NiTi thin films on a flexible substrate : Fabrication & Characterization

Abstract

NiTi (Nickel Titanium) is a shape memory alloy (SMA) known for shape memory effect and superelasticity. The shape memory effect and superelasticity allows NiTi to be strained reversibly unto 6-8%. NiTi thin films are of technological interest as actuator materials in microelectromechanical systems (MEMS) because they possess a large deformation and recovery force compared to other performance materials proposed for fabricating microactuators . Several early attempts were made to fabricate NiTi thin films and build NiTi thin film based microactuator prototypes, such as micropumps and microvalves, microgrippers and microsensors. The aim of this PhD thesis is to study the optimization of the deposition conditions for the formation of NiTi films on polyimide flexible substrates and to exploit the shape memory and superelastic property of NiTi for flexible deformable devices. In order to achieve this goal, a three-magnetron sputter deposition chamber has been used allowing to heat and to apply a bias voltage to the substrate. Prior to the deposition of NiTi, a thin layer of Cr (~10nm) was sputter deposited onto flexible polyimide substrates. Cr acts as an adhesion layer between NiTi and the flexible substrate. This equipment is mounted with a wafer curvature setup enabling an in-situ characterization of film stress during their growth. In addition to the in-situ studies, complementary ex-situ characterization techniques such as X-ray diffraction (XRD), cross-sectional transmission electron microscopy (X-TEM), Atomic force microscopy (AFM) and electrical resistivity (ER) measurements during temperature cycling have been used for a fine structural characterization. To test the maximum strain the NiTi/polyimide composite structure can withstand in-situ measurements of film resistance as a function of strain were performed. Measurements of the resistance of the samples yields insight in the strain at which the film fails.