Confined Ni film on a surface: spatially resolved XPS study of the surface reactions
Introduction
The interaction of Si with multimetal layers and the elemental distribution across the reacted layers are rather complex. They depend on many factors, such as metal–Si affinity, miscibility of the metals with the formed phases, dominant diffusing species, formation temperatures of the silicide phases, deposition sequence, etc. The formed compounds determine the local charge distribution and atomic rearrangement and thereby dominate the Schottky barrier heights [1].
Confined metal films can behave differently from continuous films, because of the possible lateral diffusion of atoms away and the edge effects at the periphery of the film. These processes occur on microscopic and mesoscopic scales, so that high lateral resolution combined with surface chemical sensitivity are required to provide detailed chemical and electronic information on a local submicrometre scale. Recently, we have demonstrated that scanning photoelectron microscopy (SPEM) fits well to experiments with confined films, produced by deposition of metal patches of chosen dimensions and thickness through a mask [2]. Using such model-confined film systems, one can simultaneously investigate: (i) the evolution of multielement interfaces, having great advantage for reference measurements of the outside region; (ii) the effect of the metal coverage, using the concentration profile across the patch edge; (iii) the effect of the surface mass transport due to thermal diffusion and electromigration, probing the spatial expansion of the confined film and boundary effects, etc.
We studied extensively several metal and bimetal/Si interfaces, and identified with submicrometre spatial resolution two-dimensional (2D) and three-dimensional (3D) phases and variations in the chemical state of the interface constituents due to chemical reactions and/or diffusion processes [2], [3], [4], [5]. We started with bimetal interfaces with noble metals (Ag+Au), which do not form stoichiometric compounds with Si [3], [4]. Our studies on the evolution of a laterally confined Au film on a interface have shown that not only the affinity of the metal to Si, but also the thermal mass transport and surface electromigration can play a rather important role [2]. Another study of bimetal system/Si interface, consisting of a transition (Ni) and a noble (Ag) metal, has revealed that the formation of Ni silicide phases is facilitated by the presence of the noble metal [5].
Here we report on the evolution of another type of bimetal interface, consisting of homologous transition metals, Ni and Pd, which form stable silicide phases [1]. The Pd/Si(111) interface is one of the most extensively studied silicide systems: Pd2Si, the only stable silicide product of the Pd–Si interaction, forms readily even at room temperature [6]. The structure obtained by mild annealing (470–570 K) of one or a few monolayers (ML) of Pd deposited on Si(111) is very similar to the Si-rich Pd3Si2 plane, and is considered as an intermediate state to Pd2Si [7]. The Ni/Si interface is a typical example of multiple silicide formation [8]. In our previous SPEM studies of Ni/Si interfaces we identified two types of 3D silicide island, of composition and electronic structure close to NiSi and NiSi2, respectively, coexisting with a dominant ordered or disordered 2D phase [9].
Section snippets
Experimental
The experiments were performed using the SPEM measurement station at the Elettra synchrotron light source. A Fresnel zone plate lens provided the microprobe in SPEM, focusing the photon beam to submicrometre dimensions. Other important parts are the specimen positioning and scanning systems and the hemispherical capacitor electron analyser with a 16-channel detector, mounted at a grazing angle with respect to the incident beam and sample normal. The SPEM works in imaging and microspectroscopy
Results and discussion
Fig. 1a–c shows Ni 3p, Pd 3d5/2 and Si 2p images centred close to the edge of the Ni patch deposited at 300 K. The reverse contrast reflects the expected attenuation of the Si 2p and Pd 3d yield by the Ni film. Here we would like to point out that these images should be considered as concentration elemental maps. They show the sum of the counts from all 16 channels and reflect the total photoemitted signal over a 4.4 eV energy window covering the corresponding core level peak. Since the mask was
Conclusions
We investigated the temperature evolution of the interface between a confined Ni film deposited on a interface using a laterally resolved photoemission. This allowed us to compare directly the evolution of the Ni-free and Ni-covered regions and to examine the edge effects during temperature-induced transformations of the laterally confined Ni film. In brief, the main findings are Pd-promoted formation of a NiSi2-like phase and local enrichment of the edge of the confined Ni film with Si
Acknowledgements
We would like to thank Diego Lonza for his excellent technical assistance. This work was supported by Sincrotrone Trieste ScpA.
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