Reduced Radius of Curvature Nitinol-on-Pt Bimorph Actuators Based on Reversible Shape Memory Effect

Abstract

In this work we discuss the design and fabrication of NiTi on Pt bimorph cantilever arrays that may be actuated by utilizing the martensite to austenite phase transformation of a sputtered thin film of equiatomic NiTi shape memory alloy (SMA). The cantilever devices were fabricated on a silicon wafer using standard micro fabrication techniques, and may therefore be applicable to microelectromechanical systems (MEMS) switch or actuator applications. This paper details the development of a co-sputtering process to yield a SMA film with controllable composition of Ni50Ti50 and transformation temperature around 60 °C. Shape memory effects were characterized and verified using Differential Scanning Calorimetry (DSC), which demonstrated a martensite-austenite phase change near 60 °C for a co-sputter deposited film onto a Si wafer at 600 °C for in-situ crystallization. We used wafer stress versus temperature measurements as additional confirmation for the repeatable measurement of reversible phase transformation which completed by 80 °C upon heating. Up to 900 MPa completely reversible stress change was available for actuation during the thermally induced phase change. The tightest curling devices were based on a 600 nm NiTi film on 20 nm Pt and were actuated between a 200 μm curl at 25 °C and flat states when heated beyond 70 °C. Using a 532 nm (green), 440 mW laser, we also characterized actuation times of NiTi on Pt cantilever actuators from 4–240 milliseconds using optical intensities ranging from 2–24 W/cm2.