Electron microscope and sample observation method

Abstract

Provided is an electron microscope capable of reducing a decrease in detection efficiency of secondary electrons even when a specimen is tilted. An electron microscope (100) includes: an electron source (10) that emits an electron beam; an electromagnetic field superposition objective lens (20) in which an acceleration tube (240) is incorporated inside a magnetic field lens (22); a correction electrode (50) disposed between the acceleration tube (240) and a specimen (S); and a specimen stage (30) that supports the specimen (S) and enables the specimen (S) to be tilted, the acceleration tube (240) is applied with a positive voltage, the correction electrode (50) is applied with a negative voltage, and a potential on an optical axis (A) between the correction electrode (50) and the specimen (S) is higher than a potential of the specimen (S).

Description

Technical Field

[0001] This invention relates to electron microscopy and methods for observing samples. Background Technology

[0002] Electromagnetic field superposition objectives are known as objectives for scanning electron microscopes. These objectives utilize an accelerating tube, where the electron beam is accelerated by a high-voltage circuit and decelerated just before reaching the sample. This reduces lens aberrations and allows for a smaller electron probe diameter. Consequently, high-resolution observation is possible.

[0003] In an electromagnetic field superposition objective lens, a decelerating electric field is formed between the objective lens and the sample. This decelerating electric field draws secondary electrons emitted from the sample into the objective lens. Therefore, a detector for detecting secondary electrons is positioned inside or above the objective lens.

[0004] The deceleration electric field is affected by the shape of the sample. Especially when the sample is tilted, the deceleration electric field deforms. Due to this deformation, secondary electrons may sometimes fail to reach the detector, leading to a decrease in detection efficiency. Therefore, for example, in Patent Document 1, the deceleration electric field is corrected by changing the sample's potential according to the tilt angle of the sample, thus reducing the decrease in detection efficiency. Existing technical documents Patent documents

[0005] [Patent Document 1] Japanese Patent Application Publication No. 10-294075 Summary of the Invention The problem the invention aims to solve

[0006] Thus, in an electron microscope equipped with an electromagnetic field superposition objective lens, it is hoped that even if the sample is tilted, the decrease in the detection efficiency of secondary electrons can be reduced. Solution for solving the problem

[0007] One embodiment of the electron microscope of the present invention includes: An electron source that emits an electron beam; Electromagnetic field superposition objective lens, which has an accelerating tube installed inside the magnetic field lens; A correction electrode is disposed between the accelerating tube and the sample; and A sample stage that supports the sample and allows the sample to be tilted. A positive voltage is applied to the accelerating tube. A negative voltage is applied to the correction electrode. The potential on the optical axis between the correction electrode and the sample is above the potential of the sample.

[0008] In such an electron microscope, by applying a negative voltage to the correction electrode, the effect of the deformation of the deceleration electric field caused by the tilt of the sample can be reduced. Moreover, in such an electron microscope, since the potential on the optical axis between the correction electrode and the sample is above the potential of the sample, secondary electrons can be drawn into the objective lens, thereby reducing the decrease in the detection efficiency of secondary electrons.

[0009] One embodiment of the sample observation method of the present invention is a sample observation method using an electron microscope, wherein the electron microscope comprises: A specimen stage that supports the specimen and allows the specimen to be tilted. An electron source that emits an electron beam; An electromagnetic field superposition objective lens, which incorporates an accelerating tube inside a magnetic field lens; and A calibration electrode is positioned between the accelerating tube and the sample. The sample observation method includes: A process for activating an optical system comprising the electromagnetic field superposition objective and the correction electrode; The process of tilting the sample; and The process of irradiating the sample with an electron beam to obtain an electron microscope image. In the process of making the optical system operate, An electron beam is accelerated by applying a positive voltage to the accelerating tube and a negative voltage to the correction electrode, and decelerated by a decelerating electric field formed between the accelerating tube and the sample. The potential on the optical axis between the correction electrode and the sample is made to be greater than or equal to the potential of the sample.

[0010] In this sample observation method, by applying a negative voltage to the correction electrode, the effect of deformation of the deceleration electric field caused by the tilt of the sample can be reduced. Furthermore, in this method, since the potential on the optical axis between the correction electrode and the sample can be made higher than the potential of the sample, secondary electrons can be drawn into the objective lens, thus reducing the decrease in the detection efficiency of secondary electrons. Attached Figure Description

[0011] Figure 1 This is a diagram showing an example of the configuration of the electron microscope according to the first embodiment. Figure 2 This is a schematic diagram showing the accelerator tube and the calibration electrode. Figure 3 It is a diagram showing the potential distribution between the objective lens and the sample. Figure 4 This is a graph showing the potential distribution between the objective lens and the sample in the electron microscope of the comparative example. Figure 5 It is a coordinate graph used to illustrate the distribution of potential along the optical axis. Figure 6 This is a flowchart illustrating an example of a sample observation method. Figure 7 This is a graph showing the relationship between the detection efficiency of secondary electrons and the voltage applied to the correction electrode. Figure 8 This is a diagram illustrating an example of the configuration of the electron microscope according to the second embodiment. Explanation of reference numerals in the attached figures 10…Electron source, 20…Electromagnetic field superposition objective lens, 22…Magnetic field lens, 24…Electrostatic lens, 30…Sample stage, 32…Sample holder, 40…Detector, 50…Correction electrode, 52…Insulation part, 54…Through hole, 60…Accelerating power supply, 70…Coil power supply, 80…Correction power supply, 90…Sample power supply, 100…Electron microscope, 110…Control unit, 200…Electron microscope, 220…Coil, 222…Upper pole, 224…Lower pole, 240…Accelerating tube. Detailed Implementation

[0012] The preferred embodiments of the present invention will now be described in detail using the accompanying drawings. Furthermore, the embodiments described below are not intended to unduly limit the scope of the invention as defined in the claims. Additionally, not all of the components described below are necessarily essential elements of the present invention.

[0013] 1. First Implementation Method 1.1. Electron Microscope First, the electron microscope of the first embodiment will be described with reference to the accompanying drawings. Figure 1 This is a diagram showing an example of the configuration of the electron microscope 100 according to the first embodiment.

[0014] like Figure 1 As shown, the electron microscope 100 includes an electron source 10, an electromagnetic field superposition objective lens 20 (hereinafter also simply referred to as the "objective lens"), a sample stage 30, a detector 40, a calibration electrode 50, an accelerating power supply 60, a coil power supply 70, a calibration power supply 80, a sample power supply 90, and a control unit 110. The electron microscope 100 is a scanning electron microscope that uses an electron beam to converge to form an electron probe and uses the electron probe to scan a sample S to obtain a scanning electron microscope image (SEM image).

[0015] The electron source 10 is, for example, an electron gun that accelerates electrons emitted from the cathode toward the anode. The electron microscope 100 has an optical system including an objective lens 20 and a correction electrode 50. Although not shown, the optical system includes a converging lens for focusing the electron beam, a deflector for deflecting the electron beam, an aperture for cutting off a portion of the electron beam, etc.

[0016] Objective lens 20 is a lens used to focus the electron probe onto the surface of sample S. Objective lens 20 includes a magnetic field lens 22 and an electrostatic lens 24. By combining the magnetic field lens 22 and the electrostatic lens 24, objective lens 20 can achieve high resolution even at low accelerating voltages.

[0017] The magnetic field lens 22 includes a coil 220, an upper pole 222, and a lower pole 224. The coil 220 generates a magnetic field. The magnetic field generated by the coil 220 is enclosed within a frame (magnetic circuit) formed by the upper pole 222 and the lower pole 224, and leaks onto the optical axis A through an opening (gap) between the upper pole 222 and the lower pole 224. The upper pole 222 forms the upper side of the frame (magnetic circuit), and the lower pole 224 forms the lower side. The opening is, for example, positioned along the optical axis A, with the upper pole 222 located on the upper side and the lower pole 224 on the lower side. The upper pole 222 and the lower pole 224 are made of magnetic material. For example, the upper pole 222 and the lower pole 224 are made of soft magnetic materials such as pure iron or Permendur alloys.

[0018] Optical axis A is a straight line passing through the center of the optical system including objective lens 20. An electron beam emitted from electron source 10 travels along optical axis A and is incident on sample S. In addition, secondary electrons emitted from sample S travel along optical axis A and are detected by detector 40.

[0019] Coil 220 is controlled by coil power supply 70. Coil power supply 70 controls the coil current flowing through coil 220. The upper pole 222 and the lower pole 224 are connected to a ground point. The ground point is the potential that serves as a reference for the device, such as ground potential (0V).

[0020] The electrostatic lens 24 includes an accelerating tube 240 and a lower electrode 224. The accelerating tube 240 is housed inside the magnetic field lens 22. The accelerating tube 240 is positioned inside the cylindrical upper electrode 222. The accelerating tube 240 is cylindrical, and its central axis is aligned with the optical axis A. The lower end of the accelerating tube 240 is located above the correction electrode 50.

[0021] The accelerating tube 240 is supplied with a positive voltage relative to its upper electrode 222 and lower electrode 224 by the accelerating power supply 60. By applying a positive voltage to the accelerating tube 240, the electron beam can be accelerated. In addition, a decelerating electric field is formed between the lower end of the accelerating tube 240 and the sample S to slow down the electron beam. The electron beam is slowed down by this decelerating electric field.

[0022] Thus, in objective lens 20, the electron beam is accelerated within the accelerating tube 240, and decelerated by the decelerating electric field, causing it to enter the sample S. Therefore, in objective lens 20, because the electron beam passing through the magnetic field lens 22 has high energy, the chromatic aberration caused by the magnetic field lens 22 can be reduced, and the diameter of the electron probe can be reduced. Moreover, in objective lens 20, the decelerating electric field acts as an electrostatic lens. The electrostatic lens is formed closer to the sample S than the magnetic field lens. Therefore, in objective lens 20, the principal surface of the lens as a whole is closer to the sample S, thus shortening the focal length and reducing the diameter of the electron probe.

[0023] The sample stage 30 supports the sample S and allows the sample S to tilt. The sample stage 30 has a tilting mechanism for tilting the sample S. The sample stage 30 may also have a moving mechanism for moving the sample S horizontally and a moving mechanism for moving the sample S vertically. The sample stage 30 is connected to a ground point.

[0024] A sample holder 32 is installed on the sample stage 30 to hold the sample S. The sample holder 32 is electrically insulated from the sample stage 30. The electron microscope 100 is equipped with a switch for switching between a state in which the sample holder 32 is electrically connected to a ground point and a state in which the sample holder 32 is electrically connected to a sample power supply 90. The sample holder 32 can be connected to the ground point or to the sample power supply 90, which outputs 0V.

[0025] Detector 40 detects secondary electrons emitted from sample S by irradiating sample S with an electron beam. The secondary electrons emitted from sample S are drawn into objective lens 20 by a decelerating electric field formed between the lower end of accelerating tube 240 and sample S. Detector 40 is positioned inside or above objective lens 20 to detect the secondary electrons drawn into objective lens 20.

[0026] The correction electrode 50 is connected to the lower pole 224 of the objective lens 20 via an insulating portion 52. The correction electrode 50 and the objective lens 20 are electrically insulated by the insulating portion 52. The insulating portion 52 can be, for example, an insulating member made of insulating material, or a space. The correction electrode 50 is made of the same magnetic material as the lower pole 224 or a magnetic material with similar properties to the lower pole 224. Therefore, the correction electrode 50 can function as part of the objective lens 20. For example, the correction electrode 50 can reduce leakage of the magnetic field generated by the magnetic field lens 22 and leakage of the electric field generated by the accelerating tube 240.

[0027] The correction electrode 50 is, for example, disk-shaped, with a through-hole 54 centered on the optical axis A. An electron beam emitted from the electron source 10 passes through the through-hole 54 and irradiates the sample S. The correction electrode 50 is subjected to a negative voltage relative to the upper electrode 222 and the lower electrode 224 by the correction power supply 80. Through the correction electrode 50, the effect of the deformation of the deceleration electric field caused by the tilt of the sample S can be reduced.

[0028] For example, the accelerating tube 240 is subjected to a voltage of + a few kV to + a dozen kV, the correction electrode 50 is subjected to a voltage of tens of V to 100 V, and the upper electrode 222, the lower electrode 224 and the sample S are connected to the grounding point.

[0029] The control unit 110 controls various components constituting the electron microscope 100. For example, the control unit 110 controls the accelerating power supply 60, the coil power supply 70, the calibration power supply 80, and the sample power supply 90. By controlling these power supplies, the control unit 110 controls the potentials of the accelerating tube 240 (electrostatic lens 24), the magnetic field lens 22, the calibration electrode 50, and the sample S. Additionally, the control unit 110 controls the sample stage 30.

[0030] The control unit 110 includes, for example, a processor such as a CPU (Central Processing Unit) and storage units (memory) such as RAM (Random Access Memory) and ROM (Read Only Memory). The storage units store programs and data used for various controls. The functions of the control unit 110 can be implemented by the processor executing the program. Alternatively, the functions of the control unit 110 can also be implemented by general-purpose circuits such as microcontrollers and microprocessors that operate according to a program, or by special-purpose circuits such as ASICs (Application Specific Integrated Circuits).

[0031] 1.2. Calibration Electrode Figure 2 This diagram schematically illustrates the accelerating tube 240 and the correction electrode 50. The correction electrode 50 has a thickness along the optical axis A. The height H of the correction electrode 50 is smaller than its inner diameter D. That is, H < D. The height H of the correction electrode 50 is the size (thickness) of the correction electrode 50 along the optical axis A. The inner diameter D of the correction electrode 50 is the diameter of the through-hole 54. The inner diameter D of the correction electrode 50 is, for example, about a few millimeters.

[0032] The decelerating electric field formed between the lower end of the accelerating tube 240 and the sample S is affected by the shape of the sample S. In particular, the decelerating electric field deforms when the sample S is tilted. For example, when the sample S is tilted, the decelerating electric field becomes asymmetric about the optical axis A. The decelerating electric field deflects electrons moving along the optical axis A. Therefore, the deformation of the decelerating electric field causes the electron beam's trajectory to deviate from the optical axis A, i.e., axis deviation occurs. In addition, secondary electrons emitted from the sample S will have their trajectories bent due to the deformation of the decelerating electric field, and therefore may fail to reach the detector 40, resulting in a decrease in detection efficiency.

[0033] The correction electrode 50 reduces the effect of deformation of the deceleration electric field caused by the tilting of the sample S. The correction electrode 50 weakens the deceleration electric field so that the deformation of the deceleration electric field has a smaller impact on the electron beam and secondary electrons. That is, a negative voltage is applied to the correction electrode 50 to weaken the deceleration electric field to a level that does not affect the electron beam and secondary electrons on optical axis A. Therefore, the effect of the deformation of the deceleration electric field on the electron beam and secondary electrons can be reduced. Thus, in the electron microscope 100, the axial deviation of the electron beam caused by tilting the sample S can be reduced. Furthermore, in the electron microscope 100, the decrease in the detection efficiency of secondary electrons caused by tilting the sample S can be reduced.

[0034] Figure 3 This is a diagram showing the potential distribution between the objective lens 20 and the sample S in the electron microscope 100. Figure 4 This is a graph showing the potential distribution between the objective lens 20 and the sample S in the electron microscope of the comparative example. Figure 3 In, satisfying H < D, in Figure 4 In the comparison example shown, H > D is satisfied. Figure 3 and Figure 4 This is the calculated result of the potential distribution under the conditions of applying +10kV to the accelerating tube 240, -100V to the calibration electrode 50, and 0V to the sample S. Figure 3 and Figure 4 For convenience, the range of -100V to +100V is indicated by color changes. Furthermore, since +10kV is applied to the accelerating tube 240, there exists a region with a potential higher than +100V, but... Figure 3 and Figure 4 In the diagram, potentials higher than +100V are indicated by the same color as +100V.

[0035] like Figure 4As shown, when the height H of the correction electrode 50 is larger than its inner diameter D (i.e., H > D), the electric field generated by the correction electrode 50 extends to the optical axis A. Therefore, a region with a potential lower than 0V exists along the optical axis A. Since secondary electrons cannot cross this region, they are pushed back to the sample S side in this region. Consequently, the detection efficiency of the detector 40 for secondary electrons decreases.

[0036] In contrast, Figure 3 When the height H of the correction electrode 50 is smaller than its inner diameter D (H < D), the potential on the optical axis A between the objective lens 20 and the sample S is always higher than 0V. Therefore, secondary electrons emitted from the sample S can be drawn into the interior of the accelerating tube 240. Consequently, the decrease in the detection efficiency of the detector 40 for secondary electrons can be reduced.

[0037] Here, as Figure 4 As shown, in the direction from electron source 10 to sample S, when the position where the potential on optical axis A changes from a positive value to a negative value (the position where the potential becomes 0V) is set as P0, the distance L between the upper end of the correction electrode 50 and position P0 is larger than the inner diameter D of the correction electrode 50. Therefore, by making the height H of the correction electrode 50 smaller than the inner diameter D of the correction electrode 50, the potential on optical axis A inside the objective lens 20 can be made to be above 0V, that is, above the potential of sample S. Therefore, by making the height H of the correction electrode 50 smaller than the inner diameter D of the correction electrode 50, secondary electrons emitted from sample S can be pulled into the interior of the accelerating tube 240.

[0038] Figure 5 It is a coordinate graph used to illustrate the distribution of the potential along the optical axis A between the objective lens 20 and the sample S. Figure 5 The coordinate graph shown has the horizontal axis representing the position on optical axis A (Z-axis) and the vertical axis representing the potential. Additionally, in... Figure 5 In the diagram, the case where H < D is shown by a solid line, and the case where H > D is shown by a dashed line. Furthermore, in... Figure 5 The diagram also includes a magnified coordinate graph within box F.

[0039] The position P0 on the Z-axis is... Figure 4 The position P0 corresponds to the position where the potential on optical axis A changes from a positive to a negative value. Additionally, position P1 is the position of the lower end of the correction electrode 50. That is, the objective lens 20 is located further inside from position P1 in the -Z direction. Position P2 is the position of the surface of the sample S. The area between positions P1 and P2 represents the optical axis A between the correction electrode 50 and the sample S. Furthermore, in Figure 5 In the coordinate graph shown, the potential of sample S is 0V.

[0040] like Figure 5 As shown, when H > D, the potential between positions P0 and P2 on the optical axis A is lower than 0V. Therefore, secondary electrons emitted from sample S are pushed back to the sample S side. In contrast, when H < D, the potential on the optical axis A is not lower than 0V. Therefore, secondary electrons travel along the optical axis A in the +Z direction and are pulled into the objective lens 20. This reduces the decrease in the detection efficiency of the detector 40 for secondary electrons.

[0041] 1.3. Sample Observation Method Figure 6 This is a flowchart illustrating an example of a sample observation method using an electron microscope 100.

[0042] First, the optical system is activated (step S100).

[0043] When a user inputs conditions for the optical system via an operation unit such as a GUI (Graphical User Interface), the control unit 110 controls the optical system, including the objective lens 20 and the correction electrode 50, based on the input conditions. The control unit 110 also controls the coil power supply 70 based on the optical system conditions. As a result, a coil current corresponding to the input optical system conditions flows through the coil 220. Furthermore, the control unit 110 controls the accelerating power supply 60 based on the optical system conditions. As a result, a voltage corresponding to the input optical system conditions is applied to the accelerating tube 240.

[0044] Furthermore, the control unit 110 controls the correction power supply 80 based on the conditions of the optical system. The storage unit of the control unit 110 stores a database that maps the conditions of the optical system to the optimal value of the voltage applied to the correction electrode 50 (hereinafter also referred to as the "correction voltage"). In the database, for example, the working distance (WD) and the voltage applied to the accelerating tube 240 are registered as conditions of the optical system.

[0045] The control unit 110 acquires information on the working distance (WD) and the voltage applied to the accelerating tube 240 as conditions for the optical system. Referring to a database stored in the storage unit, it selects the optimal value of the correction voltage based on the working distance and the voltage applied to the accelerating tube 240. The control unit 110 controls the correction power supply 80 such that the selected optimal value of the correction voltage is applied to the correction electrode 50. Furthermore, if the conditions of the optical system are inconsistent with the optical conditions registered in the database, the control unit 110 can also calculate a formula representing the relationship between the optical conditions and the correction voltage based on the data registered in the database, and use this formula to determine the optimal value of the correction voltage.

[0046] In the electron microscope 100, an electron beam is accelerated by applying a positive voltage to the accelerating tube 240 and a negative voltage to the correction electrode 50, and decelerated by a decelerating electric field formed between the accelerating tube 240 and the sample S. Furthermore, the potential on the optical axis A between the correction electrode 50 and the sample S is made greater than or equal to the potential of the sample S. That is, the minimum potential on the optical axis A between the correction electrode 50 and the sample S is made greater than or equal to the potential of the sample S.

[0047] Next, the sample S is tilted using the sample stage 30 (step S102). The control unit 110 receives the tilt angle information input by the user and controls the sample stage 30 to tilt the sample S to that tilt angle. Thus, the sample S can be tilted to the desired tilt angle.

[0048] Next, the sample S is irradiated with an electron beam to obtain a SEM image (step S104). When the user sets observation conditions such as magnification via the operation unit, the set observation conditions are reflected in the processing of the control unit 110. As a result, an SEM image at the desired magnification can be obtained.

[0049] In the electron microscope 100, an electron probe is formed by focusing an electron beam, and the sample S is scanned by the electron probe. The detector 40 detects the secondary electrons emitted from each irradiation position to obtain an SEM image.

[0050] At this time, since a negative voltage is applied to the correction electrode 50, the effect of deformation of the deceleration electric field caused by the tilt of the sample S can be reduced. Moreover, since the potential on the optical axis A between the correction electrode 50 and the sample S is above the potential of the sample S, the decrease in the detection efficiency of secondary electrons can be reduced.

[0051] The above steps are sufficient to obtain SEM images.

[0052] Furthermore, if the sample S is tilted at the first tilt angle in step S102, then even if the sample S is tilted at a second tilt angle different from the first tilt angle after step S104, the correction voltage applied to the correction electrode 50 is set to be constant. Therefore, the control unit 110 controls the correction power supply 80 in a manner that the correction voltage remains constant even if the tilt angle of the sample S is changed from the first tilt angle to the second tilt angle.

[0053] Figure 7 This is a graph showing the relationship between the detection efficiency of secondary electrons and the correction voltage applied to the correction electrode 50. Figure 7 The horizontal axis of the coordinate graph shown represents the correction voltage, and the vertical axis represents the detection efficiency of secondary electrons below 50 eV. Figure 7The calculation results are shown for the cases with a tilt angle of 0 degrees, a tilt angle of 30 degrees, and a tilt angle of 60 degrees.

[0054] like Figure 7 As shown, with a tilt angle of 60 degrees, the detection efficiency reaches its maximum at a calibration voltage of -50V. Even with a tilt angle of 30 degrees and 0 degrees, the detection efficiency remains high at a calibration voltage of -50V. Since the change in the calibration voltage that maximizes detection efficiency is small even when the tilt angle changes, the calibration voltage can be maintained even if the tilt angle changes.

[0055] 1.4. Effects The electron microscope 100 includes: an electron source 10 that emits an electron beam; an electromagnetic field superposition objective 20, which houses an accelerating tube 240 inside a magnetic field lens 22; a correction electrode 50 disposed between the accelerating tube 240 and the sample S; and a sample stage 30 that supports the sample S and allows the sample S to tilt. Furthermore, a positive voltage is applied to the accelerating tube 240, and a negative voltage is applied to the correction electrode 50. The potential on the optical axis A between the correction electrode 50 and the sample S is above the potential of the sample S.

[0056] Thus, in the electron microscope 100, the electron beam is accelerated by applying a positive voltage to the accelerating tube 240, and decelerated by the decelerating electric field formed between the lower end of the accelerating tube 240 and the sample S. Therefore, in the electron microscope 100, the aberrations of the objective lens 20 can be reduced, and the diameter of the electron probe can be reduced. In addition, in the electron microscope 100, the distortion of the decelerating electric field caused by tilting the sample S can be reduced by the correction electrode 50. Moreover, in the electron microscope 100, since the potential on the optical axis A between the correction electrode 50 and the sample S is above the potential of the sample S, secondary electrons emitted from the sample S can be drawn into the interior of the accelerating tube 240, reducing the decrease in the detection efficiency of secondary electrons.

[0057] Furthermore, in the electron microscope 100, the voltage applied to the correction electrode 50 can be varied regardless of the tilt angle of the sample S, and the voltage applied to the correction electrode 50 can be kept constant even if the tilt angle of the sample S changes.

[0058] For example, when reducing the decrease in secondary electron detection efficiency by changing the sample's potential according to its tilt angle, applying a positive voltage to the sample reduces the potential difference between the electromagnetic field superposition objective and the sample, weakening the aberration reduction effect. Furthermore, changing the sample's potential according to its tilt angle alters the energy of the electron beam reaching the sample. This change necessitates adjustments to align the electron beam with the optical axis A and to focus it. Moreover, the use of the manipulator in contact with the sample and the measurement of the absorbed current become difficult.

[0059] In contrast, in the electron microscope 100, since the voltage applied to the correction electrode 50 can be varied regardless of the tilt angle of the sample S, the potential difference between the objective lens 20 and the sample S can be kept constant even if the tilt angle of the sample S is changed. Furthermore, in the electron microscope 100, since the voltage applied to the correction electrode 50 can be varied regardless of the tilt angle of the sample S, it is not necessary to readjust the axis alignment or focus even if the tilt angle of the sample S is changed. Moreover, in the electron microscope 100, since the sample S can be maintained at a ground potential, the use of the manipulator in contact with the sample S and the measurement of the absorbed current become possible.

[0060] Thus, in the electron microscope 100, since there is no need to readjust the axis alignment or focus even if the tilt angle of the sample S is changed, and the manipulator can be used, the processing capacity can be improved, especially in devices equipped with SEM tubes and FIB tubes.

[0061] In the electron microscope 100, the size (height H) of the correction electrode 50 along the optical axis A is smaller than the inner diameter D of the correction electrode 50. Therefore, when a negative voltage is applied to the correction electrode 50 to reduce the influence of the decelerating magnetic field caused by the tilt of the sample S, the potential along the optical axis A between the correction electrode 50 and the sample S can be made higher than the potential of the sample S. Therefore, in the electron microscope 100, the decrease in the detection efficiency of secondary electrons caused by the tilt of the sample S can be reduced.

[0062] In the electron microscope 100, the electromagnetic field superposition objective lens 20 includes: a coil 220 that generates a magnetic field; and an upper pole 222 and a lower pole 224 for forming a magnetic field along the optical axis A. Additionally, a correction electrode 50 is made of a magnetic material and is connected to the lower pole 224 via an insulating portion 52. Therefore, in the electron microscope 100, the correction electrode 50 can function as part of the objective lens 20.

[0063] The electron microscope 100 includes a detector 40 for detecting secondary electrons emitted from the sample S and drawn into the electromagnetic field superimposed objective lens 20. In the electron microscope 100, since the potential on the optical axis A between the correction electrode 50 and the sample S is above the potential of the sample S, the decrease in the detection efficiency of the detector 40 for secondary electrons can be reduced.

[0064] The sample observation method using the electron microscope 100 includes: a step of operating the optical system including the objective lens 20 and the correction electrode 50; a step of tilting the sample S; and a step of irradiating the sample S with an electron beam to obtain an electron microscope image. Furthermore, in the step of operating the optical system, by applying a positive voltage to the accelerating tube 240 and a negative voltage to the correction electrode 50, the electron beam is accelerated by the accelerating tube 240, and decelerated by the decelerating electric field formed between the accelerating tube 240 and the sample S, so that the potential on the optical axis A between the correction electrode 50 and the sample S becomes greater than or equal to the potential of the sample S.

[0065] Therefore, in the sample observation method using electron microscope 100, the diameter of the electron probe can be reduced, enabling high-resolution observation. Furthermore, it reduces the decrease in detection efficiency of secondary electrons caused by the tilt of the sample S.

[0066] In the sample observation method using the electron microscope 100, there is a step of changing the tilt angle of the sample S. Even when the tilt angle of the sample S is changed, the voltage applied to the correction electrode 50 remains constant. Therefore, in the electron microscope 100, even when the tilt angle of the sample S is changed, it is not necessary to readjust the axis alignment and focusing to align the electron beam with the optical axis A.

[0067] 1.5. Variations In the above embodiment, the storage unit stores a database that maps the conditions of the optical system to the optimal value of the correction voltage. However, the storage unit may also store a relational expression representing the relationship between the conditions of the optical system and the optimal value of the correction voltage. In this case, the control unit 110 substitutes the conditions of the optical system into the relational expression to calculate the optimal value of the correction voltage. Furthermore, in the above embodiment, the working distance and the voltage applied to the accelerating tube 240 are examples of optical system conditions, but the system is not limited to these. Other optical system conditions that cause the correction voltage to vary, such as the accelerating voltage, can be used as optical system conditions. Additionally, the optical system conditions used to determine the correction voltage can be one or multiple. For example, the voltage applied to the accelerating tube 240, which has the greatest influence on the correction voltage, may be used as the optical system condition for determining the correction voltage.

[0068] 2. Second Implementation Method Next, the electron microscope of the second embodiment will be described with reference to the accompanying drawings. Figure 8 This is a diagram showing an example of the configuration of the electron microscope 200 according to the second embodiment. Hereinafter, in the electron microscope 200 of the second embodiment, components having the same function as the constituent components of the electron microscope 100 of the first embodiment will be labeled with the same reference numerals, and their detailed descriptions will be omitted.

[0069] The electron microscope 200 includes a sample power supply 90 that applies a negative voltage to the sample S. By applying a negative voltage to the sample S in the electron microscope 200, a high-resolution SEM image can be obtained by using a deceleration method that slows down the electron beam just before it is irradiated onto the sample S.

[0070] The calibration power supply 80 operates in conjunction with the sample power supply 90. For example, when the sample power supply 90 applies a negative voltage to the sample S, the calibration power supply 80 adds a value corresponding to the negative voltage applied to the sample S to apply a voltage to the calibration electrode 50. Thus, in the electron microscope 200, the potential difference between the sample S and the calibration electrode 50 can be made equal to the potential difference between the sample S and the calibration electrode 50 in the electron microscope 100 described above. Therefore, in the electron microscope 200, similar to the electron microscope 100, the calibration electrode 50 can reduce the influence of the deformation of the deceleration electric field caused by tilting the sample S, thereby reducing the decrease in the detection efficiency of secondary electrons.

[0071] When the sample power supply 90 is activated by applying a negative voltage to the sample S, the control unit 110 determines the voltage applied to the correction electrode 50 based on the voltage applied to the sample S. The control unit 110 controls the correction power supply 80 so that the determined voltage is applied to the correction electrode 50. As a result, in the electron microscope 200, similar to the electron microscope 100, the potential on the optical axis A between the correction electrode 50 and the sample S can be made to be greater than or equal to the potential of the sample S.

[0072] In electron microscope 200, the calibration power supply 80 applies a voltage to the calibration electrode 50 based on the voltage applied to the sample S by the sample power supply 90. Therefore, electron microscope 200 can achieve the same effect as electron microscope 100.

[0073] This invention is not limited to the embodiments described above and can be further modified in various ways. For example, this invention includes configurations that are substantially the same as those described in the embodiments. A substantially similar configuration refers to a configuration with the same function, method, and result, or a configuration with the same purpose and effect. Furthermore, this invention includes configurations in which non-essential parts of the configurations described in the embodiments are replaced. Additionally, this invention includes configurations that have the same effect or purpose as those described in the embodiments. Furthermore, this invention includes configurations in which known techniques are added to the configurations described in the embodiments.

Claims

1. An electron microscope, characterized in that, Include: An electron source that emits an electron beam; Electromagnetic field superposition objective lens, which has an accelerating tube installed inside the magnetic field lens; A correction electrode is disposed between the accelerating tube and the sample; and A sample stage that supports the sample and allows the sample to be tilted. A positive voltage is applied to the accelerating tube. A negative voltage is applied to the correction electrode. The potential on the optical axis between the correction electrode and the sample is above the potential of the sample.

2. The electron microscope according to claim 1, wherein, The size of the correction electrode along the optical axis is smaller than the inner diameter of the correction electrode.

3. The electron microscope according to claim 1 or 2, wherein, Include: A calibration power supply that applies a negative voltage to the calibration electrodes; and A sample power supply that applies a negative voltage to the sample. The calibration power supply applies a voltage to the calibration electrode based on the voltage applied to the sample by the sample power supply.

4. The electron microscope according to claim 1 or 2, wherein, The electromagnetic field superposition objective lens has the following characteristics: A coil that generates a magnetic field; and The upper and lower poles are made of magnetic material. The correction electrode is made of magnetic material. The correction electrode is connected to the lower electrode via an insulating portion.

5. The electron microscope according to claim 1, wherein, It includes a detector for detecting secondary electrons emitted from the sample and drawn into the electromagnetic field superposition objective.

6. A method for observing a sample, which uses an electron microscope, the electron microscope comprising: A specimen stage that supports the specimen and allows the specimen to be tilted. An electron source that emits an electron beam; An electromagnetic field superposition objective lens, which incorporates an accelerating tube inside a magnetic field lens; and A calibration electrode is positioned between the accelerating tube and the sample. The method for observing the sample is characterized by comprising: A process for activating an optical system comprising the electromagnetic field superposition objective and the correction electrode; The process of tilting the sample; and The process of irradiating the sample with an electron beam to obtain an electron microscope image. In the process of making the optical system operate, An electron beam is accelerated by applying a positive voltage to the accelerating tube and a negative voltage to the correction electrode, and decelerated by a decelerating electric field formed between the accelerating tube and the sample. The potential on the optical axis between the correction electrode and the sample is made to be greater than or equal to the potential of the sample.

7. The sample observation method according to claim 6, wherein, The process includes changing the tilt angle of the sample. Even if the tilt angle of the sample is changed, the voltage applied to the correction electrode remains constant.

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