multiparameter grating metrology using optical scatterometry

Multiparameter grating metrology using optical scatterometryChristopher J. Raymond, Michael R. Murnane, Steven L. Prins, S. Sohail, H. Naqvi, John R. McNeil, and Jimmy

W. Hosch

Citation: Journal of Vacuum Science & Technology B 15, 361 (1997); doi: 10.1116/1.589320 View online: http://dx.doi.org/10.1116/1.589320 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/15/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Measurement of residual thickness using scatterometry J. Vac. Sci. Technol. B 23, 3069 (2005); 10.1116/1.2130345 Complimentary Optical Metrology Techniques Used for Characterization of HighK Gate Dielectrics AIP Conf. Proc. 788, 129 (2005); 10.1063/1.2062950 Real-time and multipoint monitoring the dissolution rate of photoresist film by using a novel plastic optical fiberbundle Rev. Sci. Instrum. 70, 1518 (1999); 10.1063/1.1149617 Metrology applications in lithography with variable angle spectroscopic ellipsometry AIP Conf. Proc. 449, 543 (1998); 10.1063/1.56841 Metrology of subwavelength photoresist gratings using optical scatterometry J. Vac. Sci. Technol. B 13, 1484 (1995); 10.1116/1.588176

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Multiparameter grating metrology using optical scatterometryChristopher J. Raymond,a) Michael R. Murnane, Steven L. Prins, S. Sohail, H. Naqvi,b)

and John R. McNeilCenter for High Technology Materials, Electrical and Computer Engineering Bldg.,University of New Mexico, Albuquerque, New Mexico 87131

Jimmy W. Hoschc)SEMATECH, 2706 Montopolis Drive, Austin, Texas 78741-6499

~Received 24 May 1996; accepted 6 December 1996!

Scatterometry, the analysis of light diffraction from periodic structures, is shown to be a versatilemetrology technique applicable to a number of processes involved in the production ofmicroelectronic devices. We have demonstrated that the scatterometer measurement technique isrobust to changes in the thickness of underlying films. Indeed, there is sufficient information in onesignature to determine four process parameters at once, namely the linewidth and thickness of thephotoresist grating, and the thicknesses of two underlying film layers. Results from determiningthese dimensions on a 25 wafer study show excellent agreement between the scatterometrymeasurements and measurements made with other metrology instruments@top-down andcross-section scanning electron microscopy~SEM! and ellipsometer#. In particular, measurementsof nominal 0.35mm lines agree well with cross-section SEM measurements; the average bias is21.7 nm. Similarly, for nominal 0.25mm lines, the average bias is27.3 nm. In addition, therepeatability~1s! of this technique is shown to besubnanometerfor all of the parameters measured~linewidth, resist height, antireflection coating thickness, and poly-Si thickness!. © 1997 AmericanVacuum Society.@S0734-211X~97!00502-7#

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I. INTRODUCTION

The production of a microelectronic device consists omultitude of microlithographic process steps, all of whicontribute to the quality of the finished product. For contof the overall process, it is important to have an accurquantitative description of the submicron structures presat each step. At present, there is no single technique ostrument flexible enough to be implemented as a metrolfor several process steps. Ellipsometers, for example,measure film thicknesses, while scanning electron micscopes~SEMs! have become the industry standard for criticdimension~CD! measurements. Both instruments are decated to these specific process tasks and, at present, arficult to implement in an on-line arrangement for higthroughput device evaluation.

Furthermore, as the microelectronics industry pushesfaster processing speeds, the dimensions of device strucmust continue to decrease in size. This places an increademand on present metrology techniques in terms of thecuracy and precision of measurements. For example, uting advanced processing, the linewidths of metal–oxidsemiconductor field-effect transistor gates is 0.35mm. Forprocess control of these devices, the general ‘‘10% ruimplies a linewidth tolerance of 35 nm. Metrology capabities, as outlined by the semiconductor industry, shouldone-tenth of this tolerance, or one-hundredth of the li

a!Electronic mail: [email protected]!Current address: GIK Inst. of Engineering Science and Technology, Pstan.

c!Current address: Texas Instruments, P.O. Box 650311, MS 374, Dallas75265.

361 J. Vac. Sci. Technol. B 15(2), Mar/Apr 1997 0734-211X/97/

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width. By the year 2001, production linewidths are scheuled to be on the order of 0.18mm, and off-line or at-linemetrology equipment will need to detect linewidth changas small as 1.8 nm.1 Furthermore, a 3s reproducibility ofabout 0.4 nm will be required. Once linewidths reach 0.mm, scheduled to occur in 2004, it is expected that oin-line metrologies, i.e., tools measuring every wafer, wmeet the stringent requirements for linewidth control.

The task at hand, then, is to have a metrology technithat is capable of meeting or exceeding industry standafor measurement acuity while still being versatile enoughmeasure several different process parameters. For on-lineplications, an ideal metrology would be able to make a nuber of different measurements which are sufficiently acrate, repeatable, and rapid, all at the lowest possible cBecause they fulfill many of these needs, diffraction-bastechniques such as scatterometry are especially well sufor microelectronics metrology applications. Diffractionbased metrology has already proven useful in a varietymicrolithographic applications, including alignment2

overlay,3 temperature measurement,4 monitoring of latentimage focus,5 exposure,6 and postexposure bake.7 In previ-ous applications scatterometry has shown promise as ancurate, repeatable, and inexpensive alternative to ometrologies.8

We define scatterometry as the angle-resolved measment and characterization of light diffracted from periodstructures. The scattered or diffracted light pattern, oftenferred to as a ‘‘signature,’’ can be used as a fingerprintidentifying the line shape of the structure itself. We halimited our application to periodic surfaces in which the sc

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362 Raymond et al. : Multiparameter grating metrology 362

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FIG. 1. The 2-Q scatterometer experimental arrangement.

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tered light consists of distinct diffraction orders at angulocations specified by the grating equation

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whereui is the angle of incidence,un is the angular locationof the nth diffraction order,l is the wavelength of incidenlight, andd is the spatial period~pitch! of the structure. Thefraction of incident power diffracted into any order is vesensitive to the shape and dimensional parameters of thefracting structure and may be used to characterize the sture itself.9,10 In addition to the period of the structure, whiccan be determined quite easily, the thickness of the photsist, the width of the resist line, and the thicknesses of seral underlying film layers can also be measured by anaing the scatter pattern. Scatterometry offers the speedsimplicity common to optical metrologies, but with a resoltion and repeatability that is not possible using optical mcroscopes. The main difference between optical microscand scatterometry is that while microscopy involves imaing, scatterometry simply involves the measurementanalysis of the diffracted optical radiation.

The scatterometric analysis can best be defined to coof two steps. First, the diffracted light fingerprint is mesured. This is known as the forward problem. The secstep, the inverse problem, consists of determining the shof the lines of the periodic structure which diffracts the indent light. To solve this problem, the grating shapeparameterized,11 and a parameter space is defined by alloing each grating shape parameter to vary over a cerrange. A rigorous diffraction model12 is used to calculate thediffracted light fingerprint from each grating in the paramespace, and a statistical prediction algorithm is trained usthis theoretical calibration data. Subsequently this predicalgorithm is used to determine the parameters that cospond to the fingerprint found in the forward problem.

J. Vac. Sci. Technol. B, Vol. 15, No. 2, Mar/Apr 1997


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In our work discussed here, the forward problem is solvusing a 2-Q scatterometer, seen in Fig. 1. A He–Ne lasbeam,l5633 nm, is incident upon a sample after passthrough a spatial filter and some focusing optics. The samis mounted on a stage which permits it to rotate. Becausebeam itself is fixed, this rotation changes the angle of indence on the sample. Using the grating equation~1!, thedetector arm of the scatterometer is able to follow any dfracted order as the incident angle is varied. The intensitya particular diffraction order is measured as a functionincident angle~this is known as a 2-Q plot or scatter signa-ture!, and afterwards downloaded to a computer, whereinverse problem is solved.

To generate the parameter space required for the invproblem, theoretical scatter signatures were constructeding the rigorous coupled wave theory~RCWT! developed byGaylord and Moharram.13 This differential technique useMaxwell’s equations with appropriate boundary conditioat all interfaces to solve for the electric fields in each regof the sample. The total reflected intensity can then beculated, and it is comprised of discrete diffracted ‘‘orderas a result of the periodic structure. A complete overviewrigorous coupled wave theory with respect to scatteromeapplications can be found in Naqviet al.12

The scatterometry effort at the University of New Mexichas explored and reported the use of several different pretion algorithms for the solution of the inverse problem14

New algorithms are still being investigated. However, forthe results presented here, only one algorithm was useminimum mean square~MMS! error. The MMS analysisproceeds by comparing an experimental scatter signafrom a sample of unknown structure~but within the previ-ously mentioned parameter space! with each signature whichwas theoretically generated. The absolute mean squareis calculated for each comparison, and the closest match~thesignature with the minimum absolute mean square error! is


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recorded. The linewidth and/or other dimensions associawith the closest theoretical signature are taken to be therameters of the unknown structure.

The absolute mean square error between an unknownnature and a calibration signature is defined as

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wherexci is thei th data point of the calibration signature,xuiis the i th data point of the unknown file, andN is the totalnumber of data points in each file.

In addition to different algorithms, different angularanges and diffraction orders over which the data are tahave also been explored.15,16 The ultimate goal is to mini-mize the number of data points~angular measurements! nec-essary for accurate parameter determination—this not odecreases the time needed to acquire the data, but alscreases the overall run time of the comparison algorithTherefore we have focused our attention on the use ofzeroth diffraction order only, and all results presented hare for zeroth order data. As shown below, the zeroth orsignatures alone contain sufficient information to yield acrate characterization of the diffracting structure. In additioEq. ~1! shows that when the illumination wavelength to gring period ratio is 2:1, respectively, only the zeroth ordermeasurable. Thus as the microelectronics industry pushewards smaller device dimensions with smaller periodichigher orders will no longer exist for 633 nm He–Ne illumnation, making the zeroth order the only choice for scattemetric measurements at this wavelength.

With respect to the angular range over which the zerorder measurements are taken, in principle the 2-Q scatter-ometer can take data on any diffraction order for incidangles ranging from290 to190 deg. In practice, howeverwe have limited the angular range to660 deg; at anglesmuch beyond this the incident light is almost completereflected and contains little information about the patternnature of the structure~i.e., the zeroth order intensity approaches simple Fresnel reflectances for near grazingdence!. In the results presented here we have further reduthe range to span from 2 to 42 deg~the 0 and 1 deg anglecannot be measured because the detector occludes thedent beam in the current experimental arrangement!.

II. MULTIPARAMETER MEASUREMENTS:BACKGROUND

Previous work in scatterometry has used the techniqudetermine the linewidths of nominal 0.5mm resist lines.16

But with sub-0.5mm devices already in production, it iimportant that the same techniques be applicable to smCDs as well. Furthermore, there was concern that in aproduction environment variations in other aspects ofprocess~e.g., film thickness! might affect the CD measurement accuracy of scatterometry. Since the diffraction moused accounts for film thicknesses, even when they arederlying, 2-Q CD measurements should not be affected

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these changes. Ideally, the technique would be able to qtify these film thicknesses while at the same time performCD measurements.

To address these issues, the 25 wafers used in our mrecent study possess several underlying film layers whthicknesses are intentionally varied within a specified pcess latitude. These variations were chosen to simulateprocess drift. The diffracted light intensity is undeniably afected by changes in film thicknesses, and a primary goathe investigation is to demonstrate the robustness of theQtechnique to such variations. Thus it was necessary toclude the additional unknown thickness parameters intheoretical model, and from one single scatter signature,process parameters would be determined.

On each wafer in the process variation study, there isSiO2 film followed by a poly-Si layer, an antireflection coaing ~ARC!, and finally the APEX-E resist structures~Fig. 2!.The oxide and poly-Si layers were intentionally varieamong the wafers; the oxide was grown at 60, 65, and 70and the poly-Si layer was deposited at 2300, 2500, and 2Å. These thicknesses were chosen to represent the ceand extremes of a real process window. Furthermore,resist and ARC thicknesses may also vary from waferwafer, and are therefore included in the theoretical modBecause our theoretical model can accommodate multisstructures, accounting for such process variations has bstraightforward.

When these particular parameter variations were incorrated into the model, we observed that the variations ofthin oxide layers were small and have no significant effectthe scatter signatures. As is illustrated in Fig. 3, the65 Åoxide thickness variations are barely discernible—the thsignatures shown in the figure are nearly the same. Althoit is possible that a more sensitive 2-Q scatterometer coulddetect these differences, at present these variations areyond the sensitivity of our instrument. To reduce computitime the oxide thickness variations were excluded from

FIG. 2. The film process stack used on samples in this study. Theunknown parameters to be determined by scatterometry are the CD~line-width!, resist height, ARC thickness, and poly-Si thickness.


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prediction attempts, and the nominal thickness value of 6would be used in the model.

In addition to all the film thickness variations, a 939focus/exposure~F/E! matrix was printed on each wafer. Futhermore, each F/E location comprised two different linspace gratings. The first had nominal 0.35mm resist lineswith a 0.8mm period. The second had 0.25mm lines with a0.75mm period. Because of the different periods it was nessary to generate a different parameter space for eaching. Thus in total there are four parameters to be determinlinewidth, resist height, ARC thickness, and poly-Si thicness.

Because a MMS algorithm was used in this study,resolution of each measurement was limited by the stepof each parameter used to generate the set of theoreticafraction signatures. With this algorithm it is desirable to uan increment as small as possible, which requires ancreased computing time. The use of improved predictiongorithms will circumvent this problem. Furthermore, if thnominal values of some of the parameters are not knownrange of values to be covered needs to be broadened.increment size for each of the parameters must be chosea compromise between the resolution and range desirethe predictions and the maximum number of files that canreasonably generated.

Considering these factors, the linewidths for each samwere varied in 10 nm steps, from 150 to 400 nm for nomnally 250 nm lines and from 250 to 550 nm for nomina350 nm lines. The use of such a broad linewidth rangeeach grating allowed us to measure linewidths over a brexposure range. The resist height was iterated 650 to 750in steps of 10 nm. The ARC height and the poly-Si heigwere varied from 700 to 850 nm and 220 to 290 nm in stof 2.5 and 5 nm, respectively. Although the generationthese signatures can be time consuming, it needs to beformed only once. The use of optimized, dedicated hardwas well as smart algorithms can significantly reduce bothtime required for the generation of the theoretical datawell as that utilized in the MMS search. These issuescurrently being studied.

FIG. 3. The effect of65 Å oxide thickness variations on a 2-Q signature.



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III. MULTIPARAMETER MEASUREMENTS:RESULTS

In order to confirm that the parameter space used is vfor the samples being measured, it is important to checkminimum mean square error between theory and experimFor these samples a measurement result is consideredacceptable if the mean square error is less than 2%. Hever, this particular error threshold should not be consideto be a hard rule; average mean square error values coueven higher, though still acceptable, depending onsamples being measured and how well their physical stture ~including profile shape! has been accounted for in thmodel.

Figure 4 depicts what we consider to be excellent agrment between the four parameter theory and experiment~theabsolute mean square error value is 0.63%!. These particular2-Q signatures are from nominal 0.35mm lines used in partof this study. We have observed in previous applications ttypical absolute mean square errors tend to be in the 1%–range; for the work described here, most of the errors wwell below 1%. The reason for the improved errors is proably because all possible parameters that could possvary, and thus affect the signatures, have been accountein our model. In previous applications, only one parame~linewidth! had been left as an unknown. Also, since toverall resist profile will affect the scatter signature, whtheory and experiment agree well it is an indication thatresist sidewalls are vertical or near vertical.

Because this large sample set combined gratings withferent underlying film thicknesses and exposure doses, msurements could be performed and analyzed in a numbedifferent ways. First, measurements of any of the paramecould be made on different wafers with the same exposdose. This is useful for testing the ability of the techniquemeasure the different film thicknesses, and also to checthe linewidth measurements are robust to these thicknvariations. Second, measurements could be performed onsame wafer but at different exposure doses. This providebroader range of linewidths to measure while at the sa

FIG. 4. Comparison of theoretical and experimental signatures for a nom0.35mm sample.


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time tests the robustness of the film thickness measuremwhich should be approximately the same across any gwafer.

In order to test the technique in as broad a mannepossible, both of these analyses were performed for all fparameters. For simplicity the results have been categorby parameter.

A. Sub-0.5 mm linewidth measurements

Figures 5 and 6 depict linewidth measurements as a fution of exposure for wafers 12 and 17. On the figures msurement results for three different metrology techniq~scatterometry, top-down SEM, and cross-section SEM! areshown. These measurements correspond to approximat350 nm range of linewidths. Such a broad range of Cprovides a good test for the measurement technique. Aevidenced on the plots, the correlation between scatteromand cross-section SEM measurements is excellent overwhole linewidth range. The average difference between sterometry and cross-section SEM measurements for wafe~nominal 0.35mm lines! is 21.7 nm, while for wafer 17~nominal 0.25mm lines! it is 27.3 nm.

FIG. 5. Wafer 12, nominal 0.35mm lines.

FIG. 6. Wafer 17, nominal 0.25mm lines.



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The error bars on the scatterometry results~610 nm! de-picted in the figures are due to the 10 nm CD step size uin generating the theoretical signatures. This is a consetive ~large! estimate, and a smaller step size in the theoretmodel would reduce the uncertainty. The top-down SEMKLA 8000 located at SEMATECH, has a measurement ucertainty of620 nm as estimated by KLA. The resolutionthe cross-section SEM measurements, an Amray 1910 insment located at SEMATECH was68 nm, as specified by themanufacturer. Thus the scatterometry and cross-section msurements agree within the error range of the two techniq

With respect to the top-down measurements madethese two wafers, a systematic bias, one that was observprevious work,16 is clearly present. With the addition of thcross-section measurements, however, it seems clear thatop-down measurements are in fact biased. Reasons forare not clear yet. First, resist charging has long been knoto degrade SEM images and thus contribute to CD measment errors.17–19 Although the samples were measured alow accelerating voltage~1 kV!, charging is still present andcould contribute to a measurement artifact. Furthermore,use of different electron detection modes~backscatter versussecondary! can yield different CD measurements of the safeatures.20,21Note that the KLA 8000 operates in a backsctered electron mode while the Amray 1910 measuremewere performed using secondary electrons. Finally, the win which the top-down SEM measurement algorithm intprets the edge of the resist line from the raw electron sigmay also contribute to a systematic error. This effect hbeen reported for CD measurements on photomasks.22 Theuse of a theoretically modeled electron signal could hidentify the linewidth edge position more accurately on a r

TABLE I. CD bias results for all samples.

Wafer No.

Nominalfeaturesize

Average biasagainst top-down SEM

1s ofbias

12 0.25mm 21.2 nm 11.4 nm12 0.35mm 23.6 nm 8.5 nm17 0.25mm 26.0 nm 10.5 nm17 0.35mm 24.3 nm 9.1 nm22 0.25mm 25.1 nm 7.3 nm22 0.35mm 23.5 nm 7.1 nm

TABLE II. Comparison of scatterometry and SEM CD measurements.

Sample

Nominal 0.25mm lines Nominal 0.35mm lines

Scatterometry SEM Scatterometry SEM

Wafer 2 200 nm 239 nm 300 nm 340 nmWafer 7 210 nm 263 nm 310 nm 356 nmWafer 12 220 nm 254 nm 320 nm 360 nmWafer 17 200 nm 253 nm 310 nm 341 nmWafer 22 210 nm 241 nm 310 nm 290 nmAvg. bias 42.0 nm 35.4 nm1s of bias 10.4 nm 10.1 nm


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signal, and thus reduce systematic top-down SEM errorthis type.23

This bias between top-down SEM and scatterometrymeasurements has been observed on several other wfrom this same sample set. This is illustrated in Tablewhich lists the average bias and standard deviation of sterometry measurements in comparison to the top-doSEM measurements for both nominal feature sizes on thwafers. The same SEM~KLA instrument! was used for all ofthese measurements. Despite the difference, the scatteetry measurements are consistent with those performedthe SEM, as is evidenced by the low standard deviationthe bias.

For CD measurements across different wafers~and hencewafers with different film thicknesses!, the scatterometry results are summarized in Table II, along with comparisonstop-down SEM measurements performed on the sasamples. Because the measurements were performed asame focus and exposure location on each wafer, theshould essentially be the same for all five wafers. The sterometry results confirm this for both the 0.25 and 0.35mmgrating lines—for both instances the range of measured lwidths does not exceed 20 nm. Comparisons with the Sresults are good, although the previously observed biastween the two measurement techniques is still apparent.encouraging that the bias is consistent~low standard devia-tion!.

B. Resist thickness measurements

The results from measuring the resist thickness acseveral wafers are shown in Table III. Included in the tablea comparison to ellipsometer measurements. For all msurements there is 71.2 or 65.2 nm difference betweenscatterometry results and those calculated from ellipsomedata. However, it is important to note that the ellipsome

TABLE III. Comparison of scatterometry and ellipsometry resist height msurements.

Sample


Ellipsometry Scatterometry Ellipsometry Scatteromet


TABLE IV. Comparison of scatterometry and cross-section SEM resist hemeasurements.

Wafer ScatterometryCross-section

SEM

12 710 nm 696 nm17 700 nm 688 nm



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measurements were taken immediately after the resistdeposited on the wafers. Since APEX-E is a chemically aplified resist, after lithography it undergoes a postexposbake ~PEB!, a process which is known to shrink the resthickness. The difference between the two measuremmay be explained by this shrinkage.

This hypothesis was tested using a cross-section Swhich measured the thickness of the photoresist in the ceof wafers 21 and 17. These result confirm that the measments discussed earlier were biased because of resist shage, as shown in Table IV. When the resolutions of the Sand scatterometry measurements are taken into accou~8and 10 nm, respectively!, the two thickness measuremenagree well.

The resist thickness measurements for wafer 22 are shin Fig. 7. Due to the capabilities of modern resist coatitechnology, resist thickness does not vary appreciably aca given wafer. This is reflected in the scatterometry measments, which do not vary by more than 10 nm, the resolutof the parameter space. Similar results were observedother wafers as well.

C. Film stack (ARC and poly-Si) measurements

Table V shows the ARC thickness measurement resfor a series of five different wafers. Because the ARC is vthin and has an index~n51.63! close to that of the resis~n51.603! at the 633 nm wavelength used by the scatteroeter, it is more difficult for scatterometry to determine thparticular thickness. For the scatterometer, the ARC laalmost appears to be an extension of the resist. This isdenced by the range of values scatterometry determinethe thickness~as large as 15 nm for the 0.25mm lines!.

FIG. 7. Resist height across wafer 22 at each exposure location.

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TABLE V. Comparison of scatterometry and ellipsometry ARC thicknemeasurements.

Sample



Wafer 2 68.7 nm 80.0 nm 68.7 nm 77.5 nmWafer 7 68.0 nm 65.0 nm 68.0 nm 85.0 nmWafer 12 67.8 nm 75.0 nm 67.8 nm 77.5 nmWafer 17 67.5 nm 65.0 nm 67.5 nm 77.5 nmWafer 22 66.7 nm 75.0 nm 66.7 nm 85.0 nmAvg. bias 6.4 nm 12.8 nm1s of bias 3.7 nm 4.4 nm


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Despite this, agreement between the ellipsometer and sterometer measurements for both gratings is good, witbias of 6.4 nm for the 0.25mm lines and 12.8 nm for the 0.3mm lines.

Across one single wafer, No. 22, the scatterometer Athickness measurements do not show a large degree of vtion, as is illustrated in Fig. 8. Overall the ARC varies by twincrements in the parameter space, or 5 nm in total.

The poly-Si thickness measurements across the samries of five wafers, seen in Table VI, also show good agrment with ellipsometry values. Unlike the resist and ARthicknesses, which were essentially the same from wafewafer, there were three different nominal poly-Si thicknesdeposited across the five wafers. As is seen in the tascatterometry is able to consistently determine these diffethicknesses. Although there appears to be a slight offsetrespect to measurements made with the ellipsometer, thestandard deviation is evidence of consistency betweentwo measurement techniques.

For a series of poly-Si thickness measurements madethe same wafer, one would expect that, because the groprocess is well controlled across the wafer, the thicknwould be essentially the same. The scatterometry measments seen in Fig. 9 reflect this notion. Eight out of the nmeasurements show the same thickness of 2500 Å. Oneferent measurement that was made at the lowest expolocation might be because the resist profile is not squThis was confirmed by cross-section SEM examinationother wafers. The profile issue notwithstanding, it is imptant to point out that this one odd measurement differed frthe others by 10 nm, or one step size in the parameter sp

FIG. 8. ARC height across wafer 22 at each exposure location.

TABLE VI. Comparison of scatterometry and ellipsometry poly-Si thicknemeasurements.

Sample






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IV. INSTRUMENT REPEATABILITY

To determine the dynamic repeatability of the 2-Q scat-terometer for CD measurements, ten consecutive 2-Q mea-surements were made on each of the two grating samhaving the nominal underlying film thicknesses. Betweeach scan the wafer was removed, replaced and manurepositioned so that the laser spot was centered on theing. All four parameters for each of the gratings were detmined for every scan using the MMS algorithm. For easeries of scans, the average and standard deviation forparameter was then calculated. For our purposes the standeviation is defined to be

s5A( i51N ~xi2 x̄!2

N21. ~3!

The raw data~i.e., 2-Q signatures! are repeatable due tthe simplicity of the technique. In order to get differencesthe predicted dimensions a smaller step size must be usethe parameter space. Thus for this experiment a 5 Åstep sizewas used for all four parameters. The use of a small stepwill result in a large parameter space if a broad range is anecessary. However, because the repeatability was beingtermined for one location, only a small range around tparticular linewidth was utilized.

The results~3s! for the repeatability study are summarized in Table VII. For both gratings and all four parametethe repeatability~1s! is less than 1 nm. In particular, the 3srepeatability of linewidth measurements is 0.75 nm for nomnal 0.25mm lines, and 1.08 nm for nominal 0.35mm lines.For the remaining parameters the 3s value is still excellent~subnanometer in some cases!. The ARC height is fully re-peatable because, even with a 5 Åstep width, every scan ouof the ten scans performed predicts the same ARC vaOverall, the repeatability for all parameters is well within th1% tolerance specification required for a production bametrology technique, and exceeds the most recent SEMpeatability figures.24–26

s

FIG. 9. Poly-Si height across wafer 22 at each exposure location.

TABLE VII. Four parameter repeatability~3s values!.

Samplesize Linewidth

Resistheight

ARCheight

Poly-Siheight

0.25mm 0.75 nm 1.29 nm 0.0 nm 0.72 nm0.35mm 1.08 nm 0.78 nm 0.0 nm 0.78 nm


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V. CONCLUSIONS

We have demonstrated that 2-Q scatterometry is capablof measuring four process parameters simultaneounamely linewidth, resist height, ARC thickness, and polythickness. CD results in comparison to SEM measuremeboth top-down and cross section, show good consistencylinewidths ranging from 150 to 500 nm. For example, taverage difference between scatterometry and cross-seSEM measurements on nominal 0.35mm lines is21.7 nm;for 0.25mm lines, the average difference is27.3 nm.

We have shown the technique is robust to variationsunderlying film thicknesses. These thicknesses can be qtified, and show good agreement with ellipsometer measments. The repeatability of the scatterometer was showbe excellent for all four parameters measured, with a repability ~3s! on the order of 1 nm. The 3s repeatability oflinewidth measurements is 0.75 nm for nominal 0.25mmlines, and 1.08 nm for nominal 0.35mm lines. Scatterometryhas the potential to measure critical dimensions as welfilm stack properties, performing the task of two measument instruments at a substantially lower cost and higspeed.

ACKNOWLEDGMENTS

The authors thank the SRC SEMATECH for their finacial support of this research. In addition, we thank Mr. SteFarrer of Motorola/SEMATECH and Mr. Robert Elliott foproviding valuable discussion and SEM measurements.

1The National Technology Roadmap for Semiconductors~SemiconductorIndustry Association, 1994!.



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multiparameter grating metrology using optical scatterometry

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