ABSTRACT
Micro-electromechanical Systems (MEMS) refers to all those technologies that integrate
electrical and mechanical components with the size ranging from 1 micron to 1000 microns. Airbag deployment systems in automobiles successfully applied MEMS comb accelerometers. This
paper discusses design optimization of poly-silicon surface micro-machined MEMS comb
accelerometer with the help of device that utilize folded-beam structure with the main aim
enhance sensitivity where movable mass is connected to two anchors through folded-beams.
Movable masses are used in such a way that movable fingers are extruding from either sides of
movable masses with fixed comb fingers surrounding at the left and right to form a pair of
differential capacitance. When acceleration is applied in the sensitive direction, inertial force is
induced on movable mass that deflect beams and differential capacitance gap change. This
acceleration can be finding by measuring change in differential capacitance. Device sensitivity
and resonant frequency is extracts with the help of ANSYS FEM simulation by changing design
parameters. ANSYS simulation extracts dependency of different design parameter with device
sensitivity. Curves of device sensitivity versus beam width, beam length and mass width are
derived which show a good agreement with theoretical prediction. This analysis leads to
conclusion that device behavior depends on different design parameters. Desired sensitivity is
achieved by varying design parameters. From simulation results and analysis, set of optimized
design parameters for comb accelerometer is decided. ANSYS simulation shows that device has
displacement sensitivity of 3nm/g. This proposed MEMS comb accelerometer can be used for
many applications, such as automobile airbag deployment, navigations, fabrication sequence of
the comb accelerometer is also proposed. The device is to be fabricated using surface micro
machining process with sacrificial layer technique.
INTRODUCTION
MEMS are applied on small scale electronic frameworks and mechatronics related
technology for creation of smaller and little tools with the size ranging from 1 micrometer to 100
micrometers i.e. 0.001 mm to 0.1 mm. Many MEMS tools vary in size from 20 micrometers to a
1 millimeter i.e. 0.02 to 1.0 mm there for use in manufacturing advanced innovative procedure
for creation of semiconductor device that statutes a certificate of physical layers, designed by
photolithography and the fee for submitting the required forms.
In MEMS tools, Acceleration is the leading one and second largest volume of transactions
after weight sensor and used for the measurements of tilt, motion, position, vibration and sound
bombs and consists of acceleration by mass demonstration or finger brush commented on
acceptable columns tied to a stable edge. Such processes are measured by relative stress remove
or suspend the shaft.
This phenomena is the basic framework of mass attached to spring and described by
Hooke's Law i.e. F = KX and second law of motion by Newton i.e. F = ma. When this framework
experiences increasing speed, then from Newton's second law of motion, there will be equivalent
resulting energy sparingly which Lead power block either package or extend spring under
restrictions i.e. F = ma = K.X. The displacement Q on the off chance is seen to accelerate and
bring block to be removed by x = Mama / K, and realized that bloc may have seen acceleration
of K.X = / m.
Now ANSYS is used to outline accelerating device using below steps to describe
perceptions of the vibrational modes as follows.
Steps
MEMS accelerometer ground bar will be presented which is created with a single
precious silicon stone of mass in micromachining where silicon mass is associated with two
cantilevered shafts which are moored beside and finish installed. Two trees cantilevered having
same width; length and thickness accept directions of mass at edge as O (0, 0, 0). At this point
use measurements data to find directions of all main axes. For this purpose using ANSYS and
consider key focusing directions, 3-1 model of accelerometer is made. Once 3-D model is
designed, return back for cross checking of various points and make a screenshot of your 3-1
model. Modular surveys of ANSYS recurrence reproduction replay first five methods of
vibration of accelerometer and locate full relative frequencies. Liveliness of each vibration mode,
and twisting of bard in all modes in ANSYS, a screenshots for the five vibration modes and their
relative frequencies of thunder. By comparing all activities, one can distinguish among different
modes that which mode of operation of the accelerometer is full relapse.
ANSYS is used that describe accelerometer where we introduce it to our environment.
"Preprocessor" plan gadget model and apply loads then it works. "Arrangement" menu
characterize reproduction which make imaginary reconstruction efficiently that we perform and
use "General Postprocessor" for results which will be considered screenshots to other
perceptions.
Tapping ANSYS tilt selection and basic-strategy h characterize kind of review that
ANSYS perform. Modular survey of vibration mode and resounding examination of recurrence
has been place in review category named "auxiliary".
Certain types of components in ANSYS is characterized before you build gadget model,
because using 3D model one can use "Strong - 8node 185" component.
Model material is characterized with respect to its properties more for which silicon material of
small scale pump model is used. Therefore, ANSYS physical properties of silicon material are
tell in order to use these properties for reconstitution. "Props Preprocessors materials equipment
models-auxiliary Direct-Flexible-isotropic".
Currently, we are ready to build 3D model of the gadget and can manufacture any spirit
stack so as to boggle 3D model / interfacing a wide range of parallelepipeds which can cover
each other on the grounds that ultimately mix together using "Include" Boolean operation.
Adding a cuboid, we use various ways out of which one approach quickly make a cuboid to tell
ANSYS directions of its three corner askew axes (Xl,, Yl, Zl), (X2, Y2, Z2) and (X3, Y3, Z3).
Cuboid use three axes angle exceptionally which is to choose number of parallelepipeds
we do with two corners for focuses directions for each parallelepiped. In this gadget, one needs
characterize parallelepipeds 3: a cuboid bar cantilever and a cube to the ground. Directions of
three corner of each cuboid are now apart in Figure I. At present with above measures, we are
ready to do the parallelepipeds to make the gadget model. Preprocessor Demonsaation Make
volumes Cuboids by measures in the pop out window, including the guidelines for the application
of oblique corner parallelepipeds # 1 (cantilever).
So far, we did each parallelepipeds for the gadget and from now on, all individual
parallelepipeds are still isolated from each other and can move independently. They are singular
parallelepipeds separated without associating with each other; even they might be covering each
other. That is, they are partitioned segments without being raised in a complete gadget yet. To
associate all these Individual parallelepipeds together and combine them into a gadget linked
together, we must use Boolean operation (“Include") to the union together by pressing
"preprocessor$ work-Boolean-include-Volumes Demonstrations” rather than volumes ), in the
window, click "Select all". After you press "Pick All", we will see the window is closed and
nothing happens. Notwithstanding, all the individual segments (parallelepipeds) in the model
were included in general. In this sense, they are associated together and not more isolate. This is
improving to involve all the individual parts together in a gadget.
"Zoom Skillet-Tum" container unit is use to zoom and rotate 3D to look it from different angles.
One can tap decision "Top" to get in perspective, "Front" to get perspective before, so on so
compelling. One can tap huge DAB decision to enlarge the model or press the small latch grain
to zoom the model.
Currently, we have to work on machine model for which ANSYS work on your device
model into smaller units where each unit will be listed with differential correlations. Seeing all
these differential scientific proclamations with conditions beginning / end position, in concluding
from Met at some results, then ANSYS knows misshaping at each point of the central apparatus.
In case the transverse segment size is smaller (fine), the result of the reconstruction will be more
correct. However, it provides more of differential correlations Settle, and understand the reality
will surface in the long term at a very basic level more. So there is a compromise to it. 3D
Meshing of model is a bit special. Accompanying steps demonstrate methods for performing
suitable for 3D model generally accepted. For 3D model, results in a very large number of lattice
platforms for which use section Manuel came here to reduce the number of lattice platforms the
menu by clicking: Size preprocessor mounting Controls-Manuel Size Worldwide volume
controls, as shown in the following.
Now apply boundary conditions on the model. In our case, the left half of the beam is
attached to the divider. Accordingly, it must be adjusted. In other words, we characterize
dislodgement (UX, UY and UZ along X, Y, Z sections) of the line from the left edge of the
console to be constantly stable "O” (that is, it is constantly changed). This boundary condition is
characterized as "burdens". In order to see the left side of the console even more obvious, how
about we click "Plot Ctrls-Container Zoom Turn", move your model and zoom in on the left end
of the shaft.
Now apply boundary conditions to adjust volume (or many volumes) than the line (s).
With a specific end goal to choose the volumes we need to change, to ensure that we use our
"Skillet Zoom-Turn't device to rotate the 3D model of the gadget, so the volumes we need to
adjust are undoubtedly significant. To adjust the volume, we characterize dislodgement (UX, UY
and UZ along X, Y and Z bearimgs) volumes to be consistently consistent "0” (which means
they are constantly changed). We must break menu “PreprocessorLoads-DefineLoads-ApplyBasic Rem al On Volumes", select the volumes that we have to settle, and after that characterize
"All DOF" these volumes to be regular Estimate "0" as the have that follow.
Currently, we advise ANSYS this recovery we must make by clicking Sort-reconsideration
Arrangement survey
We must move forward characterize some subtle elements in the modular examination
by tapping arrangement sort-review survey Alternatives in the new window, change the "Number
of modes of focus" to 5, click well. This advises ANSYS to clear for its initial 5 vibration modes
(ANSYS will erase the vibration modes, one by one beginning from 0 Hz and recurrence end:
100000000.
Now we are ready to understand the recreation. Clicking Arrangement Current LS-tools-we are
ready to watch / read the results when all is said in done Postprocessor. We will have an overview
of all the 5 complete frequency separated gadget for your model. We can read these results in
general Postprocessor and accelerate the results to see the gadget vibration condition. Movement
gives us a correct understanding of how the gadget vibrates in its operation. Clicking General
Postprocessor-read results-First Series, ANSYS advice browse the primary results of its postprocessor arrangement. At that point click the plot CfrlsEnliven-Mode Shape.
ANSYS will carry out an activity for the gadget. We can see how the cantilever vibrates If the
main mode. We can take the screen capture of the gadget when 'Edge" comes 10 10. Most of
the distortion is greatest in this edge. In the upper left corner of the screenshot, it appears "SUB
= l", indicating that this is the first vibration mode, and "FREQ = 1303.13" shows all its
recurrence is 01 = 1303.13Hz. You can see that in this vibration mode, the cantilever twists
here and there, again and again. This is the working method of the accelerometer. We can save
these Activities.
Now General Postprocessor break-read results-Series, after the comparative progress to
the main vibration mode, you can read the following (second set) and vizier. Note that in the
top left corner, "SUB = 2" shows it is the second vibration mode, and "FREQ = 7292.21Hz"
demonstrates the thunder of recurrence is f02 = 7292.21Hz. We can see that in the second mode
of vibration, the cantilever can have different curves like a wave. Clearly this is not the method
of work of the accelerometer. Fortunately, this and other high demand vibration modes have
complete frequencies far from resounding recurring operating mode (FOI = 1403.13Hz). This
shows the higher demand vibration modes are difficult to couple the operating mode of the
accelerometer. In this way, the accelerometer of the working method is stable without
obstruction higher demand of vibration.
We continue to do activity for the other 3 modes of vibration until you reach the last
round (fifth set) results. Watch the movements between different modes of thunder to see the
distinction. Currently, we have completed the recreation recurrence of the accelerometer. The
motivation behind the modular survey (Review of reoffending) of the gadget is to discover its
dynamic attributes, and ensuring that the gadget's working method is stable without being
coupled by other modes of vibration or shaking more environmentally. The activity also allows
us to confirm whether the planned gadget carries the right to development (eg bowing all the
cantilever of this situation) as we had expected. It is essential that the recurrence of working
mode of thunder gadget is far from other adjacent vibration modes not work. This ensures higher
unwanted vibration modes are not coupled in the working method of the accelerometer and cause
unwanted vibration of the gadget. For example, if the resounding recurring operating mode is
1.01 kHz, but the recurrence following non-work mode thunder is I .02 kHz, obviously this is
not a decent insight in light of the fact that the gadget's working method is effortlessly mixed by
tracking mode of non-working vibrations. We must change the configuration to ensure that they
are isolated from each other with enough crevasse. In addition, the working mode of thunder
recurrence must also be sufficiently higher than the natural shock signals (vibrations ecological
clamor for the most complete low-recurrence in the scope of several Hz). This ensures ecological
unrest will not be coupled to the working method of the gadget as well. Recurrence of operation
thunder is also an essential parameter of the dynamic behavior of the gadget. Rarely we may
need to deliberately put the gadget to work near its full frequency to accomplish a wide
dislodgement to improve its affectability (eg for magnifying MEMS instruments), but in some
different cases, we may need to maintain a distance to let the gadget work in its recurrence
thunder dodge harm the gadget structure due to extensive removal (eg accelerometers). This is
the motivation behind our MEMS gadgets recurrence investigation.
RESULTS AND DISCUSSIONS
Experimental results showing first five vibrational modes of the designed MEMS
accelerometer.
The below figure depicts the 1st vibrational mode simulation of micro machined MEMS
accelerometer where resonant frequency (FREQ) in the figure is 1103.1 I Hz and maximum
deflection (DMX). During design of this model, mass is moving upward and downwards in the
Y-direction and beams is fixed.
STEP-I
SUB -1
The below figure shows the 2nd vibrational mode simulation of the micro machined MEMS
accelerometer where resonant frequency (FREQ) in the figure is 7202.2l Hz, also maximum
deflection (DMX). During design of this model, beams are fixed ad move up & down in X &
Y directions.
MODEL2 vibrational mode of the designed accelerometer
Below figure shows 3rd vibrational mode of simulation of the micro machined MEMS
accelerometer where we see resonant frequency (FREQ) in the figure is 17750.7 Hz and
maximum deflection (DMX) and beam move up & down in X & Y directions. From here
observe flexibility at the junction of the beams and the mass with maximum deflection than all
other modes.
SUB •3
MODEL 3 vibrational mode of the designed accelerometer.
Figure shows 4th vibrational mode of simulation of the micro machined MEMS accelerometer
which show resonant frequency (FREQ) in the figure is 13632.6 Hz and maximum deflection
(DMX) and beam move from right to left and again from left to right in Z direction which show
reduction in deflection compared to previous beams results.
MODEL 4 vibrational mode of the designed accelerometer.
Figure shows 5th vibrational mode simulation of micro machined MEMS accelerometer with
the resonant frequency (FREQ) in the figure is 61018.4 Hz and maximum deflection (DMX)
and beam move from right to left and again from left to right in X, Z directions. From the figure
we observe flexibility at the junction between beam and masses and it’s clear that the beam is
fixed with more deflection compared to previous mode.
STEP-I SUB -5
MODEL 5 vibrational mode of the designed accelerometer.
Q&A
Figure show diagram of MEMS accelerometer with measurements of diagram shown below: For
Mass:
Width Wm=6000µm,
Length Lm=4000µm,
Thickness tm=330µm.
For each beam:
Beam width Wb=1074 µm (where "abc" are the last 3 digits of our student ID)
beam length Lb= 3000µm, beam thickness tb= 16µm.
Gap between two beams is 1000 pm. Two beams are in the same level and they are located in
the central portion of the mass symmetrically and distance between top surface of the mass and
the top surfaces of the beams = distance between bottom surface of the mass and the bottom
surfaces of the beams.
Distance between front surface of the mass and the front surface of beam l distance between
back surface of the mass and the back surface of beam2
Given the density of silicon p=2.33x103kg/m3, the Young's modulus of silicon material is
E=170GPa (1Pa=l N/m2), Poisson ratio of silicon: v=0.34
1)
The two beams are connected in Parallel so the effective spring constant for two beams
is
Ktot= Kb1+Kb2= 2Kb
2)
The ratio of Mb and Mm is
2.046∗10−7
MUMm= 18.45∗10−6 = 0.0202
Yes, the mass Mb of both beams is smaller than sensing mass Mm The inertial force if
acceleration applied in upward direction is
The direction of the inertial force is Downward
3)
Resulted Deflection is
Sb= Mg/Ktot
= (9040.5µN)/(13.85 Nm) =652.7 µm
It is bend down
4)
Resonant frequency of accelerometer is
1
= [ ]𝑥
2𝜋
√13.85
[
]
18.45𝑥10−6
= 137.96 HZ
5)
Movable electrodes are placed between two fix electrodes at the rest position where
capacitance is same since distance between two plates and the over lapping area is same. If
object moves in vertical direction then central mass experience inertial force along opposite
direction hence distance changes and so as capacitance also which is directly proportional to
the change in displacement.
6)
areas
Mde
view
Figure I. Structure diagram of a silicon bulk micro machined MEMS accelerometer
9(4000, 173,879)
17(4000, 173,3500)
10(4000, 173,2500)
1 1 (7000, 173,2500)
12(7000, 173,879)
13(4000, 157,879)
24(7000, 173,5121)
7)
Designed 3D model of accelerometer in ANSYS with derived coordinates with the output is
here:
8)
Vibration mode
Resonant frequency
Description of beam
Working
mode(yes/no)?
Mode#l
FOI=1103.11hz
Yes
Mode#2
F02-7202.21hz
Mode#3
F03-17750.7hz
Beam is moving in
ydirection
Beam is moving in
xy direction
Beam is bending in y
direction
No
No
F04=13632.6hz
Mode#4
Beam is moving in x No
direction
F05-64118.4hz
Mode#5
Beam is moving in xz No
lane
The five screen shots are attached in the results section
Yes, the resonant frequency is far away from the working mode at higher vibrational modes.
Yes, for working mode the accelerometer is stable.
RE f
f 0 ( hand _ calculation )- f 0( ANSYS)
f 0 ( ANSYS)
100%
= (137.96-1403.1)/ (1403.1) *100%
= 0.90167*100%
= 90.