Capacités variables et inductances MEMS RF pour une intégration "Above-IC"

Abstract

This work was carried out in a collaborative framework between Ecole Polytechnique Federale de Lausanne and an industrial partner ST Microelectronics. RF MEMS passives are key devices for the RF front-end in modern transceiver and receiver architectures, offering significant gains in compactness, performance at GHZ frequency and low power consumption. In this context, our work deals with the development of RF MEMS tunable capacitors for above-IC integration, and the co-integration with high-Q MEMS inductors. The possibility to co-integrate L and C functions in the same platform enables the use of this technology in a number of applications such as VCOs and tunable filters. This work addresses the design, simulation and fabrication of tunable capacitors with fragmented metal (AlSi) that are laterally driven by electrothermal and electrostatic actuators. All of MEMS passives components are made with a low-temperature process (above-IC compatible) using polyimide as sacrificial layer, 4 µm AlSi as metal layer and copper. The copper layer is used for a double purpose: (i) to design and fabricate vertically integrated high Q inductors and (ii) to define lateral electrostatic actuators for the MEMS capacitors. To increase the tuning range of MEMS capacitors, we investigate solutions by varying the effective area. First, electro-thermal actuators have been demonstrated for driving suspended electrodes of tunable capacitors such as rotational and lateral actuator. We demonstrated a tuning range of 70% and 25% at 2GHz in case of MEMS capacitors with rotational and lateral actuators, respectively. These solutions evade the pull-in effect but the tuning range is less than 100% and the power consumption is high. For the purpose of power reduction, we develop tunable capacitors with fragmented electrodes that are laterally driven by electrostatic actuator: however, the fabrication process associated with these architectures appears to be very difficult and, finally, it was not possible to show full functional devices. It is worth noting that the design of MEMS capacitors has been supported by electromechanical ANSYS simulations and modeling of the fringing effects, for an optimal device design. In the AlSi layer, we also demonstrate inductors with quality factor in excess of 10 at 2GHz. Basic LC blocks are realized and characterized with a tunable frequency of 25% at 2GHz. Another particular realization are the vertically integrated, thick copper inductors, which have been successfully fabricated and measured (L = 2.5 nH, Q = 15 at 2GHz). Finally, in the same process, we fabricated and characterized WLAN and DCS bandpass filters for a demonstrator dealing with a reconfigurable radio front-end. These components are fabricated with thick copper of 10 µm