A CMOS embedded RF-MEMS tunable capacitor for multi-band/multi-mode smartphones

This paper reports on 1-chip RF-MEMS tunable capacitor that equips CMOS driver circuit in the underlying layer. A Wafer Level Chip Scale Package (WLCSP) optimized for RF-MEMS is employed to minimize the module size. The MEMS actuation voltage is generated by an Actuation Voltage Generator (AVG). The boost mechanism employed in the AVG enables instant high voltage generation and reduction of the dielectric charging. The measured noise at RF frequencies is less than −120dbm, thanks to a shield metal layer formed between MEMS and CMOS layers. To achieve high power handing and high creep immunity, we employ the previously reported techniques, the Quadruple Series Capacitor (QSC) [1] and the SiN springs [2]. The quality factor measured in the WLCSP is larger than 100 at 1GHz. The capacitance can be changed from 1.4pF to 5pF by a step of 0.45pF.

1. Introduction

To cope with the rapidly increasing communication traffic initiated by smartphones, tunable components are becoming essential in today's multiband/multimode RF front-ends. RF-MEMS tunable capacitors are suited for such systems, thanks to their low loss and excellent linearity [3]. To adopt RF-MEMS in such systems, however, several issues must be addressed. One issue is the total system size including RF-MEMS and its control circuit. While it is easier to fabricate the two components in separate chips, the resulting size and cost of the module becomes unacceptable for the state of the art smartphones. Also, the parasitic capacitance induced by the two chip composition degrades the performance of the tunable capacitor [4]. Another issue is the existence of the high voltage of about 30V required for the MEMS actuation. The high voltage generation calls for large charge pump size and large current consumption, and moreover, increases the risk of the dielectric charging. Noise generated by the charge pump is yet another threat to the RF system.