A method for measuring secondary electron yield based on a scanning electron microscope platform

Abstract

A method for testing the secondary electron yield based on a scanning electron microscope platform, with the aid of an existing scanning electron microscope and a Faraday cup, over-focuses the electron beam originally used for focused scanning and then forms a speckle to obtain the irradiated area of the electron beam for the required secondary electron yield. The current on the sample to be tested is measured under this irradiated area, and at the same time, the incident electron current is measured through the Faraday cup while ensuring that the scanning electron gun is at the same focusing parameters. The secondary electron yield at this electron beam energy can be obtained through a simple calculation of the two measured currents. This method realizes a simple, fast, and reliable test of the secondary electron yield, which is not only of great significance for related research in the field of secondary electron emission, such as the microdischarge effect of space microwave components, the performance optimization of electron multiplier tubes, and the microwave breakdown of dielectric windows, but also an effective supplement to the existing test methods.

Description

Technical Field

[0001] The present invention relates to a method for testing secondary electron yield based on a scanning electron microscope platform, belonging to the field of electronic science and technology. Background Art

[0002] The phenomenon of secondary electron emission excited by electron irradiation has always been a fundamental and key issue in the field of physical electronics. This is because the secondary electron multiplication effect caused by secondary electron emission is not only an important factor restricting the performance and reliability of space microwave components, accelerators, high-power microwave sources, nuclear fusion, etc. In addition, secondary electron emission is also the core principle of various electron multiplier tubes, scanning electron microscope imaging and detection, electron beam probe microanalysis, Auger electron spectrometers, and other electron surface analysis instruments.

[0003] The secondary electron yield (Secondary Emission Yield, SEY), also known as the secondary electron emission coefficient, is one of the most important parameters describing the secondary electron emission phenomenon. Therefore, it also directly determines technical indicators such as the microdischarge threshold of space microwave components, the breakdown threshold of high-power dielectric windows, and the gain of electron multiplier tubes. Since the secondary electron yield is closely related to the surface process of materials, the secondary electron yield is usually measured by building a vacuum system, equipping an electron gun as the incident electron source, and using the current method with a bias voltage applied to the sample or the collector method with a collection device. Summary of the Invention

[0004] The technical problem solved by the present invention is: overcoming the deficiencies of the prior art, providing a method for testing secondary electron yield based on a scanning electron microscope platform, and realizing simple, fast, and reliable testing of secondary electron yield through simple configuration of a Faraday cup and measurement of the sample stage current.

[0005] The technical solution of the present invention is: a method for testing secondary electron yield based on a scanning electron microscope platform, including:

[0006] (1) Simultaneously placing a calibration sample, a sample to be tested, and a Faraday cup in the scanning electron microscope (SEM) platform;

[0007] (2) Obtaining the secondary electron yield SEY i and the corresponding incident energy point Ep i as well as the test area ID i of electron irradiation, where i is the number of tests;

[0008] (3) Using the scanning electron microscope (SEM) to obtain a clear image on the calibration sample at the working voltage EHT i and recording the working distance WD i when a clear image is obtained at EHT i;

[0009] (4) Test the spot size of the scanning electron microscope at different positions on the calibration sample at EHT i of the sample;

[0010] (5) Calculate the relative distance ΔZ i required to be adjusted for the sample to be tested relative to WD i according to the test area ID Idi required for the sample to be tested;

[0011] (6) Use the scanning electron microscope SEM to obtain a clear image on the sample to be tested with the above EHT i and WD i , then only move the sample stage to change the sample position by ΔZ Idi . At this time, make the electron beam in the scanning electron microscope SEM in a non-scanning state and record the current I si on the sample stage;

[0012] (7) Use the scanning electron microscope SEM to obtain a clear image on the Faraday cup with the above EHT i and WD i , then make the electron beam in a non-scanning state and focus on the center position of the Faraday cup. At this time, record the current I pi on the sample stage;

[0013] (8) Calculate the secondary electron yield SEY i at the incident energy point Ep i = 1 - I si / I pi ;

[0014] (9) Repeat steps (3)-(8) to obtain a set of secondary electron yields SEY i at the required incident energy electrons Ep i .

[0015] The calibration sample is a sample that is prone to forming traces on the surface that are different from the unirradiated surface after long-term electron beam irradiation; the sample to be tested is a sample for which the secondary electron yield is desired to be obtained; the Faraday cup is a device adapted to the scanning electron microscope platform that can collect all the electron beams emitted by the electron gun.

[0016] The test area ID i is the area where the incident electrons irradiate the surface and emit secondary electrons, that is, the area where the electrons emitted by the electron gun in the SEM platform irradiate the sample. This test area ID i is not greater than the spot area of the electron gun at the maximum distance and minimum magnification of the sample in the SEM.

[0017] The working voltage EHTi Equal to the incident energy Ep corresponding to the secondary electron yield i .

[0018] Testing the beam spot size at different positions of the sample on the calibration sample with a scanning electron microscope at EHT i includes: keeping the electron beam stationary and moving the sample stage loaded with the calibration sample up or down by ΔZ ik , k = 1, 2,..., k is the number of times the sample stage moves; keeping the electron beam in the SEM in a non-scanning state for a period of time and ensuring that the calibration sample leaves a trace under the irradiation of the electron beam, and then adjusting the sample stage to WD i position to obtain a clear image, and measuring the size of the trace left on the calibration sample through the SEM scale, that is, the electron beam spot diameter is ED when the sample is at WD i under EHT i +ΔZ ik ; by changing the ΔZ ik position and repeating the above operations, a set of values corresponding to ΔZ ik and ED ik is obtained. ik

[0019] Calculating the relative distance ΔZ that the sample needs to be adjusted relative to WD i according to the required test area ID of the sample to be measured i , includes: performing data fitting on a set of measured data (ED Idi , ΔZ ik ) to obtain a fitting function of ΔZ = f(ED), and this fitting function ΔZ ik = satisfies f(ED ik ), calculating the corresponding ΔZ ik when ID i = ED Idi .

[0020] For samples with a smooth surface and consistent surface conditions or when the required test areas ID i of electron beam irradiation under different EHT i differ little, find the minimum EHT i in the step (2), that is, the SEY i corresponding to min(EHT i ); obtain a clear image at min(EHT i ) in the step (3), and record the WD i corresponding to min(EHT min(EHTi) ); calculate the relative distance ΔZ that the sample to be measured needs to be adjusted relative to WD min(EHTi) according to the steps (4) and (5) Idmin(EHTi) ​; For other different EHTs i the same WD is selected for all min(EHTi) and ΔZ Idmin(EHTi) , then steps (3)-(5) are not repeated, and only steps (6)-(8) are repeated.

[0021] For the same scanning electron microscope (SEM) platform, for the EHTs i and WDs i obtained in steps (3) and (4), the spot sizes at different corresponding positions, and the current I i and WD i of the Faraday cup in step (7) under the conditions of EHT pi , according to the stability degree of the SEM platform, for other samples to be tested within the stable range of the SEM platform, steps (3), (4), and (7) of the test do not need to be repeated.

[0022] The advantages of the present invention compared with the prior art are as follows:

[0023] (1) The present invention proposes a method for testing the secondary electron emission yield on a sample on a scanning electron microscope platform, providing a new method compared with testing the secondary electron emission yield by building a dedicated secondary electron emission test device. At the same time, the proposed method has simple test steps, high stability, good repeatability, high accuracy compared with the test results of the secondary electron emission test device, and strong feasibility;

[0024] (2) The method of the present invention can accurately select the test sample range through the electron scanning microscope platform, which is more convenient for testing the difference in secondary electron yield caused by surface states such as morphology and surface contamination at different positions on the analysis sample;

[0025] (3) The method of the present invention uses a Faraday cup to test the incident electron current, solving the error caused by the escape of high-energy electrons from the sample surface in the bias current method in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is the technical flow chart of the present invention;

[0027] Figure 2 is the comparison diagram of the test results of the Si sample on the metal secondary electron emission platform and the method of the present invention;

[0028] Figure 3 is the comparison diagram of the test results of the Au sample on the metal secondary electron emission platform and the method of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0029] The following is combined with the attached Figure 1The specific implementation manners are used to further explain and illustrate the present invention. A method for testing the secondary electron yield based on a scanning electron microscope platform of the present invention comprises the following steps:

[0030] (1) Place the Si sample to be tested, the Au sample to be tested, the Faraday cup, and the Si that has been exposed to air for a long time as a test sample into the Gemi 300SEM platform in the laboratory;

[0031] (2) It is necessary to test the SEY at the following incident energy points Ep i where i (i = 1, 2,..., 18), where

[0032] Ep = [60eV, 80eV, 100eV, 150eV, 200eV, 300eV, 400eV, 500eV, 600eV, 700eV, 800eV, 900eV, 1000eV, 1200eV, 1500eV, 2000eV, 2500eV, 3000eV];

[0033] For comparison with the test data of the secondary electron emission platform, the surfaces of the selected Si sample to be tested and the Au sample to be tested are relatively smooth and the surface states are basically the same, and the irradiated area of electrons ID> (π·1) um 2 ;

[0034] (3) Taking Ep 13 = 1000eV as an example, adjust the working voltage EHT 13 to 1keV and WD 13 to 3mm on the SEM platform to obtain a clear image of the Si that has been exposed to air for a long time;

[0035] (4) When the sample stage is lowered to ΔZ 13,1 = 5mm, after the non-scanning electron beam irradiates the Si sample that has been exposed to air for a long time for 10 minutes with focusing, move the sample stage up to the original position WD 13 , and measure that the diameter of the beam spot left on the sample is 52.23μm, and the area of this beam spot reaches the required test area ID;

[0036] (5) Since the area of the electron beam spot of the SEM reaches the required test area when WD 13 = 3mm and ΔZ 13,1 = 5mm, therefore, ΔZ Id13 = 5mm;

[0037] (6) Under the conditions of EHT 13 = 1keV and WD 13 = 3mm, obtain a clear image of the Si sample to be tested, then cancel Track Z, and lower the sample stage by ΔZ 13,1= 5 mm, click on the side bar "Specimen Current Monitor", select "Spot", and read out the current Is on the specimen stage at this time 13 = 26.2 nA;

[0038] (7) Apply EHT 13 = 1 keV, WD 13 = 3 mm to obtain a clear image, then click on "spot" to align the electron beam with the center of the Faraday cup, and read out the current Ip on the specimen stage at this time 13 = -97.1 nA;

[0039] (8) Use the formula SEY 13 = 1 - Is 13 / Ip 13 = 1.27, then the incident energy point Ep 13 = 1000 eV can be calculated, and the secondary electron yield is 1.27;

[0040] (9) On the SEM platform, adjust the working voltage EHT5 to 0.2 keV, and still keep WD5 at 3 mm to obtain a clear image. Then move the specimen stage down to ΔZ 5,1 = 5 mm. After irradiating with the focused non-scanning electron beam for 10 minutes, move the specimen stage back up to the original position WD5, and measure the irradiated spot diameter left on the specimen to be 55.32 μm;

[0041] Since the beam spot of Gemi 300SEM changes little in different voltage ranges, it can be considered that the irradiation areas under different EHT i are basically the same for specimens with relatively smooth surfaces and basically consistent surface states. Then the same ΔZ Id can be selected, and it is not necessary to repeat the test steps (3)-(5). Instead, execute step (6) with WD i +ΔZ Id = 3 mm + 5 mm, adjust EHT i , and test the current Is on the specimen stage i ;

[0042] On the Faraday cup, WD i = 5 mm, click on "spot" to align the electron beam with the center of the Faraday cup, and measure the current Ip on the specimen stage under different EHT i ; i ;

[0043] Figure 2 and Figure 3 give the results of testing three times on the scanning electron microscope using the method of this patent. Among them, the maximum deviation of the test results of the Si specimen from the mean value is 1.04%, and the maximum deviation of the test results of the Au specimen from the mean value is 0.48%.

[0044] Meanwhile, the test results on the secondary electron emission platform are given. The descriptions of the relevant equipment of the secondary electron emission platform can be found in the literature [High-performance multi-functional ultra-high vacuum metal secondary electron emission characteristic test platform, Journal of Vacuum Science and Technology, 2014, Vol. 34, No. 5, pp. 554-558].

[0045] As described above, it is only the best specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any change or replacement that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

[0046] The content not described in detail in the specification of the present invention belongs to the well-known technology of those skilled in the art.

Claims

1. A method for testing secondary electron yield based on a scanning electron microscope platform, characterized in that, Comprising: (1) A calibration sample, a sample to be measured, and a Faraday cup are simultaneously placed on a scanning electron microscope (SEM) platform; (2) Obtain the secondary electron yield SEY to be tested i and the corresponding incident energy point Ep i and the test area ID irradiated by electrons i , where i is the number of tests; (3) Use a scanning electron microscope (SEM) to obtain a clear image on the calibration sample at the working voltage EHT i and record the working distance WD at which a clear image is obtained at EHT i ; i ; (4) Test the spot size of the scanning electron microscope at different positions of the sample under EHT on the calibration sample i on the sample; (5) Calculate the relative distance ΔZ that needs to be adjusted for the sample to be measured with respect to WD according to the test area required by the sample to be measured i i Idi ; (6) Use a scanning electron microscope (SEM) with the above-mentioned EHT on the sample to be measured i and WD i to obtain a clear image, and then only move the sample stage to change the sample position by ΔZ Idi , at this time, keep the electron beam in the SEM in a non-scanning state and record the current I on the sample stage si ; (7) Using a scanning electron microscope SEM on the Faraday cup with the above-mentioned EHT i and WD i to obtain a clear image, and then keeping the electron beam in a non-scanning state and focusing it on the center position of the Faraday cup, at this time, record the current I on the sample stage pi ; (8) Calculate the incident energy point Ep i of the secondary electron yield SEY i = 1 - I si / I pi ; (9) Repeat steps (3)-(8) to obtain a set of incident energy electrons Ep required i of the secondary electron yield SEY i ; The calibration sample is a sample that is prone to forming a trace on the surface that is distinguishable from the unirradiated surface after long-term electron beam irradiation; the sample to be measured is a sample for which the secondary electron yield is desired to be obtained; the Faraday cup is a device adapted to the SEM platform and capable of completely collecting the electron beam emitted by the electron gun; The test area ID i is the area where incident electrons irradiate the surface and secondary electrons are emitted, that is, the area where the electrons emitted by the electron gun in the SEM platform irradiate the sample. This test area ID i is not larger than the spot area of the electron gun at the farthest distance and the minimum magnification of the sample in the SEM; The working voltage EHT i is equal to the incident energy Ep corresponding to the secondary electron yield i ; Test the spot size of the scanning electron microscope at different positions of the sample under EHT on the calibration sample, including: keeping the electron beam stationary, moving the sample stage loaded with the calibration sample up or down by ΔZ i , where k = 1, 2,..., k is the number of times the sample stage moves; keep the electron beam in the SEM in a non-scanning state for a period of time, and ensure that the calibration sample leaves a trace under the irradiation of the electron beam. Then, adjust the sample stage to the WD ik position to obtain a clear image, and measure the size of the trace left on the calibration sample through the SEM scale, that is, the electron beam spot diameter ED when the sample is at WD i under EHT i + ΔZ i ; by changing the ΔZ ik position and repeating the above operations, a set of corresponding values of ΔZ ik and ED ik are obtained; ik ik ​ The relative distance ΔZ to be adjusted for the sample with respect to WD is deduced according to the test area required for the sample to be measured i including: performing data fitting based on a set of measured data (ED i , ΔZ Idi ), obtaining a fitting function of ΔZ = f(ED), where this fitting function ΔZ ik = satisfies f(ED ik ), calculating the corresponding ΔZ when ID ik = ED ik ; i Idi ​​ For samples with smooth surfaces and consistent surface conditions or different EHT i The test area ID required for electron beam irradiation is i When the difference is almost the same, find the smallest EHT in step (2) i , that is, min(EHT i ) corresponding to SEY i In step (3), at min(EHT i ) to obtain a clear image and record min(EHT i ) corresponding to WD min(EHTi) According to the steps (4) and (5), the relative WD of the sample to be tested is calculated. min(EHTi) The relative distance ΔZ that needs to be adjusted Idmin(EHTi) ; For other different EHT i Use the same WD min(EHTi) and ΔZ Idmin(EHTi) , then steps (3)-(5) are not repeated, and only steps (6)-(8) are repeated; For the same scanning electron microscope (SEM) platform, for the EHT obtained in steps (3) and (4) i and WD i at different corresponding positions, the spot size, and in step (7), the current I i of the Faraday cup under the conditions of EHT i and WD pi , according to the stability of the SEM platform, for other samples to be tested, it is not necessary to repeat steps (3), (4), and (7) within the stable range of the SEM platform.

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