Transmission charged particle microscope equipped with an electron energy loss spectrometer

Abstract

To provide an improved transmissive type charge particle microscope.SOLUTION: There is provided a transmissive type charge particle microscope comprising: a charge particle beam source for discharging a charge particle beam; a sample holder for holding a sample; an illuminator for orienting the charge particle beam above the sample, the charge particle beam being emitted from the charge particle beam source; and a control unit for controlling operations of the transmissive type charge particle microscope. As described in the description, the transmissive type charge particle microscope is arranged so as to operate in at least two modes for substantially bringing a first magnification, while maintaining a state in which a focus of a diffraction pattern substantially matches. The at least two modes include: a first mode having a first setting of a final projector lens of a projection system; and a second mode having a second setting of the final projector lens.SELECTED DRAWING: Figure 8b

Description

Technical Field

[0001] Summary The present invention relates to a transmission electron microscope including an electron beam source for emitting an electron beam, a sample holder for holding a sample, an illuminator for directing the electron beam emitted from the electron beam source onto the sample, and a control unit for controlling the operation of the transmission electron microscope.

Background Art

[0002] The charged particle microscopy method is a well-known and increasingly important technique for imaging micro-objects, particularly in the form of electron microscopes. So far, the basic types of electron microscopes have evolved into several well-known devices such as transmission electron microscopes (TEMs), scanning electron microscopes (SEMs), and scanning transmission electron microscopes (STEMs), and further, for example, so-called "dual beam" devices (e.g., FIB-SEMs) that further employ a focused ion beam (FIB) enabling assisting actions such as ion beam milling or ion beam induced deposition (IBID). Those skilled in the art will be familiar with various types of charged particle microscopes.

[0003] Irradiation of a sample with a scanning electron beam promotes the emission of "secondary" radiation from the sample in the form of secondary electrons, backscattered electrons, X-rays, and cathodoluminescence (photons of infrared, visible, and / or ultraviolet light). 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), the material is exposed to an electron beam having a known narrow range of kinetic energies. Some of the electrons undergo inelastic scattering, which means that these electrons lose energy and their paths may be slightly deflected. The amount of energy loss can be measured by an electron spectrometer, and from the amount of energy loss, it is possible to elucidate what caused the energy loss. Inelastic interactions include phonon excitation, interband and intraband transitions, plasmon excitation, inner shell ionization, and Cherenkov radiation. Inner shell ionization is particularly useful for detecting the elemental components of a material. For example, it may be found that a greater number of electrons than expected pass through the material with an energy 285 eV less than the energy they had when they entered the material. This is approximately the amount of energy required to remove an inner shell electron from a carbon atom, and this can be regarded as evidence that a significant amount of carbon is present in the sample. By paying attention and considering a wide range of energy losses, it is possible to determine the type of atoms the beam impinges on and the number of each type of atom. Also, the scattering angle (i.e., the magnitude by which the path of the electron is deflected) can be measured to obtain information regarding the dispersion relation of whatever material excitation caused the inelastic scattering.

[0005] There are several basic types of EELS, which are mainly classified by shape and the kinetic energy of the incident electrons (typically measured in kilo electron volts (keV)). Perhaps the most common today is transmission EELS, where the kinetic energy is typically 100 - 300 keV and the incident electrons pass completely through the material sample. Usually, this occurs in a transmission electron microscope (TEM), particularly a scanning transmission electron microscope (STEM).

[0006] In STEM, the probe-forming optical system focuses the illumination on the sample in a small probe. The semi-convergence angle of such a probe is a balance between the desire to limit this angle in order to limit the optical aberrations that degrade the probe size, and the desire to maximize this angle in order to maximize the current in the probe and minimize the blurring due to the diffraction effect of the wave. Typically, in a STEM microscope without an aberration corrector, the probe size can be as small as about 2 Å and the semi-convergence angle can be as large as about 10 mrad. In a STEM microscope with an aberration corrector, the probe size can be as small as about 0.5 Å and the semi-convergence angle can be as large as about 40 mrad.

[0007] Normally, in STEM, since the sample is quite thin, most of the electrons in the probe do not interact with the sample. Some of the electrons in the probe may scatter elastically (without losing energy) on the sample, so significant direction changes may occur. Other electrons may scatter inelastically (without changing direction significantly) on the sample, so significant energy losses may occur. A small fraction of the electrons may undergo multiple elastic and inelastic interactions.

[0008] The distribution of the scattering angles can be observed in the back focal plane of the first imaging lens (usually the objective lens). In this plane, all electrons emerging from the sample in a particular direction are focused to a single point, regardless of the position of the electrons in the sample. This two-dimensional image of the distribution of the exit angles is called the diffraction pattern. The lenses downstream of the objective lens can transmit this pattern to some detector at the end of the microscope and be configured to form a magnified image of the diffraction pattern at this detector ("diffraction configuration"). Alternatively, in some types of electron microscopes, these lenses can also be configured to form a magnified image of the sample at this detector ("imaging configuration"). Such a detector can be, for example, a display screen, or a pixelated image detector, or, as in the present invention, an EELS spectrometer. When using an EELS spectrometer, the lenses of the microscope are usually in the diffraction configuration, because the entrance aperture of the spectrometer selects a clearly defined cone of exit angles, which facilitates a good quantitative interpretation of the EELS signal.

[0009] Electrons that are not elastically scattered exit the sample at approximately the same angle as when they entered. This portion appears as a bright disk of the exiting beam in the diffraction pattern and is called the bright field (BF) beam or BF disk. The half-angle of the BF beam is basically equal to the half-convergence angle of the probe. Elastically scattered electrons can change direction up to approximately 100 mrad. This portion appears as a low-intensity halo around the BF disk and is called the dark field (DF) beam or DF disk. Both the BF beam and the DF beam undergo energy loss and thus contain electrons that carry EELS information, and the proportion of these electrons is basically the same in the BF beam and the DF beam. Naturally, since the intensity of the BF beam is much higher, in absolute numbers, most of the EELS information is carried by the BF beam. Therefore, to obtain a high EELS signal, it is desirable to set up an optical system between the sample and the EELS spectrometer such that the EELS spectrometer captures at least the complete BF beam exiting the sample. This means that it is desirable for the EELS spectrometer to capture at least a cone of electrons exiting the sample that is the same size as the cone of electrons in the illumination probe. Conventionally, EELS has been performed on electrons with the sample exit angle and energy loss such that it is not difficult for the TEM to appropriately transmit from the back focal plane of the objective lens to the EELS spectrometer. Conventional conditions correspond, for example, to a filter collection angle corresponding to a beam energy E0 of about 200 keV, an energy loss E of about 1000 eV, an energy resolution ΔE of about 0.2 eV, and α of about 10 mrad at the sample. However, in recent years, there has been an increasing interest in EELS beyond the conventional range.

[0010] In so-called low-high tension (low HT) EELS, a smaller beam energy E0 of about 60-100 keV can be used. One of the advantages of low HT is that, generally, the damage imparted by the illumination beam to the sample is less. Another advantage of lower HT is that the energy resolution in EELS is better, which is because, firstly, the energy resolution is proportional to the beam energy, and secondly, the lower energy of the beam leads to less crosstalk between pixels in the EELS detector. This non-traditional form of EELS results in a larger energy loss (E) compared to the previously encountered beam energy (E0), which poses new requirements for the relative range E / E0 of the electron energy that can be properly transmitted from the diffraction plane to the EELS spectrometer by the microscope. Another development that leads to an increase in this range E / E0 is the development of an EELS spectrometer that can handle a higher energy range (see, for example, US9978561B2 and US10559448B2 by the present inventors). Such a very wide energy range enables the study of deep core losses, such as Si-K (1832 eV), Sn-L (3860 eV), W-L (10 keV), etc. In particular, it should be noted that in the case of these deep core losses, which generally have weak signals, the collection of these signals at a large collection angle is beneficial for good signal and good quantification.

[0011] In so-called probe-corrected EELS, a larger probe convergence angle of up to 40 mrad is used, and as a result, a larger filter collection angle α in the sample of up to 40 mrad is desirable.

[0012] In so-called ultra-high resolution (UHR) EELS, an improved energy resolution ΔE of about 0.02 eV or more is adopted. Different from conventional EELS where all EELS signals are generated in the electrons within the beam that interact with the electrons in the sample, in UHR EELS, some signals can be generated in the electrons within the beam that interact with the nucleons in the sample. Since nucleons are much heavier than electrons, these electrons that interact with nucleons are generally scattered at extremely high angles. Therefore, for these signals, it is desirable for UHR EELS to operate at a filter collection angle α of about 10 - 100 mrad in the sample. Also, in this case, the collection of these signals at a large collection angle is beneficial for good signals and good quantification.

[0013] Due to the limited size of the entrance aperture of the spectrometer, a larger collection angle requires a smaller magnification (from 10 times to 100 times) from the rear focal plane of the objective lens to the EELS spectrometer. This limitation is imposed by the optical aberration of the EELS spectrometer, which causes blurring in the EELS detector when electrons move significantly (e.g., more than 3 mm) off-axis. Such blurring impairs the energy resolution of the spectrometer. In TEM and STEM, since the lenses downstream of the sample are designed and optimized for maximum magnification (typically 100,000 - 1,000,000 times from the sample to the detector), such a low magnification causes these lenses to be used outside the range where they function with minimum aberration, and in particular, when combined with a large E / E0 and / or improved energy resolution, it may cause aberration and distortion in the EELS spectrum. SUMMARY OF THE INVENTION

[0014] In particular, in the case of a very large collection angle (corresponding to a very low magnification of the TEM imaging system), it has been found that the conventional optical system of the TEM imaging system hinders the performance of EELS. Therefore, an object of the present invention is to provide an improved transmission charged particle microscope.

[0015] For this purpose, the present invention provides a transmission charged particle microscope as defined in claim 1. The transmission charged particle microscope includes a charged particle beam source for emitting a charged particle beam, a sample holder for holding a sample, and an illuminator for directing the charged particle beam emitted from the charged particle beam source onto the sample. Further, the transmission charged particle microscope is provided with an electron energy loss spectroscopy (EELS) detector. A projection system for forming and imaging a diffraction pattern of the sample at a first magnification is provided between the sample and the EELS detector. As described above, the lens of the projection system can be set to a diffraction configuration, whereby an enlarged image of the diffraction pattern is formed at the detector at the first magnification, or the lens of the projection system can be set to an image configuration, whereby an enlarged image of the sample is formed at the detector. Typically, when the projection system is composed of a plurality of lenses, in the projection system, a plurality of intermediate images of the sample and a plurality of intermediate images of the diffraction pattern are generated. In the diffraction configuration, the intermediate images of the diffraction pattern are often of increasing size, and the final diffraction pattern at the detector is of the largest size and has the first magnification. At the same time, even in the diffraction configuration, the intermediate image of the sample has the smallest size and is of decreasing size until the final image of the sample, which is usually somewhere immediately after the last lens in the projector column and is very small and is usually regarded as a point, which is called the last crossover. In some types of electron microscopes, the projector system may be switched to an imaging configuration. In such an imaging configuration, often the intermediate images of the sample are of increasing size, and the final sample image at the detector is of the largest size. At the same time, even in the imaging configuration, the intermediate image of the diffraction pattern has the smallest size and is of decreasing size until the final image of the diffraction pattern, which is usually somewhere immediately after the last lens in the projector column and is very small and can be regarded as a point, which is called the last crossover. Often, the last crossover is simply referred to as the crossover.It should be noted that the present invention is equally applicable to a microscope equipped with a projector system that can only operate in a diffraction configuration, and a microscope equipped with a projector system that can operate in both a diffraction configuration and an imaging configuration.

[0016] As defined herein, the projection system includes at least a final projector lens which is an imaging lens arranged to form an image of the diffraction pattern of the sample. The projection system is arranged to provide a desired magnification, to provide a focus of the diffraction pattern, and to establish a crossover at a desired position. This typically requires a total of at least three imaging lenses, each due to one of the above requirements of magnification, focus, and crossover position. The projection system may include, in one embodiment, a diffraction lens, an intermediate lens, a first projector lens, and a second projector lens. In one embodiment, the final projector lens as defined herein is formed by the second projector lens. The names of these lenses reflect their main functions in a conventional TEM.

[0017] The EELS detector may include a dispersive device, an additional projection system, and a detection system, and the detection system includes, for example, a pixel array of charged particle detector elements. The dispersive device is arranged to disperse electrons according to their energy losses, and the additional projection system is arranged to magnify and image the dispersed electrons on the detection system. Specifically, the dispersive device creates an image of the crossover in a (mid) plane (spectral plane), and electrons with different energy losses form an image of the crossover at different positions in the spectral plane.

[0018] In the spectral plane, it should be noted that the dispersive device only needs to focus the image of the crossover in the dispersion direction. There is no need to focus this image in the non-dispersive direction, because the defocus in such a non-dispersive direction does not affect the energy resolution. Usually, instead of concentrating the total intensity at a certain energy loss value on a single pixel of the detector, a certain degree of defocus in the non-dispersive direction in the spectral plane is intentionally created to concentrate it on a specific column (or one or more elongated pixels) of the pixels. Therefore, the image on the detector is not a line of intensity where the intensity at each point represents the total intensity at a certain energy loss, but rather a rectangle of intensity where the intensity integrated over a certain column represents the total intensity at a certain energy.

[0019] As described above, the crossover is very small and can be approximated as a point. In this approximation, when there are no optical defects in the dispersive device, the energy resolution in the spectral plane becomes infinitely perfect. However, when the projector system introduces artifacts into the crossover (for example, due to spherical aberration or chromatic aberration of the lens), the crossover blurs to a finite size, and the blurred image in the spectral plane impairs the energy resolution.

[0020] The transmission-type charged particle microscope defined herein includes a control unit for controlling the operation of the transmission-type charged particle microscope.

[0021] As defined herein, the transmission-type charged particle microscope is configured to operate in at least two modes that substantially provide the first magnification of the diffraction pattern while maintaining the diffraction pattern substantially in focus and while maintaining the crossover substantially at a desired position. The control unit may be configured to switch between these modes, and it is considered that the user of the transmission-type charged particle microscope may set the desired mode, for example, using a graphical user interface. Other methods of setting these modes are also conceivable.

[0022] As defined herein, at least two modes include at least a first mode having a first setting of the final projector lens and at least a second mode having a second setting of the final projector lens. The second setting is substantially different from the first setting. The first and second settings are related to the excitation setting of the final projector lens.

[0023] Note that a projection system as defined herein includes a final projection lens. Of course, the projection system may include additional projector lenses, particularly provided upstream of the final projector lens, to form an image of the diffraction pattern. Further, although it is conceivable that one or more additional projector lenses are provided further downstream of the final projector lens, in view of the present disclosure, the final projector lens is, in principle, the actual final lens of the projection system in at least one of the two modes. This means that the final projection lens is arranged to form and image the diffraction pattern of the sample at a first magnification in at least one of the at least two modes.

[0024] The projection system of a transmission charged particle microscope as defined herein in two modes enables faithful transmission of the beam from the scattering plane to the EELS spectrometer. This applies not only to the conventional range of collection angles and energy losses (i.e., conventional EELS), but also to very high collection angles in combination with high energy losses (E / E0 > 2%) or in combination with extreme energy resolution. By enabling switching between a first and a second setting of the projector lens while maintaining the same first magnification of the diffraction pattern of the detector, it is possible to optimize the projection system for these different modes which may be the different EELS modes described above. Thus, using the charged particle microscope as defined herein, it becomes possible to optimize different modes in different ways by changing the excitation setting of the final projector lens. This may be done, for example, with respect to chromatic aberration or with respect to higher order aberrations. Thereby, the object of the present invention is achieved.

[0025] Note that the use of two modes, each having a different setting for the final projector lens, while maintaining the same first magnification, is different from the way EELS is implemented and performed in prior art TEMs or STEMs. In prior art TEMs or STEMs, there is only a single setting of the imaging lens for each magnification of the diffraction pattern. Thus, prior art TEMs or STEMs do not provide a range of modes for a given magnification.

[0026] Further embodiments and their advantages are described below.

[0027] In one embodiment, the first setting includes the final projector lens being substantially enabled. This allows the crossover-forming rays to be brought closer to the axis in a lens upstream of the final projector lens, which may reduce the spherical aberration of the projection system. This is particularly beneficial when used with a large acceptance angle at the limiting energy resolution, where the image at the crossover should have little blur (to reach the limiting energy resolution). Thus, in one embodiment, the first mode includes an ultra-high resolution EELS mode.

[0028] In one embodiment, the second setting includes the final projector lens being substantially disabled. Advantageously, this allows the distance between the final crossover and the final projector lens to be changed. This may require the lens excitation and / or settings of additional lenses upstream of the final projector lens to be changed as well in one embodiment. In any case, increasing the distance between the final crossover and the final projector lens reduces experimental artifacts seen in low HT (extremely large energy loss) EELS, which is mainly a result of the chromatic aberration of the projection system. Thus, in one embodiment, the second mode includes a low HT EELS mode.

[0029] In one embodiment, the projection system includes an objective lens for forming a diffraction pattern of the sample at a rear focal plane.

[0030] In one embodiment, the first magnification corresponds to an effective focal length of the projection system of about 100 mm or less. Here, the effective focal length is defined as the focal length of the first imaging lens multiplied by the magnification from the rear focal plane of this lens to the detector.

[0031] In one embodiment, the projection system is arranged to align the focus of the diffraction pattern with the diffraction pattern entrance aperture. The diffraction pattern entrance aperture may be at the entrance part of the dispersing device or may be provided upstream of the dispersing device.

[0032] According to one aspect, a method of operating a transmission electron microscope is provided, the method comprising: - providing a sample; - operating the transmission electron microscope in the first mode on the sample; - setting the transmission electron microscope to the second mode by changing the projector system setting from the first setting to the second setting; - operating the transmission electron microscope in the second mode on the same sample.

[0033] As defined herein, a transmission charged particle microscope is arranged to operate in at least two modes that substantially provide the first magnification while maintaining the diffraction pattern substantially in focus. The user may switch between these modes using a computer or other input device. Further, it is contemplated that a control unit is used for switching from the first mode to the second mode.

[0034] As defined herein, the at least two modes include at least a first mode having a first setting of the final projector lens and at least a second mode having a second setting of the final projector lens. The second setting is substantially different from the first setting. The first and second settings are related to the excitation settings of the final projector lens.

[0035] By changing the settings of the final projector lens, in one embodiment, it is possible to record different EELS spectra. This method is considered to include the step of recording a first EELS spectrum of the sample in the first mode and the step of recording a second EELS spectrum of the sample in the second mode. The first EELS spectrum is different from the second EELS spectrum.

[0036] In one embodiment, this method includes the step of switching the final projector lens basically from "on" to basically "off".

[0037] Further embodiments of this method have been discussed above with respect to transmission electron microscopes.

Brief Description of the Drawings

[0038] Here, the devices and methods disclosed herein will be described in more detail based on exemplary embodiments and the accompanying schematic drawings.

[0039]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5a

Figure 5b

Figure 6

Figure 7

Figure 8a

Figure 8b

Embodiments for Carrying Out the Invention

[0040] In the drawings, corresponding parts are indicated by corresponding reference numerals where appropriate. Note that generally the figures are not to scale.

[0041] FIG. 1 is a very schematic view of an embodiment of a transmission electron microscope M, which in this case is a TEM / STEM (however, in the context of the present disclosure, a transmission electron microscope can also be effective, for example, an ion-based microscope or a proton microscope). In FIG. 1, within a vacuum chamber E, an electron source 4 (such as a Schottky emitter, for example) generates an electron beam (B) that passes through an electron-optical illuminator 6, and the electron-optical illuminator 6 functions to direct / focus the electron beam onto a selected portion of a sample S (the sample S may be, for example, (locally) thinned / smoothed). This illuminator 6 has an electron-optical axis B', and widely includes various electrostatic / magnetic lenses, (scanning) deflectors (plural possible) D, correctors (such as an aberration corrector), etc. Typically, the illuminator 6 can also include a condenser lens system (the whole of component 6 may be referred to as a "condenser lens system").

[0042] The sample S is held on a sample holder H. As shown here, a part of this holder H (inside the housing E) is attached to a cradle A' that can be positioned / moved in multiple degrees of freedom by a positioning device (stage) A. For example, the cradle A' may be displaced (especially) in the X, Y, and Z directions (refer to the Cartesian coordinate system shown in the figure) and may be rotated about a longitudinal axis parallel to X. Due to such movements, various parts of the sample S are irradiated / imaged / inspected by an electron beam that moves along the axis B' (and / or a scanning operation is performed instead of beam scanning [using a deflector (s) D], and / or a selected part of the sample S is machined, for example, by a focused ion beam (not shown)).

[0043] The (convergent) electron beam B that moves along the axis B' interacts with the sample S to cause the sample S to emit various types of "induced" radiation, including (for example) secondary electrons, backscattered electrons, X-rays, and optical irradiation rays (cathodoluminescence). If necessary, one or more of these radiation types can be detected using a detector 22, which may be, for example, a composite scintillator / photomultiplier tube or an EDX (energy-dispersive X-ray spectroscopy) module. In such a case, the image can be constructed using basically the same principle as in SEM. However, alternatively or additionally, electrons that cross (pass through) the sample S, are emitted (released) from the sample, and continue to propagate along the axis B' (but are substantially, generally, subject to some degree of deflection / scattering) can be investigated. Such a transmitted electron beam is incident on an imaging system (a composite objective lens / projection lens) 24 that widely includes various electrostatic lenses / magnetic lenses, deflectors, correctors (such as an aberration corrector), etc.

[0044] In the normal (non-scanning) TEM mode, this imaging system 24 can focus the transmitted electron beam onto the fluorescent screen 26, which can be stored / retrieved (schematically indicated by arrow 26') so as not to interfere with axis B' if necessary. An (partial) image (or diffractogram) 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 at a suitable part of the wall of the housing E.

[0045] As an alternative to viewing the image on the screen 26, the fact that the depth of focus of the electron beam exiting the imaging system 24 is usually extremely large (e.g., about 1 meter) can instead be utilized. As a result, various types of detection devices / analytical apparatuses such as a TEM camera 30 can be used downstream of the screen 26: - A TEM camera 30. At the position of the camera 30, the electron beam can form a still image (or diffractogram), and the still image can be processed by the controller C and displayed on a display device (not shown) such as a flat panel display, for example. If not necessary, the camera 30 can be stored / retrieved (as schematically indicated by arrow 30') so as not to interfere with axis B'. - A STEM recorder 32. The output from the recorder 32 can be recorded as a function of the (X, Y) scanning position of the beam B on the sample S, and an image that is a "map" of the output from the recorder 32 as a function of X, Y can be constructed. Unlike the matrix of pixels typically present in the camera 30, the recorder 32 can include, for example, a single pixel having a diameter of 20 mm. Further, the recorder 32 generally has an acquisition rate much higher than that of the camera 30 (e.g., 10 2 images / second) (e.g., 10 6It has (a certain number of) locations / second. Similarly in this case, when not necessary, the recorder 32 can be stored / retrieved so as not to obstruct the axis B’ (as schematically shown by the arrow 32’) (however, such storage is not required for, for example, a donut-shaped annular dark-field recorder 32, but in such a recorder, the central hole allows the beam to pass through when the recorder is not in use). As an alternative to imaging using the camera 30 or the recorder 32, for example, it is also possible to drive a spectroscopic device 34 that can be an EELS module.

[0046] Note that the order / position of the components 30, 32, and 34 is not strict and many possible variations are conceivable. For example, the spectroscopic device 34 can also be integrated with the imaging system 24.

[0047] Note that the controller C (which can be a combined controller and a processor) is connected to various illustrated components via a control line (bus) C'. The controller can be connected to a computer screen 51 on which a user interface (UI) can be provided. This controller C can provide various functions such as synchronizing operations, providing set values, processing signals, executing calculations, and displaying messages / information on a display device (not shown). The controller C (schematically illustrated) can be (partially) inside or outside the housing E and can have a single or combined structure as required. It will be understood that for those skilled in the art, it is not necessary for the inside of the housing E to be maintained in a strict vacuum. For example, in a so-called "environmental TEM / STEM", a background atmosphere of a given gas is deliberately introduced / maintained inside the housing E. It will also be understood by those skilled in the art that in practice, it may be advantageous to limit the volume of the housing E. Thereby, if possible, the volume of the housing E basically extends along the axis B' and forms the shape of a small tube (for example, with a diameter of about 1 cm) through which the electron beam used passes, but it expands to accommodate structures such as the electron source 4, the sample holder H, the screen 26, the camera 30, the recorder 32, the spectrometer 34, etc.

[0048] FIG. 2 shows a more detailed embodiment of an imaging system 24 as defined herein. The imaging system 24 is provided between a sample S and an inlet aperture 3a of a dispersing device 3 (see also FIG. 3). The imaging system 24 shown includes an objective lens O and a projection system 25. The projection system 25 includes several different lenses, a total of four lenses in the embodiment shown. These lenses are, in order, a diffraction lens D, an intermediate lens I, a first projector lens P1, and a second projector lens P2. The second projector lens P2 constitutes the final projector lens P2 as defined herein. As known to those skilled in the art, an optional spherical aberration and / or chromatic aberration corrector (not shown) may be disposed between the objective lens and the projection system 25.

[0049] Referring now to FIG. 3, this shows a further detailed enlarged view of the embodiment of the spectrometer 34 of FIG. 1. In FIG. 3, an electron beam 1 (having passed through the sample S and within the imaging system 24) is shown propagating along an electron-optical axis B'. This beam 1 is incident on a dispersing device 3 (an "electron prism") and is dispersed (fanned out) into an array 5 of energy-resolved (energy-discriminated) spectral sub-beams distributed along a dispersion direction. For purposes of illustration, three of these sub-beams are labeled 5a, 5b, and 5c in FIG. 3.

[0050] Downstream of the dispersing device 3, the array 5 of sub-beams encounters a post-dispersive electron optical system 9 where, for example, it is magnified / focused and ultimately directed / projected onto a detector 11. The post-dispersive optical system may include circular lenses and / or quadrupole lenses. The detector 11 may include an assembly of sub-detectors arranged along the dispersion direction, and different sub-detectors may be adjustable to have different detection sensitivities. Note that other detector configurations for measuring an EELS spectrum are known to those skilled in the art and are equally applicable to the methods and devices disclosed herein. This method is not, in principle, limited to the use of a particular detector.

[0051] Figure 4 shows an example of an EELS spectrum. This figure represents the intensity I (in arbitrary units, a.u.) as a function of the energy loss E (in eV) of electrons that have passed through a sample containing carbon and titanium. From left to right, the main features of the spectrum are as follows: - The zero-loss peak ZLP represents electrons that pass through the sample without undergoing inelastic scattering within the sample. - The plasmon resonance peak component / section PRP, a relatively broad series of peaks / shoulders associated with one or more scatterings of the electrons of the plasmons in the sample. This typically extends over a range of about 0 - 50 eV, although there is no strict definition of its upper limit. It is characterized by peaks / shoulders such as peak 31, which result from the excitation of the collective vibrations of the valence electrons within the sample. Note that the PRP component usually has a significantly lower intensity than the ZLP. - The core-loss peak component / section CLP. This typically starts at about 50 eV (behind the PRP component), although there is no strict definition of its lower limit. Since the CLP typically has such a low intensity relative to the ZLP component / PRP component, as shown in Figure 4, this is magnified by a magnification factor (e.g., 100) to improve the visibility of this detail. As can be seen, it contains a cluster of peaks / shoulders that can be associated with specific chemical elements (such as C and Ti in this example), which are at the top of a substantial background contribution 33. The EELS spectrum shown in Figure 4 can be measured in a manner known to those skilled in the art using a device and setup as discussed in relation to Figures 1 - 3.

[0052] As discussed at the beginning, EELS has conventionally been performed on electrons having a sample exit angle and energy loss such that it is not difficult for a TEM to appropriately transmit from the rear focal plane of the objective lens to the EELS spectrometer. However, large collection angles are required for low HT (probe corrected) EELS and ultra-high resolution (UHR) EELS, and thus low magnifications are needed. Such low magnifications can introduce artifacts into the EELS spectrum, especially when combined with large E / E0 and / or improved energy resolution.

[0053] Low HT (probe corrected EELS) Figures 2 and 5a - 5b consider the aberrations that occur in low HT (probe corrected) EELS and possible settings of a transmission charged particle microscope to overcome these aberrations.

[0054] Figure 5a shows an image of the EELS spectrum of a Si sample taken in the so-called low HT (probe corrected) EELS mode. Here, a larger energy loss (E) occurs compared to the beam energy (HT), such as E / E0 = 1 - 4%. The shown EELS spectrum image is a slice image of three sub-images with different exposure times due to the large energy range and large intensity range. The EELS spectrum shows a strange reduction in the height in the non-dispersive direction in the region of 2400 eV - 2900 eV and a strange bulge with increasing intensity in the range of 2200 eV - 2600 eV. Due to the limited size of the entrance aperture of the spectrometer, it was found that at large collection angles, it is necessary to lower the magnification from the scattering plane (i.e., the rear focal plane of the objective lens) to the EELS spectrometer. Such low magnification can introduce artifacts into the EELS spectrum, especially when combined with large E / E0 as shown in Figure 5a. The artifacts prevent the EELS signal from being reliably quantified.

[0055] Returning now to Figure 2, we show the potential sources of these artifacts. In EELS, the diffraction pattern is imaged at the entrance aperture 3a of the EELS spectrometer. The magnification from the back focal plane of the objective lens to the entrance aperture 3a can be adjusted so that the entrance aperture collects a particular radius of the cone of electrons leaving the sample. A typical choice is for this cone to be equal to the cone of electrons that form the probe (so that the aperture captures exactly the so-called "bright field disk"). The magnification from the diffraction pattern to the entrance aperture 3a is called the camera length (CL) and can be interpreted as the effective focal length of the imaging system at the entrance aperture.

[0056] The image of the probe on the sample is formed at a crossover XO following the final projection lens P2. This crossover is approximately d XO = 3.5 mm below. Due to the chromatic aberration of the imaging system, electrons with energy loss E are focused slightly above this crossover XO (see dashed line in FIG. 2, with modified crossover XO′). The defocus distance dz is dz = C c (XO) (E / E0), where C c (XO) is the chromatic aberration at the XO plane. To begin with, this C c (XO) is C c (XO) =M 2 C c (spec) As a sample C c (spec) where M is the magnification from the specimen to XO.

[0057] This magnification M can be calculated as follows: The angle at the sample is α obj From FIG. 2, using the small angle approximation, the angle at the crossover XO is α XO =CLα obj Therefore, the angular magnification from the sample S to the crossover XO is equal to M α = α XO / αOBJ = CL / h. The spatial magnification from the sample S to the crossover XO is M = 1 / M α = h / CL. C c (spec) is dominated by the C of the objective lens, so C c (obj) = M c (XO) = M 2 · C c (obj) is approximated. Combining these gives dz = C c (obj) (h / CL) 2 · (E / E0) is obtained.

[0058] When dz increases and XO shifts in the P2 lens, the first-order approximation breaks down. The full simulation shows that as the energy loss E increases, XO passes through the P2 lens, through the front focal plane (FFP) of the P2 lens, and further shifts upward. For an energy loss such that XO is at the FFP of the P2 lens, the electrons exit the P2 lens parallel. Thus, at this energy loss, not only electrons with the normal scattering angle enter the spectrometer, but all electrons enter the spectrometer regardless of the scattering angle α obj This additional signal appears as a bump in the EELS spectrum in Fig. 5. At even higher energy losses, the electrons are deflected again from the parallel beam and the bump disappears from the EELS spectrum. The EELS bump can be estimated to occur when the chromatic focus blur is equal to the distance between XO and the P2 lens, i.e., when dz = d XO . (See Fig. 2). Thus, the EELS bump starts at E = E0· d XO / C c (obj) · (CL / h) 2 (Equation 1) and starts.

[0059] E0 = 120 keV, d XO = 3.5 mm, C c (obj)= 2.0 mm (ignoring the chromatic aberration from the objective lens and the image corrector, and the contribution from other imaging lenses), CL = 75 mm, h = 690 mm, this estimation results in ΔE = 120 kV·3.5 / 2.0·(75 / 690) 2 = 2.5 keV which occurs.

[0060] This is in good agreement with the experimental results shown in Fig. 5a.

[0061] Looking at Equation 1, away from the region of energy of interest, there seem to be several options to push the EELS bump to higher energies. - Increase -E0, i.e., get a higher high tension. This is not desirable for samples sensitive to the beam. - For example, by adding a Cc corrector, reduce C c (obj) This is very expensive. - Increase the camera length CL. This is not desirable as it reduces the signal collected by the entrance aperture of the EELS spectrometer. - Decrease the distance h between XO and the spectrometer. This is not desirable as it increases the magnification from XO to the spectral plane, and also, due to the finite size of the image at XO, this reduces the resolution of the EELS spectrometer. - Increase the distance d XO between the last lens and XO. This is the approach according to the present invention.

[0062] In this example, the present invention reduces the excitation of the P2 lens, and in one embodiment, by turning the P2 lens almost off, the distance d XOIt is taught to increase this. In effect, this makes P1 the last lens. FIG. 5 shows experimental evidence that this is a very effective solution, resulting in an EELS range without very large artifacts of E / E0 = 3 keV / 60 keV = 5%. FIG. 5b shows an image of the EELS spectrum of a Si sample at 60 kV, CL = 65 mm, and an entrance aperture of 5 mm (therefore, α≒35 mrad). These are composite images of three and five sub-images with different exposure times, respectively, due to a large energy range and a large intensity range. The upper part of FIG. 5b shows the conventional setup with P2 on, and the artifact starts at about 1200 eV. The lower part of FIG. 5B shows the setup according to the present invention: P2-off pushes the artifact beyond 3000 eV.

[0063] Ultimate energy resolution EELS Figures 6 to 8 consider the aberrations occurring in the extreme energy resolution EELS and the possible settings of a transmission charged particle microscope to overcome these aberrations.

[0064] Generally, the extreme energy resolution that can be obtained in a TEM is about ΔE = 15 meV at E0 = 60 keV. Such a resolution is obtained only when the minimum entrance aperture of the spectrometer is used due to the aberration of the spectrometer. This minimum aperture is 1 mm. Therefore, if a wide range of scattering angles (α>20 mrad) have to be collected by the EELS spectrometer, the magnification from the scattering plane to the spectrometer entrance has to be very small. Such a very small magnification can cause aberrations that affect the extreme energy resolution.

[0065] Figure 6 schematically shows the zero-loss peak 62 at CL = 13 mm with high resolution recorded by the EELS detector 11, and the corresponding EELS spectrum obtained by vertically integrating this image 61. Clearly, the energy resolution is blurred by some aberration (see lines 62 - 63). It was found that the blurring decreases when the camera length CL is increased (without changing anything else in the setup). This indicates that the blurring occurs somewhere in the projection system of the TEM.

[0066] Part of the resolution loss at very short camera lengths is caused by the spherical aberration C s (obj) of the imaging objective lens. This blurs the probe by d spec = 1 / 4C S (obj) α obj 3 only. As shown above with respect to Figures 2 and 5a, the probe is imaged onto XO at a magnification M = h / CL, and thus the blurring at XO is d XO = 1 / 4C S (obj)· α obj 3 (h / CL) . This can be converted to energy blurring using the apparent dispersion δ of the spectrometer at XO. The corresponding resolution loss is ΔE = 1 / 4C S (obj)· α obj 3 (h / CL) / δ.

[0067] Figure 6 was recorded using C S (OBJ) = 1.3 mm 、 α obj = 17 mrad, h = 690 mm, CL = 13 mm, and δ = 11 μm / eV at 60 kV, which gives ΔE = 8 meV. Clearly, the spherical aberration of the objective lens is important but cannot explain the broadening observed in Figure 6.

[0068] Figures 8a and 8b show two settings of the TEM imaging system, both of which transmit the diffraction pattern to the EELS detector at a very low magnification, CL = 13 mm. In the first option, P2 is almost off, and in the second option, P2 is fully on. The second option has the main drawback that in order to obtain an overall low magnification, one or more of the intermediate lenses in the column must be reduced to correct for the large magnification of P2. This results in imaging rays that are greatly off-axis in these intermediate lenses (not shown in Figure 8b), and thus these off-axis rays result in large aberrations in the image plane (in this case the EELS detector). Therefore, this second option is not normally used.

[0069] The first option, where P2 is almost off, is preferred because it is not subject to image aberrations from the intermediate lenses. Furthermore, as discussed above with respect to Figures 5 and 2, the first option has the advantage of functioning very well fortuitously at high E / E0.

[0070] In the setting where P2 is almost off (Figure 8a), XO is basically created and focused by the P1 lens. The quality of this focus is, apart from the C contribution discussed above, mainly determined by the spherical aberration of the P1 lens. The rule of thumb is that the spherical aberration of the lens is C s (obj) の about d S (P1_XO) about d im 4 / S 3 That is. In this setup, the P1 lens has an image distance d im = 80 mm and a lens spacing S = 17 mm, which gives C S (P1_XO) = 8·10 3 mm, and the full calculation gives C S (P1_XO) = 14.5·10 3From a sample giving S (proj_spec) C = C S (proj_XO) / M 4 = 0.002 mm, it can be calculated by returning to the sample.

[0071] Obviously, the contribution of the projector system 25C s (proj_spec) = 0.002 mm is much smaller than the contribution of the objective lens OC s (obj) = 1.3 mm, so it seems negligible.

[0072] However, in practice, it is important to recognize that due to mechanical shifts and tilts of the lenses, the beam may deviate from the axis by 1 - 3 mm at the entrance of the spectrometer 3a. This is usually corrected by applying a so - called "diffraction shift" using a deflector located between the objective lens O and the D lens. This causes a significant off - axis movement of the beam corresponding to an angle α=(1 - 3 mm) / CL = 80 - 230 mrad at the objective lens in the I lens and P1 lens. Although it is difficult to calculate the exact effect of such off - axis movement, a person skilled in the art will understand that this causes significant energy smearing. This can be checked by repeating the EELS measurement using a setting where the XO - forming beam is close to the axis.

[0073] In the setting below Figure 8b, since the P2 lens is on, the XO - forming beam (light in this figure) approaches the axis. As a result, the spherical aberration of the projector system is relatively low. The complete calculation gives C s (P1_XO) = 85 mm, which is 170x smaller than the setting in Figure 8a. Figure 7 shows the EELS spectrum obtained in this mode (i.e., P2 is on). In image 71 of Figure 7, there is no smearing (see lines 72 - 73).

[0074] ​From the above, a transmission type charged particle microscope is provided. In this case, the transmission type charged particle microscope is configured to operate in at least two modes that substantially provide the first magnification while maintaining the diffraction pattern substantially in focus. The at least two modes are - a first mode having a first setting of the final projector lens, - a second mode having a second setting of the final projector lens, and includes.

[0075] In one embodiment, the first mode and the second mode are different EELS modes.

[0076] The first setting may include that the final projector lens P2 is substantially enabled, for example, corresponding to the situation described above with respect to the extreme energy resolution EELS.

[0077] The second setting may include that the final projector lens P2 is substantially disabled, for example, corresponding to the situation described above with respect to low HT EELS.

[0078] In the first setting, since the final projector lens P2 is substantially enabled, a conventional EELS spectrum may also be obtained.

[0079] Note that other settings of the imaging system 24 and / or the projection system 25 may be changed between the first mode and the second mode as long as the magnifications are substantially the same. The magnification used corresponds to the effective focal length of the projection system of about 100 mm or less in one embodiment.

[0080] The desired protection is determined by the appended claims.

Claims

1. A transmission type charged particle microscope, comprising: - A charged particle beam source for emitting a charged particle beam; - A sample holder for holding a sample; - An illuminator for directing the charged particle beam emitted from the charged particle beam source towards the sample; - A projection system for forming and imaging a diffraction pattern of the sample at a first magnification, the projection system including at least a final projector lens; - An electron energy loss spectroscopy detector; - A control unit for controlling the operation of the transmission type charged particle microscope; The transmission type charged particle microscope is arranged to operate in at least two modes that substantially provide the first magnification while maintaining the diffraction pattern substantially in focus, and the at least two modes are: - A first mode having a first setting of the final projector lens; - A second mode having a second setting of the final projector lens.

2. The transmission type charged particle microscope according to claim 1, wherein the first setting includes the final projector lens being substantially activated.

3. The transmission type charged particle microscope according to claim 1 or 2, wherein the first mode includes an ultra-high resolution EELS mode.

4. The transmission type charged particle microscope according to any one of claims 1 to 3, wherein the second setting includes the final projector lens being substantially deactivated.

5. The transmission type charged particle microscope according to any one of claims 1 to 4, wherein the second mode includes a low HT EELS mode.

6. The transmission type charged particle microscope according to any one of claims 1 to 5, wherein the projection system includes an objective lens for forming a diffraction pattern of the sample at a rear focal plane.

7. The transmission type charged particle microscope according to any one of claims 1 to 6, wherein the first magnification corresponds to an effective focal length of the projection system of about 100 mm or less.

8. The transmission type charged particle microscope according to any one of claims 1 to 7, wherein the projection system is arranged to focus the diffraction pattern on a diffraction pattern entrance aperture.

9. The transmission type charged particle microscope according to any one of claims 1 to 8, wherein the projection system includes a first projection lens.

10. A method of operating a transmission electron microscope according to any one of claims 1 to 9, comprising: - providing a sample; - operating the transmission electron microscope in the first mode on the sample; - changing the setting of the projection system from the first setting to the second setting to put the transmission electron microscope in the second mode; - operating the transmission electron microscope in the second mode on the same sample.

11. The method further comprises: - recording a first EELS spectrum of the sample in the first mode; - recording a second EELS spectrum of the sample in the second mode. The method according to claim 10.

12. The method according to claim 10 or 11, further comprising switching the final projector lens from substantially "on" to substantially "off".

Images