C-Axis textured, 2–3 μm thick Al0.75Sc0.25N films grown on chemically formed TiN/Ti seeding layers for MEMS applications

Scandium-doped Aluminum Nitride, (AlSc)N, thin films have drawn large interest in the last decade as a lead-free, biocompatible, environmentally friendly piezoelectric material. Chemical stability, large thermal conductivity, large elastic modulus and compatibility with Si-based microfabrication methods make (AlSc)N thin films into on the most promising candidates for active materials for piezoelectric MEMS.

However, depositing fully oriented (001) films without ScN segregation may be challenging despite almost two decades of intensive development. This is because ScN and AlN are completely immiscible in bulk form, which is the driving force for their segregation and loss of orientation during deposition. The classical approach for deposition of (AlSc)N thin films is based on preparing highly (111)-textured seeding layers, commonly of Pt, W or Mo. These metals are chemically inert with respect to (AlSc)N even at elevated temperatures.

A protocol for successfully depositing [001] textured, 2–3 µm thick films of Al0.75Sc0.25N e_31=1.85±0.18 C/m^2, is proposed. The procedure relies on the fact that sputtered Ti is [001]-textured α-phase (hcp). Diffusion of nitrogen ions into the α-Ti film during reactive sputtering of Al0.75,Sc0.25N likely forms a [111]-oriented TiN intermediate layer. The lattice mismatch of this very thin film with Al0.75Sc0.25N is ~3.7%, providing excellent conditions for epitaxial growth. In contrast to earlier reports, the Al0.75Sc0.25N films prepared in the current study are Al-terminated. Low growth stress (