JPH0817384A - Method for aligning scanning electron microscope - Google Patents

Abstract

PURPOSE:To simplify adjustment and to increase their accuracy by automating the alignment of a scanning electron microscope. CONSTITUTION:A current through an objective lens 7 is finely varied from a value of a positive focal point to positive or negative side, and in either case line scan is performed with the direction of scanning varied radially, and data about detection during the scan is stored, and the direction and amount of movement of an image resulting from misalignment are calculated by comparison of two sets of data in the same direction of scanning, and an alignment device is controlled so that the amount of movement becomes zero. The alignment device may be an electromagnetic deflector 5 for alignment or an objective diaphragm that is mechanically moved.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for automatically adjusting the lens axis of a scanning electron microscope.

[0002]

2. Description of the Related Art In a scanning electron microscope, current center adjustment (hereinafter referred to as alignment) for adjusting an electron beam so as to pass through an axis of an objective lens is necessary in order to reduce aberrations caused by the lens. If the adjustment is not enough, the maximum performance of the scanning electron microscope cannot be realized.

Conventionally, in this alignment, the movement of the magnified image is observed while the objective lens current is finely changed in the vicinity of the positive focus, and the position of the objective aperture is adjusted to minimize the movement of the image. It was done by.

[0004]

The above-mentioned conventional method has a problem that automation is difficult because it requires a skill for adjustment work and includes a process in which a person judges movement of an image. An object of the present invention is to simplify adjustment work and improve accuracy by automating alignment.

[0005]

In order to achieve the above-mentioned problems, in the present invention, the alignment means is constructed so as to be controllable by an electric signal, and at the same time, the exciting condition of the objective lens is changed so that it can be moved in various directions. The electron beam is one-dimensionally scanned, and the detection signal obtained at this time is stored and compared to detect the movement of the image, and the adjusting means is controlled based on the detected amount.

That is, the alignment method of the present invention is
Under different excitation conditions where the exciting current of the objective lens of the scanning electron microscope is changed by a small amount between positive and negative with respect to the exciting current at positive focus, the sample is radially changed by changing the scanning direction of the electron beam at a predetermined angular interval. One-dimensional scanning is performed, the detection signal waveforms emitted from the sample during each one-dimensional scanning are stored, two detection signal waveforms obtained under the different excitation conditions obtained in the same scanning direction are compared, and two detection signals are detected. When the waveforms are relatively shifted, the one-dimensional scanning direction in which the detection signal waveform having the best overlapping portion is obtained is set as the image movement direction due to the misalignment, and the two detection signal waveforms are best overlapped. The amount of movement of the image is determined from the required relative shift amount of both detection signal waveforms, and the image movement becomes zero based on the data of the moving direction and the amount of movement of the image thus determined. It is characterized in that for controlling the alignment device so.

Completion of the alignment is confirmed by controlling the alignment device as described above, and then the difference between the two detection signal waveforms in the respective scanning directions under the different excitation conditions becomes the minimum in the vicinity of the shift amount 0, respectively. can do.

[0008]

In the present invention, the two-dimensional image information is converted into a plurality of one-dimensional information by changing the scanning direction radially, and the process of determining the movement of the image is reduced to the simple operation of comparing the one-dimensional information. Thus, the moving direction and the moving amount of the image are detected by a simple calculation. If the moving direction and moving amount of the image are obtained, accurate alignment can be realized from the known relationship between the adjustment amount by the alignment device and the moving amount of the image. This method is suitable for automation, and automation of alignment can be easily realized by existing means.

Further, the alignment method according to the present invention is
Since the completion of the adjustment can be confirmed accurately, it is easy to check the unfinished state of the adjustment due to a malfunction or the like, and highly accurate adjustment can be realized.

[0010]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram showing the overall configuration of a scanning electron microscope. The electron beam 2 generated by the electron gun 1 is converged by the condenser lens 3, the aperture angle is limited by the objective diaphragm 4, and the axis is adjusted by the alignment deflector 5, and the sample is obtained by the scanning deflectors 6, 6 ′. The sample 8 in the chamber is scanned. The objective lens 7 focuses the electron beam 2 on the sample 8. Secondary electrons generated from the sample 8 are detected by the detector 9. High-voltage power supply 1 for applying voltage to the electron gun 1.
0, the exciting power source 11 of the condenser lens 3, the exciting power source of the alignment deflector 5, the exciting power source 13 of the scanning deflectors 6 and 6 ', and the exciting power source 14 of the objective lens 7 are controlled by the CPU 16 via the interface 15, respectively. It The output signal of the secondary electron detector 9 is taken in through the interface 15 and subjected to data processing, and then a scan image is displayed on the display device 17 such as a CRT.

The objective diaphragm 4 can be arranged below the deflectors 6 and 6'or below the objective lens 7 as shown by the broken line in FIG. The alignment deflector 5 may be a one-stage deflector or a two-stage deflector. FIG. 2 is an external view of a scanning electron microscope. An operation knob is provided on the operation unit 22 of the control device 21 as shown in a circle, and when the "ALIGNMENT" switch is pressed, the alignment device 23 is operated to perform automatic alignment. Manual alignment using the X and Y knobs is also possible. The operation will be described below.

FIG. 3 shows an example of an algorithm (flow chart) for the automatic alignment operation. However, it is assumed that the focus is first. By detecting the image signal by shifting the focus by ± Δ from that point, the contrast becomes approximately the same, the image signals can be easily compared, and the image shift amount can be reliably detected. First, the exciting current I of the objective lens 7 is increased by a slight amount Δ from the value I _{0 of the} positive focus to set I = I ₀ + Δ (step 1).

Then, a saw-tooth current is passed through the X-direction scanning deflector to perform one-dimensional scanning of a length of 2N within a range of ± N, and detection signals at (2N + 1) points for each scanning length 1 are stored in a memory as data. Record. Further, the current X _{i of} the X, Y scanning deflector
And Y _i are respectively set as follows, and scanning of a length 2N is sequentially performed with respect to a plurality of scanning directions θ _j (j = 1, 2, ...), and (2N + 1) data acquisition is performed for each scanning. To do. X _i = (− N + i) cos θ _j Y _i = (− N + i) sin θ _j i = 0,1,2,
…, 2N

The scanning direction (angle) θ _j is 0 ° ≦ θ _j <1
The angle is 80 °, and as shown in FIG. 4, the range of 180 ° is divided into a plurality of portions to perform radial scanning, and detection signals are collected as data (steps 2 to 4). Each scanning direction θ _j (j
= 0,1,2, ...), i = 0,1,2,
, 2N is recorded in the memory as a ₀ , a ₁ , ..., A _{2N in the} format as shown in FIG.

Here, for simplification, the scanning length is 2N and the current change width of the X-direction scanning deflector is 2Ncosθ.
_j, is described the current change width in the Y-direction scanning deflector as 2Nsinshita _j, actually running length has a value of about 500～10Myuemu, taking current change width of the scanning deflector also accordingly value.

Next, the exciting current of the objective lens 7 is I = I ₀
Change to −Δ (step 5). Then, the currents X _i and Y _i of the scanning deflector are set as follows, respectively, and the same plural scanning directions θ _j (j = _j) as set in step 2 are set.
0, 1, 2, ...) are scanned over a length of 4N within a range of ± 2N as shown in FIG. Detected data is collected for each scanning direction θ _j (j = 0, 1, 2, ...) By (4N + 1) for each scanning length 1, and in the format as shown in FIG. 7, b ₀ , b ₁ , , _B4N is recorded in the memory (steps 6 to 8). X _i = (− 2N + i) cos θ _j Y _i = (− 2N + i) sin θ _j i = 0,1,2,
…, 4N

At the time of the first processing, step 9 is skipped and step 10 for obtaining the moving direction and moving amount of the image is skipped.
Proceed to. In the same scanning direction θ _j , (2N + 1) pieces of data when the exciting current of the objective lens is (I ₀ + Δ)
and _k, the excitation current of the objective lens and (I ₀ -Δ) consecutive (4N + 1) among the pieces of data when the (2N + 1) pieces of data b _l. Here, k and l take the following values, and indicate the addresses of the memories that store the data a _k and the data b _l . k = 0, 1, 2, ..., 2N l (m, k) = m + k

In the above equation, m is the initial address of b _l and has the following values. m = 0,1, ..., 2N Therefore, l (0, k) = 0,1, ..., 2N l (1, k) = 1,2 ,. : L (2N, k) = 2N, 2N + 1, ..., 4N, and the value A _m of the following equation is calculated for two sets of detection data of a _k and b _l , and the minimum result is obtained. Position m
Ask for.

[0019]

[Equation 1]

The above calculation is performed on the detected and recorded data in all scanning directions, and the scanning direction θ _j when the calculation result A _m becomes the minimum is obtained. As can be seen from FIGS. 6 and 7,
Similar signal waveform diagram when scanning in the moving direction of the image (a)
And data showing (b) are obtained. And FIG.
When the center of the waveform of FIG. 7A is moved with respect to the center of the waveform of FIG. 7B, the amount of movement when the two waveforms exactly overlap is the amount of movement of the image. That is, the scanning direction θ of the data that minimizes A _m is obtained as the image movement direction, and the relative address p = 1 (m, 0)-(N + 1) is obtained as the image movement amount (step 10).

Since the moving direction and the moving amount of the image are obtained as described above, the alignment deflector 5 which is determined in advance by experiments or the like and stored in the control device as a table.
Using the relationship between the drive current and the movement amount of the image, the current value given to the alignment deflector 5 is determined so as to cancel the movement of the image, and the setting is controlled (step 11).

At this time, if the alignment control is impossible, an abnormal end operation is performed to end the control (step 1
2, 13), when the alignment control is normally performed, step 1 to step 8 are repeated in that state to collect data again, and the shift amount B in the following formula is calculated based on the collected data. , The deviation amount B is a preset value γ
If it is within the range, it is determined that the adjustment can be normally performed (step 9), and the process is ended. B = Σ [( _ak− bl _{(m, k)} ) ² ] ≦ γ

If the shift amount B is larger than γ, the process proceeds to step 10 to recalculate the moving direction and the moving amount of the image by calculating A _m as described above, and realign the alignment in step 11. Then, steps 1 to 9 are repeated to confirm the result. As shown by the solid line in FIG.
When the one-stage deflector 5 is used as the alignment deflector, the electron beam 2 that has passed through the objective diaphragm 4 can be passed through the center of the objective lens. On the other hand, when the two-stage deflectors 5 and 5'are used as the alignment deflector, it is possible to perform alignment in which the axis of the objective lens and the electron beam are completely coincident with each other, as indicated by the broken line. The present invention can be applied to any of these cases.

Further, although the scanning deflector and the alignment deflector are separately arranged in the above description, the alignment deflector may be integrated with the scanning deflector or the same (current addition). Although the case where the objective diaphragm 4 is arranged above the deflector and the electromagnetic deflector is used as the alignment device has been described above, when the objective diaphragm is arranged at the center of the objective lens, the position of the objective diaphragm is set to the lens. Alignment that is set correctly at the center is required, and the mechanical movement mechanism of the objective diaphragm serves as an alignment device that replaces the above-mentioned deflector. That is, the present invention, regardless of whether it is a mechanical type or an electromagnetic type, two-dimensionally moves either the incident direction of the electron beam with respect to the objective lens, the incident position, or both to move the electron beam into the objective lens. It is applicable to any type of alignment device that matches the axis.

When the above-mentioned data acquisition is performed in a state where the alignment is completed and the image does not move, the comparison data in each scanning direction becomes minimum at the shift amount p = 0. Therefore, this can be used to confirm the end of alignment. Scan direction change step, and
The finer the number of detected data (2N) in each scan is, the more the accuracy is improved. However, if the number is too small, it takes a long time to collect and calculate the data, and the memory also needs a large capacity. Practically, the scanning direction step is 3 ° to 3 °.
It is sufficient to set 0 ° and the number of data divisions in the range of N = 16 to 256. It is desirable to change the change amount (Δ) of the objective current and the actual scanning amplitude according to the observation magnification.

[0026]

As described above, according to the present invention, the adjustment of the current center of the objective lens, which conventionally requires manual operation, can be automated with a simple structure, so that the operation is simple and the precision is high. Can be adjusted. Further, since the completion of the adjustment can be confirmed accurately, the SEM performance can be maintained at a high level, which is very effective in practical use.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the overall configuration of a scanning electron microscope.

FIG. 2 is an external view of a scanning electron microscope.

FIG. 3 is a flowchart showing an example of an alignment processing procedure.

FIG. 4 is an explanatory diagram showing electron beam scanning (2N length) during alignment.

FIG. 5 is an explanatory diagram showing electron beam scanning (4N length) during alignment.

FIG. 6 is a format of 2N long scan data.

FIG. 7 is a format of 4N long scan data.

FIG. 8 is an explanatory diagram showing movement of a scanning image.

FIG. 9 is an explanatory diagram showing an example of a detection signal.

FIG. 10 is a view for explaining the operation of the alignment deflector.

[Explanation of symbols]

1 ... Electron gun, 2 ... Electron beam, 3 ... Condenser lens,
4 ... Objective diaphragm, 5 ... Alignment deflector, 6, 6 '...
Scan deflector, 7 ... Objective lens, 8 ... Sample, 9 ... Secondary electron detector, 10 ... High voltage power supply, 11-14 ... Excitation power supply, 15
... interface, 16 ... CPU, 17 ... display device

Claims (4)

[Claims] 1. A sample is obtained by changing the scanning direction of an electron beam at a predetermined angular interval under different exciting conditions in which the exciting current of the objective lens is changed by a small amount between positive and negative with respect to the exciting current at a positive focus. Radial one-dimensional scanning is performed, the detection signal waveforms emitted from the sample at each one-dimensional scanning are stored, two detection signal waveforms obtained in the same scanning direction under the different excitation conditions are compared, and the two When the detection signal waveforms are relatively shifted, the one-dimensional scanning direction in which the detection signal waveform having the best overlapping portion is obtained is the moving direction of the image due to the misalignment, and the two detection signal waveforms are best overlapped. The amount of movement of the image is determined from the relative shift amount of both detection signal waveforms required for A method for aligning a scanning electron microscope, which comprises controlling the position. 2. After the alignment device is controlled, the difference between the two detection signal waveforms in the respective scanning directions under the different excitation conditions is minimized in the vicinity of the shift amount 0, thereby confirming that the alignment is completed. The alignment method for a scanning electron microscope according to claim 1, wherein: 3. The alignment method for a scanning electron microscope according to claim 1, wherein the alignment device mechanically moves the objective lens diaphragm in a plane perpendicular to the electron beam axis. 4. The alignment method for a scanning electron microscope according to claim 1, wherein the alignment device is an electron beam deflector.