Abstract:
A vacuum pump has a casing having an inlet port and an outlet port, a flange integral with the casing and having a peripheral inner surface defining the inlet port and an end surface for connection to a container to be evacuated, and exhaust means for drawing in a gas through the inlet port and discharging the gas through the outlet port. A coating of material having low heat conductivity is disposed on the end surface of the flange.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to vacuum pumps and, more specifically, to a turbo-molecular pump used to discharge process gas of a semiconductor manufacturing apparatus. 
     2. Description of the Related Art 
     As a result of the rapid progress in techniques for manufacturing semiconductors for an integrated circuit or the like and the demand for higher production amounts thereof, there is an increasing demand for a vacuum pump for discharging process gas from the chamber of a semiconductor manufacturing apparatus. 
     Generally speaking, as such a vacuum pump, a turbo-molecular pump in which the exhaust amount per unit time is large and which makes it possible to attain a high vacuum is used. 
     The exhaust system for discharging gas from the chamber of a semiconductor manufacturing apparatus is formed by arranging piping directly below the chamber to connect a conductance valve, and connecting a turbo-molecular pump to the conductance valve. The conductance valve is a valve for adjusting the chamber pressure. 
     By thus arranging the turbo-molecular valve in close vicinity to the chamber, the piping from the chamber to the turbo-molecular pump is shortened, whereby the reduction in conductance (easiness with which exhaust gas is conveyed) due to the piping is restrained. 
     In some cases, the turbo-molecular pump is directly connected to the chamber of the semiconductor manufacturing apparatus, without providing any conductance valve therebetween. 
     In the chamber of the semiconductor manufacturing apparatus, operations such as application of high-temperature process gas in the form of plasma to a semiconductor substrate and etching thereon are performed. 
     Such process gas is discharged by the turbo-molecular pump through the conductance valve, without being sufficiently cooled. 
     Thus, heat is imparted to the piping connected to the chamber and the conductance valve, and this heat is transmitted to the turbo-molecular pump. 
     In some cases, to enhance the reactivity of the process gas, the chamber itself is heated. Further, nowadays, in some cases, to prevent generation of deposit of the product, the conductance valve is heated. 
     As a result of heat conduction due to these factors, the temperature of the flange portion formed in the inlet of the turbo-molecular pump can exceed 60° C. 
     Inside a turbo-molecular pump, a rotor having a large number of radially arranged rotor blades rotates at a high speed of approximately several tens of thousand rpm. 
     The rotor blades are formed by an aluminum alloy or the like, which is superior in mechanical strength and lightweight. 
     However, the permissible temperature of the rotor blades is relatively low, ranging, for example, from 120° C. to 150° C. When the turbo-molecular pump is used for a long period of time at a temperature higher than this permissible temperature, the rotor blades undergo creep deformation due to the centrifugal force caused by high-speed rotation, resulting in a breakdown and a rather short period until parts replacement. 
     Further, when the flow rate of the exhaust gas is high, the temperature of the rotor blades, etc. rises due to collision of the molecules constituting the gas with the rotor blades and friction therebetween, so that, in some cases, to use the turbo-molecular pump at a temperature not higher than the permissible temperature, the amount of exhaust gas that can be continuously allowed to flow through the turbo-molecular pump (permissible flow rate) is limited. 
     It is accordingly an object of the present invention to provide a vacuum pump whose temperature rise is restrained, whereby deterioration in the vacuum pump due to temperature rise does not easily occur. 
     SUMMARY OF THE INVENTION 
     To achieve the above object, there is provided, in accordance with the present invention, a vacuum pump characterized by comprising a casing constituting an armor body, a gas inlet formed in the casing and connected to a container to be evacuated, a gas outlet formed in the casing, an exhaust means which sucks in a gas through the gas inlet and discharges the gas sucked in through the gas inlet through the gas outlet, and a bad heat conductor arranged in an end surface of the gas inlet (First Construction). 
     In the first construction, the gas inlet may be equipped with a flange, and the bad heat conductor may consist of a coating or plating formed on an opening surface of the flange (Second Construction). 
     Further, the bad heat conductor in the first construction may be a tubular member one end of which is connected to the gas inlet and the other end of which is connected to the container to be evacuated (Third Construction). The bad heat conductor in one of the first through third constructions may consist, for example, of a ceramic, resin, glass, or metal of low heat conductivity. 
     Further, in accordance with the present invention, there is provided a vacuum pump comprising a casing constituting an armor body, a gas inlet formed in the casing and connected to a container to be evacuated, a gas outlet formed in the casing, and an exhaust means which sucks in a gas through the gas inlet and discharges the gas sucked in through the gas inlet through the gas outlet, characterized in that at least a part of the casing portion from the gas inlet to the position where the exhaust means is accommodated is formed of a bad heat conductor over the entire circumference of the casing (Fourth Construction). 
     Further, to achieve the above object, there is provided, in accordance with the present invention, a vacuum pump characterized by comprising a casing constituting an armor body, a gas inlet formed in the casing and connected to a container to be evacuated, a gas outlet formed in the casing, an exhaust means which sucks in a gas through the gas inlet and discharges the gas sucked in through the gas inlet through the gas outlet, a good heat conductor arranged in the gas inlet, and a cooling means for cooling the good heat conductor (Fifth Construction). This good heat conductor may consist, for example, of aluminum or copper. The good heat conductor is, for example, a tubular member one end of which is connected to the gas inlet and the other end of which is connected to the container to be evacuated. The cooling means may consist of a cooling water supplying means for supplying cooling water to the periphery of the good heat conductor or a blowing means for supplying air flow to the periphery of the good conductor. When cooling the good conductor with air, it is possible to provide an air cooling fin in the periphery of the good conductor. The cooling means is not limited to the water cooling type and the air cooling type. It is also possible to use, for example, a device utilizing the Peltier effect, such as a Peltier element, and other methods. 
     Further, in the fifth construction, the good heat conductor is connected to the container to be evacuated through the bad heat conductor, whereby the quantity of heat transmitted from the gas inlet to the vacuum pump is reduced, and it is possible to prevent the container to be evacuated from being over-cooled by the cooling means (Sixth Construction). 
     In a vacuum pump according to one of the first through sixth constructions, the gas inlet is formed at one end of the casing, and the gas outlet is formed at the other end of the casing, and the exhaust means is a turbo-molecular pump including a rotor accommodated in the casing and rotatably supported, a plurality of rotor blades arranged radially in the periphery of the rotor, a driving means for driving the rotor to rotate it around the axis thereof, and a plurality of stator blades arranged from the inner peripheral surface of the casing toward the center of the casing (Seventh Construction). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing the construction of an exhaust system according to a first embodiment. 
     FIG. 2 is a sectional view showing the construction of the connecting surfaces of a flange of a turbo-molecular pump and a flange of a conductance valve. 
     FIGS. 3A and 3B are sectional view showing the construction of the connecting surfaces of a flange of a turbo-molecular pump and a flange of a conductance valve. FIG. 3A shows the construction of a modification of the connecting surfaces of a flange. FIG. 3B shows the case in which the flange end surface of the turbo-molecular pump is plated. 
     FIG. 4 is a sectional view of a turbo-molecular pump. 
     FIG. 5 is a schematic diagram showing the construction of an exhaust system according to a second embodiment. 
     FIG. 6 is a diagram showing a turbo-molecular pump according to a third embodiment and a part of an exhaust system connected to the turbo-molecular pump. 
     FIG. 7 is a schematic diagram showing the construction of an exhaust system according to a fourth embodiment. 
     FIG. 8 is a flowchart illustrating the operation of a temperature controller. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     First Embodiment 
     A first preferred embodiment of the present invention will now be described in detail with reference to FIGS. 1 through 4. 
     FIG. 1 is a schematic diagram showing the construction of the exhaust system of this embodiment. 
     This exhaust system is composed of piping  32 , a conductance valve  31 , and a turbo-molecular pump  1 . 
     One end of the piping  32  is connected to an opening of a vacuum device, such as the chamber of a semiconductor manufacturing apparatus, and a high temperature gas in the vacuum device flows through the piping  32 . Formed at the other end of the piping  32  is a flange, to which a flange of the conductance valve  31  is connected. 
     The connection is effected by means of bolts or a clamper, with an O-ring or a metal gasket being placed between the flanges. Due to the action of the O-ring or the gasket, the connecting portion is hermetically sealed. It is also possible to effect the connection by welding. 
     The conductance valve  31  is a valve consisting, for example, of a butterfly valve. The butterfly valve includes a cylindrical valve case, in which is provided a disc-shaped valve element  34  having a diameter that is the same as the inner diameter of the flow passage, the valve element being rotated around a diametral shaft to thereby effect opening and closing. The valve element  34  is caused to rotate from outside the conductance valve  31  to adjust the sectional area of the flow passage. In the example shown in FIG. 1, the valve element  34  is arranged inside the conductance valve  31 . 
     The conductance valve  31  is a valve for adjusting conductance (ease with which gas is allowed to flow). It is provided for the purpose of adjusting the degree to which the turbo-molecular pump  1  sucks in exhaust gas. 
     In this way, by opening and closing the conductance valve  31  for adjusting the degree to which the turbo-molecular pump  1  sucks in exhaust gas from the vacuum device, it is possible to adjust the pressure in the chamber. 
     The turbo molecular pump  1  sucks in exhaust gas through a gas inlet where a flange  2  is formed by the action of a large number of stator blades arranged on the inner peripheral surface of the casing and rotor blades arranged alternately with respect to the stator blades and adapted to rotate at high speed, and discharges the exhaust gas through a gas outlet  19 . The construction of the turbo-molecular pump  1  will be described in detail below. 
     The turbo-molecular pump  1  is used as a main vacuum pump, and an auxiliary pump is connected to the gas outlet  19 . 
     By means of the auxiliary pump, the pressure in the gas outlet  19  of the turbo-molecular pump  1  is reduced from the atmospheric state to a vacuum state, in which the turbo-molecular pump  1  functions in the normal fashion, whereby the performance of the turbo-molecular pump  1  is exerted, thereby making it possible to create a high-vacuum state inside the chamber. 
     FIG. 2 is a sectional view showing the construction of the connecting portion between the flange  2  of the turbo-molecular pump  1  and the flange  33  of the conductance valve  31 . 
     In order that the drawing may not be too complicated, the opening edge line, which ought to be visible on the back side of the plane of the drawing, or the like is omitted. In FIG. 2, the state where the flange  33  is removed from the flange  2  is shown. 
     In the flanges  2  and  33 , bolt holes (not shown) are concentrically formed. When they are connected to each other, the flange  33  is secured in position by bolts passed through the bolt holes in the direction indicated by the arrow in the drawing. 
     An annular groove is formed in the flange surface (contact surface) of the flange  2 , and an O-ring  38  is fitted in this groove. The O-ring has a round sectional configuration and is formed of synthetic rubber. When the flanges  2  and  33  are firmly connected to each other by the bolts, the round section is crushed by the pressure from both flanges, and the O-ring comes into close contact with the flanges due to the resilient force of the synthetic rubber. It is also possible, as shown in FIG. 3A, to use center rings  39  and  40  to form a groove for arranging the O-ring, whereby the same effect as described above can be obtained. 
     While in the case in which the center rings  39  and  40  are not used it is necessary to form an annular groove for attaching an O-ring to the flange  2 , the center rings  39  and  40  allow the flanges to be connected together, with the O-ring being placed between flat connection surfaces. The center ring  40  has an outwardly protruding sectional configuration, the step portions of the protrusion being fitted placed in the inner peripheries of the flanges  2  and  38 . 
     In the case of FIG. 3A, in which center rings are used, it is possible to form the center rings of a bad heat conductor, such as a resin, whereby it is possible to prevent heat from being transmitted from the flange  33 . 
     In contrast, in the case of FIG. 2, in which an O-ring groove is formed, there is no center ring, so that the flange  33  is directly connected to the flange  2 . In this case, it is impossible to intercept the heat from the flange  33 . 
     In view of this, as shown in FIG. 3B, coating or plating of a bad heat conductor is effected on the surface of the flange  2  to be connected with the flange  33 . Examples of the bad heat conductor include a fluororesin and ceramic. 
     The coating (or plating)  36  is not always necessary; it is possible to achieve the desired effect with the coating (or plating)  37  alone. In the case in which the coating (or plating)  37  alone is used, it is possible to prevent problems, such as dissipation of gas from the coating (or plating)  36  to the exhaust system. 
     It is also possible to plate the end surface of the flange  2  or the flange  33  with a substance having a lower heat conductivity than the material of the flange  2  or the flange  33 . 
     Further, while in this embodiment the O-ring  38  is used to seal the connecting portion between the flanges  2  and  33 , this should not be construed restrictively. It is also possible to use a gasket instead of the O-ring  38 . 
     Further, instead of bolts, a damper may be used for the connection of the flanges  2  and  33 . 
     The construction of the turbo-molecular pump  1  will now be described. 
     FIG. 4 is a sectional view of the turbo-molecular pump  1  taken along the direction of the rotor shaft. 
     A casing  16 , which has a cylindrical configuration, constitutes the armor body of the turbo-molecular pump  1 . 
     At the center of the casing  16 , there is provided a rotor shaft  3 . 
     In the upper portion, lower portion, and bottom portion of the rotor shaft  3  with respect to the plane of the drawing, there are provided magnetic bearing portions  8 ,  12 , and  20 , respectively. When the turbo-molecular pump  1  is operating, the rotor shaft  3  magnetically levitates in the radial direction thereof by means of the magnetic bearing portions  8  and  12  and is supported in a non-contact state; it also magnetically levitates in the thrust (axial) direction thereof and is supported in a non-contact state. 
     These magnetic bearing portions constitute a so-called 5-axis control type magnetic bearing, and the rotor shaft  3  and a rotor  11  firmly attached to the rotor shaft  3  have a degree of freedom in rotation around the axis of the rotor shaft  3 . 
     In the magnetic bearing portion  8 , four electromagnets are oppositely arranged around the rotor shaft  3  at intervals of 90 degrees. In the portion of the rotor shaft  3  opposed to these electromagnets, there is provided a target which consists of a ferromagnetic body formed through silicon steel lamination and which is attracted by the magnetic force of these electromagnets. 
     A displacement sensor  9  detects a radial displacement of the rotor  3 . When it is detected by a displacement signal from the displacement sensor  9  that the rotor shaft  3  has been radially displaced from a predetermined position, a control device  25  adjusts the magnetic force of each electromagnet and operates so as to bring the rotor shaft  3  back to the predetermined position. The adjustment of the magnetic force of the electromagnets is effected through feedback control of exciting current of each electromagnet. 
     By this feedback control by the displacement sensor  9 , the magnetic bearing portion  8 , and the control device  25 , the rotor shaft  3  magnetically levitates in the radial direction, with a predetermined clearance being maintained between it and the electromagnets in the magnetic bearing portion  8 , and is supported in space in a non-contact state. 
     The construction and operation of the magnetic bearing portion  12  are the same as those of the magnetic bearing portion  8 . 
     In the magnetic bearing portion  12 , four electromagnets are arranged around the rotor shaft  3  at intervals of 90 degrees; by the attracting magnetic force of these electromagnets, the rotor shaft  3  is supported by the magnetic bearing portion  12  in a non-contact state with in the radial direction. 
     A displacement sensor  13  detects a displacement of the rotor shaft  3  in the radial direction. 
     When it receives a displacement signal from the displacement sensor  13  indicating a radial displacement of the rotor shaft  3 , the control device  25  corrects this displacement and performs feedback control on the exciting current of the electromagnets so as to hold the rotor shaft  3  at the predetermined position. By this feedback control by the displacement sensor  13 , the magnetic bearing portion  12 , and the control device  25 , the rotor shaft  3  magnetically levitates in the radial direction in the magnetic bearing portion  12 , and is held in space in a non-contact state. 
     In this way, the rotor shaft  3  is radially held in two positions: the magnetic bearing portions  8  and  12 , so that the rotor shaft  3  is held at a predetermined position in the radial direction. 
     The magnetic bearing portion  20  provided at the lower end of the rotor shaft  3  is composed of a metal disc  18 , electromagnets  14  and  15 , and a displacement sensor  17 , and holds the rotor shaft  3  in the thrust direction. 
     The metal disc  18  is formed of a material having a high magnetic permeability. Examples of the material include iron, which is a ferromagnetic substance. The metal disc  18  is fixed at its center so as to be perpendicular to the rotor shaft  3 . The electromagnets  14  and  15  are respectively provided above and below the metal disc  18 . The electromagnet  14  magnetically attracts the metal disc  18  upwardly, and the electromagnet  15  magnetically attracts the metal disc  18  downwardly. The control device  25  appropriately adjusts the magnetic force applied to the metal disc  18  by the electromagnets  14  and  15 , causing the rotor shaft  3  to magnetically levitate in the thrust direction to hold it in space in a non-contact state. 
     A displacement sensor  17  detects a displacement of the rotor shaft  3  in the thrust direction, and transmits a signal to the control device  25 . The control device  25  monitors the displacement of the rotor shaft  3  in the thrust direction by a displacement detection signal received from the displacement sensor  13 . 
     When the rotor shaft  3  moves in either way in the thrust direction to be displaced from a predetermined position, the control device  25  performs feedback control on the exciting current of the electromagnets  14  and  15  and adjusts the magnetic force so as to correct the displacement, restoring the rotor shaft  3  to the predetermined position. By this feedback control by the control device  25 , the rotor shaft  3  magnetically levitates at a predetermined position in the thrust direction and is held at this position. 
     As described above, the rotor shaft  3  is held in the radial direction by the magnetic bearing portions  8  and  12 , and is held in the thrust direction by the magnetic bearing portion  20 , so that it only has a degree of freedom in rotation around the axis of the rotor shaft  3 . 
     In the axial direction of the rotor shaft  3 , a protective bearing  6  is provided above the magnetic bearing portion  8 , and a protective bearing  7  is provided below the magnetic bearing portion  12 . 
     The rotor shaft  3 , which is caused to magnetically levitate and held in space in a non-contact state, can be significantly deviated from the holding position as a result, for example, of a run-out around the axis of the rotor shaft  3 . In such a case, the protective bearings  6  and  7  prevent the rotor shaft  3  from coming into contact with the electromagnets of the magnetic bearing portions  8 ,  12 , and  20 , or the permanent magnet from coming into contact with the electromagnet in the motor portion  10 . 
     When the rotor shaft  3  moves from a predetermined position by an amount not smaller than a certain amount, the rotor shaft  3  comes into contact with the protective bearings  6  and  7 , thus physically restricting the movement of the rotor shaft  3 . 
     In the rotor shaft  3 , the motor portion  10  is provided between the magnetic bearing portions  8  and  12 . As described below, the motor portion  10  constitutes a DC brushless motor. 
     In the motor portion  10 , a permanent magnet is secured around the rotor shaft  3 . 
     This permanent magnet is mounted, for example, such that N- and S-poles are arranged around the axis of the rotor shaft  3  by 180 degrees. 
     On the periphery of this permanent magnet, six electromagnets, for example, spaced apart from the permanent magnet by a predetermined clearance, are arranged symmetrically with respect to the axis of the rotor shaft  3  so as to be opposed to each other at intervals of 60 degrees. 
     Further, a speed sensor  23  is mounted to the lower end of the rotor shaft  3 . By a detection signal from the speed sensor  23 , the control portion  25  can detect the speed of the rotor shaft  3 . Further, in the vicinity of the displacement sensor  13 , there is mounted a sensor (not shown) for detecting the rotation phase of the rotor shaft  3 , and the control device  25  can detect the position of the permanent magnet by using detection signals of this sensor and the speed sensor  23 . 
     According to the position of the magnetic pole detected, the control device  25  successively switches the current of the electromagnet so that the rotation of the rotor shaft  3  may be maintained. That is, by switching the exciting current for the six electromagnets, the control device  25  generates a rotation magnetic field around the permanent magnet fixed to the rotor shaft  3 , and rotates the rotor shaft  3  by causing the permanent magnet to follow this rotation magnetic field. 
     The rotor  11  is secured to the rotor shaft  3  by a bolt  5 . As the rotor shaft  3  is driven by the motor portion  10  and rotates, the rotor  11  also rotates. 
     A plurality of rotor blades  21 , inclined by a predetermined angle from the plane perpendicular to the axis of the rotor shaft  3 , are radially mounted on the rotor  11 . The rotor blades  21  are firmly attached to the rotor  11  and rotate at high speed with the rotor  11 . 
     Further, stator blades  22  are fixed to the casing  16  so as to extend toward the interior of the casing  16  and are arranged alternately with the rotor blades  21 . The stator blades  22  are fixed to the casing  16  so as to be at a predetermined angle from the plane perpendicular to the axis of the rotor shaft  3 . 
     When the rotor  11  and the rotor shaft  3  are rotated by the motor portion  10 , a gas is sucked in through a gas inlet  24  by the action of the rotor blades  21  and the stator blades  22 , and is discharged through a gas outlet  19 . 
     A flange  2  is formed on the periphery of the gas inlet  24 , making it possible to connect the turbo-molecular pump  1  to be connected to the vacuum vessel of a semiconductor manufacturing apparatus or the like. 
     The control device  25  is connected to a connector  4  of the turbo-molecular pump  1 , controlling the magnetic bearing portions  8 ,  12 , and  20  and the motor portion  10 . 
     The exhaust system constructed as described above operates as follows. 
     When turbo-molecular pump  1  operates, the rotor  11  levitates to a predetermined position by the magnetic bearing portions  8 ,  12 , and  20 , while being controlled by the control device  25 . Then, the rotor shaft  3  is driven by the motor portion  10 , and rotates around its axis, thereby causing the rotor  11  to rotate. 
     This also causes the rotor blades  21  to rotate, and, by the action of the rotor blades  21  and the stator blades  22 , the gas in the vacuum device is sucked in through the conductance valve  31  and the gas inlet  24  and discharged through the gas outlet  19 . Further, the auxiliary pump is also operated. 
     When the vacuum degree in the vacuum device has become sufficiently high, a semiconductor manufacturing process is started. A high temperature process gas is introduced into the vacuum device. This gas is discharged by the turbo-molecular pump  1  through the piping  32 , without being sufficiently cooled. The temperature of the piping  32  begins to gradually increase, and subsequently, the temperature of the conductance valve  31  also starts to increase. In some cases, the flange  33  of the conductance valve  31  attains a temperature of approximately 60° C. Due to the thermal insulation between the flange  33  and the flange  2  by the coating (or plating)  36 ,  37 , the quantity of heat transmitted to the turbo-molecular pump  1  from the flange  33  is reduced. 
     In this embodiment, the heat transmitted to the turbo-molecular pump  1  through the exhaust system can be intercepted by the coating (or plating)  36 ,  37  provided on the connecting surface of the flange  2 . 
     Thus, it is possible to suppress the increase in the temperature of the rotor blades  21 , and it is possible to mitigate the deterioration in the rotor blades  21  and other components due to creep generated as a result of the increase in temperature. 
     Second Embodiment 
     A second embodiment will now be described with reference to FIG.  5 . FIG. 5 is a schematic diagram showing the construction of an exhaust system according to the second embodiment. 
     The exhaust system comprises the piping  32 , the conductance valve  31 , a heat insulating portion  41 , and the turbo-molecular pump  1 . 
     One end of the piping  32  is connected, for example, to an opening of a vacuum device, such as the chamber of a semiconductor manufacturing apparatus, serving as an exhaust pipe for discharging high temperature gas in the vacuum device. The piping  32 , the conductance valve  31 , and the turbo-molecular pump  1  are the same as those of the first embodiment. While in the first embodiment the flange  33  is connected to the flange  2  through the center rings  39  and  40 , the connection between the flange  33  and the flange  2  in the second embodiment is effected through the heat insulating portion  41 . 
     The heat insulating portion  41  is composed of flanges  42  and  44  and a heat insulating pipe  43 . 
     The flanges  42  and  43  have circular holes at their center, and bolt holes for connection with the flange  33  and the flange  2  are formed in concentric circles around the respective circular holes. 
     The insulating pipe  43  is connected by the circular hole portions of the flanges  42  and  44  by a means suitable for the material of these members, such as adhesive, welding, or brazing. 
     Usually, the conductance valve  31 , the flange  33 , the casing  16  of the turbo-molecular pump  1 , and the flange  2  are formed of stainless steel, iron, aluminum or the like, whereas the heat insulating portion  41  is formed of a material whose heat conductivity is lower than that of these materials. Examples of such material include resin, ceramic, and a metal like chromium nickel (18Cr8Ni). 
     The flange  42  and the flange  33  are connected to each other by fastening bolts (not shown) passed through the bolt holes formed in the flanges  42  and  33 . In the connecting portion between the flange  42  and the flange  33 , there is placed an O-ring or a gasket for sealing. 
     Like the connection of the flanges  42  and  33 , the connection of the flange  2  and the flange  44  is effected by bolt fastening, with an O-ring or a gasket being placed therebetween. 
     Instead of using the bolts, these connections can be effected by using a clamper. 
     In the second embodiment, when the turbo-molecular pump  1  operates to discharge the high temperature process gas used in the vacuum device, such as the chamber of a semiconductor manufacturing apparatus, the heat transmitted through the piping  32  and the conductance valve  31  is intercepted by the heat insulating portion  41 , and the heat conduction to the turbo-molecular pump  1  can be mitigated. Further, in the second embodiment, the heat insulating pipe  43  has a length with respect to the heat transmitting direction, and is exposed to the atmosphere, so that heat radiation from the surface of the heat insulating pipe  43  to the periphery is effected while the heat is transmitted through the heat insulating pipe  43 , thereby enhancing the heat insulating effect. 
     Thus, it is possible to restrain the temperature rise due to heat conduction from the gas inlet of the turbo-molecular pump  1 , whereby it is possible to mitigate the deterioration in the turbo-molecular pump  1  which is due, for example, to the creep in the rotor blades as a result of temperature rise. 
     Further, it is also possible to form the conductance valve  31  itself of a bad conductor of heat and use it instead of the heat insulating member  41 . 
     Third Embodiment 
     A third embodiment will now be described with reference to FIG.  6 . 
     FIG. 6 is a diagram showing a turbo-molecular pump  50  according to the third embodiment and a part of the exhaust system connected to the turbo-molecular pump  50 . 
     In the exhaust system of the third embodiment, the turbo-molecular pump  1  of the first embodiment is replaced by the turbo-molecular pump  50 . 
     The armor body of the turbo-molecular pump  50  is composed of a casing  51  and a gas inlet portion  47  forming a gas inlet. 
     The casing  51  consists of a substantially cylindrical member formed of stainless steel, iron, aluminum or the like, and contains a pump main body, such as a rotor. 
     The gas inlet portion  47  is formed of a heat insulating material and connected to the casing  51  by a connecting portion  49 . 
     The construction of the turbo-molecular pump  50  is the same as that of the turbo-molecular pump  1  except that the gas inlet portion  47  is formed of a heat insulating material. 
     The gas inlet portion  47  is formed of a material whose thermal conductivity is lower than that of stainless steel, such as chromium nickel (18Cr8Ni). 
     A flange  48  formed on the gas inlet portion  47  has a circular hole at its center, and bolt holes are formed in the periphery of the circular hole. The flange  48  and the flange  33  are connected to each other by fastening bolts passed through the bolt holes, with an O-ring being placed therebetween. A connecting portion between the flange  48  and the flange  33  is sealed by the O-ring. 
     In the third embodiment, the heat transmitted through the conductance valve  31 , etc. is intercepted by the gas inlet portion  47  to thereby mitigate the heat conduction to the turbo-molecular pipe  50 . 
     Further, in the third embodiment, the material of the portion of the turbo-molecular pump  1  near the gas inlet  24  is replaced by a heat insulating material, so that the total length of the exhaust passage of the exhaust system is the same as that of the conventional exhaust system. Thus, in this embodiment, no deterioration in conductance occurs as a result of an increase in the length of the exhaust passage, and it is possible to reduce the quantity of heat transmitted to the turbo-molecular pump  1  through the piping of the exhaust system. 
     As described above, in this embodiment, it is possible to restrain flowing in of heat through the gas outlet of the turbo-molecular pump  50  while maintaining a satisfactory conductance. 
     Fourth Embodiment 
     A fourth embodiment will now be described with reference to FIG.  7 . FIG. 7 is a diagram showing the construction of the exhaust system of the second embodiment. 
     This exhaust system is composed of the piping  32 , the conductance valve  31 , a cooling portion  58 , and the turbo-molecular pump  1 . 
     One end of the piping  32  is connected to the opening of a vacuum device, such as the chamber of a semiconductor manufacturing apparatus, serving as an exhaust duct for discharging high temperature gas in the vacuum device. The piping  32 , the conductance valve  31 , and the turbo-molecular pump  1  are the same as those of the first embodiment. The gas outlet of the turbo-molecular pump  1  is connected to an auxiliary pump (not shown). 
     The cooling portion  58  is composed of flanges  55  and  57 , a heat conduction pipe  56 , and a water cooling pipe  61 . 
     The flanges  55  and  57  and the heat conduction pipe  56  are formed of a material having high heat conductivity, such as copper or aluminum. The flanges  55  and  57  have circular holes at their center, and bolt holes are formed in a concentric circle in the periphery of these circular holes. 
     The heat conduction pipe  56  is a piping for conveying exhaust gas, and flanges  55  and  57  are connected to its ends by welding or brazing. 
     The water cooling pipe  61  is spirally wound around the heat conduction pipe  56 , making it possible to effect heat exchange between the heat conduction pipe  56  and cooling water flowing through the water cooling pipe  61 . 
     Connected to the water cooling pipe  61  are an electromagnetic valve  62 , a water supply pump (not shown), and a heat exchanger. The water supply pump supplies cooling water to the water cooling pipe  61 , causing the water to circulate in the water cooling pipe  61 . The heat exchanger effects heat exchange at the cooling portion  58 ; the cooling water whose temperature has risen is sent to the cooling portion  58  again to cool it for heat exchange. 
     The electromagnetic valve  62  is electrically connected to a temperature controller  64 , and is opened and closed in response to an electric signal supplied from the temperature controller, thereby adjusting the flow of cooling water to the water cooling pipe  61 . 
     Further, the temperature controller  64  is connected to a temperature sensor  63 . The temperature sensor  63  is mounted to the flange  2  of the turbo-molecular pump  1 , and the temperature controller  64  monitors the temperature of the flange  2 . The temperature sensor  63  may consist, for example, of a thermocouple. 
     When the turbo-molecular pump  1  is operated to discharge, for example, high temperature process gas in a semiconductor manufacturing apparatus, heat is transmitted through the piping  32  and the conductance valve  31 . Thus, when the electromagnetic valve  62  is closed to stop the circulation of cooling water in the water cooling pipe  61 , the temperature of the cooling portion  58  rises, and heat is transmitted to the turbo-molecular pump  1 . When the electromagnetic valve  62  is opened to circulate the cooling water in the water cooling pipe  61 , the heat conduction pipe  56  is cooled by the cooling water, whereby it is possible to restrain heat conduction to the turbo-molecular pump  1 . 
     Due to the above construction, the temperature controller  64  opens and closes the electromagnetic valve  62  such that the temperature of the flange  2  is within a predetermined range, for example, from T 1  to T 2  (T 1 &lt;T 2 ), thereby adjusting the heat exchange at the cooling portion  58 . 
     Fifth Embodiment 
     While in the fourth embodiment the flanges  55  and  57  and the heat conduction pipe  56  are good conductors of heat, it is also possible for the flange  55  to be a bad conductor of heat and for the flange  57  and the heat conduction pipe  56  to be good conductors of heat. 
     It can happen that the product generated as a result of reaction with the process gas in the chamber adheres to the inner side of the piping when it touches the cooled pipe. The product thus adhering can, in some cases, flow reversely and adhere to the surface of the wafer in the chamber in the form of dust. The adhesion of such dust on the wafer surface results in the semiconductor manufacturing apparatus ceasing to operate in the normal manner. For example, it becomes impossible to form patterns correctly on the wafer, resulting in a reduction in yield. 
     In view of this, the conductance valve  31  and the piping  32  are thermally insulated by the flange  55  formed of a bad conductor of heat so that they may not be excessively cooled, and the heat conduction pipe  56  and the flange  57  formed of good conductors of heat are cooled, whereby it is possible to efficiently cool the flange  2  of the turbo-molecular pump  1  connected to the flange  57 . 
     It is also possible to perform temperature control in the same manner as in the fourth embodiment. 
     FIG. 8 is a flowchart for illustrating the operation of the temperature controller  64 . 
     It is to be assumed that the water supply pump and the heat exchanger connected to the water cooling pipe  61  are operating, and that heat is being transmitted through the piping  32  and the conductance valve  31  to flow into the cooling portion  58 . 
     Operation is started when the power switch of the temperature controller  64  is turned on. First, the electromagnetic valve  62  is closed (step  10 ). As a result, the cooling water in the water cooling pipe  61  does not circulate, and the cooling portion  58  transmits heat, resulting in an increase in the temperature of the flange  2 . 
     Next, the temperature controller  64  checks to see whether the power switch is off or not (step  20 ). When the power switch is on (i.e., when the answer in step  20  no), the temperature T of the flange  2  is obtained from the voltage of the temperature sensor  63  (step  30 ). 
     Next, the temperature controller  64  compares the temperature T with a predetermined temperature T 2  previously stored in a storage portion of the temperature controller  64 . When T is higher than T 2  (i.e., when the answer in step  40  is yes), the electromagnetic valve  62  is opened, and cooling water is caused to circulate in the water cooling pipe  61  (step  50 ). As a result, the heat transmitted from the conductance valve  31  is absorbed by the cooling portion  58 , and the temperature of the flange  2  starts to be lowered. 
     Next, the procedure of the temperature controller  64  returns to step  20 , and the loop from step  20  to step  50  is repeated until the power switch becomes off (the answer in step  20  is yes) or until T becomes lower than T 2  (the answer in step  40  is no). All this while, the temperature of the flange  2  continues to be lowered. 
     When the temperature controller  64  determines in step  40  that T is lower than T 2  (i.e., when the answer in step  40  is no), T is further compared with a predetermined temperature T 1  previously stored in a storage portion of the temperature controller  64  (step  60 ). T 1  is lower than T 2 . 
     When T is higher than T 1  (i.e., when the answer in step  60  is no), the procedure of the temperature controller  64  returns to step  20 , and then the operations from step  20  to step  60  are repeated unless the power switch becomes off (the answer in step  20  is yes), the electromagnetic valve  62  being maintained in the open state. That is, all this while, the temperature of the flange  2  continues to be lowered. 
     When the temperature T of the flange  2  becomes lower than T 1  (i.e., when the answer in step  60  is yes), the procedure of the temperature controller  64  returns to step  10 , and the electromagnetic valve  62  is closed. Then, the circulation of cooling water in the water cooling pipe  61  is stopped, and the temperature T of the flange  2  starts to rise. After this, the temperature controller  64  repeats the loop consisting of steps  10 ,  20 ,  30 ,  40 , and  60  until the power switch becomes off (the answer in step  20  is yes) or until T becomes higher than T 2 , maintaining the electromagnetic valve  62  in the closed state. That is, while this loop is being executed, the temperature of the flange  2  rises. 
     The temperature controller  64  repeats the above operation until the power switch becomes off (the answer in step  20  is yes), and the temperature of the flange  2  is repeatedly raised and lowered between T 1  and T 2 . 
     For example, when T 1  is 40[° C.] and T 2  is 50[° C.], the temperature of flange  2  is controlled so as to be between 40[° C.] and 50[° C.]. 
     In the fourth embodiment, the quantity of heat transmitted through the piping  32  and the conductance valve  31  to flow into the turbo-molecular pump  1  can be controlled to an appropriate value, so that it is possible to prevent a deterioration in the rotor blades  21  due to abnormal temperature rise in the rotor blades  21  of the turbo-molecular pump  1  and precipitation of the product inside the turbo-molecular pump  1  due to excessive cooling of the turbo-molecular pump  1 . 
     While in the fourth embodiment described above a water cooling system using cooling water is used as the means for cooling the cooling portion  58 , this should not be construed restrictively. It is also possible to adopt an air cooling system using an air-cooling fan. 
     While in the first through fifth embodiments described above a turbo-molecular pump is used as the vacuum pump, this should not be construed restrictively. The present invention is also applicable, for example, to a case in which a vacuum pump, such as a rotary pump or an ion pump, is thermally insulated from a vacuum apparatus. 
     In accordance with the present invention, it is possible to restrain a temperature rise in a vacuum pump due to flowing of heat into the vacuum pump through piping, making it possible to provide a vacuum pump in which deterioration in parts as a result of temperature rise is minimized.