To realize commercialization of Radio Frequency micro electro mechanical systems (RF-MEMS), development for effective packaging approach is one of the most critical issues should be solved in advanced.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 N 2 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 0.5×0.5×0.05cm 3 are designed for helium leak test based on MIT-STD-883F since real device cavity has a tiny volume of only 600×600×30µm 3 , 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 been made for comparison to evaluate the quality and reliability of packaging structure.
A novel bulk acoustic wave (BAW) resonator structure with air edge reflectors is proposed. The air reflectors provided at the border of the resonator suppress the acoustic wave leakage travelling in lateral direction. The difference of acoustic impedance at the air edge interface is so large that the reflectivity is almost unity which indicates perfect reflection. As a result of optimized lateral structure, the Q-factor at antiresonance frequency (Qa) is improved significantly to 2740. Furthermore, the effective electro-mechanical coupling coefficient (kt 2 ) which is essential to achieve wide band-width of RF filters is increased considerably by 14%. We also perform multi-physics simulation and investigate the effect of the lateral wave leakage for the BAW structures.Index Terms -film bulk acoustic wave resonator, acoustic devices, microelectromechanical devices, rf filters.
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