Experimental method for testing electron optical defocusing amount in scanning electron microscope sample immersion mode

Abstract

The invention provides an experimental method for testing the electron optical defocusing amount in a scanning electron microscope sample immersion mode, and the method comprises the steps: collecting an image under a preset condition, and recording the characteristics of a specific sample; applying a first preset sample-to-ground bias to the sample, adjusting the position of the sample until the image is refocused and positioned to the same feature, and obtaining the lateral magnification according to the pixel proportion of the feature size; imaging under a preset condition, moving the sample, and recording a magnetic lens current change value required for refocusing the image after each displacement; applying a first preset sample-to-ground bias voltage to the sample, setting the magnetic lens as a recorded current value, recording a sample displacement amount required for refocusing the sample, and obtaining a longitudinal magnification through linear fitting of the sample displacement amount before and after the bias voltage; the quantitative relation between the defocusing amount and the sample displacement is established by using the transverse magnification and the longitudinal magnification measured by experiments. The method does not need to know the electron optical characteristics of a scanning electron microscope system in advance, has no special requirements on samples, and does not involve image processing.

Description

Technical Field

[0001] This invention relates to the field of scanning electron microscopy, and in particular to an experimental method for testing electron optical defocus in scanning electron microscope sample immersion mode. Background Technology

[0002] Low-pressure scanning electron microscopy (SEM) is widely used in scientific research and industrial production due to its significant advantages such as high surface sensitivity, minimal radiation damage, and small electron interaction volume. To improve the resolution of SEMs, electromagnetic composite mirrors are used in electron optics design. This not only enhances the system's anti-interference capability but also further reduces spherical aberration and chromatic aberration coefficients during low-pressure operation, ensuring system resolution. Currently, mainstream Booster acceleration and sample deceleration technologies fall into this category. When a sample is immersed in a strong electric field, the electron trajectory is not constant before reaching the sample; therefore, the electron optical defocusing amount cannot be simply assumed to be equal to the sample displacement. Accurately understanding and testing the defocusing amount of a scanning electron microscope in immersion mode is crucial for optimizing imaging operations, imaging analysis, and intuitive simulations in practice.

[0003] Therefore, there is an urgent need for an effective testing method to test the defocusing amount under this imaging adjustment. Summary of the Invention

[0004] To achieve the above-mentioned objectives and other advantages of the present invention, the objective of the present invention is to provide an experimental method for testing electron optical defocusing in scanning electron microscope sample immersion mode, comprising the following steps: Images are acquired under preset conditions, and specific sample characteristics are recorded; Apply a first preset sample-to-ground bias voltage to the sample while keeping other parameters unchanged, adjust the sample position until the image is refocused and located on the same feature, and calculate the lateral magnification based on the pixel ratio of the feature size. Image again under the preset conditions, move the sample and record the change in magnetic lens current required to refocus the image after each displacement; Apply the first preset sample-to-ground bias voltage to the sample while keeping other parameters unchanged, set the magnetic lens to the recorded current value, record the sample displacement required to refocus the sample, and obtain the longitudinal magnification by linear fitting of the sample displacement before and after the bias voltage. Using the experimentally measured lateral and longitudinal magnification, a quantitative relationship between defocusing amount and sample displacement was established.

[0005] Furthermore, the preset conditions are configured to include the electron flight energy inside the microscope tube, the voltage of the accelerating tube to ground inside the microscope tube, the voltage of the sample to ground, and the working distance.

[0006] Furthermore, the first preset sample bias voltage to ground is greater than the sample bias voltage to ground in the preset conditions.

[0007] Furthermore, in the preset conditions, the electron's flight energy inside the microscope tube is 8100eV, the voltage of the accelerating tube to ground inside the microscope tube is 8000V, and the sample's bias voltage to ground is 0V. The beam landing voltage corresponding to these preset conditions is 100V.

[0008] Furthermore, the first preset sample has a ground bias voltage of 8000V, and its corresponding beam landing voltage is 8100V.

[0009] Furthermore, the step of adjusting the sample position until the image is refocused and located on the same feature specifically involves lowering the sample position using a high-precision sample stage until the image is refocused and located on the same feature.

[0010] Furthermore, the step of moving the sample and recording the change in magnetic lens current required to refocus the image after each displacement specifically involves moving the sample from a first preset distance to a second preset distance with a preset step size, and recording the change in magnetic lens current required to refocus the image after each displacement.

[0011] Furthermore, the first preset distance is -10µm, the second preset distance is +10µm, and the preset step size is 1µm.

[0012] Furthermore, the step of establishing a quantitative relationship between defocusing amount and sample displacement specifically involves establishing a quantitative relationship between defocusing amount and sample displacement using a formula for defocusing amount and sample displacement. The formula for defocusing amount and sample displacement is as follows: in, Indicates the ratio. Indicates the amount of defocus. Indicates sample displacement. Indicates the horizontal magnification. Indicates the vertical magnification. and Representing the sample planes respectively With virtual plane The electric potential at that point.

[0013] Compared with the prior art, the beneficial effects of the present invention are: This invention provides an experimental method for testing electron optical defocus in scanning electron microscope sample immersion mode. This method does not require prior knowledge of the electron optical characteristics of the scanning electron microscope system, has no special requirements for the sample, and does not involve image processing.

[0014] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it according to the contents of the specification, the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. Specific embodiments of the present invention are given in detail below with reference to the accompanying drawings. Attached Figure Description

[0015] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate exemplary embodiments of the invention and, together with their description, serve to explain the invention and do not constitute an undue limitation thereof. In the drawings: Figure 1 Flowchart of the experimental method for testing electron optical defocusing in scanning electron microscope sample immersion mode; Figure 2 This is a schematic diagram illustrating the experimental principle for testing electron optical defocusing in scanning electron microscope sample immersion mode. Detailed Implementation

[0016] The present invention will now be further described with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. It should be noted that, without conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0017] Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention.

[0018] The drawing numbers in this application are only used to distinguish the steps in the scheme and are not used to limit the execution order of the steps. The specific execution order is as described in the specification.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0020] This invention provides an experimental method for testing electron optical defocusing in scanning electron microscope sample immersion mode. This method is not sample-specific, requires no image processing, and does not require prior knowledge of electron optical systems.

[0021] Example 1 For thin magnetic lenses, object distance Image distance Through lens formulas and focal length Related: The refractive power of the magnetic lens is Assuming object distance Fixed, when the excitation current changes slightly At that time, Image distance change at location for: in, This represents the lateral magnification of the magnetic lens, and By this The virtual plane caused Lateral offset at the location for: here yes The electron beam half-angle at that location.

[0022] On the sample plane The corresponding horizontal offset for: in, It is the lateral magnification of the electrostatic immersion lens.

[0023] Sample plane Defocus at the point Defined as: Half angle at the sample plane Half-angle at the virtual plane Under the first-order approximation, the Lagrange-Helmholtz invariant correlation leads to the following: in, and They are respectively and The electric potential at that point.

[0024] Substituting equation (6) into equation (5), we get: If the longitudinal magnification of the electrostatic immersion lens is Then the sample displacement With current deviation The relationship is: Combining equations (7) and (8), we obtain the basic relation: This expression indicates that the ratio It depends only on the lateral and longitudinal magnification of the electrostatic immersion lens, and the square root of the ratio of the potential of the sample plane to that of the virtual plane.

[0025] Formula (9) establishes a universal relationship between defocus and sample displacement based on electro-optical properties, particularly the lateral and longitudinal magnification of electrostatic immersion lenses. For two-electrode lenses, these magnifications can be expressed by simple analytical formulas, thus demonstrating the relationship with high precision. For three-electrode lenses, numerical calculations are required based on their specific geometric configuration. Nevertheless, Formula (9) still proposes a practical experimental method for quantitatively measuring defocus, as follows (taking a two-electrode lens as an example; this method also applies to three-electrode lenses): An experimental method for testing electron optical defocusing in scanning electron microscopy sample immersion mode, such as... Figure 1 As shown, it includes the following steps: S100. Acquire images under preset conditions and record specific sample characteristics; In this embodiment, the preset conditions are configured to include the electron flight energy within the microscope tube, the voltage of the accelerating tube to ground within the microscope tube, the sample bias voltage to ground, and the working distance. Specifically, as shown... Figure 2 As shown, in the preset conditions, the electron's flight energy within the microscope tube is 8100 eV, the voltage of the booster tube to ground within the tube is 8000 V, the sample's bias voltage to ground is 0 V, and a decelerating lens is formed between the sample and the booster tube. The voltage reaching the sample, i.e., the landing voltage, depends on the potential difference between the sample and the microscope tube. Therefore, this invention determines the landing voltage by controlling the sample's bias voltage to ground. The beam landing voltage corresponding to this preset condition is 100 V. For example, images are acquired and specific sample characteristics are recorded under conditions of a sample bias voltage of 8000 V to ground, a beam landing voltage of 100 V, and a specified working distance (e.g., a = 1 mm).

[0026] S200: Apply a first preset sample-to-ground bias voltage to the sample while keeping other parameters unchanged, adjust the sample position until the image is refocused and located on the same feature, and calculate the horizontal magnification based on the pixel ratio of the feature size. In this embodiment, the first preset sample bias voltage to ground is greater than the sample bias voltage to ground in the preset conditions. Further, the first preset sample bias voltage to ground is 8000V, and its corresponding beam landing voltage is 8100V.

[0027] Specifically, keeping other parameters constant, an 8000V positive bias voltage is applied to the sample, and the sample position is lowered using a high-precision sample stage until the image is refocused and located on the same feature. The lateral magnification can be calculated based on the pixel ratio of the feature size. .

[0028] S300, Image again under the preset conditions, move the sample and record the change in magnetic lens current required to refocus the image after each displacement; In this embodiment, the step of moving the sample and recording the change in magnetic lens current required to refocus the image after each displacement specifically involves moving the sample from a first preset distance to a second preset distance with a preset step size, and recording the change in magnetic lens current required to refocus the image after each displacement. Further, the first preset distance is -10µm, the second preset distance is +10µm, and the preset step size is 1µm.

[0029] Specifically, imaging was performed again under the conditions of 0V sample-to-ground bias, 100V beam landing voltage, and the same working distance (e.g., a=1mm). The sample was moved from –10µm to +10µm in 1µm steps, and the change in magnetic lens current required to refocus the image after each displacement was recorded.

[0030] This invention calibrates the sensitivity of a magnetic lens to the longitudinal position of a sample by establishing a linear relationship between current and displacement. For example, a 1 mA change in current corresponds to approximately 0.5 µm of sample displacement, directly reflecting the longitudinal magnification.

[0031] S400. Apply the first preset sample-to-ground bias voltage to the sample while keeping other parameters unchanged, set the magnetic lens to the recorded current value, record the sample displacement required to refocus the sample, and obtain the longitudinal magnification by linear fitting of the sample displacement before and after the bias voltage. Specifically, with an 8000V positive bias applied to the sample and other parameters remaining constant, the magnetic lens is set to the current value recorded in step S300. The sample displacement required to refocus the sample is recorded. By linearly fitting the sample displacement before and after the bias, the longitudinal magnification can be obtained. .

[0032] S500, using experimentally measured lateral magnification and vertical magnification The quantitative relationship between defocusing amount and sample displacement is established by formula (9).

[0033] This invention achieves high-precision calibration of lateral and longitudinal magnification by using a motor-driven sample displacement (1 µm step) and magnetic lens current control. By adjusting the positive bias voltage (e.g., from 0 V to 8000 V), the lateral magnification of the deceleration lens between the booster and the sample can be extracted. and vertical magnification This method adapts to the imaging needs of different materials. The established relationship between defocusing and sample displacement can be extended to other SEM operating conditions (such as different beam energies and different working distances), providing a general method for instrument calibration and imaging optimization. This method does not require prior knowledge of the electron optical characteristics of the scanning electron microscope system, has no special requirements for the sample, and does not involve image processing.

[0034] The number of devices and processing scale described herein are for the purpose of simplifying the description of the invention. Applications, modifications, and variations of the invention will be readily apparent to those skilled in the art.

[0035] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

[0036] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0037] The above description is merely an embodiment of this specification and is not intended to limit the scope of one or more embodiments of this specification. Various modifications and variations can be made to one or more embodiments of this specification by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principle of one or more embodiments of this specification should be included within the scope of the claims of one or more embodiments of this specification.

Claims

1. An experimental method for testing the electron optical defocus in scanning electron microscope sample immersion mode, characterized in that, The method comprises the following steps: Collecting images under preset conditions and recording specific sample features; Applying a first preset sample-to-ground bias to the sample and keeping other parameters unchanged, adjusting the sample position until the image is refocused and positioned to the same feature, and calculating the lateral magnification according to the pixel ratio of the feature size; Imaging again under the preset conditions, moving the sample and recording the magnetic lens current change value required for refocusing after each displacement; Applying the first preset sample-to-ground bias to the sample and keeping other parameters unchanged, setting the magnetic lens to the recorded current value, recording the sample displacement required for refocusing, and obtaining the longitudinal magnification by linear fitting of the sample displacement before and after the bias; Using the experimentally measured lateral magnification and longitudinal magnification, a quantitative relationship between the defocus amount and the sample displacement is established.

2. The method of claim 1, wherein the method is performed in a scanning electron microscope in sample immersion mode. The preset conditions are configured to include the electron flight energy in the lens barrel, the accelerating tube-to-ground voltage in the lens barrel, the sample-to-ground bias, and the working distance.

3. The method of claim 2, wherein the method is performed in a scanning electron microscope in a sample immersion mode. The first preset sample-to-ground bias is greater than the sample-to-ground bias in the preset conditions.

4. The experimental method for testing electron optical defocus in scanning electron microscope sample immersion mode as described in claim 3, characterized in that: The electron flight energy in the lens barrel in the preset conditions is 8100 eV, the accelerating tube-to-ground voltage in the lens barrel is 8000 V, and the sample-to-ground bias is 0 V. The beam landing voltage corresponding to the preset conditions is 100 V.

5. The method of claim 4, wherein the method further comprises: determining the defocus of the electron beam by measuring the intensity of the backscattered electron signal at the plurality of different locations on the sample. 5 The first preset sample-to-ground bias is 8000 V, and the corresponding beam landing voltage is 8100 V.

6. The method of claim 3, wherein the method is used for testing the electron optical defocus of a scanning electron microscope in sample immersion mode. The step of adjusting the sample position until the image is refocused and positioned to the same feature is specifically lowering the sample position by a high-precision sample stage until the image is refocused and positioned to the same feature.

7. The method of claim 2, wherein the method is used for testing the electron optical defocus of a scanning electron microscope in sample immersion mode. The step of moving the sample and recording the magnetic lens current change value required for refocusing after each displacement is specifically moving the sample from a first preset distance to a second preset distance, moving at a preset step size, and recording the magnetic lens current change value required for refocusing after each displacement.

8. The method of claim 7, wherein the method is performed in a scanning electron microscope in sample immersion mode. The first preset distance is -10 µm, the second preset distance is +10 µm, and the preset step size is 1 µm.

9. The method of claim 1, wherein the method is used for testing the electron optical defocus of an electron microscope in sample immersion mode. The step of establishing a quantitative relationship between the defocus amount and the sample displacement is specifically establishing a quantitative relationship between the defocus amount and the sample displacement by a defocus amount and sample displacement formula, and the defocus amount and sample displacement formula is: wherein denotes a ratio, denotes a defocus amount, denotes a sample displacement, denotes a lateral magnification, denotes a longitudinal magnification, denotes a potential at denotes a potential at denotes a potential at denotes a potential at

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