阅读说明：本技术 具有改进的成像分辨率的电子显微镜 (Electron microscope with improved imaging resolution ) 是由 A.汉斯特拉 P.唐娜 于 2019-07-08 设计创作，主要内容包括：一种电子显微镜,其包括：-样本固持器,所述样本固持器用于固持样本；-源,所述源用于产生电子射束；-照明系统,所述照明系统用于引导所述射束以照射所述样本；-细长射束导管,所述射束被引导通过所述细长射束导管；-检测器,所述检测器用于检测响应于所述照射从所述样本发出的辐射,其中所述射束导管的至少纵向部分具有复合结构,所述复合结构包括：-由电绝缘材料构成的外管；-由导电材料构成的内表层。在替代性但相关的结构中,所述射束导管的至少纵向部分包含包括混合电绝缘材料和导电材料的聚集复合材料。(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 elongate beam guide through which the beam is directed; -a detector for detecting radiation emitted from the sample in response to the irradiating, wherein at least a longitudinal portion of the beam guide has a composite structure comprising: -an outer tube of electrically insulating material; -an inner skin layer of electrically conductive material. In an alternative but related arrangement, at least a longitudinal portion of the beam guide comprises a gathered composite material comprising a hybrid electrically insulating material and an electrically conductive material.)

1. 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 elongate beam guide 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 illumination,

wherein 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;

an inner surface layer of an electrically conductive material having a conductivity σ and a thickness t, wherein σ t<0.1Ω^-1。

2. 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 elongate beam guide 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 illumination,

characterized in that at least a longitudinal portion of the beam guide extends at least through the aberration corrector and comprises a concentrating composite material comprising

Mixing an electrically insulating material and an electrically conductive material;

wherein the beam guide has a conductivity σ and a wall thickness t_wWhere σ t_w<0.1Ω^-1。

3. The microscope of claim 1, wherein the outer tube comprises at least one material selected from the group comprising: ceramics, vitreous materials, quartz, and combinations thereof.

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

5. The microscope of claim 2, wherein the electrically insulating material is a ceramic material.

6. The microscope of claim 2 or 5, wherein the electrically conductive material is selected from the group comprising: graphite, titanium nitride and mixtures thereof.

7. According to claims 1 to 6The microscope, wherein for the conductive material, σ t<0.01Ω^-1。

8. The microscope of any one of claims 1 to 7, additionally comprising:

an imaging system for directing electrons transmitted through the sample onto the detector,

whereby the beam guide extends through the imaging system.

9. The microscope of claims 1-8, wherein the longitudinal portion extends at least between the sample holder and the aberration corrector.

10. The microscope of claim 9, wherein the aberration corrector is configured to correct at least one of spherical aberration and chromatic aberration.

11. The microscope of any one of claims 1 to 10, wherein:

a yoke outside the beam guide configured to conduct field lines into direct proximity of the beam;

the beam guide passing through an aperture in the yoke;

the inner diameter of the aperture is larger than the outer diameter of the beam guide, thereby creating a gap between the aperture and the beam guide.

12. A method of using an electron microscope according to any one of claims 1 to 11 characterised in that the beam is directed through the elongate beam guide.

Technical Field

The present invention relates to 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 elongate beam guide 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 illumination.

The invention also relates to a method of using such an electron microscope.

Background

Electron microscopy is a well-known and increasingly important technique for imaging microscopic objects. Historically, the basic genus of electron microscopy has evolved into a number of well-known device species, such as Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Scanning Transmission Electron Microscopy (STEM), as well as various subspecies, such as so-called "dual beam" devices (e.g., FIB-SEM), which additionally employ "machining" of a Focused Ion Beam (FIB), allowing supporting activities such as, for example, ion beam milling or Ion Beam Induced Deposition (IBID). More specifically:

in an SEM, the sample is illuminated by a beam of scanning electrons, accelerating the emission of "auxiliary" 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 radiation are then detected and used for image accumulation purposes.

In TEM, the electron beam used to illuminate the sample is selected to be of sufficiently high energy to penetrate the sample (for this reason, the sample will typically be thinner than in the case of SEM samples); the transmitted electrons emitted from the sample can then be used to create an image. When such a TEM is operated in scan mode (hence STEM), the image will accumulate during the scanning motion of the illuminating electron beam.

SEM may also be used in "transmissive mode", for example, when using a relatively thin sample and a relatively high incident beam energy. Such tools are commonly referred to as "TSEM" (transmission SEM) and typically have a relatively basic imaging system (e.g., a single lens and deflector) disposed between the sample and the sample post-detector.

It should be noted that the electron microscope may have other functions than imaging and performing (local) surface modification (e.g. milling, etching, deposition, etc.), such as performing spectral analysis, examining diffraction patterns, etc.

In all cases, an Electron Microscope (EM) will include at least the following components:

electron sources, such as, for example, W or LaB₆A source, a schottky gun, or a Cold Field Emission Gun (CFEG).

An illumination system/illuminator for manipulating the "raw" radiation beam from the source and performing certain operations thereon, such as focusing, aberration mitigation, cropping (using an aperture), filtering, etc. Which typically comprises one or more (charged particle) lenses and may also comprise other types of (particle) optical components. If desired, the illuminator may be provided with a deflector system which can be invoked to cause its outgoing beam to perform a scanning motion across the sample under investigation.

Sample holders-typically connected to a positioning system-on which the sample under investigation can be held and positioned (e.g. displaced, tilted, rotated). This holder can be moved, if necessary, in order to achieve a scanning motion of the sample relative to the beam. When designed to hold a cryogenic sample, the sample holder may comprise means for maintaining the sample at cryogenic temperature, for example using a suitably connected cryogen barrel.

A detector (for detecting radiation emitted from the illuminated sample), which may be unitary or compound/distributed in nature, and which may take many different forms depending on the radiation detected. Examples include photodiodes, CMOS detectors, CCD detectors, photovoltaic cells, X-ray detectors (such as silicon drift detectors and si (li) detectors), and the like. Generally, an EM may comprise several different types of detectors, the selection of which may be invoked in different situations.

In the case of a transmission microscope (such as a (S) TEM or TSEM), the EM will additionally include:

imaging systems, which essentially absorb the electrons transported through the sample (plane) and direct (focus) them onto an analytical device, such as a detector, an imaging device, a spectroscopic device (e.g. an EELS device: EELS ═ electron energy loss spectrum), etc. As with the illuminator mentioned above, the imaging system may also perform other functions, such as aberration mitigation, cropping, filtering, etc., and typically includes one or more charged particle lenses and/or other types of particle-optical components.

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 near the optical axis (of the illuminator and imaging system, if present).

While there is a continuing desire in various technical fields to further improve the imaging resolution that can be obtained with EM, this task is far from trivial. The most advanced EM's 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 various physical effects, including, for example, higher order aberrations, coulombic interactions, vacuum level fluctuations, and stray fields. One such recently discovered obstacle is electron beam interference due to "parasitic" thermal magnetic field noise along the beam path, as in Physical Review Letters PRL by s.uhlemann et al, 26 months 7 and 2013]111》，pp046101-046105 (american society of physics), wherein the authors demonstrate the thermodynamic properties of magnetic field noise observed in TEM, and take steps to cool the "conductive part of the instrument" to cryogenic temperatures (e.g., about 77k) in an attempt to reduce this phenomenon. Since the magnitude of the phenomenon varies with temperature, it implies that cooling to liquid helium temperature is required to achieve the ultimate minimization of these noise effects.

Disclosure of Invention

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 addressed without having to rely on the above-mentioned complex cooling measures.

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

-an outer tube of electrically insulating material;

-an inner surface layer of electrically conductive material having a conductivity σ and a thickness t, wherein σ t<0.1Ω^-1。

The present inventors have recognized that solutions that require cryogenic cooling of the "conductive parts" of the EM (e.g., the core/yoke in the magnetic lens, the beam guide, etc.) are cumbersome. The particle beam column of 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 excess space. In such an arrangement, it would be a cumbersome task to leave extra space for the relatively bulky cooling elements/coils/supply lines required to cool down a large part of the microscope to cryogenic temperatures. Thus, the present invention employs a completely different approach. The parasitic magnetic fields described above are largely due to Johnson-Nyquist currents in the walls of the beam guide, which traditionally consist of metal (e.g., stainless steel or titanium) tubes-the inventors decided to use electrically insulating tubes, e.g., includingSuch as zirconium oxide (ZrO)₂) Or aluminum oxide (Al)₂O₃) Etc. replace conventional beam guide designs. To prevent space charging of such a tube, its inner surface is covered by a (grounded) skin layer made of an electrically conductive material, such as a metal film-which should be relatively thin/resistive and therefore not itself a significant bay/source of parasitic currents. Such a structure appears to be disqualified by the PRL journal article mentioned above, which states that by reducing the conductivity and metal wall thickness, the noise spectrum may shift slightly from lower frequencies to higher frequencies, but the total noise power remains substantially unchanged, with a negligible/marginal net effect on image spreading. However, the inventors have shown that this demonstration has drawbacks: if the spectral shift is large enough, then the peak spectral power can be shifted to the frequency domain where the Johnson-Nyquist field no longer significantly affects the electron beam. More specifically, frequencies above the microwave cut-off frequency of the beam guide of the present invention do not substantially cause image spreading/blurring.

Based on the above discussion, those skilled in the art will appreciate that the inner skin of the composite beam guide of the present invention should (simultaneously):

sufficiently thick/conductive to effectively remove space charge (induced surface charge, e.g. caused by (spurious) electron impact);

sufficiently thin/resistive to avoid being a significant bay of the parasitic current itself.

In this regard, the product σ t of the skin conductivity σ and the skin thickness t can play an indicative role, and there is a general preference for relatively small values of σ t. For example, and providing some guidance, in an (S) TEM operating at 300kV, the inventors have found the value σ t<0.1Ω^-1Producing satisfactory results, σ t<0.01Ω^-1The results obtained are better, and σ t<0.001Ω^-1Further improvement is carried out. A given value of σ t can be achieved by selecting various (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:

relatively low t values (e.g. less than a few nm) may introduce manufacturing complexity, e.g. with respect to deposition techniques, choice of discontinuities (island formation), etc.

Relatively high t values (e.g., greater than about 100 μm order) may severely narrow the list of candidate materials that can achieve a suitable σ value.

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

- σ ═ 1/ρ, where ρ is the resistivity;

-σt＝1/R_swherein R is_sIs the sheet resistance of the surface layer, where R_s＝ρ/t。

With respect to suitable materials for the composite beam guide of the present invention, the following non-limiting examples are given for guidance purposes:

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 engineered ceramics, such as zirconia (ZrO) described above₂) And alumina (Al)₂O₃). Such materials are generally durable, temperature resistant (and therefore capable of withstanding vacuum baking), have no problems with outgassing behavior, and are relatively easy to make into tubular shapes (e.g., using casting). For good order, it should be noted that in conventional EM the beam guide typically has an inner diameter of about 6-8mm, but this range of values is not limiting.

-the inner skin layer comprises at least one material selected from the group comprising: ruthenium oxide, titanium nitrate, and combinations thereof. These materials have convenient sigma values and can be deposited relatively easily on the inner surface of the outer tube using methods like e.g. PE-CVD (plasma enhanced chemical vapour deposition), PE-ALD (plasma enhanced atomic layer deposition) and PVD (physical layer deposition). This group is not limiting: in principle, metals like W, Ti or Pt (or combinations thereof) may also be used in the inner skin layer, depending inter alia on the chosen skin thickness t.

To be goodIn order, it should be noted that the term "electrically insulating material" as referred to in the context of the present invention may also encompass materials which may conventionally be considered as semiconductors. For example, SiC is a ceramic material traditionally labeled as a semiconductor; however, its resistivity is about 10⁶Omega cm-which makes it less conductive than aluminum by about 10¹⁶Aluminum has a resistivity of about 10^-10Omega cm. In contrast, the resistivities of quartz, alumina and zirconia are about 10, respectively¹⁶Ωcm、10¹⁴Omega cm and 10⁹Omega cm. The skilled person will understand that an electrical insulator is a freely transporting material that is (substantially) free of (conducting) electrons, typically due to the presence of a relatively large band gap in such a material.

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 concentrating composite material comprising:

-mixing an electrically insulating material and an electrically conductive material;

-wherein the beam guide has a conductivity σ and a wall thickness t_wWhere σ t_w<0.1Ω^-1。

In this second related method, the beam guide comprises an aggregate composite material including a hybrid electrically insulating material and an electrically conductive material. This method is based on similar insights as described above, but it uses a beam-guide structure in which the "laminated structure" of the separate electrically insulating outer tube and electrically insulating inner skin layers described above is "effectively" deformed "into a single" monolithic "polymeric composite structure. The conductivity of this deformed structure is intermediate that of conventional conductive and insulating materials and may be functionally referred to as a "high volume resistivity" material. Simultaneously:

the insulation is sufficient to mitigate the blurring effect of parasitic Johnson-Nyquist currents in the catheter wall; however:

the electrical conductivity is sufficient to remove space charge that tends to accumulate on its inner surface.

If considered:

in the above laminated structureThe outer tube has a wall thickness t_tAnd;

the beam guide in the present polymeric structure has a wall thickness t_w，

One way of appreciating the characteristics of such deformed structures is then to consider them as "evolutions" of the above-mentioned laminated structures, in which:

-t_treduced to 0;

increase of-t to t_wThe former "skin" becomes a self-supporting wall;

-simultaneous/corresponding reduction of σ.

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

if t is about 10²By a factor of (to t)_w) (e.g., from about 10 microns to about 1mm), then;

σ will need to be reduced by about 10 accordingly²By the same factor that reduces σ to a level that can be considered intermediate between a conventional conductor and an insulator.

With respect to the construction of the polymeric composite structures described in the preceding paragraph, suitable examples of constituent materials include:

-an electrically insulating material: ceramics such as, for example, SiC and/or ZrO.

-electrically conductive material: graphite and/or titanium nitride, for example.

One way to achieve such a composite 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, it is possible to start with a conductive material and "temper" it conductively by mixing an insulating material therein. For example, the additive may be included in the receptive bulk material using methods such as diffusion or ion implantation, or by physical mixing of particles. The skilled person will be able to determine the relative amounts of the different materials to be mixed in order to obtain an aggregate composite material with a given volume resistivity, and/or he may purchase a prefabricated product. For example, the aggregate composites referred to herein are commercially available from companies such as pocogrphite, inc. 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". Such materials are discussed, for example, in the journal article "Granular electronic systems", by i.s. beloborodv et al, "modern physical reviews (rev. mod. phys.)" 79.pp.469 (april, 2007):

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

the composite beam tube structure of the invention presented above (in either approach) does not necessarily have to be used over the entire length of the (main) beam path in the microscope (although such a full length expansion is of course possible). In practice, the inventors have found that:

induced image diffusion (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;

some parts of the beam path are more susceptible to magnetic field noise than others, and of these parts the invention is most useful.

As defined herein:

the microscope comprises a multipole lens assembly configured as an aberration corrector;

the composite beam guide of the invention extends at least through this aberration corrector.

Examples of aberration correctors mentioned herein include spherical aberration (Cs) correctors, chromatic aberration (Cc) correctors, and combined spherical color (Cs-Cc) correctors, which may be used in SEM and/or TEM. For example:

in SEM or STEM, the Cs corrector may be included in the illumination system (before the sample). Such Cs correctors typically have a length (along the beam path) of about 15 cm.

In TEM, the combined Cs-Cc corrector may be comprised in an imaging system positioned downstream of the sample and used to direct the electrons transmitted through the sample onto a (imaging, diffraction and/or spectroscopy) detector. Such Cs-Cs correctors typically have a length (along the beam path) of about 45cm, but there are also Cs-Cc correctors, which are about twice this length, for example.

For example, more information about Cs and Cc correctors can be gleaned from the following references:

-U.S. Pat. No. 5,084,622;

-h.rose and w.wan, "Aberration correction in electron microscopy" (IEEE correction in Particle Accelerator Conference Proceedings of2005, usa, nuxoxvel, tennessee, pp.44-48 (2005)).

When employing an aberration corrector as described above, another relatively sensitive part of the beam path in which the invention may be utilized is the trajectory extending between the sample plane (sample holder) and the aberration corrector. This means one or both of:

a trajectory from the calibrator to the sample plane (upstream of the sample plane), e.g. in STEM;

trajectory from the sample plane to the corrector (downstream of the sample plane), e.g. in TEM.

Furthermore, it may be advantageous to apply the invention to a portion of the beam path positioned in/near the electrostatic deflection module. The skilled person will be able to decide which parts of the beam path (most) are worth applying the composite beam guide of the present invention and may decide to deploy it along (substantially) the entire main beam path. The latter solution is particularly advantageous in that it avoids the creation of a joint between two different types of beam guide-bearing in mind that the entire beam guide must be reliably maintained at high vacuum (at least) during operation of the microscope.

With the present invention, excellent STEM image resolution values can be obtained, for example, 30pm at a beam voltage of 300kV and 60pm at a beam voltage of 60kV, with a beam half-open angle of 50mrad in both cases, and without the need for the cumbersome cryogenic cooling described in the PRL journal article above. For conventional stainless steel beam catheters, the present invention generally allows image dispersion caused by Johnson-Nyquist noise to be reduced by a factor of about 10-15.

Supplementing the above-described measures of the present invention, additional measures may be taken to further reduce the deleterious imaging effects of Johnson-Nyquist noise without having to employ cryogenic cooling as described in the aforementioned PRL journal article. One such measure is to widen the aperture in one or more magnetic (e.g. iron) yokes that are used to conduct the field lines into direct proximity of the beam. Nominally, this aperture hugs/contacts the outer surface of the beam guide so as to be as close as possible to the beam axis. However, the inventors have noted that for a cylindrical bore of (inner) diameter r, the yoke causes Johnson Nyquist blur according to the 1/r dependence; thus, enlarging the hole will reduce the effect of this blurring. Increasing the aperture size in this manner will result in an empty gap between the inner surface of the aperture and the outer surface of the beam guide, but such a gap does not necessarily have a significant (overwhelming) negative aspect.

For completeness, reference is made to the following prior art documents.

US 3,787,696 a and DE 3010376 a1 disclose liners for use in scanning and/or focusing coils. US 3,634,684 a also uses a liner for the scanning coil. Here, the liner serves to oppose eddy currents originating from the scanned high frequency magnetic flux. These documents do not suggest the use of these liners as aberration correctors against johnson noise in multipole lens assemblies.

JP H0322339A discloses an aberration corrector with a conductive inner skin and an electrically insulating outer tube. Here, the conductive inner surface layer needs to apply a desired voltage to the backing tube and keep the sample grounded.

Drawings

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

FIG. 1 shows a longitudinal cross-sectional elevation of an embodiment of an EM (in this case, an (S) TEM) embodying the present invention.

Fig. 2A shows an enlarged transverse cross-sectional view of a portion of fig. 1.

Fig. 2B shows a modified version of the embodiment in fig. 2A.

FIG. 3 shows σ t (surface conductivity × surface thickness) log as an example of the present invention at different beam voltages₁₀A plot of the relative image spread (due to magnetic field noise, and compared to a conventional beam guide) of the function of (a), as shown in fig. 2A.

Fig. 4 shows an enlarged transverse cross-sectional view of an alternative (but related) embodiment shown in fig. 2A/2B.

In the figures, corresponding parts are denoted by corresponding reference signs when relevant.

Detailed Description