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Characterization of multilayer reflectors for the soft X-ray region using synchrotron radiation
Vacuum/volume 41/numbers 4-6/pages 1234 to 1236/1990 Printed in Great Britain
0042-207X/9053.00 + .00 © 1990 Pergamon Press plc
Characterization of multilayer reflectors for the soft X-ray region using synchrotron radiation M S a k u r a i a n d J Fujita, National Institute for Fusion Science, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan K Yamashita, M Ohtani, I H a t s u k a d e Osaka 560, Japan
a n d K Tamura, Department of Physics, Osaka University, Toyonaka,
and H Nagata,
Y Suzuki and S Seki, Japan Research and Development Corporation, Tokyo 100, Japan
Characterizations of multilayer reflectors were performed in the wavelength range 3 - l O n m using synchrotron radiation. Layer spacing of the multilayers, which were fabricated by the electron beam vapour deposition method, exists in the range 6 - 1 0 nm. Evaluation of multilayers was carried out by measuring the reflectivity, the wavelength resolution and the Bragg angle on the beam line for radiometric calibration (BL5B) in the UVSOR facility. The incident photon beam was monochromatized by a plane grating monochrometer. Reflectivity was measured for both s and p components using a demountable rotational stage. The peak reflectivity was found to be in the range of 1-10%, and the reflectivities of both polarization components showed the most pronounced difference at an incidence angle of 45 °, which proves that the multilayer is also useful as an X-ray polarimeter.
1. Introduction In recent years, multilayer reflectors for X-ray optics have been developed, since normal incidence optic system are now attainable for soft X-ray regions ~-3. Multilayer reflectors consist of alternate layers of low and high Z elements with constant thickness. They are not only useful for diffracting elements, but are expected to be useful in soft X-ray polarimeters. They could be applied to X-ray optic systems for plasma diagnostics, X-ray beam line optics for synchrotron radiation (SR) 4 and X-ray telescopes and microscopes 5. Characterization of the reflectivity and resolution is necessary for the development of multilayers, and the estimation of throughput of the optical systems. For plasma diagnostics, calibration of the diffracting components provides means for absolute measurements of X-ray emissions from plasma 6. For the characterization of fabricated multilayers, SR is a most suitable source to measure the reflectivity and polarizability because of its continuous spectral distribution in the X-ray to vuv range and almost linearly polarized nature. In the present paper, we report on the whole apparatus designed for the evaluation of optical components using monochromatized SR, and give some results on the reflectivity measurements of multilayers in the wavelength range of 3-10 nm.
2. Experimental The experimental setup is shown in Figure 1. It is installed on the beam line BL5B of UVSOR, Institute for Molecular Sci-
1234
CaliOration chamber
Figure 1. Experimental setup lbr reflectivity measurement. ence, Okazaki, Japan. The beam line, which belongs to the National Institute for Fusion Science, consists of a plane grating monochromator (PGM), a calibration chamber and a differential pumping section located between them. The detailed specification of the P G M is described elsewhere7. For the wavelength range with which we are concerned (3-10 nm), the PGM provides a photon flux of the order of 10 ~° s -~ at a ring current of 100 mA with a resolving power of 200. The divergence of the incident beam is ~ 10 mrad, and the beam size at the sample is 0.5 (vertical) × 3 (horizontal) mm 2. In the calibration chamber, a rotational stage (goniometer) is installed. The goniometer has six degrees of freedom: co-axial rotation of the sample and a detector, x - y translations of a sample and
M Sakurai et al." Characterization of multilayer reflectors for the soft X-ray region
C
Polarization
Figure 2. Mounting positions of goniometer. The reflecting positions are shifted to aid the eye.
interchange of samples and filters. These are driven by stepping motors which are operable in uhv. The resolution of the rotation and translation are 1.5 × 10 -5 rad and 5/~m, respectively. The goniometer is mainly made of aluminium. Three kinds of mounting positions are available, as shown in Figure 2. If the incident SR light is linearly polarized, the setup of position A provides the reflectivity measurement of the s component, and position B provides that of the p component. Position C is utilized for the characterization of the polarization of SR, where the sample is mounted at an incidence angle of 45 ° and the sample and detector are sinmltaneously driven. To cross the SR beam against the axis of rotation, the positions of the calibration chamber and the goniometer are independently adjustable in three directions. The multilayers were prepared using an electron beam deposition (EBD) apparatus s. The base pressure of EBD chamber was 1 × 10 -8 Pa, and was kept in the 10 -7 Pa range throughout deposition. Ni/C multilayers were fabricated on the substrates, commercially available float glass and home-made superpolished glass9. The deposition rate, which was normally 0.1 A s-~, was monitored by quartz oscillators mounted on both sides of the substrate. The layer spacing of multilayers (2d) is in the range of 6-10 nm, and the number of layers is 10-54. We also used some commercially available samples (W/C). During the experiment, reflected intensity and direct intensity were measured with a gas-flow type proportional counter which has a 4/~m thick polypropylene window. To avoid saturation and reduction of the gains of the counter, the direct intensity was reduced to keep the count rate of the detector less than 104cps by an additonal filter and the adjustment of the slit width of PGM. Pulse height distribution of the output signal was monitored, and the count rate of the discriminated signal was stored as a net intensity. For reflectivity measurements, either the wavelength of the incident beam or the angle of the goniometer was scanned stepwise. Beam current of the storage ring, the position of PGM and the goniometer and the detector counts are stored for each step. The control of the whole apparatus and the data processing were carried out by a microcomputer. The calibration chamber is evacuated by a turbomolecular pump (1500Is t). Typical pressure in the experiment was 5 × 10 -3 Pa due to a leakage of counter gas ( A r - 1 0 % C H 4 ) through the polypropylene window. By virtue of the differential pumping system, the pressure rise at PGM is reduced to the 10 - 7 Pa range.
3. R e s u l t s a n d d i s c u s s i o n
Figure 3 shows the wavelength dependence of reflected intensities obtained from a multilayer (Ni/C, 2d = 10.7 nm, N = 19) normalized with the direct intensity distribution shown in Figure 4. The goniometer was set in position A. For the incidence angles of 30 °, 40 ° and 50 °, Bragg peaks appear at 5.5, 7.0 and 8.2 nm, respectively. The resolving power (2/A2) is ~ 15 for all incident angles. Associated peaks around the Bragg peaks come from the interference effect among the layered structure. Since the polypropylene film cuts off the higher energy part (2 < 4.5 nm) of the incident radiation, the higher order contribution in the reflectivity measurement is negligible up to 8.9 nm, however, beyond that wavelength, we must be careful about the contribution of the higher order light. In the evaluation of reflectivities, the background intensity originating from ¢.
4
50 °
t
.':
,~'
2
8
;
~ t,
6
,o
40'
4 °
=
2
"~=a'N.,~ lo
-
8
:
30"
I
°, .o
4
2 0
•" '-,t"
.,.'o • a °°•
o
.~-.:¢.... ,""'::'.Z 7 " "
"".:" : .. ~,"'-"""
5~ -- ~. i
". :.x ";" i
4
"Z,"~
.;.
i
6
8
Wavelength
10
(nm)
3. Reflectivity of the multilayer refector (Ni/C, 2d = 10.7 nm) for various incidence angles under the goniometer mounting position A (s polarization). Figure
x.
oo3 o
\ \
'5 2
\
E
o
z
0
\
\
J 6
8
10
Wavelength (nm)
4. Normalized intensity distribution of incident radiation measured by a proportional counter with a 4am polypropylene window. Figure
1235
M Sakurai et al: Characterization of multilayer reflectors for the soft X-ray region
the zero-order tail must be subtracted from the direct intensity distribution. The pulse height distribution of the counter output pulses showed that peak channel of the direct beam for each wavelength corresponded to its energy for the wavelength range of 4.5 to 8.9 nm, i.e. higher order free regions. The peak reflectivities obtained from R - 2 curves similar to Figure 3 for various incidence angles and for both positions A and B are plotted in Figure 5. Due to the polarized incident radiation, the difference in the reflectivities of both conditions increases as the incidence angle approaches 45 ° . Taking into account the interfacial roughness, the observed peak reflectivity is approximately expressed as the following equation ~° which is analogous to the D e b y e - W a l l e r factor in X-ray diffraction,
• .,.o :'... • .... :.....•
.
(a) 6 . 5 n m
:..'4 "E
/,
.? :.° •
,,,~"
*'%',°%%
°° • •% .,%
Rp = R o exp{ -- (2nn A2'/d)2}, where Ro is the ideal peak reflectivity calculated with optical constants, n is the order of reflection and Az is the rms value of the roughness. In Figure 5, the calculated value of Rp is also shown in solid lines for each polarization. The fitting parameter Az was derived as 0.68 nm, by comparing the reflectivity measured with characteristic X-ray of A1-K, with the calculated value. The difference in peak reflectivities between the observed and calculated is possibly due to either the wavelength dependence in the roughness effect or optical constants used for calculation. At 45 ° incidence, the peak reflectivity for p polarization exceeds the calculated value, which may be explained as due to the incomplete polarization of incident radiation. The peak reflectivities for other samples with smaller 2d values ( 6 - 7 nm) were limited by less than 3%. The degradation of the reflectivity is well explained qualitatively by the roughness effect. The difference in the reflectivities of multitayers between positions A and B, i.e. s and p polarizations, amount to one order of magnitude, so it is worthwhile to examine polarization distribution of incident SR light. As shown in Figure 6,
4 =
Wavelength (nm) 6 8
,
,
=
,
=
10 =
o s-pol.
~
• p-~l.
10
._~
(b) 5 . 0 n m
• "" , :v~.%'* .
...'.""
i
I
0
i
I
i
f
30 60 Azimuthal angle (deg)
i
I
i
90
6. Intensity variation of photons reflected from multilayers with the rotating scattering plane against the optical axis of incident radiation. The samples are (a) Ni/C (2d= 10.7 nm, N = 19), and (b) W/C (2d = 7.2 nm). It is measured with the goniometer mounted in position C. Figure
reflected intensity distribution from the multilayers at the Bragg condition for 45 ° incidence was obtained as the scattering plane was rotated against the optical axis of SR. The sample for Figure 6(a) in Ni/C (2d = 10.7 nm) which has been investigated, and that for Figure 6(b) is W/C (2d = 7.2 nm, Energy Conversion Devices, Inc). Similarity of the measured profiles to a sinusoidal curve proves that the output radiation from the PGM is almost linearly polarized, however, the profile is asymmetric against both 0 ° and 90 ° positions. Since a similar result was obtained for two different samples, the observed asymmetry may come from either of the polarization properties of the incident radiation, or from imperfection in the geometrical setup of the reflectometer. In conclusion, we measured the reflectivity of multilayer reflectors in the wavelength range of 3-10 nm using synchrotron radiation. The reflectivity was measured for both s and p polarization conditions by scanning the wavelength of incident radiation at fixed scattering angle. Peak reflectivity exists at 1-10% for the samples examined here ( 2 d = 6 - 1 0 n m ) and under the s polarization condition. It is shown that multilayers are also useful for a polarimeter in the soft X-ray region. References
0.1
~ 20
f
~ ~ r 40 60 Incident angle (deg)
Figure 5. Peak reflectivity of Ni/C (2d = 10.7 nm, N = 19) for s and p polarized component. Experimental and calculated values are expressed by circles and solid lines, respectively. 1236
I j H Underwood and D T Attwood, Phys Today, 5, 44 (1984). 2T W Barbee, Proc SPIE, 563, 2 (1985). 3 K Yamashita, Proe 15th Int Syrup Space Tech Sci, Tokyo, p 1865 (1986). 4 j B Kortright, P Hag, R C C Perera, P L Cowan, D W Lindle and B Karlin, Nucl lnstrum Meth, A266, 452 (1988). 5 A B C Walker Jr, T W Barbee Jr, R B Hoover and J F Lindblom, Science 241, 1781 (1988). 6 p G Burkhalter, J V Gilfrich, D B Brown and D L Rosen, Proc SPIE, 689, 121 (1986). 7 M Sakurai, S Morita, J Fujita, H Yonezu, K Fukui, K Sakai, E Nakamura, M Watanabe, E Ishiguro and K Yamashita, Rev Scient lnstrum, 60, 2089 (1989). s K Yamashita, M R S lnt Mtg on Adv Mats, 10, 221 (1989). 9 y Namba, Optics, 13, 454 (1984) in Japanese. 1oE Spiller, AlP Conf Proc, 75, 124 (1981).
0042-207X/9053.00 + .00 © 1990 Pergamon Press plc
Characterization of multilayer reflectors for the soft X-ray region using synchrotron radiation M S a k u r a i a n d J Fujita, National Institute for Fusion Science, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan K Yamashita, M Ohtani, I H a t s u k a d e Osaka 560, Japan
a n d K Tamura, Department of Physics, Osaka University, Toyonaka,
and H Nagata,
Y Suzuki and S Seki, Japan Research and Development Corporation, Tokyo 100, Japan
Characterizations of multilayer reflectors were performed in the wavelength range 3 - l O n m using synchrotron radiation. Layer spacing of the multilayers, which were fabricated by the electron beam vapour deposition method, exists in the range 6 - 1 0 nm. Evaluation of multilayers was carried out by measuring the reflectivity, the wavelength resolution and the Bragg angle on the beam line for radiometric calibration (BL5B) in the UVSOR facility. The incident photon beam was monochromatized by a plane grating monochrometer. Reflectivity was measured for both s and p components using a demountable rotational stage. The peak reflectivity was found to be in the range of 1-10%, and the reflectivities of both polarization components showed the most pronounced difference at an incidence angle of 45 °, which proves that the multilayer is also useful as an X-ray polarimeter.
1. Introduction In recent years, multilayer reflectors for X-ray optics have been developed, since normal incidence optic system are now attainable for soft X-ray regions ~-3. Multilayer reflectors consist of alternate layers of low and high Z elements with constant thickness. They are not only useful for diffracting elements, but are expected to be useful in soft X-ray polarimeters. They could be applied to X-ray optic systems for plasma diagnostics, X-ray beam line optics for synchrotron radiation (SR) 4 and X-ray telescopes and microscopes 5. Characterization of the reflectivity and resolution is necessary for the development of multilayers, and the estimation of throughput of the optical systems. For plasma diagnostics, calibration of the diffracting components provides means for absolute measurements of X-ray emissions from plasma 6. For the characterization of fabricated multilayers, SR is a most suitable source to measure the reflectivity and polarizability because of its continuous spectral distribution in the X-ray to vuv range and almost linearly polarized nature. In the present paper, we report on the whole apparatus designed for the evaluation of optical components using monochromatized SR, and give some results on the reflectivity measurements of multilayers in the wavelength range of 3-10 nm.
2. Experimental The experimental setup is shown in Figure 1. It is installed on the beam line BL5B of UVSOR, Institute for Molecular Sci-
1234
CaliOration chamber
Figure 1. Experimental setup lbr reflectivity measurement. ence, Okazaki, Japan. The beam line, which belongs to the National Institute for Fusion Science, consists of a plane grating monochromator (PGM), a calibration chamber and a differential pumping section located between them. The detailed specification of the P G M is described elsewhere7. For the wavelength range with which we are concerned (3-10 nm), the PGM provides a photon flux of the order of 10 ~° s -~ at a ring current of 100 mA with a resolving power of 200. The divergence of the incident beam is ~ 10 mrad, and the beam size at the sample is 0.5 (vertical) × 3 (horizontal) mm 2. In the calibration chamber, a rotational stage (goniometer) is installed. The goniometer has six degrees of freedom: co-axial rotation of the sample and a detector, x - y translations of a sample and
M Sakurai et al." Characterization of multilayer reflectors for the soft X-ray region
C
Polarization
Figure 2. Mounting positions of goniometer. The reflecting positions are shifted to aid the eye.
interchange of samples and filters. These are driven by stepping motors which are operable in uhv. The resolution of the rotation and translation are 1.5 × 10 -5 rad and 5/~m, respectively. The goniometer is mainly made of aluminium. Three kinds of mounting positions are available, as shown in Figure 2. If the incident SR light is linearly polarized, the setup of position A provides the reflectivity measurement of the s component, and position B provides that of the p component. Position C is utilized for the characterization of the polarization of SR, where the sample is mounted at an incidence angle of 45 ° and the sample and detector are sinmltaneously driven. To cross the SR beam against the axis of rotation, the positions of the calibration chamber and the goniometer are independently adjustable in three directions. The multilayers were prepared using an electron beam deposition (EBD) apparatus s. The base pressure of EBD chamber was 1 × 10 -8 Pa, and was kept in the 10 -7 Pa range throughout deposition. Ni/C multilayers were fabricated on the substrates, commercially available float glass and home-made superpolished glass9. The deposition rate, which was normally 0.1 A s-~, was monitored by quartz oscillators mounted on both sides of the substrate. The layer spacing of multilayers (2d) is in the range of 6-10 nm, and the number of layers is 10-54. We also used some commercially available samples (W/C). During the experiment, reflected intensity and direct intensity were measured with a gas-flow type proportional counter which has a 4/~m thick polypropylene window. To avoid saturation and reduction of the gains of the counter, the direct intensity was reduced to keep the count rate of the detector less than 104cps by an additonal filter and the adjustment of the slit width of PGM. Pulse height distribution of the output signal was monitored, and the count rate of the discriminated signal was stored as a net intensity. For reflectivity measurements, either the wavelength of the incident beam or the angle of the goniometer was scanned stepwise. Beam current of the storage ring, the position of PGM and the goniometer and the detector counts are stored for each step. The control of the whole apparatus and the data processing were carried out by a microcomputer. The calibration chamber is evacuated by a turbomolecular pump (1500Is t). Typical pressure in the experiment was 5 × 10 -3 Pa due to a leakage of counter gas ( A r - 1 0 % C H 4 ) through the polypropylene window. By virtue of the differential pumping system, the pressure rise at PGM is reduced to the 10 - 7 Pa range.
3. R e s u l t s a n d d i s c u s s i o n
Figure 3 shows the wavelength dependence of reflected intensities obtained from a multilayer (Ni/C, 2d = 10.7 nm, N = 19) normalized with the direct intensity distribution shown in Figure 4. The goniometer was set in position A. For the incidence angles of 30 °, 40 ° and 50 °, Bragg peaks appear at 5.5, 7.0 and 8.2 nm, respectively. The resolving power (2/A2) is ~ 15 for all incident angles. Associated peaks around the Bragg peaks come from the interference effect among the layered structure. Since the polypropylene film cuts off the higher energy part (2 < 4.5 nm) of the incident radiation, the higher order contribution in the reflectivity measurement is negligible up to 8.9 nm, however, beyond that wavelength, we must be careful about the contribution of the higher order light. In the evaluation of reflectivities, the background intensity originating from ¢.
4
50 °
t
.':
,~'
2
8
;
~ t,
6
,o
40'
4 °
=
2
"~=a'N.,~ lo
-
8
:
30"
I
°, .o
4
2 0
•" '-,t"
.,.'o • a °°•
o
.~-.:¢.... ,""'::'.Z 7 " "
"".:" : .. ~,"'-"""
5~ -- ~. i
". :.x ";" i
4
"Z,"~
.;.
i
6
8
Wavelength
10
(nm)
3. Reflectivity of the multilayer refector (Ni/C, 2d = 10.7 nm) for various incidence angles under the goniometer mounting position A (s polarization). Figure
x.
oo3 o
\ \
'5 2
\
E
o
z
0
\
\
J 6
8
10
Wavelength (nm)
4. Normalized intensity distribution of incident radiation measured by a proportional counter with a 4am polypropylene window. Figure
1235
M Sakurai et al: Characterization of multilayer reflectors for the soft X-ray region
the zero-order tail must be subtracted from the direct intensity distribution. The pulse height distribution of the counter output pulses showed that peak channel of the direct beam for each wavelength corresponded to its energy for the wavelength range of 4.5 to 8.9 nm, i.e. higher order free regions. The peak reflectivities obtained from R - 2 curves similar to Figure 3 for various incidence angles and for both positions A and B are plotted in Figure 5. Due to the polarized incident radiation, the difference in the reflectivities of both conditions increases as the incidence angle approaches 45 ° . Taking into account the interfacial roughness, the observed peak reflectivity is approximately expressed as the following equation ~° which is analogous to the D e b y e - W a l l e r factor in X-ray diffraction,
• .,.o :'... • .... :.....•
.
(a) 6 . 5 n m
:..'4 "E
/,
.? :.° •
,,,~"
*'%',°%%
°° • •% .,%
Rp = R o exp{ -- (2nn A2'/d)2}, where Ro is the ideal peak reflectivity calculated with optical constants, n is the order of reflection and Az is the rms value of the roughness. In Figure 5, the calculated value of Rp is also shown in solid lines for each polarization. The fitting parameter Az was derived as 0.68 nm, by comparing the reflectivity measured with characteristic X-ray of A1-K, with the calculated value. The difference in peak reflectivities between the observed and calculated is possibly due to either the wavelength dependence in the roughness effect or optical constants used for calculation. At 45 ° incidence, the peak reflectivity for p polarization exceeds the calculated value, which may be explained as due to the incomplete polarization of incident radiation. The peak reflectivities for other samples with smaller 2d values ( 6 - 7 nm) were limited by less than 3%. The degradation of the reflectivity is well explained qualitatively by the roughness effect. The difference in the reflectivities of multitayers between positions A and B, i.e. s and p polarizations, amount to one order of magnitude, so it is worthwhile to examine polarization distribution of incident SR light. As shown in Figure 6,
4 =
Wavelength (nm) 6 8
,
,
=
,
=
10 =
o s-pol.
~
• p-~l.
10
._~
(b) 5 . 0 n m
• "" , :v~.%'* .
...'.""
i
I
0
i
I
i
f
30 60 Azimuthal angle (deg)
i
I
i
90
6. Intensity variation of photons reflected from multilayers with the rotating scattering plane against the optical axis of incident radiation. The samples are (a) Ni/C (2d= 10.7 nm, N = 19), and (b) W/C (2d = 7.2 nm). It is measured with the goniometer mounted in position C. Figure
reflected intensity distribution from the multilayers at the Bragg condition for 45 ° incidence was obtained as the scattering plane was rotated against the optical axis of SR. The sample for Figure 6(a) in Ni/C (2d = 10.7 nm) which has been investigated, and that for Figure 6(b) is W/C (2d = 7.2 nm, Energy Conversion Devices, Inc). Similarity of the measured profiles to a sinusoidal curve proves that the output radiation from the PGM is almost linearly polarized, however, the profile is asymmetric against both 0 ° and 90 ° positions. Since a similar result was obtained for two different samples, the observed asymmetry may come from either of the polarization properties of the incident radiation, or from imperfection in the geometrical setup of the reflectometer. In conclusion, we measured the reflectivity of multilayer reflectors in the wavelength range of 3-10 nm using synchrotron radiation. The reflectivity was measured for both s and p polarization conditions by scanning the wavelength of incident radiation at fixed scattering angle. Peak reflectivity exists at 1-10% for the samples examined here ( 2 d = 6 - 1 0 n m ) and under the s polarization condition. It is shown that multilayers are also useful for a polarimeter in the soft X-ray region. References
0.1
~ 20
f
~ ~ r 40 60 Incident angle (deg)
Figure 5. Peak reflectivity of Ni/C (2d = 10.7 nm, N = 19) for s and p polarized component. Experimental and calculated values are expressed by circles and solid lines, respectively. 1236
I j H Underwood and D T Attwood, Phys Today, 5, 44 (1984). 2T W Barbee, Proc SPIE, 563, 2 (1985). 3 K Yamashita, Proe 15th Int Syrup Space Tech Sci, Tokyo, p 1865 (1986). 4 j B Kortright, P Hag, R C C Perera, P L Cowan, D W Lindle and B Karlin, Nucl lnstrum Meth, A266, 452 (1988). 5 A B C Walker Jr, T W Barbee Jr, R B Hoover and J F Lindblom, Science 241, 1781 (1988). 6 p G Burkhalter, J V Gilfrich, D B Brown and D L Rosen, Proc SPIE, 689, 121 (1986). 7 M Sakurai, S Morita, J Fujita, H Yonezu, K Fukui, K Sakai, E Nakamura, M Watanabe, E Ishiguro and K Yamashita, Rev Scient lnstrum, 60, 2089 (1989). s K Yamashita, M R S lnt Mtg on Adv Mats, 10, 221 (1989). 9 y Namba, Optics, 13, 454 (1984) in Japanese. 1oE Spiller, AlP Conf Proc, 75, 124 (1981).