Pumping system including an ion pump and method for operating a pumping system including an ion pump

Abstract

A pumping system 100 having an ion pump 10 with a first electrode 50, second electrode 30, and controller 60 for selectively applying a first or second voltage of different polarity between the two el

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a pumping system including an ion pump. More in detail, the present invention relates to a pumping system in which the pressure (i.e. the vacuum level) inside the ion pump can be more precisely determined. The present invention also relates to a method for operating a pumping system including an ion pump, allowing to more precisely determine the pressure (i.e. the vacuum level) inside the pump. PRIOR ART A sputter ion pump is a capture vacuum pump which operates by sputtering a getter material. A sputter ion pump is able to create and maintain high-vacuum conditions within a confined environment. More particularly, ion pumps are commonly used in ultra-high vacuum (UHV) systems, because they can reach pressures lower than 10 11 mbar. Nowadays, the most common design of sputter ion pumps is based on the provision of one or more Penning cells, also known as Penning “traps”. The operation of the Penning trap is based on the confinement of free charged particles using an electrostatic-multipolar field combined with a magnetic-dipolar field. The magnetic field confines the particles radially, while the electrostatic field confines them in the axial direction. Accordingly, as shown in Figure 1 the basic structure of a Penning trap includes a first electrode 5, a second electrode 3 and magnets (not shown). The first electrode 5 includes a pair of plates which are arranged parallel to each other and at a distance from each other. The second electrode 3 is made a cell, for instance as a cylindrical cell with its longitudinal axis arranged perpendicular to the first electrode plates. A pair of magnets are placed on opposite sides of the Penning trap, externally to the first electrode plates and preferably outside the confined environment housing the first and second electrodes 3,5. If a voltage (typically from 3 to 9 kV) between the electrodes is applied, an intense electric field is generated between said electrodes. Said electric field, combined with the magnetic field generated by the magnets, allows to confine (i.e., to trap) the charged particles, thereby creating a non-neutral plasma. As will be explained in detail below, in order to have the Penning trap operating as an ion pump, an electron plasma is needed. In other words, the Penning cell shall work so as to “trap” electrons. In order to obtain an electron plasma, the two plates of the first electrode have to serve as cathode 5 (i.e. electrode al lower potential) and the cell of the second electrode has to serve as anode 3 (i.e. electrode at higher potential). Therefore, a voltage (typically from 3 to 9 kV) between the cathode and the anode is applied and an intense electric field is generated, this resulting in the generation of an electric discharge. Said electric field, combined with the magnetic field generated by the magnets, allows to confine (i.e., to trap) the electrons, thereby creating an electron plasma that is used to ionize the gas molecules present within the environment to be evacuated, namely within the anode cell. In detail, the plasma electrons collide with the molecules in the anode cell, thereby ionizing them. Because of the electric field, the positive ions resulting from said ionization are attracted towards the cathode 5, leaving the corresponding electrons, left in the anode cell, available for further ionization of the other gas molecules. In this respect, the presence of the magnets for generating the magnetic field allows to impart helical trajectories to electrons, thereby increasing the length of the paths of such electrons between the cathode 5 and the anode 3 and, therefore, the possibility of collision and corresponding ionization of the gas molecules present within the environment. Positive ions, meanwhile, reach the cathode 5 and collide with the surface of said cathode. The collision of positive ions with the cathode 5, which is made of a getter material (for instance, titanium) causes the phenomenon of sputtering, i.e., the emission of getter material atoms from the cathode 5. Said atoms, by covering the inner surfaces of the Penning trap, create an active film (so-called getter film) which chemically traps the gas molecules, which are therefore buried in the anode. This mechanism is named pumping at the anode or getter pumping. As mentioned above, ion pumps can reach very low pressures, lower than 10 11 mbar. The pressure inside the sputter ion pump is a function of the pump structure, the voltage applied between the electrodes and the current absorbed by the pump. Accordingly, the pump structure and the applied voltage being known, the pressure (i.e. the vacuum level) attained by the pump is usually measured by measuring the current supplied to the pump and by calculating the pressure starting from the measured current. However, the Applicant found out that the current supplied to the pump and measured is not entirely available for the pumping mechanism. Instead, the current supplied to the pump includes two fractions, namely an operating fraction or operating current, which contributes to the pumping effect, and a leakage fraction or leakage current, which is dispersed in the pumping system components (such as the controller, the electrical cables and connectors and the pump itself) and does not contribute to the pumping effect. Since the pressure inside the pump is a function of the operating current while the measured current is the overall current supplied to the pump, an inaccurate calculation of the pressure is obtained. In addition, the leakage current strongly depends on the environmental operating conditions and on the operating state of the pump (state of wear, possible malfunctioning, and the like), so that it cannot be determined during a calibration procedure of the sputter ion pump. In view of the above, an object of the present invention is to provide a pumping system including an ion pump in which the pressure or vacuum level inside the sputter ion pump can be more accurately determined. Another object of the present invention is to provide a method of operating a pumping system including an ion pump allowing to more accurately determine the pressure or vacuum level inside the ion pump. These and other objects are achieved by the pumping system and the corresponding operating method as claimed in the appended claims. SUMMARY OF THE INVENTION As mentioned above, in current arrangements the determination of the pressure is inaccurate because the pressure inside the pump is a function of the operating current while the measured current is an overall current supplied to the pump, including both the operating current and a leakage current. The invention therefore relies upon the assumption that, if it were possible to isolate the operating current, such operating current could be used for calculating the pressure and a more accurate result could be obtained. A possible way of isolating the operating current is determining the leakage current. The leakage current is absorbed by the pumping system components both when the ion pump is in an operating mode and when said pump is in a non-operating mode. As mentioned above, the ion pump is in an operating mode only when an electron plasma is generated, i.e. when a voltage with the right polarity is applied between a first electrode and a second electrode of the pump. Accordingly, the pumping system according to embodiments of the invention includes an ion pump, comprising a first electrode and a second electrode, and a controller unit for applying a voltage between said first electrode and said second electrode, and said controller unit is configured for selectively applying either a first voltage having a first polarity between said first electrode and said second electrode or a second voltage having a second polarity, opposed to said first polarity, between said first electrode and said second electrode. In a preferred embodiment invention, said controller unit comprises a piloting unit for selectively applying either a first voltage having a first polarity between said first electrode and said second electrode or a second voltage having a second polarity, opposed to said first polarity, between said first electrode and said second electrode. Preferably, the pumping system further comprises a current sensor suitable for detecting the current supplied to said ion pump. Since the first voltage and the second voltage have opposed polarities, the ion pump will be in an operating mode only when the voltage polarity allows the generation of an electron plasma between the electrodes. In this configuration, a first overall current will include both the operating current and the leakage current. On the other hand, a second overall current measured when the voltage polarity is reversed (i.e. when the voltage polarity does not allow the generation of an electron plasma between the electrodes and the ion pump is in non-operating mode) will only include the leakage current. The operating current can therefore be obtained by comparing said first overall current and said second overall current and used for the calculation of the pressure, thus allowing a very accurate determination thereof. Accordingly, the controller unit of the pumping system advantageously includes a processing unit suitable for calculating the operating current from the comparison of the first and second overall currents and for further calculating the pressure starting from the operating current thus obtained. The measurement of the leakage current itself also involves additional advantages, as it can be used for obtaining essential information about the state of the pumping system (wear, possible malfunctioning, and the like). The method for operating a pumping system including an ion pump, comprising a first electrode and a second electrode, will correspondingly include: - applying a first voltage having a first polarity between said first electrode and said second electrode; and further - applying a second voltage having a second polarity, opposed to said first polarity between said first electrode and said second electrode. Preferably, the method will further comprise: - measuring a first overall current supplied to said ion pump while said first voltage is applied between said first electrode and said second electrode; and - measuring a second overall current supplied to said ion pump while said second voltage is applied between said first electrode and said second electrode. The overall current having the lower value between the first and second overall currents corresponds to the leakage current, while the operating current corresponds to the absolute value of the difference between said first and second overall currents. The method will therefore preferably comprise calculating a pressure inside the ion pump starting from the operating current thus obtained. As mentioned above, the pressure inside the ion pump is a function of the pump structure, the voltage applied between the electrodes and the current absorbed by the pump. Accordingly, if during the operation of the pumping system the voltage applied to the electrodes is varied, a new measurement of the first and second overall currents is to be performed, in order to obtain the operating current and hence the pressure. BRIEF DESCRIPTION OF THE DRAWINGS Further features and advantages of the present invention will become more evident from the following detailed description of an embodiment of the invention, which is given by way of non-limiting example with reference to the annexed drawings, in which: Figure 1 schematically shows a Penning trap; Figure 2a schematically shows a pumping system according to a first execution example of an embodiment of the invention; and Figure 2b schematically shows a pumping system according to a second execution example of an embodiment of the invention. DESCRIPTION OF AN EMBODIMENT OF THE INVENTION Figures 2a and 2b schematically show a pumping system 100 including a sputter ion pump 10. In the embodiment shown in Figures 2a and 2b, the design of the sputter ion pump 10 is based on the provision of a plurality of Penning traps; nevertheless, such design shall not be considered as limiting the scope of the invention and other structures for the sputter ion pump can be envisaged. The sputter ion pump 10 shown in Figures 2a and 2b comprises a vacuum enclosure 20 housing at least a first electrode 50 consisting of a pair of plates, arranged substantially parallel to each other and spaced apart from each other, and a second electrode 30 consisting of a plurality of hollow cells 40 arranged between the plates of the first electrode. More particularly, the hollow cells 40 are cylindrical cells with their longitudinal axis arranged in a substantially perpendicular direction with respect to the first electrode plates. The plates of the first electrode are made of a getter material, such as titanium. The sputter ion pump 10 further comprises magnets 70, located externally to the first electrode plates and outside of the enclosure 20, for producing a magnetic field oriented parallel to the axes of the second electrode cells. The pumping system 100 further comprises a controller unit 60 for applying a voltage (potential difference) between the first and second electrodes 50,30. According to the shown embodiment of the invention, the controller unit is designed so as to selectively apply either a first voltage having a first polarity between said first electrode 50 and said second electrode 30, or a second voltage having a second polarity, opposed to said first polarity, between said first electrode 50 and said second electrode 30. By way of non-limiting example, according to the execution example shown in Figure 2a, the controller unit 60 includes a single controller 60’ that has a first operating mode in which the first voltage having the first polarity is applied between said first electrode 50 and said second electrode 30, and a second operating mode in which the second voltage having the second polarity, opposed to said first polarity, is applied between said first electrode 50 and said second electrode 30. The controller unit 60 is further provided with a piloting unit 61 for selectively making the single controller 60’ work in the first operating mode or in the second operating mode. By way of further non-limiting example, according to the execution example shown in Figure 2b, the controller unit 60 includes a first controller 60a designed to apply the first voltage having the first polarity between said first electrode 50 and said second electrode 30, and a second controller 60b designed to apply the second voltage having the second polarity, opposed to said first polarity, between said first electrode 50 and said second electrode 30. The controller unit 60 is further provided with a piloting unit 61 for selectively connecting either the first controller 60a or the second controller 60b to the sputter ion pump. With reference to both execution examples of Figures 2a and 2b, assuming that when the first voltage with the first polarity is applied the second electrode 30 is at a higher potential than the first electrode 50, the first electrode 50 will serve as cathode and the second electrode 30 will serve as anode. In this configuration, the sputter ion pump 10 is in an operating mode. If a sufficiently high voltage is applied between the electrodes (typically, 3 to 9 kV), a strong electric field region is generated between the cells 40 of the second electrode 30 (anode) and the plates of the first electrode 50 (cathode), resulting in generation of an electric discharge. The electrons then collide with gas molecules inside the cells 40, thus ionizing them. Due to the electric field, the resulting positive ions are attracted by the plates of the first electrode 50 and collide with the surface thereof. Ion collisions with the titanium plates forming the first electrode plates result in the "sputtering" phenomenon, that is, the emission of titanium atoms from the plates of the first electrode. The presence of magnets 70 for generating a magnetic field B allows for imparting helical trajectories to electrons, so as to increase the lengths of their paths between the electrodes and, consequently, the chances of colliding with gas molecules inside the cells 40 and ionizing such molecules. The controller unit 60 is provided with a current sensor 63 for detecting the overall current supplied to the sputter ion pump 10. In this configuration, a first overall current measured by the current sensor 63 includes an operating current contributing to the pumping effect as well as a leakage current. Starting from the above assumption, when the piloting unit 61 of the controller unit 60 is triggered and the second voltage with the second polarity is applied, the first electrode 50 will be at a higher potential than the second electrode 30. When the plates of the first electrode 50 serve as anode and the cells of the second electrode 30 serve as cathode, no electron plasma is generated inside the sputter ion pump. Thus, the sputter ion pump is in a non-operating mode. In this configuration, a second overall current measured by the current sensor 63 includes the leakage current only. The controller unit 60 is further provided with a processing unit 65 able to compare the first overall current and the second overall current. By comparing said overall currents, the processing unit 65 can isolate the operating current as the difference between said first overall current and said second overall current. It is evident that if the above assumption were reversed, so that the sputter ion pump 10 were in a non-operating mode when the first potential is applied and in an operating mode when the second potential is applied, the difference between the first overall current and the second overall current would have a negative value. Therefore, in general it can be said that the operating current corresponds to the absolute value of the difference between said first overall current and said second overall current. On the other hand, the leakage current corresponds to the overall current having the lower value between said first overall current and said second overall current. It is worth mentioning that, in order to correctly determine the operating current and the leakage current, first and second voltages having the same absolute value but opposed polarities have to be applied. Nevertheless, when the polarity of the applied voltage corresponds to the configuration in which the ion pump is in the non-operating mode, the power needed by the controller will be very low. Accordingly, it will be not necessary to provide a controller unit sized for supplying the same power both when applying the first voltage and when applying the second voltage. With reference to the execution example of Figure 2b, for instance, one of the controllers 60a, 60b shall be sized for supplying a power allowing to apply a proper voltage when the pump is in the operating mode, while the other one of the controllers 60, 60 could be sized so as to supply a much lower power (since it will be connected to the sputter ion pump only when the latter is in the non-operating mode. Analogously, with reference to the execution example of Figure 2a, the single controller 60’ shall not be sized for supplying the same power in both its operating modes. Advantageously, the processing unit 65 of the controller unit 60 is further able to calculate the pressure inside the sputter ion vacuum pump 10 starting from the operating current previously obtained. Such pressure is calculated according to known formulas including the voltage applied between the electrodes and the structural parameters of the pump. According to the shown embodiment, the method for operating the vacuum system 100 may include: - applying a first voltage having a first polarity between the first electrode 50 and the second electrode 30; - measuring a first overall current supplied to the sputter ion pump 10 while said first voltage is applied between said first electrode and said second electrode; - applying a second voltage having a second polarity, opposed to said first polarity between said first electrode 50 and said second electrode 30; 5 - measuring a second overall current supplied to said sputter ion pump 10 while said second voltage is applied between said first electrode and said second electrode; - obtaining a leakage current as the overall current having the lower value between said first overall current and said second overall current; 10 - obtaining an operating current as the absolute value of the difference between said first overall current and said second overall current; - calculating the pressure inside said sputter ion pump 10 starting from said operating current. It will be evident to the person skilled in the art that the 15 embodiment described above in detail should in no way be understood in a limiting sense, and that that numerous modifications and variants are possible without thereby departing from the scope of protection as defined by the appended claims.

Claims

1. A pumping system (100) comprising an ion pump (10), which comprises a first electrode (50) and a second electrode (30), and a controller (60) for applying a voltage between said first electrode (30) and said second electrode (50), characterized in that said controller unit (60) is configured for selectively applying either a first voltage having a first polarity between said first electrode (30) and said second electrode (50) or a second voltage having a second polarity, opposed to said first polarity, between said first electrode (30) and said second electrode (50).

2. The pumping system (100) of claim 1, wherein said controller unit (60) is provided with a piloting unit (61) for selectively applying either a first voltage having a first polarity between said first electrode (30) and said second electrode (50) or a second voltage having a second polarity, opposed to said first polarity, between said first electrode (30) and said second electrode (50).

3. The pumping system (100) of claim 1 or 2, wherein said controller unit (60) is provided with a current sensor (63) for detecting the overall current supplied to said ion pump (10) when a voltage is applied between said first electrode (30) and said second electrode (50).

4. The pumping system (100) of claim 3, wherein said controller unit (60) is provided with a processing unit (65) for comparing a first overall current detected by said current sensor (63) when said first voltage is applied between said first electrode (30) and said second electrode (50) and a second overall current detected by said current sensor (63) when said second voltage is applied between said first electrode (30) and said second electrode (50).

5. The pumping system (100) of claim 4, wherein said processing unit (65) calculates a first current or leakage current as the overall current having the lower value between said first overall current and said second overall current.

6. The pumping system (100) of claim 4 or 5, wherein said processing unit (65) calculates a second current or operating current as the absolute value of the difference between said first overall current and said second overall current7. The pumping system (100) of claim 4 or 5 or 6, wherein said processing unit (65) calculates a pressure inside said ion pump starting from the absolute value of the difference between said first overall current and said second overall current.

8. The pumping system (100) of any of claims 1 to 7, wherein said controller unit (60) comprises a single controller (60’) that has a first operating mode in which said first voltage having said first polarity is applied between said first electrode (50) and said second electrode (30), and a second operating mode in which said second voltage having said second polarity, opposed to said first polarity, is applied between said first electrode (50) and said second electrode (30), and wherein said single controller (60’) selectively works either in said first operating mode or in said second operating mode.

9. The pumping system (100) of any of claims 1 to 7, wherein said controller unit (60) comprises a first controller (60a) for applying said first voltage having said first polarity between said first electrode (50) and said second electrode (30), and a second controller (60b) for applying said second voltage having said second polarity, opposed to said first polarity, between said first electrode (50) and said second electrode (30), and wherein said either said first controller (60a) or said second controller (60b) is selectively connected to said ion pump (10).

10. The pumping system (100) of any of claims 1 to 9, wherein said ion pump is a sputter ion pump (10).

11. The pumping system (100) of any of claims 1 to 10, wherein said first electrode (50) comprises a pair of plates, arranged substantially parallel to eachother and spaced apart from each other, and said second electrode (30) comprises a plurality of hollow cells (40) arranged between the plates of the first electrode.

12. The pumping system (100) of claim 11, wherein said hollow cells (40) are cylindrical cells with their longitudinal axis arranged in a substantially perpendicular direction with respect to said plates.

13. A method for operating a pumping system (100) comprising an ion pump (10), which comprises a first electrode (50) and a second electrode (30), wherein a first voltage having a first polarity can be applied between said first electrode (30) and said second electrode (50), characterized in that it further comprises applying a second voltage having a second polarity, opposed to said first polarity, between said first electrode (30) and said second electrode (50).

14. The method of claim 13, further comprising measuring a first overall current supplied to said ion pump (10) while said first voltage is applied between said first electrode and said second electrode and measuring a second overall current supplied to said ion pump (10) while said second voltage is applied between said first electrode and said second electrode.

15. The method of claim 14, further comprising obtaining a first current or leakage current as the overall current having the lower value between said first overall current and said second overall current.

16. The method of claim 14 or 15, further and / or further comprising obtaining a second current or operating current as the absolute value of the difference between said first overall current and said second overall current.

17. The method of claim 14 or 15 or 16, further comprising calculating a pressure inside said ion pump (10) starting from the absolute value of the difference between said first overall current and said second overall current.

18. The method of claim 13, further comprising measuring a second overall current supplied to said ion pump (10) while said second voltage is applied between said first electrode and said second electrode, said ion pump (10) being in a non-operating mode upon application of said second voltage.

519. The method of any of claims 13 to 17, comprising applying a first voltage having a first polarity between said first electrode (30) and said second electrode (50), wherein said ion pump (10) is in an operating mode upon application of said first voltage, and subsequently applying said second voltage having said second10 polarity, opposed to said first polarity, between said first electrode (30) and said second electrode (50).

20. The method of any of claims 13 to 19, wherein said ion pump is a sputter ion pump (10).15

Images