Biomedical Engineering Reference
In-Depth Information
A third generation of piezoelectric devices, after SAW and
QCM, is based on TFBAR [297] that are able to operate up to high-
frequency range. Practical TFBARs are generally silicon-integrated
and consist of a sputtered piezoelectric thin film (e.g., AlN, GaN, or
ZnO) sandwiched between top and bottom electrodes onto which
an alternating electric field is applied to excite surface-confined
acoustic waves and produce resonating characteristics. Silicon-
based mass-sensitive TFBAR resonant sensors can be considered
as high-frequency silicon versions of the well-known quartz crystal
microbalance. Typically, QCM resonators coated with a sensitive layer
are used as mass sensors for gas detection operating at oscillating
frequencies up to 30 MHz, while SAW devices operate in the range
100-1000 MHz. The TFBAR resonance frequency (1-20 GHz) can be
increased by a thinner piezoelectric layer in the range of 1-5
µ
m.
Moreover, the mass sensitivity of TFBAR sensor can be improved,
for same film thickness, by using a piezoelectric layer with higher
acoustic velocity, e.g., AlN (11345 m/s) against ZnO (6370 m/s); or
by using a piezoelectric layer with lower density, e.g., AlN (3260 kg/
m
3
3
). Also, the mass sensitivity increases
with the square of resonance frequency. Hence, the gas sensitivity
could be dramatically enhanced with higher operating frequencies
of TFBAR compared to SAW and QCM devices for a given signal-to-
noise ratio.
Penza
) against ZnO (5665 kg/m
. [292] demonstrated a TFBAR based on vibrating
membrane of AlN/Si
et al
fabricated onto silicon substrate and
functionally characterized as gas sensor at resonating frequency of
1.045 GHz. This novel TFBAR-based gas sensor was functionalized
by a sensing nanocomposite layer, prepared by LB technique, of
SWCNTs embedded in a host matrix of organic material of cadmium
arachidate
N
3
4
High-performance gas detection of SWCNT-coated
TFBAR sensor was reported at room temperature. The sensing
device exhibited high sensitivity (e.g., acetone: 12 kHz/ppm;
ethylacetate: 17.3 kHz/ppm), fast response (within 2-3 min), slow
reversibility (within 1 h) and good repeatability (variation ≤ 5%)
of response toward tested organic vapors of acetone, ethylacetate,
and toluene. Figure 9.41 shows the schematic of TFBAR coated by
LB SWCNT nanocomposite layer and the S
.
parameter measured by
a network analyzer for the unloaded device showing the frequency
characteristics of 1.045 GHz. A typical room temperature response
to various acetone concentrations from 165 to 500 ppm using the
TFBAR sensor coated by 10 monolayers LB SWCNT nanocomposite
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