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

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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.

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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)

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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

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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

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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

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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

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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

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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

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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