Electron microscope with improved imaging resolution

Abstract

An electron microscope comprising: a sample holder for holding a sample; a source for generating an electron beam; an illumination system for directing the beam to illuminate the sample; an elongated beam guide through which the beam is directed; a detector for detecting radiation emitted from the sample in response to the illumination, wherein at least a longitudinal portion of the beam guide has a composite structure comprising: an outer tube composed of an electrically insulating material; an inner surface layer composed of an electrically conductive material. In an alternative but related arrangement, at least a longitudinal portion of the beam guide comprises an aggregate composite material comprising mixed electrically insulating and electrically conductive materials.

Description

Technical Field

[0001] The present invention relates to an electron microscope, comprising:

[0002] - a sample holder, the sample holder being used to hold a sample;

[0003] - a source for generating an electron beam;

[0004] - an illumination system for directing the beam to illuminate the sample;

[0005] - an elongated beam conduit through which the beam is guided;

[0006] - a multipole lens assembly configured as an aberration corrector;

[0007] - a detector for detecting radiation emitted from the sample in response to the illumination.

[0008] The invention also relates to a method of using such an electron microscope. Background Art

[0009] Electron microscopy is a well-known and increasingly important technique for imaging microscopic objects. Historically, the basic genus of electron microscopes has evolved into many well-known device species, such as the transmission electron microscope (TEM), the scanning electron microscope (SEM) and the scanning transmission electron microscope (STEM) as well as various subspecies, such as the so-called "dual beam" devices (e.g. FIB-SEM), which additionally employ a "machining" focused ion beam (FIB), allowing support activities such as, for example, ion beam milling or ion beam induced deposition (IBID). More specifically:

[0010] - In a SEM, the sample is irradiated by a scanning electron beam, accelerating the emission of "secondary" radiation from the sample, for example in the form of secondary electrons, backscattered electrons, X-rays and cathodoluminescence (infrared, visible and / or ultraviolet photons); one or more components of this emitted radiation are then detected and used for image accumulation purposes.

[0011] - In a TEM, the electron beam used to irradiate the sample is chosen to have a sufficiently high energy to penetrate the sample (for this purpose, the sample will generally be thinner than in the case of SEM samples); an image can then be created using the transmitted electrons emitted from the sample. When such a TEM is operated in scanning mode (hence a STEM), the image is accumulated during the scanning movement of the irradiating electron beam.

[0012] -SEM can also be used in "transmission mode", for example when using relatively thin samples and relatively high incident beam energy. Such tools are often called "TSEM" (Transmission SEM), and they usually have a relatively basic imaging system (e.g., a single lens and deflector) placed between the sample and the post-sample detector.

[0013] It should be noted that besides imaging and performing (local) surface modifications (eg milling, etching, deposition etc.) electron microscopes may also have other functionalities like performing spectroscopic analysis, examining diffraction patterns etc.

[0014] In all cases, the electron microscope (EM) will include at least the following components:

[0015] - An electron source, such as for example a W or LaB6 source, a Schottky gun or a cold field emission gun (CFEG).

[0016] - An illumination system / illuminator, which is used to manipulate the "raw" radiation beam from the source and to perform certain operations on it, such as focusing, aberration mitigation, cropping (using an aperture), filtering, etc. It usually comprises one or more (charged particle) lenses, and may also comprise other types of (particle) optical components. If necessary, the illuminator may be provided with a deflector system which can be called upon to cause its exit beam to perform a scanning motion across the sample being investigated.

[0017] - A sample holder - usually connected to a positioning system - on which the sample under investigation can be held and positioned (e.g. shifted, tilted, rotated). If necessary, this holder can be moved in order to achieve a scanning movement of the sample relative to the beam. When designed for holding cryogenic samples, the sample holder may include means for maintaining said sample at cryogenic temperature, for example using a suitably connected cryogen barrel.

[0018] - Detectors (for detecting radiation emanating from the irradiated sample), which may be unitary or composite / distributed in nature, and which may take many different forms, depending on the radiation being detected. Examples include photodiodes, CMOS detectors, CCD detectors, photovoltaic cells, X-ray detectors (such as silicon drift detectors and Si(Li) detectors), etc. In general, an EM may include several different types of detectors, the choice of which may be called for in different circumstances.

[0019] In the case of transmission type microscopy (such as (S)TEM or TSEM), EM will additionally include:

[0020] - an imaging system, which essentially absorbs the electrons transmitted through the sample (plane) and directs (focuses) them onto an analytical device, such as a detector, an imaging device, a spectroscopic device (e.g. an EELS device: EELS = Electron Energy Loss Spectroscopy), etc. As with the illuminator mentioned above, the imaging system may also perform other functions, such as aberration mitigation, cropping, filtering, etc., and it usually comprises one or more charged particle lenses and / or other types of particle optical components.

[0021] Over a significant portion of its trajectory through the microscope, the beam will propagate through an elongated (cylindrical) beam guide, which creates a relatively small volume vacuum enclosure close to the optical axis (of the illuminator and, if present, the imaging system).

[0022] While various technical fields are constantly looking to further improve the imaging resolution that can be obtained with EM, this task is far from trivial. State-of-the-art EMs have employed aberration correctors (typically comprising groups of multipole (e.g., quadrupole, hexapole, and / or octopole) lens elements) to mitigate imaging aberrations such as astigmatism, chromatic aberration, and spherical aberration, which can significantly exacerbate image degradation. However, despite such elaborate measures, EM imaging quality is still often adversely affected by a variety of physical effects, including, for example, higher-order aberrations, Coulomb interactions, vacuum level fluctuations, and stray fields. One such obstacle that has recently been discovered is due to electron beam perturbations caused by "parasitic" thermomagnetic field noise along the beam path, as reported by S. Uhlemann et al. in the 26 July 2013 issue of Physical Review Letters [PRL]. 111》 , pp. 046101-046105 (American Physical Society), in which the authors demonstrate the thermodynamic nature of the magnetic field noise observed in the TEM, and take steps to cool the "conductive parts of the instrument" to cryogenic temperatures (e.g., about 77 K) in an attempt to reduce this phenomenon. Since the magnitude of the phenomenon varies with temperature, it is implied that cooling to liquid helium temperatures is required to achieve the ultimate minimization of these noise effects. Summary of the invention

[0023] The object of the present invention is to solve these problems in an alternative way. More specifically, it is an object of the present invention to provide an EM design in which such magnetic field noise effects can be solved without having to rely on the complex cooling measures mentioned above.

[0024] In a first approach, these and other objects are achieved in an electron microscope as described in the opening paragraph above, characterized in that at least a longitudinal portion of the beam guide extends at least through the aberration corrector and has a composite structure comprising:

[0025] - an outer tube consisting of an electrically insulating material;

[0026] - an inner surface layer made of a conductive material, the inner surface layer made of a conductive material having a conductivity σ and a thickness t, wherein σt<0.1Ω -1 .

[0027] The present inventors have considered that the solution required to cryogenically cool the "conductive parts" of the EM (such as the core / yoke in the magnetic lens, the beam guide etc.) is very cumbersome. The particle beam column of the EM is already a compact space in which relatively large, ultra-high precision sub-components are positioned close to each other within tight tolerances, with little spare space. In such a setup, it would be a cumbersome task to leave additional space for the relatively bulky cooling elements / coils / supply lines required to cool large parts of the microscope to cryogenic temperatures. Therefore, the present invention takes a completely different approach. The above-mentioned parasitic magnetic fields are achieved in large part due to Johnson-Nyquist currents in the beam guide walls, which are traditionally composed of metal (such as stainless steel or titanium) tubes - the present inventors decided to replace the traditional beam guide design with an electrically insulating tube, for example, including durable ceramics such as zirconium oxide (ZrO2) or aluminum oxide (Al2O3). In order to prevent spatial charging of such a tube, its inner surface is covered by a (grounded) surface layer composed of a conductive material, such as a metal film - it should be relatively thin / resistive so that it will not itself become a significant harbor / source of parasitic currents. Such a structure would seem to be disqualified by the aforementioned PRL journal article, which argues that by reducing conductivity and metal wall thickness, the noise spectrum may shift slightly from lower frequencies to higher frequencies, but the total noise power remains essentially unchanged, resulting in a negligible / marginal net effect on image spread. However, the inventors have shown that this argument is flawed: if the aforementioned spectral shift is large enough, then the peak spectral power can be shifted to a frequency domain where the Johnson-Nyquist field no longer significantly affects the electron beam. More specifically, frequencies above the microwave cutoff frequency of the beam guide of the present invention do not substantially cause image spread / blurring.

[0028] Based on the above discussion, those skilled in the art will understand that the inner surface layer of the composite beam conduit of the present invention should (simultaneously):

[0029] - Thick enough / conductive enough to effectively remove space charge (induced surface charge, e.g. caused by (spurious) electron impacts);

[0030] - Thin enough / resistive enough to avoid being a significant harbor for parasitic currents themselves.

[0031] In this regard, the product of the surface conductivity σ and the surface thickness t, σt, can serve as an indicator, and there is a general preference for relatively small values ​​of σt. For example, and to provide some guidance, in a (S)TEM operating at 300 kV, the inventors have found values ​​of σt < 0.1 Ω -1 Produces satisfactory results, σt<0.01Ω -1 The obtained results are better and σt<0.001Ω -1 Further improvements. A given value of σt can be achieved by selecting various different (but complementary) values ​​of σ and t, respectively; however, in practice, the skilled person will understand that some practical constraints need to be considered. For example:

[0032] - Relatively low t-values ​​(eg less than a few nm) may introduce manufacturing complexities, eg regarding the choice of deposition technique, discontinuities (island formation), etc.

[0033] - Relatively high values ​​of t (eg, on the order of greater than about 100 μm) may severely narrow the list of candidate materials that can achieve appropriate σ values.

[0034] To provide some guidance, the inventors have obtained good results using t in the range of about 5-20 μm, corresponding to values ​​of σ that can be achieved using a variety of relatively common conductive materials. For the sake of good order, it should be noted that:

[0035] -σ=1 / ρ, where ρ is the resistivity;

[0036] -σt=1 / R s , where R s is the sheet resistance of the surface layer, where R s =ρ / t.

[0037] With regard to suitable materials for use in the composite beam conduit of the present invention, the following non-limiting examples are given for guidance purposes:

[0038] - The outer tube may, for example, comprise at least one material selected from the group comprising: ceramics, vitreous materials, quartz and combinations thereof. The term "ceramic" encompasses engineering ceramics, such as the above-mentioned zirconium oxide (ZrO2) and aluminum oxide (Al2O3). Such materials are generally durable, temperature resistant (and thus able to withstand vacuum baking), have no problems with outgassing behavior, and are relatively easy to make into tubular form (e.g. using casting). For the sake of good order, it should be noted that in conventional EMs, beam guide tubes typically have an inner diameter of about 6-8 mm, but this value range is not limiting.

[0039] - The inner surface layer comprises at least one material selected from the group comprising: ruthenium oxide, titanium nitrate and combinations thereof. These materials have convenient σ values ​​and can be deposited relatively easily on the inner surface of the outer tube using methods such as, for example, PE-CVD (plasma enhanced chemical vapor deposition), PE-ALD (plasma enhanced atomic layer deposition) and PVD (physical layer deposition). This group is not restrictive: in principle, metals such as W, Ti or Pt (or combinations thereof) can also be used in the inner surface layer, depending in particular on the chosen surface layer thickness t.

[0040] For the sake of good order, it should be noted that, as mentioned in the context of the present invention, the term "electrically insulating material" may also include materials that may be conventionally considered to be semiconductors. For example, SiC is a ceramic material that is conventionally labeled as a semiconductor; however, its resistivity is approximately 10 6 Ωcm - This makes it about 10 times less conductive than aluminum 16 times, the resistivity of aluminum is about 10 -10 Ωcm. In comparison, the resistivity of quartz, alumina, and zirconia is approximately 10 16 Ωcm, 10 14 Ωcm and 10 9 Ωcm. The skilled person will appreciate that an electrical insulator is a material that is (substantially) free of freely transporting (conducting) electrons, typically due to the presence of a relatively large band gap in such a material.

[0041] In an alternative solution, the electron microscope as defined herein is characterized in that at least a longitudinal portion of the beam guide extends at least through the aberration corrector and comprises a focusing composite material comprising:

[0042] - Mixing of electrically insulating and conductive materials;

[0043] wherein the beam guide has a conductivity σ and a wall thickness t w , where σt w <0.1Ω -1 .

[0044] In this second related approach, the beam conduit comprises an aggregate composite material comprising mixed electrically insulating and electrically conductive materials. This approach builds upon similar insights as above, but utilizes a beam conduit structure in which the above-described "laminated structure" of separate electrically insulating outer tubes and electrically insulating inner skin layers is effectively "deformed" into a single "monolithic" polymeric composite structure. This deformed structure has an electrical conductivity intermediate between that of conventional conductive materials and insulating materials, and may be functionally referred to as a "high volume resistivity" material. At the same time:

[0045] - The insulation is sufficient to mitigate the blurring effects of parasitic Johnson-Nyquist currents in the conduit walls; however:

[0046] - Conductive enough to remove space charge which tends to accumulate on its inner surface.

[0047] If you consider:

[0048] - The outer tube in the above laminated structure has a wall thickness t t ,as well as;

[0049] - The beam conduit in the current polymer structure has a wall thickness t w ,

[0050] One way to appreciate the properties of this deformed structure, then, is to consider it as an “evolution” of the laminated structure described above, where:

[0051] -t t Decrease to 0;

[0052] -t increases to t w , making the previous "surface" a self-supporting wall;

[0053] -σ decreases simultaneously / correspondingly.

[0054] In this analogy, for a given value of the product σt (see discussion above and below), it can be seen that:

[0055] - If t is about 10 2 The factor of increase (to t w ) (e.g., from about 10 microns to about 1 mm), then;

[0056] -σ will need to be reduced accordingly by about 10 2 It reduces σ to a level that can be considered intermediate between a conventional conductor and an insulator.

[0057] With regard to the formation of the polymeric composite structure described in the preceding paragraph, suitable examples of constituent materials include:

[0058] - Electrically insulating material: ceramic, such as, for example, SiC and / or ZrO.

[0059] - Conductive materials: graphite and / or titanium nitride, for example.

[0060] One way to achieve such a composite material is to mix a conductive material (e.g., in the form of particles or fibers) in a matrix of an insulating material (e.g., in the form of a green ceramic material); alternatively, one can start with a conductive material and "temper" its conductivity by mixing an insulating material therein. For example, the additive can be included in the receiving bulk material using methods such as diffusion or ion implantation, or by physical mixing of particles. One skilled in the art will be able to determine the relative amounts of the different materials to be mixed to obtain an aggregate composite material having a given volume resistivity, and / or he can purchase a prefabricated product. For example, the aggregate composite materials mentioned here are commercially available from companies such as PocoGraphite, Inc. of Decatur, Texas, USA. They are sometimes referred to as "ESD" materials because they are suitable for mitigating electrostatic discharge problems. Other terms sometimes used for such materials include "electroceramic" and "granular metal." For example, such materials are discussed in the journal article "Granular electronic systems" by IS Beloborodov et al., Rev. Mod. Phys. 79. pp. 469 (April, 2007):

[0061] https: / / journals.aps.org / rmp / abstract / 10.1103 / RevModPhys.79.469

[0062] The composite beam tube structure of the invention proposed above (in either approach) does not necessarily have to be used over the entire length of the (primary) beam path in the microscope (although such a full-length deployment is certainly possible). In practice, the inventors have found that:

[0063] - the induced image spread (due to magnetic field noise) at a given location in the beam guide tends to be proportional to the axial beam diameter at that location, and therefore;

[0064] - Some parts of the beam path are more susceptible to magnetic field noise than others, and it is in these parts that the invention is most useful.

[0065] As defined in this article:

[0066] - the microscope comprises a multipole lens assembly configured as an aberration corrector;

[0067] - The composite beam guide according to the invention extends at least through the aberration corrector.

[0068] Examples of aberration correctors mentioned here include spherical aberration (Cs) correctors, chromatic aberration (Cc) correctors, and combined spherical chromatic (Cs-Cc) correctors, which can be used in SEM and / or TEM. For example:

[0069] - In a SEM or STEM, a Cs corrector may be included in the illumination system (before the sample). Such a Cs corrector typically has a length (along the beam path) of about 15 cm.

[0070] - In a TEM, a combined Cs-Cc corrector may be included in the imaging system, which is positioned downstream of the sample and serves to direct the electrons transmitted through the sample onto a detector (for imaging, diffraction pattern and / or spectrum). Such Cs-Cs correctors typically have a length (along the beam path) of about 45 cm, but there are also Cs-Cc correctors with lengths of about twice this, e.g.

[0071] For example, more information on Cs and Cc correctors can be gleaned from the following references:

[0072] -U.S. Patent US 5,084,622;

[0073] - H. Rose and W. Wan, "Aberration correction in electron microscopy", IEEE Proceedings of 2005 Particle Accelerator Conference, Knoxville, Tennessee, USA, pp. 44-48 (2005).

[0074] When an aberration corrector as described above is employed, another relatively sensitive part of the beam path where the invention can be exploited is the trajectory extending between the sample plane (sample holder) and the aberration corrector. This means that one or both of:

[0075] - Trajectory from the corrector to the sample plane (upstream of the sample plane), e.g. in STEM;

[0076] - Trajectory from the sample plane to the corrector (downstream of the sample plane), e.g. in a TEM.

[0077] Furthermore, it may be advantageous to apply the invention to parts of the beam path which are positioned in / near the electrostatic deflection module. A person skilled in the art will be able to decide which parts of the beam path are (most) worthy of applying the composite beam guide of the invention, and may decide to deploy it along (substantially) the entire main beam path. The advantage of the latter approach is, among other things, that it avoids having to create a joint between two different types of beam guides - bearing in mind that the entire beam guide must reliably maintain a high vacuum (at least) during operation of the microscope.

[0078] Using the present invention, excellent STEM image resolution values ​​can be obtained, for example, 30 pm at a beam voltage of 300 kV, and 60 pm at a beam voltage of 60 kV, in both cases with a beam half-angle of 50 mrad, without having to use the cumbersome cryogenic cooling described in the above-mentioned PRL journal article. For conventional stainless steel beam guides, the present invention typically allows a reduction in image spread caused by Johnson-Nyquist noise by a factor of about 10-15.

[0079] Complementing the measures of the invention described above, additional measures can be taken to further reduce the detrimental imaging effects of Johnson-Nyquist noise without resorting to the cryogenic cooling described in the aforementioned PRL journal article. One such measure is to widen the aperture in one or more magnetic (e.g. iron) yokes used to conduct the field lines into the direct vicinity of the beam. Nominally, such an aperture hugs / contacts the outer surface of the beam guide so as to be as close to the beam axis as possible. However, the inventors have noticed that for a cylindrical aperture of (inner) diameter r, the yoke leads to Johnson Nyquist blurring according to a 1 / r dependence; therefore, enlarging the aperture will reduce the effects of this blurring. Increasing the aperture size in this way will result in an empty gap between the inner surface of the aperture and the outer surface of the beam guide, but this gap will not necessarily have a significant (overwhelming) negative aspect.

[0080] For the sake of completeness, reference is made to the following prior art documents.

[0081] US 3,787,696 A and DE 30 10 376 A1 disclose liner tubes for use in scanning and / or focusing coils. US 3,634,684 A also uses liner tubes for scanning coils. Here, the liner tubes are used to counteract eddy currents from the high-frequency magnetic flux of the scanning. These documents do not suggest using these liner tubes to counteract Johnson noise in multipole lens assemblies as aberration correctors.

[0082] JP H03 22339 A discloses an aberration corrector having a conductive inner surface layer and an electrically insulating outer tube. Here, the conductive inner surface layer is required to apply the required voltage to the backing tube and to keep the sample grounded. BRIEF DESCRIPTION OF THE DRAWINGS

[0083] The invention will now be explained in more detail based on exemplary embodiments and the accompanying schematic drawings (not to scale), in which:

[0084] Figure 1 A longitudinal cross-sectional elevation view of an embodiment of an EM (in this case, a (S)TEM) embodying the invention is given.

[0085] Figure 2A Shows Figure 1An enlarged transverse cross-sectional view of a portion of FIG.

[0086] Figure 2B Shows Figure 2A A modified version of the embodiment in .

[0087] Figure 3 is the σt(surface conductivity×surface thickness)log of the embodiment of the present invention at different beam voltages 10 A plot of the relative image spread (due to magnetic field noise and compared with a conventional beam guide) as a function of Figure 2A shown.

[0088] Figure 4 Shows Figure 2A / An enlarged transverse cross-sectional view of an alternative (but related) embodiment shown in FIG. 2B.

[0089] In the various figures, corresponding parts are indicated by corresponding reference characters where relevant. DETAILED DESCRIPTION

[0090] Example 1

[0091] Figure 1 (not to scale) is a highly schematic diagram of an EM M embodiment embodying the present invention; more specifically, it shows an embodiment of a TEM / STEM (although, in the context of the present invention, it could equally effectively be, for example, a SEM). In the figure, within a general cabinet / cover 2, there is a vacuum enclosure V, which can be evacuated by a schematically depicted vacuum pump assembly V'. Within this vacuum enclosure V, an electron source 4 generates an electron beam B, which propagates along an electron optical axis B' and traverses an illuminator system (electron beam column) 6 for guiding / focusing the electrons onto a selected portion of a sample S (typically (locally) diluting / flattening). A deflector 8 is also depicted, which can be used (among other things) to achieve a scanning motion of the beam B. Where possible, the vacuum enclosure V will typically "hug" the axis B' through (at least) the illuminator 6, in the form of a relatively narrow, elongated beam guide tube B" (e.g., about 0.5 cm in diameter), but widened where necessary to accommodate certain structures (such as, for example, items H, 26, 30, 32 and 34 discussed below).

[0092] The sample S is held on a sample holder H, which can be positioned with multiple degrees of freedom by a positioning device / stage A, which moves a bracket A' to which the holder H is (removably) attached; for example, the sample holder H may include fingers that can be moved (especially) in the XY plane (see the depicted Cartesian coordinate system), with movement parallel to Z and tilting about X / Y also generally possible. Such movement allows different parts of the sample S to be illuminated / imaged / inspected by an electron beam B traveling along an axis B' (in the Z direction), and / or allows scanning movements to be performed as an alternative to beam scanning. If (as is usually the case) the sample S is a cryogenic sample, then the sample holder H can be kept at a cryogenic temperature using a (schematically depicted) temperature control assembly T; for example, this may include a thermally conductive (e.g. metal) core that is thermally connected to the holder H and immersed in a cryogen bath, or for example a pipe system carrying a circulating cryogen.

[0093] The electron beam B will interact with the sample S in such a way that various types of "stimulated" radiation are emitted 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, nominally with the aid of an analysis device 22, which may be, for example, a combined scintillator / photomultiplier tube or an EDX (energy dispersive X-ray spectrometer) module; in this case, an image can be constructed using essentially the same principles as in a SEM. Alternatively or supplementally, however, electrons that traverse (pass through) the sample S, are emitted / emitted from it, and continue to propagate (generally, but usually with some deflection / scattering) along the axis B' can be studied. This transmitted electron flux enters an imaging system (projection lens) 24, which typically includes various electrostatic / magnetic lenses, deflectors, correctors (such as compensators), etc. In normal (non-scanning) TEM mode, this imaging system 24 can focus the transmitted electron flux onto a phosphor screen 26, which can be retracted / withdrawn (as schematically indicated by arrow 26') if necessary to move it away from the axis B'. The imaging system 24 will form an image or diffraction pattern of (a portion of) the sample S on the screen 26, and this can be viewed through viewing ports 28a, 28b located in appropriate parts of the walls of the enclosure V / cabinet 2. The retraction mechanism of the screen 26 can be, for example, mechanical and / or electrical in nature, and is not depicted here.

[0094] As an alternative to viewing the image / diffraction pattern on the screen 26, one may instead exploit the fact that the depth of focus of the electron flux leaving the imaging system 24 is typically very large (e.g., about 1 meter). Thus, various other types of analysis equipment may be used downstream of the screen 26, such as:

[0095] -TEM camera 30. At camera 30, the electron flux may form a static image or diffraction pattern, which may be processed by controller / processor 20 and displayed on a display device (not shown), such as, for example, a flat panel display. When not needed, camera 30 may be retracted / withdrawn (as schematically indicated by arrow 30') to move it away from axis B'.

[0096] - STEM camera 32. The output from camera 32 can be recorded as a function of the (X, Y) scan position of beam B on sample S, and an image can be constructed which is a "map" of the output from camera 32 as a function of X, Y. Camera 32 may include a single pixel of, for example, 20 mm in diameter, as opposed to the matrix of pixels characteristically present in camera 30. Furthermore, camera 32 will typically have a higher resolution than camera 30 (e.g., 10 pixels per second). 2 images per second) at much higher acquisition rates (e.g., 10 6 point) Again, when not needed, the camera 32 can be retracted / withdrawn (as schematically indicated by arrow 32') so that it is away from the axis B' (although in the case of, for example, an annular dark field camera 32, such retraction is not necessary; in such a camera, the central hole would allow flux to pass when the camera is not in use).

[0097] As an alternative to imaging using cameras 30 or 32 , it is also possible to call upon a spectroscopic device 34 , which can be, for example, an EELS module.

[0098] It should be noted that the order / position of the items 30 , 32 and 34 is not critical and many possible variations are conceivable. For example, the spectroscopic device 34 may also be integrated into the imaging system 24 .

[0099] Note that a controller (computer processor) 20 is connected to the various illustrated components via control lines (buses) 20'. This controller 20 may provide various functions, such as synchronizing actions, providing set points, processing signals, performing calculations, and displaying messages / information on a display device (not shown). Needless to say, the (schematically depicted) controller 20 may be (partially) inside or outside the cabinet 2 and may have a single or composite structure as desired.

[0100] A skilled person will appreciate that the interior of the enclosure V need not be maintained in a strict vacuum; for example, in a so-called "ambient TEM / STEM", a background atmosphere of a given gas is intentionally introduced / maintained within the enclosure V.

[0101] Microscope M may include one or more aberration correctors 40, each of which includes a multipole lens assembly configured to mitigate spherical (Cs) and / or chromatic (Cc) aberrations. For example, the corrector 40 depicted in illumination system 6 may be a Cs corrector, while the corrector 40 in imaging system 24 may be a combined Cs-Cc corrector (see above).

[0102] Conventionally, the beam guide tube B" comprises a metal tube. However, in the context of the present invention, this basic structure has been modified over at least a portion of the length (along the Z axis) of the beam guide tube B". Figure 2A , which shows Figure 1 From the enlarged transverse cross-sectional view of a portion of the beam conduit B" of FIG. 1 , it can be seen that, according to the invention, at least a (longitudinal) portion of the beam conduit B" has a composite form comprising:

[0103] - an outer tube 50 made of an electrically insulating material (wall thickness t t );

[0104] - An inner surface layer 52 (thickness t) of electrically conductive material.

[0105] The outer tube 50 - with an annular cross section - has an outer diameter of, for example, about 5-10 mm and an inner diameter of about 4.5-9 mm and, for example, comprises a ceramic or glassy material. On its inner (cylindrical) surface 50', a surface layer 52 has been provided / deposited, for example, having a thickness t of about 15 μm and comprising a material such as titanium nitride. During use, the inner surface layer 52 is connected to an electrical ground / earth. This composite structure can be used in the entire beam guide tube B", but it can also be applied only at selected locations thereof, such as within the correctors 40 and between each corrector 40 and the sample S. As described above, this structure protects the beam B from the influence of the Johnson-Nyquist magnetic field and thereby reduces the blurring effects of this field.

[0106] Figure 2B Shows Figure 2AA modified version of the situation in , in which there is an empty gap 51 between the outer tube 50 and the inner skin 52. In use, the gap 51 is evacuated to (substantially) the same vacuum level as the interior space of the skin 52, so that there is (substantially) no radial pressure difference across the skin 52; conversely, if the interior of the skin 52 is under a non-vacuum pressure (e.g. during manufacture, transport, maintenance, etc.), the gap 51 is also maintained at (substantially) this same pressure. In this way, the skin 52 can be effectively self-supporting, although relatively thin - although of course one or more interconnections can be provided between the items 52 and 50, for example in the form of radial struts or connected ends, for example; thus, it can also be described as a sleeve or hose, for example. The structure shown is also applicable / exploiting the insights of the current invention if the skin 52 has a relatively small σt value, for example, σt<0.1 or <0.01, for example.

[0107] Notice, Figure 2A and 2B In both cases, the surface layer 52 is "discrete" because it:

[0108] - different in structure from the insulating outer tube 50; and / or;

[0109] -exist Figure 2B In the case of , it is physically separated from the insulating outer tube 50.

[0110] Example 2

[0111] According to the present invention and Figure 2A The composite beam guide structure shown in can be manufactured in various ways. For example:

[0112] - One can start with a flat sheet of green ceramic material, such as LTCC. This will eventually become the outer tube 50 (after rolling into a cylindrical shape).

[0113] A conductive (but highly resistive) thin film is deposited on the flat surface of this plate, for example using one of the above-mentioned techniques PE-CVD, PE-ALD or PVD. This will eventually become the surface layer 52.

[0114] -The flat sheet is now rolled into a cylinder with the conductive film on the inner surface; this can be done with the help of a suitable jig / form. This is followed by isostatic pressing and sintering / baking.

[0115] In an alternative method, the tube 50 is preformed (eg, cast or rolled) into a cylindrical shape, and then the skin 52 is deposited on the interior surface thereof.

[0116] Example 3

[0117] A concise mathematical treatment of certain aspects of the invention will now be given. Electrical conductors (both magnetic and non-magnetic) passing the same thermal current generate a large fluctuating magnetic field at low frequency, which results in the well-known Johnson-Nyquist noise in resistors. Consider a non-magnetic tube of inner radius a, wall thickness t<<a, and conductivity σ. A threshold frequency is related to the skin effect and is given by:

[0118] V 表层 =(πμ0σt 2 ) -1 (3.1)

[0119] Here, μ0 is the permeability of vacuum. Now consider a loop current in a large disk of thickness t, with an average radius a>>t, and a DC resistance R0 = 2π / (σt), whereby a fluctuating magnetic field is measured at a distance a from the disk. In this case, a reasonable approximation for the frequency-dependent resistance R is:

[0120]

[0121] The current fluctuation is given by:

[0122]

[0123] in:

[0124]

[0125] The self-inductance L ind ≈μ0a, k is the Boltzmann constant, and T is the temperature. Therefore, there is a second threshold frequency v ind , which satisfies:

[0126] R(v ind )=2πμ0a v ind

[0127] in:

[0128] v ind =(μ0aσt) -1 , (3.5)

[0129] For t<<a.

[0130] The equation for the low frequency variation of the magnetic field has the following form:

[0131]

[0132] Geometry factor C geo <≈0.2. For tubes, a = tube radius. This can be extended to the spectral density:

[0133]

[0134] Because t<<a, equation (3.7) can be approximated as:

[0135]

[0136] It is integrated into:

[0137]

[0138] Therefore, the wall thickness t and the conductivity σ fall outside the equation. If σt is reduced, the spectral density at low frequencies is reduced, but all the noise power is transferred to higher frequencies.

[0139] To calculate the deflection angle caused by a fluctuating magnetic field, the so-called correlation distance ξ (along the tube axis) of the transverse magnetic field needs to be known. For the field generated by the Johnson-Nyquist current, ξ = 3a / 2 can be used. In the so-called "frozen dipole" scenario, assuming a frequency much lower than v / 2ξ, where v is the electron velocity, one obtains:

[0140]

[0141] For a tube of length L, where:

[0142] -U = accelerating voltage;

[0143] -U r = Relativistic voltage = U(1+Ue / 2mc 2 ), e and m are the electron charge and mass respectively;

[0144] -A n (z) is the normalized autocorrelation function of B(z), that is, A n (0) = 1. Possible autocorrelation functions are:

[0145] A n (z) = e -2|z| / ζ (3.11)

[0146] and

[0147]

[0148] The latter equation will be assumed below.

[0149] 3.1: "Unfrozen Dipoles"

[0150] For frequencies larger than the above-mentioned "frozen dipole" threshold v / 2ξ, the electron oscillates for multiple periods while traversing the relevant distance ξ. This greatly reduces the net deflection angle compared to the "frozen dipole" case, which (partially) explains why blackbody radiation does not contribute significantly to the image spread in the TEM. In order to include these frequencies in the Johnson-Nyquist current, a simple model consisting of a set of arbitrarily large discrete frequencies for the transverse magnetic field on the optical axis is used, without any mutual correlation between the different frequencies. The axial electrons are still assumed to be controlled by the Lorentz force, via:

[0151]

[0152] Here, γ r =1+Ue / mc 2 The correlation function of the magnetic field is allowed to vary with frequency, that is, assuming that A n (z) and ξ are frequency dependent. The deflection angle θ can then be shown:

[0153]

[0154] Where η=(e / 2m) 1 / 2 , and the exponential factor accounts for the wiggling of the electron. Equation (3.12) then yields:

[0155]

[0156] This equation states that:

[0157]

[0158] where the upper limit is roughly equal to the "frozen dipole" threshold:

[0159]

[0160] In terms of cut-off frequency:

[0161]

[0162] and the speed of light in a vacuum, c. Using equation (3.8), this yields:

[0163]

[0164] It can be concluded that if:

[0165]

[0166] A significant reduction in thermal magnetic field noise can be achieved, in which case equation (3.1.6) simplifies to:

[0167]

[0168] That is, the image diffusion is only within the range (σt) given by equation (3.1.7) 1 / 2 For example, in U max = In the specific case of an electron microscope with 300 kV, we obtain:

[0169] σt<<4 / (μ0c)=0.011Ω -1 (3.1.9)

[0170] Example 4

[0171] Figure 3 At beam (acceleration) voltages of 300 kV (black line) and 30 kV (gray line), in the embodiment of the present invention, ∑ = log 10 A plot of the relative image spread Δ (due to magnetic field noise) as a function of (σt)(surface conductivity × surface thickness). In essence:

[0172] Δ=θ(σt) inv / θ(σt) ref

[0173] Where the subscripts "inv" and "ref" mean "invention" and "reference", respectively. The conventional stainless steel beam guide is the reference for the vertical axis. The points below the horizontal dashed line (Δ=1) represent improvements with respect to the conventional beam guide. Note that at ∑=-1 (corresponding to σt=0.1), there is already an improvement, but at ∑=-2 (corresponding to σt=0.01), the improvement is stronger, and at ∑=-3 (corresponding to σt=0.001), the improvement is even stronger.

[0174] Example 5

[0175] Figure 4 An enlarged transverse cross-sectional view of an alternative (but related) embodiment shown in FIG. 2 (A and / or B) is shown. Here, the laminated composite material of FIG. 2 has been replaced by an aggregate composite material 54 comprising mixed electrically insulating and conductive materials, thereby producing a "hybrid" material having a lower conductivity σ than the skin layer 52 in FIG. 2. The conduit B" has a wall thickness t w , which is greater than the surface thickness t in Figure 2. According to the above discussion, the product σt w Relatively small, e.g. <0.01Ω -1 , or even <0.001Ω -1 .

Claims

1. An electron microscope, comprising: A sample holder, the sample holder is used to hold the sample; a source for generating an electron beam; an illumination system for directing the beam to illuminate the sample; an elongated beam conduit through which the beam is directed; a multipole lens assembly configured as an aberration corrector; a detector for detecting radiation emitted from the sample in response to the irradiation, Characterized in that at least a longitudinal portion of the beam guide extends at least through the aberration corrector and has a composite structure comprising: an outer tube composed of an electrically insulating material; The inner surface layer is composed of a conductive material, wherein the inner surface layer of the conductive material has a conductivity σ and a thickness t, where σt<0.1Ω -1 . 2 . The microscope of claim 1 , wherein the outer tube comprises at least one material selected from the group consisting of: ceramic, vitreous material, quartz, and combinations thereof.

3. The microscope according to claim 1 or 2, wherein the inner surface layer comprises at least one material selected from the group consisting of: ruthenium oxide, titanium nitrate, and combinations thereof.

4. The microscope according to claim 1 or 2, wherein for the conductive material, σt<0.01Ω -1 .

5. The microscope according to claim 1 or 2, further comprising: an imaging system for directing electrons transmitted through the sample onto the detector, Thereby, the beam guide extends through the imaging system.

6. The microscope according to claim 1 or 2, wherein the longitudinal portion extends at least between the sample holder and the aberration corrector.

7. The microscope of claim 6, wherein the aberration corrector is configured to correct at least one of spherical aberration and chromatic aberration.

8. The microscope according to claim 1 or 2, wherein: A magnetic yoke external to the beam guide tube is configured to conduct field lines into the immediate vicinity of the beam; The beam conduit passes through a hole in the yoke; The inner diameter of the hole is larger than the outer diameter of the beam guide tube, thereby creating a gap between the hole and the beam guide tube.

9. An electron microscope, comprising: A sample holder, the sample holder is used to hold the sample; a source for generating an electron beam; an illumination system for directing the beam to illuminate the sample; an elongated beam conduit through which the beam is directed; a multipole lens assembly configured as an aberration corrector; a detector for detecting radiation emitted from the sample in response to the irradiation, The invention is characterized in that at least a longitudinal portion of the beam guide extends at least through the aberration corrector and comprises a focusing composite material comprising Mixing electrical insulating and conducting materials; The beam guide has a conductivity σ and a wall thickness t w , where σt w <0.1Ω -1 .

10. The microscope of claim 9, wherein the electrically insulating material is a ceramic material.

11. The microscope according to claim 9 or 10, wherein the conductive material is selected from the group consisting of graphite, titanium nitride and mixtures thereof.

12. The microscope according to claim 9 or 10, wherein for the conductive material, σt<0.01Ω -1 .

13. The microscope according to claim 9 or 10, further comprising: an imaging system for directing electrons transmitted through the sample onto the detector, Thereby, the beam guide extends through the imaging system.

14. The microscope according to claim 9 or 10, wherein the longitudinal portion extends at least between the sample holder and the aberration corrector.

15. The microscope of claim 14, wherein the aberration corrector is configured to correct at least one of spherical aberration and chromatic aberration.

16. The microscope according to claim 9 or 10, wherein: A magnetic yoke external to the beam guide tube is configured to conduct field lines into the immediate vicinity of the beam; The beam conduit passes through a hole in the yoke; The inner diameter of the hole is larger than the outer diameter of the beam guide tube, thereby creating a gap between the hole and the beam guide tube.

17. A method of using an electron microscope according to any one of claims 1 to 16, characterized in that The beam is directed through the elongated beam conduit.

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