A conventional accelerometers have advantages of extremely small

A Review of MEMS Accelerometers Working
Principles
Sorush Salahshour Torshizi
Institute for Microsensors, -actuators and –systems (IMSAS)
Bremen, Germany
[email protected]
Abstract—This paper aims to provide a review of
MEMS-Accelerometers (accs.) different operating
principles. At first variety of acceleration sensing
and their basic principles as well as a brief overview
of their fabrication mechanism will be discussed and
lastly the paper will be focused on most
commercialized and well-known accelerometer
technique, namely, capacitive. Moreover, a
comparison table of their performance based on
acceleration sensor characteristics such as dynamic
range, sensitivity, resolution and linearity will be
depicted. finally, an evaluation of the different
sensing techniques of MEMS-Accelerometer as well
as the conclusion wraps-up the paper.
I. INTRODUCTION
Acceleration sensors are playing a vital role in
micromachined technology, moreover, the demand for
new and high-performance accelerometers is increasing
daily. The first industry which took the benefits of
MEMS-Accelerometers was the automobile industry in
2000 by utilizing MEMS-Accs as for car suspension
systems and controllability and in the same way for
safety systems such as airbags system 1. Nowadays the
application scope of accelerometers covered almost
every aspect of engineering science. MEMS-Accs
compare to conventional accelerometers have
advantages of extremely small size and ability to be
mass produced and importantly lower manufacturing
costs 1. Consequently, the application spectrum of
these acceleration sensors is not confined to car industry
while they have opened up their way in multitude branch
of science. For instance, nowadays in aviation and
aerospace industry and after the emerging the modern
technology autonomous unmanned aerial vehicles
(UAVs) the demand for highly sensitive and low-cost
accelerometers increased sharply 2. Moreover, MEMS
accelerometers are now the crucial part of space crafts
and rockets navigation systems. Moreover, they are now
an inseparable part of smart devices navigation and
tracking systems. Similarly, in Bio-engineering where
the size of the sensor is highly under magnifier for
researchers, MEMS accelerometers are used for health
monitoring with help of implanting sensors inside the
body 3. Based on aforementioned applications
different technology and principle has been used up to
now for their fabrication and operating method, the vast
majority of application employed capacitive and
piezoresistive accelerometer as their transduction
mechanism and fabrication is easier to utilize, but there
are more different working principles which will be
discussed in the next section of this paper.
II. OPERATING PRINCIPLES
As for every accelerometer, the basic working principle
is based on a fixed local inertial frame, beam, and of
course the proof mass. When an external forces apply
the mass will be displaced with respect to the local
inertial frame, the source of this force could be constant
gravity force which is called static force or it could be
caused by shock or movement which can be named as
dynamic forces 4. With reference to definition of
sensor, acceleration sensor should convert mechanical
motion which has deflected the proof mass, into
readable computer signal, for this reason there are
several transduction mechanisms which some of them
are more relevant such as Capacitive or Piezoresistive
accelerometers and also some other mechanisms like
Optical, Piezoelectric, Thermal and Tunneling,
Piezoelectric, Electromagnetic, Surface Acoustic Wave
(SAW) accelerometers. Due to content restriction and
less practical applications compare to other
mechanisms, in this paper all of the above-mentioned
principles except Electromagnetic and SAW will be
discussed.
A. Optical Accelerometers
The working principle of optical accelerometers lies
in characteristics of a beam of light. compare to wellknown
capacitive based accelerometers, optical-Accs
exhibit better sensitivity and resolution as well as higher
thermal stability which make them applicable in
hazardous environments. Optical accelerometers instead
of measuring the displacement of proof mass measure
the variation of light wave characteristics like measuring
the stress distribution among the proof mass when it is
deflected (Photoelastic effect) or determining the effect
of different forces and mass displacement on optical
signal phase (Phase modulation). Phase modulation is
normally used when the higher dynamic range is
required. The other methods are Intensity modulation
which is simple for fabrication but highly dependent on
high-quality light sources compare to Wavelength
modulation which is completely independent of light
source deviation and is highly accurate and sensitive.
The outstanding advantage of Optical-Accs is their
immunity against electromagnetic interference(EMI) 5.
Figure. 1. Optical wavelength modulation based Accelerometer
The figure.1 shows the Wavelength modulation
based Optical-Acc sensor by which the light goes
through the photonic crystal (PhC) and then enter the
photodetector for measuring the acceleration, when an
external forces applied to proof mass it will move on its
(y) axes which will cause a change of output
wavelength. Consequently, the magnitude and direction
of acceleration would be measured base on the
wavelength difference occurred.
B. Thermal Accelerometer
Thermal accelerometers compare to other
aforementioned techniques do not employ proof mass
for sensing acceleration, they utilize thermal convection
phenomenon. Thermal-Accs generally consist of silicon
etched SNx heater with two temperature sensor on both
sides of it, inside the thermal isolated encapsulated
cavity. The heater reduces the density of its surrounded
air(liquid) therefore when there is no acceleration two
temperature sensor will sense the same temperature
figure.2(A). By applying acceleration dense bubble will
move within the direction of applied acceleration which
will cause an asymmetric temperature profile for
detectors figure.2(B), consequently, this temperature
difference will be detected and amplified for converting
into a digital signal by the principle of Wheatstone
bridge. 6
Figure 2 Heat Accs,(a) rest mode (b) acceleration applied
The fabrication process of this accelerometer is simple
which means lower manufacturing cost compared to
other mechanisms. Since there is no proof mass, the
thermal accelerometer has extremely good shock
resistance and compare to capacitive sensors it has more
sensitivity, on the other hand, the dynamic range is
confined and low-frequency range makes it not suitable
for instant shocks measurements or fall sensing. 6
C. Tunnel Accelerometer
Tunneling-Accs typically consist of metal tip
connected to a proof mass which has few Nanometer
distance to a counter electrode and the working
principle lies in the quantum electron tunnelling. In
order to activate the sensor small bias voltage (around
100mV) is needed to be applied, this voltage
consequently create a small current between the metal
coated tip and counter-electrode. 7
Figure 3 Simple Schematic of Tunnel Accelerometer
When an acceleration applied the movement of proof
mass will cause the sub-angstroms displacement of the
tip which causes the change in tunnel current. The aim
of this method is to keep the tunnel current (1nA)
constant over the time, therefore, feedback forces have
applied to bring the mass back to its rest position, as a
result, the magnitude of acceleration could be measured
by closed-loop detector circuit and with help of
variation of deflection voltage. 7
The design and fabrication of Tunnel-Accs vary since
the time they introduced, Cantilevered, Lateral and
Bulk-micromachined are some of them. 7 Tunneling
accelerometers have low drive voltage supported by
wide frequency bandwidth as well as higher sensitivity
compared to capacitive. On the other hand, with
reference to the Nanoscale gap, they have complicated
fabrication process and higher production costs.
D. Piezoelectric Accelerometer
These kinds of accs. take the benefit of the inherent
piezoelectric effect of materials. A piezoelectric acc.
as shown in the figure.4 Usually, consists of a
piezoelectric material which is typically thin ZnO or
PZT which is sandwiched by two electrodes and
deposited over silicon cantilever beam. 8
Figure 4 Principle schematic of the piezoelectric
The beam is fixed to frame on one side and on
the other side there is proof mass. In the presence of
acceleration, mass displacement cause deformation of
the beam, in the same way, the piezo material
experience compression or tensile. The acceleration
then could be measured by calculating the potential
difference occurred. PZT has higher piezoelectric
constant and sensitivity but it could not be integrated
or miniaturized, On the other hand, ZnO has lower
sensitivity but easy to integrate, additionally, new
fabrication technology compatibility and its
sensitivity could be improved by miniaturization.
Overall, piezoelectric-Accs. Has high sensitivity and
compare to capacitive, lower power consumption and
lower temperature dependence as well as higher
bandwidth. 8
E. Piezoresistive Accelerometer
The first MEMS accelerometer was piezoresistive
and was developed back in 19795. It took twenty years
until the first MEMS accelerometer commercialized in
the market by a car company for their safety systems.
The backbone of this method is based on resistivity
variation of a material under the stress. Early designs of
piezoresistive-acc has a beam that holds the proof mass
and supported by a fixed frame 9, moreover, piezoresistors
were located on the special spot of the beam
where the maximum deformation and stress happens
(usually edges) and the readout circuit of them is based
on Wheatstone bridge principle, figure.5(b). acceleration
and displacement of proof mass will cause beam
deformation and consequently, the resistivity of piezoresistors
will change, resistance variation will end up
changes in the output voltage. piezoresistive
accelerometers are highly reliable and simple to
fabricate but the integration is not simple.
Figure 5 three axes piezoresistive accelerometer (a) model
view (b) equivalent Wheatstone bridge model
Up to know almost all of the papers are focused on
improving the performance and sensitivity by modifying
the geometric design and sensing mechanism or by
utilizing different fabrication technologies. For instance,
adding multiple beams instead of one flexure, figure.5(a)
or using asymmetrically gaped cantilever or ion etching
the resistors on beam instead of thermal diffusion. In
some papers, the lateral movement of mass has also
fabricated. Additionally, the length of flexure also
matters, the longer the flexure will cause, the lower
resonant frequency and therefore lower bandwidth. 9
Typically, in order to protect the sensor from high G or
instant shock, the upper and lower part of the sensor is
covered by Glass in almost most of the fabrications.
F. Capacitive Accelerometer
Capacitive-acc is the among most famous
accelerometers in MEMS sensors, ADXL series is
one the most successful accelerometers in MEMS
market. 10 Their working principle is based on
capacitance variation. The proof mass is located in a
way that has a narrow gap with fix conductive
electrodes, the displacement of the mass, therefore,
will cause a change in distance between mass and
electrodes, therefore, the capacitance will be varied.
This variation then could be transferred to a digital
signal with readout circuit. Capacitive-accs
structures could be divided into lateral, vertical or
see-saw. 1 Lateral accelerometers usually consist
of surface micromachined fix fingers as well as a
mass which shaped with sensing fingers, sensing inplane
acceleration in x-y axis, vertical structures are
usually bulk micromachined and has bigger mass
which is located between two fix electrodes, thus
they have better sensitivity and out of plane sensing
in the z-axis. The see-saw accelerometers make use
of torsional beams to suspend the mass and making
one side of structure heavier, hence, same as
verticals the have out of plane sensing mechanism.
The outstanding advantages of capacitive-accs are
high sensitivity and DC response, simple and easy to
mass produce structure, high linearity and low power
dissipation and easy to integrate. The only drawback
of capacitive accelerometers is that they are at the
mercy of electromagnetic interferences(EMI) which
require special packaging. Figure.6 Is an integrated
3-axis accelerometer with two in-plane structure for
x and z-axis and one out of plane structure for z-axis
which they connected to each other by polysilicon
connectors. The fabrication of both of them is
depicted in figure.7.
Figure 6 Three-axis single-chip micro-g accelerometer
Figure 7 Fabrication process. (a) Boron doping; (b) DRIE trench; (c)
oxide,nitride, poly deposition; (d) pattern oxide, nitride, poly; (e)
electroplate metal;(f) anisotropic etching; (g) HF release.
III. COMPARISION
There are a variety of sensors in all of aforementioned
transduction mechanisms in different dynamic range
(from micro-g ranged to hundred kilos) and sensitivity
as well as DC response or linearity. Therefore, it is not
an easy task to compare this mechanism. For the sake of
comparison six sensors which have close dynamic range
has been selected and their performance has been
written on the table1.
Range Sensitivity Resolution Non.lin
Optical + 22 3.1816 nm
g?1
n/a Linear
Thermal +10 375 mV/g 30 mg Linear
Tunnel -20-+10 133 mV/g 22.8 mg 0.6%
Piezo.res 50 3 mV/g 0.20 the mg <1% Piezo.e + 25 0.21 mV/g 0.22 mg <2% Capaci. + 27 0.5 mV/g n/a Linear Table 1 MEMS Accelerometers Comparison Table IV. CONCLUSION In this paper, six different transduction mechanism of the accelerometer, as well as their specifications with brief fabrication techniques descriptions, have been discussed. It worth to be noted that basically, each type of accelerometers has some pros and cons which make them applicable for the special and different application. For instance, EMI and temperature immunity of optical sensors for a special application which has electromagnetic fields or high sensitivity of Tunneling sensors for rocket navigation and good shock resistivity of Thermal sensors in high-g applications, simple fabrication of piezoresistive sensors for mass production and also low power consumption of piezoelectric sensors. The thing that makes the capacitive sensors different and put them in the first place in the market is that they have most of the above-mentioned advantages which make them applicable for wide variety spectrum of different applications. REFERENCES 1 N. Yazdi, F. Ayazi and K. Najafi, "Micromachined Inertial Sensors," 2 H. K. a. K. N. Junseok Chae, "An In-Plane High-Sensitivity, Low-Noise Micro-g Silicon AccelerometerWith CMOS Readout Circuitry," 2004. 3 K. Imenes, "Implantable MEMS Acceleration Sensor for Heart Monitoring Recent Development and Outlook," HiVe - Vestfold University College, Technology, Norway. 4 M. Andrejaši?c, "MEMS ACCELEROMETERS," in University of Ljubljana, 2008. 5 "A Proposal for an Optical MEMS Accelerometer Relied on Wavelength Modulation With One Dimensional Photonic Crystal," JOURNAL OF LIGHTWAVE TECHNOLOGY, vol. 34, p. 22, 2016. 6 J. B. P. M. K. G. Rahul Mukherjee, "A review of micromachined thermal accelerometers," Journal of Micromechanics and Microengineering, 2017. 7 X. G. P. Varun Kumar, "Single-Mask Field Emission Based Tunable MEMS Tunneling Accelerometer," in IEEE International Conference on Nanotechnol, Rome, 2015. 8 J.-y. W. M.-h. X. H. G. Rui-Hua HAN, "DESIGN OF A TRI-AXIAL MICRO PIEZOELECTRIC ACCELEROMETER," in Symposium on Piezoelectricity, Acoustic waves, and Device Applications, 2016. 9 A. L. R. Tarun Kanti Bhattacharyya, "MEMS Piezoresistive Accelerometers," in Micro and Smart Devices and Systems, 2014. 10 S. T. A. Albarbar, "MEMS Accelerometers: Testing and Practical Approach for Smart Sensing and Machinery Diagnostics," Springer International Publishing, vol. 2, 2017. 

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