METHOD OF EXAMINING A SAMPLE USING A CHARGED PARTICLE MICROSCOPE

Abstract

The invention relates to a method of examining a sample using a charged particle microscope, comprising the steps of providing a charged particle beam, as well as a sample; scanning said charged particle beam over said sample at a plurality of sample locations; and detecting, using a first detector, emissions of a first type from the sample in response to the beam scanned over the plurality of sample locations. Spectral information of detected emissions of the first type is used to assign a plurality of mutually different phases to said sample at said plurality of sample locations. Information relating to at least one previously assigned phase and its respective sample location is used for establishing an estimated phase for at least one other of the plurality of sample locations. Said estimated phase is assigned to said other sample location. A control unit is used to provide a data representation of said sample containing at least information on said plurality of sample locations and said phases.

Claims

1. A method of examining a sample using a charged particle microscope, comprising: providing a charged particle beam, as well as a sample; scanning said charged particle beam over said sample at a plurality of sample locations; detecting, using a first detector, emissions of a first type from the sample in response to the beam scanned over the plurality of sample locations; assigning, using spectral information of detected emissions of the first type, a plurality of mutually different phases to said sample at said plurality of sample locations; providing, by a control unit, a data representation of said sample containing at least information on said plurality of sample locations and said phases; establishing, using information relating to at least one previously assigned phase and its respective sample location, an estimated phase for at least one other of the plurality of sample locations; and assigning said estimated phase to said other sample location.

2. A method according to claim 1, comprising the step of associating phases to measured and/or expected emissions of said first type.

3. A method according to claim 1, wherein said step of establishing comprises the use of a machine learning estimator.

4. A method according to claim 3, wherein said machine learning estimator comprises one or more estimators chosen from the group comprising: Non-negative matrix factorization (NMF), Singular value decomposition (SVD), independent component analysis (ICA), latent Dirichlet allocation (LDA) and K-means.

5. A method according to claim 1, wherein a ratio between the number of measured sample locations and the number of estimated sample locations is in the range of 10:1 to 1:10, in particular wherein said ratio is in the range of 1:2 to 1:10.

6. A method according to claim 1, wherein said information relating to said at least one previously assigned phase at said respective sample location comprises spectral information of detected emissions of the first type at said sample location.

7. A method according to claim 1, wherein said first detector is an EDS detector.

8. A method according to claim 1, comprising the step of: detecting, using a second detector, emissions of a second type from the sample in response to the beam scanned over the plurality of sample locations; and wherein use is made of said detected emissions of said second type for establishing said estimated phase for said at least one other of the plurality of sample locations.

9. A method according to claim 8, wherein use is made of detected emissions of the first type and/or of detected emissions of the second type for dividing at least a part of the scanned area of the sample into multiple segments.

10. A method according to claim 9, wherein use is made of said segments for establishing said estimated phase for at least one other of the plurality of sample locations.

11. A charged particle microscope for examining a sample, comprising: an optics column, including a charged particle source, a final probe forming lens and a scanner, for focusing a beam of charged particles emitted from said charged particle source onto a specimen; a specimen stage positioned downstream of said final probe forming lens and arranged for holding said specimen; a first detector for detecting emissions of a first type originating from said specimen in response to the incidence of charged particles emitted from said charged particle source; a control unit and a processing device connected to said first detector; wherein said charged particle microscope is arranged for executing a method comprising the steps of: scanning said beam of charged particles over said specimen at a plurality of specimen locations; detecting, using the first detector, emissions of the first type from the sample in response to the beam of charged particles being scanned over the plurality of specimen locations; assigning, using spectral information of detected emissions of the first type, a plurality of mutually different phases to said specimen at said plurality of specimen locations; providing, by the control unit, a data representation of said specimen containing at least information on said plurality of sample locations and said phases; establishing, using information relating to at least one previously assigned phase and its respective specimen location, an estimated phase for at least one other of the plurality of specimen locations; and assigning said estimated phase to said other specimen location.

12. A charged particle microscope according to claim 11, wherein the charged particle microscope comprises a second detector for detecting emissions of a second type from the specimen in response to the beam scanned over the plurality of specimen locations.

13. A method according to claim 1, wherein said step of establishing comprises the use of a machine learning estimator.

14. A method according to claim 13, wherein said machine learning estimator comprises one or more estimators chosen from the group comprising: Non-negative matrix factorization (NMF), Singular value decomposition (SVD), independent component analysis (ICA), latent Dirichlet allocation (LDA) and K-means.

15. A method according to claim 11, wherein a ratio between the number of measured specimen locations and the number of estimated specimen locations is in the range of 10:1 to 1:10, in particular wherein said ratio is in the range of 1:2 to 1:10.

16. A method according to claim 11, wherein said information relating to said at least one previously assigned phase at said respective specimen location comprises spectral information of detected emissions of the first type at said specimen location.

17. A method according to claim 11, wherein said first detector is an EDS detector.

18. A method according to claim 11, comprising the step of: detecting, using a second detector, emissions of a second type from the specimen in response to the beam scanned over the plurality of specimen locations; and wherein use is made of said detected emissions of said second type for establishing said estimated phase for said at least one other of the plurality of specimen locations.

19. A method according to claim 18, wherein use is made of detected emissions of the first type and/or of detected emissions of the second type for dividing at least a part of the scanned area of the specimen into multiple segments.

20. A method according to claim 19, wherein use is made of said segments for establishing said estimated phase for at least one other of the plurality of specimen locations.

Description

[0036] The invention will now be elucidated in more detail on the basis of exemplary embodiments and the accompanying schematic drawings, in which:

[0037] FIG. 1shows a longitudinal cross-sectional view of a charged particle microscope according to a first embodiment of the invention;

[0038] FIG. 2shows a longitudinal cross-sectional view of a charged particle microscope according to a second embodiment of the invention;

[0039] FIG. 3show a schematic overview of an embodiment of the method according to the invention;

[0040] FIG. 1 (not to scale) is a highly schematic depiction of an embodiment of a charged-particle microscope M according to an embodiment of the invention. More specifically, it shows an embodiment of a transmission-type microscope M, which, in this case, is a TEM/STEM (though, in the context of the current invention, it could just as validly be a SEM (see FIG. 2), or an ion-based microscope, for example). In FIG. 1, within a vacuum enclosure 2, an electron source 4 produces a beam B of electrons that propagates along an electron-optical axis B and traverses an electron-optical illuminator 6, serving to direct/focus the electrons onto a chosen part of a specimen S (which may, for example, be (locally) thinned/planarized). Also depicted is a deflector 8, which (inter alia) can be used to effect scanning motion of the beam B.

[0041] The specimen S is held on a specimen holder H that can be positioned in multiple degrees of freedom by a positioning device/stage A, which moves a cradle A into which holder H is (removably) affixed; for example, the specimen holder H may comprise a finger that can be moved (inter alia) in the XY plane (see the depicted Cartesian coordinate system; typically, motion parallel to Z and tilt about X/Y will also be possible). Such movement allows different parts of the specimen S to be illuminated/imaged/inspected by the electron beam B traveling along axis B (in the Z direction) (and/or allows scanning motion to be performed, as an alternative to beam scanning). If desired, an optional cooling device (not depicted) can be brought into intimate thermal contact with the specimen holder H, so as to maintain it (and the specimen S thereupon) at cryogenic temperatures, for example.

[0042] The electron beam B will interact with the specimen S in such a manner as to cause various types of stimulated radiation to emanate from the specimen 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 with the aid of analysis device 22, which might be a combined scintillator/photomultiplier or EDS (Energy-Dispersive X-Ray Spectroscopy) module, for instance; in such a case, an image could be constructed using basically the same principle as in a SEM. However, alternatively or supplementally, one can study electrons that traverse (pass through) the specimen S, exit/emanate from it and continue to propagate (substantially, though generally with some deflection/scattering) along axis B. Such a transmitted electron flux enters an imaging system (projection lens) 24, which will generally comprise a variety of electrostatic/magnetic lenses, deflectors, correctors (such as stigmators), etc. In normal (non-scanning) TEM mode, this imaging system 24 can focus the transmitted electron flux onto a fluorescent screen 26, which, if desired, can be retracted/withdrawn (as schematically indicated by arrows 26) so as to get it out of the way of axis B. An image (or diffractogram) of (part of) the specimen S will be formed by imaging system 24 on screen 26, and this may be viewed through viewing port 28 located in a suitable part of a wall of enclosure 2. The retraction mechanism for screen 26 may, for example, be mechanical and/or electrical in nature, and is not depicted here.

[0043] As an alternative to viewing an image on screen 26, one can instead make use of the fact that the depth of focus of the electron flux leaving imaging system 24 is generally quite large (e.g. of the order of 1 meter). Consequently, various other types of analysis apparatus can be used downstream of screen 26, such as a TEM camera 30 and/or STEM camera 32.

[0044] At camera 30, the electron flux can form a static image (or diffractogram) that can be processed by controller/processor 20 and displayed on a display device (not depicted), such as a flat panel display, for example. When not required, camera 30 can be retracted/withdrawn (as schematically indicated by arrows 30) so as to get it out of the way of axis B.

[0045] An output from camera 32 can be recorded as a function of (X,Y) scanning position of the beam B on the specimen S, and an image can be constructed that is a map of output from camera 32 as a function of X,Y. Camera 32 can comprise a single pixel with a diameter of e.g. 20 mm, as opposed to the matrix of pixels characteristically present in camera 30. Moreover, camera 32 will generally have a much higher acquisition rate (e.g. 10.sup.6 points per second) than camera 30 (e.g. 10.sup.2 images per second). Once again, when not required, camera 32 can be retracted/withdrawn (as schematically indicated by arrows 32) so as to get it out of the way of axis B (although such retraction would not be a necessity in the case of a donut-shaped annular dark field camera 32, for example; in such a camera, a central hole would allow flux passage when the camera was not in use).

[0046] As an alternative to imaging using cameras 30 or 32, one can also invoke spectroscopic apparatus 34, which could be an EELS module, for example.

[0047] It should be noted that the order/location of items 30, 32 and 34 is not strict, and many possible variations are conceivable. For example, spectroscopic apparatus 34 can also be integrated into the imaging system 24.

[0048] In the embodiment shown, the microscope M further comprises a retractable X-ray Computed Tomography (CT) module, generally indicated by reference 40. In Computed Tomography (also referred to as tomographic imaging) the source and (diametrically opposed) detector are used to look through the specimen along different lines of sight, so as to acquire penetrative observations of the specimen from a variety of perspectives.

[0049] Note that the controller (computer processor) 20 is connected to various illustrated components via control lines (buses) 20. This controller 20 can provide a variety of functions, such as synchronizing actions, providing setpoints, 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) inside or outside the enclosure 2, and may have a unitary or composite structure, as desired.

[0050] The skilled artisan will understand that the interior of the enclosure 2 does not have to be kept at a strict vacuum; for example, in a so-called Environmental TEM/STEM, a background atmosphere of a given gas is deliberately introduced/maintained within the enclosure 2. The skilled artisan will also understand that, in practice, it may be advantageous to confine the volume of enclosure 2 so that, where possible, it essentially hugs the axis B, taking the form of a small tube (e.g. of the order of 1 cm in diameter) through which the employed electron beam passes, but widening out to accommodate structures such as the source 4, specimen holder H, screen 26, camera 30, camera 32, spectroscopic apparatus 34, etc.

[0051] The charged particle microscope M according to the invention, and of which an embodiment is shown in FIG. 1, thus comprises an optics column O, including a charged particle source 4, a final probe forming lens 6 and a scanner 8, for focusing a beam B of charged particles emitted from said charged particle source 4 onto a specimen. The apparatus further comprises a specimen stage A,H positioned downstream of said final probe forming lens 6 and arranged for holding said specimen S. The apparatus furthermore comprises a first detector 22 for detecting emissions of a first type originating from said specimen in response to the incidence of charged particles B emitted from said charged particle source 4. In the embodiment shown, the first detector 22 is the analysis device 22, whichas mentioned beforemight be a combined scintillator/photomultiplier or EDS (Energy-Dispersive X-Ray Spectroscopy) module. In a preferred embodiment, said first detector is an EDS. Furthermore, the apparatus according to the invention comprises the control unit (i.e. controller/processor) 20 that is connected (by means of lines 20) to said first detector 22 (schematically shown). According to the invention, said charged particle microscope M is arranged for executing the method according to the invention, which will be explained in more detail later.

[0052] Now first referring to FIG. 2, another embodiment of an apparatus according to the invention is shown. FIG. 2 (not to scale) is a highly schematic depiction of a charged-particle microscope M according to the present invention; more specifically, it shows an embodiment of a non-transmission-type microscope M, which, in this case, is a SEM (though, in the context of the current invention, it could just as validly be an ion-based microscope, for example). In the Figure, parts which correspond to items in FIG. 1 are indicated using identical reference symbols, and will not be separately discussed here. Additional to FIG. 1 are (inter alia) the following parts: 2a: A vacuum port, which may be opened so as to introduce/remove items (components, specimens) to/from the interior of vacuum chamber 2, or onto which, for example, an ancillary device/module may be mounted. The microscope M may comprise a plurality of such ports 2a, if desired; 10a, 10b: Schematically depicted lenses/optical elements in illuminator 6; 12: A voltage source, allowing the specimen holder H, or at least the specimen S, to be biased (floated) to an electrical potential with respect to ground, if desired; 14: A display, such as a FPD or CRT; 22a, 22b: A segmented electron detector 22a, comprising a plurality of independent detection segments (e.g. quadrants) disposed about a central aperture 22b (allowing passage of the beam B). Such a detector can, for example, be used to investigate (the angular dependence of) a flux of output (secondary or backscattered) electrons emerging from the specimen S.

[0053] Thus the charged particle microscope M as shown in FIG. 2 comprises an optics column O, including a charged particle source 4, a final probe forming lens 6, 10a, 10b and a scanner 8, for focusing a beam B of charged particles emitted from said charged particle source 4 onto a specimen S. The apparatus further comprises a specimen stage A,H positioned downstream of said final probe forming lens 6 and arranged for holding said specimen S. The apparatus furthermore comprises a first detector 22 for detecting emissions of a first type originating from said specimen in response to the incidence of charged particles B emitted from said charged particle source 4. In the embodiment shown, the first detector 22 is said analysis device 22, whichas mentioned beforemight be a combined scintillator/photomultiplier or EDS (Energy-Dispersive X-Ray Spectroscopy) module. In an alternative embodiment, the first detector 22 may be the segmented detector 22a, 22b. In a preferred embodiment, said first detector is an EDS. Furthermore, the apparatus according to the invention comprises said control unit 20 that is connected (by means of lines 20) to said first detector 22.

[0054] The apparatus shown in FIG. 1 and FIG. 2 may be used in examining a sample with a method according to the invention. Embodiments of the method according to the invention are shown in FIG. 3. In general, these methods all comprise the steps of: providing a charged particle beam B, as well as a sample S, and scanning said charged particle beam B over said sample at a plurality of sample locations; detecting, using a first detector 22, emissions of a first type from the sample in response to the beam B scanned over the plurality of sample locations; assigning, using spectral information G of detected emissions of the first type, a plurality of mutually different phases to said sample at said plurality of sample locations; and providing, by the control unit 20, a data representation of said sample S containing at least information on said plurality of sample locations and said phases.

[0055] According to the method as defined herein, the general method comprises the further steps of: establishing, using information relating to at least one previously assigned phase and its respective sample location, an estimated phase for at least one other of the plurality of sample locations; and assigning said estimated phase to said other sample location.

[0056] Now referring to FIG. 3, a first embodiment of the method according to the invention is schematically shown. FIG. 3 schematically shows, in the left hand side, an area 50 of a sample to be examined with acquired data 54a, 54b, and on the right hand side a representation 150 of the acquired data.

[0057] The area 50 of the sample is scanned with a beam of charged particles. The first detector, for example an EDS detector as described with reference to FIG. 1 and FIG. 2, is used to detect emissions of a first type from the sample in response to the beam scanned over the area 50 of the sample. These emissions are schematically indicated by data points 54a (white dots) and 54b (black dots) in FIG. 3. Data points 54a represent a different emission than data points 54b, i.e. meaning that the detector detects a different signal at these different locations. For example, this could be the result of the white points 54a representing a first material, and the black points 54b representing a second material (that is different from said first material). In this example, it is considered that the sample only contains these two materials, i.e. the sample only has the first material (white) and the second material (black). Also indicated are non-white and non-black points 54c, 54d that are representative for sample points that are undecisive yet, for example as they contain sparse spectral information. As indicated schematically in the top-left figure, point 54c seems to resemble the spectrum of the first material (54a), and point 54d seems to resemble the second material (54b). One skilled in the art will understand that the acquired data 54a, 54b can, in principle have many values (not only black or white), and that the representation values can be arbitrarily selected in dependence of the desired application. The example shown is merely an illustration of a single possibility and is not intended to be limited.

[0058] As shown in the sequence on the left-hand-side, from top to bottom, the data information 54a, 54b is relatively sparse in the beginning, and increases as scanning is continued. Some specific points are relatively sparse in terms of spectral information for that point and thus belong to an undecided material, i.e. neither black nor white (54c, 54d). In the top most step, the acquired data of the sample area 50 (left-hand side) is used to establish estimated phases 154a, 154b for the sample 150 (right-hand side). Use is made of spectral information of detected emissions 54a, 54b of the first type to establish the estimated phases of the plurality of sample locations. In the embodiment shown, the image representation 150 uses the measured points and the estimated points to represent the entire sample area 50 of the sample with data points. In other words, where no data exists, an estimated data point is used. Where inconclusive data is measured (points 54c, 54d), an estimated phase is taken in the image representation 150 as well, and hence phases are associated to the measured emissions of the first type. Where no data is present at all, the image representation has associated phases to expected emissions of the first type. It is noted that the obtained points and estimated points are shown in a scattered pattern, although a more regular, matrix-like pattern, is conceivable as well. In the embodiment shown, the data representation 150 (right-hand side) is an image representation, although any other representation, i.e. in data, is conceivable of course.

[0059] FIG. 3 shows (left-hand side, top to bottom) that scanning of the sample can continue, and that more data of the sample 50 is coming in. When more data comes in, this means that the image representation 150 (right-hand side) can be updated. Once again, sparse data points can be assigned a specific phase (either black or white, in the present case) based on measurement data that is already present. It is conceivable, in this regard, that neighboring sample points are compared to inconclusive sample points, and that based on the comparison a phase is assigned to the inconclusive sample point (54c, 54d). With increase of time, and hence incoming data, a more conclusive and reliable image representation 150 can be obtained.

[0060] It is conceivable that during data acquisition and/or processing, the scanned area 50 of the sample S is divided into multiple segments 51. As shown in FIG. 3, the scanned area 50 is divided into four regular, rectangular segments 51, although it will be understood by those skilled in the art that the number, shape, and regularity can be different. These segments can be used in establishing an estimated phase fore one or more of the pluralities of sample locations. In a particular advantageous embodiment, use is made of a second detector for establishing the segments. The second detector can be a BSE detector, as described earlier, and contours identified in the EM image can be used for segmentation. With this, it is possible to assign similar phases within a specific segment, or ensure that the estimated phase has a higher probability for a given phase that has been measured within a specific segment.

[0061] It is noted that a further division into further segments is conceivable as well. In particular, a further subdivision based on more incoming data points enables a more accurate image representation of the sample.

[0062] It is conceivable that the area 50 to be scanned is scanned multiple times to obtain the desired data quantity. Scanning multiple times may include scanning only a part of the area 50 of the sample. For example it is conceivable that, based on a first scan (or first set of scans), regions of interest and regions of non-interest are defined, and that only regions of interest are scanned in a second scan (or second set of scans). This increases the efficiency of the method. In particular, the regions of interest may be defined using the data obtained from the emissions of the second type, i.e. the EM data may be used to define regions of interest that are in particular scanned for obtaining EDS data.

[0063] It is advantageous to use a machine learning estimator for establishing estimated phases. Use of a machine learning estimator allows in particular a high ratio of measured sample locations to estimated sample locations to be obtained. As can be seen in the top right figure of FIG. 3, a ratio between the number of measured sample locations and the number of estimated sample locations is approximately 1:10, meaning that for every measured sample location it is possible to estimate a total of ten other locations. The ration can be in the range of 10:1 to 1:10, and more specifically in the range of 1:2 to 1:10. The machine learning estimator is able to predict the most likely dense spectra for individual sparse spectra at the input. The proposed machine learning estimator can maintain an inner model of phases and their spectra. With each incoming (sparse) spectrum, the model is updated and the spectrum can be evaluated to get similarity scores with the modelled phases. In other words, during data acquisition (top to bottom in FIG. 3), each newly acquired data point (left-hand side) gives information about the accuracy of the previously determined image representation (right-hand side). Hence, it is possible to update the machine learning estimator during use of the charged particle device, which makes the machine learning estimator very effective. As stated before, the machine learning estimator may comprise one or more estimators chosen from the group comprising: Non-negative matrix factorization (NMF), Singular value decomposition (SVD), independent component analysis (ICA), latent Dirichlet allocation (LDA) and K-means. These are all unsupervised learning techniques capable of component/cluster identification and characterization.

[0064] The method has been described above by means of several embodiments. The desired protection is conferred by the appended claims.