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8/2/2019 Tunneling Presentation 2004
1/15
cc 2004 ECE 449 Adil Ahmed
Tunneling
Accelerometer
By,
Adil Ahmed
Microdevices & Micromachining TechnologyECE 449
April 23, 2004
8/2/2019 Tunneling Presentation 2004
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cc 2004 ECE 449 Adil Ahmed
Table of ContentsTable of Contents FUNDAMENTALS
Conventional Accelerometer APPLICATIONS
Accelerometers
ACCELEROMETERS
Capacitive Piezoelectric
Piezoresistive
Tunneling
STM
ADVANTAGE/DISADVANTAGE
FABRICATION PROCESS
Tunneling Accelerometer
CONCLUSION
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Conventional Accelerometer:HOW IT WORKSHOW IT WORKS
Composed of the following:proof mass, spring and position detector
Proof mass will move from rest to a new position,determined by balance between its mass times
acceleration and spring Fr Acceleration traversed distance
Force feedback approach: proof mass = constant
Feedback position information to control electrodes
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Accelerometers:APPLICATIONSAPPLICATIONS
Aerospace Cost
Shuttle
Military Weapon detonation time
Automotive Industry
Air-bags deployment Suspended parallel beams that
make up an electrical capacitor,altering the amount of storedelectrical charge when subjectedto an acceleration
Signal is then elaborated by a
microchip through an algorithmthat evaluate if crash conditionhas been reached.
Key Advantages: low cost,extreme sensitiveness andreactivity related to the smalldimensions, and the reliabilitydue to the integration of the logicin the same device of the sensor.
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Accelerometers:CAPACITIVECAPACITIVE
Capacitive
Proof mass as
one plate of
capacitor and
base as other Voltage changes
when sensor
accelerated
Appliedacceleration
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Accelerometers:PIEZOELECTRICPIEZOELECTRIC
PiezoelectricElectrical charge develop due to force
W(mechanical input) W(electrical output)
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Accelerometers:PIEZORESISTIVEPIEZORESISTIVE
Piezoresistive material's resistance value
decreases when it is
subjected to a compressive
force and increases when atensile force is applied. The
piezoresistive element in
the new accelerometer is
formed by diffusing boron
into silicon. 3-Axis Si Piezoresistive AccelerometerAcceleration applied along the X- or Y-axis
causes the proof mass to incline (A), while
acceleration along the Z-axis causes the mass
to move in a downward direction (B)
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Accelerometers:TUNNELINGTUNNELING
Tunneling Metal-coated tip is
brought to within ananometer of spring-supported proof mass
Current will tunnel acrossseparation if small biasvoltage is applied
Applied accelerationcauses a relativedisplacement of spring-supported proof mass,and change in tunnelingcurrent
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ADVANTAGES/DISADVANTAGESADVANTAGES/DISADVANTAGES
Potential for long-
term drift
Sub-nano level of
sensing displacement
(extreme sensitivities)
High resolution
Tunneling
Temperature
sensitive (used inthermistors)
Not adversely
affected byelectromagnetic fields
Piezoresistive
Limited operation offrequency range
AC-response sensors
Generate ownsignals, no need to be
powered
Piezoelectric
Complex fabrication Higher sensitivities
than PR
Capacitive
DISADVANTAGESDISADVANTAGESADVANTAGESADVANTAGESACCELEROMETERACCELEROMETER
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TUNNELING Accelerometer:STMSTM
Tunneling Accelerometer
uses a general principle ofoperation that is commonly usedfor scanning tunneling microscopy(STM)
STM
a bias voltage is applied between
a sharp metal tip and a conductingsample
quantum mechanical tunnelingeffects
tunneling current is exponentiallydependent on the separation
between the tip the sample Tunneling material = Au
excellent stability
Prevents drift in the observedtunneling current over time
platinum-iridium alloys
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TUNNELING Accelerometer:FABRICATION PROCESS [I]FABRICATION PROCESS [I]
Si
Nitride
Ti-Pt-Au
Si
Nitride
SiO2
Ti-Pt-Au
Si
Nitride
1. Deposit Nitride Layer
2. Tri-layer MetalDeposition
3. Oxidation
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TUNNELING Accelerometer:FABRICATION PROCESS [II]FABRICATION PROCESS [II]
Si
p++
epi Si
SiO2
Ti-Pt-Au
Si
Nitride
SiO2
Ti-Pt-Au
Si
Nitride 4. Oxide Cavity Etch
5. CMP & Bond
6. Thin Down to Etch-
stop
p++
epi Si
SiO2
Ti-Pt-AuSi
Nitride
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TUNNELING Accelerometer:FABRICATION PROCESS [III]FABRICATION PROCESS [III]
p++ epi Si
SiO 2
Ti-Pt-Au
Si
Nitride
p++
epi Si
SiO 2
Ti-Pt-Au
Si
Nitride
p++
epi Si
SiO 2
Ti-Pt-Au
Si
Nitride
Au
7. Etch Tip Hole Through
Epitaxial Layer
8. Etch Tip Into Oxide
9. Metallize Tip &
Contact
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TUNNELING Accelerometer:FABRICATION PROCESS [IV]FABRICATION PROCESS [IV]
p++ epi Si
SiO 2
Ti-Pt-Au
Si
Nitride
Au
p++
epi Si
SiO 2
Ti-Pt-Au
Si
Nitride
Au
10. Define Cantilever
11. Oxide Etch &
Release
12. Device is ready to
be Packaged
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CONCLUSIONCONCLUSION MEMS Accelerometers
Capacitive, Piezoelectric,Piezoresistive, Tunneling
Advantages/Disadvantages
Tunneling Accelerometers
Functionality
Testing the device
Resources Micromachined Transducers
Sourcebook
MEMS & Microsystems
IEEE Journal of
Micromechanics &Microengineering
Fundamentals ofMicrofabrication
www.analog.com
www.stanford.edu