MEMS (Micro ElectroMechanical Systems) combine electrical and
mechanical components with feature sizes of the order of several
microns. MEMS technology has evolved to its present state thanks
mainly to the fabrication techniques pioneered by the semiconductor
industry. MEMS can be found in large volume applications such as
airbag-control sensors for the automotive industry; however, there
are many other MEMS applications in every day use. MEMS components
in these applications may have moving mechanical parts together
with fluidic, electrical, optical, chemical, and biomedical
MEMS for RF Applications
Passive components such as inductors, capacitors, and resistors
can take up valuable amounts of real estate on a printed-circuit
board (PCB). It is therefore desirable to integrate as many of
these components as possible into networks. Traditionally, these
networks were fabricated using thin-film techniques. Not only does
MEMS technology offer high packing densities, but the technology
also has brought with it an additional advantage in that it is
possible to make devices that are tunable. This article will focus
on such a device: an LC filter.
Simulating RF Behavior
In order to predict the behavior of an RF component, we use
computer simulation. By using a High Frequency Structure Simulator
(HFSS), it is possible to perform a full-wave electromagnetic
analysis of a 3D structure, using the finite-element method. The
simulation solves Maxwell's equations, which describe
electromagnetic phenomena completely, thus allowing us to
accurately predict the performance of the component, taking into
account electrical losses and electromagnetic-radiation effects.
HFSS is commonly used to model components such as filters,
connectors, IC packages, and antennas found in cellular telephones
and broadband communications systems. However, because it analyzes
the underlying physics of these components, you can apply HFSS with
equal success to modeling the electrical performance of MEMS
A major advantage of simulation is that you can predict and
optimize the performance of a device without having to fabricate
test structures. This capability is particularly important when the
cost of building a physical prototype is high.
The MEMS LC Filter
In this example, the main inductor is fabricated in thick copper
and isolated from the underlying silicon substrate by a twin
dielectric layer (Figure 1a). Electrically, the structure is
similar to an inductor in parallel with a small capacitor.
Figure 1: Fabrication of a tunable LC resonator
A second conductive spiral is formed on a membrane suspended
above the first (using MEMS fabrication techniques). This provides
a large parallel capacitance, which can be varied by changing the
gap between the two spirals using standard MEMS actuation methods
based on electrostatic attraction or thermal expansion (Figure
1b). The structure thus behaves as a tunable resonator.
In modeling the electrical behavior, HFSS takes into account the
losses in the underlying silicon as well as losses in the copper
tracks themselvesthis is important since these losses
directly affect the quality factor (Q) of the inductor, which is an
important parameter for RF applications.
Figure 2: Resonator performance vs. frequency for different capacitor gaps
Figure 2 shows return loss of a MEMS resonator as a
function of frequency for different capacitor gaps. The shape of
these characteristic curves is determined by the dimensions of the
spiral track and the number of turns in the spiral. Simulation lets
the engineer explore this design space and produce a structure that
achieves as high a Q factor as is possible over a required tuning
range. The use of parameterization and optimization modules to
control the simulation makes this a faster and more cost-effective
route than the manufacture of physical prototypes.
MEMS technology can be applied in a very wide range of
application areas. Particular benefits in the RF arena are the
technology's ability to produce compact, low-loss, tunable devices.
The characterization and design of such devices through physical
prototyping can be both time-consuming, and expensive.
Electromagnetic simulation offers a fast and accurate means to
predict behavior and hence develop optimum designs.
About the Author
Dr. Nick Campbell graduated in Physics at
Cambridge University and completed his Ph.D. in Electrical
Engineering in 1984. Following this, he worked for a scientific
instrument manufacturer and two engineering consultancies. His
responsibilities in these positions have been quite broadhis
main areas of technical expertise are system engineering,
electromagnetics and electron optics. He joined Ansoft Corporation
in 1997 and works as an Application Engineer at Ansoft's Northern
European Headquarters in Twickenham, London.