PhD Thesis_RuiMSMartins.pdf - RUN UNL

In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization
3.4.1.2. Ni-Ti deposited on TiN(100)/MgO(100)
As mentioned earlier, TiN films are an example of barrier-layers and they have been
used in microelectronics industry. Typically, the deposition conditions chosen to obtain a
preferred (111) grain orientation lead to a higher volume of voids between grains, whereas the
(200) textured film is denser.
The heteroepitaxial growth of TiN on MgO(100), deposited by reactive magnetron
sputtering, has been studied in situ by Bøttiger et al. [92]. Using real-time specular X-ray
reflectivity, layer-by-layer growth was observed, with the surface roughening decreasing with
an increase in the deposition temperature. TEM observations revealed atomic planes passing
through the MgO/TiN boundary. The TiN(100) films are pseudomorphic to the underlying
MgO(100) substrate. The tabulated lattice parameter for bulk TiN is 0.424 nm and the
corresponding one for MgO is 0.420 nm.
In the present work, a further experiment was carried out using a TiN buffer layer
(≈ 15 nm thick) to act as a diffusion barrier and also because it was believed to be useful as a
buffer for epitaxial growth of the Ni-Ti film on the MgO(100) substrate. The respective in-situ
characterization results are presented below.
Low angle specular reflectivity time resolved at a fixed incidence angle (θ = 1°) was
useful to determine the growth mode of the TiN buffer layer and of the Ni-Ti film (first
9min 30s) for the deposition on MgO(100) substrate (Fig. 3.46), revealing a layer-by-layer
growth mode (with the decrease in oscillation amplitude, indicating that the surface becomes
rougher during sputtering). The intensity oscillations were observed for a longer time
comparing the deposition with a TiN buffer layer and the deposition of Ni-Ti directly on
MgO(100). In both cases no V b was applied.
A single orientation of the Ni-Ti B2 phase grew during the 129 min of deposition,
with (100) planes stacking parallel to the substrate (Fig. 3.47). The first layers deposited on
the TiN(100) were constrained (compressive planar stress state) and this deformation
remained until the end of the deposition; later layers had a gradually relaxed interplanar
distance (smaller d (200) in the direction of the surface normal), indicating a gradual relaxation
of the compressive stress constraint imposed at the interface Ni-Ti/TiN (Fig. 3.48).
Chapter 3 – Results 142