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Sc nanolayers for the water window: improved performance
Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 349–353
Cr/Sc nanolayers for the water window: improved performance F. Sch.afersa,*, M. Mertina, D. Abramsohna, A. Gauppa, H.-Ch. Mertinsa, N.N. Salashchenkob a b
BESSY GmbH, Albert-Einstein-Str. 15, D-12489 Berlin, Germany Institute for Physics of Microstructures, Nizhny Novgorod, Russia
Abstract New performance data for small-period Cr/Sc multi(nano-)layers obtained with polarised synchrotron radiation are shown. The sputter-deposited multilayers were optimised for use either in normal incidence or close to 458 for polarimetry applications at photon energies around the 2p-absorption edges of Sc (399 eV) or Cr (574 eV), respectively. Due to improved stabilisation of sputter sources the reflectance, polarisance and phase retardation could be improved by up to 50% compared to results obtained three years ago. # 2001 Elsevier Science B.V. All rights reserved. PACS: 07.85; 41.50; 42.79; 78.66 Keywords: Multilayer; Reflectance; Transmission; Polarisation; Synchrotron radiation; Soft X-rays
1. Introduction In the last few years new multilayer structures on the basis of the 3d-transition metals Ti, V [1,2], Sc and Cr [3–6] and others [7], have been produced for operation in the water window range, between the C and the O k-edges. It turned out that a good performance can be reached in the vicinity of the 2p-absorption edges, where the effect of anomalous dispersion is exploited for a resonantly enhanced reflectance [2,8,9]. Normal incidence optics with reflectancies up to 10% at certain energies have now become feasible by this technique using nanostructures with individual
*Corresponding author. Tel.: +49-30-6392-2946; fax: +4030-6392-2990. E-mail address: [email protected] (F. Scha¨fers).
layer thicknesses below 1 nm [8,9]. Furthermore, polarisation sensitive optical elements were developed for operation as linear polarisers and phase plates in a polarisation detector [10,11]. The combination Cr/Sc gave best results in terms of polarising power Rs =Rp and phase retardation D ¼ ds dp at both the absorption edges of Sc (400 eV) and Cr (575 eV) [11]. Here considerably improved performance data of Cr/Sc are presented which were made possible by improved control of sputter conditions during manufacturing.
2. Experimental New Cr/Sc samples were investigated using the BESSY-reflectometer [8] at the BESSY I monochromator PM 4 and using the polarimeter [11] at the BESSY II elliptical undulator UE56/2 PGM-2
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 3 2 2 - 9
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F. Scha¨fers et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 349–353
beamline [12]. This beamline delivers high brightness soft X-ray radiation with a spectral purity E=DE of approximately 3000 and with a polarisation which is tuneable between linear (horizontal or vertical) and circular (left- or right-handed). The experimental setup enabled measurements of the reflected signal as function of the photon energy, as function of the incidence angle (y22y scans) or a combination of both to follow the Bragg-reflection peak with energy (Bragg-scan). The sample can be azimuthally rotated for measurements in s- or p-polarisation geometry. The phase-retarding properties of the transmission multilayers were determined by a complete selfcalibrating polarisation measurement [11]. The samples have been magnetron-sputtered under Ar at improved stabilised conditions on Si-wafers and on 120 nm Si3N4membranes, respectively. Details are described in Ref. [8]. The main improvements have been achieved by using denser targets of Sc and Cr, which allowed to lower the Ar-pressure down to 7 104 mbar (previously: 103 mbar). This essentially resulted in a more stable discharge and reduced the impurities. A further improvement concerned the stabilisation of the DC and RF supply units to allow for a stable deposition process.
3. Results The figures give an impression of the performance. Fig. 1 shows the near-normal incidence reflectance as function of the photon energy close to the Sc 2p edge. The maximum reflectance at the Bragg-peak at 394 eV is 11% at an angle of 858. The resolving power DE of 1.6 eV corresponds to the theoretical limit given by the number of periods of the multilayer ðE=DE ¼ 245Þ. The Kiessig fringes can be seen both in experiment and calculation, but only at the low energy side of the Bragg-peak due to the 2p absorption structure around 400 eV. Fig. 2 gives the reflectance results for the same multilayer at a fixed energy of 572 eV, close to the Cr 2p edge (see bottom inset), obtained as y22y scan from the total reflection region 08 to 558. The Bragg-peak, taken in s- and p-polarisation geometry, appears close to the Brewster angle ðy ¼ 458Þ, the optimum angle for a linear polariser. The polarising power at this energy is Rs =Rp ¼ 300. Despite of the large number of periods (245) the Kiessig fringes are clearly resolved which indicates the homogeneity of the multilayer stack. For clarity this Braggpeak region is blown up in the top inset. A fit (full curve) delivers a roughness of 0.275 nm.
Fig. 1. Near-normal incidence reflectance of a d ¼ 1:55 nm Cr/Sc multilayer close to the Sc 2p edge in comparison with calculation. The inset shows the peak reflectance obtained by a y 2y scan across the resonance region.
F. Scha¨fers et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 349–353
351
Fig. 2. Reflectance of Cr/Sc multilayer in s- and p-polarisation geometry as function of incidence angle (y22y scan) at 572 eV, close to the Cr 2p edge: measurement (points) and calculation (full curves). (The bottom inset shows the peak reflectance across the resonance). The region around the Bragg-angle at 438 is blown up in the top inset. A fit (full curve) yields a roughness of 0.275 nm. The resonant feature around 208 (not present in the calculation) corresponds to the 2nd-order Bragg-resonance from 4th-order contribution in the incident light beam. It’s shape indicates that d(Cr)>d(Sc).
Fig. 3. Polarising properties of Cr/Sc multilayer. Azimuthal scan at fixed incidence angles of 858 (394 eV) and 438 (571.7 eV), respectively, with incident linearly polarised light ðPlin ¼ 0:99Þ. s-polarisation geometry corresponds to b ¼ 08 (1808), p-geometry to b ¼ 908 (2708). The polarising power is Rs =Rp ¼ 300 close to the Brewster angle.
Resonances around 208, 308 and 358 of which the shape is determined by the Cr–Sc thickness ratio, appear due to higher order contributions in the incident light. These higher orders, however, do not impact the accuracy of the measured Bragg
peak reflectance which is three orders of magnitude lager. Fig. 3 shows the polarising properties of this multilayer obtained at the same two energies and incidence angles of Figs. 1 and 2. Reflection
352
F. Scha¨fers et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 349–353
Fig. 4. Progress in multilayer performance: Peak reflectance of Cr/Sc multilayers as function of the photon energy close to the Sc (left side) and the Cr 2p (right side) edges.
in s-polarisation geometry corresponds to an azimuth angle of b ¼ 08 (1808), p-geometry corresponds to b ¼ 908 (2708). As expected in normal incidence ðy ¼ 858Þ the reflection is insensitive to polarisation, while the polarising power is maximum close to the Brewster angle ðy ¼ 438Þ. Fig. 4 shows the progress in peak reflectance as function of the photon energy close to the Sc and Cr 2p edges for multilayers with different periods. The maximum reflectance is now 30% at 458 at 400 eV and 11% at 570 eV. This is a factor of two more than that obtained three years ago at similar samples [8], making measurements of the linear polarisation of light at these energies possible. A similar improvement in performance was obtained for a Cr/Sc phase plate. Such a multilayer is operated in transmission and it is used in a polarimeter as first optical element together with a linear polariser. The phase retarding property as function of the incidence angle is seen in Fig. 5: The phase retardation has a maximum value of D ¼ 308 at a photon energy of 399 eV, right below the Sc-edge (see inset). It changes sign at y ¼ 30:68 which corresponds to the Bragg-angle. These results are 50% higher than those obtained 3 years ago [11] and these data are thus a big step towards a quarter-wave plate for the soft X-ray
Fig. 5. Cr/Sc phase plate: Phase retardation D ¼ ds dp as function of the incidence angle. The energy of 399 eV is within the Sc 2p absorption edge (see inset).
range enabling measurements of the complete polarisation state of soft X-ray light.
4. Conclusion New Cr/Sc multilayer samples with periods in the 1–2 nm range (nanolayers) were produced under improved sputter conditions and at-wavelength tests were done using tuneable polarised synchrotron radiation in the water window range. At the 2p absorption edges of Cr and Sc a resonantly enhanced performance is observed in
F. Scha¨fers et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 349–353
terms of reflectance, polarisance and phase retardation. With a normal incidence reflectance of 11% at 400 eV this multilayer belongs to the best mirrors studied so far making normal incidence optics at this energy feasible. Cr/Sc-multilayers operated in reflection and in transmission enable to extend the classical polarisation analysis using primary optical standards (polarisers and analysers) to the soft X-ray range.
Acknowledgements This project was supported by the European Community, contract ERBFMGECT980105.
References [1] H. Grimmer, O. Zaharko, M. Horisberger, H.-Ch. Mertins, F. Sch.afers, U. Staub, SPIE Proc. 3773 (1999) 224–235.
353
[2] H.-Ch. Mertins, F. Sch.afers, H. Grimmer, D. Clemens, . P. Boni, M. Horisberger, Appl. Opt. 37 (1998) 1873–1882. [3] N.N. Salashchenko, Yu.Ya. Platonov, S.Yu. Zuev, Nucl. Instr. and Meth. A 359 (1995) 114–120. [4] N.N. Salashchenko, S.V. Gaponov et al., Proc. SPIE 2011 (1994) 402–412. [5] M. Yamamoto, K. Sakano, Y. Hotta, Inst. Phys. Conf. Ser. 159 (1999) 601–604. [6] K. Sakano, M. Yamamoto, Proc. SPIE 3767 (1999) 238–241. [7] C. Montcolm, P.A. Kearney et al., Appl. Opt. 35 (1996) 5134–5147. [8] F. Sch.afers, H.-Ch. Mertins, F. Schmolla, I. Packe, N.N. Salashchenko, E.A. Shamov, Appl. Opt. 37 (1998) 719–728. [9] F. Sch.afers, Physica B 283 (2000) 119–124. . [10] S. Di Fonzo, B.R. Muller, W. Jark, A. Gaupp et al., Rev. Sci. Instr. 66 (1995) 1513–1516. [11] F. Sch.afers, H.Ch. Mertins, A. Gaupp et al., Appl. Opt. 38 (1999) 4074–4088. [12] M. Weiss, K.J.S. Sawhney et al., Proceedings of the SRI, Stanford, 1999.
Cr/Sc nanolayers for the water window: improved performance F. Sch.afersa,*, M. Mertina, D. Abramsohna, A. Gauppa, H.-Ch. Mertinsa, N.N. Salashchenkob a b
BESSY GmbH, Albert-Einstein-Str. 15, D-12489 Berlin, Germany Institute for Physics of Microstructures, Nizhny Novgorod, Russia
Abstract New performance data for small-period Cr/Sc multi(nano-)layers obtained with polarised synchrotron radiation are shown. The sputter-deposited multilayers were optimised for use either in normal incidence or close to 458 for polarimetry applications at photon energies around the 2p-absorption edges of Sc (399 eV) or Cr (574 eV), respectively. Due to improved stabilisation of sputter sources the reflectance, polarisance and phase retardation could be improved by up to 50% compared to results obtained three years ago. # 2001 Elsevier Science B.V. All rights reserved. PACS: 07.85; 41.50; 42.79; 78.66 Keywords: Multilayer; Reflectance; Transmission; Polarisation; Synchrotron radiation; Soft X-rays
1. Introduction In the last few years new multilayer structures on the basis of the 3d-transition metals Ti, V [1,2], Sc and Cr [3–6] and others [7], have been produced for operation in the water window range, between the C and the O k-edges. It turned out that a good performance can be reached in the vicinity of the 2p-absorption edges, where the effect of anomalous dispersion is exploited for a resonantly enhanced reflectance [2,8,9]. Normal incidence optics with reflectancies up to 10% at certain energies have now become feasible by this technique using nanostructures with individual
*Corresponding author. Tel.: +49-30-6392-2946; fax: +4030-6392-2990. E-mail address: [email protected] (F. Scha¨fers).
layer thicknesses below 1 nm [8,9]. Furthermore, polarisation sensitive optical elements were developed for operation as linear polarisers and phase plates in a polarisation detector [10,11]. The combination Cr/Sc gave best results in terms of polarising power Rs =Rp and phase retardation D ¼ ds dp at both the absorption edges of Sc (400 eV) and Cr (575 eV) [11]. Here considerably improved performance data of Cr/Sc are presented which were made possible by improved control of sputter conditions during manufacturing.
2. Experimental New Cr/Sc samples were investigated using the BESSY-reflectometer [8] at the BESSY I monochromator PM 4 and using the polarimeter [11] at the BESSY II elliptical undulator UE56/2 PGM-2
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 3 2 2 - 9
350
F. Scha¨fers et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 349–353
beamline [12]. This beamline delivers high brightness soft X-ray radiation with a spectral purity E=DE of approximately 3000 and with a polarisation which is tuneable between linear (horizontal or vertical) and circular (left- or right-handed). The experimental setup enabled measurements of the reflected signal as function of the photon energy, as function of the incidence angle (y22y scans) or a combination of both to follow the Bragg-reflection peak with energy (Bragg-scan). The sample can be azimuthally rotated for measurements in s- or p-polarisation geometry. The phase-retarding properties of the transmission multilayers were determined by a complete selfcalibrating polarisation measurement [11]. The samples have been magnetron-sputtered under Ar at improved stabilised conditions on Si-wafers and on 120 nm Si3N4membranes, respectively. Details are described in Ref. [8]. The main improvements have been achieved by using denser targets of Sc and Cr, which allowed to lower the Ar-pressure down to 7 104 mbar (previously: 103 mbar). This essentially resulted in a more stable discharge and reduced the impurities. A further improvement concerned the stabilisation of the DC and RF supply units to allow for a stable deposition process.
3. Results The figures give an impression of the performance. Fig. 1 shows the near-normal incidence reflectance as function of the photon energy close to the Sc 2p edge. The maximum reflectance at the Bragg-peak at 394 eV is 11% at an angle of 858. The resolving power DE of 1.6 eV corresponds to the theoretical limit given by the number of periods of the multilayer ðE=DE ¼ 245Þ. The Kiessig fringes can be seen both in experiment and calculation, but only at the low energy side of the Bragg-peak due to the 2p absorption structure around 400 eV. Fig. 2 gives the reflectance results for the same multilayer at a fixed energy of 572 eV, close to the Cr 2p edge (see bottom inset), obtained as y22y scan from the total reflection region 08 to 558. The Bragg-peak, taken in s- and p-polarisation geometry, appears close to the Brewster angle ðy ¼ 458Þ, the optimum angle for a linear polariser. The polarising power at this energy is Rs =Rp ¼ 300. Despite of the large number of periods (245) the Kiessig fringes are clearly resolved which indicates the homogeneity of the multilayer stack. For clarity this Braggpeak region is blown up in the top inset. A fit (full curve) delivers a roughness of 0.275 nm.
Fig. 1. Near-normal incidence reflectance of a d ¼ 1:55 nm Cr/Sc multilayer close to the Sc 2p edge in comparison with calculation. The inset shows the peak reflectance obtained by a y 2y scan across the resonance region.
F. Scha¨fers et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 349–353
351
Fig. 2. Reflectance of Cr/Sc multilayer in s- and p-polarisation geometry as function of incidence angle (y22y scan) at 572 eV, close to the Cr 2p edge: measurement (points) and calculation (full curves). (The bottom inset shows the peak reflectance across the resonance). The region around the Bragg-angle at 438 is blown up in the top inset. A fit (full curve) yields a roughness of 0.275 nm. The resonant feature around 208 (not present in the calculation) corresponds to the 2nd-order Bragg-resonance from 4th-order contribution in the incident light beam. It’s shape indicates that d(Cr)>d(Sc).
Fig. 3. Polarising properties of Cr/Sc multilayer. Azimuthal scan at fixed incidence angles of 858 (394 eV) and 438 (571.7 eV), respectively, with incident linearly polarised light ðPlin ¼ 0:99Þ. s-polarisation geometry corresponds to b ¼ 08 (1808), p-geometry to b ¼ 908 (2708). The polarising power is Rs =Rp ¼ 300 close to the Brewster angle.
Resonances around 208, 308 and 358 of which the shape is determined by the Cr–Sc thickness ratio, appear due to higher order contributions in the incident light. These higher orders, however, do not impact the accuracy of the measured Bragg
peak reflectance which is three orders of magnitude lager. Fig. 3 shows the polarising properties of this multilayer obtained at the same two energies and incidence angles of Figs. 1 and 2. Reflection
352
F. Scha¨fers et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 349–353
Fig. 4. Progress in multilayer performance: Peak reflectance of Cr/Sc multilayers as function of the photon energy close to the Sc (left side) and the Cr 2p (right side) edges.
in s-polarisation geometry corresponds to an azimuth angle of b ¼ 08 (1808), p-geometry corresponds to b ¼ 908 (2708). As expected in normal incidence ðy ¼ 858Þ the reflection is insensitive to polarisation, while the polarising power is maximum close to the Brewster angle ðy ¼ 438Þ. Fig. 4 shows the progress in peak reflectance as function of the photon energy close to the Sc and Cr 2p edges for multilayers with different periods. The maximum reflectance is now 30% at 458 at 400 eV and 11% at 570 eV. This is a factor of two more than that obtained three years ago at similar samples [8], making measurements of the linear polarisation of light at these energies possible. A similar improvement in performance was obtained for a Cr/Sc phase plate. Such a multilayer is operated in transmission and it is used in a polarimeter as first optical element together with a linear polariser. The phase retarding property as function of the incidence angle is seen in Fig. 5: The phase retardation has a maximum value of D ¼ 308 at a photon energy of 399 eV, right below the Sc-edge (see inset). It changes sign at y ¼ 30:68 which corresponds to the Bragg-angle. These results are 50% higher than those obtained 3 years ago [11] and these data are thus a big step towards a quarter-wave plate for the soft X-ray
Fig. 5. Cr/Sc phase plate: Phase retardation D ¼ ds dp as function of the incidence angle. The energy of 399 eV is within the Sc 2p absorption edge (see inset).
range enabling measurements of the complete polarisation state of soft X-ray light.
4. Conclusion New Cr/Sc multilayer samples with periods in the 1–2 nm range (nanolayers) were produced under improved sputter conditions and at-wavelength tests were done using tuneable polarised synchrotron radiation in the water window range. At the 2p absorption edges of Cr and Sc a resonantly enhanced performance is observed in
F. Scha¨fers et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 349–353
terms of reflectance, polarisance and phase retardation. With a normal incidence reflectance of 11% at 400 eV this multilayer belongs to the best mirrors studied so far making normal incidence optics at this energy feasible. Cr/Sc-multilayers operated in reflection and in transmission enable to extend the classical polarisation analysis using primary optical standards (polarisers and analysers) to the soft X-ray range.
Acknowledgements This project was supported by the European Community, contract ERBFMGECT980105.
References [1] H. Grimmer, O. Zaharko, M. Horisberger, H.-Ch. Mertins, F. Sch.afers, U. Staub, SPIE Proc. 3773 (1999) 224–235.
353
[2] H.-Ch. Mertins, F. Sch.afers, H. Grimmer, D. Clemens, . P. Boni, M. Horisberger, Appl. Opt. 37 (1998) 1873–1882. [3] N.N. Salashchenko, Yu.Ya. Platonov, S.Yu. Zuev, Nucl. Instr. and Meth. A 359 (1995) 114–120. [4] N.N. Salashchenko, S.V. Gaponov et al., Proc. SPIE 2011 (1994) 402–412. [5] M. Yamamoto, K. Sakano, Y. Hotta, Inst. Phys. Conf. Ser. 159 (1999) 601–604. [6] K. Sakano, M. Yamamoto, Proc. SPIE 3767 (1999) 238–241. [7] C. Montcolm, P.A. Kearney et al., Appl. Opt. 35 (1996) 5134–5147. [8] F. Sch.afers, H.-Ch. Mertins, F. Schmolla, I. Packe, N.N. Salashchenko, E.A. Shamov, Appl. Opt. 37 (1998) 719–728. [9] F. Sch.afers, Physica B 283 (2000) 119–124. . [10] S. Di Fonzo, B.R. Muller, W. Jark, A. Gaupp et al., Rev. Sci. Instr. 66 (1995) 1513–1516. [11] F. Sch.afers, H.Ch. Mertins, A. Gaupp et al., Appl. Opt. 38 (1999) 4074–4088. [12] M. Weiss, K.J.S. Sawhney et al., Proceedings of the SRI, Stanford, 1999.