阅读说明：本技术 涡轮分子泵和用于控制处理室中压力的方法及设备 (Turbo-molecular pump and method and apparatus for controlling pressure in a process chamber ) 是由 J.A.海洛克 C.M.贝利 N.P.肖菲尔德 于 2018-12-10 设计创作，主要内容包括：公开了一种涡轮分子泵以及用于使用这种泵控制处理室中压力的方法和设备。该涡轮分子泵包括：转子,其包括在可旋转轴上的多个叶片；控制马达,其用于驱动可旋转轴；其中控制马达尺寸设计成输出以足够高的恒定速度驱动轴以提供5毫巴的排气压力所需的功率的三倍以上的功率。(A turbomolecular pump is disclosed, as well as a method and apparatus for controlling pressure in a process chamber using such a pump. The turbomolecular pump 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 more than three times the power required to drive the shaft at a constant speed high enough to provide an exhaust pressure of 5 millibar.)

1. A turbomolecular pump comprising:

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 three or more times the power required to drive the shaft at a constant speed high enough to provide an exhaust pressure of 5 mbar.

2. The turbomolecular pump of claim 1, wherein the turbomolecular pump comprises a drag stage.

3. The turbomolecular pump of claim 1, wherein the turbomolecular pump does not comprise a drag stage.

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

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

6. A turbomolecular pump according to claim 4 or 5, wherein the skirt portion comprises a magnet.

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

8. The turbomolecular pump of any of claims 4 to 7, wherein the stator of the control motor is arranged inside the skirt portion and around the shaft.

9. The turbomolecular pump of any of claims 4 to 7, wherein the stator of the control motor is arranged around the skirt portion.

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

11. A turbomolecular pump according to any of the preceding claims, comprising a further motor for driving the shaft, the further motor and shaft forming a drive spindle, and the control motor being formed around the drive spindle.

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

13. A turbomolecular pump according to any of the preceding claims, wherein the pump comprises a control circuit for controlling the pressure at the inlet of the turbomolecular pump, the control circuit being configured to control the control motor to control the speed of the rotor, thereby controlling the pressure at the inlet of the turbomolecular pump.

14. The turbomolecular pump of claim 13, wherein the turbomolecular pump comprises an exhaust valve, and the control circuit is configured to control the exhaust valve to increase the pressure at the exhaust of the turbomolecular pump in conjunction with controlling the control motor to slow the rotor.

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

16. A turbomolecular pump comprising:

a rotor including a plurality of blades mounted on a rotatable shaft;

a control motor for driving the rotatable shaft; wherein

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

17. The turbomolecular pump of claim 16, wherein the skirt portion comprises a smaller cross section adjacent to the blades of the rotor and a larger cross section distal to the blades.

18. A turbomolecular pump according to claim 16 or 17, wherein the stator of the control motor is arranged inside the skirt portion and around the shaft.

19. The turbomolecular pump of any of claims 16 to 18, wherein the stator of the control motor is arranged around the skirt portion.

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

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

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

23. The turbomolecular pump of claim 21, wherein the turbomolecular pump comprises a holweck drag stage, the skirt portion comprising a skirt of the holweck drag stage.

24. A turbomolecular pump according to any of claims 15 to 23, wherein the skirt portion comprises a magnet.

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

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

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

28. A turbomolecular pump according to any of claims 16 to 27, wherein the pump comprises control circuitry for controlling the pressure at the inlet of the turbomolecular pump, the control circuitry being configured to control the control motor to control the speed of the rotor to control the pressure at the inlet of the turbomolecular pump.

29. The turbomolecular pump of claim 28, wherein the turbomolecular pump comprises an exhaust valve, and the control circuit is configured to control the exhaust valve to increase the pressure at the exhaust of the turbomolecular pump in conjunction with controlling the control motor to slow the rotor.

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

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

32. The apparatus of claim 31, wherein the turbomolecular pump is directly connected to the process chamber.

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

accelerating a turbomolecular pump connected to the process chamber in response to determining that the pressure in the process chamber is to be reduced; and

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

Said acceleration and deceleration of said turbomolecular pump is done by a control motor dimensioned to output more than three times the power required to drive the shaft at a constant speed high enough to provide an exhaust pressure of 5 mbar.

Technical Field

Background

Turbomolecular pumps providing high and ultra-high vacuum are known. These pumps are momentum transfer pumps in which the momentum of the gas molecules entering the pump is imparted by rotating rotor blades. The pump includes a plurality of angled pairs of rows of rotor and stator vanes, the vanes of the rows of rotor vanes being angled to push the gas molecules towards the discharge end of the pump.

Turbomolecular pumps are often used to evacuate process chambers, such as semiconductor process chambers, due to their ability to provide these high and ultra-high vacuums. Various arrangements have been proposed to control the pressure in such semiconductor processing chambers. In one such arrangement, a throttle valve is disposed between the outlet of the semiconductor processing chamber and the inlet of the turbomolecular pump. Such a throttle valve has cost and reliability issues and may even more importantly be the cause of contamination in the chamber, as particles leaving the chamber via the valve may hit the surface of the valve and bounce back into the chamber.

It is therefore desirable to be able to dispense with such a throttle valve and still be able to provide effective pressure control in such a chamber.

Disclosure of Invention

A first aspect provides a turbomolecular pump comprising: 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 more than three times the power required to drive the shaft at a constant speed high enough to provide an exhaust pressure of 5 mbar.

Turbomolecular pumps provide high and ultra-high vacuum by rotating at extremely high speeds. Furthermore, because they do not operate well at higher exhaust pressures, they often have drag stages (dragstages) integral with the turbomolecular pump and these drag stages have significant inertia and weight. Therefore, changing the rotational speed of such pumps requires very high energy and there is a technical prejudice to provide sufficient rotational speed control for such pumps to allow their use for controlling the pressure in the chamber.

However, the inventors of the present invention have realised that whilst the use of a turbomolecular pump to control the pressure in the process chamber is not easy due to the high rotational speed and inertia of the rotor, if a large control motor is provided for the turbomolecular pump, and not only is a slightly larger motor in this regard, but a significantly larger motor than is normally required to generate the necessary pressure differential, then such control may be feasible and the pump will be able to accelerate and decelerate fast enough to provide the required pressure control. In this regard, the motor for the turbomolecular pump is typically sized such that the exhaust pressure is kept at 5 mbar or less, since above this value the turbomolecular pump will not operate well. However, turbomolecular pumps are typically installed in confined spaces, and the size of the motor is similarly limited. Therefore, there is a technical prejudice for increasing the size of the motor, in particular by a significant amount.

However, the inventors have realised that if pressure control is provided by the pump, intermediate elements such as a throttle valve can be omitted and cost savings of these elements can be achieved and an improved system provided in which contamination due to bouncing of particles on the throttle valve is avoided.

It should be noted that the conventional size of the motor used in a turbomolecular pump depends on the pump and its pumping capacity. For any particular pump, the engineer will determine the size of the motor required to provide the required exhaust pressure, which will typically be below 5 mbar. In this case, a motor is provided which is 3 times as large, and in some cases 5 or even 10 times as large, as that required for normal operation of a typical pump of similar size, and in this way a sufficiently fine control of the inlet pressure is achieved without the need for a throttle valve. In this respect, the acceleration power (short-term power) of standard turbomolecular pumps and pumps according to the present application is about 1.5 times the maximum steady-state power. For example, a pump of 3000 liters/second typically requires about 1 kilowatt of steady state motor power. The turbomolecular pump of the present application provides a motor that provides 3 or more times the standard continuous power of a typical turbopump and 3 or more times the short term power. In this regard, the pump provides 3 times the power of a typical turbine pump for a given inlet speed, so for a 3000 liter/second pump, there is typically a 1 kilowatt motor, and thus, the pressure controlled pump motor would be 4.5 kilowatts or more.

In some embodiments, the turbomolecular pump comprises a drag stage, while in other embodiments it does not comprise a drag stage.

It should be noted that turbomolecular pumps are often provided with drag stages, since they do not operate well at higher exhaust pressures, and additional stages behind the pump reduce the exhaust pressure. However, they can also be operated with a separate backing pump connected to the exhaust of the turbomolecular pump, and when the drag stage is integral with the turbomolecular pump, it necessarily increases the weight and inertia of the rotor. In such a case, when it is desired to provide control of the inlet pressure by controlling the speed of the rotor, it may be advantageous to reduce the weight and inertia of the rotor by not having a drag stage, and to provide a system in which the inlet pressure can be more easily controlled and does not require such a large motor.

As previously mentioned, if the inlet pressure is to be controlled by the speed of the rotor, a larger motor is required than is typically used with turbomolecular pumps. One way of providing such a large motor, which provides direct drive of the rotor and which is still relatively compact, is to have the rotor of the pump include a skirt portion coaxial with and surrounding its shaft. The skirt portion may serve as a rotor for the motor, providing direct drive of the motor and providing an increased size of the motor rotor.

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

As previously mentioned, it may be advantageous for a turbomolecular pump to have a drag stage, although this increases the inertia of the rotor, but in the presence of 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 may be incorporated into the turbomolecular pump, while preserving the drag stage of the turbomolecular pump and providing an effective and efficient pumping system.

In some embodiments, the skirt portion comprises a magnet.

It should be noted that the skirt portion may be formed of metal, or it may be formed of carbon fiber. The magnets may be mounted on the skirt portion, allowing the skirt to be driven in response to the rotating magnetic field generated by the windings of the stator of the motor.

In some embodiments, the skirt portion includes a smaller cross-section adjacent the blades of the rotor and a larger cross-section distal the blades.

When the stator of the motor is incorporated within the skirt, it may be advantageous for the portion of the skirt where the stator is located to have a relatively large cross-section. In some cases, the skirt may be cylindrical and may be cylindrical with a relatively large cross-section, while in other cases the cross-section may be smaller near the vanes, thereby providing a larger path for the gas at this point, the cross-section being larger away from the vanes, the increase in volume providing increased space for the motor.

In some embodiments, the stator of the control motor is disposed within the skirt portion and around the shaft, while in other embodiments, the stator of the control motor is disposed around the skirt portion.

When the motor is outside the skirt, a skirt having a cylindrical shape and a relatively small cross-section may be advantageous, possibly similar to or slightly larger than the inner diameter of the rotor blade.

In some embodiments, the pump includes a pump housing, and the control motor is located within the pump housing.

Although the control motor may need to be of 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 that can fit within the pump housing.

In some embodiments, the turbomolecular pump may comprise only one motor, i.e. a control motor that provides acceleration and deceleration under steady state operation. In other embodiments, the turbomolecular pump comprises a further motor for driving the shaft, the further motor and shaft forming a drive spindle, and the control motor and skirt portion being formed around the drive spindle.

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

In some embodiments, the pump comprises control circuitry for controlling the pressure at the inlet of the turbomolecular pump, the control circuitry being configured to control the control motor to control the speed of the rotor, thereby controlling the pressure at the inlet of the turbomolecular pump.

As previously mentioned, providing a large control motor for the pump allows the rotational speed, and thus the inlet pressure, of the turbomolecular pump to be controlled accurately. Thus, a control circuit 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, the turbomolecular pump comprises an exhaust valve, and the control circuitry is configured to control the exhaust valve to increase the pressure at the exhaust of the turbomolecular pump in conjunction with controlling the control motor to slow the rotor.

In addition to controlling the speed of the rotor of the pump, it may be advantageous to have an exhaust valve on the turbomolecular pump, and this may be controlled in conjunction with slowing the rotor to increase the pressure at the exhaust of the turbomolecular pump, as this helps the rotor to slow down and increase the efficiency and reaction speed of the pump.

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

Additionally and/or alternatively, increasing the pressure at the exhaust port using a valve purge gas may be used to increase the pressure at the exhaust port and facilitate slowing of the rotor. In this regard, the valves are potentially expensive components, require maintenance, and may become contaminated. The addition of an inert purge gas inlet may be a more convenient way to control the pressure at the exhaust of the turbomolecular pump.

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

Providing pressure control at the inlet of a turbomolecular pump by controlling the rotational speed of the pump can be problematic due to the high speed of the turbomolecular pump and its corresponding inertia, such that the change in rotational speed requires high energy and is difficult to manage without significant delay. However, in the event that the control motor has sufficient power, then accurate control of the inlet pressure can be provided. Such a control motor can be provided in a compact manner by using a skirt portion on the rotor on the pump as the rotor of the control motor, so that direct drive of the rotor is provided by a motor having a rotor of significant size.

In some embodiments, the skirt portion includes a smaller cross-section adjacent the blades of the rotor and a larger cross-section distal the blades.

In some embodiments, the stator of the control motor is disposed inside the skirt portion and surrounds the shaft.

In some embodiments, the stator of the control motor is disposed about the skirt portion.

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

For the purposes of this patent application, substantially equal to within 10% of what is considered to be the inner diameter. In some embodiments, the skirt portion is adjacent to the most downstream blade of the turbomolecular pump and extends perpendicular to it away from the other blades.

In some embodiments, the turbomolecular pump comprises a drag stage.

In other embodiments, the turbomolecular pump does not comprise a drag stage.

In some embodiments, the turbomolecular pump comprises a holweck drag stage, the skirt portion comprising a skirt of the holweck drag stage.

In some embodiments, the skirt portion comprises a magnet.

In some embodiments, the pump includes a pump housing, and the control motor is located within the pump housing.

In some embodiments, the control motor comprises an annular shape having an inner diameter greater than a maximum inner diameter of the rotor blade and less than an outer diameter of the rotor blade, and an outer diameter greater than 90% of the diameter of the rotor blade.

In some embodiments, the control motor comprises an annular shape having an inner diameter greater than the maximum inner diameter of the rotor blade but less than 1.2 times the maximum inner diameter and an outer diameter less than or equal to the outer diameter of the rotor blade.

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

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

In some embodiments, the pump comprises control circuitry for controlling the pressure at the inlet of the turbomolecular pump, the control circuitry being configured to control the control motor to control the speed of the rotor, thereby controlling the pressure at the inlet of the turbomolecular pump.

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

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

In some embodiments, the turbomolecular pump is directly connected to the process chamber.

Controlling the pressure in the process chamber by controlling the rotational speed of a turbomolecular pump connected to the process chamber eliminates the need for intermediate pressure control such as a throttle valve. This reduces the cost of the equipment and also reduces maintenance requirements and contamination that may occur due to contaminants being expelled through the throttle valve and being impeded as they exit and possibly bouncing back into the process chamber.

A fourth aspect of the invention provides a method of controlling pressure in a process chamber, the method comprising: accelerating a turbomolecular pump connected to the process chamber in response to determining that the pressure in the process chamber is to be reduced; and in response to determining that the pressure in the process chamber is to be increased, decelerating a turbomolecular pump connected to the process chamber; wherein said acceleration and deceleration of said turbomolecular pump is accomplished by a control motor sized to output more than three times the power required to drive said shaft at a constant speed high enough to provide an exhaust pressure of 5 mbar.

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

Where a device feature is described as being operable to provide a function, it will be understood that this includes a device feature that provides the function or is adapted or constructed to provide the function.

Drawings

Embodiments of the invention will now be further described with reference to the accompanying drawings, in which:

figure 1 shows a turbomolecular pump without a drag stage according to a first embodiment;

FIG. 2 shows a turbomolecular pump according to a second embodiment;

FIG. 3 shows a turbomolecular pump according to a second embodiment, configured to evacuate a process chamber;

FIG. 4 illustrates a turbomolecular pump with a drag stage and a combined control motor, according to one embodiment; and

fig. 5 shows another embodiment of a turbomolecular pump with a holweck drag stage and a combined control motor.

Reference numerals

10 processing chamber

12 control circuit

14 exhaust valve

16 control motor

17 stator of control motor

18 control rotor of motor

22 pump rotor

23 rotor shaft

30 additional drive motor

34 magnets.

Detailed Description

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

A turbomolecular pump is provided that has a motor that is significantly larger than that typically required to operate a conventional turbomolecular pump of the same size. The larger motor serves to provide effective acceleration and deceleration of the pump's rotor, allowing it to be used to control the pressure at the inlet of the pump and hence the pressure of any chamber connected thereto.

The turbomolecular pump may have one large motor, or it may have a large motor and an additional motor similar to the ordinary drive motor on a conventional turbomolecular pump, for stable operation. The additional motor drives the shaft and is mounted within the drive spindle. Additional larger motors are mounted around the drive spindle and provide the acceleration and deceleration required for pressure control.

The rotor of a 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 the motor is a part, to rotate. The stator of the motor may be inside the rotor and in this way be protected from the process gas, or it may surround 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 a process chamber, having 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 circuit may additionally control a valve at the exhaust of the turbomolecular pump and/or the input of purge gas at or near this point.

Fig. 1 shows a turbomolecular pump without a drag stage, comprising a rotor 22 mounted on a shaft 23 and driven by a motor 16. The rotor 22 has a skirt portion 18 extending from a lower portion of the rotor, the skirt portion forming the rotor of the motor 16. The skirt portion 18 includes magnets that 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 the skirt portion 18. In this position helps to protect the stator of the motor from the process flow. In practice, motors with an inside out type arrangement are used, where the stator is provided inside the rotor. In this embodiment, in order to have sufficient space for the stator, the skirt portion 18 has an increased cross-section such that it meets the lower portion at a location near the inner diameter of the blade, but then extends outwardly to a larger diameter.

Fig. 2 shows a similar embodiment of the turbomolecular pump without a drag stage, but in this embodiment the stator of the motor is external to the skirt portion 18. In this case, the motor stator 17 may be larger than when it is inside the skirt portion, and thus a higher power motor may be provided, however, it may be contaminated by the process gas. It is advantageous if it is still fitted in the pump housing of the turbomolecular pump.

Fig. 3 shows the turbomolecular pump of fig. 2 evacuating a process chamber 10, in which process chamber 10, for example, semiconductor processing may be performed. A control circuit 12 is provided for controlling the pressure within the process chamber. The control circuit 12 controls the pressure within the process chamber 10 by controlling the motor 16 and, in this embodiment, the exhaust valve 14. Thus, the control circuit 12 will monitor the pressure in the process chamber 10 and also receive a control signal indicative of the desired pressure. It will then control the motor 16 to rotate the rotor 22 of the turbomolecular pump at a suitable speed to generate the required pressure. It may also control the exhaust valve 14 to increase the pressure at the exhaust port in the event that it is desired to increase the pressure in the process chamber and slow the pump down. Instead of an exhaust valve (not shown), a purge gas inlet for the input of an inert gas such as nitrogen may be used to increase the pressure at the exhaust of the turbomolecular pump.

The large size of the motor 16 and its use to directly drive the rotor through the skirt portion extending downwardly from the rotor blades enables sufficient power to be applied to accelerate and decelerate the rotor and enables the pressure within the process chamber to be precisely controlled, thereby eliminating the need for a throttle between the input of the pump and the process chamber, allowing the input to be wide and without constrictions. This improves performance and reduces contamination that may occur due to contaminants being blocked by the throttle valve.

Fig. 4 shows an alternative embodiment with a drag stage on the pump. Having a drag stage on the pump may improve performance but may increase the inertia of the rotor. Thus, the motor size required to provide the required acceleration and deceleration is larger than if no drag stage were present. However, it has been recognised that the skirt of the holweck drag stage may also have a dual function as a rotor for the motor, and in this case an efficient turbomolecular pump with a large control motor which provides effective pressure control in the process chamber may be provided. In the embodiment of fig. 4, the stator of the motor is within the skirt of the holweck drag stage and is therefore protected from the process gas. The holweck drag stage will include a magnet 34 allowing it to be driven by the rotating magnetic field of the stator of the motor. In this embodiment, in addition to having such a large control motor, an additional motor 30 is provided at a conventional location on the pump shaft 23, the motor 30 being used to drive the shaft 23 during steady state operation. The control motor 16, which is a large motor, provides acceleration and deceleration to control changes in the chamber pressure.

Fig. 5 shows an alternative embodiment in which the stator 17 of the motor is located outside the holweck skirt, which acts as the rotor of the motor. This is similar to the embodiment of fig. 4 and for fig. 4 there is an additional motor 30 mounted on the shaft 23. It will be appreciated that in some embodiments there may be no additional motor 30 and that 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 illustrated embodiment the motors each have a rotor comprising a skirt portion extending from the pump motor, other pump motors may be used as long as they are sufficiently large, i.e. three or more times larger than a conventionally sized motor.

Although illustrative embodiments of the present invention have been disclosed in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, 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.