MULTI-BEAM PARTICLE MICROSCOPE WITH IMPROVED BEAM CURRENT CONTROL

Abstract

A multi-beam particle microscope can provide improved beam current control. Excess electrons discharged from one or just a few regions of an absorber layer provided on a multi-aperture array can be measured via an ammeter. The measured currents can be used as controlled variables in a closed loop control. The measurement can be large-area and low-noise. The multi-aperture array can be specifically structured to also realize a direction sensitive detection, for example via a quadrant detector or a tertial detector.

Claims

1. A multi-beam particle microscope, comprising: a beam generating system comprising a particle source, an extractor electrode and an anode, the beam generating system configured to produce a first charged particle beam; a multi-beam generator comprising a multi-aperture array, the multi-beam generator configured to produce a first field of a plurality of first individual charged particle beams from the first charged particle beam, a first side of the multi-aperture array comprising an absorber layer configured to absorb charged particles, the absorber layer connected to a ground electrode to discharge excess electrons; a first beam current measuring mechanism configured to measure the discharged excess electrons generated by charged particles impinging on the multi-aperture array in an outer region around the openings in the multi-aperture array; a condenser lens system between the beam generating system and the multi-beam generator; a first particle optical unit having a first particle optical beam path, the first particle optical unit configured to direct the first individual particle beams at a sample so that the first individual particle beams strike the sample at incidence locations, which form a second field; a detection system; a second particle optical unit having a second particle optical beam path, the second particle optical unit configured to image second individual particle beams, which emanate from the incidence locations in the second field, onto the detection system; a particle optical objective lens configured to have the first and the second individual particle beams pass therethrough; a beam switch in the first particle optical beam path between the multi-beam generator and the objective lens, the beam switch in the second particle optical beam path between the objective lens and the detection system; and a controller configured to control the beam generating system, the condenser lens system, the particle optical objective lens, the first particle optical unit, the second particle optical unit, and the detection system, wherein the controller is configured to control: the beam generating system based on a measurement made using the first beam current measuring mechanism; and/or the condenser lens system based on a measurement made using the first beam current measuring mechanism.

2. The multi-beam particle microscope of claim 1, wherein the first beam current measuring mechanism is configured to measure the discharged excess electrons generated by charged particles impinging on the multi-aperture array in an inner region comprising the openings in the multi-aperture array.

3. The multi-beam particle microscope of claim 1, wherein: the absorber layer comprises exactly two separate regions that are isolated from one another; each of the exactly two regions of the absorber layer are connected to ground; a first region of the exactly two regions of the absorber layer is an inner region comprising the openings of the multi-aperture array; a second region of the exactly two regions of the absorber layer is the outer region around all of the openings in the multi-aperture array; and the first beam current measuring mechanism is configured to measure the excess charged particles discharged only from the second region.

4. The multi-beam particle microscope of claim 1, wherein: the absorber layer comprises two separate regions isolated from one another; each of the two regions is connected to ground; and the first beam current measuring mechanism is configured to measure the excess electrons discharged from each region separately.

5. The multi-beam particle microscope of claim 4, wherein: the absorber layer comprises an inner region and an outer region; the inner region comprises the openings of the multi-aperture array; the outer region is around all of the openings in the multi-aperture array; the outer region comprises four separate regions defining a direction indicating quadrant detector; and the first beam current measuring mechanism is configured to measure the excess electrons discharged from each of the four regions separately.

6. The multi-beam particle microscope of 4, wherein: the absorber layer comprises an inner region and an outer region; the outer region comprises three separate regions configured to define a direction indicating tertial detector; and the first beam current measuring mechanism is configured to measure the excess electrons discharged from each of the three regions separately.

7. The multi-beam particle microscope of claim 1, further comprising a double deflector in a region of the condenser lens system, wherein the controller is configured to control the double deflector based on the measurement made using the first beam current measuring mechanism.

8. The multi-beam particle microscope of claim 1, wherein the first beam current measuring mechanism comprises an ammeter.

9. The multi-beam particle microscope of claim 1, wherein at least 60% of the beam current reaching the multi-aperture array is used for the beam current measurement.

10. The multi-beam particle microscope of claim 1, wherein at least 90% of the beam current reaching the multi-aperture array is used for the beam current measurement.

11. The multi-beam particle microscope of claim 1, wherein an active beam measurement surface of the absorber layer is configured to absorb charged particles and to discharge electrons for the beam current measurement, and the active beam measurement surface is at least 60% of an entire area of the first surface of the multi-aperture array.

12. The multi-beam particle microscope of claim 1, wherein an active beam measurement surface of the absorber layer is configured to absorb charged particles and to discharge electrons for the beam current measurement, and the active beam measurement surface is at least 90% of an entire area of the first surface of the multi-aperture array.

13. The multi-beam particle microscope of claim 1, wherein the multi-beam particle microscope is configured so that, during use, an average single beam current of the plurality of the first individual particle beams is at most 1% of the entire beam current measured by the first beam current measuring mechanism.

14. The multi-beam particle microscope of claim 1, wherein: the absorber layer comprises an absorber coating; and/or the absorber layer comprises at least one member selected from the group consisting of gold, silver, titanium, and platinum.

15. The multi-beam particle microscope of claim 1, wherein the multi-aperture array is a first multi-aperture array downstream of the condenser lens system, and the multi-aperture array is configured to divide the first charged particle beam into the plurality of first individual particle beams.

16. The multi-beam particle microscope of claim 1, wherein the multi-aperture array is not a first multi-aperture array downstream of the condenser lens system.

17. The multi-beam particle microscope of claim 1, wherein the controller is configured to set a voltage supplied to the extractor electrode to control the beam generating device.

18. The multi-beam particle microscope of claim 1, wherein the controller is configured to set a temperature of the particle source to control the beam generating device.

19. A multi-beam particle microscope, comprising: a beam generating system comprising a particle source, an extractor electrode and an anode, the beam generating system configured to produce a first charged particle beam; a multi-beam generator comprising a pre-aperture plate and a multi-aperture array, the multi-beam generator configured to produce a first field of a plurality of first individual charged particle beams from the first charged particle beam, the multi-aperture array downstream and adjacent to the pre-aperture plate, a first side of the multi-aperture array comprising an absorber layer configured to absorb charged particles, the absorber layer connected to a ground electrode to discharge excess electrons, a first side of the pre-aperture plate comprising a pre-aperture plate absorber layer configured to absorb charged particles, the pre-aperture plate absorber layer connected to a ground electrode to discharge excess electrons; a first beam current measuring mechanism configured to measure the discharged excess electrons generated by charged particles impinging on the pre-aperture plate; a condenser lens system between the beam generating system and the multi-beam generator; a first particle optical unit having a first particle optical beam path, the first particle optical unit configured to direct the first individual particle beams at a sample so that the first individual particle beams strike the sample at incidence locations, which form a second field; a detection system; a second particle optical unit having a second particle optical beam path, the second particle optical unit configured to image second individual particle beams, which emanate from the incidence locations in the second field, onto the detection system; a particle optical objective lens configured to have the first and the second individual particle beams pass therethrough; a beam switch in the first particle optical beam path between the multi-beam particle source and the objective lens, the beam switch in the second particle optical beam path between the objective lens and the detection system; and a controller configured to control the beam generating system, the particle optical objective lens, the first particle optical unit, the second particle optical unit, and the detection system, wherein the controller is configure to: drive the beam generating system based on a measurement made using the first beam current measuring mechanism; and/ or control the condenser lens system based on a measurement made using the first beam current measuring mechanism.

20. The multi-beam particle microscope of claim 19, further comprising a double deflector in a region of the condenser lens system, wherein the controller is configured to control the double deflector based on the measurement made using the first beam current measuring mechanism.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In this context, the disclosure will be understood even better with reference to the accompanying figures, in which:

[0051] FIG. 1: shows a schematic representation of a multi-beam particle microscope (MSEM);

[0052] FIG. 2: schematically illustrates a beam current measurement;

[0053] FIGS. 3A-3B: schematically illustrate multi-aperture arrays with an absorber layer on its upper side;

[0054] FIG. 4: compares a single beam current and a current generated by excess electrons discharged from an absorber layer of a multi-aperture array;

[0055] FIG. 5: schematically illustrates another beam current measurement;

[0056] FIG. 6: schematically illustrates a quadrant detector;

[0057] FIG. 7: schematically illustrates another quadrant detector;

[0058] FIG. 8: shows a schematic representation of an adjustment of the beam cone of the illuminating beam upon incidence on a multi-aperture array;

[0059] FIG. 9: shows a schematic representation of an electrostatic double deflector in the region of a condenser lens system;

[0060] FIG. 10: schematically shows a multi-beam particle microscope having closed-loop beam current control via and compensators that are controlled via a controller;

[0061] FIG. 11: schematically shows details about a beam current control;

[0062] FIG. 12: schematically illustrates details of another beam current control based on a measurement of X-rays; and

[0063] FIG. 13: schematically illustrates a beam current measuring via using X-rays converted into NIR radiation.

DETAILED DESCRIPTION

[0064] FIG. 1 is a schematic representation of a particle beam system 1 in the form of a multi-beam particle microscope 1, which uses a plurality of particle beams. The particle beam system 1 generates a plurality of particle beams which strike an object to be examined in order to generate there interaction products, e.g., secondary electrons, which emanate from the object and are subsequently detected. The particle beam system 1 is of the scanning electron microscope (SEM) type, which uses a plurality of primary particle beams 3 which are incident on a surface of the object 7 at a plurality of locations 5 and generate there a plurality of electron beam spots, or spots, that are spatially separated from one another. The object 7 to be examined can be of any desired type, e.g., a semiconductor wafer or a biological sample, and comprise an arrangement of miniaturized elements or the like. The surface of the object 7 is arranged in a first plane 101 (object plane) of an objective lens 102 of an objective lens system 100.

[0065] The enlarged excerpt I1 in FIG. 1 shows a plan view of the object plane 101 having a regular rectangular field 103 of incidence locations 5 formed in the first plane 101. In FIG. 1, the number of incidence locations is 25, which form a 5?5 field 103. The number 25 of incidence locations is a number chosen for reasons of simplified illustration. In practice, the number of beams, and hence the number of incidence locations, can be chosen to be significantly greater, such as, for example, 20?30, 100?100 and the like.

[0066] In the depicted embodiment, the field 103 of incidence locations 5 is a substantially regular rectangular field having a constant pitch P1 between adjacent incidence locations. Exemplary values of the pitch P1 are 1 micrometer, 10 micrometers and 40 micrometers. However, it is also possible for the field 103 to have other symmetries, such as a hexagonal symmetry, for example.

[0067] A diameter of the beam spots shaped in the first plane 101 can be small. Exemplary values of the diameter are 1 nanometer, 5 nanometers, 10 nanometers, 100 nanometers and 200 nanometers. The focusing of the particle beams 3 for shaping the beam spots 5 is carried out by the objective lens system 100.

[0068] The primary particles striking the object generate interaction products, e.g., secondary electrons, back-scattered electrons or primary particles that have experienced a reversal of movement for other reasons, which emanate from the surface of the object 7 or from the first plane 101. The interaction products emanating from the surface of the object 7 are shaped by the objective lens 102 to form secondary particle beams 9. The particle beam system 1 provides a particle beam path 11 for guiding the plurality of secondary particle beams 9 to a detector system 200. The detector system 200 comprises a particle optical unit with a projection lens 205 for directing the secondary particle beams 9 at a particle multi-detector 209.

[0069] The excerpt I2 in FIG. 1 shows a plan view of the plane 211, in which individual detection regions of the particle multi-detector 209 on which the secondary particle beams 9 are incident at locations 213 are located. The incidence locations 213 lie in a field 217 with a regular pitch P2 from one another. Exemplary values of the pitch P2 are 10 micrometers, 100 micrometers, and 200 micrometers.

[0070] The primary particle beams 3 are produced in a beam generating apparatus 300 comprising at least one particle source 301 (e.g., an electron source), at least one collimation lens 303, a multi-aperture arrangement 305 and a field lens 307. The particle source 301 produces a diverging particle beam 309, which is collimated or at least substantially collimated by the collimation lens 303 in order to shape a beam 311 which illuminates the multi-aperture arrangement 305.

[0071] The excerpt I3 in FIG. 1 shows a plan view of the multi-aperture arrangement 305. The multi-aperture arrangement 305 comprises a multi-aperture plate 313, which has a plurality of openings or apertures 315 formed therein. Midpoints 317 of the openings 315 are arranged in a field 319 that is imaged onto the field 103 formed by the beam spots 5 in the object plane 101. A pitch P3 between the midpoints 317 of the apertures 315 can have exemplary values of 5 micrometers, 100 micrometers, and 200 micrometers. The diameters D of the apertures 315 are smaller than the pitch P3 between the midpoints of the apertures. Exemplary values of the diameters D are 0.2?P3, 0.4?P3, and 0.8?P3.

[0072] Particles of the illuminating particle beam 311 pass through the apertures 315 and form particle beams 3. Particles of the illuminating beam 311 which strike the plate 313 are absorbed by the latter and do not contribute to the formation of the particle beams 3.

[0073] On account of an applied electrostatic field, the multi-aperture arrangement 305 focuses each of the particle beams 3 in such a way that beam foci 323 are formed in a plane 325. Alternatively, the beam foci 323 can be virtual. A diameter of the beam foci 323 can be, for example, 10 nanometers, 100 nanometers and 1 micrometer.

[0074] The field lens 307 and the objective lens 102 provide a first imaging particle optical unit for imaging the plane 325, in which the beam foci 323 are formed, onto the first plane 101 such that a field 103 of incidence locations 5 or beam spots arises there. Should a surface of the object 7 be arranged in the first plane, the beam spots are correspondingly formed on the object surface.

[0075] The objective lens 102 and the projection lens arrangement 205 provide a second imaging particle optical unit for imaging the first plane 101 onto the detection plane 211. The objective lens 102 is thus a lens that is part of both the first and the second particle optical unit, while the field lens 307 belongs only to the first particle optical unit and the projection lens 205 belongs only to the second particle optical unit.

[0076] A beam switch 400 is arranged in the beam path of the first particle optical unit between the multi-aperture arrangement 305 and the objective lens system 100. The beam switch 400 is also part of the second optical unit in the beam path between the objective lens system 100 and the detector system 200.

[0077] Further information relating to such multi-beam particle beam systems and components used therein, such as, for instance, particle sources, multi-aperture plate and lenses, can be obtained from the international patent applications WO 2005/024881 A2, WO 2007/028595 A2, WO 2007/028596 A1, WO 2011/124352 A1 and WO 2007/060017 A2 and the German patent applications DE 10 2013 016 113 A1 and DE 10 2013 014 976 A1, the disclosure of which in the full scope thereof is incorporated by reference in the present application.

[0078] FIG. 2 schematically illustrates a beam current measurement. A multi-aperture array 304 is shown in cross-sectional view. The multi-aperture array 304 comprises on its upper side an absorber layer 341 which can absorb charged particles. In the depicted embodiment, the entire upper side of the multi-aperture array 304 is covered by the absorber layer 341. In the present embodiment, the multi-aperture plate is arranged as the first multi-aperture array downstream of a condenser lens system (not shown in FIG. 2). The illuminating particle beam 311 is incident on the multi-aperture array 304. Most of the incident particles of the illuminating particle beam 311 impinge on the absorber layer 304a and only a small portion of all particles pass through the openings 304a thus generating the plurality of first individual charged particle beams 3. A part of the illuminating particle beam 311 impinges on the outer region 366 of the multi-aperture array 304, exemplarily, particle beams 311b are indicated in FIG. 2. Other portions of the illuminating particle beam 311 impinge on the multi-aperture array 304 in an inner region 367. Some of these particles are indicated exemplarily with a reference sign 311a in FIG. 2. In the present example, the multi-aperture array 304 is not structured. Therefore, all particles impinging on the absorber layer 341 generate excess electrons that are discharged from the absorber layer 341 and are measured by the first beam current measuring mechanism 370 which is embodied as a picoamperemeter in the present case. The measured value is communicated to the control 10 and is used for example for controlling the beam generating system, for example a voltage applied to the extractor electrode or by setting a temperature of the particle source. Other control loops are also possible.

[0079] In the example shown only one picoamperemeter is applied which is positioned between the connection to the absorber layer 341 on the one hand and the ground electrode on the other hand. Therefore, the whole area of the absorber layer 341 contributes to the measured value, this comprises a measurement of discharged excess electrons generated by charged particles impinging the multi-aperture array 304 in the outer region 366 as well as in the inner region 367. The depicted principle of measurement is a measurement over a large area which ensures a very good signal-to-noise ratio. In the example shown, an average single beam current of the plurality of the first individual particle beams 3 is equal to or less than 1/1000 of the beam current entirely measured by the first beam current measuring mechanism 370. It is noted that the dimensions in FIG. 2 are not true to scale.

[0080] FIGS. 3A-3B schematically illustrate multi-aperture arrays 304 with an absorber layer 341 on its upper side. In FIG. 3A the multi-aperture array 304 as already depicted in FIG. 2 is shown in a top view. It is noted that there is no structuring of the absorber layer 341, but the entire surface of the absorber layer 341 can be used for a measurement of excess electrons.

[0081] In contrast thereto, FIG. 3B depicts a multi-aperture plate 304 that is structured into two separate regions that are isolated from one another. The first region is identical with the outer region 366 which is defined as a region around all of the openings 304a in the multi-aperture array 304. The second region is the inner region 367 which comprises all openings 304a in the multi-aperture array 304. In the example shown the plurality of apertures 304a is arranged in a hexagonal way. Therefore, the structuring 368 or the isolation 368 is also provided as a hexagon. Of course, other shapes of an isolation could still be chosen, even if the overall arrangement of apertures 304a is chosen hexagonal. It is for example possible to choose a circle or to choose a rectangle, for example. It is noted that also in FIG. 3B the entire surface of the multi-aperture array 304 is provided with an absorber layer 341, wherein reference sign 341a indicates the absorber layer in the outer region 366 and reference sign 341b indicates the absorber layer in the inner region 367. The absorber layer in the inner region 367 and in the outer region 366 can be chosen to be made from an identical material; however, the material can be also chosen differently. According to an example, the absorber layers can comprise or consist of any one of the following: gold, silver, titanium, platinum. In general, noble metals with a good conductivity can be used.

[0082] According to the embodiment depicted in FIG. 3B, both absorber layers 341a and 341b can be connected to ground electrodes, respectively. The excess electrons discharged from the absorber layer 341a are measured by an ammeter 370 in any case. In contrast thereto, a measurement of the excess electrons discharged from the absorber layer 341b with another ammeter is optional. It is noted that there is no further structuring in the inner region 367 of the multi-aperture array 304. Therefore, formation of the plurality of first individual beams 3 is not at all disturbed by the presence of any structuring or electrodes on the multi-aperture array 304.

[0083] FIG. 4 compares a single beam current and a current generated by excess electrons discharged from an absorber layer 341 of a multi-aperture array 304. The curve indicated by reference sign C1 depicts the current measured with the first beam current measuring mechanism 370. Reference sign C2 indicates a single beam current (shifted in the graph) that was measured for example with a second beam current measuring mechanism, for example via a Faraday cup temporarily put on a stage during a calibration of the entire multi-beam particle microscope. A finding of this comparison is that the variations and fluctuations occurring in the single beam current (curve C2) are also reflected in the curve C1 and thus in a measurement that does not at all target a measurement of a single beam current, but is in general an ensemble measurement. This finding is a decisive base for allowing the change of measurement principle according to the present disclosure: it is not longer the aim to measure as many as possible single beam currents separately with separately provided additional detectors near respective apertures in the multi-aperture array 304. Instead, the target is a measurement with a very good signal-to-noise ratio that can be accomplished by an ensemble measurement and more precisely with a measurement over a large area on the multi-aperture array 304. As a side remark, it is noted that the desired proportionality between the single beam current on the one hand and the measured overall coating current cannot automatically be found by large area measurements carried out at other apertures within the system: an ensemble measurement carried out at an extractor aperture or at an aperture of the anode of the source did not show the desired proportionality between the two parameters.

[0084] FIG. 5 schematically illustrates another beam current measurement according to another embodiment of the disclosure: in this example, the multi-beam generator comprises a pre-aperture plate 380 and the multi-aperture array 304. As before, the entire surface of the multi-aperture array 304 is covered by an absorber layer 341 which is connected to ground. However, directly upstream of the multi-aperture array 304, the pre-aperture plate 380 is provided. Basically, this pre-aperture plate 380 covers or blocks the outer region 366 of the multi-aperture array 304 and generates its own outer region 366a. Particles impinging on the pre-aperture plate 380 are absorbed by the absorber layer 341a and converted into excess electrons that are discharged from the layer 341a and transported to the ground electrode. Within this line to ground, a first beam current measuring mechanism 370 in terms of a single pico amperemeter is provided. The measurement value is communicated to the controller 10. Once again, based on this measurement result, the controller 10 is configured for driving for example the beam generating system or for controlling the condenser lens system. Although this is not depicted in FIG. 5, it is optionally also possible to arrange a further constituent part of the first beam current measuring mechanism 370 in the line leading from the absorber layer 341 to ground and therefore basically to measure excess electrons generated by impinging particles in the inner region 367 onto the multi-aperture array 304 as well.

[0085] FIG. 6 schematically illustrates a quadrant detector: according to the shown embodiment, the multi-aperture plate 304 is structured into five separate regions 351, 352, 353, 354 and 367 that are isolated from one another. Each region 351, 352, 353, 354 and 367 is connected to ground. The first beam current measuring mechanism 370 comprises five constituent parts 370a, 370b, 370c, 370d and 370e in the embodiment shown. In each case, excess electrons are measured and the measurement result is communicated to the control 10. It is noted that the inner region 367 comprises all openings in the multi-aperture array 304. Therefore, the inner region 367 is not disturbed at all by any structuring or by separately provided detectors. This ensures a very good beam current quality of the generated individual particle beams 3. The outer region 366 is subdivided into the four quadrants 351, 352, 353 and 354. The quadrants 351 and 353 have an area that is identical in size. The same holds for the larger areas of regions 352 and 354. If the beam cone of the illuminating particle beam 311 impinges centered onto the multi-aperture array 304, the signals generated by the measurement of regions 351 and 353 shall indicate the same signal strength. The same holds for signals generated by measurements on the regions 352 and 354. In the different scenario when the beam cone of the illuminating particle beam 311 is shifted into one direction, the signal generated by each quadrant 351, 352, 353 and 354 shows a variation that allows for identifying the direction of the shift. This shift can be corrected, for example by controlling a double deflector in the region of the condenser lens system which allows for parallel shifting of the entire illuminating beam cone 311.

[0086] Of course, the quadrant detector depicted in FIG. 6 can in general be realized in a different way. The shape of the quadrants can be changed, so can be the arrangement of apertures itself which is in the present example depicted to be hexagonal.

[0087] In general, a directional variation of the entire illuminating beam cone 311 can already be identified by a detector that comprises only three outer regions: an example is a direction indicating tertial detector wherein the outer region 366 is subdivided into three different regions, such as spanning about 120 degrees of the outer region.

[0088] Of course, it also possible to further structure the outer region 366 into more than four separate regions. However, it is to be born in mind that any structuring or isolation provided on top of the multi-aperture array 304 bears the potential risk of deteriorating the beam quality of the plurality of first individual particle beams 3 which should be avoided. Furthermore, the bigger the area for a measurement is, the better is the signal-to-noise ratio that can be achieved for this kind of measurement. Optionally, the entire number of separate regions on a multi-aperture array 304 is not bigger than six regions, optionally, it is only exactly four or five separate and isolated regions.

[0089] In FIG. 6 the excess electrons stemming from the inner region 367 are measured by the first beam current measuring mechanism 370e. However, this measurement is only optional, it is not necessary in any case to provide a first beam current measuring mechanism 370e. Instead, the central region 367 can only be connected to the ground electrode without any further measurement put in between.

[0090] FIG. 7 schematically illustrates another quadrant detector with regions 355, 356, 357 and 358. Once again, the entire surface of the multi-aperture plate 304 is provided with an absorber layer 341. However, the example depicted in FIG. 7 has the disadvantage that there also exists a structuring/isolation within the inner region of the multi-aperture array 304 which bears the risk of unwanted deteriorations of the beam quality of the individual charged particle beams 3. The depicted example is therefore less advantageous, even though the desire to measure over a large area is still met.

[0091] FIG. 8 shows a schematic representation of an adjustment of the beam cone of the illuminating beam 311 upon incidence on a multi-aperture array 313. The beam current per individual particle beam 3 can be adjusted by adjusting the beam cone. Initially, particles or a divergent particle beam 309 are emitted by a source 301. The divergent particle beam 309 passes through a collimation lens system or condenser lens system 303, which comprises two condenser lenses 303.1 and 303.2 in the present example. FIG. 8 now shows two different settings of the condenser lens system 303: In a first setting, the condenser lens 303.1 is activated and the condenser lens 303.2 is deactivated. As a result, the particles of the divergent particle beam 309 are collimated in the condenser lens 303.1 and strike the multi-aperture array 313 as an illuminating particle beam 311.1 with the diameter d1. In the second case, the condenser lens 303.1 is deactivated and the condenser lens 303.2 is activated. Hence, the divergent particle beam 309 expands further and is only collimated in the second condenser lens 303.2 such that an illuminating particle beam 311.2 with the diameter d2 is incident on the multi-aperture plate 313. The number of particles incident on the multi-aperture array 313 is the same in both cases but the density differs. Thus, individual particle beams 3 with different beam current intensities that depend on the diameter of the illumination spot are formed when the multi-aperture array 313 with its openings 315 (not shown) is traversed.

[0092] In the example shown, the condenser lenses 303.1 and 303.2 are magnetic lenses in each case. However, it is also possible to replace one or both of the magnetic lenses with an electrostatic condenser lens. Moreover, it is possible to change the number of condenser lenses in the condenser lens system 303 overall, that is to say provide only one lens or else provide three or more lenses. Moreover, one or more deflectors can be provided for the adjustment of the illuminating beam 311. These adjustment approaches and the type of condenser lens(es) have an influence on how quickly the illumination spot can be adjusted. This will be discussed in more detail below, within the scope of this patent application. Initially, all that should be illustrated here is how the different beam currents of the individual particle beams arise when different illumination spots are used.

[0093] FIG. 9 illustrates further design options for a closed-loop beam current control mechanism. FIG. 9 depicts a ray of the divergent particle beam 309, which runs along the optical axis 105 and was generated via the beam generating system 301. It passes through the condenser lens system 303 having the first condenser lens 303.1 and the second condenser lens 303.2. Each one is a magnetic lens in the depicted example. An electrostatic double deflector with constituent parts 345 and 346 is arranged in the region of the condenser lens system 303. In relation to the particle optical beam path, the constituent part 345 is downstream of the first condenser lens 303.1 and the constituent part 346 is downstream of the second condenser lens 303.2 in the example shown. However, other arrangements of the double deflector in the region of the condenser lens system 303 are possible; by way of example, both constituent parts 345, 346 can be arranged downstream of the second condenser lens 303.2 in relation to the particle optical beam path.

[0094] The beam 311 can be offset in parallel by way of the double deflector. Upon incidence on the multi-aperture plate 313, the beam 311 is offset in relation to the optical axis 105 by the vector V. In this case, the electrostatic double deflector 345, 346 can be driven quickly and it is suitable for a high-frequency correction of an offset when the multi-aperture array 313 is illuminated. In turn, the double deflector 345, 346 can be driven on the basis of current values measured via a first beam current measuring mechanism, for example measured via the sensors 370 on the surface of the multi-aperture plate 313. This feedback loop can also be used for fast closed-loop current control during an image recording procedure.

[0095] Moreover, it is possible to form one of the condenser lenses 303 as an electrostatic condenser lens 303. This electrostatic condenser lens 303 can also be driven quickly and quasi instantaneously, in order to vary the diameter d of the illumination spot upon incidence on the multi-aperture plate 313 as a result. Once again, driving can be implemented in the form of a feedback loop based on current measurements which, in turn, have been determined for example via sensors 370 on the upper side of the multi-aperture array 313.

[0096] FIG. 10 schematically shows a multi-beam particle microscope 1 having closed-loop beam current control mechanisms and compensators that are driven via a controller 10. The controller 10 can be formed in one part or in many parts, the entire multi-beam particle microscope 1 being able to be controlled via the controller 10. In particular, the controller 10 controls the beam generating system 301, the components of the first particle optical unit, of the second particle optical unit, of the detection system 200 and further components of the multi-beam particle microscope 1, which may or may not be explicitly depicted. In the schematic representation of FIG. 10, only certain control elements and aspects in the context of the present disclosure are represented by connecting lines to selected particle optical components.

[0097] Initially, the beam current is measured via various beam current measuring mechanisms and the measured values are transmitted to the controller 10. In the example shown, a first beam current measuring mechanism which is configured to measure at least the discharged excess electrons generated by charged particles impinging the multi-aperture array in an outer region around all of the openings in the multi-aperture array can be connected to the micro-optics 306 comprising a multi-aperture array 313. In this case, this could be a detection arrangement as illustrated in FIG. 2, 3, 5, 6 or 7, for example. Additionally, an overall beam current is measured in the example shown via a sensor system arranged on or assigned to a beam stop 111. In this case, a multi-beam deflector 390 is used to steer the individual particle beams 3 onto the beam stop 111, which is arranged upstream of the objective lens 102 and level with a cross-over plane in the first particle optical beam path. In particular, the controller 10 can be configured to direct the first individual particle beams 3 into the beam stop 111 during a line jump or during an image jump when scanning over a sample surface. Thus, the overall beam current can be measured during an image recording procedure. Alternatively or additionally, the beam current of individual particle beams can be measured using a Faraday cup or an array of Faraday cups provided on the sample stage 503 for calibration purposes.

[0098] The components of the multi-beam particle microscope 1 are driven in a manner known per. This includes adjusting the extractor voltage in the beam generating system 301 and also driving the condenser lens system 303. The deflector 330 which is additionally depicted in FIG. 10 serves for static adjustment of the illuminating beam 311 upon incidence on the micro-optics 306. However, the multi-beam particle microscope 1 can comprises further components and control elements for low-frequency or high-frequency driving for the purposes of controlling the beam current:

[0099] Additionally or as an alternative, a condenser lens of the condenser lens system 303 can be designed as a fast electrostatic condenser lens and likewise be driven quickly. As a result, it is possible to quickly correct the diameter of the beam incident on the micro-optics 306.

[0100] For a fast correction of a lateral offset of the illumination spot, one or more electrostatic deflectors, in particular an electrostatic double deflector as depicted in FIG. 8 for example, may be additionally or alternatively provided in the condenser lens system 303. These deflectors can likewise be driven by way of a feedback signal based on a measured current value via the first beam current measuring mechanism.

[0101] FIG. 11 schematically shows details about a beam current control. More particularly, details of a source control loop are shown. A current monitoring processor 840 is configured for the control loop. The input signal for the control loop is the measurement carried out by the first beam current measuring mechanism which is configured to measure at least the discharged excess electrons generated by charged particles impinging the multi-aperture array 304 in an outer region around all of the openings in the multi-aperture array 304. The first beam current measuring mechanism which can be embodied by an ammeter and in particular a picoammeter is not shown in FIG. 11. However, schematically, the absorber layer 341 provided on the upper side of the multi-aperture array 304 is shown. The multi-aperture array 304 is part of the multi-aperture arrangement 305 which furthermore comprises a second multi-aperture 306 plate which can, for example, comprise a lens array, a deflector array and/or a stigmator array as well as a final multi-aperture plate 310. Other configurations are also possible.

[0102] The current monitoring processor 840 is part of the entire control 10 of the multi-beam particle microscope 1. The current monitoring processor 840 is configured for controlling the beam generating system 301 and/or the condenser lens system 303 on the basis of the measurement via the first beam current measuring mechanism 370. Other particle optical components can be controlled as well.

[0103] The beam generating system 301 comprises several parts. In the example shown, the beam generating system 301 comprises a source tip 301.1, a suppressor electrode 301.2 and an extractor electrode 301.3. The current monitoring processor 840 can for example be configured for controlling the beam generating device 301 by setting a voltage supplied to the extractor electrode 301.3. Additionally or alternatively, the controller 840 can be configured for controlling the beam generating device 301 by setting a temperature of a particle source 301.1, in particular by setting a heating current or heating voltage. Additionally or alternatively, a voltage supplied to the suppressor electrode 301.2 can be set.

[0104] Additionally or alternatively, the controller 840 can control the condenser lens system 303 which comprises in the present case three condenser lenses 303.a, 303.b and 303.c. They can be controlled for setting the focal length and also for setting the diameter of the illuminating particle beam 311 impinging on the multi-aperture arrangement 304 and more precisely impinging on the first multi-aperture array 304 in the example shown.

[0105] In the depicted embodiment, a double deflector 303.d, in particular an electrostatic double deflector 303.d, is provided in the region of the condenser lens system 303. The controller 840 is configured to control the double deflector 303.d on the basis of the measurement via the first beam current measuring mechanism 370.

[0106] Optionally, the controller 840 can also control the electrode 307.1 generating an immersion field in the first multi-aperture array 304. Optionally, a controlled multi-pole electrode for tilt correction can also be provided and controlled by the controller 840.

[0107] According to the above-described embodiments, the controlled variable in each case is a current generated by of the discharged excess electrons, the discharged excess electrons being generated by charged particles impinging the multi-aperture array 304 in an outer region 366 around all of the openings in the multi-aperture array 304. However, it is also possible to use another controlled variable which is not the current generated by discharged excess electrons: according to an alternative solution, the controlled variable is an X-ray detection.

[0108] FIG. 12 schematically illustrates details of another beam current control based on a measurement of X-rays 900. In the depicted embodiment, an X-ray detector 950 is provided instead of the amperemeters measuring discharged excess electrons. The X-rays 900 are generated by the charged particles impinging the absorber layer 341 on the upper side of the multi-aperture array 304. Experiments carried out by the inventors have shown that the amount of X-rays or number of X-ray photons measured by the X-ray detector 950 is proportional to the beam current of the first individual particle beams striking the sample at incidence locations. In the present case, the X-ray detector 950 is provided as a ring-shaped scintillator element in the circumference of the multi-aperture array 304. By this arrangement a good signal-to-noise ratio can be achieved. The controller 840 is then configured to control the beam generating system 301 on the basis of the measurement via the X-ray detector 950. The remaining elements of the current control by X-ray detection are identical with the elements already depicted and further described in FIG. 11; the same reference signs indicate the same elements. In order to avoid superfluous repetitions, explicit reference is made to FIG. 11 for further explanations.

[0109] FIG. 13 schematically illustrates another realization of a beam current measuring mechanism using X-rays 900 converted into NIR (near infrared) radiation. In the depicted example, the multi-aperture array 304 comprises a quartz plate 905 that is coated with the absorber layer 341. Instead of the quartz plate 905 other plates made from transparent materials can be used, such as PMMA, for example. The quartz plate is doped with a fluorescent material which serves as a scintillator. Charged particles such as electrons impinging the absorber layer 341 are converted into X-rays 900 first. Inside the quartz plate 905, the X-rays 900 are converted into photons or near infrared radiation 901. The photons 901 are guided inside the quartz plate 905 by internal reflection and are finally detected by one or more light detectors 910 arranged at the periphery of the quartz glass plate 905. By way of example, a point T at which total reflection of the photons 901 occurs is depicted in FIG. 13. The signals measured by the one or more light detectors 910 are communicated to the controller 10 (or its component 840, for example) and are used for controlling the beam generating system 301 and/or the condenser lens system 303. The other kinds of control schematically shown in FIG. 12 can also be carried out. Explicit reference is made to FIG. 12 and also to FIG. 11 in this respect.

[0110] Also according to this embodiment, the desired proportionality between the beam current of the individual particle beams striking the sample on the one hand and the near infrared radiation detected via the light detectors 910 shows the proportionality.

[0111] A multi-beam particle microscope with improved beam current control is disclosed. Excess electrons discharged from one or just a few regions of an absorber layer provided on a multi-aperture array are measured via an ammeter. The measured currents are used as controlled variables in a closed loop control. The measurement is large-area and low-noise. The multi-aperture array can be specifically structured to also realize a direction sensitive detection, for example via a quadrant detector or a tertial detector.

Example 1

[0112] A multi-beam particle microscope, comprising the following: [0113] a beam generating system comprising a particle source, an extractor electrode and an anode and configured to produce a first charged particle beam; [0114] a multi-beam generator having a multi-aperture array, the multi-beam generator being configured to produce a first field of a plurality of first individual charged particle beams from the first charged particle beam, the multi-aperture array comprising on its upper side an absorber layer which absorbs charged particles, the absorber layer being connected to at least one ground electrode to discharge excess electrons; [0115] an X-ray detector configured to detect X-rays generated by the charged particles impinging the absorber layer of the multi-aperture array; [0116] a first particle optical unit with a first particle optical beam path, configured to direct the generated first individual particle beams at a sample such that the first individual particle beams strike the sample at incidence locations, which form a second field; [0117] a detection system; [0118] a second particle optical unit with a second particle optical beam path, which is configured to image second individual particle beams, which emanate from the incidence locations in the second field, onto the detection system; [0119] a particle optical objective lens, through which both the first and the second individual particle beams pass; [0120] a beam switch, which is arranged in the first particle optical beam path between the multi-beam particle source and the objective lens and which is arranged in the second particle optical beam path between the objective lens and the detection system; and [0121] a controller which is configured to control the beam generating system, the particle optical objective lens, the first particle optical unit, the second particle optical unit, and the detection system, and [0122] with the controller being configured for driving of the beam generating system on the basis of the measurement via the X-ray detector, and/ or [0123] with the controller being configured for controlling the condenser lens system on the basis of the measurement via X-ray detector.

Example 2

[0124] The multi-beam particle microscope according to example 1, wherein the X-ray detector is provided as a ring-shaped scintillator element upstream of and in the circumference of the multi-aperture array.

Example 3

[0125] A multi-beam particle microscope, comprising the following: [0126] a beam generating system comprising a particle source, an extractor electrode and an anode and configured to produce a first charged particle beam; [0127] a multi-beam generator having a multi-aperture array, the multi-beam generator being configured to produce a first field of a plurality of first individual charged particle beams from the first charged particle beam, the multi-aperture array comprising on its upper side an absorber layer which absorbs charged particles, the absorber layer being connected to at least one ground electrode to discharge excess charged particles; [0128] an X-ray conversion mechanism for converting X-rays generated by the charged particles impinging the absorber layer of the multi-aperture array into NIR radiation; [0129] a light guide for guiding the NIR radiation to a light detector; [0130] the light detector configured for detecting NIR radiation [0131] a first particle optical unit with a first particle optical beam path, configured to direct the generated first individual particle beams at a sample such that the first individual particle beams strike the sample at incidence locations, which form a second field; [0132] a detection system; [0133] a second particle optical unit with a second particle optical beam path, which is configured to image second individual particle beams, which emanate from the incidence locations in the second field, onto the detection system; [0134] a particle optical objective lens, through which both the first and the second individual particle beams pass; [0135] a beam switch, which is arranged in the first particle optical beam path between the multi-beam particle source and the objective lens and which is arranged in the second particle optical beam path between the objective lens and the detection system; and [0136] a controller which is configured to control the beam generating system, the particle optical objective lens, the first particle optical unit, the second particle optical unit, and the detection system, and [0137] with the controller being configured for driving of the beam generating system on the basis of the measurement via the light detector, and/ or [0138] with the controller being configured for controlling the condenser lens system on the basis of the measurement via the light detector.

Example 4

[0139] The multi-beam particle microscope according to example 3, [0140] wherein the light guide comprises a quartz glass plate doped with a scintillating material for converting X-rays into NIR radiation; and [0141] wherein the light detector is arranged at the periphery of the quartz glass plate.

LIST OF REFERENCE SIGNS

[0142] 1 Multi-beam particle microscope [0143] 3 Primary particle beams (individual particle beams) [0144] 5 Beam spots, incidence locations [0145] 7 Object [0146] 9 Secondary particle beams [0147] 10 Computer system, controller [0148] 11 Secondary particle beam path [0149] 13 Primary particle beam path [0150] 25 Sample surface, wafer surface [0151] 100 Objective lens system [0152] 101 Object plane [0153] 102 Objective lens [0154] 103 Field [0155] 105 Optical axis of the multi-beam particle microscope [0156] 108 Cross-over [0157] 110 Collective scan deflector [0158] 111 Beam stop with a second current measuring mechanism [0159] 200 Detector system [0160] 205 Projection lens [0161] 207 Detection region [0162] 208 Deflector for adjustment purposes [0163] 209 Particle multi-detector [0164] 211 Detection plane [0165] 212 Cross-over [0166] 213 Incidence locations [0167] 214 Aperture filter [0168] 215 Detection region [0169] 216 Active element [0170] 217 Field [0171] 218 Deflector system [0172] 220 Multi-aperture corrector, individual deflector array [0173] 222 Collective deflection system, anti-scan [0174] 300 Beam generating apparatus [0175] 301 Particle source, beam generating system [0176] 303 Collimation lens system [0177] 304 multi-aperture array [0178] 304a opening [0179] 305 Multi-aperture arrangement [0180] 306 Micro-optics [0181] 307 Field lens [0182] 308 Field lens [0183] 309 Diverging particle beam [0184] 311 Illuminating particle beam [0185] 313 Multi-aperture plate, multi-aperture array [0186] 315 Openings in the multi-aperture plate [0187] 316 Hexagon [0188] 317 Midpoints of the openings [0189] 319 Field [0190] 323 Beam foci [0191] 325 Intermediate image plane [0192] 326 Field lens system [0193] 330 Deflector [0194] 340 Tip [0195] 341 absorber layer [0196] 342 Extractor electrode [0197] 343 Anode [0198] 345 Deflector [0199] 346 Deflector [0200] 351 Region [0201] 352 Region [0202] 353 Region [0203] 354 Region [0204] 360 Beam current intensity representation [0205] 366 outer region [0206] 367 inner region [0207] 368 structuring, isolation [0208] 370 First beam current measuring mechanism, ammeter, picoampere meter [0209] 380 pre-aperture plate [0210] 390 Multi-beam deflector [0211] 400 Beam switch [0212] 420 Magnetic element [0213] 500 Sample stage [0214] 503 Voltage supply for the sample [0215] 900 X-ray [0216] 901 Photon, NIR radiation [0217] 905 Quartz plate [0218] 910 Light detector [0219] 950 X-ray detector [0220] d1 Beam cone diameter [0221] d2 Beam cone diameter [0222] V Displacement between beam cone midpoint and multi-aperture array midpoint [0223] T point of total reflection