A method for manufacturing a protective layer for protecting an intermediate structural layer against etching with hydrofluoric acid, the intermediate structural layer being made of a material that can be etched or damaged by hydrofluoric acid, the method comprising the steps of: forming a first layer of aluminium oxide, by atomic layer deposition, on the intermediate structural layer; performing a thermal crystallization process on the first layer of aluminium oxide, forming a first intermediate protective layer; forming a second layer of aluminium oxide, by atomic layer deposition, above the first intermediate protective layer; and performing a thermal crystallization process on the second layer of aluminium oxide, forming a second intermediate protective layer and thereby completing the formation of the protective layer. The method for forming the protective layer can be used, for example, during the manufacturing steps of an inertial sensor such as a gyroscope or an accelerometer.