JP2003068612A - Apparatus for inspecting superposition - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus for inspecting superposition that accurately inspects the mount of slippage of superposition by properly correcting the influence of distortion aberrations of an optical system. SOLUTION: The device for inspecting overlap, measuring the slippage amount of overlap of first and second marks, is provided with lighting systems (1-9) for illuminating a overlap mark composed of the first and second marks formed on the surface (11) of an object, imaging optics (9, 16) for forming an image of the overlap mark on an image surface, and detecting systems (17 and 18) for detecting the image of the overlap mark formed on the image surface. Based on distortion aberrations of the imaging optics and decentering of an optical axis, the positions of feature points of the first and second marks are compensated according to a coordinate within the image surface, and the slippage amount of overlap is found, based on the position of the compensated feature point.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overlay inspection apparatus, and more particularly to a pattern overlay inspection in a manufacturing process of a semiconductor element or a liquid crystal display element.

[0002]

2. Description of the Related Art In the manufacture of semiconductor elements and liquid crystal display elements,
A photoresist is applied on the substrate, and the pattern is transferred by an exposure device to form a predetermined pattern. And
An element having a predetermined function is formed by repeating a process of performing processing such as etching or vapor deposition on the pattern. At this time, it is necessary to accurately form a resist pattern for the next process on the previously formed pattern.

In the overlay inspection, it is inspected whether or not the resist pattern is correctly formed with respect to the previous pattern. In the conventional overlay inspection, the outer mark formed in the previous process and the inner mark formed on the resist by exposure are magnified and observed with a microscope, and the misalignment between the outer mark and the inner mark is determined by the amount of overlay misalignment. Is detected as.

[0004]

However, in the magnified observation with the microscope in the conventional overlay inspection, the image is deformed depending on the position of the visual field due to the aberration of the optical system, particularly the distortion aberration, and the overlay misalignment amount is accurately measured. There was the inconvenience of not being able to do it. Therefore, a specially designed optical system that suppresses various aberrations including distortion is adopted to minimize image distortion in the field of view.

In recent years, the line width of a pattern of a semiconductor device or the like has been about 0.1 μm, and the line width is 1 of the overlay accuracy.
An accuracy of about / 3, that is, an overlay accuracy of about 30 nm is required. In order to perform the overlay inspection with such accuracy, a sensitivity of 3 nm to 5 nm is required as the detection sensitivity of the overlay deviation amount. On the other hand, the size of the outer mark used for overlay inspection is 20
The image distortion is 0.01% because it is about 30 μm
It is necessary to keep below. However, there is a limit in designing and manufacturing with suppressed distortion in a microscope having a high magnification for observing such a small mark.

The present invention has been made in view of the above-mentioned problems, and an overlay inspection apparatus capable of properly correcting the influence of distortion aberration of an optical system and accurately inspecting an overlay deviation amount. The purpose is to provide.

[0007]

In order to solve the above-mentioned problems, according to the present invention, an illumination system for illuminating an overlay mark composed of a first mark and a second mark formed on an object surface, An image forming optical system for forming an image of the overlay mark on an image plane, and a detection system for detecting an image of the overlay mark formed on the image plane,
In an overlay inspection apparatus that measures an overlay deviation amount between the first mark and the second mark, the first mark and the second mark are detected based on the distortion aberration of the imaging optical system and the eccentricity of the optical axis. There is provided an overlay inspection apparatus characterized in that the position of a feature point is corrected according to the coordinates in the image plane, and the overlay deviation amount is obtained based on the corrected position of the feature point.

According to a preferred aspect of the present invention, the magnification of the image forming optical system is β, the distortion aberration coefficient of the image forming optical system is D, and the coordinates indicating the center of the optical axis on the image plane (on the image plane). When the corrected coordinate origin in () is (Ξ, Η), the coordinates (x ', y') on the object plane of the feature point corrected with respect to the coordinates (X, Y) in the image plane are expressed as x '= (X-Ξ) / β-D × (X-Ξ) × [(X-Ξ) ² ＋ (Y-Η) ² ] / β ³ y' ＝ (Y-Η) / β-D × ( Y-Η) x [(X-Ξ) ² + (Y-Η) ² ] / β ³ is calculated.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In an overlay inspection apparatus,
Since the imaging optical system and the imaging surface (image surface) are fixed,
The image distortion is uniquely determined by the position of the visual field. Therefore, in the present invention, the overlay deviation amount is calculated after correcting the image of the overlay mark formed via the imaging optical system according to the position in the field plane. At this time, the coordinates (X, Y) of the image are corrected coordinates (x ′,
By converting into y ′), the influence of the distortion aberration of the imaging optical system can be removed. In other words, the coordinates (x ', y') on the object plane of the feature point corrected with respect to the coordinates (X, Y) in the image plane can be obtained according to the following equation.

X '= (X-Ξ) / β-D × (X-Ξ) × [(X-Ξ) ² + (Y-Η) ² ] / β ³ y' = (Y-Η) / β -D × (Y-Η) × [(X-Ξ) ² + (Y-Η) ² ] / β ³ where β is the magnification of the imaging optical system and D is the distortion aberration of the imaging optical system. It is a coefficient (distortion coefficient). Further, (Ξ, Η) is a coordinate (correction coordinate origin on the image plane) indicating the center of the optical axis on the image plane.

Embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a diagram schematically showing a configuration of an overlay inspection apparatus according to an embodiment of the present invention. The overlay inspection apparatus of FIG. 1 includes an observation system for observing a mark on the wafer 11 which is an object to be inspected. Here, the observation system includes an observation light source 1 such as a halogen lamp. The illumination light supplied from the observation light source 1 is condensed by the collector lens 2 and then the wavelength selection filter 3
Incident on.

The illumination light whose desired wavelength range (for example, 530 nm to 800 nm) is selected by the wavelength selection filter 3 is incident on the half mirror 7 via the first relay lens 4, the second relay lens 5 and the condenser lens 6. To do. The illumination light reflected by the half mirror 7 enters the dichroic mirror 8. Dichroic mirror 8
Has a characteristic of transmitting the illumination light whose wavelength is selected through the wavelength selection filter 3. Therefore, the illumination light transmitted through the dichroic mirror 8 illuminates the wafer 11 placed on the stage 10 via the objective lens 9.

In the optical path between the first relay lens 4 and the second relay lens 5, the angular range of the illumination light beam incident on the wafer 11 is located at a position substantially optically conjugate with the pupil plane of the objective lens 9. (Ie, wafer 11
Aperture stop 12 for limiting the luminous flux that contributes to the illumination of
Is provided. A field stop 13 for defining the shape of the illumination area formed on the wafer 11 is provided at a position that is optically conjugate with the wafer 11 in the optical path between the second relay lens 5 and the condenser lens 6. It is provided.

Further, the stage 10 is constructed so as to be movable along the optical axis of the objective lens 9 and two-dimensionally movable along a plane perpendicular to the optical axis of the objective lens 9. The wafer 11 is vacuum-sucked by the wafer holder 14 attached to the stage 10. Further, the wafer holder 14 is configured to be rotatable about a rotation shaft 14c arranged near the center of the wafer 11. For this purpose, a rotation adjusting motor 15 is attached to the rotating shaft 14c.

Light (reflected light, diffracted light, scattered light) from the illuminated mark on the wafer 11 enters the half mirror 7 through the objective lens 9 and the dichroic mirror 8. The light transmitted through the half mirror 7 forms a mark image on the wafer 11 on the image pickup surface of the image pickup device 17 via the second objective lens 16. The image pickup device 17 is configured by laying out substantially square pixels of a CCD (charge coupled device) in a direction orthogonal to each other. The mark image detected by the image sensor 17 is photoelectrically converted, an image signal corresponding to the light intensity is converted into a digital signal, and the digital signal is supplied to the image processing system 18.

The overlay inspection apparatus of FIG. 1 detects the focus shift of the wafer 11 and adjusts the height position of the wafer holder 14 so as to keep the distance between the objective lens 9 and the wafer 11 constant. It has an auto focus system. Here, the autofocus system is a light source that supplies light having a wavelength different from the observation light, for example, the infrared LED light source 1
9 is equipped. The infrared illumination light supplied from the AF light source 19 illuminates the field stop 21 via the collector lens 20. The infrared illumination light transmitted through the slit-shaped opening (light transmitting portion) of the field stop 21 enters the dichroic mirror 8 via the half mirror 22 and the relay lens 23.

The dichroic mirror 8 is the AF light source 1
9 has the property of reflecting infrared illumination light. Therefore, the infrared illumination light reflected by the dichroic mirror 8 illuminates the wafer 11 via the objective lens 9. In this way, a slit-shaped illumination area is formed on the wafer 11 (that is, a slit-shaped infrared light flux is irradiated). In this case, the reflected light from the wafer 11 irradiated with the slit-shaped infrared light flux enters the dichroic mirror 8 via the objective lens 9.

The light reflected by the dichroic mirror 8 enters the half mirror 22 through the relay lens 23. The light reflected by the half mirror 22 enters the pupil division prism 25 via the relay lens 24. The pupil division prism 25 is a pupil division element for dividing the incident light flux into two light fluxes in the vicinity of the pupil conjugate plane of the objective lens 9 and guiding the light fluxes in different directions. In this way, the light beam divided into two by the pupil division prism 25 is formed by the imaging lens 26.
And, two images are formed on the image pickup surface of the one-dimensional image pickup device (line sensor) 28 via the cylindrical lens 27.

Here, the pupil splitting prism 25 splits the autofocus light into two and guides the split light in predetermined directions corresponding to the lateral direction of the projected slit light flux. Then, the short-side component of the two slit images divided by the imaging lens 26 forms an image on the one-dimensional image sensor 28. Further, the longitudinal spread of the slit image is compressed by the cylindrical lens 27 and projected on the one-dimensional image pickup device 28 to improve the detection sensitivity of the surface of the wafer 11. The output of the one-dimensional image pickup device 28 is supplied to the AF processing system 29.

In this way, the AF processing system 29 detects the focus shift amount of the wafer 11 with respect to the object plane of the objective lens 9 based on the interval between the two slit images formed on the one-dimensional image pickup device 28. Then, the stage 10 (and thus the wafer holder 14) is driven based on the detected focus shift amount, and the surface of the wafer 11 is moved to the objective lens 9
Is set to the in-focus state.

The autofocus system of this embodiment employs a method of determining the amount of movement of the wafer holder 14 by measuring the distance between the pupil-divided slit images, but this method is used for the autofocus mechanism. There is no limitation. For example, a slit image projection method in which a slit image is obliquely projected to detect a shift in the focus position, and the wafer holder 14 is moved up and down by an amount corresponding to the shift amount,
An image processing method or the like may be used in which an image at a position where the contrast of the image captured by moving the stage in the vertical direction is maximized is used as the image signal.

FIG. 2 is a view showing an overlay mark composed of an outer mark and an inner mark formed on the wafer. Note that FIG. 2 shows a state in which the outer mark and the inner mark are displaced only in the x direction for the sake of clarity. Referring to FIG. 2, the overlay mark is
An outer mark having a square shape of L on one side formed on the first layer and an inner mark having a square shape of S having one side formed on the second layer are overlapped with each other. Here, the size L of the outer mark is about 20 μm to 30 μm, and the size S of the inner mark is about 10 μm to 15 μm.

The overlay deviation amount δx of the inner mark with respect to the outer mark in the x direction is determined by the coordinates (XoL, Y), (XoR, Y) of the edge of the image of the outer mark and the coordinate (XiL, of the edge of the image of the inner mark. From Y), (XiR, Y),
It is expressed by the following equation (1). δx = (XiL + XiR−XoL−XoR) / 2 (1)

Similarly, the overlay deviation amount δy of the inner mark with respect to the outer mark in the y direction is determined by the coordinates (X, YoD), (X, YoU) of the edge of the image of the outer mark and the coordinate of the edge of the image of the inner mark. (X, YiD), (X, YiU)
Therefore, it is expressed by the following equation (2). δy = (YiD + YiU-YoD-YoU) / 2 (2)

However, the coordinates (x, y) on the wafer 11
The point at is due to the imaging optics (9,8,7,16)
A point (X (x, y), on the image plane (image plane) of the image sensor 17
An image is formed on Y (x, y). At this time, if the imaging optical system has distortion, the points on the image plane of the image sensor 17 are expressed by the following equations (3) and (4). Here, β represents the magnification of the imaging optical system, ξ and η represent the decentering of the optical axis, and D represents the distortion aberration coefficient. X (x, y) = β {(x−ξ) + D (x−ξ) [(x−ξ) ² + (y−η) ² ]} (3) Y (x, y) = β {(y −η) + D (y−η) [(y−ξ) ² + (y−η) ² ]} (4)

At the time of measuring the overlay deviation amount, the position coordinates (x, y) of the outer mark can be read when the image is captured. Distortion aberration coefficient D of the imaging optical system,
And the amount of deviation of the optical axis center from the coordinates on the screen ξ, η
Can be considered to be constant unless the imaging optical system is changed. And when the distortion is not so large,
By converting the coordinates X and Y of the approximately captured image into the corrected coordinates (x ′, y ′) on the object surface by the following equations (5) and (6), the distortion aberration of the imaging optical system The effect can be eliminated.

X ′ = (X-Ξ) / β-D × (X-Ξ) × [(X-Ξ) ² + (Y-Η) ² ] / β ³ (5) y ′ = (Y-Η ) / β-D × (Y-Η) × [(X-Ξ) ² ＋ (Y-Η) ² ] / β ³ (6) where (Ξ, Η) is the optical axis on the image plane. Coordinates indicating the center, that is, the corrected coordinate origin. Strictly speaking, the position where the optical axis passes through the image plane is represented by the coordinate system of the image plane. On the other hand, (ξ, η) represents the position where the optical axis passes through the image plane in the coordinate system of the object plane.

An image of the overlay mark in which the center of the outer mark and the center of the inner mark are displaced by δx, δy is imaged when the center of the outer mark is at the coordinates (x, y), and the overlay displacement amount Δx, Measure Δy. The overlay deviation amount δx in the x-axis direction is calculated by averaging the x-coordinates (x + δx−S / 2) and (x + δx + S / 2) of the two edges of the inner mark, and calculating the x-coordinate (x− of the two edges of the outer mark). L /
It is obtained by subtracting the average of 2) and (x + L / 2). However, when the mark image is captured via the image forming optical system having distortion, the image forming position is displaced according to the object height (x, y) from the optical axis as described above, and X is shifted. (X, y), Y (x, y). Therefore, the measured overlay deviation amounts Δx and Δy are expressed by the following equations (7) and (8).

[0029]

[Equation 2]

That is, the original overlay deviation amounts δx, δ
A term that is proportional to the distortion aberration coefficient D and that depends on the coordinates (x-ξ, y-η) from the optical axis of the mark is added to y. The correction coordinate origin (Ξ,
Η) corrected overlay deviation amount Δx ′, Δ
y'is calculated according to the following equations (9) and (10).

[0031]

[Equation 3]

Assuming that the image forming optical system and the image pickup surface (image surface) do not move because they are fixed, the position coordinates (X, Y) of the mark are the outer marks within the visual field (image surface). It is possible to obtain from the position of the edge of the, and a system using a high-precision stage and interferometer to determine the absolute mark position in the field of view and a pattern for strictly measuring the distortion aberration are necessary. There is no.

Since the correction can be performed by using the edge position in the coordinate system in the image plane, the original displacement is measured by measuring the overlay deviation amounts Δx and Δy at a plurality of different positions in the field plane for the same mark. Since the overlay deviation amounts δx and δy of the marks on the wafer are constant regardless of the visual field position of the optical system, the corrected overlay deviation amounts Δx ′ and Δy ′.
The distortion aberration coefficient D and the correction coordinate origin (Ξ, Η) can be statistically fitted by the least square method or the like to be simultaneously obtained. That is, the correction equations (9) and (1
0), Δx ′ and Δy ′ are calculated so that the residual error with respect to the true overlay deviation amounts δx and δy is minimized, the correction overlay deviation amounts Δx ′ and Δy ′, the distortion aberration coefficient D, and the correction coordinate origin (Ξ , Η) and can be obtained. At this time, it is desirable that the position measured as the center of the mark has a random value centered on the corrected coordinate origin (Ξ, Η). Specifically, when measuring a plurality of shift amounts Δx and Δy, the stage 10 is controlled to move the same mark within the field of view of the image pickup device 17, and, for example, the object to be inspected while being observed. Deviations Δx, Δ at the first position in the field of view
After measuring y, the shift amounts Δx and Δy are further measured at a second position different from the first position, and this operation is repeated at least three times or more while changing the position within the visual field plane.

In the above method, it takes time and labor to measure the same mark a plurality of times, but since the variation factor relating to the imaging optical system is calculated and corrected every time one mark is measured, the imaging optical system and the mechanical system are corrected. Even if the stability of is not sufficient, highly accurate measurement can be performed.

[0035]

As described above, according to the present invention, the influence of the distortion aberration of the optical system (imaging optical system) can be satisfactorily corrected, and the overlay deviation amount can be accurately inspected. In addition, the distortion aberration coefficient D is determined by using the overlay inspection mark.
Since it is possible to obtain the correction coordinate origin (Ξ, Η) on the image plane, it is also possible to determine these coefficients for each actual process wafer including factors that have a complicated influence on the correction formula. Further, if the processing time has a margin, it is possible to measure the overlay deviation amount at a plurality of positions within the field of view for the same overlay mark and obtain the true overlay deviation amount for each overlay mark.

[Brief description of drawings]

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

FIG. 2 is a diagram showing an overlay mark composed of an outer mark and an inner mark formed on a wafer.

[Explanation of symbols] 1 Observation light source 2 collector lens 3 wavelength selection filter 6 condenser lens 7 Half mirror 8 dichroic mirror 9 Objective lens 10 stages 11 wafers 12 Aperture stop 13 Field stop 14 Wafer holder 15 Motor for rotation adjustment 16 Second objective lens 17 Image sensor 18 Image processing system 19 AF light source 20 collector lens 21 Field stop 22 Half mirror 25 pupil division prism 26 Imaging lens 27 Cylindrical lens 28 One-dimensional image sensor (line sensor) 29 AF processing system

Continued front page    F term (reference) 2F065 AA03 AA20 BB27 BB28 CC17                       CC19 CC25 EE08 FF04 FF10                       GG02 GG07 JJ02 JJ26 LL00                       LL20 LL22 LL47 MM03 MM04                       QQ24 QQ25 QQ31                 5F046 ED01 FA11 FB17 FC04

Claims (5)

[Claims] 1. A first mark and a second mark formed on an object surface.
An illumination system for illuminating an overlay mark composed of a mark, an imaging optical system for forming an image of the overlay mark on an image plane, and an overlaying system for the overlay mark formed on the image plane. An overlay inspection apparatus, comprising: a detection system for detecting an image, for measuring an overlay deviation amount between the first mark and the second mark, wherein distortion of the imaging optical system and decentering of an optical axis are detected. The positions of the characteristic points of the first mark and the second mark are corrected based on the coordinates in the image plane based on the above, and the overlay deviation amount is obtained based on the corrected position of the characteristic points. Overlay inspection device. 2. The magnification of the image forming optical system is β, the distortion aberration coefficient of the image forming optical system is D, and the coordinates indicating the center of the optical axis on the image plane (correction coordinate origin on the image plane) are defined. When (Ξ, Η), the coordinates (x ', y') on the object plane of the feature point corrected with respect to the coordinates (X, Y) in the image plane are x '= (X-Ξ) / β-D × (X-Ξ) × [(X-Ξ) ² ＋ (Y-Η) ² ] / β ³ y '＝ (Y-Η) / β-D × (Y-Η) × [( The overlay inspection apparatus according to claim 1, wherein the overlay inspection apparatus is obtained according to an equation of X-Ξ) ² + (Y-Η) ² ] / β ³ . 3. The center position of the overlay mark and the overlay deviation amounts Δx, Δy of the overlay mark are measured at a plurality of different positions within the visual field of the imaging optical system, and the distortion aberration coefficient D and the image plane are measured. Upper correction coordinate origin (Ξ,
Η) and the calculated distortion aberration coefficient D and the correction coordinate origin (Ξ, Η) are used as correction coefficients, and the measured overlay deviations Δx and Δy are measured on the image plane of the overlay mark. The overlay inspection apparatus according to claim 2, wherein the correction is performed according to the position of. 4. The first mark is a square mark having a side length of L, and the second mark has a side length of S.
Is a square mark, the magnification of the image forming optical system is β, the distortion aberration coefficient of the image forming optical system is D, and the coordinates indicating the center of the optical axis on the image plane (correction coordinates on the image plane). When the origin is (Ξ, Η) and the measured overlay deviations are Δx, Δy, the overlay deviations Δ corrected using the corrected coordinate origin (Ξ, Η).
x ′ and Δy ′ are as follows: The overlay inspection device according to claim 3, wherein the overlay inspection device is obtained according to 5. A corrected overlay shift amount corrected by measuring the center position of the overlay mark and overlay shift amounts Δx and Δy of the overlay mark at a plurality of different positions within the visual field of the imaging optical system. Δx ', Δy',
The overlay inspection apparatus according to claim 4, wherein the distortion aberration coefficient D and the correction coordinate origin (Ξ, Η) on the image plane are obtained by fitting.