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 high-frequency micro electromechanical system (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 SiOx dielectric frame is improved by an Al 2 O 3 interlayer by 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 m width was done at 300 ºC. 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 8×10-16Pa m3/s. The isolation of open signal lines was measured up to 10 GHz for different designs of the sealing ring and SiOx 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 m and the thickness of SiOx dielectric frame is larger than 10 m. The developed wafer-level packaging technology is ready for applications to an radio frequency (RF) MEMS switch. K E Y W O R D SALD (Atomic Layer Deposition), feedthrough, metal bonding, RF MEMS, wafer-level packaging FOREWORDThe main feature of wafer-level packaging of MEMS (micro electromechanical system) is hermetic or vacuum sealing while retaining internal space. 1 With high-frequency (RF) MEMS switches, internal space is usually retained in vacuum to prevent degradation of electric contacts, or to maintain switching speed through suppression of aid damping. [2][3][4][5][6][7][8][9][10][11][12][13][14] In so doing, leads must be drawn while maintaining vacuum, which is a problem especially in RF MEMS using low-resistance thick Au lines.Cap wafer bonding using glass frit, 5,15,16 feedthrough glass substrates, 7,17 anodic bonding of LTCC (Low Temperature Cofired Ceramic) substrates, 19,20 and other methods are long known techniques for wafer-level hermetic packaging of RF MEMS. The technique using glass frit is simple and convenient. Specifically, glass frit fusion allows for some unevenness of bonding surface; therefore, bonding surface (seal ring) may traverse feedthrough wiring, while requirements for surface roughness are not stringent. However, there are concerns that gases generated from glass frit 21 and high temperature during bonding (normally above 450 • C) would adversely affect device performance and reliability. Degree of vacuum in a cavity after glass frit bonding is reported to be 900-2,000 Pa when no getter is used. Besides, the width of seal ring using screen print is normally 100-150 m or greater; taking into 44

Study on the Wetting Mechanism between Hot-Melt Nano Glass Powder and Different Substrates

“…Nano glass powder or the glass frit inter-layer packaging has the advantages of a high tolerance to the surface roughness of the bonding interface, suitable for various materials in the MEMS, electrical insulation characteristics to simplify the electrode lead extraction process and patterning without an additional lithography process by using screen printing [ 3 , 4 , 5 ]. It has been widely used in the packaging of the MEMS pressure switch [ 6 , 7 ], MEMS gyroscope [ 8 ] and accelerometer [ 9 ]. Many scholars only describe the packaging principle, packaging process and packaging results of nano glass powder, but there is no report on both the mechanism of infiltration and flow process of hot-melt glass on the substrate.…”