Cryogenic cell for mounting a specimen in a charged particle microscope

#The techniques disclosed herein relate to methods of examining a cryogenic specimen in a Charged Particle Microscope, comprising: Providing the specimen in a cryogenic cell on a specimen holder; Directing a charged particle beam from a source and along an axis through an evacuated beam conduit of an illuminator system so as to irradiate at least a portion of the specimen therewith; Using a detector to detect radiation emanating from the specimen in response to said irradiation. The disclosed techniques also relate 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 −150° C. 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: A first aperture allows access of the irradiating beam to the specimen; A (discretionary) second aperture allows radiation emanating from the specimen to reach said detector. 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: In a SEM, irradiation of a specimen by a scanning electron beam precipitates emanation of “auxiliary” radiation from the specimen, in the form of secondary electrons, backscattered electrons, X-rays and cathodoluminescence (infrared, visible and/or ultraviolet photons), for example; one or more components of this emanating radiation is/are then detected and used for image accumulation purposes. In a TEM, the electron beam used to irradiate the specimen is chosen to be of a high-enough energy to penetrate the specimen (which, to this end, will generally be thinner than in the case of a SEM specimen); the transmitted electrons emanating from the specimen can then be used to create an image. When such a TEM is operated in scanning mode (thus becoming a STEM), the image in question will be accumulated during a scanning motion of the irradiating electron beam. 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. 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.

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

#FIG. 1 renders a longitudinal cross-sectional elevation view of an embodiment of a CPM in which the present invention can be implemented.

#FIG. 2A renders a magnified view of part of the subject of FIG. 1, more specifically a vicinity of the specimen/cryogenic cell.

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

#FIG. 1 (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 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 produces a beam B of electrons that propagates along an electron-optical axis B′ and traverses an illuminator system (charged particle beam column) 6, 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 8, 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. 0.5 cm in diameter) through the illuminator 6, but widening out where necessary to accommodate certain structures (such as the items H, 26, 30, 32, and 34 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 specimen holder H can be maintained at a cryogenic temperature using a (schematically depicted) temperature control assembly T. This may, for example, comprise a conductive (e.g. metallic) wick that is thermally connected to the holder H and is immersed in a bath of cryogen, or a pipe system carrying a circulating cryogen, for example. The specimen holder H can be provided with a (cooled) cryogenic cell C, in which the specimen S is seated, with beam entry/exit apertures disposed along axis B′.