Ternary and quaternary aluminum oxynitrides - TeQuAION

Aluminum (silicon) oxynitride, Al-(Si-)O-N, is a material that can be fabricated as transparent, hard coatings applicable to protect objects against wear, impact and corrosion. In the project presented here, thin films of Al-(Si-)O-N were deposited by reactive unbalanced closed field direct current magnetron sputtering (R-UCFDC-MS) and investigated for their chemical, microstructural and mechanical properties.
The R-UCFDC-MS process applied for the deposition of Al-(Si-)O-N is a Physical Vapor Deposition (PVD) process that was conducted with elemental Al and Si targets and the reactive gases O2 and N2. Working with O2 is not trivial due to the high reactivity of this gas. To maintain process control, the sputter setup used was therefore modified. Two separate gas inlets were installed for the two reactive gases, such that N2 was introduced directly at the targets and O2 remote from the latter at the substrate. This promoted nitration and avoided oxidation of the targets and allowed the stable and reproducible deposition of transparent Al-(Si-)O-N films with adjustable compositions. The O content in the films was varied through the O2 flow fed into the process and the Si content was varied through the power applied to the Si target.
A number of analytical techniques were applied to assess the properties of the Al-(Si-)O-N coatings deposited typically onto Si(100) wafers and glass. The chemical compositions of the coatings were determined by Rutherford Backscattering Spectrometry (RBS), Elastic Recoil Detection Analysis (ERDA) and Helium Elastic Recoil Detection (He-ERD). Thin films of AlN, of Al-O-N containing up to 60% O and of Al-Si-O-N containing up to 65% O and 20% Si were obtained.
The micro- and nanostructure of the coatings were characterized by X-Ray Diffraction (XRD) to determine the crystallinity, by Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) for images of film cross sections and by X-Ray Photoelectron Spectroscopy (XPS) for information on chemical states of the coatings. Combining the results of these analytical methods led to the determination of a microstructural evolution with changing chemical composition in the films.
Al-O-N coatings in the regime 0-8% O were found to consist of a crystalline solid solution made up of (002) fiber textured wurtzite crystallites, into which O incorporates in anionic lattice positions substituting N. In this regime, the wurtzite crystal lattice was observed to shrink with increasing O content, which was attributed to V(Al) vacancies generated by extra electrons (e-) from O(N) replacements. Simultaneously, a decrease in the crystallite size (CS) was observed, as O hinders crystallite growth because its valence e- configuration mismatches the crystalline structure of wurtzite. In the range 8-30%, Al-O-N coatings exhibit nanocomposite formation, during which the O-saturated wurtzite crystallites are progressively encapsulated in an amorphous Al2O3 matrix. While nanocomposites in the regime with 8-16% O maintain (002) wurtzite fiber texture, those in the regime with 16-30% O only have a preferred texture. With increasing O content, the crystalline fraction reduces with a concurrent CS reduction and the amorphous fraction increases. Al-O-N coatings with more than 30% O form an X-ray amorphous solid solution. An equivalent evolution has previously been found in the Al-Si-N system upon increasing Si Content [121, 122, 123, 124, 125], driven also by an additional e- from Si replacing Al [129]. Al-Si-O-N represents the quaternary combination of the two ternary systems Al-O-N and Al-Si-N and was found to exhibit a more complex structural evolution.
The performance of the coatings was assessed by determining the optical properties by ellipsometry, the hardness (HD) and Young's modulus (E) by nanoindentation and the residual stress state (sigma) from curvatures of thin coated substrates. The adhesion of the films to the substrates during the tests was strong, such that delamination was never observed. It was found that the transparent Al-(Si-)O-N coatings exhibit a linear decrease of the refractive index n from 2.1 to 1.6 with increasing O content from 0 to 65% independent of the Si content. In the same O content range, HD of the coatings decreases from 26 to 8 GPa and E from 330 to 150 GPa, and the residual stress remains below 1 GPa. Al-O-N coatings exhibit a dip in HD due to hydrogen (H) incorporation exclusively in the fiber textured nanocomposite regime containing 8-16% O.
The V(Al) vacancies found experimentally through the crystal lattice shrinkage in crystallites were supported by ab initio Density Functional Theory (aiDFT) calculations. In a supercell of 192 atoms, O(N) substitutions and V(Al) were positioned in different concentrations and configurations. The lattice parameters calculated upon these cell modifications are in good agreement with those measured experimentally.
The enthalpy H additionally obtained from aiDFT was combined with the entropy S obtained from a combinatorial calculation of the possible microstates to yield the Gibbs free energy G of the coatings. This result was complimented with high temperature experiments, for which Al-O-N films were equilibrated at temperatures up to 1600°. It was found that coatings containing crystalline wurtzite solid solutions including O are metastable, forming because the conditions in a R-UCFDC-MS process are far from thermodynamic equilibrium.
The Al-(Si-)O-N coatings were tested in several protective and functional applications. It was found that the films protect substrates such as Si(100) and glass against influences such as weathering and force impact. Due to the variability of the refractive index n with the O content in the films, protective coatings with reduced interference coloration could be fabricated. Additionally, the films could be used as a transparent matrix for the inclusion of gold (Au) nanoparticles, which resulted in decorative red, purple and blue films.