The present invention relates to a pump and a method of reducing a time invariant stray magnetic field of the pump.

BACKGROUND

Pumps, such as turbomolecular pumps, are often employed as a component of a vacuum system used to evacuate devices such as scanning electron microscopes (SEMs), lithography devices and other apparatus.

The performance of scanning electron microscopes and other electrical equipment is highly susceptible to mechanical vibrations and stray magnetic fields emitted from turbomolecular pumps. These stray magnetic fields may have a time varying AC field component and a time invariant DC field component. The stray magnetic field is generally mitigated using correction magnets mounted on the rotor and configured to counteract the stray field in general.

In this regard although the DC field component of the stray magnetic field is often larger than the AC field component, for many applications it is the time varying nature of the field that causes the problem and thus, it is this in particular that is mitigated. However, there are applications where the time invariant DC field is a particular problem.

It would be desirable to provide a means to mitigate at least one time invariant component of a stray magnetic field emitted from a pump.

SUMMARY

A first aspect provides a method of reducing at least one time invariant component of a stray magnetic field of a pump, said pump comprising a motor assembly and a magnetic bearing assembly, said method comprising: determining said at least one time invariant component of said stray magnetic field generated at least partly by said magnetic bearing assembly at at least one reference point; providing at least one correction magnet for mounting at a fixed position with respect to a stator of said pump, said at least one correction magnet being configured with magnetic characteristics such that a magnetic field generated by said correction magnet counteracts said determined at least one time invariant component of said stray magnetic field; and mounting said at least one correction magnet in a fixed position with respect to said stator.

The inventor of the present invention recognised that vacuum pumps such as turbomolecular pumps that are used to generate high vacuums often use high speed rotating magnets during operation in, for example, the permanent magnetic bearings used to support the shaft. These generate stray magnetic fields, that is magnetic fields that extend beyond their useful operational region and which may interfere with the operation of other components in the vicinity of the bearings and the pumps. The stray magnetic field emitted by the pump when the rotor is not rotating is typically a static, time invariant field known also as ‘DC’ field. This field has a direction and can be determined as a vector. If the rotor is rotating and if the magnetisation of the bearing magnets is not uniform, the magnetic field measured in any position in space changes with time. Such a time varying field is also known as ‘AC’ field and can also be determined as a vector with time varying components. Thus, when the rotor is rotating, the stray magnetic field is essentially constituted of, the time varying and time invariant components . Typically, the amplitude of the time-varying (AC) field is smaller than the amplitude of the static (DC) field.

Generally, it is the time varying magnetic field that is problematic and various techniques are used to reduce this field. These techniques may be used to reduce both the AC and DC fields in combination and generally involve mounting one or more correction magnets on the rotor. However, the DC component is often significantly larger than the AC one and may only need to be addressed in some situations, therefore it was recognised that it may be advantageous to address it separately. Embodiments do this by providing a static correction magnet that is configured to counteract at least one time invariant component of the stray magnetic field generated at least partly by the magnetic bearings, and in some embodiments solely by the magnetic bearings at a reference point or in a reference area or volume. The reference point may be selected as the point where it is important that the time invariant stray magnetic field is low. Thus, where the pump is to be used in conjunction with other devices that are sensitive to magnetic fields then this point may be at or close to such a device. In some embodiments, it may not be a point at or close to the device it may be an area such as a plane or a volume at or close to the device.

A static correction magnet that is not mounted on the rotor is easier to install and to determine the characteristics of and does not generally require the same robustness as one that is mounted on the rotor would.

The time invariant stray magnetic field comprises a vector and in some embodiments it is the component in one particular direction, perhaps in a direction towards a sensitive piece of apparatus that the correction magnet is configured to mitigate. In other embodiments, the correction magnet is configured to mitigate a plurality, in some cases all, of the time invariant components of the stray magnetic field.

Although the correction magnet may be mounted in any fixed position with respect to the pump, in some embodiments said correction magnet is mounted attached to said stator. Attaching the magnet to the stator allows the position of the magnet with respect to the magnetic bearings of the pump to be fixed and independent of any movement of the pump.

In some embodiments the method of reducing the stray magnetic field comprises a method of assembling or building a pump with a correction magnet mounted at a fixed position with respect to the pump, in some embodiments on the stator. Alternatively, it may be a method of servicing or optimising a pump, wherein an additional correction magnet for counteracting the stray magnetic field is mounted at a fixed position with respect to the pump, in some embodiments on the stator.

In some embodiments, said step of mounting said correction magnet on said stator comprises mounting said magnet on said stator at a position that is not part of the magnetic bearing assembly, in some embodiments within a recess in a central portion of said stator at an inlet end of said pump.

The stator of a pump such as a turbomolecular pump, may have a recess in a central portion towards an inlet of the pump, such a position may be ideal for a correction magnet configure to mitigate a time invariant stray magnetic field generated by the pump and affecting an apparatus that the pump may be evacuating. Towards the pump inlet is toward the apparatus and a central position is effective for any stray field generated by magnetic bearings supporting the rotor of the pump.

In some embodiments, said method comprises providing a plurality of correction magnets and said step of mounting comprises mounting said plurality of correction magnets at different fixed positions with respect to or on said stator

It may be advantageous to have more than one correction magnet, particularly where the regions requiring stray field reduction are complex and located across different areas or volumes.

As an example, the plurality of correction magnets may comprise a first correction magnet and a second correction magnet, wherein said method comprises mounting said first correction magnet at a different fixed position compared to said second correction magnet. ln some embodiments, said plurality of correction magnets have different magnetic characteristics and/or different shapes.

As an example, said plurality of correction magnets may comprise a first correction magnet and a second correction magnet, wherein said first correction magnet has a different shape compared to said second correction magnet. As a further example, the plurality of correction magnets may comprise a first correction magnet with a first magnetic characteristic and a second correction magnet with a second magnetic characteristic, wherein the first magnetic characteristic is different to the second magnetic characteristic.

The different magnetic characteristics may include different directions of magnetization and field strength.

In some embodiments, said step of providing said at least one correction magnet comprises forming said at least one correction magnet by bonding powder, at least some of said powder being magnetic powder, said magnetic characteristics of said correction magnet being produced by variations in density of said magnetic powder within a volume of said magnet.

The magnet may be formed by bonding powder of different magnetic densities in particular arrangements to provide the desired magnetic characteristics, perhaps by sintering or by manufacturing a correction magnet with a predetermined volume and said magnetic characteristics using an additive manufacturing technique.

Additive manufacturing techniques allow a magnet to be generated by bonding powder, some of the powder being magnetic powder, in a controlled fashion. This allows the local density of the magnetic powder to be controlled and in this way a magnet with particular magnetic characteristics can be generated. ln some embodiments, the magnet may be formed of plastic bonded powder using for example FFF (fused filament fabrication) or FDM (fused deposition modelling) techniques while in other embodiments it may be formed at least partially from metallic powder perhaps by selective laser melting. In either case the powder is joined securely to form the magnet.

In other embodiments, said step of providing said at least one correction magnet comprises selecting a correction magnet with said magnetic characteristics from a plurality of magnets of a same predetermined volume and with different magnetic characteristics, each of said plurality of magnets being manufactured using an additive manufacturing technique.

The magnet that is used to counteract at least one component of the stray magnetic field may be generated following determination of the stray magnetic field and may be configured for this. In other embodiments a plurality of magnets with different magnetic characteristics may be generated using additive manufacturing techniques and the step of providing may be simply choosing one of the magnets with appropriate magnetic characteristics to counteract the determined at least one component of the time invariant stray magnetic field.

In some embodiments, said correction magnet is mounted on said stator within a protective shell. A magnet formed from bonded powder may be less robust than a magnet formed in some other way and thus, it may be advantageous to mount the magnet in a shell. Mounting the magnet in a shell may also help in the mounting of the magnet on the stator.

A second aspect provides a method of manufacture of at least one correction magnet for mounting at a fixed position with respect to a stator of a pump, said pump comprising a motor assembly and a magnetic bearing assembly, said method comprising: determining at least one time invariant component of a stray magnetic field generated at least partly by said magnetic bearing assembly at at least one reference point; and determining magnetic characteristics of said at least one correction magnet which when mounted at a fixed position with respect to said stator would counteract said determined at least one time invariant component of said stray magnetic field; manufacturing said at least one correction magnet with said determined magnetic characteristics, said correction magnet being formed from bonded powder at least some of said powder being magnetic powder, a local density of said magnetic powder being controlled to generate said magnetic characteristics.

In some embodiment the method comprises manufacturing said at least one correction magnet using additive manufacturing,

A further aspect provides, a pump comprising a motor assembly and a magnetic bearing assembly, said pump further comprising: at least one correction magnet mounted at a fixed position with respect to a stator of said pump, said at least one correction magnet being configured with magnetic characteristics such that a magnetic field generated by said correction magnet counteracts at least one time invariant component of a stray magnetic field generated at least partly by said magnetic bearing assembly at at least one reference point.

In some embodiments, said at least one correction magnet is mounted on said stator.

In some embodiments, said at least one correction magnet is mounted within a recess in a central portion of said stator at an inlet end of said pump.

In some embodiments, said pump comprises a plurality of correction magnets each mounted at a different place on said stator

As an example, the plurality of correction magnets may comprise a first correction magnet and a second correction magnet, wherein the first correction magnet is mounted at a different place compared to said second correction magnet. ln some embodiments, said plurality of correction magnets have at least one of a different shape and different magnetic characteristics.

As an example, said plurality of correction magnets may comprise a first correction magnet and a second correction magnet, wherein said first correction magnet has a different shape compared to said second correction magnet. As a further example, the plurality of correction magnets may comprise a first correction magnet with a first magnetic characteristic and a second correction magnet with a second magnetic characteristic, wherein the first magnetic characteristic is different to the second magnetic characteristic.

In some embodiments, said at least one correction magnet is formed from bonded powder, at least some of said powder being magnetic powder.

In some embodiments, said at least one correction magnet comprises variations in local density of magnetic powder across a volume of said magnet, said variations generating a magnetic field which counteracts said at least one time invariant component of said stray magnetic field.

Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: Figure 1 shows a pump with a correction magnet according to an embodiment; Figure 2 schematically shows a magnetic bearing for mounting a rotor of a pump; Figure 3 schematically shows the reduction in the time invariant magnetic field provided by a correction magnet according to an embodiment;

Figure 4 schematically shows a flow diagram illustrating a method of reducing the time invariant stray magnetic field of a pump with magnetic bearings according to an embodiment; and

Figure 5 schematically shows a flow diagram illustrating a method of manufacturing a correction magnet according to an embodiment.

DESCRIPTION OF THE EMBODIMENTS

Before discussing the embodiments in any more detail, first an overview will be provided.

Vacuum pumps, especially vacuum pumps using passive magnetic bearings, emit a magnetic field (‘stray’ field) that can be an issue in certain applications.

The addition of a magnet in a fixed relationship to the pump, perhaps on the stator of the pump creates a time constant or time invariant (‘DC’) magnetic field, whose orientation and amplitude can be chosen to reduce or cancel at least one component (i.e. vectorial component) of the DC stray magnetic field generated by the pump in a chosen position in space.

Although the compensation may be provided by a single magnet and be effective in a particular small region, in some embodiments improved results are obtained by using more than one compensating magnet, the area of stray magnetic field that is mitigated may be increased with the use of such additional correction magnets.

Reduction of the time varying (‘AC’) stray magnetic field emitted by a pump during operation (i.e. running) has been the focus of much work whilst the time constant (‘DC’) stray field has not in the past been a concern, however there are increasing numbers of applications where this is becoming an issue.

The addition of a compensating magnet in a fixed relation with respect to the pump, perhaps mounted on the stator of a pump can be a simple way to reduce its DC stray magnetic field and make it fit for purpose for certain applications. The stator of the pump is the portion of the pump that does not move (rotate).

Figure 1 shows the implementation to a pump. The pump may be a vacuum pump and in particular, it may be a turbomolecular pump (‘TMP’), where a compensating magnet is added to the stator, removed from but in relatively close proximity to the permanent magnets forming the passive magnetic bearing (PMB) and in proximity to the pump’s inlet.

In this embodiment the PMB is formed with pairs of rotor and stator permanent magnet rings, axially magnetised, where each pair has opposing directions of magnetisation. Figure 2 schematically shows an example of such a magnetic bearing. There may be a DC stray magnetic field emitted by the bearing when it is stationary and a time varying magnetic field when it is rotating.

The compensating or correction magnet 32 will create its own magnetic field signature thus providing a reduction I cancellation of the time invariant stray magnetic field or at least one component of the field in one position in space.

Figure 1 illustrates schematically the arrangement of the pump assembly 10 according to one embodiment. The pump assembly comprises a rotor 2 that is driven by a motor 12, 1 such that it rotates about axis 9. The rotor is mounted via rotor bearing magnets 30, that interact with stator bearing magnets 20 to maintain the rotor at a substantially fixed distance from the stator as it rotates around it.

The motor assembly comprises a motor stator magnet coils 12, comprising an annular ring housing a set of coils that is coaxially-aligned with a rotor magnet 1. The rotor 2 has a stem portion which receives the motor rotor magnet 1 and a head portion extending therefrom. The head portion defines a cylindrical, recessed, void portion on an end face of the rotor 2. A radially-inner surface 2B of the head portion or hub which defines the void receives a first part of a magnetic bearing assembly formed by an outer stack of rotor magnets 30. In this embodiment, the outer stack of rotor magnets 30 comprises three ring magnets stacked atop one another, with adjacent ring magnet having opposing polarities. An upper portion of the stator 4 receives a second part of the magnetic bearing assembly formed by an inner stack of stator magnets 20. In this embodiment, the inner stack of stator magnets 20 comprises three ring magnets stacked atop one another, with adjacent ring magnet having opposing polarities. However, it will be appreciated that more or fewer magnets may be provided to form the magnetic bearing assembly.

In operation, the rotor 2 is rotated at one end by the motor rotor magnet 1 in response to switching by the motor stator magnet coils 12 and is supported at the other end by the magnetic bearing assembly. Such an arrangement is particularly suited to turbomolecular pumps.

In this embodiment there is a correction magnet 32 mounted in a recess on the stator 4 towards the pump inlet. The correction magnet 32 is formed by an additive manufacturing technique and is configured to provide the function of correction magnet for the time invariant stray magnetic field.

Figure 3 shows the field reduction effect with the pump arrangement shown in Figure 1 , where the compensating magnet is positioned at the inlet. In this example the target is to reduce the magnetic field in the axis of the pump, 40 mm above the inlet. The plot shows the reduction of the total field in such position, which in this case is purely axial (i.e. the vertical component) compared to the case where there is no correction. In the desired position the field is orders of magnitude lower, less than 1/10 of the original field for a distance of ±5 mm from the targeted position. However, closer to the inlet, the field increases.

The compensation can in principle be improved by using a combination with more than one compensating magnets having different shapes, non co-located and/or with different magnetisations

Figure 4 illustrates a method of reducing the stray magnetic field according to an embodiment. At step S10 at least one time invariant component of a stray magnetic field generated at least in part by the magnetic bearings of a pump is determined at a point. This may be done by measuring the stray magnetic field or a vector component of this field at that point.

At step S20 the magnetic characteristics of one or more magnets to counteract the stray field is provided. This may be done by selecting the one or more magnets from a plurality of magnets with the desired magnetic characteristics, or it may be done by manufacturing the one or more magnet with the desired characteristics, perhaps by bonding powder, at least some of it being magnetic powder, to form the magnet(s). This bonding may be done using 3D printing or other additive manufacturing techniques. The one or more correction magnets are then mounted on the stator, at step S30, perhaps in a protective shell.

Figure 5 shows a flow chart schematically illustrating steps in a method of manufacturing a magnet to be used to counteract a stray magnetic field. The initial step S40 is the same as step S10 of the method of Figure 4, that is the at least one time invariant component of a stray magnetic field generated at least in part by the magnetic bearings of a pump is determined at a point. At step S50 the magnetic characteristics of a magnet which when mounted on the stator would counteract at least one component of the determined time invariant stray magnetic field is determined and then at step S60 a magnet with these characteristics is manufactured using additive manufacturing. Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

REFERENCE SIGNS

1 motor rotor magnet

2 rotor

2B inner diameter of rotor hub 4 pump stator

9 axis of rotation

10 pump

12 motor stator magnet coils

20 stator bearing magnets 30 rotor bearing magnets

32 correction magnet