Aluminum oxide from trimethylaluminum and water by atomic layer deposition: The temperature dependence of 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, the understanding of stress and mechanical properties such as elastic modulus, hardness and adhesion of thin film is crucial. In this work a comprehensive characterization of the stress, elastic modulus, hardness and adhesion of ALD aluminum oxide (Al₂O₃) films grown at 110–300 °C from trimethylaluminum and water is presented. Film stress was analyzed by wafer curvature measurements, elastic modulus by nanoindentation and surface-acoustic wave measurements, hardness by nanoindentation and adhesion by microscratch test and scanning nanowear. The films were also analyzed by ellipsometry, optical reflectometry, X-ray reflectivity 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 stress decreased strongly with increasing ALD temperature. The stress was stable during storage in air. Elastic modulus and hardness of ALD Al₂O₃ saturated to a fairly constant value for growth at 150 to 300 °C, while ALD at 110 °C gave softer films with lower modulus. ALD Al₂O₃ films adhered strongly on cleaned silicon with SiO_x termination. - Highlights: • The residual stress of Al₂O₃ was tensile and stable during the storage in air. • Elastic modulus of Al₂O₃ saturated to at 170 GPa for films grown at 150 to 300 °C. • At 110 °C Al₂O₃ films were softer with high residual hydrogen and lower density. • The Al₂O₃ adhered strongly on the SiO_x-terminated silicon