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  • Summer 2002 Yield Management Solutions 35

    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

  • Summer 2002 Yield Management Solutions36

    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

    S P E C I A L F O C U S

    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.

  • Summer 2002 Yield Management Solutions 37

    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

  • Summer 2002 Yield Management Solutions38

    S P E C I A L F O C U S

    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

  • Summer 2002 Yield Management Solutions 39

    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 firs