Biosensing Detection - Bioinspired Approaches for Human-Centric Technologies - page 101

on the substrate, keeps a curved equilibrium position, at a fixed height with respect
to the substrate. If the cantilever is under the action of any type of physical force
along its length direction, it bends increasing or decreasing its curvature (up to
complete flattening). This approach turns out to be very robust against deformation
and fast with respect to real time to external stimuli. A micro-strain gauge sensor,
strongly attached on the cantilever, senses the deformation caused by an applied
force: due to this variation, negative or positive, the sensor is able to distinguish the
orientation of the applied force. Such architecture makes the operation of the
devices very similar to the natural hair cell.
The technological principle for the realization of such artificial hair cells relies
on the material properties of the constituent layers, namely, the mismatch of atomic
sizes between different layered materials grown by heteroepitaxy and/or a gradient
in lattice mismatch developed during single-layer material deposition or thermal
cycling of the chip during micro-fabrication (Hu 1991 ). The generated deformation
causes a release of stress in the multilayered cantilever beam which in turn induces
an intrinsic upward (out-of-plane) bending of the cantilever. Previous designs
related to this approach account for single-layer beam such as CVD (chemical
vapor deposition) silicon nitride (Wang et al. 2007 ), polycrystalline silicon (Zhang
et al. 2010 ), and silicon dioxide (Zhang et al. 2010 ). Noteworthy, these examples
are inherent to intrinsic stress developed during growth and are hardly controlled.
More recently, Qualtieri et al. (Qualtieri et al. 2011 , 2012 ; Rizzi et al. 2013 )
developed a new design for stress-driven flow sensors based on multilayered
cantilevers on a sacrificial layer. Once the sacrificial layer underneath is removed,
the unbalanced stresses relax, bending upward the cantilever beam. The difference
among internal stresses and/or crystalline lattice of each layer of the cantilever
beam can be controlled through the layer thicknesses and by the lithographic
patterning dimensions (beam length, beam width), resulting in a tight control of
the moment and of the bending curvature and height of the device. Figure 3.9 shows
the scanning electron microscope (SEM) pictures of the bent cantilevers: a 200 m

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long aluminum nitride (AlN)/molybdenum (Mo) beam, equipped by a 50 m long

strain gauge on the left (Fig. 3.9a ). On the right (Fig. 3.9b ), the upward bending of
the beam (radius of curvature and tip height) is shown to be dependent from the
beam length.
For waterproofing and mechanical properties tuning, a parylene C conformal
coating was deposited by room temperature chemical vapor deposition. The
parylene coating has been realized on the SiN/Si cantilever beam to control directly
the cantilever beam flexural stiffness and the device sensitivity. Figure 3.10 shows a
SEM image of the cantilever. The upward bending through the residual stress inside
the cantilever beam allows reaching a height up to 1.2 mm. The parylene coating
layer, virtually unstressed (Harder et al. 2002 ), does not affect the bending and
radius of curvature of the cantilever (i.e., the dynamic range) but the stiffness of the
cantilever. Two thicknesses of parylene have been investigated (0.5

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m
on both cantilever faces) in order to explore how a different cantilever flexural
stiffness influences the sensitivity to flow sensing and the dynamic range (Rizzi
et al. 2013 ).

m and 2

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