Biosensing Detection - Bioinspired Approaches for Human-Centric Technologies - page 94

Fig. 3.5 Array 2 2of
piezoelectric transducers,
the stress release of the AlN
polycrystalline layer leads
the dome shape of the
circular part

S ij

P l ¼ μ ijkl ∂

x k

∂

where P is the polarization,

μ ijkl the flexoelectric coefficient, S ij the components of
the elastic strain, and x k the direction of the strain S gradient.
The flexoelectric effect is due to a discontinuity in crystal lattice, and it is
experimentally observed on AlN dome since the pressure depresses the dome,
thus producing a strain gradient as also confirmed by finite element method
(FEM) simulations.
In Fig. 3.6a , the polarization is recorded as a capacitance variation generated by
the application of a load on the dome. Loads from 1 g up to 20 g were applied for
transducers with diameters d ranging between 500 and 800 m. The investigated

μ

pressure range goes from 10 kPa up to 1 MPa.
The dome is designed and simulated by finite element method (Fig. 3.6b ,c)with
diameter and height experimentally measured by optical microscope and
profilometry. The simulation is performed by applying a parametric pressure on
top of the sensor dome . In the case of smaller domes ( d

m),
the x and y strains are reported as a function of the distance from the edge of the
structure, and the presence of a strain gradient across the layer is confirmed
(Fig. 3.6b ). The strain gradient is enhanced by applying higher forces, which is
consistent with the capacitance variation observed experimentally (Fig. 3.6c ). The
model confirms the hypothesis that AlN layer is elastically deformed under load and
subjected to a growing strain.
This flexoelectric effect increases the sensitivity of flexible AlN on Kapton in
sensing application for both dynamic and static forces of small medium intensity
(Petroni et al. 2011 , 2012 ). The resulting transducers are very elastic and the
deformations of the substrate do not affect the electrical performances of the

¼ 500 m and h ¼ 32

μ μ