Patent Publication Number: US-7896625-B2

Title: Vacuum pumping system and method of operating a vacuum pumping arrangement

Description:
FIELD OF THE INVENTION 
     The present invention relates to a vacuum pumping system comprising a vacuum pumping arrangement and a method of operating a vacuum pumping arrangement. 
     BACKGROUND OF THE INVENTION 
     A known vacuum pumping arrangement for evacuating a chamber comprises a molecular pump which may include: molecular drag pumping means; or turbomolecular pumping means; or both molecular drag pumping means and turbomolecular pumping means. If both pumping means are included the turbomolecular pumping means are connected in series with the molecular drag pumping means. The pumping arrangement is capable of evacuating the chamber to very low pressures in the region of 1×10 −6  mbar. The compression ratio achieved by the molecular pump is not sufficient to achieve such low pressures whilst at the same time exhausting to atmosphere and therefore a backing pump is provided to reduce pressure at the exhaust of the molecular pump and hence permit very low pressures to be achieved at the inlet thereof. 
     The turbomolecular pumping means of a molecular pump comprises a circumferential array of angled blades supported at a generally cylindrical rotor body. During normal operation the rotor is rotated between 20,000 and 200,000 revolutions per minute during which time the rotor blades collide with molecules in a gas urging them towards the pump outlet. Normal operation occurs therefore at molecular flow conditions at pressures of less than about 0.01 mbar. As it will be appreciated, the turbomolecular pumping means does not work effectively at high pressures, at which the angled rotor blades cause undesirable windage, or resistance to rotation of the rotor. This problem is particularly acute at start up conditions close to or at atmospheric pressure, when it is difficult if not impossible to rotate the rotor of the turbomolecular pumping means at high speed. Therefore, it is desirable to evacuate the turbomolecular pumping means to relatively low pressures by operating the backing pump before starting rotation of the molecular pump. An alternative but undesirable solution to the problem of turbo stage start-up, would be the provision of a much more powerful motor for driving the rotor, that would be able to overcome the windage caused by the angled rotors blades at atmospheric pressure. This solution is undesirable because, generally, a molecular pump, especially when used in the semiconductor processing industry, is kept running most of the time, and is shut down only during power failures, for servicing etc. Accordingly, a powerful motor would be needed only for a relatively small amount of the pump&#39;s operating time and therefore the increased cost of such a motor cannot be justified. 
     Hereto, a molecular pump and a backing pump thereof are separate units of the same vacuum pumping arrangement, the pumps being associated with respective drive shafts which are driven by respective motors. As described above, it is desirable initially to operate the backing pump to evacuate the molecular pump, prior to start-up of the molecular pump. Clearly, this would be possible only if the two pumps can be driven separately. 
     It is desirable to provide an improved vacuum pumping system and method of operating a vacuum pumping arrangement. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a vacuum pumping system comprising a vacuum pumping arrangement comprising: a drive shaft; a motor for driving said drive shaft; a molecular pumping mechanism comprising turbomolecular pumping means; and a backing pumping mechanism, wherein said drive shaft is for driving said molecular pumping mechanism and said backing pumping mechanism, and the system comprises evacuation means for evacuating at least said turbomolecular pumping means. 
     The present invention also provides a method of operating a vacuum pumping arrangement comprising: a drive shaft; a motor for driving said drive shaft; a molecular pumping mechanism comprising turbomolecular pumping means; and a backing pumping mechanism, said drive shaft being for driving said molecular pumping mechanism and said backing pumping mechanism, the method comprising the steps of operating an evacuation means connected to the arrangement to evacuate the arrangement to a predetermined pressure and operating the motor to start rotation of the drive shaft. 
     Other aspects of the present invention are defined in the accompanying claims. 
     In order that the present invention may be well understood, some embodiments thereof, which are given by way of example only, will now be described with reference to the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a vacuum pumping arrangement shown schematically; 
         FIG. 2  is an enlarged cross-sectional view of a portion of a regenerative pump of the arrangement shown in  FIG. 1 ; 
         FIG. 3  is a diagram of a control system; 
         FIG. 4  is a schematic representation of a vacuum pumping system; 
         FIG. 5  is a schematic representation of another vacuum pumping system; and 
         FIGS. 6 to 8  are cross-sectional views of further vacuum pumping arrangements all shown schematically. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a vacuum pumping arrangement  10  is shown schematically, which comprises a molecular pumping mechanism  12  and a backing pumping mechanism  14 . The molecular pumping mechanism comprises turbomolecular pumping means  16  and molecular drag, or friction, pumping means  18 . Alternatively, the molecular pumping mechanism may comprise turbomolecular pumping means only or molecular drag pumping means only. The backing pump  14  comprises a regenerative pumping mechanism. A further drag pumping mechanism  20  may be associated with the regenerative pumping mechanism and provided between drag pumping mechanism  18  and regenerative pumping mechanism  14 . Drag pumping mechanism  20  comprises three drag pumping stages in series, whereas drag pumping mechanism  18  comprises two drag pumping stages in parallel. 
     Vacuum pumping arrangement  10  comprises a housing, which is formed in three separate parts  22 ,  24 ,  26 , and which houses the molecular pumping mechanism  12 , drag pumping mechanism  20  and regenerative pumping mechanism  14 . Parts  22  and  24  may form the inner surfaces of the molecular pumping mechanism  12  and the drag pumping mechanism  20 , as shown. Part  26  may form the stator of the regenerative pumping mechanism  14 . 
     Part  26  defines a counter-sunk recess  28  which receives a lubricated bearing  30  for supporting a drive shaft  32 , the bearing  30  being at a first end portion of the drive shaft associated with regenerative pumping mechanism  14 . Bearing  30  may be a rolling bearing such as a ball bearing and may be lubricated, for instance with grease, because it is in as part of the pumping arrangement  10  distal from the inlet of the pumping arrangement. The inlet of the pumping arrangement may be in fluid connection with a semiconductor processing chamber in which a clean environment is required. 
     Drive shaft  32  is driven by motor  34  which as shown is supported by parts  22  and  24  of the housing. The motor may be supported at any convenient position in the vacuum pumping arrangement. Motor  34  is adapted to be able to drive simultaneously the regenerative pumping mechanism  14 , and the drag pumping mechanism  20  supported thereby, and also the molecular pumping mechanism  12 . Generally, a regenerative pumping mechanism requires more power for operation than a molecular pumping mechanism, the regenerative pumping mechanism operating at pressures close to atmosphere where windage and air resistance is relatively high. A molecular pumping mechanism requires relatively less power for operation, and therefore, a motor selected for powering a regenerative pumping mechanism is also generally suitable for powering a molecular pumping mechanism. Means are provided for controlling the rotational speeds of the backing pumping mechanism and the molecular pumping mechanism so that pressure in a chamber connected to, or operatively associated with, the arrangement can be controlled. A suitable control system diagram for controlling speed of the motor  34  is shown in  FIG. 3  and includes a pressure gauge  35  for measuring pressure in a chamber  33  and a controller  37  connected to the pressure gauge for controlling the pump&#39;s rotational speed. 
     Regenerative pumping mechanism  14  comprises a stator comprising a plurality of circumferential pumping channels disposed concentrically about a longitudinal axis A of the drive shaft  32  and a rotor comprising a plurality of arrays of rotor blades extending axially into respective said circumferential pumping channels. More specifically, regenerative pumping mechanism  14  comprises a rotor fixed relative to drive shaft  32 . The regenerative pumping mechanism  14  comprises three pumping stages, and for each stage, a circumferential array of rotor blades  38  extends substantially orthogonally from one surface of the rotor body  36 . The rotor blades  38  of the three arrays extend axially into respective circumferential pumping channels  40  disposed concentrically in part  26  which constitutes the stator of the regenerative pumping mechanism  14 . During operation, drive shaft  32  rotates rotor body  36  which causes the rotor blades  38  to travel along the pumping channels, pumping gas from inlet  42  in sequence along the radially outer pumping channel, radially middle pumping channel and radially inner pumping channel where it is exhausted from pumping mechanism  14  via exhaust  44  at pressures close to or at atmospheric pressure. 
     An enlarged cross-section of a single stage of the regenerative pumping mechanism is shown in  FIG. 2 . For efficient operation of the regenerative pumping mechanism  14 , it is important that the radial clearance “C” between rotor blades  38  and stator  26  is closely controlled, and preferably kept to no more than 200 microns or less, and preferably less than 80 microns, during operation. An increase in clearance “C” would lead to significant seepage of gas out of pumping channel  40  and reduce efficiency of regenerative pumping mechanism  14 . Therefore, regenerative pumping mechanism  14  is associated with the lubricated rolling bearing  30  which substantially resists radial movement of the drive shaft  32  and hence rotor body  36 . However, if there is radial movement of the drive shaft at an end thereof distal from the lubricated bearing  30 , this may also cause radial movement of the rotor of the regenerative pumping mechanism, resulting in loss of efficiency. In other words, bearing  30  may act as a pivot about which some radial movement may take place. To avoid loss of efficiency, the rotor  36  of the regenerative pumping mechanism is connected to the drive shaft  32  so as to be sufficiently close to the lubricated bearing  30  (i.e. the pivot) so that radial movement of distal end of the drive shaft translates substantially to axial movement of the rotor blades relative to respective circumferential pumping channels  40 . Preferably, the bearing  30  is substantially axially aligned with the circumferential pumping channels so that any radial movement of the rotor blades  38  does not cause significant seepage. As shown, the stator  26  of the regenerative pumping mechanism  14  defines the recess for the bearing  30  and the rotor body  36  is, as it will be appreciated, adjacent the stator  26 . Accordingly, the bearing  30 , which resists radial movement, prevents significant radial movement of the rotor body  36  and also hence of the rotor blades  38 . Therefore, clearance “C” between the rotor blades  38  and stator  26  can be kept within tolerable limits. 
     Extending orthogonally from the rotor body  36  are two cylindrical drag cylinders  46  which together form rotors of drag pumping mechanism  20 . The drag cylinders  46  are made from carbon fibre reinforced material which is both strong and light. The reduction in mass when using carbon fibre drag cylinders, as compared with the use of aluminium drag cylinders, produces less inertia when the drag pumping mechanism is in operation. Accordingly, the rotational speed of the drag pumping mechanism is easier to control. 
     The drag pumping mechanism  20  shown schematically is a Holweck type drag pumping mechanism in which stator portions  48  define a spiral channel between the inner surface of housing part  24  and the drag cylinders  46 . Three drag stages are shown, each of which provides a spiral path for gas flow between the rotor and the stator. The operation and structure of a Holweck drag pumping mechanism is well known. The gas flow follows a tortuous path flowing consecutively through the drag stages in series. 
     The molecular pumping mechanism  12  is driven at a distal end of drive shaft  32  from the regenerative pumping mechanism  14 . A back up bearing may be provided to resist extreme radial movement of the drive shaft  32  during, for instance, power failure. As shown, the lubricant free bearing is a magnetic bearing  54  provided between rotor body  52  and a cylindrical portion  56  fixed relative to the housing  22 . A passive magnetic bearing is shown in which like poles of a magnet repel each other resisting excessive radial movement of rotor body  52  relative to the central axis A. In practice, the drive shaft may move about 0.1 mm. 
     A small amount of radial movement of the rotor of a molecular pumping mechanism does not significantly affect the pumping mechanism&#39;s performance. However, if it is desired to further resist radial movement, an active magnetic bearing may be adopted. In an active magnetic bearing, electro magnets are used rather than permanent magnets in passive magnetic bearings. Further provided is a detection means for detecting radial movement and for controlling the magnetic field to resist the radial movement.  FIGS. 6 to 8  show an active magnetic bearing. 
     A circumferential array of angled rotor blades  58  extend radially outwardly from rotor body  52 . At approximately half way along the rotor blades  58  at a radially intermediate portion of the array, a cylindrical support ring  60  is provided, to which is connected drag cylinder  62  of drag pumping mechanism  18 . Drag pumping mechanism  18  comprises two drag stages in parallel with a single drag cylinder  62 , which may be made from carbon fibre to reduce inertia. Each of the stages is comprised of stator portions  64  forming with the tapered inner walls  66  of the housing  22  a spiral molecular gas flow channel. An outlet  68  is provided to exhaust gas from the drag pumping mechanism  18 . 
     During normal operation, inlet  70  of pump arrangement  10  is connected to a chamber, the pressure of which it is desired to reduce. Motor  34  rotates drive shaft  32  which in turn drives rotor body  36  and rotor body  52 . Gas in molecular flow conditions is drawn in through inlet  70  to the turbomolecular pumping means  16  which urges molecules into the molecular drag pumping means  18  along both parallel drag pumping stages and through outlet  68 . Gas is then drawn through the three stages in series of the drag pumping mechanism  20  and into the regenerative pumping mechanism through inlet  42 . Gas is exhausted at atmospheric pressure or thereabouts through exhaust port  44 . 
     Regenerative pumping mechanism  14  is required to exhaust gas at approximately atmospheric pressure. Accordingly, the gas resistance to passage, of the rotor blades  38  is considerable and therefore the power and torque characteristics of motor  34  must be selected to meet the requirements of the regenerative pumping mechanism  14 . The resistance to rotation encountered by the molecular pumping mechanism  12  is relatively little, since the molecular pumping mechanism operates at relatively low pressures. Furthermore, the structure of the drag pumping mechanism  18  with its only moving part being a cylinder rotated about axis A does not suffer significantly from gas resistance to rotation. Therefore, once power and torque characteristics for motor  34  have been selected for regenerative pumping mechanism  14 , only a relatively small proportion of extra capacity is needed so that the motor also meets the requirements of molecular pumping mechanism  12 . In other words, a 200 w motor, which is typically used for a molecular pumping mechanism, is significantly less powerful than motor  34  which preferably is a 2 kw motor. In the prior art, the typical motor is not powerful enough so that pressure change in a chamber can be controlled by controlling the rotational speed of the pump. However, since a powerful motor is selected to drive regenerative pumping mechanism  14 , the additional power can also be used to control rotational speed of the molecular pumping mechanism and thereby control pressure. 
     A typical turbomolecular pumping means is evacuated to relatively low pressures before it is started up. In the prior art, a backing pumping mechanism is used for this purpose. Since the backing pumping mechanism and turbomolecular pumping means are associated with the same drive shaft in vacuum pumping arrangement  10 , this start up procedure is not possible. Accordingly, the vacuum pumping arrangement forms part of a vacuum pumping system which comprises additional evacuation means to evacuate at least the molecular pumping mechanism  12  prior to start up to a predetermined pressure. Preferably, the molecular pumping mechanism is evacuated to less than 500 mbar prior to start up. Conveniently, the whole vacuum pumping arrangement is evacuated prior to start up, as shown in  FIGS. 4 and 5 . The evacuation means may be provided by an additional pump, although this is not preferred since an additional pump would increase costs of the system. When the pumping arrangement  10  is used as part of a semi-conductor processing assembly, it is convenient to make use of a pump or pumping means associated with the system such as the pump for the load lock chamber.  FIG. 4  shows the arrangement of a semiconductor processing system, in which the load lock pump  74  is, in normal use, used to evacuate pressure from load lock chamber  76 . A valve  78  is provided between load lock chamber  76  and load lock pump  74 . Load lock pump  74  is connected to the exhaust of pumping arrangement  10  via valve  80 . A further valve  82  is provided downstream of exhaust  44  of pumping arrangement  10 . During start up, valve  78  and valve  82  are closed whilst valve  80  is opened. Load lock pump  74  is operated to evacuate gas from arrangement  10  and therefore from turbomolecular pumping means  16 . During normal operation, valves  82  and  78  are opened whilst valve  80  is closed. Arrangement  10  is operated to evacuate pressure from vacuum chamber  84 . 
     Alternatively, vacuum pumping arrangement  10  can be started up as described with reference to  FIG. 5 . The additional evacuation means comprises a high pressure nitrogen supply which is connected to an ejector pump  90  via valve  88 . Valve  88  is opened so that high pressure nitrogen is ejected to evacuate arrangement  10  and therefore turbomolecular pumping means  16 . Nitrogen is a relatively inert gas at normal operating temperatures of the system and does not contaminate the system. 
     Although the pumping arrangement  10  may be evacuated prior to start up, it is also possible to evacuate the arrangement after or during start up, since the arrangement can be started but will not reach suitable rotational speeds until evacuation is performed. However, if the arrangement and in particular the turbomolecular pumping means is started prior to or during evacuation, torque of the motor is preferably limited to prevent overloading until evacuation is performed. 
     There now follows a description of three further embodiments of the present invention. For brevity, the further embodiments will be discussed only in relation to the parts thereof which are different to the first embodiment and like reference numerals will be used for like parts. 
       FIG. 6  shows a vacuum pumping arrangement  100  comprising an active magnetic bearing in which a cylindrical pole of the magnetic bearing  54  is mounted to the drive shaft  32  with a like pole being positioned on housing  22 . The rotor body  52  of the turbomolecular pumping means  16  of the molecular pumping mechanism, is disc-shaped and the overall size of the arrangement  100  is reduced as compared with the first embodiment. 
     In  FIG. 7 , a vacuum pumping arrangement  200  is shown in which the turbomolecular pumping means  12  comprises two turbomolecular pumping stages  16 . A stator  92  extends radially inwardly from housing part  22  between the two turbo stages  16 . 
     In  FIG. 8 , a vacuum pumping arrangement  300  is shown in which molecular drag pumping mechanism  20  has been omitted.