Calorimetric evidence of asymmetry in the nucleation of CuMg 2 in CuÕMg multilayers M. Gonzalez-Silveira and M. T. Clavaguera-Mora Departamento de Fı ´sica, Grup de Fı ´sica de Materials I, Universitat Auto `noma de Barcelona, 08193 Bellaterra, Spain F. Pi Departamento de Fı ´sica, Grup O ` ptica, Universitat Auto `noma de Barcelona, 08193 Bellaterra, Spain J. Rodriguez-Viejo* Departamento de Fı ´sica, Grup de Fı ´sica de Materials I, Universitat Auto `noma de Barcelona, 08193 Bellaterra, Spain Received 21 October 2003; revised manuscript received 22 December 2003; published 29 March 2004 We report the observation of temperature-dependent nucleation and lateral growth behavior of the interme- diate product phase CuMg 2 in Cu/Mg multilayers depending if the interface is Mg on Cu or Cu on Mg. The addition of an intermediate Fe layer inhibiting CuMg 2 nucleation on one of the Cu/Mg interfaces demonstrates the important role of the nature of the interface on the nucleation events of the intermediate phase. This unexpected behavior is related to the mode of growth of the thin films. A kinetic model with independent nucleation and lateral growth stages at each interface is used to fit the calorimetric curves. DOI: 10.1103/PhysRevB.69.113411 PACS numbers: 68.65.Ac, 68.60.Dv, 68.35.Ct Intermetallic compounds formed by solid-state reactions in thin-film or multilayer systems are of fundamental impor- tance in a number of applications such as heat and corrosion resistant coatings, and metallization layers in microelectronic devices or in magnetic devices. The knowledge of the inter- face structures or the intermediate phases that form upon heat treatment either at low or high temperature and the ki- netics and thermodynamics involved in such processes are therefore of interest to the scientific community. Differential scanning calorimetry DSC is particularly suited for such analysis because it maps the temporal evolution of the sys- tem being sensitive to processes activated at different tem- peratures. However, despite the important number of works devoted to the analysis of intermetallic formation in multi- layers systems, 1–4 the understanding of the effects of inter- faces on the nucleation of intermediate compounds is still incomplete, due in part to the large number of parameters that may influence the nucleation process. It has been previ- ously shown that the product phase can nucleate either at the A / B interface 5 or heterogeneously at triple points with grain boundaries 6,7 or inside grain boundaries, 2 followed in all cases by lateral growth until coalescence of the nuclei. Pre- vious studies by Thompson and co-workers 8 have also shown that the nucleation behavior at an amorphous-Si/V interface is different from that at a crystalline interface. The influence of oxygen at the interface has also been considered by Ma et al., 7 who reported variations in the nucleation and lateral growth of Al 3 Ni due to the presence of interfacial impurities, such as oxygen. Several works have already ad- dressed the issue of asymmetry of the different interfaces depending if they are A on B or B on A type. In particular, recent measurements on Al/transition-metal thin films by Buchanan et al. 9 show the intermixing lengths are remark- ably different depending if the film sequence is Al on X or X on Al. Similar results have been also observed by Bigault et al. 10 in Ni/Au multilayers, by Luo et al. 11 in NiFe/Cu mul- tilayers, or by Gupta et al. 12 in Fe/Ag multilayers. Asymme- try of interface roughness has also been reported in several multilayer systems such as Mo/Si. 13 The origin of this differ- ence in roughness is typically related to the microstructure of the films which might be different for layer A or B depending on the deposition conditions and the nature of the film itself. For instance, it may be expected that if the polycrystalline film shows a zone-2-type microstructure, its lower and upper parts may exhibit significant variations with respect to the density of high-angle grain boundaries reaching the inter- face. However, to our knowledge no evidence of nucleation dependence on the type of interface, A on B or B on A, in polycrystalline multilayers, has been reported yet. Focusing on the Cu/Mg multilayer system, past studies proposed and demonstrated that CuMg 2 is always the first phase to form for any composition ratio. 14,15 They also showed that the formation of this intermetallic phase is di- vided into two stages, nucleation and growth along the inter- face until coalescence is achieved followed by vertical growth until depletion of one of the species. Thus, if the composition ratio is 1Cu:2Mg, CuMg 2 will be the unique phase to form, and a calorimetric study of this system would produce two exothermic peaks, one for lateral and another for vertical growth. 15 We have previously shown, 16 however, the existence of two overlapped peaks, both corresponding to a nucleation and lateral growth process. A nonsimultaneous nucleation for the two kinds of interfaces Mg on Cu and Cu on Mg was then suggested to explain this unexpected behavior. Here we report on specific calorimetric experiments of the intermetallic phase formation of CuMg 2 and unambiguously show that the origin of the two low-temperature overlapped peaks in the calorimetric curve of Cu/Mg multilayers is re- lated to a differentiated nucleation and lateral growth on both types of interfaces, Mg on Cu and Cu on Mg. To isolate the nucleation events in both interfaces we add a new layer ei- ther on top of the Cu films or on top of the Mg layers. This barrier acts as a buffer layer, preventing nucleation of the intermetallic compound in the corresponding interface. Fe was chosen due to the reduced solubility with both Cu and PHYSICAL REVIEW B 69, 113411 2004 0163-1829/2004/6911/1134114/$22.50 ©2004 The American Physical Society 69 113411-1