VACUUM GENERATING SYSTEM

Abstract: A vacuum evacuation system comprising a non-­ positive displacement type vacuum pump called molecular pump (6) which obtains high pumping speed in a high vacuum range, and an oil-free, positive displacement type vacuum pump called screw vacuum pump (3). The non-positive displacement type vacuum pump and the positive displacement type vacuum pump are connected to each other such that the non-positive displacement type vacuum pump is located on a high vacuum side and the positive displacement type vacuum pump is located on a low pressure or atmospheric side, to thereby prevent oil from being penetrated into a system to be evacuated.

Description: The invention relates to a vacuum evacuation system comprising a first vacuum pump having a rotary component and a suction port and an exhaust port, in which gas molecules are caused to collide with said rotary component rotating at high speed so as to be given a momentum in a direction of linear velocity of said rotary component so that a gas flow is produced in a given direction, a second vacuum pump including a casing provided with a suction port and an exhaust port to produce a differential pressure between said suction and exhaust ports provided in said casing, and piping means for connecting said exhaust port of said first vacuum pump to said suction port of said second vacuum pump, said suction port of said first vacuum pump being disposed on a vacuum side, and said exhaust port of said second vacuum pump being disposed on an atmospheric side. The US-A-3 104 802 discloses a vacuum pump in which on a single drive shaft formed by flexible coupled shaft sections first and second pumps of the same type (siegbahn type) and coaxially with them, a sliding vane prevacuum pump are mounted. With this construction it is impossible to accomplish vacuum in the range from vaccuum level to the atmospheric pressure by only two pump stages. The US-A-3 677 664 discloses, among other things, dry running coupled screw compressors on a single set of shafts and operating as a two stage vacuum pump. There exist small clearances between the relatively moving parts, thus avoiding in case that such a vacuum pump would be used in semiconductor manufacturing apparatus that the oil which is needed for sealing and lubrication of the rotors will diffuse reversely as the pumps are not dry pumps. As semiconductors tend to become high in integration and density, such reverse diffusion on a molecular level would cause substantial reduction of yield in the manufacture of products. Additonally, oil is deteriorated in quality due to the contact of the reaction gas therewith within a short period of time to thereby need substantial maintenance. The invention is based on the problem to provide a vacuum evacuation system which can accomplish vacuum of 1 Pa or less while to an even greater extent preventing oil from being penetrated into a working chamber and a system to be evacuated, at the same time enabling the designer the selection of the most favorable respective rotational speeds and furthermore guaranteeing a compact arrangement. Starting out from the vaccum evacuation system of the generic kind, this problem is solved by a system having the features of the characterizing part of claim 1. Advantageously the first vacuum pump includes an outer case, an outlet, a rotor blade stage having stator blades fixedly secured to said outer case and rotor blades arranged in facing relation to said stator blades, for causing molecules to collide with each blade to produce a gas flow in a downstream direction, and a screw pump stage including a helically grooved rotor having an outer peripheral surface facing said outer case, said outer peripheral surface being formed with a helical groove, for delivering the gas from said rotor blade stage in the downstream direction by said helical groove. Preferably the drive means include first drive means fixedly secured to said outer case for driving said first vacuum pump means, and second drive means for driving said screw rotors of said second vacuum pump means. In a preferred embodiment said helically grooved rotor of said first vacuum pump means is in the form of an annulus having an upper end wall, the annular portion surrounding said first drive means. It is advantageous, if the helically grooved rotor is fixedly mounted to said first drive means at that upper end wall. With the vacuum evacuation system of the invention vaccuum of 1 Pa or less can be accomplished. As both pumps are dry pumps, oil is prevented from being penetrated into a working chamber so that there is no back-diffusion of oil to a system to be evacuated. The auxiliary pump of the above-described combined arrangement, that is, the second vacuum pump is of the type in which a pair of male and female screw rotors are supported within a casing by respective bearings with a slight gap maintained between the inner surface of the casing and the screw rotors, and the pair of screw rotors are rotated in synchronized relation by timing gears with a slight gap maintained between the screw rotors, to produce a differential pressure between a suction and an exhaust port provided in the casing. It is unnecessary to lubricate the working chamber formed by the screw rotors and the casing. In addition, lubricating oil supplied to the bearings supporting the respective screw rotors is prevented from being penetrated into the working chamber by respective seal assemblies each comprised of a labyrinth seal, a screw type seal, a floating labyrinth seal and the like. Thus, the second vacuum pump is of an oil-free construction. For the reasons described above, the combination of the second vacuum pump with the molecular pump can provide a vacuum evacuation system which is clean and has high pumping speed in a high vacuum range. The invention is further explained by means of drawings in which Fig. 1 is a perspective view showing a vacuum evacuation system in accordance with an embodiment of the invention; Fig. 2 is a perspective view showing an internal construction of a molecular pump incorporated in the system illustrated in Fig. 1; Fig. 3 is a longitudinally cross-sectional view showing a screw vacuum pump apparatus incorporated in the system illustrated in Fig. 1; Fig. 4 is a longitudinally cross-sectional view showing a screw vacuum pump element of the apparatus illustrated in Fig. 3; and Fig. 5 is an enlarged cross-sectional view of a seal assembly illustrated in Fig. 4. Fig. 1 is a perspective view of a vacuum evacuation system in accordance with an embodiment of the invention, Fig. 2 is a perspective view of an internal construction of a molecular pump incorporated in the system illustrated in Fig. 1, Fig. 3 is a longitudinally cross-sectional view of a screw vacuum pump apparatus incorporated in the system of Fig. 1, Fig. 4 is a cross-sectional view of a screw vacuum pump element of the apparatus illustrated in Fig. 3, and Fig. 5 is a cross-sectional view of a seal assembly illustrated in Fig. 4. The vacuum evacuation system shown in Fig. 1 comprises a base 1 and a gear case 2 fixedly mounted thereon. Attached in a cantilevered manner to the respective sides of the gear case 2 are a screw vacuum pump element 3 forming a second vacuum pump, and a motor 4 for driving the screw vacuum pump element 3, to constitute an atmospheric-side pump. A frame 5 is mounted on the base 1 so as to straddle the atmospheric-side pump. A molecular pump 6 forming a first vacuum pump and at an angle of 90° to the screw pump is mounted to an upper portion of the frame 5, to constitute a vacuum-side pump. Piping 9 is provided for connecting an exhaust port 7 of the molecular pump 6 to a suction port 8 of the screw vacuum pump 3. The molecular pump 6, i.e., the vacuum-side pump and the screw vacuum pump 3, i.e., the atmospheric-side pump are supplied with electric power from an electric power supply device (not shown) and are operated by a control panel (not shown). The illustrated vacuum evacuation system has a suction port which is a suction port 10 of the molecular pump 6, and an exhaust port which is an exhaust port 11 of the screw vacuum pump 3. The molecular pump 6 forming the first vacuum pump will first be described in detail with reference to Fig. 2. As shown in Fig. 2, a pump drive motor comprises a motor stator 13 fixedly mounted vertically within a housing 12. Within the motor stator 13, a motor rotor 14 and a rotary shaft 15 fitted thereinto are supported vertically. The rotary shaft 15 has an upper portion thereof extending from the housing 12. A multiplicity of rotor blades 16 are fixedly secured to the peripheral surface of an upper section of the extending portion of the rotary shaft 15. A rotor 17 is fixedly mounted between the rotor blade assembly and the housing 12 so as to cover or surround the same. The rotor 17 is comprised of an upper end wall 17B and an annular portion 17C connected thereto. A helical groove 17A of a trapezoidal cross-section is formed in the outer peripheral surface of the annular portion 17C. A stator 18 forms an outer case of the molecular pump 6, and a slight gap is maintained between the stator 18 and the outer peripheral surface of the rotor 17. Inside an upper portion of the stator 18, stator blades 19 are fixedly secured at positions overlapping the rotor blades 16. As electric current is caused to pass through the coils of the stator 13 from the electric power supply device (not shown), a rotary component comprised of the rotary shaft 15, motor rotor 14, rotor blades 16 and rotor 17 is rotated at high speed so that gas molecules introduced through the suction port 10 are mechanically blown off by the rotor blades 16 and the trapezoidal helical groove 17A and are discharged through the exhaust port 7, to thereby produce a pumping action. However, if the pressure on the exhaust side is high, the molecular pump 6 cannot be operated, because extremely high power is required. The molecular pump 6 can be operated if the pressure at the exhaust port 7 is brought to a level equal to or less than 2 Torr. The screw vacuum pump forming the second vacuum pump (auxiliary pump) will next be described with reference to Figs. 3 through 5. A speed increasing gear 20 is disposed within the gear case 2 and is fixedly mounted on an output shaft 4a of the motor 4. The speed increasing gear 20 is in mesh with a male-rotor-side timing gear 21. Within a casing 22 of the screw vacuum pump element 3, a pair of male and female screw rotors 23 and 24 are supported with a slight gap maintained between an inner surface of the casing 22 and the screw rotors 23 and 24. These screw rotors 23 and 24 are in mesh with each other by means of the male-rotor-side timing gear 21 and a female-rotor-side timing gear 25 with a slight gap maintained between the screw rotors 23 and 24. As shown in Fig. 3, the casing 22 is provided with a suction port 8′ and an exhaust port 11′. A seal assembly 26 illustrated in Fig. 4 is provided for each of shaft portions of the respective male and female screw rotors 23 and 24. As shown in detail in Fig. 5, the seal assembly 26 is comprised of a bearing 27, a labyrinth seal 28, a screw type seal 29 and a floating labyrinth seal 30. Rotation of the motor 4 is increased by the speed increasing gear 20 to rotate the pair of male and female screw rotors 23 and 24. As the screw rotors are rotated, gas drawn through the suction port 8′ is delivered toward the exhaust side (right side in Fig. 3), while being maintained confined within a closed chamber formed by the helical grooves of the respective screw rotors and the inner surface of the casing 22. The delivered gas is discharged through the exhaust port 11′. The volume of the above-mentioned closed chamber at completion of the suction is different from the volume of the closed chamber just before the discharge, and the latter volume is made smaller than the former volume by an amount corresponding to the compression ratio, so that a pumping action is produced. The bearings 27 respectively supporting the screw rotors are lubricated forcibly or in a splashing manner through lubricating piping (not shown) by an oil supply device (not shown). However, the triple seals as shown in Fig 5 prevent the oil from being penetrated into the working chamber. The operation of the entire vacuum evacuation system in accordance with the embodiment of the invention will next be described with reference to Fig. 1. In case where the illustrated vacuum evacuation system is operated from the point of time the pressure on the suction side of the system, that is, the pressure at the suction port 10 of the molecular pump 6 is higher than a predetermined pressure, the screw vacuum pump 3 is first operated, and the molecular pump is subsequently operated after the pressure at the exhaust port 7 of the molecular pump 6 is reduced to a level equal to or less than a predetermined pressure (about 2,66 x 10² Pa). If the system is operated to cause the gas to flow when the pressure at the exhaust port 7 of the molecular pump 6 is equal to or less than the predetermined pressure, both pumps are operated. The screw vacuum pump 3 compresses the gas of the flow rate taken in by the molecular pump 6, from the pressure at the exhaust port 7 to the atmospheric pressure, and discharges the compressed gas through the exhaust port 11. Control of the operation of the pumps is automatically effected by pressure sensors and a control device (both not shown). According to the illustrated embodiment, it is possible to cause the gas to flow at high flow rate in the high vacuum range, as compared with the conventional mechanical booster (the ultimate pressure is on the order of 1,33 x 10⁻² Pa, and the design pumping speed is obtained in the vicinity of 1,33 to 1,33 x 10² Pa, because the illustrated embodiment is so arranged as to comprise the combination of the oil-free screw vacuum pump 3 and the molecular pump 6 (the ultimate pressure is 1,33 x 10⁻⁸ Pa, and the flow rate is on the order of 200 liter/sec. at 1,33 x 10⁻¹ to 1,33 x 10⁻⁸ Pa. Moreover, both the molecular pump 6 on the vacuum side and the screw vacuum pump element 3 on the atmospheric side are of a construction in which the working chamber has therein no oil and, therefore, there is provided a vacuum evacuation system which is clean and which is extremely low in back-diffusion of the oil to the vacuum side. This avoids the necessity of a foreline trap for oil adsorption which has conventionally been used to even slightly relieve the back-diffusion of the oil from the oil-sealed rotary vacuum pump. Furthermore, many of the gases employed in the semiconductor manufacturing apparatuses have such nature as to immediately degrade oil. Accordingly, it has been necessary for the conventional oil-sealed rotary vacuum pump to waste great labor in maintenance of the oil. In the illustrated embodiment, however, since there is almost no contact between the gas and the oil, it is made possible to considerably reduce labor wasted in the maintenance.

Claims: A vacuum evacuation system comprising: a first vacuum pump (6) having a rotary component (15) and a suction port (10) and an exhaust port (7), in which gas molecules are caused to collide with said rotary component (15) rotating at high speed so as to be given a momentum in a direction of linear velocity of said rotary component (15) so that a gas flow is produced in a given direction, a second vacuum pump (3) including a casing (22) provided with a suction port (8') and an exhaust port (11') to produce a differential pressure between said suction and exhaust ports (8', 11') provided in said casing (22), and piping means (9) for connecting said exhaust port (7) of said first vacuum pump (6) to said suction port (8') of said second vacuum pump (3), said suction port (10) of said first vacuum pump (6) being disposed on a vacuum side, and said exhaust port (11') of said second vacuum pump (3) being disposed on an atmospheric side, characterized in that the second pump (3) comprises a pair of dry running male and female screw rotors (23, 24) supported within said casing (22) with a slight gap maintained between said casing (22) and said screw rotors (23, 24), said pair of male and female screw rotors (23, 24) being rotated with a slight gap maintained between, separate drive means (13, 14; 4) including motors for respectively driving said first and second pump means (6; 3) and disposed at an angle of about 90° are provided and sealing means (26) are disposed between the casing (22) and the respective shaft portions of said screw rotors (23, 24). A vacuum evacuation system according to claim 1, wherein the first vacuum pump means (6) includes an outer case (18), a rotor blade stage having stator blades (19) fixedly secured to said outer case (18) and rotor blades (16) arranged in facing relation to said stator blades (19), for causing molecules to collide with each blade to produce a gas flow in a downstream direction, and a screw pump stage including a helically grooved rotor (17) having an outer peripheral surface facing said outer case (18), said outer peripheral surface being formed with a helical groove (17A), for delivering the gas from said rotor blade stage in the downstream direction by said helical groove (17A). A vacuum evacuation system according to claim 2, wherein the drive means include first drive means (13, 14) fixedly secured to said outer case (18) for driving said first vacuum pump means (6), and second drive means (4) for driving said screw rotors (23, 24) of said second vacuum pump means (3). A vacuum evacuation system according to claim 2 or 3 wherein said helically grooved rotor (17) of said first vacuum pump means (6) is in the form of an annulus (17C) having an upper end wall (17B), the annular portion (17C) surrounding said first drive means (13, 14). A vacuum evacuation system according to claim 4 wherein the helically grooved rotor (17) is fixedly mounted to said first drive means (13, 14) at that upper end wall (17B).

Inventors: HAYAKAWA, TADASHI, MITSUHASHI, SINJI, NAYA, KOTARO, SHIINOKI, KAZUAKI
Applicants: HITACHI, LTD.
Application year: 1987
Main classification: F04C25
IPC codes: F04B37, F04D17, F04C25, F04D19, F04C23