Effects of O and N impurities on the nanostructural evolution during growth of Cr/Sc multilayers
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Eric. M. Gullikson Lawrence Berkeley National Laboratory, Berkeley, California 94720-8199
Ulrich Kressig Institut fu¨r Ionenstrahlphysik und Materialforschung, Forschungzentrum (FZD), Rossendorf, D-01314 Dresden, Germany
Manfred Beckers, Lars Hultman, and Jens Birch Department of Physics, Chemistry, and Biology (IFM), Linko¨ping University, S-58183 Linko¨ping, Sweden (Received 19 May 2008; accepted 15 September 2008)
Transition metal multilayers are prime candidates for high reflectivity soft x-ray multilayer mirrors. In particular, Cr/Sc multilayers in the amorphous state have proven to give the highest reflectivity in the water window. We have investigated the influence of impurities N and O as residual gas elements on the growth, structure, and optical performance of Cr/Sc multilayers deposited in high vacuum conditions by a dual cathode direct current magnetron sputter deposition. Multilayer structures with the modulation periods in the range of 0.9–4.5 nm and Cr layer to bilayer thickness ratios in the range of 0.17–0.83 were deposited with an intentionally raised base pressure (pB), ranging from 2 107 to 2 105 Torr. Compositional depth profiles were obtained by elastic recoil detection analysis and Rutherford backscattering spectroscopy, while the structural investigations of the multilayers were carried out using hard x-ray reflectivity and transmission electron microscopy. By investigating stacked multilayers, i.e., several multilayers with different designs of the modulation periods, stacked on top of each other in the samples, we have been able to conclude that both N and O are incorporated preferentially in the interior of the Sc layers. At pB 2 106 Torr, typically 3 107 Torr, nitrogen is the main constituent of the residual gas. Because the RGA scans were made in-between the deposition runs of Cr/Sc multilayers, we believe that oxygen and water gettered by highly reactive Sc and Cr, just deposited on the chamber walls, is the cause of the low relative levels of these gases. The calculated deposition rates for the three pB used in this work are listed in Table I. At the higher background pressure, pB(C), a 30% increase in rCr and the
FIG. 2. Residual gas partial pressures of the most abundant species; nitrogen, oxygen, and water vapor versus the chamber background pressure pB when air with 0.87 wt% H2O was introduced to control pB. The values for nitrogen and oxygen represent the sums of the molecular and atomic partial pressures.
TABLE I. Deposition rates, r, for Cr and Sc at the three different deposition series A, B, and C with different pB. Deposition series A B C
pB (Torr) 7
2 10 2 106 2 105
rCr (nm/s)
rSc (nm/s)
0.0254 0.0275 0.0326
0.0375 0.0346 0.0264
same relative decrease in rSc was determined compared to pB(A). This change in the deposition rates when going to high pB may be due to a combined effect of target poisoning and multilayer density alteration when residual gas elements are incorporated into the film. We believe that the latter effect is the dominant h
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