JPH0765771A - Focusing position detecting device - Google Patents

Abstract

PURPOSE:To reduce the focusing detecting time by controlling the sampling cycle of the signal to indicate the focusing regulating condition shorter/longer according to the inside/the outer side of the scope of a specific neighboring area of the scheduled focusing position stored in the memory. CONSTITUTION:The variable scope of the image forming position of an object lens 4 is divided into the A zone inside the scope of a specific neighboring area of the scheduled focusing position, and the B zone other than the A zone. And the C zone is set in the A zone as a focusing position allowable scope. The displacement amount of the object lens 4 to be the sampling cycle is set shorter in the A zone and longer in the B zone. The CPU 11 increases the image forming position of the lens 4 gradually from 0mum to a specific long displacement amount in the B zone, evaluates the focusing condition, and stores the result in a memory 14. In the A zone, the same process is carried out in a specific shorter displacement amount. When the maximum evaluation value is in the C zone after a specific scope is sampled, the position is made as the focusing position, and when it is not in the C zone, the focusing position detecting operation is carried out again.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an in-focus position detecting device, and more particularly to an in-focus position detecting device suitable for an optical microscope, an electron microscope or their applied devices.

[0002]

2. Description of the Related Art A conventional focus position detecting device will be described by taking a case of observing a pattern of a wafer with a scanning electron microscope as an example. The focusing position detection device of the scanning electron microscope changes the image forming position of the objective lens in the optical axis direction by adjusting the exciting current of the objective lens, and acquires a signal indicating the focus adjustment state at a plurality of image forming positions, The focus position of the objective lens is determined based on this signal.

Even if the wafers have the same size, there is a maximum difference in thickness of about 100 μm due to dimensional tolerances, warpage, and various processes. Therefore, the wafer is placed on the stage, and the imaging position of the objective lens is set to 100 at maximum.
The focus position can be detected by changing it by about μm. Here, when observing the pattern of the wafer, the observation magnification is tens of thousands times, and therefore the depth of focus of the scanning electron microscope is about 2 μm. Therefore, the change width of the image forming position of the objective lens is set to 2 μm, and the image forming position is changed by 100 μm while acquiring the signal indicating the focus adjustment state at each image forming position. Then, the focus position is detected based on the signal indicating the focus adjustment state at each image forming position.

[0004]

However, in the conventional in-focus position detecting device, in order to detect the in-focus position with high accuracy, it is necessary to acquire many signals representing the focus adjustment states. That is, in the above example, the change amount of the image formation position is 100
Since it is .mu.m and the change width is 2 .mu.m, it is necessary to acquire the signal indicating the focus adjustment state 51 times. Further, in order to detect the in-focus position with higher accuracy, it is necessary to further reduce the change width of the image formation position and acquire more signals representing the focus adjustment state. Therefore, there is a problem that it takes time to detect the in-focus position.

Therefore, it is an object of the present invention to provide a focusing position detecting device which shortens the focusing position detecting time while maintaining a high focusing accuracy.

[0006]

In order to solve the above-mentioned problems, the in-focus position detecting apparatus of the present invention uses an image formation of an objective lens (4) in order to solve the above-mentioned problems. Changing means (4, 11, 13) for changing the relative position between the position and the sample (6) in the optical axis direction of the objective lens (4), and a plurality of relative positions when the relative position is changed. Sampling means (7, 10, 11, 13) for sampling a signal indicating a focus adjustment state for the sample (6), and based on the signal indicating the focus adjustment state, a focus position for the sample (6) is determined. In the focus position detection device to be determined, a storage device (14) for storing a planned focus position in advance and a sample for performing focus position detection,
The sampling cycle of the signal indicating the focus adjustment state is shortened within a predetermined vicinity range of the planned focus position stored in the storage device (14), and the sampling cycle of the signal indicating the focus adjustment state is outside the predetermined vicinity range. Sampling cycle control means (4, 11, 13, 14) for lengthening

[0007]

[Operation] The in-focus position detecting device of the present invention comprises a storage device (1
The planned focusing position is stored in advance in 4), and the sampling cycle control means (4, 11, 13, 14) is caused to perform the following control based on the stored focusing position. That is, since it is assumed that the focus position of the sample for which focus detection is performed is within the predetermined vicinity range of the planned focus position, within the predetermined vicinity range of the planned focus position stored in the storage device (14). The sampling period of the signal indicating the focus adjustment state is shortened, and the sampling period of the signal indicating the focus adjustment state is controlled to be long outside the predetermined vicinity range.

As a result, while maintaining high focusing accuracy,
Focus detection time can be shortened.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below by taking a scanning electron microscope for observing a semiconductor wafer as an example. FIG. 1 is a schematic diagram of an electron microscope of the present invention. In FIG. 1, an electron beam emitted from an electron gun 1 passes through a condenser lens 2, a deflector 3 and an objective lens 4, and irradiates a wafer 6 mounted on a stage 5. The secondary electron detector 7 detects secondary electrons generated from the wafer 6 by the irradiation, and the detection signal of the secondary electron detector 7 is converted into digital data by the A / D converter 9 through the amplifier 8. And stored in the frame memory 10. The digital data is used as an image signal for displaying the secondary electron signal on the CRT 12 and a focus detection signal for determining the in-focus position. The image signal and the focus detection signal are physically the same.

The CPU 11 performs various calculations for detecting the in-focus position and controls the electron optical system drive circuit 13. The electron optical system drive circuit 13 drives the electron gun 1, the condenser lens 2, the deflector 3, the objective lens 4 and the like according to a command from the CPU 11. Further, the CPU 11 stores the image formation position of the objective lens 4 when the objective lens 4 is focused on the wafer 6 in the memory 14 as a planned focus position, and this planned focus position is used for detecting the focus position of the next wafer. used.

Next, the operation of this embodiment will be described. The focus position detection operation of the present invention will be described with reference to the flowcharts of FIGS. 2 and 3 showing the image formation position of the objective lens 4. Even if the wafers have the same size, there is a maximum difference in thickness of about 100 μm due to dimensional tolerances, warpage, and various processes. Therefore, in this embodiment, as shown in FIG. 2, the image forming position of the objective lens 4 is changed from 0 μm to 100 μm. Here, the position of 0 μm is the lower limit position of the imaging position when the size of the wafer is the same.
Further, the planned focus position stored in the memory 14 is, for example, 42 μm in which the objective lens 4 is focused during the previous observation of the wafer.

Then, the variable range of the image forming position of the objective lens 4 is divided into a predetermined vicinity range and a predetermined vicinity range of the planned focus position. In this embodiment, ± 42 from the planned focus position of 42 μm
A position of 16 μm (26 μm to 58 μm) is set as a predetermined near range A zone of the planned focus position, and a portion outside the A zone is set as a B zone outside the predetermined near range. Further, the C zone is set in the A zone as an in-focus position allowable range described later (described in step 210). In the present embodiment, a position within ± 10 μm (32 μm to 52 μm) from the planned focus position is set as the focus position allowable range.

In step 201, the CPU 11 changes the image forming position of the objective lens 4 by 0 μm to 100 μm.
In addition, the deflector 3 is set to 1 in order to shorten the focus position detection time.
Electron optical system drive circuit 13 so as to scan only the line
Send a control signal to. The CPU 11 executes step 202
The previous focus position stored in the memory 14 is used as the planned focus position. In this embodiment, as described above, 42
The position of μm is used as the planned focus position.

Although various methods have been proposed for determining the in-focus position from the signal indicating the focus adjustment state, in the present embodiment, the difference between the maximum value and the minimum value of the signal indicating the focus adjustment state is the largest. The focus position is defined as the focus position. CP
U11 sets an initial value for detecting the in-focus position in step 203. The initial value is the displacement amount P1 = 2μ when the image forming position of the objective lens 4 changes in the zone A of FIG.
m, displacement amount P2 = 10 μm when the image forming position of the objective lens 4 changes in the B zone in FIG. 2, and image forming position F = 0 μm for changing the image forming position of the objective lens 4 from 0 μm. The amount of displacement when the imaging position of the objective lens 4 changes is a sampling cycle.

In step 204, the CPU 11 obtains an evaluation value representing the focus adjustment state when the image forming position of the objective lens 4 is 0 μm in the following procedure. The CPU 11 sets the image forming position of the objective lens 4 to 0 via the electron optical system drive circuit 13.
Instruct to move to the μm position. The electron beam emitted from the electron gun 1 has an image forming position of the objective lens 4 of 0 μm.
The wafer 6 is irradiated in the state of m, and the deflector 3 scans only one line. The secondary electrons generated from the wafer 6 by the scanning of the electron beam are detected by the secondary electron detector 7 as a signal indicating the focus state as described above, and are digitalized via the amplifier 8 and the A / D converter 9. Becomes data,
It is stored in the frame memory 10. Then, CPU11
Calculates the difference between the maximum value and the minimum value of this digital data (this difference is called an evaluation value) and stores it in a memory (not shown).

The CPU 11 at step 205
It is determined whether or not the displacement amount of the objective lens 4 can be set to P1. In this embodiment, it is determined whether the image formation position F is 26 or more and less than 58. Currently F = 0, so
The determination in step 205 is NO, and the process proceeds to step 207.

In step 207, the CPU 11 adds P2 (10 μm) to the current image forming position F = 0 to set the next image forming position F = 10. In step 208, the CPU 11 determines that the value of the image formation position F is the range 10 of change of the image formation position of the objective lens 4.
Judge that it does not exceed 0. Since F = 10 at present, the determination in step 208 is YES, and the process returns to step 204.

Hereinafter, when F = 10 and F = 20, F =
Steps 205, 207, 20 of the same loop as when 0
8 and return to step 204. The CPU 11 acquires the evaluation value when F = 30 in step 204 and proceeds to step 205. Since F = 30, step 205
The determination is YES, and the process proceeds to step 206.

In step 206, the CPU 11 adds P2 = 2 to the current image forming position F to obtain the next image forming position F.
Hereinafter, F is in order from 32, 34, 36, 38, ... 5
Steps 204, 205, 206 up to 2, 54, 56,
Repeat the loop. When F = 56, the CPU 11
Is set to F = 58 in step 206, and step 204
Return to.

The CPU 11 makes F = 58 in step 204.
The evaluation value at the time of is acquired and it progresses to step 205. F = 5
8, the determination in step 205 is NO,
Go to step 207. The CPU 11 executes step 207.
The next image forming position is determined by. Since the image forming position has already exceeded the range of the zone A, 10 is added to the current image forming position F to obtain the next image forming position. That is, since F = 58 at present, the next F becomes 68, and the routine proceeds to step 208.

Since F = 68, the determination in step 208 is YES, and the process returns to step 204. Thereafter, the loop of steps 204, 205, 207 and 208 is repeated from the image forming position 68 to 78, 88 and 98 in order.
When F = 98, the next F becomes 108 in step 207, and the routine proceeds to step 208. Since F = 108,
The determination in step 208 is NO, and step 209
Proceed to.

The CPU 11 at step 209
The maximum evaluation value is selected from the stored evaluation values, and the image formation position of the objective lens 4 corresponding to the selected evaluation value is stored as a focus position Fmax in a memory (not shown), and step 210
Proceed to. In step 210, the CPU 11 determines whether or not the value of the focus position Fmax obtained in step 209 is in the C zone of the focus position allowable range. That is, the focus position allowable range is a parameter for determining the focus accuracy of the focus position Fmax. It can be seen that when the focus position Fmax is in the C zone of FIG. 2, the number of samplings near the focus position Fmax is large, and therefore the focus accuracy is high. However, when the in-focus position Fmax is not in the C zone of FIG. 2, the number of samplings near the in-focus position Fmax is small, and the reliability of the in-focus accuracy is poor. Therefore, the CPU 11 stores the focus position Fmax in the memory 14 and causes the focus position detection operation to be performed again.

When the focus position Fmax is in the C zone,
The determination in step 210 is YES, and the CPU 11
Can focus the wafer 6 by adjusting the exciting current of the objective lens 4 through the electron optical system drive circuit 13 so that an image is formed at the focus position Fmax. If the in-focus position Fmax is not in the C zone, the determination in step 210 is NO and the process proceeds to step 211.

In order to perform the focus position detection operation again, the CPU 11 stores the focus position Fmax as the planned focus position in the memory 14 in step 211, and returns to step 202. In step 202, the CPU 11 recognizes the previous focus position of the objective lens 4 as the focus position Fmax, and thereafter performs the focus position detection operation in the same manner as described above. As a result, when the second focus position is detected, a large number of evaluation values near the first focus position Fmax are acquired, and the focus accuracy is improved.

If there is a lot of noise or one line scan is performed in a place where there is no wafer pattern, step 2
Since the determination of 12 is always NO and the in-focus position detecting operation may not be completed, the steps 210 to 202 are executed.
It is also possible to provide a loop counter on the route returning to the above and to forcibly end the focus position detection operation when the route has been passed a certain number of times. Further, in order to avoid such an error, the focus position detection operation can be performed by calculating the evaluation value from the image information of the entire screen instead of the one-line scanning.

In the flow chart of FIG. 3, if the determination in step 205 is NO, in step 207 P2 = 10 is added to the current F to obtain the next F. That is, as described above, when F = 20, the determination in step 205 is NO, and the next F is set to 30 in step 207. In the present invention, high focusing accuracy is maintained by shortening the sampling cycle within a predetermined vicinity of the planned focusing position. Therefore, the first value in the A zone, 26, and 2
If the evaluation value with respect to the next imaging position 28 obtained by adding P1 = 2 to 6 is not acquired, the reliability of focusing accuracy is lacking. Therefore,
The flow chart of FIG. 4 additionally includes determination item steps 205A and 205B for determining whether the change amount of the objective lens 4 should be P2 = 10 while going from step 205 to step 207 of the flow chart of FIG.
That is, the CPU 11 determines in step 205A whether the image formation position F is larger than 16 and smaller than 58.

When F = 20, the determination at step 205A is YES and the routine proceeds to step 205B. CPU11
In step 205B, the next F is set to 26, which is the start value of the A zone, and the process returns to step 204. The CPU 11 obtains the evaluation value when F = 26 in step 204, proceeds to step 205, and the next F becomes 28 in step 205. Since the evaluation value when F = 28 can be obtained in step 204, high focusing accuracy can be maintained.

The flowchart of FIG. 4 is the same as the flowchart of FIG. 3 except that steps 205A and 205B are added. When the focusing operation is completed, the CPU 11 causes the deflector 3 to two-dimensionally scan a predetermined area on the wafer 6 via the electron optical system drive circuit 13, and takes in a secondary electron signal for one screen into the frame memory 10,
After the image processing, the CRT 12 displays the secondary electron image of the wafer 6.

In the flowchart of FIG. 4, when the image forming position of the objective lens 4 is changed from the lower limit position by 100 μm,
The evaluation value is acquired when the image forming position of the objective lens 4 is 0, 10,
20, 26, 28, 30, 32, 34, 36, 38, 4
0, 42, 44, 46, 48, 50, 52, 54, 5
Since only 24 points of 6, 58, 68, 78, 88, 98 are required, the focus position detection time can be shortened.

In the present embodiment, the image forming position of the objective lens 4 is set to a fixed value from the lower limit position 0 μm to the upper limit position 100 μm in advance. The image forming positions may be distributed. In the present embodiment, the value at which the objective lens 4 was focused at the previous time of observing the wafer was stored in the memory 14 as the planned focus position. The focus position is stored in the memory 14, and the memory 14 is stored according to the size of the wafer to be observed.
You can also read the planned focus position from. In addition to the wafer, the focus position of the focused observation sample may be stored in the memory 14 as the planned focus position, and the planned focus position stored in the memory 14 may be read when the next observation is performed. Good.

[0031]

As described above, according to the present invention, the sampling period of the signal indicating the focus adjustment state is shortened within the predetermined vicinity of the planned focus position stored in the storage device, and the predetermined value stored in the storage device is set. Outside the near range, the sampling period of the signal indicating the focus adjustment state is set long, so that the focusing position detection time can be shortened while maintaining high focusing accuracy.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a scanning electron microscope that is an embodiment of the present invention.

FIG. 2 is a diagram showing a variable range of an image forming position of an objective lens 4.

FIG. 3 is a flowchart for executing a focus position detection operation.

FIG. 4 is a flowchart for executing a focus position detection operation.

[Explanation of symbols]

 4 Objective Lens 6 Wafer 7 Secondary Electron Detector 10 Frame Memory 11 CPU 13 Electron Optical System Drive Circuit 14 Memory

Claims (1)

[Claims] 1. A changing means for changing a relative position between an image forming position of an objective lens and a sample in an optical axis direction of the objective lens, and the sample at a plurality of relative positions when the relative position is changed. A sampling means for sampling a signal indicating a focus adjustment state for the sample, and a focus position detecting device for determining a focus position for the sample based on the signal indicating the focus adjustment state. For the storage device that stores the focus position and the sample that performs the focus position detection, within a predetermined vicinity of the planned focus position stored in the storage device, the sampling cycle of the signal indicating the focus adjustment state is shortened. A sampling period control means for lengthening the sampling period of the signal indicating the focus adjustment state outside the predetermined vicinity range. Focus position detection device.