Surface Acoustic Wave (SAW) Sensors: Physics, Materials, and Applications

Abstract

Surface acoustic waves (SAWs) are the guided waves that propagate along the top surface of a material with wave vectors orthogonal to the normal direction to the surface. Based on these waves, SAW sensors are conceptualized by employing piezoelectric crystals where the guided elastodynamic waves are generated through an electromechanical coupling. Electromechanical coupling in both active and passive modes is achieved by integrating interdigitated electrode transducers (IDT) with the piezoelectric crystals. Innovative meta-designs of the periodic IDTs define the functionality and application of SAW sensors. This review article presents the physics of guided surface acoustic waves and the piezoelectric materials used for designing SAW sensors. Then, how the piezoelectric materials and cuts could alter the functionality of the sensors is explained. The article summarizes a few key configurations of the electrodes and respective guidelines for generating different guided wave patterns such that new applications can be foreseen. Finally, the article explores the applications of SAW sensors and their progress in the fields of biomedical, microfluidics, chemical, and mechano-biological applications along with their crucial roles and potential plans for improvements in the long-term future in the field of science and technology.

Keywords: AlN; GaAs; GaN; IDT; Lamb wave; Love wave; MEMS; Rayleigh wave; SAW devices; SAW wave; SH-wave; ZnO; biosensors; chemical sensors; crystal cut; interdigitated electrodes; lithium niobate; lithium tantalate; point-of-care (POC); surface acoustic waves (SAW).

Figure 1

Classification of SAW applications and their detection parameters.

Figure 2

Classification of the SAW sensing parameters.

Figure 3

Schematic of the Rayleigh waves propagating through a substrate [91].

Figure 4

Schematic of the shear horizontal (SH) waves propagating through a substrate [91].

Figure 5

Schematic of the Lamb waves propagating through a substrate. Mathematical equations are those used in deriving the Lamb wave dispersion [91].

Figure 6

Schematic and mathematics of the Love waves propagating through a surface [91].

Figure 7

SAW generation phenomena in piezoelectric wafer [4].

Figure 8

Different orientational cuts of the Lithium Niobate crystal (top) and the Eulerian angles (bottom).

Figure 9

The delay line configuration setup with and without resonator [17].

Figure 10

Schematic representation of the bi-directional electrodes [2].

Figure 11

Schematic representation of the split electrode configuration [2].

Figure 12

Schematic representation of the SPUDT electrode configuration [2].

Figure 13

Schematic representation of the DART electrode configuration [2].

Figure 14

Schematic representation of the FEUDT electrodes configuration, where electrodes are numbered, 1, 2, 3, 4 designated by light blue, grey, red and dark blue colors, respectively in FEUDT layout [2].

Figure 15

Schematic representation of the dispersive delay line electrode configuration [153].

Figure 16

Schematic representation of the tapered electrode configuration [2].

Figure 17

Focused electrode configuration and F-SAW on liquid manipulation [77]. (a) a droplet is placed into the directed area on which the focused SAWs radiate into the droplet. Jet formation of the droplet at time (b) t = 0 s, (c) t = 0.67 ms and (d) t = 1.33 ms due to focused SAW exposure.

Figure 18

Love-wave sensing platform with PDMS microchannels for biosensing [167].

Figure 19

Using a similar philosophy, different applications of Love-wave based SAW sensing where input frequencies and output frequencies are compared to detect the change in the bio-functionalized layer [171,172,173,174,175,176].

Figure 20

A fundamental schematic showing chemical and gas sensing applications using SAW devices [179].

Figure 21

Different applications and methods of chemical and gas sensing using SAW devices [84,179,180,181,182,183].

Figure 22

Schematic of the pentagon-shaped sensor cell and the gas delivery system [184].

Figure 23

A generic approach for SAW devices with microfluidics [185].

Figure 24

(A) FSAW platform for microfluidic mixing and (B) experimental setup [191].

Figure 24

(A) FSAW platform for microfluidic mixing and (B) experimental setup [191].

Figure 25

Various applications of SAW devices with microfluidics [194,195,196,197,198,199].

Figure 26

Various mechano-biological applications using SAW devices [37,200,201,202]. (A) SAWs for the purpose of cell lysis on a 128° YX-cut lithium niobate wafer, (B) controlling the cell–cell interactions by administering four orthogonal IDTs at a 45° angle to the X-axis of the 128 Y-cut lithium niobate substrate, (C) ZnO/Si thin films surface acoustic waves for the manipulating and 3D patterning of yeast cells and microparticles, (D) 15° tilted microfluidic channel with interdigitated electrodes on a 128° YX-cut lithium niobate wafer to generate a standing SAW.