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 Potential lower cost 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

Auditory Brainstem Implants Cochlear Implants Auditory Brainstem Implants Source: http://www.nidcd.nih.gov/health/hearing/coch.asp

Piezoelectric Devices Operation Advantage: inert in a magnetic field Disadvantage: Power output directly related to size of crystal. Example: Middle Ear Transducer (MET) Pass current into Piezoceramic Crystal Crystal changes volume Vibratory signal produced

Middle Ear Transducer Translates electrical signals into mechanical motion to directly stimulate the ossicles

Middle Ear Transducer Remote MET Implant Charger

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 Pass current into Electric Coil Magnetic Flux created Drives adjacent magnet

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 layers of Electrodes, oxide, and photoresist (as shown) Deposit Silicon Nitride Layer 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 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

Fabrication Make Poly-Si diaphragm base thicker Release oxide layers

Performance and Optimization

Speaker Mechanics Fspring Felect Force balance: + +/- where and Setting

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.

Observed Acoustic Power Sound intensity decays quadratically with distance  This results in limited effective speaker range

Comparison of Acoustic Sound Power Situation and sound source sound power Pac watts 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!

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