Method of operating a cold emission electron source

Abstract

A method of operating a cold emission electron source (8). The method comprises the steps of measuring the emission current using an emission current detector (15), specifying the zero value of the emission current noise using an evaluation unit (12), specifying the emission current noise as the difference between the measured emission current and the zero value of the emission current noise, evaluating the emission current noise, and stopping the electron emission if the emission current noise exceeds the limit value (21 ) of the magnitude of the emission current noise stored in the memory of the evaluation unit (12).

Description

[0001] Method of Operating a Cold Emission Electron Source

[0002] Technical Field

[0003] The invention relates to a method of operating a cold emission electron source in electron microscopy.

[0004] Background of the Invention

[0005] In focused electron beam systems, such as a scanning electron microscope (SEM), transmission electron microscope (TEM), or scanning transmission electron microscope (STEM), an electron current is used to image the sample, which is shaped into a narrow beam by various methods and components of the device. The electron source can be, for example, a field emission gun (FEG), which exists in two types, namely a Schottky source (described, e.g., in EP1592040B1 ) or a cold field emission gun (CFEG, described, e.g., in US11158479B2). The FEG utilizes the quantum tunneling effect to emit electrons from the cathode surface, where a strong electric field is created around the cathode, which is terminated by a sharp tip, usually made of tungsten, which causes the emission of electrons from the cathode tip. The result is an electron beam with higher intensity and less scattering than when using a conventional thermionic electron cathode. In the case of the Schottky source, the source needs to be heated to a temperature of about 1800 K to emit electrons, while in the case of the CFEG source, the electrons are emitted at room temperature.

[0006] Images acquired using the FEG source achieve an order of magnitude higher resolution and quality than those acquired using thermionic sources. A longer lifetime of the source and higher stability of the electron beam are also an advantage. Among commonly used electron sources, the CFEG source achieves the best resolution because it has the smallest virtual source size and the lowest energy scattering of the emitted electrons. Therefore, the CFEG source is used in the most advanced imaging devices.

[0007] A disadvantage of the CFEG sources is their susceptibility to impurity accumulation on the cathode surface, which leads to a deterioration of the quality of the electron current (emission current). Therefore, this source needs to be operated at so-called ultra-high vacuum of 10’8to 10’11Pa. Nevertheless, rapid contamination of the source still occurs as gases are continually released from the components of the work chamber, the column, and the source itself or from organic impurities present in the chamber, the tube or the source. These gases accumulate on the cathode tip as well as in its vicinity, where they form additional, unwanted tips from which electrons are released. This leads to a disturbance in the stability of the emission current and to its changes (noise), where the electron emission gradually increases, resulting in the damage of the tip of the cold cathode (described, e.g., in YEONG, K. S. and THONG, J. T. L. Life cycle of a tungsten cold field emitter. Online. Journal of Applied Physics. 2006, vol. 99, no. 10. ISSN 0021 - 8979. Available from: 2197267. [cit. 2024-07-17].).

[0008] Therefore, the CFEG source needs to be freed of the accumulated impurities, which is done by short-term heating to a high temperature (the term flashing is used in English). This will degas the impurities accumulated on and around the cathode tip. A disadvantage of this method is the necessity to interrupt the operation of the whole device until the source is cleaned and cooled down to operating temperature.

[0009] It is therefore desirable to set the intervals between the cleanings of the CFEG source such that the device may be operated for as long as possible, but at the same time such that the source is cleaned before it becomes damaged. A commonly used method is, e.g., a determination of regular time intervals for cleaning of the CFEG source (e.g. US8426835B2). The analysis of the emission current noise, the fluctuations of which may precede source damage, is addressed marginally in the document US8736170B1 ; the aim of this patent is to reduce the emission current noise and increase the stability of the CFEG source.

[0010] A disadvantage of these methods is that the full potential of the CFEG source is not utilized, as the interval-based cleaning occurs earlier than at a moment when the cleaning is strictly necessary, and in the case of unexpectedly higher contamination, the cleaning may take place too late and the source may become damaged. The method of monitoring the fluctuations of the emission current noise (described, e.g., in US2009260112A1 or US2011221360A1 ) without monitoring the magnitude of these fluctuations, is disadvantageous in that it is not possible to determine which fluctuation of the emission current noise is already dangerous to the source, and further, the time difference between the recorded fluctuation and the damage of the cathode is too short and it is difficult to shut down the CFEG source in time to prevent damage.

[0011] It would therefore be suitable to continuously monitor the contamination of the CFEG source such that the cleaning occurs only when the contamination exceeds a specified limit, thereby extending the service life of the CFEG source while simultaneously eliminating the risk of its damage.

[0012] Summary of the Invention

[0013] The above problems are solved by the presented method of operating a device with a charged particle beam comprising a cold emission electron source comprising a tip-shaped cathode, wherein the device further comprises an emission current detector and an evaluation unit comprising a memory comprising a limit of the emission current noise, characterized in that the method comprises the steps of: measuring the emission current using the emission current detector, specifying the zero value of the emission current noise using the evaluation unit, specifying the emission current noise as the difference between the measured emission current and the zero value of the emission current noise using the evaluation unit, evaluating the emission current noise using the evaluation unit, stopping the electron emission if the emission current noise exceeds the limit value of the magnitude of the emission current noise stored in the memory of the evaluation unit, wherein the magnitude of the emission current noise is specified as the sum of the emission current noise exceeding the limit of the emission current noise. This method allows a reliable determination of the moment when the source needs to be shut down and cleaned to prevent its damage.

[0014] In a preferred embodiment of the method, the memory of the evaluation unit comprises the limit value of the relative value of the emission current noise and, further, the length of the time window. The step of evaluating the emission current noise using the evaluation unit then comprises the steps of: a) specifying the value of the noise-free component of the emission current using the evaluation unit as the lower envelope function of the measured emission current, b) specifying the absolute value of the emission current noise using the evaluation unit as the difference between the currently measured value of the emission current and the value of the noise-free component of the emission current according to the lower envelope function of the measured emission current at the same point in time, c) specifying the relative value of the emission current noise using the evaluation unit as the ratio between the absolute value of the emission current noise and the value of the noise-free component of the emission current according to the lower envelope function of the measured emission current at the same point in time, d) specifying the magnitude of the emission current noise using the evaluation unit as the definite integral of the relative value of the emission current noise exceeding the limit value of the relative value of the emission current noise over a certain time window stored in the memory of the evaluation unit.

[0015] In another preferred embodiment of the method, the step of stopping the electron emission is performed when the magnitude of the emission current noise over a certain time window is greater than the limit value of the magnitude of the emission current noise over a certain time window.

[0016] The advantage of the described procedure lies in monitoring the relative value of the emission current noise, which allows the use of the same limit of the noise value at different values of the emission current. Calculating the magnitude of the emission current noise using the relative value of the emission current noise allows monitoring of the gradual increase in noise, which indicates the approaching end of the source cathode service life, and shutting down the source at the moment, when the magnitude of the noise exceeds a pre-specified limit. Thus, there is no premature shutdown of the source in the event of occasional fluctuations in the emission current, allowing the source to be used for longer periods of time.

[0017] Description of Drawings

[0018] A summary of the invention is further clarified using exemplary embodiments thereof, which are described with reference to the attached drawings. For better clarity, only those parts of the device that are important from the standpoint of the principle of the present invention are shown in the drawings.

[0019] Fig. 1 is a device with a charged particle beam.

[0020] Fig. 2 is a CFEG source.

[0021] Fig. 3 is a chart of the detected emission current.

[0022] Fig. 4 is a chart of the relative value of the emission current noise.

[0023] Fig. 5 is a chart of the relative value of the emission current noise in a time window.

[0024] Fig. 6 is a chart of the relative value of the emission current noise in a time window.

[0025] Fig. 7 is a chart of the magnitude of the emission current noise not exceeding the limit.

[0026] Fig. 8 is a chart of the magnitude of the emission current noise exceeding the limit. Exemplary Embodiments of the Invention

[0027] Said embodiments show exemplary variants of the embodiments of the invention, which, however, have no limiting effect from the point of view of the scope of protection.

[0028] The present method is implemented in a device 1. with at least one charged particle beam. The device 1. is a scanning electron microscope (SEM), a transmission electron microscope (TEM), a scanning transmission electron microscope (STEM), a microscope combining an electron beam and a focused ion beam, or another similar device utilizing a charged particle beam.

[0029] In the first exemplary embodiment, the device 1_ comprises at least one column 2 connected to a work chamber 3, inside which there is a device 4 for positioning and holding the sample, at least one signal particle detector 5, and other commonly used components of the work chamber 3. Column 2 contains elements 6 for shaping and directing charged particles into a charged particle beam, for example lenses, astigmatism correctors, electrodes at potential, apertures, and other commonly used components for particle optics.

[0030] In an alternative exemplary embodiment of the device 1., the device 1. is equipped with two or more columns 2 connected to the work chamber 3.

[0031] The column 2 further comprises a chamber 7 of a CFEG source 8, in which the CFEG source 8 is located.

[0032] The CFEG source 8 comprises a cathode 9 with a tip from which electrons are emitted. The tip of the cathode 9 has a diameter of less than 500 nm, most preferably less than 100 nm. The cathode 9 is made of a single crystal of a base metal, which can be W, Ta, Re, Mo, lr or other metals or their alloys, provided that the base metal is conductive, non-magnetic and may be heated to a high temperature without a significant increase of the radius of the tip. The base material of the cathode 9 may be coated with a coating material, which are typically compounds, e.g., compounds of O, N, or C with, e.g., Zr, Ti, Hf, Nb, V, and others. The purpose of the coating material is to reduce the adsorption of gases around the cathode 9 and reduce the instability of the emission current or reduce the output work of the material of the cathode 9 to facilitate the electron emission. In the first exemplary embodiment of the CFEG source 8, the cathode 9 is rigidly connected to at least one filament 10 that maintains the cathode 9 in a needed position and heats it when needed. In an alternative exemplary embodiment of the CFEG source 8, the cathode 9 is connected to two or more filaments 10. The filament 10 of both the first and the alternative exemplary embodiment of the CFEG source 8 is connected by the ends thereof to a heating current source 11 , which is connected to an evaluation unit 12 that controls the length, amplitude, and width of the heating pulse at the moment of the cleaning of the CFEG source 8.

[0033] The CFEG source 8 further comprises an extraction electrode 13 positioned opposite the cathode 9 in an optical axis 14 of the column 2 in the direction of the electron current. The extraction electrode 13 has a positive potential compared to the cathode 9, e.g., with a potential difference of several kilovolts between the tip of the cathode 9 and the extraction electrode 13. The potential difference generates an electric field around the tip of the cathode 9, where due to this electric field, electron emission from the tip of the cathode 9 occurs. The extraction electrode 13 is connected to the voltage source of the extraction electrode (not shown).

[0034] The CFEG source 8 also includes devices for maintaining vacuum connected to the chamber of the CFEG source 8.

[0035] The device 1. further comprises the evaluation unit 12.

[0036] The device 1. further comprises an emission current detector 15 connected to the extraction electrode 13 and also connected to the evaluation unit 12 that processes data obtained from the emission current detector 15, evaluates the fluctuations of the emission current, and controls the process of cleaning the CFEG source 8.

[0037] The CFEG source 8 has a specific emission current cycle. Shortly after cleaning the CFEG source 8, the emission current is high but decreases rapidly (within minutes to hours) after impurities accumulate on the surface of the cathode 9 of the CFEG source 8. Subsequently, the emission current is stable for a period of time (usually from a few hours to several tens of hours). However, during this time, additional impurities accumulate on the surface of the cathode 9 of the CFEG source 8, creating additional tips on the surface of the cathode 9, from which electrons are emitted. Therefore, the emission current starts to fluctuate, and simultaneously, the total value of the emission current increases. If the CFEG source 8 is not cleaned at this time, the cathode 9 gets damaged (melted) due to the emission current being too high.

[0038] During the use of the device 1, the emission current is measured by the emission current detector 15 and evaluated by the evaluation unit 12. The emission current is continuously evaluated by the evaluation unit 12 in the time window 19. The time window 19 has a preset arbitrary length that is stored in the memory of the evaluation unit 2. At each moment, the time window 19 comprises the last measured value of the emission current, i.e. , it continuously shifts and always comprises the current value of the emission current as well as the values of the emission current retrospectively over a period corresponding to the length of the time window 19.

[0039] The measured emission current 16 is recorded in the chart (fig. 3). Within the time window 19 during certain time segments, which may be of any length, the evaluation unit 12 identifies the lower points of the measured values of the emission current 16 in the chart in each time segment. The lower points from all time segments within the time window 19 are combined by the evaluation unit 12 into a lower envelope function 17. The lower envelope function 17 is evaluated by the evaluation unit 12 as the noise-free component of the emission current 16.

[0040] The evaluation unit 12 then specifies the absolute value of the emission current noise.

[0041] The absolute value of the emission current noise (Nabs) is specified as the difference between the value of the measured emission current 16 (I) and the value of the noise-free component of the emission current 16 according to the lower envelope function 17 (lenv) at the same point in time.

[0042] The evaluation unit 12 then specifies the relative value of the emission current noise.

[0043] The relative value of the emission current noise (Nrei) is specified as the ratio between the absolute value of the emission current noise (NabS) and the value of the noise-free component of the emission current 16 according to the lower envelope function 17 (lenv) for the same point in time for which the absolute value of the emission current noise was determined.

[0044] The relative value of the emission current noise is recorded by the evaluation unit 12 in the chart. A limit value 18 of the relative value of the emission current noise (hereinafter referred to as limit L1 ) (fig. 4) is stored in the memory of the evaluation unit 12 as a value whose exceeding is monitored by the evaluation unit 12, as will be described below.

[0045] The relative value of the emission current noise is monitored by the evaluation unit 12 in the time window 19. If it exceeds the limit 18 L1 , the evaluation unit 12 calculates the magnitude of the relative value of the emission current noise exceeding the limit 18 L1 in the given time window 19 (hereinafter referred to as the magnitude of the emission current noise). The calculation is carried out using the definite integral of the relative value of the emission current noise exceeding the limit 18 L1 in the time window 19. Thus, for the present method, the decisive factor is thus the area 20 enclosed between two curves, namely the curve formed by the points representing the relative value of the emission current noise exceeding the limit 18 L1 and the curve formed by the limit 18 L1 . If the limit 18 L1 is exceeded multiple times within the time window 19, the magnitude of the emission current noise is calculated as the sum of the obtained areas 20 (fig. 5).

[0046] An increase in the magnitude of the emission current noise in the time window 19 does not in itself indicate the approaching end of the service life of the cathode 9, since an increase in the emission current noise may occur for a short time at any time during the use of the cathode 9.

[0047] Only a significant increase in the magnitude of the emission current noise manifested by an increase in the area 20 enclosed by the curve formed by the points representing the relative value of the emission current noise exceeding the limit 18 L1 and the curve formed by the limit 18 L1 in the time window 19 indicates dangerous contamination of the cathode (fig. 6). For this reason, a limit value 21 of the magnitude of the emission current noise (hereinafter referred to as the limit L2) is stored in the memory of the evaluation unit 12. If the magnitude of the emission current noise exceeds the limit 21 L2 in the given time window 19 (fig. 8), the electron emission is stopped because the contamination is already high and the cathode 9 is in danger of being damaged. In a particular exemplary embodiment, the device 1. comprises one column 2 connected to the work chamber 3, inside which the device 4 for positioning and holding the sample and the signal particle detector 5 are located. In the column 2, the elements 6 for shaping and directing the charged particles into the charged particle beam are located. The column 2 further comprises the chamber 7 of the CFEG source 8, in which the CFEG source 8 is located. The CFEG source 8 comprises the cathode 9 with a tip. The cathode 9 is rigidly connected to a single filament 10, which is connected by the ends thereof to the heating current source 1_1_> which is connected to the evaluation unit 12. The CFEG source 8 further comprises the extraction electrode 13, which is connected to the voltage source of the extraction electrode. The CFEG source 8 also includes devices for maintaining vacuum connected to the chamber of the CFEG source 8. The device 1. further comprises the evaluation unit 12 and further comprises the emission current detector 15, which is connected to the extraction electrode 13 and also to the evaluation unit 12.

[0048] Data necessary for the correct implementation of the present method are stored in the memory of the evaluation unit 12, namely the length of the time window 19, the limit 18 L1 , and the limit 21 L2. The emission current is evaluated by the evaluation unit 12 in the time window 19 having a duration of 30 minutes.

[0049] During the operation of the CFEG source 8, the emission current is measured by the detector 15 and recorded in the chart by the evaluation unit 12. The evaluation unit 12 connects the lower points of the measured values of the emission current in the lower envelope function 17 (fig. 3).

[0050] The evaluation unit 12 then specifies the absolute value of the emission current noise at a certain point in time. For the same point in time, it specifies the relative value of the emission current noise.

[0051] The relative values of the emission current noise are recorded by the evaluation unit 12 into the chart. The evaluation unit 12 monitors whether the relative value of the emission current noise exceeds the limit 18 L1 in the time window 19.

[0052] In the time window 19 in fig. 5, the limit 18 L1 was exceeded. Therefore, the evaluation unit 12 calculates the magnitude of the emission current noise as the area 20 enclosed by the curve formed by the points representing the relative value of the emission current noise exceeding the limit 18 L1 and the curve formed by the limit 18 L1 . Since the limit 18 L1 was in the time window 19 in fig. 5 exceeded twice, the calculation of the area enclosed between these curves will be performed twice. The resulting magnitude of the emission current noise is determined as the sum of the corresponding areas 20.

[0053] The evaluation unit 12 then in the time window 19 compares the magnitude of the emission current noise with the limit 21 L2. Since the limit 21 L2 was not exceeded in the time window 19 in fig. 7, the CFEG source 8 can continue to be used.

[0054] In the time window 19 in fig. 6, the limit 18 L1 was also exceeded. Therefore, the evaluation unit 12 calculates the magnitude of the emission current noise. It then compares the magnitude of the emission current noise with the limit 21 L2. In the time window 19 in fig. 8, the limit 21 L2 was exceeded. Therefore, the evaluation unit shuts down the CFEG source 8.

[0055] List of Reference Signs

[0056] 1 - device

[0057] 2 - column

[0058] 3 - work chamber

[0059] 4 - device for positioning and holding the sample

[0060] 5 - signal particle detector

[0061] 6 - elements for shaping and directing the charged particles

[0062] 7 - chamber of the CFEG source

[0063] 8 - CFEG source

[0064] 9 - cathode

[0065] 10 - filament

[0066] 11 - heating current source

[0067] 12 - evaluation unit

[0068] 13 - extraction electrode

[0069] 14 - axis of the column

[0070] 15 - emission current detector

[0071] 16 - measured emission current

[0072] 17 - lower envelope function of the emission current

[0073] 18 - limit value of the relative value of the emission current noise

[0074] 19 - time window

[0075] 20 - area

[0076] 21 - limit value of the magnitude of the emission current noise

Claims

CLAIMS1. A method of operating a device (1 ) with a charged particle beam comprising a cold electron emission source (8) comprising a tip-shaped cathode (9), wherein the device (1 ) further comprises an emission current detector (15) and an evaluation unit (12) comprising a memory comprising a limit value of the emission current noise, characterized in that the method comprises the steps of: measuring the emission current using the emission current detector (15), specifying the zero value of the emission current noise using the evaluation unit (12), specifying the emission current noise as the difference between the measured emission current and the zero value of the emission current noise using the evaluation unit (12), evaluating the emission current noise using the evaluation unit (12), stopping the electron emission if the emission current noise exceeds the limit value (21 ) of the magnitude (20) of the emission current noise stored in the memory of the evaluation unit (12), wherein the magnitude (20) of the emission current noise is specified as the sum of the emission current noise exceeding the limit value of the emission current noise.

2. The method of operating the device (1 ) according to claim 1 , characterized in that the memory of the evaluation unit (12) comprises the limit value (18) of the relative value of the emission current noise and further comprises the length of the time window (19), and, further, in that the step of evaluating the emission current noise using the evaluation unit (12) comprises the steps of: a) specifying the value of the noise-free component of the emission current using the evaluation unit (12) as the lower envelope function (17) of the measured emission current (16),b) specifying the absolute value of the emission current noise using the evaluation unit (12) as the difference between the currently measured value of the emission current (16) and the value of the noise-free component of the emission current according to the lower envelope function (17) of the measured emission current (16) at the same point in time, c) specifying the relative value of the emission current noise using the evaluation unit (12) as the ratio between the absolute value of the emission current noise and the value of the noise-free component of the emission current according to the lower envelope function (17) of the measured emission current (16) at the same point in time, d) specifying the magnitude (20) of the emission current noise using the evaluation unit (12) as the definite integral of the relative value of the emission current noise exceeding the limit value (18) of the relative value of the emission current noise over a certain time window (19) stored in the memory of the evaluation unit (12).

3. The method of operating the device (1 ) according to claims 1 and 2, characterized in that the memory of the evaluation unit (12) comprises the length of the time window (19) and, further, in that the step of stopping the electron emission is performed when the magnitude (20) of the emission current noise over a certain time window (19) is greater than the limit value (21 ) of the magnitude (20) of the emission current noise over a certain time window (19).

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