Surface acoustic wave mass sensor

A piezoelectric mass sensor is a device that measures the amount of material adsorbed on its surface by the effect of the adsorbed material on the propagation of acoustic waves. Piezoelectric devices work by converting electrical energy to mechanical energy. There are a number of different piezoelectric mass sensors. Thickness shear mode sensors measure the resonant frequency of a quartz crystal. Surface acoustic wave mode sensors measure the amplitude or time delay. Flexure mode devices measure the resonant frequency of a thin Si3N4 membrane. In shear horizontal acoustic plate mode sensors, the resonant frequency of a quartz crystal is measured. 

J.W. Grate, S.N. Kaganove, V,R, Bhethanabotla Examination of mass and modulus contributions to thickness shear mode and surface acoustic wave vapour sensor responses using partition coefficients. Pp. 259-83. 

MEMS-Based Acoustic Wave Biosensors Introduction A MEMS-based acoustic wave biosensor is a chemical sensor which detects changes in resonant frequency of a mechanical resonator when biomolecules are adsorbed on the surface of a biologically active membrane. Since frequency change can be measured very precisely, very small mass changes can be measured. This leads to high sensitivity of the biosensors. Typical acoustic wave biosensors are bulk acoustic wave (BAW) and surface acoustic wave (SAW) sensors. 

Ricco AJ, Martin SJ (1992) Thin metal film characterization and chemical sensors monitoring electronic conductivity, mass loading and mechanical properties with surface acoustic wave devices. Thin Solid Films 206 94-101 Ricco AJ, Martin SJ, Zipperian TE (1985) Surface acoustic wave gas sensor based on film conductivity changes. Sens Actuators 8 319-333... 

Both propagation modes of acoustic waves may be utilized to construct mass-sensitive chemical sensors. Wave propagation through the bulk is the basis of so-called bulk acoustic wave (BAW) devices. Propagation along the surface is used in surface acoustic wave (SAW) sensors. 

A new chemical sensor based on surface transverse device has been developed (99) (see Sensors). It resembles a surface acoustic wave sensor with the addition of a metal grating between the tranducer and a different crystal orientation. This sensor operates at 250 mH2 and is ideally suited to measurements of surface-attached mass under fluid immersion. By immohi1i2ing atra2ine to the surface of the sensor device, the detection of atra2ine in the range of 0.06 ppb to 10 ppm was demonstrated. 

Fig. 21. A surface acoustic wave dual-delay line oscillator. The sensitise layer is placed in the propagation path of one of the two SAW devices. The differenee in Ireqnency (At) between the two channels provides a dtrecl result of the mass loading and electric field effects associated w ith the sensor layer...

There are several applications of ZnO that are due to its excellent piezoelectric properties [28,164]. Examples are surface-acoustic wave (SAW) devices and piezoelectric sensors [28,165-167]. Typically, SAW devices are used as band pass filters in the tele-communications industry, primarily in mobile phones and base stations. Emerging field for SAW devices are sensors in automotive applications (torque and pressure sensors), medical applications (chemical sensors), and other industrial applications (vapor, humidity, temperature, and mass sensors). Advantages of acoustic wave sensors are low costs, ruggedness, and a high sensitivity. Some sensors can even be interrogated wirelessly, i.e., such sensors do not require a power source. 

The addition of mass provides the means of transduction for many chemical sensors, including surface acoustic wave (SAW) devices, quartz crystal microbalances (QCM), and microcantilevers. In all these devices, the mass addition either perturbs the vibration, oscillations, or deflection within the transducer. The mode of transduction in an optical interferometer can also be linked to mass addition the sensor s response is altered by refractive index changes in the material being monitored. It is possible that this change can be elicited solely from refractive index changes without the addition of mass, although in sensing a particular... 

Another state-of-the-art detection system contains a surface acoustic wave (SAW) device, which is based on a piezoelectric crystal whose resonant frequency is sensitive to tiny changes in its mass—it can sense a change of 10-1° g/cm2. In one use of this device as a detector it was coated with a thin film of zeolite, a silicate mineral. Zeolite has intricate passages of a very uniform size. Thus it can act as a molecular sieve, allowing only molecules of a certain size to pass through onto the detector, where their accumulation changes the mass and therefore alters the detector frequency. This sensor has been used to detect amounts of methyl alcohol (CH3OH) as low as 10 9 g. 

The adsorption and desorption isotherms of an inert gas (classically N2 at 77 K) on an outgassed sample are determined as a function of the relative pressure (Prei = p/Po/ the ratio between the applied pressure and the saturation pressure. The adsorption isotherm is determined by measuring the quantity of gas adsorbed for each value of p/po by a gravimetric or a volumetric method (less accurate but simpler). A surface acoustic wave device can also be used as a mass sensor or microbalance in order to determine the adsorption isotherms of small thin films samples (only 0.2 cm of sample are required in the cell) [42,43]. 

Surface acoustic waves (SAW), which are sensitive to surface changes, are especially sensitive to mass loading and theoretically orders of magnitude more sensitive than bulk acoustic waves [43]. Adsorption of gas onto the device surface causes a perturbation in the propagation velocity of the surface acoustic wave, this effect can be used to observe very small changes in mass density of 10 g/cm (the film has to be deposited on a piezoelectric substrate). SAW device can be useful as sensors for vapour or solution species and as monitors for thin film properties such as diffusivity. They can be used for example as a mass sensor or microbalance to determine the adsorption isotherms of small thin film samples (only 0.2 cm of sample are required in the cell) [42]. 

---