Sputter-Deposited Shape-Memory Alloy Thin Films: Properties and Applications
- PDF / 767,641 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 28 Downloads / 238 Views
Sputter-Deposited Shape-Memory Alloy Thin Films: Properties and Applications
Akira Ishida and Valery Martynov Abstract Shape-memory alloy (SMA) thin films formed by sputter deposition have attracted considerable attention in the last decade. Current intensive research demonstrates that unique fine microstructures are responsible for the superior shape-memory characteristics observed in thin films as compared with bulk materials. Simultaneously, much effort has been undertaken to develop and fabricate micro devices actuated by SMA thin films. This article reviews the research to date on shape-memory behavior and the mechanical properties of SMA thin films in connection with their peculiar microstructures. Promising applications such as microvalves are demonstrated, along with a focused discussion on process-related problems. All of the results indicate that thin-film shape-memory actuators are ready to contribute to the development of microelectromechanical systems. Keywords: intermetallic alloys, phase transformation, MEMS, shape memory, thin films.
Introduction In the last decade, shape-memory alloy (SMA) thin films formed by sputtering have been attracting great interest as powerful actuators in microelectromechanical systems (MEMS) such as microvalves, microfluid pumps, and micromanipulators, since SMA thin films possess a large force and displacement as compared with the other actuator types (electrostatic, electromagnetic, and piezoelectric). Typically, the available stress and elongation limits for SMA actuators are 600 MPa and 5%, as compared with 40 MPa and 0.1% for piezoelectric actuators. SMA actuators have the advantage of a relatively large power output for a small size1 (Figure 1). In addition, the slow response, which is regarded as a drawback for SMA actuators of normal size, is enhanced by a large surface-to-volume ratio. Furthermore, they have potential compatibility with the batch-processing technology of silicon micromachining, so the
MRS BULLETIN/FEBRUARY 2002
actuators could be powered and controlled electrically by resistive heating of the film, utilizing the potential of integrated microelectronics. Such micro devices fabricated by the silicon micromachining process are in high demand in a variety of fields such as medicine, biotechnology, the semiconductor industry, and other applications. The first microvalve with a thin-film SMA actuator was reported in 1992.2 Since then, intensive research work has been done to clarify the properties of SMA thin films, and simultaneously, many efforts to fabricate other micro devices driven by SMA thin films have been made. This article reviews the properties of SMA thin films and introduces some promising applications.
Sputtering Process SMA thin films with thicknesses ranging from 0.5 m to 20 m can be produced by
magnetron sputtering. In the sputtering process, atoms removed from a target material by the sputtering gas condense onto various substrates such as silicon wafers, polyimide films, and glass plates to form a thin film. The early diffic
Data Loading...
