Integration of AlN piezoelectric thin films on ultralow fatigue TiNiCu shape memory alloys
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Integration of AlN piezoelectric thin films on ultralow fatigue TiNiCu shape memory alloys Sabrina M. Curtis1 , Niklas Wolff2, Duygu Dengiz3, Hanna Lewitz3, Justin Jetter3, Lars Bumke3, Patrick Hayes3, Erdem Yarar3, Lars Thormählen3, Lorenz Kienle2, Dirk Meyners3, Eckhard Quandt3,a) 1
Chair for Inorganic Functional Materials, Faculty of Engineering, Kiel University, Kiel 24143, Germany; and Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA 2 Chair for Synthesis and Real Structure, Faculty of Engineering, Kiel University, Kiel 24143, Germany 3 Chair for Inorganic Functional Materials, Faculty of Engineering, Kiel University, Kiel 24143, Germany a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. Received: 24 December 2019; accepted: 7 April 2020
Biomagnetic field sensors based on AlN/FeCoSiB magnetoelectric (ME) composites desire a resonant frequency that can be precisely tuned to match the biomagnetic signal of interest. A tunable mechanical resonant frequency is achieved when ME composites are integrated onto shape memory alloy (SMA) thin films. Here, high-quality c-axis growth of AlN is obtained on (111) Pt seed layers on both amorphous and crystallized TiNiCu SMA thin films on Si substrates. These composites show large piezoelectric coefficients as high as d33,f = 6.4 pm/V ± 0.2 pm/V. Annealing the AlN/Pt/Ta/amorphous TiNiCu/Si composites to 700 °C to crystallize TiNiCu promoted interdiffusion of Ti into the Ta/Pt layers, leading to an enhanced conductivity in AlN. Depositing AlN onto already crystalline TiNiCu films with low surface roughness resulted in the best piezoelectric films and hence is found to be a more desirable processing route for ME composite applications.
Introduction Thin-film magnetoelectric (ME) composites are attractive candidates for use in biomagnetic sensors, energy harvesters [1], highly efficient power converters, magnetometers, RF tunable inductors, and mechanical antennas [2, 3, 4, 5]. Heterostructures with aluminum nitride (AlN) serving as the piezoelectric layer and amorphous iron-cobalt silicon boron (FeCoSiB) alloy as the magnetostrictive layer have demonstrated ME coefficients in vacuum as large as 20 kV/cm Oe [6] pffiffiffiffiffiffi with limits of detection as low as 1 pT= Hz [6, 7, 8]. The ME voltage and sensitivity can be enhanced by 1–2 orders of magnitude when driven at the mechanical resonant frequency [3, 9, 10]; therefore, many applications could benefit from a ME composite with a tunable resonant frequency. For example, deep brain stimulation (DBS) is a medical treatment used on patients suffering from tremors or dystonia. In this treatment, an array of electrodes are implanted deep in the patient’s brain and stimulated at a well-defined frequency between 130 and 170 Hz [11]. For localization of the stimulated area near the
electrode, it would be necessary to precisely tune the mechanical resonant frequency of a ME sensor to match the stimulation frequency o
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