METHOD FOR DETERMINING A PRESSURE AND PRESSURE SENSOR

Description

[0001] The present invention relates to a method for determining a pressure in a vacuum system, a vacuum pressure sensor, a device for carrying out the method, an application of the method, and a use of the pressure sensor.

[0002] Vacuum pressure sensors or vacuum gauges with which pressures significantly below normal pressure can be determined are well known. Among the known vacuum pressure sensors, so-called ionization vacuum gauges have a particularly wide measuring range. They measure pressure via gas ionization. First, the ionizability of the residual gas and thus the gas density in the vacuum system is determined. The electrons required to ionize the gas are generated either by a hot cathode (hot cathode ionization vacuum gauge) or in an independent gas discharge between cold electrodes (cold cathode vacuum gauge). Since the independent gas discharge between cold electrodes extinguishes at pressures around 10 -3 < mbar, the arrangement according to patent DE 716 712 by FMPenning extended the measuring range of cold cathode ionization gauges to pressures in the high vacuum range by using a magnetic field that significantly lengthens the electron trajectories in the self-contained electrical gas discharge and thus increases the ion yield. These Penning vacuum gauges are widely used and have been technically improved over the years. The configuration frequently used today is that of an inverted magnetron proposed by Hobson and Redhead (Redhead, P, A, (1959), The magnetron gauge, a cold cathode vacuum gauge, Can. J. Phys. 37, 1260-1271). This is capable of covering a pressure range from approximately 10 -3 < mbar up to the ultra-high vacuum range. Penning vacuum gauges, as well as cold cathode vacuum gauges based on a magnetron or inverted magnetron arrangement, have in common that an electric field is essentially perpendicular to a magnetic field.

[0003] Another vacuum pressure sensor is known from US 6 351 131 B1.

[0004] One of the main problems with known configurations is the limitation of the pressure range to high pressures. Due to a change in the dominant component in the discharge, an ambiguity arises in the current measurement. This is due to the fact that both electrons and ions contribute to the measured current. The nature of the resulting plasma changes depending on the pressure. A typical pressure-current calibration curve of a cold cathode vacuum gauge, for pressures below approximately 10 -3 < mbar, exhibits a range in which the current increases monotonically with increasing pressure. This is the usable measuring range in which the pressure can be unambiguously determined from a measured current. In this range, ring currents with electrons dominate. The pressure-current calibration curve typically exhibits a current maximum at approximately 10 -2 < mbar. At pressures above this maximum, the current decreases with increasing pressure. In this pressure range, the plasma dominates, i.e.the mixture of electrons and positively charged ions. Conventionally, the usable measuring range cannot be extended beyond the stated maximum current.

[0005] The object of the present invention was to find improved methods for determining vacuum pressure compared to the prior art. A further object of the present invention was to provide an alternative vacuum pressure sensor, specifically with an expanded usable measuring range.

[0006] According to the invention, this object is achieved by a method according to claim 1.

[0007] The method according to the invention is a method for determining pressure in a vacuum system. The method comprises the following steps: a) generating a plasma in a sample chamber which is fluid-dynamically connected to the vacuum system and wherein the plasma is in electrical contact with a first electrode and a second electrode; b) measuring a current intensity of an electric current flowing through the plasma between the first electrode and the second electrode; c) measuring a first radiation intensity of electromagnetic radiation emitted from the plasma in a first wavelength range, wherein the first wavelength range contains at least one first emission line of a first plasma species of a first chemical element;d) measuring a second radiation intensity of electromagnetic radiation emitted from the plasma in a second wavelength range, wherein the second wavelength range includes a second emission line of the first plasma species of the first chemical element or of a second plasma species of the first chemical element, and wherein the second emission line lies outside the first wavelength range; and e) determining the pressure in the vacuum system as a function of the measured current, the measured first radiation intensity, and the measured second radiation intensity.

[0008] The generated plasma produces electromagnetic radiation and charged particles. Based on the charged particles generated in the plasma, the particle density in the sample chamber can be determined by measuring a current flowing in the plasma using an ammeter. The pressure in the sample chamber can be determined from the particle density in the sample chamber. The sample chamber is fluid-dynamically connected to the vacuum system so that pressure equalization takes place between the vacuum system and the sample chamber and the pressure determined in the sample chamber corresponds to the pressure in the vacuum system. The sample chamber extends between the mutually insulated electrodes in such a way that a plasma generated in the sample chamber can be in contact with both the first electrode and the second electrode. The ammeter can be any charge rate meter, e.g. an ammeter or an electron counter.A wide variety of plasma sources can be used to generate the plasma. Electron cyclotron resonance (ECR) ion sources, Penning discharges, inductively coupled plasmas (ICP), or glow discharge sources, etc., are suitable for the inventive method. Penning, magnetron, and inverted magnetron arrangements are suitable for generating the plasma because these arrangements can be constructed very compactly and ensure sufficient ion yield even at low pressures, especially at pressures as low as 10 -8 mbar. The last three plasma sources are collectively referred to as ExB sources.

[0009] The inventors have recognized that by measuring the intensities in two different, cleverly selected wavelength ranges of the electromagnetic radiation emanating from the plasma, in particular electromagnetic radiation in the optical range, additional information about the pressure in the sample space can be obtained, by means of which an ambiguity in the assignment of a pressure to the measurement result of a current measurement can be resolved.

[0010] The measured intensities of electromagnetic radiation refer, for example, to radiation intensities in the optical range, i.e., radiation intensities of visible light, ultraviolet radiation, or infrared radiation, particularly in the near-infrared range. Atoms and ions in a plasma emit radiation in this wavelength range, which exhibits emission lines characteristic of the respective chemical element. The wavelengths of these characteristic emission lines are known from the field of atomic emission spectrometry (AES) and can be looked up for a gas to be measured. Depending on the application, a gas to be measured can be, for example, nitrogen, oxygen, argon, or helium.

[0011] According to the invention, a first radiation intensity of electromagnetic radiation in a first wavelength range is measured, and a second radiation intensity of electromagnetic radiation in a second wavelength range is measured. The first and second wavelength ranges are selected such that the first wavelength range contains a first emission line, but a second emission line is excluded. In the second wavelength range, however, the second emission line is observable. The first and second wavelength ranges can be selected to be very narrow, i.e., for example, only slightly wider than the natural linewidths of the first and second emission lines, respectively, so that essentially only the first and second emission lines lie in the respective wavelength range. The second wavelength range can be selected to be large enough that the first emission line also lies in the second wavelength range.The first and second emission lines are emission lines of plasma species of the same chemical element, which is referred to here as the first chemical element. They can be a first and second emission line of the same first plasma species. Alternatively, the second emission line can be an emission line of a second plasma species that is different from the first plasma species, but which is a plasma species of the same first chemical element. By plasma species we mean gaseous atoms and chemical compounds that are characterized by a chemical structural formula, their charge state (e.g., neutral, singly ionized, doubly ionized), and possibly their excited state. A plasma species of an element contains this element in its structural formula.For example, the neutral nitrogen molecule N2, the neutral nitrogen atom NI, a singly ionized nitrogen atom NII and a doubly ionized nitrogen atom NIII are four different plasma species of the chemical element nitrogen. As a further example, the neutral argon atom Ar and the argon ion Ar+< are two different plasma species of the chemical element argon. By observing two different emission lines from one or two plasma species of the same chemical element according to the invention, a further pressure-dependent parameter can be determined in addition to the current strength. In the event that emission lines from plasma species with a structural formula with more than two atoms are observed, it is advantageous if the emission lines belong to plasma species that not only have a common chemical element, but also have as large parts of the structural formula in common as possible.In this way, the intensity ratio of the measured first and second intensities depends primarily on the particle density in the plasma, i.e., on the pressure. This reduces the dependence on the composition of the residual gas. Preferably, the first and second emission lines are emission lines of the same gas, i.e., emission lines of the same atom or molecule in the gaseous or plasma state.

[0012] The final step of the process is to determine the pressure in the vacuum system as a function of the measured current, the measured first radiation intensity, and the measured second radiation intensity. For example, the first and second radiation intensities can be calculated to obtain a radiation intensity ratio, and the pressure can then be determined based on the measured current and the calculated radiation intensity ratio.

[0013] The measurements of the current strength and the first and second radiation intensities can be performed simultaneously or staggered. Once the plasma has been generated, these measurements can be repeated periodically or continuously, for example, to monitor changes in the pressure of the vacuum system periodically or continuously. For this purpose, the plasma must be maintained by supplying energy. Variants of the procedure in which a plasma is generated before each measurement are also conceivable.

[0014] Variants of the method result from the features of dependent claims 2 and 3.

[0015] In a variant of the method, in step e) of determining the pressure in the vacuum system, an estimated pressure value is determined based on the measured first radiation intensity and the measured second radiation intensity. A definition domain of a pressure-current calibration curve is restricted to a pressure range that contains the estimated value and in which the pressure-current calibration curve is monotonic. The pressure in the vacuum system is determined based on the pressure-current calibration curve in the restricted definition domain and based on the measured current.

[0016] This variant of the method allows the determination of a pressure across a large measurement range, even if the pressure-current calibration curve exhibits monotonically rising and monotonically falling sections throughout the entire measurement range. It is not necessary to determine an exact pressure from the two measured radiation intensities. It is sufficient if an estimated pressure value can be determined, with the knowledge of which the relevant pressure values ​​can be restricted with sufficient certainty to a monotonic range of the pressure-current calibration curve. Then, the correlation of the pressure to the measured current becomes unambiguous, and the pressure can be determined with the high precision typical of current measurement.

[0017] In a variant of the method, a logarithm of the estimated value p0 of the pressure is calculated using the formula log p0 = a I_1 / I_ 2 + b where a and b are pre-determined coefficients that depend on the choice of emission lines, the setup used to generate the plasma, and the base of the logarithm.

[0018] This variant of the method enables a very simple and robust estimation of the magnitude of the pressure based on a linear function of the intensity ratio of the first and second radiation intensities. A pressure range around this estimated value can then be used as a restricted definition range for the pressure. For example, if the estimated value p0 estimates the true pressure with an error tolerance of Δp, the restricted definition range for the pressure can be chosen from (p0-10*Δp) to (p0+10*Δp).

[0019] The coefficients a and b can be determined by calibration measurements at a known pressure. Changing the base of the logarithm (e.g., logarithm to base e or logarithm to base 10) changes the coefficients a and b by the same factor. Furthermore, the object is achieved by a vacuum pressure sensor according to claim 4.

[0020] The vacuum pressure sensor according to the invention comprises: a sample chamber in which a plasma can be generated, the sample chamber having electrical contact with a first electrode and with a second electrode, a current measuring device which is electrically connected to the first and second electrodes and is connected in series to the sample chamber, a wavelength-selective element, and a first and a second detector element for measuring a radiation intensity of electromagnetic radiation.

[0021] The wavelength-selective element, the first detector element, and the second detector element are arranged such that only electromagnetic radiation of a first wavelength range emanating from the sample chamber can enter the first detector element, and only electromagnetic radiation of a second wavelength range emanating from the sample chamber can enter the second detector element. At least one first emission line of a first plasma species of a first chemical element lies in the first wavelength range. A second emission line of the first plasma species of the first chemical element or of a second plasma species of the first chemical element lies in the second wavelength range. The second emission line lies outside the first wavelength range.

[0022] The vacuum pressure sensor according to the invention is suitable for performing the measurements according to steps b), c), and d) of the inventive method. The wavelength-selective element and the first and second detector elements enable the measurements of the radiation intensity from the first and second wavelength ranges, respectively, with these two wavelength ranges being selected as discussed above in connection with the inventive method.

[0023] The wavelength-selective element can, for example, be an optical filter with narrowband, wideband, or bandpass characteristics. Such an optical filter can, for example, be mounted directly in front of one of the detector elements, so that the electromagnetic radiation from the plasma arriving at the corresponding detector element must pass through the optical filter. Such a filter can, for example, be an interference filter that reflects in certain wavelength ranges and is transparent to radiation in others. The filter can be a color filter with a transmission of more than 95% in a selected wavelength range. Thus, the radiation in the beam path downstream of the filter primarily contains radiation with the wavelengths passed through the filter.The wavelength-selective element can also be an optical grating or a prism, for example, so that electromagnetic radiation is diffracted or refracted in different directions depending on the wavelength. The detector elements can then be positioned in the corresponding direction.

[0024] The vacuum pressure sensor can, for example, have a connection opening, e.g., as the connection opening of a measuring chamber. Through such a connection opening, the sample volume can be fluid-dynamically connected to a vacuum system. In this case, the vacuum pressure sensor can be manufactured and maintained independently of the vacuum system. The connection opening can, for example, be surrounded by a standard vacuum flange. Alternatively, the vacuum pressure sensor can also be integrated into a vacuum system.

[0025] The sample space can, for example, be surrounded by a measuring chamber. At least one of the first and second electrodes can be partially or completely formed as a wall of a measuring chamber of the vacuum pressure sensor.

[0026] Embodiments of the vacuum pressure sensor result from the features of claims 5 to 12.

[0027] One embodiment of the vacuum pressure sensor comprises a measuring chamber surrounding the sample chamber. The measuring chamber has a window in a wall of the measuring chamber or as a wall of the measuring chamber. The window is transparent in an optical wavelength range. A continuous first radiation path is defined, which originates from the sample chamber and traverses the window and ends in the first detector element. A continuous second radiation path is defined, which originates from the sample chamber and traverses the window and ends in the second detector element.

[0028] In this embodiment, the sample chamber and the two detector elements are located on different sides of the window. The window can be made of sapphire or quartz glass, for example. Windows made of sapphire or quartz glass are transparent across virtually the entire optical wavelength range, are chemically inert—i.e., they are not attacked by the plasma or any process gases—have high mechanical strength, and are vacuum-compatible. Sapphire, in particular, has a very high transmission in the wavelength range from 250 to 1000 nanometers. With a window made of sapphire or quartz glass, the optical detector elements can be separated from the vacuum, or from the plasma in the sample volume, or from any process gases, without having to accept any loss in the detectable radiation intensity emanating from the sample volume.The optical detector elements can be installed directly behind the window, as seen from the sample volume, which on the one hand results in high sensitivity to radiation intensity from the sample volume and on the other hand enables a very compact design of the entire pressure sensor.

[0029] In the embodiment with a window, the first and second detector elements can be arranged outside the measuring chamber. In this case, the first and second detector elements do not need to meet any requirements regarding vacuum compatibility.

[0030] The window can, for example, have two flat, parallel boundary surfaces, with a first boundary surface facing a vacuum side and a second boundary surface facing a detector side. The window can additionally also have the function of an electrical feedthrough, in particular a high-voltage feedthrough, in particular for feeding a central anode into the sample chamber. Furthermore, one or more boundary surfaces of the window can be curved and thus have a focusing effect on electromagnetic radiation paths. In particular, the geometry of the boundary surfaces can be designed such that radiation paths emerging from the plasma are focused on at least one of the detector elements. An increase in radiation intensity on the first and / or second detector element and thus more precise evaluation of the spectral lines can be achieved with this variant of the embodiment.In this way, the window can be seen, for example, as an electro-optical feedthrough with a lens effect.

[0031] In one embodiment of the vacuum pressure sensor, the first detector element and / or the second detector element is a photodiode, a phototransistor, a charge-coupled device (abbreviated CCD), a multi-channel plate (abbreviated MCR) or a channel electron multiplier (abbreviated CEM).

[0032] With a photodiode or a phototransistor, the optical wavelength range relevant for the evaluation of the emission lines of the gas to be measured, i.e. infrared, visible range and ultraviolet, can be well covered. With photodiodes or phototransistors on

[0033] For example, silicon-based photodiodes can cover a sensitivity range for wavelengths from approximately 190 to 1100 nanometers. Germanium-based photodiodes or phototransistors, for example, can cover a sensitivity range for wavelengths from approximately 400 to 1700 nanometers. The use of individual photodiodes has the advantage that they can cover a relatively large entrance area, thus significantly increasing sensitivity, compared to, for example, the use of a spectrometer. Photodiodes or phototransistors are relatively inexpensive detector elements. Compared to other detector elements, photodiodes or phototransistors require less space, thus allowing for a compact design of the vacuum pressure sensor.

[0034] In one embodiment, the vacuum pressure sensor comprises a miniature spectrometer having a detector array, and the first detector element and the second detector element are elements of the detector array.

[0035] A miniature spectrometer can, for example, have a slit or a hole where the first radiation path enters the miniature spectrometer. An optical grating or a prism ensures that the incident radiation is diffracted or deflected in different directions and falls on the various elements of a detector array. In this way, an intensity spectrum of the radiation emitted from the sample volume can be determined. Miniature spectrometers with geometric dimensions in the order of 20 mm x 10 mm x 10 mm are commercially available. This enables a compact design of the vacuum pressure sensor. A detector array can, for example, be designed as a photodiode array or an array of phototransistors. A detector array can, for example, be designed as an array of CCD elements. The first and second wavelength ranges can each be covered by a single element of the detector.It is also possible to add the measured intensities of several, e.g. several neighboring, elements to the measured first or second radiation intensity.

[0036] In one embodiment, the vacuum pressure sensor comprises a device for generating a magnetic field in the sample space.

[0037] The device for generating the magnetic field in the sample volume can be, for example, a coil. The device for generating a magnetic field in the sample volume can also be, for example, an array of permanent magnets. Additionally, ferromagnetic elements can also be used to guide the magnetic fields generated by the device in order to achieve the desired magnetic field distribution in the sample volume. In this embodiment, the electric fields generated by the first and second electrodes and the magnetic fields generated by the device for generating a magnetic field in the sample volume can be arranged such that a plasma is generated very efficiently.The electric fields accelerate the electrons to energies greater than the ionization potential and the magnetic fields on the one hand lengthen the trajectories, which gives a higher probability of collision with another particle and on the other hand keep the electrons in orbits with about hundreds of electron volts, where the maximum cross section with molecules and atoms lies.

[0038] In one embodiment of the vacuum pressure sensor with a device for generating a magnetic field in the sample chamber, the arrangement of the first electrode, second electrode, and the device for generating a magnetic field in the sample chamber is configured such that, by applying an electrical voltage to the electrodes, an electric field can be generated that is oriented substantially perpendicular to the magnetic field in the sample chamber. In this embodiment, the arrangement of the first electrode, second electrode, and the device for generating a magnetic field can be configured, in particular, as a magnetron arrangement, inverted magnetron arrangement, or Penning arrangement, i.e., it can be a so-called ExB plasma source.

[0039] In an ExB plasma source, the device for generating the magnetic field in the sample volume must be arranged so that the field lines of the magnetic and electric fields in the sample volume intersect essentially at a right angle.

[0040] The inventors have recognized that magnetron arrays, inverted magnetron arrays, and Penning arrays have proven particularly effective. Effective in this context means that the region of the sample volume with maximum photon emission remains spatially stable over a large pressure range, for example, several orders of magnitude in pressure. For example, with an inverted magnetron array, the region of the sample volume with maximum photon emission remains spatially stable over more than 10 orders of magnitude in pressure. Whether the discharge is ignited at 10 -9 < mbar or at 10 mbar, the volumes with the greatest emission are always located largely at the same location, or rather, in a torus-shaped region with a constant diameter. Furthermore, this spatial region is easily accessible for observation in the aforementioned electrode arrays.This allows the use of a very simple detector element: photodiodes with color filters (and possibly a small focus lens), for example, mounted directly adjacent to the window around a central anode. Instead of diodes, a miniature spectrometer, for example, can be installed directly adjacent to the window.

[0041] In one embodiment, the vacuum pressure sensor comprises an energy source for supplying energy to a plasma in the sample space.

[0042] Maintaining a plasma requires an energy source, which can, in principle, be provided independently of the vacuum pressure sensor according to the invention, but can also affect the sample chamber. In the embodiment discussed, the energy source is part of the vacuum pressure sensor, resulting in particularly easy-to-handle and compact arrangements. Energy can be supplied from the energy source to the plasma via electrically conductive contacts, capacitively, inductively, or via radiation, such as laser radiation or microwave radiation.

[0043] In one embodiment of a vacuum pressure sensor with an energy source for supplying energy to a plasma in the sample space, the energy source comprises a high-voltage source which is electrically conductively connected to the first and second electrodes and which is connected in series with the ammeter.

[0044] In one embodiment of a vacuum pressure sensor with an energy source for supplying energy to a plasma in the sample space, the energy source comprises an alternating current source and an induction coil, wherein the induction coil is electrically connected to the alternating current source and is designed to generate an alternating magnetic field in the sample space when alternating current flows through the induction coil.

[0045] Features of embodiments of the vacuum pressure sensor can be combined as desired, provided they do not contradict each other.

[0046] The invention further relates to a device according to claim 13. This device is for carrying out the method according to the invention. The device comprises a vacuum pressure sensor according to the invention and a processing unit. The vacuum pressure sensor can have the features of one of the aforementioned embodiments. The processing unit is operatively connected to the current measuring device and the first and second detector elements for electromagnetic radiation for transmitting the measured current, the measured first radiation intensity, and the measured second radiation intensity. The processing unit is designed to determine the pressure in the vacuum system as a function of the measured current, the measured first radiation intensity, and the measured second radiation intensity.

[0047] The processing unit can comprise a digital computer, an analog computer, or a corresponding electronic circuit. A suitable electronic circuit can, for example, comprise two amplifiers with a logarithmic response function, each connected upstream of an input of an operational amplifier. With such an electronic circuit, for example, the logarithmic ratio of the first and second radiation intensities can be formed when the output signal of the first and second detector elements is each fed to an input of one of the two amplifiers. The processing unit can, in particular, comprise a microprocessor or a digital signal processor.The processing unit can, in particular, be programmed such that method step e) of the method according to the invention or a variant of the method is carried out if the measured current intensity, the measured first radiation intensity, and the measured second radiation intensity are available as input values. These input values ​​can be transmitted to the processing unit as an analog or digital signal, via electrical lines, or wirelessly. The processing unit can be installed in a housing of a vacuum pressure sensor. The processing unit can comprise storage means for storing calibration data, in particular pressure-current calibration curves or coefficients a and b according to a variant of the method.

[0048] Furthermore, the invention is directed to an application according to claim 14.

[0049] This involves an application of the inventive method for extending the pressure measurement range of a vacuum pressure sensor based on measuring a current through a plasma. In particular, this can be a vacuum pressure sensor based on the functional principle of a cold cathode vacuum gauge. The pressure measurement range is extended to a pressure measurement range that includes both pressures below and pressures above an extreme value of a pressure-current characteristic curve of the vacuum pressure sensor.

[0050] If the pressure-current characteristic curve of a vacuum pressure sensor, which is based on measuring a current through a plasma, exhibits an extreme value, i.e., a minimum or a maximum, the assignment of a measured current to a pressure is not clear. Typically, one must restrict oneself to a pressure measurement range that either includes only pressures below the pressure at which the extreme value occurs, or only pressures above the pressure at which the extreme value occurs. By applying the method according to the invention, this ambiguity can be eliminated, and an extended pressure measurement range on both sides of the extreme value can be covered with the vacuum pressure sensor.

[0051] The application of the method according to the invention thus enables, for example, a wide-range cold cathode vacuum gauge.

[0052] The use according to claim 15 is also within the scope of the invention.

[0053] The use according to the invention is the use of a vacuum pressure sensor according to the invention in a method according to the invention. Embodiments of the vacuum pressure sensor can be combined arbitrarily with variants of the method, provided they do not contradict each other.

[0054] Embodiments of the present invention are explained in more detail below with reference to figures. Fig. 1 a flow chart of the method according to the invention; Fig. 2 a schematic representation of a vacuum pressure sensor according to the invention; Fig. 3 a graphical representation of the relationship between pressure and current intensity or pressure and radiation intensity ratio in an embodiment of the vacuum pressure sensor; Fig. 4 a schematic cross-section through part of an embodiment of a vacuum pressure sensor; Fig. 5a schematic nitrogen emission spectrum with examples for a first and a second wavelength range; Fig. 6 a schematic cross-section through a detail of an embodiment of a vacuum pressure sensor; Fig. 7 a schematic cross-section through an embodiment of a vacuum pressure sensor; Fig. 8 an example of a definition of a first and second wavelength range using a wavelength-selective element, in sub-figures 8.a) and 8.b) each efficiency curve as a function of the wavelength.

[0055] Figure 1 shows the steps of the method 100 for determining a pressure in a vacuum system as blocks in a flowchart. The method begins with the step of generating 101 a plasma in a sample chamber 20, which is fluid-dynamically connected to the vacuum system and which is in electrical contact with a first electrode and a second electrode.

[0056] Three measurement processes 102, 103, and 104 are performed in parallel or at different times, as represented by the slightly offset blocks in the flowchart. All three measurement processes refer to measurements on the previously generated plasma. These are the following steps: Measuring 102 a current intensity C_plasma of an electric current flowing through the plasma between the first electrode and the second electrode, measuring 103 a first radiation intensity I_1 of electromagnetic radiation of a first wavelength range emitted from the plasma, measuring 104 a second radiation intensity I_2 of electromagnetic radiation of a second wavelength range emitted from the plasma.

[0057] The first and second wavelength ranges are defined as described above for the method according to the invention.

[0058] As a final step, based on the measurement results C_plasma, I_1, and I_2 of the measurement processes, the pressure p in the vacuum system is determined 105 as a function of the measured current C_plasma, the measured first radiation intensity I_1, and the measured second radiation intensity I_2. This relationship is expressed by the formula p = f( C_plasma, I_1, I_2 ), where f symbolizes a mathematical function or a mathematical procedure, which in variants of the method can process additional inputs in addition to the three measured values, e.g., calibration coefficients, a calibration curve, or a calibration surface.

[0059] Figure 2shows a schematic, highly simplified, and partially cross-sectional view of a vacuum pressure sensor 10 according to the invention. A sample chamber 20 is located at the center. A plasma—indicated by dotted lines—can be generated in this sample chamber. A first electrode 1 and a second electrode 2 are in electrical contact with the plasma in the sample chamber. An ammeter 42 is electrically connected to the first and second electrodes and connected in series with the sample chamber. A wavelength-selective element 51 is mounted in front of a first detector element 31 such that radiation 21 of a first emission line (shown as a dashed arrow with short dashes) can reach the detector element 31, but radiation 22 of a second emission line (shown as a dashed arrow with long dashes) is blocked. Radiation 21, 22 of the first and second emission lines reaches a second detector element 32.The arrows represent a possible radiation path from the plasma to the detector elements 31, 32.

[0060] Figure 3The upper graphic shows a typical calibration curve of the current C_plasma in amperes flowing through the plasma as a function of the pressure p in the sample chamber of a vacuum pressure sensor. The lower graphic shows a typical calibration curve for the ratio I_1 / I_2, i.e. the ratio of the measured first radiation intensity I_1 to the measured second radiation intensity I_2, as a function of the pressure p in the same vacuum pressure sensor. In this case, these are calibration curves for a type of vacuum pressure sensor that is designed as a cold cathode vacuum gauge. The horizontal axis with the pressure p in Torr applies to both curves. The pressure is plotted on a logarithmic scale, so that the dependence on pressure can be read off over 8 orders of magnitude from 10 -8 < Torr to 10 0 < Torr.The current C_plasma is also plotted on a logarithmic scale, which ranges over 5 orders of magnitude from 10 -8< Ampère to 10 -3< Ampère.

[0061] The current C_plasma has a maximum at a pressure close to 10 -2 < Torr. For a current C_plasma in the range between approximately 2*10 -6 < Ampere and approximately 3*10 -4 < Ampere, there are two pressure values ​​p that can lead to this current strength. The ratio I_1 / I_2 is plotted on a linear scale. The zero point and slope are not fixed for the ratio I_1 / I_2 shown. It can be seen that there is a linear relationship between the logarithm of the pressure and I_1 / I_2, which can be described by a slope a and a constant term b. If the ratio I_1 / I_2 is known, the ambiguity of the relationship between current C_plasma and pressure p can be resolved.

[0062] In Figure 4A cross-section through part of one embodiment of a pressure sensor is shown. The second electrode is formed as the wall of the measuring chamber 3 and has essentially the shape of a hollow cylinder. In this arrangement, the second electrode can be operated, for example, as a cathode. The first electrode 1, in the form of a rod, lies on the axis of the hollow cylinder. The first electrode can be operated, for example, as an anode. The embodiment shown comprises a device 8 for generating a magnetic field in the sample volume, which is designed as a permanent magnet. "N" and "S" denote the position of the north and south poles of the permanent magnet ring. Magnetic elements 9 form a magnetic return path for the magnetic fields generated by the device 8. In the sample volume 20, in which a plasma can be generated, there is a central region 20' in which the maximum luminosity in the sample volume occurs when a plasma is generated.From this region 20', radiation emitted from the plasma can travel along the first radiation path 21 and along a second radiation path 22 through the window 5 to the first detector element 31 and the second detector element 32, respectively. A wavelength-selective element in the form of a filter 51 is arranged upstream of the first detector element 31 in the direction toward the sample chamber 20. The filter 51 transmits radiation in the wavelength range of a first emission line and blocks at least the wavelength range of a second emission line. The material of the window 5 is selected such that radiation from the wavelength range of the first and second emission lines is transmitted. The window can be formed, for example, by a sapphire plate or a quartz glass plate. In the illustrated embodiment, the detector elements are arranged on a circuit board 43 which supports the detector elements.A further circuit board 44, adjacent to the first electrode, for supplying high voltage is arranged below the circuit board 43. An arrow indicates the possible position of an optional connection opening 4 for connection to a vacuum system. The connection opening and vacuum system are not shown in this figure. In the embodiment shown, the window 5 also forms a vacuum-tight electrical feedthrough for the first electrode 1. The first electrode can be glazed into the window, for example, by glazing it with a solder glass ring, and thus be connected to the window in a vacuum-tight manner. In the embodiment shown, the window is delimited both on the vacuum side and on the side with the detector elements 31 and 32 by a flat boundary surface. The window 5 is connected to the second electrode 2 in a vacuum-tight manner at its outer circumference. This connection can also be implemented, for example, by glazing it with a solder glass ring.

[0063] Figure 5.a) shows a schematic representation of an emission spectrum of plasma species of the element nitrogen. Intensities I in arbitrary units (au) are plotted against the wavelength λ in nanometers (nm). Groups of emission lines in the range 300-400 nm belong partly to molecular nitrogen N2 and atomic nitrogen NI. Groups of emission lines in the range 600-800 nm belong partly to singly and doubly (NIII) ionized nitrogen. Strictly speaking, the lines of N2, NI, NII, and NIII occur intermixed in the frequency ranges. Nitrogen is an important residual gas in vacuum systems. The method according to the invention can, for example, relate to plasma species of the chemical element nitrogen. Three possibilities for defining the first and second wavelength ranges are described in the Figures 5.b), 5.c) and 5.d) shown, where the wavelength ranges are each on the wavelength axis of Fig. 5.a) relate.

[0064] As in Fig. 5.b) As shown, the first wavelength range W1 can, for example, cover a range of 300-400 nm and include, among other things, several emission lines from plasma species of N2 and Ni. The second wavelength range W2 in this case covers a range of 600-800 nm and thus contains exclusively emission lines that are not included in the first wavelength range. Fig. 5.c) shows an example with a narrowly selected first W1' and second W2' wavelength range, each comprising a single emission line of a plasma species from the group N2 and N. Fig. 5.d) shows a further example in which the second wavelength range W2" completely overlaps the first wavelength range W1" and additionally covers a large wavelength range comprising several emission lines of plasma species N2 and N.

[0065] Figure 6shows a cross-sectional detail of an embodiment in which a miniature spectrometer 30 is mounted directly adjacent to the window 5. The first radiation path 21 leads through the window 5 through a slot 53 in a housing of the miniature spectrometer onto a grating 54. This creates fanned-out radiation paths, each of the fanned-out radiation paths corresponding to a specific wavelength. The radiation intensities of the individual wavelengths are measured by a detector array 39 with a plurality of detector elements. One of the fanned-out radiation paths is the first radiation path 21, which ends in the first detector element 31. A second detector element 32 is arranged at the end of a second radiation path 22.

[0066] Figure 7shows a cross-section of an embodiment based on a vacuum pressure sensor, as described in detail in the publication CH 707 685 A1. This vacuum pressure sensor comprises a permanent magnet arrangement 9 for generating a magnetic field in the sample chamber, wherein the magnetic field is largely shielded from the outside. The embodiment shown has a high-voltage feedthrough 5 that is transparent in the optical range and thus functions as a window 5. Two photodiodes D1 and D1, represented by their circuit symbol, are mounted so that the light passing through the high-voltage feedthrough (symbolized here with two arrows) can be observed. A filter 51 is arranged between the high-voltage feedthrough and photodiode D2 as a wavelength-selective element. Light falling from the sample chamber through the high-voltage feedthrough onto photodiode D1 remains unfiltered.

[0067] Thus, the photodiode D2 functions as the first detector element 31, and the photodiode D1 functions as the second detector element 32, with which at least one second emission line from the unfiltered wavelength range can be observed. The filter characteristic of the filter 51 and the sensitivity of the photodiodes can be selected in the embodiment shown, for example, as described below. Figure 8 explained. For the stability of the acquired signals, it is advantageous to use photodiodes of the same type. Alternatively, both photodiodes can be fitted with a filter, each adapted to the first and second wavelength ranges to be measured. On the right side of the figure, a standard vacuum flange can be seen, with which the vacuum pressure sensor can be connected to a vacuum system, so that the sample chamber 20 is fluid-dynamically connected to the vacuum system.

[0068] Figure 8.a)shows, for the wavelength range from 400 nm to 1000 nm plotted on the horizontal axis, the sensitivity 81 of a photodiode D1, the transmission characteristic 82 of a filter 51, and the combined sensitivity curve 83 resulting from a photodiode D2 identical to photodiode D1 with a filter 51 connected upstream. In the example shown, the filter has a stopband below the cutoff wavelength of 600 nm.

[0069] Figure 8.b) shows for the same wavelength range as Fig. 8.a)a sensitivity distribution 84, which results from the difference between the unfiltered sensitivity and the sensitivity achieved with an upstream filter. Using the first sensitivity distribution 83 thus generated, a first wavelength range of approximately 600-900 nm can be selected, and using the second sensitivity distribution 84, a first wavelength range of approximately 400-600 nm can be selected. A first radiation intensity can thus be measured as a voltage across photodiode D2, and a second radiation intensity can be measured as a voltage across photodiode D1. With the configuration described here, in the case of a nitrogen plasma, a first and a second radiation intensity are measured integrally across a plurality of emission lines of plasma species. List of reference symbols

[0070] 1 first electrode 2 second electrode 3 measuring chamber 4 connection opening 5 window 5' window area (facing the sample chamber) 6 wall of the measuring chamber 7 electrical connections 8 permanent magnet arrangement 9 magnetic element 10 pressure sensor 20 sample chamber 20' area of ​​maximum illumination in the sample chamber 21 electromagnetic radiation of a first wavelength range 22 electromagnetic radiation of a second wavelength range 30 miniature spectrometer 31 first detector element 32 second detector element 33, 34, 35, 36, 37,38 further detector elements 39 detector array 41 high voltage source 42 current measuring device 43 circuit board (carries detector element) 44 circuit board (carries voltage supply) 51 filter 52 lens 53 slit 54 optical grating 81 sensitivity of a photodiode 82 transmission characteristic of a filter 83 first sensitivity distribution 84 second sensitivity distribution 100 method for determining a pressure 101 method step generating a plasma 102 method step measuring a current 103 method step measuring a first radiation intensity 104 method step measuring a second radiation intensity 105 method step determining the pressure N, S north / south poles of a permanent magnet arrangement I intensity I_1 first radiation intensity I_2 second radiation intensity C_plasma current through the plasma N2, NI, NII, NIII plasma species of nitrogen p pressure p0 estimated value for the pressure W1, W1', W1" first wavelength range W2, W2',W2" second wavelength range λ wavelength START starting point of a process END end point of a process,

Claims

1. Method (100) for determining a pressure in a vacuum system, wherein the method comprises the steps of: a) generating (101) a plasma in a sample chamber (20) which is fluid-dynamically connected to the vacuum system and wherein the plasma is in electrical contact with a first electrode and a second electrode; b) measuring (102) a current intensity (C_plasma) of an electrical current flowing through the plasma between the first electrode and the second electrode; c) measuring (103) a first radiation intensity (I_1) of electromagnetic radiation of a first wavelength range which is emitted from the plasma, wherein the first wavelength range contains at least a first emission line of a first plasma species of a first chemical element; d) measuring (104) a second radiation intensity (I_2) of electromagnetic radiation of a second wavelength range, which is emitted from the plasma, wherein the second wavelength range contains a second emission line of the first plasma species of the first chemical element or of a second plasma species of the first chemical element, and wherein the second emission line lies outside the first wavelength range; and e) determining (105) the pressure (p) in the vacuum system as a function of the measured current intensity (C_plasma), the measured first radiation intensity (I_1) and the measured second radiation intensity (I_2).

2. Method (100) according to claim 1, wherein in step e) of determining (105) the pressure in the vacuum system, based on the measured first radiation intensity and the measured second radiation intensity, an estimated value (p0) of the pressure is determined, wherein a definition range of a pressure-current intensity calibration curve is restricted to a pressure range which contains the estimated value and in which the pressure-current intensity calibration curve is monotonic, and wherein based on the pressure-current intensity calibration curve in the restricted definition range and based on the measured current intensity, the pressure in the vacuum system is determined.

3. Method (100) according to claim 2, wherein a logarithm of the estimated value (p0) of the pressure is determined using the formula log p0 = a I_1 / I_ 2 + b , wherein a and b are pre-determined coefficients that depend on the choice of emission lines, the arrangement used to generate the plasma, and the basis of the logarithm.

4. Vacuum pressure sensor (10), comprising: - a sample chamber (20) in which a plasma can be generated, wherein the sample chamber has electrical contact with a first electrode (1) and with a second electrode (2), - a current measuring device (42) electrically connected to the first and to the second electrodes and connected in series with the sample chamber, - a wavelength-selective element (51, 54), and - a first (31) and a second detector element (32) for measuring a radiation intensity of an electromagnetic radiation, wherein the wavelength-selective element, the first detector element and the second detector element are arranged such that in the first detector element only electromagnetic radiation of a first wavelength range emanating from the sample chamber can arrive, and that in the second detector element only electromagnetic radiation of a second wavelength range emanating from the sample chamber can arrive, wherein at least a first emission line of a first plasma species of a first chemical element lies in the first wavelength range, wherein a second emission line of the first plasma species of the first chemical element or of a second plasma species of the first chemical element lies in the second wavelength range, and wherein the second emission line lies outside the first wavelength range.

5. Vacuum pressure sensor (10) according to claim 4, comprising a measurement chamber (3) surrounding the sample chamber, having a window (5) in a wall or as a wall of the measurement chamber, wherein the window is transparent in an optical wavelength range and wherein a continuous first radiation path is defined which, starting from the sample chamber, traverses the window and ends in the first detector element and wherein a continuous second radiation path is defined which, starting from the sample chamber, traverses the window and ends in the second detector element.

6. Vacuum pressure sensor (10) according to one of claims 4 or 5, wherein the first detector element (31) and / or the second detector element (32) is a photodiode, a phototransistor, a charge coupled device, a multi-channel plate, or a channel electron multiplier.

7. Vacuum pressure sensor (10) according to one of claims 4 to 6, wherein the vacuum pressure sensor comprises a miniature spectrometer (30) having a detector array (39) and that the first detector element (31) and the second detector element (32) are elements of the detector array.

8. Vacuum pressure sensor (10) according to one of claims 4 to 7, wherein the vacuum pressure sensor comprises a device (8) for generating a magnetic field in the sample chamber (20).

9. Vacuum pressure sensor (10) according to claim 8, wherein the arrangement of first electrode (1), second electrode (2) and the device (8) for generating a magnetic field in the sample chamber is designed such that by applying an electric voltage to the electrodes an electric field can be generated which is aligned in the sample chamber substantially perpendicular to the magnetic field, in particular wherein the arrangement is designed as a magnetron array, inverted magnetron array or Penning array.

10. Vacuum pressure sensor (10) according to one of claims 4 to 9, further comprising an energy source for supplying energy to a plasma in the sample chamber.

11. Vacuum pressure sensor (10) according to claim 10, wherein the power source comprises a high-voltage source (41) which is electrically conductively connected to the first and second electrodes and which is connected in series to the current measuring device.

12. Vacuum pressure sensor (10) according to claim 10, wherein the power source comprises an AC power source and an induction coil, wherein the induction coil is electrically connected to the AC power source and adapted to generate an alternating magnetic field in the sample chamber when AC power is passed through the induction coil.

13. Device for carrying out the method according to one of claims 1 to 3, comprising a vacuum pressure sensor according to one of claims 4 to 12, and a processing unit, wherein the processing unit is operatively connected to the current intensity measuring device (42), the first (31) and a second detector element (32) for electromagnetic radiation for transmitting the measured current intensity (C_plasma), the measured first radiation intensity (I_1) and the measured second radiation intensity (I_2), and is adapted to determine as a function thereof the pressure (p) in the vacuum system.

14. Application of the method according to one of claims 1 to 3 for extending the pressure measurement range of a vacuum pressure sensor based on a measurement of a current intensity by a plasma, in particular a vacuum pressure sensor based on the operating principle of a cold cathode vacuum gauge, to a pressure measurement range comprising both pressures below and pressures above an extreme value of a pressure-current intensity characteristic curve of the vacuum pressure sensor.

15. Use of a vacuum pressure sensor (10) according to one of claims 4 to 12 in a method according to one of claims 1 to 3.