A low temperature, hermetic wafer level packaging method for RF MEMS switch

In this paper, a low temperature hermetic wafer level packaging (WLP) scheme for RF-MEMS devices such as micro-switches is presented. The real component with size 1mm×1mm is composed of two parts: cap substrate and device substrate, cap substrate has a via-in-cavity structure with cavity depth of 20μm. High aspect ratio via hole is fabricated by inductive coupled plasma-reactive ion etching (ICP-RIE) and electroplated with Cu for electrical feed-through. Eutectic bonding is still the most commonly used packaging technology at present. For the purpose of hermetic sealing, Au-Sn multilayer metallization with a close square loop of 100μm width have been sputtered onto cap wafer surface as soldering system. Deposition of cap wafer metallization should be finished in one high vacuum chamber process in order to prevent oxidation of Sn layer during producing process. And Ti-Ni-Au combination structure is deposited and patterned on device wafer in accordance with the sealing and interconnection areas in cap wafer. Bonding is performed in wafer level using eutectic bonder (TPS-2000A, BNP science) at a relative low temperature of 280°C for heating in static N₂ ambience for a period of time. As-bonded wafers are then diced into pieces and subjected to a series of performance test for evaluation. Shear strength of two bonded interfaces are measured for sample cells by shear tester ROYCE 552 100K to evaluate mechanical property. RF characteristics insertion loss at 2GHz has measured by HP 8510C network analyzer probe station, a total packaging insertion loss less than 0.05DB could be achieved. For hermeticity test, specific test vehicles which have a large cavity of 0.5×0.5×0.05cm³ are designed for helium leak test based on M1T-STD-883F since real device cavity has a tiny volume of only 600×600×30μm³, test vehicles indicate a maximum equivalent leak rate in air of 1.6×10^-8 mbar.l/sec. Also residual gas analysis (RGA) test is performed for bonded device sample. Reliability tests like thermal shock and high temperature, high humidity storage test are also performed according to MIL-STD-883F. For samples before and after reliability tests, measurements also have b- een made for comparison to evaluate the quality and reliability of packaging structure.