Compound electromagnetic objective, scanning electron beam imaging device and imaging method

Abstract

The present application belongs to the technical field of scanning electron beam imaging system, and particularly relates to a composite electromagnetic objective lens, a scanning electron beam imaging device and an imaging method, comprising a first magnetic lens, a second magnetic lens and a floating pole shoe; the first magnetic lens and the second magnetic lens have a shared pole shoe, the first magnetic lens and the floating pole shoe form a radial gap on the side close to the sample; the second magnetic lens and the floating pole shoe form an axial gap along the optical axis direction; the floating pole shoe is arranged at the end of the shared pole shoe and is electrically insulated from the shared pole shoe at high voltage; the floating pole shoe is configured to be capable of being applied with an independent bias to form an electrostatic lens. The present application is compatible with low landing energy CDSEM surface layer imaging and high landing energy HVSEM bottom / or shallow layer under pattern imaging in the same electron optical structure, and controls the return trajectories of SE and BSE and the detector distribution ratio, so that signal electrons of different types or different sources can be collected layer by layer.

Description

Technical Field

[0001] This invention belongs to the technical field of scanning electron beam imaging systems, and specifically relates to a composite electromagnetic objective, a scanning electron beam imaging device, and an imaging method. Background Technology

[0002] In the semiconductor integrated circuit manufacturing process, it is necessary to measure the surface pattern, high aspect ratio bottom structure, and the overlay accuracy of multilayer patterns. In scanning electron beam imaging, secondary electrons (SE) have low energy and shallow escape depth, mainly reflecting the surface morphology; backscattered electrons (BSE) have high energy and large escape depth, reflecting the contrast of the underlying or bottom material. Therefore, low landing energy CDSEM is suitable for measuring critical surface dimensions, while high landing energy HVSEM is more suitable for measuring high aspect ratio bottom patterns and multilayer pattern overlay measurements.

[0003] Existing low-landing-energy scanning electron beam imaging systems typically employ a negative sample bias to create a repulsion field, combined with a boost electrode or half-stroke boost structure, to maintain high energy transmission of the main electron beam within the microscope tube and decelerate it upon landing near the sample, thus achieving both low sample damage and high resolution. However, when the object of investigation expands from surface patterns to high aspect ratio aperture structures, shallow subsurface patterns, or overlaid targets, low-landing-energy modes alone are insufficient to obtain adequate underlying information. While high-voltage electron microscopes or high-landing-energy scanning electron beam imaging devices can enhance backscattered electron signals, the wide range of backscattered electron escape angles and the difficulty in controlling their trajectories typically require complex detector arrangements and electron optics designs, resulting in high equipment structure, debugging, and maintenance costs.

[0004] Chinese Patent Publication No. CN115714080A discloses a scanning electron beam imaging device and imaging method. It forms two magnetic lenses through a first ring coil and a second ring coil, and obtains magnetic field distributions on different axes by adjusting the current direction and ratio of the two sets of coils to adapt to high-energy or low-energy electron beam imaging. At the same time, the pole piece can also be extended to form a metal electrode, and a positive high voltage is applied as a boosting electrode or boosting lens.

[0005] The above-mentioned scheme can improve the focusing of the main electron beam and the imaging adaptability under different energy conditions. However, its control focus is mainly on the on-axis magnetic field distribution and focusing state of the main electron beam. For signal electrons such as SE and BSE generated after the sample is bombarded, especially signal electrons with different energies, different escape angles, and different generation positions, there is still a lack of fine-grained return trajectory control. When HVSEM detects deep holes, deep trenches, or multi-layer structures, the BSE emitted from the wide angle is difficult to stably guide into the predetermined detection area, and a large number of stray secondary electrons generated on the sidewalls can easily overwhelm the weak effective signal at the bottom, resulting in blurred bottom imaging and insufficient contrast.

[0006] Furthermore, to address the issue of strong stray secondary electrons on the sidewalls and weak effective signals at the bottom in high aspect ratio structures, it is also necessary to solve the problem of controlling the initial divergence and return paths of the stray secondary electrons on the sidewalls and the weak effective signals at the bottom, thereby improving the image clarity and signal-to-noise ratio at the bottom. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a composite electromagnetic objective lens that is compatible with both low-landing-energy CDSEM surface imaging and high-landing-energy HVSEM bottom imaging within the same electro-optical structure. It also controls the return trajectories of SE and BSE and the detector allocation ratio, enabling the acquisition of signal electrons of different types or sources in a layered manner.

[0008] The composite electromagnetic objective lens includes: a first magnetic lens, a second magnetic lens, and a suspended pole piece; The first magnetic lens includes a first pole piece, a common pole piece, and a first coil. The first coil is located between the first pole piece and the common pole piece to provide a first magnetic field. The first pole piece and the suspended pole piece form a radial gap on the side close to the sample. The second magnetic lens includes a second pole piece, a common pole piece, and a second coil. The second coil is located between the common pole piece and the second pole piece to provide a second magnetic field. An axial gap is formed between the second pole piece and the suspended pole piece along the optical axis. The first coil and the second coil are connected to an excitation control circuit that can adjust the magnitude and direction of their excitation currents respectively, so that the first magnetic field and the second magnetic field can be formed independently, superimposed in the same direction, or superimposed in opposite directions. The suspended pole piece is located at the end of the common pole piece and is electrically insulated from the common pole piece by a high voltage; the suspended pole piece is configured to be able to be independently biased to form an electrostatic lens.

[0009] Preferably, the suspended pole piece extends obliquely from the lower end of the shared pole piece toward the center of the optical axis and is located within the objective lens exit area formed by the first pole piece and the second pole piece.

[0010] Preferably, a first electrostatic lens is formed between the suspending pole piece and the adjacent pole piece to adjust the main electron beam energy in the end region of the objective lens; a second electrostatic lens is formed between the suspending pole piece and the sample to adjust the landing energy of the main electron beam and to provide a traction electric field toward the interior of the objective lens for the signal electrons excited by the sample.

[0011] Furthermore, in order to address the problem of how to control the initial divergence and return paths of stray secondary electrons on the sidewalls and the weak effective signals at the bottom, thereby improving the clarity and signal-to-noise ratio of bottom imaging, this application also includes a field control electrode, which is coaxially arranged between the suspended pole piece and the sample. The field control electrode voltage is independently adjustable, which is used to fine-tune the equipotential line distribution on the sample surface, change the secondary electron deflection trajectory, and achieve selective separation and collection of secondary electrons and backscattered electrons.

[0012] Another object of the present invention is to provide a scanning electron beam imaging device, including an electron beam emitter, a scanning deflection system, a sample stage, a detector system, a power control system, and the above-mentioned composite electromagnetic objective lens; The electron beam emitter is used to emit an initial electron beam; The scanning deflection system is used to achieve electron beam grating scanning; The composite electromagnetic objective is located at the end of the electron optical path near the sample and is used to perform electron beam focusing, energy regulation and signal electron traction. The detector system is arranged in a gradient along the optical axis to collect secondary electrons and backscattered electrons at different scattering angles in layers. The excitation control circuit of the power control system is electrically connected to the first coil, the second coil, the suspended pole piece, and the sample stage, respectively, and is used to adjust the excitation current of the first coil and the second coil, as well as the bias voltage of the suspended pole piece and the sample.

[0013] The composite electromagnetic objective also includes a voltage-independently adjustable field control electrode, which is coaxially arranged between the suspending pole piece and the sample. The field control electrode is used to change the proportion of secondary electrons that pass through at least one detector central aperture or are intercepted by the detector.

[0014] Another object of the present invention is to provide a scanning electron beam imaging method, which, using the above-described imaging apparatus, includes the following steps: The electron beam emitter emits an electron beam, which is then scanned on the sample surface by a scanning deflection system; The imaging device selects the excitation combination of the first magnetic lens and the second magnetic lens according to the target imaging conditions to form a magnetic field distribution on the target axis; The imaging device applies an independent bias voltage to the suspended pole piece to form a first electrostatic lens, and applies a bias voltage to the sample to form a second electrostatic lens. The first electrostatic lens increases the electron beam energy, which can effectively reduce chromatic aberration; the second electrostatic lens controls the landing energy of the main electron beam and the trajectory of the emitted electrons. The main electron beam bombards the sample to excite signal electrons, and the magnetic field and electrostatic field coupling effect is used to pull the signal electrons upward and back; Multi-mode imaging is achieved by classifying and collecting signal electrons within each return trajectory range using multiple detectors arranged along the optical axis.

[0015] Preferably, the first electrostatic lens and the second electrostatic lens work together to perform electrostatic focusing: In the low landing energy mode, the suspended pole shoe applies a positive high voltage and the sample applies a negative bias voltage. The two electrostatic fields work together to form a strong deceleration electric field, which enables the main electron beam to land at low energy. Together with the aforementioned magnetic lens, it achieves an electron optical system with low spherical aberration and chromatic aberration, thus meeting the requirements for high-resolution electron beam imaging at low landing energy. In the high landing energy mode, the suspended pole shoe is grounded or a negative bias is applied. The bias applied to the sample is a negative bias that is relatively small compared to the incident energy of the electron beam, so that the main electron beam is incident on the sample at a landing energy higher than that in the low landing energy mode, and an electric field is formed on the sample surface to guide the signal electrons back into the objective lens. The magnitude of the bias is only required to accelerate the signal electrons escaping from the sample surface back into the objective lens, so that the high-energy main electron beam does not lose too much energy and still bombards the sample with a relatively high landing energy. Preferably, the imaging device controls the first magnetic lens to be excited individually, so that the on-axis magnetic field peak is close to the sample, forming an immersion strong magnetic field; The first electrostatic lens applies a positive high voltage to the descending main electron beam for pre-acceleration, reducing electron beam energy dispersion and chromatic aberration; the second electrostatic lens uses the sample negative bias voltage to form a strong repulsion deceleration field, decelerating the electron beam to a low energy landing.

[0016] Preferably, the imaging device controls the second magnetic lens to be excited separately, and the on-axis magnetic field peak is far away from the sample, forming a weak immersion or non-immersion magnetic field; The first electrostatic lens reduces or changes the positive bias voltage of the floating pole piece; the second electrostatic lens reduces the negative bias voltage of the sample, weakens the repulsion electric field, and enables the electron beam to land at a high energy.

[0017] Preferably, the imaging device controls the first magnetic lens and the second magnetic lens to be excited in the same direction, and adjusts the excitation ratio of the first magnetic lens and the second magnetic lens so that the two magnetic fields are superimposed along the optical axis, so as to enhance the main electron beam focusing ability under high landing energy conditions and increase the proportion of backscattered electrons entering the objective lens.

[0018] Preferably, the imaging device controls the first magnetic lens and the second magnetic lens to be excited in opposite directions, so that the first magnetic field and the second magnetic field cancel each other out in at least a part of the axial region, thereby reducing the proportion of backscattered electrons entering part of the detector and changing the distribution ratio of secondary electrons among multiple detectors.

[0019] Furthermore, the method for configuring the excitation ratio of the composite electromagnetic objective is as follows: Calculate the third-order aberration coefficients of the electron optical system based on the third-order aberration theory of electron optics; To evaluate the beam spot size, considering the contributions of the Gaussian spot and the diffraction spot, a set of curves showing the relationship between the beam spot size and the NI1 / NI2 excitation ratio were found, where NI1 is the excitation value of the first magnetic lens and NI2 is the excitation value of the second magnetic lens. Obtaining the collection rate of signal electrons: Based on electron optical simulation, NI1 and NI2 are obtained, and the return trajectory of signal electrons is simulated to obtain the collection rate and separation rate of signal electrons on each detector; Select an excitation combination that meets both the resolution requirements and the requirements for various signal separation and detection.

[0020] Furthermore, the voltage of the field control electrode is adjusted according to the excitation mode of the first magnetic lens and the second magnetic lens to change the capture ratio of secondary electrons and backscattered electrons on multiple detectors.

[0021] For example, in the low landing energy mode, the field control electrode is applied with a positive or negative voltage relative to the ground; the field control electrode, together with the high voltage of the suspension pole piece which is positive relative to the ground, adjusts the equipotential lines on the sample surface, constrains the divergence trajectory of low-energy secondary electrons, suppresses the proportion of backscattered electrons, and improves the clarity of fine line edges. In high landing energy mode, the field control electrodes are applied with positive, zero, or weakly negative potentials, which can adjust the trajectory of high-energy backscattered electrons or secondary electrons with large opening angles.

[0022] Preferably, in the dual magnetic lens co-excitation mode, the field control electrode is given a positive potential, which, together with the immersion magnetic field of the first magnetic lens, enhances the collection of backscattered electrons escaping at a large angle, increases the proportion of signal electrons reflected back into the lens tube, and, together with the detectors arranged in an axial gradient, enables the layered acquisition of SE and BSE, thereby improving the overlay measurement accuracy. Preferably, in the dual magnetic lens reverse excitation mode, the field control electrode is subjected to an adjustable negative bias voltage to weaken the surface electric field, thereby weakening the convergence effect on secondary electrons and backscattered electrons with large opening angles, causing them to be blocked by the bottom detector, while the weak signal electrons with small opening angles escaping from the bottom are still converged near the optical axis and captured by the upper and middle detectors, thereby improving the contrast of the bottom image of the high aspect ratio structure.

[0023] The beneficial effects of this invention are as follows: This invention achieves compatible imaging for both low-landing-energy CDSEM and high-landing-energy HVSEM by incorporating independently adjustable first and second magnetic lenses and a levitating pole piece within the same composite electromagnetic objective. By individually, in the same direction or opposite directions, excitation of the two magnetic lenses allows adjustment of the on-axis magnetic field distribution, constraining wide-angle escaped electron beams (BSEs) and altering their detector capture ratio. Adjustment of the near-sample electric field via the levitating pole piece, sample bias, and field control electrodes distinguishes between sidewall stray secondary electrons and weak effective signals from the bottom. Combined with multi-stage detectors for layered acquisition, this invention improves the imaging clarity and overlay measurement capabilities of deep holes and deep trenches, while reducing the production and maintenance costs associated with separate CDSEM and HVSEM configurations.

[0024] This invention, by placing a field control electrode between the suspended electrode and the sample, can adjust the distribution of equipotential lines in the near-sample region. Utilizing the characteristic that the electric field's ability to converge signal electrons varies with incident angle and energy differences, the size of the trajectory envelope of the returning signal electrons in the central aperture of each detector level can be adjusted, thus achieving feature signal separation. This solution can be applied to: multi-layer patterned sample scenarios, utilizing energy differences to achieve the separation and detection of low-energy SE signals on the surface and high-energy BSE signals at the bottom layer on detectors at each level; or to multi-layered patterned high aspect ratio structures, utilizing escape angle differences to effectively separate weak signals with small escape angles at the bottom of the high aspect ratio structure at the detector end, improving the purity of the target image signal, thereby improving the clarity of each layer of the pattern, and the clarity and contrast of imaging the bottom of high aspect ratio structures such as deep holes and deep grooves. Attached Figure Description

[0025] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a schematic diagram of the structure of the composite electromagnetic objective lens of the present invention; Figure 2 This is an on-axis magnetic field distribution diagram of the dual magnetic lens under different excitation modes in one embodiment of the present invention; Figure 3 This is an equipotential line and electric field distribution diagram under different landing energy modes in one embodiment of the present invention; Figure 4 This is a schematic diagram of the layout and signal coverage of the detector system of the present invention; Figure 5 This is a trajectory simulation diagram of the signal coverage of the detector system of the present invention; Figure 6 This is the BSE and SE trajectory envelope diagram corresponding to Embodiment 8 of the present invention; Figure 7 This is the BSE and SE trajectory envelope diagram corresponding to Embodiment 9 of the present invention; Figure 8 This is the BSE and SE trajectory envelope diagram corresponding to Embodiment 10 of the present invention; Figure 9 This is the BSE and SE trajectory envelope diagram corresponding to Embodiment 11 of the present invention; Figure 10 This is a schematic diagram of a composite electromagnetic objective lens that uses field control electrodes to control the field on the sample surface. Figure 11 This is a schematic diagram of the SE trajectory under different field control electrode voltages in embodiment 12 of the present invention in a high-voltage application scenario; Figure 12 This is a schematic diagram of the BSE and SE trajectory adjustment under the condition of 0kV bias voltage of the field control electrode in embodiment 13 of the present invention. Figure 13 This is a schematic diagram of the BSE and SE trajectory adjustment under the condition of -5kV bias voltage of field control electrode in embodiment 14 of the present invention. Figure 14 This is a schematic diagram of the overall module layout of the scanning electron beam imaging device according to Embodiment 15 of the present invention.

[0026] The diagram is marked as follows: 001, Electron beam scanning imaging system; 010, Electron beam emitter; 100, Anode; 101, Main electron beam; 201, Condenser lens; 202, Aperture stop; 203, Wien filter; 205, Deflector; 206, Deflector; 002. Silicon wafer transmission system; 003. Electrical control system; 301. Install the detector; 1021, the signal electron received by the upper detector; 1021(BSE), the BSE received by the upper detector; 1021(SE), the SE received by the upper detector; 302. Medium detector; 1022, the signal electron received by the detector; 1022(BSE), the BSE received by the detector; 1022(SE), the SE received by the detector; 303. Lower detector; 1023, the signal electron received by the lower detector; 1023(BSE), the BSE received by the lower detector; 1023(SE), the SE received by the lower detector; 1024, Signal electrons that did not pass through the detector; 1024(BSE), BSE that did not pass through the detector; 1024(SE), SE that did not pass through the detector; 1020, signal electrons that have passed the upper detector; 1020(BSE), BSE that has passed the upper detector; 1020(SE), SE that has passed the upper detector; 1021A, signal electrons captured by an upper detector with off-axis deflection; 400, Composite electromagnetic objective; 401, First pole piece; 402, Second pole piece; 403, Common pole piece; 401C, First coil; 402C, Second coil; 403A, Suspended pole piece; 404, Field control electrode; 4011, Radial slit; 4012, Axial slit; 500. Sample stage; 501. Sample; 601. Voltage source one; 602. Voltage source two. Detailed Implementation

[0027] like Figure 1 As shown, this embodiment discloses a composite electromagnetic objective lens, including a first magnetic lens, a second magnetic lens, and a suspended pole piece 403A.

[0028] The first magnetic lens includes a first coil 401C, a first pole piece 401, and a common pole piece 403. The first coil 401C is located between the first pole piece 401 and the common pole piece 403 to provide a first magnetic field. The first pole piece 401 and the suspended pole piece 403A form a radial gap 4011 on the sample side, which forms an immersion magnetic field near the sample after being energized.

[0029] The second magnetic lens includes a second coil 402C, a second pole piece 402, and a common pole piece 403. The second coil 402C is located between the second pole piece 402 and the common pole piece 403 to provide a second magnetic field. An axial gap 4012 is formed between the second pole piece 402 and the suspended pole piece 403A along the optical axis. After being electrically excited, a weak immersion or non-immersion magnetic field is formed away from the sample.

[0030] A suspended pole piece 403A is disposed at the end of the common pole piece 403 and is electrically isolated from the common pole piece 403 by an insulating medium. The term "suspended" refers to the fact that its potential can be independently set relative to the adjacent pole piece. The suspended pole piece 403A extends obliquely from the lower end of the common pole piece towards the center of the optical axis and is located within the objective lens exit region formed by the first and second pole pieces. The suspended pole piece 403A can be independently biased by voltage source 601 to form a first electrostatic lens; the sample 501 can be negatively biased by voltage source 602 to form a second electrostatic lens together with the suspended pole piece 403A. The levitation pole piece 403A of this application is not a standalone boosting electrode, but is located at the intersection of three regions: the exit of the first magnetic lens, the paraxial side of the second magnetic lens, and the near-field electric field region of the sample. Its axial and radial arrangement causes the magnetic field control region and the electrostatic field control region to overlap near the sample near field, so that the excitation mode of the dual magnetic lens, the levitation pole piece bias voltage and the sample bias voltage can work together to affect the signal electron return trajectory. It can simultaneously affect the main electron beam focusing, the BSE return angle and the off-axis divergence of the SE without changing the objective lens mechanical working distance.

[0031] First, as a foundation, this application utilizes two magnetic lenses to individually, in the same direction, or in opposite directions, thereby adjusting the on-axis magnetic field distribution. It should be noted that the four magnetic lens excitation modes in Examples 1-4 are not only used to change the focusing strength of the main electron beam, but also to change the return trajectory of the signal electrons excited by the sample inside the objective lens.

[0032] Example 1. First magnetic lens is excited alone. This embodiment describes a low-landing-energy application scenario. Because the electron beam in low-landing-energy mode has a low initial energy, its imaging performance is typically limited by initial energy differences, space charge effects, and external excitation perturbations (all categorized as chromatic aberration). The conventional approach is to use a magnetic immersion objective lens in conjunction with a repulsion field to suppress the effects of chromatic aberration and achieve high-resolution imaging at low landing energy. For example... Figure 2 As shown in Figure a, the imaging device controls the first magnetic lens to be excited independently, and the magnetic lens field immerses the sample. The horizontal axis in the figure corresponds to the axial position of the compound electromagnetic objective, and the vertical axis represents the on-axis magnetic field strength. Since the pole shoe slits of the first magnetic lens are mainly arranged radially, the on-axis leakage magnetic field extends towards the sample region, causing the main electron beam 101 to be continuously focused before landing. Therefore, the peak of the curve in the figure is close to the sample position, that is, the magnetic field is close to the sample, forming a strongly constrained immersion magnetic field. The electron beam can achieve good focusing at a short working distance, with a small beam spot, high resolution, and high SE / BSE collection rate, which is suitable for CD-SEM.

[0033] Example 2. Second magnetic lens is excited separately. This embodiment is applicable to high-landing-energy electron beam imaging. High-energy electron beams require very strong magnetic fields for focusing, but nearly half of the magnetic field in the immersion field is located behind the sample, which does not aid in focusing. If only the immersion field of the first lens is used, a very high excitation current is required. Power consumption becomes an engineering challenge, and Embodiment 1 can be considered an embodiment based on superconducting excitation. This embodiment, however, is based on a currently economically feasible method, utilizing only a second magnetic lens. Figure 2 As shown in b, the peak value of the second magnetic field is far from the sample, indicating a non-immersion magnetic field or a weakly immersion magnetic field. Since the pole piece gaps of the second magnetic lens are mainly arranged axially, the resulting on-axis magnetic field is relatively far from the wafer or sample surface, resulting in a larger equivalent working distance. Consequently, key electron optical performance indicators, such as spherical aberration and chromatic aberration, are relatively poor, and the beam spot is also larger. Therefore, when the second magnetic lens works alone, it can provide high-energy electron beam focusing imaging. However, since the sample is not immersed in the magnetic field, it is difficult to use this to increase the proportion of BSE entering the lens and achieve in-tube detection. Therefore, simply using in-tube detection imaging yields poor results.

[0034] Example 3. Co-directional excitation of dual magnetic lenses like Figure 2 As shown in Figure c, the magnetic field peaks of the first and second magnetic lenses superimpose to form a strong and concentrated magnetic field, resulting in high excitation efficiency. This compensates for the resolution loss in Example 2, and power consumption is not a major concern. Furthermore, the sample is immersed in a strong magnetic field; the closer the magnetic field is to its peak, the better it is at re-constraining and guiding the large-angle escaped BSE back into the lens tube. The SE and BSE are then clearly separated by detectors at different positions, making it suitable for overlay measurements or imaging of underlying patterns.

[0035] Example 4. Reverse excitation of dual magnetic lenses The currents in the first magnetic lens and the second magnetic lens are in opposite directions. This embodiment, for example... Figure 2 As shown in d, the magnetic field strength along the axis is opposite in direction, which reduces the overall magnetic excitation efficiency. The electron optical performance is generally not as strong as that of the same-direction superposition mode, but it can more flexibly change the motion path of the returning signal electrons, especially the BSE.

[0036] The principle is as follows: BSEs have high energy and are relatively less affected by electrostatic fields, but they are affected by the direction and spatial distribution of magnetic fields. A reverse magnetic field alters the gyration, convergence, or divergence state of the BSE during its return to the microscope tube. Large-angle BSEs are blocked by pole shoe apertures or sidewalls, preventing them from entering certain detectors, or causing BSEs with different escape angles to be redistributed to different detection areas. Therefore, this embodiment does not pursue high collection rates for various signals, but rather filters out large-angle BSEs through the distribution of the reverse magnetic field, resulting in higher angular sensitivity of the BSEs intercepted on different detectors.

[0037] In summary, Example 1 generates a strong immersion field near the sample, suitable for low landing energy and high-resolution surface topography imaging; Example 2 generates a high-energy main electron beam focusing field far from the sample, suitable for high landing energy imaging; Example 3 has two magnetic fields superimposed in the same direction, suitable for high landing energy imaging, while also ensuring high collection and separation rates (i.e., signal purity) of BSE and SE on their respective independent detectors; Example 4 has two magnetic fields in opposite directions, with the BSE undergoing two focusing processes, allowing for greater trajectory adjustment, suitable for filtering large-angle BSE, high-sensitivity detection of small-angle BSE, and separation of SE.

[0038] This application does not simply change the focusing state of the main electron beam, but rather, by adjusting the position of the levitating pole piece, it allows the magnetic field modulation to directly affect the return BSE / SE trajectory and the multi-detector capture ratio. The following describes the equipotential line distribution and on-axis electric field distribution formed by the dual electrostatic lenses in the composite mirror under different landing energy modes.

[0039] Example 5. Bias configuration combination of electrostatic lenses Figure 3 The green curve in the figure can be interpreted as an equipotential line, reflecting how the potential transitions between the objective pole piece, the suspension pole piece 403A, and the sample 501. The red curve above represents the change in electric field intensity along the electron optical axis, reflecting the degree of acceleration or deceleration experienced by the electron beam as it approaches the sample. This figure illustrates that by adjusting the electron gun accelerating voltage Vacc, the bias voltage Vb of the suspension pole piece 403A, and the sample bias voltage Vs, it is possible to switch between low-landing energy mode and high-landing energy mode within the same composite mirror structure, further influencing the trajectory separation of the SE and BSE after their return to the mirror tube.

[0040] Figure 3Mode a represents the low landing energy mode. A positive bias is applied to the levitation magnetic pole 403A, and a negative bias is applied to the sample, creating a large potential difference between the levitation magnetic pole and the sample. In one specific embodiment, the electron gun accelerating voltage is approximately Vacc = -6kV, the levitation pole shoe 403A is positively biased by Vb = 8kV, and the sample 501 is negatively biased by Vs approximately -5kV, corresponding to a landing energy of approximately LE = e(Vs - Vacc) = 1keV. The electric field is concentrated in a narrow region between the two, causing a significant peak in the on-axis electric field near the sample. This electric field allows the main electron beam to obtain higher energy when entering the end region of the objective lens to reduce chromatic aberration, and it is slowed down by a repulsion field as it approaches the sample, thus incident on the sample with low landing energy, avoiding damage to the photoresist or surface devices caused by high-energy electrons. Simultaneously, since the initial energy of low-energy electrons (SEs) is typically very low, this strong near-surface electric field can accelerate and guide low-energy secondary electrons excited by the sample into the lens barrel, making it easier for the SEs to enter the detector region near the optical axis, improving secondary electron collection efficiency.

[0041] Figure 3 In mode b, a high landing energy mode, a negative bias is applied to both the levitation magnet 403A and the sample. In one example, the electron gun accelerating voltage is approximately Vacc = -40kV, the levitation magnet 403A is subjected to a negative bias of Vb = -5kV, and the sample is subjected to a negative bias of Vs = -4kV, corresponding to a landing energy of approximately LE = e(Vs - Vacc) = 36keV. Since both the levitation magnet 403A and the sample are negatively biased, and their potential difference is relatively... Figure 3 The a value is much smaller, so the main electron beam is only subject to limited deceleration, allowing it to bombard the sample with higher landing energy. In this mode, the on-axis electric field is mainly used to maintain the sample surface's attraction to signal electrons, and, in conjunction with the magnetic lens, controls the trajectory of backscattered electrons, enabling high-energy BSEs to enter detectors at different positions within the mirror tube. Through these two electric field distribution modes, this invention can achieve compatibility between low-landing-energy CDSEM imaging and high-landing-energy HVSEM / overlay measurement imaging within the same composite mirror structure.

[0042] Example 6. Three-detector layout (without off-axis deflection device) In one example of the present invention, the detector system is configured with three detectors: a lower detector 303, a middle detector 302, and an upper detector 301, as shown below. Figure 4 As shown in (a). These detectors can be direct electronic detectors (such as Si detectors, MCP detectors), indirect detectors (such as Robinson detectors, Everhart-Thornley (ET) detectors), or any combination thereof.

[0043] Specifically, the detectors are distributed along the optical axis at different positions within the lens, such as the lower, middle, and upper positions. The signal detected by each detector varies depending on the operating conditions. Generally, the main electron beam passes along the optical axis through the outer-axis detector or the inner aperture of the ring detector, undergoing focusing and scanning before reaching the sample surface. This excites two types of electrons (secondary electrons and backscattered electrons), collectively referred to here as signal electrons 102. Due to differences in energy and escape angle, the angle at which they return into the lens tube also differs, resulting in varying probabilities of being captured by each detector. Figure 4 The returned signal electrons are subdivided into the following five parts according to this situation: 1. Signal electrons 1024 that did not pass through the detector are large-angle BSEs, which were blocked by the pole shoes or unable to enter the lens tube due to magnetic field force; 2. Signal electrons 1023 received by the lower detector 303, including BSE1023(BSE) and SE 1023(SE) received by the lower detector, which will be distinguished as 1023(BSE) and 1023(SE) thereafter (other signals are distinguished in the same way). From the design perspective of HVSEM, the higher the 1023(BSE) received by the lower detector 303, the better, and the lower the 1023(SE), the better; 3. Signal electrons 1022 received by the middle detector pass through the lower detector 303 and arrive at the middle detector. From the design perspective of HVSEM, it is desirable for 1022(SE) to be the main component, with little or no 1022(BSE); 4. Signal electrons 1021 received by the upper detector pass through the middle detector and arrive at the upper detector 301. From the design perspective of HVSEM, detector 301 receives predominantly 1021 (SE) electrons, with ideally zero or very little 1021 (BSE) electrons. Both 1021 and 1022 signals should ideally be dominated by SE electrons. The difference between 1021 (SE) and 1022 (SE) lies in their initial angular range. 1021 (SE) collects secondary electrons escaping from the central aperture of detector 302, and has a smaller initial escape angle than 1022 (SE), representing bottom information in a high aspect ratio structure. 5. Signal electrons 1020 that pass through the upper detector, almost without deviating from the optical axis and passing through the central aperture of the upper detector, are mainly SE electrons. In this case, this portion of the signal cannot be detected and imaged.

[0044] HVSEM focuses on the separation and detection of BSE and SE by detectors at various locations, forming image representations that express different morphologies, materials, and layers of the sample. CDSEM, on the other hand, focuses more on the surface features of the image, and the signal separation of detectors at various locations is not particularly important; in general, the images can be merged.

[0045] The inner and outer diameters of the ring detector are also key factors affecting the interception ratio of various signals. Once fixed, the interception ratio can also be influenced by the lens's electric field, magnetic field, and electron beam energy. In this invention patent, the inner diameter is selected from 0.5mm to 6mm; in some embodiments of this invention, the inner diameters of the upper, middle, and lower detectors are chosen to be 1mm, 5.6mm, and 2mm, respectively. The outer diameter depends on space constraints and requirements. For example, the lower detector receives the signal electrons with the largest incident angle, so its size is the largest; the outer diameters of other detectors generally do not need to be too large. In some embodiments of this invention, the outer diameters of the upper, middle, and lower detectors are chosen to be 9mm, 9mm, and 24mm, respectively.

[0046] Figure 5 a and Figure 5 b are respectively Figure 4 The simulation results of the trajectories of BSE and SE in the signal electrons under Example 6 are presented. In an optional embodiment, the initial emission energies of BSE and SE excited by the main electron beam on the sample surface are taken as 26 keV and 5 eV, respectively. On the cross section through the optical axis, with the optical axis as a reference (the normal direction of the sample surface), the angle θ between the direction of SE and BSE escaping from the sample and the normal is theoretically distributed in the range of (+90~-90), and the relationship between the yield n and the emission angle follows a cosine distribution, i.e., n=cos(θ).

[0047] Figure 5 a represents the simulation results for BSE: the BSE component in signal electrons 1024 that did not pass through the detector due to being blocked by the pole shoes or suppressed by the magnetic field, i.e., 1024(BSE) accounts for 18%; the BSE component in signal electrons 1023 received by the lower detector 303, i.e., 1023(BSE) accounts for 65%; the BSE component in 1022 received by the middle detector, i.e., 1022(BSE) accounts for 9%; the BSE component in 1021 received by the upper detector 301, i.e., 1021(BSE) accounts for 4%; and the BSE that leaked out from the central hole of the upper detector, i.e., 1020(BSE), accounts for 4%.

[0048] Figure 5 b is the simulation result for SE: 1024 (SE) accounts for zero, that is, all low-energy secondary electrons are accelerated into the lens tube and the diffusion is very small, confined to the vicinity of the optical axis; therefore, the lower detector 303 also intercepts very little, accounting for only 3% of 1023 (SE); 1022 (SE) accounts for 17% on the middle detector; 1021 (SE) accounts for 26% on the upper detector; 54% of 1020 (SE) is not detected at all.

[0049] In Example 6 above, the sum of 1020 (BSE) and 1020 (SE) accounts for 58% of the total signal electrons (i.e., 4% + 54%), which is still a considerable signal loss. To solve the problem of signal electrons being lost when crossing the optical axis, a Wien filter needs to be introduced.

[0050] Example 7. At least three detectors combined with an off-axis deflection device To further improve the collection efficiency of secondary electrons at small opening angles and reduce the 1020 (SE) loss along the axis, one embodiment of the present invention adopts... Figure 4 Option b. Figure 4 In step b, an off-axis deflection device, such as a Wien filter, is introduced between the upper detector 301 and the middle detector 302. The Wien filter essentially consists of a pair of electrostatic deflectors and a pair of magnetic deflectors, generating a set of orthogonal diode electric and magnetic fields (ExB) in a plane perpendicular to the optical axis. For a velocity of... v The main electron beam needs to operate under the following direct-axis conditions: E = v x B At this point, the electric and magnetic forces acting on the main electron beam are equal in magnitude and opposite in direction. However, for the upward-moving signal electrons, the electric and magnetic forces are in the same direction, causing them to be deflected off-axis and detected by one side of the detector. Figure 4 In b, theoretically, the upper detector 301 will intercept all electrons passing through the middle detector, that is... Figure 4 The total amount of 1021A signal electrons captured by the upper detector with off-axis deflection in b is equal to Figure 4 The sum of signal electrons 1021 received by the upper detector and signal electrons 1020 that pass through the upper detector under the off-axis deflection device in a is, that is, the total amount of signal electrons of 1021A = the amount of signal electrons of 1021 + the amount of signal electrons of 1020.

[0051] Figure 5 c and Figure 5 d are respectively Figure 4 The simulation results of the trajectories of BSE and SE in the signal electrons under one embodiment (b). This embodiment does not change the trajectory of the signal electrons below the upper detector 301, but only affects the detectivity of the upper detector. All signal electrons 1021A passing through the central aperture of the middle detector are filtered by the Wien filter. E x B The field is deflected off the optical axis and intercepted by one side of a detector arranged on one side or a ring detector. This solves the problem of signal loss.

[0052] Of course, the Wien filter in this embodiment can also be placed above the upper detector 301, and a fourth detector can be introduced above it. Therefore, this embodiment includes at least three detectors and at least one deflection device.

[0053] The aforementioned Embodiments 6 and 7 are two basic embodiments of the detector system layout of this patent. Their difference lies in whether an off-axis deflection device is introduced into the on-axis signal electronics to achieve off-axis detection. Given the obvious functional differences between these two basic embodiments, for simplicity and without hindering the description of the fundamental aspects of this disclosure, the following embodiments are based solely on… Figure 4 The basic layout of the detector system of embodiment a is described in embodiment 6. As for the case of another basic layout embodiment 7, it can be obviously extended based on the relevant knowledge in the art, so it will not be described in detail.

[0054] Examples 1-5 illustrate a variety of excitation methods for compound mirrors. These different combinations of excitation methods not only affect the performance of the electron-optical system of the compound mirror, but also affect the return trajectory of signal electrons and their distribution on each detector.

[0055] Examples 6-7 illustrate two configurations of at least three detectors. These two sets of examples are interconnected. For instance, in the dual magnetic lens section of the composite electromagnetic objective of the present invention, by changing the excitation method of the first and second magnetic lenses, the return trajectories of BSE and SE in the composite objective can be altered, thereby changing the capture ratio of BSE and SE by different detectors. Furthermore, the composite electromagnetic objective of the present invention is also equipped with a dual electrostatic lens structure. By applying a bias voltage Vb to the suspended pole piece 403A and a bias voltage Vs to the sample, various electrostatic field combinations can be formed to adapt to the different requirements of low-landing-energy imaging and high-landing-energy imaging. This also affects the trajectory of the returning signal electrons, thus affecting the collection efficiency of each detector.

[0056] To facilitate understanding of the electron optical performance evaluation and discussion in the subsequent embodiments, an embodiment of the electron microscope structure used in this invention is first introduced below. See Figure 14The electron microscope 001 includes: an electron source 010 (its core parameters are the virtual source size S0 and the energy discrete ΔE), which is suspended on a negative bias voltage Vacc and faces a grounded anode 100 (not shown in the figure), providing initial energy E=eVacc to the main electron beam 101; at least one condenser lens 201, which collimates and focuses the main electron beam, causing it to reach the surface of the aperture 202 at a specific convergence angle; the aperture 202, which passes through the optical axis, has a limiting membrane aperture that allows the main electron beam with a corresponding divergence angle to pass through; the beam current of the main electron beam can be adjusted by adjusting the excitation of the condenser lens in front of the aperture; the main electron beam passes through a Wien filter 203, which has a reasonable ratio of voltage and current driving, so that the electric and magnetic forces acting on the main electron beam cancel each other out, allowing the main electron beam to pass through the Wien filter in a straight line; and a compound electromagnetic objective lens 400 focuses the main electron beam onto the sample 5. On the surface of the sample (01), a negative potential is applied, thus creating a repulsion field for the main electron beam. Simultaneously, an electric or magnetic deflection system exists in front of the objective lens; the figure shows an electrostatic deflection system composed of two deflectors, 205 and 206. This enables grating scanning of the main electron beam on the sample surface. The scanning focus point of the main electron beam on the sample excites two types of signal electrons, 102 (SE) and 102 (BSE) (not labeled in the figure), which escape at an initial angle θ to the optical axis, where θ ranges from -90° to θ < 90°. At this point, the electric field on the sample surface accelerates the signal electrons 102 that have already left the surface, guiding them into the lens barrel. The lower detector 303, middle detector 302, and upper detector 301 are sequentially distributed in the lower, middle, and upper sections inside the lens barrel, collecting electrons of different angles θ and different types (SE and BSE) for subsequent independent channel imaging or selective synthetic imaging.

[0057] The electro-optical performance of a lens is mainly determined by its spherical aberration coefficient. and color difference coefficient Characterization, respectively determining the circle of confusion of spherical aberration. and color difference dispersion circle Here and below, This is half the convergence angle of the electron beam focused on the sample surface, typically between 1 and 10 mrad. Besides spherical aberration and chromatic aberration, factors affecting the beam spot size also include the Gaussian beam pattern. Contribution It is a constant determined by the beam current and the brightness of the electron gun; there is also the diffraction spot. The aberrations (Gaussian source, spherical aberration, chromatic aberration, diffraction) are spatially independent and circularly symmetrical. The overall point spread function is the convolution of each component, so the synthesized spot can be simply synthesized using RMS (root mean square) as the formula. =

[0058] Given the beam conditions, namely Vacc, ΔE, and C0, and the objective lens design, the optimal beam spot is determined by the objective lens excitation. Traditional electron microscopes use a single excitation source objective lens and do not consider the detector's separation detection and collection efficiency for SE and BSE. A second condenser lens can be used to allow the objective lens to be fitted to an optimal image-side convergence angle. To obtain the optimal beam pattern.

[0059] This invention relates to dual magnetic lens objectives and dual electric lens objectives. Electric lenses are relatively simple, primarily used to alter landing energy and accelerate signal electrons into the lens tube. Magnetic lenses, on the other hand, offer a wider variety of configurations, affecting not only optimal resolution but also the efficiency of signal electron collection. This diversity also leads to different focuses, such as prioritizing SE detection, prioritizing BSE / SE separation detection, focusing on high energy, or focusing on low energy, among other application scenarios.

[0060] The following examples will be described in more detail based on high-energy electron microscopy and low-energy electron microscopy applications.

[0061] Examples 8-11 are all based on high landing energy applications. For example, the electrostatic conditions are: LE = 26 keV, electron gun accelerating voltage Vacc = -30 kV, levitation magnetic pole 403A applies Vb = 8 kV, and sample 501 applies Vs = -4 kV. The selection of these voltages mainly considers the following factors: 1. Electron gun accelerating voltage Vacc: This determines the energy of the electron beam at the electron gun exit. The actual landing energy LE = e(Vs - Vacc) depends on the Vs bias voltage. If Vs = 0, then LE = eVacc. The higher the LE, the deeper the BSE (Body Sequence Array) excited by the main electron beam within the sample, making it possible to characterize the material contrast pattern deeper below the surface through the BSE. Conventional HVSEM refers to LE of 30keV to 50keV, with the BSE reaching submicron or even micron depths below the silicon sample surface. LE also tends to be higher, with its upper limit depending on the tolerance of the sample / device under test to high-energy damage. For generality, this application uses -40kV and -30kV in some cases.

[0062] 2. The sample bias voltage Vs is generally considered redundant in laboratory electron microscopy, but it is essential in this invention. It is necessary for compatibility with LVSEM design, utilizing the same set of detectors distributed at different positions along the optical axis within the lens for detection and imaging. Maintaining this bias voltage serves two purposes: first, to attract signal electrons (SE and BSE) into the microscope tube for detection and imaging using the same set of detectors; and second, to provide sufficient acceleration energy for secondary electrons to reach the energy threshold of the detector response. For example, one embodiment of this invention uses a PIN detector with a signal energy threshold of 3 keV; below this value, the output signal is extremely weak. Therefore, the sample bias voltage is at least -3 kV. Without loss of generality, -4 kV is also used in some embodiments.

[0063] 3. The levitation magnetic pole bias voltage Vb is a mandatory option for LVSEM. In this embodiment, Vb = 8kV is inherited from the LVSEM setting. Vb can be adjusted in HVSEM scenarios, and can be different, negative, positive, or zero. The purpose of setting it to 8kV here is to accelerate and collect more of the large-angle BSE into the lens, so Vb itself is also a means of adjusting the signal collection rate of each detector.

[0064] 4. Given the voltage setting of the electron beam, there are multiple options for the excitation of the composite mirrors, primarily the selection of NI1 and NI2. Firstly, considering the overall excitation distribution, four major excitation modes need to be covered: independent excitation mode of the first magnetic lens, co-directional excitation mode of the two lenses, reverse excitation mode of the two lenses, and independent excitation mode of the second magnetic lens. Within these four modes, the electron optical performance, signal electron collection rate, and separation rate must be balanced, and the excitation ratio of NI1 and NI2 must be optimized to obtain a better excitation ratio for each mode. Electron optical performance is calculated based on the third-order aberration theory of electron optics, calculating the third-order aberration coefficients of the entire electron optical system, such as spherical aberration and chromatic aberration coefficients. The beam spot size is evaluated, taking into account the contributions of the Gaussian spot and diffraction spot, thus finding a set of curves relating the beam spot size to the NI1 / NI2 excitation ratio. Next, the signal electron collection rate needs to be considered: based on the NI1 and NI2 excitation values ​​obtained from electron optical simulation, the return trajectory of the signal electrons is simulated using multiphysics to obtain the collection rate and separation rate of the signal electrons on each detector. Taking all factors into consideration, an excitation combination was found that satisfies both resolution requirements and various signal separation and detection requirements. Here, only a few typical dual-magnetic lens (NI1 and NI2) excitation combinations are selected as examples, as shown in Table 1, to cover four cases: single-lens independent excitation, dual-lens co-directional excitation, and reverse excitation. The detection characteristics of the signal electrons of each detector are also explained.

[0065] To facilitate the explanation of the effect of the lens field on signal electrons in subsequent embodiments, the force situation of electrons during the process of passing through the magnetic lens will be analyzed and introduced first.

[0066] On-axis magnetic field distribution of a magnetic lens They often exhibit a bell-shaped field pattern, characterized by a strong center and low sides. For ease of understanding, and without losing generality, let's consider... Figure 2 Taking the field distribution shown in figure a as an example, an electron is incident into the magnetic field region from the right side at a certain off-axis tilt angle α, that is, along the -z direction.

[0067] The electron initially already possessed v r and v z Two velocity components. After entering the magnetic field region, a radial magnetic field B exists along the path traversed. r and axial magnetic field B z Two directional components; axial motion v z and B r The action produces rotational motion, resulting in an angular velocity v. θ Since we have angular velocity v θ It will cut the magnetic field lines B r and B r This generates forces in two other directions: (1) V θ Cutting magnetic field lines B z Generates radial force F r During the entire process of passing through the lens's magnetic field, F r Both are pointed towards the optical axis, but in the two regions before and after the peak of the magnetic field on the axis, F r The force acting on it first increases and then decreases. This is why a magnetic lens is always a focusing lens; (2) V θ Cutting magnetic field lines B r Generates radial force F z : ,and ), )for The higher-order components of the field are very small and can be ignored in the paraxial case. The direction of the field is from Decision. The first half of the zone, F z The direction of this motion is opposite to the axial motion of the electron, thus acting as an axial deceleration effect; at the peak point... F Z ≈0; in The latter half of the zone, F z The direction of the motion is the same as the axial direction, thus providing axial acceleration. The above explanation suggests that for signal electrons, during their entry into the lens from the sample side, in addition to being focused, they also encounter a repulsive force exerted by the lens's magnetic field, pointing towards the sample. For signal electrons with large opening angles and high initial velocities, i.e., v... r >vz , v r It will generate a very strong angular velocity v θ This led to the arrival The peak value points towards the sample, indicating an increased repulsive force. If it cannot be confined to the paraxial region, i.e., it is not very small, higher-order quantities The repulsive force becomes increasingly stronger as the axial energy of the signal electrons becomes insufficient to overcome this reverse force. If the axial energy of the signal electrons is insufficient to overcome this reverse force, the signal electrons cannot enter or pass through the magnetic lens. This is the fundamental problem to consider when probing signal electrons within a lens.

[0068] The most direct way to reduce this negative effect is to minimize it. The goal is to immerse the sample as close to the peak point as possible within the specified range. Therefore, an immersion field magnetic lens satisfies this improvement direction. Additionally, providing an accelerating field to the signal electrons allows them to gain higher energy to overcome the reverse magnetic field force. Clearly, the booster effect also aligns with this objective. Thus, the technical solution of low-energy electron microscopy perfectly matches these two requirements, making in-lens detection in LVSEM not a significant challenge.

[0069] For HVSEM, the energy of the BSE in the signal electrons is very high. If the BSE still exits at a large angle, even if the size is reduced... The range may not be sufficient to overcome the reverse magnetic field force; if the sample is to be completely immersed in... Within a certain range, the excitation efficiency would be greatly sacrificed, leading to engineering or feasibility issues related to power consumption and heat dissipation. For SEs, due to their low energy and slow speed, they are confined to the vicinity of the paraxial axis by the combined effects of the accelerating field formed by Vb and Vs and the focusing effect of the magnetic lens. The reverse magnetic field force is not very large, and entry into the lens is not a problem. Therefore, for HVSEMs, in addition to achieving specific electron-optical performance indicators, the composition of the magnetic field and its impact on the collection rate of BSEs are the focus of the following embodiments.

[0070] Table 1. Comparison of excitation conditions, electron-optical performance parameters, and SE / BSE detection rates for Examples 5-8.

[0071] Example 8 This embodiment provides an operating mode in which only the first magnetic lens is excited.

[0072] Please refer to the following: Figure 1 The compound electromagnetic objective lens shown and such Figure 4The multi-detector system is shown. In this embodiment, the electron beam scanning imaging system operates in a high landing energy mode, causing the electron beam to bombard the sample surface with a landing energy of approximately 26 keV. The first magnetic lens is excited alone, while the second magnetic lens is not excited. The excitation amount NI1 of the first magnetic lens is 5424 AT, and the excitation amount NI2 of the second magnetic lens is 0 AT. This results in a smaller beam spot on the sample surface by the main electron beam, leading to higher resolution.

[0073] Under this excitation condition, the first magnetic lens generates a strong immersion magnetic field near the sample. The on-axis magnetic field distribution is referenced. Figure 2 a. The peak value is near the front surface of the sample. This field distribution exhibits excellent optical performance for the main electron beam: the electron optical beam spot size d is approximately 0.54 nm, the spherical aberration coefficient Cs is approximately 1.32 mm, and the chromatic aberration coefficient Cc is approximately 1.67 mm (see Table 1). The specific principles and theoretical basis are basic industry knowledge and will not be elaborated here.

[0074] In this embodiment, the peak value of the on-axis magnetic field is relatively close to the sample surface. The BSE enters a weak defocusing region immediately upon activation, and experiences a relatively weak reverse magnetic force. (Reference) Figure 6 Simulations show that almost no BSE returns to the sample; a portion is mainly blocked by the upper pole shoe aperture or bounced off the sidewalls. This portion, 1024 (BSE), accounts for 28%. The remaining incoming signal electrons can all be detected by the detector. Among them: LDET, that is, the proportion of BSE 1023 (BSE) received by the lower detector is about 55%, and the proportion of SE 1023 (SE) received by the lower detector is 3%; MDET, that is, the proportion of BSE 1022 (BSE) received by the middle detector is about 9%, and the proportion of SE 1022 (SE) received by the middle detector is 17%; UDET, that is, the proportion of BSE 1021 (BSE) received by the upper detector is about 4%, and the proportion of SE 1021 (SE) received by the upper detector is 26%; the proportion of BSE 1020 (BSE) that leaks through the upper detector on the axis is 4%, and the proportion of SE 1020 (SE) that leaks through the upper detector is 54%; if we consider that the Gavin filter can be returned to the upper detector 301, then the proportion of 1021A (BSE) is 8%, and the proportion of 1021A (SE) is 80%.

[0075] The advantage of this embodiment is that, under high landing energy conditions, the strong immersion magnetic field of the first magnetic lens improves the electron optical performance of the main electron beam and the BSE introduction efficiency, making it suitable for detection scenarios with high requirements for contrast signals of underlying materials or deep structural information. The disadvantage is that the high excitation intensity presents a challenge in heat dissipation.

[0076] Example 9 This embodiment uses the same composite electromagnetic objective, multi-detector system, and electrostatic operating conditions as Embodiment 8. The difference lies in that the first magnetic lens and the second magnetic lens are simultaneously excited, and they are excited in opposite directions. In one optional implementation, the excitation amount NI1 of the first magnetic lens is 1782 AT, and the excitation amount NI2 of the second magnetic lens is -1495 AT. This choice aims to achieve a relatively balanced power consumption and excitation efficiency. Under the premise of achieving an acceptable resolution (e.g., around 1 nm), a specific application mode is found. This invention does not exclude the possibility of choosing other excitation amounts for the first and second magnetic lenses, as long as the above conditions are met.

[0077] Reference Figure 7 Because the magnetic fields of the first and second magnetic lenses intersect at zero points along the axis, forming two independent strong focusing lens regions, they exert a strong effect on the BSE. The large-angle BSE formed by overfocusing is blocked by the objective lens pole piece or adjacent structures before entering the microscope tube. The capture rate of the lower detector 303 for the BSE is about 15%, while the capture rates of the middle and upper detectors 301 for the BSE are close to 0. At this time, the lower detector can be considered as a small-angle BSE detector, and the image reflects the bottom features. At the same time, because the initial energy of the SE is low and mainly affected by the electrostatic field near the sample, it can still be accelerated and introduced into the microscope tube, forming a relatively dispersed distribution among the multiple detectors.

[0078] In this embodiment, a portion of the BSE signal, namely 1024 (BSE), accounting for 85%, fails to reach or pass through the detector. The percentage of BSE 1023 (BSE) received by the lower detector is approximately 15%, and the percentage of SE 1023 (SE) received by the lower detector is 23%; the percentage of BSE 1022 (BSE) received by the middle detector is approximately 0%, and the percentage of SE 1022 (SE) received by the middle detector is 47%; the percentage of BSE 1021 (BSE) received by the upper detector is approximately 0%, and the percentage of SE 1021 (SE) received by the upper detector is 14%; the percentage of BSE 1020 (BSE) that misses the upper detector on the axis is 0%, and the percentage of BSE 1020 (SE) that misses the upper detector is 16%; if we consider that the Gavin filter can return the signal to the upper detector 301, then the percentage of 1021A (BSE) is 0%, and the percentage of 1021A (SE) is 30%.

[0079] In this embodiment, the electron optical beam size d is approximately 1.06 nm, the spherical aberration coefficient Cs is approximately 2.32 mm, and the chromatic aberration coefficient Cc is approximately 6.24 mm. Although the electron optical performance in this mode is slightly lower than that in Embodiment 8, the lower detector has a high detection rate for small-angle BSEs, and the total excitation of the magnetic lens is low, resulting in lower power consumption. Furthermore, it allows the SEs to be distributed across different detectors according to different exit angles or return trajectories.

[0080] Clearly, this mode masks most of the BSE signal, and the detector collects a relatively pure SE signal. Therefore, this mode is suitable for imaging surface topography, voltage contrast, or SE signals at the bottom of high aspect ratio structures where the SE signal is dominant.

[0081] Example 10 This embodiment uses the same composite mirror structure, multi-detector arrangement, and electrostatic operating conditions as Embodiment 8. The difference lies in that the first magnetic lens and the second magnetic lens are simultaneously excited in the same direction. The excitation amount NI1 of the first magnetic lens is 3857 AT, and the excitation amount NI2 of the second magnetic lens is 712 AT. This choice aims to overcome the shortcomings of Embodiment 9, namely, the high BSE loss rate, while balancing resolution and power distribution.

[0082] Reference Figure 8 When the first and second magnetic lenses are excited in the same direction, their on-axis magnetic fields are superimposed. The first magnetic lens provides an immersion magnetic field close to the sample, while the second magnetic lens provides additional focusing capability. The overall focal length or working distance is relatively short, ensuring high electron optical performance. In this embodiment, the electron optical beam size d is approximately 0.62 nm, the spherical aberration coefficient Cs is approximately 2.31 mm, and the chromatic aberration coefficient Cc is approximately 2.63 mm, exhibiting good electron optical performance and moderate magnetic excitation power consumption.

[0083] In this embodiment, the sample surface maintains a strong surface field, which strongly focuses the emitted BSE and SE, resulting in almost no signal returning to the sample. The strong excitation of the second magnetic lens also ensures that the BSE passing through the levitated pole piece is still focused, with most of it not blocked by the pole piece and directed towards the detector. Specifically, only a small portion of the BSE signal, namely 1024 (BSE), accounting for 3%, fails to be directed towards the detector; the remaining signal electrons are subsequently received by the detector. Of these, approximately 90% of BSE 1023 (BSE) is received by the lower detector, and 9% of SE 1023 (SE) is received by the lower detector; approximately 6% of BSE 1022 (BSE) is received by the middle detector, and 21% of SE 1022 (SE) is received by the middle detector; approximately 0.5% of BSE 1021 (BSE) is received by the upper detector, and 18% of SE1021 (SE) is received by the upper detector; 0.5% of BSE 1020 (BSE) and 52% of SE1020 (SE) are missed by the upper detector on the axis; if a Wien filter is considered, the portion missed on the axis can be retrieved to the upper detector 301, at which point 1021A (BSE) is 1% and 1021A (SE) is 70%. SE is mainly distributed in the middle and upper detectors 301, totaling 91%. BSE is mainly distributed in the lower detector 303. The BSE and SE are clearly separated in the detector space, enabling high-sensitivity imaging respectively.

[0084] The purpose of this embodiment is to simultaneously improve the BSE collection rate and SE detectability rate under high landing energy conditions through the co-directional composite excitation of the first and second magnetic lenses. This allows detectors (or combinations thereof) at different locations on the same device to work collaboratively to acquire high-purity BSE or SE signals respectively. In this embodiment, the lower detector acquires 90% BSE and the remaining 9% SE. In the total detected signal, the purity of BSE is approximately 90% / (90% + 9%). On the middle and upper detectors, considering the adoption of a Wien filter, a total of 91% SE and 7% BSE are acquired; the purity of SE is 91% / (91% + 7%) = 94%. Therefore, this mode is particularly suitable for semiconductor multilayer pattern overlay measurement, deep structure identification, and detection scenarios requiring simultaneous independent imaging of SE / BSE.

[0085] Example 11 This embodiment uses the same composite mirror structure, multi-detector system, and electrostatic operating conditions as Embodiment 5. The difference is that in this embodiment, only the second magnetic lens is excited, and the first magnetic lens is not excited. The excitation amount NI1 of the first magnetic lens is 0 AT, and the excitation amount NI2 of the second magnetic lens is 1305 AT. This embodiment mainly illustrates the application prospects under non-magnetic immersion or weak magnetic immersion conditions. This is because for samples containing magnetic materials, such as MRAM (magnetoresistive random access memory), it is necessary to avoid the influence of the magnetic field interacting with the sample.

[0086] In this embodiment, due to the non-immersion magnetic lens, the sample working distance is long, resulting in relatively large spherical and chromatic aberrations, and a significant sacrifice in optical performance. The electron optical beam size d is approximately 1.06 nm, the spherical aberration coefficient Cs is approximately 22.5 mm, and the chromatic aberration coefficient Cc is approximately 6.53 mm.

[0087] Reference Figure 9 Since the magnetic field of the second magnetic lens is mainly distributed at a position relatively far away from the sample, and there is a lack of strong immersion magnetic field near the sample, the BSE excited by the sample diverges at a large angle during the return process. Some BSEs are blocked by the objective lens pole piece or adjacent structures, and the proportion of BSEs that can enter the lens tube and be captured by the detector is low.

[0088] In this embodiment, most of the BSE signals, i.e., 1024 (BSE), accounting for 76%, fail to reach or pass through the detector, with only 24% entering the detectable channel. The percentage of BSE 1023 (BSE) received by the lower detector is approximately 12%, and the percentage of SE 1023 (SE) received by the lower detector is 93%; the percentage of BSE 1022 (BSE) received by the middle detector is approximately 6%, and the percentage of SE 1022 (SE) received by the middle detector is 4%; the percentage of BSE 1021 (BSE) received by the upper detector is approximately 4%, and the percentage of SE 1021 (SE) received by the upper detector is 1.5%; the percentage of BSE 1020 (BSE) that misses the upper detector on the axis is 2%, and the percentage of SE 1020 (SE) that misses the upper detector is 1.5%; if we consider the Gavin filter recovering the signals on the axis to detector 301, then 1021A (BSE) is 6%, and 1021A (SE) is 3%. SE (Sequence Detection) is mainly distributed on the lower detector 303, reaching 93%. Therefore, it can be considered as an SE imaging mode.

[0089] As can be seen from Examples 8 to 11 above, under the same composite mirror structure and the same multi-detector arrangement, the trajectory envelopes of SE and BSE within the mirror tube and the detector capture ratio can be significantly changed simply by altering the excitation state, excitation direction, and excitation ratio of the first and second magnetic lenses. When the first magnetic lens is excited alone, a strong immersion magnetic field and high BSE acquisition capability can be obtained, but the power consumption is high. When the first and second magnetic lenses are excited in opposite directions, the magnetic excitation power consumption can be reduced and the distribution of SE among the detectors can be changed, but the BSE capture rate decreases. When the first and second magnetic lenses are excited in the same direction, high landing energy focusing capability, efficient BSE collection, and SE / BSE separation imaging can be balanced, making it a preferred mode for overlay measurement and multilayer structure detection. When only the second magnetic lens is excited, a strong immersion magnetic field is lacking near the sample, resulting in a low BSE capture rate, which can be used as an SE-priority imaging mode.

[0090] The above results demonstrate that the dual magnetic lenses in this embodiment are not conventional focusing components that operate independently of each other. Instead, they form an adjustable composite magnetic field in the objective lens exit region through the radial and axial slits defined by the suspended pole piece 403A, causing the signal electrons to be converged, expanded, or redistributed to varying degrees during their return to the lens barrel. Therefore, the dual magnetic lenses of this application are not simply an addition of a lens, but rather achieve multi-mode scanning electron beam imaging without changing the objective lens structure or significantly altering the working distance.

[0091] Example 12 In the above embodiments, the trajectory of backscattered electrons (BSE) is mainly altered by adjusting the magnetic field distribution ratio of the two magnetic lenses. For secondary electrons (SE), due to their lower energy, their trajectory is primarily determined by the electrostatic field of the sample surface. Given a specific structural size, the electrostatic field strength of the sample surface mainly depends on the bias voltage Vb and the sample voltage Vs. Although the Vb voltage can be used to adjust the SE trajectory, its adjustment sensitivity is low because the electrode applying this voltage is relatively far from the sample.

[0092] To enhance the control over the electron beam (SE), a more sensitive field control mechanism is needed during the acceleration phase of the electric field on the sample surface. To this end, this invention further proposes an improved objective lens structure design. A typical implementation is as follows: Figure 10 As shown. Compared to Figure 1This design incorporates a field control electrode 404 within the first electrostatic lens. The field control electrode 404 is coaxially positioned between the levitating electrode shoe and the sample. Its voltage is independently adjustable, allowing for fine-tuning of the equipotential line distribution on the sample surface. This influences the initial stage of secondary electron emission, adjusting its spatial expansion, changing the secondary electron deflection ratio, and thus adjusting the ratio of electrons reaching the middle detector 302 and the lower detector 303. This achieves selective separation and acquisition of secondary electrons and backscattered electrons. This satisfies the requirement for selective detection of secondary electrons at different emission angles under secondary electron voltage contrast imaging conditions.

[0093] It is important to emphasize that the field control electrode 404 is used because the SE has low energy and is most susceptible to the influence of the electric field near the sample in the initial stage of escape. By placing the field control electrode between the levitating shoe and the sample, the spread angle and axial / radial distribution of the SE can be changed when it just escapes, thereby changing its interception rate by the upper, middle and lower detectors.

[0094] Based on Example 10, this example introduces a field control electrode 404. In one example, without changing the detector position and structural dimensions, the field control electrode potential is changed from... Figure 11 The 0V in b increases to Figure 11 The simulation showed that the BSE trajectory envelope was almost unaffected (not illustrated), while the proportion of SE passing through the central aperture of the lower detector 303 increased from 91% to 99%, allowing for better interception by the upper and middle detectors. The lower detector 303 then focused more on BSE detection, thus enabling better separation and imaging of the BSE and SE signals.

[0095] The foregoing embodiments primarily focus on high landing energy scenarios, while embodiments 13-14 below illustrate low landing energy application scenarios of the present invention. In one optional embodiment, an electrostatic lens is used as a reference (but not limited thereto). Figure 3 The configuration shown in a has the following optional parameters: LE=1keV, Vacc=-6kV, Vb=8kV, Vs=-5kV.

[0096] Example 13 In this embodiment, a field control electrode 404 is configured, with its bias voltage set to Vfc=0. The magnetic lens configuration and electro-optical performance are shown in Table 2. As can be seen from the table, this embodiment only excites the first magnetic lens. Table 2: Excitation conditions, electro-optical performance parameters, and SE / BSE detection rate of Embodiment 13.

[0097] See Figure 12Under low landing energy conditions, almost 100% of SE and BSE signals enter the telescope tube, with only 1% of SE 1023 (SE) received by the lower detector. This means that SE can be completely detected. This is because SE has a low initial energy, is strongly influenced by the electric field, and is less likely to diffuse to areas far from the axis. Therefore, most of it enters the regions of the middle and upper detectors through the central aperture of the lower detector 303. However, the lower detector 303 intercepts about 1 / 4 of the BSE. BSE has a higher energy (approximately 1 keV here), and under the influence of a strong accelerating electric field, the focusing on large-angle BSE is too strong, causing some BSE to diffuse to areas far from the axis and thus be intercepted by the lower detector 303. Overall, the mixed signals of BSE and SE can be imaged by the upper and middle detectors. Nearly half of the BSE and SE signals pass through the central aperture of the upper detector 301. This portion of 1020 (BSE) and 1020 (SE) signals escapes at small angles and better reflects the bottom information under the high aspect ratio structure (HAR). If you are interested in bottom imaging of this type of HAR, you can combine it with the extended example of embodiment 7, and configure another detector above the upper detector 301, and configure a Wien filter between the two to achieve off-axis deflection collection imaging.

[0098] In this embodiment, the electron optical beam size d is approximately 1.57 nm, the spherical aberration coefficient Cs is approximately 0.24 mm, and the chromatic aberration coefficient Cc is approximately 0.23 mm. The electron optical performance is excellent. Although the electron optical performance in this mode is slightly lower than that in Embodiment 8, it is still excellent for applications requiring low landing energy.

[0099] Example 14 As described in the previous embodiments, applying a bias voltage to the field control electrode 404 can also be used to adjust the trajectory of the BSE in low landing energy scenarios. This embodiment provides an example of Vfc = -5kV, which can also be extended to the case where the field control electrode voltage is close to or equal to the sample bias voltage. Under this condition, its negative bias voltage partially cancels the effect of the bias voltage Vb on the boost electrode 403A, the acceleration field on the sample surface weakens, the focusing effect on the BSE decreases, and by the time it reaches the surface of the lower detector 303, it has already diffused, reducing the proportion that passes through the central hole, thus enabling efficient detection. In this embodiment, see [reference needed]. Figure 13 The lower detector 303 achieves a 99% detection rate for BSE, intercepting only 10% of SE; while the combined detection rate of all detectors above the lower detector 303 reaches 90% for SE. BSE and SE are clearly separated. Therefore, this embodiment can achieve SE and BSE separation detection imaging in low-pressure scenarios.

[0100] The excitation conditions, optical performance, and SE / BSE detection rate of this embodiment are shown in Table 3. Table 3. Comparison of excitation conditions, electron-optical performance parameters, and SE / BSE detection rates in Example 14

[0101] In the foregoing embodiments, by reducing the surface field strength of the sample, the distribution of backscattered electrons (BSE) on the surface of the lower detector 303 is expanded. Similarly, by adjusting the excitation ratio of the first and second magnetic lenses, or even changing the relative polarity of the excitation, the magnetic field distribution along the BSE path can be adjusted, thereby promoting the diffusion of BSE on the surface of the lower detector 303. Other feasible methods will not be listed here.

[0102] Example 15 This embodiment provides a scanning electron beam imaging device, such as... Figure 14 As shown, the system includes an electron beam scanning imaging system 001, a silicon wafer transport system 002, and an electronic control system 003. The electron beam scanning imaging system adopts a vertically coaxial electron optical structure, consisting of an electron beam emitter 010, a scanning deflection system, a detector system, a compound electromagnetic objective lens 400, and a sample stage 500, arranged from top to bottom. The electron beam emitter 010 is used to emit the initial main electron beam 101; the scanning deflection system is used to achieve electron beam grating scanning. The compound electromagnetic objective 400 is located at the end of the electron optical path near the sample to perform electron beam focusing, energy regulation, and signal electron traction; the detector system is arranged in a gradient along the optical axis to collect secondary electrons and backscattered electrons at different scattering angles in layers.

[0103] This device also includes an anode 100, a condenser lens 201, and a confinement membrane aperture 202. The anode 100 is positioned below the electron beam emitter 010 to accelerate the emitted main electron beam 101, providing it with a predetermined initial acceleration energy. At least one condenser lens 201 is located below the anode 100 and is used to converge and calibrate the accelerated main electron beam, adjusting its beam angle and beam current intensity. At least one confinement membrane aperture 202 is arranged below the condenser lens 201 to filter out stray electrons in the optical path, constrain the cross-sectional size of the effective electron beam, and improve the imaging signal-to-noise ratio. The scanning deflection system includes two sets of deflectors 205 and 206 arranged along the optical axis, capable of driving the main electron beam to perform a two-dimensional grating scan on the sample surface, achieving full-coverage imaging of the target area on the silicon wafer.

[0104] The suspended pole piece 403A of the composite electromagnetic objective lens is installed at the end of the common pole piece 403 and maintains a high-voltage electrical insulation state between it and the common pole piece 403. The suspended pole piece 403A can be applied with an independently adjustable bias voltage and cooperates with the peripheral ground pole piece to form a first electrostatic lens.

[0105] The sample 501 carried by the sample stage 500 can be subjected to a negative bias voltage, which, together with the levitation pole piece 403A, forms a second electrostatic lens. The two-stage electrostatic lens and the double magnetic lens are spatially highly combined to work together to complete the focusing, acceleration and deceleration adjustment of the main electron beam.

[0106] The sample stage 500 adopts a high-precision three-axis moving platform structure, which has precise positioning capabilities in the XYZ three-dimensional direction. A vacuum adsorption structure is set on the stage surface, which can stably support the silicon wafer sample and move the sample to the designated imaging position under the objective lens, maintaining a small working distance to ensure imaging resolution and signal collection efficiency.

[0107] In other embodiments, a field control electrode 404 may also be included. The field control electrode 404 is coaxially arranged between the levitation shoe 403A and 501, and its voltage can be independently adjusted. It is used to finely control the electric field distribution near the sample surface and optimize the emission trajectories of secondary electrons and backscattered electrons.

[0108] The electronic control system 003 includes a high-voltage power supply module, a magnetic lens excitation drive module, an electrostatic voltage source module, a scanning control module, and a signal acquisition and processing module. The high-voltage power supply module provides a stable high-voltage output for the electron gun and the electrostatic lens. The magnetic lens excitation drive module is used to independently control the magnitude and direction of the excitation current of the two sets of magnetic lenses, realizing the switching between individual excitation, unidirectional excitation, and reverse excitation. The electrostatic voltage source module provides adjustable bias voltages for the levitation pole piece, the field control electrode, and the sample, respectively. The scanning control module drives the deflector to complete the precise scanning of the electron beam. The signal acquisition and processing module amplifies, acquires, and processes the output signals of the multi-stage detectors, and finally outputs image data that can be used for critical dimension calculation and overlay accuracy analysis.

[0109] As can be seen from the above embodiments, the composite electromagnetic objective of this application does not merely change the focusing conditions of the main electron beam through two magnetic lenses. Instead, it utilizes the axial and radial composite position of the levitation pole piece 403A to simultaneously participate in the formation of the radial magnetic gap, axial magnetic gap, and near-sample electrostatic field modulation region. The first magnetic lens, the second magnetic lens, the levitation pole piece bias voltage, the sample bias voltage, and the field control electrode 404 jointly determine the landing energy of the main electron beam and the return trajectory of the signal electrons. By combining and controlling the above electromagnetic parameters, different capture ratios can be formed between the lower detector 303, the middle detector 302, and the upper detector 301 for BSE, SE, and signal electrons from different source positions. This enables various modes such as low-landing-energy CDSEM surface imaging, high-landing-energy HVSEM bottom imaging, multi-layer overlay measurement, and high aspect ratio bottom signal enhancement imaging.

[0110] In particular, in high aspect ratio structural imaging scenarios, the field control electrode 404 can directly adjust the distribution of equipotential lines near the sample surface without changing the main objective structure and detector position, enabling stray secondary electrons from the sidewalls and weak signal electrons from the bottom to form different return paths during the initial divergence stage. This precise control of the near-sample electric field, combined with the dual magnetic lens return trajectory adjustment and multi-detector layered acquisition, can improve the detectability of weak signals at the bottom and enhance SE / BSE separation imaging capabilities.

[0111] The term "sample" as used herein includes, but is not limited to, semiconductor wafers, semiconductor components, and other components such as memory disks. It should be noted that the directional terms such as "upper" and "lower" used above are only used to indicate approximate relative positions and are not intended to limit the relevant structures in the embodiments of this application.

[0112] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A composite electromagnetic objective lens, characterized in that, Includes a first magnetic lens, a second magnetic lens, and a suspended pole piece; The first magnetic lens includes a first pole piece, a common pole piece, and a first coil. The first coil is located between the first pole piece and the common pole piece to provide a first magnetic field. The first pole piece and the suspended pole piece form a radial gap on the side close to the sample. The second magnetic lens includes a second pole piece, a common pole piece, and a second coil. The second coil is located between the common pole piece and the second pole piece to provide a second magnetic field. An axial gap is formed between the second pole piece and the suspended pole piece along the optical axis. The first coil and the second coil are connected to an excitation control circuit that can adjust the magnitude and direction of their excitation currents respectively, so that the first magnetic field and the second magnetic field can be formed independently, superimposed in the same direction, or superimposed in opposite directions. The suspended pole piece is located at the end of the common pole piece and is electrically insulated from the common pole piece by a high voltage; the suspended pole piece is configured to be able to be independently biased to form an electrostatic lens.

2. The composite electromagnetic objective lens according to claim 1, characterized in that, The suspended pole piece extends obliquely from the lower end of the shared pole piece toward the center of the optical axis and is located within the objective lens exit area formed by the first pole piece and the second pole piece. A first electrostatic lens is formed between the suspended pole piece and the adjacent pole piece; a second electrostatic lens is formed between the suspended pole piece and the sample. The sample bias voltage is used to adjust the landing energy of the main electron beam and to provide a traction electric field toward the interior of the objective lens for the signal electrons excited by the sample.

3. The composite electromagnetic objective lens according to claim 1, characterized in that: It also includes a field control electrode, which is coaxially arranged between the suspended electrode shoe and the sample; The field control electrode voltage is independently adjustable, which is used to fine-tune the equipotential line distribution on the sample surface, change the secondary electron deflection trajectory, and achieve selective separation and collection of secondary electrons and backscattered electrons.

4. A scanning electron beam imaging device, characterized in that: Includes an electron beam emitter, a scanning deflection system, a sample stage, a detector system, a power control system, and the composite electromagnetic objective lens as described in any one of claims 1-3; The electron beam emitter is used to emit an initial electron beam; The scanning deflection system is used to achieve electron beam grating scanning; The composite electromagnetic objective is located at the end of the electron optical path near the sample and is used to perform electron beam focusing, energy regulation and signal electron traction. The detector system is arranged in a gradient along the optical axis to collect secondary electrons and backscattered electrons at different scattering angles in layers. The excitation control circuit of the power control system is electrically connected to the first coil, the second coil, the suspended pole piece, and the sample stage, respectively, and is used to adjust the excitation current of the first coil and the second coil, as well as the bias voltage of the suspended pole piece and the sample.

5. A scanning electron beam imaging method, characterized in that: The imaging apparatus of claim 4 is used in the following steps: The electron beam emitter emits an electron beam, which is then scanned on the sample surface by a scanning deflection system; The imaging device selects the excitation combination of the first magnetic lens and the second magnetic lens according to the target imaging conditions to form a magnetic field distribution on the target axis; The imaging device applies an independent bias voltage to the suspended pole shoe to form a first electrostatic lens, and applies a bias voltage to the sample to form a second electrostatic lens. The first electrostatic lens and the second electrostatic lens work together to control the landing energy of the main electron beam. The main electron beam bombards the sample to excite signal electrons, and the magnetic field and electrostatic field coupling effect is used to pull the signal electrons upward and back; Multi-mode imaging is achieved by classifying and collecting signal electrons within each return trajectory range using multiple detectors arranged along the optical axis.

6. The scanning electron beam imaging method according to claim 5, characterized in that, The first electrostatic lens and the second electrostatic lens work together to perform electrostatic focusing: In the low landing energy mode, the suspended pole shoe applies a positive high voltage and the sample applies a negative bias voltage. The two electrostatic fields work together to form a strong deceleration electric field, which enables the main electron beam to land at low energy. In the high landing energy mode, the levitating pole shoe is grounded or a negative bias is applied. The bias applied to the sample is a negative bias that is smaller than the incident energy of the electron beam, so that the main electron beam is incident on the sample at a higher landing energy than in the low landing energy mode, and an electric field is formed on the sample surface to guide the signal electrons back into the objective lens.

7. The scanning electron beam imaging method according to claim 6, characterized in that: In high landing energy mode: The imaging device controls the first magnetic lens to be excited separately. The first magnetic lens forms a strong immersion magnetic field near the sample, which causes the main electron beam to be focused on the sample surface and causes the large-angle BSE excited by the sample to be constrained by the magnetic field and enter the lens tube. Alternatively, the imaging device controls the second magnetic lens to be excited separately, and the lack of a strong immersion magnetic field near the sample reduces the proportion of backscattered electrons entering the objective lens, and secondary electrons are captured by the detector to form an SE imaging mode. Alternatively, the imaging device controls the first magnetic lens and the second magnetic lens to be excited in the same direction, and adjusts the excitation ratio of the first magnetic lens and the second magnetic lens to make the two magnetic fields superimposed along the optical axis, the first magnetic lens provides an immersion magnetic field close to the sample, and the second magnetic lens provides additional focusing capability to enhance the main electron beam focusing capability under high landing energy conditions and increase the proportion of backscattered electrons entering the objective lens. Alternatively, the imaging device controls the first magnetic lens and the second magnetic lens to be excited in opposite directions, so that the first magnetic field and the second magnetic field cancel each other out in at least a part of the axial region, thereby reducing the proportion of backscattered electrons entering part of the detector and distributing secondary electrons to each detector according to different emission angles or return trajectories.

8. The scanning electron beam imaging method according to any one of claims 5 to 7, characterized in that: The excitation ratio configuration method of the composite electromagnetic objective is as follows: Calculate the third-order aberration coefficients of the electron optical system based on the third-order aberration theory of electron optics; To evaluate the beam spot size, considering the contributions of the Gaussian spot and the diffraction spot, a set of curves showing the relationship between the beam spot size and the NI1 / NI2 excitation ratio were found, where NI1 is the excitation value of the first magnetic lens and NI2 is the excitation value of the second magnetic lens. Obtaining the collection rate of signal electrons: Based on electron optical simulation, NI1 and NI2 are obtained, and the return trajectory of signal electrons is simulated to obtain the collection rate and separation rate of signal electrons on each detector; Select an excitation combination that meets both the resolution requirements and the requirements for various signal separation and detection.

9. The scanning electron beam imaging method according to any one of claims 5 to 7, characterized in that: The composite electromagnetic objective also includes a voltage-independently adjustable field control electrode, which is coaxially arranged between the suspending pole piece and the sample. The field control electrode is used to change the proportion of secondary electrons that pass through at least one detector central aperture or are intercepted by the detector. The imaging device adjusts the first and second magnetic lenses to confine backscattered electrons back into the objective lens, and adjusts the voltage of the field control electrode to change the capture ratio of secondary electrons and backscattered electrons on different detectors.

10. The scanning electron beam imaging method according to claim 9, characterized in that: In high aspect ratio structural imaging mode, the distribution of equipotential lines near the sample surface is changed by adjusting the field control electrode voltage, so that stray secondary electrons on the sidewalls and weak signal electrons at the bottom form different return trajectories, and are collected layer by layer by detectors at different axial positions.

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