Methods for measuring overlays

A method for measuring overlay includes receiving a first image of a first overlay mark captured using light having a first wavelength. The method includes receiving a second image of a second overlay mark captured using light having a second wavelength different from the first wavelength. The method includes measuring a displacement between a central portion of the first image and a central portion of the second image, wherein the first and second overlay marks are disposed on different levels.

TECHNICAL FIELD

The present inventive concept relates to a method for measuring overlays and, more particularly, to a method for measuring overlay errors using image based overlay measurement techniques.

DISCUSSION OF RELATED ART

In semiconductor manufacturing processes, there is a need for measuring and controlling specific wafer parameters. Overlay error is one of the wafer parameters. Overlay error can be referred to as a relative displacement between structures formed on different layers in the wafer. The larger the overlay error is between structures, the greater the misalignment is between the structures. The yield and performance of semiconductor devices can be decreased due to overlay error.

SUMMARY

Exemplary embodiments of the present inventive concept provide a method for measuring overlay errors using different wavelengths.

Exemplary embodiments of the present inventive concept provide a method for measuring overlay errors in which an image corresponding to overlay marks formed on a lower layer is acquired using light having a longer wavelength and an image corresponding to another overlay mark formed on an upper layer is acquired using light having a shorter wavelength.

According to an exemplary embodiment of the present inventive concept, a method for measuring overlay includes receiving a first image of a first overlay mark captured using light having a first wavelength and a second image of a second overlay mark captured using light having a second wavelength different from the first wavelength. The method for measuring overlay includes measuring a displacement between a central portion of the first image and a central portion of the second image, wherein the first overlay mark and the second overly mark are disposed on different levels.

According to an exemplary embodiment of the present inventive concept, the first overlay mark may be provided on a lower level than the second overlay mark, and the first wavelength may be longer or shorter than the second wavelength.

According to an exemplary embodiment of the present inventive concept, at least one of the first and second wavelengths may be included in a range of visible light, a range above the range of visible light, or a range below the range of visible light.

According to an exemplary embodiment of the present inventive concept, the first overlay mark may be disposed on a first layer of a wafer, and the second overlay mark may be disposed on a second layer over the first layer of the wafer.

According to an exemplary embodiment of the present inventive concept, the first and second overlay marks may be provided on a scribe lane of the wafer.

According to an exemplary embodiment of the present inventive concept, a method for measuring overlay may include receiving a first image corresponding to a first overlay mark captured using light having a first wavelength, the first overlay mark disposed on a first layer of a wafer. The method for measuring overlay may include receiving a second image corresponding to a second overlay mark captured using light having a second wavelength different from the first wavelength, the second overlay mark disposed on a second layer over the wafer. The method for measuring overlay may include receiving a combined image in which the first and second images are overlapped. The method for measuring overlay may include calculating a displacement between a central portion of the first image and a central portion of the second image in the combined image to measure an overlay between the first overlay mark and the second overlay mark.

According to an exemplary embodiment of the present inventive concept, the first wavelength may be longer or shorter than the second wavelength.

According to an exemplary embodiment of the present inventive concept, the method may further include receiving a third image corresponding to a third overlay mark captured using light having a third wavelength. The third overlay mark may be provided on a third layer over the second layer. The third wavelength may be different from the first and second wavelengths.

According to an exemplary embodiment of the present inventive concept, the first wavelength may be longer or shorter than the second wavelength, and the second wavelength may be longer or shorter than the third wavelength.

According to an exemplary embodiment of the present inventive concept, the first overlay mark may be disposed on the first layer that corresponds to a scribe lane of the wafer. The second overlay mark may be disposed on the second layer that corresponds to the scribe lane of the wafer.

According to an exemplary embodiment of the present inventive concept, one of the first and second overlay marks may be horizontally spaced apart from the other and may be not vertically overlapped with the other.

According to an exemplary embodiment of the present inventive concept, the second layer may be directly on the first layer, or an additional layer may be further disposed between the first and second layers.

According to an exemplary embodiment of the present inventive concept, the first overlay mark may include a plurality of first parallel lines that are spaced apart. The second overlay mark may include a plurality of second parallel lines that are spaced apart.

According to an exemplary embodiment of the present inventive concept, receiving the first image may include selecting the first parallel lines and obtaining an image of the first parallel lines using the light having the first wavelength.

According to an exemplary embodiment of the present inventive concept, capturing the second image may include selecting the second parallel lines and obtaining an image of the second parallel lines using the light having the second wavelength.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments of the present inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which some exemplary embodiments of the present inventive concept are shown. However, the present inventive concept should not be construed as limited to the exemplary embodiments set forth herein and may be embodied in different forms.

In the drawings, the thicknesses of layers and regions may be exaggerated for clarity. Like reference numerals in the drawings and specification may denote like elements.

FIG. 1is a top plan view of an overlay mark according to an exemplary embodiment of the present inventive concept.

Referring toFIG. 1, an overlay mark100may be formed on a scribe lane of a wafer to be used for determining an overlay between two or more layers stacked on the wafer or between two or more separate patterns on a single layer of the wafer. For ease of discussion, the overlay mark100may be mainly used to determine an overlay between two or more layers stacked on a wafer. It should be noted, however, that this is not a limitation and that the overlay mark100may also be used to determine an overlay between two or more separate patterns on a single layer of a wafer.

The overlay mark100may be arranged within an optical perimeter105set by a field of view that defines an area available for capturing an image by a metrology tool used to measure overlay. The overlay mark100may comprise a plurality of zones configured to determine overlay errors in X and Y directions between two layers on the wafer. For example, the overlay mark100may comprise first zones111,112,113and114that may be provided on a first layer of the wafer and second zones121,122,123and124that may be provided on a second layer over the first layer of the wafer. The first zones111to114may be horizontally spaced apart from the second zones121to124and do not vertically overlap the second zones121to124. The second zones121to124may be positioned closer to a center (represented by a cross) of the optical perimeter105than the first zones111to114.

The first zones111to114may include a first pattern111, a second pattern112, a third pattern113, and a fourth pattern114which are rotationally symmetric, for example ±90°, 180°, 270°, 360° around the center of the optical perimeter105. Each of the first to fourth patterns111to114may include a plurality of first overlay marks110. The first overlay marks110may be a plurality of parallel lines arranged periodically on the first layer of the wafer. The first overlay marks110included in the first and third patterns111and113may be provided to measure an overlay in the Y direction while the first overlay marks110included in the second and fourth patterns112and114may be provided to measure an overlay in the X direction.

Similarly, the second zones121to124may include a first pattern121, a second pattern122, a third pattern123, and a fourth pattern124which are rotationally symmetric, for example ±90°, 180°, 270°, 360° around the center of the optical perimeter105. Each of the first to fourth patterns121to124may include a plurality of second overlay marks120. The second overlay marks120may be a plurality of parallel lines arranged periodically on the second layer of the wafer. In exemplary embodiments of the present inventive concept, the second layer may be disposed directly on the first layer. An additional layer (e.g., additional layer32ofFIG. 4) may be further disposed between the first and second layers. The second overlay marks120included in the first and third patterns121and123may be provided to measure an overlay in the Y direction while the second overlay marks120included in the second and fourth patterns122and124may be provided to measure an overlay in the X direction.

FIG. 2is a flow diagram illustrating a method for measuring overlay according to an exemplary embodiment of the present inventive concept.

Referring toFIG. 2, a method for measuring an overlay between the first and second layers of the wafer may begin at a first step S110where a first image (e.g.,110pofFIG. 3A) corresponding to the first zones111to114is captured using light having a first wavelength. After obtaining the first image110p, the process may proceed to a second step S120where a second image (e.g.,120pofFIG. 3B) corresponding to the second zones121to124is captured using light having a second wavelength. After obtaining the second image120p, the process may proceed to a third step S130where a combined image (e.g.,130pofFIG. 3C) of the first and second images110pand120pis acquired. After obtaining the combined image130p, the process may proceed to a fourth step S140where an overlay error is measured by calculating a displacement or offset between a center (e.g.,110cofFIG. 3A) of the first image110pand a center (e.g.,120cofFIG. 3B) of the second image120p. When an overlay error is present, a step S150may be performed to correct the overlay error. After the overlay correction, the first to fourth steps S110to S140may be selectively repeated. The overlay measurement may be further described in detail below.

FIGS. 3A to 3Fare top plan views illustrating a method for measuring overlay according to an exemplary embodiment of the present inventive concept.FIG. 3Dis a top plan view illustrating a portion ofFIG. 3C.FIG. 3Fis a top plan view illustrating a portion ofFIG. 3E.FIG. 4is a diagram of an overlay measurement system according to an exemplary embodiment of the present inventive concept.

Referring toFIGS. 1 and 3A, a first image110pmay be acquired by selectively capturing the first zones111to114each having the first overlay marks110from the overlay mark100. The first image110pmay include a first pattern image111pcorresponding to the first pattern111, a second pattern image112pcorresponding to the second pattern112, a third pattern image113pcorresponding to the third pattern113, and a fourth pattern image114pcorresponding to the fourth pattern114. The first to fourth pattern images111pto114pmay be rotationally symmetric, for example ±90°, 180°, 270°, 360° around a first image center110cof the first image110p.

The first image center110cmay be defined at a cross point between a horizontal line110xand a vertical line110y. The horizontal line110xmay run through a middle of the first image110palong the X direction, e.g., between the first pattern image111pand the third pattern image113p. The vertical line110ymay run through a middle of the first image110palong the Y direction, e.g., between the second pattern image112pand the fourth pattern image114p. The horizontal line110xmay be used to measure an overlay along the Y direction while the vertical line110ymay be used to measure an overlay along the X direction.

Referring toFIGS. 1 and 3B, a second image120pmay be acquired by selectively capturing the second zones121to124each having the second overlay marks120from the overlay mark100. The second image120pmay include a first pattern image121pcorresponding to the first pattern121, a second pattern image122pcorresponding to the second pattern122, a third pattern image123pcorresponding to the third pattern123, and a fourth pattern image124pcorresponding to the fourth pattern124. The first to fourth pattern images121pto124pmay be rotationally symmetric, for example 190°, 180°, 270°, 360° around a second image center120cof the second image120p.

The second image center120cmay be defined at a cross point between a horizontal line120xand a vertical line120y. The horizontal line120xmay run through a middle of the second image120palong the X direction, e.g., between the first pattern image121pand the third pattern image123p. The vertical line120ymay run through a middle of the first image110palong the Y direction, e.g., between the second pattern image122pand the fourth pattern image124p. The horizontal line120xmay be used to measure an overlay along the Y direction while the vertical line120ymay be used to measure an overlay along the X direction.

The first image110pand the second image120pmay be acquired by an overlay measurement system or metrology tool10that can select two or more wavelengths as described, for example, inFIG. 4. With reference toFIG. 4, the overlay measurement system10may include a light source11for emitting incident light21towards a wafer30, a filter13for selectively passing a portion of output light23scattered from the wafer30, and a camera15for generating an image of the overlay mark100based on filtered output light25. The filter13may be configured to selectively allow particular colors to pass such as red, yellow, green, or blue. The overlay mark100may be provided on the wafer30. The first overlay marks110may be formed on a first layer and the second overlay marks120may be formed on a second layer over the first layer of the wafer30. An additional layer32may be interposed between the first and second layers of the wafer30. The additional layer32may be a single layer or multiple layers.

The first overlay marks110may be disposed below one or more additional layers32. Therefore, the first image110pmay be acquired based on light that passes through one or more additional layers32. The second image120pmay be acquired based on light that does not pass through the additional layer32. There may be thickness differences between the first and second layers of the wafer30. Different wavelengths may be used to generate clear first and second images110pand120p, respectively.

According to an exemplary embodiment of the present inventive concept, the first image110pmay be obtained by selecting and capturing the first overlay marks110based on light having a first wavelength. The second image120pmay be obtained by selecting and capturing the second overlay marks120based on light having a second wavelength different from the first wavelength. For example, the second wavelength may be shorter than the first wavelength. In other words, light used to obtain the first image110pmay have a wavelength longer than that of light used to obtain the second image120p. Alternatively, the first wavelength may be shorter than the second wavelength.

For example, the light used to capture the first image110pmay have red color in a range of visible light and the light used to capture the second image120pmay have blue color in the range of visible light. Alternatively, at least one of the light for capturing the first image110pand the light for capturing the second image120pmay be from a range of visible light, a range (e.g., infrared rays) longer than that of visible light, or a range (e.g., ultraviolet rays) shorter than that of visible light. The first image110pmay be acquired by entirely selecting and capturing the overlay mark100based on light having the first wavelength, for example light having red color. In this case, clearness of an image corresponding to the first overlay marks110may be greater than that of an image corresponding to the second overlay marks120. The second image120pmay be acquired by entirely selecting and capturing the overlay mark100based on light having the second wavelength, for example light having blue color. In this case, clearness of an image corresponding to the second overlay marks120may be greater than that of an image corresponding to the first overlay marks110.

Referring toFIGS. 3C and 3D, there may be acquired a combined image130pin which one of the first and second images110pand120plies on top of the other. If there is zero overlay error, the first image center110cmay coincide with the second image center120c.

Referring toFIGS. 3E and 3F, if there is an overlay error, the position of the first image center110cmay be inconsistent with the position of the second image center120c. An X-directional overlay error ΔX and a Y-directional overlay error ΔY may be determined by calculating a displacement between the first image center110cand the second image center120c.

FIG. 5is a top plan view of an overlay mark according to an exemplary embodiment of the present inventive concept.FIG. 6is a top plan view of an overlay mark according to an exemplary embodiment of the present inventive concept.

Referring toFIG. 5, an overlay mark200may include a first overlay mark210in a shape of an open-centered box and a second overlay mark220in a shape of a closed box. For example, the first overlay mark210may be disposed on a first layer of a wafer and the second overlay mark220may be disposed on a second layer over the first layer of the wafer. The first overlay mark210may surround the second overlay mark220.

A first image of the first overlay mark210may be acquired based on light having a first wavelength, a second image of the second overlay mark220may be acquired based on light having a second wavelength different from the first wavelength. A combined image may be acquired by overlapping the first and second images. An overlay error may be determined by calculating displacements between lines C1and C2corresponding to opposing inner edges of the first overlay mark210and lines C3and C4corresponding to opposing outer edges of the second overlay mark220. The first wavelength may be longer or shorter than the second wavelength.

For example, if a distance between the lines C1and C3is substantially identical to a distance between the lines C2and C4, an overlay error along an X direction may be zero. An overlay error along a Y direction may also be determined using the above technique.

Referring toFIG. 6, an overlay mark300may include first overlay marks310provided on a first layer of a wafer, second overlay marks320provided on a second layer over the first layer, and third overlay marks330provided on a third layer over the second layer. The first overlay marks310may include a plurality of lines which extend along an X direction and are spaced apart at regular intervals along a Y direction and a plurality of lines which extend along the Y direction and are spaced apart at regular intervals along the X direction. Shapes and arrangements of the second and third overlay marks320and330may be identical or analogous to those of the first overlay marks310. The first to third overlay marks310to330may be horizontally spaced apart from each other and might not vertically overlap with each other.

A first image corresponding to the first overlay marks310may be acquired based on light having a first wavelength. A second image corresponding to the second overlay marks320may be acquired based on light having a second wavelength. A third image corresponding to the third overlay marks330may be acquired based on light having a third wavelength. A combined image may be acquired by overlapping at least two of the first to third images. An overlay error may be determined by calculating displacements between at least two image centers of the overlapped two images. The first to third wavelengths may be different. For example, the second wavelength may be shorter than the first wavelength, and the third wavelength may be shorter than the second wavelength. Alternatively, the first wavelength may be shorter than the second wavelength, and the second wavelength may be shorter than the third wavelength. The second wavelength may be shorter than the first and third wavelengths, and one of the first and third wavelengths may be longer or shorter than the other.

According to exemplary embodiments of the present inventive concept, variation of light wavelength depending on the position of the overlay mark may increase image clarity. Accordingly, reliable determinations of overlay errors may increase yield and improve electrical characteristics of semiconductor devices. Moreover, overlay errors may be monitored in real time and the occurrence of manufacturing errors may be reduced.