Reduction In Intrinsic Stress Research Articles

Understanding of the development of in-plane residual stress in sol-gel-derived metal oxide thin films

The in-plane residual stress in thin films greatly affects their properties and functionality as well as the substrate bending, and hence is an important factor to be controlled. In order to obtain general knowledge on the development of residual stress in sol-gel-derived oxide thin films, the in-plane residual stress was measured for yttria stabilized zirconia gel films on Si(100) wafers as a function of firing temperature by measuring the substrate curvature. The films showed a rather complex variation in residual stress, and the mechanism of the residual stress evolution was discussed, referencing the intrinsic stress and the x-ray diffraction data. At low annealing temperatures of 100–200 °C, the residual tensile stress decreased and became compressive partially due to the structural relaxation occurring during cooling. When the firing temperature was increased over 200 °C, the residual stress turned tensile, and increased with increasing annealing temperature, which was attributed to the increase in intrinsic stress due to film densification as well as to the reduced structural relaxation due to the progress of densification. The residual tensile stress slightly decreased at firing temperatures of 500–600 °C, which was attributed to the reduction in intrinsic stress due to thermally activated atomic diffusion as well as to emergence of thermal stress. At firing temperature over 600 °C, the residual tensile stress increased again, which was attributed to the increase in thermal stress generated during cooling due to the increased Young’s modulus of the film. Although appearing to be complicated, the whole variation of residual stress with firing temperature could be understood in terms of film densification, structural relaxation, atomic diffusion, progress of crystallization and thermal strain. The illustration presented in the work may provide a clear insight on how the residual stress could be developed in a variety of functional sol-gel-derived, crystalline oxide thin films.

Reduction of intrinsic stress in cubic boron nitride films

Several techniques reported to be effective for the preparation of thin films of cubic phase boron nitride (c-BN) are reviewed. In order to maintain integrity and adhesion for such films at a useful thickness, it is critical to minimize their intrinsic stress. A method is detailed for stress reduction by MeV ion irradiation following the deposition of c-BN films. Based on these successful processes, a new method is proposed, in which the production of stress-reducing atom displacements is achieved during ion-assisted c-BN deposition, by concurrent bombardment of the growing film with energetic ions, typically at keV energies.

Ion induced stress generation in arc-evaporated TiN films

The influence of Ti ion bombardment on the intrinsic stress and microstructure of TiN films during deposition by arc evaporation of Ti in pure N2 has been investigated. Ions with an average charge of +1.6 were accelerated from the arc discharge by a negative substrate bias Vs between 5 and 540 V which yielded a steady-state substrate temperature between 300 and 600 °C, respectively. The compressive intrinsic stresses in the films, as determined by the x-ray-diffraction (XRD) sin2 ψ method after subtracting the thermal stress contribution at room temperature, changed abruptly from 1.9 to a maximum of 6.5 GPa as Vs increased from 5 to 100 V. The compressive stress then decreased monotonically to ∼1.6 GPa as Vs increased to 540 V. Broadening of XRD peaks (β) showed accompanying inhomogeneous strain with a maximum values for Vs=100 V. Cross-sectional transmission electron microscopy showed a dense columnar film microstructure. Electron microdiffraction showed a distorted structure within the same columns for Vs=100 V and better defined grains for Vs=500 V. The observed reduction in intrinsic stress with increasing Ti ion energy is in addition to what is normally observed in sputter-deposited TiN films with the same ion energy range of predominantly Ar-ion bombardment. A comparison with published work of magnetron sputtered TiN showed that defects created by Ti ions are more easily annihilated than defects created using Ar ions. Ar atoms tend to be strongly bound to lattice defects and forming thermally stable complexes and prevent stress relief at high Vs as is observed for arc-evaporated films. The obtained results are compared to models for stress generation during steady-state conditions in the competition between collisionally induced point defect formation and defect annihilation during deposition.