CZ2014373A3 - Method of measuring energy distribution of electron emission from cathodes with a small virtual source and energy spectrometer for making the same - Google Patents

Abstract

The electron beam (1) in the vacuum environment is initially focused on the first magnetic field (2), the size of the aperture angle of the electron beam (1) being defined, which is then deflected laterally by the second magnetic field (5). The bent electron beam (1) widens with the third magnetic field (6). The line spectrum produced by the expanded electron beam (1) on the scintillation vista screen (7) is then subtracted and evaluated. The energy spectrometer comprises a cathode electron gun (8) with a small virtual source to which a condenser lens (9) is assigned to focus the electron beam (1) emitted from the electron gun (8). Behind it, in the direction of the optical axis (10) of the energy spectrometer, a diaphragm (3) is arranged to optimize the diameter of the electron beam (1) before it enters the magnetic prism (11) to deflect the electron beam (1) arranged behind the diaphragm (3) in the direction of the optical axis (10) of the energy spectrometer. Behind it, in the direction of the optical axis (10) of the energy spectrometer, a first scintillation vista screen (4) is arranged to enable the focusing of the electron beam (1). A projector lens (12) is arranged downstream of the magnetic prism (11) in the direction of the optical axis (10) by the magnetic prism (11) of the deflected electron beam (1) to enlarge the spectrum. A second scintillation vista screen (7) is arranged downstream of the projector lens (12) to display the bar spectrum of the electron emission energy distribution.

Description

Method for measuring the energy distribution of electron emission from small virtual source cathodes and energy spectrometer for carrying out this method

The invention relates to a method for measuring the energy width of emitted electrons from sources whose virtual size does not exceed 100 nm.

BACKGROUND OF THE INVENTION

Contemporary electron microscopy, both scanning and scanning, uses mainly cathodes as electron sources, whose emission mechanism is based on the effects of purely tunnel or tunnel stimulated by increasing the temperature of the emitter. Such electron sources exhibit many times the directional current density of the electron beam, thereby making it possible to achieve a significantly better signal-to-noise ratio when forming an electron-optical image. Moreover, these emitters are, in principle, point sources from which the electrons seem to emanate. The size of this apparent point, called the "virtual source", is in the range of 1 to 100 nm, depending on the particular arrangement.

Such small dimensions of the virtual source are advantageous in the field of radiation electron microscopy because the coherence of the illuminating electron beam is high and allows optimal phase contrast values to be achieved. In the field of scanning electron microscopy, the small size of a virtual source is an essential parameter to minimize the number of electron-optical scaling elements and thus obtain a more favorable ratio between the final dimension of the focused beam and its electron current.

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The small dimension of the virtual source arises whenever, in addition to the electron emission thermal process, the electron tunneling process from the cathode surface to the vacuum is utilized due to the high electrostatic field gradient in front of the emitter surface. Such a high electrostatic field gradient is achieved by tipping the top of the cathode into a cone-shaped cone tipped with a spherical cap having a radius ranging from R = 10 nm to R = 1000 nm. Electrons emitted from the spherical canopy spherical surface are first released into a vacuum by a large gradient of the electrostatic field from the cathode metal material (due to the tunneling process by a narrowed potential barrier) and then accelerated on a very short path in the radial direction to the spherical canopy. The cathodes thus formed are sometimes called spikes. Electrons virtually fly out of the fictional center of the spherical canopy. The dimension of this point (virtual source) is a function of the actual radius and shape of the spherical canopy and, moreover, the temperature of the cathode at which the emission of electrons occurs. A larger canopy radius and a higher cathode temperature inevitably increase the size of the virtual source. The most commonly used cathodes currently have virtual source dimensions ranging from 2 nm (sharp autoemission cathodes operating at or below room temperature) to 25 nm (thermoautoemission cathodes with a tip radius of 1000 nm operating at 1800 K).

All “tip” cathodes operate in direct emission mode. That is, they do not use spatial electrostatic charges to form a real electron beam intersection, but work directly with the size of the virtual source. It follows that even an electron optical system with a reduction of only 1: 1 is able to produce an electron beam at its output in the principle of nanometer dimensions. Despite this advantage, the energy width of the emitted electrons is still an important parameter. This energy scattering from 0.3 eV (cold cathodes) to eg 5 eV (very current-loaded thermo-auto-emission cathodes) causes deterioration of the electron beam parameters (blur) as a result of the chromatic defect of electron-optical elements.

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For this reason, it is necessary to deal with the energy distribution of electrons in the beam, both illumination and imaging, and look for methods to reduce this adverse effect. The first prerequisite for evaluating the emitters' energy width is to construct a metrological device (energy spectrometer) capable of measuring the energy width of an electron beam with a unit resolution of up to tens of mV. Such a metrological device would then serve not only to evaluate individual types of emitters, but also to verify the function of possible monochromators designed to limit the energy width of the emitted electrons and thus to limit the influence of chromatic defect of electron optics on the imaging properties of the system.

Currently used devices of this type, achieving the necessary energy resolution in the region close to 5 mV, are very expensive and are usually not designed to measure the characteristics of electron sources. One possible solution is, for example, the use of the Energy Loss Spectrometer (EELS), which is equipped with radiation or scanning radiation microscopes of the highest price category. These spectrometers, however, are designed to investigate the energy loss of the primary beam caused by the interaction of electrons with the material of the study preparation, but they are not intended for routine development work in the field of electron sources.

Several devices and methods for energy loss of electrons are known, usually in electron microscopes. For example, EP 2 708 874 of the FEI Company discloses a method for generating a tomographic image of a sample in a particle microscope in which the sample tilts against an electron beam of electrically charged particles, typically protons or ions. The images obtained in this first set are then mathematically combined to obtain a composite image. In the following procedure, a second set of images is taken with a spectral detector at different tilt of the sample to obtain spectral maps, which are then used to obtain a composite image. Quality is not examined here

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... .. ·· ··· ····· electron beam, but up to a signal already containing the sample image. Thus, this method cannot be used to measure the energy distribution of electron emission from cathodes with a small virtual source in sufficient resolution.

EP 2 387 062 of the FEI Company discloses detectors for an electron-optical system enabling the detection of electrons in different energy ranges, and a method for analyzing a sample in which the electrons passed through the sample are divided into two energy bands and detected by two detectors. downstream of the diffuser and its signal is proportional to the number of electrons in the second energy band, the second detector being positioned between the diffuser and the electron optics used to project the electrons emitted by the sample to the first detector. Even this method cannot be used to measure the energy distribution of electron emission from cathodes with a small virtual source in sufficient resolution, because here too the energy scattering of electrons passed through the sample is investigated.

From JP H06310063 Hitachi Ltd. an electron optical device measuring the energy loss spectrum of electrons after passing through a sample through a photodiode array or a linear CCD sensor is known. This device does not examine the quality of the electron beam, but detects the spectral image of the sample. Therefore, even this method cannot be used to measure the energy distribution of electron emission from cathodes with a small virtual source in sufficient resolution.

From JP H0721966 Hitachi Ltd. An analytical scanning electron microscope is known which, in addition to observing a sample image, also enables elementary analysis of a small portion of a sample. This microscope is able to analyze the energy loss of electrons in a small part of the sample with an electromagnet and simultaneously perform spectral analysis by a flame detector. Also, this device does not examine the quality of the electron beam, but detects the spectral image of the sample. Therefore, neither this

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-5 the method cannot be used to measure the energy distribution of electron emission from cathodes with a small virtual source in sufficient resolution.

SUMMARY OF THE INVENTION

The above-mentioned drawbacks of the prior art are largely eliminated by the method of measuring the energy distribution of electron emission from small virtual source cathodes arranged in a vacuum, the principle being that the electron beam in a vacuum environment is first focused and the aperture size is limited to minimize aberration. the angle of the electron beam, which is then avoided laterally by the second magnetic field, the bent electron beam is widened by the third magnetic field, and the line spectrum generated by the expanded electron beam on the scintillation viewing screen is read and evaluated.

In a preferred embodiment of the method, before evaluating the expanded electron beam line spectrum on a scintillation viewing screen, the line spectrum is optically increased.

These drawbacks of the prior art are also largely overcome by an energy spectrometer for measuring the energy distribution of electron emission from small virtual source cathodes arranged in a vacuum, the nature of which is described below. This energy spectrometer contains a cathode with a small virtual source. It is associated with a condenser lens for focusing an electron beam emitted from a cathode with a small virtual source. Downstream of the condenser lens, a diaphragm is arranged in the direction of the optical axis of the energy spectrometer to optimize the diameter of the electron beam before it enters the magnetic prism to deflect the electron beam arranged behind the diaphragm in the direction of the optical axis of the energy spectrometer. Behind the magnetic prism is in the direction of the optical axis

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6 · · “·“.................... _The first scintillation sightglass is provided on the energy spectrometer to enable the focusing of the electron beam. A projector lens is arranged downstream of the magnetic prism in the direction of the optical axis by the magnetic prism of the deflected electron beam to increase the spectrum dimension. A second scintillation viewing screen is provided downstream of the projector lens in the direction of the optical axis of the deflected electron beam to display a line of energy distribution of the electron emission.

In a preferred embodiment of the energy spectrometer according to the invention, an optical objective is arranged downstream of the second scintillation viewing screen for displaying the energy spectrum of the electron emission distribution.

In another preferred embodiment of the energy spectrometer according to the invention, a photographic camera is then assigned to the optical lens.

BRIEF DESCRIPTION OF THE DRAWINGS

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in more detail in the accompanying drawings, in which: Figure 1 schematically illustrates a method for measuring the energy distribution of electron emission from cathodes with a small virtual source; and Figure 2 schematically illustrates an energy spectrometer for measuring energy distribution of electron emission from an electron gun.

DETAILED DESCRIPTION OF THE INVENTION

At present perspective electron sources consist of spike cathodes of types of autoemission, respectively thermo-autoemission, and in this area it is in principle guaranteed that the dimensions of virtual emission sources do not exceed

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100 nm. This is a relatively advantageous situation because the extremely small size of the virtual source offers a relatively simple spectrometer solution based on the dispersion properties of the magnetic sector field or prism.

The purpose of the invention is to provide an electron spectrometer arrangement which allows the energy width of the emitted electrons of any electrode source operating with a spike cathode to be measured, either directly or after monochromatization of the electron beam.

That is, if the size of the virtual source is less than 100 nm, three basic electron optical elements, i.e., an auxiliary condenser, a magnetic prism, and a projector, can be arranged to operate at an energy resolution in the present case, depending on the virtual source's own size, of the order of mV.

This possibility of a very simplified yet effective arrangement of the energy spectrometer results from the fact that even due to the very low energy dispersion of magnetic prisms, of the order of μηι / V units, the resolution of up to 1000 lines can be achieved in the dispersion plane per volt, which represents a theoretical resolution of 1 mV.

Figure 1 schematically illustrates a method of measuring the energy distribution of electron emission from small virtual source cathodes arranged in a vacuum. In a vacuum environment, the electron beam 1 is in focus and the aperture angle of the electron beam 1 is defined to minimize aberration with the aperture 3. The electron beam 1 is then focused on the first scintillation viewing screen 4. After focusing the electron beam 1, the second magnetic field 5 by which the electron beam 1 is avoided laterally. The electron beam 1 is then removed

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On May 28, 2014 the third magnetic field 6 is expanded, whereupon the line spectrum created by the expanded electron beam 1 on the second scintillation transparent screen 7 is subtracted and evaluated. This line spectrum created by the extended electron beam 1 on the second scintillation transparent screen 7 can then be optically enlarged for better resolution.

Fig. 2 schematically shows an energy spectrometer for measuring the energy distribution of electron emission from an electron gun 8 containing a small virtual source cathode arranged in a vacuum. In the exemplary embodiment, a tip cathode placed on a vertical optical axis at any distance from the spectrometer and operating in an optimum operating mode was used. Assigned to it is a condenser lens 9 for focusing the electron beam 1 emitted from a small virtual source of the electron gun 8. Behind the condenser lens 9 is arranged in the direction of the optical axis 10 of the energy spectrometer an orifice 3 to optimize the electron beam diameter 1 before it enters the magnetic prism 11 a deflection of the electron beam 1 arranged behind the orifice plate 3 in the direction of the optical axis 10 of the energy spectrometer. The diaphragm 3 is a replaceable diaphragm to optimize the magnitude of the input angle to the spectrometer to eliminate optical prisms of the magnetic prism.

A first scintillation viewing screen 4 is arranged downstream of the magnetic prism 11 in the direction of the optical axis 10 of the energy spectrometer to allow the focusing of the electron beam 1 to be checked. A prism 12 is arranged behind the magnetic prism 11 in the direction of the optical axis 10. spectrum. Downstream of the projector lens 12, in the direction of the optical axis 10 of the deflected electron beam 1, a second scintillation viewing screen 7 is provided to display a line of energy distribution of the electron emission. Second scintillation

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The viewing screen 7 is followed on the horizontal optical axis by an appropriate light-optical magnification and registration system for photometric evaluation of the obtained electron spectra.

The advantage of the proposed arrangement is the relative simplicity of the whole device, which can lead to its better availability. As already described, the core of the spectrometer is a rectangular magnetic prism 11 with perpendicular inlet and outlet of electron beams rotated at 90 ° to each other. Such a magnetic prism 11 provides a homogeneous magnetic field which has the ability to focus electrons in a plane perpendicular to the direction of the homogeneous magnetic field and, conversely, not to focus electrons in the direction of the magnetic field. This physical mechanism creates the basic conditions for the formation of the line electron spectrum because electrons with different energy are focused in another plane of the spectral surface in a plane perpendicular to the direction of the magnetic field. For, for example, two different energies of the electron beam 1, two spectral lines are formed after passing through the magnetic prism 11, the mutual distance of which is proportional to their energy difference. The ability to separate individual energy beams is called energy dispersion.

A major problem is that the energy dispersion of the prisms 11 is relatively small and makes pm to V. Sensing of such a spectrum by methods of direct electron current measurement in individual spectral lines is very disadvantageous, since the dimensions of the respective selective orifices or detectors would have to be realized. nanometer dimensions, if it is necessary to achieve energy resolution in the order of mV. This circumstance leads to an arrangement that utilizes a further degree of magnification of the entire spectrum by electron optical path. Placing a powerful projector lens 12 beyond the output edge of the magnetic prism 11 allows the spectral plane image magnified up to 50 times to be transferred to the viewing area of the scintillation screen

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- 107 high resolution. This creates the conditions for the next scanning step by means of photometric methods.

The energy spectrometer for measuring the electron emission energy distribution from small virtual source cathodes is assembled in a high vacuum vacuum chamber equipped with independent pumping means. The second scintillation viewing screen 7 is already provided on the atmospheric side with a holder (not shown) of a suitable optical lens followed by a camera.

The inlet side of this energy spectrometer, above the condenser lens 9, is provided with a vacuum adapter 13, which is an auxiliary flange, allowing the connection of various types of electron guns 8 meeting the conditions of a small virtual emission source.

The electronic equipment provides the possibility of independent excitation of both the magnetic lenses and the magnetic prism 11 so as to accommodate measurements in a wide range of energies, typically from 1 to 30 to V.

The distance of the emitter itself, i.e. the electron gun 8, from the inlet orifice 3 of the spectrometer is not limited as well as the operating mode of the electron gun 8, which can produce a divergent, parallel or convergent electron beam 1. The spectrometer in the proposed configuration can include measurements over a wide range electron guns 8 and electron guns 8, respectively, supplemented with a monochromator of any type.

The object of the present invention is to use one universal condenser lens 9 for adjusting the parameters of the input electron beam 1 into the spectrometer, from different types of electron guns 8 operating in different modes, followed by a slit diaphragm 3 to define the aperture angle of the electron beam 1 before entering the magnetic field

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The magnetic prism 11, based on its energy dispersion properties, where the angle of the electron beam 1 is 90 °, produces a line electron spectrum at its output, the spectral line thickness for one given energy being roughly equal to the size of the virtual source. of the emitter of interest, i.e. the electron gun 8, and will therefore be in nm. Due to the dispersing power of the magnetic prism 11, the distance of the spectral line with energy of 1 eV will be higher by several pm from the original. Thus, it can be argued that 1 eV of energy change can be investigated with a potential resolution of up to 1000 lines per 1 eV.

However, such a compressed electron spectrum is difficult to monitor. It is therefore possible to magnify the spectrum by the respective electron projection lens, for example 50 times, so that, after projection on the respective high-resolution viewing screen, a spectrum image is readable already by means of light microscopy in cooperation with the respective camera. Due to the limited possibilities of magnification by the light-optical element and the camera, a value of 5 mV can be considered as a reasonable achievable limit of the energy resolution.

The actual electron-optical enlargement of the primary spectrum also leads to an enlargement of the virtual source image, but does not lead to a reduction in the dispersion ratio. The virtual source image is enlarged just as many times as the dispersion range.

It is further evident that the magnetic prism 11 with perpendicular incidence and output of the electron beam 1 focuses the electron beam 1 only in a plane perpendicular to the direction of the homogeneous magnetic field. In a plane parallel to the direction of the magnetic field, the magnetic prism 11 does not focus and thus retains the natural divergence of the electron beam 1 corresponding to the situation before entering the magnetic prism 11. This is therefore the mechanism of the generation of the line spectrum. The big advantage is that the photometric measurement of the width of the spectral lines can be repeated many times, that is, at each point of the spectral line, and

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• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • statistically process the obtained results and thus clean up the final result from the effects of statistical noise. It is easy to make measurements, for example, in each of the 1024 columns of the monitor to obtain a relatively large statistical file.

It is advantageous to perform the energy calibration of the electron spectrometer before each measurement of the spectral line width. For different excitation settings of both the condenser lens 9 and the projector lens 12 or the lens associated with the electron gun 8 being measured, the calibration value will be different. The calibration method of the described system is based on simply changing the electron beam energy by, for example, +/- 5 eV and detecting the number of pixels by the two spectral lines dividing. This process gives a unique calibration constant in volts per pixel for the current system setting.

It turns out that most progress in the field of electron microscopy in the last 30 years has been achieved thanks to the development of new types of electron sources. These are mainly high brightness sources capable of delivering the maximum possible electron current to the minimum beam diameters. This circumstance suggests that the physical boundaries of these developmental steps lie in the area of electron-electron interaction, which not only spatially “expands” the electron beam, but radically alters its energy width. The change of this important parameter depends on the way the electron emission itself originates, but also on the way the electron beam is guided behind the emitter. Examples of risky solutions include redundant beam crossings, beam propagation areas in parallel mode, and reducing total beam energy, which is increasingly relevant to various types of applications, eg for the semiconductor industry, genetic structure observation, and the like.

In the future, it is very important to verify the effect of newly developed systems for monochromatization of electron beams. These will increasingly be used as correctors of chromatic aberration, which is one of the main causes of the inability to achieve higher resolution in electron

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The microscopes of both the scanning and scanning and scanning scanning types, respectively.

Industrial applicability

The use of the above described assembly of the electron spectrometer is expected in laboratories closely cooperating with groups of electron source development.

Claims (5)

Patent claims Method for measuring the energy distribution of electron emission from small virtual source cathodes arranged in a vacuum, in which the electron beam (1) in the vacuum environment first focuses on the first magnetic field (2), the aperture angle of the electron beam (1) is defined; characterized in that the treated electron beam (1) is avoided laterally by the second magnetic field (5), the bent electron beam (1) is widened by the third magnetic field (6), and the line spectrum produced by the extended electron beam (6) is read and evaluated. 1) on a scintillation sight glass (7). Method for measuring the energy distribution of electron emission from small virtual source cathodes according to claim 1, characterized in that the line spectrum is optically enlarged before evaluating the line spectrum generated by the expanded electron beam (1) on the scintillation viewing screen (7). 3. A condenser lens (9) for focusing the electron beam (1) emitted from the electron beam (1) is coupled to an electron gun (8) with a small virtual source cathode for measuring the electron emission energy distribution from a small virtual source cathode arranged in vacuum. A nozzle (3) is arranged behind it in the direction of the optical axis (10) of the energy spectrometer, to optimize the diameter of the electron beam (1), characterized in that the orifice (3) is placed before the entrance D3528 May ^9, 2014 9 electron beam (1) into magnetic prism (11) for deflection of electron beam (1), arranged behind the aperture (3) a first scintillation transparent screen (4) is arranged in the direction of the optical axis (10) of the energy spectrometer and downstream of it in the direction of the optical axis (10) of the energy spectrometer to allow focusing of the electron beam (1); in the direction of the optical axis (10) by the magnetic prism (11) of the deflected electron beam (1), a projector lens (12) is arranged to enlarge the spectrum, and a second scintillation perspective is arranged in the direction of the optical axis (10) a screen (7) for displaying a line spectrum of the electron emission energy distribution. Energy spectrometer for measuring the electron emission distribution from small virtual source cathodes according to claim 3, characterized in that an optical objective is arranged downstream of the second scintillation viewing screen (7) for displaying the electron emission distribution energy spectrum. Energy spectrometer for measuring the energy distribution of the electron emission from small virtual source cathodes according to claim 4, characterized in that a photographic camera is associated with the optical lens.