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 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
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).
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.
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 doublesided 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,
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
The heart of the MEMS microphone is the variable
capacitor formed by a fixed back plate and a flexible
Sound pressure deflects the membrane, causing a
change in capacitance. This change in capacitance
causes a change in voltage according to:
C = Capacitance [Farads]
Q = Charge [Coulumbs]
V = Voltage [Volts]
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.
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
When the diaphragm vibrates, the coil moves in the
magnetic field, producing a varying current in the coil
through electromagnetic induction.
A piezo microphone uses the phenomenon of
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
Images of the acoustic test chamber used for testing the 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 &
MEMS Microphones integrate signal
amplication & analog to digital conversion on
the same chip. The digital output is typically
in the pulse-densitymodulation
Most versions of the digital MEMS microphone
allow fortwo mics to multiplex on the same data
line. (This is oftenuse in noise-cancelling circuits)
Greater reliability due to fewer parts
Higher tolerance to mechanical vibrations
Smaller component height & footprint
Integrated signal conditioning & analog-to-digital conversion
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
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.