Tantalum oxide thin films as protective coatings for sensors

Reactively sputtered tantalum oxide thin-films have been investigated as protective coating for aggressive media exposed sensors. Tantalum oxide is shown to be chemically very robust. The etch rate in aqueous potassium hydroxide with pH 11 at 140/spl deg/C is lower than 0.008 /spl Aring//h. Etching in liquids with pH values in the range from pH 2-11 have generally given etch rates below 0.04 /spl Aring//h. On the other hand patterning is possible in hydrofluoric acid. Further, the passivation behaviour of amorphous tantalum oxide and polycrystalline Ta/sub 2/O/sub 5/ is different in buffered hydrofluoric acid. By ex-situ annealing in O/sub 2/ the residual thin-film stress can be altered from compressive to tensile and annealing at 450/spl deg/C for 30 minutes gives a stress-free film. The step coverage of the sputter deposited amorphous tantalum oxide is reasonable, but metallisation lines are hard to cover. Sputtered tantalum oxide exhibits high dielectric strength and the pinhole density for 0.5 /spl mu/m thick films is below 3 cm/sup -2/.