Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
Modulation Instability in RF MEMS Devices
Romolo Marcelli 1 , Giancarlo Bartolucci 1,2 , Giorgio De Angelis 1 , Andrea Lucibello 1 and Emanuela Proietti 1
1 CNR-IMM Roma, Via Fosso del Cavaliere 100, 00133 Roma, Italy
2 Dept. of Electronic Engineering, University of Roma “Tor Vergata”, Via del Politecnico 1, 00133 Roma, Italy
Abstract- Modulation instability generated by mechanical
frequencies in RF MEMS switches is predicted and its
potential contribution to the RF signal degradation is
discussed. In particular, evaluations have been performed for
double clamped configurations in shunt capacitive devices. As
a conclusion, it is evidenced the possibility for the excitation of
satellites affecting as noise sources higher than -40 dB the
spectral purity of microwave sources.
I. INTRODUCTION
High power effects in RF MEMS devices could be a
limiting factor in their performances because of selfactuation
of micro-switches [1][2][3] or non-linear response
and excitation of satellite frequencies [4][5][6][7]. In the
first case we have a failure of the device related to the unwanted
actuation, eventually caused by micro-welding due
to the increase of temperature during and after the bridge
collapse. In the second case, the RF power carried by the
signal flowing through the microwave transmission line can
excite transversal and longitudinal mechanical modes,
contributing to the degradation of the signal.
In this paper the reconstruction of the spectrum due to the
presence of mechanical resonances of the beam of a shunt
connected RF MEMS switch is presented, with the aim to
evaluate the contribution of inter-modulation products to the
RF signal processed by the switch.
II. RF SIGNAL PROCESSING IN RF MEMS
Micro-electromechanical switches for Radio Frequency
applications (RF MEMS switches) are movable microsystems
which commute from an ON to an OFF state by
means of the collapse of a metalized beam [8]. They can be
actuated in several ways but, generally, the electrostatic
actuation is preferred because no current is flowing in the
device nor power absorption has to be involved in the
process. In a coplanar waveguide (CPW) configuration, like
that shown in Fig. 1 and Fig. 2, the bias DC voltage signal is
usually separated with respect to the RF signal for
application purposes. Anyway, in the simplest mechanical
model, a voltage difference V is imposed between the metal
bridge, connected to the ground plane of a coplanar
waveguide (CPW) structure, and the central conductor of
the CPW, which also carries the high frequency signal.
Under these circumstances, the switch will experience an
electrostatic force which is balanced by its mechanical
stiffness, measured in terms of a spring constant k. The
balance is theoretically obtained until the bridge is going
down approximately (1/3) of its initial height. After that, the
bridge is fully actuated, and it needs a value of V less than
the initial one to remain in the OFF (actuated) position,
because contact forces and induced charging effects help in
maintaining it in the down position.
Fig. 1. Typical capacitive RF MEMS shunt switch in CPW configuration
Fig. 2. Cross section of the switch structure, where the metal bridge is
suspended by means of dielectric anchors on a multilayer composed by: (i)
the air gap g with respect to (ii) a metal thin layer at a floating potential
(FM) to be used for improving the capacitance definition in the down
position, (iii) a dielectric layer with thickness d deposited onto (iv) the
metal M of the central conductor of the CPW, and finally (v) the SiO 2
thermally grown layer onto the high resistivity silicon wafer.
The RF power carried by the signal flowing in the central
conductor of the CPW can be written as:
RF
out
1
2
in
2
RF
RF
IVP
RF
Pin
2 Z0
(1)
[ ]
M
1 V
+−=
PPPP
r
===
Where: V RF and I RF are the effective RF voltage and
current respectively, and Z 0 is the characteristic impedance
of the line. P out is the output power, which is obtained from
the input power P in decreased by the power coupled to the
mechanical structure P M and the reflected power P r . The
necessity to distinguish between the last two contributions
depends on the different nature of the transferred power: the
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