EUV Mask Blank Fabrication & Metrology

Phil Seidel

International SEMATECH, Lithography Division, 2706 Montopolis Drive, Austin TX, 78741, USA

Abstract Extreme Ultraviolet (EUV) lithography is under development to succeed 157nm lithography for commercial 1Cmanufacturing for the 45nm technology node as defined by the ITRS 2001 Lithography Roadmap. EUV masks pose manymanufacturing and technical challenges to meet the future commercial needs. Although some EUV mask manufacturingprocesses can be extended from current optical masks, many new issues arise due to transitioning to all reflective multi-layermirror system with patterned features versus conventional optical masks. EUV lithography operation at 13.4 - 13.5nmwavelength requires optimal multi-layer performance including peak reflectance, wavelength matching to the optical system, andvery low defect levels. The Low Thermal Expansion Material (LTEM) that is used as a substrate for the multi-layer reflector alsorequires demanding performance levels including Coefficient of Thermal Expansion (CTE), flatness, and roughness to supportEUV mask needs. Performance improvements as large as several orders of magnitude are needed for some of these parameters.To aid these developments, specialized metrology tools are needed. These tools fall into two categories: Manufacturing processinspections tools include flatness interferometry, atomic force microscopy, EUV reflectometry, defect inspection, and others.Analytical tools include scanning electron microscopy, X-Ray Diffraction, ion beam milling, and others used for problemsolving. Both metrology types will play a major role in the successful development of EUV masks to meet the 45nm noderequirements. This paper will review many applicable metrology techniques in addition to those listed, describe the application toEUV mask blank development or manufacturing, show problem solving examples of the techniques, and highlight particularproblems or areas of need.

INTRODUCTION

Extreme ultraviolet lithography (EUVL) is aleading next generation lithography technology forintegrated circuit (1C) manufacturing for the 45nmtechnology node and below. EUV technology relies onreflective optics and masks that utilize multilayer (ML)structures. The EUV masks have patterned absorbersfeatures to create the 1C features during the exposureprocess. The ITRS Roadmap for EUV calls forsubstantial improvements in the quality of both themask substrates as well as the mask ML film stacks [1].

Two SEMI standard are now available thatdefine the production needs of EUV masks. Onestandard is for the substrate and another for the MLfilms. Commercial suppliers are developing polishingprocesses for Low Thermal Expansion Materials(LTEM) for the mask substrates.

Commercial EUV mask blank suppliers arealso developing ML deposition processes for thereflective ML and absorber films. There are severalperformance requirements that must be improved tomeet the production requirements. Most important isthe improvement in defects in the ML coating process

that is a main concern of mask blanks to attain zeroprintable defects.

The production requirements of EUV blanksand resulting masks need to support thermal loadings atthe mask of up to 6 watts for high system throughputsthat require LTEM use. The mask blanks ML reflectiveperformance must match that of the EUV exposure toolprojection optic to maintain exposure process windowsas well as overall system throughputs. Since EUVL anall reflective optical system the mask focal plane mustbe maintained in to stringent tolerances therefore maskblank and mask flatness must be on the order of 50nmPeak to Valley.

EUV MASK BLANK PHYSICS

EUV lithography wavelengths are highlyattenuating especially at the desired wavelengths of13.4nm to 13.5nm and only multilayer structures can beused to reflect the energy throughout the exposuresystem. The EUV mask also uses these ML reflectiveproperties. For reflective properties to work, EUVoptics and masks utilize quarter wavelength stack or"Bragg" ML reflection principals. The ML reflectorsconstruction uses alternating pairs of selected materials

CP683, Characterization and Metrology for VLSI Technology: 2003 International Conference,edited by D. G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, S. Zollner, R. P. Khosla, and E. M. Secula

© 2003 American Institute of Physics 0-7354-0152-7/03/$20.00371

to optimize the refractive index at the EUV wavelengthof choice. Alternating layers of Molybdenum (Mo) andSilicon (Si) with thickness of ~4.1nm and ~2.8nmrespectively are deposited to build up to 40 or morepairs each having thickness of 6.9nm for 13.40nm EUVwavelength operation. The individual layer thicknessand pair pitches can be optimized to center thewavelengths from about 12.0 nm up to 15.0 nm.

Various literatures describe in detail thephysics of such ML reflective films [2], [3]. Figure 1describes the ML stack and the reflective parameters ofinterest.

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FIGURE 1. Reflective ML stack parameters with incomingEUV radiation a0 angle of incidence (<|>), film thickness (d1?d2), ML pair "d spacing" (A), reflected energy bn, and layerrefractive index (n) with both reflective (8) and absorption (p)coefficients

Depending on the wavelength angle ofincidence compared to the critical angle by Equation (1)both reflection and transmission is encountered at eachMo and Si interface.

(1)

Braggs Law (2) relationship provides thewavelength and ML pair d spacing interaction.

(2)

The photon reflected amplitude from the firstML interface is described by equation (3).

And the transmitted photon amplitude energyto the next ML interface is described by equation (4).

(4)

The photon reflectance bn and transmittance anamplitudes of each successive ML interface isdescribed similarly to equations (3) and (4) until the last"nth" interface is reached. The corresponding reflectedor transmitted photon intensities are simply the squaresof equations (3) and (4). The reflected energy from eachinterface interferes constructively 180° in phase. Thesummation of all reflected photon energies addconstructively from the ML interfaces can reach up to>63%. Increasing the number of pairs, optimizing MLinterface properties, and tailoring other potential MLmaterials have shown EUV peak reflectivity >70%.

EUV MASK BLANK MANUFACTURING

There are many processing steps required tomanufacture high performance EUV mask blanks fromthe raw LTEM to the final coated and inspected blank.Figure 2 describes the general processing steps in sucha process. A particular blank supplier may usealternative process steps however most of these stepsare consistent from supplier to supplier. Within theprocess flows the first half of the processes supportgetting the LTEM into a format ready for multilayerdepositions. The definition of this unit is called the"mask substrate". There are many specifications thatthe substrate must meet and are defined in the SEMIP37-1101 Specification for Extreme UltravioletLithography Mask Substrates [4].

The second half of the process defines the finalmask blank with ML depositions, defect inspections,repair strategies, and other steps. There are manyspecifications that the ML films must meet and aredefined in the SEMI P38-1102 Specification forAbsorbing Film Stacks and Multilayers on ExtremeUltraviolet Lithography Mask Blanks [5].

The combination of P37-1101 and P38-1102standards together total over 30 separate specificationssome of which allow particular levels or "grades" ofperformance. The needed specification improvementsin many of these specifications will require thesuccessful use of capable process related metrology and

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analytical inspection technologies in the developmentstoward commercial performance needs. The processflow represented in Figure 2 contains many steps inwhich inspection or verification is needed after aspecific step. Additional analytical metrology toolswould be used in both initial process development aswell as troubleshooting problems during manufacturing.

Table 1 outlines most of the key inspectionmetrology technologies required for the measurementsneeded and whether it is targeted as a process monitoror an analytical tool to be used for problem solving.Many of the specific metrology tools in Table 1 will bedescribed in detail and its application.

LTEMFIGURE 2. Generic EUV mask blank process flow with substrate and blank process steps identified

TABLE 1. EUV Mask Blank Process FlowMetrology System

Dilatometer

Optical microscopeTensile testSEM with EDXFlatness InterferometerPhase Contrast Micro (PMM)Atomic Force Micro (AFM)Defect InspectionTrans. Elect. Micro. (TEM)EUV ReflectometerX-Ray diffractionEllipsometer

Performance / Need

LTEM CTE mean & uniformity

Gross defectsLTEM stiffnessDefect analysisSubstrate & blank flatness (LSF)Substrate mid spatial frequency (MSF)Substrate roughness & defect analysisDefect count, location, size, otherML film structure, defect analysisML film EUV reflectivity performanceML film structures, film thickness uniformityML film thickness

Process•

••

••••

•••

Analytical

•

••

•••

••

METROLOLGY TOOLS

The following section will highlight the majormetrology tools that are used to support the EUV maskblank manufacturing developments. Most, but not all ofthe entire specific tools listed in Table 1 will bementioned. A brief overview of the metrologyoperation, performance metrics and impacts to EUVLoperations, and examples where appropriate todemonstrate the tools effectiveness will be provided.

Dilatometer

Dilatometers are used to measure a materialsimpact to thermal loadings through changes in lengthdimensions. The rate at which materials expand orcontract under temperature changes is defined as it'sCoefficient of Thermal Expansion (CTE). EUV masksas well as projection optics require LTEM materialswith very low CTE. Dimensional length changes fromheat loadings (up to 6 Watts for masks) in productionEUVL systems will create image placements errors ordistortions on the printed wafer. The development ofvery low CTE materials to meet "Class A"requirements of + 5 ppb/°K along with a spatialuniformity of 6 ppb/°K requires dilatometer methodswith uncertainty of at least less than 5 ppb/°K andpreferably 1-2 ppb/°K.

Dilatometer Operation

Although dilatometers basic operation isreasonably straightforward the implementation can bedemanding. Many variations of dilatometers have beendiscussed in literature with potentials and limitations[6]. Dilatometers major components include use ofreference surfaces between the two ends of the materialunder test, a measurement system to record linearchanges, and the frame that connects the measurementsystem to the reference surfaces.

Laser Hght

Figure 3 shows one of the many dilatometerconcepts for potential EUV LTEM CTE measurementaccuracies to meet P37-1101 requirements. Using Fabry-Perot narrow band pass filter strategy laser lighttransmitted when the LTEM resonance matches that ofthe wavelength can be directly correlated to the LTEMchange in length. Very accurate temperature controlledfluid bath provides control of the LTEM test sampleand cavity.

Flatness InterferometerFlatness interferometers are used to measure

the substrate and mask blank flatness error. Flatness orfigure interferometers have been used in optical lensmanufacturing for many decades. Evolutionaryimprovements over time have allowed this technique tobe used today. Due to the tight tolerances of the EUVmask needing to be held within the focal plane in an all-reflective system, masks need to have flatness controlof 50nm (peak-to-valley) (P-V). Masks having flatnesscontrol exceeding this level will introduce imagedistortions and image placement errors on the printedwafer. The performance of flatness needs to bemeasured over a spatial frequency range of 100mmdown to 1mm.

Multiple types of interferometer systems havebeen designed to measure substrate flatness. Fizeau,Twyman-Green, Shack, Haidinger or otherinterferometer variants have been improved upon asdescribed by Malacara [7]. The general systemscomprise of a light source that is reflected off a testsunit whose wavefront is intentionally interfered againsta reference flat and additionally through a beam splitter.The resulting interference wavefront is a measure oftest surface flatness error and then is captured on acamera for analysis (e.g. CCD).

FIGURE 3: Dilatometer concept using Fabry-Perot

FIGURE 4: Interferometer diagram based on Fizeauarchitecture as described by Chiayu Ai [8]. Elements ofsystem are laser (10), spatial filter (16), collimator optics (18),reference flat optic (20), test piece (22), cubic beam splitter(24), CCD camera (26), and zoom lens (28).

Flatness interferometers are used in severalpoints within the EUV substrate and mask blank

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process as initial rough lapping steps, followed bydouble sided polishing, and any sub aperture finishingsteps. Incoming part flatness quality to a particularfinishing step is very important especially when localareas need to be identified for local figuring.

In order to achieve needed substrate flatnessperformance for EUV blanks of 50nm P-V or better theflatness interferometers require overall systemaccuracies of better than 20nm P-V including condenseroptics, fixturing, temperature and airflow controls.

FIGURE 5: Interferometer 3D surface profile of LTEMsubstrate with a) 660nm P-V fronstide surface variation afterinitial coarse figuring and b) 300nm P-V after final figure andpolishing.

Phase Contrast / Measuring Microscope

A Phase Measuring Microscope (PMM) isused to measure the substrates "Mid Spatial FrequencyRoughness" (MSFR). This attribute in EUVL is themain contributor to system flare or light scattering dueto rough surfaces. Excessive flare will degrade the pointspread function or aerial image of the printed feature onthe wafer. Figure 6 shows a generic description of aPMM that utilizes phase contrast from the test surfaceagainst a reference surface in which a diffractionpattern is captured by a camera. This figure is a conceptdescribed by Leslie Deck in U.S. patent #5,402,234.Additional system variations can take the form ofMichelson, Mirau, Linnik, and Fizeau based traditionalinterferometers described in detail by Malacara [10].

The performance of MSFR for EUV projectionoptics is less than 0.2nm rms and is to be held over aspatial frequency between 1mm and 1 um. For EUV

masks and blanks MSFR error is additionally importantas it contributes to surface local slope changes. Largelocal slope variations will cause localized imageplacements introduced by the reflective masks asdescribed by SEMI P37-1101 standards. RecentlyMSFR errors in the mask blank can contribute toprinted features "line edge roughness" (LER). Figure 7shows PMM surface maps of the same substratemeasured using three different aperture magnifications.

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FIGURE 6: Description of Phase Measuring Microscopeconcept outlined by L. Deck [9]

Atomic Force Microscopy (AFM)

AFMs are used to measure the substrates "High SpatialFrequency Roughness" (HSFR). AFM descriptions oroperations can be found in various literature. The HSFRattribute in EUVL is a main contributor to maintainingsystem reflectivity. As discussed previously the desireto have well defined ML boundaries for good contrastprovides high reflectivity. As the surface on whichML's are deposited become rougher this can impact theML as small surface perturbations can be replicatedthroughout the ML stack. The performance of HSFRfor EUV mask substrates calls for less than 0.15nm rmsover a spatial frequency region of 10 um down to about20nm.

AFM's can also show very high definitionsurface quality of LTEM such as processing artifacts(e.g. sleeks) or material inhomogeneity such asinclusions that have penetrated the surface.Improvements to LTEM substrate processing usingAFM feedback can dramatically improve surfacequality that may not show up in HSFR rms valuesalone. The following Figure 8 shows AFM scans withsuch results.

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This relationship further supports that throughconventional ML deposition technologies such asconventional Ion Beam Deposition (IBD) or magnetronsputtering that HSFR for masks need to be held to lessthan O.lSnmrms.

FIGURE 7: Three different PMM 2D surface maps ofsame LTEM substrate: a) 2D surface map using a 2.5xobjective measuring a 0.50nm rms error, b) 2D surfacemap using a lOx objective measuring 0.26 nm rmserror, c) 2D surface map using 50x objective measuring0.1 2nm rms error (apparent sleeks caused by polishingprocess).

The amount of HSFR has been correlated tooverall EUV reflectivity loss in ML through the"Debye- Waller" factor.

(5)

FIGURE 8: Two separate AFM scans on two differentLTEM that both have HSFR of 0.15nm rms howeverimproved polishing processes have eliminated sleeks.

Defect Inspection

Particle defects and defect detection on theLTEM substrate surface or on the ML blank are themost critical to EUV performance. Defects that are on

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or near the surface of the ML blank usually will absorbthe EUV wavelengths and cause a printed defect on thewafer. This type of defect is termed an "amplitude"defect. The other type of defects is ones that are eitheron the LTEM substrate surface or near the bottom ofthe ML film stack. These defects disrupt the uniformML thickness locally and can cause unwanted surfacepropagations. Since the local ML films in the vicinity ofsuch a defect are either affected in thickness, d spacing,or A/4 periods, EUV reflected light could be driven outof an 180° in phase condition, cause destructiveinterference, and produce a "phase" defect. Defectprintability simulations have shown that sizes down toabout 25nm in diameter have high probability to printby E. M. Gullikson et al [11] (see Figure 9).

50 100

Gaussian FWHM (nm)150

FIGURE 9: Comparison between initial starting defect sizeand resulting surface ML defect height with the defectprintability threshold [11].

There are two types of defect inspectiontechnologies for EUV blanks one using opticalwavelength inspection and the other is "actinic" or atwavelength EUV inspection. Both approaches havetheir merits and limitations and trade-offs betweendefect size detection sensitivity and throughput. Opticalinspection techniques generally have higher throughputat the expense of defect size sensitivity compared toactinic techniques.

Optical Defect Inspection

Optical defect inspection techniques varywidely however two such techniques will be discussedhere. One is based on light scattering principals asdiscussed by Seong-Ho Yoo et al [12]. The othertechnique uses confocal microscopy. Figure 10 showsthe comparisons of the two approaches. Paramount ineither inspection technology are the algorithms used

once either scattering or imaging information iscaptured by the cameras or other sensors (ChargedCoupled Devise -CCD). Current inspection systems canoutput information in discrete defect coordinatelocations, defect maps, and size histograms as seen inFigure 11.

FIGURE 10: Two types of optical defect inspectiontechnologies used for EUV substrate and ML blankinspection, a) scattering technology Yoo et al [12]. b)confocal microscopes.

Actinic Defect Inspections

Actinic defect inspection may be required ifthere is a higher occurrence of printed defects on thewafer that is not detected through optical techniques.There continues to be debate whether actinic inspectionis required in EUV blank manufacturing. Currentlyactinic defect inspection studies have been performedusing modified optical systems on research synchrotronbeamlines (e.g. Lawrence Berkeley National Labs).

Using synchrotron radiation by modifying it to13.4nm wavelengths is accomplished throughundulators. EUV wavelength are scanned at nearnormal angles on the mask blanks and resulting scatteris recorded both in "dark field" and bright field" modesimultaneously. Such techniques can be seen by Park etal[13]andYietal[14].

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I fieri*; *FIGURE 11: Figure showing an operator interface output fileof detected defects on an inspected EUV mask blanks at adefect sensitivity of 150nm PSL and higher with; a) defectmap and b) defect size histogram.

it m m m mFIGURE 12: Concept of dark field and bright field actinicinspection from Park et al [13] with one representative defectfound in dark field mode providing a 18.7% signal.

X-Ray Diffraction

Shallow hard X-ray reflectivity (0.154nm)analysis or diffraction can be used to obtain to valuableinformation about ML interfaces, individual filmthickness, and d-spacing periods. X-ray diffractionanalysis uses the property of crystal lattices to diffractmonochromatic X-ray light. This involves theoccurrence of interferences of the waves scattered at thesuccessive planes within the ML. With the use ofBragg's Law equation (2) and understanding the MLmaterial property d spacing, individual film thickness,and calculated EUV reflectivity can be generated.

It is important to note that although XRD is avery useful tool the reflectivity performance at 0.154nmwavelength as seen in Figure 13 does not assure EUVwavelength performances at 13.4nm. Filmcontamination or other effects would not influence the0.154nm radiation. XRD reflectivity curves such asFigure 15 can provide useful information as the "BraggPeaks" occurring at the correct angle of incidence canbe well correlated to the Ion Beam Sputter (IBS)deposition process.

d

10

FIGURE 13: An example of XRD data obtained from aMo/Si multilayer sample. The layer thicknesses are: Tsi=3.209 nm, TMo/Si= 1.001 nm, TMo=2.294 nm, and Tsi/Mo=0-373 nm. Bragg Peaks M=l,3,5, etc provideinformation on Mo/Si interface performance while M=2,4,6,etc. provides ML thickness asymmetry information [15].

Transmission Electron Microscope (TEM)

High resolution TEM analysis of the MLstructures can be very informative as an analytical

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inspection technique to understand issues within ML.The first area is the analysis of the interface betweenMolybdenum and Silicon layers within the ML stack.As mentioned previously keeping the ML interfacesuniform with sharp contrast supports high EUVreflectivity. Effect of crystalline or silicide diffusion ofSiO2 between the Silicon and Moly can be readily seenin TEM's. Figure 14 shows when Mo/Si ML are heatedabove 200°C there is intermixing and the Mo thicknessappears to increase causing a change in Mo / Si "d-spacing.

LTEM substrate can affect the Mo/Si ML very sharply.Depending on the IBSD process smoothing or lack of itsuch a defect would become a printable "phase" defect.

FIGURE 14: Cross-sectional HRTEM images of Mo/Simultilayer samples, (c) IBS sputtered Mo/Si below thetransition at rMo = 1.89±0.10 nm. (d) IBS sputtered Mo/Siabove the transition at TMo=2.14±0.10 nm. The thicker Mo inbottom picture effects overall "d" spacing and EUVreflectivity performance [15]

The second important information that TEM'sprovide is the effects of buried defect within the MLstack and how conformal or smoothing the ML is oversuch defects. Such IBS process improvementexperiments can greatly aid to understanding sucheffect. Figure 15 shows how a buried defect on the

FIGURE 15: Cross-sectional transmission electronmicroscopy image of a Mo/Si multilayer coating deposited byion beam sputtering at off-normal incidence on a ~60 nmdiameter gold spheres. [ 16]

EUV Reflectometer

A EUV reflectometer measures the absolutepeak reflectivity and spectral output performances ofthe Mo/Si ML stacks. Reflectometers have beendeveloped using two approaches one utilizingsynchrotron electron storage rings with filters andundulators to modify the hard X-ray wavelengths intoEUV wavelengths (e.g. 13.4nm) and the other using aplasma generated source. For dedicated use and highutilization in a blank manufacturing line it is anticipatedthat the smaller dedicated plasma or discharge sourceEUV reflectometers will be used. There are somecommercial suppliers developing such dedicatedsystems.

Figure 16 is a diagram showing a genericreflectometer for dedicated high utilization. The majorsubcomponents of such systems use a laser that strikesa solid metal target either of Sn, Au, or other element tocreate plasma in which EUV radiation is generated.This energy is collected and filtered through use ofcollimator and spherical grating elements. The energythen is measured once before it is reflected off of thetest piece and then after. Due to the energy variation ofeach plasma discharge on the solid target the reflectedenergy measurement accuracy requires both the

379

incoming energy "pre" and the reflected energy "post"intensities. Description details of EUV reflectometeroperation are found in Gullikson et al [17].

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The measured output of EUV mask blank asrecorded by reflectometers provides the full EUVspectra, peak reflectivity, median, centroid, and fullwidth half maximum (FWHM) performance. Themeasurement spot size is on the order of 1 - 3 mm andreflectivity uniformity of the test piece is accomplishedthough multiple measurements across the plate. Figure17 shows a typical EUV reflected spectral measurementfrom an EUV reflectometer.

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FIGURE 17: Typical spectral output of reflected EUV asmeasured by an EUV reflectometer.

SUMMARY

Metrology systems will continue to play amajor role in the further development of EUV maskblanks to meet production specifications described bySEMI P37 and P38. Most important are the systems thatdetect defects and provide information on the location,composition, and impacts to the ML performance andpotential printability in the exposure systems.

REFERENCES

[I] International Technology Roadmap for Semiconductors,2001 Edition, Lithography Chapter.[2] E. Spiller, "Soft X-Ray Optics", SPIE Optical EngineeringPress, Bellingham WA, pp. 39-58, 101-138.[3] D. Attwood, "Soft X-Rays and Extreme UltravioletRadiation", Cambridge University Press 1999, pp. 98-122.[4] Semiconductor Equipment and materials International(SEMI); Microlithography Volume, P37-1101 "Specificationfor Extreme Ultraviolet Lithography Mask Substrates ";[5] Semiconductor Equipment and Materials International(SEMI); Microlithography Volume, P38-1102 "Specificationfor Absorbing Film Stacks and Multilayers on ExtremeUltraviolet Lithography Mask Blanks "; www.semi.org[6] V. Badami, M. Linder, "A hot topic for small parts;Future applications demand precise measurement of thermalexpansion; Optical Engineering Magazine, SPIE Dec 2002.[7] D. Malacara, "OpticalShop Testing", 2nd Ed., John Wiley& Sons Inc. '92pp. 1-77.[8]Chiayu Ai, US Patent #5,452,088, Sept 19 1995, prior art.[9] L. Deck, U.S. Patent #5,402,234, "Method and apparatus

for the rapid acquisition of data in coherence scanninginterferometry", March 28th, 1995.[10] Daniel Malacara, "Optical Shop Testing ",2nd edition,John Wiley & Sons, '92, pp 697-703.[II]E. M. Gullikson et al; "Practical approach for modelingextreme ultraviolet lithography mask defects ", Journal ofVacuum Sci. & Tech. B:, Vol. 20, No. 1, pp. 81-86, Jan '02[12] Seong-Ho Yoo etal\ "Identification and sizing ofparticle defects in semiconductor-wafer processing", Journalof Vacuum Sci. & Tech. B:, V 19, No. 2, March '01[13] Park etal; "Characterization of extreme ultravioletlithography mask defects by actinic inspection withbroadband extreme ultraviolet illumination ", J. Vac. Sci.Technol. B 20 (6), Nov / Dec 2002[14] Yi et al, "High sensitivity actinic detection of nativedefects on extreme ultraviolet lithography mask blanks ", J.Vac. Sci. Technol. B 19 (6), Nov / Dec 2001[15] S. Bajt et al; "Investigation of the amorphous-to-crystalline transition in Mo/Si multilayers ", Journal ofApplied Physics, Vol. 90, No. 2, pp. 1017-1025, 15 July 2001[16] P. B. Mirkarimi et al; "Technique employing goldnanospheres to study defect evolution in thin films "; J. Vac.Sci. Technol. B 19 (3), May / Jun 2001.[17] Gullikson et al; "A softx-Ray /'EUV'reflectometer basedon laser produced plasma"', J. X-Ray Sci. Tech. V 3 '92

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