Patent Description:
An overlay measurement generally specifies how accurately a first patterned layer aligns with respect to a second patterned layer disposed above or below it or how accurately a first pattern aligns with respect to a second pattern disposed on the same layer. The overlay error is typically determined with an overlay target having structures formed on one or more layers of a work piece (e.g., semiconductor wafer). The structures may take the form of gratings, and these gratings may be periodic. If the two layers or patterns are properly formed, then the structure on one layer or pattern tends to be aligned in a specific measurable orientation relative to the structure on the other layer or pattern. If the two layers or patterns are not properly formed, then the structure on one layer or pattern tends to be offset or misaligned with respect to this specific orientation.

<CIT> discloses a measurement system and measurement method.

<CIT> describes a pattern recognition and metrology structure for an x-initiative layout design.

There continues to be a need for improved techniques and apparatus for measuring and determining overlay.

In one embodiment, a method of performing overlay metrology upon a target having at least two layers formed thereon as recited in claim <NUM> is disclosed.

In an alternative embodiment, the invention pertains to a charged particle beam system for performing overlay metrology upon a target having at least two layers formed thereon as recited in claim <NUM>.

These and other aspects of the invention are described further below with reference to the figures.

In general, an overlay error between two process layers or a shift between two sets of structures on the same layer may be determined using specially designed overlay targets, for which the target structures are designed with a known relationship between their symmetry characteristics and discrepancies from such symmetry correspond to overlay error in such targets. As used herein, a layer may refer to any suitable materials, such as a semiconductor or a photoresist layer that are generated and patterned for fabrication of a wafer device or test structure. Although the following target examples are shown to have structures on two (or more) layers for measuring overlay, it is readily apparent that each target may include two (or more) sets of structures on the same layer for determining a shift error between such set of structures. Additionally, techniques of the present invention may be applied to any type of periodic targets, and such targets may be formed in an inactive area (e.g., scribe line) or in an active region of a die on of a production wafer (e.g., as part of a device portion of a die). Techniques that are described herein may also be applied to targets for determining other types of characteristics, such as critical dimension (CD), CD uniformity (CDU), edge placement error (EPE), pattern fidelity, etc..

<FIG> is a top plan view of two overlay targets for measuring overlay error between two different process layers in an X and Y direction, respectively. As shown, a first target <NUM> is arranged for measuring an overlay error between a set of first structures 106a and 106b in a first layer and a set of second structures 108a and 108b in a second layer with respect to an X direction. A second target <NUM> is arranged for measuring an overlay error between a set of first structures 112a and 112b in a first layer and a set of second structures 114a and 114b in a second layer with respect to a Y direction.

In this embodiment, each of the X and Y targets are designed so that its first structures have a same <NUM>° rotational center of symmetry as its second structures although the X direction target <NUM> is designed to have a center of symmetry (COS) <NUM> that has a different location than the Y direction target <NUM> COS <NUM>. For example, the X direction target <NUM> has first structures that are divided into two groups 106a and 106b that are positioned with respect to each other so that if they were rotated <NUM>° about a center of symmetry <NUM>, the first structures would have a same appearance before and after such rotation. The X direction target <NUM> also includes second structures that are divided into two groups 108a and 108b that are positioned with respect to each other so that if they were rotated <NUM>° about a center of symmetry <NUM>, these second structures would have a same appearance before and after such rotation. In this illustration, the COS of the first structures is at the same position as the COS of the second structures. When an overlay error is present within a target, the COS of the first structures of such target is shifted from the COS of the second structures. This shift is called the overlay error.

In some targets, the overlay error in separate X and Y targets may be determined based on a priori knowledge that each target is designed to have structures in each layer that have a <NUM>° rotational symmetry about a same COS. Any shift between the COS's of the first and second layer structures may be imaged and measured as an overlay error. In alternative embodiments, the X and/or Y targets of <FIG> may be arranged so that the first and second structures have a COS with a known offset. In this case, if the shift does not match the known offset, the amount of variance corresponds to the overlay error.

In general, a measurement on overlay targets can be performed with an optical measurement system after or during a photoresist development inspection (ADI) step. If overlay is assessed after development of the photoresist, for example, the wafer can be reworked if the overlay results are out of specification. As the device feature size scales down, however, the pattern of an optical target will tend to be much larger than the real device pattern and not provide reliable results. Thus, a scanning electron microscope (SEM), which can resolve and image much smaller patterns, becomes more attractive for overlay measurement applications.

SEM overlay (SEM-OVL) or e-beam overly (eOVL) measurements have mainly been used after etch inspection (AEI), which is also called after clean inspection (ACI), to calibrate any non-zero bias between scribe line ADI measurements of optical overlay and the overlay in real devices after etch. Of course, IC manufacturers are also interested in applying SEM-OVL/eOVL in ADI applications. Although the following overlay determination techniques may utilize electron beam scanning metrology systems, it is contemplated that similar techniques may be used with any suitable type of charged beam system.

For SEM or e-beam ADI applications, high landing energy (LE) may be needed to allow primary electrons not only to penetrate the layer under the photo-resist (PR) layer to reach the target structures of the underlying layer, but also to allow backscattered electrons (BSE) to also reach the detector. Because the BSE signal usually is significantly lower than the secondary electron (SE) signal, higher beam current is preferred. However, an e-beam with higher LE and higher beam current may also cause charging and e-beam induced PR damage.

<FIG> illustrates asymmetric imaging of a grating target portion <NUM> caused by scanning in an X direction (specifically, in a west and east direction) with a charged particle beam. As shown, a charged particle beam is raster scanned in a scan pattern <NUM> that is in an "east" direction relative to the lines of the target <NUM>. For each scan, the beam scans along a plurality of scan paths <NUM> that are each in a direction that is perpendicular to the line structures of the grating target <NUM>. <FIG> illustrates similar asymmetric imaging of a grating target <NUM> that is caused by scanning in a single Y (or south) direction with a charged particle beam.

In either the X or Y target examples, the line structures asymmetrically retain charge from the charged particle beam as it scans across the line structures and, as a result, the periodic line image is asymmetric. For instance, leading edge 204a of line <NUM> may have a thicker white edge in the resulting image than the trailing edge 204b of the same line <NUM>. This asymmetry may be caused by uneven positive charging of the line edges as the beam moves across such lines, which results in unequal amounts of secondary electrons from the different edges reaching the detector and contributing to the image.

Certain embodiments of the present invention provide symmetric scanning patterns that result in symmetric charging effects for the image generated for the sample, which also results in symmetric PR damage. In one example, a charged particle beam (e.g., e-beam) is simply scanned back across the line structures so as to have two opposite-direction scans for each scanned line position (or each scanned swath). <FIG> illustrates a process for symmetrically scanning the charged particle beam back across a Y direction grating target in a direction that is opposite of the charged particle beam scan illustrated in <FIG> to result in target image <NUM>. As shown, e-beam scans <NUM> will tend to result in asymmetry between the leading and lagging edges in the target image <NUM>, which is opposite the asymmetry in the target image <NUM> that is caused by asymmetric beam scans <NUM> as shown in <FIG>. Combining images from the bidirectional (opposite or symmetrical) scans will tend to result in symmetrical charging effects in the combined image.

<FIG> illustrates a scanning pattern that includes symmetrical beam scans that are perpendicular to the Y direction grating structures, which can be used to form a symmetric combined target image <NUM> in accordance with one embodiment of the present invention. Said in another way, a charged particle beam is scanned symmetrically in both a "north" direction (up) and a "south" direction (down) with respect to the longitudinal axis of each line in the grating that is oriented in an east and west direction.

A similar technique can be implemented with respect to an X direction overlay target. For example, the target image <NUM> of <FIG> was obtained from grating lines that each have a north and south aligned longitudinal axis. In this example, a beam may also be scanned in a west direction that is opposite the east-direction beam scan <NUM> to thereby achieve symmetrical east and west direction scans relative to the overlay target grating <NUM>. The images from the symmetric scans (east and west) may then be combined to form a symmetric image.

In sum, e-beam patterns that scan in both +X and -X (east and west) directions or in both +Y and -Y (north and south) directions can be used to form combined symmetric target images for X and Y direction grating structures, respectively. The asymmetries in the two different images that were formed by the two directional, but symmetrical, scans may then be combined to form a symmetric image. The symmetric image may then be analyzed for accurate overlay (or other measurements, such as CD) determination.

Although symmetric (or bi-directional) beam scans that are perpendicular to the grating targets work well for single direction (X or Y) targets, a different orientation of the symmetric scans may be used with respect to a multidirectional targets (e.g., X and Y target). <FIG> is a top plan view of a combination XY overlay mark <NUM> for which an alternate symmetric beam scan pattern may be implemented in accordance with a specific embodiments of the present invention as described further below. Unlike the targets of <FIG>, the overlay mark <NUM> of <FIG> is configured to measure overlay in two separate directions. As such, mark <NUM> obviates the need to have one mark for each direction in which overlay needs to be measured. Overlay mark <NUM> is shown in a configuration that results when the tested layers of a wafer are in perfect alignment. The overlay mark <NUM> is generally provided to determine the relative shift between two or more successive layers of a wafer or between two or more separately generated patterns on a single layer of a wafer. For ease of discussion, the overlay mark <NUM> will be described in context of measuring overlay between different layers of a substrate. It should be noted, however, that the overlay mark in this figure (or any other target described herein) may also be used to measure two or more separately generated patterns on a single layer of a substrate.

The overlay mark <NUM> includes a plurality of working zones <NUM> for determining the registration error between two wafer layers in two different directions. In the illustrated embodiment, the overlay mark <NUM> includes eight rectangular shaped working zones <NUM>, which are configured to substantially fill its perimeter <NUM>. The working zones <NUM> represent the actual areas of the mark that are used to calculate alignment between different layers of the wafer. The working zones <NUM> are spatially separated from one another so that they do not overlap portions of an adjacent working zone. In this particular configuration, some of the working zones are separated via exclusion zones while other working zones are positioned next to an adjacent working zone. For example, working zone 332B is separated from working zones 332E and 332F via an exclusion zone <NUM> while working zones 332E and 332F are positioned next to one another at their edges without an exclusionary zone there between.

To facilitate discussion, the working zones <NUM> are grouped into a first working group <NUM> and a second working group <NUM>. The first working group <NUM> includes four working zones 332A-D that are configured to provide overlay information in a first direction. By way of example, the first direction may be the Y direction. Of the four working zones 332A-D, two of them 332A and 332D are disposed in the first layer and two of them 332B and 332C are disposed in the second layer (the first layer is represented by cross hatching, the second layer is represented by no cross hatching). As should be appreciated, for this mark configuration and in the case of zero overlay error (as shown), the centers of symmetry <NUM> of working zones 332A&D and working zones 332B&C coincide exactly. The second working group <NUM> includes four working zones 332E-H configured to provide overlay information in a second direction that is perpendicular to the first direction. By way of example, the second direction may be the X direction. Of the four working zones 332E-H, two of them 332E and <NUM> are disposed in the first layer and two of them 332F and <NUM> are disposed in the second layer (the first layer is represented by cross hatching, the second layer is represented by no cross hatching). Similarly to the above, for this mark configuration and in the case of zero overlay (as shown), the centers of symmetry <NUM> of working zones 332E&H and working zones 332F&G coincide exactly. In this target or any targets described herein, the centers of symmetry may also be offset by a predefined amount and a deviation from such predefined offset indicates an overlay error.

As should be appreciated, each of the groups <NUM> and <NUM> represents an "X" - configured mark (albeit offset). For example, working group <NUM> includes working zones 332A&D, which are on the same first layer and in diagonally opposed positions relative to one another, and working zones 332B&C, which are on the same second layer and in diagonally opposed positions relative to one another. Further, working zones 332A&D are angled relative to working zones 3322B&C. Further still, working zone 332A is spatially offset from working zone 332D, and working zone 332B is spatially offset from working zone 332D.

In addition, working group <NUM> includes working zones 332E&H, which are on the same first layer and in diagonally opposed positions relative to one another, and working zones 332F&G, which are on the same second layer and in diagonally opposed positions relative to one another. Further, working zones 332E&H are angled relative to working zones 332F&G. Further still, working zone 332E is spatially offset from working zone <NUM>, and working zone 332F is spatially offset from working zone <NUM>. In essence, this particular mark produces two "X" configured marks that are positioned orthogonal to each other, i.e., working group <NUM> and working group <NUM>.

To elaborate further, a working zone on one layer is generally juxtaposed relative to a working zone on another layer. For example, in the first working group, working zone 332A is juxtaposed relative to working zone 332B and working zone 332C is juxtaposed relative to working zone 332D. Similarly, in the second working group, working zone 332E is juxtaposed relative to working zone <NUM> and working zone 332F is juxtaposed relative to working zone <NUM>. Of the two juxtaposed pairs, the working zone on the second layer is typically positioned closer to the center than the working zone on the first layer. For example, working zones 332B and 332C and working zones 332F and <NUM> are positioned closer to the center <NUM> of the region of interest <NUM> than their juxtaposed working zones 332A and 332D and working zones 332E and <NUM>, respectively. Furthermore, within each of the working groups, the juxtaposed pairs are positioned in an opposed relationship (e.g., diagonal) relative to the other juxtaposed pair in the group. For example, juxtaposed pairs 332A&B are positioned opposite juxtaposed pairs 332C&D, and juxtaposed pairs 332E&F are positioned opposite juxtaposed pairs <NUM>&H.

As should be appreciated, in this particular mark, the configuration of the working zones is rotationally symmetric (+<NUM>, <NUM>, <NUM>, <NUM> degrees around the center of the mark). This is typically done to reduce the impact of radial and axial variations across the field of view of the metrology tool, as for example, radial and axial variations caused by non-uniform optical aberrations and illumination that may cause tool induced shifts (TIS). Radial variations generally refer to variations that radiate from the center of the mark to the outer regions of the mark. Axial variations generally refer to variations that occur in directions along the axis of the mark, as for example, in the X direction from the left to the right portions of the mark, and in the Y direction from the lower to the upper portions of the mark.

Each of the working zones 332A-H includes a periodic structure <NUM> comprised of a plurality of coarsely segmented lines <NUM>. The linewidths, D, and spacings, s, of the coarsely segmented lines may be widely varied. As shown, each of the periodic structures <NUM> substantially fills the perimeter of its corresponding working zone <NUM>. As should be appreciated, the periodic structures <NUM> are also disposed on the layer of its corresponding working zone <NUM>.

For ease of discussion, the periodic structures <NUM> may be broken up into a first periodic structure 338A that is associated with the first working group <NUM> and a second periodic structure 338B that is associated with the second working group. As shown, the first periodic structures 338A are all oriented in the same direction, i.e., the coarsely segmented lines <NUM> are parallel and horizontally positioned relative to each other. The second periodic structures 338B are also all oriented in the same direction (albeit differently than the first periodic structures), i.e., the coarsely segmented lines <NUM> are parallel and vertically positioned relative to each other. As such, the periodic structures 338A in the first working group <NUM> are orthogonal to the periodic structures 338B in the second working group <NUM>.

In one example, the coarsely segmented lines of juxtaposed periodic structures are aligned with one another, (e.g., if we ignore the different layers, they appear to be continuous gratings). For example, the coarsely segmented lines of working zone 332A may align with the coarsely segmented lines of working zone 332B and coarsely segmented lines of working zone 332C may align with the coarsely segmented lines of working zone 332D. In addition, the coarsely segmented lines of working zone 332E may align with the coarsely segmented lines of working zone 332F and coarsely segmented lines of working zone <NUM> may align with the coarsely segmented lines of working zone <NUM>.

For a multidirectional target, such as the XY target <NUM> of <FIG>, a symmetrical X or Y beam scan pattern would result in the e-beam being scanned along at least a portion of the target lines' longitudinal axis, which is generally avoided. For instance, an e-beam that is scanned bi-directionally along the X axis (east and west) would form scan paths that are perpendicular to the vertical lines of the second working group <NUM> and parallel with the horizontal lines of the first second working group <NUM>. Likewise, an e-beam that is scanned bi-directionally in the Y direction (north and south) would form scan paths that were perpendicular to the horizontal lines of the first working group <NUM> and parallel to the vertical lines of the second working group <NUM>.

In order to avoid scanning parallel along any of the lines of a multi-directional target, certain embodiments of the present invention include charged particle beam scan patterns that form symmetrical scan paths having angles with respect to the scanned edges that are at least <NUM>° or in a range between about <NUM>° and <NUM>° from any of the lines of the target.

The scan paths may include pairs of scan paths at multiple rotations so as to maximize the symmetry in the final combined image. <FIG> illustrates an alternative beam scan pattern having symmetric scan paths that are <NUM>° with respect to a combination XY target having both X and Y lines (e.g., target <NUM> of <FIG>) in accordance with a specific implementation of the present invention. In this example, multiple pairs of bi-directional line scans at two different rotations (or <NUM> different scan angles) symmetrically cover the target.

Any suitable technique may be used to symmetrically scan a periodic target. <FIG> is a flowchart illustrating a general procedure <NUM> for determining overlay with symmetric beam scanning in accordance with one embodiment of the present invention. Initially, a periodic target for measuring overlay in at least one direction is provided in operation <NUM> of <FIG>. For example, the target may include structures for only measuring overlay in a single direction (e.g., X or Y) or a target that includes structures for measuring overlay in two or more directions (e.g., X and Y), like the XY target <NUM> of <FIG>.

A charged particle beam may then be scanned in a first direction across a plurality of scan swaths of the target and at a first tilt with respect to the target so that each scanned edge of the target is scanned at an angle in operation <NUM>. Additionally, the charged particle beam can then be scanned in a second direction, which is opposite the first direction, across the same plurality of scan swaths and at a second tilt that is <NUM>° from the first tilt in operation <NUM>. For a target that only includes X or Y overlay structures, the pattern of scans may include either scanning in north and south directions or scanning in east and west directions, respectively, across a plurality of scanned swaths.

For other types of targets such as a combination XY target, the scanning operations may be repeated at new tilts so as to achieve symmetry in the scans with respect to the target. As shown in the combination XY target example of <FIG>, the beam is scanned with respect to the target along sets of bidirectional swaths at tilts of <NUM>°, <NUM>°, <NUM>°, and <NUM>°. That is, scanned swaths for each first pair <NUM> are at <NUM>°and <NUM>°, and scan swaths of each second pair <NUM> are at <NUM>°and <NUM>°.

These scan swaths may be achieved by scanning the charged particle beam in two translational directions while the target is moved in a translational and rotational manner. For instance, the target is moved in a linear and rotational manner under the charged particle beam so as to scan the target in four frames. Said in another way, the target is moved perpendicular to the beam's scan direction so as to scan a first set of swaths in a first direction and at a first tilt (e.g., <NUM>°). The target may then be rotated to a second tilt (e.g., <NUM>°), which is <NUM>° from the first tilt, so that the beam will effectively scan in a second opposite direction with respect to the first set of swaths. The target is then translated in a direction that is perpendicular to the beam's scan direction so as to scan the same first set of swaths in the second direction that is opposite the first direction. This process may then be repeated for the other two tilt pair <NUM>°and <NUM>°.

Of course, the charged particle may be scanned in two opposite linear directions, and the target may be rotated to only two tilt positions for such bidirectional beam scans. At each tilt position and beam direction, the target is translated under the beam in a direction that is perpendicular to the beam scan's direction so as to raster scan multiple lines in the beam's direction. For example, the target may be rotated and translated with respect to the charged particle beam so that the beam scans across a plurality of lines/swaths in two directions and at a first pair of tilts <NUM>°and <NUM>° with respect to the target. Next, the target is then rotated once more, and the target is translated so that the beam then scans across a plurality of lines/swaths in two directions and at a second pair of tilts <NUM>°and <NUM>° with respect to the target.

Symmetric charged particle scan patterns may likely result in improvements for the edge sharpness and symmetry for a wide variety of target configurations, such as an XY overlay target. In another embodiment, the beam is only scanned in opposite directions along either scan swaths <NUM> or scan swaths <NUM>, with or without rotating the target to achieve the two symmetric directions along the same scan swaths.

In order to reduce damage, e.g., to the photoresist material, each sequential scan may skip swaths (or lines) in accordance with an alternative embodiment of the present invention. <FIG> is a diagrammatic representation of a symmetric scanning process that includes skipping lines in accordance with an alternative embodiment of the present invention. In contrast to a scan pattern <NUM> that is composed of sequentially scanned swaths that result in swath images that abut or overlap each other, the scan pattern <NUM> includes sequentially scanned lines that skip swaths or lines. For example, <NUM> lines are skipped between each sequential swath scan. Of course, the scan pattern would also include scans in the opposite direction (from right to left) that trace over the illustrated scan lines <NUM>. Additionally, the skipped lines would be left unscanned, and overlay may be determined based on a combination of the signals or images that are generated in response to the beam moving across each of the scanned lines <NUM> in two opposite symmetrical directions. For example, symmetric image portions obtained in response to the scanned lines <NUM> are combined to form a combination image that excludes image data from skipped lines of the target.

Any suitable number of lines may be skipped so as to minimize damage to the scanned material and obtain enough data for accurately determining overlay or the like. In general, a scan pattern that skips lines causes less damage to certain materials such as photoresist, which tends to shrink in areas that are scanned by a charged particle beam. The number of skipped lines may be selected so that the amount of shrinkage will tend to be comparable to line edge roughness and not significantly affect the overall line edges. In one example, <NUM> or more lines are skipped. In other embodiments, <NUM> or more lines are skipped, for example, for an <NUM> by <NUM> field of view (FOV).

Regardless of whether skipping occurs during the symmetric scans, each swath scan results in generation of signals or images, for example, by the system's detector and processor system. Referring back to the illustrated process of <FIG>, the images from the first and second direction scans may then be combined into a combined image in operation <NUM>. <FIG> shows a representation of a combined image for a Y direction target portion in which no lines are skipped. If lines are skipped, the image portions for the individually scanned lines may be compressed together so that they abut each other in the combined image. If <NUM> lines are skipped by way of example, the resulting combined image will be <NUM>/<NUM> the size of an image that is formed without skipping lines.

In general, images are combined as each pair of scanned swaths of the target are collected. Alternatively, images may be combined after the images for all of the pairs of swaths for a particular overlay direction are collected or after the images for all pairs of swaths for all overlay directions are collected.

The combined image may then be analyzed to determine overlay (or another characteristic, such as CD) in operation <NUM>. Each combined image for each scan swath may be analyzed separately or a single combined image for all the swaths may be analyzed.

It is then determined whether a process is out of specification in operation <NUM>. One may determine whether the targets are within specification based on a combined image produced by embodiments of the present invention in any suitable manner, as further described herein. If a process is not out of specification, the procedure ends.

If a process is out of specification, a number of techniques may be implemented to alleviate the problem. In a first technique, in accordance with the invention, a subsequent process is adjusted to compensate for the process that is out of specification in operation <NUM>. For example, if it is determined that the photoresist pattern is misaligned in any portion, the photoresist may then be stripped and reapplied in a corrected pattern to eliminate the misalignment. The subsequent process is then performed so as to continue fabrication of the same wafer in operation <NUM>. For example, the wafer may be patterned. In a second technique, processing of the wafer may be halted and the wafer may then be discarded in operation <NUM>. The process that is out of specification may then be adjusted for subsequent wafers in operation <NUM>.

<FIG> is a flow chart illustrating the operation <NUM> of <FIG> for determining whether a target is out of specification in accordance with a specific implementation of the present invention. Although this procedure is described with respect to a target having structures with a <NUM>° rotational COS, of course, this procedure may be easily modified for structures with mirror or other type of symmetry.

In the illustrated example of <FIG>, the center of the imaged target structures are initially moved to the center of the FOV of the inspection tool in operation <NUM>. The region of interests (ROI's) of each layer are then determined in operation <NUM>. The X target structures of <FIG> will be used to illustrate the procedure of <FIG>. For example, four ROI's may be formed for the X direction target structures 106a, 106b, 108a and 108b of <FIG>, as represented by the dotted lines. The dotted line <NUM> may represent the FOV of the inspection tool, while the cross <NUM> represents the center of the X target structures.

The COS for each set of structures <NUM> and <NUM> from the first and second layers, respectively, may be determined using any suitable technique. For example, an edge technique may be utilized to determine COS for the structures in each layer. In the illustrated embodiment, the outside edges of each ROI of each layer are used to determine the COS for each layer in operation <NUM>. For the structures <NUM> and <NUM>, the outside edges of each ROI may be determined and then the edges are then used to find a center position between the outside edges of each set of structures (e.g., between structures 106a and structures 106b). For structures having sub-resolution features (e.g., target of <FIG>, which is described below), the edge of each set of sub-resolution lines (e.g., fine lines <NUM> that form part of each course set of lines <NUM>) would be measured as a single edge.

Another COS determination technique is referred to as the correlation technique. In this technique, an initial COS position is estimated between the ROI's of the structures of each layer in operation <NUM>. As shown for the structures <NUM>, an initial estimate of COS <NUM> may be positioned between structures 106a and 106b. Two linear arrays are then obtained by measuring across the two sets of structures at positions that are equal distances from the initial COS. The structures 106a and 106b will tend to each result in a periodic signal with three peak intensity values. The two obtained linear arrays are then flipped horizontally and vertically and matched and a metric of correlation such as the product is calculated. The arrays are moved with respect to one another and the metric is calculated for each offset. The metric is then plotted and the correct COS is located by finding the maximum of the correlation metric. Intelligent searching algorithms (e.g., a binary search) may also be used to efficiently locate the correct COS position.

Said in another way, for each ROI set of each layer, its <NUM>° rotation counterpart is automatically placed based on the initial COS in operation <NUM>. The COS is continually moved for each layer until the best correlation is found between the rotated image and original images of each layer in operation <NUM>. After the best correlation is found, the COS is found.

After the COS is found using any suitable technique, it is then determined whether the COS of the first layer structures differs from the COS of the second layer structures by more than a predetermined value in operation <NUM>. If they do not differ by more than the predetermined value, it is determined that the target under analysis in not out of specification in operation <NUM>. However, if they do differ by more than the predetermined amount, it is determined that the target under analysis is out of specification in operation <NUM>. The procedure for determining whether the target is out of specification then ends.

Opposite direction and tilted (with respect to the overlay direction or longitudinal axis of the target lines) scan patterns may be applied to any suitable type of combination X and Y targets, besides the example of <FIG>. Several periodic overlay targets are further described in <CIT>, which patent is incorporated herein by reference. <FIG> illustrate a plurality of different combination XY targets for which symmetric beam scanning techniques may be implemented in accordance with various embodiments of the present invention.

<FIG> is a top plan view of another multi-directional overlay mark <NUM>. In this particular embodiment, the coarsely segmented lines <NUM> are formed by a plurality of finely segmented elements <NUM>.

<FIG> is a top plan view of an overlay mark <NUM>, in accordance with an alternate target. By way of example, the overlay mark <NUM> may generally include X and Y line gratings with the addition of a box in box overlay structure <NUM>. Similarly to the overlay mark <NUM> of <FIG>, overlay mark <NUM> contains eight working zones 412A-H for determining the registration error between two wafer layers in two different directions (one layer is represented by cross-hatching, the other is not). Each of the working zones includes a periodic structure <NUM> comprised of a plurality of coarsely segmented lines <NUM>. The working zones <NUM> are arranged to accommodate additional structure <NUM> in the center of the mark <NUM>. In the illustrated embodiment, the working zones 412A-H are disposed around the outer region of the mark, while the additional structure <NUM> is disposed in the center of the mark. The additional structure <NUM> may represent a standard box in box overlay structure.

<FIG> is a top plan view of an overlay mark <NUM>, in accordance with an alternate target. Like the mark of <FIG>, the overlay mark <NUM> of <FIG> is configured to measure overlay in two separate directions. In contrast to the mark of <FIG>, the mark <NUM> includes triangularly shaped working zones <NUM>.

A first set of working zones 422A-D are configured to provide overlay information in a first direction. By way of example, the first direction may be the Y direction. Of the four working zones 422A-D, two of them 422A and D are disposed in the first layer and two of them <NUM> B and 422C are disposed in the second layer. As should be appreciated, for this mark configuration and in the case of zero overlay (as shown), the centers of symmetry <NUM> of working zones 422A&D and working zones 422B&C coincide exactly. A second set of working zones 422E-H are configured to provide overlay information in a second direction that is perpendicular to the first direction. By way of example, the second direction may be the X direction. Of the four working zones 422E-H, two of them 422E and <NUM> are disposed in the first layer and two of them 422F and <NUM> are disposed in the second layer. Similarly to the above, for this mark configuration and in the case of zero overlay (as shown), the centers of symmetry <NUM> of working zones 422E&H and working zones 422F&G coincide exactly. In addition, and all of the working zones <NUM> are equally positioned relative to the center of the mark. Each of the working zones <NUM> includes a periodic structure <NUM> comprised by a plurality of coarsely segmented lines <NUM>. Although not shown, each coarsely segmented line may be formed by a plurality of finely segmented elements to further enhance this mark or any mark described herein.

<FIG> is a top plan view of an overlay mark <NUM>, in accordance with an alternative target. As shown, mark <NUM> has the same general layout and characteristics as mark <NUM> of <FIG>, i.e., eight triangularly shaped working zones. Mark <NUM> differs from mark <NUM>, however, in that it biases the center of the mark with a grating pattern <NUM> formed on one of the two layers. The grating pattern <NUM> is typically used in cases where the mark quality in one layer is poorer than the mark quality in the other layer due to contrast or graininess. That is, the information (e.g., edges) in a layer where contrast is low is increased. Alternatively, biasing the center of the FOV with one layer may further protect it from process damage. The grating pattern <NUM> may be widely varied. For example, grating pattern may include any number of lines in any number of distributions and sizes. In this particular embodiment, the grating pattern is formed on the second layer and includes groups of two coarsely segmented lines <NUM> that alternate in direction (e.g., X and Y directions) around the center of the mark.

<FIG> is a top plan view of an overlay mark <NUM>, in accordance with an alternate target. Like the overlay mark of <FIG>, overlay mark <NUM> is configured to measure overlay in two separate directions. The overlay mark <NUM> includes a plurality of working zones <NUM> for determining the registration error between two wafer layers in two different directions. In the illustrated embodiment, the overlay mark <NUM> includes sixteen square shaped working zones <NUM>, which are configured to substantially fill its perimeter. Each of the working zones <NUM> includes a periodic structure of coarsely segmented lines. Of the <NUM> working zones, <NUM> of the working zones 442A are oriented in the X direction and <NUM> of the working zones 442B are oriented in the Y direction (as shown by the periodic structures disposed therein). Of the <NUM> working zones <NUM>, in any given orientation (A or B), <NUM> of the working zones <NUM>' are printed in a first layer (represented by cross hatching) while <NUM> of the working zones <NUM>" are printed in a second layer (not represented by cross hatching).

<FIG> is a top plan view of an overlay mark <NUM>, in accordance with an alternate target. Like the overlay mark of <FIG>, overlay mark <NUM> is configured to measure overlay in two separate directions. The overlay mark <NUM> includes a plurality of working zones <NUM> for determining the registration error between two wafer layers in two different directions. Each of the working zones <NUM> includes a periodic structure of coarsely segmented lines. Of the <NUM> working zones, <NUM> of the working zones 452A are oriented in the X direction and <NUM> of the working zones 452B are oriented in the Y direction (as shown by the periodic structures disposed therein). Of the <NUM> working zones <NUM>, in any given orientation (A or B), <NUM> of the working zones <NUM>' are printed in a first layer (represented by cross hatching) while <NUM> of the working zones <NUM>" are printed in a second layer (not represented by cross hatching).

<FIG> is a top plan view of an overlay mark <NUM>, in accordance with an alternate target. Like the overlay mark of <FIG>, overlay mark <NUM> is configured to measure overlay in two separate directions. The overlay mark <NUM> includes a plurality of working zones <NUM> for determining the registration error between two wafer layers in two different directions. Each of the working zones <NUM> includes a periodic structure of coarsely segmented lines. Of the <NUM> working zones, <NUM> of the working zones 462A are oriented in the X direction and <NUM> of the working zones 462B are oriented in the Y direction (as shown by the periodic structures disposed therein). Of the <NUM> working zones <NUM>, in any given orientation (A or B), <NUM> of the working zones <NUM>' are printed in a first layer (represented by cross hatching) while <NUM> of the working zones <NUM>" are printed in a second layer (not represented by cross hatching).

<FIG> is a top plan view of an overlay mark <NUM>, in accordance with an alternate structure. Like the overlay mark of <FIG>, overlay mark <NUM> is configured to measure overlay in two separate directions. The overlay mark <NUM> includes a plurality of working zones <NUM> for determining the registration error between two wafer layers in two different directions. Each of the working zones <NUM> includes a periodic structure of coarsely segmented lines. Of the <NUM> working zones, <NUM> of the working zones 482A are oriented in the X direction and <NUM> of the working zones 482B are oriented in the Y direction (as shown by the periodic structures disposed therein). Of the <NUM> working zones <NUM>, in any given orientation (A or B), <NUM> of the working zones <NUM>' are printed in a first layer (represented by cross hatching) while <NUM> of the working zones <NUM>" are printed in a second layer (not represented by cross hatching). Furthermore, of the <NUM> working zones <NUM>, in any given orientation (A or B), <NUM> of the working zones <NUM> have a periodic structure M with a first period (represented by thinner lines) while <NUM> of the working zones <NUM> have a periodic structure N with a second period that is different than the first period (represented by wider lines).

<FIG> is a top plan view of an overlay mark <NUM>, in accordance with an alternate structure. Like the overlay mark of <FIG>, overlay mark <NUM> is configured to measure overlay in two separate directions. However, overlay mark <NUM> is also configured to determine the relative shift between three successive layers of a wafer or between three separately generated patterns on a single layer of a wafer. In the illustrated embodiment, the overlay mark <NUM> includes sixteen square shaped working zones <NUM>. Each of the working zones <NUM> includes a periodic structure of coarsely segmented lines.

Of the <NUM> working zones <NUM>, <NUM> of the working zones <NUM>' are printed in a first layer (represented by cross hatching), <NUM> of the working zones <NUM>" are printed in a second layer (represented by white fill), and <NUM> of the working zones <NUM>‴ are printed in a third layer (represented by black fill). In this particular embodiment, the first layer (also represented by a single prime) is disposed over the second layer (also represented by a double prime) and the second layer is disposed over the third layer (also represented by a triple prime). By way of example, the first layer may represent a resist layer, the second layer may represent a first metal layer, and the third layer may represent a second metal layer. It should be noted that the above configuration may be widely varied. For example, of the <NUM> working zones in any given orientation (A or B), <NUM> may be printed in a first layer, while each additional pair of gratings may be printed in up to any of <NUM> previous layers.

<FIG> is a top plan view of an alternative combination XY overlay mark <NUM> having overlapping periodic line structures for which symmetric beam scan techniques may be implemented in accordance with specific embodiments of the present invention. This target <NUM> includes complex patterns of sub-resolution X and Y features, with some of the X structures interleaved with the Y structures. For example, working group 502A contains interleaved X and Y sub-resolution features for a first layer as shown in expanded area <NUM>. Some of the working zones also include an overlaid working zone for a second layer that is formed after and on top of the first layer. For instance, working zone 502B contains X and Y sub-resolution periodic features in the first layer with a second layer of X course periodic structures formed over the first layer periodic structures. Overlay may generally be determined by the difference between centers of symmetry/gravity of the currently formed layer patterns and previous layer patterns.

<FIG> is another target <NUM> having XY overlay structures for which symmetric beam scan techniques may be implemented in accordance with specific embodiments of the present invention. As shown, working group 602A contains Y overlay grating structures in three different layers (white, black and cross-hashing), while working group 602B contains X overlay grating structures in the same three different layers (white, black, and cross-hatching). In this example, the structures for each layer in each working group are arranged to have a same center position <NUM> when there is zero overlay error. Overlay may generally be determined by the difference between centers of gravity (or deviation from predefined offset between centers of gravity) of the currently formed layer patterns and previous layer patterns.

For each of the above-described XY targets, a scan pattern may generally include tilted bidirectional scans with respect to the line edges and symmetrical with respect to the target, with or without skipping lines in between sequential scans. For instance, the beam may be scanned with respect to combination XY target along sets of swaths that are tilted at <NUM>°, <NUM>°, <NUM>°, and <NUM>° or any suitable combination of symmetrical angles that are tilted with respect to the overlay target's orientation. For instance, the tilt angles can include <NUM>° to <NUM>° and <NUM>° to <NUM>° so the e-beam can scan perpendicular to the top layer pattern (e.g., <NUM>) in <FIG>.

The scanning techniques described herein may also be applied to targets having periodic structures that include sub-structures that are tilted in directions with respect to each other, other than perpendicular to each other like the XY combination targets of <FIG>. <FIG> is a top plan view of an alternative target portion <NUM> having periodic line structures in different layers and directions that are tilted with respect to each other at an angle that excludes a perpendicular or parallel angle. That is, the tilt angle between the two gratings is greater than <NUM>° and less than <NUM>°. In this illustration, the target has a set of lines in a first layer, which are denoted by cross-hatched shading, and a set of lines in a second layer, which are denoted by white shading. The second layer lines are tilted with respect to the first layer lines at an angle that is about <NUM>°.

In the example of <FIG> as well as other types of targets, the charged particle beam can be scanned in pairs of symmetrical scans at any suitable symmetrical combination of tilted angles. Each pair of scans include two opposite directional set of scans that are symmetrical with respect to each other, and the resulting scan paths of all scan pairs are not parallel to any longitudinal axis of the target. In general, the beam scan pattern may include scans that are either perpendicular to each of the line sets or tilted with respect to each of the line sets. For the target <NUM> by way of example, the charged particle beam would not be scanned in east and west directions since this would result in the horizontal lines of the second layer being scanned along their longitudinal axis. Overlay for the target of <FIG> may generally be determined by the difference between centers of gravity of the currently formed layer patterns and previous layer patterns.

In other embodiments, the target does not include separated X and Y structures, but integrated XY structures. <FIG> is a top plan view of an alternative overlay mark <NUM> formed from a plurality of contact structures for which X and Y overlay may be determined. As shown, the overlay mark <NUM> is configured to measure overlay in two separate directions. As such, mark <NUM> obviates the need to have separately positioned or offset structures for each X and Y direction in which overlay is to be measured. Overlay mark <NUM> is shown in a configuration that results when the tested layers of a wafer are in perfect alignment.

The overlay mark <NUM> includes a plurality of working zones <NUM> for determining the registration error between two wafer layers in two different directions. In the illustrated embodiment, the overlay mark <NUM> includes four square shaped working zones <NUM>, which are configured to substantially fill a field of view (not shown) of the metrology tool used to image the overlay mark <NUM>. The working zones <NUM> represent the actual areas of the mark that are used to calculate alignment between different layers of the wafer. As mentioned previously, the working zones <NUM> are spatially separated from one another so that they do not overlap portions of an adjacent working zone of the second layer.

In this example, the working zones are configured to provide overlay information in two directions, as for example, in the X and Y directions. Of the four working zones 812A-D, two of them 812A and 812D are disposed in the first layer and two of them 812B and 812C are disposed in the second layer (the first layer is represented by solid fill, the second layer is represented by no fill). Working zones 812A and 812D, which are disposed on the same first layer, are positioned opposite one another at a first vertical angle while working zones 812B and 812C, which are disposed on the same second layer, are positioned opposite one another at a second vertical angle (e.g., diagonally). These cross-positioned structures form an "X" shaped pattern.

Each of the working zones <NUM> contains an individual periodic structure <NUM>, as for example, periodic structures 814A-D. As shown, each of the periodic structures <NUM> substantially fills the perimeter of its corresponding working zone <NUM>. As should be appreciated, each of the periodic structures <NUM> is formed in the layer of its corresponding working zone <NUM>. The periodic structures <NUM> include coarsely segmented elements <NUM> that are arranged in spaced apart rows and columns. Optionally, each of the coarsely segmented elements <NUM>, in turn, may also be formed by finely segmented elements <NUM>. The finely segmented elements <NUM> are also arranged in spaced apart rows and columns. The individual coarsely segmented elements <NUM> and finely segmented elements <NUM> may be configured with a variety of sizes, shapes and distributions. In the illustrated embodiment, both the coarsely segmented elements <NUM> and finely segmented elements <NUM> are square shaped and equally spaced from an adjacent element. As should be appreciated, overlay mark <NUM> can be used to measure the misregistration value in two separate directions that are perpendicular to each other since the mark <NUM> has the same repeating structural pattern in orthogonal directions.

The overlay contact array target <NUM> may be scanned in north, south, east, and west directions, only north and south directions (for Y overlay), or only eat and west directions (for X overlay). Any scan pattern may include skipping lines. In another combination example (not illustrated), the target may include an array of cross-shaped structures. In this later example, the beam may be scanned at tilted angles with respect to the target, like the combination XY targets described herein.

Any suitable combination of hardware and/or software may be used to implement any of the above described techniques. <FIG> is a diagrammatic representation of a scanning electron microscopy (SEM) overlay metrology system in accordance with one embodiment of the present invention. Scanning in symmetrical directions across the periodic structures may avoid image asymmetries caused by buildup of electrons, and as a result, minimize inaccurate overlay measurements (or the like).

In some embodiments, the system <NUM> may include, but is not limited to, a defect-review (DR) SEM tool with SEM overlay option, a critical-dimension (CD) SEM tool with SEM overlay option, a standalone SEM tool, a lithography/etch tool with integrated SEM overlay metrology, or a lithography/etch metrology cluster with features such as imaging optical overlay, scatterometry optical overlay, scatterometry CD, and CDSEM with SEM overlay option. The system <NUM> may be configured to scan a sample <NUM> such as, but not limited to, a wafer (e.g., semiconductor wafer) having two or more layers formed thereon with an electron beam <NUM> in order to determine overlay error (e.g., a misalignment or spatial offset between at least two layers of interest).

The system <NUM> may operate in any scanning mode known in the art. For example, the system <NUM> may operate in a swathing mode when scanning an electron beam <NUM> across the surface of the sample <NUM>. In this regard, the system <NUM> may scan an electron beam <NUM> across the sample <NUM>, while the sample is moving, with the direction of scanning being nominally perpendicular to the direction of the sample motion. By way of another example, the system <NUM> may operate in a step-and-scan mode when scanning an electron beam <NUM> across the surface of the sample <NUM>. In this regard, the system <NUM> may scan an electron beam <NUM> across the sample <NUM>, which is nominally stationary when the beam <NUM> is being scanned.

The system <NUM> may include an electron beam source <NUM> for generating one or more electron beams <NUM>. The electron beam source <NUM> may include any electron source known in the art. For example, the electron beam source <NUM> may include, but is not limited to, one or more electron guns. In some embodiments, a computing system <NUM> or controller may be communicatively coupled to the electron beam source <NUM>. The computing system <NUM> may be configured to adjust one or more electron source parameters via a control signal to the electron beam source <NUM>. For example, the computing system <NUM> may be configured to vary the beam current for the electron beam <NUM> emitted by source <NUM> via a control signal transmitted to control circuitry of the electron beam source <NUM>.

The sample <NUM> may be disposed on a sample stage <NUM> configured to support the sample <NUM> during scanning. In some embodiments, the sample stage <NUM> is an actuatable stage. For example, the sample stage <NUM> may include, but is not limited to, one or more translational stages suitable for selectably translating the sample <NUM> along one or more linear directions (e.g., x-direction, y-direction and/or z-direction). By way of another example, the sample stage <NUM> may include, but is not limited to, one or more rotational stages suitable for selectably rotating the sample <NUM> along a rotational direction. By way of another example, the sample stage <NUM> may include, but is not limited to, a rotational stage and a translational stage suitable for selectably translating the sample along a linear direction and/or rotating the sample <NUM> along a rotational direction.

In some embodiments, the computing system <NUM> or controller is communicatively coupled to the sample stage <NUM>. The computing system <NUM> may be configured to adjust one or more stage parameters via a control signal transmitted to the sample stage <NUM>. The computing system <NUM> may be configured to vary the sample scanning speed and/or control the scan direction via a control signal transmitted to control circuitry of the sample stage <NUM>. For example, the computing system <NUM> may be configured to vary the speed and/or control the direction with which sample <NUM> is linearly translated (e.g., x-direction or y-direction) relative to the electron beam <NUM>. As discussed in further detail herein, the sample <NUM> may be scanned in a tilted orientation relative to feature placement (e.g., perpendicular or tilted otherwise with respect to a longitudinal axis of the pattern lines) of target structures forming an overlay metrology target or mark on the sample <NUM>.

The system <NUM> may further include a set of electron-optic elements <NUM>. The set of electron-optics may include any electron-optic elements known in the art suitable for focusing and/or directing the electron beam <NUM> onto a selected portion of the sample <NUM>. In one embodiment, the set of electron-optics elements includes one or more electron-optic lenses. For example, the electron-optic lenses may include, but are not limited to, one or more condenser lenses <NUM> for collecting electrons from the electron beam source. By way of another example, the electron-optic lenses may include, but are not limited to, one or more objective lenses <NUM> for focusing the electron beam <NUM> onto a selected region of the sample <NUM>. In some embodiments, the electron beam <NUM> may be directed to the sample <NUM> at a controlled angle to the sample grating. Because wafer system of coordinates does not necessarily coincide with SEM system of coordinates, controlling a fine scan angle may improve matching between the coordinate systems and significantly contribute to sampling performance/accuracy.

In some embodiments, the set of electron-optics elements includes one or more electron beam scanning elements <NUM>. For example, the one or more electron beam scanning elements <NUM> may include, but are not limited to, one or more scanning coils or deflectors suitable for controlling a position of the beam relative to the surface of the sample <NUM>. In this regard, the one or more scanning elements <NUM> may be utilized to scan the electron beam <NUM> across the sample <NUM> in a selected scan direction or pattern. For example, the sample <NUM> may be scanned in tilted or perpendicular bidirectional scans relative to feature placement (e.g., at bidirectional directions and angled with respect to target lines) of target structures forming an overlay metrology target or mark on the sample <NUM>. The computing system <NUM> or controller may be communicatively coupled to one or more of the electron-optic elements <NUM>, such as the one or more scanning elements <NUM>. Accordingly, the computing system may be configured to adjust one or more electron-optic parameters and/or control the scan direction via a control signal transmitted to the one or more communicatively coupled electron-optic elements <NUM>.

The system <NUM> may further include a detector assembly <NUM> configured to receive electrons <NUM> from the sample <NUM>. In some embodiments, the detector assembly <NUM> includes an electron collector <NUM> (e.g., secondary electron collector). The detector assembly may further include an energy filter <NUM> based, for example, on retarding field principle. In this regard, the energy filter <NUM> may be configured to stop low energy secondary electrons while passing high energy secondary electrons (i.e., backscattered electrons). If the energy filter <NUM> is not activated, all secondary electrons are detected according to collection efficiency of the detection system. By subtracting high energy electron image from overall electron image, low energy secondary electron image can be obtained. The detector assembly <NUM> may further include a detector <NUM> (e.g., scintillating element and PMT detector <NUM>) for detecting electrons from the sample surface (e.g., secondary electrons). In some embodiments, the detection system <NUM> may include several electron detectors, such as, for example, one or more Bright Field (BF) detectors <NUM> and one or more Dark Field (DF) detectors <NUM>. In some embodiments, there may be from <NUM> to <NUM> (or even more) DF detectors <NUM>. The BF detector <NUM> detects electrons with low (according to wafer normal) emission angles, while DF detectors <NUM> provide information carried by the electrons with higher emission angles. In some embodiments, the detector <NUM> of the detector assembly <NUM> includes a light detector. For example, the anode of a PMT detector of the detector <NUM> may include a phosphor anode, which is energized by the cascaded electrons of the PMT detector absorbed by the anode and may subsequently emit light. In turn, the light detector may collect light emitted by the phosphor anode in order to image the sample <NUM>. The light detector may include any light detector known in the art, such as, but not limited to, a CCD detector or a CCD-TDI detector. The system <NUM> may include additional/alternative detector types such as, but not limited to, Everhart-Thornley type detectors. Moreover, the embodiments described herein are applicable to single detector and multiple-detector arrangements.

In some embodiments, the computing system <NUM> or controller is communicatively coupled to the detector assembly <NUM>. The computing system <NUM> may be configured to adjust one or more image forming parameters via a control signal transmitted to the detector assembly <NUM>. For example, the computing system may be configured to adjust the extraction voltage or the extraction field strength for the secondary electrons. Those skilled in the art will appreciate that "the computing system <NUM>" may include one or more computing systems or controllers, such as one or more processors configured to execute one or more instruction sets embedded in program instructions stored by at least one non-transitory signal bearing medium. The computing system <NUM> may control various scanning or sampling parameters such as, but not limited to, those described herein.

While the foregoing description focused on the detector assembly <NUM> in the context of the collection of secondary electrons, this should not be interpreted as a limitation on the present invention. It is recognized herein that the detector assembly <NUM> may include any device or combination of devices known in the art for characterizing a sample surface or bulk with an electron beam <NUM>. For example, the detector assembly <NUM> may include any particle detector known in the art configured to collect backscattered electrons, Auger electrons, transmitted electrons or photons (e.g., x-rays emitted by surface in response to incident electrons). In some embodiments, as discussed above, the detected electrons are differentiated (e.g., secondary electrons vs. backscattered electrons) based upon the energy levels and/or emission angles of the detected electrons, and by subtracting high energy electron image from overall electron image, low energy secondary electron image can be obtained.

The computing system <NUM> may be configured to receive and/or acquire data or information (e.g., detected signals/images, statistical results, reference or calibration data, training data, models, extracted features or transformation results, transformed datasets, curve fittings, qualitative and quantitative results, etc.) from other systems by a transmission medium that may include wireline and/or wireless portions. In this manner, the transmission medium may serve as a data link between the computing system <NUM> and other systems (e.g., memory on-board metrology system, external memory, reference measurement source, or other external systems). For example, computing system <NUM> may be configured to receive measurement data from a storage medium (e.g., internal or external memory) via a data link. For instance, results obtained using the detection system may be stored in a permanent or semipermanent memory device (e.g., internal or external memory). In this regard, the results may be imported from on-board memory or from an external memory system. Moreover, the computing system <NUM> may send data to other systems via a transmission medium. For instance, qualitative and/or quantitative results, such as overlay values, determined by computing system <NUM> may be communicated and stored in an external memory. In this regard, measurement results may be exported to another system.

Computing system <NUM> may include, but is not limited to, a personal computer system, mainframe computer system, workstation, image computer, parallel processor, or any other device known in the art. In general, the term "computing system" may be broadly defined to encompass any device having one or more processors, which execute instructions from a memory medium. Program instructions may be stored in a computer readable medium (e.g., memory). Exemplary computer-readable media include read-only memory, a random access memory, a magnetic or optical disk, or a magnetic tape.

The metrology tool may be designed to make many different types of measurements related to semiconductor manufacturing. Certain embodiments of the invention for determining quality and/or quantitative values may utilize such measurements. Additional metrology techniques for determining specific target characteristics may also be combined with the above-described quality determination techniques. For example, in certain embodiments the tool may obtain signals/images and determine characteristics of one or more targets (e.g., overlay, critical dimensions, sidewall angles, film thicknesses, process-related parameters (e.g., focus and/or dose). The targets can include certain regions of interest that are periodic in nature, such as for example gratings in a memory die. Targets can include multiple layers (or films) whose thicknesses can be measured by the metrology tool. Targets can include target designs placed (or already existing) on the semiconductor wafer for use, e.g., with alignment and/or overlay registration operations. Certain targets can be located at various places on the semiconductor wafer. For example, targets can be located within the scribe lines (e.g., between dies) and/or located in the die itself. In certain embodiments, multiple targets are measured (at the same time or at differing times) by the same or multiple metrology tools. The data from such measurements may be combined. Data from the metrology tool may be used in the semiconductor manufacturing process, for example, to feed-forward, feed-backward and/or feed-sideways corrections to the process (e.g. lithography, etch) and therefore, might yield a complete process control solution.

Computational algorithms are usually optimized for metrology applications with one or more approaches being used such as design and implementation of computational hardware, parallelization, distribution of computation, load-balancing, multi-service support, dynamic load optimization, etc. Different implementations of algorithms can be done in firmware, software, FPGA, programmable optics components, etc..

Claim 1:
A method (<NUM>) of performing overlay metrology upon a target having at least two layers formed thereon, the method comprising:
providing a target having a plurality of periodic structures for measuring overlay in at least two overlay directions (<NUM>);
scanning a charged particle beam in a first direction across a plurality of scan swaths of the target and at a first angle with respect to the target so that each edge of the periodic structures is scanned at an angle (<NUM>);
scanning the charged particle beam in a second direction, which is opposite the first direction, across the plurality of scan swaths and at a second angle that is <NUM>° from the first angle (<NUM>);
repeating the first and second direction scanning operations for different first and second angles and a different plurality of scan swaths of the target so that the target is scanned symmetrically;
combining images that are generated by the first and second direction scanning operations to form a combined image (<NUM>);
determining and reporting an overlay error of the target based on analyzing the combined image (<NUM>); and
determining whether a process is out of specification based on the overlay error (<NUM>);
adjusting a subsequent process to compensate for the process that is determined to be out of specification (<NUM>);
performing the subsequent process to continue fabrication of the same wafer (<NUM>).