Biomedical Engineering Reference
In-Depth Information
devices. This makes the integration of FBAR arrays for parallel detection both feasible and
low cost. Gabl et al. (2004) used a label-free FBAR gravimetric biosensor with a high oper-
ating frequency of 2 GHz based on a ZnO film to detect DNA and protein molecules. Its
sensitivity of 2400 Hz cm
2
/ng is about 2500 times higher than that of a conventional QCM
device with a frequency of 20 MHz. A recent study using an Al/ZnO/Pt/Ti FBAR design
gave a sensitivity of 3654 kHz cm
2
/ng with a good thermal stability (Lin et al., 2008; Yan
et al., 2007).
Conventional (0001) textured ZnO FBARs operate using a longitudinal wave and can-
not be used for sensing in a liquid environment. In contrast, a (11-20) textured ZnO film
exhibits pure shear modes waves that can propagate in a liquid with little damping effect.
A ZnO shear mode FBAR device has been used in a water-glycerol solution, with a high
operating frequency of 830 MHz and a sensitivity of 1000 Hz cm
2
/ng (Link et al., 2007).
Weber et al. (2006) fabricated a ZnO FBAR device that operated in a transversal shear
mode, using a ZnO film with 16° off
c
-axis crystal orientation. For an avidin/anti-avidin
system, the fabricated device had a high sensitivity of 585 Hz cm
2
/ng and a mass detection
limit of 2.3 ng/cm
2
. This shear wave FBAR device has also been reported to have a stable
TCF (Link et al., 2006).
Compared with ZnO films, AlN is a promising material for FBAR devices owing to
its advantageous characteristics, including high BAW velocity, moderate electromechani-
cal coupling constant, high electric resistivity, low dielectric loss, good chemical stabil-
ity, good thermal stability, and a wide band gap (Chiu, 2007, 2008). PE AlN-based FBAR
devices have already been successfully commercialized (Kim et al., 2001; Tadigadapa
et al.,
2009).
For liquid FBAR sensing, a good idea is to develop ZnO or AlN films in which the
c
-axis
is inclined relative to the surface normal, thus allowing both longitudinal and shear wave
modes to be generated (with one example shown in Figure 8.20a) (Weber, 2006). Another
popular method is to use lateral field excitation (LFE) of the PE layer, requiring both signal
and ground electrodes being in-plane and parallel on the exposed surface of the ZnO or
AlN film. A laterally excited ZnO thickness shear mode resonator has been reported that
(a)
(b)
Al
Al
g
ZnO
Network
analyzer
SiO
2
In
Out
W
AlN
Al
SiO
2
W
Si
Liquid
Si
SiO
2
W
Si
FIGURE 8.20
(a) Schematic illustration of a shear mode FBAR resonator together with a microfluidic transport system.
(From Weber et al.,
Sens. Actuators
, A128, 84-88, 2006. With permission.) (b) A laterally excited ZnO thickness
shear mode resonator that is both extremely simple to fabricate and highly sensitive to surface perturbations.
Resonator configuration consists of a laterally excited, solidly mounted ZnO thin film resonator that incorpo-
rates use of an acoustic mirror. (From Dickherber et al.,
Sens. Actuators A
, 144, 7-12, 2008; Corso et al.
J. Appl.
Phys.
, 101, 054514, 2007. With permission.)
