25 SAW and Integrated Devices

Dr. Monika Tomar and Dr. Ayushi Paliwal

1. Introduction

Surface Acoustic wave (SAW) is an acoustic wave travelling on the surface of elastic material. It can be generated on the surface of piezoelectric material by inter digital transducers via inverse piezoelectric effect. SAW devices find applications in filters, delay lines, resonators, pressure sensor, torque sensor, temperature sensors and oscillators. On integration with some suitable sensing layer they can be used as chemical sensors, biosensors, UV detectors, Magnetic field sensors and so on. SAW based sensors are frequency based and hence, the response of the sensor can be continuously transmitted and received at some far place allowing the wireless detection. Moreover, SAW devices are passive devices and requires no power for their operation. The sensitivity of SAW sensors is very high and they are capable of detecting very low concentrations of the analytes as compared to other sensors.

2. Brief History:

• 1885-John William Strutt Lord Rayleigh first proposed the existence of a surface acoustic wave
• 1915-First acoustic wave transducer was designed
• 1920-Acoustic Oscillators were developed
• 1960-It was suggested that Acoustic Waves useful in electronics industry
• 1965-Inter Digital Transducers were developed
• Today-More than 3 billion SAW Devices are being produced annually for various applications.

3. Working:

Before starting with the working of SAW device, we must know what is piezoelectricity and waves.

a) Piezoelectricity

The phenomenon of generation of electric field on applying stress to a special class of material is called as piezoelectricity.

There exist some materials (non-centrosymmetric) in which the application of applied stress leads to the separation of positive and negative charges on two different sides which leads to the generation of electric field. Such phenomenon is called as piezoelectric effect. For example Quartz crystal, is a piezoelectric material (figure 1(a)). Application of stress on quartz crystal leads to the generation of electric field but the direction of electric field is opposite in expansion (figure 1(b)) as compare to compression(figure 1(c)). Reverse is also true, on applying electric field stress is produced in piezoelectric materials. Such phenomenon is called as inverse piezoelectric effect.

Figure 1: (a) Quartz crystal, (b) Generation of electric field in Quartz crystal on application of expansion stress (c) Generation of electric field in Quartz crystal on application of compressive stress

b) Waves

Waves are the disturbances which travel through the medium (or vacuum in case of electromagnetic waves) via virtue of elasticity and inertia.

Basically waves are of two types:

i) Transverse wave: Waves in which the motion of the particle is perpendicular to the direction of propagation of the wave as shown in figure 2. Examples are vibrations in strings, waves on water and electromagnetic waves.

ii) Longitudinal wave: Waves in which the motion of the particle is parallel to the direction of propagation of the wave as shown in figure 3. Example is sound waves.

Surface Acoustic Waves: These waves are the special kind of waves travelling on the surface of materials exhibiting elasticity with amplitude decaying exponentially with the depth of the material.

Surface acoustic waves are further classified on the basis of their direction of propagation and the motion of particles as: Rayleigh waves, Shear horizontal waves, Love waves, Lamb waves, Acoustic plate waves, Schezawa waves and Bulysteien- Guley waves. Rayleigh waves are the most basic waves known to exist in all elastic materials, other waves are the special kind of waves generated under specific conditions only. In this module we will focus on Rayleigh waves, other waves are beyond the scope of this text. In Rayleigh waves the motion of the particle is along both directions i.e. transverse as well as longitudinal thereby each particle traversing a elliptical path with the amplitude decaying with the depth as shown in figure 4.

4. Generation of Surface Acoustic Waves:

In order to generate surface acoustic waves inter digital transducers (IDTs) of suitable metal needs to be fabricated on piezoelectric material (Quartz in this module) using lithography technique (process of patterning) as shown in figure 5.

On applying the Electromagnetic field to the input of Inter digital transducers of the SAW device, In the first cycle all the electrodes connected to one side gets charged negatively while others gets charged positive as shown in figure 6.

Due to this alternate regions of compression and expansions get setup in the piezoelectric material via inverse piezoelectric effect. In the next cycle negatively charged rods becomes positive and positively charged rods become negative leading to the expansion of compressed parts and compressions of the expanded parts. Since the polarity of the electric field is changing continuously, acoustic waves got generated on the surface of piezoelectric material. On reaching of Surface acoustic waves at output IDTs they got converted back to the oscillating EM field via piezoelectric effect.

Electromechanical coupling coefficient (k2): Percentage of applied electrical energy converted to mechanical (acoustic) energy and is given as:

v is the velocity of surface acoustic wave in piezoelectric material

v’ is the velocity of surface acoustic wave in piezoelectric material with top surface metalized

During this journey of oscillating electric field from input IDTs to output IDTs some interesting things happen:

a) Out of all range of frequencies applied at input IDTs only a small band of frequencies do appear at output with one centre frequency which resonates with SAW device frequency i.e. f=v/λ where f is the centre frequency v is the velocity of acoustic wave in that piezoelectric material λ is the acoustic wavelength i.e. periodicity of the electrodes.Hence SAW device can be used as band pass filter.

b) On increasing the number of electrode pairs in IDTs upto certain limit, the bandwidth becomes very small implying SAW devices can be used as resonators.

c) The velocity of EM wave is of order of 108m/s, while the velocity of acoustic wave is of order of 103m/s. During its role as acoustic wave propagating over surface of elastic medium the velocity of EM wave become 105 times smaller implying SAW devices can be used in delay lines.

d) If there is some change in the ambient temperature, the velocity of the acoustic wave changes (because of the temperature coefficient of delay of the material) in turn changing the output centre frequency. Hence SAW devices can also be used as temperature sensor. In similar way SAW devices can be used as pressure sensors.

5. Applications of SAW devices:

Apart from the above mentioned applications, small modifications (coating of some sensing thin film over propagating path) in SAW device can be useful for other applications such as chemical sensors, biosensors, UV detectors and magnetic field sensors. For the sensing basically three mechanisms are responsible:

a) Mass Loading: Due to the adsorption of external stimulas on the sensing surface, the velocity of the wave changes because of the added mass on the sensing layer causing the shift in the frequency.

b) Elastic Loading: Due to the interaction of some external stimulus with the sensing surface the elasticity changes leading to the shift in centre frequency of SAW devices.

c) Acousto- Electric interaction: Due the change in electric parameters of sensing layer, and interaction of propagating acoustic wave with charge carriers, the SAW velocity get shifted, hence the change in its operating frequency.

Some of the examples are:

1) Chemical sensors: On coating some suitable thin film such that it will selectively adsorbs desired chemical species. On adsorption of chemical species on sensing film, the shift in operating frequency of SAW device is observed due to mass loading, elastic loading or acousto-electric interactions. As can be seen from the figure 7 NO2 molecules get adsorb on a suitable sensitive thin film (present between the both input and output IDTs).

Biosensor: Surface acoustic wave devices can be used to fabricate biosensor. A suitable sensitive coating, for the immobilization of selective bioreceptor needs to be deposited between input and output IDTs. The antigen was then immobilized on this sensitive coating via suitable method. On interaction of antibody with antigen, shift in centre frequency of SAW biosensor can be observed because of the mass loading. Schematic for the biosensor is shown in figure 8

3) UV detectors: On coating the devices with semiconductors of desired band gap SAW UV detectors can be realized. When UV radiations of desired wavelength interact with the semiconducting layer, electrons move from valence band to conduction band leading to the change in conductivity. Due to the change in conductivity, the acousto-electric interaction leads to the shift in the centre frequency of SAW detectors. Schematic for the UV detector is shown in figure 9.

5. Towards Wireless:

On integrating the SAW device with suitable RF Amplifier, oscillator can be made. The necessary conditions for the oscillator is

Aβ>=1

Where A is the gain of the amplifier, β is the loss of the SAW device, and total phase difference should be 0 or 3600. If the phase difference of the SAW device is 1800 then the schematic for the oscillator can be seen from figure 10. The SAW device is connected directly in the feedback loop of the RF amplifier (Since the phase difference of 1800 is introduced by the amplifier). The frequency of the oscillator is defined by the feedback loop i.e. by the centre frequency of the SAW device.

However if the phase of the SAW device is not equal to 1800, a phase shifter can also be integrated with the above circuit in order to meet the condition of phase 00 or 3600 to produce oscillations. The schematic with phase shifter is shown in figure 11.

In order to avoid any false alarming due to environmental factors, two identical oscillators can be designed (one having SAW device with sensing layer and another without sensing layer) and the difference in the frequencies of two oscillators can be taken as output. The change in frequency of both oscillators due to change in environment factors will be identical and not affect the differential output. Schematic for the same is given in figure 12.

Wireless Sensors:

On integrating the SAW device with RF amplifier to form oscillator, wireless sensing can be performed. SAW devices with suitable sensing layer as discussed above can be operated in oscillator form. The frequency signals can be transmitted wirelessly and can be received using receiver at far places. The wireless sensors can be installed at remote area and the sensing can be performed.

6. Summary:

It can be seen that Surface Acoustic wave devices find applications in various fields. Not only as filters, resonators and oscillators, SAW device find applications in various sensors. The sensitivity of these kinds of sensors is very  high and detection limit is low. Moreover wireless sensing can be performed using these sensors.