First lithographic results from the extreme ultraviolet Engineering ...xrm.phys.northwestern.edu/research/pdf_papers/2001/chapman_jvstb_2001.pdf · that has been developed to demonstrate

First lithographic results from the extreme ultraviolet EngineeringTest Stand

H. N. Chapmana)

Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550

A. K. Ray-Chaudhuri,b) D. A. Tichenor, W. C. Replogle, R. H. Stulen, G. D. Kubiak,P. D. Rockett, L. E. Klebanoff, D. O’Connell, A. H. Leung, and K. L. JeffersonSandia National Laboratories, P. O. Box 969, Livermore, California 94551

J. B. WronoskySandia National Laboratories, P. O. Box 5800, Albuquerque, New Mexico 87185

J. S. Taylor, L. C. Hale, K. Blaedel, E. A. Spiller, G. E. Sommargren, J. A. Folta,and D. W. SweeneyLawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550

E. M. Gullikson, P. Naulleau, K. A. Goldberg, J. Bokor, and D. T. AttwoodLawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720

U. Mickan and R. HanzenASML, de Run 1110, 5503 LA Veldhoven, The Netherlands

E. Panning, P.-Y. Yan, C. W. Gwyn, and S. H. LeeIntel Corporation, 2200 Mission College Boulevard, Santa Clara, California 95052

~Received 14 June 2001; accepted 4 September 2001!

The extreme ultraviolet~EUV! Engineering Test Stand~ETS! is a step-and-scan lithography toolthat operates at a wavelength of 13.4 nm. It has been developed to demonstrate full-field EUVimaging and acquire system learning for equipment manufacturers to develop commercial tools. Theinitial integration of the tool is being carried out using a developmental set of projection optics,while a second, higher-quality, projection optics is being assembled and characterized in a paralleleffort. We present here the first lithographic results from the ETS, which include both static andscanned resist images of 100 nm dense and isolated features throughout the ring field of theprojection optics. Accurate lithographic models have been developed and compared with theexperimental results. ©2001 American Vacuum Society.@DOI: 10.1116/1.1414017#

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I. INTRODUCTION

Extreme ultraviolet~EUV! lithography is an attractivenext generation lithographic technology since it will suppimaging dense 1:1 line–space features smaller than 30and could be introduced as early as the 70 nm mode.EUV Engineering Test Stand~ETS! is an alpha-class toothat has been developed to demonstrate full-field printingEUV images and to acquire system learning for equipmmanufacturers to develop commercial tools.1 The ETS hasbeen developed and integrated following an acceleraschedule, using a developmental projection optics b~POB1!. A second, higher quality, projection system~POB2!is currently being assembled and characterized,2 which willreplace POB1 in the ETS in early 2002. The ETS has ungone its initial setup, which involved determining the reticand wafer plane locations, exposure time, illumination cditions, and scan-stage velocity. Static and scanning imahave been printed in photoresist and examined by scanelectron microscopy. This article presents the first imathat have been obtained. Since the primary purpose ofinitial ETS experiments with the developmental set of opt

a!Electronic mail: [email protected]!Present address: KLA-Tencor, 160 Rio Robles, San Jose, CA 95134.

2389 J. Vac. Sci. Technol. B 19 „6…, Nov ÕDec 2001 1071-1023Õ200

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is to assess the quality of the printed images and to determtheir dependence on the various subsystems of the ETS,important that the lithographic results be compared wmodels. We give a detailed description of the method to copute aerial images and show initial comparisons withprinted images and with critical dimension~CD! measure-ments.

II. THE ENGINEERING TEST STAND „ETS…

The ETS is a step-and-scan system that operateswavelength of 13.4 nm. The POB is a multilayer-coatefour-mirror, ring-field system with a 0.1 numerical apertu~NA!, and a 4:1 image reduction.3 The field is a 30° sector othe 211 mm radius ring field~96 mm chord length3 6 mmwidth at the reticle!. The reticle is a reflective multilayercoated ULE substrate that is patterned with absorbfeatures.4 A schematic of the ETS, including the vacuuenclosure, support structures, optics, stages, and sourcsembly, is shown in Fig. 1.

A. EUV source

The EUV radiation is generated from a laser-producplasma by focusing a pulsed Nd:YAG beam onto a xen

23891Õ19„6…Õ2389Õ7Õ$18.00 ©2001 American Vacuum Society

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2390 Chapman et al. : First lithographic results from the EUV Engineering Test 2390

cluster target.5 The source diameter is approximately 3mm. The initial lithography results presented here were pformed with a 40 W laser, which delivers 0.11 mW/cm2 EUVpower at the wafer. Tests of a higher-power laser, develoby the TRW Corp., have been performed in a separate sodevelopment facility adjacent to the ETS. Measuremeshow that 10–20 mW/cm2 can be expected at the wafer plain the ETS, depending on source conditions, following ingration of this high-power source. This compares withdesired EUV power density of 22.6 mW/cm2 at the waferthat would support ten 300 mm wafer/s operations.

B. Illuminator

The condenser system collects the EUV light from tsource and provides quasistationary illumination acrossring field, with a six-channel pupil fill.6 The collection solidangle is 1.68 sr. The collector and subsequent condensetics are divided into six channels. Each channel proviKohler-critical illumination that produces an arc-shapbeam footprint on the mask to match the ring field of tprojection optics at the reticle plane, and an angular illumnation of approximatelysV50.3 andsH50.015. The inten-sities of all six channels overlap at the reticle plane, butangular distribution of each is displaced to give an effectpupil fill of s50.7. The measured reticle plane illuminatiointensity and uniformity are shown in Fig. 2. The modelpupil fill, for the central field point, is shown in Fig. 3~a!.Note that as the field point moves azimuthally around theeach channel in the condenser exit pupil rotates by theresponding angle. Thus, the open-frame pupil fill, whichthe fill integrated over all field points, shows channels tare bow-tie shaped. Figure 3~b! shows the open-frame pupfill, observed experimentally on a scintillator located at tprojection optics pupil.

FIG. 1. Solid-model schematic of the EUV ETS.

J. Vac. Sci. Technol. B, Vol. 19, No. 6, Nov ÕDec 2001

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C. Projection optics

The developmental projection optics box~POB1! wasassembled1 and characterized by both visible-light and EUinterferometry.7 With both techniques, the system wave froerror was measured at 45 field points throughout the rfield. A contour plot of the rms wave front error across tring field, as measured by visible-light interferometry,shown in Fig. 4~a!. The mean rms wave front error across tfield is 1.20 nm~l/11!, in a 36-term Zernike polynomiadecomposition, and varies between 0.90 and 1.4 nm. Onerage, the difference between the EUV and visible-light msurements was 0.25 nm rms. This is an indication ofaccuracy of the wave front measurement. The visible-liinterferometer measures wave front aberrations in the protion optics pupil up to a spatial frequency of 54 cycles/puradius ~i.e., per NA!. The measured aberration map at tcentral field point is shown in Fig. 4~c!. The rms wave fronterror for this map is 2.02 nm. At this field point, the rmwave front error in a 36-term Zernike decomposition is 0.nm, so the residual power~up to a frequency of 54/NA! is1.79 nm. This structure, which is quite noticeable in F4~c!, is not an artifact of the measurement but is due topolishing process of manufacturing the mirrors. The wafront error in these frequencies will lead to short-range flawhich will cause a loss of image contrast, as discussedSec. III. Initial measurements of POB2 show a residual rerror of ;1.0 nm.

Aberrations at spatial frequencies greater than 54/have been inferred statistically, using EUV scattering msurements of the individual mirrors of the POB. The scatt

FIG. 2. ~a! Open frame resist exposure at the wafer and~b! deviation of thescan-averaged intensity from the mean. The width of the illumination ismm at the wafer.

FIG. 3. ~a! Modeled pupil fill for the central field point.~b! Open frame pupilillumination, measured on a scintillator at the pupil of the POB. The cirdenotess50.7.

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FIG. 4. POB1 is extensively characterized:~a! rms wave front error as afunction of field location at the wafer plane, as measured by visible-liinterferometry.~b! Vector plot of distortion at the wafer plane, determineby Leica IPRO measurements of printed fiducials. The mean distortion inm, and the maximum is 80 nm.~c! Aberration map at the central fieldpoint. ~d! Power spectral density of the system aberration as a functionormalized frequency~number of cycles per pupil radius!, determined fromscattering measurements of the mirrors~thin line! and from the aberrationmap in~c! ~thick line!. The integrated power in frequencies 0–54/NA is 2.nm, and the total integrated power is 2.33 nm.

JVST B - Microelectronics and Nanometer Structures

ing measurements are compiled8 into a system wave fronPSD, as shown in Fig. 4~d!. These higher frequencies leadlong-range flare, which also reduces image contrast.

The distortion~or image-placement error!, shown in Fig.4~b!, was measured from static prints made in the ETPrinted images of fiducial patterns were examined usinLeica LMS IPRO wafer inspection instrument. IPRO mesurements were made on many exposures printed at valocations across a wafer to average random error in the IPand error due to wafer nonflatness. The measurement ewas estimated to be 4 nm. The distortion could be furtimproved~to a mean of 27 nm, with a maximum error of 5nm! by adjusting the tilts of the reticle and wafer planes. Tstatic distortion field impacts the quality of scanned imagas is described in Sec. IV C. In a scan, the distortion cauimage motion during the exposure, which results in a blring of the printed image.

D. Scanning stages

Scanned images are formed by scanning the reticleconstant velocity through the illuminated field of the projetion optics. The wafer is synchronously scanned at a quaof the mask velocity so that the scanned image appearstionary on the wafer. Any deviation from true synchroniztion, either in speed or direction, will cause a blur of tprinted image. A feedback control system provides the schronization of the stages and dynamically references thto the POB structure.9 In terms of stage operation, statimaging is simply scanning at zero velocity. The stagesstill actively controlled in this case and dynamically trackto the POB structure. Several full-field scanned images, emeasuring 24 mm332.5 mm, have been printed. The reticscan speed was approximately 40mm/s ~limited by the EUVintensity!. At zero velocity and 40mm/s the jitter of thereticle stage is below 5 nm rms and the jitter of the wastage is below 10 nm. The stages have also been testespeeds of 20 mm/s, and were seen to meet the sameperformance.

III. IMAGE MODELING

A primary goal of the initial lithographic tests of the ETSusing the developmental set of projection optics, is to undstand the quality of the printed images and how the varisubsystems impact those images. By comparing imagesmeasurements of image metrics~such as linewidth! to mod-els we can identify and resolve problems that arise in ingrating the components of the ETS.

It was found that a conventional model based on odimensional periodic objects could not accurately moETS images. The sparse pupil fill, coupled with the higspatial-frequency nature of the wave front map~see Figs. 3and 4!, leads to significant differences in the printing of ondimensional~1D! periodic objects versus 2D elbow pattern

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In the 1D periodic object case, the discrete diffraction ordonly sample small regions of the projection optics aberratmap, even when considering all illumination directions frothe condenser. A small change in linewidth completchanges the sampled regions of the aberration and mayto a completely different effect on the image. The 2D elbpattern produces a continuous diffraction pattern and socomputation does not exhibit these effects. As an examthe predicted horizontal–vertical focal shift caused by asmatism varies strongly and erratically with linewidth in th1D calculation. A 0.5 nm addition of astigmatism to the wafront gave a focal shifts of 0.9, 0.5, and 0mm for linewidthsof 80, 100, and 200 nm, respectively. This behavior is ctrary to that observed in the ETS, and clearly the 1D callations are not useful for estimating the degree of astigmtism in printed images~see Sec. IV A!.

The 2D images are computed as the sum of adeterminis-tic and astatisticalcomponent. The deterministic componeis a partially coherent image that is computed from a givobject pattern, a measured wave front error map~dependingon the field point being considered!, and an illumination pu-pil fill. The wave front map includes spatial frequencies up54/NA, which corresponds to a maximum extent of tpoint-spread function~PSF! of 7.2 mm, for the 0.1 NA and13.4 nm wavelength. An example of such a computed aeimage is given in Fig. 5~a!. When modeling images of largeextent, or objects that are in an extended bright field,must include aberrations of higher spatial frequency. Thare characterized by the power spectral density~PSD!,shown in Fig. 4~d!. It is assumed that at these higher frequecies the wave front error is homogeneous and isotroacross the pupil and across the field. This implies thatimage caused by scattering from these frequencies doedepend on pupil fill.8 Hence, this image component may bcomputed as a convolution of the object pattern withlong-range intensity PSF~which is directly proportional tothe PSD!.

The two components of the image computation can abe categorized as high and low resolution. In general, a laarea of the mask may contribute intensity to the image regof interest due to the high-frequency aberration componof the optics. In this case, the low-resolution componencomputed over that large area, with a large pixel samplingreduce computation time. The appropriate subregion ofimage is then extracted and extrapolated to the hi

FIG. 5. Computed images of 100 nm elbow patterns:~a! high-resolutionpartially coherent image,~b! low-resolution incoherent image, based on tPSD, and~c! sum of ~a! and ~b!. The contrast of the images has bereduced~gamma of 3%! to show the short-range flare.


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resolution image, as shown in Fig. 5~b!. The low-resolutioncomputation may be further split into various resolutiongimes, so it is possible to include intensity contributiofrom the entire illuminated field. In this way the effectsflare ~that is, high-frequency aberrations! can be accuratelymodeled for any mask pattern. The final~high-resolution!computed image is the sum of the components, as showFig. 5~c!.

IV. INITIAL TOOL SETUP

A. In situ aberration correction

Our initial imaging tests on the ETS were performedmonths after interferometric characterization of POB1. Neertheless, the initial static imaging results indicated a remaable mechanical stability of the POB, with the only chanbeing a small drift in astigmatism. This can be seen in F6~a!, which displays some of the first static resist imagfrom the ETS. The degree of astigmatism was found by crelating printed images of elbow patterns at various linwidths, foci, and dose, with partially coherent modeled iages. The best correlation was found when20.5 nm rms of0°–90° astigmatism was added to the model. Modeled reimages, based on a simple intensity threshold, are showFig. 6~b!. For our off-axis ring-field POB, astigmatism is thmost sensitive aberration that is induced by any motionany of the mirrors. Such a small value of astigmatism implthat none of the mirrors could have drifted by more thanmrad over the 9 month period since the POB was aligned

FIG. 6. Through-focus resist images of 100 nm dense elbow patterns~a!prior to aberration correction compred with modeled images with20.5 nmadded astigmatism~b!. ~c! Resist images after correction of the projectiooptics by 1.0 nm of astigmatism compared with modeled images with10.5nm added astigmatism~d!.

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FIG. 7. Selection of static resist imageof 100 nm elbow patterns and star paterns from various locations throughout the ring field, as denoted by thcircled numbers and the map of thring field. The scale bar refers only tothe star patterns.

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characterized. Furthermore, analysis shows that other abtions such as coma and spherical aberration could not hchanged by more than 0.02 nm. To first order, a tilt of anythe four mirrors can be used to correct the observed astigtism. The mirrorM4 was chosen since this has the loweimpact on image placement error.

Modeling, in fact, showed that the optimum valueastigmatism for the 100 nm features is10.5 nm rms. There-fore a correction of 1 nm rms astigmatism~uniform acrossthe field! was induced by tiltingM4 by 6.0mrad. The result-ing images after adjustment and model predictions are shin Figs. 6~c! and 6~d!, respectively.

B. Full-field static images

Static images were examined throughout the field, acompared with computed images, to ensure that the illunation was best aligned to the well-corrected field ofprojection optics. The wafer was tilted to correct for the fcus variation across the ring field. However, further waand mask adjustments must still be carried out to furtreduce the static distortion field~this is a consequence onontelecentricity at the reticle plane!. A sample of resist im-ages of elbow and star patterns from various locatithroughout the field is shown in Fig. 7. The trends obserin the prints agree with the modeled images. For examthe known increased wave front aberration at the right eof the field leads to the degraded image performance the

C. Scanned images

To print scanned images, the velocity~i.e., speed and di-rection! of the wafer relative to that of the reticle must boptimized. The residual velocity error of the wafer, whichthe vector difference between the velocity of the scanimage and the wafer stage, can be characterized by itsthogonal components: stage magnification error and sskew error. A stage magnification error results in a blur fution that extends in the scan direction, whereas a skew eleads to image motion~relative to the scanned wafer! in thecross-scan direction. The skew error depends on image


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tion caused by the projection optics and error in the paralism of the stages, and was determined from a series ofposures where the stage skew is varied. The stmagnification error was predicted from the IPRO static dtortion measurements@see Fig. 4~b!# to be 682 ppm. Theoptimum stage skew and stage magnification correctiwere found, from a series of prints where both paramewere varied, to be2680 mrad and175 ppm, respectively.After subtracting the magnification error from the static dtortion field, the predicted amount of the blur due to tprojection optics distortion for the central field point is aproximately 10 nm.

Scanned printed images from the ETS are shown in F8. All of these images are from the center of the field andscan direction is vertical on the page. Analysis of the linwidth and contrast of the scanned images has not yet bcompleted. However, the scanned 100 nm elbow printed

FIG. 8. Scanned resist images of elbow patterns, from the center of the fiThe scans were acquired with the full 1.5 mm width of the ring field illminated. The dose was optimized separately for each linewidth. Each imhas been separately scaled in size.

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terns are almost indistinguishable from the static imagThis shows that scanning has minimal impact on the imquality, as is predicted given the 10 nm blur~Gaussian halfwidth! caused by static distortion.

V. STATIC LITHOGRAPHIC PERFORMANCE

Measurement of the width of lines in images of 1line:space elbow patterns, printed at constant dose in 100thick Shipley 2D positive-tone resist, results in the CD liearity plot shown in Fig. 9. It is seen that there is an isdense bias of about 40 nm, and that the printed CD increas 80% of the coded CD of the pattern. The aberrationsshort-range flare of the projection optics can explain boththese effects, and the results are in good agreementmodeled images. It is well understood that a 1 nm ~l/13!36-term Zernike aberration leads to greater contrast losisolated lines than dense lines. The flare in the projecoptics exacerbates this contrast loss. The flare~in combina-tion with the particular mask pattern used in these measments! also causes the reduction of linewidth with increasCD. The elbow patterns are dark lines in a bright L-shapisland in an almost entirely dark field. Thus, only pointsthe bright L-shaped island contribute scattered light intoline patterns. The area of this bright region increases assquare of the CD. That is, larger patterns have a higher ctribution of flare, and so larger CD patterns require less dto print. For example, as shown in Fig. 10, at the requidose for 100 nm features, the 300 nm features will be ovexposed and the cleared lines in resist will be smaller tideal.

An additional observation from the printed images is thhorizontal dense lines print approximately 20 nm wider thvertical dense lines. This may be due to vibration in t

FIG. 9. Measured and modeled linewidths of the horizontal and vertidense, and isolated features of elbow patterns printed at a constant do


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projection optics. This will be investigated further as soonadditional diagnostics, including an aerial-image monitor,implemented this year.

IV. CONCLUSIONS

The EUV ETS is operational in scanning and static imaing modes. The tool integration and initial testing were acomplished very quickly. The first EUV images were print6 months ahead of our original schedule, and scanned imfollowed only 3 weeks later. Part of the initial setup involvean in situ correction of astigmatism. It was found that the 0nm rms observed astigmatism could easily and accuratelcorrected with a controlled tilt of one the POB mirrors. Staresist images of dense 1:1 elbow patterns, ranging infrom 100 to 300 nm, show as-expected image quality acrthe entire ring field of the projection optics. The 100 npatterns, including both dense and isolated features, areresolved throughout the entire field, and 80 nm patternswell resolved at the center of the field, where aberrationslowest. Currently, the only deviation from the expected Eperformance is a 20 nmH –V bias in the static printed patterns. Full-field~24332.5 mm2! scanned images have beeacquired. The scanned images of dense 100 nm elbowterns are almost indistinguishable from the static imagproving that neither stage motions nor the POB static distion field impacts the image quality.

Lithographic characterization of the ETS has only jubegun, and the near-term plans include the measuremeimage contrast and line-edge roughness of static imagescharacterization of scanned images, and measuremenflare throughout the field. We will continue to compare mesurements with simulations to provide a firm foundationsystem learning. These will be followed by the full-powsource upgrade and then the installation of the higher quaprojection optics.

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FIG. 10. Modeled aerial images of elbow patterns of vairous sizes showthat, for these particular mask patterns, flare increases with increasingture size.

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ACKNOWLEDGMENTS

This work was performed by the University of CalifornLawrence Livermore National Laboratory under the auspiof the U.S. Departemnt of Energy, Contract No. W-740ENG-48, by Sandia Naitonal Laboratories under the aucies of the U. S. Department of Energy, Contract No. DAC04-94AL85000, and by the Lawrence Berkeley NationLaboratory under the auspices of the U. S. DepartmenEnergy Office of Basic Energy Sciences. Funding was pvided by the Extreme Ultraviolet Limited Liability Corporation under a Cooperative Research and Development Agment.


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1D. A. Tichenoret al., Proc. SPIE~in press!.2P. Naulleau, K. A. Goldberg, P. Batson, S. Rekawa, P. Denham, anBokor, J. Vac. Sci. Technol. B, these proceedings.

3D. W. Sweeny, R. Hudyma, H. N. Chapman, and D. Shafer, Proc. S3331, 2 ~1998!.

4S. Hector and P. Mangat, J. Vac. Sci. Technol. B, these proceedings5G. D. Kubiak, L. J. Bernardez, K. Krenz, and W. C. Sweatt, Proc. SP3676, 669 ~1999!.

6W. C. Sweatt, inOSA Proceedings on Soft X-Ray Projection Lithograph,edited by A. M. Hawryluk and R. H. Stulen, 1994, Vol. 18, pp. 70–72

7K. A. Goldberg, P. Naulleau, P. Batson, P. Denham, E. Anderson,Chapman, and J. Bokor, J. Vac. Sci. Technol. B18, 2911~2000!.

8D. G. Stearns, D. P. Gaines, D. W. Sweeney, and E. M. Gullikson, J. APhys.84, 1002~1998!.

9J. B. Wronoskyet al., Proc. SPIE3997, 829 ~2000!.

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First lithographic results from the extreme ultraviolet Engineering ...xrm.phys.northwestern.edu/research/pdf_papers/2001/chapman_jvstb_2001.pdf · that has been developed to demonstrate