JPH0471151A - Method of reducing astigmatism of scanning type electron microscope - Google Patents

Abstract

PURPOSE:To keep the distortion of an image beam resulting from astigmatism to a minimum by applying a prestored current to astigmatism correcting coil in association with the movement of a stage while impressing a prestored voltage upon each of stage high-precision control motors to reproduce an astigmatism corrected magnetic- field in its initial state. CONSTITUTION:An observer moves a stage in order to observe an image in a desired position. Then the judges which one of divided image areas corresponds to the area of the desired-image position from coordinate values given according to the movement of the stage. When the judgment result corresponds to NO, then a current value or a voltage value prestored for a reference image-area is fed to an astigmatsm correcting coil 5, a coil 20 and stage high-precision control motors 40 to 43 respectively to reproduce a stigmatic magnetic-field. When a difference between a magnetic field measured by each of magnetic-field sensors 1 to 4 and the prestored stigmatic magnetic-field obtained at the time of the reproduction exceeds a specified value, such magnetic-field reproducing sequence that a correction signal corresponding to the difference may be fed to the stage high-precision control motors 40 to 43 can further be executed so as to automatically reduce the astigmatism.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for reducing astigmatism in a scanning electron microscope, which reduces astigmatism when a sample to be observed is irradiated with an electron beam focused by an objective lens. It is something.

[Conventional technology]

Generally, a scanning electron microscope has an objective lens shown in FIG. This objective lens usually has a cylindrical symmetrical shape with the electron beam path 43 as a common axis.

The magnetic path 41 connecting the magnetic poles 41-1 and 41-2 is made of a magnetic material such as pure iron, and a magnetic field is created between the magnetic poles 41-1 and 41-2 by passing current through a coil 44 that provides magnetomotive force. It is occurring. The electron beam passing through this magnetic field is subjected to a convergence effect and collides with an observation surface 46 of a wafer (semiconductor substrate) 42 to be observed, generating secondary electrons. The secondary electrons generated in this way are subjected to the action of both the vertical magnetic field generated by the objective lens and the electric field applied in the axial direction (Lorentz force), and do not move spirally along the axial direction. It separates from the wafer and is captured by the detector. The secondary electrons captured by the detector are image-processed and displayed on a cathode ray tube.

Further, FIG. 10 shows a schematic configuration of a scanning electron microscope filed by the present inventors (Patent No. 244392, 1999).

The basic operation will be explained in detail using the scanning electron microscope shown in FIG.

In FIG. 10, an electron beam 12 emitted from an electron gun 11 is aligned by an alignment coil 13, converged by a focusing lens 14, and incident on an objective lens 17 via an aperture 16. The electron beam further focused by the objective lens 17 irradiates the wafer 22, and this irradiation point is scanned by the deflection coil 25, and the secondary electrons emitted from the wafer 22 are detected by the secondary electron detector 824.
A so-called secondary electron image is displayed by modulating the brightness on a display (not shown).

Next, the configuration and operation of a conventional scanning electron microscope will be described in detail.

In FIG. 10, the magnetic pole 19 is a magnetic pole for constructing a magnetic loop for the flat magnetic pole 21. In FIG.

The coil 20 applies a magnetomotive force to the objective lens 17 by passing a current therethrough, thereby generating a lens action.

The flat magnetic pole 21 is a flat magnetic pole that can move in parallel to the opposing magnetic pole 18 .

The wafer 22 is a sample to be observed placed on the flat magnetic pole 21 .

The holding stand 23 is a stand that holds the flat magnetic poles 21 in parallel.

The secondary electron detector 24 collects and detects secondary electrons generated by irradiating the wafer 22 with an electron beam.

The deflection coil 25 scans the electron beam onto the wafer 22.

Adjustment screws 26.26' are attached to fixtures 31.26 on holding base 23.
This is a mechanism for adjusting the parallelism of the flat magnetic pole 21 to which the wafer is fixed at 31° with respect to the magnetic pole 18 (that is, the parallelism with respect to a plane perpendicular to the axial direction of the objective lens).

The moving tables 27 and 28 are tables capable of moving the holding table 23 within a plane parallel to a plane perpendicular to the axial direction of the objective lens.

The sample chamber 30 is a vacuum chamber that houses the wafer 22 and the like.

In FIG. 10, a coil 2 constituting an objective lens 17 is shown.
By passing a current through 0, a magnetomotive force is generated, and a magnetic circuit is formed in a loop shape as shown by the dotted line in the figure.

As a result, a magnetic field is generated in the inner interval (between the magnetic pole 18 and the flat magnetic pole 21) and the outer interval (between the magnetic pole 19 and the flat magnetic pole 21) of the magnetic poles.

The only magnetic field that effectively acts as an objective lens on the electron beam is an axially symmetrical magnetic field generated between the magnetic pole 18 and the flat magnetic pole 21. The gap between the magnetic pole 19 and the flat magnetic pole 21 is for forming a magnetic circuit. here,
The flat magnetic pole 21 acts on the electron beam between the magnetic pole 18 and the flat magnetic pole 21 when the wafer 22 mounted on the flat magnetic pole 21 is moved in parallel to move the electron beam irradiation position for observation. Wafer 2 is placed so that the magnetic field does not change.
It is configured to have a size that is sufficiently larger than the size of 2.

Further, a wafer 22 to be observed is placed closely on the flat magnetic pole 21 and fixed by fixtures 31 and 31'. Further, the flat magnetic pole 21 is configured to be in close contact with a holding base 23 having sufficient thickness and strength to maintain flatness. The holding base 23 is equipped with adjustment screws 26 and 26 in order to adjust the parallelism of the parallel magnetic pole 21 to the magnetic pole 18.
Movable platform 27.28 in lll3i adjustable by °
(Moveable on x and y axes) are fixed. The adjustment screw 26.26° is also provided in a direction perpendicular to the straight line connecting 26 and 26', allowing adjustment of the two axes, X and Y. The adjustment screw 26.26° is adjusted while observing the secondary electron image or using an optical microscope so that the height of the observation point does not change when the moving table 27.28 is moved. There is. Further, the adjustment screws 26 and 26' are adjusted and fixed so that the amount of astigmatism does not increase due to movement of the moving table. This is because the astigmatism of the objective lens is caused by the non-uniformity of the objective lens magnetic field in the paraxial region of the electron beam trajectory. Therefore, maintaining the parallelism between the magnetic poles 18 and 19 and the flat magnetic pole 21 and forming a uniform magnetic field is as follows.
Very important.

As explained above, in a scanning electron microscope, in order to stably obtain the best image that the scanning electron microscope has, it is necessary to
It is essential to reduce the astigmatism of the objective lens with a parallelism of 1.

[Problem to be solved by the invention]

However, with the above configuration, there is no mechanism to automatically reduce astigmatism, so there is a problem that the operator has no choice but to correct the astigmatism while visually checking the image displayed on the display. . Further, the parallelism between the first magnetic poles 18 and 19 and the second flat magnetic poles 21 largely depends on mechanical processing accuracy and assembly accuracy, and the only way to adjust them is by manual alignment.

In view of this point, an object of the present invention is to provide a scanning electron microscope capable of automatically reducing astigmatism.

[Means to solve the problem]

The present invention includes a first magnetic pole 18 that is axially symmetrical, has a hole in the center through which an electron beam passes, and is conical and has a flat tip, and a plane perpendicular to the axis between the first magnetic poles 18. an objective lens 17 having a second flat magnetic pole 21 movable therein;
It is equipped with an astigmatism correction coil 5 for correcting astigmatism generated by the objective lens 17, and stage height control motors 40 to 43 for controlling the height and inclination of the movable stage, and is mounted on the movable stage. The sample to be observed is mounted on the second flat magnetic pole 21, and the movable table is moved to perform astigmatism correction in advance on arbitrary points (for example, coordinate values (x, y)) of the sample to be observed. The current values (voltage values) flowing through the astigmatism correction coil 5, the coil 20 of the objective lens 17, and the indicated values of the stage height control motors 40 to 43 are stored respectively, and the sample to be observed is moved and observed. At times, the stored current value (voltage value) is applied to the astigmatism correction coil 5 (or the astigmatism correction coil 5 and the coil 20 of the objective lens 17).
) and supplies signals corresponding to the stored instruction values to stage height control motors 40 to 43. The scanning electron microscope is also equipped with a magnetic field sensor to detect the asymmetry of the magnetic field of the objective lens 17 and supply a correction signal to the stage height control motors 40 to 43.

[Effect]

With the above-described configuration, the present invention provides a current value (voltage f [)
is supplied to the astigmatism correction coil 5 (or the astigmatism correction coil 5 and the coil 20 of the objective lens 17), and a signal corresponding to the indicated value is supplied to the stage height control motors 40 to 43 to reduce the astigmatism. If necessary, the amount of deviation of the magnetic field detected by the magnetic field sensors 1 to 4 is further determined, and a correction signal generated based on this amount of deviation is supplied to the stage height control motors 40 to 43 to perform astigmatism reduction. ing. These reductions in astigmatism make it possible to always observe the sample under observation in the best condition.

[Embodiment] Next, the configuration and operation of an embodiment of the present invention will be explained in detail using FIGS. 1 to 8.

FIG. 1 shows a schematic sectional view of the scanning electron microscope of the present invention, and FIG. 2 shows a sectional view of the main structure of the present invention.

In FIGS. 1 and 2, magnetic field sensors 1 to 4 are sensors (for example, Hall elements, see FIG. 8) for measuring the strength of the magnetic field generated between the magnetic field 8i1B of the objective lens 17 and the flat magnetic pole 21. It is.

These magnetic field sensors 1 to 4 are, as shown in FIG.
Two are provided in each of the X and Y directions of the orthogonal coordinate system, and the magnetic field strength of the Z-axis (center axis of the lens) direction component is measured.

The astigmatism correction coil 5 is a coil that corrects astigmatism generated by the objective lens 17, and is, for example, an 8-pole air-core (having no magnetic material) coil with an N pole and an N pole (or an S pole). It is a coil with opposing poles facing each other, such as S and S poles (or N and N poles).

The magnetic field sensors 6 to 9 are connected to the magnetic field i outside the objective lens 17.
This is a sensor (for example, a Hall element) for measuring the strength of the magnetic field generated between the magnetic pole 19 and the flat magnetic pole 21. These magnetic field sensors 6 to 9 are provided two each in the X and Y directions of the orthogonal coordinate system, and measure the magnetic field strength of the component in the direction of the Z axis (center axis of the lens). This is for adjusting the degree.

The stage height control motors 40 to 43 are used to adjust the height of the movable stage 27'' mounted on the movable stage 27 (arbitrary adjustment in the x and y directions).

Next, the astigmatism reduction method will be explained in detail using FIG. Here, steps 1 through 2 are performed only once during initial adjustment, and steps 1 through 2 are performed each time during image observation.

In FIG. 3, ■ indicates that the wafer 22 is divided into nine areas. This divides the observable range of the wafer 22, which is the sample to be observed, into nine areas consisting of a 3×3 matrix, for example, using stage coordinates.

(2) The operator performs astigmatism correction at the center of each area.

In this process, an operator manually performs astigmatism correction at the center of each of the nine divided areas.

(2) Detects magnetic field information with a magnetic field sensor and stores it in memory. This is because when the operator manually performs astigmatism correction in each of the 9 areas divided into 2, the magnetic field strength measured by magnetic field sensors 1 to 4 is stored in a memory prepared for each area in advance (
memory area).

(2) indicates the coil 20, the astigmatism correction coil 5, the indicated values of the stage height control motors 40 to 43, and the stage coordinates (x, y
) in memory. As a result, for all divided areas, the current value flowing through the astigmatism correction coil 5 in the state of astigmatism correction, the magnetic field strength measured by the magnetic field sensors 1 to 4, the current value of the coil 20 (the current value of the objective lens 17) , and the command values of the stage height control motors 40 to 43 are stored in memory.

The above process completes the initial training.

Next, processing during image observation will be explained.

In FIG. 3, ■ moves the stage.

This moves the stage in order for the observer to observe the image at the desired position.

[Phase] determines the area from the stage coordinates. This corresponds to the movement of the stage in (2) and determines which area is divided from the coordinate values.

In (2), it is determined whether the area is the same as before the movement.

If YES, perform image observation work with @ and perform [phase]. On the other hand, if NO (if the area is different from before moving)
To reproduce the current value or voltage value stored in the corresponding area in [phase], supply it to the astigmatism correction coil 5, coil 2Q, and stage height control motors 40 to 43, and further reproduce the magnetic field. Magnetic field reproduction that supplies a correction signal (correction voltage) corresponding to the difference between the magnetic field measured by the sensors 1 to 4 and the stored magnetic field to the stage height control motors 40 to 43 when the difference is greater than a specified value. Sequence (4th
(described later using figures), and then perform [phase].

■ Performs image observation work.

[Phase] determines whether observation has ended or not. If YES, image observation ends. If NO, repeat the steps after [phase].

Through the above processing, the current values flowing through the astigmatism correction coil 5 and coil 20, and the voltage values applied to the stage height control motors 40 to 43 are automatically corrected to the previously saved state as the stage moves, Furthermore, when the difference detected by the magnetic field sensors 1 to 4 during reproduction is equal to or greater than the specified value, a correction voltage corresponding to the difference is applied to the stage height control motor 40.
43 to reduce astigmatism, it becomes possible to automatically reduce astigmatism as the sample to be observed moves.

An example of the magnetic field reproduction sequence of FIG. 3 [phase] will be explained in detail using FIG. 4.

In FIG. 4, [phase] calls the information of that area from the memory to the memory on the CPU. Here, we will use the magnetic field B3 of the corresponding area that has been saved in memory. .

(i=1.2.3.4) etc. are extracted to the memory on the CPU.

[Phase] controls and reproduces the coil 20, the astigmatism correction coil 5, and the stage height control motors 40 to 43. This means that the value of the initial state 1 @ (O) called in
43 to reproduce the initial state.

■Detects magnetic field information by magnetic field sensors 1 to 4,
Save in memory. This is the magnetic field B detected by the magnetic field sensors 4 during the current state of the stage.

Save (i-1, 2, 3, 4) in memory.

[Phase] compares the magnetic field information at the time of initial adjustment with the latest information and calculates ΔB,i. This is ΔB,,=B,,-B,. r (i=1.2.3.4
) is calculated.

[Phase] determines whether ΔB1, <ΔB, (standard +1). This is the difference ΔB2. between the magnetic field information B2° at the time of initial adjustment and the current magnetic field information Bxi. is smaller than the standard value ΔBS. If YES, the difference ΔB,
Since it is determined that □ is smaller than the standard value ΔB8, it is assumed that the astigmatism has been reduced, the process ends, and the process proceeds to FIG. 3 [Phase]. On the other hand, in the case of No, the difference ΔB1 is the standard value ΔB,
Since it is determined that the value is larger than , it is assumed that astigmatism correction has not been performed, and [phase] and 0 are performed.

[Phase] is ΔB, = Δ13mg−ΔB.

Calculate ΔBy=ΔE3g, -ΔB114. This is the magnetic field sensor 1 shown in Figure 2 (b).
The difference between the amount of deviation of the magnetic field in the X direction by the magnetic field sensor 2 is ΔB
Similarly, the difference in the amount of deviation of the magnetic fields in the Y direction between the magnetic field sensor 3 and the magnetic field sensor 4 is calculated as ΔB7.

(2) corrects the voltage of the stage height control motors 40 to 43 in the direction of reducing ΔB8 and ΔB (ΔB).

, ΔB, and the voltage supplied to the stage height control motors 40 to 43 are measured and determined in advance). [Phase] to [Phase] are repeated in this corrected state.

Through the above processing, in response to the movement of the stage, the state is restored to the state in which the astigmatism was corrected at the time of the initial adjustment, and furthermore, when the current magnetic field deviation amount ΔBX, ΔB, from the time of the initial adjustment is the specified value 65 or more, it is pre-measured. By supplying the correction voltage obtained in advance to the stage height control motors 40 to 43 to reduce astigmatism, it becomes possible to automatically reduce astigmatism in response to the movement of the stage.

Next, another astigmatism reduction method will be explained using FIG.

In FIG. 5, ■ divides the wafer into nine areas.

Similar to Figure 3 ■, this is the wafer 2 that is the sample to be observed.
The observable range of 2 is divided into 9 areas consisting of, for example, a 3×3 matrix using stage coordinates.

0, the operator performs astigmatism correction at the center of each area.

In this case, as in FIG. 3@, the operator manually performs astigmatism correction at the center of each of the nine divided areas.

■, [Phase] detects magnetic field information with a magnetic field sensor and stores it in memory. This saves the magnetic field strength measured by magnetic field sensors 1 to 4 in the memory (memory area) prepared for each area in advance when the operator manually performs astigmatism correction in each area divided into 9 areas. .

[Phase] is the magnetic field strength near the curve in the coordinate system of the stage. For example, as described on the right side, BgiI = Ifi (X% y) (1 = 1.2)
...(1) Bgj' r, (x, y) (j=3.4)
...-(2+ and the magnetic field intensities B, i, and B wJ are respectively approximated by the curves.

Here, the magnetic field strength B1 indicates the magnetic field strength indicated by the magnetic field sensors i and j, and both are functions of the (x, y) coordinates.

[Phase] stores the proximal expression in memory. This machine is [stock]
The equation (1+, (2)) approximated by the curve is saved in memory.

The above process completes the initial adjustment.

Next, processing during image observation will be explained.

In FIG. 5, ■ moves the stage.

This moves the stage in order for the observer to observe the image at the desired position.

[Co., Ltd.] calculates magnetic field information at that position from the stage coordinates and an approximation formula. This is the current stage coordinates (
magnetic field information at the current position is calculated from the proximal equations (1) and (2).

[Phase] controls the voltage of the stage height control motors 40 to 43 and reproduces the magnetic field at that position. This reproduces the magnetic field at the time of initial adjustment by controlling the stage height control motors 40 to 43 so that the magnetic field information calculated in [phase] is equal to the magnetic field information measured by magnetic field sensors 1 to 4 at the current position. do.

@ performs image observation work.

(2) determines whether the observation has ended or not. If YES,
Image observation ends. In the case of No, perform 0 or more kidneys.

Through the above processing, the voltages of the stage height control motors 40 to 43 are controlled so as to reproduce the state of the magnetic field information calculated based on the approximation formula obtained and stored in advance as the stage moves. Astigmatism can be reduced automatically as the observation sample moves.

In the above embodiments, a method has been described in which astigmatism is reduced by storing magnetic field information after astigmatism correction in advance and reproducing the stored magnetic field state as much as possible. This is because normally, astigmatism cannot be completely corrected simply by forming a uniform symmetrical magnetic field in the axial direction. However, if astigmatism can be reduced practically enough simply by forming a uniform axially symmetrical magnetic field due to improvements in processing and assembly accuracy, then the astigmatism reduction method shown in Figure 6 is extremely practical and effective. It becomes a useful method. This will be explained below using FIG. 6.

FIG. 6 shows a flowchart of the stigma reduction operation of the present invention.

In FIG. 6, () is used to adjust the parallelism using adjustment screws 26.26° during stage assembly adjustment.

This is adjusted by adjusting screws 26, 26'' in FIG. 1 so that the flat magnetic pole 21 mounted on the holding base 23 and the magnetic poles 18, 19 are parallel to each other.

By making the above adjustments when assembling the stage, magnetic pole 1
8 and the flat magnetic pole 21 are parallel to each other, and astigmatism caused by the spatial positional relationship of the magnetic field is minimized.

Next, 0 measures the magnetic field strength using a magnetic field sensor during actual observation. This is because when the stage is moved to position the sample to be observed at a desired position, the magnetic field B□ (i=1.2
．． 3.4) Measure.

Q is the axially symmetric component B. Calculate the amount of deviation from

Here, the axis-symmetric component B1° is the amount of deviation ΔB, 1 (i=1.2.3.4) is ΔB, l=
B,, -Byo. (i=1, 2.3.4)+41.

[Phase] is B2° and ΔB, = (i = 1.2.3.4>
Determine whether or not it is within the specifications. If YES,
Finish automatic stigma reduction work. If NO, perform a correction operation (correction by stage control motor, etc.),
■Do the following.

With the above processing, the second
Figure (b) Controlling the stage control motor so that the amount of deviation from the axially symmetrical component measured by the magnetic field sensors 1 to 4 is within the standard, and forming a uniform and axially symmetrical axial magnetic field B1 with high precision. This makes it possible to automatically reduce astigmatism. Forming a uniform magnetic field that is uniform and axially symmetrical means nothing but improving the parallelism between the magnetic poles 18 and 19 and the second flat magnetic pole 21 macroscopically.

Therefore, by placing the following operation shown in FIG. 7 between 0 and 0 in FIG. 6, it is possible to quickly perform the astigmatism reduction operation when there is a large deviation in parallelism.

FIG. 7 shows an explanatory diagram of correction of magnetic pole parallelism according to the present invention.

Part 7 (a) shows the state before parallel correction, and the flat magnetic pole 2
1 is tilted downward to the right as shown in the figure. In this state, B8゜'=B□=B21, B'16>
B f? becomes. Here, the subscript 6.7 represents two magnetic fields of the magnetic pole 19 in the X direction, and the subscript 8.9 represents two magnetic fields of the magnetic pole 19 in the Y direction.

Figure 7 (b) shows the state after parallel correction, and the flat magnetic pole 2
1 are parallel as shown. In this state, B. '=B,,=B,,=Bm@=BzQ
becomes. Here, .

In addition, in the astigmatism reduction method described above, in order to simplify the explanation, a case has been described in which the area is divided into nine areas and four magnetic field sensors are used, but the present invention is limited to this. Alternatively, the area may be divided into arbitrary plurality of areas and a larger number of magnetic field sensors may be used.

FIG. 8 shows an example of a magnetic field sensor according to the present invention.

FIG. 8(a) shows a state in which a magnetic field sensor 32 is provided at the M1 pole 18 of the objective lens.

FIG. 8(b) shows an enlarged view of the magnetic field sensor 32. This magnetic field sensor 32 has a plurality of Hall elements (for example, 256) arranged in a circular shape on a semiconductor substrate, and these Hall elements are arranged one by one or in groups of a predetermined number (for example, in the case of FIG. 2(B)). (summarized into four parts). In this way, since the Hall element is created on a semiconductor substrate using LSI technology, the magnetic field sensor 32 has uniform magnetic field detection characteristics.
can be easily created. This created magnetic field sensor 32 is
For example, the magnetic field in the axial direction between the magnetic pole 18 and the flat magnetic pole 21 (
It is also possible to easily create a device that measures the magnetic field (longitudinal magnetic field) or the magnetic field perpendicular to the axial direction.

〔Effect of the invention〕

As explained above, according to the present invention, one of the magnetic poles constituting the objective lens is a movable flat plate magnetic pole, and the current flowing through the astigmatism correction coil 5 and the coil 20 after performing stigma correction in advance. The voltage applied to the stage height control motors 40 to 43 is stored, and as the stage moves, the stored current is passed through the stigma correction coil 5 (or the stigma correction coil 5 and the coil 20). By applying the stored voltage to the stage height control motors 40 to 43 and reproducing the initial state magnetic field with astigmatism correction (adjusted by the operator), astigmatism correction and further magnetic field adjustment can be performed as necessary. The system employs a configuration that performs reproduction magnetic field correction based on the amount of magnetic field deviation detected by the sensor, minimizing distortion due to astigmatism and allowing stable observation with the best image quality that a scanning electron microscope has. be able to. Therefore, the practical effects of the present invention are very large.

[Brief explanation of drawings]

FIG. 1 shows a conceptual cross-sectional view of a scanning electron microscope according to the present invention. FIG. 2 shows a sectional view of the main components of the present invention. FIG. 3 is a flowchart of the astigmatism reduction method according to the present invention (
The l) is shown below. FIG. 4 shows an example of a magnetic field reproduction sequence according to the present invention. FIG. 5 is a flowchart of the astigmatism reduction method according to the present invention (
Part 2) is shown. FIG. 6 shows a flowchart of the main operations for reducing astigmatism according to the present invention. FIG. 7 shows an explanation of correction of magnetic pole parallelism according to the present invention. FIG. 8 shows an example of a magnetic field sensor according to the present invention. FIG. 9 and FIG. 1O show explanatory diagrams of the prior art. In the figure, 1 to 4 are magnetic field sensors that measure the magnetic field of the magnetic pole 18, 5 is an astigmatism correction coil, 6 to 9 are magnetic field sensors that measure the magnetic field of the magnetic pole 19, 17 is an objective lens, and 18.19
is a magnetic pole, 20 is a coil, 21 is a flat magnetic pole, 22 is a wafer, 23 is a holding table, 24 is a secondary electron detector, 26.26°
is the adjustment screw, 27.27°, 28 is the moving table, 31.31
° represents a fixture, and 40 to 43 represent stage height control motors.

Claims (1)

[Scope of Claims] (1) A scanning electron microscope that irradiates a sample to be observed with an electron beam focused by an objective lens and observes its image, which has an axially symmetrical hole through which the electron beam passes through the center; and an objective lens having a first magnetic pole having a conical shape and a flat tip, and a second flat magnetic pole facing the first magnetic pole and movable in a plane perpendicular to the axis; It is equipped with an astigmatism correction coil that corrects astigmatism that occurs and a stage height control mechanism that controls the height and inclination of the moving table, and the sample to be observed is placed on the second flat magnetic pole attached to the moving table. The above-mentioned astigmatism correction coil (5) is equipped with the above-mentioned astigmatism correction coil (5) when carrying out astigmatism correction in advance for arbitrary plural points (for example, coordinate values (x, y)) of the observed sample by moving the movable table; The current value (voltage value) flowing through the objective lens coil and the indicated value of the stage height control mechanism are memorized, respectively, and the memorized current value (voltage value) is used when moving and observing the sample to be observed. is supplied to the astigmatism correction coil (or the astigmatism correction coil and the objective lens coil), and a signal corresponding to the stored instruction value is supplied to the stage height control mechanism. scanning electron microscope. (2) providing a magnetic field sensor near the axis of the objective lens to detect the asymmetry of the magnetic field of the objective lens;
The sample to be observed is mounted on the second flat magnetic pole attached to the moving table, and the moving table is moved to preliminarily measure arbitrary points (for example, coordinate values (x, y)) on the sample to be observed. When performing astigmatism correction, the current value flowing through the astigmatism correction coil (5), the coil of the objective lens, the instruction value of the stage height control mechanism, and the magnetic field of the objective lens detected by the magnetic field sensor are stored. Then, when moving and observing the sample to be observed, supply the memorized current value to the astigmatism correction coil (5) (or the astigmatism correction coil (5) and the objective lens coil), and When a signal corresponding to the stored instruction value is supplied to the stage height control mechanism, the difference between the magnetic field detected by the magnetic field sensor and the stored magnetic field is equal to or greater than a predetermined value. 2. The scanning electron microscope according to claim 1, wherein the scanning electron microscope is configured to supply a correction signal corresponding to the stage height control mechanism to the stage height control mechanism. (3) As the above arbitrary points, the observable range of the sample to be observed is divided into a plurality of meshes, and the astigmatism correction coil (5) and the objective lens are The value of the current flowing through the coil, the indicated value of the stage height control mechanism, and if necessary, the magnetic field of the objective lens detected by the magnetic field sensor are stored, and when the position is within this mesh during observation, the stored value (current value,
The present invention is characterized by being configured to use values (current value, voltage value, magnetic field) calculated for the current position based on an approximate curve determined from values stored in advance at the time of observation. A scanning electron microscope according to claims (1) and (2).