Radio-Controlled Microactuator Based on Shape-Memory-Alloy Spiral-Coil Inductor

This paper reports a bulk-micromachined shape-memory-alloy (SMA) actuator in the form of a spiral coil that constitutes an inductor-capacitor resonant circuit. The out-of-plane actuation of the SMA spiral-coil inductor is wirelessly controlled using external radio frequency (RF) magnetic fields. The resonant circuit is used as a frequency-selective wireless heater in which the SMA inductor produces heat to activate its own actuation when resonated with the RF magnetic field. The direct integration of bulk-micromachined nitinol SMA with a threshold temperature of 65 ^°C into a planar microfabrication process is enabled to build the 3-D spiral-coil SMA actuator in a self-assembled manner using a SiO₂ reset layer patterned on the SMA coil. The fabricated SMA structure yields an out-of-plane displacement of 466 μm in the cold state. The full actuation to the flat state is reached at 70°C upon tuning the field frequency to ~ 230 MHz with an RF output power of 0.7 W. The developed actuator is demonstrated to provide a maximum force of 30 mN. The temporal response of the actuator is revealed to be two to three times faster than that of previously reported wireless SMA actuators with separate heat sources.