The present invention relates to the field of measuring devices, in particular devices for measuring the diameter and shape of an ion or electron beam, in particular in tools such as an ion implanter or an electron beam welding device.
Electron beam machines have found various applications, in particular in the fields of welding, surface modification, X ray generation, electron beam lithography, electronic microscopy, and many others. With these applications came the need for precise control of the focus and alignment of the beam, as well as a particular need to determine the distribution of energy in an electron beam.
The production of reproducible electron beams can be done independently of the machine or operator if the energy distribution of the beam can be controlled and known precisely. The traditional methods for adjusting the energy distribution are based on the know-how of an operator specialized in welding who visually adjusts the beam on a secondary target. The operator looks at the intensity of the radiation from the light given off rather than a direct measurement of the energy distribution of the beam. This old method is inherently imprecise, requiring an experienced operator and precise judgment to focus the beam correctly. Clearly each operator can adjust the parameters differently depending on the interpretation each has of what he sees.
The current density in question is influenced by many variables, such as the shape of the cathode, focus adjustments, the firing distance, the value of the current, the accelerating voltage, the vacuum level, and the alignment of the electrodes. A variation of these parameters can cause a variation in the distribution of the current density of the beam, which can have a significant effect on the penetration of the welding, the width of the weld seam and the quality of the surface of the objects welded by the electron beam.
The traditional methods for adjusting the beam are not fully satisfactory. Various devices have been developed to determine various characteristics of the electron or ion beams, such as the configuration of the beam, the current density, etc. among the different devices, the rotary yarn probe, pinhole devices, the modified Faraday cage, the modified and improved Faraday cage, the double rotary slotted disk scanner.
These quantitative diagnostic means were intended to better determine the distribution of the current density, and thereby provide better monitoring the conditions of the focus of the beam. To obtain the most realistic image of the current density of the electron beam, the beam should ideally be located at rest without scanning above an analysis tool (this scanning is, however, used with tools of the rotary yarn type and the modified and improved Faraday cage).
The invention aims to propose a device and a method for analyzing a beam of charged particles, in particular an electron or ion beam, making it possible to determine the distribution of the current density, therefore of the power, in a section of the beam. Furthermore, the invention aims to propose a solution not using tomographic reconstruction.
According to a first object of the invention, such a device for analyzing the current density in an incident beam of charged particles is characterized in that it comprises a moving target, the target being positioned to project the beam there during analysis, at least one hole passing through the moving target and arranged so that each hole passes through a section of the beam along a respective path when the target moves and only a fraction of the incident beam passes through the target via the hole, and means for measuring the current density of the fraction.
Preferably, the target rotates around an axis, and the travel of each hole is an arc of circle centered on the axis. The target advantageously comprises at least one reference hole, the section of the reference hole being noticeably different from, preferably twice, the section of the at least one hole. The target can comprise several holes, the arcs of circle passed through by the holes preferably being regularly radially spaced apart. The arcs of circle passed through by the holes can also be radially closer in an intermediate zone and radially further apart on either side of said intermediate zone. The holes may or may not be regularly angularly spaced apart from one another.
Preferably, the means for measuring the current density of the fraction comprise at least one Faraday cage.
The device can also comprise a primer target, preferably cooled. The device can then comprise means for deflecting the beam to be analyzed, incident, toward the primer target.
According to a second object of the invention, a method for analyzing the current density in a section of an incident beam of charged particles is characterized in that it comprises steps for:
To measure the profile, it is possible to move a target pierced with at least one hole between the beam and means for measuring the current density in the fractions, and in that each fraction is an instantaneous fraction of the beam passing through one of the holes.
The target can be a target rotating around an axis pierced with at least one series of several holes, the holes preferably being regularly distributed, radially and/or angularly, relative to the axis, the holes having an identical section to one another. The target advantageously comprises a reference hole having a different section from that of the other holes, so that the route of said reference in the analyzed section causes an anomaly in the profile, so that one verifies that the analysis is complete while ensuring that two anomalies appear in the profile. The appearance of two successive anomalies can be used to determine the actual speed of rotation of the target during the analysis.
The method according to the invention can also comprise steps to:
The method can also comprise at least one step for calculating at least one parameter of the beam among the full width at half-maximum, the full width at height 1/e2, the maximum surface power density.
Such a method can be used in a method for determining wear of an electrode of a charged particle beam generator.
Such a method can also be used in a method for determining an alignment flaw of an electrode of a charged particle beam generator.
Such a method can also be used to determine an optimal focus of the beam.
Several embodiments of the invention will be described below, as non-limiting examples, in reference to the appended drawings, in which:
In the example illustrated in
In the illustrated example, the device 1 comprises a motorized target 2, and a motor 3 to move said target. The device also comprises a window 4, through an opening 6 from which the beam is fired, a Faraday cage 8 to receive a filtered part dF of the beam F, and a secondary primer target 9.
The motorized target 2 is essentially made up of a plate 2 in the shape of a disk. The disk 2 is mounted rotating around an axis of revolution X2. The axis X2 is substantially combined with the geometric axis of the disk. The axis X2 is positioned substantially parallel to the firing direction of the beam F, i.e. substantially vertically. The motor 3 is provided to rotate the disk 2 around the axis X2. The motor 3 is also provided to rotate the target 2 at a constant speed during the analysis of the beam F.
The window 4 forms an opening 6, above the target 2, near the periphery of the disk. The window 4 forms, around the opening 6, a protection for the target 2 when the beam is not in position to be analyzed, i.e. when the beam is not oriented vertically in the space delimited by the opening 6.
The target is pierced with “pinholes” 7, i.e. it is pierced with through holes 7 with small dimensions relative to the diameter of the beam F to be analyzed. In the illustrated example, the holes 7 are regularly positioned, such that during a complete rotation of the target 2, each travels through the opening 6 along an arc of circle specific to it, the arcs of circle being radially spaced apart from one another, regularly, i.e. the distance between two neighboring arcs is constant. In the illustrated example, the dimensions of the opening are such that the arcs of circle can potentially be likened to straight segments if the required precision is not too high. The holes are also angularly spaced apart regularly, i.e. the angle, relative to the axis X2, between two successive holes is constant. Preferably, the holes are regularly distributed over 360 degrees around the axis X2. Thus, the orifices 7 are positioned in a spiral shape. The holes are advantageously positioned so that there is never more than one hole 7 passing through the opening 6 at the same time. In particular, the size of the opening 6 is chosen so that the analyzed beam only sees one rotating hole at a time.
All of the holes 7 have circular horizontal sections substantially identical to one another, except one orifice 7A. The orifice 7A is a hole forming a reference. It has different dimensions from those of the other holes, so that it provides an indication of the relative position of the target 2 relative to the beam, each time that hole 7A passes through the opening 6. Thus, if the reference 7A has a section substantially larger than that of the other holes 7, it will also receive a noticeably larger quantity of electrons. For example, if the reference has an inlet section twice that of the other holes, it will receive substantially twice as many electrons as expected. In the illustrated example, the dimensions of the reference 7A differ from those of the other holes in that they are doubled in the direction of the corresponding arc. Preferably, the reference is positioned so that it travels through a central region of the window. One thus avoids it passing through a peripheral region, not very large, with low intensity, or without the analyzed beam.
The rotation of the target 2 makes it possible to distribute, over a large surface area, the energy transmitted to the target by the beam F during the measurements. Nevertheless, the material used for the disk is very advantageously of the refractory type. It can in particular be made from tungsten, molybdenum or graphite. If it is made from graphite, it can include only graphite or comprise, on its upper surface, a deposition or projection of a refractory material such as tungsten or molybdenum. The deposition or projection of a refractory metal leads to increasing the number of secondary electrons (“electrons bouncing” from the surface), thereby reducing the thermal charge of the disk, which makes it possible to use the device 1 to analyze the beams F with higher powers.
Advantageously, the device is equipped with a current collector (not shown in the figures) of the coal type in contact with the target, to continuously evacuate the current intercepted during the analysis.
As particularly illustrated in
The centering means are formed by a piece 13 adjusted on the shaft, between the clamps 14, 15. The piece comprises an annular part 18 with a reduced thickness that bears against the inner edge 16 of the target 2.
The upper clamp 15 and the centering piece 13 can be disassembled from the shaft 11, in particular to make it possible to change the target 2.
Preferably, the contact of the clamps and the centering piece are not continuous along the inner edge 16, so that this limits the thermal transmission. To the same end, empty spaces are also provided on either side of the annular zone 18, between it and each of the clamps 14, 15.
As particularly shown in
The Faraday cage 8 is positioned under the target 2, opposite the opening 6, so that, irrespective of the hole 7 passing through the opening, the filtered part dF that it diffuses is still absorbed by the Faraday cage. The Faraday cage 8 has an inner configuration in the form of a conical well 8A so that the electrons are trapped there. In the example illustrated in
As illustrated in
In the illustrated example, the target 9 is a cooled target. During priming thereof, the beam is deflected there for long enough to stabilize, then, after measurement, its extinction time. One thus limits the exposure time of the rotary target to the time needed for measurements.
The device 1, in particular formed by the motorized target 2, the motor 3, the window 4, the Faraday cage 8 and the primer target 9, therefore constitutes a scanner for the electron beam F. Means can be provided in the device 1 to deflect the incident beam F toward the primer target. It is also possible to use deflecting means provided in the source of the beam F. The deflection means can be magnetic field means.
Although not shown in the figures, the device 1 according to the invention can advantageously comprise acquisition means for the signal coming from the output S and means for processing that signal; in particular, these means can respectively comprise acquisition software and processing software for the specific signal. Such a device 1 can also comprise control means for the motor and, if applicable, control means for the deflection means of the incident beam F. Preferably, all of the operations—rotation of the disk, triggering of the acquisition, priming and extinguishing the beam, deflection of the beam, etc.—are managed by a programmable machine.
The device can also comprise means to be moved along several axes, so as to be positioned relative to the beam F.
We will now describe the successive steps of analyzing the beam F using the illustrated device.
First, the target 2 being in rotation, the beam is kept non-deflected and at a low power, toward the target 2. The centering of the opening 6 of the window 4, on the non-deflected beam, is verified, then potentially corrected.
One then deflects the beam F toward the cooled target 9, the time needed to prime it, i.e. the time needed for it to reach its nominal power and a thermal equilibrium.
Then, one stops the deflection of the beam F, so that it strikes the rotary target 2, in a zone delimited by the window 4, inside the opening 6, as illustrated in
One then deflects the beam F again toward the cooled target 9, the time for it to be extinguished.
The signal S can be processed as it is acquired or deferred after prior recording.
The steps for acquiring and processing the signal S, in particular those illustrated in
We will now describe, in reference to
In this embodiment, there are two detection devices, here two Faraday cages 8, the second Faraday cage 82 being positioned at 90 degrees, relative to the axis X2, from the first Faraday cage 81, as illustrated in
Furthermore, in this second embodiment, and as particularly illustrated in
In particular when such a target 2, with a variable radial pitch, is used with a device 1 of the first embodiment, as illustrated in
Owing to the second embodiment, and to the use of the two Faraday cages 81, 82 positioned at 90 degrees relative to one another, it is therefore possible to scan, i.e. analyze, the beam F in two substantially perpendicular directions. Each analysis has corresponding routes A7 respectively illustrated by
In the example illustrated in
Preferably, means are provided (not shown in the figures) to position the device 1, with sufficient precision in light of the precision desired for the analysis, so that the analysis is always done when the beam F is not deflected, whether with the first or second Faraday cage. Thus, the first window 61 is positioned in the axis of the non-deflected beam F, then, a first series of measurements being done, the device is moved to position the second window in the same axis of the non-deflected beam, before performing a second series of measurements.
The positioning is preferably precise enough to be able to directly superimpose the two series of measurements. Nevertheless, a correction can be provided, for example by comparing measurements that should be substantially identical, in particular at the intersections of the routes, during the superposition.
Of course, the invention is not limited to the examples described above.
In particular, if in the illustrated example the measurements are done on two complete revolutions, it is also possible to detect a first and second appearance of the measurement peak corresponding to the reference, and to immediately interrupt the measurement after the appearance of the second peak. It is thus possible to limit the exposure of the target to the beam.
In the illustrated example, the reference travels through a region of the beam F specific to it, i.e. the measurements that are done through said hole participate, after normalization, in the analysis. Nevertheless, the reference can also be provided to travel through a region that may or may not already have been traveled through by another hole, so that the reference no longer participates in the analysis, but only performs its reference function.
The device can also comprise means for measuring speed variations on the disk during an analysis, so as to be able to then correct the measurements done as a function of these speed variations.
The holes can have a solely tapered shape, i.e. not comprise a cylindrical portion.
The rotary target can comprises several series of holes and each series can form a respective spiral. Each of these spirals can be radially offset and have holes with different dimensions relative to one another. In that case, the Faraday cage can have sufficient dimensions to be used for all of the spirals. A specific Faraday cage can also be provided for each of the spirals. Each of the spirals can have a use dedicated, for example, to a type of beam with a specific diameter or intensities, or according to the desired precision.
The angular spaces between the holes, relative to the axis, may or may not be regular. The radial distance between the holes can increase or decrease relative to the axis.
An irregular radial or angular spacing can in particular make it possible to process, with a same target and a same spiral, beams having different characteristics; for example, a portion of the spiral having a wide spacing making it possible to analyze wide beams, such as natural or unfocused beams, and another of the spiral have a smaller spacing making it possible to analyze thin beams. Thus, the interval between the holes may, but non-limitingly, be larger at the beginning and end of the spiral, and may be smaller in an intermediate part of the spiral, as illustrated in the second embodiment.
A spacing table for the holes is advantageously used to reconstruct the intensities of the beam, so that an irregular spacing does not affect the result. Although all or part of the irregularity results from tolerance during the machining of the target, it can thus be compensated by a precise geometric reading reflected by the table.
Advantageously, the rotary target is interchangeable, for example to be able to use targets whereof the holes have a spacing, radial or angular, and/or diameters, that are different according to each target, depending on the desired precision and/or the transverse dimensions of the beam to be analyzed.
Number | Date | Country | Kind |
---|---|---|---|
10 60194 | Dec 2010 | FR | national |