Hermetic metallization of aluminum nitride for RF windows

Aluminum nitride (AlN) is a leading candidate for replacing BeO or high-purity alumina as RF window material for high power, multi-megawatt (20 MW-100 MW) microwaves both at cryogenic temperatures, i.e., 2 K and room temperature (300 K) because of its high thermal conductivity (200 W/m°K), high dielectric strength (>15 kV/mm), low dielectric loss (tangent 6=0.0005), and good mechanical strength (400-450 MPa). Ceramic RF windows must be metallized and brazed to a metallic structure, usually copper, and must form hermetic seals. Sienna Technologies Inc., has successfully developed a refractory Mo-Mn thick film metallization for AlN with a sheet resistance of 35 Ω/□ and an adhesive strength of 96 MPa using sound thermochemical and thermomechanical principles. We have also demonstrated the metallized AlN RF windows can be brazed to copper and Kovar structures to form hermetic seals (better than 1×10^-9 atm·cm³/sec) using industry standard braze alloys.