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Apparatus and methods for determining overlay of structures having rotational or mirror symmetry

Apparatus and methods for determining overlay of structures having rotational or mirror symmetry description/claims

This application claims priority of U.S. Provisional Patent Application No. 60/698,535 (Attorney Docket No. KLA1P149P), entitled APPARATUS AND METHODS FOR DETERMINING OVERLAY STRUCTURES HAVING ROATIONAL OR MIRROR SYMMETRY, filed 11 Jul. 2005 by Mark Ghinovker, which application is incorporated herein by reference in its entirety for all purposes.

The present invention relates generally to overlay measurement techniques, which are used in semiconductor manufacturing processes. More specifically, the present invention relates to techniques for measuring alignment error between different layers or different patterns on the same layer of a semiconductor wafer stack.

The measurement of overlay error between successive patterned layers on a wafer is one of the most critical process control techniques used in the manufacturing of integrated circuits and devices. Overlay accuracy generally pertains to the determination of how accurately a first patterned layer aligns with respect to a second patterned layer disposed above or below it and to the determination of how accurately a first pattern aligns with respect to a second pattern disposed on the same layer. Presently, overlay measurements are performed via test patterns that are printed together with layers of the wafer. The images of these test patterns are captured via an imaging tool and an analysis algorithm is used to calculate the relative displacement of the patterns from the captured images.

The most commonly used overlay target pattern is the “Box-in-Box” target, which includes a pair of concentric squares (or boxes) that are formed on successive layers of the wafer. The overlay error is generally determined by comparing the position of one square relative to another square.

To facilitate discussion, FIG. 1 is a top view of a typical “Box-in-Box” target 10. As shown, the target 10 includes an inner box 12 disposed within an open-centered outer box 14. The inner box 12 is printed on the top layer of the wafer while the outer box 14 is printed on the layer directly below the top layer of the wafer. As is generally well known, the overlay error between the two boxes, along the x-axis for example, is determined by calculating the locations of the edges of lines c1 and c2 of the outer box 14, and the edge locations of the lines c3 and c4 of the inner box 12, and then comparing the average separation between lines c1 and c3 with the average separation between lines c2 and c4. Half of the difference between the average separations c1&c3 and c2&c4 is the overlay error (along the x-axis). Thus, if the average spacing between lines c1 and c3 is the same as the average spacing between lines c2 and c4, the corresponding overlay error tends to be zero. Although not described, the overlay error between the two boxes along the y-axis may also be determined using the above technique.

This type of target has a same center of symmetry (COS) for both the x and y structures, as well as for the first and second layer structures. When the target structures are rotated 180° about their COS, they maintain a same appearance. Conventionally, it has been a requirement that both the x and y structures and both the first and second layer structures have a same COS. However, these requirements may be too restrictive under certain conditions. For example, space may be limited on the wafer and a target having x and y structures (or first and second layer structures) with the same COS may not fit into the available space. Additionally, it may be desirable to use device structures for determining overlay, and device structures are not likely to meet this strict requirement.

Although this conventional overlay design has worked well, there are continuing efforts to provide improved techniques for determining or predicting overlay in device structures. For example, targets or device structures that have more flexible symmetry characteristics, as well as techniques for determining overlay with such structures, are needed.

In general, overlay targets having flexible symmetry characteristics and metrology techniques for measuring the overlay error between two or more successive layers of such targets or a shift between two sets of structures on the same layer are provided. In one embodiment, a target includes structures for measuring overlay error (or a shift) in both the x and y direction, wherein the x structures have a different center of symmetry (COS) than the y structures. In another embodiment, one of the x and y structures is invariant with a 180° rotation and the other one of the x and y structures has a mirror symmetry. In one aspect, the x and y structures together are variant with a 180° rotation. In yet another example, a target for measuring overlay in the x and/or y direction includes structures on a first layer having a 180 symmetry and structures on a second layer having mirror symmetry. In another embodiment, a target for determining overlay in the x and/or y direction includes structures on a first layer and structures on a second layer, wherein the structures on the first layer have a COS that is offset by a known amount from the COS of the structures on the second layer. In a specific implementation, any of the disclosed target embodiments may take the form of device structures. In a use case, device structures that have an inherent 180° rotational symmetry or a mirror symmetry in each of the first and second layers are used to measure overlay in a first layer and a second layer. Techniques for imaging targets with flexible symmetry characteristics and analyzing the acquired images to determine overlay or alignment error are disclosed.

In one embodiment, a semiconductor target for determining a relative shift between two or more successive layers of a substrate or between two or more separately generated patterns on a single layer of a substrate is disclosed. This target includes a plurality of first structures having a first center of symmetry (COS) or first line of symmetry (LOS) and being arranged to determine the relative shift in an x direction by analyzing an image of the first structures. This target further includes a plurality of second structures having a second COS or second LOS and being arranged to determine the relative shift in an x direction by analyzing an image of the second structures. The first COS or LOS has a different location than the second COS or LOS.

In a further aspect, the first structures have a first LOS about which the first structures have a mirror symmetry or the first structures have a 180° rotational symmetry with respect to the first COS, and the second structures have a first LOS about which the second structures have a mirror symmetry or the second structures have a 180° rotational symmetry with respect to the second COS. In another aspect, the first and second structures are in the form of device structures. In a further embodiment, a one of the first or second structures has a 180° rotational symmetry with respect to its COS and the other one of the first or second structures' has a mirror symmetry with respect to its LOS. In yet a further implementation, the first structures and the second structures together are variant with a 180° rotational asymmetry or together have a mirror asymmetry.

In an alternative embodiment, a semiconductor target for determining an overlay error between two or more successive layers of a substrate is disclosed. This target comprises a plurality of first structures formed in a first semiconductor layer and having a first center of symmetry or first line of symmetry (LOS) and a plurality of second structures formed in a second semiconductor layer and having a second COS OR LOS. The first COS OR LOS is designed to have a known offset from the second COS or LOS so that the overlay error can be determined by acquiring an image of the first and second structures and then analyzing a shift between the first and second COS's or LOS's in the image and comparing the shift to the known offset.

In a specific implementation, the first structures have a first LOS about which the first structures have a mirror symmetry or the first structures have a 180° rotational symmetry with respect to the first COS, and the second structures have a first LOS about which the second structures have a mirror symmetry or the second structures have a 180° rotational symmetry with respect to the second COS. In yet a further aspect, the first and second structures are in the form of device structures. In another implantation, a one of the first or second structures has a 180° rotational symmetry with respect to its COS and the other one of the first or second structures' has a mirror symmetry with respect to its LOS. In a further implementation, the first structures and the second structures together are variant with a 180° rotational asymmetry or together have a mirror asymmetry.

In another embodiment, the invention pertains to a method for determining the relative shift between two or more successive layers of a substrate or between two or more separately generated patterns on a single layer of a substrate. A first image is acquired of a plurality of first structures having a first center of symmetry (COS) or first line of symmetry (LOS) and being arranged to determine the relative shift in an x direction by analyzing an image of the first structures. A first image is acquired of a plurality of second structures having a second COS or second LOS and being arranged to determine the relative shift in an x direction by analyzing an image of the second structures. The first COS or LOS has a different location than the second COS or LOS. The first image of the first structures' COS is analyzed to determine whether the first structures have a shift in the x direction that is out of specification, and the second image of the second structures' COS is analyzed determine whether the second structures have a shift in the y direction that is out of specification.

In a specific aspect, the first and second images are acquired together in a same field of view. In another aspect, analyzing the first image comprises (i) when it is determined that the first structures fail to have a 180 rotational or mirror symmetry, determining that the first structures are out of specification; and (ii) when it is determined that the second structures fail to have a 180 rotational or mirror symmetry, determining that the second structures are out of specification. In another feature, analyzing the first image and analyzing the second image each includes (i) using outside edges of each region of interest of the first or second image to determine a COS or LOS for a first set of substructures and a COS or LOS for a second set of substructures, and (ii) when the COS or LOS of the first set of substructures differs from the COS or LOS of the second set of substructures by more than a predetermined amount, determining that the corresponding structures are out of specification. In a further aspect, the first set of substructures are formed from a first layer and the second set of substructures are formed from a second layer.

In yet another implementation, analyzing the first image and analyzing the second image each includes (i) for a first set of substructures, selecting an initial COS or LOS between a plurality of regions of interest, (ii) for the first set of substructures, automatically placing its 180 degree or mirror counterpart based on the initial COS or LOS, respectively, for each of the first and second images, (iii), for the first set of substructures, continuing to move the initial COS or LOS until a best correlation is found between the first substructures and their counterpart, (iv) repeating operations (i) through (iii) for a second set of substructures, (v) when a best correlation is found for both the first and second substructures and when the COS or LOS of the first set of substructures differs from the COS or LOS of the second set of substructures by more than a predetermined amount, determining that the corresponding first structures are out of specification. In a further aspect, the first set of substructures are formed from a first layer and the second set of substructures are formed from a second layer.

In a further method embodiment, the overlay error between two or more successive layers of a substrate is determined. An image is acquired of a plurality of first structures formed in a first semiconductor layer and having a first center of symmetry (COS) or line of symmetry (LOS) and a plurality of second structures formed in a second semiconductor layer and having a second COS or LOS. The first COS or LOS is designed to have a known offset from the second COS or LOS so that the overlay error can be determined by acquiring an image of the first and second structures and then analyzing a shift between the first and second COS's or LOS's in the image and comparing the shift to the known offset. The image of the first and second structures' COS or LOS is analyzed to determine whether there is an overlay error between the first and second structures that is out of specification.

In a specific implementation, the first structures have a first LOS about which the first structures have a mirror symmetry or the first structures have a 180° rotational symmetry with respect to the first COS, and the second structures have a first LOS about which the second structures have a mirror symmetry or the second structures have a 180° rotational symmetry with respect to the second COS. In another implementation, the first and second structures are in the form of device structures. In another embodiment, a one of the first or second structures has a 180° rotational symmetry with respect to its COS and the other one of the first or second structures' has a mirror symmetry with respect to its LOS. In another aspect, the first structures and the second structures together are variant with a 180° rotational asymmetry or together have a mirror asymmetry.

These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.

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