We fabricate thermally stable and low resistance Ru ohmic contacts to $n-ZnO:Al(3 imes10^{18} extrm{cm}^{-3})$, grown by specially designed dual target sputtering system. It is shown that the as-deposited Ru contact produces a specific contact resistance of $2.1{ imes}10^{-3}{Omega} extrm{cm}^2$. Annealing of the Ru contacts leads to the improvement of current-voltage characteristics. For example, annealing of the contact at $700^{circ}C$ for 1 min produces a contact resistance of $3.2{ imes}10^{-5}}{Omega} extrm{cm}^2$. furthermore, the metallisation scheme is found to be thermally stable: the surface of the contact is fairly smooth with a rms roughness of 1.4 nm upon annealing at $700^{circ}C$. These results strongly indicate that the Ru contact represents a suitable metallisation scheme for fabrication of high-performance ZnO-based optical devices and high-temperature devices. In addition, possible mechanisms are suggested to describe the annealing temperature dependence of the specific contact resistance.