PhD Thesis_RuiMSMartins.pdf - RUN UNL

In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization
volume ratios allow a rapid heat transfer improving thus response speed. As films they have
the additional potential for batch fabrication and, correspondingly, Ni-Ti films are attractive
materials for microfabrication and integration in micro-miniature systems composed of
mechanical elements, actuators, sensors and electronics made on one chip [28-30]. Due to
their high power density, and due to the large displacement and actuation force, most
applications of Ni-Ti films are focused on micro-actuators, such as micropumps,
microgrippers, microvalves, micropositioners etc. [10]. Because of their sensitivity to
environmental changes, e.g. thermal, and/or to stress they are also ideal materials for
applications in microsensors [31].
In a general way, for the production of films of a certain material, the choice of the
deposition technique is normally based on fulfilling the requirements put forth by the sample
properties as well on practical limitations such as cost, area of coverage and vacuum
requirements. In this study, magnetron co-sputtering has been applied for depositing the Ni-Ti
samples.
Nonthermal growth techniques, such as sputtering, have proved to be among the most
promising regarding the control of morphology and crystalline quality during film growth.
This technique can lead to unusual and often advantageous growth kinetics and can force a
film to grow under conditions far from thermal equilibrium. Melt-spinning is considered as
well a non-equilibrium technique (the result may strongly depend upon the cooling rate) and it
has been used by some researchers for the production of Ni-Ti ribbons [32, 33]. It allows also
to obtain fine grains (rapid quenching) but it may be difficult to obtain reproducible results
among different apparatus or among different researchers. Typically the thickness of meltspun
ribbons is 30-40 µm. It is also possible to make strips as thin as 30 µm by cold rolling
and, thus, melt-spinning technique and rolling technique may become competitive in the
future [16]. Nevertheless, considering potential applications in the field of MEMS, sputter
deposition technique is much more suited because the Ni-Ti films can be deposited directly on
semiconductors or IC, and patterning can be as well incorporated using photoresist technique.
A brief introduction to the sputtering technique, including growth and morphology of
films in general, is presented in this section. There are also some considerations concerning
the effects of ions bombardment, which are linked to this deposition technique, as well as the
occurrence of stress in the films. Furthermore, the type of texture and their driving forces are
discussed.
Chapter 1 - Background and Literature Review 18