Thermo-mechanical properties of alumina films created using the atomic layer deposition technique

Abstract

Interdigitated humidity sensors with atomic layer deposited (ALD) coatings of aluminum oxide demonstrated no leakage current relative to uncoated sensors stored in the ambient, indicating Al 2O 3 may be used to limit the effects of H 2O and other chemical species in miniaturized mechanical- and electronic-devices. The long term durability of such coatings is not known, but may be predicted from the related material characteristics. The modulus and hardness of Al 2O 3 were therefore measured by nanoindentation using a Berkovich tip. Because the coatings are brittle and possess a significant tensile stress, the influence of film stress on the indentation measurements was quantified using a numerical analysis protocol, which also considered the effect of substrate compliance. The film stress and coefficient of thermal expansion for Al 2O 3 were determined using the wafer curvature method. Film stress was characterized using thermal cycling up to 500 °C. Separate Si/SiO 2/Si microcantilever arrays demonstrated a stress variation according to the thickness of Al 2O 3 coatings. Fracture toughness was examined by indentation with a cube-corner tip; the estimates are subject to film stress and the material-dependent geometry factor.