IEEE Spectrum December, 2016 - 50

Vacuum

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Top electrode

Silicon membrane

Top electrode

Piezo layer
Bottom
electrode

Silicon
membrane

Insulator

Silicon wafer
- Bottom electrode/Ground

Silicon
wafer
-Ground

BiG-BenD tHeory: ultrasound transducers create sound by flexing a membrane. in a capacitive micromachined
ultrasound transducer (CMut), the bottom electrode pulls the top electrode to launch the ultrasonic pulse, but that
bottom electrode also limits the membrane's motion [left]. in a piezoelectric micromachined ultrasound transducer
(PMut), the bottom electrode flexes with the voltage, giving the membrane more room to move [right].

transducers, however, are making it possible to use ultrasound in the consumer world.
MEMS technology has already made a big splash in the area
of miniature microphones. Acoustic MEMS components first
broke into the cellphone market in 2003, when the popular
Motorola Razr phone incorporated a MEMS microphone from
Knowles Acoustics. Today, MEMS microphones-which are
smaller, consume less power, and include more onboard signal
processing than traditional electret condenser microphones-
are in virtually all smartphones. Such microphones are currently being produced by Akustica, Cirrus Logic, Infineon,
InvenSense, Knowles, STMicroelectronics, and others.
More recently, a few companies have begun to exploit the
ability of MEMS microphones to receive near-audio-band
ultrasound. While manufacturers' data sheets specify performance in the audio band (up to approximately 20 kilohertz), the MEMS transducers inside these microphones are
often capable of receiving signals at over twice this frequency.
The first MEMS-based ultrasound application to make it to
the mass consumer market was Qualcomm's Snapdragon digital pen, which is incorporated in the HP Slate line of tablets.
Qualcomm picked up this technology when it acquired the
Israeli ultrasound-technology company EPOS Development
in 2012. This ultrasonic stylus can be tracked at a distance
from the surface of the tablet, allowing the user to write on
a paper pad adjacent to the tablet.
An example of another ultrasound innovation to come to
consumer devices is a product named Beauty, introduced
this year by Elliptic Labs, of Norway. Beauty is a softwareonly ultrasound approach that uses a smartphone's existing
earpiece and MEMS microphone to replace the IR proximity
sensor that detects when the phone is close to your ear, triggering a disabling of the touch screen and display.
These early applications of consumer ultrasound use conventional MEMS microphones that do fine when receiving
ultrasound signals. These microphones, however, are based
on capacitive transducers, which are ill suited to transmitting ultrasound in air.
A capacitive microphone transducer consists of two capacitor
plates-the backplate and the membrane-separated by a small
air gap (on the order of 1 micrometer wide). This type of trans50

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ducer receives sound by detecting the change in capacitance
that occurs when the membrane deflects under an incident
sound wave. Normally, this is all that microphones do-receive
sound. But they can be made to transmit sound by reversing
this process, deflecting the membrane to launch a sound wave.
The problem here is that a good receiver requires a very
small air gap between the membrane and backplate. That's
because the receiver's acoustic sensitivity scales with the
inverse square of the gap, so increasing the gap by, say, a
factor of three reduces the sensitivity by a factor of nine.
While small gaps are good for receiving sound, they pose a
problem for transmitting sound because a small gap limits
the membrane displacement and therefore the maximum
sound pressure level (SPL) that can be transmitted. The SPL
is proportional to the product of the acoustic impedance
of the surrounding medium-air-and the frequency and
amplitude of the membrane's motion. Capacitive transducers, such as capacitive micromachined ultrasonic transducers (CMUTs), work well in medical applications where the
surrounding medium is fluid, the ultrasound frequency is
greater than a few megahertz, and high voltage is available
to drive the transducer.
However, high-frequency ultrasound is quickly attenuated
in air, with absorption loss increasing from about 1 decibel
per meter at 40 kHz to 100 dB/m at 800 kHz. For this reason,
air-coupled ultrasound transducers typically operate at frequencies from 40 to 200 kHz. At these frequencies, due to the
much lower acoustic impedance of air than fluid, an ultrasound transducer must vibrate more than 1 micrometer to
transmit at a sound pressure level sufficient to allow echoes
to be measured from objects more than a few centimeters
from the transducer. CMUTs capable of this much vibration
amplitude require large gaps and therefore high voltages
(greater than 100 volts) to operate.
So producing a high-pressure sound wave in a low-voltage
device that is also a sensitive receiver requires that the membrane displacement not be limited by a nearby backplate.
The answer is piezoelectricity. Piezoelectricity, first discovered by the Curie brothers in 1880, refers to the ability of certain materials to produce an electrical charge when they are
mechanically deformed. In a piezoelectric micromachined

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Table of Contents for the Digital Edition of IEEE Spectrum December, 2016