APPLYING DIPOLE ILLUMINATION TO CHARACTERIZE THE IMAGING PERFORMANCE OF 193 NM PHOTORESISTS FOR THE 100 NM NODE

Bert Vleeming, Barbra Heskamp, Jo Finders (ASML, Veldhoven, The Netherlands) Patrick Jaenen (IMEC, Leuven, Belgium))

This paper was first presented at the Arch Chemicals Seminar,

November 2000San Diego, California, U.S.A.

APPLYING DIPOLE ILLUMINATION TO CHARACTERIZE THE IMAGING PERFORMANCE OF 193 NM PHOTORESISTS FOR THE 100 NM NODE

Bert Vleeming, Barbra Heskamp, Jo Finders (ASML, Veldhoven, The Netherlands) Patrick Jaenen (IMEC, Leuven, Belgium))

ABSTRACT

A comparison is made between state-of-the-art 193 nm photoresists in their ability to print 100 nm dense lines. To ob-tain an aerial image contrast suitable for imaging 100 nm dense lines (L:S=1:1), an ASML PAS 5500/950[1] with dipoleillumination is used. The PAS 5500/950 has a maximum Numerical Aperture (NA) of 0.63. The combination of this NAwith dipole illumination gives the same aerial image contrast as annular illumination with an NA of 0.75. In this way, itis now possible to test thoroughly the ability of a photoresist to print 100 nm dense lines, without the use of expensiveand immature Phase Shift Masks (PSM).

By comparing the experimentally obtained exposure latitudes with the theoretical aerial image contrast, an indicationof the maturity of the photoresists is obtained. This work will give the current status of the 193 nm photoresists in termsof readiness for the 100 nm node. In addition, this work shows that by using dipole illumination, it is possible to do thesetests now, providing an opportunity for resist, etch and further process optimization.

1. INTRODUCTION

Over the last two years, there has been significantprogress in 193 nm lithography. Currently, 193 nm re-sists exposed on 0.63 NA Argon Fluorine (ArF)Step & Scan systems have process windows compara-ble to, or larger than those of 0.7 NA, 248 nm lithogra-phy[2]. However, because of the maturity of the 248 nmlithography process, it is believed that for most applica-tions of the upcoming 130 nm technology node, high NAKrF lithography using appropriate enhancement tech-niques will be the technology of choice.

Alternatively, for the 100 nm node, 193 nm lithographywill be needed and it is therefore crucial to have 193 nmlithography ready in time.

The ITRS timetable shows that the 100 nm technologynode should be in place on an R&D level in approxi-mately six months. The development of high NA,Step & Scan 193 nm lithography tools is progressingrapidly and next year 193 nm exposure systems with0.75 NA will be available in time to meet the needs of the100 nm node. The 193 nm photoresists have been sig-nificantly improved over the last year, but it has not yetbeen proved that these photoresists will fulfil the re-quirements of the 100 nm node. While 100 nm isolatedand semi-dense lines can be tested on the currentlyavailable, lower NA, 193 nm systems, a higher NA, or al-ternating PSM, are necessary to print 100 nm denselines (L: S=1:1). Therefore, 193 nm photoresists werenot tested so far in their ability to print these critical100 nm dense lines (L:S=1:1).

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Figure 1 (A) shows the Normalized Image Log Slope(NILS), simulated with ProLith (v.6.1.2.0.). It is shown asa function of linewidth for fully dense (L:S=1:1) lines fora 0.63 NA, 193 nm system in four different illuminationmodes. In decreasing order of Critical Dimensions (CD),the preferred illumination modes are: first conventional,then QUASAR, and, finally, annular illumination for thehighest resolution[3]. In some applications, dipole illumi-nation can be used to enhance the aerial image con-trast. Dipole illumination shows similar characteristics toalternating PSM, with NILS being constant down to100 nm dense lines. This can easily be understood fromFigure 2, where the pupil fillings are shown for annularand dipole illumination[3]. The aerial image contrast isdetermined by the amount of first order diffracted lightthat is captured by the lens pupil. For dipole illumination,this is 100% for resolutions down to 100 nm dense lines.It is clear that for annular illumination, the reduction inaerial image contrast is more gradual. The shape of theNILS versus pitch plot makes dipole illumination suitableto investigate the imaging performance for 100 nmdense lines using a 0.63 NA exposure tool. An ideal re-sist should give a constant exposure latitude for denselines down to 100 nm.

In Figure 1 (B), the NILS for a 0.75 NA system is shownfor conventional, QUASAR and annular illumination. Inthis plot, the NILS for an 0.63 NA system with dipole il-lumination is added. It shows that for 100 nm denselines, the combination of dipole illumination and 0.63 NAgives a similar contrast to that of QUASAR or annular il-lumination on a 0.75 NA system. This makes it possibleto test photoresists in their ability to print 100 nm dense

lines at the present time. Figure 3 shows the results ofexperiments and ProLith simulation for dipole and annu-lar illumination for a 248 nm resist[3]. It is clear that forboth illumination modes, experiment and theory coin-cide well. This illustrates that 248 nm resists are matureenough to push 248 nm lithography to its limits.

2. EXPERIMENTAL CONDITIONS

The experiments were done on an ASML, PAS5500/950 ArF Step & Scan system, which has a 0.63 NAZeiss StarlithTM 950 projection lens[1]. The same expo-sure system was used for all experiments. A TEL Act 8track and a FSI Polaris 2000 track were used.

CD measurements were done on a Hitachi S-8C40,top-down CD-SEM. Cross-sectional SEM pictures weremade with a Philips XL30 SEM.

Data analysis of the CD measurements was done withFINLE ProData (v. 2.0.0b). FINLE ProLith (v.6.1.2.0)was used for aerial image simulations.

Three advanced, 193 nm resists were evaluated in Ta-ble 1. Resists A and B are single-layer resists on top ofa thin organic BARC. These resists have been shown tohave a good performance in the 0.13 mm node[2]. Thedifference between photoresists A and B is mainly in re-sist contrast. Resist B has an expected higher resistcontrast and is designed for dense line applications.Photoresist C is a new bi-layer resist where the imagingtakes place in a 265 nm thick top layer. In the experi-ments described here, the 400 nm thick underlayer wasnot etched, and only the results directly after develop-ment are shown. For all three resists, the process con-ditions used were those recommended by theapplicable resist supplier.

(a)

(b)Figure 1: Normalized Image Log Slope (NILS) simulated

with ProLith. In (A) NILS is shown for four illumination modes and NA=0.63. In (B) the dipole NILS for NA=0.63 is drawn together with the other three illumination modes for NA=0.75.

Figure 2: Pupil fillings for annular and dipole illumination versus CD. The overlap of the first order light with the pupil is given as well.

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Figure 3: Experimentally obtained exposure latitude and simulated NILS versus CD for 248 nm and dipole and annular illumination.

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3. EXPERIMENTAL RESULTS

Both annular and dipole illumination were used, with amaximum NA of 0.63 and maximum inner and outer sig-mas of σo=0.87 and σi=0.57. This was done to get max-imum aerial image contrast at the lower CDs. The samereticle was used for all experiments. This reticle, charac-terized by SEM, has several feature sizes for one pointin the field. In addition, long cleavable lines are present.

Two different jobs were exposed. One was a Focus Ex-posure Matrix (FEM) for one point in the field. From thisjob, Exposure Defocus (ED) windows were measured.The second job is an FEM for the long cleavable lines.This job was used to get cross-sectional data.

ED windows were measured for 90, 100, 110, 120, 130and 150 nm dense lines (L:S=1:1). After analysis withProData, values for the Maximum Exposure Latitude(Max EL) and Maximum Depth Of Focus (Max DOF)were obtained. A summary of these results is given inTable 2.

Clear differences in Max EL between the three resistsare observed. Resist B has the largest EL for featuresdown to 110 nm with dipole illumination. It is not, howev-er, able to resolve the small CDs achieved by the otherresists. Resist C is even capable of printing 90 nmdense lines with dipole illumination. For dipole illumina-tion, Max DOF is at a maximum for 110 nm dense lines,which can be seen from the pupil fillings shown in Figure2. The DOF increases when the zeroth order light andfirst order light are more symmetrical with respect to theaxis of the projection system[3].

From the exposures of the long cleavable lines,cross-sections have been made for different CDs at bestfocus and energy-to-size. Figure 4 shows cross-section-al pictures for photoresists B and C with dipole and an-nular illumination. It can be seen that with resist C anddipole illumination, it is possible to print 90 nm dense(1:1) lines. With annular illumination, it is possible toprint 110 nm lines. Resist B had adhesion problems atthe high resolutions. Pattern collapse occurred at100 nm for dipole illumination and at 110 nm for annularillumination. It is possible that process optimizationcould improve the performance of resist B.

Photoresist Type SubstrateResist Thickness

(nm)Substrate

Thickness (nm)

A Single layer Organic BARC 330 81

B Single Layer Organic BARC 330 81

C Bi-layer Underlayer 265 400

Table 1: The photoresists that are investigated here.

Dipole NA 0.63, σo 0.87, σi 0.57 Annular NA 0.63, σo 0.87, σi 0.57

CD (nm) Max EL (%) Max DOF (µm) Max EL (%) Max DOF (µm)

A B C A B C A B C A B C

150 15.3 19.5 16.8 0.4 0.4 0.5 14.0 17.1 15.7 0.6 0.7 0.7

130 15.0 17.9 16.8 0.5 0.5 0.5 12.8 14.8 12.5 0.5 0.6 0.6

120 14.4 18.9 15.6 0.6 0.6 0.5 12.9 14.6 10.8 0.4 0.5 0.5

110 14.7 19.9 16.0 0.7 0.5 0.6 4.4 0 8.5 0.2 0 0.5

100 11.5 0 12.3 0.4 0 0.5 0 0 0 0 0 0

90 0 0 8.4 0 0 0.3 0 0 0 0 0 0

80 0 0 0 0 0 0 0 0 0 0 0 0

Table 2: Experimental results of maximum exposure latitude and maximum depth of focus versus CD for the three resists and two illumination modes.

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Figure 4: Cross-sectional pictures of photoresist B and C at best focus and best energy for annular and dipole illumination.

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To get a good comparison between the resists, the ELand the NILS were normalized to their respective valuesfor 150 nm lines. The normalized EL and NILS areshown as a function of CD in Figure 5.

It can be seen that with dipole illumination, there areclear differences in the resolution of the resists. For an-nular illumination, the differences are less pronounced.This may be due to the fact that the energy steps in theFEM were too large to derive a finite value of EL at thesmaller CDs. With annular illumination, the aerial imagecontrast becomes much smaller for the smaller CDs, incomparison to dipole illumination. As the simulationspredicted, when dipole illumination is used, EL is almostconstant for all three resists from 150 nm down to110 nm. Below 110 nm, the behaviour of the three re-sists starts to deviate from the ideal. Resist B is not able

to print 100 nm dense lines with a finite EL and DOF. Forresists A and C, EL at 100 nm reduces by a similaramount, while NILS remains constant. Resist A doesnot print below 100 nm. Resist A has good adhesion, butit is not possible to open 90 nm fully dense lines.Resist C is the only resist that has a distinct EL for90 nm dense lines. This is also shown in Figure 6 whereCD is plotted as a function of energy at best focus for90 nm dense lines. Figure 7 shows the correspondingEL as a function of DOF. The combination of thePAS 5500/950 lens and resist C is very similar to thetheoretical NILS curve in Figure 1. From these data, itcan be concluded that, of the three photoresists, resistC is the most promising material for 100 nm technology.

4. DISCUSSION

Based on the data presented in the previous section, itcan be concluded that dipole illumination is a powerfultool for judging the imaging performance of photoresistsfor the 100 nm technology node. Clear differences in theperformance of resists A, B and C are observed. ResistB, which is currently seen as an excellent resist for the130 nm node[2], is not able to print 100 nm dense lines,even with an aerial image contrast which is suitable toimage 100 nm fully dense lines. This shows that it is veryimportant to expose current photoresists with a suitableaerial image contrast to investigate their imaging perfor-mance for the future 100 nm node.

The question arises which parameters contribute to theobserved differences between the different resists. Allthree resists were exposed on the same tool and underthe same conditions, so the parameters have to befound on the resist/processing side. For example, resistcontrast, absorption, adhesion, line-edge roughness,substrate interaction and diffusion lengths may all havean impact on the resolution of the resist. The cross-sec-tions shown in Figure 4 and the excellent EL for the larg-er features for resist B give the impression that resist Bis able to print smaller features. However, adhesionproblems degrade its performance. The cause of thefailure of resist A is different. Here adhesion is not aproblem, but at the smaller CDs, it is not possible toopen the lines. Amongst other differences in the resistparameters, this may be due to the expected lower resistcontrast for resist A when compared to the other two re-sists. The failure may also be caused by a prematureclose-up, due to, for example, “scumming” or substrateinteraction.

Figure 5: Experimental results of exposure latitude for the three resists and simulated NILS versus linewidth of fully dense (1:1) lines. Results are normalized for its value at 150 nm linewidth. In (A) dipole illumination is shown and in (B) annular illumination.

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5. CONCLUSION

Three state-of-the art 193 nm photoresists have beenevaluated in their capabilities to print 100 nm fully dense(L:S=1:1) lines. An ASML PAS 5500/950 with dipole illu-mination was used to get an aerial image contrast suit-

able for imaging 100 nm dense lines. It is shown thatdipole illumination is a very powerful technique to dothese tests when using the currently available 0.63 NAexposure systems. The possibility to do these tests atthe present time provides an opportunity for resist, etchand further process optimization.

Clear differences in the performance of the three resistsare observed. The exposure latitudes are measured forseveral feature sizes and then compared to the NILSsimulated with ProLith. The combination of the ASMLPAS 5500/950 and photoresist C approaches the idealsituation simulated with ProLith NILS. Of the three pho-toresists, resist C is the most promising for the 100 nmdesign rule. Resist C even resolved 90 nm fully denselines with 8.4% EL and 0.3 µm DOF using dipole illumi-nation. The other two resists have less resolution due todifferent failure mechanisms. Resist B has adhesionproblems, which might be improved with process optimi-zation. Resist A has no adhesion problems, but is notable to open up 90 nm dense lines with dipole illumina-tion. This might have to do with the expected lower resistcontrast of resist A.

Figure 6: CD versus energy at best focus for 90 nm dense (1:1) lines, showing the capabilities of Resist C using dipole illumination.

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Figure 7: Process window of 90 nm dense lines for resist C with dipole illumination

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6. ACKNOWLEDGEMENTS

The authors would like to thank Kurt Ronse, Geert Van-denberghe, Diziana Vangoidsenhoven and Peter deBisschop from IMEC for the availability of the exposuretool, track and SEM and for assisting in the experiments.We thank Eddy van der Heijden and Youri van Dom-melen for support in the experiments. The resist suppli-ers are thanked for the opportunity to use their resists.We thank Simon Duerden, Vivian Kim and the illustra-tors from ASML publications department for their assis-tance in the preparation of this manuscript.

REFERENCES

[1] J. Mulkens et al., “ArF Step & Scan exposuresystem for 0.15 µm and 0.13 µm technologynode?”, Proc. SPIE, 3679 (1999), p. 506

[2] Kurt Ronse et al., “Status of ArF lithography for the130 nm technology node”, Proc. SPIE 4000(2000), p.410

[3] Jo Finders et al., “DUV lithography (KrF) for130 nm using off-axis illumination and assistingfeatures”, Semicon Japan (1999)

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