Biosensing Detection - Bioinspired Approaches for Human-Centric Technologies - page 92

temperature sputtering on polyimide with a discrete crystal orientation manifests
piezoelectricity (Akiyama et al. 2007 , 2008 ). The inclusion of AlN/polymer struc-
tures into the family of flexible piezoelectric devices paves the way to sensors with
augmented mechanical

and electrical properties. Sputtered Mo/AlN/Mo

heterostructures on 25 m thick Kapton foils result in very well-oriented polycrys-

talline layers which can be patterned in circular shape to obtain large elastic domes
due to the compressive stress of AlN over Kapton. The organization of the crystal in
the dome makes the system very elastic under load, with the capability to undergo
higher strains than rigid AlN thin films on silicon. The system is sensitive to two
different types of mechanical stimuli: vibration through the piezoelectric effect,
recorded as time-varying voltage, and pressure as capacitive variation due to the
flexoelectric effect. The last contribution is due to the strain gradient generated in
the crystal. By applying a pressure to the dome , a further polarization is generated,
which is detected as capacitance variation (Fig. 3.3 ).
The analogy with mechanoreceptors is remarkable: the flexible AlN allows to
sense time-varying stimuli as Pacinian corpuscles and Merkel disks do in human
fingers (Fig. 3.4 ).
Despite the hybrid nature of such devices, the AlN films are crack-free and
exhibit a good adhesion on the plastic Kapton film. The crystalline structure of Mo
and AlN films is indeed preserved as evidenced by the X-ray diffraction patterns in
the inset of Fig. 3.3 .
The force transducers device can be fabricated in one lithographic step by a
(SU8-25) negative photoresist. The Mo top electrode and the AlN are dry etched by
inductively coupled plasma (gas mixture SiCl 4 ,N 2, and Ar). Upon detaching the
device from the starting rigid substrate, a free-standing foil of circular transducers is
obtained. Each transducer transforms itself into a dome-shaped 3D structure,
because of the residual stress of the crystal layers on the polymer (Fig. 3.5 ). The
structure is elastic and does not crack even under large deformations.
The piezoelectricity of AlN is well known and allows the detection of time-
varying signals in a wide frequency range as the Pacinian mechanoreceptors. The
application of an oscillating voltage V generates an out-of-plane deformation; the
relation between the z displacement and the applied voltage depends on the
piezoelectricity of the material and corresponds to d 33 . The measured d 33 in these
devices is 4.7

μ

0.5 pm/V, which is consistent with the values reported for AlN
layers grown on silicon and silicon-based substrates (Xu et al. 2001 ; Dubois and
Muralt 2001 ).
In addition to piezoelectricity, the application of a load to the dome generates a
strain gradient in the film. The strain gradient is responsible for a polarization
known as flexoelectricity which manifests itself through a capacitance variation.
The flexoelectric effect is the property of piezoelectric (and dielectric) materials to
generate charge separation when subjected to elastic strain gradients (Tagantsev
1986 ). The flexoelectric polarization is described by the following equation: