the outer wall to the inner wall of the spacer. 
   Furthermore, the fixing structure between the stator column and the spacer may be adopted the construction in which the spacer is fixed to the stator column by fastening through a fixing hole provided in the stator column in the axial direction of the rotor shaft. 
   The vacuum pump according to the present invention may have a turbo-molecular pump mechanism portion wherein a plurality of rotor blades are integrally formed on the outer wall of the rotor and a plurality of stator blades are integrally formed on the outer wall of the rotor. The rotor blades and the stator blades are alternately disposed in the pump case. 
   According to the present invention, the vacuum pump has a structure in which a spacer having an outer-wall shape based on the inner-wall shape of the rotor and detachably fixed to the outer circumferential surface of the stator column. With this structure, even when the vacuum pump is required to pump a large amount of gas, and thus the rotor having the large-diameter rotor blades is mounted on the stator so as to form a small predetermined gap between the inner wall of the rotor and the outer wall of the stator column, the rotor is not required to have a thick part, or the expensive stator column is not required to be manufactured depending on the size of the gap, but to exchange the spacer only, thereby leading to a dramatic reduction in manufacturing cost of the vacuum pump. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a vertical sectional view illustrating the entire structure of a vacuum pump according to the present invention; 
     FIGS.  2 ( a ) and  2 ( b ) illustrate a first embodiment of a spacer fixing structure wherein FIG.  2 ( a ) is a vertical sectional view of the vacuum pump and FIG.  2 ( b ) is a cross-sectional view taken along the line II—II indicated in FIG.  2 ( a ); 
     FIGS.  3 ( a ) and  3 ( b ) illustrate a second embodiment of a spacer fixing structure wherein FIG.  3 ( a ) is a vertical sectional view of the vacuum pump and FIG.  3 ( b ) is a cross-sectional view taken along the line III—III indicated in FIG.  3 ( a ); 
       FIG. 4  is a vertical sectional view illustrating a third embodiment of a spacer fixing structure of the vacuum pump shown in  FIG. 1 ; 
       FIG. 5  is a vertical sectional view of the vacuum pump according to the present invention, having large-diameter rotor blades disposed therein; 
       FIG. 6  is a vertical sectional view illustrating the entire structure of a conventional vacuum pump; and 
       FIG. 7  is a vertical sectional view illustrating disadvantages of the conventional vacuum pump, shown in  FIG. 6 , having large-diameter rotor blades disposed therein. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of a vacuum pump according to the present invention will be described in detail with reference to the accompanying drawings. 
     FIG. 1  is a vertical sectional view illustrating the entire structure of a vacuum pump P 1  according to the present invention. As shown in the figure, the vacuum pump P 1  has a composite type pump mechanism formed by a turbo molecular pump mechanism portion PA and a thread groove pump mechanism portion PB, both being accommodated in a pump case  11 . 
   The pump case  11  is composed of a cylindrical portion  11 - 1  and a base member  11 - 2  mounted at the lower end thereof. The upper surface of the pump case  11  is opened and serves as a gas suction port  12 . To the gas suction port  12 , a vacuum vessel such as a process chamber (not shown) is fixed to a flange of the pump case  11  with a screw. The lower side surface of the pump case  11  has a gas exhaust port  13 , to which a gas exhaust pipe  23  is mounted. 
   The lower bottom of the pump case  11  is covered with a bottom cover  11 - 3 , above which a stator column  14  being provided so as to be erected toward the inside of the pump case  11  is fastened to the base member  11 - 2 . The stator column  14  has a rotor shaft  15 , which passes through the end faces of the stator column and is rotatably journaled by radial electromagnets  22  and axial electromagnets  23 , both serving as magnetic bearings, which are provided in the stator column  14 , in the radial and axial directions of the rotor shaft  15 . A ball bearing  17  coated with a dry lubricant prevents the contact between the rotor shaft  15  and the electromagnets  22  and  23  to support the rotor shaft  15  at the power failure of a magnetic bearing composed of the radial electromagnet  22  and the axial electromagnet  23 , being in non-contact with the rotor shaft  15  in normal operation. 
   In the pump case  11 , a rotor  16  is disposed so as to surround the stator column  14 . The top end of the rotor  16  extends upwards close to the gas suction port  12  and the rotor  16  is fixed to the rotor shaft  15  with screws in the axial direction L of the rotor shaft  15 . A drive motor  19  such as a high-frequency motor disposed between the rotor shaft  15  and the stator column  14  in the substantially central part of the rotor shaft  15  with respect to the axial direction L so that the drive motor  19  drives the rotor shaft  15  and the rotor  16  to rotate at high speed. 
   In addition, the rotor  16  has a plurality of rotor blades  17  integrally formed therewith on the upper outer wall thereof such that the blades  17  are disposed starting from the vicinity of the gas suction port  12  and coming down along the axial direction L. The cylindrical portion  11 - 1  in the pump case  11  has a plurality of stator blades  18  fixed to the inner wall thereof such that the rotor blades  17  and the stator blades  18  are alternately disposed. This structure forms the turbo molecular pump mechanism PA in which gas molecules from the gas suction port  12  are fed into the lower stage of the pump mechanism PA by the interaction between the high-speed rotating rotor blades  17  and the stationary stator blades  18 . 
   The lower outer wall of the rotor  16  is a smooth cylindrical surface. The base  11 - 2  in the pump case  11  has a cylindrical threaded stator  20  fixed thereto and opposing the cylindrical surface of the lower outer wall of the rotor  16  with a small gap therebetween. The threaded stator  20  has a plurality of thread grooves  21 , indicated by a dotted line in the figure, formed on the inner surface thereof. This structure forms the thread groove pump mechanism portion PB in which the gas molecules fed from the turbo molecular pump mechanism PA are compressed from an intermediate flow state to a viscous flow state by the interaction between the cylindrical surface of the lower outer wall of the rotor  16  rotating at high-speed and the thread grooves  21  on the inner wall of the stationary threaded stator  20  and then are exhausted from the gas exhaust port  13  in the subsequent stage of the pump mechanism PB. 
   A spacer S is provided between the lower inner wall of the rotor  16  and the outer wall of the stator column  14  opposing thereto, spacer which has an outer-wall shape Sb based on an inner-wall shape  16   a  of the rotor  16 . The spacer S is preferably composed of a high-thermal-conductivity metal material. Accordingly, the spacer S is formed by machining a light metal, such as an aluminum alloy, which is a relatively soft metal and is easily processed, and further has a relatively large specific tensile strength, or an iron-base metal, such as a stainless steel or a nickel steel, into a predetermined shape and then is detachably fixed to the peripheral outer surface of the stator column  14 . Although various types of detachable fixing structures between the spacer S and the stator column  14  are considered, for example, those shown in  FIGS. 2  to  4  may be adopted. 
   The fixing structure of a spacer S 1  in a vacuum pump P 1  shown in  FIG. 2  is adopted as a structure in which a part of the outer wall of the spacer S 1  is cut out so as to form a flange  31  and the spacer S 1  is fixed to the stator column  14  by clamping the flange  31  with a connecting member such as a bolt  33 . More particularly, as shown in FIG.  2 ( b ), the spacer S 1  is generally ring-shaped and has a clamping structure. In order to have an inner diameter conforming to an outer circumferential surface or outer-wall shape  14   a  of the stator column  14  and also an outer diameter conforming to the inner-wall shape  16   a  of the rotor  16 , a radial cut or pass-through groove  32  is formed at a part the spacer S 1  from the outer wall to the inner wall (e.g., outer and inner circumferential surfaces) thereof to define a pair of arm portions  90 ,  92  having aligned through-holes  90   a ,  90   a , respectively, which are aligned with one another. The flange  31  is formed by cutting out a part of the outer wall of the spacer S 1  in the vicinity of the pass-through groove  32  so as to have an L-sectional shape. The spacer S 1  is clamped to the stator column  14  by inserting the bolt  33  through the through-hole  90   a ,  90   a  of the arm portion  90 ,  92 , respectively, from the flange  31  so that the bolt  33  extends orthogonal to the pass-through groove  32 . In this manner, the spacer S 1  is tightened and thereby removably integrally connected to the outer circumferential surface of the stator column  14 . 
   The fixing structure of a spacer S 1  in a vacuum pump P 3  shown in  FIG. 3  is adopted as a structure in which the spacer S 2  is fixed to the stator column  14  by fastening a connecting member or setscrew  41  screwed from the outer wall to the inner wall thereof. More particularly, as shown in FIG.  3 ( b ), in this fixing structure that is a fastening structure, in order to have an inner diameter based on an outer-wall shape  14   a  of the stator column  14  and also an outer diameter based on the inner-wall shape  16   a  of the rotor  16 , a threaded hole  42  is formed in a part thereof so as to extend from the outer wall to the inner wall of the cylindrical spacer S 2  having ring-shape cross section, and thus the spacer S 2  is fastened to the stator column  14  from the side surface thereof with the setscrew  41  inserted through the threaded hole  42  so that the setscrew  41  contacts the outer circumferential surface of the stator column  14 . 
   The fixing structure of the spacer S 3  in a vacuum pump P 4  shown in  FIG. 4  is adopted as a structure in which the spacer S 3  is fixed to the stator column  14  by fastening a bolt  54  placed in a fixing hole  52  provided in the stator column  14  in the axial direction L of the rotor shaft  15 . More particularly, in this fixing structure, in order to have an inner diameter based on the outer-wall shape  14   a  of the stator column  14  and also an outer diameter based on the inner-wall shape  16   a  of the rotor  16 , a fixing step portion  53  is formed on a part of the outer wall of the spacer S 3  having a ring-shape cross section so as to have an L-sectional shape, a fixing through-hole  51  is formed in the fixing step  53  portion in the axial direction L of the rotor shaft  15 , the fixing hole  52  is formed in the stator column  14  so as to agree with the fixing through-hole  51 , and thus the spacer S 3  is fastened to the stator column  14  in the axial direction L of the rotor shaft  15  with the bolt  54  inserted and screwed through the fixing through-holes  51  and  52  in this order. 
   With these fixing structures shown in  FIGS. 2  to  4 , the cylindrical spacers S 1  to S 3  disposed around the outer circumferential surface of the cylindrical stator column  14  are firmly fixed to the stator column  14 . In addition, the spacers S 1 , S 2 , and S 3  are easily detached from the stator column  14  only by unfastening the bolt  33 , the setscrew  41 , and the bolt  54 , respectively. 
   Referring now to  FIG. 5 , an operation of the vacuum pump according to the present invention will be described. 
     FIG. 5  is a vertical sectional view of a turbo molecular pump Pn in which a rotor  16 - n  having rotor blades  17 - n  which have a larger outer diameter Ln than the outer diameter L 1  of the rotor blades  17  shown in  FIG. 1  is mounted on the rotor shaft  15  shown in FIG.  1 . The same members are identified by the same reference numerals shown in FIG.  1  and their detailed description will be omitted. Also, since the composite-type pump mechanism composed of the turbo molecular pump PA and the thread groove pump mechanism portion PB is substantially the same as the conventional vacuum pump, the operation of the pump mechanism will not be described. 
   As shown in  FIG. 5 , since the rotor  16 - n  has the rotor blades  17 - n  having the larger outer diameter Ln than the outer diameter L1 of the rotor blades  17  shown in  FIG. 1 , a larger gap gn is formed between the inner wall of the rotor  16 - n  and the outer wall of the stator column  14 , than the gap g 1  shown in FIG.  1 . To solve this problem, a spacer Sn having a larger diameter than that of the spacer S shown in  FIG. 1  is disposed on and fixed to the stator column  14  in this embodiment. More particularly, the spacer Sn has inner-wall shape Sna and outer-wall shape Snb based on the outer-wall shape  14   a  of the stator column  14  and inner-wall shape  16 - na  of the rotor  16 - n , respectively, and is detachably fixed to the peripheral outer surface of the stator column  14  such that the fixed spacer Sn and the rotor  16 - n  have the predetermined small gap g 1  between the outer wall of the fixed spacer Sn and the inner wall of the rotor  16 - n . Since the spacer Sn is fixed to the peripheral outer surface of the stator column  14  which is stationary during operation of the vacuum pump, the spacer Sn is not displaced by the centrifugal force of the rotating cylindrical body composed of the rotor  16 - n  and the rotor blades  17 - n  and thus always maintains a predetermined gap from the inner wall of the rotor  16 - n.    
   Thus, as shown in  FIG. 5 , the cylindrical rotation body composed of the rotor  16 - n  and the rotor blades  17 - n  under an elevated temperature state caused by the heat of gas compression during operation of the vacuum pump is cooled by feeding a high-thermal-conductivity purging gas such as nitrogen gas (i.e., N 2  gas) into the pump case  11  from the outside. More particularly, as  FIG. 5  shows the flows of the vacuum-pumping gas and the purging gas indicated by the dotted and solid arrow lines, respectively, the purging gas flows along a passage Rn, which is in communication with the gap between the outer wall of the rotor shaft  15  and the inner wall of the stator column  14  and with the other gap between the outer wall of the spacer Sn and the inner wall of the rotor  16 - n , and exits from the gas exhaust port  13 , thereby the purging gas transferring the heat of gas compression stored in the rotor  16 - n  by conduction from the inner wall of the rotor  16 - n  to the outer wall of the stator column  14  and also to the outer wall of the spacer Sn. With this structure, a thermal boundary layer, which would be formed in the large gap between the outer wall of the stator column  14  and the inner wall of a rotor  16 - n  if the spacer Sn is not disposed in the gap, is not formed in the small gap between the outer wall of the spacer Sn and inner wall of the rotor  16 - n . Accordingly, the purging gas is prevented from having a lowered thermal conductivity and effectively transfers the heat of gas compression by conduction so as to discharge the heat outside the vacuum pump. 
   Furthermore, when the rotor  16 - n  having the rotor blades  17 - n  which have the large outer diameter Ln is mounted on the stator column  14  so as to have the predetermined small gap g 1  between the inner wall of the rotor  16 - n  and the outer wall of the stator column  14 , the rotor  16 - n  is neither required to be formed so as have a thick lower part, nor the stator column  14  accommodating expensive electrical components and the like is required to be manufactured depending on the size of the gap. The only thing to do is to exchange the spacer Sn and fix it to the stator column  14 . As a result, a dramatic cost reduction in manufacturing the vacuum pump can be expected in comparison with the manufacturing cost of the conventional vacuum pump. 
   In the above-described embodiment, in the thread groove pump mechanism portion PB, the outer wall of the rotor  16  is a smooth cylindrical surface and the thread grooves  21  are formed on the inner wall, opposing the cylindrical surface, of the threaded stator  20 . Alternatively, the thread grooves  21  may be formed on the outer wall of the lower part of the rotor  16  and the threaded stator  20  may have an inner wall, opposing this outer wall, formed so as to be a smooth cylindrical surface. In this case, the effect of the interaction between the thread grooves  21  on the outer surface of the rotor  16  and the cylindrical surface of the threaded stator  20  can also be expected in the same fashion as that in the above described embodiment. 
   Although a turbo molecular pump is used in the foregoing embodiments by way of example, the present invention is also applicable to a groove pump and a vortex pump whose structures are well known, in addition, to a molecular pump which is a combination of the turbo molecular pump, the groove pump, and the vortex pump. 
   As described above in detail, according to the present invention, the vacuum pump has a structure in which a spacer having an inner-wall shape based on an outer-wall shape of the stator column and an outer-wall shape based on the inner-wall shape of the rotor and detachably fixed to the peripheral outer surface of the stator column is detachably fixed to the outer circumferential surface of the stator column. With this structure, a thermal boundary layer is not formed in the gap between the outer wall of the stator column and the inner wall of a rotor. Accordingly, a lowered thermal conductivity can be prevented and effective heat transfer can be achieved. Also even when it is required to form a small predetermined gap between the inner wall of the rotor and the outer wall of the stator column, the rotor is not required to have a thick part, or the expensive stator column is not required to manufactured depending on the size of the gap, but to exchange the spacer only, thereby leading to a dramatic reduction in manufacturing cost of the vacuum pump.