MEMS Presentation_Sehrawat

Piezoelectric Devices
• Operation
Pass current into Piezoceramic Crystal Crystal changes volume
Vibratory signal produced
• Advantage: inert in a magnetic field • Disadvantage: Power output directly related to size of crystal. Example: • Middle Ear Transducer (MET)
Note: system parameters can be tailored to be significantly below the resonant frequency.
Observed Acoustic Power
• Sound intensity decays quadratically with distance
This results in limited effective speaker range
18 x 10
-12
Acoustic Performance Device Output Hearing Threshold
Io = δ 2 ⋅ 2πf ⋅ ρaircsound
Sound Power [W]
– Downsides:
• • • • Require surgery Much larger Cost Complexity
Implantable Hearing Devices
Cochlear Implants Auditory Brainstem implants Implantable Middle-ear devices
• Other methods of securing the diaphragm
– “Spring” attachment
• Decrease the mass of the diaphragm by altering fabrication process
QUESTIONS
Sound Power [W] 2 x 10 2.5
-11
Acoustic Performance Device Output Hearing Threshold
10 W 1W 0.1 W 0.01 W 0.001 W
1.5
1
Decreasing frequency
0.5
10−5 W 10−7 W 10-10 W 10-12 W
Situation and sound source Rocket engine Turbojet engine Siren Machine gun Jackhammer Chain saw Helicopter Loud speech, vivid children Usual talking, Typewriter Refrigerator (Auditory threshold at 2.8 m) (Auditory threshold at 28 cm) sound power Pac watts 1,000,000 W 10,000 W 1,000 W
– Some piezoelectric speakers – Bone conduction speaker for above the ear: 1 inch long – CMOS MEMS speakers exits, and are being developed
• Several hearing devices
MEMS Rigid Diaphragm Speaker
Scott Maghy Tim Havard Sanchit Sehrawat
Macro-scale
Try to make MEMS device based on same concept
Motivation
• Few similar products • Small size
Vibrant Soundbridge
Magnet surrounded by coil
Ridged Diaphragm MEMS Speaker
Materials
• Polysilicon: structural material for cantilever and diaphragm • Silicon Oxide: for sacrificial layers • Silicon Nitride: isolation of wafer • Gold: electrodes and electrical connections
Charger
Electromagnetic Devices
• Operation
Pass current into Electric Coil Magnetic Flux created Drives adjacent magnet
• 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
0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 Distance from User [m]
Device is in the threshold of human hearing!
Improvements
• Implement a process that allows for sealing of speaker cone to support
16 14 12
I = Io
1 distance
2
10 8 6 4 2 0 -2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Distance from User [m] 0.8 0.9 1
Pacoustic = I ⋅ Area
Comparison of Acoustic Sound Power
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 Young’s modulus that would accomplish this but fabrication is suspect
Deposit oxide 3 as shown
Remove photoresist and deposit Poly-Si
Fabrication
Make Poly-Si diaphragm base thicker
Release oxide layers
Performance and Optimization
– 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
– Clandestine – Privacy – Low power
• Potential lower cost • Highly customizable performance • No surgery!
Current Hearing Devices
• Few speakers that fit completely inside the ear
Deposit Oxide 2 layer
Fabrication
Etch oxide 2, and make Poly-Si columns
Coat columns with Photoresist and etch away remaining oxide 2 Remove photoresist from electrode 2
Speaker Mechanics
Force balance:
Ewt Fspring = k ⋅ δ = ⋅δ 3 4L
Q Felect = 2εA
2
3
+ +/-
where
Q = CV
and
C=
εA
g
Felect =
εAV
2g
2
Setting
Fspring = Felect
2 L3εA 2 δ= ⋅V 3 gwt E
Acoustic Modeling
Sinusoidal input voltage:
V = Av ⋅ sin(2πfT )
diaphragm displacement [m]
10 9 8 7 6 5 4 3 2 1