euv lithography—the successor to optical...

EUV Lithography—The Successor to Optical Lithography? 1

EUV Lithography—The Successor to Optical Lithography?

John E. Bjorkholm Advanced Lithography Department, Technology and Manufacturing Group, Santa Clara, CA.

Intel Corporation

Index words: EUV lithography, lithography, microlithography

AbstractThis paper discusses the basic concepts and current stateof development of EUV lithography (EUVL), a relativelynew form of lithography that uses extreme ultraviolet(EUV) radiation with a wavelength in the range of 10 to14 nanometer (nm) to carry out projection imaging.Currently, and for the last several decades, opticalprojection lithography has been the lithographictechnique used in the high-volume manufacture ofintegrated circuits. It is widely anticipated thatimprovements in this technology will allow it to remainthe semiconductor industry’s workhorse through the 100nm generation of devices. However, some time aroundthe year 2005, so-called Next-Generation Lithographieswill be required. EUVL is one such technology vying tobecome the successor to optical lithography. This paperprovides an overview of the capabilities of EUVL, andexplains how EUVL might be implemented. Thechallenges that must be overcome in order for EUVL toqualify for high-volume manufacture are also discussed.

IntroductionOptical projection lithography is the technology used toprint the intricate patterns that define integrated circuitsonto semiconductor wafers. Typically, a pattern on amask is imaged, with a reduction of 4:1, by a highlyaccurate camera onto a silicon wafer coated withphotoresist. Continued improvements in opticalprojection lithography have enabled the printing of everfiner features, the smallest feature size decreasing byabout 30% every two years. This, in turn, has allowedthe integrated circuit industry to produce ever morepowerful and cost-effective semiconductor devices. Onaverage, the number of transistors in a state-of-the-artintegrated circuit has doubled every 18 months.

Currently, the most advanced lithographic tools used inhigh-volume manufacture employ deep-ultraviolet (DUV)radiation with a wavelength of 248 nm to print featuresthat have line widths as small as 200 nm. It is believed

that new DUV tools, presently in advanced development,that employ radiation that has a wavelength of 193 nm,will enable optical lithography to print features as smallas 100 nm, but only with very great difficulty for high-volume manufacture. Over the next several years it willbe necessary for the semiconductor industry to identify anew lithographic technology that will carry it into thefuture, eventually enabling the printing of lines as smallas 30 nm. Potential successors to optical projectionlithography are being aggressively developed. These areknown as “Next-Generation Lithographies” (NGL’s).EUV lithography (EUVL) is one of the leading NGLtechnologies; others include X-Ray lithography, ion-beam projection lithography, and electron-beamprojection lithography. [1]

In many respects, EUVL may be viewed as a naturalextension of optical projection lithography since it usesshort wavelength radiation (light) to carry out projectionimaging. In spite of this similarity, there are majordifferences between the two technologies. Most of thesedifferences occur because the properties of materials inthe EUV portion of the electromagnetic spectrum arevery different from those in the visible and UVwavelength ranges. The purpose of this paper is toexplain what EUVL is and why it is of interest, todescribe the current status of its development, and toprovide the reader with an understanding of thechallenges that must be overcome if EUVL is to fulfill itspromise in high-volume manufacture.

Why EUVL?In order to keep pace with the demand for the printing ofever smaller features, lithography tool manufacturershave found it necessary to gradually reduce thewavelength of the light used for imaging and to designimaging systems with ever larger numerical apertures.The reasons for these changes can be understood fromthe following equations that describe two of the mostfundamental characteristics of an imaging system: its

Intel Technology Journal Q3’98


resolution (RES) and depth of focus (DOF). Theseequations are usually expressed as

RES = k1 λ / NA (1a)

and

DOF = k2 λ / (NA)2, (1b)

where λ is the wavelength of the radiation used to carryout the imaging, and NA is the numerical aperture of theimaging system (or camera). These equations show thatbetter resolution can be achieved by reducing λ andincreasing NA. The penalty for doing this, however, isthat the DOF is decreased. Until recently, the DOF usedin manufacturing exceeded 0.5 µm, which provided forsufficient process control.

The case k1 = k2 = ½ corresponds to the usual definitionof diffraction-limited imaging. In practice, however, theacceptable values for k1 and k2 are determinedexperimentally and are those values which yield thedesired control of critical dimensions (CD’s) within atolerable process window. Camera performance has amajor impact on determining these values; other factorsthat have nothing to do with the camera also play a role.Such factors include the contrast of the resist being usedand the characteristics of any etching processes used.Historically, values for k1 and k2 greater than 0.6 havebeen used comfortably in high-volume manufacture.Recently, however, it has been necessary to extendimaging technologies to ever better resolution by usingsmaller values for k1 and k2 and by accepting the need fortighter process control. This scenario is schematicallydiagrammed in Figure 1, where the values for k1 andDOF associated with lithography using light at 248 nmand 193 nm to print past, present, and future CD’sranging from 350 nm to 100 nm are shown. The“Comfort Zone for Manufacture” corresponds to theregion for which k1 > 0.6 and DOF > 0.5 µm. Alsoshown are the k1 and DOF values currently associatedwith the EUVL printing of 100 nm features, which willbe explained later. As shown in the figure, in the verynear future it will be necessary to utilize k1 values thatare considerably less than 0.5. Problems associated withsmall k1 values include a large iso/dense bias (differentconditions needed for the proper printing of isolated anddense features), poor CD control, nonlinear printing(different conditions needed for the proper printing oflarge and small features), and magnification of mask CDerrors. Figure 1 also shows that the DOF valuesassociated with future lithography will be uncomfortablysmall. Of course, resolution enhancement techniquessuch as phase-shift masks, modified illuminationschemes, and optical proximity correction can be used toenhance resolution while increasing the effective DOF.

However, these techniques are not generally applicable toall feature geometries and are difficult to implement inmanufacturing. The degree to which these techniquescan be employed in manufacturing will determine howfar optical lithography can be extended before an NGL isneeded.

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DUV248nm DUV193nm EUVL13.4nm

k1

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Figure 1: The k1 and DOF values associated with 248nm and 193 nm lithographies for the printing of CD

values ranging from 350 nm down to 100nm assumingthat k2 = k1 and NA = 0.6

EUVL alleviates the foregoing problems by drasticallydecreasing the wavelength used to carry out imaging.Consider Figure 2. The dashed black line shows thelocus of points corresponding to a resolution of 100 nm;the region to the right of the line corresponds to evenbetter resolution.

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Figure 2: The region between the lines shows thewavelength and numerical aperture of cameras

simultaneously having a resolution of 100 nm or betterand a DOF of 0.5 µm or better



The solid red line shows the locus of points for which theDOF is 0.5 µm; in the region to the left of that line theDOF values are larger. Points in the region between thetwo lines correspond to situations in which the resolutionis 100 nm or better, and the DOF is 0.5 µm or longer. Asshown, to be in this favorable region, the wavelength ofthe light used for imaging must be less than 40 nm, andthe NA of the imaging system must be less than 0.2. Thesolid circle shows the parameters used in current imagingexperiments. Light having wavelengths in the spectralregion from 40 nm to 1 nm is variously referred to asextreme uv, vacuum uv, or soft x-ray radiation.Projection lithography carried out with light in thisregion has come to be known as EUV lithography(EUVL). Early in the development of EUVL, thetechnology was called soft x-ray projection lithography(SXPL), but that name was dropped in order to avoidconfusion with x-ray lithography, which is a 1:1, near-contact printing technology.

As explained above, EUVL is capable of printing featuresof 100 nm and smaller while achieving a DOF of 0.5 µmand larger. Currently, most EUVL work is carried out ina wavelength region around 13 nm using cameras thathave an NA of about 0.1, which places the technologywell within the “Comfort Zone for Manufacture” asshown in Figure 1 by the data point farthest to the right.

EUVL TechnologyIn many respects, EUVL retains the look and feel ofoptical lithography as practiced today. For example, thebasic optical design tools that are used for EUV imagingsystem design and for EUV image simulations are alsoused today for optical projection lithography.Nonetheless, in other respects EUVL technology is verydifferent from what the industry is familiar with. Most ofthese differences arise because the properties of materialsin the EUV are very different from their properties in thevisible and UV ranges.

Foremost among those differences is the fact that EUVradiation is strongly absorbed in virtually all materials,even gases. EUV imaging must be carried out in a nearvacuum. Absorption also rules out the use of refractiveoptical elements, such as lenses and transmission masks.Thus EUVL imaging systems are entirely reflective.Ironically, the EUV reflectivity of individual materials atnear-normal incidence is very low. In order to achievereasonable reflectivities near normal incidence, surfacesmust be coated with multilayer, thin-film coatings knownas distributed Bragg reflectors. The best of these functionin the region between 11 and 14 nm. EUV absorption instandard optical photoresists is very high, and new resist

and processing techniques will be required forapplication in EUVL.

Because EUVL utilizes short wavelength radiation forimaging, the mirrors that comprise the camera will berequired to exhibit an unprecedented degree of perfectionin surface figure and surface finish in order to achievediffraction-limited imaging. Fabrication of mirrorsexhibiting such perfection will require new and moreaccurate polishing and metrology techniques.

Clearly, then, there are a number of new technologyproblems that arise specifically because of the use ofEUV radiation. Intel has formed a consortium called theEUV, LLC (the LLC), which currently also includesAMD and Motorola, to support development of theseEUV-specific technologies. The bulk of this developmentwork is carried out by three national laboratoriesfunctioning as a single entity called the Virtual NationalLaboratory (VNL). Participants in the VNL areLawrence Livermore National Laboratory, SandiaNational Laboratories, and Lawrence Berkeley NationalLaboratory. Development work is also carried out byLLC members, primarily on mask fabrication andphotoresist development. Recently, additional supportfor some of this work has come from Sematech. Thework described in the following sections was carried outwithin this program, primarily by workers within theVNL.

Multilayer ReflectorsIn order to achieve reasonable reflectivities, the reflectingsurfaces in EUVL imaging systems are coated withmultilayer thin films (ML’s). These coatings consist of alarge number of alternating layers of materials havingdissimilar EUV optical constants, and they provide aresonant reflectivity when the period of the layers isapproximately λ/2. Without such reflectors, EUVLwould not be possible. On the other hand, the resonantbehavior of ML’s complicates the design, analysis, andfabrication of EUV cameras. The most developed andbest understood EUV multilayers are made of alternatinglayers of Mo and Si, and they function best forwavelengths of about 13 nm. Figure 3 shows thereflectivity and phase change upon reflection for anMo:Si ML that has been optimized for peak reflectivity at13.4 nm at normal incidence; similar resonance behavioris seen as a function of angle of incidence for a fixedwavelength. While the curve shown is theoretical, peakreflectivites of 68% can now be routinely attained forMo:Si ML’s deposited by magnetron sputtering.



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Figure 3: Curve showing the normal incidencereflectivity and phase upon reflection of an Mo:Si ML as

a function of wavelength; the coating was designed tohave peak reflectivity at 13.4 nm

This resonance behavior has important implications forEUVL. A typical EUVL camera is composed of at leastfour mirrors, and light falls onto the various mirrors overdifferent angular ranges. As a consequence, the periodsof the ML’s applied to the various mirrors must bedifferent so that all the mirrors are tuned to reflect thesame wavelength. Proper matching of the peakwavelengths is crucial for achieving high radiationthroughput and good imaging performance. The range ofangles of incidence over a single mirror surface must alsobe considered. For some optical designs, the angularranges are small enough that ML’s with a uniform periodover the surface can be used. In other designs, theangular ranges are so large that the ML period must beaccurately varied over the surface in order to achieveuniform reflectivity. There are optical designs in whichthe angular ranges are so large that ML reflectors can notbe utilized.

The effects on imaging performance due to the variationsof ML reflectivity and phase with wavelength and anglehave been extensively modeled. The effects have beenshown to be minimal for cameras of interest to us. Theprimary perturbations of the wavefront transmitted by thecamera are described as a simple tilt and defocus.

In our work we are fabricating two types of EUVcameras. The first is a small field, microstepper-likedesign that utilizes two mirrors and that images with areduction factor of 10. We call it the “10X camera.” Thiscamera has been used extensively in our earlyinvestigations of EUV imaging. One of the mirrors inthis camera requires a strongly graded ML coating.Three of these cameras have been fabricated and havebeen shown to perform well. (Examples of the imaging

performance of these cameras are shown later in thispaper.) The second camera, currently being fabricated, isa prototype lithography camera with a ring field of 26mm X 1.5 mm. This camera was designed so that it willperform well with uniform ML coatings. The VNL hasdemonstrated the ability to achieve the ML matching,uniformity, and grading requirements of EUVL camerascurrently of interest.

EUV Cameras Designing an all-reflective camera that achieveslithographic-quality imaging is more difficult thandesigning a refractive imaging system because mirrorshave fewer degrees of freedom to vary than do lenses. Asa result, most of the mirrors in an EUVL camera willhave aspheric surfaces. The detailed reasoning that leadsto this conclusion was first discussed in 1990. [2]

A schematic of a four-mirror camera that the VNL is inthe process of fabricating is shown in Figure 4. Themirror segments shown in blue are the pieces actuallybeing fabricated, while the full, on-axis “parent” mirrorsare shown in red. This camera will become part of an

“engineering test stand,” so it is called the ETS camera.

Figure 4: Schematic diagram of the 4-mirror ETScamera

It has an NA = 0.1 and is designed to be used with Mo:SiML’s at a wavelength of 13.4 nm. Mirror 3 is spherical,and the other three mirrors are aspheres. Some of themost important features of this camera are as follows:

• Its resolution is better than 100 nm over a 26 mm x1.5 mm, ring-shaped field.

• It images with a reduction factor of 4.

• The departures of the aspheres from a best-fit sphereare less than 10 µm.

The camera is intended for use in a step-and-scanlithography system. In actual operation, the mask andwafer are simultaneously scanned in opposite directions,with the mask moving four times faster than the wafer, as

Wafer

Mask

M1

M2M3

M4 M1, M2 and M4are aspheric



done in current DUV step-and-scan systems. The designof this camera has been optimized so that the effectivedistortion when scanning (about 1 nm) is considerablyless than the distortion obtained for static printing (15nm).

Because short wavelength radiation is used to carry outthe imaging, the surfaces of the mirrors are required toexhibit unprecedented perfection. In order to achievediffraction-limited imaging at 13.4 nm, the root-mean-square (rms) wavefront error of the camera must be lessthan 1 nm. Assuming that the surface errors on themirrors are randomly distributed, this means that thesurface figure (basic shape) of each mirror must beaccurate to 0.25 nm (2.5 angstroms!) rms, or better.Until recently, achieving this kind of surface figureaccuracy was out of the question, even for spheres.Furthermore, aspheres are much more difficult tofabricate than are spheres. We have been workingclosely with optics fabricators to address this issue, anddramatic progress has been made over the last 18months.

The figure of a surface refers to its basic shape. Stringentrequirements must also be placed on the roughness of thesurfaces. For our purposes, we define surface figureerrors as those errors that have a spatial wavelength scaleof 1 mm or longer; such errors are typically measureddeterministically using instruments such asinterferometers. We define surface roughness as surfaceerrors with a spatial wavelength scale shorter than 1 mm.Typically such surface errors are described and measuredstatistically. We define roughness with wavelengths inthe range of 1 mm through 1 µm as mid-spatialfrequency roughness (MSFR). Roughness in thisfrequency range causes small-angle scattering of light offthe mirror surfaces. This scattering causes a reduction inthe contrast of images because it scatters light frombright regions of the image plane onto regions intendedto be dark. This scattering is often called flare. Becausethe effects of scatter scale as 1/λ2, the deleterious effectsof flare are becoming more evident as the wavelengthsused for lithography continue to be reduced. For a givensurface roughness, the amount of scattering at 13.4 nm isapproximately 340 times larger than that at 248 nm. Inorder to keep flare to manageable levels in EUVL, theMSFR must be 0.2 nm rms, or less. Until recently, eventhe best surfaces exhibited MSFR of 0.7 nm rms.Roughness with spatial wavelengths less than 1 µm iscalled high-spatial-frequency roughness (HSFR), and itcauses large angle scattering off the mirrors. Lightscattered at such angles is typically scattered out of theimage field and represents a loss mechanism for light.We require HSFR to be less than 0.1 nm rms. Optical

fabricators have for some time been able to use “super-polishing” techniques to produce surfaces with HSFReven better than this. A well-polished silicon wafer alsoexhibits such HSFR.

The challenge for a fabricator of optics for EUVL is toachieve the desired levels of figure accuracy and surfaceroughness simultaneously. The manufacturer we havebeen working with has made exceptional progress in thisregard. As a measure of the progress that has been made,the first copy of Mirror 3 has been completed, and itssurface has been measured and found to have thefollowing characteristics:

• Surface figure: 0.44 nm rms

• MSFR: 0.31 nm rms

• HSFR: 0.14 nm rms

This result demonstrates excellent progress towards thesurface specifications that we need to achieve.

MetrologyThe progress made in optics fabrication described abovecould not have been achieved without access toappropriate metrology tools. Some of the required toolswere recently developed by workers within the VNL.

Two very significant advances have been made in themeasurement of figure. Previous to these advances, notools existed that could measure figure to the accuracy werequire. The first of these innovations is theSommargren interferometer, which uses visible light toachieve unprecedented accuracy. [3] In this version of a“point-diffraction interferometer,” the wavefront to bemeasured is compared with a highly accurate sphericalwave generated by an optical fiber or by an accurate,small pinhole. Interferogram stitching algorithms havebeen developed that allow aspheric surfaces to bemeasured without the need for null optics, which aretypically the weak link in such measurements. Anaccuracy of 0.25 nm rms has already been demonstrated,and an engineering path exists for improvements down toone half that value. Four versions of the interferometerhave been supplied to our optics manufacturer for use inthe fabrication of the four individual mirrors of the ETScamera. The interferometer can also be configured tomeasure the wavefront quality of an assembled camera.However, visible light does not interact with MLreflectors in the same manner as EUV light. Thus it is ofgreat importance to be able to characterize an EUVcamera using light at the wavelength of intendedoperation. To this end, an EUV interferometer has beendeveloped which will be used to characterize thewavefront quality of assembled EUV cameras and to



guide final adjustments of the camera alignment. [4] Thissystem has been shown to have an innate rms accuracy ofbetter than 0.003 waves at the EUV wavelength! Itsaccuracy is far better than needed to qualify an EUVcamera as diffraction-limited.

Several commercial instruments have been used tomeasure surface roughness. An interference microscopewas used to measure MSFR, and an atomic forcemicroscope (AFM) was used to measure HSFR. Therelevance of these measurements was verified by makingdetailed precision measurements of the magnitude andangular dependence of EUV scattering off of surfacescharacterized with the other instruments. Excellentagreement has been obtained between the directscattering measurements and the predictions based on themeasurements of MSFR and HSFR.

MasksEUVL masks are reflective, not transmissive. Theyconsist of a patterned absorber of EUV radiation placedon top of an ML reflector deposited on a robust and solidsubstrate, such as a silicon wafer. Membrane masks arenot required. The reflectance spectrum of the mask mustbe matched to that of the ML-coated mirrors in thecamera. It is anticipated that EUVL masks will befabricated using processing techniques that are standardin semiconductor production. Because a 4:1 reduction isused in the imaging, the size and placement accuracy ofthe features on the mask are achieved relatively easily.

Nonetheless, there are a number of serious concernsabout mask development. The foremost is the fact thatthere is no known method for repairing defects in an MLcoating. Since masks must be free of defects, a techniquemust be developed for depositing defect-free MLreflectors. The defect densities in ML coatings producedby magnetron sputtering have been found to be adequatefor camera mirrors, but far too high for mask blanks. Asa result, a much cleaner deposition system that uses ion-beam sputtering has been constructed. A reduction ofabout 1000 in the density of defects larger than 130 nm,to a level of better than 0.1/cm2, has been obtained withthis system, but further improvement will certainly berequired. Present defect detection techniques use visiblelight, and it is all but certain that the density of defectsprintable with EUV light is higher. Defects can take theform of amplitude or phase perturbations, and the propertools for detecting EUV-printable defects are currentlybeing developed. Initially it will be necessary to inspectthe mask blanks using EUV radiation. In the long run, itis hoped that experience will show that adequateinspection can be carried out with commercially availablevisible-light and e-beam inspection tools.

Finally, in current practice, pellicles are used to protectmasks from contamination. The use of pellicles in EUVLwill not be possible because of the undesirable absorptionthat would be encountered. Other methods for protectingEUV masks are under development.

Sources of EUV RadiationA number of sources of EUV radiation have been used todate in the development of EUVL. Radiation has beenobtained from a variety of laser-produced plasmas andfrom the bending magnets and the undulators associatedwith synchrotrons. Our work has used a succession ofcontinually improved laser-produced plasma sources.Work is also being done on the development of dischargesources that might be able to provide adequate power inthe desired wavelength range. Eventually a source willbe required that reliably provides sufficient power to yieldadequate wafer throughput in a manufacturing tool.

ResistsThe main problem to be confronted in developing asatisfactory photoresist for EUVL is the strong absorptionof EUV radiation by all materials. The absorption depthin standard organic resists used today is less than 100nm. EUV resists will most likely be structured so thatprinting occurs in a very thin imaging layer at the surfaceof the resist. Resist types being actively worked oninclude silylated single-layer resists, refractory bi-layerresists, and tri-layer resists. A resist acceptable for highvolume manufacture must exhibit high contrast forprinting in combination with a sensitivity that will yieldan acceptable throughput. A resist sensitivity of 10mJ/cm2 is our goal since it represents a good compromisebetween the need for high throughput and the desire tominimize the statistical fluctuations due to photon shotnoise. Of course, a successful resist must also possessexcellent etch resistance. As the features printed in resisthave continued to shrink, the roughness at the edges ofresist lines has begun to be a serious problem for alllithographies. While not strictly an EUVL problem, asuccessful EUV resist will be required to solve the line-edge roughness (LER) problem.

Experimental ResultsOur imaging experiments to date have been carried outusing the 10X EUVL microstepper. These experimentshave allowed us to evaluate the EUV imagingperformance of the camera and to relate it to themeasured surface figure and surface roughness of itsmirrors. The imaging performance also correlated wellwith the camera wavefront as measured directly with theEUV interferometer. Additionally, these experimentshave been used to investigate various resists and masks



and to help us understand a number of system issues.Three cameras have been built for this system, all ofwhich image with a 10X reduction. The camera itself isa simple Schwarzschild design and is comprised of twospherical mirrors. A schematic diagram of this camera isshown in Figure 5. As shown in the lower part of thefigure, we used off-axis portions of the full mirrors toavoid obscuration of the light by the mirrors; the NAused was 0.07 or 0.08.

Figure 5: Schematic of the 10X EUVL camera

The cameras were originally aligned using visibleinterferometry. Subsequent EUV interferometry revealedthat the at-wavelength measurements yielded nearlyidentical results. Not all camera designs allow foralignment with visible light.

Figure 6 shows the cross-sectioned profiles of dense linesand spaces printed in resist with the 10X camera. Thefigure shows resist profiles of lines and spaces withwidths of 200 nm, 150 nm, and 100 nm. As can be seen,the resist profiles are well defined. From a series ofmeasurements like this it is possible to demonstrate theexcellent linearity of the printing.

0.100 µm0.150 µm0.200 µm

Figure 6: Resist profiles of line and space patternsimaged by the 10X camera for line and space widths of

200 nm, 150 nm, and 100 nm

That is, the width of the resist image is equal to theintended size as written on the mask. Figure 7demonstrates excellent linearity for dense lines andspaces from a line width of 250 nm down to 80 nm.

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Figure 7: Linearity of printing by the 10X camera inresist for line and space patterns with linewidths from

200 nm down to 80 nm

Exposures such as the above can also be used todemonstrate the large DOF inherent in EUVL. Figure 8presents the data from such a series of exposures: itshows how the line width of a 130 nm line (theremaining resist) varies as the camera image is defocusedon the wafer. As seen, the line width only changes byabout 5% as the wafer is moved from best focus to aposition 2 µm away from best focus. This observation isin reasonable agreement with the behavior predicted byEquation 1. In manufacturing of high-performance IC’s,it is desired to control the critical line widths to +/- 10%or better.

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as the wafer is defocused by 2 µm

Finally, in Figure 9, we show cross-sectioned resistimages of 80 nm lines and spaces (with a line space ratioof 1:2). This demonstrates the resolving power of the

28 MM

Clear apertureN.A. ~ 0.07

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10X camera and our ability to print such fine features inresist.

Figure 9: Printing of 80 nm lines and spaces (with a 1:2pitch) by the 10X camera

While the 10X camera has been of great use in ourprogram, we look forward to the completion of the ETScamera so that we can explore EUV imaging with acamera of the kind needed for production-typelithography.

ConclusionSuccessful implementation of EUVL would enableprojection photolithography to remain the semiconductorindustry’s patterning technology of choice for years tocome. However, much work remains to be done in orderto determine whether or not EUVL will ever be ready forthe production line. Furthermore, the time scale duringwhich EUVL, and in fact any NGL technology, has toprove itself is somewhat uncertain. Several years ago, itwas assumed that an NGL would be needed by around2005 in order to implement the 0.1 µm generation ofchips. Currently, industry consensus is that 193 nmlithography will have to do the job, even though it will bedifficult to do so. There has recently emerged talk ofusing light at 157 nm to push the current opticaltechnology even further, which would further postponethe entry point for an NGL technology. It thus becomescrucial for any potential NGL to be able to address theprinting of feature sizes of 50 nm and smaller! EUVLdoes have that capability.

The battle to develop the technology that will become thesuccessor to 193 nm lithography is heating up, and itshould be interesting to watch!

AcknowledgmentsThe work described in this paper was done by a largenumber of people, too numerous to mention individually,

within the VNL and the LLC. I am indebted to all ofthem for allowing me to play a small role in the overalleffort and to summarize their work here.

References[1] For readers interested in digging deeper, Irecommend the following sources:

For a compilation of papers on EUVL see “OSA Trendsin Optics and Photonics Vol. 4,” Extreme UltravioletLithography, G.D. Kubiak and D.R. Kania, eds. (OpticalSociety of America, Washington, DC 1996).

For recent papers on the various NGL’s and on opticallithography see J. Vac. Sci. Technol. B15, Nov./Dec.1997.

[2] T.E. Jewell, J.M. Rodgers, and K.P. Thompson, J.Vac. Sci. Technol. B8, 1509 (1990).

[3] G.E. Sommargren, Laser Focus World 32, 61 (1996).

[4] E. Tejnil, et al., J. Vac. Sci. Technol. B15, 2455(1997).

Author’s BiographyJohn E. Bjorkholm has been a Principal Scientist at IntelCorporation in Santa Clara, CA since 1996. In 1994,after 28 years at Bell Laboratories, he retired as aDistinguished Member of the Technical Staff. He was akey player there in the work on laser cooling andtrapping of atoms. He received a BSE with highesthonors in EE-Physics from Princeton University in 1961,and in 1966 he received a Ph.D in Applied Physics fromStanford University. He is a Fellow of the OSA and theAPS. He served as an OSA Director-at-Large (1988-90)and as the OSA Treasurer (1992-96). He also served as aTrustee of Princeton University (1991-95). His emailaddress is [email protected].

euv lithography—the successor to optical...

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