ICMCTF 2023 Session HP-ThP: Advanced Characterization Techniques for Coatings, Thin Films, and Small Volumes (Symposium H) Poster Session

XPS depth profiling is a widely employed analytical technique to determine the chemical composition of thin films, coatings and multi-layered structures, due to its ease of quantification, good sensitivity and chemical state information. Since the introduction of XPS as a surface analytical technique more than 50 years ago, depth profiles have been performed using ion beam sputtering. However, many organic and inorganic materials suffer from ion beam damage, resulting in incorrect chemical compositions to be recorded during the depth profile. This problem has been resolved for most polymers through the use of argon gas cluster ion beams (GCIBs), but the use of GCIBs does not solve the issue for inorganics. A prototype XPS depth profiling instrument has been constructed which employs a femtosecond laser rather than an ion beam for XPS depth profiling purposes. This novel technique has shown the capability of eradicating chemical damage during XPS depth profiling for all initial inorganic, compound semiconductor and organic materials examined. The technique is also capable of profiling to much greater depths (10s - 100s microns) and is much faster than sputter XPS sputter depth profiling. FESLA XPS results will be shown for selected bulk, thin films and oxidised surfaces and the outlook for this new technique discussed.

Magnetron sputtering deposition is one of the main PVD deposition techniques, for a wide range of materials ranging from metals to semiconductors and insulators. Current research efforts in thin film growth and characterization are often hindered by extraneous interferences and impurities, stemming from film surface contact with the atmosphere between the film elaboration and ex situ characterization steps.

In order to overcome these challenges, the MISSTIC (Multilayers and Interfaces Sputtered-deposition on STructured substrates and In-situ Characterization) vacuum experimental setup was developed. This setup (Fig-A) is composed of a deposition chamber and an analysis chamber, connected via a load-lock mechanism through which samples can be transferred under vacuum conditions. This internal sample transfer between chambers avoids contact with the atmosphere, thereby avoiding interfering phenomena such as film ageing and adsorbed species from air. As such, by eliminating the need for capping (protective) layers on top of the deposited metallic films, in situ X-Ray Photoemission Spectroscopy (XPS) in the analysis chamber can characterize the deposited film surface chemistry, without any depth profiling necessary. Sample annealing under vacuum, with an electron beam setup located in the same chamber, can even be used for studying elemental diffusion in thin film stacks. Finally, surface characterizationusing in situ Low Energy Electron Diffraction (LEED) is used for the study of crystallized surfaces, and for studying thin film epitaxial growth – such as for the case of Ag film sputtering on ZnO underlayers.

Real-time characterization techniques are another kind of measurement, present in the MISSTIC deposition chamber. These measurements are carried out during film deposition, and allow for the detection of different stages of film growth: they include Surface Differential Reflectance Spectroscopy (SDRS), film electrical resistance, and a new method for film mechanical stress measurements (PReMC, Refs[1,2]. For the study of Ag film growth on silica substrates during magnetron sputtering deposition, real-time measurements allow for the detection of the nucleation, growth, coalescence, percolation, and continuous film formation threshold thicknesses. Combinations of these techniques are used for studying the effects of different parameters of the sputtering deposition process, on the Ag thin film growth mechanism.

Ref1: Sergey Grachev et al 2022 Nanotechnology 33 185701

Ref2: Quentin Hérault et al. Acta Materialia 221 (2021) 117385

Space-based optical glass sensors are sensitive to the harsh radiation of Earth’s Van Allen belts, and one approach to protect these sensors is by using thick optical coatings. However, thick films of radiation absorbing materials, such as silver or gold, often exhibit increased surface roughness compared to the thin films used for high reflectivity mirrors. This surface roughness degrades the desired reflectivity and increases the optical scatter as film thickness increases. Alloying Ag with Al is an approach towards improving the surface roughness of these thick optical films, but alloying can lead to high residual stresses. These residual stresses are compositionally dependent and can be detrimental to surface figure and film adhesion. In this work, in-situ stress of sputtered AgAl alloy thick optical films (≈1 μm thickness) was monitored to better understand stress evolution. Films of varying composition were deposited on pre-characterized Si substrates via RF magnetron sputtering to investigate the interplay of alloy content on microstructure, stress, and optical performance. For the AgAl alloys, the focus was on compositions within the intermetallic region, as this has been shown to yield films with greatly improved optical properties. All AgAl thick films showed improved surface roughness and scatter performance compared to their thick undoped Ag counterparts. In-situ stress measurements for all compositions revealed compressive films, developing into a more compressive state with increasing thickness. However, ex-situ stress measurements revealed films with tensile residual stresses. Monitoring the film stress post-deposition and during venting showed that the films underwent a stress reversal as they cooled. Various venting conditions were explored to further investigate this phenomenon.

For hexagonal structured materials, anisotropic properties are well known due to their specific lattice distortion with respect to basal and prismatic planes. In the case of magnetron sputtered transition metal diborides direction-dependent hardness was reported for WB_2-z, ZrB₂, as well as for TiB₂ [1-3]. In more detail, the anisotropic behaviour of hexagonal AlB₂ structured diborides is explained by a more difficult dislocation movement due to energetically less preferred slip systems. For TiB_2+z it was shown that a preferred crystal growth in 0001 direction predominates super-hardness (> 40 GPa) over any stoichiometry variations [3,4]. Furthermore, the growth orientation of TiB_2+z is highly dependent on process parameters, especially the pressure, which was suggested by Neidhardt et al. [5] and experimentally confirmed in a recent publication [3].

For state-of-the art protective coatings not only a high hardness is beneficial, but also specifically low residual stress states are preferred. By varying the pressure during the growth of TiB_2+x coatings, the mechanical properties have been tailored by structural adaptions throughout the film cross-section. In addition, the incorporation of metallic Ti interlayers is an interesting tool for stress management within these films. To study the progression of the stress sates as well as orientation relations throughout the film cross-section, X-ray nano-diffraction synchrotron experiments (beamline P03 at PETRA III) have been performed. Furthermore, the structure-mechanical properties were described by a broad set of characterization techniques such as SEM, Nanoindentation, or micro-mechanical testing techniques.

Keywords: Transition Metal Diborides, Anisotropy, Residual Stresses, Synchrotron Investigations, PVD

[1]B. Hunter et al., Investigations into the slip behavior of zirconium diboride, J. Mater. Res. 31 (2016) 2749–2756.

[2]C. Fuger et al., Influence of Tantalum on phase stability and mechanical properties of WB2, MRS Communications. 9 (2019) 375–380.

[3]C. Fuger et al., Revisiting the origins of super-hardness in TiB2+z thin films – Impact of growth conditions and anisotropy, Surf. Coat. Technol. 446 (2022) 128806.

[4]P.H. Mayrhofer et al., Self-organized nanocolumnar structure in superhard TiB2 thin films, Appl. Phys. Lett. 86 (2005) 131909.