WO2015118316A1

WO2015118316A1 - System and method for measuring properties of a charged particle beam

Info

Publication number: WO2015118316A1
Application number: PCT/GB2015/050287
Authority: WO
Inventors: Colin RIBTON
Original assignee: The Welding Institute
Priority date: 2014-02-04
Filing date: 2015-02-03
Publication date: 2015-08-13
Also published as: GB201401892D0; TW201535458A

Abstract

A system is disclosed for measuring properties of a charged particle beam (8) output by a charged particle beam generator (15). The system comprises: a probe assembly (21), a beam deflection control module (23) and a detection module (22). The probe assembly comprises a plurality of probes (30) arrayed across a plane on a mount, each probe comprising at least two elongate, electrically conductive elements (32, 33, 34) arranged such that their respective elongate directions make a non-zero angle with one another in the plane of the array. The beam deflection control module is adapted to control the deflection of the charged particle beam along a measurement path which crosses sequentially at least two of the elongate, electrically conductive elements of at least one of the probes. The detection module is connected to the electrically conductive elements of each of the plurality of probes, and is adapted to detect electric signals output sequentially by the electrically conductive elements of each probe upon intersection of the charged particle beam therewith. The detected electric signals from each probe are indicative of properties of the charged particle beam when directed to the location of the respective probe across the probe assembly by the charged particle beam generator. A corresponding method of measuring properties of a charged particle beam output by a charged particle beam generator is also disclosed.

Description

SYSTEM AND METHOD FOR MEASURING PROPERTIES

OF A CHARGED PARTICLE BEAM

The present invention relates to systems and methods for measuring properties of charged particle beams, including electron beams and ion beams, such as their width and/or intensity profile.

Charged particle beams have many industrial applications, including materials processing tasks such as welding, additive layer manufacturing, 3-D printing and drilling, cutting, curing, melting, evaporation or other treatments whereby a material or workpiece is modified or treated using the beam. Characteristics of the beam, such as its dimensions and intensity, will affect the results of the process and it is therefore useful to be able to measure such properties of the beam in order to check whether the beam meets the desired criteria for the process and, if not, to enable adjustment of the beam properties accordingly.

Various probe devices for measuring properties of charged particle beams are known. In a first category, a probe comprises a masking plate having a plurality of non-parallel slits therethrough, across which the beam is deflected. The portion of the beam which passes through each slit is collected by a Faraday cup and the resulting electric signals can be used to reconstruct an intensity map of the beam. An example of this approach is disclosed in US-B-7348568. US-B- 7,875,860, meanwhile, uses a plurality of Faraday cups, each of which receives a portion of a scattered beam, in place of slits. A similar technique disclosed in "Electron beam diagnostics: a new release of the Diabeam system", Dilthey et al, Vacuum Volume 62, Issues 2-3, 15 June 2001, Pages 77-85, makes use of a pinhole in a plate over a Faraday cup, over which the beam is rastered. A variant of this approach is described in US-B-6977382, in which the beam is rastered over a pin head and the signal reflected from it is collected by backscattered electron detectors.

Another form of probe comprises one or more wires which are arranged to intersect the beam during relative movement between the beam and the wire(s). Examples are disclosed in GB-B-1209034. Part of the beam current is collected on the wire(s) and the resulting electric signals output by the wire(s) are detected. The pulses derived from the probe represent an approximate energy density profile of the beam at the point(s) of intersection.

All of the known probe devices, including those described above, are designed to measure the properties of the beam along, or very close to, the axis of the beam generator (e.g. the axis of an electron gun column). Electron beam processing machines, for example, historically have been used to weld and melt material with the beam at most only slightly deflected by typically a few millimetres from the main axis. For example, typical beam deflections in traditional processing applications are of less than 1 degree and over less than 10mm in width. In accordance with a first aspect of the present invention, a system for measuring properties of a charged particle beam output by a charged particle beam generator comprises:

a probe assembly comprising a plurality of probes arrayed across a plane on a mount, each probe comprising at least two elongate, electrically conductive elements arranged such that their respective elongate directions make a nonzero angle with one another in the plane of the array;

a beam deflection control module adapted to control the deflection of the charged particle beam along a measurement path which crosses sequentially at least two of the elongate, electrically conductive elements of at least one of the probes; and

a detection module connected to the electrically conductive elements of each of the plurality of probes, adapted to detect electric signals output sequentially by the electrically conductive elements of each probe upon intersection of the charged particle beam therewith, the detected electric signals from each probe being indicative of properties of the charged particle beam when directed to the location of the respective probe across the probe assembly by the charged particle beam generator. By providing an array of multiple probes in this way, the properties of the beam can be measured over a wide range of beam deflection angles (corresponding to the extent of the array) and not solely at or near its on-axis position. Each probe enables the measurement of the beam's properties (e.g. width and intensity) as exhibited when it is deflected to the location of that probe, thereby enabling any changes in the properties of the beam arising upon deflection to an off-axis position to be detected and (if desired) corrected for, such that the beam properties remain substantially uniform over a range of deflection angles. This is of significant importance for example in emerging processes such as electron beam additive layer manufacturing or 3D printing and electron beam surface texturing, which deflect the beam over wide areas and require the beam quality to be maintained even when the beam is deflected several hundreds of millimetres from the electron gun column axis. For instance, the beam may be required to be swept over angles of up to 25 degrees across workpieces of up to 400mm width.

The array of probes preferably extends across a working area over which the beam will be moved during operation of the device in which it is incorporated. The array therefore allows measurement of the beam properties at each of various positions (corresponding to the discrete locations of the probes) over the working area, and thus can be used to provide assurance that the beam properties are consistent over the working area, and/or to provide feedback so that adjustment of beam optical parameters can be made to ensure this. This type of beam assurance is required, for example, by aerospace manufacturers, where there are stringent quality controls. The probe array allows measurement of the beam properties over the entire working area without requiring any manual adjustment of probe parts, so in the case of a vacuum chamber the beam characterisation can be carried out quickly and within a single pump down cycle of the machine if desired. Moreover, there are no moving parts, with the result that the system is robust and reliable.

By forming each probe of electrically conductive elements and detecting the electric signals output by each element as it is crossed by the beam, each probe is substantially insensitive to the landing angle of the beam. This is important since, as noted above, the angle at which the beam strikes each probe will vary significantly, between approximately zero degrees for a probe adjacent the main axis, up to e.g. 25 degrees for a probe at the periphery of the array. Most conventional probe types, in contrast, are only suitable for use in measuring the properties of a beam which strikes the probe substantially along its normal (i.e. at approximately zero degrees). For example, probe devices based on slits or a pin-hole require the slits or hole to be narrow and formed through a thick substrate, in order to achieve good resolution without the substrate melting. As such, if the beam strikes the probe at an angle significantly away from zero, the slit or hole walls will obstruct the passage of the beam therethrough and prevent the detection of any signal. An electrically conductive element, however, will output a signal irrespective of the angle at which it is intersected by the beam, even if this is significantly away from zero. Each of the disclosed probes is therefore fully operative at any position across the array, enabling consistent measurement of the beam properties at each probe location. Moreover, all of the probes can be identical to one another and can be interchanged or moved from one location to another within the array without detriment.

By providing each probe with at least two elongate, electrically conductive elements arranged such that their respective elongate directions make a nonzero angle with one another in the plane of the array (which will preferably be substantially perpendicular to the axis of the beam generator, e.g. the electron gun column) - i.e. such that the elements are non-parallel - the properties of the beam can be measured in at least two directions at each probe location, thereby enabling for example a measure of the beam size and shape (e.g. ovality) at each probe. For example, given knowledge of the beam speed and angle at which the measurement path crosses each element, the width of the beam in the direction perpendicular to the element's elongate direction can be calculated from the duration of the electric signal output by the element. Thus, the at least two directions in which the properties are measured at each probe will be determined by the elongate directions of the elements and whilst these could differ from one probe to the next, preferably the elements of all the probes making up the array are arranged along the same at least two directions so that the beam properties are measured in the same at least two directions at each probe location. Further preferred features of the individual probes will be discussed below.

The detected signals could be utilised without knowledge of which element or probe is the source of each signal, e.g. by checking that each signal meets one or more predetermined criteria, in which case it can be concluded that the properties of the beam are uniform to an acceptable degree at each of the probe positions crossed by the beam. However, more information can be obtained if the signals are each attributed to the probe, and preferably the particular element within the probe, from which it originates, since the properties of the beam can then be correlated with its deflection position (i.e. the probe location) and against the absolute directions of the elements. Preferably, therefore, the detection module is further adapted to identify which elongate, electrically conductive element of which probe is the source of each detected electric signal. This could be achieved, for example, by configuring the detection module with multiple channels, one for each element, such that the incoming signals are automatically distinguished from one another. In this case, the elements themselves would need to be electrically isolated from one another to avoid cross-talk.

However, in more preferred embodiments, for each probe, the elongate, electrically conductive elements are in electrical contact with one another. For example, the elements may make physical contact with one another, or be connected by another conductive component, or could even be integral with one another. For example, the at least two elongate elements could be different portions of a single conductive wire or other filament, manipulated such that each portion is aligned along a different direction, e.g. by winding the wire around a suitable holder. Placing the elements of each probe in electrical contact with one another simplifies the construction of the probe assembly since only one electrical connection need be made to each probe. The signals from each probe could then be received on different channels by the detection module. More preferably, however, the elongate, electrically conductive elements of each probe are connected in parallel or in series to those of the other probes and the detection module is adapted to receive the electric signals output by the electrically conductive elements of the probe assembly on a single channel. This is particularly beneficial since only a single feed-though between the detection module and the probe assembly is required and this significantly simplifies the deployment of the system, particularly when used in a vacuum chamber (as will often be the case), where each conduit passing between the interior and exterior of the chamber must be carefully engineered to maintain the vacuum.

Where the detection module receives signals from more than one probe or element on a single channel, the detection module preferably comprises a correlation section adapted to identify which electrically conductive element of which probe is the source of each detected electric signal, based on knowledge of the measurement path (which is preferably pre-determined) and either the time at which each electric signal is detected and/or the sequence in which the electric signals are detected. For example, if the path followed by the beam across the elements is known, since the signals will be received in the same order as that in which the elements are intersected by the beam, each signal can be attributed to an element by virtue of the order in which it is detected or by a time stamp corresponding to each signal. The knowledge of the measurement path can be obtained in a number of ways. In one example, the system could rely on stored information as to the path which the beam should follow and hence, preferably, a memory is provided, having stored measurement path data, the stored measurement path data preferably including: the starting location of the beam, the path to be followed by the beam, and optionally the speed of the beam at least at the points of intersection with the electrically conductive elements. However for greater accuracy it is preferable to base the identification on the real-time deflection of the beam and hence, advantageously, the knowledge of the measurement path is provided by a beam location monitoring section which is adapted to receive information as to the present deflection of the beam from the charged particle beam generator and/or the beam deflection control module. In some cases, a combination of the two approaches may be appropriate. Deflecting the beam along the (preferably pre-determined) measurement path could be achieved in a number of ways under the control of the beam deflection control module. Typically, the charged particle beam generator (e.g. an electron gun) will comprise a primary beam deflection unit and a primary controller adapted to generate and output a primary control signal to the primary beam deflection unit for directing the beam to a selected location or along a primary beam path, which during normal operation will act to move the beam along the intended material processing path, e.g. a weld line or the like. In some cases the beam deflection control module of the presently disclosed system could be implemented by the primary controller of the charged particle beam generator, programmed as appropriate to move the beam along the desired measurement path. However, this approach is not preferred since many beam generators incorporate automatic beam adjustments for distortion such as astigmatism, which are angle-dependent, and hence if the primary controller is programmed to deflect the beam along the measurement path, the beam will undergo automatic adjustments meaning that its properties measured at each element will be different and not precisely representative of the beam when directed to the probe location. In addition such controllers typically define a beam path digitally, i.e. as a series of discrete locations which will form a continuous line if the beam is moved between them sufficiently slowly. However at the high speeds at which the beam is preferably moved across the probe elements in order to avoid melting of the elements, the digital nature of the path may result in discontinuities in the locations irradiated by the beam which could result in variation in the speed of the beam as it passes over the probe elements, which would cause a distortion of the probe signal. Although this could be corrected by post-processing of the signal, this is undesirable as it complicates the signal processing and interpretation.

Therefore, in one preferred implementation, the beam deflection control module comprises a secondary controller configured to output a secondary control signal which is superimposed on the primary control signal output by the primary controller, the secondary control signal defining a secondary beam path, such that the primary beam deflection unit directs the beam along the measurement path which is a combination of the secondary beam path with any primary beam path. In this way, the primary controller is "unaware" of the additional deflection imposed on the beam by the secondary control signal, with the result that no automatic adjustment of the beam due to the additional deflection is performed, such that the beam properties remain substantially the same as the beam is moved across the elements of any one probe, and the measured properties accurately represent those of the beam when directed to the probe location. Further, the secondary control signal is preferably analogue and therefore does not suffer from the aforementioned problems associated with digital control. Alternatively, the beam deflection control module may comprise a secondary beam deflection unit disposed downstream of the primary beam deflection unit and a secondary controller configured to output a secondary control signal to the secondary beam deflection unit, the secondary control signal defining a secondary beam path, such that the primary and secondary beam deflection units in combination direct the beam along the measurement path which is a combination of the secondary beam path with any primary beam path. Again, in this way the additional deflection is imposed on the beam without affecting any adjustment performed by the beam generator. This implementation is particularly well suited to retrofitting of the system to an existing charged particle beam generator. An example of a device comprising the probe assembly and secondary beam deflection unit which can readily be installed in such machines without requiring modification of the machine is described below.

It should be noted that in some cases, the primary and/or secondary (if provided) beam deflection units may not achieve precisely the deflection instructed by the control signal(s) - i.e. there may be a deflection discrepancy - in which case the beam may not follow exactly the desired measurement path (at least initially). Techniques for taking account of this are discussed below. During the measurement process, typically the coarse movement of the beam from one probe position to the next will be performed by the beam generator, e.g. defined by the primary beam path described above. The beam could be moved continuously from one probe to the next (at constant or varying speed), or could be directed towards discrete positions, one corresponding to each probe, with the beam switched off at intermediate positions. The fine movement of the beam by the beam deflection control module causes the beam to deviate from the primary path to result in the desired measurement path. This deviation could be applied continuously, or may be synchronised to the primary path based on knowledge of the beam speed and timings, but most preferably, the beam deflection control module comprises a monitoring section adapted to receive information as to the present deflection of the beam from the charged particle beam generator, preferably the primary control signal from the primary controller, and to output the secondary control signal in response to the received information. This enables the beam to be deviated from its primary path at selected points along the path, in order to form the measurement path.

Preferably, the beam deflection control module is adapted to output the secondary control signal in response to the beam deflection reaching one or more pre-defined trigger positions, each pre-defined trigger position preferably corresponding to the location of a different one of the probes. For example, the secondary control signal could define a substantially circular or arcuate path which is superimposed on the primary path at the selected trigger positions, causing the beam to cross the at least two elongate electrically conductive elements of each of a plurality of the probes corresponding to the trigger positions. As noted above the primary control signal may be configured to hold the beam stationary at each trigger position in sequence, such that upon reaching each position, the beam is moved under the control of the secondary control signal only to define the measurement path.

Preferably, the measurement path followed by the beam in within each probe remains close to the centre of the probe, e.g. to within 2 to 5 mm, so that the deflection from the probe centre in order to follow the measurement path is small. As such the measured properties of the beam at the measurement path can be taken to be closely representative of the beam at the centre of the probe.

However the beam deflection is implemented, the measurement path preferably comprises at least one segment crossing at least two, preferably all, of the elongate, electrically conductive elements of one of the probes, which segment is substantially circular or follows a substantially circular arc. For example the circle or arc may preferably be centred on the probe centre and may advantageously have a small radius of e.g. 2 to 4 mm in order that the deflection of the beam away from the probe centre is small. Preferably the circular or arcuate segments are deflection paths defined by the secondary control signal output by the secondary controller, and most preferably are output having a constant beam speed. Circular or arcuate paths are preferred since, due to the nature of the deflection means which rely upon analogue, waveform inputs, a circular or arcuate control signal can be more accurately generated than other path forms which involve straight-line sections or corners. A constant beam speed is preferred in order that the detected electric signals from each element can be directly compared against one another. However, if necessary, variations in beam speed can be corrected for during post-processing. Circular or arcuate paths are also particularly useful for detecting any deflection discrepancy in the beam, as discussed below.

For landing angles well away from normal, the actual path traversed by the beam also may not correspond to the intended path since the plane of the probe array will effectively pass through the beam path at an angle (irrespective of any deflection discrepancy). For example, where the beam is instructed by the controller(s) to process about a circle, although the processional angular velocity of the beam will be constant, at high angles (i.e. towards the periphery of the probe array), the beam will trace out an ellipse rather than a circle on the probe assembly. As such, the speed of the beam will vary as it proceeds along the path, which will lead to different durations of the detected signals from each element, even if the beam is perfectly symmetric. To avoid this, the secondary controller is preferably adapted to modify the secondary control signal in dependence on the deflection angle of the beam, preferably by applying a correcting elliptical deflection for each probe location such that the path followed by the beam on the probe assembly is substantially circular or arcuate (and hence the beam speed remains constant) at each probe of the array.

The measurement path could intersect the elements of a single probe if the properties of the beam are only to be measured at one location. However, in order to determine the beam properties at a variety of positions it is preferred that the measurement path comprises a plurality of segments, each segment crossing at least two, preferably all, of the elongate, electrically conductive elements of one of the probes, each segment intersecting a different one of the probes. Most preferably, the measurement path is configured to intersect all of the probes in sequence. As previously noted, the measurement path could be discontinuous (i.e. the beam switched off) between the segments. Advantageously, the start and/or end points of the or each segment are not coincident with any of the elongate, electrically conductive elements. This is beneficial since it reduces the risk of the beam dwelling on one of the elements, either during low speed movement or whilst stationary, which could melt the element.

The identification of the element from which each signal originates could be performed based entirely on knowledge of the measurement path as previously described. However in an advantageous modification, each of the probes further comprises a marker element which is electrically conductive and of greater width than that of the elongate electrically conductive elements in the direction crossed by the measurement path, the measurement path additionally crossing the marker element, whereby an electric signal output by each marker element upon intersection of the charged particle beam therewith has a greater peak amplitude than those output by the elongate elements, and wherein the detection module is adapted to identify which elongate, electrically conductive element within a probe is the source of each detected electric signal based at least in part on the electric signal output by the marker element. Since the positions of the elongate, electrically conductive elements in the probe relative to the marker element will be known, this assists in identifying the signals: for example, the first signal received after the high-amplitude marker signal can be attributed to the element adjacent the marker element along the measurement path, and so on. This is particularly useful where the measurement path is formed by imposing a secondary control signal on a primary control signal applied by the beam generator (as discussed above) since if the secondary control signal is cyclic, its phase may not be the same at each probe location, meaning that the elements of one probe may not be crossed by the beam in the same order as those in a different probe.

The provision of a marker signal in this way enables the detection module to monitor only the present deflection of the beam according to the beam generator (e.g. from the primary controller), which will enable the module to attribute each signal to the probe from which it originates, and then the marker signal enables attribution of each signal to the respective individual element within the identified probe. Thus, in this case the detection module is adapted to identify the probe which is the source of each detected electric signal based on knowledge of the measurement path and either the time at which each electric signal is detected and/or the or sequence in which the electric signals are detected, and to identify the elongate electrically conductive element of the identified probe which is the source of each detected electric signal based at least in part on the electric signal output by the marker element.

Each individual probe could be of various constructions. In particularly preferred embodiments, the at least two elongate, electrically conductive elements of each probe comprise at least first and second elongate electrically conductive elements arranged such that their respective elongate directions are substantially orthogonal to one another in the plane of the array. By collecting signals from orthogonal elements, the width of the beam in two orthogonal directions can be measured and/or compared, thereby providing an indication of beam spot shape, e.g. ovality, or to detect the intensity profile of the beam in each of the two directions. Most preferably, the two directions are aligned with the x and y axes of the beam generator. Two directions is also the minimum required in order to build up a two-dimensional intensity map of the beam; however better resolution will be achieved by providing each probe with more elements, aligned along different directions. Hence, preferably, each probe comprises at least three, more preferably at least four, elongate, electrically conductive elements arranged such that their respective elongate directions make non-zero angles with one another in the plane of the array, the respective different elongate directions preferably being substantially equally angularly spaced from one another. For example, where there are three elements they may be angled at 60 degrees to one another and where there are four elements they may be angled at 45 degrees to one another. Many more elements could potentially be provided in each probe, each with a different orientation, in order to further increase the resolution.

Advantageously, the at least two elongate, electrically conductive elements of each probe have at least one intersection between them (e.g. where they cross one another or are joined to one another), at which the elements are preferably in electrical contact. For example, all of the elements of any one probe may intersect one another in the centre or at another position of the probe. This can provide additional physical support for each element. In a particularly preferred embodiment, the at least two elongate, electrically conductive elements of any one probe are arranged such that their elongate directions extend radially from an intersection between them and are preferably equally angularly spaced from one another about the intersection. In combination with a measurement path which is designed to follow a circular or arcuate path which crosses the elements and which is centred on the intersection, this enables any deflection discrepancy is the beam to be detected and corrected for as discussed below. In a particularly preferred embodiment, the at least two elongate, electrically conductive elements of each probe comprise at least two conductive filaments which are crossed with one another at a non-zero angle, the conductive filaments optionally being different portions of one continuous conductive filament. This provides a particularly straightforward construction and is also robust. The conductive filaments could comprise wires, ribbons or strips for example.

Advantageously, the elongate, electrically conductive elements have a width in the plane of the array equal to or greater than their thickness in the direction perpendicular to the array. This minimises the sensitivity of the probes to the incident angle of the beam, since any particles received by the side faces of the elements is minimal compared by that received by the top surface (facing the beam source). For instance, the elements could be of substantially square or circular cross-section, or rectangular if the width is greater than the thickness. (The width of an element defined as the dimension perpendicular to the elongate direction, in the array plane). Strip elements with a substantially flat surface are particularly preferred since this will reduce the degree of backscattering compared to circular wires, for example.

The width of the elongate, electrically conductive elements will have an impact on the precision with which the beam properties can be measured. If the width of each element is equal to or greater than the width of the beam, it will not be possible to deduce any information as to the intensity profile of the beam, although the width of the beam would still be measurable. Therefore, it is preferable that the width of each elongate, electrically conductive element in the plane of the array is less than the diameter of the charged particle beam at the array plane, preferably between one third and two thirds the diameter of the charged particle beam. The narrower the element, the higher the resolution with which the beam profile can be measured. However, in practice if the element is too narrow, it will be liable to failure by overheating upon irradiation by the beam. Typical beam widths at the plane in which processing is to take place (and in which the probe assembly should be disposed) are of the order of 0.5mm and hence in preferred examples the elements each have a width in the range 5 to 200 microns. The width of all the elements need not be the same but this preferred to avoid the need to take account of this in the signal processing.

In order that the electric signals from each element are received discretely and can be distinguished from one another, the beam should intersect only one element at any one time as it moves along the measurement path. Hence, preferably, the at least two elongate, electrically conductive elements are spaced from one another, at least along a portion of their lengths, by a distance at least equal to, preferably greater than, the diameter of the charged particle beam at the array plane. The length of this portion should also be at least equal to or greater than the beam diameter so that the full width of the beam can be accommodated without intersecting any other elements. Hence in preferred cases, the at least two elongate, electrically conductive elements are spaced from one another, at least along a portion of their lengths, by at least 0.5mm, more preferably at least 1 mm. In a particularly preferred implementation, each probe comprises a frame configured to support the elongate, electrically conductive elements at at least two points across an aperture defined by the frame. The frame may or may not fully enclose the aperture. By providing support to the elements in this way, the use of narrower, less robust elements is enabled since the elements are not required to be self-supporting. In one advantageous embodiment, the frame comprises a tube section, e.g. a bobbin, preferably a cylindrical tube section, the elongate, electrically conductive elements being arranged across one or both (flat) ends of the tube section. The conductive component(s) can be wound around the frame to form the desired arrangement of elements with different elongate directions.

Preferably, the frame comprises an electrically conductive portion configured to connect the at least two elongate, electrically conductive elements to a circuit connected to the detection module. This provides a straightforward and reliable means for achieving an electrical connection between all of the elements in each probe, and for connecting them to the detection module. The probes could be arranged according to any desired layout across the array but in general it is preferred that the plurality of probes are arranged according to a regular grid (e.g. orthogonal, hexagonal etc.) across the array, and preferably include one probe on the axis of the charged particle beam generator, so that the beam properties can be measured at a series of locations spaced regularly across the working area. However in some cases where the machine in question is dedicated for use in a particular task it may be desirable to arrange the probes according to some other scheme in order to measure the beam properties at locations of particular importance to the task. Preferably, the mount comprises a plate having a plurality of apertures in which the plurality of probes are disposed, the plate preferably being conductive and optionally configured to provide an electrical connection between the probes and a circuit connected to the detection module. The array of probes preferably lies in a plane substantially perpendicular to the axis of the charged particle beam generator.

The detected signals are indicative of properties of the beam and therefore could be utilised "raw", by comparison with voltage, current or time thresholds or the like. However, preferably the system further comprises a signal processor adapted to calculate properties of the beam based on the detected electrical signals and knowledge of the beam measurement path. The calculated properties may be output to a user of the system, e.g. by display on a monitor or the like, or otherwise utilised.

In particularly preferred examples, the signal processor is adapted to calculate the width of the beam in at least two directions, at at least one probe location, based on the detected electric signals from the at least two elongate electrically conductive elements of the respective probe, and knowledge of the speed of the beam across the elements. For example, the total duration of each signal will correspond to the time taken by the beam to cross the respective element and given knowledge of the beam speed this can be converted to the distance travelled by the beam in that time. This will correspond to the beam width plus the element width (in the direction of the measurement path), which is known thereby permitting calculation of the beam width in the direction perpendicular to the elongate direction of the element.

Also preferably, the signal processor may be adapted to calculate the intensity profile of the beam in at least two directions, at at least one probe location, based on the detected electric signals from the at least two elongate electrically conductive elements of the respective probe, and knowledge of the width of the beam. For example, as the beam crosses an element, the detected signal will vary in magnitude depending on the width of the beam in the elongate direction of the element and on any beam intensity variations. Since the width of the beam can be measured separately as discussed above, the intensity profile of the beam in the direction perpendicular to the elongate direction of the element can then be calculated. In another preferred embodiment, the signal processor is adapted to calculate an intensity map of the beam in two dimensions, at at least one probe location, by tomographic reconstruction of the detected electric signals from the at least two elongate electrically conductive elements of the respective probe. This can be achieved using known reconstruction methods such as using an inverse Radon function to de-convolute the collected signals from all the elements within one probe. The more elements that are provided in each probe, the better the resolution that can be achieved. Preferably, the signal processor is adapted to calculate properties of the beam in at least two directions at each of a plurality of the probe locations, preferably the width of the beam, the intensity profile of the beam and/or the intensity map of the beam. As mentioned above, in practice the beam may suffer a deflection discrepancy, meaning that the actual location to which the beam is directed by the deflection unit(s) does not precisely match that instructed. If this is not identified, the measured properties may be distorted due for example to the beam crossing each element at some angle different from that expected, or even (in an extreme case), in some different sequence. Therefore, in a preferred embodiment, the the signal processor is further adapted to (before calculating the properties of the beam), detect any deflection discrepancy of the beam, at at least one probe location, based on the spacing between adjacent detected electric signals from the at least two elongate electrically conductive elements of the respective probe, and to generate and output a deflection discrepancy feedback signal to the charged particle beam generator to thereby correct the deflection discrepancy. The expected (temporal) spacing between each signal will preferably be known based on the pre-determined measurement path and the probe geometry, at least in terms of relative spacing between adjacent pairs of signals (if the beam speed is not accurately known), or in terms of absolute spacing (if the beam speed is known). The signal processor can therefore compare the actual spacing measured between one signal and the next, and compare this against the expected spacing. If these match, the beam is known to be following the expected measurement path and the property measurements can be made as described above. If the actual spacing (either absolute or relative) between signals does not match that expected, this is indicative of a deflection discrepancy. The signal processor outputs a signal to the charged particle beam generator to enable correction of the beam deflection such that the measurement path of the beam is displaced so that it is aligned with the probe. In a particularly preferred embodiment, the signal generator performs this calibration by: (a) detecting any deflection discrepancy of the beam, at at least one probe location, based on the spacing between adjacent detected electric signals from the at least two elongate electrically conductive elements of the respective probe; (b) generating and outputting a deflection discrepancy feedback signal to the charged particle beam generator to thereby correct any deflection discrepancy of the beam calculated in step (a), after which the properties of the beam can be calculated based on the detected electrical signals output after correction of any deflection discrepancy.

In an especially preferred case this technique is implemented using a probe construction having at least two elongate, electrically conductive elements arranged such that their elongate directions extend radially from an intersection between them and are equally angularly spaced from one another about the intersection. The measurement path is configured to comprise at least one segment crossing at least two, preferably all, of the elongate, electrically conductive elements of the probe, which segment is substantially circular or follows a substantially circular arc and is centred on the location of the intersection between the elements from which the elongate directions extend. As a result, if the beam follows the measurement path accurately, the temporal spacing between the detected signals from the elements of the probe should be equal. As such, any difference in spacing between adjacent pairs of detected electric signals from the elements of the probe is indicative of a deflection discrepancy. For example, the path may be incorrectly centred or may not be sufficiently circular, which if uncorrected would result in the beam intersecting the elements at unknown angles. In this case, preferably, the deflection discrepancy is corrected by adjusting the deflection of the beam until the difference in spacing between adjacent pairs of detected electric signals from the elements of the probe is substantially zero. The above technique enables correction of any deflection discrepancy and hence calibration of the beam generator and can preferably be performed automatically by the system without operator intervention. For example, a feedback algorithm can be implemented to adjust the deflection control signals based on the feedback signal in order to correct for any discrepancy detected.

The outputs from the system could be used to provide the user with diagnostic information or assurance, but in preferred embodiments the system additionally or alternatively further comprises a feedback module for generating and supplying a feedback signal to the charged particle beam generator to thereby adjust the properties of the charged particle beam based on the properties measured by the system. For example, feedback based on the measured width (diameter) or shape of the beam may be used to adjust the beam focus achieved by the beam generator. Again, this can be performed automatically by the system through the provision of appropriate feedback algorithms.

It should be noted that the system according to the first aspect of the invention need not include the charged particle beam generator. For example, the system can be retrofitted to an existing beam generator such as an electron gun. According to a second aspect of the invention, an apparatus is provided comprising a charged particle beam generator and a system in accordance with the first aspect of the invention, configured to measure properties of the charged particle beam output by the charged particle beam generator. The charged particle beam generator preferably comprises a charged particle source, a particle accelerating unit for accelerating charged particles from the source along an axis to form a charged particle beam, a primary deflection unit adapted to deflect the charged particle beam away from the axis and a primary controller adapted to output a primary control signal for control of the deflection applied by the primary deflection unit.

The diagnostic system disclosed herein is particularly well adapted for use in materials processing applications where a workpiece or a material (including gaseous material) is modified by application of the charged particle beam. Hence, a third aspect of the present invention further provides a materials processing tool comprising an apparatus as described above. In particularly preferred embodiments, the tool is one of:

an electron beam welding tool, the charged particle beam being adapted for the welding of materials;

an additive layer manufacturing tool, the charged particle beam being adapted for treatment of a powder material, preferably the fusion thereof;

a curing tool, the charged particle beam being adapted for curing of a workpiece;

a cutting tool, the charged particle beam being adapted for cutting of materials;

a melting or evaporation tool, the charged particle beam being adapted for melting and/or evaporation of materials;

the tool is a gas treatment tool, the charged particle beam being adapted for the treatment of gaseous substances, preferably combustion fumes;

a sterilisation tool, the charged particle beam being adapted for the sterilisation of solids or liquids;

a drilling tool, the charged particle beam being adapted for the drilling of a workpiece; or

a materials texturing tool, the charged particle beam being adapted for the forming of protrusions or structures on a workpiece.

A fourth aspect of the present invention provides a method of measuring properties of a charged particle beam output by a charged particle beam generator, the method comprising:

providing a probe assembly comprising a plurality of probes arrayed across a plane on a mount, each probe comprising at least two elongate, electrically conductive elements arranged such that their respective elongate directions make a non-zero angle with one another in the plane of the array; controlling the deflection of the charged particle beam to follow a measurement path which crosses sequentially at least two of the elongate, electrically conductive elements of at least one of the probes; and

detecting electric signals output sequentially by the electrically conductive elements of each probe upon intersection of the charged particle beam therewith, the detected electric signals from each probe being indicative of properties of the charged particle beam when directed to the location of the respective probe. As discussed above in relation to the first aspect of the invention, such a technique enables the properties of the beam such as its width and intensity profile to be measured at each of the probe locations. The method can be implemented by the system of the first aspect of the invention and may include preferred features and steps corresponding to any of the features of the system discussed above.

In accordance with a fifth aspect of the invention, a probe for measuring properties of a charged particle beam is provided, the probe comprising a frame defining an aperture and at least two elongate, electrically conductive elements supported by the frame and crossing the aperture, arranged such that their respective elongate directions make a non-zero angle with one another in the plane in which the frame lies.

The system of the first aspect of the invention can be implemented with a plurality of probes of this sort in a particularly preferred embodiment. As previously mentioned, by arranging the elements on a frame across an aperture, the elements are supported (preferably at at least two points), allowing the use of thinner and less robust elements. This in turn improves the resolution of the measurements. The at least two elongate, electrically conductive elements are preferably in electrical contact with one another and optionally are integral with one another. The elements could cross the aperture on different sides of the frame but most preferably lie in the same plane as one another.

The probe can have any of the preferred features discussed above with respect to the first aspect of the invention.

In accordance with a sixth aspect of the invention, a device for measuring properties of a charged particle beam output by a charged particle beam generator is provided, comprising: a probe assembly comprising a plurality of probes arrayed across a plane on a mount, each probe comprising at least two elongate, electrically conductive elements arranged such that their respective elongate directions make a non-zero angle with one another in the plane of the array; a beam deflection unit adapted to deflect the charged particle beam along a measurement path; wherein the beam deflection unit is supported on the probe assembly by a support assembly disposed therebetween, the beam deflection unit being orientated such that in use the charged particle beam can be deflected across the probe assembly by the beam deflection unit. In this way, the probe assembly and secondary deflection unit can be installed straightforwardly into a machine having a charged particle beam generator, without needing to modify the machine. The beam deflection unit is connected in use to a controller such as the beam deflection control module as described above, and receives inputs such as the secondary beam control signal already discussed in order to control the beam deflection. The probe assembly is connected in use to a detection module such as that previously described, which is adapted to detect the electric signals output by the probes. Preferably, the probe assembly, beam deflection unit and support assembly are supplied as a unit such that installation requires only placing the unit into the machine in the correct alignment, and making the appropriate signal corrections. In other cases the device could be supplied as a kit comprising the probe assembly, deflection coil and support assembly, and put together in situ.

The device could be used in the system of the first aspect of the invention to provide the probe assembly and to implement the beam deflection (at least partially). The probe assembly could have any of the features discussed with respect to the first aspect of the invention and the probes could be constructed to include any of the features mentioned above. The beam deflection unit may comprise a beam deflection coil.

Preferably, the support assembly comprises one or more support arms connected between the probe assembly and the beam deflection unit. Advantageously, the arrangement of the components is such that the axis of the beam deflection unit is substantially perpendicular to the plane of the probe assembly.

Examples of systems and methods for measuring the properties of a charged particle beam in accordance with the present invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 schematically depicts an exemplary materials processing tool including an apparatus which comprises an exemplary charged particle beam generator and a system in accordance with a first embodiment of the invention;

Figure 2 shows an exemplary probe assembly suitable for use in the system shown in Figure 1 , and schematically illustrates a circuit suitable for collecting electric signals therefrom;

Figure 3 illustrates an exemplary probe suitable for use in the probe assembly shown in Figure 2;

Figure 4 is a schematic diagram illustrating electrical connections between probe elements of a portion of the probe assembly shown in Figure 2, according to one embodiment;

Figure 5 schematically shows a portion of the probe assembly of Figure 2, on which is depicted an exemplary measurement path of the beam;

Figure 6 shows a portion of an exemplary elongate conductive element suitable for use in the probe assembly shown in Figure 5, and the positions of the beam as it crosses the element in one embodiment;

Figure 7 is a plot showing an exemplary electric signal as may be detected from one of the elongate conductive elements of Figure 5;

Figure 8(a) is a schematic plot showing an exemplary series of electric signals as may be detected from the probe assembly of Figure 5 in one embodiment, Figures 8(b) and (c) illustrating corresponding beam shapes at two different probe locations;

Figure 9 (a) schematically illustrates the passage of a beam across an exemplary elongate conductive element in another embodiment, Figure 9(b) showing a plot of an exemplary electric signal output by the element in response to intersection by the beam; Figure 10 schematically depicts an exemplary primary beam path (Figure 10(a)), secondary beam path (Figure 10(b)) and resulting measurement path (Figure 10c), in one embodiment;

Figure 1 1 schematically depicts an exemplary primary beam path (Figure 1 1 (a)), secondary beam path (Figure 1 1 (b)) and resulting measurement path (Figure 1 1c), in another embodiment;

Figure 12 shows selected components of an apparatus in accordance with a second embodiment of the invention;

Figure 13 schematically depicts an exemplary materials processing tool including an apparatus which comprises an exemplary charged particle beam generator and a system in accordance with a third embodiment of the invention; Figure 13(a) schematically depicts a device in accordance with another embodiment of the invention;

Figure 14(a) illustrates two probes forming part of a probe assembly forming part of a system in accordance with a fourth embodiment of the invention, on which is depicted an exemplary measurement path of the beam, Figure 14(b) showing a corresponding plot of exemplary electric signals that may be detected;

Figure 15(a) shows selected components of a probe in accordance with a further embodiment of the invention, Figure 15(b) showing additional components of the same probe;

Figures 16(a) and (b) schematically depict two further exemplary probes in accordance with embodiments of the invention, on which are depicted exemplary measurement paths of the beam;

Figure 17 shows another exemplary probe in accordance with an embodiment of the invention; and

Figure 18 is a plot showing an exemplary series of electric signals as may be detected from a probe such as that depicted in Figure 15.

The description below will largely focus on examples in which the charged particle beam whose properties are to be measured is an electron beam. However it will be appreciated that the disclosed systems and methods could equally be utilised in conjunction with beams of positive ions. The presently disclosed systems and methods can be used to diagnose beam properties in many different industrial applications, including for example electron beam welding, ion beam drilling, surface texturing of materials and the like. One particular application which makes use of particle beams deflected over large working envelopes at a variety of incidental angles is additive manufacturing. As such, the presently disclosed systems and methods are particularly well suited for use in additive manufacturing tools and the following description will utilise this example. However it will be appreciated that the systems and methods could be transferred to any other type of equipment which makes use of a charged particle beam.

One such form of additive manufacturing using particle beams is described in US-A-2012/0234671 and uses a high power beam, preferably an electron beam, to manufacture complex objects. Figure 1 hereto depicts an example of a materials processing tool 1 which here is an additive manufacturing tool of the sort disclosed in US-A-2012/0234671 , and which incorporates a beam diagnosis system 20 in accordance with a first embodiment of the present invention, as well as a charged particle beam generator 10, e.g. an electron gun. As described in US-A-2012/0234671 , the tool 1 uses a powder bed 5 carried on a work table 3 to manufacture metal objects in layers 5a that are melted by a deflected electron beam B. The equipment comprises a vacuum work chamber 2 onto which an electron gun 15 is mounted. The electron gun comprises a source 16 of charged particles such as a thermionic emitter (e.g. heated cathode) or a plasma source such as that disclosed in WO-A-2013/051296, an accelerator 17 such as an anode, and deflection means 18 such as magnetic deflection coils for controlling the direction of the generated beam B in accordance with instructions from a controller 1 1. Within the chamber 2 there is a powder bed 5, which can be lowered via pedestal 4 to allow production of each layer, and typically powder hoppers and a rake system (not shown) to spread powder for each layer.

The electron gun 15 typically has a wide angle deflection system 18 so that a powder bed 5 of up to 400mm diameter can be addressed at a working distance of some 500mm. The components that can be addressed are limited in size by the angle Θ of beam deflection away from the gun axis A that can be achieved without excessive beam spot distortion (which would affect the part build quality). For the accurate calibration of an electron beam 3D printing machine of this sort it is necessary to characterise the properties of the beam spot over the powder bed 5.

The system 20 enables measurement of the beam properties across the working area during a calibration process in order to achieve this aim. The system 20 comprises a probe assembly 21 which, in use, is mounted in the normal position F-F' of the powder bead of the machine. Suitable mounting points typically exist in the machine for mounting of a conventional calibration plate (i.e. a plate with an array of holes used to provide manual adjustment of the beam deflection to ensure it is accurate). The probe assembly 21 carries an array of probes spaced across the working area, each of which is configured to enable the measurement of beam properties at that location, as detailed below. Signals from the probe assembly 21 are output by a signal conduit 21 such as one or more cables to a detection module 22 which is adapted to capture signals output by the probes as the beam B is passed across each one. The beam is controlled to move along a measurement path by a beam deflection control module 23 which in this example is implemented by the deflection coils 18 of the beam generator 15, although alternative techniques are also envisaged as discussed below. The detected signals are indicative of properties of the beam B at each probe location. For example, the beam deflection may be triggered and a scope card collects the signal and the data is transferred to a signal processor or computer 24 for post processing and storage. Through synchronisation, the pulses collected can be correlated with X and Y measurements at each position across the powder bed where a probe element is mounted. The system 20 may also optionally include a correlation module 22a for attributing each detected signal to the probe (and optionally the element within the probe, as discussed below) from which it originated, and a signal processor configured to calculate parameters such as the beam width or intensity profile from the raw signals. The system 20 may also include a memory provided with information as to the measurement path followed by the beam during the calibration process and/or a monitoring module 26a for monitoring the real-time position of the beam based on feedback from the controller 1 1 of the beam generator and/or from the beam deflection control module 23. A feedback module 26b for generating and supplying feedback signals based on the measured properties to the beam generator may also be provided. Each of these optional components will be discussed below.

The operational principles of the probe assembly 21 will be explained with reference to an exemplary probe assembly shown in Figure 2. This comprises a mount 27 such as a plate, which carries a plurality of probes 30 arrayed across the mount, preferably in a regular grid arrangement. In Figure 2, only three of the probes 30a, 30b, 30c are labelled for clarity but it will be seen that a total of 16 probes are provided in this example, arranged in a 4 x 4 orthogonal grid. Any number (2 or more) of probes could be provided and most preferably will include one at the centre of the array (not shown in this example), which will be positioned on the gun column axis A in use. For example a 5x5 array may be particularly advantageous. The probes are typically spaced from one another across the plate 27 by between 1 and 10 cm - for example in this embodiment the spacing L_x along the machine's x-axis is about 5 cm and the spacing L_y along the machine's y-axis is about 5 cm. The mount may comprise for example a conductive (e.g. aluminium) plate with an aperture for each probe position. Signals from the probes are output on a line 28 and taken to ground G via a resistor 29a (e.g. a 50 Ohm resistor), the voltage across which is detected by a suitable meter 29b. The measured signals are then taken by a screened lead 21 a to a coaxial vacuum feed-through to allow capture outside the vacuum chamber at detection module 22.

An example of one of the probes 30 is shown in Figure 3. This comprises at least two elongate, electrically conductive elements 32 which are arranged such that their elongate directions make a non-zero angle with one another (i.e. are non-parallel) in the plane of the array - i.e. the plane perpendicular to gun column axis A, here the x-y plane. In this example, as is preferred, one element 33 is aligned along the machine's x-axis direction and the other element 34 is aligned along the machine's y-axis direction such that the two are substantially orthogonal. This is desirable so that the beam characteristics can be related directly to the machine directions, but is not essential. Each element 32 can be formed for example of a conductive filament such as a wire, ribbon or strip, supported if necessary by a frame 31 , e.g. a hollow tube section. For instance, in this embodiment the elements are formed of tungsten wires, crossed with one another at intersection 38. It should be noted that the elements 32 may or may not be in electrical contact with one another, depending on how the signals are to be collected and processed. In one example, the elements 32 could each be isolated from one another and their signals output to the detection module on separate channels. However it is preferred to utilise a single channel output in order to simplify deployment of the system and so the elements 32 can be electrically connected to one another, e.g. by contact at intersection 38 and/or by means of frame 31 which may be conductive or comprise at least a conductive portion or pathway. The signals from the elements 32 can then be connected to the detection module by a lead 39 connected to the frame or to any one of the elements 32.

The signals from each probe 30 could be collected by a separate dedicated input channel at the detection module 22 but as already indicated a single channel is preferred and so preferably all the probes are connected together, in series or in parallel, e.g. by means of suitable tracks or wiring provided in mounting plate 27 or by forming the plate 27 of a conductive material. A schematic diagram illustrating a suitable scheme for connecting an array of nine probes, each having two crossed elements 33, 34 as shown in Figure 3, is depicted in Figure 4. In this case, the probe elements of the array are all connected in parallel to single feed-through 21 which connects the probe assembly 21 to detection module 22. An electric signal is output by each element 32 when it receives charged particles from the beam B passing over it. Thus, the beam B is controlled by module 23 to follow a measurement path which crosses at least two of the elongate elements 32 provided in the probe 30. In some cases it may be needed to measure the properties of the beam at only one of the probe locations, in which case the measurement path can be restricted to a single probe, but more typically it will be desirable to measure the beam properties at several of the probe locations, preferably all of the probe locations. An example of a suitable measurement path MP is shown in Figure 5, superimposed on an enlarged portion of probe assembly 21 shown in Figure 2. Each of the probes 30 is of the sort shown in Figure 3, having two elongate elements 33, 34 aligned with the x- and y- axes of the machine. The measurement path starts with the beam directed towards probe 30a at the position marked B. The beam traverses a substantially circular arc segment of the path MP during which it crosses first the element 33a and then the element 34a, sequentially. The elements are arranged such that they are sufficiently spaced from one another (at least along their portions which will be intersected by the beam) that the beam will only strike one element at any one time. That is, the space S between adjacent elements along the measurement path is at least equal to and preferably greater than the beam width BW. Typical beam widths are of the order of 0.5 mm and so preferably the space S is at least 1 mm, more preferably at least 2mm. The dimensions of the probe are such that the path followed by the beam in order to cross the elements sequentially in this way does not require a large deflection away from the nominal probe location (e.g. the centre of the probe), so that the properties of the beam will be substantially the same at the positions at which it crosses each element within one probe. By making the probe small the beam is probed in close proximity to specific positions over the working area so that the probe results are representative of the beam at each central position of the probe on the working area. For example, the length l_x, l_y of the two wires forming the elongate elements in this case may be between 5 and 10mm, e.g. 8mm in a particular example, such that a path along a circular arc of radius around 2.5 mm will provide the necessary sequential signals. In preferred embodiments the radius of the measurement path followed within each probe is small, e.g. between 2 and 4mm, so that the corresponding angular displacement of the beam from the centre of the probe is also small. In the example shown, the path segment crosses each element 32 only once, but in other cases the path segment could be substantially circular in which case each element of each probe will be crossed twice, in this example. This can help improve the accuracy of the measured properties since an average of the two signals from each element can be used to compute the measurement. The beam could also be arranged to circuit each probe multiple times, if desired. The use of circular or arcuate measurement path segments for intersecting the elements of each probe is not essential but is preferred for reasons discussed below.

Upon completion of the arcuate segment, the beam continues along the measurement path MP to the next probe 30b. It should be noted that the beam may be switched off for this movement so as not to damage the plate 27, such that the path MP is effectively discontinuous. Upon reaching the next probe 30b, the beam is controlled to follow another arcuate segment in order to cross element 33b and 34b of probe 30b. The beam is controlled to continue along the measurement path in this way until it has crossed the elements of all of the probes at which locations the properties of the beam are to be measured. Examples of properties which can be measured using the signals obtained in this way will now be described with reference to Figures 6 to 9. Figure 6 shows an illustrative elongate, conductive element - for example element 33a of probe 30a shown in Figure 5 - and the path of the beam B across it in the Figure 5 example. Specifically, B(i) represents the position of the beam at which it first strikes the element 33a, and B(ii) represents its position at the point where the beam moves off the element. The element will output an electrical signal beginning when the beam is at position B(i) and ending when the beam is at position B(ii), i.e. when it has moved through a distance D which will be equal to the sum of the element width w plus the beam width BW in the direction perpendicular to the element's elongate direction - i.e. the width in the y-axis direction, in this case. It should be noted that in the more general case where the measurement path MP does not cross the elongate direction of the element perpendicularly, the effective width of the element contributing to the signal will be greater and this will be accounted for in the signal processing based on the known angle of crossing.

Figure 7 shows an exemplary signal output by the element 33a: here this is a plot of voltage vs. time, although the signal current could be measured instead. The duration T of the signal is obtained and converted to find the corresponding distance D travelled by the beam B, based on the known speed of the beam. Having obtained the distance D, the beam width BW can be calculated since the element width w is known. Hence the signal from element 33a can be processed to give a measure of the beam width BW in the x-axis at the location of probe 30a, and preferably a signal processor 24 is provided which is adapted to perform this calculation. Similarly, the signal subsequently output when the beam intersections element 34a can be processed to obtain a measurement of the beam width in the y-axis direction at the location of probe 30a. Together, these provide an indication of the shape or ovality of the beam spot when it is directed towards probe 30a.

It will be seen that, in order to interpret the detected signals, it is necessary to be able to identify which element is the source of each signal and this can be done in a number of ways. As mentioned previously, the signals from each element could be output on separate channels, in which case they will be inherently distinguished from one another. However, it is preferred to connect all elements together such that the signals are output on a single channel and as such the detection module 22 is adapted to attribute each signal to its source element based on knowledge of the measurement path MP and either the sequence in which the signals are detected, or the time at which each signal is detected.

Knowledge of the measurement path can be obtained in different ways. In one embodiment, the detection module may be provided with a memory 25 (Figure 1 ) in which is stored details of the pre-determined path to be followed by the beam, e.g. the x-y co-ordinates at which it will strike the probe assembly 21 at each instant over a selected period of time, or corresponding vector data. The information preferably also includes the speed of the beam. However, the precision with which a beam can be controlled is not exact and so, for better accuracy it is preferred to provide the detection module with a monitoring module 26 which monitors the present (e.g. real-time) beam position based on the deflection currently being applied to the beam. This may involve receiving feedback from controller 1 1 and/or beam deflection control module 23 (although as discussed below there could still be a discrepancy between the intended beam position as output by controller 11 and/or beam deflection control module 23 and techniques for dealing with this will be provided). Given this knowledge of the beam path, sequential signals received on a single channel can be correctly allocated to the element from which each derives.

For example, Figure 8 schematically shows an output trace from the probe assembly 21 shown in Figure 5 upon movement of the beam along the illustrated beam path. The first signal (1 ) is detected at time ti and can be attributed to element 33a of probe 30a either by virtue of it being the first signal detected or given knowledge of the distance which will have been travelled by the beam from its starting point to the element 33a, which will correspond to time ti multiplied by the known beam speed. Alternatively, the real-time beam location at time ti can be checked by reference to the monitoring module 26. The duration T_33a of the signal corresponds to the width of the beam in the x-direction (x_30a) at probe location 30a, as previously discussed. The next signal (2) can similarly be attributed to the element 34a of probe 30a and hence its duration T_34a provides the width of the beam in the y-direction (y_30a) at the same location. As such, the shape or at least ovality of the beam spot when directed to the location of probe 30a can be ascertained. As shown in Figure 8(b), in this example, the first signal (1 ) is found to be of longer duration than the second signal (2), with the result that the beam spot is found to be elongate in the x direction and not precisely circular.

Continuing along the trace in Figure 8(a), the third and fourth signals (3) and (4) can be correlated to elements 33b and 34b of the next probe 30b, by applying the same principles. The signal durations T_33b and T_34b provide the widths of the beam spot when directed to probe 30b in the x and y directions respectively (X30b, y30b) and since in this case the durations are similar, as shown in Figure 8(c), here the beam spot is found to be substantially circular. Of course, these calculations assume that the width of each element 33, 34 is equal. This need not be the case but, if not, their different widths will need to be taken into account during the processing. The detected signals can also be used to obtain a profile of the beam intensity (i.e. current density) in the direction perpendicular to the elongate direction of each element. This is illustrated in Figure 9, where Figure 9(a) shows the beam B starting to intersect an element 32 along a measurement path MP, and Figure 9(b) shows a plot of the detected signal (i) and the contributions to that signal made (ii) as a result of the increasing width of the beam B crossing the element in the direction along its length (y-direction), and (iii) deriving from variations in the beam intensity along the x-direction. As the beam B crosses the element 32, at any one time, a strip portion of the beam will be incident upon the element (line-hatched portion shown in Figure 9(a)) and the signal measured at that instant will be proportional to the current density of the beam across the collected portion. As the relative movement progresses, different strip portions of the beam spot will be sampled by the element 32, and the detected signal will vary accordingly. However, due to the substantially circular shape of the beam spot, the length I of the element 32 which is radiated by the beam will vary over time according to a function l(t) which can be estimated if the beam shape is known. The detected signal (i) shown in Figure 9(b) is effectively the sum of the variation in received signal due to the function l(t), indicated by the dashed line (ii) and that due to the actual intensity variation in the beam, indicated by (iii). Therefore, since the function l(t) can be estimated (based for example on width measurements made as discussed with reference to Figures 6 to 8), the intensity profile (iii) can be calculated from the detected signal (i).

The intensity profile will be measured along the direction perpendicular to the elongate direction of the element and hence using the apparatus shown in Figure 5, can be obtained in the x and y directions utilising elements 33a and 34b respectively. This can also be repeated at each probe location.

If desired, the signals can be processed further to obtain a two-dimensional intensity map of the beam spot. This is achieved by performing topographic reconstruction on the profiles obtained along multiple directions to de-convolute the signals. For example, the well-known inverse Radon function can be used for this purpose. A minimum of two profiles obtained along different directions through the beam spot is required for this purpose but better resolution will be achieved where more than two such profiles are available. Probes with additional elements particularly suitable for this purpose are described below with reference to Figures 15 and 16.

The measured properties can be used to adjust the beam generator to modify the beam properties as desired. For example, in many applications it is desirable for the properties of the beam to be substantially uniform across the working area. Thus the system may include a feedback module 26b adapted to generate and output a feedback signal to the beam generator (e.g. controller 1 1 ) which a can be used to adjust for example the optical system of the electron gun in order to modify the beam width, shape or intensity as appropriate. This process can preferably be automated by providing the generator and/or system with a suitable algorithm which, given the feedback signal, can adjust the beam to achieve the desired outputs without further operator input. The beam deflection control module can be implemented in a number of ways. In some embodiments, the controller 1 1 (hereinafter referred to as the "primary" controller) of the beam generating apparatus could be used to move the beam along the desired measurement path, by programming the primary controller accordingly. However, this is not preferred since in many cases, the primary controller will apply adjustments to the beam to correct for distortions such as astigmatism. Since these are angle-dependent, if the primary controller is instructed to deflect the beam away from the probe location (e.g. the centre of the probe) at which the beam properties are to be measured, the beam will be automatically adjusted to take account of the deviation and hence the beam which crosses each probe element will not have the same properties as those of the beam when directed to the intended location. Further, the primary controller 1 1 typically outputs a digital control which at the high speeds at which the beam is preferably deflected over the probe elements may result in unintentional discontinuities in the beam path. The beam deviations giving rise to the desired measurement path are therefore preferably imposed upon the beam by bypassing the primary controller 1 1. The primary controller is programmed to convey the beam along a primary beam path Pi which moves the beam from one probe location to the next. The beam deflection control module then a secondary beam path (or "deviation") P₂ onto the primary beam path. The measurement path comprises the superposition of the secondary beam path P₂ onto the primary beam path Pi (i.e. Μ Ρ = Ρι + P₂). Figure 10 shows an example of primary and secondary beam paths used to form a measurement path similar to that discussed with reference to Figure 5, except that in this case the beam traverses a substantially circular path segment at each probe location. Figure 10(a) shows the primary beam path Pi output by the primary controller 11. The beam B is moved from one probe 30a to the next 30b, at each one being located at the probe centre. This is the probe location against which the measured properties will be recorded. Each of the sequential positions is recorded as a trigger position 40a, 40b, 40c. The secondary beam path P₂ is shown in Figure 10(b) and comprises a short straight movement away from the probe centre, followed by a substantially circular segment designed to cause the beam to cross the elements in one probe as previously described.

In operation, the beam is moved to position 40a by primary controller 1 1. The beam deflection control module 23 receives information as to the current position of the beam according to the primary beam path from the primary controller via monitoring module 26. Since position 40a is recorded as a trigger position, in response to the beam reaching that position, the beam deflection module 23 outputs the secondary beam path control signal representing path P₂, which causes the beam B to be deflected away from the centre of probe 30a and to follow the circular path segment around probe 30a as shown in Figure 10(c). Upon completion (e.g. after a pre-set delay), the primary controller moves the beam to the next probe 30b at position 40b and again this triggers the output of secondary beam control signal P₂. The resulting measurement path is shown in Figure 10(c), where the dashed lines represent discontinuities in the path where the beam is switched off (as built into the primary beam path P^. The start and end points of each circular segment are preferably located away from any of the elongate elements, as shown in the Figure 10 example, so that the beam does not rest on any element for a significant period of time, which could cause the element to overheat and melt. For example, the beam deflection may be static at a position of 270 degrees around the probe centre until triggered to carry out one procession.

Another example of a suitable measurement path and the primary and second beam path contributions are shown in Figure 1 1. Here, the primary beam path Pi (Figure 1 1 (a)) is a continuous straight line from one probe to the next, preferably with the beam offset from the probe centre to avoid the intersection between the elements. The secondary beam path P₂ is a substantially circular segment (Figure 1 1 (b)). In this case there are no trigger points and the beam deflection control module need not receive feedback as to the current location of the beam along the primary beam path. The secondary beam control signal is output continuously with the result that the measurement path MP follows a spiral as shown in Figure 1 1 (c). Whilst the path no longer crosses the elements of each probe at the same positions or in the same order, with knowledge of the path (e.g. from the monitoring module), the detection module can still correctly attribute each pulse to the respective element. In order to prevent damage to the plate 27, the beam B may be switched off by the primary controller along sections between the probes. As previously mentioned, circular or arcuate segments of the measurement path are preferred and one reason for this is that, with the elements arranged as shown to extend radially from a central intersection, the path intersects each element at the same angle, i.e. perpendicularly. In addition, the speed of the beam can be maintained constant by applying a constant processional angular velocity as the secondary control signal, which simplifies processing of the detected signals. However, for landing angles well away from normal (e.g. at the peripheries of the array), although the processional angular velocity of the beam will be constant, the beam will trace out an ellipse rather than a circle due to the angle at which the array plane intersects the beam. As such, the speed of the beam will vary as it processes. This will lead to different widths of pulse on the X and Y traces, even when the beam is perfectly symmetric. This distortion can be corrected if the secondary control signal is made dependent on the beam deflection angle (known from the monitoring section 26). For example, the beam deflection control module 23 can be configured to produce a correcting elliptical deflection for each probe position such that the path of the beam around the probe is maintained to be circular, and hence of constant beam speed. Alternatively, the correction may be carried out in post processing of the signal. Further advantages of circular or arcuate measurement paths will be discussed below.

The secondary beam path can be imposed on the beam in a number of ways. Figure 12 shows selected components of the tool already described with reference to Figure 1 . Here, the deflection coils 18 of the electron gun 15 are themselves used to apply all of the required deflections to the beam. The primary beam path control signal Pi is output by the primary controller 1 1 and typically passed through a digital to analogue converter 12. The secondary beam path control signal P₂ is generated and output by the beam deflection control module 23 and superimposed onto the primary beam path control signal Pi to form the measurement path signal which is then received by the deflection coils 18. The secondary beam path control signal may for example be produced by two arbitrary function generators synchronised to produce a single sine wave output and phase shifted by 90 degrees to produce a nominally circular deflection.

An alternative implementation is shown in Figure 13, which depicts a modified version of the additive layer manufacturing tool 1 ' already discussed with reference to Figure 1 . Components labelled with like reference numerals are the same as described previously and will not be discussed again. In this embodiment, the beam deflection control module 23 further includes a secondary deflection unit 23a, e.g. a second set of deflection coils, disposed downstream of the primary deflection unit 18 along the gun axis A. Thus, the primary deflection unit 18 controls the beam in accordance with the (unmodified) primary control signal from controller 1 1 , whilst secondary deflection unit 23a imposes a deviation on the beam B in accordance with the secondary beam control signal from module 23. This has the advantage of requiring no change to the machine electronics, which is useful when retrofitting the probe system to existing machines.

In one particularly advantageous implementation, the secondary deflection unit 23a and probe assembly 21 are provided in one integral device, an example of which is shown in Figure 13a. The device 50 comprises a probe assembly 21 such as that described above, and a beam deflection unit 23a, such as a deflection coil. The beam deflection unit 23a is supported on the probe assembly by a support assembly 51 , which here comprises four support arms 51 , one disposed at each corner of the probe assembly (although any number could be provided). Preferably the support assembly 51 fixedly connects the deflection unit to the probe assembly so that the device as a whole can be straightforwardly placed into a machine such as tool 1 ' when the properties of the beam are to be measured. Alternatively the device could be supplied as a kit and assembled in-situ.

To install the measurement system, the device 50 is placed in the vacuum chamber 2 of the machine in alignment with the beam axis A. The probe assembly 23 can be fitted to the workpiece table 3 using fittings typically provided in such machines for mounting manual calibration plates. The deflection unit is thus automatically positioned by way of the support assembly. The deflection unit 23a is connected by two coaxial feed-throughs to a controller such as beam deflection control module 23 in order to implement the secondary beam control signal, e.g. a circular deflection.

The probe outputs are connected in the same manner as previously described to a detection module 22 for detection and processing of the electric signals output by the probe elements.

This approach enables the presently disclosed system and method to be readily deployed into existing machines making use of charged particle beams without modifying the beam generator control system of the machine. The modular unit can be temporarily fitted into the machine without the necessity of changing the system itself. The support assembly can for example comprise support arms such as dowels and pre-machined holes, or any other appropriate structure which positions the beam deflection unit correctly.

In the embodiments described so far, the detected signals are attributed to each probe element based entirely on knowledge of the beam path. As already discussed, in order to achieve good accuracy, in practice this requires monitoring of the present deflection of the beam using a monitoring section 26 which receives positional information from the primary controller 11 and preferably also from the beam deflection control module 23. In theory, if the secondary beam path is output upon the beam reaching known trigger positions and its exact shape and phase is known, the system could rely solely on feedback as to the primary beam path which would permit identification of the probe from which each signal originates, and then rely upon the order of the signals from within one probe to ascertain which element of the probe is the source of which signal. However in practice the secondary beam path may not be exactly the same, and in particular may not start at the same position around the probe at each instant at which it is output. Similarly in the case of a continuous secondary deflection signal as in the Figure 11 embodiment, it will typically be a different one of the elements in each probe which is crossed first by the beam.

Figure 14(a) shows a portion of a probe assembly according to a further embodiment which is modified to assist in identifying the source of each signal. Here, the construction of each probe 30a, 30b is largely as previously discussed with reference to Figure 5 and like reference numerals indicate the same components. Each probe is provided with a marker element 36, 36 which comprises a conductive element which is wider in the direction of the measurement path than the elongate elements 33, 34. For example, the marker element may comprise a peg or a protrusion of the probe frame. The marker element 36 is electrically connected to the detection module in the same manner as each of the elongate elements 33, 34. Due to its greater width, when struck by the beam B, the marker element will generate an electric signal of greater peak amplitude than those originating from the elongate elements 33, 34 and can thus be used as a marker signal to identify the other signals based on their order relative to the marker. Figure 14(b) is a plot showing an exemplary series of electric signals detected as the beam B follows the measurement path MP illustrated in Figure 14(a). It should be noted that not all of the signals that will be generated are shown, for clarity. The first three signals (1 ), (2) and (3) are attributed to probe 30a by means of feedback from monitoring section 26. The second signal (2) is of substantially greater magnitude than the other signals and is thus known to originate from marker element 36a. As a result, based on knowledge of the probe geometry, pulse (1 ) can be attributed to element 33a, and pulse (3) to element 34a. When the beam reaches probe 30b, it will be seen from Figure 14(b) that the beam starts at a different position around the probe such that the elements will be crossed in a different order. This is identified by the presence of marker pulse (4) as the first pulse attributable to probe 30b, allowing the subsequent pulses (5) and (6) to be matched to elements 34b and 33b respectively. Figure 15 shows a further embodiment of a probe 30' suitable for use in the presently disclosed system. As shown in Figure 15(a), the probe is based on a frame 31 a which as in previous examples is a cylindrical tube section, akin to a bobbin, and defines a central aperture. The frame 31 is preferably electrically conductive and may be formed of stainless steel for example. The outer surface of the frame is provided with eight axial grooves equally spaced around the frame and used to hold elongate electrically conductive elements 32 in position. For example, the elements 32 could be formed by wrapping a continuous wire around the frame using the grooves so that it crosses over itself as shown to provide the differently orientated elements. In this example, a total of four different directions are accommodated: elements are provided along the x and y directions, and in the two directions equidistant between them. By providing additional measurement directions in this way, the width of the beam spot can be measured in more directions and hence its shape more accurately determined. Further, a higher resolution intensity map can be reconstructed. It should be noted that all of the elements preferably lie in the same flat plane so that each measures the properties of the beam at the same height relative to the working surface, although this is not essential. The elements 32 in this embodiment are formed of 18 micron tungsten wire held in a frame 31a of 12mm outside diameter and 8mm inside diameter. The frame 31 is mounted into a ceramic holder 31 b which is itself mounted at a specific position in the working area of the machine by fixing into a hole bored in the array plate 27.

In the examples depicted so far, the probe elements are formed with an intersection at the probe centre, e.g. as a cross-hair arrangement. However this is not essential and Figure 16 shows some further examples of exemplary probe configurations which could be used. In both cases, the probe is based on a tubular frame 31 with circular cross section but this is not essential and the frame could take any shape defining a central aperture. In the Figure 16(a) example, four elongate elements are provided and arranged to form a square or rectangular shape. Elements 33 and 33' are aligned with the x-axis of the machine and elements 34 and 34' with the y-axis of the machine. The elements are preferably in electrical contact with each other and/or with the frame 31 at the corner intersections: for example the elements may be formed by winding a wire around four pegs provided at the top surface of frame 31 to define the four corners. An arrangement such as this has the advantage that no elements are present at the centre of the probe 30" meaning that there is no need to immediately deflect the beam away from the probe centre to avoid melting of the elements: the beam can rest at the centre of the probe without detriment. The beam can intersect any two or more of the elements, e.g. by following a circular measurement path MP as shown. In this case the measurement path will cross the elements at various different angles and this will need to be taken into account in the signal processing as previously mentioned.

In the Figure 16(b) example, three elements 33, 34 and 35 are provided and are arranged at arbitrary angles, non-parallel to one another. Again, no elements are present at the centre of the probe 30"' which is advantageous for the reasons described above. The beam can take any measurement path which crosses at least two of the elements. In this case, a right-angled measurement path MP made of two straight line portions is illustrated, from which the same data as previously described can be obtained. However, paths such as these are more difficult to accurately implement than substantially circular or arcuate paths such as those previously described, particularly by analogue control means since the control signals require the superposition of many high- frequency components to approximate to the straight lines and/or sharp corners intended. In contrast, circular or arcuate beam deflections can be readily achieved at high speed without distortion.

The probes utilised in the disclosed system are advantageous and particularly well suited to the present application whereby the properties of the beam are to be measured at widely varying incident angles, because the probes are substantially insensitive to the landing angle of the beam B. This is due to the use of conductive elements 32 for sampling the beam and outputting the detected signal. The conductive elements will generate a signal irrespective of the beam angle incident upon them. As a result, all of the probes in the array can be of the same construction and are interchangeable since each can deal with the whole range of landing angles experienced at different positions over the working area. The multi-positional probe thus enables the melting/material processing performance to be measured consistently over a distributed working area where the particle beam is deflected to each position. To further remove any remaining angle-sensitivity it is preferred that the elongate, electrically conductive elements have a width in the plane of the array (i.e. perpendicular to the gun column axis A) which is equal to or greater than their thickness in the direction perpendicular to the array. For example, this criteria is fulfilled by a wire of circular cross section or a flattened wire, strip or tape which has its thin dimension arranged normal to the probe axis. This minimises the sensitivity of the probes to the incident angle of the beam, since any particles received by the side faces of the elements is minimal compared by that received by the top surface (facing the beam source. Strip elements with a substantially flat surface are particularly preferred since this will reduce the degree of backscattering compared to circular wires, for example. A further example of a probe 30^IIIV of this sort is shown in Figure 17 and here the elements 33, 34 are formed integrally with one another from a conductive foil which is suspended across frame 31.

As mentioned previously, the beam deflection achieved by the deflection unit 18 (and/or 17, if provided) may not correspond exactly to the intended deflection instructed by the controller 1 1 and/or module 23, i.e. there may be a deflection discrepancy. In this case, the beam would not follow the intended measurement path exactly and this could lead to distortions in the measured properties, due for example to the beam not crossing each element at the expected angle. The system therefore preferably includes means for detecting and correcting any such deflection discrepancy and advantageously this is implemented by signal processor 24 based on the temporal spacing between the detected electric signals.

A suitable technique will be described for the exemplary scenario in which a circular measurement path is utilised in combination with a probe of the sort shown in Figure 15, i.e. having four crossed conductive filaments which effectively form eight elements having their elongate directions extending radially from the intersection point. The circular measurement path is designed to be centred on the intersection point so that it crosses each element at the same angle (90 degrees). In an experiment, the beam was driven to follow this measurement path with a constant frequency of 10kHz (i.e. completing 10,000 circuits of the path each second). The beam was designed to have a beam current of 0.25 mA, at 100 kV and a sharp focus. Figure 18 is a plot which depicts (i) the secondary deflection signal supplied to the deflection unit(s) in the y-axis direction, in terms of volts vs. time (showing the period of the circular deflection), and (ii) the detected electric signals. Region C corresponds to one circuit of the path, during which the elements will be intersected eight times and hence eight electric signals (1 ) to (8) will be detected.

The signal processor measures the temporal spacing ΔΤ between each adjacent pair of signals. For example, the spacing between signals (1 ) and (2) is ΔΤ₁₂. If the beam is following the desired measurement path accurately, the spacing ΔΤ between each adjacent pair of signals should be equal, i.e. ΔΤ₁₂ = ΔΤ₂3 = ΔΤ₃₄, etc. It will be seen from the plot depicted that in the case of Figure 18, the spacings are not equal which indicates that the beam is not following the intended measuring path exactly - for example, it may be off-centre, or it may not be sufficiently circular, i.e. there is a discrepancy between the intended beam deflection (encoded in the deflection control signals) and that actually achieved. As a result, the signal widths and peak values also differ from one another and, whilst to some degree this may be indicative of the beam's properties varying according to the direction, this will also be a result of the beam crossing each element at some unknown angle which is not 90 degrees.

To correct for this, the signal processor preferably generates and outputs a feedback signal based on the differences in ΔΤ to the deflection units(s). The deflection control is adjusted based on the feedback via a suitable algorithm in order to reduce any difference between the measured ΔΤ values, preferably to zero at which point the beam will be known to be following the intended path. The beam properties can then be measured accurately based on the subsequently detected electrical signals using the principles discussed above.

Whilst it is especially preferred to implement this technique utilising a circular measurement path and radially extending probe elements as described, it is not essential. For example, the expected values of (absolute or relative) ΔΤ between each adjacent pair of signals can be calculated based on the planned measurement path and known probe geometry, and the feedback algorithm can be designed to adjust the beam deflection until these expected values are attained. However the computation is simplified by the above described implementation since all the expected ΔΤ values will be equal. The correction calculated and applied by this technique can also be stored and used during future processing performed by the tool in which the system is incorporated, whereby the tool is calibrated. Some of the advantages of preferred embodiments of the disclosed system and method are as follows:-

• Quantified measurement of beam properties such as width, shape and/or intensity is enabled at many, distributed positions across a working area, including for high beam deflection angles.

• Each probe is preferably small so it measures the beam in the close locality of the probe location on the working area.

• Using elongate conductive elements such as wires allow measurements to be independent of beam landing angle and as such the probes function accurately even for high deflection angles.

• The probes can all be connected together simplifying vacuum chamber connection: only a single feed-through is required.

• The system allows measurement over the entire working area without requiring any manual adjustment of probe parts or operator intervention, so in the case of a vacuum chamber the beam characterisation can be carried out quickly and within a single pump down cycle of the machine.

• The integration of beam deflection and data collection provides fully automatic measurement; no operator input is required once the measurement path is initiated.

• Collection of beam measurements can be very fast (e.g. less than 1

second for tens of positions).

• All of the measurements can be made in a single machine pump down cycle which allows for a short overall measurement time.

• The preferred circular beam deflections allow high speed deflection to be used with accurate beam positioning and speed, enabling high power beams to be probed.

• No moving parts, straightforward operator set-up.

• The probe system can be readily retrofitted to existing beam generators or integrated into material processing tools.

Claims

1. A system for measuring properties of a charged particle beam output by a charged particle beam generator, comprising:

a detection module connected to the electrically conductive elements of each of the plurality of probes, adapted to detect electric signals output sequentially by the electrically conductive elements of each probe upon intersection of the charged particle beam therewith, the detected electric signals from each probe being indicative of properties of the charged particle beam when directed to the location of the respective probe across the probe assembly by the charged particle beam generator.

2. A system according to claim 1 , wherein the detection module is further adapted to identify which elongate, electrically conductive element of which probe is the source of each detected electric signal.

3. A system according to claim 1 or claim 2, wherein for each probe, the elongate, electrically conductive elements are in electrical contact with one another and optionally are integral with one another.

4. A system according to claim 3, wherein the elongate, electrically conductive elements of each probe are connected in parallel or in series to those of the other probes and the detection module is adapted to receive the electric signals output by the electrically conductive elements of the probe assembly on a single channel.

5. A system according to claim 3 or claim 4, wherein the detector module comprises a correlation section adapted to identify which electrically conductive element of which probe is the source of each detected electric signal, based on knowledge of the measurement path and either the time at which each electric signal is detected and/or the sequence in which the electric signals are detected.

6. A system according to claim 5, wherein the knowledge of the measurement path is provided by a memory having stored measurement path data, the stored measurement path data preferably including: the starting location of the beam, the path to be followed by the beam, and optionally the speed of the beam at least at the points of intersection with the electrically conductive elements.

7. A system according to claim 5, wherein the knowledge of the measurement path is provided by a beam location monitoring section which is adapted to receive information as to the present deflection of the beam from the charged particle beam generator and/or the beam deflection control module.

8. A system according to claim 5, wherein the knowledge of the measurement path is provided by a memory having stored measurement path data and by a beam location monitoring section which is adapted to receive information as to the present deflection of the beam from the charged particle beam generator and/or the beam deflection control module, in combination.

9. A system according to any of the preceding claims, wherein the charged particle beam generator comprises a primary beam deflection unit and a primary controller adapted to generate and output a primary control signal to the primary beam deflection unit for directing the beam to a selected location or along a primary beam path, and the beam deflection control module comprises a secondary controller configured to output a secondary control signal which is superimposed on the primary control signal output by the primary controller, the secondary control signal defining a secondary beam path, such that the primary beam deflection unit directs the beam along the measurement path which is a combination of the secondary beam path with any primary beam path.

10. A system according to any of claims 1 to 8, wherein the charged particle beam generator comprises a primary beam deflection unit and a primary controller adapted to generate and output a primary control signal to the primary beam deflection unit for directing the beam to a selected location or along a primary beam path, and the beam deflection control module comprises a secondary beam deflection unit disposed downstream of the primary beam deflection unit and a secondary controller configured to output a secondary control signal to the secondary beam deflection unit, the secondary control signal defining a secondary beam path, such that the primary and secondary beam deflection units in combination direct the beam along the measurement path which is a combination of the secondary beam path with any primary beam path.

1 1. A system according to claim 9 or claim 10, wherein the beam deflection control module comprises a monitoring section adapted to receive information as to the present deflection of the beam from the charged particle beam generator, preferably the primary control signal from the primary controller, and to output the secondary control signal in response to the received information.

12. A system according to claim 1 1 , wherein the beam deflection control module is adapted to output the secondary control signal in response to the beam deflection reaching one or more pre-defined trigger positions, each predefined trigger position preferably corresponding to the location of a different one of the probes.

13. A system according to claim 12, wherein the primary control signal is configured to direct the beam to a plurality of discrete locations in sequence, each corresponding to a different probe and each being a pre-defined trigger position, whereby the beam is deflected by the beam deflection control module along the measurement path corresponding to the secondary beam path, upon reaching each discrete location.

14. A system according to any of claims 9 to 13, wherein the secondary control signal output by the secondary controller defines a substantially circular or arcuate deflection path, preferably having a constant beam speed.

15. A system according to claim 14, wherein the secondary controller is adapted to modify the secondary control signal in dependence on the deflection angle of the beam, preferably by applying a correcting elliptical deflection for each probe location such that the path followed by the beam on the probe assembly is substantially circular or arcuate at each probe of the array.

16. A system according to any of the preceding claims, wherein the measurement path comprises at least one segment crossing at least two, preferably all, of the elongate, electrically conductive elements of one of the probes, which segment is substantially circular or follows a substantially circular arc.

17. A system according to any of the preceding claims, wherein the measurement path comprises a plurality of segments, each segment crossing at least two, preferably all, of the elongate, electrically conductive elements of one of the probes, each segment intersecting a different one of the probes.

18. A system according to claim 17, wherein the measurement path is discontinuous between the segments.

19. A system according to any of claims 16, 17 or 18, wherein the start and/or end points of the or each segment are not coincident with any of the elongate, electrically conductive elements.

20. A system according to any of the preceding claims, wherein each of the probes further comprises a marker element which is electrically conductive and of greater width than that of the elongate electrically conductive elements in the direction crossed by the measurement path, the measurement path additionally crossing the marker element, whereby an electric signal output by each marker element upon intersection of the charged particle beam therewith has a greater peak amplitude than those output by the elongate elements, and wherein the detection module is adapted to identify which elongate, electrically conductive element within a probe is the source of each detected electric signal based at least in part on the electric signal output by the marker element.

21. A system according to claim 20 and any of claims 5 to 8, wherein the detection module is adapted to identify the probe which is the source of each detected electric signal based on knowledge of the measurement path and either the time at which each electric signal is detected and/or the or sequence in which the electric signals are detected, and to identify the elongate electrically conductive element of the identified probe which is the source of each detected electric signal based at least in part on the electric signal output by the marker element.

22. A system according to any of the preceding claims, wherein the at least two elongate, electrically conductive elements of each probe comprise at least first and second elongate electrically conductive elements arranged such that their respective elongate directions are substantially orthogonal to one another in the plane of the array.

23. A system according to any of the preceding claims, wherein each probe comprises at least three, more preferably at least four, elongate, electrically conductive elements arranged such that their respective elongate directions make non-zero angles with one another in the plane of the array, the respective different elongate directions preferably being substantially equally angularly spaced from one another.

24. A system according to any of the preceding claims, wherein the at least two elongate, electrically conductive elements of each probe have at least one intersection between them at which the elements are preferably in electrical contact.

25. A system according to claim 24, wherein the at least two elongate, electrically conductive elements are arranged such that their elongate directions extend radially from an intersection between them and are preferably equally angularly spaced from one another about the intersection.

26. A system according to claim 25 and claim 26, wherein the measurement path comprises at least one segment crossing at least two, preferably all, of the elongate, electrically conductive elements of one of the probes, which segment is substantially circular or follows a substantially circular arc and is centred on the location of the intersection between the elements from which the elongate directions extend.

27. A system according to any of the preceding claims, wherein the at least two elongate, electrically conductive elements of each probe comprise at least two conductive filaments which are crossed with one another at a non-zero angle, the conductive filaments optionally being different portions of one continuous conductive filament.

28. A system according to claim 27, wherein the conductive filaments comprise wires, ribbons or strips.

29. A system according to any of the preceding claims, wherein the elongate, electrically conductive elements have a width in the plane of the array equal to or greater than their thickness in the direction perpendicular to the array.

30. A system according to any of the preceding claims, wherein the width of each elongate, electrically conductive element in the plane of the array is less than the diameter of the charged particle beam at the array plane, preferably between one third and two thirds the diameter of the charged particle beam.

31. A system according to any of the preceding claims, wherein the width of each elongate, electrically conductive element in the plane of the array is between 5 and 200 microns.

32. A system according to any of the preceding claims, wherein the at least two elongate, electrically conductive elements are spaced from one another, at least along a portion of their lengths, by a distance at least equal to, preferably greater than, the diameter of the charged particle beam at the array plane.

33. A system according to any of the preceding claims, wherein the at least two elongate, electrically conductive elements are spaced from one another, at least along a portion of their lengths, by at least 0.5mm, more preferably at least 1 mm.

34. A system according to any of the preceding claims, wherein each probe comprises a frame configured to support the elongate, electrically conductive elements at at least two points across an aperture defined by the frame.

35. A system according to claim 34, wherein the frame comprises a tube section, preferably a cylindrical tube section, the elongate, electrically conductive elements being arranged across one or both ends of the tube section.

36. A system according to claim 34 or 35, wherein the frame comprises an electrically conductive portion configured to connect the at least two elongate, electrically conductive elements to a circuit connected to the detection module.

37. A system according to any of the preceding claims, wherein the plurality of probes are arranged according to a regular grid across the array and preferably include one probe on the axis of the charged particle beam generator.

38. A system according to any of the preceding claims, wherein the mount comprises a plate having a plurality of apertures in which the plurality of probes are disposed, the plate preferably being conductive and optionally configured to provide an electrical connection between the probes and a circuit connected to the detection module.

39. A system according to any of the preceding claims, wherein the array of probes lies in a plane substantially perpendicular to the axis of the charged particle beam generator.

40. A system according to any of the preceding claims, further comprising a signal processor adapted to calculate properties of the beam based on the detected electrical signals and knowledge of the beam measurement path.

41. A system according to claim 40, wherein the signal processor is adapted to calculate the width of the beam in at least two directions, at at least one probe location, based on the detected electric signals from the at least two elongate electrically conductive elements of the respective probe, and knowledge of the speed of the beam across the elements.

42. A system according to claim 40 or 41 , wherein the signal processor is adapted to calculate the intensity profile of the beam in at least two directions, at at least one probe location, based on the detected electric signals from the at least two elongate electrically conductive elements of the respective probe, and knowledge of the width of the beam.

43. A system according to any of claims 40 to 42, wherein the signal processor is adapted to calculate an intensity map of the beam in two dimensions, at at least one probe location, by tomographic reconstruction of the detected electric signals from the at least two elongate electrically conductive elements of the respective probe.

44. A system according to any of claims 40 to 43, wherein the signal processor is adapted to calculate properties of the beam in at least two directions at each of a plurality of the probe locations, preferably including the width of the beam, the intensity profile of the beam, the intensity map of the beam and/or of the beam.

45. A system according to any of claims 40 to 44, wherein before calculating the properties of the beam, the signal processor is adapted to detect any deflection discrepancy of the beam, at at least one probe location, based on the spacing between adjacent detected electric signals from the at least two elongate electrically conductive elements of the respective probe, and to generate and output a deflection discrepancy feedback signal to the charged particle beam generator to thereby correct the deflection discrepancy.

46. A system according to any of the preceding claims, further comprising a feedback module for generating and supplying a feedback signal to the charged particle beam generator to thereby adjust the properties of the charged particle beam based on the properties measured by the system.

47. A system according to any of the preceding claims, wherein the charged particle beam is an electron beam or an ion beam.

48. An apparatus comprising a charged particle beam generator and a system according to any of the preceding claims configured to measure properties of the charged particle beam output by the charged particle beam generator.

49. An apparatus according to claim 48, wherein the charged particle beam generator comprises a charged particle source, a particle accelerating unit for accelerating charged particles from the source along an axis to form a charged particle beam, a primary deflection unit adapted to deflect the charged particle beam away from the axis and a primary controller adapted to output a primary control signal for control of the deflection applied by the primary deflection unit.

50. A materials processing tool comprising an apparatus according to claim 48 or claim 49, wherein the tool is one of:

a cutting tool, the charged particle beam being adapted for cutting of materials; a melting or evaporation tool, the charged particle beam being adapted for melting and/or evaporation of materials;

51. A method of measuring properties of a charged particle beam output by a charged particle beam generator, the method comprising:

detecting electric signals output sequentially by the electrically conductive elements of each probe upon intersection of the charged particle beam therewith, the detected electric signals from each probe being indicative of properties of the charged particle beam when directed to the location of the respective probe.

52. A method according to claim 51 , further comprising identifying which elongate, electrically conductive element of which probe is the source of each detected electric signal.

53. A method according to claim 51 or claim 52, wherein at least all the electric signals output by the at least two elongate, electrically conductive elements of one probe are detected on a single channel.

54. A method according to claim 53, wherein all of the electric signals output by the plurality of probes are detected on a single channel.

55. A method according to claim 53 or 54, further comprising correlating each detected electric signal to the electrically conductive element which is its source, based on knowledge of the measurement path and either the time at which each electric signal is detected and/or the sequence in which the electric signals are detected.

56. A method according to claim 55, comprising retrieving stored measurement path data to provide the knowledge of the measurement path, the stored measurement path data preferably including: the starting location of the beam, the path to be followed by the beam, and optionally the speed of the beam at least at the points of intersection with the electrically conductive elements.

57. A method according to claim 55, comprising monitoring the present deflection of the beam to provide the knowledge of the measurement path.

58. A method according to claim 55, comprising both retrieving stored measurement path data and monitoring the present deflection of the beam to provide in combination the knowledge of the measurement path.

59. A method according to any of claims 51 to 58, wherein the charged particle beam generator comprises a primary beam deflection unit and a primary controller adapted to generate and output a primary control signal to the primary beam deflection unit for directing the beam to a selected location or along a primary beam path, and controlling the deflection of the charged particle beam to follow the measurement path comprises outputting a secondary control signal which is superimposed on the primary control signal output by the primary controller, the secondary control signal defining a secondary beam path, such that the primary beam deflection unit directs the beam along the measurement path which is a combination of the secondary beam path with any primary beam path.

60. A method according to any of claims 51 to 59, wherein the charged particle beam generator comprises a primary beam deflection unit and a primary controller adapted to generate and output a primary control signal to the primary beam deflection unit for directing the beam to a selected location or along a primary beam path, and the controlling the deflection of the charged particle beam to follow the measurement path comprises outputting a secondary control signal to a secondary beam deflection unit disposed downstream of the primary beam deflection unit, the secondary control signal defining a secondary beam path, such that the primary and secondary beam deflection units in combination direct the beam along the measurement path which is a combination of the secondary beam path with any primary beam path.

61. A method according to claim 59 or 60, further comprising monitoring the present deflection of the beam and wherein the outputting of the secondary control signal is in response to the monitored deflection.

62. A method according to claim 61 , wherein the secondary control signal is output in response to the beam deflection reaching one or more pre-defined trigger positions, each pre-defined trigger position preferably corresponding to the location of a different one of the probes.

63. A method according to claim 62, wherein the primary control signal directs the beam to a plurality of discrete locations in sequence, each corresponding to a different probe and each being a pre-defined trigger position, whereby the beam is deflected along the measurement path corresponding to the secondary beam path, upon reaching each discrete location.

64. A method according to any of claims 51 to 63, wherein the measurement path comprises at least one segment crossing at least two, preferably all, of the elongate, electrically conductive elements of one of the probes, which segment is substantially circular or follows a substantially circular arc.

65. A method according to claim 64, wherein the at least two elongate, electrically conductive elements are arranged such that their elongate directions extend radially from an intersection between them and are preferably equally angularly spaced from one another about the intersection, and the measurement path comprises at least one segment crossing at least two, preferably all, of the elongate, electrically conductive elements of one of the probes, which segment is substantially circular or follows a substantially circular arc and is centred on the location of the intersection between the elements from which the elongate directions extend.

66. A method according to any of claims 51 to 65, wherein the measurement path comprises a plurality of segments, each segment crossing at least two, preferably all, of the elongate, electrically conductive elements of one of the probes, each segment intersecting a different one of the probes.

67. A method according to claim 66, wherein the measurement path is discontinuous between the segments.

68. A method according to any of claims 64 to 67, wherein the start and/or end points of the or each segment are not coincident with any of the elongate, electrically conductive elements.

69. A method according to any of claims 51 to 68, further comprising calculating properties of the beam based on the detected electrical signals and knowledge of the beam measurement path.

70. A method according to claim 69, wherein calculating properties of the beam comprises calculating the width of the beam in at least two directions, at at least one probe location, based on the detected electric signals from the at least two elongate electrically conductive elements of the respective probe, and knowledge of the speed of the beam across the elements.

71. A method according to claim 69 or 70, wherein calculating properties of the beam comprises calculating the intensity profile of the beam in at least two directions, at at least one probe location, based on the detected electric signals from the at least two elongate electrically conductive elements of the respective probe, and knowledge of the width of the beam.

72. A method according to any of claims 69 to 72, wherein calculating properties of the beam comprises calculating an intensity map of the beam in two dimensions, at at least one probe location, by tomographic reconstruction of the detected electric signals from the at least two elongate electrically conductive elements of the respective probe.

73. A method according to any of claims 69 to 72, wherein properties of the beam are calculated in at least two directions at each of a plurality of the probe locations, the properties preferably including the width of the beam, the intensity profile of the beam and/or the intensity map of the beam.

74. A method according to any of claims 51 to 73, further comprising generating and supplying a feedback signal to the charged particle beam generator to thereby adjust the properties of the charged particle beam based on the measured properties of the charged particle beam.

75. A method according to any of claims 69 to 74, comprising:

(a) detecting any deflection discrepancy of the beam, at at least one probe location, based on the spacing between adjacent detected electric signals from the at least two elongate electrically conductive elements of the respective probe;

(b) generating and outputting a deflection discrepancy feedback signal to the charged particle beam generator to thereby correct any deflection discrepancy of the beam calculated in step (a); and then

(c) calculating the properties of the beam based on the detected electrical signals output after correction of any deflection discrepancy.

76. A method according to claim 75, wherein the at least two elongate, electrically conductive elements of the respective probe are arranged such that their elongate directions extend radially from an intersection between them and are equally angularly spaced from one another about the intersection, and the measurement path comprises at least one segment crossing at least two, preferably all, of the elongate, electrically conductive elements of the probe, which segment is substantially circular or follows a substantially circular arc and is centred on the location of the intersection between the elements from which the elongate directions extend, whereby in step (a) any difference in spacing between adjacent pairs of detected electric signals from the elements of the probe is indicative of a deflection discrepancy.

77. A method according to claim 76, wherein in step (b) the deflection discrepancy is corrected by adjusting the deflection of the beam until the difference in spacing between adjacent pairs of detected electric signals from the elements of the probe is substantially zero.

78. A method according to any of claims 51 to 77, wherein the charged particle beam is an electron beam or an ion beam.

79. A probe assembly for measuring properties of a charged particle beam comprising a plurality of probes arrayed across a plane on a mount, each probe comprising:

a frame defining an aperture; and

at least two elongate, electrically conductive elements supported by the frame and crossing the aperture, arranged such that their respective elongate directions make a non-zero angle with one another in the plane in which the frame lies.

80. A device for measuring properties of a charged particle beam output by a charged particle beam generator, comprising:

a beam deflection unit adapted to deflect the charged particle beam along a measurement path; wherein the beam deflection unit is supported on the probe assembly by a support assembly disposed therebetween, the beam deflection unit being orientated such that in use the charged particle beam can be deflected across the probe assembly by the beam deflection unit.

81. A device according to claim 80, wherein the support assembly comprises one or more support arms connected between the probe assembly and the beam deflection unit.

82. A device according to claim 80 or 81 , wherein the axis of the beam deflection unit is substantially perpendicular to the plane of the probe assembly.