Surface acoustic waves

Introduction
• a wave propagating along the surface of an
elastic substrate
• In a crystalline solid a sound wave is
transmitted as a result of the displacement of
the lattice points about their mean position

WACHIRA J.
NDUNG'U/MEMS/MECHATRONIC
ENGINEERING/JKUAT

Types of acoustic waves
• Bulk acoustic waves- propagated through the
bulk substrate material
• transverse wave- motions of the matter
particles conveying the wave are
perpendicular to the direction of propagation
of the wave itself
• longitudinal wave- motion of particles is back
and forth along the direction of propagation
• Surface acoustic waves propagate along the
surface of a substrate
WACHIRA J.
ENGINEERING/JKUAT

Saw in Mems
• Mechanical attributes: ruggedness, light
weight, small size.
• Electrical merits : signal processing at
selected frequencies in the range from
about 10 MHz up to a current reported
value of 11 GHz

WACHIRA J.
ENGINEERING/JKUAT

Working principle
• device consists of two IDTs on a piezoelectric
substrate. The input IDT launches and the
output IDT receives the waves.

WACHIRA J.
ENGINEERING/JKUAT

• operation of the SAW device is based on
acoustic wave propagation near the surface of
a piezoelectric solid
• the wave can be trapped or otherwise
modified while propagating
• The displacements decay exponentially away
from the surface, so that the most of the wave
energy (usually more than 95 %) is confined
within a depth equal to one wavelength
• The surface wave can be excited electrically by
means of an interdigital transducer (IDT)
WACHIRA J.
ENGINEERING/JKUAT

• The commonly used substrate crystals are:
quartz, lithium niobate, lithium tantalate, zinc
oxide and bismuth germanium oxide
• The wave velocity is a function of the
substrate material and is in the range of 1500
m/s to 4800 m/s, which is 105 times lower
than the electromagnetic wave velocity. . This
enables the construction of a small size delay
line of a considerable delay
• IDTs are only used as converters of electrical
to mechanical signals, and vice versa
WACHIRA J.
ENGINEERING/JKUAT

Saw Sensors
• modulation of surface acoustic waves to sense a physical
phenomenon
• The sensor transduces an input electrical signal into a
mechanical wave which, unlike an electrical signal, can be
easily influenced by physical phenomena
• The device then transduces this wave back into an electrical
signal. Changes in amplitude, phase, frequency, or timedelay between the input and output electrical signals can
be used to measure the presence of the desired
phenomenon.
• Conventional fields of application – communications and
signal processing
Other application - as identification tags, chemical and
biosensors, and as sensors of different physical quantities

WACHIRA J.
ENGINEERING/JKUAT

Analysis of SAW’s
consider that longitudinal vibrations
are excited in a rod

Hooke’s law:

WACHIRA J.
ENGINEERING/JKUAT

• Mechanical vibrations propagate along the rod
as an elastic wave. The longitudinal
displacement is given by the sum of waves
that propagate in opposite directions
• The displacement causes the longitudinal
mechanical stress given by
• The speed of the longitudinal wave is given by

WACHIRA J.
ENGINEERING/JKUAT

Advantages of SAW Sensors
• The SAW sensors are passive elements (they do
not need power supply)
• can be accessed wirelessly, enabling remote
monitoring in harsh environment. They work in
the frequency range of 10 MHz to several GHz.
• They have the rugged compact structure,
outstanding stability, high sensitivity, low cost,
fast real time response, extremely small size
(lightweight).

WACHIRA J.
ENGINEERING/JKUAT

Examples
• Biosensor

. SAW delay line biosensor integrated in a microfluidic channel. The surface between
the IDTs is coated with antibodies sensitive to the analyte to be detected. The
analyte
molecules binding to the immobilized antibodies on the sensor surface influence
the velocity of the SAW and hence the output signal generated by the driving
electronics
WACHIRA J.
ENGINEERING/JKUAT

SAW’s – Other Applications
• components in satellite receivers, remote
control units, keyless entry systems, television
sets to identification tags
• Other emerging applications of SAW
resonators include gas sensors
, chemical, temperature and pressure sensors

WACHIRA J.
ENGINEERING/JKUAT

Fabrication
• standard photolithography
• Integration of acoustic elements and
electronic circuitry on a single silicon chip

WACHIRA J.
ENGINEERING/JKUAT

Surface acoustic waves

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