Dry pumps are widely used in, for example, the semiconductor manufacturing industry to provide clean, low pressure (often vacuum) environments in a process chamber in which wafers can be manufactured. The pump mechanisms of dry pumps come in a variety of forms, one of which generally comprises one or more rotors housed by a stator. The rotors are suitably shaped so as to draw gas from the process chamber into the pump inlet, and exhaust the pumped gas from the pump outlet. Often the rotor may comprise a number of rotor elements arranged in series along the axis of rotation of a common rotary shaft.

One example of a dry pump mechanism is the Roots mechanism. In a Roots mechanism, each Roots rotor element has two or more substantially similarly shaped and sized lobes, each lobe extending radially from the axis of rotation of the rotary shaft. The lobes are arranged in rotational symmetry about the rotational axis of the shaft. In early stages of the pump (i. e. those nearer the pump inlet), there may be fewer lobes on the rotor elements. For instance, two lobes are commonly used in the early stages of the pump. Pairs of rotors may be arranged in close contact with each other but not touching. Each rotor element is arranged at an angle to the opposing rotor element to form a stage such that, when the two rotary shafts rotate, the lobes from one rotor element pass through the spaces between the lobes of the other.

Another mechanism is known as the Northey, or"claw"mechanism ; In this mechanism, the lobes described for the Roots stage above are replaced by sickle or claw shapes arranged rotationally symmetrically about the rotational axis of the rotary shaft.

A third mechanism operating on similar principles to the Roots and claw mechanism is sometimes referred to as a ball and socket arrangement. In this arrangement, the rotors are aligned in pairs in parallel. The first rotor comprises one or more rotor elements, each having a cross section which is generally circular but has a number of sockets cut into the circumference of the circle and equally spaced radially about the rotational axis of the shaft. The second rotor comprises one or more corresponding rotor elements each of a generally circular cross section and having protrusions extending from their circumference, these protrusions being shaped and located so as to intermesh with the sockets of the immediately adjacent rotor element on the first rotor. Each pair of cooperating rotor elements is called a stage.

In each of the foregoing examples, as the two rotors rotate at a steady frequency of rotation, periodic pressure fluctuations occur. The rotor elements of each stage trap slugs of gas between the rotors and the stator and exhaust some to the pump outlet. As the shafts rotate at a constant speed in opposite directions and due to the rotational symmetry of the rotor elements, the release of trapped gas occurs at regular intervals. Thus, there is generated a pulsation having a regular period. If these rotor elements are positioned in the final or exhaust stages of the pump, this periodic pulsation can be transmitted through connecting pipes between the pump and containment vessels and can be heard in the surrounding environment. Such pulsation can cause various problems, for example, it can be very distracting to persons working in the environment to have to hear such a regular pulse for prolonged periods. There is also a danger that, should the pulses reach the harmonic frequency of components in the containment vessels, these components may be excited and vibrate vigorously, which may result in excessive noise.

In the past, this noise problem has been addressed by introducing a stage with multi-lobed, rotationally symmetrical rotor elements at the final stage of the pump, adjacent the exhaust (or outlet). This multi-lobed rotor element has more lobes than the rotor elements in the preceding stages and serves to reduce the amplitude of pulses generated by the previous stages of the pump mechanism.

Nevertheless, in use it generates its own regular beat at a higher frequency than the other stages. This can excite ducts used to direct the exhaust gases to a suitable vent space and cause the noise problems previously mentioned.

In addition, or as an alternative to the above-mentioned solution, it is known to use silencers or mufflers to reduce the noise occurring at the exhaust of a pump. In certain circumstances, the use of silencers is not desirable. Some semi- conductor manufacturing processes create dusty or condensable materials which can be hazardous if they collect in significant quantities and are exposed to air (for example during service or maintenance of the pump). If used in such environments, silencers and mufflers can result in an accumulation of such hazardous end products making the servicing or maintenance issues more difficult.

The present invention seeks to provide a vacuum pump which alleviates some of the previously described problems associated with pulsation at the pump exhaust.

In accordance with the present invention there is provided a vacuum pump comprising a chamber housing first and second intermeshing rotor elements located on respective shafts and adapted for counter-rotation within the chamber, at least one of the rotor elements comprising a plurality of protrusions extending from a centre of rotation, the protrusions being rotationally asymmetrically arranged around the centre of rotation of the rotor element.

The protrusions may, for example, take the form of lobes of a Roots rotor element, claws of a claw rotor element or the protrusions (or spaces between sockets) of a ball and socket arrangement.

It will be appreciated that each protrusion has an apex radially spaced from the centre of rotation. Desirably, the sector angle between each pair of apices as subtended at the centre of rotation is different to the equivalent angle between the adjacent pair of apices. The sector angles between the apices may be randomly

selected. Desirably each of the sector angles is different to all others, though this is not essential.

The plurality of protrusions is desirably three or more, more preferably between 3 and 9. For example, the plurality may comprise 4,5, 6 or 7 protrusions. The protrusions may be formed integrally with the rotor element or, alternatively, the protruding components may be formed separately and assembled to subsequently form the rotor element.

The rotationally asymmetrical rotor element is desirably positioned as the last rotor element of the pump, in use, adjacent to the exhaust (or outlet) of a pump. The pump may also comprise intermeshing, rotationally symmetrical rotor elements.

In another aspect, the invention provides a vacuum, or dry, pump including a rotor comprising at least one rotationally asymmetrical Roots rotor element including a plurality of protrusions rotationally asymmetrically arranged around the centre of rotation of the rotor element.

The present invention also provides a vacuum pump comprising a pump inlet for receiving fluid to be pumped, a pump outlet for exhausting pumped fluid, and a stator housing first and second shafts adapted for counter-rotation within the stator and each having located thereon a plurality of rotor elements such that the rotor elements on the first shaft intermesh with the rotor elements on the second shaft, wherein each of the intermeshing rotor elements proximate the outlet comprises a plurality of protrusions extending from a centre of rotation, the protrusions being rotationally asymmetrically arranged around the centre of rotation of the rotor element.

For the purposes of exemplification, some embodiments of rotor elements in accordance with the invention are now described with reference to the Figures in which:

Figure 1 shows one embodiment of a multi-stage pump; Figure 2 shows a cross section of Figure 1 taken along line XX; Figure 3 shows a configuration of a single stage of the pump of Figure 1; and Figure 4 shows an alternative embodiment of a single stage of the pump of Figure 1.

Figure 1 shows schematically an embodiment of a multi-stage vacuum pump. In the illustrated embodiment, the pump comprises two parallel aligned rotary shafts 2a, 2b each carrying a rotor assembly 1 a, 1 b in the form of a series of rotor elements. The shafts 2a, 2b are mounted in bearings 3a, 3b, 3c, 3d. The shaft 2b is driven by a drive mechanism 3, a timing gear arrangement (not shown) connecting the two shafts 2a, 2b to ensure that the two shafts counter-rotate in synchronisation. The rotor elements 1 a, 1 b are located within a housing unit 4, which defines a stator element comprising a series of chambers each housing a respective pair of rotor elements 1 a, 1 b.

In this embodiment, each of the rotor elements 1 a, 1 b has a Roots-type profile.

The shafts 2a, 2b are separated by a distance which is less than twice the maximum radius of a rotor so that, as shaft 2b rotates, the rotors intermesh such that the lobes of the rotor elements 1 b pass through the spaces between the lobes of the rotor elements 1 a.

Figure 2 shows the axial gas flow path through the pump illustrated in Figure 1. A pump inlet 7 provided in the housing 4 communicates directly with the first stage of the pump, that is, with the chamber housing the pair of intermeshing rotor elements 1 a, 1 b located closest to the inlet 4, to convey gas received from, for example, a process chamber to the first stage of the pump. Channels 6 are provided between adjacent stages to direct the pumped gas from the outlet of one stage to the inlet of the next. A pump outlet 5 is provided in communication with

the exhaust stage, that is, with the pair of intermeshing elements located closest to the outlet 5, to allow the pumped gas to be exhausted from the pump.

Figure 3 illustrates Roots rotor elements for forming the exhaust stage of the pump in Figure 1. Each element has a centre of rotation R from which radially extends five lobes. Each lobe has an apex A1, A2, A3, A4, A5 the radii of which are oriented around the centre of rotation of the rotor element. The sector angle (i. e. the internal angle between the radii of adjacent apices) is shown in the diagram.

By separating the lobes by different rotational angles, the frequency of pulses at the exhaust becomes irregular. Consequently, no regular pulse is heard.

Instead, any noise produced is spread over a range of frequencies. Such random noise is unlikely to be amplified by ducting in the pump system or to excite frequencies within the duct system, thereby reducing the occurrence of the problems previously discussed.

Similar advantages are associated with the configuration of rotor elements for the exhaust stage of a pump utilising a ball and socket pumping mechanism as shown in Figure 4. The ball and socket stage comprises a first,"ball"rotor element generally designated 21 and a second,"socket"rotor element generally designated 22. The ball rotor element comprises a central piece 23 of substantially circular cross section and five protrusions 24a, 24b, 24c, 24d, 24e that are spaced radially at unequal angular separations about the centre of rotation R of the central piece 23. These protrusions may be manufactured separately and then joined to the central piece during subsequent assembly of the rotor.

Alternatively they may be formed as an integral part of the central piece. The socket rotor element comprises a single piece 25 which comprises a piece of substantially circular cross section having cut therefrom five sockets 26a, 26b, 26c, 26d, 26e to leave five protrusion 27a, 27b, 27c, 27d, 27e spaced radially at unequal angular separations about the centre of rotation R of the piece 25, so that, during use, the protrusions 24a, 24b, 24c, 24d, 24e intermesh with the sockets 26a, 26b, 26c, 26d, 26e to draw the pumped gas through the pump.

It is to be understood that the foregoing describes just two embodiments of the invention and is not intended to be limiting on the true scope of the invention as claimed in the appended claims.