WO2022254213A1

WO2022254213A1 - Holweck drag pump

Info

Publication number: WO2022254213A1
Application number: PCT/GB2022/051397
Authority: WO
Inventors: Daniel SUAREZ ARIAS
Original assignee: Edwards Limited
Priority date: 2021-06-04
Filing date: 2022-06-01
Publication date: 2022-12-08
Also published as: TW202305244A; CN117795203A; GB2607339A; GB202108014D0; EP4348055A1

Abstract

The present invention provides a Holweck drag pump comprising a non-ferromagnetic helically grooved surface for conveying a gaseous medium, said surface being provided by a rotor shaft-facing surface of a motor stator of the Holweck drag pump or by an outer surface of a rotor shaft, preferably by a rotor shaft-facing surface of a motor stator of the Holweck drag pump or motor stator-facing outer surface of a rotor shaft. The present invention also provides a permanent magnet motor for use in a vacuum pump, and a vacuum pump comprising such a Holweck drag pump and/or permanent magnet motor.

Description

Holweck Drag Pump

Field of the Invention

[001] The invention relates to a Holweck drag pump and in particular a Holweck drag pump with a non-metallic helically grooved surface for conveying a gaseous medium. The invention further relates to a permanent magnet motor for use in a vacuum pump, and a vacuum pump comprising said permanent magnet motor and/or said Holweck drag pump.

Background

[002] Known vacuum pumps, including for example a turbomolecular pump mechanism, comprise a housing providing an inlet and an outlet. The housing may house one or more pump mechanisms. A turbomolecular pump mechanism may form one of these mechanisms and generally comprises at least one rotor having a plurality of axially spaced, annular arrays of inclined rotor blades. Typically, the blades are regularly spaced within each array, and extend radially outwards from a rotor shaft. A stator of the pump surrounds the rotor, and comprises at least one annular array of inclined stator blades which alternate in an axial direction with the arrays of rotor blades. Each pair of adjacent rotor blade arrays and stator blade arrays is referred to herein as a level. The turbomolecular pump mechanism may comprise a plurality of such levels.

[003] Typically, a motor is arranged within the housing to rotate the rotor shaft during operation of the pump. The motor may be an induction motor or a permanent magnet motor. As the rotor rotates, the rotor blades impact incoming molecules of gaseous medium. This impact transfers the mechanical energy of the blades into gas molecule momentum, which molecules are directed through the pump towards the pump outlet.

[004] A vacuum pump may often comprise a turbomolecular pump stage followed by at least one molecular drag stage such as a Holweck drag pump or Siegbahn pump. Known Holweck molecular drag stages comprise a rotating cylinder arranged adjacent a stator with a gap therebetween, referred to herein as a pass, wherein either the stator or the cylinder has a helical groove or a thread. The impact of the gas molecules with the rotating rotor imparts a stimulus velocity thereon, resulting in the displacement of gaseous medium along the pass. [005] To improve the efficiency of the molecular drag stage, and thereby the compression ratio of the vacuum pump as a whole, several concentric molecular drag stage passes may be provided about the rotor shaft. It is therefore desirable to provide a plurality of passes in the molecular drag stage. Often, a limiting factor on the number of molecular drag passes that can be included in a vacuum pump is the total available size for the pump. A vacuum pump is typically part of a larger system with a volume set aside for accommodating the vacuum pump. Accordingly, providing vacuum pumps of reduced size may be desirable.

[006] As the molecular drag stage passes are arranged concentrically, when the vacuum pump is in use the direction of gas flow through reverses between each adjacent pass. Thus, for practical purposes, usually only an odd number of molecular drag stages is used such that the outlet of the final pass is located away from the turbomolecular pump mechanism, where the pump outlet may easily be situated. Accordingly, the design freedom of current Holweck molecular drag pumps is to an extent constrained, which may reduce the efficiency of the pump system.

[007] There is an ongoing need for improved Holweck molecular drag stages which are more efficient, simpler, and cheaper to manufacture, which improve the compression ratio of the vacuum pump, and provide a high degree of freedom in design.

[008] The present invention addresses, at least in part, these and other problems with the prior art.

Summary of the Invention

[009] Accordingly, in a first aspect, the present invention provides a Holweck drag pump comprising a non-ferromagnetic, helically grooved surface for conveying a gaseous medium. The surface is provided by a rotor shaft-facing surface of a motor stator of the Holweck drag pump or by an outer surface of a rotor shaft. Preferably, the surface is provided by a rotor shaft-facing surface of a motor stator of the Holweck drag pump or motor stator-facing outer surface of a rotor shaft.

[010] Preferably, the surface provides the innermost and/or final pass of the Holweck drag pump. The final innermost and/or final pass may be directly fluidly connected to an outlet of the Holweck drag pump. [011] The motor stator is preferably configured to generate an electromagnetic drive field for transmitting a torque onto the rotor. The motor stator can have a core comprising windings preferably set about a central magnetic, in particular soft-magnetic, and/or metallic material. Typically, the windings are metallic. Preferably, the Holweck drag pump comprises a motor stator surrounding a rotor shaft and there is substantially no ferromagnetic material located between a rotor shaft-facing surface of the coil-wrapped core of the motor stator and a metallic surface of the rotor shaft.

[012] Between the motor stator and the rotor shaft is a gap, in particular a radially extending gap. Preferably, the gap extends along the motor stator in an axial direction, i.e. in a direction parallel to the axis of rotation of the rotor. The electromagnetic drive field extends across the gap to transmit drive torque to the rotor shaft.

[013] The present inventors have found that it may be disadvantageous to employ a ferromagnetic helically grooved surface, particularly on the motor stator. In use, such a ferromagnetic surface may be present in the electromagnetic drive field generated by the motor of the Holweck drag pump. The presence of a ferromagnetic surface may disrupt the electromagnetic drive field by forming parasitic eddy currents, and therefore may reduce the efficiency of the motor. Furthermore, the parasitic eddy currents induced in such a surface when present in an electromagnetic field may result in localised temperature increases and thermal expansion of the surface. Thermal expansion of the surface is undesirable as increased tolerances must be provided which may in turn reduce the efficiency of the motor.

[014] Advantageously, the non-ferromagnetic helically grooved surface can be present in the electromagnetic drive field generated by the motor of the Holweck drag pump with minimal interference on the electromagnetic drive field. The non-ferromagnetic helically grooved surface may provide additional pump capacity for the Holweck drag pump. Accordingly, providing such a non-ferromagnetic surface may enable increased pumping performance without negatively impacting motor efficiency.

[015] For the purposes of the present invention, a helically grooved surface may be a substantially concave helical groove defined in a surface. For the avoidance of doubt, the, or each helical groove may be defined by one or more corresponding convex helical threads formed on the surface. Preferably, all the grooves are non-ferromagnetic. Preferably the groove(s) are monolithic. The relative thickness of the groove(s) may be chosen depending on the intended use. [016] The helically grooved surface may comprise one or more helical groove(s), for example from about 1 to about 15 helical grooves. The helical groove(s) may be a substantially constant pitch helix, or alternatively may be a variable pitch helix. The helical groove(s) may have a substantially rectangular, trapezoidal, semi-circular, or other cross- sectional convex profile. The cross-sectional profile and/or size of the helical groove(s) may be uniform or may vary along the length of the helical groove(s).

[017] Advantageously, as the surface comprising the helical groove is non ferromagnetic, there is a high degree of design freedom available for the arrangement of the helical groove. In contrast to a helical groove in a ferromagnetic surface, providing the helical groove in a non-ferromagnetic surface results in insignificant disruption to the electromagnetic drive field of the motor. Thus, the present invention improves the design freedom for the helical grooved surface, whilst allowing efficient operation of the pump.

[018] Preferably, the at least one helical groove extends axially from a first end to a second end of the surface of the motor stator or for a distance the length of the motor stator of the rotor shaft.

[019] In embodiments, the Holweck drag pump may comprise a conduit defining an axial passage extending through the Holweck drag pump. The axial passage may extend from an opening in a base of the pump to an opening axially beyond a pump inlet. The rotor shaft and motor stator may extend around the conduit. The conduit may be coaxial with the axis of rotation of the rotor shaft.

[020] Preferably, the conduit defining the axial passage may be located radially inwardly of the non-ferromagnetic helically grooved surface. Preferably, the conduit defining the axial passage may be located within the rotor shaft.

[021] Advantageously, the axial passage through the Holweck drag pump may provide a central pathway for connectors, a mounting shaft, a power supply, and/or other componentry, to pass through the Holweck drag pump, as is discussed in greater detail in relation to embodiments of a further aspect defined herein. This may aid in providing uniform gas flow, particularly when the inlet of the Holweck drag pump is connected to an outlet of a turbomolecular pump mechanism. [022] In a further aspect, the present invention provides a permanent magnet motor for use in a vacuum pump. The permanent magnet motor comprises a rotor shaft of the vacuum pump and a motor stator. A rotor shaft-facing surface of the motor stator and a motor stator-facing surface of the rotor shaft form a pass of a Holweck drag pump. One of said surfaces is non-ferromagnetic, and provides helical groove for conveying a gaseous medium.

[023] Preferably, the vacuum pump comprising the permanent magnet motor may comprise a Holweck drag pump mechanism. More preferably, the pass of the Holweck drag pump of the permanent magnet motor may be fluidly connected to said Holweck drag pump mechanism.

[024] Preferably, the surface provides the innermost and/or final pass of the Holweck drag pump. The final innermost and/or final pass may be fluidly connected to an outlet of the Holweck drag pump.

[025] Preferably, a rotor shaft-facing surface of a motor stator is both helically grooved for conveying a gaseous medium and non-ferromagnetic. The motor stator-facing surface of the rotor shaft may be substantially smooth. For the purposes of the present invention, substantially smooth may define a surface without a helical groove. Advantageously, balancing of the rotor may easier if the rotor has a smooth (e.g., featureless) surface and the helical groove is located on a stator.

[026] As set out in relation to the preceding aspect, the motor stator is preferably configured to generate an electromagnetic drive field for transmitting a torque onto the rotor. The motor stator can have a core comprising windings preferably set about a central magnetic, in particular soft-magnetic, and/or metallic material. Typically, the windings are metallic. Preferably, there is substantially no ferromagnetic material located between the rotor shaft-facing surface of the coil-wrapped core of the motor stator and the metallic surface of the rotor shaft.

[027] Between the motor stator and the rotor shaft is a gap, in particular a radially extending gap. Preferably, the gap extends along the motor stator in an axial direction, i.e. in a direction parallel to the axis of rotation of the rotor. The electromagnetic drive field extends across the gap to transmit drive torque to the rotor shaft. [028] Typically, the rotor shaft is radially surrounded by the motor stator and the rotor shaft and motor stator are radially separated by a gap. Preferably, the gap extends through the motor in an axial direction, i.e. in the direction of the axis of rotation of the rotor shaft.

[029] As discussed hereinbefore, the present inventors have found that it may be disadvantageous to employ a ferromagnetic helically grooved surface, particularly on the motor stator due to the disruption of the electromagnetic drive field of the motor and the formation of parasitic eddy currents in the surface.

[030] Advantageously, the non-ferromagnetic helically grooved surface can be present in the electromagnetic drive field generated by the permanent magnet motor with minimal interference on the electromagnetic drive field. It has been found that the gas compression provided by the pass of the Holweck drag pump is suitable to outweigh any increased gas friction between the rotor shaft-facing surface of the motor stator and the motor stator-facing surface of the rotor shaft. Accordingly, providing such a non ferromagnetic helically grooved surface may enable increased pumping performance with little impact on the efficiency of the motor. Such a motor may efficiently provide drive torque to the rotor shaft whilst improving the compression ratio of the pump.

[031] Advantageously, because the surface comprising the helical groove is non ferromagnetic, there may be a high degree of design freedom available for the arrangement of the helical groove. In contrast to a helical groove in a ferromagnetic surface, providing the helical groove in a non-ferromagnetic surface results in insignificant disruption to the electromagnetic drive field of the motor. Thus, the present invention improves the design freedom for the helical grooved surface, whilst allowing efficient operation of the pump.

[032] Preferably, the at least one helical groove extends axially from a first end to a second end of the surface of the motor stator or for a distance the length of the motor stator of the rotor shaft.

[033] The following features can apply to the Holweck drag pump and/or permanent magnet motor according to any preceding aspect or embodiment.

[034] Preferably, the non-ferromagnetic surface has a magnetic permeability less than about 1.0x1 O ⁴ H/m, preferably less than about 1.260^c10^~6 H/m. Additionally, or alternatively, the non-ferromagnetic surface may have a relative permeability less than about 10, preferably less than about 1.05.

[035] The non-ferromagnetic surface may have an electrical resistivity less than about 2.0x1 O ⁸ Wth, preferably less than about 6.9x1 O ⁷ Wth.

[036] Preferably, the helically grooved surfaced is provided by injection moulding, an insert, co-moulding and/or coating attached to a rotor or stator.

[037] Preferably, the helically grooved surface may be manufactured via injection moulding, extrusion, additive manufacturing, and/or machining. A further finishing step, for example a machining step, may be performed. Most preferably, the helically grooved surface may be injection moulded, followed by a machining step.

[038] In embodiments wherein the helically grooved surface is provided by an insert, the insert may be coupled to a motor stator or rotor shaft of a vacuum pump by a fixture, an interference fit, a snap-fit, and/or one or more gaskets. Advantageously, such an insert may be separable from the Holweck drag pump or permanent magnet motor and therefore replaceable. Furthermore, such an insert may be retrofitted to an existing Holweck drag pump or permanent magnet motor. The insert may comprise a substantially tubular member configured to be inserted in a gap between the motor stator and rotor shaft. Preferably, the insert may be configured to couple to the motor stator and a radially inwardly facing surface thereof may define the helically grooved non-metallic surface.

[039] The insert may be unitary or, preferably, comprise a plurality of segments configured to provide the non-ferromagnetic surface when assembled in situ, e.g. within the Holweck drag pump or permanent magnet motor. Advantageously, providing the insert as a plurality of interconnectable segments may enable easier assembly of the insert. Furthermore, it may enable the insert to be retrofitted to existing Holweck drag pumps or permanent magnet motors.

[040] In embodiments wherein the helically grooved surface is provided by a co moulding and/or insert, the co-moulding and/or insert may provide the additional benefit of isolating the magnet core of the motor stator. During operation of the motor, gaseous medium may convey through the gap of the motor. The gaseous medium may carry particulate matter that may degrade the windings if unprotected. Therefore, providing the helically grooved surface as a co-moulding and/or insert may prevent damage to the magnet core by particulate contamination during operation of the vacuum pump. Thus, the magnet core may require servicing and replacement less frequently.

[041] In embodiments wherein the helically grooved surface is provided by a coating, said coating may preferably be attached to a stator, more preferably to the motor stator of the permanent magnet motor.

[042] Preferably, the helically grooved surface is integrally formed with the rotor or stator.

[043] Preferably, the non-ferromagnetic surface comprises a polymeric material, ceramic material, or composite material. Most preferably, the non-ferromagnetic surface comprises a polymeric material. Preferred polymers are selected from group consisting of thermoplastic materials or thermosets. In particular, thermoplastic materials classified as high-performance thermoplastic materials are preferred due to their mechanical and thermal properties. Suitable thermoplastic polymers include polyether ether ketone (PEEK), polytetrafluorethylene (PTFE), polyamide, polyphenylene sulfide (PPS), and derivatives and copolymers thereof. PEEK is particularly preferred. Suitable thermoset polymers may be cured epoxy resins.

[044] Advantageously, such materials may reduce interference with the magnetic field of the motor. Furthermore, they are relatively mechanically stiff, thermally stable, and have a low cost. They may also be injection moulded, and is easily machinable and/or injection mouldable, enabling straightforward production.

[045] The polymeric material may additionally include one or more from the group consisting of anti-statics, antioxidants, mould release agents, flame-proofing agents, lubricants, colorants, flow enhancers, fillers, including nanofillers, light stabilizers and ultraviolet light absorbers, pigments, anti-weathering agents and plasticizers.

[046] Additionally, or alternatively, the polymeric material may comprise one or more reinforcing element, for example glass fibres, carbon fibres, or particulates.

[047] Preferably, the non-ferromagnetic surface is monolithic. Particularly, the non ferromagnetic surface is substantially provided by a single, unitary body of material. Advantageously, this may simplify the manufacture of the non-metallic surface. For example, the non-ferromagnetic surface, or body comprising the same, may be injection moulded, providing a relatively low-cost, fast, and uniformly replicable manufacturing method.

[048] Optionally, the body comprising the non-metallic surface may comprise a through channel fluidly connected to an outlet of the Holweck drag pump mechanism. Accordingly, the direction of conveyance of the gaseous medium through the Holweck pass can be substantially towards or away from the outlet of the Holweck drag pump mechanism.

[049] In some embodiments, the permanent magnet motor may comprise a conduit defining an axial passage extending through the motor. The axial passage may extend from an opening in a base of the motor to an opening axially beyond the opposite end to the base. Preferably, the conduit may be defined within the rotor shaft. The axial passage may be coaxial with the axis of rotation of the rotor shaft. Advantageously, the axial passage through the motor may provide a central pathway for connectors, a mounting shaft, a power supply, and/or other componentry, as discussed in relation to embodiments of a further aspect defined herein.

[050] In a further aspect, the present invention provides a vacuum pump comprising a permanent magnet motor and/or Holweck drag pump according to any preceding aspect or embodiment.

[051] Preferably, the Holweck drag pump mechanism comprises at least one further pass arranged concentrically about the rotor shaft. More preferably, the Holweck drag pump mechanism comprises from about 2 to about 8 further passes, wherein the passes are arranged concentrically about the rotor shaft.

[052] When the Holweck drag pump mechanism is in operation, in subsequent passes, gaseous medium may be pumped in opposite directions. These directions may be substantially parallel to the axis of rotation of the rotor shaft. In prior art vacuum pumps, having an even number of stages complicates the design by having gaseous medium exiting the final stage within the pump mechanism, requiring additional channels or drillings to facilitate gaseous medium movement to the outlet of the vacuum pump. Thus, usually only an odd number of molecular drag stages was possible such that the last molecular drag stage ends towards the outlet of the vacuum pump, thereby limiting the freedom of the design of the vacuum pump. [053] Advantageously, in the pump according to the present invention, the final pass of the Holweck drag pump mechanism may be provided by the non-ferromagnetic helically grooved surface. This additional pass, which is preferably adjacent the rotor shaft, may increase the compression ratio of the pump, and allow for more efficient use of space to provide additional passes.

[054] Preferably, the vacuum pump further comprises at least one level of a turbomolecular pump mechanism fluidly connected to the Holweck drag pump mechanism. Preferably, the vacuum pump comprises from about 2 or more turbomolecular pump stages fluidly connected to the Holweck drag pump mechanism. The at least one turbomolecular pump mechanism may comprise one or more levels. Preferably, each turbomolecular pump mechanism comprises from about 2 to about 8 levels, for example 5 levels.

[055] Preferably, the at least one turbomolecular pump mechanism (or stage) is arranged fluidly upstream of the Holweck drag pump mechanism (or stage). During operation of the vacuum pump, gaseous medium may be conveyed from an outlet of the turbomolecular pump stage to an inlet of the Holweck drag pump mechanism.

[056] Preferably, the at least one turbomolecular pump stage and Holweck drag pump mechanism may be arranged on a single drive shaft. Preferably, the at least one turbomolecular pump stage and Holweck drag pump mechanism may be driven by the same motor during operation of the vacuum pump. For example, the at least one turbomolecular pump stage and Holweck drag pump mechanism may be driven by a permanent magnet motor as set out in a preceding aspect.

[057] Preferably, the vacuum pump may be a turbomolecular pump as produced by Edwards Limited.

[058] Preferably, the vacuum pump may comprise a conduit defining an axial passage extending through the vacuum pump. The conduit may extend from an opening in a base of the pump to an opening axially beyond the pump inlet. The rotor shaft and motor stator may extend around the conduit. Preferably, the axial passage may be coaxial with the axis of rotation of the drive shaft. [059] Preferably, the conduit defining the axial passage may be located radially inwardly of the non-ferromagnetic helically grooved surface. Preferably, the conduit defining the axial passage may be located within the drive shaft.

[060] The vacuum pump may further comprise a plate configured to obscure the inlet of the turbomolecular pump mechanism (i.e. the pump inlet). The plate may be configured to obscure the pump inlet by a controllable amount. Thereby, the plate may control an inlet conductance of the vacuum pump. The vacuum pump may further comprise a shaft for mounting the plate. The shaft may be hollow and extend around the passage. The plate may comprise a hole in a central portion through with the conduit extends. The plate may be a valve plate.

[061] Preferably, the conduit defining the axial passage may be located within the single drive shaft of the turbomolecular pump stage and Holweck drag pump mechanism.

[062] Preferably, the conduit may extend axially through the entire vacuum pump, including both the Holweck drag pump mechanism and turbomolecular pump mechanism. Providing such a “hollow” vacuum pump may enable the use of a poppet valve at the pump inlet, which may be the inlet to the turbomolecular pump. Advantageously, this may be used for pressure control and/or isolation.

[063] The pump inlet may be fluidly connected to a vacuum chamber during use. For many processes performed within vacuum chambers, a substrate may be mounted within said chamber, such as semiconductor etch processes. These processes may require uniform gas flow across a substrate if a high-quality result is to be achieved. Mounting the substrate above the pump inlet may help to provide uniform gas flow across the substrate. However, in prior art systems, the support member to some extent, and more importantly the power and control supplied to the substrate may disturb the gas flow and decrease the uniformity of gas flow across the substrate. The conduit of the present invention may enable a substrate mounting shaft to be fed through the vacuum pump. Additionally, or alternatively, the conduit may enable connectors, power supply, control wiring, and/or cooling supply to be fed through the vacuum pump such that they are centrally aligned beneath the substrate. This may enable increased uniformity of airflow around the substrate during operation.

[064] Furthermore, the valve plate obscuring the pump inlet by a controllable amount may provide fast pressure control and reduced particle generation and shedding. Additionally, it may provide for symmetric flow around the plate and may reduce non uniformities in the gas flow across a wafer mounted within the chamber. It should be noted that although the plate may be a flat circular structure, it may also have other forms, such as a conical form. In general, it is advantageous if the plate has a circular cross section, as this may improve the uniformity of the gas flow.

[065] A hollow pump having a plate at least partially obscuring the pump inlet, with axial movement of the plate providing inlet conductance control, may provide an effective and fast control of the inlet conductance to the pump and thus, the pressure within the chamber and a more uniform gas flow over the substrate.

[066] The plate may at least partially obscure the inlet. Movement of the plate may cause the inlet conductance to vary as the gas flows around the edge of the plate. In some cases, the plate may extend beyond the pump’s inlet in all axial positions. In others, the plate may extend beyond the inlet in some axial positions. Axial movement may be defined as movement substantially parallel to the rotor shaft of the pump. The plate may be mounted on a shaft. Preferably, the shaft may be hollow and may extend around the conduit defining the passage.

[067] In some embodiments, the plate may be mounted on an end of the drive shaft of the vacuum pump towards the inlet of the turbomolecular pump. The drive shaft may be movable in an axial direction with respect to the motor stator. The drive shaft may be mounted to or positioned within the pump via electro-magnetic bearings. A current supplied to electro-magnets associated with said bearings may control the axial position of the drive shaft and thereby the plate. The pump may comprise an actuator associated with the drive shaft for varying the axial position of the shaft and the plate.

[068] In alternative embodiments, the pump may comprise a mounting shaft to which the plate is mounted, the mounting shaft being separate to the drive shaft. The mounting shaft may be configured not to rotate. The mounting shaft may be movable in an axial direction relative to the motor stator. The pump may comprise an actuator associated with the mounting shaft for varying the axial position of the shaft and the plate.

[069] In some embodiments, the plate may comprise an O-ring on a surface facing the pump inlet. In alternative embodiments, the plate may not comprise an O-ring. [070] The pump may comprise control circuitry configured to control the axial position of the plate. The control circuitry may comprise an input configured to receive a signal indicative of a pressure produced by the vacuum pump. The control circuitry may be configured to control the axial position of the plate according to the signal.

[071] An advantage with having a passage through the vacuum pump may be that any connections to a substrate mount, such as a cathode, may pass through the pump and arrive underneath the substrate mount towards its centre. In some embodiments, an end of the conduit towards the pump inlet may comprise mounting means for mounting a substrate mount such as a cathode. This may allow the substrate to be mounted centrally above the pump inlet without the need for separate substrate supports which themselves may disrupt uniform gas flow.

[072] Vacuum pumps suitable for use herein are described in WO 2020/012154, which is incorporated herein by reference. Particularly, with reference to Figures 1-3 which illustrate vacuum pumps having an axial passage.

[073] Although in the example described the vacuum pump comprises a Holweck pump mechanism and Turbomolecular pump mechanism, other vacuum pumps with a central shaft such as a Holweck pump alone or in combination with an alternative pump are also applicable and may comprise a passage through the centre and an axially movable valve plate.

[074] In a further aspect, the present invention provides the Holweck drag pump, permanent magnet motor and/or vacuum pump according to any preceding aspect or embodiment, wherein the non-ferromagnetic helically grooved surface is provided by an injection moulding, extrusion, additive manufacture, and/or machining process.

[075] Optionally, the process may include one or more finishing steps. Such a finishing step may include, for example, a machining step, application of a coating, and/or surface treatment.

[076] For the avoidance of doubt, all aspects and embodiments described hereinbefore may be combined mutatis mutandis.

Brief Description of Figures [077] Preferred features of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

[078] Figure 1 shows a cross-sectional schematic view of an embodiment of a vacuum pump according to the present invention.

[079] Figure 2 shows a cross-sectional schematic view of an alternative embodiment of a pump stage according to the present invention.

[080] Figure 3 shows a graph comparing the compression ratio and backing pressure of different vacuum pumps.

Detailed Description

[081] Figure 1 illustrates a cross-sectional schematic view of an embodiment of a vacuum pump according to the present invention.

[082] The vacuum pump (1) comprises a turbomolecular pump mechanism (2) and a Holweck drag pump mechanism (3). The turbomolecular pump mechanism (2) is fluidly connected to the Holweck drag pump mechanism (3). The vacuum pump (1) comprises a housing (4) that substantailly surrounds the turbomolecular pump mechanism (2) and Holweck drag pump mechanism (3).

[083] The turbomolecular pump mechanism (2) comprises a rotor. The rotor has a plurality of axially spaced, annular arrays of inclined rotor blades (5). The blades are regularly spaced within each array (5), and extend radially outwards from a rotor shaft (6). A stator surrounds the rotor arrays (5), and comprises a plurality of annular arrays of inclined stator blades (7) which alternate in an axial direction with the arrays of rotor blades (5). Each adjacent pair of arrays of rotor and stator blade arrays form a level of the turbomolecular pump stage (2).

[084] The vacuum pump (1) further comprises a Holweck drag pump stage (3). The Holweck drag pump mechanism (3) is fluidly connected to the turbomolecular pump mechanism (2). The Holweck drag pump mechanism (3) comprises a rotor (8). The rotor (8) is directly connected to the rotor shaft (6). The rotor (8) comprises a first cylinder (9) and a second cylinder (10). The first cylinder (9) and second cylinder (10) are concentrically arranged about, and connected to, the rotor shaft (6). [085] The Holweck drag pump mechanism (3) further comprises a first stator (11). The first stator (11) has a helically grooved surface which faces the radially outwardly facing surface of the first cylinder (9). The first stator (11) and radially outwardly facing helically grooved surface of the first cylinder (9) define a first pass of the Holweck drag pump mechanism (3). The radially inwardly facing helically grooved surface of the first stator (11) faces a radially outwardly facing surface of the second stator (12) to define a second pass of the Holweck drag pump mechanism (3). The radially outwardly facing surface of the second cylinder (10) faces a radially inwardly facing helically grooved surface of the second stator (12) to define a third pass of the Holweck drag pump mechanism (3). The radially inwardly facing surface of the second cylinder (10) faces a radially outwardly facing helically grooved surface of a third stator (13) to define a fourth pass of the Holweck drag pump mechanism (3).

[086] The Holweck drag pump mechanism (3) further comprises a permanent magnet motor (14) configured to provide a drive torque to the rotor shaft (6) during operation of the vacuum pump (1). The rotation of the rotor shaft (6) is configured to rotate both the rotor blade arrays (5) of the turbomolecular pump mechanism (2), and the rotor(s) (8) of the Holweck drag pump mechanism (3). The permanent magnet motor (14) comprises a motor stator (15). The motor stator (15) comprises a magnet core (16) which comprises windings. The windings are wire coiled about a laminated soft iron magnetic core. The motor stator (15) further comprises a non-ferromagnetic portion that defines a helically grooved surface (17) facing the rotor shaft (6). The surface of the rotor shaft (6) and the non-ferromagnetic helically grooved surface (17) define a fifth and final pass of the Holweck drag pump mechanism (3).

[087] During operation, the permanent magnet motor (14) imparts a drive torque to the rotor shaft (6), which in turn rotates the rotor blade arrays (5) of the turbomolecular pump mechanism (2) and rotor (8) of the Holweck drag pump mechanism (3). Gaseous medium conveyed through the turbomolecular pump mechanism (2) enters the Holweck drag pump mechanism (3). The first and second cylinders (9,10) impact gas molecules and transfer the mechanical energy into gas molecule momentum. Thereby, rotation of the first and second cylinders (9,10) relative to the helically grooved surfaces of the first, second and third stators (11,12,13) conveys the gaseous medium through the first, second, third and fourth passes of the Holweck drag pump mechanism (3), respectively. Finally, the gaseous medium is conveyed through the fifth pass as a result of the rotation of the rotor shaft (6) relative to the non-ferromagnetic helically grooved surface (17) of the motor stator (14).

[088] In the illustrated example, the non-ferromagnetic helically grooved surface (17) is polymeric, for example epoxy resin. Advantageously, this protects the magnet core (16) from particulate damage during operation of the vacuum pump (1). The non-ferromagnetic helically grooved surface (17) is monolithic.

[089] The fifth pass of the Holweck drag pump mechanism (3) improves the compression ratio of the vacuum pump (1). This increased compression ratio outweighs any increased friction resulting from the introduction of the fifth pass of the Holweck drag pump mechanism (3). Moreover, because it is non-ferromagnetic it does not substantially negatively affect the performance of the motor.

[090] Figure 2 illustrates a cross-sectional schematic view of an alternative embodiment of a vacuum pump according to the present invention. Figures 1 and 2 have many corresponding features, which will not be repeated and for which the same reference numerals will be used.

[091] In this embodiment, the motor stator (14) further comprises an insert (18). The insert (18) comprises a radially inwardly facing helically grooved surface (19). The helically grooved surface (19), with the radially outwardly facing surface of the rotor shaft (6), defines the fifth pass of the Holweck drag pump mechanism (3). The insert (18) is a non ferromagnetic material. Preferably, the insert is polymeric, for example epoxy resin, polytetrafluoroethylene or polyether ether ketone.

[092] The insert (18) is coupled to a body of the motor stator (14) by fixing means, for example an O-ring (20). The insert (18) may be removable from the motor stator (14) and replaced if worn. Advantageously, this may avoid the need for replacement of the entire rotor stator. Furthermore, the insert (18) may be retrofitted to pre-existing vacuum pumps to improve the compression ratio.

[093] Figure 3 provides a graph comparing the compression ratio and backing pressure of different vacuum pumps. Line (21) shows the compression ratio at different backing pressures for a vacuum pump comprising a turbomolecular pump mechanism followed by a Holweck drag pump mechanism, wherein the rotation of the rotors of the turbomolecular pump mechanism and Holweck drag pump mechanism are driven by a permanent magnet motor according to the prior art, i.e. without a Holweck drag pump pass within the permanent magnet motor. Line (22) shows the compression ratio at different backing pressures for a vacuum pump that is identical to that of line (21) except that the vacuum pump also includes a Holweck drag pump pass within the permanent magnet motor, i.e. the vacuum pump illustrated in Figure 1.

[094] It can be seen that for corresponding backing pressures, the compression ratio of line (22) is greater than that of line (21). Thus, the additional pass of the Holweck drag pump within the permanent magnet motor increases the overall compression ratio of the vacuum pump.

[095] For the avoidance of doubt, features of any aspects or embodiments recited herein may be combined mutatis mutandis. It will be appreciated that various modifications may be made to the embodiments shown without departing from the spirit and scope of the invention as defined by the accompanying claims as interpreted under patent law.

Claims

1. A Holweck drag pump comprising a non-ferromagnetic helically grooved surface for conveying a gaseous medium, said surface being provided by a rotor shaft-facing surface of a motor stator of the Holweck drag pump or by an outer surface of a rotor shaft, preferably by a rotor shaft-facing surface of a motor stator of the Holweck drag pump or motor stator-facing outer surface of a rotor shaft.

2. A permanent magnet motor for use in a vacuum pump, the permanent magnet motor comprising a rotor shaft of the vacuum pump and a motor stator, wherein a rotor shaft facing surface of the motor stator and a motor stator-facing surface of the rotor shaft form a pass of a Holweck drag pump, wherein one of said surfaces is non ferromagnetic, and provides helical groove for conveying a gaseous medium.

3. The Holweck drag pump and/or permanent magnet motor according to claim 1 or 2, wherein the non-ferromagnetic surface has a magnetic permeability less than about 1.0x1 O⁴ H/m, preferably less than about 1.260x10^~6H/m, and/or a relative permeability less than about 10, preferably less than about 1.05.

4. The Holweck drag pump and/or permanent magnet motor according to any preceding claim, wherein the non-ferromagnetic surface has an electrical resistivity less than about 2.0x1 O ⁸ Wth, preferably less than about 6.9x1 O⁷ Wth.

5. The Holweck drag pump and/or permanent magnet motor according to claims 1 to 4 wherein the helically grooved surface is provided by injection moulding, an insert, co moulding and/or coating attached to a rotor or stator.

6. The Holweck drag pump and/or permanent magnet motor according to any preceding claim, wherein the helically grooved surface is integrally formed with the rotor or stator.

7. The Holweck drag pump and/or permanent magnet motor according to any preceding claim, wherein the non-ferromagnetic helically grooved surface comprises a polymeric material.

8. The Holweck drag pump and/or permanent magnet motor according to claim 7, wherein the polymeric material is selected from an epoxy resin, polytetrafluorethylene, polyether ether ketone, polyamide or polyphenylene sulfide.

9. The Holweck drag pump and/or permanent magnet motor according to claim 7 or 8, wherein the polymeric material comprises one or more reinforcing elements.

10. The Holweck drag pump and/or permanent magnet motor according to any preceding claim, wherein the helically grooved surface is monolithic.

11. A vacuum pump comprising a permanent magnet motor and/or Holweck drag pump according to any of claims 1 to 10.

12. The vacuum pump according to claim 11 , wherein the Holweck drag pump mechanism comprises at least one further pass, preferably from about 2 to about 8 further passes, wherein the passes are arranged concentrically about the rotor shaft.

13. The vacuum pump according to either claim 11 or 12, further comprising at least one level of a turbomolecular pump mechanism stage fluidly connected to the Holweck drag pump mechanism.

14. The Holweck drag pump, permanent magnet motor and/or vacuum pump according to any preceding claim, wherein the non-ferromagnetic helically grooved surface is provided by an injection moulding, extrusion, additive manufacture, and/or a machining process.

15. The Holweck drag pump, permanent magnet motor and/or vacuum pump according to claim 14, wherein the process includes one or more finishing steps.