EP3465732B1 - Magnet-free generation of ion pulses - Google Patents

Description

The invention relates to a device for generating, storing and releasing ions from a residual gas atmosphere with the features of claim 1 and a method therefor with the features of claim 10.

Devices and methods for generating ions from a residual gas atmosphere are already known from the state of the art. Physically, the effect of impact ionization is usually used for this purpose, in which electrons hit the particles of the residual gas atmosphere and ionize them.

In a first example, known as EBIT (electron beam ion trap) and described in the US patent US$6,717,155 In the method disclosed, an electron beam is used to generate ions. High-energy electrons are used, with acceleration voltages in the range of more than 15 kV and electron currents of more than 20 mA. In order to focus the electron beam on the smallest and most sharply focused beam cross-section possible, complex magnetic fields with the corresponding coil devices are required, which must ensure a magnetic field strength in the range of more than 200 mT. The ions are generated from the neutral gas particles in the electron beam by the well-known process of impact ionization. The ions generated are stored in a cylindrically symmetrical structure, with the ions being enclosed in the radial direction via space charge forces and in the axial direction via additional electrodes.

The EBIT process has a number of disadvantages. A first disadvantage is the considerable equipment required to generate and focus the electron beam, which is particularly evident in the magnetic field device. EBIT devices therefore take up a relatively large amount of space and, due to the high magnetic field strengths, cannot easily be combined with devices in the vicinity of the system that are sensitive to ambient magnetic fields. Accordingly, appropriate Shielding or a minimum structural distance is required, which further increases the space required by the device.

In addition, the EBIT process mainly produces multiply ionized ions due to the high-energy electrons, and the molecules present in the residual gas atmosphere are also broken down into smaller fragments to a considerable extent. However, such multiply charged ions and molecular fragments are a significant problem for downstream analytical processes, particularly for the purpose of residual gas analysis. This is because the ionized molecular fragments are in an excited, multiply ionized state. Their properties cannot therefore be easily compared with the data stored in relevant databases. In addition, when original compounds in the residual gas are broken down, the original molecular aggregates present there are destroyed, so that they can no longer be detected directly, but only indirectly. This causes a certain analytical detection uncertainty. Another disadvantage is the high emission current required of >20mA. The high heating power required to generate this emission current results in a high power and heat input into the entire arrangement and the adjacent vacuum chamber.

In another example, from the US patent US$4,904,872 In the known methods, an electron beam is also used for ionization. The electron beam is focused there via an arrangement of repeller electrodes. To store the ions, an additional electrode arrangement is used which generates a potential that is attractive for ions by means of an additional storage electrode within the ionization volume and in particular within the ion storage space. The method disclosed there has the disadvantage that the ions generated can only have a short lifespan because they are attracted to the storage electrode and neutralized. The ion currents that can be generated, i.e. ultimately the number of stored ions, are therefore proportional to the pressure of the residual gas atmosphere. Although the arrangement disclosed there is free of magnetic fields, the arrangement shown there is also characterized by a complicated storage electrode structure, with the acceleration voltages required at the electron source being in the range of 500 to 1000 V.

In both known methods, the energy distribution of the ions produced is also significantly higher than the thermal distribution in the residual gas atmosphere. This is particularly disadvantageous with regard to subsequent residual gas analysis, for example by measuring the time of flight of the ion packet, since the individual mass-separated ion packets diverge greatly in space during the flight, which generally reduces the resolution of the apparatus and requires longer flight distances.

EP 0 676 792 A2 discloses a device for generating, storing and releasing ions from a residual gas atmosphere, comprising an electron source for releasing electrons and an ion storage space which, solely as a result of the negative space charge distribution generated by the electrons, has a spatial potential distribution that is attractive for the ions generated by ionization of the residual gas atmosphere, wherein the particles of the residual gas can be ionized by the negative space charge and an attractive potential can be formed for the positive ions formed. The ions are extracted by an electrode after a predetermined time.

This method is not always sufficiently sensitive for small partial prints.

The aim is therefore to remedy the disadvantages mentioned. In particular, a method and a device for generating, storing and releasing ions from a residual gas atmosphere are to be specified, in which the number of ions generated is essentially independent of the pressure of the residual gas atmosphere. The method should also be able to be carried out at a low pressure in the residual gas and should deliver the largest possible quantity of ions for reliable ion detection. The arrangement for carrying out the method should have a simple structure and be able to be carried out without the additional influence of magnetic fields.

In addition, the process should ensure that the energy distribution within the quantity of ions produced is as thermal as possible. The energy distribution of the ions should therefore not be given a falsifying or additional signature by the process of ion production itself. Finally, the process of ion production and the device used should leave the molecules within the residual gas atmosphere as intact as possible even during ionization, and only singly charged ions should be produced.

The above objects are achieved with a device for generating, storing and releasing ions from a residual gas atmosphere having the features of claim 1 and with a corresponding method for generating, storing and releasing ions from a residual gas atmosphere with the features of claim 10. The subclaims contain expedient or advantageous embodiments of the device and the method.

The device claimed according to the invention for generating, storing and releasing ions from a residual gas atmosphere comprises an electron source for releasing electrons, an anode which is permeable to the electrons released by the electron source and has a negative space charge distribution formed by the electrons within an ion storage space at least partially surrounded by the anode, and a pulse electrode which is electrically insulated from the anode for extracting the ions from the storage space. According to the invention, there are no further electrodes within the ion storage space and the ion storage space has a spatial potential distribution which is attractive for the ions generated by ionization of the residual gas atmosphere and stores ions solely as a result of the negative space charge distribution generated by the electrons. According to the invention, the anode and the pulse electrode are at the same potential during the storage process.

According to the invention, the particles of the residual gas can be ionized by the negative space charge and an attractive potential is created for the positive ions formed, whereby the negative space charge forms a storage area for the ions generated.

As an essential component of the device according to the invention, a means for regulating the time interval between successive switching operations of the pulse electrode to a fixed predetermined strength of the measured released ion packet is also provided.

The device for generating, storing and releasing ions from a residual gas atmosphere thus comprises an electron source for releasing electrons and an anode permeable to the electrons released by the electron source with a negative space charge formed by the electrons within an ion storage space at least partially surrounded by the anode.

The ion storage space for the ions generated by ionization of the residual gas atmosphere has an attractive spatial potential distribution due to the negative space charge. Furthermore, a junction that closes the ion storage space in the emission direction and is based on an electrical Potential switchable and perforated pulse electrode provided for releasing an ion packet from the ion storage space.

The device according to the invention is based on the idea of ionizing and storing the particles of the residual gas in the attractive potential of a negative space charge cloud. In contrast to the known EBIT method, in which an electron beam that is strongly collimated by a magnetic field is used, the negative space charge cloud or electron density is formed without further bundling or collimation and without the increase of additional electrodes in the axial direction. The anode, which is permeable to the emitted electrons, serves to form the negative space charge cloud. The emitted electrons collect in the area of the permeable anode and form the space charge cloud, particularly in its interior.

The negative space charge cloud has a dual function: firstly, it ionizes the particles of the residual gas and secondly, it creates an attractive potential for the positive ions formed in it. The positive ions accumulate in this attractive potential, so that the negative space charge cloud forms a storage area for the ions produced.

In one embodiment, the anode and the pulse electrode are at the same potential during the storage process.

In one embodiment of the device, the electron source is designed as a hot cathode in the form of a ring filament that surrounds the electron-permeable anode. This means that the electron-permeable anode is exposed from all sides to the electrons emitted from the ring filament. Other designs of the electron source are also conceivable in which the electrons are emitted from one or more sources and impact the anode at the appropriate points. In the end, the decisive factor is the negative space charge distribution generated within the anode.

Compared to the known prior art methods, the acceleration voltage for the electrons can be significantly reduced. In one embodiment, the acceleration voltage applied to the hot cathode that is effective for the emitted electrons is a maximum of 200 volts.

Compared to the known EBIT processes, the electron current can be significantly reduced. In one embodiment, the electron current emerging from the hot cathode is no more than 10mA; in particular, 2mA is sufficient for typical applications. This significantly reduces the power consumption and heat input in the sensor.

In one embodiment, an electrostatic arrangement of focusing electrodes and/or a repeller surrounding the electron source is provided for additional alignment and shaping of the electron emission. The focusing electrodes and/or the repeller serve in particular to direct the electrons not emitted in the direction of the electron-permeable anode in the direction of the electron-permeable anode, thus supporting the formation of the negative space charge.

During the storage process, the negative space charge distribution forms a potential well with respect to the anode and pulse electrode potential, which, if the negative space charge distribution is not compensated with ions, acts attractively on ions in the ionization volume and forms an electrostatic exit barrier for ions acting in all directions, thus enabling storage of ions until the negative space charge is compensated.

For ion extraction, the pulse electrode can be switched to a negative potential compared to the anode, whereby the collected ions can be extracted in the direction of the pulse electrode.

The frequency for switching the pulse electrode is preferably a minimum of 0.1 Hz and a maximum of 1 MHz, in particular a minimum of 1 Hz and a maximum of 100 kHz.

In particular, the electron current generated by the electron source is a minimum of 1µA and a maximum of 15 mA, in particular a minimum of 5µA and a maximum of 2mA.

The acceleration voltage effective for the electrons generated at the electron source (1) due to the potential difference between the electron source (1) and the anode (2) is a minimum of 30V and a maximum of 400V, in particular a minimum of 70V and a maximum of 150V.

In a further embodiment, the anode that is permeable to the emitted electrons has a cylindrically symmetrical structure. The outer surface of the cylinder can be designed parallel to the circumferential ring filament of the electron source, while the pulse electrode forms one of the two cover surfaces of the cylindrically symmetrical anode. The cylindrically symmetrical structure promotes the multiple passage of the electrons through the ionization space, which increases the electron yield compared to directed beam paths and increases the storable charge with the same emission current due to the increased space charge in the storage space.

In addition, a detector arranged in the direction of flight of the ions can be provided to measure the ion current.

The method for generating, storing and pulsed releasing ions from a residual gas atmosphere is carried out with the following method steps essential to the invention:
Electrons are emitted from an electron source and accelerated towards the ionization space through the permeable anode arrangement.

A negative space charge cloud is generated within the ionization space due to the electrons moving through the ionization space

According to the invention, the anode arrangement and a pulse electrode are connected to the same potential during the storage process.

Impact ionization of gas molecules and/or gas atoms occurs within the ionization space and the generated positively charged ions are stored in the attractive potential of the negatively charged space charge cloud as a positively charged ion reserve.

The pulse electrode is then switched to a potential relative to the anode potential and the ion supply located in the potential of the space charge cloud is accelerated out of the ionization space.

This results in the extraction of at least part of the stored ion pool in the form of an ion packet.

As part of the method according to the invention, a determination of a total pressure is carried out in connection with this. The time interval between successive switching operations of the pulse electrode is regulated to a fixed predetermined strength of the measured released ion packet, wherein the time interval between successive switching operations of the pulse electrode is a measure of the total pressure to be measured.

This results in electrons being emitted from an electron source and accelerated towards the permeable anode arrangement. If the electrons do not die on the anode grid, they pass through the ionization space (possibly several times) and form a negatively charged space charge cloud.

At the same time, impact ionization of gas molecules and/or gas atoms occurs within the negative space charge cloud. The positively charged ions generated are collected in the attractive potential of the negatively charged space charge cloud and form a positively charged ion reserve stored there.

This is followed by switching an ion-permeable pulse electrode to a negative potential and accelerating the ion supply in the potential of the space charge cloud in the direction of the pulse electrode. In connection with this, at least one part of the ion supply accelerated out of the attractive potential in the field of the pulse electrode is released in the form of an ion packet through the pulse grid.

The basic idea of the process is to use a negative space charge cloud formed from released electrons for the impact ionization of neutral gas particles and to use the negative potential of the Space charge cloud is used simultaneously to collect and store the ions generated. This negative potential is gradually filled with the positive ions generated until this potential is essentially balanced. Because the depth of the negative potential is essentially not dependent on the pressure of the surrounding gas atmosphere and this potential is constantly filled during the ionization of the gas particles, the number of ionized gas particles in the negative potential is largely independent of the pressure of the gas atmosphere, so that a pressure-independent ion packet can also be released. The energy distribution of the ions within the storage potential is essentially thermal, the ions themselves are usually only positively charged due to the weak, low kinetic electron energy, with larger molecules essentially not being split into smaller fragments. The process does not require complex magnetic focusing of an electron beam and can be operated at relatively low electron energies compared to the state of the art. Complex storage electrodes for collecting the ions generated are also not required.

In an advantageous embodiment of the method, the electrons are emitted from the electron source from a hot cathode that surrounds the transmissive anode arrangement in a ring, whereby the electrons are accelerated in the field of the transmissive anode arrangement and form the negative space charge cloud. The transmissive anode arrangement therefore only serves the purpose of concentrating the emitted electrons in a certain spatial region and thereby generating the ionizing and simultaneously storing space charge cloud.

In one embodiment of the method, the strength of the released ion pulse is adjusted over a time interval between successive switching operations of the pulse electrode, so that the strength of the ion pulse is proportional to the length of the time interval.

In a modification of the method, in addition to the release of the ion packet, a determination of a total pressure can also be carried out. In this case, the strength of the released ion current is determined at a fixed time interval between successive switching operations of the pulse electrode. measured, using the strength of the ion current as a measure of the total pressure to be measured.

Essential to the invention, as part of the method, in addition to releasing the ion packet, a total pressure is also determined. As mentioned, the time interval between successive switching operations of the pulse electrode is regulated to a fixed predetermined strength of the measured released ion packet, whereby the time interval between successive switching operations of the pulse electrode is a measure of the total pressure to be measured. Due to the strength of the ion packet being independent of the pressure, the ion source can be operated in a very wide pressure range from 1e-12 mbar to 2e-2 mbar.

The device and the method will be explained in more detail below using exemplary embodiments. The following are used for clarification: Figures 1 to 14 The same reference symbols are used for identical or equivalent parts.

Fig. 1

eine Gesamtdarstellung der Vorrichtung zu Ionenerzeugung, -speicherung und -freisetzung im Schnitt,

Fig. 2

eine Darstellung der Vorrichtung aus Fig. 1 in einer perspektivischen Ansicht,

Fig. 3

eine Darstellung des Vorgangs der Ionenerzeugung, -speicherung und - freisetzung in der erfindungsgemäßen Vorrichtung,

Fig. 4a und 4b

verschiedene Darstellungen einer beispielhaften Pulselektrode mit einem Pulsgitter,

Fig. 5

eine beispielhafte Darstellung der Vorrichtung zur Ionenerzeugung in Verbindung mit einer Sensorik für eine Flugzeit-Spektroskopie,

Fig. 6

eine beispielhafte Darstellung verschiedener Potentialverteilungen im Ionenspeicherraum infolge der durch die Elektronen bewirkten negativen Raumladung,

Fig. 7

eine Darstellung der Abhängigkeit der Anzahl der speicherbaren Ionen vom Emissionsstrom an der Elektronenquelle für zwei verschiedene auf die Elektronen wirkende Beschleunigungsspannungen,

Fig. 8

eine Darstellung der Abhängigkeit der speicherbaren Ladung vom Emissionsstrom an der Elektronenquelle für zwei verschiedene auf die Elektronen wirkende Beschleunigungsspannungen,

Fig. 9

eine Darstellung des zeitlichen Füllvorgangs für das attraktive Potential in Abhängigkeit von zwei verschiedenen Drücken,

Fig. 10

eine Darstellung der simulierten Fülldauer in Abhängigkeit vom Druck,

Fig. 11

eine Darstellung eines simulierten Sammeleffektes und einer sich daraus ergebenden Fülldauer für einen Druck von 10^-5 mbar,

Fig. 12

eine Darstellung der experimentell bestimmten Fülldauer in Abhängigkeit vom Druck,

Fig. 13

beispielhafte Bestimmungen der gesammelten Ladung in Abhängigkeit vom der Sammelzeit und des Totaldrucks über eine Auswertung eines Integrals über ein Messsignal in einem Faraday-Becher,

Fig. 14

eine Darstellung der Flugzeiten von Ionen unterschiedlicher Massen.

It shows:

Fig.1

an overall view of the device for ion generation, storage and release in section,

Fig.2

a representation of the device Fig.1 in a perspective view,

Fig.3

a representation of the process of ion generation, storage and release in the device according to the invention,

Fig. 4a and 4b

various representations of an exemplary pulse electrode with a pulse grid,

Fig.5

an exemplary representation of the device for ion generation in conjunction with a sensor for time-of-flight spectroscopy,

Fig.6

an exemplary representation of various potential distributions in the ion storage space due to the negative space charge caused by the electrons,

Fig.7

a representation of the dependence of the number of storable ions on the emission current at the electron source for two different accelerating voltages acting on the electrons,

Fig.8

a representation of the dependence of the storable charge on the emission current at the electron source for two different accelerating voltages acting on the electrons,

Fig.9

a representation of the temporal filling process for the attractive potential depending on two different pressures,

Fig.10

a representation of the simulated filling time depending on the pressure,

Fig. 11

a representation of a simulated collection effect and a resulting filling time for a pressure of 10 ^-5 mbar,

Fig. 12

a representation of the experimentally determined filling time as a function of pressure,

Fig. 13

exemplary determinations of the collected charge as a function of the collection time and the total pressure via an evaluation of an integral over a measuring signal in a Faraday cup,

Fig. 14

a representation of the flight times of ions of different masses.

Fig.1 shows an overall view of the device for ion generation, storage and release in section. The device contains an electron source 1, which in the example presented here is designed as a hot cathode in the form of a ring filament. The ring filament surrounds an anode 2 that is permeable to electrons. Within the anode and in the space area of the ring filament, a negative ionizing space charge 3 is generated by the emitted electrons, which is shown in the figure presented here by a dashed line. line. The negative space charge extends in particular into an ion storage space 4, which is located inside the anode 2. The anode 2 is electrically insulated from the pulse electrode 5.

In the example given here, the arrangement is additionally surrounded by focusing electrodes 6 and shielded from the outside. The focusing electrodes in particular align the electrons emitted by the electron source in the direction of the anode 2, which is permeable to the electrons.

Fig.2 shows the Fig.1 shown arrangement in a perspective view. The anode 2, which is permeable to electrons, is constructed here with cylindrical symmetry. It contains a surface area 7, which is designed as a sufficiently fine-meshed grid, sieve or conductive fabric. The front side of the surface area 7, ie one of the base surfaces of the cylindrical anode, is formed by the pulse electrode 8.

The entire anode arrangement, including the outer surface 7 and the pulse electrode 8, is gas-permeable and at the same time partially transparent to electrons.

In the example presented here, the electron source is a ring filament 9 in the form of a ring-shaped hot cathode running parallel to the lateral surface 7 at a certain distance, which emits electrons at high temperature by means of thermal emission.

The electrons emitted from the hot cathode are emitted from all sides, including towards the outer surface 7 of the anode, and accelerated and penetrate the outer surface 7, penetrating the interior of the anode and ionizing the gas particles present there via the process of impact ionization. At the same time, the electrons form a negative space charge cloud, which represents an attractive potential for the positively charged ions. The ions are thus collected in this attractive potential within the ion storage space 4 of the anode. By switching the pulse electrode 8 to a negative potential, the ions can now be released from the ion storage space to the outside through the are accelerated through a pulse grid. An ion packet is extracted. The extraction direction of the ion packet is indicated by an arrow.

In an alternative embodiment, the pulse electrode can be switched to a potential that is positive compared to the anode in order to accelerate the collected ions in the opposite direction to the pulse electrode. If in this case the part of the anode that is in the direction of flight of the ions is also designed to be permeable to the ions, the collected ions can be extracted from the ion storage space in the opposite direction to the pulse electrode by a repulsive electrostatic interaction. In such a case, the pulse electrode can be designed to be impermeable to the ions and drives the positive ions out through the body of the anode.

Fig.3 shows the process of ion generation and storage in a series of process steps A to D. In state A, the electron source 1 is inactive. The anode 2 is at a positive potential V _AN , which is essentially location-independent and constant over the cross section of the anode in its interior ion storage space 4.

In state A, the anode and the pulse electrode are at the same potential V _AN , so that a constant potential V _AN is established in the ionization volume.

In state B, the electron source 1 is activated. The emitted electrons penetrate the anode and form a negative space charge cloud 3, particularly in the ion storage space 4. As a result, the potential within the anode takes on a location-dependent value. In the central area of the space charge cloud and thus also of the ion storage space, a potential well with a local potential minimum V _min is formed.

State C takes into account the impact ionization of the gas particles present in the ion storage space 4 due to the influence of the emitted electrons. The positively charged ions are moved to the local minimum of the potential in the ion storage space and accumulate within the negative space charge cloud. Due to the attractive potential well, the ions are kept within the ionization volume, cannot collide with the electrodes surrounding the ionization space and are neutralized. The potential is equalized over time, the depth of the local potential minimum within the ion storage space decreases and the potential minimum becomes increasingly flatter.

In state D, the potential minimum is filled by the positive ions. The negative space charge of the electrons is completely compensated by the positive accumulated ions. With regard to the potential inside the anode, a relationship is now established as was the case in state A: an essentially location-independent constant electrostatic potential inside the anode. The ions stored there can now be accelerated towards the pulse electrode under the attractive influence of the now activated pulse electrode 8 and released to the outside as an ion packet. For this purpose, the pulse electrode is switched from the anode potential V _AN to a more negative potential, whereby the generated ions are withdrawn from the ion storage space.

As a result, state B is again assumed, in which there is an empty, unfilled potential minimum within the ion storage space and which can be filled again and emptied of the ions stored there by passing through states C and D again. This cyclic operation thus makes it possible to generate ion packets or ion pulses, i.e. spatially limited ion accumulations. The time from state B to complete space charge compensation in state D is referred to below as the filling time. The storage capacity of the ion source results from the positive ion charge stored in state D.

The Figures 4a and 4b show a representation of the pulse electrode 8 with a pulse grid 5 incorporated therein. A sheet metal disk serves as the basis for the pulse electrode 8. In the example presented here, a hexagonal honeycomb structure 10 is incorporated into the inner surface of the sheet metal disk, in particular cut into it by means of laser radiation, which has a diameter D _PG . This increases the permeability of the pulse grid and ensures sufficient shielding from external fields.

The pulse electrode can be electrically contacted via metallic leads. The pulse electrode can thus be activated via an external circuit On the one hand, it is necessary to keep the pulse electrode at a constant potential V _PG = V _AN as shown in Fig.3 to ensure the formation of the potential minimum in the anode chamber. On the other hand, the attractive pulses V _Puls of a pulse generator (not shown here), which are required to extract the accumulated ions, are also transmitted to the pulse electrode.

Fig. 4b shows a detailed view of an example of a grid structure of the pulse grid 5. In the example given here, the grid structure 10 consists of hexagonal openings 11 arranged in a honeycomb pattern, which are separated from one another by webs 12. The geometric dimensions of the grid structure of the pulse grid are chosen in such a way that the extraction of the ions from the anode space is as loss-free as possible, with optimal shielding from external fields. Corresponding transmission losses, which arise because ions neutralize themselves at the grid webs, can be minimized by appropriate dimensioning of the web width S and the mesh width G of the hexagonal openings 11.

The functionality of the device and the method will be explained in more detail below. The following explanation of the ion source is given in conjunction with the explanation of a time-of-flight spectroscopy device.

The Fig.5 The schematic structure of the cylindrically symmetrical arrangement of the ion source and the time-of-flight spectroscopic device shown essentially consists of three components. These components are the ion source, consisting of the electron source 1, the anode 2 and the pulse electrode 8. A repeller 13 is also provided and, as a second component, a time-of-flight (TOF) mass separator 14 connected to the ion source with a detector unit as the third component, which is designed here as a Faraday cup 15. The entire arrangement has the compact design of conventional ionization vacuum meters.

Electrons are released from the ring-shaped filament of the electron source by thermal emission, which are guided into the anode chamber inside due to the attractive potential of the anode. Then, collisions between the electrons and the neutral residual gas particles create positive Ions which, as mentioned, can be accumulated in the anode to a certain extent.

By switching the pulse electrode to a potential V _Pulse that is attractive to the ions, the accumulated ions are drawn out of the anode. This creates a potential that is attractive to the positive ions, which extracts and accelerates the collected ions into the time-of-flight mass separator. In this, the ions are separated from one another according to their mass using the principle of TOF mass spectroscopy.

If the ions are separated according to their mass, temporally separated signals can be detected at the detector, which here is designed in the form of a Faraday cup 15.

The advantage of time-of-flight mass separation is that it makes it possible to record an entire mass spectrum with one and the same detector using a single ion pulse. The time t _TOF required for the ions to travel a given distance S _TOF between the release of the ion pulse and their impact on the detector is measured. Because all ions have the same charge and the accelerating potential is the same for all ions, it can be assumed that all ions have the same kinetic energy. More massive ions will travel at a lower speed than less massive ions and will therefore arrive at the detector later, i.e. after the less massive ions. After time-resolved detection on the Faraday cup, a mass is assigned to the respective flight times and a conclusion is drawn about the residual gas composition prevailing in the vacuum chamber.

The ions separated along the flight time according to their mass can also be electrically detected with other types of detectors. A Faraday cup offers the advantage of a simple structure. This consists of a metallic cup 15c, which is kept at a constant potential V _FC = 0V. Inside the cup is the metal plate 15b. A positive ion striking the metal plate now generates an additional net charge. This positive charge is then neutralized by an electron flowing towards the metal plate. The current I _FC , which flows during The current flowing through the metal plate during discharge is therefore directly proportional to the number of incident ions.

As already described above, the time-of-flight mass separator separates light ions from heavier ones. Since light ions are accelerated more strongly, they reach the Faraday cup at an earlier point in time. Individual peaks become visible at the Faraday cup depending on the time of detection. If individual masses differ sufficiently, the corresponding signals are shown separately in time in the spectrum, otherwise they will partially overlap.

The sum of the entire temporal signal of an ion pulse at the detector provides information about the total pressure within the chamber, while the analysis of the individual signal peaks allows a conclusion about the respective partial pressure.

The following section will provide a theoretical characterization of the ion source. To this end, the next sections will provide a computational estimate of selected characteristic variables, such as the field distribution within the anode, the storage capacity or the ion current and the resulting pressure-dependent filling time.

Since the anode is kept at a constant positive potential V _AN , the electrons are, as already explained, directed into the anode due to the potential that is attractive to them. Here they cause a potential minimum due to negative space charge effects, which distorts the initially constant potential.

The negative space charge is simplified below as an electron density with a given radius r _0. The field distribution in the presence of the electron density is divided into two areas. The first area is formed by the space within the electron density with the radius r < r ₀ , the second area is formed by the environment outside the electron density with r ₀ < r < r _AN , where the boundary condition is V (r = r _AN ) = V _AN . The potential distribution V (r) for these two areas can be described as Dependence on r. For a given distance r _AN the potential can be calculated in Fig.6 shown graphically.

The potential, which depends on the radius r, can be approximately represented by the following curve:
Within the electron density with radius r ₀ the potential for r < r ₀ takes the following course: $$V\left(r\right)=V_{\mathit{AT}}-V_e\cdot \left(2\ln \left(\frac{r_{\mathit{AT}}}{r}\right)+\left(1-\frac{r^2}{{\mathrm{<figure-callout\; id="0"\; label="r"\; filenames="imgf0006.png,imgf0013.png"\; state="\{\{state\}\}">r</figure-callout>}}_0^2}\right)\right)$$

Outside the electron density, for r ≥ r ₀ , the potential can be set as follows: $$V\left(r\right)=V_{\mathit{AT}}-2V_e\cdot \ln \left(\frac{r_{\mathit{AT}}}{r}\right)$$

It is clearly visible that the depth of the potential minimum is strongly dependent on the radius of the electron density. For example, an electron density of radius r ₀ = 6mm results in a potential minimum of V _AN of approximately 2V, whereas an electron density focused on radius r ₀ = 1mm already generates a potential minimum of V _AN of approximately 8V. It is therefore an advantageous design to focus the electrons emitted by the filament to a sufficient extent to ensure effective storage of the ions within the potential minimum. In Fig.1 the focusing electrodes 6 are indicated schematically. If the focusing electrode 6 is placed at a potential less than or equal to the potential of the electron source 1, the electrons emerging from the electron source 1 are focused in the direction of the anode.

To calculate the maximum number of ions that can be stored, ie the storage capacity N ₊ of the storage ion source, it can be assumed to be a good approximation that an electron density of current density j _e = ρ _e · v _e contains a negative charge of Q _e = ρ _e · V within a volume V = A · ΔL. A is the lattice area through which the electrons in the anode space, and ΔL corresponds to the length traveled by the electrons within the anode.

Taking into account the kinetic energy of the electrons after leaving the filament, the charge density of the electrons can be written as: $${\rho }_e=\frac{j_e}{{\nu }_e}=\frac{I_e}{A}\cdot \sqrt{\frac{m_e}{2{\mathit{eV}}_e}}$$

Here V _e = V _AN - V _Fil is the acceleration voltage acting on the electrons due to the potential difference between the anode and the filament. Furthermore, the following applies to the negative charge density: $${\rho }_e=\frac{Q_e}{V}=\frac{N_{e}e}{V}$$

If both equations are equated and solved for the number of electrons N _e , we obtain: $$N_e=\sqrt{\frac{m_e}{2{\mathrm{<figure-callout\; id="3"\; label="e"\; filenames="imgf0001.png"\; state="\{\{state\}\}">e</figure-callout>}}^3}\cdot \mathrm{\Delta }L\cdot \frac{I_e}{\sqrt{V_e}}}$$

The maximum storage capacity is achieved when the number of ions N+ produced is equal to the number of electrons N _e and thus N+ = N _e . Fig.7 shows the dependence of the number of storable ions Q ₊ = N+e on the emission current I _e for two different acceleration voltages acting on the electrons. The upper graph shows the curve for a voltage of V _e = 70 V, the lower graph for a voltage of V _e = 120 V.

As can be seen from equation (A), the storage capacity N+ of the storage ion sources depends on the emission current I _e . By increasing the emission current, the space charge density increases, which means that more ions can be stored in the negative space charge potential. This relationship is for the resulting storable charge Q ₊ = N+e depending on two different acceleration voltages of the electrons V _e in Fig.8 shown graphically.

With an emission current of I _e = 1mA, an acceleration voltage of the electrons of V _e = 70V and a length of the storage volume of L = 1 cm, the number of ions that can be stored is N+ ≈ 1.3 · 10 ⁷ . This corresponds to a charge of Q ₊ = N+ e ≈ 2.0 · 10 ^-12 C, which can be stored within an electron density with a current of I _e = 1mA at most. In general, the electron current generated by the electron source 1 is a minimum of 1µA and a maximum of 15 mA, in particular a minimum of 5µA and a maximum of 2mA. The heating power required to generate these emission currents is sufficiently low to cause only a small input of power and heat into the entire arrangement and the adjacent vacuum chamber. The generated charge quantities of Q ₊ ≈ 2.0 · 10 ^-12 C are sufficiently high to be detected by simple detectors (e.g. Faraday cup type) with sufficient signal-to-noise ratio.

Furthermore, Fig.8 that when the acceleration voltage is increased from 70 V to 130 V, the storage capacity is reduced because the faster electrons generate a smaller negative space charge. In addition, with the selected acceleration voltages and an emission current in the range of 10µA to 5mA, a charge of approx. 10 ^-14 C to 10 ^-11 C can be stored. These charge quantities can be detected with a simple Faraday cup and a good signal-to-noise ratio without additional complex equipment, such as a secondary electron multiplier.

The filling time, ie the time in which the negative electron space charge is completely compensated by stored ions, depends on the prevailing total pressure in the vacuum chamber. If the collection time t _collection is varied, ie the time in which the ion collection process takes place unhindered, a signal dependent on this collection time will be generated, which will increase accordingly with the progress of t _collection until the filling time t _fill is reached, as in Fig.10 shown.

From a certain point in time t _fill the potential minimum is completely compensated with positive charges and the measuring signal V _FC assumes a constant value.

Furthermore, the filling time t _fill also depends on the prevailing pressure p, since at higher pressures there are correspondingly more neutral gas particles which can fill the potential minimum more quickly after ionization. With an increasing pressure p ₁ > p ₂ a decreasing filling time t _fill , ₁ < t _fill , ₂ can be observed, whereby the qualitative course of these curves will be similar.

At this point, it should be emphasized that the maximum number of ions released during a pulse depends only very weakly on the pressure. This is because the storage capacity of the ion source is determined exclusively by the depth of the potential formed by the negative space charge. As shown in the experimental data from Fig. 13 As can be seen, the maximum storable charge changes by about half while the pressure is varied by about 3 decades from about 5E-6 mbar to about 5E-9 mbar. Furthermore, the change in the storable charge decreases with decreasing pressure. This enables the ion source to be used over a very wide pressure range without a significant loss of measurement sensitivity.

After the potential well is filled, further ions are formed which displace the ions already stored. Ions with a higher mass number are generally more difficult to displace than ions with a lower mass number. This leads to an enrichment of heavy ions and a displacement of light ions, while the sum of the ions and the stored charge remain the same. On the one hand, this effect can be exploited to increase the detection ability of the arrangement for ions with a higher mass number. On the other hand, this effect can be minimized by ion extraction before the potential well is completely filled.

also schreiben: $$j_e=\frac{I_e}{A}={\rho }_e\cdot \nu =n_e{\mathit{e\nu }}_e$$

For the theoretical estimation of the filling time t _Fill mentioned above, one can again use basic relationships. The starting point is again the electric current density j, which describes the ratio of the current intensity I to the cross-sectional area A available to it. Furthermore, the current density can be expressed using the space charge density ρ = n · e and the average drift velocity v of the respective charge carriers. In relation to the electrons emitted by the filament with V _Fil towards the anode, one can use the relationship $${\nu }_e=\sqrt{\frac{2e\left(V_{\mathit{AT}}-V_{\mathit{Fil}}\right)}{m_e}}$$

so write: $$j_e=\frac{I_e}{A}={\rho }_e\cdot \nu =n_e{\mathit{e\nu }}_e$$

If you rearrange the above equation for n _e and multiply it by the volume V, you get the number of free electrons N _e within the volume in question. As described above, these generate a negative charge Q _e = N _e e. In order to compensate for this charge with a corresponding positive charge Q ₊ , ie to fill the potential minimum, the positive ion current I+ must flow over a corresponding time t. It can therefore be summarized as follows: $$Q_e=N_{e}e\stackrel{!}{=}{Q}_{+}=I_{+}\cdot t$$

The time t _fill required to fill the negative charge of the electrons with corresponding positive ions is therefore: $$t_{\mathit{Fill}}=\frac{Q_e}{I_{+}}=\frac{Q_e}{I_e\cdot \frac{\sigma \cdot \mathrm{\Delta }L}{k_{B}T}\cdot p}$$

This clearly shows the dependence of the filling time on the total pressure. A graphical representation of this dependence is shown in Fig. 11 shown.

For the pressure-dependent filling time, it is clear that at pressure ranges of p = 10 ^-7 ... 10 ^-3 mbar, ion currents of approximately I+ = 10 ^-10 ... 10 ^-6 A are to be expected. With an emission current of I _e = 1 mA, the potential minimum in the anode chamber will therefore be filled after approximately t _fill = 10 ^-3 ... 10 ^-7 s.

By switching the pulse electrode from the anode potential to a negative extraction potential, for example, the electric field within the anode is manipulated in such a way that the collected ions are accelerated out of the ionization volume and detected at the Faraday cup. If the pulse electrode is switched back to the anode potential, the original state is restored: electrons create a potential minimum in which ions are generated and The length of time that ions are collected until they are extracted by switching the pulse electrode is the collection time.

If you wear, as in Fig. 11 , the stored ion charge is plotted over the corresponding collection time, the assumption is confirmed that at a defined pressure a corresponding number of generated ions slowly fill the negative charge Q _e ≈ -1·10 ^-12 C of the potential minimum as the collection time progresses. The collected positive ion charge Q ₊ thus increases steadily and changes to a constant value after a corresponding time t _fill . In the example chosen here, one can therefore deduce from the diagram in Fig. 11 Determine the filling time as t _fill ≈ 50 µs.

With the storage ion source developed here, it is possible to generate ion pulses of varying strengths depending on the collection time in the anode. Storage was achieved by a low-energy electron density, due to space charge effects, causing a sufficiently low potential minimum within the anode, which otherwise lies at the constant potential V _AN .

In further experiments, the clear dependence of the time required until the potential minimum is completely compensated with positive ions, the so-called filling time, on the prevailing pressure could be shown, as in Fig. 12 is shown.

This makes it possible to use the storage ion source as a total pressure sensor and to use the required filling time, which is easily accessible by measurement, as a measure of the pressure. As in Fig. 13 As can be seen, there is a clear connection between the time between two extraction pulses, the ion charge collected during this time and the prevailing total pressure, provided the operating parameters remain the same. Using appropriate evaluation electronics, a conclusion can be drawn about the prevailing total pressure from the total extracted ion charge and the set time between the extraction pulses.

Likewise, Fig. 13 It can be seen that the required collection time to generate e.g. 25 nVs signal at the detector is less than 1 ms for pressures p≥ 1E-7 mbar This device is therefore suitable for use as a "fast" total pressure sensor for detecting rapid pressure changes with response times < 1 ms.

As in Fig. 14 As shown, it is possible to separate the helium signal from the other residual gas components in time. Therefore, it is possible to use the total pressure sensor simultaneously as a helium detector, which also enables a helium leak test. The measured values in Fig. 14 The underlying flight distance is only 2 cm, so that the sensor is very compact.

The device discussed here is highly compact, provides reliable total pressure determination and helium mass separation. The dimensions of the sensor correspond to those of a conventional ionization vacuum gauge.

Furthermore, the possibility of subsequent data processing allows the stored ion charge per pulse to be calculated directly by determining the integral over the entire measurement curve, which considerably facilitates the comparison and evaluation of different spectra with diverging experimental parameters.

The device according to the invention and the method according to the invention have been explained using exemplary embodiments. Further embodiments are possible within the scope of protection of the claims within the scope of expert actions. These also arise from the subclaims.

List of reference symbols

1

Electron source

2

Electron-permeable anode

3

Negative ionizing space charge

4

Ion storage space

5

Pulse grid

6

Focusing electrode

7

Shell surface

8th

Pulse electrode

9

Ring filament

10

Honeycomb structure

11

Hexagonal breakthroughs

12

Bridges

13

Repeller

14

Time-of-flight mass separator

15

Faraday cup

15a

Shielding grid

15b

Metal plates

15c

metallic cup

Claims (14)

1. Device for generating, storing and releasing ions from a residual gas atmosphere,

   comprising

   an electron source (1) for releasing electrons,

   an anode (2) permeable to the electrons released by the electron source (1) with a negative space charge distribution (3) formed by the electrons within an ion storage space (4) at least partially surrounded by the anode (2) and a pulse electrode (8) electrically insulated from the anode (2) for extracting the ions from the storage space, wherein no further electrodes are located within the ion storage space (4) and the ion storage space (4) has a spatial potential distribution which is attractive for the ions generated by ionization of the residual gas atmosphere solely as a result of the negative space charge distribution generated by the electrons and stores ions and the anode (2) and the pulse electrode (8) are at the same potential during the storage process,

   wherein

   the particles of the residual gas can be ionized by the negative space charge and an attractive potential can be formed for the positive ions formed, wherein the negative space charge forms a storage area for the ions generated,

   with a means for regulating the time interval between successive switching operations of the pulse electrode to a fixed predetermined strength of the measured released ion packet.
2. Device according to claim 1,
   characterized in that
   the negative space charge distribution during the storage process forms a potential well with respect to the anode and pulse electrode potential which, if the negative space charge distribution is not compensated with ions, acts attractively on ions in the ionization volume and forms an electrostatic exit barrier for ions in all directions and thus enables ions to be stored until the negative space charge is compensated.
3. Device according to one of the preceding claims,
   characterized in that
   for ion extraction the pulse electrode (8) can be switched to a negative potential compared to the anode, whereby the collected ions can be at least partially extracted in the direction of the pulse electrode.
4. Device according to one of the preceding claims,
   characterized in that
   the frequency for switching the pulse electrode (8) is at least 0.1 Hz and at most 1 MHz, in particular at least 1 Hz, and at most 100 kHz.
5. Device according to one of the preceding claims,
   characterized in that
   the electron current generated by the electron source (1) is at least 1 µA and at most 15 mA, in particular at least 5 µA and at most 2 mA.
6. Device according to one of the preceding claims,
   characterized in that
   the electron source (1) is designed as a glow cathode surrounding the electron-permeable anode (2) in the form of a ring filament (9).
7. Device according to claim 1,
   characterized in that
   an electrostatic arrangement of focusing electrodes (6) and/or a repeller (13) surrounding the electron source (1) is provided for additional alignment and shaping of the electron emission.
8. Device according to one of the preceding claims,
   characterized in that
   the anode (2), which is permeable to the emitted electrons, has a cylindrically symmetrical structure.
9. Device according to one of the preceding claims,
   characterized in that
   a detector arranged in the direction of flight of the ions is provided for measuring the ion current.
10. Method for the generating, storing and pulsed releasing of ions from a residual gas atmosphere, comprising the method steps of:

    - emitting electrons from an electron source (1) and accelerating them in the direction of the ionization space (4) through the permeable anode arrangement (2),

    - generating a negative space charge cloud within the ionization space due to the electrons moving through the ionization space (4),

    - applying the anode arrangement (2) and a pulse electrode (8) to the same potential during the storage process,

    - impact ionization of gas molecules and/or gas atoms within the ionization space (4) and storage of the generated positively charged ions in the attractive potential of the negatively charged space charge cloud as a positively charged ion supply,

    - switching the pulse electrode (8) to a potential with respect to the anode potential and accelerating the ion supply located in the potential of the space charge cloud out of the ionization space,

    - extracting at least part of the stored ion supply in the form of an ion packet,

    wherein a determination of a total pressure is carried out,

    in which the time interval between successive switching operations of the pulse electrode is regulated to a fixed predetermined strength of the measured released ion packet, wherein the time interval between successive switching operations of the pulse electrode is a measure of the total pressure to be measured.
11. Method according to claim 10,
    characterized in that
    the electrons are emitted from the electron source from a glow cathode surrounding the transmissive anode arrangement in the form of a ring, wherein the electrons are accelerated in the field of the transmissive anode arrangement, pass the anode several times and form the negative space charge cloud in the ionization space (4).
12. Method according to one of claims 10 or 11,
    characterized in that
    the electrical charge of the released ion packet is adjusted over a time interval between successive switching operations of the pulse electrode, wherein the electrical charge of the ion packet is proportional to the length of the time interval.
13. Use of a device for generating, storing and releasing ions from a residual gas atmosphere according to claim 1 and a method according to claim 10 for determining a total pressure.
14. Use of a device according to claim 13 as
    a helium detector, wherein a helium signal is separated from the other residual gas components and detected.

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