MEMS MICROPHONE

BY :NIDHI TRIVEDI

MEMS MICROPHONES

INTRODUCTION

ABOUT MEMS

What is MEMS?

∙Micro – Small size, micro fabricated structures

∙Electro – Electrical signals/control(in/out)

∙Mechanical- Mechanical functionality(out/in)

∙Systems- structures, devices, system control

What is size of MEMS?

They range in size from the sub micron level to the millimeter level and there can be any number, from a few to millions in particular system.

MEMS MICROPHONE

MEMS microphones are similar to the standard ECMs (electret condensor microphones) found in modern consumer electronics,except that the components are built onto a single chip using CMOS technology (material deposition & etching), rather than assembled from discrete parts.There are two main categories of MEMS microphones:

Analog MEMS Microphones convert a sound pressure input into an analog voltage output.

Digital MEMS Microphones convert a sound pressure input into a digital output signal, typically in pulse density modulation format (PDM).

ALL ABOUT MICROPHONES

Microphone Sensing: Principle

A microphone is an acoustic to electric transducer that converts sound into an electrical signal.

Microphones capture sound waves with a thin, flexible diaphragm. The vibrations of this element are then converted by various methods into an electrical signal that is an analog of the original sound.

Most microphones in use today use electromagnetic generation (dynamic microphones), capacitance change (condenser microphones) or piezo-electric generation to produce the signal from mechanical vibration.

MEMS MICROPHONE

Microphone Fabrication

A piezoelectric microphone based on PZT thin films was designed in order to investigate its potential use as a passive sensing element in a photoacoustic resonant cell.

The microphone fabrication process began with a double-sided polished silicon wafer and used several different types of deposition systems .

A plasma-enhanced chemical vapor-deposited (PECVD) silicon dioxide thin film (1 μm thick) was deposited with a Plasma-Therm 790 reactor using a mixture of SiH4, He, and N2O.

Microphone Fabrication

The oxide served as the membrane structural layer and was chosen to be 1 μm thick. After deposition, the film was annealed in an A.G. Associates Heatpulse 610 rapid thermal anneal (RTA) furnace at 700 °C for 60 seconds in a nitrogen atmosphere.

This annealing removes the trapped hydrogen within the film and causes the film to obtain a slightly tensile stress, which aids in producing a planar membrane and a high performance microphone.

FABRICATION PROCESS

DC Magnetronsaputtering

Sol-Gel Deposition

PECVD

Transducer Physics

The heart of the MEMS microphone is the variable capacitor formed by a fixed back plate and a flexible membrane.Sound pressure deflects the membrane, causing a change in capacitance. This change in capacitance causes a change in voltage according to: C = Q / VC = Capacitance [Farads]Q = Charge [Coulumbs]V = Voltage [Volts]

Types of microphone

Condenser Microphones

In a condenser microphone, the diaphragm acts as one plate of a capacitor, and the vibrations produce changes in the distance between the plates.

Since the plates are biased with a fixed charge (Q), the voltage maintained across the capacitor plates changes with the vibrations in the air.

Condenser Microphone

Dynamic Microphone

In a dynamic microphone, a small movable induction coil, positioned in the magnetic field of a permanent magnet, is attached to the diaphragm.

When sound enters through the windscreen of the microphone, the sound wave vibrations move the diaphragm.

When the diaphragm vibrates, the coil moves in the magnetic field, producing a varying current in the coil through electromagnetic induction.

Dynamic Microphone

PZT Microphone

A piezo microphone uses the phenomenon of piezo-electricity

It is widely used to amplify acoustic instruments for live performance to record sounds in unusual environments

Two different sound sources were used to characterize the performance of the PZT microphones

PZT Microphone

Images of the acoustic test chamber used for testing the PZT microphone

PZT Microphone

The BNC mounts provide electrical contact to the packaged microphone whereas the acrylic plate on top provides a connection for the acoustic tube.

Signal Conditioning & Output

MEMS Microphones integrate signal amplication & analog to digital conversion on the same chip. The digital output is typically in the pulse-densitymodulationMost versions of the digital MEMS microphone allow fortwo mics to multiplex on the same data line. (This is oftenuse in noise-cancelling circuits)

Product Distinctions

Advantages:Greater reliability due to fewer parts Higher tolerance to mechanical vibrations Smaller component height & footprintIntegrated signal conditioning & analog-to-digital conversion

Challenges:If using digital-output versions, many require the use of a separate codec to translate the PDM signal into a sound wave.Only available as a surface mount component difficult to breadboard.

CONCLUSION

The medical, wireless technology, biotechnology, computers, automotive, aerospace industries are only a few that will benefit greatly from mems.

This enabling technology promises to cerate entirely new categories of products.

MEMS will be indispensable factor for advancing technology in the 21 century.