JP3678910B2 - Scanning electron microscope - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a scanning electron microscope in which the movement of an observation field is eliminated by tilting a sample when the sample is tilted.
[0002]
[Prior art]
In a scanning electron microscope, an electron beam is scanned two-dimensionally on a sample, secondary electrons and reflected electrons generated from the sample are detected, and a detection signal is supplied to a cathode ray tube to obtain a scanned image of the sample. Yes.
[0003]
In such a scanning electron microscope, the sample is placed on a stage that can be moved horizontally, rotated, moved in the Z direction (moving the working distance), and moved in an inclined manner. By performing these movements, visual field movement, rotation observation, tilt observation, and the like are performed.
[0004]
By the way, when the tilt stage is driven to perform the tilt observation of the sample, it is necessary to maintain the condition that the center of the observation screen is located on the tilt axis, that is, the eucentric condition. If the sample deviates from the eucentric condition, the field of view to be observed moves with an inclination.
[0005]
FIG. 1 shows such a state. FIG. 1A shows a state where the sample s satisfies the eucentric condition, and the observation center of the sample 1 in the horizontal state (the sample on the optical axis O). Position) The point A and the tilt axis P coincide with each other. In this case, even if the sample in the horizontal state is tilted so as to be in the dotted line state, the sample position A on the optical axis O does not change and the visual field does not move with the tilt.
[0006]
FIG. 1B shows a state where the sample s does not satisfy Eucentric conditions, that is, the center of observation (sample position on the optical axis O) A of the sample s in the horizontal state with the height of the sample s shifted. And the tilt axis P is shifted in the Z direction. In this case, if the sample in the horizontal state is tilted to be in the dotted line state, the observation center A point in the horizontal state is shifted from the optical axis O to A ′ point, and as a result, the observation field of view changes.
[0007]
[Problems to be solved by the invention]
Considering the above, normally, when mounting the sample on the stage so that the center of the observation field is at the eucentric position, alignment is done visually so that the height of the sample is at the eucentric position. Do. However, the alignment of the sample in the Z direction by visual inspection is limited in accuracy and is troublesome.
[0008]
The present invention has been made in view of these points, and an object thereof is to realize a scanning electron microscope capable of automatically aligning the height position of a sample so as to satisfy Eucentric conditions. It is in.
[0009]
[Means for Solving the Problems]
In the scanning electron microscope according to the present invention, a sample is placed on an inclined stage on a Z-direction moving stage, an electron beam finely focused on the sample is two-dimensionally scanned, and the sample is scanned with the electron beam. In a scanning electron microscope configured to display an image based on the obtained signal, a measuring means for measuring the Z-direction position of the sample by detecting the Z-direction position of the Z-direction moving stage, and autofocus of the electron beam The difference between the Z direction position of the sample measured by the measuring means and the Z direction position of the sample obtained based on the autofocus operation is obtained, and the Z direction of the sample is determined according to the difference. It is characterized in that the position is adjusted.
[0010]
In the present invention, the position in the Z direction (height direction) of the sample is measured by detecting the Z direction position of the Z direction moving stage, and the Z direction position of the sample is calculated based on the autofocus operation. The position in the height direction of the sample is corrected according to the difference between the two kinds of Z-direction positions.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 2 shows a scanning electron microscope to which the present invention is applied. In FIG. 1, reference numeral 1 denotes an electron gun. An electron beam EB generated from the electron gun 1 and accelerated is finely focused on the sample 4 by the condenser lens 2 and the objective lens 3. Although the auxiliary coil 5 is disposed close to the objective lens 3, the objective lens 3 is driven by the drive circuit 6, and the auxiliary coil 5 is driven by the drive circuit 7.
[0012]
Further, the electron beam EB is scanned in the X and Y directions by the deflection coil 8. A scanning signal is supplied to the deflection coil 8 from the scanning circuit 9, and the scanning circuit 9 is controlled by a scanning control circuit 10.
[0013]
Secondary electrons generated from the sample by irradiation of the sample 4 with the electron beam EB are detected by the secondary electron detector 11. The detection signal of the secondary electron detector 11 is supplied to a cathode ray tube (not shown) and also supplied to the memory 13 via the AD converter 12 and stored therein. The signals stored in the memory 13 are integrated in the integration circuit 14 for each signal for one screen.
[0014]
The sample 4 is placed on an inclined stage 16 provided on the Z-direction moving stage 15. The Z direction moving stage 15 is driven by the drive circuit 17 to move the sample 4 in the Z direction. The tilt stage 16 is driven by the drive circuit 18 to tilt the sample 4. The Z direction position of the sample is measured by the Z position detecting means 19.
[0015]
The scanning control circuit 10, the objective lens driving circuit 6, the auxiliary coil driving circuit 7, the Z-direction moving stage driving circuit 17, and the tilt stage driving circuit 18 are controlled by a computer 20. Further, the integrated value from the integrating circuit 14 is supplied to the computer 20.
[0016]
The computer 20 performs an autofocus operation, which will be described later, and a signal at the Z-direction position of the sample obtained based on the autofocus operation is supplied to the comparison circuit 21. The Z position signal measured by the Z direction position detection means 19 is also supplied to the comparison circuit 21. The computer 20 is provided with a memory 22 in which the amount of misalignment in the Z direction obtained by the comparison circuit 21 is stored. The operation of such a configuration will be described next.
[0017]
First, an ordinary scanning electron microscope image is observed by focusing the electron beam EB generated from the electron gun 1 and accelerated on the sample 4 by the condenser lens 2 and the objective lens 4. Further, by supplying a scanning signal from the scanning circuit 9 to the deflection coil 8 in accordance with the scanning speed and magnification set from the scanning control circuit 10, the electron beam is scanned two-dimensionally in a predetermined region on the sample 4.
[0018]
Secondary electrons generated by irradiation of the sample 4 with the electron beam EB are detected by the secondary electron detector 11. Since the detection signal of the detector 11 is supplied to a cathode ray tube (not shown), a scanning image of the sample is displayed on the cathode ray tube.
[0019]
Next, a position correction operation for satisfying the Eucentric condition regarding the position of the sample will be described. First, after placing the sample 4 on the stage, measuring the Z position of the sample by the Z-position detecting means 19. The Z position signal of the measured sample is supplied to the computer 20 and the comparison circuit 21.
[0020]
The computer 20 controls the objective lens driving circuit 6 based on the supplied Z position signal, and drives the objective lens 3 with the excitation intensity corresponding to this Z position. In this state, the computer 20 executes an autofocus operation. As shown in FIG. 3, the computer 20 controls the scanning control circuit 10 so that the autofocus operation is performed on the image D in a range S along the tilt axis P.
[0021]
Note that before performing this autofocus operation, always perform hysteresis removal of the objective lens 3, is the accuracy of the focus height position requires high Kusuru. In the autofocus, the auxiliary coil drive circuit 7 is controlled by the computer 20, and an excitation current on the step is supplied to the auxiliary coil 5, so that the focus of the electron beam on the sample is shifted for each step. The area S on the sample is scanned once by the electron beam for each step.
[0022]
Based on the scanning of the electron beam for each step, the secondary electron detection signal detected by the detector 11 is converted into a digital signal by the AD converter 12 and then sent to the memory 13 for storage. The detection signals for one screen stored in the memory 13 are integrated by the integration circuit 14. The integration signal of the integration circuit 14 is supplied to the computer 20 and stored in correspondence with the exciting current of the auxiliary coil 5 at that time.
[0023]
When such scanning of the electron beam in the range S on the sample and integration processing and storage of the secondary electron detection signal are performed for each stepwise excitation current change to the auxiliary coil 5, the computer 20 includes the auxiliary coil. 5 is obtained for each excitation current. The computer 20 compares the respective integration results, acquires the excitation current at which the maximum peak value of the integration value is obtained, and supplies this excitation current to the auxiliary coil 5 or corrects the excitation current of the objective lens 5. End autofocusing.
[0024]
With this autofocus operation, the computer 20 calculates the Z direction position of the sample 4 from the maximum peak value of the integrated value. The calculated Z-direction position is supplied to the comparison circuit 21. In the comparison circuit 21, a difference ΔZ between the Z direction position of the sample obtained by the autofocus operation supplied from the computer 20 and the Z direction position measured by the Z position detecting means 19 is obtained.
[0025]
The value of the difference ΔZ is supplied to the computer 20, and the computer 20 controls the Z direction stage drive circuit 17 based on this value and moves the Z direction moving stage 15 to correct the height of the sample 4. By this operation, the sample 4 satisfies the eucentric conditions, and even when the sample is tilted, the field of view does not escape.
[0026]
Note that the value of the difference ΔZ obtained by the computer 20 is sent to the memory 22 and stored therein. The ΔZ stored in the memory 22 is used as a default value for the movement error of the sample in the Z direction when the stage is subsequently moved.
[0027]
Although one embodiment of the present invention has been described above, the present invention is not limited to this embodiment. For example, secondary electrons are detected, but reflected electrons may be detected.
[0028]
【The invention's effect】
In the present invention, the position in the Z direction (height direction) of the sample is measured by detecting the Z direction position of the Z direction moving stage, and the Z direction position of the sample is detected based on the autofocus operation. Since the position in the height direction of the sample is corrected according to the difference between the two Z-direction positions, the Eucentric position can be automatically detected, and the height position of the sample can be automatically detected with high accuracy. Can be set to the eucentric position. As a result, the allowable range of error when setting the sample on the sample stage is widened. Further, it is possible to automatically realize a eucentric operation with high accuracy when the 5-axis stage for moving, rotating, and tilting the sample is controlled by a motor.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining Eucentric conditions;
FIG. 2 is a diagram showing an example of a scanning electron microscope according to the present invention.
FIG. 3 is a diagram for explaining a scanning range of a specimen by an electron beam during autofocus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Electron gun 2 Condenser lens 3 Objective lens 4 Sample 5 Auxiliary coils 6, 7, 17, 18 Drive circuit 8 Deflection coil 9 Scan circuit 10 Scan control circuit 11 Secondary electron detector 12 AD converter 13 Memory 14 Integration circuit 15 Z Direction moving stage 16 Inclined stage 19 Z position detecting means 20 Computer 21 Comparison circuit 22 Memory

Claims (1)

A sample is placed on an inclined stage on a Z-direction moving stage, an electron beam finely focused on the sample is scanned two-dimensionally, and an image is obtained based on a signal obtained by scanning the sample with the electron beam. In the scanning electron microscope, the measuring means is provided with measuring means for measuring the Z-direction position of the sample by detecting the Z-direction position of the Z-direction moving stage, and means for performing autofocus of the electron beam. Scanning in which the difference between the Z direction position of the sample measured by the means and the Z direction position of the sample obtained based on the autofocus operation is obtained, and the Z direction position of the sample is adjusted according to the difference electronic microscope.

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