Abstract

The miniaturization of engineering devices has created interest in new actuation methods capable of large displacements and high frequency responses. Shape memory alloy (SMA) thin films have exhibited one of the highest power densities of any material used in these actuation schemes and can thermally recovery strains of up to 10%. Homogenous SMA films can experience reversible shape memory effect, but without some sort of physical biasing mechanism, the effect is only one-way. SMA films mated in a multi-layer stack have the appealing feature of an intrinsic two-way shape memory effect (SME). In this work, we developed a near-equiatomic NiTi magnetron co-sputtering process and characterized shape memory effects. We mated these SMA films in several “bimorph” configurations to induce out of plane curvature in the low-temperature Martensite phase. We quantify the curvature radius vs. temperature on MEMS device structures to elucidate a relationship between residual stress, recovery stress, radius of curvature, and degree of unfolding. We fabricated and tested laser-irradiated and joule heated SMA MEMS actuators to enable rapid actuation of NiTi MEMS devices, demonstrating some of the lowest powers (5–15 mW) and operating frequencies (1–3 kHz) ever reported for SMA or other thermal actuators.

Highlights

  • One of the earliest reported thin film version of Nickel-Titanium shape memory alloy was done in 1990 [1]

  • The first several accounts of Shape memory alloy (SMA) film characterization on Si wafers showed measureable shape memory effects, but all transformations happened below ambient conditions, in part due to the fact that the films tended to be Ni-rich in composition if starting from equiatomic NiTi sputter target, due to the different sputter yields of Ni and Ti

  • By shrinking the volume and heat capacity of the SMA MEMS actuators, we showed for the first time, reversible actuation beyond 1 kHz frequencies, verifying that the heat transfer, could happen more than 1000 times per second

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Summary

Introduction

One of the earliest reported thin film version of Nickel-Titanium shape memory alloy was done in 1990 [1]. The first several accounts of SMA film characterization on Si wafers showed measureable shape memory effects, but all transformations happened below ambient conditions, in part due to the fact that the films tended to be Ni-rich in composition if starting from equiatomic NiTi sputter target, due to the different sputter yields of Ni and Ti. Ni has a higher sputter rate than Ti, and Ti has a tendency to react with any residual oxygen in the deposition chamber. In order to make high performance thermal actuators, it is necessary to undergo the necessary processing to ensure transformations are measured above ambient conditions [2]. This is no easy task, but can be done by carefully controlling Ni/Ti ratio and thermal processing (i.e., annealing). One of the first SMA-based MEMS actuators was reported out of Case Western University (CWU) in 2001, based on a Advanced Functional Materials sputtered NiTi film capable of recovering about 250 MPa according to their stresstemperature loop on 4” Si wafer [2]

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