Metal-Bonding-Based Hermetic Wafer-Level MEMS Packaging Technology Using In-Plane Feedthrough

—Hermeticity and High Frequency Characteristics of Thick Gold Film Feedthrough—

Abstract

Au-Au-bonding-based wafer-level vacuum packaging technology using in-plane feedthrough of thick Au signal lines was developed for RF MEMS. Compared with conventional technology based on glass frit bonding, the developed technology is advantageous in terms of smaller width of sealing frames, lower process temperature and smaller amount of degas. To guarantee the hermetic sealing, the adhesion between the thick Au lines and a SiO_x dielectric frame is improved by an Al₂O₃ interlayer by ALD (Atomic Layer Deposition). The steps of the dielectric frame above the thick Au lines are absorbed by an electroplated Au seal ring planarized by fly cutting. The thermocompression bonding of the Au seal rings of 20∼100 µm width was done at 300ºC. A cavity pressure of about 500 Pa or lower was measured by “zero balance method” using Si diaphragms. Vacuum sealing was maintained for more than 19 months, and the leak rate is less than 8×10^-16 Pa m³/s. The isolation of open signal lines was measured up to 10 GHz for different designs of the sealing ring and SiO_x dielectric frame. The influence of the in-plane feed through to the isolation is as low as 2∼3 dB, if the width of the sealing ring is 20 µm and the thickness of SiO_x dielectric frame is larger than 10 µm. The developed wafer-level packaging technology is ready for applications to an RF MEMS switch.