Thin film nickel titanium shape memory alloy microactuators

In this thesis, thin film (micrometer thickness) shape memory alloy (SMA) micro actuators are presented. The thin film SMA with composition of approximately 50:50 Nickel Titanium (NiTi) is sputter-deposited onto a silicon wafer in an ultra high vacuum system. Transformation temperatures of the NiTi film are determined by measuring the residual stress as a function of temperature. The transformation temperature is independent of the presence of Chromium (Cr) used as an adhesion layer, or being exposed to air before annealing. A mixture of Hydro Fluoric Acid (HF), Nitric Acid (HN03) and deionized (DI) water is used to etch the film. Different etch masks are evaluated to protect the NiTi film during the etching. Among the masks tested, a thick photoresist (AZ-4620) produces the best result. The NiTi membrane is hot-shaped into a 3-D dome shape using a stainless-steel jig. Results indicate the membrane exhibits two-way effect. The performance of the SMA micro actuator is characterized with a laser measurement system for deflection vs. input power and frequency response.; Manufacturing issues related to thin film NiTi shape memory alloy (SMA) microactuator (i.e. microwrapper) have also been investigated using both wet and dry etching techniques. Results show that wet etching the amorphous film produces a cleaner pattern than the crystallized film. However, this process produces breakage in the NiTi film at the sacrificial layer steps. This is believed to be due to residual stresses developed between the film and substrate during sputtering. The film breakage is overcome by dry etching the film with an ion-milling technique. Curvature in the microwrapper arms is induced using either a bi-layer material (i.e. polyimide and NiTi) or a functionally graded NiTi film. Results show that when heated the microwrapper arms flatten due to shape memory effect and curl up to form a cage-like structure when cooled.