Rapid and low power laser actuation of sputter-deposited NiTi shape memory alloy (SMA) MEMS thermal bimorph actuators

Abstract

NiTi SMA thermal bimorph actuators have potential as high-force, high-displacement MEMS actuators. Historically, even microscale SMA actuation response has been limited to a maximum of ˜100 Hz. As NiTi film and device dimensions are scaled down into the micro and nano scales, heat transfer, and thus device cycling speeds can be significantly improved upon. We have used an in-situ annealed nickel-titanium (NiTi) shape memory alloy (SMA) sputter deposition process to sputter equiatomic NiTi films at 600 °C. We characterized our thin film (270 nm NiTi – 1.6 μm NiTi) material and verified its reversible shape memory effects (SME) using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and wafer bow versus temperature measurements. Upon release, the NiTi material exhibited a reversible phase change around 60 °C with a hysteresis of ˜3 °C. For the substrate confined case (i.e. NiTi on Si or Pt), hysteresis was much larger (i.e. ˜40 °C) with the phase change completed at ˜80 °C. In addition to SMA material characterization, we fabricated NiTi/Pt bimorph actuators at several (100 nm to 1.2 μm) NiTi and Pt thicknesses. Free-standing bimorph actuators were produced via a dry etch release, and temperature dependent curvature of these cantilevers was investigated. To address the low power, and high response time aspects, we performed a dynamic characterization using a 440 mW, 532 nm “green” laser to irradiate devices from 2 to 24 W/cm2 while measuring actuation response times that varied from 3 ms at the highest irradiation fluxes to 240 ms at the lowest. Our results showed decreased actuation powers and faster heating or actuation times compared to past works with NiTi microactuators. The NiTi films with 600 nm thickness on top of 20 nm Pt films exhibited the greatest change in curvature from 200 μm to flat states, and actuated in under 3 ms due to the very small volume of SMA requiring to be heated. These results suggest that NiTi/Pt bimorphs have potential applications as lower-power, faster-response, laser-activated micro shutters or thermal switches without needing a traditional wired power source.