A highly reliable single-crystal silicon RF-MEMS switch using Au sub-micron particles for wafer level LTCC cap packaging

Abstract

This paper presents a small packaged, reliable and low-voltage driven RF-MEMS switch for wireless communication. We developed novel fabrication process of an RF-MEMS switch using a PZT actuator without a backside cavity to realize wafer-level-packaging (WLP). We also developed organic-free WLP process using sub-micron Au particles simultaneously to contact electrically and to seal hermetically with a low temperature co-fired ceramics (LTCC) wafer as a cap on a multilevel height RF switch. As a result, we obtained a small packaged SPST switch, which is in the small size of 1.4 x 0.9 x 0.8 mm. The RF-MEMS switch operated keeping a low contact resistance (<3 Ω) up to 1 billion cycles at a low-power condition (DC 1 mW). IntroductionRF-MEMS switches are attractive due to their low insertion loss, high isolation, and excellent linearity[1]. However, none of the MEMS switches have been commercially assembled for mobile phones yet because switches in mobile applications are required for a low-voltage operation less than 10-20 voltage and long switch lifetime more than 10^9-10^10 cycles.Although highly reliable MEMS switches using electrostatic actuation have been realized [2,3], their actuation voltages are 60-90 V. This is because a large contact force is necessary for sustaining a low contact resistance through quite a lot of switching cycles. A large restoring force is also required to avoid the stiction problem, so reducing the stiffness of the movable part is not allowed.In the other hand, several piezoelectric actuated MEMS switches with low-voltage actuation of less than 15 V have been demonstrated [4,5], however the switch lifetime is referred only a little. One of the reason is that the beams of piezoelectric actuators are SiO2 thin films, therefore it is difficult to realize a large restoring force.We have already reported an RF-MEMS switch that had a stiff movable beam based on a single crystal silicon (SCS) using a silicon-on-insulator (SOI) wafer and with a piezoelectric actuator to realize high reliability [6]. However, we formed a cavity using backside deep RIE in a handle substrate for our conventional switch, and it was then impossible for it to be hermetically sealed with a cap wafer.In this work, we developed novel fabrication process without a backside cavity and realized WLP using an LTCC cap wafer.

Integrated Magnetic MEMS Relays: Status of the Technology

“…Work has been published on low cost packaging solutions by using backside integration techniques [22], optimized packaging processes for minimal influence on the device performance [23], innovative liquid crystal polymer packaging [69], and wafer level Low Temperature Co-fired Ceramic (LTCC) cap packaging techniques [70]. Novel packaging techniques are continually being sought after to help ensure the required functionality and dependability, while avoiding detrimental effects such as excessive damping and stiction [71], and equally important, minimizing the overall production costs.…”