APPARATUS AND METHOD FOR HIGH-PERFORMANCE CHARGED PARTICLE DETECTION

Abstract

A detection apparatus and method for detecting charged particles. The device relies on a detection assembly comprising microchannel plates. The useful surface of the microchannel plate device is maximized in time through the use of beam deflection means upstream of the detection front.

Claims

1. A detection apparatus for detecting charged particles, the detection apparatus comprising: a charged particle beam inlet; a detection front comprising an entry face of at least one microchannel plate (MCP) assembly, wherein the entry face extends along a first direction, wherein the MCP assembly is configured for receiving, along a second direction perpendicular to said first direction, a plurality of beams of charged particles that impinge on the entry face and for generating, for each impinging charged particle, a corresponding amplified detection signal on an opposite exit face; at least one read-out anode for collecting said amplified detection signals, the anode being arranged at a distance to, and in parallel with the exit face of said at least one MCP assembly, a beam deflection means arranged downstream of said charged particle beam inlet at a distance from said entry face and configured for selectively deflecting incoming beams of the charged particles along said first direction, so that the corresponding charged particles selectively reach different portions of the entry face of the at least one MCP assembly along said first direction; wherein the charged particle beam inlet, the beam deflection means, the detection front, and the at least one read-out anode extend along said second direction.

2. The apparatus according to claim 1, wherein said beam deflection means comprise a charged particle optics unit and a control unit, the control unit being configured to dynamically control a deflection angle to be applied to a propagation direction of the charged particle beam by the charged particle optics unit.

3. The apparatus according to claim 2, wherein said charged particle optics unit comprises a pair of deflection plates.

4. The apparatus according to claim 1, wherein said beam deflection means comprise a charged particle optics unit and a control unit, the control unit being configured to dynamically control an opening angle within which a propagation direction of the charged particle beam is deflected by the charged particle optics unit.

5. The apparatus according to claim 4, wherein said charged particle optics unit comprises a Bradbury Nielsen Grid.

6. The apparatus according to claim 1, wherein the entry face of said at least one MCP assembly extends over 2 to 3 cm along said first direction

7. The apparatus according to claim 1, wherein the at least one MCP assembly includes a plurality of MOP assemblies that include a plurality of entry faces, wherein the entry faces of the plurality of MCP assemblies extend over an aggregated length of at least 15 cm in said second direction.

8. The apparatus according to claim 7, further comprising one dedicated read-out anode for each of the MCP assemblies, and wherein said read-out anode extends along the corresponding exit face of the MCP assembly.

9. The apparatus according to claim 1, wherein the at least one read-out anode includes at least one of a delay-line anode, a pixelated anode array, a resistive anode, a shaped anode, and a single anode.

10. The apparatus according to claim 1, further comprising biasing means configured for applying a positive or negative floating electric potential to components of the apparatus, including the beam deflection means.

11. The apparatus according to claim 1, wherein said charged particles comprise ions.

12. The apparatus according to claim 1, wherein said charged particles comprise electrons.

13. A mass spectrometer for dispersing ions along a focal plane in accordance with their mass/charge ratio, the spectrometer comprising the detection apparatus of claim 1, the detection front of the detection apparatus being arranged on said focal plane so that said dispersed ions impinge on the detection front, wherein said first direction is perpendicular to the plane in which the ions are dispersed.

14. The mass spectrometer in accordance with claim 13, wherein the deflection means of said detection apparatus are arranged so as to deflect all dispersed ions exiting a mass filtering unit of the mass spectrometer.

15. The mass spectrometer in accordance with claim 13, wherein the detection front of said detection apparatus spans said focal plane so that any ions dispersed by a mass filtering unit of the mass spectrometer impinge there upon.

16. The mass spectrometer according to claim 14, wherein the mass spectrometer is a Mattauch-Herzog type device.

17. A method for detecting charged particles, using the apparatus of claim 1, the method comprising the following steps: i) providing the plurality of charged particle beams; ii) using the beam deflection means of the device, deflecting said charged particle beams in accordance with a predetermined deflection angle along said first direction in which the at least one MCP assembly extends; iii) using the at least one read-out anode of the device, reading out the amplified detection signals, as provided by said at least one MCP assembly; and iv) repeating steps ii) and iii) at least once using a different predetermined deflection angle.

18. A method of using the mass spectrometer in accordance with claim 13, the method comprising the following steps: a) dispersing ion species comprised in a secondary ion beam using a mass filtering unit of said spectrometer device, thereby generating a plurality of ion beams; b) using the beam deflection means of the spectrometer deflecting said plurality of ion beams in accordance with a predetermined deflection angle along said first direction in which the at least one MCP assembly extends; c) using the at least one read-out anode of the spectrometer, reading out the amplified detection signal, as provided by said at least one MCP assembly; and d) repeating steps b) and c) at least once using a different predetermined deflection angle.

19. The method according to claim 18, wherein steps b) and c) are repeated so as to scan the charged particle beam over the extent of the entry face of said at least one MCP assembly along said first direction.

20. The method according to claim 18, wherein the beam deflection means comprise a charged particle optics unit and a control unit, the control unit being configured to dynamically control a deflection angle to be applied to a propagation direction of the charged particle beam by the charged particle optics unit, and wherein between two successive iterations of step b), the deflection angle is altered so that the spot generated during a first iteration by a deflected beam on the entry face of said at least one MCP assembly does not overlap with the spot generated during a second iteration by the same deflected beam.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Several embodiments of the present invention are illustrated by way of figures, which do not limit the scope of the invention, wherein:

[0065] FIG. 1 is a schematic illustration of a perspective view of a charged particle beam hitting a detector assembly;

[0066] FIG. 2 is a schematic illustration of a lateral cut through a detection apparatus in accordance with a preferred embodiment of the invention;

[0067] FIG. 3 is a schematic illustration of a top view of a detection apparatus in accordance with a preferred embodiment of the invention;

[0068] FIG. 4 is a schematic illustration of a lateral cut through a detection apparatus in accordance with a preferred embodiment of the invention;

[0069] FIG. 5 is a schematic illustration of a charged particle deflection unit in accordance with a preferred embodiment of the invention;

[0070] FIG. 6 is a schematic illustration of a mass spectrometer device in accordance with a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0071] This section describes features of the invention in further detail based on preferred embodiments and on the figures, without limiting the invention to the described embodiments. Unless otherwise stated, features described in the context of a specific embodiment may be combined with additional features of other described embodiments. Throughout the description, similar reference numerals will be used for similar or the same concept across different embodiments of the invention. For example, references 100, 200, 300, 400 and 500 each describe a detection apparatus in accordance with the invention, but in two respective embodiments thereof.

[0072] The description puts focus on those aspects of the proposed detector apparatus that are relevant for understanding the invention. It will be clear to the skilled person that the device also comprises other commonly known aspects, such as for example an appropriately dimensioned power supply, or a mechanical holder frame for holding the various elements of the apparatus in their respectively required positions, even if those aspects are not explicitly mentioned.

[0073] FIG. 1 illustrates a charged particle beam 10. By way of a non-limiting example, the beam may correspond to an ion beam exiting the magnetic filtering sector of a mass spectrometer. The ion beam hits a charged particle detector 110 that would typically extend along the focal plane of the mass spectrometer in the direction indicated by the letter X. The detector's front 112 is comprised of entry faces of channels of a microchannel plate, MCP, assembly. The channels are used to amplify the detected ions that enter them into a measurable detection signal that is generated on the corresponding exit face 114. In known MCP-based detectors, the ion beam 10 (solid lines) always hits the detector 110 on the same spot 12 along the Z direction, which is a perpendicular direction to the X direction. The 1D pixel size along the detector in the X-direction does not depend on the active width of the detector in the vertical direction Z. The wider the active width in the Z-direction is, the larger the number of the channels potentially contained in one 1D pixel. A typical ion beam in a magnetic sector mass spectrometry instrument is well focused onto a small spot size of few hundreds μm in the X-direction by a few thousands μm (typically less than 2000 μm) in the Z-direction. The beam therefore typically hits only a part of the available active width of the detector in the Z-direction, resulting in a limited actual number of amplifying channels in each 1D pixel involved in detecting the ion beam.

[0074] If the ion beam 10, 10′, 10″ is scanned along the Z-direction (see the dotted and dashed lines in FIG. 1) within the active width of the MCP as illustrated, the total number of the actual MCP channels involved in the detection of the ion beam is significantly increased and therefore the detectable maximum local count rate and dynamic range is significantly improved while keeping the same detector 1D resolution along the X-direction. As the spot 12, 12′, 12″ illuminates different parts of the detector's front 112 at different times, the aggregated active width of the MCP in the Z-direction (vertical) involved in this detection process can be extended to more than 20 mm, resulting in at least an order of magnitude of improvement as compared to known detectors. This helps to improve the local count rate and dynamic range by more than an order of magnitude compared to the case according to which the ion beam is not scanned on the MCP assembly's front face 112. This scanning mechanism also helps to further reduce the probability for multiple ions hitting the same channel, and therefore to further improve the detection efficiency of the MCP detector assembly.

[0075] FIG. 2 provides a schematic view of an apparatus 100 in accordance with a preferred embodiment of the invention. The detection apparatus 100 comprises an opening or inlet 102 through which a beam 10 carrying charged particles such as ions or cluster ions is able to enter the apparatus. It is appreciated that due to the lateral cut view, the inlet extends along the X direction as shown in FIG. 1, forming a slit. Multiple charged ion beams enter the apparatus 100, scattered along the X direction and propagating towards the deflection and detection means. In the example that is illustrated, by way of a non-limiting example, an incoming charged particle beam 10 travels along the direction Y towards the inlet. Beam deflection means 130 are arranged downstream of the inlet. The beam deflection means may comprise deflection plates, a device of the Bradbury-Nielsen Gate type, or other units known in the art for selectively acting on the direction of the charged particle beam's direction. Depending on the selected magnitude of deflection, which is controlled by a control unit 140, the charged particle beam 10 that traverses the deflection means is deflected by a corresponding deflection angle θ with respect to its initial propagation direction Y. The deflection angle is typically a function of the strength of the electric field in which the charged particle beam evolves while traversing the deflection means 130. Once the charged beam 10 exits the deflection means, it continues its deviated trajectory in a straight line, the entire apparatus being contained in a vacuum enclosure that is not illustrated. Further downstream of the deflection means 130, a microchannel plate, MCP, assembly 110 is arranged. The MCP assembly comprises a plurality of microchannels forming a detection front 112, for receiving the charged particle beam. Depending on the deflection angle θ, the spot generated by the beam on the detection front 112 illuminates a different set of channels 12 along the Z direction. For each charged particle entering a channel, a corresponding amplified electrical signal is generated on the exit face 114 of the detector. These detection signals are collected by at least one read-out anode 120, which is arranged at a distance to, and in parallel with the exit face 114 of the MCP assembly. The read-out anode 120 is operatively coupled to data processing means, which are not illustrated. The data processing means are configured for storing the detection counts provided by the read-out anode in a memory element and/or for further processing the recorded data.

[0076] FIG. 3 shows a top view of a cut through a detection apparatus 200 in accordance with a preferred embodiment of the invention. For the sake of explaining the various concepts, the illustrated dimensions are not at scale and some distances have been exaggerated for easier appreciation of the figure. The apparatus comprises a front area that extends along a principal direction X, and along the perpendicular Z direction. The detection front 212 comprises the respective entry faces of a plurality of microchannel plate, MCP, assemblies 210 arranged side by side along the X direction. The front faces 212 of the MCP assemblies make up the front area and the same numerals will be used to designate both. An exception is given by the physical gaps or interstices G that separate the MCP assemblies. In the example shown, three MCP assemblies are used. Of course, other pluralities are possible without departing from the scope of the present invention. Depending on the available MCP sizes and on the desired total length in the X direction (e.g., from less than 1 cm to over 100 cm for a typical Mattauch-Herzog type mass spectrometer) of the detection front, an appropriate number of MCP assemblies is integrated into the device 200. In the example of FIG. 3, MCP assemblies extending over different lengths are arranged side by side. Each MCP assembly provides a differently sized entry face. The depth H of each MCP assembly 210 is preferably the same, so that the amplification characteristics are uniform along the length of the detector in the X direction. By way of a non-limiting example, the depth H is typically of about 0.5 to 1 mm for a single MCP assemblies, 1-2 mm for a Chevron type stacked MCP assembly, and 1.5-3 mm for a Z-stacked type MCP assembly. Ideally, the gaps G are of neglectable size, but they should in practice not be larger than 1 mm. Indeed, a charged particle (i.e. ions, electrons) impinging on the front face 212 can only be detected if it hits a front face 212 of one of the MCPs, the gaps G forming dead spots in the detection range. Preferably, the gaps between any two adjacent MCP assemblies are of uniform size. Each impinging charged particle generates a corresponding amplified signal at the exit face 214 of the MCP assembly whose entry face 212 it hit. The gap G extends to the respective exit faces of two adjacent MCP assemblies. In order to detect the amplified signals, at least one read-out anode 220 is arranged in parallel to the exit faces of the MCP assemblies 210, at a distance d (typically but not limited to 2-5 mm) therefrom. Irrespective of the use of a single MCP assembly, or of multiple MCP assemblies, the read-out anode(s) is/are operatively coupled to non-illustrated data processing means, which store the detection counts read by the anode(s), together with their position along the X axis and the Z-axis of the detector. The position of a detection event along the Z direction is dictated by the deflection that is inflicted on the charged particle beams 10 by the beam deflection means 230 arranged upstream from the MCP assemblies 210. Information describing the configuration of the detection apparatus at the time of recording, i.e. a deflection voltage, angle or a deflection distance on the MCP's entry face, is also stored together with the detection counts for each detection event. The values of these parameters are available at the non-illustrated control unit determining the deflection angle applied by the deflection means 230. By doing so, spectral data of the incoming charged particles, 10 is produced. The spectral data is preferably stored in a memory element for further processing thereof, or for displaying the same on a display device. The processing means may for example comprise a data processor having read/write access to a memory element. While the data processor may comprise specific circuitry designed for reading out the anode(s) 220, it may alternatively comprise a programmable processor, programmed to perform this task by an appropriate software code. All described components are held in place by a non-illustrated holder frame which may for example be a machined frame. Different types of anodes 220 may be considered for collecting the electrons leaving the MCP assemblies 210. The first type is a single anode, which is typically a single metal plate placed behind the MCP. This anode plate collects total number of electrons leaving the entire MCPs and therefore detects the total signal intensity hitting the MCPs (analog current or number of particles). The second type of anode is related to the position sensitive anode readouts, which can return both the position and the intensity of multiple events hitting the MCPs. Different types of position sensitive anode readout are currently used for the MCP-based detectors such as delay line, DL, anode, resistive anode, pixelated anode array, shaped anode array, etc. . . . .

[0077] If the detection front is composed of a plurality of MCP assemblies along the X axis, all MCP assemblies may preferably have substantially the same channel size and amplification gain characteristics. Further, all MCP assemblies may have the same width extending perpendicularly to said principal direction, i.e. along the Z axis. The apparatus further comprises non-illustrated biasing means configured for applying an electric potential difference between the respective entry and exit faces of each MCP assembly. The biasing means may for example comprise a source of electricity. In accordance with a preferred embodiment of the invention, the biasing means are configured for applying a positive or negative floating electric potential to the detector's front face, and for applying a common electric potential to the respective exit faces of all MCP assemblies.

[0078] In accordance with a preferred embodiment of the invention, the deflection means of the proposed detection apparatus are provided by a device comprising deflection plates, as illustrated in FIG. 4. FIG. 4 shows a lateral cut view through an apparatus 300 in accordance with an embodiment of the invention, comprising an inlet 302, MCP assemblies 310 as previously described having entry and exit faces 312, 314, and beam deflection means 330 comprising an ion beam deflector 332, 334, all extending in the X direction.

[0079] The two plates 332 in an ion beam deflector extend in parallel and define a passageway or gap for the charged particle beam 10 in between themselves. The deflector plates 332 are biased to opposite polarities to deflect the ion beams in a direction that is perpendicular to the plane in which the deflector plates extend. To this end, a source of electricity, voltage biasing unit or control unit 340 is used. The dimensions, relative arrangement and biasing electric potentials of the deflector 332, 334 are in practice optimised, for example through numerical simulation, to best match a given target application. By way of non-limiting example, the deflection plate width along the Y direction is preferably in the range from 2 mm to 8 mm. The distance between the plates 332 along the Z direction is preferably in the range from 3 mm to 5 mm. The gap between the plates 332 and the outer electrodes 334 (on both sides of the deflector to confine the fringe field regions to smaller distances) is in the range from 1 mm to 4 mm. Preferably, the distance between the plates is of about 4 mm, the gap between the plates and the outer electrodes is of about 2 mm, while the width of the plates along the Y direction is of about 6 mm. The distance between the outer electrodes, defining the inlet 302 of the apparatus, if preferably of about 6 mm. Preferably, the deflection plates 332 and outer electrodes 334 extend along the X axis, which is perpendicular to both Y and Z axes, by about 100 mm to 150 mm, and preferably by about 130 mm. Generally, the deflection length of the ion beams along the Z axis, i.e. the maximum span than can be scanned on the MCP assembly's entry face, increases with increasing deflection plate width. The mass resolution and transmission values decrease with increasing transmission plate width. The target of the deflection of 20 mm can be achievable for various combinations of the geometric and voltage values with the constraints of the mass resolution (at least up to half of the mass resolution observed at 0 V) and transmission (70%).

[0080] The working principle of the charged particle deflector 300 is as follows: the deflection distance along the Z axis, D, depends on the distance d between the deflection plates 332, their length L and the electrical potential difference (ΔV) that is applied to the pair of plates (±V). When an ion enters with an energy of eV.sub.0 into the electric field created between the two plates, (E=−ΔV/d) it will be deflected with a deflection angle θ.

[0081] Each MCP channel of an MCP assembly, whether of a single MCP, Chevron or Z-stack assembly, is characterized by recovery time, during which the channel cannot amplify a newly impinging charged particle. After a given response time, a saturated channel becomes once operational again. In accordance with a preferred embodiment, a control unit of the deflection means is therefore synchronized with the response time of the MCP assembly's micro-channels. The control unit effectively aims at steering the charged particle beam at all times, through deflection as previously described, to areas of the detection front that comprise operational MCP channels at the time of deflection. The speed of a continuously and linearly evolving scanning pattern is therefore preferably synchronized with the response time of the MCP channels, so that the charged particle beam illuminates the same channel only once it has completely recharged from the depletion caused by its previous illumination.

[0082] In an alternate embodiment, the control unit uses a sawtooth signal to scan multiple charged particle beams that are initially scattered along the X direction, periodically along the Z direction of the MCP assembly's detection front. A low-power high voltage operational amplifier scans the voltage of the ion deflection plates with a scan rate in the range from 1 Hz to 10 kHz. The scanning signal (sawtooth signal) consists of a combination of multiple scans of step voltages. The number of voltage steps depends on the size of the ion beams and the required deflection length of the ion beams in the vertical axis (Z) of the detector. A higher signal rate (>1 kHz) is desirable to achieve an enhanced signal count rate of the detector. The scanning rate is established in practice as a function of the recuperation time of a saturated channel of the MCP assembly When a measurement starts, the control unit sends a scanning signal to the high-voltage amplifier, and synchronously starts/triggers a Time-to-digital converter, TDC. Each time the step voltage changes, a pulse will pass to the TDC on a second line to increase an internal counter. During the acquisition, data is collected from the TDC. The raw data has acquired by the read-out anode and TDC has a format of {x position, y position, number of the voltage step, and iteration of the scan}. After multiple scans, there may be an integration time on each mass spectrum at different step voltages to combine all the mass spectra.

[0083] FIG. 5 shows a lateral cut view through an apparatus 400 in accordance with an embodiment of the invention, comprising an inlet 402, MCP assemblies 410 as previously described having entry and exit faces 412, 414, and beam deflection means 430 comprising an ion beam deflector 432, 434.

[0084] The deflection means 430 of the proposed detection apparatus comprise a Bradbury-Nielsen Gate, BNG, device. A BNG device consists of two interleaved sets of wires or strips 433, 434 extending along the Y direction, which are equally spaced and are biased at opposite polarities so that the deflecting field region is limited to short distances, typically twice of the diameter of the wires or length of the strips. The strip width is for example in the range from 0 to 1.6 mm, each strip having a thickness of 50-100 μm. The distance between the strips is in the range from 0.4 mm to 1 mm. By way of example, for a charged particle beam comprising ions at 3 keV, the voltage on the strips may be ramped from 0V up to ±800 V by a corresponding non-illustrated control unit of the deflection means. The deflection length of the ion beams increases with increasing strip width. The transmission values decrease with increasing strip width. Depending on the applied voltages, the BNG scatters charged particle beams within an angle having a voltage-dependent opening along the Z direction.

[0085] In all presented embodiments, the detector device is able to be floated to a high voltage of up to 10 kV, while the floating potential may have either positive or negative polarity. The floating potential may preferably be applied to all components of the detection apparatus, including the beam deflection means.

[0086] FIG. 6 illustrates components of a Mattauch-Herzog type spectrometer device in accordance with an aspect of the invention. Ions are generated by an ionization technique (not discussed here) and extracted from a sample under analysis. The ion beam 10 is then focused and analysed using different entities of the spectrometer. The functioning of such spectrometers, which use an electrostatic sector 20 followed by a magnetic sector 30 for filtering an incoming ion beam 10 is well understood in the art and will not be explained in further details in the context of this description. The magnetic sector 30 comprises an exit plane 32 through which the ions comprised in the initial beam exit, dispersed along a principal direction X in accordance with their respective mass-to-charge ratios. The spectrometer also comprises a detection apparatus in accordance with the present invention, as previously described. The detection apparatus 500 comprises beam deflection means 530, for example the previously described deflection plates, controlled by an operatively coupled control unit 540. The detection apparatus also comprises a detection area or front that features the entry face 512 of at least one MCP assembly 510. The detection front is arranged so as to be located on the focal plane of the spectrometer. It is appreciated in this example, that the entry face 512 of the MCP assembly 510 may extend at angle with respect to the deflection means 530. Also, the ion beams traverse the deflection means 430 at different angles along the X direction, resulting in traversing trajectories of different, but known lengths for each position along the X direction. For each out of N applied deflection voltages, i.e., for each deflection angle of all the filtered ion beams along the Z axis, perpendicular to X axis, a full mass spectrum 40 (i.e. a mass spectrum for all m/z ratios) of the sample is obtained by recording the correspondingly amplified detection signals on the non-illustrated read-out anode(s), as previously described. The data describing N corresponding partial mass spectra (i.e. partial counted detection events) may be combined into an aggregated mass spectrum of the sample (i.e., aggregated counted detection events for all m/z ratios) by a data processing unit having access to said memory element. The detection apparatus is high vacuum/ultra-high vacuum, HV/UHV, compatible and covers the full focal plane of the Mattauch-Herzog mass spectrometer, which is typically from several centimetres to several tens of centimetres. It provides one-dimensional (horizontal) spatial resolution of better than 100 μm, a local count rate of better than 10.sup.6 cps, a dynamic range better than 10.sup.5.

[0087] The N voltage/deflection steps are predetermined so that the correspondingly deflected spots generated by the deflected beam on the MCP entry faces span most of the entry face along the Z direction. Further, the response time of the MCP channels is taken into account: a given voltage step is applied during a time that corresponds at most to the time during which the MCP channels are capable of detection charged particles, until they saturate. Then the next voltage step is applied, wherein the difference between two steps is such that the deflected beam next illuminates a different set of micro-channels, which that are not in saturated mode. These measures maximize the availability of MCP channels for accurately counting detection events, thereby increasing the count rate of the apparatus. The corresponding calibration data depends on the type of MCP assembly that is used, on the dimensions of the apparatus, and on other factors. These data are provided on assembly of the apparatus and made available in a memory element to which the control unit 540 has read access.

[0088] At each of the N deflection steps, a corresponding detection signal is recorded using the read-out anode. Mass spectrum data for all m/z ratios is therefore recorded at each deflection step. The data corresponding to all steps is then combined, for example by adding up the successively read-out ion detection counts, to generate a full mass spectrum.

[0089] It should be understood that the detailed description of specific preferred embodiments is given by way of illustration only, since various changes and modifications within the scope of the invention will be apparent to the skilled person. The scope of protection is defined by the following set of claims.