TRANSMISSION CHARGED PARTICLE MICROSCOPE WITH AN ELECTRON ENERGY LOSS SPECTROSCOPY DETECTOR

Abstract

The invention relates to a transmission 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 onto the sample, and a control unit for controlling operations of the transmission charged particle microscope. As defined herein, the transmission charged particle microscope is arranged for operating in at least two modes that substantially yield a first magnification whilst keeping said diffraction pattern substantially in focus. Said at least two modes comprise a first mode having first settings of a final projector lens of a projecting system; and a second mode having second settings of said final projector lens.

Claims

1. A transmission 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 onto the sample; a projecting system for forming and imaging a diffraction pattern of the sample at a first magnification, wherein said projecting system comprises at least a final projector lens; an Electron Energy-Loss Spectroscopy detector; and a control unit for controlling operations of the transmission charged particle microscope; wherein the transmission charged particle microscope is arranged for operating in at least two modes that substantially yield said first magnification whilst keeping said diffraction pattern substantially in focus, wherein said at least two modes comprise: a first mode having first settings of said final projector lens; and a second mode having second settings of said final projector lens.

2. Transmission charged particle microscope according to claim 1, wherein said first settings comprise said final projector lens being substantially enabled.

3. Transmission charged particle microscope according to claim 1, wherein said first mode comprises an ultra high resolution EELS mode.

4. Transmission charged particle microscope according to claim 1, wherein said second settings comprise that said final projector lens is substantially disabled.

5. Transmission charged particle microscope according to claim 1, wherein said second mode comprises a low-HT EELS mode.

6. Transmission charged particle microscope according to claim 1, wherein said projecting system comprises an objective lens for forming a diffraction pattern of the sample at a back focal plane.

7. Transmission charged particle microscope according to claim 1, wherein said first magnification corresponds to an effective focal distance of said projecting system of approximately 100 mm or less.

8. Transmission charged particle microscope according to claim 1, wherein said projecting system is arranged for bringing said diffraction pattern in focus at a diffraction pattern entrance aperture.

9. Transmission charged particle microscope according to claim 1, wherein said projecting system comprises a first projecting lens.

10. Method of operating a transmission electron microscope, comprising the steps of: providing a sample; operating a transmission electron microscope comprising a projector system in a first mode on said sample; bringing said transmission electron microscope to a second mode by changing a projector system settings of said projector system from first settings to second settings; and operating said transmission electron microscope in said second mode on said same sample.

11. Method according to claim 10, wherein said method comprises: recording a first EELS spectrum of said sample in said first mode; and recording a second EELS spectrum of said sample in said second mode.

12. Method according to claim 11, wherein said second mode comprises a low-HT EELS mode.

13. Method according to claim 11, wherein said first mode comprises an ultra high resolution EELS mode.

14. Method according to claim 10, wherein said method comprises the step of switching said final projector lens from essentially “on” to essentially “off”.

15. Method according to claim 14, wherein said transmission electron microscope operates in the first mode while said final projector lens is substantially enabled.

16. Method according to claim 14, wherein said transmission electron microscope operates in the second mode while said final projector lens is substantially disabled.

17. Method according to claim 10, wherein said projecting system comprises an objective lens for forming a diffraction pattern of the sample at a back focal plane.

18. Method according to claim 10, wherein said projecting system is arranged for bringing said diffraction pattern in focus at a diffraction pattern entrance aperture.

19. Method according to claim 10, wherein the transmission charged particle microscope is arranged such that, while operating in either the first mode or the second mode, the transmission charged particle microscope substantially yields a first magnification whilst keeping said diffraction pattern substantially in focus.

20. Method according to claim 19, wherein said first magnification corresponds to an effective focal distance of said projecting system of approximately 100 mm or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

[0037] FIG. 1 shows a longitudinal cross-sectional view of a charged-particle microscope;

[0038] FIG. 2 shows an enlarged longitudinal cross-sectional view of projecting system as defined herein;

[0039] FIG. 3 shows an enlarged cross-sectional view of a spectroscopic apparatus including the projecting system;

[0040] FIG. 4 shows an example of an EELS spectrum;

[0041] FIG. 5a shows an image on the EELS detector of an EELS spectrum according to the prior art;

[0042] FIG. 5b shows an image on the EELS detector of an EELS spectrum according to the prior art, and according to a mode of the invention;

[0043] FIG. 6 shows an example of the Zero Loss Peak of an EELS spectrum according to the prior art;

[0044] FIG. 7 shows an example of the Zero Loss Peak of an EELS spectrum obtained with the invention;

[0045] FIGS. 8a and 8b show the projecting system at different settings of the final projecting lens.

[0046] In the Figures, where pertinent, corresponding parts are indicated using corresponding reference symbols. It should be noted that, in general, the Figures are not to scale.

DETAILED DESCRIPTION OF EMBODIMENTS

[0047] FIG. 1 is a highly schematic depiction of an embodiment of a transmission charged particle microscope M, which, in this case, is a TEM/STEM (though, in the context of the present disclosure, it could just as validly be an ion-based or proton microscope, for example). In FIG. 1, within a vacuum enclosure E, an electron source 4 (such as a Schottky emitter, for example) produces a beam (B) of electrons that traverse an electron-optical illuminator 6, serving to direct/focus them onto a chosen part of a sample S (which may, for example, be (locally) thinned/planarized). This illuminator 6 has an electron-optical axis B′, and will generally comprise a variety of electrostatic/magnetic lenses, (scan) deflector(s) D, correctors (such as stigmators), etc.; typically, it can also comprise a condenser system (the whole of item 6 is sometimes referred to as “a condenser system”).

[0048] The sample S is held on a sample holder H. As here illustrated, part of this holder H (inside enclosure E) is mounted in 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 (inter alia) be displaceable in the X, Y and Z directions (see the depicted Cartesian coordinate system), and may be rotated about a longitudinal axis parallel to X. Such movement allows different parts of the sample S to be irradiated/imaged/inspected by the electron beam traveling along axis B′ (and/or allows scanning motion to be performed as an alternative to beam scanning [using deflector(s) D], and/or allows selected parts of the sample S to be machined by a (non-depicted) focused ion beam, for example).

[0049] The (focused) electron beam B traveling along axis B′ will interact with the sample S in such a manner as to cause various types of “stimulated” radiation to emanate from the sample S, including (for example) secondary electrons, backscattered electrons, X-rays and optical radiation (cathodoluminescence). If desired, one or more of these radiation types can be detected with the aid of detector 22, which might be a combined scintillator/photomultiplier or EDX (Energy-Dispersive X-Ray Spectroscopy) module, for instance; in such a case, an image could be constructed using basically the same principle as in a SEM. However, alternatively or supplementally, one can study electrons that traverse (pass through) the sample S, emerge (emanate) from it and continue to propagate (substantially, though generally with some deflection/scattering) along axis B′. Such a transmitted electron flux enters an imaging system (combined objective/projection lens) 24, which will generally comprise a variety of electrostatic/magnetic lenses, deflectors, correctors (such as stigmators), etc.

[0050] In normal (non-scanning) TEM mode, this imaging system 24 can focus the transmitted electron flux onto a fluorescent screen 26, which, if desired, can be retracted/withdrawn (as schematically indicated by arrows 26′) so as to get it out of the way of axis B′. An image (or diffractogram) of (part of) the sample S will be formed by imaging system 24 on screen 26, and this may be viewed through viewing port 28 located in a suitable part of a wall of enclosure E. The retraction mechanism for screen 26 may, for example, be mechanical and/or electrical in nature, and is not depicted here.

[0051] As an alternative to viewing an image on screen 26, one can instead make use of the fact that the depth of focus of the electron flux emerging from imaging system 24 is generally quite large (e.g. of the order of 1 meter). Consequently, various types of sensing device/analysis apparatus can be used downstream of screen 26, such as:

[0052] TEM camera 30. At camera 30, the electron flux can form a static image (or diffractogram) that can be processed by controller C and displayed on a display device (not depicted), such as a flat panel display, for example. When not required, camera 30 can be retracted/withdrawn (as schematically indicated by arrows 30′) so as to get it out of the way of axis B′.

[0053] STEM recorder 32. An output from recorder 32 can be recorded as a function of (X,Y) scanning position of the beam B on the sample S, and an image can be constructed that is a “map” of output from recorder 32 as a function of X,Y. Recorder 32 can comprise a single pixel with a diameter of e.g. 20 mm, as opposed to the matrix of pixels characteristically present in camera 30. Moreover, recorder 32 will generally have a much higher acquisition rate (e.g. 10.sup.6 points per second) than camera 30 (e.g. 10.sup.2 images per second). Once again, when not required, recorder 32 can be retracted/withdrawn (as schematically indicated by arrows 32′) so as to get it out of the way of axis B′ (although such retraction would not be a necessity in the case of a donut-shaped annular dark field recorder 32, for example; in such a recorder, a central hole would allow beam passage when the recorder was not in use).

[0054] As an alternative to imaging using camera 30 or recorder 32, one can also invoke spectroscopic apparatus 34, which could be an EELS module, for example.

[0055] It should be noted that the order/location of items 30, 32 and 34 is not strict, and many possible variations are conceivable. For example, spectroscopic apparatus 34 can also be integrated into the imaging system 24.

[0056] Note that the controller (which may be a combined controller and processor) C is connected to various illustrated components via control lines (buses) C′. Controller can be connected to a computer screen 51, which may be provided with a user interface (UI). This controller C can provide a variety of functions, such as synchronizing actions, providing setpoints, processing signals, performing calculations, and displaying messages/information on a display device (not depicted). It will be understood that the (schematically depicted) controller C may be (partially) inside or outside the enclosure E, and may have a unitary or composite structure, as desired. The skilled artisan will understand that the interior of the enclosure E does not have to be kept at a strict vacuum; for example, in a so-called “Environmental TEM/STEM”, a background atmosphere of a given gas is deliberately introduced/maintained within the enclosure E. The skilled artisan will also understand that, in practice, it may be advantageous to confine the volume of enclosure E so that, where possible, it essentially hugs the axis B′, taking the form of a small tube (e.g. of the order of 1 cm in diameter) through which the employed electron beam passes, but widening out to accommodate structures such as the source 4, sample holder H, screen 26, camera 30, recorder 32, spectroscopic apparatus 34, etc.

[0057] FIG. 2 shows a more detailed embodiment of the imaging system 24 as defined herein. The imaging system 24 is provided in between the sample S and an entrance aperture 3a of a dispersing device 3 (see also FIG. 3). The imaging system 24 shown comprises an objective lens O and a projecting system 25. The projecting system 25 comprises a number of different lenses, and in the embodiment shown a total of four lenses. These lenses are, subsequently, 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. In between the objective lens and the projecting system 25, an optional spherical and/or chromatic aberration corrector (not shown) may be placed, as known to those skilled in the art.

[0058] Turning now to FIG. 3, this shows an enlarged and more detailed view of an embodiment of the spectroscopic apparatus 34 in FIG. 1. In FIG. 3. a flux 1 of electrons (which has passed through sample S and imaging system 24) is shown propagating along electron-optical axis B′. This flux 1 enters a dispersing device 3 (“electron prism”), where it is dispersed (fanned out) into an energy-resolved (energy-differentiated) array 5 of spectral sub-beams, which are distributed along a dispersion direction; for illustration purposes, three of these sub-beams are labelled 5a, 5b and 5c in FIG. 3.

[0059] Downstream of the dispersing device 3, the array 5 of sub-beams encounters post-dispersion electron optics 9, where it is magnified/focused, for example, and ultimately directed/projected onto detector 11. The post-dispersion optics may comprise round lenses and/or quadrupole lenses. The detector 11 may comprise an assembly of sub-detectors arranged along the dispersion direction, with different sub-detectors being adjustable so as to have different detection sensitivities. It is noted that other detector configurations for measuring EELS spectra are known to those skilled in the art and are applicable in the method and device as disclosed herein as well. The method is in principle not limited to the use of a specific detector.

[0060] FIG. 4 shows an example of an EELS spectrum. The Figure renders intensity I (in arbitrary units, a.u.) as a function of energy-loss E (in eV) for electrons that have traversed a sample containing Carbon and Titanium. From left to right, the main features of the spectrum are:

[0061] A Zero-Loss Peak ZLP, representing electrons that traverse the sample without undergoing inelastic scattering therein;

[0062] A Plasmon Resonance Peak component/section PRP, a relatively broad series of peaks/shoulders associated with single or multiple scattering of electrons on plasmons in the specimen. This typically extends from about 0 to 50 eV, although there is no strict definition of its upper limit. It is characterized by peaks/shoulders resulting from excitations of collective vibrations of the valence electrons in the sample, such as peaks 31. Note that the PRP component usually has a significantly lower intensity than the ZLP.

[0063] A Core Loss Peak component/section CLP. This typically starts at about 50 eV (after the PRP component), although there is no strict definition of its lower limit. It is typically of such a low intensity relative to the ZLP/PRP components that, as rendered in FIG. 4, it is enlarged by a multiplication factor (e.g. 100) to improve visibility of its details. As can be seen, it contains (clusters of) peaks/shoulders that can be associated with certain chemical elements (such as C and Ti, in the current example), seated on top of a substantial background contribution 33. The EELS spectrum shown in FIG. 4 can be measured in ways known to those skilled in the art, using the device and set up as discussed with reference to FIGS. 1 to 3.

[0064] As discussed in the introduction, EELS is traditionally done on electrons with specimen-exit-angles and energy losses which are not difficult for the TEM to transfer properly from the back focal plane of the objective lens to the EELS spectrometer. However, low-HT (probe corrected) EELS and ultra-high resolution (UHR) EELS require large collection angles, which in turn requires low magnification. Such low magnification can give artefacts in the EELS spectrum, especially when it is combined with large E/E0 and/or improved energy resolution.

[0065] Low-HT (Probe Corrected EELS)

[0066] FIG. 2 and FIG. 5a-5b discuss the aberrations that occur in low-HT (probe corrected) EELS, and possible settings for a transmission charged particle microscope to overcome these aberrations.

[0067] FIG. 5a shows an image of an EELS spectrum of a Si sample taken in so called low-HT (probe corrected) EELS mode. Here, a larger energy loss (E) compared to the beam energy (HT) is encountered, such as E/E0=1-4%. The image of the EELS spectrum shown is a sliced image of three sub-images each with different exposure time because of the large energy range and the large intensity range. The EELS spectrum exhibits a strange shrinking of height in the non-dispersive direction in the region of 2400 eV-2900 eV, and a strange bump of increased intensity in the range of 2200 eV-2600 eV. It was found that the large collection angles require low magnification from scattering plane (that is, the back focal plane of the objective lens) to the EELS spectrometer, as the entrance aperture of the spectrometer is limited in size. Such low magnification can give artefacts in the EELS spectrum, especially when it is combined with large E/E0 as shown in FIG. 5a. The artefacts make that the EELS signal can no longer be quantified reliably.

[0068] Now turning back to FIG. 2, a potential cause for these artifacts will be indicated. In EELS, the diffraction pattern is imaged at the EELS spectrometer entrance aperture 3a. The magnification from objective back focal plane to entrance aperture 3a can be tuned such that the entrance aperture collects a specific radius of cone of electrons exiting the specimen. A typical choice is that this cone equals the cone of electrons that form the probe (then the aperture precisely captures the so-called ‘bright-field disc’). The magnification from diffraction pattern to entrance aperture 3a is called the camera length (CL) and can be interpreted as the effective focal distance of the imaging system at the entrance aperture.

[0069] An image of the probe at the specimen is formed at a cross-over XO following the final projection lens P2. This cross-over is located about d.sub.XO=3.5 mm below this last imaging lens P2. Due to the chromatic aberration of the imaging system, electrons with energy loss E will be focused somewhat above this cross-over XO (see dashed lines in FIG. 2, having modified cross-over XO′). The defocus distance dz is given by dz=C.sub.e.sup.(XO).Math.(E/E0), where C.sub.c.sup.(XO) is the chromatic aberration at the XO plane. In first order, this C.sub.c.sup.(XO) is related to the chromatic aberration at the specimen C.sub.c.sup.(spec) as C.sub.c.sup.(XO)=M.sup.2.Math.C.sub.c.sup.(spec) where M is the magnification from specimen to XO.

[0070] This magnification M can be calculated as follows. The angle at the specimen is α.sub.obj. From FIG. 2 it follows that, using a small angles approximation, the angle at the cross-over XO is equal to α.sub.XO=CLα.sub.obj/h. Hence, the angular magnification from the specimen S to the cross-over XO is M.sub.α=α.sub.XO/α.sub.obj=CL/h. The spatial magnification from the specimen S to the cross-over XO is M=1/M.sub.α=h/CL. Since C.sub.c.sup.(spec) is dominated by the C.sub.c.sup.(obj) of the objective lens, we approximate C.sub.c.sup.(XO)=M.sup.2.Math.C.sub.c.sup.(obj). Combining these gives dz=C.sub.c.sup.(obj).Math.(h/CL).sup.2.Math.(E/E0).

[0071] The first order approximation breaks down when dz gets that large that the XO shifts in the P2 lens. Full simulations show that as the energy loss E increases, the XO shifts upwards through the P2 lens, through the front focal plane (F.F.P.) of the P2 lens, and further upwards. When the energy loss is such that the XO is at the F.F.P. of the P2 lens, the electrons are exiting the P2 lens in a parallel way. Thus, at this energy loss, not only electrons with the usual scattering angles enter the spectrometer, but all electrons enter the spectrometer, irrespective of their scattering angle α.sub.obj. This additional signal pops up in the EELS spectrum as the bump in FIG. 5. At even higher energy losses, the electrons are again deflected away from the parallel beam and the bump disappears from the EELS spectrum. We can estimate that the EELS bump occurs when the chromatic defocus becomes equal to the distance between XO and P2 lens, dz=d.sub.XO (see FIG. 2). Thus the EELS bump starts at

E=E.sub.0.Math.d.sub.XO/C.sub.c.sup.(obj).Math.(CL/h).sup.2  (Equation 1)

[0072] With E.sub.0=120 keV, d.sub.XO=3.5 mm, C.sub.c.sup.(obj)=2.0 mm (is chromatic aberration from objective lens plus image corrector, neglect contribution from other imaging lenses), CL=75 mm, h=690 mm, this estimate results in:

ΔE=120 kV.Math.3.5/2.0.Math.(75/690).sup.2=2.5 keV  (Equation 2)

[0073] This is in good agreement with the experimental results shown in FIG. 5a.

[0074] Looking at Equation 1, there seem several options to push the EELS bump to higher energies, away from the region of energies of interest:

[0075] Increase E.sub.0, that is, going to higher high tension. This is not desirable for beam-sensitive specimens.

[0076] Reduce C.sub.c.sup.(obj), for example, by adding a Cc corrector. This is very expensive.

[0077] Increase camera length CL. This is undesirable because it reduces the signal collected by the entrance aperture of the EELS spectrometer.

[0078] Decrease the distance h between XO and spectrometer. This is undesirable because it increases the magnification from XO to spectrum plane, and, because of the finite size of the image at the XO, this deteriorates the resolution of the EELS spectrometer.

[0079] Increase the distance d.sub.XO between the last lens and the XO. This is the approach according to the present invention.

[0080] The present invention teaches, in this example, to increase the distance d.sub.XO by reducing the excitation of the P2 lens, and in an embodiment by nearly switching the P2 lens off. This effectively makes that P1 becomes the last lens. FIG. 5 shows experimental proof that this is indeed a very effective solution, yielding an extremely large artefact-free EELS range of E/E0=3 keV/60 keV=5%. FIG. 5b shows images of EELS spectrum of Si sample at 60 kV, CL=65 mm, 5 mm entrance aperture (thus α≈35 mrad). These are spliced images of three and five sub-images each with different exposure time because of the large energy range and the large intensity range. The top of FIG. 5b shows a traditional set-up with P2-on, artefact starts around 1200 eV. The bottom of FIG. 5B shows a set-up according to the invention: P2-off pushes the artefact beyond 3000 eV.

[0081] Ultimate Energy Resolution EELS

[0082] FIGS. 6-8 discuss the aberrations that occur in ultimate energy resolution EELS, and possible settings for a transmission charged particle microscope to overcome these aberrations.

[0083] The ultimate energy resolution that can be obtained in a TEM in general is about ΔE=15 meV at E.sub.0=60 keV. Such resolution is only obtainable when the smallest entrance aperture of the spectrometer is used because of spectrometer aberrations. This smallest aperture is 1 mm. Therefore, the magnification from scattering plane to spectrometer entrance must be very small when a large range of scattering angles (α>20 mrad) has to be collected by the EELS spectrometer. Such very small magnification can cause aberrations that affect the ultimate energy resolution.

[0084] FIG. 6 schematically shows the zero loss peak 62 at CL=13 mm at high resolution as recorded on EELS detector 11, and the corresponding EELS spectrum obtained by integrating this image 61 vertically. Clearly, the energy resolution is blurred (see lines 62-63) by some aberration. It was found that the blur decreases when the camera length CL is increased (without changing anything else in the set-up). This indicates that the blur originates somewhere in the projection system of the TEM.

[0085] Part of the resolution loss at very low camera length is caused by the spherical aberration C.sub.s.sup.(obj) of the imaging objective lens: This blurs the probe by d.sub.spec=¼C.sub.s.sup.(obj) α.sub.obj.sup.3. As indicated above with respect to FIGS. 2 and 5a, the probe is imaged to the XO with a magnification M=h/CL, hence the blur at the XO is d.sub.XO=¼C.sub.s.sup.(obj).Math.α.sub.obj.sup.3.Math.(h/CL). This can be translated to an energy blur using the apparent dispersion δ of the spectrometer at the XO. The corresponding resolution loss is

ΔE=¼C.sub.s.sup.(obj).Math.α.sub.obj.sup.3.Math.(h/CL)/δ  (Equation 3)

[0086] FIG. 6 has been recorded with C.sub.s.sup.(obj)=1.3 mm, α.sub.obj=17 mrad, h=690 mm, CL=13 mm, δ=11 μm/eV at 60 kV, and this gives ΔE=8 meV. Clearly, the spherical aberration of the objective lens is significant but cannot explain the broadening observed in FIG. 6.

[0087] FIGS. 8a and 8b show two settings for the TEM imaging system that both transfer the diffraction pattern to the EELS detector with very low magnification, CL=13 mm. The first alternative has P2 almost off, the second alternative has P2 fully on. The second alternative has the main drawback that, in order to obtain a total low magnification, one or more of the intermediate lenses in the column must be de-magnifying in order to compensate for the large magnification of P2. This yields image-forming rays (not sketched in FIG. 8b) which are far off-axis in these intermediate lenses and these far off-axis rays yield correspondingly large aberrations at the image plane (in this case the EELS detector). Therefore, this second alternative is normally not used.

[0088] The first alternative which has P2 almost off is preferred because it does not suffer from image aberrations coming from the intermediate lenses. Furthermore, it has the benefit that it happens to perform very good at high E/E0, as discussed above with respect to FIG. 5 and FIG. 2.

[0089] In the setting with P2 almost off (FIG. 8a), the XO is essentially created and focused by the P1 lens. The quality of this focus is, apart from the above discussed contribution of C.sub.s.sup.(obj), mainly determined by the spherical aberration of the P1 lens. A rough rule of thumb is that the spherical aberration of a lens is C.sub.s.sup.(P1_XO)˜d.sub.im.sup.4/S.sup.3. In this set-up, the P1 lens has image distance d.sub.im=80 mm and lens gap S=17 mm and this gives for C.sub.s.sup.(P1_XO)=8.Math.10.sup.3 mm; a full calculation gives C.sub.s.sup.(P1_XO)=14.5.Math.10.sup.3 mm at the XO position. We can calculate this back to the specimen as a spherical aberration at the specimen: C.sub.s.sup.(proj_spec)=C.sub.s.sup.(proj_XO)/M.sup.4=0.002 mm using M=h/CL=53× as magnification from specimen to XO.

[0090] Clearly, the contribution of the projector system 25 C.sub.s.sup.(proj_spec)=0.002 mm is much smaller than the contribution of the objective lens O C.sub.s.sup.(obj)=1.3 mm so it seems that it can be neglected.

[0091] However, it is important to realize that, in practice, due to mechanical shifts and tilts of the lenses, the beam can be off-axis by 1 . . . 3 mm at the entrance of the spectrometer 3a. This is normally corrected by applying a so-called ‘diffraction shift’ using the deflectors located between the objective lens O and the D-lens. This causes significant off-axis travel of the beam in the I and P1 lenses, equivalent to an angle at the objective of α=(1 . . . 3 mm)/CL=80 . . . 230 mrad. The precise effect of such off-axis travel is difficult to calculate but the skilled artisan will understand that this causes noticeable energy blur. This can be checked by repeating the EELS measurement using a setting in which the XO-forming rays are close to the axis.

[0092] The lower setting in FIG. 8b has P2 lens on, thus bringing the XO-forming rays (light in this figure) close to the axis. As a consequence, the spherical aberration of the projector system is relatively low; a full calculation gives C.sub.s.sup.(P1_XO)=85 mm, which is 170× less than in the setting in FIG. 8a. FIG. 7 shows the EELS spectrum obtained in this mode (i.e. with P2 on). The blur is absent in the image 71 of FIG. 7, see lines 72-73.

[0093] From the above it follows that a transmission charged particle microscope is provided, wherein said transmission charged particle microscope is arranged for operating in at least two modes that substantially yield said first magnification whilst keeping said diffraction pattern substantially in focus, wherein said at least two modes comprise: a first mode having first settings of said final projector lens; and a second mode having second settings of said final projector lens. The first mode and the second mode are different EELS modes, in an embodiment. The first setting may comprise that the final projector lens P2 is substantially enabled, and corresponds for example to the situation as described above with respect to Ultimate Energy Resolution EELS. The second setting may comprise that the final projector lens P2 is substantially disabled, and corresponds for example to the situation as described above with respect to low-HT EELS. In the first setting, with the final projector lens P2 set to substantially enabled, a traditional EELS spectrum may be obtained as well.

[0094] It is noted that other settings of the imaging system 24 and/or the projecting system 25 may be changed in between the first mode and the second mode, as long as the magnification is substantially the same. The magnification used corresponds, in an embodiment, to an effective focal distance of said projecting system of approximately 100 mm or less.

[0095] The desired protection is determined by the appended claims.