edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 3 8 that grow and create grain boundaries. This process induces tensile stress as grains attract each other. Nix and Clemens’ model estimates this stress based on surface and grain boundary energies, showing it is proportional to the root of the reciprocal of grain size: (Eq 3) where σT is the tensile stress, Y is Young’s modulus, ν is the Poisson’s ratio, d is the grain size, and γs and γgb are the surface and grain boundary energies of the film, respectively. This model indicates that the tensile stress is proportional to the root of the reciprocal of the grain size. Experimental data in Fig. 5 aligns with this model. Elastic modulus measurements for ITO films indicate consistent crystal structure and composition across thicknesses. For a 45 nm ITO film, the surface and grain boundary energy difference is calculated to be 6.4 ± 1.3 J/m². During columnar grain growth, compressive stress dominates, stabilizing at -0.4 GPa. The steady-state residual compressive stress was also observed in high-mobility materials like Cu and Ag, which is closely tied to the material and growth conditions.[6,7] SUMMARY The study investigated electrical resistivity, thickness, and residual stress of RF-sputtered ITO films. Nine films of varying thicknesses were prepared under identical conditions. The electrical resistivity results revealed a significant spatial distribution of resistivity, which was influenced by the growth conditions of the films. This variation is attributed to energetic negative oxygen ions formed during RF sputtering, which cause metal depletion through re-sputtering. The findings emphasize the importance of monitoring the uniformity of electronic properties during the film deposition process. It was concluded that assessing thickness or sheet resistance alone is insufficient for effective ITO film process control. Instead, both factors must be combined to accurately calculate resistivity. This comprehensive approach provides a better understanding of the uniformity of electrical properties, offering valuable insights into improving the deposition process. On the other hand, the residual stress result suggests that ITO films exhibit a transition of stress from tensile to compressive as the thickness of ITO films is increased. The tensile stress originated from the impingement and coalescence of newly deposited equiaxed grains, while the observed compressive stress was attributed to the incorporation of excess material in the boundaries of columnar grains. The ability to monitor electric resistivity distribution and stress evolution in sputtered ITO films on glass substrates provides valuable insights for process control in ITO device manufacturing. Understanding and controlling the electric resistivity and residual stress of ITO films are crucial for ensuring the performance and reliability of devices utilizing these films. REFERENCES 1. G.G. Stoney: “The Tension of Metallic Films Deposited by Electrolysis,” Proc. R. Soc. London, 1909, 82, p. 172. 2. N.M. Ahmed, et al.: “The Effect of Post Annealing Temperature on Grain Size of Indium-Tin-Oxide for Optical and Electrical Properties Improvement,” Results Phys. 2019, 13, p. 102159. 3. K. Norrman, P. Norby, and E. Stamate: “Preferential Zinc Sputtering during the Growth of Aluminum Doped Zinc Oxide Thin Films by Radio Frequency Magnetron Sputtering,” J. Mater. Chem. C, 2022, 10, p. 14444. 4. S. Khan and E. Stamate: “Comparative Study of Aluminum-Doped Zinc Oxide, Gallium-Doped Zinc Oxide and Indium-Doped Tin Oxide Thin Films Deposited by Radio Frequency Magnetron Sputtering,” Nanomaterials, 2022, 12, p. 1539. 5. J. Liang, et al.: “Quantitative Study of the Thickness-dependent Stress in Indium Tin Oxide Thin Films,” Thin Solid Films, 2024, 788, p. 140163. 6. E. Chason, et al.: “Origin of Compressive Residual Stress in Polycrystalline Thin Films,” Phys. Rev. Lett., 2022, 88, p. 156103. 7. F. Spaepen: “Interfaces and Stresses in Thin Film,” Acta Mater., 2000, 48, p. 31. Fig. 5 Stress as a function of the root of the reciprocal of the grain size. The red dashed line indicates the fitting line using Nix’s theoretic model. The inserted graph shows the elastic modulus values of the ITO films measured with nanoindentation.[5] ITO films have been observed by stylus profilometry to exhibit a transition of stress from tensile to compressive as the thickness of ITO films is increased. This analysis provides valuable insights into the evolution of stress versus thickness of ITO films and can serve as a reference for monitoring and modulating the stress during the production process control of ITO devices.
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