Multi-beam charged particle imaging apparatus

Abstract

A charged particle imaging apparatus comprising: A specimen holder, for holding a specimen; A particle-optical column, for: Producing a plurality of charged particle beams, by directing a progenitor charged particle beam onto an aperture plate having a corresponding plurality of apertures within a footprint of the progenitor beam; Directing said beams toward said specimen,
wherein: Said aperture plate comprises a plurality of different zones, which comprise mutually different aperture patterns, arranged within said progenitor beam footprint; The particle-optical column comprises a selector device, located downstream of said aperture plate, for selecting a beam array from a chosen one of said zones to be directed onto the specimen.

Claims

1. A charged particle imaging apparatus comprising: a specimen holder, for holding a specimen; and a particle-optical column, configured to: produce a plurality of charged particle beams, by directing a progenitor charged particle beam onto an aperture plate having a corresponding plurality of apertures within a footprint of the progenitor charged particle beam; and direct the plurality of charged particle beams toward said specimen; characterized in that: said aperture plate comprises a plurality of different zones arranged within said progenitor charged particle beam footprint, wherein the each of the plurality of different zones comprise mutually different aperture patterns; and the particle-optical column comprises a selector device, located downstream of said aperture plate, wherein the selector device is configured to select a beam array from a chosen zone of the plurality of different zones to be directed onto the specimen.

2. An apparatus according to claim 1, wherein at least two of said plurality of different zones have an essentially identical aperture distribution.

3. An apparatus according to claim 1, wherein one of said plurality of different zones comprises a singular through-hole, configured to allow passage of a portion of said progenitor charged particle beam without subdividing it.

4. An apparatus according to claim 3, wherein at least one of a variable opening in said selector device, and an adjustable lens assembly disposed between said aperture plate and said selector device, is used to curtail a beam current value of a beam passing through said singular through-hole in said aperture plate.

5. An apparatus according to claim 1, wherein said selector device comprises a masking plate with a restrictive opening that can be positioned so as to allow only the selected beam array from the chosen zone to pass to the specimen.

6. An apparatus according to claim 5, wherein said selector device further comprises a deflector assembly, located prior to the masking plate, wherein the deflector assembly is configured to deflect said plurality of beams relative to said masking plate so that only the selected beam array from the chosen zone is directed through said restrictive opening of the masking plate.

7. An apparatus according to claim 1, comprising a scanning assembly for producing a relative scanning motion of the specimen and beam array.

8. An apparatus according to claim 1, selected from a group comprising a charged particle microscope and a charged particle lithography imaging system.

9. A method of using a charged particle imaging apparatus comprising: a specimen holder, for holding a specimen; a particle-optical column, configured to: produce a plurality of charged particle beams by directing a progenitor charged particle beam onto an aperture plate, wherein the aperture plate comprises: a corresponding plurality of apertures within a footprint of the progenitor charged particle beam; and a plurality of different zones which comprise mutually different aperture patterns arranged within the progenitor beam charged particle footprint; direct said plurality of charged particle beams toward said specimen; and select using a selector device located downstream of said aperture plate, a beam array from a chosen zone of said plurality of different zones to be directed onto the specimen.

10. A method according to claim 9, wherein: in a first use session, a first of said plurality of different zones is selected to irradiate the specimen with a beam array in which each beam has a first beam current value; in a second use session, a second of said plurality of different zones is selected to irradiate the specimen with a beam array in which each beam has a second, different beam current value.

11. A method according to claim 9, wherein: said aperture plate is configured to comprise a zone having a singular through-hole, for allowing passage of a portion of said progenitor charged particle beam without subdividing it; and said progenitor charged particle beam is directed onto said through-hole, so as to produce a single-beam operational mode of said apparatus.

12. An apparatus according to claim 2, wherein the apertures of a first zone of the at least two of the plurality of different zones have a first diameter, and the apertures of a second zone of the at least two of the plurality of different zones have a second diameter that is different from the first diameter.

13. A method according to claim 10, wherein said first plurality of different zones and said second plurality of different zones have an essentially identical aperture distribution.

14. A method according to claim 13, wherein the apertures of the first plurality of different zones have a first diameter, and the apertures of the second plurality of different zones have a second diameter that is different from the first diameter.

15. A method according to claim 11, wherein the selector device is configured to change the operational mode of said apparatus from a multibeam mode of operation to the single beam mode of operation by selecting beam array from the zone having a singular through-hole.

Description

(1) The invention will now be elucidated in more detail on the basis of exemplary embodiments and the accompanying schematic drawings, in which:

(2) FIG. 1 renders a longitudinal cross-sectional elevation view of an embodiment of a charged particle imaging apparatusin this case, an electron microscopein which the present invention is implemented.

(3) FIG. 2 illustrates a manner in which an electron beam array can be generated from a single progenitor beam.

(4) FIG. 3 shows elevation and plan views of various components disposed along the particle-optical axis in an embodiment of a charged particle apparatus according to the invention.

(5) FIGS. 4A and 4B show alternative embodiments of possible aperture plates that can be used in embodiments of the present invention.

(6) In the Figures, where pertinent, corresponding parts are indicated using corresponding reference symbols.

EMBODIMENT 1

(7) FIG. 1 (not to scale) is a highly schematic depiction of an embodiment of charged particle imaging apparatus in which the present invention is exploited; more specifically, it shows an embodiment of a SEM-though, in the context of the current invention, it could also be a STEM or a lithography imaging system, for example. The microscope M comprises a particle-optical column/illuminator 1, which produces an electron beam (charged particle beam) that propagates along a particle-optical axis B. The particle-optical column 1 is mounted on a vacuum chamber 3, which comprises a specimen holder 17 and associated stage/actuator 19 for holding/positioning a specimen S. The vacuum chamber 3 is evacuated using vacuum pumps (not depicted). With the aid of voltage source 21, the specimen holder 17, or at least the specimen S, may, if desired, be biased (floated) to an electrical potential with respect to ground.

(8) The particle-optical column 1 comprises an electron source (charged particle source) 5 (such as a Schottky emitter, cold FEG or LaB.sub.6 filament, for example), lenses 11, 13 to focus the electron beam onto the specimen S, and a deflection unit 15 to perform beam deflection/scanning of the beam. By scanning an electron beam over the specimen S, output radiationcomprising, for example, a flux of X-rays, infrared/visible/ultraviolet light, secondary electrons and/or backscattered electronsemanates from the specimen S. Detectors 23, 27 can be chosen from a variety of possible detector types that can be used to examine different types/modalities of such output radiation. In the apparatus depicted here, the following detector choices have been made: Detector 23 is a segmented electron detector, comprising a plurality of independent detection segments (e.g. quadrants) disposed about a central aperture 25 (encompassing the optical axis B). Such a detector can, for example, be used to investigate the angular dependence of a flux of electrons emerging from the specimen S. Detector 27 is, for example, an X-ray detector, which can be used to register X-rays emanating from the specimen S, and thus perform Energy-Dispersive X-ray Spectroscopy (EDX). It could alternatively be a cathodoluminescence detector, for example.
Alternatively/supplementally, use could be made of a backscattered electron detector as set forth in the aforementioned co-pending patent application EP18176596.7, for example. Since the detected output radiation is position-dependent (due to said scanning motion), the information obtained from the detectors 23, 27 will also be position-dependent, and can thus be used to assemble an image that is basically a map of detector output as a function of scan-path position on the specimen S. The signals from the detectors 23, 27 pass along control lines (buses) 29, are processed by the controller 29, and displayed on display unit 31. Such processing may include operations such as combining, integrating, subtracting, false colouring, edge enhancing, and other processing known to the skilled artisan. In addition, automated recognition processes (e.g. as used for particle analysis) may be included in such processing.

(9) Various refinements and alternatives of such a basic set-up will be known to the skilled artisan, including, but not limited to: The use of dual primary beam speciesfor example an electron beam for imaging and an ion beam for machining (or, in some cases, imaging) the specimen S; The use of a controlled environment at the specimen Sfor example, maintaining a pressure of several mbar (as used in a so-called Environmental SEM) or by admitting gases, such as etching or precursor gases.

(10) Of importance to the present invention is the fact that a single primary electron beam as used in a conventional SEMis here replaced by a multi-beam array. FIG. 2 shows how such a multi-beam array can be generated, using a relatively small modification to FIG. 1. A progenitor electron beam B.sub.o leaves the electron source 5, traverses a series of (extraction/acceleration) electrodes 7, and impinges upon an aperture plate 9, which contains a plurality of apertures 9; such a structure 9 can also be referred to as an Aperture Lens Array (ALA)since the apertures 9 have a lensing effectand can, for example, be manufactured by using thin film (MEMS) technology to etch an array of small holes in a silicon sheet (see the aforementioned PhD thesis). As a result of impinging on the ALA 9, the progenitor beam B.sub.o is converted into a plurality B of sub-beams/beamlets/component beams B, in the same geometric configuration as the plurality of holes 9 used to generate them. This beam array B then follows its course along axis B through the illuminator 1, which directs it onto the specimen S (see FIG. 1).

(11) In the current invention, the ALA 9 takes a special form, in that it comprises a plurality of different zones {Z} having mutually different aperture patternsarranged within the beam footprint (upon the ALA 9) of progenitor beam B.sub.o (so that multiple zones are concurrently illuminated/traversed by beam B.sub.o). An example of such a scenario is illustrated in FIG. 3, in which: The left hand side of the Figure shows an elevation view of certain components disposed along the electron path (refer also to FIGS. 1 and 2); The right hand side of the Figure shows separate plan views of two particular components not to the same scalenamely the ALA 9 and (part of) the selector device 33.
Starting with the inventive ALA 9, it is seen in the current example that this comprises five different zones, namely: Four essentially square quadrant zones Z.sub.1, Z.sub.2, Z.sub.3, Z.sub.4. These each have a 1414 orthogonal configuration of beam apertures, but they are mutually different as regards the diameters/widths of these apertures. More specifically, zone Z.sub.1 has the largest apertures and zone Z.sub.4 has the smallest, with zones Z.sub.2 and Z.sub.3 exhibiting a progression between these two extremal cases. For clarity, the perimeter of zone Z.sub.1 is marked by a dashed outline; the other quadrant zones Z.sub.2, Z.sub.3, Z.sub.4 are similarly defined, but their perimeters are not dashed, so as to avoid cluttering the Figure. In the set-up illustrated here, the beam apertures are circular, but this does not necessarily have to be the case, and other shapes (such as elliptical) are also possible. A central zone Z.sub.5 (located at the intersection/common corner of zones Z.sub.1-Z.sub.4), which here takes the form of a (relatively large) circular through-hole. This zone Z.sub.5 allows use of the depicted apparatus in single-beam mode, if desired, as will be explained in more detail below.
Moving now to the inventive selector device 33, it is seen that this is located downstream of the ALA 9 (refer also to FIG. 1) and, in the current example, comprises two sub-components, namely a masking plate 331 and a beam deflector assembly 332. In this particular situation, the masking plate 331 comprises a movable strip 331, which can be positioned/moved within the beam path by an actuator 331, such as an electric motor. The illustrated strip 331 (which may, for example, be comprised of a thin sheet of (MEMS-processed) silicon) has (in this particular case) seven different regions, 35a, 35b, . . . , 35g, each of which has a corresponding restrictive opening 37a, 37b, . . . 37g. These openings (windows) 37a-37g have the following form: In the case of four regions 35a, 35b, 35c, 35d, the corresponding openings 37a, 37b, 37c, 37d take the form of an open quadrant, though the relative positions of these open quadrants 37a-37d differs between the regions 35a-35d. Comparing the regions 35a-35d to zones Z.sub.1-Z.sub.4 of ALA 9, it is seen that openings 37a (upper right), 37b (upper left), 37c (lower left), 37d (lower right) positionally correspond to zones Z.sub.1, Z.sub.2, Z.sub.3 and Z.sub.4, respectively. The sizes of openings 37a-37d are matched to the sizes of zones Z.sub.1-Z.sub.4 at the plane in which selector device 33 is located. In the case of three regions 35e, 35f, 35g, the corresponding openings 37e, 37f, 37g take the form of a central round hole, though the size of these round holes 37e-37g differs between the regions 35e-35g. Comparing the regions 35e-35g to ALA 9, it is seen that all openings 37e-37g positionally correspond to zone Z.sub.5. The sizes of openings 37e-37g are tailored to the size of zone Z.sub.5 at the plane in which selector device 33 is located, whereby: Largest opening 37e corresponds exactly to the size of Z.sub.5 at this plane, or is slightly larger. Openings 37f, 37g are progressively smaller than opening 37e.
As regards the operation of the beam selector 33 in concert with the ALA 9, this essentially involves two aspects, namely: Moving masking plate 331 so as to place a chosen one of the restrictive openings 37a-37g close to/upon the particle-optical axis B; Invoking the beam deflector assembly 332which may, for example, comprise a set of electrostatic deflection electrodesto position (a beam array/beam cross-section portion emerging from) a selected one of the zones Z.sub.1-Z.sub.5 upon the restrictive opening in question.
It is evident that using masking plate 331 and deflector assembly 332 in this way (e.g. according to positional entries in a lookup table) will result in admission (and passage onward toward specimen S) of a beam array from a chosen one of the zones Z.sub.1-Z.sub.5. More specifically: In the case of zones Z.sub.1-Z.sub.4, since each of these zones contains apertures of a different width, they will each result in a different beam current value for the various beams constituting the orthogonal 1414 multi-beam arrays concerned. In the case of zone Z.sub.5, the beam current of this single-beam mode can be curtailed by selecting opening 37f or 37g rather than opening 37e.
The skilled artisan will understand that the specific one-to-one correspondence set forth above does not have to be rigidly adhered to; for example, a magnetic lens (assembly) located upstream of the masking plate 331 can be used to rotate the beam array impinging thereupon, thus potentially allowing (depending on the chosen degree of rotation) various different (quadrant-shaped) zones to be directed through a given restrictive opening, one at a time.

(12) The particular dimensioning of the components 9 and 331, and of the apertures therein, will depend on various factors, such as the specific charged particle column design, desired beam current values, etc., and the skilled artisan will be well able to select values that suit the needs of a given situation. However, by way of providing some general guidance, the following non-binding examples may be considered: Typical diameters of individual apertures in zones Z.sub.1-Z.sub.4: ca. 1-50 m. Typical diameter of through-hole in zone Z.sub.5: ca. 50-300 m. Typical diameters of restrictive openings 37a-37d: ca. 0.1-1.5 mm. Typical diameter of restrictive openings 37e-37g: ca. 4-200 m. Typical area of ALA 9 impinged upon by progenitor beam Bo: up to ca. 1.51.5 mm.sup.2.

(13) As regards the ALA 9 and selector device 33, it should be explicitly noted that the examples illustrated and described here are non-binding, and that many alternatives are possible, e.g. as regards the number and relative positioning/arrangement of zones {Zi} in ALA 9, the manner in which such zones mutually differ from one another, the numbers/shapes of openings in strip 331, etc. See, in this regard, the next Embodiment, for example.

EMBODIMENT 2

(14) FIGS. 4A and 4B illustrate alternative embodiments of possible aperture plates that can be used in embodiments of the present invention. More specifically: FIG. 4A shows an aperture plate/ALA that is similar to ALA 9 in FIG. 3, in that it contains four quadrant-like zones with an orthogonal multi-beam configuration, nested about a single-beam through-hole. However, the mutual separation between the quadrant-like zones is bigger here, allowing a larger through-hole in the middle. Such an arrangement is a roomier alternative to the ALA 9 in FIG. 3. FIG. 4B shows a situation in which the single-beam through-hole has been moved from a central position and located instead in a quadrant of its own. Such an arrangement allows simplification of the attendant masking plate, if desired, since all zones of the ALA can now be serviced by a (movable) square or circular restrictive opening.