Mechanical/Structural Properties of ALD Zirconium Oxide (ZrO2) Thin Films for High-Tech Applications

Among the transition metal oxides, zirconium oxide (zirconia) is considered the most extensively studied because of its remarkable properties such as high melting point, resistance against oxidation, and high refractive index [1-3]. Monoclinic crystal structure exists at room temperature and in the temperature range of up to 1170 °C whereas in the temperature range of 1170-2370 °C the dominant crystal structure is tetragonal. As the temperature increases above 2370 and up to 2680 °C, the crystal structure becomes cubic. Due to these various properties, zirconium oxides have been used in protective coatings, optical devices, laser mirrors, biomedical and in microelectronic applications. ZrO₂ is being used in artificial hip joints replacement due to its high hardness and excellent mechanical properties and in dental implants due to its exceptional resistance to bacterial infection.  ZrO₂ films of 200, 300, and 500, cycles were deposited on p-type Si (100) substrate using Atomic layer deposition (ALD) technique. The 300 ALD cycle samples were further annealed at 600 °C. We tested these films for structural, surface morphology, and nanomechanical properties using nanoindentation. The structural and surface properties were explored using X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). We discuss the influence of the deposition technique on the structure and properties of the films citing the superiority of the ALD technique that is particularly used in the fabrication of the films that are presented in this study. The nanoindentation results indicate that the films become consistently softer as the number of cycles increase and the film thickness grow larger, Figure 1. Additionally, further annealing of the films enhanced the hardness and fracture toughness of the films. Figure 2 depicts radial cracks of shorter length of the annealed sample (left) compared to the other one (right) under the same applied stress.      

References

1. J. Ciosek, W. Paszkowicz, P. Pankowski, J. Firak, U. Stanislawek, Z. Patron, Vacuum 72 (2004) 135.
2. Meriani S, editor. Proceedings of the InternationalConference Zirconia ’88. Advances in Zirconia Science and Technology. New York: Elsevier; 1989.
3. Venkataraj S, Kappertz O, Weis H, Drese R, Jayavel R, Wuttig M. J Appl Phys 2002;92(7):3599–607.