Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
Design of the silicon membrane of high fidelity and
high efficiency MEMS microspeaker
Iman Shahosseini, Elie Lefeuvre, Emile Martincic,
Marion Woytasik, Johan Moulin, Souhil Megherbi
Univ. Paris Sud – CNRS
Institut d'Electronique Fondamentale
91405 Orsay Cedex, France
Abstract- This study presents a novel approach to MEMS
microspeakers design aiming to tackle two main drawbacks of
conventional microspeakers: their poor sound quality and
their weak efficiency. For this purpose, an acoustic emissive
surface based on a very light but very stiff structured silicon
membrane was designed. This architecture, for which the
membrane undesirable vibration modes were reduced to only
five within the microspeaker bandwidth, is promising to let the
microspeaker produce high sound quality from 300 Hz to 20
kHz. This silicon membrane is suspended by a whole set of
silicon springs designed to enable out-of-plane displacements
as large as 300 µm. Different geometries of springs were
considered and the material maximum stress was analyzed in
each case by finite element modeling. The proposed structure
promises an efficiency of 10 -4 , that is to say ten times higher
than that of conventional microspeakers.
I. INTRODUCTION
The broad development of mobile electronic devices
embedding audio function is now strongly increasing the
demand for higher sound level and better sound quality.
From this point of view, the problem mainly comes from
the poor quality of available microspeakers. Thus, more and
more attention is being paid to acoustic performances of the
microspeakers used for instance in mobile phones, which
represent more than one billion units per year market. This
explains why significant research efforts are currently
focused on improvement of the performances of
microspeakers [1, 2]. Until now, such microspeakers are not
MEMS: they are manufactured using conventional
"macroscopic" machining technologies. But limits of
conventional technologies in terms of integration and sound
quality are not far to be reached, and MEMS technologies
present a very promising potential for overcoming these
limitations, as shown by recent studies [3]. Indeed,
microtechnologies bring outstanding dimensional precision
and good reproducibility which are needed for
manufacturing high quality sound transducers. Moreover,
thanks to batch process the fabrication costs may be kept
reasonably low.
Another critical issue of mobile electronic devices is the
autonomy of batteries. Taking again the example of cell
Romain Ravaud and Guy Lemarquand
Université du Maine – CNRS
Laboratoire d'Acoustique de l'Université du Maine
72085 Le Mans, France
phones, nearly one quarter of the total power consumption
is due to the audio system when used in free-hand mode.
Analysis of the components usually used in the sound
reproduction chain of mobile devices shows that D-A
converters have very little consumption. Amplifiers have
pretty good efficiencies, typically between 50% and 90%.
From the efficiency point of view, the weakness is mainly
due, again, to the microspeakers. Indeed, efficiency of the
electrical-to-acoustic power conversion remains typically
lower than 0.001%. So it is clear that improvement of the
efficiency of microspeakers is the best approach to increase
significantly the overall efficiency of the audio chain. For
instance, improvement of the microspeaker efficiency by a
factor of ten, that is to say reaching 0.01% efficiency, will
roughly divide the consumption of the sound reproduction
chain by the same factor ten. The total power consumption
will thus be notably reduced, with significant gain in term
of energy autonomy of mobile devices.
The approach developed in this paper aims at improving
both the efficiency and the sound quality of microspeakers.
Until now, few works on MEMS microspeakers have been
reported in literature. Transduction principles such as
piezoelectric, electrostatic, electrostrictive, electrodynamic
and thermoacoustic actuation, which are achievable using
MEMS technologies, have been proposed [4]. But nonlinear
response of piezoelectric, electrostrictive and
thermoacoustic materials is a major drawback for high
fidelity transduction. Electrostatic principle, which is
broadly used for MEMS actuators because of its
technological simplicity, has however low power density
and requires relatively high driving voltages. So, although it
requires magnets whose integration into MEMS is not very
developed yet, electrodynamic actuation principle is the best
way to meet the objectives in terms of linearity, power
density and efficiency. This actuation principle lies on the
Lorentz force which appears on a conductor, usually coil
shaped, driven by an electrical current and surrounded by a
magnetic field, usually created by a permanent magnet.
Predictions developed in this paper, based on analytical and
finite element method (FEM) modeling of the microspeaker
show that the targeted efficiency of 0.01% is reachable
using a planar copper coil and ring-shaped magnets with
axial magnetization of 1.5 T.
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