Surface acoustic wave based wireless mems actuators

Surface Acoustic Wave based
Wireless MEMS Actuators for
Biomedical Applications
by
Sukanta Bhattacharyya
Registration # 1651210007

Agenda
Introduction
Surface Acoustic Wave (SAW) sensor
Device Design
SAW sensor operation
SAW based wireless microactuator
Material selection
Fabrication process flow
Operation
Advantage and disadvantage
Applications
Conclusion
References
Surface Acoustic Wave based wireless MEMS actuator

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Thursday, November 07, 2013

Introduction
Micro-Electro-Mechanical Systems (MEMS) technology has
made it possible to fabricate small size, and high performance
implantable devices to meet critical medical and biological
needs such as in–vivo drug delivery, Lab–on–a–Chip
(LoC), surface acoustic wave devices, polymerase chain
reaction (PCR) etc.
Actuators are one of the important components in BioMEMS, especially for fluid manipulation
Surface Acoustic Wave (SAW) devices are used to develop
micromachines such as ultrasonic micromotors and fluid
transfer methodologies such as flexural micropumps

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Surface acoustic wave sensor is a class of MEMS device which
is based on the modulation of surface acoustic waves to sense
a physical phenomenon.


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Device Design
The basic surface acoustic wave device consists of a
piezoelectric substrate, an input interdigitated transducer
(IDT) on one side of the surface of the substrate, and a
second, output interdigitated transducer on the other side of
the substrate. The space between the IDTs, across which the
surface acoustic wave will propagate, is known as the delayline.


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Fig 1: Surface Acoustic Wave sensor

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SAW sensor operation
SAW technology is based on the piezoelectric effect.

Fig 2: Surface Acoustic Wave sensor operation

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SAW sensor operation contd..
Input Interdigitated Transducer (IDT) transduces electric
signal to acoustic waves when the two ends of IDT are
subjected to sinusoidal signal.
Alternating polarity(+-) develops between the fingers of IDT
resulting in generation of electric field.
Direction of electric field changes alternatively between the
adjacent set of fingers creating alternate regions of tensile
strain and compressive strain (mechanical vibrations) thus
producing mechanical waves.
Waves produced at the surface of the piezoelectric substrate
known as surface acoustic waves.

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SAW sensor operation contd..
Acoustic waves propagate through the region of delay line
between the two IDTs.
At the output end the propagated waves are picked up by the
output IDT.
Waves are converted back to electric signal by piezoelectric
effect which is then measured and calibrated.


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SAW based wireless microactuator

Fig 3: SAW based wireless microactuator


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Material selection
The SAW based microactuator is similar to the normal SAW
sensor only differing in the actuator part.
SAW substrate: The SAW substrate is generally made of
quartz. Lithium Niobate (LiNbO3) is also used as it is best
suited for Rayleigh wave propagation.
IDT: Made of highly conductive low cost materials such as
copper (Cu), aluminum (Al), gold (Au), tungsten (W), and
titanium (Ti).


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Material selection contd..
Microactuator: A thin conductive plate (conductive material) is
placed over the output IDT with a small air gap in between
that acts as the actuator.
Also made of materials such as Silicon (Si) or Silicon Nitride
(Si3N4) and the bottom surface of the microactuator can be
coated with a thin conductive material such as Gold, Platinum
or Aluminium.
Input IDT is connected to a micro-antenna for wireless
interrogation.


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Fabrication Process flow


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Operation
Working principle is similar to that of normal SAW sensor. Only
difference is that the wireless SAW uses the RF waves as the
exciting source for the generation of Rayleigh waves at the
input IDT that propagate in the forward direction towards the
output IDT.
RF is an electromagnetic wave having a frequency between 3
kHz to 300 GHz
The RF signal is fed to the SAW device through the microstrip
antenna


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Rayleigh waves are a type of surface acoustic wave that travel
on solids. They can be produced in materials in many
ways,
such
as
by
a
localized
impact
or
by piezoelectric transduction.
Rayleigh waves include both longitudinal and transverse
motions that decrease exponentially in amplitude as distance
from the surface increases.

Fig 4: Ray Leigh waves

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Actuation mechanism: electrostatic actuation
A thin conductive plate is placed on top of the output
IDT, which is separated by an air–gap.
The conductive plate does not alter the mechanical boundary
conditions of the SAW substrate, but causes the surface to be
equipotential and the propagating electric potential to be
zero at the surface of the conductive plate. As a result, an
electrostatic force is generated between the conductive plate
and the output IDT in the SAW device causing micro
deformations or microactuation in the conductive plate

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Advantages
•
•
•
•
•
•
•

High biocompatibility
Reliable
Low power operation
Small size
Simplicity fabrication
Cost effectiveness
Fast fluid actuation


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Disadvantage
One disadvantage of SAW is that Rayleigh waves are surfacenormal waves, making them poorly suited for liquid sensing.
When a SAW sensor is contacted by a liquid, the resulting
compressional waves cause an excessive attenuation of the
surface wave.


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Applications

Widely used in micro fluidics for studying the manipulation of
fluids.
Also used to develop micromachines such as ultrasonic
micromotors and fluid transfer methodologies such as flexural
micropumps


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Conclusion

A brief introduction about the SAW sensor is highlighted at
the beginning stating the device layout and working principle
which is followed by a detailed explanation about the wireless
based SAW microactuator. We talked about the fabrication
process that is involved in the making of such a device and
later we focused on some merits and demerits of the SAW
based microactuator.


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References
1.

2.
3.
4.
5.

Surface Acoustic Wave Based Wireless MEMS Actuators for Biomedical
Applications Don W. Dissanayake, Said Al Sarawi and Derek
Abbott, The School of Electrical and Electronic Engineering, The
University of Adelaide Australia. SA 5005
A Fabrication Study of a Surface Acoustic Wave Device for Magnetic
Field Detection by Matthew L. Chin
www.google.com
www.wikipedia.com
Google images


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THANK YOU

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Surface acoustic wave based wireless mems actuators

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