handbook of modern sensors

17.5 Complex Sensors 517
frequency (wavelength) of the acoustic sensor. Therefore, the oscillating sensor converts
the mass value into a frequency shift. Because frequency and time are the easiest
variables to measure by electronic circuits, the entire sensor’s accuracy is determined
virtually by the ability to assure that the coefficient S m is known and does not change
during the measurement (see Fig. 17.18 as an example of this type of a sensor).
Molecules or larger particles of a chemical compound deposit on the surface of
the crystal increasing its mass and, subsequently, lowering its resonant frequency.
An electronic circuit measures the frequency shift, which is almost a linear measure
of the chemical concentration in the sampled gas. Thus, this method is sometimes
called a microgravimetric technique, as added mass is extremely small. The absolute
accuracy of the method depends on such factors as the mechanical clamping of the
crystal, temperature, and so forth; therefore, the over-the-range calibration is usually
required.
Oscillating sensors are extremely sensitive. For instance, a typical sensitivity is in
the range of 5 MHz cm 2 /kg, which means that 1 Hz in frequency shift corresponds to
about 17 ng/cm 2 added weight. The dynamic range is quite broad: up to 20 µg/cm 2 .To
assure a selectivity, a crystal is coated with a chemical layer specific for the material
of interest.
Another type of a gravimetric detector is a surface-acoustic-wave (SAW) sensor.
The SAW is a phenomenon of propagating mechanical waves along a solid surface
which is in a contact with a medium of lower density, such as air [21]. These waves
are sometimes called Reyleigh waves, after the man who predicted them in 1885.
As with a flextural plate, the SAW sensor is a transmission line with three essential
components: the piezoelectric transmitter, the transmission line with a chemically
selective layer, and the piezoelectric receiver. An electrical oscillator causes the electrodes
of the transmitter to flex the substrate, thus producing a mechanical wave. The
wave propagates along the transmission surface toward the receiver. The substrate
may be fabricated of LiNbO 3 with a high piezoelectric coefficient [22]. However, the
transmission line does not have to be piezoelectric, which opens several possibilities
of designing the sensor of different materials, like silicon. The transmission surface
interacts with the sample according to the selectivity of the coating, thus modulating
the propagating waves. The waves are received at the other end and converted back
to an electric form. Often, there is another reference sensor whose signal is subtracted
from the test sensor’s output.
Typical designs of the acoustic sensors which can be adapted for measuring mass
are covered in Section 12.6 of Chapter 12. Here, we briefly describe the gravimetric
SAW sensor which is adapted for sensing gas concentrations (Fig. 17.14).The sensor is
designed in the form of a flextural thin silicon plate with two pairs of the interdigitized
electrodes deposited by use of the sputtering technology. A thin piezoelectric ZnO
thin film is deposited beneath the electrodes, so that the plate can be mechanically
excited by the external electronic circuit. The piezoelectric film is needed to give
piezoelectric properties to the silicon substrate. The top surface of the sensing plate
is coated with a thin layer of a chemically selective material (or glue, if the sensor
is intended to detect air pollutants). The entire sensor is positioned inside a tube
where the sampled gas is blown through. The left and right pairs of the electrodes are