MEMS Rigid Diaphragm Speaker

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MEMS Rigid Diaphragm Speaker

• Scott Maghy
• Tim Havard
• Sanchit Sehrawat

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Macro-scale

• Try to make MEMS device based on same concept

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Motivation

• Few similar products
• Small size
• Clandestine
• Privacy
• Low power
• Potential lower cost
• Highly customizable performance
• No surgery!

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Current Hearing Devices

• Few speakers that fit completely inside the ear
• Some piezoelectric speakers
• Bone conduction speaker for above the ear 1 inch
  long
• CMOS MEMS speakers exits, and are being developed
• Several hearing devices
• Downsides
• Require surgery
• Much larger
• Cost
• Complexity

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Implantable Hearing Devices

• Cochlear Implants
• Auditory Brainstem implants
• Implantable Middle-ear devices
• Piezoelectric devices
• Electromagnetic devices

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Cochlear Implants
Auditory Brainstem Implants
Source http//www.nidcd.nih.gov/health/hearing/co
ch.asp
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Piezoelectric Devices

• Operation
• Advantage inert in a magnetic field
• Disadvantage Power output directly related to
  size of crystal.
• Example
• Middle Ear Transducer (MET)

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Middle Ear Transducer

• Translates electrical signals into mechanical
  motion to directly stimulate the ossicles

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Middle Ear Transducer
Remote
MET Implant
Charger
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Electromagnetic Devices

• Operation
• Small magnet is attached to vibratory structure
  in ear
• Only partially implantable coil must be housed
  externally. Sizes of coil magnet restricted by
  ear anatomy.
• Power decreases as the square of the distance
  between coil magnet coil magnet must be
  close

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Vibrant Soundbridge
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Ridged Diaphragm MEMS Speaker
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Materials

• Polysilicon structural material for cantilever
  and diaphragm
• Silicon Oxide for sacrificial layers
• Silicon Nitride isolation of wafer
• Gold electrodes and electrical connections

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Fabrication
Deposit layers of Electrodes, oxide, and
photoresist (as shown)
Deposit Silicon Nitride Layer
Pattern photoresist then etch electrodes
oxide using RIE
Deposit Oxide 2 layer
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Fabrication
Coat columns with Photoresist and etch away
remaining oxide 2 Remove photoresist from
electrode 2
Etch oxide 2, and make Poly-Si columns
Deposit oxide 3 as shown
Remove photoresist and deposit Poly-Si
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Fabrication
Make Poly-Si diaphragm base thicker
Release oxide layers
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Performance and Optimization
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Speaker Mechanics
Force balance
Fspring

/-
Felect
where
and
Setting
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Acoustic Modeling
Sinusoidal input voltage
Drives diaphragm displacement
Which causes sound intensity
Acoustic power can then be obtained
Note system parameters can be tailored to be
significantly below the resonant frequency.
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Observed Acoustic Power

• Sound intensity decays quadratically with
  distance
• ? This results in limited effective speaker range

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Comparison of Acoustic Sound Power
Situationandsound source sound powerPacwatts
Rocket engine 1,000,000 W
Turbojet engine 10,000 W
Siren 1,000 W
Machine gun 10 W
Jackhammer 1 W
Chain saw 0.1 W
Helicopter 0.01 W
Loud speech,vivid children 0.001 W
Usual talking,Typewriter 10-5 W
Refrigerator 10-7 W
(Auditory threshold at 2.8 m) 10-10 W
(Auditory threshold at 28 cm) 10-12 W
Decreasing frequency
Device is in the threshold of human hearing!
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Improvements

• Implement a process that allows for sealing of
  speaker cone to support
• This would give better acoustic properties
• Could be accomplished by CMOS MEMS procedure
• Fabricate cone shape with stamping method to
  achieve better shape and more cost effective
  fabrication

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Improvement Cont.

• Further research into materials for the
  cantilevers to decrease stiffness of cantilevers
• This would allow greater diaphragm displacement
  and therefore greater intensity
• Other materials exist with lower Youngs modulus
  that would accomplish this but fabrication is
  suspect
• Other methods of securing the diaphragm
• Spring attachment
• Decrease the mass of the diaphragm by altering
  fabrication process

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QUESTIONS