Stress Control for Highly Doped Aluminum Scandium Nitride Films

Recently, highly Sc doped aluminum nitride films have been used for variety of applications ranging from microphones to PMUT sensors. Deposition on different substrate surfaces such as Pt, Mo, W, Si, SiO2 and SiN (both patterned and un-patterned)lead to requirement to be able to control stress over 1 GPa range. In order to obtain zero stress on different surfaces, equivalent process condition on Si wafers requires large stress offset. Stress is typically controlled by adjusting deposition pressure and/or substrate bias. For concentrations above 20%atomic Sc, stress control using substrate bias results in non-uniform stress across wafer. Stress control using adjustment in deposition pressure can produce good stress control across a wafer but can only produce stress adjustment with good thickness uniformity control over a limited stress range. Furthermore, at Sc concentrations above 30%, sporadic crystal growth is observed on more tensile depositions. In this presentation, we demonstrated stress control over 1 GPa range, by using a secondary magnetic field that is easily adjusted from outside of the magnetron and vacuum environment. We were able to obtain stress variation across 200mm wafer below 100MPa. No sporadic crystal growth was observed between 20% Sc and 32% Sc range.