Biomedical Engineering Reference
In-Depth Information
(a)
6000
5500
V
t
Sezawa2
5000
V
R
Sezawa1
4500
4000
3500
Rayleigh
3000
V
V
R
2500
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Normalized thickness,
hk
(b)
80
300 nm
750 nm
500 nm
400 nm
250 nm
G
S
G
AlN
ZnO
60
Pt
ZnO
Pt
ZnO
Pt
Si substrate
40
‚eory model
Experimental data
20
0
0.0
0.5
1.0
1.5
t
ZnO
(µm)
FIGURE 8.10
(a) Phase velocities for
Rayleigh and Sezawa modes on different thickness of ZnO film. A film normalized thick-
ness,
hk
, is used to describe film thickness effect (
k
= 2
π
/
λ
: wave vector,
h
: film thickness). (From Du, X.Y., PhD
thesis, University of Cambridge, 2008. With permission.) (b) Resonant frequency of ZnO FBAR devices vs. film
thickness. (From Yan et al.,
Appl. Surf. Sci.
, 90, 9372-9380, 2007. With permission.)
(d) For the ZnO/Si SAW devices, the metallization ratio (ratio of thickness of metal
and ZnO film) of the IDTs affects the generation of the guided mode and its
harmonics. For example, if the metallization ratio changes from 0.5 to 0.4 (or
to 0.7) in the ZnO/Si devices, many higher-order odd harmonic waves, such as
the 3rd or 5th Rayleigh mode harmonic, or 3rd Sezawa mode harmonic can be
realized together with the fundamental one (Brizoual et al., 2006, 2008). These
higher mode harmonic waves can reach frequencies of a few GHz using con-
ventional photolithography that potentially makes it unnecessary to use high
acoustic velocity substrate materials (such as diamond) or advanced lithogra-
phy processes (such as e-beam or deep-UV lithography) to make IDTs with
submicron arm widths.
