Ion bombardment of shape memory alloy (SMA) Ni-Ti films

Ni-Ti films were deposited by magnetron cosputtering using a chamber installed into the six-circle goniometer of the Rossendorf BeamLine at ESRF. The in-situ XRD studies enable us to identify different steps of the structural evolution during growth and annealing. The use of diverse type of substrates and the effect of the application of a bias voltage to the substrate during film deposition were major tools in this study. When a negative bias voltage is applied, it causes a higher energetic ion bombardment of the growing film, resulting in an enhanced surface-atom mobility, but at sufficiently high voltage, leads to defect formation. Cross-sectional TEM and SEM micrographs have shown a change in the morphology of the films linked with different levels of bias voltages (0, -45 and -90V in this study). The transformation behaviour of the sputtered Ni-Ti films (as shown by electrical resistivity measurements) is influenced by the energy of the bombarding ions. The microstructure details resulting from the deposition using different bias values are discussed.
Additionally, it has been commissioned an ion gun allowing post-deposition ion irradiation or ion bombardment during sputter deposition. In this first series of experiments, a Ni-Ti film was irradiated with He ions after deposition (without exposing the film to the atmosphere, i.e. reduced surface oxide formation) thus modifying deliberately the microstructure of the film locally. A layer of approximately 50% of the thickness of the film was selectively removed at the irradiated region (6 x 6 mm²).
This type of studies will be very useful for the easier design and miniaturization of Ni-Ti films based devices in the field of micro-electro-mechanical systems (MEMS).