Turbo-molecular pump

Abstract: The present invention relates to a turbo-molecular pump comprising a pump body (2) housing a gas intake port (3) and a gas exhaust port (4), pumping assemblies (56a, 56b) located within said pump body and formed by a plurality of stators (5a, 5b) and rotors (6a), these latter being alternated with the former and secured to a rotation shaft (7) that is supported by at least one rotatable support means (8, 9) and driven by a motor (10), wherein one of said stators is provided with at least one pumping spiral channel (12) adapted to push and eject the gas from an area proximal to the shaft (7) toward an area distal from said shaft or vice versa.

Description: The present invention relates to a turbo-molecular pump. More particularly the invention refers to a turbo-molecular pump provided with pumping stages with a high compression rate, of the type used in the manufacturing of semiconductor devices. Turbo-molecular pumps have been widely used in the manufacturing of semiconductor devices since the introduction of dry etching methods in the manufacturing of integrated circuits (chips). One of the main problems when using turbo-molecular pumps in semiconductor industry is the low resistance to corrosion of the conventional ball bearings and magnetic media when exposed to corrosive gas mixtures, such as HCl, HBr, CL 2 , Fl 2 , etc.., normally used in the processing of integrated circuits. This is due to the build up of not negligible amounts of gas because of diffusion phenomena trough the pumping stages of the pump. Said gases can irremediably alter both the steel of the end supporting members and the material of the cages in the bearings, and the lubricant contained therein, thus causing a failure of the bearings. In order to avoid a quick damaging of the turbo-molecular pumps, it has been necessary to develope special pumps resistant to corrosion, known as "CP" (Corrosion Proof) pumps. In this kind of pump, a inert gas flow, admitted into the space housing the bearings, forms a barrier against the entry of corrosive substances produced in the processes for IC manufacturing. Figure 1 is a schematic view showing a longitudinal cross section of a conventional turbo-molecular pump 1' equipped with a first assembly 56a' of pumping stages having rotor disks with blades 6a', and a second assembly 56b' of pumping stages having smooth rotor disks 6b'. A pump of this type has been disclosed in EP-A 0 445 855, in the name of the present applicant. The above mentioned pump has been modified to allow the admission of an inert gas for being used in the above mentioned application. Referring to Figure 1, the arrows schematically show the path of an inert gas, admitted under pressure through a radial hole 16' into the gaps between the motor assembly 10' and the pump body 2', and directed towards the bearing 8'. In a pump of the above mentioned type, for conveyng the flow of inert gas towards the bearing 8' a circular plate 11', separating the bearing 8' from the second pumping assembly 56b' of pumping stages having smooth rotor disks 6b', is provided with three radial channels 14' angularly spaced at 120° from each other and communicating with as many axial holes 17', provided in the body 2' of the pump 1'. Thus said channels 14' communicate with the openings or ports existing between the body 2' of the pump 1' and the motor assembly 10'. In spite of a relevant decrease of the corrosive gases, this solution still has a number of drawbacks. First, it requires a source of dry nitrogen, a connecting circuit for admitting the gas - provided with valves and adjusting means - and a main pump having a larger flow rate for pumping the inert gas. Additionally, the admission of an extraneous gas into the manufacturing process could contaminate or alter the initial mixture of gases used in the manufacturing cycle, while at the same time reducing the pump final pressure. To solve the above mentioned problems, L. Mathieu and J.M. Gruffat of ALCATEL CIT, have proposed a solution known as "Inverted Dynamic Seal". A detailed description of such solution can be found in "Vacuum", Volume 44, numbers 5-7, at pages 701 to 703, Pergamon Press Ltd, 1993. Referring to Figure 2, which schematically illustrates the above solution, the "inverted dynamic seal" substantially comprises a screw 101 located inside a cylindrical chamber 102 having a smooth wall and formed in the body 103 of the rotor assembly 100 of the pump. The rotating motion, and the reduced gap between the walls of the chamber 102 and the screw 101 generate a pressure difference that can be used to achieve the so-called "inverted dynamic seal" as well as for pumping the gas. The so obtained sealing is implemented in a turbo-molecular pump 100, and is used for pumping the gas into the space housing the bearings 105. However such solution too presents a number of drawbacks. Firstly, the inverted dynamic seal is poorly effective when the gases to be pumped are lighter than Ar (40), e.g. HF (20), HCl (36), and for low flow rate of the gases to be pumped. Additionally the inverted dynamic seal is not suitable for applications wherein the input pressures are higher than 10 -2 Pa, that is for high pumping flow rates, when on the contrary a maximum protection against corrosive gases would be required. The last but not least drawback of the inverted dynamic seal is the difficult manufacturing of the screw located within the chamber housing the bearings. There are also known turbo-molecular pumps provided with spiral-shaped pumping stages of the so-called Siegbahn type. Such pumps have only been realized in a single-stage configuration with the gas pumping occuring exclusively within the spirally-shaped channel, and no gas pumping would be possible without this channel. An example of pump assembly equipped with a plurality of siegbahn stages whose stators are provided with spiral grooves is disclosed in document DE 4314418. According to the teaching of this document siegbann stages are used as pumping stages for rising the compression ratio of the pump, thereby allowing the use of a small and economic forepump. US 4,732,529 refers to a pump assembly comprising rotors having on their circumferential surface a plurality of spiral grooves and ring grooves. Additionally, stator rings are provided with helical grooves on the top face and bottom face respectively. The purpose of the grooved rotors and stators is to obtain a compression ratio raising section within the pump assembly, thereby increasing the pumping operation at lower vacuum. A solution to the problem of how to protect the pump components from dust particles is disclosed in EP 0408791 which refers to a pump assembly comprising a labyrinth seal between the volume containing the motor of the pump and the pumping stages. Channels are also provided for admitting a gas in said volume, the gas flows through the labyrinth seal and evacuates through a hole thereby cleaning the volume containing the motor of the pump. The labyrinth sealings however, are "static" devices since they do not use the rotation of moving parts for achieving the sealing effect, but only use the geometrical effect of a path increase. Their efficiency in avoiding diffusion of gases towards the motor and bearings is therefore limited. The main object of the present invention is to provide a turbo-molecular pump equipped with a dynamic seal that achieves the advantages of the known solutions, while at the same time avoiding the drawbacks thereof. Another object of the present invention is to provide a turbo-molecular pump having pumping stages with a high compression ratio. A further object of the present invention is to provide a dynamic seal that is easy and economical to be achieved. These and other objects are accomplished by the present invention as defined in the attached claims. The present invention will now be described in detail referring to the annexed drawings, in which: Figure 1 is a schematic cross section view of a turbo-molecular pump of the prior art; Figure 2 is a schematic cross section view of a turbo-molecular pump incorporating an inverted dynamic seal of the prior art; Figure 3 is a cross section view of a turbo-molecular pump according to the invention; Figure 4 is a plan view of the plate in which a spiral seal with a single channel is formed; Figure 5 is a plan view of the plate in which a spiral seal with four channels is formed; Figure 6 is a plan view of the plate, as seen from the opposite side of that of Figures 4 or 5; Figure 7 is a cross section view of one of the stators of the pump shown in Figure 3; Figure 8 is a plan view of the stator shown in Figure 7; Figure 9 is a plan view, from the opposite side in respect to that shown in Figure 8, of the stator of Figure 7. With reference to Figure 3, the turbo-molecular pump 1 of the present invention comprises a substantially cylindrical pump body 2 provided with an axial intake port 3 and a radial exhaust port 4 for the gases. Within said pump body 2, in the portion of the pump facing the inlet port 3, there is fitted a first pumping assembly 56a, formed by a plurality of stators 56a and rotors 6a, these latter being provided with blades, with the stators and rotors being coplanar (i.e. substantially laying in a same plane) and alternating with each other. A second pumping assembly 65b, formed by a plurality of stators 5b and rotors 6b that are smooth (i.e. without blades), coplanar and alternating with each other are fitted within the pump body 2, near the exhaust port 4, and axially aligned with said first pumping assembly 56a. Rotors 6a and 6b are secured to a same rotation shaft 7, which is supported by a pair of bearings 8 and 9 with a motor assembly 10 located therebetween. A circular plate 11 is provided between the bearing 8 and the second pumping assembly 56b, and a spiral channel 12 is formed in the plate surface facing the first rotor 6b of said pumping assembly 56b. As better seen in Figure 4, the spiral channel 12 is designed so that, when the rotors 6a and 6b rotate in the direction of arrow 13, the gases contained in the channel are pushed and ejected from the area proximal to the shaft 7 towards an area distal from said shaft 7. This way channel 12 forms an effective pumping stage with its own characteristic compression ratio and pumping speed. As better seen in Figure 6, the plate 11 is also provided with three radial channels 14, located at 120° in respect of each other, and each communicating with one of three axial holes 17 in the pump body 2 that open into the space 15 housing the motor 10. A radial hole 16 passes through the pump body 2 and allows the admission of an inert gas into the space 15. As schematically illustrated by arrows in Figure 3, the inert gas flows from the space 15 through the axial holes 17 and the radial channels 14 into the gap between the plate 11 and the adjacent rotor 6b on which a spiral seal is formed by the channel 12. Thanks to the pumping action produced by the spiral channel 12 and the action of the inert gas present in correspondence of the plate 11 within said channel 12, the corrosive gases are pushed away from the bearing 8 and ejected towards the gas exhaust port 4 of the pump 1. From tests that have been carried out, it has been verified that the amount of inert gas, e.g. N 2 used for protecting the bearings in pumps equipped with a spiral channel as above described is lower than the amount required in pumps without such spiral sealing. Figure 5 illustrates another embodiment of the spiral sealing of the invention in which the spiral sealing formed on the surface of the plate 21 comprises four spiral channels 20 extending in the same direction. In the last illustrated embodiment it has been experimentally achieved a 30% reduction of the amount of inert gas required for maintaining the bearings of the turbo-molecular pump free from corrosive gases. The spiral sealing according the present invention can be advantageously used even between two pumping stages of the type with flat rotor disk 6b to achieve an increased compression ratio of the pumping stages in which said rotors 6b are located. Referring again to Figures 3 and 7 to 9, one of the stators 5b is provided with a double spiral sealing each cooperating with the corresponding adjacent rotor 6b. Said double spiral sealing is obtained by means of a single spiral channel 18 located on a face of the stator 5b, and by means of four spiral channels 19, located on the opposite face of the adjacent stator 5b. Said channels 18 and 19 are oriented in such a manner as to generate a counter-pumping effect with respect to the pumping flow generated by the pumping stage, such counter-pumping contrasting the natural movement of the escaping gas molecules towards the stages with higher pressure, through the ports located between the plane of the rotor disks 6b and the plane of the stator disks 5b. Figures 8 and 9 illustrate the spiral orientation with respect to motion of the rotor disk, with the rotating direction indicated by the arrow 13. In this way, an outwardly directed pumping of the gases is achieved, thus increasing the sealing of the pumping stages and their compression ratio. The choice between the configuration with a single channel 18 and that with four channels 19 is based upon the fact that the sealing results of the single spiral channel are better at low pressures, typically about 10 -1 Pa, while the four channels sealing presents better results at high pressures, typically about 10 Pa, that should be present in proximity of the gas exhaust port of the pump. In this configuration, the spiral sealing is similar to the one known as labyrinth sealing. However the object of a labyrinth sealing is to geometrically increase the length of the interstitial paths between the static and rotating parts of the turbo-machines to reduce the conductances, and therefore the losses due to blow-by. Thus the labyrinth sealings are "static" devices since they do not use the rotation of moving parts for achieving the sealing effect, but only use the geometrical effect of a path increase. On the contrary, the spiral sealing of this invention, besides contributing to geometrically increase the length of the escaping ways, dynamically operates by pumping away the gas which tends to enter the ducts. Another embodiment of the present invention provides for reversing the orientation of the single spiral channel, or the four spiral channels, in respect to what previously described and shown in the above embodiments. In case the spiral channel(s) is (are) incoming in respect of the rotation of the rotor disc, and the channel inlets are located in correspondence of the outlets of the pumping channels in the pumping assembly, there is achieved a pumping effect with the same direction of the pumping stage with smooth rotors. With this solution in which the gas is pushed into the spiral channel end ejected toward the pump interior, one obtains an increased compression ratio of the pumping stage due to the pumping effect of the spiral channel(s). A preferred embodiment - particularly suitable in presence of corrosive gases - provides for arranging three spiral seals in series, positioned as illustrated in Figure 3, with the first one having a single channel and pumping outwardly, formed on the surface of plate 11 facing rotor 6b; the second one, having a single channel and pumping inwardly, formed on the surface of the stator 5b facing the plate 11; and the third one, having four channels and pumping outwardly, formed on the other face of the same stator 5b. In a pump according to this configuration, it has been experimentally found that the pumping stage incorporating such spiral channel has a compression ratio K = 10. By using three spiral seals in series it was possible to achieve a compression ratio close to k = 1000. In respect to the inverted dynamic seal illustrated in Figure 2, the sealing obtained through the present invention, when the punp sizes are equal, advantageously operates at higher peripheral speeds, typically 200 m/sec instead of 70 m/sec, being formed on a plane rather than on a cylinder located within the rotors.

Claims: A turbo-molecular pump (1) comprising: a pump body (2) having a gas intake port (3) and a gas exhaust port (4); pumping assemblies (56a, 56b) located within said pump body and formed by a plurality of stators (5a, 5b) and rotors (6a, 6b), these latter being alternated with the former and secured to a rotation shaft (7) driven by a motor; at least one rotatable support means (8, 9) for supporting said shaft; a plate (11) separating the space (15) housing the support means (8,9) from said pumping assemblies (56a, 56b), characterized in that at least one spiral channel (12) is formed in the plate (11) surface facing a first rotor (6b) of said pumping assemblies (56b), whereby a dynamic spiral sealing is formed between said plate and facing rotor which avoids diffusion of the gas towards said support means. A turbo-molecular pump according to claim 1, characterized in that said at least one spiral channel (12) is designed so that when the rotors (6a, 6b) rotate the spiral channel (12) forms a pumping channel tending to push and eject the gases contained in the channel (12) from an area proximal to the shaft (7) towards an area distal from said shaft (7). A turbo-molecular pump according to claim 1, characterized in that said plate (11) is provided with a plurality of radial channels (14) adapted to allow the admission of an inert gas into the gap between the plate (11) and the facing rotor (6b). A turbo-molecular pump according to claim 3, characterized in that said body (2) is provided with axial holes (17) adapted to allow the passage of the gas flowing from a radial intake hole (16) into said radial channels (14). A turbo-molecular pump according to claim 4, characterized in that each of said axial holes (17) opens into the space (15) housing the motor (10) of the pump and in that said radial hole (16) communicates with said space (15). A turbo-molecular pump according to claim 3, characterized in that said plurality of radial channels (14) comprises three channels located at 120° in respect of each other. A turbo-molecular pump according to any claim from 2 to 6, characterized in that it includes a second dynamic spiral sealing, provided on the surface of one of said stators (5b) facing the rotating support means (8, 9) and formed by at least one channel (18) which defines a pumping channel adapted to pump the gases from a distal area of the shaft (7) toward an area proximal to said shaft; and a third dynamic spiral sealing, provided on the opposite face of the stator (5b) on which said second spiral sealing is located, said third spiral sealing being formed by at least one pumping channel adapted to push and eject the gases from an area proximal to the rotation shaft (7) towards an area distal from said shaft and towards the exhaust port (4). A turbo-molecular pump according to claim 7, characterized in that said third dynamic spiral sealing comprises four pumping channels adapted to push and eject the gases from an area proximal to the rotation shaft (7) towards an area distal from said shaft and towards the exhaust port (4). A turbo-molecular pump according to any of the preceding claims, characterized in that said plate (11) is of substantially circular shape.

Inventors: CERRUTI ROBERTO, LEVI GIAMPAOLO, CERRUTI, ROBERTO, LEVI, GIAMPAOLO
Applicants: VARIAN SPA, VARIAN S.P.A.
Application year: 1996
Main classification: F04D17
IPC codes: F04D19, F04D17