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Integrated Circuit Interface for SAW Biosensors Applications

by Aggour, Khaled, MS

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Chapter 2: Theoretical Background 8

Chapter 2

Theoretical Background

2.1 Introduction

There are many types of sensors with different interfacing circuits depending on the
application. This chapter presents a number of sensors technologies and interfacing
relevant to this work. The discussions focus on smart sensors, Biosensors, SAW
devices, using SAW devices as biosensors and SAW resonator modelling.

2.2 Sensors

The main energy forms include electrical, thermal, radiation, mechanical, magnetic,
kinetic and chemical energies. Energy transformation is widely used in diverse
applications. In nature, green plants convert light energy to chemical energy in the
form of food. Chemical to kinetic energy conversion takes place in most vehicle
motors while gas heating systems utilise chemical to thermal energy transformation.

A transducer is defined as a conversion device that is used to convert between
different forms of energy (Pallas-Areny et al., 2001). When the conversion takes
place from a non-electrical energy to electrical output; the transducer is defined as a
sensor. When it happens in the opposite direction, from electrical energy to nonelectrical
energy, the transducer is named an actuator. Nowadays, sensors and
actuators play a vital role in our daily life. Electric heating is an example of an
actuator that produces thermal energy when supplied with electricity.

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Chapter 2: Theoretical Background 9

There are several reasons for sensors to dominate the transducers world.
Pallas-Areny et al. (2001) summarised these reasons as follows:

- Sensors can be designed for any quantity with a non-electrical nature by

selecting a suitable material
- Since the sensing system is able to amplify electrical signals; there is no

energy drained in the sensing process.
- The availability of Integrated Circuits (ICs) which performs the

transformation of the sensor electric output signal to another appropriate
electric signal.
- The availability of several data display and save forms for electrical signals.
- Electric signals are more adaptable to transmission other than other signals.

2.3 Smart Sensors

Before the microelectronics era, sensor outputs were monitored manually by an
observer (Frank et al., 2000). If the sensor output requires any calibration it had to
be done manually. It was a tedious process and consumed time and effort. There was
a need for sensors providing more readable outputs. This was achieved by sensors
producing an electrical output. The output could be interfaced to provide unattended
measurements and control. The main vital intelligence required by sensors is the
ability to interface with a digital processor, a Micro-Controller Unit (MCU), a
Digital Signal Processor (DSP) or an Application Specific Integrated Circuit (ASIC)
(Frank et al., 2000). This sort of sensor is referred to as a smart sensor.

Changing the sensor output to a digital format enables the user to benefit from
the digital world advantages such as easy and cheap data storage, autonomous
working, meaningful output display, automatic control, and automatic easy
calibration. The smart sensor requires minimal human monitoring and intervention.

Sensor output varies according to the nature of the measured quantity and the
required sensitivity and resolution. A sensor’s output is usually a small analogue
signal which needs to be amplified and adapted by a signal condition circuitry. The
task of the signal conditioning circuit is to convert this signal to a digital signal
suitable for digital interface applications.

From the previous discussion it can be seen that in most sensors the sensor
output is only the first step towards smart sensing. To achieve the smartness of a
sensing system; the sensor output has to pass through several operations in order to
produce a digital (smart) output.

Sensor Interfacing or Signal Conditioning is defined as the conversion of the
sensor output to an output which is more suitable for display, recording and

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Chapter 2: Theoretical Background 10

processing. Signal conditioning is usually an electronic circuit (either integrated or
discrete). The block diagram of the smart sensing system is shown in figure 2-1.

Figure 2- 1 Smart Sensing system

Sensors can be classified according to their sensing technology as follows:

- Resistive Sensors: These sensors are defined by Pallas-Areny et al. (2001) as
sensors in which their electric resistance varies with the measured quantity.
This type is widely used because many materials electric resistances are
sensitive to physical quantities variations. The most common applications for
resistive sensors are strain gauges and gas sensors.

Wheatstone bridge is a common technique for resistive sensors signal
conditioning. One or more bridge arms resistance change with sensed
quantity variation. The change in the resistance is reflected at the bridge
output voltage or current. The output is amplified and converted to digital.
Jordana and Pallas-Areny (2006) integrated four strain-gauges in a four-arm
bridge for pressure sensing.

- Capacitive Sensors: The measured quantity affects the dielectric of a
material. The capacitance between two parallel plates and sensing material is
a function of the sensed quantity. These sensors are commonly used for
sensing humidity, pressure, or displacement.

For interfacing capacitive sensors, a capacitance to digital circuit has to be
implemented. This can be done in several ways. The most widely used
technique is to convert the capacitance to a voltage via an integrator,
afterwards converting this voltage to a digital word as done in Jaervinen et al.
(2008) and O'Dowd et al. (2005).

- Self Generating Sensors: Self-generating sensors are defined as sensors
which are capable of producing an electric output signal from a measured
quantity without electric supply (Pallas-Areny et al., 2001). This type of
sensor is used mainly in chemical sensing, temperature measurement
(thermocouples), and piezoelectric sensors.

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Chapter 2: Theoretical Background 11

The output from these sensors is usually either an electric current or a voltage
proportional to the measured quantity. This current or voltage might be very
small or with a very small frequency (Pallas-Areny et al., 2001).
Accordingly, amplification, filtering, or DC level shifting may be
implemented for interfacing the sensor output to a digital circuit.

- Quasi or semi-digital Sensors: This type of sensors was named semi-digital
or quasi-digital because their output is a variable period or frequency which
can be easily converted to digital without complex circuitry. The most
common example for this type of sensor is the Surface Acoustic Wave
(SAW) sensor which produces an output frequency sensitive to the change in
the sensing quantity.

- Digital Sensors: Digital sensors are sensors that produce discrete outputs.
They are more likely to perform smarter than other sensors types because of
their output digital nature.

2.4 Biosensors

The biosensor is a device that integrates both a biological sensing element and a
signal transducer (Hall, 1990). The biosensor is composed of two main entities, a
receptor and a transducer. The receptor has biological components that act together
with the biological input. The transducer responds with the electrical output
according to the receptor interaction (Harsanyi, 2000). This can be illustrated by
figure 2-2.

Figure 2- 2 General block diagram of biosensors

Source: Harsanyi, G., 2000. Sensors in biomedical applications: Fundamentals, technology
and applications, CRC Press, pp 224.

Harsanyi (2000) classified biosensors according to their receptor types as follows:

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Chapter 2: Theoretical Background 12

- Enzymatic biosensors: They are biosensors based on catalysts or substances

that allow the biochemical interactions.
- Affinity biosensors: This type of sensor is based on chemical binding.
- Living Biosensors: This kind of biosensor utilises microorganisms or tissues

as receptors for sensing.

Due to the wide diversity of enzymes in living bodies, the immune system
produces a large number of antibodies against antigens. Accordingly, there is a large
number of available receptor molecules, so the most challenging aspect in biosensors
is not the receptor availability. The immobilisation techniques, operation stability
and transducer choice are the most difficult parts (Harsanyi, 2000).

The following transducer types are employed in biosensors:

- Calorimetric: Its principle is to measure the heat caused by metabolic activity

of the bio-components via a thermal transducer (Kress-Rogers, 1997).
- Electrochemical: They include potentiometric, amperometric and

conductimetric electromechanical cells.
- Fibre-Optic Transducers.
- Gravimetric resonant transducers: In this type, the biological quantity affects

the resonant frequency of a piezoelectric resonator. The most widely used
transducer type in this category is the Surface Acoustic Wave (SAW)
resonator which is described in detail in the following section.

2.5 Surface Acoustic Wave (SAW) sensors

2.5.1 Introduction

The role of physical and chemical microsensors has become increasingly important
in physical and chemical property measurement (Gardner et al., 2001). For this
family of microsensors; SAW sensors play an important role.

Surface acoustic wave sensors have many advantages such as their capabilities
in wireless measurements (Gardner et al., 2001), the ability to operate at high
frequencies, sensitivity to different phenomena and the feasibility of production in
mass. Moreover, SAW devices produce a quasi-digital sensor output, which make
them favourable for smart sensors systems. SAW device operation is based on the
piezoelectric effect; the piezoelectricity phenomenon can be illustrated as follows

When an electric field is applied on a piezoelectric material, either a tensile or
compression force occurs depending on the direction of the applied field. The
direction of the applied electric field in figure 2-3 causes tension force. Reversing the
electric field polarity causes compression force to the structure. If tensile or

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Chapter 2: Theoretical Background 13

compressive force is applied on a piezoelectric crystal, a reversible phenomenon
occurs and a charge is produced on the sides of this crystal depending on the type
and the direction of the applied force.

Figure 2- 3 Piezoelectric effect

Source: Gardner, J.W., Varadan, V.K., Awadelkarim, O.O., 2001. Microsensors, MEMS, and Smart Devices, John
Wiley and Sons, p. 306.

The Inter-Digital Transducer (IDT) consists of two metal electrodes placed on
a piezoelectric substrate. The surface waves are generated using the IDT electrodes
by applying a time varying voltage across the IDTs. This will result a synchronous
deformation to the piezoelectric material and consequent production of surface
waves (Harsanyi, 2000). Each IDT finger is a combination of multiple strips. Each
couple of opposite strips constitutes a finger-pair. Two main types of fingers exist.
The first one is called a solid or single finger in which the IDT period consists of a
couple of strips. In the case of the presence of four strips in each IDT period; the IDT
is named split-finger or double finger (Hashimoto, 2000). The structure of the two
types is shown in figure 2-4.

Figure 2- 4 The main two Interdigital Transducers (IDT) structures

Source: Hashimoto, K., 2000. Surface acoustic wave devices in telecommunications, Springer, p 47

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Chapter 2: Theoretical Background 14

The principle of the SAW operation depends on the symmetry of the IDT
pairs, when stimulating the SAW, each IDT period generates a surface acoustic
wave. Choosing the IDT period as an integer multiple of the surface acoustic wave
length causes the excited waves to be summed together and produce a standing

Three important factors define the IDT characteristics (Hashimoto, 2000):

- The width and the length of the fingers.
- The number of IDT finger pairs
- The piezoelectric substrate material

2.5.2 Acoustic Wave sensors:

Acoustic wave sensors can be classified into three categories according to their wave
pattern propagation (Gardner et al., 2001) Rayleigh Surface Acoustic Waves:

Figure 2- 5 Surface acoustic wave generation in Quartz by IDTs
Source: Gardner, J.W., Varadan, V.K., Awadelkarim, O.O., 2001. Microsensors, MEMS, and Smart Devices,
John Wiley and Sons, pp. 309.

Rayleigh waves are the most common used surface acoustic waves. The acoustic
wave is generated by the physical disturbance caused by the electric signal, and the
wave propagation pattern is decided by the particular IDT fingers as shown in figure

In Rayleigh SAWs, the piezoelectric substrate and the crystal cut establish the
wave velocity and consequently the frequency. In these SAW devices; energy is
confined close to the substrate surface. The Rayleigh SAW shear component couples
with the contacting medium, so it is not suitable for liquid sensing. Instead it finds
their application in gas detection and high frequency filters.

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Chapter 2: Theoretical Background 15 Shear Horizontal Surface Acoustic Waves (SH-SAW)

Shear Horizontal acoustic waves are similar to Rayleigh SAWs but use a thinner
wafer. In SH-SAW; the travelling waves are parallel to the surface plane and
perpendicular to the wave propagation direction. The IDTs generate acoustic waves
in the material bulk at an angle to the surface. The wave velocity is decided by the
finger spacing and substrate.

Waves are able to propagate when making a contact with a liquid without
coupling acoustic wave energy into the liquid (Zhang et al., 2001). Moreover, they
are sensitive to mass loading so have found their application mainly in liquid
detection as shown by Kogai et al. (2008). Love Surface Acoustic Waves

The Love Surface Acoustic Waves are defined as SAW waves that propagate in
infinite thickness in a waveguide mode using a second layer of metal as shown in
figure 2-6. The Love wave energy is located at the surface and near it.

Figure 2- 6 Schematic of a Love wave propagation region and relevant layers

Source: Gardner, J.W., Varadan, V.K., Awadelkarim, O.O. 2001. Microsensors, MEMS, and Smart Devices,
John Wiley and Sons, pp. 313.

2.6 SAW Devices as Biosensors

As indicated in section 2.4, gravimetric biosensors based on resonators are widely
used. The two main types of resonators are the Quartz Crystal Microbalance (QCM)
and SAW devices (Ripka et al., 2007). QCMs operate at a few Mega-Hertz range.
SAW devices have the advantage of higher frequency operation and consequently

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Chapter 2: Theoretical Background 16

more selectivity. Therefore SAW biosensors can be found in many biological
sensing applications.

2.6.1 SAW Biosensor Applications

The high sensitivity of acoustic wave devices in bio-related molecules detection
made them an attractive choice for sensors in biological applications (Rocha-Gaso et
al., 2009). As indicated by Rocha-Gaso et al. (2009) and Länge et al. (2008), the
common applications for SAW biosensors are DNA, proteins, sugar, bacteria and
viruses, molecules, and cells detection.

The basic setup for a SAW biosensor is shown in figure 2-7. The SAW device
generates and detects the acoustic wave by the IDT on the piezoelectric substrate
surface. Bio-specific molecules are immobilised on the SAW device to catch specific
biological molecules (Länge et al., 2008). The coating of the SAW device with a
bio-specific layer corresponding to a specific biological stimulus strongly depends
on the substrate material and the substrate guiding layer (if exists).

Figure 2- 7 Basic SAW biosensor setup exemplified by a SAW immunosensor

1. The flow of the liquid sample 2. Piezoelectric crystal
3. IDTs 4. The surface acoustic wave
5. Immobilized antibodies 6. Analyte molecules in the sample
7. The driving electronics 8. The output signal

Source: Länge, K., Rapp, B.E., Rapp, M., 2008. Surface acoustic wave biosensors: a review, Analytical and
Bioanalytical Chemistry, vol. 391, p.1512

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Chapter 2: Theoretical Background 17

DNA detection was carried out by Sakong et al. (2007) utilising SH-SAW on
LiTaO3 substrate coated with a gold guiding layer. The DNA was immobilised on
the gold layer. Hur et al. (2005) developed a SAW sensing system based on mass
loading of DNA hybridisation on a gold-coated surface of a LiTaO3 substrate.
Papadakisa et al. (2009) used an acoustic Love wave device to evaluate the DNA
molecules geometrical properties.

SAW devices have found many applications in protein detection. Proteins
allow the oriented immobilisation of anti-bodies (Dubrovsky et al., 1996 cited Länge
et al. 2008, p.1513). Glucose oxidase immobilisation was carried out by Wessa et al.
(1999) on a SH-SAW device based on LiTaO3 substrate to detect polyclonal antiglucose
oxidase antibodies.

The detection of bacteria and viruses was successfully implemented using
SAW immunosensors (Länge et al., 2009). A SH-SAW device based on langasite
substrate was designed for the detection of Escherichia coli bacterium by Berkenpas
et al. (2006). Kwon et al. (2004) developed a SH-SAW sensor based on LiTaO3
substrate to detect human-immuno-globulin protein molecules antigens. Deobagkar
et al. (2005) utilised a bulk acoustic wave based device based on quartz substrate to
detect Escherichia coli bacteria cells in water through immunochemical reactions on
polystyrene coated device surfaces.

The SAW biosensors can be classified according to their IDT structure into
two main types; the Delay Line SAW and SAW Resonator. Both types are discussed
in the next two sections.

2.6.2 SAW Delay-Line Biosensor

Figure 2- 8 Schematic of a delay-line arrangement with inter-digitated transducers
on a piezoelectric substrate

Source: Varadan, V.K., Vinoy, K.J., Gopalakrishnan, S., 2006. Smart Material Systems and MEMS: Design
and Development Methodologies, John Wiley and Sons, p. 97.

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