Hermetic wafer-level packaging development for RF MEMS switch

Abstract

This paper presents a low temperature «350°C) hermetic solution to fully package at wafer level a RF MEMS switch connecting upwards. The switch has a piezoelectric actuation and an electrostatic hold. In this architecture, the packaging is actually part of the switch itself and shall meet many requirements:• Use of Thru-Silicon Via (TSV) for DC and RF connections with minimum via resistance • Electrical connection between both wafers • Hermetic sealing under controlled atmosphere with 5 +1-0.5 /lm gap between the switch and the cap wafers • No degradation of the switch performances • Low temperature «350°C) packaging process to preserve the moving part materials • Electrode and dielectric for electrostatic hold • No sticking of the moving contact Two main processes were developed and implemented together: Au-Sn eutectic bonding under atmospheric pressure with 5/lm spacers to ensure the gap, and 'post-bonding' TSV. The complete process flow of the cap wafer, bonding and TSV process is presented. The solder material, made of 80wt.%Au and 20wt.%Sn, is only 5/lm thick and is electroplated. SEM, XPS, EDX analyses and shear tests have been performed. Hermeticity evaluation tests have been set-up, and a standard leak rate lower than 1.2 x 10-1 2 mbar.lls has been demonstrated using the membrane de flexion method. TSV and Au-Sn bump resistance is less than I4mn and IOmn with a yield of 92% and 98% respectively across the 200mm wafer. The resistance between 2 via is more than 500Mn at 5V. As to the packaged switch, its insertion losses at 2 GHz are 0.74dB and its off-state isolation is 43.6dB. At last, it has been demonstrated that the substrates resistivity has a great influence on the insertion losses. IntroductionTargeting the mobile communication market, a RF MEMS piezoelectric switch connecting upwards with low-voltage actuation and electrostatic hold has been developed [1] (Figure 1). Indeed, RF MEMS switches compared to semiconductor switches are known for their potential great performances at the GHz frequency range such as lower insertion losses, better isolation and better cut-off frequency.Piezoelectric actuation is a good means to have low operation voltage, and electrostatic hold enables strong pressure on the contact, producing low contact resistance and low RF losses. PZT has been selected for its high piezoelectric constants. Requiring a 5/lm gap enables

Metal‐bonding‐based hermetic wafer‐level MEMS packaging technology using in‐plane feedthrough: Hermeticity and high frequency characteristics of thick gold film feedthrough

“…[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. 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.…”

3-D Integration and Through-Silicon Vias in MEMS and Microsensors

“…For simplicity, TSVs locate in the cap wafer [303]- [305], but it is also possible to be in the substrate [306]. For both cases, the TSVs are used for RF signal transmission through the hermetically-sealed substrate or cap.…”

Zero-level packaging for (RF-)MEMS implementing TSVs and metal bonding

“…This paper presents a 0-level package based on chip capping. Two features in particular make the package very attractive and distinct from previous implementations [7][8][9][10]:…”

“…The length of the feedthroughs varies with the width of the sealing ring and the design is such that the microstrip line keeps a characteristic impedance identical to the CPW (typically 50 ) [6]. (3) Vertical via's, implemented either in the MEMS substrate [7,8,9] or in the Cap [10] as shown in Fig. 1, present a more compact solution (smaller footprint) than the horizontal feedthrough designs, but, the process is more complex as through-wafer hole etching is required.…”