Kinetics of voiding and agglomeration of copper nanolayers on silica

Abstract

We report the kinetics of annealing-induced dewetting of sub-$60\text{\ensuremath{-}}\mathrm{nm}$-thick Cu films on silicon dioxide based upon in situ electrical resistance, ex situ scanning electron microscopy and atomic force microscopy measurements. Cu films become discontinuous upon annealing between $\ensuremath{\sim}300$ and $600\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ through void nucleation and island formation. Films dewet via two kinetically limiting sequential processes: void nucleation by grain boundary grooving (activation energy ${E}_{a}=1.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) followed by void growth and islanding through surface diffusion of Cu at the $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$ interface, i.e., surface spreading, (activation energy ${E}_{a}=0.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$). The kinetic pathway for dewetting is film-thickness dependent. For film thinner than $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, complete dewetting occurs between 300 and $450\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and is limited by surface diffusion of Cu at the $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$ interface, while for thicker films $(>20\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ dewetting is governed solely by grain boundary grooving. This thickness-dependent dewetting is described by a phenomenological model validated by the evolution of mean roughness and lateral correlation length of the Cu surface. This work provides a framework for evaluating the morphological stability on ultrathin metal films on dielectric materials, in particular those being considered for use in micro- and nanodevice structures.