Tunneling Accelerometer By, Adil Ahmed Microdevices & - - PowerPoint PPT Presentation

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

Table of Contents Table of Contents

FUNDAMENTALS

Conventional Accelerometer

APPLICATIONS

µAccelerometers

µACCELEROMETERS

Capacitive Piezoelectric Piezoresistive Tunneling

STM

ADVANTAGE/DISADVANTAGE FABRICATION PROCESS

Tunneling µAccelerometer

CONCLUSION

cc 2004 ECE 449 Adil Ahmed

Conventional Accelerometer:

HOW IT WORKS HOW 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 α traversed distance Force feedback approach: proof mass = constant

Feedback position information to control electrodes

cc 2004 ECE 449 Adil Ahmed

µAccelerometers:

APPLICATIONS APPLICATIONS

• 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 stored electrical charge when subjected to an acceleration

• Signal 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:

CAPACITIVE CAPACITIVE

Capacitive

Proof mass as

• ne plate of

capacitor and base as other

Voltage changes

when sensor accelerated

Applied

acceleration

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µAccelerometers:

PIEZOELECTRIC PIEZOELECTRIC

Piezoelectric

Electrical charge develop due to force W(mechanical input) ↔ W(electrical output)

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µAccelerometers:

PIEZORESISTIVE PIEZORESISTIVE

Piezoresistive

material'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:

TUNNELING TUNNELING

Tunneling

Metal-coated tip is

brought to within a nanometer of spring- supported proof mass

Current will tunnel across

separation if small bias voltage is applied

Applied acceleration

causes a relative displacement of spring- supported proof mass, and change in tunneling current

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ADVANTAGES/DISADVANTAGES ADVANTAGES/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 fabrication Higher sensitivities

than PR Capacitive DISADVANTAGES DISADVANTAGES ADVANTAGES ADVANTAGES µ µACCELEROMETER ACCELEROMETER

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TUNNELING µAccelerometer:

STM STM

• Tunneling Accelerometer

uses a general principle of

• peration that is commonly used

for scanning tunneling microscopy (STM)

• STM

a bias voltage is applied between

a sharp metal tip and a conducting sample

quantum mechanical tunneling

effects

tunneling current is exponentially

dependent on the separation between the tip the sample

• Tunneling material = Au

excellent stability Prevents drift in the observed

tunneling current over time

platinum-iridium alloys

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TUNNELING µAccelerometer:

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 Metal

Deposition

• 3. Oxidation

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TUNNELING µAccelerometer:

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-Au Si Nitride

cc 2004 ECE 449 Adil Ahmed

TUNNELING µAccelerometer:

FABRICATION PROCESS [III] FABRICATION PROCESS [III]

p++ epi Si SiO2 Ti-Pt-Au Si Nitride p++ epi Si SiO2 Ti-Pt-Au Si Nitride p++ epi Si SiO2 Ti-Pt-Au Si Nitride Au

• 7. Etch Tip Hole Through

Epitaxial 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-Au Si Nitride Au p++ epi Si SiO2 Ti-Pt-Au Si Nitride Au

• 10. Define Cantilever
• 11. Oxide Etch &

Release

• 12. Device is ready to

be Packaged

cc 2004 ECE 449 Adil Ahmed

CONCLUSION CONCLUSION

• MEMS Accelerometers

Capacitive, Piezoelectric,

Piezoresistive, Tunneling

Advantages/Disadvantages

• Tunneling µAccelerometers

Functionality Testing the device

• Resources

Micromachined Transducers

Sourcebook

MEMS & Microsystems IEEE Journal of

Micromechanics & Microengineering

Fundamentals of

Microfabrication

www.analog.com www.stanford.edu