Scatterfield microscopy for extending the limits of image-based optical metrology

Scatterfield microscopy for extending the limits ofimage-based optical metrology

Richard M. Silver,* Bryan M. Barnes, Ravikiran Attota, Jay Jun, Michael Stocker, Egon Marx,and Heather J. Patrick

National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA

*Corresponding author: [email protected]

Received 25 October 2006; revised 16 March 2007; accepted 19 March 2007;posted 22 March 2007 (Doc. ID 76383); published 20 June 2007

We have developed a set of techniques, referred to as scatterfield microscopy, in which the illuminationis engineered in combination with appropriately designed metrology targets to extend the limits ofimage-based optical metrology. Previously we reported results from samples with sub-50-nm-sized fea-tures having pitches larger than the conventional Rayleigh resolution criterion, which resulted in imageshaving edge contrast and elements of conventional imaging. In this paper we extend these methods totargets composed of features much denser than the conventional Rayleigh resolution criterion. For theseapplications, a new approach is presented that uses a combination of zero-order optical response andedge-based imaging. The approach is, however, more general and a more comprehensive set of analysesusing theoretical methods is presented. This analysis gives a direct measure of the ultimate size anddensity of features that can be measured with these optical techniques. We present both experimentalresults and optical simulations using different electromagnetic scattering packages to evaluate theultimate sensitivity and extensibility of these techniques.

OCIS codes: 110.0180, 050.1940, 120.3940.

1. Introduction

Recently, there has been significant research inves-tigating potential technologies to meet the challengesof imaging very small device-sized targets used infeature-to-feature overlay metrology and linewidthmeasurements for the semiconductor and nanoelec-tronics manufacturing industries [1,2]. Tighter man-ufacturing tolerances are driving the need to usedevice-sized targets, which more closely emulate thecharacteristics of the actual features of interestprinted on the wafers. There also remains a lingeringquestion as to how much longer optical methods willprovide accurate metrology outputs on very smallfeature sizes [3]. Often, the high throughput andlower cost of optical systems make them the measure-ment techniques of choice, provided they are capableof robust measurement of the features of interest.

Most obstacles encountered in leading-edge bright-field optical metrology can be divided into two sepa-

rate, related areas: (1) the challenge of whether or nota feature can be resolved and (2) whether one canmake a robust optical image-based measurement.The first challenge is whether a feature or signal canbe resolved or imaged and whether there is appropri-ate sensitivity to changes in the measurand. Thislends itself to using physics-based electromagneticscattering models and the necessity to evaluate theoptical response and sensitivity to changes in an im-aging target. The second challenge relates to the con-tinued application of edge- and image-based opticaltechniques as opposed to more complex optical signa-tures that require model-based interpretation.

In previous publications we described our imple-mentation of the scatterfield microscopy technique,which generally involves engineering the illumina-tion in a Köhler illuminated bright-field microscopein combination with structured samples [4,5]. To in-vestigate the limitations of this approach we havemodeled targets with linewidth dimensions down to10 nm. Fundamentally there are no significant road-blocks to measuring isolated features 1�20th the size0003-6935/07/204248-10$15.00/0

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of the measurement wavelength and, using well-engineered illumination and target designs, verygood signal-to-noise ratios can be achieved. A numberof examples presented in [6] utilize both theoreticaltechniques and experimental implementations to de-velop and demonstrate excellent optical response tosmall subwavelength features. The problem, how-ever, becomes more challenging when the featuresbecome densely positioned. The optical response fromisolated small features is quite different from that ofsmall features in dense arrays.

A. Zero-Order Imaging

In this paper we present a new approach to opticalmeasurements of very dense arrays. As an example,we analyze 50 nm size features, which are spaced250 nm apart with 546 nm wavelength light. Thetypical optical response to an array filling the field ofview at the 546 nm measurement wavelength is aflat or constant intensity pattern with no optical in-tensity variation. The cause of this constant intensitypattern is that only specular or zero-order lightis reflected. That is, the collection optics are onlycapturing the specular reflected component; eitherhigher-order terms do not exist or they are outsidethe collection aperture. We have developed an ap-proach to image the sample in this situation. If fea-tures are placed close enough together they locallyreflect only the zero-order component, resulting in aflat intensity response across the image plane. How-ever, if the target actually has a finite overall dimen-sion within the field of view, then the edges of thetarget will diffract higher-order light. Several targetscan be co-placed in the field of view having this char-acteristic. This results in a conventional image whereeach small array is delineated only by its boundaryedges, although the internal signal from within theedges is attributed to only zero-order reflected light.We refer to this as zero-order imaging.

The result is that small groupings of features withsubwavelength dimensions can give an excellentlocalized optical response as a function of positionacross the image plane. Their optical response is acombination of the higher-order edge response and azero-order central intensity. These targets have ap-plications in feature-to-feature overlay and criticaldimension linewidth metrology. For overlay appli-cations the arrayed targets can act as individualobjects, which are well suited for edge detection al-gorithms and correlation functions commonly used todetermine relative feature positions for image-basedtechniques.

B. Back Focal Plane Scanning

In addition to the imaging of dense features as de-scribed above, a more complex approach is to scan anaperture in a conjugate back focal plane, resulting inillumination of the sample at specific chosen angles.More rigorously, in the Köhler illumination configu-ration used in our hardware, a small cone of planewaves illuminates the sample from a selected direc-tion (the primary angle of illumination), which is

ultimately limited in our current design by thecollection numerical aperture. As the aperture isscanned in a conjugate illumination back focal plane,the primary angle of illumination changes and im-ages are captured at each illumination angle. Theimages captured are the result of two different phe-nomena. Either a dense array is imaged and only aconstant intensity is captured at the image plane, orfor less dense targets the image will contain higher-order optical response information and appear as aconventional optical image of an array. In either case,the intensity image is captured and a total or averageintensity as a function of illumination angle or aper-ture position is determined. This approach is similarto that used in scatterometry and is essentially acombination of scatterometry and bright-field opticalmicroscopy (giving rise to the name scatterfield mi-croscopy). Figure 1 shows the basic hardware config-uration. The hardware and instrumentation used inthe research reported in this paper are described indetail elsewhere [7]. Figure 1 is a schematic repre-sentation of the illumination configuration and com-bined collection and illumination optical train. Theactual optical column involves a number of relay lensgroups and magnification lens groups designed toprovide an adequate conjugate back focal plane sizewith an acceptable level of aberration.

We first present applications of the technique tolarge continuous arrays of features and evaluatetheir sensitivity to changes in feature geometry andlinewidth. Then we present results from the mea-surement of a group of arrays for use in combinedoptical metrology applications. Although comparisonof theoretical analysis and experimental data will bepresented, this is not intended to be a comprehensivetreatment of experimental and theoretical compari-sons. The results presented here do, however, givea concrete indication as to what the ultimate sen-sitivity and resolution limits are for optical-basedmetrology.

Fig. 1. (Color online) Simplified schematic of the optical configu-ration. The upper figure is a conceptual drawing of the illumina-tion path demonstrating how access to a conjugate back focal planeenables selective angular illumination or more general illumina-tion engineering. The lower part of the figure shows the layout ofthe laboratory implementation.

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C. Test Sample Characterization

To develop the methods described in this paper,etched silicon target arrays were fabricated atSEMATECH with different process stacks. These tar-gets include a variety of large arrays having overalldimensions from 20 �m � 20 �m up to 100 �m �100 �m. The large array data presented in this paperare from the 100 �m � 100 �m arrays. These targetsare etched 230 nm deep in silicon and have side-wall angles measured using atomic force microscopy(AFM). These targets are referred to as the etchedsilicon targets and were designed as part of the Over-lay Metrology Advisory Group (OMAG 3 reticle set)[8]. A second set of arrayed targets were preparedhaving a similar set of array designs and dimensionsbut were prepared by etching a 100 nm thick amor-phous silicon layer on a 2 nm gate oxide. These wa-fers (OMAG 4 reticle set) resulted in better controlof the sidewall geometry and allowed for overetchingthe lines, yielding smaller critical dimensions (CDs).The OMAG 4 reticle set also has a set of arrayedoverlay targets, the National Institute of Standardsand Technology (NIST) high-resolution overlay tar-gets, presented in Section 5 of this paper.

The wafers were exposed using a focus exposurematrix (FEM). This results in linewidths that variedby less than 1 nm from die to die and provides excel-lent targets for testing sensitivity to small changes infeature size. The FEM did result in some variation ofthe sidewall angles. The data presented in the follow-ing sections are from selected die that yield a totalchange in linewidth across the dies chosen of 6 nm forthe 300 nm pitch arrays and 12 nm for the 600 nmpitch arrays.

AFM and scanning electron microscopy (SEM)were used to measure the linewidths and pitch valuesfor use in determining the starting-point simulationparameters. There are minor disagreements betweenthe SEM measurements and the AFM measure-ments, which remain unresolved. Depending onthe linewidth and sidewall angle in particular, thedisagreement ranged from insignificant to as muchas 25 nm. However, both methods showed approxi-mately the same variation in linewidth across a givenrange of die but offset with respect to one another.The discrepancies may at least in part be due to thevariation in sidewall angle and the different mea-surement algorithms used.

2. Background Compensation and SignalNormalization

Data were acquired using the configuration shownschematically in Fig. 1. An aperture is scanned in aconjugate back focal plane of the objective in the il-lumination optical path. An image is acquired forselected positions of the aperture across the backfocal plane. Thirty-one positions were routinely used,which spanned illumination angles greater than theobjective numerical aperture (NA) could accept. Animage is obtained for each aperture position with thecorresponding illumination configuration having a

small cone of plane waves nominally 0.11 NA in size.That is, a small aperture of 200 �m, equivalent to anon-axis NA of 0.11, is scanned across the back focalplane, which is approximately 2 mm in size. Therange of angles for the cone of illuminating planewaves is ��6°, and this is scanned from more than�60° to �60° (the primary angle) with respect to theoptical axis. The collection NA is 0.95.

There are several contributions to the target imageand background signal that require normalization.First, there is a dark current that is removed fromeach image. There are target-independent variationsin the illumination intensity across the field of view.There are also variations in the overall intensity ateach aperture position, due to the angular depen-dence of the illumination and transmission in themicroscope optics. To correct for these effects, at eachoccurrence of an acquisition (for each illuminationangle), a blank background image is acquired at anearby site on the silicon. The intensity of a targetimage is then normalized to that of its backgroundimage using a pixel-by-pixel array method.

The use of a bare silicon background image as ref-erence enables us to calculate a correction for the po-larization and angular dependence of the illuminationand the transmission through the microscope optics.This correction is then applied to the target image toconvert the intensity of a target image at a given illu-mination angle to reflectance of the target at that an-gle. This approach works well for zero-order imaging,where, for both the target and for bare silicon, eachillumination angle maps to a single specular reflectionangle. However, the correction calculated from baresilicon has limited application to targets with higherdiffraction orders because in this case the target dif-fracts light at multiple angles for a single illuminationangle, and the transmission correction for the higherorders is not obtained from the bare silicon correction.

Figure 2 shows the normalization sequence forevaluating the polarization-dependent transmissionthrough the imaging system. The upper panels showunpolarized and polarized intensity as a function ofangle, for a blank area of silicon, measured at theimage plane with a CCD camera. In the lower panelsthe theoretical curves for silicon reflectivity as a func-tion of angle are shown. The theoretical curves arecalculated from Fresnel reflectance theory, using theoptical constants of silicon [9]. The difference be-tween the upper and lower panels is the direct effectof polarization-dependent transmission through theoptics as a function of angle. The TE polarization isnearly flat on average, whereas the TM polarizationis closer to the expected nominal behavior. The ex-perimental data for a target can only be compared totheory after this effect has been properly compen-sated. For each polarization, a correction curve isderived by dividing the lower (calculated) curve forsilicon shown in Fig. 2 by the upper (measured) curve.The reflectance of a target versus illumination angleis then given by its measured intensity versus angle,multiplied by the correction curve. Figure 3 showstypical target data as acquired and following the cor-


rection. The data in Fig. 3 are normalized on a point-by-point basis by correcting for the intensity errorobserved at each angle relative to that based on thatobserved. The finite angular illumination approachallows us to accurately measure and quantify theangular dependence of the illumination and trans-mission through the optical train. This is an impor-tant effect to quantify any time optical images arecompared to theory since this polarization depen-dence results in a substantial error, particularly forthe TE-polarized light. In subsequent measurementsthis has been confirmed through measurements ofthe polarized light directly on the exit of the objectivelens.

3. Full-Field Angle Scans

In the first part of this section the data describedwere acquired from OMAG 3 arrays, which fill thefield of view. The data were acquired by collectingconventional images at the image plane with a CCDcamera. A window or kernel is placed in the imageand an average intensity per unit area determined.This mean intensity is evaluated for each angle ofillumination. Two types of results are observed. Thefirst result occurs when the pitch values are large

enough for higher-order optical information to bescattered and collected by the optics. The second,alternative result is a flat intensity across the fieldcontaining only zero-order optical information. Theaperture is scanned in the back focal plane across alarge enough distance to clip the aperture at the ex-tremes, and as a result the intensity falls off for po-sitions in the back focal plane that correspond toangles of illumination greater than 60°.

Figure 3 shows four sets of curves, the top two arefor TE polarization and the bottom two for TM polar-ization data. This figure shows the 300 nm pitch ar-ray with linewidths varying from 150 to 156.5 nmbased on the SEM values. The data shown on the leftside of the figure are not yet normalized for the siliconreflectivity although they have been processed forbackground and dark current compensation. Thecurves show clear sensitivity to nanometer changesin linewidth. These data are repeatable and show amonotonic response to linewidth changes.

The data on the right side of Fig. 3 show the sameresults after they have been corrected for the siliconreflectivity. The correction effectively scales the data,which in this case increases the dynamic range in the

Fig. 2. (Color online) Background intensity data is acquired for each image as a function of angle, plotted in the upper panel for theunpolarized case and both polarizations. A point-by-point matrix correction is used to normalize the angle-resolved background data toaccount for the angle-dependent intensity variation. The reflected light is then normalized using the theoretical angle-dependent siliconreflectivity curves shown in the lower part of the figure. This approach is essential to normalize the angle-resolved data correctly so it canbe compared to simulations.


graph and makes the curves appear closer to eachother in value relative to the raw, uncorrected data.

A similar set of data is shown in Fig. 4 for lines witha nominal pitch of 600 nm. In this case the initialimage shows an optical response that includes inten-sity variation throughout the field of view. The sameapproach of applying a window and running an algo-rithm to calculate an average integrated intensity perunit area can be used, followed by graphing the av-erage integrated intensity as a function of angle. Re-sults are shown for both experimental data and aseries of simulations. The simulations cover a broadrange of linewidths based on the range of SEM andAFM measurements. In both cases there are offsetsbetween the SEM and AFM data, which is the reasonsuch a large range of linewidth parameters were usedin the initial simulations.

Comparison of the TE data in Fig. 4 shows qualita-tive agreement in intensity patterns between the ex-perimental range of linewidths and those in thesimulation near 155 nm. Similarly, comparing theTM data and simulations show a qualitative trend ofagreement with the simulation data in the 145–155 nm range. These simulations are intended to

identify the feature dimensions to within 10 nm,where a more comprehensive analysis can be run,which includes a parametric evaluation of the param-eter space to determine the effects of other physicalparameters such as small variations in line height,sidewall angle, or optical constants. These paramet-ric effects are discussed in more detail in [10], wherea fractional factorial approach was used to optimizethe simulation parameters.

The theoretical methods used have been describedin detail elsewhere [11,12]. The first method usedhere was the rigorous coupled waveguide analysis(RCWA). Briefly, the RCWA model uses a modalmethod called the analytic waveguide method withan infinite grating. It calculates the eigenmodes in awaveguide decomposition of the scattering essen-tially exactly. The fields are expanded in terms of theeigenmodes and then calculated analytically withoutuse of a Fourier series. A matrix relates the general-ized Fourier series of the fields in one z layer to thosein an adjacent z layer using boundary conditions [10].The finite-difference time domain computation in-volves a time-based leapfrog method where updatingequations alternate between electric field and mag-

Fig. 3. Experimental data before and after normalization. TE polarization is on top and TM is shown on the bottom. The vertical axesare units of normalized intensity and the horizontal axes are illumination angle in degrees. The data show good sensitivity to nanometerchanges in linewidth. The data on the left are presented prior to normalization and show good sensitivity to small changes in line width.The data on the right side have been normalized to the background. The normalization process changes the dynamic range and resultsin the apparent loss of sensitivity, which can be easily regained by focusing on a smaller range of angles. The normalization process isessential when comparing theory to experiment.


netic field calculations. To do so, Maxwell’s equationsare transformed from their vector, differential forminto difference equations. For each time step, x, y, andz components of the E field are evaluated at the timestep. Similarly, the three H-field components areevaluated between time steps, requiring a total of sixcalculations [12].

There are a number of parameters that affect theoptical image and the optical response of a target toa particular illumination configuration. The opticalconstants n and k have a well-known effect on theelectromagnetic scattering from a target. The illumi-nation NA as well as the collection NA also havesubstantial effects on the intensity response and res-olution. We have worked extensively to advance andquantify the detailed theoretical understanding ofthese numerous parametric effects on the opticalscattering and imaging processes. One importantadvantage of using the scanned aperture approachdescribed here is that errors introduced to theillumination and imaging process can be evaluated ina simplified form and as a function of illuminationangle. This has allowed us to quantify and normalizethe substantial polarization effects in the opticaltrain.

In addition, we have previously shown that theillumination NA can play a significant role in enhanc-ing the optical response. The measurement wave-length affects the conventional resolution seen in an

image but also plays a crucial role in the more com-plex optical phenomena seen in [6]. In this work, lightis scattered off an array of lines and strong elec-tromagnetic field oscillations in the intensity areobserved above the sample. The details of theseoscillations in the electromagnetic intensities can bemeasured and attributed directly to the line geome-try. The target pitch dimension in this work was alsoshown to couple strongly with measurement wave-length and to have a similar effect on the opticalresponse. The work presented here builds directly onthose concepts.

Analysis of these data shows nanometer sensitiv-ity to the small differences in the die-to-die varia-tion in linewidth. The simulations show that, giventhe parametric inputs used, the lines are closest tothe 145–155 nm linewidths, in qualitative agreementwith the values measured by SEM. The simulationsalso show significant theoretical sensitivity to geo-metrical changes for these targets and that thistechnique is a candidate for measuring changes ingeometrical shape.

4. High-Resolution Arrayed Targets

Although the above discussion and results were fo-cused on sensitivity to changes in linewidth, geome-try, and the ability to obtain an optical signal fromsmall features, these results have significant impli-cations for optical overlay metrology. In developing

Fig. 4. (Color online) Data on the left are simulation results and the right side shows experimental results. The vertical axes are unitsof normalized intensity. The simulations span the range of CD values measured with the AFM and SEM. The experimental data are siliconcorrected and compare most favorably with the simulations between 145 and 155 nm. Subsequent work with higher-resolution simulationsteps confirms these results.


applications of this high-resolution optical method itis strongly desirable to enable both direct image-based measurements when desired (e.g., for feature-to-feature overlay) while permitting more complexmodel-based signal analysis for critical dimensionand linewidth applications. In this section we willapply the same scanning aperture techniques de-scribed above to an array of targets within the field ofview.

An additional set of targets were analyzed from theOMAG 4 wafer set and used to obtain the resultsshown in the next section. The etched amorphoussilicon process allowed us to achieve smaller line-widths. We did not prepare a characterized set offocus exposure die since this part of the study wasintended to test and evaluate the technique atsmaller feature dimensions. The features were, how-ever, well characterized again by AFM and SEM todetermine a linewidth window and sidewall geometry.

A. Overlay Targets

A set of targets was designed that consisted of anarray of small line arrays as shown in Fig. 5. Unlikethe previous targets, these targets are intended toallow measurement of the relative position or overlaybetween two layers that can also be imaged using thezero-order imaging approach. In the figure, the outerline sets are from one photolithographic level and theinner line sets are from a second photo level. Thecurrent target designs are sets of eight lines with avariety of line lengths, line set separations, and linewidths to allow investigation of these parameters on

target performance. This figure shows one variationof the target design. For this target design, the opticalresponse to individual line sets includes high-orderoptical content reflecting from the central part of thelines, as shown in Fig. 5(b). Alternatively, a secondset of targets is imaged in the zero-order mode asshown in Fig. 6. In this figure, only zero-order spec-ular reflection occurs from the central target region ofan individual line set, although the individual linesets do reflect higher-order optical content from theiredges.

Figures 5 and 6 show typical applications of thealgorithms, which select and generate the window orkernel for use in analysis. The algorithm applied usesa horizontal window as shown in the figures, which istypically 2–5 �m in height (100–250 pixels verti-cally), which is then collapsed into a single profile.This algorithm is applied to every image acquired ateach angle. A resulting profile from one image isshown in Fig. 5(c). Note the number of oscillations inthe central region for each line set. In Fig. 6 the threebright oscillations are an interference effect involvingthe eight lines and the edges that comprise the target,not higher-order optical content from the interior re-gion of the eight lines.

An interesting enhancement to the measurementscan be made by illuminating at high angles. This cancontribute, depending on the target and measurementwavelength, higher-order diffraction content to theimage. A theoretical example is shown in Fig. 7, wherethe sample is illuminated at four different angles, fromnormal incidence to approximately 60°, resulting at

Fig. 5. (a) Schematic of the target designs. This is one example of several variations of this design. (b) Image and a set of profiles for atarget, which reflects higher-order optical content. On the wafer there are five target pitches and three offset values, 0, ��10 nm.


the highest illumination angles in images with signif-icantly increased high-frequency image content. Usingthis approach with dipole illumination for single-axisoverlay metrology or quadrapole illumination for two-dimensional measurements can enable symmetric im-age profiles with higher-order content.

B. Arrayed Target Scanning Measurements

Acquiring data as described in Section 3 by incre-menting the illumination angle and acquiring imagesas the aperture is stepped across the back focal planeallows the entire set of line arrays to be analyzedsimultaneously. Using the scanned aperture ap-proach in Section 3 with these targets enables simul-taneous analysis of the nine arrays of lines. Toevaluate this approach and test sensitivity, a specialhigh-resolution target array was designed, which iscalled the arrayed linearity target. This target hasbuilt-in CD variations across the field of view. Eachrow of line sets in the target has a different designCD. Figure 8 shows the normal incidence 0° illumi-nation image data with profiles for three differentdesign CDs as described above. In these data, thedesign increments are 5 nm and the average nominal

Fig. 6. (Color online) An image with zero-order scattering characteristics for all targets is shown on the left. The data windows from the imageare plotted on the right side. This is a standard target with no designed-in CD variation. The intensity oscillations, observed as the four barsin the top row and bottom row of targets, is an edge-based interference effect. This is the result of the optical proximity of the array edges,which interfere with one another. Larger target arrays with the same linewidth dimensions and spacing demonstrate the expected constantintensity in the central array region.

Fig. 7. (Color online) Different panels showing the image as afunction of illumination angle, from normal incidence to 60° in 20°increments. The vertical axes are normalized intensity and thehorizontal axes are lateral position in micrometers. At the highillumination angles, the higher-order diffraction orders are rockedinto the collection optics. A dipole or alternative symmetric illumi-nation can yield symmetric profiles or overlay measurements withhigher-order optical content, which would otherwise only showzero-order response.


CDs measured by AFM and SEM are 55, 60, and65 nm. From this image an average intensity is de-termined for the central region for each profile. Ex-perimental data are shown on the top, and the bottompanels show theoretical results. This is not meant tobe a rigorous comprehensive modeling treatment forthis target, but rather a demonstration of principle.From each image acquired at a given illuminationangle, the resulting profile is analyzed to determine

an average integrated intensity (AII) value for thecentral target region. Both the experimental resultsand simulation data show approximately 10 percentchanges in the AII for each increment in linewidth.These curves are normalized to background althoughno silicon reflectivity corrections have been applied.

A complete set of angle-resolved data for the centeris shown in Fig. 9. The upper data set is for a nonin-cremented 3 � 3 overlay target and the lower set isfor an incremented linearity target. The upper datashow no substantial variation in optical responseacross the target, whereas the lower set shows a clearmonotonic response to the changes in linewidthacross the target. The data from Fig. 8 are represen-tative of the 0° illumination angle from the center ofthe p-polarized plot. The data clearly indicate theimportance of polarization with respect to sensitivityto change in linewidth.

5. Discussion

Although gaps remain in the agreement between thetheory and experiment, the sensitivity and resolutionare well predicted by the simulations and observedexperimentally. The importance of the angle scan-ning technique is that we can now measure differ-ences in targets with resolution limits well beyondthe conventional Rayleigh limit. In fact, the funda-mental resolution limits are unclear at this time. Thespecular component contains quantitative informa-tion specific to the feature dimensions, and muchrecent work in scatterometry shows that measure-ment resolution and sensitivity will ultimately ap-proach the molecular level.

Fig. 8. (Color online) Normalized intensity versus position fromthree windows in a linearity target having only zero-order re-sponse form the eight line target arrays. Simulation results areshown on the bottom and experimental data on top. The meanintensity from the central part of each profile can be analyzed forsensitivity to changes in CD. The experimental results and simu-lations show the same nominal change in average intensity.

Fig. 9. (Color online) Angle-resolved intensity plots for the center locations in the target patterns. The upper set of panels are for targetswith no CD variations designed in and the lower set show 5 nm increments in CD across the target. The p polarization shows the bestsensitivity. These are complete angle scan data sets.


Significant research has occurred over the years toclose the gap in the theoretical understanding of theimaging and electromagnetic scattering processes.Inaccuracies in the model inputs are one significantcause of experiment to model discrepancies. An im-portant component of this work is the ability to mea-sure the illumination as a function of angle, whichallows a quantitative evaluation of the polarizationdependence of the light as it traverses the opticaltrain. Likewise, the ability to structure and measurethe illumination has provided significant insight intothe degree that Köhler illumination is achieved in theoptical microscope. Additionally, the simplified illu-mination fields become an important tool to evaluatethe effects of aberration and misalignment on theillumination fields and the resulting optical images oroptical signatures in the case of a scanned aperturemeasurement.

6. Summary and Conclusions

In this paper we have presented a new technique thatcombines traditional optical imaging techniques andscanned illumination fields. This was accomplished byscanning an aperture in a conjugate illumination backfocal plane. We applied these methods to full-field ar-rays as well as a new generation of high-resolutiontargets that can serve as combined overlay and line-width metrology targets. The new target designs werefabricated by SEMATECH and included arrayed fea-tures at device sizes as small as 50 nm with targetdensities from isolated to 1–2 line-space ratios. Ourimaging approach was demonstrated on features sig-nificantly smaller than predicted by conventionalRayleigh resolution limits. A set of specialized linear-ity targets having designed-in CD variations wereanalyzed using both experimental and theoreticalanalysis.

In analyzing the data from these targets we gaindirect insight into the fundamental optical limitsfrom both experimental and theoretical perspec-tives. This measurement technique, based on anoptical signature rather than an edge-based ap-proach, has allowed us to substantially extend themeasurement limits with respect to feature dimen-sions, feature-to-feature distance, and the ability toresolve the difference between two similar targetgeometries. The data show that by engineering theillumination fields and target designs, accuratemeasurements of sub-20-nm-sized features may wellbe attainable.

We acknowledge useful discussions with TomGermer and Nien Fan Zhang of NIST. We are grate-ful for metrology, wafer fabrication, and AFM andSEM support from Pete Lipscomb and Mike Bishopfrom SEMATECH and also acknowledge MarkDavidson for useful discussions and modeling sup-port. We thank the Office of Microelectronics Pro-grams and the Scatterfield Competence Project, bothfrom NIST, for the financial support of this effort.

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