1 Laser Annealing of Amorphous Ni-Ti Shape Memory Alloy Thin Films Xi Wang, Zhenyu Xue, Joost J. Vlassak Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA, U.S.A. Yves Bellouard Micro- & Nano-Scale Engineering, Mechanical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands Introduction Shape memory alloy (SMA) thin films, especially those based on Ni-Ti, have received considerable attention since it was demonstrated that they can undergo a nearly perfect shape memory effect [1-7]. When SMA films are used as actuators in MEMS devices, biasing springs are generally needed to restore the initial state in order to achieve a two- way shape memory effect. As a result, use of SMA actuator in MEMS has been limited mainly to bimorph-like mechanisms. Recently, laser annealing of shape memory alloys (LASMA) emerged as a promising approach for the fabrication of planar mechanisms [8,12]. This technique has the advantage that shape memory properties can be spatially distributed: material crystallized by laser irradiation has shape memory properties and can be used as an actuator, while untransformed material is passive and provides a restoring force. In this paper, we present the results of an experimental study of the laser annealing process for NiTi thin films, along with a computational model. Experiments NiTi thin films with a thickness of approximately 1.5 µm were prepared by co-sputtering a NiTi alloy target and an elemental Ti target. The substrates consisted of 1mm thick fused quartz. The back- ground pressure of the deposition chamber was less than 5.0x10 -8 Torr and the Ar working pressure was 1.5 mTorr. The nominal target-substrate distance was 100 mm; the deposition rate was approximately 20 nm/min. during deposition. In order to get a uniform film thickness and composition, the substrates were rotated at a speed of 20 RPM. The composition of the films was measured to be 50.5 ± 0.2 at. %Ti by means of electron microprobe analysis (EMPA). The as-deposited films were amorphous as confirmed by X-ray diffraction. In the laser annealing process, samples were annealed by scanning a laser beam over the surface of the films. A fiber-injected CW near-IR laser diode (coherent/925nm) was used to obtain the results presented in this study. The laser beam had a Gaussian power distribution and a diameter of approximately 0.9mm. The specimen was mounted on a platform capable of planar translational and rotational motions with micron resolution. In the experiment, the laser power was varied from 5 to 9.4 W; the scan speed was varied from 1 to 8 mm/s. All scans were performed in air in a thermally stabilized environment. During laser annealing a thin oxide layer was formed on top of the NiTi film. As we will show later in this paper, this oxide plays an important role in the laser annealing process. Experimental results The laser beam was scanned across the film surface in a straight-line pattern. The annealed section of the film was then investigated using optical microscopy. As shown in Fig. 1, the annealed area is rough and shows surface relief in contrast to the shiny unannealed area. The width of the annealed line is a few hundred microns. The structure of the annealed films was investigated systematically as a function of laser power density and scan speed using X-ray diffraction (XRD) and transmission electron microscopy (TEM); the results are