wavelength dependence of carbon contamination on mirrors with different capping layers

Wavelength dependence of carbon contamination on mirrors with different capping layers

Petros Thomas1, Leonid Yankulin1, Yashdeep Khopkar1, Rashi Garg1, Chimaobi Mbanaso1, Alin Antohe1, Yu-Jen Fan1, Gregory Denbeaux1, Samir Aouadi2, Vibhu Jindal3, Andrea

Wüest3

1College of Nanoscale Science and Engineering, University at Albany, NY, 2Southern Illinois University at Carbondale, IL , 3SEMATECH, Albany, NY

Keywords: EUV Lithography, carbon contamination, capping layers, Ruthinium, TiO2, ZrO2, wavelength dependence of contamination

Abstract

Optics contamination remains one of the challenges in extreme ultraviolet (EUV) lithography. In addition to the desired wavelength near 13.5 nm (EUV), plasma sources used in EUV exposure tools emit a wide range of out-of-band (OOB) wavelengths extending as far as the visible region. We present experimental results of contamination rates of EUV and OOB light using a Xe plasma source and filters. Employing heated carbon tape as a source of hydrocarbons, we have measured the wavelength dependence of carbon contamination on a Ru-capped mirror. These results are compared to contamination rates on TiO2 and ZrO2 capping layers.

1. Introduction

Contamination of EUV optics by the interaction of radiation with residual hydrocarbons is still one of the major concerns with EUV exposure tools [1-3, 7-12]. It is well known that the carbonaceous contamination on the surfaces of EUV mirrors is caused only in the region of the mirror exposed to light. The contamination layer grows on the surface from the dissociation of residual hydrocarbons in this light-exposed region. However, whether the dissociation of the hydrocarbons is predominantly photon induced [1] or secondary electron induced [2] is still unknown. Both the photoabsorption cross section of a typical hydrocarbon and the calculated secondary electron yield for a carbon-contaminated surface show significantly lower values near the EUV wavelength (11–17 nm) than the out-of-band wavelength (>17 nm) with peak values near 60 nm [3]. Whether the dissociation of hydrocarbons is dominated by photons or secondary electrons, the rate of dissociation of hydrocarbons is higher due to out-of-band light than EUV light, which could imply a faster contamination rate for out-of-band than EUV radiation.

Extreme Ultraviolet (EUV) Lithography, edited by Bruno M. La Fontaine, Proc. of SPIE Vol. 7636,76361X · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.847015

Proc. of SPIE Vol. 7636 76361X-1

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ranges were further selected using Xe and Ne gases in the beam line. Table 1 describes the filters and their corresponding wavelength ranges used in our experiment.

Table 1 Spectral filters used in the experiments and their corresponding wavelength range

Filters Wavelength range (nm)

Zr 11–17

Al 17–80

Al + Xe gas 17–40

Al + Ne gas 57–80

The contaminant species was the vapor of a heated carbon tape admitted into the main chamber through an aperture. The carbon tape is a double sided tape from SPI® which is typically used for mounting samples in scanning electron microscopes. Heated carbon tape was chosen as a contaminant species after it was found to have a faster contamination rate than the other species we tested [4]. During exposure with the heated carbon tape, the main chamber pressure was in the range of low 10-4 Torr to mid 10-5 Torr (the base pressure of the chamber was in the low 10-7 Torr). Pressures were measured using an ion gauge from MKS.

The Light intensity in the sample plane was measured every 2 mm along two axes (i.e., an areal map of the intensity) with an AXUV-20 photodiode from International Radiation Detectors. The measured intensities for EUV (11–17 nm) and out-of-band radiation (17–80 nm) were less than 1 mW/cm2 and 0.2 mW/cm2, respectively. The typical exposure times were between 3 to 5 hours.

The samples used in these contamination experiments are sputter-deposited Ru, TiO2, and ZrO2–capped on single Si/Mo layer from Southern Illinois University at Carbondale.

The areal map of carbonaceous contamination thickness on the samples was analyzed using X-ray photoelectron spectroscopy (XPS) (every 2 mm along two axes) with an analysis spot size of 600 microns. To determine the carbon thickness from the XPS data, we used the attenuation of the Ru 3d-5/2 electrons (in Ru-capped samples) through the carbon film relative to the unexposed point on the sample (carbon thickness = - λsin(θ)Ln(I/I0), where λ is the attenuation length [6], θ is take-off angle, I is the intensity of the contaminated region and I0 is the intensity of unexposed region). For TiO2- and ZrO2-capped samples, we used the attenuation of Ti 2p-3/2 and Zr 3d-5/2 electrons, respectively.

The contamination rates were determined by dividing point by point the measured carbonaceous thickness by the measured dose (see representative data in Fig. 2).



Fig. 2 Areal m

3.1 Com

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Fig 5 showsperformed bsample planin two axevariations, contaminaticapping layRu-capped had a slight

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4.1 Dose m

The photodTechnologyFor the 17–

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s the contamby exposing ane. The capabs guaranteedto compare on on the saers experiencand TiO2-caply higher rate

Fig. 5 Contami

4

measurement

iode used to y (NIST). For t80 nm wavele

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4. Discuss

t

measure the the 11–17 nmength, the me

us contamin

on three cap sample and

nslation stageaneous plac

of contaminatmeasured usie contaminatios exhibited the

Ru, TiO2, and

ions of our

intensity of ligm wavelength,easured quan

nation rates o

pping layers: either a TiO

e to scan the lement of twtion. After exng XPS. Withon rate from Ee same conta

ZrO2-capped m

assumptio

ght was calibr, the measure

ntum efficienc

of three diffe

Ru, TiO2, anO2 or ZrO2-cap

light intensity wo samples,

xposure, the hin the error EUV radiation

amination rate

mirrors using E

ons and erro

rated at the Ned quantum ecy for 40 nm w

erent capping

nd ZrO2. Thespped sample on the sampeach with kthickness o

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EUV and out-of

or estimates

National Instituefficiency for 1was used. Due

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quantum efficiency across the wavelength range, these assumptions can cause up to a 10% error in the dose measurement.

The experimental setup included a shallow angle mirror (~10o) that reflects the light towards the sample plane. Because this mirror was contaminated, the intensity of light reaching the sample changes during an exposure. We took an average of the intensity measurements before and after each exposure. As a result of this linearity assumption, we estimate an error of about 15%.

4.2 Thickness measurement

Using XPS, thickness measurements at several points on a given sample gave an error of up to 1 nm due to sample tilt. We have reduced this error to below 0.5 nm by fitting the data and subtracting out the slope from the sample tilt.

Systematic thickness measurement errors that could change the measured thickness can be ignored in all the samples when comparing different wavelengths, capping layers, and intensities. For example, the error due to the assumption of electron attenuation length from the inelastic mean free path calculation (IMFP) for XPS analysis is about 15–30% and the assumption of density for the carbonaceous film to calculate IMFP [6] could be off by a factor of 2 to 3.

5. Conclusion

We have experimentally measured carbonaceous contamination rates resulting from EUV (11–17 nm) and out-of-band (17–80 nm) radiation on Ru-capped Si/Mo single layer mirrors. The contamination rate per dose from out-of-band light is 7.4 times higher than from EUV light; assuming an average photon energy of 92 eV for EUV and 31 eV for out-of-band gives about a 2.6 times higher contamination rate per photon with the out-of-band light. We also measured the contamination rates on three different capped Si/Mo single layer mirrors. Using EUV radiation, we found no difference in the contamination rate on Ru, TiO2, and ZrO2-capped mirrors. For the out-of-band radiation, the contamination rates on Ru and TiO2 were the same while ZrO2 had a slightly higher contamination rate. Moreover, in the range of our intensity level (less than 1 mW/cm2 and less than 0.2 mW/ cm2 for out-of-band), the carbonaceous contamination thickness was found to be linear with intensity.

The authors would like to acknowledge very useful suggestions from B. V. Yakshinskiy (Rutgers, The State University of New Jersey) on XPS analysis for carbon thickness measurements on Ru and TiO2 surfaces.

6. References

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[2] J. Hollenshead and L. Klebanoff, J. Vac. Sci. Technol. B, 24, 64 (2006).



[3] V. Jindal, R. Garg, G. Denbeaux, and A. Wuest, Proc.SPIE 7271, 72713Q (2009).

[4] L. Yankulin, MS thesis, CNSE, University at Albany, SUNY (2009).

[5] Database of optical constants in the EUV at http://henke.lbl.gov/optical_constants, maintained by the Center for X-ray Optics, Lawrence Berkeley National Laboratory (http://www-cxro.lbl.gov).

[6] S. Tanuma, C. J. Powell and D. R. Penn, Surface and Interface Analysis 11, 577 (1988).

[7] B. Mertens, B. Wolschrijn, R. Jansen, N. Koster, M. Weiss, M. Wedowski, R. Klein, R. Bock, and R. Thornagel, Proc. SPIE 5037, 95 – 102 (2003).

[8] Y. Fan et al., Proc. SPIE, Vol. 7271, 72713U (2009).

[9] Y. Fan et al., Journal of Vacuum Science and Technology B 28 (2), (2010).

[10] A. Barty and K.A. Goldberg, Proc. SPIE 5037, 450–459 (2003).

[11] S. Matsunari, T. Aoki, K. Murakami, Y. Gomei, S. Terashima, H. Takase, M. Tanabe, Y. Watanabe, Y. Kakutani, M. Niibe, and Y. Fukuda, Proc. SPIE 6517, 6512X (2007).

[12] B.V. Yakshinskiy, I. Nishiyama, A. Wuest, and T.E. Madey, Proc. SPIE 6921, 69213E (2008).



wavelength dependence of carbon contamination on mirrors with different capping layers

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