Multi-stage turbomolecular pump

Abstract

A vacuum pump with a turbomolecular stage and a drag stage, the vacuum pump comprising a stator and a rotor. The rotor comprises a turbomolecular rotor and a drag rotor attached together. The turbomolecular rotor comprises a hub from which a plurality of blades extend, the hub comprising a mounting portion for mounting to a spindle of a motor and a hollow cylindrical portion, the hollow cylindrical portion extending from the mounting portion toward an outlet end of the turbomolecular stage. The drag rotor comprises a cylindrical skirt and a mounting portion extending away from the cylindrical skirt, the mounting portion extending into the hollow cylindrical portion of the hub of the turbomolecular rotor, the mounting portion being attached at a point closer to the mounting portion than to the outlet end of the turbomolecular rotor.

Description

[Technical field]

[0001] The field of the invention relates to vacuum pumps with turbomolecular and drag stages. [Background technology]

[0002] Turbomolecular pumps are used to provide high vacuum, for example, the high vacuum required for semiconductor processing. These pumps are expensive pumps designed to operate at high tip speeds. The rotor of a turbomolecular pump is rotatably mounted on magnetic bearings to eliminate the need for lubrication, reduce vibration, and allow operation in clean rooms.

[0003] Turbomolecular pumps do not operate satisfactorily at high pressures and are therefore not exhausted to atmosphere, so they typically have some form of backing pump stage to reduce the pressure at the exhaust of the turbo stage. These backing stages are typically downstream of one or more turbomolecular stages and comprise a drag stage integrated within the pump and mounted on the same shaft. The pump may also have an additional backing pump connected remotely from the vacuum pump. Summary of the Invention [Problem to be solved by the invention]

[0004] There is an increasing desire to operate turbomolecular pumps at higher temperatures. For example, in semiconductor processes, pumps must be maintained at high temperatures to prevent condensation of process by-products. As gas flows through the pump system and pressure increases, the pump temperature and the risk of condensate formation increases. Traditionally, turbomolecular pump rotors are cast from aluminum, with the drag and turbo stages cast as one unit, providing a structurally robust rotor suitable for rotation at high speeds. Aluminum loses much of its strength above 130°C, which limits turbopump operation to temperatures below 130°C.

[0005] It would be desirable to provide a vacuum pump with turbo and drag stages that are at least partially suitable for high temperature operation. [Means for solving the problem]

[0006] A first aspect provides a vacuum pump having a turbomolecular stage and a drag stage, the vacuum pump comprising a stator and a rotor, the rotor comprising a turbomolecular rotor and a drag rotor mounted together, the turbomolecular rotor comprising a hub having a plurality of blades extending therefrom, the hub comprising a mounting portion for mounting to a spindle of a motor and a hollow cylindrical portion, the hollow cylindrical portion extending from the mounting portion towards an outlet end of the turbomolecular stage, the drag rotor comprising a cylindrical skirt and a mounting portion extending away from the cylindrical skirt, the mounting portion extending within the hollow cylindrical portion of the hub of the turbomolecular rotor and mounted at a point closer to the mounting portion than the outlet end of the turbomolecular rotor.

[0007] The inventors of the present invention have recognized that as pressure increases through a turbomolecular pump, the risk of condensation of the process gas also increases. Thus, there is a desire to operate the pump at increasingly higher temperatures to avoid condensation, but this problem is more severe in the drag stage than in the turbo stage. Thus, one way to mitigate the problem of condensation in such a vacuum pump is to operate the two stages at different temperatures with some thermal insulation between the two stages. Thus, an embodiment of a vacuum pump is formed with a rotor made from two parts, the rotor of the drag stage being attached to the rotor of the turbo stage via a mounting that extends longitudinally away and upwardly from the skirt of the drag rotor to the inner hub of the turbo rotor. The mounting may be attached at a point away from the outlet end of the turbo stage, such that the main heat path between the hotter drag stage and the cooler turbo stage is through the mounting point via this mounting element. This reduces the thermal conductivity between the two parts of the rotor, allowing the two rotor parts to operate at different temperatures, with the drag stage operating at a higher temperature than the turbo stage, reducing condensation at higher pressures within this stage.

[0008] Additionally, forming the rotor in two components allows different materials to be selected for the two components, with a material selected for the drag stage rotor that has properties suited to high temperature operation, while a material selected for the turbo rotor is suited to higher tip speeds.

[0009] In some embodiments, the drag rotor is formed from a material that can withstand higher temperatures than the material forming the turbomolecular rotor.

[0010] The drag stage of a turbomolecular pump operates at higher pressures than the turbomolecular stage, and therefore may be desirable to operate at higher temperatures. When the drag and turbomolecular rotors are formed of different parts attached together, an opportunity exists to form them of different materials. Traditionally, the drag rotor and turbomolecular rotor have been cast as a single element and therefore constrained to be formed of the same material. Forming the rotor in two parts provides greater flexibility in material choices, allowing the drag rotor to be formed of a material that can withstand higher temperatures than the material forming the turbomolecular rotor.

[0011] Additionally and / or alternatively, the drag rotor is formed from a material having a lower thermal conductivity than the material forming the turbomolecular rotor.

[0012] Because vacuum pumps are configured such that the drag stage can operate at a higher temperature than the turbo stage to reduce condensation in the drag stage, it is advantageous to reduce heat flow from the drag rotor to the turbomolecular rotor if the hotter drag rotor is at least somewhat thermally isolated from the turbomolecular rotor. Thus, in some embodiments, it may be advantageous to make the drag rotor out of a material that has a lower thermal conductivity than the material forming the turbomolecular rotor.

[0013] Although the drag rotor can be formed from a number of materials, in some embodiments the drag rotor is formed from steel, which is a robust material that can withstand high temperatures, is relatively easy to cast, and is relatively inexpensive.

[0014] In some embodiments, the drag rotor is formed from stainless steel.

[0015] Stainless steel has a particularly low thermal conductivity of about 18 W / mK, making it a particularly effective material for forming a drag rotor that is resistant to corrosion and high temperatures. In this regard, both steel and stainless steel can operate at temperatures up to 300°C.

[0016] In some embodiments, the turbomolecular rotor is formed from aluminum.

[0017] Turbomolecular rotors are traditionally made of aluminum, which has a low density and is therefore suitable for the high tip speeds at which they operate, and is robust and can be cast. However, aluminum has a thermal conductivity of 200 W / mK, which is significantly higher than steel or stainless steel. Aluminum is therefore suitable for turbomolecular rotors, but the ability to make the drag rotor of a different material with higher heat resistance and lower thermal conductivity allows the drag and turbo stages of the pump to operate at different temperatures, allowing the drag rotor to operate at a higher temperature to reduce condensation while keeping the turbomolecular rotor at a lower temperature suitable for aluminum. In this regard, when aluminum operates at temperatures above 130° C., it begins to lose strength.

[0018] In some embodiments, the attachment portion is attached to the mounting portion of the turbomolecular rotor.

[0019] The attachment can be mounted on different parts of the turbomolecular rotor, provided it is not too close to the outlet end, thereby providing some thermal insulation, but it is particularly advantageous to mount the attachment on a mounting part of the turborotor that is remote from the outlet, which allows the attachment to be particularly long and provides a suitable surface for mounting the attachment.

[0020] In this regard, the mounting portion extends substantially parallel to the blades of the turbomolecular rotor and perpendicular to the cylinder.

[0021] The mounting portion is perpendicular to the cylinder and therefore forms a convenient surface for mounting the attachment portion of the drag rotor.

[0022] In some embodiments, the mounting portion has a thermal conductivity of less than 50 W / mK, preferably less than 20 W / mK.

[0023] By providing mountings with low thermal conductivity, the turbomolecular rotor can be kept at a significantly lower temperature than the drag rotor. This is important because the turbomolecular rotor operates at a particularly high vacuum and it is not easy to remove heat from this part of the pump. Therefore, if the two parts of the rotor are to be kept at significantly different temperatures, the thermal conductivity between the two parts must be kept low.

[0024] In some embodiments, the attachment portion is thin, having a thickness of 3mm or less.

[0025] A thin attachment is believed to be advantageous to reduce thermal conductivity between the drag rotor and the turbomolecular rotor. In this regard, the attachment must be relatively robust in order to allow the rotor to rotate at high speeds and maintain the rigidity of the two parts. It has been found that attachments having a thickness of less than 3 mm, and in some cases 2 mm or less, have suitable strength and the required thermal conductivity, especially when made from materials such as steel or stainless steel.

[0026] In some embodiments, there is a thermal barrier between the attachment and the turbomolecular rotor at the attachment point, which can be in the form of a ceramic washer, in other embodiments there is no intermediate portion, and in some embodiments the attachment is welded or brazed to the turbomolecular rotor and there is no intermediate portion between the turbomolecular rotor and the attachment.

[0027] In some embodiments, the mounting portion comprises a cylinder of smaller diameter than the hollow cylindrical portion of the hub of the turbomolecular rotor such that a gap exists between the cylinder of the mounting portion and the cylindrical portion of the hub.

[0028] In order to be physically robust yet be able to fit within the hub of the turbomolecular rotor, the mounting portion may have a cylindrical shape with a diameter smaller than that of the turbomolecular rotor such that there is an air gap between them.

[0029] In this regard, the skirt of the drag rotor may have the same diameter as the cylinder of the attachment, or may have a wider diameter with a step between the two.

[0030] In some embodiments, the turbomolecular rotor comprises a high emissivity coating.

[0031] As mentioned above, as a result of the high vacuum, it can be difficult to remove heat from the turbomolecular stage of the pump. It may be advantageous to coat the rotor with a high emissivity coating to promote radiation and thereby increase heat flow from the rotor.

[0032] In some embodiments, the turbomolecular stator comprises a high emissivity coating.

[0033] For similar reasons, it is advantageous for the turbomolecular stator to be provided with a high emissivity coating.

[0034] In some embodiments, the stator comprises a turbomolecular stage and a drag stage stator, the turbomolecular stage stator extending around the rotor, and the drag stage stator mounted within and thermally isolated from the turbomolecular stage stator.

[0035] Because the drag stage of the pump may operate at a higher temperature than the turbomolecular stage, it may be advantageous to provide some degree of thermal isolation of the drag stage stator from the turbomolecular stage stator to reduce heat flow between the two. In this regard, this may result in the implementation of an internal insulation layer comprised of insulating material disposed between the two.

[0036] In some embodiments, the vacuum pump includes a heater for heating the drag stage stator.

[0037] Because the drag stages of turbomolecular pumps operate at higher pressures, problems can arise when pumping process gases from processes such as semiconductor manufacturing due to condensation of particulate matter from these gases at the higher pressures. It can therefore be important to maintain the drag stage at a higher temperature than the turbomolecular stage of the pump, and to do this the drag stage may in some embodiments have a heater associated with the stator. In this case, thermal insulation between the drag stator and the turbomolecular stator is important, as well as some thermal insulation between the drag stage rotor and the turbomolecular stage rotor.

[0038] To maintain the process gas at a temperature where the process by-products will not condense, the heater may maintain the temperature of at least the portions of the stator and rotor in contact with the process gas in the drag stage above 130° C., preferably above 150° C., and in some embodiments between 160-180° C. These temperatures are sufficient to maintain the process gas by-products above their condensation temperature at the operating pressure of the drag pump without weakening the steel components.

[0039] 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.

[0040] It should be understood that where features of an apparatus are described as operable to provide a functionality, this description includes features of an apparatus that provide that functionality or that are adapted or configured to provide that functionality.

[0041] Embodiments of the invention will now be further described with reference to the accompanying drawings. [Brief description of the drawings]

[0042] [Figure 1] 1 illustrates a schematic diagram of a vacuum pump according to an embodiment; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] Before discussing embodiments in more detail, an overview will first be provided.

[0044] The vacuum pump includes a turbomolecular stage and a drag stage, the rotor being formed in two pieces. The drag stage rotor is attached to the turbomolecular stage rotor by a mounting that extends upward from the drag stage skirt in the turbomolecular stage rotor. The mounting is configured to have low thermal conductivity so that the drag stage operates at a higher temperature than the turbomolecular stage, thereby preventing condensation of the process gas. Heat flow from the hotter drag stage rotor to the turbomolecular rotor is restricted by the low thermal conductivity of the mounting that connects the two. To keep the turbomolecular rotor from heating, any heat flow that passes along the mounting should be less than or equal to the amount that can be dissipated from the turbomolecular rotor. In this regard, most of the heat dissipated from the turborotor as a result of the high vacuum operation of this stage of the pump is via radiation and is therefore quite small. A high emissivity coating on the turborotor can increase radiative heat losses. This coating can take the form of a black coating in some embodiments.

[0045] FIG. 1 shows a vacuum pump according to one embodiment. The vacuum pump comprises a turbomolecular stage and a drag stage. The vacuum pump has a main turbo rotor 20 mounted on a drive spindle 22 in a motor and magnetic bearings 70. The magnetic bearings allow the rotor to rotate at high speed with very low friction and no lubrication is 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. The turbo stator 80 extends around the entire vacuum pump and forms part of the pump housing. Within this pump housing is a stator of a drag stage 40 mounted to the turbomolecular stator 80 via a thermal insulation member 50. The drag stage 40 is heated to maintain it at a temperature selected to be sufficient to inhibit condensation of the process gas 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. The drag stage rotor is skirt-shaped and extending from the top surface is a thin mounting section 30. The thin mounting section 30 extends to the cylindrical hub 12 of the turbomolecular rotor and is attached to the underside of the upper part of the cylindrical hub. In some cases, the thin mounting section 30 may be braided or welded to the top, and in other cases, it may be attached by some bolting means, and there may be insulation between the mounting element and the turbomolecular portion of the rotor.

[0046] The mounting element 30 is in the form of a cylinder having a diameter smaller than the inner diameter of the cylindrical hub 12 of the turbomolecular rotor, so that there is an air gap between the two.

[0047] During operation, the drag stage of a vacuum pump will operate at higher temperatures and pressures than the turbomolecular stage. Operating at higher pressures increases the likelihood of condensation of particles from the process gas being pumped. Maintaining the drag stage at a higher temperature reduces the likelihood of such condensation appearing. The use of a stainless steel rotor 60, which is more robust to high temperatures, allows for this high temperature operation, while the attachment element 30 has a significant length to the turbomolecular rotor and is formed from a material with low thermal conductivity, providing low thermal conductance between the high temperature drag stage rotor and the low temperature turbomolecular stage rotor, allowing operation at different temperatures.

[0048] Previously, the drag and turbomolecular stages were formed as one piece, making it difficult to maintain a temperature difference between them. An embodiment of the present invention forms the rotor in two pieces to allow for the use of different materials. Furthermore, the two pieces are attached to each other such that, although the two parts of the rotor are adjacent to each other, they are attached using long attachment elements that extend into the turbo stage rotor. In this way, some thermal insulation is provided between the two stages of the rotor, allowing for different operating temperatures.

[0049] Although exemplary embodiments of the present invention have been disclosed in detail herein with reference to the accompanying drawings, it is understood that the present invention is not limited to the precise embodiments, and various changes and modifications can be made by those skilled in the art without departing from the scope of the present invention as defined by the appended claims and their equivalents. [Explanation of symbols]

[0050] 10 Turbo rotor blades 12 Cylindrical hub 20 Turbo rotor 22 Drive spindle 30 Mounting part 40 Drag stage stator 50 Insulation materials 60 Drag stage rotor 70 Magnetic bearings and drive motors 80 Turbomolecular Stator

Claims

1. 1. A vacuum pump comprising a turbomolecular stage and a drag stage, The vacuum pump comprises a stator and a rotor, the rotor comprising a motor spindle and one turbomolecular rotor and one drag rotor mounted on the spindle; the turbomolecular rotor comprises a hub having a plurality of blades extending therefrom, the hub having a mounting portion for mounting to the spindle and a hollow cylindrical portion, the hollow cylindrical portion extending from the mounting portion toward an outlet end of the turbomolecular stage; The drag rotor comprises a cylindrical skirt and a mounting portion extending away from the cylindrical skirt, the mounting portion extending into the hollow cylindrical portion of the hub of the turbomolecular rotor and being attached at a location closer to the mounting portion than the outlet end of the turbomolecular rotor.

2. 2. The vacuum pump of claim 1, wherein the drag rotor is formed from a material that is more resistant to high temperatures than a material forming the turbomolecular rotor.

3. 3. A vacuum pump according to claim 1, wherein the drag rotor is formed from a material having a lower thermal conductivity than a material forming the turbomolecular rotor.

4. 4. A vacuum pump as claimed in claim 1, wherein the drag rotor is formed from steel.

5. 5. A vacuum pump according to claim 1, wherein the drag rotor is made of stainless steel.

6. 6. A vacuum pump according to claim 1, wherein the turbomolecular rotor is made of aluminium.

7. 7. A vacuum pump according to claim 1, wherein the attachment portion is attached to the mounting portion of the turbomolecular rotor.

8. 8. A vacuum pump according to claim 1, wherein the mounting extends parallel to the blades of the turbomolecular rotor and perpendicular to the spindle.

9. 9. A vacuum pump according to claim 1, wherein the mounting portion has a thermal conductivity of less than 50 W / mK.

10. 10. A vacuum pump as claimed in claim 1, wherein the mounting portion is thin, having a thickness of 3 mm or less.

11. 11. A vacuum pump according to claim 1, wherein the mounting portion comprises a cylinder of smaller diameter than the hollow cylindrical portion of the hub of the turbomolecular rotor, such that a gap exists between the cylinder of the mounting portion and the cylindrical portion of the hub.

12. 12. A vacuum pump according to any one of claims 1 to 11, wherein the turbomolecular rotor is provided with a high-emissivity coating.

13. A vacuum pump as claimed in any one of claims 1 to 12, wherein the stator is provided with a high emissivity coating.

14. 14. A vacuum pump according to claim 1, wherein the stator comprises a turbomolecular stage stator and a drag stage stator, the turbomolecular stage stator extending around the rotor, and the drag stage stator mounted within and thermally isolated from the turbomolecular stage stator.

15. 15. The vacuum pump of claim 14, wherein the vacuum pump comprises a heater for heating the drag stage stator.

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