Source: http://www.google.com/patents/US7616313?dq=4740761
Timestamp: 2016-05-26 16:50:53
Document Index: 252928266

Matched Legal Cases: ['Application No. 60', 'Application No. 60', 'Application No. 60', 'Application No. 60', 'Application No. 60', 'Application No. 60', 'Application No. 60', 'Application No. 20040233441']

Patent US7616313 - Apparatus and methods for detecting overlay errors using scatterometry - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsEmbodiments of the invention include a scatterometry target for use in determining the alignment between substrate layers. A target arrangement is formed on a substrate and comprises a plurality of target cells. Each cell has two layers of periodic features constructed such that an upper layer is arranged...http://www.google.com/patents/US7616313?utm_source=gb-gplus-sharePatent US7616313 - Apparatus and methods for detecting overlay errors using scatterometryAdvanced Patent SearchPublication numberUS7616313 B2Publication typeGrantApplication numberUS 11/525,320Publication dateNov 10, 2009Filing dateSep 21, 2006Priority dateMar 31, 2006Fee statusPaidAlso published asUS20070229829, WO2007126559A2, WO2007126559A3Publication number11525320, 525320, US 7616313 B2, US 7616313B2, US-B2-7616313, US7616313 B2, US7616313B2InventorsDaniel Kandel, Walter D. Mieher, Boris GolovanevskyOriginal AssigneeKla-Tencor Technologies CorporationExport CitationBiBTeX, EndNote, RefManPatent Citations (14), Non-Patent Citations (2), Referenced by (7), Classifications (4), Legal Events (2) External Links: USPTO, USPTO Assignment, EspacenetApparatus and methods for detecting overlay errors using scatterometry
6. The target arrangement of claim 5 wherein
the first set of target cells includes four cells, and
the second set of target cells includes four cells.
7. The target arrangement of claim 6 wherein
the four cells of each set of cells has an offset characterized by the spatial periodicity value (p) and the free parameter (f0) in accordance with following relation:
in a first cell in the first set of cells the first layer is offset relative to the second layer by a value of +P/4+f0,
in a second cell in the first set of cells the first layer is offset relative to the second layer by a value of +P/4−f0,
in a third cell in the first set of cells the first layer is offset relative to the second layer by a value of −P/4+f0,
in a fourth cell in the first set of cells the first layer is offset relative to the second layer by a value of −P/4−f0; and
in a first cell in the second set of cells the third layer is offset relative to the fourth layer by a value of +P/4+f0,
in a second cell in the second set of cells the third layer is offset relative to the fourth layer by a value of +P/4−f0,
in a third cell in the second set of cells the third layer is offset relative to the fourth Layer by a value of −P/4+f0,
in a fourth cell in the second set of cells the third layer is offset relative to the fourth layer by a value of −P/4−f0;
and wherein the free parameter is defined by a value greater than zero and less than P/4.
8. The target arrangement of claim 2 wherein
the first layer of the first set of cells is the same as the third layer of the second set of cells; and
the second layer of the first set of cells is the same as the fourth layer of the second set of cells.
9. The target arrangement of claim 2 wherein
the first set of cells includes six cells, and
the second set of cells includes six cells.
10. The target arrangement of claim 9 wherein
each cell defines a spatial periodicity having a value (p) associated with a greatest common denominator for the pitches associated with each cell;
the offset of each cell is associated with the value (p) of the cell; and
wherein the six cells of each set of cells has an offset characterized by the value (p) in accordance with following relation:
in a first cell in the first set of cells the first layer is offset relative to the second layer by a value of +p�( 1/12),
in a second cell in the first set of cells the first layer is offset relative to the second layer by a value of +p�( 3/12),
in a third cell in the first set of cells the first layer is offset relative to the second layer by a value of +p�( 5/12),
in a fourth cell in the first set of cells the first layer is offset relative to the second layer by a value of −p�( 1/12),
in a fifth cell in the first set of cells the first layer is offset relative to the second layer by a value of −p�( 3/12),
in a sixth cell in the first set of cells the first layer is offset relative to the second layer by a value of −p�( 5/12); and
in a first cell in the second set of cells the third layer is offset relative to the fourth layer by a value of +p�( 1/12),
in a second cell in the second set of cells the third layer is offset relative to the fourth layer by a value of +p�( 3/12),
in a third cell in the second set of cells the third layer is offset relative to the fourth layer by a value of +p�( 5/12),
in a fourth cell in the second set of cells the third layer is offset relative to the fourth layer by a value of −p�( 1/12),
in a fifth cell in the second set of cells the third layer is offset relative to the fourth layer by a value of −p�( 3/12),
in a sixth cell in the second set of cells the third layer is offset relative to the fourth layer by a value of −p�( 5/12).
11. The target arrangement of claim 1 wherein the target arrangement the disambiguation features comprise coarse symmetric disambiguation features suitable for resolving ambiguities caused by signals produced by illuminating the cells, wherein the symmetric configuration of the disambiguation features enables coarse alignment of the disambiguation features in two perpendicular axes.
a substrate having at least two layers formed thereon, the layers comprising a second layer formed above a first layer;
a target arrangement formed on the substrate enabling the measurement of the overlay error between the first and the second layer, the target arrangement comprising a plurality of periodic target cells configured such that each cell has two arrays of periodic features a first array in the first layer and a second array in the second layer arranged such that there is a predetermined offset between the periodic features of first and second arrays of each target cell, each cell configured to generate an overlay signal when illuminated; and
a plurality of coarse, optically resolvable disambiguation features configured to resolve alignment ambiguities in the overlay signal generated by the illuminated target cell wherein at least a first one of the coarse disambiguation features includes a set of coarse overlay bars configured such that the sides of the bars are exposed to allow each edge to be imaged and is arranged in spaces between adjacent target cells of the arrangement and wherein a second one of the coarse disambiguation features defines the periphery of the target arrangement and surrounds the plurality of periodic target cells.
13. The target arrangement recited in claim 12 wherein the target cells are configured so that the periodic features of the first array have a different pitch than the periodic features of the second array wherein the relationship between the pitches is arranged to generate a periodic signal for the target when the target cell is exposed to an illumination source.
14. The target arrangement recited in claim 12 wherein the target cells are configured so that the periodic features of the first array have the same pitch as the periodic features of the second array.
15. The target arrangement of claim 12 wherein each disambiguation feature has at least two perpendicular edges.
16. The target arrangement of claim 12 wherein the periodic features of the top Layer of the target cells have a first pitch p1 and an associated value n1 and the periodic features of the bottom layer have a second pitch p2 and an associated value n2 wherein the relationship between the pitches is defined by the relationship n1p1=n2p2, and wherein a spatial periodicity (P) is defined for the target cell in accordance with the relation n1p1=n2p2=P and wherein the pitches are selected so that the fraction p1/p2 is a rational number.
17. The target arrangement of claim 16 wherein the predetermined offset between the periodic features of the bottom layer and the periodic features of the top layer is associated with the greatest common divisor of the values for p1 and p2.
18. The target arrangement of claim 16 wherein the target cells have a size dimension of at least fifteen times the spatial periodicity (P) for the cell.
19. The target arrangement of claim 12 wherein the coarse disambiguation features comprise at least two optically resolvable disambiguation features arranged between the periodic target cells and configured to resolve signal ambiguities caused by the generation of overlay signal when the target arrangement is exposed to the illumination source.
20. The target arrangement of claim 19 wherein the target arrangement has a center of symmetry and the at least two disambiguation features are such configured such that the disambiguation features are capable of being symmetrically rotated 180 degrees about axis of symmetry.
21. The target arrangement of claim 20 wherein,
the first set of periodic target cells of the target arrangement are arranged on the substrate in a two by two arrangement of four target cells; and
the second set of periodic target cells of the target arrangement are arranged on the substrate and positioned adjacent to the first set in another two by two arrangement of four target cells; and
the at least two disambiguation features are arranged so that,
22. The target arrangement of claim 20 wherein each disambiguation feature has at least two perpendicular edges.
23. The target arrangement of claim 22 wherein each disambiguation feature is positioned in its entirety in a space between two adjacent target cells.
24. The target arrangement of claim 23 wherein each disambiguation feature is rectangular in shape.
25. The target arrangement of claim 12 wherein the targeting arrangement is further configured such that:
the plurality of periodic target cells include,
a first set of periodic target cells arranged so that the periodic features of the first and second layers of the first set of periodic target cells are oriented in a first configuration and arranged to enable the targeting arrangement to determine overlay error in a first direction;
a second set of periodic target cells arranged so that the periodic features of the first and second layers of the second set of periodic target cells are oriented in a second configuration that is perpendicular to the first configuration and arranged to enable the targeting arrangement to determine overlay error in a second direction that is perpendicular to the first direction; and
the disambiguation features arranged between the periodic target cells, the disambiguation features configured to resolve signal ambiguities caused by the generation of the overlay signal when the target arrangement is exposed to the illumination source.
26. The target arrangement of claim 25 wherein,
the first set of periodic target cells includes six target cells; and
the second set of periodic target cells includes six target cells.
27. The target arrangement of claim 25 wherein,
the first set of periodic target cells includes four target cells; and
the second set of periodic target cells includes four target cells.
28. A method for determining an overlay error between layers of a sample, the method comprising:
providing a substrate having a target arrangement formed thereon, the target arrangement comprising a plurality of target cells constructed such that each cell has a first and a second layer, where the plurality of target cells includes a set of coarse overlay bars configured such that the sides of the bars are exposed to allow each edge to be imaged and each of the first and second layer includes a set of periodic features configured such that the periodic features of the first layer have an offset relative to the periodic features of the second layer and also includes a plurality of symmetric coarse disambiguation features located within the targeting arrangement, the coarse disambiguation features being arranged so that they have a common center of symmetry with an associated group of target calls and configured such that the coarse disambiguation features are large enough to be optically imaged;
illuminating the target cells with the illumination source to generate overlay signals for each of the target cells;
obtaining a plurality of overlay signals, at least one such signal associated with each illuminated target cell;
processing the overlay signals associated with each illuminated target cell to obtain a plurality of ambiguous overlay measurements;
optically imaging the coarse disambiguation features to obtain a plurality of projection signals associated with the coarse disambiguation features;
processing the plurality of projection signals to obtain a plurality of coarse measurements of the center of symmetry for each of the coarse disambiguation features and each associated group of target cells;
using the coarse measurements of the center of symmetry to clarify the ambiguous overlay measurements; and
processing the corrected overlay measurements to determine any overlay error between the first and second layers, using information obtained from the signal obtained for each of the target cells.
29. The method of claim 28, wherein the providing of the substrate having a target arrangement includes providing a target arrangement constructed such that the periodic features of the first layer have a different pitch than the periodic features of the second layer.
30. The method of claim 28, wherein, providing the targeting arrangement further includes providing a targeting arrangement having a first set of target cells having periodic features oriented in a first direction and a second set of target cells having periodic features oriented in a second direction and wherein the periodic features of both sets of target cells are configured so that said first and second layers each have a different pitch, wherein the relationship between the pitches is arranged to generate a periodic signal the target cells are exposed to an illumination source.
31. A target arrangement for determining overlay alignment on a substrate, the target arrangement comprising:
a substrate having at least two layers formed thereon, including a second layer formed above a first layer;
a target arrangement formed on the substrate and configured to include,
a plurality of spaced apart target cells configured to enable the generation of periodic overlay signals associated with the overlay of the first layer relative to the second layer, and
a plurality of optically resolvable disambiguation features arranged, at least in part, between the target cells of the target arrangement, where the plurality of optically resolvable disambiguation features includes a set of coarse overlay bars configured such that the sides of the bars are exposed to allow each edge to be imaged and the disambiguation features are configured to enable resolution of signal ambiguity in the periodic overlay signals, the plurality of target cells being surrounded by at Least one of the plurality of optically resolvable disambiguation features.
32. A target arrangement as in claim 31 wherein the a plurality of optically resolvable disambiguation features are configured to include,
a first disambiguation feature arranged between the target cells in one of the first and second layers, and
a second disambiguation feature arranged between the target cells in the other of the first and second layers.
33. A target arrangement as in claim 31 wherein the a plurality of optically resolvable disambiguation features are configured to include,
a second disambiguation feature arranged around the target cells in the other of the first and second layers.
34. A target arrangement as in claim 31 wherein the target cells of the arrangement comprise four spaced apart target cells configured to enable overlay metrology measurements in two perpendicular axes,
wherein the first disambiguation feature has a cruciform shape arranged between the target cells; and
wherein the second disambiguation feature is arranged around the target cells and the first disambiguation feature.
35. A compact target arrangement for determining overlay alignment on a substrate, the target arrangement comprising:
a target arrangement arranged to enable measurement of overlay alignment between said at least two layers, the target arrangement comprising,
a first set of optically resolvable target features arranged in a pattern that defines a space between said optically resolvable features, the first set of optically resolvable target features configured to enable coarse measurements of overlay between said at least two layers;
a second set of periodic high resolution target features arranged within the space defined by the first set of optically resolvable target features;
wherein said periodic high resolution target features are configured to enable generation of periodic overlay signals associated with the overlay alignment between said at least two layers and wherein the optically resolvable target features are configured to enable resolution of ambiguities in said periodic overlay signals to enable accurate measurement of overlay,
where the target arrangement includes a set of coarse overlay bars configured such that the sides of the bars are exposed to allow each edge to be imaged.
36. A compact target arrangement for determining overlay alignment on a substrate, the target comprising:
a target arrangement, the target arrangement arranged to enable measurement of overlay alignment between said at least two layers, the target arrangement comprising,
a first set of optically resolvable target features, the first set of optically resolvable target features configured to enable coarse measurements of overlay between said at least two layers;
a second set of periodic high resolution target cells, the first set of optically resolvable target features and the second set of periodic high resolution target cells situated in close proximity to one another, such that a distance between the first set of optically resolvable target features and the second set of periodic high resolution target cells are smaller than a dimension of one of the second set of periodic high resolution target cells;
where the target arrangement includes a set of coarse overlay bars configured such that the sides of the bars are exposed to allow each edge to be imaged. Description
This application is related to and claims priority to Application No. 60/788,005, filed Mar. 31, 2006, entitled “APPARATUS AND METHODS FOR DETECTING OVERLAY ERRORS USING SCATTEROMETRY”, by Daniel Kandel, et al. The above application being incorporated by reference in its entirety for all purposes.
(1) Application No. 60/431,314, entitled METHOD FOR DETERMINING OVERLAY ERROR BY COMPARISON BETWEEN SCATTEROMETRY SIGNALS FROM MULTIPLE OVERLAY MEASUREMENT TARGETS, by Walter D. Mieher et al., filed 5 Dec. 2002, (2) Application No. 60/440,970, entitled METHOD FOR DETERMINING OVERLAY ERROR BY COMPARISON BETWEEN SCATTEROMETRY SIGNALS FROM MULTIPLE OVERLAY MEASUREMENT TARGETS WITH SPECTROSCOPIC IMAGING OR SPECTROSCOPIC SCANNING, by Walter D. Mieher, filed 17 Jan. 2003, (3) Application No. 60/504,093, entitled APPARATUS AND METHODS FOR DETECTING OVERLAY ERRORS USING SCATTEROMETRY, by Walter D. Mieher, filed 19 Sep. 2003, (4) Application No. 60/449,496, entitled METHOD AND SYSTEM FOR DETERMINING OVERLAY ERRORS BASED ON SCATTEROMETRY SIGNALS ACQUIRED FROM MULTIPLE OVERLAY MEASUREMENT PATTERNS, by Walter D. Mieher, filed 22 Feb. 2003, (5) Application No. 60/498,524, filed 27 Aug. 2003, entitled “METHOD AND APPARATUS COMBINING IMAGING AND SCATTEROMETRY FOR OVERLAY METROLOGY”, by Mike AdeI, and (6) Application No. 60/785,430, filed 23 Feb. 2004, entitled “APPARATUS AND METHOD FOR DETECTING OVERLAY ERRORS USING SCATTEROMETRY”, and published as Application No. 20040233441 on 25 Nov. 2004, by Walter D. Mieher, et al. These applications are herein incorporated by reference in their entirety. TECHNICAL FIELD
The invention described herein relates generally to methods and apparatus for determining the alignment of overlay structures formed in single or multiple layers. More particularly, it relates to using improved targets and methods for determining overlay based on diffraction of radiation interacting with such structures.
In accordance with the principles of the present invention, an improved overlay target arrangement and methods for its fabrication and use are disclosed.
In one brief example illustration, a set of four targets is provided with each target having two sets of structures on two different layers which are offset from each other. The structures define gratings that can be used to determine overlay alignment. In a specific implementation, an offset may be defined as the sum or the difference of two separate distances: a first distance F and a second distance f0, with F being greater than f0. Denoting the four targets as “target A”, “target B”, “target C” and “target D”, predetermined offsets for each of these targets may be defined as follows for an example of one target design:
FIG. 2( a) is a side cross-section view illustrating an example patterned grating in a top layer L2 having an offset F from a patterned grating of a bottom layer L1. Each layer L1 and L2 is patterned into a set of structures. The structures may include any suitable feature, such as a line, trench or a contact. The structures may be designed to be similar to a semiconductor device feature. The structures may also be formed from a combination of different features. In general these structures are configured as diffraction gratings. Further, the structures may be located on any layer of the sample, e.g., either above the top layer of the sample, within any layer of the sample, or partially or completely within a layer of the sample. In the illustrated embodiment of FIG. 2( a), layer L1 includes the complete structures 204 a-c, while layer L2 includes the complete structures 202 a-c. Construction of scatterometry overlay targets structures and methods for producing them are described in U.S. patent application, having application Ser. No. 09/833,084, filed 10 Apr. 2001, entitled “PERIODIC PATTERNS AND TECHNIQUE TO CONTROL MISALIGNMENT”, by Abdulhalim, et al., which application is herein incorporated by reference in its entirety.
A incident radiation beam is directed towards each of the four targets A, B, C, and D to generate four spectra SA, SB, SC, and SD from the four targets. Examples of optical systems and methods for measuring scatterometry signals to determine overlay may be found in (1) U.S. patent application, having patent Ser. No. 09/849,622, filed 4 May 2001, entitled “METHOD AND SYSTEMS FOR LITHOGRAPHY PROCESS CONTROL”, by Lakkapragada, Suresh, et al. and (2) U.S. patent application, having application Ser. No. 09/833,084, filed 10 Apr. 2001, entitled “PERIODIC PATTERNS AND TECHNIQUE TO CONTROL MISALIGNMENT”, by Abdulhalim, et al. These applications are herein incorporated by reference in their entirety. This cell configuration is illustrated in FIG. 1. The spectra can be captured, measured, processed, and compared to measure overlay error. Many approaches can be employed, with one particular approach being described in detail in U.S. Patent Publication No. US 2004/0233441, filed 23 Feb. 2004, entitled “APPARATUS AND METHOD FOR DETECTING OVERLAY ERRORS USING SCATTEROMETRY”, by Walter D. Mieher, et al., which application is incorporated by reference in its entirety for all purposes.
The following paragraph illustrates a few particularized embodiments for practicing the present invention. In one embodiment, (which can comprise two sets of four target cells i.e., eight target cells) a particularly suitable set of desirable offsets for each of the four cells of a target for a given pitch (p) (also referred to as the spatial periodicity of the target cell) is as follows: for pitch (p) suitable offsets are p/4+ƒ0, −p/4+ƒ0, p/4−ƒ0 and −p/4−ƒ0, where ƒ0 is a free parameter in the range of 0<ƒ0<p/4. FIGS. 3( a) and 3(b) provides examples of simplified schematic depictions of a pair of four-cell target arrangements suitable for use in some embodiments of the invention.
In one embodiment, such a periodic function can be obtained where there is a specific predetermined relationship between the pitches of the layers. This relationship can be satisfied by the following condition: n1p1=n2p2 and where the relationship p1/p2 defines a rational number. In such case, values for “nx” are selected as integer values (such as in an example embodiment where p1 and p2 are given as integers values, for example, in nanometers). Thus, both n1 and n2 comprise integer values. The values for “nx” are also selected so that they satisfy the relationship n1p1=n2p2.
In other words, the relationship p1/p2=n2/n1 is selected to be a rational number. In one embodiment, n1 and n2 are selected as “Minimal Integers” that satisfy the relation P=n1p1=n2p2, where P is defined as the “spatial periodicity” of the combined structure of the two gratings (or the cell associated with the two gratings) also referred to as the periodicity of the cell.
Generally, the periodicity of any scatterometry signal is not equal to “spatial periodicity” P of the combined structure of the two gratings.
Thus, using the information of paragraphs [0037] and [0064] above, the optimal values of the intentional offsets of the 4 cells of the target are p/4+ƒ0, −p/4+ƒ0, p/4−ƒ0 and −p/4−ƒ0, where p=GCD(p1,p2) and where ƒ0 is less than p/4. In the preceding case p=(GCD)=100 nm. Therefore p/4=25 nm and ƒ0 should be chosen as some value less than 25 nm. This is depicted in the simplified diagram of FIG. 2( h) which shows two layers of periodic features 261, 262 arranged in different layers of a substrate. The spacing between the features of a first layer 261 of periodic features defines the pitch p1 for the first layer. Similarly, the spacing between the features of a second layer 262 of periodic features defines the pitch p2 for a second layer on the target. In this depiction the pitch p1 for the first layer is greater than the pitch p2 for the second layer. Additionally, a portion of the offset F is depicted. This value F is measured for example from the centerline of a feature in the first layer to the centerline of a feature in the second layer. Additionally, the other portion of the offset f0 is shown as added to (or subtracted from) F. The offsets in the present invention are determined as above (+F+ƒ0, +F−ƒ0, −F+ƒ0, −F−ƒ0) and used to define a set of cells defining a targeting arrangement. Generally, this means eight cells per arrangement, four for each direction (of which the directions are perpendicular to each other).
The effectiveness of this approach is enhanced in embodiments having a sufficiently large cell size. The spatial periodicity P of the cell plays a role here. Additionally, a large number of spatial periods P should be contained in the illumination spot directed onto the target. In preferred embodiments, a cell size of about 15P or greater is generally preferred. Such a cell size can be used to in order to avoid finite size effects and poor repeatability. In another embodiment, each cell is at least 20P across (“across” in this context means in the direction perpendicular to the length of the features forming the cell.)
Referring to Step 252 of the flow diagram (FIG. 2( g)), a method embodiment for determining overlay in accordance with one embodiment of the present invention can include the following. A first set of target cells includes four targets A, B, C, and D which are designed to have offsets Xa through Xd. The target offsets are, for each target respectively, Xa=p/4+ƒ0, Xb=−p/4+ƒ0, Xc=p/4−ƒ0 and Xd=−p/4−ƒ0, where spatial periodicity value for the resultant target signal (Sp)=p=GCD(p1, p2) and where f0 is less than p/4.
In Step 254, an incident radiation beam is directed towards each of the four targets A, B, C, and D to generate four spectra SA, SB, SC, and SD from the four targets in which can be measured. The spectra generation operations may be carried out sequentially or simultaneously depending on the measurement system's capabilities. The incident beam may be any suitable form of electromagnetic radiation, such as laser or broadband radiation. Examples of optical systems and methods for measuring scatterometry signals to determine overlay may be found in (1) U.S. patent application, having patent Ser. No. 09/849,622, filed 4 May 2001, entitled “METHOD AND SYSTEMS FOR LITHOGRAPHY PROCESS CONTROL”, by Lakkapragada, Suresh, et al. and (2) U.S. patent application, having application Ser. No. 09/833,084, filed 10 Apr. 2001, entitled “PERIODIC PATTERNS AND TECHNIQUE TO CONTROL MISALIGNMENT”, by Abdulhalim, et al. These applications are herein incorporated by reference in their entirety.
Many embodiments of suitable measurement systems and their use for determining overlay error are further described in detail in the applications incorporated above. But, in short, the spectra are measured and processed to obtain overlay error information (Step 256). The inventors contemplate that in various embodiments of the present invention, the spectra SA, SB, SC, and SD (and any additional spectra that may be present) could include any type of spectroscopic ellipsometry or reflectometry signals, including: tan(Ψ), cos(Δ), Rs, Rp, R, α (spectroscopic ellipsometry “alpha” signal), β (spectroscopic ellipsometry “beta” signal), ((Rs−Rp)/(Rs+Rp)), etc.
Spectrum SB (−p/4+ƒ0) can then be subtracted from spectrum SA (+p/4+ƒ0), and spectrum SD (−p/4−ƒ0) can then be subtracted from spectrum SC (+p/4−ƒf0) to form two associated difference spectra D1 and D2. Next, a difference spectrum property Prop1 is obtained from the difference spectra D1 and a difference spectrum property Prop2 is obtained from the difference spectrum D2. The difference spectra properties Prop1 and Prop2 are generally obtained from any suitable comparable characteristics of the obtained difference spectra D1 and D2. The difference spectra properties Prop1 and Prop2 may also each simply be a point on the each difference spectra D1 or D2 at a particular wavelength. By way of other examples, difference spectra properties Prop1 and Prop2 may be the result of an integration or averaging of the difference signal, equal the average of the SE alpha signal, equal a weighted average which accounts for instrument sensitivity, noise or signal sensitivity to overlay, or many other parameters. As is known to those having ordinary skill in the art, the comparison spectra can be obtained and processed using many different methods.
The SCOL Ambiguity Problem
Such techniques as described above are useful and accurate for a determination of an overlay error. However, as also mentioned briefly above, the periodicity of SCOL signals commonly leads to an ambiguity of n�p/2 in the overlay, where n has an integer value. The inventors address this issue by adding disambiguation features to enhance the SCOL target. Such disambiguation features enable coarse measurements of overlay offset. Such coarse measurements are enabled by adding disambiguation target structures in the two layers and using, for example, a box-in-box type algorithm to generate a coarse measurement of the overlay error to clarify the ambiguity in the cell measurements.
To resolve ambiguity in the periodic spectra signals obtained by illuminating the target cells (e.g., 311 b), a set of disambiguation features are added to the target. The disambiguation features can comprise a set of coarse overlay features arranged symmetrically about an axis of symmetry 301. A first set 330 of coarse overlay “bars” is arranged in one of the layers (for example, the same layer as the bottom features of the cells), while a second set 332 of coarse overlay “bars” is arranged in the other one of the layers. Such structures should be sized so that they can be optically resolved with an imaging system used with the apparatus. The idea being that the features are large enough to be optically resolvable with a common imaging system thereby enabling easy and efficient navigation to the relevant sites (one example of a suitable navigation optical system can be included on a SCOL system of the present invention). Also, coarse overlay measurements made in accordance with the principles of the invention generally need not be more accurate than slightly less than p/4 (where p=GCD (p1,p2) for the cell) or in some cases p/12. Generally, this means that the coarse overlay measurements need only be accurate to about 10-20 nanometers to satisfy the accuracy requisite to resolve the ambiguity generated by the cells. The bars (330, 332) are generally sized and positioned in the space available. In the depicted implementations the bars are located between the cells and at the outer edge of the target. Such positioning is convenient and does not waste additional space. As mentioned above, the only true requirement as to the size of the bars is that they be large enough to be optically resolved with the imaging system conducting the measurements. Bars on the order of about 0.5μ (micrometers) to about 1μ are easily suitable for the purposes of the invention. However, both bigger and smaller dimensions for the bars are contemplated by the inventors.
FIG. 3( b) depicts an alternative target 340 embodiment also having two sets 350, 360 of four cells each (351 a, 351 b, 351 c, & 351 d and 361 a, 361 b, 361 c, & 361 d, respectively). As before, the features of each set of cells are perpendicular to the features of the other set of cells. Again a set of disambiguation features is added to the target. As before the disambiguation features can comprise coarse overlay features arranged symmetrically about an axis of symmetry 341. As depicted here, a first set 370 a, 370 b of coarse overlay “bars” is arranged in one of the layers. Also, a second set 372 of coarse overlay “bars” is arranged in the other of the layers. In this embodiment, the second set 372 of coarse overlay is in a “domino”-type configuration with one of the bars bisecting the cells of the target. As before, the disambiguation features should be sized so that they can be optically resolved with the imaging system used. The bars are generally sized as described in FIG. 3( b).
FIG. 5 provides a brief illustration of one embodiment used to disambiguate SCOL measurements with coarse disambiguation features. FIG. 5 depicts a target 500 embodiment also having sets of target cells 501 arranged as discussed in previously described embodiments. A set of disambiguation features is added to the target (510, 520). Here, each of the disambiguation features comprise coarse overlay features arranged symmetrically about center of symmetry 502. In this case, such symmetry means that the disambiguation features can be rotated 180 degrees about the center of symmetry 502 and maintain the same pattern and orientation. As depicted here, a first set 510 of coarse overlay “bars” is arranged in one of the layers. Also, a second set 520 of coarse overlay “bars” is arranged in another layer.
The following describes one example for using the coarse overlay to obtain a measurement in the X-direction. Using an imaging tool the coarse overlay “bars” 510, 520 are imaged using a sensor (for example a CCD array). Regions of interest (ROI) are the defined for the two layers. The two ROIs correspond to portions of the overlay “bars” 510, 520. For example, ROI (defined by the dotted line) 521 corresponds to the second set 520 of coarse overlay “bars”. And another ROI (defined by the dotted/dashed line) 511 corresponds to the left hand portion of the first set 510 of coarse overlay “bars”. Whereas, ROI (defined by the dotted/dashed line) 512 corresponds to the right hand portion of the first set 510 of coarse overlay “bars”. The bars are illuminated by the inspection tool and scattered light signals are received at the sensor. The received signals are then processed for each ROI. For example, the signals can be received by a CCD sensor, and then each CCD pixel is processed in a summing operation for each ROI (projection operation) to obtain a one dimensional measurement for each ROI. This results in three one-dimensional projected signals (one for each ROI 511, 512, 521).
Referring to FIG. 6, the inventors describe one embodiment 600 of a target that can achieve reduced cross-talk from adjacent features without sacrificing additional space. As depicted here, a first set 601 of coarse overlay “bars” is arranged in one of the layers. Also, a second set 602 of coarse overlay “bars” is arranged in the other of the layers. In this embodiment, the coarse overlay bars are smaller (shorter) than those previously disclosed. In particular, all sides of the bars are exposed to allow each edge to be imaged and used for disambiguation.
The following describes one example for using the coarse overlay to obtain a measurement in the X-direction. Using an imaging tool the coarse overlay bars are imaged using a sensor. ROI's are the defined for the two layers. For example, here two ROIs 704 a, 704 b correspond to portions of the overlay “bars” 703, 703′ (defined by the dashed/dotted line 704 a, 704 b). And, two other ROIs 705 a, 705 b correspond to portions of additional overlay bars 706, 706′ (defined by the dashed/dotted line 705 a, 705 b) formed in another layer. As before, the bars are illuminated by the inspection tool and scattered light signals are received at the sensor. The received signals are then processed for each ROI. For example, a summing operation for each ROI (projection operation) to obtain a one dimensional measurement for each ROI (704 a, 704 b, 705 a, 705 b). This results in four one-dimensional projected signals.
In the embodiment depicted in FIG. 8, a pair of six-cell sets are configured to form a twelve cell target. Much the same as the previously described embodiments a set of cells is described hereinbelow. FIG. 8 depicts an target 800 embodiment also having two sets 810, 820 of six cells each (811 a, 811 b, 811 c, 811 d, 811 e & 811 f and 821 a, 821 b, 821 c, 821 d, 821 e & 821 f, respectively). As described with respect to the eight cell arrangements, the features of each set of cells are perpendicular to the features of the other set of cells (i.e., set 810 is perpendicular to set 820). Again a set of disambiguation features can be added to the target. As before the disambiguation features can comprise coarse overlay features arranged symmetrically about an axis of symmetry 830. As depicted here, a first set 840 a, 840 b of coarse overlay “bars” is arranged in one of the layers. Also, a second set 850 of coarse overlay “bars” is arranged in the other of the layers. In this embodiment, the second set 850 of coarse overlay is in a “domino”-type configuration with one of the bars bisecting the cells of the target. As before, the disambiguation features should be sized so that they can be optically resolved with the imaging system used. The bars are generally sized as described elsewhere herein.
The present invention has been particularly shown and described with respect to certain preferred embodiments and specific features thereof. However, it should be noted that the above-described embodiments are intended to describe the principles of the invention, not limit its scope. Therefore, as is readily apparent to those of ordinary skill in the art, various changes and modifications in form and detail may be made without departing from the spirit and scope of the invention as set forth in the appended claims. Other embodiments and variations to the depicted embodiments will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims. In particular, it is contemplated by the inventors that many different arrangements and configurations can be established for constructing overlay targets configured in accordance with the principles of the invention. Although only a few configurations are expressly disclosed herein, it should be appreciated by anyone having ordinary skill in the art that, using the teachings disclosed herein, many different target configurations can be implemented and still fall within the scope of the claims. Further, reference in the claims to an element in the singular is not intended to mean “one and only one” unless explicitly stated, but rather, “one or more”. Furthermore, the embodiments illustratively disclosed herein can be practiced without any element which is not specifically disclosed herein.
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