Classifications

• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
  • G03F7/70—Microphotolithographic exposure; Apparatus therefor
  • G03F7/70483—Information management; Active and passive control; Testing; Wafer monitoring, e.g. pattern monitoring
  • G03F7/70605—Workpiece metrology
  • G03F7/70616—Monitoring the printed patterns
  • G03F7/70633—Overlay, i.e. relative alignment between patterns printed by separate exposures in different layers, or in the same layer in multiple exposures or stitching

Definitions

• 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.
• an improved overlay target arrangement and methods for its fabrication and use are disclosed.
• the present invention is directed toward target arrangements that include target cells having overlaid tiers of periodic features set with possibly different pitches. Further embodiments enhance the target arrangements through the addition of disambiguation features that enable resolution of ambiguities in the periodic signals generated by the target cells. Method of using these target arrangements are also disclosed.
• One embodiment of the invention comprises a target arrangement for determining overlay alignment.
• the target arrangement is formed on a substrate to be inspected such that the arrangement includes at least two layers that are capable of being illuminated by an illumination source to produce a resulting spectrum (signal).
• the target arrangement comprises a plurality of periodic target cells configured such that each cell has a top layer arranged above a bottom layer.
• the layers are configured such that the top layer has periodic features and the bottom layer has periodic features and such that there is an offset between the top layer and the bottom layer of the cell.
• the target cells can be configured so that the periodic features of the top layer have a different pitch than the periodic features of the bottom layer.
• an inventive target arrangement can include embodiments that include disambiguation features, which upon illumination and analysis, enable the resolution of ambiguities in the spectra generated by the periodic target cells.
• a target arrangement configured similar to the above embodiments is arranged so that the values of offsets provide improved accuracy and repeatability of overlay measurements.
• the target cells are configured so that the periodic features of the top layer have the same pitch as the periodic features of the bottom layer.
• the periodic features of the top layer of a target cell can be arranged to have a different pitch than the periodic features of the bottom layer of the cell.
• the pitch ratio for the cell can be chosen such that each target cell (comprising the two layers) is periodic.
• the target arrangement comprises a plurality of periodic target cells formed on a substrate such that the cells each include at least two layers that are capable of being illuminated by an illumination source to produce a resulting spectrum (signal).
• the target arrangement comprises a plurality of periodic target cells configured such that each cell has a top layer arranged above a bottom layer.
• the layers are configured such that the top layer has periodic features and the bottom layer has periodic features and such that there is an offset between the top layer and the bottom layer of the cell.
• the values of offsets are chosen for optimized accuracy and repeatability of the overlay measurement.
• the target cells are configured so that the periodic features of the top layer have a different pitch than the periodic features of the bottom layer. The pitch ratio being chosen so that the complete structure of the two layers is periodic.
• the invention includes a method of determining overlay error using the aforementioned target arrangements.
• a substrate is provided having a target arrangement formed thereon.
• the target arrangement includes a plurality of target cells each having first and second layers with a set of periodic features configured such that the periodic features of the first layer have a different pitch than the periodic features of the second layer.
• the target arrangement is also provided having disambiguation features arranged between the target cells, the disambiguation features being configured to enable resolution of signal ambiguities caused by the generation of the spectra during illumination of the cells.
• the target cells are illuminated to obtain spectra associated with each of the target cells.
• the disambiguation features are illuminated to obtain a signal associated with the disambiguation features.
• the spectra information is used to determine the overlay error between the first and second layers. Any ambiguity in this determination is resolved by using information obtained from the signal associated with the disambiguation features.
• Fig. 1 illustrates the relative distribution of designed overlay offsets
• FIG. 2(a) is a side view illustration of a patterned top layer L2 being offset by an amount +F from a patterned bottom layer Ll in accordance with one embodiment of the present invention.
• Fig. 2(b) is a side view illustration of a patterned top layer L2 being offset by an amount -F from a patterned bottom layer Ll in accordance with one embodiment of the present invention.
• Fig. 2(c) is a side view illustration of a patterned top layer L2 being offset by an amount +F+fo from a patterned bottom layer Ll in accordance with one embodiment of the present invention.
• Fig. 2(d) is a side view illustration of a patterned top layer L2 being offset by an amount -F+fo from a patterned bottom layer Ll in accordance with one embodiment of the present invention.
• Fig. 2(e) is a side view illustration of a patterned top layer L2 being offset by an amount +F+fo+E from a patterned bottom layer Ll in accordance with one embodiment of the present invention.
• Fig. 2(f) is a side view illustration of a patterned top layer L2 being offset by an amount -F+fo+E from a patterned bottom layer Ll in accordance with one embodiment of the present invention.
• Fig. 2(g) is a simplified flow diagram illustrating an embodiment of determining overlay error in accordance with the present invention.
• Fig. 2(h) is a set of simplified side view illustrations showing an example offset and the pitch differentials between a first and second layer of periodic features of a target cell constructed in accordance with the principles of the present invention.
• Figs. 3 (a) & 3(b) are simplified plan view depictions of SCOL targets constructed in accordance with the principles of the invention.
• Fig. 4 is a simplified schematic side section view of a portion of the target depicted in Fig. 4 and constructed in accordance with the principles of the invention.
• Fig. 5 is a simplified plan view depiction of a SCOL target constructed in accordance with the principles of the invention and showing ROI' s.
• Fig. 6 is a simplified plan view depiction of another SCOL target embodiment constructed in accordance with the principles of the invention.
• Fig. 7 depicts a SCOL target such as embodied in Fig. 6 depicting
• FIG. 8 is another simplified plan view depiction of a SCOL target embodiment constructed in accordance with the principles of the invention.
• like reference numerals designate like structural elements. Also, it is understood that the depictions in the Figures are not necessarily to scale.
• the present invention encompasses scatterometry targets and methods for their use in determining overlay alignment errors.
• a SCOL target commonly consists of two sets of four (4) cells (one set for each direction). Each cell includes two overlapping gratings generally set in two overlapping layers. The design (e.g., the pitch and grating configuration) of each set includes four (4) cells that are identical except for an intentional offset between the top and bottom gratings of each cell.
• a set of four or more scatterometry overlay targets are formed on a sample, such as a semiconductor device.
• the targets are used to provide a measure of the placement accuracy of various structures comprised in the device.
• placement accuracy is characterized by measurement of an overlay error between two different layers of the semiconductor device.
• 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.
• an offset may be defined as the sum or the difference of two separate distances: a first distance F and a second distance fo, with F being greater than f 0 .
• predetermined offsets for each of these targets may be defined as follows for an example of one target design:
• the offsets for X 3 through Xa may be any suitable value for practicing the techniques of the present invention so as to determine overlay.
• Fig. 1 illustrates the distribution of offsets X a , Xb, X c and Xa along the x-axis. As shown, the offsets X a and X c are both positive with X 3 being larger than Xc. Offsets X b and X d are both negative with Xa being more negative than X b - [0037]
• the number of targets and the magnitude and sense of their corresponding offsets may be chosen in any suitable manner so that the techniques of the present invention may be practiced to determine overlay error. An illustrative example set of targets and their corresponding offsets are described below in relation to Figures 2(a) through 2(f).
• 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 Ll.
• Each layer Ll 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.
• layer Ll includes the complete structures 204a-c, while layer L2 includes the complete structures 202a-c.
• Construction of scatterometry overlay targets structures and methods for producing them are described in U.S. patent application, having application no. 09/833,084, filed 10 April 2001, entitled “PERIODIC PATTERNS AND TECHNIQUE TO CONTROL MISALIGNMENT", by Abdulhalim, et al., which application is herein incorporated by reference in its entirety.
• the structures of the top layer L2 are offset by an amount F from the structures of the bottom layer Ll.
• the structures of the two offset layers may be located within adjacent layers or have any suitable number and types of layers disposed in between the two offset layers.
• FIG. 2(a) also shows three films Tl 1 T2, and T3 between patterned layers Ll and L2 and their corresponding structures. To the extent that any other layers exist between the two layers having the structures, these other layers exhibit at least a minimum degree of transmission for electromagnetic radiation to permit propagation of the radiation between the layers . having the structures.
• Fig. 2(b) is a side view illustration of a patterned top layer L2 being offset by an amount -F from a . patterned bottom layer Ll in accordance with one embodiment of the present invention.
• Fig. 2(c) is a side view illustration of a patterned top layer L2 being offset by an amount +F + fo from a patterned bottom layer Ll in accordance with one embodiment of the present invention.
• FIG. 2(d) is a side view illustration of a patterned top layer L2 being offset by an amount +F+fo from a patterned bottom layer Ll in accordance with one embodiment of the present invention.
• Fig. 2(e) is a side view illustration of a patterned top layer L2 being offset by an amount +F + fo +E from a patterned bottom layer Ll in accordance with one embodiment of the present invention.
• Fig. 2(f) is a side view illustration of a patterned top layer L2 being offset by an amount -F + fo +E from a patterned bottom layer Ll in accordance with one embodiment of the present invention.
• an error offset E may be determined by analyzing at least the measured spectra from four or more targets A, B, C 5 and D each having offsets between two patterned layers, such as offsets X a through Xd. This analysis can be performed without comparing any of the spectra to a known or reference spectra i.e., in the absence of calibration.
• the four targets A, B, C, and D are designed to have offsets X 3 through Xa as described above. That is, target A is designed with offset +F + fo; target B with offset -F + f 0 ; target C with offset +F - f 0 ; and target D with offset - F - f 0 .
• a incident radiation beam is directed towards each of the four targets
• Fig. 1 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 February 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.
• Aspects of the invention can be used to improve the design of the foregoing.
• Embodiments of the invention can be used to optimize the number of cells in a target and their respective offsets.
• Other embodiments of the invention can be used to define a range of pitch relationships that characterize the relationship between the pitch of the top and bottom gratings where the pitches of the two layers are not the same. Additionally, values for the intentional offsets (F, fo) between the gratings for given pair of layers having differing pitches can be determined. Also, target structures can be enhanced using added elements to reduce the inherent ambiguity in overlay measurements obtained using SCOL. 10045] Heretofore, typical SCOL targets used the same pitch for both the top and bottom gratings, i.e., the pitch of the top and bottom gratings are identical. The following disclosure specifically identifies some implementation parameters that are highly suitable for use in accordance with the principles of the invention.
• Embodiments of the invention include methods and target arrangements configured to take advantage of specially constructed target cells. Such cells have alignment features arranged in different layers of the cell.
• the alignment features are embodied in a series of periodically repeating structures that are constructed so that the pitch of the two layers of repeating features are different.
• Some aspects of the invention are directed to defining a set of acceptable pitch relationships between the layers of a cell and determining corresponding offsets for such cells.
• the invention establishes a set of allowed pitch relationships and the optimal offsets that correspond to the pitch relationships.
• a particularly suitable set of desirable offsets for each of the four cells of a target for a given pitch (p) is as follows: for pitch ( p ) suitable offsets are p/4 +f 0 , — p/4 + f 0 , p/4 — / 0 and - p/4 — / 0 , where / 0 is a free parameter in the range of 0 ⁇ f Q ⁇ 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.
• a pair six target cell sets i.e., twelve target cells
• a suitable set of desirable offsets for a given pitch (/? ) is - p/12, p/4, —p/4, p/12 and — 5p/ ⁇ 2 for each one of the six cells.
• the inventors point out that for such targets, the free parameter is not needed.
• Fig. 8 provides a simplified schematic depiction of a six cell target arrangement suitable for use in some embodiments of the invention.
• One set of six cells is arranged in each perpendicular axis of the target.
• other offset values are generally employed.
• SCOL signal Due to the periodicity of SCOL signals, the resulting signals produced by a given overlay scheme is somewhat ambiguous (i.e., a selected set of phase shifted patterns can look identical to one another). This means that SCOL signal is generally determinable up to an integer multiple ofp/2. In many cases this may not pose a serious problem. However, in many other cases this ambiguity produces a degree of uncertainty that produces an overlay estimate of insufficient accuracy. In such case the signal ambiguity is unacceptable.
• this ambiguity is resolved by adding some box-like structures (sometimes referred to as (BiB) or box-in-box structures) that can be employed to obtain a rough measurement of the overlay offset.
• box-like structures sometimes referred to as (BiB) or box-in-box structures
• Such measurements are less accurate than the periodic measurements made above, they are generally sufficiently accurate to resolve a P/2 ambiguity in the previously described approach.
• Some embodiments of the present invention utilize specific designs for box-like structures to enable the resolution of such ambiguity.
• a substrate having layers formed thereon is provided (Step 252).
• Such substrates are of a type known and used by those of ordinary skill.
• the substrate of this embodiment includes 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 with a set of periodic features configured such that the periodic features of the first layer have a different pitch than the periodic features of the second layer.
• the layers of the target cells are generally associated with the layers of the substrate.
• the target cells are illuminated by an illumination source to obtain spectra for each of the target cells (Step 254).
• This illumination of the target cells produces a number of spectra which are distinct and depend on which target cell is illuminated and the extent of any misalignment in the layers.
• the spectra are received by sensors which then provide an associated signal.
• These signals are processed using a processor and any overlay error between the first and second layers is determined using information obtained using the spectra obtained for each of the target cells (Step 256).
• this is the end of the process. Due to the nature of the foregoing Steps 542-256 and the periodic nature in the signal obtained, there can be some ambiguity in the results and a certain indeterminacy in the degree of overlay error can result.
• the following optional process steps can be used to clarify the resulting ambiguities and in some embodiments this may be a desirable result.
• Step 252 includes providing a target arrangement that further includes disambiguation features arranged between the target cells, the disambiguation features configured to enable signal ambiguities caused by the generation of the periodic signal to be resolved.
• the accuracy obtainable with the disambiguation features alone is quite a bit less than that obtainable within the target cells described above, it is generally enough to resolve the periodic ambiguity inherent in the target cells.
• Step 258 includes illuminating the disambiguation features with the illumination source to obtain a signal associated with the disambiguation features. Generally, this signal is an image signal.
• Step 259 the ambiguity is resolved by a comparison of disambiguation signals to determine the extent of the overlay error.
• a disambiguation feature can be imaged and then measurements taken as the feature is rotated about an axis of symmetry and then a rough estimate of overlay error determined.
• Embodiments ' of the . invention can include SCOL targets having different grating pitches for the top and bottom gratings.
• the differing grating pitch values for the bottom and top gratings pitch can be defined as, p x and p 2 , respectively.
• the relationship between the pitch values are selected to enable the resulting scatterometry signal to remain periodic as a function of the overlay.
• 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.
• the periodicity of any scatterometry signal is not equal to "spatial periodicity" P of the combined structure of the two gratings.
• the following case illustrates a method of determining/?.
• the example begins with pitch values (e.g., pi, pi).
• the bottom grating has a pitch of 200 nm (nanometers) and the top grating has a pitch of 300 nm.
• nj can equal 3 and «.? can equal 2 and thereby satisfy the condition.
• P — n ⁇ p x ⁇ n 2 p 2 to define a spatial periodicity P.
• the spatial periodicity P of the target (or cell) is 600nm.
• the Greatest Common Divisor (GCD) of p ⁇ and p 2 is 100 nm (100 being the greatest divisor common to 200 and 300).
• this GCD defines the periodicity of the signal /7 as lOOnm. .
• the optimal values of the intentional offsets of the 4 cells of the target are p/4 + / ⁇ ,
• p GCD(j> ⁇ , p 2 ) and where fo is less than p/4 .
• Fig. 2(h) 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 pi for the first layer.
• the spacing between the features of a second layer 262 of periodic features defines the pitch /? 2 for a second layer on the target.
• the pitch pi for the first layer is greater than the pitch P2 for the second layer.
• 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.
• the other portion of the offset fo is shown as added to (or subtracted from) F.
• the offsets in the present invention are determined as above (+F+fo, +F-fo, -F+fo, -F-fo) 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).
• 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.
• each cell is at least 2QP across ("across" in this context means in the direction perpendicular to the length of the features forming the cell.)
• a first set of target cells includes four targets A, B, C, and D which are designed to have offsets X a through X ⁇ j.
• 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 S D 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 No. 09/849,622, filed 04 May 2001, entitled “METHOD AND SYSTEMS FOR LITHOGRAPHY PROCESS CONTROL", by Lakkapragada, Suresh, et al. and (2) U.S. patent application, having application no.
• the spectra S A , S B , SC, and SD could include any type of spectroscopic ellipsometry or reflectometry signals, including: tan( ⁇ ), cos( ⁇ ), Rs, Rp, R, ⁇ (spectroscopic ellipsometry "alpha” signal), ⁇ (spectroscopic ellipsometry "beta” signal),((Rs-Rp)/(Rs+Rp)), etc.
• Spectrum SB (-p/4+f ⁇ ) can then be subtracted from spectrum SA
• a difference spectrum property Propi is obtained from the difference spectra Dj and a difference spectrum property Prop 2 is obtained from the difference spectrum D 2 .
• the difference spectra properties Propi and Prop 2 are generally obtained from any suitable comparable characteristics of the obtained difference spectra Di and D2.
• the difference spectra properties Propi and PrOp 2 may also each simply be a point on the each difference spectra Dj or D 2 at a particular wavelength.
• difference spectra properties Propi and Prop 2 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.
• the comparison spectra can be obtained and processed using many different methods.
• the magnitude and direction of an overlay error E can then be calculated directly from the difference spectra properties Propi and Prop 2 -
• a linear approximation can be performed based on the difference spectra properties Propi and Prop 2 to determine the overlay error E, while in another technique the difference spectra properties Propi and PrOp 2 can be used to approximate a sine wave function or other periodic function which is then used to determine the overlay error E.
• linear regression techniques can be used.
• the overlay result may be obtained by a statistical calculation (e.g. averaging or weighted averaging) of overlay results obtained from properties of multiple wavelengths or multiple wavelength ranges.
• Such techniques as described above are useful and accurate for a determination of an overlay error.
• 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.
• Such added features provide a disambiguation quality for the target arrangement.
• these disambiguation features need only provide a coarse alignment and be arranged symmetrically about an axis of symmetry for the target (by symmetric, the inventors mean that the target should be symmetric if the target is rotated through 180°).
• the added target structures can be large bar-like structures.
• the bar can comprise raised structures or trenches. Any large easily imageable structure will suffice. Two examples of structures which can be used for this purpose are shown in the target arrangements shown in Figures 3 (a) and 3(b).
• Fig. 3(a) depicts a target 300 having two sets 310, 320 of four cells each (31 Ia, 31 Ib, 31 Ic, & 31 Id and 32Ia 3 321b, 321c, & 321d, respectively).
• the features of each set e.g., 31 Ia 5 311b, 311c, & 31 Id
• the features of the other set e.g., 321a, 321b, 321c, & 32Id.
• each of the cells are depicted as being square, this need not be the case.
• the cells of each set are grouped together next to one another. The inventors point out that this need not be the case.
• the cells are sized as explained above. In this example, cell 311b extends about 2OP in a direction perpendicular to the direction the features extend.
• 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 bars (330, 332) are generally sized and positioned in the space available.
• 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.
• 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 0.5 ⁇ (micrometers) to about 1 ⁇ 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 (351a, 351b, 351c, & 351d and 361a, 361b, 361c, & 36 Id, respectively).
• the features of each set of cells are perpendicular to the features of the other set of cells.
• a set of disambiguation features is added to the target.
• the disambiguation features can comprise coarse overlay features arranged symmetrically about an axis of symmetry 341.
• a first set 370a, 370b of coarse overlay "bars" is arranged in one of the layers.
• a second set 372 of coarse overlay "bars" is arranged in the other of the layers.
• the second set 372 of coarse overlay is in a "domino"-type configuration with one of the bars bisecting the cells of the target.
• 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).
• a small portion 373 of one of the targets 340 is illustrated in simplified side sectional view in Fig. 4. Which shows the features and pitch p 2 of periodic features 401 comprising a portion of a lower layer of cell 36 Id.
• 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).
• each of the disambiguation features comprise coarse overlay features arranged symmetrically about center of symmetry 502.
• symmetry means that the disambiguation features can be rotated 180 degrees about the center of symmetry 502 and maintain the same pattern and orientation.
• a first set 510 of coarse overlay "bars” is arranged in one of the layers.
• a second set 520 of coarse overlay "bars” is arranged in another layer.
• ROI Regions of interest
• the two ROIs correspond to portions of the overlay "bars" 510, 520.
• ROI (defined by the dotted line) 521 corresponds to the second set 520 of coarse overlay "bars”.
• another ROI (defined by the dotted/dashed line) 511 corresponds to the left hand portion of the first set 510 of coarse overlay "bars".
• ROI (defined by the dotted/dashed line) 512 corresponds to the right hand.
• 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.
• 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 ROL. This results in three one- dimensional projected signals (one for each ROI 511, 512, 521). [0082]
• the projected signals are used to find the position of the center of symmetry for each set of coarse overlay bars for each layer.
• the center of the symmetry for the outer portions of the coarse overlay bars 510 is found from the projected signals obtained from the outer ROI' s (511, 512), while the center of symmetry of the inner coarse overlay bar 520 is found from the projected signal obtained from the inner ROI 521.
• Commonly used algorithms known to those having ordinary skill in the art can be used to find the centers of symmetry based on the signals obtained.
• the signals are processed using standard correlation algorithms.
• a degree of alignment . between the two layers can be determined, for example, by comparing the distance between the determined centers of symmetry for the inner coarse overlay bar 520 and outer coarse overlay bars 510.
• the first set 510 of coarse overlay bars is generally assumed to produce a signal symmetric as reflected about a Y-axis for the target. Any asymmetry in this signal will bias the coarse overlay measurement.
• a major source of such asymmetry is the optical interaction of the bar 510 with its surroundings. In the case of a SCOL target, the surroundings include gratings on the right and left hand sides of the bar (501), which differ from each other. Additionally, the semiconductor pattern itself (which is formed around the target) introduces bias in the symmetry which is difficult to accurately compensate for.
• a standard overlay algorithm can be used to reduce the ambiguity of such signals but the inventors believe additional accuracy can be obtained using the features of the following further described embodiments.
• the previously discussed approaches involve finding the center of symmetry of each of the coarse overlay structures along their narrow sides.
• the inventors posit that it is possible and advantageous to find the center of symmetry of the coarse overlay structures along their wide sides as well as the narrow sides. This offers the ability to determine these measurements at an orientation of 90° from the measurements on the narrow sides. This offers an opportunity to reduce the cross-talk from adjacent periodic features without a significant increase in target size.
• a first set 601 of coarse overlay "bars” is arranged in one of the layers.
• a second set 602 of coarse overlay "bars” is arranged in the other of the layers.
• the coarse overlay bars are smaller (shorter) than those previously disclosed.
• all sides of the bars are exposed to allow each edge to be imaged and used for disambiguation.
• this cross talk distorts the optical signal to the extent that it can appear to the shift the centers of symmetry for the target.
• the centers of symmetry of horizontal bars are shifted almost only in the vertical direction
• the centers of symmetry of vertical bars are shifted almost only in the horizontal direction.
• the measurement consists of finding the centers of symmetry of horizontal structures along the horizontal direction and of the vertical structures along the vertical direction. Since the positions of these centers of symmetry are not significantly affected by the cross talk effect, the bias in their positions should be small, and the inventors can take advantage of the fact that such a measurement would have an accuracy to within about 10 — 20 nanometers, good enough to achieve the requisite accuracy needed in the coarse overlay measurement.
• Fig. 7 depicts a target 700 embodiment having sets of target cells (701, 702 and so one) arranged as discussed in previously described embodiments.
• the disambiguation features (703, 706, and so on) are shortened not extending beyond the edges of the target cells.
• Sets of disambiguation features are added to the target.
• the disambiguation features comprise coarse overlay features arranged symmetrically about an axis of symmetry 712. As depicted here, a first set 710 of coarse overlay bars is arranged in one of the layers (these are depicted in the cross-hashed boxes 710).
• a second set 711 of coarse overlay bars is arranged in another layer (these are depicted in the speckle-filled boxes 711).
• the following describes one example for using the coarse overlay to obtain a measurement in the X-direction.
• the coarse overlay bars are imaged using a sensor.
• ROFs are the defined for the two layers.
• two ROIs 704a, 704b correspond to portions of the overlay "bars" 703, 703' (defined by the dashed/dotted line 704a, 704b).
• two other ROIs 705a, 705b correspond to portions of additional overlay bars 706, 706' (defined by the dashed/dotted line 705a, 705b) formed in another layer.
• 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 (704a, 704b, 705a, 705b). This results in four one- dimensional projected signals.
• the projected signals are used to find the position of the center of symmetry for each set of coarse overlay bars for each layer.
• the center of the symmetry for the coarse overlay bars 703, 703' is found from the projected signals obtained from the ROFs (704a, 704b), while the center of symmetry of the other coarse overlay bars 706, 706' is found from the projected signal obtained from the ROI's (705a, 705b).
• commonly used algorithms known to those having ordinary skill in the art can be used to find the centers of symmetry based on the signals obtained.
• the signals are processed using " standard correlation algorithms. A degree of alignment between the two layers can be determined by comparing the positions of the determined centers of symmetry for each set of measurements. [0094J As shown in Fig.
• the reason for this improvement in accuracy is that the different gratings (target cells 701, 702, for example) in the vicinity of the affected ROI (in this example 703) are located above and below the affected ROI rather than to the left and right. Since the gratings (target cells) are configured to be homogeneous along the X-direction they do not induce asymmetry in the signals along the X-direction. This improves the symmetry of the signals considerably and gives rise to a more accurate coarse overlay measurement.
• Fig. 8 depicts a simplified implementation of a six-cell target.
• the inventors note that such a six-cell target can be used together with another six-cell target to form a larger twelve-cell target, much in the same manner that the two (2) four-cell targets can be employed to form an eight cell target arrangement.
• One embodiment of such an implementation is what is 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 a simplified implementation of a six-cell target.
• the inventors note that such a six-cell target can be used together with another six-cell target to form a larger twelve-cell target, much in the same manner that the two (2) four-cell targets can be employed to form an eight cell target arrangement.
• One embodiment of such an implementation is what is depicted in Fig. 8.
• a pair of six-cell sets are configured to form a twelve cell target.
• FIG. 8 depicts an target 800 embodiment also having two sets 810, 820 of six cells each (811a, 811b, 811c, 81 Id, 8 l ie & 81 If and 821a, 821b, 821c, 82 Id, 82 Ie & 82 If, respectively).
• 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).
• a set of disambiguation features can be added to the target.
• the disambiguation features can comprise coarse overlay features arranged symmetrically about an axis of symmetry 830.
• a first set 840a, 840b of coarse overlay "bars” is arranged in one of the layers.
• a second set 850 of coarse overlay "bars” is arranged in the other of the layers.
• the second set 850 of coarse overlay is in a "domino"-type configuration with one of the bars bisecting the cells of the target.
• 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.
• a suitable set of desirable offsets for the cells is 5p/l2 , - p/l2 , p/4 , - p/4, p/12 and ⁇ Spj ⁇ 2.
• p a suitable set of desirable offsets for the cells
• no free parameter is required.
• other offset values are generally employed.

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• Physics & Mathematics (AREA)
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• Exposure And Positioning Against Photoresist Photosensitive Materials (AREA)
• Exposure Of Semiconductors, Excluding Electron Or Ion Beam Exposure (AREA)
• Testing Or Measuring Of Semiconductors Or The Like (AREA)
• Length Measuring Devices By Optical Means (AREA)

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• 2006
  • 2006-09-21 US US11/525,320 patent/US7616313B2/en active Active
• 2007
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  • 2007-03-08 WO PCT/US2007/006031 patent/WO2007126559A2/en not_active Ceased
• 2013
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