Vacuum pump

Abstract: A vacuum pump of the present invention comprises a housing (103) including a suction port (101) and an exhaust port (102), a stator (105) fixed in the housing (103), a rotor (104) rotatably supported in the housing (103), and a cooling jacket (109) provided adjacent to the stator (105). Gas sucked from the suction port (101) and having a pressure substantially equal to or close to the atmospheric pressure is exhausted from the exhaust port (102). A cooling fluid having a thermal conductivity smaller than that of water, for example, of 0.08 to 0.25 Kcal/m·h·°C flows through the cooling jacket (109). When the gas contains aluminum chloride, the cooling fluid flows into the cooling jacket (109) in such a manner that the temperature inside a gas conduit is maintained to be higher than the sublimation temperature of aluminum chloride. Lubrication oil (110) may be supplied as the cooling fluid to the cooling jacket (109) from the same supply line as lubrication oil supplied to oil lubricating bearings (107a,107b) provided below a pump mechanism unit. A closed-loop line may be comprised of the cooling jacket (109), a tank, the supply line (111), and a return line, to thereby circulate the cooling fluid by means of a pump (113).

Description: The invention relates to a vacuum pump used as a multi-stage exhaust pump in a semiconductor manufacturing apparatus, which vacuum pump comprises a housing including a suction port and an exhaust port through which gas sucked from the suction port is exhausted to have a pressure substantially equal to or close to atmospheric pressure, a stator fixed to an inner wall of said housing, a rotor attached to a rotary shaft and constituting by mating relation with the stator a multi-stage pump mechanism unit, a motor housing mounted below the pump mechanism unit including a motor chamber and a motor for rotating the rotary shaft, an upper bearing and a lower bearing for rotatably supporting the rotary shaft in the housing and motor housing, respectively, and a cooling jacket on the outer periphery of the stator provided with a coolant supply. Such a vacuum pump as illustrated in Fig. 5 comprises a rotor 4 rotatably supported by bearings 6 in a casing 3 including a suction port 1 and an exhaust port 2, and a stator 5 securely fixed in the casing 3. Gas sucked from the suction port 1 is successively compressed in multi-stage due to the compression function of a pump mechanism unit comprised of the rotor 4 and the stator 5, and is then discharged via the exhaust port to the atmosphere. In the compressing process of the gas, heat is generated by compressing the gas and the amount of the compression heat of the gas becomes larger as the gas arrives nearer the exhaust port 2. For the purpose of removing this compression heat, a cooling jacket 7 is provided on the outside of the stator 5 for cooling the stator 5 by water supplied from a water supply port 8. An example of this kind of conventional technique is disclosed, for example, in unexamined Japanese Patent Publication No. 62-29796 or unexamined Japanese Utility Model Publication No. 64-46495. Water has so large specific heat and so large thermal conductivity that its cooling effect is very preferable. However, in case where gas sucked by a vacuum pump is one whose sublimation temperature is high, i.e., which is liable to be solidified and deposited even at a low temperature, the gas is transferred into the solid phase if the interior of the pump is cooled excessively, and the gas is solidified to adhere to or accumulate on the interior of the pump as a reaction product so that a conduit in the pump is clogged and a rotor is unfavorably locked. For avoiding this, the temperature of a stator is kept, as disclosed in unexamined Japanese Utility Model Publication No. 64-46495, at a predetermined value by controlling an amount of the cooling water as circulated. However, if an amount of the cooling water is decreased to be less than a predetermined amount, the overall of the pump cannot be cooled uniformly, which results in a problem that an efficiency of the vacuum pump is degraded. Further, a flow meter is required for controlling the amount of the cooling water. Since bleaching powder precipitates at a narrow portion of the flow meter, there also occurs a problem that the temperature of the pump cannot be controlled reliably. Incidentally, though it is suggested to provide a heater only at an exhaust port of the vacuum pump so as to prevent the sublimate gas from solidification, the method of heating the gas by provision of the heater is disadvantageous in that the heater sometimes is not reliable in operation. US-4 283 167 describes an oil sealed mechanical rotary vacuum pump having high and low vacuum stages for a pumped gas, each stage being located in a housing immersed in an oil pool. The oil used also for sealing and lubricating purposes is in direct contact with the pumped gas. Heat generated in the pump and transferred to the oil is removed by means of a substantially laminar forced convection air flow contacting the outer surface of the housing. According to DE 28 04 653 Al, a vacuum pump is completely immersed in a liquid for cooling and noise reduction purposes. As liquid, water is mentioned. For better sound or noise reduction, use of glycol as liquid is recommended. Journal of Vacuum Science and Technology, 8(1990) May/ June, No. 3, "High throughput tandem turbomolecular pump for extreme high vacuum", pages 2668-2771 discloses a turbomolecular pump for achieving extreme high vacuum comprising two high throughput turbomolecular pumps arranged in tandem. The pump can discharge a large volume of process gases from semiconductor manufacturing and causes no contamination for the manufacturing equipment with oil, as both vacuum pumps are of the perfect magnetic suspension type and use no lubricant at all. FR-A-2 602 834 shows a turbomolecular pump, the rotary shaft of its rotor being supported by gas bearings acting as a dynamic seal. It is the object of the present invention to provide a vacuum pump of compact and simple design which, even if gas of a high sublimation temperature is sucked into the pump, reliably prevents solidification of the gas and deposition of reaction products. This object is obtained with the vacuum pump of the generic kind in that the coolant is a lubrication oil, in that the coolant supply is a closed loop system comprising an oil pump and an oil cooler as well as flow passages for feeding the lubrication oil also to the bearings, and in that seal gas means is provided between the pump mechanism unit and the upper bearing for preventing lubrication oil from entering the pump mechanism unit and gas fed from the suction port from entering the motor chamber. Preferably, the vacuum pump comprises a rib formed on the inner surface of the cooling jacket for upwardly revolving the cooling lubrication oil around the stator. The vacuum pump according to the invention has a compact and simple design, as the lubrication oil and its closed loop supply system allows circulation cooling of the stator as well as lubrication of the bearings using one and the same fluid, i.e. the lubrication oil. As the lubrication oil has a specific heat and a thermal conductivity which are smaller than those of water, uniform cooling and maintaining of the pump parts at a temperature which is higher than the sublimation temperature of the sucked process gas is possible, which will therefore be maintained in its gaseous state and will not be solidified or deposited to adhere to or accumulate on conduits and pump parts. An embodiment of the present invention will be hereinafter described in detail with reference to the attached drawings. Fig. 1 is a vertical cross-sectional view showing an embodiment of a vacuum pump according to the invention; Fig. 2 is an explanatory schematic view illustrative of flow of a coolant in the embodiment shown in Fig. 1; Figs. 3 and 4 are respectively graphs showing a characteristic curve of sublimation temperature of aluminum chloride (AlCl 3 ) and a temperature of a stator at each stage of the invention, in comparison with that of the prior art; Fig. 5 is a vertical cross-sectional view of a vacuum pump according to the prior art. As shown in Fig. 1 a housing or casing 103 comprises a cylindrical portion 103a and upper and lower end plates 103b and 103c. The upper end plate 103b is formed with a suction port 101, and the lower end plate 103c is formed with an exhaust port 102. A motor housing 130 is provided below the lower end plate 103c. In the housing 103 including the suction port 101 and the exhaust port 102, there is installed a pump mechanism unit 106 including a rotor 104 and a stator 105. The rotor 104 is supported by upper and lower bearings 107a and 107b and driven by a motor 108 within the motor housing 130, and the stator 105 is provided to surround the rotor 104. Gas sucked from the suction port 101 is successively compressed in multi-stage due to the compression function of the rotor 104 and the stator 105, and then the compressed gas is discharged via the exhaust port 102 to the atmosphere. A cooling jacket 109 is provided on the outer peripheral side of the stator 105. Lubrication oil 110 which has collected in a bottom portion of the motor housing 130 is supplied via an oil supply port 111 to the cooling jacket 109 by means of an oil pump 113. Heat generated when the gas sucked from the suction port 101 is compressed is carried away by the oil 110 supplied to the cooling jacket 109. A rib 109a is formed on the inner surface of the cooling jacket 109 so that the cooling fluid (oil) supplied tc a lower portion of the jacket will flow upwardly revolving round the stator 105 in the peripheral direction thereof until it is discharged from an upper portion of the cooling jacket 109 to thereby make uniform the temperature distribution of the stator 105 in the peripheral direction. As seen from the drawing, the cooling jacket 109 does not cover the final stage of the rotor and stator. This is because it is necessary to keep the temperature high at a high pressure region of the pump, and because the final stage of the rotor and stator which is cooled by seal gas can be prevented from being cooled excessively. Fig. 2 is an schematic view of the supply of the lubrication oil 110 to the cooling jacket 109. As shown in this figure, the lubrication oil supply system is a closed-loop system. The oil 110 which has absorbed the gas compression heat at the cooling jacket 109 and increased in temperature is cooled by cooling water or the like in an oil cooler 117, and thereafter the oil is supplied again to the cooling jacket 109 by the oil pump 113. The temperature of the lubrication oil is controlled by the oil cooler 117. As shown in Fig. 1, the oil pump 113 also serves to supply the lubrication oil to the rolling bearings 107a and 107b. The flow passages of the lubrication oil to the bearings are composed of the common closed-loop line with the flow passage of the cooling medium to the cooling jacket. That is to say, part of the lubrication oil discharged from the oil pump 113 flows through oil supply ports 112a and 112b so as to be fed to the upper and lower bearings 107a and 107b, respectively. With this arrangement, the cooling medium line can also serve as the lubrication oil line to thereby make the whole apparatus compact. A shaft seal portion 114 is formed between the pump mechanism unit 106 and the upper bearing 107a, and seal gas is supplied to this shaft seal portion 114 through a seal gas supply port 115 from the outside of the apparatus. For example, dry nitrogen is used as such seal gas so that it will not react with the gas sucked from the suction port 101. The seal gas discharged from the seal gas supply port 115 toward the surface of the rotor 104 is divided into upward and downward flows. Part of the seal gas flows into the pump mechanism unit 106 and is discharged from the exhaust port 102 with the gas fed from the suction port 101, whereas the rest of the seal gas flows through the upper bearing 107a into a motor chamber 116 and is discharged from a seal gas discharge port 119. These two flows of the seal gas can prevent the lubrication oil fed to the bearings from entering the pump mechanism unit 106, and can also prevent the gas fed from the suction port 101 from entering the motor chamber 116. In operation the gas sucked from the suction port 101 is successively compressed in multi-stage in a conduit of the pump mechanism unit 106 including the rotor 104 and the stator 105, and thereafter the compressed gas is discharged from the exhaust port 102 into the atmosphere. when the gas is discharged, it is heated to have a high temperature in a region where the rotor 104 is rotated at high speeds, and this heat is transmitted to the stator 105. If such a condition is unchanged, the gas temperature is increased, and consequently, the high-temperature gas degrades compression performance of the pump mechanism unit 106, thus deteriorating its pumping function, while it causes thermal expansion which brings the rotor 104 and the stator 105 into contact with each other. In the present invention, however, the stator 105 can be cooled by the cooling jacket 109 through which the lubrication oil is made to flow, and can be maintained at a certain temperature by reliable cooling operation. For example, when the suction port 101 of the vacuum pump is connected with a reactor of an aluminum dry etching device of a semiconductor manufacturing apparatus, aluminum chloride (AlCl 3 ) is generated as a reaction product after etching. Fig. 3 shows a graph of temperatures relative to pressures where a characteristic curve A of sublimation temperature of aluminum chloride represents a boundary line between a solid-phase side and a gaseous-phase side. In Fig. 3, a curve 18 denotes data of a conventional example, and a curve 19 denotes data of a particular embodiment of the present invention. If water cooling is conducted by supplying water to the cooling jacket 109, the temperature inside the stator 105 will be on the solid-phase side of the characteristic curve A of sublimation temperature of aluminum chloride. Therefore, AlCl 3 will be solidified and adhere to or accumulate on the inner wall of the stator 105. In the embodiment according to the invention, the oil is supplied to the cooling jacket 109 so as to cool the stator 105. Since the thermal conductivity of oil is as small as about 1/5 of that of water, the temperature inside the stator 105 can be made higher by oil when water and oil having the same temperature are used. As a result, the temperature inside the stator 105 can be kept on the gaseous-phase side of the characteristic curve A of sublimation temperature of AlCl 3 to thereby prevent the reaction product from adhering to the inner wall of the stator 105. The function of the present invention will be described more specifically with reference to Fig. 4. In this graph, a curve 18 denotes data of a conventional example, and a curve 19 denotes data of a particular embodiment of the present invention. If water cooling is conducted by supplying water to the above-described cooling jacket 109, the temperature inside the stator 105 will be on the solid-phase side of the characteristic curve A of sublimation temperature of AlCl 3 , and therefor, AlCl 3 will adhere to or accumulate on the inner wall of the stator 105. The thermal conductivity of water at a temperature of 40°C is 0.63 W/m·K and larger than that of oil. In the present invention, the cooling medium is a lubrication oil having a thermal conductivity of 0.093 to 0.29 W/m·K. Suitable lubrication oils are 90 turbine oil, #140 turbine oil, vacuum oil (of alkyldiphenyl ether, of perfluoropolyether), mineral oil, synthetic oil, and the like. The thermal conductivity of the lubrication oil is as small as about 1/5 of that of water, and consequently, the temperature of the lubrica tion oil can be kept higher when water and the lubrication oil having the same temperature are used, so that the temperature inside the stator 105 can be made higher by the lubrication oil, and that the temperature inside the stator 105 can be kept on the gaseous-phase side of the characteristic curve A of sublimation temperature of AlCl 3 . As a result, the reaction product can be prevented from adhering to the inner wall of the stator 105. If a cooling medium having a thermal conductivity of 0.29 W/m·K is used, the temperature of the stator 105 varies from its first stage to the eighth stage, as indicated by a curve 19a in Fig. 4, and part of the curve 19a is quite close to the characteristic curve A of sublimation temperature of AlCl 3 . Accordingly, if a cooling medium having a large thermal conductivity is used, AlCl 3 may be solidified. In order to prevent AlCl 3 from being solidified, therefore, a cooling medium having a thermal conductivity of 0.29 W/m·K or less is used. On the other hand, if a cooling medium having a thermal conductivity of 0.093 W/m·K is used, the temperature of the stator 105 can be maintained substantially as indicated by a curve 19b in Fig. 4. If a cooling medium having a small thermal conductivity is used, however, the stator 105 will not be cooled sufficiently, and will have a high temperature. In case it exceeds about 250°C, sealing material interposed between mating faces of the stator 105 may be broken, or cooling of compressed gas may become insufficient, thus deteriorating the compression performance. The stator 105 should be maintained at a temperature not more than 250°C, and therefore, a cooling medium having a thermal conductivity of 0.093 W/m·K or more is used. In the embodiment shown in Fig. 1, the oil cooler 117 is provided outside of the motor housing 130. Alternatively, the oil cooler 117 may be provided inside the motor housing 130.

Claims: A vacuum pump used as a multi-stage exhaust pump in a semiconductor manufacturing apparatus, which vacuum pump comprises a housing (103) including a suction port (101) and an exhaust port (102) through which gas sucked from the suction port is exhausted to have a pressure substantially equal to or close to atmospheric pressure, a stator (105) fixed to an inner wall of said housing (103), a rotor (104) attached to a rotary shaft and constituting by mating relation with the stator (105) a multi-stage pump mechanism unit (106), a motor housing (130) mounted below the pump mechanism unit (106) including a motor chamber (116) and a motor (108) for rotating the rotary shaft, an upper bearing (107a) and a lower bearing (107b) for rotatably supporting the rotary shaft in the housing (103) and motor housing (130), respectively, and a cooling jacket (109) on the outer periphery of the stator (105) provided with a coolant supply characterized in that the coolant is a lubrication oil (110), the coolant supply is a closed loop system comprising an oil pump (113), and an oil cooler (117) as well as flow passages (112a, 112b) for feeding the lubrication oil (110) also to the bearings (107a, 107b) and seal gas means (114, 115) is provided between the pump mechanism unit (106) and the upper bearing (107a) for preventing lubrication oil (110) from entering the pump mechanism unit (106) and gas fed from the suction port (101) from entering the motor chamber (116). Vacuum pump according to claim 1, characterized by a rib (109a) formed on the inner surface of the cooling jacket (109) for upwardly revolving the cooling lubrication oil (110) around the stator (105). A vacuum pump according to claim 2, wherein said coolant is one selected from lubrication oil, vacuum oil, mineral oil, synthetic oil, ethylene glycol and ethyl alcohol. A vacuum pump according to claim 3, wherein said lubrication oil is either #90 turbine oil or #140 turbine oil. A vacuum pump according to claim 3, wherein said vacuum oil is alkyldiphenyl ether based or perfluoropolyether based.

Inventors: MASE MASAHIRO, NAGAOKA TAKASHI, NAKAMORI KAZUAKI, TANIYAMA MINORU, MASE, MASAHIRO, NAGAOKA, TAKASHI, NAKAMORI, KAZUAKI, TANIYAMA, MINORU
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: F04D19
IPC codes: F04C29, F04D29, F04D19