Sample carrier and use thereof

Abstract

The present disclosure relates to a sample carrier for a charged particle microscope. The sample carrier includes a planar or substantially planar body, an opening, and at least one protrusion. The opening is provided in the planar or substantially planar body. The protrusion extends into the opening within a plane defined by the planar or substantially planar body. The protrusion is configured to hold a charged particle microscope sample.

Description

Background Art

[0001] The present disclosure relates to a sample carrier for a charged particle microscope and a method of using the sample carrier in a charged particle microscope. The present embodiment also relates to an apparatus and method for sample preparation for an imaging system. Summary of the invention

[0002] Charged particle microscopy is a well-known and increasingly important technique for imaging microscopic objects, particularly in the form of electron microscopes. Historically, the basic category of electron microscopes has evolved into a number of well-known equipment categories, such as transmission electron microscopes (TEM), scanning electron microscopes (SEM) and scanning transmission electron microscopes (STEM), and also into various subcategories, such as so-called "dual beam" equipment (e.g., FIB-SEM), which additionally employs a "machining" focused ion beam (FIB), for example, thereby allowing supporting activities such as ion beam milling or ion beam induced deposition (IBID).

[0003] In a SEM, illumination of the sample by a scanning electron beam promotes emission of "auxiliary" radiation from the sample, for example in the form of secondary electrons, backscattered electrons, X-rays, and photoluminescence (infrared, visible and / or ultraviolet photons); one or more components of this emitted radiation flux are then detected and used for image accumulation purposes.

[0004] 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); the flux of transmitted electrons emitted from the sample can then be used to create an image. When such a TEM is operated in scanning mode (hence a STEM), the image in question will be accumulated during the scanning movement of the irradiating electron beam.

[0005] For example, more information on some of the topics covered here can be gleaned from the following Wikipedia links:

[0006] http: / / en.wikipedia.org / wiki / Electron_microscope

[0007] http: / / en.wikipedia.org / wiki / Scanning_electron_microscope

[0008] http: / / en.wikipedia.org / wiki / Transmission_electron_microscopy

[0009] http: / / en.wikipedia.org / wiki / Scanning_transmission_electron_microscopy[TP346944USORG1]

[0010] As an alternative to using electrons as the irradiation beam, charged particle microscopy can also be performed using other kinds of charged particles. In this regard, the phrase "charged particles" should be interpreted broadly to include, for example, electrons, positive ions (such as Ga or He ions), negative ions, protons and positrons. Regarding ion-based microscopy, for example, some further information can be gleaned from sources such as:

[0011] http: / / en.wikipedia.org / wiki / Scanning_Helium_Ion_Microscope

[0012] W. H. Escovitz, T. R. Fox, and R. Levi-Setti, Scanning Transmission Ion Microscope with a Field Ion Source, Proc. Nat. Acad. Sci. USA 72(5), pp. 1826-1828 (1975).

[0013] It should be noted that in addition to imaging, charged particle microscopes may also have other functions, such as performing spectroscopic analysis, examining diffraction patterns, performing (local) surface modifications (e.g. milling, etching, deposition), etc.

[0014] In order to prepare samples for charged particle beam processes (such as detection by transmission electron microscopy (TEM), scanning electron microscopy (SEM), etc.), the samples have been milled into thin slices. Some techniques for preparing TEM samples may involve cleavage, chemical polishing, mechanical polishing, or wide beam low energy ion milling. Combinations of these techniques are also possible. These techniques generally require the starting material to be cut into smaller and smaller pieces, thereby destroying most of the original sample. Other techniques, commonly referred to as "peeling" procedures, use a focused ion beam (FIB) to cut samples from a substrate while greatly limiting or eliminating damage to the surrounding areas of the substrate. For example, these techniques are useful for analyzing the results of semiconductor manufacturing.

[0015] A charged particle microscope (CPM) may include at least the following components: a radiation source, an illuminator, a sample holder and a detector. The radiation source may be a radiation gun such as a Schottky electron source or an ion gun. The illuminator may be used to manipulate the "raw" radiation beam from the source and perform certain operations such as focusing, mitigating aberrations, cropping (with holes), filtering, etc. The illuminator typically includes one or more (charged particle) lenses and may also include other types of (particle) optical components. If necessary, the illuminator may be provided with a deflector system that may be called upon so that its output beam performs a scanning motion over the sample being studied. The sample being studied may be held and positioned (e.g. tilted, rotated) by a sample holder. If necessary, the holder may be moved so as to achieve a scanning motion of the beam about the sample. Generally, such a sample holder will be connected to a positioning system, such as a mechanical stage. A detector may be used to detect radiation emitted from the irradiated sample, which detector may be single or compound / distributed in nature, [TP346944USORG1]

[0016] And the detector may take many different forms, depending on the radiation being detected. Examples include photomultiplier tubes (including solid state photomultipliers, SSPMs), photodiodes, CMOS detectors, CCD detectors, photovoltaic cells, etc., which may be used, for example, in conjunction with scintillating films, for example.

[0017] In the case of a transmission-type microscope (e.g. such as a (S)TEM), the CPM may also include an imaging system that essentially collects the charged particles transmitted through the sample (plane) and directs (focuses) them onto an analytical device, such as a detection / imaging device, a spectroscopic device (such as an EELS module), etc. As with the illuminator mentioned above, the imaging system may also perform other functions, such as mitigating aberrations, cropping, filtering, etc., and typically includes one or more charged particle lenses and / or other types of particle optical components.

[0018] In addition to imaging, an important aspect of working with CPM is sample preparation. This is particularly (though not exclusively) the case in transmission-type CPM, where samples are typically very thin (e.g. of the order of 1 nm-100 nm), and therefore relatively brittle / fragile, and therefore (very) difficult to handle. Once such a sample has been (preliminarily) mounted on (the sample mounting area of) a holder (e.g. using adhesive, or a mechanical clamping mechanism such as clips, flanges, screws, etc.), it is highly desirable not to have to remove it until strictly necessary.

[0019] However, after mounting, a number of operations (alterations, finishing) may have to be performed on the sample for the purpose of thinning, surface modification, etc., such as ion milling, ion beam induced deposition (IBID), electron beam induced deposition (EBID), etc. Many such functions can be obtained in situ in CPM, but their applicability / usefulness is limited in many cases by the suboptimal operability of the sample holder. Against this background, the inventors have conducted intensive work to identify the shortcomings in conventional holder designs and to effectively address these shortcomings in order to produce better performance.

[0020] Samples for electron microscopy imaging require certain preparation for observation under transmitted light or electron radiation. For example, thin slices (or slices) of the sample are usually cut or milled from the overall sample in a grid or tube. Cutting or milling can be performed by a focused ion beam (FIB) system, or in a dual beam system including both FIB and electron microscope. Examples of such dual beam systems include the Quanta3D dual beam system from FEI Company (Hillsboro, OR, USA).

[0021] The process of excavating a sample from a workpiece and attaching the excavated sample to a grid can be learned from, for example, MAYER, Joachim et al., "TEM Sample preparation and FIB-induced damage", Materials Research Society Bulletin (MRS Bulletin), Vol. 32, May 2007, pp. 400-407, more specifically page 401: "In-situ Lift-out / microsampling".

[0022] [TP346944USORG1]

[0023] MAYER describes how to dig a sample in situ from a wafer, thin it and attach it to a grid in a focused ion beam (FIB) machine. Alternatively, dig the sample, then attach it to a grid and finally thin it.

[0024] The thinned sample is usually called a lamella. Therefore, in this application, the phrases "lamella" and "sample" are used interchangeably. The resulting lamella is thinned to be transparent to electrons and typically has a thickness of 50 nm or less. The area of ​​the lamella is typically rectangular with a side length of a few microns to tens of microns, for example a rectangle with dimensions of 5 microns by 15 microns. After preparing the lamella and attaching the lamella to the peeling grid, the peeling grid is positioned so that electron microscopy can be performed, i.e., it can be transferred from the focused ion beam machine to the TEM.

[0025] A problem with known peel-off grids is that when the sample is welded to the grid, a large part of the emission solid angle is blocked by the grid, especially near the sample / grid interface. As a result, a large part of the X-rays cannot be detected, resulting in extended processing time and / or a wrong assessment of the amount of X-rays emitted from the sample. Note that the sample is typically only a few microns long and the foil is typically 25 microns thick, resulting in a large shadowing effect, blocking almost 50% of the emission solid angle of the X-rays.

[0026] A similar problem arises when making X-ray or electron beam tomograms, where the sample is rotated (tilted) on an axis perpendicular to the electron beam, and images are made at a large number of these tilt angles. These images at different tilt angles are then used to reconstruct a 3D image of the sample. Note that tomograms typically use thicker samples, between 70nm and 1μm thick (depending on sample composition).

[0027] Since part of the sample is shaded by the grid (either downstream or upstream of the electron beam), the tomograms are limited to tilt angles where the sample is not or barely shaded. Furthermore, the shade depends on the tilt angle used. Therefore, the 3D reconstruction is limited in terms of resolution (since only a fraction of the mechanically possible tilt angles can produce a useful detection signal), in particular for sample parts near the grid / sample interface.

[0028] Another problem is the transfer of the grid including the lamellae into the C-shaped clamp ring, which may lead to mechanical and thermal instabilities.

[0029] Therefore, there is a need for a specimen carrier that provides a tomographic tilt range of + / - 70 degrees without obscuring the lamella and that can also be positioned without the use of a c-clamp ring.

[0030] In one aspect, the present disclosure provides a sample carrier for a charged particle microscope. The sample carrier includes a planar or substantially planar body, an opening, and at least one protrusion. The opening is disposed in the planar or substantially planar body. The protrusion extends into the opening within a plane defined by the planar or substantially planar body. The protrusion is configured to hold a charged particle microscope sample.

[0031] [TP346944USORG1]

[0032] In another aspect, the present disclosure provides a method for preparing a sample for detection in a charged particle microscope. The method includes: providing a sample carrier; connecting the sample carrier to a mechanical stage device of the charged particle microscope; providing a sample; and connecting the sample to a grid member of the sample carrier. The sample carrier includes a planar or substantially planar body, an opening disposed in the planar or substantially planar body, and at least one protrusion extending into the opening within a plane defined by the planar or substantially planar body. The protrusion is configured to hold a charged particle microscope sample.

[0033] Other aspects of the disclosure will become apparent by consideration of the detailed description and accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a schematic diagram of a top view of a sample carrier according to the present disclosure.

[0035] Figure 2 is a schematic diagram of a close-up view of a sample carrier. DETAILED DESCRIPTION

[0036] Before explaining any embodiments of the present disclosure in detail, it should be understood that the present disclosure is not limited in its application to the details of construction and arrangement of components set forth in the following description or shown in the following drawings. The present disclosure is capable of other embodiments and can be practiced or carried out in various ways.

[0037] Hereinafter, the present disclosure may be explained by way of example in the specific context of electron microscopy. However, such simplifications are only for clarity / illustrative purposes and should not be interpreted as limiting.

[0038] The present disclosure aims to solve at least some of the above problems by providing a sample carrier 1 for a charged particle microscope. Figure 1 A top view of a sample carrier 1 within the scope of the present disclosure is shown. As shown, the sample carrier 1 comprises an opening 2 with three protrusions 3 on opposite sides of the opening 2. A sample 4 is positioned on the ends of some of the protrusions 3. The sample carrier 1 shown is substantially circular and has a raised outer edge 5 which may serve as a support profile. Figure 2 A close-up of the protrusion 3 is provided and the positioning of the sample 4 on the protrusion 3 is illustrated.

[0039] The sample carrier 1 comprises a planar or substantially planar body, an opening 2 arranged in the planar or substantially planar body, and at least one protrusion 3 extending in a plane defined by the planar or substantially planar body into the opening 2. The protrusion 3 is configured to hold a charged particle microscopy sample 4.

[0040] The charged particle microscope may be any charged particle microscope suitable for imaging an object. In particular, the charged particle microscope may be an electron microscope, such as a transmission electron microscope (TEM), a scanning electron microscope (SEM) or a scanning transmission electron microscope (STEM), including various subcategories, such as [TP346944USORG1]

[0041] Such as so called "dual beam" instruments (e.g. FIB-SEM) which additionally employ a "machining" focused ion beam (FIB), allowing supporting activities such as ion beam milling or ion beam induced deposition (IBID). It may be preferred that the charged particle microscope is a TEM, such as a cryo-TEM.

[0042] The sample carrier 1 of the present disclosure has a planar or substantially planar body. The term "planar or substantially planar" means that the carrier shape can be substantially flat / horizontal and uninterrupted, or can have openings, recesses or other interruptions therein, and / or can be curved or bent, as long as they do not deviate from and / or affect the plane of the carrier, as will be understood by those skilled in the art, such defects may occur during the manufacturing process.

[0043] The planar or substantially planar body may be any shape suitable for use as a sample carrier 1. For example, the planar or substantially planar body may be circular (substantially circular), elliptical (substantially elliptical), square (substantially square) or rectangular (substantially rectangular). In a preferred aspect, the shape of the sample carrier 1 may be circular.

[0044] As used herein, the terms "substantially circular," "substantially elliptical," "substantially square," or "substantially rectangular" are intended to mean that the planar or substantially planar body has a shape that appears to be circular, elliptical, square, or rectangular, but may deviate slightly from the mathematical definition of a circle, ellipse, square, or rectangle, as will be appreciated by those skilled in the art, as such imperfections may occur during the manufacturing process.

[0045] The planar or substantially planar body may be of any size suitable for use as a sample carrier 1. For example, the planar or substantially planar body may have a diameter of about 1 mm to about 5 mm, such as about 2.5 mm to about 4.5 mm or about 3 mm to about 4 mm. Preferably, the planar or substantially planar body may have a diameter of about 3.5 mm.

[0046] In one aspect of the present disclosure, the planar or substantially planar body may have a convex outer edge 5. That is, the outer edge 5 of the sample carrier 1 may be thicker than a portion inside the outer edge 5 (hereinafter may be referred to as a "central portion").

[0047] The central portion is preferably thinner than the outer edge 5 of the plane or substantially plane body. The central portion can be of any thickness. However, it will be appreciated by those skilled in the art that the thickness is designed so that when the sample 4 is positioned on at least one protrusion, the central portion does not obscure the sample.

[0048] In one aspect of the present disclosure, the thickness ratio between the outer edge 5 and the central portion of the planar or substantially planar body may be from about 1.1:1 to about 10:1, such as from about 1.5:1 to about 5:1.

[0049] [TP346944USORG1]

[0050] In the sample carrier 1 of the present disclosure, an opening 2 is provided in a planar or substantially planar body. The term "opening" means that the planar or substantially planar body includes a region in which the planar or substantially planar body does not exist.

[0051] Typically, the opening 2 may be located in the centre of the planar or substantially planar body.

[0052] The opening 2 can be any shape. For example, the opening 2 can be substantially circular, substantially elliptical or substantially rectangular. It will be appreciated by those skilled in the art that the opening 2 should be as small as possible to provide mechanical and thermal stability, and large enough to increase the visible tilt range area.

[0053] In one aspect of the present disclosure, the opening 2 may be rectangular (substantially rectangular) and located at the center of a circular planar or substantially planar body. In this aspect, the circular or substantially circular planar body may have a diameter of about 3 mm to about 4 mm (such as about 3.5 mm), and / or the rectangular (substantially rectangular) opening 2 may have a width of about 0.1 mm to about 1 mm and a length of about 0.5 mm to about 1.5 mm.

[0054] In the sample carrier 1 of the present disclosure, at least one protrusion 3 extends into the opening 2 within a plane defined by the planar or substantially planar body.

[0055] The term “protrusion” is intended to mean an area or section of a planar or substantially planar body which extends from the edge 5 of the opening 2 towards the centre of the opening 2 but does not extend completely across the opening 2 .

[0056] The protrusion 3 may be of any shape, but in a preferred aspect, it may be in the shape of a cube or a finger, wherein one end of the cube or finger is connected to the edge 5 of the opening 2 and the opposite end extends into the opening 2 .

[0057] The protrusion 3 is configured to hold a charged particle microscopy sample 4. That is, the protrusion 3 must have a sufficient size to hold a charged particle microscopy sample 4, in particular an exfoliated sample 4, such as an exfoliated cryo-TEM sample 4.

[0058] The protrusions 3 may be of any size, but those skilled in the art will appreciate that the protrusions must preferably be large enough to hold the desired sample 4, while being small enough to avoid obscuring the sample 4. Any means may be used to attach the sample 4 to the protrusions 3. As a non-limiting example, the sample 4 may be attached by ion beam induced deposition (IBID).

[0059] Typically, in the sample carrier 1 of the present disclosure, the opening 2 may include at least two protrusions 3 extending into the opening 2 within a plane defined by the planar or substantially planar body. For example, the sample carrier 1 may include one, two, three, four, five or six protrusions 3.

[0060] In case there are more than one protrusion 3, the protrusions 3 may be located on the same side of the opening 2 or may be located on opposite sides of the opening. For example, the sample carrier 1 may include one, two, three, four, five or six protrusions 3 on one side of the opening 2 [TP346944USORG1], or alternatively, the sample carrier 1 may include one, two, three, four, five or six protrusions 3 on one side of the opening 2 and one, two, three, four, five or six protrusions 3 on opposite sides of the opening 2.

[0061] In a preferred aspect, the sample carrier 1 may include one, two or three protrusions 3 on one side of the opening 2 and one, two or three protrusions 3 on the opposite side of the opening 2. For example, the sample carrier 1 may include a circular planar or substantially planar body with a centrally located rectangular opening 2, wherein the opening 2 includes three protrusions 3 on one long edge 5 of the rectangular opening 2 and three protrusions 3 on the opposite long edge 5 of the rectangular opening 2.

[0062] In the case where there are more than one protrusion 3 (ie, at least two protrusions 3), there may generally be a gap (space) between each protrusion. The gap between each protrusion 3 may be the same or different. In a preferred aspect, the gap between each protrusion 3 may be the same.

[0063] The gaps between the protrusions 3 may be of any size suitable to avoid shielding the sample 4. Typically, the gap size between each protrusion 3 may be about 0.01 mm to about 0.1 mm, such as about 0.04 mm to about 0.07 mm. In the case where there are more than one protrusion 3 (i.e., at least two protrusions 3), it may be preferred that the protrusions 3 are staggered relative to each other. "Staggered" is intended to mean that the end of the protrusion 3 that is not connected to the edge 5 of the opening 2 has extended into the opening 2 a different distance than other protrusions 3 extending from the same edge 5, and / or the protrusions 3 are positioned at different heights along the edge 5 of the opening 2.

[0064] It may be preferred that, in the case where two or more protrusions 3 are positioned on one side of the opening 2, the end of each protrusion 3 that is not connected to the edge 5 of the opening 2 extends in a stepwise manner to a farther distance into the opening 2 than the protrusion 3 directly adjacent thereto. For example, in the case where the opening 2 is rectangular, it may be preferred that the protrusion 3 located at the centermost on the longest edge 5 of the rectangular opening 2 is the smallest (extends the shortest / smallest distance into the opening 2), wherein the protrusions 3 gradually extend to a farther distance into the opening 2, wherein the protrusion 3 located closest to the short edge 5 of the rectangular opening 2 is the longest (extends the longest distance into the opening 2), or vice versa.

[0065] It will be appreciated that the distance that the protrusions 3 extend into the opening 2 will depend on the size of the opening 2. For example, the longest protrusion 3 may extend half the distance of the opening 2.

[0066] In one aspect of the present disclosure, the protrusions 3 may increase or decrease the amount by which they extend into the opening 2 by the same amount each time. In the case where there are more than one protrusion 3 (i.e., at least two protrusions 3), it may be preferred that the protrusions 3 are not mirror images of each other. That is, they are not reflection symmetrical (i.e., they are not positioned directly opposite each other on either side of the opening 2). However, the arrangement of the protrusions 3 in the sample carrier 1 may be rotationally symmetrical. In the case where more than one protrusion 3 is located on opposite sides of the opening 2 [TP346944USORG1], the protrusions 3 may be reverse mirror images on opposite sides of the opening 2. Additionally or alternatively, in the case where there are more than one protrusion 3 (i.e., at least two protrusions 3), the protrusions 3 may be staggered in a direction substantially orthogonal to the planar body or in a direction substantially parallel to the planar body. At least two protrusions 3 located on opposite sides may be positioned not directly opposite. The at least two protrusions 3 are staggered in a direction substantially parallel to the planar body.

[0067] The position of the protrusion 3 directly affects the positioning of the sample 4 attached thereto. As mentioned above, the known sample carrier 1 has problems associated with a restricted tilted position of the sample 4 and / or shielding.

[0068] The unique positioning of the protrusions 3 within the sample carrier 1 allows the sample 4 (such as a peeled sample) to be observed with a larger tilt range than expected. In particular, the protrusions 3 of the sample carrier 1 as defined above can provide a total tilt range of + / - 70 degrees or 140 degrees. The tilt range is configured so as not to obscure the thin slices. All thin slices are positioned so as not to be aligned with each other to minimize the risk of damaging another thin slice when working on the thin slice. In addition, for example, where there is more than one protrusion 3, it will be possible to analyze more than one sample 4 in different orientations. The sample 4 can be positioned on each protrusion 3 so that the sample can be observed in a "normal view" and a "plan view" without having to reposition the sample carrier 1.

[0069] The sample carrier 1 may be formed of any material suitable for use in a charged particle microscope, such as molybdenum or copper. For example, the sample carrier 1 may be formed of a single sheet of metal, such as copper. The carrier 1 may comprise an integral mechanical support profile.

[0070] The present disclosure also provides a method for preparing a sample 4 for detection in a charged particle microscope, the method comprising: providing a sample carrier 1 as defined above, connecting the sample carrier 1 to a mechanical stage device of the charged particle microscope, providing a sample 4, and connecting the sample 4 to a grid member of the sample carrier 1.

[0071] An advantage of the sample carrier 1 as defined above is that it can be positioned within a C-shaped clamp ring. In particular, the raised edge 5 acts as a support profile and allows the sample carrier 1 to be positioned in a charged particle microscope without using a C-shaped clamp ring that is standard in electron microscopy. Thus, the method disclosed herein can be performed without a C-shaped clamp ring or other similar holding device.

[0072] Various features and advantages of the disclosure are set forth in the following claims.

Claims

1. A sample carrier for a charged particle microscope, the sample carrier comprising: A flat or substantially flat body; an opening disposed in the planar or substantially planar body; and At least one protrusion extends into the opening within a plane defined by the planar or substantially planar body, wherein the protrusion is configured to hold a charged particle microscopy sample.

2. A sample carrier according to claim 1, wherein the opening comprises at least two protrusions extending into the opening within a plane defined by the planar or substantially planar body. The sample carrier according to claim 2 , wherein the at least two protrusions are staggered. The sample carrier according to claim 2 , wherein the at least two protrusions are located on the same side of the opening.

5. The sample carrier of claim 2, wherein the at least two protrusions are located on opposite sides of the opening.

6. The sample carrier of claim 5, wherein the at least two protrusions are mirror images on opposite sides of the opening.

7. The sample carrier according to claim 5, wherein the at least two protrusions located on opposite sides are positioned not directly opposite.

8. The sample carrier of claim 3, wherein the at least two protrusions are staggered in a direction substantially orthogonal to the planar body.

9. The sample carrier of claim 3, wherein the at least two protrusions are staggered in a direction substantially parallel to the planar body.

10. The sample carrier of claim 1, wherein the opening is centrally located in the planar or substantially planar body.

11. The sample carrier of claim 1 , wherein the opening is substantially rectangular.

12. The sample carrier of claim 1, wherein the charged particle microscopy sample is an exfoliation sample.

13. The sample carrier of claim 1, wherein the protrusion provides a tilt range of 140 degrees.

14. The sample carrier of claim 1, wherein the charged particle microscope is a transmission electron microscope.

15. The sample carrier of claim 1, wherein the carrier is formed from a single sheet of metal and includes an integral mechanical support profile.

16. The sample carrier of claim 15, wherein the metal sheet is copper.

17. A method of preparing a sample for examination in a charged particle microscope, the method comprising: providing a sample carrier comprising a planar or substantially planar body, an opening disposed in the planar or substantially planar body, and at least one protrusion extending into the opening within a plane defined by the planar or substantially planar body, wherein the protrusion is configured to hold a charged particle microscopy sample; connecting the sample carrier to a mechanical stage assembly of the charged particle microscope; providing the charged particle microscopy sample; and The sample is connected to the grid member of the sample carrier. The method of claim 17 , wherein the sample is a peeled sample.

19. The method of claim 17, wherein the charged particle microscope is a transmission electron microscope.

20. The method of claim 19, wherein the transmission electron microscope is a cryo-TEM.

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