Instrumentation & Measurement Magazine 26-3 - 15

Fig. 1. (a) A schematic drawing of the rotary comb-based electrostatic cantilever-type MEMS energy harvester; (b) Detailed figure of the design parameters.
to maximize the travel range and minimize the operational
frequency and overall size. The proposed MEMS design can
be incorporated in embedded systems such as wearable devices
where high availability of the energy source is required.
The use of an advanced conditioning circuit, such as the Bennet
doubler, is intended to increase the performance of the
device. To the authors' best knowledge, this is the first time a
combination between a rotary comb-drive and a Bennet doubler
as a conditioning circuit is proposed for energy harvesting
applications.
Design and Analytical Model
The proposed electrostatic MEMS EH consists of two stationary
electrodes and a movable cantilever-type electrode. The beam
supports a rotary comb-drive with a total of 16 interdigitated
fingers attached from both sides and a proof-mass attached to
its tip. The rotary comb-drive will convert the kinetic energy of
the in-plane vibration from ambient sources and then store it
as an electrical charge using a developed circuit. A schematic
drawing of the rotary comb-based cantilever-type electrostatic
EH is shown in Fig. 1a. As the external source of vibration applies
on the movable comb-drive, the cantilever beam deforms
in the in-plane direction, resulting in an engagement between
its electrodes and one of the stationary comb-drive. This action,
in fact, results in an opposite motion on the other side where
the rotary comb-drive moves away from the second set of fixed
comb-drive. The movable and the two sets of stationary electrodes
constitute two out-of-phase variable capacitances.
The movable comb-drive beam has a length of lb = 600 μm
and a width of b = 3 μm. It carries 16 interdigitated electrodes
with a length that varies from 26.96 and 44.81 μm and a width
of fb = 2 μm. A proof mass with a length of ltip
width of btip
= 50 μm and a
= 80 μm is also attached at the tip of the cantilever
beam, as shown in Fig. 1b. The overlapping angle between
the movable comb-drive electrodes and the stationary combdrive
that carries 18 stationary electrodes is set to
 5o
May 2023
 while
the separation gap between each pair of the fingers is set to
g = 2 μm. The structural device layer thickness is h = 40 μm. The
device is made of Boron-doped silicon with a Young Modulus
E = 166 GPa and a density ρ = 2332 kg/m3
and a standard
Silicon on Insulator fabrication process is utilized to fabricate
the cantilever-based energy harvester. The wafer is a
p-type < 100 > low resistivity Silicon on Insulator (SOI). It has
a structural layer thickness of 40 μm, the buried oxide layer
thickness is 1 μm and the handle layer thickness is 550 μm.
The wafer is initially cleaned using RCA to remove all types
of contamination from the top surface. Then, a seed layer made
of Chromium is deposited, followed by a metallization layer of
Gold deposited. After that, a layer of photoresist is spun onto the
wafer and a physical mask (Mask-1) is used to pattern photoresist
on the contact pads. This results in contact pads being used
for actuation. Another layer of photoresist is spun onto the wafer
and patterned using a second mask (Mask-2) to define the device
area. Next, the silicon layer is etched all the way to the buried oxide
layer using Deep Reactive Ion Etching, and the unexposed
photoresist is stripped away. Finally, the device is released by
etching the buried oxide layer using HF. A cross-section for the
last fabrication step that is used to fabricate the device, showing
the back-side etching step, is illustrated in Fig. 2.
The equation of motion of the cantilever-type EH has been
derived using the Euler-Bernoulli beam theory. The end-proof
mass supported by the cantilever beam is modeled as rigid
bodies with a concentrated point mass and its rotary inertia
and elastic deformation are ignored. The movable interdigitated
fingers are also modeled as concentrated point mass.
Then, a standard Reduced-Order Model (ROM), that uses
straight beam mode shapes as basis functions, is developed
to solve the equation of motion. Following the methodology
developed by Alneamy et al. [7], one can write the nondimensional
equation of motion describing the in-plane motion of
the energy harvester, carrying a proof mass and 16 fingers, and
accounting for the electrostatic force and the base excitation as:
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Instrumentation & Measurement Magazine 26-3

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