Tantalum oxide thin films as protective coatings for sensors

Abstract

Reactively sputtered tantalum oxide thin-films have been investigated as protective coatkg 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 "C is lower than 0.008 &h. Etching in liquids with pH values in the range from pH 211 1 have generally given etch rates below 0.04 &h. On the other hand patterning is possible in hydrofluoric acid. Further, the passivation behaviour of amorphous tantalum oxide and polycrystalline Ta20s is different in buffered hydrofluoric acid. By ex-situ annealing in O2 the residual thin-film stress can be altered from compressive to tensile and annealing at 450 "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 pm thick films is below 3 cm-2.

Nested potassium hydroxide etching and protective coatings for silicon-based microreactors

“…Typical causes of pinhole formation are incomplete surface coverage (because of insufficient film thickness) and trace particle contamination [26]. We found that evaporator cleanliness, in addition to film thickness, was a key process variable determining pinhole density.…”

“…For example, nickel, aluminum, and copper, with thickness of up to approximately 1 m, are commonly deposited by e-beam evaporation. Sputtered tantalum oxide thin films were reported for use as protective coatings in the packaging of microfabricated sensors (e.g., piezoresistive pressure sensors) [26]. These techniques may be complemented with electroless and electrolytic metal plating to deposit thicker metallic coatings or create entirely metallic structures.…”

Aggressive media exposed differential pressure sensor with a deposited membrane

“…The wafers were annealed again, this time at 400°C for 30 min. The annealing ensures good electrical contact between the metallization and the polysilicon interconnects and reduces (TR of the a-TaO from approximately -250 MPa to roughly 0 MPa [2]. The residual stress of the sputtered silicon is only marginally influenced by this heat treatment.…”

“…Sputtered amorphous tantalum oxide (a-TaO) has shown promise as coating material due to excellent chemical and mechanical properties [2]. The step coverage of the sputtered film is poor.…”

“…The step coverage of the sputtered film is poor. It is difficult to coat patterns with steep sidewalls and sharp concave corners, such as polysilicon piezoresistors and interconnects [2]. The design presented here enables coatings on both sides of the membrane.…”