tunneling presentation 2004

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cc 2004 ECE 449 Adil Ahmed

Tunneling

Accelerometer

By,

Adil Ahmed

Microdevices & Micromachining TechnologyECE 449

April 23, 2004


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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

tunneling presentation 2004

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