Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/26—Mass spectrometers or separator tubes
  • H01J49/34—Dynamic spectrometers
  • H01J49/40—Time-of-flight spectrometers
  • H01J49/405—Time-of-flight spectrometers characterised by the reflectron, e.g. curved field, electrode shapes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/0004—Imaging particle spectrometry

Definitions

• the present invention relates to a mass analysis device with wide angular acceptance comprising a reflectron.
• angular acceptance is understood as the capacity for the device integrating the reflectron, to treat ions emitted from a source such as the surface of a sample to be analyzed, with a large angular dispersion.
• the present invention is particularly applicable to the field of time-of-flight spectrometers, and more particularly to mass analysis devices such as mass spectrometers and time-of-flight atomic probes equipped with electrostatic mirrors, or reflectrons.
• a time of flight mass spectrometer or TOF makes it possible to determine the mass of ions torn off from a sample by measuring the flight time of the ions from a given position, real or virtual, from the ion source, to their impact on a detector, through an analysis chamber.
• the time of flight of an ion through an electrostatic field is proportional to the square root of the mass-to-charge ratio of this ion for a given kinetic energy.
• the mass resolution of a time-of-flight spectrometer depends, in addition to the precision with which the start and impact times of the ions can be measured, of the energy dispersion of the ions; Indeed, ions of an identical charge-to-charge mass ratio but with different charge-energy ratios have different flight times from the ion source to the detector.
• a known method making it possible to eliminate, at least in first approximation, the dependence of the flight time of an ion on its energy, and thus to improve the mass resolution of a time-of-flight spectrometer, is to incorporate in the analysis chamber of the mass spectrometer an ion mirror device.
• This method has been proposed for the first time by Alikhanov, and implemented by Mamyrin.
• the ion mirrors used in reflectron time-of-flight mass analyzers typically incorporate homogeneous and uniform homogeneous particle retardation fields, that is homogeneous in defined spatial areas.
• Ionic mirrors commonly consist of a main electrode, a particular geometry and excited by an electrical potential, and a gate electrode of similar geometry and excited by a different electrical potential.
• the electrostatic field generated by these electrodes is contained in the space separating these electrodes, and its characteristics can for example be adjusted as a function of the excitation potentials.
• mass spectrometers rely on various ionic emission methods such as field desorption, laser desorption or secondary ion emission, themselves known from the state of the art.
• a characteristic of the ion beams resulting from these techniques is a strong angular dispersion of emitted ions, which can wait for values of the order of 90 ° or more. It is important not to restrict ion beams with wide angular dispersion in order to maximize the sensitivity of the mass spectrometer.
• the analysis of ions emitted with wide angular dispersion may be of major interest
• the atomic probes also called atomic probe microscopes, in which an increase in the angular acceptance is synonymous with an enlargement of the field of view of the microscope, since different angles of emission correspond to different positions on the surface of the sample to which the ions are torn off.
• Ion mirrors of conventional piecewise homogeneous electrostatic reflectrons can not accept an angular dispersion of the ion beam greater than about 10 °.
• curved geometry mirrors have been proposed in the article by Vialle et al., Rev. Sci. Instrum., 68 (1997) 2312.
• a curved geometry reflectron is proposed in International Patent Application WO2006 / 120428. This type of reflectron performs a transformation of the ion beam diverging from a sample of reduced size into a substantially parallel beam, capable of being admitted by a detector of reasonable dimensions.
• the plane of the detector is substantially perpendicular to the ion beam, in order to avoid increasing the dimensions of the detector, which would otherwise be unavoidable.
• a device In addition to its spatial focusing properties, and focusing in terms of flight time as a function of ion energy, such a device has spatial focusing properties as a function of ion energy, and can thus be used to obtain spatially resolved images of a sample in atomic probe microscopes.
• Such a reflectron has some disadvantages.
• the angular acceptance of such a device can not exceed 90 ° for reasons simply related to the geometry of the device.
• the angular acceptance of such a reflectron is further reduced by an indispensable inclination with respect to the plane of the detector, the surface on which is made the focusing in terms of flight time as a function of energy.
• Another disadvantage of this type of reflectron is related to the fact that the intersection between most of the trajectories of the ions and the direction normal to the input gate electrode of the reflectron mirror is made at relatively open angles, which considerably increases the dispersion of the ions at the local inhomogeneities of electric field generated by the grid.
• An object of the present invention is to overcome at least the aforementioned drawbacks, by proposing a new reflectron design, capable of allowing a large angular acceptance of ions emitted from the reduced surface of a sample in at least one direction.
• the subject of the invention is a time-of-flight mass analysis device, particularly of the mass spectrometer or atomic probe type, characterized in that it comprises: means for receiving a sample, means for extracting ions from the surface of the sample, a detector, - an ion mirror producing an electrostatic field with a toroidal geometry whose equipotential lines are defined by a first curvature in a first direction comprised in the radial plane of the device mass analysis and a first center of curvature, and a second curvature in a second direction perpendicular to the first direction in the transverse plane of the mass analysis device and a second center of curvature, the sample being disposed at a distance from the first center of curvature less than a quarter of the first radius of curvature.
• the time-of-flight mass analysis device may be characterized in that the detector is disposed at a distance from the spatial focusing point of the ions emitted from the sample in the first direction. , after reflection by the ion mirror, less than a quarter of the first radius of curvature.
• the time-of-flight mass analysis device may be characterized in that the detector is disposed downstream of the spatial focusing point of the emitted ions of the sample in the first direction, after reflection by the ion mirror.
• the time of flight mass analysis device can be characterized in that the detector is sensitive to the two-dimensional position of the impact of the ions on its surface.
• the time-of-flight mass analysis device can be characterized in that the detector is able to be displaced along the main axis of the mass analysis device.
• the time-of-flight mass analysis device may be characterized in that the ion mirror comprises a rear electrode and a gate electrode, the electrostatic field being formed between the back electrode and the grid electrode.
• the time of flight mass analysis device may be characterized in that the rear electrode and the gate electrode have a cylindrical surface.
• the time-of-flight mass analysis device can be characterized in that the rear electrode and the gate electrode are of spherical surface.
• the time of flight mass analysis device may be characterized in that it comprises additional means capable of varying the electrostatic field produced by the ion mirror.
• the time-of-flight mass analysis device can be characterized in that the sample is able to be moved in all directions and / or to be rotated.
• the time of flight mass analysis device may be characterized in that the ion extraction means pull out the ions from the surface of the sample by means of field desorption and / or or laser desorption, or secondary ion emission.
• a particular reflectron time-of-flight mass analyzer geometry is proposed for this purpose.
• the sample to be analyzed is placed at a distance substantially close to the center of curvature of the curved-field mirror in the direction of the emitted ion field.
• the ions emitted, after reflection on the curved-field mirror, are then focused in the direction considered, at a conjugate point located at an opposite position, with respect to the center of curvature, of the position of the sample.
• the detector can be arranged, depending on the physical problem that arises, either at the point of focus or downstream from this point.
• the shift from the position of the detector focus points in terms of flight time as a function of ion energy for all directions of ionic emission, is minimal.
• the angular image of the sample can be resolved at a detector of reasonable dimensions.
• An advantage of the present invention is that these properties remain valid for theoretically unlimited angular dispersion, i.e. up to 180 °.
• Another advantage of the invention lies in the fact that, irrespective of the angular dispersion, the angles of intersection of the ion trajectories with respect to the normal to the surface of the gate-entry electrode of the mirror for the considered direction , remain low, thus allowing a reduction of the angular dispersion of the ions at this level.
• the spatial energy dispersion offered by the reflectron time-of-flight mass analyzer according to the invention is not zero at the detector.
• FIG. 1 is the sectional view in the radial plane of a mass analysis device, of a reflectron geometry known from the state of the art; - Figure 2, the sectional view in the radial plane of a mass analysis device, an example of reflectron geometry according to one embodiment of the present invention;
• FIG. 3 is a perspective view, of an example of the image formed at a position-sensitive detector, of ions emitted from a sample in different directions in the radial plane and in the transverse plane; according to an embodiment of the present invention;
• FIG. 4 is a sectional view in the radial plane of a mass analysis device of an example of a reflectron geometry with a detector placed at the conjugated point of focus of the point at which the sample is located, according to another embodiment of the present invention.
• Figure 1 shows the sectional view in the radial plane of a mass analysis device, a reflectron geometry known from the state of the art, as presented in the aforementioned patent application WO2006 / 120148.
• a mass analyzer 100 comprises a sample 101 of small size, for example in the form of a tip, from which ions are emitted and accelerated by extraction electrodes 102.
• the emitted ions follow in the analysis chamber of the analyzer.
• the ions are reflected in an ionic mirror 103 forming a curved equipotential surface electrostatic field 104.
• the equipotential lines have a center of curvature 105.
• the ion mirror 103 consists of a rear electrode 107. and a gate electrode 106.
• a detector 108 collects the ions.
• the detector 108 is sensitive to the position of the point of impact of the ions on its surface.
• the center of curvature 105 of the equipotential lines of the field generated by the ion mirror 103 is typically at a greater distance from the mirror 103 than from the sample 101.
• the ion mirror 103 allows divergent ion trajectories from the sample 101 to become substantially less divergent or even slightly convergent after reflection. Thus, at a great distance from the mirror 103, the ion trajectories can still be picked up by the detector 108 whose size can remain reasonable. This large spacing of the trajectories allows the ions to have a sufficient flight time to give the mass analyzer 100 a high resolution mass.
• the intensity of the electrostatic field inside the ion mirror 103, and hence the length of the trajectories within the ion mirror 103, is chosen so that ions emitted from the sample in the same direction, but with different energies, along the paths 109 and 110, reach the detector 108 essentially at the same time; that is, the focus in terms of flight time with respect to ion energy is ensured.
• the distance between the ion mirror 103 and the detector 108 is chosen so that ions emitted from the sample in the same direction, but with different energies, reach the detector 108 at essentially the same point of impact; that is, the spatial focus with respect to ion energy is ensured.
• Figure 2 shows the radial sectional view of a mass analysis device, an example of reflectron geometry according to an embodiment of the present invention.
• the sample 101 is disposed near the center of curvature 105 of the equipotential lines 104 of the electrostatic field generated by the ion mirror 103.
• the electrode 107 of the electrostatic mirror 103 has a spherical geometry, and that the gate electrode 106.
• the equipotential lines 104 of the electrostatic field have a spherical symmetry.
• the ions are emitted from the surface of the sample 101 and accelerated by the extraction electrodes 102, then reflected by the ion mirror 103.
• ions pass through a point 1 1 1, conjugate of the point at which the sample 101 forming a tip can be assimilated into a first approximation. Downstream of the point 1 1 1, the ions reach the detector 108, sensitive to the position of the impact points with its surface.
• the electrostatic field prevailing inside the ion mirror 103, and therefore the length of the trajectories of the ions within the ion mirror 103, are chosen so that ions emitted from the surface of the sample 101, in the same direction. direction but with different energies, following paths 109 and 1 10, reach the detector 108 essentially at the same time; that is, the focus in terms of flight time with respect to ion energy is ensured.
• the focus in terms of flight time relative to the energy of the ions can not be rigorously achieved in practice at the detector 108, since the surface on which the condition of such a focus is filled is of substantially spherical shape. , with a center located at the conjugate point 1 1 1.
• this surface is substantially parallel to the central region of the surface of the detector 108, thus the dependence of the flight time of an ion on its energy remains low for a relatively large emission angular dispersion, this dependence increasing as the square the distance separating the center of the detector 108 to the point of impact of the ion considered on the surface of the detector 108. Since the sample 101 is close to the center of curvature
• particular configuration makes it possible to reduce the effects of ion dispersion caused by the inhomogeneities of the local electrostatic field at the proximity of the gate electrode 106.
• the difference between the trajectories 109 and 110 of ions leaving from the surface sample 101 in the same direction, but having different energies, remains reduced after reflection by the ion mirror 103; this gap tends to zero when the sample 101 tends towards the center of curvature 105 of the equipotential lines 104 of the electrostatic field produced by the ion mirror 103.
• the coincidence, at the level of the detector 108, trajectories of ions of the same initial direction but having different energies is not perfect, it remains excellent if the energy dispersion of the ions remains relatively low. Space chromatic aberration is still said to be weak. In this way, ions with different emission directions can be solved at the detector 108 with good accuracy.
• the radius of curvature of the rear electrode 107 may for example be equal to 400 mm, the distance from the sample 101 to the center of curvature 105 may be equal to 30 mm, and the distance from the detector 108 to the focusing point 1 1 1 could be equal to
• FIG. 3 shows the perspective view, of an example of the image formed at a position-sensitive detector, of ions emitted from a sample in different directions in the radial plane and in the transverse plane, according to an embodiment of the present invention.
• This embodiment of the invention allows the analysis of ions emitted from the surface of the sample 101 with a large angular dispersion, in theory up to ⁇ radians, using a finite size detector 108.
• the angular acceptance is even greater than the center of the detector 108 is close to the point 1 1 1 conjugate of the point at which the sample 101 is assimilated.
• this embodiment of the invention allows large mass resolution with wide angular acceptance, as well as good spatial resolution, due to low spatial chromatic aberration.
• this embodiment of the invention is particularly suitable for an atomic probe microscope type application.
• the aperture or energy focusing can be realized differently in the radial plane and in the transverse plane.
• FIG. 4 shows the perspective view of an example of the geometry of a reflectron with a detector 108 placed at the conjugated point of focus of the point at which sample 101 is located, according to another embodiment of the present invention. invention.
• the intensity of the electrostatic field generated by the ion mirror 103 can be chosen so as to allow focussing in terms of flight time relative to the energy of the ions, at the detector 108.
• This particular embodiment may be advantageous if a spatial resolution of the ions is not necessary.
• This embodiment allows a high mass resolution for ions emitted from the surface of the sample 101 with a large angular dispersion. This characteristic can be obtained by placing the detector at a position coinciding with the focusing point 11 in terms of flight time relative to the energy of the ions.
• the reflectron geometry can be simplified by using a gate electrode 106 and a rear electrode 107 of cylindrical surfaces.
• the electrodes of the ion mirror 103 can be equipped with additional mechanical alignment means and / or additional electrode sets for adjusting the shape of the electrostatic field. It is also advantageous, for a better adjustment of the performance of the mass analyzer 100, to allow a displacement of the detector 108 along the main axis of the analysis device 100 and / or the sample 101 along the three axes. It may also be advantageous to provide the sample holding mechanism with means for tilting the sample in order to correct tilting defects of the sample and / or the sample holder.

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• Physics & Mathematics (AREA)
• Spectroscopy & Molecular Physics (AREA)
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