JP2007178764A - Automatic focusing method and automatic focusing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an automatic focusing method and an automatic focusing device capable of automatically focusing an image formed by using charged-particle lines, in which a focus current is changed with a predetermined width, so as to obtain a plurality of line profiles from samples; the sums of the absolute values of the values obtained by differentiating the respective line profiles are calculated respectively, and a point where these sums are maximum or a point where adjacent sums are approximately equal is decided as a focused focal point, so as to obtain the image, so that a focused image having high reproducibility is automatically obtained. <P>SOLUTION: The automatic focusing method has: a step of obtaining the line profiles from the samples respectively, when the current or voltage of an objective lens is successively changed with the predetermined width; a step of differentiating the obtained respective profiles and calculating the sums of the absolute values respectively; and a step of adjusting to the current or voltage of the objective lens when the calculated sums are maximum or the adjacent sums are approximately equal. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an autofocus method and an autofocus apparatus that perform autofocus on an image generated using a charged particle beam.

  Conventionally, when a focused image is acquired from a mask or LSI pattern with a scanning electron microscope or the like, the operator visually observes that the current image becomes sharpest by changing the current of the objective lens little by little. The image at that time is determined and acquired, or the image of the point that becomes the sharpest is acquired by line scanning the image.

  However, if the former operator adjusts the focus of the image as described above, there is a problem that if it takes a lot of time, variations may occur due to collocation and lack of reproducibility.

  In addition, since the image of the sharpest point is acquired by performing line scanning of the latter image described above, an image with a focus having sufficient reproducibility is acquired when the line waveform is gentle. There was a problem that it was difficult.

  In order to solve these problems, the present invention obtains a plurality of line profiles from a sample by changing the focus current by a predetermined width, calculates the sum of absolute values of values obtained by differentiating these line profiles, respectively, The image is acquired by determining the maximum point or the point where adjacent sums are substantially equal as the focal point.

  The present invention obtains a plurality of line profiles from a sample by changing the focus current by a predetermined width, calculates the sum of absolute values of values obtained by differentiating these line profiles, and calculates the point where the sum is the maximum or the adjacent sum By determining an approximately equal point as a focal point and acquiring the image, it is possible to automatically acquire a focused image with high reproducibility.

  The present invention obtains a plurality of line profiles from a sample by changing the focus current by a predetermined width, calculates the sum of absolute values of values obtained by differentiating these line profiles, and calculates the point where the sum is the maximum or the adjacent sum We decided to acquire the image by determining the point that is almost equal as the in-focus point, and automatically acquiring the focused image with high reproducibility.

FIG. 1 shows a system configuration diagram of the present invention.
In FIG. 1, a lens barrel 1 is a known lens barrel. In the case of a scanning electron microscope, for example, an electron gun that generates an electron beam that is a charged particle beam, and a generated electron beam are focused. A focusing lens, a deflection coil that deflects in two stages so as to irradiate the surface of the sample 4 with a finely focused electron beam beam (scanning in the x and Y directions), and an electron beam that is finely focused on the surface of the sample 4 It comprises an objective lens 2 for forming (focusing) a beam, a detector for detecting secondary electrons and reflected electrons, and the like. The lens may be an electrostatic type. Further, the charged particle beam may be not only an electron beam beam but also various charged particle beams such as an ion beam.

  The objective lens 2 irradiates the surface of the sample 4 with an electron beam focused by a focusing lens (not shown) constituting the lens barrel 1 and focused on the surface of the sample 4. is there. In a state in which the surface of the sample 4 is narrowly irradiated with the electron beam, a deflection coil (not shown) constituting the lens barrel performs plane scanning (scanning in the X direction and Y direction), and the secondary beam emitted at that time is emitted. Electrons, light, reflected backscattered electrons, and absorbed absorbed electrons are detected by a known detector (not shown), and an image of the surface of the sample 4 is displayed on the screen of a display device (not shown).

The sample chamber 3 is a chamber that is evacuated to a vacuum (10 ⁻⁴ Torr or less) by a vacuum exhaust system (not shown) that houses the sample 4.

  The sample 4 is a sample to be observed and inspected, and is, for example, a mask on which an LSI pattern is printed.

  The PC 11 is a personal computer, and performs various processing according to a program. From the line profile acquisition unit 12, the focus current adjustment unit 13, the evaluation value calculation unit 14, the pattern specification unit 15, the image file 16, and the like. It is composed.

  The line profile acquisition unit 12 issues a command to a control device (not shown) that controls the lens barrel 1 and acquires an image (line profile) of a line at a specific location on the sample 4. The acquired line profile is temporarily stored in the image file 16 and used for various processes (described later with reference to FIGS. 2 to 4).

  The focus current adjusting unit 13 adjusts the current (focus current) of the objective lens 2 (which will be described later with reference to FIGS. 2 to 4).

  The evaluation value calculation means 14 differentiates the line profile and calculates the sum of absolute values (described later with reference to FIGS. 2 to 4).

  The pattern specifying unit 15 specifies a place where the pattern exists on the sample 4, for example, a mask (described later with reference to FIGS. 2 to 4).

  The image file 16 stores a line profile acquired from the sample 4.

Next, the operation of the configuration of FIG. 1 will be described in detail according to the order of the flowchart of FIG.
FIG. 2 shows a flowchart for explaining the operation of the present invention.

In FIG. 2, S2 gives an appropriate current value to channels 1 to 15. For example, in order from 1 to 15 channels,
I, I + Δi, I + 2Δi,... I + 14Δi
give. I is the current value of the starting point, and Δi is the current value of the step width (initially a normal mA value, a current value that is set by experimentation because it varies depending on the apparatus).

  In S3, the applied current is set as a focus current. This is based on the current given in S2 (the respective values set for the 1 to 15 channels) as the focus current (the current that is supplied to the objective lens 2 for focusing).

  In S4, the mask is scanned to obtain an image waveform (line profile). This is because the focus current given in S3 (focus current set for channels 1 to 15) is sequentially supplied to the objective lens 2 and the electron beam is focused on the mask which is the sample 4, and the deflection is not shown in the figure. For example, the secondary electrons emitted at that time are detected and amplified to obtain the line profiles (FIG. 3B) of the secondary electrons.

  In S5, an evaluation value is obtained by performing a differentiation process. For example, after obtaining the value of FIG. 3C by differentiating the line profile of FIG. 3B acquired in S4, for example, the sum of the absolute values of the values (of FIG. 3D) is obtained. (Evaluation value) is calculated.

S6 stores the evaluation value (E).
In S7, it is determined whether N = 15. It is determined whether S1 to S6 are repeated from N = 1 for channels 1 to 15 and N = 15. In the case of YES, each of the line profiles is acquired with the focus current set to channels 1 to 15 in S2 (S4), differentiation processing (S5), and the sum of the absolute values is calculated as an evaluation value (S6) processing Since all the processes are completed, the process proceeds to S8. In the case of NO, S3 and subsequent steps are repeated for the focus current value set for the next channel. .
With the above S2 to S7 YES, for example, at a certain place in the pattern of FIG. 3A described later, the electron beam is made into the pattern with a focus current value in which the current of the objective lens 2 is set to 1 to 15 channels. This means that the line profiles when the focus is adjusted are sequentially acquired and differentiated, and the sum of the absolute values of the differentiated values can be calculated as the evaluation value.

  In S8, the maximum value Emax (1) is searched from the evaluation values. The current value Io of N (any one of channels 1 to 15) corresponding to Emax (1) is stored.

In step S9, Io is given to the 8 channels, and the 8 channels are reset. This is because the maximum value Emax is searched for from the evaluation values of the line profiles respectively acquired with the focus current values set for the channels 1 to 15, the focus current value Io of the line profile of the maximum value Emax (1) is extracted, and the current Set the value Io to 8 channels and reset. For example, 1 channel: Io-7Δi
...
6 channels: Io-2Δi
7 channels: Io-Δi
8 channels: Io
9 channels: Io + Δi
10 channels: Io + 2Δi
11 channels: Io + 3Δi
...
15 channels: Io + 7Δi
And reset.

  S10 calculates | requires the evaluation value with respect to each N, and memorize | stores Nmax (2). In the same manner as S4 to S6, line profiles are acquired for the reset channels 1 to 15, respectively, differentiated and the sum of absolute values is calculated as an evaluation value, and the maximum evaluation value is set to Nmax (2). Remember as.

  In S11, it is determined whether Nmax (1) = Nmax (2). In the case of YES, since it has been found that the point at which the evaluation value at the current step (Δi) is maximum has been detected, the process proceeds to S12. On the other hand, in the case of NO, since it has been found that the point at which the evaluation value at the current step (Δi) is maximum has not been detected yet, S2 and subsequent steps are repeated, and the maximum evaluation value at the current step (Δi) is obtained. Repeat until obtained.

  In S12, it is determined that the maximum evaluation value point (focus current value) has been obtained in the current step (Δi) in YES of S11. For example, it is determined whether it is 100 μA or less. In the case of YES, since it has been found that the focus current value at the maximum evaluation value at the step (Δi) below the limit has been detected, the current value of each channel is fixed in S14. As a result, the objective current value of the just focus is fixed to the channel of the maximum evaluation value among the channels 1 to 15 and the current value of the objective lens 2 is fixed (auto focus and fixed) to a state where a just focus image is obtained. The Rukoto. On the other hand, in the case of NO in S12, since step (Δi) is not less than the limit, Δi (step size of pretension width) is set to 0.1 times (Δi = 0.1 × Δi). Thereby, S2 to S12 are repeated with Δi reduced to 1/10, and the focus current value with the maximum evaluation value is calculated with the updated Δi. When Δi becomes less than the limit (for example, 100 μA or less), The focus current value is determined and fixed as the just focus current value, and the focusing is completed (although it is set to 0.1 times, the present invention is not limited to this, and an appropriate value may be determined and set by experiment).

  As described above, the step Δi (step value of the swing width of the objective lens 2) is determined to be appropriate (default), and the line profile is acquired and differentiated by setting the channel 1 to 15 in the step Δi, and the value of the value is differentiated. The sum of absolute values is calculated as an evaluation value, and the current value of the objective lens 2 at the maximum evaluation value Emax (1) is set to approximately the center, for example, 8 channels, and is reset at the step Δi. The maximum evaluation value Emax (2) is obtained and repeated until Emax (1) = Emax (2). When Emax (1) = Emax (2), the maximum evaluation value of the step Δi is found. Therefore, step Δi is multiplied by 0.1 and steps S2 to S12 are repeated in the same manner. When step Δi is below the limit (for example, 100 μA or less), the maximum evaluation value at that time To determine the current value of the objective lens is set to the channel as the current value just focus is fixed, and thus completing the automatic focus. As a result, the line profile is differentiated, the sum of absolute values of the differentiated values is calculated as an evaluation value, and the current value of the objective lens that is the maximum evaluation value is automatically calculated and fixed. Since the evaluation value is calculated with the information involved and the fine profile of the just focus is determined, it is possible to perform the automatic focus adjustment with good accuracy and good reproducibility.

FIG. 3 is an explanatory diagram of the present invention.
FIG. 3A shows an example of a pattern on the sample (mask) 4. Since there are a portion with a pattern and a portion without a pattern on the mask which is the sample 4, it is desirable that the scanning line crosses the portion with the pattern in order to operate the automatic focus of the present invention efficiently. A state in which a scanning line (location) crossing the line is specified is shown.

  FIG. 3B shows the line profile of the secondary electron image acquired when the scanning line crosses the pattern of FIG. 3A (the horizontal axis indicates the distance corresponding to the length direction of the scanning line, the vertical The axis indicates the profile and waveform expressed by the brightness at that time. Here, the signal luminance is small outside the pattern of FIG. 3A, the signal luminance is large inside the pattern as shown, and a curve as shown is drawn when entering the pattern from outside the pattern. (Draws a characteristic known curve obtained when detecting and amplifying secondary electrons obtained by scanning the electron beam with the scanning line of FIG. 3A).

  FIG. 3C shows a state in which values obtained by differentiating the line profile of FIG. 3B are plotted. The evaluation value is calculated as the sum of absolute values of the values obtained by differentiating the line profile.

  FIG. 3D shows the transition state of the evaluation value when the focus current value is on the horizontal axis and the evaluation value is on the vertical axis. This is because the sum of absolute values of values obtained by differentiating the line profile of FIG. 3C is obtained as an evaluation value, and the obtained evaluation value is plotted in correspondence with the current value of the objective lens 2 at this time. A state in which a curve passing through the plotted points is conceptually shown is shown.

  -The portion described as the left overfocus is the transition of the evaluation value of the line profile when the objective lens current value (focus current value) of the objective lens 2 in FIG. Shows the state.

  The portion described as the central positive focus is the transition of the evaluation value of the line profile when the objective lens current value (focus current value) of the objective lens 2 in FIG. Shows the state.

  The portion described as the underfocus on the right side is the transition of the evaluation value of the line profile when the objective lens current value (focus current value) of the objective lens 2 in FIG. Shows the state.

  As described above, when the current value of the objective lens is sequentially changed from the hyperfocal side or the underfocus side by a predetermined step Δi and the evaluation value of the line profile obtained at that time is calculated and plotted, the overfocus is changed to the normal focus. Since the focus shifts from underfocus (or conversely from normal focus to overfocus), the point at which the evaluation value is maximized (YES in S11 in FIG. 2 and YES in S12). As a result, it is possible to automatically adjust the focus current value of the positive focus with high accuracy and good reproducibility.

FIG. 4 is an explanatory diagram (pattern identification) of the present invention.
FIG. 4A shows a flowchart.

  In FIG. 4A, scanning is performed in S21. For example, an arbitrary place is scanned with an electron beam on the mask of FIG.

  In step S22, a profile is acquired. This is because the line profile of the secondary electron beam when an arbitrary place on the mask is scanned with the electron beam in S21 (the secondary electron beam is detected and amplified, the scanning direction is the horizontal axis, and the luminance at that time A line profile image with the vertical axis as the vertical axis.

In S23, differential processing is performed.
In S24, an evaluation value is calculated. In S23 and S24, the above-described differentiation processing is performed on the line profile of the certain place acquired in S22 (the line profile of the portion having the pattern shown in FIG. 3B described above or the portion having no pattern). The sum of the absolute values is obtained as an evaluation value (S5 in FIG. 2).

  In S25, it is determined whether or not the evaluation value calculated in S24 is a reference value or more. In the case of YES, since the profile evaluation value is equal to or higher than the reference value and the profile of the portion having the pattern is found, the process proceeds to S26. On the other hand, in the case of NO, since it has been found that the profile evaluation value is equal to or less than the reference value and there is no pattern, the process ends here.

S26 determines that there is a pattern if YES in S25.
Through the above S21 to S26, it is specified whether or not there is a pattern in the portion of the scanning line on the mask scanned by the electron beam, and when it is specified that there is a pattern, there is a pattern specified by the preprocessing. By performing the automatic focus adjustment described above with reference to FIG. 2, the automatic focus adjustment can be performed more reliably, with good accuracy, and with good reproducibility.

  In step S27, autofocus is performed. This implements S1 to S14 of FIG. 2 described above.

  In S28, measurement is performed. This is because an image including a pattern to be measured is acquired in a state in which the autofocus is performed in S27 and the current of the just focus at that time is supplied to the objective lens 2, and the pattern measurement (width, length, etc.) is performed on the image. Measurement).

  As described above, a place where a pattern on the mask which is the sample 4 is found (YES in S21 to S25), a line profile is acquired at that place, and the current supplied to the just focus objective lens 2 is determined. By supplying a just focus current to the objective lens 2 to obtain a just focus image and measuring a dimension of the pattern on the image, it is possible to obtain a measurement result with good accuracy and good reproducibility.

  FIG. 4B schematically shows a state when searching for a place with a pattern on the mask.

  FIG. 4B-1 shows an entire image of the mask. In this state, the pattern on the mask is fine and where it exists is unknown.

  (B-2) of FIG. 4 shows a state where an arbitrary area A (or a measurement target area) is enlarged in the entire image of the mask of (b-1) of FIG. On the region obtained by enlarging the region A in FIG. 4B-2, here, the scanning line 15 is scanned in S21 in FIG. The evaluation value is calculated (S21 to S24), the location of the scanning line whose evaluation value is equal to or greater than the reference value is specified as the location with the pattern, and the just focus current value is determined in accordance with the flowchart of FIG. An image of the area (b-2) in FIG. 4 is acquired with the focus current value, and the pattern is accurately measured on the acquired image. As a result, a place (scan line) with a pattern in the area A in (b-2) of FIG. 4 is specified, and the just focus current value is calculated with the specified scan line, and the normal focus ( It is possible to automatically determine (e-2) in FIG. 3 and acquire an image with high reproducibility that is accurately focused.

  The present invention obtains a plurality of line profiles from a sample by changing the focus current by a predetermined width, calculates the sum of absolute values of values obtained by differentiating these line profiles, and calculates the point where the sum is the maximum or the adjacent sum The present invention relates to an autofocus method and an autofocus device that determine an approximately equal point as a focal point, acquire an image thereof, and automatically acquire an image focused with high reproducibility.

It is a system configuration diagram of the present invention. It is an operation | movement explanatory flowchart of this invention. It is explanatory drawing of this invention. It is explanatory drawing (specification of a pattern) of this invention.

Explanation of symbols

1: Lens barrel 2: Objective lens 3: Sample chamber 4: Sample 11: PC (PC)
12: Line profile acquisition means 13: Focus current adjustment means 14: Evaluation value calculation means 15: Pattern specifying means 16: Image file

Claims (6)

In an autofocus method for autofocusing an image generated using a charged particle beam,
Acquiring each line profile from the sample when the current or voltage of the objective lens is sequentially changed by a predetermined width; and
Differentiating each acquired line profile and calculating the sum of its absolute values respectively;
Adjusting the current or voltage of the objective lens when the calculated sum is maximum or adjacent sums are substantially equal; and
An autofocus method.   On the sample, line profiles are acquired sequentially from arbitrary different locations, the acquired line profiles are differentiated to calculate the sum of their absolute values, and the line profile when the calculated sum is greater than a predetermined reference value. An autofocus method characterized in that a place where the pattern is acquired is determined as a place with a pattern, and the line profile according to claim 1 is acquired for the determined position.   As the current or voltage of the objective lens is sequentially changed by a predetermined width, the predetermined width is initially set to be larger, and the predetermined width is sequentially reduced when the calculated sum is maximum or the adjacent sums are substantially equal. 3. The autofocus method according to claim 1, wherein the focus adjustment is repeated until a predetermined threshold value is reached, and fine adjustment is sequentially performed from rough adjustment.   The autofocus method according to any one of claims 1 to 3, wherein the current or voltage of the objective lens is sequentially changed to a value set for a predetermined number of channels as the current or voltage is sequentially changed with a predetermined width.   The width of the value set for the predetermined number of channels is initially set to a larger value for the predetermined width, and when the calculated sum is the maximum or the adjacent sums are almost equal, the predetermined width is sequentially decreased. The autofocus method according to any one of claims 1 to 4, wherein the autofocus method is set and repeated until a predetermined threshold value is reached, and focus adjustment is sequentially performed from rough adjustment to fine adjustment. In an autofocus device that autofocuses an image generated using a charged particle beam,
Means for acquiring each line profile from the sample when the current or voltage of the objective lens is sequentially changed by a predetermined width;
Means for differentiating each acquired line profile and calculating the sum of absolute values thereof;
Means for adjusting the current or voltage of the objective lens when the calculated sum is maximum or adjacent sums are substantially equal;
An autofocus device characterized by comprising:

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