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U. Schmid, M. Schneider:
"Advanced Aluminium Nitride Thin Films for Piezoelectric MEMS";
Hauptvortrag: Surfcoat Korea 2019, Incheon, Korea (eingeladen); 27.03.2019 - 29.03.2019; in: "Surfcoat Korea 2019 - Book of Abstracts", (2019), S. 31.

Electromechanical transducers based on the piezoelectric
effect are continuously finding their
way into micro-electromechanical systems
(MEMS), typically in the form of thin films. Piezoelectric
transducers feature a linear voltage response,
no snap-in behaviour and can provide
both attractive and repulsive forces. This removes
inherent physical limitations present in the commonly
used electrostatic transducer approach
while maintaining beneficial properties such as
low-power operation. Furthermore, piezoelectric
materials can serve for both actuation and sensing
purposes, thus enabling pure electrical excitation
and read-out of the transducer element in combination
with a compact design. Based on these
outstanding features, piezoelectric transducers are
operated most beneficially in a large variety of
different application scenarios, ranging from resonators
in liquid environment, advanced acoustic
devices to sensors in harsh environments. In order
to exploit the full potential of piezoelectric
MEMS in the future, interdisciplinary research
efforts are needed ranging from investigations of
advanced piezoelectric materials over the design
of novel piezoelectric MEMS sensor and actuator
devices, to the integration of PiezoMEMS devices
into full low-power systems.
In this talk, we will highlight latest results on the
electrical, mechanical and piezoelectrical characterization
of sputter-deposited aluminium nitride
(AlN) including the impact of sputter parameters,
film thickness and substrate pre-conditioning
[1,2]. We will present the impact of doping of
AlN with scandium, which leads to an increase of
the moderate piezoelectric coefficient of AlN up
to a factor of four.
In a next step, these films are implemented into
fabrication processes of cantilever-type MEMS
devices. In combination with a tailored electrode
design, resonators are realized featuring in liquids
Q-factors up to about 300 in the frequency range
of 1-2 MHz. This enables the precise determination
of the viscosity and density of fluids up to
dynamic viscosity values of almost 300 mPas [3].
Besides this application, such high Q factors are
useful when targeting mass-sensitive sensors,
thus paving the way to e.g. particle detection even
in highly viscous media.
Given the low increase in permittivity of ScAlN
compared to AlN, another field of application for
this functional material class are vibrational energy
harvesters, where the benefit of ScAlN compared
to pure AlN is demonstrated [4].
Finally, we will present some selected results of
ScAlN thin films within SAW devices ranging
from high temperature applications to droplet
manipulation in microfluidics [5].