In these heterogeneous films XAS data can be usefully compared with transport experiments that return an average value at a definite temperature or the behavior vs. temperature. The behavior of the resistivity vs. thickness showed in Figure 1
confirms that Mo films grown through the sputtering process contain small amounts of insulating oxide phases. In the following we will discuss the characterization of the samples listed in Table 1
using the XAS technique performing experiments at the Mo K-edge. Consequently, to interpret the macroscopic behavior and to reproduce the effective conductivity of these films, it is necessary to identify the amount of the insulating phases present in each sample.
The XAS technique is a powerful method capable of investigating the local structural properties at the atomic level, such as geometry and coordination numbers in ordered and disorder systems, including surfaces, coatings, or interfaces, but also sensitive to the electronic structure [30
]. XAS measurements were carried out at B18, the core XAS beamline of the Diamond Light Source, the UK synchrotron radiation facility operating at the electron energy of 3 GeV with a current of ~250 mA in the top-up mode. Spectra were collected using a double-crystal monochromator equipped with two Si (111) crystals and coupled to a Pt-coated mirror in the focusing mode illuminating the sample with a spot of ~200 μm × 200 μm [31
]. This spot does not allow the resolution of a single phase domain, but simultaneously probes thousands of small domains. Indeed, these Mo metallic films are complex multiphase systems where molybdenum may exist in several oxidation states: Mo4+
, and Mo6+
. The thermal annealing in vacuum at moderate temperature does not affect the crystal structure, but may trigger the motion of oxygen atoms changing the oxygen contents and the oxidation state of Mo ions tuning the electrical conductivity. The acquisition of the Mo K-edge spectra has been performed in the continuous scan mode using a nine-element Ge detector with XSPRESS-II acquisition electronics in the fluorescence detection mode. XAS experiments have been performed at grazing incidence to enhance the signal associated to the surface layers of the Mo coatings to probe their structural composition. This approach was already discussed in a preliminary investigation of two relatively thick Mo films where the combined analysis of transport experiments and structural data showed that the amount of ordered phases play a limited role to the conductivity of these metallic films [15
]. In that case the annealing at 300 °C slightly increased the conductivity. Actually, the conductivity of a Mo film may depend on the composition, the substrate, the morphology of the coatings, the oxygen content, the local order, the grain size, etc., and differences in the resistivity can be correlated also to the ratio between grain size and layer thickness [32
Local Electronic Properties
XAS spectroscopy is a technique that, in addition to the local structure, probes the local and partial empty density of states around the photo-absorber. At the Mo K-edge we excite 1s core level electrons and, neglecting the possible correlation phenomena due to d
electrons, the transition probes the p
-projected empty density of states at the Mo site because of the dipole selection rule. Looking at the experimental spectra in Figure 2
and as discussed in [15
] Mo K-edge spectra of these films are similar to the Mo metal spectrum and the small differences observed at the rising edge of each spectra points out tiny changes of the empty density of states of p
character. In other words, the small difference in the partial empty density of states implies a different number of occupied electrons and a different hybridization [33
]. In addition, the presence of a small shift of the K-edge points out a concurrent shift of the Fermi level, a parameter that can be correlated to the transport properties.
To this purpose, the comparison among spectra in Figure 2
is extremely interesting. In the two panels we compared the spectra of Mo films of different annealed thicknesses for one hour at 300 °C (left panel) and at 600 °C (right panel). At first sight all spectra are similar, pointing out that the local structure around molybdenum atoms is very similar. Weak changes appear only in the X-ray Absorption Near Edge Structure (XANES) region for samples annealed at 600 °C. However, after a careful normalization the differences among these spectra show a contribution near the edge that is correlated with the empty density of states of p
character due to the contributions of dipolar and quadrupolar transitions, respectively. Taking as a reference the thicker films (900 nm), the empty density of states increases with thickness. The differences of the XAS spectra (green and brown curves) are in good agreement with the differences in the resistivity show in Table 1
left panel: green ≥ 20 μΩ∙cm, brown ≥ 85 μΩ∙cm; Figure 2
right panel: green ≥ 45 μΩ∙cm, brown ≥ 105 μΩ∙cm). In fact, for these films the spectra shows a negligible amount of insulating oxide phases. The differences among the spectra demonstrates that the edge of the thicker films shifts slightly towards a lower energy, corresponding to a change of the Fermi energy actually compatible with a weak contribution of MoO3
(see the bottom panel in Figure 3
As underlined above, the conductivity of Mo metallic films depends on many parameters. For this reason, for large scale applications, not only for high-performance accelerator components, is it important to characterize all properties: morphology, composition, structure and resistivity, and substrate. As an example, on crystalline substrates we may grow oriented films with a negligible strain and for thicknesses up to 600 nm with a resistivity <100–150 μΩ∙cm. In this framework, in recent years, modeling and simulation tools have become more and more important, being effective alternatives to long and costly experimental tests. However, for improved RF devices, it is mandatory to study the growth of Mo films not only on oriented surfaces of insulating or semiconductor substrates as discussed above, but on metallic substrates, such as copper.
We deposited Mo films by sputtering on oxygen-free high thermal conductivity (OFHC) copper surfaces with a high roughness (~700 nm). Atomic force microscopy (AFM, MSNL-10 of Veeco Instruments, New York, NY, USA) images showed that the surface morphology of the Mo coatings preserve the initial roughness, and even without annealing, the molybdenum layer may slightly improve the original roughness. However, since Cu and Mo have different crystal structures (f.c.c. vs. b.c.c.) and different thermal expansion coefficients, the Mo layer deposited on top of the Cu surface changes with temperature and its structure above 900 °C. We investigated the deposition of thin Mo layers sputtered on Cu surfaces annealed at 750 °C for 10 min [5
]. A previous study pointed out that a sputtering thickness of 180 nm is not suitable enough to obtain a homogeneous coating surface and, due to the low contacting force and the intense thermal stress, this thin coating annealed at 600 °C for two hours shows a clear separation of the Mo layer from the substrate. Indeed, the thermal expansion coefficients of Cu and Mo are quite different, i.e., 16 × 10−6
and 5 × 10−6
μm/m/°C, respectively. Rutherford backscattering spectrometry (RBS, 3.5 MeV Singletron accelerator, High Voltage Engineering Europa B.V., Amersfoort, The Netherlands) experiments performed with a 4
ion beam characterized the depth profile of Mo on the Cu substrates [5
]. Data showed an inter-diffusion of the Mo at the Cu-Mo interface, a process that tend to form a homogeneous mixture at the interface not observed for films growth on SiO2
. RBS also pointed out a Mo concentration lower than a pure Mo film, a behavior compatible with the presence of oxide phases in the deposited film. The non-uniform oxygen distribution and the resistivity, the latter higher by about two orders of magnitude compared to a pure film of Mo, are also compatible with the presence of insulating oxide phases [5
These Mo metallic films are more heterogeneous and disordered than those grown by magnetron sputtering on Al2
substrates, as shown in Figure 3
(upper panel), where we compare the XAS spectra at the K-edge of Mo of two films of ~300 nm and ~600 nm grown on a clean OFHC Cu surface. The spectrum of the 300 nm film is characterized at the edge by a very broad feature, characteristic of a strongly-disordered local structure, while the spectrum of the 600 nm film starts to show the typical shape of Mo metal with a shift of 1.5 (±0.2) eV towards low energy, which points out a lowering of the Fermi level with respect to the thinner disordered Mo film. The difference (blue curve) clearly points out a significant enhancement of the empty density of states in the 300 nm disordered film, compatible with a small contribution from insulating oxide phases.
To clarify the behavior of the XANES spectra of the Mo film grown on Al2
and copper substrates, we simulated the Mo K-edge XANES spectra of Mo metal and of the two main oxide phases: MoO2
. Spectra were simulated using the full multiple scattering (FMS) theory as implemented in FEFF9.6 code [34
]. The self-consistent field (SCF) method was employed for potential calculations with Hedin-Lundqvist exchange and correlation potential. The radius of atomic clusters for SCF and FMS calculations are, respectively, 5 Å and 10 Å. The crystallographic structures were adopted as starting models. Neither an additional energy shift nor extra broadening convolution were applied in all calculations. In Figure 3
(bottom panel) are compared to the simulated XANES spectra of the main oxides phases (MoO2
metallic and MoO3
insulating). Both spectra are different from the spectra of Mo coatings, in agreement with the description of these films: heterogeneous systems containing metallic domains and negligible contributions of insulating oxide phases.