EP3485168A1

EP3485168A1 - Method for lowering the pressure in a loading and unloading lock and associated pumping unit

Info

Publication number: EP3485168A1
Application number: EP17735085.7A
Authority: EP
Inventors: Eric MANDALLAZ; Christophe SANTI
Original assignee: Pfeiffer Vacuum SAS
Current assignee: Pfeiffer Vacuum SAS
Priority date: 2016-07-13
Filing date: 2017-06-29
Publication date: 2019-05-22
Anticipated expiration: 2037-06-29
Also published as: TWI723186B; CN109477485B; WO2018010970A1; KR20190022880A; TW201804083A; CN109477485A; KR102404612B1; FR3054005B1; EP3485168B1; FR3054005A1

Abstract

The invention relates to a method for lowering the pressure in a lock for loading and unloading a substrate at atmospheric pressure by a pumping unit (1) including a primary vacuum pump (2) and a secondary vacuum pump (3) arranged upstream from said primary vacuum pump (2) in the direction of flow of the gases to be pumped. During the lowering of the pressure until the pressure in the loading and unloading lock reaches a predefined low-pressure threshold, the speed of rotation of the secondary vacuum pump (3) is controlled as a function of an operating parameter of the secondary vacuum pump (3) in order to increase the flow rate generated by the secondary vacuum pump (SoR) so that the flow rate generated by the secondary vacuum pump (SoR) is comprised in a range of which the upper value corresponds to six times the flow rate generated by the primary vacuum pump (Sol) and the low value corresponds to 1.3 times the flow rate generated by the primary vacuum pump (Sol). The invention also relates to a pumping unit for implementing said pressure-lowering method.

Description

Pressure lowering method in a loading and unloading chamber and associated pumping unit

The present invention relates to a method of descent pressure in a lock of loading and unloading (or "load-lock" in English) of a substrate, such as a flat panel display ("or flat panel display" in English) or a photovoltaic substrate, from atmospheric pressure to a low pressure for loading and unloading the substrate into a treatment chamber maintained at low pressure. The present invention also relates to an associated pumping unit for implementing said pressure-lowering method.

In some manufacturing processes, an important step is to treat a substrate under controlled atmosphere at very low pressure in a process chamber. To maintain an acceptable rate and to avoid the presence of any impurity and any pollution, the atmosphere surrounding the substrate is first lowered at low pressure by a loading lock and unloading communicating with the process chamber.

For this, the airlock comprises a sealed chamber, a first door communicates the interior of the chamber with an atmospheric pressure zone, such as a clean room, for loading at least one substrate. The chamber of the lock chamber is connected to a pumping unit enabling the pressure in the chamber to be lowered until a suitable low pressure is obtained similar to that prevailing in the process chamber so that the substrate can be transferred to the process chamber. . The airlock further includes a second door for unloading the substrate into the process chamber after being evacuated. This same airlock is generally also used for the rise in pressure of the substrate at the end of its treatment, and its discharge at atmospheric pressure.

However, it is understood that each loading or unloading of substrates requires to lower and then back up alternately the pressure in the enclosure of the lock, which involves the frequent intervention of the pumping unit. It is also understood that the establishment of the vacuum in the chamber of the lock is not instantaneous and that this is a limit to the overall speed of the manufacturing process. This limit is even more sensitive when the substrates are large. This is particularly the case for the manufacture of flat displays or photovoltaic substrates, the chamber of the chamber necessarily having the appropriate volume to contain one or more flat screens. For example, at present, the airlock enclosures used for the manufacture of flat screens have large volumes, generally of the order of 500 to 1000 liters, and sometimes exceeding 5000 liters, which must therefore be pumped as quickly as possible. For this purpose, particularly powerful pumping units are used, in particular to ensure the pumping at the opening of the airlock, when the pressure in the chamber is at atmospheric pressure.

Generally, the pumping unit comprises one or more primary vacuum pumps and a secondary vacuum pump, such as a Roots single-stage vacuum pump. The secondary vacuum pump is arranged upstream of the primary pump in the direction of flow of the gases to be pumped. Its main purpose is to "boost" the overall pumping speed of the low pressure pumping unit.

The flow rate generated by the secondary vacuum pump can be of the order of five times the flow rate generated by the primary pump. Because of the strong gas flow at the opening of the airlock, a significant pressure is generated at the discharge of the secondary vacuum pump, up to 4 bar (or 3 bar). This high overpressure causes a very high consumption of the secondary vacuum pump and a clogging at the suction of the primary vacuum pump, constituting a risk of malfunction for both the primary vacuum pump and the secondary vacuum pump.

To avoid this, a known solution consists of arranging a pipe connecting the inlet of the primary vacuum pump to the inlet of the secondary vacuum pump. The pipe is equipped with a recirculation valve (or "bypass" in English), calibrated to open when the pressure difference between the suction and discharge of the secondary vacuum pump becomes too high, usually tared for s open for a maximum pressure difference between 50 and 80 mbar. Thus, the recirculation valve opens at the beginning of the descent in pressure, to circulate the surplus of the gas flow of the discharge to the suction of the secondary vacuum pump. Then, for an upstream / downstream pressure difference of the secondary vacuum pump lower than 50 or 80 mbar, the recirculation valve closes. At high pressure, the pressure drop is only ensured by the primary vacuum pump, the role of the secondary vacuum pump is then only to participate in the "recirculation" of the gas flow.

The recirculation valve thus makes it possible to protect the primary vacuum pump by diverting the surplus gas flow. This recirculation also makes it possible to thermally protect the secondary vacuum pump by preventing its discharge pressure from becoming too great.

The lowering of the pressure in the airlock then generates the pressure reduction at the discharge of the secondary vacuum pump and the closure of the recirculation valve, thus allowing the secondary vacuum pump to start compressing the gases to be pumped from the pump. a pressure in the airlock generally of the order of 200 mbar.

This device of the state of the art may, however, have certain disadvantages. At the start of the pressure drop, the initial overall pumping speed of the pumping unit is low because pumping is only performed by the primary vacuum pump.

In addition, until the pressure in the airlock reaches a few mbar, the power consumed by the secondary vacuum pump is large and lost due to the recirculation of the gas flow.

Another problem lies in the fact that the recirculation valve operates in a pulsating manner, opening and closing cyclically very quickly, in particular because of the cyclical pump principle of the volumetric secondary vacuum pump. From this may result risks of premature mechanical wear of the recirculation valve and therefore the risk of leakage. Also, the pulsating operation of the recirculation valve may cause unwanted noise.

Furthermore, the gases flowing in the recirculation valve pipe are hot, due to the compression of the secondary vacuum pump. These recycled hot gases can also contribute to the heating of the secondary vacuum pump.

One of the aims of the present invention is therefore to propose a pressure descent method in a loading and unloading chamber and an associated pumping unit which at least partially solve the problems of the state of the art, in particular by allowing to increase the pumping speed at the start of the pressure drop while reducing the power consumed by the secondary vacuum pump.

Another object of the present invention is to protect the primary vacuum pump and the secondary vacuum pump from the risk of damage due to excess gas flow at the opening of the air pressure chamber.

Another object of the present invention is to limit the risks of wear of the recirculation valve and the heating of the secondary vacuum pump by the "recycled" hot gases.

For this purpose, the subject of the invention is a method of descent in pressure in a chamber for loading and unloading a substrate at atmospheric pressure by a pumping unit comprising a primary vacuum pump and a secondary vacuum pump arranged in upstream of said primary vacuum pump in the direction of flow of the gases to be pumped, characterized in that during the descent in pressure and until the pressure in the loading chamber and unloading reaches a threshold of predefined low pressure, the rotation speed of the secondary vacuum pump is controlled according to an operation parameter of the secondary vacuum pump to increase the flow rate generated by the secondary vacuum pump so that the flow rate generated by the pump secondary vacuum is included in a range whose high value corresponds to six times the flow generated by the primary vacuum pump and the low value to 1, 3 times the flow generated by the primary vacuum pump.

According to one or more characteristics of the pressure reduction process, taken alone or in combination,

the operating parameter of the secondary vacuum pump is a motor parameter of the secondary vacuum pump,

the speed of rotation of the secondary vacuum pump is started as a function of an operation parameter of the secondary vacuum pump when it is detected that the value of an operation parameter of the secondary vacuum pump exceeds one predefined launch threshold for a first predefined duration,

if the value of an operation parameter of the secondary vacuum pump is greater than a predefined safety threshold beyond a second predetermined duration, the decrease in the speed of rotation of the secondary vacuum pump is forced,

if the value of an operating parameter of the secondary vacuum pump is below a predefined waiting threshold beyond a third predetermined time, the rotation speed of the secondary vacuum pump is checked at a speed of reduced fixed rotation.

The invention also relates to a pumping unit comprising a primary vacuum pump and a secondary vacuum pump, said secondary vacuum pump being arranged upstream of said primary vacuum pump in the direction of flow of the gases to be pumped and having a frequency converter, characterized in that the secondary vacuum pump comprises a control unit connected to the frequency converter, configured to control the rotational speed of the secondary vacuum pump as a function of a signal representative of a parameter of operation of the secondary vacuum pump, so that during the descent in pressure and until the pressure in the loading and unloading chamber reaches a preset low pressure threshold, the flow rate generated by the pump the secondary vacuum is increased and included in a range whose high value corresponds to six times the flow generated by the primary vacuum pump and the value b 1, 3 times the flow it is generated by the primary vacuum pump.

According to a particular embodiment, the primary vacuum pump comprises a load shedding module of a pumping stage. The signal representative of an operating parameter of the secondary vacuum pump is for example a parameter of the motor of the secondary vacuum pump, such as the current or the power.

According to an exemplary embodiment, the pumping unit comprises a recirculation pipe connecting the inlet of the primary vacuum pump to the inlet of the secondary vacuum pump, the recirculation pipe comprising a discharge module configured to open. as soon as the suction pressure of the primary vacuum pump exceeds the suction pressure of the secondary vacuum pump by a preset overshoot value of between 100 and 400 mbar.

The secondary vacuum pump is for example a vacuum pump type ROOTS.

By maintaining the flow rate generated by the secondary vacuum pump greater than 1, 3 times the flow rate generated by the primary vacuum pump and less than six times the flow generated by the primary vacuum pump during the pressure reduction and up to in that the pressure in the loading and unloading airlock reaches a predefined low pressure threshold, the ratio of the flow rates generated between the primary vacuum pump and the secondary vacuum pump is adapted to be optimal. More specifically, a flow rate generated by the secondary vacuum pump is maintained for the strong initial gas flow, that is to say less than six times the flow rate generated by the primary vacuum pump. Simultaneously, an optimized generated flow rate is maintained for the primary vacuum pump, that is to say greater than 1.3 times the flow rate generated by the primary vacuum pump to ensure at the earliest possible compression of the gases.

The pressure difference between the suction and the discharge of the secondary vacuum pump then remains below a value between 150 and 300 mbar. The pipe and the recirculation valve of the device of the prior art, calibrated to open for a differential pressure of the secondary vacuum pump of between 50 and 80 mbar, can then be eliminated. For safety reasons, the pumping unit may however comprise a discharge module configured to open as soon as the pressure difference between the suction and the discharge of the secondary vacuum pump exceeds a higher value, between 100 and 400 mbar. , depending on the value of the flow ratio that is selected and according to the mechanical safety settings, to protect the vacuum pumps including the time that the speed control is effective.

As soon as the rotation speed of the secondary vacuum pump is controlled, the secondary vacuum pump is no longer "short-circuited", as it was above a pressure of 200mbar in the airlock of the airlock. state of the art, but it is used as if it were the real first pumping stage of the primary vacuum pump. The operating characteristics of the secondary vacuum pump are thus adapted to the capacities of the primary vacuum pump so that the Secondary vacuum pump can be effective almost at atmospheric pressure. This results in a significant decrease in the power consumed as well as an increase in the overall pumping speed of the pumping unit at the beginning of the pressure drop, and therefore a reduction in the pressure drop time in the airlock. For example, in the pressure range of 1000 mbar to 20mbar, the pumping speed increases by 20 to 50%, compared to the pumping speed of the devices of the prior art. Likewise, the overall pressure drop time in the enclosure of a 500-liter airlock between a pressure close to 1000 mbar and a transfer pressure of the order of 0.1 mbar increases from 25 to 20 seconds, ie a reduction of around 20%.

In addition, since the discharge module opens at a higher pressure than the recirculation valve of the state of the art and then the flow ratio between the primary and secondary vacuum pumps is optimized, the The discharge pressure of the secondary vacuum pump decreases rapidly, so that the discharge module is only very shortly open. Being little solicited, the discharge module wears less quickly and is less noisy. In addition, little gas circulates in the recirculation line, which prevents the secondary vacuum pump from overheating by hot compressed gases.

Other features and advantages of the invention will emerge from the following description given by way of example, without limitation, with reference to the accompanying drawings, in which:

FIG. 1 represents a schematic view of a pumping unit according to the invention,

FIG. 2 is a graph illustrating a pressure drop in a loading and unloading chamber connected to the pumping unit of FIG. 1, with the abscissa being the pressure in the chamber of the airlock (in mbar), on the ordinate right: the rotation frequency of the secondary vacuum pump (in Hz) and on the left ordinate: the power consumed by the secondary vacuum pump (in kW), Figure 3 is a graph similar to Figure 2 with on the right ordinate: the ratio of the generated flow rate of the secondary vacuum pump to the generated flow rate of the primary vacuum pump, and

FIG. 4 is a graph illustrating the pumping speeds (in m ³ / h) of a primary vacuum pump alone, the pumping unit according to the invention and a pumping device of the state of the art, during a pressure drop as a function of the pressure in the chamber of the loading and unloading chamber (in mbar).

In these figures, the identical elements bear the same reference numbers. "Atmospheric pressure" is defined as the surrounding pressure outside the loading and unloading chamber of the substrate, such as the pressure prevailing in a room in which the clean room operators operate, that is to say a pressure of the order of 10 ⁵ Pascal (1000 mbar) or slightly higher to favor the direction of flow to the outside of the enclosure.

The volume corresponding to the volume driven by the rotors of the vacuum pump multiplied by the number of minute revolutions is defined by "generated flow rate" (or generated volume).

FIG. 1 shows an example of a pumping unit 1 intended to be connected to a loading and unloading chamber enclosure (or "load lock" in English) via an isolation valve (not shown).

In a manner known per se, the airlock for loading and unloading includes a sealed chamber, the first door of which communicates the interior of the chamber with a zone under atmospheric pressure, such as a clean room, for charging at least one large volume substrate, such as a flat panel display ("flat panel display" in English) or a photovoltaic substrate. Such locks have a volume generally between 500 and 5000 liters.

The airlock further comprises a second door for the unloading of the substrate in the process chamber after evacuation, and a device for introducing a neutral gas, in particular for the return to atmospheric pressure after treatment of the substrate.

The pumping unit 1 comprises a primary vacuum pump 2 and a secondary vacuum pump 3 arranged upstream of the primary vacuum pump 2 in the direction of flow of the gases to be pumped.

The primary vacuum pump 2 comprises for example a multi-stage dry vacuum pump with rotary lobes such as Roots type with two or three lobes (bi-lobes, tri-lobes). According to other embodiments not described, the primary vacuum pump comprises several pumps in series or in parallel. Furthermore, other conventional pumping principles can be used for the primary vacuum pump.

The primary vacuum pump 2 shown diagrammatically in FIG. 1 comprises, for example, five pumping stages T1, T2, T3, T4, T5 connected in series one after the other with a decreasing flow rate generated with the position of the stage. pumping in the series, and between which circulates a gas to be pumped between an intake inlet 4 and a discharge 5.

In general, a rotary lobe vacuum pump "Roots" comprises two rotors of identical profiles, carried by two shafts extending in the pump stages Tl, T2, T3, T4, T5 and driven by a pump motor. primary vacuum 2 (not shown) to rotate inside a stator in opposite directions. During rotation, the sucked gas is trapped in the free space between the rotors and the stator, then it is repressed. The operation is carried out without any mechanical contact between the rotors and the stator of the primary vacuum pump 2, which allows the total absence of oil in the pump stages T1, T2, T3, T4, T5.

In the illustrated example, the first pump stage T1 of the primary vacuum pump 2 has a generated flow rate Sol of the order of 600 m ³ / h, the second pump stage T2 has a generated flow rate So2 of the order of 400 m ³ / h, the third pump stage T3 has a generated flow So3 of the order of 200m ³ / h and the two last pump stages T4 and T5 have a generated flow So4, So5 of the order of 100m ³ / h. The flow rates generated vary according to the pressure range, these values correspond to the maximum values, constant pumping flow, with a rotation speed of the primary vacuum pump 2 in fixed operation and of the order of 65 Hz.

The primary vacuum pump 2 further comprises a nonreturn valve 6 at the outlet of the last pump stage T5, at the outlet 5, to prevent the return of the pumped gases into the primary vacuum pump 2.

The secondary vacuum pump 3 is, like the primary vacuum pump 2, a volumetric vacuum pump, that is to say a vacuum pump which, with the aid of pistons, rotors, vanes, valves, sucks, transfers and represses the gas to be pumped.

The secondary vacuum pump 3 is for example a single-stage vacuum pump (having only a single pumping stage), with rotors such as Roots type or a similar principle, such as Claw type.

In operation, the maximum generated flow rate SoR of the secondary vacuum pump 3 is, for example, of the order of 3000 m ³ / h at maximum rotation speed (ie around 70 Hz), in the optimum pressure range.

The secondary vacuum pump 3 comprises a motor 7, such as an asynchronous motor, a frequency converter 8 for driving the motor 7 driving the rotors and a control unit 9 connected to the frequency converter 8.

During the pressure drop in the loading and unloading lock at atmospheric pressure and until the pressure in the airlock reaches a predefined low pressure threshold, the control unit 9 is configured to control the speed of the rotation of the rotors of the secondary vacuum pump 3 according to a signal representative of an operating parameter of the secondary vacuum pump 3 to increase the flow rate generated so that the flow rate generated by the secondary vacuum pump SoR is included in a range whose high value corresponds to six times the flow rate generated by the primary vacuum pump So and the low value to, 3 times the flow rate generated by the primary vacuum pump So. The preset low pressure threshold is for example 20 mbar. Below this, the rotational speed of the secondary vacuum pump 3 is controlled at its maximum value, that is to say 70 Hz in the present example.

The pressure difference between the suction and the discharge of the secondary vacuum pump then remains below a value between 150 and 300 mbar.

The signal representative of an operating parameter is, for example, the discharge pressure of the secondary vacuum pump PI or a parameter of the motor 7 of the secondary vacuum pump 3.

In the latter case, the parameter of the motor 7 of the secondary vacuum pump 3 may be the current, representative of the power consumed, or directly the power consumed. These signals can be received from the variable speed drive 8 connected to the motor 7. Thus, the control of the secondary vacuum pump 3 is autonomous because it does not require any information from the airlock for loading and unloading, nor adding a pressure sensor to intake inlet 4 of the primary vacuum pump 2.

Controlling the speed of rotation of the rotors of the secondary vacuum pump 3 according to a signal representative of an operating parameter of the secondary vacuum pump 3 is a closed-loop control: when the discharge pressure PI or the motor current 7 or the power increases and that the generated flow approaches or exceeds the high value of the authorized range, the speed of rotation is slowed down, or even decreased.

The pumping unit 1 further comprises a pipe 10 connecting the inlet 4 of the primary vacuum pump 2 to the intake inlet 11 of the secondary vacuum pump 3.

The pipe 10 comprises a discharge module, such as a valve 12 or a valve controlled by the treatment unit 9, configured to open as soon as the pressure difference between the suction and the discharge of the secondary vacuum pump 3 exceeds a preset value of ΔΡ, between 100 and 400 mbar, the ΔΡ overflow value being defined according to the ratio of the flow rates generated and according to the mechanical safety settings.

For example, for a ratio of the maximum generated flow rates of the order of 4, 5, the pressure difference of the secondary vacuum pump 3 always remains lower than a pressure of the order of 250 mbar. The discharge module is therefore configured to open as soon as the suction pressure of the primary vacuum pump PI exceeds the suction pressure of the secondary vacuum pump Pasp with a predetermined value of ΔΡ, for example at 300 mbar. .

Furthermore, to absorb the strong initial gas flow from the vacuum of the air pressure chamber, it is expected that the primary vacuum pump 2 is designed to be able to absorb and transfer this strong gas stream with the power consumed the weakest possible. For this, for example, the primary vacuum pump 2 comprises a load shedding module of a pumping stage.

Indeed, although the generated flow rate of the secondary vacuum pump SoR is adapted to correspond to the generated flow rate of the primary vacuum pump Sol, that is to say the flow generated by the first pump pump stage T1 of the pump With a primary vacuum 2, the second or the third pump stage T2, T3 can, in turn, limit the overall generated flow rate of the primary vacuum pump 2. Thus, for the primary vacuum pump 2 to be able to absorb pumping streams from time to time. in this example, a pumping pressure PI limited to the opening pressure of the discharge module, that is to say 300 mbar, the load shedding module is connected to the output of a pumping stage low pressure, such as the second pump stage T2.

The load shedding module comprises for example a channel 13 connecting the output of the low pressure stage (T1 or T2) to the discharge 5 of the primary vacuum pump 2. The channel 13 is provided with a valve 14.

Reference is now made to the graphs of FIGS. 2, 3 and 4, illustrating an example of descent in pressure in a 500 liter loading and unloading lock.

In the initial state, the rotation speed of the secondary vacuum pump 3 is at a reduced fixed rotation speed, for example of the order of 30 Hz in order to limit the power consumption.

Then, after the loading of a substrate at atmospheric pressure in the chamber of the loading and unloading chamber, the airlock opens the isolation valve insulating the atmospheric pressure chamber of the pumping unit 1 (tl).

During a relatively short period of time, of the order of a few seconds, the secondary vacuum pump 3 compresses the excess gas from the enclosure, increasing the discharge pressure of the secondary vacuum pump PI and lowering the speed rotation (curve V in FIG. 2).

As soon as the pressure difference between the suction and the discharge of the secondary vacuum pump 3 exceeds 300 mbar, the discharge module of the pipe 10 opens, thus limiting the increase of the discharge pressure of the pump to the pump. secondary vacuum PI. The gas flow is absorbed by the first two pump stages T1, T2 of the primary vacuum pump 2, and is then discharged at the outlet of the second pump stage T2, to the discharge 5 of the primary vacuum pump 2 by the module. offloading.

When an operation parameter of the secondary vacuum pump 3, such as the power consumed by the secondary vacuum pump 3 (curve P in FIG. 2), exceeds a predefined launch threshold for a first predefined duration, the unit treatment 9 can start a pressure descent cycle. The processing unit 9 then controls the speed of rotation of the secondary vacuum pump 3 (curve V in FIG. 2) as a function of an operating parameter of the secondary vacuum pump 3, such as the power consumed by the motor 7, (curve P in FIGS. 3), to increase the flow rate generated by the secondary vacuum pump SoR so that the flow rate generated by the secondary vacuum pump SoR remains greater than 1.3 times the flow rate generated by the primary vacuum pump Sol and less than 4, 5 times the flow generated by the primary vacuum pump Sol in the example shown in Figure 3 (curve R).

Since the power consumed by the secondary vacuum pump 3 increases, but the flow generated by the secondary vacuum pump SoR remains less than 4, 5 times the flow generated by the primary vacuum pump Sol, the treatment unit 9 controls the increase in the speed of rotation (curve V in FIG. 2 between t1 and t2), causing the ratio of the flow rates generated from 1 to 3 to be increased to 5. 5. The power consumption then stabilizes around 17kW (Figure 3). This consumed power is due to maintaining an effective compression at the discharge of the secondary vacuum pump 3 and, mechanically and thermally acceptable, for the primary vacuum pump 2 and the secondary vacuum pump 3.

In addition, it is possible to provide for safety a ceiling for the power consumed by the secondary vacuum pump 3. If the value of the engine parameter 7 of the secondary vacuum pump 3 is greater than a predefined safety threshold beyond one second predetermined duration, it forces the decrease in the rotational speed of the secondary vacuum pump 3. This precaution applies more particularly in the case of large airlock, for example greater than 100m ³ , for which the pumping groups have been dimensioned for small volumes of the order of 2m ³ to 20m ³ . This protects against thermal overheating of the secondary vacuum pump 3.

It can be seen that between the atmospheric pressure (tl) and the predefined low pressure threshold, such as 20 mbar (t2), the ratio of the flow rate generated by the secondary vacuum pump SoR to the flow rate generated by the primary vacuum pump Sol remains between 1, 3 and 4, 5.

Maintaining the ratio of the flow rates generated below 4, 5 allows the flow generated by the secondary vacuum pump SoR to be admissible by the primary vacuum pump 2. This limits the overconsumption and the secondary vacuum pump 3 still ensures a compression. The pressure difference between the suction and the discharge of the secondary vacuum pump 3 remains below a value of between 50 and 350 mbar.

The secondary vacuum pump 3 is no longer "short-circuited" as it was in a device of the state of the art.

By way of comparison, FIG. 4 shows the pumping speeds during the descent in pressure in an airlock for a pumping group 1 (curve A), for a primary vacuum pump 2 alone (curve B). and for a device of the state of the art comprising a pump primary and secondary vacuum similar to those of the pumping group 1 according to the invention, but having a recirculation valve calibrated at 60 mbar and a rotational speed of the fixed secondary vacuum pump (curve C).

It can be seen that for the device of the state of the art, between 200mbar and the atmospheric pressure, the secondary vacuum pump does not improve the overall pumping speed, the lowering pressure being solely provided by the primary vacuum pump. The role of the secondary vacuum pump whose speed of rotation is controlled at a maximum fixed speed is then only to participate in the recirculation of the overconsumer gas flow (curves B and C between tl and ta).

In contrast, the secondary vacuum pump 3 of the pumping unit 1 according to the invention is used as if it were the real first pumping stage of the primary vacuum pump 2 thanks to the adapted ratio of the generated flow rates SoR and Sol. The secondary vacuum pump 3 is therefore effective almost from atmospheric pressure (curve A in Figure 4 from tl). The efficiency of the secondary vacuum pump of the device of the state of the art catches that of the secondary vacuum pump 3 of the pumping group 1 that around 5 mbar (tb).

This results in a significant decrease in the power consumed by the pumping group

1 and an increase in its overall pumping speed at the beginning of the pressure drop, and therefore a reduction in the pressure drop time in the airlock. In the example, at 200 mbar, the overall pumping speed increases by 40% compared to the device of the state of the art.

In addition, since the discharge module is only slightly stressed, the discharge module wears less quickly and is less noisy. Similarly, little gas circulates in the recirculation pipe 10, which prevents the superheating of the secondary vacuum pump 3 by these previously hot compressed gases.

Then, when the pressure in the airlock reaches the predefined low pressure threshold, (t2 on the curve B of FIG. 4), the setpoint of the rotation speed of the secondary vacuum pump 3 is set at its maximum value 70 Hz. The discharge pressure of the secondary vacuum pump PI decreases, reducing the power consumed by the secondary vacuum pump (curve P of Figures 2 and 3). At these low pressure values in the airlock, the power consumed is of the order of 2 kW. Below this predefined low pressure in the airlock, the pumping by the primary and secondary vacuum pumps 2, 3 can be carried out conventionally without adapting the rotation speed of the secondary vacuum pump 3 because the pumping flows and the power consumed are weak.

At very low pressure (beyond t3), for example waiting for the airlock to open to the process chamber for the transfer of the substrate, if the value of the engine parameter of the secondary vacuum pump 3 remains below a second predefined threshold beyond a second predetermined duration, for example 2 kW for a few minutes, it is possible to control the speed of rotation of the secondary vacuum pump 3 at a reduced fixed rotational speed (called "standby" in English), lower than the maximum speed 70Hz, so as to limit the power consumption.

Claims

A method for descent pressure in a chamber for loading and unloading a substrate at atmospheric pressure by a pumping unit (1) comprising a primary vacuum pump (2) and a secondary vacuum pump (3) arranged in upstream of said primary vacuum pump (2) in the direction of flow of the gases to be pumped, characterized in that during the descent in pressure until the pressure in the loading chamber and unloading reaches a predefined low pressure threshold, the speed of rotation of the secondary vacuum pump (3) is controlled according to an operating parameter of the secondary vacuum pump (3) to increase the flow rate generated by the secondary vacuum pump ( SoR) so that the flow generated by the secondary vacuum pump (SoR) remains in a range whose high value corresponds to six times the flow generated by the primary vacuum pump (Sol) and the low value to 1, 3 times the flow generated by the mpe vacuum primary (ground).

Pressure reduction method according to one of the preceding claims, characterized in that the operating parameter of the secondary vacuum pump (3) is a parameter of the motor (7) of the secondary vacuum pump (3).

3. Pressure reduction method according to one of the preceding claims, characterized in that begins to control the rotational speed of the secondary vacuum pump (3) according to an operating parameter of the vacuum pump secondary (3) when it detects that the value of an operating parameter of the secondary vacuum pump (3) exceeds a predefined launch threshold for a first predefined duration.

4. Pressure reduction method according to one of the preceding claims, characterized in that if the value of an operating parameter of the secondary vacuum pump (3) is greater than a predefined safety threshold beyond a second predetermined time, it reduces the speed of rotation of the secondary vacuum pump (3).

5. Pressure reduction method according to one of the preceding claims, characterized in that if the value of an operating parameter of the secondary vacuum pump (3) is less than a predefined waiting threshold beyond At a third predetermined time, the rotational speed of the secondary vacuum pump (3) is controlled at a reduced fixed rotational speed.

Pumping unit comprising a primary vacuum pump (2) and a secondary vacuum pump (3), said secondary vacuum pump (3) being arranged upstream of said primary vacuum pump (2) in the direction of flow of the gases to be pumped and comprising a frequency converter (8), characterized in that the secondary vacuum pump (3) comprises a control unit (9) connected to the frequency converter (8), configured to control the speed of rotation of the pump to secondary vacuum (3) as a function of a signal representative of an operating parameter of the secondary vacuum pump (3), so that during the descent in pressure until the pressure in the airlock of loading and unloading reaches a predefined low pressure threshold, the flow generated by the secondary vacuum pump (SoR) is increased and included in a range whose high value corresponds to six times the flow generated by the primary vacuum pump (Sol ) and the low value, to 1, 3 times the flow generated by the primary vacuum pump (Sol).

7. Pumping unit according to claim 6, characterized in that the primary vacuum pump (2) comprises a load shedding module of a pumping stage (T1, T2).

Pumping unit according to one of Claims 6 or 7, characterized in that the signal representative of an operating parameter of the secondary vacuum pump (3) is a parameter of the motor (7) of the vacuum pump. secondary (3).

Pumping unit according to Claim 8, characterized in that the motor parameter (7) of the secondary vacuum pump (3) is the current.

Pumping unit according to Claim 8, characterized in that the engine parameter (7) of the secondary vacuum pump (3) is the power.

Pumping unit according to one of Claims 6 to 10, characterized in that it comprises a recirculation pipe (10) connecting the intake inlet (4) of the primary vacuum pump (2) to the inlet (11) of the secondary vacuum pump (3), the recirculation pipe (10) having a discharge module configured to open as soon as the suction pressure of the primary vacuum pump (PI) exceeds the suction pressure of the secondary vacuum pump (Pasp) with a predefined value (ΔΡ) exceeding 100 to 400 mbar.

Pump unit according to one of Claims 6 to 11, characterized in that the secondary vacuum pump (3) is a ROOTS type vacuum pump.