Transcript MEMS Acoustics - University of California, San Diego

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
– Clandestine
– Privacy
– Low power
• Potential lower cost
• Highly customizable performance
• No surgery!
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
Implantable Hearing Devices
 Cochlear Implants
 Auditory Brainstem implants
 Implantable Middle-ear devices
– Piezoelectric devices
– Electromagnetic devices
Cochlear Implants
Source: http://www.nidcd.nih.gov/health/hearing/coch.asp
Auditory Brainstem Implants
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)
Middle Ear Transducer
• Translates electrical signals into mechanical
motion to directly stimulate the ossicles
Middle Ear Transducer
Remote
MET Implant
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
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
Fabrication
Deposit Silicon Nitride Layer
Pattern photoresist & then etch
electrodes & oxide using RIE
Deposit layers of Electrodes, oxide,
and photoresist (as shown)
Deposit Oxide 2 layer
Fabrication
Etch oxide 2, and make Poly-Si columns
Deposit oxide 3 as shown
Coat columns with Photoresist and
etch away remaining oxide 2
Remove photoresist from electrode 2
Remove photoresist and deposit Poly-Si
Fabrication
Make Poly-Si diaphragm base thicker
Release oxide layers
Performance and Optimization
Speaker Mechanics
Force balance:
3
Ewt
Fspring  k   

3
4L
2
Q
Felect 
2A
where
Setting
Q  CV
Fspring  Felect
+
+/-
and
C
A
g
Felect 
AV 2
2g
2 L3A 2

V
3
gwt E
Acoustic Modeling
Sinusoidal input voltage:
-10
V  Av  sin( 2fT )
2 L A 2

V
3
gwt E
3
Which causes sound intensity:
Io   2  2f  aircsound
Diaphragm Vibration
9
8
diaphragm displacement [m]
Drives diaphragm displacement:
10
x 10
7
6
5
4
3
2
1
Acoustic power can then be obtained:
Pacoustic  I  Area
0
0
1
2
time [s]
3
-4
x 10
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
-12
18
Io    2f  aircsound
14
2
Pacoustic  I  Area
Sound Power [W]
12
1
distance
Device Output
Hearing Threshold
16
2
I  Io
Acoustic Performance
x 10
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
Comparison of Acoustic Sound Power
Situation
and
sound source
sound power
Pac
watts
Rocket engine
-11
x 10
Device Output
Hearing Threshold
2.5
1,000,000 W
Turbojet engine
Acoustic Performance
10,000 W
2
1,000 W
Machine gun
10 W
Jackhammer
1W
Chain saw
0.1 W
Helicopter
0.01 W
Loud speech,
vivid children
Usual talking,
Typewriter
Sound Power [W]
Siren
1.5
1
Decreasing frequency
0.001 W
0.5
10−5 W
Refrigerator
10−7 W
(Auditory threshold at 2.8 m)
10-10 W
(Auditory threshold at 28 cm)
10-12 W
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
– 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
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
• Other methods of securing the diaphragm
– “Spring” attachment
• Decrease the mass of the diaphragm by altering
fabrication process
QUESTIONS