Biomedical Engineering Reference
In-Depth Information
morphologies. Figure 8.5 shows the cross-sectional SEM images of the ZnO film for dif-
ferent film thicknesses. Columnar grains of ZnO can be observed that are perpendicular
to the surface. This is because ZnO crystals typically grow as long hexagonal rods along
the c-axis, which results in columnar grain structures.
c
-Axis ZnO structures are the pre-
ferred structures for SAW devices used for microfluidics, as these normally require a wave
displacement perpendicular to the surface. X-ray diffraction (XRD) spectra of the ZnO
films of different thickness indicate that most of the films have a single peak at 34.2° which
corresponds to the diffraction from the (002) plane of the ZnO. Atomic force microscopy
(AFM) images of samples grown under different conditions are shown in Figure 8.6. The
surface roughness values for these films are approximately of the order of tens of nano-
meters, indicating that the films are reasonably smooth. Both grain sizes and roughness
increase with thickness. The sputtered ZnO films show significant compressive stress that
is made up of both the intrinsic and thermal components and presents a major challenge
to the SAW devices. The thermal stress is related to the difference between the deposi-
tion temperature and operational temperature of the devices. However, the main cause of
internal stress in ZnO film is compressive and is significantly greater than the thermal stress
with typical values of 1 GPa. This internal compressive stress is a consequence of the high-
energy ion bombardment on the film surface, which can be decreased significantly with
(a)
(b)
1.2 µm
0.27 µm
Acc.V
6.00 kV
Spot
3.0
Magn
06392x
WD
7.0
Exp
1
Acc.V
6.00 kV
Det
SE
Spot
3.0
Magn
34292x
Det
TLD
WD
6.1
Exp
1
200 nm
1 µm
(c)
(d)
6.6 µm
5.2 µm
Acc.V
5.00 kV
Spot
3.0
Magn
9204x
Det
SE
WD
6.3
Exp
1
Spot
3.0
Magn
7738x
Det
TLD
2 µm
Acc.V
5.00 kV
WD
0.4
Exp
1
2 µm
FIGURE 8.5
SEM images of cross section of ZnO films of different thickness. (a) 0.27
μ
m; (b) 1.2
μ
m; (c) 5.2
μ
m; and (d) 6.6
μ
m.
(From Du, X.Y., PhD thesis, University of Cambridge, 2008. With permission.)
