JPH06175009A - Autofocusing controller - Google Patents

Abstract

PURPOSE:To provide an autofocusing controller capable of performing accurate focusing by irradiating a proper position on a sample with measuring light for detecting focus. CONSTITUTION:While the reflected light image of a laser beam radiated from a laser light source 1 toward the surface 5a of the sample through an objective lens 4 is oscillated by an oscillation mirror 7, it is received by a front focus detection part 10, a rear focus detection part 11 and a focus detection part 12, and the lens 4 is focused on the surface 5a of the sample based on the change of the intensity of the reflected light beam for measuring by them. The image of the surface 5a of the sample irradiated with the light beam from a light source 101 is fetched in a frame memory 109 through a CCD camera 107. An MPU 110 judges the change state of the surface 5a of the sample in the irradiation range by the laser beam based on the gradation difference of the image data fetched in the frame memory 109. When the change state is not within an allowable range, the irradiating position of the laser beam is changed to a place where the change state is within the allowable range.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic focus control device for focusing an objective lens of a semiconductor inspection device or an optical microscope on a sample.

[0002]

2. Description of the Related Art As an apparatus of this type, Japanese Patent Laid-Open No. 62-909.
The one shown in Japanese Patent Publication is known. FIG. 7 shows a schematic configuration of the optical system. In the figure, 1 is a laser light source, 2 is a beam expander that expands the luminous flux of the laser light from the laser light source 1, 3a and 3b are half mirrors, and the laser light that has passed through the lower half mirror 3b is the objective lens 4 And is imaged on the measurement surface 5a on the stage 5 (for example, the surface of the sample mounted on the stage 5). The reflected light of the laser light on the measurement surface 5a is reflected by the half mirror 3b and guided to the focus detection device 6. The focus detection device 6 reciprocally oscillates the reflected light image of the laser light guided from the half mirror 3b by the oscillating mirror 7 at a constant cycle, and through the imaging lenses 8 and 9, the front pin detection unit 10 and the rear pin detection unit. 11 and the focusing pin detection unit 12, and these apertures A11 to A1
The photoelectric detectors D11 to D1n, D21 to D2n, and D detect the intensities of the reflected light that has passed through n, A21 to A2n, and A. In the figure, reference numerals 13 to 16 and M11 to M1n and M21 to M2n are half mirrors or mirrors for guiding the reflected light to the apertures A11 to A1n, A21 to A2n and A, respectively.

The aperture A of the focusing pin detector 12 is
It is at a position conjugate with the measurement surface 5 a with respect to the objective lens 4 and the imaging lens 9. Aperture A of front pin detector 10
11 to A1n are located in front of the position conjugate with the measuring surface 5a with respect to the objective lens 4 and the imaging lens 8, and the amount of deviation increases from the aperture A11 near the imaging lens 8 to the farthest aperture A1n. . Rear pin detector 1
The apertures A21 to A2n of 1 are behind the position conjugate with the measurement surface 5a with respect to the objective lens 4 and the imaging lens 8, and the amount of deviation is close to the aperture A.
It becomes larger from 21 toward the farthest aperture A2n.

In the above apparatus, the intensity of the reflected light passing through the apertures A11 to A1n, A21 to A2n, A is increased or decreased in synchronization with the vibration of the vibrating mirror 7, and the amplitude thereof matches the focal position of the objective lens 4. Maximum on top. The lens control device 17 utilizes such a property to detect the photoelectric detector D11.
.About.D1n, D21 to D2n, D, the defocus amount of the objective lens 4 is calculated from the output waveforms, and the focus adjustment is performed. FIG. 8 shows an example of the output waveforms of the photoelectric detectors D11 to D1n, D21 to D2n, D when the objective lens 4 is focused on the measurement surface 5a, and FIGS. The photoelectric detectors D1n, D11, (c) of the detector 10 are photoelectric detectors D of the focusing pin detector 12, and (d), (e) are output waveforms of the photoelectric detectors D21, D2n of the rear-pin detector 11, respectively. Is.

[0005]

By the way, when the objective lens 4 is focused on the surface of the sample placed on the stage 5 in the above-mentioned apparatus, there is a shape change portion such as an edge in the laser light irradiation range on the sample. The material is required to be uniform. For example, when irradiating a silicon substrate with laser light, if a resist layer is included in the irradiation range, reflected light is affected and disturbed, and accurate focus adjustment cannot be performed. Incidentally, FIGS. 9 (a) to 9 (e) show examples in which the reflected light intensity waveforms shown in FIGS. 8 (a) to 8 (e) are disturbed by the foreign portion existing in the irradiation range of the laser light.

When focusing on a semiconductor substrate in the above apparatus, the area available for focus control is steadily decreasing with the recent miniaturization and high integration of circuit patterns, and the measuring light for focus control is being used. It is becoming difficult to irradiate an appropriate position with. For this reason, it is very likely that focus adjustment is deteriorated by performing focus adjustment while irradiating the measurement light to an inappropriate position.

An object of the present invention is to provide an automatic focus control device capable of irradiating an appropriate position with measurement light for focus detection to perform accurate focus adjustment.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to FIG. 1 showing an embodiment. In the present invention, an objective lens 4 facing a sample and a reflected light of a measurement light irradiated toward the sample are received. Then, the automatic focus control device including focus adjusting means 6 and 17 for adjusting the focus of the objective lens 4 based on the change in the measured value when the intensity of the reflected light is measured at a specific position while vibrating the received light image. Applied to. Then, the above-mentioned object is to take in a certain range on the sample including the irradiation range of the measurement light as image data represented by the gradation corresponding to the reflectance of each part of the sample within the range.
08 and the sample state determination means 1 for determining the change state of the surface of the sample in the irradiation range of the measurement light based on the image data captured by the image capturing means 107, 108.
10 and the irradiation position determining means 110 that determines the irradiation position of the measurement light based on the judgment result of the sample state judging means 110.

[0009]

When the image capturing means 106 and 107 captures image data of a certain range on the sample including the measurement light irradiation range, the sample state determination means 110 based on the image data determines the sample in the measurement light irradiation range. The change state of the surface is determined, and the irradiation position of the measurement light is determined based on the determination result.

Incidentally, in the section of means and action for solving the above problems for explaining the constitution of the present invention, the drawings of the embodiments are used for the purpose of making the present invention easy to understand. It is not limited to.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. The same parts as those in the conventional example shown in FIG. 7 are designated by the same reference numerals, and the description thereof will be omitted. In FIG. 1, reference numeral 101 denotes a light source for image processing using a halogen lamp or the like, the illumination light of which is modulated by an optical system 102 and then reflected by a mirror 103 to form half mirrors 104, 3a,
After passing through 3b, it enters the objective lens 4 and forms an image on the measurement surface 5a. The range in which the light source 101 irradiates the measurement surface 5a is set to be a circle having a predetermined radius centered on the irradiation position of the laser light from the laser light source 1. The reflected light of the illumination light of the light source 101 on the measurement surface 5a passes through the objective lens 4 and the half mirrors 3b and 3a, is reflected by the half mirror 104, is expanded by the reflected light optical system 105, and is formed on the image forming surface 106. To reach. As a result, the image of the measuring surface 5a is formed on the image forming surface 106.

The image on the image plane 106 is a CCD camera 107.
Is taken into. The output of the CCD camera 107 is converted into a digital signal by the AD converter 108 and taken into the frame memory 109. The operation of taking image data into the frame memory 109 is controlled by a microprocessor unit (hereinafter referred to as MPU) 110. MP
The U 110 performs predetermined image processing based on the stored contents of the frame memory 109 and the text memory 111.
The frame memory 109 and the text memory 11
1 is a memory of the same size. A stage drive circuit 112 is connected to the MPU 110. Stage drive circuit 1
12 responds to the drive command from the MPU 110 and outputs the stage 5
To drive. The movement of the stage 5 changes the irradiation position of the laser light on the measurement surface 5a.

FIG. 2 shows an example of a correspondence relationship between an image formed on the image forming surface 106 and an image captured in the frame memory 109. In this example, the silicon substrate Bs
An image of the sample TP having the resist pattern Pr formed thereon is formed on the image forming surface 106, and a range Zm surrounded by the dotted line is taken into the frame memory 109. The image data captured in the frame memory 109 is a two-dimensional image in which the brightness of the image on the image plane 106 is expressed in gradations corresponding thereto. For example, when the frame memory 109 is an 8-bit memory, the range of the sample TP. Zm is captured in the frame memory 109 as a two-dimensional image in which 0 is black (hatched portion in the figure) and 255 is white. The point F in the figure is the irradiation position of the laser light.

The number of pixels on the CCD camera 107 corresponding to the image range captured in the frame memory 109 is m × n.
(For example, 640 × 480), which is 1 on the measurement surface 5a.
The pixel size is calculated in advance and stored in the internal memory of the MPU 110. In this embodiment, one pixel is the measurement surface 5
It is 1 μm square on a. The sizes of the frame memory 109 and the text memory 111 are 640 × 480 × 8 bits, and the addresses thereof are 0 to 639 in the horizontal direction (X direction) of the image data from left to right and the upper direction in the vertical direction (Y direction). From 0 to 479.

In this embodiment, when image processing is performed by the MPU 110, the irradiation range of laser light is set to the frame memory 1.
The number of pixels of the image data captured in 09 is determined as follows. The irradiation range of the laser light on the measurement surface 5a increases as the amount of defocation of the objective lens 4 increases. Assuming that the defocus amount is Z, the wavelength of the laser light is λ, and the numerical aperture of the projection lens is NA, the diameter W of the irradiation range of the laser light is W = 2√ ((λ / π · NA) ² + (Z・ NA) ² ). In this embodiment, the laser light source 1 has a wavelength λ = 63.
A He-Ne laser of 2.8 nm was used, and the numerical aperture of the objective lens 4 was NA = 0.55. FIG. 3 shows the relationship between the defocus amount and the laser beam diameter under such conditions. Here, assuming that the variation in the height direction of the measurement surface 5a of the stage 5 (the surface of the sample TP) is 5 μm at the maximum, the objective lens 4 when the measurement is repeated while moving the stage 5 is performed.
The fluctuation range of the defocus amount is ± 5 μm. As shown in FIG. 3, the diameter of the laser light when the defocus amount is 5 μm is 6 μm with the fractional part rounded up. Therefore, the irradiation range of the laser light on the sample TP is a circle having a maximum diameter of 6 μm. Since a 1 μm square on the measurement surface 5a corresponds to one pixel of the image data of the frame memory 109, the above irradiation range is 6 pixels in the X direction and the Y direction in the image data, that is, a total of 6 × 6. This corresponds to a range of 36 pixels.

Next, the operation of the apparatus of this embodiment will be described with reference to FIGS. FIG. 5 is a flowchart showing an image processing procedure in the MPU 110. The image of the measurement surface 5a illuminated by the illumination light of the light source 101 is displayed in the frame memory 109.
5 is started. In the first step S1, the secondary differential value of each pixel of the image data fetched in the frame memory 109 is calculated, and the absolute value is stored in the text memory 111 at the same address (Xi, Yj). This secondary differential value is obtained by applying the differential filter of FIG. 4 to the gradation of the pixel at the address (Xi, Yj) of the frame memory 109 and the gradations of the eight pixels surrounding it. That is, the gradation of the address (Xi, Yj) is I (X
i, Yj) and the differential value is D (Xi, Yj): D (Xi, Yj) = 8I (Xi, Yj) -I (Xi-1, Yj-
1) -I (Xi, Yj-1) -I (Xi + 1, Yj-1) -I (Xi-1, Yj) -I (Xi + 1, Yj) -I (Xi-1, Yj + 1) ) -I (Xi, Yj + 1) -I (Xi + 1, Yj + 1). Address of text memory 111 (Xi, Y
In j), the absolute value | D (Xi, Yj) | of the calculation result is stored. The outermost pixel (i = 0, 63) of the image data
9, j = 0,479) is processed as D (Xi, Yj) = 0.

When the processing in step S1 is completed, the data in the text memory 111 is then divided into blocks of a predetermined size in step S2, and the address of each block is defined. Such processing will be described with reference to FIG. In step S1, the data as shown in FIG.
Suppose that it was taken into. In the figure, the center position of the irradiation range of the laser light is at the point F, and 6 × 6 pixels around the point F are the irradiation range of the laser light described above. The center position of the irradiation range of the laser light is given to the MPU 110 in advance,
In step S2, the data in the text memory 111 is divided into blocks of 6 × 6 pixels with reference to 6 × 6 pixels centered on the point F. The dividing position is indicated by an arrow B in FIG. Then, as shown in FIG. 6 (b), the blocks of each block are moved in the order of moving away from the laser beam center position F while tracing each block counterclockwise around the block in the laser beam irradiation range. Define the address.

After the processing of step S2 is completed, the address of the block to be judged is set to an initial value of 1 in step S3, and 2 of the 36 pixels forming the block corresponding to the address specified in step S4 are set. The secondary differential value is read from the text memory 111. In step S5, the read second-order differential value of each pixel is compared to select the maximum value M. In the next step S6, it is determined whether or not the maximum value M is less than or equal to a preset allowable value TH. The allowable value TH is empirically determined according to the material of the sample TP. In the example of FIG. 6, it is set to 10.

When it is determined in step S6 that the maximum value M exceeds the allowable value TH, 1 is added to the designated value of the block address in step S7, and the process returns to step S4.
When step S6 is affirmed, the process proceeds to step S8,
The stage 5 is driven so that the irradiation position of the laser beam moves to the center position of the block corresponding to the designated address, and the process is ended.

In the above processing, since the gradation of each pixel of the image data fetched in the frame memory 109 in step S1 is differentiated, the data fetched in the text memory 111 is the same as the image fetched in the frame memory 109. The greater the change in gradation, that is, the greater the change in reflected light intensity from the measurement surface 5a, the greater the value. Therefore, if there is a portion where the reflected light intensity greatly changes in the blocks divided in step S2, step S6 is denied,
Step S6 is affirmed for the first time when a block whose reflected light intensity hardly changes is designated. In the case of the example in FIG. 6A, the maximum value M of the second-order differential values exceeds 10 in blocks up to, so step S6 is denied, and step S6 is affirmed when the block is designated. Then, in step S8, the irradiation position of the laser light moves to the center position G of the block. Since the size of the blocks divided in step S2 is adjusted to the irradiation range of the laser light when the defocus amount of the objective lens 4 is maximum,
As long as the center position G of the block is the irradiation position of the laser light, the position where the reflected light intensity greatly changes within the irradiation range of the laser light, in the example of FIG. 2, the boundary position between the silicon substrate Bs and the resist pattern Pr and the silicon. The position of the step on the substrate Bs is not included.

As described above, in the present embodiment, in step S6, the change state of the measurement surface 5a within the laser light irradiation range is discriminated to determine the suitability as the laser light irradiation position. Measurement surface 5a until a large area is found
Since the determination of the change state is repeated, it is possible to always obtain high focusing accuracy by irradiating the appropriate position where the surface state of the sample TP hardly changes with laser light.

Further, when the first block is judged to be unsuitable, the suitability is judged in the order of blocks close to this, and so on. Therefore, the amount of deviation between the selected focus detection position and the first irradiation position is small, and these The deterioration of the focusing accuracy due to the displacement of the position is minimized. By the way, Figure 6
In the example of (a), when the focus detection is performed at the point G and the reflected light intensity of the sample TP by the laser light is measured at the point F, if the distance between the points F and G is large, the measurement surface between them is large. Due to the change in the shape of 5a, the focusing accuracy at the point F may be reduced, and the measurement accuracy at the point F may be affected.

When the block is not suitable in the example of FIG. 6, if the block to be judged next is shifted within the range of the number of pixels of 5 or less, an appropriate position as the focus detection position is found at a position closer to the point G. There is a possibility. However, if the first block is unsuitable, the measurement surface 5a often changes greatly even in the immediate vicinity thereof, and searching for only the vicinity of the point F may unnecessarily increase the processing time. Therefore, the division position of each block needs to be determined in consideration of the size of one pixel on the measurement surface 5a and the degree of change in the surface shape of the sample TP.

If the set value of the size of one pixel on the measurement surface 5a is changed, the number of pixels corresponding to the irradiation range of the laser beam (6 × 6 pixels in the embodiment) changes, and if the number of pixels is small, the processing speed is reduced. However, there is a possibility that the resolution may be deteriorated and inconvenience may occur in a semiconductor having a fine line width. Therefore, the size of one block needs to be determined in consideration of processing speed and resolution. When the processing of FIG. 5 is executed after the focus of the objective lens 4 is once adjusted by the lens controller 17 regardless of the state of the measurement surface 5a, the correspondence between the irradiation range of laser light and the number of pixels of image data The variation range of the defocus amount of the objective lens 4 that should be considered when calculating
Since it is possible to compress from the shape error range of the measurement surface 5a to the error range of the focusing accuracy of the objective lens 4, the number of pixels corresponding to the irradiation range of the laser light is reduced to compress the amount of data to be processed, thereby improving the processing speed. Can be planned.

In the embodiment, the image data captured in the frame memory 109 is differentiated to emphasize the change of the measurement surface 5a. However, when the image data having a large gradation difference is obtained at the stage of the frame memory 109, it is processed as it is. May be. In addition, the processing may be appropriately changed according to the state of the image data, such as determining the presence or absence of an edge after binarizing the image data in the frame memory 109. In the embodiment, the data of the block corresponding to the irradiation range of the laser light is fetched from the text memory 111 each time the designated range is determined to be unsuitable, and the determination is repeated. However, the data of the frame memory 109 and the text memory 111 are fetched at once. The position where the change in reflected light intensity is small may be determined with.

In the above embodiment, the focus detection device 6 and the lens control device 17 serve as the focus adjustment means and the CCD camera 1
08 and the frame memory 109 constitute image capturing means, and the MPU 110 constitutes sample state determining means and irradiation position determining means. Further, in the embodiment, the laser light from the laser light source 1 corresponds to the measurement light. However, the present invention is also applicable to a device that uses light other than laser light as the measurement light.

[0027]

As described above, according to the present invention, the state of the surface of the sample in the irradiation range of the measurement light is judged from the image data of the fixed range including the irradiation range of the measurement light. If the position where the change in the surface condition of the sample is within the predetermined allowable range is selected as the irradiation position of the measurement light, accurate focus adjustment can always be performed.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of an apparatus according to an embodiment of the present invention.

FIG. 2 is a measurement surface 5a and a frame memory 10 of the apparatus shown in FIG.
7 is a diagram showing a correspondence relationship with images captured in FIG.

FIG. 3 is a diagram showing a relationship between a defocus amount of an objective lens 4 of the apparatus of FIG. 1 and a size of an irradiation range.

FIG. 4 is a diagram showing a differential filter used in the MPU 110 of FIG.

5 is a diagram showing an image processing procedure in the MPU 110 of FIG.

FIG. 6 is a diagram showing a correspondence between data in the text memory 111 of FIG. 1 and addresses of blocks obtained by dividing the data.

FIG. 7 is a diagram showing an outline of an optical system of a conventional automatic focus control device.

FIG. 8 is a diagram showing a normal example of an output waveform of the photoelectric detector of FIG.

9 is a diagram showing an example of an abnormal output waveform of the photoelectric detector shown in FIG.

[Explanation of symbols]

 1 Laser Light Source 4 Objective Lens 5a Stage Measurement Surface (Sample Surface) 6 Focus Detection Device 17 Lens Control Device 101 Light Source for Image Processing 107 CCD Camera 109 Frame Memory 110 MPU TP Sample

Claims (1)

[Claims] 1. An objective lens facing a sample, and receiving reflected light of measurement light irradiated toward the sample,
In the automatic focus control device including a focus adjustment unit that adjusts the focus of the objective lens based on a change in a measurement value when the intensity of the reflected light is measured at a specific position while vibrating the received light image, the measurement A certain range on the sample including the irradiation range of light,
Image capturing means for capturing as image data represented by gradation corresponding to the reflectance of each part of the sample within the range, and of the sample in the irradiation range of the measurement light based on the image data captured from the image capturing means. Automatic focus control, comprising: sample state determination means for determining a change state of the surface; and irradiation position determination means for determining the irradiation position of the measurement light based on the determination result of the sample state determination means. apparatus.