Elsevier

Thin Solid Films

Volume 520, Issue 3, 30 November 2011, Pages 986-993
Thin Solid Films

Surface acoustic wave-based gas sensors

https://doi.org/10.1016/j.tsf.2011.04.174Get rights and content

Abstract

A brief review of the Surface Acoustic Wave (SAW)-based gas sensors and their physical operation background in a dual-delay line oscillator system is presented together with some own achievements dealing with SAW layered sensor structures.

Introduction

Surface Acoustic Waves were first explained in 1885 by Lord Rayleigh, who described the surface acoustic mode of propagation and predicted its properties in his classic paper [1]. A surface acoustic wave (SAW) is an acoustic wave traveling along the surface of a material exhibiting elasticity, with an amplitude that typically decays exponentially with depth into the substrate. The main properties of SAW of the Rayleigh type are as follows: penetration depth (~ 1 wavelength) — the great majority of energy is localised in the surface region — high energy density, elliptical particle trajectory, propagation wave velocity (typical 10 5 of light), at the same frequency the acoustic wavelength is 10 5 times that of electromagnetic waves. In a similar way the surface displacements are 10 5 times that of SAW wavelength. The high energy density in the surface region of the substrate is the key feature for the high sensitivity of the SAW-based gas sensors.

In the case of a piezoelectric substrate, which is mainly used in sensor technology, an electric field is also associated with SAW propagation. The perpendicular direction dependence of this field is the source of the acoustoelectric coupling and can be an important factor in sensor sensitivity for some specially designed sensor elements like bilayer structures — discussed in detail in 5 Layered sensor structures, 6 Conclusions and future trends.

Named after their discoverer, Rayleigh waves have a longitudinal and vertical shear component that can couple with any media in contact with the surface —Fig. 1. This coupling strongly affects the amplitude and velocity of the wave, allowing SAW sensors to directly sense mass, mechanical and electrical properties (in case of piezoelectric substrates). The Rayleigh SAW macroscopic visualization in a delay line configuration is presented in Fig. 1, whereas the visualization of the other types of acoustic waves like: acoustic plate mode (APM), Lamb wave, surface transverse wave (STW) and Love wave can be found in Ref. [2].

Here must be pointed out that the piezoelectric substrate is connected only with the ease of exciting the surface wave by interdigital transducers (IDTs), which have been discovered by Voltmer and White in 1965 [3]. If we excite the surface wave on a non-piezoelectric substrate then we can create a sensor based only on mechanical effects.

The place of the Rayleigh SAW sensors in a whole Acoustic Wave Sensors “family” is presented in Fig. 2. We can distinguish two main branches with bulk and surface waves. From the point of view of the different configuration of the sensor elements, the Rayleigh SAW-based gas sensors can be made with single and layered sensor structures — with different materials and manufacturing technology — discussed in more details in Section 5 “Layered sensor structures”.

The first review paper on “Surface Acoustic Wave Sensors” by R.White appeared one hundred years after their discovery [4]. Next (in 1989), we can find the first review paper on SAW sensors for atmospheric gas monitoring by Ch.Fox and J.Alder [5]. In 1993 report papers appear by J. Grate et al. on Acoustic Wave Microsensors [6]. In 1999 — a review paper by J. Cheeke and Z. Wang [7] and in 2001, by Dorozhkin and Rozanov [8] on Acoustic wave gas sensors. We can find an excellent book position entitled: “Surface-Launched Acoustic Wave Sensors” by M.Thomson and D. Stone [9]. Recently we can find interesting reviews on Bioanalytical applications by T. Gronewold [10] and biosensors by K. Laenge, B. Rapp and M. Rapp [11]. Table 1 shows the most important historical steps in acoustic wave sensors by the publication year, journal, name(s), device type with working area and some important details.

The main idea of the SAW-based gas sensor is shown in Fig. 3. On a piezoelectric substrate where the surface wave is excited, a thin sensor layer is created using any depositing technique. If the sensor material can absorb gas molecules from the surrounding atmosphere, then the boundary conditions for the propagating surface wave are changed and consequently the velocity and attenuation of the wave undergo a change. These changes can be further detected with great accuracy in an electronic system — mainly in oscillators. The velocity of propagation of the surface wave in a system shown in Fig. 1 depends on various factors (on quite a number of factors), the most important of which are presented in relation to the parameters of the substrate and the layer. Particularly in the case of sensitive thin semiconductor layers the propagation of the waves is much perturbed due to adsorption of the particles of some toxic gases at the surface of the layer.

Surface acoustic wave gas sensors are very attractive because of their remarkable sensitivity which are due to changes in the boundary conditions for the propagating wave, introduced by the interaction of active material with specific gas molecules. This unusual sensitivity results from the simple fact that most of the wave energy is concentrated near the crystal surface within one or two wavelengths. Consequently, the surface wave is in its first approximation highly sensitive to any changes of the physical or chemical properties of the thin active layer previously placed on the crystal surface. As long as the thickness of the sensor material, h, is substantially less than the surface wave wavelength, λ, we can speak of a perturbation of the Rayleigh wave. Otherwise, we have to take into account other types of waves, such as Love waves, which can propagate in layered structures [9], [17], [18].

The essence of gas sensors can be presented in the form of three basic couplings, which are applicable not only for SAW-based gas sensors. We can distinguish the first coupling between gas molecules and sensor element parameters, which is the most important from the sensor point of view. Next is the second one — between sensor element parameters and the detection system. Here, important is the fact that the detection system must “feel” the subtle changes of the sensor element parameters. The third coupling relies on signal analysis and conversion to gas concentration —Fig. 4.

In the case of SAW-based gas sensors the second coupling is indirect — we can observe the changes of the sensor elements parameters as a result of wave velocity and/or wave attenuation modulation.

According to the three above mentioned basic couplings, there are three main directions of evolution using SAW-based gas sensors. The first direction is concerned with improvement of the first coupling, like new sensor materials of the sensor elements. For instance: cyclodextrin for organic vapours made by self-assembled monolayers (SAM) and sol–gel technology [20]. Metal ion/cryptand-22 complex like Ag(I)/cryptand-22 was found to be a very good cover for alkene and alkyne detection [21]. The UV crosslinked polysiloxanes with different side group substitution for organic vapours detection [22]. The rf sputtered InOx and PECVD SiNx films for H2 and O3 detection at different operation temperatures [23].

Besides these, carbon nanotubes (CNT) have been used for carbon dioxide [24] and organic vapours sensing as well [25]. Lately a modified diamond nanoparticles (DNPs) were used as coatings for volatile chemicals detection [26].

The second coupling improvement is connected with sensor parameters modulation and the influence on the velocity and attenuation of the SAW. Here we can lately distinguish between multi-strip couplers (MSC) suppressing bulk acoustic waves [27], single phase unidirectional transducer (SPUDT) — not entirely a new idea, but applicable in SAW sensors improving the wave excitation [28], and layered sensor structures enabling acoustoelectric interactions based on the conductivity effect — described in Section 5 “Layered sensor structures”.

The third coupling relies on signal analysis of the SAW sensor and conversion to gas concentration. Here we can use the well-known methods of pattern recognition or neural analysis — not discussed in this review.

Section snippets

Factors which perturb the propagation of the surface wave

In this point the main factors which perturb the surface acoustic wave propagation will be described.

With reference to the material of the substrate, these factors may be divided into two fundamental groups, viz. interior and exterior ones. To the former belongs, first of all, the influence of the substrate temperature on the velocity of propagation of the surface wave. This influence is, of course, connected with the temperature coefficients of the piezoelectric crystals, differing from each

SAW-based gas sensor in dual-delay line oscillator

In practice, the experimental set-up is often based on frequency changes in a surface acoustic wave dual-delay line oscillator system shown in Fig. 5, which is nowadays well known [9], [38], [39], [40], [41], [42], [43], [44].

On a piezoelectric substrate, usually LiNbO3 or ST-cut quartz, two identical acoustic paths are formed using interdigital transducers. Next, an active thin sensor layer structure is formed in the measuring line, for example by a vacuum deposition process or any other

Response profiles

The changes in measuring frequency can have different response profiles, which can be shown according to [9] in Fig. 8. We can distinguish an ideal response connected with a pure mass effect, irreversible chemisorption, combined mass and elastic effects and large elastic or acoustoelectric response. Here the response profiles have been expanded to the large acoustoelectric interaction, which is especially expected for layered sensor structures.

The examples of the practically achieved own SAW

Layered sensor structures

In SAW-based gas sensors, mainly the single sensor structures, were used to detect specific gas concentrations [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58]. The example is a single palladium film, well known for hydrogen molecules detection [38]. The similar own result has been shown in Fig. 10, where for palladium thin film 20 nm, only mass effect is observed, which is very small in case of light hydrogen molecules.

The layered sensor structures (like bilayers of

Conclusions and future trends

Surface acoustic wave-based gas sensors are very attractive because of their remarkable sensitivity which can be adjusted by the film thickness in a layered nanostructure sensor system. The layered nanostructure (semiconductor + metal) creates new possibilities for gas sensing in a SAW sensor with the use of an acoustoelectric coupling between the surface wave and the sensor structure. The “work point” of such a structure must be shifted to the high sensitivity region, where even small variations

Acknowledgements

The author is very grateful to Prof. J. Szuber and Prof. M. Urbańczyk from SUT Gliwice, Poland, for helpful discussions and Dr. E. Maciak and Dr. K. Gut also from SUT Gliwice, Poland, for technological help.

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