Multifunctional Sputtered NiTi Thin Film Stretchable Electrodes for Wearable Electronics

The next generation of wearable sensors, actuators, and power microsystems require reversible elastic deformation of at least 30% - 100% macroscopic strain, to match the elasticity of human skin. To achieve this, devices are often fabricated through the “island-plus-bridge” method where an array of active rigid device micro-islands are separated by stretchable metal interconnects, patterned into serpentine geometries. Through design, traditional thin-film metals in serpentine microstructures undergo geometrical reconfigurations to relieve stress, achieving unusually large macroscopic strains of tens to several hundreds of percent [1].

Sputtered thin film Nickel – Titanium (Ni_XTi_1-X) based alloys with austenitic finish temperatures below room temperature are attractive for stretchable systems. They are capable of pseudoelastic recovery from remarkable intrinsic strains of up to 8% due to stress induced martensitic transformations, significantly higher than strains achieved by traditional metals (~1-3%). Additionally, they exhibit exceptional mechanical strength and efficient electrical conductivity, making them ideal stretchable electrode material candidates for wearable electronics.

Here we present a fabrication process which allows for the release of sputtered NiTi films in stretchable serpentine geometries, using a double sided polished (100) Si wafer. Preliminary tensile testing of a 2 µm thick NiTi film in 50 µm wide curved corner and rectangle serpentine traces demonstrated large strain-to-ruptures of 130% and 162% respectively. We report on NiTi thin films that exhibit superelastic behavior at room temperature, their cyclic tensile performance, and their resistivity to evaluate the film’s stretchable mechanical and electrical fatigue performance.

[1] Zhang, Yihui, et al. Soft Matter 9.33 (2013): 8062-8070.