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EUV multilayer optics
Microelectronic Engineering 83 (2006) 703–706 www.elsevier.com/locate/mee
EUV multilayer optics Torsten Feigl *, Sergiy Yulin, Nicolas Benoit, Norbert Kaiser Fraunhofer Institut fu¨r Angewandte, Optik und Feinmechanik, Albert-Einstein-Strasse 7, D-07745 Jena, Germany Available online 30 January 2006
Abstract According to the optics requirements of an EUVL tool, the accurate deposition of high reflective and laterally graded multilayers on ultraprecise polished substrates can be regarded as one of the major challenges of EUV lithography development today. To meet these requirements, a new dc magnetron sputtering system NESSY and technologies to coat laterally graded EUV multilayers on curved optics were developed. The major characteristics of the deposition tool and results of sputtered multilayer optics are presented in this paper. 2006 Elsevier B.V. All rights reserved. Keywords: EUV; EUVL; Mo/Si; Optics; Multilayer mirrors; Broadband; Narrowband; Magnetron sputtering
1. Introduction The demand to enhance the optical resolution, to structure and observe ever smaller details, has pushed the way towards the EUV and soft X-rays. Induced mainly by the production of more powerful electronic circuits with the aid of projection lithography, optics developments in recent years can be characterized by the use of electromagnetic radiation with smaller wavelength. The good prospects of the EUV and soft X-rays for next generation lithography systems at a wavelength of k = 13.5 nm, microscopy in the ‘‘water window’’ (k = 2.3–4.4 nm), solar physics (k = 5–60 nm), spectroscopy, plasma diagnostics and EUV/soft X-ray laser research have led to considerable progress in the development of different multilayer optics. Since optical systems in the EUV/soft X-ray spectral region consist of several mirror elements a maximum reflectivity of each multilayer is essential for a high throughput of the system. Moreover, most applications of multilayer mirrors in EUVL requires also a long-term and thermal stability, often at elevated temperatures. This requirement is most important in the case of the first mirror of the illumination system close to the EUV source (C1) where a shorttime decrease of reflectivity is most probable. *
Corresponding author. Tel.: +49 3641 807 240; fax: +49 3641 807601. E-mail address: [email protected] (T. Feigl).
0167-9317/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.12.033
A serious drawback of multilayer coatings for their application in EUV optics is their limited range of reflectivity in the spectral range; the spectral FWHM (full width at half maximum) of typically 0.5 nm covers only a small part of the output of some EUV sources, e.g. the spectrum of a broadband Xe source [1]. In all cases where maximum peak reflectivity is not required, e.g. in EUV metrology, astronomy and microscopy, broadband mirrors provide useful applications. Whereas the tailoring of the spectral properties of optical components is developed to a high level and widely used for the hard X-ray range, the UV, VIS and IR, it is rarely used by now in the EUV range. However, some papers focus on the use of depth-graded multilayer mirrors in the hard Xray region [2] and in neutron optics [3]. So-called ‘‘supermirrors’’ with a broadband reflectivity have been developed on the basis of a period thickness variation by a power law [4]. Specially depth-graded multilayer mirrors [5] are used for telescopes, beam collimators, and X-ray scanners [6]. 2. Experimental setup The dc magnetron sputtering system NESSY (Fig. 1) is equipped with four rectangular magnetrons, 600 mm · 125 mm each. The maximum substrate diameter is about 650 mm. Two B 450 mm substrates or three B 300 mm substrates can be
704
T. Feigl et al. / Microelectronic Engineering 83 (2006) 703–706
3. Experimental results
Fig. 1. DC magnetron sputtering system NESSY.
coated simultaneously. The target-substrate-distance is variable and allows the installation of moving shutters to realize lateral thickness gradients of the sputtered multilayer. The system operates with a UHV load lock system (Fig. 2). The base pressure is well below 8 · 109 mbar. Special effort was made to construct the cathodes. Different configurations of the magnets were successfully realized in order to assure highest flexibility for different coating materials in terms of homogeneity requirements and target utilization. The lateral layer thickness distribution was optimized with specially formed shadowing masks fixed close to the cathodes. A homogeneity of ±0.1% on 150 mm and ±0.2% on 300 mm and a reflectivity of R > 68.5% @ 13.5 nm is routinely achieved with Mo/Si multilayers on both flat and curved substrates.
Mo/Si multilayers with different thin film designs were realized. Beside the maximization of the peak reflectivity using a periodic multilayer design, the maximization and minimization of the FWHM were designed and realized using special broadband and narrowband multilayer designs, respectively. Normal incidence reflection measurements were performed with synchrotron radiation at the PTB Berlin (BESSY II), Germany. All mirrors were measured in the wavelength range 12–15 nm with a wavelength resolution of 0.02 nm and an accuracy of DR = 0.5%. The incident angle of the beam was fixed at 1.5, the spot had a diameter of about 1.5 mm at the sample surface. Fig. 3 compares the measured reflectance of Mo/Si multilayer mirrors with a periodic, a broadband and a narrowband design. The main results of the optical characterization of the periodic, the broadband and the narrowband multilayer design are summarized in Table 1. 3.1. Broadband design A non-periodic design was used to obtain the broadband reflection in the EUV range. The design is based on a stochastic optimization process of all layer thicknesses. A desired spectral reflectivity R0(k) in the wavelength range between kmin and kmax is used as a so-called ‘‘target function’’. Numerical calculations are used to optimize the design of a multilayer stack by a stochastic variation of each layer thickness to minimize the deviation between
0.7
0.6
periodic design broadband design
reflectivity
0.5
narrowband design
0.4
0.3
0.2
0.1
0 12.5
13.01
3.51
4.0
14.5
15.01
5.5
wavelength, nm
Fig. 3. Measured EUV reflectivity of Mo/Si multilayers.
Table 1 Measured reflectivity, peak wavelength and FWHM of periodic, broadband and narrowband multilayer designs
Fig. 2. Substrate loading.
R (%) k (nm) FWHM (nm)
Periodic
Broadband
Narrowband
68.8 13.5 0.50
20 13–15 2.33
14.6 13.5 0.077
T. Feigl et al. / Microelectronic Engineering 83 (2006) 703–706
R0(k) and the reflectivity R(k) of the multilayer design. For this purpose, the merit function Z k max 2 MF ¼ ðRðkÞ R0 ðkÞÞ dk is minimized. For all calculations, we used a commercial thin film design program (SCI Film WizardTM). The optical constants of molybdenum (Mo) and silicon (Si) from the Henke tables [7] were imported to this program. Before the start of the calculation the number of layers and limits for the maximum and minimum film thickness have to be specified. The result of the optimization strongly depends as well on a reasonable choice for dmin and dmax as on a target function R0(k) with reflectivity values that are in an achievable range. A mirror with a constant (±1%) and as high as possible reflectivity in the wavelength range from 13 nm to 15 nm was designed. The design consists of 101 layers of Mo and Si in the thickness range between 2.8 nm and 4.5 nm. The optimized thickness distribution of this multilayer and the theoretical performance demonstrated in Fig. 4. An advantage of the non-periodic design is the possibility to provide constant reflectivity over wide spectral range. 3.2. Narrowband design It has been shown that the FWHM of Mo/Si multilayer mirrors can be adjusted by variation of the absorber layer thickness ratio C. Since this method is very limited the FWHM variation of Mo/Si multilayer mirrors was realized using high reflectance orders. Simulation results for Mo/Si multilayer mirrors with N = 50 periods used at normal incidence and optimized for maximum reflection at k = 13.5 nm are presented in Fig. 5. According to the Bragg condition (2d sin H ffi mk) for the application of high orders (m = 2,3. . .), the multilayer period d has to be increased. The FWHM off the reflection is reduced by a factor that is almost equivalent to the order
70 Mo/S, N=50 O
=1.5 60 (theory)
RINT = 54 Rp = 74.6 % FWHM=0.63 nm
Reflectivity, %
50 40
RINT = 79 R ~ 31 % FWHM=2.31 nm
30 20
80 70
1 st order 2 nd order 3 rd order 5 th order 10 th order
Mo/Si N=50
60
Reflectivity, %
k min
705
50 40 30 20 10 0 13.0
13.2
13.4
13.6
13.8
14.0
Wavelength, nm
Fig. 5. Theoretical performance of a Mo/Si multilayer with 50 periods optimized for high reflectivity in high reflection orders.
of reflection m. The calculated theoretical reflectivity of the multilayers decrease due to the higher absorption caused by the increasing overall thickness. However, the decrease in their bandwidths is much stronger. For example, in the 3rd reflection order the FWHM is reduced by a factor of 2.89 in comparison to the 1st order mirror, whereas the peak reflectivity is only reduced by a factor of 1.32 compared to the 1st order mirror. In high reflection orders a strong reduction of the FWHM can be achieved, e.g. in the 10th reflection order a reduction by a factor of almost 8 is possible. 4. Summary High reflective Mo/Si multilayer mirrors for different applications in the EUV wavelength range were designed, deposited by dc magnetron sputtering and characterized. The maximum near normal incidence reflectivity of R = 68.8% @ k = 13.45 nm were measured at the PTB reflectometer. To enhance the reflectivity and stability of the multilayer mirrors, further work will concentrate on optimized diffusion barrier layers for Mo/Si. Acknowledgments This work was financed by the German BMBF under contract No. 13N8405. The authors also thank Frank Scholze, Christian Laubis and Heike Wagner for the EUV reflectivity measurements at the Physikalisch-Technische Bundesanstalt Berlin. References
10 0 12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0
Wavelength, nm Fig. 4. Theoretical performance of a broadband multilayer design in comparison to a periodic multilayer mirror (dashed line).
[1] K. Bergmann, G. Schriever, O. Rosier, M. Mu¨ller, W. Neff, R. Lebert, Highly repetitive, extreme ultraviolet radiation source based on a gasdischarge plasma, Appl. Opt. 38 (1999) 5413–5417. [2] I.V. Kozhevnikov, I.N. Bukreeva, E. Ziegler, Design of X-ray supermirrors, Nucl. Instr. Meth. Phys. Res. A 460 (2001) 424–443. [3] P. Bo¨ni, Supermirror-based beam devices, Physica B 234–236 (1997) 1038–1043.
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T. Feigl et al. / Microelectronic Engineering 83 (2006) 703–706
[4] K.D. Joensen, P. Voutov, A. Szentgyorgyi, J. Roll, P. Gorenstein, P. Hoghoj, F.E. Christensen, Design of grazing-incidence multilayer supermirrors for hard X-ray reflectors, Appl. Opt. 34 (1995) 7935–7944. [5] Z. Wang, J. Cao, A.G. Michette, Depth-graded multilayer X-ray optics with broad angular response, Opt. Commun. 177 (2000) 25–32.
[6] V.V. Protopov, V.A. Kalnov, X-ray multilayer mirrors with an extended angular range, Opt. Commun. 158 (1998) 127–140. [7] B.L. Henke, E.M. Gullikson, J.C. Davis, X-ray interactions: photoabsorption, scattering, transmission, and reflection at E = 50–30,000 eV, Z = 1–92, Atom. Data Nucl. Data 54 (2) (1993) 181–342.
EUV multilayer optics Torsten Feigl *, Sergiy Yulin, Nicolas Benoit, Norbert Kaiser Fraunhofer Institut fu¨r Angewandte, Optik und Feinmechanik, Albert-Einstein-Strasse 7, D-07745 Jena, Germany Available online 30 January 2006
Abstract According to the optics requirements of an EUVL tool, the accurate deposition of high reflective and laterally graded multilayers on ultraprecise polished substrates can be regarded as one of the major challenges of EUV lithography development today. To meet these requirements, a new dc magnetron sputtering system NESSY and technologies to coat laterally graded EUV multilayers on curved optics were developed. The major characteristics of the deposition tool and results of sputtered multilayer optics are presented in this paper. 2006 Elsevier B.V. All rights reserved. Keywords: EUV; EUVL; Mo/Si; Optics; Multilayer mirrors; Broadband; Narrowband; Magnetron sputtering
1. Introduction The demand to enhance the optical resolution, to structure and observe ever smaller details, has pushed the way towards the EUV and soft X-rays. Induced mainly by the production of more powerful electronic circuits with the aid of projection lithography, optics developments in recent years can be characterized by the use of electromagnetic radiation with smaller wavelength. The good prospects of the EUV and soft X-rays for next generation lithography systems at a wavelength of k = 13.5 nm, microscopy in the ‘‘water window’’ (k = 2.3–4.4 nm), solar physics (k = 5–60 nm), spectroscopy, plasma diagnostics and EUV/soft X-ray laser research have led to considerable progress in the development of different multilayer optics. Since optical systems in the EUV/soft X-ray spectral region consist of several mirror elements a maximum reflectivity of each multilayer is essential for a high throughput of the system. Moreover, most applications of multilayer mirrors in EUVL requires also a long-term and thermal stability, often at elevated temperatures. This requirement is most important in the case of the first mirror of the illumination system close to the EUV source (C1) where a shorttime decrease of reflectivity is most probable. *
Corresponding author. Tel.: +49 3641 807 240; fax: +49 3641 807601. E-mail address: [email protected] (T. Feigl).
0167-9317/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.12.033
A serious drawback of multilayer coatings for their application in EUV optics is their limited range of reflectivity in the spectral range; the spectral FWHM (full width at half maximum) of typically 0.5 nm covers only a small part of the output of some EUV sources, e.g. the spectrum of a broadband Xe source [1]. In all cases where maximum peak reflectivity is not required, e.g. in EUV metrology, astronomy and microscopy, broadband mirrors provide useful applications. Whereas the tailoring of the spectral properties of optical components is developed to a high level and widely used for the hard X-ray range, the UV, VIS and IR, it is rarely used by now in the EUV range. However, some papers focus on the use of depth-graded multilayer mirrors in the hard Xray region [2] and in neutron optics [3]. So-called ‘‘supermirrors’’ with a broadband reflectivity have been developed on the basis of a period thickness variation by a power law [4]. Specially depth-graded multilayer mirrors [5] are used for telescopes, beam collimators, and X-ray scanners [6]. 2. Experimental setup The dc magnetron sputtering system NESSY (Fig. 1) is equipped with four rectangular magnetrons, 600 mm · 125 mm each. The maximum substrate diameter is about 650 mm. Two B 450 mm substrates or three B 300 mm substrates can be
704
T. Feigl et al. / Microelectronic Engineering 83 (2006) 703–706
3. Experimental results
Fig. 1. DC magnetron sputtering system NESSY.
coated simultaneously. The target-substrate-distance is variable and allows the installation of moving shutters to realize lateral thickness gradients of the sputtered multilayer. The system operates with a UHV load lock system (Fig. 2). The base pressure is well below 8 · 109 mbar. Special effort was made to construct the cathodes. Different configurations of the magnets were successfully realized in order to assure highest flexibility for different coating materials in terms of homogeneity requirements and target utilization. The lateral layer thickness distribution was optimized with specially formed shadowing masks fixed close to the cathodes. A homogeneity of ±0.1% on 150 mm and ±0.2% on 300 mm and a reflectivity of R > 68.5% @ 13.5 nm is routinely achieved with Mo/Si multilayers on both flat and curved substrates.
Mo/Si multilayers with different thin film designs were realized. Beside the maximization of the peak reflectivity using a periodic multilayer design, the maximization and minimization of the FWHM were designed and realized using special broadband and narrowband multilayer designs, respectively. Normal incidence reflection measurements were performed with synchrotron radiation at the PTB Berlin (BESSY II), Germany. All mirrors were measured in the wavelength range 12–15 nm with a wavelength resolution of 0.02 nm and an accuracy of DR = 0.5%. The incident angle of the beam was fixed at 1.5, the spot had a diameter of about 1.5 mm at the sample surface. Fig. 3 compares the measured reflectance of Mo/Si multilayer mirrors with a periodic, a broadband and a narrowband design. The main results of the optical characterization of the periodic, the broadband and the narrowband multilayer design are summarized in Table 1. 3.1. Broadband design A non-periodic design was used to obtain the broadband reflection in the EUV range. The design is based on a stochastic optimization process of all layer thicknesses. A desired spectral reflectivity R0(k) in the wavelength range between kmin and kmax is used as a so-called ‘‘target function’’. Numerical calculations are used to optimize the design of a multilayer stack by a stochastic variation of each layer thickness to minimize the deviation between
0.7
0.6
periodic design broadband design
reflectivity
0.5
narrowband design
0.4
0.3
0.2
0.1
0 12.5
13.01
3.51
4.0
14.5
15.01
5.5
wavelength, nm
Fig. 3. Measured EUV reflectivity of Mo/Si multilayers.
Table 1 Measured reflectivity, peak wavelength and FWHM of periodic, broadband and narrowband multilayer designs
Fig. 2. Substrate loading.
R (%) k (nm) FWHM (nm)
Periodic
Broadband
Narrowband
68.8 13.5 0.50
20 13–15 2.33
14.6 13.5 0.077
T. Feigl et al. / Microelectronic Engineering 83 (2006) 703–706
R0(k) and the reflectivity R(k) of the multilayer design. For this purpose, the merit function Z k max 2 MF ¼ ðRðkÞ R0 ðkÞÞ dk is minimized. For all calculations, we used a commercial thin film design program (SCI Film WizardTM). The optical constants of molybdenum (Mo) and silicon (Si) from the Henke tables [7] were imported to this program. Before the start of the calculation the number of layers and limits for the maximum and minimum film thickness have to be specified. The result of the optimization strongly depends as well on a reasonable choice for dmin and dmax as on a target function R0(k) with reflectivity values that are in an achievable range. A mirror with a constant (±1%) and as high as possible reflectivity in the wavelength range from 13 nm to 15 nm was designed. The design consists of 101 layers of Mo and Si in the thickness range between 2.8 nm and 4.5 nm. The optimized thickness distribution of this multilayer and the theoretical performance demonstrated in Fig. 4. An advantage of the non-periodic design is the possibility to provide constant reflectivity over wide spectral range. 3.2. Narrowband design It has been shown that the FWHM of Mo/Si multilayer mirrors can be adjusted by variation of the absorber layer thickness ratio C. Since this method is very limited the FWHM variation of Mo/Si multilayer mirrors was realized using high reflectance orders. Simulation results for Mo/Si multilayer mirrors with N = 50 periods used at normal incidence and optimized for maximum reflection at k = 13.5 nm are presented in Fig. 5. According to the Bragg condition (2d sin H ffi mk) for the application of high orders (m = 2,3. . .), the multilayer period d has to be increased. The FWHM off the reflection is reduced by a factor that is almost equivalent to the order
70 Mo/S, N=50 O
=1.5 60 (theory)
RINT = 54 Rp = 74.6 % FWHM=0.63 nm
Reflectivity, %
50 40
RINT = 79 R ~ 31 % FWHM=2.31 nm
30 20
80 70
1 st order 2 nd order 3 rd order 5 th order 10 th order
Mo/Si N=50
60
Reflectivity, %
k min
705
50 40 30 20 10 0 13.0
13.2
13.4
13.6
13.8
14.0
Wavelength, nm
Fig. 5. Theoretical performance of a Mo/Si multilayer with 50 periods optimized for high reflectivity in high reflection orders.
of reflection m. The calculated theoretical reflectivity of the multilayers decrease due to the higher absorption caused by the increasing overall thickness. However, the decrease in their bandwidths is much stronger. For example, in the 3rd reflection order the FWHM is reduced by a factor of 2.89 in comparison to the 1st order mirror, whereas the peak reflectivity is only reduced by a factor of 1.32 compared to the 1st order mirror. In high reflection orders a strong reduction of the FWHM can be achieved, e.g. in the 10th reflection order a reduction by a factor of almost 8 is possible. 4. Summary High reflective Mo/Si multilayer mirrors for different applications in the EUV wavelength range were designed, deposited by dc magnetron sputtering and characterized. The maximum near normal incidence reflectivity of R = 68.8% @ k = 13.45 nm were measured at the PTB reflectometer. To enhance the reflectivity and stability of the multilayer mirrors, further work will concentrate on optimized diffusion barrier layers for Mo/Si. Acknowledgments This work was financed by the German BMBF under contract No. 13N8405. The authors also thank Frank Scholze, Christian Laubis and Heike Wagner for the EUV reflectivity measurements at the Physikalisch-Technische Bundesanstalt Berlin. References
10 0 12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0
Wavelength, nm Fig. 4. Theoretical performance of a broadband multilayer design in comparison to a periodic multilayer mirror (dashed line).
[1] K. Bergmann, G. Schriever, O. Rosier, M. Mu¨ller, W. Neff, R. Lebert, Highly repetitive, extreme ultraviolet radiation source based on a gasdischarge plasma, Appl. Opt. 38 (1999) 5413–5417. [2] I.V. Kozhevnikov, I.N. Bukreeva, E. Ziegler, Design of X-ray supermirrors, Nucl. Instr. Meth. Phys. Res. A 460 (2001) 424–443. [3] P. Bo¨ni, Supermirror-based beam devices, Physica B 234–236 (1997) 1038–1043.
706
T. Feigl et al. / Microelectronic Engineering 83 (2006) 703–706
[4] K.D. Joensen, P. Voutov, A. Szentgyorgyi, J. Roll, P. Gorenstein, P. Hoghoj, F.E. Christensen, Design of grazing-incidence multilayer supermirrors for hard X-ray reflectors, Appl. Opt. 34 (1995) 7935–7944. [5] Z. Wang, J. Cao, A.G. Michette, Depth-graded multilayer X-ray optics with broad angular response, Opt. Commun. 177 (2000) 25–32.
[6] V.V. Protopov, V.A. Kalnov, X-ray multilayer mirrors with an extended angular range, Opt. Commun. 158 (1998) 127–140. [7] B.L. Henke, E.M. Gullikson, J.C. Davis, X-ray interactions: photoabsorption, scattering, transmission, and reflection at E = 50–30,000 eV, Z = 1–92, Atom. Data Nucl. Data 54 (2) (1993) 181–342.