Patent Description:
For the purposes of this disclosure, the following abbreviations are explained herein: SEM (scanning electron microscope), STEM (scanning transmission electron microscope), TEM (transmission electron microscope), ED (electron diffraction), EDS being equivalent with EDX (energy-dispersive X-ray spectroscopy), sCMOS (scientific complementary metal-oxide-semiconductor), AES (atomic emission spectroscopy), EELS (electron energy loss spectroscopy), BSE (back-scattered electrons), EM (electromagnetic), ES (electrostatic), MS (magnetostatic).

Conventional TEM design is connected with increased demands for space and conditions, high energy consumption and consumption of supporting media, mainly cooling water. Its installation is complicated and often requires construction work. Electromagnetic components, such as electromagnetic coils, often used together with polepieces, are conventionally used in transmission electron microscopes for focusing and magnifying, which requires a space-demanding external cooling system. Therefore, their operation is limited to special laboratories and means a significant ecological load.

It has been known that lower electron energy (e. below <NUM> kV) in low-voltage electron microscopes avoids in many cases sample damage, increases robustness of a microscope, allows for miniaturisation of the whole device and lowers its sensitivity to vibrations. Equally, magnetostatic and electrostatic optics in electron microscopes does not create any heat loss, thus avoiding cooling and allowing further miniaturisation as opposed to conventionally used electromagnetic optics. However, a further trend in miniaturisation is hindered by other design aspects, such as arranging detectors and optics together in positions necessary for good detection efficiency.

Moreover, electron microscopy has seen another trend in providing electron microscope capable of multiple analytic modes at the same time or at least within the same microscope to exploit potential analytical outcomes of a given sample while reducing time and equipment required for such an analysis.

What is more, insulating the optics from the accompanying electronics of the microscope and its negative impact on the analysis has also caused problems in miniaturisation of electron microscopes.

A multi-mode low-voltage electron microscope operative in the accelerating voltage range of <NUM> to <NUM> kV and enabling STEM, TEM and ED sample analysis is known as LVEM25 (by DELONG, see e.g. https://delongamerica. com/lvem25). The accelerating voltage of <NUM> kV is used for TEM analysis and the accelerating voltage of <NUM> to <NUM> kV is used for STEM analysis, thus securing sufficient beam penetration of a sample, better imaging properties, higher contrast and the possibility to analyse both stained and non-stained samples. It comprises a magnetostatic condenser lens, a magnetostatic objective lens and an electrostatic projective lens, which removes the need to cool down these lenses and results in a compact microscope. Each magnetostatic lens, i. the magnetostatic condenser lens and the magnetostatic objective lens, comprises at least one permanent magnet and at least one polepiece in order to create a field of magnetostatic lens. Each electrostatic lens, i. the electrostatic projective lens, comprises at least one shaped electrode connected to supply voltage or ground potential in order to create a field of electrostatic lens. An electron beam source is a <NUM> kV Schottky-type field emission gun providing high brightness and spatial coherence with high contrast. A sample stage can be moved in a precise way by means of joystick-controlled piezoelectric actuators. A combination of a diaphragm pump and a turbomolecular pump assures long maintenance-free operation and an ion getter pump provides vibration-free environment. The TEM mode uses an sCMOS camera as a detector. A disadvantage of the LVEM25 microscope is the absence or further detection modes.

A similar multi-mode low-voltage electron microscope operative in the accelerating voltage of <NUM> kV and enabling SEM, STEM, TEM and ED sample analysis is known as LVEM5 (by DELONG). The LVEM5 microscope differs from the above-mentioned LVEM25 microscope in the electron beam source being a <NUM> kV Schottky-type field emission gun, providing accelerating voltage of <NUM> kV for SEM, STEM and TEM analysis. The TEM mode uses an sCMOS camera as a detector and the SEM mode uses a detector for back-scattered electrons. A disadvantage of the LVEM5 microscope is the absence or further detection modes.

Another multi-mode electron microscope is disclosed in <CIT>, enabling SEM, STEM, EDX, AES, EELS, Auger spectroscopy analysis and quantitative/qualitative elemental analysis modes. It is operative in accelerating voltage below <NUM> kV (e. <NUM> kV or <NUM> kV) in the SEM and STEM modes, securing high contrast and electron beam stability and avoiding charge accumulation and sample damage. This document also discloses a transmission electron microscope used in comparative examples. A disadvantage of the microscope disclosed in <CIT> is the absence or further detection modes. Furthermore, the embodiment of EDX detection is not sufficiently disclosed in terms of structural features.

EDX detectors, including a collimator, are conventionally mounted onto a microscope chamber, which requires a great deal of precision in order to direct the collimator such that it detects as much desired signal originating from the sample as possible and at the same time, as little parasitic signal not originating from the sample as possible. The aspect of precision gains importance in trends in miniaturisation of microscope chambers or if another detector, such as an SEM detector, is present in the vicinity.

Further related multi-mode electron microscopes are disclosed in <CIT> (SEM, STEM and TEM modes, accelerating voltage not mentioned) and <CIT> (SEM, STEM and TEM modes, accelerating voltage being <NUM> to <NUM> and it can be lowered as low as <NUM> kV).

An arrangement of an electron microscope and an EDS detector for detecting a signal of energy-dispersed X-ray radiation in an electron microscope is disclosed in <CIT>. Based on <FIG> and par. <NUM> and <NUM> of said US patent application, the electron microscope comprises an objective polepiece and an EDS detector comprising a support ring (being a part of a collimator) attached to the objective polepiece.

The publication of <NPL> and Fig. <NUM> a SEM having an EDX detector which is arranged between objective lens polepieces essentially co-planarly and laterally with respect to a sample holder and which does not comprise a collimator.

As seen from above, known multi-mode electron microscope operative in accelerating voltage range of less than <NUM> kV allow for an analysis of a sample only in certain modes, being either STEM, TEM and ED modes, or SEM, STEM and EDX modes. Based on the prior art, there is a need to provide a multi-mode electron microscope operative in accelerating voltage range of less than <NUM> kV that would combine more analysis modes and that would provide an analysis of a sample in these modes essentially (almost or actually) simultaneously.

It is therefore an object of this invention to provide a multi-mode electron microscope operative in accelerating voltage range of <NUM>-<NUM> kV, which combines an EDS mode with at least one of STEM, TEM, ED and optionally, SEM modes.

According to the invention, the above-mentioned object is achieved by a multi-mode low-voltage electron microscope according to independent claim <NUM> and dependent claims <NUM> to <NUM>. The electron microscope is operative in the accelerating voltage range of <NUM>-<NUM> kV, such as at <NUM> kV, <NUM> kV, <NUM> kV, <NUM> kV, <NUM> kV or <NUM> kV, and comprises in the following order based on the direction of a primary electron beam: an electron beam source to generate the primary electron beam, a first magnetostatic condenser lens means, a second magnetostatic condenser lens means, a condenser aperture, a sample holder, a magnetostatic objective lens means, an objective aperture, a first electrostatic projective lens means, and an end detection system. The second magnetostatic condenser lens means and the magnetostatic objective lens means together comprise a first objective polepiece and a second objective polepiece with a sample holder arranged between the respective first and second objective polepieces.

The end detection system comprises a detection screen (commonly denoted as fluorescent screen in TEM applications or scintillator screen in SEM applications) and at least one detector selected from a STEM detector configured to detect a signal of transmitted electrons, a TEM detector configured to detect a signal of transmitted electrons and/or an ED detector configured to detect a signal of diffracted electrons.

The underlying idea of this invention is that an EDS detector configured to detect a signal of energy-dispersed X-ray radiation (EDS signal) is arranged between the first objective polepiece and the second objective polepiece, essentially co-planarly and laterally with respect to the sample holder. The EDS detector comprises a collimator attached to the second objective polepiece. Under "attached" a mechanical attachment is herein understood. The advantage of mounting the collimator to the second objective polepiece instead of mounting it to the EDS detector further mounted to a microscope chamber is an easier, more flexible, yet more precise way of providing all the necessary structural elements for EDS detection. Importantly, such an advantage allows miniaturisation of the whole immersion objective, and by extension of the whole electron microscope. The EDS detector having the collimator according to the invention is robust enough to provide a sufficiently noise-free EDS signal originating from the sample in a miniaturised immersion objective assembly.

In general, collimators are commonly attached to EDS detectors mounted on the microscope chamber and serve to limit the incidence of EDS signals, which do not originate directly from a sample, on the detector. A collimator is usually tubular in shape, i. an enclosure with at least two (and usually two) open ends, and is usually made of a single element material, whose spectral lines can be easily subtracted in the analysis, such as zirconium, gold or pure graphite.

The first and second magnetostatic condenser lens means serve to alter the properties of a light signal in the end detection system and, as it is generally known from prior art, each magnetostatic condenser lens means comprises at least one permanent magnet and at least one polepiece in order to create a field of magnetostatic condenser lenses. Similarly, and as it is known from prior art, the magnetostatic objective lens means comprises at least one permanent magnet and at least one polepiece in order to create a field of magnetostatic objective lens. The first and second objective polepieces together with an assembly of permanent magnets create a strong magnetic field in an immersion objective. The magnetic field of the immersion objective has two parts, where the part ahead of the sample functions as the second magnetostatic condenser lens and the part behind the sample as the magnetostatic objective lens. The condenser aperture can be embodied as a condenser multi-aperture sheet including a plurality of condenser apertures of different diameters. Similarly, the objective aperture can be embodied as an objective multi-aperture sheet including a plurality of objective apertures of different diameters. The first electrostatic projective lens means serves to magnify the image and its projection on the detection screen and, as it is generally known from prior art, the first electrostatic projective lens means comprises at least one shaped electrode connected to supply voltage or ground potential in order to create a field of electrostatic projective lens.

As an example, the accelerating voltage can be <NUM> kV for TEM mode, <NUM> or <NUM> kV for STEM/SEM modes, <NUM>, <NUM> or <NUM> kV for EDS mode. Different detection modes are achieved by different processing and parameters of the primary electron beam.

Preferably, the multi-mode low-voltage electron microscope comprises a SEM detector configured to detect a signal of back-scattered electrons (BSE) and arranged between the first objective polepiece and the sample holder.

Preferably, the TEM detector and the ED detector are constructed as a combined TEM/ED detector comprising a camera, wherein in order to change between these modes, the rear focal plane, in which the diffraction signals are detected, needs to be re-focused by means of the electrostatic projective leans means. Preferably, the STEM detector comprises a photomultiplier tube.

Preferably, the STEM detector and/or the TEM detector is/are configured to detect transmitted electrons in a bright field detection mode and in a dark field detection mode. For TEM mode, the bright and dark field detection modes are switched by beam inclination and subsequent selection of a suitable diffracted beam. For STEM mode, the bright and dark field detection modes are switched by changing the position of a detector aperture.

Preferably, the multi-mode low-voltage electron microscope comprises a tilting mirror arranged in the end detection system between the detection screen and the STEM, TEM and ED detectors such that the tilting mirror allows a light signal generated by the detection screen to pass to the TEM and/or ED detector in a first position and to the STEM detector in a second position. As a result, it is no longer needed to move the detectors in a demanding manner when changing modes.

Preferably, the sample holder is vertically adjustable.

Preferably, the multi-mode low-voltage electron microscope comprises an electrostatic condenser lens means arranged between the electron beam source and the first magnetostatic condenser lens means. The electrostatic condenser lens means is adjustable and, as it is generally known from prior art, the electrostatic condenser lens means comprises at least one shaped electrode connected to supply voltage or ground potential in order to create a field of electrostatic condenser lens.

Preferably, the multi-mode low-voltage electron microscope comprises a second electrostatic projective lens means arranged between the first electrostatic projective lens means and the end detection system. The second electrostatic projective lens means allows to increase the extent of magnification and, as it is generally known from prior art, the second electrostatic projective lens means comprises at least one shaped electrode connected to supply voltage or ground potential in order to create a field of electrostatic projective lens.

Preferably, the end detection system comprises a light objective arranged between the detection screen and at least one detector, or when the tilting mirror is present, between the detection screen and the tilting mirror. The light objective assures a decrease in sensitivity to undesired vibrations and electromagnetic interference with accompanying electronics. The light objective further assures that the signal of transmitted electrons reaches the STEM and/or TEM detectors.

Preferably, the multi-mode low-voltage electron microscope comprises integrated control electronics and high voltage supply and a cooling means, such as a fan or a plurality of fans. The remaining part of the electron microscope (i. the column with the features specified in previous paragraphs) is electromagnetically shielded from the control electronics and high voltage supply and the cooling means by means of magnetic shielding and/or thermally shielded from the control electronics and high voltage supply and the cooling means by means of thermal shielding and/or vibrationally shielded from the control electronics and high voltage supply and the cooling means by means of a cooling means damper and/or a column damper and/or a camera damper. The microscope is preferably at least partially shielded by means of an X-ray shield, inter alia comprising the camera damper. In particular the microscope column, including the EDS detector, is shielded by the X-ray shield in order to protect the operator from X-ray radiation.

In addition, zone power control of the control electronics and high voltage supply and the cooling means can be provided in order to prevent heat transfer therefrom to the remaining part of the electron microscope (i. the column). A system of thermal shielding and zone heat control conducting the heat from the electronics enables to connect the column and the electronics into a single structure, thus saving more space.

Preferably, the multi-mode low-voltage electron microscope comprises an ion pump configured to create vacuum and integrally coupled with at least one recovery baking element connected to a baking unit for automatized vacuum recovery (e. after transport or vacuum break). The baking unit controls and powers heating of the ion pump by means of at least one recovery baking element arranged integrally with the ion pump, which can easily put the microscope into operation in several hours. The heating of the ion pump causes the release of adsorbed molecules (usually water from air moisture) from the respective inner surfaces, which is then followed by evacuating these molecules by the ion pump. Since only a part of the microscope is heated, the baking unit refers to a soft baking function. Because the recovery baking elements arranged integrally with the ion pump operate on low, secure voltage with precisely defined power, no temperature sensor regulation is necessary, and the system is simple and robust.

In the second, non-claimed aspect which does not from part of the invention, the above-mentioned object is achieved by an arrangement of an electron microscope and an EDS detector for detecting a signal of energy-dispersed X-ray radiation in the electron microscope. The electron microscope of that non-claimed generally comprises an objective polepiece and the EDS detector comprises a collimator attached to the objective polepiece. The advantages of such mounting are mentioned above and apply also in other electron microscopes aiming to provide EDS detection mode and comprising a general objective polepiece. As an example, the collimator can be attached to an objective polepiece behind a sample holder based on the direction of a primary electron beam, such as in the multi-mode low-voltage electron microscope according to the invention. As another example, the collimator can be attached to an objective polepiece ahead of a sample holder based on the direction of a primary electron beam, such as in scanning electron microscopes generally having the sample holder arranged behind the objective polepiece(s).

An optics diagram of a conventional transmission electron microscope is shown in <FIG>. For the purposes of clarity, the diagram is shown upside down compared to a real microscope. The TEM comprises in the following order based on the direction of a primary electron beam (bottom to top): an electron beam source <NUM> to generate the primary electron beam, an electromagnetic condenser lens means <NUM>, a sample holder <NUM> for holding a sample, an electromagnetic objective lens means <NUM>, an objective aperture <NUM>, an assembly of intermediate lens means <NUM>, an electromagnetic projective lens means <NUM> and a detection screen <NUM>. Each electromagnetic element produces undesirable heat and requires cooling, which is often spatially demanding.

An optics diagram of the electron microscope according to the present invention as well as the prior art electron microscope titled LVEM25 (by DELONG) is shown in <FIG>. The electron microscope comprises in the following order based on the direction of a primary electron beam <NUM> (bottom to top, the beam <NUM> is also shown in <FIG>): an electron beam source <NUM> to generate the primary electron beam <NUM>, an electrostatic condenser lens means <NUM>, a first magnetostatic condenser lens means <NUM>, a second magnetostatic condenser lens means <NUM>, a condenser aperture <NUM>, a sample holder <NUM> for holding a sample, a magnetostatic objective lens means <NUM>, an objective aperture <NUM>, a first electrostatic projective lens means <NUM>, a second electrostatic projective lens means <NUM> and a detection screen <NUM>. The electrostatic and magnetostatic elements do not produce undesirable heat, therefore do not need to be cooled, which can advantageously lead to miniaturisation of the microscope.

A detailed arrangement of various detectors of the electron microscope according to the present invention is shown in <FIG>. The second magnetostatic condenser lens means <NUM> and the magnetostatic objective lens means <NUM> together comprise a first objective polepiece <NUM> (bottom) and a second objective polepiece <NUM> (top). The sample holder <NUM> is arranged between the objective polepieces <NUM>, <NUM>, which together with an assembly of permanent magnets create a strong, two-part magnetic field in an immersion objective. The part ahead of the sample holder <NUM> acts as the second magnetostatic condenser lens and the part behind the sample holder <NUM> as the magnetostatic objective lens.

An EDS detector <NUM> configured to detect a signal of energy-dispersed X-ray radiation is arranged between the first objective polepiece <NUM> and the second objective polepiece <NUM>. The EDS detector <NUM> is arranged essentially co-planarly and laterally with respect to the sample holder <NUM> (i. on the side of the sample holder <NUM>). The EDS detector <NUM>, itself attached to a microscope chamber, comprises a tubular collimator <NUM> attached to the second objective polepiece <NUM>. Moreover, a SEM detector <NUM> configured to detect a signal of back-scattered electrons is arranged between the first objective polepiece <NUM> and the sample holder <NUM>.

The detection screen <NUM> for generating a light signal <NUM> is comprised in an end detection system <NUM> together with a light objective <NUM>, a tilting mirror <NUM>, a STEM detector <NUM> comprising a photomultiplier tube and configured to detect a signal of transmitted electrons and a combined TEM/ED detector <NUM> comprising a camera (such as of sCMOS type) and configured to detect a signal of transmitted and diffracted electrons. The tilting mirror <NUM> is arranged between the light objective <NUM> and the STEM, TEM and ED detectors <NUM>, <NUM> such that it allows a light signal <NUM> generated by the detection screen <NUM> and modified by the light objective <NUM> to pass to the TEM/ED detector <NUM> in a first position and to the STEM detector <NUM> in a second position.

Results from an analysis of a gallium nitride lamella sample are shown in <FIG>. <FIG> and <FIG> show a sample overview and detailed sample overview in TEM mode under low magnification at <NUM> kV. <FIG> shows a sample part in TEM bright field mode at <NUM> kV. <FIG> and <FIG> show a sample part in TEM dark field mode at <NUM> kV at two different diffraction maxima. <FIG> shows an electron diffraction pattern of the sample at <NUM> kV, corresponding with TEM dark field mode in <FIG> and <FIG> shows a sample part in STEM bright field mode at <NUM> kV. <FIG> shows a sample part in SEM mode at <NUM> kV. <FIG> and <FIG> show a sample part in EDS mode at <NUM> kV with atom mapping (<FIG> for gallium, <FIG> for nitrogen, <FIG> for silicon). <FIG> shows a graphical representation of elemental sample analysis in EDS mode at <NUM> kV, showing characteristic Kα transitions (peaks from left to right: a typical peak region for nitrogen at approximately <NUM> keV, a dominant peak for gallium at approx. <NUM> keV, a dominant peak for silicon at approx. <NUM> keV, a dominant peak for gallium at approx. <NUM> keV).

An overall view of the electron microscope, including the features of integrated design are shown in <FIG>. The bottom part of the microscope comprises control electronics and high voltage supply <NUM>, which produces heat and needs to be cooled with a cooling means, such as fans, arranged on cooling means dampers <NUM>. The microscope column arranged in the middle must be electromagnetically, thermally and vibrationally shielded from the control electronics and high voltage supply <NUM> by means of magnetic shielding <NUM>, thermal shielding <NUM> and column dampers <NUM>. The camera arranged in the top and connected via cables <NUM> secured with cable clamping <NUM> to the control electronics <NUM> is also protected with a camera damper <NUM>. Overall, the whole microscope has an external acoustic cover <NUM> and anti-vibrational standing blocks <NUM>. There is also an ion pump <NUM> with recovery baking elements <NUM> and a baking unit <NUM> for automatized vacuum recovery, e. after transport or vacuum break.

An example electron microscope together with associated electronics is schematically shown in <FIG>. Only electronically-controlled components of the electron microscope are shown, i. without the permanent magnets, which are not electronically controlled. The electronically-controlled components include electronics-optics components, i. a gun chamber connected with conditioning (COND), two ion pumps (IP-A, IP-B) and a microscope chamber comprising a sample stage, an aperture stage, a projective, an octupole, two lenses and connected to gauge vacuum, an EDS detector, an ion pump (IP-C) and a turbomolecular pump (TMP). The electronically-controlled components also include light-optics components, i. a light objective, a STEM detector, a camera (a combined TEM/ED detector). The gun chamber is powered and controlled by a gun high voltage power supply unit. The ion pumps are powered and controlled by an ion pump power supply unit, and further controlled by a baking unit (denoted as "soft baking") for automatized vacuum recovery, further connected to a hardware security unit. The sample stage and the light objective is powered and controlled by a combined sample stage and light objective control unit. The aperture stage and is powered and controlled by an aperture stage control unit. The lenses are powered and controlled by a high voltage power supply unit. The octupole is powered and controlled by an octupole and scan control unit and a STEM and scan control unit. The EDS detector is powered and controlled by an EDS control unit, which in turn also controls the STEM and scan control unit. The STEM detector is powered a high voltage power supply unit and controlled by a STEM and scan control unit. All of the above mentioned units are further connected to a general power supply unit and a general communication and control system. The camera is also powered by a general power supply unit. The turbomolecular pump and the gauge vacuum are powered by a general power supply unit and controlled by a general communication and control system. The camera, the EDS control unit, the STEM and scan control unit and the general communication and control system are digitally connected to a computer.

Claim 1:
A multi-mode low-voltage electron microscope operative in the accelerating voltage range of <NUM>-<NUM> kV and comprising in the following order based on the direction of a primary electron beam (<NUM>):
an electron beam source (<NUM>) to generate the primary electron beam (<NUM>),
a first magnetostatic condenser lens means (<NUM>),
a second magnetostatic condenser lens means (<NUM>),
a condenser aperture (<NUM>),
a sample holder (<NUM>),
a magnetostatic objective lens means (<NUM>),
an objective aperture (<NUM>),
a first electrostatic projective lens means (<NUM>), and
an end detection system (<NUM>) comprising a detection screen (<NUM>) and at least one detector selected from a STEM detector (<NUM>) configured to detect a signal of transmitted electrons, a TEM detector configured to detect a signal of transmitted electrons and/or an ED detector configured to detect a signal of diffracted electrons,
wherein the second magnetostatic condenser lens means (<NUM>) and the magnetostatic objective lens means (<NUM>) together comprise a first objective polepiece (<NUM>) and a second objective polepiece (<NUM>) with the sample holder (<NUM>) arranged therebetween,
characterised in that an EDS detector (<NUM>) configured to detect a signal of energy-dispersed X-ray radiation is arranged between the first objective polepiece (<NUM>) and the second objective polepiece (<NUM>), essentially co-planarly and laterally with respect to the sample holder (<NUM>), wherein the EDS detector (<NUM>) comprises a collimator (<NUM>) attached to the second objective polepiece (<NUM>).