Patent Description:
The invention also relates to a charged particle microscope in which such a method can be enacted.

The term "cryogenic" should be interpreted as referring to temperatures at or below -<NUM>. Typical examples of cryogenic fluids ("cryogens") include liquid nitrogen, liquid helium, liquid ethane, liquid propane, liquid oxygen, and mixtures hereof. A cryogenic cell ("cryo box") as referred to here is a cell (container/capsule/box) in which a specimen can be held at cryogenic temperature, and in which:.

The cell will typically be comprised of material with good thermal conductivity (e.g. comprising suitable metal) that is thermally connected to a vat of cryogen, for example.

Charged-particle microscopy is a well-known and increasingly important technique for imaging microscopic objects, particularly in the form of electron microscopy. Historically, the basic genus of electron microscope has undergone evolution into a number of well-known apparatus species, such as the Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), and Scanning Transmission Electron Microscope (STEM), and also into various sub-species, such as so-called "dual-beam" apparatus (e.g. a FIB-SEM), which additionally employ a "machining" Focused Ion Beam (FIB), allowing supportive activities such as ion-beam milling or Ion-Beam-Induced Deposition (IBID), for example. More specifically:.

More information on some of the topics elucidated here can, for example, be gleaned from the following Wikipedia links: http://en. org/wiki/Electron microscope http://en. orq/wiki/Scanning electron microscope http://en. orq/wiki/Transmission electron microscopy http://en. orq/wiki/Scanning transmission electron microscopy.

As an alternative to the use of electrons as irradiating beam, charged particle microscopy can also be performed using other species of charged particle. In this respect, the phrase "charged particle" should be broadly interpreted as encompassing electrons, positive ions (e.g. Ga or He ions), negative ions, protons and positrons, for instance. As regards non-electron-based charged particle microscopy, some further information can, for example, be gleaned from references such as the following: https://en. org/wiki/Focused ion beam http://en. org/wiki/Scanning Helium Ion Microscope.

It should be noted that, in addition to imaging and performing (localized) surface modification (e.g. milling, etching, deposition, etc.), a charged particle microscope may also have other functionalities, such as performing spectroscopy, examining diffractograms, etc..

In all cases, a Charged Particle Microscope (CPM) will comprise at least the following components:.

In the case of a transmission-type microscope (such as a (S)TEM, for example), a CPM will additionally comprise:.

In what follows, the invention may - by way of example - sometimes be set forth in the specific context of electron microscopy; however, such simplification is intended solely for clarity/illustrative purposes, and should not be interpreted as limiting.

An example of a cryogenic specimen is a vitrified biological sample, for instance, in which water content has been congealed into an amorphous solid form that is different from conventional, crystalline water ice. Such vitrification can occur using, for example, a rapid cooling / plunge cooling technique as set forth in <CIT>, <CIT>and <CIT> (all assigned to the assignee of the present application).

Non-biological examples include, for instance, cooled ceramic/metal/semiconductor materials for superconductivity studies, cooled metals (or other construction materials) for low-temperature brittleness studies, etc..

When studying a cryogenic specimen in a CPM, the usable lifetime of the specimen is currently limited by the deposition rate of environmental water-ice contamination on the specimen, due to the inevitable presence of unwanted water molecules in the CPM's vacuum chamber. In many cases, a specimen will be unusable after a time period of the order of just a day. Since many specimens are rare and/or expensive and/or have been subjected to time-consuming and costly preparation processes (such as TEM specimen lift-out and milling in a FIB-SEM), such rapid deterioration due to water-ice contamination is very frustrating.

<CIT> discloses an anticontaminator comprising a metallic cryogenic shield, a tank for holding a cryogenic liquid, and a thermal pathway between the shield and the tank.

<CIT> discloses a specimen placed in a cylindrical portion and two inner ring plates of a cold trap for preventing contamination build-up.

It is an object of the invention to address this issue. More specifically, it is an object of the invention to provide a modified architecture/method for cryogenic specimen study in a CPM that can lead to a significant increase in usable lifetime of the specimen in the CPM.

These and other objects are achieved in a method as set forth in the opening paragraph above, and as defined in claim <NUM>, and an apparatus as defined in claim <NUM>.

As already mentioned above, gaseous water in the CPM's vacuum chamber is responsible for water-ice contamination on a cryogenic specimen. When the specimen is shielded in a cryogenic cell, water in the vacuum of the beam conduit is the main contributor to such contamination. The elongate tube (snout) of the present invention has a two-pronged effect on such contamination, in that:.

To give a non-limiting example: assuming a cryogenic cell with a height (along the beam axis) of ca. <NUM>, and using an elongate tube - according to the present invention - with a height of ca. <NUM>-<NUM> and a diameter of ca. <NUM> (internal) / <NUM> (external), the inventors have observed a reduction in water ice contamination of a specimen within the cryogenic cell by a factor of five - which represents a huge extension of usable specimen lifetime.

A further effect of the elongate tube of the present invention is that it tends to increase the stiffness of the cryogenic cell assembly, thus giving it a higher eigenfrequency. As a result, it tends to suffer less from vibration effects, and associated electrical eddy currents.

In an embodiment of the invention, the elongate tube is configured to be demountable from the cryogenic cell. Such a construction is advantageous in that it makes it easier to place/remove the cryogenic cell within the cramped specimen space below (and within) the illuminator system. One way of achieving such a demountable structure is to provide an extremity of the tube and a receptive portion of the cryogenic cell with cooperating screw threads, thereby allowing the tube to be screwed onto / off of the cell at will. In an alternative approach, the tube is thermally clamped into a cooperating cavity in the cell, by virtue of a difference in thermal coefficient of expansion between said extremity/cavity; in such a structure, the tube and cavity can be easily separated at room temperature, but become tightly clamped together when cooled, for example. Complementary material pairs for this latter embodiment include, for example, aluminum / titanium.

An elongate tube as used in the present invention can, for example, be manufactured by conventional machining techniques, such as drilling a longitudinal bore along the cylindrical axis of a solid metallic cylinder, for example. Alternatively, one could fold/roll a metal sheet in upon itself, so as to transform it into a cylindrical form, for instance. Yet another alternative would be to cast the tube using an annular-cylindrical mold into which liquid metal is poured. The skilled artisan will be able to select a manufacturing technique that is suited to the needs of a given situation.

The invention will now be elucidated in more detail on the basis of exemplary embodiments and the accompanying schematic drawings, in which:.

In the Figures, where pertinent, corresponding parts are indicated using corresponding reference symbols.

<FIG> (not to scale) is a highly schematic depiction of an embodiment of a CPM M in which the present invention is implemented; more specifically, it shows an embodiment of a TEM/STEM (though, in the context of the current invention, it could just as validly be an ion-based microscope or a SEM, for example). In the Figure, within a general cabinet/cover <NUM>, 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 <NUM> produces a beam B of electrons that propagates along an electron-optical axis B' and traverses an illuminator system (charged particle beam column) <NUM>, serving to direct/focus the electrons onto a chosen part of a specimen S (which will generally be (locally) thinned/planarized). Also depicted is a deflector <NUM>, which (inter alia) can be used to effect scanning motion of the beam B. Where possible, the vacuum enclosure V will generally "hug" the axis B', taking the form of a relatively narrow beam conduit B" (e.g. of the order of ca. <NUM> in diameter) through the illuminator <NUM>, but widening out where necessary to accommodate certain structures (such as the items H, <NUM>, <NUM>, <NUM>, and <NUM> discussed below, for example).

The specimen S is held on a specimen holder H that can be positioned in multiple degrees of freedom by a positioning device / stage A, which moves a cradle A' into which holder H is (removably) affixed; for example, the specimen holder H may comprise a finger that can be moved (inter alia) in the XY plane (see the depicted Cartesian coordinate system), with motion parallel to Z and tilt about X/Y also typically being possible. Such movement allows different parts of the specimen S to be illuminated / imaged / inspected by the electron beam B traveling along axis B' (in the Z direction), and/or allows scanning motion to be performed as an alternative to beam scanning. When the specimen S is a cryogenic specimen, then:.

The electron beam B will interact with the specimen S in such a manner as to cause various types of "stimulated" radiation to emanate from the specimen 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 nominally detected with the aid of analysis device <NUM> (when the cryogenic cell is not deployed), which might be a combined scintillator/photomultiplier or EDX (Energy-Dispersive X-Ray Spectroscopy) module, for instance; in such a case, an image could be constructed using basically the same principle as in a SEM. However, alternatively or supplementally, one can study electrons that traverse (pass through) the specimen S, exit/emanate from it and continue to propagate (substantially, though generally with some deflection/scattering) along axis B'. Such a transmitted electron flux enters an imaging system (projection lens) <NUM>, which will generally comprise a variety of electrostatic / magnetic lenses, deflectors, correctors (such as stigmators), etc. In normal (non-scanning) TEM mode, this imaging system <NUM> can focus the transmitted electron flux onto a fluorescent screen <NUM>, which, if desired, can be retracted/withdrawn (as schematically indicated by arrows <NUM>') so as to get it out of the way of axis B'. An image or diffractogram of (part of) the specimen S will be formed by imaging system <NUM> on screen <NUM>, and this may be viewed through viewing ports 28a, 28b located in suitable parts of the walls of enclosure V / cabinet <NUM>. The retraction mechanism for screen <NUM> may, for example, be mechanical and/or electrical in nature, and is not depicted here.

As an alternative to viewing an image/diffractogram on screen <NUM>, one can instead make use of the fact that the depth of focus of the electron flux leaving imaging system <NUM> is generally quite large (e.g. of the order of <NUM> meter). Consequently, various other types of analysis apparatus can be used downstream of screen <NUM>, such as:.

It should be noted that the order/location of items <NUM>, <NUM> and <NUM> is not strict, and many possible variations are conceivable. For example, spectroscopic apparatus <NUM> can also be integrated into the imaging system <NUM>.

Note that controller (computer processor) <NUM> is connected to various illustrated components via control lines (buses) <NUM>'. This controller <NUM> can provide a variety of functions, such as synchronizing actions, providing setpoints, processing signals, performing calculations, and displaying messages/information on a display device (not depicted). Needless to say, the (schematically depicted) controller <NUM> may be (partially) inside or outside the cabinet <NUM>, and may have a unitary or composite structure, as desired.

The skilled artisan will understand that the interior of the enclosure V does not have to be kept at a strict vacuum; for example, in a so-called "Environmental TEM/STEM", a background atmosphere of a given gas is deliberately introduced / maintained within the enclosure V.

Turning now to <FIG>, this shows a magnified view of the vicinity of the cryogenic cell C of <FIG>. This cryogenic cell C is essentially a box with thermally conducting (e.g. metallic) walls that are maintained at cryogenic temperatures, by virtue of intimate thermal contact with cooled specimen holder H and as a result of a dedicated flow of cryogen through a cooling tube in intimate contact with said walls. The cryogenic cell C may, for example, take the form of a pillbox / squat cylinder, e.g. with a diameter (in the XY plane) of ca. <NUM> and a height parallel to Z (and B') of ca. It includes:.

To this end, both apertures <NUM>, <NUM> are disposed (e.g. centered) upon beam axis B', and can have a diameter of ca. <NUM>, for example.

In <FIG>, the set-up of <FIG> has been modified in accordance with (an embodiment of) the present invention. To this end, an elongate tube (snout) <NUM> now extends from the upper (beam entry) side of the cryogenic cell C and protrudes into beam conduit B", thereby reaching upward into illuminator system <NUM>. This tube <NUM> encloses (embraces) beam axis B', and effectively displaces beam entry aperture <NUM> up into the illuminator <NUM>, creating a new entry aperture <NUM> at an elevated location relative to the old aperture <NUM>. The elongate tube <NUM> has thermally conducting (e.g. metallic) walls that connect intimately to the cryogenic cell C, as a result of which the tube <NUM> is also cooled to cryogenic temperatures. By way of example, the tube <NUM> can have a height (parallel to Z) of ca. <NUM>, and an internal diameter (in the XY plane) of ca. <NUM> - though other values are, of course, possible. As set forth above, the tube <NUM> may, if desired, be demountable/detachable from the cryogenic cell C; alternatively, it can be permanently attached to the cell C, e.g. using a solder joint, or adhesive connection, for instance.

Claim 1:
A method of examining a cryogenic specimen (S) in a Charged Particle Microscope, comprising:
- Providing the specimen (S) in a cryogenic cell (C) on a specimen holder (H);
- Directing a charged particle beam from a source and along an axis (B') through an evacuated beam conduit (B") of an illuminator system so as to irradiate at least a portion of the specimen therewith;
- Using a detector (<NUM>, <NUM>, <NUM>) to detect radiation emanating from the specimen (S) in response to said irradiation,
- Configuring said cell to comprise an elongate tube (<NUM>) that extends within said beam conduit into said illuminator system and encloses said axis;
- Maintaining said tube at a cryogenic temperature at least during said irradiation,
wherein:
- Said specimen holder is maintained at a cryogenic temperature using a temperature control assembly (T);
- Said elongate tube has thermally conducting walls that connect intimately to said cryogenic cell, as a result of which the tube is cooled to a cryogenic temperature; characterized in that
- Said cryogenic cell is maintained at a cryogenic temperature by virtue of intimate thermal contact with said specimen holder.