PhD Thesis_RuiMSMartins.pdf - RUN UNL

In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization
between the intermediate layer of poly-Si and the crystallization behaviour, as well as the
final structure of the deposited films, was discussed. It has been shown that the presence of a
poly-Si buffer layer enhances the crystallization process of the Ni-Ti films, because of the
higher number of nucleation sites provided by the rougher surface morphology of the poly-Si
layer, when compared with a naturally oxidized Si(100) substrate. The in-situ studies of the
structural development have been limited to post-deposition heat treatments and it seems
essential to follow in situ the evolution of the structure of the growing film. Nowadays,
despite several methods proposed for resolving some of the processing difficulties, the
deposition of Ni-Ti films with definite stoichiometry and high purity remains a challenge.
Moreover, it is clear from the published works that important issues like the formation of film
texture and its control are still unresolved.
The aim of this PhD thesis is to study the optimisation of the deposition conditions for
the formation of Ni-Ti films in order to obtain the material fully crystallized at the end of the
deposition, and to establish a clear relationship between the substrates and texture
development. In order to achieve this goal, a two-magnetron sputter deposition chamber has
been used allowing to heat and to apply a bias voltage to the substrate. This equipment can be
mounted into the six-circle diffractometer of the Rossendorf Beamline (ROBL) at the
European Synchrotron Radiation Facility (ESRF), enabling an in-situ characterization by X-
ray diffraction (XRD) of the films during their growth and annealing. In addition to the in-situ
studies, complementary ex-situ characterization techniques such as XRD at a laboratory
source, Rutherford backscattering spectroscopy (RBS), Auger electron spectroscopy (AES),
cross-sectional transmission electron microscopy (X-TEM), scanning electron microscopy
(SEM) and electrical resistivity (ER) measurements during temperature cycling have been
used for a fine structural characterization.
The thesis consists of five principal chapters: (1) Background and literature review, (2)
Experimental details, (3) Results, (4) Discussion and (5) Conclusions and future work.
Chapter 1 is divided into three sections. First a brief overview of the Ni-Ti system is
given that supports future discussions on the composition control, the phases present in the
films and phase transformations. In this section, the SME and the SE are also described. The
second section is dedicated to the phenomena involving the deposition of thin films in general
by sputtering. A short background of the sputtering process, including growth and
morphology of the films is presented. The effects of ion bombardment and some
considerations about stress in the films are also presented. In this thesis, since, the relationship
Introduction 3