Two stage primary dry pump

Abstract: An evacuating apparatus includes a first pump section disposed in a sucking opening side, a second pump section for exhausting a smaller amount of gas than the first pump section, the second pump being disposed in an exhaust opening side, a plurality of rotors accommodated in a housing, bearings for supporting rotation of the rotors, a sucking opening of fluid and a first exhaust opening thereof both formed on the housing, and a plurality of motors for driving the rotors. In the apparatus, the first pump section is a pump of positive displacement type formed by utilizing a volume change of a space defined by the rotors and the housing, and the second pump section is a viscous-type pump formed by utilizing a relative moving surface formed in a small gap defined between the rotors and the housing.

Description: The present invention relates to an evacuating apparatus, particularly to a vacuum pump used to exhaust gas from a vacuum chamber of semiconductor-manufacturing equipment. A vacuum pump for generating vacuum environment is essential to a CVD apparatus, a dry etching apparatus, a sputtering apparatus and the like to be used in the process for manufacturing semiconductor. The demand for the vacuum pump having improved performance is growing higher and higher in recent years in correspondence with a highly integrated and fine semiconductor-manufacturing process. The vacuum pump is required to provide a high degree of vacuum, be clean, compact, and easy to perform maintenance. With the development of a composite semiconductor-manufacturing process, so-called multi-chamber system in which each of a plurality of vacuum chambers is separately evacuated is mainly employed in semiconductor-manufacturing equipment. Accordingly, the number of vacuum pumps to be used in semiconductor-manufacturing equipment is increasing. In order to comply with the demand for the improved evacuating system of the semiconductor-manufacturing equipment, a roughing dry vacuum pump is broadly used instead of a conventional oil-sealed rotary vacuum pump so as to obtain cleaner vacuum. But such a vacuum pump has the following disadvantages: (A) A great amount of energy is consumed; (B) Great noise and vibration is generated; and (C) The degree of ultimate vacuum pressure is insufficient. Firstly, detailed description of the above disadvantage (A) is made below. The operation period of time of the vacuum pump used in the semiconductor-manufacturing equipment is divided into the following two processes: (Process 1) Period of time required to exhaust a great amount of gas inside a vacuum chamber; and (Process 2) Period of time required to maintain a vacuum pressure which has been attained. The ratio of the period of time of process 2 to that of process 1 is very great. Since the vacuum pump does not carry out the work of transporting gas in the process 2, no work is done by the vacuum pump in principle. However, the conventional vacuum pump consumes a great amount of power both in processes 1 and 2. Attention is paid to the reason a great amount of power is consumed in the process 2. Referring to Fig. 2 showing the power consumed by a conventional screw type roughing pump with respect to pressure of sucked gas, the following points (1) and (2) are noted. Point (1) Consumed power is 4.OKW when the pressure of suck gas is in the vicinity of 10 3 torr, i.e., when the vacuum pump starts exhausting gas of a great weight flow from a vacuum chamber. Point (2) Consumed power is 3.2KW when the pressure of the sucked gas has dropped enough. The ratio of the consumed power of the point (2) to the point (1) is approximately 80%. Much power which is not contributed to effective operations is wasted by tens or hundreds of dry vacuum pumps operating simultaneously in the semiconductor-manufacturing factory. The reason power is wasted is described below in detail by exemplifying a screw vacuum pump of twin rotor type. As shown in Fig. 7, a conventional screw vacuum pump of twin rotor type (screw type with a thread groove) comprises two rotors 600a and 600b accommodated in a casing 602 and rotating in opposite directions with grooves 608a and 608b engaging each other. Gas is sucked from a sucking opening 601 and discharged from an exhaust opening 602. The vacuum pump further comprises rotary shafts 603a and 603b integrally connected with the rotors 600a and 600b; ball bearings 605a, 605b and 606a, 606b for supporting the rotary shafts 603a and 603b; and timing gears 607a and 607b for obtaining the synchronous rotation of the two rotors 600a and 600b. Normally, in this kind of dry pump, a delivery valve (check valve) is not formed on the exhaust opening 602 so as to reduce fluid resistance in exhaust. Figs. 8A, 8B, and 8C are model views showing each process (N = 0 through 4) of the suction, transporting, and exhaust of the above-described pump. The portions indicated by chain lines denote thread grooves 608a and 608b formed on the back surface which cannot be seen from the front. Reference symbols S (shown in Figs. 8A, 8B, 8C, and 7) at the center and both edges denote portions to form sealing lines as a result of the engagement between the thread grooves of the rotors 600a and 600b. Accordingly, in the pump of twin rotor type having thread grooves, a fluid-transporting space for transporting fluid from the suction side to the exhaust side is constituted of the sealing line S, the thread grooves 608a and 608b, and the casing 602. Let it be supposed that the left half of the fluid-transporting space is denoted as n = 1 through 5, and the right half thereof is denoted as n' = 1 through 5. How the transporting space formed in the rotor 600a of the twin rotor transports fluid is described below with attention paid to the fluid-transporting space in the left half. First process: N=0 shows the state in which sucking process has just started. Attention is paid to gas sucked from the suction side and accommodated in a groove of n=1 as shown by arrows in Fig. 8A. Second process: At the rotation of N=1, gas is moved to a groove of n=2 and enclosed in a space cut off from the suction side. The description of N=2, 3 is omitted herein. Third process: At the rotation of N=4, immediately after a part of a groove of n=5 communicates with the discharge side, gas on the high pressure discharge side flows back to the groove of n=5. Then, gas which has flowed into the groove of n=5 is exhausted to the exhaust side again with the progress of the entire process. As described above, the reason a great power is required although the pressure on the suction side of the dry vacuum pump having no delivery valve (check valve) has reached a sufficiently low degree of vacuum is because the third process is included in the entire process. The operation of the screw type vacuum pump to be performed after the rotation of N=4 is described below by replacing the screw type vacuum pump with a close coupled type pump. Referring to Fig. 9, the pump comprises a vacuum chamber 700; a cylinder 701; a fluid-transporting space 702 on the suction side of the pump; a fluid-transporting space 703 on the exhaust side thereof; a piston 704; a piston rod 705; a sucking pipe 706; an exhaust pipe 707; an adsorption tower 708 for processing reactive gas; and a factory pipeline 709. The pressure in the fluid-transporting space 702 is sufficiently low and the pressure in the fluid-transporting space 703 is approximately to the atmospheric pressure (P=1kg/cm 2 ). Accordingly, as shown in Fig. 9, in this process, the difference in pressure applied to the front and rear of the piston 704 is as large as approximately ΔP=1kg/cm 2 . The piston 704 is required to move to the right against the pressure difference (external load). In this manner, energy which is not contributed to effective operations is lost. This disadvantage is common to vacuum pumps of positive displacement type although description has been made by way of the close coupled type pump. It is conceivable to provide the vacuum pump with a delivery valve (check valve) as used in a compressor so as to prevent the back flow of gas from the exhaust side to the suction side in the exhaust process. But the following disadvantages arise. The exhaust pressure of the vacuum pump is lower (close to atmospheric pressure) than that of the compressor and the volume flow rate of the vacuum pump is greater than that of the compressor. A great exhaust amount (for example, equal to or more than 500 liter/min) is required in the semiconductor-manufacturing equipment. Because of the above disadvantages, it is necessary that the passage area of the delivery valve is sufficiently large when it is opened to the greatest extent. To this end, it is necessary to make the lift (moving amount) of the delivery valve sufficiently large, i.e., the use of a large delivery valve is required. The large has, however, slow response. Thus, it is difficult to compose the delivery valve in conformity to a screw type vacuum pump, claw type vacuum pump or a scroll type vacuum pump. In addition, noise is increased by compound vibration of the delivery valve and fluid even though the delivery valve is provided, as previously described as the disadvantage (B). EP-A-401 741 (corresponding to DE 690 00 990 T2) discloses a two-stage vacuum pump having a first pumping section of a larger capacity in the form of a positive displacement pump with two screw-type rotors engaging each other. The second pumping section of a smaller capacity is in the form of a viscous-type pump having only one screw-type rotor. Both rotors of the first pumping section are coupled by gears driven by a common motor and the rotor of the second pumping section is coupled to one of the rotors of the first stage. In the description it is mentioned that the main reason for having a two-stage pump is to avoid unsymmetric stresses and deformations of the parts of the pump due to high power consumption. From EP-A-472 933 a one-stage vacuum pump is known, having two rotors which are driven by two separate motors synchronized electronically. The main pump is a positive displacement pump, whereas an additional drag pump in the form of a viscous-type pump is arranged upstream the main pump. GB-A-1 248 032 discloses a composite pump comprising a dry screw-type pump as a first stage and an oil-sealed screw-type pump as a second stage (as a backing pump). Each of the two pump sections has two rotors engaging each other, and corresponding rotors of both pumps are mounted on common shafts. The two shafts are mechanically coupled by gears, and one of the shafts is driven by driving means. This known composite pump is provided with a dry screw-type pump as a first stage in order not to contaminate the vacuum with oil, whereas the second stage uses an oil-sealed screw-type pump in order to obtain a high vacuum. It is an object of the present invention to provide an evacuating apparatus with low power consumption and low noise and vibration, while attaining a sufficient degree of ultimate vacuum pressure quickly. In accomplishing the object of the invention, an evacuating apparatus comprises: a housing having a fluid suction opening and a fluid exhaust opening, a first pump section mounted in said housing and having its suction side connected with said fluid suction opening, said first pump section being a positive displacement pump having a first and a second rotor accommodated in said housing and which positively displace fluid by utilizing a volume change of a space defined by said rotors and said housing, a second pump section having its suction side connected with the exhaust side of said first pump section and its exhaust side with said fluid exhaust opening, for exhausting a smaller amount of fluid than said first pump section, said second pump section having a first and a second rotor, the first rotors of both pump sections being coupled with each other and driven by a first motor, and the second rotors of both pump sections being coupled with each other and driven by a second motor, and detecting means for detecting a rotational angle of each motor and/or the rotational speed thereof, the rotors being rotated synchronously in cooperation of the motors each for independently driving each of the rotors, based on signals outputted from the detecting means. Preferable embodiments are defined in the dependent claims. Accordingly, the evacuating apparatus comprises a first pump section providing a large amount of exhaust and a second pump section having a small amount of exhaust but providing a low degree of vacuum. The first pump section and the second pump section are connected to each other in series. Immediately after the vacuum pump starts evacuation, the first pump section works efficiently and exhausts gas in a large weight flow rate. If the volume of the vacuum chamber connected with the upstream side of the vacuum pump is small, normally, the pressure inside the vacuum chamber drops to a sufficiently low degree of vacuum in less than several seconds. In this state, the second pump section communicates with the atmospheric side (exhaust side). Accordingly, torque to be determined by the exhaust amount (pressure-receiving area) is small. A pump of positive displacement type or viscous type can be used as the second pump section because a large exhaust amount is not required for the second pump section. The first pump section can be composed of rotors of thread groove type pump or rotors of screw type pump, and then the rotors can be rotated synchronously by an electronic control means. In this manner, the rotors can be rotated at a high speed. As a result, internal leakage from the atmospheric side of the second pump section to the upstream side can be decreased, and consequently, the pressure in the downstream side of the first pump section can be kept at a low degree of vacuum. Therefore, the vacuum pump can be driven by a small torque. Preferably, a valve is placed between a portion intermediate between the first pump section and the second pump section, and the exhaust opening. In this manner, gas can be exhausted from the vacuum chamber in a short period of time. In the state in which the vacuum pump has just started evacuation, the first pump section can exhaust gas in a large weight flow rate and gas can be exhausted from the first exhaust opening through the valve disposed at the portion intermediate between the first pump section and the second pump section. When the pressure on the suction side of the first pump section has reached a sufficiently low degree of vacuum, the valve can be closed and only the second exhaust opening disposed on the downstream side of the second pump section can communicate with the exhaust side disposed outside the vacuum pump. At this time, the pressure on the exhaust side of the first pump section can be at a sufficiently low degree of vacuum by the operation of the second pump section. Accordingly, greatly reduced power can suffice for driving the first pump section. In addition, since the exhaust amount of the second pump section is small, only a small amount of power is required for the exhaust. Therefore, greatly reduced power can suffice for driving the first and second pump sections. In the above cases, the rotation of the first pump section can be equivalent to the rotation in vacuum. Therefore, unlike the conventional dry vacuum pump, gas can not flow back from the exhaust side to the suction side and hence a cyclic pulsation sound is not generated. In addition, the blade of the thread groove (screw) can not generate wind noise during the high speed rotation of the rotor. When the vacuum pump is driven by rotating the rotors synchronously by the above-described electronic control means, timing gears (for example, 607a and 607b of Fig. 7) do not generate contact sound. Therefore, the cause of noise can be greatly reduced unlike the conventional roughing pump. These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which: Figs. 1A and 1B are views, showing the principle of a two-stage pump, in which a valve is not provided; Fig. 1C is a view showing the principle of a two-stage pump in which a valve is open; Fig. 1D is a view showing the principle of the pump in Fig. 1C, in which the valve is closed; Fig. 2 is a graph showing the relationship between consumed power and intake pressure of a pump according to an embodiment of the present invention and a conventional pump; Fig. 3 is a view showing an evacuating system, including the evacuating apparatus according to the present invention, used in a semiconductor-manufacturing factory; Fig. 4 is a view showing a vacuum pump of non-contact and synchronous control type to which the vacuum pump according to a first embodiment is applied; Fig. 5 is a sectional view showing the vacuum pump of Fig. 4; Fig. 6 is a block diagram showing an electronic control means used to perform the synchronous rotation of rotors; Fig. 7 is a sectional view showing a conventional vacuum pump of screw type; Figs. 8A, 8B, and 8C are model views showing the process of sucking, transportation, and exhaust of a conventional screw pump; Fig. 9 is a model view showing the principle of one-stage conventional vacuum pump; and Fig. 10 is a sectional view showing a vacuum pump according to a second embodiment of the present invention. Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings. Evacuating apparatuses according to embodiments of the present invention will be described below with reference to the drawings. [1] Principle of the present invention Regarding the principle of the present invention, description is made on the case in which [1-I] a valve is not provided and the case in which [1-II] a valve is provided. [1-I] Case in which valve is unprovided Figs. 1A and 1B are model views showing a close coupled type vacuum pump constituting an evacuating apparatus according to this case. More specifically, Fig. 1A shows a state in which the exhaust of gas in a vacuum chamber has just started, and Fig. 1B shows a state in which the pressure in the vacuum chamber has reached a sufficiently low degree of vacuum. A first vacuum pump section in the vacuum pump is constituted of a vacuum chamber 1, a cylinder 2, a fluid-transporting space 3 in its suction side, a fluid-transporting space 4 in its exhaust side, a piston 5, and a piston rod 6. The evacuating apparatus comprises a second vacuum pump section in the vacuum pump 7; an adsorption tower 12 for processing reactive gas; and a factory pipeline 13. (1) State in which exhaust of gas in vacuum chamber has just started: The first vacuum pump section sucks a large amount of gas thereinto from the vacuum chamber 1 and the same amount of gas is exhausted from the exhaust side. At that time, since the exhaust volume of the second pump section 7 is small, the second pump section 7 is incapable of discharging a large amount of gas therefrom and thus the gas in the suction side (fluid-transporting space 4) of the second vacuum pump section 7 is compressed. Consequently, there is a possibility that the temperature of the second vacuum pump section 7 rises. In a semiconductor-manufacturing process such as a dry etching process, the pressure in the vacuum chamber 1 attains to a sufficiently low value in several seconds to several tens of seconds when the volume of the vacuum chamber of the vacuum pump of Figs. 1 and 2 is 10 to 20 liters. Therefore, heat generated by compressed gas causes no practical problems in operation. Gas discharged from a micro-pump (second vacuum pump section 7) is transported to the factory pipeline 13 via the adsorption tower 12. (2) State in which pressure in vacuum chamber has attained to sufficiently low degree of vacuum: At this time, the weight flow of gas to be sucked from the vacuum chamber 1 into the first vacuum pump section is very small. When reactive gas which pressure is approximately 1 atm is introduced into the vacuum chamber 1, the weight flow of the reactive gas to be sucked from the vacuum chamber 1 into the first vacuum pump section is as small as Q = 50 to 150cc/min. The exhaust amount of the second vacuum pump section 7 is very small, while the second vacuum pump section 7 is constructed so that a sufficiently low ultimate vacuum can be obtained. Accordingly, in the evacuating apparatus according to the present invention, gas hardly flows back from the exhaust side to the fluid-transporting space 4 in the exhaust process unlike the conventional pump. The pressure in the fluid-transporting space 4 is very low and the difference in the pressure between the front of the piston 5 and the rear thereof is slight. Accordingly, the energy loss of the first vacuum pump section can be reduced greatly. [1-II] Case in which a valve is provided When the volume of the vacuum chamber 1 is large and thus when gas is to be exhausted in a very short period of time, in an evacuating apparatus of the second case, a valve 11 may be provided in parallel with the second pump section 7 as shown in Figs. 1C and 1D. The other construction of the evacuating apparatus in Figs. 1C and 1D is the same as that in Figs. 1A and 1B. The principle of the present invention is the same as that of [1-I]. Referring to Figs. 1C and 1D, the evacuating apparatus comprises a first exhaust passage 8 disposed between the first vacuum pump section and the second vacuum pump section 7, a second exhaust passage 9 disposed on the exhaust side of the second vacuum pump section 7, a connecting portion 10 for connecting the passages 8 and 9 with each other, and the valve 11 disposed in the first exhaust passage 8. (1) State in which exhaust has just started: The valve 11 is open and a large amount of gas is discharged through the valve 11 as shown in Fig. 1C. (2) State in which pressure in vacuum chamber has reached sufficiently low degree of vacuum: The first exhaust passage 8 is closed by the valve 11. The second vacuum pump section 7 transports a slight amount of gas with a great pressure difference between its suction side and its exhaust side maintained. As will be described later, a viscous-type pump or a screw pump having a shallow groove may be used as the second vacuum pump section 7. The exhaust amount of the second vacuum pump section 7 is smaller than that of the first vacuum pump section. Accordingly, a much smaller torque is sufficient for driving the second vacuum pump section 7 than the torque required to drive the first vacuum pump section. Thus, the evacuating apparatus of the present invention can be driven by a much smaller amount of power compared with the amount of power consumed by the conventional one. Fig. 2 shows an example of the characteristics of power to be consumed relative to the pressure of gas sucked into the vacuum pump according to the embodiment in comparison with a vacuum pump according to the prior art. Referring to Fig. 3, an evacuating system in a semiconductor manufacturing equipment comprises a vacuum chamber 100; a load-locking chamber 101; a gate 102 disposed between the vacuum chamber 100 and the load-locking chamber 101; a gate 103 disposed at the atmospheric air side of the load-locking chamber 101; a throttle valve 104; a first valve 105; a second valve 106; a third valve 107; a roughing vacuum pump 108 constituted as the evacuating apparatus according to the embodiment; a source 109 of reactive gas; a mass-flow controller 110; an N 2 gas source 111; a change-over valve 112; a turbo-molecular pump 113; an adsorption tower 114; and a factory pipeline 115. The operation procedure of the evacuating system is performed as follows: First process (1): When the operation of the apparatus starts, the gates 102 and 103 are cut off, and then the roughing pump 108 is operated to discharge gas inside the vacuum chamber 100 and in the load-locking chamber 101. The detail evacuating system of the load-locking chamber 101 is not shown in Fig. 5. The second valve 106 is opened, with the third valve 107 cut off in this process. Second process (2) : When the pressure inside the vacuum chamber 100 has dropped sufficiently, the second valve 106 is closed and the third valve 107 is opened to drive the turbo-molecular pump 113 with the roughing pump 108 being driven. Third process (3): After the pressure in the vacuum chamber 100 reaches a predetermined degree of vacuum, a slight amount of N 2 gas is introduced into the vacuum chamber 100 so as to exhaust gas (including H 2 O) remaining in the vacuum chamber 100 therefrom. The load-locking chamber 101 is evacuated similarly. Then, the gate 102 is opened to introduce a wafer into the vacuum chamber 100. Fourth process (4): After the gates 103 and 102 are cut off, the reactive gas 109 is introduced into the vacuum chamber 100. The amount of gas is controlled by the mass-flow controller 110 while the pressure inside the vacuum chamber 100 is being detected. When the wafer has been processed, N 2 gas is introduced into the vacuum chamber 100 again to exhaust the reactive gas therefrom. Fifth process (5): The gate 102 is opened to take out the wafer from the vacuum chamber 100 and return the wafer to the load-locking chamber 101. So long as the production continues in the above-described process, the operation returns to the second process (2) from the fifth process (5) to repeat the production in the same procedure. Loads are applied to the roughing pump 108 in the above-described process as follows: The roughing pump 108 transports a great amount of gas only in the first process (1), namely, only in the stage of exhausting air inside the vacuum chamber 100 therefrom. The first process (1) is completed for several seconds. In the stage after the second process (2) is completed, namely, in the stage in which the turbo-molecular pump 113 and the roughing pump 108 are simultaneously used, the roughing pump 108 is used to drop the pressure on the exhaust side of the turbo-molecular pump 113, and gas to be transported is slight in quantity. The quantity of N 2 gas and reactive gas to be transported in the second to fourth processes (2), (3), and (4) is as slight as Q = 50 to 150cc/min. Accordingly, as described above, in the semiconductor-manufacturing process, the ratio of the period of time in which the roughing pump 108 transports gas having a high density to the total operation period of time of the evacuating apparatus is slight. In most cases, the roughing pump is used to maintain the pressure difference between the atmospheric air and the pressure in the vacuum chamber or reduce the pressure in the exhaust side of the turbo-molecular pump disposed in an upper stage. As described previously, with the adoption of multi-chamber method (evacuation in each vacuum chamber) in semiconductor-manufacturing equipment in recent years, the number of vacuum pumps to be used in semiconductor-manufacturing equipment is increasing, and an amount of exhausting gas from the vacuum pump is increasing. The vacuum pump according to the present invention can save energy in a great amount in semiconductor-manufacturing equipment. An evacuating apparatus according to a first embodiment is described below with reference to Figs. 4 and 5. In this embodiment, the evacuating apparatus is applied to a broad-band vacuum pump in which a pair of rotors rotate synchronously without contact. The present inventors proposed a vacuum pump comprising a plurality of rotors; a plurality of motors; and detecting means. Each rotor is driven by an independent motor synchronously rotated without contact. The detecting means such as a rotary encoder is used to detect the rotational angle of each motor and/or the number of rotation thereof. This vacuum pump may be used as a roughing pump which is maintenance-free, clean, compact, space-saving and in addition, the rotors rotates at a high speed. A broad-band vacuum pump which produces from the atmospheric pressure to a high degree of vacuum can be obtained by providing a pump producing a high degree of vacuum on the shaft of one of the rotors of the vacuum pump proposed by the present inventors. The vacuum pump proposed by the present inventors can be greatly improved as follows by applying the evacuating apparatus of the present invention thereto. The vacuum pump comprises a housing 201; a first fixed sleeve 203 accommodating a first rotary shaft 202 vertically; a second fixed sleeve 205 accommodating a second rotary shaft 204 vertically; and cylindrical rotors 206 and 207 disposed coaxially with each of the rotary shafts 202 and 204. The rotary shafts 202 and 204 are supported by each a pair of ball bearings 236 and 237 and a pair of bearings 238 and 239. Thread grooves 208 and 209 serving as fluid-transporting grooves and engaging each other are formed on the peripheral surfaces of the rotors 206 and 207. The engaging portion of each of the thread grooves 208 and 209 serves as the structure section 190 (first pump section) of a positive displacement type vacuum pump. A cylindrical rotary sleeve 210 integrally connected with the rotor 206 is disposed on an upper portion of the first rotary shaft 202. Fixed cylinders 222 and 223 are disposed on the casing 201 so that the rotary sleeve 210 is accommodated between the fixed cylinders 222 and 223 in one direction. Spiral drag grooves 211 and 212 are formed on the moving inside and outside surfaces of the rotary sleeve 210. The portion formed of the sleeve 210 and the fixed cylinders 222 and 223 is denoted as a structure section 191 (third pump section) of a drag pump for evacuating the vacuum chamber from an intermediate to a high degree of vacuum. The third pump section has a function of exhausting gas mainly in a molecular flow region or an intermediate flow region. That is, due to the drag action of the spiral grooves 211 and 212, gas which has flowed from a sucking opening 213 disposed in a high degree of vacuum side is exhausted to a space 214 accommodating the positive displacement type screw vacuum pump. The gas which has flowed into the positive displacement type screw vacuum pump is exhausted from a discharge opening 215. It is possible to suck gas from a sucking opening 240 (shown by two-dot chain line) of positive displacement type vacuum pump when the pressure in the vacuum chamber is close to the atmospheric pressure after the pump is actuated and then suck gas from the sucking opening 213 when the pressure in the vacuum chamber has approached to a vacuum pressure. Contact preventing gears 216 and 217 for preventing the contact between the thread grooves are formed on the peripheral surface of the lower end of each of the rotors 206 and 207. A solid lubricating film is formed on the gears 216 and 217 so as to withstand some friction between metals. The backlash between the engaging portion of the gears 216 and 217 is set to be smaller than that of the engaging portion of the thread grooves 208 and 209 formed on the peripheral surfaces of the rotors 206 and 207. Accordingly, the gears 216 and 217 do not contact each other when the rotary shafts 202 and 204 are synchronously rotating, while if the rotary shafts 202 and 204 are unsynchronously rotating, the gears 216 and 217 contact each other before the thread grooves 208 and 209 contact each other. In this manner, the thread grooves 208 and 209 can be prevented from contacting each other. The gears 216 and 217 may be used as the second pump section (gear pump). This construction may eliminate the provision of the viscous-type pumps 241a and 241b constituted of the spiral groove which will be described later. The first rotary shaft 202 and the second rotary shaft 204 are rotated at a speed as fast as several tens of thousands of revolutions per minute by AC servo-motors 218 and 219 disposed at lower portions of the rotary shafts 202 and 204. The control of the synchronous rotation of the rotary shafts 202 and 204 is accomplished as follows: Rotary encoders 220 and 221 are disposed at the lower ends of the rotary shafts 202 and 204. Pulses outputted from the rotary encoders 220 and 221 are compared with a predetermined instruction pulse (target value) of a virtual rotor as shown by the block diagram of Fig. 6. The deviation between the target value and the values (number of rotations and rotational angle) outputted from each of the rotary shafts 201 and 204 are calculated by each phase difference counter, and the rotation of each of the servo-motors 218 and 219 disposed on the rotary shafts 202 and 204 is controlled to erase the deviation. In this embodiment, a laser type encoder utilizing the diffraction and interference of a laser beam and having high resolution and high response is used instead of a magnetic encoder or an optical encoder. The evacuating apparatus further comprises second pump sections 241a and 241b, of viscous type pump, formed coaxially with the rotors 206 and 207; a second exhaust opening 242; a control valve 243 constituted of a spring 243a, a spool 243b, and the like. A second embodiment is described below with reference to Fig. 10. A screw (thread groove) pump of positive displacement type is used as the second pump section in contrast to the first embodiment, and a control valve is not provided. The vacuum pump comprises micro-screws 250a and 250b and an exhaust opening 251. The following effect is obtained when the evacuating apparatus according to the embodiments of the present invention is applied to a vacuum pump in which two rotors are rotated synchronously by an electronic means: (Effect 1) High speed operation allows vacuum pump to be driven by a small amount of power. Most of power required to drive the vacuum pump of the embodiments is determined by a torque required to drive the first pump section as described previously. The lower the pressure in vacuum on the downstream side (upstream side of second pump section) of the first pump section is, the smaller torque is required. The smaller the ratio of (inner leakage/exhaust capability) is, the smaller the vacuum pressure can be generated on the upstream side of the second pump section. (Effect 2) High speed operation generates low ultimate vacuum. In the structure portion 191 (third pump section) of the drag pump, the rotary portion (sleeve 210) rotates in a low pressure space when the pressure in the suction side 213 has reached a low degree of vacuum pressure. Therefore, a small load due to the pressure is applied to the vacuum pump and thus torque required to drive the first pump section becomes small. Utilizing this point, the pump of this embodiment accomplishes the following operations. (Operation 1) First, the vacuum pump is driven at approximately 10,000 rpm and the first pump section of positive displacement type is fully operated to reduce the pressure in the vacuum chamber to as low as 10 -2 to 10 -3 torr. (Operation 2) When the operation 1 has been completed, the pressure in the downstream side of the first pump section (screw pump) has become sufficiently low. Therefore, the power consumed by the motor is very small. Then, the number of rotations of the vacuum pump is increased to, for example, 20,000 to 30,000 rpm. The vacuum pump can be rotated at a high speed even by a small motor because of the reduction of the required torque. As a result, the exhaust efficiency of the pump 191 (third pump section) of kinetic type is increased and consequently, the ultimate vacuum can be reduced to as low as equal to or less than 10 -8 torr. (Effect 3) A broad-band vacuum pump can be installed directly on a vacuum chamber in a semiconductor-manufacturing factory. The evacuating apparatus according to the present invention comprises the first pump section providing a large amount of exhaust amount and the second pump section having a small exhaust amount but providing a sufficiently low degree of vacuum which are combined with each other. The following effects can be obtained by using the vacuum pump in the evacuating system of a semiconductor-manufacturing equipment: [I] A great amount of energy can be saved during a steady drive of the vacuum pump. The driving torque of the pump is proportional to the theoretical exhaust volume of the vacuum pump. Therefore, when the pressure on the suction side has reached a low pressure, the torque of the first pump section can be greatly reduced. The second pump section provides a sufficiently low degree of vacuum and exhausts a slight amount of gas, and thus a small torque is required to drive the second pump section. Thus, a greatly reduced power suffices for driving the vacuum pump. [II] Vacuum pump provides a very low degree of vacuum. In the vacuum pump, an exhaust amount is disproportional to an ultimate degree of vacuum. If a large exhaust amount is to be obtained, a favorable ultimate degree of vacuum cannot be obtained. If a desired degree of vacuum is to be obtained, the exhaust amount becomes small. The present invention is characterized in that two pumps having different properties are combined with each other: (1) First pump section (upstream pump section) providing a large amount of exhaust amount; and (2) Second pump section (downstream pump section) having a small exhaust amount but providing a low degree of vacuum. In the vacuum pump, a large passage area can be formed in the exhaust side by providing a sufficiently large exhaust opening between the first pump section and the second pump section, and thus the exhaust amount is not limited. When the pressure on the suction side has reached a low degree of vacuum, the exhaust opening is closed and the first pump section and the second pump section communicate with each other. Due to the existence of the second pump section, the pressure on the exhaust side of the first pump section becomes sufficiently small. As a result, the leakage of a high pressure gas of the first pump section toward the upstream side is decreased. That is, in the evacuating apparatus of the present invention, a high ultimate degree of vacuum can be obtained by the effect generated by the combination of the first and second pump sections.When the volume of the vacuum chamber connected with the suction side of the pump is sufficiently small, it is unnecessary to provide the valve and the exhaust opening between the first and second pump sections. Thus, the construction of the vacuum pump is simple.The first and second pump sections are not necessarily independent pump sections. For example, the entire length of the rotor can be shortened by using a pump of positive displacement type in which thread grooves (or screw curve) continuously change. [III] A further improved performance noise reduction can be obtained by synchronously rotating rotors by electronic control means. The effect of the present invention can be obtained by combining the first pump section, previously proposed by the present inventors, driven by the electronic control means for the synchronous rotation of the rotors and the second pump section according to the present invention with each other. One of the reasons is that the number of rotations of the pump can be greatly increased by the electronic control means used to perform the synchronous rotation of the rotors.For example, the pump composed of the combined pump sections according to the conventional art and the present invention can be rotated tens of thousands of times per minute whereas the conventional first pump section is rotated thousands of times per minute. As a result, the following effects can be obtained: (1) The internal leakage of gas from the exhaust side of the pump to the upstream side thereof can be reduced and thus the vacuum pressure of the second pump section can make lower on the upstream side thereof, thus making torque for which driving the pump lower. (2) The ultimate degree of vacuum of the first pump section can make lower due to the high speed drive of the pump. A vacuum pressure as low as 10 -8 Torr can be generated by a composite pump in which the first pump section is combined with a drag pump (third pump section), because the exhaust pressure of the drag pump is very low. Further, in addition to the feature of the electronic control to perform the synchronous rotation of the rotors that no noise is generated from the contact between the timing gears, the following effect of preventing noise can be obtained. That is, according to the vacuum pump of the present invention, the rotors of the first pump section having a great exhaust amount rotate in a space having a low pressure both in the exhaust side and the suction side. Consequently, no noise is generated by the rotation of rotors in a particular configuration, for example, a screw configuration. In addition, the back flow of gas from the exhaust side of the pump to the interior thereof does not occur or no re-outflow occurs and hence no pulsation sound is generated. Accordingly, the pump according to the present invention can reduce the generation of noise to a much smaller degree than the conventional roughing pump by 10 to 20dB.

Claims: An evacuating apparatus comprising: a housing (201) having a fluid suction opening (213, 240) and a fluid exhaust opening (242; 251); a first pump section (208, 209) mounted in said housing and having its suction side connected with said fluid suction opening, said first pump section being a positive displacement pump having a first and a second rotor (208, 209) accommodated in said housing and which positively displace fluid by utilizing a volume change of a space defined by said rotors and said housing; a second pump section (241b, 214a; 250b, 250a) having its suction side connected with the exhaust side of said first pump section and its exhaust side with said fluid exhaust opening, for exhausting a smaller amount of fluid than said first pump section, said second pump section having a first and a second rotor (241b, 241a; 250b, 250a); the first rotors (208, 241b; 208, 250b) of both pump sections being coupled with each other and driven by a first motor (218), and the second rotors (209, 241a; 209, 250a) of both pump sections being coupled with each other and driven by a second motor (219); and detecting means (220, 221) for detecting a rotational angle of each motor (218, 219) and/or the rotational speed thereof, the rotors being rotated synchronously in cooperation of the motors each for independently driving each of the rotors, based on signals outputted from the detecting means. An apparatus as claimed in claim 1, wherein said second pump section (250b, 250a) is a positive displacement pump. An apparatus as claimed in claim 1, wherein said second pump section (241b, 241a) is a viscous-type pump. An apparatus as claimed in any of claims 1-3, further comprising a third pump section (191) of a viscous type between the fluid suction opening (213) and the inlet (214) of the first pump section (208, 209), the third pump section (191) being disposed on a shaft of at least one of the rotors (209). An apparatus as claimed in any of claims 1-4, further comprising a by-pass (215) to the second pump section (241b, 241a) including a control valve (243) for opening the by-pass at the initial stage of pumping action.

Inventors: ABE YOSHIKAZU, IKEMOTO YOSHIHIRO, MARUYAMA TERUO, TAKARA AKIRA, ABE, YOSHIKAZU, IKEMOTO, YOSHIHIRO, MARUYAMA, TERUO, TAKARA, AKIRA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: F04C23
IPC codes: F04C23, F04C29, F04C28, F04C18, F04D19, F04D25