Published in Proceedings of the 2005 Materials Research Society Spring Meeting: Symposium O, Volume 875: Thin Films—Stresses and Mechanical Properties XI, April 1, 2005.
Microactuators fabricated with NiTi thin films take advantage of this material’s large energy density (~5-10 joules/cm3) and high strain recovery (~8%). Microelectromechanical Systems (MEMS) devices designed with these actuators can serve as biosensors, micro-fluidic pumps or optical switches. However, the fundamental mechanical properties stemming from the crystalline structure of these shape memory NiTi films have not been fully characterized with MEMS scale test structures. Understanding the relationship of crystalline structure, film stoichiometry, and phase transformation temperatures is crucial when designing a MEMS actuator. In addition, force displacement curves for both the martensite and austenite phases of NiTi are necessary to know how an actuator will perform. Atomically equal NiTi thin films were deposited on silicon wafers by co-sputtering a NiTi and a Ti target at 250W and 75W respectively. These power settings yielded a deposition rate of 12 angstroms per second. Pre-sputtering the NiTi and Ti targets effectively reduced the base pressure and cleaned the target surfaces, which made oxygen undetectable in the films. A 100 nm chromium layer provided excellent adhesion to Si, but poor adhesion to SiO2. The films will eventually be used in a cantilever test structure in conjunction with a surface profilometer to measure their mechanical and actuation properties.
Materials Science and Engineering
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