Silicon nitride substrate technology for extreme environment electronics packaging

A multilayer dielectric and an encapsulation glass for Si₃N₄ substrates has been successfully developed. Both multilayer dielectric and encapsulation glass showed good wettability, no cracking, and no bubbling. They showed good adhesion to the Si₃N₄ substrates as evidenced by high pull strengths of 7.2-8.3 ksi for the multilayer dielectric and 7.6-10.8 ksi for the encapsulation glass. The leakage currents were measured with a Keithley 6517A Electrometer/High resistance meter at temperatures of -180°C to 125°C. As expected the leakage current decreased with decreasing temperature. At all the measurement temperatures, the leakage currents were less than 0.1 nA (10^-10 A). Test vehicles (TV) were designed and fabricated to characterize the robustness of the substrate metallization and dielectric, the die attach material and process, and wire bonds as a function of aging and thermal cycling. The Au conductor has strong adhesion on Si₃N₄ before and after storage tests. While the dielectric has strong adhesion to the thick film Au, the adhesion of the dielectric to Si₃N₄ was 60 lbs, but does not significantly change upon storage at -180°C or +125°C. Semiconductor die attach with indium performs and Au thermosonic wire bonding was also studied. The die shear strength was measured initially and after aging at 125°C. High temperature aging was selected for the test as intermetallics grow more rapidly at elevated temperatures. The die shear strength actually increased with aging. Wire bond pull strength, ball shear and electrical resistance were also measured initially and after aging at 125°C. The ball shear strength and resistance remained stable with aging, while the wire pull strength decreased slightly due to annealing of the Au wire. A SiGe Butterworth filter was designed, fabricated, and tested using the thick film Si₃N₄ substrate technology assembled in a 14-pin DIP-style multilayer package. The successful operation of this circuit demonstrated both device and package functionalities over the temperature range of -180°C to +125°C.