This review presents an overview of 20 years of worldwide development in the field of biosensors based on special types of surface acoustic wave (SAW) devices that permit the highly sensitive detection of biorelevant molecules in liquid media (such as water or aqueous buffer solutions). 1987 saw the first approaches, which used either horizontally polarized shear waves (HPSW) in a delay line configuration on lithium tantalate (LiTaO3) substrates or SAW resonator structures on quartz or LiTaO3 with periodic mass gratings. The latter are termed “surface transverse waves” (STW), and they have comparatively low attenuation values when operated in liquids. Later Love wave devices were developed, which used a film resonance effect to significantly reduce attenuation. All of these sensor approaches were accompanied by the development of appropriate sensing films. First attempts used simple layers of adsorbed antibodies. Later approaches used various types of covalently bound layers, for example those utilizing intermediate hydrogel layers. Recent approaches involve SAW biosensor devices inserted into compact systems with integrated fluidics for sample handling. To achieve this, the SAW biosensors can be embedded into micromachined polymer housings. Combining these two features will extend the system to create versatile biosensor arrays for generic lab use or for diagnostic purposes.

Surface acoustic wave biosensors: a review

A review of miniaturised humidity sensors is presented. Recent achievements in capacitive-, hygrometric-, gravimetric-, optical- and integrated sensors are discussed. Attention is paid to a general perspective of miniaturised humidity sensors, emphasising on integration issues and technological challenges. A table summarising most of the reviewed devices is included.

Recent achievements in miniaturised humidity sensors—a review of transduction techniques

https://www.repository.cam.ac.uk/bitstream/1810/264501/1/Fu_et_al-2017-Progress_in_Material_Science-VoR.pdf

Advances in piezoelectric thin films for acoustic biosensors, acoustofluidics and lab-on-chip applications

Abstract Surface acoustic wave (SAW) devices offer many attractive features for application as vapour phase chemical microsensors. This paper describes the characteristics of SAW devices and techniques by which they can be employed as vapour sensors. The perturbation of SAW velocity by polymeric coating films is investigated both theoretically and experimentally. Highest sensitivity can be achieved when the device is used as the resonating element in a delay line oscillator circuit. A simple equation has been developed from theoretical considerations which offers reasonably accurate quantitative predictions of SAW device frequency shifts when subjected to a given mass loading. In this mode the SAW device behaves very like conventional bulk-wave quartz crystal microbalances except that the sensitivity can be several orders of magnitude higher and the device size can be several orders of magnitude smaller. Detection of mass changes of a few femtograms by a SAW device having a surface area of 10−4 cm2 is theoretically possible.

Mechanism of operation and design considerations for surface acoustic wave device vapor sensors

A brief overview of acoustic wave sensor physics, materials, sensor types, and applications is presented in this paper. Emphasis is placed on the different types of acoustic wave sensors, their respective advantages, and their specific applications in industry.

Acoustic wave technology sensors

Surface acoustic wave (SAW) devices have been studied for the last twenty years as highly sensitive yet relatively inexpensive microsensors for applications ranging from temperature and stress to gas and biological sensing. This wide range of applications is due to the SAW microsensors' high sensitivity to several physical parameters including mass, temperature, stress, and conductivity. Their low cost results from the use of standard batch microelectronic fabrication techniques for their manufacture. In this paper several chemical sensing applications for SAW devices are described. These include: gas detection; thin-film polymer characterization; dew-point measurements; surface energy measurements; and as a method to measure surface cleanliness. Experimental results are presented along with comparisons to other measurement techniques.

http://iopscience.iop.org/article/10.1088/0964-1726/6/6/002/pdf

Surface acoustic wave microsensors and applications

Acoustic temperature sensors have the advantages of a high-resolution frequency output and ease of integration with other acoustic sensors but require hermetic packaging to prevent sensor contamination. Surface-skimming bulk-wave (SSBW) devices have been found to be much less sensitive to surface contamination than other acoustic devices, and although their temperature response has been studied extensively, they have not been studied specifically as temperature sensors. Surface acoustic wave (SAW) based chemical sensors requiring temperature measurement or control are susceptible to temperature measurement error because the temperature cannot be measured in the same location as the chemical sensor. The objectives of this work were to examine the temperature characteristics and performance of a SSBW temperature sensor when integrated with a SAW condensation and humidity sensor in a novel design. The SSBW temperature sensor had over an order of magnitude less sensitivity to condensation and water uptake in certain polyimide films than an integrated SAW gas sensor indicating that this design is practical for sensing films in the delay path where film thickness is carefully considered.

A novel integrated acoustic gas and temperature sensor

Surface acoustic wave (SAW) devices have been studied for the last twenty years as highly sensitive yet relatively inexpensive microsensors for applications ranging from gas and biological sensing to thin film and surface characterization. This wide range of applications is due to SAW microsensors high sensitivity to several physical parameters including mass, conductivity, permittivity, stress, temperature and electric fields. Their low cost results from the use of standard batch microelectronic fabrication techniques for their manufacture. In this work several SAW sensing applications are described. These include: gas detection; thin film polymer characterization; dew-point measurements; surface energy measurements; and as a method to measure surface cleanliness. Experimental results are presented along with comparisons to other measurement techniques.

Acoustic temperature sensors have the advantages of high-resolution, frequency output, and ease of integration with other acoustic sensors but require hermetic packaging to prevent sensor contamination This slows sensor response time and increases cost Surface skimming bulk wave (SSBW) devices have been found to be much less sensitive to surface contamination than other acoustic devices however they have not been studied as temperature sensors The objectives of this work were to study the contamination sensitivity of a 124 MHz quartz SSBW device as a temperature sensor, as well as calibration methods The SSBW device had over an order of magnitude less sensitivity to mass loading than a similar SAW device The temperature coefficient of frequency (TCF) was 32 PPM/°C with a deviation from a second order curve fit of ±022°C over a 78°C temperature range

Temperature measurement using surface skimming bulk waves

The increased use of thin film polymers in microelectronic applications has resulted in the need to better understand their chemical, thermal, mechanical, and electrical properties. Of particular interest are changes in mass and viscoelasticity during curing of new high temperature polymers. A highly sensitive technique that can monitor mass and viscoelastic changes in thin polymer films during curing to high temperature is needed. In this work a surface acoustic wave (SAW) based system was developed that was capable of measuring the mass loss due to water outgassing during cure of thin polymer films in a temperature range of 20 to 400/spl deg/C. It also could measure the apparent glass transition temperature of acoustically thin films, and film resonance for acoustically thick films. The principle limitations of the system are the limited accuracy of temperature compensation and the limited ability to separate mass loss effects from viscoelastic effects.

R. D. Mileham

Papers

Surface acoustic wave microsensors and applications

A novel integrated acoustic gas and temperature sensor

Surface acoustic wave microsensors and applications

Temperature measurement using surface skimming bulk waves

Surface acoustic wave thermogravimetric measurements of thin polymer films