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cc 2004 ECE 449 Adil Ahmed Tunneling μAccelerometer By, Adil Ahmed Microdevices & Micromachining Technology ECE 449 April 23, 2004
cc 2004 ECE 449 Adil Ahmed
Tunneling µAccelerometer
By,Adil AhmedMicrodevices & Micromachining TechnologyECE 449April 23, 2004
cc 2004 ECE 449 Adil Ahmed
Table of ContentsTable of ContentsFUNDAMENTALS
Conventional AccelerometerAPPLICATIONS
µAccelerometersµACCELEROMETERS
Capacitive Piezoelectric PiezoresistiveTunneling
STMADVANTAGE/DISADVANTAGEFABRICATION PROCESS
Tunneling µAccelerometerCONCLUSION
cc 2004 ECE 449 Adil Ahmed
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 FrAcceleration α traversed distanceForce feedback approach: proof mass = constant
Feedback position information to control electrodes
cc 2004 ECE 449 Adil Ahmed
µAccelerometers:APPLICATIONSAPPLICATIONS
Aerospace↓CostShuttle
MilitaryWeapon detonation time
Automotive IndustryAir-bags deployment
Suspended parallel beams that make up an electrical capacitor, altering the amount of stored electrical charge when subjected to an accelerationSignal is then elaborated by a microchip through an algorithm that evaluate if crash condition has been reached.
Key Advantages: low cost, extreme sensitiveness and reactivity related to the small dimensions, and the reliability due to the integration of the logic in the same device of the sensor.
cc 2004 ECE 449 Adil Ahmed
µAccelerometers:CAPACITIVECAPACITIVE
CapacitiveProof mass as one plate of capacitor and base as otherVoltage changes when sensor accelerated
Applied acceleration
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µAccelerometers:PIEZOELECTRICPIEZOELECTRIC
PiezoelectricElectrical charge develop due to forceW(mechanical input) ↔ W(electrical output)
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µAccelerometers:PIEZORESISTIVEPIEZORESISTIVE
Piezoresistivematerial's resistance value decreases when it is subjected to a compressive force and increases when a tensile force is applied. The piezoresistive element in the new accelerometer is formed by diffusing boron into silicon. 3-Axis Si Piezoresistive Accelerometer
Acceleration 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)
cc 2004 ECE 449 Adil Ahmed
µAccelerometers:TUNNELINGTUNNELING
TunnelingMetal-coated tip is brought to within a nanometer of spring-supported proof massCurrent will tunnel across separation if small bias voltage is appliedApplied acceleration causes a relative displacement of spring-supported proof mass, and change in tunneling current
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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 in thermistors)
Not adversely affected by electromagnetic fields
Piezoresistive
Limited operation of frequency range
AC-response sensors
Generate own signals, no need to be powered
Piezoelectric
Complex fabricationHigher sensitivities than PR
Capacitive
DISADVANTAGESDISADVANTAGESADVANTAGESADVANTAGESµµACCELEROMETERACCELEROMETER
cc 2004 ECE 449 Adil Ahmed
TUNNELING µAccelerometer:STMSTM
Tunneling Accelerometeruses a general principle of operation that is commonly used for scanning tunneling microscopy (STM)
STM a bias voltage is applied between a sharp metal tip and a conducting samplequantum mechanical tunneling effectstunneling current is exponentially dependent on the separation between the tip the sample
Tunneling material = Auexcellent stabilityPrevents drift in the observed tunneling current over timeplatinum-iridium alloys
cc 2004 ECE 449 Adil Ahmed
TUNNELING µAccelerometer: FABRICATION PROCESS [I]FABRICATION PROCESS [I]
Si
Nitride
Ti-Pt-Au
Si
Nitride
SiO2
Ti-Pt-AuSi
Nitride
1. Deposit Nitride Layer
2. Tri-layer MetalDeposition
3. Oxidation
cc 2004 ECE 449 Adil Ahmed
TUNNELING µAccelerometer: FABRICATION PROCESS [II]FABRICATION PROCESS [II]
Si
p++ epi Si
SiO2
Ti-Pt-AuSi
Nitride
SiO2
Ti-Pt-AuSi
Nitride 4. Oxide Cavity Etch
5. CMP & Bond
6. Thin Down to Etch-stop
p++ epi Si
SiO2
Ti-Pt-AuSi
Nitride
cc 2004 ECE 449 Adil Ahmed
TUNNELING µAccelerometer:FABRICATION PROCESS [III]FABRICATION PROCESS [III]
p++ epi Si
SiO2
Ti-Pt-AuSi
Nitride
p++ epi Si
SiO2
Ti-Pt-AuSi
Nitride
p++ epi Si
SiO2
Ti-Pt-AuSi
Nitride
Au
7. Etch Tip Hole ThroughEpitaxial Layer
8. Etch Tip Into Oxide
9. Metallize Tip &Contact
cc 2004 ECE 449 Adil Ahmed
TUNNELING µAccelerometer:FABRICATION PROCESS [IV]FABRICATION PROCESS [IV]
p++ epi Si
SiO2
Ti-Pt-AuSi
Nitride
Au
p++ epi Si
SiO2
Ti-Pt-AuSi
Nitride
Au
10. Define Cantilever
11. Oxide Etch &Release
12. Device is ready tobe Packaged
cc 2004 ECE 449 Adil Ahmed
CONCLUSIONCONCLUSIONMEMS Accelerometers
Capacitive, Piezoelectric, Piezoresistive, TunnelingAdvantages/Disadvantages
Tunneling µAccelerometersFunctionalityTesting the device
ResourcesMicromachined Transducers SourcebookMEMS & MicrosystemsIEEE Journal of Micromechanics & MicroengineeringFundamentals of Microfabricationwww.analog.comwww.stanford.edu
