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The Sidewall Chrome Alternating Aperture Mask (SCAAM), a next generation alternating Phase Shift Mask (alt-PSM) structure, has printed 75 nm semi-dense lines (220 nm pitch) without characteristic PSM anomalies, thus offeringthe potential for sub-100 nm imaging with 248 nm light. The even-lower-cost Phase Phirst! paradigm would employready-to-write SCAAM blanks with pre-patterned surface topography, chrome and resist, eliminating the cost of writinga custom phase pattern on every plate. Circuit designers, however, would have to place every minimum-sized circuit featureat a predefined phase-step location. This system is economically superior to other advanced lithography schemes when standard pre-patterned substrates can be mass-produced using wafer fab techniques, which requires standardization ofdesign grids. Using a conventional or attenuated phase-shift trim mask in a two-exposure lithography scheme facilitatesarbitrary interconnections.
Pattern Transfer/ShrinksS P E C I A L F O C U S
Exposing the SCAAM
Theory, Characterization, and Confirmation of theValidity of an Innovative Optical Extension Technique
Marc D. Levenson, M.D. Levenson Consulting, Takeaki (Joe) Ebihara, Canon USA Inc., Sunil Desai and Sylvia White, KLA-Tencor Corporation
It has long been known that alternatingaperture phase-shifting masks (alt-PSMs)can project images with pitches down to0.5/NA (about 170 nm for 248 nm light)and almost unlimitedly small dark lines.(The current record is 9 nm1) and low CDvariation. However, widespread implemen-tation of alt-PSM technology has beendelayed by various challenges, includingimaging artifacts and the high cost of pro-duction-quality reticles. By addressing themanufacturability issues of alt-PSMs, wehave found a mask structure and productiontechnology that realizes the full theoreticalresolution and CD control potentials ofthese reticles and promises low cost imple-mentation.2
Low cost is important, as roughly half of allreticles are used for chip designs that haveproduction runs under 600 wafers.3 In such
short production runs, the reticle cost already dominatesall other factors at 250 nm and the high projected costof sub-100 nm reticles cannot be borne by this industrysegment. The Phase Phirst! PSM paradigm discussedhere can result in lower overall cost of production forchips with wafer runs of one thousand 200 mm-equiva-lent and below. However, certain chip-design constraintsare necessary to achieve the necessary economies ofscale, and it has proved difficult to interest the designcommunity in implementing these design rules.
The key innovation is the Sidewall Chrome AlternatingAperture Mask (SCAA mask or SCAAM), a next gener-ation alternating Phase Shift Mask (alt-PSM) structureshown in Figure 1(a).2, 4 The SCAAM process etches thephase topography first and then sputters an opaquechrome layer over the phase layer, finally coating withresist. A second write step then forms transparentopenings in the conformal chrome layer to define theimage. The great optical advantage of this structure is that the physical environment is the same for all
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openings in the chrome, independent of phase. Thatimmediately eliminates most of the causes of the asym-metries that plague other alt-PSM structures.3, 5
Figure 1 compares the electric field amplitudes and theaerial image intensities for three different mask struc-tures, as calculated using KLA-Tencors ProMAX/2Dand PROLITH/2 process window simulation software. Itis clear that the E-fields of both the 0 and 180 aper-tures are the same at the chrome surface of the SCAAmask. That is not true of the dual trench structure,where the bottoms of the trenches affect the amount oflight transmitted and the trench walls alter the phase.2
The net result is a dimmer, asymmetrical image, andone which varies with focus because of a trench-widthdependent error in the effective phase. The idealizedundercut structure produces less asymmetry and phaseerror, but the 80+nm undercut of the chrome edgessignificantly reduces the chrome layer adhesion. In theSCAA mask structure, all chrome is supported and all
trench walls are covered. The SCAAM symmetrybetween 0 and 180 spaces significantly reduces thecomplexity of the mask design (i.e. OPC) process andhelps achieve the optical performance predicted bysimple theories.4, 6
Imperfect fabrication is less of an issue for the SCAAmask structure than for other alt-PSM designs. Figure 2shows that many classes of phase defects are simplyburied under the chrome and thus cannot print.4
Pinholes, protrusions, mouse-bites and other chromepatterning errors can be repaired using conventionaltechniques since the chrome layer is in contact withthe substrate everywhere. Errors in the chrome layercannot induce unrepairable phase defects, since thephase layer is patterned first in the SCAA maskprocess. An inspection between phase patterning andphase etch has been shown to detect all printabledefects except tiny phase pits in 0 spaces.7 It is evenconceivable that FIB tools may be able to repair phase
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Figure 1. Reticle structures, rigorous electric field simulations at the reticle plane and aerial images through focus for 100 nm line100 nm space
patterns for a SCAA mask (a), dual trench PSM (b) and undercut PSM (c) as imaged at 248 nm, NA=0.744, 4x and =0.2 (k1=0.30). These
calculations were performed using ProMAX/2D and ProLITH/2 from KLA-Tencor, Inc.
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errors on completed masks by machining entire win-dows to a 180 or 360 phase level and then etching orre-depositing opaque material to closely approximatethe correct transmission. Because strong-PSMs suppressthe MEEF, these repairs need not be made to the preci-sion required for COG masks intended to projectimages with the same dimensions. Thus, since inspec-tion and repairs are feasible, it may be that SCAAmasks will prove more economical than other strongPSM structures at the 100 nm node and beyond.
ExperimentSubwavelength lithography requires an exposure toolwith minimal aberrations and a highly capable resistprocess as well as an appropriate photomask technology.The test mask (prepared by DNP, Ltd.)contained >180 line-space targets with awide variety of CDs and pitches.2 A 4xCanon FPA-5000 ES3 step-and-scanexposure tool (with total aberrations
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753 nm lines over a 0.6 m range of focus: a k1 factorof 0.19 (at NA=0.63), below the theoretical minimumfor equal line-space patterns. Note that the widths ofthe spaces are constant over the focus range. Had therebeen significant phase or amplitude errors, adjacentspaces would have had visibly different widths. Carefulmeasurements revealed a shift 50%)iso-dense bias is characteristic of uncorrected alternating-PSM designs.9 Proper iso-dense correction may beachieved in dual-exposure trim-mask PSM systems eitherby sizing the windows bracketing isolated PSM lines cor-rectly or by using an all-dense pattern on the PSM, eras-ing unwanted lines with a trim mask.1,9,10 Systematicallycomparing the 1000 nm wide spaces on either side of theisolated lines revealed a through-focus shift of
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fall on a characteristic straight line. The fact that zeroshift occurs at a focus level of~0.1 m (best focus withinresist stack) implies that there is no transmission dif-ference between 0 and 180 spaces. Figure 6b plotsthe measured focus dependent line shift coefficient (inunits of nanometers of shift per micrometer of defocus)as a function of spacewidth. The small values observedeven for the somewhat anomalous 220 nm pitch case
indicates that line shift will not be a problem forSCAA mask imaging within a 0.5 m CD process win-dow for pitches between 220 nm and 400 nm. Theexperimental resist CDs printed using the SCAA maskagreed with those predicted by an aerial image modelfor NA=0.63, 0.68 and 0.73 and demonstrated theunimportance of the residual 0.016 wave aberrations inthe ES3 projection lens.4
Figure 5. Bossung plots for the 150 nm spaces with 0 and 180 phase shifts in the 220 nm pitch pattern. The relatively small slope of these plots
for doses above threshold (~230 J/m2) implies the effective phase shift is very near 180.
S P E C I A L F O C U S
Figure 6. The shift of the center dark line is measured as 1/4 of the dif ference of pitch values measured to the left and right of the center line as
shown in the inset. The measured shift of lines in 250 nm pitch patterns is linear in focus for all fully developed exposures (a). The measured focus
dependent line shift correlates with space width for most sites on the SCAA test mask (b).10
The optical proximity effect, however, continues toaffect imaging with the SCAA mask. Figure 7 showsthe measured resist CDs for 100 nm geometrical (1x)mask features with various pitches at 320J/m2. Clearly,the densest line space pattern here prints the finest fea-tures, with a >40% shift between 300 nm and 500 nmpitch. Printing equally narrow lines in the many envi-ronments characteristic of a real chip might prove ratherdifficult. However, designs are possible in which all thefine lines are in semi-dense arrays on the PSM and theunwanted features are erased using the trim mask. Suchdesigns would not require extensive optical proximitycorrection. In the case of isolated lines, the printed line-width depends on the width of the transparent windowon either side of the mask feature.10 With proper design,there may be little need to print unwanted assist features.
The wide variety of test patterns on the first SCAAmask permitted the analysis of linearity and the maskerror enhancement factor (MEEF) shown in Figure 8.8
As expected, the resist CD for the isolated line became
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insensitive to the mask CD around ~150 nm (@1x),leading to a MEEF < 0.1 for resist CDs near 100 nm.The behavior of resist CDs for lines in ~300 nm pitchline-space arrays was entirely different. The resist CDscorrelated closely with the mask CDs until the spacesbetween the dark lines became too small to print. Theresult was a correlated MEEF (i.e. a MEEF in which allfeatures grow or shrink together, keeping pitch constant)of ~0.8 for resist CDs between 75 nm and 160 nm.Such a value is close enough to unity to allow OPCprograms to correct for residual interactions. However,fully exploiting this linearity requires placing featuresin well-defined arrays, at least on the alt-PSM used in atwo-exposure trim-mask system.
Phase Phirst!If the phase-shifted features need to be in regular arraysfor imaging reasons, the optimum phase patterns canbe pre-imprinted on generic SCAA mask substrates,greatly simplifying the PSM fabrication process. Suchready-to-write SCAAM blanks are at the core of thePhase Phirst! PSM paradigm, as illustrated in Figure 9.In this scheme, the mask house would hold a variety ofPhase Phirst! substrates in inventory and when aPhase-Phirst job came in, the appropriate plate wouldbe taken out and the chrome openings written usingconventional technology. The mask maker would doone write, one development, one etch, one inspection
Figure 7. The proximity effect for 100 nm nominal lines of various
pitch on the SCAA mask. The densest line-space patterns printed the
smallest lines for 320 J/m2 exposure even at NA=0.63.
Figure 8. Linearity and correlated MEEF for isolated and grouped lines
with pitch near 300 nm at 320 J/m2 exposure.
Figure 9. The three steps needed to fabricate a SCAA mask using a
ready-to-write Phase Phirst! substrate.
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and (possibly) one repair. The overall process flowshown in Figure 9 would be nearly identical to that fora COG mask. Turn-around time should be three daysor less, as for COG plates today. A trim-mask would bewritten at the same time to complete the dual exposuredarkfield mask set. The cost to the customer should besimilar to that of two COG masks!4
The key question is whether designers would be willingto place small circuit features at the designated phase-shift locations across an entire reticle. Current designsdo not conform to any such restriction, and thus simplyshrinking current layouts is not compatible with PhasePhirst! Neither is post-processing an existing design;circuit elements will have to be moved to conform tothe phase pattern, and that is not possible during tape-out. On the other hand, current designs can be madecompatible with a variety of phase patterns as shown inFigure 10.
Larger featureswhich would not require phase-shift toprint properlywould be placed randomly as requiredon a trim mask. The second exposure using that COGor attenuated PSM would create the connection patternallowing the chip to function. In the fab, this PhasePhirst! Paradigm would resemble the darkfield dualexposure PSM lithography methods that are alreadybeing applied.12, 13 Figure 10 shows how the same cir-cuit cell can be made using various phase patterns. The
inverse is also truea single substratephase pattern can produce many dif-ferent circuit structures with differentchrome openings and blockout masks.It seems likely that a small number ofphase patterns will prove adequate for90% of ASIC devices. The most feasi-ble starting point may be a simplephase stripe pattern with all gates ori-ented in the same direction and spacedon a 250 nm center-to-center grid.The circuits designed for such sub-strates may also be patterned success-fully (but perhaps more expensively)using dipole illumination, and attenu-ated-PSMs! The table suggests opti-mum phase-step spacings (featurepitches) for various tool parameters.Since the exposure tools will demagnifyby 4x or 5x, the substrate features willbe 1000 nm today (and 240 nm in2007), easily fabricated with todaysoptical tools.
Circuit density may be another concern. While it mightappear that a rigid coarse-grid rule would require circuitfeatures to be spaced further from one another than in aless constrained design, Figure 11 shows that this maynot be the case. On the left appears an SEM of the gatelayer of an existing microprocessor design, which hasbeen post processed for PSM. Most fine features arealready placed properly on the Phase Phirst! grid lines,but two are not. The center and right pictures showthat this SEM can be modified using Powerpoint toplace all the gates on the grid in two different ways. Inone case, the density does decrease, but only by a fewpercent. In the other example, the density increases asthe disciplined design eliminates apparently wastedspace. Of course, strong PSM allows considerable circuitshrinkage in any case. Further partnerships with the
S P E C I A L F O C U S
Figure 10. The same circuit cell can be printed with the same transmission windows and trim-mask
using any of several substrate phase patterns in the Phase Phirst! PSM paradigm.
Wavelength Numerical Aperture Phase Step Spacing Circuit Half-Pitch
248 nm 0.60-0.68 250 nm 125 nm
248 nm 0.73-0.80 200 nm 100 nm
193 nm 0.75-0.78 150 nm 75 nm
157 nm 0.78-0.85 120 nm 60 nm
Table. Optimum Phase Phirst! substrate phase-edge spacings for different
exposure wavelengths and projection lens NAs along with the resulting
minimum circuit half pitches for ASIC devices. The wafer-scale pitch for
the phase step pattern is twice the indicated spacing and the mask
dimension would be 4 or 5 times larger, depending on demagnification.
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design community need to be carried out to test whetherthe Phase Phirst! design paradigm is feasible for anentire chip and whether density increases or decreases.
Substrate and reticle manufacturingPhase Phirst! substrate production is a new business,best undertaken by manufacturers of reticle blanks.The optimum process flow appears in Figure 12. ThePhase Phirst! blanks begin as fused silica plates as doconventional COG substrates, but a new high-techtopography creation/inspection process is inserted afterthe final polishing step and before chrome coating.2
The proven way to make this topography is to spin ona resist film, pattern it in a stepper or (1x scanner)using a photomask that defines the phase pattern andthen etch the silica substrate to the prescribed depth.Alternatively, interferometric lithography could be usedto define a simple phase stripe pattern.14 A spin-on orCVD silica layerwhich etches more rapidly than thesubstratecan be applied to the polished surface priorto lithography to improve phase-shift accuracy.15 Thesurface topography would then be inspected to insurepattern fidelity and the absence of killer defects. Rejectplates would be polished flat and re-used. However, if asubstrate was perfect except for a few isolated anomalies,the locations of these phase-defects could be recordedin a database and the plate coated with chrome andresist, inspected and shipped. Since most of the dark-
field PSM will remain covered by the opaque film ofchrome, it should be possible to use the defect databaseto match substrate and chip design so that no trans-parent windows are written in defective areas.
The process flow in Figure 12 requires specialized capitalequipmentsuch as a stepper equipped to handle sixinch square substrates and avoid stitching errors.Recovering the capital investment requires mass production of identical substrates. That gives rise to a chicken and egg problem: Which comes first, the
Figure 11. Image-processing an SEM of an existing chip to eliminate off-grid gates and conform to Phase Phirst! layout rules may allow for increased
Figure 12. Process flow for fabricating Phase Phirst! substrates. A
defect database may lower costs by permitting the use of less than
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chip designs that demand Phase Phirst! substrates, orthe economical substrates that can make those designsworth the effort?
The reticle industry has not yet realized the economiesof scale that have allowed the semiconductor industryto reduce prices consistently for 30 years. The resulthas been a dramatic increase in both the cost of pro-duction and the sales price of advanced reticles. As wemove into the sub-wavelength era, designers, reticlesuppliers and fab operators must transcend old practicesand seek more efficient methods. By allowing simplifiedOPC, higher fab yields and lower reticle cost based onthe mass production of identical, defect free phase patterns, Phase Phirst! could be part of the answer.However, Phase Phirst! is economically unfeasible forlow volume mask production.
Figure 13 shows the estimated sales price for strong-PSMs produced in different ways as a function of theproduction volume. The conventional alternating PSMprocesses (with separate write and etch steps for thecustom-designed phase and chrome layers) do notexhibit economies of scale. They do not require special-ized apparatus, but there is a firm lower limit for thereticles produced by such processes, independent ofvolume, estimated here as $35,000 per plate. Low yieldsmay increase the price of production-quality PSMs wellabove that limit, at least until the processes mature.
SCAA masks can be produced using Phase Phirst! withthree different exposure tools: a 5x i-line stepper, a 1x
projection aligner or an interferometric lithographysystem. The stepper is most expensive to install, but itsmaster reticles are relatively inexpensive. In contrast, thecapital cost of a refurbished aligner is low, but its masterreticles would be quite costly, reducing the economies ofscale. The interferometric lithography tool would also beinexpensive and would require no reticles, but it couldonly pattern simple linear phase arrays. Its total costwould be similar to the case of the aligner with 1-3 phasedesigns. Figure 13 also assumes dedicated etch andinspection tools that are the same in each case and thatall other costs are comparable to COG mask making.
Figure 13 shows clearly the unfeasibly high pricesrequired for PSMs manufactured using Phase Phirst!when the volumes are less than about 30 substrates.These high initial expenditures constitute a seriousimpediment to the implementation of this system.However, when the capital costs and learning can beamortized over 100-300 reticles, the Phase Phirst! costfalls below the minimum possible with custom phase-layer production. At even higher volumes, the fixedcosts become unimportant and the SCAA mask manu-facturing cost approaches that of a COG mask. However,since SCAA masks have low MEEF (see Figure 8), CDcontrol and chrome layer defectivity may be less chal-lenging than for corresponding COG masks. We estimate that the Phase Phirst! production costs willbecome asymptotic to that of the previous generationCOG masks for volumes >10,000 plates per year.
There are other mask-making economics issues:Roughly 700,000 reticles are made each year, buttoday
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alt-PSM structures. The optical advantages predictedfor alt-PSMs by basic theory have been achieved andthe design and OPC tasks simplified. The SCAA maskstructure also facilitates chrome repair and literally buriesmany types of phase defects. SCAA masks are also morestable physically than alt-PSM structures in which thequartz walls are undercut beneath the chrome edges tominimize the trench walls effects, and can be inspectedeasily using top-down SEM technology.10 These nextgeneration PSMs can be built, and make KrF lithographyfeasible for half-pitches corresponding to k1=0.28 andresist linewidths down to 75 nm. Resist and processtrimming techniques could push the final CD to 30 nmand below, with acceptable CD variation.
Future work with higher NA and finer image pitchesshould demonstrate similar process windows for sub-100 nm half pitches and 60 nm linewidths, and evenless at 193 nm. The SCAAM may prove the mostviable alt-PSM structure for the 100 nm node. Multipleresist processes with SCAA masks having line to spaceratios of 1:3 and a pitch near 280 nm may even print70 nm equal line-space patterns if the overlay controland resist process are good enough to place the narrow(70 nm) dark line of the second exposure exactly in themiddle of the wide (210 nm) bright space of the first.
The Phase Phirst! system is economically superior toother advanced lithography schemes when standardpre-patterned substrates can be mass produced usingwafer fab techniques.2,4 A small number of predefinedphase grids will be sufficient for the vast majority ofhigh-speed ASIC designs, but numerous designers mustadopt this system if the economies of scale are to berealized. Arbitrary interconnections are possible whenthe SCAAM is made using the Phase Phirst! substratealong with a conventional or attenuated phase-shift trimmask in a two-exposure lithography scheme. Successfulimplementation of this PSM paradigm will requireincreased R&D participation by potential users.
AcknowledgementsToo many individuals and companies have contributedto this work to acknowledge them all here by name.However, the authors wish to especially thank NaoyaHayashi, Yasutaka Morikawa and Haruo Kokubo ofDai Nippon Printing Co. for making the test mask.Phase Phirst! is a trademark of M. D. LevensonConsulting. This paper is based on material originallyintended for presentation at Interface 2001.
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