Residual Stress Control by Ion Beam Assisted Deposition
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"*Materials Science and Engineering, University ABSTRACT
The origin of residual stresses were studied in both crystalline metallic films and amorphous oxide films made by ion beam assisted deposition (IBAD). Monolithic films of A12 0 3 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 and were characterized in terms of composition, thickness, density, crystallinity, microstructure and residual stress. The stress was a strong function of ion beam parameters and gas content and compares to the behavior of other amorphous compounds such as MoSix and WSi 2 .2. With increasing normalized energy (eV/atom), residual stress in crystalline metallic films (Mo, W) increases in the tensile direction before reversing and becoming compressive at high normalized energy. The origin of the stress is most likely due to densification or interstitial generation. Residual stress in amorphous films (A120 3 , MoSix and WSi 2 .2 ) is initially tensile and monotonically decreases into the compressive region with increasing normalized energy. The amorphous films also incorporate substantially more gas than crystalline films and in the case of A120 3 are characterized by a high density of voids. Stress due to gas pressure in existing voids explains neither the functional dependence on gas content nor the magnitude of the observed stress. A more likely explanation for the behavior of stress is gas incorporation into the matrix, where the amount of incorporated gas is controlled by trapping. ORIGIN OF STRESSES IN ION BEAM ASSISTED DEPOSITION Tensile Stresses The origins of residual stresses (and strains) in films produced by ion beam assisted deposition are numerous. In the case of formation of a single crystal film on a single crystal substrate, coherency can give rise to strains in both film and substrate. Otherwise, there is no clear reason why an as-deposited film should be formed directly in a strained state. [1] In order for a stress to develop, atomic rearrangement after film formation is required. More often than not, rearrangement involves diffusion after deposition. The driving force for diffusion is the nonequilibrium state of the as-deposited film. Thus, films in which the equilibrium structure can be reached during the deposition process will not display growth stresses. A condition for the development of these stresses is a low surface mobility which prevents the film from reaching its equilibrium structure during deposition. [2] Models describing the development of stresses fall broadly into two categories, those which relate to the development of a tensile stress and those which address the formation of a residual compressive stress. In the former category are models based on structural relaxation; densification, grain growth and phase changes. [2] Structural relaxation occurs in order to minimize the free energy of the system and results in a reduction of the specific volume of the film as long as the film is unconstrained. However, if
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