Keywords

1 Introduction

Surface Acoustic Waves (SAW) can be generated at the free surface of an elastic solid [1]. When the solid is a piezoelectric the waves are coupled to potential fields as well. Through interactions with metal electrodes deposited on the solid surface and through digital sampling of these waves, a variety of signal processing operations can be realized. This field is broadly referred as acousto-electronics or ultrasonic-electronics. The foundation for this field was laid way back in 1885 by Lord Raleigh [2] who stated that the behavior of waves upon the plane free surface of infinite homogeneous isotropic elastic solid is such that the disturbance is confined to a superficial region of thickness comparable with the wavelength. Surface acoustic waves are mechanical (acoustic) rather than electromagnetic in nature. However it was only in 1965 when White and Voltmer [1, 2] first exploited the phenomenon of SAW propagation for its application to electronic device. With the invention of interdigital transducer (IDT) that real development of this field began in right earnest [2, 3]. IDT is a metallic comb like structure carved on metallic film deposited on the piezoelectric substrate. SAW can be generated by applying RF voltage to IDT placed on the surface of the piezoelectric substrate. In a SAW device two IDTs are required and are called as input and output IDT. Input IDT converts the high frequency voltage signal into mechanical surface acoustic waves and output IDT converts mechanical SAW vibrations into electrical signal. Surface outside the IDT regions should be elastic, not necessarily being piezoelectric. In SAW devices the piezoelectric material is used to provide the electromechanical coupling. Velocity of SAW waves is of the order of about 105 times less than electromagnetic waves and wavelength is of the order 3 \(\upmu \)m at the frequency of 1 GHz. Because of slow speed the acoustic waves enable one to delay a time varying signal by using very small space. Most common type of SAW devices are delay lines, band pass filters and resonators [4].

SAW propagation is sensitive to a number of different parameters such as temperature, humidity, pressure, etc. These parameters change the electrical characteristics of the SAW devices. Hence SAW devices are widely used in sensing applications. The word sensor is derived from sentire meaning ‘to perceive’. ‘Sensor’ is a device that detects a change in a physical or chemical stimulus and turns it in to signal, which can be measured or recorded. It is like a transducer but the difference between sensor and transducer is to use ‘sensor’ for the sensing element and ‘transducer’ for sensing element plus any associated circuitry. The use of acoustic wave devices for sensor applications has been investigated since the early 1960s [3]. The first acoustic wave sensors were based on bulk acoustic waves. SAW sensors have the potential to offer fully integrated low cost, high performance, solid-state devices for a variety of physical, chemical and biological sensing applications. SAW physical sensors are used to measure the physical changes like acceleration, stress, strain, temperature, pressure, humidity, vacuum and motion [5]. SAW chemical sensors are used to sense the presence of almost any chemical, with detection limits extending down to few parts per trillion [5]. SAW biological sensors are used in medicines, food product industry, environment and defense. Biosensors are sensitive to monitor ingredients of food and drink, food additives, toxic components and contaminants [5]. In SAW devices the whole operation takes place on the surface, the characteristics of the device is susceptible to environmental perturbations on or near the surface.

A SAW device can be imparted sensitivity for a particular chemical species by coating the device surface with a selective adsorbent film. Absorption of chemical species into the sensor film causes change in its properties such as (mass or visco-elasticity). As a result SAW velocity and amplitude are changed. This process of chemical transduction is translated into shift in phase and amplitude of the signal. SAW chemical sensors can be further used in

  • Detection of environmental pollutants [5].

  • Detection of chemical warfare agents in trace level [5].

  • Detection in explosive and narcotics of extremely low amount [5].

  • Process parameter monitoring and control [5].

  • Detection of hazardous chemicals in industrial environment [5].

  • Passive remote sensing at hazardous and inaccessible locations [5].

Various techniques are used to measure the change in SAW properties via phase and amplitude. Wiring the SAW device in an oscillator loop as a frequency-determining element is the simplest and effective method for sensing [6]. In this case the change in phase is visualized as shift in frequency of the oscillator. The oscillator should have low noise, low power consumption, high stability, capable of sustaining mass loading and easy to handle. Electronic oscillators realized using LC tank circuit or RC network in the feedback loop of an amplifier have low Q factor in the range of \(\sim \!\!10^{3}\) and hence stability is less. Oscillator employing piezoelectric crystals have high Q factor of the order of \(\sim \!\!10^{5}\) and hence it shows higher stability and can be used in applications where demand on stability is more [5]. Schematic of SAW sensor has been shown in Fig. 1. It consists of two identical SAW devices one is used for sensing toxic agent and another is used for reference to compensate the temperature and pressure error.

Fig. 1
figure 1

Schematic diagram of interface electronics circuit for SAW sensor

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1.1 Acoustic Wave Sensors

Piezoelectric effect allows transduction of electrical energy into acoustic energy and vice versa [7]. This is exploited to realize a variety of device configurations on piezoelectric crystals for sensor applications. An acoustic wave device in general is comprised of electro-acoustic transducers fabricated on crystal surface. Typically two transducers one for input and other for output are used. The characteristic of acoustic waves (velocity, power) propagating between two transducers is affected by electrical/physical boundary conditions on the surface and the surrounding environment [8]. This is precisely what makes the basis for realizing an acoustic wave sensor. Changes or a perturbation in surface conditions or immediate environment under physical, chemical or biological stimulus causes changes in acoustic wave properties, which can be monitored as electrical signals [5].

1.1.1 Physical Sensor

All acoustic wave sensors are sensitive, to varying degrees, to perturbations from many different physical parameters. As a matter of fact all acoustic wave devices manufactured for the telecommunication industry must be hermetically sealed to prevent any disturbances because they will be sensed by the device and cause an unwanted change in output [5]. But for sensor application requirement is just reversed. Acoustic wave sensors become pressure, torque, shock, strain and force etc. detectors under an applied stress that changes the dynamics of the propagating medium and called as physical sensor.

1.1.2 Chemical Sensor

An acoustic wave sensor becomes chemical sensor when a thin film is applied on piezoelectric substrate that adsorbs only specific chemical. Other techniques involve detection of change in thermal conductivity, electrical conductivity, etc. due to chemical interaction by SAW devices [5].

1.1.3 Biological Sensor

Biosensors are those devices that include as a “front end” bio recognition element that responds to only one biological parameter. In a biosensor the scope for a very high selectivity arises from 3-D structures of the biomolecules and the specific arrangement and location of functional groups. The functional groups located in 3-D structure engage in selective (mostly polar) interactions with molecules having complementary functional groups in proper orientation. Sensitive and selective detection of biochemically active compounds can be achieved by employing antigen–antibody, enzyme–substrate and other receptor–proteins pairs. When coating on acoustic wave sensor absorbs specific biological chemicals in liquid, they become a biosensor [5].

1.2 Surface Acoustic Wave Device

Besides discrete surface acoustic wave sensors which exploit the SAW fundamental principle, SAW resonators can be used as sensing devices. This device consists of a pair of electrodes positioned in a resonant cavity formed by two distributive SAW reflector arrays patterned on crystal surface. These reflector arrays consist of half—wavelength wide metal strips on grooves, when the surface wave is incident on these periodic structures, small amount of wave energy reflected from each discontinuity adds to a given nearly complete reflection (99 %). The high Q and low insertion loss that results make these devices extremely stable when incorporated in an oscillator circuit, resulting in lower detection limits. However reflector arrays add significantly to substrate area occupied by sensor and complicate coating of device surface.

1.3 Advantages of SAW Sensor

SAWs are well suited for either fixed or wireless sensor applications as the specific sensor constituent to be measured typically varies one or more of the SAW’s physical parameters. Surface acoustic wave (SAW) based sensors and sensor systems have following advantages:

Sensitivity(S) of SAW sensor are very high as SAW devices can operate at very high frequency(f) hence we can achieve very high sensitivity \((\mathrm{S}\, \alpha \,\mathrm{f}^{2})\). SAW sensors:

  • have low noise operation hence we can achieve low detection limit

  • fabrication is based on IC planar technology, hence it can be mass producible and low cost [4]

  • are in small size, rugged, reproducible, and reliable

  • have fast response time and they are suitable for individual use

  • can be placed in confined and inaccessible places

  • are small and light weight

  • operate in severe environments such as explosive, corrosive, radiated environments

  • are hermetically sealed so there is no effect of the environmental condition.

1.4 Development of SAW Device for Vapor Sensing

Vapor sensor based on SAW devices were first reported in 1979, most of them rely on the mass sensitivity of the detector, in conjunction with a chemically selective coating that adsorbs the vapor of interest and result in an increased mass loading of the device [5]. In its simplest form SAW sensor consists of a delay line, which consists of an IDT at each end of an appropriate piezoelectric substrate. One IDT acts as transmitter and other acts as the receiver of acoustic energy which travels along the substrate surface. The propagating acoustic wave is free to interact with matter in contact with delay line surface. This wave film interaction results in alteration of wave characteristic such as amplitude, phase, velocity, harmonic contents etc. which are the basis of SAW chemical sensors. SAW delay line sensors are most easily configured to monitor either changes in SAW amplitude or SAW velocity. Amplitude measurements are made using an apparatus in which the Rayleigh wave is excited using an RF power source and the power at receiving end of SAW delay line is measured with RF power meter [5]. Measurement of SAW velocity is made indirectly by using SAW delay line in oscillator configuration. The resonant frequency of device is altered by changes in velocity of acoustic wave and can be measured accurately using a digital frequency counter. Experience has shown that measurement of SAW velocity provides a superior precision compared to SAW amplitude perturbation so we concentrate more on velocity measurement techniques.

1.5 Issues and Approaches for SAW Sensor Realization

The following issues are to be approached for the realization of SAW vapor sensor

  • Stability

  • Sensitivity

  • Selectivity

  • Response time

  • Reversibility

  • Reliability

  • Temperature and pressure effects.

Sensing device should be fairly stable so that change in frequency can easily be distinguished due to chemical interaction. For sensing application highly stable systems are not required [5]. Several design considerations must be satisfied when selecting and applying the chemically sorptive coating. Ideally, the coating film should be completely reversible, meaning that it will absorb and then completely desorbs the vapor when purged with clean air. The rate at which the coating absorbs and desorbs should be fairly quick, \(<\)1 s, for instance. The coating should be robust enough to withstand corrosive vapors. It should be selective, absorbing only very specific vapors while rejecting others. The coating film must operate over a practical temperature range. It should be stable, reproducible, and sensitive. And finally, its thickness and uniformity are very important [9]. Temperature, pressure and humidity affect the performance of SAW sensor. To tackle temperature and pressure effects we can use one device as sensor and another device as reference, which effectively minimizing the effects of temperature and pressure variations. For humidity control we incorporate a filter, through which the desired vapor passes and which absorbs the moisture from the sample.

1.6 Application of SAW Sensor

SAW sensors are extremely versatile devices that are just beginning to realize their commercial potential. They are competitively priced, inherently rugged, very sensitive, and intrinsically reliable, and can be interrogated passively and wirelessly. The range of phenomenon that can be detected by SAW devices can be greatly expanded by coating the devices with materials that undergo changes in their mass, elasticity, or conductivity upon exposure to some physical or chemical stimulus. These sensors become pressure, torque, shock, and force detectors under an applied stress that changes the dynamics of the propagating medium. They become mass or gravimetric sensors when particles are allowed to contact the propagation medium, changing the stress on it. They become vapor sensors when a coating is applied that absorbs only specific chemical vapors. These devices work by effectively measuring the mass of the absorbed vapor. If the coating absorbs specific biological chemicals in liquids, the detector becomes a biosensor.

2 Surface Acoustic Waves

Surface acoustic wave is guided along the surface, with its amplitude decaying exponentially with depth. The wave is strongly confined, with typically 90 % of energy propagating within one wavelength of the surface. It is non-dispersive, with a velocity of typically 3,000 m/s [10]. A bounded medium also supports many other type of acoustic waves, and the boundary condition can substantially affects the nature of the waves. On the other hand, a medium with dimensions much larger than the wavelength can support waves with characteristic similar to those of wave in an infinite medium. The term bulk wave is often used to describe waves, which are not bounded to the medium. Surface acoustic waves can be generated at the free surface of elastic solid. In the SAW devices the generation of such waves is achieved by application of a voltage to metal film interdigital transducer (IDT) deposited on the surface of a piezoelectric solid [5, 9].

2.1 Interdigital Transducer

The most efficient and widely used means of generating and detecting surface elastic waves for electronic applications is the interdigital transducer shown in Fig. 2. In its simplest form it consists of a series of parallel metal plate electrodes periodically spaced on the surface of a piezoelectric substrate. The transducer is a three-port device: one electrical and two acoustic. When an alternating voltage is applied to its electrical port, time varying electric fields are set up within the substrate, and these excite alternating stress pattern via the piezoelectric effect. The electric field is reversed at each electrode, and therefore at frequencies for which the periodic length lambda of the array is equal to the wavelengths of elastic surface waves (synchronous), the waves are launched most efficiently in both directions normal to the electrodes [10].

Fig. 2
figure 2

Inter digital transducer (IDT)

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The space between them known is the wavelength of the mechanical wave \((\lambda )\). We can express the operating frequency \(f_{0}\) of the device with SAW velocity \((v_{p})\) and wavelength \((\lambda )\) as:

$$\begin{aligned} f_{o} = \frac{V_{p}}{\lambda } \end{aligned}$$
(1)

3 Fabrication of SAW Device for Toxic Gas Detection

A SAW device is fabricated on a ST (42.75\(^\circ \) Y-X) piezoelectric quartz substrate. The quartz substrate is known to have high electromechanical coupling coefficient \((K^{2})\) and low-temperature coefficient of frequency (TCF) [2]. A SAW device was comprised of an input interdigital transducer (IDT), a mass loading area and an output IDT, as shown in Fig. 1. The dimension of the one electrode is 11.26 \(\upmu \)m and wave length is 45.06 \(\upmu \)m. An Al thin film with a thickness of 200 nm was evaporated on a cleaned ST-quartz substrate by thermal evaporation technique and then, the IDT pattern was formed by standard photolithography process and steps for the SAW device fabrication are shown in Fig. 3. Once the IDTs are formed on the substrate, sensitive film of the desired gas has been deposited on the mass loading area. Terminals are connected for the testing of the device. Two identical devices have been fabricated.

Fig. 3
figure 3

SAW device fabrication flow chart

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3.1 Deposition of Sensing Layer on SAW Device

In SAW sensor, for obtaining significant sensitivity and selectivity for the measurement of a given analyte requires a chemical interface, henceforth referred to as “the coating”. The coating, which should be physically or chemically bound to the sensor surface, may consist of a solid adsorbent, a chemical reagent, or a sorptive liquid or polymer [5, 11]. The coating acts as a chemically sensitive and selective element that immobilizes a finite mass of some chemical species from the environment. Resultant changes in physical and/or chemical properties of the coating, in turn perturb the underlying acoustic wave device. Perturbations of acoustic waves resulting from interactions of the coating with one or more analytes constitute the basis for detection and quantification [12].

3.2 General Coating Requirements

An attractive feature of acoustic-wave-based chemical sensors is that they impose relatively few constraints on the materials that can be used as chemically selective coatings. The film should have following properties to work as an excellent chemical interface:

  • Uniformity

  • Adherent

  • Thin chemically and physically stable when in contact with its working medium (gas or liquid)

  • Non conductive.

Typically, uniformity in film thickness is not crucial, but can be important in some circumstances. Assuming all parts of the film fall within the acceptable thickness limits discussed in the following, and that the particular film being examined has been calibrated, then under conditions of equilibrium between the analytes in the ambient phase and the film, variations in film thickness are unimportant. If, however, transient measurements are of interest- one means of identifying a chemical species is its rate of permeation through a given material-then uniformity becomes very important: non-uniformities in thickness will “smear out” the transient response, making identification difficult. Also, if device-to-device reproducibility is important, then the film must be quite uniform, unless aluminum films can be fabricated with the same set of non-uniformities. Uniform coverage of the acoustic wave path is of some importance as well. In the case of SAW delay line, the propagating wave front is typically fairly linear; a film that covers some parts of the wave path to a greater extent than others causes parts of the wave front adversely affects the signal-to-noise ratio. The selected materials must adhere to the device surface in such a manner that it moves synchronously with the acoustic wave and must maintain this adhesion in the presence of expected analytes and interferants. The adhesion of thin film to many types of surfaces, including those that are chemically very dissimilar to the coating material is also very crucial. Often, adhesion promoting inter layers have been developed for general classes of problems, such as securing a highly monopolymer film to a very polar substrate [13]. Highly conductive coatings (i.e., all metals and most semiconductors) must, of course, be electrically insulated from IDT electrodes in order to prevent shorting; this is not a concern with the planar electrodes of TSM quartz resonators [14]. This is readily accomplished by either deposition using a line-of-sight technique, e.g., vacuum evaporation, with a protective stencil interposed between the vice and the source; or depositing over the entire surface, and then chemically removing the conductive film from the IDTs (the film’s chemical nature differ considerably from that of the IDT material in this case [15]. A final constraint is that the coating be acoustically thin. A somewhat standard rule of thumb is that thickness of the film less than 1 % of the acoustic wavelength are appropriate [5, 16]. In fact, whether or not a particular coating thickness is acoustically thin depends critically upon the acoustic properties of that material under the particular set of conditions (temperature, nature and concentration of contacting gaseous species, etc.) being evaluated. In other words, it is the acoustic wavelength in the coating film that is relevant; this can differ appreciably from the acoustic wavelength in the device substrate, particularly in materials such as rubbery polymers, which have vastly different sound velocities than, for example, quartz. In practice, coating thickness from a few angstroms to several \(\upmu \)m have been utilized in sensing applications [17].

3.3 Coating Methods

Numerous methods have been described in the literature for depositing coatings onto piezoelectric acoustic sensors. They generally fall into three categories [5, 18]:

  • Solvent casting techniques,

  • Vacuum deposition techniques, and

  • Vapor-phase deposition techniques.

Solvent Casting Techniques: Solvent casting is perhaps the simplest coating method. It requires that the coating material be soluble in a solvent that does not chemically attack the piezoelectric sensor device and its transducers. Once the coating material is dissolved the solution can be spread over the device and the solvent evaporated to leave the desired coating material. Popular techniques in this category include [5, 19]:

  • Syringe deposition,

  • Painting with small brushes or Q-tips,

  • Dipping,

  • Spraying, and

  • Spin casting.

The coating reproducibility that is achievable with syringe deposition and painting can range from poor to good depending on the material used and the skill of the person applying the coating.

In any of the solvent casting techniques, less than a few micrograms of coating material are sufficient to completely coat the sensor device with a film of the appropriate thickness. Thus, the solutions used are usually quite dilute, the sequence of which is that solvent purity and equipment cleanliness must be carefully considered. Spray coating is performed by aspirating a dilute solution of the coating through an atomizing nozzle using a compressed-gas propellant (an inexpensive tool available at art supply stores, the air brush, is often utilized for this process). The fine, atomized mist of solution droplets is propelled toward the device they impact, stick, and evaporates, thereby leaving the non-volatile coating. Like the syringe- and paint-brush-deposited films, the coatings formed by this procedure often have somewhat irregular texture and coverage, but good reproducibility in thickness is possible, particularly if the acoustic sensor is operating during deposition process: monitoring the sensor output signal during deposition allow the apparent thickness of the coating to be measured in real time [20]. Spin casting generally offers the highest degree of film uniformity; and the greatest film-to-film thickness reproducibility. A commercially available “spinner” of the type used in the deposition of photoresist films for lithography, holds the substrate face-up on a motor-driven vacuum chuck that can be spun at hundreds to several thousand rpm. The surface of the device is then “flooded” with a viscous solution of the coating. When the motor is turned on, centrifugal and aerodynamic forces cause all but a thin layer of the solution to “fly” off the device surface; spinning continues long enough (10–60 s) for the vast majority of the solvent to evaporate, inhibiting recoalescence of film droplets. The uniformity and thickness reproducibility of the resulting film are often excellent. Film thickness is controlled by varying spinning speed and the solution viscosity through choice of solvent and concentration of coating material [21]. Dip coating, particularly using the Langmuir-Blodgett (LB) technique can be very reproducible. However, this method does require that the coating have ambiphilic properties (i.e., the individual molecules must possess a polar end and a nonpolar end separated by an intervening chain or body of a few atoms) and be somewhat water insoluble in order to form a stable monolayer at the air/water interface of the LB deposition trough. The required trough is commercially available but relatively expensive. For coating materials that do form stable monolayers, the LB deposition technique affords exquisite control of film thickness, since only a single monolayer of coating molecules is transferred to the device surface with each dip through the air/water interface. LB film have the advantage over other materials that they are highly ordered in two dimensions, a result of the alignment of all the molecules at the air/liquid interface to film deposition. The consequence of this is more predictable, precisely controlled chemical and physical properties than is available with the molecularly disordered materials produced with many other deposition techniques [22].

Fig. 4
figure 4

Non uniform polymer film deposited on the SAW device

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3.4 Interface Coating on SAW Device

The device has been coated with a solution having polymer “bisphenol” which is sensitive for DMMP nerve agent. Before formation of thin coating film, the device was cleaned with de ionized (DI) water and acetone to remove any particulate if sitting on the device. The device was coated through a process of “drop coating” using a micro-syringe. The liquid solution of \(\sim \)1 \(\upmu \)l is deposited on the surface and the device is then baked for 5 h at \(110\,^{\circ }\mathrm{C}\). Unfortunately, thin film is not uniformly coated on the SAW device, which is very necessary for desired electrical characteristics of the sensor. It is clearly observed in the Fig. 4 that the coating of polymer over the device is non-uniform. The non-uniform coating of sensing film over the SAW device may occur due to improper cleaning of the device surface. The uncleaned particles may reduce the surface energy of the device for proper adhering of the film. To improve the uniformity, the surface of the device was then plasma cleaned with the help of plasma cleaning machine (Make ROTH & RAU). Plasma is generated by ionizing argon gas using microwave. This ionized gas will strike the surface of the device with very high kinetic energy removing any particle on the surface. Immediately after plasma cleaning, we coat the polymer on SAW device and the film was baked at \(110\,\,^{\circ }\mathrm{C}\) for 5 h. The device surface with uniform coating suitable for sensing application is shown Fig. 5.

Fig. 5
figure 5

Uniform polymer film deposited on the SAW device

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4 Characterization of SAW Device

The packaged device has been characterized by NA (Network Analyzer) model no 3577A and 35677A using S-parameter test set-up to determine the center frequency and insertion loss of the device which is the important parameter of SAW sensor and required for its oscillator design. The response of the packaged device is shown in Fig. 6. The central frequency is around 70 MHz. SAW oscillators have been successfully used in a number of applications where high frequency and low noise are important conditions. SAW oscillators are of high spectral purity, small power consumption and size for fundamental frequency of operation up to GHz range i.e. the oscillators can be realized in the range from about 20 MHz to 3 GHz. SAW oscillators have high value of quality factor (defined as reciprocal of fraction of energy stored to fraction of energy dissipated per radian). It is now possible to construct SAW oscillators with Q value of 1,00,000. For a fundamental frequency of 100 MHz it is possible to have \(\mathrm{Q} = 10^{5}-10^{7}\) (instead of only 4,000–5,000 for the simple delay line). The power dissipation is comparably very low in a SAW oscillator. SAW oscillator uses a SAW device in the feedback circuit. These oscillators are called SAW oscillators because SAW device in the feedback circuit is used for frequency control. The SAW device is connected to a wide band amplifier in a feedback loop and provides stability through frequency selective feedback. The sensor electronics includes the signal processing and the display units.

Fig. 6
figure 6

Response of the SAW device for its central frequency

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4.1 Condition for Oscillation

The two necessary conditions required to obtain oscillations are:

Fig. 7
figure 7

SAW oscillator

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Total phase shift around the loop must be equal to a multiple of \(2\pi \). Amplifier gain must exceed the combined loss of the delay element. Schematic of SAW oscillator is shown below in Fig. 7. The output of the amplifier is connected to transmitter IDT and the input of amplifier is connected to receiver IDT. To maintain oscillation the product of amplifier gain ‘A’ and feedback gain ‘\(\beta \)’ should be equal to unity i.e.

$$\begin{aligned} \left| {A\beta } \right| =1\end{aligned}$$
(2)
$$\begin{aligned} \angle A\beta =2n\pi \end{aligned}$$
(3)

These conditions must be satisfied simultaneously for obtaining the oscillations. For realizing the circuit, the methodology adopted has been shown in Fig. 7 and every aspect of that is discussed in the forth-coming headings.

4.2 Amplifier Design

The chosen SAW device work in RF range hence to make an oscillator, the role of the RF amplifier is critical. It needs to have enough gain over the required frequency range of operation and should be stable in itself [23]. An attempt has been made to design an amplifier, which may be capable of supporting oscillations with 70 MHz. SAW filter device is mounted in a SAW package. Keeping a speculation in mind that the total losses may not exceed 30 dB, the amplifier has been designed using INA12063 amplifier IC [24].

RF Amplifier: It is one of the major building blocks of modern RF circuits. The amplifier is usually required to provide low noise gain with low distortion at both small and large signal levels. It should also be stable, i.e. not generate unwanted spurious signals, and the performance should remain constant with time. The input and output match is also desirable. Thus for amplifier we require:

  • Low noise

  • Low distortion

  • Stable operation

  • Filtering of unwanted signal

  • High bandwidth.

For SAW oscillator the gain of the amplifier should be greater than the loss of the device so that sustained oscillation should be generated. Amplifier should be properly biased as per application. Careful attention must be given to the design of the SAW filter and its RF amplifier. Optimal performance can be accomplished by having a large number of pairs of interdigital electrodes and series inductor accurately tuned with them to achieve maximum power transfer with the RF amplifier. The RF amplifier itself should be operated with moderate gain and should have low noise figure. Proper heat sinking of the RF amplifier will help to minimize thermally induced electronic phase shift variations [25]. By keeping above issues in the mind amplifier has been designed with INAl2063 which is a 1.5 GHz low noise self-biased transistor amplifier. The salient features of the INA12063 have been listed below:

  • Integrated active bias circuit

  • Single positive supply voltage

  • Adjustable current from 1 to l0 mA

  • 2 dB noise figure at 900 MHz

  • 25 dB gain at 100 MHz.

The amplifiers have tuning facility through an inductor and capacitors [26]. But for tuning, most of the times only experience plays an important role. For fixing \(\mathrm{R}_{\text {bis}}\) the formula is used is:

$$\begin{aligned} \mathrm{R}_{\text {bis}}= 10(\mathrm{V}_{\mathrm{d}}-0.8)/{\mathrm{I}_\mathrm{c}} \end{aligned}$$
(4)

where \(\mathrm{V}_{\mathrm{d}}\) is device voltage and \(\mathrm{I}_{\mathrm{c}}\) is the desired collector current.

For our circuit design we have chosen \(\mathrm{V}_{\mathrm{d}}= 3\mathrm{V}, \mathrm{I}_{\mathrm{c}}= 5\,\mathrm{mA}\), Hence \(\mathrm{R}_{{\text {bis}}}=~4.4\,\mathrm{K}\Omega \)

Because \(4.4\,\mathrm{K}\Omega \) resistance was not available so we are using \(4.6\,\mathrm{K}\Omega \) resistance for biasing. The amplifier has provision to be tuned on both the input and output side. But tuning cannot be done simply with formula:

$$\begin{aligned} \mathrm{F}=1/2\pi \surd \mathrm{{LC}} \end{aligned}$$
(5)

It is also done with trial and error and experience.

4.3 Mixer

The difference in frequency between the sensor oscillator and the reference one is the signal of our interest, which carries information about chemical species. In order to find out the frequency difference between two oscillators, first both the frequencies are mixed and then the difference frequency is extracted using a low pass filter. Mixer is a non-linear resistance having two sets of inputs terminals and one set of output terminals. When two signals of different frequencies are applied to two terminals, the output terminal will have several frequencies including the difference frequency between the two input frequencies. The most common type of mixers utilized are bipolar transistors, FET, dual gate MOSFET and integrated circuits. Depending on the application, one can choose any of them [27].

4.4 Low Pass Filter

Out of various mixed frequencies, the information carrying frequency is the difference frequency. In order to separate it from the other frequencies available at the output of the mixer, a low pass filter is needed. It is expected that the difference frequency will be of the order of a few kHz only. In order to accommodate the default difference between the reference and sensor oscillator, the cutoff frequency of the filter has been kept at 1 MHz. A passive low pass filter of the form shown in Fig. 8 has been utilized for this purpose. This filter did not require any complicated design techniques. The components values have been chosen as per the formula:

$$\begin{aligned} \mathrm{f}_{\mathrm{c}} = 1/ (2\pi \mathrm{RC}) \end{aligned}$$
(6)
Fig. 8
figure 8

First order low pass filter circuit

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However, by using higher order filter response can be improved further. The loss in the passive filter is expected to be in limit. Otherwise also an amplifier following the filter can make up the thing. It is expected that the loss in the passive filter will be affordable as the mixer has an in-built amplifier. The cutoff frequency is around 1 MHz.

4.5 Testing of Amplifier

The RF amplifier has been designed which is capable of supporting oscillations at 70 MHz frequency with SAW filter device mounted in the package. Keeping a speculation in mind that the total losses may not exceed 30 dB, the amplifiers have been designed using INA12063 instrumentation type amplifier IC. INAl2063 which is an 1.5 GHz low noise self-biased transistor amplifier have 25 dB gain at 100 MHz. The performance of the amplifier has been determined experimentally and the frequency response of the amplifier has been shown in Fig. 9. From the graph, we have observed that the peak is at around 70 MHz and gain is almost flat (23 dB) from 60 to 90 MHz range. Thus frequency range from 60 to 90 MHz has been used for analyzing the performance of the SAW device in presence of vapor molecules to be detected. 70 MHz SAW device is connected in feedback of amplifier circuit and we have recorded the oscillator signal on spectrum analyzer. The output of the spectrum analyzer is shown in Fig. 10.

Fig. 9
figure 9

Frequency response curve of the RF amplifier

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Fig. 10
figure 10

70 MHz SAW oscillator output

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5 Working of SAW Sensor

SAW device, when coated with polymer coating behaves as a SAW sensor. SAW devices as a vapor sensor are attractive because of their small size (\(<\!0.1\,\mathrm{cm}^{3}\)), ruggedness, low cost, electronic output, sensitivity and adaptability to a wide variety of vapor phase analytical problems. The presence of thin organic film on the SAW device surface can potentially cause attenuation of the wave through interaction with both longitudinal and the vertical shear components. Attenuation increases with the length of the SAW device and also with the inverse of the wavelength (i.e. operating frequency) in relation with oscillator circuit performance. If the adjacent medium is thick enough (i.e. its thickness is greater then \(\lambda \)), then compressional wave is generated. Ambient gases easily meet this condition but for film, which is very thin (less than \(\lambda \)/100) compressional wave cannot exist and can therefore be neglected. Thus, the attenuation produced by thin organic coating film will be caused primarily through interactions with the shear component. In this situation the viscosity, elastic modulus and thickness of the organic coating will significantly affect the attenuation of the shear component. For organic film whose thickness is less than one percent of the acoustic wavelength attenuation is very small. This level of attenuation can be quite significant for SAW delay line oscillator since the loss introduced by the film make it more difficult to sustain oscillation in the device. The coating, through either physisorption or chemisorption, must interact selectively with the vapor of interest to cause a change in mass or mechanical property of the coating. While the SAW oscillator is easily capable of detecting monolayer adsorption on to its surface, greater sensitivity can be obtained by using a thicker coating film through which the vapor can diffuse and interact with greater number of sorption sites than on a smooth surface. Vapor diffusion rate and device response time is closely related. Clearly film that is very dense will not permit easy diffusion and the device will respond very slowly. For polymeric coating rapid diffusion is most easily obtained if the polymer is highly amorphous and well above its glass transition temperature. For such a film the shear modulus is probably small enough to make it mechanical contribution to SAW oscillator frequency a negligible quantity. Thus, incremental changes in film density or thickness (i.e., mass per unit area) will result in a corresponding resonant frequency change. Volatility of most liquids is high enough to cause a steady baseline drift due to evaporation. Interaction involving chemisorption will usually be quite selective but irreversible under normal conditions and will be most useful in dosimetry applications where an integral signal related to the concentration and time of exposure to a particular vapor is desired. The large dynamic range capability of SAW oscillator and the low cost of the device permit irreversible interaction to be considered as practical system. For reversible responses, interaction-involving physisorption will be required. Unfortunately the relatively low energies of adsorption associated with physisorption processes could permit several different vapors with similar properties to interact with the coating film. The sensitivity and size of SAW delay oscillator sensors is directly related to their resonant frequencies. Frequency shift for a given mass loading will increase with the square of the operating frequency. Frequency shift obtained for a given mass loading is independent of the length of the SAW delay line. Mass loading effects is given by Sauerbrey’s equation [28, 29]:

$$\begin{aligned} \Delta f = -{\mathcal {C}}\frac{f_{0}^{2}}{A} \Delta m \end{aligned}$$
(7)

Where \(\Delta \mathrm{m}\) is the mass change of quartz surface, \(\Delta \mathrm{f}\) is the frequency change caused by \(\Delta \mathrm{m}\), \(f_{0}\) is the fundamental resonant frequency of SAW device in the absence of the deposited mass, A is the geometric area of electrode and C is constant. The area of a SAW delay line with constant IDT impedance from device to device and constant number of wavelengths will vary inversely with the square of the operating frequency [30]. The reductions of the device area also have implication with regard to the minimum mass change that can be detected by a SAW delay line oscillator. Since the device is sensitive to mass per unit area, reductions in device area result in a corresponding reduction in the minimum detectable mass change.

6 Development of a Sensor Head

Sensor head basically a covering for the SAW devices so that they could be exposed only to the chemical intended. It is shown in Fig. 11. The 70 MHz SAW sensor is basically designed to test the presence of a nerve agent called ‘Sarin’. Since it is very harmful and dangerous for testing at laboratory level, another material called DMMP (Di-methyl methyl phosponate) has been used for testing. DMMP is less harmful and has the same properties as Sarin.

Fig. 11
figure 11

Schematic diagram of the sensor cell

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7 Detection Mechanism

The detection of chemical analytes can be based on changes in one or more of physical characteristics of thin film or layer in contact with the device surface. Some of the intrinsic film properties that can be utilized for detection include mass/area, elastic stiffness (modulus) viscoelasticity, viscosity electrical conductivity, and permittivity. In addition, changes in extrinsic variables such as temperature and pressure also produce a sensor response [31, 32]. SAW sensor works as a mass sensor, when analytes of interest comes in contact of polymer film, which is coated, on SAW device. Desired toxic gas is exposed to the device through the carrier gas such as \(\mathrm{N}_{2}\) and then gas molecules are desorbed [5, 33, 34]. Device has been tested for determining the characteristics such as (i) frequency shift with different concentration of vapor (ii) transient response for response and recovery times (iii) transient response for reproducibility of the sensor. Since arrangement has been made to compensate the

error due to temperature variation with signal conditioning circuit, effect of temperature variation has not been studied. Figure 12 shows the scheme of the experimental setup and Fig. 13 shows the photograph of the sensor head with sensor and signal conditioning circuit.

Fig. 12
figure 12

Block diagram of the experimental set up

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Fig. 13
figure 13

Photograph of the sensor cell with detection electronics circuit

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8 Results

Due to adsorption of chemical analytes on the device, the frequency of SAW oscillator is shifted. The frequency shift is then recorded in the PC through data acquisition system. Initially the sensor was refreshed by passing \(\mathrm{N}_{2}\) gas and then exposed to a certain concentration of analytes. The response curve for several cycles at fixed concentration of 0.5 ppm is shown in Fig. 14. During refreshing, the device output is returned to its initial oscillation frequency. The output of the device is highly reproducible. From the transient curve shown in Fig. 15 the response time of the sensor is 16 s, recovery time is 45 s and base line is very stable. There is almost 700 Hz frequency change due to change in vapor concentration of 0.5 ppm. Results show that there is significant change is frequency due to very small change is analytes. Thus the device has the potential for detecting chemical war fare agents. Response

curve with the variation of toxic gas in different concentration is shown in Fig. 16. It is observed that as the concentration level increases, the frequency shift of the detection electronics circuit increases. But the increase in frequency shift is nonlinear. This is normally the situation that most of the SAW devices show nonlinear response curve. This may be due to the fact that adsorption and desorption phenomenon do not occur in similar rate. For direct display, this nonlinearity can be compensated by analog or digital method of compensation.

Fig. 14
figure 14

Transient response curve of the SAW sensor for determining reproducibility

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Fig. 15
figure 15

Transient response curve for determining response and recovery time

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Fig. 16
figure 16

Frequency response of the sensor at different concentration of DMMP

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9 Conclusion

The present work deals with theory, design and application of SAW device for chemical war fare agent detection. A CW sensor with high frequency SAW device has been realized and tested. For this purpose SAW device has been fabricated on STX-quartz using standard photolithography. Fabricated device has been packaged and characterized using vector network analyzer. A high frequency amplifier has been designed and fabricated for SAW oscillator realization. SAW oscillator has been tested using the spectrum analyzer. For realizing the sensor a selective interface (polymer) film has been optimized on SAW device. A sensor head had been developed for the packaging of the device with the provision of gas inlet and out let port. Sensing experiments with DMMP a (stimulant of CW agents) had been carried. Sensor shows very fast response time which is the prime requirement for CW detection. Also sensor shows good sensitivity as it can detects in sub ppm range with good amount of frequency shift and it also shows good base line stability as it return to its base frequency after purging the sensor. Hence this work developed insight about integral approach for a product realization for CW agent detection.