How to use a charged particle microscope to examine a sample and obtain an Electron Energy Loss Spectroscopy (EELS) spectrum

Abstract

To provide a method of performing EELS with a charged particle microscope.SOLUTION: A method comprises: a step 101 of providing a charged particle beam, as well as a sample; a step 102 of scanning the charged particle beam over the sample at a plurality of sample positions; a step 103 of acquiring an EELS spectrum for each sample position of the plurality of sample positions; a further step 104 of scanning, once more, the charged particle beam over the sample at the plurality of sample positions; a further step 105 of acquiring a further EELS spectrum for each sample position of the plurality of sample positions; and a further step 106 of combining, for each sample position of the plurality of sample positions, the EELS spectrum with the further EELS spectrum. With this, it is possible to acquire rapid information on the sample being investigated, allowing for faster performing processing of samples.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a method for inspecting a sample using a charged particle microscope, the method comprising the steps of providing a charged particle beam as well as a sample, scanning the charged particle beam across the sample at a plurality of sample locations, and obtaining an Electron Energy Loss Spectroscopy (EELS) spectrum for each of the plurality of sample locations. [Background technology]

[0002] Charged particle microscopy is a well-known and increasingly important technique for imaging microscopic objects, especially in the form of electron microscopes. So far, the basic type of electron microscope has evolved into several well-known instruments, such as transmission electron microscopes (TEM), scanning electron microscopes (SEM) and scanning transmission electron microscopes (STEM), and also into various auxiliary instruments, such as so-called "dual beam" instruments (e.g., FIB-SEM), which further employ a "machining" focused ion beam (FIB) to allow auxiliary actions, such as ion beam milling or ion beam induced deposition (IBID). Those skilled in the art will be familiar with the different types of charged particle microscopes.

[0003] Irradiation of a sample by a scanning electron beam stimulates emission of "auxiliary" radiation from the sample in the form of secondary electrons, backscattered electrons, x-rays, and cathodoluminescence (infrared, visible, and / or ultraviolet photons). One or more components of this emitted radiation can be detected and used for sample analysis.

[0004] One way to analyze a sample is to utilize an Electron Energy Loss Spectroscopy (EELS) module. In Electron Energy Loss Spectroscopy (EELS), a material is exposed to a beam of electrons with a known narrow range of kinetic energies. Some of the electrons undergo inelastic scattering, meaning that they lose energy and their paths are deflected slightly and randomly. The amount of energy loss can be measured by an electron spectrometer, and from the amount of energy loss it can be worked out what the cause of the energy loss was. Inelastic interactions include phonon excitation, inter- and intra-band transitions, plasmon excitation, inner shell ionization, and Cherenkov radiation. Inner shell ionization is particularly useful for detecting the elemental composition of a material. For example, one may notice that a larger number of electrons than expected enter the material with 285 eV less energy than they had when they entered the material. This is roughly the amount of energy required to remove an inner shell electron from a carbon atom, which can be taken as evidence that a significant amount of carbon has entered the sample. With sufficient care and considering the wide range of energy losses, it is possible to determine the types of atoms the beam strikes, and the number of atoms of each type. The scattering angle (i.e., the amount the electron's path is deflected) can also be measured, giving information about any dispersion relations regardless of the material excitation that caused the inelastic scattering.

[0005] In EELS, a specific location on the sample is illuminated with a beam and the spectral information of the emission detected at that location is collected to form an EELS spectrum for that specific sample location. The EELS spectrum may contain valuable information like chemical composition, phase, electronic material properties of the sample location under investigation. During EELS, the charged particle beam is moved to a first sample location and an EELS spectrum is acquired for that first sample location. A predefined dwell time is used to acquire good statistics of the EELS spectrum for that first location. The beam is then moved to a neighboring sample location and an EELS spectrum is acquired for that neighboring sample location. The EELS spectra acquired for multiple sample locations can be combined into a so-called spectral cube (or Scube) that contains both positional (x,y) and spectral (energy) information. As a post-processing step, data analysis is performed, including for example extensive multiple linear least squares (MLLS) fitting over thousands of spectra, to extract hard to obtain information. This is often a very slow and tedious process that depends on the skills of the user and the desired information to be derived, and is usually done offline at a later stage. Summary of the Invention

[0006] With the above in mind, it is an object of the present invention to provide improved methods for performing EELS in charged particle microscopes, in particular for obtaining EELS spectral data more quickly, more accurately, and / or in an improved manner.

[0007] For this purpose, a method for inspecting a sample using a charged particle microscope is provided as defined in claim 1. In this method, a sample and a charged particle beam are provided. The charged particle beam is scanned over the sample at a plurality of sample positions. For each of these positions, an EELS spectrum is acquired. Thereby, a Scube (comprising position information and an associated EELS spectrum) is acquired. The Scube allows for the extraction of one or more first characteristics of the sample at the plurality of sample positions. According to the method defined herein, the method comprises a further step of scanning the charged particle beam over the sample at the plurality of sample positions. Thus, in other words, the same sample position is scanned at least once more. For each of the plurality of sample positions, a further EELS spectrum is acquired. Thus, a further Scube (comprising position information and an associated further EELS spectrum) is acquired. The further Scube allows for the extraction of one or more second characteristics of the sample at the plurality of sample positions. For each of the plurality of positions, this further EELS spectrum is combined with the already acquired EELS spectrum. The combining step may comprise adding the acquired EELS spectra, i.e. by simply adding a further Scube to the Scube. However, the combining step may also comprise alternative or further steps, such as correction or averaging. The combining step may in one embodiment be performed by the control unit.

[0008] Indeed, the method defined herein allows the full EELS spectrum of a selected location of a sample to be gradually accumulated by scanning the sample multiple times. With each scan, information about the sample can be obtained, which can be, for example, analyzed and displayed to the user. In this way, rather than spending a lot of valuable tool time on one location, the available tool time is used to scan multiple locations with a shorter dwell time per location. This shorter dwell time gives less information per sample location (compared to a longer dwell time on one sample location), but allows valuable information about the sample to be obtained and allows the user to be given feedback about the sample. This gives quick insight into the sample while it continues to be investigated under the microscope, i.e., real-time feedback during data acquisition, rather than post-acquisition feedback. The information obtained during the investigation of the sample can then be used by the user to modify the details of the acquisition. For example, if after several rapid scans of a relatively large size or area, certain material interfaces become evident, the user can crop the view towards only these areas, allowing for much faster, more informed acquisition that is interesting for high throughput applications, for example in semiconductor metrology and materials science. Thus, the user can continue acquisition, stop acquisition, or move acquisition to another part of the sample, allowing for a more efficient way of performing EELS in a charged particle microscope.

[0009] As an additional advantage, the relatively short dwell time allows drift corrections to be applied during the acquisition, which may also improve the quality of the method.

[0010] Thus, the objectives as defined herein are achieved.

[0011] Advantageous embodiments are described below.

[0012] In one embodiment, the method comprises providing, by a control unit, an image representation of the sample, the image representation comprising at least one information relating to the EELS spectrum, the further EELS spectrum, and / or the combined EELS spectrum, in particular the image representation can comprise features extracted from the EELS spectrum, the further EELS spectrum, and / or the combined EELS spectrum.

[0013] In one embodiment, the method includes using the EELS spectrum to extract one or more first features of the sample, hi one embodiment, the method includes using the further EELS spectrum to extract one or more second features of the sample.

[0014] In one embodiment, a first Scube is acquired, the first Scube including the EELS spectra of the plurality of sample locations acquired during a first scan. The method may include acquiring a second Scube by acquiring additional EELS spectra of the plurality of sample locations by performing a second scan. The first and second Scubes may be combined and analyzed to extract one or more features of the sample. The extracted features may be displayed in a pictorial representation of the sample.

[0015] In one embodiment, the method includes associating a corresponding number of different hues with the first and second features and / or the extracted features relative to an HSV color space, and the image representation includes the different hues. The HSV color space is a color space that describes attributes of a particular color using hue (often written as "color", e.g., red, green, blue), saturation (often written as the intensity or purity of a color), and color value (often written as the lightness or darkness of a color). The HSV color space can be represented as a cylinder shape with hue and its angle dimension starting with the red primary at 0°, passing through the green primary at 120° and the blue primary at 240°, before wrapping back to red at 360°. The central vertical axis includes neutral, achromatic, or gray colors ranging from black at the bottom with a value of 0 to white at the top with a value of 1. The larger the radius, the more saturated the color.

[0016] In principle, any color can be described in the HSV color space, regardless of the actual color space used. Thus, the methods described herein are not limited to the use of any particular color space, but can also be applied to the RGB color space (as all RGB colors can also be described with reference to the HSV color space), or any other color space as the situation requires.

[0017] By providing an image representation of a sample, where the image representation includes color information associated with features based on the acquired EELS data, it is possible to provide a user (particularly a human operator) with a real-time semi-quantitative structural / chemical mapping tool. The image representation including color information can be updated with each new scan, i.e. after the acquisition of a further Scube.

[0018] In one embodiment, the method comprises a step of denoising the EELS spectrum and / or the further EELS spectrum. It is conceivable that the method comprises a step of denoising the combined EELS spectrum. This allows denoising of the already acquired spectrum to obtain better noise characteristics of the resulting spectrum sent to post-processing, which may for example include post-processing of the image representation as described above and / or post-processing of the data analysis as described below. The method may also comprise a step of drift correction.

[0019] The methods described herein are very useful for obtaining informative EELS spectra of dose-sensitive samples. Thus, in one embodiment, - / Å 2 Electron dose not exceeding 5e - / Å 2 The use of electron doses not exceeding 1000 nm is used, which prevents damage to the sample. These extremely low dose scanning applications are particularly useful in life sciences, for example for thin samples and / or atomic resolution use cases.

[0020] In one embodiment, the method includes a step of analyzing the EELS spectrum and / or the further EELS spectrum. In particular, the method can include a step of using multivariate analysis, in particular using multivariate analysis on one or more of the acquired Scubes. Multivariate analysis (MVA) techniques can be applied to the Scube (or one or more Scubes in combination) to look for potential features in the Scube without any analysis or modeling of the individual spectra. The MVA technique is therefore relatively fast. The use of MVA allows real-time feature extraction even from noisy data. This allows the visualization of qualitative features - structure maps, phase maps or chemical maps - with real-time EELS-derived tools. Especially when combined with color coding of features, this is useful information for the user.

[0021] It should be noted that the use of MVA techniques is known in principle, but only as a post-processing technique. Using MVA on acquired Scubes allows for rapid analysis of the input data, making it possible to perform real-time data analysis.

[0022] Additionally, using MVA in combination with the low dose applications defined above provides a way to obtain data on dose sensitive samples.

[0023] The method defined herein, in one embodiment, integrates and integrates concepts from unsupervised multivariate techniques such as Principal Component Analysis (PCA), Independent Component Analysis (ICA), Non-negative Matrix Factorization (NMF) and / or clustering (e.g., k-means) to process the Scube (or Scubes) as a whole (without the need to subtract background and model each individual spectrum) and extract spectral features. This provides a fast and accurate qualitative representation that can be linked to intrinsic properties such as structure, phase, or composition. While these features may not be actual individual atomic elemental representations, they are quite easy to infer and therefore useful to assist expert users in manipulating or assessing regions of interest in real time.

[0024] According to one embodiment, there is provided a charged particle microscope for examining a sample using the methods disclosed herein. The charged particle microscope comprises: an optical column including a charged particle source, a final probe forming lens, and a scanner for focusing a charged particle beam emitted from the charged particle source onto a sample; a sample stage positioned downstream of said final probe forming lens and adapted to hold said sample; a first detector configured to detect a first type of emission from the sample in response to the incidence of charged particles emitted from the charged particle source; a control unit and a processing device connected to said first detector, The charged particle microscope is adapted to carry out the methods disclosed herein.

[0025] The microscope disclosed herein is configured to perform multiple scans on a sample and acquire an EELS spectral datacube for each scan.

[0026] The control unit and / or the processing device are arranged to extract features from the acquired Scube. In one embodiment, this may include pre-processing of the acquired Scube and conversion of the Scube into a standardized reconstruction matrix. Features may be extracted from this standardized reconstruction matrix by using multivariate analysis techniques that may be performed by the control unit and / or the processing device. The results, which may include color-coding information in the image representation, may be displayed on a screen device.

[0027] The above-described method and device, in one embodiment, can be broadly divided into three main steps and / or units that can be defined by the following blocks a to c: a) Acquire the EELS Scube in multiple scans; b) Processing the obtained Scube to extract features; and and c) visualizing (in real time) the acquired features.

[0028] Block (a) controls the EELS detector and the scanning module as well as possible image alignment techniques (drift correction). Multiple short scans taken continuously provide new Scubes to refine the EELS representation.

[0029] Block (b) is the Scube preprocessing module. The Scube preprocessing module removes all unwanted spiky events from the Scube that may skew the results. It flattens the Scube into a matrix, where each row contains a spectrum. The Scube is mean-centered (each column) so that the matrix is ​​reconstructed.

[0030] Block (b) can also transform the reconstructed higher dimensional matrix into a lower dimensional space represented by the principal components. Further MVA techniques (such as ICA / component rotation) can be applied to this data to rotate the principal components to optimally extract features.

[0031] Block (d) combines the obtained features in a specific way. These features are inverse transformed and visualized as an image to obtain a colorEELS representation. Each spectrum can then be reconstructed from a lower dimensional representation, which then shows an improved spectral quality that can be further exploited in conventional ways for fine-scale quantification.

[0032] The methods and devices disclosed herein will now be explained in more detail on the basis of exemplary embodiments and the accompanying schematic drawings. [Brief description of the drawings]

[0033] [Figure 1] 1 shows a longitudinal section of a charged particle microscope. [Diagram 2] 1 shows an enlarged longitudinal cross-sectional view of a spectroscopic device. [Diagram 3] An example of an EELS spectrum is shown. [Figure 4] 1 illustrates a schematic diagram of one embodiment of the method disclosed herein. [Figure 5a] 1 shows an example of an image that can be obtained with the method described herein. [Figure 5b] 1 shows an example of an image that can be obtained with the method described herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] In the drawings, corresponding parts have been indicated using corresponding reference numerals where appropriate, and it should be noted that the drawings are generally not to scale.

[0035] FIG. 1 is a highly schematic illustration of an embodiment of a transmission charged particle microscope M, which in this case is a TEM / STEM (although in the context of the present disclosure a transmission charged particle microscope could just as well be, for example, an ion source microscope or a proton microscope). In the figure, within a vacuum enclosure E, an electron source 4 (such as, for example, a Schottky emitter) emits an electron beam (B) which passes through an electron flood illuminator 6, which serves to direct / focus the electron beam onto a selected portion of a sample S (which can, for example, be (locally) thinned / flattened). This illuminator 6 has an electron projection axis B' and broadly includes a wide variety of electrostatic / magnetic lenses, (scanning) deflector(s) D, correctors (such as astigmatism correctors), etc., and typically the illuminator 6 can also include a focusing lens (the entire set of parts 6 may be denoted as "focusing lens system").

[0036] The sample S is held in a sample holder H. As shown here, a part of this holder H (inside the housing E) is mounted on a cradle A' which can be positioned / moved in multiple degrees of freedom by a positioning device (stage) A, e.g. the cradle A' can be displaced (among other things) in the X, Y and Z directions (see the illustrated Cartesian coordinate system) and rotated about a longitudinal axis parallel to X. Such a movement allows different parts of the sample S to be illuminated / imaged / inspected by the electron beam moving along the axis B' (and / or allows a scanning movement instead of beam scanning [using deflector(s) D] and / or allows selected parts of the sample S to be processed, e.g. by a focused ion beam (not shown)).

[0037] The (converging) electron beam B moving along the axis B' interacts with the sample S, causing various types of "stimulated" radiation to be emitted from the sample S, including (for example) secondary electrons, backscattered electrons, X-rays, and light radiation (cathodoluminescence). If necessary, one or more of these radiation types can be detected with the aid of a detector 22, which can be, for example, a combined scintillator / photomultiplier tube, or an EDX (Energy Dispersive X-ray Spectroscopy) module, in which case the image can be constructed using essentially the same principles as in an SEM. However, alternatively or supplementarily, one can study the electrons that pass through the sample S, are emitted from the sample, and continue to propagate along the axis B' (although in effect, usually subject to significant deflection / scattering). Such a transmitted electron bundle is incident on an imaging system (combined objective / projection lens) 24, which broadly includes a wide variety of electrostatic / magnetic lenses, deflectors, correctors (such as astigmatism correctors), etc. In normal (non-scanning) TEM mode, this imaging system 24 can focus the transmitted electron beam onto a phosphor screen 26, which can be stored / retracted away from axis B', if required (schematically indicated by arrow 26'). An image (or diffractogram) of (a part of) the sample S is formed on the screen 26 by the imaging system 24, and this image can be viewed through a viewing port 28 located in a suitable part of the wall of the housing E. The retraction mechanism of the screen 26 can be, for example, mechanical and / or electrical in nature, but is not depicted in the drawings.

[0038] As an alternative to viewing an image on the screen 26, one can instead take advantage of the fact that the focusing depth of the electron beam emerging from the imaging system 24 is typically quite large (e.g., about one meter). As a result, various types of detectors / analyzers can be used downstream of the screen 26, such as a TEM camera 30 and a STEM recorder 32: -TEM camera 30. At the position of camera 30 the electron flux can form a still image (or diffractogram) which can be processed by controller C and displayed on a display device (not shown), for example a flat panel display. When not required, camera 30 can be retracted / retracted (as shown diagrammatically by arrow 30') to move the camera off axis B'. - a STEM recorder 32. The output from the recorder 32 can be recorded as a function of the scanning position (X,Y) of the beam B on the sample S, and an image can be constructed that is a "map" of the output from the recorder 32 as a function of X,Y. The recorder 32 is distinct from the pixel matrix that is characteristically contained in the camera 30, and can comprise a single pixel, for example 20 mm in diameter. Furthermore, the recorder 32 is typically a single pixel that is different from the pixel matrix that is characteristically contained in the camera 30 (for example 10 2 Much higher acquisition rates (e.g., 10 images / sec) 6 points / sec). Again, when not needed, recorder 32 can be retracted (as shown diagrammatically by arrow 32') to move the recorder away from axis B' (such retraction is not required in the case of a doughnut-shaped annular dark field recorder 32, for example, where a central hole allows the beam to pass when the recorder is not in use). As an alternative to using the camera 30 or recorder 32 to perform imaging, it is also possible to drive a spectroscopic device 34, which may for example be an EELS module.

[0039] It should be noted that the order / position of components 30, 32, and 34 is not strict and many possible variations are possible. For example, spectroscopic device 34 could be integrated with imaging system 24.

[0040] It should be noted that a controller C (which may be a combined controller and processor) is connected to the various components shown via control lines (bus) C'. The controller may be connected to a computer screen 51 which may have a user interface (UI). This controller C may perform a wide variety of functions such as synchronizing the processes, providing set points, processing signals, performing calculations and displaying messages / information on a display device (not shown). It should be understood that the controller C (shown diagrammatically) may be (partly) inside or outside the housing E and may have a unitary or composite structure as required. Those skilled in the art will understand that the interior of the housing E does not need to be kept in an airtight vacuum state, but rather that for example in a so-called "Environmental TEM / STEM" an ambient atmosphere of a given gas is intentionally introduced / maintained inside the housing E. Those skilled in the art will also appreciate that in practice it may be advantageous to confine the volume of the housing E so that it takes the form, if possible, of a small diameter tube (e.g., about 1 cm in diameter) substantially surrounding the axis B' through which the electron beam employed passes, but which extends outwards to accommodate structures such as the source 4, sample holder H, screen 26, camera 30, recorder 32, spectroscopic device 34, etc.

[0041] Reference is now made to Figure 2, which shows a more detailed enlarged view of an embodiment of the spectroscopic device 34 of Figure 1. In the figure, an electron bundle 1 (having passed through the sample S and the imaging system 24) is shown propagating along an electron-optical axis B'. This bundle 1 enters a dispersive element 3 ("electron prism"), which disperses (fans out) it into a high energy resolution (energy differential imaging) array 5 of auxiliary beams (schematically shown in Figure 2 using dashed lines) whose spectrum is distributed along a dispersion direction X, of which for illustrative purposes three auxiliary beams are labelled 5a, 5b and 5c in the figure. It should be noted in this regard that conventionally propagation is considered to take place along the Z direction, and as such the illustrated Cartesian coordinate system is "co-deflected" with bundle 1 in the dispersive element 3.

[0042] Downstream of the dispersive element 3, the auxiliary beam array 5 encounters an adjustable / retractable slit (letterbox) 7, which can be used in EFTEM mode, for example, to select / allow a certain part of the array 5 and discard / block other parts of the array, and for this purpose the slit 7 is connected to an actuation device 7a, which can drive the slit 7 (the slit opening) to open / close / move as required. In EELS mode, this slit 7 is normally (fully) open / retracted. The skilled person will understand that the slit 7 is advantageously provided at or near the dispersive plane of the spectroscopic device 34, and similarly the detector 11 is also advantageously located at or near such plane. If required, the spectral auxiliary beam array 5 impinging on the slit 7 can be directed / shifted, for example by appropriately adjusting the (electrical signal to) the dispersive element 3 and / or a drift tube / deflector (not shown) provided, for example, between the dispersive element 3 and the slit 7.

[0043] After passing through the slit 7, the array 5 (selected portion of the array) passes through the electron optics 9 after the dispersive element, where the array is, for example, magnified / focused, and finally directed / projected onto the detector 11. The detector 11 may include an auxiliary detector assembly aligned along the dispersion direction X, where different auxiliary detectors are adjustable to have different detection sensitivities. For illustrative purposes, three of these auxiliary detectors are labelled in the figure with 11p, 11q and 11r. Each of these auxiliary detectors (e.g. 11p, 11q, 11r) may, for example: - an avalanche photodiode, or a linear array of such avalanche photodiodes extending along Y, - It can be a pixel of a CMOS or CCD sensor (e.g. a 3T pixel), or a linear array of such pixels extending along Y.

[0044] The sensitivity of a given auxiliary detector (eg, 11p, 11q, 11r) can be matched to match the intensity of the portion of array 5 that impinges on the auxiliary detector.

[0045] It should be noted that other detector configurations for measuring EELS spectra are known to those skilled in the art and are equally applicable to the method disclosed herein, which is not limited in principle to the use of any particular detector.

[0046] Figure 3 shows an example of an EELS spectrum. The figure shows the intensity I (in arbitrary units, au) as a function of the energy loss E (in eV) of the electrons passing through a sample containing carbon and titanium. From left to right, the main features of the spectrum are: - Zero loss peak ZLP represents electrons passing through the sample without undergoing inelastic scattering in the peak. - Plasmon Resonance Peak component / section PRP (sometimes denoted as Valence Loss component), a relatively broad series of peaks / shoulders associated with one or more scatterings of plasmon electrons in the sample. It usually extends from about 0 to 50 eV, but there is no strict definition of its upper limit. It is characterized by peaks / shoulders resulting from outer shell scattering events in the sample, such as peak 31. Note that the PRP component usually has a significantly lower intensity than the ZLP. - Core loss peak component / section CLP. This usually starts at about 50 eV (behind the PRP component), but there is no strict definition of its lower limit. Since it is usually of such low intensity relative to the ZLP / PRP components, it is magnified by a factor (e.g. 100) to improve the visibility of this detail, as represented in FIG. 3. As can be seen, it contains (clusters of) peaks / shoulders that can be associated with certain chemical elements (such as C and Ti in this example), on top of a substantial background contribution 33.

[0047] The EELS spectrum shown in Figure 3 can be measured by methods known to those skilled in the art. In a particular embodiment, use is made of the spectroscopy device 34 of Figure 2, The auxiliary beam 5a impinges on an auxiliary detector 11r which contains (part of) the ZLP spectral components and is adjusted to have a relatively low detection sensitivity. The auxiliary beam 5b impinges on an auxiliary detector 11q which contains (a part of) the PRP spectral component and is adjusted to have an intermediate detection sensitivity. The auxiliary beam 5c contains (a component part of) the CLP spectral components and impinges on an auxiliary detector 11p which is adjusted to have a relatively high detection sensitivity.

[0048] Since acquiring an EELS spectrum for the entire sample can take a long time, the method disclosed herein provides an alternative way to perform EELS on a sample. This method is shown diagrammatically in FIG. - providing a charged particle beam and a sample (101); - scanning (102) the charged particle beam across the sample at a plurality of sample positions; acquiring (103) an EELS spectrum for each of the plurality of sample positions; - again scanning (104) the charged particle beam across the sample at a plurality of sample positions; - acquiring (105) a further EELS spectrum for each of the plurality of sample positions; - for each sample position of the plurality of sample positions, combining (106) the EELS spectrum with the further EELS spectrum.

[0049] EELS spectra acquired for multiple sample locations during one scan can be combined into a so-called spectral cube (or Scube) that contains positional (x,y) and spectral (energy) information. In a sense, a first Scube is acquired by steps 103 and 104 defined above for scanning a first time to acquire an EELS spectrum. A second Scube is acquired by steps 104 and 105 for scanning a second time to acquire an EELS spectrum. It is noted that the method can include acquiring one or more further Scubes by performing step 104 of scanning at least once more for each sample location of the plurality of sample locations and step 105 of acquiring at least once more further EELS spectra for each sample location of the plurality of sample locations. Data analysis can be performed on one or more acquired Scubes, such as one Scubes or a combination of Scubes.

[0050] In effect, this method allows relatively sparse EELS spectra to be acquired for multiple sample locations and then further (relatively sparse) EELS spectra to be acquired for multiple sample locations. Data analysis can be performed during each individual scan or on one or more of the EELS spectra acquired for each individual scan. The acquired information can be combined to obtain, in particular, a total EELS spectrum for multiple sample locations. In this way, for each full scan, information about the sample can be obtained from the acquired EELS spectra or from the data analysis performed on said spectra. The information can be combined and displayed to a user. This allows the user, in particular a human operator, to evaluate the acquired information and decide how to continue with the data acquisition.

[0051] In one embodiment, the method described herein allows for providing an image representation of the sample, which may include the combined EELS spectrum or may include at least one information of the combined EELS spectrum.

[0052] Figures 5a and 5b show examples of such image representation embodiments. The image representation is based on the acquired Scubes, each Scube acquired during one scan of the sample. The acquired Scubes are transformed and reconstructed into a standardized matrix, and a multivariate analysis technique (MVA) in the form of principal component analysis is performed on the standardized matrix. Further MVA methods can be performed on the results of the MVA method to extract one or more features of the sample under investigation. In the embodiment shown in Figure 5a, a total of three different features are acquired and imaged individually.

[0053] The acquired features provide a qualitative representation that can be linked to intrinsic properties such as structure, phase, or composition. Although these features may not be actual individual atomic elemental representations, they are quite easy to infer and therefore useful to assist expert users to manipulate or evaluate areas of interest in real time. The individual features can be combined and inversely transformed as images (as shown in FIG. 5a) and visualized in one image to obtain the final image representation (FIG. 5b). This final image representation can be displayed on a computer screen 51. Each spectrum can then be reconstructed from a lower dimensional representation, which then shows improved spectral quality that can be further exploited in conventional ways for detailed quantification.

[0054] It should be noted that the final image representation in FIG. 5b is shown in black and white, but can contain color-coding information. In particular, each feature can contain a specific hue relative to the HSV color space (HSV=hue, saturation, color value). As an example, FIG. 5a can contain (from left to right) a red hue, a green hue, and a blue hue. The final image representation shown in FIG. 5b can contain a combination of these hues, resulting in a final image that is, for example, purple and blue. It can be seen that the image can contain different saturations of the specific hue, and that the image also contains color value information. This association of the HSV to the final image based on the feature information acquired in the data acquisition allows more relevant information to be displayed to the user, especially in real time.

[0055] The images may be enhanced by using one or more image processing techniques. In particular, noise reduction techniques and / or drift correction may be applied to the images or to the raw data.

[0056] The desired protection is provided by the appended claims.

Claims

1. 1. A method for examining a sample using a charged particle microscope, comprising the steps of: - providing a charged particle beam and a sample; - scanning the charged particle beam across the sample at a number of sample positions; - acquiring an EELS spectrum for each of said plurality of sample locations to obtain a spectral cube comprising position information and spectral information; - the further step of scanning said charged particle beam across said sample at said plurality of sample positions; - a further step of acquiring a further EELS spectrum for each sample location of said plurality of sample locations to obtain a further spectral cube comprising position information and spectral information; - a further step of combining, for each sample position of said plurality of sample positions, said spectral cube with said further spectral cube; analyzing the EELS spectrum and / or the further EELS spectrum, the analysis using multivariate analysis; A method comprising:

2. The method of claim 1 , comprising providing, by a control unit, an image representation of the sample, the image representation including the combined spectral cube.

3. The method of claim 1 or 2, comprising using the EELS spectrum to extract one or more first features of the sample.

4. A method according to any one of claims 1 to 3, comprising using the further EELS spectrum to extract one or more second features of the sample.

5. Providing, by a control unit, an image representation of the sample, the image representation including the combined spectral cubes; using the EELS spectrum to extract one or more first features of the sample; using the further EELS spectrum to extract one or more second features of the sample; and 2. The method of claim 1, further comprising: associating a corresponding plurality of different hues with the first feature and the second feature relative to an HSV color space, the image representation including the different hues.

6. The method of any one of claims 1 to 5, comprising the step of denoising the EELS spectrum and / or the further EELS spectrum.

7. The method of any one of claims 1 to 6, comprising the step of denoising the combined spectral cube.

8. The method according to any one of claims 1 to 7, wherein an electron dose of less than 20 e- / Å2, or less than 5 e- / Å2, is used.

9. The method of any one of claims 1 to 8, comprising applying a drift correction to the EELS spectrum and / or the further EELS spectrum.

10. A charged particle microscope for examining a sample using the method according to any one of claims 1 to 9, comprising: an optical column for focusing the charged particle beam emitted by said charged particle source onto a sample, said optical column including a charged particle source, a final probe forming lens, and a scanner; a sample stage positioned downstream of said final probe forming lens and adapted to hold said sample; a first detector for detecting a first type of emission emanating from said sample in response to the incidence of charged particles emitted by said charged particle source; a control unit and a processing device connected to said first detector, Charged particle microscope, wherein the charged particle microscope is arranged to carry out the method according to any one of claims 1 to 9.

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