FIELD OF THE INVENTION

The field of the invention relates to a vacuum pump with a turbomolecular stage and a drag stage.

BACKGROUND

Turbomolecular pumps are used to provide high vacuums, for example to provide the high vacuum required for semiconductor processing. They are expensive pumps designed for operation at high tip speeds. Their rotors are rotatably mounted on magnetic bearings to avoid the need for lubrication and to reduce vibrations, allowing clean room operation.

Turbomolecular pumps do not exhaust to atmosphere as they do not operate well at higher pressures and so generally these pumps have some form of backing pump stages to decrease the pressure at the exhaust of the turbo stages. These backing stages generally comprise a drag stage downstream of the

turbomolecular stage or stages, integrated within the pump and mounted on the same shaft. The pump may also have additional backing pump(s) remote from and connected to the vacuum pump.

There is an increasing desire to operate turbomolecular pumps at higher temperatures. Semiconductor processes for example require pumps to be maintained at high temperatures to prevent process by-products from

condensing. The temperature of a pump and the risk of condensates forming increases as the gases flow through the pumping system and pressures increase. Conventionally the rotors of turbomolecular pumps have been cast from Aluminium, with the drag and turbo stages being cast as one unit which provides a structurally robust rotor suitable for rotation at high speeds.

Aluminium loses much of its strength above 130°C and this limits turbo pump operation to temperatures at or below 130°C. It would be desirable to provide a vacuum pump with a turbo and drag stage that is suitable for at least partial higher temperature operation.

SUMMARY

A first aspect provides a vacuum pump comprising a turbomolecular stage and a drag stage, said vacuum pump comprising a stator and a rotor, said rotor comprising a turbomolecular rotor and a drag rotor attached together; wherein said turbomolecular rotor comprises a hub from which a plurality of blades extend, said hub comprising a mounting portion for mounting to a spindle of a motor and a hollow cylindrical portion, said hollow cylindrical portion extending from said mounting portion towards an outlet end of said turbomolecular stage; and said drag rotor comprises a cylindrical skirt and an attachment part extending away from said cylindrical skirt, said attachment part extending within said hollow cylindrical portion of said hub of said turbomolecular rotor and being attached thereto at a point that is closer to said mounting portion than to said outlet end of said turbomolecular rotor.

The inventors of the present invention recognised that as the pressure increases through a turbomolecular pump so too does the risk of condensation of process gases. Thus, although there is a desire to operate pumps at increasingly higher temperatures to avoid condensation, this problem is more acute in the drag stage than it is in the turbo stage. Thus, one way to reduce condensation problems within such a vacuum pump might be to operate the two stages at different temperatures with some degree of thermal isolation between the two stages. Thus, the vacuum pump of embodiments is formed with a rotor made in two parts, the rotor of the drag stage being attached to the rotor of the turbo stage via an attachment part that extends longitudinally away from the skirt of the drag rotor and up into the inner hub of the turbo rotor. The attachment part can then be attached at a point that is remote from the outlet end of the turbo stage, such that the main thermal path between the hotter drag stage and the cooler turbo stage is via this attachment piece and through the point of attachment. This reduces the thermal conductivity between the two parts of the rotors and allows the two rotor parts to operate at different temperatures, such that the drag stage may be operated at a higher temperature than the turbo stage and condensation at the higher pressures within this stage is reduced.

Forming the rotors in two pieces also allows different materials to be selected for the two pieces so that materials with properties suitable for higher temperature operation can be selected for the drag stage rotor, while those more suitable for high tip speeds can be selected for the turbo rotor.

In some embodiments, said drag rotor is formed of a material that is resistant to higher temperatures than a material forming said turbomolecular rotor.

The drag stage of a turbomolecular pump operates at a higher pressure than the turbomolecular stage and it may be desirable to run it at a hotter temperature. Where the drag and turbomolecular rotor are formed of different parts attached together there is an opportunity to form them of different materials.

Conventionally the drag and turbomolecular rotor has been cast as a single piece and as such has been constrained to be formed of the same material. Forming the rotor in two parts provides greater flexibility in the choice of materials allowing the drag rotor to be formed of a material that is more resistant to higher temperatures than the material forming the turbomolecular rotor.

Additionally and/or alternatively said drag rotor is formed of a material with a lower thermal conductivity than a material forming said turbomolecular rotor.

As the vacuum pump is configured to allow the drag stage to operate at a higher temperature than the turbo stage to reduce condensation in the drag stage, it is advantageous if the hotter drag rotor is thermally isolated, at least to some extent, from the turbomolecular rotor to reduce heat flowing from the drag rotor to the turbomolecular rotor. Thus, it may advantageous to make the drag rotor of a material with a low thermal conductivity, in some embodiments of a material with a lower thermal conductivity than the material forming the turbomolecular rotor. Although the drag rotor may be formed of a number of materials, in some embodiments the drag rotor is formed of steel. Steel is a robust material that is resistant to high temperatures and is relatively easy to cast and is also relatively inexpensive.

In some embodiments, said drag rotor is formed of stainless steel.

Stainless steel may make a particularly effective material for forming the drag rotor having a particularly low thermal conductivity of about 18 W/mK and being resistant to corrosion and higher temperatures. In this regard, both steel and stainless steel can operate at temperatures up to 300°C.

In some embodiments, said turbomolecular rotor is formed of Aluminium.

Turbomolecular rotors are conventionally formed of Aluminium which has a low density, and is therefore suitable for the high tip speeds that turbomolecular rotors operate at, it is also robust and can be cast. Aluminium does however have a significantly higher thermal conductivity than steel or stainless steel having a thermal conductivity of 200W/mK. Thus, although it is suitable for a turbomolecular rotor, being able to form the drag rotor of a different material that is both more thermally resistant and has a lower thermal conductivity allows the drag and turbo stages of the pump to operate at different temperatures, allowing the turbomolecular rotor to stay at a lower temperature suitable for Aluminium while the drag rotor operates at a higher temperature that reduces condensation. In this regard, if aluminium operates at a temperature in excess of 130°C then it starts to lose its strength.

In some embodiments, said attachment part is attached to said mounting portion of said turbomolecular rotor. Although the attachment part can be mounted to different parts of the

turbomolecular rotor provided they are not too close to the outlet end thereby providing some thermal isolation, it may be particularly advantageous to attach the attachment part to the mounting portion of the turbo rotor this being remote from the outlet. This allows the attachment part to be particularly long and also provides a suitable surface for attaching the attachment part.

In this regard, said mounting portion extends substantially parallel to said blades of said turbomolecular rotor and perpendicular to said cylinder.

As the mounting portion is perpendicular to the cylinder it forms a convenient surface for attachment of the attachment part of the drag rotor.

In some embodiments, said attachment part has a thermal conductivity of less than 50W/mK, preferably less than 20W/mK.

Providing an attachment part with a low thermal conductivity allows the turbomolecular rotor to be maintained at a significantly lower temperature than the drag rotor. This is important as the turbomolecular rotor operates at a particularly high vacuum so that removing heat from this portion of the pump is not easy. Thus, if the two portions of the rotor are to be maintained at significantly different temperatures thermal conductivity between the two must be kept low.

In some embodiments, said attachment part is thin and has a thickness of 3mm or less.

In order to reduce the thermal conductivity between the drag rotor and the turbomolecular rotor, it may be advantageous if the attachment part is thin. In this regard, the attachment part must be relatively robust to enable the rotor to spin at a high speed and for the two portions to maintain rigidity. An attachment part with a thickness of less than 3mm in some cases 2mm or less has been found to have suitable strength and the required thermal conductivity, in particular when formed of a material such as steel or stainless steel. in some embodiments there is a thermal break between the attachment part and the turbomolecular rotor at the point of attachment. This may be in the form of a ceramic washer In other embodiments there is no intermediate part and in some embodiments the attachment part is welded or braised to the turbomolecular rotor and there is no intermediate part between the turbomolecular rotor and the attachment part.

In some embodiments, said attachment part comprises a cylinder of a smaller diameter than said hollow cylindrical portion of said hub of said turbomolecular rotor such that there is a gap between said cylinder of said attachment part and said cylindrical portion of said hub.

In order to be physically robust and yet able to fit within the hub of the

turbomolecular rotor, the attachment part may have a cylindrical form with a diameter that is smaller than the diameter of the turbomolecular rotor such that there is an air gap between them.

In this regard, the skirt of the drag rotor may have the same diameter as the cylinder of the attachment part or it may have a wider diameter there being a step between the two.

In some embodiments, said turbomolecular rotor comprises a high emissivity coating.

As mentioned previously, it may be difficult to remove heat from the

turbomolecular stage of the pump owing to the high vacuum. It may be

convenient to coat the rotor with a high emissivity coating to encourage radiation and thereby increase heat flow from the rotor. ln some embodiments, said turbomolecular stator comprises a high emissivity coating.

For similar reasons it may also be advantageous for the turbomolecular stator to have a high emissivity coating.

In some embodiments, said stator comprises a turbmolecular stage and a drag stage stator, said turbomolecular stage stator extending around said rotor and said drag stage stator being mounted within and thermally isolated from said turbomolecular stage stator.

As the drag stage of the pump may operate at a higher temperature than the turbomolecular stage, in order to reduce heat flow between the two it may be advantageous for the stator of the drag stage to be thermally isolated to some extent from the turbomolecular stage stator. In this regard, it may be mounted within it with a thermal break comprised of a thermally insulating material located between the two.

In some embodiments, said vacuum pump comprises a heater for heating said drag stage stator.

As the drag stage of the turbomolecular pump operates at a higher pressure there may be problems when pumping process gasses from processes such as semiconductor fabrication due to the condensation of particulates from these gasses at the higher pressures. Thus, it may be important to maintain the drag stage at a higher temperature than the turbomolecular stage of the pump and in order to do this the drag stage may in some embodiments have a heater associated with the stator. Where this is the case thermal insulation between the drag stator and the turbomolecular stator is important, as is some degree of thermal isolation between the drag stage rotor and the turbomolecular stage rotor. ln order for the process gases to be maintained at a temperature where process by-products do not condense then the heater may maintain the temperature of at least the portions of the stator and rotor that contact the process gases within the drag stage above 130°C and preferably above 150°C and in some embodiments between 160 - 180°C. These temperatures do not weaken the steel components and are sufficient to maintain the process gas by-products above their

condensation temperatures at the pressure of operation of the drag pump.

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 schematically illustrates a vacuum pump according to an embodiment.

DESCRIPTION OF THE EMBODIMENTS

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

A vacuum pump is provided with a turbomolecular stage and a drag stage the rotor for which is formed in two parts. The drag stage rotor is attached to the turbomolecular stage rotor by an attachment part that extends upwardly from the drag stage skirt inside the turbomolecular stage rotor. The attachment part is configured to have a low thermal conductivity such that the drag stage can run at higher temperatures than the turbomolecular stage thereby impeding condensation of process gases. Heat flow from the hotter drag stage rotor to the turbomolecular rotor is constrained by the low thermal conductivity of the attachment part connecting the two. In order for the turbomolecular rotor not to heat up, any heat flow that does pass along the attachment part should be less than, or of an amount that can be dissipated from the turbomolecular rotor. In this regard owing to the high vacuum operation of this stage of the pump most of the heat dissipated from the turbo rotor is through radiation and is thus, quite small. A high emissivity coating to the turbo rotor may increase radiation heat loss. This coating may in some embodiments take the form of a black coating.

Figure 1 shows a vacuum pump according to an embodiment. This vacuum pump comprises a turbomolecular stage and a drag stage. The vacuum pump has a main turbo rotor 20 which is mounted by a drive spindle 22 within a motor and magnetic bearings 70. The magnetic bearings allow the rotor to rotate at high speeds with very low friction such that lubricants are not required. The main turbo rotor 20 comprises turbo pump blades 10 and a central cylindrical hub 12 from which the blades extend. The turbo stage of the rotor has a stator 80 which also has blades corresponding to the rotor blades. Turbo stator 80 extends around the whole of the vacuum pump to form a part of the pump housing.

Within this pump housing is the stator of the drag stage 40 that is mounted to the turbomolecular stator 80 via thermal insulating members 50. The drag stage 40 is heated to maintain it at a temperature selected to be sufficient to inhibit condensation of the process gasses being pumped. The drag stage of the pump has a drag stage stainless steel rotor 60 which in this embodiment is a Holweck drag stage rotor. This drag stage rotor has a skirt form and extending from the upper surface is a thin attachment part 30. The thin attachment part 30 extends up into the cylindrical hub 12 of the turbomolecular rotor and is attached to the under surface of the upper part of the cylindrical hub. In some cases it may be braised or welded to the upper part, in other cases it may be attached with some bolting means and there may be a thermal insulator between the attachment piece and turbomolecular part of the rotor. The attachment piece 30 is in the form of a cylinder that has a smaller diameter than the inner diameter of the cylindrical hub 12 of the turbomolecular rotor. In this way there is an air gap between the two.

During operation the drag stage of the vacuum pump will operate at a higher temperature and pressure than the turbomolecular stage. As it operates at a higher pressure there is an increased likelihood of condensation of particles from process gasses being pumped. Maintaining the drag stage at a higher temperature reduces the chance of such condensates appearing. The use of a stainless steel rotor 60 that is more robust to higher temperatures allows this higher temperature operation while the attachment piece 30 having a significant length moving up into the turbomolecular rotor and being formed of a material with a low thermal conductivity, provides low thermal conduction between the higher temperature drag stage rotor and the lower temperature turbomolecular stage rotor allowing them to operate at different temperatures.

Conventionally the drag stage and turbomolecular stage have been formed as a single piece such that differences in temperatures between the two are difficult to maintain. Embodiments of the present invention form the rotor in two parts such that different materials can be used. Furthermore, although the two parts are attached together this is done in a way that despite the two parts of the rotor being adjacent to each other they are attached using a long attachment piece that extends within the turbo stage rotor. In this way, a certain degree of thermal isolation between the two stages of the rotor is provided allowing different temperatures of operation.

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 turbo rotor blades

12 cylindrical hub

20 turbo rotor

22 drive spindle

30 attachment part

40 drag stage stator

50 thermal isolator

60 drag stage rotor

70 magnetic bearings and drive motor

80 turbomolecular stator