An electromechanical device that measures acceleration forces

1. CAPACITIVE
MEMS ACCELEROMETER
 WALEED WAQAS
 MAZHAR TANVEER AFZAL NAZ
 MUHAMMAD HASAN
 FARHAN ALI

2. INTRODUCTION
• An electromechanical device that measures acceleration forces
• First designed in 1979 at Stanford University.
• MEMS accelerometers revolutionized the automotive industry.
• Huge commercial potential.
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3. ACCELROMETER?
• An accelerometer is a device that
measures the vibration, or
acceleration of motion of a
structure. The force caused by
vibration or a change in motion
(acceleration) causes the mass to
"squeeze" the piezoelectric material
which produces an electrical charge
that is proportional to the force
exerted upon it.
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4. 4
How?
Measure Acceleration
Forces
Measure Proper
Acceleration
Acceleration of a body in
its own instantaneous rest
frame
Static Dynamics

5. 5
StaticAcceleration
Marine
Mobile Tilt
AirBagActivation
Saving Data

6. 66
DynamicAcceleration Navigation
Flight Stabalization
S

7. DESIGN APPROACH
• Piezoelectric
• Piezo-resistive
• Mechanical
• Capacitive
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8. MECHANICAL ACCELEROMETER
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CAPACITIVE ACCELEROMETER
PIEZOELECTRIC ACCELEROMETER PIEZORESISTIVE ACCELEROMETER

9. CAPACTIVE ACCELEROMETER
• In bulk micro machined capacitive accelerometers the sensing element
typically comprises of a proof mass which can move freely between fixed
electrodes.
• The fixed electrode forms a capacitor with the proof mass which acts as a
common center electrode.
• The differential change in capacitance between the capacitors is proportional
to the deflection of the proof mass from the center position.
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10. WORKING PRINCIPLE
• External acceleration displaces the support frame relative to the proof mass,
which in turn changes the internal stress in the suspension spring. Both this
relative displacement and the suspension-beam stress can be used as a
measure of the external acceleration.
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13. SENSING MECHANISM
Two Types:
In-Plane Sensing
 Lateral sensing (a)
Change in overlapping length
 Transverse sensing (b)
 Change in finger gap
Out-of-Plane Sensing
Change in overlapping width (w).
(a)
(b)
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14. 14

15. DEVICE FABRICATION
• Poly-silicon surface-micromachining
I. MEMS microstructure
II. CMOS circuitry
• In this way aluminum inter connect in CMOS circuitry will not
be damaged due to the high-temperature process in MEMS
fabrication

16. THE FABRICATION STEPS
a) Pure silicon substrate.
b) Silicon nitride is deposited as the insulation layer.
(a) Silicon wafer
(b) Deposit Si3N4

17. THE FABRICATION STEPS
c) The Boron Silicate Glass is used as the
sacrificial layer
2µm thick
Etching
d) Application of photoresist and exposure to UV
light
(c) Deposit BSG sacrificial layer (2µm)
(d) Applying photoresist

18. THE FABRICATION STEPS
e) After exposing to the UV rays, the photo resist
is stripped from the wafer.
f) Poly silicon layer (3µm thick)
(e) Pattern BSG Layer for
anchors and fixed fingers
(f) Deposit poly-Si Structure layer

19. THE FABRICATION STEPS
g) The above two steps are repeated on the
poly silicon layer and etching is performed.
h) Removal of sacrificial layer to release movable
microstructures.
Cleaning
Packaging
(g) Pattern poly-Si layer for
devices structure
(h) Remove BSG sacrificial layer

20. THE FABRICATION STEPS
Removal of BSG sacrificial layer
using wet etching.
Stiction

21. SEM OF MEMS ACCELROMETERS
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[1] HTTPS://PDFS.SEMANTICSCHOLAR.ORG/50F1/85C6A7DCD1F4A70DB77CAAB4F9428111E444.PDF
[2] HTTP://MAFIJA.FMF.UNI-LJ.SI/SEMINAR/FILES/2007_2008/MEMS_ACCELEROMETERS-KONCNA.PDF

22. AVAILABLE IN MARKET
 MMA7361l from Freescale  ADXL345 from Analog Devices
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23. CHOOSING RIGHT ACCELEROMETER
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HTTPS://WWW.ANALOG.COM/EN/ANALOG-DIALOGUE/ARTICLES/CHOOSING-THE-MOST-SUITABLE-MEMS-ACCELEROMETER-FOR-YOUR-
APPLICATION-PART-1.HTML

24. VIBRATION AND SHOCK MEASUREMENT
Vibration measurement is a pure industrial
application
• Industrial machinery vibration control
• Preventive maintenance
• Railway security, safety and comfort
• Structure monitoring (bridges, building,
wind turbine towers…)
• Testing (automotive, truck, planes…)
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25. APPLICATIONS
Automotive
Crash detection & Air bag
deployment.
Consumer Electronics
 hard disk protection(laptops)
 screen rotation (mobile)
 Image stabilization (camera)
Industrial
 Vibration detection (machine)
 Crack detection (pulley)
Aerospace & Defence
 Navigation
 Missile guidance
 Thrust detection
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26. THANK
YOU
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