Improved contrast and reflectivity of multilayer reflective optics for wavelengths

beyond the Extreme UV

Tim Tsarfati1,a, Erwin Zoethout1, Eric Louis1, Robbert van de Kruijs1, Andrey Yakshin1,

Stephan Müllender2, and Fred Bijkerk1,3

1FOM Institute for Plasma Physics Rijnhuizen, 3430 BE Nieuwegein, The Netherlands 2Carl Zeiss SMT AG, Oberkocken, Germany

3MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands

ABSTRACT

We present a computational and experimental study on interface passivation of B4C/La multilayers for

photolithography at wavelengths beyond 13.5 nm. We successfully applied N-plasma treatment to form

interface-localized BN and LaN layers, preventing LaB6 and LaC2 interlayer formation and increasing the optical

contrast. Experiments suggest an improvement of absolute reflection by up to 20% for 200 period multilayers,

with a best-so-far result of 41.5 % at near-normal incidence of 6.7 nm.

Keywords: B4C/La, multilayer, nitridation, XPS, reflectometry, lithography

1. INTRODUCTION

In earlier work we addressed the applications and first experimental results on B/La and B4C/La multilayers for

near normal incidence reflection of wavelengths just above 6.7 nm1. This is a candidate wavelength for next

generations of advanced photolithography. The multilayer performance at this wavelength critically determines

the overall lithography system performance, and, as such, substantial experimental data is needed before the full

potential can be assessed. We focus here on first steps in the multilayer development work.

Appropriate multilayer systems for λ = 6.7 nm require individual layer thicknesses of ~1.7 nm, which are

observed to suffer from significant intermixture and interlayer formation, resulting in a loss of optical contrast

and reflectivity. Thin layer growth, intermixture and the use of diffusion barriers have earlier been discussed for

the 13.5 nm wavelength region2,3,4,5. The small layer thickness and low optical contrast in the 6.7 nm wavelength

regime however limits the possibilities of applying diffusion barriers to limit intermixing and interlayer

a T.Tsarfati@rijnhuizen.nl, +31306096987

Alternative Lithographic Technologies, edited by Frank M. Schellenberg, Bruno M. La Fontaine Proc. of SPIE Vol. 7271, 72713V · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.824434

Proc. of SPIE Vol. 7271 72713V-1

formation. We will present a first computational and experimental feasibility study on thin, yet stable B- and La-

compounds that can prevent further interlayer formation. We successfully incorporate interface passivation by

nitridation to limit interlayer formation, increasing both chemical and optical contrast and greatly enhancing

reflectance. This control of the optical contrast increases the perspective for such multilayers for

photolithography at wavelengths beyond 13.5 nm, 6.7 nm being the main candidate1,6.

In order to passivate the interfaces against the detrimental LaB6 and LaC2 interlayer formation1,7,8 via

264 LaC 3 LaB 4 CB 6 La 7 +→+

we consider the compound formation enthalpy (∆Hfor). Table 1 shows the ∆Hfor, the real refractive index (n) and

the absorption constant (β) at λ = 6.7 nm for all relevant elements and compounds.

Table 1) Formation enthalpy and optical constants for 6.7nm multilayer relevant elements and

compounds.

Compound LaN BN LaB6 LaC2 B4C La

∆Hfor (kJ/mol) -303 -255 -130 -89 -71 0

n 0.981 0.995 0.992 0.986 0.999 0.984

β (10-3) 1.420 0.894 0.853 0.996 0.528 1.075

The high ∆Hfor values shown in table 1 suggest that nitridation can prevent formation of LaB6 and LaC2 via eq.

(1)6. The mentioned species are widely discussed in literature9,10,11,12,13. Nitridation of La also enhances the

optical contrast in n and β, as shown in table 1, increasing the theoretical reflectance and bandwidth14.

Fig. 1) Calculated reflection curves for several nitridated 200 period B4C/La multilayer reflectors.

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Figure 1 shows the simulated reflection curves at 1.5° angle of incidence for several partially nitridated 200

period B4C/La multilayers. Based on in-depth XPS analysisError! Bookmark not defined.4, the interface diffuseness is

assumed to be 0.3 nm, with BN and LaN interface films of 0.7 nm thick, compared with a standard B4C/La

multilayer with 1.5 and 0.5 nm thick interlayers at the B4C-on-La and La-on-B4C interface resp.1 The reflection

curves are calculated for optimum combined La and LaN fraction (Ґ) of 0.45.

IMD15 calculations show a more than 70% relative increase in both peak reflection and bandwidth when La is

replaced by LaN to prevent interlayer formation. The multilayer even performs better than B4C/La multilayers

without any LaB6 and LaC2 interlayer formation, due to the higher optical contrast between B4C and LaN. This

mainly accounts for the major calculated gain obtained by nitridation of La, aside from the expected

improvement in chemical and thermal stability.

2. EXPERIMENT

First multilayer samples, meant to study interface phenomena, have been grown by magnetron deposition onto

natively oxidized super polished Si (100) substrates in a modified Leybold Optics A1105 UHV DC-magnetron

and e-beam deposition facility with a base pressure of 1·10-6 Pa. Both complete and partial layer nitridation were

achieved by Kaufmann N-plasma treatment during and after B4C deposition resp. A Thermo Theta Probe

monochromated Al-Kα XPS setup was used also for 0.5 keV Ar+ depth profiling to determine the in-depth

material distribution and compound formation in grown multilayers. Although the relocation range of the ions

can occasionally exceed the calculated penetration depth, the low energy sputtering should result only in

moderate ion beam mixing compared to the XPS probing depth16. The bulk densities of 6.73, 6.17, 2.25 and 2.37

kg m-3 were assumed for respectively LaN, La, BN and B4C layers and films17.

3. RESULTS

Figure 2 shows the XPS depth profile of a standard 51 period 3.540 nm d-spacing B4C/La multilayer with a Ґ of

0.52; thicker than the targeted optimum 0.45 value. The moderate amplitudes for B and C modulation and shifts

in B1s and C1s binding energy suggest diffusion into La and formation of a LaB6 interlayer. The B4C-on-La

interface appears more diffused than La-on-B4C interfaces, which can be attributed to atomic B and C deposition

and surface segregation into the La due to the lower surface free energy of La. For individual layer thicknesses

up to 5.0 nm, no crystallization is visible in XRD1 and CS-TEM18. Peak reflection is measured to be 5.94% at λ =

7.00 nm in these interface test series of samples. This can be modeled by 1.5 and 0.5 nm thick interlayers at the

B4C-on-La resp. La-on-B4C interfaces with 0.47 nm diffuseness. The interface thicknesses are in good agreement

with our in vacuo Angular Resolved XPS (ARPES)1 and grazing incidence x-ray reflectometry (GIXR) results,

Proc. of SPIE Vol. 7271 72713V-3

as well as results of J.-M. André et. al.19. A 200 period multilayer would then yield 37% reflection at λ = 7.00

nm, in accordance with results obtained by Y. Platonov et. al.20.

Fig. 2) XPS depth profile of a La/B4C multilayer. Brackets denote chemical bonds to other elements.

To reduce the LaB6 and LaC2 interlayer formation observed above, N-ion beam assisted deposition (NIBAD) of

B4C and/or La is now applied in a second, third and fourth multilayer series, following the in vacuo ARPES

results shown in Fig. 1.

We observe very poor periodicity and reflection results for our third case, when NIBAD of B4C is applied. The

fourth case, NIBAD of only the La layers, as shown in Fig. 3, reveals a ~25% lower La content, of which about

60% forms LaN as a result of NIBAD. A slight improvement in localization compared to the non-nitrided

multilayer in Fig. 2 is visible. The multilayer d-spacing decreases to 3.388 nm and Ґ to well below 0.3,

attributable to the lower La content. We measure a peak reflectance of 11.52% at λ = 6.74 nm, a significant

increase that is modeled to yield 43% reflection at λ = 6.72 nm and 38% at λ = 7.00 nm for 200 multilayer

periods with adjusted d-spacing. Instead of NIBAD, local nitridation and passivation of only the interfaces will

now be explored by applying a 60 s N-plasma treatment after completion of each layer, as is shown in Fig. 4,

representing our fifth case.

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Fig. 3) XPS depth profile of a La/B4C multilayer with N-plasma treatment during each La layer.

Fig. 4) XPS depth profile of a La/B4C multilayer with 60 s N-plasma treatment after each layer.

Figure 4 reveals a high degree of localization for the B4C, with BN at both the La-on-B4C and B4C-on-La

interfaces. The d-spacing increases to 4.34 nm and Ґ is 0.30, with a peak reflectance of 15.37% at 8.47 nm.

Taking into account the ARPES and depth profiling results of Fig. 2, 3, and 8, a 200 period multilayer is modeled

to yield 51% reflection at λ = 6.72 nm and 44% at λ = 7.00 nm.

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Figure 5 shows the XPS depth profile of a multilayer where 60 s N-plasma treatment is applied only after growth

of each B4C layer, our sixth case.

Fig. 5) XPS depth profile of a La/B4C multilayer with 60 s N-plasma treatment after each B4C layer.

Figure 5 shows a much lower BN content than Fig. 4, mainly located at the La-on-B4C interface. Remarkably, a

slightly larger ratio of the La is now nitridated, again poorly localized towards the bottom of the La layer. The d-

spacing is 4.190 nm, Ґ is 0.44 and peak reflectance is measured to be 11.48% at λ = 8.22 nm. A 200 period

multilayer with adjusting the d-spacing is modeled to reflect 29% at λ = 6.72 nm and 23% at λ = 7.00 nm.

The depth profile of a B4C/La multilayer where 60 s N-plasma treatment was applied after growth of each La

layer, our seventh case, is shown in Fig. 6. It shows that about 60% La forms LaN throughout the layer, with also

more localized BN at the La-on-B4C interface, attributable to N implantation deep into the La layer. The d-

spacing is 4.000 nm and Ґ is 0.46 for this multilayer, indicating that the La layer thickness is unchanged upon N

treatment and that the increase in d-spacing is solely attributable to the BN formation that can be observed in Fig.

6. The peak reflectance is 15.53% at λ = 7.85 nm. A 200 period multilayer with adjusted d-spacing is modeled to

yield 45% reflection at λ = 6.72 nm and 39% at λ = 7.00 nm. The above findings have been used to produce first

200 period multilayers for λ = 6.7 nm reflection, mainly exploiting the fifth and seventh method described above.

A first, very initial result showed a reflectivity of 41.5% at near-normal incidence of 6.7 nm radiation. A

deviation from the targeted value of over 60% is still present and caused by minor instrumental imperfections in

the long run deposition process. These factors are identified and scheduled to be solved in next series of

deposition sets.

Proc. of SPIE Vol. 7271 72713V-6

Fig. 6) XPS depth profile of a La/B4C multilayer with 60 s N-plasma treatment after each La layer.

4. CONCLUSION

In conclusion, we performed an experimental feasibility study of multilayer reflective optics for use in the 6.7

nm wavelength range. The standard La/B4C combination offers a high theoretical reflection, but is observed to

yield chemical reactivity and interlayer formation at the interfaces, reducing optical contrast and reflection. A

systematic survey of growth and layer diffusion kinetics and optical properties reveals that a major increase in

chemical and optical contrast can be achieved by nitridation, exemplified by results of seven different N-

treatment cases. While NIBAD of especially B4C yields poor results, significantly better layer localization is

observed when post N-treatment is applied on deposited B4C and La layers. This yields local BN and more

diffuse LaN formation, with some exchange of N between the layers. In effect, we greatly improved chemical

and optical contrast in La/B4C multilayers by applying N-plasma treatment after individual layer growth to create

BN and LaN layers that prevent formation of optically detrimental LaB6 interlayers. This technique will enable

the growth of La/B4C multilayers with a major increase in reflection maximum and bandwidth. From these early

and very initial results, it is evident that substantial further options for improvement of the optical performance of

such multilayer coatings are likely to exist. The nitridation is a first method to improve on multiayer performance

in the beyond EUV range.

ACKNOWLEDGEMENTS

This work is part of the FOM Industrial Partnership Programme I10 (‘XMO’) which is carried out under contract

with Carl Zeiss SMT AG, Oberkochen and the ‘Stichting voor Fundamenteel Onderzoek der Materie (FOM)’,

Proc. of SPIE Vol. 7271 72713V-7

the latter being financially supported by the ‘Nederlandse Organisatie voor Wetenschappelijk Onderzoek

(NWO)’.

REFERENCES

1 T. Tsarfati, E. Zoethout, R. W. E. van de Kruijs, F. Bijkerk, submitted.

2 T. Tsarfati, E. Zoethout, R. W. E. van de Kruijs, F. Bijkerk, J. Appl. Phys. (2009, accepted)

3 I. Nedelcu, R. W. E. van de Kruijs, A. E. Yakshin, F. Bijkerk, Appl. Opt. 48, 2, 155-160 (2009).

4 T. Tsarfati, E. Zoethout, R. W. E. van de Kruijs, F. Bijkerk, submitted.

5 T. Tsarfati, E. Zoethout, R. W. E. van de Kruijs, F. Bijkerk, Surf. Sci. (2009, accepted)

6 T. Tsarfati, E. Zoethout, R. W. E. van de Kruijs, F. Bijkerk, submitted.

7 A. Goldstein, Y. Yeshurun, A. Goldenberg, J. of the European Ceramic Society 27, 695–700 (2007).

8 S. Ordan’yan, O. Yurchenko, S. Vikhman, Russian J. of Appl. Chem. 78, 2, 333-335 (2005).

9 N. Ooi, V. Rajan, J. Gottlieb, Y. Catherine, and J.B. Adams, Modelling Simul. Mater. Sci. Eng. 14, 515-535 (2006).

10 I. Narita, T. Oku, Diamond and related materials 12, 1912-1917 (2003).

11 G. Vaitheeswaran, V. Kanchana, M. Rajagopalan, Solid State Communications 124, 97–102 (2002).

12 G. Kalpana, B. Palanivel, M. Rajagopalan, Phys. Rev. B 50, 12318 (1994).

13 G. Kalpana, B. Palanivel, M. Rajagopalan, Phys. Rev. B 52, 4 (1995).

14 E. Spiller, “Soft x-ray optics”, SPIE Optical Engineering Press (1994).

15 D. Windt. IMD - Software for Modeling the Optical Properties of Multilayers.

16 S. Tanuma, C. J. Powell, D.R. Penn, Surface and Interface Analysis 17 (1991).

17 D. L. Perry, S. L. Phillips, Handbook of Inorganic Compounds, Electronic Database.

18 C. Michaelsen, J. Wiesmann, R. Bormann, C. Nowak, C. Dieker, S. Hollensteiner, and W. Jäger, Optics. Lett. 26, 11

(2001).

19 J.-M. André, P. Jonnard, C. Michaelsen, J. Wiesmann, F. Bridou, M.-F. Ravet, A. Jérome, F. Delmotte, E. O.

Filatova, X-Ray Spectrom. 34, 203–206 (2005).

20 Y. Platonov, L. Gomez, D. Broadway, SPIE 12, 4782-152 (2002).

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