JPH08148108A - Automatic focus adjustment - Google Patents

Abstract

PURPOSE: To shorten time for determining an in-focus point of an objective lens by setting drive current for an objective lens at an predicted in-focus current and then so setting that an evaluation relation showing an in-focus degree may be maximum by varying the drive current. CONSTITUTION: Drive current for an objective lens is set at a predicted focussing current, so that the focal point position is set at a predicted position. The drive current is varied finely in a region including a predicted focussing current, and it is varied coarsely in region outside it. An image obtained from charged particles arising from a specimen is processed, so that an evaluation function showing the focusing degree is obtained. A current when the evaluation function becomes a maximum, is set a at drive current for an objective lens. This procedure is repeated by an operator watching an image shown on a CRT display until the image becomes most clear.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic focus adjusting method suitable for automatically adjusting the focal position of an objective lens in, for example, a scanning electron microscope.

[0002]

2. Description of the Related Art In a scanning electron microscope used for observing a fine structure of a sample such as a semiconductor element, for example, when the sample is exchanged or the observation field of view is switched on the sample, When the height changes, the obtained image becomes unclear, so an automatic focus adjustment mechanism for automatically setting the focus position of the objective lens on the sample is provided.

In one example of the conventional adjusting method in the automatic focus adjusting mechanism, first, the driving current of the objective lens is sequentially changed with a coarse step amount to change the focus position within a predetermined adjusting range, for example, in units of several 100 μm. An evaluation amount indicating the degree of focus is obtained, and a focus position where the evaluation amount is maximum is obtained. Next, in the vicinity of the focus position thus obtained, the focus position is changed in finer units in the direction in which the evaluation amount increases, and the evaluation amount indicating the degree of focus is obtained,
The focus position where this evaluation amount is maximum is obtained. Then, while gradually increasing the resolution of the amount of change in the focus position, the in-focus point is finally detected with a resolution of, for example, about 10 μm.

[0004]

As described above, in the conventional automatic focus adjustment method, after performing the rough adjustment in which the focus position is first changed in a certain coarse step to obtain the evaluation amount indicating the degree of focus, In the vicinity of the focus position where the evaluation amount is maximum, the fine adjustment was performed by changing the focus position in finer fixed steps to obtain the evaluation amount, so it takes a long time to finally determine the in-focus point. There was an inconvenience of this.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide an automatic focus adjustment method which can shorten the time until the focus is determined on average.

[0006]

A first automatic focusing method according to the present invention is, for example, as shown in FIGS. 1 to 3, a charged particle beam is passed through an objective lens (4, 6) through a sample (5). In the method of adjusting the focal position of the objective lens (4, 6) in the device for irradiating the object, the drive current (IR + IF) of the objective lens (4, 6) is set to the expected focusing current for setting the focal position to the expected position. First step (step 103)
And the drive current of the objective lens (4, 6) is changed densely in the first region including the expected focusing current, and in the second region outside the first region at coarse intervals. Second step of processing an image obtained from the charged particles generated from the sample (5) to obtain an evaluation function indicating the degree of focus (step 10)
4 to 108) and a third step (step 114) of setting the drive current of the objective lens (4,5) to the drive current when the evaluation function obtained in the second step becomes maximum, Next, when the focus adjustment is performed, the predicted position in the first step is set as the focus position set in the previous steps, and the first to third steps are repeated.

In this case, when the drive current when the evaluation function becomes maximum after the end of the second step is in the second region away from the predicted focusing current, the evaluation function becomes maximum. The driving currents of the objective lenses (4, 6) are densely changed in the region including the driving current at that time to obtain their evaluation functions (steps 111 to 113), and then the third step (step 114) is performed. Is desirable.

Further, when the evaluation function monotonously increases or decreases with respect to the drive current of the objective lens (4, 6) after the completion of the second step, the expected focusing current in the first step is obtained. After shifting (step 110), it is desirable to execute the second step (steps 104 to 108). Further, the second automatic focusing method according to the present invention is, for example, as shown in FIGS. 1 to 3, an objective lens (4) in an apparatus for irradiating a sample (5) with a charged particle beam through the objective lens (4). In the method of adjusting the focal position of (4), a correction lens (6) for correcting the focal position of the objective lens (4) within a predetermined range is used, and the drive current of the correction lens is set to the midpoint (DF) of the variable range of the drive current. ₁ ) to set the drive current of the objective lens (4) to the expected focusing current (DR ₁ ) for setting the focus position of the combined system including the objective lens (4) and the correction lens (6) to the expected position. In the first step (step 103) and the first area (20A) including the midpoint of the variable range, the area becomes dense, and the second area (20B) outside the first area becomes coarse. Change the drive current of the correction lens (6) at intervals, Re sample (5) by processing an image obtained from the charged particles generated from the second step of obtaining an evaluation function f indicating the degree of focus (DF) (step 104-10
8) and a third step (step 114) of setting the drive current of the correction lens (6) to the drive current when the evaluation function f (DF) obtained in the second step becomes maximum,
Next, when the focus adjustment is performed, the predicted position in the first step is set as the focus position set in the previous steps, and the first to third steps are repeated.

[0009]

According to the first automatic focus adjusting method of the present invention, the interval when the drive current (focus adjusting current) of the objective lens is changed in a predetermined step to change the focus position is:
Instead of a fixed interval, use a predetermined predicted position (for example, the focus position at the stage when the automatic focus adjustment instruction is received) as the center, and set finely in the area near it and coarsely in the area away from the predicted position. ing. Therefore, the higher the probability that the in-focus point exists, the closer the search can be performed, and the efficiency of in-focus point detection can be improved. Further, the evaluation function indicating the degree of focus can be more accurately obtained in a region in which the probability of existence of the focus is higher. As a result, it is easy to simultaneously realize high speed and high accuracy of the automatic focus adjustment.

Further, the drive current when the evaluation function becomes maximum after the end of the second step is the second current which is far from the expected focusing current.
When it is in the area of, the search for the focal point is rough. Therefore, in order to obtain the in-focus point more accurately, the drive currents of the objective lenses (4, 6) are densely changed in a region of a predetermined width including the drive current when the evaluation function becomes maximum, and the focus position is changed. The search interval should be fine. Since it is unlikely that the in-focus point exists in a region away from the expected position, the time until the in-focus point is detected hardly changes on average even if such a step of finely searching again is added.

When there is no peak of the evaluation function, that is, when the evaluation function monotonically increases or decreases with respect to the drive current of the objective lens (4, 6), the first
After shifting the expected focusing current in the process in the direction in which the evaluation function increases, by performing the second process,
You can search the focal point. Next, according to the second automatic focus adjustment method of the present invention, the drive current (focus coarse adjustment current) of the objective lens (4) is adjusted roughly when the focus position is adjusted, and the focus position is adjusted. In order to search, the drive current (focus fine adjustment current) of the correction lens (6) for finely adjusting the focus position is adjusted. That is, first, when the focus position reaches the predicted position, the drive current of the objective lens (4) is determined so that the drive current of the correction lens (6) becomes substantially the midpoint (DF ₁ ) of the adjustable range, By changing the driving current of the correction lens (6) by different step amounts, the focus position is finely changed in a region where the existence probability of the in-focus point is high, that is, in the vicinity of the predicted position, and the focus position is changed in the region away from the predicted position. Change roughly. Thereby, the focus position can be detected on average at high speed and with high accuracy. Furthermore, since the driving current of the correction lens is small, the focal position can be detected at a high response speed.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the automatic focusing method according to the present invention will be described below with reference to the drawings. In the present embodiment, the present invention is applied to the case where the focus position of the objective lens is automatically adjusted by the automatic focus adjustment mechanism provided in the scanning electron microscope. FIG. 2 is a block diagram of the main components of the scanning electron microscope of this embodiment. In FIG. 2, between the electron gun 1 and the sample table 16, the condenser lens 3 and the X direction are arranged in order from the electron gun 1 side. A deflection coil 7, a Y-direction deflection coil 8, an objective lens 4, and a focus correction lens 6 are arranged,
The sample 5 to be observed is placed on the sample table 16. Here, the Z axis is taken parallel to the optical axis of the electron optical system of FIG. 2, and the X axis is parallel to the paper surface of FIG. 2 in a plane perpendicular to the Z axis.
The Y axis is taken perpendicular to the plane of FIG.

The electron beam 2 emitted from the electron gun 1 is
A two-stage electromagnetic converging lens system including a condenser lens 3 and an objective lens 4 converges on the surface of the sample 5.
At this time, the focus position of the electron beam 2 (converging position in the Z direction) is adjusted by adjusting the focus adjustment current of the objective lens 4. The focus correction lens 6 arranged in the vicinity of the objective lens 4 performs focus adjustment at a higher speed within a narrower range than the focus adjustment range of the objective lens 4 in order to assist the focus adjustment function of the objective lens 4. is there. Therefore, the control circuit 14 for controlling the drive current of each lens
The focus adjustment current supplied from the control lens 14 to the objective lens 4 is called a focus coarse adjustment current IR, and the focus adjustment current supplied from the control circuit 14 to the focus correction lens 6 is a focus fine adjustment current I.
Call F.

Further, an X scanning drive current IX and a Y scanning drive current IY are supplied from the control circuit 14 to the X direction deflection coil 7 and the Y direction deflection coil 8, respectively, so that the electron beam 2 causes the X direction deflection coil 7 to move. , And Y-direction deflection coils 8 on the surface of the sample 5 in the X-direction and Y-direction, respectively.
Direction is two-dimensionally scanned. By irradiating the surface of the sample 5 with the electron beam 2, the secondary electrons 9 generated from the sample 5 are captured by the secondary electron detector 10 and converted into a current signal, and then the current / voltage (I It becomes a voltage image signal via the (/ V) converter 11. The voltage image signal output from the I / V converter 11 is passed through an analog / digital converter (not shown) as the image data S to the image memory 12
Is supplied to. The image memory 12 is also supplied from the control circuit 14 with information on the X-direction address indicating the scanning position (X, Y) of the electron beam 2 on the sample 5 and the Y-direction address (X / Y address). The image data S corresponding to the address specified by the X / Y address in 12 is stored. The image data S read from the image memory 12 is converted into a video signal and supplied to the CRT display 13, and an image (electron microscope image) in the observation visual field on the sample 5 is displayed on the display screen of the CRT display 13. .

In this example, the central processing unit 15 which controls the operation of the entire apparatus can read out the necessary image data S from the image memory 12 when calculating the evaluation function indicating the degree of focus as described later. It is like this. Further, when performing automatic focus adjustment, observation of the sample 5, and the like, the central processing unit 15 controls the operation of the control circuit 14 to control the drive current of each lens. Further, the operator can send an automatic focus adjustment start command or the like to the central processing unit 15 via the keyboard 18, and the sample stage 16 can be moved in the X direction and Y direction via the sample stage controller 17.
It can be moved in any direction.

In the scanning electron microscope of the present example configured as described above, when manually adjusting the focus,
Follow the sequence below. First, the operator is CR
While observing the electron microscope image displayed on the T display 13, the central processing unit 15 is subjected to coarse adjustment of the focus position,
Alternatively, a command for instructing the direction of fine adjustment and the adjustment amount is given. The central processing unit 15 determines the current setting value D of the objective lens 4.
R and the current setting value DF of the focus correction lens 6 are always stored and held as 10-bit digital data, and these current setting values are corrected according to the instruction of the operator. After that, the central processing unit 15 supplies the corrected current set values DR and DF to the control circuit 14. The control circuit 14 supplies the objective lens 4 and the focus correction lens 6 with a focus coarse adjustment current IR corresponding to the supplied current setting value DR and a focus fine adjustment current IF corresponding to the current setting value DF. As a result, the focal position of the electron beam 2 is shifted in the Z direction by a predetermined distance.

After that, the operator looks at the electron microscope image displayed on the CRT display 13 and repeats the above operation until the electron microscope image becomes the clearest. As a result, the focus position is adjusted to the in-focus point. Next, in the scanning electron microscope of the present example, an example of the operation when performing automatic focus adjustment will be described with reference to the flowchart of FIG.

First, in FIG. 2, it is assumed that the sample 5 to be observed is placed on the sample table 16 and the focus position is adjusted on the surface of the sample 5 by the manual adjustment described above in the initial state. From this initial state, step 10 of FIG.
1, the operator operates the sample stage 16 via the sample stage controller 17 of FIG. 2 to move the observation field of view on the sample 5 to another region. At this time, the sample table 16
Due to the influence of the inclination of, the flatness of the sample 5 itself, etc.
If the electron microscope image on the CRT display 13 is out of focus, the operator at step 102
An instruction for automatic focus adjustment is given to the central processing unit 15.

In response to this, in step 103, the central processing unit 15 leaves the focus position of the composite system consisting of the objective lens 4 and the focus correction lens 6 of FIG. 2 set at the position set by the manual adjustment. , The current setting value DF of the focus correction lens 6 is DF ₁ at approximately the middle point of the variable range.
So that the value of the current setting value DR of the objective lens 4 becomes D
Reset to R ₁ . An example of calculation of the current set value DR ₁ will be shown below.

[0020]

[Table 1]

Table 1 shows the number of windings of the objective lens 4 and the focus correction lens 6 used in this embodiment and the variable range of the current. In Table 1, the number of windings of the objective lens 4 is 2000 turns. The number of windings of the focus correction lens 6 is 100 turns. Further, the current setting value DR of the objective lens 4 is a 10-bit binary number (digital data) in the range of values 0 to 1023 (decimal number display), and the 10-bit current setting value DR is the control of FIG. 0 to 1.0 depending on circuit 14
The focus coarse adjustment current IR up to A is converted. Therefore, the maximum value of the product (ampere turn) [A turn] of the current showing the strength of the magnetic field for focus adjustment of the objective lens 4 and the number of windings is 2000 [A turn], and the minimum bit of the binary number. (L
The value of ampere turn per SB) is 1.955 [A turn / LSB].

On the other hand, the current setting value DF of the focus correction lens 6
Is a 10-bit binary number in the range of 0 to 1023, the 10-bit current setting value DF is converted into a focus fine adjustment current IF of 0 to 0.2 A by the control circuit 14 of FIG. It Therefore, the maximum value of the ampere turn of the focus correction lens 6 is 20 [A turn], and the value of ampere turn per LSB is 0.019 [A turn / LS].
B]. Therefore, the maximum value of the magnetic field of the focus correction lens 6 is 1% of the maximum value of the magnetic field of the objective lens 4, and the focus adjustment range by the focus correction lens 6 is about 1% of the focus adjustment range of the objective lens 4. is there. Further, the resolution of the focus adjustment by the focus correction lens 6 is approximately 1/100 of the resolution of the focus adjustment by the objective lens 4, and the objective lens 4 is used by using the focus correction lens 6. Compared with the case, the focus position search can be performed in a narrow range with finer resolution.

Here, when there is an instruction to perform automatic focus adjustment, the current setting value DR of the objective lens 4 is set to 71.
Assuming that the current setting value DF of the focus correction lens 6 is 0, the total value ΣAT of ampere turns at that time is as follows.

[0024]

[Equation 1] ΣAT = 1.0 × (710 ÷ 1023) × 2000 + 0.2 × (2
30 ÷ 1023) × 100 = 1392.57 [A turn] Next, the current setting value DF is reset to DF ₁ by setting 530 as substantially the midpoint DF ₁ of the variable range of the current setting value DF of the focus correction lens 6. Sometimes, the sum of the ampere turns of the objective lens 4 and the focus correction lens 6 is Σ
DR the current setting value DR of the objective lens 4 when it becomes AT
_{When set to 1} , the following equation holds.

[0025]

[Equation 2] 1.0 × (DR ₁ ÷ 1023) × 2000 + 0.2 × (530 ÷
1023) × 100 = 1392.57 [A turn] When this equation is solved for the current setting value DR ₁ , it becomes as follows.

[0026]

[Equation 3] DR ₁ = 707 Therefore, the current setting value DR of the objective lens 4 is set to DR ₁ (= 70
7), set the current setting value DF of the focus correction lens 6 to DF ₁ (= 5
Even if the value is set to 30) again, since the ampere-turn values of the combined system are equal, the focal position of the combined system hardly changes.

In this example, the focus correction lens 6 can adjust the focus at a higher speed and with higher accuracy than the objective lens 4, so that the current setting value of the focus correction lens 6 is set when the automatic focus adjustment is performed. It is desirable to adjust the focus by changing only Therefore, in step 103, as described above, the current setting value DR of the objective lens 4 is reset so that the current setting value DF of the focus correction lens 6 is substantially at the midpoint of the variable range. Automatic focus adjustment can be performed.

FIG. 3A shows a current setting value D of the objective lens 4.
The variable range of R, FIG. 3B shows the variable range of the current setting value DF of the focus correction lens 6, and when step 103 is executed, the current setting values DR and DF are respectively DR ₁ and DF in FIG. Set to ₁ . Next, at step 104, the current setting value DF of the focus correction lens 6
Is set to one end (for example, 0) within the variable range of 0 to 1023. This means that the convergence point (focus position) of the electron beam 2 in the Z direction in FIG. 2 is set to the lower limit or the upper limit of the range for searching the in-focus point. In the next step 105, the electron beam 2 of FIG. 2 is scanned on the sample 5 along a predetermined locus to capture the image data S into the image memory 12 via the secondary electron detector 10 and the like. Then, in step 106, the captured image data S is subjected to arithmetic processing to calculate an evaluation function f (DF) indicating the degree of focus. This evaluation function f (DF) is calculated by the focus correction lens 6
It is calculated each time the current setting value DF of is changed.

Specifically, FIG. 4 shows a scanning locus of the electron beam 2 on the sample 5. In FIG. 4, the electron beam traces loci 19A and 19B parallel to the X axis and these loci. Trajectories 19C, 19D, 19 obtained by rotating the respective directions by approximately 30 °, 60 °, and 120 °
Scan along E, 19F, and 19G, 19H.
Therefore, the eight loci 19A to 19H are designated by an integer i that changes in the range from 1 to 8, and the j-th locus (j = 1, 2, 3, ..., N) obtained along the i-th locus. The image data S of is represented by S (i, j). For example, n is an integer with a value of 512. Then, the difference between the image data obtained from the adjacent points on the same locus {S (i, j) -S (i, j-
The value obtained by dividing the sum of absolute values of 1)} by the number N of all image data is defined as an evaluation function f (DF).

[0030]

F (DF) = Σ | S (i, j) -S (i, j-1) | / N In this (Formula 4), the sum symbol Σ is 1 for the subscript i.
8 to 8 and the sum of the range from 2 to n for the subscript j. Evaluation function f (DF) represented by (Equation 4)
Is a function that has the largest value at the focal position.

Next, at step 107 in FIG. 1, the current setting value DF is the other end (= 102) of the variable range in FIG.
3) is determined, and if not reached, the process proceeds to step 108, the current setting value DF of the focus correction lens 6 is changed by a predetermined step amount, and the focus position of the electron beam 2 in FIG. Is changed in the Z direction by a predetermined distance. In this case, in this example, the focus position is changed at a close interval in the area near the focus position that has been set up to that point (the focus point up to that point), and the coarser the area that is farther from the focus point that has been set so far. The focus position is changed at regular intervals.

For that purpose, the change amount of the current setting value DF and
It is assumed that the amount of change in the focus position in the Z direction is approximately proportional to
The current setting value DF at the position of the approximately midpoint shown in FIG.
DF ₁Near the position of the position DF₁Or
It may be changed at a coarse interval in a region distant from it. concrete
In this example, as shown in FIG. 3B, the current setting value DF
Variable range of position DF₁Including the value of 420 to 619
Area 20A, values 220-41 outside this area 20A
Area 20B of 9 and 620 to 819, and this area 20
Area outside B of values 0-219 and 820-1023
Divide into 20C. Then, the area around the current setting value DF
In 20A, it is changed in ΔD steps and the area outside
In 20B, it is changed in 2.ΔD steps, and the outermost area is changed.
In the area 20C, it is changed in 4 · ΔD steps. in this case
The step amount ΔD is, for example, 4 LSB (ie, 1
Set to 0) in 4).

As a result, assuming that the focal position in the area 20A changes in the Z direction at intervals of, for example, approximately 10 μm,
In the area 20A, the focus position changes at intervals of about 20 μm,
In the area 20C, the focus position changes at intervals of approximately 40 μm. If the focus position is changed as described above, step 105
Then, the operation of sampling the image data and calculating the evaluation function f (DF) in step 106 is repeated. When the current setting value DF reaches the other end of the variable range in step 107, step 1
In step 09, the peak position of the evaluation function f (DF) is obtained.

FIG. 5 shows the current setting value D of the focus correction lens 6.
Evaluation function f showing the degree of focus obtained when F is changed
An example of (DF) is shown in FIG.
The value DF _max of the current setting value DF when the value of (DF) reaches a peak is approximately 440. That is, since there is a peak in FIG. 5, the process further moves to step 111, and it is checked whether or not the peak is in the central region 20A of FIG. 3B. In the case of FIG. 5, the peak value DF _max is in the region 2
Since it is within 0 A, the process proceeds to step 114, and the current setting value DF of the focus correction lens 6 is set to the peak value DF _max.
Set to. With this, the automatic focus adjustment is completed, and FIG.
At, the surface of the sample 5 becomes the focal point. Therefore, the process proceeds to step 115 and the sample 5 is observed.

Next, at step 109, when there is no peak of the evaluation function f (DF) within the variable range of the current setting value DF of FIG. 5, the routine proceeds to step 110, where the value of the evaluation function f (DF) is In the direction of increasing
The current setting value DR of the objective lens 4 is changed by a predetermined amount. The amount of change in the focus position at this time is determined by the focus correction lens 6
The width is slightly smaller than the width of the variable range of the focal position due to. After that, the process proceeds to step 104 and the in-focus point is searched again. Since the probability that the evaluation function f (DF) does not have a peak in step 109 is extremely low, the probability that step 110 is executed is also extremely low. However, when the evaluation function f (DF) has no peak in step 109, an alarm may be issued to the operator.

If the peak position of the evaluation function f (DF) is not in the central region 20A in step 111, the process proceeds to step 112 and the current setting value DF required for observing the sample 5 is set. Change interval (accuracy required)
, It is checked whether the peak is rougher than the set interval (step accuracy) of the focal position in the region (20B or 20C) where the peak exists. Then, when the required accuracy is rougher than the step accuracy, the routine proceeds to step 114, where the current setting value DF of the focus correction lens 6 is set to the value DF _max at which the evaluation function f (DF) reaches its peak.

On the other hand, in step 112, when the required accuracy is smaller than the step accuracy, the routine proceeds to step 113, where a predetermined width centered on the value DF _max at which the evaluation function f (DF) reaches a peak. In the area, the current setting value DF of the focus correction lens 6 is changed with a step accuracy smaller than the required accuracy to calculate the evaluation function f (DF) showing the degree of focus, and then the process proceeds to step 114 to set the current. Set the value DF to the peak position. In this case, the time to detect the in-focus point is long and the operation speed is slow, but the frequency is low, so that it is acceptable.

As described above, according to the present embodiment, the set intervals of the focus positions within the predetermined search range are not fixed intervals, but the focus positions at the stage when the automatic focus adjustment instruction is received, as the center. The area in the vicinity is finely divided, and the farther from the center, the coarser the area. That is, since the focus position is densely changed and the search is performed in a range in which the in-focus point is higher, the efficiency of in-focus point detection can be improved and the evaluation function f (DF) near the in-focus point. Can be created more accurately.

In the above-described embodiment, the focus correction lens 6 is provided in addition to the objective lens 4 in order to finely adjust the focus position at a high response speed. The invention is applicable. When focusing is performed using only the objective lens 4, the focus adjustment current IR of the objective lens 4 is changed in fine steps in the vicinity of the focus point so far, and in a coarse step in a region away from the focus point. You can change it with. However, when focusing is performed using only the objective lens 4, the objective lens 4 in Table 1 is used.
It is desirable that the range of the current set value DR of is wider than 10 bits. Further, the case of using only the objective lens 4 is also equivalent to the case of collectively considering the objective lens 4 and the focus correction lens 6 in FIG. 2 as one objective lens.

Further, the present invention can be applied not only to a scanning electron microscope but also to all devices for converging a charged particle beam on a sample via an objective lens, such as a line width measuring instrument. As described above, the present invention is not limited to the above-described embodiments, and various configurations can be adopted without departing from the gist of the present invention.

[0041]

According to the first automatic focus adjustment method of the present invention, the focus position is obtained by changing the focus position densely in the vicinity of the predicted position of the focus point and roughly in the region away from the predicted value. Therefore, there is an advantage that the time to determine the focus point can be shortened on average. In this case, when the drive current when the evaluation function becomes maximum after the end of the second step is in the second region away from the expected focus current, the region including the drive current when the evaluation function becomes maximum After densely changing the drive current of the objective lens with each to obtain its evaluation function,
In the case of shifting to the third step, there is an advantage that the in-focus point can be detected with high accuracy even if the in-focus point is far away from the expected position.

When the evaluation function monotonously increases or decreases with respect to the driving current of the objective lens after the completion of the second step, the expected focusing current in the first step is shifted to perform the second step. When executed, there is an advantage that the in-focus point can be accurately detected even when the in-focus point is not within the search range of the first in-focus point. Furthermore, according to the second automatic focus adjustment method of the present invention, by changing the drive current of the correction lens, the focus position is densely near the predicted position of the in-focus point and roughly in the area away from the predicted value. Since the in-focus point is obtained by changing, there is an advantage that the time until the in-focus point is determined can be shortened on average. Further, since it is a method of controlling the drive current of the correction lens, there is an advantage that the response speed of focus adjustment is fast.

[Brief description of drawings]

FIG. 1 is a flowchart showing an embodiment of an automatic focus adjustment method according to the present invention.

FIG. 2 is a configuration diagram showing a scanning electron microscope used in Examples.

3 is a diagram showing a variable range of a current setting value DR for the objective lens 4 and a current setting value DF for the focus correction lens 6 of FIG.

FIG. 4 is a diagram showing an example of a scanning trajectory of an electron beam on a sample for obtaining an evaluation function of a focus degree.

FIG. 5 is a diagram showing an example of changes in the evaluation function f (DF) when the current setting value DF of the focus correction lens 6 is changed.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Electron gun 2 Electron beam 3 Condenser lens 4 Objective lens 5 Sample 6 Focus correction lens 7 X direction deflection coil 8 Y direction deflection coil 9 Secondary electron 10 Secondary electron detector 11 Current / voltage converter 12 Image memory 13 CRT display 14 control circuit 15 central processing unit 16 sample stage 17 sample stage control unit

Claims (4)

[Claims] 1. A method of adjusting a focal position of an objective lens in an apparatus for irradiating a sample with a charged particle beam through an objective lens, comprising: a predictive value for setting a drive current of the objective lens to a predicted position. A first step of setting a pyroelectric current; a drive current of the objective lens is densely arranged in a first area including the expected focus current and coarsely arranged in a second area outside the first area. A second step of changing and processing an image obtained from charged particles respectively generated from the sample to obtain an evaluation function indicating the degree of focus; and a drive current of the objective lens is an evaluation function obtained in the second step. And a third step of setting the drive current at the time of maximum, and the next time the focus adjustment is performed, the predicted position in the first step is set as the focus position set in the previous steps. 1st process to 3rd Automatic focusing method characterized by repeated up degree. 2. A method of adjusting a focal position of the objective lens in an apparatus for irradiating a sample with a charged particle beam through an objective lens, wherein a correcting lens for correcting the focal position of the objective lens within a predetermined range is used, Prediction for setting the drive current of the correction lens to the midpoint of the variable range of the drive current and setting the drive current of the objective lens to the expected position of the focus position of the composite system including the objective lens and the correction lens A first step of setting a focusing current; densely in a first region including the midpoint of the variable range,
In the second region outside the first region, the drive current of the correction lens is changed at a coarse interval, and an image obtained from the charged particles generated from the sample is processed to evaluate the degree of focus. A second step of obtaining a function; and a third step of setting the drive current of the correction lens to the drive current when the evaluation function obtained in the second step is maximum; At this time, the automatic focus adjustment method is characterized in that the predicted position in the first step is used as the focus position set in the previous steps and the first to third steps are repeated. 3. When the drive current when the evaluation function becomes maximum after the end of the second step is in the second region away from the expected focusing current, when the evaluation function becomes maximum 2. The driving current of the objective lens is densely changed in a region including the driving current of 1 to obtain the evaluation function, and then the process proceeds to the third step.
The described automatic focus adjustment method. 4. When the evaluation function monotonously increases or decreases with respect to the drive current of the objective lens after the end of the second step, the expected focusing current in the first step is shifted to change the estimated focus current. The automatic focus adjusting method according to claim 1, wherein the second step is executed.