Time resolved x-ray reflectivity study of interfacial reactions in $\mathrm{Cu}∕\mathrm{Mg}$ thin films during heat treatment

We present an in-situ time-resolved synchrotron x-ray reflectivity (XRR) analysis of the thermal stability of $\mathrm{Cu}∕\mathrm{Mg}∕\mathrm{Cu}$ trilayers during heating ramps up to $300°\mathrm{C}$ at $2°\mathrm{C}∕\min$. We demonstrate that under some simplifying assumptions the temporal evolution of XRR scans can be used to follow the kinetics of formation of the $\mathrm{Cu}{\mathrm{Mg}}_2$ intermetallic phase. The simultaneous refinement of selected parameters of 70 reflectivity scans measured during the heat treatment permits an accurate analysis with respect to the as-deposited state. A gradual damping of the long period Kiessig fringes of the trilayer structure is observed upon heating up to $190°\mathrm{C}$. Above this temperature the XRR is reminiscent of a single thin film of $\mathrm{Cu}{\mathrm{Mg}}_2$. The evolution of interface interdiffusion and/or roughness and thickness of the initial layers allowed to identify a differentiated nucleation and lateral growth process of the intermetallic $\mathrm{Cu}{\mathrm{Mg}}_2$ phase depending on the type of interface, Cu on Mg and Mg on Cu. These results are compared to experimental measurements obtained from differential scanning calorimetry.

Figure 1

Schematics of the experimental setup: (1) bending magnet source; (2) primary slits; (3) Si(111) monochromator with sagittally bent second crystal (horizontal focusing); (4) beam; (5) secondary slits; (6) sample slits; (7) detector slits; (8) detector; (9) furnace chamber with sample; (10) vacuum cap with kapton windows; (11) Mo plate with sample; (12) resistive element and thermocouples; (13) cooling circuit; (14) vacuum connection; (15) goniometer.

Figure 2

In-situ x-ray reflectivity scans obtained at different temperatures. Circles correspond to experimental data and the continuous line to their best fitting ($30°\mathrm{C}$ fitting performed with Mg-approx and the rest with $\mathrm{Cu}{\mathrm{Mg}}_2$-approx).

Figure 3

Cross-section SEM pictures of a $\mathrm{Cu}∕\mathrm{Mg}$ multilayer as-prepared (a) and heated up to a middle point of the vertical growth stage (b). In the as-prepared sample, Mg layers correspond to the dark fringes and Cu to the bright ones. In the case of the heat-treated sample, the intermetallic (identified with the same color as the Cu layers) penetrates into the Mg layer defining an irregular growth front.

Figure 4

Temperature evolution of the parameters resulting from the fitting of the XRR scans during the heat treatment. In the case of thickness (a) and density (c), the symbols correspond to: upper Cu (circles), $\mathrm{Mg}∕\mathrm{Cu}{\mathrm{Mg}}_2$ (squares) and bottom Cu (up triangles). For roughness and/or interdiffusion (b), squares represent Cu on Mg interface and up triangles Mg on Cu interface. In all cases void symbols correspond to Mg-approx and filled symbols to $\mathrm{Cu}{\mathrm{Mg}}_2$-approx. Inset: Detail of the thickness of both Cu layers calculated with Mg-approx between $60°\mathrm{C}$ and $130°\mathrm{C}$ (circles, bottom Cu, triangles: upper Cu). A vertical offset has been introduced between both curves to clarify the differences.

Figure 5

(a) Thickness evolution vs time of the upper (circles) and bottom (up triangles) Cu layers obtained through the $\mathrm{Cu}{\mathrm{Mg}}_2$-approx. The continuous line corresponds to a mathematical fitting. (b) Time derivative of the thickness evolution of both Cu layers, upper (continuous line) and bottom (dashed line). This derivative is related to the transformation rate which is proportional to the heat released by the sample during the transformation (Ref. 30). (c) Calorimetric signal of a $\mathrm{Cu}∕\mathrm{Mg}$ multilayer sample with eight stacks of nominal thicknesses $\mathrm{Cu}:20\mathrm{nm}$ and $\mathrm{Mg}:80\mathrm{nm}$ heated at $2°\mathrm{C}∕\min$. This signal corresponds to the heat released by the sample as a function of time.

Figure 6

Detail of a cross-section TEM image of an as-deposited $\mathrm{Cu}∕\mathrm{Mg}$ multilayer with thicknesses corresponding to $\mathrm{Mg}:80\mathrm{nm}$ and $\mathrm{Cu}:20\mathrm{nm}$. The white arrows indicate the Mg on Cu interface, with a small grain microstructure in the Mg layer. The black arrows are pointing to the vertical grain boundaries that intersect the Cu on Mg interface.