Probing Atomic Migration in Nanostructured Multilayers: Application of X-Ray Standing Wave Fields

X-ray standing wave fields, excited in periodic nanostructured multilayers during Bragg diffraction, have been used to probe atomic migration in multilayers. Ion beam induced migration of Fe impurity atoms from the C layers to the Pt layers in a $\mathrm{Pt}(\mathrm{Fe})/\mathrm{C}(\mathrm{Fe})$ multilayer, where each layer is about 2 nm thick, has been detected. With a depth resolution better than 0.2 nm of this technique, the direction of Fe migration (here outward) and the change of Fe concentration in C (also Pt) layers have been determined. The results of such measurements are important for understanding the properties of multilayers, for example, the evolution of ferromagnetism in the present example [Dev et al., Microelectron. Eng. 83, 1721 (2006)].

Figure 1

(a) Illustration of the formation of x-ray standing wave patterns in a periodic multilayer during the first order Bragg diffraction from the multilayer. (b) Variation of Fe $K\alpha$ fluorescence yield curve when an atomic layer of Fe migrates from position 0.5 (center of a C layer) to $0.4,0.3,\dots ,0.0$ (center of a Pt layer), etc. (inward) and (c) to $0.6,0.7,\dots ,1.0$ (center of a Pt layer), etc. (outward). (See text for details.)

Figure 2

When x-ray absorption effect for the whole multilayer stack is included, the curves in Figs. 1b, 1c give the modified curves (a) and (b), respectively. Following whether the fluorescence yield curve evolves like those in (a) or in (b), still inward and outward migration are quite distinguishable. (See text for details.)

Figure 3

(a) The experimental setup for the x-ray reflectivity and standing wave measurements and the sample (SA) are schematically shown. (b) First order Bragg peak from the virgin sample (solid black circle) and Pt $L\alpha$ [dark gray (blue) symbols: $V$, open circle; $B$, diamond; $C$, triangle up; $D$, triangle right; $F$, cross] and Fe $K\alpha$ [light gray (red) symbols: $V$, square; $B$, plus; $C$, triangle down; $D$, triangle left; $F$, character A] fluorescence yields from the virgin and the irradiated samples along with the fitted curves. Pt and Fe fluorescence yields from the irradiated samples are shifted vertically for clarity.

Figure 4

The change in the normalized Fe concentration in the C layers with ion fluence is shown.