L. J. Parfitt

Residual stresses in amorphous alumina films synthesized by ion beam assisted deposition

A set of experiments was conducted to determine the origin of residual stresses in amorphous Al2O3 films formed by ion beam assisted deposition. Samples were deposited during bombardment by Ne, Ar, or Kr over a narrow range of energies, E, and a wide range of ion‐to‐atom arrival rate ratios, R. Films were characterized in terms of composition, thickness, density, crystallinity, microstructure, and residual stress. Film composition was independent of ion beam parameters and residual stress was independent of thickness over the range 200–1200 nm. Stress varied strongly with ion beam parameters and gas content. Residual stress and gas content saturated at a normalized energy of ∼20 eV/atom or an R of ∼0.05. Where residual stress varied linearly with RE1/2, results are consistent with an atom peening model, but saturation at high R or RE1/2 is inconsistent with such a model. Stress due to gas pressure in existing voids explains neither the functional dependence on gas content nor the magnitude of the observed...

Role of ion beam assisted deposition in the synthesis and fracture of metal-ceramic multilayers

Abstract Multilayer metal-ceramic films have the potential to serve as strong, tough and environmentally resistant films and coatings for a wide variety of applications. They derive their properties from the multilayer structure (architecture), the microstructure and, hence, mechanical properties of the individual layers and the stress state of the film. However, in order to realize the potential of microlaminates, these features (architecture, microstructure and stress) must be controlled. Ion beam assisted deposition (IBAD) holds the promise to provide this control. In an effort to understand how IBAD can control mechanical properties of films, single trilayer and five-bilayer metal-ceramic, Al-Al 2 O 3 films were fabricated on ductile metal substrates using IBAD over a range of thicknesses and normalized energies. Results of bending and tension experiments revealed that the stress state is critical in determining the fracture strain (ductility) of the film. A residual compressive stress is beneficial and can be formed in the oxide phase by bombardment of the film with Ar during deposition. The behavior of film stress correlates well with Ar gas incorporation and the film consists of a high density of small cavities. Gas incorporation into the cavities or the surrounding matrix may be responsible for the observed residual compressive stress. The density of surface cracks at high strains is a function of the film architecture, film strength and the interfacial shear strength. Use of a multilayer structure reduces the crack density over a monolithic oxide film by increasing the strain needed to form through-thickness cracks and by increasing the intrinsic strength of the brittle layers by decreasing their thickness. Ion bombardment of the metal layers resulted in radiation damage and grain size refinement, both of which result in a stronger film and a lower crack density. It was shown that architecture, microstructure and stress are the key ingredients in microlaminate properties and are uniquely controllable by IBAD.

Synthesis and properties of microlaminate structures by ion beam assisted deposition

Abstract Films of Al, Al 2 O 3 , Mo and MoSi x were formed by ion beam assisted deposition (IBAD) at R ratios between 0.004 and 0.1 and film thicknesses between 150 and 1100 nm. Al films were crystalline with a strong (111) fiber texture becoming more pronounced and azimuthally oriented with increasing R ratio. Mo films were crystalline with strong (110) texture and a distinct azimuthal texture indicative of planar channeling of the ion beam along (110)_planes. The microstructure of Al films is characterized by large columnar grains at R = 0 with breakup starting at R = 0 04, while that of Mo films showed little change with increasing R ratio. Al 2 O 3 and MoSi x films are amorphous under all deposition conditions. The average stress in oxide and silicide films is tensile at R = 0 and becomes compressive with increased values of the normalized energy, saturating at ∼ 15 eV/atom. The average stress in Mo films is tensile at R = 0, increases to a maximum value of 0.63 GPa and becomes compressive with increasing normalized energy.

Microstructure and Residual Stresses in Al2O3 Prepared by Ion Beam Assisted Deposition

Alumina films synthesized by ion beam assisted deposition (EBAD) were characterized in terms of their microstructure and residual stress. Normalized energy per deposited atom, En, ranged from 0 to 130 eV/atom. The microstructure of PVD films (E n =0) is a mixture of crystalline (γ-Al 2 O 3 ) and amorphous phases and IBAD films are amorphous. Density and stoichiometry vary between 2.6 and 3.1 g/cm 3 and 1.3 and 1.6, respectively. Neither are dependent on either ion-to-atom arrival rate ratio, R, or E n . The film porosity is in the form of small (4-6 nm) voids of density 10 17 - 10 18 cm -3 . Bombarding gas is incorporated with 80% efficiency to levels of 4-5 at. %. A tensile residual stress of 0.3 GPa exists in PVD films. A rapid transition to high compressive stresses occurs with increased E n , with a saturation of -0.4 GPa occurring at high E n There is a strong correlation between gas incorporation and residual film stress. However, no existing models are capable of providing a quantitative explanation of the results.

Native Oxide and the Residual Stress of Thin Mo and Ta Films

Residual stress changed substantially between 2.5 and 80 nm film thickness in very thin Mo and Ta coatings sputtered onto Si substrates with native oxide. For both Mo and Ta, the thinnest films had a high compressive stress on the order of 2 to 3 GPa, and the stress relaxed, or became slightly tensile, with increasing film thickness. The coatings were examined using a variety of advanced characterization techniques including Auger Electron Spectroscopy (AES), Grazing Incidence X-ray Scattering (GIXS), and High Resolution Transmission Electron Microscopy (HRTEM). AES showed the presence of 0 and C contamination at the interface between the substrate and the film; these impurities originated from the adsorbed species and the native oxide on the surface of the wafers. GIXS analysis of the Mo films showed that the lattice of the thinnest layers was considerably expanded compared to the interplanar spacing of bulk, pure Mo. This expansion was caused by incorporation of impurities from the substrate into interstitial sites, which caused the very high compressive stress. HRTEM showed that the sputter deposition process did not alter the thickness of the native oxide layer, suggesting that the Mo and Ta films interacted with only the adsorbed impurities and the top one or two layers of the SiO 2 . Mo films deposited onto clean W layers were tensile, which supported the hypothesis that impurities caused the high compressive stresses in the films deposited onto SiO 2 . Grain growth and phase transformations contributed to the relaxation in stress observed in the thicker films.