Improved transmission electron microscopy

Abstract

A scanning transmission electron microscope (STEM) having a sample plane, the STEM comprising a primary electron beam source arranged to provide a primary electron beam to a sample located at the sample plane of the STEM. A STEM detector, wherein the sample plane is located between the primary electron beam source and the STEM detector. And a first secondary electronic SE detector located between the primary electron beam source and the sample plane of the STEM. And the second SE detector is positioned between the sample plane and the STEM detector. And a signal acquisition circuit configured to simultaneously acquire a first signal from the first SE detector, a second signal from the second SE detector, and a third signal from the STEM detector. There is also a method for generating an image from a STEM.

Description

Technical Field

[0001] The present invention relates to Scanning Transmission Electron Microscopy (STEM) and in particular to STEM which detects secondary electrons emitted from features within a sample. Background Art

[0002] In existing scanning transmission electron microscopes (STEMs), a beam of electrons is delivered to a sample. Figure 1 The STEM 10 is shown in an existing configuration.

[0003] The electron source 15 generates an electron beam 20 which is focused by a focusing element such as an upper electron objective pole piece 40. The electron beam 20 is directed toward a specimen or sample plane 30. The primary electron beam 25 which has passed through the sample plane is collected by a lower objective pole piece 70 and detected by a STEM detector 80. The upper objective pole piece 40 directs the electron beam 20 through the sample plane 30, typically in a raster pattern, with the STEM detector 80 generating a signal at each position or continuously throughout the scan. These data are used to form a STEM image.

[0004] As with the transmitted electrons that form the primary electron beam 25, the secondary electrons 60 may also be scattered from features within the sample. These secondary electrons 60 may provide additional information about the sample and may be generated by a beam located above the sample plane 30 (e.g., Figure 1 The secondary electron detector 50 is located on the side of the sample plane adjacent to the electron source 15 and opposite to the STEM detector 80. The secondary electron signal collected in this manner can be used to visualize the morphology of a particular sample. Thus, the secondary electron detector 50 is used to detect the secondary electrons from the sample surface (e.g., Figure 1 The secondary electron detector 50 is intended to provide additional data about features on the upper surface of a thick or bulk sample, for example. In this example STEM 10, data is either collected using the STEM detector 80 (e.g., for a thin sample) or from the secondary electron detector 50 (e.g., to obtain information about features on the surface of a thick or bulk sample).

[0005] Figure 2 Schematically, a sample 100 is shown with individual features 110. An electron beam 20 is directed at the sample from above, such as Figure 2 An example STEM image (collected from data acquired by the STEM detector 80) is shown as Figure 2120 in the transmission STEM image 120. The image formed by the data collected by the secondary electron detector 50 is shown as image 125. As can be seen from the two generated images, the same features can be identified in each image, but they have different intensities. Each sample feature 110 shown in the transmission STEM image 120 has a similar signal intensity because they are formed by sample features having the same composition in this example. In contrast, the image 125 collected using the secondary electron detector 50 shows the same sample feature 110 with different intensity values. This is because some of the sample features 110 generate more detected secondary electrons than other sample features. For example, feature 130 is located at the surface of sample 100 and therefore generates more secondary electrons (SE), which can be collected by the secondary electron detector 50 with complete morphological information. Feature 140 is embedded below the surface of sample 110 and therefore shows a lower signal than the signal generated by feature 130, as collected by the secondary electron detector 50. Features 150 within sample 100 are further embedded within the sample, including those features on the side of the sample opposite to the secondary electron detector 50. These features exhibit much lower secondary electron activity, as detected by secondary electron detector 50. Therefore, the morphological information provided by secondary electron image 125 is further suppressed for these features. As can be seen from this illustration, the attenuation depends on the depth of feature 110 within sample 100.

[0006] Using this type of existing STEM 10 provides little or no morphological information for features deeply embedded in the sample (especially thick samples). In addition, it can only provide limited depth information for features, especially for those features with low atomic numbers as the components, which generate fewer secondary electrons.

[0007] Therefore, there is a need for methods and systems that overcome these problems. Summary of the invention

[0008] A scanning transmission electron microscope (STEM) provides an electron beam to a sample. The electron beam is formed by an electron source and directed toward the sample (e.g., by an electric field). For example, the electron beam may have an energy of about 200 keV. Two secondary electron (SE) detectors or sensors are disposed above and below the sample. These detectors or sensors detect secondary electrons that scatter back toward the electron beam source and forward from the sample and away from the source. The secondary electrons may have an energy in the range of 2 keV to 10 keV. A STEM detector or sensor is also provided to detect a signal for generating a STEM image. A scanning coil is also provided to guide the electron beam through the sample.

[0009] For example, the sample may take the form of a thick sheet, and features may be located in the bulk of the sample, closer to one surface of the sheet than to an opposing surface. When the sample sheet is located at or about the sample plane of the STEM, one surface will face or be closer to one SE detector, and the other surface of the sample will face or be closer to another SE detector.

[0010] When the sample (located at or around the sample plane) is scanned by the electron beam, the STEM detector collects data. At the same time, the SE detector detects any secondary electrons. The SE detector can detect signals from specific features. For each detector, the signal at each point in the sample scan is also recorded. Therefore, the specific signal of each point in the sample from three or more different detectors can be compared. However, for the same feature, the signal detected from a separate SE detector may have different amplitude values. For example, a feature close to the surface of the sample sheet or at the surface of the sample sheet should generate a higher signal at the SE detector on that side of the sample than the SE detector on the other side of the sample. This may be due to the body of the sample attenuating (i.e., absorbing) the secondary electrons when the secondary electrons pass through the sample. Therefore, comparing the signals collected by the SE detector for the same point in the scan provides depth information (e.g., on the Z axis of the STEM or in the direction of the electron beam). The depth information can be merged with the image generated by the STEM detector. If the material of the sample is known together with any attenuation characteristics of the material, the accuracy of the depth information can be further improved. Data can be extracted or separated from the detector data and provide information describing different layers of the sample. Thus, virtual sectioning of the sample can be achieved.

[0011] In this context and according to a first aspect, there is provided a scanning transmission electron microscope (STEM) having a sample plane, the STEM comprising:

[0012] a primary electron beam source arranged to provide a primary electron beam to a sample located at a sample plane of the STEM;

[0013] a STEM detector, wherein a sample plane is located between a primary electron beam source and the STEM detector;

[0014] a first secondary electron SE detector, the first SE detector being located between the primary electron beam source and the sample plane of the STEM;

[0015] a second SE detector located between the sample plane and the STEM detector; and

[0016] A signal acquisition circuit, the signal acquisition circuit being configured to simultaneously acquire:

[0017] a first signal from a first SE detector,

[0018] a second signal from a second SE detector, and

[0019] A third signal from a STEM detector. STEM can be used to determine the three-dimensional location of features within a sample using a single scan (especially for thick samples where secondary electrons from features within the sample cannot be detected by the upper SE detector alone). In addition, having a second SE detector or a lower SE detector allows data to be collected simultaneously from secondary scattering of features located on opposite sides of the sample.

[0020] Preferably, the STEM may further include a data processing circuit, wherein the data processing circuit is configured to:

[0021] When the primary electron beam scans the sample, a first signal, a second signal, and a third signal are received, and a first data set, a second data set, and a third data set are generated from the received first signal, second signal, and third signal, respectively.

[0022] Advantageously, the data processing circuitry may be further configured to receive position information of the primary electron beam corresponding to when the first, second and third signals are detected, and to incorporate the position information into the generated first, second and third data sets. Thus, the signals (e.g., amplitude or relative amplitude) may be related to xy, horizontal or sample plane positions and to each other. The position information may be specific coordinates in the sample plane or information that may be used to derive such coordinates.

[0023] Preferably, the first data set, the second data set and the third data set may be image data. Other data types may be used. For example, the data set may include information for generating image data.

[0024] Preferably, the data processing circuit can be further configured to:

[0025] Depth information of one or more features identified in the third dataset is generated based on the first signal data and the second signal data in the first dataset and the second dataset, with corresponding position information in the first dataset and the second dataset. The depth information may be z-axis data, where the sample plane is in the xy plane.

[0026] Preferably, the depth information may be perpendicular to the sample plane.

[0027] Optionally, the SE detector may be an annular detector.Other detector types and shapes may be used.

[0028] Preferably, the STEM may further comprise an upper objective pole piece (on the beam side of the sample) and a lower objective pole piece (after the beam has passed through the sample plane), the upper objective pole piece and the lower objective pole piece being configured to scan the primary electron beam source through the sample plane.

[0029] Optionally, the STEM detector can be a dark field DF, high angle dark field HAADF, DF, bright field BF or a pixelated detector. Other detector types may be used.

[0030] Optionally, the first SE detector, the second SE detector and the STEM detector may be coaxial with the electron beam provided by the primary electron beam source.Other configurations may be used.

[0031] According to a second aspect, there is provided a method for generating an image from a scanning transmission electron microscope (STEM) having a primary electron beam source, a STEM detector, a first secondary electron SE detector and a second SE detector, the method comprising the following steps:

[0032] receiving a first signal from a first SE detector, a second signal from a second SE detector, and a third signal from a STEM detector, wherein the first signal, the second signal, and the third signal are detected simultaneously while the primary electron beam scans the sample;

[0033] receiving position information of the primary electron beam corresponding to when the first signal, the second signal, and the third signal are detected;

[0034] generating a first image data set, a second image data set and a third image data set from the received first signal, the second signal and the third signal, respectively, when the primary electron beam scans the sample, and incorporating the position information into the first data set, the second data set and the third data set;

[0035] generating depth information perpendicular to the sample plane for one or more features identified in a third image dataset based on the first signal data and the second signal data in the first image dataset and the second image dataset, having corresponding position information in the first dataset and the second dataset; and

[0036] The depth information is combined with the third image data set to generate an output image.

[0037] Preferably, the method may further comprise the step of merging the first image data set and the second image data set with the output image to generate a merged output image.

[0038] Optionally, the step of generating depth information of one or more features may further include comparing the relative intensities of the first signal data and the second signal data having corresponding position information of the one or more features. This may provide qualitative or quantitative depth information, depending on the degree of knowledge of the characteristics of the sample, beam and detector.

[0039] Optionally, the step of generating the first image dataset, the second image dataset and the third image dataset may further include: normalizing the first image dataset, the second image dataset and / or the third image dataset. This may help adjust or compensate for differences in detector sensitivity, etc.

[0040] Advantageously, the step of generating the first image data set and the second image data set may further comprise estimating a signal noise floor of the first signal and the second signal.

[0041] Preferably, the position information may be parallel to the sample plane.

[0042] The method described above can be implemented as a computer program comprising program instructions for operating a computer. The computer program may be stored on a computer-readable medium.

[0043] The computer system may include one or more processors (e.g., local, virtual, or cloud-based), such as a central processing unit (CPU), and / or a single graphics processing unit (GPU) or a collection thereof. The processor can execute logic in the form of a software program. The computer system may include a memory including volatile and non-volatile storage media. A computer readable medium may be included to store logic or program instructions. Different parts of the system can be connected using a network (e.g., a wireless network and a wired network). The computer system may include one or more interfaces. For example, the computer system can include a suitable operating system such as UNIX, Windows (RTM), or Linux.

[0044] It should be noted that any of the features described above can be used with any aspect or embodiment of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The invention can be practiced in many ways and embodiments will now be described by way of example only and with reference to the following drawings, in which:

[0046] Figure 1 A schematic diagram of a prior art scanning transmission electron microscope (STEM) is shown;

[0047] Figure 2 Shown by Figure 1 Schematic diagram of samples and images collected by STEM;

[0048] Figure 3 shows a schematic diagram of a STEM according to an example embodiment;

[0049] Figure 4 Shown by Figure 3 Schematic diagram of samples and example images collected by STEM;

[0050] Figure 5shows the Figure 3 A flowchart of an example method for generating a STEM image from STEM collected data;

[0051] Figure 6 shows the electrons interacting with features within the sample and the Figure 3 Schematic diagram of example resulting images collected by STEM;

[0052] Figure 7 Shown with Figure 5 The same samples and Figure 5 Image of Figure 3 Schematic diagram of the signal intensity collected by STEM;

[0053] Figure 8 Shown by Figure 3 Schematic diagram of the signals and images generated by different detectors within a STEM; and

[0054] Fig. 9 Shown in Figure 1 and Figure 3 Secondary electron detector used within a STEM.

[0055] It should be noted that the figures are presented for simplicity and are not necessarily drawn to scale. Similar features are provided using the same reference numerals. DETAILED DESCRIPTION

[0056] Figure 3 A schematic diagram of an example implementation of a scanning transmission electron microscope (STEM) 200 that provides additional information and data about a sample is shown. In particular, STEM 200 provides three-dimensional information about features embedded within the sample. Figure 1 Component features similar to those described in connection with STEM 10 have the same reference numerals. Figure 1 Same as described in STEM 10, Figure 3 The STEM 200 shown has an electron source 15 that produces an electron beam 20 that is focused by an upper objective lens pole piece 40. In this example implementation, the primary electron beam 20 has an energy of 200 keV or about 200 keV. However, this may be varied, or other energies may be used. The STEM 200 also has a STEM detector 80. This may be of any suitable type (e.g., the STEM detector may be annular and / or may be a high angle annular dark field (HAADF) detector).

[0057] Other STEM detectors may be used. These may include pixelated detectors such as the Ceta-D camera, Falcon 4i camera, and Electron Microscope Pixel Array Detector (EMPAD) camera available from ThermoFisher Scientific.

[0058] Secondary electrons (SE) 60 scattered back toward the electron source 15 are detected by the upper secondary electron detector 50. However, secondary electrons 210 scattered in the same direction as the primary electron beam 20 (i.e., in the direction toward the STEM detector 80) are detected by the second or lower scattered electron detector 220.

[0059] Circuitry within the STEM 200 controls the electron source 15 and the upper objective pole piece 40 to direct the electron beam 20 through the sample plane 30. At the same time, circuitry (not shown in this figure) collects data from the STEM detector 80, the first or upper scattered electron detector 50, and the second or lower secondary electron detector 220.

[0060] The detected signals are stored together or in association with the specific location at the sample plane 30 that was illuminated at that time. Thus, image data can be generated for each of these at least three detectors and can be compared between them. Such data can be stored within the STEM 200 or elsewhere (e.g., within a computer system not shown in this figure).

[0061] Figure 4 Schematically illustrated is how these image data may appear for a particular example sample 100 having features 110. An incident electron beam 20 generates secondary electrons 60 which scatter in different directions from the features within the sample. The electrons are transmitted through the sample as bright field (BF) or primary beam 25 and dark field (DF) 310 electrons.

[0062] Figure 4 The image can be Figure 2 , which does not have a second secondary electron detector below the sample plane 30. The image generated from the STEM data collected by the STEM detector 80 (image 120) and the image 125 obtained from the data collected by the upper or first secondary electron detector 50 are therefore comparable to those obtained using Figure 1 The example images (120, 125) generated by the STEM 10 are identical (both use the same example sample). However, Figure 4Yet another image generated from data obtained by the second or lower secondary electron detector 220 is shown. This image 340 has features 350, 360, 370 that also correspond to some of the features 110 on the sample 100. However, the intensities of the features shown in this figure 340 are different from those shown in the image 125 generated by the data collected by the secondary electron detector 50. Image features 130 and 140 show high image intensity on the image 125, while the corresponding feature 370 in the image 340 has a lower image intensity or no image intensity. This is because these features are located at or just below the top surface of the sample. Therefore, secondary electrons passing in the direction toward the electron source 15 are slightly attenuated because they pass through less bulk material of the sample 100. Any secondary electrons from these features that pass through the bulk of the sample 100 away from the electron source 15 will be more strongly attenuated, and therefore their intensity will be much lower as shown in the image 340. In contrast, image features 350 and 360 shown in image 340 have much higher intensity because they are located on the bottom surface of sample 100 and are collected by the second secondary electron detector below sample plane 30, and are therefore only slightly attenuated by any sample material.

[0063] Thus, further information about the depth of a particular sample feature 110 may be obtained, as well as morphological information that would not otherwise be available in a prior art STEM 10 having only a single secondary electron detector 50 located above the sample plane 30. Furthermore, direct comparison of the data collected by all three detectors of the STEM 200 may yield additional spatial and morphological information about the sample.

[0064] Figure 4 Feature 130 on the image 125 (first SE detector 50 image) shown is visible (has the highest amplitude) because the feature is located at the upper surface (closest to the first SE detector 50). Therefore, the signal from the feature (SE signal) provides complete information about the morphology or structure of the feature.

[0065] Figure 4 The feature 140 on the image 125 shown (the first SE detector 50 image) is visible, but has a lower amplitude or contrast because the feature is located at a depth Z from the surface, where Z <L e (SE escape depth). Some of the morphological information provided by this signal is suppressed. The signals from the first SE detector 50 obtained from features 150 are further attenuated because these features are located deeper within the bulk sample (i.e., up to and including the bottom of the bulk sample, where Z>L e ).

[0066] Figure 4The image 340 of FIG. 340 shows the inverse relationship for the same feature because the image 340 is generated from data collected from the second SE detector 220. Thus, feature 350 located on the bottom or lower surface of the bulk sample displays complete morphological information in the SE generated image 340, but feature 370 contains little or no morphological information (Z>L e ).

[0067] STEM 200 can be operated in a variety of ways to obtain data about a particular sample. Figure 5 A flow chart of an example method 500 (e.g., a computer-implemented method) for using these collected data to generate a STEM image with additional information is shown. Once the sample is placed at the sample plane 30 of the STEM 200, the method 500 begins. At step 510, the primary beam 20 scans the sample using the upper objective lens pole piece 40 (e.g., by applying an appropriately varying electric field to the beam path 20). As shown in steps 520, 530, and 540, simultaneous signals are received from the first SE detector 50, the second SE detector 220, and the STEM detector 80, respectively, at a processing circuit (not shown). These signals are received at each point of the sample scan, and the specific position (e.g., a pre-calibrated position) is also recorded and stored as the primary beam scans the sample. Therefore, the position and amplitude information can be correlated during subsequent processing. In addition, the beam energy can be changed depending on the sample type and sample thickness.

[0068] Image data is generated from the received signals at step 550 (from the first SE detector 50), at step 560 (from the second SE detector 220), and at step 570 (from the STEM detector 80). The STEM image corresponds to Figure 4 The first SE image corresponds to image 125, such as Figure 4 The second SE image corresponds to Figure 4 Image 340 is shown.

[0069] Depth information is generated using the first SE image and the second SE image at step 580. The depth information may be generated from intensity values ​​at specific points in the image, but other parameters may also be considered, such as properties of a specific sample and / or features within the sample. This may include an attenuation coefficient for the bulk sample and / or a scattering efficiency or cross section of a specific feature. The calculation of the depth information may also use beam energy and pre-stored calibration data.

[0070] Even without this additional data, depth information can be discerned. At step 590, the depth information for each specific point in the scan is merged with the STEM image 120. This merged data is provided as an output image, and the depth information can be displayed using any suitable technique, such as, for example, colorization, contrast, brightness, or highlighting.

[0071] The STEM 200 can be calibrated for depth information. For example, one or more test samples can be used for calibration (e.g., having known bulk and feature composition and having features placed at known depths in the sample). The incident electron beam energy can be varied to generate multiple images and / or data from each detector. The resulting STEM / SE images or data can be analyzed to determine when secondary electrons are no longer detected from features at a specific depth in the test sample. Therefore, when a sample having properties similar to the test sample is studied by STEM 200, the depth of a specific feature found in the image in the bulk sample can be determined (e.g., by comparing the amplitude data from the test sample with the amplitude data of the calibration sample). Therefore, virtual slicing of the sample (i.e., the ability to obtain images and composition information at different layers of the bulk sample) can be achieved. This also enhances resolution and contrast and provides three-dimensional sample information. The signal background noise can also be estimated in advance for each detector.

[0072] Background removal techniques can be used instead of or in addition to signal noise floor compensation. Examples of suitable background removal techniques include:

[0073] Histogram shape-based methods, where, for example, the peaks, valleys, and curvature of a smoothed histogram are analyzed. These methods may make certain assumptions about the image intensity probability distribution (i.e., the shape of the histogram);

[0074] Clustering-based methods, where grayscale samples are clustered into two parts: background and foreground;

[0075] Entropy-based methods, which produce algorithms that use the entropy of foreground and background regions, cross entropy between the original image and the binarized image, etc.

[0076] Object attribute-based methods, which search for similarity measures between grayscale and binary images, such as fuzzy shape similarity, edge coincidence, etc.;

[0077] Spatial methods, which use higher-order probability distributions and / or correlations between pixels.

[0078] The data used to generate the first image data set, the second image data set, and / or the third image data set may also be normalized. This may help adjust or compensate for detector sensitivity and experimental variations.

[0079] The depth of sample features within the sample can be determined based on the simultaneously collected data (from the STEM detector 80, the first SE detector 50, and the second SE detector 220). The z-coordinate (i.e., depth) information can be determined by analyzing the differences between the signals collected from each detector. Thus, an improved three-dimensional description of the sample structure can be determined. The morphology can also be determined simultaneously from the opposite (i.e., top and bottom) surfaces of the sample (e.g., lamelas).

[0080] The method can be implemented as a computer-implemented method, where data is received by a separate processor or computer. Alternatively, the STEM 200 may include processing circuitry to perform these method steps. The results may be saved to a storage device (e.g., a hard drive), stored remotely, and / or displayed on a display screen attached to the STEM 200 or computer. The storage device may also store calibration data for different sample types.

[0081] Figure 6 It is schematically shown how features 110 of the sample 100 can interact with the incident electron beam 20, and how the first and second SE detectors 50, 220 and the STEM detector 80 generate data forming separate images 125, 140 and 120, respectively.

[0082] Cross section A schematically shows a sample 100, with sample features 110 shown numerically as 1, 2, 3, 4, and 5. An incident electron beam 20 is shown interacting with each of these sample features. Secondary electrons 60 are (schematically) shown scattered back toward the incident electron beam 20, and the number and density of the arrows (curved arrows) indicate the intensity of the secondary electrons 60, as detected by the first SE detector 50.

[0083] In this example, because sample feature 1 is located on the surface of sample 100, the first SE detector 50 detects more secondary electrons than the second SE detector 220 because little or no attenuation occurs through any bulk of sample 100. In contrast, sample feature 5 is located on the opposite surface of sample 100, and in this example, the bulk material of sample 100 completely attenuates all secondary electrons, and therefore no secondary electrons are detected by the first or upper SE detector 50.

[0084] Secondary electrons scattered by sample features and detected by the second SE detector 220 (located below the sample plane 30 and on the side with the STEM detector) are also detected in Figure 6 2. The secondary electrons 210 detected by the second SE detector 220 are also shown in this figure as curved arrows, the number and density of which indicate the amplitude or signal level recorded by the second SE detector 220.

[0085] In this example, sample feature 5 shows the maximum intensity of these secondary electrons 210, as detected by the second SE detector 220. Sample feature 1 has no secondary electrons detected by the second SE detector 220 because these electrons are attenuated or scattered away by the bulk material of the sample 100. Sample features 2, 3, and 4 show intermediate signal levels depending on their location within the sample 100.

[0086] It should be noted that secondary electrons may be emitted in all directions, but the arrows indicate only those secondary electrons detected by the two SE detectors 50 , 220 .

[0087] Image 120 (B) shows an example top view STEM image obtained from the same sample 100. Again, the same sample features are identified by numbers 1-5. Since the primary electron beam 25 passing through the sample plane 30 has a much higher intensity (10 to 100 times) than the secondary electrons scattered off each sample feature, there is little or no noticeable attenuation in the STEM image 120. In this example, the characteristics of each sample feature 1-5 are similar, and these sample features have the same contrast and characteristics. Therefore, the STEM image 120 does not contain morphological or depth information for this particular example sample 100.

[0088] Example image 125(C) shows image data generated from signals captured by the first or upper SE detector 50. The intensity of the signal corresponds to the amplitude of the secondary electrons captured from the secondary electrons 60 scattered back toward the electron source 15. Sample features closer to the top surface (as shown in this figure) have higher contrast and signal levels (in image 125(C)) than sample features located deeper within the body of the sample 100. Sample 1 has the highest signal in this image, and sample feature 5 generates little or no signal at all.

[0089] In contrast, example image 340(D) ​​schematically illustrates image data generated from signals collected by second or lower SE detector 220. Sample features located closer to the bottom surface of sample 100 have higher contrast signals than those sample features located toward or at the top surface of sample 100.

[0090] Figure 7A representation of the data collected by each of the three detectors is shown, but in graphical form. A cross-sectional or side view of the sample 100 is shown in row A. Row B shows a graph of the signal intensity in that direction as the scan moves across the axis (from left to right in the figure) at each point being irradiated or exposed to the electron beam 20. The signal obtained by the first SE detector 50 is shown as row C, where the same points on the vertical axis are linked by dotted lines to indicate that the data is aligned with the sample position. Similarly, row D shows example data collected from the same sample 100 by the second SE detector 220.

[0091] therefore, Figure 7 shows how to collect at least one line of image data and compare it with Figure 6 The images (120, 125, 340) shown correspond.

[0092] Figure 8 Some additional processing is shown, the application of which can account for system noise and other factors that cause output errors. The figure schematically shows the processing of specific features or regions of interest. Column A shows data collected by the STEM detector 80. Column B shows data and image output shown for the same features, where the data was collected by the first SE detector 50. Column C shows data and image output derived from data collected by the second or lower SE detector 220.

[0093] The graphs in the second row of the Figure correspond to the signals acquired by the different detectors. Figure 8 The specific features shown are similar to those from Figure 7 2 (shown in side view in this figure). As with the previous method, each image and data signal is collected simultaneously when the same point or location is exposed to the electron beam.

[0094] Each detector (and in particular the SE detector) can be characterized before studying the sample. Based on this detector characterization process, the detector noise (and / or background noise) can be determined. The signal from each detector is processed by applying the function ( Figure 8 ) to compensate for anomalies found during detector characterization. These anomalies (as well as any determined functions, normalized functions, or calibration functions) may be stored prior to processing each sample. Figure 8The sixth row of shows a function applied to a signal that can change the contrast and intensity values ​​of the resulting image. For example, a signal intensity ratio can be applied to a signal. Such mapping can be implemented at different points in the scan, where, for example, these data can then be used to create depth distribution information for a specific feature with greater accuracy. The function can change the contrast and intensity values ​​of the resulting image in different ways. For example, if the material is known, the function can account for the amount or percentage of secondary electrons that are expected to pass through the corresponding thickness of the sample (which will have the added benefit of allowing the function to determine the thickness of the feature). Another example can be a function that uses the known thickness of the sample and the relative intensities found in images B and C to determine the expected depth in the sample for the location of the feature. The function can also take into account the beam intensity and expected emission rate of the secondary electrons for a specific feature material.

[0095] Fig. 9 A schematic diagram of a SE detector that can be used for one or both of the first SE detector 50 and the second SE detector 220 is shown. The SE detector takes the form of an Elastar column, but other SE detectors (e.g., a Nicol column) can also be used. The sample 100 emits secondary electrons 60. At Fig. 9 In the configuration shown, the SE detector is a first SE detector 50, where secondary electrons 60 are scattered back towards the electron source 15. The same but inverted detector may be used as the second SE detector 220.

[0096] The reflector 400 provides an electric field (eg, -15V) to reflect the secondary electrons. The push electrode 410 (eg, -100V) guides the secondary electrons 60 toward the scintillator 420. The bias tube applies a bias voltage (eg, +50V) to the secondary electrons 60.

[0097] It will be appreciated by those skilled in the art that details of the embodiments described above can be varied without departing from the scope of the present invention as defined by the appended claims.

[0098] For example, different detector types may be used. The SE detector may also be annular.

[0099] Many combinations, modifications or variations of the features of the above embodiments will be apparent to those skilled in the art and are intended to form a part of the present invention. Any of the features described in particular in relation to one embodiment or example can be used in any other embodiment by making appropriate changes.

Claims

1. A scanning transmission electron microscope (STEM) having a sample plane, the STEM comprising: a primary electron beam source arranged to provide a primary electron beam to a sample located at the sample plane of the STEM; a STEM detector, wherein the sample plane is located between the primary electron beam source and the STEM detector; a first secondary electron SE detector, the first secondary electron SE detector being located between the primary electron beam source and the sample plane of the STEM; A second SE detector is located between the sample plane and the Between STEM detectors; and A signal acquisition circuit, wherein the signal acquisition circuit is configured to simultaneously acquire: a first signal from said first SE detector, a second signal from the second SE detector, and A third signal from the STEM detector.

2. The STEM according to claim 1, further comprising a data processing circuit, wherein the data processing circuit is configured to: When the primary electron beam scans the sample, the first signal, the second signal, and the third signal are received, and a first data set, a second data set, and a third data set are generated from the received first signal, second signal, and third signal, respectively.

3. The STEM of claim 2, wherein the data processing circuit is further configured to receive position information of the primary electron beam corresponding to when the first signal, the second signal, and the third signal are detected, and to incorporate the position information into the generated first data set, second data set, and third data set.

4. The STEM of claim 2, wherein the first data set, the second data set, and the third data set are image data.

5. A STEM according to any preceding claim, wherein the data processing circuit is further configured to: Depth information of one or more features identified in the third data set is generated based on first signal data and second signal data in the first data set and the second data set having corresponding position information in the first data set and the second data set. The STEM of claim 5 , wherein the depth information is perpendicular to the sample plane.

7. A STEM according to any preceding claim, wherein the SE detector is an annular detector.

8. A STEM according to any preceding claim, further comprising upper and lower objective pole pieces, the upper and lower objective pole pieces being configured to scan the primary electron beam source across the sample plane.

9. A STEM according to any preceding claim, wherein the STEM detector is a dark field DF, high angle angular dark field HAADF, DF, bright field BF or a pixelated detector.

10. A STEM according to any preceding claim, wherein the first SE detector, the second SE detector and the STEM detector are coaxial with the electron beam provided by the primary electron beam source.

11. A method for generating an image from a scanning transmission electron microscope (STEM) having a primary electron beam source, a STEM detector, a first secondary electron (SE) detector and a second SE detector, the method comprising the steps of: receiving a first signal from the first SE detector, a second signal from the second SE detector, and a third signal from the STEM detector, wherein the first signal, the second signal, and the third signal are detected simultaneously while the primary electron beam scans a sample; receiving position information of the primary electron beam corresponding to when the first signal, the second signal, and the third signal are detected; generating a first image data set, a second image data set and a third image data set from the received first signal, the second signal and the third signal respectively when the primary electron beam scans the sample, and incorporating the position information into the first data set, the second data set and the third data set; generating depth information perpendicular to the sample plane for one or more features identified in the third image dataset based on first signal data and second signal data in the first image dataset and the second image dataset having corresponding position information in the first dataset and the second dataset; as well as The depth information is combined with the third image data set to generate an output image.

12. The method of claim 11, further comprising the step of merging the first image data set and the second image data set with the output image to generate a merged output image.

13. The method according to claim 11 or claim 12, wherein the step of generating depth information of the one or more features further comprises: The relative strengths of the first signal data and the second signal data having the corresponding position information of the one or more features are compared.

14. The method according to any one of claims 11 to 13, wherein the step of generating the first image data set, the second image data set and the third image data set further comprises: The first image data set, the second image data set and / or the third image data set are normalized.

15. The method according to any one of claims 11 to 14, wherein the step of generating the first image data set and the second image data set further comprises: A signal noise floor of the first signal and the second signal is estimated.

16. The method according to any one of claims 11 to 15, wherein the position information is parallel to the sample plane.

17. A computer program comprising program instructions which, when executed on a computer, cause the computer to perform the method according to any one of claims 11 to 16.

18. A computer-readable medium comprising instructions which, when executed by a computer, cause the computer to perform the method according to any one of claims 11 to 16.

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