ALD Al2O3 from TMA and water on Si: residual stress, elastic modulus, hardness and adhesion

Use of atomic layer deposition (ALD) in microelectromechanical systems (MEMS) has increased as ALD enables conformal growth on 3-dimensional structures at relatively low temperatures. For MEMS device design and fabrication, understanding of the residual stress and mechanical properties of thin film is crucial as these influence directly the device properties and performance. Al2O3 from Me3Al and H2O is one of the most often used materials, but even for that, a detailed study of the mechanical properties as a function of ALD temperature is missing. In this work a comprehensive study of the stress, elastic modulus, hardness and adhesion of atomic layer deposited (ALD) Al2O3 films grown at 110 - 300 °C from trimethylaluminum and deionized water is presented. Film stress was analysed by wafer curvature measurements and Stoney's equation, elastic modulus by nanoindentation and surface-acoustic wave measurements, hardness by nanoindentation, and adhesion by microscratch testing and scanning nanowear measurements. The films were also analysed by ellipsometry, optical reflectometry, x-ray refl ectivity and time-of-flight elastic recoil detection for refractive index, thickness, density and impurities. The ALD Al2O3 films were under tensile stress, in the scale of hundreds of MPa. The magnitude of the residual stress decreased strongly with increasing ALD temperature. Growth-induced stress accounted for most of the stress at low ALD temperature and its importance decreased with increasing ALD temperature. The stress was independent of the type of ALD reactor used. Films grown at 150 to 300 °C had a fairly constant elastic modulus about 170 GPa and hardness of 10-11 GPa. Films grown at 110 °C were softer with a lower elastic modulus, which can at least partly explained by the higher residual hydrogen content in the films. ALD Al2O3 films adhered strongly on RCA-cleaned silicon with SiOx termination. The large set of data obtained in this work as a function of ALD temperature allowed a more detailed observation of the trends in the measured properties than has been possible before. For example, a continuously increasing elastic modulus as a function of ALD temperature was not observed in this study. Instead, after initial increase at low temperature elastic modulus settled to approximately constant value at 150 °C.