EP1243795B1 - A two-stage vacuum pump - Google Patents

Abstract

In a vacuum pumping system according to the invention, the Roots or claw multistage dry primary pump discharges into an outlet stage including an additional piston or membrane pump connected in parallel with a preliminary evacuation pipe including a check valve. The outlet stage very significantly reduces heating of the primary pump and thereby enables the vacuum pumping system to pump efficiently and without damage gases with a low thermal conductivity, such as argon or xenon.

Description

The present invention relates to pumping systems using multi-stage Roots type dry pump vacuum or type multi-lobe "claw", in which the inlet of the primary pump receives the gases to be pumped and the output of the primary pump discharges gases pumped to the atmosphere or to a recycling system pumped gases.

In various industries such as the semiconductor industry, we use atmospheric manufacturing processes controlled at low pressure, in a vacuum enclosure connected to a vacuum pumping system.

To achieve and maintain the vacuum in the vacuum enclosure, the vacuum pumping system must first pump a relatively large gas flow to create the vacuum; in one second, the vacuum pumping system extracted from the enclosure to empties the residual gases or the treatment gases introduced voluntarily in the vacuum enclosure during the various stages of manufacturing processes in a controlled atmosphere. Gas flows to pumping by the vacuum pumping system are then lower.

A permanent concern, especially in the semiconductor industry, is to maintain a high purity of the gases contained in the vacuum vessel. To this end, the retrograde pollution from the vacuum pumping system. it prohibited, in particular, the use of vacuum pumping systems including liquid ring pumps. In the techniques modern vacuum pumping systems are pump based Roots or claw type dry.

On the other hand, the treatment gases introduced voluntarily in the vacuum vessel are frequently gases expensive, and there is an advantage in recycling these gases at the outlet of the vacuum pumping system, by a gas recycling system pumped, to then reintroduce them in a controlled manner into the vacuum vessel. It is therefore necessary not to contaminate these gas as they pass through the vacuum pumping system, and that's a second reason why we have to use pumps Roots or claw dry primers, rather than pumps traditional oil seal primers.

Thus, in known pump vacuum pumping systems dry primary type Roots or claw, the inlet of the primary pump receives the gases to be pumped, either directly from the vacuum enclosure, either indirectly by a secondary pump which can be a pump turbomolecular. The primary pump delivers the pumped gases directly to the atmosphere or directly to a system of recycling of pumped gases.

Various industries are required to pump and recycle pure gases with low thermal conductivity such as argon or xenon. This is particularly the case in the semiconductor industry where these gases are used in light sources emitting in the deep ultraviolet to make equipment for photolitography for the manufacture of circuits new generation electronics.

In this type of application, these very pure gases are used at low pressure in the vacuum vessel, and are evacuated by a multi-stage primary dry pump pumping system Roots type or multi-lobe claw type. So the document US 4,504,201 A describes a Roots type multi-stage pump and two claw floors. The top floor pushes the atmosphere.

In a multi-stage pump, the gas to be evacuated is sucked by the first stage of the pump then compressed in the stages following until reaching a pressure slightly higher than the atmospheric pressure at the exit of the top floor and so be released to the atmosphere or returned to a recycling system for pumped gases.

It has previously been found that pumping systems using known vacuum with multi-stage Roots or multi-lobe dry pump claw have a serious drawback in the case where pure gases at low thermal conductivity such as argon or xenon are introduced into the vacuum enclosure during the process steps. Indeed, the presence in the pumped gases of a high gas content pure with low thermal conductivity such as argon or xenon causes very rapid blocking and destruction of the pump dry primary.

Rapid blocking and destruction of the pump is due blockage of the last stage of the pump, stage which drives the gas at a pressure close to atmospheric pressure.

The explanation lies in the following analysis: in a multi-stage dry pump, whatever its technology, the gas undergoes several successive compressions in the various stages of the pump since the suction pressure at the inlet of the first stage up to atmospheric pressure at the outlet of the last stage. At each compression stage the gas heats up and heats the neighboring pump parts. However, this compression is not regular, and it is in the last stage that the strongest compression occurs. Generally, a compression greater than 5.10 ⁴ Pa is achieved in the last stage. It is therefore in the last stage that the gas heats up most and that most of the energy must therefore be dissipated in the form of calories .

However, the structure of the primary dry pumps includes a stator in which rotate two mechanically coupled rotors and laterally offset from each other. The rotors are held by bearings, and are separated from the stator by the gas slide contained in the mechanical clearances between the rotor and the stator or pump body. The dissipation of calories in a stage of the pump is carried out, for a very small part, by conduction across the axis of the rotor towards the pump body, and for a preponderant part by conduction through the gas slide between the rotor and the stator.

In the case of pumping low conductivity gases thermal, the gas opposes the thermal transfer between the rotor and the stator. This results in the last stage of the pump multi-stage primary, an increase in rotor temperature very large and rapid, which results in dilation of the rotor such that the latter comes into contact with the stator, causing blocking and destruction of the primary pump.

To avoid such a phenomenon, we have already proposed a solution consisting in injecting into the intermediate stages of the pumps a gas with high thermal conductivity such as nitrogen or helium. However, these additive gases are then mixed with pure gas, and prevent simple recycling.

Another known solution is to increase voluntarily the top floor functional games to lower its compression rate and thus reduce the calories to be evacuated. But the pump is then no longer capable of achieving performance required, and then the loss of compression ratio must be distributed over a large number of additional floors, which leads to design a complex and bulky pump.

We also know from document DE 37 10 782 A two-stage vacuum pump to pump a mixture of gases and steam. The first stage is of the sliding pump type. The second floor is a diaphragm pump. We control the two pump stages so as to keep the intermediate pressure between the stages below of the dew point pressure of the vapor. The goal is reduce the vacuum pressure achieved.

The problem proposed by the present invention is to design a new vacuum pumping system structure to avoid destruction of the dry primary pump in the case of pumping gas with low thermal conductivity, in using known multi-stage dry primary pumps without modify them, also keeping the same technique possible recycling, thus avoiding the development of a new pump.

To reach these and other objects, a system vacuum pump according to the invention comprises a primary pump Roots or claw multi-stage dryer, pump inlet primary receiving the gases to be pumped and the pump outlet primary pumping the pumped gases towards the atmosphere or towards a pumped gas recycling system. According to the invention, the system vacuum pumping system includes an additional pump whose inlet is connected to the output of the primary pump and whose output back to the atmosphere or to the gas recycling system pumps. A vacuum hose is connected in parallel on the additional pump, and includes a non-return valve allowing gas from the primary pump to pass. The pump additional is a dry pump of technology other than Roots or claw and adapted to safely support the elevation of temperature due to the final compression of the pumped gases.

According to a first embodiment, the pump additional is a diaphragm pump.

According to another embodiment, the additional pump is a piston pump.

The additional pump must be sized to be able to pump all of the gas flow through the system vacuum pumping during the vacuum pumping steps at low pressure, for example to pump the process gas flow during the low pressure manufacturing process steps in a vacuum enclosure.

Preferably, the additional pump can be sized to be just capable of pumping said flow of gas during the steps of pumping a vacuum at low pressure. We can thus use a small, inexpensive and nevertheless sufficient to remove the problem of destruction of the primary pump dries.

The drain line must be dimensioned way to let through the important gas flow during the stages for vacuuming an empty enclosure.

The vacuum pumping system according to the invention can be connected to a vacuum enclosure containing or in which are injected gases with low thermal conductivity.

Low thermal conductivity gases may include argon or xenon.

Advantageously, the pumped gases are discharged at the outlet of the vacuum pumping system in a gas recycling system pumps. The pumped gas recycling system extracts and recycles said gases with low thermal conductivity, to reinject them from controlled way in the vacuum chamber.

• la figure 1 est une vue générale schématique d'un système de pompage à vide selon un mode de réalisation de l'invention, connecté à une enceinte à vide ;
• la figure 2 est une vue de côté en coupe longitudinale illustrant une structure possible de pompe Roots multi-étagée ; et
• la figure 3 est une vue de côté en coupe longitudinale d'une structure possible de pompe à membranes.

Other objects, characteristics and advantages of the present invention will emerge from the following description of particular embodiments, given in relation to the attached figures, among which:

• Figure 1 is a general schematic view of a vacuum pumping system according to one embodiment of the invention, connected to a vacuum enclosure;
• FIG. 2 is a side view in longitudinal section illustrating a possible structure of a multi-stage Roots pump; and
• Figure 3 is a side view in longitudinal section of a possible membrane pump structure.

In the embodiment illustrated schematically on Figure 1, a vacuum pumping system according to the invention includes a primary pump 1 dry multi-stage Roots type or claw, whose inlet 2 receives the gases to be pumped from a vacuum enclosure 3, and the outlet 4 of which discharges the gases pumped to an output stage 5 comprising an additional pump 6 and a drain line 7.

The additional pump 6 has an inlet 8 connected to the outlet 4 of the primary pump 1, and has an outlet 9 which back to the outside atmosphere or to a system of recycling of pumped gases 10.

Pre-hose 7 is connected in parallel on additional pump 6, i.e. its input is connected to input 8 of the additional pump 6 and to output 4 of the primary pump 1, and its output is connected to the output 9 of the additional pump 6 and to the atmosphere or to the recycling of pumped gases 10. The evacuation pipe 7 includes a non-return valve 11, which allows the gases of entry to exit while prohibiting their movement from the exit to the entrance. Thus, the non-return valve 11 allows passage gases from outlet 4 of the primary pump 1.

The additional pump 6 is a dry pump of technology different from the Roots or claw technologies used for primary pump 1, and is adapted to support without damage the temperature rise due to the final compression of the gases pumped before being discharged to the atmosphere or to the pumped gas recycling system 10.

A first example of an additional pump that can suitable is a diaphragm pump, as shown schematically in Figure 3. It is understood that such a pump membranes is a dry pump, that is to say in which the seal sealing of the pump is not achieved by a liquid volume. It is also understood that the membrane pump structure does not does not have a rotor isolated from the stator by the blade of pumped gases.

A second example of an additional pump that may be suitable is a piston pump, which is a well-known structure in the state of the art. In such a piston pump, there is no rotor isolated from the stator either by a blade of pumped gases.

As a result, in both of the technologies piston pump or diaphragm pump, all parts of the pump can be cooled by conduction from the body outside of the pump which is itself cooled by a circuit of forced cooling, so that such an additional pump is able to dissipate the large amount of heat resulting from the final compression of the pumped gases.

The additional pump 6 must be dimensioned so as to be able to pump the entire process gas flow through the vacuum pumping system during the pumping steps from vacuum to low pressure. During these stages where the pumped gas is low pressure, the gas flow is relatively low. So it is enough that the additional pump be sized to be fair capable of pumping said gas flow, so that inlet 8 of the additional pump 6 is at a much lower pressure at atmospheric pressure, and the primary pump 1 must thus achieve a reduced compression ratio which consequently reduces the heating of the gases passing through it and the heating which results on its constituent parts. To ensure a reduction satisfactory gas pressure at inlet 8 of the pump additional 6, it is sufficient that the additional pump 6 is capable of pumping the entire gaseous flow of the operating regime normal, the non-return valve 11 ensuring the maintenance of the pressure difference between inlet 8 and outlet 9 of the pump additional 6.

Pre-hose 7 is required to allow pass the gas flow at a higher flow rate than the primary pump 1 must evacuate at the start of emptying a vacuum chamber 3. In this case, the gases being pumped generally do not include low gas thermal conductivity, and the compression that the last stage of primary pump 1 is lower than that the vacuum pumping system must perform in operating mode normal, i.e. when the pressure in the vacuum vessel 3 is very low. The primary pump 1 is thus capable of ensuring that only the step of vacuuming the vacuum chamber 3, through the drain line 7, and the additional pump 6 has not significant effect on the functioning of the system. The pipeline 7 should be dimensioned so that the significant gas flow during the steps of the feed the vacuum chamber 3.

In the embodiment illustrated in FIG. 1, the pumped gas recycling system 10 generates a gas flow recycled. The flow of recycled gas is sent through a pipeline of recycling 110 to a piloted gas source 12 which is itself connected to the vacuum enclosure 3 by an injection pipe 13 to inject into the vacuum vessel 3 appropriate quantities of gas during programmed operating steps.

The primary pump 1 is for example a multi-stage dry pump Roots type, as illustrated more clearly on Figure 2. In such a multi-stage Roots pump, the stator 14 defines a succession of compression chambers, for example the compression chambers 15, 16 and 17, in which rotate Roots compression lobes carried by two rotors parallel such as the rotor 20 mechanically coupled, with gas passage pipes to let gases pass successively between the adjacent compression chambers.

Rotors such as rotor 20 are mounted parts rotating on bearings, and a play is necessarily present between the compression lobes and the walls of the stator 14. A blade therefore exists between the compression lobes of the rotors and the stator mass 14. In the case of low gas pumping heat conduction, the gas blade effectively insulates the lobes compression of the rotors relative to the stator, and therefore opposes the passage of heat energy from the rotors to stator 14. It this results in the heating of rotors such as rotor 20.

This heating is more accentuated in the last floor 17 of the primary pump, stage where the most compression occurs important gas.

The vacuum pumping system as illustrated in the figure 1 according to the invention makes it possible to lower the pressure at outlet 4 of the primary pump 1, thereby reducing overheating of the top stage of the primary pump 1.

This effect is particularly advantageous during pumping gas with low thermal conduction, and prevents destruction primary pump 1.

The operation of the system according to the invention is the following: at the start of pumping of the gases present in an enclosure at vacuum 3, the primary pump 1 sucks the gases at its inlet 2 and the compresses to discharge them at its outlet 4 at pressure close to the atmospheric pressure. The gas flow is important, and the pumped gas mixtures generally contain good gases coefficient of thermal conduction. Primary pump type 1 Multi-stage Roots is thus capable of ensuring the pumping of this gas flow, during a vacuuming step. The gas driven back to its outlet 4 mainly pass through the hose 7 to through the non-return valve 11, to escape towards the atmosphere. The additional pump 6 sees only a weak pass portion of the discharged gas flow, its pumping capacity being scaled down.

When the low pressure is established in the enclosure to vacuum 3, the process steps can be carried out under vacuum, by example for the manufacture of semiconductors. During these stages, i.e. during normal operation, process are injected into the vacuum enclosure 3 by the gas source 12 through the injection pipe 13. These process gases can be insulating gases such as argon or xenon, in the stages where these gases are used for example in sources of light emitting in the deep ultraviolet. The gas flows pumped being weak, the additional pump 6 is capable of ensuring the pumping of all the gas flow leaving the primary pump 1 by outlet 4, and no flow flows through the drain line 7. As a result, the additional pump 6 produces a lowering of the pressure at its inlet 8, i.e. at the outlet 4 of the primary pump 1. The primary pump 1 is thus capable of withstand the presence of gases with low thermal conductivity such than argon or xenon in the flow of pumped gases, without exaggerated heating of its elements.

Generally, low conductivity pumped gases are expensive gases, which it is worth recycling. This is the reason why, at the outlet of the system, the gases are returned to the pumped gas recycling system 10, which itself returns the recycled gas through recycling line 110 to the gas source 12, for subsequent reinjection into the vacuum chamber 3.

The present invention is not limited to the modes of which have been explicitly described, but it includes the various variants and generalizations that are contained in the appended claims.

Claims (8)

1. A vacuum pumping system including a Roots or claw multistage dry primary pump (1), in which the inlet (2) of the primary pump (1) receives gases to be pumped and the outlet (4) of the primary pump (1) discharges pumped gases to the atmosphere or to a pumped gas recycling system (10), characterized in that:

   the inlet (2) of the primary pump (1) is connected to a vacuum enclosure (3) containing, or into which are injected, low thermal conductivity gases,

   an additional pump (6) comprises an inlet (8) connected to the outlet (4) of the primary pump (1) and comprises an outlet (9) that discharges to the atmosphere or to the pumped gas recycling system (10),

   a preliminary evacuation pipe (7) is connected in parallel with the additional pump (6) and includes a check valve (11) adapted to pass gases coming from the primary pump (1),

   the additional pump (6) is a dry pump using a technology other than Roots or claw and not having a rotor isolated from the stator by a thin layer of pumped gases, to withstand without damage the temperature rise due to the final compression of the pumped gases.
2. A vacuum pumping system according to claim 1 characterized in that the additional pump (6) is a membrane pump.
3. A vacuum pumping system according to claim 1 characterized in that the additional pump (6) is a piston pump.
4. A vacuum pumping system according to any of claims 1 to 3 characterized in that the additional pump is rated to be capable of pumping all of the flow of gas passing through the vacuum pumping system when pumping a vacuum at low pressure.
5. A vacuum pumping system according to claim 4 characterized in that the additional pump (6) is rated to be just capable of pumping said flow of gas when pumping a vacuum at low pressure.
6. A vacuum pumping system according to any of claims 1 to 5 characterized in that the preliminary evacuation pipe (7) is rated to pass the high gas flow during preliminary evacuation of a vacuum enclosure (3).
7. A vacuum pumping system according to any of claims 1 to 6 in which the low thermal conductivity gases include argon or xenon.
8. A vacuum pumping system according to any of claims 1 to 7 characterized in that the pumped gases are discharged into a pumped gas recycling system (10) which extracts and recycles said low thermal conductivity gases.

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