EP3724510A1

EP3724510A1 - A turbomolecular pump and method and apparatus for controlling the pressure in a process chamber

Info

Publication number: EP3724510A1
Application number: EP18819358.5A
Authority: EP
Inventors: James Alexander Haylock; Christopher Mark Bailey; Nigel Paul Schofield
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2017-12-12
Filing date: 2018-12-10
Publication date: 2020-10-21
Also published as: WO2019116012A1; US20200347851A1; CN111527311A; CN111527311B; GB2569314A; GB201720705D0

Abstract

A turbomolecular pump and method and apparatus for controlling the pressure in a process chamber using such a pump is disclosed. The turbomolecular comprises: a rotor comprising a plurality of blades on a rotatable shaft; a control motor for driving the rotatable shaft; wherein the control motor is sized to output a power that is more than three times larger than a power required to drive the shaft at a constant velocity that is sufficiently high to provide an exhaust pressure of 5 millibars.

Description

A TURBOMOLECULAR PUMP AND METHOD AND APPARATUS FOR CONTROLLING THE PRESSURE IN A PROCESS CHAMBER

FIELD OF THE INVENTION

The field of the invention relates to turbomolecular pumps and to a method and apparatus for controlling the pressure in a process chamber using such a turbomolecular pump.

BACKGROUND

Turbomolecular pumps which provide high and ultrahigh vacuums are known. These pumps are momentum transfer pumps in which gas molecules entering the pump are given momentum by rotating rotor blades. The pump comprises multiple angled rotor and stator blade row pairs, the blades of the rotor blade rows are angled to push the gas molecules towards the exhaust end of the pump.

Owing to their ability to provide these high and ultrahigh vacuums

turbomolecular pumps are often used to evacuate process chambers such as semiconductor processing chambers. Various arrangements have been proposed for controlling the pressure in such a semiconductor process chamber. In one such arrangement, a throttle valve is provided between the outlet of the semiconductor processing chamber and the inlet of the

turbomolecular pump. Such throttle values have cost and reliability issues and perhaps even more importantly can be a cause of contamination in the chamber as particles exiting the chamber via the valves may hit surfaces of the valves and bounce back into the chamber.

It would therefore be desirable to be able to dispense with such throttle valves and yet still be able to provide effective pressure control in such chambers.

SUMMARY

A first aspect provides a turbomolecular pump comprising: a rotor comprising a plurality of blades on a rotatable shaft; a control motor for driving said rotatable shaft; wherein said control motor is sized to output a power that is more than three times larger than a power required to drive said shaft at a constant velocity that is sufficiently high to provide an exhaust pressure of 5 millibars.

Turbomolecular pumps provide high and ultrahigh vacuums by rotating at extremely high speeds. Furthermore, as they do not operate well with higher exhaust pressures they often have drag stages integral with the turbomolecular pump and these have a significant inertia and weight. Thus, changing the speed of rotation of such a pump requires very high energy and there is a technical prejudice against providing such pumps with sufficient rotational speed control to allow this to be used to control pressure in a chamber.

However, the inventors of the present invention recognised that although controlling the pressure in a process chamber using a turbomolecular pump is not straightforward owing to the high speed of rotation and the inertia of the rotor, were the turbomolecular pump to be provided with a large control motor and in this regard not just a slightly larger motor but one that was significantly larger than the motor generally required to generate the necessary pressure differences, then such a control might be possible and the pump would be able to be accelerated and decelerated sufficiently fast to provide the pressure control needed. In this regard, a motor for a turbomolecular pump is generally sized such that the exhaust pressure is kept to 5m illibars or below as above this the turbomolecular pump will not operate well. However, turbomolecular pumps are often mounted in constrained spaces and the size of the motor is similarly constrained. Thus, there is a technical prejudice against increasing the size of the motor and in particular of increasing it by a significant amount.

However, the inventors recognised that if the pressure control were provided by the pump, intermediate elements such as throttle valves could be dispensed with and a cost saving of these elements would be made as well as an improved system provided, in which contamination due to bounce back of particles on the throttle valve is avoided. It should be noted that the conventional size of motor used in a turbomolecular pump is dependent on the pump and on its pumping capacity. For any particular pump an engineer would determine the size of motor required to provide the required exhaust pressure which would generally be below 5 millibars. In this case, a motor that is 3 times larger than that required for normal operation of a typical pump of a similar size is provided and in some cases 5 times larger or even 10 times larger and in this way sufficiently fine control of the inlet pressure is achieved without the need to have a throttle valve. In this regard, the acceleration power (short term power) for a standard turbomolecular pump and for a pump according to the application is about 1.5 times the maximum steady state power. For example a 3000 I/s pump would conventionally require a steady state motor power of about 1 KW. The turbomolecular pump of the application provides a motor that provides 3 or more times the standard continuous power of a typical turbo pump and 3 or more times the short term power. In this regard the pump provides 3 times the power for a typical turbo pump of a given inlet speed, so typically one has a 1 KW motor for a 30001/s pump and thus, the motor of the pressure control pump would be 4.5KW or more.

In some embodiments, said turbomolecular pump comprises a drag stage while in other embodiments it comprises no drag stage.

It should be noted that turbomolecular pumps are often provided with drag stages as they do not function well with higher exhaust pressures and an additional stage on the back of the pump reduces the exhaust pressure.

Flowever, they could also be operated with separate backing pumps connected to the exhaust of the turbomolecular pump and where a drag stage is integral with the turbomolecular pump it does necessarily increase the weight and inertia of the rotor. In this case, where it is desired to provide control of the inlet pressure by controlling the speed of the rotor, reducing the weight and inertia of the rotor by having no drag stage may be advantageous and provide a system where the inlet pressure can be controlled more easily and without the requirement for such a large motor. As noted previously, a larger motor than is generally used for turbomolecular pumps is required if the inlet pressure is to be controlled by the speed of the rotor. One way of providing such a large motor that provides direct driving of the rotor and yet is still relatively compact is for the rotor of the pump to comprise a skirt portion which is coaxial with its shaft and surrounds the shaft. This skirt portion can be used as the rotor of the motor providing direct driving of the motor and providing an increased sized motor rotor.

In some embodiments, said turbomolecular pump comprises a Holweck drag stage, said skirt portion comprising a skirt of said Holweck drag stage.

As noted previously, it may be advantageous for turbomolecular pumps to have a drag stage and although this increases the inertia of the rotor, where there is such a stage it may be advantageous to use the skirt of the drag stage as the rotor of the control motor. In this way, a large motor can be incorporated into the turbomolecular pump while retaining the drag stage of the turbomolecular pump and providing an effective and efficient pumping system.

In some embodiments, said skirt portion comprises magnets.

It should be noted that the skirt portion may be formed of metal or it may be formed of carbon fibre. Magnets may be mounted on the skirt portion allowing the skirt to be driven in response to the rotational magnetic field produced by the windings of the stator of the motor.

In some embodiments, said skirt portion comprises a smaller cross section adjacent to said blades of said rotor and a larger cross section remote from said blades. Where the stator of the motor is incorporated within the skirt, then it may be advantageous to have a relatively large cross section for the portion of the skirt where the stator is located. In some cases the skirt may be a cylinder and may be a cylinder with a relatively large cross section, while in others the cross section may be smaller adjacent to the blades to provide a larger path for the gas at this point, the cross section being larger remote from the blades the increase in volume providing an increased space for the motor.

In some embodiments, said stator of said control motor is arranged inside said skirt portion and around said shaft, while in other embodiments, said stator of said control motor is arranged around said skirt portion.

Where the motor is outside of the skirt, then a skirt with cylinder shape and a relatively small cross section may be advantageous, perhaps similar or slightly larger than that of the inner diameter of the rotor blades.

In some embodiments, said pump comprises a pump housing and said control motor is located within said pump housing. Although, the control motor may need to be of a relatively large size in order to provide the acceleration and deceleration required to control the pressure in the process chamber it may be advantageous if it is of a size where it can fit within the pump housing. In some embodiments, the turbomolecular pump may only comprise one motor, that is the control motor which provides the acceleration and the deceleration under steady state operation. In other embodiments, said turbomolecular pump comprises a further motor for driving said shaft said further motor and shaft forming a drive spindle and said control motor and skirt portion being formed around said drive spindle.

In some embodiments, said further motor is configured to drive said rotor during steady state operation and said control motor is configured to drive said rotor during acceleration and deceleration of said rotor of said turbomolecular pump.

In some embodiments, said pump comprises control circuitry for controlling a pressure at an inlet of said turbomolecular pump, said control circuitry being configured to control said control motor to control a speed of said rotor and thereby a pressure at an inlet of said turbomolecular pump. As noted previously, providing the pump with a large control motor allows the speed of rotation to be accurately controlled and thereby the inlet pressure of the turbomolecular pump. Thus, control circuitry may be provided to control the inlet pressure of the pump and in this way the pressure in any chamber being evacuated by controlling the speed of the control motor.

In some embodiments, said turbomolecular pump comprises an exhaust valve, said control circuitry being configured to control said exhaust valve to increase pressure at an exhaust of said turbomolecular pump in conjunction with controlling said control motor to slow said rotor.

In addition to controlling the speed of the rotor of the pump it may be

advantageous if there is an exhaust valve on the turbomolecular pump and this may be controlled to increase the pressure at the exhaust of the turbomolecular pump in conjunction with slowing the rotor, as this helps the rotor to slow and improves the efficiency and speed of reaction of the pump.

Additionally and/or alternatively, the turbomolecular pump may comprise a purge gas inlet at the exhaust, wherein the control circuitry is configured to control addition of purge gas to the exhaust in conjunction with controlling the control motor to slow the rotor.

Additionally and/or alternatively to increasing the pressure at the exhaust using a valve purge gas may be used to increase the pressure there and help in the slowing of the rotor. In this regard, a valve is a component that can be expensive, requires maintenance and may become contaminated. The addition of an inert purge gas inlet may be a more convenient way of controlling pressure at the exhaust of the turbomolecular pump.

A second aspect of the present invention provides a turbomolecular pump comprising: a rotor comprising a plurality of blades mounted on a rotatable shaft; a control motor for driving said rotatable shaft; wherein said rotor further comprises a skirt portion extending in an axial direction beyond said blades of said rotor and having an annular cross section, said skirt portion forming a rotor of said control motor and being coaxial with and surrounding said shaft.

Providing pressure control at an inlet of a turbomolecular pump by controlling the speed of rotation of the pump can be problematic due to the high speed of such a pump and its corresponding inertia such that changes in the speed of rotation require high energy and are difficult to manage without significant delay. However, where a control motor is provided with sufficient power then accurate control at the inlet pressure may be provided. Such a control motor may be provided in a compact way by using a skirt portion on the rotor on the pump as a the rotor of the control motor such that direct driving of the rotor is provided by a motor with a significant sized rotor.

In some embodiments, said skirt portion comprises a smaller cross section adjacent to said blades of said rotor and a larger cross section remote from said blades.

In some embodiments, said stator of said control motor is arranged inside said skirt portion and around said shaft.

In some embodiments, said stator of said control motor is arranged around said skirt portion.

In some embodiments, said skirt portion has an inner diameter substantially equal to an inner diameter of the most downstream blade of said turbomolecular pump.

For the sake of this patent application substantially equal is taken to be within 10% of the inner diameter. In some embodiments, the skirt portion is adjacent to the most downstream blade of turbomolecular pump and extends

perpendicular to it away from the other blades.

In some embodiments, said turbomolecular pump comprises a drag stage. ln other embodiments, said turbomolecular pump does not comprise a drag stage.

In some embodiments, said turbomolecular pump comprises a Holweck drag stage, said skirt portion comprising a skirt of a Holweck drag stage.

In some embodiments, said skirt portion comprises magnets.

In some embodiments, said pump comprises a pump housing and said control motor is located within said pump housing.

In some embodiments, said control motor comprises a ring shape, an inner diameter of which is larger than a largest inner diameter of said rotor blades and smaller than an outer diameter of said rotor blades, and an outer diameter of which is greater than 90% of a diameter of said rotor blades.

In some embodiments, said control motor comprises a ring shape, an inner diameter of which is larger than a largest inner diameter of said rotor blades but smaller than 1.2 times said largest inner diameter, and an outer diameter of which is less than or equal to an outer diameter of said rotor blades.

In some embodiments said turbomolecular pump comprises: a further motor for driving said shaft, said further motor and shaft forming a drive spindle and said control motor and skirt portion being formed around said drive spindle.

In some embodiments, said motor is configured to drive said rotor during steady state operation and said control motor is configured to drive said rotor during acceleration and deceleration of said rotor of said turbomolecular pump.

In some embodiments, said pump comprises control circuitry for controlling a pressure at an inlet of said turbomolecular pump, said control circuitry being configured to control said control motor to control a speed of said rotor and thereby a pressure at an inlet of said turbomolecular pump. ln some embodiments, said turbomolecular pump comprises an exhaust valve, said control circuitry being configured to control said exhaust valve to increase pressure at an exhaust of said turbomolecular pump in conjunction with controlling said control motor to slow said rotor.

In some embodiments, said turbomolecular pump comprises a turbomolecular pump according to any preceding claim, the turbomolecular pump being connected to said process chamber, said pressure of said process chamber being controlled by controlling a speed of rotation of said turbomolecular pump.

A third aspect provides an apparatus for controlling the pressure in a process chamber, the apparatus comprising a turbomolecular pump according to any preceding claim, the turbomolecular pump being connected to said process chamber, said pressure of said process chamber being controlled by a controller configured to control a speed of rotation of said turbomolecular pump.

In some embodiments, said turbomolecular pump is connected directly to said process chamber. Controlling the pressure in the process chamber controlling the speed of rotation of the turbomolecular pump connected to the chamber removes the need for intermediate pressure control such as a throttle valve. This reduces the cost of the apparatus and also reduces the service requirements and contamination that might occur due to contaminates exiting via the throttle valve and being impeded in their exit and perhaps being bounced back into the process chamber.

A fourth aspect of the present invention provides a method of controlling the pressure in a process chamber, the method comprising: in response to determining a pressure is to be decreased in said process chamber and accelerating a turbomolecular pump connected to said process chamber; and in response to determining a pressure is to be increased in said process chamber decelerating a turbomolecular pump connected to said process chamber;

wherein said acceleration and deceleration of said turbomolecular pump is done by a control motor that is sized to output a power that is more than three times larger than a power required to drive said shaft at a constant velocity that is sufficiently high to provide an exhaust pressure of 5 millibars. 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 turbomolecular pump without a drag stage according to first embodiment;

Figure 2 illustrates a turbomolecular pump according to a second embodiment; Figure 3 illustrates a turbomolecular pump according to the second embodiment configured to evacuate a process chamber;

Figure 4 illustrates a turbomolecular pump with a drag stage and combined control motor according to an embodiment; and

Figure 5 illustrates a further embodiment of a turbomolecular pump with

Flolweck drag stage and combined control motor.

DESCRIPTION OF THE EMBODIMENTS

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

A turbomolecular pump with a motor that is significantly larger than is generally required to operate the same sized conventional turbomolecular pump is provided. The larger motor is used to provide effective acceleration and deceleration of the rotor of the pump allowing it to be used to control the pressure at the inlet of the pump and thus, the pressure of any chamber to which it is connected.

The turbomolecular pump may have one large motor or it may have a large motor and an additional motor similar to the usual drive motor on a conventional turbomolecular pump, such an additional motor being used for steady operation. This additional motor drives the shaft and is mounted within a drive spindle.

The additional larger motor is mounted around the drive spindle and provides the acceleration and deceleration required for pressure control.

The rotor of the large control motor may be a skirt extending from the rotor and comprising magnets such that the rotating magnetic field generated by the stator of the motor causes the rotor of the motor and thus, the rotor of the pump of which it is a part, to rotate. The stator of the motor may be within the rotor and in this way be protected from the process gases, or it may be around the rotor in which case it may have a significantly increased size.

A method and apparatus for controlling the pressure in a process chamber is also provided using such a turbomolecular pump connected to the process chamber with control circuitry operable to determine the pressure in the chamber and the required pressure and to control the speed of the pump accordingly. The control circuitry may additionally control a valve at the exhaust of the turbomolecular pump and/or the input of purge gas at or close to this point.

Figure 1 shows a turbomolecular pump without a drag stage comprising a rotor

22 mounted on shaft 23 and driven by motor 16. The rotor 22 has a skirt portion 18 extending from the lower part of the rotor, the skirt portion forming the rotor of motor 16. The skirt portion 18 comprises magnets which cause the rotor to rotate under the rotating magnetic field generated by the motor stator 17. In this embodiment, the motor stator is located outside and around the shaft

23 of the rotor but within skirt portion 18. Being in this position helps protect the stator of the motor from the process flow. In effect a motor that has an inside out type arrangement where the stator is provided within the rotor is used. In order for there to be sufficient space for the stator in this embodiment the skirt portion 18 has an increasing cross section such that it meets the lower part at a place close to the inner diameter of that blade, but then extends out to a larger diameter.

Figure 2 shows a similar embodiment of a turbomolecular pump without a drag stage but in this embodiment the stator of the motor is outside of the skirt portion 18. In this case, the motor stator 17 can be larger than when it is within the skirt portion and thus, a higher power motor may be provided, however, it may be subject to contamination by the process gases. It is advantageous if it still fits within the pump housing of the turbomolecular pump.

Figure 3 shows the turbomolecular pump of Figure 2 evacuating a process chamber 10 in which for example semiconductor processing may be performed. Control circuitry 12 is provided for controlling the pressure inside the process chamber. The control circuitry 12 controls the pressure inside process chamber 10 by controlling motor 16 and also in this embodiment exhaust valve 14. Thus, control circuitry 12 will monitor the pressure in process chamber 10 and also receive control signals indicating the required pressure. It will then control motor 16 to rotate rotor 22 of the turbomolecular pump at a suitable speed to generate the pressure required. It may also control exhaust valve14 to increase the pressure at the exhaust where it is required that the pressure in the process chamber increases and the pump is being slowed. As an alternative to the exhaust valve (not shown) a purge gas inlet for inputting an inert gas such as nitrogen may be used to increase the pressure at the exhaust of the

turbomolecular pump.

The large size of motor 16 and the use of it to directly drive the rotor via the skirt portion extending down from the rotor blades enables sufficient power to be imparted to accelerate and decelerate the rotor and enable the pressure within the process chamber to be accurately controlled and thereby avoid the need for a throttle valve between the input of the pump and the process chamber allowing this input to be wide and without constrictions. This improves performance and reduces contamination that might arise due to contaminates being impeded by the throttle valve.

Figure 4 shows an alternative embodiment with a drag stage on the pump. Having a drag stage on the pump may improve performance but increases the inertia of the rotor. Thus, the size of motor required to provide the required acceleration and deceleration is larger than when no drag stage is present. However, it has been recognised that the skirt of the Holweck drag stage may also have a dual function as the rotor of the motor and in such case an efficient turbomolecular pump with a large control motor that provides effective pressure control in a process chamber can be provided. In the embodiment of Figure 4 the stator of the motor is within the skirt of the Holweck drag stage and as such is protected from the process gases. The Holweck drag stage will comprise magnets 34 allowing it to be driven by the rotating magnetic field of the stators of the motor. In this embodiment, in addition to having this large control motor there is an additional motor 30 provided on the conventional position on the shaft 23 of the pump which acts to drive the shaft 23 during steady state operations. The control motor 16 which is a larger motor provides the acceleration and deceleration for controlling changes in the pressure within the process chamber.

Figure 5 shows an alternative embodiment where the stator 17 of the motor is outside the Holweck skirt which acts as the rotor of the motor. This is similar to embodiment of Figure 4 and as for Figure 4 has the additional motor 30 mounted on the shaft 23. It will be appreciated that the additional motor 30 may not be present in some embodiments and the driving of the rotor is performed entirely by the control motor both in steady state and during acceleration and deceleration. It should be noted that although in the embodiments shown the motors have each had a rotor comprising a skirt portion extending from the pump motor, other pump motors may be used provided they are sufficiently large, that is three or more times larger than a conventionally sized motor. 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

10 process chamber 12 control circuitry

14 exhaust valve

16 control motor

17 stator of control motor

18 rotor of control motor

22 rotor of pump

23 rotor shaft

30 additional drive motor

34 magnets

Claims

1. A turbomolecular pump comprising:

a rotor comprising a plurality of blades on a rotatable shaft;

a control motor for driving said rotatable shaft; wherein

said control motor is sized to output a power that is three or more times larger than a power required to drive said shaft at a constant velocity that is sufficiently high to provide an exhaust pressure of 5 millibars.

2. A turbomolecular according to claim 1.wherein said turbomolecular pump comprises a drag stage.

3. A turbomolecular according to claim 1.wherein said turbomolecular pump does not comprise a drag stage.

4. A turbomolecular pump according to any preceding claim, said rotor further comprising a skirt portion extending in an axial direction beyond said blades of said rotor and having an annular cross section coaxial with and surrounding said shaft, said skirt portion forming a rotor of said control motor.

5. A turbomolecular pump according to claim 4 when dependent on claim 2, wherein said turbomolecular pump comprises a Holweck drag stage, said skirt portion comprising a skirt of said Holweck drag stage.

6. A turbomolecular pump according to claim 4 or 5, wherein said skirt portion comprises magnets.

7. A turbomolecular pump according to any one of claims 4 to 6, wherein said skirt portion comprises a smaller cross section adjacent to said blades of said rotor and a larger cross section remote from said blades.

8. A turbomolecular pump according to any one of claims 4 to 7, wherein said stator of said control motor is arranged inside said skirt portion and around said shaft.

9. A turbomolecular pump according to any one of claims 4 to 7, wherein said stator of said control motor is arranged around said skirt portion.

10. A turbomolecular pump according to any preceding claim, wherein said pump comprises a pump housing and said control motor is located within said pump housing.

11. A turbomolecular pump according to any preceding claim, said

turbomolecular pump comprising a further motor for driving said shaft, said further motor and shaft forming a drive spindle and said control motor being formed around said drive spindle.

12. A turbomolecular pump according to claim 11 , wherein said further motor is configured to drive said rotor during steady state operation and said control motor is configured to drive said rotor during acceleration and deceleration of said rotor of said turbomolecular pump.

13. A turbomolecular pump according to any preceding claim, wherein said pump comprises control circuitry for controlling a pressure at an inlet of said turbomolecular pump, said control circuitry being configured to control said control motor to control a speed of said rotor and thereby a pressure at an inlet of said turbomolecular pump.

14. A turbomolecular pump according to claim 13, wherein said

turbomolecular pump comprises an exhaust valve, said control circuitry being configured to control said exhaust valve to increase pressure at an exhaust of said turbomolecular pump in conjunction with controlling said control motor to slow said rotor.

15. A turbomolecular pump according to claim 13 or 14, said turbomolecular pump comprising a purge gas inlet at an exhaust, wherein said control circuitry is configured to control addition of a purge gas at said exhaust in conjunction with controlling said control motor to slow said rotor.

16. A turbomolecular pump comprising:

a rotor comprising a plurality of blades mounted on a rotatable shaft; a control motor for driving said rotatable shaft; wherein

said rotor further comprises a skirt portion extending in an axial direction beyond said blades of said rotor and having an annular cross section, said skirt portion forming a rotor of said control motor and being coaxial with and surrounding said shaft.

17. A turbomolecular pump according to claim 16, wherein said skirt portion comprises a smaller cross section adjacent to said blades of said rotor and a larger cross section remote from said blades.

18. A turbomolecular pump according to claim16 or 17, wherein said stator of said control motor is arranged inside said skirt portion and around said shaft.

19. A turbomolecular pump according to any one of claims 16 to 18, wherein said stator of said control motor is arranged around said skirt portion.

20. A turbomolecular pump according to any one of claims 16 to 19, wherein said skirt portion has an inner diameter substantially equal to an inner diameter of the most downstream blade of said turbomolecular pump.

21. A turbomolecular pump according to any one of claims 16 to 20, wherein said turbomolecular pump comprises a drag stage.

22. A turbomolecular according to any one of claims 16 to 20, wherein said turbomolecular pump does not comprise a drag stage.

23. A turbomolecular pump according to claim 21 , wherein said

turbomolecular pump comprises a Holweck drag stage, said skirt portion comprising a skirt of a Holweck drag stage.

24. A turbomolecular pump according to any one of claims 15 to 23, wherein said skirt portion comprises magnets.

25. A turbomolecular pump according to any one of claims 15 to 24, wherein said pump comprises a pump housing and said control motor is located within said pump housing.

26. A turbomolecular pump according to any one of claims 15 to 25, said turbomolecular pump comprising a further motor for driving said shaft, said further motor and shaft forming a drive spindle and said control motor and skirt portion being formed around said drive spindle.

27. A turbomolecular pump according to claim 26, wherein said motor is configured to drive said rotor during steady state operation and said control motor is configured to drive said rotor during acceleration and deceleration of said rotor of said turbomolecular pump.

28. A turbomolecular pump according to any one of claims 16 to 27, wherein said pump comprises control circuitry for controlling a pressure at an inlet of said turbomolecular pump, said control circuitry being configured to control said control motor to control a speed of said rotor and thereby a pressure at an inlet of said turbomolecular pump

29. A turbomolecular pump according to claim 28, wherein said

30. A turbomolecular pump according to claim 28 or 29, said turbomolecular pump comprising a purge gas inlet at said exhaust, wherein said control circuitry is configured to add a purge gas at said exhaust in conjunction with controlling said control motor to slow said rotor.

31. An apparatus for controlling the pressure in a process chamber, the apparatus comprising a turbomolecular pump according to any preceding claim, the turbomolecular pump being connected to said process chamber, said pressure of said process chamber being controlled by a controller configured to control a speed of rotation of said turbomolecular pump.

32. An apparatus according to claim 31 , wherein said turbomolecular pump is connected directly to said process chamber.

33. A method of controlling the pressure in a process chamber, the method comprising:

in response to determining a pressure is to be decreased in said process chamber accelerating a turbomolecular pump connected to said process chamber; and

in response to determining a pressure is to be increased in said process chamber decelerating a turbomolecular pump connected to said process chamber; wherein

said acceleration and deceleration of said turbomolecular pump is done by a control motor that is sized to output a power that is more than three times larger than a power required to drive a shaft at a constant velocity that is sufficiently high to provide an exhaust pressure of 5 millibars.