Ti multilayers

Ti multilayers

ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 562 (2006) 389–392 www.elsevier.com/locate/nima New interface solution for Ni...

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ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 562 (2006) 389–392 www.elsevier.com/locate/nima

New interface solution for Ni/Ti multilayers M. Aya,, C. Schanzera, M. Wolffb,c, J. Stahna a

Laboratory for Neutron Scattering ETH & PSI, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland b Department of Physics, Ruhr-University Bochum, D-44780 Bochum, Germany c Institut Laue-Langevin, F-38042 Grenoble Cedex 9, France Received 19 December 2005; accepted 27 February 2006 Available online 22 March 2006

Abstract Nickel/titanium multilayers (ML) and supermirrors (SM) were fabricated by DC magnetron sputtering on glass or Si (1 0 0) substrates. It was possible to increase the reflectivity of ML and SM by the deposition of an ultrathin Cr layer at the Ni/Ti interfaces. Interdiffusion decreases when the Ni and Ti are separated by a covering Cr layer (3 A˚), whereas, in the range where the Cr interlayer thickness is higher than the Ni/Ti interdiffusion depth (approximately 6 A˚) or the surface roughness, the growth of a closed Cr layer is prohibited reducing the reflectivity. The presence of a 10 A˚ Cr interlayer on some Ti layers also results in enhanced reflectivities due to a smoothing effect of the interface for the following layers. The reflectivities of two monochromators for qz ¼ 0:069 A˚ and a bandwidth (Dl/l) of 6% and 12% reach a value of approximately 0.90. r 2006 Elsevier B.V. All rights reserved. PACS: 68.35.ct; 68.35.fx; 68.65.ac; 81.15.cd Keywords: Multilayer mirror; Neutron supermirror; Multilayer monochromator; Band pass filter

1. Introduction Modern neutron optical elements are desired to further improve the performance of neutron experiments. The demand on this can be framed: (i) to provide more useful neutrons by extracting the proper phase space from the source, (ii) to provide more freedom by selecting the neutron spin states and (iii) to form compact beams by focusing techniques. Artificial multilayers (ML) have been proven to be very useful for the fabrication of neutron optical elements as their properties can be widely tailored. Regarding the focusing of neutron beams, linear tapered devices or advanced geometries like parabolic and elliptic guides [1] require supermirror (SM) coatings with a high reflectivity range when the guide cross-section becomes small at their ends. The upper limit qcSM of the reflection of a SM is measured relative to the critical angle of Ni (qcSM ¼ mqcNi , i.e., m times the critical angle of the total Corresponding author. Fax: +41 56 310 5380.

E-mail address: [email protected] (M. Ay). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.02.188

reflection of natural Ni). SM with m ¼ 2 having high reflectivity can be considered as standard at present. Beyond this range the preparation of SM becomes significantly more complex, in particular, due to the drastically increasing number of layers (E4 m4 for Ni/Ti [2]). Having a large number of layers, interfacial roughness and internal strain become crucial for the performance, i.e., they reduce the reflectivity and the stability of the SM. Here, we report on the first results to suppress the evolution of interfacial roughness in high m SM using alternative materials for selected layers of the multilayer sequence. 2. Experiments The preparation of Ni/Ti ML and SM with interlayers of Al, Si, Fe, and Cr at dedicated positions in the layer sequence was carried out using a Leybold Z600 DC magnetron sputtering system (max. sample area 400  500 mm2) [3]. Only the Cr interlayer systems shows an increase of reflectivity so far. Hence, we focus our

ARTICLE IN PRESS M. Ay et al. / Nuclear Instruments and Methods in Physics Research A 562 (2006) 389–392

attention on the results using Cr interlayers, here. The base pressure in the sputtering chamber was less than 1  106 mbar while during the deposition the pressure was approximately 1  103 mbar. The sputtering was performed under an Ar atmosphere for Ti and Cr layers and under an Ar/N2 mixture for the Ni layers. Partial pressures of Ar and N2 were maintained in the sputtering chamber by controlling their flow rates. The substrates were translated below the targets in order to deposit large areas. The thickness of the layers was controlled by the velocity of the translation of the substrates below the targets. The sputtering rate was determined from X-ray reflectivity measurements on calibration films. The X-ray measurements were carried out using a Seifert X-ray diffractometer with CuKa radiation. Neutron reflectivity measurements were performed using the reflect˚ ometers MORHHEUS ðl ¼ 4:743 AÞ at SINQ, Paul ˚ at ILL. Scherrer Institut and ADAM ðl ¼ 4:41 AÞ The samples were prepared on glass (70  500 mm2) or Si (1 0 0) substrates and can be grouped into the following categories: 1. Series A: multilayers with Cr interlayers on each Ni and Ti layer, series A1: [Ti (70 A˚)/Cr (tCr)/Ni (70 A˚)/Cr (tCr)]10 with tCr ¼ 0220 A˚ and series A2: [Ti (70 A˚)/Cr ˚ (tCr)/Ni (70 A˚)/Cr (tCr)]20 with tCr ¼ 028 A. 2. Series B: supermirrors m ¼ 2 with 60 Ti/Ni periods and ˚ Cr interlayers on each Ni and Ti layer with tCr ¼ 028 A. 3. Series C: multilayer monochromators with bandwidths Dl/l of 6% and 12% at qz ¼ 0:069 A˚ and numbers of Ti/Ni bilayers of 78 and 165, respectively. Cr interlayers ˚ were only deposited on every 25th Ti layer. ðtCr ¼ 10 AÞ The idea was to reduce the interface roughnesses with a smoothening Cr interlayer [4].

3. Experimental results and discussion Fig. 1 shows specular neutron reflectivities in the region of the first superlattice (SL)-Bragg reflection of multilayers [Ti (70 A˚)/Cr (tCr)/Ni (70 A˚)/Cr (tCr)]10 with different Cr interlayer thicknesses in the range of tCr ¼ 0220 A˚ (series A1). The position of the SL Bragg peak shifts towards lower qz with increasing tCr as a consequence of the increasing period (d-spacing) of the ML. In order to correct the inherent qz dependence of the intensity of the SL Bragg peak, we have normalized the reflectivities by a simulated reflectivity of a corresponding NiCrTi alloy. ˚ We The Bragg peak of the Cr-less ML is at qz ¼ 0:059 A. have defined the intensity of this reflex to be 100%, as listed in the graphic. For a Cr interlayer of nominally tCr ¼ 3 A˚ the intensity of the SL Bragg peak rises to 117%. The shift of the peak position indicates an increase of the d-spacing to 116.5 A˚ compared to 106.7 A˚ for the Cr-less sample. The ML with Cr interlayers of tCr ¼ 6 A˚ shows a gain of 11% and a d-spacing of 118.5 A˚. Beyond this interlayer thickness

Cr interlayer & reflectivity [ICr=0Å] 0Å 3Å 6Å 10Å 20Å

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qz [Å-1] Fig. 1. Corrected specular neutron reflectivities of [Ti (70 A˚)/Cr (tCr)/Ni (70 A˚)/Cr (tCr)]10 multilayers covering the angular range of the first supperlattice Bragg peak. The Cr interlayer thicknesses are in the range 0–20 A˚.

the intensity decreases to 95% and 82% for tCr ¼ 10 A˚ ˚ and tCr ¼ 20 A˚ ðd ¼ 143:8 AÞ, ˚ respectively. ðd ¼ 131:7 AÞ Fig. 2 shows diffuse scattering data of the series of multilayers [Ti(70 A˚)/Cr(tCr)/Ni(70 A˚)/Cr(tCr)]20 with Cr interlayer thicknesses of (a) 0, (b) 4, and (c) 8 A˚ (series A2). The intensity maps are plotted as a function of ki+kf and kikf with kiðf Þ ¼ 2p=l sin ðaiðf Þ Þ, where ai(f) is the angle of the incident (final) beam. In Fig. 2a the specular (ki  kf ¼ 0) SL peaks are visible up to the third order, and in Fig. 2b and c up to the fourth order. The appearance of the second-order peak (Fig. 2a) at Cr-less ML indicates that the thicknesses of Ni and Ti layers are not exactly identical. The diffuse scattering exhibits distinct features. Some of the diffuse intensity is concentrated in horizontal sheets through the specular SL Bragg peaks (Holy Bananas or resonant diffuse scattering, RDS sheets). Another streak of diffuse intensity is observed along the direction of 451 through the first-order Bragg peak. This streak is known as Bragg like line since the Bragg condition of the superlattice is fulfilled for the incident beam in this case. The intersection of the Bragg like line with the RDS sheet results in enhanced intensity appearing as a peak in the reciprocal space map. The existing surface and interface roughness gives rise to weak Yoneda wings. Actually, the observed features of the diffuse scattering are expected to be symmetric with respect to the specular rod. The asymmetry in the measured intensity is a consequence of the experimental resolution, which varies across the reciprocal space. The presence of the RDS sheets strongly indicates a correlated interfacial roughness. The intensity of the diffuse scattering originating from correlated roughness increases when adding Cr interlayers. We conclude that the Cr layers prevent the intermixing of Ni and Ti leading to sharper interfaces [5,6].

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The main interest for neutron mirrors is to achieve high reflectivity. For this reason we focus to the specular reflectivity in the region of the first Bragg reflection (Fig. 2). The results of series A2 are consistent with series A1. It turns out that for tCr ¼ 4 A˚ the intensity increases by approximately 3%, and for tCr ¼ 8 A˚ it decreases by approximately 5%. Based on these results we expect to achieve higher reflectivities for SM and ML monochromators. The layer thicknesses are calculated using the formalism of Hayter [7]. Fig. 3 shows the reflectivities of series B, Ti/Cr/Ni/Cr m ¼ 2 supermirrors with 60 Ti/Ni periods and Cr interlayers with tCr ¼ 028 A˚ at every interface. The Cr-less SM shows a reflectivity of 0.91 at m ¼ 2. The use of Cr interlayers increases the reflectivity up to 0.93 for tCr ¼ 2 A˚ and 3 A˚. A further increase of the Cr interlayer thickness ˚ leads to a decrease of the reflectivity: 0.92 for tCr ¼ 4 A, ˚ ˚ ˚ 0.91 for 5 A, and 0.87 for 6 A and 8 A of tCr. These results are consistent with the observations from the multilayers of series A, where diffusion barriers in the range of a closed monolayer lead to a rise of intensity. However, when the interlayer thickness is larger than the interdiffusion layer between Ni and Ti (E6 A˚) the benefit of the Cr interlayers is lost and the reflectivity even decreases. The application of Cr interlayers at every boundary for supermirrors with 200 layers and more could not show an improvement of the reflectivity up to this day. We think, this is due to the fact that the higher roughness due to the larger number of layers prohibits the growth of closed Cr layers with thicknesses of 2–4 A˚ (only a fraction of the surface is covered with Cr). We suppose that this leads to a negative effect because of interdiffusion of Ni and Ti and a variation

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1.2 qz=0.069 Å-1 1.0 0.9 reflectivity

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of the scattering length density profile (lateral and perpendicular) caused by the clustering of the Cr. For different layer systems it is known that thin Al [9] or C [8] films, in the range of one monolayer to 40 A˚, lead to a surface smoothening. For this reason, we have produced monochromators with a modified layer sequence. We have deposited a 10 A˚ thick Cr layer after every twenty-fifth period on the Ti layer, with the aim to provide a smooth surface for the growth of the following layers. Fig. 4 shows the corresponding reflectivities of two ML monochromators and a state-of-the-art m ¼ 3:6 SM. The monochromators have their center at qz ¼ 0:069 A˚ and a bandwidth of 6% and 12%. The sample properties are committed by the demand for experiments at FUNSPIN (PSI). The d-spacing of Ni/Ti bilayers gradually varies in order to select the desired qz range. The monochromators consist of 159 and 336 single layers and d-spacings in the range from 86.6 to 92.0 A˚ and 82.7 to 93.3 A˚ to obtain a ˚ bandwidth of 6% and 12%, respectively. At qz ¼ 0:069 A, they have reflectivities of 0.90 and 0.89, respectively. For comparison, a state-of-the-art SM and monochromators shows at the same qz-value a reflectivities of approximately 0.75 and 0.80, respectively.

A new solution to control the interfaces of Ni/Ti multilayers and SM, which were fabricated by DC magnetron sputtering, enables to improve the performance of neutron optical devices. We demonstrate the possibility to increase the reflectivity by adding covering Cr layers ˚ at the Ni/Ti interfaces, which prevent the ðtCr ¼ 3 AÞ intermixing of Ni and Ti and make the interfaces between the layers sharper. However, surface roughness can prohibit the growth of a closed Cr layer resulting in clustering of the Cr and therefore even increasing the interfacial roughness. Interlayers of Cr with tCr ¼ 10 A˚ on every twenty-fifth period on the Ti layer increase also the reflectivity due to a smoothening effect of the surface for the growth of the following layers. The reflectivities of two monochromators with 159 and 336 single layers, a bandwidth of 6% and 12%, and the center at ˚ reach approximately 0.90. Compared with a qz ¼ 0:069 A, state-of-the-art monochromators with approximately 0.80 at this qz-value this means an enormous improvement for these monochromators. A successful implementation of this technique for high m supermirrors will push their application for neutron optics to a next level. Acknowledgements We gratefully acknowledge financial support by EU [NMI3-JRA3 (Neutron Optics)]. References [1] C. Schanzer, P. Bo¨ni, U. Filges, T. Hils, Nucl. Instr. and Meth. A 529 (2004) 63. [2] P. Bo¨ni, Physica B 276–278 (2000) 6. [3] O. Elsenhans, P. Bo¨ni, H.P. Friedli, H. Grimmer, P. Buffat, K. Leifer, J. So¨chtig, I.S. Anderson, Thin Solid Films 246 (1994) 110. [4] Z.-J. Liu, Y.G. Shen*, L.P. He, T. Fu, Appl. Surf. Sci. 226 (2004) 371. [5] C. Sella, M. Kaˆabouchi, M. Miloche, M. Maˆaza, R. Krishnan, Appl. Surf. Sci. 60–61 (1992) 781. [6] C. Sella, M. Maaza, M. Miloche, M. Kaabouchi, R. Krishnan, Surf. Coat. Technol. 60 (1–3) (1993) 379. [7] J.B. Hayter, H.A. Mook, J. Appl. Crystallog. 22 (1989) 35. [8] H. Takenaka, H. Ito, K. Nagai, Y. Muramatsu, E. Gullikson, R.C.C. Perera, Nucl. Instr. and Meth. A 467–468 (2001) 341. [9] Z.-J. Liu, Y.G. Shen, L.P. He, T. Fu, Appl. Surf. Sci. 226 (2004) 371.