MEMS Rigid Diaphragm Speaker
Scott MaghyTim 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
Source: http://www.nidcd.nih.gov/health/hearing/coch.asp
Cochlear Implants Auditory Brainstem Implants
Piezoelectric Devices• Operation
• 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
MET Implant
Charger
Remote
Middle Ear Transducer
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
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 Deposit layers of Electrodes, oxide, and photoresist (as shown)
Pattern photoresist & then etch electrodes & oxide using RIE Deposit Oxide 2 layer
Fabrication
Etch oxide 2, and make Poly-Si columns
Coat columns with Photoresist and etch away remaining oxide 2Remove photoresist from electrode 2
Deposit oxide 3 as shown Remove photoresist and deposit Poly-Si
Fabrication
Make Poly-Si diaphragm base thicker Release oxide layers
Performance and Optimization
Speaker Mechanics
A
QFelect
2
2
CVQ g
AC
3
3
4L
EwtkFspring
g
AVFelect
2
2
23
32V
Egwt
AL
electspring FF
where and
Setting
++/-
Force balance:
Acoustic Modeling
AreaIPacoustic
)2sin( fTAV v
soundaircfIo 22
23
32V
Egwt
AL
Sinusoidal input voltage:
Which causes sound intensity:
Drives diaphragm displacement:
Acoustic power can then be obtained:0 1 2 3
x 10-4
0
1
2
3
4
5
6
7
8
9
10x 10
-10
time [s]
diap
hrag
m d
ispl
acem
ent
[m]
Diaphragm Vibration
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
AreaIPacoustic
soundaircfIo 22
2tancedisII o
1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
-2
0
2
4
6
8
10
12
14
16
18x 10
-12 Acoustic Performance
Distance from User [m]
Sou
nd P
ower
[W
]
Device Output
Hearing Threshold
Comparison of Acoustic Sound PowerSituationandsound source
sound powerPac
wattsRocket engine 1,000,000 WTurbojet engine 10,000 WSiren 1,000 WMachine gun 10 WJackhammer 1 WChain saw 0.1 WHelicopter 0.01 WLoud 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
Device is in the threshold of human hearing!
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05
0
0.5
1
1.5
2
2.5
x 10-11 Acoustic Performance
Distance from User [m]
Sou
nd P
ower
[W
]
Device Output
Hearing Threshold
Decreasing frequency
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
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