performance optimization of si/gd extreme ultraviolet ...performance optimization of si/gd extreme...

Performance optimization of Si/Gdextreme ultraviolet multilayers

David L. Windt,1,* Jeffrey A. Bellotti,1 Benjawan Kjornrattanawanich,2

and John F. Seely3

1Reflective X-ray Optics LLC, 1361 Amsterdam Ave, Suite 3B, New York, New York 10027, USA2Universities Space Research Association, Brookhaven National Laboratory, Upton, New York 11973, USA

3Naval Research Laboratory, 4555 Overlook Avenue S.W., Washington, D.C. 20375, USA

*Corresponding author: [email protected]

Received 17 July 2009; revised 14 September 2009; accepted 15 September 2009;posted 15 September 2009 (Doc. ID 114408); published 1 October 2009

We compare the performance, stability and microstructure of Si/Gd multilayers containing thin barrierlayers of W, B4C, or SiNx, and determine that multilayers containing 0:6nm thick W barrier layers ateach interface provide the best compromise between high peak reflectance in the extreme ultraviolet nearλ ¼ 60nm and good stability upon heating. The Si/W/Gd films have sharper interfaces and also showvastly superior thermal stability relative to Si/Gd multilayers without barrier layers. We find that thesestructures have relatively small compressive film stresses, and show good temporal stability thus far. Wemeasured a peak reflectance of 29.7% at λ ¼ 62:5nm, and a spectral bandpass ofΔλ ¼ 9nm (FWHM), foran optimized Si/W/Gd multilayer having a period d ¼ 32:0nm. © 2009 Optical Society of America

OCIS codes: 230.4170, 310.1620, 310.6860, 340.0340, 350.1260.

1. Introduction

Reflective multilayer films having nanometer-scaleperiods are now widely used for a variety of scientificand industrial applications in the extreme ultra-violet (EUV) region of the electromagnetic spectrum.For solar physics applications in particular, multi-layer coatings operating near normal incidence inthe EUV have enabled the construction of a numberof spaceborne instruments over the past two decadesof research, for both monochromatic imaging usingtelescopes coated with narrowband multilayers (e.g.,[1]) and high-resolution spectroscopy using multi-layer-coated gratings (e.g., [2]). There are severalavailable multilayer material combinations, suchas Mo/Y, Si/Mo, SiC/Si, and Si/Sc, that provide goodoptical performance in the EUV, and that have suffi-ciently high stability and low stress for use in solarinstrumentation. But, while currently available mul-

tilayer coatings can be tuned to a number of differ-ent wavelengths in the approximate range 9 < λ <50nm, corresponding to emission from particularspectral lines (or line complexes) that originate fromionized atoms in the solar atmosphere, all of thebright, well-isolated emission lines in this wave-length band arise from plasma formed at either low(e.g., He II, λ ¼ 30:4nm, logT ¼ 4:7) or high tem-peratures (Fe VIII, λ ¼ 13:1nm, logT ¼ 5:9; Fe IX,λ ¼ 17:1nm, log ¼ 6:0; etc.) Thus, in practice, exist-ing multilayer coatings are ill suited for observingwell-isolated emission lines formed at intermediatetemperatures in the solar atmosphere (i.e., in the ap-proximate range of 4:7 < logT < 5:9), and so thereremains an observational “gap,” in that the solarplasma cannot yet be well sampled over a wide rangeof temperatures using currently available multi-layers.

To fill the need for an efficient narrowband multi-layer coating that can be used to observe an inter-mediate-temperature solar emission line, we haverecently developed the Si/Gd multilayer system [3]

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5502 APPLIED OPTICS / Vol. 48, No. 29 / 10 October 2009

for use near the O V line at λ ¼ 62:97nm, which isformed at logT ¼ 5:4 in the solar atmosphere. Fromdetailed measurements of the Gd optical constants inthe EUV, we discovered that this material has a nar-row transmission “window” in the vicinity of the O Vline, owing to unfilled 4f and 5d sublevels [4]. Whenused in combination with Si, the low absorption of Gdin the wavelength region near λ ¼ 63nm can beexploited to produce highly efficient, narrowbandSi/Gd multilayers.We have already reported on the initial devel-

opment of the Si/Gd multilayer system [2]; that ef-fort included a comparison of various materials (i.e.,tungsten, boron carbide, and silicon nitride) thatwere tested for use as diffusion barriers at the Si–Gd interfaces to improve performance and stability.In this paper we describe a subsequent effort to sys-tematically optimize the performance and stabilityof the Si/Gd system, using normal-incidence EUV re-flectometry, grazing-incidence x-ray reflectometry(XRR), thermal annealing, transmission electron mi-croscopy, and film stress measurements to guide thedesign of the thin barrier layers used at each inter-face so as to minimize diffusion. In the sections thatfollow, we describe our experimental techniques, pre-sent our results, and discuss their significance.

2. Experimental Procedures

The multilayer films discussed here were preparedby DC magnetron sputtering using a deposition sys-tem that has been described previously [5]. This sys-tem comprises three cathodes of two different typesthat each use solid, rectangular targets: the two lar-ger cathodes used targets of Si (99.999% purity) andGd (99.8% purity, excluding Ta), measuring 50 cm×9 cm × 0:6 cm, while the third, smaller cathode,needed only for samples containing W or B4C barrierlayers, used rectangular targets of either W (99.95%purity) or B4C (99.5% purity), measuring 43 cm×7:4 cm × 0:6 cm. The cathodes were all operated inregulated power mode, with 600W applied to theSi target, and 400Wapplied to the Gd, W, or B4C tar-gets. The vacuum chamber is cryopumped, and thebackground pressure in the chamber prior to deposi-tion was in the range of 1–3 × 10−7 Torr in all cases.The sputter gas was Ar (99.999% purity), and the gaspressure was held constant at 1:60� 0:01mTorr dur-ing deposition. For samples containing silicon nitridebarrier layers, reactive sputtering of Si with anAr=N2 gas mixture was used during barrier layer de-position. In that case, the sputter gas mixture wasmodulated during deposition, with pure Ar used forthe Si and Gd layers, and an Ar=N2 gas mixture forthe barrier layers. The Ar=N2 gas pressure wasmaintained at 1:6mTorr during barrier layer deposi-tion: the N2 flow rate was 25 sccm and the Ar flowrate was 250 sccm, thus, the flowing gas contained9.0% N2 by volume. As the exact compositional stoi-chiometry of these nitride layers is unknown, wehenceforth designate the reactively deposited mate-rial as SiNx.

Multilayer films were deposited onto 75mm dia-meter, prime-grade Si ð100Þ wafers. Individual layerthicknesses were adjusted by varying the computer-controlled rotation rate, and hence the exposuretime, of the substrate as it passes over each mag-netron cathode. The effective deposition rates, com-puted using layer thicknesses determined fromXRR measurements (described below) divided by theknown exposure times, were found to be ∼0:11nm=sfor Si, ∼0:27nm=s for Gd, ∼0:052nm=s for W,∼0:0017nm=s for B4C, and ∼0:010nm=s for SiNx.

XRRmeasurements were made in the θ–2θ geome-try using a four-circle x-ray diffractometer having asealed-tube Cu source and a Ge ð111Þ crystal mono-chromator tuned to the Cu K − α line (λ ¼ 0:154nm,E ¼ 8:04keV). The angular resolution of this systemis estimated to be δθ ∼ 0:015°. Fits to the XRR data(performed using IMD software [6]) were used to de-termine the film thickness of single-layer films usedfor deposition rate calibrations, and to determine theperiod of multilayer films, with an estimated preci-sion of δd∼�0:01nm.

Transmission electron microscopy of selected sam-ples was performed by Evans Analytical Group, Sun-nyvale, California. Cross-sectional samples wereprepared by the wedge polishing technique and thenion milled to electron transparency using 3kV Arþions and a Gatan Model 691 Precision Ion PolishingSystem. Measurements were made using a JEOLmodel 2010 microscope operating at 200keV.

Film stress was measured using a Toho Technolo-gies Flexus model 2320S wafer curvature system.The curvature of the Si wafer substrates (0:4mmnominal thickness) was measured along two orthog-onal directions before and after film deposition. Thetotal film thickness as determined by XRR wasused to compute film stress, following the standardformalism based on the Stoney equation [7]. Inaddition, stress-versus-temperature measurements,from room-temperature (25 °C) to 300 °C, weremade on selected multilayer samples using the sameinstrument.

The reflectance of selected samples was measuredusing synchrotron radiation at an incidence angle of5° from normal, using the Naval Research Labora-tory reflectometer on beamline X24C at the NationalSynchrotron Light Source (NSLS), BrookhavenNational Laboratory. A dual-element monochroma-tor utilizing a Au mirror and a 600 line=mm gratingwas used to generate a tunable, monochromatic pen-cil beam over the energy range E ¼ 11–465 eV. TheEUV reflectance was computed from the ratio of thereflected beam current to the incident beam cur-rent, both measured using the same Si photodiodeafter being normalized against the storage ringcurrent decay.

3. Results and Discussion

During our previous, initial investigation [2], wecompared the performance of Si/Gd multilayers withand without barrier layers of W, B4C, or SiNx, as a

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function of barrier layer thickness. (Identical barrierlayers were deposited at both the Si-on-Gd andthe Gd-on-Si interfaces.) Based on those results, abarrier layer thickness of 1nm was selected forthe thermal stability investigations that we discusspresently.

Shown in Fig. 1 are the measured reflectance-versus-wavelength curves for the four different (as-deposited) Si/Gd multilayers studied here, all havingnominal 10:49nm thick Si layers and 23:02nm thickGd layers, i.e., ΓSi ¼ dSi=ðdSi þ dGdÞ∼ 0:3; multi-layers with barrier layers of W, B4C, or SiNx areshown in Fig. 1 along with the Si/Gd multilayer con-taining no barrier layers. The addition of either W orSiNx barrier layers increases the peak reflectance by∼1%, while the film having B4C barrier layers has apeak reflectance that is ∼3% lower than Si/Gd. Allthree multilayers containing barrier layers have asomewhat larger spectral bandpass than Si/Gd mul-tilayers without barrier layers.

Shown in Fig. 2 are the measured reflectance datafor three samples of each of these same four filmdesigns after annealing, on a hot plate in air, for1h at 100 °C, 200 °C, and 300 °C, relative to theas-deposited films. Heating to 100 °C had no measur-able effect on EUV reflectance in all four cases. At200 °C, however, the Si/Gd, SiB4C=Gd, and Si=SiNx=Gd films all show shifts in peak wavelength; the Si/Gd and Si=B4C=Gd films show significant decreasesin peak reflectance, as well. On the other hand, theSi/W/Gd film is essentially unchanged after heating

Fig. 1. (Color online) Measured normal-incidence reflectance ofSi/Gd multilayers containing W, B4C and SiNx barrier layers asmarked.

Fig. 2. (Color online) Measured normal-incidence reflectance for (a) Si/Gd, (b) Si/W/Gd, (c) Si=B4C=Gd, and (d) Si=SiNx=Gd multilayersthat have been annealed for 1h in air at 100 °C, 200 °C, and 300 °C, as marked.


to 200 °C. Upon heating to 300 °C, the Si/Gd andSi=SiNx=Gd films show no Bragg peaks whatsoever,and the SiB4C=Gd film peak height has decreased bymore than a factor of 2. The Si/W/Gd film still showsrelatively high reflectance after 300 °C annealing,with only a slight (∼0:7nm) shift toward shorterwavelengths. The changes in peak height and multi-layer period apparent in the EUV data of Fig. 2closely follow the peak height and period changes ob-served from XRR measurements, which we do notpresent here.Based on the results of Fig. 2, we focused our at-

tention on Si/W/Gd multilayers for the subsequentstudies that are now described. Shown in Fig. 3are high-resolution transmission electron micro-scopy (HRTEM) images of the as-deposited Si/Gdand Si/W/Gd multilayers shown in Figs. 1 and 2.The Si layers in these films are amorphous, whilethe Gd layers are polycrystalline, with a strong crys-tallite orientation in the direction of growth, as evi-denced by the predominance of in-plane latticefringes. Amorphous interlayers are clearly visible atboth types of interfaces in the Si/Gd film; the appar-ent interlayer thickness is∼3:2nm, as indicated. Theapparent Si and Gd layer thicknesses in the Si/Gdfilm are ∼9:2 and ∼18:7nm, respectively, and aresubstantially smaller than the nominal values listedabove, presumably resulting, in part, from the forma-tion of the amorphous interlayers just mentioned.Comparable interlayers are not evident in the Si/W/Gd film. However, the W barrier layers in this filmhave an apparent thickness of 1:8nm, which is signif-icantly larger than the 1:0nm nominal value, sug-gesting that these layers may contain some Gd orSi in addition to W. In any case, the apparent Siand Gd layer thicknesses in the Si/W/Gd film are∼11:1 and ∼21:0nm, respectively, values muchcloser to nominal.

Shown in Fig. 4 are HRTEM images of these samefilms after 300 °C annealing. While the layer struc-ture is still evident after annealing in the Si/Gd film,the “pure” Si and Gd layers are much thinner, andthe contrast between the Si and Gd layers is greatlyreduced: the Si–Gd interlayers have apparentlygrown substantially, consuming most of the “pure”Si and Gd layers in the process. In contrast, the an-nealed Si/W/Gd film appears essentially unchanged,commensurate with the EUV results shown in Fig. 2and discussed above. The thin W layers in this struc-ture are evidently quite effective barriers against dif-fusion, at least up to 300 °C.

Shown in Fig. 5 are stress-versus-temperature re-sults for Si/Gd and Si/W/Gd multilayer films identi-cal to those just described in Figs. 3 and 4. For thisexperiment the samples were heated to 300 °C at arate of 5°=min, then held at 300 °C for a period of30 min, and finally allowed to cool to room tempera-ture. As can be seen from Fig. 5, the stress in bothfilms increases nearly linearly with temperature,up to about 90 °C, owing to the mismatch in thermalexpansion coefficients between the film and sub-strate. Above 90 °C the film begins to relax, a changein stress that has been observed in many other Si-basedmultilayer systems, such as Si/Mo [8], and thathas been attributed to viscous flow and possibly todensity changes, as well. In any case, the stress de-creases further in the Si/W/Gd film up to 300 °C, andthen, upon cooling, the stress decreases nearly line-arly, again due to the thermal expansion coefficient

Fig. 3. (Color online) HRTEM images of as-deposited Si/Gd (left)and Si/W/Gd (right) multilayers.

Fig. 4. (Color online) HRTEM images of Si/Gd (left) and Si/W/Gd(right) multilayers after annealing for 1h at 300 °C.

Fig. 5. (Color online) Film stress measured as a function of tem-perature for Si/Gd and Si/W/Gd multilayers.


mismatch. In contrast, the Si/Gd film without W bar-rier layers displays a more complicated behavior ofstress with temperature above ∼200 °C: unlike theSi/W/Gd film, above 200 °C the stress in the Si/Gdmultilayer remains approximately constant up to∼270 °C, and then the stress abruptly decreases by∼175MPa on further heating to 300 °C; upon coolingto room temperature, the stress again decreaseslinearly, as in the Si/W/Gd film. The large differencein stress–temperature behavior between the Si/Gdand Si/W/Gd films upon heating above 200 °C is pre-sumably associated with the growth of the Si–Gdinterlayers suggested by the HRTEM images ofFig. 4; the EUV reflectance data of annealed Si/Gdfilms shown in Fig. 2—which suffer large perfor-mance change above 200 °C—are also consistentwith the hypothesized interlayer growth at thesetemperatures.

We next attempted to further optimize the perfor-mance of Si/W/Gd multilayers by systematicallyvarying the W barrier layer thicknesses at each ofthe two types of interfaces independently. Specifi-cally, we fabricated an array of 25 different Si/W/Gd multilayers, all having nominal Si and Gd layersof 10.49 and 23:02nm thickness, respectively, inwhich the W layer thicknesses at the Gd-on-Si inter-face, denoted as W1, and at the Si-on-Gd interface,denoted as W2, were varied over the range of0:3–1:5nm, in increments of 0:3nm. We measuredboth the normal-incidence reflectance and the filmstress of each of the 25 resultant samples.

Shown in Figs. 6 and 7 are contour plots of peakreflectance and film stress, respectively, as a functionof the two W barrier layer thicknesses, W1 and W2.From Fig. 6 we see that the peak reflectance variesover the range of 16:5% < Rmax < 22:5% for thesesamples, with a maximum in reflectance for W1 ¼

Fig. 6. (Color online) Measured peak reflectance as a function ofthe W barrier layer thickness at the Gd-on-Si and the Si-on-Gdinterfaces, W1 and W2, respectively. The small filled rectangles in-dicate the points at which measurements were made, while thesmooth contours were computed using bilinear interpolation be-tween these points.

Fig. 7. (Color online) Measured film stress as a function of the Wbarrier layer thickness at the Gd-on-Si and the Si-on-Gd inter-faces, W1 andW2, respectively. The small filled rectangles indicatethe points at which measurements were made, while the smoothcontours were computed using bilinear interpolation betweenthese points.

Fig. 8. (Color online) Measured normal-incidence reflectance for Si/W/Gd multilayers that have been annealed for 1h in air at 100 °C,200 °C, and 300 °C, as marked. The films in (a) have barrier layer thickness W1 ¼ 0:6nm and W2 ¼ 0:3nm, while the films in (b) haveW1 ¼ W2 ¼ 0:6nm.


0:6nm andW2 ¼ 0:3nm. Figure 7 reveals only a slowvariation in film stress with W1 and W2, with rela-tively small compressive stresses (less than−600MPa) in all cases.Based on the results shown in Figs. 6 and 7, we

selected the two Si/W/Gd film designs having thehighest peak reflectance for further thermal anneal-ing studies: both films haveW1 ¼ 0:6nm, but one hasW2 ¼ 0:3nm and the other has W2 ¼ 0:6nm. As forthe films discussed above and shown in Fig. 2, forthese two Si/W/Gd film designs, we performed 1hthermal anneals on a hot plate in air, for three nom-inally identical samples of each of the two designs, attemperatures of 100 °C, 200 °C, and 300 °C, respec-tively. Each film was characterized after annealing(along with the as-deposited films) using XRR andnormal-incidence EUV reflectometry. As is evidentfrom the EUV results shown in Fig. 8, both filmsshow no Bragg peak after heating to 300 °C, unlikethe Si/W/Gd film with W1 ¼ W2 ¼ 1:0nm shown inFig. 2(b) that shows only a small change in reflec-tance after heating to 300 °C. However, while both

films shown in Fig. 8 still have high reflectance afterheating to 200 °C, the film having W2 ¼ 0:3nmshows a significant wavelength shift (∼0:8nm, to-ward shorter wavelengths), whereas the film havingW2 ¼ 0:6nm is unchanged after heating to 200 °C.(Once again, peak height and period changes ob-served from XRRmeasurements obtained from thesefilms are consistent with the EUV measurementsshown in Fig. 8, but are not presented here.) In lightof this last result, we have selected the design withW1 ¼ W2 ¼ 0:6nm as the best compromise betweenhigh peak reflectance (for as-deposited films) andgood thermal stability.

Having identified the optimal W barrier layerthicknesses as just described, we next fabricated aseries of seven Si/W/Gd films with periods in therange d ¼ 25:6–44:8nm. All films have W1 ¼W2 ¼ 0:6nm, and all have a constant Si to Gd layerthickness ratio, namely dSi=ðdSi þ dGdÞ ¼ 0:3. Shownin Fig. 9 are the normal-incidence reflectance dataobtained from these seven films. It is obvious fromFig. 9 that the peak reflectance and spectral band-pass vary strongly with multilayer period, owing tothe large variation in Gd optical constants over thisrange of wavelengths. The highest peak reflectance isfound for the film having d ¼ 32:0nm, which givesRmax ¼ 29:7% at λ ¼ 62:5nm, with a spectral band-pass of Δλ ¼ 9nm (FWHM). For larger or smallermultilayer periods, the peak reflectance is lower, andthe spectral bandpass higher. In any case, the filmhaving d ¼ 32:0nm is the most suitable of the seriesfor monochromatic imaging or high-resolution spec-troscopy of the O V line at λ ¼ 63nm near normalincidence.

Finally, to assess the temporal stability of Si/Gdmultilayers, we show in Fig. 10 a plot of the peak re-flectance as a function of time, measured periodicallyover a span of approximately 14 months, for the Si/Gd, Si/W/Gd, Si=B4C=Gd, and Si=SiNx=Gd multi-layers shown in Fig. 1. All four films, which werestored in air at room temperature, show similar be-havior: the peak reflectance steadily decreases by afew percent over the first ∼8 months, and thenremains essentially constant over time. No shift inpeak wavelength was found (within experimentaluncertainty). The initial reduction in reflectanceshown in Fig. 10 is likely due to the growth of a thina-SiO2 surface layer at the topmost Si layer in thesecoatings. In any case, while we will continue to moni-tor the performance of these coatings over the comingyears, our initial results as shown in Fig. 10 providelittle evidence that these coatings will suffer fromsignificant degradation over longer periods of time.

4. Summary and Conclusions

We have described our experimental efforts to char-acterize the performance, stability, and microstruc-ture of Si/Gd multilayers containing thin barrierlayers of W, B4C, or SiNx. We have determinedthat Si/W/Gd multilayers containing 0:6nm thickW barrier layers at each interface provide the best

Fig. 9. (Color online) Reflectance versus wavelength of Si/W/Gdmultilayers as a function of multilayer period d over the ranged ¼ 25:6–44:8nm, as indicated.

Fig. 10. (Color online) Peak reflectance measured periodically asa function of time for Si/Gd, Si/W/Gd, Si=B4C=Gd, and Si=SiNx=Gdmultilayers as indicated.


compromise between high peak reflectance in theEUV near λ ¼ 60nm and good stability upon heating.HRTEM indicates that these films contain amor-phous Si layers and polycrystalline, oriented Gdlayers. While Si/Gd multilayers without barrierlayers show relatively large Si–Gd interlayers, filmscontaining thin W barrier layers have significantlysharper interfaces and slightly higher peak reflec-tance. The Si/W/Gd films also show vastly superiorthermal stability relative to Si/Gd multilayers with-out barrier layers. We find that these structures haverelatively small compressive film stress (less than−600MPa), and show good temporal stability thusfar, with a drop in peak reflectance of a few percentoccurring within the first eight months of storage inair, presumably due to the growth of a-SiO2 at the topSi layer, and then no subsequent changes over time.We find an as-deposited peak reflectance of 29.7% atλ ¼ 62:5nm for an optimized Si/W/Gd multilayerhaving a period of d ¼ 32:0nm, and a spectral band-pass of Δλ ¼ 9nm (FWHM). Based on our investiga-tions, we propose that Si/W/Gd multilayers can beused for high-resolution solar imaging and spectro-scopy applications that target the O V line at λ ¼63nm (logT ¼ 5:4), which is presently the best can-didate emission line for probing the solar atmosphereat intermediate temperatures in the EUV.

This research was sponsored by NASA SmallBusiness Innovation Research contract numbersNNM07AA41C and NNM08AA24C.

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