at-wavelength interferometry for extreme ultraviolet ...1997).pdf · at-wavelength interferometry...

At-wavelength interferometry for extreme ultraviolet lithographyEdita Tejnil,a) Kenneth A. Goldberg,b) SangHun Lee,a) Hector Medecki, Phillip J. Batson,and Paul E. DenhamCenter for X-Ray Optics, Lawrence Berkeley National Laboratory, Berkeley, California 94720

Alastair A. MacDowellAdvanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720

Jeffrey Bokora),c) and David Attwooda)

Center for X-Ray Optics, Lawrence Berkeley National Laboratory, Berkeley, California 94720

~Received 27 May 1997; accepted 9 July 1997!

A phase-shifting point diffraction interferometer is being developed for at-wavelength testing ofextreme ultraviolet lithographic optical systems. The interferometer was implemented tocharacterize the aberrations of a 103 Schwarzschild multilayer-coated reflective optical system atthe operational wavelength of 13.4 nm. Chromatic vignetting effects are observed and theydemonstrate the influence of multilayer coatings on the wave front. A subaperture of the optic witha numerical aperture of 0.07 was measured as having a wave front error of 0.090 wave~1.21 nm!root mean square~rms! at a 13.4 nm wavelength. The wave front measurements indicatemeasurement repeatability of60.008 wave (60.11 nm) rms. Image calculations that include theeffects of the measured aberrations are consistent with imaging performed with the 103Schwarzschild optic on an extreme ultraviolet exposure tool. ©1997 American Vacuum Society.@S0734-211X~97!01306-1#

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I. INTRODUCTION

Wave front measuring interferometry plays a key rolethe fabrication, alignment, and qualification of optical sytems, including lithographic stepper lenses. Interferomecharacterization of extreme ultraviolet~EUV! projection li-thography optics is necessary to achieve the near diffractlimited optical performance required for lithography at crical dimensions of 0.1mm and below. Such EUV lithographioptical systems operate at a wavelength of about 13 nmnumerical apertures of around 0.1. To characterize the arations, interferometry with subnanometer wave front msuring accuracy is required. In addition, measurements aoperational wavelength of 13 nm are needed to charactethe system EUV wave front, which is produced both by tfigure of mirror surfaces and by multilayer coatinproperties.1 Common-path techniques, such as point diffration interferometry2 or shearing interferometry,3 are requiredfor at-wavelength wave front characterization due to the lof long-coherence-length light sources at EUV waveleng

Point diffraction interferometry is suitable for direct mesurement of wave front aberrations at EUV wavelengthscause of its applicability over a wide spectral range, compdesign, and relaxed temporal coherence lenrequirements.2 This technique utilizes the generation ofhigh-quality spherical reference wave front by diffractiofrom a ‘‘subresolution’’ pinhole. This reference wave frointerferes with the unknown wave front under test. We harecently developed a phase-shifting point diffraction interf

a!Also at Dept. of Electrical Engineering and Computer Sciences, Univerof California, Berkeley, CA 94720.

b!Also at Dept. of Physics, University of California, Berkeley, CA 94720c!Electronic mail: [email protected]

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ometer~PS/PDI! that enables EUV wave front characteriztion with both high efficiency and a phase-shiftincapability.4–6

In this article, we describe recent measurements o103 demagnification, multilayer-coated, Schwarzschild ojective, designed for a prototype EUV lithography exposutool7 using the new phase-shifting point diffraction interfeometer. The repeatability of the interferometric measuments is characterized. The aberrations of the optical sysare described, and a demonstration of chromatic aberratdue to multilayer coatings is also presented. Finally, thetical performance predicted by the interferometry is verifiby imaging experiments utilizing the EUV exposure tool fwhich the optic was designed.

II. INTERFEROMETER CONFIGURATION FORTESTING THE 103 SCHWARZSCHILD SYSTEM

The phase-shifting point diffraction interferometer for awavelength testing of EUV lithographic optics operatesthe undulator beamline 12.0 at the Advanced Light Sou~ALS! at Lawrence Berkeley National Laboratory. Thbeamline optics include a grating monochromator thatused to select the desired wavelength, and a KirkpatricBaez~KB! illuminator that is designed for optimum transfeof spatially coherent radiation to the interferometer.

The Schwarzschild test optic, designed for 103 reductionEUV projection lithography experiments, consists of twnearly concentric spherical mirrors.7 Both mirrors are coatedwith molybdenum–silicon multilayer reflective coatings wipeak transmission at 13.4 nm wavelength. While the annuconcave secondary is coated with a multilayer of nearlly uform thickness, the convex primary has a graded multilacoating designed to compensate for the varying angles

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2456 Tejnil et al. : At-wavelength interferometry for EUV 2456

incidence across its surface.8 An off-axis aperture stop tharests on the primary mirror can select a circular portionthe annular clear aperture for imaging experiments. Thistatable aperture stop contains three separate subapercorresponding to different selectable numerical apertu~NA! of 0.06, 0.07, and 0.08. The image plane of the opdetermined during the assembly of the system, is definedthree balls attached to the optical housing.

The configuration of the phase-shifting point diffractiointerferometer for testing the 103 Schwarzschild optical system is illustrated in Fig. 1~a!. The optic is tested in its in-tended vertical operational orientation. The EUV beamsteered into the optic with an adjustable, multilayer-coat45° turning mirror. This mirror enables small angular adjuments in the beam direction that are needed to optimizeillumination of the optic. The KB optics focus the beam onthe object plane of the 103 Schwarzschild system, where

FIG. 1. ~a! Components of the phase-shifting point diffraction interferomefor testing the 103 demagnification Schwarzschild optical system.~b! ASEM image of the spatial filter used in the image plane of the Schwarzscoptic. Two subresolution pinholes with diameters of 130 nm~top! and 165nm ~right! are next to a 5-mm35-mm sq window.

J. Vac. Sci. Technol. B, Vol. 15, No. 6, Nov/Dec 1997

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subresolution pinhole is placed. The pinhole selects spaticoherent radiation from the beam and spatially filters itilluminate the test optic with a spherical wave front. Thobject–plane pinholes used in these measurements aremercially available, laser-drilled pinholes with nominal dameters of 0.5mm. These pinholes are significantly smallthan the diffraction-limited resolution of the test optic on tobject side~subresolution!, ensuring a high-quality illumina-tion wave front.9 This pinhole is placed in a kinematic mounattached to a computer-controlled, three-axis stage thalows alignment of the pinhole within the beam.

A coarse diffraction grating, placed between the objecplane pinhole and the Schwarzschild optic, serves as a langle beamsplitter by dividing the wave front into multipdiffractive orders. The 18-mm-pitch grating consists of a225-nm-thick patterned gold absorber supported by a 1nm-thick silicon nitride membrane. On propagation throuthe test optic, the aberrations of interest are introducedthe spherical illumination beams. Two of the diffractive oders are selected with a spatial filter, shown in Fig. 1~b!,placed in the image plane of the optic. The zero diffractorder is chosen as the test beam and is transmitted throthe large 5-mm35-mm sq window, which is significantlylarger than the focal spot size. One of the first diffractiorders is spatially filtered by a subresolution pinhole to pduce a spherical reference wave front over the numeraperture of the measurement. The choice of the zero difftive order for the test beam ensures that aberrations duthe grating line placement are not introduced into the msured wave front. Translation of the grating in the directiperpendicular to its lines controls the relative phase shifttween the test and reference beams necessary to perphase-shifting interferometry. The grating pitch is chosenprovide a 4.5mm separation between the test and the refence wave foci in the image plane. This corresponds todistance between the subresolution pinhole and the centethe window. Two separate reference pinholes, placed inorthogonal directions from the center of the test-beam wdow, allow measurement with two different grating orienttions.

The image–plane pinhole apertures were drilled by acused ion beam into a membrane structure consisting ofnm of silicon nitride and 100 nm of indium antimonide asorber. After the aperture definition, an additional 70 nlayer of indium antimonide absorber was deposited on eside of the membrane to increase the absorption and alsdecrease the pinhole size. The pinholes fabricated withmethod and used in experiments reported here range f130 to 165 nm in diameter. The pinholes are aligned afixed to be at the center of the image plane by means okinematic mount that rests on the three balls that defineimage plane.

The interference of the test and reference beams iscorded with a back-thinned, back-illuminate102431024-pixel, 1-in. sq, silicon charge coupled devi~CCD! detector, placed 12.7 cm beyond the image planethe Schwarzschild optic with its surface normal to the cen

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ray of the off-axis beam. The exposure time, typically 5–s, is controlled with a compact, high-speed shutter placefront of the 45° turning mirror, as shown in Fig. 1~a!.

In its EUV configuration, the PS/PDI records the interfeence between test and reference beams without reimaoptics. To prevent beam overlap in the image plane, theeral separation of the two beam foci is designed to be qlarge. As a result, in addition to the aberrations of interethe recorded interference fringes contain a large tilt indirection of the beam separation. The geometrical effecthe beam separation also produces various orders of comthe fringe pattern. For the beam separations and numeapertures used in this experiment, only the third order cois relevant. This systematic effect can be removed frommeasured aberrations because its orientation is known tperpendicular to the tilt fringes and its magnitude scales wthe measurement numerical aperture in a known way. Cbining the measurements performed with two orthogograting orientations makes removal of this coma effstraightforward.

Alignment of the test and reference beams throughsystem is accomplished by high-precision translation ofoptic and the attached pinhole apertures on a bearing inlateral directions. The focus is controlled by translationthe object–plane pinhole along the beam propagation dition. When aligned for data acquisition, the interferomeremains in alignment for several hours, and it has demstrated good mechanical stability despite the fact thattemperature of the system is not controlled. Measuvacuum chamber temperature fluctuations typically doexceed60.5 °C over 24 h.

To mitigate the carbon contamination problem reporrecently,6 several improvements have been made to theterferometer vacuum system, including the cleaning of mof the interferometer components and increasing the puing speed to achieve a base pressure of 531027 Torr. Inoperation during exposure to EUV light, oxygen gas is binto the chamber to reduce carbon contamination buildmaintaining a pressure of 1024 Torr. These measures havnearly eliminated the contamination problem.

III. RESULTS OF INTERFEROMETRICMEASUREMENTS

Numerous experiments have been done to characterizaberrations in the Schwarzschild optic and to evaluatecapabilities of the interferometer. The measurements inclcharacterization of several regions of the annular full apture, evaluation of aberrations at different wavelengths,determination of measurement repeatability.

One important measurement is to characterize the ltransmitted through the uniformly illuminated test optic. Ttransmitted intensity of 13.4 nm radiation is shown in F2~a!, which reveals localized contamination of the opticsurfaces. Some of the contaminants, most likely particulaon the mirror surfaces, cause complete loss of transmissOther contaminants, possibly a residue from a wet-cleanprocess of the optical substrates or the coated surfaces,

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to reduction in the transmitted intensity. The contaminaregions are on the order of 100mm in size. When the measurement is repeated at a visible wavelength of 632.8shown in Fig. 2~b!, the transmitted intensity is quite different. The particulate contamination produces a loss of tramission, but the other residue is nearly imperceptible. Ovall, the contamination effects are more pronounced at Ewavelengths than at visible wavelengths. Mirror surface ctamination present in this Schwarzschild optic scatters ration to moderate angles and contributes to wave front abrations in the midspatial-frequency regime. In PS/Pmeasurements, some of the scatter in the reference beatransmitted through the test-beam window. Although tcorrupts the measurement in the vicinity of the contaminaregions in the aperture, it does not affect the ability of tinterferometer to measure low-order aberrations.

The measurements reported here utilize phase-shiftingterferometry techniques that combine multiple interfegrams with different relative phase shifts between theand reference beams. Analysis of individual phase-shiftdata series is performed using an adaptive least-squalgorithm,10 modified to compensate for irregular phasecrements caused by a coarsely controlled grating translastage. The presence of numerous blemishes on the ocomplicates both the phase unwrapping and the Zernpolynomial fitting. The unwrapping of the raw phase mauses a highly filtered, continuous version of the phase mas a guide to determine the correct phase increments.11 Theunwrapped phase maps are fitted to the first 37 Zernikecular polynomials12 that describe the low-spatial-frequencaberrations of interest. Fitting utilizes an intermediate sepolynomials, orthogonal over the domain of valid data poithat excludes the blemish regions.13,14Following the Zernikefit, the geometrical coma effect is removed by combiniseparate measurements performed with two orthogonal ging orientations.

Three different regions of the annular aperture of t103 Schwarzschild optic, corresponding to the three subertures in the aperture stop, were characterized at 13.4wavelength. The wave front aberrations of the three subertures, reconstructed from the Zernike polynomial fit, ashown in Fig. 3. Figure 3~a! shows the relative positions othe different subapertures in the aperture stop. The lar

FIG. 2. Transmission of light through the Schwarzschild optic at two diffent wavelengths:~a! 13.4 and~b! 632.8 nm. The contamination of the optcal surfaces appears quite different at the two wavelengths.

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subaperture is noncircular due to a fabrication error. Taberrations of the 0.08, 0.07, and 0.06 NA subaperturesshown in Figs. 3~b!–3~d!, respectively. Visible-light interfer-ometry, performed during assembly of the optic, was usealign the least aberrated region of the optic with the 0.07 Nsubaperture. With the tilt and defocus removed, the msured wave front errors of the 0.08, 0.07, and 0.06 NA sapertures are, respectively, 0.260, 0.090, and 0.043 wroot mean square~rms! at 13.4 nm. Figure 3 shows the wavfront aberrations with small a amount of defocus reintduced to illustrate that the aberrations in fact follow the anulus of the optic and seem to correspond to a zonal facation error.

One primary advantage of at-wavelength interferometryits ability to characterize the overall EUV wave front prduced by both the mirror surface figure and the multilaycoatings. Owing to the fact that, upon a change in walength, the aberrations due to multilayers change wherthose due to surface errors do not, multilayer effects canobserved directly via wave front measurements over a raof wavelengths. In this experiment, both transmitted intensand the wave front phase were measured at several wlengths. Transmission through the 0.07 NA subaperturethe optic at 13.0, 13.2, 13.4, and 13.6 nm is shown in F4~a!. Transmission through different portions of the apertureveals a zonal effect that follows the annular full apertuWithin the coating pass band, at 13.2 and 13.4 nm, the tramission is quite uniform, being lower only near the edgesthe aperture. The measured transmission along the centthe annulus is peaked at;13.4 nm wavelength, in agreemewith the coating design,8 but the transmission peak is shifteto ;13.3 nm on the inner edge of the annulus and;13.35 nm on the outer edge of the annulus. This indica

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FIG. 4. Measured chromatic aberration produced by multilayer reflective coatings.~a! As the wavelength is changed, transmission through the optic varies.overall transmission is peaked at 13.4 nm.~b! The measured differences between the aberrations at the indicated wavelengths and the 13.4 nm wademonstrate the presence of multilayer-coating phase effects.

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that the multilayer-coating period deviates from its intendvalue, which is designed to achieve transmission uniformbetter than 99% at 13.4 nm. At the edges of the pass ban13.0 and 13.6 nm, the transmission is very nonuniform. Tbehavior is not unexpected, even for perfect multilayers,cause the coatings are designed to accommodate a ranincidence angles in a limited spectral band. Outside thesign pass band, the differences in incidence angles acrosoptic are amplified because the coating properties varyidly outside the central transmission lobe.

The chromatic phase effects resulting from reflectionmultilayer mirrors are illustrated in Fig. 4~b!, which showsthe difference between the aberrations measured at 113.2, 13.4, and 13.6 nm and the aberrations measure13.4 nm. Within the coating pass band, the differences inmeasured wave fronts are small because the wavelechange results primarily in a constant phase offset that ismeasurable by interferometry. At the pass band edges, wnonuniformities in the coating properties are accentuatedmeasured phase difference over the aperture is consiwith a radial nonuniformity in the multilayer-coating thickness.

The quality of the interferometry was characterizedrepeatability measurements performed on the 0.07 NA saperture of the Schwarzschild optic. The aberrations wfirst characterized in a series of measurements perforover several weeks with multiple object–plane input pholes, several image–plane reference pinholes, and twothogonal gratings. Following this series of experiments,other two subapertures of the optic were measured. Suquently, the 0.07 NA subaperture of the Schwarzschild owas measured again in a second series of experiments.cessive measurements of each subaperture required remof the optic from the vacuum chamber, followed by repotioning of the optic and complete realignment of the interfometer. The wave front aberration map and the correspoing Zernike polynomial fit, determined at 13.4 nwavelength from 23 separate measurements includingseries, are shown in Figs. 5~a! and 5~b!, respectively. Thewave front error of 0.09060.008 wave (1.2160.11 nm) rmsand 0.53160.046 wave (7.1160.61 nm) peak-to-valley, isdominated by astigmatism. The aberrations appear to icate a zonal fabrication error described earlier. The relativsmall aberration magnitude indicates the nearly diffractilimited quality of the optic. The error bars of the Zernikpolynomial coefficients correspond to the rms variationeach term over the 23 measurements. The standard dtions of the 23 measured rms and peak-to-valley wave fraberrations indicate repeatability to within60.008 wave(60.11 nm) rms and60.046 wave (60.61 nm) peak-to-valley. The absolute accuracy of the measurements isrently under investigation.

IV. VERIFICATION WITH IMAGING EXPERIMENTS

Perhaps the most significant value of interferometric wafront measurements is their ability to predict the imagiperformance of optical systems. The interferometry p


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formed here on the 103 Schwarzschild optical system predicts good EUV imaging quality for the 0.07 NA subapeture. The image quality was verified with photoresexposure experiments performed on the 103I EUV imagingsystem7 at Sandia National Laboratories in Livermore, Cafornia. The Sandia exposure tool utilizes imaging optidentical in optical design and housing construction toSchwarzschild system described here. In this tool, EUV lifrom a laser-produced plasma is collected by an ellipsocondenser and directed via a 45° turning mirror onto aflective mask/object at near-normal incidence. The masklumination is of the Ko¨hler type, with a partial coherencfactor of approximately 0.5. The image of the mask patteproduced with the 103 Schwarzschild optic, is recorded onphotoresist-coated wafer.

Prior to the exposure experiments, the interferometry don the 0.07 NA subaperture were used to calculate thepected image intensities for several test patterns. The imawere calculated using the programSPLAT,15 which simulates

FIG. 5. ~a! Wave front aberrations and~b! the Zernike polynomial fit coef-ficients of the 0.07 NA subaperture of the Schwarzschild optic determifrom multiple measurements at 13.4 nm wavelength. The Zernike cocients correspond to polynomials scaled to have a peak magnitude of 1magnitudes of astigmatism~coefficients 4 and 5!, coma~6 and 7!, sphericalaberration~8!, and triangular astigmatism~9 and 10! are indicated. Thedominant aberration is astigmatism.


FIG. 6. ~a! Image intensities of a bright-field star resolution pattern calculated using the measured aberrations of the Schwarzschild optic.~b! SEM images ofthe same test pattern recorded experimentally in SAL 601 photoresist show excellent agreement with the calculated prediction.

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partially coherent image formation. The calculations wedone for a numerical aperture of 0.07, an exposure walength of 13.4 nm, an illumination partial coherence factor0.5, and the measured aberrations of the Schwarzschild oFigure 6~a! shows the calculated image intensity of a brigfield star resolution pattern through focus. The imagingquite good in all directions at best focus. As is expected ooptic having astigmatism as the dominant aberration, sowhat out of focus, the resolution of the optic improves in odirection but degrades in the orthogonal direction. Onother side of focus, the behavior is similar but with the twdirections reversed.

The presence of the aberrations measured inSchwarzschild system was subsequently verified with phresist exposure experiments. Negative-tone, chemicallyplified, 100-nm-thick, SAL 601 photoresist was usedrecord images produced by the optic. Figure 6~b! showsscanning electron microscopy~SEM! photographs of the developed photoresist features of the star resolution pattThe direction of the best resolution changes with defocusexcellent agreement with the predictions of astigmatic iming performance. Some of the differences apparent atcenter of the patterns are produced by variations in the ptoresist exposure dose from image to image.

In summary, the measured aberration magnitude of 0.wave rms for the 0.07 NA subaperture predicts nediffraction-limited optical performance of the Schwarzschoptic. This was observed with the printing of several t


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V. CONCLUSIONS AND FUTURE WORK

The aberrations of a 103 Schwarzschild optic, designefor EUV lithography experiments, were characterized withphase-shifting point diffraction interferometer at EUV wavlengths. Measurements of several subregions of the annaperture indicate the presence of zonal fabrication errThe 0.07 NA subaperture was found to have relatively smaberrations of 0.090 wave~1.21 nm! rms at 13.4 nm wave-length. The presence of phase aberrations due to multilacoatings was observed directly with wave front measuments at different wavelengths.

Measurements of the Schwarzschild optic have aserved to evaluate the performance of the PS/PDI designimplementation. The measurement repeatability is60.008wave (60.11 nm) rms. Although, determination of the abslute accuracy of the interferometer is still in progress, optiperformance predicted from interferometry, evaluated wimage calculations that include the effects of the aberratimeasured, is consistent with the image quality observedperimentally in the imaging experiments.

ACKNOWLEDGMENTS

The authors would like to thank Senajith B. Rekawa aCharles D. Kemp of the Center for X-Ray OpticsLawrence Berkeley National Laboratory for mechanical e

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gineering support, and Joshua Cantrell of the UniversityCalifornia at Berkeley for software support. The authors aacknowledge the staff members at the Advanced LiSource at Lawrence Berkeley Laboratory for providing aliable source of photons. The authors are grateful to AvijitRay-Chaudhuri and Kevin Krenz of Sandia National Labratories for the opportunity to perform imaging experimeon their exposure tool and for their help with the expements. The authors also wish to thank Richard H. Livengoof the Intel Corporation for providing access to the focusion beam tool for pinhole fabrication and Charles H. Fieof the University of California at Berkeley for his help witpinhole fabrication. This research was supported by the ICorporation, by the SRC under Contract No. 96-LC-460,the DARPA Defense Advanced Lithography Program, byDOE Office of Basic Energy Sciences, and by an Intel Fodation graduate fellowship.

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