Structural and Chemical Characterisation of Ni/Ti Multilayers with TEM
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Abstract Ni/Ti multilayers were prepared by argon sputtering. We observe changes of lattice parameter, strain, grain size and texture depending on the single layer thickness and the pressure of the sputter gas. Moreover the quality of the layers can be improved by including impurities during the growth of the Ni and the Ti layers. Generally the results show that such additions lead to more planar layers and to structural changes within the individual layers. Although we observe hexagonal Titanium in the as-deposited films, some of the specimens which were prepared for TEM observation reveal an fcc-Ti structure.The chemical composition of the layers has been investigated in a field emission microscope using a probe diameter of nmm.Composition profiles were determined by evaluating EELS spectra taken at each point in an automated scan, In this way it was possible to determine residual concentrations of Ni in Ti layers lower than 0.5%. In the samples studied the interface width appears to be of the order of 4-5nm and the concentration profiles of Ni-Ti interfaces appear to be different from those of the Ti-Ni interfaces.
I. Introduction We present in this paper a series of TEM characterizations of Ni/Ti multilayers which have interest as neutron reflectors . Although classical neutron mirrors, made of Ni films, show total reflectivity up to a critical angle 0C, 0c is very low (1.73 mrad/A). Using multilayers the angle of total reflection can be increased 1,. As Ni/Ti multilayers containing only the pure materials did not show the optimal performance 3, hydrogen 4 '5,vanadium 4 ,carbon 6 and 0-N 4,7 containing multilayers were produced. Enhanced neutron reflectivity found in these layers could be correlated with the planarity of the layers, the interface sharpness and the density of the layers 3 All samples are deposited at growth speeds of 2-3nm/s onto Si substrates in a DC magnetron sputtering chamber with a base pressure of 2xl0 7 mbar. The characterization was carried out as a function of the number of bilayers N (15 to 30), the single layer thickness d (dTi=dNi=2.5-20nm) and the argon pressure (0.7x10-3 to 9xlO- 3mbar). The results were compared to similar measurements made on a NiNxOy/TiVz specimen with gradually increasing single layer thicknesses (dTi=7-20nm,dNi=7-35nm) between the substrate and the surface. The TEM specimens were prepared by cross sectional techniques by mechanically grinding down the specimen to a thickness of 10-50gm and final thinning by ion bombardment at LN 2 temperature. TEM was performed on a Philips EM430ST and a Hitachi HF2000 (field emission gun, probe diameter 1.1-1.7nm (FWHM) at a probe current of InA).
II. Results A) Morphology Careful inspection of fig. 1 reveals the overall structure of a typical Ni/Ti multilayer. The individual layers are polycrystalline with an amplitude of the interfacial roughness (for the layers close to the substrate) which increases from 0.5 to 0.8 nm as the single layer thickness increases from 5 to 20nm. The Ni and the Ti layers are separated by a lnm
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