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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