Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J41/00—Discharge tubes for measuring pressure of introduced gas or for detecting presence of gas; Discharge tubes for evacuation by diffusion of ions
  • H01J41/12—Discharge tubes for evacuating by diffusion of ions, e.g. ion pumps, getter ion pumps
  • H01J41/18—Discharge tubes for evacuating by diffusion of ions, e.g. ion pumps, getter ion pumps with ionisation by means of cold cathodes
  • H01J41/20—Discharge tubes for evacuating by diffusion of ions, e.g. ion pumps, getter ion pumps with ionisation by means of cold cathodes using gettering substances

Definitions

• the present invention relates to a method for operating an ion pump.
• the present invention relates to a method for operating an ion pump allowing the pressure inside the ion pump to be more accurately estimated.
• the present invention also relates to a pumping system including an ion pump and suitable for implementing the above method.
• 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.
• UHV ultra-high vacuum
• the basic structure of a Penning trap includes a cathode and an anode received in a vacuum tight housing.
• the cathode includes a pair of plates which are arranged parallel to each other and at a distance from each other.
• the anode is made as a cell, for instance as a cylindrical cell with its longitudinal axis arranged perpendicular to cathode plates.
• a pair of magnets are placed on opposite sides of the Penning trap, externally to the cathode plates and preferably outside the confined housing in which the cathode and the anode are received.
• One of the electrodes is grounded, while the other electrode is fed with a high voltage (either positive or negative), so that an intense electric field is generated between the electrodes. More specifically, in a first possible configuration the cathode plates are grounded, while the anode cell is fed with positive high voltage, typically 3 - 7 kV, so that an intense electric field is generated between said electrodes (so called "diode configuration").
• a vacuum tight housing 110 is arranged between the poles of a magnet 112 and a pair of cathode plates 114, 114' of getter material, for example titanium, are provided inside the housing 110; an anode 116 formed of a plurality of cylindrical hollow cells 118 is secured between the pair of cathode plates 114 and 114'.
• the two cathode plates 114, 114' are grounded, while the anode cells 118 of the anode 116 are connected to the positive pole of a potential difference source 120.
• getterable gases some gases, such as nitrogen (N 2 ), oxygen (O 2 ), carbon monoxide (CO) or carbon dioxide (CO 2 ) can form chemical compounds with getter materials, such as titanium, and are therefore called “getterable gases”.
• gases such as nitrogen (N 2 ), oxygen (O 2 ), carbon monoxide (CO) or carbon dioxide (CO 2 ) can form chemical compounds with getter materials, such as titanium, and are therefore called “getterable gases”.
• the anode cell is grounded (and the walls of the vacuum tight housing are grounded as well), while the cathode plates are fed with negative high voltage, typically - 3 - - 7 kV, so that an intense electric field is generated between said electrodes (so called "triode configuration").
• a vacuum tight housing 210 is arranged between the poles of a magnet 212 and a pair of cathode plates 214, 214' of getter material, for example titanium, are provided inside the housing 210; an anode 216 formed of a plurality of cylindrical hollow cells 218 is secured between the pair of cathode plates 214 and 214'.
• the anode cells 218 of the anode 216, as well as the walls of the housing 210 are grounded, while the cathode plates 214, 214' are connected to the negative pole of a potential difference source 220.
• This configuration is mainly used for pumping inert gases, such as noble gases.
• the aim of the Penning cell is to increase as much as possible the path of these electrons from the cathode to the anode, hence increasing the probability of a collision between the electrons and gas molecules of the residual gas inside the pump. Because of such collisions, the trapped electrons can ionize the gas molecules, thus creating positive ions and extracted electrons: the ions, positively charged, are attracted towards the cathodes, while the extracted electrons are trapped as well and are available for further ionization of the other gas molecules.
• This mechanism can be considered as the basic principle for the operation of Penning cells and therefore, for ion pumps.
• an ion pump is a static device as it has no moving parts, which avoids both vibrations and the need for any lubricant (which could be a source of contamination).
• ion pumps are often used as pressure indicators, thanks to the well-known almost linear dependence of pressure on the ion current, as long as the cell geometry, the number of cells, the voltage and the magnetic field are taken as fixed parameters.
• I k ⁇ P
• sensitivity a parameter (so-called "sensitivity" of the pump) depending upon the geometrical dimensions and distances of the Penning cell, the number of Penning cells, the magnetic field, and the applied voltage.
• the only parameter that can change is the voltage applied. If the applied voltage changes, the sensitivity k changes.
• leakage current spurious currents
• the leakage current does not actually affect the pumping efficiency at all, but when this phenomenon occurs, the ion pump becomes poorly reliable in pressure reading because the pressure is proportional to the ion current only, while the measured current always results from the sum of the ion current and the leakage current (not depending form the pressure).
• the leakage current can arise from both external sources (such as power supply, connecting cables, and so on) and internal sources (e.g., the metallization of ceramic insulators).
• the main cause of leakage current is the field electron emission (FEE) from cathodes, by which free electrons are emitted from the titanium surface when strong electric field is applied.
• FEE field electron emission
• N(W x )dW x be the electron supply function or the number of electrons within a metal having energy between W x and W x +dW x and D(W x ) be the electron transmission coefficient, or the probability that an electron with an energy W x escapes the potential barrier.
• Microprotrusion or "whiskers” present on the electrodes are responsible for the FEE current due to the high electric field that is generated locally at the tip.
• E ⁇ Vd ⁇ 1
• V the applied voltage
• d the electrode spacing
• ⁇ a "shape factor" depending on the height and width of the protrusion.
• the presence of the leakage current can limit the use of an ion pump as a pressure indicator, especially in low pressure applications (lower than 10 -9 mbar), in which it is more likely to have a not negligible component of leakage current compared to the ion current-
• an object of the present invention is to provide a method for operating an ion pump allowing to provide an accurate estimation of the pressure inside the pump, even when the leakage current is not negligible.
• a further object of the invention is to provide a pumping system including an ion pump and suitable for implementing such method.
• FEE current the current due to field electron emission
• the applied voltage strongly affects the FEE current and decreasing the applied voltage is an effective way to decrease the FEE current, so that the FEE current may be negligible if the applied voltage is low enough.
• the voltage value at which the FEE current becomes negligible might depend, for instance, from the geometry of the ion pump.
• This risk can be reduced by supplying the pump electrodes with a higher voltage.
• the method of the invention allows to estimate the contribution of the leakage current to the overall current absorbed by the pump when the pump is supplied with higher voltages, so that the actual ion current can be more accurately estimated and, consequently, the pressure can be accurately estimated.
• the method according to the invention relies on two assumptions:
• the pressure in the ion pump is substantially independent from the applied voltage.
• the sensitivity depends on the applied voltage only.
• the method according to embodiments of the invention comprises the following steps:
• the measured current at said first voltage substantially coincides with the ion current at said first voltage.
• a voltage threshold at which the leakage current is substantially negligible is set and the first voltage is selected equal to or lower than said threshold.
• Such threshold might depend, for instance, from the constructional parameters of the ion pump (such as the geometry of the ion pump) and/or from the operating parameters of the ion pump.
• the estimated current at said second voltage takes into account the ion current only, as it is calculated on the basis of a theoretical relationship, while the measured current at said second voltage is the result of both the contribution of the ion current and of the leakage current, if any, at said second voltage.
• a leakage current at said second voltage can be estimated as the difference between the measured current at said second voltage and the estimated current at said second voltage.
• the method further includes the step of calculating the pressure in the ion pump according to the well-known linear relationship between the ion current and the pressure.
• the leakage current is substantially negligible and the measured current coincides with the ion current, so that the pressure can be correctly estimated starting from said measured current at said first voltage.
• the ion pump can be operated at higher voltages and it still provides for an accurate estimation of the pressure inside the pump.
• leakage current in ion pumps generally increases as the time during which the pump is operated elapses.
• the first voltage can be applied again and the above-described steps can be repeated, so that it is possible to verify whether the leakage current at the second voltage is still accurately estimated and the corresponding correction factor is still correctly determined and, if needed, it is possible to correct the estimation of the leakage current at said second voltage and adjust and update the correction factor to be used in calculating the pressure during the subsequent time interval.
• the length of the time interval can be set by the user.
• the length of the time interval can be determined upon the operating conditions of the pump.
• the length of the time intervals can be adjusted depending on the accuracy of the estimation of the leakage current: if the leakage current at said second voltage turns out to be accurately estimated, the length of the time intervals could be increased, while if the leakage current at said second voltage turns out to be inaccurately estimated, the length of the time intervals should be decreased.
• the first voltage is about 3 kV.
• the second voltage is selected between about 5kV and about 7 kV.
• At least one further, higher voltage can be set and, for said further voltage or for each of said further voltages, the method further provides the steps of:
• a correction factor can be determined and applied in order to accurately estimate the pressure in the ion pump starting from the estimated leakage current at said further voltage.
• the first voltage is about 3 kV
• the second voltage is about 5kV
• a further voltage is about 7 kV.
• the measured current at said second voltage is different from the estimated current at said second voltage, then the measured current at said second voltage is inputted into an algorithm comparing said measured current with a theoretical model based on the equation of Fowler-Nordheim. Thanks to the comparison with such theoretical model following the Fowler-Nordheim equation, an indicator of the reliability of the estimation of the leakage current at said second voltage is provided.
• information concerning the source of said leakage current can be obtained. More specifically, it can be ascertained whether the leakage current at said second voltage is due to filed electron emission or to other internal or external factors.
• the leakage current can arise from both external sources and internal sources.
• the main cause of leakage current is the field electron emission (FEE) from cathodes, by which free electrons are emitted from the cathode surface when strong electric field is applied.
• FEE field electron emission
• FEE current The leakage current due to field electron emission, so-called “FEE current” is strongly dependent on the electrode spacing and, once the construction and geometry of an ion pump is set, on the applied voltage.
• the FEE current for an applied voltage 7 kV is 1.2.10 -3 A; a reduction of the applied voltage to 5 kV (at the same electrode spacing) gives a FEE current of 1.4 ⁇ 10 -5 A; a further reduction of the applied voltage to 3 kV gives 0.93 ⁇ 10 -9 A, i.e. a reduction of more than four order to magnitudes when compared to an applied voltage of 5 kV and of more than six order of magnitudes when compared to an applied voltage of 7 kV.
• the FEE current is absolutely negligible (even considering a non-favorable surface structure of the electrodes), while it becomes more and more relevant as the applied voltage increases.
• the leakage current (or at least its main component) is negligible and the measured overall current substantially coincides with the ion current, i.e. with the pressure-dependent current.
• a sufficiently low voltage e.g., about 3 kV or below such value - the leakage current (or at least its main component) is negligible and the measured overall current substantially coincides with the ion current, i.e. with the pressure-dependent current.
• the method of the invention allows to reliably estimate the pressure in the ion pump even if the higher voltage is applied.
• the method according to a preferred embodiment of the invention essentially comprises the following steps:
• the ion pump is kept running at said second, higher voltage for a predetermined time interval ⁇ t.
• the current is measured (step 65) and the pressure inside the ion pump is calculated starting either from said measured current at said second voltage or from said measured current at said second voltage and said correction factor (step 70).
• the ion pump can be operated at higher voltages and it still provides for an accurate estimation of the pressure inside the pump.
• the first voltage is applied again and the steps 15 - 60 are repeated, so as to verify whether the leakage current at said second voltage is still accurately estimated and, in case, adjust and update the correction factor to be applied.
• the length of the time interval ⁇ t can be set by the user.
• the length of the time interval ⁇ t can be determined upon the operating conditions of the pump.
• the length of the time interval can be longer at the beginning of the pump life, when the cathode surface is smoother, and it can be shortened as the wear of the pump increases, i.e. as the cathode surface becomes rougher and the presence of unevenness on the cathode surface is likely to generate a higher leakage current due to field electron emission.
• the length of the time interval can be iteratively determined depending on the accuracy of the estimation of the leakage current at said second voltage. If the correction factor calculated at the end of the time interval is substantially the same as the previously calculated correction factor, the length of the time interval could be increased; conversely, if the correction factor calculated at the end of the time interval is sensibly different from the previously calculated correction factor, the length of the time intervals should be decreased.
• one or more further higher voltages can be applied, and for each further voltage a corresponding leakage current can be estimated and a corresponding correction factor can be calculated.
• the measured current at said second voltage differs from the estimated current at said second voltage, then the measured current at said second voltage is compared with a theoretical model based on the equation of Fowler-Nordheim (not shown in the flow chart of Figure 3 ).
• the method according to a preferred embodiment of the invention can be implemented in a pumping system 1 as schematically shown in Figure 4 .
• the pumping system 1 includes a ion pump 3 having the general structure shown in Figure 1a or in Figure 1b .
• the pumping system further comprises a potential difference source 5 for applying a voltage between the electrodes of the ion pump 3.
• the pumping system 1 further includes one or more sensors 7 for measuring the current absorbed by the ion pump 3.
• the pumping system 1 further includes a controller 9 for controlling the operation of the ion pump 3.
• the controller 9 is suitable for controlling the potential difference source 5 for applying a selected voltage to the electrodes of the ion pump 3.
• controller 9 is equipped with a memory unit 11 in which the theoretical relationship between the ion current and the applied voltage for the ion pump 3 (i.e. the geometrical and magnetic parameters of the ion pump being fixed) are stored.
• a theoretical model of the FEE current in the ion pump 3 according to the Fowler Nordheim equation is also stored in the memory unit 11.
• the controller 9 is further equipped with a processing unit 13 which is adapted to calculate the estimated currents at different voltages starting from the theoretical relationship stored in the memory unit 11, receive the values of the measured currents coming from the sensor(s) 7, compare the estimated current with the measured current at the applied voltage, calculate a correction factor based on the difference between the estimated current and the measured current (if any), and calculate the pressure starting from the measured current and the correction factor (if applicable).
• a processing unit 13 which is adapted to calculate the estimated currents at different voltages starting from the theoretical relationship stored in the memory unit 11, receive the values of the measured currents coming from the sensor(s) 7, compare the estimated current with the measured current at the applied voltage, calculate a correction factor based on the difference between the estimated current and the measured current (if any), and calculate the pressure starting from the measured current and the correction factor (if applicable).
• the processing unit 13 also includes an algorithm for comparing the measured current with the theoretical model of the FEE current stored in the memory unit 11 and providing an indicator of the reliability of the calculated correction factor based on such comparison.

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• Control Of Positive-Displacement Pumps (AREA)

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• 2025
  • 2025-09-19 EP EP25203447.5A patent/EP4645366A3/de active Pending

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