Stability and residual stresses of sputtered wurtzite AlScN thin films

Scandium-alloying of aluminum nitride (AlScN) enhances the piezoelectric properties of the material and increases the performance of piezoelectric microelectromechanical systems (MEMS). However, this enhancement is caused by the destabilization of the wurtzite phase and so far the stability of AlScN thin films has not been sufficiently studied. Stability is especially important for piezoelectric devices because changes to the film microstructure or residual stress can lead to drastic changes in the device behavior. The stability of AlScN is investigated by annealing sputtered films and characterizing the resulting changes. It is found that the wurtzite phase of ${\mathrm{Al}}_{0.7}{\mathrm{Sc}}_{0.3}\mathrm{N}$ is stable at least up to $1000{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ and annealing increases the crystal quality, reaching a maximum at $800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. When annealed for more than 100 h at $1000{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, argon used in sputtering segregates into the grain boundaries and causes compressive strains and formation of rock-salt phase. Additionally, annealing at $1000{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ for 5 h reduces the average tensile stress by approximately 1 GPa.