Distinctive Imaging Techniques in Scanning Transmission Charged Particle Microscopy

Abstract

Differential imaging technique in a scanning transmission charged particle microscope. A method for imaging a sample in a scanning transmission charged particle microscope, comprising the following steps: - providing a sample on a sample holder; - providing a charged particle beam directed from a source through an illuminator to illuminate the sample; - providing a segmented detector for detecting a flux of charged particles passing through the sample, the flux forming a beam footprint on the detector; - causing the beam to scan the surface of the sample, combining signals from different segments of the detector to produce a vector output from the detector at each scanning position, and compiling the data to produce an imaging vector field; - mathematically processing the imaging vector field by performing a two-dimensional integration operation on the imaging vector field to produce an integrated vector field image of the sample, specifically comprising: - using a restricted sub-region of the beam footprint to produce the vector output, and the accompanying imaging vector field and integrated vector field image.

Description

Technical Field

[0001] The present invention relates to a method for imaging a sample in a scanning transmission charged particle microscope, comprising the following steps:

[0002] - providing a sample on a sample holder;

[0003] - providing a charged particle beam directed from a source through an illuminator to irradiate a sample;

[0004] - providing a segmented detector for detecting a flux of charged particles passing through the sample, the flux forming a beam footprint on said detector;

[0005] - scanning the beam over the surface of the sample, combining the signals from different sections of the detector to produce a vector output from the detector at each scan position, and compiling the data to produce an imaging vector field;

[0006] - mathematically processing the imaging vector field by performing a two-dimensional integration operation on the imaging vector field, thereby generating an integrated vector field image of the sample.

[0007] The invention also relates to a scanning transmission charged particle microscope (STCPM) capable of carrying out the method. Background Art

[0008] Charged particle microscopy, particularly in the form of electron microscopy, is a well-known and increasingly important technique for imaging microscopic objects. Historically, the basic class of electron microscopes has evolved into a number of well-known device types, such as the transmission electron microscope (TEM), the scanning electron microscope (SEM) and the scanning transmission electron microscope (STEM), as well as various sub-classes such as the so-called "dual beam" tools (e.g., FIB-SEM) which additionally employ a "machining" focused ion beam (FIB), e.g. allowing supporting activities such as ion beam milling or ion beam induced deposition (IBID). More specifically:

[0009] - In a SEM, irradiation of a sample with a scanning electron beam results in the emission of "secondary" radiation in the form of, for example, secondary electrons, backscattered electrons, X-rays, and cathodoluminescence (infrared, visible and / or ultraviolet photons); one or more components of this emitted radiation are then detected and used for image accumulation purposes.

[0010] - In a TEM, the electron beam used to irradiate the sample is chosen to have a sufficiently high energy to penetrate the sample (for this reason, it is usually thinner than a SEM sample); the transmitted electrons emitted from the sample can then be used to create an image. When such a TEM is operated in scanning mode (thus becoming a STEM), the image in question is accumulated during the scanning movement of the irradiating electron beam.

[0011] As an alternative to using electrons as the irradiation beam, charged particle microscopy can also be performed using other kinds of charged particles. In this regard, the phrase "charged particles" should be interpreted broadly to include, for example, electrons, positive ions (e.g., Ga or He ions), negative ions, protons and positrons. Regarding non-electron-based charged particle microscopy, some further information can be gleaned from references such as the following:

[0012] https: / / en.wikipedia.org / wiki / Focused_ion_beam

[0013] http: / / en.wikipedia.org / wiki / Scanning_Helium_Ion_Microscope

[0014] - W. H. Escovitz, T. R. Fox and R. Levi-Setti, "Scanning Transmission Ion Microscope with a Field Ion Source," Proc. Nat. Acad. Sci. 72 (5), pp. 1826-1828 (1975)

[0015] http: / / www.ncbi.nlm.nih.gov / pubmed / 22472444

[0016] It should be noted that besides imaging and performing (local) surface modifications (such as milling, etching, deposition, etc.), charged particle microscopes can also have other capabilities, such as performing spectroscopy, examining diffraction patterns, studying ion channeling / ion backscattering (Rutherford backscattering spectroscopy), etc.

[0017] Typically, in all cases, a charged particle microscope (CPM) will include at least the following components:

[0018] - a particle source, such as a W or LaB6 source, a Schottky gun or a cold field emission gun (CFEG) in the case of electrons, or a liquid metal ion source (LMIS) or a nanoaperture ion source (NAIS) in the case of ions.

[0019] - an illuminator (illumination system; illuminating particle-light column) which manipulates the "raw" radiation beam coming from the source and performs certain operations on it, such as focusing, aberration mitigation, cropping (with diaphragms), filtering, etc. It will usually include one or more (charged particle) lenses and may also include other types of (particle) optical components. If necessary, the illuminator can be provided with a deflector system that can be called up so that its outgoing beam performs a scanning movement over the sample being studied.

[0020] - a sample holder on which the sample to be investigated can be fixed and positioned (e.g. tilted, rotated). If necessary, this holder can be moved to achieve a scanning movement of the sample relative to the beam. Generally, such a sample holder will be connected to a positioning system. When designed to accommodate cryogenic samples, the sample holder may include means for keeping said sample at a cryogenic temperature, for example using a suitably connected cryobarrel.

[0021] - Detectors (for detecting radiation emanating from the irradiated sample) which may be unitary or composite / distributed in nature and may take many different forms, depending on the radiation being detected. Examples include photodiodes, CMOS detectors, CCD detectors, photovoltaic cells, X-ray detectors (such as silicon drift detectors and silicon (lithium) detectors), etc. In general, a CPM may include several different types of detectors, the selection of which may be invoked in different circumstances. In the context of the present invention, the detector employed will be segmented (having a subdivided detection surface), for example as in the case of quadrant detectors, pixelated detectors (camera sensors), position sensitive detectors (PSDs), etc.

[0022] Transmission Charged Particle Microscopy (TCPM; e.g. TEM) will specifically include:

[0023] - An imaging system (imaging particle-optical column), which basically collects the charged particles transmitted through the sample (plane) and directs (focuses) them onto an analytical device, such as a detection / imaging device, a spectroscopic device (such as an EELS device; EELS = Electron Energy Loss Spectrometer), etc. As with the illuminator mentioned above, the imaging system may also perform other functions, such as mitigating aberrations, cropping, filtering, etc., and typically includes one or more charged particle lenses and / or other types of particle optical components.

[0024] In the following, by way of example, the invention may sometimes be described in the specific context of an electron microscope; however, this simplification is for clarity / illustrative purposes only and should not be construed as limiting.

[0025] The method proposed in the above opening paragraph is utilized in so-called integral vector field (iVF) imaging, which is described in US 9,312,098 and US 2016 / 307729 A1, both of which have the same inventors as the present invention and are incorporated herein by reference. Although this recently developed imaging technique has produced excellent results so far, the inventors have attempted to improve it further. The result of this effort is the subject of the present invention. Summary of the invention

[0026] It is an object of the present invention to produce an improved iVF imaging technique. More specifically, it is an object of the present invention that the imaging technique should be usable to improve the quality of iVF imaging of multi-element samples.

[0027] These and other objectives are achieved by the method described in the first paragraph above, characterized in that:

[0028] - generating the vector output using a restricted sub-region of the beam footprint, and accompanying imaged vector field and integrated vector field images.

[0029] The present invention exploits the recognition that the beam footprint on the detector (essentially a bright field (BF) circle or a convergent beam electron diffraction (CBED) disk) is typically not uniform, but has an internal structure (essentially associated with diffraction pattern fringe effects, resulting in local intensity maxima / minima). In order to "explore" this internal structure using iVF imaging, the present invention is able to "zoom in" on a specific sub-region of the footprint, and compile the imaging vector field for that specific sub-region rather than for the entire footprint; thus, the associated iVF image will contain amplitude / phase information only from the specific sub-region of interest. As will be explained in more detail below, this creates the possibility to construct a composite "global" iVF image from many contributing "local" iVF images obtained from various different sub-regions within the beam footprint - typically creating a "the whole is greater than the sum of its parts" situation. For example, in the case of a sample containing various elemental constituents of different atomic numbers Z, it is found that:

[0030] - relatively low-Z components tend to preferentially scatter emitted beam particles into the central region of the beam footprint on the detector;

[0031] - Relatively high-Z components tend to preferentially scatter transmitted beam particles into peripheral regions of the beam footprint on the detector.

[0032] Being able to image separately and, if desired, subsequently combine iVF images from different sub-regions of the beam footprint allows the signals from the lower Z and higher Z components to be "unmixed" with each other and better represented than in conventional non-differentiated iVF imaging. The technician will be able to select the location, size and / or shape of the particular sub-region of the beam footprint that interests him.

[0033] In a specific embodiment of the invention:

[0034] - the detector employed is embodied as a pixelated detector comprising an array of pixels;

[0035] -The vector output is compiled by:

[0036] • comparing pixel values ​​to determine the position of the intensity centroid of said sub-region of the beam footprint on the detector; and

[0037] ●The coordinate position of the center of gravity is indicated on the detector.

[0038] Because the individual pixels in a pixelated detector are relatively small, and are typically present in relatively large numbers (e.g., in a 2048×2048 array), such an embodiment allows for relatively high discriminating resolution in its ability to select / isolate relatively small sub-regions of the beam footprint and their associated intensity centroids. For example, the centroids of selected sub-regions can be relatively easily located using (proprietary) image analysis software. It is also relatively easy to select / adjust specific sub-regions of the footprint, for example by using a mouse-controlled reshape function on a visual user interface. See, for example, Figure 3A .

[0039] In an alternative embodiment of the invention:

[0040] - the employed detector is configured to comprise an annular detection surface which is subdivided into a plurality of sectors of substantially equal area;

[0041] - Compiling the vector output by determining weighted signals from different sectors.

[0042] This device is essentially a derivative of the conventional quadrant detector, using an annular rather than a "pie-shaped" detection surface. Seeing that the beam footprint on the detector plane is essentially a disk, the use of an annular detection surface more easily allows signal to be collected from specific sub-areas of the footprint (rather than the entire footprint). In a further extension of this concept, the detector is configured to include a set of nested concentrically arranged annular detection surfaces.

[0043] In such a configuration, for example, one can observe / predict how the beam footprint "sits" on the detector, and then choose to use detection signals from (one or more) rings that are hit by a desired sub-region of the footprint. See, for example Figure 3B .

[0044] In any of the embodiments discussed in the previous two paragraphs, the selection of a specific sub-region may include the use of an aperture plate in the flux between the sample and the detector. Such an aperture plate may be used to adjust the size / shape / position of a specific sub-region of the beam footprint that is allowed to impinge on the detector. The aperture plate may, for example:

[0045] - an adjustable masking device, such as a diaphragm, or an XY selector similar to a set of reticle masking blades; and / or

[0046] - The flux (beam path) can be moved in / out using a switch mechanism such as a turntable, robotic arm, etc. If desired, a selection of different well plates can be stored in a library / rack, allowing the selection of sub-areas of different sizes / shapes / positions by selecting the appropriate well plate and positioning it appropriately between the sample and the detector.

[0047] The aperture plates described herein may also be used in conjunction with a "conventional" multi-sector detector (eg a quadrant detector) to select specific sub-regions of the detector that will be allowed entry.

[0048] In a specific embodiment of the invention:

[0049] - The samples studied include various elemental components with different atomic numbers (Z);

[0050] - Selecting selected sub-regions of the beam footprint in order to distinctively record sub-ranges of atomic numbers.

[0051] As discussed above, elements with higher Z values ​​tend to scatter further from the (nominal) particle-optical axis (the center of the beam footprint on the detector) preferentially than elements with lower Z values; thus, separate iVF imaging of different radial sub-regions of the beam footprint (e.g., central and peripheral sub-regions) will tend to produce images that are (at least partially) "biased" toward a sub-range of given Z values ​​(e.g., relatively low Z and higher Z). This effect can be exploited to more effectively image various components of a variety of samples: see, for example, Figures 4A-4D .

[0052] In a further generalization of the embodiment set out in the previous paragraph, the iVF image obtained using the first sub-area of ​​the beam footprint is combined with at least one of:

[0053] - an iVF image obtained using a second different sub-region of the beam footprint;

[0054] - iVF images acquired using the entire beam footprint;

[0055] -Annular dark field image of the sample.

[0056] The "composite image" resulting from this combination tends to exploit the specific individual imaging sensitivities of the various imaging modalities and then "overlay" these to obtain a more detailed composite image. See, for example, Figure 4D .

[0057] Those skilled in the art will appreciate that the post-sample beam footprint on the imaging detector in a STCPM is typically formed after the beam passes through one or more clean-up holes, the purpose of which is, for example, to prevent portions of the beam from striking the detection electronics along the periphery of the detector detection surface, etc. Such clean-up holes typically "scrape off" only a relatively minimal portion of the beam, and typically maximize the size of the beam footprint allowed on the available detection surface - thereby producing a "nominal" (or "default") beam footprint. The present invention can be considered to focus on one or more sub-regions of this "nominal" beam footprint.

[0058] As a general comment, it should be noted that if one only observes BF images (for example) instead of iVF images, one cannot exploit the mechanisms of the present invention. In the present invention, the iVF image can be considered as the convolution of the contrast transfer function (CFT) and the object-dependent Fourier transform, whereby the CTF is essentially object-independent. In principle, this CTF can be adjusted so that there are no (or only very few) zero crossings - this helps to avoid discontinuities (and associated artifacts). On the other hand, in BF imaging, one cannot talk in the same way as the object-independent CTF; instead, one must deal with an object-dependent function, which cannot be optimized in the same way to minimize zeros. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The present invention will now be described in detail based on exemplary embodiments and the accompanying schematic drawings, in which:

[0060] Figure 1 A longitudinal cross-sectional view is given of an embodiment of a STCPM embodying the present invention.

[0061] FIG. 2A shows a plan view of a specific embodiment of a segmented detector (quadrant detector), which may be Figure 1 The subjects were used according to conventional iVF imaging techniques.

[0062] FIG. 2B shows a plan view of another embodiment of a segmented detector (pixelated detector) which may be Figure 1 Another conventional iVF imaging technique was used in the subject.

[0063] Figure 3A A modification of the subject matter of FIG. 2B is shown according to an embodiment of the present invention.

[0064] Figure 3B A modification of the subject matter of FIG. 2A is shown according to another embodiment of the present invention.

[0065] Figure 4A , 4B , 4C and 4D show various STCPM images of SrTiO3 crystals and illustrate various aspects of embodiments of the present invention.

[0066] Figure 5A and 5B CBED disks of two different samples are shown, clearly showing the internal intensity inhomogeneities.

[0067] In the drawings, corresponding reference numerals may be used to indicate corresponding parts where relevant.

[0068] Example 1

[0069] Figure 1 (not to scale) is a highly schematic diagram of an embodiment of a scanning transmission charged particle microscope (STCPM)M in which the present invention may be implemented; more specifically, a STEM is shown (however, in the context of the present invention it may effectively just be, for example, an ion-based microscope). In the figure, within a vacuum housing 2, an electron beam B generated by an electron source 4 propagates along an electro-optical axis B" and passes through an electro-optical illuminator 6, directing / focusing the electrons onto a selected portion of a sample S (e.g., which may be (locally) thinned / planarized). A deflector system 8 is also depicted, which may (among other things) be used to achieve a scanning motion of the beam B.

[0070] The sample S is fixed to a sample holder H which can be positioned with multiple degrees of freedom by a positioning device / stage A which moves a cradle A′ into the (removably) attached holder H; for example, the sample holder H may include a finger that can move (among other things) in the XY plane (see the coordinate system depicted; typically, movement parallel to Z and tilting about X / Y are also possible). This movement allows different parts of the sample S to be illuminated / imaged / inspected by an electron beam B traveling along the B” axis (in the Z direction) - and / or allows scanning movements to be performed as an alternative to beam scanning. If desired, an optional cooling device (not depicted) can be brought into close thermal contact with the sample holder H in order to, for example, maintain it (and the sample S thereon) at cryogenic temperatures.

[0071] The electron beam B will interact with the sample S in such a way that various types of "stimulated" radiation are emitted from the sample S, including (for example) secondary electrons, backscattered electrons, X-rays and optical radiation (cathodoluminescence). If desired, one or more of these radiation types can be detected by means of an analysis device 22, which can be, for example, a combined scintillator / photomultiplier tube or an EDX (energy dispersive X-ray spectroscopy) module; in this case, an image can be constructed using essentially the same principles as for the SEM. However, alternatively / complementarily, electrons that pass through (through) the sample S, are emitted / emitted from the sample S, and continue to propagate (generally, although there is usually some deflection / scattering) along the B" axis can be studied. This transmitted electron flux enters an imaging system (projection lens) 24, which will typically include various electrostatic / magnetic lenses, deflectors, correctors (such as an stigmator), etc. In normal (non-scanning) TEM mode, this imaging system 24 can focus the transmitted electron flux onto a fluorescent screen 26, which can be retracted / withdrawn (as indicated by arrow 26') if desired, so that it is away from the B" axis. An image (or diffraction pattern) of (a portion of) the sample S will be formed by the imaging system 24 on the screen 26, and this can be viewed through a viewing port 28 located in a suitable portion of the wall of the housing 2. The retraction mechanism of the screen 26 is mechanical and / or electrical in nature and is not depicted here, for example.

[0072] As an alternative to viewing the image on the screen 26, use can instead be made of the fact that the depth of focus of the electron flux leaving the imaging system 24 is typically quite large (e.g. of the order of 1 meter). Therefore, various other types of detectors / analyzers can be used downstream of the screen 26, such as:

[0073] -TEM camera 30. At camera 30, the electron flux can form a static image (or diffraction pattern) which can be processed by controller / processor 20 and displayed on a display device (not shown), such as a flat panel display, for example. When not needed, camera 30 can be retracted / withdrawn (as indicated by arrow 30') to move it away from the B" axis.

[0074] - STEM camera / detector 32. The output of camera 32 can be recorded based on the (X, Y) scan position of beam B on sample S, and an image can be constructed based on X, Y, which is a "map" of the output of camera 32. Camera 32 may include a single pixel with a diameter of, for example, 20 mm, rather than the pixel matrix characteristic of camera 30. In addition, camera 32 generally has a much higher acquisition rate (e.g., 10 per second). 6 points) will usually be faster than the camera 30 (e.g., 10 per second 6Likewise, when not needed, the camera 32 can be retracted / withdrawn (as indicated by arrow 32') to move it away from the B" axis (although such retraction is not necessary in the case of, for example, an annular dark field camera (ADF) 32; in such cameras, the central hole will allow flux passage when the camera is not in use).

[0075] As an alternative to imaging using a camera 30 or an imager 32 , it is also possible to call upon a spectroscopic device 34 , which can be, for example, an EELS module.

[0076] It should be noted that the order / position of items 30 , 32 and 34 is not critical and many possible variations are conceivable. For example, the spectroscopic device 34 may also be integrated into the imaging system 24 .

[0077] It should be noted that a controller (computer processor) 20 is connected to the various illustrated components via control lines (buses) 20'. This controller 20 may provide a variety of functions, such as synchronizing actions, providing set points, processing signals, performing calculations, and displaying messages / information on a display device (not depicted). Needless to say, the (schematically depicted) controller 20 may be (partially) located inside or outside the housing 2, as desired, and may have a unitary or composite structure.

[0078] Those skilled in the art will appreciate that the interior of the enclosure 2 need not necessarily be kept in a strict vacuum; for example, in so-called "environmental TEM / STEM", a background atmosphere of a given gas is intentionally introduced / maintained within the enclosure 2. Those skilled in the art will also appreciate that, in practice, it may be advantageous to limit the volume of the enclosure 2 so that, where possible, it is substantially contiguous to the axis B", and is in the form of a small tube (e.g., of about 1 cm in diameter) through which the electron beam used passes, but is widened to accommodate structures such as the source 4, the sample holder H, the screen 26, the camera 30, the camera 32, the spectroscopic device 34, etc.

[0079] In the specific context of iVF imaging, the following additional points deserve further clarification:

[0080] - The detector 32 used (see Figure 1 ) is embodied as a segmented detector, for example, which may include a quadrant sensor, a pixelated CMOS / CCD / SSPM detector or a PSD. Specific embodiments of these detectors are shown in FIGS. 2A / 2B and Figure 3A This is shown schematically in plan view in FIG. 3B and will be discussed below.

[0081] - If the beam B propagating along the particle-optical axis B″ passes through the sample S without experiencing any scattering / deflection in the sample, it will impinge (substantially) symmetrically on the center / origin O of the detector 32 and give a (substantially) “null” reading. This is shown in more detail in FIGS. 2A and 2B , which show coordinate axes X, Y with origin at point O, with a dashed circle at its center, which schematically represents the impact footprint F′ of the (ghost) charged particle beam B, with intensity centroid C′, such that:

[0082] ■ In Fig. 2A, the coverage area F' covers symmetrically over the detection quadrants (electrodes) Q1, Q2, Q3, Q4. If the detection signals (currents) from these quadrants are represented by S1, S2, S3, S4 respectively, then this will produce zero difference signals S1-S3 and S2-S4 between the opposite pairs of quadrants.

[0083] ■ In Fig. 2B, an orthogonal matrix of detection pixels p is depicted (eg in a CMOS detector, possibly with overlapping scintillation layers), with zero deviation between the chosen origin O of said pixel matrix and the intensity centroid C'.

[0084] - On the other hand, if the charged particle beam B experiences some scattering / deflection in the sample S, it will land on the detector 32 at a position offset from the origin O and / or its intensity distribution in its cross section will change (see Figure 5A , 5B , for reference). In this context, FIGS. 2A and 2B schematically illustrate a beam footprint F with an intensity centroid C that is no longer centered at O. The position of point C relative to O defines a vector V, which has an associated magnitude (length) and direction (e.g., pointing angle relative to the X-axis). This vector V can be expressed as the coordinates of point C (X C , Y C ) indicates that it can be detailed as follows:

[0085] ■ In Figure 2A, X can be derived using the following formula C , Y C The (basic) estimator of:

[0086]

[0087] The term in the denominator can (more correctly) be replaced by the total charged particle dose at each pixel location; however, when absorption effects in the sample are relatively insignificant (e.g. for thin samples), this total dose will be essentially the same as the denominator in equation (1) [or proportional to it if absorption does not vary significantly from point to point].

[0088] ■ In Figure 2B, X can be derived by examining the output signal from each pixel p C , Y CThe value of C is determined by the beam footprint F, since pixels p illuminated by the beam footprint F will give a different output signal (e.g. resistance, voltage or current) to pixels p outside the footprint F. The position of C can then be deduced directly by recording the coordinates of the specific pixels producing the extreme value signal, or indirectly by mathematically calculating the intensity centroid of the cluster of pixels p hit by B, or by a hybrid technique combining e.g. both approaches.

[0089] Those skilled in the art will appreciate that, for example, Figure 1 The so-called “camera length” of STCPM is used to change the size of the beam footprint F.

[0090] - When the beam B is scanned over the sample S so as to describe a two-dimensional scan path (area), the method described in the previous item can be used to obtain the value of V for each coordinate position along the scan path. This allows to compile a "map" of the vector V as a function of the scan position on the sample S, which is equivalent to a mathematical field (and a physical field, since the vector V can be assigned a (proportional) physical meaning, such as an electrostatic field vector).

[0091] - The vector field resulting from the previous step can now be integrated in two dimensions in order to obtain an integrated vector field (iVF) image (which represents the electrostatic potential map in the sample S).

[0092] More information on these (and related) aspects may be obtained, for example, from the above-mentioned US 9,312,098.

[0093] Now go to Figure 3A and 3B , respectively showing modifications of FIG. 2B and FIG. 2A according to an embodiment of the present invention. More specifically:

[0094] -from Figure 3A Initially, this shows a (centrally located) sub-area F of coverage area F. S , which has an associated intensity center C S and vector V S (Connect origin O to C S As mentioned above, since the coverage area F (and sub-area F S ) usually does not have a uniform / isotropic intensity distribution, so the sub-region F S The center of gravity C S It is usually not coincident with the center of gravity C of the coverage area F, so the vector V S Usually not equal to vector V in FIG2B. Sub-region F S The center of gravity C S The sub-region F can be calculated S The following pixel outputs are determined by weighted sums. Note that Figure 3AThe detector in FIG. 2B may be substantially the same as the detector in FIG. 2B - in essence, the only difference between the figures relates to the detector used to select the sub-region F. S size / shape / position and determine its strength center of gravity C S The selection algorithm (specific parameter values) of

[0095] - Figure 3B A modification of the architecture of the detector in FIG2A is schematically shown. More specifically, Figure 3B A set R1, R2, R3 of individually selectable annular detection surfaces in a nested concentric arrangement is shown, each annular detection surface R i is subdivided into multiple sectors of substantially equal area {S i1 , S i2 , S i3 , ...} (although this is not necessarily the case). In the middle of this nested arrangement is a pie-shaped detection surface R0, having four quadrant-shaped sectors S 01 , S 02 , S 03 , S 04 2A; since R0 is actually a ring with an inner radius of zero, the four detector regions R0, R1, R2, R3 may be referred to herein as rings. One or more of the rings R0, R1, R2, R3 may be arbitrarily selected to serve as the detection surface, and in a given ring R i The components of the sectors {S ij}. Note that in the current architecture:

[0096] ● Each of the rings R0, R1 has four sectors;

[0097] ● Ring R2 has six sectors;

[0098] ●Ring R3 has eight sectors,

[0099] Although this is not necessarily the case, and other options are possible (e.g. depending on the selected inner / outer radius values ​​for each ring). It should also be noted that the rings do not necessarily have to have the same radial extent (outer radius minus inner radius).

[0100] Similar to Figure 3A In the situation, Figure 3B The (centrally located) sub-area F of the coverage area F is shown S , which has an associated intensity center C S and vector V S (Connect origin O to C S ); for example, appropriate (adjustable and / or retractable) aperture plates / spatial filters (e.g. Figure 1 The plate 32a in the embodiment selects such a sub-area FS Similarly, due to the coverage area F / sub-area F S Usually does not have a uniform / isotropic intensity distribution, so the sub-region F S The center of gravity C S It is usually not coincident with the center of gravity C of the coverage area F, so the vector V S Usually not equal to vector V (see Figure 2A). As shown here, sub-region F S falls within the boundaries of ring R1, but this does not necessarily have to be the case, and it can span multiple rings. Similar to the situation in equation (1) above, V can therefore be calculated using the following formula S The (basic) estimator (X CS , Y CS ):

[0101]

[0102] More generally, it is possible to determine the subregion F from which it is superimposed S Ring R i Different sectors in {S ij}, for example based on the output voltage or current from each corresponding sector. Note that other rings can be used to study other different sub-areas: for example, ring R2 can be used to study a sub-area near the periphery of coverage area F [larger (X, Y)], while ring R0 can be used to study a sub-area near the inner periphery of coverage area F [smaller (X, Y)].

[0103] As a general illustration, it should be recognized that the magnitudes of the displacements shown in FIGS. 2 and 3 (e.g., F′ and F; C and C S ; V and V S The differences between the two scenarios are exaggerated to clearly illustrate the potential impacts, which are likely to be small.

[0104] Example 2

[0105] Figure 4A , 4B 4C and 4D show various STCPM images of SrTiO3 (strontium titanate) crystals and illustrate various aspects of embodiments of the present invention. SrTiO3 has a so-called perovskite structure, and the atomic numbers (Z) of its various components are Sr = 38, Ti = 22 and O = 8. More specifically:

[0106] - Figure 4A A "standard" iVF image is shown which is not focused on a specific sub-region of the beam footprint, here considered to have a radius r o(It can be defined, for example, as the radius value where the coverage intensity is below a given threshold, such as 10% of the light-dark difference.) The low Z oxygen atoms (e.g., see white squares) are relatively faint, while the high Z Sr / Ti atoms (e.g., see white circles) are relatively bright. In this image, there is no clear distinction between Sr and Ti.

[0107] - Figure 4B An iVF image produced in accordance with the present invention is shown, resulting from focusing on a sub-region of the beam footprint - in this case a radius of 0.3 r o The central disk of the image. The low-Z oxygen atoms (e.g., white squares) are now relatively bright, while the high-Z Sr / Ti atoms (e.g., white circles) are relatively faint. Again, there is no clear distinction between Sr and Ti in this image.

[0108] - Figure 4C It shows that when r>r o An annular dark field (ADF) image is produced at Figure 1 The detector 30 in FIG. 1 is implemented as an ADF detector. Figure 4A and 4B The image in has less "depth", but it more clearly shows the distinction between Sr and Ti atoms (see, for example, the white pentagons / hexagons).

[0109] - Figure 4D is by combining (e.g. corresponding to Figure 4A , 4B 4C) by summing the corresponding pixel values ​​of the images in 4C. Note that in this color image, the three different types of atoms in the crystal, St, Ti, and O, can be clearly distinguished from each other.

[0110] Example 3

[0111] Figure 5A and 5B The CBED disks (beam footprint) of two different samples are shown, clearly showing the internal intensity inhomogeneities. Both disks are related to GaN

[110] , but Figure 5A The sample thickness in is 2.4 nm, while Figure 5B The sample thickness in the Figures is 7.5 nm; in both cases the initial (incident) electron beam was focused at a subsurface depth of 2 nm. The accompanying figures clearly show the intensity "fringe" within the disks and illustrate how / why different centers of gravity for the intensities are typically obtained when considering different sub-regions of each disk.

Claims

1. A method for imaging a sample in a scanning transmission charged particle microscope, comprising the following steps: - providing a sample on a sample holder; - providing a charged particle beam directed from a source through an illuminator to irradiate a sample; - providing a segmented detector for detecting a flux of charged particles passing through the sample, the flux forming a beam footprint on said detector; - scanning the beam over the surface of the sample, combining the signals from different sections of the detector to produce a vector output from the detector at each scan position, and compiling the vector output to produce an imaging vector field; - mathematically processing the imaging vector field by performing a two-dimensional integration operation on the imaging vector field to produce an integrated vector field image of the sample, Features: - generating the vector output using a restricted sub-region of the beam footprint, and accompanying imaged vector field and integrated vector field images.

2. The method according to claim 1, wherein: - the detector is embodied as a pixelated detector comprising an array of pixels; -The vector output is compiled by: comparing pixel values ​​to determine the position of the intensity centroid of said sub-region of the beam footprint on the detector; The coordinate position of the center of gravity is indicated on the detector.

3. The method according to claim 1, wherein: - the detector is configured to include an annular detection surface subdivided into a plurality of sectors having substantially equal areas; - Compiling the vector output by determining weighted signals from different sectors. The method according to claim 3 , wherein the detector is configured to include a set of nested concentrically arranged annular detection surfaces.

5. The method of claim 1, wherein the selection of the sub-regions comprises using an aperture plate in the flux between the sample and the detector.

6. The method according to any one of claims 1 to 5, wherein: - the sample comprises various elemental components having a range of different atomic numbers; - selecting said sub-area of ​​the beam footprint so as to distinctively record a sub-range of atomic numbers.

7. The method according to claim 6, wherein: - said sub-area of ​​the beam coverage area is a restricted central area of ​​said coverage area; - said subrange of atomic numbers comprises relatively low atomic numbers.

8. The method according to claim 6, wherein: - said sub-area of ​​the beam footprint is a restricted peripheral area of ​​said footprint; - said subrange of atomic numbers comprises relatively high atomic numbers.

9. The method according to any one of claims 1 to 8, wherein: - in a first imaging period, the first sub-area is used as a basis for generating a first integrated vector field image; and - In a second imaging period, a second, different sub-region is used as a basis for generating a second integrated vector field image.

10. The method according to any one of claims 1 to 9, wherein the integrated vector field image obtained using the first sub-area of ​​the beam footprint is combined with at least one of: - an integrated vector field image obtained using a second different sub-region of the beam footprint; -Integrated vector field image obtained using the entire beam footprint; -Annular dark field image of the sample.

11. A scanning transmission charged particle microscope, comprising: - a sample holder for fixing the sample on a sample plane; - an illuminator for directing the beam of charged particles from the source onto the sample; - a segmented detector for detecting a flux of charged particles passing through the sample, the flux forming a beam footprint on said detector; - a controller configured to: · achieving a scanning motion of the beam over the sample surface; combining signals from different sections of the detector to produce a vector output from the detector at each scan position, and compiling the vector output to produce an imaging vector field; mathematically processing the imaging vector field by performing a two-dimensional integration operation on the imaging vector field to produce an integrated vector field image of the sample, Characterized in that the controller is configured to: - generating the vector output using a restricted sub-region of the beam footprint, and accompanying imaged vector field and integrated vector field images.

12. A detector assembly for detecting charged particles, comprising: - a set of nested, concentrically arranged, individually selectable annular detection surfaces, each annular detection surface being subdivided into a plurality of sectors of substantially equal area; - a processor configured to compile a vector output by calculating weighted signals from different sectors and to process the vector by performing a two-dimensional integration operation on the vector.

Images