Patent Application
20060093473
The present invention relates generally to the field of vacuum pumping, and more particularly, to a momentum transfer pump of the hybrid or compound type having two or more sections of different operational mode for improving the operating range or throughput of the pump.
Certain research and manufacturing processes require the use of a process chamber with high vacuum. For example, in semiconductor wafer processing, vacuum is used during the thin-film deposition and etching operations, primarily to reduce contamination. In such processes, pumps capable of producing a “high vacuum” of 10−6 torr or lower are useful to assure adequate pumping speed at process pressure, and to allow for a low base pressure for cleanup between steps.
Several currently-available vacuum pump configurations are capable of maintaining a high vacuum. One design, the turbo-molecular vacuum pump, is frequently used in both manufacturing processes and in research instrumentation. A conventional stage arrangement of a turbo-molecular vacuum pump includes a stack of alternate rotors and stators. Each stage effectively comprises a solid disc with a plurality of blades depending (nominally) radially inwardly or outwardly therefrom. The blades are evenly spaced around the circumference of the disc and angled “about” radial lines out of the plane of the disc in the direction of rotation of the rotor stage.
The rotor and stator blades have positive and negative gradients respectively when viewed from the side in a radial line from the disc. That arrangement has the effect, in molecular flow conditions, of causing a movement of molecules through the pump.
The turbo-molecular vacuum pump is inefficient or inoperable outside the molecular flow realm. For that reason, a commercially available vacuum pump may contain, in addition to several turbo-molecular stages, one or more molecular drag stages and one or more regenerative stages placed between the turbo-molecular stages and the pump outlet.
Referring to
The turbo-molecular section 50 also includes a stator 56 which is formed integrally with a body 22. Extending radially inwardly from the stator 56 are stator vanes 58 defining three spaced arrays of vanes, each array including about 20 vanes. The vanes 54 of the rotor interleaf with the vanes 58 of the stator and are angled relative to each other as is known in the turbo-molecular vacuum pump art.
The rotor 9 also comprises the rotor portion of the regenerative section 1. That portion includes a series of concentric rows of airfoils such as airfoil 10 that induce motion to the gas contained in channels 11. The channels 11 are stationary and are attached to the body 22 and stator 56. The regenerative portion of the pump is arranged to exhaust through the pump outlet 32.
The molecular drag section of the exemplary prior art pump illustrated in
A high-speed drive (not shown) is connected to a shaft 60 to drive the rotating components of the pump. The shaft, rotors and drive are supported by high speed, low friction bearings (not shown) such as dry ceramic race ball bearings or magnetic levitation bearings.
Operation of the turbo-molecular section of the above-described system requires high relative velocity of the turbo-molecular stators and rotors. On the other hand, the angular velocity of the rotor is limited by stresses placed on the components by centripetal acceleration, and also by maximum operating speeds of the other pump sections. Special fasteners and larger component cross sections are sometimes used to reduce stress and to permit a higher rotational speed.
The pumping speed requirements of the turbo-molecular section are often met by increasing the overall diameter of the pump, imparting a greater tangential velocity to the outer portions of the rotor vanes for a given rotational speed. That solution, however, increases the overall package size.
To start the above-described pump, the enclosed volume of the turbo-molecular section must first be partially evacuated. That is done to permit rotation of the opposing vanes in that section at the required high speeds. An additional downstream “roughing” pump is sometimes used for that purpose. A valved bypass line may be provided to permit the system to be pumped down without the turbo offering a restriction.
There is therefore presently a need to provide an improved compound or hybrid vacuum pump including a turbo-molecular pump section. Particularly, the improved pump should provide a solution to the conflicting speed requirements, and should reduce the need to evacuate the pump at start-up using a roughing pump. To the inventor's knowledge, there is currently no such technique available.
The present invention addresses the needs described above by providing an improved vacuum exhaust pump and a method for controlling the pump. In one embodiment, a vacuum exhaust pump having a turbo-molecular pump section and a regenerative pump section is provided. The pump includes a regenerative rotating group mounted for rotation about a pump axis, a turbo rotating group connected to the regenerative rotating group for rotation therewith, and a first drive connected to the regenerative rotating group and the turbo rotating group for rotating those groups in a first rotational direction. The pump also includes a turbo counter-rotating group mounted for rotation about the pump axis, and a second drive connected to the turbo counter-rotating group for rotating the turbo counter-rotating group in at least a second rotational direction opposite the first rotational direction.
The turbo counter-rotating group may be an outer rotor of a turbo-molecular pump section, or may be an inner rotor. The turbo counter-rotating group may be mounted on magnetic levitation bearings.
The vacuum exhaust pump may also include a pump controller for controlling the first and second drives. That controller is configured to activate the first drive during start-up to rotate the regenerative rotating group and the turbo rotating group in the first direction without activating the second drive.
The second drive may be for rotating the turbo counter-rotating group in both the first rotational direction and the second rotational direction.
The vacuum exhaust pump may include a drive control configured to rotate the turbo counter-rotating group in the first rotational direction during start-up, and to rotate the turbo counter-rotating group in the second rotational direction after a sufficient vacuum is drawn. The controller may further be configured to progressively transition the turbo rotating group from rotation in the first rotational direction to rotation in the second rotational direction.
The pump may have a regenerative stator group having a close running clearance with the regenerative rotating group to form the regenerative pump section. A pump inlet may enter the turbo pump section, and a pump outlet may leave the regenerative pump section. The pump may also include a molecular drag rotating group connected to the regenerative rotating group for rotation therewith.
In another embodiment of the invention, a vacuum exhaust pump includes at least a regenerative section, a turbo-molecular section, a first rotor common to both sections, and a separately rotating second rotor of the turbo-molecular section.
The vacuum exhaust pump may also include a first drive to drive the first rotor in a first rotational direction, and a second drive to drive the second rotor in a direction opposite the first. The pump may further include a controller for activating the first drive during startup before activating the second drive.
In another embodiment, a method is provided for controlling a vacuum exhaust pump having a turbo-molecular pump section and a regenerative pump section. The method includes the steps of rotating in a first rotational direction a first rotor supporting elements of both pump sections, and, after sufficient vacuum has been drawn in the turbo-molecular pump section, rotating in a second rotational direction, opposite the first, a second rotor supporting additional elements of the turbo-molecular pump section.
The method may include the step of, before sufficient vacuum has been drawn in the turbo-molecular pump section, rotating the second rotor in the first rotational direction. The step of rotating the second rotor in the first rotational direction may include driving the second rotor with a drive.
The step of rotating the second rotor in the first rotational direction may include driving the first rotor with a drive, whereby the second rotor is driven in the first direction by rotating air in the turbo-molecular pump section.
The step of rotating the second rotor in the second rotational direction may further comprise progressively slowing the second rotor from rotation in the first direction, stopping the rotor and progressively accelerating the rotor in rotation in the second direction.
The method may also include the step of controlling a pressure of a chamber connected to the pump by controlling a relative rotational speed of the first and second rotors.
According to the present invention, a compound vacuum pump includes two rotors that are driven independently by two drives. One of the rotors includes forward-rotating arrays of turbo vanes as well as the rotor portions of the regenerative and/or drag sections. The other rotor includes counter-rotating arrays of vanes that replace the stator vanes of the prior art discussed above. By utilizing counter-rotating vanes in the turbo-molecular section of the pump, the operating angular velocity of the forward-rotating rotor may be reduced without degrading the performance of the turbo section, ameliorating some of the problems discussed above.
Referring now to
Two operational sections of the compound vacuum pump 200 are shown: a turbo-molecular pump section 209 and a regenerative section 219. A molecular drag section (not shown) is also included in a preferred embodiment of the invention. That section has been eliminated from the view of
The compound vacuum pump 200 of the invention further has a first rotor 201 and a second rotor 202. The rotors 201, 202 support the rotating portions of the turbo-molecular pump section 209 and the regenerative section 219 as described below.
The first rotor 201 is driven by a drive 230 and suspended on bearings 225, 226. The drive 230 is preferably an inverter drive as is commonly used in the vacuum pump art. The bearings 225, 226 may be lubricated or dry, and preferably have ceramic rolling elements to reduce wear and micro pitting. The drive 230 is capable of rotating the first rotor 201 at a speed of about 18,000 RPM.
The first rotor 201 includes a regenerative rotor 220 of the regenerative section 219. The regenerative rotor rotates with the rotor first 201 and acts in concert with a regenerative stator 221. The regenerative stator 221 is attached to the body 204 and remains stationary during operation of the pump. The regenerative section inlet is an internal transfer port 214 communicating with the turbo-molecular section 209. The regenerative section exhausts through the pump exhaust port 206.
The first rotor 201 also includes an outer turbo-molecular rotor 212 of the turbo-molecular pump section 209. The outer turbo-molecular rotor 212 rotates with the first rotor 201 and includes radially-inwardly-extending vanes (not shown) as described above with reference to the vanes 58 of
The second rotor 202 is supported on bearing 208 and driven by drive 207. The bearing 208 and drive 207 are preferably integral parts of a magnetic levitation drive as is frequently used in the art. The non-contact magnetic levitation drive permits high speed rotation without excessive bearing wear and frictional heat buildup, and with a minimum of vibration. The drive 207 is capable of driving the second rotor 202 at about 18,000 RPM.
The second rotor includes an inner turbo-molecular rotor 210 connected for rotation therewith. The inner turbo-molecular rotor includes radially-outwardly-extending vanes (not shown) as discussed above with respect to the vanes 54 shown in
While the outer turbo-molecular rotor has been described as connected to the first pump rotor and the inner turbo-molecular rotor has been described as connected with the second pump rotor, those relationships may be reversed. In other words, in an alternate embodiment, the inner turbo-molecular rotor may rotate with the regenerative rotor, and the outer turbo molecular rotor may be driven in counter-rotation.
In normal, steady-state operation, the first and second rotors turn in opposite directions, each at approximately 18,000 RPM. The regenerative rotor and regenerative stator therefore operate at a relative speed of 18,000 RPM, while the inner and outer turbo-molecular rotors operate at a relative speed of 36,000 RPM.
The above-described rotor configuration yields several advantages in steady-state operation. The turbo-molecular section of the pump is operated at a high relative speed, while subjecting the radial vanes to stresses induced by a rotation of the rotors that is only about one half the relative speed of the interleaved vanes. That allows a less bulky design of the vanes, and also permits the turbo-molecular section to be constructed with a smaller outer diameter than would otherwise be necessary to maximize tangential velocity.
Further, the turbo-molecular section of the pump may be operated a relative speed that is much greater than the relative speed of the stator and rotor of the regenerative section. That allows each section to be designed for a more optimal speed range. A molecular drag section of the pump, if included, also rotates with the first pump rotor and would also benefit from a design aimed at a lower, more optimal speed range than would otherwise be necessary to operate a turbo-molecular section with stators.
While the two rotors are driven independently, the drive controllers are connected to allow communication between them, as illustrated by the communication system 300 of
The vacuum pump controller 310 coordinates the start-up and steady-state operation of the vacuum pump. For example, as illustrated by the velocity diagram 400 of
During an initial period 450, the first rotor is accelerated by its drive in a first rotational direction (shown as the positive direction on the velocity axis 401). The regeneration rotor and the outer turbo-molecular rotor are thereby brought up to speed during that period 450.
During that same period 450, the second rotor, including the inner turbo-molecular rotor, also accelerates in the positive rotational direction. In one embodiment of the invention, the acceleration of the second rotor is induced by the frictional forces of the gasses in the turbo-molecular pump section on the vanes of the inner and outer turbo-molecular rotors. In other words, while the outer vanes of the turbo are driven, the inner vanes are allowed to freely rotate, and are induced by driven gasses in the turbo to rotate in the same rotational direction as the outer vanes. While the velocity profile of
In an alternative embodiment, the drive for the second rotor is reversible, and the second rotor is accelerated in the positive rotational direction shown during period 450 by the drive.
Because the inner and outer turbo-molecular rotors are rotating in the same direction at start-up, the pump may be run at full speed while gas still remains in the turbo-molecular section. That is not possible with a turbo rotor/stator arrangement, because high stresses are placed on the vanes by the unevacuated gas at high relative rotational speeds of the rotor and stator. With a rotor/stator arrangement, it is therefore necessary to at least partially evacuate the turbo-molecular section before bringing the pump up to speed.
After the first rotor 201 (
In the next start-up phase 470, the vacuum pump controller 310 (
In one embodiment of the invention, the reversal phase 470 is started at a predetermined time after the first rotor is started; i.e., a timer is used to sequence the control. In another embodiment, a pressure sensor in the turbo-molecular section is monitored, and the controller 310 signals for reversal of the second rotor only after the turbo-molecular section has been sufficiently evacuated to begin counter-rotation of the vanes.
Phase 480 of the vacuum pump operation is the steady-state operation of the pump. It can be seen that the relative rotational speed 486 of the turbo-molecular rotors is greater than the rotational speed of either pump rotor. Further, the relative rotational speed 486 may be adjusted independently of the rotational speed 485 of the regenerative section by adjusting the rotational speed 484 of the second rotor. A pressure of an evacuated chamber connected to the turbo section can therefore be controlled by controlling the relative rotational speed of the first and second rotors.
It can also be seen from the diagram of
A method 500 for controlling a vacuum exhaust pump having a turbo-molecular pump section and a regenerative pump section is illustrated in
Initially, the first rotor is rotated (step 510) in a first rotational direction. The second rotor may also be rotated (step 520) in the first direction by separately driving in the first direction with its own drive. Alternatively, the second rotor may simply be permitted to be rotated by gasses in the turbo-molecular section driven by elements supported by the first rotor.
After sufficient vacuum has been drawn in the turbo-molecular pump section, the second rotor is rotated (step 530) in a second rotational direction, opposite the first. The existence of sufficient vacuum may be detected using sensors, or may be estimated by time elapsed.
The foregoing Detailed Description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the Description of the Invention, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. For example, while the system is described in connection with a high vacuum pump having a turbo molecular section and drag and/or regenerative sections, other vacuum pump geometries and functions may be incorporated. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.
