Surface Acoustic Wave Humidity Sensor: A Review

Abstract

The Growing demands for humidity detection in commercial and industrial applications led to the rapid development of humidity sensors based on different techniques. Surface acoustic wave (SAW) technology is one of these methods that has been found to provide a powerful platform for humidity sensing owing to its intrinsic features, including small size, high sensitivity, and simple operational mechanism. Similar to other techniques, the principle of humidity sensing in SAW devices is also realized by an overlaid sensitive film, which serves as the core element whose interaction with water molecules is responsible for overall performance. Therefore, most researchers are focused on exploring different sensing materials to achieve optimum performance characteristics. This article reviews sensing materials used to develop SAW humidity sensors and their responses based on theoretical aspects and experimental outcomes. Herein the influence of overlaid sensing film on the performance parameters of the SAW device, such as quality factor, signal amplitude, insertion loss, etc., is also highlighted. Lastly, a recommendation to minimize the significant change in device characteristics is presented, which we believe will be a good step for the future development of SAW humidity sensors.

Keywords: SAW; humidity sensing materials; humidity sensor; performance characteristics.

Figure 1

Humidity sensors categories based on sensing mechanism. (a) Resistive (reproduce with permission from Ref. [7], Copyright 2012 Elsevier). (b) Capacitive (reprinted from Ref. [5]). (c) Optical (reproduce with permission from Ref. [10], Copyright 2019 Elsevier). (d) Acoustic wave based (reprinted from Ref. [13]).

Figure 2

Schematic of SAW humidity sensors based on different configurations. (a) Delay line (b) One port resonator (c) Two port resonator.

Figure 3

Illustration of the operating mechanism of the SAW humidity sensor. (a) Sensing film coated on delay path; (b) Application of electrical signal; (c) Sensing layer and water molecules Interaction; (d) Wave propagation through sensing film with absorbed water molecules; (e) Propagating wave at output IDTs.

Figure 4

Sensing materials classification for SAW humidity sensors with SEM images. Reproduce with permission from Ref. [38], Copyright 2015 Elsevier, Ref. [46], Copyright 2012 Elsevier, Ref. [47], Copyright 2002 AIP Publishing and Ref. [39], Copyright 2020 Elsevier.

Figure 5

(a) The effect of electrospray time (b) The effect of concentration of electrospray solution on the frequency response of SAW humidity sensor. Reprinted with permission from Ref. [59] Copyright 2010 Elsevier.

Figure 6

Dynamic responses of SAW humidity sensors. (a) Metal oxide films; (b) SiO₂ films. Reprinted with permission from Ref. [38], Copyright 2015 Elsevier.

Figure 7

(a) SEM images. (b) AFM images of Metal oxides and SiO₂ films. Reprinted with permission from Ref. [38], Copyright 2015 Elsevier.

Figure 8

(a) Respiration monitoring by utilizing a SAW humidity sensor. (b) Real-time breath monitoring of two volunteers. (c) Frequency response to humidity for twelve volunteers (reprinted with permission from Ref. [39] Copyright 2020 Elsevier).

Figure 9

(a) S₁₁ parameters of the bare and SiO₂ sensing film-coated SAW resonators (reprinted with permission from Ref. [40] Copyright 2021 John Wiley & Sons). (b) S₂₁ parameters (c) Time response of the bare SAW device and devices coated with SiO₂, PVA/SiO₂, and 3DAG/PVA/SiO₂ (reprinted with permission from Ref. [39], Copyright 2020 Elsevier). (d) Humidity response of GO-coated SAW sensor (e) Humidity response of MoS₂/GO-coated SAW sensor (reprinted with permission from Ref. [74], Copyright 2022 Elsevier). (f) Humidity response of PVA film coated on IDC reflectors (reprinted with permission from Ref. [79], Copyright 2015 John Wiley & Sons). (g,h) insertion loss as a function of ambient humidity level of PVA and PVP-coated SAW sensors, respectively (reprinted with permission from Ref. [31], Copyright 2011 Elsevier).

Figure 10

Mass sensitivity variation of SiO₂ films. (a) Two port SAW resonator with three IDTs; (b) Measured frequency responses of SiO₂ deposited on the lateral IDTs; (c) Measured frequency responses of SiO₂ deposited at the central IDTs; (d) Mass sensitivity for different areas on SAW resonators, reprinted from Ref. [82].