Title of the invention: Device and method for monitoring a deposit of reaction by-products for the discharge of a vacuum pump

The present invention relates to a device and a method for monitoring a deposit of reaction by-products for the discharge of a vacuum pump. The invention relates also to a vacuum pump provided with said monitoring device.

In vacuum applications, in particular in the semiconductor industry or in thin-film deposition processes, vacuum pumps convey various types of gases and evaporated substances which can, because of changes in the pressure or temperature conditions or changes in the nature of the chemical reactions, be deposited on the inner surfaces of the vacuum pump.

The deposits of reaction by-products can be solids, polymers or even dusts. These deposits tend to accumulate, particularly in the high-pressure zones or cold zones of the vacuum pumps. They reduce the gas passage section, which can reduce the pumping performance. The reduction of the dimensions of the gas passage section also creates an increase in pressure which can, by cascade effect, provoke an even greater deposition of by-products.

Regular maintenance must therefore be scheduled to frequently clean the vacuum pump. Such maintenance is however incompatible with the production rate imperatives. It is consequently sought to monitor the formation of deposits in the vacuum pumps in order to space apart the intervals between maintenance operations as much as possible. One of the difficulties however is that it is not possible to observe the interior of the vacuum pumps without having to stop to wholly or partly dismantle them. Furthermore, in some applications, the exposure of the interior of the vacuum pump to the open air can be hazardous.

Many known sensor technologies allow these deposits, and their growth in vacuum pumps, to be monitored.

In the case of turbomolecular vacuum pumps, one known method consists in measuring the current of the motor or the positions of the magnetic levitation rotor in order to determine the possible presence of by-products. Changes in the motor current or in the positions of the magnetic levitation rotor can provide information on the presence of deposits. This strategy may however not be sufficiently accurate, notably because the increase in the current is generally detected much too late, only a few seconds or fractions of seconds before the crash, without the possibility of intervening in time.

One of the objects of the present invention is therefore to propose a device and a method for monitoring deposits of by-products, which at least partially resolve one of the abovementioned drawbacks.

To this end, the subject of the invention is a device for monitoring a deposit of reaction by products for the discharge of a vacuum pump, characterized in that it comprises:

- a thermal flowmeter comprising:

- a first temperature probe placed at an upstream location in the direction of flow of the gases at the discharge,

- a second temperature probe placed at a downstream location,

- a heating element interposed between the temperature probes,

- a substrate insulating the temperature probes and the heating element from one another, and

- a processing unit configured to perform a measurement by the thermal flow meter in order to determine the presence of a deposit of reaction by-products at the discharge as a function of the difference between the flow rate measured by the thermal flowmeter and an estimated value of the flow rate of gas pumped by the vacuum pump.

The monitoring device thus allows the presence of deposits at the discharge of the vacuum pump to be detected more accurately and at the earliest possible moment.

The monitoring device can further comprise one or more of the features described hereinbelow, taken alone or in combination.

The thermal flowmeter can be a MEMS component.

The monitoring device can further comprise a pressure sensor configured to determine the pressure at the discharge of the vacuum pump, the processing unit being configured to estimate the pumped gas flow rate on the basis of information on a power parameter of the motor and on the measurement from the pressure sensor. The estimated value of the pumped gas flow rate can then be obtained from information available via the vacuum pump alone, that is to say without having access to the information on the quantities and nature of the gases introduced upstream of the vacuum pump.

The power parameter of the motor of the vacuum pump can be the electrical current.

The processing unit can be configured to communicate with a process chamber depressurized by means of the vacuum pump, to estimate the pumped gas flow rate. The information transmitted by the process chamber to the processing unit can then allow the value of the pumped gas flow rate to be accurately estimated.

Also a subject of the invention is a vacuum pump comprising:

- a stator comprising an inlet orifice and an outlet orifice,

- at least one rotor arranged in the stator and configured to drive a gas to be pumped between the inlet orifice and the outlet orifice, characterized in that the vacuum pump further comprises a monitoring device as described previously, the thermal flowmeter being arranged inside the vacuum pump.

The monitoring device thus allows the presence of deposits in the discharge of the vacuum pump to be detected more accurately and at the earliest possible moment, which makes it possible to better manage the scheduling of the maintenance interventions. The monitoring can be performed in situ, that is to say without having to dismantle the vacuum pump. The measurement device is non-invasive. It generates no pressure or sealing losses. It has no moving parts, which limits the possibilities of malfunctions.

The thermal flowmeter is, for example, arranged in a duct of the discharge.

The vacuum pump is, for example, a turbomolecular vacuum pump.

According to another example, the vacuum pump is a rough-vacuum pump comprising two rotors configured to revolve synchronously in reverse directions in at least one pumping stage to drive a gas to be pumped between the inlet orifice and the outlet orifice.

Yet another subject of the invention is a method for monitoring a deposit of reaction by products for the discharge of a vacuum pump by means of a monitoring device as described previously, in which a measurement is performed by the thermal flowmeter to determine the presence of a deposit of reaction by-products at the discharge as a function of the difference between the flow rate measured by the thermal flowmeter and an estimated value of the flow rate of gas pumped by the vacuum pump.

The heating element of the thermal flowmeter can be powered to perform a measurement at intervals spaced apart by more than 10 hours, such as a daily measurement. The duration of a measurement by the thermal flowmeter can be less than a few minutes, such as less than two minutes, even less than one minute. The highest observed deposition rates in turbomolecular vacuum pumps, notably in semiconductor fabrication processes, such as etching equipment, are generally less than 1 mm per week, i.e. approximately 5 mΐti per hour, so a relatively low measurement frequency can suffice to observe the appearance of a deposit. Limiting the duration of the measurements to a few seconds per day prevents the measurement performed by the thermal flowmeter from being able to falsify the results, by preventing the deposition of condensable species at the point of the flowmeter, because of the input of heat from the heating element. In fact, the deposits are reduced, even nonexistent, at high temperature.

The thickness of a deposit can be assessed as a function of the value of the deviation of the measurement from the thermal flowmeter.

The method for monitoring a deposit can comprise a preliminary calibration step in which at least one measurement from the thermal flowmeter, obtained for a predetermined gas flow rate in a vacuum pump, is recorded. The various data that can be collected in the preliminary calibration step can allow for better interpretation of the values measured by the thermal flowmeter, notably as a function of the pumped gas flow rates, of the pumped gaseous species, of the nature of the deposit and of the thicknesses of the deposit.

These measurements can be performed for the flow rate and nature values of the gases defined in the recipes which are to be carried out in the process chamber connected to the vacuum pump, notably for characteristic steps of these recipes.

For example, when the monitoring method provides a measurement performed on a particular deadline and the gas flow rate and the nature of the gas pumped at that moment are known, the preliminary calibration step can record the measurement obtained by the thermal flowmeter for the gas flow and nature values of that specific operating point. Description of the drawings

Other objects, features and advantages of the present invention will emerge from the following description of particular embodiments, given with reference to the attached drawings in which:

[Fig. 1] Figure 1 shows a schematic view of a process chamber of fabrication equipment, connected to a vacuum line.

[Fig. 2] Figure 2 shows a schematic view of a thermal flowmeter arranged in a discharge pipe of the vacuum line of Figure 1 and on which a thermal distribution in the absence of gas flow and in the absence of deposits is schematically represented.

[Fig. 3] Figure 3 shows a view of the thermal flowmeter similar to Figure 2 in the presence of a gas flow.

[Fig. 4] Figure 4 shows a view of the thermal flowmeter similar to Figure 3 in the presence of a gas flow and a deposit.

[Fig. 5] Figure 5 shows a schematic cross-sectional view of a turbomolecular vacuum pump.

[Fig. 6] Figure 6 shows a partial schematic view of elements of another exemplary embodiment of a vacuum pump.

In the figures, elements that are identical bear the same references. The drawings are simplified to facilitate understanding.

The following embodiments are examples. Although the description refers to one or more embodiments, that does not necessarily mean that each reference relates to the same embodiment, or that the features apply only to a single embodiment. Simple features of different embodiments can also be combined or interchanged to provide other embodiments.

“Upstream” is understood to mean an element which is placed in front of another with respect to the direction of flow of the gas. On the other hand, “downstream” is understood to mean an element placed after another with respect to the direction of circulation of the gas to be pumped.

Figure 1 illustrates an example of equipment 101 for fabricating, for example, flat display screens or photovoltaic substrates or semiconductor substrates (wafers).

The equipment 101 comprises a process chamber 102 connected to a vacuum line comprising a turbomolecular vacuum pump 1 which is itself arranged upstream of a rough-vacuum pump 100 by a discharge pipe 103.

Also in Figure 1, it can be seen that the vacuum line comprises a device 200 for monitoring a deposit of reaction by-products at the discharge 7 of the vacuum pump 1.

The monitoring device 200 comprises a thermal flowmeter 20 and a processing unit 22.

The thermal flowmeter 20 can be arranged at the discharge 7 of the turbomolecular vacuum pump 1, in the turbomolecular vacuum pump 1 itself as will be seen later or in the discharge pipe 103 connected to the outlet of the turbomolecular vacuum pump 1 as represented in Figure 1, or can be arranged at the discharge of the rough-vacuum pump 100, in the rough-vacuum pump 100 or in a pipe connected to the outlet of the rough-vacuum pump 100.

In the first example of Figure 1, the thermal flowmeter 20 is arranged in the discharge pipe 103 connected to an outlet orifice 8 of the turbomolecular vacuum pump 1.

The thermal flowmeter 20 is used to measure the flow rate of a gas flowing in a duct or pipe. The principle of a thermal flowmeter 20 is based on the propagation of heat by convection through a fluid. As is known per se, a thermal flowmeter 20 comprises two temperature probes: a first temperature probe 23 placed at an upstream location in the direction of flow of the gases at the discharge 7 and a second temperature probe 24 placed at a downstream location (Figure 2).

The thermal flowmeter 20 also comprises a heating element 25 interposed between the temperature probes 23 and 24, and a substrate 26 insulating the temperature probes 23 and 24 and the heating element 25 from one another. The heating element 25 is, for example, a heating resistor. The temperature probes 23, 24 are, for example, thermistors. The substrate 26 encapsulates, for example, the temperature probes 23, 24 electrically and thermally insulating them from one another and protecting them from possible attack from the gases.

The temperature probes 23, 24 are, for example, disposed equidistant from the heating element 25. The temperature probes 23, 24 and the heating element 25 can be aligned along a straight line parallel to the axis of the discharge pipe 103 in which the thermal flowmeter 20 is arranged.

To perform a measurement by the thermal flowmeter 20, power is supplied to the heating element 25 which heats up, for example to 100°C, and the temperature difference between the temperature probes 23, 24 is measured.

When no gas flows in the discharge pipe 103, the heat diffused by the heating element 25 is uniformly distributed around the heating element 25 (Figure 2). The temperature probes 23, 24 allow a first temperature difference to be measured, which is zero when the probes 23, 24 are equidistant from the heating element 25.

When a gas flow flows in the discharge pipe 103, the heat convection lowers the temperature measured by the first temperature probe 23 placed upstream and raises the temperature measured by the second temperature probe 24 placed downstream (Figure 3). A temperature difference is then observed between the temperature probes 23, 24 that is greater than that that can be observed in the absence of circulation of a gas flow. This difference allows a measurement of the gas flow rate to be deduced therefrom.

The thermal flowmeter 20 can be a MEMS (MicroElectroMechanical Systems) component, manufactured from semiconductor materials. The dimensions of the thermal flowmeter 20 are then less than a centimetre.

The processing unit 22 comprises a controller or microcontroller or computer or programmable logic controller and computer programs configured to implement a method for monitoring a deposit of reaction by-products at the discharge of the vacuum pump 1. The processing unit 22 is, for example, the controller of the vacuum pump 1 notably allowing the frequency of rotation of the rotor of the vacuum pump 1 to be controlled.

The processing unit 22 is configured to perform a measurement by the thermal flowmeter 20 in order to determine the presence of a deposit of reaction by-products at the discharge 7 as a function of the difference between the flow rate measured by the thermal flowmeter 20 and an estimated value of the flow rate of gas pumped by the vacuum pump 1.

The estimated value of the pumped gas flow rate can be obtained from information available from the vacuum pump 1 alone, that is to say without having access to the information on the quantities and nature of the gases introduced upstream of the vacuum pump 1.

For that, according to an exemplary embodiment, the monitoring device 200 comprises a pressure sensor 21 configured to determine the pressure at the discharge 7 of the vacuum pump 1 (Figure 1).

The processing unit 22 is then configured to estimate the pumped gas flow rate from information on a power parameter of the motor 16 of the vacuum pump 1 and from the measurement of the pressure sensor 21.

The power parameter of the motor 16 of the vacuum pump 1 is, for example, the electrical current. The current consumed by the motor 16 and the pressure at the discharge 7 of the vacuum pump 1 depend on the gas flow rate and on the nature of the gas pumped. By measuring the pressure at the discharge 7 and knowing the current consumed by the motor 16, it is possible to estimate a pumped gas flow rate value that can be compared to the value measured by the thermal flowmeter 20.

According to another exemplary embodiment, the processing unit 22 is configured to communicate with the process chamber 102 depressurized by means of the vacuum pump 1 to estimate the pumped gas flow rate. The process chamber 102 uses recipes defining the durations, the nature, the flow rates and the pressures of the gases introduced into the chamber. These recipes, or elements of these recipes, are information which can be transmitted by the process chamber 102 to the processing unit 22 which can then accurately estimate the pumped gas flow rate value. The information transmitted by the chamber 102 can be a digital signal or a dry contact, or the like.

A variation in the difference between the value measured by the thermal flowmeter 20 and the estimated value of the pumped gas flow rate allows the presence of a deposit 27 of reaction by-products in the pipe 103 (figure 4) to be determined.

In fact, in the absence of deposits, for one and the same flow and one and the same nature of gas, identified for example by the pressure measurement and the motor current consumed, the difference between the temperatures measured by the temperature probes 23, 24 is the same.

However, when a deposit 27 appears on the internal walls of the discharge pipe 103, and in particular on the thermal flowmeter 20, then a variation in the difference in the measured temperatures is observed. The layer of deposit 27 deposited on the temperature probes 23, 24 reduces the transfer of heat to the second temperature probe 24 placed downstream (Figure 4). The temperature measured by the second temperature probe 24 decreases in relation to the situation without deposit for the same flow of the same gas (Figure 3). The flow rate measured by the thermal flowmeter 20 is therefore different in the presence of a deposit for the same flow rate of the same gas.

The difference observed, between the flow rate measured by the thermal flowmeter 20 and the estimated value of the pumped gas flow rate, by means of the pressure measurement and the value of the current consumed for example, makes it possible to conclude on the presence of a deposit 27 of reaction by-products.

The duration of a measurement by thermal flowmeter 20 can be less than two minutes, such as less than one minute. A measurement is for example performed by the thermal flowmeter 20 at intervals spaced apart by more than 10 hours, such as a daily measurement.

The highest deposition rates observed in turbomolecular vacuum pumps 1, notably in semiconductor fabrication processes, such as etching equipment, are generally less than 1 mm per week, i.e. approximately 5 mhi per hour, so a relatively low measurement frequency can suffice to observe the appearance of a deposit. Limiting the duration of the measurements to a few seconds per day prevents the measurement performed by the thermal flowmeter 20 from being able to falsify the results, by preventing the deposition of the condensable species at the point of the flowmeter 20, because of the input of heat from the heating element 25. In fact, the deposits are reduced, even nonexistent, at high temperature.

It is also possible to assess the thickness of the deposit 27 as a function of the value of the deviation of the measurement given by the thermal flowmeter 20. The greater the deviation in the temperature difference measured by the thermal flowmeter 20 with respect to the expected value, the greater the thickness of the deposit 27.

The monitoring method can also comprise a preliminary calibration step in which at least one measurement from the thermal flowmeter 20, obtained for a predetermined flow rate of gas, pumped by the vacuum pump 1, is recorded in the processing unit 22.

Several measurements from the thermal flowmeter 20 for different gas flow rates and/or different gaseous species pumped by the vacuum pump 1 are for example recorded.

These measurements can be performed for the flow rate and nature values of the gases defined in the recipes which are to be carried out in the process chamber 102 connected to the vacuum pump 1.

These measurements can be performed for characteristic steps of these recipes.

For example, when the monitoring method provides a measurement performed on a particular deadline and the gas flow rate and the nature of the gas pumped at that moment are known, the preliminary calibration step can record the measurement obtained by the thermal flowmeter 20 for the flow rate values and the nature of the gases at that specific operating point.

These measurements can be performed in the presence of a deposit 27 on the internal walls of the discharge pipe 103, for example for several thicknesses of deposit 27, to allow the thickness of the deposit 27 to be assessed as a function of the valuation of the deviation of the measurement from the thermal flowmeter 20.

These measurements can also be performed in the absence of deposit, for example after each maintenance operation, on startup, when the discharge pipe 103 is free of deposits. The measurements performed during the monitoring method can then be compared to these reference values.

The various data that can be collected during the preliminary calibration step can allow better interpretation of the values measured by the thermal flowmeter 20, notably as a function of the flow rates of the gases pumped, of the gaseous species pumped, of the nature of the deposit and of the thicknesses of the deposit.

It is understood from what has just been described that the monitoring method and device allow the presence of deposits at the discharge 7 of the vacuum pump 1 to be detected more accurately and at the earliest possible moment.

Figure 5 illustrates a second exemplary embodiment in which the thermal flowmeter 20 is arranged inside the turbomolecular vacuum pump 1.

As can be seen more specifically in this figure, the turbomolecular vacuum pump 1 comprises a stator 2 and a rotor 3 arranged in the stator 2 and configured to drive a gas to be pumped between an inlet orifice 6 and an outlet orifice 8 of the stator 2 in a direction of flow of the gases that is represented by the arrows in Figure 5.

The vacuum pump 1 comprises a turbomolecular stage 4 and a molecular stage 5 situated downstream of the turbomolecular stage 4 in the direction of flow of the gases. The pumped gases enter through the inlet orifice 6, pass first of all through the turbomolecular stage 4, then through the molecular stage 5, and then the discharge 7, to be then dispelled through the outlet orifice 8 of the vacuum pump 1. The outlet orifice 8 is connected to a rough-vacuum pump.

In the turbomolecular stage 4, the rotor 3 comprises at least two stages of blades 9 and the stator 2 comprises at least one stage of fins 10. The stages of blades 9 and of fins 10 follow one another axially along the axis of rotation l-l of the rotor 3 in the turbomolecular stage 4. The rotor 3 comprises, for example, more than four stages of blades 9, such as, for example, between four and eight stages of blades 9 (six in the example illustrated in Figure 1).

Each stage of blades 9 of the rotor 3 comprises inclined blades which extend substantially radially from a hub 11 of the rotor 3 fixed to a shaft 12 of the turbomolecular vacuum pump 1. The blades 9 are evenly distributed on the periphery of the hub 11.

Each stage of fins 10 of the stator 2 comprises a crown ring from which extend, in a substantially radial direction, inclined fins, evenly distributed on the inner perimeter of the crown ring. The fins of a stage of fins 10 of the stator 2 are engaged between the blades of two successive stages of blades 9 of the rotor 3. The blades of the rotor 3 and the fins of the stator 2 are inclined to guide the pumped gas molecules to the molecular stage 5.

According to an exemplary embodiment, the rotor 3 comprises a Holweck skirt 13 in the molecular stage 5, formed by a smooth cylinder, which revolves opposite helical grooves of the stator 2. The helical grooves allow the pumped gases to be compressed and guided to the discharge 7.

The rotor 3 is fixed to the shaft 12 which is driven in rotation at high axial rotation speed in the stator 2, for example with a rotation at more than twenty thousand revolutions per minute, by means of a motor 16 of the turbomolecular vacuum pump 1. The motor 16 is, for example, arranged under a cover of the stator 2, which is itself arranged under the Holweck skirt 13 of the rotor 3. The rotor 3 is guided laterally and axially by magnetic or mechanical bearings 18. The vacuum pump 1 can also comprise backup rolling bearings 19.

The thermal flowmeter 20 is placed at the discharge 7 inside the vacuum pump 1, which corresponds to the volume contained between the outlet of the rotor 3, that is to say, here, the end of the Holweck skirt 13, and the outlet orifice 8, at the point where there is no longer compression of the gases but where the pressures are the highest and the risk of deposition is the greatest in the vacuum pump 1.

The thermal flowmeter 20 is, for example, arranged in a duct 14 of the discharge 7, that is to say a pipe which generally has the standard diameter of vacuum couplings and which emerges through the outlet orifice 8.

As in the preceding example, the processing unit 22 is configured to perform a measurement by the thermal flowmeter 20 in order to determine the presence of a deposit of reaction by-products at the discharge 7 of the vacuum pump 1 as a function of the difference between the flow rate measured by the thermal flowmeter 20 and an estimated value of the flow rate of gas pumped by the vacuum pump 1.

As previously, the estimated value of the pumped gas flow rate can be obtained from information available from the vacuum pump 1 alone, that is to say without having access to the information on the quantities and nature of the gases introduced upstream of the vacuum pump 1.

For that, according to one exemplary embodiment, the monitoring device 200 comprises a pressure sensor 21 configured to determine the pressure at the discharge 7 of the vacuum pump 1. The pressure sensor 21 is, for example, also arranged in the duct 14 of the stator 2.

The difference observed between the flow rate measured by the thermal flowmeter 20 and the estimated value of the pumped gas flow rate, by means of the pressure measurement and the value of the current consumed, for example, makes it possible to conclude on the presence of a deposit 27 of reaction by-products.

The monitoring device 200 thus allows the presence of deposits in the discharge 7 of the vacuum pump 1 to be detected more accurately and at the earliest possible moment, which allows for better management of the scheduling of the maintenance interventions.

The monitoring can be performed in situ, that is to say without needing to dismantle the vacuum pump 1. The measurement device is non-invasive. It does not generate pressure or sealing losses. It does not have any moving parts, which limits the possibilities of malfunctions. Figure 6 illustrates a third exemplary embodiment in which the thermal flowmeter 20 is arranged inside the rough-vacuum pump 100.

As represented in this figure, the rough-vacuum pump 100 comprises a stator 2 forming at least one pumping stage, such as between two and ten pumping stages, here five, mounted in series between an inlet orifice 6 and an outlet orifice 8 of the stator 2 and in which a gas to be pumped can circulate.

The pumping stage communicating with the inlet orifice 6 of the vacuum pump 100 is the first pumping stage or lowest pressure stage and the pumping stage communicating with the outlet orifice 8 is the last pumping stage or highest pressure stage. The vacuum pump 1 further comprises two rotors 300 arranged in the stator 2 and configured to revolve synchronously in reverse directions in the pumping stages to drive a gas to be pumped between the inlet orifice 6 and the outlet orifice 8. The rotors 300 have, for example, lobes of identical profiles, for example of “Roots” type with two lobes, three lobes or more, or of “claw” type, or based on another similar volumetric vacuum pump principle. In operation, the rotors 300 are driven in rotation by a motor, for example arranged at an end of the vacuum pump 1, such as on the side of the outlet orifice 8.

During rotation, the gas sucked from the inlet orifice 6 is captured in the volume created by the rotors 300 and the stator 2 of the pumping stage, then is compressed and driven to the outlet and to the next stage. The vacuum pump 100 is said to be “dry” because, in operation, the rotors 300 revolve inside the stator 2 with no mechanical contact between them or with the stator 2, but with very small clearances, which allows for the absence of oil in the compression chambers.

In this embodiment, the discharge 7 is defined by the volume contained between the outlet of the rotors 300 of the last pumping stage and the outlet orifice 8, at the point where there is no longer compression of the gases but where the pressures are the highest and the risk of deposition the greatest.

The thermal flowmeter 20 is, for example, arranged in a duct of the discharge 7, that is to say in a pipe which generally has the standard diameter of vacuum couplings linking the outlet of the rotors 300 of the last pumping stage to the outlet orifice 8.