Tungsten interconnects in the nano-scale regime

The resistivity of tungsten interconnect structures manufactured by a damascene CVD process was studied for a wide range of line widths (40-1000 nm). A significant electrical size effect was observed: The relative increase reaches values up to 65% for a 50 nm line compared to wide lines. The root cause for the size effect is the reduced mobility of the charge carriers as critical dimensions decrease. For the first time the size effect in tungsten nano-interconnects is studied on the basis of a broad experimental data set. It was found for a temperature range down to 6 K that the size effect is independent of temperature. An excellent fit of the resistivity increase using our size effect model has been obtained. Based on SEM and TEM images a line width dependence of the median grain size was found. The median grain size was used to quantify the modeling parameter for scattering at grain boundaries. For grain boundary scattering a reflection coefficient R of 0.25 was found. For the contribution of surface scattering, the specularity parameter p was 0.3. The bulk value of resistivity and the mean free path of charge carriers were 8.7 μΩ cm and 33 nm.