Insight in the thermal degradation phenomena and the relation to phase transformation is presented for cobalt disilicide (CoSi2) on narrow polysilicon lines (linewidth ranging from 30nmto1μm) using electrical and morphological analysis in the temperature range 650–900°C. When polysilicon lines are scaled laterally to 30nm, an abrupt CoSi2 sheet resistance increase (>1000Ω∕sq) is observed, which is attributed to silicide agglomeration. At localized positions along the 30-nm Co-silicided lines, Si/silicide layer inversion is observed. At low formation temperature (650°C), no agglomeration phenomena occur, but incidentally the transition from CoSi into CoSi2 is delayed on the 30-nm-wide polysilicon structures to such an extent that the CoSi2 growth virtually stops. Only using nitrogen implantation through CoSi and a CoSi2 formation temperature of 900°C, low sheet resistance values are obtained for 35-nm-wide polysilicon lines. Based on the results obtained in the present work, we propose that the abrupt increase in the CoSi2 sheet resistance for the narrow polysilicon lines is a consequence of a decrease in the availability of nucleation sites leading to the reduction of the CoSi∕CoSi2 transformation rate, which, in turn, results in the agglomeration of Co silicide at elevated temperature.