M. Caron and S. Dahl
A critical need in the development of silicon based microelectromechanical systems (MEMS) is to obtain forms of actuation which are compatible with both the materials and processing technologies used in silicon micro-electronics. Furthermore, actuation would ideally be powered and controlled electrically to fully exploit the potentiel of integrated MEMS. Thin films of shape memory alloys (SMAs) are considered as attractive candidates able to fulfil these stringent requirements.
SMAs form a group of metals that have a typical shape-recovery characteristic when heated. When these alloys are deformed while below the martensitic finish temperature, they recover their original, undeformed shape, when heated above the austenite temperature (Fig. 1).
A magnetron sputtering process for TiNi shape memory alloys is presently under research at LISA. This alloy contains titanium and nickel, materials which are compatible to IC processing facilities. Furthermore, the heat energy required to initiate and maintain the shape recovery mechanism can be provided by resistive heating of the SMA itself. This allows for the use of electrical energy as the source of both power and control for the actuator.
TiNi thin films sputtered under different conditions and treated thermally at different temperatures for different times have been fully characterized by SEM, EDS, AES, XPS and XRD. These films have the appropriate Ni/Ti ratio but still contain a small amout of oxygen that will decrease the phase transformation temperature. The resistivity have been also measured in the -100 deg.C to 100 deg.C temperature range in order to characterize the phase tranformation. Another technique used at LISA is the measurement of the stress induced in the film using the substrate curvature method. Stress was measured for a temparature cycle between 25 deg.C and 600 deg.C in order to follow the crystallization and phase transformation. Figure 2 is an example of such an experiment, showing that the minimum in the heating curve is related to the film crystallization.
Moreover, it as been shown that annealing under a mixture of N2 and H2 induce the formation of a 100nm oxyde layer which have the capability to be etched by Reactive Ion Etching.
The shape recovery capabilities of our films are demonstrated using the device shown in Figure 3. Cyto-toxicity tests are currently performed for bio-actuator applications.
Fig. 1: The stress as a function of the temperature and the deformation for a SMA phase transition.
Fig. 2: The induced stress cycle of as deposited amorphous TiNi films.
Figure 3: Characterization device to demonstrate the shape recovery of TiNi films