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