WO2006008458A1

WO2006008458A1 - Sensor

Info

Publication number: WO2006008458A1
Application number: PCT/GB2005/002723
Authority: WO
Inventors: Alan Edward Holme; Matthew Key; Nicholas Daniel Hutton; Waleed Ahmed Qader
Original assignee: The Boc Group Plc
Priority date: 2004-07-15
Filing date: 2005-07-12
Publication date: 2006-01-26
Also published as: TW200615529A; GB0415862D0

Abstract

A corrosion sensor comprises a corrodible member which is maintained at a substantially constant temperature. An electrical property which is indicative of the extent of the corrosion member is monitored, and a signal is output from the sensor depending upon the value of the monitored electrical property.

Description

SENSOR

The present invention relates to the field of sensors, in particular sensors for determining the level of exposure of a mechanism to a corrosive medium.

Vacuum pumps serving semiconductor process tools are, increasingly, being exposed to harsher materials both during the processing activity and during cleaning of the tool. Many corrosive gases pass through the pump during its operation, especially during the clean cycle of the tool. One such gas, for example, is fluorine, which is used to clean the interior surface of the process chamber and is then pumped away by the vacuum pump. Prolonged exposure to these gases can lead to corrosion of components within the pump. The consequences of any internal components of the vacuum pump failing during operation of the tool can be significant and may lead to loss of any product contained within the process chamber. This expensive loss of product is compounded by the fact that the tool is taken off line whilst the pump is either replaced with a new device or the damaged components within the pump are replaced.

Visual inspection of all components at each maintenance interval would be too time consuming, and so an indirect monitoring mechanism is preferable. It is, therefore desirable to implement a sensor that monitors the conditions within the pump to indicate the level of corrosion that has taken place. In this way, damaged components can be replaced or reconditioned during scheduled maintenance shut downs thus reducing the likelihood of pump component failure during active processing and thus protecting any product contained within the process chamber.

According to one aspect of the present invention there is provided a corrosion sensor comprising a corrodible member; means for monitoring an electrical property indicative of the extent of the corrosion of the corrodible member; and means for outputting a signal dependent upon the value of the monitored electrical property, wherein the corrodible member is formed from a material selected to form during corrosion thereof a by-product which does not adhere to the surface of the corrodible member.

An advantage of this aspect of the present invention is that since the material of the corrodible member is selected such that a by-product formed during corrosion thereof does not adhere to the surface of the corrodible member, the corrodible member will remain fully exposed to the corrosive fluid and will therefore react at an optimum level.

The electrical property being monitored may be one of resistance, capacitance and inductance. The electrical property may be of the corrodible member itself or of a circuit containing the corrodible member. The corrodible member may be one of a filament, a bar and at least one plate.

The sensor may comprise a means for maintaining the corrodible member at a substantially constant temperature. Accordingly there is provided, in another aspect of the present invention a corrosion sensor comprising a corrodible member; means for maintaining the corrodible member at a substantially constant temperature; means for monitoring an electrical property indicative of the extent of the corrosion of the corrodible member; and means for outputting a signal dependent upon the value of the monitored electrical property, wherein the corrodible member is formed from a material selected to form during corrosion thereof a by-product which does not adhere to the surface of the corrodible member.

Since temperature of the sensing component is maintained at a substantially constant value, the effect of fluctuations in the ambient temperature has a minimal effect on the temperature of the sensing component. Resistivity, p, of a material is a temperature dependent parameter and therefore changes in resistance occur as a result of thermal changes in the ambient conditions within the pump. As the sensor does not experience any fluctuations in temperature, it is not necessary to incorporate a temperature compensation device or the like into the sensor, thus a simple device may be provided.

The corrodible member may be heated to a temperature above ambient temperature, preferably by passing an electrical current therethrough. Alternatively, the corrodible member may be heated by an external heating source such as via a thermally conductive support member using the conduction properties of the sensor components or by any other heating means.

The corrodible member may be configured such that the cross-sectional area decreases continuously when exposed to the corrosive medium. The material of the corrodible member is preferably chosen such that any corrosion by- product is a fluid, preferably a gaseous fluid. This allows a sensor to be provided that is capable of monitoring continuous corrosion damage and hence is able to determine a cumulative level of corrosion. The corrodible member may be formed from a material that is selected to react with a halogenated chemical, preferably a fluoro-chemical (that is, any species comprising fluorine) for example, F₂. This material preferably comprises tungsten, and optimally a further component to form a tungsten-rich material, for example a tungsten alloy. This further component is preferably selected to enhance an electrical property of the corrodible member, such as the electrical resistivity. An example of such an alloy is an alloy of tungsten and rhenium. The quantity of rhenium in the corrodible member is typically 26% (by weight) or less, but preferably 10% (by weight) or less. Most preferably the quantity of rhenium in tungsten is between 3% and 5% (by weight).

According to another aspect of the present invention there is provided a sensor for determining the cumulative level of exposure of a pump to a corrosive fluid passing therethrough, the sensor comprising a corrodible member locatable within the pump and formed from a material selected to react with the corrosive fluid such that the size of the corrodible member continuously decreases with exposure to the corrosive fluid; means for monitoring an electrical property of the corrodible member, which varies with the size of the corrodible member; and means for outputting a signal dependent on the value of the monitored electrical property. In another aspect the present mvenWon provides a vacuum pump comprising a sensor as aforementioned.

According to another aspect of the present invention there is provided a method for detecting corrosion due to exposure to a corrosive fluid comprising the steps of: providing a corrodible member within the path of a stream of corrosive fluid; supplying heat to the corrodible member, to actively maintain the temperature thereof at a substantially constant value; monitoring an electrical property indicative of the extent of corrosion of the corrodible member; and outputting a signal indicative of the corrosion of the corrodible member, depending on the value of the monitored electrical property.

According to a further aspect of the present invention there is provided use of a tungsten-rich member within a vacuum pump to determine, from corrosion thereof, the condition of the pump as a result of exposure to fluorine passing through the pump.

Features described above in relation to sensor aspects of the invention are equally applicable to method and use aspects, and vice versa.

Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a filament sensor; Figure 2 is a schematic representation of the sensing element of an inductive sensor;

Figure 3 is a schematic representation of the sensing element of a capacitive sensor; and Figure 4 is a schematic representation of one way of implementing the sensor.

A vacuum pump is typically exposed to harsh chemicals during the cleaning cycle of the process whereby the chemical, for example, fluorine is introduced into the process chamber to remove any by-products of the process from the walls of the process chamber. This chemical together with any released residue is drawn from the chamber by activating the vacuum pump. These chemicals also serve to reduce any residue build up on the internal components of the pump but their action goes beyond residue removal, and indeed they may cause corrosion of various internal surfaces of the vacuum pump.

Certain parts of a vacuum pump are more vulnerable to damage caused by corrosion. For example, in a multi-stage vacuum pump, the sealing components which are located between adjacent stages of the stator can be severely affected. These seal components, typically elastomeric o-rings, may be degraded by prolonged exposure to cleaning fluids. The bulk material of the stator and rotor components is also prone to corrosion damage when exposed to such harsh chemicals such as fluorine, chlorine and bromine. When the bulk material, typically iron, is exposed to these chemicals the products from the resulting chemical reaction have a significantly greater volume than the unreacted material. Consequently there may be a reduction in the clearances between rotor and stator components which can inhibit relative motion therebetween, potentially leading to seizure of the pump. The mating faces of adjacent stator components can also be severely affected by corrosion. When these surfaces are undamaged and remain smooth there is a minimal leakage path between the two components such that the o-ring seals are well protected. However, as the process and cleaning chemicals react with the mating surfaces the higher volume by-products force the stator components apart, thus increasing the leakage path to the o-rings such that they suffer an increased level of exposure to the process and cleaning fluids.

Furthermore, the components that are most susceptible to corrosion are those positioned at the exhaust of the vacuum pump as pressures and temperatures in this region are much greater than those in the earlier stages of the pump. Typically exhaust pressures are at approximately atmospheric levels and these conditions are therefore significantly more damaging than those associated with sub-atmospheric pressure levels.

By placing sensors in the fluid flow path which are exposed to the same process and cleaning fluids as the internal pump components the exposure, and therefore potential corrosion damage to these internal components can be assessed. Such a sensor is illustrated in Figure 1. The sensor 1 comprises a probe 2 having a corrosion resistant, electrically conducting component 3, and a corrodible sensing component 4. In this particular example, the sensing component 4 is provided by a filament, however a plate or a bar may be used instead. The probe 2 is typically positioned in the exhaust region of the vacuum pump within the fluid flow path to experience the harshest environment, as discussed above. However, especially where space is limited, the probe 2 may be positioned in a fore line or an exhaust line of the vacuum pump.

The corrosion resistant, conducting component 3 is connected to a controller 5 and is supported by a rigid, support member 6 by which it is fixed within the vacuum pump. A temperature sensor 7 is provided as part of the sensor. In Figure 1 the temperature sensor 7 is positioned in thermal communication with the support member 6, although the sensor 7 may be positioned remotely from the probe 2 and support member 6, in direct thermal communication with the process and cleaning fluids. In use, the probe 2 is exposed to a fluid flow stream 8, typically, within the exhaust region of the vacuum pump. In this example, the sensing component 4 is made from tungsten. When tungsten is exposed to fluorine a chemical reaction takes place, the result of which is a gaseous product which is readily dissipated when positioned in a fluid flow stream thus leaving the bulk tungsten material exposed to the fluid flow. In other words, no inert coating is formed on the surface of the tungsten that might prevent further reactions from occurring. Consequently, when the tungsten sensing component 4 is exposed to a gaseous, fluorine cleaning fluid, the exposed surface of the tungsten will react with the fluorine and any by-products from that reaction will be removed by the fluid stream 8. This causes the cross sectional area of the filament to be reduced.

Where pure tungsten is used, a very thin filament needs to be provided in order to achieve a suitable level of resistivity. However, a thin filament is corroded rapidly and, therefore, it may be desirable to provide a thicker filament. In order to prevent the resistivity of the device from becoming too low an alternative material may be used. The electrical properties of pure tungsten may be enhanced by adding small quantities of other elements to provide a tungsten alloy. Consequently, a thicker sensing component 4 can be provided which, in turn, can corrode for a longer period of time before its integrity is breached. In one example, a small quantity (approximately 3 % to 5% by weight) of rhenium is added to form a tungsten-rich alloy with enhanced electrical resistivity. Tungsten-rhenium alloys having up to 26% rhenium have been shown to be beneficial in increasing the resistivity of the corrodible member.

Returning to Figure 1 , the sensing component 4, here a filament, is suspended between two fixed supports formed by the corrosion resistant component 3. Consequently, the overall length of the sensing component 4 remains unchanged in use. An electrical current is passed through the probe 2 from controller 5 and, in this particular embodiment, the electrical resistance of the probe is monitored. In use, the cross sectional area of the filament of the sensing component 4 becomes diminished due to reaction with the fluid stream. As the length of the member is maintained, the resistance through the probe 2 will steadily increase in direct proportion to the decrease in cross-sectional area of the sensing component 4. This increase in resistance can therefore be used as an indicator of the total level of corrosion that has been experienced by the sensing component 4 due to its exposure to the fluid stream 8. Where the sensor component material is carefully selected to yield a gaseous product when it reacts with a corrosive medium, the cross sectional area of the sensing component 4 will continue to diminish with prolonged exposure to the corrosive medium rather than form an inert layer that would serve to protect the filament from further corrosion. Thus, cumulative effects of corrosion damage can be monitored with a high degree of accuracy. The level of exposure to a corrosive medium can be regarded as increasing under two different conditions. Firstly where there is an increased duration of exposure and secondly where the corrosive medium is supplied with an increased concentration.

The temperature of the fluid stream 8 is likely to fluctuate due to a number of varying parameters experienced by the pump. Examples of such parameters are differences in ambient temperature, quantity and type of fluid being displaced by the pump, build up of residue and therefore variation in clearances within the pump. The fluctuation in temperature of the fluid stream 8 causes a corresponding variation in temperature of the sensing component 4. The resistivity, p, of a material is dependent on temperature and so a variation in temperature of the sensing component will result in a corresponding fluctuation in resistance of the sensing component. This can lead to erroneous readings which indicate that corrosion has taken place to a greater or lesser degree than is actually the case. To overcome this, the temperature of the sensing component 4 is preferably actively maintained at a substantially constant value, typically above the ambient temperatures experienced within the pump, rather than allowing the temperature of the sensing component to fluctuate as it is passively heated by the fluid stream 8. Preferably, the sensing component 4 is heated directly by passing an electric current through the probe 2 components. The temperature of the probe is monitored using temperature sensor 7 and this reading is used to control the magnitude of electrical current that is passed to the probe. By maintaining the temperature of the sensing component 4 at a constant value the fluctuations in resistance due to fluctuating temperature are avoided. Consequently, any further changes in resistance can be regarded as being due to the changing condition (notably due to corrosion) of the sensing component 4 and a truer indication thereof is provided.

Alternative indirect techniques that may be used to heat the sensing component 4 include heating the support member 6 with a separate heating element. In this configuration, the support member 6 and the corrosion resistant component 3 would be formed from thermally conductive materials to allow the sensing component 4 to be heated through thermal conduction between components. Conversely, a radiative mechanism may be used to control the temperature of the sensing component 4. For example, the probe 2 may be positioned out of the primary fluid flow path in a chamber through which a part of the corrosive fluid is recirculated. This chamber may be surrounded by a heating (or even a cooling) element such that the probe within the chamber is subjected to a radiative thermal source (or sink). A further heating technique that may be used is electron bombardment of the sensing component 4.

In the main embodiment described above, the parameter monitored to determine the cumulative level of corrosion is the resistance of the sensing component 4. This is especially effective where the sensing component 4 is provided by a filament. Other configurations of sensing component, such as a bar or a sheet, are also envisaged. Figure 2 illustrates an embodiment where a bar 14 is utilised. The bar 14 is located within a hollow pipe 11 which is surrounded by a wire coil 12 such that an inductor is formed. The wire coil is connected to a controller 15 which, in use, receives a signal indicative of any monitored parameters, here inductance. The fluid stream 8 is passed through the pipe such that it is brought into contact with the bar 14. As the bar 14 is corroded the volume of material reduces (Figure 2b) and the inductive properties will change such that an indication of the level of corrosion is provided.

Figure 3 illustrates an embodiment where the sensor component is configured into a pair of corrodible plates 24, between which a layer of dielectric material 21 is placed, thus a capacitor is formed. The capacitor is supported by a pair of corrosion resistant springs 22. Each plate 24 of the capacitor is connected to a controller 25 to allow the capacitance of the device to be monitored. As the plates 24 are eroded (Figure 3b) the area of the capacitor will be reduced and the capacitance will be affected. An indication of the level of corrosion experienced within the pump can thus be provided by monitoring the value of capacitance returned to the controller 25 from the plates 24.

The sensor may form a part of a Wheatstone bridge circuit as illustrated in Figure 4 where a constant voltage is provided to the circuit from source 33. The sensor component 34 is located within the bridge as shown and, in use, device 32 indicates a level of corrosion experienced by the sensor component 34.

Claims

1. A corrosion sensor comprising: a corrodible member; means for monitoring an electrical property indicative of the extent of the corrosion of the corrodible member; and means for outputting a signal dependent upon the value of the monitored electrical property, wherein the corrodible member is formed from a material selected to form, during corrosion thereof, a by-product which does not adhere to the surface of the corrodible member.

2. A sensor according to Claim 1 , wherein the monitored parameter is one of the group of resistance, capacitance and inductance.

3. A sensor according to any Claim 1 or Claim 2, wherein the corrodible member is one of the group of a filament, a plate and a bar.

4. A sensor according to any preceding claim, comprising means for maintaining the corrodible member at a substantially constant temperature.

5. A corrosion sensor comprising: a corrodible member; means for maintaining the corrodible member at a substantially constant temperature; means for monitoring an electrical property indicative of the extent of the corrosion of the corrodible member; and means for outputting a signal dependent upon the value of the monitored electrical property, wherein the corrodible member is formed from a material selected to form, during corrosion thereof, a by-product which does not adhere to the surface of the corrodible member.

6. A sensor according to Claim 4 or 5, wherein the maintaining means comprises heating means.

7. A sensor according to Claim 6, wherein the heating means comprises means for supplying an electrical current to the corrodible member.

8. A sensor according to Claim 6, wherein the heating means comprises an external heating source.

9. A sensor according to Claim 8, wherein the external heating source is configured to heat a thermally conductive support member of the corrodible member whereby the corrodible member is heated by conduction.

10. A sensor according to any preceding claim, wherein the corrodible member is configured such that a cross-sectional area of the corrodible member continuously decreases with corrosion thereof.

11. A sensor according to any preceding claim, wherein the by-product is a fluid.

12. A sensor according to Claim 11 , wherein the by-product is a gas.

13. A sensor according to any preceding claim, wherein the material of the corrodible member is selected to react with a halogenated chemical.

14. A sensor according to Claim 13, wherein the material of the corrodible member is selected to react with a fluoro-chemical.

15. A sensor according to Claim 14, wherein the material of the corrodible member is selected to react with fluorine.

16. A sensor according to Claim 14 or 15, wherein the material of the corrodible member comprises tungsten.

17. A sensor according to Claim 16, wherein the material of the corrodible member comprises a further component selected to increase the electrical resistance of the corrodible member.

18. A sensor according to Claim 17, wherein the further component comprises rhenium.

19. A sensor according to Claim 18, wherein the corrodible member comprises less than 26% rhenium.

20. A sensor according to Claim 19, wherein the corrodible member comprises between 3% and 5% rhenium.

21. A sensor for determining the cumulative level of exposure of a pump to a corrosive fluid passing therethrough, the sensor comprising a corrodible member beatable within the pump and formed from a material selected to react with the corrosive fluid such that the size of the corrodible member continuously decreases with exposure to the corrosive fluid; means for monitoring an electrical property of the corrodible member, which varies with the size of the corrodible member; and means for outputting a signal dependent on the value of the monitored electrical property.

22. A vacuum pump comprising a sensor according to any preceding claim.

23. A method for detecting corrosion due to exposure to a corrosive fluid comprising the steps of: providing a corrodible member within the path of a stream of corrosive fluid; supplying heat to the corrodible member, to actively maintain the temperature thereof at a substantially constant value; monitoring an electrical property indicative of the extent of corrosion of the corrodible member; and outputting a signal indicative of the corrosion of the corrodible member, depending on the value of the monitored electrical property.

24. Use of a tungsten-rich member within a vacuum pump to determine, from corrosion thereof, the condition of the pump as a result of exposure to fluorine passing through the pump.