Tantalum oxide thin films as protective coatings for sensors

Reactively sputtered tantalum oxide thin films have been investigated as pr otective coatings for aggressive media exposed sensors. Tantalum oxide is shown to be chemically very robust. The etch rate in aque ous potassium hydroxide with pH 11 at 140 degrees C is lower than 0.008 Ang strom h(-1). Etching in liquids with pH values in the range from pH 2 to 11 have generally given etch rates below 0.04 Angstrom h(-1). On the other ha nd patterning is possible in hydrofluoric acid. Further, the passivation be haviour of amorphous tantalum oxide and polycrystalline Ta2O5 is different in buffered hydrofluoric acid. By ex situ annealing in O-2 the residual thin-film stress can be altered fr om compressive to tensile and annealing at 450 degrees C for 30 minutes giv es a stress-free film. The step coverage of the sputter deposited amorphous tantalum oxide is reas onable, but metallization lines are hard to cover. Sputtered tantalum oxide exhibits high dielectric strength and the pinhole density for 0.5 mu m thick films is below 3 cm(-2).