Characterization of Sputtered Nickel-Titanium (NiTi) Stress and Thermally Actuated Cantilever Bimorphs Based on NiTi Shape Memory Alloy (SMA)

Abstract : Nickel-titanium (NiTi) based thin films are the most commonly used material for shape memory alloys (SMAs). We report on our NiTi sputter process and results for fabricating thin-film NiTi microelectromechanical system (MEMS) cantilevers. To produce a repeatable and optimal process for depositing NiTi thin films, we focused on varying the sputter parameters during NiTi deposition, such as thickness, substrate temperature during deposition and anneal, and argon pressure during deposition. We recorded equilibrium heating and cooling videos of released NiTi cantilevers, which were used to study the cantilever bending as a function of temperature. We determined that on heating, the cantilevers gradually curled upward from 30 to 74 deg C, at which point they rapidly folded flat. On cooling, the cantilevers rapidly curled at 63 deg C and remained curled until 40 deg C, at which point they gradually flattened as cooled to room temperature. We measured stress as a function of temperature for NiTi on silicon (Si), obtaining room temperature residual stresses as low as -29 MPa. In addition, we achieved a residual stress differential of up to 882 MPa, corresponding to useful actuation. Further research is needed to optimize the film properties and produce a consistent process for achieving low residual stress MEMS cantilevers.