MEMS Rigid Diaphragm Speaker Scott Maghy Tim Havard

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 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: Fspring + +/- where Setting and Felect

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 source sound power Pac watts Rocket engine 1, 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 Usual talking, Typewriter 0. 001 W Decreasing frequency 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!

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