In Situ, Real-Time Characterization of Silicide Nanostructure Coarsening Dynamics by Photo-Electron Emission Microscopy

Abstract

Photo-electron emission microscopy (PEEM) was used to observe the growth and coarsening dynamics of transition metal (TM) silicide and rare earth (RE) silicide nanostructures on silicon surfaces in real-time with high lateral resolution during in situ annealing. Evidence was obtained indicating that the coarsening of the silicide islands is strongly influenced by local variations in the size, shape, and number of nanostructures on the surface.

The titanium, hafnium, and zirconium silicide nanostructures were observed to grow via Ostwald ripening and attractive migration and coalescence (AMC) at temperatures as high as ˜1200°C. Ostwald ripening is a classic coarsening process in which larger nanostructures grow at the expense of smaller surrounding structures as per the Gibbs-Thompson relation. Attractive migration and coalescence is a newly discovered coarsening pathway where nearby islands are observed to migrate attractively towards each other and subsequently coalesce in response to local adatom concentration variations on the surface. A shape distortion of the normally compact and rounded TM silicide islands has been observed during these coarsening processes. The shape distortion suggests that the nanostructures are exchanging mass with each other via diffusion limited processes and these observations support the AMC model.

The dysprosium and erbium silicide nanostructures exhibit a distinct faceted morphology and primarily coarsen via Ostwald ripening. The RE silicides form highly elongated nanowires and compact rectangular nanostructures on Si(001) and triangular or hexagonal structures on Si(111). Although the seemingly one-dimensional growth of the metallic nanowires is potentially promising for future microelectronics applications, we show that the wires are metastable structures which decay in favor of the larger rectangular islands at high temperatures. Additionally, the rectangular shape and faceted morphology of the RE silicide nanostructures greatly influences their coarsening dynamics.

A separate PEEM study explored the thermal stability of thin films of TM oxides (TiO2, ZrO2, HfO2) grown on ultra-thin SiO2 buffer layers on Si(001) surfaces. The decomposition of the TM oxides was observed in the PEEM during annealing at ˜870°C, ˜900°C, and ˜1000°C for the TiO2, ZrO2, and HfO2 films, respectively. The degradation of the oxide films is attributed to a two-step reaction process which is initiated at defects in the TM oxide⁄SiO2⁄Si stack.