Abstract:
A vacuum pump apparatus has a casing and a base connected to the casing. The base has an opening communicating with an interior space of the casing. A heater is mounted on the base for heating the base to maintain a gas discharge path at a temperature high enough to prevent solidification of processed gas in the vacuum pump. A cover member covers the opening of the base. The cover member has a first surface communicating with the interior space of the casing and a second surface exposed to the exterior of the vacuum pump. A substrate is connected to the first surface of the cover member and has at least one circuit for controlling operation of the vacuum pump.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a vacuum pump and, more particularly, to a vacuum pump which is used when a process gas for a semiconductor manufacturing system, for example, is sucked and exhausted. 
   2. Description of the Related Art 
   In recent years, semiconductor devices such as memory and integrated circuit devices have been used extensively along with the development of electronics. Therefore, the demand for semiconductor manufacturing systems has experienced a sudden increase. 
   The semiconductor manufacturing system is provided with a high vacuum chamber in which etching or other work is performed. Generally, a vacuum pump is frequently used to evacuate the vacuum chamber. 
   The semiconductor device manufacturing processes include processes in which various kinds of process gases are applied to a substrate of a semiconductor, so that the vacuum pump is used not only to evacuate the vacuum chamber but also to suck and exhaust these process gases. 
   These process gases are sometimes introduced into the chamber in a high-temperature state to enhance the reactivity. However, these process gases are cooled during exhaustion, and thereby a chemical reaction takes place to form a solid product, which may adhere and accumulate in the vacuum pump. 
   For example, when silicon chloride (SiCl 4 ) is used as a process gas for an aluminum etching apparatus, in a low-vacuum region of 760 [torr] to 10 −2  [torr] containing much water, the chemical reaction of silicon chloride is promoted, and thus aluminum chloride (AlCl 3 ) is precipitated as a solid product, and adheres and accumulates in the vacuum pump. In a low-temperature region of about 20° C., the chemical reaction of silicon chloride is further promoted. 
   In the vacuum pump, a rotor provided with a large number of rotor blades rotates at a high speed of several ten thousand revolutions per minute. If precipitates accumulate on a stator blade disposed on the inner peripheral surface of a casing of the vacuum pump, for example, a disadvantage of contact with the rotor blade may occur. Also, in some case, the accumulated precipitates narrow a gas discharge path, which remarkably degrades the performance of the vacuum pump. 
   Thereupon, methods for restraining the precipitation of a solid product in the vacuum pump have so far been proposed. 
   Generally, there is used a method in which heating is performed from the outside to increase the internal temperature of the vacuum pump, by which the adhesion of process gas is restrained. An example of this method is briefly explained with reference to a turbo-molecular pump shown in  FIG. 2. A  location at which the solid product of process gas is precipitated most easily in the turbo-molecular pump is a base  101  which has a high pressure and moreover is close to a water cooled tube  102  (for temperature control). Therefore, the base  101  is heated by using a heater  103  so as to be kept at a high temperature. 
   However, the above-described method using a heater presents a problem with a heat conduction path. 
   The conduction path of heat generated by the heater  103  is indicated by the arrow marks in FIG.  2 . Thus, the heat generated by the heater  103  is transferred to a motor housing  106  and a substrate  104  located inside the vacuum pump through the base  101 . Since a motor section  105  disposed in the motor housing  106  and the inside substrate  104  have a design limit temperature set considering reliability, the vacuum pump must be used in the value range of design limit temperature when the vacuum pump is operated. In particular, the design limit temperature of the substrate  104  is as low as 80° C. 
   Thus, in the conventional construction, if a heater is used for heating, the motor section and the inside substrate, which are not desired to be heated, are also heated. Therefore, the temperature of the substrate disposed in the motor housing increases undesirably. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide a vacuum pump in which a discharge path for process gas in the vacuum pump is kept at a higher temperature than before, and in which an inside substrate is cooled effectively. 
   To achieve the above object, the invention of a first aspect provides a vacuum pump including a body which has a casing and a base having an opening communicating with the casing and is provided with a gas intake port and a gas discharging port; a rotor pivotally supported in the body so as to be rotatable; a motor for driving the rotor; a motor housing for supporting the motor; gas transfer means, which is provided between the casing and the rotor, for transferring gas sucked through the gas intake port to the gas discharge port; heating means for heating a gas discharge path for the gas transferred by the gas transferring means; a back cover for covering the opening of the base; and a substrate which is arranged on the motor housing side of the back cover. 
   The heating means is composed of, for example, a heater disposed around the base or the casing or in the vacuum pump. 
   To achieve the above object, in the invention of a second aspect, the vacuum pump further includes cooling means for cooling the back cover. 
   To achieve the above object, in the invention of a third aspect, the back cover is fixed via a heat insulating material. 
   To achieve the above object, in the invention of a fourth aspect, the cooling means is a water cooled tube provided on the back cover to circulate cooling water. 
   To achieve the above object, in the invention of a fifth aspect, the heat insulating material is formed of a heat insulating ceramic material or resin. 
   According to the present invention, by arranging the substrate on the inside of the back cover, the inside substrate can be cooled efficiently. 
   Also, by providing the cooling means for cooling the back cover, the efficiency in cooling the substrate is improved. 
   Further, by arranging the heat insulating material in the connecting portion between the back cover and the base, the heat of the base heated intentionally by the heater can be prevented from conducting to the back cover. 
   Thus, if the efficient cooling of the inside substrate is effective, the temperature in the pump can be increased further as compared with the conventional vacuum pump. Therefore, the temperature of the gas discharge path for process gas can be made higher than before, and hence the accumulation of solid product in the vacuum pump can be restrained further. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view of a turbo-molecular pump in accordance with the present invention; and 
       FIG. 2  is a sectional view of a conventional turbo-molecular pump. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A preferred embodiment of the present invention will now be described in detail with reference to FIG.  1 . 
     FIG. 1  is a sectional view of a turbo-molecular pump in accordance with the present invention, showing a cross-section in the axial direction of a rotor shaft  2 . 
   Although not shown in  FIG. 1 , a gas inlet or intake port  3  of a turbo-molecular pump  1  is connected to a vacuum chamber of a semiconductor manufacturing system via a conductance valve (a valve for regulating conductance, i.e., flowability of exhaust gas by changing the cross-sectional area of flow path of pipe) and the like, and a gas outlet or discharge port  4  is connected to an auxiliary pump. 
   A casing  5  that forms a casing for the turbo-molecular pump  1  has a cylindrical shape, and a rotor shaft  2  is disposed in the center thereof. The casing  5  constitutes a body  31  of the turbo-molecular pump  1  together with a base  6 . The casing  5  has an interior space and the base  6  has an opening communicating with the interior space. 
   At the upper part, lower part, and bottom part in the axial direction of the rotor shaft  2 , there are provided magnetic bearing portions  7 ,  8  and  9 , respectively. The rotor shaft  2  is supported in the radial direction (radial direction of the rotor shaft  2 ) in a non-contact manner by the magnetic bearing portions  7  and  8 , and is supported in the thrust direction (axial direction of the rotor shaft  2 ) in a non-contact manner by the magnetic bearing portion  9 . These magnetic bearing portions  7 ,  8  and  9  constitute what is called a five-axis control type magnetic bearing, and the rotor shaft  2  has only the degree of freedom of rotation around the axis of the rotor shaft  2 . 
   In the magnetic bearing portion  7 , four electromagnets are arranged at 90° intervals around the rotor shaft  2  so as to be opposed to each other. The rotor shaft  2  is formed of a material with high magnetic permeability (for example, iron), and hence is attracted by a magnetic force of the electromagnet. 
   A displacement sensor  10  detects displacement in the radial direction of the rotor shaft  2 . When detecting displacement of the rotor shaft  2  in the radial direction from a predetermined position by means of a displacement signal sent from the displacement sensor  10 , a control section, not shown, operates to return the rotor shaft  2  to the predetermined position by regulating the magnetic force of each electromagnet. Thus, the magnetic force of electromagnet is regulated by feedback controlling the exciting current of each electromagnet. 
   The control section carries out feedback control of magnetic force of the magnetic bearing portion  7  by means of a signal sent from the displacement sensor  10 . Thereby, the rotor shaft  2  is magnetically levitated in the radial direction in the magnetic bearing portion  7  with a predetermined clearance being provided with respect to the electromagnets, and is held in a space in a non-contact manner. 
   The construction and operation of the magnetic bearing portion  8  are the same as those of the magnetic bearing portion  7 . 
   In the magnetic bearing portion  8 , four electromagnets are arranged at 90° intervals around the rotor shaft  2 , and the rotor shaft  2  is held in the radial direction in the magnetic bearing portion  8  in a non-contact manner by a suction force of magnetic force of the electromagnets. 
   A displacement sensor  11  detects displacement in the radial direction of the rotor shaft  2 . 
   Upon receipt of a displacement signal in the radial direction of the rotor shaft  2  from the displacement sensor  11 , the control section, not shown, carries out feedback control of the exciting current of electromagnet so as to hold the rotor shaft  2  at a predetermined position by correcting the displacement. 
   The control section carries out feedback control of magnetic force of the magnetic bearing portion  8  by means of a signal sent from the displacement sensor  11 . Thereby, the rotor shaft  2  is magnetically levitated in the radial direction in the magnetic bearing portion  8  with a predetermined clearance being provided with respect to the electromagnets, and is held in a space in a non-contact manner. 
   Thus, since the rotor shaft  2  is held in the radial direction at two places of the magnetic bearing portions  7  and  8 , the rotor shaft  2  is held at the predetermined position in the radial direction. 
   The magnetic bearing portion  9  provided at the lower end of the rotor shaft  2  is composed of a disk-shaped metallic disk  12 , electromagnets  13  and  14 , and a displacement sensor  15 , and holds the rotor shaft  2  in the thrust direction. 
   The metallic disk  12 , which is formed of a material with high magnetic permeability such as iron, is fixed perpendicularly to the rotor shaft  2  in the center thereof. Above and below the metallic disk  12 , the electromagnet  13  and the electromagnet  14  are provided respectively. The electromagnet  13  attracts the metallic disk  12  upward by means of the magnetic force, and the electromagnet  14  attracts the metallic disk  12  downward. The control section suitably regulates the magnetic force applied to the metallic disk  12  by the electromagnets  13  and  14  so that the rotor shaft  2  is magnetically levitated in the thrust direction and held in a space in a non-contact manner. 
   The displacement sensor  15  detects displacement in the thrust direction of the rotor shaft  2 , and sends the detection signal to the control section, not shown. The control section detects displacement in the thrust direction of the rotor shaft  2  based on the displacement detection signal received from the displacement sensor  11 . 
   When the rotor shaft  2  moves either way in the thrust direction and is displaced from a predetermined position, the control section operates so that the magnetic force is regulated by feedback controlling the exciting currents of the electromagnets  13  and  14  so as to correct the displacement, by which the rotor shaft  2  is returned to the predetermined position. The control section continuously carries out this feedback control so that the rotor shaft  2  is magnetically levitated in the thrust direction at the predetermine deposition and held there. 
   The rotor shaft  2  is provided with a motor section  16  disposed in a motor housing  24  and between the magnetic bearing portions  7  and  8 . In this embodiment, the motor section  16  is assumed to be formed of a dc brushless motor as an example. 
   In the motor section  16 , a permanent magnet is fixed around the rotor shaft  2 . This permanent magnet is fixed so that, for example, the N pole and S pole are arranged 180° apart around the rotor shaft  2 . Around this permanent magnet, for example, six electromagnets are arranged at 60° intervals symmetrically and opposingly with respect to the axis of the rotor shaft  2  with a predetermined clearance being provided with respect to the rotor shaft  2 . 
   Also, at the lower end of the rotor shaft  2 , a rotational speed sensor, not shown, is installed. The control section, not shown, can detect the rotational speed of rotor shaft  2  based on the detection signal from the rotational speed sensor. Also, for example, near the displacement sensor  11 , a sensor, not shown, is installed to detect the phase of rotation of the rotor shaft  2 . The control section detects the position of the permanent magnet by using the detection signals of this sensor and the rotational speed sensor. 
   At the upper end of the rotor shaft  2 , a rotor  17  is installed with a plurality of bolts  18 . 
   As described below, a portion ranging from a substantially middle position of the rotor  17  to the gas intake port  3 , that is, a substantially upper half portion in  FIG. 1  is a molecular pump section, and a substantially lower half portion in the figure, that is, a portion ranging from a substantially middle position of the rotor  17  to the gas discharge port  4  is a screw groove pump section. 
   In the molecular pump section located on the gas intake port side of the rotor  17 , rotor blades  19  are installed at a plurality of stages radially from the rotor  17  so as to be inclined through a predetermined angle from a plane perpendicular to the axis of the rotor shaft  2 . The rotor blade  19  is fixed to the rotor  17  so as to be rotated at a high speed together with the rotor shaft  2 . 
   On the gas intake port side of the casing  5 , stator blades  20  are arranged toward the inside of the casing  5  alternately with the rotor blades  19  so as to be inclined through a predetermined angle from a plane perpendicular to the axis of the rotor shaft  2 . 
   When the rotor  17  is driven by the motor section  16  and is rotated together with the rotor shaft  2 , exhaust gas is sucked through the gas intake port  3  by the action of the rotor blades  19  and the stator blades  20 . 
   The exhaust gas sucked through the gas intake port  3  passes between the rotor blade  19  and the stator blade  20 , and is sent to the screw groove pump section formed in the lower half portion in the figure. At this time, the temperature of the rotor blade  19  is increased by friction between the rotor blade  19  and the exhaust gas and the conduction of heat generated in the motor section  16 . This heat is transferred to the stator blade  20  by radiation or gas molecule of exhaust gas. 
   A spacer  21  is a ring-shaped member, and is formed of a metal such as aluminum, iron, stainless steel, copper, or an alloy containing these metals as components. 
   The spacer  21  is interposed between stages of the stator blades  20  to keep the stage formed by the stator blades  20  at a predetermined interval, and holds the stator blades  20  at predetermined positions. 
   The spacers  21  are connected to each other in the outer peripheral portion, and form a heat conduction path for conducting the heat that the stator blade  20  receives from the rotor blade  19  and the heat generated by friction between the exhaust gas and the stator blade  20 . 
   The screw groove pump section formed on the gas discharge port side of the rotor  17  is composed of a rotor  17  and a screw groove spacer  22 . 
   The screw groove spacer  22  is a cylindrical member formed of a metal such as aluminum, copper, stainless steel, or iron, or an alloy containing these metals as components, and has a plurality of spiral screw grooves  23  formed in the inner peripheral surface thereof. 
   The direction of spiral of the screw groove  23  is a direction such that when molecules of exhaust gas move in the rotation direction of the rotor  17 , the molecules are transferred to the gas discharge port  4 . 
   When the rotor  17  is driven and rotated by the motor section  16 , the exhaust gas is transferred from the molecular pump section in the upper half portion in the figure to the screw groove pump section. The transferred exhaust gas is transferred toward the gas discharge port  4  while being guided by the screw groove  23 . 
   A heater  29  is mounted on the outer peripheral surface of the base  6 . The heater  29  is formed of an electrical heating member such as a nichrome wire, and is supplied with electric power from a temperature controller, not shown. The heater generates heat when being supplied with electric power, and heats the base  6 . By heating the base  6 , the temperature in a gas discharge path for process gas is kept at a high temperature, and thus the precipitation of solid product in the pump is restrained. 
   In the embodiment of the present invention, the heater  29  is mounted on the outer peripheral surface of the base  6  to heat the interior of a gas discharge path near the base  6 , which meets the conditions (low temperature, high pressure) for easy precipitation of solid product of process gas. Therefore, even if the heater  29  is mounted on the outer peripheral surface of the casing  5 , in which case the interior of gas discharge path can be heated, an effect of restraining the precipitation of solid product of process gas can be achieved. Also, the heater  29  can be incorporated directly in the turbo-molecular pump to heat the gas discharge path. 
   A cover member or back cover  26  is installed at the bottom of the base  6  for covering the opening of the base  6 . Since it is exposed to the outside air, the back cover  26  is in a relatively low temperature state in the turbo-molecular pump. 
   On the inside of the back cover  26 , there is arranged a substrate  25  in which various types of information on the vacuum pump are recorded. The substrate  25  comprises a substrate having circuits for controlling operation of the vacuum pump and in which pump operation time, error history, setting temperature for temperature control, etc. are stored. These circuits use a large number of semiconductor parts. Since the design limit temperature for the semiconductor part is set considering reliability, the semiconductor part must be used within the range of setting value of design limit temperature when the vacuum pump is operated. The design limit temperature is set at a value considering the guaranteed value of the parts maker and a margin. 
   The difference in arrangement position of the substrate  25  from that in the conventional vacuum pump can be seen by making comparison with the arrangement position of the substrate  104  in FIG.  2 . 
   Since the substrate  104  in the conventional vacuum pump is attached to a magnetic bearing portion  107 , the heat generated by a heater is transferred to the substrate  104  through a base  101 , a motor housing  106 , and the magnetic bearing portion  107 , or the heat from a motor  106  is transferred to the substrate  104 . 
   However, by locating the substrate  25  to the back cover  26 , the aforementioned heat conduction path for heating the substrate  25  can be cut off. Thereby, a rise in temperature of the substrate  25  can be restrained. 
   Since the pump substrate  25  in the present invention is arranged on the inside of the back cover  26 , the wires for the substrate  25  are designed so as to be longer than those in the conventional vacuum pump considering the efficiency of work for assembling the vacuum pump. 
   Although an example of a turbo-molecular pump using a magnetic bearing as a bearing has been described in the embodiment of the present invention, the present invention can also be applied to the case where, for example, a mechanical bearing is used. 
   As described above, the substrate  25  must be kept at a low temperature because of the parts mounted thereon. For this reason, a water cooled tube  30  is installed on the outside of the back cover  26 , on which the substrate  25  is arranged, to forcedly cool the back cover  26  by circulating cooling water in the water cooled tube  30 . Another water cooled tube  28  is arranged on a lower surface of the base  6  for cooling the base  6 . 
   An effect of cooling the back cover  26  can also be achieved by providing a forced air cooling device such as a fan in place of the water cooled tube  30 . 
   In a connecting portion  27   a  and a contacting portion  27   b  between the back cover  26  and the base  6 , a heat insulating material  27  with low heat conductivity is arranged. The heat insulating material  27 , which is an element for improving an effect of cooling the back cover  26  and an effect of heating the base  6 , serves to interrupt the transfer of the heat of the base  6  heated by the heater  29  to the back cover  26 . The heat insulating material is formed of a heat insulating ceramic material (for example, KO, nTiO, CaO, or SiO) or resin (for example, fluorine contained resin, acrylic resin, epoxy resin, or other high-temperature resins), or a metal with low heat conductivity (for example, stainless steel or chromium-nickel alloy (18Cr 2 Ni). 
   The process gas sucked through the gas intake port  3  moves in the gas discharge path toward the gas discharge port  4  while the temperature thereof decreases. However, since the base  6  is heated by the heater  29 , the process gas can be prevented from adhering and accumulating near the base  6  as a solid product. 
   Also, since the substrate  25  is arranged on the inside of the back cover  26 , the back cover  26  can be cooled efficiently by being cooled forcedly from the outside.