JP2594715B2 - Automatic focusing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for automatically adjusting the focus of an optical system when magnifying an image of a semiconductor wafer having scribe lines formed thereon in various measuring apparatuses such as a film thickness measuring apparatus. .

[0002]

2. Description of the Related Art Conventionally, as this type of method, an automatic focusing method utilizing the contrast (brightness / darkness ratio) of a captured image is known. Hereinafter, the conventional method will be described with reference to FIG. FIG. 8 is an enlarged image of the semiconductor wafer taken by a television camera via the epi-illumination type microscopic optical system. In the drawing, S _X and S _Y indicate scribe lines formed on the semiconductor wafer.

When a semiconductor wafer having a scribe line formed thereon is observed by an epi-illumination type microscopic optical system, the edge of the scribe line becomes darker than other regions, and a contrast is obtained in an observed image. This is because the pattern area where the circuit element is formed has one or more films formed more than the scribe line area, so that the edge of the scribe line has a step due to a height difference from the pattern area. For this reason, it is inclined on the substrate surface. That is, in the epi-illumination type microscopic optical system, the light illuminating the substrate is limited to the light emitted from the objective lens, and when the illumination light is applied to the edge of the scribe line, the edge of the scribe line is inclined. for,
Reflected out of the entrance pupil of the epi-illumination type microscopic optical system,
The edge part of the scribe line becomes dark in the observation image.

[0004] Therefore, the captured image as shown in FIG. 8 obtained through the epi-illumination type microscope optics, detection line L _X of orthogonal vertical and horizontal, set L _Y, the detection line L _X,
Differentiating the image data along the L _Y, obtaining the peak value of the derivative data. This peak value takes the largest value when the epi-illumination type microscopic optical system is in focus. Therefore, the focal point of the optical system can be automatically adjusted by changing the distance between the epi-illumination type microscopic optical system and the semiconductor wafer and finding the position where the peak value becomes maximum.

[0005]

However, the prior art having such a configuration has the following problems. As described above, in the conventional method, the automatic focusing is performed by differentiating the image data along the orthogonal vertical and horizontal detection lines L _X , L _Y , so that the scribe lines S _X , S _Y and the detection line L _X ,
When L _Y overlaps, the detection lines L _X and L _Y do not intersect the edge of the scribe line, so that it is not possible to detect contrast on the detection line, so that automatic focusing cannot be performed. . In such a case, according to the conventional method, the detection line is moved or the semiconductor wafer is moved, and again,
It is necessary to perform the processing along the detection line, and the processing efficiency is reduced accordingly.

The present invention has been made in view of such circumstances, and provides a method capable of reliably detecting a contrast and performing automatic focusing without correcting movement of a detection line. It is intended to be.

[0007]

The present invention has the following configuration to achieve the above object. That is, the automatic focusing method according to the present invention captures an enlarged image of the substrate surface by enlarging the image of the substrate surface on which the scribe line is formed through an epi-illumination type microscopic optical system, and closes the image in the captured image. A detection line for forming a figure is set, the contrast of the captured image is detected along the detection line, and the most contrast is obtained from among the contrasts obtained by varying the distance between the substrate surface and the microscope optical system. The position where a clear contrast is obtained is set as the focal position.

[0008]

According to the present invention, since the detection line for forming the closed figure is set on the enlarged captured image of the substrate surface, the detection line always intersects the edge of the scribe line. When the edge of the detection line and the edge of the scribe line intersect, the contrast is detected at the intersection,
Automatic focusing is ensured.

[0009]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of a film thickness measuring apparatus using the automatic focusing method according to the present invention. In FIG. 1, reference numeral W denotes a substrate such as a semiconductor wafer having scribe lines formed on the surface. The substrate W is
It is mounted on a Z stage 1 that moves along the optical axis of an epi-illumination type microscopic optical system described later. Reference numeral 2 denotes an objective lens arranged to face the substrate W, and the illumination light emitted from the light source 3 falls on the substrate W via the half mirror 4 and the objective lens 2. The reflected light from the substrate W enters the perforated mirror 5 via the objective lens 2 and the half mirror 4. The aperture mirror 5 is a total reflection mirror having a small hole 5a formed on the optical axis. The reflected light from the substrate W passes through the small hole 5a of the perforated mirror 5 and the spectroscope 10 for film thickness measurement.
And is reflected by the perforated mirror 5 and enters the television camera 6. An epi-illumination type microscopic optical system is configured by a series of optical means including the objective lens 2, the light source 3, the half mirror 4, and the perforated mirror 5.

[0010] The substrate W captured by the television camera 6
Is converted to C through the image processing unit 7.
By being provided to the RT 8, an enlarged image of the surface of the substrate W is displayed on the CRT 8. The image processing unit 7
The focus position of the optical system is detected by performing an automatic focusing process described later on the input multi-tone image on the substrate surface, and the detection signal is given to the Z stage drive circuit 9. The Z stage drive circuit 9 drives the Z stage 1 based on the detection signal, thereby performing focusing of the optical system. After such focusing, the substrate W is aligned to set the film thickness measuring spot at a desired position on the substrate W, and the spectroscope 10 enters the reflected light from the film thickness measuring spot. As a result, the film thickness is measured.

The automatic focusing process of the apparatus according to the present embodiment will be described below with reference to the flowcharts shown in FIGS. Step S1: The count values m and n of the counter in the image processing unit 7 are reset. The count value m is the number of times of image capturing when the Z stage 1 is set at a certain height. In addition, the count value n is set in the Z stage 1
Is the number of times the height is set.

Step S2: The count value m of the counter,
After resetting n, the enlarged multi-tone image of the substrate W captured by the television camera 6 is taken into the image processing unit 7. Since the image captured by the image processing unit 7 is an image captured through the epi-illumination type microscopic optical system as described above, contrast is obtained at the edge of the scribe line.

Step S3: A detection line for forming a closed figure is set for the image data taken into the image processing unit 7, and the image data is differentiated along the detection line. Here, as shown in FIG. 4, the detection line L ₁ ~L ₄ diamond is set. By setting such a diamond-shaped detection line, the scribe lines S _X , S
Regardless of the orientation of _Y , the detection line always intersects the edge of the scribe line, so that the contrast can be detected at the intersection.

The symbol P in FIG. 4 is a spot for measuring the film thickness. In the apparatus of the present embodiment, since the substrate surface is imaged by the television camera 6 via the perforated mirror 5, a portion corresponding to the small hole 5a of the perforated mirror 5 in the captured image, that is, the film thickness measurement spot The part appears dark. If the vertical and horizontal detection lines L _X and L _Y passing through the center of the image are set for such an image as in the conventional method, the detection lines L _X and L _Y pass through the film thickness measuring spot P. As a result, the portion of the measurement spot P is taken in as contrast data, resulting in an erroneous focusing operation of the optical system. However, in this embodiment, since the set of detection lines L ₁ ~L ₄ to include a measurement spot P therein, detection lines L ₁ ~L ₄
Does not pass through the measurement spot P, and the influence of the measurement spot P can be avoided.

[0015] Step S4: FIG. 5 is a waveform diagram of the differential data along the detection line L ₁ ~L ₄ obtained by the process of step S3. In step S4, obtaining the maximum vertical variation width d _m from the differential data. The maximum vertical fluctuation range d _m is utilized as contrast data for automatic focusing.

Steps S5 to S7: count value m of the counter
Is counted up, and steps S2 to S2 are performed until m = 3.
By repeating the process of S4 three times, the maximum vertical fluctuation widths d ₁ , d ₂ , and d ₃ are obtained.

Step S8: The maximum vertical fluctuation width d ₁ ,
An average value D _n of d ₂ and d ₃ is calculated. Such averaging processing is performed to reduce the influence of noise in the image data.

Steps S9 to S11: The count value n of the counter is counted up. If the count value n is "1", the Z stage 1 is raised by a predetermined distance H, and the process returns to step 2. , by performing the process up to step S8, and calculates the average value D ₂ of the maximum vertical fluctuation range in the new set position of the Z stage 1.

Steps S12 and S13: If the count value n of the counter is "2", the Z stage 1 raised in step SS11 is lowered by a distance 2H, and then step S12 is performed.
Returning to 2, and calculates the average value D ₃ of the maximum vertical variation width in the same manner as described above a new setting position.

Steps S14 and S15: The average value of the maximum vertical fluctuation width at each set position of the Z stage 1 obtained in the above steps (hereinafter simply referred to as "variation width").
The magnitude relationships of D ₁ , D ₂ and D ₃ are compared. Z stage 1
For variation width D ₁ in the initial setting position is greater than the other variation width D _2, D _3, the variation range D _1, D _2, D ₃
Can be approximately represented as a point on a quadratic curve, as shown in FIG. This quadratic curve shows the relationship between the set position of the Z stage 1 and the maximum vertical fluctuation width of the differential data along the detection line. As described above, when the Z stage 1 is at the focal position, the contrast appears most clearly, that is, the maximum vertical fluctuation width has a maximum value. Therefore, conversely, by obtaining the maximum value D _MAX of the quadratic curve, it is possible to calculate the focus position. Upon obtaining such a local maximum value D _MAX, if the variation range D ₁ is greater than the other variation width D _2, D ₃ (the state shown in FIG. 6), be determined with relatively accuracy the maximum value D _MAX it can. On the other hand, if the larger of D ₂ or D ₃ than the variation width _{D 1, D 1, D 2} , D 3 left than the maximum value D _MAX, or so biased to the right, the maximum value D _MAX detection accuracy Gets worse. Therefore, in such a case, the process proceeds to step S16.

Step S16: Here, Z stage 1
Is set to a larger value (H + ΔH).
Then, the processing of steps S1 to S13 described above is repeated.
And satisfies the conditions of steps S14 and S15
Fluctuation range D₁, D_Two, D _ThreeIs detected.

Step S17: Steps S14 and S15
When the fluctuation widths D ₁ , D ₂ , and D ₃ satisfying the following condition are detected, these values are substituted into the following approximate polynomial to obtain the position of the Z stage 1 having the maximum value D _MAX , that is, the focal position. Find H ₀ .

The approximate polynomial for obtaining a maximum value D _MAX is represented by the following formula (1). y = AX ² + BX + C (1) In the above equation (1), y is the maximum vertical fluctuation width, X is the set position of the Z stage 1, and A, B, and C are unknown coefficients (A <0).
It is.

The focal position H _{0 at} which the maximum value D _MAX is obtained is obtained by the following equation (2) since the first derivative of the above equation (1) becomes “0”. H ₀ = −B / 2A (2)

To determine the unknown coefficients A, B, C,
When the set positions of the fluctuation ranges D ₁ , D ₂ , D ₃ and the Z stage 1 are respectively substituted into the expression (1), the following is obtained.
Here, the initial setting position of the Z stage 1 is X = 0,
The ascending position is X = H, and the descending position is X = -H. D ₁ = C (3) D ₂ = AH ² + BH + C (4) D ₃ = AH ² -BH + C (5)

By solving the above equations (3) to (5), the coefficients A and B can be expressed as follows. A = (D ₂ + D ₃ −2D ₁ ) / 2H ² (6) B = (D ₂ −D ₃ ) / 2H (7)

Substituting equations (6) and (7) into equation (2)
And the maximum value D_MAXFocus position H ₀Is given by the following equation (8)
Is represented as H₀= ｛(D_Three-D_Two) / (D_Two+ D_Three-2D₁)｝ × H / 2 (8)

That is, the detected fluctuation ranges D ₁ , D ₂ ,
By applying D ₃ and the displacement width H of the Z stage 1 to the above equation (8), the Z stage 1 having the maximum value D _MAX is obtained.
, That is, the focal position H ₀ can be obtained.

In the above-described embodiment, a diamond-shaped detection line is used as an example of a detection line for forming a closed figure. However, the present invention is not limited to this. For example, as shown in FIG. can be used to detect line forming a circular or detection line L _5, such as the detection line L ₆ triangles as shown in FIG. (b), any closed figure.

The method for obtaining the contrast of the captured image along the detection line is not particularly limited. That is, in the above-described embodiment, the change in the maximum vertical fluctuation width associated with the movement of the Z stage 1 is approximated by a quadratic polynomial. However, by using a stricter polynomial and calculating the maximum value of the approximate polynomial, The focus position may be obtained. Conversely, when the focus position does not require much accuracy, it is not necessary to determine the focal position by using an approximate polynomial as in the embodiment. For example, the initial position of the Z stage 1 and the vertical displacement The maximum vertical fluctuation widths D ₁ , D ₂ , and D ₃ may be obtained at the respective positions, and the position of the Z stage 1 having the maximum value among them may be the focal position.

Further, in the embodiment, a film thickness measuring apparatus is taken as an example of an apparatus to which the automatic focusing method according to the present invention is applied. It is needless to say that the present invention can also be applied to various devices that need to be focused, such as a line width measuring device.

[0032]

As is apparent from the above description, according to the present invention, since the detection line for forming the closed figure is set on the enlarged captured image of the substrate surface, the edges of the detection line and the scribe line are set. Always intersect, and the contrast is detected at the intersection, so that it is not necessary to correct the movement of the detection line as in the conventional method, and the focusing can be performed reliably and quickly.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of a film thickness measuring apparatus as an example using an automatic focusing method according to the present invention.

FIG. 2 is a flowchart showing a procedure of a focusing process of the embodiment apparatus.

FIG. 3 is a flowchart illustrating a procedure of a focusing process of the apparatus according to the embodiment.

FIG. 4 is an explanatory diagram of a contrast detection line set on a captured image.

FIG. 5 is a waveform diagram of differential data obtained by differentiating image data along a detection line.

FIG. 6 is a diagram for explaining a method of obtaining a focal position from each data of the maximum vertical fluctuation width of differential data.

FIG. 7 is a diagram illustrating another example of setting a detection line.

FIG. 8 is a diagram for explaining a conventional automatic focusing method.

FIG. 9 is a diagram for explaining a problem of the conventional method.

[Explanation of symbols]

 W ... Substrate 1 ... Z stage 2 ... Objective lens 3 ... Light source 4 ... Half mirror 5 ... Perforated mirror 6 ... TV camera 7 ... Image processing unit 8 ... CRT 9 ... Z stage drive circuit 10 ... Spectroscope

Continuation of the front page (56) References JP-A-62-245102 (JP, A) JP-A-4-95811 (JP, A) JP-A-63-7626 (JP, A)

Claims (1)

(57) [Claims] 1. A magnified image of a substrate surface on which a scribe line is formed is taken through an epi-illumination type microscopic optical system to capture an enlarged image of the substrate surface, and a detection line for forming a closed figure in the captured image is provided. Setting, detecting the contrast of the captured image along this detection line, and the position where the sharpest contrast is obtained from the respective contrasts obtained by varying the distance between the substrate surface and the microscope optical system. An automatic focusing method, wherein is a focal position.