2022

Ylivaara, Oili M. E.; Langner, Andreas; Ek, Satu; Malm, Jari; Julin, Jaakko; Laitinen, Mikko; Ali, Saima; Sintonen, Sakari; Lipsanen, Harri; Sajavaara, Timo; Puurunen, Riikka L.

In microelectromechanical system devices, thin films experience thermal processing at temperatures some cases exceeding the growth or deposition temperature of the film. In the case of the thin film grown by atomic layer deposition (ALD) at relatively low temperatures, post-ALD thermal processing or high device operation temperature might cause performance issues at device level or even device failure. In this work, residual stress and the role of intrinsic stress in ALD Al₂O₃ films grown from Me₃Al and H₂O, O₃, or O₂ (plasma ALD) were studied via post-ALD thermal processing. Thermal expansion coefficient was determined using thermal cycling and the double substrate method. For some samples, post-ALD thermal annealing was done in nitrogen at 300, 450, 700, or 900 °C. Selected samples were also studied for crystallinity, composition, and optical properties. Samples that were thermally annealed at 900 °C had increased residual stress value (1400-1600 MPa) upon formation of denser Al₂O₃ phase. The thermal expansion coefficient varied somewhat between Al₂O₃ made using different oxygen precursors. For thermal-Al₂O₃, intrinsic stress decreased with increasing growth temperature. ALD Al₂O₃ grown with plasma process had the lowest intrinsic stress. The results show that ALD Al₂O₃ grown at 200 and 300 °C is suitable for applications, where films are exposed to post-ALD thermal processing even at temperature of 700 °C without a major change in optical properties or residual stress.