Dry Etching Techniques of Amorphous Silicon for Suspended Metal Membrane RF MEMS Capacitors
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Dry Etching Techniques of Amorphous Silicon for Suspended Metal Membrane RF MEMS Capacitors Raphaël Fritschi1, 2, Cyrille Hibert1, Philippe Flückiger1 and Adrian M. Ionescu2 1 Center of Micro-Nano-Technology (CMI), 2Electronics Laboratory (LEG) Swiss Federal Institute of Technology, Lausanne (EPFL), CH-1015 Lausanne, Switzerland
ABSTRACT A novel dry etching technique of amorphous silicon is proposed to suspend the metal membrane of RF MEMS tunable capacitors. The proposed fabrication process is simple and excludes all the drawbacks related to a wet process. Moreover, it has the advantage of being fully compatible with CMOS post-processing. Experimental results demonstrate that the capacitor suspension beams design with meanders can reduce the tuning voltage to less than 5 to 10 V.
INTRODUCTION Nowadays, passive components, such as tunable capacitors, integrated on silicon or SOI substrates are subject of increased interest for programmable RF ICs. Conventional on-chip tunable capacitors are solid-state devices that suffer from excessive series loss, caused by large series resistance, and a low quality factor. MEMS technologies seem very promising for alleviating these problems [1]. The realization of MEMS capacitors requires the releasing of suspended conductive membranes, which act as movable electrodes. Usual techniques for releasing structures are: (i) standard polysilicon surface micromachining process, which involves a silicon dioxide sacrificial layer wet etch and a supercritical carbon dioxide drying process [2] and (ii) organic sacrificial layers released in oxygen plasma [3, 4]. We present results on the development of novel dry etching techniques that use amorphous silicon as sacrificial layer for suspended metal (Al) membrane RF MEMS capacitors. The proposed fabrication process is fully compatible with CMOS post-processing.
PRINCIPLE OF OPERATION As shown in figure 1a, the MEMS tunable capacitor consists of one suspended top membrane and one fixed bottom electrode, with an overlap area. When a bias voltage, VDC, is applied between the two electrodes, the suspended membrane is electrostatically displaced by x towards the bottom electrode and the air-gap is reduced from the initial value, d, to (d-x). Neglecting the fringing effects, the capacitance C between the two electrodes is given by: C=
ε 0A d−x
(1)
where ε0 is the dielectric constant of air, A is the overlap area. U2.7.1 Downloaded from https:/www.cambridge.org/core. Columbia University Libraries, on 14 Jun 2017 at 23:14:12, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-729-U2.7
It is well-known that there is an equilibrium between the electrostatic force and the elastic force of the suspension beams until the suspended membrane reaches about one third of the initial air-gap. Then, for increased applied voltage, the suspended membrane snaps down and the controllable capacitance tuning is lost (the device acts as an electrostatic switch). The applied bias voltage at
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